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NASA NTRS1981

Study of radiation induced deep-level defects and annealing effects in the proton irradiated AlGaAs-GaAs solar cells

The radiation induced deep-level defects and the recombination parameters in the proton irradiated AlGaAs-GaAs p-n junction solar cells were investigated over a wide range of proton energies (from 50 KeV to 10 MeV) and proton fluences (from 10 to the 10th to 10 to the 13th P/sq cm), using DLTS, I-V, C-V, and SEM-EMIC measurement techniques. The measurements were used to determine the defect and recombination parameters such as defect density and energy level, carrier lifetimes, and the hole diffusion lengths in the GaAs LPE layers. Results show that a good correlation was obtained between the measured defect parameters and the dark recombination current as well as the performance parameters of the solar cells. The most damages to the cell were produced by the 200 KeV protons. In addition, the effects of low temperatures (200 to 400 C) thermal annealing on the deep-level defects and the dark current of the 200 KeV proton irradiated samples were examined.

Li, S. S.↗

NASA NTRS1984

Efficient AlGaAs shallow-homojunction solar cells

Shallow-homojunction n+/p/p+ solar cells with one-sun, AM1 conversion efficiencies as high as 12.9 percent have been fabricated in Al0.2Ga0.8As epitaxial layers grown by organometallic chemical vapor deposition on single-crystal GaAs substrates. For these cells, which have n+ layers thinned by anodic oxidation to about 500 A, the quantum efficiencies in the short-wavelength portion of the spectrum are as high as the best reported for AlGaAs cells with high band-gap window layers.

Gale, R. P.↗

NASA NTRS1984

Direct-current polarization characteristics of various AlGaAs laser diodes

Polarization characteristics of AlGaAs laser diodes having various device geometries have been measured. Measurements were performed with the laser diodes operating under dc conditions. Results show that laser diodes having different device geometries have optical outputs that exhibit varying degrees of polarization purity. Implications of this result, with respect to incoherent polarization-beam combining, are addressed.

Fuhr, P. L.↗

NASA NTRS1985

High-efficiency AlGaAs-GaAs Cassegrainian concentrator cells

AlGaAs-GaAs heteroface space concentrator solar cells have been fabricated by metalorganic chemical vapor deposition. AMO efficiencies as high as 21.1% have been observed both for p-n and np structures under concentration (90 to 100X) at 25 C. Both cell structures are characterized by high quantum efficiencies and their performances are close to those predicted by a realistic computer model. In agreement with the computer model, the n-p cell exhibits a higher short-circuit current density.

Werthen, J. G.↗

NASA NTRS1986

Average power constraints in AlGaAs semiconductor lasers under pulse-position-modulation conditions

In some optical communications systems there are advantages to using low duty-cycle pulsed modulation formats such as pulse-position-modulation. However, because of intrinsic limitations of AlGaAs semiconductor lasers, the average power that they can deliver in a pulsed mode of operation is lower than in a CW mode. The magnitude of this problem and its implications are analyzed in this letter, and one possible solution is mentioned.

Katz, J.↗

NASA NTRS1986

Field-dependent linewidths and photoluminescence energies in GaAs-AlGaAs multiquantum well modulators

Photoluminescence linewidths and transition energies have been measured in GaAs-AlGaAs multiple quantum wells with large (equal to or greater than 160 A) barrier widths as a function of applied transverse electric field. The experimental data agree well with values calculated by using a recently developed variational technique. It is apparent that heterointerface roughness is the dominant line broadening mechanism. The emission intensity decreases rapidly with field, principally due to carrier tunneling at high fields. At 80 kV/cm a shift of 20 meV in the emission energy is observed.

Juang, F.-Y.↗

NASA NTRS1986

AlGaAs growth by OMCVD using an excimer laser

AlGaAs has been grown on GaAs by laser assisted OMCVD using an excimer laser, wavelength 193 nm, and a Cambridge OMCVD reactor. Films were grown at temperatures of 450 and 500 C with the laser beam parallel to the surface and impinging onto the surface at 15 deg from parallel. The samples were heated by RF coils while the laser beam was perpendicular to the gas flow. Typical gas flow parameters are 12 slm of H2, 15 sccm of Ga(CH3)3, 13 sccm of Al(CH3)3, and a pressure of 250 mbar. The initial energy density of the beam at the surface was 40 mJ/sq cm, the pulse rate was 20 pps, and the growth time was 7 min. The films were analyzed by Auger electron spectroscopy for the aluminum concentration and by TEM for the surface morphology.

Warner, Joseph D.↗

NASA NTRS1987

Performance of AlGaAs, GaAs and InGaAs cells after 1 MeV electron irradiation

Electron irradiations were made on three different types of III-V cells. AlGaAs, GaAs, and InGaAs cells with bandgaps of approximately 1.72, 1.43, and 1.1 eV, respectively, were tested. All of the cells were concentrator cells and performance data from one sun to beyond 100x AMO were taken. The total 1 MeV electron fluence was 3 times 10 to the 15th power e/square cm with data taken at several intermediate fluences. Cell performance is presented as a function of electron fluence for various concentration ratios and two different temperatures (25 and 80 C). Since these three cell types are potential candidates for the individual cells in a cascade structure, it is possible to calculate the loss in performance of cascade cells under 1 MeV irradiation. Data are presented which show the calculated performance of both series-connected and separately connected cascade cells.

Curtis, Henry B.↗

NASA NTRS1987

A ten-element array of individually addressable channeled-substrate-planar AlGaAs diode lasers

The fabrication of arrays of channeled-substrate-planar (CSP) AlGaAs diode lasers which emit up to 150 mW CW in a single spatial mode and are applicable to mulitchannel optical recording systems is described. The CSP diode lasers are incorporated in ten-array geometry, and each array is 1.95 nm in width and 100 microns in thickness and is cleaved to have a cavity length of 200 microns and coated to produce 90-percent reflectivity on the back facet and 10-percent reflectivity on the front facet. The array is attached to a thermoelectrically cooled submount. The optical output power versus input current characteristics for the array are evaluated, and the lateral far-field intensity profiles for each of the lasers (at 30 mW CW) and CW spectra of the lasers are analyzed.

Carlin, D. B.↗

NASA NTRS1987

AlGaAs-InGaAs SHEP's for space applications

The authors propose a novel class of cascade photovoltaics which are intended to overcome two problems with current cascade solar cell designs: poor upper subcell performance and excessive joule losses in the intercell ohmic contact (IOC). The devices feature multijunction upper subcells which improve spectral response by reducing the mean collection distance for photogenerated carriers. They circumvent the need for a monolithic IOC by using npn and pnp three-terminal cells arranged in a complementary configuration. Emphasis is on the AlGaAs-InGaAs system, which allows optimal partitioning of the solar spectrum to attain maximum possible efficiency for a two-gap cascade design. Computer simulations of these cells show that one-sun beginning-of-life efficiencies in excess of 27 percent are possible.

Educato, J. L.↗

NASA NTRS1988

High-power AlGaAs channeled substrate planar diode lasers for spaceborne communications

A high power channeled substrate planar AlGaAs diode laser with an emission wavelength of 8600 to 8800 A was developed. The optoelectronic behavior (power current, single spatial and spectral behavior, far field characteristics, modulation, and astigmatism properties) and results of computer modeling studies on the performance of the laser are discussed. Lifetest data on these devices at high output power levels is also included. In addition, a new type of channeled substrate planar laser utilizing a Bragg grating to stabilize the longitudinal mode was demonstrated. The fabrication procedures and optoelectronic properties of this new diode laser are described.

Connolly, J. C.↗

NASA NTRS1989

High-power single spatial mode AlGaAs channeled-substrate-planar semiconductor diode lasers for spaceborne communications

A high power single spatial mode channeled substrate planar AlGaAs semiconductor diode laser was developed. The emission wavelength was optimized at 860 to 880 nm. The operating characteristics (power current, single spatial mode behavior, far field radiation patterns, and spectral behavior) and results of computer modeling studies on the performance of the laser are discussed. Reliability assessment at high output levels is included. Performance results on a new type of channeled substrate planar diode laser incorporating current blocking layers, grown by metalorganic chemical vapor deposition, to more effectively focus the operational current to the lasing region was demonstrated. The optoelectronic behavior and fabrication procedures for this new diode laser are discussed. The highlights include single spatial mode devices with up to 160 mW output at 8600 A, and quantum efficiencies of 70 percent (1 W/amp) with demonstrated operating lifetimes of 10,000 h at 50 mW.

Connolly, J. C.↗

NASA NTRS1989

AlGaAs phased array laser for optical communications

Phased locked arrays of multiple AlGaAs diode laser emitters were investigated both in edge emitting and surface emitting configurations. CSP edge emitter structures, coupled by either evanescent waves or Y-guides, could not achieve the required powers (greater than or similar to 500 mW) while maintaining a diffraction limited, single lobed output beam. Indeed, although the diffraction limit was achieved in this type of device, it was at low powers and in the double lobed radiation pattern characteristic of out-of-phase coupling. Grating surface emitting (GSE) arrays were, therefore, investigated with more promising results. The incorporation of second order gratings in distribute Bragg reflector (DBR) structures allows surface emission, and can be configured to allow injection locking and lateral coupling to populate 2-D arrays that should be able to reach power levels commensurate with the needs of high performance, free space optical communications levels. Also, a new amplitude modulation scheme was developed for GSE array operation.

Carlson, N. W.↗

NASA NTRS1989

Optically controlled reflection modulator using GaAs-AlGaAs n-i-p-i/multiple-quantum-well structures

An optically controlled reflection modulator has been demonstrated that consists of a combination of a GaAs-AlGaAs n-i-p-i doping structure with a multiple-quantum-well structures on top of a distributed Bragg reflector, all grown by MBE. A modulation of approximately 60 percent is obtained on the test structure, corresponding to a differential change of absorption coefficient in the quantum wells of approximately 7500/cm. Changes in reflectance can be observed with a control beam power as low as 1.5 microW. This device structure has the potential of being developed as an optically addressed spatial light modulator for optical information processing.

Law, K.-K.↗

NASA NTRS1990

Direct detection optical intersatellite link at 220 Mbps using AlGaAs laser diode and silicon APD with 4-ary PPM signaling

A newly developed 220 Mbps free-space 4-ary pulse position modulation (PPM) direct detection optical communication system is described. High speed GaAs integrated circuits were used to construct the PPM encoder and receiver electronic circuits. Both PPM slot and word timing recovery were provided in the PPM receiver. The optical transmitter consisted of an AlGaAs laser diode (Mitsubishi ML5702A, lambda=821nm) and a high speed driver unit. The photodetector consisted of a silicon avalanche photodiode (APD) (RCA30902S) preceded by an optical interference filter (delta lambda=10nm). Preliminary tests showed that the self-synchronized PPM receiver could achieve a receiver bit error rate of less than 10(exp -6) at 25 nW average received optical signal power or 360 photons per transmitted information bit. The relatively poor receiver sensitivity was believed to be caused by the insufficient electronic bandwidth of the APD preamplifier and the poor linearity of the preamplifier high frequency response.

Sun, Xiaoli↗

NASA NTRS1989

High-power low-threshold graded-index separate confinement heterostructure AlGaAs single quantum well lasers on Si substrates

A high-power low-threshold graded-index separate confinement heterostructure AlGaAs single quantum well laser on Si substrates has been demonstrated for the first time by a hybrid growth of migration-enhanced molecular beam epitaxy followed by metalorganic vapor phase epitaxy. The quantum well laser showed an output power of more than 400 mW per facet under pulsed conditions. A room-temperature threshold current of 300 mA was obtained with a differential quantum efficiency of 40 percent without facet coating. The threshold current density was 550 A/sq cm for a cavity length of 500 microns. These results show the highest peak power reported to date for low-threshold lasers on Si substrates. The full width at half maximum of the far-field pattern parallel to the junction was 6 deg. Threshold current densities as low as 250 A/sq cm were obtained for lasers on GaAs substrates.

Kim, Jae-Hoon↗

NASA NTRS1991

A field induced guide-antiguide modulator of GaAs-AlGaAs

A guide-antiguide modulator of GaAs-AlGaAs using the electric-field-induced waveguide concept was demonstrated. The device was formed with a central waveguide electrode sandwiched between two antiguide electrodes on the surface of a p-i-n multiple quantum well (MQW). Switching between lateral guiding and antiguiding was accomplished by reverse biasing either the central electrode or the adjacent electrodes to increase the index beneath these respective regions. The on-off ratio was measured to be 20:1 with a propagation loss of the on-state of about 5 dB/mm.

Huang, T. C.↗

NASA NTRS1990

AlGaAs top solar cell for mechanical attachment in a multi-junction tandem concentrator solar cell stack

Free-standing, transparent, tunable bandgap AlxGa1-xAs top solar cells have been fabricated for mechanical attachment in a four terminal tandem stack solar cell. Evaluation of the device results has demonstrated 1.80 eV top solar cells with efficiencies of 18 percent (100 X, and AM0) which would yield stack efficiencies of 31 percent (100 X, AM0) with a silicon bottom cell. When fully developed, the AlxGa1-xAs/Si mechanically-stacked two-junction solar cell concentrator system can provide efficiencies of 36 percent (AM0, 100 X). AlxGa1-xAs top solar cells with bandgaps from 1.66 eV to 2.08 eV have been fabricated. Liquid phase epitaxy (LPE) growth techniques have been used and LPE has been found to yield superior AlxGa1-xAs material when compared to molecular beam epitaxy and metal-organic chemical vapor deposition. It is projected that stack assembly technology will be readily applicable to any mechanically stacked multijunction (MSMJ) system. Development of a wide bandgap top solar cell is the only feasible method for obtaining stack efficiencies greater than 40 percent at AM0. System efficiencies of greater than 40 percent can be realized when the AlGaAs top solar cell is used in a three solar cell mechanical stack.

Dinetta, L. C.↗