Showing papers on "Band offset published in 1986"

Unified disorder induced gap state model for insulator–semiconductor and metal–semiconductor interfaces

Abstract: The energy location for the interface state density N s s minimum of the insulator–semiconductor (I–S) interface and the Fermi‐level pinning position at the metal–semiconductor (M–S) interface are shown to coincide and to lie at the same position of 5.0 eV from the vacuum level for major tetrahedral semiconductors. Neither the unified defect model nor the metal induced gap statemodel can explain the novel striking correlation between the I–S and M–S interfaces. The correlation as well as the observed peculiar photoionization behavior of the I–S interface are explained by the novel unified disorder induced gap state (DIGS) model where DIGS pin or restrict the movement of the surfaceFermi level. The above characteristic energy, E HO, is shown to be the Fermi energy of the DIGS spectrum which is given by the hybrid orbital energy of the s p 3 bond of the host. The DIGS model explains remarkably well the behavior of the M–S interface formed on the bare or oxide covered semiconductor surface as well as the various features of N s s distribution of the I–S interface. The correlation between the DIGS‐free heterojunction (S–S) interface and M–S/I–S interface is explained by the fact that E HO is a universal reference energy level of the host which is invariant under any off‐diagonal interactions, as is evidenced by the alignment of transition metal deep levels, DX centers and EL2 with respect to E HO. Band offset at the S–S interface is proposed to be determined by the alignment of E HO which inevitably involves formation of interface dipole when two E HO levels lie at different positions from the vacuum level.

Staggered band alignments in AlGaAs heterojunctions and the determination of valence‐band offsets

Abstract: Photoluminescence spectra from Al0.37Ga0.63As/AlAs multiple quantum well structures with staggered band alignments are presented which provide the first direct optical measure of the valence‐band offset at a semiconductor heterojunction. The experiment takes advantage of the crossover occurring at a critical aluminum concentration above which the indirect X minima in the AlAs become the lowest energy conduction bands in the system, and recombination occurs across the interface. The resulting emission fixes the valence‐band offset at ΔEv =342±4 meV for this structure.

Calculation of phonon‐assisted tunneling between two quantum wells

Abstract: We have calculated the longitudinal optical phonon scattering between two subbands of a system of two coupled quantum wells. Our model takes the delocalization of the wave function over the whole structure into account, thus avoiding the shortcomings of earlier computations for thin barriers. The dependence on the various parameters (width of the wells and of the barrier, band offset, voltage drop between the wells, temperature, and effective mass of carriers) is discussed and illustrated.

Pressure dependence of band offsets in an InAs-GaSb superlattice.

Abstract: Using magneto-optical methods, we have measured the pressure dependence of the energy difference between subbands in an InAs-GaSb superlattice associated with the GaSb valence and the InAs conduction bands, respectively. The experimental results allow a determination of the pressure dependence of the energy separation between the InAs conduction band and the GaSb valence band which is found to decrease at a rate of 5.8 me/kbar. This result shows that both the conduction- and the valence-band offsets are pressure dependent. Therefore these experiments constitute a critical test for different theories of band lineup.

Understanding and controlling heterojunction band discontinuities

Abstract: We discuss two recent results on the microscopic nature and control of the band lineup at semiconductor-semiconductor interfaces. First, we identified a correlation between measured heterojunction band discontinuities and Schottky barrier heights of the corresponding semiconductors, as predicted by several theoretical models. Second, we found that ultrathin metal intralayers modify the band lineup of polar interfaces by several tenths of an electron volt. At least in principle, this degree of freedom can be exploited to tailor heterojunction devices.

Tight-binding calculation of the band offset at the Ge-GaAs (110) interface using a local charge-neutrality condition.

Abstract: A tight-binding Green's-function calculation of the electronic properties of the Ge-GaAs (110) interface is presented The atomic charges in the vicinity of the interface are determined by use of a continued-fraction expansion technique Approximate self-consistency is achieved through the use of a local neutrality condition which is shown to be justified in this context Finally the band offset is calculated and compared to experimental and other calculated values

Well size dependence of Stark shifts for heavy‐hole and light‐hole levels in GaAs/AlGaAs quantum wells

Abstract: Stark shifts for heavy‐hole and light‐hole levels in an isolated GaAs/AlGaAs quantum well have been analyzed by an exact numerical calculation within the envelope function approximation. The calculated results predict that for wells thicker than about 100 A the Stark shift for heavy hole is larger than that for light hole; however, for thinner wells this tendency is reversed. It also predicts that this well size dependence strongly depends on the band offset ratio.

Confinement in the GaSb-InAs(001) superlattice

Abstract: The authors have used non-local pseudopotentials with spin-orbit coupling to study the processes of confinement in a GaSb-InAs(001) superlattice of period 36 AA as a function of the difference between the average bulk crystal potentials. They show that short-wavelength scattering is the prime mover in the physical processes leading to the confinement of charge. Although long-wavelength scattering processes act largely as a delocalisation mechanism which tends to increase the interaction between different states, their presence is essential in the formation of a stable electronic system. In addition, they have shown the form of the confined states that exist in the absence of a band offset and have indicated the effects of band mixing on confinement. They have shown that although a superlattice state may not show confinement in the conventional sense (i.e. in the sense of the envelope function approximation), confinement may take place in particular networks of bonds. They have probed the different optical properties that may be exhibited by the various superlattice states with varying band offsets and have highlighted the link between these optical properties and the form of the confined states. In particular, they indicate the different optical properties associated with resonant states in type I and type II superlattices. They also show that even for GaSb-InAs systems comprising ultra-thin layers, the envelope function model Hamiltonian remains a useful practical tool for states lying closest to the band edges.

Double heterojunction field effect transistor data storage device

Abstract: Reversible control of conduction in a band offset heterojunction field effect transistor device is achieved by .an asymmetric barrier (10) which can position a potential well across the Fermi level to produce conduction and away from the Fermi level for a non-conducting condition and to retain that position in the absence of a signal. The device can be a GaAs channel FET with a multilayer gate comprising, in order of proximity to the GaAs channel, a gate layer (3) of GaA1As, a charge storage layer (8) of GaAs, an asymmetric barrier layer (10) of GaA1As graded towards the GaAs storage layer and an ohmic adapting layer (13) of GaAs.

An effective dipole model for predicting band offsets in semiconductor heterojunctions

Abstract: A simple model to predict the band offsets in semiconductor heterojunctions is presented The model is based on the formation of an effective dipole induced by the penetration of bulk band states into a quantum barrier in the neighboring semiconductor Application of the model to a large number of heterojunctions gives band offset values in good agreement with experiment

Lattice mismatch and band offsets in strained layers

Abstract: Lattice mismatch effects are obscured by the standard envelope wave‐function approach to electronic states because the lattice potential does not appear. A theoretical analysis of the first‐order effect of lattice mismatch is presented using the deformable ion approximation. It is found that a potential barrier of height Δ, normally associated with a band offset, acquires an additional energy (E) dependent term becoming Δ+η(Δ−E) where η is the fractional lattice mismatch in the growth direction. For strained layers, Δ includes the effect of deformation in the strained layer. Quantitatively, the lattice mismatch effect can be comparable to or larger than the deformation potential effect on the band offset. A scattering approach is employed.

Semiconductor ballistic electron velocity control structure

Abstract: A semiconductor device where an emitter material composition and doping profile produces an electron gas in a base adjacent a band offset heterojunction interface, the electrons in the electron gas in the base are confined under bias by a low barrier and the ballistic carriers have their kinetic energy controlled to prevent intervalley scattering by an electrostatic barrier that under influence of bias provides an essentially level conduction band in the portion of the base adjacent the collector.

Photovoltaic properties and anomalous effects in the ZnSe-GaAs heterojunction

Abstract: The photoelectric properties of the n ZnSe- p GaAs heterojunction obtained by deposition of ZnSe on GaAs in the MOCVD process have been investigated. The current-voltage characteristics of the illuminated solar cells are shifted to lower voltages than those expected from the superposition principle. The dependence of quantum efficiency and C – V curves on light intensity and wavelength has been observed. A heterojunction model is proposed for both light and dark conditions. We assumed that near the interface a compensated ZnSe layer exists with negative charged deep centers. As a result electric current in the heterojunction is limited by a barrier in the conduction band. Variation of charge in the compensated layer is connected with electron transfer from deep centers to the conduction band. This process explains the anomalous effects for such a heterojunction. The characteristics calculated by this model are close to the experimental curves.

Orbital mismatch in semiconductor quantum wells

Abstract: A generalised effective-mass model is developed that, in addition to describing the band structure from the Gamma point to approximately half-way to the Brillouin zone boundary, retains the essential orbital information of a tight-binding calculation. The model is used to compare the effects of 'band offset' and 'orbital mismatch' across a semiconductor heterojunction. When applied to the case of a GaAs quantum well in AlxGa1-xAs it is found that for broad wells the effects of mismatch of the bonding nature of the orbitals across an interface are comparable with those of band offset. In addition, it is found that in narrow wells, differences in the valence orbitals can have a dominant effect due to admixture of evanescent zone-boundary states.

A transistor device

Abstract: The composition and doping profile of the emitter (4) produces an electron gas (16) in the base (3) adjacent a band offset heterojunction interface be- 'tween the emitter and the base. When a suitable bias is applied, the electron gas is confined adjacent to the interface by a low barrier (produced by layer 10). The kinetic energy of ballistic electrons crossing the base to the collector (2) is controlled to prevent intervalley scattering by an electrostatic barrier (7) that under influence of bias provides an essentially level conduction band in the portion of the base adjacent the collector.

Strain Modification of Alloy Fluctuations in CdTe/(Cd,MnTe) Superlattice Systems

Abstract: We have calculated the effect of strains on the valence band offset and bandgap in the CdTe/(Cd,Mn)Te superlattice system for various growth directions and alloy compositions. We find that strain can modify the effect of alloy fluctuations on band offset and bandgap. The consequences of this for the formation of interface states will be discussed.

Far Infrared Magnetoabsorption in Hg1-xMnxTe/HgTe Superlattice

Abstract: Hg-based superlattices (SL)-e.g., the HgTe/CdTe SL–exhibit interesting and important physical properties. Among these are the tunability of the band gap, achieved by varying the thicknesses of the two constituent materials; and the existence of the vajeqnce band offset, on which the details of the valence bands of the SL largely depen. Recently a new type of Hg-based SL, the Hg 1-x Mn x Te/HgTe SL, has been successfully prepared. In addition to the ability of varying its band gap by controlling the Mn content, this SL has the advantage that some of its important parameters, including the band offset, can be tuned by an external magnetic field because of the spin-spin exchange interaction in the Hg 1-x Mn x Te layers. So far, the electronic and optical properties of this type of SL have not been explored.