T. George

Visible luminescence from silicon wafers subjected to stain etches

Etching of Si in a variety of solutions is known to cause staining. These stain layers consist of porous material similar to that produced by anodic etching of Si in HF solutions. We have observed photoluminescence peaked in the red from stain‐etched Si wafers of different dopant types, concentrations, and orientations produced in solutions of HF:HNO3:H2O. Luminescence is also observed in stain films produced in solutions of NaNO2 in HF, but not in stain films produced in solutions of CrO3 in HF. The luminescence spectra are similar to those reported recently for porous Si films produced by anodic etching in HF solutions. However, stain films are much easier to produce, requiring no special equipment.

Photonic crystal enhanced narrow-band infrared emitters

We have experimentally and theoretically developed a unique thermally stimulated midinfrared source that emits radiation within a narrow range of wavelengths (δλ/λ⩽0.2). The emission wavelengths are defined by the periodicity of a metal coated silicon–air photonic crystal etched into the emitter surface. The lattice of the holes in the metal mediate the coupling of light into discrete surface plasmon states. This yields surfaces with spectrally nonuniform infrared reflection properties where over much of the IR 90+% of photons are reflected yet, in a narrow spectral region, 90% absorption is observed. Transfer matrix calculations simulate well the position and strength of the absorption features. This technology will afford tunable infrared emitters with high power in a narrow spectral band that are critical for sensing, spectroscopy, and thermophotovoltaic applications.

Electronic structure of light‐emitting porous Si

Characterization of light‐emitting porous Si films with x‐ray photoelectron spectroscopy is reported. Only traces of O are detected on HF‐etched samples, in contradiction to an earlier report that oxides are a significant component of porous Si. Si 2p and valence‐band measurements demonstrate that the near‐surface region of high porosity films which exhibit visible luminescence consists of amorphous Si.

Microstructural investigations of light-emitting porous Si layers

The structural and morphological characteristics of visible‐light‐emitting porous Si layers produced by anodic and stain etching of single‐crystal Si substrates are compared using transmission electron microscopy and atomic force microscopy (AFM). AFM of conventionally anodized, laterally anodized and stain‐etched Si layers show that the layers have a fractal‐type surface morphology. The anodized layers are rougher than the stain‐etched films. At higher magnification 10 nm sized hillocks are visible on the surface. Transmission electron diffraction patterns indicate an amorphous structure with no evidence for the presence of crystalline Si in the near‐surface regions of the porous Si layers.

GaN/AlN digital alloy short‐period superlattices by switched atomic layer metalorganic chemical vapor deposition

In this letter we report the fabrication of GaN/AlN short‐period superlattices using switched atomic layer epitaxy. Superlattice structures with GaN well thicknesses ranging from 2.6 to 20.8 A (with AlN barrier thicknesses of 2.5, 7.5, and 15 A) were deposited over basal plane sapphire and characterized for their structure, crystallinity, and optical properties. Cross‐sectional transmission electron micrographs indicate GaN/AlN interfaces and the superlattice surfaces are atomically smooth. The structures exhibit strong room temperature photoluminescence and a sharp absorption edge indicating a high optical quality.

Leak-tight piezoelectric microvalve for high-pressure gas micropropulsion

This paper describes the results of our development of a leak-tight piezoelectric microvalve, operating at extremely high upstream pressures for microspacecraft applications. The device is a normally closed microvalve assembled and fabricated primarily from micromachined silicon wafers. The microvalve consists of a custom-designed piezoelectric stack actuator bonded onto silicon valve components (such as the seat, boss, and tether) with the entire assembly contained within a stainless steel housing. The valve seat configurations include narrow-edge seating rings and tensile-stressed silicon tethers that enable the desired, normally closed, leak-tight operation. Leak testing of the microvalve was conducted using a helium leak detector and showed leak rates of 5/spl times/10/sup -3/ sccm at 800 psi (5.516 MPa). Dynamic microvalve operation (switching rates of up to 1 kHz) was successfully demonstrated for inlet pressures in the range of 0/spl sim/1000 psi. The measured static flow rate for the microvalve under an applied potential of 10 V was 52 sccm at an inlet pressure of 300 psi. The measured power consumption, in the fully open state, was 3 mW at an applied potential of 30 V. The measured dynamic power consumption was 180 mW for 100 Hz continuous operation at 100psi.

Vertical–cavity stimulated emission from photopumped InGaN/GaN heterojunctions at room temperature

We report the observation of room temperature violet (415 nm) stimulated emission in the vertical cavity mode from photopumped GaN/In0.25Ga0.75N heterojunctions. The InGaN/GaN heterojunction was deposited over sapphire substrates using low‐pressure metalorganic chemical vapor deposition and was of high enough optical quality to achieve room‐temperature stimulated emission. The observed emission intensity was found to be a nonlinear function of incident optical pump power density. At threshold we observe a clear line narrowing of the output optical signal from 20 to 1.5 nm full width at half‐maximum.

Temperature‐mediated phase selection during growth of GaN on (111)A and (1̄1̄1̄)B GaAs substrates

GaN layers having the zinc blende and wurtzite structures can be selectively deposited on (111)A and (111)B GaAs substrates by varying the growth temperature. Using the growth temperature as a variable, layers having the two structures have been sequentially deposited. The as‐grown structures have been examined by cross‐sectional high resolution electron microscopy. Results indicate that the two phases once formed are structurally stable in the temperature range examined. Furthermore, the transition from GaN (zinc blende) to GaN (wurtzite) is sharp, whereas a faulted region is observed during the reverse transition. Arguments have been developed to rationalize these observations.

Low pressure metalorganic chemical‐vapor deposition of cubic GaN over (100) GaAs substrates

We report on the low pressure metal organic chemical‐vapor deposition of single crystal cubic GaN films over (100) GaAs substrates. Using photoluminescence and direct optical absorption measurements we estimate the band gap for c‐GaN at room temperature to be 3.3 eV. Reflection high energy electron diffraction, x‐ray, transmission electron microscopy, optical absorption, and room‐temperature photoluminescence data are presented to establish the quality of a 0.8‐μm‐thick cubic GaN film over (100) GaAs substrate. Preliminary measurement results for the carrier density and mobility of the as‐deposited c‐GaN film are also presented.

Elemental boron-doped p(+)-SiGe layers grown by molecular beam epitaxy for infrared detector applications

SiGe/Si heterojunction internal photoemission (HIP) detectors have been fabricated utilizing molecular beam epitaxy of p+‐SiGe layers on p−‐Si substrates. Elemental boron from a high‐temperature effusion cell was used as the dopant source during molecular beam epitaxy (MBE) growth, and high doping concentrations (≳5×1020 cm−3) have been achieved. Strong infrared absorption, mainly by free‐carrier absorption, was observed for the degenerately doped SiGe layers. The use of elemental boron as the dopant source allows a low MBE growth temperature (350 °C), resulting in improved crystalline quality and smooth surface morphology of the Si0.7Ge0.3 layers. Nearly ideal thermionic emission dark current characteristics have been obtained. Photoresponse of the HIP detectors in the long‐wavelength infrared regime has been demonstrated.