Hydrogenated amorphous silicon (a-Si:H) thin-film-transistors (TFT’s) and logic circuits have been fabricated using a simple 3-mask process. Inverted staggered TFT structures were prepared in a multichamber UHV-compatible system that provides: 1) sequential deposition of dielectrics, and intrinsic and doped a-Si:H by remote plasma-enhanced chemical-vapor deposition (Remote PECVD) in a single deposition chamber; 2) in-situ surface analysis by Auger Electron Spectroscopy (AES); and 3) dry etching of deposited thin films by Remote plasma-enhanced etching, Remote PEE. A field-effect electron mobility of 0.74 cm2/V-s, and a threshold voltage of 2.5 Volts have been measured at room temperature in the TFTs. The on/off current ratio of the TFT exceeds 106. The a-Si:H TFT logic circuits, 2-transistor inverters and addressable 6-transistor static memory cells, operate at a supply voltage of as low as 8 Volts. The experimental results indicate that high performance a-Si:H thin film devices can be fabricated by sequential Remote plasma processing in a multichamber integrated system.

Thin films of AlN and GaN are deposited primarily via the common forms of organometallic vapor phase epitaxy (OMVPE) and molecular beam epitaxy (MBE). Sapphire is the most common substrate; however, a host of materials have been used with varying degrees of success. Both growth techniques have been employed by the authors to grow AlN and GaN primarily on 6H-SiC(0001). The mismatch in atomic layer stacking sequences along the growth direction produces inversion domain boundaries in the AlN at the SiC steps; this sequence problem may discourage the nucleation of GaN. Films of AlN and GaN grown by MBE at 650°C are textured; monocrystalline films are achieved at 1050°C by this technique and OMVPE. Donor and acceptor doping of GaN has been achieved via MBE without post growth annealing. Acceptor doping in CVD material requires annealing to displace the H from the Mg and eventually remove it from the material. High brightness light emitting diodes are commercially available; however, numerous concerns regarding metal and nitrogen sources, heteroepitaxial nucleation, the role of buffer layers, surface migration rates as a function of temperature, substantial defect densities and their effect on film and device properties, ohmic and rectifying contacts, wet and dry etching and suitable gate and field insulators must and are being addressed.

Silicon carbide (SiC) and aluminum nitride (AlN) thin films have been grown on 6HSiC(0001) substrates by gas-source molecular beam epitaxy (GSMBE) at 1050°C. Step flow, step bunching and the deposition of 6HSiC occurred at the outset of the exposure of the (1 × 1) vicinal substrate surface to C2H4Si2H6 gas flow ratios of 1, 2 and 10. Subsequent deposition resulted in step flow and continued growth of 6H films or formation and coalescence of 3CSiC islands using the gas flow ratio of one or the ethylene-rich ratios, respectively. The (3 × 3) surface reconstruction observed using the former ratio is believed to enhance the diffusion lengths of the adatoms, which in turn promotes step flow growth. Essentially atomically flat monocrystalline AlN surfaces were obtained using on-axis substrates. Island-like features were observed on the vicinal surface. The coalescence of the latter features at steps gave rise to inversion domain boundaries (IDBs) as a result of the misalignment of the SiC bilayer steps with the AlN bilayers in the growing film. The quality of thicker AlN films is strongly influenced by the concentration of IDBs. Undoped, highly resistive (102 Ω · cm) and Mg-doped, p-type (0.3 Ω · cm) monocrystalline GaN films having a thickness of 0.4–0.5 μm have also been grown via the same technique on AlN buffer layers without post-processing annealing.

Organometallic vapor phase epitaxy (OMVPE) and molecular beam epitaxy (MBE) are the most common methods for the growth of thin films of A1N and GaN. Sapphire is the most common substrate; however, a host of materials have been used with varying degrees of success. Both growth techniques have been employed by the authors to grow AIN, GaN and AlxGa1−xN thin films primarily on 6H-SiC(0001). The mismatch in atomic layer stacking sequences along the growth direction produces double positioning boundaries in A1N and the alloys at the SiC steps; this sequence problem appears to discourage the two-dimensional nucleation of GaN. Films of these materials grown by MBE at 650°C are textured; monocrystalline films are achieved between 850°C (pure GaN) and 1050°C (pure A1N) by this technique and OMVPE. Donor and acceptor doping of GaN has been achieved via MBE without post growth annealing. Acceptor doping in CVD material requires annealing to displace the H from the Mg and eventually remove it from the material. High brightness light emitting diodes are commercially available; however, numerous concerns regarding metal and nitrogen sources, heteroepitaxial nucleation, the role of buffer layers, surface migration rates as a function of temperature, substantial defect densities and their effect on film and device properties, ohmic and rectifying contacts, wet and dry etching and suitable gate and field insulators must and are being addressed. Selected issues surrounding the growth of these materials with particular examples drawn from the authors' research are presented herein.

The morphology and interface chemistry occurring during the initial deposition of BN, AlN and GaN films via metal evaporation and N2 decomposition under UHV conditions have been determined. FTIR spectroscopy and TEM revealed the consecutive deposition of an initial 20Å layer of a-BN, 20-60Å of oriented h-BN, and a final layer of polycrystalline c-BN. This sequence is attributed primarily to increasing intrinsic compressive stress in the films. XPS analysis revealed the growth of GaN on sapphire to occur via the Stranski-Krastanov mode; growth on SiC showed characteristics of three-dimensional growth. AlN grew layer-by-layer on both substrates. Vicinal 6H-SiC(0001) substrate surfaces contain closely spaced, single bilayer steps. During deposition of Si and C at 1050°C, 6H layers initially form and step bunching occurs. The latter phenomenon results in more widely spaced steps, the nucleation of 3C-SiC both on the new terraces and at the larger steps and formation of double position boundaries. The C/Si ratio in the gaseous reactants also affects the occurrence of these three phenomena.

Fabrication of optoelectronic devices from III-N materials, operable in the blue and ultraviolet regions of the spectrum, has been a goal of many groups since the first infrared and red devices were commercially produced. Commercially viable blue-light-emitting diodes have now been achieved in these materials, and numerous investigators are now entering tills field. Microelectronic devices are also of considerable interest.While much of the original research on GaN and InN centered around films with the wurtzite structure, the zincblende structure nitrides have recently been receiving increasing interest because of their potential inherent advantages, e.g. increased carrier mobility. Recent results from several groups around the world have successfully addressed sonic of the traditional problems associated with growth of these materials.Chemical vapor deposition (CVD) has been a traditional growth technique for nitride growth and continues to be very successful. However, many researchers, including those growing III-V nitrides, have turned to plamsa-enhanced molecular beam epitaxy (MBE) for its important advantages in purity and in situ, analytical capabilities. This paper reviews the recent developments in MBE growth of cubic boron nitride, aluminum nitride and the two polytypes of gallium nitride.

This study presents the results of surface investigation of the heteroepitaxial AIN/SiC interface. The analytical tools employed included UPS, XPS, Auger spectroscopy, and LEED. The surface electronic states were characterized by uv photoemission obtained at surface normal. Conclusions drawn from this study are that the AIN/SiC structure results in a negative electron affinity surface which is extremely sensitive to defect density. The surface Fermi level is found to be near the middle of the AlN gap, and a possible band alignment between the AlN and SiC is presented.

Time-resolved reflectivity and transmission measurements were made on a-Si:H thin films, with a 50 femto-second, fs, time resolution, and in a spectralrange from 1.2 to 2.3 eV. Analysis of pump/probe data for photo-excited carrier densities of ∼1018 cm−3 to ∼1020 cm−3 demonstrates that carrier motion by diffusive transport applies over at least this range of optical excitation, and that the hopping distance for the diffusive transport is approximately equal to the SiSi inter-atomic spacing of ∼2.3 Å.

We present a study of spectral- and time-resolved reflectivity and transmission measurements on hydrogenated amorphous silicon (a-Si:H) with 50 fs time resolution. Electron-hole pairs are photoexcited into the extended states of a-Si:H by an ultrashort pump pulse at ℏω=2 eV. The temporal evolution of the photoinduced optical response is studied for a broad range of excitation densities from 1018 up to 1020 cm−3, and on a timescale of up to 200 ps. The temporal evolution of the optical response is discussed in terms of recombination and trapping mechanisms for carriers in the extended states of a-Si:H.

The properties of native defects and impurities in diamond and SiC are investigated via large-scale band structure and Car-Parrinello calculations. In diamond, the activation energy for self-diffusion is very high in the intrinsic material (9 eV) but decreases by up to 3 eV in either p- or n-type material. Phosphorus, lithium and sodium are shallow donors, but their solubilities are very low, which makes them unsuitable for incorporation into diamond via in-diffusion. Instead, kinetic trapping during growth or ion implantation must be used. Considering the stability at the dopant site, substitutional phosphorus is expected to diffuse by the vacancy mechanism and to have a high activation energy by analogy to self-diffusion. Both lithium and sodium diffuse through the interstitial channel. Lithium is a relatively fast diffuser while sodium should be stable up to moderately high temperatures. For nitrogen in diamond, the well known (111) distortion is found to be due to the interaction of the fully occupied nitrogen lone pair with the dangling bond of the C(111) atom. The single electron associated with the center resides in an antibonding orbital formed from the dangling hybrid and the nitrogen lone pair. This orbital has most of its amplitude on the carbon atom. In SiC, the lowest energy defect in n-type and intrinsic material is the electrically inactive silicon antisite, while the lowest energy defect in p-type SiC is the doubly positive carbon vacancy. The electrons released by the vacancies compensate acceptor dopants, leading to strong self-compensation effects when doping occurs during crystal growth. In carbon-rich SiC, the dominant defect for all Fermi level positions is the electrically inactive carbon antisite. In boron-doped SiC, BC is preferred for silicon-rich material while, in carbon-rich SiC, BC and BSi have similar formation energies.

The microscopic mechanisms for Zn diffusion in GaAs and Zn-induced interdiffusion in GaAs/AlAs superlattices are investigated by ab initio molecular dynamics. Among the various proposed mechanisms for Zn diffusion, kick-out by Ga interstitials has the lowest activation energy. Zn in-diffusion generates nonequilibrium group-III interstitials, which are bound to Zn by Coulomb forces. The interstitials follow the Zn diffusion front and disorder the superlattice. The calculated activation energies for these processes are in good agreement with the experimental data.

The microstructure and electrical properties of μc-Si and μc-Si,C prepared by remote plasma-enhanced chemical-vapor deposition, PECVD, are reviewed. The microstructure has been characterized by transmission electron microscopy, TEM, infrared, IR, absorption and Raman scattering. The electrical properties were characterized by temperature-dependent dark-conductivity measurements. These studies have explained significant quantitative differences between the carrier transport properties of μc-Si and μc-Si,C alloys in terms of a band offset model for the interfacial potential steps between the amorphous and crystalline constituents of these material systems.

Films of undoped and doped Si:H and Si,C:H alloys were deposited by remote plasma-enhanced chemical vapor deposition onto thermally grown silicon oxide layers at a substrate temperature of 250 °C. The microstructure of the films, including the degree of crystallinity, and the distribution of carbon within the Si,C alloys films were characterized by transmission electron microscopy, Raman scattering, and infrared absorption spectroscopy. The degree of crystallinity depends on both the doping level and on the presence of carbon. For two-phase μc-Si or μc-Si,C alloy films, the results indicate that (i) the crystallites are Si, and (ii) the amorphous encapsulating materials are a-Si:H for μc-Si,H, and a-Si,C:H for the μc-Si,C:H alloys. A relationship between microstructure, doping levels, and the measured dark conductivity is discussed.

We have extended the remote PECVD process to the deposition of intrinsic and doped, amorphous and microcrystalline silicon,carbon alloy films, a-Si,C:H and µc-Si,C, respectively. The electrical and optical properties of a-Si,C:H deposited by remote PECVD are comparable to those of films deposited by the glow discharge or GD process. The degree of crystallinity in the µc-Si,C alloys, as determined from the relative intensities of crystalline and amorphous features in the Raman spectra, is lower than that of µc-Si films deposited under comparable deposition conditions. The Raman spectra indicate that the crystallites in the µc-Si,C alloys are Si, while the infrared measurements establish that the intervening amorphous component is an a-Si,C:H alloy.

An analysis of the room-temperature dark conductivities and activation energies for doped µc-Si and µc-Si,C is used to develop a band alignment model which shows that the maximum attainable dark conductivities in these materials are determined by transport through, or over interfacial potential barriers between Si crystallites, c-Si, and the intervening amorphous regions: a-Si:H or a-Si,C:H, respectively. For all levels of doping in µc-Si,C, the transport is limited by thermionic emission over interfacial barriers at the c-Si/a-Si,C:H interface, placing a significant constraint on applications requiring both high optical transparency and high conductivity.

Current-voltage voltage characteristics of heterojunctions formed by remote plasma enhanced chemical vapor deposition (PECVD) of heavily doped µc-Si onto doped c-Si have been studied, as well as capacitance-voltage characteristics of MOS capacitor structures using heavily doped remote PECVD µc-Si and a-Si films as gate electrodes on thermally oxidized crystalline Si. Shifts in the flat-band voltages of MOS devices using the µc-Si and a-Si as gate electrodes relative to that of a reference Al/SiO2/c-Si structure are measured and explained in terms of a band structure model for the µc-Si and a-Si. Rectification and a photovoltaic effect observed in the pn heterojunctions are also explained in context of the same model.

Electronically active defects in hydrogenated amorphous silicon thin films, deposited by the conventional glow discharge process in the temperature range between about 225 and 325 °C with ∼10–15 at. % hydrogen, undergo a thermally activated relaxation during film deposition. We determine the kinetics of this relaxation process in films with similar hydrogen concentrations deposited by reactive magnetron sputtering at a substrate temperature of ∼40 °C, and annealed at temperatures greater than 150 °C. We present a quantitative relationship between the relaxation time, and the deposition and/or annealing conditions required to produce low defect density material.

Remote plasma-enhanced chemical-vapor deposition (remote PECVD) was used to deposit photovoltaic (PV)-grade intrinsic and n- and p-type a-Si:H, heavily doped n- and p-type μc-Si thin films, and a photovoltaic material, a highly photoconductive intrinsic μc-Si material. This material is deposited by compensating the native defects in the as-deposited, undoped materials with a relatively small amount of boron. The dark conductivity of this compensated intrinsic material is reduced significantly with respect to the undoped μc-Si, and the films display a level of photoconductivity comparable to that of PV-grade intrinsic a-Si:H. In addition, the material shows no degradation in photoconductivity after long-term exposure to intense illumination (~50 mW/cm2 for 6 h)

We have prepared unhydrogenated and hydrogenated ([H] = 14 at.%) amorphous silicon thin films using magnetron sputtering with substrate temperature, TS = 40°C. After deposition, the films were annealed at temperatures between 150 and 200°C and conductivity was measured as a function of anneal time. We find that for that for both materials, the conductivity changes non-exponentially with annealing time. The characteristic time constant for annealing at 175°C is approximately the same in unhydrogenated films (τ≈100min) as found in films containing 14 atomic % hydrogen (τ≈200min).