Patent Application
20150111370
This application claims priority to and the benefit of GB Application No. GB1318420.5, filed Oct. 17, 2013. The entire contents of all of these are incorporated herein by reference.
This invention relates to methods for producing gallium nitride materials, and semiconductor templates for producing gallium nitride materials.
Gallium nitride materials are semiconductor compound materials that are typically grown on a substrate, for example silicon (Si), sapphire or silicon carbide. Common examples of gallium nitride materials include gallium nitride (GaN) and the alloys indium gallium nitride (InGaN), aluminium gallium nitride (AlGaN) and aluminium indium gallium nitride (AlInGaN).
In typical growth processes, layers of the GaN are successively deposited onto the substrate. There is a problem however that in many cases, the GaN will have a different thermal expansion co-efficient than the substrate. This may lead to cracking of the GaN during cooling, especially where the nitride layer is relatively thick. A further problem arises since the lattice constants of GaN and the substrate are usually different, i.e. mismatched, which can lead to defect formation in the deposited GaN layers.
It has been proposed to address these problems by the inclusion of at least one intermediate layer between the substrate and the subsequently deposited GaN, i.e. forming a semiconductor template comprising a substrate and an additional layer formed over the substrate, over which the GaN may be formed.
In the particular case of silicon substrates, which exhibit particularly large differences in both thermal expansion co-efficient and lattice constant to GaN, it has been proposed to use intermediate transition layers of graded composition between the silicon and the GaN, and this is schematically shown in
There are two general types of grading employed within the transition layer: a “continuous” grading, in which the concentration of gallium (for the sake of example) increases smoothly from the bottom to the top of the layer, and “discontinuous” grading, in which the concentration increases in a step-wise manner from the bottom to the top of the layer.
However, both the continuous and discontinuous techniques have disadvantages. With discontinuous schemes, at the point of discontinuity, there is a large lattice mismatch, which can lead to defect formation from the interface and extended to the overgrown AlGaN. With continuous schemes, the effect of strain engineering—particularly in introducing the compressive strain is much more difficult to achieve. The gradient profile of the continuously graded layer is very difficult to control due to the binding energy and gas phase reaction of Al and Ga with NH3. The Ga concentration increases exponentially in the initial stage of linear GaN concentration ramping, and leave the later stage of Ga profile nearly flat. This phenomenon is particularly pronounced for the concentration difference of the initial and final Ga exceeding 30%.
It has also been proposed to use superlattice structures to reduce internal stresses. As is well-known in the art, a superlattice is a periodic structure of layers of at least two materials, typically each layer being in the nanometer scale of thickness.
A problem with this superlattice technique is the initial strain is retained and the strain engineering effect of introducing compressive strain is limited.
As prior art may be mentioned U.S. Pat. No. 6,659,287 and its continuation U.S. Pat. No. 6,617,060 which disclose various continuous and discontinuous GaN layering schemes, including use of discontinuous superlattices. Its claim 1 for example is directed to a semiconductor material comprising: a silicon substrate; an intermediate layer comprising aluminium nitride, an aluminium nitride alloy, or a gallium nitride alloy formed directly on the substrate; a compositionally-graded transition layer formed over the intermediate layer; and a gallium nitride material layer formed over the transition layer, wherein the semiconductor material forms a FET. Its claim 2 meanwhile is directed to the semiconductor material of claim 1, wherein the composition of the transition layer is graded discontinuously across the thickness of the layer.
As other prior art may be mentioned US 20020020341 which discloses the use of continuous-grade GaN layering. Its claim 1 for example is directed to a semiconductor film, comprising: a substrate; and a graded gallium nitride layer deposited on the substrate having a varying composition of a substantially continuous grade from an initial composition to a final composition formed from a supply of at least one precursor in a growth chamber without any interruption in the supply.
It is an aim of the present invention to overcome the problems noted above, and to provide improved methods for forming gallium nitride materials. This aim is achieved by using transition layers in various controlled schemes.
In accordance with a first aspect of the present invention there is provided a method for producing gallium nitride material, comprising the steps of:
The Al concentration difference between the two plateaux may be less than or equal to 30% of the Al concentration at depth z1.
The Al concentration difference between the two plateaux may be less than or equal to 30% of the Al concentration at depth z2.
The compositional grading function f(z) may include at least one additional plateau at a respective depth zn where df(zn)/dz=0.
Between depths z1 and z2 the Al concentration function f(z) may decrease linearly.
Between depths z1 and z2 the Al concentration function f(z) may decrease non-linearly.
The method may further comprise the step of forming a buffer layer between the substrate and the transition layer.
The method may further comprise the step of forming a buffer layer between the transition layer and the gallium nitride material layer.
The transition layer may comprise a superlattice.
With the stepwise semi-continuous transition and maintaining the concentration difference between two neighbouring plateau less or equal to 30%, there is no abrupt interface to introduce the interface lattice mismatch related defects, and the gradient profile of the continuously decreasing region is much more easy to control with better strain engineering effect.
The metal layer may comprises Al.
The thickness of metal layer may be in the range from 1-2 monolayers.
The method may further comprise the step, intermediate steps a) and b), of forming an AlN layer over the substrate.
The AlN layer may be formed over the metal layer.
The substrate may comprise silicon.
In accordance with a second aspect of the present invention there is provided a method for producing gallium nitride material, comprising the steps of:
The method may further comprise the step, intermediate steps a) and b), of forming an AlxGa(1-x)N layer with 0.1<x<0.9 over the substrate, and wherein in step b) the superlattice transition layer is formed over the AlxGa(1-x)N layer.
Step b) may be repeated at least once.
Steps b) and c) may be repeated at least once.
The method may further comprise the step of forming a buffer layer between the substrate and the superlattice transition layer.
The method may further comprise the step of forming a buffer layer between the superlattice transition layer and the gallium nitride material layer.
The metal layer may comprise Al.
The thickness of metal layer may be in the range from 1-2 monolayers.
The method may further comprise the step, intermediate steps a) and b), of forming an AlN layer over the substrate.
The AlN layer may be formed over the metal layer.
The substrate may comprise silicon.
In accordance with a third aspect of the present invention there is provided a method for producing gallium nitride material, comprising the steps of:
Step b) may be repeated at least once.
Steps b) and c) may be repeated at least once.
The method may further comprise the step, intermediate steps a) and b), of forming an AlxGa(1-x)N layer with 0.1<x<0.9 over the substrate, and wherein in step b) the superlattice transition layer is formed over the AlxGa(1-x)N layer.
The metal layer may comprise Al.
The thickness of metal layer may be in the range from 1-2 monolayers.
The method may further comprise the step, intermediate steps a) and b), of forming an AlN layer over the substrate.
The AlN layer may be formed over the metal layer.
The substrate may comprise silicon.
In accordance with a fourth aspect of the present invention there is provided a method for producing gallium nitride material, comprising the steps of:
One of the transition layers may comprise AlGaN.
One of the transition layers may comprise SiN.
Steps d) and e) may be repeated at least once.
The metal layer may comprise Al.
The thickness of metal layer may be in the range from 1-2 monolayers.
The method may further comprise the step, intermediate steps a) and b), of forming an AlN layer over the substrate.
The AlN layer may be formed over the metal layer.
The substrate may comprise silicon.
In accordance with a fifth aspect of the present invention there is provided a method for producing gallium nitride material, comprising the steps of:
Step d) may be repeated at least once.
Steps d) and e) may be repeated at least once.
Step d) may comprise forming at least two additional transition layers, such that transition layers of AlGaN and SiN are alternately formed.
Each transition layer may be formed at a higher temperature than the previous transition layer.
The transition layers may comprise a superlattice.
The method may further comprise the step of forming a buffer layer between the substrate and the first transition layer.
The method may further comprise the step of forming a buffer layer between the second transition layer and the gallium nitride material layer.
The metal layer may comprise Al.
The thickness of metal layer may be in the range from 1-2 monolayers.
The method may further comprise the step, intermediate steps a) and b), of forming an AlN layer over the substrate.
The AlN layer may be formed over the metal layer.
The substrate may comprise silicon.
In accordance with a sixth aspect of the present invention there is provided a method for producing a substrate material, the method comprising the steps of:
The laser treatment may comprise stealth laser treatment.
The bowing may be concave.
The bowing may be convex.
The substrate may comprise silicon.
In accordance with a seventh aspect of the present invention there is provided a semiconductor template for producing a gallium nitride material, comprising a substrate with a metal layer formed over the substrate, and a transition layer formed over the substrate, the transition layer being compositionally graded such that the composition of the transition layer at a depth (z) thereof is a function f(z) of that depth;
wherein the Al compositional grading function f(z) of the transition layer has a profile including two plateaux at respective depths z1 and z2 where df(z1)/dz=df(z2)/dz=0, and wherein the function decreases continuously between z1 and z2.
In accordance with a eighth aspect of the present invention there is provided a semiconductor template for producing a gallium nitride material, comprising a substrate with a metal layer formed over the substrate, and a superlattice transition layer formed over the substrate, the superlattice transition layer being compositionally graded such that the Al composition of the superlattice transition layer at a depth (z) thereof is a function f(z) of that depth;
wherein the Al compositional grading function f(z) of the superlattice transition layer decreases continuously throughout the thickness of the superlattice transition layer.
In accordance with an ninth aspect of the present invention there is provided a semiconductor template for producing a gallium nitride material, comprising a substrate with a metal layer formed over the substrate, a first transition layer formed over the substrate and a second transition layer formed over the first transition layer, wherein the second transition layer is formed at a higher temperature than the first transition layer.
In accordance with a tenth aspect of the present invention there is provided a 45. A semiconductor template for producing a gallium nitride material, comprising a substrate with a metal layer formed over the substrate, with a layer of AlGaN and a layer of SiN formed over the substrate.
The substrate may comprise silicon.
Other aspects of the present invention are as set out in the accompanying claims.
The invention will now be described with reference to the accompanying drawings, in which:
a-3e schematically show known grading schemes for an insertion layer, with
a, 5b and 5c schematically show semi-continuous grading schemes according to respective embodiments of the present invention;
a to 9 schematically show cross-sectional views of exemplary structures formed in accordance with aspects of the present invention; and
a and 10b schematically show a laser treated substrate in plan and sectional views respectively, including a convex bowing.
In a first embodiment, gallium nitride material is produced using a structure similar to that shown in
In more detail, a transition layer comprising AlGaN for example is formed over the substrate, and is compositionally graded such that the composition of the transition layer at a depth (z) thereof is a function f(z) of that depth, wherein the Al compositional grading function f(z) of the transition layer grown in step b) has a profile including at least two plateaux at respective depths z1 and z2 where df(z1)/dz=df(z2)/dz=0, and wherein the function increases continuously between z1 and z2. In fact,
a shows an example where the grading function f(z) varies linearly between depths z1 and z2.
Conveniently, the grading function may indicate the concentration of aluminium at each depth (z) of the transition layer. Although aluminium is particularly suitable, the concentration of other substances may alternatively be so varied.
In a first embodiment, shown in
This method reduces dislocation density in both XRC (X-Ray Crystallography) (102) and (002) axes.
The transition layers could comprise AlGaN for example, or, similarly to the embodiment below, may comprise AlGaN and SiN in alternate, paired, layers.
This example relates to that shown in
In a modification of this embodiment (not shown), a (111) Silicon substrate of about 2, 4, 6 or 8 inches in diameter is loaded in the MOCVD. A thin Al layer is deposited for about 10 seconds after the thermal desorption at 1050° C. under H2, followed by the deposition of undoped AlN of 20-200 nm. Then an Al0.25Ga0.75N layer is deposited. The first transitional is grown with the Al0.9Ga0.1N of thickness around 15 nm plus a thin Si3N4 layer, then a GaN layer of around 0.5 to 0.75 urn is grown, and the transitional layer process is repeated three times. Finally a layer of n-GaN of thickness around 1 to 4 μm is grown. The epitaxial growth of the full device is continued in the MOCVD reactor. A typical LED structure formed comprises the following layers: InGaN/GaN MQW active region (30 Å/120 Å, 2-8 pairs), AlGaN:Mg capping layer (˜200 Å), p-type Mg-doped GaN (0.1-0.3 μm). The electron and hole concentration in the GaN:Si and GaN:Mg layers are about 8×1018 cm−3 and 8×1017 cm−3, respectively.
c shows a further example, in which the process is similar to that of Example 2, except that an extra AlxGal-xN layer 23 with 0.1<x<=0.3 is grown on top of the MN, then followed by the growth of a layer 24 of GaN and a layer 45 of SiN with a further GaN layer 24 on top of that. Multiple transitional layers 46 (followed by a further GaN layer 24), 47 (followed by a further GaN layer 24), and 48 of AlxGal-xN with 0.1<x<1, are then successively grown, with each layer grown at a different temperature. In this example layers 46, 47, and 48 are grown at 850, 890 and 9.40° C. respectively. A final layer 39 of GaN is then grown.
In a further embodiment, shown in
As in the previous embodiment, successive transition layers could be formed at successively higher temperatures.
b shows a further example. Here, the process is similar to that of Example 2 except that a layer 23 of AlGaN 25% is grown on top of the layer 22 of AlN. A layer 24 of GaN is grown followed by multiple transitional layers comprising a pair of alternating AlGaN layer 36 with Al>=50% and SiNx layer 38 of thickness less than 10 nm. Following growth of each such pair, a further GaN layer 24 is grown, followed by another transitional layer pair. In total, there are three sets of GaN layer plus associated paired transitional layers.
The transition layer here may optionally comprise a superlattice.
In another embodiment, a template structure generally similar to that of
The grading function f(z) may decrease linearly or non-linearly throughout the thickness of the superlattice transition layer as appropriate.
a and 10b show a further embodiment a six inch (for the sake of example only) silicon (111) substrate 41 of about 1000 um thickness is pre-treated with 942 nm laser beam application to create a pattern within the substrate to cause the substrate to bend, creating a convex “bow” having a displacement depth of around 10-35 um. The laser ablated patterned area 42 is located inside the wafer at a depth of approximately 125 um. The pattern used is a square pattern of 1×1 mm gap between each laser scribe.
Such a bowed substrate may for example be used to benefit subsequent MOCVD growth processes. The temperature of the bottom of the wafer during the heating up is always higher than the top surface, particularly with fast and high power heating to around 1000° C. (such as with GaN growth). This tends to cause a concave bowing in the wafer, which causes an uneven deposition thickness on the surface. However, with a pre-formed convex bow obtained using this laser process, during the heating up, the subsequent bending causes the wafer to flatten out for better uniform deposition.
The above-described embodiments are exemplary only, and other possibilities and alternatives within the scope of the invention will be apparent to those skilled in the art. For example, with any of the schemes or structures outlined above, one or more buffer layers may be provided, for example between the substrate and lower transition layer, or between the upper transition layer and the grown gallium nitride materials layer.
In general, use of silane doping will increase the tensile stress quite significantly. However a three step growth process as described above provides a significant improvement in the tensile stress gradient produced by silane doping. The transition layer or layers may optionally be doped with silane or carbon for the purpose of forming full devices. In this case, it has been found that silane doping concentrations of up to about 6×1018/cm3 can maintain a reasonable compressive stress even with a single transition layer thickness of over 4 μm.
| Number | Date | Country | Kind |
|---|---|---|---|
| GB1318420.5 | Oct 2013 | GB | national |
