Showing papers on "Heterojunction published in 1978"

Electron mobilities in modulation‐doped semiconductor heterojunction superlattices

Abstract: GaAs‐AlxGa1−xAs superlattice structures in which electron mobilities exceed those of otherwise equivalent epitaxial GaAs as well as the Brooks‐Herring predictions near room temperature and at very low temperatures are reported. This new behavior is achieved via a modulation‐doping technique that spatially separates conduction electrons and their parent donor impurity atoms, thereby reducing the influence of ionized and neutral impurity scattering on the electron motion.

Polar heterojunction interfaces

Abstract: A study of heterojunction interface geometry based on our measured differences in $3d$ core-state binding energies for germanium and gallium at Ge-GaAs heterojunctions of different crystallographic orientations is reported. For the interfaces which have been studied, i.e., (110), (100) Ga, (100) As, (111) Ga, and ($\overline{1}\overline{1}\overline{1}$) As, orientation-dependent variations in dipole contributions to valence-band discontinuities of about 0.2 eV have been observed. From electrostatic considerations we deduce the simplest interface geometries consistent with the facts that the differences are small and no large charge accumulations can occur at the junction. An abrupt planar junction is allowed for the (110) interface, but the polar interfaces require at least two transition planes of atoms with compositions which are deduced from the two conditions above. The electrostatic calculations were based upon the differences in nuclear charge and are unaffected by the resulting polarization of the bonds if that polarization is described in an "electronegativity" approximation. In this approximation there would in fact be no dipole shift for the ideal geometries proposed. An improved treatment of the bond polarization based upon the bond-orbital model gives residual dipole shifts somewhat smaller than those observed, and in poor agreement with our measurements. Inclusion of lattice-distortion effects at the interface also fails to account for the observed dipole shifts. We conclude that the experimentally prepared junctions must contain deviations from the ideal atom arrangements. The number of these deviations required to account for the observed shifts is on the order of one for every fifteen interface atoms.

Heterostructure lasers. Part A. Fundamental principles

Abstract: The book consists of two parts. Part A contains chapters entitled: Introduction; Optical Fields and Wave Propagation; Stimulated Emission in Semiconductors; and Heterojunctions. (TFD)

Photovoltaic properties of ZnO/CdTe heterojunctions prepared by spray pyrolysis

Abstract: An extended investigation has been made of the electrical and photovoltaic properties of heterojunctions prepared by spray‐pyrolysis deposition of thin ZnO films on single‐crystal p‐type CdTe. The principal experimental variables were the substrate temperature and the postdeposition temperature for annealing in H2. Under actual sunlight the optimum cell showed an open‐circuit voltage of 0.54 V, a short‐circuit current of 19.5 mA/cm2, and a solar efficiency (referred to the active area) of 8.8%, the highest value obtained to date for an authentic heterojunction on CdTe. The nature of the forward transport mechanism has been investigated, and a tunneling model in which bulk and interface deep traps control the forward characteristics is shown to provide good correlation with the experimental data.

InAs-GaSb superlattice energy structure and its semiconductor-semimetal transition

Abstract: We performed a band calculation, based on the LCAO (linear combination of atomic orbitals) method, for the InAs-GaSb superlattice. Of possible semiconductor combinations the InAs-GaSb is of special interest because of indications that at a heterojunction interface the bottom of the conduction band of InAs lies below the top of the valence band of GaSb. Our results show that the InAs-GaSb superlattice is a semiconductor when the layers of the constituent materials are thin, and becomes a semimetal when the layer thicknesses are increased. The critical InAs thickness for this transition is approximately 115 \AA{}, over which the electrons from the valence band of GaSb "flood" the conduction band of InAs. For the thick-layer limit, we treat the problem in a Fermi-Thomas approximation.

Raman spectroscopy—A versatile tool for characterization of thin films and heterostructures of GaAs and AlxGa1−xAs

Abstract: It is shown that Raman spectroscopy can provide useful information on characteristic properties of thin crystalline films of compound semiconductors. Crystal orientation, carrier concentration, scattering times of charge carriers, composition of mixed crystals and depth profiles can be studied in thin layers and heterostructures of GaAs and AlxGa1−xAs. The advantages and disadvantages of Raman scattering compared to conventional characterization methods are discussed.

Electrical current in solids with position-dependent band structure

Abstract: Conduction in materials with a position-dependent band structure, such as graded band gap semiconductors and graded heterojunctions, is considered. A brief discussion of the meaning of spatially varying band structure, based on an extension of Wannier's theorem, is given. Using this extended theorem, it is shown that the conventional Boltzmann transport equation is inadequate for the situation under discussion; a generalized form of the Boltzmann transport equation is derived. From this the complete electron and hole currents are obtained. Besides drift, chemical, and thermal diffusion, they contain two extra terms. One of these involves the gradient of the electron affinity and of the band gap, while the other one depends on the gradient of the density of states. The results thus extend previous results by van Ruyven and Williams and by van Overstraeten et al. It is further shown that the results can be transformed to a form which resembles the conventional current equations, providing a modified position-dependent mobility and diffusivity are introduced. A discussion of the realm of applicability of these results is also presented.

SnO2/Si solar cells—heterostructure or Schottky‐barrier or MIS‐type device

Abstract: The performance of SnO2/Si solar cells has been measured and analyzed in detail. This has given us an insight into cell characteristics never revealed in previous studies. Though the device is a junction between two semiconducting materials, SnO2 and Si, it performs like an MIS device and in the thin‐oxide limit as a Schottky‐barrier device. The dark I‐V characteristics are attributed to a combination of thermionic and diffusion processes. The diode constant associated with the former is close to 2 or higher and that associated with the latter is unity. It has been demonstrated that under certain conditions the diode constant can be independent of the reverse‐saturation current. An MIS‐type model can explain the spectral‐response curve, the short‐circuit photocurrent, the open‐circuit photovoltage, and the light and dark I‐V characteristics. A theoretical limit of the efficiency is estimated for the device.

Ge–GaAs(110) interface formation

Abstract: The heterojunction chemistry for Ge grown by molecular beam epitaxy (MBE) on in situ cleaved GaAs exhibits significant interdiffusion in short times at growth temperatures T G of 430°C (significantly lower critical T G than that reported for moderate-vacuum physical vapor deposition). This results in profound changes in the electronic properties of the interface as probed by synchrotron-radiation-excited 3d core electron photoemission. Even when there is significant alloying of the two lattice-matched semiconductors, there is nearly equal probability for Ge to bond to either a Ga or an As atom at the initial stage. As Ge becomes the dominant species, we find As preferentially diffusing toward the Ge side of the junction. This As is distributed throughout the overlayer in contrast to metal-semiconductor interface formation where the diffusing constituent resides only on the free, growing surface. We show that these behaviors are consistent with the kinetic and thermodynamic properties of the atomic species. The valence band discontinuity is negligible over atomic dimensions, while for an abrupt interface (T G = 350°C) we measure ΔE v = 0.7± 0.3 0.05 eV. The photoemission changes character rapidly with temperature, indicating an activation barrier for the diffusion below which simple expressions for attenuation of the photoelectrons by electron—electron scattering are applicable. In that case we deduce an escape depth of 7.0 ±0.5 A, indicating uniform growth of Ge, with composition changing abruptly from GaAs over ~1 bond length in the (110) direction. A negligible (<0.2 eV) localized interface dipole layer is formed in the process.

VPE Growth of III/V Semiconductors

Abstract: In recent years research on the epitaxial growth of IIIjV alloys has con­ centrated on those systems capable of producing lattice-matched hetero­ structures. The most important alloy of this type is AlxGal-xAs because the nearly exact lattice-parameter match between GaAs (ao = 5.654) and AlAs (ao = 5.661) allows the growth of lattice-matched heterostructures with nearly ideal interfaces and direct bandgap energies covering an important range for optoelectronic devices from 1 .43 to ,..", 2.0 eV. Other systems for which the lattice parameter of the epilayer can be made to match that of the substrate at a given composition such as GaxInl-xP / GaAs, or for a range of compositions, such as GaxIn 1 xAsyP 1 y/InP, are in much earlier stages of development. The ability to grow high quality lattice-matched AlxGa1-xAs heterostructures has resulted in dramatic improvements in the performance of such optoelectronic devices as lasers, high radiance IR LEDs for optical communication applications, visible LEDs, and integrated optics elements and circuits (1). In addition, two-layer devices where the top AlxGal-xAs layer acts to reduce surface recombination and to transmit effectively bandgap light to the junction have resulted in solar cells with air mass zero efficiencies of 19% (2) versus 14% for the best Si devices. If these could be produced cheaply, they could have significant impact on the feasibility of terrestrial photo voltaic solar energy converSlOn. A number of techniques used for the epitaxial growth of IIIjV semi­ conductors might be applied to the growth of AlxGal-xAs, including liquid phase, vapor phase, and molecular beam expitaxial growth (LPE, VPE, and MBE, respectively). Nearly all of the devices listed above are fabricated in AlxGal-xAs grown by the LPE technique. The LPE tech­ nique for a multilayer structure consists of a series of individual growth

The influence of bulk nonradiative recombination in the wide band‐gap regions of molecular beam epitaxially grown GaAs‐AlxGa1−xAs DH lasers

Abstract: It is shown that bulk nonradiative recombination near the heterojunctions in the wide‐band‐gap regions is probably an important contributor to the higher threshold current densities of the best state‐of‐the‐art molecular beam epitaxial GaAs‐AlxGa1−xAs DH lasers over similar geometry liquid‐phase epitaxial lasers. This recombination probably is at least part of the cause of the large recombination velocity (measured as interfacial recombination velocity) in molecular beam epitaxial heterostructures.

Semiconductor light-emitting device

Abstract: A semiconductor light-emitting device comprising a sapphire substrate, an epitaxial layer of monocrystalline semiconductor p-type gallium nitride deposited on the substrate, and a layer of semiconductor-type aluminum nitride, deposited on the semiconductor gallium nitride layer and forming an injecting heterojunction therewith. The device emits bright blue light.

An InGaAs detector for the 1.0–1.7‐μm wavelength range

Abstract: We report an InGaAs/InP heterojunction detector having high quantum efficiencies in the 1.0–1.7‐μm wavelength range. Conditions for growth of the lattice‐matched composition by liquid‐phase epitaxy at various temperatures are described.

Low‐threshold continuous laser operation (300–337 °K) of multilayer MO‐CVD AlxGa1−xAs‐GaAs quantum‐well heterostructures

Abstract: Data are presented on multilayer AlxGa1−xAs‐GaAs quantum‐well heterostructures showing that cw 300–337 °K laser operation is possible at photoexcitation threshold levels (≲1.2×103 W/cm2, Jth≲500 A/cm2) comparable to better LPE double heterojunctions and much lower than all previous single or multiple quantum‐well heterostructures. These quantum‐well heterostructures are grown by metalorganic chemical vapor deposition (MO‐CVD) and consist of four 80–90‐A GaAs active layers coupled by three 80–90‐A AlxGa1−xAs (x∼0.35) barriers, all of which are sandwiched between 1‐ and 0.3‐μm AlxGa1−xAs (x∼0.40) confining layers.

Angle‐resolved photoemission measurements of band discontinuities in the GaAs‐Ge heterojunction

Abstract: The conduction- and valence-band discontinuities for the (110) GaAs-Ge heterojunction have been measured as ΔE c = 050 eV and ΔE v = 025 eV by the angle-resolved ultraviolet photoemission (ARUPS) technique These values are in good agreement with the theoretical predictions of Pickett et al

Low‐resistivity ZnCdS films for use as windows in heterojunction solar cells

Abstract: Low‐resistivity ZnxCd1−xS films have been obtained by a multisource evaporation method. The films have been doped with In during the deposition. The resistivity of such films varies from 2×10−3 Ω cm for x=0 up to 2 Ω cm for x=0.3 and rises up to 1012 Ω cm for x=1. For energies lower than the energy gap, the transparency of these films, when corrected for the reflection loss, can reach a value of almost 100%. In the range of an x variation between 0 and 0.4 these films, because of their low resistivity and their high transparency, can be used as windows in heterojunction solar cells.

Double heterojunction solar cells

Abstract: Photovoltaic cell comprising two heterojunctions between three component semiconductors Ga 1-x AL x As with x varying from 0 to 0.9, GaAs, and Ge which have respective bandgaps of 0.66, 1.43 and 2.4 eV, lattice constants matching at 0.07% and expansion coefficients matching at 1.7%. The cell is mounted in a cell device comprising a parallelepipedal unit, a Fresnel lens, a tapering cavity within said unit, partially filled up with a lens shaped antireflecting transparent material and a radiator, said device forming a sunlight concentrator.

Fabrication and characterization of indium tin oxide (ITO)/polycrystalline silicon solar cells

Abstract: Efficient indium tin oxide (ITO)/polycrystalline silicon heterojunction solar cells have been fabricated utilizing neutralized ion‐beam sputtering techniques. These cells were fabricated on single‐pass float‐zone‐refined silicon. Conversion efficiencies of 6.25% under AM1 illumination have been observed. Cells were analyzed by I‐V characteristics and a scanning laser photoresponse technique. Qualitative minority‐carrier lifetime has been mapped using the EBIC mode of a SEM. This has revealed a reduced photoresponse at the grain boundaries independent of grain size, and also at defect clusters within individual grains. Surface blemishes and etch pits are not important in reducing the cell photoresponse. It appears that the low‐temperature processing inherent in semiconductor‐insulator‐semiconductor solar cells is applicable to polycrystalline material.

CdS/InP and CdS/GaAs heterojunctions by chemical‐vapor deposition of CdS

Abstract: CdS/InP and CdS/GaAs heterojunctions were prepared by an in situ H2S/H2 etching process and H2 transport of CdS onto the InP and GaAs substrates. The etching involves a reaction of the III‐V compound surface to the corresponding sesquisulfides and simultaneous or subsequent removal by reduction in H2 gas. The etching process changes the junction characteristics considerably. The junction on p‐GaAs shows a pinning of the Fermi level at the interface approximately 0.75 eV above the GaAs valence band. The junction on n‐GaAs is Ohmic.

The effects of grain boundary and interface recombination on the performance of thin-film solar cells

Abstract: The effects of grain boundary and interface minority carrier recombination on polycrystalline thin-film photovoltaic heterojunctions are presented. The grain boundary is modeled accounting for interface states, due to low-, medium- and high-angle grain boundaries. The dark and illuminated diffusion potentials are calculated as functions of interface state densities and carrier concentrations. These are used to estimate the recombination velocities and minority carrier lifetimes. These parameters are, in turn, correlated with the short-circuit currents and open-circuit voltages. The dependence of Voc upon grain diameter is predicted. Junction interface states are discussed in terms of a dislocation model. The dependence of Voc, affected primarily through dark reverse current, on the recombination velocity is indicated. The combined effects of grain boundary and interface recombination mechanisms on Jsc is discussed. Data are presented to verify the model based upon the CdS/CuInSe2 photovoltaic heterojunction.

Electrical and photovoltaic characteristics of indium-tin oxide/silicon heterojunctions

Abstract: Measurements on indium-tin-oxide/Si heterojunctions indicate that the conduction band discontinuity at the interface, ΔEc, is on the order of 0.45 eV for ITO with 9 mole-% SnO2. This value is inappropriate for use of ITO/Si heterojunction cells for photovoltaic energy conversion. It is too large for ITO/p-Si cells and too small for ITO/n-Si devices. For either type of device the resultant built-in voltage is inadequate for high efficiency solar cells. The maximum solar conversion efficiencies obtained are on the order of 0.3%. These results are in disagreement with previous work on ITO/Si heterojunctions.

The influence of interface state and energy barriers on the efficiency of heterojunction solar cells

Abstract: A heterojunction solar cell, in which interface recombination and arbitrary energy barriers at the interface occur, is analysed to find the optimum structure. Thermionic emission theory is used; it is assumed that one type of carriers are majority carriers at the interface and that their concentration can be known a priori. It is shown that these assumptions do not seriously limit the applicability of the theory. The results are discussed in the case when one semiconductor is much more heavily doped than the other, which is a condition that certainly improves the efficiency of the solar cell.

Liquid‐phase epitaxial growth of lattice‐matched InGaAsP on (100)‐InP for the 1.15–1.31‐μm spectral region

Abstract: The distribution coefficients for the growth of lattice‐matched InGaAsP on (100) ‐InP substrates in the 1.15–1.31‐μm spectral range have been determined. These results have been used in the growth of heterojunction photodiodes with quantum efficiencies ⩾48% at 1.27 μm.

Bandfilling in metalorganic chemical vapor deposited AlxGa1−xAs‐GaAs‐AlxGa1−xAs quantum‐well heterostructure lasers

Abstract: Data are presented showing that quantum‐well (Lz∼200 A) AlxGa1−xAs‐GaAs‐AlxGa1−xAs heterostructures, grown by metalorganic chemical vapor deposition, can be operated as lasers on confined‐particle transitions over an unusually large range (Δλ≳1000 A). The bandfilling properties of these quantum‐well heterostructures, which can easily be excited to carrier densities as high as n≳1019/cm3, are described. Quantum‐well laser diodes (x∼0.5) are described that operate (300 K) from the Γ band edge to wavelengths as short as 7700 A (ΔE∼185 meV). Narrow photopumped samples (15–30 μm) are shown to operate (77 K) as lasers on clearly defined confined‐particle transitions from the band edge to 6980 A (ΔE∼270 meV). On samples from another wafer, laser operation has been observed to 6885 A (ΔE≡hν−Eg=293 meV). The photoluminescence spectra of these heterostructures extend well into the region of the recently determined L band minima of GaAs.

Beamwidth approximations for the fundamental mode in symmetric double-heterojunction lasers

Abstract: The beamwidth for the fundamental TE 0 mode in symmetric DH lasers is approximated within 4 percent by analytical formulas covering wide ranges of cavity thickness and refractive index steps. By using a Gaussian approximation for near-field distributions, and numerically calculated beamwidth curves, we find a Gaussian beam-like expression for the beamwidth, over the range 1.5 , where D is the normalized guide thickness. In the nonGaussian region 0 , the beamwidth is estimated by using a corrected Dumke-like asymptotic approximation.

Use of oxygen‐doped AlxGa1−xAs for the insulating layer in MIS structures

Abstract: Metal‐insulator‐semiconductor capacitors were prepared by doping an Al0.5Ga0.5As layer with oxygen in order to demonstrate a lattice‐matched single‐crystal insulator‐semiconductor heterojunction. Crystal growth was by molecular‐beam epitaxy. Interface trap effects were not observed in capacitance‐voltage measurements, while the current‐voltage measurements show space‐charge‐limited currents for forward and reverse biases.