P. Cooke

Multi‐quantum‐well zero‐gap directional coupler with disordered branching waveguides

A nonlinear switch formed by the integration of an overmoded multi‐quantum‐well (MQW) section with disordered input and output branching waveguides is presented. The area‐selective disordering of GaAs/AlGaAs MQWs is achieved by diffusion of group III vacancies generated by etching of the surface oxide. The absorption edge of the disordered MQW regions was shifted by 71 nm and the disordered ridge waveguides had a loss figure of 8 dB/cm. Time‐resolved optical pump‐probe measurements were performed on an integrated switch that had more than a 5:1 power split between the two output ports. The measured signal recovery of the switch had a time constant of 110 ps.

INTERMEDIATE TEMPERATURE MOLECULAR BEAM-EPITAXY GROWTH FOR DESIGN OF LARGE-AREA METAL-SEMICONDUCTOR-METAL PHOTODETECTORS

Large‐area metal‐semiconductor‐metal (MSM) photodetectors are fabricated on molecular beam epitaxy (MBE) grown GaAs material at growth temperatures ranging from 250 to 500 °C. It is shown that materials grown at intermediate temperatures are a suitable choice for large‐area, high photocurrent detectors. Particularly, MSM devices made from material grown at around 350 °C have a dark current of the same magnitude as those grown at lower temperatures while having a substantially larger photocurrent. Higher low‐field mobility at intermediate temperatures should give these devices speed advantage as well. A change of close to 4 orders of magnitude in dark current and more than 2 orders of magnitude in light response is observed for this temperature range.

Current transport in as‐grown and annealed intermediate temperature molecular beam epitaxy grown GaAs

Molecular beam epitaxy (MBE) GaAs grown in the intermediate temperature range of about 400 °C may provide combination of low lifetime, high resistivity, and high mobility. We compare current conduction in unannealed and annealed material grown at 400 °C by fabricating photodetectors on substrates grown between temperature ranges of 250–500 °C. The unannealed version of the device grown at 400 °C shows substantial difference of conduction properties in dark and under light. It is shown that while at low biases the unannealed material may be semi‐insulating, at high biases more current is conducted than in annealed material. We attribute this to the effect of intergap states on current conduction and suggest that defect state assisted tunneling is the dominant current transport mechanism in these ranges. Quenching of response by light suggests that occupancy of traps can eliminate their role in current conduction.

Simultaneous blue- and red-shift of light-hole and heavy-hole band in a novel variable-strain quantum well heterostructure

We have designed and studied a new type of strained semiconductor quantum well structure, variable‐strain quantum well. The strain within the quantum well is graded from compressive to tensile in order to obtain mutually opposing slopes for the heavy‐ and light‐hole band edges, causing the heavy and light holes to experience opposite fields created by the same strain. A unique bias controlled crossover with a simultaneous red and blue quantum confined Stark shift for the heavy‐ and light‐hole transitions, respectively, has been observed by electroreflectance spectra. This results in a bias controlled change of the polarization properties and the transition energies suitable for polarization controllable photonic devices.

Broad-bandwidth, high-responsivity intermediate growth temperature GaAs MSM photodetectors

In this letter, we report the design, fabrication, parametric testing, and analysis of a intermediate growth temperature (IGT) GaAs MSM photodetectors. The broad-area, ultrafast photodetector displays linear and nonlinear photocurrent generation characteristics as a function of applied bias and optical power. The photocurrent/dark current ratio 7/spl times/10/sup 3/ is significantly larger than normal growth temperature or low growth temperature 200/spl deg/C based GaAs MSMs. The optical responsivity of 130 mV/pJ Is the highest ever reported for similar ultrafast MSM photodetectors. This device is uniquely suited for high-speed, high-photocurrent applications, such as optoelectronic trigger sources for phased-array antenna systems.

Lateral-field-effect optoelectronic waveguide devices employing multiquantum wells

Lateral carrier transport has been used to enhance the recovery time of bandgap resonant nonlinear transmission changes in multiple quantum well waveguide structures. Recovery times on the order of 90 ps have been measured in our samples. Such technique is applicable to all-optical and optoelectronic integrated optic switches.

High-performance GaAs metal-semiconductor-metal photodetectors grown at intermediate temperatures

Planar metal-semiconductor-metal (MSM) devices fabricated on gallium arsenide (GaAs) are promising candidates for use as photodetectors in coherent optical communications and millimeter-wave phased-array applications. Their primary features are broad bandwidth, large responsivity, high power-handling capability, and compatibility with monolithic optoelectronic integrated circuits. We have characterized the performance of an interdigitated GaAs MSM photodetector grown by molecular beam epitaxy at 350 degree(s)C using a fast sampling technique in the time domain. A key factor for undoped GaAs material grown at this temperature is the optimal combination of both low dark current and high photocurrent. Experimental measurements are made of the temporal response of the MSM detector to optical impulses generated by a mode-locked titanium-sapphire (Ti:Al2O3) laser. Speed and responsivity are characterized over a range of optical powers and DC bias voltages. Results demonstrate that this device can switch up to 69% of the applied DC bias voltage under high optical pulsed power. Results also indicate responsivities exceeding 80 mV/pJ and bandwidths approaching 20 GHz. This high-efficiency, broad-bandwidth photodetector may find critical applications in the optical production of millimeter-wave signals by frequency conversion (mixing) and harmonic generation.

Deep defects in intermediate temperature molecular beam epitaxy grown GaAs and their role in current transport

Current transport in molecular beam epitaxy (MBE) GaAs grown at low and intermediate growth temperatures is strongly affected by defects. A model is developed here that shows that tunneling assisted by deep defect states can dominate, at some bias ranges, current transport in Schottky contacts to un-annealed GaAs material grown at the intermediate temperature range of about 400/spl deg/C. It is suggested that reduction of these defects by thermal annealing can explain lower current conduction at high biases in the annealed device as well as higher current conduction at low biases due to higher lifetime. Quenching of current by light in the as-grown material is shown and can also be explained based on occupancy of trap states. Identification of this mechanism can lead to its utilization in making ohmic contacts, or its elimination by growing tunneling barrier layers. This material finds wide-spread use in microwave and optical applications.

Metal-semiconductor-metal photodetectors on intermediate temperature MBE grown GaAs for lightwave/millimeter-wave applications

Intermediate growth temperature (IGT) GaAs Metal-Semiconductor-Metal (MSM) photodetectors enable an optimal combination of large dynamic range and speed. In addition these devices are suitable for monolithic integration. The static and temporal response of GaAs MSMs grown by Molecular Beam Epitaxy (MBE) at 350/spl deg/C has been measured.

Lateral carrier sweep-out in multi-quantum well optoelectronic devices

The frequency response of multi-quantum well (MQW) optoelectronic devices is governed by the rate of carrier injection and removal from the active regions. In a conventional p-i-n structure vertical rate limiting carrier transport mechanism is primarily due to thermionic emission of holes and electrons from the quantum wells. However, in waveguide structures the p and n contacts can be in-diffused on either side of the waveguide. With an applied reverse electrical bias, lateral transport of the carriers by diffusion and drifting is accomplished without the need to overcome any energy potential barrier. Since the waveguide geometry can be very small (/spl sim/1 /spl mu/m), the time taken for the carriers to traverse that region is inherently fast and hence high speed devices can readily be achieved.