PLATFORM OF LARGE METAL NITRIDE ISLANDS WITH LATERAL ORIENTATIONS AND
LOW-DEFECT DENSITY
Technical Field
[001] The present invention relates to a platform of large metal nitride islands that are
laterally-oriented with respect to a crystallographic template substrate and with lowdefect density. The present invention also relates to a process for a single-step
preferential nucleation and growth of defect-free platform of large metal nitride islands,
with lateral orientations.
Background of the invention
[002] Group III nitrides that are made up of nitrides of aluminum (AlN), gallium (GaN)
and indium (InN) and their alloys are used in a number of semiconductor devices. The
usefulness of gallium nitride (GaN) and its ternary and quaternary compounds
incorporating aluminum and indium (AlGaN, InGaN, AlInGaN) has been well established
for the fabrication of visible and ultraviolet optoelectronic devices, high-frequency
devices and high-power electronic devices. These devices are typically grown epitaxially
by techniques including molecular beam epitaxy (MBE), metal-organic chemical vapor
deposition (MOCVD), or hydride vapor phase epitaxy (HVPE).
[003] However, the growth of Group III-A nitrides, is typically performed on foreign
substrates, such as sapphire, silicon carbide and silicon, primarily due to the lack of largearea single crystal substrates of gallium nitride (GaN) or aluminum nitride (AlN) (<2 inch
diameter) and their expensive cost. This normally leads to very high defect densities
(mostly in the form of dislocations) in the as-grown films due to the lattice mismatch and
thermal mismatch between the nitride thin films and the substrates, and in extreme cases
due to film de-lamination and cracking.
[004] In order to manage these stresses and reduce defects, in a typical top-down
process, buffer layers with spatially varying compositions are used, on which layers
constituting the active device are then deposited. The devices themselves occupy small
regions over the entire wafer, and are patterned using known lithographic techniques, while the rest of the material across the substrate/wafer constitutes a mechanical and
thermal support. Whereas, a bottom-up approach to fabricating devices would, on the
other hand, involve material growth only at the areas on the substrate where the devices
are desired, which would in turn give rise to significant material savings. While this can be
achieved by using techniques such as the vapor-liquid-solid method which is used to grow
nanowires at pre-selected locations, the material thus grown is typically very small in size,
tens or hundreds of nanometers in lateral dimensions, and hence impose severe
restrictions on the device areas that can be realized. Furthermore, the high aspect ratio of
such structures (vertical:lateral sizes) also precludes the possibility of planar processing
which is the mainstay of the semiconductor industry.
[005] Hence, there is a pressing need to develop bottom-up fabrication techniques to
grow nitrides of device with relevant dimensions, at pre-patterned locations and over
large-areas on a substrate.
[006] The typical dislocation densities for growth of nitride thin films on sapphire, SiC
and Si are 10
8
cm
-2
, 10
8
-10
9
cm
-2
and 10
9
cm
-2
, respectively. The need to reduce the defect
densities is critical to improve the performance and reliability of devices made of these
materials. Defect reduction schemes such as lateral epitaxial overgrowth or pendeoepitaxy are employed, which can bring down the defect density selectively to 10
6
cm
-2
but
with added process complexities. These schemes typically involve multiple lithography
steps with interruption of the growth process in between to realize “pockets” of these
low defect density structures, which are then used to fabricate devices. In addition, in
these methods there is also a need to use a mask layer with openings. In such methods,
limited areas of the defect-free material growth are obtained, with the material growth
happening all over the substrate, in an uncontrolled manner. Therefore, there is a need to
reduce defect density in hetero-epitaxial nitride thin films over larger areas.
[007] Nanostructuring offers a way to reduce defect densities in metal nitride
structures, where nanowires of GaN and other semiconductors have been grown with
near perfect crystallinity on foreign substrates. [008] The vapor–liquid–solid method (VLS) is a mechanism for the growth of onedimensional structures, such as nanowires, from chemical vapor deposition. In
VLS method, a metal particle, which is used as a seed, having catalytic properties, is
heated in an environment containing vapors of gaseous precursor molecules. Due to
heating, a eutectic melt between the precursor species and the metal catalyst is formed
in the seed particle. When the material in the seed particle reaches a critical saturation
concentration, it precipitates out of the seed particle in a given crystallographic
orientation. In case, the gaseous precursor is replaced with a liquid phase precursor, the
method is then called Liquid-Liquid-Solid (LLS) technique. The other related methods of
VLS include vapour-solid-solid (VSS) and liquid-solid-solid (LSS) where the catalyst remains
a solid instead of forming a eutectic liquid phase.
[009] The use of bottom-up techniques such as the vapour-liquid-solid (VLS) technique,
allows for precise selectivity and control over the location of growth of these nanowires
with limited deposition at other locations thus saving on material costs. However, the
limited area of these devices and their large aspect ratio, precludes the possibility of
planar processing of semiconductor devices, which is essential for their integration with
existing semiconductor process flows.
[0010] In a known vapor-liquid-solid (VLS) technique, the size of the catalysts, which are
used for the growth of metal nitride structures, is typically less than one micron and the
corresponding metal nitride growth structures also correspond to the size of the catalyst.
[0011] In a known lateral epitaxial growth of metal nitride layers, mask layers are used
with pre-defined openings, for defect density reduction, even though the presence of
mask layers have no other significant functional attributes. Additionally, the growth
process needs to be interrupted in order to remove the sample from the growth
chamber, followed by patterning the mask layer using at least one lithographic step. The
sample is then reloaded into the growth chamber and further deposition is initiated,
which results in areas of low defect density material obtained over the masked regions.
Such growth interruptions give rise to a large process turnaround time and the interface
between the material deposited in the first and second growth steps is not atomically pristine as the surface is exposed to atmospheric contaminants upon removal from the
growth chamber.
[0012] In consideration of the above, there is a need to provide a structure with large
lateral areas of metal nitride layers, with reduced dislocation densities and a process for
preparing such structures, which does not have the limitations of methods such as VLS,
lateral epitaxial overgrowth (LEO) and where mask layers are used during the growth of
structures.
Objects of the present invention
[0013] The primary object of the present invention is to provide a platform of large metal
nitride islands that are selectively grown from an array of catalyst sites of a substrate with
a crystallographic surface orientation and where the lateral dimensions of the metal
nitride islands are larger than the dimensions of the catalyst sites.
[0014] An object of the present invention is to provide a platform of large metal nitride
islands that are grown selective from an array of catalyst sites of a substrate having a
crystallographic surface orientation, where the array of catalyst sites is formed with a
single type of catalyst material or different types of catalyst materials.
[0015] Another object of the present invention to provide a platform of large metal
nitride islands that are grown selectively from an array of catalyst sites of a substrate on
which an electrically and optically-active device is arranged.
[0016] It is also an object of the present invention to provide a process for a single-step
preferential nucleation and growth of defect-free platform of large metal nitride islands,
without using a mask.
[0017] Yet another object of the present invention is to provide a process for a singlestep material growth for fabricating a platform for the growth of metal nitride islands that
are laterally-oriented with respect to a crystallographic template substrate and with a
low-defect density, where the integration of the platform with the devices, is performed in a single lithography step, at an initial stage and without any further growth
interruption.
[0018] These and the other objects and the appurtenant advantages of the embodiments
herein will be understood easily by studying the following specification with the
accompanying drawings.
Brief description of the drawings
[0019] The accompanying drawings illustrate various exemplary embodiments of the
invention. It will be appreciated that illustrated element boundaries, for instance shapes
and lines in the drawings, represent an exemplary instances of the boundaries.
[0020] FIG.1 is a schematic drawing depicting lateral orientation of exemplary gallium
nitride islands, grown on a substrate, through an array of catalyst sites with typical sizes
less than 1 μm and catalyst site separation distance of more than 1 μm.
[0021] FIG.2 is a schematic drawing depicting lateral orientation of exemplary gallium
nitride islands, grown on a substrate, through an array of catalyst sites with typical sizes
less than 1 μm and catalyst site separation distance of more than 1 μm.
[0022] FIG.3(a) is a schematic cross-sectional view depicting lateral orientation of
exemplary gallium nitride islands, grown on a substrate, through an array of catalyst sites,
with reduced dislocation density.
[0023] FIG.3(b) is a schematic cross-sectional view depicting lateral orientation of
exemplary gallium nitride islands, grown on a substrate, through an array of catalyst sites,
with reduced dislocation density and along with active devices.
[0024] FIG.4 is a scanning electron microscope (SEM) image of an exemplary GaN island
that is laterally grown on sapphire c-plane substrate through an array of catalyst sites.
[0025] FIG.5 is an optical micrograph of two exemplary hexagonal single crystal gallium
nitride islands as part of an array grown on a sapphire substrate. 7
[0026] FIG.6 is a cross-sectional scanning transmission electron microscope image of the
as-grown gallium nitride island.
[0027] FIG.7 is a schematic depiction of dislocation bending during lateral growth of the
metal nitride islands.
[0028] FIG.8 is a schematic depiction of the presence of different catalyst materials on
the same substrate at pre-defined locations, which can be used to grow low-defect
density islands of varying compositions and orientations.
[0029] FIG.9 is a flow drawing indicating the process steps for the preparation of platform
of metal nitride islands with lateral orientations and reduced dislocation density.
Summary of the invention
[0030] The present invention provides a metal nitride platform for semiconductor
devices, comprising, a pre-defined array of catalyst sites, disposed on a substrate. The
metal nitride islands with lateral to vertical size ratios of at least greater than one (1) are
disposed on the array of catalyst sites, where the surfaces of the metal nitride islands are
with reduced dislocation densities and side walls with bending of dislocations. The
platform of metal nitride islands is further used to build electrically and optically-active
devices. The present invention also provides a process the preparation of a metal nitride
platform, selectively, on the array of catalyst sites, in the presence of a reactive gas and
precursors and under preferred reaction conditions, to grow metal nitride islands with
lateral to vertical size ratios of at least greater than one (1).
Detailed description of the invention:
[0031] Accordingly, the present invention provides a platform with an array of catalyst
sites incorporated on a crystallographic template substrate, to bear metal nitride islands,
where the lateral dimensions of the metal nitride islands are larger than the dimensions
of the catalyst sites.
[0032] In the present invention, as an exemplary aspect, growth of metal nitride islands
with reduced dislocation density on a substrate, where the lateral dimensions of the metal nitride islands are larger than the dimensions of the catalyst sites, is demonstrated
with c-plane polar orientation. However, it is understood that by changing the orientation
of the substrate, the growth of metal nitrides can be performed on non-polar oriented
substrates, with reduced dislocation density.
[0033] In an aspect of the present invention, catalyst sites, which are lithographically
defined on a selected substrate, are used to obtain site-selective nuclei, which then act as
focal sites, for enhancing the lateral growth of metal nitride islands, from these catalyst
sites.
[0034] As initially presented in FIG.1, the platform 100 includes a substrate 101, which is
typically sapphire, silicon, silicon carbide. The selected substrate 101 can be of any
suitable size, preferably in the range of 2 to 12 inches and more preferably in the range of
4 to 8 inches and with a thickness in the range of 100 microns-2mm. The crystal
orientations of a preferred silicon substrate are (100), (111), (110) and whereas the
crystal orientations of a preferred silicon carbide substrate are (4H), (6H), (3C). The
preferred sapphire substrate is with c-plane, r-plane, a-plane or m-plane orientations.
[0035] A pre-defined array of catalyst sites 102 is arranged on the selected substrate 101.
The catalyst sites 102 are formed from materials such as gold, nickel or a layered
material, preferably graphene, molybdenum disulphide (MoS2) or tungsten disulphide
(WS2) or a combination of these materials. The catalyst sites 102 are advantageously
arranged lithographically on the substrate 101, considering the required dimensions of
the semiconductor devices, that are to be built on these catalyst sites 102. The size of an
individual catalyst site is smaller in dimension, than preferably about 1 micron. The
smaller size of the catalyst site, supports much larger lateral growth of metal nitride
islands, even without increasing the size of the catalyst site. In this aspect, it also
understood that in the pre-defined array of catalyst sites 102, the separation between
each of the catalyst sites can be suitably varied to suit the size of the desired
semiconductor device. The catalyst sites 102 are employed to merely obtain site-selective
nuclei, which then act as focal sites for enhancing the lateral growth of metal nitride
islands 103 as illustratively shown in FIG.2, where the sizes of the catalyst sites 102 are relatively smaller than the metal nitride islands 103, indicating the enhancement of the
lateral dimension of the metal nitride islands 103 as compared to the size of the catalyst
sites 102. The material for the array of catalyst sites 102 is selected from a metal,
preferably gold, nickel or a layered material such as graphene, molybdenum disulphide
(MoS2) or tungsten disulphide (WS2), or a combination of these materials.
[0036] In another aspect of the present invention, the array of catalyst sites 102 is
formed with a single type of material. However, it is within the purview of this invention
to form an array of catalysts sites by using more than one type of desired materials as
particularly shown in FIG.8. Such an arrangement of catalyst sites 102 with more than one
type of metallic materials facilitate the growth of metal nitride islands of different
compositions under each type of catalyst size. For instance, in case of two catalysts with
different types of metallic materials, the composition of metal nitride islands 103 thus
obtained under each type of catalyst are based on the general formula AlxGayIn1-x-yN and
AlpGaqIn1-p-qN where p≠x and q≠y, where the actual values of p, q, x and y are determined
by the incorporation of the precursors into the metal catalysts.
[0037] The metal nitride islands 103 with variable lateral dimensions that are larger than
the dimensions of the catalyst sites 102, are grown on the array of catalyst sites 102. The
material for the metal nitride island 103 that is grown on the catalyst sites 102 is selected
from gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN) or a combination
of any these metal nitrides. The lateral growth of the metal nitride islands 103 is achieved
by confining the selectivity of the metal nitride island to the catalyst site 102, which is
followed by their enhanced lateral growth, to render large-area metal nitride islands 103.
The lateral dimensions of the metal nitride islands 103 are preferably in the range of 1
micron to 1 mm with the sizes and spacings determined by the area of the device
required to be fabricated. An exemplary metal nitride island having a maximum lateral
dimension of 20 microns is shown in FIGs.4 and 5 with the spacing that is provided
between the islands is about 100 microns. The variations in the lateral growth of the
metal nitride islands 103 can be achieved by varying the growth time and process
parameters such as temperature, pressure, flow rates and V/III ratios. It is understood here that the size of these islands 103 can also be suitably increased beyond this
preferable range.
[0038] The metal nitride islands 103 either incorporate the catalyst particles 102 within
their thickness as shown in FIG.3(a) or at their surface as shown in FIG.1. Such
incorporation is dependent on the choice of the catalyst, the metal nitride to be grown
and process parameters used for the deposition. The laterally-grown metal nitride islands
103 are in the form of a single domain with all regions of a given metal nitride islands 103
being oriented in one particular orientation with reference to the substrate 101. The ratio
of the lateral dimension of the metal nitride islands 103 and the size of catalyst site 102 is
at least in the range of 10:1 with the lateral dimension of the required device determining
the size of the final metal nitride islands. However, the size of the catalyst sites, is
advantageously retained at < 1 μm.
[0039] The laterally-grown metal nitride islands 103 are controlled precisely, where such
precise control refers to obtaining the metal nitride islands 103 centered only at the
catalyst sites 102, with the lateral and vertical dimensions of the said metal nitride islands
103 are precisely determined by the growth time and process parameters such as
temperature, pressure, flow rate and V/III ratio.
[0040] The surfaces 103a of the metal nitride islands 103 exhibit substantially low
defects, where dislocations originating from the lower portions, thread through vertically,
to reach the surfaces 103a of the metal nitride islands 103. Concurrently, the dislocations,
which encounter the side walls 103b of the metal nitride islands 103, undergo lateral
deviations, instead of extending vertically, as schematically illustrated in FIG.7, which
shows the bending of those dislocations that encounter the side facets and whereas
those dislocations encountering the top surface thread through to higher thicknesses.
Accordingly, the dislocation density of the metal nitride island 103 is substantially
reduced at the surface 103a, by virtue of deviation of the dislocations laterally. In an
embodiment of the present invention, the reduced dislocation density on the surfaces
103a of the metal nitride islands 103 is equal to or less than 10
6
/cm
2
. The exemplary GaN
islands with reduced dislocation density is as shown in FIG.6, which is a cross sectional TEM image, where the dislocation density at the surface is practically zero as can be seen
from the presence of only two dislocation lines intersecting the surface over the entire
island image. The bending of the dislocations encountering the side walls of the growing
island, are to the schematically as shown in FIG. 7.
[0041] In another aspect of the present invention, the metal nitride islands 103 are
grown in the absence of any masking materials such as silicon nitride, silicon oxide, which
are commonly used to grow the low defect density metal nitride structures with lateral
epitaxy. The substrates once loaded into the growth chamber are not subject to removal
from the chamber during any intermediate stage of the complete growth process i.e.,
during the nucleation of metal nitride at catalyst sites and their preferential lateral
growth to form large area metal nitride islands. All material growth occurs directly on the
substrate and not over any masking layer as is the case for the known lateral epitaxial
overgrowth techniques.
[0042] The surfaces 103a of the metal nitride islands 103 are very smooth as shown in
FIG.4 and are provided with root mean square (RMS) surface roughness is less than 1 nm.
[0043] In yet another aspect of the present invention, as shown in FIG.3(b), at least an
intermediate layer or an epitaxial device layer 104 is arranged on the top surface 103a of
the metal nitride island 103 where these layers can be selected from one of AlxGayIn1-x-yN
where 0≤x, y≤1 and typically constitute a multitude of such layers with alternating
compositions.
[0044] The metal nitride platform 100 with reduced dislocation density can be built on
any substrate, which is suitable for III-nitride growth, including obtaining non-polar metal
nitride films, which are particularly relevant for constructing opto-electronic device stacks
by varying the orientation of the substrate 101.
[0045] In the present invention, as an exemplary aspect the orientation of III-nitrides with
c-plane polar orientation is obtained. However, it is by changing the orientation of the
substrate 101 non-polar oriented templates can be grown, with reduced dislocation
density [0046] In yet another aspect of the present invention, a semiconductor device, which is
built on the platform 100 of the present invention, includes a substrate 101. A predefined array of catalyst sites 102 are arranged on the substrate 101. Metal nitride islands
103, with variable lateral dimensions, are grown from the catalyst sites 102, where the
variable lateral dimensions that are larger than the dimensions of said catalyst sites 102
with reduced dislocation densities at the surface and bending of dislocations at side walls
of said metal nitride islands. Electrically and optically-active devices such as high electron
mobility transistor devices, quantum well light emitting diodes etc., are arranged on the
metal nitride islands either directly or with appropriate intermediate layers 104.
[0047] The device(s) can be fabricated on this low defect density platform 100 either
directly or after the deposition of further epitaxial layer 104, enabling the realization of
both electronic devices (such as high electron mobility transistors) and opto-electronic
devices such as LEDs and laser diodes 105, as shown in FIG.3(b). A further growth of
additional epitaxial layers can be performed should there be any such requirement.
Exemplary devices 105, which can be built on the platform 100, include high electron
mobility transistors and light emitting diodes having alternating layers of metal nitride
alloy of aluminumxgalliumyindium(1-x-y)nitride (AlxGayIn1-x-yN), where where x and y are in the
range of 0-1.
[0048] Therefore, the metal nitride platform 100 for arranging active devices comprises,
the pre-defined array of catalyst sites 102, with intervening distances, disposed on the
substrate 101. The large metal nitride islands 103 with lateral to vertical size ratios of at
least greater than one (1) are disposed on the array of catalyst sites 102. The surfaces
103a of the metal nitride islands 103 are with reduced dislocation densities and side walls
103b with bending of dislocations.
[0049] The present invention also provides a process for a single-step preferential
nucleation and growth of a platform of large metal nitride islands with reduced
dislocation density is as shown in FIG.9. [0050] In the process steps of the present invention, initially, the cleaned substrates are
deposited with pre-patterned array of catalyst sites, preferably by lithography and the
substrates bearing the catalyst sites are loaded into a growth chamber having an
environment that is conducive for metal organic chemical vapor deposition (MOCVD),
molecular beam epitaxy (MBE), or other any other suitable methods, where the
substrates are heated to a high temperatures, where the catalyst sites undergo dewetting to typically form spherical blobs. In the process steps of the present invention, in
an exemplary aspect, MOCVD reaction chamber is used in the presence of precursors
such as Al, Ga and In and ammonia for deposition of the metal nitride films or layers.
[0051] In yet another aspect of the process steps of the present invention, subsequent to
the de-wetting of the catalyst sites, the growth chamber is ramped to the metal nitride
growth conditions and the growth of metal nitride islands is performed, by regulating the
process parameters, to confine the growth of the metal nitride islands to the catalyst sites
and to ensure that no residual growth of metal nitride islands is permitted on the bare
areas of the substrate.
[0052] In yet another aspect of the process steps of the present invention, growth
conditions during the growth of metal nitride islands are maintained or varied to promote
lateral growth of the initial seed islands, in order to expand the lateral dimensions of
these islands.
[0053] In still another aspect of the process steps of the present invention, the size of
each of the metal nitride islands thus obtained is in the range of 20 microns and the
height of about 5 microns, where size and height of the islands can be suitably increased
by a simple expedient of increasing the growth time of the metal nitride islands in the
growth chamber.
[0054] It is also an aspect of the process steps of the present invention, the defect-free
nature of these metal nitride islands is due to the fact that the defects are accommodated
primarily at the growth front, whereby, the amount of dislocations that thread through to
the top surface, where the active device layers are located, is substantially reduced. [0055] In a further aspect of the process steps of the present invention, the large metal
nitride islands are used as templates to initiate growth of further layers and ultimately for
the fabrication of active devices.
[0056] In the process steps of the present invention for preparation of a metal nitride
platform, initially the selected substrate is cleaned and treated to remove impurities such
as native oxides and organic and metallic contaminants. Thereafter, a pre-defined array of
catalyst sites is deposited on the substrate. The substrate with the array of catalyst
islands is transported into a reaction chamber to grow metal nitride islands, selectively,
on the array of catalyst sites, in the presence of a reactive gas, which is preferably a
mixture of nitrogen and hydrogen. Metal precursors preferably in the form of gallium,
aluminum, indium and nitrogen or tri-methyl gallium, tri-methyl aluminum, tri-methyl
indium and ammonia, are permitted under preferred reaction conditions, to grow metal
nitride islands on the array of catalyst sites. Thereafter, the metal nitride islands are
grown with lateral to vertical size ratios of at least greater than one (1), under desired
reaction conditions. The preferred reaction conditions for the reaction chamber are
temperature in the range of about 500°C to 1200°C, a pressure in the range of 40-900
mbar and metal organic precursors (V/III ratio) in the range of 50-5000. Metal nitride
epitaxial layers, are grown optionally on the metal nitride islands for further arrangement
of metal nitride electronic and opto-electronic devices. Alternately, the metal nitride
electronic and opto-electronic devices can also be arranged directly on the metal nitride
islands.
[0057] In an aspect of the process steps of the present invention, the indicated
temperature and pressure conditions of the reaction chamber is maintained for the
substrates that are deposited with catalyst sites, in the presence of a reactive gas,
preferably hydrogen or nitrogen, such that the temperature condition is less than the
melting point of the selected catalyst material.
[0058] In yet another aspect of the process steps of the present invention, the substrates
are held at the indicated temperature and pressure conditions for about 5-180 minutes
prior to introduction of the growth precursors [0059] In yet another aspect of the process steps of the present invention, the metal
nitride islands are grown only at the catalyst sites, without any growth or deposition on
the bare areas of the substrates, where catalyst sites are not located. For the formation
of metal nitride islands, metal-organic growth precursors are introduced into the reaction
chamber. In the present invention, the preferred metal-organic precursors are tri-methyl
gallium and ammonia is preferred for gallium nitride (GaN) deposition; tri-methyl
aluminum and ammonia is preferred for aluminum nitride (AlN) deposition, and trimethyl indium and ammonia is preferred for indium nitride (InN) deposition. The
composition of the metal nitride, for the indicated combinations of AlN, GaN and InN
islands, is based on the general formula AlxGayIn1-x-yN where x0≤, y≤1.
[0060] In further aspect of the process steps of the present invention, once the desired
lateral growth of the metal nitride islands is obtained, further epitaxial layers, including
the active device layers are deposited on the laterally-grown metal nitride islands, by
introducing the flow of the desired metal-organic precursors into the reaction chamber.
[0061] In the process steps of the present invention, a single-step growth of metal nitride
islands is performed without any growth interruptions, such as having to unload samples
from the reaction chamber for the deposition of masking layers.
[0062] It is understood here that the aforementioned process parameters such as
temperature, pressure and V/III ratio are exemplary in nature and will vary depending on
the configuration of the reaction chamber. Any suitable variation in the combination of
stated process parameters may be used so long as the desired metal nitride islands are
obtained preferentially at the catalyst sizes, with lateral sizes larger than the catalyst
dimensions, and the lateral to vertical dimensions of the islands greater than 1.
[0063] The preferred embodiments of the subject matter of the invention are now
described in the form of the following examples, which are exemplary and non-limiting in
nature and shall not be construed as limiting the scope of the present invention. Example 1
Growth of GaN islands on c-plane sapphire substrates using Au catalysts and a single set
of process conditions
[0064] A two-dimensional array of catalyst sites are first patterned on bare c-plane
oriented sapphire substrates using lithography where the catalyst dimension is chosen to
be 200 nm x 200 nm and the spacing between individual catalyst sites is set to 100
microns. Gold films are formed only over the patterned regions by a lift-off technique.
The samples are then loaded into the growth chamber and the temperature is ramped up
to 900°C, which is well below the melting point of gold and in a nitrogen ambient. The
substrates are maintained at this temperature for 180 minutes, to enable them to
completely de-wet. The growth conditions are then ramped to a temperature of 1000°C,
a pressure of 150 mbar and in a hydrogen ambient. The growth precursors in the form of
ammonia and tri-methyl gallium are introduced, to form islands of c-oriented GaN only
over the catalyst sites. The same conditions are maintained to continue the lateral growth
of these islands only from the initial array of catalyst sites. The islands with lateral
dimensions 20 microns and with vertical dimensions of 5 microns are obtained only over
the pre-patterned catalyst sites as illustratively shown in FIG.1 and FIG.5. These GaN
islands are with a reduced dislocation density of <10
6
cm
-2
at the surface and with a
smooth surface having RMS roughness of <1 nm as shown FIG.4 (SEM image).
Example 2
Growth of GaN islands on c-plane sapphire substrates using Ni catalysts
[0065] A two-dimensional array of catalyst sites are first patterned on bare c-plane
oriented sapphire substrates using lithography, where the catalyst dimension is chosen to
be 200 nm x 200 nm and the spacing between individual catalyst sites is set to 100
microns. Nickel films are obtained only over the patterned regions of the catalyst sites,
using a lift-off technique. The samples are then loaded into the growth chamber and the
temperature is ramped up to 1100°C, which is well below the melting point of nickel and
in a nitrogen ambient. The substrates are maintained at temperature for 180 minutes to enable them to completely de-wet. The growth conditions are then ramped to a
temperature of 1050°C, a pressure of 150 mbar and in a hydrogen ambient. The growth
precursors in the form of ammonia and tri-methyl gallium are introduced, to form islands
of c-oriented GaN only over the catalyst sites. The same conditions are maintained to
continue the lateral growth of these GaN islands only from the initial array of catalyst
sites. The GaN islands are with lateral dimensions of 20 microns and vertical dimensions
of 5 microns and the islands are obtained only over the pre-patterned catalyst sites as
shown in FIG.1 and FIG.5. These islands are with reduced dislocation density of <10
6
cm
-2
at the surface and with a smooth surface having RMS roughness of <1 nm, as shown in
FIG.4.
Example 3
Growth of GaN islands on c-plane sapphire substrates using Au catalysts and a multiple
set of process conditions
[0066] A two-dimensional array of catalyst sites are first patterned on bare c-plane
oriented sapphire substrates using lithography with intended spacings and sizes. Gold
films are formed only over the patterned regions using a lift-off technique. The substrates
are then loaded into the growth chamber and the temperature is ramped up to 1000°C,
which is well below the melting point of gold in a nitrogen ambient. The substrates are
maintained at this temperature for 90 minutes to enable their complete de-wetting. The
temperature is then ramped to 1020°C, under a pressure of 150 mbar and in a hydrogen
ambient. The growth precursors in the form of ammonia and tri-methyl gallium are
introduced to form islands of c-oriented GaN only over the array of catalyst sites. The
growth temperature is then ramped to 1040°C to promote lateral growth of GaN islands
only from the initial catalyst sites. The GaN islands with lateral dimensions 20 microns and
vertical dimensions of 5 microns are obtained only over the pre-patterned catalyst sites
as shown in FIG.1 and FIG.5. These GaN islands are with dislocation density of <10
6
cm
-2
at the surface and with a smooth surface having RMS roughness of <1 nm, as shown in Growth of GaN islands on r-plane sapphire substrates using Au catalysts
[0067] A two-dimensional array of catalyst sites are first patterned on bare r-plane
oriented sapphire substrates using lithography with the intended sizes and spacings. Gold
films are formed only over the patterned regions using a lift-off technique. The substrates
are then loaded into the growth chamber and the temperature is ramped up to 1000°C, in
a nitrogen ambient. The substrates are maintained at this temperature until they are
completely de-wetted. The growth conditions are then ramped to a temperature of
1000°C, a pressure of 150 mbar and in a hydrogen ambient. The growth precursors in the
form of ammonia and tri-methyl gallium are introduced, to form islands of non-polar
GaN, only over the catalyst sites. The same growth conditions are maintained to continue
lateral growth of these non-polar GaN islands only from the initial catalyst sites as
schematically illustrated in FIGs.1, 2 and 3.
Example 5
Growth of high electron mobility transistor devices on the reduced defect density,
bottom-up GaN growth platform
[0068] A two-dimensional array of gold catalyst sites are first patterned on bare c-plane
oriented sapphire substrates using lithography as described in earlier examples. The
substrates are then loaded into the growth chamber and c-plane oriented, reduced defect
density GaN islands of large lateral dimensions are obtained only over the pre-patterned
catalyst sites as described earlier and as particularly shown in FIGs.1 and 5. Following this,
the growth conditions are ramped to a temperature of 1050°C and a pressure of 40 mbar.
The growth precursors in the form of tri-methyl gallium, tri-methyl aluminium and
ammonia are introduced to form a layer of AlxGa1-xN, directly over the GaN layer thus
forming a two-dimensional electron gas at this interface constituting a known high
electron mobility transistor but deposited on the reduced defect density GaN growth platform. Example 6
Growth of InGaN quantum well light emitting diodes on the reduced defect density,
bottom-up GaN growth platform
[0069] A two-dimensional array of gold catalyst sites are first patterned on bare c-plane
oriented sapphire substrates using lithography as described in earlier examples. The
substrates are then loaded into the growth chamber and an array of c-plane oriented and
with reduced defect density GaN islands of large lateral dimensions, are obtained only
over the pre-patterned catalyst sites as described earlier and shown in FIGs. 1, 4 and 5.
Following this, the growth conditions are ramped to a temperature of 700°C, a pressure
of 200 mbar and growth precursors in the form of tri-methyl gallium, tri-methyl indium
and ammonia are introduced to form a layer of InxGa1-xN over the GaN layer. This is
followed by stopping the flow of the indium precursor to obtain another layer of GaN,
sandwiching the InxGa1-xN film thus forming a quantum well. This process is repeated as
required, in order to grow a multi-period quantum well structure that constitutes a lightemitting diode over the reduced defect density growth platform.
[0070] The present invention advantageously obtains a single-step growth platform for
metal nitride electronic and opto-electronic devices, without any masking layers. The
platform of large metal nitride islands is with reduced dislocation density (<10
6
cm
-2
),
which can be grown on, on a variety of substrates, using a bottom-up process that leads
to material savings by using a nucleation assisted epitaxial technique at pre-defined
catalyst sites. These defect-free, large-area islands render themselves to conventional
planar processing to fabricate semiconductor devices. The process for obtaining such as
platform can be advantageously utilized without any growth interruptions or deposition
of masking layers, when compared to conventional lateral overgrowth schemes, which
leads to lower process complexity and better turn-around times.
[0071] It will thus be seen that the embodiments as set forth above, are efficiently
attained and since certain changes may be made in carrying out the present invention
without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings be
interpreted as illustrative and not in a limiting sense.
[0072] It is also understood that the following claims are intended to cover all the generic
and specific features of the invention herein described and all statements of the scope of
the invention, which as a matter of language might be said to fall there-between. We Claim:
1. A metal nitride platform 100, said metal nitride platform comprising:
− a pre-defined array of catalyst sites 102, with intervening distances,
disposed on a substrate 101;
− large metal nitride islands 103 with lateral to vertical size ratios of at least
greater than one (1), disposed on said array of catalyst sites 102; and
− surfaces 103a of said metal nitride islands 103 are with reduced dislocation
densities and side walls 103b with bending of dislocations.
2. The platform 100 as claimed in claim 1, wherein the material for said array of
catalysts sites 102 is selected from a metal, preferably gold, nickel or a layered
material, preferably graphene, molybdenum disulphide (MoS2) or tungsten
disulphide (WS2) or a combination of these materials.
3. The platform 100 as claimed in claim 1, wherein lateral to vertical ratios between
said large metal nitride islands 103 and said array of catalyst sites 102 are the
range of 10:1.
4. The platform 100 as claimed in claim 1, wherein said substrate 101 is silicon (100),
(111), (110), silicon carbide (4H), (6H), (3C) and sapphire c-plane, r-plane, a-plane
and m-plane.
5. The platform 100 as claimed in claim 1, where the material for said metal nitride
islands 103 is aluminum nitride, gallium nitride, indium nitride or a combination
thereof.
6. The platform 100 as claimed in claim 1, wherein the reduced dislocation density at
said surface 103a of said metal nitride islands 103 is equal to or less than 10
6
/cm
-2
.
7. The platform 100 as claimed in claim 1, wherein the root mean square (RMS)
roughness of the metal nitride islands 103 is less than 1 nm.
8. The platform 100 as claimed in claim 1, wherein at least an epitaxial intermediate
layer 104 disposed on said metal nitride islands 103.The platform 100 as claimed in claim 1, where an electrically and optically-active
device 105, disposed on said metal nitride islands 103 or on said epitaxial layer
104.
10. The platform 100 as claimed in claim 1, wherein said an electrically and opticallyactive device 105 is formed with a metal nitride alloy of
aluminumxgalliumyindium(1-x-y)nitride (AlxGayIn(1-x-y)N), where x and y are in the
range of 0-1.
11. A process the preparation a metal nitride platform for active devices, said process
comprising the steps of:
(a) depositing a pre-defined array of catalyst sites with intervening distances, on a
substrate;
(b) growing initial seed metal nitride islands, selectively, only on said array of
catalyst sites, by placing said substrate in a reaction chamber, in the presence
of a reactive gas and precursors and under preferred reaction conditions, to
grow metal nitride islands;
(c) growing said metal nitride islands with lateral to vertical size ratios of at least
greater than one (1), with an enhanced lateral to vertical ratios between said
large metal nitride islands and said array of catalyst sites and under preferred
reaction conditions; and
(d) growing, optionally, an epitaxial layer on said metal nitride islands.
12. The process as claimed in claim 12, wherein said reactive gas is a mixture of
hydrogen and nitrogen.
13. The process as claimed in claim 12, where said precursors are selected from
gallium, aluminum, indium and nitrogen, preferably tri-methyl gallium, tri-methyl
aluminum, tri-methyl indium and ammonia.
14. The process as claimed in claim 12, wherein said preferred reaction conditions are
temperature in the range of about 500°C to 1200C°, pressure in the range of 40-
900 mbar and V/III ratio in the range of 50-5000. The process as claimed in claim 12, wherein said lateral to vertical ratios between
said large metal nitride islands and said array of catalyst sites are the range of
10:1.
16. The process as claimed in claim 12, wherein said large metal nitride islands 103
are formed without a mask.
17. The process as claimed in claim 12, wherein electrically and optically-active
devices, disposed on said metal nitride islands or on said epitaxial layer.
18. The process as claimed in claim 17, wherein said an electrically and optically-active
device is formed with a metal nitride alloy of aluminumxgalliumyindium(1-xy)nitride(AlxGayIn(1-x-y)N), where x and y are in the range of 0-1.