Showing papers on "Band offset published in 1989"

Strain effects and band offsets in GaAs/InGaAs strained layered quantum structures

Abstract: Strained single quantum wells composed of GaAs/InGaAs/GaAs were grown by molecular beam epitaxy and characterized at room temperature by photoreflectance and at 6 and 77 K by photoluminescence spectroscopy. For the InGaAs/GaAs heterojunction, utilizing a band offset ratio of 85:15 (conduction band:valence band) for the intrinsic (nonstrained) interface and a contribution of the hydrostatic compression to the valence band movement corresponding to the pressure sensitivity of the spin orbit band, excellent agreement is found between calculated excitonic transition energies and those found by experiment at all temperatures studied. Our analysis indicates that material parameters and the combined strain components used to calculate band structure are not temperature dependent to our degree of sensitivity. An empirical equation, which differs slightly from that for bulk InGaAs crystals, describing the nonstrained band‐gap energy as a function of In fraction at 77 K is presented. The difference between band offset ratios for the intrinsic and strained heterojunction are found to be significant and the relative merits of each are discussed.

Band‐edge discontinuities of strained‐layer InxGa1−xAs/GaAs heterojunctions and quantum wells

Abstract: The conduction‐band discontinuity (ΔEc ) and the band‐gap offset (ΔEgh) of InxGa1−xAs/GaAs multiple quantum wells grown on GaAs substrates by molecular beam epitaxy are investigated for 0

Optical and electrical properties of isotype crystalline molecular organic heterojunctions

Abstract: We have studied the electrical and optical characteristics of isotype p‐P organic heterojunctions (HJ) consisting of CuPc (copper phthalocyanine) and PTCDA (3,4,9,10‐perylenetetracarboxylic dianhydride). It is found that the charge‐transport properties of the heterojunction are limited by thermionic emission of holes over the energy barrier at the CuPc/PTCDA heterojunction at low forward and reverse bias, and by series resistance at high voltage. The heterojunction energy barrier at the CuPc/PTCDA valence‐band edge was measured using both current‐voltage and capacitance‐voltage analysis and was found to be ΔEvC,P=0.48±0.05 eV. Similar measurements made for HJs consisting of CuPc and PTCDA in combination with another perylene‐based material, 3, 4, 9, 10‐perylenetetracarboxylic‐bis‐benzimidazole (PTCBI), suggest that the band offsets for these three materials follow a transitive relationship. That is, ΔEvC,P=ΔEvC,B−ΔEvB,P, where subscripts C, P, and B refer to CuPc, PTCDA, and PTCBI, respectively. The resul...

Band‐edge alignment in heterostructures

Abstract: Conduction‐band and valence‐band energies are presented for ternary III‐V compounds in a novel way. These data are used to evaluate new material combinations for heterostructure devices.

Electronic structure of cadmium-telluride-zinc-telluride strained-layer superlattices under pressure.

Abstract: The pressure dependence of the low-temperature photoluminescence of CdTe/ZnTe strained-layer superlattices is reported, up to the phase transition of the structure at about 6 GPa. The superlattices can be simultaneously type I for heavy holes and type II for light holes. A theoretical fit to the pressure dependence of the type-I and type-II luminescence lines confirms a number of bulk and superlattice parameters. In particular, the results are consistent with a treatment of the band structure in the framework of the envelope-function approach. The ground valence-band state is found to be the first light-hole valence subband. The valence-band offset has been fitted to be pressure dependent; we found 75+4.5P meV. Under pressure, a type-I\char21{}type-II transition is observed, due to a crossover in the valence band of the superlattice.

Tunnel spectroscopy of the AlGaAs-GaAs heterostructure interface

Abstract: The material interface of a molecular beam epitaxy grown Al0.5Ga0.5As‐GaAs heterostructure is investigated on a cross‐sectional (110) cleavage plane using tunnel spectroscopy. The depleted n‐type region and the electron confinement layer adjacent to the interface are identified with local current‐voltage spectroscopy. The spatial width of these layers is close to 15 nm. The spectroscopy can be interpreted with the valence‐band offset in the interface, and a value of 0.35 eV is found for this quantity.

Can We Tune the Band Offset at Semiconductor Heterojunctions

Abstract: The long-standing problem of determining which interface-specific properties affect the band offset at semiconductor heterojunctions is readdressed using a newly developed theoretical approach. The actual interface is considered as a perturbation with respect to a reference periodic system (virtual crystal). By comparison with state-of-the-art self-consistent calculations, we show that linear-response theory provides a very accurate description of the electronic structure of the actual interface in a variety of cases, and sheds light on the mechanisms responsible for the band offset. Results are presented for a number of lattice-matched junctions, both isovalent and heterovalent. It is shown that—within linear response theory—band offsets are genuine bulk properties for isovalent interfaces, whereas they do depend on the atomic structure of the junction for polar interfaces between heterovalent semiconductors. In the latter case, however, the interface-dependent contribution to the offset can be calculated—once the microscopic geometry of the junction is known—from such simple quantities as the lattice parameters and dielectric constants of the constituents. Perspectives for extending the theory to non-lattice-matched systems are also briefly discussed.

Doping and p-n function formation in InAs 1-x Sb x /InSb SLS's by MOCVD

Abstract: Diethylzinc, dimethylcadmium, hydrogen selenide, silane, dimethyltellurium, and di-ethyltellurium were investigated as dopants for InSb and InAs1-xSbx Carrier concentrations between 5 x 103 and 5 x 1019 cm-3 have been achieved for bothn- and p-type dopants by using dilute mixtures, 10 to 50 ppm, of dimethylcadmium and dimethyltellurium in hydrogen The 77 K Hall mobilities of p-type InSb ranged from 7000 to 200 cm2/Vs and of n-type from 55,000 to 1700 cm2/Vs An InAs017Sbo083/InSb SLS infrared photodiode has been fabricated with a wavelength response cutoff of 102 μm at 77 K The zero bias, external current responsivity and infrared absorption of this device were measured The predicted optical transitions using a type II heterojunction band offset closely match the observed absorption The minority carrier diffusion length, perpendicular to the growth planes, is approximately 05 μm

Resonant tunneling of holes in AlSb/GaSb/AlSb double‐barrier heterostructures

Abstract: We have made the first observations of resonant tunneling in the AlSb/GaSb material system. Double‐barrier p‐type heterostructures exhibit two distinct features in their current‐voltage characteristics, indicating resonant tunneling via confined valence‐band states. The measured energy level positions are consistent with a substantial valence‐band offset of approximately 0.4 eV.

Electric fields and valence-band offsets in n+n [001] and [110] ZnSe/GaAs, GaAs/Ge, and ZnSe/Ge superlattices

Abstract: Calcul par la methode du potentiel gele des discontinuites de la bande de valence dans les superreseaux. Comparaison avec les resultats obtenus par une nouvelle methode fondee sur les deplacements des potentiels des atomes individuels. La discontinuite de bande de valence n'est pas affectee par les champs electriques internes resultant de la separation de charge a l'interface des superreseaux polaires [001]

Commutativity of the GaAs/AlAs (100) band offset

Abstract: X-ray photoelectron spectroscopy is used to measure the valence-band offset in situ for GaAs/AlAs (100) heterojunctions grown by molecular beam epitaxy. Ga 3d and Al 2p core level to valence-band edge binding energy differences are measured in GaAs (100) and AlAs (100) samples, respectively, and the Al 2p to Ga 3d core level binding energy difference is measured in GaAs–AlAs (100) and AlAs–GaAs (100) heterojunctions. Measurements of the Al 2p to Ga 3d core level energy separations indicate that the band offset for GaAs/AlAs (100) is commutative; the value we obtain is DeltaEv=0.46±0.07 eV. Our observation of commutativity is believed to be a consequence of the high quality of our GaAs/AlAs (100) heterojunctions, and of the inherent commutativity of the GaAs/AlAs (100) band offset.

Conduction‐band offset in strained Al0.15Ga0.85As/In0.15Ga0.85As/GaAs pseudomorphic structures

Abstract: We report a first determination of the conduction‐band offset in the strained‐layer Al0.15Ga0.85As/In0.15Ga0.85As/GaAs pseudomorphic structure. Two‐dimensional electron density and its effective mass are independently measured by Shubnikov–de Haas and cyclotron resonance experiments for a series of samples with a range of spacer thickness from 30 to 100 A. Using a charge transfer model, the conduction‐band offset at the Al0.15Ga0.85As/In0.15Ga0.85As interface is found to be (255±35) meV.

Temperature dependence of the mercury telluride-cadmium telluride band offset

Abstract: After reviewing the experimental data on the valence‐band offset for HgTe‐CdTe heterojunctions, we support previous suggestions of an extreme temperature dependence for this offset with a calculation based on a bond charge model The model predicts the T dependence of the valence‐band offset to be 77% of the difference in the band‐gap temperature dependence of the heterojunction constituents In the HgTe‐CdTe system, the opposite signs of the band gap T variations yield an anomalously large increase in the offset of 213 meV between 0 and 300 K

Far-infrared determination of effective mass and valence-band offset in the HgTe/CdTe superlattice

Abstract: On analyse les effets de plasma de l'infra-rouge lointain dans deux super-reseaux de HgTe/CdTe pour obtenir les masses effectives paralleles aux plans. L'analyse fournit egalement les epaisseurs de couches et le contenu en Hg dans les couches des barrieres nominales de CdTe

Superlattices and multilayers in hydrogenated amorphous-silicon devices

Abstract: An investigation of the physical properties of ultrathin multilayer structures that consist of hydrogenated amorphous silicon (a-Si:H) and silicon-based compounds such as a-Si/sub 1-x/N/sub x/:H and a-Si/sub 1-x/C/sub x/:H prepared by either plasma chemical vapor deposition (CVD) or direct photo-CVD is discussed. X-ray interference in the multilayers shows that the heterojunction interface is atomically flat and abrupt. The band offset is successfully determined by X-ray photoelectron spectroscopy. The energy band profile in the superlattices can therefore be designed. The carrier confinement in the ultrathin a-Si:H layers causes a blue shift of the optical bandgap and luminescence spectrum due to the quantization effects. Better understanding of the quantum size effects in the multilayers permits the introduction of superlattice structures to devices such as solar cells, thin-film transistors, and light-emitting diodes. Some of the experimental results are discussed. >

Valence‐band barrier formation in graded Hg1−xCdxTe heterojunctions with a valence‐band offset included

Abstract: In two previous papers [J. Appl. Phys. 6 2, 3267 (1987); 6 4, 6373 (1988)], we used a highly accurate numerical model to solve the problem of equilibrium barrier formation in graded Hg1−x Cd x Te heterojunctions. However, we concentrated solely on narrow‐gap p on wide‐gap nheterojunctions in which the barrier forms in the conduction band. The present calculation is an extension of that work, but with emphasis on wide‐gap p on narrow‐gap nheterojunctions. We perform a full parametric study, varying doping concentrations (symmetric and asymmetric), cadmium compositions, and cadmium grading widths. All band profiles are calculated with and without a valence‐band offset. Clear trends are observed. Without the valence‐band offset, for the larger grading widths, there is a tendency for the valence band to bow down on the n side. The band bowing acts as a potential barrier for the minority carriers. When the valence‐band offset is included though, the existence of a barrier to minority carriers depends on the grading width. For the most narrowly graded junctions, a potential well for the minority carriers is present. In the present study, we assume a valence‐band offset of 300 meV for HgTe:CdTe. The effects of elevated temperatures and interdiffusion of dopants are also studied, but appear to be inconsequential for the present cases.

Evidence for a large valence-band offset at HgTe-CdTe heterojunctions.

Abstract: Most theoretical and experimental studies of HgTe-CdTe interfaces have found a valence-band offset of \ensuremath{\lesssim}0.35 eV, and in many cases much lower. Here it is suggested that the available theoretical calculations can equally well be interpreted as indicating a valence-band offset of nearly 0.5 eV, which is apparently consistent with the limited experimental evidence available.

Determination of band offsets and effective masses in quantum wells using optical data

Abstract: A type‐I semiconductor quantum well inherently consists of two quantum wells, one each in the conduction and valence bands. A new method to decouple and independently analyze these two wells is presented. The interband 3e‐3h and 1e‐1h transitions are decoupled by using the commonly observed 1e‐3h transition. The resulting intraband 3h‐1h and 3e‐1e transitions are used to independently estimate the band offset and carrier mass in each well. Decoupling of the wells in each band adds a degree of freedom to the problem and allows an independent calculation of the average well width to be made. In addition, an accurate linear approximation is developed for use in place of the usual trigonometric solution to the quantum‐well problem. For Al0.30Ga0.70As/GaAs quantum wells, results are found to be highly consistent with Qc =0.605, and m*hh =0.321 and m*e =0.0671 in the well.

Type II to type I conversion of pseudomorphic GaAs on InP dependent on growth direction

Abstract: Predictions of band offsets assign a type‐II offset for the GaAs–InP interface without inclusion of strain effects, with the conduction‐ and valence‐band‐edge offsets approximately given by 0.4 and 0.3 eV, respectively. It has been demonstrated recently that pseudomorphic GaAs grown on [100]‐oriented InP also has a type‐II interface, but with the conduction‐band offset reduced by the hydrostatic deformation potential of GaAs. Poisson’s ratio in GaAs for [111]‐directed uniaxial stress is smaller by 40% than the value for [100]‐directed stress. The lattice constant perpendicular to the growth interface is therefore not reduced as much for [111] growth compared with the [100]‐growth case, leading to a consequently larger shift of the conduction‐band‐edge energy. Based on the recently measured value of −9 eV for the conduction‐band‐edge hydrostatic deformation potential in GaAs, the band offset for pseudomorphic GaAs grown on (111) InP has been calculated to be type I. This material system should therefore show type II to type I conversion dependent on the growth direction of the pseudomorphic layer. The issues involved in the growth and measurement of GaAs grown on (111) InP are discussed briefly.

Photoluminescence Study of Type I and Type II GaAs/GaP Strained-Layer Superlattices Grown on GaAs Substrates

Abstract: We studied the photoluminescence emission from short-period GaAs-GaP strained-layer superlattices grown by Atomic Layer Molecular Beam Epitaxy (ALMBE) on GaAs substrates. The observed peaks as functions of the temperature, excitation power and design parameters (period and strain accommodated in the constituent layers) were studied. Amongst the samples under study we found spatially direct (type I) and indirect (type II) superlattices. Good agreement between experiments and calculations were found for a conduction-band offset of 0.4 eV.

Crossover of Direct and Indirect Transitions in (GaAs)m/(AlAs)5 Superlattices (m=1-11)

Abstract: Energy band structures of short period superlattices (GaAs) m /(AlAs) 5 with m ranged from 1 to 11 are calculated by using the tight-binding approximation, where the second-nearest neighbor interactions are included in addition to the nearest neighbor interactions. The calculated lowest direct and indirect energy gaps are found to depend on the valence band discontinuity. Momentum matrix elements are also calculated for the transitions between the top valence bands and several lowest conduction bands in order to clarify the optical transition, allowed or forbidden. It is found that the transition for the lowest direct gap in the superlattices with m 7.

Equilibrium Barrier Formation In p-on-N And P-on-n Graded HgCdTe Heterojunctions

Abstract: We present results of our calculations of the equilibrium barrier formation in graded Hg1_xCdxTe heterojunctions using a highly accurate numerical model. Results for wide gap-p on narrow gap-n structures (Pn) are presented together with a review of our earlier results on narrow gap-p on wide gap-n (pN) heterojunctions [J. Appl. Phys. 62, 3267 (1987); 64, 6373 (1988)1 in which the barrier forms only in the conduction band. All band profiles are calculated with and without (common anion rule) a valence band offset; clear trends are observed. In the case of the narrow gap-p on wide gap-n heterostructures, the band profiles calculated with and without the valence band offset do not differ significantly. On the other hand, for the wide gap-p on narrow gap-n heterostructures, and using the common anion rule, the valence band tends to bow down on the n-side for the larger grading widths. The band bowing acts as a potential barrier for the minority carriers. When the valence band offset is included, though, the existence of a barrier to minority carriers depends upon the grading width: for the most narrowly graded junctions, a potential well for the minority carriers is present. In our calculations, we assume a valence band offset of 300 meV for HgTe:CdTe. We cannot make general predictions with regard to conditions needed to support the formation and growth of a barrier either in the conduction or valence band; instead; we find the band profiles to be a complex function of all the junction design parameters.

Band offset at InAs/GaAs interfaces

Abstract: The electronic band structures of InAs/GaAs superlattices are calculated and band offsets determined from the local density of states

Offset reduction by modulation doping in abrupt heterojunction bipolar transistors

Abstract: Conventional heterojunction bipolar transistors exhibit large δVCE offset voltages in common emitter characteristics: We experimentally show that the replacement of the large gap emitter by a thin modulation‐doped GaAlAs barrier in an otherwise all GaAs structure results in a large reduction of the offset, without degradation of the gain.

Relation Between Schottky Barrier Heights, Band Offsets and the Energy Levels of Transition Metal Impurities

Abstract: It is first shown that the Schottky barrier heights and the band offsets can be related through the energy of dangling bonds. At Schottky barriers, they become resonant states and the Friedel sum rule imposes that the dangling bond energy aligns with the metal Fermi energy. For heterojunctions the condition of negligible charge transfer corresponds to an alignment of the same dangling bond energies. The proposed theory thus allows the identification of Tersoff’s midgap level with the everage dangling bond energy. Detailed calculations are worked out in a charge dependent tight binding scheme and show good correlation with experimental data.

The Pressure Dependent Band Offset in a Type II Superlattice, a Test for Band Line-Up Theories

Abstract: The heterojunction band offset, i.e. the position of the bandedges in one semiconductor relative to those in another one in close contact, presents a problem in solid state physics which is neither experimentally nor theoretically well understood. Yet this quantity is of growing and crucial interest for the characterisation and design of novel heterostructure devices, which can now be grown with near perfection by modern growth techniques such as MBE) and MOCVD)1–3