W. F. Tseng

An interdigitated stacked p-i-n multiple-quantum-well modulator

We demonstrate low-voltage operation of a strained InGaAs-GaAs interdigitated hetero n-i-p-i modulator (or stacked SEED) that is grown and fabricated using a shadow-mask growth technique for making the metal contacts to the n- and p-layers separately. An absorption change of 6/spl times/10/sup 3/ cm/sup -1/ with an applied bias as low as /spl sim/1 V is observed in an unoptimized structure. Optical switching of the unbiased structure is also demonstrated.

Scaling of the nonlinear optical cross sections of GaAs-AlGaAs multiple quantum-well hetero n-i-p-i's

We study the dependence of the Stark shift optical nonlinearity of GaAs-AlGaAs multiple quantum-well hetero n-i-p-is on the number of quantum wells per intrinsic region in otherwise identical hetero n-i-p-is. We determine that /spl sigma//sub eh/, the nonlinear absorption cross section, is proportional to the number of quantum wells per intrinsic region. A study of the fluence dependence of /spl sigma//sub eh/ shows that the saturation carrier density is inversely proportional to the number of wells per intrinsic region. We find that the turn-on time of the nonlinear absorption change in our samples is independent of the number of quantum wells per intrinsic region. All of these results are consistent with the absence of retrapping of photogenerated carriers. >

Defects in III-V materials and the accommodation of strain in layered semiconductors

High resolution monochromatic synchrotron-radiation diffraction images of five, high quality epitaxial heterojunctions on silicon, gallium arsenide, and indium phosphide substrates display several forms of accommodation to lattice mismatch. From the images, we deduce a coherent set of factors for the loss of crystalline order in layered semiconducting crystals. Lattice mismatch is demonstrated in each of the systems by warping after layer deposition. Nevertheless, local lattice orientation is maintained across each layer interface. In two of the systems, one severely mismatched while the other is not, no arrays of dislocations appear. Sets of mixed linear lattice mismatch dislocations, consistent with identification as 60° dislocations, are found in two of the other systems with intermediate degrees of mismatch. A set of pure edge dislocations penetrating all layers is found in a system with a grid structure. These observations indicate that the formation of extensive arrays of dislocations during uniform one micrometer layer deposition depends not only on the extent of lattice mismatch and layer thickness but also on the degree of crystalline order of the substrate. Establishment of a nonpseudomorphic layer mismatched with the substrate by several tenths of a percent is an important factor, as previously determined. However, localized absence of crystalline order, e.g. in the form of scratches or dislocations in the substrate, appears also to be required for the formation of arrays of interface mismatch dislocations. Where these criteria are not fulfilled, the formation of dislocations in uniform layered systems is inhibited. Localized residual stress can initiate dislocation formation even where it would not appear in uniform layers. The images show also that crystalline disorder in state-of-the-art indium phosphide differs markedly from that in comparable gallium arsenide. Understanding of crystalline order in both monolithic materials is extended by this work.

Controlled Interface Roughness in GaAs/AlAs Superlattices

We report the results of our study of controlled interface roughness in low-order GaAs/AlAs superlattices. Samples were prepared using either the interrupted growth or the migration-enhanced epitaxy (MEE) technique. The samples were prepared with m atomic planes of GaAs and m atomic planes of AlAs ( m × m ) per modulation wavelength and repeated p times. For this study, m = 1 or 3. The samples were studied using X-ray diffraction. The interrupted growth samples both showed a split in one diffraction line indicating layers were not of integral order while the MEE samples showed no splitting, indicating integral order layers.