Parvez N. Uppal

Molecular beam epitaxial growth of GaAs on Si(211)

The molecular beam epitaxial growth of GaAs on Si(211) has been investigated. Theoretical considerations had suggested the (211) orientation to be particularly suitable for the nucleation and growth of a zincblende‐type compound semiconductor on a diamond‐type elemental one. The experimental results support the theoretical prediction. Morphologies of thin (≤0.1 μm) (211) layers are substantially better than for (100) layers, which nucleate poorly and require large layer thicknesses (≂1μm) to yield good morphologies. When the (211) layer growth is initiated with a thin (GaAs/Al, Ga)As superlattice buffer (0.1 μm), consisting of 10 periods of 5+5 nm, the (211) morphology rivals that of GaAs(100) homoepitaxial growth. Chemical etching studies as well as transmission electron microscope investigations show the layers to have the (211)B orientation and to be free of antiphase domains, both as predicted. The (211) layers show strong photoluminescence at 4 K. Not intentionally doped layers are n type, with elect...

High-performance, 0.1 μm InAlAs/InGaAs high electron mobility transistors on GaAs

This letter describes the material characterization and device test of InAlAs/InGaAs high electron mobility transistors (HEMTs) grown on GaAs substrates with indium compositions and performance comparable to InP-based devices. This technology demonstrates the potential for lowered production cost of very high performance devices. The transistors were fabricated from material with room temperature channel electron mobilities and carrier concentrations of /spl mu/=10000 cm/sup 2//Vs, n=3.2/spl times/10/sup 12/ cm/sup -2/ (In=53%) and /spl mu/=11800 cm/sup 2//Vs, n=2.8/spl times/10/sup 12/ cm/sup -2/ (In=60%). A series of In=53%, 0.1/spl times/100 /spl mu/m/sup 2/ and 0.1/spl times/50 /spl mu/m/sup 2/ devices demonstrated extrinsic transconductance values greater than 1 S/mm with the best device reaching 1.074 S/mm. High-frequency testing of 0.1/spl times/50 /spl mu/m/sup 2/ discrete HEMTs up to 40 GHz and fitting of a small signal equivalent circuit yielded an intrinsic transconductance (g/sub m,i/) of 1.67 S/mm, with unity current gain frequency (f/sub T/) of 150 GHz and a maximum frequency of oscillation (f/sub max/) of 330 GHz. Transistors with In=60% exhibited an extrinsic g/sub m/ of 1.7 S/mm, which is the highest reported value for a GaAs based device.

Dual band QWIP MWIR/LWIR focal plane array test results

We report on the results of laboratory and field tests on a pixel-registered, 2-color MWIR/LWIR 256 X 256 QWIP FPA with simultaneous integrating capability. The FPA studied contained stacked QWIP structures with spectral peaks at 5.1 micrometer and 9.0 micrometer. Normally incident radiation was coupled into the devices using a diffraction grating designed to operate in both spectral bands. Each pixel is connected to the read-out integrated circuit by three bumps to permit the application of separate bias levels to each QWIP stack and allow simultaneous integration of the signal current in each band. We found the FPA to have high pixel operability, well balanced response, good imaging performance, high optical fill factor, and low spectral crosstalk. We present data on measurements of the noise-equivalent temperature difference of the FPA in both bands as functions of temperature and bias. The FPA data are compared to single-pixel data taken on devices from the same wafer. We also present data on the sensitivity of this FPA to polarized light. It is found that the LWIR portion of the device is very sensitive to the direction of polarization of the incident light. The MWIR part of the device is relatively insensitive to the polarization. In addition, imagery was taken with this FPA of military targets in the field. Image fusion techniques were applied to the resulting images.

MBE growth of GaInAsSb p/n junction diodes for thermophotovoltaic applications

Abstract This paper reports recent progress in the development of quaternary III–V thermophotovoltaic (TPV) devices based on MBE grown Ga x In 1− x As y Sb 1− y . TPV is of great interest for a variety of applications (1st and 2nd NREL Conf. on Thermophotovoltaic Generation of Electricity, AIP Conf. Proc. 321 (1994), 358 (1995)). The objective of this work is to develop a TPV cell which is “tunable” to the emission spectrum of a heated blackbody, at temperatures in the range of 1200–1473 K. One aspect of this “tuning” is to match the band gap, E gap , of the photovoltaic device to the peak output of the heat source. An advantage of the quaternary III–V semiconductor systems is that devices can be fabricated by molecular beam epitaxy on a suitable binary substrate, such as GaSb or InAs, and the band gap and lattice constant can be adjusted more or less independently, to match requirements. Quaternary cells, with band-gaps in the 0.5–0.72 eV range, have been fabricated and tested. For 0.54 eV devices we obtained open circuit voltage V OC = 0.3 V and short circuit current I SC = 1.5 A/cm 2 under infrared illumination of a 1200 K blackbody. Under high illumination levels the V OC and I SC ranged from 0.5 V at 3 A/cm 2 for 0.72 eV devices to 0.31 V at 1.2 A/cm 2 for 0.5 eV devices, indicating good photovoltaic device characteristics over the range of bandgaps. The diode ideality factor for 0.54 eV devices ranged from 2.45 at low illumination indicating tunneling-dominated dark current, to 1.7 at high illumination intensity indicating recombination-generation dominated dark currents.

Room temperature photoluminescence from modulation doped AlGaAs/InGaAs/GaAs quantum wells

The photoluminescence (PL) spectral shape and position from single, modulation doped, and undoped AlGaAs/InGaAs/GaAs quantum wells have been studied at room temperature (RT) with the purpose of evaluating the usefulness of the PL technique for verifying device material structures. Starting with a general expression for the line shape, we can qualitatively predict the spectral shape and position by evaluation of the squared overlap integrals of the four possible transitions between the two lowest states in the valence and conduction band wells. A self‐consistent calculation is used to determine the equilibrium wave functions and the energies of the bound states in the quantum well. Good agreement is found between the experimental and theoretical peak positions, and the Stark shift in the low‐energy spectral onset between doped and undoped structures also can be closely reproduced. The accuracy of the calculations has been verified by comparing structures with varying layer widths and constant In compositio...

High‐indium modulation doped field effect transistors on GaAs substrates

We have grown InxGa1−xAs modulation doped field effect transistors (MODFETs) using novel buffer layer schemes on GaAs substrates with x values up to 0.5. The MODFET active layers were grown at substrate temperatures between 500 and 520 °C and characterized by x‐ray diffraction, photoluminescence, and Hall effect measurements. MODFET devices with 0.15 μm gate length were fabricated, and dc and rf tested. Our work has indicated that the electron mobility in the MODFETs is a function of the growth mode, i.e., two‐dimensional (2D) layer‐by‐layer versus three‐dimensional island growth. MODFET performance, the ability to maintain a streaky reflection high‐energy electron diffraction pattern, as well as morphology had a very strong correlation. With these observations in mind we used a buffer layer scheme to give 2D growth fronts for InxGa1−xAs with x values up to 0.5, which is a modification of the buffer scheme described in K. Maezawa and T. Mizutani, IEEE Trans. Electron Devices 37, 1416 (1990). For In0.34Ga0...

Optical characterization of Al0.5Ga0.5As1−xSbx buffer layers with modulation doped AlInAs/InGaAs structures

We present results of photoluminescence studies of Al0.5Ga0.5As1−xSbx grown by molecular beam epitaxy on GaAs substrates. We compared the photoluminescence (PL) peak position as a function of composition with the band gap prediction using the established formalism developed for other quaternaries. We found a discrepancy of up to 0.2 eV indicating that an extra bowing factor needs to be included in the band gap composition equation for this material system. InGaAs/AlInAs structures grown on top of the Al0.5Ga0.5As1−xSbx were also characterized and compared with similar structures grown on InP and were shown to exhibit similar PL intensities and Hall mobilities, indicating that the quaternary buffer layer can be used for growth of high-In concentration InGaAs device structures on GaAs.

Cell configuration‐induced strain in quaternary films

This article describes two effects that can cause serious bending in GaSb and InAs substrates used for quaternary compounds. Understanding of both effects is required in order to produce flat wafers. We have observed a very large bending with both positive and negative curvature in wafers with mismatched films, both at growth and room temperature. We therefore postulate that misfit dislocation formation is suppressed in these materials and that the bending is due to the lattice mismatch. However, bending may also occur in wafers where the mismatch in the center of the wafer is negligible. We propose that this effect is due to a mismatch due to nonuniform mole fraction variations across the wafer and throughout the film which in turn are induced by the way the flux distributions combine to create a particular mole fraction. The conclusion that must be drawn from these observations is that production of flat wafers require the intentional introduction of a slight mismatch during the growth process.

Development of high efficiency thermophotovoltaics for space power applications

This paper reports recent progress on thermophotovoltaic (TPV) devices for space power applications. Previous investigations (Schock 1994 and Ewell 1993) indicate that the TPV approach may offer greater efficiency than conventional thermoelectric conversion. We discuss an approach based on photovoltaic (PV) devices made from ternary and quaternary III‐V alloys, using dielectric and plasma filters. The objective is to develop a TPV cell and filter which is ‘‘tunable’’ to the emission spectrum of radioisotope or reactor heat sources, at temperatures in the range of 1273–1473 K. An advantage of quaternary III‐V semiconductors is that devices can be fabricated by molecular beam epitaxy (MBE) on a suitable binary substrate, such as GaSb or InAs, and the band gap and lattice constant can be adjusted independently to match requirements. Energy gaps from 0.5 to 0.72 eV can be obtained. The use of Al as one component of a III‐V quaternary provides exceptional adjustability of the gap, while the use of Sb as one co...