D.R. Calawa

Molecular‐beam epitaxy growth of high‐performance midinfrared diode lasers

Recent advances in the performance of GaInAsSb/AlGaAsSb quantum‐well diode lasers have been directly related to improvements in the quality of the molecular‐beam epitaxy (MBE)‐grown epitaxial layers. These improvements have been based on careful measurement and control of lattice matching and intentional strain, changes in shutter sequencing at interfaces, and a generally better understanding of the growth of Sb‐based epitaxial materials. By using this improved MBE‐grown material, significantly enhanced performance has been obtained for midinfrared lasers. These lasers, which are capable of ∼2‐μm emission at room temperature, presently exhibit threshold current densities of 143 A/cm2, continuous wave powers of 1.3 W, and diffraction‐limited powers of 120 mW. Such high‐performance midinfrared diode lasers are of interest for a wide variety of applications, including eye‐safe laser radar, remote sensing of atmospheric contaminants and wind turbulence, laser surgery, and pumping of solid‐state laser media.

Evolution of surface structure and phase separation in GaInAsSb

Atomic force microscopy was used to study changes in the surface step structure of GaInAsSb layers with varying degrees of phase separation. The layers were grown by organometallic vapor phase epitaxy on (001) GaSb substrates with 2{sup o} miscut angles toward (-1-11)A, (1-11)B, and (101). Alloy decomposition was observed by contrast modulations in plan-view transmission electron microscopy, and broadening in x-ray diffraction and photoluminescence peaks. GaInAsSb layers with a minimal degree of phase separation exhibit a step-bunched step structure. A gradual degradation in the periodicity of the step structure is observed as the alloy decomposes into GaAs- and InSb-rich regions. The surface eventually develops trenches to accommodate the local strain associated with composition variations, which are on the order of a few percent. The surface composition is affected by substrate miscut angle, and although phase separation cannot be eliminated, its extent can be reduced by growing on substrates miscut toward (1-11)B.

Effect of lattice-mismatched growth on InAs/AlSb resonant-tunneling diodes

Nominally identical InAs/AlSb resonant-tunneling diodes are fabricated on InAs and GaAs substrates to ascertain the effect of dislocations on the resonant-tunneling process. Although the diode on the GaAs substrate had a much higher dislocation density, as evidenced by X-ray diffraction measurements, it displayed only a small decrease in peak-to-valley current ratio. >

Self-organized vertical superlattices in epitaxial GaInAsSb

Self-organized superlattices are observed in GaInAsSb epilayers grown nominally lattice matched to vicinal GaSb substrates. The natural superlattice (NSL) is detected at the onset of growth and is inclined by an additional 4° with respect to the (001) terrace of the vicinal GaSb substrate. This tilted NSL intersects the surface of the epilayer, and the NSL period is geometrically correlated with the periodicity of surface undulations. While the underlying driving force for this phase separation arises from solution thermodyamics, the mechanism for the self-organized microstructure is related to local strain associated with surface undulations. By using a template with surface undulations, the tilted NSL can be induced in layers with alloy compositions that normally do not exhibit this self-organized microstructure under typical growth conditions.

Self-Organized Superlattices in GaInAsSb Grown on Vicinal Substrates

Self-organized superlattices are observed in GaInAsSb epilayers grown lattice matched to vicinal GaSb substrates. The natural superlattice (NSL) is oriented at a slight angle of about 4{sup o} with respect to the vicinal (001) GaSb substrate. This vertical composition modulation is detected at the onset of growth. Layers in the NSL are continuous over the lateral extent of the substrate. Furthermore, the NSL persists throughout several microns of deposition. The NSLs have a period ranging from 10 to 30 nm, which is dependent on deposition temperature and GaInAsSb alloy composition. While the principle driving force for this type of phase separation is chemical, the mechanism for the self-organized microstructure is related to local strains associated with surface undulations. By using a substrate with surface undulations, the tilted NSL can be induced in layers with alloy compositions that normally do not exhibit this self-organized microstructure under typical growth conditions. These results underscore the complex interactions between compositional and morphological perturbations.