It has been observed that aluminum nitride (AIN) ceramics exhibit a significant photo-darkening when exposed to UV or X-ray radiation, though the origin of this effect has never been understood. In this study, the optical character of these defects is investigated utilizing a UV-pump, visible-probe arrangement, where very large changes ΔT in the probe transmission T (induced absorption) are observed after excitation (Δ/T-0.60). These experiments reveal that the photo-darkening is due to the creation of light induced defects, with an energy level deep in the AIN bandgap. Utilizing these results, a light induced defect formation model is proposed which is consistent with the known defect chemistry of this material.

Gallium Nitride films were grown on (111) Gallium Arsenide substrates using reactive rf magnetron sputtering. Despite a 20% lattice mismatch and different crystal structure, wurtzite GaN films grew epitaxially in basal orientation on (111) GaAs substrates. Heteroepitaxy was observed for growth temperatures between 550–600°C. X-ray diffraction patterns revealed (0002) GaN peak with a full-width-half-maximum (FWHM) as narrow as 0.17°. Possible surface reconstructions to explain the epitaxial growth are presented.

A1N films were grown on the (100) plane of 3C-SiC/Si and the (0001) plane of A12O3 substrates by metalorganic chemical vapor deposition (MOCVD) using trimethylaluminum (TMA) and ammonia (NH3) as the precursors. The deposited films were characterized by X-ray diffraction (XRD) and a Read thin film camera. At 1150°C, preferentially oriented polycrystalline AlN films were obtained on both substrates and the crystal structure was wurtzite. The epitaxial relations were (1010)AlN//(100)SiC//(100)Si and (0001)AlN// (0001)Al2O3. The attempt to grow cubic AlN on 3C-SiC/Si was not successful.

Thin films of aluminum nitride were grown epitaxially on Si(111) by ultra-high-vacuum dc magnetron reactive sputter deposition. Epitaxy was achieved at substrate temperatures of 600° C or above. We report results of film characterization by x-ray diffraction, transmission electron microscopy, and Raman scattering.

AlN ceramics were plastically deformed using uniaxial compression under hydrostatic pressure between room temperature (RT) and 800°C. Deformation microstructures have been studied by Transmission Electron Microscopy (TEM) using the weak beam technique. The deformation substructure at RT is characterized by perfect glide loops with 1/3<1120> Burgers vector in (0001) elongated in the screw direction. When deformation temperature increases, the screw character is associated to cross slip events and dislocation dipolesare found. In the investigated temperature range, slip of dislocations with 1/3<1120> Burgers vector is also evidenced on prismatic planes. Weak beam observations failed to evidence any dislocation splitting. Some of these dislocation properties, similar to those of III-V compound semiconductors, suggest that electronic doping effects could be used to control plastic behaviour of covalent ceramics.

The dielectric functions ε = ε1+iε2 of AlAs were determined from 1.5 eV to 5.0 eV, by spectroscopie ellipsometry (SE), from room temperature (RT) to ∼577 °C in an ultrahigh vacuum (UHV) chamber. Molecular beam epitaxy (MBE)-grown AlAs was covered by a thin GaAs layer, which was passivated by arsenic capping to prevent oxidation. The arsenic cap was desorbed inside the UHV chamber. SE measurements of the unoxidized sample were made, at various temperatures. Temperature dependent optical constants of AlAs were obtained by mathematically removing the effects of the GaAs cap and substrate. Quantitative analyses of the variations of critical-point energies with temperature, by using the harmonic oscillator approximation (HOA), indicate that the E1 and E1+Δ1 energies decrease -350 meV as temperature increases from RT to 500 °C.

GaP epllayers grown at temperatures ranging from 420 to 500°C had smooth surfaces and streaky RHEED patterns. The decomposition of group-III sources of TEGa limits the growth rates of GaP at lower substrate temperatures(<390 °C ). The growth rate of GaP epitaxial layers was efficiently enhanced by N2∼laser irradiation at lower substrate temperatures.

GaP epllayers grown at temperatures ranging from 420 to 500°C had smooth surfaces and streaky RHEED patterns. The decomposition of group-III sources of TEGa limits the growth rates of GaP at lower substrate temperatures(<390 °C ). The growth rate of GaP epitaxial layers was efficiently enhanced by N2∼laser irradiation at lower substrate temperatures.

Both SiC and ZnS exist in the cubic diamond-like (zinc blende) structure. A polytype is derived from the cubic stacking of atomic double layers by having a regularly repeated pattern of stacking faults. Dozens of such polytypes are known for SiC and ZnS.

A study with several co-workers has been completed into the origin of this phenomenon. Are the polytypes genuine stable phases frozen in from some higher equilibrium temperature? Or are they inherently metastable structures born of some growth mechanism?

Quantum mechanical calculations have shown that SiC polytypes formed from bands of 2 and 3 cubically stacked layers have a lower energy than other structures, in particular lower than the cubic or wurtzite structures. That already suggests that the polytypes can be stable phases. Further calculations of the phonon free energy and relaxation of the bond lengths rounds out the phase diagram. In this connection it is somewhat surprising that the material normally grows initially in the metastable cubic modification, transforming subsequently to a mixture of polytypes, but the calculations also suggest why this is so.

The answers for ZnS turn out to be different from those for SiC.

Monocrystalline, epitaxial cubic (100) SiC films have been grown on monocrystalline (100) Si substrates at 750°C, the lowest epitaxial growth temperature reported to date. The films were grown by low-pressure chemical vapor deposition, using methylsilane, SiCH3H3, a single precursor with a Si:C ratio of 1:1, and H2. The films were characterized by means of transmission electron microscopy, single- and double-crystal X-ray diffraction, infra-red absorption, ellipsometry, thickness measurements, four-point probe measurements, and other methods. Based on X-ray diffractometry, the crystalline quality of our films is equivalent to that of commercial films of similar thickness. We describe the novel growth apparatus used in this study and the properties of the films.

A low temperature process of silicon carbide deposition using the pyrolysis of di-tert-butylsilane has been explored for formation of emitter structures in silicon heterojunction bipolar transistors. Near stoichiometric amorphous silicon carbide films were achieved at 775°C. Doping and annealing of these films resulted in resistivity as low as 0.02 ohm-cm.

Deformation experiments were carried out on 6H-SiC single crystals and the deformed samples were examined by electron-optical techniques to verify any evidence for stress-induced polytypic transformation.

A comparison of several simple hydrocarbon gases, with H:C ratios ranging from 1 to 4, as precursors for the carbonization of Si is presented. The growth experiments were performed by RTCVD at reactor pressures of 760 and 5 Torr. For AP-RTCVD, we have found that C3H8, C2H4 and C2H2 have very similar dependence of growth rate on hydrocarbon partial pressure in the chamber. At 1300°C, this involved a maximum in film thickness being obtained at a hydrocarbon flow rate of 8–10 seem, representing a transition hydrocarbon fraction (in H2) of ∼ 5×10-4. CH4 carbonization produces a peak growth rate at a significantly higher fraction, ∼ 4×10-3. For LP-CVD at 5 Torr, the transition carbonization fraction increases by approximately an order of magnitude. The AP-RTCVD carbonization activation energy for C3H8, C2H4 and C2H2 at higher temperatures (∼1200–1300°C) has a common value of ∼ 0.8 eV, while for lower temperatures it depends on the hydrocarbon.

3C-SiC layers were grown on Si(111) substrates by chemical vapor deposition (CVD) using SiH4-CH3CI-H2 gas mixture. 3C-SiC(111) heteroepitaxial layers were obtained with smooth surfaces and reduced warpage. All the epilayers were n- type, and the carrier density and Hall mobility were 2.1×1016∼2.8×1017 cm-3 and 120∼180 cm2/Vs at room temperature, respectively. Temperature dependences of the electrical properties of the self-supported 3C-SiC(111) epilayers were measured between 15 and 300 K for the first time. 3C-SiC(111) epilayers showed a similar temperature dependence of carrier density to 3C-SiC(001) heteroepitaxial layer. Hall mobility was maximum (∼360 cm2/Vs) around 100 K.

Stoichiometric films of 3C SiC, 50 to 1000 nm thick were deposited on Si wafers by laser ablation of ceramic stoichiometric SiC targets. Films grown at substrate temperatures above 1000° C on [001] and above 900° C on [111] show orientation epitaxial to the Si substrate along the film normal. Depending on the deposition conditions, the oriented crystallite dimension along this direction ranges from 20 nm to over 100 nm. The crystallite dimensions in the film plane range from 20 to 70 nm. Raman spectra show the expected TO and LO lines from SiC but indicate that the films sometimes contain other material, for example (30 to 50 Å) graphitic inclusions or small amounts of polycrystalline silicon.

Epitaxial silicon carbide films have been produced on Si (100) substrates by CVD with 90% of the carbon supplied by methane and 10% by propane as compared to 100% by propane (or 100% by any carbon source more reactive than methane). This implies a methane to propane mole ratio of thirty. Among possible carbon gases, methane is the purest coimnercially available hydrocarbon source, But methane has not been commonly used for growth of silicon carbide due to its low chemical reactivity. Our process demonstrates the feasibility of achieving high SiC growth rates while using a carbon source that is predominantly methane. We have established that silicon carbide films grown at 1350 °C in a CVD reactor using the above carbon source ratio results in quality single crystalline films at similar growth rates and lower carrier concentrations than fikus grown from propane and silane.

The main tools used to characterize the grown films are X-ray and electron diffraction, optical microscopy, surface profilometry, Hall mobility measurements, and thickness measurements.

Fabrication processes of metal-oxide semiconductor (MOS) capacitors on n-type, Si-face, 6H-SiC were studied. The effects of thermal oxidation conditions at temperatures between 1100 and 1250°C on the electrical properties of MOS capacitors were determined. The wafers were annealed under argon to improve the C-V characteristics. C-V characteristics of AI-SiO2-SiC metal-oxide-semiconductor were measured at high frequency in the dark and under illumination. In the dark inversion does not occur, probably owing to the absence of minority carriers due to the large band gap of 6H-SiC. The accumulation, depletion, and inversion regions were clearly observed when the C-V measurements were made under illumination for both wet and dry thermally grown oxides. The interface trap densities and emission time constants of fast states were determined by ac conductance measurements. From the analysis of data we obtained a total of Fixed charges and the slow interface traps, Nf + NssSlow of 1.5 to 3.3 × 1012 cm-2, fast interface trap densities, NssFast of 0.5 to 1.7 × 1011 cm-2 eV-1 and emission times constant of 0.3 to 1.4 μsec for wet oxidation. For dry oxidation, Nf + N, ssSlow of 3.5 to 11.2 × 10cm-2, NssFast of 0.7 to 1.25 × 1010 cm-2 eV-1 and emission time constants of 0.6 to 2 μsec were obtained.

Chemical and electrical studies were performed to determine the characteristics of contacts to 6H-SiC. Both elemental metals (Ni, Mo) and suicides (MoSi2, TaSi2, TiSi2) were studied. Chemical analysis by Auger Electron Spectroscopy (AES) was performed to examine interface reactions caused by heat treatment. Electrical measurements (current-voltage and capacitance-voltage) were made during annealing sequences to determine the rectifying or ohmic characteristics of the contacts. Where possible, barrier height and contact resistance values were calculated.

In order to fabricate high temperature sensors and other devices, it is necessary to develop ohmic contact metallizations that can withstand elevated temperatures. A variety of ohmic contact metallizations were investigated with contact resistivity measured as a function of anneal time in air. The metallizations were based on Ti and W ohmic contacts, which have contact resistivities as low as 10-4 Ω-cm2. Several of the contact metallizations were stable after 10 hrs. at 650°C, while one system, based on a Ti ohmic contact, was able to withstand > 20 hrs. at 650°C with only a 30–40% increase in contact resistivity.

New x-ray diffraction measurements performed on bonm nitride films deposited by pulsed excimer laser deposition are presented. The x-ray data, taken with both a molybdenum rotating anode source and synchrotron radiation, indicate that the epitaxial cBN films are ≤ 200 Å thick. We also report the successful growth of oriented crystalline diamond on the (001) surface of cBN/Si substrates using the method of pulsed laser deposition. X-ray diffraction measurements indicate that the diamond layer is 200 Å thick with a lattice constant of 3.56 Å. The structures of metastable films (cBN and diamond) are very sensitive to growth conditions: we present evidence that an epitaxial-crystalline to incoherent phase transition occurs when the thickness of the films exceeds a critical value (∼ 200 Å for our present growth conditions).