H. M. Hobgood

Effects of stoichiometry on thermal stability of undoped, semi‐insulating GaAs

We provide independent evidence of the effects of melt composition on the semi‐insulating behavior and thermal stability of undoped GaAs crystals pulled from pyrolytic boron nitride crucibles by the liquid‐encapsulated Czochralski technique. Semi‐insulating crystals pulled from stoichiometric or As‐rich melts show excellent stability to heat treatments, in contrast to semi‐ insulating Ga‐rich material which exhibits p‐type conversion after prolonged annealing at 860 °C. Deep level transient spectroscopy and Hall‐effect measurements suggest that p‐conversion is caused by outdiffusion of native As and/or Ga defects associated with the EL2 deep donors, resulting in uncompensated residual acceptors, predominantly carbon impurities at the surface. The measured mobility of undoped, semi‐insulating GaAs is found empirically to be a sensitive parameter for qualifying substrates for direct ion implantation.

On the compensation mechanism in high‐resistivity 6H–SiC doped with vanadium

A model is presented which describes the compensation mechanism resulting in semi‐insulating 6H silicon carbide by vanadium doping. Undoped 6H–SiC crystals grown by physical vapor transport methods frequently contain between 1×1017 and 5×1018 cm−3 uncompensated boron acceptors. Upon addition of vanadium, the 3d1 electron of the vanadium donor compensates the holes of the boron centers. It is shown that when vanadium is present in concentrations greater than that of boron, the Fermi level is pinned to the vanadium donor level. From temperature dependent Hall effect measurements, this donor level has been determined to reside 1.35 eV below the conduction band minimum. Thermally stimulated current measurements on V‐doped SiC crystals show that boron is the major compensating center for the vanadium impurity.

Compensation of residual boron impurities in extrinsic indium-doped silicon by neutron transmutation of silicon

Infrared‐sensitive focal plane arrays based on extrinsic silicon, which integrate the detection and signal‐processing functions onto a single chip, are currently being developed at several laboratories. For imaging in the 3–5‐μm atmospheric window, highly doped Si : In is a leading candidate due to its spectral range, quantum efficiency, and moderate cooling requirements (50–60 K). The effects of residual boron impurities in the Si : In detector must, however, be compensated by donor concentrations to achieve these operational temperatures, so that precision compensation is a key factor for the production of uniform high‐responsivity detector material. We report here the successful use of thermal‐neutron irradiation for transmuting a small fraction of the silicon atoms into a known concentration of phosphorus donors in order to compensate Si : In detector material. Czochralski‐grown Si : In starting material of 〈100〉 orientation was evaluated by variable‐temperature Hall effect studies to contain NIn=2.5×...

ZnGeP2 grown by the liquid encapsulated Czochralski method

The growth of ZnGeP2 by the liquid encapsulated Czochralski method is reported for the first time herein. Large boules of ZnGeP2, with diameters up to 40 mm and weights up to 400 gm were grown by Czochralski pulling from B2O3 encapsulated melts under high pressure (20 atm Ar) using axial gradients ≤120 °C/cm. Boules pulled at ≤4 mm/h exhibited large (50×20×15 mm3) monocrystalline grains of α‐phase ZnGeP2 with room temperature electrical properties of p‐type conduction, carrier concentrations ranging from 1012 to 1016 cm−3, and mobilities of 20 cm2/V s or less. Optical samples exhibited broad IR transmission (0.7 to 12.5 μm), second harmonic generation at 4.7 μm with 7.2% conversion efficiency, a broad subband gap photoluminescence signature, and near band‐edge absorption similar to that observed in Bridgman‐grown ZnGeP2.

Evidence of the role of boron in undoped GaAs grown by liquid encapsulated Czochralski

We provide experimental evidence of electrical activity correlated with residual boron impurities and native point defects in undoped liquid‐encapsulated Czochralski GaAs crystals. In Ga‐rich samples containing ≥1017 cm−3 boron, an 0.073‐eV acceptor level is observed in which the concentration increases with Ga and B content. An approximately quadratic increase in the concentration of the 0.073‐eV defect acceptor is observed with increasing boron concentration, suggesting that a complex involving boron with an intrinsic defect (VAs, Gai, or GaAs ) is responsible for the observed acceptor behavior. No evidence of electrically active boron or boron complexes was found in semi‐insulating GaAs pulled from stoichiometric or As‐rich melts.

An atomic force microscopy study of super-dislocation/micropipe complexes on the 6H-SiC(0 0 0 1) growth surface

We have used atomic force microscopy (AFM) to study the (0 0 0 1) growth surface of a 6H-SiC single crystal at the points where micropipes emerge on the growth surface. All of the micropipes examined are origins of spiral steps, indicating that dislocations intersect the surface at these points. The dislocations observed at the surface/micropipe intersections have Burgers vectors of at least 4b0, where b0 is the Burgers vector of a unit screw dislocation aligned along the c-axis (b0 = 15.19A). Single and double unit dislocations were also observed, but they are not associated with micropipes. Micron-scale deposits of a heterogeneous phase were observed in the vicinity of the micropipes. The curvature of growth steps around these heterogeneities indicates that they impeded step motion while the crystal was growing. Based on our observations, we propose a model for the formation of super-dislocation/micropipe complexes that involves the coalescence of unit screw dislocations that are forced towards one another as large steps grow around heterogeneous material on the surface.

The doping concentrations of indium‐doped silicon measured by Hall, C‐V, and junction‐breakdown techniques

Hall‐effect measurements are routinely used to determine the doping concentrations in semiconductor material. We show that for Si : In such measurements give erroneously high results. The Hall concentrations, determined by curve fitting, are typically twice those determined from C‐V and junction‐breakdown measurements, which measure the doping concentrations directly. We consider the latter data more reliable.

OPTICAL AND ELECTRICAL CHARACTERIZATION OF BORON IMPURITIES IN SILICON CARBIDE GROWN BY PHYSICAL VAPOR TRANSPORT

Undoped SiC crystals grown by physical vapor transport have been characterized by temperature dependent Hall effect and near infrared optical absorption measurements. Crystals with reduced nitrogen content were found to exhibit p‐type conductivity with carrier concentrations in the 5×1014–1×1016 cm−3 range at room temperature. The Fermi level position determined from Hall effect measurements at elevated temperatures was 0.35 eV above valence band. The primary acceptor‐type impurity was identified as substitutional boron with total concentration of uncompensated acceptors in the 1×1017–5×1018 range. This interpretation was confirmed by near infrared absorption spectra, which were dominated by a broad photoionization band with a threshold at 0.7 eV and a maximum at 1.75 eV. The shape of the band was fitted, and the thermal ionization energy of the defect was found to be in the 0.3–0.4 eV range. A correlation between the photoionization band intensity, and the uncompensated boron content was used to determin...

Vanadium related near‐band‐edge absorption bands in three SiC polytypes

Low‐temperature optical absorption experiments have been performed on a variety of n‐type, p‐type, and high‐resistivity silicon carbide samples, including the polytypes: 4H, 6H, and 15R. These experiments reveal a set of absorption band close to the band edge with a fine structure depending upon the polytype. Each sample exhibits a spectrum with the number of lines corresponding to the number of inequivalent substitutional lattice sites contained in the polytype. A correlation of these lines with the neutral vanadium 2E→2T2 intracenter transition indicates that the initial state for the near‐band‐gap absorption lines is the 2E state of the 3d1 configuration of vanadium. The near‐band‐edge absorption lines were interpreted as due to an exciton bound to a vanadium donor with an electron occupying an atomic‐like d state. The position of the vanadium acceptor level was estimated to be, at most, 250 meV from the conduction band for the cubic site in 6H SiC.