Showing papers on "Band offset published in 1992"

Empirical fit to band discontinuities and barrier heights in III–V alloy systems

Abstract: We present a figure summarizing the variation of conduction band discontinuity, valence band discontinuity, and gold Schottky barrier height for binary and ternary III–V semiconductors. This figure, which applies to unstrained material, makes use of the property of transitivity in band alignments, and the observed experimental correlation between barrier heights and band gap discontinuities, to consolidate a wide range of data. The figure should be very useful in the design of novel heterostructure electronic and optical devices.

Band Offsets in Semiconductor Heterojunctions

Abstract: Publisher Summary This chapter presents an overview of several theoretical approaches and experimental measurement techniques for determining band offset values and discuss the experimental and theoretical data reported for a number of specific heterojunction systems. It also evaluates the credibility and accuracy of the experimental measurements and provides a tabulation of reliable band offset values for as many heterojunctions as possible. Among the most important physical parameters for a given heterojunction system are the conduction- and valence-band offsets; indeed, the quality and even the feasibility of heterojunction device concepts often depend crucially on the values of these band offsets. The band offset is defined simply as the discontinuity in the band edge at the interface between two semiconductors. A number of current theories seem to yield band offset values in reasonable agreement with experiment, even though the physical ideas underlying these theories can be quite different. These ideas include electron affinities, Schottky barrier heights, bulk band structures on the same energy scale, and the definition of effective midgap energies corresponding to charge neutrality for each bulk constituent.

Band offset determination in analog graded parabolic and triangular quantum wells of GaAs/AlGaAs and GaInAs/AlInAs

Abstract: The band offset parameter Qc = ΔEc/ΔEg for both GaAs/AlGaAs (lattice matched to GaAs), and GaInAs/AlInAs (lattice matched to InP) was extracted from the optical interband transition energies obtained from both triangular and parabolic quantum well shapes of various widths. The wells were grown using continuous analog compositional grading as opposed to the discrete, superlattice (digital) grading used by previous researchers. Electron beam electroreflectance (EBER) was the primary technique used to measure the interband transition energies. By combining the theoretical energies from quantum mechanical potential well calculations with EBER measured energies, it was possible to extract band offset values in a self‐consistent manner. Qc values obtained were 0.658±0.009 and 0.650±0.001 for GaAs/AlGaAs and GaInAs/AlInAs, respectively. Measurements also revealed that Qc was both temperature and concentration independent within the range of composition studied.

Elimination of heterojunction band discontinuities by modulation doping

Abstract: Heterojunction band discontinuities have been an active field of research during the last decade’ and made possible the realization of new device concepts such as modulation-doped transistors, heterobipolar transistors, and quantum-well lasers. The physical principles of these devices are based on heterojunction band discontinuities. In other device structures, however, heterojunction band discontinuities impede the flow of charge carriers across the junction. These structures include the optical distributed Bragg reflector which consists of alternating layers of two semiconductors with different refractive index, each having a thickness of a quarter wavelength. If distributed Bragg reflectors are used for current conduction, the constituent heterojunction band discontinuities impede the current flow, which is a highly undesired concomitant effect. It is the purpose of this publication to demonstrate that unipolar heterojunction band discontinuities can be eliminated by modulation doping and compositional grading of heterojunctions. The charge carrier transport across a heterojunction is illustrated in Fig. 1, which shows the band diagram of two semiconductors “A” and “B.” Band discontinuities occur in the conduction and valence band since the fundamental gap of semiconductor B is larger than the gap of A. Such discontinuities are usually referred to as type-1 heterojunctions, which contrast to type-11 (staggered) and type-III (broken gap) heterostructures. Transport across the heterojunction barrier can occur via thermal emission or via tunneling as schematically illustrated in Fig. 1. For sufficiently thick and high barriers, tunneling and thermal emission of carriers are not efficient transport mechanisms across the barrier. It is therefore desirable to eliminate such heterojunction band discontinuities in the conduction or valence band. Modulation doping of a parabolically graded heterojunction will next be shown to result in a flat-band-edge potential. The band diagram of a parabolically graded conduction-band edge is shown in Fig. 2 (a). The energy of the band edge increases parabolically with a positive second derivative between the points z, and z,. The band edge further increases parabolically with a negative second derivative between z2 and zs. The energy of the band edge can be expressed as / -&(z,) + 2(zf~z,)‘iz - zd’

Charge transfer and low‐temperature electron mobility in a strained Si layer in relaxed Si1−xGex

Abstract: Calculated results for charge transfer and low‐temperature electron mobility in strained silicon grown epitaxially on relaxed Si1−xGex are presented versus the thickness of an undoped spacer layer and other structural and materials parameters. The indicated conduction band offset for Si on relaxed Si0.7Ge0.3 is 180±15 meV. Scattering by the remote doping impurities that supply the carriers is found to be the dominant scattering mechanism in high‐mobility samples. Samples with enhanced interface scattering are expected to have a stronger temperature dependence of mobility.

Room‐temperature 1.3 μm electroluminescence from strained Si1−xGex/Si quantum wells

Abstract: We report the first room‐temperature 1.3 μm electroluminescence from strained Si1−xGex/Si quantum wells. The electroluminescence is due to band‐edge carrier recombination, and its intensity increases linearly with the forward current up to 1700 A/cm2. The internal quantum efficiency is estimated to have a lower limit of 2×10−4. As the temperature is increased from 77 to 300 K, luminescence from the silicon increases relative to that from the Si1−xGex wells. A minimum band offset is required to have effective room‐temperature luminescence from the Si1−xGex quantum wells.

Optical and structural investigation of SiGe/Si quantum wells

Abstract: In this letter we report photoluminescence and structural results obtained on asymmetrically strained Si0.7Ge0.3/Si single and multiple quantum wells epitaxially grown by low pressure chemical vapor deposition. Well‐resolved peaks were obtained which can be attributed to quantum well excitons and their transversal optical phonon replica. A good correlation between peak properties and structure results was found. From the photoluminescence peak energies a valence band offset of 0.27 eV and an effective hole mass of 0.25 were estimated.

Conduction‐band discontinuity in InGaP/GaAs measured using both current‐voltage and photoemission methods

Abstract: Both current‐voltage and photoemission measurements of the conduction‐band discontinuity of the same InGaP/GaAs p‐n heterojunction have been carried out. Interpretation of the current‐voltage results using thermionic emission theory applied to a heterojunction bipolar transistor have resulted in a conduction‐band offset value of 0.21 eV in the case of a compositionally abrupt junction. This figure has been confirmed by performing independent photoemission measurements on the same junction.

Interface measurements of heterojunction band lineups with the Vanderbilt free-electron laser

Abstract: We used optical pumping by the Vanderbilt free-electron laser and the technique of internal photoemission to measure with high accuracy the conduction-band discontinuity of semiconductor heterojunction interfaces. The experiment is the first application to our knowledge of a free-electron laser to interface research.

Interfacial atomic structure and band offsets at semiconductor heterojunctions

Abstract: The relationship between interfacial atomic structure and band offsets at semiconductor heterojunctions is explored through first‐principles local density functional calculations. In particular, the effects of variations in interfacial geometry are analyzed for (001) interfaces between III–V/III–V materials. For the AC/BC case of a common atom, isovalent A–B intermixing in the noncommon atomic planes near the interface does not affect the band offset, even in the case of large lattice‐mismatched systems. For quaternary AB/CD systems, there are two possible chemically abrupt interfaces (A–D or B–C); these can have offsets that differ by up to 80 meV. In those cases where the chemically abrupt AB/CD offset depends on the interfacial identity, intermixing leads to offset variations which are directly related to the offset difference between the chemically abrupt A–D and B–C interfaces. The differing behavior of common‐atom versus noncommon‐atom systems is analyzed in terms of the symmetry of the nearest‐neig...

Band alignments in Zn(Cd)S(Se) strained layer superlattices

Abstract: The authors estimate, by reviewing the available photoluminescence data, the band alignments at heterojunctions of Zn(Cd)S(Se) semiconductors. The photoluminescence peak energies of ZnSe-ZnS, CdS-ZnS and CdSe-ZnSe strained layer superlattices (SLS), with a range of well and barrier widths, have been calculated using Kronig-Penney theory and assumed values of the band offsets which yield acceptable fits to both the experiments and data taken from the literature. For ZnSe-ZnS SLS, fair agreement is obtained between calculation and experiment by assuming, in line with previous work, a negligible conduction band offset. At the same time, good estimates of the photoluminescence peak energies of CdS-ZnS and CdSe-ZnSe superlattices are obtained by assuming a negligible valence band offset in each case. The authors construct a band alignment diagram which predicts that CdSe-CdS and CdS-ZnSe are type-II SLS with electron confinement in the CdS layers; these predictions are borne out by experiment.

Estimation of critical thicknesses and band lineups in ZnCdSe/ZnSSe strained-layer system for design of carrier confinement quantum well structures

Abstract: For the fabrication of practical single and multiple semiconductor quantum wells (QWs) capable of confining carriers in the well layers, it is essential that the compositions and thicknesses of constituent layers be suitably chosen to yield sufficiently large conduction and valence band offsets between well and barrier layers, and to avoid lattice relaxation. In this paper we present a guideline for the design of strained-layer ZnCdSe/ZnSSe carrier confinement QWs which satisfy such requirements, based on the estimations of critical thicknesses and band lineups of the structures. From the results obtained here, ZnCdSe/ZnSSe QWs are demonstrated to be eminently suitable for applications to optoelectronic devices operating in the blue-green region of the spectrum.

Heavy- and light-hole character of optical transitions in InAs/GaAs single-monolayer quantum wells.

Abstract: We examine the nature of the optical transitions in InAs/GaAs single-monolayer quantum wells grown by molecular-beam epitaxy. The heavy- and light-hole character of the observed transitions is revealed by monitoring the linear polarization of the emitted luminescence and by selective excitation of the states by linearly polarized light. In addition, these experiments provide information about the band alignment between InAs and GaAs.

Microscopic capacitors and neutral interfaces in III-V/IV/III-V semiconductor heterostructures.

Abstract: We examined band discontinuities in AlAs-Ge-GaAs(001) and GaAs-Ge-AlAs(001) single-quantum-well structures, as well as individual isolated Ge-GaAs(001), Ge-AlAs(001), GaAs-Ge(001), and AlAs-Ge(001) heterojunctions. We found that well-defined in- equivalent neutral interfaces are established in III-V/IV/III-V structures for Ge coverages as low as 1--2 monolayers. Deviations from the transitivity and commutativity rules of heterojunction behavior reflect inequivalent local interface environments rather than charged interfaces.

The characteristics of an In0.5Ga0.5P and In0.5Ga0.5P/GaAs heterojunction grown on a (100) GaAs substrate by liquid‐phase epitaxy

Abstract: The growth of a high‐quality In0.5Ga0.5P/GaAs heterostructure on a (100) GaAs substrate by liquid‐phase epitaxy is demonstrated. This has been achieved by controlling the vaporizing time of phosphorus after the melt saturation procedure. The photoluminescence spectra of In0.5Ga0.5P/GaAs heteroepitaxial layers show that the major residual acceptor impurity is either carbon or silicon. The measured values of the conduction‐band discontinuity ΔEc and the fixed interface charge density σi for a In0.5Ga0.5P/GaAs heterostructure are 110 meV and 1×1011 cm−2, respectively. Only one electron trap with a thermal activation energy of Ea=0.32 eV, which is thought to be related to the anion vacancy, is found in Sn‐doped In0.5Ga0.5P (n∼1×1017 cm−3) layers.

Measurement of the conduction‐band discontinuity in pseudomorphic InxGa1−xAs/In0.52Al0.48As heterostructures

Abstract: Compositional dependence of the conduction‐band discontinuity ΔEc in InxGa1−xAs/In0.52Al0.48As pseudomorphic heterostructures has been measured as a function of InAs mole fraction over the range of 0.44≤x≤0.64 using both current‐versus‐voltage‐versus‐temperature and capacitance‐versus‐voltage measurements on semiconductor‐insulator‐semiconductor structures. The results show a monotonic increase of effective ΔEc with InAs mole fraction x according to ΔEc≊0.384+0.254x for x≤0.54 and an abrupt shift to ΔEc≊0.344+0.487x for x≥0.58. The effects of the conduction‐band nonparabolicity and the lattice strain on the Fermi potential have been taken into account in deducing ΔEc from the measured barrier height across the InxGa1−xAs/In0.52Al0.48As heterojunction.

Magneto-optical study of CdTe/Cd1-xMnxTe multiple quantum wells with low potential barriers

Abstract: The authors present an optical study of multiple quantum wells of CdTe/Cd1-xMnxTe grown with a low manganese concentration in the barrier layers, typically x approximately=0.05. The structures grow pseudomorphically to the substrate so that the strains in the CdTe and CdMnTe layers are precisely known. In an attempt to determine the valence band offset the energy difference between the light- and heavy-hole excitons has been carefully analysed. They find that in these quantum wells with small barrier heights, and hence with small confinement energies, the exciton binding energies may account for a substantial fraction of the splitting. Thus a precise determination of the valence band offset would need a very accurate calculation of these binding energies. The transition from a type I to a type II band structure with an applied magnetic field has been investigated. Various simulations show that the large decrease of energy expected for the heavy-hole excitonic gap at this transition can be almost completely compensated by a simultaneous decrease of the exciton binding energy, which explains why no significant features at the transition were observed in their samples.

Optical and structural properties of ternary alloy AlxGa1−xAs/AlAs multiple‐quantum‐well structures

Abstract: The linear and nonlinear optical properties of a series of ternary alloy AlxGa1−xAs/AlAs multiple‐quantum‐well structures have been investigated and related to the multilayer configuration. The direct energy gap was found to scale with the AlAs mole fraction as predicted by Lee and Yuravel [Phys. Rev. B 21, 659 (1980)] and the band offset ratio to depend on the alloy composition. Exciton absorption bleaching was observed at room temperature and the nonlinear absorption cross sections were estimated for the first two confined excitonic states. Finally, the possibility of achieving optical gain for the type‐II band alignment along the growth direction as well as in the layer plane is demonstrated.

Microscopic Manipulation of Homojunction Band Lineups

Abstract: We tested the theoretical prediction that the band structures on the opposite sides of a homojunction can be artificially displaced in energy with respect to each other by means of double intralayers of atomiclike thickness, producing band discontinuities of potential interest for practical applications. Evidence of such discontinuities was found when Ga‐As, Al‐As, Ga‐P, or Al‐P intralayers were inserted between Si and Si or Ge and Ge.

Electronic structure and total energy of diamond/BeO interfaces

Abstract: Electronic structure calculations are used to study the bonding at diamond/BeO interfaces. The {110} interface between zinc blende BeO and diamond is used as a representative model for general reconstructed interfaces characterized by an equal amount of Be–C and O–C bonds. The interface energy is calculated to be 2 J/m2 and combined with the estimated free surface energies to obtain an estimate of the adhesion energy. It is found to be close to the adhesion of BeO to itself, but somewhat lower than that of diamond to itself. The effects of the 7% lattice mismatch on the total energy and the band structure for a biaxially strained pseudomorphic diamond film are investigated. The effect of misfit dislocations, expected to occur for thicker films, on the adhesion energy is estimated to be lower than 10%. The bulk properties, such as equilibrium lattice constant, bulk modulus, cohesive energy, and band gap of BeO are shown to be in good agreement with experimental values and previous calculations. The valence-band offset is calculated to be 3.9 eV and found to take up most of the large band gap discontinuity. The nature of the bonding is discussed in terms of the local densities of states near the interface. The interface localized features are identified in terms of Be–C and O–C bonding and antibonding states.

Electronic Structures of Hetero-Crystalline Semiconductor Superlattices

Abstract: Crystal structures of hetero-crystalline superlattices made of materials with different crystalline structures are proposed and band structures of (hexagonal-diamond Si)/(cubic-diamond Si) and (wurzite ZnS)/(zinc-blend ZnS) superlattices are calculated using the first-principles pseudopotential method within the local-density approximation. It is shown that Si superlattices have indirect band gaps of energy around 1.1 eV, while ZnS superlattices have direct band gaps of energy around 3.8 eV. In both superlattices, the states around the fundamental gap show charge densities localized in one constituent layers, indicating that these superlattices give quantum well systems of good quality. Band offsets are also evaluated.

High valence‐band offset of GaSbAs‐InAlAs quantum wells grown by molecular beam epitaxy

Abstract: Photoluminescence measurements were made at 2–300 K on GaSbAs‐InAlAs single quantum wells lattice matched to InP grown by molecular beam epitaxy. The experimental exciton transition energy from the first electron subband to the first heavy hole subband was analyzed with calculated values obtained from the envelope function method. The quantum well exciton transition energy for well widths of 10–300 A is fitted to a large valence‐band offset (ΔEv) of 0.93 ΔEg. The exciton linewidth increases with decreasing well width. The two dominant exciton line broadening mechanisms were found to be monolayer fluctuations of the well width and the band filling of electron.

Valence Band Offset and Electronic Structures of Zinc-Compound Strained Superlattices

Abstract: Electronic structures and band offsets of Zinc-compound (001) strained superlattices are calculated by the first-principles pseudopotential method. It is shown that strained superlattices become quantum well systems of good quality for the states around the fundamental gap. With varying the strain surroundings, band offset drastically changes due to the energy-level shift by strain and charge transfer at the interface, mechanism of which is analyzed in order to predict the general feature of the variation of offset in strained superlattices.

Transport and Microstructure of Microcrystalline Silicon Alloys

Abstract: The microstructure and electrical properties of μc-Si and μc-Si,C prepared by remote plasma-enhanced chemical-vapor deposition, PECVD, are reviewed. The microstructure has been characterized by transmission electron microscopy, TEM, infrared, IR, absorption and Raman scattering. The electrical properties were characterized by temperature-dependent dark-conductivity measurements. These studies have explained significant quantitative differences between the carrier transport properties of μc-Si and μc-Si,C alloys in terms of a band offset model for the interfacial potential steps between the amorphous and crystalline constituents of these material systems.

Long wavelength infrared photocurrent study of Si-SiGe heterostructures

Abstract: We report on an experimental long wavelength infrared photocurrent study of a series of a Si‐SiGe double‐heterostructure samples. The active region is a thin heavily p‐type doped SiGe layer, and the photoexcited holes due to free‐carrier absorption are collected over the potential barrier resulting from the Si‐SiGe valence‐band offset. Photocurrent spectra with different cutoff wavelengths are observed for samples with different SiGe compositions, arising from internal photoemission in the Si‐SiGe heterojunction. Photocurrents at finite biases and at zero bias (i.e., photovoltaic operation) are studied. Optimizing device parameters may lead to detector structures for large focal plane arrays.

Determination of the valence-band offset of GaAs-(Ga,In)P quantum wells by photoreflectance spectroscopy.

Abstract: This work reports on growth, characterization, and calculation of the electronic structure of GaAs-(Ga,In)P quantum wells, where the two semiconductors share neither a common anion nor a common cation. Metal organic molecular-beam epitaxy was the growth method that we used. The composition of the disordered alloy was close to 50 at. % indium. We have determined the valence-band offset in such structures. We have found ΔE c /ΔE υ =0.4, and have calculated the influence of the spin-orbit split-off states on the light-hole confinement. We found that these deep valence states significantly reduce the light-hole confinement