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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

Interfacial recombination at /AlGa/As/GaAs heterojunction structures

Experiments were conducted to determine the interfacial recombination velocity at Al0.25Ga0.75As/GaAs and Al0.5Ga0.5As/GaAs heterojunctions. The recombination velocity was derived from a study of the injected minority-carrier lifetime as a function of the junction spacing. It is found that for heterojunction spacings in excess of about 1 micron, the interfacial recombination can be characterized by a surface recombination velocity of 4,000 and 8,000 cm/sec for the two types of heterojunctions, respectively. For double-heterojunction spacings below 1 micron, the constancy of the minority-carrier lifetime suggests that the interfacial recombination velocity decreases effectively. This effect is technologically very important since it makes it possible to construct very low-threshold injection lasers. No such effect is observed in single-heterojunction diodes.

Ettenberg, M.↗
NASA NTRS1977

Heterojunctions in photovoltaic devices

Heterojunctions have interesting optical properties which make them attractive for solar cells. Several types of heterostructure solar cells have been investigated: heteroface, abrupt heterojunction, and graded-gap heterojunction solar cells. The primary advantage of heterostructure cells is the enhanced short-wavelength response although there is potential for low-cost cells with polycrystalline material. The improvement in solar-cell performance for heterojunctions depends upon the selection of semiconductors with useful energy gaps that are closely matched in lattice-spacing and thermal-expansion coefficients. The importance of the heterojunction interface and its dependence on material properties is discussed. Various fabrication methods are discussed and their application to the different types of heterostructures. Recent performance data for material systems representative of each type of heterostructure cell are discussed and considered in terms of maximum expected performance.

Feucht, D. L.↗
NASA NTRS1990

New heterojunction LWIR detector options

Researchers investigate the heterojunction internal photoemission (HIP) approach that potentially offers long wavelength infrared (LWIR) photovoltaic detector performance (single pixel) that is competitive with the best of other approaches being considered. Most significantly, this approach offers a relatively simple device technology that promises producible and uniform FPA's. Researchers emphasize an exciting process based on intervalence band absorption. They investigate both III-V and Si-based heterojunctions grown by molecular beam epitaxy (MBE) in which the barrier can be tailored to the desired cutoff wavelength. In addition, MBE allows one to optimize the device structure with precise control of doping profiles and layer thicknesses, and perform band structure engineering by control of composition and heterojunction strain. Researchers also consider free carrier absorption in heterojunctions. Acceptable absorption coefficients can be achieved in very heavily n(exp +) doped semiconductor layers (approx. equals 10(exp 20)cm(exp -3). However, in this case the appreciable filling of conduction band states leads to a Schottky-like photoresponse with a gradual (quadratic) turn-on above threshold. A more satisfactory approach would be to use p(exp +) doping so that with the higher density of states in the heavy hole valence band there would be a narrow band of occupied states. This gives the desirable effect of a more rapid (linear) turn-on above threshold. Unfortunately, the higher hole effective mass also reduces (inversely) the free carrier absorption. For this and other reasons, the intervalence band absorption process looks much more promising.

Maserjian, Joseph↗
NASA NTRS1976

A theoretical study of heterojunction and graded band gap type solar cells

The theory of heterojunction and graded bandgap solar cells was studied to help determine the material compositions and device dimensions needed for high efficiency solar cells. Because of the involved analysis of the fundamental equations describing solar cell operation, a general numerical device analysis program was used. A major part of the initial work was involved in modifying an existing silicon solar cell analysis program to account for the unique features of graded bandgap and heterojunction solar cells. The most successful III-V solar cells have so far been constructed in the GaAs and Gal-xAlxAs material systems; this project was concerned with such solar cells. The most efficient solar cell so far evaluated is an abrupt heterojunction cell with a pure AlAs layer at the surface with a GaAs substrate. The predicted efficiency for this cell is slightly larger than that of a graded bandgap Gal-xAlxAs solar cell.

Hauser, J. R.↗
NASA NTRS1977

Auger depth profiling of Au-Al/x/Ga/1-x/As interfaces and LPE Al/x/Ga/1-x/As-GaAs heterojunctions

An attempt is made to report Auger depth profiles produced in a UHF ion-milling Auger system for intentionally 'graded' Al(x)Ga(1-x)As-GaAs heterojunctions, 'abrupt' growth heterojunctions of the same material, and Au-Al(x)Ga(1-x)As interfaces. It is found that the interface widths of the 'abrupt' growth heterojunctions did not fluctuate greatly from an average value of 110 A and that 'graded' growth samples had interface widths of about 525 to 1500 A. The results for the Au-containing samples indicate that an oxide layer was present between the Au and the Al(x)Ga(1-x)As.

Garner, C. M.↗
NASA NTRS1978

The recombination velocity at III-V compound heterojunctions with applications to Al (x) Ga(1-x)As-GaAs(1-y)Sb(y)

Interface recombination velocity in AlxGa1-xAs-GaAs and A10.85 Ga0.15As-GaAs1-ySby heterojunction systems was studied as a function of lattice mismatch. The results are applied to the design of highly efficient III-V heterojunction solar cells. A horizontal liquid-phase epitaxial growth system was used to prepare p-p-p and p-p-n double heterojunction test samples with specified values of x and y. Samples were grown at each composition, with different GaAs and GaAsSb layer thicknesses. A method was developed to obtain the lattice mismatch and lattice constants in mixed single crystals grown on (100) and (111)B oriented GaAs substrates.

Kim, J. S.↗
NASA NTRS1977

Design curves for double-heterojunction laser diodes

This paper reviews theoretical double-heterojunction laser design curves relating the structure with the transverse mode order able to propagate, the degree of radiation confinement to the active region, the maximum field intensity within the device, and the radiated beam shape. The curves presented are general and applicable to any double-heterojunction structure where the spacing of the heterojunctions and the refractive index in the relevant regions are known.

Butler, J. K.↗
NASA NTRS1992

SNS Heterojunctions With New Combinations Of Materials

New combinations of materials proposed for superconductor/normal-metal/superconductor (SNS) heterojunctions in low-temperature electronic devices such as fast switches, magnetometers, and mixers. Epitaxial heterojunctions formed between high-temperature superconductors and either oxide semiconductors or metals. Concept offers alternative to other three-layer heterojunction concepts; physical principles of operation permit SNS devices to have thicker barrier layers and fabricated more easily.

Vasquez, Richard P.↗
NASA NTRS1994

Si 1-X Ge X /Si Heterojunction Internal Photoemission Long Wavelength Infrared Detector

Long wavelength Si 1-X Ge X /Si heterojunction internal photoemission (HIP) infrared detectors have been successfully demonstrated utilizing the growth of degenerately boron doped Si 1-X Ge X layers on Si. Recently, Si 0.7 GE 0.3 /SI HIP detectors with either a Si 1-X Ge X single layer or a Si 1-X Ge X /Si multilayer have been demonstrated with cutoff wavelengths out to 23 µm. Near-ideal thermionic emission dark current characteristics were measured and the electrical potential barriers were determined by the Richardson plot. A photoresponse model, similar to the modified Fowler Equation has been developed for the Si 1-X Ge X /Si heterojunction internal photoemission infrared detector at wavelengths corresponding to photon energies less than the Fermi energy. The optical potential barriers, the corresponding cutoff wavelengths, and the emission coefficients, C 1 , for the HIP detectors have been determined from the measured spectral responses using the photoresponse model. Similar optical and thermal potential barriers were obtained.

infrared↗
NASA NTRS1975

A theoretical study of heterojunction and graded band gap type solar cells

Heterojunction and graded band gap type solar cells are theoretically investigated. A computer program is developed to account for energy band gap variations and the resulting built-in electric fields which result from heterojunctions and graded energy band gaps. This program is used in studying solar cell operation under various optical irradiation conditions. Results are summarized.

Hauser, J. R.↗
NASA NTRS1975

Glass-Si heterojunction solar cells

An increase in efficiency from 2% to over 7% was achieved by a solar cell that used polycrystal silicon heterojunction with SnO2 glass. For Single crystal substrates, efficiencies close to 10% were measured. These cells had the same structures as polycrystalline cells. Work on p+/n heterojunction using In2O3 glass resulted in cells of 3% efficiency on polycrystalline material and 4% efficiency on single crystal material.

Ahrons, R. W.↗
NASA NTRS1975

Applied research on 2-6 compound materials for heterojunction solar cells

Several II-VI heterojunctions show promise for photovoltaic conversion of solar energy. The three of greatest interest are p-CdTe/n-CdS, p-CdTe/n-ZnSe, and p-ZnTe/n-CdSe. Several p-CdTe/n-CdS heterojunction cells have been prepared by close spaced transport deposition of p-CdTe on single crystal n-CdS, and by two source vacuum evaporation of n-CdS on single crystal p-CdTe. Both types of cells, in an experimental stage, are quite comparable, exhibiting values of quantum efficiency between 0.5 and 0.9, open circuit voltages between 0.50 and 0.66 V, fill factors between 0.4 and 0.6, and solar efficiencies up to 4 percent. Cells of p-ZnTe/n-CdSe have also been made by close spaced vapor transport deposition of n-CdSe on single crystal p-ZnTe.

Bube, R. H.↗
NASA NTRS1982

Mode characteristics of nonplanar double-heterojunction and large-optical-cavity laser structures

Mode behavior of nonplanar double-heterojunction (DH) and large-optical-cavity (LOC) lasers is investigated using the effective index method to model the lateral field distribution. The thickness variations of various layers for the devices discussed are correlated with the growth characteristics of liquid-phase epitaxy over topographical features (channels, mesas) etched into the substrate. The effective dielectric profiles of constricted double-heterojunction (CDH)-LOC lasers show a strong influence on transverse mode operation: the fundamental transverse mode (i.e., in the plane perpendicular to the junction) may be laterally index-guided, while the first (high)-order mode is laterally index-antiguided. The analytical model developed uses a smoothly varying hyperbolic cosine distribution to characterize lateral index variations. The waveguide model is applied to several lasers to illustrate conditions necessary to convert leaky modes to trapped ones via the active-region gain distribution. Theoretical radiation patterns are calculated using model parameters, and matched to an experimental far-field pattern.

Butler, J. K.↗
NASA NTRS1984

Microcrystalline silicon growth for heterojunction solar cells

A single source of evaporation with B mixed with highly doped Si is used instead of the coevaporation of separate Si and B sources to reduce possible carbon contamination. The results of both the heterojunction or heteroface structures, however, are similar when evaporation is used. The best Voc of the heterojunction is about 460mV and no improvement in Voc in the heteroface structure is observed. Slight Voc degradation occurred. A study of the p m-Si/p c-Si structure showed a negative Voc in many cases. The interface properties between the two materials are such that instead of repelling minority carriers from the substrate carrier, collection actually occurred. Another study of cells made in the part of substrates not covered by n-Si results in performance lower than the controls. This indicates possible substrate degradation in the process.

Iles, P. A.↗
NASA NTRS1984

Microcrystalline silicon growth for heterojunction solar cells

Microcrystalline Si (m-Si) films with a 1.7eV energy bandgap and crystal size of several hundred A were e-beam evaporated on single crystalline Si (c-Si) to form a heterojunction with the substrate, or a window layer to a single crystalline p-n junction (heteroface structure). The goal was to enhance Voc by such uses of the larger bandgap m-Si, with the intriguing prospect of forming heterostructures with exact lattice match on each layer. The heterojunction structure was affected by interface and shunting problems and the best Voc achieved was only 482mV, well below that of single crystal Si homojunctions. The heteroface structure showed promise for some of the samples with p m-Si/p-n structure (the complementary structure did not show any improvement). Although several runs with different deposition conditions were run, the results were inconsistent. Any Voc enhancement obtained was too small to compensate for the current loss due to the extra absorption and poor carrier transport properties of the m-Si film.

Leung, D. C.↗
NASA NTRS1986

Low-Resistivity Zinc Selenide for Heterojunctions

Magnetron reactive sputtering enables doping of this semiconductor. Proposed method of reactive sputtering combined with doping shows potential for yielding low-resistivity zinc selenide films. Zinc selenide attractive material for forming heterojunctions with other semiconductor compounds as zinc phosphide, cadmium telluride, and gallium arsenide. Semiconductor junctions promising for future optoelectronic devices, including solar cells and electroluminescent displays. Resistivities of zinc selenide layers deposited by evaporation or chemical vapor deposition too high to form practical heterojunctions.

Stirn, R. J.↗
NASA NTRS1989

Calculation of the electron wave function in a graded-channel double-heterojunction modulation-doped field-effect transistor

Three double-heterojunction modulation-doped field-effect transistor structures with different channel composition are investigated theoretically. All of these transistors have an In(x)Ga(1-x)As channel sandwiched between two doped Al(0.3)Ga(0.7)As barriers with undoped spacer layers. In one of the structures, x varies from 0 from either heterojunction to 0.15 at the center of the channel quadratically; in the other two, constant values of x of 0 and 0.15 are used. The Poisson and Schroedinger equations are solved self-consistently for the electron wave function in all three cases. The results showed that the two-dimensional electron gas (2DEG) concentration in the channel of the quadratically graded structure is higher than the x = 0 one and slightly lower than the x = 0.15 one, and the mean distance of the 2DEG is closer to the center of the channel for this transistor than the other two. These two effects have important implications on the electron mobility in the channel.

Mui, D. S. L.↗
NASA NTRS1989

The photovoltaic properties of an Al In As/InP heterojunctions grown by LPE method

Work is presented on heterojunction solar cells which were studied under the NASA/Arizona State University intern program. The heterojunction solar cells were fabricated by the liquid phase epitaxy method. The basic conversion efficiency was measured at 5 percent. It was determined that a thicker epilayer is needed, and that the density of recombination center should be reduced to give a smaller saturation current and hence a larger open-circuit voltage.

Wang, Edward Y.↗