The electron counting rule and passivation of compound semiconductor surfaces

A brief review is presented of the role of electron counting rule (ECR) in explaining structural stability and passivation of compound semiconductor surfaces. While III-V(1 1 0) and majority of III-V(0 0 1) and III-V(1 1 1) surfaces reconstruct in accordance with the ECR, there are a few low- and high-index surfaces which disobey the ECR but stabilize by sustaining significant elastic deformation in the surface region. We explain the latter scenario with the help of computational results for the geometric and electronic structure of GaSb(0 0 1)-(1×3), GaSb(0 0 1)-c(2×6), InP(1 1 1)A-(), and GaAs(1 1 1)B-Sb(1×3). We also discuss hydrogen passivation of these surfaces. It is pointed out that the recently observed stable InP(1 1 1)A-() surface can be both chemically and electronically passivated by exposing it to a hydrogen gas of one quarter of a monolayer coverage.     

Interface models and processing technologies for surface passivation and interface control in III-V semiconductor nanoelectronics

Interface models and processing technologies are reviewed for successful establishment of surface passivation, interface control and MIS gate stack formation in III-V nanoelectronics. First, basic considerations on successful surface passivation and interface control are given, including review of interface models for the band alignment at interfaces, and effects of interface states in nanoscale devices. Then, a brief review is given on currently available surface passivation technologies for III-V materials, including the Si interface control layer (ICL)-based passivation scheme by the authors’ group. The Si-ICL technique has been successfully applied to surface passivation of nanowires and to formation of a HfO₂ high-k dielectric/GaAs interfaces with low values of the interface state density.     

Mechanism of atomic-scale passivation and flattening of semiconductor surfaces by wet-chemical preparations

Atomic arrangements of Si(001), Si(110) and 4H-SiC(0001) surfaces after wet-chemical preparations are investigated with scanning tunneling microscopy. Their passivated structures as well as the surface formation mechanisms in aqueous solutions are discussed. On both Si(001) and Si(110) surfaces, simple 1 × 1 phases terminated by H atoms are clearly resolved after dilute HF dipping. Subsequent etching with water produces the surfaces with 'near-atomic' smoothness. The mechanisms of atomic-scale preferential etching in water are described in detail together with first-principles calculations. Furthermore, 4H-SiC(0001), which is a hard material and where it is difficult to control the surface structure by solutions, is flattened on the atomic scale with Pt as a catalyst in HF solution. After a mechanism is proposed based on electroless oxidation, the flattened surface mainly composed of a 1 × 1 phase is analyzed. The obtained results will be helpful from various scientific and technological viewpoints.     

A study of the galvanic replacement reaction at surfaces and the role of lateral charge propagation

The galvanic replacement of isolated nanostructures of copper and silver on conducting supports as well as continuous films of copper with gold is reported. The surface morphology was characterized by scanning electron microscopy and the replacement with gold was confirmed by EDX analysis. It was found that lateral charge propagation during the replacement reaction had a significant effect in all cases. For the isolated nanostructures the deposition of gold was observed not only at the sacrificial template but also at the surrounding unmodified areas of the conducting substrate. In the case of copper films the role of lateral charge propagation was also confirmed by connecting it to an ITO electrode through an external circuit upon which gold deposition was also observed to occur. Interestingly, by inhibiting the rate of charge propagation, through the introduction of a series resistor, the morphology of gold on the copper substrate could be changed from discrete surface decoration with cube like nanoparticles to a more porous rough surface.     

On the electronic potential and ionic relaxation at metal surfaces

Some results, obtained by a simple method which gives the main effect of the ions on the surface potential of metals, are advanced. A general expression is given for the change in the surface dipole barrier. The ionic relaxation of (110) and (111) faces of Al are obtained self-consistently, tending to support one of the two current interpretations of LEED diagrams.     

Scanning tunneling spectroscopy of charge effects on semiconductor surfaces and atomic clusters

We have used scanning tunneling microscopy and scanning tunneling spectroscopy at liquid helium temperature to study the electronic structure of in situ cleaved, (110) oriented surfaces of InAs single crystals. Both unperturbed, atomically flat areas and areas with an atomic-size defect cluster have been investigated. We show that the anomalous behavior of the local tunneling conductivity, which indicates a pronounced enhancement of the semiconductor band gap for the flat areas, is consistent with band bending induced by charges localized at the apex of the tip. Atomic-size defect clusters contain additional charges which modify the band bending; this explains the different behavior of the tunneling conductivity near the defect cluster. The experimentally observed oscillations of the tunneling conductivity near the band gap edges can be directly related to resonant tunneling through quantized surface states which appear because of the band bending. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 2, 130–135 (25 January 1998) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Image charge potential at the interface of two semi-infinite media: Thomas-Fermi solution

The image charge potential is found for a parallel plate capacitor geometry in the infinite barrier Thomas—Fermi approximation to Poisson's equation. The material composition of the semi-infinite media are taken to be plasma-like although the jellium-dielectric interface is discussed in detail. Approximations such as single imaging (Newns) or that due to Gomer and Swanson are found to be reasonable in the center of large gaps but inaccurate for separations on the order of molecular dimensions. The dipole image potential is derived from the Green function solution for this geometry.     

Image charge approximations of reaction fields in solvents with arbitrary ionic strength

In this paper, we report a new development of image charge approximations of reaction fields for a charge inside a dielectric spherical cavity immersed in an ionic solvent with arbitrary ionic strength. This new development removes the requirement of low ionic strength of the solvent in a previous result [S. Deng, W. Cai, Extending the fast multipole method for charges inside a dielectric sphere in an ionic solvent: high-order image approximations for reaction fields, J. Comput. Phys. 227 (2007) 1246–1266], thus extending the applicability of the image charge approximations of reaction fields in the modeling of biomolecular solvation.     

On the role of the interface charge in non-ideal metal–semiconductor contacts

The bias dependent interface charge is considered as the origin of the observed non-ideality in current–voltage and capacitance–voltage characteristics. Using the simplified model for the interface electronic structure based on defects interacting with the continuum of interface states, the microscopic origin of empirical parameters describing the bias dependent interface charge function is investigated. The results show that in non-ideal metal–semiconductor contacts the interface charge function depends on the interface disorder parameter, density of defects, barrier pinning parameter and the effective gap center. The theoretical predictions are tested against several sets of published experimental data on bias dependent ideality factor and excess capacitance in various metal–semicoductor systems.     

Surface and near-surface passivation, chemical reaction, and Schottky barrier formation at ZnO surfaces and interfaces

Using a combination of depth-resolved cathodoluminescence spectroscopy, electronic transport, and surface science techniques, we have demonstrated the primary role of native defects within ZnO single crystals as well as native defects created by metallization on metal-ZnO Schottky barrier heights and their ideality factors. Native defects and impurities resident within the ZnO depletion region as well as defects extending into the bulk from the intimate metal-ZnO interface contribute to barrier thinning of, carrier hopping across, and tunneling through these Schottky barriers. Chemical reactions at clean ZnO-metal interfaces lead to metal-specific eutectic or oxide formation with pronounced transport effects. These results highlight the importance of bulk crystal quality, surface cleaning, metal interaction, and post-metallization annealing for controlling Schottky barriers.     

The role of ionic quadrupolar deformation in atom-surface potential

We show that the quadrupolar deformation of surface ions gives appreciable contribution to the potential energy between the atom and an ionic surface. Calculations are presented for a number of alkali halides. The validity of Fumi and Tosi crystal radii in the surface region is discussed in connection with the depth and the corrugation parameter.     

Chaotic potential on semiconductor boundary under conditions of the partial self-organization of the surface ion charge

The possibility of varying the parameters of electrostatic chaotic potential on a semiconductor surface with the association of point defects is investigated. Two models of surface dipole structures are considered. The amplitude and type of chaotic potential spread are determined under conditions of the partial self-organization of the ion charge. A reduction in the degree of chaos is obtained that depends on a structural parameter of a system of dipoles (their surface concentration).     

Determination of the effective charge on muonium during its reaction in aqueous solution

The rate of reaction of muonium atoms with solutes of either charge is unaffected by the addition of a high concentration of an inert salt, therefore the effective charge on the muonium at the point of reaction is essentially zero.     

The potential of mean force for an infinitely dilute ionic solution

The potential of mean force between two ions immersed in a dipolar solvent is calculated in the linearized hypernetted chain approximation. The results are found to be in good agreement with Monte Carlo computations. The variation with dipole strength is shown.     

Analysis of semiconductor surfaces and interfaces

Recent development in the experimental and theoretical analysis of semiconductor surfaces is described. Special attention is given to the Secondary Ion Mass Spectroscopy technique and to its use in the ultrasensitive elemental analysis of semiconductors. Applications to III–V compounds are described.     

Theoretical investigations on the formation of wurtzite segments in group III-V semiconductor nanowires

Structural trends in group III-V semiconductor nanowires (NWs) are systematically investigated based on Monte-Carlo simulations using our empirical potential calculations. The calculated NW stacking sequences for the selective area growth demonstrate that the averaged periodicity between wurtzite segments, which is independent of the NW size, decreases with increasing ionicity of semiconductors f_i. It is also found that the periodicity is affected by the nucleus size of NWs: The calculated periodicity in InP (InAs) NWs with the nucleus size consisting of ∼ 10 atoms are 0.76 (0.86) nm, reasonably consistent with the experimentally reported one. On the other hand, the nucleus size to reproduce the experimentally reported periodicity in GaAs NWs is estimated to be more than 70 atoms. These results thus imply that the nucleus size as well as f_i is of importance in determining the averaged periodicity between wurtzite segments.