Shallow Electronic States Research Articles

Shallow electron states of bounded intrinsic stacking faults in silicon

The electronic structure of bounded intrinsic stacking faults in silicon is studied. Especially the influence of the stacking fault width on the electronic states in the band gap is investigated. The extended defect studied comprises an intrinsic stacking fault with two reconstructed 90\ifmmode^\circ\else\textdegree\fi{} partials as boundaries. The atomic structure is determined by different valence force fields. These are the Keating potential, the bond-charge model, and an anharmonic version of the bond-charge model. The electronic structure is calculated by linear combinations of atomic orbitals. Ten Gaussian-type atomic orbitals of s-, p-, and d-type are used, and up to fourth-nearest-neighbor interactions are taken into account. The levels in the band gap are evaluated by the recursion method for nonorthogonal basis functions, and by a continued fraction representation of the local density of states.

Photochemical desorption from chlorinated Si(100) and Si(111) surfaces — Mechanisms and models

Understanding the mechanisms controlling the anisotropy of microetching is particularly critical as the scale of semiconductor devices shrink. Defining complex, dynamic chemical systems such as halogen etching require microscopic measurements combining kinetics, dynamics, surface layer composition and micromorphology on prototypical surfaces. This study is concerned with two important variables in addition to spontaneous chemical etching, the role of electronic defects induced by high level doping in producing site-specefic reaction and the enhancement of etching by irradiation at low fluences. Substitutional defects introduced by selective doping significantly influence the rate of chlorine etching by forming shallow electronic states that are ionized at room temperature 1. We have shown that chlorine sticking coeficients as well as laser-assisted etching are significantly affected by doping at very high dopant levels. Enhancement for n-type doping is consistent with the simple assumption that holes at the surface should enhance Si-Si surface bond breaking and in disagreement with the fact that heavily p-doped silicon has a higher chlorine sticking coefficient than n-doped material 2. Carrier effects generated by photoirradiation with above bandgap photons are considerably more complex than simple doping. A depletion layer and associated electric field are set up at the surface and minority carriers are preferentially swept to the surface. The type of photocarrier present at the surface is determined by both the doping and the photoirradiation. Using photoinduced etching of heavily doped Si(100) and Si(111) by chlorine at low laser fluences, we studied the mechanism of photostimulated desorption using core-level photoemission and time-of-flight measurements of the photoproducts 2. These results will be interpreted in terms of field-modified electron-hole transport together with carrier-modified chlorine adsorption and desorption.

Injection spectroscopy of localized states in thin insulating layers on semiconductor surfaces

An overview of charge injection techniques for studies of localized electron states in insulators is presented. These states control most important electrical properties of the materials, especially their thin layers and the multilayered structures containing them. Electronic properties of thin insulating layers are usually different from those of bulk material because in vicinity of an interface the thin film is subjected to action of structural strain, spatial confinement, extra energy release during growth or deposition, e.c. All these factors substantially affect the spectrum of localized electron states (intrinsic defects, dopants, impurities) and influence their spatial distribution. The interfaces often exhibit an unusual behaviour relative the defect generation and an impurity transport and segregation comparing with a bulk of insulator. The characterization of these states in thin-layered systems suffers from low sensitivity of both electron and optical spectroscopies to shallow electron states those have to be detected on the background of the signal from substrate. Nevertheless, the problem may be resolved by means of charge carrier injection into the insulating layer followed by monitoring of the charge becomes trapped. The localized states are detected by this way as a potential wells for the injected a mobile charge carriers. In thin layers the mean path of injected electron (hole) with respect to interaction with such a trap may greatly exceed thickness of the layer. In offers possibility to use linear modes for the trapping and detrapping analysis. We will show that under these conditions the charge-injection methods provide the most important phenomenological parameters of a localized state: the density per unit area, in-depth location, cross section for capture of carriers or recombination, energy position of the trapped carrier level in the insulator bandgap, e.c. On the base of these possibilities a non-destructive methods for characterization of extrinsic and intrinsic defects states are developed. Various charge injection and charge detection methods are compared. It shows unambiguously the advantages of using a semiconducting substrate for the insulator trapping studies. The limits of the linear models applicability are discussed in detail and the effects arising from the non-linear correlation phenomena are analyzed.

Electronic properties of a complex Cu-related acceptor with a bound exciton at 2.3423 eV in ZnTe.

A detailed study of the electronic structure of a deep complex acceptor in ZnTe is reported, based on experimental data from optical spectroscopy of a corresponding bound exciton (BE) at 2.3423 eV (2 K). Only one electronic BE state is observed in the bandgap, quite unlike the situation with substitutional acceptors. From an evaluation of Zeeman data taken at 10 T for the BE line it is concluded that the hole in the neutral acceptor ground state, as well as the second hole in the BE two-hole state, are essentially spinlike, with a g value ${g}_{h}$ close to +2. The electron is shallow with an isotropic g value of ${g}_{e}$\ensuremath{\simeq}-0.40 and a quadratic Zeeman shift c\ensuremath{\simeq}7.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}6}$ eV/${\mathrm{T}}^{2}$, typical for shallow electron states in ZnTe. A residual anisotropic contribution of about 15% to the hole g value is observed, and explained as induced by the spin-orbit interaction for the bound hole. A realistic general description of magnetic properties of holes bound at complex acceptors is presented, without resort to the effective-mass formalism. The quenching of orbital angular momentum is observed for nondegenerate hole states in a strongly hole-attractive potential. A strong spin-orbit interaction may prevent such a quenching however, a quite common observation for other complex defects in ZnTe. The defect symmetry in this case is ${C}_{1h}$ from the Zeeman data, and the identity is tentatively suggested as a ${\mathrm{Si}}_{\mathrm{Te}}$-${\mathrm{Cu}}_{\mathrm{i}}$ pair. The axis is distorted about 8\ifmmode^\circ\else\textdegree\fi{} off the trigonal 〈111〉 direction, as a result of a relaxation of the ${\mathrm{Cu}}_{\mathrm{i}}$ interstitial ion towards one of the nearby Te atoms. Such a distorted configuration of the interstitial ion has recently been concluded also for other defects in ZnTe. The ionicity of ZnTe might be a sufficient reason for this relaxation of the ${\mathrm{Cu}}_{\mathrm{i}}$ interstitial ion.