Arvind Baliga

High-performance 770-nm AlGaAs-GaAsP tensile-strained quantum-well laser diodes

Experimental results on tensile-strained Al/sub 0.3/Ga/sub 0.66/As-GaAs/sub 0.78/P/sub 0.22/ separate-confinement-heterostructure single-quantum-well (SCH-SQW) laser diodes are reported. Threshold current density as low as 260 A/cm/sup 2/ for broad-stripe lasers with a cavity length of 1500 /spl mu/m has been observed. Broad-stripe devices have operated cw at room temperature with output power as high as 620 mW/facet. Ridge-waveguide lasers have exhibited cw threshold currents as low as 13.5 mA and output power of 90 mW. The output of the tensile-strained lasers is TM polarized.<<ETX>>

Dependence of polarization mode and threshold current on tensile strain in AlGaAsGaAsP quantum well lasers

Abstract Trends in threshold current and other device characteristics were measured in a series of tensile-strained Al 0.34 Ga 0.66 As GaAs 1−y P y separate confinement heterostructure single quantum-well (SCH-SQW) lasers. Tensile strain in the wells was varied between 0 and 1% by changing the phosphorous composition from 0 to 30%. For short (300 μm) cavity length, devices with 15% P demonstrated the lowest threshold current density of 330 A/cm2. For longer cavities, the threshold was lowest (195 A/cm2) for nearly lattice-matched devices, and increased slightly with higher P compositions up to 18% (222 A/cm2). The output of the tensile-strained lasers is transverse magnetic (TM) polarized for devices with ≥9% P in the well.

Tensile strain and threshold currents in GaAsP-AlGaAs single-quantum-well lasers

The effects of tensile strain on threshold current in GaAsP-AlGaAs quantum well lasers are studied theoretically and experimentally. A comprehensive model for the light-current characteristics of separate-confinement strained-layer lasers, which is based on a six-band Luttinger-Kohn valence dispersion model, is first developed. Theoretical and experimental results for broad stripe single-well laser diodes with a constant well width of 115 /spl Aring/ are then presented. Experimentally observed variations in threshold currents and TE/TM polarization switching are accurately described by the model for phosphorus compositions in the quantum-well ranging from 0 to 0.30 and cavity lengths ranging from 300 to 1500 /spl mu/m. Constant-gain contours generated from the theoretical model are shown to provide a simple and powerful guide to various regimes of operation. Our studies show that tensile-strain-related effects lower threshold currents in GaAsP-AlGaAs only in the high gain (short cavity) regime, and suggest more generally that the threshold advantages offered by tensile strain are conditional.

POLARIZED EXCITATION LUMINESCENCE OF SEMICONDUCTOR QUANTUM-WELLS

A new photoluminescence excitation (PLE) spectroscopy technique for the characterization of quantum‐well structures is described. The technique, which requires a standard PLE setup modified to allow for variable control of the excitation beam polarization, provides an unusually simple and unambiguous means for distinguishing between electron‐to‐light‐hole (e‐lh) and electron‐to‐heavy‐hole (e‐hh) near‐gap excitonic optical transitions. Transition types are identified by inspection of the quotient of two PLE spectra obtained using orthogonal incident beam polarizations. In this letter we describe the experimental setup and procedure for this technique, discuss the theoretical foundation for our interpretation scheme, and present experimental results for tensile‐strained GaAsyP1−y‐AlxGa1−xAs and lattice‐matched GaAs‐AlxGa1−xAs quantum‐well structures.

Tensile‐strained barrier GaAsP/GaAs single quantum‐well lasers

Device characteristics of tensile‐strained barrier AlGaAs/GaAs1−yPy/GaAs separate‐confinement heterostructure single‐quantum well (SCH‐SQW) broad area lasers are reported. Three phosphorous compositions of 9%, 15%, and 22% were used for the tensile‐strained GaAs1−yPy barrier. For a 1000 μm long cavity device with a wellwidth of 113 A and 15% P in the barrier, the threshold current density was as low as 233 A/cm2. The threshold current densities are slightly higher than those observed on similar device structures with tensile‐strained GaAsP wells and unstrained GaAs wells with AlGaAs barriers.

Valance band offsets in strained GaAs 1−x P x /GaAs heterojunctions

Valence band offsets at [100]-oriented heterojunctions between tensile-strained GaASj1−xPx and unstrained GaAs are studied experimentally and theoretically. Light-hole (LH) and heavy-hole (HH) offsets are first extracted from the well-width dependence of valence subband splittings observed in luminescence spectra of tensile-strained GaAs1−xPx/GaAs quantum wells of various compositions (x = 0.06,0.09, and 0.19). This data is then combined with results from two other laboratories, yielding a set of 30 independent experimental offset values for junctions with compositions throughout the range 0.06≤x ≤0.32. The data are found to be highly consistent, with linear fits δELH = −140x (meV) and δEHH= −401x (meV) describing the measured offsets to within less than 5 meV on average. Experimental results are then compared with theoretical predictions for the GaAs1−x Px/GaAs system obtained from a tight-binding model for strained heterojunctions. Predictions from the tight-binding calculations are found to lie within experimental scatter for the LH offsets, which define the valence band edge in these heterostructures, while magnitudes of the tight-binding HH offsets exceed measured values by ~20% on average.

Identification of band-edge optical transition types in tensile strained quantum wells

A new characterization technique based on photoluminescence excitation spectroscopy (PLE) which allows simple and direct identification of band-edge transition types in quantum wells is developed. A ratio curve is generated by pointwise division of one PLE spectrum by a second PLE spectrum. The two PLE spectra are obtained using orthogonal polarizations of the excitation beam, which is incident on the sample surface at an oblique angle, and the transition types near the band edge are identified by simple visual inspection of the ratio curve. The authors describe and assess the theoretical foundation for the ratio method, detail the experimental procedure, present PLE spectra and ratio curves for several quantum wells, and determine optimum experimental conditions and the physical origin of features in the ratio curve through investigation of the influence of several experimental parameters. >

InAsSb/InAlAsSb quantum-well diode lasers emitting beyond 3 um

The current status of InAsSb/InAlAsSb quantum-well (QW) lasers emitting between 3 and 4 micrometer is described. QW lasers grown on GaSb substrates, with emission wavelengths at approximately 3.9 micrometer, have operated pulsed up to 165 K. At 80 K, cw power of 30 mW/facet has been obtained. Ridge-waveguide lasers have operated cw up to 128 K. QW lasers grown on InAs have emission wavelengths between 3.2 and 3.55 micrometer. Broad- stripe lasers have operated pulsed up to 225 K and ridge-waveguide lasers have operated cw to 175 K. Theoretical analysis of the laser gain using a 6 by 6 k (DOT) p model to calculate the valence subband structure is reported.

Quantum well structures with almost-degenerate valence bands

A novel technique for bringing the light- and heavy-hole valence bands in a quantum well, (QW), into approximate degeneracy is described and demonstrated. It utilizes pseudomorphic tensile strain in the barriers generated by lattice mismatch between the barrier and the substrate material. An important consequence of this strain is that the splitting of the light- and heavy-hole valence band energies at the Brillouin zone center, due to the quantum confinement effect, is approximately cancelled. Unlike a similar result in systems with tensily strained wells, this degeneracy is not sensitive to the exact QW width (for QW widths greater than 5 nm) or the precise strain present in the layer. It is thus more amenable to the growth and fabrication of devices which should simultaneously exhibit the polarization isotropy of bulk structures and the enhanced performance of QWs. The technique is demonstrated by an optical investigation of GaAs/GaAs1 - yPy quantum wells grown on GaAs substrates by metalorganic chemical vapor deposition.

Single-mode bi-directional links for fiber-in-the-loop and optical networks

This paper is a review of the present status of an ARPA sponsored program to develop low cost serial links. In particular, this team has focused its efforts on the development of bi-directional links which offer the special feature of sending information in both directions over a single fiber in order to eliminate half the cost of the fiber hardware in the network. Furthermore, this program leverages on the portfolio of technologies provided by the team members to provide a demonstration of a new design for the modules and a new way of manufacturing the modules in order to substantially reduce cost. Team member AMP Incorporated provides manufacturing methods for silicon waferboard assembly, connectors and packaging, Digital Optics provides novel holograms for a bi-directional optic as well as high speed ASIC design, GTE Laboratories provides advice on network requirements and silicon waferboard technology invented at GTE, BroadBand Technologies provides customer input for module requirements for their Digital Flex network for fiber-in-the-loop, the University of Colorado provides modeling for the modules and software for computer integrated manufacturing, Lasertron provides coolerless semiconductor laser die suitable for passive alignment, and Write Laboratories provides oversight and advice on behalf of ARPA.