Band Edge Discontinuity Research Articles

Theory of Semiconductor Heterojunctions

A short review of characteristic electronic properties of heterojunction interfaces is given. Band edge discontinuities and interface band structures for lattice-matched junctions are discussed in detail. The examples presented include non-polar and polar junctions as well as overlayer systems. The results of involved calculations are interpreted in terms of simple physically appealing pictures by directly relating the changes in bonds across an interface to the resulting bands in the interface band structure. The meaning of the results for the transport properties of semiconductor heterojunctions is briefly assessed.

Enhancement of electron impact ionization in a superlattice: A new avalanche photodiode with a large ionization rate ratio

The first superlattice avalanche photodiode (APD) is reported. The high field region of this p-i-n structure consists of 50 alternating Al0.45Ga0.55As (550 Å) and GaAs (450 Å) layers. A large ionization rate ratio has been measured in the field range (2.1–2.7)×105 V/cm, with α/β≃10 at a gain of 10 giving a McIntyre noise factor Fn = 3. The ionization rate ratio enhancement with respect to bulk GaAs and AlGaAs is attributed to the large difference in the band edge discontinuities for electrons and holes at the heterojunction interfaces. The superlattice APD is a new device concept which can be used to develop low noise APD’s in a variety of III-V materials including long wavelength 1.3–1.6-μm semiconductors.

Ideal vacancy induced band gap levels in lattice matched thin superlattices: The GaAs–AlAs(100) and GaSb–InAs(100) systems

Results of the ideal cation and anion vacancy induced deep levels in thin GaAs–AlAs(100) and GaSb–InAs(100) superlattices are reported. The deep levels are obtained as a function of (a) the spatial location of the vacancy with respect to the interface and (b) the magnitude of the appropriate band edge discontinuity characterizing these superlattices. The theoretical method employed is the Koster–Slater Green’s function technique combined with a LCAO-tight binding description of the electronic structure of perfect superlattices. The tight binding matrix elements, up to second nearest neighbors, are obtained via fitting to the band structure of individual bulk materials involved with the added requirement of reproducing the photothreshold which ensures a correct qualitative ordering of their ionicity. In an attempt to gain an estimate of the importance of self-consistency in the determination of vacancy induced levels, results are also obtained for inequivalent values of the diagonal tight binding energies for As atoms corresponding to the two normally equivalent As interfaces characteristic of GaAs–AlAs(100) superlattice. The effect is estimated to be about 10%. It is argued that these results also provide a simulation of and insight into the possible presence of local potential fluctuations and inequivalent band edge discontinuities at the two interfaces arising from the nature of superlattice growth.

Electronic structure of Ge-GaAs(111) and ( [formula omitted]) interfaces

The interface states of Ge-GaAs(111) and ( 111 ) heterojunctions are calculated by applying extended Hückel theory to a superlattice with alternating Ge and GaAs atomic layers. The band-edge discontinuity, interface bands, and local densities of states are presented. It is found that no interface states are revealed in the fundamental gaps of Ge and GaAs and that there is an appreciable difference in electronic structure between both kinds of interface.

Partial dislocations, columnar growth, clustering, and pinhole formation in ultrathin film semiconductor heterostructures

Island growth of partially ionic semiconductor films (such as GaAs or CuBr) on covalent substrates (such as Ge) may produce not only misfit dislocations but also partial dislocations and antiphase boundaries because of randomization of the translational diatomic phase registry of the ionic phase during multiple island nucleation. It is suggested that such partial dislocations may play an important role in the asymmetry of epitaxial crystal growth of Ge on GaAs compared to GaAs on Ge observed in recent MBE experiments, and may explain nontransitivity of band-edge discontinuities.

The electronic structure of Si/GaP(110) interface and superlattice

It is argued that the (110) interface between group IV and III–V semiconductors are more likely to lend themselves to fabrication via MBE techniques. Results for the electronic structure of Si-GaP(110) superlattice are reported for the first time. Both, interface states and two dimensionally confined states are found. Sensitivity of the energy and charge distribution of these states to the interfacial geometry and the band edge discontinuity is investigated. The results are contrasted with the situation found for lattice matched III–V compound semiconductor systems such as GaAs/AlAs and GaSb/InAs.

Energy-band profile and interface potential step in the theory of semiconductor heterojunctions

Abstract The dependence of the energy-band profile at the interface on the interface potential step and bulk properties of the two semiconductors in epitaxial contact is investigated. In order to clarify the main concepts we develop firstly a one-dimensional model of a heterojunction between monatomic and diatomic semiconductors and calculate from first principles −eV0, ΔEc and ΔEv. The natural pass to a rigorous three-dimensional treatment is performed on the basis of the two-band narrow-gap approach to the theory of heterojunctions between semiconductors with gaps opened in the centre or the edges of the Brillouin zones. Relations are again derived between the potential step and the relevant band-edge discontinuities.

Study of the electronic structure of model (110) surfaces and interfaces of semi-infinite III-V compound semiconductors: The GaSb-InAs system

This paper presents the results of a study of the electronic structure of model (110) surfaces and interfaces of semi-infinite III-V compound semiconductors, with specific application to the GaSb-InAs system. The description of the individual solids is sought within the empirical tight-binding method and combined with the Green's-function technique to account for their semi-infinite nature. Surface states are found to exist for ideal (110) surfaces, consistent with previous results obtained via the use of the finite-slab method. Results presented for the interface between two semi-infinite III-V semiconductors are the first to be reported and indicate the possible importance of the nature of the band-edge discontinuity of the heterojunction. It is shown that the expected unusual band lineup of the GaSb-InAs system gives rise to a finite density of states at the interfacial atomic planes, throughout the region of the bulk fundamental band gaps. Consequently, no interfacial barrier exists in this heterojunction. It is noted that this behavior of the GaSb-InAs heterojunction occurs in a regime of thickness of the individual materials which, in practice, is not accessible via the usual finite-slab method. Results of the influence of the interface geometry on the interface electronic structure are reported. For the interface characterized by interfacial chemical bonds of bulk bond strengths, we find no interface states in the fundamental-band-gap regions. This is found to be the case for both usual and unusual band lineup. Variations in the interface bond strengths, simulating the influence of the interface geometry, show that interface states first appear for unusually large departures from bulk bond strengths. It is thus suggested that interface states in lattice-matched III-V semiconductor heterojunctions are unlikely to arise solely due to variations in intrinsic chemical bonding effects. Results for interface states associated with dangling-bond are presented. It is found that the energy position of such states is relatively insensitive to those changes in surrounding geometry which do not appreciably alter the surface bond strengths of the host materials. These results suggest that the most likely origin of interface states for the class of systems investigated here is the presence of defects involving relaxation of the surrounding interfacial atoms.

Impact ionisation in multilayered heterojunction structures

Calculations are reported showing that in multilayered heterojunction structures the effective impact ionisation rates for electrons and holes can be very different, even if they are the same in the basic bulk materials. The reason for this is the difference in the band-edge discontinuities for electrons and holes and the lower phonon mean free path for holes in quantum well structures.

Two-dimensional effects and effective masses of the InAs/GaSb (001) superlattices

The tight binding method is employed to study the two-dimensional aspects of the electronic structure of InAs/GaSb(100) superlattices. Results are presented which show the spatially confined nature of the light hole, heavy hole and conduction bands. The hole states are found to be confined primarily on the GaSb side, whereas the conduction bands are confined to the InAs side. Effective masses in the plane of the superlattice are calculated. The conduction band mass is found to compare well with the appropriate value extracted from recent magnetoresistance measurements. Optical absorption data is used to determine the InAs conduction band to GaSb valence band edge discontinuity at the Γ-point. It is found that the former lies below the latter, qualitatively consistent with the electron affinity rule, but only by ∠ 60 meV. Reduction to this value from the affinity value of ∠ 140 meV is consistent with the affect of charge transfer. Finally, some comments regarding the variation, with thickness, of the relative and absolute positions of energies are made, particularly in light of the prevalent, though often ambiguous, practice of comparing projected regions of bulk energies with finite slab or superlattice band structures.

Self-consistent calculation of the electronic structure of the (110) GaAs-ZnSe interface

The abrupt (110) interface of GaAs-ZnSe is studied using the self-consistent pseudopotential method. No interface states are found in the fundamental gap, but interface states do split off from bulk states in some other regions. The band-edge discontinuity found in the present calculation does not satisfy the simple electron affinity rule; the calculated discontinuity of the conduction-band minima is 0.7 eV compared with the electron-affinity difference of 0.02 eV. The role of ionicity in the electronic structure of the interface is discussed.

Energy-band profile and interface states in semiconductor heterojunctions

The simultaneous appearance of both band-edge discontinuities and interface states in the energy-band profile of semiconductor heterojunction is discussed on the basis of a model that is as simple as possible, consistent with it remaining useful. An attempt is thus made to bridge the gap in the treatment of the two most important features that characterise the energy-band structure of a heterojunction.

Theoretical study of relaxation at the (110) Ge-GaAs interface

A simple relaxation of the Ge-GaAs (110) interface is studied theoretically using self-consistent pseudopotentials. The relaxation is one in which the distance between the Ge and GaAs atomic planes at the interface is increased by 20%. Band edge discontinuities are found to be insensitive to this type of relaxation in spite of large distortions of the electronic charge density.

Ge-GaAs (110) Interface: A Self-Consistent Calculation of Interface States and Electronic Structure

Self-consistent calculations allowing electron redistribution in a wide region around a Ge-GaAs(110) interface reveal six types of interface states. The charge densities and local density of states at the interface indicate Ge-Ga bonding states in the gap (but below the valence-band maximum) and a variety of other states at lower energy. Some estimates based on these pseudopotential results give information on band-edge discontinuities and the character of bonds across a nonpolar interface.

Elementary theory of heterojunctions

An LCAO theory of heterojunction band-edge discontinuities is formulated and tested for approximate self-consistency. It leads to a table of valence-band maxima for all tetrahedral semiconductors; discontinuities can be obtained from the table directly by subtraction. The discrepancies with the current scattered data do not appear significantly larger than the uncertainty in those data, a few tenths of electron volts. A pseudopotential theory of such discontinuities is also formulated, based upon self-consistent atomic pseudopotentials. This leads to valence-band maxima reasonably consistent with the LCAO theory, except for junctions between materials of significantly different bond length. It also suggests that the Frensley–Kroemer scheme does produce self-consistency for systems of matching lattice constant, but produces incorrect trends with mismatch in lattice constant. The goal in any case is taken to be a table of valence-band maxima. LCAO values seem a better standard than photoelectric thresholds, though a comparison of the two indicates them to be roughly consistent for treating junctions if both sides are homopolar or if both sides are polar.