David L. Spears

GaN avalanche photodiodes grown by hydride vapor-phase epitaxy

Avalanche photodiodes have been demonstrated utilizing GaN grown by hydride vapor-phase epitaxy. Spatially uniform gain regions were achieved in devices fabricated on low-defect-density GaN layers that exhibit no microplasma behavior. A uniform multiplication gain up to 10 has been measured in the 320–360 nm wavelength range. The external quantum efficiency at unity gain is measured to be 35%. The electric field in the avalanche region has been determined from high-voltage C–V measurements to be ∼1.6 MV/cm at the onset of the multiplication gain. Electric fields as high as 4 MV/cm have been measured in these devices. Response times are found to be less than 5 μs, limited by the measurement system.

NH3 spectral line measurements on Earth and Jupiter using a 10 μm superheterodyne receiver

Measurements of absolute line positions and shapes of eleven absorption lines of NH3 were made using an infrared heterodyne spectrometer. Line profiles were obtained with 5 MHz resolution. NH3 line positions relative to 12C16O2 and 13C16O2 local oscillator laser line emission frequencies were determined to ±3 MHz. Using the obtained data a search list for NH3 spectral lines on Jupiter was generated. Observations of Jovian polar regions in search of NH3 aR(1,1) and sQ(2,1) auroral emission revealed possible strong nonthermal features. The line widths were consistent with low local kinetic temperatures. The relatively high line intensities, determined by local pumping mechanisms, corresponded to temperatures of several hundred Kelvins. Experimental details and implications of these measurements are discussed.

X-Ray Lithography: A Complementary Technique to Electron Beam Lithography

X-ray lithography provides a means of replicating, in a single large-area exposure, submicron linewidth patterns made by scanning electron beam lithography. The technique is complementary to existing electron beam technology, and provides a number of unique advantages: (i) it is simple and inexpensive; (ii) the penetrating character of x-rays makes it relatively insensitive to contamination; (iii) both positive and negative type resists can be used; and (iv) because of the absence of backscattering effects, both positive and negative type patterns can be made with equal facility. Exposure times of seven minutes have been achieved for 3 μ mask-sample gaps. This can be decreased to less than one minute by using a rotating anode, or by reducing the mask-sample gap. The most recent results in x-ray lithography are reported, including the fabrication of surface wave devices. The elements of a multiple-mask alignment system are described. This system should permit the rapid and automatic superposition on a subs...

HIGH-PERFORMANCE GAINASSB THERMOPHOTOVOLTAIC DEVICES WITH AN ALGAASSB WINDOW

A large increase in the quantum efficiency (QE) and open-circuit voltage Voc of GaInAsSb thermophotovoltaic (TPV) devices is obtained by the use of an AlGaAsSb window layer compared with devices without a window layer. The TPV structure, grown on GaSb substrates by organometallic vapor phase epitaxy or molecular beam epitaxy, consists of a 1-μm-thick n-GaInAsSb base layer, a 3-μm-thick p-GaInAsSb emitter layer, a 100-nm-thick AlGaAsSb window layer, and a 25-nm-thick GaSb contacting layer. The band-gap energy of the lattice-matched GaInAsSb is 0.53–0.55 eV. The peak internal QE of the TPV cells with the window is >90%, compared with less than 60% for those without the window. At a short-circuit current density of ∼1000 mA/cm2, Voc of ∼300 meV is obtained for cells with the window layer, compared with less than 220 meV without the window layer. These increases are attributed to a substantial decrease in the surface recombination velocity with the window layer. Based on a standard calculation, the electron d...

Planar HgCdTe quadrantal heterodyne arrays with GHz response at 10.6 μm

Abstract Planar quadrantal arrays of GHz response HgCdTe photodiodes for application in CO2 laser radar have been fabricated using ZnS as a diffusion mask for an indium-mercury diffusion. Quantum efficiencies over 50% have been obtained with a uniformity within the array of better than ± 3%. Electrical isolation between array elements was better than 35 dB out to 1.5 GHz, yet negligible optical dead space was found between the photodiodes.

Determination of absolute frequencies of 12C16O and 13C16O laser lines

The absolute frequencies of one 13C16O and seven 12C16O laser lines have been determined with an uncertainty of about ±5 MHz by measuring the beat frequencies between these CO lines and second harmonics of CO2 laser transitions generated in a CdGeAs2 frequency‐doubling crystal. Beat frequencies up to 11 GHz have been measured using a high‐speed HgCdTe detector.

HgCdTe varactor photodiode detection of cw CO2 laser beats beyond 60 GHz

The simultaneous operation of a HgCdTe photodiode as an optical heterodyne receiver and a varactor microwave harmonic generator and frequency downconverter has made possible the detection of CO2 laser beats to beyond 60 GHz.

Laser radar component technology

The development of the laser soon led to recognition of its potential in radar applications. As radar systems were designed and implemented, however, it became clear that major refinement of the laser sources was needed to meet specific radar transmitter requirements in terms of waveforms, spectral purity, stability, beam quality, and power. The sensitivity and bandwidth of receivers were also in some cases insufficient, and new enabling detector technologies had to be created. This article provides a framework for much of the component development by describing the wideband high-power carbon dioxide (CO/sub 2/) imaging radar system at the Firepond Laser Radar Research Facility. The paper also details the critical components developed for the Firepond CO/sub 2/ range-Doppler imaging laser radar transmitter as well as transmitters for short-range CO/sub 2/ radars.

Anomalous noise behavior in wide‐bandwidth photodiodes in heterodyne and background‐limited operation

When the frequency response of a semiconductor photodiode is limited by carrier diffusion to the junction, anomalous noise behavior can result. Although the signal current decreases, the noise current remains constant since the junction shot noise is only a function of the dc current flow. Under these conditions, there is a marked decrease in high‐frequency sensitivity and the noise spectrum is not a true measure of the frequency response.

Lattice-matched epitaxial GaInAsSb/GaSb thermophotovoltaic devices

The materials development of Ga{sub 1{minus}x}In{sub x}As{sub y}Sb{sub 1{minus}y} alloys for lattice-matched thermophotovoltaic (TPV) devices is reported. Epilayers with cutoff wavelength 2--2.4 {micro}m at room temperature and lattice-matched to GaSb substrates were grown by both low-pressure organometallic vapor phase epitaxy and molecular beam epitaxy. These layers exhibit high optical and structural quality. For demonstrating lattice-matched thermophotovoltaic devices, p- and n-type doping studies were performed. Several TPV device structures were investigated, with variations in the base/emitter thicknesses and the incorporation of a high bandgap GaSb or AlGaAsSb window layer. Significant improvement in the external quantum efficiency is observed for devices with an AlGaAsSb window layer compared to those without one.