Kezhong Hu

High contrast ratio asymmetric Fabry–Perot reflection light modulator based on GaAs/InGaAs multiple quantum wells

We report the realization of both high contrast ratio (66:1) and dynamic range (30%) at room temperature in a strained GaAs/InGaAs(100) multiple quantum well based asymmetric Fabry–Perot reflection modulator. The p‐i‐n configuration modulator also acts as a photodetector and exhibits a high quantum efficiency (∼80%).

Inverted cavity GaAs/InGaAs asymmetric Fabry-Perot reflection modulator

We report the realization of an inverted cavity (through‐substrate) reflection modulator based on an asymmetric Fabry–Perot configuration that utilizes the transparency of the GaAs substrate for operation at wavelengths appropriate for strained‐layer GaAs/InGaAs multiple quantum wells. At room temperature, a contrast ratio of 12:1 is realized along with a dynamic range of 20% at an operating wavelength of 9565 A.

High‐contrast optically bistable optoelectronic switch based on InGaAs/GaAs (100) asymmetric Fabry–Perot modulator, detector, and resonant tunneling diode

The realization at room temperature of a high contrast ratio (20:1) and an on‐state reflectivity of 46.5% in an optically bistable switch involving strained InGaAs/GaAs (100) multiple‐quantum‐well‐based asymmetric Fabry–Perot reflection modulator, detector, and InGaAs/AlAs‐based resonant tunneling diode and an Si field‐effect transistor is reported.

Picosecond time‐resolved measurements of electroabsorption in an InGaAs/GaAs multiple quantum well p‐i‐n modulator

We report the first picosecond time‐resolved measurements of electroabsorption in a strained InGaAs/GaAs multiple quantum well p‐i‐n structure as a function of applied bias at various optical excitation densities. The data above 4 V are explained by a model which assumes a rise time dominated by a single carrier sweep‐out time and a decay dominated by lateral diffusion. A bias independent electro‐absorption rise time of 10±2 ps is obtained and appears to be limited by the transit time of carriers across the structure. The decay time is also ≊10 ps. Below 4 V the decay time increased to 80 ps as the bias was reduced to 0 V. This increase may be related to the change in depletion length with bias in the intrinsic region.

Sweep‐out times of electrons and holes in an InGaAs/GaAs multiple quantum well modulator

The switch‐on and switch‐off times of a p‐i‐n diode containing In0.13Ga0.87As/GaAs multiple quantum wells in the i region have been characterized by a time‐resolved pump/probe technique. We observe bias‐insensitive switch‐on times and dramatically increasing switch‐off times with decreasing bias. We use a simple model invoking the cross‐well motion of holes as well as electrons to explain the experimental result. The effective drift velocities of both electrons and holes across the multiple quantum wells at different bias voltages are deduced from this model.

Realization of high mobilities at ultralow electron density in GaAs‐Al0.3Ga0.7As inverted heterojunctions

We report the first realization of extremely low free‐carrier concentration (≤4×1010 cm−2) and high LN2 electron mobilities (∼1.8×105 cm2/V s) in the dark in inverted Al0.3Ga0.7As /GaAs(100) modulation‐doped structures. The obtained results are all the more remarkable since the structures do not involve any superlattice or graded barrier, δ doping, or large spacer layer thicknesses. We attribute the observed properties to the high quality of the ambient in the molecular beam epitaxy system and the use of optimized growth kinetics and procedure as determined from reflection high‐energy electron diffraction intensity behavior.

EXTERNALLY DEPOSITED PHASE-COMPENSATING DIELECTRIC MIRRORS FOR ASYMMETRIC FABRY-PEROT CAVITY TUNING

The wavelength dispersion of the phase shift on reflection that is inherent in dielectric Bragg mirrors can be used to phase compensate resonant‐cavity‐based devices such as multiple quantum well asymmetric Fabry–Perot spatial light modulators and vertical cavity surface‐emitting lasers. We demonstrate the post‐growth ability to accurately fine‐tune the location of the Fabry–Perot minima in a resonant cavity by employing either a normal or inverted dielectric mirror configuration. The dielectric multilayer mirrors are composed of alternating quarter‐wave layers of MgF2 and Sb2S3, and exhibit broadband reflectivities.

High contrast ratio self-electro-optic effect devices based on inverted InGaAs/GaAs asymmetric Fabry-Perot modulator

We demonstrate a new class of ‘‘normally off’’ high contrast ratio asymmetric Fabry–Perot (ASFP) reflection modulators based on a blue‐shift of the Fabry–Perot mode under bias. The negative differential resistance (NDR) exhibited by the photocurrent‐bias behavior was exploited to implement self‐electro‐optic devices (SEED) using as the load (a) a photodiode (D‐SEED), and (b) a phototransistor (T‐SEED). In the D‐SEED scheme, a contrast ratio of ∼50:1 with ∼20% throughput was realized. The T‐SEED configuration was found to offer nearly as high contrast ratio but with a ∼200:1 gain from the control light beam. Moreover, the ASFP modulators used belong to the inverted configuration which offers considerable convenience in device integration and optical interconnections.

Post‐growth tuning of inverted cavity InGaAs/AlGaAs spatial light modulators using phase compensating dielectric mirrors

A novel method is demonstrated for the correction of cavity thickness deviations imposed by technological limitations in the growth process of a resonant cavity spatial light modulator. This method is based on cavity phase compensation through the use of an externally‐deposited dielectric Bragg mirror and provides an effective means of optimizing the device characteristics. In particular, such mirrors can significantly relax otherwise stringent epitaxial growth requirements in the fabrication of hybrid silicon/compound‐semiconductor spatial light modulators incorporating Fabry–Perot cavities. We further demonstrate deposition of a conductive, index‐tunable indium tin oxide (ITO) antireflection coating that is designed to maximize the contrast ratio and throughput of the inverted‐cavity modulator configuration.

Growth of high quality strained AlxGa1−xAs/In0.26Ga0.74As/AlzGa1−zAs quantum wells and the effect of silicon nitride encapsulation and rapid thermal annealing

We report on the realization of high quality single quantum wells (SQWs) in the highly strained AlxGa1−xAs/In0.26Ga0.74As/AlzGa1−zAs system with 0≤(x, z)≤1.0. Photoluminescence (PL) linewidths of 5.5±0.4 meV for 0.30≤(x and z)≤0.70 have been achieved. Use of both alloys and short period multiple layer structures as the well and/or barrier layers has been examined. The silicon nitride encapsulation as well as rapid thermal annealing (RTA) induced changes in the PL properties of the as‐grown SQW structures have been examined. Deposition of the nitride is found to induce a blue shift in the exciton peak. RTA induces a further blue shift, though not as large as that induced by RTA of unencapsulated (i.e., as‐grown) structures. The RTA induced changes indicate interdiffusion of the group III atoms at the GaAs/InGaAs and InGaAs/AlGaAs interfaces.