This research program applies photoluminescence (PL), photoluminescence excitation spectroscopy, time-resolved PL and PL imaging to the characterization of defects and impurities in bulk and epitaxial semiconductor materials; the composition, doping, thickness, interfaces, and uniformity of layered semiconductor heterostructures; rare earth-doped semiconducting glasses; and the effects of patterning and fabrication processing steps on the electronic and optical properties of photonic and optoelectronic devices.

This program investigates III-V semiconductor quantum-wire (QWR) structures grown by solid and gas source molecular beam epitaxy. Spontaneous strain-induced lateral layer ordering process which occurs during growth of short-period superlattice heterostructures will be used to prepare the QWR structures with the wire size of less than 100;aoA ;ts 100;aoA. The influence of the growth conditions, such as the growth temperature, the V/III flux ratio, and the magnitude of the strain on structural and optical properties of the QWR structures will be studied.

Multiple-layer heterostructure devices such as HEMTs and HBTs have shown great promise recently as devices for optoelectronic integrated circuit (OEIC) receiver applications. In this project, various heterojunction OEIC structures will be prepared, characterized, and optimized. The short wavelength (0.85 ;gmm) and long wavelength (1.3 ;sl 1.55 ;gmm) OEIC receivers based on HEMT and HBT technologies will be grown by solid source and gas source MBE, respectively, using either the highly developed AlGaAs/GaAs materials system or the advanced AlInAs/GaInAs/InP and GaInAsP/InP heterostructure materials systems.

It is still a challenge to integrate the GaAs-based light-emitting devices and the well-established Si-based transistor technology. It has been our continuous effort to improve the reliability, although it remains a key concern for fabricating monolithic GaAs laser structures on Si substrates. Hybrid integration provides a promising alternative to the monolithic approach. We are studying the availability of physically attaching the readily made light-emitting devices and/or photodetectors on the Si substrates by flip chip bonding technique. Environmental concern leads to the development of a silver-tin-based alloy containing no lead as the solder bump. Study is under way to realize optimum processing conditions.

This investigation focuses on exploring the relationship between strain and phase separation in III-V semiconductors with the intention of using phase separation to achieve nanostructure devices. In situ strain can be introduced by either strain-balanced short-period superlattice technique in InGaP/GaAs and InGaAs/InP systems or by an excess incorporation of As in AlGaAs grown at low temperatures. 1-D or 2-D compositional modulation have been observed. Work is underway to realize modulation in ternary alloys by applying ex situ strain through growth of lattice mismatched cap layers. Control of modulation is of great technological importance.

The objective of this project is to develop an OEIC photoreceiver integrating a wide-bandwidth multistage I nGaAs/InP HBT-based amplifier with an InGaAs(P) p-i-n [fj photodiode for application to 1.3 and 1.55 ;gmm fiber-optic communication networks. The device structures are grown by chemical beam epitaxy utilizing either C-doped or Be-doped base regions in the HBTs. The lower section of the HBT structure (base to subcollector) comprises the p ⁺ -i-n photodiode structure, thereby providing both electrical and optical device structures necessary for integration in a single growth.

The objective of this project is to develop the chemical beam epitaxy (CBE) growth technique for the deposition of high-quality GaAs and InGaP for high-speed heterojunction bipolar transistors (HBTs). Research under this project involves material characterization using a variety of techniques as well as electrical characterization of devices fabricated from the grown layers. The new sources including TEIn and TIBGa will be investigated as possible sources for the growth of high-quality InGaP. Additionally, the effects of growth parameters on the quality of interfaces will be investigated in order to produce hyper-abrupt composition as well as dopant changes.

This project entails the growth and material characterization of high-quality InGaAs(P) layers latticed matched to InP for the development of HBTs and PINs using two novel growth techniques, GSMBE and CBE. Initial work will concentrate on the development of HBTs with a C-doped base using CCl;i4 and CBr;i4. Carbon is the preferred dopant for the development of high-reliability HBTs because of its low diffusivity. PIN detectors will be integrated in order to develop OEICs for 1.3 and 1.55 ;gmm fiberoptic communication networks.

The goal of this project is the realization of high-speed circuits based on 100-GHz InGaP/GaAs:C heterojunction bipolar transistor (HBT) technology. The material will be grown by LP-MOCVD. Device structures implementing thin collector widths and graded base designs will be used to reach the 100-GHz goal. Self-aligned collector and base etches will also be used to minimize parasitic effects. A device model will be developed for designing and demonstrating high-speed A/D circuit components.

The primary thrust of this program is the growth and characterization of heteroepitaxial materials employing native oxide layers. A variety of optoelectronic structures are being grown by MOCVD including AlGaAs/GaAs, InAlP/GaAs, and InAlP/InGaP double heterostructures. Currently under investigation are the minority carrier lifetime in the active regions, the interface recombination velocity between the active and oxide regions, and the effect of various oxidation conditions upon interface abruptness and impurity distributions. The results of this research will enable further advances in VCSEL (laser), field-effect transistor (MOSFET), and other technologies utilizing native oxide layers.

Research under this project involves the investigation into alternative contacting structures for InGaP/GaAs HBTs. Currently, latticed-mismatched InGaAs layers are used to form low-resistance contacts to InGaP/GaAs HBTs. These contacts, however, have shown reliability problems from the strain induced by the lattice-mismatch. N+-GaAs and N+-InGaP grown using the CBE growth technique and SiBr;i4 as the n-type dopant source will be investigated as an alternative to InGaAs. Subsequently, the effect of the new contacting layers on device performance will be evaluated.

This program is aimed at developing InP/InGaAs heterojunction bipolar transistors that utilize heavily doped InP layers as low-resistance contacting layers. InP has improved thermal conductivity over InGaAs, the material used currently for HBT contacting layers. Under this project, a fully self-aligned, high-frequency fabrication process will be developed. High-frequency electrical characterization will be used to evaluate the effect of InP contacting layers on device performance.

L uminescence and Laser Studies in III-V Semiconductors
N. Holonyak, Jr., P. Evans, J. Wierer, D. Kellogg
National Science Foundation, ECS 82-00517
(In conjunction with the Department of Physics)

Heterojunctions in Al x Ga _{1- x} As-GaAs and related materials are being examined. Quantum size effects have been observed and have led to single and multiple active layer quantum-well diode light emitters and lasers. Stimulated emission, absorption, disorder, alloy clustering, carrier scattering, phonon processes, tunneling effects, and impurity diffusion in these structures are being studied. Impurity-induced disordering and Al-bearing native oxides are being studied and used to form stripe-geometry lasers and more complicated array structures. Quantum well lasers have been operated in an external grating cavity in an extended wavelength range. Newer forms of quantum-well lasers have been realized, including native-oxide-defined lasers and waveguides.

The fundamental properties of III-V heterostructures grown by vapor phase epitaxy are being studied. On quantum-well MOCVD AlGaAs-GaAs heterostructures, laser operation 400 meV above E g (GaAs) has been observed, the first cw 300 K laser operation has been achieved, laser operation on phonon-sidebands below the confined-particle states has been observed, and alloy disorder and clustering in quantum well heterostructures have been identified. Impurity-induced disordering of quantum-well hetero structures and Al-bearing native oxides, e.g., the native oxide of Al;yxGa;i1 -x As formed at 400° to 500°C with H;i2O + N;i2, are being examined via TEM and photoluminescence studies. This project is the first (1977) to realize p-n quantum-well lasers and to ``coin'' the name ``QW lasers.''