Method of making an icosahedral boride structure

Abstract

A method for fabricating thin films of an icosahedral boride on a silicon carbide (SiC) substrate is provided. Preferably the icosahedral boride layer is comprised of either boron phosphide (B12P2) or boron arsenide (B12As2). The provided method achieves improved film crystallinity and lowered impurity concentrations. In one aspect, an epitaxially grown layer of B12P2 with a base layer or substrate of SiC is provided. In another aspect, an epitaxially grown layer of B12As2 with a base layer or substrate of SiC is provided. In yet another aspect, thin films of B12P2 or B12As2 are formed on SiC using CVD or other vapor deposition means. If CVD techniques are employed, preferably the deposition temperature is above 1050° C., more preferably in the range of 1100° C. to 1400° C., and still more preferably approximately 1150° C.

Description

BACKGROUND OF THE INVENTION

[0003] The icosahedral borides, such as boron phosphide (B12P2) and boron arsenide (B12As2), are hard and chemically inert solids that exhibit exceptional radiation tolerance due, at least in part, to the strong bonding within the boron icosahedra. It has been suggested that if these wide bandgap materials could be suitably doped, they would be useful for a variety of applications, in particular those applications requiring radiation hardness and/or high temperature capabilities. Early work has indicated that a high background impurity concentration will degrade the luminescence properties of B12P2 while crystalline imperfections are expected to degrade the electrical transport properties of the material. It is expected that B12As2 and B12P2 will exhibit similar electrical and optical behavior because of the structural similarity of these two materials.

[0004] Crystalline perfection and background impurity issues are linked as crystalline imperfections cause increased contamination incorporation through accelerated diffusion. Additionally, crystalline imperfections provide natural locations for accommodating such contaminants. Therefore it is anticipated that the intrinsic electrical, optical and other properties of B12P2 and B12As2 will best be revealed in high crystalline quality samples that have a low background impurity concentration.

[0005] In order to obtain the desired icosahedral boride material, a number of parties have produced B12P2 and B12As2 thin films using chemical vapor deposition (CVD) techniques. For example, in 1973 Hirayama et al. published a note entitled “Hetero-Epitaxial Growth of Lower Boron Arsenide on Si Substrate Using Ph3-B2H6-H2 System” (Jap. J. Appl. Phys., 12 (1973)1504-1509) in which it was shown that B12As2 could be deposited using dilute hydride sources of diborane (B2H6) and arsine (AsH3) in a hydrogen ambient environment. The B12As2 films were deposited on silicon substrates with three different orientations, (100), (110) and (111). The film morphology was found to be orientation dependent. Electron reflection diffraction analysis indicated that the films were single crystal, epitaxial B12As2 thin films containing patches of polycrystalline material.

[0006] Years later, in an article entitled “Chemical Vapor Deposition of Boron Subarsenide Using Halide Reactants” (Reactivity of Solids, 2 (1986)203-213), Correia et al. demonstrated that B12As2 films could be grown by CVD on a variety of substrates (i.e., tungsten, nickel, fused quartz, Si(111) and Si(100)) using the halide sources BBr3 and AsCl3. The authors established that the film crystallinity was dependent on growth conditions, especially growth temperature and source flow rate, and showed how changing these conditions could yield either amorphous films or polycrystalline films. They also found that during deposition on a silicon substrate, intermixing occurred between the B12As2 and silicon, with up to 4% Si being found in the B12As2 film.

[0007] In 1997 Kumashiro et al. published an article entitled “Epitaxial Growth of Rhombohedral Boron Phosphide Single Crystalline Films by Chemical Vapor Deposition” (J. Solid State Chem., 133 (1997)104-112) reporting the results of B12P2 film growth on silicon using CVD techniques. The authors confirmed the sensitivity of film crystallinity in B12P2 to the growth conditions and found that polycrystalline B12P2 was obtained at a growth temperature of 1050° C. while single crystal B12P2 was obtained at a temperature of 1100° C. They also confirmed earlier findings that reactant gas flow is the most important parameter in determining the quality of the grown crystal.

[0008] Although it appears that the growth conditions for B12As2 and B12P2 have been optimized, the desired film crystallinity and impurity concentrations have not yet been achieved. Accordingly, what is needed in the art is a method for achieving the desired film crystallinity and impurity concentrations in icosahedral boride materials. The present invention provides such a method and the desired resultant material.

SUMMARY OF THE INVENTION

[0009] The present invention provides a method for fabricating thin films of crystalline icosahedral boride on a silicon carbide (SiC) substrate. Preferably the crystalline icosahedral boride layer is comprised of either boron phosphide (B12P2) or boron arsenide (B12As2). The method provides improved film crystallinity and lowered impurity concentrations.

[0010] In one aspect of the invention, an epitaxially grown layer of B12P2 which is in crystallographic registry with a base layer or substrate of SiC is provided.

[0011] In another aspect of the invention, an epitaxially grown layer of B12As2 which is in crystallographic registry with a base layer or substrate of SiC is provided.

[0012] In yet another aspect of the invention, thin films of B12P2 or B12As2 are formed on SiC using CVD or other vapor deposition means. If CVD techniques are employed, preferably the deposition temperature is above 1050° C., more preferably in the range of 1100° C. to 1400° C., and still more preferably approximately 1150° C.

[0013] A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIG. 1 is an illustration of a structure in accordance with the present invention;

[0015] FIG. 2 is an illustration of the complex unit cell of B12As2;

[0016] FIG. 3 is a scale drawing of four boron icosahedra overlaying a SiC basal plane atomic structure;

[0017] FIG. 4 is an x-ray diffraction pattern for a B12As2 thin film deposited on a <0001>6H-SiC substrate;

[0018] FIG. 5 is a high resolution TEM micrograph showing the interface between a B12As2 thin film and a SiC substrate; and

[0019] FIG. 6 is an electron diffraction pattern along the [1210] zone axis for a B12As2 film deposited on the “on-axis”<0001>6H—SiC substrate.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

[0020] Silicon carbide (SiC) offers a variety of characteristics that make it an ideal candidate for a base substrate for the epitaxial growth of icosahedral boride layers in general, and boron arsenide (B12AS2) and boron phosphide (B12P2) layers in particular. First and foremost is the lattice parameter of SiC, which closely matches that of B12As2 and B12P2. By matching lattice parameters, epitaxial film strain and the associated strain energy can be minimized.

[0021] If the epitaxial film strain associated with mismatched lattice parameters is maintained to a level below approximately 2 percent, typically a thin film can be grown as a uniform 2-dimensional layer. As the film thickness increases, however, the strain energy also increases, eventually being large enough to create misfit dislocations. If the lattice mismatch strain is larger than approximately 2 percent, the deposited material may rearrange itself from a uniform 2-dimensional film to form an array of 3-dimensional islands. Depending upon the actual strain value, formation of 3-dimensional islands can occur at the start of deposition or after some thickness of 2-dimensional film growth has occurred. When 3-dimensional growth occurs, the resultant film will typically be polycrystalline and have a rough surface morphology.

[0022] As a result of the close match in lattice parameters between two unit cells of SiC and one unit cell of B12As2, and to a lesser degree B12P2, lattice mismatch strain is minimized. Accordingly, films of B12As2 and B12P2 can be grown on SiC which exhibit improved crystallinity and surface morphology.

[0023] In addition to favorable lattice parameters, SiC offers both high thermal and chemical stability. As a consequence of these material characteristics, SiC is suitable for use in a high temperature, or otherwise aggressive, deposition environment. Accordingly, icosahedral boride layers exhibiting negligible contamination can be exitaxially grown on SiC substrates.

[0024] As illustrated in FIG. 1, and in accordance with the invention, a thin film 101 of the desired icosahedral boride material (e.g., B12As2 or B12P2) is deposited onto a base substrate 103 comprised of SiC. Due to the close match of the lattice parameter of substrate 103 with layer 101, negligible lattice mismatch strain occurs. As a result, a uniform 2-dimensional film is formed.

[0025] As previously noted, to achieve a high quality film, it is desirable to match as close as possible the lattice parameter of the substrate to that of the deposited film. For B12As2 and Bl2P2 thin films, the lattice parameters are extremely close to that of twice the unit cell of SiC. For example, the basal-plane, lattice parameter of B12As2 (aB12As2=6.145 Å) is approximately (to within <0.14%) equal to twice the basal plane lattice parameter of SiC (aSiC=3.077 Å). As a result of this lattice match-up, it is possible to epitaxially deposit a layer of B12As2 onto a SiC substrate. Although the lattice match-up between B12P2 and SiC is not as good as that between B12As2 and SiC (approximately 2.8% versus less than 0.14%), the lattice parameters are still close enough to generally allow epitaxial growth of B12P2 layers.

[0026] FIGS. 2 and 3 illustrate the epitaxial relationship between B12As2 and SiC. FIG. 2 shows the complex unit cell of B12As2, illustrating the 12-atom, boron icosahedra with four of the boron icosahedra at the base of the unit cell numbered 201-204. FIG. 3 is a scale drawing showing boron icosahedra 201-204 overlaying the SiC basal plane atomic structure. Boron icosahedra 201-204 are shown in FIG. 3 in their normal, unstrained relative positions. For clarity, only the four icosahedra and one monolayer of SiC are shown. The excellent lattice match between the two crystal structures is readily apparent in this figure.

[0027] Although it is expected that a variety of different substrate orientations can be used without severely affecting the resultant film quality, substrate misorientation is preferably not much more than 3.5 degrees off of <0001>, more preferably less than 3.5 degrees off of <0001>, and still more preferably oriented along <0001>.

[0028] In the preferred embodiment, the desired icosahedral boride films are grown on the SiC base substrate using chemical vapor deposition (CVD). It will be appreciated, however, that other epitaxial growth techniques are equally applicable to the present invention (e.g., molecular beam epitaxy or MBE).

EXAMPLES

[0029] B12As2 films were deposited on 6H <0001>and 3.5° off <0001>6H—SiC substrates in an RF-heated, horizontal-geometry CVD reactor. The SiC substrates were approximately 300 μm thick and were n-type with a bulk resistivity of approximately 0.1 Ω-cm. The substrates were degreased and then dried under nitrogen gas before being loaded into the CVD reactor. CVD films were grown using dilute sources of diborane (1% B2H6 in H2) and arsine (1% AsH3 in H2), which provided boron and arsenic respectively. The flow rates for each of the source gases was 50 sccm with a hydrogen carrier gas flow rate of 5 slm. In order to obtain crystalline films, the deposition temperature should be above 1050° C., preferably in the range of 1100° C. to 1400° C., and more preferably approximately 1150° C. At a deposition temperature of 1150° C. and a pressure of 100 torr, a B12As2 growth rate of 0.2 μm/hr was achieved.

[0030] X-ray diffraction patterns were measured for the films grown on both the on-axis SiC and 3.5 degree off-axis SiC substrates. Similar spectra were obtained. FIG. 4 is a typical x-ray diffraction pattern obtained for these films. This pattern unambiguously confirms that the deposited films are comprised of B12As2.

[0031] FIG. 5 is a cross-sectional micrograph taken using transmission electron microscopy (i.e., TEM), the micrograph showing the interface between the B12As2 thin film 501 and the SiC substrate 503. As shown, B12As2 thin film 501 contains oriented, polycrystals of B12As2 with a mosaicity (i.e., a grain to grain misorientation) of up to 3 degrees.

[0032] The lattice matching relationship between B12As2 and SiC can be expressed mathematically as aB12As2=2aSiC. This relationship is confirmed from the electron diffraction micrograph of FIG. 6 which shows the (1010) B12As2 reflections occurring exactly at the midpoint between the (1010) SiC reflections.

[0033] As previously noted, B12As2 thin films grown on silicon substrates exhibited a high silicon background concentration. This silicon concentration has been attributed to the decomposition of the substrate during CVD growth. It is recognized that a high background impurity concentration can significantly degrade the optical and electrical properties of the icosahedral boride thin film. Careful analysis of the high resolution TEM image in FIG. 5 shows that the SiC lattice is regular and undisturbed to within one monolayer of the B12As2/SiC interface, thus indicating that the SiC substrate remained stable and did not decompose during the CVD process, resulting in a significantly less contaminated B12As2 epilayer.

[0034] As will be understood by those familiar with the art, the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosures and descriptions herein are intended to be illustrative, but not limiting, of the scope of the invention which is set forth in the following claims.

Claims

1. A method for fabricating a semiconductor device, comprising the steps of: providing a SiC substrate; and epitaxially growing an icosahedral boride layer on at least one surface of said SiC substrate.

2. The method of claim 1, further comprising the step of selecting B12P2 as said icosahedral boride layer.

3. The method of claim 1, further comprising the step of selecting B12As2 as said icosahedral boride layer.

4. The method of claim 1, further comprising the step of orienting said SiC to less than 3.5 degrees off of <0001>, wherein said orienting step is performed prior to said epitaxially growing step.

5. The method of claim 1, further comprising the step of orienting said SiC to <0001>, wherein said orienting step is performed prior to said epitaxially growing step.

6. The method of claim 1, further comprising the step of selecting a deposition temperature of above 1050° C., said deposition temperature associated with said epitaxially growing step.

7. The method of claim 1, further comprising the step of selecting a deposition temperature within the range of 1100° C. to 1400° C., said deposition temperature associated with said epitaxially growing step.

8. The method of claim 1, further comprising the step of selecting a deposition temperature of approximately 1150° C., said deposition temperature associated with said epitaxially growing step.

9. The method of claim 1, wherein said step of epitaxially growing said icosahedral boride layer utilizes a chemical vapor deposition technique.

10. The method of claim 1, further comprising the steps of: degreasing said SiC substrate; and drying said SiC in a flowing nitrogen gas environment, wherein said steps of degreasing and drying are performed prior to said epitaxially growing step.