J. M. DePuydt

p‐type ZnSe by nitrogen atom beam doping during molecular beam epitaxial growth

A novel approach to producing p‐type ZnSe epitaxial layers is reported which involves nitrogen atom beam doping during molecular beam epitaxial growth. Net acceptor concentrations as large as 3.4×1017 cm−3 have been measured in nitrogen atom beam doped ZnSe/GaAs heteroepitaxial layers which represents the highest acceptor concentration reported to date for ZnSe:N epitaxial material grown by molecular beam epitaxy. In addition, light‐emitting diodes based on ZnSe:N/ZnSe:Cl, p‐n homojunctions have been found to exhibit dominant electroluminescence in the blue region of the visible spectrum at room temperature.

Heavily doped p‐ZnSe:N grown by molecular beam epitaxy

The growth of p‐ZnSe:N films by molecular‐beam epitaxy, employing a free radical nitrogen source, has been investigated. Using this technique we have obtained p‐type ZnSe with net acceptor concentrations up to 1.0×1018 cm−3, as measured by capacitance–voltage(C–V) profiling−this is the highest ever reported for p‐type ZnSe. By adjusting the flux of active nitrogen and the substrate temperature, films with net acceptor concentrations from 1.0×1016 to 1.0×1018 cm−3 were grown. Evidence of compensation was found in the low temperature photoluminescence and C–V measurements; the degree of compensation depends on the amount of nitrogen incorporated into the film. The dependencies of nitrogen density, net acceptor concentration, and degree of compensation upon the flux of active nitrogen and the substrate temperature are discussed.

Degradation of II‐VI based blue‐green light emitters

We have carried out the first detailed structural studies of degradation in II‐VI blue‐green light emitters. Electroluminescence and transmission electron microscopy studies carried out on light emitting diodes fabricated from quantum well laser structures and electroluminescence studies on stripe laser structures show that degradation occurs by the formation and propagation of crystal defects. The studies indicate that room temperature cw lasing in such structures is possibly prevented by the rapid formation of such defects at the high current densities required for lasing.

Role of stacking faults as misfit dislocation sources and nonradiative recombination centers in II‐VI heterostructures and devices

We have investigated the role of stacking faults in high quality ZnSxSe1−x heterostructures and a ZnSxSe1−x/CdxZn1−xSe based II‐VI blue‐green quantum well laser structure grown on GaAs substrates. We find that these stacking faults, which originate at the epilayer/substrate interface during the initial stages of the growth, act as sources for misfit dislocation formation in the quantum well region of ZnSxSe1−x/CdxZn1−xSe based devices. We have analyzed the formation mechanism of these dislocations. We also show through cathodoluminescence microscopy that these stacking faults act as nonradiative recombination centers which therefore reduce the luminescence of these devices.

Low‐threshold buried‐ridge II‐VI laser diodes

Blue‐green (λ=511 nm) separate confinement laser structures based on lattice‐matched MgZnSSe‐ZnSSe‐CdZnSe have been grown by molecular beam epitaxy. Wide stripe gain‐guided devices have been fabricated from several such wafers. These devices exhibit room‐temperature pulsed threshold current densities as low as 630 A/cm2 and threshold voltages less than 9 V. Using a novel self‐aligned process that results in a planar surface, buried‐ridge laser diodes have also been fabricated. These devices have demonstrated room‐temperature threshold currents as low as 2.5 mA, which is more than a factor of 50 lower than that of any previously reported II‐VI laser diode. Room‐temperature operation at duty factors up to 50% has been demonstrated. The far‐field patterns from these devices indicate single lateral mode operation, suitable for diffraction‐limited applications, such as optical data storage.

Blue‐Green Diode Lasers

The development of compact, reliable and inexpensive short‐wavelength lasers is certain to have profound effects on virtually any technology that uses coherent visible light. Although the impact of such devices will be far‐reaching, the primary driving force behind efforts to develop blue‐green diode lasers is without question optical recording. The demand for increased data storage capabilities is continually forcing the recording industry to increase storage densities.The development of compact, reliable and inexpensive short‐wavelength lasers is certain to have profound effects on virtually any technology that uses coherent visible light. Although the impact of such devices will be far‐reaching, the primary driving force behind efforts to develop blue‐green diode lasers is without question optical recording. The demand for increased data storage capabilities is continually forcing the recording industry to increase storage densities.

〈100〉 dark line defect in II‐VI blue‐green light emitters

We have carried out a microstructural study of the 〈100〉 dark line defect that forms during degradation of II‐VI blue‐green light emitters. We find that these defects lie in or near the ZnCdSe quantum well and do not correspond to a readily observable dislocation network in transmission electron microscopy studies. We speculate that they may consist of point defects or small point defect complexes. We have also carried out estimates of point defect migration rates during device operation that can give rise to such degradation.

Dependence of the density and type of stacking faults on the surface treatment of the substrate and growth mode in ZnSxSe1−x/ZnSe buffer layer/GaAs heterostructures

A systematic dependence of the density and type of stacking fault defects with substrate surface chemistry and film growth mode was observed in ZnSe‐based films grown on GaAs substrates. Namely, the density of Frank‐type stacking faults is very large for films grown on Ga‐rich surfaces, but is very low for films grown on As‐stabilized surfaces exposed to Zn prior to the growth of the film. In contrast, the density of Shockley‐type stacking faults increases for films grown by 3D growth mode at the initial stages of growth, but decreases greatly if the films are grown by the layer‐by‐layer growth mode. Films with stacking fault densities as low as 104/cm2 were obtained by growing the films by the layer‐by‐layer growth on GaAs epilayers with As‐stabilized surfaces that were exposed to Zn for 1–2 min prior to the growth of the films.

Electrical characterization of p‐type ZnSe

ZnSe epitaxial layers doped with lithium have been found to be p type with resistivities as low as 2.9 Ω cm. The majority‐carrier types of these films were determined using both capacitance‐voltage and potential profiling techniques. The sample resistivities were obtained using ac resistivity measurements and potential profiling. Uncompensated acceptor densities have been measured to be as high as 8×1016 cm−3 using capacitance‐voltage profiling. Current‐voltage traces taken with evaporated and sputtered gold contacts typically show reverse breakdown which is consistent with avalanche breakdown in these materials.

Molecular beam epitaxy of low defect density (⩽1×104 cm−2) ZnSSe on GaAs

We study the growth of pseudomorphic ZnSSe layers on GaAs. The dependence of the epilayer quality on Zn exposure to the GaAs surface is investigated. Zn treatment prior to the ZnSe buffer growth on the As‐rich GaAs surface results in the lowest defect density. Transmission electron microscopy studies show that an atomic scale smooth interface is formed. Based on the etch pit density and photoluminescence image analysis, ZnSSe layers with defect density ≤1×104 cm−2 can be reproducibly obtained.