U.S. patent application number 11/533036 was filed with the patent office on 2007-01-18 for doped semiconductor nanocrystal layers and preparation thereof.
This patent application is currently assigned to Group IV Semiconductor Inc.. Invention is credited to Steven E. Hill, Edward A. Irving, Peter Mascher, Jacek Wojcik.
Application Number | 20070012907 11/533036 |
Document ID | / |
Family ID | 32771927 |
Filed Date | 2007-01-18 |
United States Patent
Application |
20070012907 |
Kind Code |
A1 |
Hill; Steven E. ; et
al. |
January 18, 2007 |
Doped Semiconductor Nanocrystal Layers And Preparation Thereof
Abstract
The present invention relates to a doped semiconductor
nanocrystal layer comprising (a) a group IV oxide layer which is
free of ion implantation damage, (b) from 30 to 50 atomic percent
of a semiconductor nanocrystal distributed in the group IV oxide
layer, and (c) from 0.5 to 15 atomic percent of one or more rare
earth element, the one or more rare earth element being (i)
dispersed on the surface of the semiconductor nanocrystal and (ii)
distributed substantially equally through the thickness of the
group IV oxide layer. The present invention also relates to a
semiconductor structure comprising the above semiconductor
nanocrystal layer and to processes for preparing the semiconductor
nanocrystal layer.
Inventors: |
Hill; Steven E.; (Castle
Rock, CO) ; Mascher; Peter; (Dundas, CA) ;
Wojcik; Jacek; (Dundas, CA) ; Irving; Edward A.;
(Hamilton, CA) |
Correspondence
Address: |
ALLEN, DYER, DOPPELT, MILBRATH & GILCHRIST P.A.
1401 CITRUS CENTER 255 SOUTH ORANGE AVENUE
P.O. BOX 3791
ORLANDO
FL
32802-3791
US
|
Assignee: |
Group IV Semiconductor Inc.
McMaster University
|
Family ID: |
32771927 |
Appl. No.: |
11/533036 |
Filed: |
September 19, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10761409 |
Jan 22, 2004 |
|
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11533036 |
Sep 19, 2006 |
|
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60441413 |
Jan 22, 2003 |
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Current U.S.
Class: |
257/9 |
Current CPC
Class: |
B82Y 20/00 20130101;
B82Y 30/00 20130101; C09K 11/7706 20130101; B82Y 15/00 20130101;
H01S 3/09403 20130101; C09K 11/77 20130101 |
Class at
Publication: |
257/009 |
International
Class: |
H01L 29/06 20060101
H01L029/06 |
Claims
1-45. (canceled)
46. A process for preparing a doped semiconductor nanocrystal
layer, the process comprising: (a) introducing (i) a gaseous
mixture of a group IV element precursor and molecular oxygen, and
(ii) a gaseous rare earth element precursor, at the same time in a
plasma stream of a Plasma Enhanced chemical Vapor Deposition
(PECVD) instrument, whereby the rare earth element and the group IV
element are deposited onto a substrate simultaneously to form a
semiconductor rich group IV oxide layer doped with a rare earth
element, and (b) annealing the semiconductor rich group IV oxide
layer doped with a rare earth element at a temperature of from
600.degree. C. to 1000.degree. C., whereby atomic excess of the
group IV element is converted into semiconductor nanocrystals; and
whereby the rare earth elements are dispersed through the
semiconductor rich group IV oxide layer when the semiconductor
nanocrystals are formed, and whereby the rare earth elements are
localized on the surface of the nanocrystals.
47. A process according to claim 46, wherein the group IV element
precursor is a hydride of a group IV element.
48. A process according to claim 46, wherein the group IV element
precursor comprises silicon, germanium, tin or lead.
49. A process according to claim 46, wherein the group IV element
precursor is silane.
50. A process according to claim 46, wherein the ratio of the group
IV element precursor and of the molecular oxygen is selected to
obtain the semiconductor rich group IV oxide layer with 30 to 50
atomic percent of excess semiconductor.
51. A process according to claim 46, wherein the rare earth element
precursor comprises a rare earth element selected from cerium,
praseodymium, neodymium, promethium, gadolinium, erbium, thulium,
ytterbium, samarium, dysprosium, terbium, europium, holmium,
lutetium, and thorium.
52. A process according to claim 46, wherein the rare earth element
precursor comprises erbium, thulium or europium.
53. A process according to claim 46, wherein the rare earth element
precursor comprises a ligand selected from
2,2,6,6-tetramethyl-3,5-heptan-edione, acetylacetonate,
flurolacetonate,
6,6,7,7,8,8,8-heptafluoro-2,2-di-methyl-3,5-octanedione,
i-propylcyclopentadienyl, cyclopentadienyl, and
n-butylcyclopentadienyl; whereby the rare earth element precursor
with the ligand forms a compound, which is volatile and enters the
gaseous phase at a relatively low temperature without changing the
chemical nature of the compound, and comprises organic components
that, upon exposure to the plasma in the PECVD apparatus, will form
gaseous by-products that can be removed through gas flow or by
reducing the pressure within the PECVD apparatus.
54. A process according to claim 46, wherein the rare earth element
precursor is selected from
tris(2,2,6,6-tetramethyl-3,5-heptanedionato) erbium(III), erbium
(III) acetylacetonate hydrate, erbium (III) flurolacetonate,
tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedi-onate)erbium
(III), tris(i-propylcyclopentadienyl)erbium (III),
Tris(cyclopentadienyl)erbium (III), and
tris(n-butylcyclopentadienyl)erbi-um (III).
55. A process according to claim 46, wherein the semiconductor rich
group IV oxide layer is annealed at a temperature of from 800 to
950.degree. C.
56. (canceled)
57. The process according to claim 46, wherein step (b) is carried
out under an oxygen atmosphere to insure oxidation of the rare
earth element, or under a reduced pressure in order to facilitate
the removal of any volatile by-products that are produced.
58. The process according to claim 46, further comprising heating
the rare earth element precursor to ensure it is in a gaseous
state.
59. The process according to claim 58, wherein the rare earth
element precursor is heated in an oven at between 80.degree. C. and
110.degree. C.
60. The process according to claim 46, wherein the gaseous rare
earth element precursor is introduced to the plasma stream with an
inert carrier gas.
61. The process according to claim 46, wherein the gaseous rare
earth element precursor is introduced to the plasma at a position
that is below a position where the group IV element containing
compound is introduced to the plasma.
62. The process according to claim 46, wherein step a) includes use
of a dispersion ring to assist in the dispersion of the gaseous
rare earth element precursor in the plasma.
63. The process according to claim 46, wherein step a) includes
placing the substrate on a rotating scepter to obtain a more even
deposition of the semiconductor rich group IV oxide layer.
64. The process according to claim 46, further comprising producing
the plasma in an Electron Cyclotron Resonated (ECR) reactor,
wherein electrons have a spiral motion caused by a magnetic field,
which allows a high density of ions in a low-pressure region,
whereby a rare earth metal component of the rare earth element
precursor is stripped of organic components of the rare earth
element precursor and incorporated uniformly and in a high
concentration.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a divisional application of U.S.
patent application Ser. No. 10/761,409, filed on Jan. 22, 2004,
entitled "DOPED SEMICONDUCTOR NANOCRYSTAL LAYERS", assigned to the
assignee of the present application, and claims the benefit of U.S.
Provisional Patent application Ser. No. 60/441,413, filed Jan. 22,
2003 entitled "PREPARATION OF TYPE IV SEMICONCUDTOR NANOCRYSTALS
DOPED WITH RARE-EARTH IONS AND PRODUCT THEREOF", the contents of
which are hereby incorporated by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to semiconductor nanocrystal
layers doped with rare earth elements, to semiconductor structures
comprising these semiconductor nanocrystal layers, and to processes
for preparing the semiconductor nanocrystal layers doped with rare
earth elements.
BACKGROUND OF THE INVENTION
[0003] Silicon has been a dominant semiconductor material in the
electronics industry, but it does have a disadvantage in that it
has poor optical activity due to an indirect band gap.
[0004] This poor optical activity has all but excluded silicon from
the field of optoelectronics. In the past two decades there have
been highly motivated attempts to develop a silicon-based light
source that would allow one to have combined an integrated digital
information processing and an optical communications capability
into a single silicon-based integrated structure. For a
silicon-based light source (silicon Light Emitting Diode (LED)) to
be of any practical use, it should (1) emit at a technologically
important wavelength, (2) achieve its functionality under practical
conditions (e.g. temperature and pump power), and (3) offer
competitive advantage over existing technologies.
[0005] One material that has gathered much international attention
is erbium (Er) doped silicon (Si). The light emission from Er-doped
Si occurs at the technological important 1.5 micron (gm)
wavelength. Trivalent erbium in a proper host can have a
fluorescence of 1540 nm due to the .sup.4113/.sup.2->
.sup.4115/2 intra-4f transition. This 1540 nm fluorescence occurs
at the minimum absorption window of the silica-base
telecommunication fibre optics field. There is great interest in Er
doping of silicon as it holds the promise of silicon based
optoelectronics from the marriage of the vast infrastructure and
proven information processing capability of silicon integrated
circuits with the optoelectronics industry.
[0006] Theoretical and experimental results also suggest that Er in
Si is Auger-excited via carriers, generated either electrically or
optically, that are trapped at the Er-related defect sites and then
recombine, and that this process can be very efficient due to
strong carrier-Er interactions. However, if this strong carrier-Er
interaction is attempted in Er-doped bulk Si, the efficiency of the
Er.sup.3+ luminescence is reduced at practical temperature and pump
powers.
[0007] Recently, it has been demonstrated that using silicon-rich
silicon oxide (SRSO), which consists of Si nanocrystals embedded in
a SiO.sub.2 (glass) matrix, reduces many of the problems associated
with bulk Si and can have efficient room temperature Er.sup.3+
luminescence. The Si nanocrystals act as classical sensitizer atoms
that absorb incident photons and then transfer the energy to the Er
.sup.3+ ion, which then fluoresce at the 1.5 micron wavelength with
the following significant differences. First, the absorption cross
section of the Si nanocrystals is larger than that of the Er.sup.3,
ions by more than 3 orders of magnitude. Second, as excitation
occurs via Auger-type interaction between carriers in the Si
nanocrystals and Er.sup.3+ ions, incident photons need not be in
resonance with one of the narrow absorption bands of Er.sup.3+.
However, existing approaches to developing such Si nanocrystals
have only been successful at producing concentrations of up to 0.3
atomic percent of the rare earth element, which is not sufficient
for practical applications.
[0008] In general, manufacture of type IV semiconductor
nanocrystals doped with a rare earth element is done by ion
implantation of silicon ions into a silicon oxide layer, followed
by high temperature annealing to grow the silicon nanocrystals and
to reduce the ion implantation damage. The implantation of Si ions
is followed by an ion implantation of the rare earth ions into the
annealed silicon nanocrystal oxide layer. The resulting layer is
again annealed to reduce the ion implant damage and to optically
activate the rare-earth ion.
[0009] There are several problems with this method: i) it results
in a decreased layer surface uniformity due to the ion
implantation; ii) it requires an expensive ion implantation step;
iii) it fails to achieve a uniform distribution of group IV
semiconductor nanocrystals and rare-earth ions unless many
implantation steps are carried out; and iv) it requires a balance
between reducing the ion implant damage by thermal annealing while
trying to maximise the optically active rare-earth.
[0010] To diminish the above drawbacks, Plasma Enhanced Chemical
Vapor Deposition (PECVD) has been utilised to make type IV
semiconductor nanocrystal layers. The prepared layers are then
subjected to a rare-earth ion implantation step and a subsequent
annealing cycle to form the IV semiconductor nanocrystals, and to
optically activate the rare-earth ions that are doped in the
nanocrystal region. Unfortunately, the layers prepared with this
method are still subjected to an implantation step, which results
in a decrease in surface uniformity.
[0011] Another PECVD method that has been used to obtain a doped
type IV semiconductor crystal layer consists of co-sputtering
together both the group IV semiconductor and rare-earth metal. In
this method, the group IV semiconductor and a rare-earth metal are
placed into a vacuum chamber and exposed to an Argon ion beam. The
argon ion beam sputters off the group IV semiconductor and the
rare-earth metal, both of which are deposited onto a silicon wafer.
The film formed on the silicon wafer is then annealed to grow the
nanocrystals and to optically activate the rare-earth ions. As the
rare earth metal is in solid form, the argon ion beam (plasma) is
only able to slowly erode the rare earth, which leads to a low
concentration of rare earth metal in the deposited film. While
higher plasma intensity could be used to more quickly erode the
rare earth metal and increase the rare earth concentration in the
film, a higher intensity plasma damages the film or the group IV
semiconductor before it is deposited. The plasma intensity is
therefore kept low to preserve the integrity of the film, therefore
limiting the rare earth concentration in the film. The doped group
IV semiconductor nanocrystal layers made through this method have
the drawbacks that: i) the layer does not have a very uniform
distribution of nanocrystals and rare-earth ions, ii) the layer
suffers from upconversion efficiency losses due to rare-earth
clustering in the film, and iii) the concentration of rare earth
metal in the layer is limited by the plasma intensity, which is
kept low to avoid damaging the layer.
[0012] The concentration of the rare earth element in semiconductor
nanocrystal layers is preferably as high as possible, as the level
of photoelectronic qualities of the film, such as
photoluminescence, is proportional to the concentration. One
problem encountered when a high concentration of rare earth element
is present within the semiconductor layer is that when two rare
earth metals come into close proximity with one another, a
quenching relaxation interaction occurs that reduces the level of
photoelectronic dopant response observed. The concentration of rare
earth element within a semiconductor film is thus balanced to be as
high as possible to offer the most fluorescence, but low enough to
limit the quenching interactions.
SUMMARY OF THE INVENTION
[0013] In one aspect, the present invention provides a doped
semiconductor nanocrystal layer, the doped semiconductor
nanocrystal layer comprising (a) a group IV oxide layer which is
free of ion implantation damage, (b) semiconductor nanocrystals
distributed in the group IV semiconductor oxide layer, and (c) from
0.5 to 15 atomic percent of one or more rare earth elements. The
one or more rare earth element are: (i) dispersed on the surface of
the semiconductor nanocrystal and (ii) distributed substantially
equally through the thickness of the group IV oxide layer.
[0014] In another aspect, the present invention provides a
semiconductor structure comprising a substrate, on which substrate
is deposited one or more of the doped semiconductor nanocrystal
layer described above.
[0015] In another aspect, the present invention provides a process
for preparing a doped semiconductor nanocrystal layer, the process
comprising:
[0016] (a) introducing (i) a gaseous mixture of a group IV element
precursor and molecular oxygen, and (ii) a gaseous rare earth
element precursor, in a plasma stream of a Plasma Enhanced chemical
Vapor Deposition (PECVD) instrument to form a semiconductor rich
group IV oxide layer doped with a rare earth element, and
[0017] (b) annealing the semiconductor rich group IV oxide layer
doped with a rare earth element at a temperature of from
600.degree. C. to 1000.degree. C.
[0018] The above and other objects, features and advantages of the
present invention will become apparent from the following
description when taken in conjunction with the accompanying figures
which illustrate preferred embodiments of the present invention by
way of example.
DESCRIPTION OF THE FIGURES
[0019] Embodiments of the invention will be discussed with
reference to the following Figures:
[0020] FIG. 1 is a diagram of a semiconductor structure comprising
a substrate, a doped semiconductor nanocrystal layer, and a current
injection layer;
[0021] FIG. 2 is a diagram of a superlattice semiconductor
structure comprising a substrate and alternating doped
semiconductor nanocrystal layers and dielectric layers; and
[0022] FIG. 3 is a diagram of a Pulse Laser Deposition
apparatus.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Doped Semiconductor Nanocrystal Layer
[0023] The doped semiconductor nanocrystal layer of the invention
comprises a group IV oxide layer in which is distributed
semiconductor nanocrystals. The group IV element used to prepare
the layer is preferably selected from silicon, germanium, tin and
lead, and the group IV semiconductor oxide layer is more preferably
silicon dioxide. The group IV oxide layer preferably has a
thickness of from 1 to 2000 nm, for example of from 80 to 2000 nm,
from 100 to 250 nm, from 30 to 50 nm, or from 1 to 10 nm.
[0024] The semiconductor nanocrystals that are dispersed within the
group IV semiconductor oxide layer are preferably the nanocrystals
of a group IV semiconductor, e.g. Si or Ge, or a group II-VI
semiconductor, e.g. ZnO, ZnS, ZnSe, CaS, CaTe or CaSe, or of a
group III-V semiconductor, e.g. GaN, GaP or GaAs. The nanocrystals
are preferably from 1 to 10 nm in size, more preferably from 1 to 3
nm in size, and most preferably from 1 to 2 nm in size. Preferably,
the semiconductor material is present within the group IV
semiconductor oxide layer in a concentration of from 30 to 50
atomic percent, more preferably in a total concentration of 37 to
47 atomic percent, and most preferably in a concentration of from
40 to 45 atomic percent.
[0025] The one or more rare earth element that is dispersed on the
surface of the semiconductor nanocrystal can be selected to be a
lanthanide element, such as cerium, praseodymium, neodymium,
promethium, gadolinium, erbium, thulium, ytterbium, samarium,
dysprosium, terbium, europium, holmium, or lutetium, or it can be
selected to be an actinide element, such as thorium. Preferably,
the rare earth element is selected from erbium, thulium, and
europium. The rare earth element can, for example, take the form of
an oxide or of a halogenide. Of the halogenides, rare earth
fluorides are preferred as they display more intense fluorescence
due to field distortions in the rare earth-fluoride matrix caused
by the high electronegativity of fluorine atoms. Most preferably,
the rare earth element is selected from erbium oxide, erbium
fluoride, thulium oxide, thulium fluoride, europium oxide and
europium fluoride.
[0026] The one or more rare earth element is preferably present in
the group IV semiconductor oxide layer in a concentration of 0.5 to
15 atomic percent, more preferably in a concentration of 5 to 15
atomic percent and most preferably in a concentration of 10 to 15
atomic percent. While such a high concentration of rare earth
element has led to important levels of quenching reactions in
previous doped semiconductor materials, the doped semiconductor
nanocrystal layer of the present invention can accommodate this
high concentration as the rare earth element is dispersed on the
surface of the semiconductor nanocrystal, which nanocrystal offers
a large surface area. The reduced amount of quenching reactions
between the rare earth element and the proximity of the rare earth
element to the semiconductor nanocrystal provide the basis for a
doped semiconductor nanocrystal layer that offers improved
optoelectronic properties.
Semiconductor Structure
[0027] Using the doped semiconductor nanocrystal layer described
above, a multitude of semiconductor structures can be prepared. For
example, a semiconductor structure is shown in FIG. 1, in which one
or more layers 33 of the doped semiconductor nanocrystal layer are
deposited on a substrate 31.
[0028] The substrate on which the semiconductor nanocrystal layer
is formed is selected so that it is capable of withstanding
temperatures of up to 1000.degree. C. Examples of suitable
substrates include silicon wafers or poly silicon layers, either of
which can be n-doped or p-doped (for example with 1.times.10.sup.20
to 5.times.10.sup.21 of dopants per cm.sup.3), fused silica, zinc
oxide layers, quartz and sapphire substrates. Some of the above
substrates can optionally have a thermally grown oxide layer, which
oxide layer can be of up to about 2000 nm in thickness, a thickness
of 1 to 20 nm being preferred. The thickness of the substrate is
not critical, as long as thermal and mechanical stability is
retained.
[0029] The semiconductor structure can comprise a single or
multiple doped semiconductor nanocrystal layers, each layer having
an independently selected composition and thickness. By using
layers having different rare earth elements, a multi-color emitting
structure can be prepared. For example, combining erbium, thulium
and europium in a single semiconductor structure provides a
structure that can fluoresce at the colors green (erbium), blue
(thulium), and red (europium).
[0030] When two or more doped semiconductor nanocrystal layers are
used in a single semiconductor structure, the layers can optionally
be separated by a dielectric layer. Examples of suitable dielectric
layers include silicon dioxide, silicon nitrite and silicon oxy
nitrite. The silicon dioxide dielectric layer can also optionally
comprise semiconductor nanocrystals. The dielectric layer
preferably has a thickness of from 1 to 10 nm, more preferably of 1
to 3 nm and most preferably of about 1.5 nm. The dielectric layer
provides an efficient tunnelling barrier, which is important for
obtaining high luminosity from the semiconductor structure.
[0031] The semiconductor structure can also have an Indium Tin
Oxide (ITO) current injection layer (34) overtop the one or more
doped semiconductor nanocrystal layers. The ITO layer preferably
has a thickness of from 150 to 300 nm. Preferably, the chemical
composition and the thickness of the ITO layer is such that the
semiconductor structure has a conductance of from 30 to 70 ohms
cm.
[0032] The thickness of the semiconductor structure is preferably
2000 nm or less, and the thickness will depend on the thickness of
the substrate, the number and thickness of the doped semiconductor
nanocrystal layers present, the number and the thickness of the
optional dielectric layers, and the thickness of the optional ITO
layer.
[0033] One type of preferred semiconductor structure provided by an
embodiment of the present invention is a superlattice structure,
shown by way of example in FIG. 2, which structure comprises
multiple layers of hetero-material 20 on a substrate 11. Multiple
doped semiconductor nanocrystals layers having a thickness of from
1 nm to 10 nm are deposited on the substrate 12 and 14, and the
doped semiconductor nanocrystals layers can comprise the same or
different rare earth elements. Optionally, the doped semiconductor
nanocrystal layers are separated by dielectric layers 13 of about
1.5 nm in thickness, and an ITO current injection layer (not shown)
can be deposited on top of the multiple layers of the superlattice
structure. There is no maximum thickness for the superlattice
structure, although a thickness of from 250 to 2000 nm is preferred
and a thickness of from 250 to 750 nm is more preferred.
Preparation of the Doped Semiconductor Nanocrystal Layer
[0034] The preparation of the doped semiconductor nanocrystal layer
comprises the following two general steps:
[0035] (a) the simultaneous deposition of a semiconductor rich
group IV oxide layer and of one or more rare earth element; and
[0036] (b) the annealing of the semiconductor rich group IV oxide
layer prepared in (a) to form semiconductor nanocrystals.
[0037] The semiconductor rich group IV oxide layer comprises a
group IV oxide layer, which group IV oxide is preferably selected
from SiO.sub.2 or GeO.sub.2i, in which group IV oxide layer is
dispersed a rare earth element and a semiconductor, which
semiconductor can be the same as, or different than, the
semiconductor that forms the group IV oxide layer.
[0038] By "semiconductor rich", it is meant that an excess of
semiconductor is present, which excess will coalesce to form
nanocrystals when the semiconductor rich group IV oxide layer is
annealed. Since the rare earth element is dispersed within the
oxide layer when the nanocrystals are formed, the rare earth
element becomes dispersed on the surface of the semiconductor
nanocrystals upon nanocrystal formation.
[0039] Since the semiconductor rich group IV oxide layer and the
one or more rare earth element are deposited simultaneously, ion
implantation of the rare earth element is avoided. As such, the
group IV oxide layer surface is free of the damage associated with
an implantation process. Also, since the rare earth element is
deposited at the same time as the semiconductor rich group IV oxide
layer, the distribution of the rare earth element is substantially
constant through the thickness of the group IV oxide layer.
[0040] The deposition of the semiconductor rich group IV oxide
layer doped with one or more rare earth elements is preferably
carried out by Plasma-Enhanced Chemical Vapor Deposition (PECVD) or
by Pulse Laser Deposition (PLD). The above two methods each have
their respective advantages for preparing the semiconductor rich
group IV oxide layer doped with one or more rare earth elements,
and the methods are described below.
Pulse Laser Deposition
[0041] Pulse laser deposition is advantageous for the deposition of
the semiconductor rich group IV oxide layer doped with one or more
rare earth elements as it permits the deposition of a wide variety
of semiconductors and a wide variety of rare earth elements.
[0042] Referring now to FIG. 3, which shows by way of a diagram a
typical set up of a pulse laser deposition apparatus, the pulse
laser deposition apparatus consists of a large chamber 41, which
can be evacuated down to at least 10.sup.-7 bars or pressurized
with up to 1 atmosphere of a gas such as oxygen, nitrogen, helium,
argon, hydrogen or combinations thereof. The chamber has at least
one optical port 42 in which a pulse laser beam 45 can be injected
to the chamber and focused down onto a suitable target 44. The
target is usually placed on a carrousel 43 that allows the
placement of different target samples into the path of the pulse
laser focus beam. The carrousel is controlled so that multiple
layers of material can be deposited by the pulse laser ablation of
the target. The flux of the focused pulse laser beam is adjusted so
that the target ablates approximately 0.1 nm of thickness of
material on a substrate 47, which can be held perpendicular to the
target and at a distance of 20 to 75 millimetres above the target.
This flux for instance is in the range of 0.1 to 20 joules per
square cm for 248 nm KrF excimer laser and has a pulse width of
20-45 nanosecond duration. The target can be placed on a scanning
platform so that each laser pulse hits a new area on the target,
thus giving a fresh surface for the ablation process. This helps
prevent the generation of large particles, which could be ejected
in the ablation plume 46 and deposited on to the substrate. The
substrate is usually held on a substrate holder 48, which can be
heated from room temperature up to 1000.degree. C. and rotated from
0.1 to 30 RPM depending on the pulse rate of the pulse laser, which
in most cases is pulsed between 1- 10 Hz. This rotation of the
substrate provides a method of generating a uniform film during the
deposition process. The laser is pulsed until the desired film
thickness is met, which can either be monitored in real time with
an optical thickness monitor or quartz crystal microbalance or
determined from a calibration run in which the thickness is
measured from a given flux and number of pulses. Pulse laser
deposition can be used for depositing layers of from 1 to 2000 nm
in thickness.
[0043] For the preparation of a semiconductor rich group IV oxide
layer doped with one or more rare earth elements, the target that
is ablated is composed of mixture of a powdered group IV binding
agent, a powdered semiconductor that will form the nanocrystal, and
a powdered rare earth element. The ratio of the various components
found in the doped semiconductor nanocrystal layer is decided at
this stage by controlling the ratio of the components that form the
target. Preferably, the mixture is placed in a hydraulic press and
pressed into a disk of 25 mm diameter and 5 mm thickness with a
press pressure of at least 500 Psi while being heated to
700.degree. C. The temperature and pressure can be applied, for
example, for one hour under reduced pressure (e.g. 10.sup.-3 bars)
for about one hour. The press pressure is then reduced and the
resulting target is allowed to cool to room temperature.
[0044] The group IV binding agent can be selected to be a group IV
oxide (e.g. silicon oxide, germanium oxide, tin oxide or lead
oxide), or alternatively, it can be selected to be a group IV
element (e.g. silicon, germanium, tin or lead). When the group IV
binding agent is a group IV oxide, the binding agent, the
semiconductor and the rare earth element are combined to form the
target, and the pulse laser deposition is carried out in the
presence of any one of the gases listed above. If a group IV
element is used as the group IV binding agent instead, the pulse
laser deposition is carried out under an oxygen atmosphere,
preferably at a pressure of from 1.times.10.sup.-4 to
5.times.10.sup.-3 bar, to transform some or all of the group IV
element into a group IV oxide during the laser deposition process.
When the semiconductor element which is to form the nanocrystals is
selected to be a group II-VI semiconductor (e.g. ZnO, ZnS, ZnSe,
CaS, CaTe or CaSe) or a group III-V semiconductor (e.g. GaN, GaP or
GaAs), the oxygen concentration is kept high to insure that all of
the group IV element is fully oxidized. Alternatively, if the
nanocrystals to be formed comprise the same group IV semiconductor
element that is being used as the binding agent, the oxygen
pressure is selected so that only part of the group IV element is
oxidized. The remaining non-oxidized group IV element can then
coalesce to form nanocrystals when the prepared semiconductor rich
group IV oxide layer is annealed.
[0045] The powdered rare earth element that is used to form the
target is preferably in the form of a rare earth oxide or of a rare
earth halogenide. As mentioned above, the rare earth fluoride is
the most preferred of the rare earth halogenides.
[0046] Pulse laser deposition is useful for the subsequent
deposition of two or more different layers. Multiple targets can be
placed on the carrousel and the pulse laser can be focussed on
different targets during the deposition. Using this technique,
layers comprising different rare earth elements can be deposited
one on top of the other to prepare semiconductor structures as
described earlier. Different targets can also be used to deposit a
dielectric layer between the semiconductor rich group IV oxide
layers, or to deposit a current injection layer on top of the
deposited layers. Pulse laser deposition is the preferred method
for preparing the superlattice semiconductor structure described
above.
[0047] Preparation of the semiconductor rich group IV oxide layer
doped with one or more rare earth elements can of course be carried
out with different pulse laser deposition systems that are known in
the art, the above apparatus and process descriptions being
provided by way of example.
Plasma Enhanced Chemical Vapor Deposition
[0048] PECVD is advantageous for the deposition of the
semiconductor rich group IV oxide layer doped with one or more rare
earth element, as it permits the rapid deposition of the layer. The
thickness of the semiconductor rich group IV oxide layer doped with
one or more rare earth element prepared with PECVD is 10 nm or
greater, more preferably from 10 to 2000 nm.
[0049] Formation of anon-doped type IV semiconductor nanocrystal
layer through chemical vapor deposition has been described, for
example, by J. Sin, M. Kim, S. Seo, and C. Lee [Applied Physics
Letters, (1998), Volume 72, 9, 1092-1094], the disclosure of which
is hereby incorporated by reference.
[0050] In this embodiment, the doped semiconductor nanocrystal
layer is prepared by incorporating a rare-earth precursor into the
PECVD stream above the receiving heated substrate on which the
semiconductor film is grown. PECVD can be used to prepare the doped
semiconductor nanocrystal layer where the semiconductor nanocrystal
is a silicon or a germanium nanocrystal, and where the rare earth
element is a rare earth oxide.
[0051] In the PECVD process, a group IV element precursor is mixed
with oxygen to obtain a gaseous mixture where there is an atomic
excess of the group IV element. An atomic excess is achieved when
the ratio of oxygen to group IV element is such that when a group
IV dioxide compound is formed, there remains an excess amount of
the group IV element. The gaseous mixture is introduced within the
plasma stream of the PECVD instrument, and the silicon and the
oxygen are deposited on a substrate as a group IV dioxide layer in
which a group IV atomic excess is found. It is this excess amount
of the group IV element that coalesces during the annealing step to
form the group IV nanocrystal. For example, to prepare a silicon
dioxide layer in which silicon nanocrystals is dispersed, a silicon
rich silicon oxide (SRSO) layer is deposited on the substrate.
[0052] The group IV element precursor can contain, for example,
silicon, germanium, tin or lead, of which silicon and germanium are
preferred. The precursor itself is preferably a hydride of the
above elements. A particularly preferred group IV element precursor
is silane (SiH.sub.4).
[0053] The ratio (Q) of group IV element precursor to oxygen can be
selected to be from 3:1 to 1:2. If an excess of group IV element
precursor hydride is used, the deposited layer can contain
hydrogen, for example up to approximately 10 atomic percent
hydrogen. The ratio of the flow rates of the group IV element
precursor and of oxygen can be kept, for example, between 2:1 and
1:2.
[0054] Also introduced to the plasma stream is a rare earth element
precursor, which precursor is also in the gaseous phase. The rare
earth precursor is added to the plasma stream at the same time as
the group IV element precursor, such that the rare earth element
and the group IV element are deposited onto the substrate
simultaneously. Introduction of the rare earth precursor as a
gaseous mixture provides better dispersion of the rare earth
element within the group IV layer.
[0055] Preferably, presence of oxygen in the plasma stream and in
the deposited layer leads to the deposition of the rare earth
element in the form of a rare earth oxide.
[0056] The rare earth element precursor comprises one or more
ligands. The ligand can be neutral, monovalent, divalent or
trivalent. Preferably, the ligand is selected so that when it is
coordinated with the rare earth element, it provides a compound
that is volatile, i.e. that enters the gaseous phase at a fairly
low temperature, and without changing the chemical nature of the
compound. The ligand also preferably comprises organic components
that, upon exposure to the plasma in the PECVD apparatus, will form
gaseous by-products that can be removed through gas flow or by
reducing the pressure within the PECVD apparatus. When the organic
components of the ligand are conducive to producing volatile
by-products (e.g. CO.sub.2, O.sub.2) less organic molecules are
incorporated into the deposited layer. Introduction of organic
molecules into the deposited layer is generally not beneficial, and
the presence of organic molecules is sometimes referred to as
semiconductor poisoning.
[0057] Suitable ligands for the rare earth element can include
acetate functions, for example 2,2,6,6-tetramethyl-3,5
heptanedione, acetylacetonate, flurolacetonate,
6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedione,
I-propylcyclopentadienyl, cyclopentadienyl, and
n-butylcyclopentadienyl. Preferred rare earth metal precursor
include tris(2,2,6,6-tetramethyl-3,5-heptanedionato) erbium(III),
erbium (III) acetylacetonate hydrate, erbium (III) flurolacetonate,
tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate)erbium
(III), tris(i-propylcyclopentadienyl)erbium (III),
Tris(cyclopentadienyl)erbium (III), and
tris(n-butylcyclopentadienyl)erbium (III). A particularly preferred
rare earth element precursor is
tris(2,2,6,6-tetramethyl-3,5-heptanedionato) erbium(III) (Er.sup.+3
[(CH.sub.3).sub.3CCOCH=COC(CH.sub.3).sub.3].sub.3), which is also
referred to as Er+.sup.3 (THMD).sub.3.
[0058] If the rare earth element precursor is not in the gaseous
phase at room temperature, it must be transferred to the gaseous
phase, for example, by heating in an oven kept between 80.degree.
C. and 110.degree. C. The gaseous rare earth element precursor is
then transferred to the plasma stream with an inert carrier gas,
such as argon. The gaseous rare earth element precursor is
preferably introduced to the plasma at a position that is below a
position where the group IV element containing compound is
introduced to the plasma. Use can be made of a dispersion
mechanism, for example a dispersion ring, to assist in the
dispersion of the gaseous rare earth element precursor in the
plasma.
[0059] In order to obtain a more even deposition of the doped type
IV oxide layer, the substrate can be placed on a sceptre that
rotates during deposition. A circular rotation of about 3 rpm is
suitable for increasing the uniformity of the layer being
deposited.
[0060] An Electron Cyclotron Resonated (ECR) reactor is suitable
for producing the plasma used in the PECVD method described above.
ECR is a particular method of generating plasma, where the
electrons have a spiral motion caused by a magnetic field, which
allows a high density of ions in a low-pressure region. The high
ion density with low pressure is beneficial for deposition, as the
rare earth metal precursor can be stripped of its organic
components and incorporated uniformly and in a high concentration.
The plasma used in the PECVD method can comprise, for example,
argon, helium, neon or xenon, of which argon is preferred.
[0061] The PECVD method is carried out under a reduced pressure,
for example 1.times.10.sup.-7 torr, and the deposition temperature,
microwave power and scepter bias can be kept constant. Suitable
temperature, microwave and scepter bias values can be selected to
be, for example, 300.degree. C., 400 W and -200V.sub.DC,
respectively.
[0062] The semiconductor rich group IV oxide layer doped with one
or more rare earth element can be grown at different rates,
depending on the parameters used. A suitable growth rate can be
selected to be about 60 nm per minute, and the semiconductor rich
group IV oxide layer can have a thickness of from 10 to 2000 nm,
more preferably of from 100 to 250 nm.
[0063] Preparation of the semiconductor rich group IV oxide layer
doped with one or more rare earth elements can of course be carried
out with different plasma enhanced chemical vapor deposition
systems that are known in the art, the above apparatus and process
descriptions being provided by way of example.
Annealing Step
[0064] After the semiconductor rich group IV oxide layer doped with
one or more rare earth element has been prepared, the doped type IV
oxide layer is annealed, optionally under flowing nitrogen
(N.sub.2), in a Rapid Thermal Anneal (RTA) furnace, at from about
600.degree. C. to about 1000.degree. C., more preferably from
800.degree. C. to 950.degree. C., from 5 minutes to 30 minutes,
more preferably from 5 to 6 minutes. It is during the annealing
step that the atomic excess of semiconductor is converted into
semiconductor nanocrystals.
[0065] When PECVD is used to prepare the semiconductor rich group
IV oxide layer doped with one or more rare earth element, the
annealing step can also be carried out under an oxygen atmosphere
to insure oxidation of the rare earth element, or under a reduced
pressure in order to facilitate the removal of any volatile
by-products that might be produced.
[0066] The amount of excess semiconductor in the group IV oxide
layer and the anneal temperature dictate the size and the density
of the semiconductor nanocrystal present in the final doped
semiconductor nanocrystal layer.
[0067] Since the rare earth element is well dispersed through the
deposited group IV semiconductor oxide layer, when the nanocrystals
are formed during the annealing step, the rare earth element
becomes localised on the surface of the nanocrystals. Since the
nanocrystals provide a large surface area on which the rare earth
element can be dispersed, the concentration of the rare earth
element can be quite elevated, while retaining good photoelectronic
properties.
[0068] The following examples are offered by way of illustration
and not by way of limitation.
EXAMPLES
Example 1
[0069] Silane (SiH.sub.4) and Oxygen (O.sub.2) are added to an
argon plasma stream produced by an Electron Cyclotron Resonated
(ECR) reactor via dispersion ring. The ratio (Q) of silane to
oxygen has been varied between 3:1,1.7:1,1.2:1,1:1.9, and 1:2. An
erbium precursor (Tris(2,2,6,6-tetramethyl-3,5-heptanedionato)
erbium(III) [Er.sup.+3(THMD).sub.3]) is placed in a stainless steel
oven held between 90 and 110.degree. C.
[0070] A carrier gas of Ar is used to transport the Er precursor
from the oven through a precision controlled mass-flow controller
to a dispersion ring below the Silane injector and above the heated
substrate. The instrument pressure is kept at about
1.times.10.sup.-7 torr. The substrates used are either fuse silica
or silicon wafers on which is thermally grown an oxide layer of
2000 nm thickness. The deposition temperature, the microwave power
and the sceptre bias are kept constant at 300.degree. C., 400 W and
-200V.sub.DC. The SiH.sub.4, Ar flow rates were adjusted while
keeping the O.sub.2 flow rate at 20 militorr sec.sup.-1 for the
various excess silicon content. The Er/Ar flow rate was adjusted to
the vapor pressure generated by the temperature controlled oven for
the desired erbium concentration. The film is grown at a rate of 60
nm per minute and thickness has been grown from 250 nm to 2000 nm
thick. The scepter was rotated at 3 rpm during the growth to help
in uniformity of film. After deposition, the samples are annealed
at 950.degree. C. -1000.degree. C. for 5-6 minutes under flowing
nitrogen (N.sub.2) in a Rapid Thermal Anneal (RTA) furnace.
Example 2
[0071] An ablation target is fabricated by combining powdered
silicon, powdered silicon dioxide and powdered erbium oxide, the
prepared powder mixture comprising 45% silicon, 35% silicon oxide
and 20% erbium oxide. Each powder component has a size of about 300
mesh. The mixture is placed into a ball mill and ground for
approximately 5 to 10 minutes. The mixture is then placed into a 25
mm diameter by 7 mm thick mould, placed into a hydraulic press, and
compressed for 15 minutes at 500 psi. The obtained target is then
placed into an annealing furnace and heated to 1200.degree. C. in a
forming gas atmosphere of 5% H.sub.2 and 95% N.sub.2 for 30
minutes. The Target is cooled down to room temperature and then
reground in a ball mill for ten minutes. The mixture is then again
placed in a mould, compressed and annealed as described above. The
obtained target is placed onto a target holder inside a vacuum
chamber. A silicon substrate [n-type, <110> single crystal,
0.1-0.05 .OMEGA.cm conductivity] of 50 mm diameter and 0.4 cm
thickness is placed on a substrate holder parallel to and at a
distance of 5.0 cm above the surface of the target. The substrate
is placed onto a substrate support that is heated at 500.degree.
C., and the substrate is rotated at a rate of 3 rpm during the
deposition. The vacuum chamber is evacuated to a base pressure of
1.times.10.sup.-7 torr and then back filled with 20.times.10.sup.-3
torr of Ar. An excimer laser (KrF 248 nm) is focused on to the
target at an energy density of about 10 JCm.sup.-2 and at a
glancing angle of 40.degree. to the vertical axis, such that a 0.1
nm film is generated per pulse. The target is rotated at 5 rpm,
during deposition in order to have a fresh target surface for each
ablation pulse. After a 100 nm layer is deposited on the substrate,
the newly deposited film is annealed at temperature of from
900.degree. C. to 950.degree. C. for 5 minutes to form silicon
nanocrystals in the Silicon Rich Silicon Oxide (SRSO).
[0072] The substrate is reintroduced in the vacuum chamber, 20 and
the target is replaced with an Indium Tin Oxide (ITO) target. The
atmosphere inside the vacuum chamber is set to
2.times.10.sup.-3torr of O.sub.2, and the substrate is heated to
500.degree. C. and rotated at 3 rpm. A 100 nm ITO layer is
deposited on top of the annealed rare earth doped SRSO film.
[0073] All publications, patents and patent applications cited in
this specification are herein incorporated by reference as if each
individual publication, patent or patent application were
specifically and individually indicated to be incorporated by
reference. The citation of any publication is for its disclosure
prior to the filing date and should not be construed as an
admission that the present invention is not entitled to antedate
such publication by virtue of prior invention.
[0074] Although the foregoing invention has been described in some
detail by way of illustration and example for purposes of clarity
of understanding, it is readily apparent to those of ordinary skill
in the art in light of the teachings of this invention that certain
changes and modifications may be made thereto without departing
from the spirit or scope of the appended claims.
[0075] It must be noted that as used in this specification and the
appended claims, the singular forms "a", "an", and "the" include
plural reference unless the context clearly dictates otherwise.
Unless defined otherwise all technical and scientific terms used
herein have the same meaning as commonly understood to one of
ordinary skill in the art to which this invention belongs.
* * * * *