Thin films of silicon carbide have been formed on silicon field emitters by chemical reaction with ethylene (C2H4) at temperatures of 850 to 950°C using ethylene gas pressures up to 5×10−3 Torr. By controlling the reaction time and temperature, we have made SiC coatings of from ~20 A thickness to complete transformation of tips to SiC (1-2 µm). The electron diffraction pattern of the SiC layers show the expected 20% lattice mismatch with silicon and, for those emitters completely transformed, a polycrystalline 3C-SiC polytype was identified. The small radius of curvature was maintained for both the coated and completely transformed tips, although some defects and surface roughness was introduced during the treatments.

Article Physico-Chemical Considerations in the Preparation of New Compound Semiconductor Solar Cells was published on December 1, 1975 in the journal Zeitschrift für Physikalische Chemie (volume 98, issue 1-6).

The average optical properties of an ultra-thin surface reaction layer (SRL) during growth by pulsed chemical beam epitaxy (PCBE) can be quantitatively accessed by p-polarized reflectance spectroscopy (PRS), as demonstrated on the example of heteroepitaxial GaP growth. Under PCBE growth conditions, the surface of the substrate is exposed to pulsed ballistic beams of tertiarybutyl phosphine [TBP, (C 4 H 9 )PH 2 ] and triethylgallium [TEG, Ga(C 2 H 5 ) 3 ]. The pulsed precursor supply causes a period in composition and thickness modulated SRL, monitored as a fine structure that is superimposed on interference oscillations, resulting from back reflection at the substratelayer interface with increasing layer thickness. The amplitude of this fine structure undergoes a period amplitude modulation and exhibits turning points at which the response to the first precursor pulse changes sign. The turning points can be characterized by the expression R 4 (Φ max )=R 4 (Φ min ), which describes the maximal and minimal values of the temporally modulated phase factor in the SRL, using a four layer stack description. The positions of these turning points are not affected by the thickness of the SRL, which allows the computation of the average complex dielectric function of the SRL independent of its thickness. In the next step, the average thickness of the SRL can be extracted from the amplitude of the observed fine structure.

Double beam photoconductivity experiments are reported for the system Cd1−xMnxTe. The technique allows a separation of surface and bulk contributions, respectively. Bulk effects dominate for manganese rich material and reveal a sharp peak at the band gap energy while surface conductivity reveals a step function type spectral behavior. The growth of a native oxide on the surface causes an increase in the surface recombination velocity and a change from surface to bulk conduction. An oxide related trap state was discovered that is located at approximately 400 meV above the valence band edge.

ZnGeP2 can be transported by chemical vapor transport (CVT) using phosphorus or ZnP2 as transporting agent. Since germanium has a seven order of magnitude smaller partial pressure than zinc and phosphorus solely the formation of a volatile GePy (y≥l) species can explain the CVT-growth which was observed in a temperature range from 973 to 1573 K. To stabilize this species phosphorus pressures in the bar range are needed. Using absorption spectroscopy to analyze the equilibrium vapor phase composition over P, GeP and ZnGeP2, absorption lines were found adjacent to P2 band heads in the wavelength range from 180 to 260 nm which were addressed as band heads of an unknown GePy species. Quadrupole mass spectroscopy (QMS) measurements revealed that this species is not stable at lower pressures decomposing presynthesized GeP and ZnGeP2 in a Knudsen cell. In the temperature range from 550 to 800 K at total pressures of 10−7 to 10−6 mbar, ZnGeP2 decomposes into Zn and P4 whereas Zn is the dominant gas phase species. Under vacuum ZnGeP2 starts to decompose at 600 K while in a N2 atmosphere decomposition occurs at 923 K. By adding ZnP2 (5mg/cm3) to polycrystalline ZnGeP2, which was placed in an evacuated and closed quartz glass ampoule, red transparent crystals were yielded in a temperature gradient ΔT= 1073-1023 K.

The growth of diamond films on various polycrystalline metal and (001) Si substrates by biased hot-filament chemical vapor deposition is discussed. The deposited films have been characterized by scanning electron microscopy, x-ray diffraction, Auger electron spectroscopy, and Raman spectroscopy. Films grown on Si, Ni, and W exhibited the best quality according to Raman sp3/sp2 peak intensity ratios and the full width at half maximum of the 1332 cm−1 Raman peak. The relationship between this quality and substrate properties such as surface energy and lattice parameter is discussed. Also, the residual stress in the film as measured by the Raman peak shift is correlated with the thermal expansion coefficient of the substrate.

Steady state and time-resolved photoluminescence (PL) investigations on ZnGeP 2 crystals grown from the vapor phase by high pressure physical vapor transport (HPVT) and from the melt by gradient freezing (GF) are reported. The luminescence spectra reveal a broad infrared emission with peak position at 1.2 eV that exhibits features of classical donor-acceptor recombination. The hyperbolic decay characteristic over a wide energy range, investigated from 1.2 eV up to 1.5eV, suggest that this broad emission band is related to one energetic recombination center. Higher energetic luminescence structures at 1.6eV and 1.7eV were revealed after annealing of ZnGeP 2 crystals in vacuum for a longer period of time. The emission decay behavior in this energy range is characterized by two hyperbolic time constants, viewed as the supercomposition of the decay from the broad emission center peaked at 1.2eV and additional donor-acceptor recombination emissions at 1.6eV and 1.7eV, respectively. ZnGeP 2 crystals grown under Ge-deficient conditions by HPVT show an additional emission structure at 1.8 eV with sharp emission fine structures at 1.778 eV related to the presence of additional donor states.

Thick GaN layers as well as AlGaN/GaN and AlN/GaN heterostructures grown by metalorganic vapor phase epitaxy have been photoelectrochemically (PEC) etched in various dilute electrolytes, and bandgap-selective etching has been demonstrated in heterostructures. This result is a significant step forward in the fabrication of group III-nitride devices and one-dimensional photonic bandgap (PBG) structures in the deep UV. Based on initial results from thick GaN layers, a method was developed to achieve self-stopping selective etching of thin GaN layers in AlGaN/GaN and AlN/ GaN heterostructures. Selective PEC etching requires the use of a suitable light source with photon energies larger than the bandgap of GaN, but smaller than that of AlGaN or AlN, thus enabling selective hole generation in the GaN layers to be etched. Additionally, it is imperative to use an electrolyte that supports PEC etching of GaN without chemically etching AlGaN or AlN.

GaN whiskers with nanoscale dimensions have been fabricated by photoelectrochemical (PEC) etching in dilute H3PO4 electrolyte. Etching in lower concentration H3PO4 electrolyte for 1 h or for a short time of 5 min at a higher concentration results in individual whiskers with a density of ∼2×109cm−2 and diameters to 15nm. It is observed that ∼10% of them have formed nearly perfect hexagonal plates on the top of the whiskers, which appear to evolve into flowerlike features upon extended etching to 12 min. Such hexagonal plates have not been reported previously in the PEC etching of GaN. The presence of a dislocation along the central axis of the needles is clearly demonstrated, and the etch pattern is suggested to be related to the growth mechanism for GaN on sapphire. When etched for times >30min, these whiskers are typically arranged in clusters with a density of 2–5×107cm−2 and have ten or more whiskers contributing to the central top of the cluster.

A procedure for calculating homolytic dissociation rate constants is reported for modeling organometallic vapor-phase epitaxy (OMVPE) of III-V compounds for all pressure regimes. Reaction rate constants were predicted following a semiclassical approach based on quantum mechanical calculations and transition-state theory. The critical configuration was determined using linear interpolations for the geometry of the intermediate structures, Morse potentials for the intermediate electronic energies, and Hase's relationship for the vibrational frequencies that become annihilated. Low-pressure rate constants were calculated from Rice-Ramsperger-Kassel-Marcus (RRKM) theory following the Troe approach. The calculations were compared with experimental values for the dissociation of one methyl radical from the closed-shell molecules Al(CH3)(3), Ga(CH3)(3), and In(CH3)(3) and the radical molecules Ga(CH3)(2) and In(CH3) and for the dissociation of one hydrogen atom from NH3, PH3, and AsH3. A simplified system of reactions for the homolytic dissociation of In(CH3)(3) was modeled in an OMV reactor designed for the pressure range 10(-2) to 10(2) atm using computational fluid dynamics coupled with chemical kinetics. The steady-state simulations were carried out at 1000 K and at N-2 pressures of 1 and 20 atm.

We have investigated the origin of contrast features observed in coalesced GaP islands, deposited by chemical beam epitaxy on (001) Si, by high resolution transmission electron microscopy and conventional dark field electron microscopy. Our results indicate that these features are antiphase boundaries (APBs) lying on {110} planes. Image simulations have been performed to show that APBs can only be seen under specific defocus conditions in high resolution lattice images. The observed contrast is attributed to the presence of Ga–Ga and P–P wrong bonds at APBs. A model is proposed to show that the coalescence of GaP islands on the same Si terrace may not produce APBs, and the formation of such boundaries may require the presence of monoatomic steps, separating the coalescing islands.

The coalescence of GaP islands, grown on Si(001), Si(111), Si(110) and Si(113) surfaces by chemical beam epitaxy, has been investigated by high-resolution transmission electron microscopy. Stacking faults and first-order twins are observed within islands before coalescence and result from stacking errors during growth on the smaller P-terminated {111} facets of GaP islands. Upon island coalescence, complex moire fringes are observed contiguous to highly faulted {111} planes within epitaxial layers grown on all four Si substrate orientations and are attributed to multiple twinning. Second-and third-order twins are also observed within (111) and (110) layers and their formation is attributed to successive twinning on differently inclined {111} facets. Amongst the four orientations, coalesced growths on the Si(111) surface are the most defective and this may be caused by a higher density of P-terminated {111} facets on islands grown on the Si(111) surface.

The structural, electronic, and thermochemical properties of indium compounds which are of interest in halide transport and organometallic chemical vapor deposition processes have been studied by ab initio and statistical thermodynamic methods. The compounds reported include: indium halides and hydrides (InF, InCl, InCl3, InH, InH2, InH3); indium clusters (In-2, In-3); methylindium, dimethylindium, and their hydrogen derivatives [In(CH3), In(CH3)H, In(CH3)H-2, In(CH3)(2), In(CH3)(2)H]; dimethylindium dimer [In-2(CH3)(4)] and trimethylindium [In(CH3)(3)]; dehydrogenated methyl-, dimethyl-, and trimethylindium [In(CH3)(2)CH2, In(CH3)CH2. In(CH2)]; trimethylindium adducts with ammonia, trimethylamine and hydrazine [(CH3)(3)In:NH3, (CH3)(3)In:N(CH3)(3), (CH3)(3)In:N(H-2)N(H-2)]; dimethylamino-indium and methylimino-indium [In(CH3)(2)(NH2). In(CH3)(NH)]; indium nitride and indium nitride dimer (InN, In2N2); indium phosphide, -arsenide, and -antimonide (InP, InAs, InSb). The predicted electronic properties are based on density functional theory calculations; the calculated thermodynamic properties are reported following the format of the JANAF (Joint Army, Navy, NASA, Air Force) Tables. Equilibrium compositions at two temperatures (298 and 1000K) have been analyzed for groups of competing simultaneous reactions.

We analyze the phase-matching conditions for second-harmonic generation (SHG) and optical parametric oscillation (OPO) in birefringent nonlinear semiconductor waveguides and apply these results to the model system of ZnGeP2 on a GaP substrate. The analyses and numerical results show that phase matching can be achieved for OPO and SHG for reasonable guide thicknesses throughout much of the infrared, indicating significant potential applications for nonlinear birefringent waveguides. For the fundamental mode of a relatively thick guide the region of phase matching and the phase-matching angles are similar to those in bulk material. However, the waveguide has the added flexibility that phase-matched coupling can occur between the various modes of the guide. For example, the phase-matching region for SHG can be considerably extended by coupling the pump into the guide in the fundamental, m = 0, mode and phase matching to the m = 2 mode of the second harmonic. Significantly, the results indicate, among other things, that ZnGeP2 waveguides with harmonic output in the m = 2 mode can be used for efficient SHG from input radiation in the 9.6-10.6-microm region where bulk efficiencies in this wavelength range are too small to be useful.

: As part of the DoD-MURI research program entitled 'Modeling and control of Advanced Chemical Vapor Deposition Processes: the Control of Defects in Mixed III-V Compound Semiconductors' (Grant F49620-95-1-0447), DURIP funding was provided to establish a real-time optical characterization facility to study thin-film growth processes extended to super-atmospheric pressures. The extension of the chemical vapor deposition (CVD) to operating conditions at super-atmospheric pressures mandated validation of: (a) Simulations of surface reaction kinetics; (b) Simulations of homogeneous gas phase reactions coupled to transport in laminar high density vapor flows; and (c) the development of experimental methods for the detection of onset of turbulence, that is, verification of laminar flow under conditions of high pressure OMCVD.

GaP islands grown on selected surfaces of Si and their coalescence behavior have been investigated by transmission electron microscopy. These layers were grown by chemical beam epitaxy. A number of significant observations emerge from this study. First, planar defect formation has been shown to be related to stacking errors on the smaller P-terminated {111} facets of GaP islands. Amongst the four orientations, (111) epilayers have a higher density of stacking faults and first order twins because of more P-terminated ({111} facets per island. Second, multiple twinning on exposed ({111} facets can produce tilt boundaries and irregular growths when islands coalesce. Third, inversion domain boundaries lying on {110} planes have been shown to form during GaP island coalescence across monatomic steps on (001) Si. Image simulations have been performed to show that these boundaries can be seen in high resolution lattice images and the observed contrast is attributed to the presence of wrong Ga-Ga and P-P bonds at the inversion boundary.

Evolution of gallium phosphide epitaxial islands, grown on the (001), (111), (110) and (113) surfaces of Si by chemical beam epitaxy, has been investigated by p-polarized reflectance spectroscopy, transmission electron microscopy and atomic force microscopy. The growth nucleates as faceted three dimensional islands on the (001) and (111) Si surfaces because of the polar nature of the heterointerface which increases the interfacial energy. A more two-dimensional-like growth mode is seen on the (110) and (113) surfaces which is attributed to the absence of charge build up at the GaP-Si heterointerface for these orientations, thereby reducing the interface energy. Islands grown on (001) Si become more faceted and larger in size with increase in growth temperature. This is due to a lower incubation time and enhanced atomic mobility at high temperatures. Wurtzite GaP has been observed to coexist with the zincblende polytype in some of the islands grown on (111) Si at 560 degrees C. Arguments have been developed to rationalize these observations.