U.S. patent application number 10/814292 was filed with the patent office on 2004-09-23 for single electron devices.
This patent application is currently assigned to BTG International Limited. Invention is credited to Deppert, Knut Wilfried, Samuelson, Lars Ivar.
Application Number | 20040183064 10/814292 |
Document ID | / |
Family ID | 10822439 |
Filed Date | 2004-09-23 |
United States Patent
Application |
20040183064 |
Kind Code |
A1 |
Samuelson, Lars Ivar ; et
al. |
September 23, 2004 |
Single electron devices
Abstract
A single electron tunnelling device is formed by positioning
between first and second electrodes a particle formed of a material
having a first conductivity characteristic having a surface layer
of a material of a second conductivity characteristic, the
thickness of said layer being sufficiently small to support quantum
mechanical tunnelling therethrough.
Inventors: |
Samuelson, Lars Ivar;
(Malmo, SE) ; Deppert, Knut Wilfried; (Lund,
SE) |
Correspondence
Address: |
MILES & STOCKBRIDGE PC
1751 PINNACLE DRIVE
SUITE 500
MCLEAN
VA
22102-3833
US
|
Assignee: |
BTG International Limited
|
Family ID: |
10822439 |
Appl. No.: |
10/814292 |
Filed: |
April 1, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10814292 |
Apr 1, 2004 |
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09563200 |
May 2, 2000 |
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6744065 |
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09563200 |
May 2, 2000 |
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PCT/GB98/03429 |
Nov 13, 1998 |
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Current U.S.
Class: |
257/14 ;
257/E21.408; 257/E29.322 |
Current CPC
Class: |
B82Y 10/00 20130101;
B82B 3/00 20130101; B82Y 40/00 20130101; B82Y 30/00 20130101; H01L
29/66469 20130101; C30B 29/605 20130101; H01L 29/7613 20130101;
C30B 33/00 20130101 |
Class at
Publication: |
257/014 |
International
Class: |
H01L 029/06 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 21, 1997 |
GB |
9724642.5 |
Claims
1. A single electron tunnelling device comprising a particle
together with first and second electrodes positioned adjacent to
said particle to facilitate the flow of current therebetween
characterised in that said particle is formed of a material having
a first conductivity characteristic having a surface layer of a
material of a second conductivity characteristic, the thickness of
said layer being sufficiently small to support quantum mechanical
tunnelling therethrough.
2. A single electron tunnelling device according to claim 1
characterised in that said device includes a plurality of said
particles to define a current path between said first and second
electrodes.
3. A single electron tunnelling device according to claim 1
characterised in that said material having said first conductivity
characteristic is substantially homogenous.
4. A single electron tunnelling device according to any one of
claims 1 to 3 characterised in that said surface layer is
semiconducting.
5. A single electron tunnelling device according to any one of
claims 1 to 3 characterised in that said surface layer is
insulating.
6. A single electron tunnelling device according to claim 1
characterised in that said surface layer is gallium arsenide.
7. A single electron tunnelling device according to claim 1
characterised in that said surface layer is indium oxide.
8. A single electron tunnelling device according to claim 1
characterised in that said surface layer is indium arsenide
phosphide.
9. A single electron tunnelling device according to claim 1
characterised in that said surface layer is silica.
10. A method of fabricating single electron devices comprising the
steps of forming a plurality of particles forming a layer of a
thickness sufficiently small to support quantum mechanical
tunnelling on the surface of said particles and positioning at
least one of said particles between a pair of electrodes to form a
single electron device.
11. A method of fabricating single electron devices according to
claim 10 characterised in that a further step of selecting
particles of predetermined size takes place prior to the step of
forming said layer.
12. A method of fabricating single electron devices according to
claim 10 or 11 characterised in that said plurality of particles is
formed as an aerosol.
13. A method of fabricating single electron devices according to
claim 10 or 11 characterised in that said layer is formed by the
chemical modification of the surface of said particles.
14. A method of fabricating single electron devices according to
claim 10 or 11 characterised in that said layer is formed by the
expitaxial deposition of a material on the surface of said
particles.
15. A method of fabricating single electron devices according to
claim 10 characterised in that the positioning of said particle is
performed by means of an atomic force microscope.
16. A method of fabricating a single electron device comprising the
steps: forming a plurality of particles; forming on the surface of
each particle a peripheral layer of a thickness sufficiently small
to support quantum mechanical tunnelling therethrough; providing a
pair of electrodes and positioning at least one of said particles
between said pair of electrodes to form a single electron
device.
17. A method of fabricating a single electron device according to
claim 16, wherein a further step of selecting particles of
predetermined size takes place prior to the step of forming said
peripheral layer.
18. A method of fabricating a single electron device according to
claim 16, wherein said plurality of particles is formed as an
aerosol.
19. A method of fabricating a single electron device according to
claim 16, wherein said peripheral layer is formed by chemical
modification of the surface of each of said particles.
20. A method of fabricating a single electron device according to
claim 16, wherein said peripheral layer is formed by the epitaxial
deposition of a material on the surface of each of said
particles.
21. A method of fabricating a single electron device according to
claim 16, wherein the positioning of said particle is performed by
means of an atomic force microscope.
22. A method of forming a single electron tunnelling device
comprising: forming a particle of a material having a first
conductivity characteristic, forming on the particle a
semiconducting surface layer of a second conductivity
characteristic, the thickness of said layer being sufficiently
small to support quantum mechanical tunnelling therethrough; and
positioning said particle between first and second electrodes to
provide a current path between the electrodes.
23. A method of forming a single electron tunnelling device,
comprising: forming a particle of a material having a first
conductivity characteristic; forming on the surface of the particle
a surface layer of gallium arsenide, the thickness of said layer
being sufficiently small to support quantum mechanical tunnelling
therethrough; and positioning said particle between first and
second electrodes to provide a current path therebetween.
24. A method of forming a single electron tunnelling device,
comprising: forming a particle of a material having a first
conductivity characteristic; forming on the surface of the particle
a peripheral layer of indium oxide, the thickness of said layer
being sufficiently small to support quantum mechanical tunnelling
there through; positioning said particle between first and second
electrodes to provide a current path therebetween.
25. A method of forming a single electron tunnelling device,
comprising: forming a particle of a material having a first
conductivity characteristic; forming on the surface of the particle
a peripheral layer of indium arsenide phosphide, the thickness of
said layer being sufficiently small to support quantum mechanical
tunnelling therethrough; positioning said particle between first
and second electrodes to provide a current path therebetween.
26. A method of forming a single electron tunnelling device,
comprising: forming a particle of a material having a first
conductivity characteristic; forming on the surface of the particle
a peripheral layer of silica, the thickness of said layer being
sufficiently small to support quantum mechanical tunnelling
therethrough; positioning said particle between first and second
electrodes to provide a current path therebetween.
27. A method forming an electronic device, comprising: forming at
least one nanoparticle having an inner core of a conductive
material of predetermined size of nanometer dimensions; forming on
the inner core, an outer shell of a controlled thickness of
nanometer dimensions and of a further material which is different
from that of the inner core; and providing first and second
electrodes, and providing a current flow path therebetween
comprising said at least one particle, the characteristics of
current flow in the current flow path being determined by electron
tunnelling via said outer shell and inner core.
28. A method according to claim 27, wherein said further material
is insulating.
29. A method according to claim 28, wherein said further material
is an oxide of one of: silicon, indium, aluminium.
30. A method according to claim 27, wherein said further material
is semiconducting.
31. A method according to claim 30, wherein said semiconducting
material contains one of the following: indium, silicon.
32. A method according to claim 27, wherein said conductive
material contains one of: silicon, germanium, indium, gallium.
33. A method according to claim 27, comprising providing a
multiplicity of said nanoparticles stacked adjacent one another
whereby to provide said current flow path.
34. A method of forming a nanocrystal in the form of a particle
that is defined by a size of nanometer dimensions, the method
comprising: forming, in an aerosol, a core particle of an
electrically conductive material and having a size of predetermined
nanometer dimensions; and forming epitaxially on the core particle,
by the action of gas on the aerosol, an outer shell of a further
material that is different from that of the core, and having a
controlled thickness of nanometer dimensions.
35. A method according to claim 34 wherein said conductive material
contains one of: silicon, germanium, indium, gallium.
36. A method according to claim 34, wherein said further material
is semiconducting.
37. A method according to claim 34, wherein said conductive
material contains one of the following: indium, germanium, gallium;
and said further material is semiconducting and comprises one of
the following: indium, silicon, aluminium.
38. A method of forming a nanocrystal in the form of a particle
that is defined by a size of nanometer dimensions, the method
comprising: forming in an aerosol, a core particle of an
electrically conductive material and having a size of predetermined
nanometer dimensions; and exposing the gas to an aerosol, the gas
reacting with the surface of the core particle to form an outer
shell of a further material that is different from the material of
the core particle, and that has a controlled thickness of nanometer
dimensions.
39. A method according to claim 38, wherein the gas reacts with the
surface of the core particle to form an oxide of the core
material.
40. A method according to claim 39, wherein the core particle
contains one of: silicon, indium.
41. A method according to claim 38, wherein the gas reacts with the
surface of the core particle by an exchange process, wherein atoms
in the surface of the core particle are exchanged for atoms in the
gas.
42. A method.according to claim 41, wherein the core material is a
compound semiconductor, and the gas comprises molecules containing
phosphorus.
43. A method according to claim 42, wherein the core material
comprises indium arsenide, and arsenic atoms are replaced by
phosphorus atoms.
44. A method of forming a nanocrystal in the form of a particle
that is defined by a size of nanometer dimensions, the method
comprising: forming from an aerosol, a core particle of an
electrically conductive material and having a size of predetermined
nanometer dimensions; and forming epitaxially on the core, by the
action of gas on the aerosol, an outer shell of a further material
that is different from that of the core, and having a controlled
thickness of nanometer dimensions; and reacting the outer shell to
form an oxide of the further material.
45. A method according to claim 44, wherein the conductive material
is gallium arsenide, the further material is aluminium arsenide,
and said oxide is aluminium oxide.
46. A method of forming nanocrystals comprising: a) forming an
aerosol of particles of a predetermined conductive material and
diameters of nanometer dimensions; b) filtering said aerosol of
particles to provide particles with a narrow predetermined spread
of diameters; and c) processing the filtered aerosol with a vapour
of a material in order to form a shell of a further material on
each aerosol particle and of a controlled thickness, said further
material being different from said predetermined conductive
material.
47. A method according to claim 46, wherein said processing in step
c) comprises forming by an epitaxial process said shell.
48. A method according to claim 47, wherein said conductive
material contains one of: indium, germanium, gallium; and said
further material includes one of: indium,silicon, aluminium.
49. A method according to claim 46, including the further step of
reacting the further material of the outer shell to form an
oxide.
50. A method according to claim 49, wherein said inner core
comprises gallium arsenide, said further material comprises
aluminium arsenide, and said oxide comprises aluminium oxide.
51. A method according to claim 46, wherein in said step c) said
vapour reacts with the surface of the particle in order to form
said shell by modification of the surface of the particle.
52. A method according to claim 51, wherein the modification is an
exchange process.
53. A method according to claim 51, wherein the particle material
is a compound semiconductor, and the vapour comprises molecules
containing phosphorus.
54. A method according to claim 53, wherein the particle material
comprises indium arsenide, and arsenic atoms are replaced by
phosphorus atoms so that the material of the shell is indium
arsenide phosphide.
55. A method according to claim 51, wherein the modification is
formation of an oxide.
56. A method according to claim 55, wherein the material of the
particle is one of: silicon, indium.
57. A method according to claim 46, wherein said step a) comprises:
forming an aerosol of group III metallic particles, and filtering
the aerosol to provide particles with a narrow predetermined
dimensional spread; and reacting the aerosol with a group V
precursor gas, in order to provide an aerosol of particles
consiting og III-V semiconductor material.
Description
[0001] This invention relates to electronic components and, in
particular, to so-called single-electron devices and to methods of
manufacture thereof.
[0002] The field of single electron devices emerged from
investigations of the tunnel junction, which consists of two
electrodes of a conducting material, separated by a thin layer of
an insulating material having a thickness of about one nanometre.
According to the laws of quantum mechanics, electrons have a small
probability of tunnelling through such an insulating layer. If a
voltage is applied across the junction, electrons will tunnel
preferentially in one particular direction through the insulator.
Hence, they will carry an electric current through the junction.
The magnitude of the current depends on both the thickness of the
insulating layer and the material properties of the conducting
electrodes.
[0003] In early 1985, Averin and Likharev attempted to predict the
behaviour of very small tunnel junctions with superconducting
electrodes but the equations were too complex to be easily solved.
However, for a small tunnel junction with electrodes of normal
conductors, if a constant electric current is passed through a
junction, it will induce a voltage that oscillates periodically in
time. These periodic oscillations have a frequency equal to the
current divided by the charge of an electron. This frequency is
totally independent of any other parameters of the system. An
alternative view is that each oscillation represents the response
of the device as a single electron tunnels through the insulating
layer. The phenomenon is known as single-electron tunnelling (SET)
oscillations.
[0004] To understand this effect, one must appreciate how electric
charge moves through a normal conductor such as an aluminium wire.
An electric current can flow through the conductor because some
electrons are free to move through the lattice of atomic nuclei.
Despite the motion of the electrons, any given volume of the
conductor has virtually no net charge because the negative charge
of the moving electrons is always balanced by the positive charge
of the atomic nuclei in each small region of the conductor. Hence,
the important quantity is not the charge in any given volume but
rather how much charge has been carried through the wire. This
quantity is designated as the "transferred" charge. This charge may
take practically any value, even a fraction of the charge of a
single electron. The reason for this is that charge is proportional
to the sum of shifts of all the electrons with respect to the
lattice of atoms. Because the electrons in a conductor can be
shifted as little or as much as desired, this sum can be changed
continuously, and therefore so can the transferred charge.
[0005] If a normal conductor is interrupted by a tunnel junction,
electric charge will move through the system by both a continuous
and a discrete process. As the transferred charge flows
continuously through the conductor, it will accumulate on the
surface of the electrode against the insulating layer of the
junction (the adjacent electrode will have equal but opposite
surface charge). This surface charge Q may be represented as a
slight continuous shift of the electrons near the surface from
their equilibrium positions. On the other hand, quantum mechanics
shows that the tunnelling can only change Q in a discrete way: when
an electron tunnels through the insulating layer, the surface
charge Q will change exactly by either +e or -e, depending on the
direction of tunnelling. The interplay between continuous charge
flow in conductors and discrete transfer of charge through tunnel
junctions leads to several interesting effects. These phenomena can
be observed when the tunnel junctions are very small and the
ambient temperatures are very low. (Low temperatures reduce thermal
fluctuations that disturb the motion of electrons.) In this case,
if the charge Q at the junction is greater than +e/2, an electron
can tunnel through the junction in a particular direction,
subtracting e from Q. The electron does so because this process
reduces the electrostatic energy of the system. (The energy
increases in proportion to the square of the charge and does not
depend on the sign of the charge.) Likewise, if Q is less than
-e/2, an electron can tunnel through the junction in the opposite
direction, adding e to Q, and thus again decrease the energy. But
if Q is less than +e/2 and greater than -e/2, tunnelling in any
direction would increase the energy of the system. Thus, if the
initial charge is within this range, tunnelling will not occur.
This suppression of tunnelling is known as the Coulomb
blockade.
[0006] If the surface charge Q is zero initially, then the system
is within the Coulomb blockade limits, and tunnelling is
suppressed. Therefore, the current flowing from the source through
wires will start to change the charge Q continuously. For
convenience, assume that the deposited charge rate is positive
rather than negative. If the charge reaches and slightly exceeds
+e/ 2, tunnelling becomes possible. One electron will then cross
the junction, making its charge slightly greater than -e/2. Hence,
the system is within the Coulomb blockade range again, and
tunnelling ceases to be possible. The current continues to add
positive charge to the junction at a constant rate, and Q grows
until it exceeds +e/2 again. The repetition of this process
produces the single-electron tunnelling (SET) oscillations: the
voltage changes periodically with a frequency equal to the current
divided by the fundamental unit of charge, e:
[0007] To produce SET oscillations, tunnel junctions must be of a
very small area and cooled to ensure that the thermal energy does
not influence tunnelling. Typically, the device must be cooled to
temperatures of about a tenth of a degree above absolute zero if
the junction is 100 nanometres in length and width.
[0008] European Patent Application EP 0 750 353 discloses a single
electron tunnel device of this invention which includes a multiple
tunnel junction layer including multiple tunnel junctions; and
first and second electrodes for applying a voltage to the multiple
tunnel junction layer, wherein the multiple tunnel junction layer
includes an electrically insulating thin film and metal particles
and/or semiconductor particles dispersed in the electrically
insulating thin film.
[0009] The electrically insulating thin film may be made of an
oxide and the particles may be of at least one type of metal
selected from the group consisting of gold (Au), silver (Ag),
copper (Cu), platinum (Pt), and palladium (Pd).of the particles.
Their diameter may be 50 nm or less.
[0010] Fabrication of suitable structures to support single
electron tunnelling has proved difficult. In particular, it has
proved difficult to form films having the size and disposition
which are suitable for tunnelling. However, we have now devised a
method suitable for the fabrication of arrays of these devices.
[0011] According to one aspect of the present invention there is
provided a single electron tunnelling device comprising a particle
of a material having a first conductivity characteristic having a
surface layer of a material of a second conductivity
characteristic, the thickness of said layer being sufficiently
small to support quantum mechanical tunnelling therethrough
together with first and second electrodes positioned adjacent to
said particle to facilitate the flow of current therebetween.
[0012] Said first and second electrodes may be superconducting.
[0013] In a preferred embodiment of the invention a plurality of
such particles is positioned between said first and second
electrodes.
[0014] There is also provided a method of fabricating single
electron devices comprising the steps of forming a plurality of
particles forming a layer of a thickness sufficiently small to
support quantum mechanical tunnelling on the surface of said
particles and positioning at least one of said particles between a
pair of electrodes to form a single electron device.
[0015] The invention will be particularly described by way of
example with reference to the accompanying drawings, in which
[0016] FIG. 1 is a flow chart illustrating the method according to
one aspect of the present invention;
[0017] FIG. 2 is shows in schematic form apparatus for producing
nanocrystals suitable for use for the fabrication of
single-electron devices;
[0018] FIG. 3 shows the structure of various nanoparticles;
[0019] FIG. 4 is a micrograph of a nanocrystal produced by the
process of aerotaxy
[0020] FIG. 5 is a schematic diagram of a prior art tunnel
junction
[0021] FIG. 6 is a schematic diagram showing a barrier between a
nanoparticle and a conducting substrate;
[0022] FIG. 7 is a diagram illustrating the principle of movement
of a nanoparticle by means of an atomic force microscope; and
[0023] FIGS. 8a and 8b are schematic diagrams illustrating the
principles of device construction in accordance with the
invention
[0024] Referring now to FIG. 1 of the drawings, this illustrates a
process for controlled formation of simple and multi-layered
metallic and semiconducting nanocrystals or nanoparticles suitable
for single electron device fabrication. Ultra-fine particles of a
Group III element are formed as an aerosol. These are then filtered
to select those of a predetermined size. A Group V precursor is
then added and the mixture processed to form nanocrystals of a
III-V semiconductor.
[0025] FIG. 2 shows an aerosol production unit in accordance with a
specific embodiment of the invention. This comprises a furnace F1
which generates metallic particles by sublimation. These particles
are then carried in a transport gas stream through a charger to a
particle size filter DMA1 and thence to a second furnace F2 where
the gas stream is mixed with the hydride of a Group V element and
heated to form nanoparticles of a II-V semiconductor. The
nanocrystals are then filtered to select those of a predetermined
size which are then deposited on to a substrate, which, preferably
is a semiconducting wafer. in a deposition chamber DC. An
electrometer E1 and pump Pu are connectable to the flow line to
create and measure the pressure therein.
[0026] In one embodiment, a semiconducting core nanocrystal is
coated by a surface layer of a material with different properties,
e.g. with a larger fundamental band-gap, fabricating nanocrystals,
the composition and size of which is tightly controlled. The
approach is unusual in that we have managed to form, in an aerosol
phase, metallic nanoparticles (or droplets) having a narrow
dimensional spread. The particles of elements from the third column
in the periodic table are later allowed to react with a vapour
containing selected atoms or molecules from the fifth column in the
periodic table, resulting in the production of nanocrystals of
III-V semiconductors of uniform size. This control requires a
completely saturated conversion of the primary metallic
nanoparticle into the corresponding III-V nanocrystal.
[0027] Gallium arsenide nanocrystals, of approximate diameter 10
nm, have been produced and deposited on various substrates. The
fabrication route allows the production of nanocrystals with a very
narrow size distribution. It utilises the formation of ultrafine
gallium particles and their self-limiting reaction with arsine at
elevated temperatures. The kinetics of the reaction of gallium to
produce gallium arsenide depends on the temperature and the arsine
flow. The temperature at which the reaction began was found to be
as low as 200.degree. C. This permitted the production of
nanocrystals of compound semiconductors of predeternined size in a
simple, reliable, and efficient way.
[0028] An important feature of a further embodiment of this
invention is a new technique for controllable formation of a
surface layer of a different semiconducting or insulating material
on these original nanocrystals. They may have a homogeneous core
and a surface layer of a second composition with an appropriate
electronic structure for the single-electron device operation.
[0029] After a size selection, the semiconducting or metallic
nanocrystal is exposed to a reacting gas environment while being
maintained in the aerosol phase. In one embodiment, a mono-disperse
aerosol of silicon nanocrystals is allowed to react with oxygen
under closely controlled conditions, leading to a controlled
thickness of the silicon particle being converted to silica.
SiO.sub.2 is an insulator with ideal and well characterised
interfaces with silicon. In a second embodiment, mono-disperse
nanocrystals of compound semiconductors, such as indium arsenide,
are allowed to interact with phosphorus-containing gaseous
molecules, an interaction which results in exchange processes by
which arsenic atoms in a finite depth surface layer are replaced by
phosphorus atoms, hence transforming the surface to a surface layer
of In(As)P. In a third embodiment, pre-fabricated nanoparticles of
indium react with oxygen to form a skin of InO. In this embodiment,
the simplest single-electronic building block is formed by
producing a homogenous particle, exemplified by a spherical
monodisperse particle shown in FIG. 3.
[0030] The second embodiment involves direct epitaxial deposition
of a different material on the surface of a primary core, often
called hetero-epitaxy. The art of hetero-epitaxy on flat surfaces
is at a very advanced stage but the use of nanoparticles as
"substrates" for aerosol-phase epitaxial crystal growth is very
novel. For the application of nanoparticles in single-electronics,
however, this is of great importance. Examples are the coating of a
small band-gap semiconductor with a thin epitaxial layer of a
larger band-gap material, such as indium phosphide on the surface
of indium arsenide or silicon on the surface of a core of
germanium. Finally there is a hetero-epitaxy based mechanism for
formation of semiconductor particles surrounded by very well
controlled insulating layers, which can be achieved by surrounding
a nanocrystal of gallium arsenide (for example) with a few
monolayers of epitaxially grown aluminium arsenide. At a late
stage, this aluminium arsenide layer is allowed to react with
oxygen to form a layer of aluminium oxide, most probably
Al.sub.2O.sub.3, which is an excellent insulator. Hence, the ideal
hetero-expitaxial process will lend itself indirectly to the
formation of a few mono-layer-thick insulating layer on
semiconductor particles. (FIG. 3)
[0031] FIG. 4 is a TEM image of an 8 nm indium phosphide particle
produced by the process of aerotaxy.
[0032] In the mechanism of single-electron devices, the most
important fundamental property is the existence of a central
conductive island which is coupled by tunnelling to source and
drain electrodes and coupled capacitively to a gate electrode. The
size-related capacitance of the central island should be
sufficiently low that the electrostatic charging energy
E=e.sup.2/2C is much larger than kT and in an energy range suitable
for device and circuit biasing. The dimensional requirements can be
described as:
[0033] for particle size, the diameter for room temperature
operation should be in the range 2-4 nm, corresponding to charging
energy of a few hundred meV, to be compared with kT (.about.26 meV
at room temperature).
[0034] for tunnelling gaps, the distances between conducting leads
and conducting particle, and the distance between connected
particles should support tunnelling, that is it should be in the
range 1-3 nm.
[0035] In most prior demonstrations of single-electron phenomena,
low temperatures at or below liquid helium boiling temperature (4K)
have been employed. The tolerance for lithographic definition of
the island size is much relaxed. In these studies, tunnelling
distances are often defined by an aluminium film, which is
converted by controlled oxidation into an insulating thin film,
placed in between the conductors.
[0036] Experiments performed at elevated temperatures, such as the
boiling point of liquid nitrogen (77K) or at room-temperature
(300K) have been performed with the use of small metallic (or
semiconducting) particles but with the tunnelling distances
controlled by a thin insulating film on which the particle rests
and, for the second electrode spacing, by a tunnelling distance
which is controlled by a scanning tunnelling microscope.
[0037] We have been able to fabricate planar single-electron
devices which are controllably created by a "nano-robot", an atomic
force microscope (AFM), by manipulation of size-selected nanometre
sized particles relative to pre-fabricated contacts. In this
approach capacitances are accurately controlled by the exact
particle fabrication (by aerosol technique) and tunnelling gaps are
governed by the controlled positioning of the nano-particles to
create the proper tunnelling current levels.
[0038] FIG. 5 illustrates a conventional thin film tunnel junction
device. The surface of a deposited film 11 is oxidise to form a
thin tunnel barrier 13 and a further conductor 15 is deposited
thereon. An analogous device based on a small metallic particle is
illustrated in FIG. 6. A thin oxide layer 17 is formed on a
conductive substrate 19. and small metallic particle 21 is
positioned thereon. Contact is made by means of the tip of a
scanning tunnelling microscope 23. This principle is extended in
the device illustrated in FIG. 7 in which a small metallic particle
25 is positioned between a source electrode 27 and a drain
electrode 29 by means of an atomic force microscope.
[0039] A key feature of one aspect of the present invention is the
prefabrication of particles in such a way that they provide the
conducting core as well as tightly controlled tunnel-gap, building
a network of identical capacitances and tunnelling rates permits
randomness in lateral location within an ensemble of
nano-particles.
[0040] The significance of the above aerosol-based fabrication of
granular single-electron circuits is illustrated in FIGS. 8a and 8b
which show a two-dimensional arrangement of nano-particles P
between two electrodes E1, E2 with non-identical (FIG.8a) vs
identical cores (FIG. 8b) as well as with random vs well-controlled
tunnel barriers 31. The tunnel barrier in most cases is exactly
twice the shell thickness, in two-dimensional as well as
three-dimensional randomly arranged arrays. The key feature is
that, due to nature of the single-electron tunnelling
characteristic, for a macroscopic device the number of
nanoparticles (in either two or tlree dimensions) between the
electrodes is not critical.
* * * * *