U.S. patent application number 11/832020 was filed with the patent office on 2008-08-07 for preparation method of a coating of gallium nitride.
This patent application is currently assigned to Centre National De La Recherche Scientifique (CNRS). Invention is credited to Nicolas GRANDJEAN, Jean MASSIES, Fabrice SEMOND.
Application Number | 20080188065 11/832020 |
Document ID | / |
Family ID | 8851146 |
Filed Date | 2008-08-07 |
United States Patent
Application |
20080188065 |
Kind Code |
A2 |
SEMOND; Fabrice ; et
al. |
August 7, 2008 |
PREPARATION METHOD OF A COATING OF GALLIUM NITRIDE
Abstract
The invention concerns a monocrystalline coating crack-free
coating of gallium nitride or mixed gallium nitride and another
metal, on a substrate likely to cause extensive stresses in the
coating, said substrate being coated with a buffer layer, wherein:
at least a monocrystalline layer of a material having a thickness
ranging between 100 and 300 nm, preferably between 200 and 250 nm,
and whereof crystal lattice parameter is less than the crystal
lattice parameter of the gallium nitride or of the mixed gallium
nitride with another metal, is inserted in the coating of gallium
nitride or mixed gallium nitride with another metal. The invention
also concerns the method for preparing said coating. The invention
further concerns electronic and optoelectronic devices comprising
said coating.
Inventors: |
SEMOND; Fabrice; (Mougins Le
Haut, FR) ; MASSIES; Jean; (Valbonne, FR) ;
GRANDJEAN; Nicolas; (Nice, FR) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
UNITED STATES
703-413-3000
703-413-2220
patentdocket@oblon.com
|
Assignee: |
Centre National De La Recherche
Scientifique (CNRS)
3, rue Michel Ange
Paris
FR
75016
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20080050894 A1 |
February 28, 2008 |
|
|
Family ID: |
8851146 |
Appl. No.: |
11/832020 |
Filed: |
August 1, 2007 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10/297,494 |
Sep 25, 2007 |
7273664 |
|
|
PCTFR0101777 |
Jun 8, 2001 |
|
|
|
11832020 |
Aug 1, 2007 |
|
|
|
Current U.S.
Class: |
438/492 ;
257/E21.097; 257/E21.121; 257/E21.127 |
Current CPC
Class: |
H01L 21/02505 20130101;
Y10T 428/12528 20150115; C30B 25/02 20130101; H01L 21/02661
20130101; H01L 21/0237 20130101; C30B 23/02 20130101; H01L 21/0254
20130101; H01L 21/02458 20130101; H01L 21/02631 20130101; C30B
29/406 20130101; C30B 29/403 20130101; H01L 21/02381 20130101 |
Class at
Publication: |
438/492 ;
257/E21.097 |
International
Class: |
H01L 21/20 20060101
H01L021/20 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 9, 2000 |
FR |
0007417 |
Claims
1-21. (canceled)
22. A method of forming a substantially crack free monocrystalline
layer of gallium nitride or a mixed nitride of gallium and another
metal on a substrate that could cause tension stresses in the
layer, the method comprising the steps of: a) providing a
substrate; b) disposing a buffer layer on the substrate; c) forming
a nitride layer on the buffer layer, the nitride layer including at
least one of (i) gallium nitride and (ii) a mixed nitride of
gallium and another metal; and d) disposing at least one
monocrystalline intermediate layer within the nitride layer,
wherein the monocrystalline intermediate layer (i) is
monocrystalline upon deposition, (ii) has a thickness between about
100 and about 300 nm and (iii) has a lattice parameter smaller than
a lattice parameter of the nitride of the nitride layer.
23. The method of claim 22, wherein a temperature during growth of
the intermediate layer is substantially high.
24. The method of claim 23, wherein the temperature is between
about 800.degree. C. and 1000.degree. C.
25. The method of claim 24, wherein the temperature is between
about 900.degree. C. and about 950.degree. C.
26. The method of claim 22, wherein growth of one or more of the
buffer layer, the nitride layer and the monocrystalline
intermediate layer includes a process selected from the group
consisting of (i) molecular beam epitaxy, (ii) metalorganic
chemical vapor deposition and (iii) hydride vapor phase
epitaxy.
27. The method of claim 22, wherein the substrate includes material
selected from the group consisting of silicon, sapphire and
SiC.
28. The method of claim 22, wherein the substrate is deoxidized
prior to disposing a buffer layer on the substrate.
29. The method of claim 22, wherein the buffer layer includes
AlN.
30. The method of claim 22, wherein the buffer layer is between
about 10 nm and about 50 nm in thickness.
31. The method of claim 22, wherein the monocrystalline
intermediate layer includes material selected from the group
consisting of AlN and AlGaN.
32. The method of claim 22, wherein the nitride layer includes at
least one of AlGaN and InGaN.
33. The method of claim 22, wherein different portions of the
nitride layer are made of the same material.
34. The method of claim 22, further comprising disposing a second
intermediate layer within the nitride layer.
35. The method of claim 34, further comprising: a) disposing a
third intermediate layer within the nitride layer; b) disposing a
fourth intermediate layer within the nitride layer; c) disposing a
fifth intermediate layer within the nitride layer; and d) disposing
a sixth intermediate layer within the nitride layer.
36. The method of claim 35, wherein the second, third, fourth,
fifth and sixth intermediate layers include material selected from
the group consisting of GaN, AlGaN and InGaN.
37. The method of claim 22, wherein the nitride layer is at least
about 3 .mu.m thick.
38. The method of claim 22, wherein the composite layer of nitride
layer is at least about 5 .mu.m thick.
Description
[0001] The invention relates to monocrystalline layers, and
particularly to thick layers of gallium nitride or of a mixed
nitride of gallium and another metal, and their preparation
process.
[0002] The invention also relates to electronic or opto-electronic
devices comprising such layers.
[0003] The technical field of the invention may be defined in
general as the preparation of semiconducting material layers based
on nitrides on a substrate.
[0004] Semiconducting materials based on nitrides of elements in
groups III to V in the periodic table already occupy an important
place in the electronic and opto-electronic fields, and this place
will become increasingly important. The application field of these
semiconducting materials based on nitrides actually covers a wide
spectrum ranging between laser diodes to transistors capable of
operating at high frequency and high temperature, and including
ultraviolet light detectors, devices with acoustic surface waves
and light emitting diodes (LEDs).
[0005] In these components, the most frequently used substrate for
the growth of nitrides is sapphire, and to a lesser extent silicon
carbide, SiC. These two materials, and particularly sapphire, have
a number of disadvantages.
[0006] The disadvantages of sapphire are that it is an electrical
insulator and a bad conductor of heat, while the disadvantages of
SiC are that it is expensive and its quality is variable.
Therefore, it was proposed to replace sapphire and SiC by silicon
that has obvious economic and technical advantages compared with
the two materials mentioned above.
[0007] The attractive properties of silicon include the fact that
it is a good conductor of heat and that it can easily be eliminated
chemically.
[0008] Furthermore, silicon is the preferred substrate for low cost
mass production, since there is already a technological system
based on silicon that is perfectly controlled on the industrial
scale, and that its cost is significantly less than the cost of
sapphire and sic.
[0009] The growth of nitrides such as gallium nitride on silicon
substrates is hindered by problems related to the large differences
between lattice parameters and the coefficient of thermal expansion
of the substrate and the nitride. Therefore in order to grow a good
quality nitride layer such as GaN, it is commonly accepted that a
thin layer, for example of AlAs, SiC or AlN, called the "buffer
layer" with a thickness of a few tens of nanometers, should be
deposited on the silicon substrate before starting. This layer
continuously covers the substrate and enables the GaN layer to grow
two-dimensionally. However, the GaN layers on the silicon substrate
are in tension, due to the mismatch mentioned above between the
lattice parameter and the coefficient of thermal expansion between
the silicon and GaN. The value of this tension stress generated
during cooling after growth, increases with the thickness of the
nitride layer such as GaN, and if this layer is thicker than a
critical thickness, usually more than 1 .mu.m, cracks are formed in
it.
[0010] Consequently, nitride layers such as GaN layers epitaxied on
a silicon substrate, are either thin and crack-free, or thick and
cracked.
[0011] In both cases, it is difficult to use these layers.
[0012] The same problem arises for the growth of nitride layers,
such as GaN, on substrates made of silicon carbide SiC.
[0013] It is obvious that increasing the thickness of the GaN layer
also improves the structural, optical and electrical properties.
Consequently, it would be very useful to be able to prepare thick
and crack-free nitride layers, and particularly GaN layers, on a
silicon substrate.
[0014] The document by S. A. NIKISHIN, N. N. FALEEV, V. G. ANTIPOV,
S. FRANCOEUR, L. GRAVE DE FERALTA, G. A. SERYOGIN, H. TEMKIN, T. I.
PROKOFYEVA, M. HOLTZ and S. N. G. CHU, APPL. PHYS. LETT. 75, 2 073
(1999) indicates that the growth mode for the thin AlN buffer layer
deposited on the silicon substrate, must transit as quickly as
possible towards a two-dimensional growth mode, if cracking is to
be eliminated and the tension stress in GaN layers on silicon is to
be limited. Crack free GaN layers with a thickness of up to 2.2
.mu.m are thus obtained.
[0015] It is found that this increase in the critical thickness at
which cracks appear is due to pre-treatment of the silicon
substrate surface that consists of exposing the deoxidised silicon
surface for a few seconds to ammonia before depositing a single
layer of aluminium and beginning the growth of the AlN buffer
layer.
[0016] Furthermore, with this pre-treatment, growth of the AlN
buffer layer transits more quickly to a two-dimensional growth
mode.
[0017] However, the layers prepared in this manner still have a
small residual tension stress and the layer will crack if it is
more than 2 .mu.m thick.
[0018] Therefore, there is a need for monocrystalline layers of
gallium nitride and gallium alloys, and particularly thick and
continuous layers, in other words in which there are no cracks, on
a substrate that could cause tension stresses in the layers.
[0019] There is also a need for a reliable, simple, reproducible
and inexpensive process for preparation of such layers on a
substrate.
[0020] The purpose of this invention is to satisfy all the needs
mentioned above, and others, and to provide a layer and a process
for preparation of this layer, that do not have the disadvantages,
defects, limitations and disadvantages of layers and processes
according to the prior art.
[0021] More precisely, the purpose of this invention is to provide
crack free monocrystalline layers of gallium nitride and its
alloys.
[0022] This and other purposes are achieved according to the
invention by a crack-free monocrystalline layer of gallium nitride
or of a mixed nitride of gallium and another metal, on a substrate
that can cause tension stresses in the layer, said substrate being
covered with a buffer layer; in which at least one monocrystalline
layer of a material with a thickness of between 100 and 300 nm and
preferably between 200 and 250 nm, for which the lattice parameter
is smaller than the lattice parameter of gallium nitride or of a
mixed nitride of gallium and another metal, is inserted in the
layer of gallium nitride or of a mixed nitride of gallium and
another metal.
[0023] According to the invention, the lattice parameter of the
material used for the intermediate monocrystalline layer is smaller
than the lattice parameter of the gallium nitride or of the mixed
nitride of gallium and another metal.
[0024] For the required purpose, regardless of the intermediate
layer and regardless of the monocrystalline layer of nitride, the
lattice parameter of the intermediate monocrystalline layer must be
smaller than the lattice parameter of the nitride layer.
[0025] The layer according to the invention may have any thickness,
for example it may be 1 .mu.m thick or even less. In particular,
the layer according to the invention may be a thick layer; a thick
layer according to the invention usually means a layer with a
thickness of 2 .mu.m or more, for example from 2 to 5 .mu.m, and
preferably more than 2 to 5 .mu.m, or even better from 3 to 5
.mu.m.
[0026] The layers according to the invention are fundamentally
different from layers according to prior art in that they have at
least one intermediate monocrystalline layer with a lattice
parameter smaller than the lattice parameter of gallium nitride or
of the mixed nitride of gallium and another metal, and in that this
layer is specifically and fundamentally a monocrystalline layer and
not a polycrystalline layer or even an amorphous layer, and finally
in that the specific thickness of the intermediate layer is 100 to
300 nm. The objective according to the invention is to vary the
stress, and therefore "thick" intermediate layers are used, whereas
thin layers a few tens of nanometers thick are used in prior art,
in which defects are exploited.
[0027] According to the invention, it is essential and fundamental
that the layer according to the invention should comprise not only
an inserted intermediate layer with a lattice parameter smaller
than the lattice parameter of GaN, but furthermore also that this
inserted intermediate layer should be a specific monocrystalline
layer, and finally that this layer should have a specific
thickness.
[0028] This layer is usually made under precise and defined
conditions, particularly at high temperature, namely usually at a
temperature between 800 to 1000.degree. C., and preferably between
900 and 950.degree. C.
[0029] The only way to successfully solve the specific technical
problem of crack formation, is to use a specifically
monocrystalline intermediate layer, advantageously made at high
temperature according to the invention.
[0030] It has never been suggested or mentioned in prior art that a
monocrystalline intermediate layer (for example made of AlN),
advantageously made at high temperature, could help to make thick
uncracked GaN layers.
[0031] The mismatch in the lattice parameter (namely the lattice
parameter of the intermediate layer material being smaller than the
lattice parameter of gallium nitride or of the mixed nitride of
gallium and another metal) is such that the higher nitride layer(s)
is (are) in compression during growth.
[0032] This compression compensates or even cancels out the tension
stress that occurs during cooling, and surprisingly the layers of
gallium nitride or of a mixed nitride of gallium and another metal
may be thick and crack free, in other words they may be continuous
on a substrate that could cause tension stresses in the layer.
[0033] Unlike the layers according to the document by NIKISHIN et
al., the layers according to the invention comprise the buffer
layer, but also another intermediate layer within the material made
of gallium or of the mixed nitride of gallium and another metal,
which effectively eliminates cracks that the buffer layer by itself
cannot prevent.
[0034] In other words, in particular the intermediate layer
according to the invention significantly increases the critical
thickness at which cracks appear, above the critical thickness
obtained when a buffer layer alone is used.
[0035] This means that the intermediate layer makes it possible to
impose a higher compressive stress that precisely or partially,
compensates for the tension stress formed during cooling.
Consequently, the crack free thickness of the layers according to
the invention has never been achieved before, since in the document
mentioned above, the maximum thickness of crack free layers is only
2 .mu.m, whereas it may be is much as 3 .mu.m or even more (up to 5
.mu.m) according to the invention.
[0036] However, note that the invention is not limited to "thick"
layers, but is applicable to all layers with a structure according
to the invention, for example layers will have a thickness of 1
.mu.m or more.
[0037] Furthermore, the intermediate layer is fundamentally a
monocrystalline layer, and is not an amorphous or polycrystalline
layer.
[0038] Advantageously, the mixed gallium nitrides are chosen among
mixed nitride of gallium and aluminium or indium.
[0039] The substrate may be any substrate used in the industry that
could create tension stresses in the layer, but it will preferably
be a substrate chosen from among silicon and silicon carbide
substrates.
[0040] More precisely, according to the invention, we now have
crack free layers, and particularly thick layers, on these
substrates. There are many advantages with these substrates, but in
the prior art the disadvantages related to the generated stresses
were such that these substrates could not be used, despite these
advantages.
[0041] Silicon carbide substrates prove to be particularly suitable
for the types of components to be made with nitrides.
[0042] The silicon substrate is preferably a silicon substrate
oriented along the (111) plane, and also preferably the silicon is
deoxidised.
[0043] The buffer layer is preferably an AlN layer, and also
preferably this layer is a thin layer, usually 10 to 50 nm
thick.
[0044] The intermediate layer may also be made from the same
material as the buffer layer, or it may be made from a different
material. However, the intermediate is usually a layer of AlN or
AlGaN. According to the invention, this layer is a monocrystalline
layer.
[0045] The thickness of said intermediate layer is usually between
100 and 300 nm, and preferably between 200 and 250 nm.
[0046] The layer according to the invention may comprise 1 to 5
intermediate layers.
[0047] The invention also relates to a process for the preparation
of a crack free monocrystalline layer of gallium nitride or of a
mixed nitride of gallium and another metal on a substrate that
could create tension stresses in the layer, said process comprising
the following steps in sequence: [0048] a) possible exposure of the
heated substrate surface to ammonia; this treatment is carried out
if the substrate is made of silicon; [0049] b) deposition of a
single atomic layer of aluminium; [0050] c) deposition of a buffer
layer; [0051] d) growth of a deposit of gallium nitride or of a
mixed nitride of gallium and another metal; [0052] e) interruption
of the growth of the deposit of gallium nitride or of the mixed
nitride of gallium and another metal; [0053] f) growth of an
intermediate monocrystalline layer of a material for which the
lattice parameter is smaller than the lattice parameter of gallium
nitride or of the mixed nitride of gallium and another metal, and
the thickness of which is between 100 and 300 nm, and preferably
between 200 and 250 nm; [0054] g) repetition of steps d) to f) if
necessary; [0055] h) continuation of the growth of the deposit of
gallium nitride or of the mixed nitride of gallium and another
metal, until the final required thickness of the layer of gallium
nitride or of the mixed nitride of gallium and another metal;
[0056] i) cooling of the substrate and the layer of gallium nitride
or of the mixed nitride of gallium and another metal.
[0057] The fundamental step in the process according to the
invention that essentially differentiates the invention from
processes according to prior art, is step f).
[0058] As already mentioned above, the fact of inserting at least
one monocrystalline layer of a material with a lattice parameter
(see above) different from the lattice parameter for gallium
nitride, in other words a lattice parameter smaller than the
lattice parameter of gallium nitride or smaller than the lattice
parameter of mixed gallium nitride in the GaN layer or of the mixed
nitride of gallium and another metal, compensates or even cancels
out the tension stress that develops during cooling, for example
down to ambient temperature, and consequently by using the process
according to the invention, continuous crack free layers, and in
particular thick layers, can be prepared on a substrate that can
cause or induce tension stresses in the layer.
[0059] Step f) for growth of an intermediate layer that is
specifically and fundamentally monocrystalline, makes it possible
to impose a higher compressive stress than the stress created in
the process according to prior art, described in the document by
NIKISHIN et al.
[0060] This compressive stress precisely or partially compensates
for the tension stress that is generated during cooling, with the
result that the layers produced can be relaxed, and the critical
thickness of these layers at which cracks appear is increased
significantly, as mentioned above.
[0061] The state of stress in the layers obtained is different,
depending on their thickness. Thin layers, less than 1 .mu.m thick,
are relaxed, whereas layers more than 1 .mu.m thick are in tension
again, but they do not crack unless they are more than 3 .mu.m
thick.
[0062] It was absolutely not predictable that the addition of this
additional step f) for growth of an intermediate, fundamentally
monocrystalline layer with a specific thickness, to the known steps
a), b), c), d) according to the process according to prior art,
could solve the disadvantages of prior art.
[0063] By growing an intermediate layer, in addition to the surface
pre-treatment a) according to prior art, thick layers (and
particularly thick relaxed layers or uncracked stressed layers) can
be prepared according to the invention.
[0064] According to the invention, it is also essential that the
intermediate layer that is grown should be a monocrystalline layer
which, alone, makes it possible to obtain the final layers, and
particularly thick crack free layers.
[0065] It is also important that the specific thickness of the
intermediate layer should be between 100 and 300 nm, so that the
stress can be varied.
[0066] In other words, although steps a), b), c) and d) of the
process according to the invention are similar to the process
according to prior art mentioned above, a comparison between the
values of the residual stress and the maximum possible crack free
thickness which is usually 3 .mu.m or more with this process,
compared with 2 .mu.m in prior art, demonstrates that the
additional step in the process according to the invention performs
an essential and decisive role in increasing the compressive stress
and further compensates for the tension that occurs during
cooling.
[0067] Conditions for growth of the intermediate layer according to
the invention are important and must be chosen precisely, so that
this layer is monocrystalline.
[0068] In particular, the temperature used during growth of the
intermediate layer is advantageously a high temperature, and
usually 800 to 1000.degree. C. and preferably from 900 to
950.degree. C.
[0069] The other particular parameters or growth conditions that
usually have to be chosen precisely are the growth rate and the
thickness of the intermediate layer to be grown.
[0070] Furthermore, the process according to the invention is
simple, reliable and reproducible, thus for example it has been
demonstrated that reproducibility of the process for about 20
layers produced is 100%.
[0071] As mentioned above, the substrate may be any substrate that
could cause tension stresses in the layer. This is one of the
advantages of the process according to the invention, in that a
priori it no longer depends on the substrate and it can be very
broadly applied. The essential step in which a monocrystalline
layer, for example an AlN layer, has to be inserted in the layer of
gallium nitride or of the mixed nitride of gallium and another
metal, is completely independent of the substrate. The preferred
substrates were described above.
[0072] Furthermore, the process according to the invention may
indifferently use any growth technique for deposition of the buffer
layer, the growth of deposits of gallium nitride or of the mixed
nitride of gallium and another metal, and for growth of the
intermediate layer. However, the growth conditions of this layer
must be such that it is monocrystalline.
[0073] Therefore, Molecular Beam Epitaxy (MBE), already mentioned
above, or Metalorganic Chemical Vapour Deposition (MOCVD), or
Hydride Vapour Phase Epitaxy (HVPE) can also be used for growth of
these layers and deposits. The preferred process is MBE,
particularly under the conditions described below in which growth
of a monocrystalline intermediate layer is possible: temperature
from 800 to 1000.degree. C., and preferably from 900 to 950.degree.
C., and growth rate from 0.1 to 0.5 .mu.m/h.
[0074] With the process according to the invention, it is also
possible to deposit several intermediate layers, for example of AlN
in the gallium nitride layer or the layer of a mixed nitride of
gallium and another metal, in other words as mentioned in g), steps
d) to f) may be repeated, for example between 1 and 5 times, thus
leading to the deposition of 2 to 6 intermediate monocrystalline
layers.
[0075] Therefore, the process according to the invention is
extremely flexible, both from the point of view of choosing the
substrate, and the process for the growth of deposits and
layers.
[0076] This is not the case for the process according to prior art
as described in the document by NIKISHIN et al., since it is
obvious that in this process, the speed of the transition from
three-dimensional to two-dimensional growth of the AlN layer will
be dependent on the substrate and the growth technique used.
[0077] Furthermore, this process can only apply to the interface
between the substrate and the AlN buffer layer once. Therefore the
process according to the invention has the advantage that it can be
applied and transposed unchanged, for example when using silicon
carbide substrates instead of silicon substrates, as we have
already seen these SiC substrates are particularly suitable for the
types of components that we would like to make with nitrides.
[0078] The invention also relates to an electronic and/or
opto-electronic device comprising at least one thick and crack free
monocrystalline layer of gallium nitride or of a mixed nitride of
gallium and another metal, according to the invention, as described
above.
[0079] For example, these devices may be laser diodes, transistors
capable of operating at high frequency and high temperature,
ultraviolet light detectors, acoustic surface wave devices, light
emitting diodes, etc.
[0080] Inclusion of layers according to the invention in these
electronic devices significantly improves performances and it is
obvious that increasing the thickness of the gallium nitride layer
or of a mixed nitride of gallium and another metal, can improve the
structural, optical and electrical properties at the same time. For
example, the only existing document that describes the manufacture
of transistors based on nitride heterostructures on a silicon
substrate, by A. T. SCHREMER, J. A. SMART, Y. WANG, O. AMBACHER, N.
C. MacDONALD and J. R. SHEALY, Appl. Phys. Lett. 76, 736 (2000),
mentions mobility values of between 600 and 700 cm.sup.2/Vs,
whereas if layers according to the invention are used, values
higher than 1500 cm.sup.2/Vs are obtained at 300 K. This is
essentially due to the fact that in this document, it is impossible
to make layers thicker than 0.7 .mu.m without the formation of
cracks. Thus, the quality of heterostructures according to this
document are not as good as hererostructures made according to the
invention on layers at least 2 .mu.m thick.
[0081] This example dealing with the improvement of performances
applies to transistors, but in reality, regardless of the
application, the products that will be made from thick layers
according to the invention will have better characteristics than
products made with thin layers, simply because as the distance
between the interface and the substrate increases, the quality of
the epitaxied material will also improve.
[0082] The invention will now be described more precisely in the
detailed description given below for illustrative and
non-limitative purposes, with reference to the appended drawings in
which:
[0083] FIG. 1 shows a chart that diagrammatically shows the
different steps in the process according to the invention for
deposits and growth by MBE, showing the temperature T (.degree. C.)
as a function of the time (minutes) in these various steps;
[0084] FIG. 2 shows a diagrammatic section through a GaN layer
according to the invention;
[0085] FIGS. 3A and 3B are optical microscope photographs of a 2
.mu.m thick layer of GaN that does not comprise an intermediate
layer (not conform with the invention), and a 2 .mu.m thick GaN
layer that does comprise an intermediate layer (conform with the
invention);
[0086] FIGS. 4A and 4B are photo luminescence and reflectivity
spectra respectively, obtained with 1 .mu.m thick GaN without an
intermediate layer (not conform with the invention) and a GaN layer
of the same thickness comprising an intermediate layer (conform
with the invention) respectively. The ordinates are in arbitrary
units (u.a) and the abscissas represent the photon energy E
(eV).
[0087] The process according to the invention and the layers
according to the invention are described below.
[0088] The substrates used are preferably silicon wafers in the
(111) plane.
[0089] Before starting growth of the layer itself, the native oxide
layer that covers the silicon substrate is evaporated in situ in
the growth chamber by fast thermal annealing, for example up to
950.degree. C.; this is called the "deoxidation" step (step 0)
which is not shown in FIG. 1.
[0090] The next step is to begin the process according to the
invention (at time 0 in FIG. 1). The temperature of the substrate
is usually fixed, for example, at 600.degree. C. and the silicon
surface is exposed to ammonia for a few seconds (usually from 2 to
10 seconds) (step 1).
[0091] The substrate temperature is then increased, generally up to
about 830.degree. C. The temperature of the substrate is then
lowered, generally to 600.degree. C., and a single atomic layer of
aluminium is then deposited (step 2).
[0092] The growth of the buffer layer, usually AlN, is then
initiated by increasing the temperature to 900.degree. C. (step
3).
[0093] Note that the process described in this document is the MBE
process (Molecular Beam Epitaxy).
[0094] After a deposition of 10 to 50 nanometers of the buffer
layer, for example for 30 minutes, growth is stopped and the
temperature generally drops to 780.degree. C. The GaN or a mixed
nitride of gallium and another metal is grown at this temperature
on the buffer layer, for example made of AlN (step 4).
[0095] The growth of GaN or of the mixed nitride of gallium and
another metal, is generally interrupted after deposition of a 100
to 300 nm thickness (for example 250 nm in FIG. 1) and the
temperature is generally increased to 900.degree. C. for growth of
an intermediate monocrystalline layer of AlN, in the case in which
the buffer layer and the intermediate layer are both composed of
AlN. The thickness of this monocrystalline intermediate layer is
usually between 100 and 300 nm.
[0096] After growth of this monocrystalline intermediate layer, the
temperature is lowered again generally to 780.degree. C. as before,
to restart growth of the GaN or of the mixed nitride of gallium and
another metal until reaching the required final thickness (step 6)
which is usually from 2 to 5 .mu.m.
[0097] Note that the different parameters such as the temperature,
duration, reagents used, etc., for the deposition and growth steps
of the buffer or GaN layer or of the layer of the mixed nitride of
gallium and another metal, and for example the surface treatment of
the substrate are known and/or may easily be determined by those
skilled in the art. The values given above are given for
guidance.
[0098] However, the conditions for deposition of the intermediate,
monocrystalline layer, essential to the invention are specific,
precisely to enable growth of a monocrystalline layer.
[0099] FIG. 1 mentioned above diagrammatically shows the different
steps in the process according to the invention and after
deoxidation of the substrate, while FIG. 2 shows a diagrammatic
section of the structure of the layer thus made according to the
invention; for example with a substrate (21) made of Si (111), a
buffer layer made for example of AlN (22) and a layer of GaN (23)
in which there is an intermediate layer of AlN (24).
[0100] The invention will now be described with reference to the
following examples given for illustrative and non restrictive
purposes.
EXAMPLE 1
[0101] In this example, Molecular Beam Epitaxy (MBE) is used to
prepare a 2 .mu.m thick layer of GaN according to the invention on
a silicon substrate, in other words with a 250 .mu.m thick
intermediate AlN monocrystalline layer.
[0102] The silicon substrate is heated to about 600.degree. C. for
about 10 hours under an ultra vacuum, to degas it. It is then
placed in the growth chamber and the temperature is quickly
increased to about 950.degree. C. in order to remove the silicon
oxide layer on the surface. The temperature is then lowered to
about 600.degree. C. to expose the silicon surface to an ammonia
flow for 2 seconds.
[0103] The temperature is then increased to 820.degree. C. and is
then lowered to about 600.degree. C. again. At this temperature, a
single layer of aluminium (equivalent to a 10-second deposit) is
deposited, and the temperature is then increased to 600.degree. C.
At this temperature, ammonia and aluminium are added simultaneously
to make the buffer layer of aluminium nitride (AlN). The
temperature is gradually increased during the first 2 minutes to
reach a temperature of the order of 900.degree. C. and this
temperature is maintained to make the 50 nanometers of AlN grow to
form the buffer layer (namely about 20 minutes under our growth
conditions). Once the buffer layer is finished, the temperature is
lowered to about 780.degree. C. and the growth of gallium nitride
(GaN) is started. A 250 nanometer layer of GaN is grown (requiring
about 15 minutes under our growth conditions).
[0104] Once this 250 nm thickness has been reached, the growth of
the intermediate monocrystalline layer of AlN is started. The
beginning of this growth takes place at 780.degree. C., but the
temperature is quickly increased to 900.degree. C. (the temperature
rise takes 2 minutes). The thickness of the intermediate AlN layer
is 250 nm (which takes about 2 hours under our growth conditions).
Once this layer is finished, the temperature is lowered to about
780.degree. C. and growth of the 2 micrometer thick GaN layer is
started, and lasts for about 2 hours.
EXAMPLE 2
Comparison
[0105] In this example, Molecular Beam Epitaxy (MBE) is used to
prepare a layer of GaN with the same thickness as the layer in
example 1 (2 .mu.m), under the same conditions as in example 1 but
without the intermediate layer of AlN, for comparison purposes.
[0106] The silicon substrate is heated to about 600.degree. C. for
about 10 hours under an ultra vacuum to degas it. It is then placed
in the growth chamber and its temperature is increased quickly to
the order of 950.degree. C. in order to remove the layer of silicon
oxide located on the surface. The temperature is then lowered to
about 600.degree. C. to expose the silicon surface to the ammonia
flow for 2 seconds.
[0107] The temperature is then increased to 820.degree. C. and it
is lowered again to about 600.degree. C. A single layer of
aluminium is deposited at this temperature (equivalent to a 10
second deposit) and the temperature is then increased to
650.degree. C. At this temperature, ammonia and aluminium are added
simultaneously to make the buffer layer of aluminium nitride (AlN).
During the first two minutes, the temperature is gradually
increased up to the order of 900.degree. C. and is kept at this
value to grow the 50 nanometers of AlN in the buffer layer (which
is about 20 minutes under our growth conditions). Once the buffer
layer is finished, the temperature is lowered to about 780.degree.
C. and growth of the 2 micrometer thick GaN layer is started, and
lasts for about 2 hours.
[0108] Observation of the layers prepared in examples 1 and 2 using
an optical microscope (plates in FIGS. 3A and 3B) at a
magnification of .times.100 shows that the GaN layer made without
an intermediate AlN layer (example 2, FIG. 3A, for comparison and
not conform with the invention) is completely cracked whereas the
layer of GaN made with the intermediate monocrystalline layer
(example 1, FIG. 3B, conform with the invention) is continuous and
entirely crack free.
EXAMPLE 3
[0109] In this example, a 1 micrometer thick layer of GaN is
prepared by Molecular Beam Epitaxy according to the invention under
the same conditions as in example 1, in other words with a 250 nm
thick intermediate monocrystalline layer of AlN.
[0110] The silicon substrate is heated under an ultra vacuum for
about 10 hours at about 600.degree. C. to degas it. It is then
inserted into the growth chamber and the temperature is quickly
increased to about 950.degree. C. in order to remove the silicon
oxide layer on the surface. The temperature is then lowered to
about 600.degree. C. to expose the silicon surface to the ammonia
flow for about 2 seconds.
[0111] The temperature is then increased to 820.degree. C. and then
lowered again to about 600.degree. C. At this temperature, a single
layer of aluminium is deposited (equivalent to a 10 second deposit)
and the temperature is then increased to 650.degree. C. At this
temperature, ammonia and aluminium are added simultaneously to make
the buffer layer of aluminium nitride (AlN). For the first 2
minutes, the temperature is increased gradually to reach a
temperature of the order of 900.degree. C. and this temperature is
maintained to make the 50 nanometers of AlN in the buffer layer
grow (this takes about 20 minutes under our growth conditions).
Once the buffer layer is finished, the temperature is lowered to
about 780.degree. C. and growth of gallium nitride (GaN) is
started. A 250 nanometer thick GaN layer is grown (which takes
about 15 minutes under our growth conditions).
[0112] Once the 250 nm thickness has been reached, growth of the
intermediate AlN monocrystalline layer is started. Growth begins at
780.degree. C., but the temperature is quickly increased to
900.degree. C. (the temperature rise lasts for 2 minutes). The
thickness of the intermediate monocrystalline AlN layer is 250 nm
(which is equivalent to about 2 hours under our growth conditions).
Once this layer is finished, the temperature is lowered to about
780.degree. C. and growth of the 1 .mu.m thick GaN layer is
started, which is the equivalent of about 1 hour.
EXAMPLE 4
Comparison
[0113] A layer of GaN with the same thickness as that in example 3
(1 .mu.m) is prepared under the same conditions as in example 3,
but omitting the intermediate monocrystalline layer of AlN, for
comparison purposes.
[0114] The silicon substrate is heated to about 600.degree. C. for
about 10 hours under an ultra-vacuum to degas it. It is then placed
in the growth chamber and the temperature is quickly increased to
about 950.degree. C. in order to remove the silicon oxide layer on
the surface. The temperature is then lowered to about 600.degree.
C. to expose the silicon surface to the ammonia flow for 2
seconds.
[0115] The temperature is then increased to 820.degree. C. and is
then lowered again to about 600.degree. C. A single layer of
aluminium is deposited at this temperature (equivalent to a 10
second deposition), and the temperature is then increased to
650.degree. C. Ammonia and aluminium are added simultaneously at
this temperature, to make the buffer layer of aluminium nitride
(AlN). For the first two minutes, the temperature is increased
gradually to reach a temperature of the order of 900.degree. C.,
and this temperature is maintained to enable growth of the 50
nanometers of AlN in the buffer layer (about 20 minutes under our
growth conditions). Once the buffer layer is finished, the
temperature is lowered to about 780.degree. C. and growth of the 1
micrometer thick GaN layer is started, which takes about 1
hour.
[0116] In order to quantify the stress present in the GaN layers,
we combined the photo luminescence and reflectivity experiments
carried out on the layers in examples 3 and 4. These experiments
are used conventionally in the physics of semiconductors to
determine the precise stress state of materials. The spectra
obtained on the two layers of GaN with equal thickness (1 .mu.m),
one made without the intermediate layer (example 4) and the other
made with the intermediate layer of AlN (example 3) are shown in
FIGS. 4B and 4A. An analysis of these spectra shows that the layer
of GaN obtained without the intermediate layer of AlN is in
tension, whereas the layer of GaN obtained with the intermediate
monocrystalline layer of AlN is relaxed (the objective is to
precisely determine the prohibited band of the GaN material by the
energy position of the free exciton A).
* * * * *