Charles Horton

Manufacture of solar cells on the moon

In support of the space exploration initiative a new architecture for the production of solar cells on the lunar surface is devised. The paper discusses experimental data on the fabrication and properties of lunar glass substrates, evaporated lunar regolith thin films (antireflect coatings and insulators), and preliminary attempts in the fabrication of thin film (silicon/II-Vl) photovoltaic materials on lunar regolith substrates. A conceptual design for a solar powered robotic rover capable of fabricating solar cells directly on the lunar surface is provided. Technical challenges in the development of such a facility and strategies to alleviate perceived difficulties are discussed. Finally, preliminary cost benefit ratio analysis for different in situ solar cell production scenarios (using exclusively in-situ planetary resources or hybrid) are discussed.

DONOR-ACCEPTOR-PAIR SPECTROSCOPY OF GAAS GROWN IN SPACE ULTRAVACUUM

Selective pair luminescence has been used to identify shallow acceptor levels in undoped GaAs epilayers grown by molecular beam epitaxy (MBE) in the ultrahigh vacuum of space generated in the wake of the free-flying Wake Shield Facility satellite (Space Shuttle Endeavor mission STS69). The low-temperature photoluminescence spectra are typical of high-purity GaAs. Unlike for conventional MBE-grown undoped GaAs where the near-band-edge excitonic luminescence is dominated by acceptor-bound excitons, the space-grown GaAs near-band-edge luminescence was found to be dominated by donor-bound excitons. The comparison of measured ground (1S3/2) and excited acceptor states to published acceptor spectra leads to the identification of CAs as the main residual acceptor impurity in the layer. Furthermore, the existence of a donor–acceptor-pair emission band demonstrates that a second acceptor is electronically active. Excited-state spectroscopy clearly identifies the second residual acceptor as ZnGa, which is scarcely ...

Model calculations of optical properties for disordered GaAs/AlAs superlattices

We have investigated the role of disorder in effecting the optical properties of short period superlattices. A one‐dimensional, two atoms per unit cell, tight binding model of a superlattice is solved exactly for the ordered structure and for a given disorder in the growth direction. A difference vector and the resulting disorder structure factor are proposed to characterize the disorder. The density of states, participation number, and optical absorption for both the ordered and disordered superlattice models are calculated as a function of energy. The results show that introduction of disorder into an indirect band gap material enhances the optical transitions near the indirect band edge.

Effect of disorder on the optical properties of short period superlattices

The optical properties of disordered short period superlattices are studied using a one‐dimensional tight‐binding model. A difference vector and disorder structure factor are proposed to characterize the disordered superlattice. The density of states, participation number, and optical absorption coefficients for both ordered and disordered superlattices are calculated as a function of energy. The results show that introduction of disorder into an indirect band gap material enhances the optical transition near the indirect band edge.

Solar cell development on the surface of Moon from in-situ lunar resources

The use of the indigenous resources of the Moon can result in the development of a power system on the Moon based on the fabrication of solar cells by thin film growth technology in the vacuum environment of the Moon. This can be accomplished by the deployment of a moderately-sized (/spl sim/200kg) crawler/rover on the surface of the Moon with the capabilities of preparation of the lunar regolith for use as a substrate, evaporation of the appropriate semiconductor material for the solar cell structure, and deposition of metallic contacts and interconnects. This unique process will allow for the emplacement of a lunar electric power system that can reach a 1 MW capacity level in several years of crawler operation. This approach for the emplacement of an electric power system on the Moon would require the transportation of a much smaller mass of equipment to the Moon than would otherwise be required to install a complete electric power system brought to the Moon and emplaced there. It would also result in an electric power system that was repairable/replaceable through the simple fabrication of more solar cells.

Interesting aspects of reflection high‐energy electron diffraction oscillations during growth of GaAs(100)

The intensities of several reflection high‐energy electron diffraction (RHEED) beams have been recorded during molecular‐beam epitaxial growth of GaAs(100) using a novel video intensity measurement system that records multiple RHEED beam intensities simultaneously. The RHEED beam intensities were recorded at varying angles of incidence and crystal substrate azimuth angles. Strong oscillations in the intensities in specular and nonspecular beams with the same period but varying phases have been measured. As noted by other investigators, the phase relationship of the oscillations of the various beams has been found to vary with incident and azimuthal angle. The results are examined with regard to recent studies of the role of Kikuchi processes on the phase of the specular beam. In contrast to other reports, it is found that although the diffracted intensities in the vicinity of the elastically diffracted beams are influenced by inelastically scattered electrons from the Kikuchi lines, these effects can not ...

Thin film growth in the ultra‐vacuum of space: The second flight of the Wake Shield Facility

The Wake Shield Facility (WSF) created to characterize and utilize the ultra‐vacuum of space has flown its second mission on Endeavor, STS‐69 in September, 1995. In its second flight the WSF flew free behind the Orbiter at a nominal distance of 25 nmi, grew four semiconductor thin film samples by molecular beam epitaxy, and one oxide thin film sample through the use of LEO atomic oxygen. The semiconductor thin films were in the III‐V semiconductor area with focus on AlGaAs/GaAs system. The WSF also generated an ultra‐vacuum consistent with past predictions of space wake vacuum levels. In addition, eleven cooperative experiments were flow on the WSF yielding additional information on the low earth orbit space environment.

Vacuum and flow field results from the wake shield facility flight experiment

The Wake Shield Facility (WSF) created to characterize and utilize the ultra‐vacuum of space has corroborated the formation of a wake region behind an orbiting vehicle. In its maiden flight on the remote manipulator arm of STS‐60 in February, 1994, the WSF generated the highest vacuum ever measured in low earth orbit space. In addition, details of wake formation and flow field characterization have been obtained with the added benefit of the characterization of the contaminating effects of the Orbiter on the low earth vacuum environment.

III‐V compound semiconductor film growth in low earth orbit on the wake shield facility

Gallium Arsenide (GaAs) films, both silicon doped and undoped, have been deposited by Molecular Beam Epitaxy (MBE) in Low Earth Orbit (LEO) in an ultra vacuum environment created by the Wake Shield Facility (WSF). The WSF is a 12 foot diameter stainless steel disk that sweeps out a volume of space thus creating an ultra vacuum in its wake. It was developed specifically to take advantage of the ultra vacuum for the deposition of thin film materials. The WSF was flown for the first time on STS‐60 in February, 1993. The mission objectives were to measure the unique wake vacuum environment and to epitaxially deposit GaAs thin films. In this paper, we describe the films deposited and report on the characterization performed to date. Films were deposited in two basic structures. The first structure consisted of undoped GaAs films of thicknesses ranging from 2 to 4 μm with a thin (≊200 nm) highly silicon doped layer (n≊5×1017/cc) on top. This is basically a metal‐semiconductor field effect transistor (MESFET) st...

Photoluminescence of GaAs grown by metallorganic molecular beam epitaxy in space ultra-vacuum

Abstract Low-temperature photoluminescence and selective pair luminescence has been used to identify shallow acceptor levels in undoped GaAs epilayers grown by molecular beam epitaxy and metallorganic molecular beam epitaxy (triethylgallium/As4) in the ultra-vacuum of space generated in the wake of the free flying Wake Shield Facility satellite (Space Shuttle Colombia mission STS 80). The low-temperature photoluminescence spectra are typical of high-purity GaAs. The near-band-edge excitonic luminescence are found to be dominated by donor bound excitons. Carbon CAs appears as the main residual acceptor impurity and excited state spectroscopy clearly identifies the presence of zinc residual acceptor (ZnGa). Finally, the absence of a Be impurity, introduced in large quantities to the growth environment prior to the MOMBE growth, suggest a minimized memory effect on the free flyer.