K. J. Bachmann

Sellmeier parameters for ZnGaP2 and GaP

Sellmeier parameters are determined for the birefringent material ZnGaP2 and the singly refractive material GaP by the minimization of chi-square employing the Levenberg–Marquardt method. The distinguishing feature of the present work is that all five Sellmeier parameters are treated as adjustable. In previous work the Sellmeier parameter related to the restrahlen frequency was fixed and set equal for both ordinary and extraordinary waves in the birefringent material. The fitted results show there is approximately an 8% difference between the two. Taking this parameter as adjustable allows for a better fit on all other parameters.

Modification of Si field emitter surfaces by chemical conversion to SiC

Silicon field emitters have been modified by coating with a thin SiC film through a chemical conversion process. Silicon carbide was formed on Si emitter surfaces by reacting with ethylene gas at temperatures between 850 and 950 °C using pressures as high as 5×10−3 Torr. The thickness of the coatings ranged from 2 to 500 nm, determined by a combination of reaction time, pressure, and temperature. Stable emission currents above 10 μA were measured from individual SiC coated emitters.

Antiphase boundaries in GaP layers grown on (001) Si by chemical beam epitaxy

Abstract 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.

P-polarized reflectance spectroscopy: A highly sensitive real-time monitoring technique to study surface kinetics under steady state epitaxial deposition conditions

This paper describes the results of real-time optical monitoring of epitaxial growth processes by p-polarized reflectance spectroscopy (PRS) using a single wavelength application under pulsed chemical beam epitaxy (PCBE) condition. The high surface sensitivity of PRS allows the monitoring of submonolayer precursor coverage on the surface as shown for GaP homoepitaxy and GaP on Si heteroepitaxy as examples. In the case of heteroepitaxy, the growth rate and optical properties are revealed by PRS using interference oscillations as they occur during growth. Superimposed on these interference oscillations, the PRS signal exhibits a fine structure caused by the periodic alteration of the surface chemistry by the pulsed supply of chemical precursors. This fine structure is modeled under conditions where the surface chemistry cycles between phosphorus supersaturated and phosphorus depleted surfaces. The mathematical model describes the fine structure using a surface layer that increases during the tertiarybutyl phosphine (TBP) supply and decreases during and after the triethylgallium (TEG) pulse, which increases the growing GaP film thickness. The imaginary part of the dielectric function of the surface layer is revealed from the turning points in the fine structure, where the optical response to the first precursor pulse in the cycle sequence changes sign. The amplitude of the fine structure is determinated by the surface layer thickness and the complex dielectric functions for the surface layer with the underlying bulk film. Surface kinetic data can be obtained by analyzing the rise and decay transients of the fine structure.

NATIVE DEFECT RELATED OPTICAL PROPERTIES OF ZNGEP2

We present photoluminescence, photoconductivity, and optical absorption spectra for ZnGeP2 crystals grown from the melt by gradient freezing and from the vapor phase by high pressure physical vapor transport (HPVT). A model of donor and acceptor related subbands in the energy gap of ZnGeP2 is introduced that explains the experimental results. The emission with peak position at 1.2 eV is attributed to residual disorder on the cation sublattice. The lower absorption upon annealing is interpreted in terms of both the reduction of the disorder on the cation sublattice and changes in the Fermi level position. The n‐type conductivity of ZnGeP2 crystals grown under Ge‐deficient conditions by the HPVT is related to the presence of additional donor states.

Heteroepitaxy of lattice‐matched compound semiconductors on silicon

The heteroepitaxial overgrowth of silicon by nearly lattice‐matched compound semiconductors is reviewed in the context of the separation of the chemical problems associated with the initial sealing of the silicon surface by a contiguous epitaxial compound film from the problems associated with the generation of strain during heteroepitaxial growth. Of the mixed compound systems available dilute solid solutions of composition AlxGa1−xNyP1−y and ZnSySe1−y as well as ZnSixGe1−xP are suitable candidates for the exactly lattice‐matched epitaxial overgrowth of silicon. Real‐time process monitoring by nonintrusive methods is important for gaining an understanding of the epitaxial overgrowth mechanism and for controlling the film properties. A new method, p‐polarized reflectance spectroscopy is introduced that provides detailed information about the growth rate per cycle, the bulk optical properties of the film and its topography. Submonolayer resolution is accomplished for thousands of A of film growth by pulsed...

Orientation mediated self-assembled gallium phosphide islands grown on silicon

Abstract 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°C. Arguments have been developed to rationalize these observations.

The phase relations in the system Cu,In,Se

The phase relations in the system Cu,In,Se are evaluated by DTA and direct observations of the solid-liquid equilibria in the temperature range 800 ≤ T ≤ 1300 K. Five distinct miscibility gap regions where two liquids coexist in equilibrium with a solid phase are observed. Four of these regions extend from monotectics on the Cu-Se and In-Se binaries into the ternary field and tie to the homogeneity ranges about Cu2Se, In2Se3 and In4Se3, respectively. The fifth miscibility gap regions ties to the homogeneity range about CuInSe2 at ∼ 943 K. Further clarification concerning the phase relations on the Cu2Se-In2Se3 pseudobinary is presented for the Cu2Se-rich section where four temperature invariant 3-phase lines are observed. In addition to the γ-δ transition a second solid state transformation δ-ζ exists for stoichiometric CuInSe2 in the disordered high temperature phase. A correction of the previously published phase relations at XIn2Se3 < 0.2 is needed because of reactive interactions of the melt with unprotected fused silica enclosures used in the earlier work. The position of the second congruently melting ternary compound C on the Cu2Se-CuInSe2 cut is slightly shifted from the previously reported composition Cu5InSe4 to Cu18In4Se15, and the existence of an additional compound P is suggested that we tentatively link to a peritectic reaction at 1258 K between liquid at x = 0.105 and saturated solid solution of In2Se3 in Cu2Se referred to as α-phase. A eutectic exists between P and C at 1208 K.

Photoluminescence Studies of CuInS2-CuInSe2 Alloy Crystals

Photoluminescence spectra are presented for single crystals of CuInS2, CuInSe2 and CuInS2ySe2-2y alloys. The PL spectrum of stoichiometric CuInSe2 is dominated by free exciton emission of 9 meV FWHM at 1.03 eV, but structure at 090, 0.94 and 0.97 eV is observed due to transitions involving residual native defect states. For pure CuInS2 broad deep luminescence bands are obtained that involve several deep native defect states, e.g. donors at 45 and 160 meV below the CBE and acceptors at 85 meV above the VBE. These defects persist in S-rich CuInS2/CuInSe2 alloys, but excitonic emission is observed, in addition to the deep luminescence. Surprisingly CuInS2ySe2-2y crystals at y ≳ l are totally dominated by free exciton emission. This result shows that an increase of at least 20% over the bandgap of CuInSe2 and excellent crystal quality can be achieved by partial substitution of Se by S.

Real-time optical monitoring of epitaxial growth: pulsed chemical beam epitaxy of GaP and InP homoepitaxy and heteroepitaxy on Si

We present a study of the real-time monitoring of the homoepitaxial growth of GaP, InP, and the growth of InP/GaP and GaP/Si(001) heterostructures, combining single wavelength p-polarized reflectance (PRS), reflectance-difference spectroscopy (RDS), and laser light scattering (LLS) during pulsed chemical beam epitaxy with tertiarybutylphosphine, triethylgallium, and trimethylindium sources. The growth rate and the bulk optical properties are revealed by PRS with submonolayer resolution over 1000A of film growth. The surface topography is monitored by LLS providing additional information on the evolution of the surface roughness as well as the nucleation/growth mechanism. The optical surface anisotropy, which is related to surface reconstruction and/or surface morphology, is monitored by RDS and compared with the results of PRS and LLS. The results are discussed with respect to the deposition kinetics, in particular as a function of the V:III flux ratio. The pulsed supply of chemical precursors causes a periodic alteration of the surface composition, which is observed as correlated periodic changes in the RD and PR signals, confirming the high sensitivity of both methods to surface chemistry.