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NASA NTRS1989

Composite Semiconductor Substrates

Epitaxial structure of three semiconductor materials - silicon, gallium arsenide, and cadmium telluride - makes possible integrated monolithic focal-plane arrays of photodectors. Silicon layer contains charge-coupled devices, gallium arsenide layer contains other fast electronic circuitry, and cadmium telluride layer serves as base for array of mercury cadmium telluride infrared sensors. Technique effectively combines two well-established techniques; metalorganic chemical-vapor deposition (MOCVD) and molecular-beam epitaxy (MBE). Multilayer structure includes HgCdTe light sensors with Si readout devices and GaAs signal-processing circuits. CdTe layer provides base for building up HgCdTe layer.

Nouhi, Akbar↗

DOE OSTI2025

Unveiling the Interfacial Reconstruction Mechanism Enabling Stable Growth of the Delafossite PdCoO 2 on Al 2 O 3 and LaAlO 3

Delafossites, composed of noble metal (A + ) and strongly correlated sublayers (BO 2 – ), form natural superlattices with highly anisotropic properties. These properties hold significant promise for various applications, but their exploitation hinges on the successful growth of high-quality thin films on suitable substrates. Unfortunately, the unique lattice geometry of delafossites presents a significant challenge to thin-film fabrication. Different delafossites grow differently, even when deposited on the same substrate, ranging from successful epitaxy to complete growth suppression. These variations often lack a clear correlation to obvious causes like lattice mismatch. Unidentified stabilization mechanisms appear to enable growth in certain cases, allowing these materials to form stable thin films or act as buffer layers for subsequent delafossite growth. This study employs advanced scanning transmission electron microscopy techniques to investigate the nucleation mechanism underlying the stable growth of PdCoO 2 films on Al 2 O 3 and LaAlO 3 substrates grown via molecular-beam epitaxy. Our findings reveal the presence of a secondary phase within the substrate surface that stabilizes the films. This mechanism deviates from the conventional understanding of strain relief mechanisms at oxide heterostructure interfaces and differs significantly from those observed for Cu-based delafossites.

36 MATERIALS SCIENCE↗

DOE OSTI2025

Conditions of coherent-phonon excitation in SrMnSb2 films and crystals

Excitation of coherent phonons has the potential to dramatically alter the electronic structure of Dirac and Weyl semimetals, enabling sub-picosecond control of their optical and electronic properties. The Dirac semimetal SrMnSb2 is a candidate for such control, with a coherent-phonon mode that is predicted to close and reopen a gap at the Dirac node. Here, through a series of ultrafast pump-probe experiments, we establish suitable samples and conditions for driving the coherent phonon to high amplitude and attempting to observe the gap’s closure. Films of SrMnSb2 grown by molecular-beam epitaxy are shown to have phononic properties matching those of bulk crystals. We find that the phonon can be strongly excited by pump pulses with wavelength near 1500 nm, which will excite a 30-nm film almost uniformly and will penetrate the arsenic capping layers that protect the films. We find that samples withstand pump pulses of fluence up to 20 mJ/cm2, and we demonstrate the potential for sequences of pulses to amplify the oscillation while suppressing other phonon modes. Armed with our new knowledge of the conditions for exciting the desired coherent phonon, future experiments will be well prepared to measure its motion and to observe phononic control of the Dirac-point gap.

Rai, Manita↗

NASA NTRS1985

Preparing InSb Substrates

Molecular-beam epitaxial substrates passivated by chloride film growth. Chloride layer produced by direct chlorination, by chloride radicals in solutions, or by formation of preliminary oxide layer followed by chloride substitution.

Vasquez, R. P.↗

NASA NTRS1984

High-efficiency V-band GaAs IMPATT diodes

Double-drift GaAs IMPATT diodes were designed for V-band frequency operations and fabricated using molecular-beam epitaxy. The diodes were fabricated in two configurations: (1) circular mesa diodes with silver-plated (integrated) heat sinks: (2) pill-type diodes bonded to diamond heat sinks. Both configurations utilized a miniature quartz-ring package. Output power greater than 1 W CW was achieved at V-band frequencies from diodes on diamond heat sinks. The best conversion efficiency was 13.3 percent at 55.5 GHz with 1 W output power.

Ma, Y. E.↗

NASA NTRS1984

Computer modeling results on all-Si cascade solar cells

The properties of a cascade solar cell made entirely of silicon are investigated numerically with the goal of developing an optimal silicon solar cell grown by molecular-beam epitaxy. The cascade cell is modeled as two standard back-surface field cells with abrupt junctions connected by a tunnel junction. A cascade cell would have approximately twice the open-circuit voltage of a single cell. If the minority carriers generated in the front cell can be reflected before reaching the tunnel junction, then the cascade cell will show an increase in efficiency over a single cell by a percentage point.

Sparks, P. D.↗

NASA NTRS1986

Electron-Diffraction Analysis of Growth of GaAs

Report describes experiments that used reflection high-energy-electron diffraction (RHEED) to investigate behavior of GaAs surfaces during and after growth by molecular-beam epitaxy (MBE). Experimental results show dynamic RHEED measurements useful both as probes of surface and growth kinetics and as methods for determining and reproducing surface and growth conditions.

Lewis, B. F.↗

NASA NTRS1986

Lateral coherence properties of broad-area semiconductor quantum well lasers

The lateral coherence of broad-area lasers fabricated from a GaAs/GaAlAs graded index waveguide separate confinement and single quantum well heterostructure grown by molecular-beam epitaxy was investigated. These lasers exhibit a high degree of coherence along the junction plane, thus producing a stable and very narrow far field intensity distribution.

Larsson, A.↗

NASA NTRS1986

Deep levels in as-grown and Si-implanted In(0.2)Ga(0.8)As-GaAs strained-layer superlattice optical guiding structures

Trap levels in about 2-micron In(0.2)Ga(0.8)As(94 A)/GaAs(25 A) strained-layer superlattices, suitable for optical waveguides, have been identified and characterized by deep-level transient spectroscopy and optical deep-level transient spectroscopy measurements. Several dominant electron and hole traps with concentrations of approximately 10 to the 14th/cu cm, and thermal ionization energies Delta-E(T) varying from 0.20 to 0.75 eV have been detected. Except for a 0.20-eV electron trap, which might be present in the In(0.2)Ga(0.8)As well regions, all the other traps have characteristics similar to those identified in molecular-beam epitaxial GaAs. Of these, a 0.42-eV hole trap is believed to originate from Cu impurities, and the others are probably related to native defects. Upon Si implantation and halogen lamp annealing, new deep centers are created. These are electron traps with Delta-E(T) = 0.81 eV and hole traps with Delta-E(T) = 0.46 eV. Traps occurring at room temperature may present limitations for optical devices.

Dhar, S.↗

NASA NTRS1986

Variable angle of incidence spectroscopic ellipsometry Application to GaAs-Al(x)Ga(1-x)As multiple heterostructures

The sensitivity of spectroscopic ellipsometry data to multilayer model parameters is shown to be a strong function of the angle of incidence. A quantitative study of sensitivity versus angle of incidence is performed for a GaAs-Al(x)Ga(1-x)As-GaAs substrate structure, showing that maximum sensitivity to layer thicknesses and AlGaAs composition occurs near the wavelength-dependent principal angle. These results are verified by experimental measurements on two molecular-beam epitaxy grown samples.

Snyder, Paul G.↗

NASA NTRS1987

Anisotropic transport in modulation-doped quantum-well structures

Anisotropic electron transport has been observed in GaAs modulation-doped quantum wells grown by molecular-beam epitaxy on a thick (001) Al(0.3)Ga(0.7)As buffer grown at 620 C. Thicker quantum wells (150, 200, and 300 A) show progressively less anisotropy, which vanishes for a 300-A quantum well. The degree of anisotropy is also reduced or eliminated by suspending growth of the Al(0.3)Ga(0.7)As for a period of 300 s prior to growing the GaAs quantum well. Growing the Al(0.3)Ga(0.7)As buffer at higher temperatures (680 C) also reduces the degree of anisotropy. Higher two-dimensional electron gas sheet densities result in less anisotropy.The anisotropy is eliminated by replacing the thick Al(0.3)Ga(0.7)As buffer with a periodic multilayer structure comprising 15 A of GaAs and 200 A of Al(0.3)Ga(0.7)As. The degree of anisotropy is related to the thickness and growth parameters of the Al(0.3)Ga(0.7)As layer grown just prior to the growth of the GaAs.

Radulescu, D. C.↗

NASA NTRS1988

Stacked Metal Silicide/Silicon Far-Infrared Detectors

Selective doping of silicon in proposed metal silicide/silicon Schottky-barrier infrared photodetector increases maximum detectable wavelength. Stacking layers to form multiple Schottky barriers increases quantum efficiency of detector. Detectors of new type enhance capabilities of far-infrared imaging arrays. Grows by molecular-beam epitaxy on silicon waferscontaining very-large-scale integrated circuits. Imaging arrays of detectors made in monolithic units with image-preprocessing circuitry.

Maserjian, Joseph↗

NASA NTRS1987

InGaAs/GaAs multiquantum-well electroabsorption modulator with integrated waveguide

A monolithically integrated guided-wave modulator has been realized by using molecular-beam epitaxial regrowth and ion-milling techniques. The guiding and modulating regions consist, respectively, of In-doped GaAs and GaAs/In(0.34)Ga(0.66)As strained-layer multiquantum wells. Modulation is achieved by field-enhanced electroabsorption in the multiquantum wells. The insertion loss of the modulator is 0.9 dB, and the transmission loss in the guides is less than or equal to 1 dB/cm. The temporal response of similar GaAs/InGaAs as-grown photodiodes to pulsed laser excitation is characterized by a rise time of 115 psec.

Das, Utpal↗

NASA NTRS1988

Ultralow threshold graded-index separate-confinement heterostructure single quantum well (Al, Ga) As lasers

Broad area graded-index separate-confinement heterostructure single quantum well lasers grown by molecular-beam epitaxy (MBE) with threshold current density as low as 93 A/sq cm (520 microns long) have been fabricated. Buried lasers formed from similarly structured MBE material with liquid phase epitaxy regrowth had threshold currents at submilliampere levels when high reflectivity coatings were applied to the end facets. A CW threshold current of 0.55 mA was obtained for a laser with facet reflectivities of about 80 percent, a cavity length of 120 micron, and an active region stripe width of 1 micron. These devices driven directly with logic level signals have switch-on delays less than 50 ps without any current prebias. Such lasers permit fully on-off switching while at the same time obviating the need for bias monitoring and feedback control.

Derry, P. L.↗

NASA NTRS1988

Variable angle spectroscopic ellipsometry - Application to GaAs-AlGaAs multilayer homogeneity characterization

Variable angle spectroscopic ellipsometry has been applied to a GaAs-AlGaAs multilayer structure to obtain a three-dimensional characterization, using repetitive measurements at several spots on the same sample. The reproducibility of the layer thickness measurements is of order 10 A, while the lateral dimension is limited by beam diameter, presently of order 1 mm. Thus, the three-dimensional result mainly gives the sample homogeneity. In the present case three spots were used to scan the homogeneity over 1 in of a wafer which had molecular-beam epitaxially grown layers. The thickness of the AlGaAs, GaAs, and oxide layers and the Al concentration varied by 1 percent or less from edge to edge. This result was confirmed by two methods of data analysis. No evidence of an interfacial layer was observed on top of the AlGaAs.

Alterovitz, Samuel A.↗

NASA NTRS1989

Strain-Layer-Superlattice Light Modulator

Conceptual device combines resonant reflection and photovoltaic action to enable one light beam to impose spatial and temporal modulation on another light beam. Such spatial light modulator, with high speed and multiplicity of parallel signal channels, used in image processing or similar computation requiring high data-throughput rates. Microstructures of GaAs and InAs with multiple quantum wells and compositional superlattices grown by molecular-beam epitaxy. Enhanced electro-optical properties of arrangement of alternating layers enables writing light beam to modulate reading light beam.

Maserjian, Joseph↗

NASA NTRS1989

Metastable defects in Be-doped Al(x)Ga(1-x)As

Deep-level transient spectroscopy has been used to study metastable defects in Be-doped Al(x)Ga(1-x)As grown by molecular-beam epitaxy. The metastability manifests itself by the appearance of different spectra depending upon whether the sample is cooled from a high temperature with zero bias or a reverse bias applied to it. The defects are found in concentrations of 10 to the 15th/cu cm in a sample doped with 10 to the 18th Be/cu cm and in much lower concentrations in a 10 to the 17th Be/cu cm sample. Isochronal annealing experiments indicate that the defect is multistable and that it is best modeled as a mobile interstitial which can reside at several sites near acceptor. The activation energies for these defects are between 0.2 and 0.5 eV above the valence band.

Magno, R.↗

NASA NTRS1989

Deep-level transient spectroscopy of Al(x)Ga(1-x)As/GaAs using nondestructive acousto-electric voltage measurement

The amplitude and the transient time constant of the acoustoelectric voltage were measured as a function of temperature to determine the activation energy of deep levels in Al(x)Ga(1-x)As/GaAs grown by molecular-beam epitaxy. In comparison to other methods based on monitoring the capacitance transient, deep-level transient spectroscopy has several advantages. The technique is nondestructive and highly sensitive, and, because of the dependence of the polarity of the acoustoelectric voltage on the carrier type, it yields information about the charge of the transient carriers and the type of deep traps involved in the release or trapping of these carriers.

Tabib-Azar, Massood↗