W. Q. Li

Photoluminescence and electro‐optic properties of small (25–35 nm diameter) quantum boxes

The luminescence and electro‐optic properties of buried 25–35 nm quantum boxes have been measured. The quantum boxes were defined by a combination of molecular beam epitaxial growth and regrowth, electron beam lithography, and dry etching. The photoluminescence from 35 nm boxes shows a blue shift of ∼15 meV compared to the bulk luminescence and an enhancement, taking into account the fill factor. An enhanced effective linear electro‐optic coefficient, rl, is observed for the quantum boxes.

Improved performance of strained InGaAs/GaAs photodiodes grown on patterned GaAs substrates by molecular beam epitaxy

We have experimentally studied the photoresponse characteristics of strained InxGa1−xAs (0.05≤x≤0.20) p‐i‐n photodiodes grown in small holes etched in GaAs substrates where the thickness of the i region exceeds the critical thickness. The diodes exhibit enhanced quantum efficiency and no degradation of the temporal response compared with devices grown on planar substrates, indicating that reduced area growth may be a useful technique for making optoelectronic devices with strained‐layer structures.

Molecular beam epitaxial GaAs/AlGaAs heterojunction bipolar transistors on

The authors have investigated the characteristics and reproducibility of Si-doped p-type

Breakdown voltage improvement in strained InGaAlAs/GaAs FETs

GaAs MESFETs with a surface layer of pseudomorphic InGaAlAs have been fabricated. The compressive strain and wide bandgap in the InGaAlAs layer reduce the impact ionization rate in this layer and improve the breakdown voltage of the device. A 1 mu m*75 mu m gate device with the pseudomorphic surface layer showed an improvement in gate-to-drain breakdown of over 55% and an improvement in channel breakdown of 50% as compared to a similar device without the pseudomorphic layer. Both devices had a peak transconductance of about 190 mS/mm and a saturation current of about 265 mA/mm.<<ETX>>

Transparent emitter contact HBT's for direct optical injection locking of oscillators

Direct optical injection locking and tuning of high frequency oscillators made with GaAs-AlGaAs heterojunction bipolar transistors (HBTs) have been investigated. A new HBT technology using transparent indium-tin-oxide (ITO) emitter contacts for convenient optical access has been developed. Optical injection locking and tuning experiments have been performed on 6 GHz HBT oscillators. A locking range up to 2.5 MHz, and a tuning range up to 25 MHz have been measured with the injection of optical RF power at 30 dB below the oscillator power level.<<ETX>>

High-speed Al/sub 0.2/Ga/sub 0.8/As/GaAs multi-quantum-well phototransistors with tunable spectral response

The authors have measured the high-frequency characteristics and temporal response of a GaAs/AlGaAs heterojunction phototransistor with a GaAs/Al/sub 0.2/Ga/sub 0.8 /multi-quantum-well collector. The quantum wells offer tunability of the photoresponse (10 nm for a bias change of 4 V) and a negative differential resistance in the photocurrent-voltage characteristics. Measured values of f/sub T/ and f/sub max/ are 20 and 6 GHz, respectively. The temporal response to short-pulse optical excitation is characterized by a linewidth of 46 ps. Such devices are attractive candidates for making optically induced oscillators.<<ETX>>

Multiple-Quantum-Well-Based Spatial Light Modulators For Electro-Optical Implementation Of Neural Metworks

There has been a resurgence of interest in artificial neural networks. However, dedicated hardware implementation is still a bottleneck for the development of significant applications. The neurons and the synapses are the two key components in a neural network. Synapses are used to provide massive interconnections between different neurons. The computational capability of a neural network depends on its storage capacity and speed. The storage capacity is related to the number of synapses (interconnections) between individual neurons. The information is stored in the strength and the distributed pattern of the interconnections. The 2-D nature of optics with inherent capability to provide large connectivity is particularly attractive for implementing neural network models.

Demonstration of dual gain mechanism in an InGaAs/InAlAs superlattice photodiode

A high‐gain photodiode in which the internal gain can result from either potential barrier lowering or mass filtering action, depending on device geometry and bias conditions, is proposed and demonstrated. The photodiode structure is similar to a modulated barrier diode and uses In0.53Ga0.47As and InGaAs/InAlAs superlattice absorption regions. The superlattice helps to reduce the dark current and aids in mass filtering. The devices reported here were made with multilayered InP‐based materials grown by molecular beam epitaxy and exhibit responsivity as high as 1000 A/W.

Direct optical injection locking of InP-based MODFET and GaAs-based HBT oscillators

We have investigated direct optical injection locking and tuning of high frequency oscillators made with GaAs/AlGaAs Heterojunction Bipolar Transistors (HBTs) and 0.25 /spl mu/m gate InGaAs/InAlAs Modulation Doped Field Effect Transistors (MODFETs). We have developed an HBT technology using transparent indium-tin-oxide (ITO) emitter contacts for convenient optical access. Optical injection locking and tuning experiments have been performed on 6 GHz microwave oscillators made with the HBTs. Locking ranges up to 2.5 MHz and tuning ranges up to 25 MHz have been measured with the injection of the optical RF power at 30 dB below the oscillator power level. Similarly, direct subharmonic optical injection locking at 10 and 19 GHz has been achieved with the MMIC InGaAs/InAlAs MODFET oscillators.<<ETX>>

Integrated Quantum Well Bipolar Devices for Tunable Detection and Optical Logic Applications

The properties of exciton-based quantum well devices suitable for low-power photonic switching and logic applications and wavelength selective detection will be described. The basic device is a heterojunction bipolar transistor with MQW collector region monolithically integrated with a p-i(MQW)-n modulator, realized by single-step epitaxy. The integrated controller-modulator device, by virtue of its unique photocurrent-voltage characteristics, exhibits integrating-thresholding properties. These properties have been exploited to demonstrate cascadability, fan-out, optoelectronic amplification, special logic functions, programmable optical/electronic memory capabilities, and wavelength selective detection with GaAs/AlGaAs heterostructure devices.