Silicon carbide (SiC) is the strongest near-term semiconducting candidate material for use in electronic and opto-electronic devices and circuits designed to operate at temperatures from 300.degree. to 700.degree. C. and above. The electronic and thermal transport properties of SiC, such as the carrier saturation velocity, the breakdown electric field and the thermal conductivity are significantly superior to those of Si and GaAs, making SiC very attractive for use in high power and high frequency applications. SiC is also more resistant to ionizing radiation than Si and GaAs. SiC exists in the cubic, .beta. phase as well as in about 170 hexagonal phases or polytypes. Each phase has a somewhat different energy band structure and thus a different bandgap and carrier mobilities. Bulk SiC, however is not available in sizes above 1 inch and in high quality and purity, as required for practical applications. On the other hand, high quality and high purity Si is available in sizes up to 8 inches (200 mm) in diameter. Therefore, there have been intensive efforts to deposit SiC epitaxially, as a thin monocrystalline film, on a foreign single-crystalline substrate, primarily on Si. In this context, a single- or monocrystalline film has only one grain, with one orientation, while a polycrystalline film consists of a multitude of grains of differing orientations. Either detailed physical characterization (e.g. by transmission electron microscopy, TEM) or device data (e.g. carrier mobility) are needed to confirm single crystallinity. A film is considered to be epitaxial with the substrate if there are well-defined relationships between the orientations of the crystal planes in the film and those in the substrate, e.g. (100) SiC // (100) Si and [100]SiC // [100]Si. A preferentially oriented polycrystalline film can be epitaxial, but for high-performance electronic device applications, it is necessary to obtain a single-crystalline film.
Methods which have been used to deposit SiC films include chemical vapor deposition (CVD), evaporation in high vacuum, molecular beam epitaxy (MBE) in ultra-high vacuum (UHV), sputtering and laser ablation. Chemical vapor deposition has the unique advantage of producing conformal coatings, i.e. films which have near-constant thickness across the substrate, regardless of non-planar features on the substrate. Films deposited by CVD are conformal because in CVD, the gaseous precursor materials are thermally decomposed over, and only over the heated substrate; cooler areas in the deposition chamber will not, in principle, be deposited on. Thus, if the substrate is maintained at constant temperature, other conditions being equal, a uniform film will be obtained. Another very important advantage of CVD is the ability to dope the film in-situ n- or p-type, using appropriate precursor species. In the case of SiC, nitrogen, phosphorus or arsenic dopes the film n-type, and aluminum or boron (e.g. from diborane) dopes the film p-type. CVD can also be scaled up in size more readily than the other film deposition techniques and does not require UHV conditions.
Single-crystalline, epitaxial SiC films have been produced on Si by the CVD technique in the past and electronic devices have been fabricated in such films [see R. F. Davis, in "The Physics and Chemistry of Carbides, Nitrides and Borides", R. Freer, ed., Kluwer Academic Publishers: Dordrecht, the Netherlands, p. 589 (1990)]. However, most device-quality SiC films deposited on Si single crystals have been grown at relatively high temperatures of 1300.degree.-1380.degree. C. and at atmospheric pressure, using separate precursors for Si and C. For instance, SiH.sub.4 or Si.sub.2 H.sub.6 have been used for Si and CH.sub.4 or C.sub.3 H.sub.8 have been used for C [P. Liaw and R. F. Davis, J. Electrochem. Soc. 132, 642 (1985)]. The relatively high growth temperatures are detrimental to the quality of the films and the performance of the devices fabricated in them, due to the large mismatches in thermal expansion coefficients and lattice parameters of SiC and Si. For example, the thermal expansion coefficients of SiC and Si, averaged between 25.degree. and 1000.degree. C. are 4.45.times.10.sup.-6 and 3.8.times.1O.sup.-6 per .degree. C., respectively. The lattice parameters of cubic .beta. SiC and Si at room temperature are 4.3596 and 5.431 .ANG., respectively. Due to these large mismatches in thermal expansion coefficients and lattice parameters of SiC and Si, SiC films grown on Si at such high temperatures are under high tensile stress and contain high concentrations of crystalline lattice defects. These defects degrade the performance of devices fabricated in such films. For instance, carrier mobilities are reduced and leakage currents are increased by the presence of electrically active defects. These defects originate at the SiC/Si interface and propagate through the growing layer. The defect concentration generally diminishes with increasing layer thickness, but the layers tend to crack if they are too thick. Thus, in order to fabricate electronic devices in the top .apprxeq.0.5 .mu.m region of the film, a film up to 30 .mu.m thick must be grown. Since the growth rates for SiC on Si are generally below 2 .mu.m/h, relatively long deposition runs are required, which are impractical and costly from a manufacturing standpoint. Ideally, films 0.2-2 .mu.m in thickness would be desirable. Deposition at such high temperatures is also more costly (a) in terms of the power required to heat the substrate, since the power required is proportional to T.sup.4 (where T is in K) in this temperature region and (b) in terms of the materials used in constructing the deposition vessel, since quartz, for example, can only be used up to 1150.degree. C. Furthermore, the lower the deposition temperature, the easier it is to maintain the purity of the deposition chamber and to minimize autodoping effects in the growing film, which are due to impurities from the growth environment. Finally, as the sizes of electronic circuits and devices shrink and their density increases, it becomes more important to reduce the processing temperatures used in fabrication, in order to preserve sharp junction interfaces and reduce diffusion and smearing of dopants and metal contacts. This is especially true when a SiC film is used as part of a device made in a lower bandgap and less refractory material, such as Si (e.g. SiC emitter in heterojunction bipolar transistor). Therefore, it is highly desirable to significantly reduce the temperature required for the deposition of SiC epitaxial layers. To summarize, the advantages of a lower deposition temperature are lower tensile stress in the films, lower defect concentrations, higher purity, improved device performance, reduced smearing of dopant profiles and junctions, faster growth time, lower power requirements, and lower equipment costs.
The use of separate precursors for Si and for C in the growth of SiC thin films is generally also undesirable, since it tends to result in small departures from stoichiometry, i.e. excess of Si or C, in the films. The stoichiometry of a SiC film produced from two separate gaseous precursors for Si and for C will depend primarily on the ratio of the two corresponding flow rates and also on the sticking coefficient of each entity on the surface of the substrate at the deposition temperature. Flow rates can only be controlled to a precision in the order of about 0.1% or 1000 ppm using state-of-the-art mass flow controllers, so that the ratio would have an uncertainty of at least about 0.2% or 2000 ppm. In addition, the Si precursor tends to decompose more readily than the C precursor at typical deposition temperatures of SiC. Thus, since the SiC material system cannot tolerate even small departures from stoichiometry (where Si/C=1), if excess Si or C atoms arrive at the surface, they result in Si or C interstitials, vacancies, anti-site defects or microscopic inclusions and precipitates, depending on the departure from stoichiometry. Since the dopant concentrations required to operate typical MOSFET or MESFET devices are in the range of 5.times.10.sup.16 -5.times.10.sup.18 atoms/cm.sup.3, i.e. 1-100 ppm, the concentration of electrically active defects should be kept well below this range. In this respect, SiC differs from GaAs, where large excesses of arsenic are tolerated, as only the amount required to react with gallium is used, and the remainder does not stick to the surface. Thus, the growth of GaAs thin films can be accomplished well using two separate precursors for As and Ga, but in the case of SiC the use of two precursors is believed to degrade the quality of the film.
To date, most device-quality SiC films deposited on Si substrates have been produced by CVD processes which have suffered from one or more of these disadvantages: (1) high deposition temperature of 1300.degree.-1380.degree. C.; (2) use of separate precursors for Si and for C; and/or (3) need for a pre-deposition carbonization treatment step for the Si. Prior art attempts to produce device-quality epitaxial SiC thin films on Si at temperatures lower than 1300.degree.-1380.degree. C. used in CVD, or using a single source precursor for both Si and C, or under avoidance of a surface carbonization pretreatment step have generally not been successful.
Accordingly, it is an object of the present invention to provide a process for depositing a single crystal stoichiometric SiC film on a foreign substrate, in particular a Si substrate, at a temperature lower than heretofore thought practicable.
It is a further object of the invention to produce such SiC films from a single precursor containing silicon and carbon in a ratio of unity, which precursor is free of halogen atoms.
Lastly, it is an object of the invention to produce such SiC films at a temperature below 900.degree. C., and specifically as low as 750.degree. C. for cubic single-crystalline SiC.