U.S. patent application number 12/003741 was filed with the patent office on 2013-06-13 for nanostructures and methods for manufacturing the same.
This patent application is currently assigned to QuNano AB. The applicant listed for this patent is Bjorn Jonas Ohlsson, Lars Ivar Samuelson. Invention is credited to Bjorn Jonas Ohlsson, Lars Ivar Samuelson.
Application Number | 20130146835 12/003741 |
Document ID | / |
Family ID | 30118388 |
Filed Date | 2013-06-13 |
United States Patent
Application |
20130146835 |
Kind Code |
A1 |
Samuelson; Lars Ivar ; et
al. |
June 13, 2013 |
NANOSTRUCTURES AND METHODS FOR MANUFACTURING THE SAME
Abstract
A resonant tunneling diode, and other one dimensional
electronic, photonic structures, and electromechanical MEMS
devices, are formed as a heterostructure in a nanowhisker by
forming length segments of the whisker with different materials
having different band gaps.
Inventors: |
Samuelson; Lars Ivar;
(Malmo, SE) ; Ohlsson; Bjorn Jonas; (Malmo,
SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Samuelson; Lars Ivar
Ohlsson; Bjorn Jonas |
Malmo
Malmo |
|
SE
SE |
|
|
Assignee: |
QuNano AB
|
Family ID: |
30118388 |
Appl. No.: |
12/003741 |
Filed: |
December 31, 2007 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10613071 |
Jul 7, 2003 |
7335908 |
|
|
12003741 |
|
|
|
|
60393835 |
Jul 8, 2002 |
|
|
|
60459982 |
Apr 4, 2003 |
|
|
|
Current U.S.
Class: |
257/13 |
Current CPC
Class: |
C30B 11/12 20130101;
C30B 11/00 20130101; H01L 29/66318 20130101; H01L 31/068 20130101;
Y10S 977/763 20130101; C30B 25/00 20130101; H01L 29/20 20130101;
C30B 25/02 20130101; H01L 29/882 20130101; C30B 29/605 20130101;
C30B 25/183 20130101; Y02E 10/547 20130101; C30B 25/02 20130101;
C30B 29/605 20130101; C30B 29/605 20130101; Y10S 977/762 20130101;
C30B 25/18 20130101; H01L 29/7371 20130101; C30B 29/406 20130101;
C30B 25/14 20130101; H01L 31/035227 20130101; C30B 29/403 20130101;
C30B 11/00 20130101; H01L 31/048 20130101; H01L 29/068 20130101;
H01L 29/0665 20130101; H01L 33/04 20130101; H01L 29/0673
20130101 |
Class at
Publication: |
257/13 |
International
Class: |
H01L 33/04 20060101
H01L033/04 |
Claims
1. A photonic crystal, comprising a substrate, and an array of
one-dimensional nanoelements extending from one side of the
substrate, each element extending upright from the substrate, and
having a substantially constant diameter of nanometer dimension,
wherein the array of nanoelements is arranged in a two-dimensional
lattice, whereby to provide a photonic band gap for incident
electromagnetic radiation; and wherein the nanoelements are spaced
apart by a distance of about 300 nm.
2. A photonic crystal according to claim 1, wherein the diameter of
each nanoelement is not greater than about 100 nm.
3. (canceled)
4. A photonic crystal according to claim 1, wherein each
nanoelement comprises a nanowhisker having a plurality of
lengthwise segments of a first type, comprised of a material having
a first refractive index and having a first predetermined length,
said segments of said first type alternating with at least one
segment of a second type, comprised of a material having a second
refractive index and having a second predetermined length, said
first and second refractive indices and said first and second
predetermined lengths being selected to form a three dimensional
photonic crystal.
5. A photonic crystal according to claim 1, wherein each
nanoelement comprises a semiconductor light emitting
nanowhisker.
6-9. (canceled)
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Divisional of U.S. application Ser.
No. 10/613,071, filed on Jul. 7, 2003, which claims the benefit of
the priority of U.S. Provisional Application No. 60/393,835, filed
Jul. 8, 2002, and of U.S. Provisional Application No. 60/459,982,
filed on Apr. 4, 2003, each of which is incorporated herein by
reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates generally to structures, essentially
in one-dimensional form, and which are of nanometer dimensions in
their width or diameter, and which are commonly known as
nanowhiskers, nanorods, nanowires, nanotubes, etc.; for the
purposes of this specification, such structures will be termed
"one-dimensional nanoelements". More specifically, but not 20
exclusively, the invention relates to nanowhiskers, and to methods
of forming nanowhiskers.
[0004] 2. Brief Description of the Prior Art
[0005] The basic process of whisker formation on substrates, by the
so-called VLS (vapor-liquid-solid) mechanism is well known. A
particle of a catalytic material, usually gold, for example, on a
substrate is heated in the presence of certain gases to form a
melt. A pillar forms under the melt, and the melt rises up on top
of the pillar. The result is a whisker of a desired material with
the solidised particle melt positioned on top--see "Growth of
Whiskers by the Vapour-Liquid-Solid Mechanism"--Givargizov--Current
Topics in Materials Science Vol. 1, pages 79-145--North Holland
Publishing Company 1978. The dimensions of such whiskers were in
the micrometer range.
[0006] International Application WO 01/84238 discloses in FIGS. 15
and 16 a method of forming nanowhiskers wherein nanometer sized
particles from an aerosol are deposited on a substrate and these
particles are used as seeds to create filaments or nanowhiskers.
For the purposes of this specification the term nanowhiskers is
intended to mean one dimensional nanoelements with a diameter of
nanometer dimensions, the element having been formed by the VLS
mechanism.
[0007] Typically, nanostructures are devices having at least two
dimensions less than about 1 .mu.m (i.e., nanometer dimensions).
Ordinarily, layered structures or stock materials having one or
more layers with a thickness less than 1 .mu.m are not considered
to be nanostructures, although nanostructures may be used in the
preparation of such layers, as is disclosed below. Thus the term
nanostructures includes free-standing or isolated structures having
two dimensions less than about 1 .mu.m which have functions and
utilities that are different from larger structures and are
typically manufactured by methods that are different from
conventional procedures for preparing somewhat larger, i.e.,
microscale, structures. Thus, although the exact boundaries of the
class of nanostructures are not defined by a particular numerical
size limit, the term has come to signify such a class that is
readily recognized by those skilled in the art. In many cases, an
upper limit of the size of the dimensions that characterize
nanostructures is about 500 nm.
[0008] Where the diameter of a nanoelement is below a certain
amount, say 50 nm, quantum confinement occurs where electrons can
only move in the length direction of the nanoelement; whereas for
the diametral plane, the electrons occupy quantum mechanical
eigenstates.
[0009] The electrical and optical properties of semiconductor
nanowhiskers are fundamentally determined by their crystalline
structure, shape, and size. In particular, a small variation of the
width of the whisker may provoke a considerable change in the
separation of the energy states due to the quantum confinement
effect. Accordingly, it is of importance that the whisker width can
be chosen freely, and, of equal importance, is that the width can
be kept constant for extended whisker lengths. This, together with
the possibility of positioning whiskers at selected positions on a
substrate, will be necessary if an integration of whisker
technology with current semiconductor component technology is to be
possible. Several experimental studies on the growth of GaAs
whiskers have been made, the most important reported by Hiruma et
al. They grew III-V nano-whiskers on III-V substrates in a metal
organic chemical vapor deposition--MOCVD--growth system--K. Hiruma,
M. Yazawa, K. Haraguchi, K. Ogawa, T. Katsuyama, M. Koguchi, and H.
Kakibayashi, J. Appl. Phys. 74, 3162 1993; K. Hiruma, M. Yazawa, T.
Katsuyama, K. Ogawa, K. Haraguchi, M. Koguchi, and H. Kakibayashi,
J. Appl. Phys. 77,4471995; E. I. Givargizov, J. Cryst. Growth 31,20
1975; X. F. Duan, J. F. Wang, and C. M. Lieber, Appl. Phys. Lett.
76, 1116 2000; K. Hiruma, H. Murakoshi, M. Yazawa, K. Ogawa, S.
Fukuhara, M. Shirai, and T. Katsuyama, IEICE Trans. Electron. E77C,
1420 1994; K. Hiruma, et al, "Self-organised growth on GaAs/InAs
heterostructure nanocylinders by organometallic vapor phase
epitaxy", J. Crystal growth 163, (1996), 226-231. Their approach
relied on annealing a thin Au film to form the seed particles. In
this way, they achieved a homogeneous whisker width distribution,
the mean size of which could be controlled by the thickness of the
Au layer and the way this layer transforms to nanoparticles. With
this technique, it is difficult to control the size and surface
coverage separately, and it is virtually impossible to achieve a
low coverage. The correlation between film thickness and whisker
thickness was not straightforward, since the whisker width also
depended on growth temperature, and there were even signs of a
temperature-dependent equilibrium size of the Au particles. The
authors also noticed a strong correlation between the size of the
Au droplets de-posited from a scanning tunneling microscope tip and
the resulting whisker width. For the free-flying Si whiskers grown
by Lieber et al.,--Y. Cui, L. J. Lauhon, M. S. Gudiksen, J. F.
Wang, and C. M. Lieber, Appl. Phys. Lett. 78, 2214, 2001--a clear
particle-whisker size correlation has been shown.
[0010] It is necessary, if whiskers are to be used as electrical
components, that there should be well-defined electrical junctions
situated along the length of a whisker, and much work has been
directed at achieving this--see for example Hiruma et al, "Growth
and Characterisation of Nanometer-Scale GaAs, AlGaAs and GaAs/InAs
Wires" IEICE Trans. Electron., Vol. E77-C, No. 9 Sep. 1994, pp
1420-1424. However, much improvement is necessary.
[0011] Much work has also been carried out on carbon nanotubes
(CNTs). Despite progress, research has been frustrated by a lack of
control of the conductivity-type of CNTs and an inability to form
1D heterostructures in a controlled manner. Randomly formed
interfaces as kinks between metallic and semiconducting parts of
CNTs have been identified and studied (Yao et al, Nature, 1999,
402, 273) as have doping (pn) junctions in semiconducting CNTs
(Derycke et al, Nano Letters, 2001, 1,453) and transitions between
CNTs and semiconductor (Si and SiC) nanowhiskers (Hu et al, Nature,
1999, 399, 48).
[0012] In a separate trend of development, attempts to fabricate 1D
devices have been made since the late 1980s by top-down methods, as
pioneered by Randall, Reed and co-workers at Texas Instruments--M.
A. Reed et al., Phys. Rev. Lett. 60, 535 (1988). Their top-down
approach, which still represents the state of the art for this
family of quantum devices, is based on epitaxial growth of
multi-layers defining the two barriers and the central quantum
well. Electron-beam lithography is then used to define the lateral
confinement pattern, together with evaporation of the metallic
layers to form the top contact. A lift-off process is then used to
remove the e-beam-sensitive resist from the surface, and reactive
ion etching removes all the material surrounding the intended
narrow columns. Finally, the devices are contacted via the
substrate and from the top using a polyimide layer. In the studies
of devices fabricated by this bottom-up technique, 100-200 nm
diameter columns have been observed, however, with rather
disappointing electrical characteristics and peak-to-valley
currents at best around 1.1:1. An alternative approach to realizing
low-dimensional resonant tunneling devices has been reported more
recently, employed strain-induced formation of self-assembled
quantum dots (I. E. Itskevich et al., Phys. Rev. B 54, 16401
(1996); M. Narihiro, G. Yusa, Y. Nakamura, T. Noda, H. Sakaki,
Appl. Phys. Lett. 70, 105 (1996); M. Borgstrom et al., Appl. Phys.
Lett. 78, 3232 (2001)).
SUMMARY OF THE INVENTION
[0013] The invention comprises a method of forming nanowhiskers,
one-dimensional semiconductor nanocrystals, in which segments of
the whisker have different compositions, for example indium
arsenide whiskers containing segments of indium phosphide, wherein
conditions for growth allow the formation of abrupt interfaces and
heterostructure barriers of thickness from a few monolayers to
hundreds of nanometers, thus creating a one-dimensional landscape
along which electrons can move. In a preferred method of chemical
beam epitaxy method (CBE), rapid alteration of the composition is
controlled by the supply of precursor atoms into a eutectic melt of
seed particle and substrate, supplied as molecular beams into the
ultra high vacuum chamber. The rapid switching between different
compositions is obtained via a sequence where growth is interrupted
or at least reduced to an insignificant amount, and supersaturation
conditions for growth are reestablished; at least, change of
composition and supersaturation is changed faster than any
appreciable growth. With abrupt changes in material of the whisker,
stresses and strains arising from lattice mismatch are accommodated
by radial outward bulging of the whisker, or at least by lateral
displacement of the atoms in the lattice planes near the
junction.
[0014] Further, the invention includes a technique for the
synthesis of size-selected, epitaxial nano-whiskers, grown on a
crystalline substrate. As catalysts, size-selected gold aerosol
particles are used, which enables the surface coverage to be varied
completely independently of the whisker diameter. The whiskers were
rod shaped, with a uniform diameter between 10 and 50 nm,
correlated to the size of the catalytic seed. By the use of
nano-manipulation of the aerosol particles, individual
nano-whiskers can be nucleated in a controlled manner at specific
positions on a substrate with accuracy on the nm level. The method
of the invention enhances width control of the whisker by virtue of
choice of nanoparticle. The nanoparticle may be an aerosol or a
liquid alloy on the substrate may be made by starting from gold
rectangles formed on the substrate which when melted form accurate
diameter balls. Other materials may be used instead of gold as the
seed particle, e.g. Gallium.
[0015] Whilst it is desirable in many applications to have
nanowhiskers which are essentially constant in diameter, the shape
of the whisker, and other attributes, may be varied by selectively
changing the diffusion constant (diffusion coefficient) of the
group III material, e.g. Ga, during whisker formation. This can be
done by: [0016] Lowering the temperature of the process--this
produces whiskers tapered towards their free ends; [0017]
Increasing the pressure of the group V material; [0018] Increasing
the pressure of both group V and group III materials.
[0019] More specifically, the invention provides a method of
forming a nanowhisker comprising:
[0020] depositing a seed particle on a substrate, and exposing the
seed particle to materials under controlled conditions of
temperature and pressure such as to form a melt with the seed
particle, so that the seed particle melt rises on top of a column
whereby to form a nanowhisker, the column of the nanowhisker having
a diameter with a nanometer dimension;
[0021] wherein during the growth of the column, selectively
changing the compositions of said materials whereby to abruptly
change the composition of the material of the column at regions
along its length, whilst retaining epitaxial growth, whereby to
form a column having along its length at least first and second
semiconductor segment lengths, the first semiconductor segment
being of a material having a different band gap from that of the
second semiconductor segment.
[0022] Functional 1D resonant tunneling diodes and other components
and structures have been obtained via bottom-up assembly of
designed segments of different semiconductor materials in III/V
nanowhiskers. Electronic and photonics components comprising
nanowhiskers have also been formed as heterostructures, with a
single crystal formation, wherein length segments of the
nanowhisker are of different materials, so as to create well
defined junctions in the whisker between different band gap
materials, whereby to create a component with a desired
function.
[0023] Thus, the invention provides in general terms a
heterostructure electronic or photonics component, comprising a
nanowhisker having a column of a diameter with a nanometer
dimension, the column having disposed along its length a plurality
of length segments of different material composition with
predetermined diametral boundaries between adjacent segments
extending over a predetermined length of the nanowhisker column,
such as to give desired band gap changes at the boundaries, in
order to enable the component to carry out a desired function.
[0024] In a general aspect, the invention provides an electronic or
photonic component, comprising a nanowhisker having a column with a
diameter, which has a nanometer dimension,
[0025] the column comprising along its length at least first and
second length segments of different materials with an abrupt
epitaxial composition boundary disposed between the first and
second segments, wherein lattice mismatch at the boundary is
accommodated by radial outward expansion of the nanowhisker at the
boundary.
[0026] In another general aspect, the invention provides an
electronic or photonic component, comprising a nanowhisker having a
column with a diameter, which has a nanometer dimension,
[0027] the column comprising along its length at least first and
second length segments of different materials with an abrupt
epitaxial diametral material boundary disposed between the first
and second segments, wherein the transition between the composition
of the different materials of the first and second segments occurs
over an axial distance of not more than eight diametral lattice
planes. Preferably, the transition between the composition of the
first and second segment occurs over an axial distance of not more
than 6, lattice planes, preferably not more than 5 lattice planes,
still more preferably not more than 4 lattice planes, still more
preferably not more than 3 lattice planes, still more preferably
not more than 2 lattice planes and most preferably not more than
one lattice plane.
[0028] In a further aspect, the invention provides an electronic or
photonic component, comprising a nanowhisker having a column with a
diameter which has a nanometer dimension, the column comprising
along its length at least first and second length segments of
different materials, the first segment having a stoichiometric
composition of the form A.sub.1-xB.sub.x, and the second segment
having a stoichiometric composition of the form A.sub.1-yB.sub.y,
where A and B are selected substances, and x and y are variables,
wherein an epitaxial composition boundary disposed between the
first and second segments, comprises a predetermined gradual change
from the variable x to the variable y over a predetermined number
of diametral lattice planes. In a similar embodiment the
compositions of the first and second segments of a nanowhisker of
the invention can be represented by the formulas A.sub.1-xB.sub.xC,
and A.sub.1-yB.sub.yC, respectively, wherein A and B represent
elements of one group, e.g., group III, of the periodic table, and
C represents an element of another group, e.g., group V, of the
periodic table. The variables x and y may assume a value between 0
and 1, and represent different numbers within that range. Thus,
such a nanowhisker is formed of a compound semiconductor that may
vary in composition along its length, thereby incorporating a
heterojunction. An example of such a compound semiconductor is
Al.sub.xGa.sub.1-xAs. A nanowhisker of the invention may be
constructed to have, e.g., two lengthwise segments, a first segment
having a composition Al.sub.1-yGa.sub.yAs, wherein the variable x
has a given value between 0 and 1 and a second segment having a
composition Al.sub.1-yGa.sub.yAs, wherein the variable y has a
second value different from the value of x. Between the two
segments is an interface within which the composition varies
continuously from the composition of the first segment to that of
the second segment, i.e., the value of the variable x changes
continuously, and usually monotonically, to the value of the
variable y. This interface thus constitutes a heterojunction. The
transition may be made to occur over a predetermined number of
diametral lattice planes by adjusting the conditions under which
the whiskers are grown, as will be explained in more detail below.
Furthermore, the growth conditions can be periodically adjusted to
produce a plurality of such heterojunctions along the length of the
nanowhisker.
[0029] The diameter of the nanowhisker is controlled by the
invention to be essentially constant along the length of the
nanowhisker, or having a defined variation, such as a controlled
taper. This ensures precise electrical parameters for the
nanowhisker, the controlled taper being equivalent to producing a
voltage gradient along the length of the nanowhisker. The diameter
may be small enough such that the nanowhisker exhibits quantum
confinement effects. Although the diameter is precisely controlled,
there will be small variations in the diameter arising from the
processing method, in particular a radial outward bulging of the
nanowhisker at a composition boundary in order to accommodate
lattice mismatch in the epitaxial structure. In addition the
diameter of one segment may be slightly different from that of
another segment of a different material, because of the difference
in lattice dimensions.
[0030] According to the invention the diameter of the nanowhiskers
preferably will not be greater than about 500 nm, preferably not
greater than about 100 nm, and more preferably not greater than
about 50 nm. Furthermore, the diameter of the nanowhiskers of the
invention may preferably be in a range of not greater than about 20
nm, or not great than about 10 nm, or not greater than about 5
nm.
[0031] The precision of formation of the nanowhisker enables
production of devices relying on quantum confinement effects, in
particular a resonant tunneling diode. Thus, an RTD has been
developed wherein the emitter, collector and the central quantum
dot are made from InAs and the barrier material from InP. Ideal
resonant tunneling behavior, with peak-to-valley ratios of up to
50:1, was observed at low temperatures.
[0032] In a specific aspect, the invention provides a resonant
tunneling diode, comprising a nanowhisker having a column of a
diameter with a nanometer dimension, such as to exhibit quantum
confinement effects,
[0033] the column comprising along its length first and second
semiconductor length segment forming respectively an emitter and a
collector, and, disposed between the first and second semiconductor
segments, third and fourth length segments of material having a
different band gap from that of the first and second semiconductor
segments, and a fifth central length segment of a semiconductor
material having a different band gap from that of the third and
fourth segments, disposed between the third and fourth segments and
forming a quantum well.
[0034] A problem which arises with an electrical or photonic
component formed from a nanowhisker is that of making efficient
electrical contacts to the nanowhisker.
[0035] One method is to remove the nanowhisker from its substrate,
by a mechanical scraping process, and to deposit the nanowhiskers
on a further substrate, on their side lengthwise on the substrate.
Metallised bond pads may then be formed over the ends of the
nanowhisker, or alternatively the nanowhisker can be manipulated to
be positioned over preformed contact pads.
[0036] Alternatively, in a method which may be better suited to
mass-production, the nanowhiskers may be left on the substrate,
with their base ends having been formed on an electrical contact.
Once formed, the nanowhiskers may be encapsulated in a resin or
glassy substance, and then contact pads may be formed over the
surface of the encapsulation in contact with the free ends of the
nanowhiskers. To assist in this, the catalytic particle melt,
towards the end of the formation of the nanowhisker, may have extra
conductive substances injected into it, so as to improve the
electrical contact with the bond pads.
[0037] Further specific components are set forth in the appended
claims, and described below. In particular, these include a
heterobipolar transistor, and light emitting diodes and
photodetectors.
[0038] Light emitting diodes are well suited to the present
invention, since it is possible to construct them with an emission
wavelength which can be selected at will from a continuous range of
wavelengths over the UV, visible, and infrared regions.
[0039] The present invention provides a light emitting diode,
comprising a nanowhisker having a column of a diameter with a
nanometer dimension, such as to exhibit quantum confinement
effects,
the column comprising along its length in sequence first, second
and third semiconductor length segments comprising respectively an
emitter, quantum well active segment and collector, said second
segment having a different band gap from that of the first and
second segments, and forming an active area of the light emitting
diode.
[0040] One particular application of a light emitting diode is for
emission of single photons. This is of use in various applications,
but in particular in quantum cryptography, where unauthorised
interception of a photon stream will inevitably cause destruction
or modification of the photon, in accordance with quantum theory,
and thus corruption of the transmitted signal--see P. Michler, A.
Imamoglu, M. D. Mason, P. J. Carson, G. F. Strouse, S. K. Buratto,
Nature 406, 968 (2000); C. Santori, M. Pelton, G. Solomon, Y. Dale,
Y. Yamamoto, Phys. Rev. Lett. 86, 1502 (2001).
[0041] The invention provides a single photon light source,
comprising a one dimensional nanoelement, having disposed along its
length a volume of optically active material sufficiently small to
form a quantum well, with tunneling barriers formed on either side
of the quantum well, such that in use the quantum well is capable
of emitting a single photon at a time.
Another form of light source in accordance with the invention is
designed for terahertz radiation, beyond the far infrared. Much
work has been done on superlattices, pioneered by Capasso and
co-workers at Lucent Technologies. Their `quantum cascade` lasers
utilise intersubband photon emission in InGaAs/InAlAs/InP
heterostructures, and have achieved room temperature (pulsed mode)
operation at wavelengths up to 17 microns. See for example IEEE
Spectrum July 2002, pages 23,24, "Using Unusable Frequencies" and
F. Capasso, C. Gmachl, D. L. Sivco, and a. Y. Cho, "Quantum cascade
lasers" Physics Today, May 2000, pp. 34-39.
[0042] The invention provides a source of terahertz radiation,
comprising a nanowhisker having a column of a diameter with a
nanometer dimension, the column including a multiplicity of layers
of a first band gap semiconductor interleaved with a multiplicity
of layers of a second band gap material, whereby to form a
superlattice, the dimensions being such that electrons can move
with a wave vector such as to radiate terahertz radiation.
[0043] In components, structures and processes according to the
invention, an array of a large number of nanowhiskers may be formed
extending from a substrate, essentially parallel to one another.
There are various methods of forming such arrays, for example
positioning an array of aerosol particles on the substrate to
provide catalytic seed particles, depositing particles on the
substrate from a colloidal solution, or forming on the substrate by
a nanoimprint lithography (NIL) process (or by any other
lithography process, e.g. e beam, UV, or X-ray), an array of areas
of predetermined shape (rectangular or other shape) and thickness,
which when heated, form balls of a desired volume to permit the
nanowhisker growth process to proceed.
[0044] Such arrays may be employed as photonic crystals, solar
cells comprised of a large number of photodetectors, field emission
displays (FED), converters to convert an infrared image to a
visible light image, all as described herein below. A further
application is that of a polarisation filter.
[0045] In processes of the invention, an array of a large number of
nanowhiskers may be employed to create a layer of an epitaxial
material on a wafer substrate of a cheaper substance, for example
silicon. A long-standing problem in the art is the formation of
single crystal wafers of expensive III-V materials, from which
chips can be formed. Much research has been made into forming
single crystal layers on silicon wafer substrates--see for example
WO 02/01648. However further improvements are desirable.
[0046] In accordance with the invention, a substrate of silicon or
other substance is provided on which is grown a mask material,
resistant to epitaxial growth, for example a dielectric material
such as SiO.sub.2, or Si.sub.3N.sub.4. An array of
nanometer-dimensioned apertures is formed in the mask material,
such as by a NIL process, and catalytic seed-forming material is
deposited in the apertures. Alternatively an array of seed forming
material areas is deposited on the substrate, and a layer of mask
material is then deposited over the substrate and the seed particle
areas. Application of heat causes melting of the seed particle
areas to create the seed particles, and then growth of the
nanowhiskers of the desired III-V or other material is initiated.
After growth of the nanowhiskers, growth of the desired material
continues, using the whiskers as nucleation centres, until a single
continuous layer of the material is formed. The material is single
crystal epitaxial. As preferred, the seed particle melt at the end
of the nanowhiskers is removed at a convenient opportunity to avoid
contamination of the epitaxial layer.
[0047] In a modification, mass growth of the epitaxial layer is
initiated, using the seed particle melts as nucleation points,
prior to formation of the nanowhiskers, and while the growth
underneath the seed particles is still in the liquid phase.
[0048] In a further modification, microscopic V-grooves are formed
in the upper surface of the silicon surface, for example
<111> etchings in a <100> substrate. The seed particle
forming areas are formed on the surfaces of the V-grooves, whereby
the nanowhiskers grow at an angle to the substrate, and cross one
another at the grooves. This makes for a more efficient growth of
the epitaxial layer from the nanowhisker nucleation centres.
Further, grain boundaries between domain areas with different
growth phases are avoided; which has been a problem with prior
processes.
[0049] The present invention thus provides in a further aspect a
method for forming an epitaxial layer of a desired material on a
substrate of a different material, the method comprising forming on
a substrate a configuration of seed particle material areas,
forming a layer of mask material around the seed particle areas,
growing nanowhiskers from the seed particles areas of said desired
material, and continuing to grow said desired material, using the
nanowhiskers as growth sites, whereby to create an epitaxial layer
of said desired material extending over said substrate.
[0050] In a further aspect of the invention, processes have been
developed for forming nanowhiskers of III-V material extending in
the <100> direction, as opposed to the usual <111>
direction for nanowhiskers. This has important applications,
particularly for nitride materials which tend to grow in the
<111> direction, but with many stacking faults, as the
material alternates between a zinc blende and wurtzite
structure.
[0051] The invention provides a method of forming nanowhiskers
comprising providing a substrate, forming a configuration of seed
particles on the upper surface, growing nanowhiskers from said seed
particles which extend from the substrate initially in a
<111> direction, and forming a short segment of a barrier
material in said nanowhiskers such as to change their direction of
growth to a <100> direction.
In a further aspect, the invention provides method of forming
nanowhiskers, a method of forming nanowhiskers, comprising
providing a substrate, forming a configuration of seed particles on
the upper surface, growing nanowhiskers from said seed particles
which extend from the substrate initially in a <111>
direction, and changing the growth conditions of said nanowhiskers
such as to change their direction of growth to a <100>
direction. The present invention also relates to one-dimensional
nanoelements incorporated in MEMS devices--micromechanical
devices.
[0052] In one aspect a substrate, for example of silicon, has a
matrix of electrical contact areas formed on one surface. On each
contact area, one, or a number, of nanowhiskers are formed from,
for example, gold catalyst particles so as to be upstanding from
the substrate's surface. Each nanowhisker, or group of nanowhiskers
may therefore be individually addressable by electrical signals.
Such a structure may make contact with the end of a nerve or
perhaps the nerves in the retina of an eye, and the electrodes may
be activated so as to provide a repairing or artificial function
for enabling the nerve. Thus for example, when applied in the
retina of an eye, the structure may overcome certain blindness
problems.
[0053] In another aspect a nanowhisker is provided, which may
function as a nerve electrode or in other applications, wherein the
whisker is formed of silicon or of a metal which may be oxidised,
and the whisker is oxidised to form a layer of oxide along its
length. The particle melt at the end of the whisker however
including gold or other non-oxidisable material remains free of
oxide and may therefore be used to form an electrical contact. This
arrangement provides more precise electrical characteristics than
nanowhiskers with exposed conductive material along their lengths
and such nanowhiskers may be used as nerve electrodes or as devices
where the capacitance of the nanowhisker is of importance. As an
alternative, other materials may be used as the outer layer for
example higher bandgap shells, for example where the whisker is
formed of gallium arsenide, the outer layer may be gallium
phosphide.
[0054] An important application of nanostructures is in
micromechanical cantilever beams where a beam fixed at one end
projects into space and may be subject to an external force, for
example, electrical or weight or an external object or a chemical
force, to give a bending of the cantilever. This bending may be
detected for example by a change in electrical capacitance of the
structure.
[0055] In a further aspect the present invention provides one or
more nanowhiskers, which may or may not be oxidised in accordance
with the above-mentioned aspect of the invention along their length
to provide a cantilever or an array of cantilevers formed as a row
or parallel beams. Such an arrangement may provide an order of
magnitude or more sensitivity than a previous arrangement where an
etching process has been used to produce the beams.
[0056] One application for such cantilevers is where the whiskers
are formed with a material with a coating which is sensitive to
certain organic molecules or biological molecules, such that a
molecule, when making contact with a cantilever beam undergoes a
certain chemical reaction. This produces certain stresses on the
cantilever beam and causes bending of the beam, which may be
detected by optical or electrical monitoring.
[0057] In a further specific aspect, a nanowhisker is formed on a
substrate projecting up into an aperture of a layer of material,
which is essentially insulative. The upper surface of the
insulative layer has an electrically conductive material formed
thereon. This electrically conductive material is roughly the same
height from the substrate as the tip of the nanowhisker, which has
a conductive seed particle melt thereon. By appropriate activation
of the conductive material, the whisker may be made to mechanically
vibrate within the aperture at a certain eigen frequency, for
example, in the gigahertz range. During the period of a single
vibration, a single electron is transferred from one side of the
conductive material to the other via the seed particle melt. This
creates a current standard generator, where the current I through
the conductive material is equal to product of the frequency of
vibration and the charge e of an electron: I=fe.
[0058] If the whisker is sensitised to attract molecules of a
certain type, then the deposition of a molecule onto the whisker
will change the inertial characteristics of the whisker and
therefore its natural frequency of vibration. This may therefore be
detected by electrical activation of the conductive material. This
technique may be used to calculate the weight of a molecule to a
very accurate degree.
BRIEF DESCRIPTION OF THE DRAWINGS
[0059] Preferred embodiments of the invention will be now be
described merely by way of example with reference to the
accompanying drawings, in which:
[0060] FIG. 1 is a schematic depiction of a fabrication technique
according to the invention, for forming nanowhiskers: (a)
deposition of size-selected Au aerosol particles onto a GaAs
substrate; (b) AFM manipulation of the particles for the
positioning of the whiskers; (c) alloying to make a eutectic melt
between Au and Ga from the surface; (d) GaAs whisker growth.
[0061] FIG. 2. (a) TEM micrograph of GaAs nano-whiskers grown from
10 nm Au aerosol particles. (b) SEM micrograph of a
GaAs<111>B substrate with GaAs whiskers grown from nm Au
aerosol particles. (c) High-resolution electron microscope image
400 kV of GaAs whisker grown from an Au cluster. The inset shows a
magnified part of the whisker.
[0062] FIG. 3 is a schematic diagram of apparatus for carrying out
the methods of this invention.
[0063] FIG. 4. Composition profile of an InAs nanowhisker,
according to an embodiment of the invention, containing several InP
heterostructures, using reciprocal space analysis of lattice
spacing. (a) High-resolution TEM image of a whisker with a diameter
of 40 nm. (b) Power spectrum of the image in (a). (c) An inverse
Fourier transform using the information closest to the InP part of
the split 200 reflection. InP (bright) is located in three bands
with approximately 25, 8 and 1.5 nm width, respectively. (d)
Superimposed images, using an identical mask over the InP and InAs
parts of the 200 reflection, respectively.
[0064] FIG. 5. Analysis of InP heterostructures inside InAs
nanowhiskers. (a) TEM image of InP barriers (100, 25, 8, and 1.5
nm) inside a 40 nm diameter InAs nanowhisker. (b) Magnification of
the 8 nm barrier region, showing crystalline perfection and the
interface abruptness on the level of monolayers. (c) Simulated
band-structure diagram of the InAs/InP heterostructures, including
(left edge) ideal formation of ohmic contacts to InAs. (d) Ohmic
I-V dependence for a homogeneous InAs whisker, contrasted by the
strongly nonlinear I-V behavior seen for an InAs whisker containing
an 80 nm InP barrier. (e) Arrhenius plot showing measurements of
thermionic excitation of electrons across the InP barrier (at a
bias of 10 mV), yielding a barrier height of 0.57 eV.
[0065] FIG. 6. Evaluation of transport mechanisms for single
barriers of various thick nesses, for use in resonant tunneling
diodes of the invention. (A) A SEM image of a whisker on the growth
substrate. (Scale bar depicts 1 .mu.m.) (B) An InAs/InP nanowhisker
contacted by two alloyed ohmic contacts. (Scale bar depicts 2
.mu.m) (C) TEM image of an InAs whisker with an 8 nm InP segment
perpendicular to the long axis of the whisker. (D) The
current-voltage characteristics for three different barrier
situations;
[0066] FIG. 7. High-resolution TEM imaging. (A) A TEM image of an
InAs whisker grown in the <111> direction with two InP
barriers for forming a first embodiment of the invention. (Scale
bar depicts 8 nm.) (B) A one-dimensionally integrated profile of
the boxed area in A. The width of the barrier is about 5.5 nm (16
lattice spacing), and the interface sharpness is of the order of
1-3 lattice spacings, judged by the jump in image contrast.
[0067] FIG. 8. A resonant tunneling diode (RTD) forming an
embodiment of the invention.
[0068] (A) TEM image of the top end of a whisker with the double
barrier clearly visible, in this case with a barrier thickness of
about 5 nm (scale bar depicts 30 nm).
[0069] (B) The principle of the energy band diagram for the device
investigated with the characteristic electronic states in the
emitter region indicated (left).
[0070] (C) Current-voltage data for the same device as shown in A
and B revealing a sharp peak in the characteristics, reflecting
resonant tunneling into the ground state, E1z, with a voltage width
of about 5 mV. This width can be translated into an energy width of
the transition of about 2 meV, corresponding to the width of the
shaded energy band in the emitter from which electrons tunnel. The
device characteristics are shown in the inset, which provides a
magnified view of the resonance peak for increasing voltage and for
decreasing voltage.
[0071] FIG. 9 is a schematic representation of the preferred
embodiment of the resonant tunneling diode according to the
invention;
[0072] FIG. 10 is a schematic representation of a further
embodiment of the invention including a wide band gap insulating
segment;
[0073] FIG. 11 is a schematic representation of a further
embodiment of the invention comprising a hetero bipolar transistor
(HBT);
[0074] FIG. 12 is a band gap diagram of the HBT correlated with the
HBT structure;
[0075] FIG. 13 is a diagram showing band gap variation with
compositional change of a ternary compound;
[0076] FIGS. 14A and 14B are diagrams showing band gap versus
lattice dimensions for a variety of semiconductor compounds;
[0077] FIG. 15 is a schematic representation of an embodiment of
the invention comprising a light emitting diode and laser;
[0078] FIG. 16 is a schematic representation of a further
embodiment of the invention comprising the application of a laser
to detection of individual molecules of desired species;
[0079] FIG. 17 is a schematic representation of a further
embodiment of the invention comprising the application of an array
of lasers to patterning photoresists in a NIL process;
[0080] FIG. 18A is a schematic representation of a further
embodiment of the invention comprising a photodetector, and FIGS.
18B and 18C are variants thereof;
[0081] FIG. 19A is a schematic representation of a further
embodiment of the invention comprising a solar cell, and FIG. 19B
is a variant thereof;
[0082] FIG. 20 is a schematic representation of a further
embodiment of the invention comprising a radiation source of
terahertz radiation;
[0083] FIGS. 21A-C are schematic representations for explaining an
embodiment of the invention comprising a photonic crystal, and FIG.
21D is a variant thereof for forming a 3-D photonics crystal;
[0084] FIGS. 22A-G are schematic representations of a further
embodiment of the invention for forming a layer of material
epitaxial with a substrate, wherein the lattices are not matched to
one another;
[0085] FIGS. 23A-C are schematic representations for explaining a
further embodiment of the invention for forming a layer of material
epitaxial with a substrate, wherein the lattices are not matched to
one another;
[0086] FIGS. 24A-B are schematic representations for explaining a
further embodiment of the invention, for forming whiskers, which
extend in a<100> direction, as opposed to the usual
<111> direction;
[0087] FIGS. 25A-B are schematic representations of a further
embodiment of the invention comprising a field emission display
(fed), wherein the individual elements of the display are
nanowhiskers and are individually addressable;
[0088] FIG. 26 is a schematic representation of a further
embodiment of the invention comprising an arrangement for
upconverting an image in the infrared region to a visible light
region;
[0089] FIG. 27 is a schematic representation of a further
embodiment of the invention comprising an antenna for infrared
radiation;
[0090] FIG. 28 is a schematic representation of a further
arrangement comprising a ferromagnetic whisker for spintronics
applications;
[0091] FIG. 29 is a schematic view of a further embodiment of the
invention comprising an array of selectively addressable electrodes
for implantation into a nerve;
[0092] FIG. 30 is a schematic view of a further embodiment of the
invention comprising a nanowhisker with an oxidised outer surface
along its length;
[0093] FIG. 31 is a schematic view of a further embodiment
comprising a row of nanowhiskers upstanding from a substrate and
forming a cantilever arrangement;
[0094] FIG. 32 is a schematic view of a further embodiment of the
invention comprising a nanowhisker arranged for oscillation and
providing precise measurements of weight and frequency; and
[0095] FIG. 33 is a schematic view of a further embodiment of the
invention, comprising the tip of a Scanning Tunneling
Microscope.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0096] Methods of producing nanowhiskers, in accordance with the
invention will now be described. Such methods may be suitable for
production of resonant tunneling diodes described below, and other
electronic and/or photonic components, as will become apparent.
[0097] Whiskers are highly anisotropic structures, which are
spatially catalyzed by molten metallic droplets, often
unintentionally introduced as contaminants, on a crystal surface.
Gold is commonly chosen as catalyst, or seed particle, since it
forms eutectic alloys with semiconductor materials or constituents
such as Si, Ga, and In. The melting points of these eutectic alloys
are lower than the usual growth temperatures for Si and group III-V
materials. The molten metal droplet serves as a miniature, liquid
phase, epitaxy system, where precursors are fed into it in the form
of a vapour or, in this case, by a molecular beam in vacuum. The
growth is usually referred to as vapour-liquid-solid growth. The
electrical and optical properties of semiconductor nanowhiskers are
fundamentally determined by their crystalline structure, shape, and
size. In particular, a small variation of the width of the whisker
provokes a considerable change in the separation of the energy
states due to the quantum confinement effect. Accordingly, it is of
importance that the whisker width can be chosen freely, and, of
equal importance, is that the width can be kept constant for
extended whisker lengths. This, together with the possibility of
positioning whiskers at selected positions on a substrate, is
necessary for an integration of whisker technology with current
semiconductor component technology.
[0098] According to the invention, techniques have been developed
for the synthesis of size-selected, epitaxial nanowhiskers, grown
on a crystalline substrate. The Chemical Beam Epitaxy apparatus
employed in the techniques described below is shown schematically
in FIG. 3.
[0099] Chemical Beam Epitaxy (CBE) combines a beam epitaxial
technique like Molecular Beam Epitaxy (MBE) and the use of chemical
sources similar to Metal Organic Chemical Vapor Deposition (MOCVD).
In MOCVD or related laser ablation techniques, the pressure inside
the reactor is usually greater than 10 mbar and the gaseous
reactants are viscous, which means that they have a relatively high
resistance to flow. The chemicals reach the substrate surface by
diffusion. CBE reduces the pressure to less than 10.sup.-4 mbar and
the mean free path of the diffusants then becomes longer than the
distance between the source inlet and the substrate. The transport
becomes collision free and occurs in the form of a molecular beam.
The exclusion of the gas diffusion in the CBE system means a fast
response in the flow at the substrate surface and this makes it
possible to grow atomically abrupt interfaces.
[0100] The CBE apparatus shown in FIG. 3 consists of a UHV
(ultra-high vacuum) growth chamber 1001 where the sample 1021 is
mounted on a metal sample holder 1041 connected to a heater 1061.
Around the chamber there is a ring 1081 filled with liquid nitrogen
that is called the cryoshroud. The cryoshroud pumps away species
that don't impinge or that desorb from the substrate surface. It
prevents contamination of the growing surface layer and reduces the
memory effect. Vacuum pumps 1101 are provided.
[0101] The sources 1121 for CBE are in liquid phase and they are
contained in bottles which have an overpressure compared to the
chamber. The sources are usually as follows: TMGa, TEGa, TMIn,
TBAs, TBP. The bottles are stored in constant-temperature baths and
by controlling the temperature of the liquid source, the partial
pressure of the vapor above the liquid is regulated. The vapor is
then fed into the chamber through a pipe complex 1141 to, in the
end of the pipe just before the growth chamber, a source injector
1161. The source injector is responsible for injection of the gas
sources into the growth chamber 1001, and for generation of a
molecular beam with stable and uniform intensity. The III-material,
from the metal organic compounds TMIn (trimethylindium), TMGa
(trimethylgallium) or TEGa (triethylgallium), will be injected by
low temperature injectors to avoid condensation of the growth
species. They will decompose at the substrate surface. The
V-material is provided by the metal-organic compounds, TBAS
(tertiarybutylarsine) or TBP (tertiarybutylphosphine). As opposed
to the decomposition of the III-material, the V-material will be
decomposed before injection into the growth chamber 1001, at high
temperatures, in the injectors 1161. Those injectors 1161 are
called cracking cells and the temperatures are kept around
900.degree. C. The source beam impinges directly on the heated
substrate surface. Either the molecule gets enough thermal energy
from the surface substrate to dissociate in all its three alkyl
radicals, leaving the elemental group III atom on the surface, or
the molecule get desorbed in an undissociated or partially
dissociated shape. Which of these processes dominates depends on
the temperature of the substrate and the arrival rate of the
molecules to the surface. At higher temperatures, the growth rate
will be limited by the supply and at lower temperatures it will be
limited by the alkyl desorption that will block sites.
[0102] This Chemical Beam Epitaxy method permits formation of
heterojunctions within a nanowhisker, which are abrupt, in the
sense there is a rapid transition from one material to another over
a few atomic layers.
[0103] For the purposes of this specification, "atomically abrupt
heterojunction", is intended to mean a transition from one material
to another material over two or less atomic monolayers, wherein the
one material is at least 90% pure on one side of the two
monolayers, and the other material is at least 90% pure on the
other side of the two monolayers. Such "atomically abrupt
heterojunctions" are sufficiently abrupt to permit fabrication of
heterojunctions defining quantum wells in an electrical component
having a series Of heterojunctions and associated quantum
wells.
[0104] For the purposes of this specification, "sharp
heterojunction", is intended to mean a transition from one material
to another material over five or less atomic monolayers, wherein
the one material is at least 90% pure on one side of the five
monolayers, and the other material is at least 90% pure on the
other side of the five monolayers. Such "sharp heterojunctions" are
sufficiently sharp to permit fabrication of electrical components
having one, or a series of, heterojunctions within a nanoelement,
where the heterojunctions have to be accurately defined. Such
"sharp heterojunctions" are also sufficiently sharp for many
components relying on quantum effects.
[0105] As an illustration, in a compound AB used in the
nanowhiskers of the invention, where A represents one or more
selected elements of a first group and B represents one or more
selected elements of a second group, the total proportions of the
selected element(s) in the first group and the selected element(s)
in the second group are predetermined to constitute a semiconductor
compound designed to provide desired properties. The compound AB is
considered 90 pure when the total proportion of the selected
element(s) within each group is at least 90% of its predetermined
proportion.
Example 1
[0106] FIGS. 1 and 3 show whiskers of predetermined sizes grown
from several III-V materials, in particular, GaAs whiskers with
widths between 10 and 50 nm. These whiskers can be grown rod shaped
with a uniform diameter, in contrast to earlier reports on
epitaxially grown nano-whiskers, which tended to be tapered,
narrowing from the base towards the top. As catalysts,
size-selected gold aerosol particles were used, whereby the surface
coverage can be varied independently of the whisker diameter.
[0107] The whisker width in general is slightly larger than the
seed particle diameter. This is mainly due to two factors: First,
the gold particle incorporates Ga and possibly As from the
substrate, which makes the particle grow. Second, when the particle
melts, the base diameter of the liquid cap will be determined by
the wetting angle between the alloy and the substrate surface.
Simple assumptions give a widening of up to 50%, depending on
temperature and particle diameter, and introduce a reproducible
correlation between the particle diameter and the width of the
whisker.
[0108] A GaAs<111>B substrate 10 was used, etched in
HCL:H.sub.2O, 1:10 to remove any native oxide and surface
contaminants before aerosol deposition. The size-selected Au
particles 12 were made in a locally constructed aerosol facility
situated in a glove box 14 with ultra pure N.sub.2 atmosphere. The
particles are created in a tube furnace 16 by the
evaporation/condensation method, at a temperature of about
1750.degree. C., and are electrically charged by UV light at 18.
The particles are size selected by means of a differential mobility
analyzer DMA 20. The DMA classifies the sizes of charged aerosol
particles by balancing their air resistance against their mobility
in an electric field. After size classification, the particles were
heated to 600.degree. C., in order to make them compact and
spherical. The setup gives an aerosol flow with a narrow size
distribution, the standard deviation being <5% of the mean
particle diameter. Still charged, the particles were deposited on
the substrate 10 by means of an electric field E. Size-selected
aerosol particles in the range between 10 and 50 nm were used to
grow whiskers.
[0109] After aerosol deposition, some samples were transferred to
an AFM Topometrix Explorer 24, also situated inside the glove box,
and connected to the aerosol fabrication equipment. Thus, these
samples were exposed only to sub-ppm levels of H.sub.2O and O.sub.2
during the deposition and manipulation phases. With the AFM tip,
specific particles 12 were selected and placed in a predetermined
configuration, giving complete control of the positioning of
individual seed particles.
[0110] The GaAs substrate 10 with Au aerosol particles 12, either
arranged or as deposited, was then transferred into a chemical beam
epitaxy CBE chamber. In a CBE configuration, GaAs growth occurs
under vacuum/molecular beam conditions and with metal organic
sources, in this case, triethylgallium TEG and tertiarybutylarsine
TBA. The TBA is thermally pre-cracked to predominantly As.sub.2
molecules, while the TEG usually cracks after impinging on the
surface of the substrate. The growth is typically performed with a
slight As.sub.2 over-pressure, which means that the Ga flow
determines the growth rate. Just before growth, the substrate was
heated by a heater to 600.degree. C. for 5 min, while exposed to an
As.sub.2 beam. In this step, the Au droplet can form an alloy with
the GaAs constituents, whereby the Au particle absorbs some of the
Ga from the substrate. The Au/Ga alloy forms at 339.degree. C.
However, this step also works as a deoxidizing step, taking away
any new native oxide layer, originating from the transport to and
from the glove box system. The oxide is expected to evaporate at
590.degree. C., although this is not always the case. The
volatility of the oxide can be followed with reflective high-energy
electron diffraction RHEED. With a successful transfer, a streaky
diffraction pattern, indicating a crystalline, reconstructed
surface, can be seen already at temperatures lower than 500.degree.
C. Often, however, the oxide stays stable up to 590.degree. C.,
sometimes as high as 630.degree. C. The whisker growth was
performed at substrate temperatures between 500 and 560.degree. C.,
with a TEG pressure of 0.5 mbar and a TBA pressure of 2.0 mbar.
After growth, the samples were studied by scanning and transmission
electron microscopy SEM and TEM.
[0111] The resulting whiskers were rod shaped and fairly
homogeneous in size, although their lengths varied slightly. The
size homogeneity was clearly dependent on the volatility of the
surface oxide. For samples with a hard oxide, as seen with RHEED,
the size homogeneity was decreased. An oxygen-free environment is
therefore to be preferred for reproducible results. At the growth
temperatures described, no tapering of the whiskers was observed,
irrespective of particle size. For whiskers grown below 500.degree.
C., however, there were clear signs of tapering. The growth of
either rod-shaped or tapered whiskers, depending on temperature, is
explained by the absence or presence of uncatalyzed growth on the
surfaces parallel to the long axis of the whisker. The simplest
surfaces of this orientation are <110> facets. Under ordinary
CBE growth conditions, close to the ones used in these experiments,
<110> facets are migration surfaces. However, at lower
temperature, the Ga diffusion constant decreases, which initiates
growth on the <110> facets. In MOCVD growth the Ga migration
length is even smaller, which explains the typically tapered
whiskers of prior workers.
[0112] In FIG. 2a, a TEM image of a truss of 10.+-.2-nm-wide
whiskers grown from 10 nm particles is shown. The relatively low
density of whiskers is illuminated by the SEM image in FIG. 2b,
which is of a GaAs<111>B substrate with GaAs whiskers grown
from 40 nm Au aerosol particles. In FIG. 2c, a single 40-nm-wide
whisker is shown in a high-resolution TEM micrograph. The growth
direction is perpendicular to the close-packed planes, i.e., 111 in
the cubic sphalerite structure, as found by other groups. Twinning
defects and stacking faults can also be observed, where the whisker
alternates between cubic and hexagonal structure. Most of the
whisker has the anomalous wurzite structure W, except for the part
closest to the Au catalyst, which always is zinc blende Z.
SF=stacking fault, T=twin plane. The change in image contrast at
the core is due to the hexagonal cross-section.
[0113] Such a growth method is used in the method described below
with reference to FIGS. 4 to 6 for forming whiskers with segments
of the whisker with different compositions. The method is
illustrated by InAs whiskers containing segments of InP.
Example 2
[0114] Conditions for growth of nanowhiskers allow the formation of
abrupt interfaces and heterostructure barriers of thickness from a
few monolayers to 100s of nanometers, thus creating a
one-dimensional landscape along which the electrons move. The
crystalline perfection, the quality of the interfaces, and the
variation in the lattice constant are demonstrated by
high-resolution transmission electron microscopy, and the
conduction band off-set of 0.6 eV is deduced from the current due
to thermal excitation of electrons over an InP barrier.
[0115] In this method, the III-V whiskers are grown by the
vapor-liquid-solid growth mode, with a gold nanoparticle
catalytically inducing growth, in the manner described above.
Growth occurs in an ultrahigh vacuum chamber 100, FIG. 3, designed
for chemical beam epitaxy (CBE). The rapid alteration of the
composition is controlled by the supply of precursor atoms into the
eutectic melt, supplied as molecular beams into the ultrahigh
vacuum chamber. The rapid switching between different compositions
(e.g., between InAs and InP) is obtained via a sequence where
growth is interrupted as the indium source (TMIn) is switched off,
followed by a change of the group III sources. Finally; the
supersaturation conditions, as a prerequisite for reinitiation of
growth, are reestablished as the indium source is again injected
into the growth chamber.
[0116] For the abruptness of the interfaces, FIG. 4 shows TEM
analysis of an InAs whisker containing several InP heterostructure
barriers. In FIG. 4a, a high-resolution image of the three topmost
barriers is shown, recorded with a 400 kV HRTEM (point resolution
0.16 nm). FIG. 4b shows a nonquadratic power spectrum of the HREM
image, showing that the growth direction is along [001] of the
cubic lattice. The reflections show a slight splitting due to the
difference in lattice constants between InAs and InP. FIG. 4c shows
an inverse Fourier transform, using a soft-edge mask over the part
of the 200 reflection arising from the InP lattice. A corresponding
mask was put over the InAs part of the reflection. The two images
were superimposed as in FIG. 4d.
[0117] FIG. 5a shows a TEM image of an InAs/InP whisker. The
magnification of the 5 nm barrier in FIG. 5b shows the atomic
perfection and abruptness of the heterostructure interface. Aligned
with the 100 nm thick InP barrier, the result of a 1D Poisson
simulation (neglecting lateral quantization, the contribution of
which is only about 10 mev) of the heterostructure 1D energy
landscape expected to be experienced by electrons moving along the
whisker is drawn (FIG. 5c). This gives an expected band offset
(q1/4B) in the conduction band (where the electrons move in n-type
material) of 0.6 eV. This steeplechase-like potential structure is
very different from the situation encountered for electrons in a
homogeneous InAs whisker, for which ohmic behavior (i.e., a linear
dependence of the current (I) on voltage (V)) is expected and
indeed observed (indicated curve in FIG. 5d). This linear behavior
is dramatically contrasted by the indicated I-V curve measured for
an InAs whisker containing an 80 nm thick InP barrier. Strongly
nonlinear behavior is observed, with a voltage bias of more than 1V
required to induce current through the whisker. This field-induced
tunnel current increases steeply with increasing bias voltage, as
the effective barrier through which the electrons must tunnel
narrows. To test whether the ideal heterostructure band diagram
within the 1D whisker is valid, the temperature dependence of the
current of electrons overcoming the InP barrier via thermionic
excitation was measured. The result is shown in FIG. 5e, where the
logarithm of the current (divided by T.sup.2) is plotted as a
function of the inverse of the temperature in an Arrhenius fashion,
measured at a small bias voltage (V) 10 mV) to minimize
band-bending effects and the tunneling processes described above.
From the slope of the line fitted to the experimental data points
an effective barrier height, q1/4B, of 0.57 ev may be deduced, in
good agreement with the simulation.
[0118] An added benefit of this approach to realizing
heterostructures within 1D whiskers is the advantageous condition
for combining highly mismatched materials, provided by the
efficient strain relaxation by the proximity to the open side
surface in the whisker geometry. In comparison, only a few atomic
layers may be epitaxially grown in transitions between materials
like InAs and InP with different lattice constants before either
islanding or misfit dislocations occur, thereby preventing
formation of ideal heterointerfaces.
Resonant Tunneling Diodes and Heterobipolar Transistors
[0119] The present invention also comprises, at least in preferred
embodiments, functional 1D (one-dimensional) resonant tunneling
diodes (RTDs) obtained via bottom-up assembly of designed segments
of different semiconductor materials in III/V nanowires. Such RTDs
comprise, in order, an emitter segment, a first barrier segment, a
quantum well segment, a second barrier segment, and a collector
segment. As is known to those skilled in the art, the barrier
segments in RTDs are made thin enough that significant quantum
tunneling of charge carriers is possible under conditions that
favor such tunneling. In RTDs according to the invention,
fabricated in nanowitres, the nanowhiskers may be made thin enough
so that the central quantum well is effectively a quantum dot. In a
concrete example, the emitter, collector and the central quantum
dot may be made from InAs and the barrier material from InP. In an
example, excellent resonant tunneling behavior, with peak-to-valley
ratios of up to 50:1, was observed.
[0120] According to the invention 1D heterostructure devices were
fabricated utilizing semiconductor nanowhiskers. The whiskers were
grown by a vapor-liquid-solid growth mode, size controlled by, and
seeded from, Au aerosol particles, as more fully described above in
Examples 1 and 2. Growth takes place in a chemical beam epitaxy
chamber under ultra-high-vacuum conditions where the
supersaturation of the eutectic melt between the Au particles and
the reactants acts as the driving force for whisker growth.
[0121] The incorporation of heterostructure segments into the
whiskers is achieved via the following switching sequence (more
fully described above); the group III-source beam is switched off
to stop growth, and shortly thereafter the group V-source is
changed. Once the group III-source is reintroduced into the
chamber, the supersaturation is re-established and growth
continues. In examples described below the material system used was
InAs for the emitter, collector and dot, and InP as the barrier
material. The aerosol particles were chosen so that the final
whisker diameter was 40-50 nm. In order to prepare contacted
electronic devices with single nanowhiskers as the active elements,
the whiskers were transferred from the growth substrate to a
SiO.sub.2-capped silicon wafer, on top of which large bond pads
were predefined by Au metal evaporation through a transmission
electron microscope (TEM) grid mask. In FIG. 6B a scanning electron
microscope (SEM) image of a nanowire device is shown, displaying
the alignment capability in the e-beam lithography system, allowing
positioning of metallic electrodes on the nanowires with an
accuracy that is better than 100 nm. FIG. 6D shows the
current-voltage (I-V) characteristics of a set of single-barrier
devices, as the thickness of the InP barrier was varied from 80 nm
down to zero. The thicker InP segments act as ideal tunneling
barriers for electron transport, allowing only thermal excitation
over this barrier (measured to be about 0.6 eV (23)) or tunneling
made possible by the effective thinning of the barrier when a large
bias is applied to the sample. In FIG. 6D it can be seen that
almost no current flows through the thick InP barrier. In samples
containing thinner single barriers (FIG. 2C), quantum tunneling is
possible and electrons can penetrate barriers thinner than about 10
nm in thickness. In the extreme case with zero barrier thickness,
the I-V characteristics are perfectly linear down to at least 4.2
K. In order to verify the crystalline quality and to evaluate the
abruptness of the heterointerfaces high-resolution TEM
investigations were performed. In FIG. 7A a magnification of a 5.5
nm thick InP barrier in a <111>-InAs nanowhisker is shown,
where the (111) lattice planes can be clearly seen. From the
integrated profile of the area in FIG. 7A the sharpness of the
interfaces was determined to be 1-3 lattice spacings. The average
spacing between the lattice fringes in the lighter band is 0.344
nm, corresponding well to d111=0.338 nm of InP. FIG. 7B is a
one-dimensionally integrated profile of the boxed area in A. The
width of the barrier is about 5.5 nm (16 lattice spacings), and the
interface sharpness is of the order of 1-3 lattice spacings, judged
by the jump in image contrast. The background is not linear due to
bend and strain contrast around the interfaces. The difference in
lattice spacing between the InP and the InAs is 3.4%, which
corresponds well with the theoretical value of the lattice mismatch
(3.3%).
[0122] Since the heterointerfaces were determined to be abrupt
enough for making high quality quantum devices, double-barrier
resonant tunneling devices may therefore be envisaged. A barrier
thickness of about 5 nm was chosen. In FIG. 8A a TEM image of such
a double barrier device structure formed inside a 40 nm wide
nanowhisker can be seen. The barrier thickness is roughly 5 nm on
either side of the 15 nm thick InAs quantum dot. Below the TEM
image (FIG. 8B) the energy band diagram expected for the device is
shown, with the longitudinal confinement (z-direction) determined
by the length of the dot and the lateral confinement (perpendicular
direction) depending on the diameter of the whisker. For this
device only the lowest transverse quantized level was occupied
(splitting of the order of 5 meV), with the Fermi energy indicated,
determining the highest occupied longitudinal states filled with
electrons. In between the two InP barriers the fully quantized
levels of the central quantum dot are indicated, with the same
sequence as schematically indicated in the emitter region for the
transverse quantised levels, but with a greater splitting (of the
order of 100 mev) between the longitudinal quantized states in the
quantum dot and an approximate quantization energy for the ground
state of Elz=40 meV. At zero applied bias, the current should be
zero since no electronic states in the emitter are aligned with any
states in the central dot because of the difference in energy
quantization between the dot and the emitter. As the bias is
increased the states in the dot will move towards lower energy and,
as soon as the lowest dot-state is aligned with the Fermi level,
the current starts to increase (here the Fermi level is assumed to
lie between the two lowest states in the emitter). When the
dot-state falls below the energy level of the first emitter state
the current again drops to zero, resulting in the characteristic
negative differential resistance.
[0123] The electrical properties of this 1D DBRT device are
presented in FIG. 8C, showing almost ideal I-V characteristics, as
expected for such a device. The I-V trace shows no current below a
bias of around 70 mV, corresponding to the bias condition for which
electrons must penetrate both barriers plus the central InAs
segment to move from the emitter to the collector. At a bias of
about 80 mV a sharp peak is seen in the I-V characteristics, with a
half-width of about 5 my in bias (which can be translated into an
energy sharpness of the resonance of about 1-2 meV). The
peak-to-valley ratio of the 80 mV peak is extremely high, about
50:1, and was seen in different samples investigated. After the
deep valley, the current increases again for a bias of about 100
mV, with some unresolved shoulder features observed on the rising
slope. Note that the I-V trace for increasing bias voltage
coincides with that for decreasing bias voltage indicating that the
device characteristics are highly reproducible and exhibit
negligible hysteresis effects. In addition, the 80 mV appears
similarly in the reverse bias polarity. In this case the peak is
only slightly shifted (5 mV) suggesting a high symmetry of the
device structure. Accordingly, these results report the
investigation of the materials and barrier properties of single
heterostructure barriers inside semiconductor nanowires, bridging
the gap from thick barriers, for which only thermal excitation
above the barrier is possible, down to single barrier thickness,
for which tunneling through the barrier dominates.
[0124] With this approach one-dimensional, double-barrier resonant
tunneling devices have been prepared, with high-quality device
properties, and an energy sharpness of about 1 meV and
peak-to-valley current ratio of 50:1.
[0125] Referring now to FIG. 9, a preferred embodiment of a
resonant tunneling diode is shown, having a nanowhisker extending
between collector and emitter contacts 42, 44, 2 microns apart.
First and second InAs portions 46, 48 of the whisker make
electrical contact with respective contacts 42, 44. Barrier
portions 50,52 of InP separate a central quantum dot or quantum
well portion of InAs, 54, from the emitter and collector portions.
The length of the portion 54 is around 30 nm. The precise
dimensions will be selected in dependence upon bandgap barrier
height, etc., in order to achieve appropriate quantum
confinement.
[0126] The diode operates in the conventional way of RTDs; for an
explanation of the theory of operation; see, for example, Ferry and
Goldnick, Transport in Nanostructures, CUP 1999, pp 94 et seq.
[0127] In the RTD of FIG. 9, the segments 50, 52 may be replaced by
a wide band gap insulating material, in the manner shown in FIG.
10. Referring to FIG. 10, an embodiment is shown having an
insulating segment. A germanium whisker 100 is grown by the
processes described above, having a short segment 102 of silicon.
Lattice mismatch is accommodated by radial outward expansion of the
whisker. This silicon dot is oxidised by heat to give a large
silicon dioxide spacer 104 within the germanium whisker. This has
an extremely stable large bandgap offset. Aluminium can be used
instead of silicon. This embodiment can be used for example for
tunneling effects, in the embodiment of FIG. 9.
[0128] As regards making electrical contacts with the collector and
emitter portions of the embodiment of FIG. 9, this can be done in
different ways. The whisker may be positioned across large
metallised bond pads, as shown in FIG. 9. Alternatively, the
nanowhisker may be positioned on a substrate, its position
identified by a suitable scanning method, and then bond pads may be
formed over the ends of the whisker by a metallization process.
Another alternative is to leave the nanowhisker extending from the
substrate, where it makes contact at its base with an electrical
contact, to encapsulate the whisker in a resin or glassy substance,
and then form an electrode over the encapsulation, making
electrical contact with the whisker tip. This latter method may be
more suitable for integration with other electrical components and
circuits.
[0129] Referring now to FIGS. 11 to 14, an embodiment of the
invention is disclosed which comprises a heterojunction bipolar
transistor (heterobipolar transistor; HBT); this differs from the
conventional bipolar transistor in that different band gap
materials are used in the transistor. For example, a nanowhisker
110 may have an emitter segment 112 of GaP, connected to a base
segment 114 of p-doped Si, which is in turn connected to an n-doped
collector segment 116 of Si. Metallisation electrodes 118 make
contact with the respective segments 112, 114, and 116. FIG. 12
shows a band gap diagram for the HBT. By reason of the relatively
wide band gap of the emitter, minority current flow from the base
to the emitter is inhibited. The depletion area between the base
and collector is characterized by a gradual change in doping from
p-type to n-type. As an alternative, the base and collector may be
formed of ternary or quaternary materials, being a stoichiometric
composition, and the composition gradually changes over a large
number of lattice planes, say 100 to 1000, to give the required
depletion region field. Change in energy band gap with composition
is shown in FIG. 13 for the ternary mixture
Al.sub.xGa.sub.1-xAs.
[0130] FIG. 14 shows variation in bandgap energy and lattice
parameters for a variety of III-V materials. It will be appreciated
that with the method of forming nanowhiskers according to the
invention, it is possible to form heteroepitaxial junctions of
materials with widely different lattice parameters, e.g. GaN/AlP,
the lattice mismatch being accommodated by radial bulging of the
whisker.
[0131] Photonics Components
[0132] Referring to FIG. 15, this shows schematically an extremely
small LED capable of single photon emission. Single photon emission
is of importance, for example for quantum photography or detection
of individual molecules of molecular species. A whisker 150 has
anode and cathode outer regions 152 of indium phosphide either side
of an inner region 156 formed of indium arsenide, so as to define a
quantum well. Regions 152 are connected to respective anode and
cathode electrical contacts, formed as metallisation areas 158. In
contrast to planar devices, where because of the need for lattice
matching and for relieving mismatch strain, only certain
wavelengths are possible, an important point of this embodiment is
that the wavelength of the LED is fully variable since the
materials making up the diode may be of any desired composition to
achieve a desired wavelength of emission (see FIG. 14 discussed
above), since lattice mismatch is accommodate by radial outward
bulging of the whisker. Since the materials may be stoichiometric
compositions, the wavelength is continuously variable across the
range from 1.5 ev to 0.35 ev. A one-dimensional structure requires
much less processing than prior art layered structures and is made
by a self-assembly process, with the whole structure between the
electrical contacts. If a laser construction is required, Fabry
Perot (FP) cleavage planes 159 are formed spaced an appropriate
distance apart. As an alternative, regions 159 are formed as
mirrors comprising superlattices. The superlattices may be formed
as alternating sequences of InP/InAs, the sequence alternating over
segments of only a few lattice planes, as is known to those skilled
in the art.
[0133] LEDs, lasers, and other micro cavity structures are often
fabricated with gallium nitride (GaN). Whilst nitrides have a
number of advantages, particularly in optics, problems with
nitrides are that firstly they are filled with dislocations and
that secondly there is a lack of suitable substrates (sapphire
being one commonly used substrate). Whiskers can be made with
defect-free nitrides, and there is not a problem of lattice
matching to a substrate. A regular FP laser can be made, with the
structure of FIG. 15, with dimensions less than 300 nm, preferably
of the order of 100 nm. It is a bottom up structure, which is well
suited to reading DVDs and writing thereto. Nitride systems are
quite well suited for whisker growth.
[0134] The light source-emitting region 156 can be made as small as
about 20 nm.sup.3. This represents an extreme example of a point
source and can be used, as indicated schematically in FIG. 16 to
locally excite individual biological cells 160. The light source
156 provides a near field 162 (exponentially decaying) which
excites the cell 160 since the physical spacing between the light
source and object is a fraction of a wavelength. It is of use in
DNA sequencing, and, as shown, the source 156 may be mounted in a
groove 164 of a glass capillary tube 166. The cell flows along the
tube as part of a fluid mixture, and flows past the source 156.
[0135] Referring to FIG. 17, this shows an embodiment of the
invention adapted for Nano Imprint Lithography (NIL), where an
array 170 of whiskers 156, providing point sources of light, are
individually addressable by an energisation source 172. The array
is mounted on a carriage 174 movable over the surface of a resist
material 176. The carriage is movable in steps of 20 nm, and at
each step, the whiskers 156 are selectively energised in order to
illuminate the material 176 with near field light, and to create a
desired developable pattern in the resist 176.
[0136] Referring to FIG. 18A, a photodetector is shown in
accordance with the invention. For example, a nanowhisker 180 may
extend between metallised contact pads 182. There is typically a
high contact resistance, between 10KO to 100KO, arising from small
contact areas between pads 182, and whisker 180. The whisker may
comprise an n-doped indium phosphide portion 184, and a p-doped
indium phosphide portion 186, with a p-n junction 188 between,
which may be abrupt, or may extend over a large number of lattice
planes. This arrangement is suitable for detecting light with
wavelengths 1.3 micron or 1.55 microns. As indicated in FIG. 14,
any desired compositional "match" may be used, and therefore the
materials can be modified for detection of any wavelength, from
1.55 microns or less. As an alternative, a PIN or Schottky diode
structure may be used. A PIN structure, as shown in FIG. 18B has an
intrinsic semiconductor material segment 188 between the two
semiconductor portions 184 and 186. The whisker is constructed as
described with reference to FIG. 10. A Schottky diode structure, as
shown in FIG. 18C has a base portion 189 formed as a metallisation
contact from which the whisker extends; the interface between the
contact and the whisker forms the Schottky diode. The lower
frequency limit on detection of radiation is in the terahertz
region of the electromagnetic spectrum.
[0137] Referring to FIG. 19A, a solar cell application is shown for
the photodetector structures of FIG. 18. Millions of whiskers 190,
each having p- and n-doped portions 191, 192 are formed on a
substrate 193, doped (P+). The whiskers are formed by growth using
gold, or other, nanoparticles, deposited onto substrate 193, e.g.,
from an aerosol. The whiskers may be encapsulated in plastics 194
and have a transparent tin oxide electrode 196 on the upper
surface, which makes contact with the free ends of the whiskers to
permit electrical current to flow along the length of the whiskers.
The structure is extremely efficient in trapping light since each
whisker is 100% reliable. The overall efficiency is between 35 and
50% and is of use in multi-bandgap solar cells. By contrast
amorphous silicon grown at 300.degree. C. gives an efficiency of
about 10%. Crystalline silicon gives an efficiency of about 15% and
special purpose III-V solar cells for space applications are grown
at 400.degree. C. and have an efficiency of up to 25%. Gratzel
solar cells for space applications have titanium dioxide
nanoparticles painted on solar panels, with an appropriate dye;
such cells have an efficiency up to about 8%.
[0138] Referring to the modification shown in FIG. 19B, each
whisker of the solar cell array is modified to the form shown 197,
with different segments of different materials 198 along its
length. These materials are selected so that the p-n junctions
absorb light at different wavelengths. The point along the whisker
at which the whisker is most sensitive to light of a particular
wavelength depends on the precise structure of the solar cell and
factors such as reflection and refraction within the structure.
[0139] The embodiment of FIGS. 19A-B is inexpensive, since the
growth conditions are inexpensive, and further only very small
quantities of expensive materials are required. In alternative
constructions, the whiskers can be silicon (least expensive) or
germanium. The length of the whiskers is 1 or 2 microns. A PN
junction is achieved by doping the whisker along part of its
length, or by forming Schottky barriers, as indicated in FIG. 18C
at the base of the whisker.
[0140] Referring to FIG. 20, an embodiment is shown, which is a
source of very long wavelength infrared radiation, e.g., at
terahertz frequencies. An indium phosphide nanowhisker 200 has a
series of very thin indium arsenide stripes 202, separated by
spacer stripes 204 of indium phosphide. The stripes are grown by
the process described above. Each stripe 202, 204 is a few lattice
planes wide, and the stripes create a superlattice 206. By applying
a voltage across electrode contacts 208, electrons move across the
superlattice. The superlattice creates a series of quantum well
bandgaps (potential wells) which, according to the Bloch theorem
will give a conduction band with allowable regions of electron wave
number or momentum k-these allowable regions correspond to
terahertz frequencies, thereby to create terahertz emission.
[0141] FIGS. 21A-21D illustrate an embodiment of the invention,
implemented as a photonic crystal. Photonic crystals are well
known--see for example copending application WO 01/77726. In the
main, prior methods of forming photonic crystals involve etching
air holes in a substrate according to a predetermined lattice
pattern. A concept of this embodiment is to use a patterning
technique for defining a crystal lattice pattern on a substrate,
but to grow nanowhiskers to define the crystal, rather than etching
holes. This has numerous advantages in that etching techniques are
not as reliable (etching harms the substrate surface) as a bottom
up technique of growing whiskers. Therefore the whisker technique
is more accurate and gives higher quality; and simplicity, as well
as economy in that fewer process steps are required.
[0142] Referring to FIG. 21A, a substrate 210 has a triangular
lattice pattern of square patches 212 of gold about 300 nm.sup.2,
spaced apart by a distance of 300 nm, the patches having been
formed by ebeam lithography, UV lithography or a nanoimprint
lithography (NIL) process. The substrate is initially prepared
before gold deposition as a clean substrate without oxide
contaminants. The substrate is heated to melt the gold rectangles
so that they form balls 214, about 100 nm diameter, as shown in
FIG. 21B, which are then annealed. Whiskers 216 are then grown by
the process as described in Example 1, about 100 nm wide to form a
photonics crystal, as shown in FIG. 21C.
[0143] It is possible in accordance with the invention to define
three-dimensional photonic crystals by whisker formation. This can
be done as indicated in FIG. 21D by forming each whisker with a
sequence of segments 217, 218 of different materials, for example
an alternating sequence of III-V materials such as InAs/GaAs, or
group IV materials such as Ge/Si, in accordance with the method of
Example 2, so that at intervals along each whisker, segments are
provided with an appropriate refractive index to form a photonic
band gap.
[0144] Single Crystal Layers of III-V Materials
[0145] Referring to FIGS. 22A-22G, an embodiment of the invention
is shown for growing epitaxial layers of a desired material on a
substrate. As shown in FIGS. 22A & B, a silicon or gallium
arsenide substrate 220 has formed on an upper surface rectangles
222 of gold, indium or gallium, which are positioned on the
substrate by a stamp 223 in a NIL process or as described in
Example 1. An epitaxial mask deposit 224 a few nanometers wide of
dielectric material, for example, silicon dioxide or silicon
nitride, are formed over the substrate 220 and around rectangles
222. Heat is applied to anneal the rectangle to balls 226, FIG.
22C, and whiskers 228, FIG. 22D, are grown of for example InP or
GaAs. Alternatively a carbon-based material is used as the deposit
224 (a carbon based material stabilises the particle when the ball
is formed by annealing, the dielectric material being desorbed).
The balls are used as seed openings for bulk growth i.e. a layer of
the desired material. The dielectric layer prevents atomic bonding
and lattice mismatch effect between the substrate and the crystal
layer. The whiskers grow together with a bulk layer of InP or GaAs
229, FIG. 22E. There are gradual changes in growth conditions from
the whisker to the layer. Thus there is nucleation on the whiskers
without creating defects. There are small nucleation steps and
strain effects do not appear to give dislocations. Where the
substrate is a III-V material, the important advantage is to create
a lattice-mismatched layer on the substrate without getting misfit
dislocations.
[0146] In a variation, as shown in FIG. 22F, gold balls 226 are
deposited on the surface from an aerosol, in accordance with the
method of Example 1. The epitaxial mask deposit 224 is formed over
the balls. Whiskers are then grown, as in FIG. 22D.
[0147] In a further development in accordance with the invention,
it is known that whiskers tend to grow preferentially in the
<111>B direction because for gallium arsenide (a zinc blende
lattice), the arsenic atom is at the apex of a pyramid with gallium
ions at the base of the pyramid, see FIG. 23A. A preferred
embodiment of the invention is illustrated in FIG. 23B, where a
substrate 230 of silicon has a serrated surface having V-grooves
232 of microscopic dimensions etched to expose <111> planes.
Gold particles 234 are deposited on the surfaces of the V-grooves.
GaAs whiskers 236, shown in ghost form in FIG. 23C, and grown in
accordance with Example 1, will extend perpendicular to the walls
of the serrations. These whiskers provide nucleation points for
bulk growth of a GaAs layer 238. There are gradual changes in
growth conditions from the whisker to the layer. Thus there is
nucleation on gallium arsenide without creating defects. Any small
nucleation steps and strain effects do not appear to give
dislocations. The direction of the whiskers, in <111>
directions at an angle to the substrate, forces epitaxial growth in
a certain direction, and takes away the problem of antiphase
domains, which has been a problem. Thus this provides a way of
integrating III-V compounds onto silicon (or other Group IV)
substrates, and is cheaper than existing methods--see for example
PCT Published Patent Application No. WO 02/01648.
[0148] A further advantage of a V-grooved substrate arises in
connection with the solar cell application of FIG. 19, in that the
serrated substrate provides multiple reflections of incident light,
and hence an increased probability of photon capture.
[0149] Referring now to FIG. 24, a preferred embodiment is
described for controlling the orientation of whiskers. Normally, as
described above, whiskers of III-V compounds grow in the
<111>B direction. A problem here is that such whiskers change
more or less randomly between hexagonal (wurtzite) (FIG. 24A) and
cubic (zinc blende) (FIG. 23A) structures. This gives rise to many
stacking faults. Stacking faults are always a problem particularly
for optical properties, but also for electrical characteristics. By
applying strain to the whisker during formation, by change of
growth conditions, the direction of growth of the whisker can be
changed to the <100> direction, which gives a cubic lattice
structure (zinc blende), which does not have stacking faults.
[0150] In FIG. 24B, a silicon substrate 240 with a <100>
surface has whiskers 242 of, e.g., InP, grown on it. The whiskers
start to grow as at 244 in the <111> direction, but shortly
after initial growth, operating conditions are changed, by
increasing the rate of growth and increasing the temperature and
pressure within the CBE apparatus, so that the whisker continues to
grow as at 246 in the <100> direction. The point 248 at which
direction changes is a <110> facet. The whisker at the
transition maintains its epitaxial crystalline nature. The
structure of the crystal in segment 246 is hexagonal close packed,
which significantly reduces the problem of stacking faults.
[0151] In an alternative method of growth, a short barrier segment
of a wide band gap material, e.g. InAs, is grown at point 248; this
has the same effect of changing the subsequent orientation of the
whisker.
[0152] This embodiment is therefore particularly suitable for the
growth of nitrides, e.g. GaN, which preferentially grow as
hexagonal lattices, and which are particularly prone to stacking
faults. By "forcing" the nitride crystal to grow in cubic form,
stacking faults are reduced. Further, where structures are made in
accordance with Example 2 with segments of different material along
the whisker, micro-cavity structures for gallium nitride lasers can
be developed. Nitride systems are quite well suited for whisker
growth. The problem with nitrides is that they are filled with
dislocations and the lack of suitable substrates. Whiskers can be
made with defect-free nitrides, and the problem of lattice matching
is not there. A regular FP laser can be made in a nanowhisker less
than 300 nm length, of the order of 100 nm. It is a bottom up
structure, which is well suited to reading and writing to DVDs.
[0153] Referring now to the embodiment shown in FIG. 25, this
embodiment relates to field emission tips or Spindt cathodes. These
are of use in field emission displays (FED), and many methods have
been proposed for making such displays. One prior art arrangement
as shown in FIG. 25A comprises a silicon substrate 250, with a
surface 252, which is patterned by laser ablation, or the like, to
form microscopic or nanometric tips 253. A phosphor screen 254 is
positioned adjacent the tips, and a voltage between the tips and
the screen generates extremely high field strengths at the tips,
which causes current flow into the screen, and thus radiation of
visible light from the screen.
[0154] In FIG. 25B, an embodiment of the invention is shown,
comprising an FED, wherein the elements of the display are
individually addressable. Etched contact metallisation areas 256
are formed on a silicon substrate 250. Gold seed particles 258 are
positioned on each metallisation area, by the method as described
in Example 1. The gold particles are used as seeds for whisker
growth, in order to grow Si whiskers 259, each whisker extending
from a respective metallisation area. A single whisker, as shown,
or a group of nanowhiskers, forming a single display element may
extend from a respective metallisation area. In addition to being
individually addressable, an advantage of this embodiment is that
the FED is 100% reliable, in contrast to prior methods, e.g. carbon
nanotubes (CNT).
[0155] FIG. 26 discloses an embodiment for infrared to visible
light up-conversion. An image 260 of infrared radiation with a
wavelength of 1.55 or 2.5 .mu.m is shone on the base surface of a
gallium arsenide substrate 262--a relatively large band gap
material which will not interact with the radiation. The other side
of the substrate has indium arsenide projecting whiskers 264, grown
as described in Example 1, and having a relatively small band gap,
which will cause absorption of the photons of the radiation.
Whiskers 264 are not however individually addressable, in contrast
to FIG. 25. A voltage of about 20-50 volts is applied between the
ends of the whiskers and a nearby fluorescent screen 266, and
electrons are generated from the indium arsenide whiskers. Indium
arsenide has a bandgap corresponding to 3 microns, and will
therefore produce electrons in response to radiation shorter than 3
microns. Gallium phosphide may be used as an alternative, but this
has a visible light bandgap. The emitted electrons cause
fluorescence to give visible light 268 emitted from the fluorescent
screen, and a version of the image, but up-converted to visible
light wavelength. The applied voltage may be raised sufficiently to
induce avalanche effects.
[0156] FIG. 27 discloses an embodiment of the invention in which a
whisker 270, 400 nm long of GaAs (made in accordance with Example
1) extends from a metallisation contact area 272 on a silicon
substrate 274. This dimension is 1/4 of a wavelength of 1.55 micron
radiation, and hence the whisker provides a ?/4 resonant antenna
for 1.55 micron radiation. Contact area 272 provides a ground
plane. The antenna may be positioned to receive radiation 276 in
free space; alternatively, it may be positioned adjacent the end of
a silica fibre link 278 for detection of radiation in the third
optical window.
[0157] Referring now to FIG. 28, an embodiment of the invention is
shown for use in the field of spintronics. Spintronics is a
technical field where the properties of electronic devices rely on
the transport of electron spin through the device--see for example
Scientific American June 2002 pp 52-59, "Spintronics", David D.
Awschalom et al. In FIG. 28, a whisker 280, formed by the process
of Example 1, of a magnetic or semi-magnetic material such as
manganese gallium arsenide (semi-magnetic) or manganese arsenide
(ferromagnetic) is formed on a Si substrate 281. Under an applied
voltage V, spin polarised electrons 283 are emitted from the tip of
the whisker, which makes electrical contact with an electrical
contact 284 disposed on a substrate 286. The spin polarised
electrons 283 are used for reading and writing magnetic storage
devices 288 disposed on substrate 286.
[0158] In a further development of this embodiment, a problem is
overcome, which is that, with ferromagnetism, there is normally a
lower limit on ferromagnetic domain width, about 10-15 nm, below
which the ferromagnetism changes to super-paramagnetism. However
when incorporated in a nanowhisker, in accordance with the method
of Example 1, the domain diameter can be reduced, because of the
reduced possibilities for symmetrical alignment in a 1-dimensional
system, which makes it more difficult for the ions of the material
to have more than one orientation. The material of the whisker can
be iron, cobalt, manganese, or an alloy thereof.
[0159] Referring now to FIG. 29, a further embodiment of the
invention is shown comprising a substrate with an array of
electrodes for implantation into a nerve for repairing a nerve
function, for example the retina of an eye. The electrodes are
individually addressable. Etched contact metallisation areas 350
are formed on silicon substrate 352. Gold seed particles 354 are
positioned on each metallisation area, by the method as described
above. The gold particles are used as seeds for whisker growth, in
order to grow silicon whiskers 358, each whisker extending from a
respective metallisation area. A single whisker, as shown, or a
group of nanowhiskers, forming a single electrode element may
extend from a respective metallisation area. In addition to being
individually addressable, an advantage of this embodiment is that
the electrodes are 100% reliable.
[0160] Referring now to FIG. 30, a further embodiment is shown
comprising a nanowhisker 360 formed by the method described above.
The whisker is formed of silicon and has a gold particle melt at
one end 362. Subsequent to formation of the whisker, the whisker is
exposed to an atmosphere at a suitable temperature for oxidation of
the silicon. This forms an outer shell 364 of silicon dioxide
surrounding the whisker and extending along its length. The gold
particle melt 362 remains in an unoxidised condition. This
therefore provides a structure highly suitable for the electrode
assembly shown in FIG. 29, wherein the electrode has very precise
electrical characteristics. The silicon material may be replaced by
any other material that can be oxidised.
[0161] As an alternative, the whisker 360 may be exposed to an
atmosphere of a suitable material for forming a high band gap
material as an alternative to the oxidation layer 364.
[0162] Referring now to FIG. 31, this shows a further embodiment of
the invention comprising a silicon base member 370. This base
member may be a planar substrate, or just a bar. In any event, a
row of nanowhiskers 372 is formed from one edge surface of the bar
or substrate. The nanowhiskers are regularly spaced apart and
project into space. The nanowhiskers may have a coating formed on
them for absorbing certain molecular structures. In any event the
cantilever beam arrangement may be used for any of the well-known
applications for cantilever arrangements for measuring molecular
species etc.
[0163] Referring to FIG. 32 this shows a further embodiment of the
invention comprising a molecular sensing device. A substrate 380,
e.g., of silicon nitride, has an insulating layer 382 formed
thereon, with a conductive surface 384, for example gold. An
aperture 386 is formed within the layers 382, 384 and a nanowhisker
388 is formed within the aperture.
[0164] This is done essentially by a self-assembly process, since
the aperture is formed in insulating layer 382 and the gold layer
384 is subsequently deposited. Gold is therefore in consequence
deposited on the base of the aperture, indicated at 389, and upon
heating forms a gold particle melt which enables formation of a
nanowhisker with appropriate conditions. The gold particle melt 389
resides on top of the nanowhisker in the finished nanowhisker. The
nanowhisker height is such that the particle melt 389 is at least
approximately co-planar with the gold surface layer 384.
[0165] The natural resilience of the nanowhisker implies that it
has a characteristic frequency of vibration from side to side in a
direction transverse to its length. Oscillation of particle melt
389 can be detected by voltage or current signals being created in
conductive layer 384. This therefore provides a means of detecting
the frequency of vibration of the nanowhisker 388.,
[0166] By appropriate activation of the conductive material with an
applied voltage, the whisker may be made to mechanically vibrate
within the aperture at a certain eigen frequency, for example, in
the gigahertz range. This is because, in view of the small
dimensions and low currents involved, during the period of a single
vibration, a single electron is transferred from one side of the
conductive material to the other via the seed particle melt. This
creates a current standard generator, where the current I through
the conductive material is equal to product of the frequency of
vibration f and the charge e of an electron: I=fe. Thus a known
reference signal is generated which can be used in appropriate
circumstances.
[0167] In addition, the particle melt 389 may be coated with a
receptor substance so as to permit certain molecular species to be
absorbed on the surface of the particle melt 389. This will cause a
change in characteristic frequency of the nanowhisker. This change
in frequency may be detected and provides a means of computing the
weight of the molecular species absorbed on the surface of the melt
389.
[0168] FIG. 33 shows the tip of a Scanning Tunneling Microscope
(STM) as comprising a nanowhisker 392 of InP formed on the end of a
flexible beam 394 of Silicon. Beam 394 is formed by etching from a
substrate or bar.
* * * * *