David A. Cammack

Structural properties of the ZnSe/GaAs system grown by molecular‐beam epitaxy

We report the results of an investigation of the structural properties and relaxation of misfit stress with transmission electron microscopy and x‐ray diffraction techniques. Epitaxial films of ZnSe were grown on GaAs by molecular‐beam epitaxy of thicknesses ranging from 0.05 to 4.9 μm. The films contain stacking fault defects up to thicknesses of about 150 nm. Above 150 nm perfect misfit dislocations are generated from surface sources and the stacking fault defects to accommodate the lattice mismatch. The majority of dislocations observed are of the 60° type with Lomer edge type dislocations observed in a much lower concentration. The density of misfit dislocations increases with increasing epilayer thickness. Above about 1 μm the films exhibit biaxial tension which we believe is due to thermal expansion differences of ZnSe and GaAs. Good agreement is observed between microscopic and diffraction measurements of the relaxation phenomena.

Correlation between radiative transitions and structural defects in zinc selenide epitaxial layers

We present low‐temperature photoluminescence and transmission electron microscopy data to show that two transitions I0V at ∼2.774 eV and Y0 at ∼2.60 eV, frequently observed in unintentionally doped zinc selenide epitaxial layers, are directly related to structural defects. It is shown that these transitions are strong in those samples which have very low background impurities and high density of structural defects and weak in those cases that have either high background impurities or low density of structural defects.

Non‐Gaussian fundamental mode patterns in narrow‐stripe‐geometry lasers

Near‐ and far‐field patterns of narrow (⩽10 μm) stripe‐geometry lasers are reported which differ significantly from the Gaussian patterns predicted and observed in wider‐stripe lasers. The field patterns are explained by using a waveguide model which takes into account the finite extent of the stripe. High‐output‐power operation in a single fundamental transverse and lateral mode (up to 1 W in 25‐ns pulses) has been observed in devices of this type.

Effect of lattice mismatch in ZnSe epilayers grown on GaAs by molecular beam epitaxy

We report a detailed study of the effect of lattice mismatch on ZnSe epilayers grown on 〈001〉 GaAs by molecular beam epitaxy using photoluminescence (PL), x‐ray diffraction, and transmission electron microscopy (TEM) techniques. We find that our samples are of high quality, exhibiting sharp and strong bound excitons, and that these bound excitons shift to higher energies due to tetragonal distortion as the thickness of the ZnSe epilayer is systematically reduced from ∼1 to 0.1 μm. Fairly good agreement is found between PL and x‐ray data for the total strain relaxation as a function of layer thickness. TEM measurements are also used to estimate an inelastic component of the strain relaxation in the layers.

Stimulated emission via inelastic exciton‐exciton scattering in ZnSe epilayers

Stimulated emission at 6 K by optical excitation has been investigated in ZnSe epilayers. A detailed examination of the spectra below threshold indicates that the stimulated emission is due to inelastic exciton‐exciton scattering, in contrast with some recent reports that lasing is due to electron‐hole plasma luminescence modified by self‐absorption. A red shift of the lasing line at high pump intensities has also been modeled in terms of the exciton‐exciton and band filling mechanisms, giving excellent agreement with the data.

ELECTRICAL CHARACTERIZATION OF P-TYPE ZNSE:LI EPILAYERS GROWN ON P+-GAAS BY MOLECULAR-BEAM EPITAXY

Small‐signal ac admittance data are presented, and shown to provide accurate and unambiguous evidence of p‐type behavior in Li‐doped ZnSe epilayers grown on p+‐GaAs. The devices measured are two‐terminal vertical‐transport structures, with unequal‐area contacts in order to distinguish effects arising from the top metal Schottky barrier from those arising from the heterojunction. Qualitative features of the data alone are sufficient to determine the (positive) sign of the mobile carriers in the ZnSe. The data are analyzed using an equivalent‐circuit model shown to be reliable for n‐type ZnSe, from which the net doping and the ZnSe resistivity are extracted. Using these quantities, the ZnSe hole mobility is found to be about 28±4 cm2/V s. Schottky barrier and heterojunction barrier heights are also determined.

Exciton line broadening in ZnSxSe1−x epilayers grown on GaAs by molecular‐beam epitaxy

ZnSxSe1−x epilayers grown on GaAs substrates by molecular‐beam epitaxy were examined for exciton line broadening in photoluminescence as functions of x(0.0≤x≤0.49) and temperature (6–300 K). The variation of linewidth with x is partly explained by alloy broadening effects and partly attributed to the density of defects near the epilayer surface, which in turn is a function of x and layer thickness. The increase in linewidth with temperature is modeled up to 200 K by taking into account scattering due to acoustic and optical phonons. The band gap is observed to be a quadratic function of x at both low (6 K) and high (300 K) temperatures.

Low‐temperature growth of ZnSe by molecular beam epitaxy using cracked selenium

The growth of zinc selenide by molecular beam epitaxy using a cracked selenium source is studied. It is found that high quality growth can be achieved at substantially lower substrate temperatures than has been possible using uncracked selenium sources. It is determined from reflection high‐energy electron diffraction observations that the use of cracked selenium produces growth dominated by a two‐dimensional mechanism at substrate temperatures as low as 225 °C and that exposure of the GaAs substrate to cracked selenium prior to the initiation of growth has a substantial effect on the GaAs substrate and the early stages of ZnSe growth.

dc and ac transport in molecular‐beam‐epitaxy‐grown metal/ZnSe/GaAs heterojunction structures

The room‐temperature electrical transport properties of ZnSe epilayers grown above the critical layer thickness on n+‐GaAs substrates by molecular‐beam epitaxy have been studied. dc current versus voltage and small‐signal ac admittance versus voltage and frequency measurements were made on Schottky contacts (Au or Hg). A novel method of analysis is presented which, extending a previous model to include the three‐dimensional effects of current spreading as a function of frequency, allows the ZnSe epilayer resistivity to be obtained. Combining this with the doping concentration obtained from analysis of the C‐V characteristics, the ZnSe mobility may be calculated. Mobilities thus measured compare well with our own and other workers’ published Hall mobility data on similar samples, as well as with theoretical calculations. We believe that our results represent a unique and detailed characterization of an epilayer heterostructure on a highly conducting substrate.

Interface barrier height in ZnSe/GaAs structures

Abstract The electronic transport characteristics of Schottky metal/ n -ZnSe/ n + GaAs structures are presented. The small-signal-admittance characteristics of these structures show certain distinct features which are located at a bias point dependent on the interface barrier height. In most cases these features are sharp slope changes in the characteristics and their shapes are determined by the modulation of the effective hetero-interface barrier height with bias. In some samples, these features are seen in the form of sharp minima, and they correspond to cases where the interface pinning is strong. From the bias location of these features, the interface barrier height in thick n -ZnSe/ n + GaAs(100) structure is found to be 0.4–0.5 eV for electrons injected into ZnSe.