The crystal structures of several icosahedral boron-containing compounds have been refined using MoKα intensity data. Preparation of crystals differs from previous procedures, with boron carbide grown from molten copper and boron arsenide by arsenic vapor transport. The structures consist of icosahedral units bonded together with direct B-B bonds as well as other linkage units (B-C-B and As-As chains).

The bonding features of α-boron were studied using electron density analysis procedures. Deformation density maps and valence density were calculated and the structure analysed by so-called multipole refinements, yielding R - 0.0119. The refinement model correctly describes the bonding and results in a difference density distribution free from any meaningful residual peaks.

Silicon carbide (SiC), with a favorable combination of semiconducting and refractory properties, has long been a candidate for high temperature semiconductor applications. Research on processes for producing the needed large-area high quality single crystals has proceeded sporadically for many years. Two characteristics of SiC have aggravated the problem of its crystal growth. First, it cannot be melted at any reasonable pressure, and second, it forms many different crystalline structures, called polytypes. Recent progress in the development of two crystal growth processes will be described. These processes are the modified Lely process for the growth of the alpha polytypes (e.g. 6H SiC), and a process for the epitaxial growth of the beta polytype (i.e. 3C or cubic SiC) on single crystal silicon substrates. A discussion of the semiconducting qualities of crystals grown by various techniques will also be included.

Single crystals of cubic SiC were hetero-epitaxially grown on Si by chemical vapor deposition (CVD) method. A carbonized buffer layer on Si is utilized to overcome the large lattice mismatch of 20 %. Optimum conditions to make the buffer layers and those structures are discussed. Crystal quality of the CVD grown cubic SiC is analyzed by using X-ray analyses and microscopic observations. Electrical properties controlled by impurity doping during epitaxial growth are described together with fundamental electronic devices.

Cubic SiC was grown on Si substrates by a combination of carbonization and consecutive chemical vapor deposition. Grown layers on the (100) and (111) substrates were single crystalline cubic SiC. Those on the (110) and (211) were poly-crystalline. A model for the mechanism of carbonization was proposed as a result of these observations. Based on this model, the origin of antiphase boundaries were made clear, and antiphase boundaries were expected to be eliminated by controlling atomic steps of the Si surface. In fact, antiphase boundaries were eliminated by introduction of off orientation of Si(lO0) substrates. The relationship between generation of antiphase boundaries and off orientation of the surface was investigated by using spherically polished Si(100) substrates. Off orientation towards (011) was found to be effective for elimination of antiphase boundaries. Off orientation except for towards (011) resulted in generation of antiphase boundaries.

ArF excimer laser CVD was performed in a very-low pressure reactor. C2H2 and Si2H6 were used as source gases and carrier gases were not used. Epitaxial 3C-type SiC(111) grew parallel to α-Al2O3 (0001) substrate. It had a twin structure. Mismatch between the lattice parameters was estimated to be 12%. The epitaxial growth occurred down to 980°C and the films about 0.5μm in thickness grew at 1150°C for 30 minutes. Unirradiated films were polycrystalline or spontaneously peeling-off epitaxial films, while irradiated ones were strongly adherent epitaxial films.

Ion implantation of 14N and Rapid Isothermal Annealing (RIA) were employed to achieve n-type doping in epitaxial-grown 6H-SiC layers. The electrical properties of the implanted films were investigated by Hall effect measurements in order to optimize the annealing parameters. In comparison with standard furnace annealing (1470°C/7min), the annealing parameters for the RIA process could be considerably reduced (1050°C/4min). Based on planar technique, implanted p-n junctions were fabricated. The temperature dependence of I-V characteristics and of the quantum efficiency of photodiodes were studied. The maximum of the quantum efficiency at γ=330 nm reaches values of 35% at 400°C.

Epitaxial thin films of cubic-SiC (100) have been grown at temperatures ranging from 1340–1400°C by using a reduced pressure chemical vapor deposition system. In this work, we will report on the doping of SiC using trimethylalumium (TMA), phosphine, and nitrogen. The electrical measurements on the SiC layers have been performed (Hall, CV, DLTS, etc.) and will be discussed in reference to SiC MESFETs and other SiC device structures.

A short historical introduction concentrates on key experimental and theoretical developments prior to the mid-seventies which form the basis of our present understanding of the semiconducting and optical properties of the polytypes of SiC. Selected research of the last decade is reviewed. Finally, we discuss our on-going surface and defect studies in both epitaxial single crystal films of 3C SiC and other hexagonal SiC polytypes.

A theoretical study of both the bulk phonon dispersion relations in cubic SiC and the vibrational properties induced by native defects is presented. The Keating Hamiltonian is used for the quadratic potential energy of the system, and the supercell approach is employed to treat the defect problem. The defect complexes considered are antisite defects, divacancy defects, and three different types of carbon di-interstitial defects.

A β-SiC MESFET structure with functional DC characteristics has been fabricated and evaluated. The MESFET employs an epitaxial n on p SiC layer grown by chemical vapor deposition on a p-type Si(100) substrate. Modulation of the n-type channel current is achieved with a Au Schottky barrier gate. A transconductance of 2.3 mS/mm was obtained using a 5 micron gate length.

Monocrystalline β-SiC films have been chemically vapor deposited on Si(100) and c-SiC(0001) at 1660K-1823K and 0.1 MPa using SiH4 and C2H4 carried in H2. Films grown directly on Si(100) contained substantial concentrations of dislocations, stacking faults and antiphase boundaries (APB); those on α-SiC(0001) contained double positioning boundaries. Both the APBs and the double positioning boundaries were eliminated by using off-axis orientations of the respective substrates. Films produced on Si(100) have also been doped during growth and via ion implantation with B or Al (p-type) or P or N (n-type) at LN, room and elevated temperatures. Results from the former procedure showed the ionized dopant/total dopant concentration ratios for N, P, B and Al to be 0.1, 0.2, 0.002 and 0.01, respectively. The solubility limits of N, P and B at 1660K were determined to be ∼ 2E20, 1E18 and 8E18 cm−3, respectively; that of Al exceeds 2E19 cm−3. High temperature ion implantation coupled with dynamic and post annealing resulted in a markedly reduced defect concentration relative to that observed in similar research at the lower temperatures. Schottky diodes, p-n junctions, and MOSFET devices have been fabricated. The p-n junctions have the characteristics of insulators containing free carriers and deep level traps. The MOSFETs show very good I-V characteristics up to 673K, but have not been optimized.

Inversion-type n-channel MOSFET's of cubic-SiC were successfully fabricated. MOSFET's were fabricated on an antiphase-domains free layer grown on Si(100) by carbonization and subsequent chemical vapor deposition. Ion implantation technique was used to form source and drain of MOSFET's. A gate oxide of SiO2 was formed by thermal oxidation of SiC. Inversion mode operation was confirmed for the first time. Annealing temperature dependence of electrical characteristics of P+ and N2+ implanted layer and characteristics of p-n junction diodes fabricated using ion implantation technique were also investigated.

Electron Spin Resonance (ESR), temperature dependent Hall effect measurements and photoluminesence (PL) are used to examine the assumption that the residual donor in β-SiC films is nitrogen. At low temperatures the ESR has a three line isotropic spectrum which is characteristic of a central hyperfine interaction with nitrogen. The temperature dependence of the intensity of the nitrogen ESR signal correlates with the concentration of un-ionized donors measured by the Hall effect. Donor-Acceptor pair PL spectra are used to establish that the binding energy of the dominant donor in the films is the same as the nitrogen donor observed in Lely-grown samples. Neither PL nor ESR provide any evidence for the presence of a shallower donor.

Electrical and luminescent properties of nondoped, and N-doped n-type 3C-SiC layers epitaxially grown on Si(100) by chemical vapor deposition were studied. Nondoped n-type epilayers with carrier concentration of 1×1016cm−3 and the Hall mobility of 750cm2/Vs at room temperature have the activation energy of donors, Ed=2OmeV, which is different from that of the donors in the N-doped layers. The photoluminescence spectra of nondoped layers are different from those of N-doped ones. These results suggest that the donors in the unintentionally doped n-type 3C-SiC are not due to N impurities. 45–70 % of N-donors in the N-doped epilayers are compensated.

Schottky-barrier and MOS-type field-effect transistors have been fabricated from 3C-SiC. The transistor operations of MESFETs and MOSFETs were studied at elevated temperatures up to 440°C. Transconductances of 1.7mS/mm and 0.15mS/mm for MESFET and 0.8 and 0.05mS/mm for MOSFET at room temperature and 440°C, respectively, were obtained. The drain currentvoltage characteristics of both the FETs at room temperature did not change in the least after heating up to 440°C in the air.

The reaction of thin Ti films deposited, in situ, onto (100) 3C-SiC has been studied by Auger Electron Spectroscopy, and Low Energy Electron Diffraction. The effects of reaction temperature and SiC surface condition were investigated as part of a program to obtain a high temperature electrical contact for this wide band gap semiconductor.

The results indicate that the interface reaction is dominated by the formation of TiC in SiC, and the liberation of Si which diffuses to the surface. The amount of liberated Si depended on the relative amounts of free surface C and of deposited Ti. In the presence of excess carbon, the reaction of Ti with SiC was suppressed and a (TiC + C) surface layer formed which was stable after 500 hours at 800°C.

Beta SiC is an important semiconductor whose development has been slowed by synthesis difficulties. The physical and electronic properties which make β-SiC desirable for high speed and high power electronics are discussed, with special emphasis on field effect transistor (FET) applications. A history of synthesis efforts is presented to illuminate the obstacles encountered in the growth of semiconductor device quality P-SiC. A new approach to single crystal epitaxy of β-SiC, using TiC as a substrate, is described. The properties of TiC which make it a uniquely suitable substrate for β-SiC epitaxial growth are discussed, and procedures used to prepare TiC surfaces for β-SiC epitaxy are described. The growth process employed at our laboratory, chemical vapor deposition (CVD), is described, and experimental observations of the effects of the CVD growth environment on β-SiC epitaxial growth are presented. Based on these observations, we propose to synthesize β-SiC in a singlesource reaction, using molecules which decompose directly to SiC units. This contrasts with current approaches, which introduce Si and C separately, in molecules which must decompose and subsequently react to form SiC.

After Nichrome, Ni, Cr, and Au/Ta films are deposited onto single crystals of cubic SiC, their reactivities at the metal-semiconductor interface are studied by Auger Electron Spectroscopy. For all metals except Ni, metal carbides are detected at the interfaces. Initially these carbides tend to promote adhesion and limit the reaction with the substrate. Annealing at or above 450°C causes Ni to diffuse into the SiC and the Ta and Cr films to migrate through the capping material and away from the SiC, thereby delaminating. Development of the secondary phases at interface severely complicates the interpretation of the depth profiling data and with the uniformity of the contact film. These phases seem to be initiated at SiC defect sites where the reaction kinetics are considerably different and may be minimized with improved SiC substrates.

The β-SiC formation process through post-implantation annealing of Si-C mixtures fabricated on Si by C-ion implantation at room temperature is studied by means of infrared absorption spectroscopy. It is shown that the formation of B-SiC from the Si-C mixtures is greatly enhanced by the subsequent irradiation of other energetic ions prior to the thermal annealing. The continuous amorphization of the Si-C mixed layers is considered to be the dominant cause for the enhancement of the B-SiC formation. The activation energy of the β-SiC formation process which is 5.3 eV without irradiation is reduced to 4.0 eV by the irradiation of 150 keV, 1 × 1017/cm2 Ar ions.

The field of amorphous hydrogenated silicon-carbon alloys and thin films has witnessed, since the pioneering work of Anderson and Spear [1], rapid development and has attracted scientific attention and technological interest [2,3]. However, relatively little information is known [4] about the physical mechanisms that govern the inclusion of C and H in the silicon matrix, the nature of the chemical bonds involved, and the structural changes that result in the amorphous phase. Clearly, further fundamental studies are needed to achieve complete understanding of such amorphous systems with variable disorder.