J. M. Gaines

Blue‐green injection lasers containing pseudomorphic Zn1−xMgxSySe1−y cladding layers and operating up to 394 K

We describe the performance of blue‐green injection lasers containing Zn1−xMgxSySe1−y cladding layers. The devices have yielded the lowest reported threshold current densities (500 A/cm2) and the highest reported pulsed output powers (500 mW) at room temperature. Lasing has been observed at temperatures as high as 394 K. The room temperature and 85 K lasing wavelengths are 516 and 496 nm, respectively. The use of Zn1−xMgxSySe1−y, instead of ZnSzSe1−z, cladding layers provides a clear improvement in optical confinement, demonstrated by the widening of the far‐field pattern in the direction perpendicular to the layers. The lasers are separate‐confinement heterostructures with a ZnS0.06Se0.94 waveguiding region and a single Cd0.2Zn0.8Se strained quantum well. The entire structure is pseudomorphic with the GaAs substrate.

SCANNING TUNNELING MICROSCOPY COMPARISON OF GAAS(001) VICINAL SURFACES GROWN BY MOLECULAR BEAM EPITAXY

We report the first scanning tunneling microscope observations of molecular beam epitaxy grown GaAs(001) vicinal surfaces cut 2° towards (111)A and 2° towards (111)B. The A‐type step edges are found to be relatively straight, with 16 A kinks occurring typically every 100 A along the step. In contrast, the B‐type step edges are found to be very ragged. On both surfaces, the terrace widths varied considerably. The details of the two step structures are dominated by the structure of the (2×4) unit cell.

On the construction of an Fe3O4 based all-oxide spin-valve

Abstract The progress made in constructing an all-oxide spin valve based on the intrinsic, fully spin-polarized electron transport in Fe 3 O 4 is discussed. Two possible oxidic spacer layers, MgO and Mn 3 O 4 , have been investigated. Interlayer coupling studies indicate that MgO is the more suitable spacer layer of the two. Thus far a limited magnetoresistive effect ⩽0.4% is found in the all oxide, Fe 3 O 4 -based, spin-valves which we have made. Possible causes for this low magnetoresistive effect are discussed.

Structural properties of ZnSe films grown by migration enhanced epitaxy

We describe the structural properties of ZnSe grown on (001) GaAs by migration enhanced epitaxy, and show that the heterointerface plays a significant role in determining the structural properties of the ZnSe films. Films were grown with thicknesses ranging from 900 to 10 000 A. Transmission electron microscopy and high‐resolution x‐ray diffraction measurements show that the resulting structure is highly dependent on how growth is initiated. Nearly perfect films are obtained, for thicknesses up to about 2500 A [considerably thicker than the critical thickness for molecular beam epitaxy (MBE)‐grown films], when growth begins with an initial exposure of the GaAs substrate to Zn. However, if growth begins with an initial high‐temperature Se exposure, then stacking fault densities are greatly increased, and more rapid relaxation occurs. Comparison to MBE‐grown films shows greater defect densities and a faster rate of relaxation of the misfit strain for the MBE‐grown films.

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.

Effect of N doping on the structural properties of ZnSe epitaxial layers grown by molecular beam epitaxy

The structural properties of ZnSe doped with N, in the concentration range of 1×1018–2×1019 cm−3, were characterized by transmission electron microscopy, x‐ray diffraction, and Raman spectroscopy techniques. The relaxation of the lattice mismatch induced compressive strain between ZnSe and GaAs is less for N doped layers for a given ZnSe thickness. The smaller amount of strain relaxation with N doping results in layers that contain residual compressive strain up to thicknesses of at least 1.7 μm. In addition, the misfit dislocation array becomes a regular rectangular grid when N is incorporated in ZnSe layers. The ZnSe lattice constant, as measured by x‐ray diffraction, decreases as the N concentration increases. The reduction in lattice constant, however, is greater than can be explained by the shorter Zn‐N bond distance of model predictions. We attribute the excess lattice contraction to the generation of point defects accompanying N doping. The Raman spectra display a broadening of the linewidth as the...

MBE growth on vicinal GaAs(001) surfaces studied by scanning tunneling microscopy

We have imaged several different vicinal GaAs(001) surfaces, grown by molecular beam epitaxy (MBE), with the scanning tunneling microscope (STM). All the samples were grown under arsenic-rich conditions and had a (2 × 4)/c(2 × 8) surface reconstruction. The structure of the steps was found to be determined by the structure of the (2 × 4) unit cell. Atomic resolution images of the steps show that there is no step edge reconstruction and that the steps are built up from complete (2 × 4) unit cells. Vicinal GaAs(001) surfaces cut 2° towards (111)A and 2° towards (111)B and grown under step-flow growth conditions were compared. The A-type steps are relatively straight whereas the B-type steps are very ragged. Changing the angle of misorientation of the sample was not found to have a major effect on the step structure. A-type stepped surfaces grown under 2D nucleation growth conditions were found to be less regular than those grown under step-flow growth conditions. The results are discussed in terms of the growth mode of GaAs(001) and the prospects for growing sufficiently regular stepped surfaces for use in device structures such as quantum wires are considered.

ZnSe(100): The surface and the formation of Schottky barriers with Al and Au

A study of the ZnSe(100) surface and of its interfaces with Al and Au is presented. We find that the (2×1) reconstructed Se‐rich surface is terminated with a full monolayer of dimerized Se, whereas the C(2×2) reconstructed Zn‐rich surface corresponds to a half‐monolayer of nondimerized Zn atoms. These atomic configurations and corresponding surface electron affinities are consistent with the requirement of dangling bond saturation and fully accounted for by the electron counting rule. For the metal/ZnSe interfaces, we find that Au forms an abrupt junction, whereas Al reacts and forms Al2Se3, releasing Zn in the process. The stabilized Fermi level position is 2.1 eV above the valence band maximum for Al and 1.15 eV for Au, irrespective of doping type and in qualitative agreement with the Schottky limit for the barrier heights. The Au/p‐ZnSe Schottky barrier height can be reduced by 0.25 eV by introducing a 2–3 ML Se interlayer between the metal and the semiconductor.

Structural characterization of II‐VI separate confinement heterostructure lasers with Zn1−xMgxSySe1−y cladding layers

We have investigated the structural characteristics of II‐VI separate confinement heterostructure lasers grown on GaAs substrates and containing Zn1−xMgxSySe1−y quaternary cladding layers, ZnSe or ZnSySe1−y guiding layers and Zn1−zCdzSe quantum well active layers. The study was performed with a combination of transmission electron microscopy and high resolution x‐ray diffraction techniques. We found that the quaternary cladding layers remain pseudomorphic to the GaAs substrate although they can be lattice mismatched up to 0.1%. When the 0.5‐μm‐thick optical guiding layer contains ZnSe, there is partial relaxation of the laser structure by misfit dislocations at the lower cladding‐guiding layer interface and the threading dislocation density in the Zn1−zCdzSe quantum well active region is about 107 cm−2. However, when lattice matched (to GaAs)ZnSySe1−y is used as the guiding layer the entire laser structure is pseudomorphic and the threading dislocation density is <106 cm−2. The combination of low defect d...

Characterization of low defect density blue-green lasers

Abstract We have used a combination of techniques to characterize low defect density (≤ 10 5 cm −2 ) blue-green separate confinement heterostructure lasers. The limits of lattice mismatch between the substrate and quaternary cladding layers that result in a pseudomorphic laser structure were determined by X-ray diffraction and transmission electron microscopy to be ≤ 0.0015. The determination of defect density (stacking faults, threading dislocations, etc.) in the active layer was performed by optical imaging. Photoluminescence imaging is nicely suited for defect observation, because of the nonradiative transitions associated with the defects and can easily be performed over large areas. Propagation of defects during device operation (device degradation) was monitored in real time with optical imaging. The degradation was observed to start at grown-in defect sites (in the active layer) that emanate from 〈100〉 dark line defects. From the direct observation of device degradation, a mechanism involving the creation of new defects from nonradiative recombination at existing defects sites is proposed.