WO2000062331A2

WO2000062331A2 - Semiconductor heterostructures with crystalline silicon carbide alloyed with germanium

Info

Publication number: WO2000062331A2
Application number: PCT/US2000/008671
Authority: WO
Inventors: James Kolodzey; Gary Katulka; Cyril Guedj
Original assignee: University Of Delaware
Priority date: 1999-04-02
Filing date: 2000-03-31
Publication date: 2000-10-19
Also published as: WO2000062331A3; WO2000062331A9; AU5866100A

Abstract

A semiconductor heterostructure (a) is formed by mixing the elemental semiconductor germanium (Ge) with the compound semiconductor silicon carbide (SiC) to form an alloy of silicon carbide: germanium (SiC:Ge). The alloy (SiCGe) could be used alone or in multilayered structures with other semiconductors to improve the performance of electronic and optical devices and circuits.

Description

SEMICONDUCTOR HETEROSTRUCTURES WITH CRYSTALLINE SILICON CARBIDE ALLOYED WITH GERMANIUM

Government License Rights

The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided by the terms of research grant N00014-93-1-0393 awarded by the Office of Naval Research.

Background of Invention

Various materials have been tried to function as suitable semiconductor heterostructures . It would be desirable to provide a semiconductor that has sufficiently high electrical conductivity, proper optical adsorption, proper lattice parameter (atomic spacing) and compatibility with conventional Sic.

A. Uddin and T. Uemoto, "Trap centers in germanium- implanted and in as-grown 6H-SiC," Jpn. J. Appl . Phys . , Vol. 34, pp. 3023-3029, 1995, describes the properties of SiC with extremely small amounts of Ge (about 1000 less density than used with the present invention) . The implanted Ge behaved as a defect or contaminants . This report lacked the success that found with Ge in SiC with the present invention, and it made no mention of the electrical and optical advantages of the present invention.

In addition, the present invention uses larger amounts of Ge that form a three-part alloy of SiC (in equal parts) with Ge, for a total ratio of 1.1.x, where x is the portion of Ge. The inventive alloy can also be written as:

(SiC)-_|___χGe_χ, where x is the Ge mole fraction, and (1-x) is the mole fraction of silicon carbide.

Summary of Invention

An object of this invention is to provide an improved material for semiconductor heterostructures .

In accordance with this invention a new alloy is fabricated by mixing elemental semiconductor germanium (Ge) with the compound semiconductor silicon carbide (SiC) . The new alloy can be used alone or in multi- layered structures with other semiconductors to improve the performance of electrical and optical devices and circuits . The Drawings :

Figure 1 is a graph for Bragg (0004) reflections comparing a) SiC:Ge as-implanted sample (thick solid line) b) pure SiC (dashed line) and c) SiC:Ge sample after RTA anneal at 1000 C for 1 minute, which indicated improved crystallinity of the annealed SiC:Ge compared to as- implanted sample;

Figure 2 are graphs for: a) 2 eV He+ ion backscattering spectroscopy of pure SiC substrate prior to Ge implantation showing scattering from Si near 1.2 MeV and C near 0.4 MeV. b) 2MeV He+ ion backscattering spectroscopy of Ge implanted SiC:Ge showing the Ge peak near 1.6 MeV. c) 2MeV He+ ion backscattering spectroscopy of SiC:Ge after RTA annealing. The spectra relative ion intensity remain unchanged after annealing compared to the as-implanted SiC:Ge;

Figure 3 is a graph of Raman spectra of a) SiC:Ge

after 800°C 1 minute RTA anneal, b) pure SiC, and c)

SiC:Ge after 1000°C 1 minute anneal. Measurements show large amplitude Raman signal in most of the spectra for

the sample annealed at 800°C probably due to implantation

induced defects, which is greatly reduced after the

1000°C RTA anneal. The pure SiC sample show distinct

peaks at 205, 770, and 970 cm^"1;

Figure 4 is a view along [100] of a substitutional Ge into 3C-SiC. Molecular dynamics computation is used to relax the 512 atoms supercell isotopically . C, Si and Ge are represented with spots of increasing diameter, (a) is obtained when Ge replaces Si , while (b) corresponds to the substitution of C by Ge . A modified Keating model which takes into account the anharmonicity of C is used to compute the atomic interactions;

Figure 5 is a graph showing the evolution of the lattice parameter after incorporation of substitutional Ge in 3C-SiC.

(a) is obtained when Ge replaces only Si in a random manner .

(b) is computed using a linear combination of the lattice parameters of Si, Ge and diamond, pondered by the relative concentrations

(Vegard' s law) . (c) is obtained when Ge replaces Si or C randomly;

Figure 6 is a graph showing localized phonon spectra around Ge calculated by the recursion method. Ge is assumed to replace only Si into the zinc-blende 3C-SiC crystal. Curves (a) to (f) are calculated for increasing substitutional Ge concentrations (respectively 0.2, 4.9, 9.4, 14.8, 18.4 and 50 %) ;

Figure 7 is a graph showing the current-voltage characteristics prior to contact annealing, of p-type 4H- SiC wafer with regions implanted with Ge and N per the following wafer quadrants of Figure 8. Region 1 has Ge and N; region 2 has Ge only; region 3 has N only; region 4 is p-4H-SiC with no implants;

Figure 8 illustrates an undoped wafer of p type 4H- SiC. The 4H-SiC implant regions are:

1. Implanted with 8xl0²⁰ cm^"3 of Ge and lxlO²⁰ cm^"3 of N.

2. Implanted with 8xl0²⁰ cm^"3 of Ge .

3. Implanted with lxlO²⁰ cm^"3 of N.

4. Unimplanted (p type 4H-SiC) .

The wafer is from Chris Clarke, Graeme Eldridge and Rowan Messham of Northrop- Grumman; Figure 9 is an X-ray diffraction from HBT structure with p-type 4H-SiC substrate implanted with: Ge at 1.6E16 cm-2, 750 KeV, and N at 1.45E12 cm-2, at 200 KeV for the base region, and Ga 2.75 E16 cm-2, 225 KeV, as an emitter region;

Figure 10 shows the four regions of a wafer where region 1 is SiC with Ge,N, region 2 is with SiC with Ge, Region 3 is SiC with N, region 4 is SiC; and

Figure 11 is a series of graphs showing capacitance voltage characteristics showing change in built-in voltage with Ge and with N doping.

Detailed Description

The present invention in its broadest aspect may be considered as a composition of matter: a new alloy fabricated by mixing the elemental semiconductor germanium (Ge) with the compound semiconductor silicon carbide (SiC). The new alloy, silicon carbide: germanium (SiC:Ge) , can be used alone, or in multilayer structures with other semiconductors, to improve the performance of electronic and optical devices and circuits. We have produced Samples of SiC:Ge can be produced by ion- implanting Ge atoms into substrates of crystalline SiC. It should be possible, however, to fabricate it by other means, as later described. The SiC:Ge alloy has new properties that are distinct from either SiC or Ge, but it is chemically compatible with both of these and also with the semiconductor silicon (Si) that is used in integrated circuits (ICs) for computers. SiC:Ge is particularly attractive as a heterostructure material when used in conjunction with conventional silicon carbide because no other material has a lattice constant near enough to that of SiC to permit the interfaces to be nearly defect-free. Compatibility is highly important because pure silicon carbide is unique for making circuits that can sustain extreme operating conditions at high powers and high temperatures .

The invention is thus a novel alloy or solid solution of the pure compound semiconductor SiC with the elemental semiconductor Ge. The novel features include: 1) a higher electrical conductivity than pure SiC; 2) changes in optical absorption; 3) a lattice parameter (atomic spacing) larger than that of SiC and smaller than that of Ge, and 4) ; inherent compatibility with conventional SiC (due to its similar lattice parameter) . Pure semiconducting SiC is presently under intense interest for fabricating electronic circuits that can operate under the extreme conditions of high-powers and high-temperatures . The reason for this capability is that SiC is chemically highly stable with a high melting point and is mechanically hard. It is even used mechanically in sandpaper because it is so robust. Unfortunately, no other semiconductors are compatible with SiC because of its relatively small lattice parameter. Until now, no other material could be layered onto pure SiC to form the multilayer heterojunction devices that make possible so many modern devices including lasers in CD players and cellular telephone circuits. In principle, regions of SiC:Ge can be fabricated onto semiconductor structures of pure SiC, Si, or Ge in order to create heterostructure devices and circuits with significantly enhanced capabilities compared to other alloys. In particular , SiC:Ge can improve the capabilities of circuits made from silicon carbide by providing adjacent regions or layers with slightly different conductivity, bandgap energy and chemical etching behavior. Until now, the lack of compatible materials has been a major limitation to the usefulness of SiC for commercial applications.

The extended properties that will be made available by SiC:Ge will revolutionize the SiC circuit industry.

Although layers of SiC:Ge can be added to SiC devices to make new structures with enhanced capabilities, SiC:Ge is a new alloy with its own inherent properties and it can also be used independently of pure SiC.

SiC:Ge alloys have been prepared using the technique of ion-implanting Ge atoms into a substrate of SiC (hexagonal, 4H type) . It may also be possible to fabricate SiC:Ge by chemical vapor deposition (CVD) and by molecular beam epitaxy (MBE) . CVD is the presently standard technique for fabricating SiC.

The alloy SiC:Ge has the potential: 1) for deposition onto conventional SiC for better electrical contacts; 2) for differences in chemical reactivity that can be exploited in materials processing (e.g. etch-stop layers); and 3) as strain-relieving layers with a lattice constant intermediate between those of pure SiC, pure Si, and pure Ge. Layers of SiC:Ge could behave as a bridge between conventional Si for integrated circuits, and with SiC, which is robust for high-power, high-temperature circuits.

As explained later, the availability of differences in properties is often more important than the magnitude of the property itself.

Presently, pure SiC is attractive for fabricating circuits that operate at high temperatures such as on automotive or jet aircraft engines, high power converters for traction motors, and in high temperature chemical process monitoring. It is expected that layers or regions of SiC:Ge can be added to these circuits to improve their performance, when used as listed above.

Possible uses include: a. layers with higher electrical conductivity fabricated onto other semiconductors, such as pure SiC, for improved active regions of transistors and other electronic devices, which may result in lower power consumption and increased electrical efficiency. b. layers with lower bandgap energies than pure SiC, forming energy barriers and quantum wells that can confine the charge carriers (electrons and holes) producing heterostructure devices with improved properties. Pairs of heterostructures in other semiconductor systems include: SiGe/Si and GaAs/GaAlAs . We propose SiC/SiC :Ge multilayers. c. layers to control the etching characteristics of structures (i.e. as etch-stop layers) due to different chemical reactivities with etchants compared to neighboring layers . d. regions with higher electrical conductivity for improving the performance of electrical and metallic contacts to the devices. The lack of good contacts is presently a severe limitation for pure SiC. e. the smaller energy bandgap compared to pure SiC may produce light emitting diodes with new emission colors. Conventional light emitting diodes fabricated from SiC contain a small quantity of aluminum, forming a chemical complex that yields blue light. With the variable bandgap energy of SiC:Ge (due to variable Ge content) , light emitting diodes fabricated from SiC:Ge may yield a range of emission colors. f. as intermediate layers for joining two other semiconductors having different lattice parameters, to relieve strain or to reduce the formation of dislocation defects in the crystal structure. g. as multi-layer elements for the reflection and diffraction of x-rays. A possible application of this capability is x- ray lithographic masks for the fabrication of integrated circuits. h. as photodetectors . The relatively smaller bandgap of SiC:Ge extends the photoresponse to longer wavelengths compared to pure SiC . i. the etching rate differences between SiC and SiC:Ge allows micro-electromechanical systems (MEMS) to be made from SiC with greater robustness than with softer materials such as Si. Si is doped with materials that change its etch rate to selectively remove mechanical regions such as gears or optical elements from the unwanted background. With etch differences between SiC:Ge and SiC, it should be possible to make MEMS with SiC.

A possible limitation is a reduction in thermal stability and ability to withstand high temperatures compared to pure SiC. At very high temperatures, the alloy may decompose, perhaps by precipitation, into separated regions of the constituents: SiC and Ge . It is expected that this is not a severe limitation because we have already annealed the SiC:Ge alloy up to temperatures of 1000°C with Ge remaining substitutional. Even higher temperatures may be possible. This may not be a limitation because this temperature is at the upper range of normal process temperatures .

Evidence indicates that SiC:Ge has electrical and optical properties different from pure SiC. Therefore, regions of SiC:Ge can be fabricated onto semiconductor structures of pure SiC, Si, or Ge in order to create heterostructure devices and circuits with significantly enhanced capabilities compared to homogeneous materials. In particular , SiC:Ge can improve the capabilities of circuits made from silicon carbide that are useful for operation under high-temperature, high-power conditions. Many types of heterojunction devices can be made with the intrinsic advantages of SiC (high temperature, high power, as described above) .

The following discusses work done in evaluating the electrical and optical properties of Ge-implanted 4H-SiC.

The structural, electronic, and optical properties of single crystalline n-type 4H-SiC implanted with Ge atoms have been investigated through x-ray diffraction (XRD) , Rutherford backscattering spectroscopy (RBS) , Raman spectroscopy, and sheet resistivity measurements. Ge atoms are implanted under the conditions of a 300-keV ion beam energy with a dose of 2 x 10 ¹⁶ cm^"2. X-ray diffraction of the Ge-implanted sample showed broadening of the Bragg peaks. A shoulder on the (0004) reflection indicated an increase in the lattice constant corresponding to substitutional Ge and implantation induced lattice damage, which was repaired through thermal

annealing at 1000° C. The diffraction pattern after

annealing indicated improved crystal structure and a peak shift to a lower reflection angle of 35.2°. The composition of Ge detected through XRD was reasonably consistent with RBS measurements that indicated 1.2% Ge in a 1600 A thick layer near the SiC surface. Raman spectroscopy also showed fundamental differences in the spectra obtained for the Ge-implanted SiC (SiC:Ge) compared to a pure sample of SiC. Sheet resistivity measurements indicate a higher conductivity in the Ge implant by a factor of 1.94 compared to unimplanted SiC. These results have demonstrated the possibility of substitutional implantation of Ge atoms into the crystalline lattice of 4H-SiC substrates. The change in composition and properties may have numerous electronic device applications including high-power, high- temperature, optoelectronic, as well as high-frequency device structures.

The single crystalline SiC substrate investigated for the Ge implantation is n-type nitrogen doped to 2.5 x 10¹⁸ cm^"3, is 421.6 μm thick, and is from Cree Research, Inc. The sample was cleaned prior to implantation and analysis with a standard chemical rinse of methanol, acetone, and deionized water. The SiC substrate was ion implanted uniformly with Ge atoms from a hot filament electron bombardment ion source with an ion mass spectrometer for a period of 2000 seconds. The ion energy during the implant was 300 keV and the fluence was 2 x 10 ¹⁶ cm^"2. The ion current was 1.5 μA providing a current density of about 1

μA/cm². It is estimated that the SiC substrate reached a

steady state temperature of 50°C during the implantation process. Post implantation annealing was performed with an AG Associates Heatpulse 610 rapid thermal annealer (RTA) with forming gas ambient consisting of 85% H₂ and 15% N₂. Experimental characterization of the SiC and SiC:Ge samples was carried out with x-ray diffraction, RBS spectroscopy, Raman spectroscopy, and sheet resistivity measurements .

X-ray diffraction (XRD) measurements were made with a Philips X-pert diffractometer utilizing the Cu K_αl wavelength in the symmetrical Bragg configuration at low resolution as described previously. (Appl . Phys . Lett . , 71, 26 (1997)) XRD results of the pure SiC and SiC:Ge implanted samples have indicated distinct differences in

the region representing the (0004) Bragg reflection (2θ=

35.7°) of the hexagonal 4H-SiC. The XRD data of the 4H-SiC

and Ge implanted 4H-SiC (SiC:Ge) is similar for the two samples including an intense X-ray peak at 35.695°. This peak is associated with the (0004) plane of 4H-SiC which has also demonstrated strong reflection characteristics in XRD analysis by other researchers. (J. Crystal Growth 144 (1994)) For the as-implanted SiC:Ge sample a XRD

plot of Figure 1 shows a subtle feature near 35.2°.

Analysis of this feature indicates a defective 4H-SiC layer with slight compressive strain induced by the Ge in

the lattice. For the SiC:Ge sample annealed at 1000°C for

a period of 1 minute, the X-ray pattern of Figure 1 shows a sharpened peak centered around 35.695° indicating an improved SiC:Ge layer with fewer defects. The shift toward the direction of 35.2° indicates a substitutional Ge content of about 4% applying Vegard's law. This value of Ge concentration is somewhat larger than that obtained from RBS which indicate a Ge content of approximately 1.2%.

Rutherford backscattering spectroscopy (RBS) data with 2 MeV He+ ions was obtained for the pure SiC and SiC:Ge samples along with theoretical results from Rutherford Universal Manipulation Program (RUMP) simulations. (Nucl. Instrum Methods Phys . Res. B, 9,344 (1985) ) Fitting the experimental RBS data to RUMP simulations was carried out interactively for the purposes of determining the content of Ge atoms in the SiC:Ge layer as well as the layer thickness of SiC:Ge on the surface of the 4H-SiC, both of which are important for determining the electrical and optical properties. RBS results for the pure SiC, as implanted SiC:Ge, and SiC:Ge annealed at

1000°C, are given in Figure 2. The Ge concentration and

SiC:Ge layer thickness were determined to be 1.2% and 1600 A, respectively, as indicated by RUMP simulation results shown in Figure 2b which closely match the experimental data obtained by RBS as well as calculations performed with TRIM based on the implant conditions. There was no detectable change in the RBS data taken on the annealed SiC:Ge compared to the as implanted sample (see Figure 2c) .

Raman spectra for pure and Ge ion-implanted SiC are illustrated in Figure 3 which were obtained with incident polarized laser light having a wavelength of 785 nm. Figure 3 shows three different spectra having varied sample properties. The spectra for pure 4H-SiC has distinct Raman peaks at 205 cm^"1, 770 cm^"1, and 970 cm^"1. These well-defined bands are also strongly evident in the Ge implanted SiC:Ge sample and they are believed to be associated with Si and C vibrational modes. Similar Raman spectra have also been reported for Si and C solid state structures previously. (Macromolecules , 29, 22 (1996)) The two other spectra in the figure are for the SiC:Ge

sample after an RTA cycle at 800°C for 1 minute, and the

other after annealing the SiC:Ge at 1000°C for 1 minute.

The Raman spectrum of the as-implanted SiC:Ge sample prior to RTA annealing were badly distorted by broadening and band amplification across the entire region of interest possibly due to fluorescence of the damaged material surface, and was not included here. Annealing with the RTA did have a strong influence on the resultant Raman spectra and the degree of fluorescence observed in the SiC:Ge sample, illustrated by the noticeably large differences in the Raman spectra shown in Figure 3. The reduction of the fluorescence effect is believed to be attributable to thermally repairing crystalline damage induced by the ion implantation process itself. The repairing of implantation induced crystalline damage through high temperature annealing in SiC has been well documented elsewhere. (Jpn J. Appl . Phys., 34 (1995) and J. Elec . Mats. (1997)) Without additional experiments, it is difficult to determine the degree to which lattice damage has been sufficiently repaired and can be neglected as well as the point at which optimal Ge substitutional implantation has occurred.

The sheet resistivity of the SiC:Ge sample was determined qualitatively by four point probe measurements. The four point probe measurements were performed in two different spatial regions of the SiC:Ge sample and compared to results obtained on a pure SiC sample. The measurements were made on the SiC:Ge sample just after Ge implantation and prior to annealing. For the pure SiC and the SiC:Ge, the apparatus indicted an average sheet resistance of 584.75 and 301.35 ohms/square, respectively, indicating the conductivity of the Ge implanted material is nearly twice that of SiC.

The substitutional implantation of Ge in SiC may play an important role in the electronic and optical properties required for several electronic device applications including those of high power, high frequency, and optoelectronics. The experimental observations to date on the ion implanted SiC:Ge sample investigated here include fundamental differences compared to those of pure SiC. For example, our measurements have shown that the x-ray diffraction pattern near the (0004) reflection in ion implanted SiC:Ge is significantly modified in comparison to that of the pure sample of 4H-SiC. The altered x-ray pattern is believed to be caused by the implantation of substitutional Ge atoms and the subsequent introduction of strain into the SiC lattice. The affect of thermal

annealing at 1000°C for 1 minute has resulted in improved

crystallinity compared to the as-implanted sample and the shift near the (0004) reflection toward 35.2 degrees is still evident. However, additional measurement with annealed samples will be required in order to separate the affects of substitutional Ge implantation and implantation induced crystalline damage. RBS measurements taken immediately after ion implantation of Ge indicate a shallow layer of approximately 1600 A containing 1.2% Ge atoms, which are not altered by the RTA annealing that was performed. Raman spectroscopy has revealed surface damage in the as implanted SiC:Ge, but there is also strong evidence that lattice damage, as indicated by fluorescence in the Raman spectra, is greatly reduced by thermally annealing the material after implantation. Finally, the as implanted SiC:Ge sample has been shown to have a surface conductivity nearly twice that of SiC. This result was determined from standard four point probe resistivity measurements.

The following is a discussion of substitutional Ge in 3C-SiC.

The incorporation of substitutional Ge_.into 3C-SiC alloys is studied theoretically with an anharmonic Keating model specifically adapted to the computation of the structural properties and the lattice dynamics of Si_x-..- _yGe_xC_y alloys. Basic energy calculations show that the substitution of Si by Ge is more probable than the substitution of C by Ge in the zinc-blende silicon carbide crystal. If Ge replaces only Si, then the lattice parameter equals to (0.43593±0.00002) +

(0.000337+0.000002) y , where y stands for the Ge content. Hence, Vegard's law is not applicable. The alloy is characterized by a distinct phonon spectrum whose maximum peak position in cm^"1 is best described by the exponential decay (243±1) + (27±2) exp (-y/ (7.5±1.2) ) up to the zinc- blende GeC compound. Silicon carbide (SiC) based electronics is suitable for high power and high frequency applications, as well as for severe environments, including high temperature and high radiation. (Mater. Sci. Eng. B, 1, 77 (1988) and Vol. 185 of Nato Advanced Study Institute, Series E: Appl . Sci. (Kluwer, Dordrecht, 1990)) Silicon carbide's ability to function under such extreme conditions is expected to enable significant improvements to a far ranging variety of applications and systems. Substitutional Ge may modify the structural, electrical and optical properties of SiC based heterostructures, therefore Ge incorporation into SiC-based microelectronics and optoelectronics may provide further device opportunities through bandgap and strain engineering. The crystal growth might not be simple, mainly because of the difference in lattice parameters and covalent radii between silicon germanium and diamond, but Ge seems to have a beneficial effect in the epitaxy of single-crystalline 3C-SiC on silicon. (Appl. Phys. Lett, 70(11) (1997)) There are few if any data available about substitutional Ge in 3C-SiC . Because of the broad technological importance of IV-IV materials and devices, as well as the increasing ability to grow highly metastable alloys, it is essential to develop theoretical predictions of the physical properties of this new material .

The understanding of the lattice-vibrational properties is important to explain various interesting properties of SiC, among them mechanical, thermal and structural ones. For example, phonons may stabilize the polytypism via several contributions to the free energy. (J. Phvs. Condens. Matter, 3, 539 (1991)) Here the structural properties and lattice dynamics of substitutional Ge in 3C-SiC are determined using a modified anharmonic Keating model specially adapted to the computation of lattice parameters and phonon spectra of Si₁-_x__yGe--C_y alloys. First, the theoretical model will be briefly exposed, then the structural effects of substitutional Ge in 3C-SiC and the corresponding localized vibrational spectra will be successively detailed .

Our theoretical approach is based upon a valence force field model derived from Keating (Phys . Rev. , 145, 637 (1966) ) and taking into account the effects of carbon anharmonicity. The interactions between atoms have been modeled with an interatomic potential similar to the one of Rύcker et al . (Phvs. Rev. B, 53, 1302 (1996)) with the exception of adjusting our force coefficients to yield the correct lattice parameters and phonon modes of Si, Ge, Si-_L-_j-Ge--, diamond and 3C-SiC at 300- K. Our model gives a localized vibrational mode of C in pure Ge situated at 531 cm^"1, in agreement with Hoffman et al . (Phys . Rev. B , 55, 1167 (1997)) . The very high precision and reliability of these experimental data, which can be obtained by x-ray diffractometry, Raman spectrometry and absorption spectroscopy, justify our approach. In addition, our set

of parameters enables the computation of the L0(T) mode

from 3C-SiC, which cannot be obtained from the parameters given by Rύcker et al . (Phys . Rev. B , 53, 1302 (1996)) to our knowledge. This model is therefore ideally suited to a precise computation of lattice parameters and phonon spectra of tetrahedrally coordinated Sii.-----Ge.-C-. alloys. The molecular dynamics relaxation is calculated from a 512 atom supercell by time increments of 3 fs, until the total potential energy reaches a stable minimum. We have computed isotropic relaxations, where no external pressure is applied to the computational box, to simulate the structural properties of the alloy. In a further stage, the local phonon density around carbon is computed, using the recursion method detailed in A. Hairie, F. Hairie, G. Nouet, E. Paumier, A. P. Sutton, "Polycristalline Semiconductors III- Physics and Technology", ed. H. P. Strunk, J. H. Werner, B. Fortin, O. Bonnaud, Vol. 37-38, 91 (1993) Copyright 1994 Scitec Publications Ltd, Switzerland, member of the Trans Tech Group of Publishers, ISBN 3-908450-04-7, Volumes 37-38 of Solid State Phenomena (Pt. B of Diffusion and Defect Data - Solid State Data ISSN 0377-6883) distributed by Trans Tech Publications Ltd, Hardstrasse 13, CH-4714 Aedermannsdorf , Switzerland. This algorithm enables the computation of the local phonon density around chosen atoms. The calculated spectra can be compared with absorption spectroscopy and Raman spectrometry results after application of proper selection rules. Numerous atomic configurations have been tested, in order to simulate the substitution of Ge into the 3C-SiC crystal .

The application of these theoretical tools gives an insight into the structure of the substitutional alloy. In the case of a single substitutional Ge into the zinc- blende SiC matrix, the atomic positions after isotropic relaxation are depicted in Figure 4. The substitution of C by Ge induces a higher lattice distortion than the substitution of Si, which is logical because Ge is close to Si in term of atomic radius, electronegativity and elastic properties. The substitution of C requires at least 38 times more energy, therefore it is highly unprobable. Silicon carbide is more rigid than silicon, therefore the distortions are confined across few atomic distances. This strain modifies the crystal dimensions, in a manner depending on the statistical atomic distribution. The lattice parameter is obviously proportional to the mean size of the supercell after relaxation, and the results are displayed in Figure 5. If one assumes that Ge replaces Si only (Figure 5(a)), then the lattice parameter equals to (0.43593±0.00002) + (0.000337±0.000002) y , where y stands for the Ge content . A Vegard' s law

(Figure 5(b)) between Ge and cubic SiC would give a lattice parameter of 0.43596 + 0.0012994 x. Even in the hypothesis of a random substitution of Si or C by Ge

(Figure 5(c)), Vegard' s law does not apply. In the case of a random substitution, the increase of lattice parameter is rapid, but the corresponding increase of elastic energy is unlikely to be observed experimentally, because more stable atomic arrangements will occur instead. In any case, the modification of lattice parameter induced by substitutional Ge in 3C-SiC should be easily probed by x- ray diffraction.

The computation of phonon spectra give a further tool to investigate this new material. In Figure 6 (a) to (f) , the localized vibrational spectra around Ge are computed for increasing Ge content. The substitutional incorporation of Ge in 3C-SiC is characterized by a distinct phonon spectrum whose maximum peak position in cm^"1 is best described by the exponential decay (243±1) + (27±2) exp(-y/ (7.5±1.2) ) up to the zinc-blende GeC compound. Because of the strong local elastic heterogeneity, the calculated spectrum is usually complex, and the clear maximum obtained in Figure 6 (e) is marked by an arrow. In the real crystal, a smoothing of the spectrum should naturally occur because of the very high number of atoms involved. Hence, our fit assumes an additional gaussian smoothing. Physically, these spectra could be considered as a result of complex multiphonon processes involving the nearest neighbors of Ge . The Ge-C bond is stabilized in the zinc-blende GeC compound, therefore it is reasonable that the fitted phonon energy decreases when the Ge concentration increases. According to the lattice dynamics calculations, this peak should be Raman active. Hence, proper substitutional Ge incorporation into 3C-SiC could be assessed by Raman spectrometry.

In summary, we have performed molecular dynamics simulations using a valence force field model to simulate the incorporation of Ge into substitutional sites of 3C- SiC. The silicon site is energetically favored over its C counterpart for the Ge substitution, and the lattice parameter is expected to deviate from Vegard' s law. If Ge replaces only Si, then the calculated lattice parameter equals to (0.43593±0.00002) + (0.000337±0.000002) y , where y stands for the Ge content. The alloy is characterized by a distinct phonon spectrum whose maximum peak position in cm^"1 is best described by the exponential decay (243±1) + (27±2) exp (-y/ (7.5±1.2) ) up to the zinc- blende GeC compound. This mode should be Raman active. This novel compound, which can be probed by x-ray diffraction and Raman spectrometry, might play an important role into future IV-IV microelectronics and optoelectronics .

Claims

What is Claimed is :

1. A semiconductor heterostructure comprising elemental semiconductor germanium mixed with the compound semiconductor silicon carbide to form the alloy of silicon carbide: germanium (SiC:Ge).

2. The heterostructure of claim 1 wherein said alloy is in the form of a layer.

3. The heterostructure of claim 3 wherein said layer is part of a multilayer structure with at least one other semiconductor structure as a further layer.

4. The heterostructure of claim 3 wherein one of said layers is made of SiC, and said alloy layer having a lattice constant near enough to that of said SiC layer to permit the interfaces thereof to be nearly defect-free.

5. The heterostructure of claim 4 wherein said alloy layer has a higher electrical conductivity than pure SiC, said alloy layer having a lattice parameter larger than that of SiC and smaller than that of Ge, and said alloy layer having compatibility with SiC.

6. The heterostructure of claim 3 wherein said alloy layer is fabricated onto a semiconductor structure of pure SiC.

7. The heterostructure of claim 3 wherein said alloy layer is fabricated onto a semiconductor structure of pure Si .

8. The heterostructure of claim 3 wherein said alloy layer is fabricated onto a semiconductor structure of pure Ge .

9. The heterostructure of claim 3 wherein said alloy layer has lower bandgap energies than pure SiC thereby forming energy barriers and quantum wells to

confine charge carriers.

10. The heterostructure of claim 3 wherein said alloy layer is an intermediate layer for joining two other semiconductor layers having different lattice parameters.

11. The heterostructure of claim 3 wherein said alloy layer is incorporated in an X-ray lithographic mask for the fabrication of integrated circuits.

12. The heterostructure of claim 1 wherein said heterostructure is part of an electronic circuit.

13. The heterostructure of claim 1 wherein said heterostructure is incorporated in a circuit for a machine selected from the group consisting of automotive engines, jet aircraft engines, high power converters for traction motors, and high temperature chemical process monitoring.

14. The heterostructure of claim 1 wherein said heterostructure is incorporated in a light-emitting diode having smaller energy bandgap than pure SiC.

15. The heterostructure of claim 1 wherein said alloy is incorporated in a photodetector having shorter wavelengths than pure SiC.

16. The heterostructure of claim 1 wherein said alloy is incorporated in micro-electromechanical systems and is harder than pure Si.

17. The heterostructure of claim 1 wherein said alloy is incorporated in a transistor.

18. A method of making a semiconductor heterostructure comprising mixing elemental semiconductor germanium with the compound semiconductor silicon carbide to fabricate an alloy of silicon carbide: germanium.

19. The method of claim 18 including incorporating the alloy in a transistor or in electronic or optical devices and circuits.

20. The method of claim 18 wherein the alloy is incorporated as a layer of a multilayer structure with at least one other semiconductor structure for the device.

: :ODMA\MHODMA\CB; 85141;!