# Patent

## Title
Group III-nitrides on Si substrates using a nanostructured interlayer

## Abstract

A layered group III-nitride article includes a single crystal silicon substrate, and a highly textured group III-nitride layer, such as GaN, disposed on the silicon substrate. The highly textured group III-nitride layer is crack free and has a thickness of at least 10 μm. A method for forming highly textured group III-nitride layers includes the steps of providing a single crystal silicon comprising substrate, depositing a nanostructured InxGa1-xN (1≧x≧0) interlayer on the silicon substrate, and depositing a highly textured group III-nitride layer on the interlayer. The interlayer has a nano indentation hardness that is less than both the silicon substrate and the highly textured group III-nitride layer.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application incorporates claims benefit and incorporates by reference in its entirety Provisional Patent Application Ser. No. 60/712,922 entitled “GROUP III-NITRIDES ON SI SUBSTRATES USING A NANOSTRUCTURED INTERLAYER” filed on Aug. 31, 2005.


STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
The United States Government may have certain rights in this invention pursuant to a grant from the U.S. Air Force grant number FA8650-04-2-1619.


BACKGROUND
GaN and related materials continue to grow in importance for optical and electronic devices. As in other semiconductor systems, epitaxial growth of GaN ideally occurs on GaN substrates cut from bulk GaN single crystals. Bulk crystal growth of GaN, however, requires extremely high pressure to maintain the nitrogen content in the crystal, rendering bulk growth extremely difficult. For this reason, the high volume production of large size, bulk GaN is improbable in the near future and the search for alternative substrates continues.
Two of the main factors associated with substrate choice are cost and resulting GaN epilayer quality. Silicon is increasingly being used as a substrate for GaN deposition because Si substrates are available at comparatively low cost, high quality, large area, and large quantity, thus presenting many manufacturing advantages over other available substrates for GaN, such as sapphire and SiC.
The disadvantages of Si as a substrate for GaN heteroepitaxy include an a-plane +20.5% misfit which led to the conclusion that growth of GaN directly on silicon was unfeasible. Moreover, the thermal expansion misfit between GaN (5.6×10−6 K−1) and Si (6.2×10−6 K−1) of 9.6% can lead to cracking upon cooling in films grown at high temperature, and, at elevated temperature, melt-back etching between Ga and the Si substrate during the initial stages of growth or at stress is known to induce cracks that form in GaN films during GaN deposition.
Traditionally these issues trigger polycrystalline GaN growth on Si substrates. Typically, thin AlN buffer layers are used to absorb the lattice mismatch between the GaN film and the Si substrate. The subsequent deposition of GaN introduces significant strain into the structure due the large lattice mismatch along with the resultant high density of defects that introduce additional tensile stress into the film. This tensile stress is exacerbated during cool down from growth temperature with macro-crack formation customary for GaN films thicker than 1 μm.
To overcome GaN cracking problems, different techniques have been used including use of multiple AlN interlayers, AlGaN graded layers, patterned Si, and in situ SiN masking (non-uniform deposition). These methods were reported to provide some decrease in bowing and cracking, but no method successfully produced crack-free thick (e.g. >10 μm) GaN films likely because there still remains excessive tensile stress, as well as strong cohesion between GaN (or AlN buffer layer) and Si. Although  ̃7 μm thick crack-free GaN on Si has been reported by incorporating multiple AlN interlayers, the maximum thickness of a commercially available crack-free GaN layer on Si is about 1 μm.
Cracks can be generated during growth or cooling due to the excess tensile stress caused by large lattice and thermal expansion differences. It has been observed by the present Inventors that the cracks penetrate through the Si substrate and separation occurs inside the Si substrate. The strong cohesion between GaN and Si (or AlN and Si in GaN/AlGaN/AlN/Si template case), as well as the brittleness of Si, are responsible for cracking to take place in pre interior of the Si wafer. The bond strength of Si—Si is 7 eV which is lower than and the Ga—N (8.9 eV) or Al—N (11.5 eV) and Si—N (10.5 eV). The bond strength of Si—Si is the weakest. The nano-indentation hardnesses of the GaN, AlN, and Si are 20, 18 and 14 GPa, respectively. Therefore, the cracking penetration to the Si substrate observed by the present Inventors was expected. This brittleness of Si added with the large tensile stress created by the lattice mismatch and thermal expansion differences makes the growth of crack-free GaN on Si even more challenging.
SUMMARY
A layered group III-nitride article comprises a single crystal silicon comprising substrate and a highly textured crystal group III-nitride layer disposed on the silicon substrate. The highly textured group III-nitride layer is crack free and has a thickness of at least 10 μm, such as 15 to 50 μm. As used herein, the term “highly textured” as applied to the group III-nitride layer refers to a layer which provides (i) a full width half maximum (FWHM) X-ray ω-scan rocking curve of no more than 20 arc-min, more preferably less than 10 arc-min, and most preferably less than 7 arc-min, and (ii) an XRD pole figure (Phi scan) that provides a highly non-uniform pole density that is clustered around the number of points characteristic of the particular orientation. For example, regarding the XRD, highly textured GaN will evidence only 2 peaks being at (002) and (004) in a LRXRD spectrum. Crack free is defined herein refers to an area of at least 25 mm2 being “crack-free” as confirmed by SEM, AFM, or TEM.
The silicon substrate can be (111), (100) or other orientations. In a preferred embodiment, the highly textured group III-nitride layer comprises GaN. In this preferred embodiment, the article can further comprise a thin layer of a In—Ga—N alloy at an interface between the GaN layer and the silicon substrate.
A method for forming highly textured group III-nitride layers comprises the steps of providing a single crystal silicon comprising substrate, depositing a nanostructured InxGa1-xN (1≧x≧0) interlayer on the silicon substrate, and depositing a highly textured group III-nitride layer on the interlayer. The interlayer has a nano indentation hardness that is less than both the silicon substrate and the highly textured group III-nitride layer. The group III-nitride layer can be GaN and the interlayer can be InN. The step of depositing a highly textured group III-nitride layer preferably comprises a first group III-nitride layer deposition at a first temperature followed by a second group III-nitride layer deposition at a second temperature, wherein the first temperature is below a decomposition temperature of the InxGa1-xN (1≧x≧0) and the second temperature is at least 150° C. greater than the first temperature.
The highly textured group III-nitride layer is generally crack free and has a thickness of at least 10 μm. The nanostructured InxGa1-xN (1≧x≧0) interlayer can comprise a columnar film or nanorods having an average rod diameter of 300 to 700 nm. The silicon substrate is preferably (111) oriented.



BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1(a) and (b) are schematic diagrams of an exemplary method according to the invention to grow thick, substantially crack-free, Group III-Nitrides film on Si substrates using a nanostructured interlayer (a) randomly oriented, and b) well aligned nanorods.
FIG. 2 shows InN nanorods grown by H-MOVPE on Si(111) at T=600° C.: a,b—scanned SEM images; c—LRXRD spectrum; d—AES spectrum.
FIG. 3 shows LT-GaN deposited on top of InN nanorods on Si: a, b—SEM images; c—EDS spectrum, d—LRXRD spectrum.
FIG. 4 shows a thick (28 μm) substantially crack free GaN grown on Si using nanorod interlayer: a—plane view scanned SEM image; b—scanned XSEM, c—LRXRD spectrum; d—free-standing GaN.
FIG. 5(a) shows a wide area scanned SEM plan-view of 30 min LT-GaN grown on InN nanorods. FIG. 5(b) is a scanned cross-sectional view that shows that the thickness of the LT-GaN was  ̃4 μm and that voids were formed at the interface. FIG. 5(c) is an XRD θ-2θ scan showing GaN (002), InN (002), and InN (101) peaks, as well as the Si (111) substrate peak.
FIG. 6 is a low resolution XRD θ-2θ of freestanding and crack-free GaN film (28 μm thick) grown on InN nanorods (d=500 nm)/Si (111) substrates according to the invention. The low resolution XRD shows GaN (002), (004), and (103) peaks. No InN was detected.



DETAILED DESCRIPTION OF THE INVENTION
It has been found by the inventors that a suitably nanostructured InxGa1-xN (1≧x≧0) interlayer relieves much of the stress at the Si substrate-group III-nitride film interface during group III-nitride film deposition and thus prevents crack formation during growth of thick (≧1 μm) group III-nitride films thereon. A layered group III-nitride article comprises a single crystal silicon comprising substrate and a highly textured crystal group III-nitride layer disposed on the silicon substrate. The highly textured group III-nitride layer is crack free and has a thickness of at least 10 μm, such as 15 to 50 μm.
The silicon substrate is preferably (111) oriented. However, the substrate can be other orientations, such as (100).
The structure of the nanostructured InxGa1-xN (1≧x≧0) interlayer used is preferably nanorods from 300 to 700 nm in diameter. Such nanorods are nanocrystalline single crystal dislocation free having a [00.1] growth axis.
In a preferred embodiment, the highly textured group III-nitride layer is selected from the group consisting of GaN, InN, AlN and their solid solutions. However, more generally, and other semiconductor materials, such as II-VI and IV-VI materials, as well as carbon nanotubes, can be disposed on the nanostructured interlayer.
The nanostructured InxGa1-xN (1≧x≧0) can comprise a plurality of crystalline nanorods, which can be randomly oriented or aligned with one another. A thickness of the nanostructured interlayer is generally from 0.1 to 3.0 μm.
A method for forming textured group III-nitride layers comprises the steps of providing a single crystal silicon comprising substrate, the silicon substrate preferably having a thin (native; about 15 to 20 angstrom) silicon dioxide layer disposed thereon, depositing a nanostructured InxGa1-xN (1≧x≧0) interlayer on the silicon substrate, and depositing a highly textured group III-nitride layer on the interlayer. Thus, in a preferred embodiment, the native oxide layer on the silicon substrate is not removed prior to depositing the nanostructured interlayer.
The interlayer growth generally is performed on the Si substrate at low temperature, such as T<600° C. The indium mole fraction can be varied to obtain a desired value in the entire compositional range (0≦x≦1). In practice, different mole fractions of In can be controlled by varying the ratio of the inlet flow rate of an In comprising reagent, such as trimethyl indium (TMIn) to the total group III flow rate. When the Ga comprising reagent is triethyl gallium (TEGa), the total group III flow rate is the flow rate of TMIn plus TEGa, and the flow ratio noted above is (TMIn/[TMIn+TEGa]).
The step of depositing a highly textured group III-nitride layer can comprise a first group III-nitride layer deposition at a first temperature followed by a second group III-nitride layer deposition at a second temperature, wherein the second temperature is generally 150° C. greater than the first temperature. Two exemplary approaches are provided for the annealing procedure:

    
    
        I: in situ slowly cooling down after the growth when reactor is off from the growth to room temperature, and
        II: after growth annealing in N2 or NH3 at the growth temperature (T=850 C, 15 to 30 min).
    
    


Free-standing III-Nitrides can also be grown using the invention by first depositing a nanostructured InxGa1-xN (1≧x≧0) at the interface along with a certain post grown annealing procedure. Free-standing as used herein refers to relatively thick (≧50 μm) film that is separated from the substrate.
The invention is expected to provide improved provide improved group III-nitride-based devices. For example, the invention can provide improved RF and microwave components for wireless industry, such as those based on high-electron mobility transistors (HEMTs).
EXAMPLES
It should be understood that the Examples described below are provided for illustrative purposes only and do not in any way define the scope of the invention.
In the Examples described below, two types of reactors were used to deposit GaN. The primary reactor used was a quartz hot-wall merged-hydride reactor that can alternate between MOCVD and HVPE. H-MOVPE has several advantages. The quick switching of precursors made it possible to grow InN nanorods and GaN without taking the wafers out from the reactor. It can also ease regulation of the amount of precursors which has been found to control the diameter, length, and density of the nanorods.
Si(111) substrates were cleaned by trichloroethylene, acetone, and methanol followed by DI water rinse and N2 dry. Etching was purposely not carried out to take advantage of native oxide layer for nanorod growth. As used herein, the phrase “native oxide layer” refers to the silicon dioxide layer that forms on the surface of a silicon wafer from exposure to oxygen at or near room temperature.
It was believed that a thin native SiO2 layer may help the nucleation of nanorods. After loading, the substrate was heated to 600° C. at a rate of 15° C./min. The precursors for the InN nanorod growth for the interlayer were trimethylindium (solution TMIn, Epichem), hydrogen chloride (10% HCl, 90% N2, Air Products), and ammonia (grade 5 anhydrous NH3, Matheson-Tri gas) with N2 carrier gas. Reactor pressure P=760 Torr, T=600° C., NH3/TMIn=250, HCl/TMIn=4. Flow rates: TMIn=0.7 sccm; HN3=175 sccm, HCl (10%)=28 sccm, N2=1600 sccm.
After growth of the InN nanorods, the reactor was cooled and the low temperature (LT) GaN initial layer was deposited at 560° C. for 15 to 60 minutes with an approximate LT GaN thickness of thickness of 0.1-3.0 μm min. The temperature of the reactor was then increased to 850° C. after the LT-GaN growth. During the heating and cooling processes N2 was always provided. High temperature (HT) GaN (850° C.) was subsequently grown on top of LT-GaN and thick GaN (>20 μm) was obtained without cracking. After growth, the reactor was cooled in N2 ambient. The cooling rate was about −15° C./min.
FIGS. 2(a) and (b) are scanned SEM plane views of the deposited InN nanorods. The average diameter of InN nanorods was around 100 nm and the length was around 1 μm. Hexagonal, well-faceted features show that individual nanorods had very good crystal quality. An XRD θ-2θ scan shows typical InN crystal patterns with Si(111) substrate peak (see FIG. 2 (c)). A SiO2 layer was observed by XRD. As noted above, the native oxide was intentionally not removed by etching to facilitate the nucleation of nanorods. Based on AES results shown in FIG. 2 d, the In:N ratio was determined to be 1:0.85. Carbon and oxygen were detected and origin was due to the chemosorption. No Cl was detected using the AES technique.
FIG. 3 is a scanned SEM micrograph of LT-GaN grown on top of the InN nanorods. An interesting embossed pattern was observed. The diameter of the individual cell is around 1 μm, which is close to the length of InN nanorods. EDS results show the existence of elemental Cl. The growth temperature was 560° C., which is lower than the InCl3 boiling temperature (586° C.). As a result InCl3 may be formed during LT-GaN growth because excess HCl was provided as HCl/TMGa ratio=2. An XRD θ-2θ scan shows Si, InN, and GaN peaks that evidence the co-existence of InN nanorods and LT-GaN film.
FIG. 4 is a scanned SEM showing crack-free 28.4 μm thick GaN film grown on a single crystal Si(111) wafer. No cracks were observed by SEM over the entire film as seen in FIG. 4 (b). High crystalline quality of the GaN film was demonstrated using XRD θ-2θ scan. The FWHM of GaN (002) peak was 341 arc sec. No Si(111) peak was detected due to screening by the thick GaN film.
Experiments regarding InN nanostructured buffer growth on Si is now described. InN columnar film, small nanorods (d=250 nm), large nanorods (d=500 nm), and microrods were grown on Si substrates at different conditions listed in the Table below and used as templates for thick GaN growth.










Growth conditions for InN buffer interlayer on Si.











Growth T (° C.)
Cl/In
N/In
Time
Feature















560
1
2500
1
hr
(a) Columnar Film


600
4
250
20
min
(b) Nanorods (d = 250 nm)


600
4
250
1
hr
(c) Nanorods (d = 500 nm)


650
5
250
1
hr
(d) Microrods









Experiments regarding low-temperature GaN growth on InN nano/Si templates are now described. Low temperature (560° C.)-GaN was grown on the 4 different InN interlayers/Si for comparison of the structural effects of InN. LT-GaN was grown on various InN crystals for 10, 20, and 30 min. The growth conditions for LT-GaN were set as T=560° C., Cl/In=1.5, and N/In=570. It was observed that only InN columnar films and larger nanorods (d=500 nm) provided uniform coverage of LT-GaN, while the smaller nanorods (d=250 nm) and microrods demonstrated non-uniform coverage of LT-GaN. It was found that GaN deposition occurred mainly on the InN surface, rather than on the Si substrate.
FIG. 5(a) shows a wide area scanned SEM plan-view of 30 min LT-GaN grown on InN nanorods. LT-GaN started to coalescence after 30 min, and an interesting embossed pattern could be seen. The scanned cross-sectional view (FIG. 5(b)) shows that the thickness of the LT-GaN was  ̃4 μm and that voids were formed at the interface. XRD θ-2θ scan (FIG. 5(c)) showed GaN (002), InN (002), and InN (101) peaks, as well as the Si (111) substrate peak. This data shows that InN nanorods still exist after 30 min LT-GaN growth. This was expected since 560° C. is well below the decomposition temperature of InN. No In metal was detected by the XRD.
High-temperature GaN growth on LT-GaN/InN/Si(111) then followed. Thick GaN films were grown at 850° C. for 2 hr after the 30 min LT-GaN growth on InN crystals described above. The growth conditions for HT-GaN were as follows: Cl/Ga=1.5, N/Ga=570, and T=850° C. N2 was used as the carrier gas. Thick GaN films having a thickness of about 20 μm were deposited. Thick (about 20 μm) GaN films were grown on LT-GaN for each of an InN columnar film, small InN nanorods (d=250 nm), large InN nanorods (d=500 nm), and InN microrods. Thick GaN grown on the InN columnar film/Si showed significant peeling of the GaN film, but no cracks. Thick GaN grown on smaller InN nanorods (d=250 nm) over Si(111) showed cracks likely due to the non-uniform deposition of InN nanorods and LT-GaN. Thick GaN film on InN microrods on Si showed cracks.
The best result for crack-free thick GaN was found for growth on the large InN nanorod (d=500 nm) interlayer. Although the surface was found to be is still rough, the data obtained makes it clear that dense and uniform InN nanorods can provide a good structure for thick, crack-free GaN growth on Si(111).
The GaN film occasionally self-separated from the Si substrates without effort. SEM and XRD were obtained for freestanding and crack-free GaN film (28 μm thick) grown on InN nanorods (d=500 nm)/Si (111) substrates. No cracks were observed by SEM over the wide range of the film. Low resolution XRD shown in FIG. 6 shows GaN (002), (004), and (103) peaks. No InN was detected presumably because at the growth temperature (850° C.), the InN dissolved into the InxGa1-xN. The film was polycrystalline, but highly textured along the [002] axis with an FWHM of 341 arcsec.
A cross-sectional SEM view of crack-free thick (40 μm) GaN grown on Si substrate using dense and large (d=500 nm) InN nanorods was obtained. In this case, self-separation did not occur. InN nanorods at the interface were not visible after HT-GaN. InN likely dissolved into GaN to form InxGa1-xN alloy at the interface.
This invention can be embodied in other forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be had to the following claims rather than the foregoing specification as indicating the scope of the invention.

## Claims

1. A layered group III-nitride article, comprising:
a single crystal silicon comprising substrate;
a nanostructured interlayer; said nanostructured interlayer comprising a nanostructured feature selected from the group consisting of a columnar film, a plurality of nanorods, a plurality of microrods, and a combination thereof;
and a highly textured crystal group III-nitride layer, said highly textured group III-nitride layer being crack free and having a thickness of at least 10 μm wherein said nanostructured interlayer is disposed between said silicon substrate and said highly textured group III-nitride layer.

2. The article of claim 1, wherein said silicon substrate is (111) or (100) oriented.

3. The article of claim 2, wherein said highly textured group III-nitride layer comprises GaN.

4. The article of claim 3, wherein said interlayer comprises an InGaN alloy.

5. The article of claim 1, wherein said nanostructured interlayer comprises nanorods, microrods, or a combination thereof.

6. The article of claim 1, wherein said nanostructured interlayer comprises nanorods, said nanorods being well aligned.

7. The article of claim 1, wherein said nanostructured interlayer comprises nanorods, said nanorods being randomly oriented.

8. The article of claim 1, wherein said nanostructured interlayer comprises nanorods, said nanorods having an average rod diameter of 300 to 700 nm.

9. The article of claim 1, wherein said nanostructured interlayer comprises nano rods, said nanorods comprising InN.

10. The article of claim 1, wherein said nanostructured interlayer comprises InxGa1-xN, wherein 1>x>0.

11. The article of claim 1, wherein said nanostructured interlayer has a thickness of 0.1 to 3.0 μm.

12. The article of claim 1, wherein said silicon substrate is (111) oriented.