Sokrates T. Pantelides

Acceptor doping in ZnSe versus ZnTe

It is a long‐standing puzzle that ZnSe is difficult to dope p type, while ZnTe—which is very similar to ZnSe—is very easily doped p type. We report ab initio calculations which show that the solubilities of Li and Na acceptors are much greater in ZnTe than the solubilities of the same acceptors in ZnSe. We trace the origin of this difference to the bonding properties of the acceptors with the neighboring chalcogens. Our results also explain the experimentally observed dependence on dopant concentration of the dislocation density in p‐type ZnSe epilayers grown on GaAs.

Effect of hydrogen on shallow dopants in crystalline silicon

Passivation of shallow impurities by H has been attributed to H‐impurity pairing in both p‐type and n‐type Si. We show that existing interpretations of data were based on contradictory assumptions and that a coherent interpretation of all the data can only be obtained if one assumes that diffusing H has a donor level in the gap. A novel interpretation emerges: In p‐type material, passivation is due to direct compensation, so that pairing is a consequence, not a cause of passivation; in n‐type material, passivation is indeed due to pairing, but is suppressed by H2 formation and possibly other reactions. Several predictions are made and new experiments are proposed as tests.

Energy-gap reduction in heavily doped silicon: Causes and consequences

Abstract The authors review briefly the existing theoretical treatments of the various effects that contribute to the reduction of the energy gap in heavily doped Si, namely electron-electron and electron-impurity interactions and the effect of disorder in the impurity distribution. They then turn to the longstanding question why energy-gap reductions extracted from three different types of experiments have persistently produced values with substantial discrepancies, making it impossible to compare with theoretical values. First, they demonstrate that a meaningful comparison between theory and experiment can indeed be made if theoretical calculations are carried out for actual quantities that experiments measure, e.g. luminescence spectra, as recently done by Selloni and Pantelides. Then, they demonstrate that, independent of any theoretical calculations, the optical absorption spectra are fully consistent with the luminescence spectra and that the discrepancies in the energy-gap reductions extracted from the two sets of spectra are caused entirely by the curve-fitting procedures used in analyzing optical-absorption data. Finally, they show explicitly that, as already believed by many authors, energy-gap reductions extracted from electrical measurements on transistors do not correspond to true gap reductions. They identify two corrections that must be added to the values extracted from the electrical data in order to arrive at the true gap reductions and show that the resulting values are in good overall agreement with luminescence and absorption data. They, therefore, demonstrate that the observed reduction in emitter injection efficiency in bipolar transistors is not strictly due to a gap reduction, as generally believed, but to three very different effects.

The electronic structure of impurities and defects in SiO2

Abstract We describe a simple tight-binding model which gives qualitative understanding and quantitative estimates of the electronic energy levels of several classes of impurities, defects and impurity complexes in SiO 2

CONTINUOUS-RANDOM-NETWORK MODELS FOR THE Si-SiO2 INTERFACE*

ABSTRACT Continuous-Random-Network (CRN) models have been constructed in order to simulate the atomic arrangement of the Si-SiO2 interface. It was found that models could be constructed with or without an SiOx layer between the crystalline Si and the amorphous stoichiometric SiO2. The atomic coordinates were relaxed using a computer program and a simple force model. In order to probe the width w of the SiOx layer, additional O atoms were gradually inserted, thus reducing w, and a normalized elastic energy was monitored. Using a wide range of choices for the force constants, it was found that the elastic energy was always lowered as w → 0.

Native defects and diffusion in amorphous silicon -- a revisit

Abstract Recent theoretical and experimental results are assessed as they relate to the structure and properties of the dominant native paramagnetic point defect (D center) and the mechanisms of H diffusion. These results lead us to propose that the D center is neither a dangling or a floating bond, but an intermediate configuration which we label “frustrated bond”. Experimental results by Jackson et al. set an upper bound for the diffusivity of D centers, but are not incompatible with D centers mediating H motion. Doping- and illumination-enhanced H diffusion can be accounted for in a unifying manner in terms of excess D centers that are mobile and mediate H motion via the kick-out mechanism, without any new assumptions.

Some properties of the oxides of the tetrahedral semiconductors and the oxide–semiconductor interfacesa)

Continuous‐random‐network models have been constructed for the Si–SiO2 interface. It is found that an abrupt interface with no SiOx layer is posible. A simple tight‐binding model is described that is applicable for the calculation of the electronic properties of the bulk oxides and the oxide–semiconductor interfaces. Results are given only for selected bulk properties, namely the photoemission and x‐ray emission spectra, and the dielectric constants of SiO2, GeO2, and various ABO4‐type oxides.

Critique of the empirical tight‐binding method for semiconductor surfaces and interfaces

The empirical tight‐binding method has been used extensively over the last several years to study the electronic and atomic structure of semiconductor surfaces and interfaces. The purpose of this paper is to provide a critical review of both the method itself and the variety of techniques that have been used to implement the basic idea. The foundations of the method are examined, identifying its strengths, its limitations, the types of properties it can describe reliably, its range of applicability and its overall usefulness. Individual techniques e.g., slab techniques, transfer‐matrix techniques, Koster–Slater techniques, etc.), which often introduce additional assumptions and approximations, are discussed and compared. Selected applications of the method, both to electronic‐ and atomic‐structure problems, are described and discussed in order to illustrate the general conclusions.

First‐principles mesoscopic dynamics in heterogeneous materials

A systematic link between microscopic and macroscopic theories of matter has been lacking in the case of heterogeneous materials (polycrystals, composites, etc.). The properties of such materials are largely determined by their collective microstructure, which defines several intermediate or mesoscopic length scales. A microscopic description is presented and the principles of statistical mechanics are used to derive a set of mesoscopic field dynamical equations that describe microstructure evolution under external stresses, temperature gradients, or electric current. The macroscopic dynamical equations of continuum mechanics are recovered. The cross‐coupled phenomena of elastic deformations and inelastic diffusive and slip processes are present, setting the stage for a first‐principles theory of dislocation dynamics and plasticity.

Point defects in crystalline silicon, their migration and their relation to the amorphous phase

Abstract Vacancies and selfinterstitials in silicon have been studied extensively both experimentally and theoretically and many of their properties have been established. We review briefly some of the recent results obtained by ab initio calculations and by atomistic simulations using empirical interatomic potentials. These results yield a comprehensive picture of defect structure, migration paths, and relative contributions to self-diffusion. We elucidate the details of the atomic migration process by analyzing one of the migration paths of the self-interstitial in terms of the lattice normal modes (phonons). Finally, we report new calculations to clarify further similarities between crystals containing large concentrations of point defects and amorphous silicon. In addition to a notable trend in the density of crystals containing increasingly higher self-interstitial concentrations, we find strong similarities in the vibrational and structural properties.