Influence of the As/Ga flux ratio on diffusion of Be in MBE GaAs layers

Beryllium (Be) diffusion after rapid thermal annealing experiments is studied in heavily doped GaAs structures grown by MBE. SIMS measurements show that in p/p⁺ structures, Be diffusion is reduced by increasing the As₄/Ga flux ratios. In contrast, no effect is observed in p/p⁺/p structures. Furthermore, Be concentration profiles measured after annealing experiments performed at 770 and 850°C for 30 s indicate that Be redistribution is almost independent of the annealing temperature. These results are discussed in terms of a substitutional interstitial diffusion mechanism.     

Point defects in silicon, first-principles calculations

Native defects, vacancies and self-interstitials, play a fundamental role in determining mechanical and electronic properties of silicon materials used for electronics devices. The important phenomena include diffusion of dopant impurities and formation of extended defects. First-principles calculations of electronic and ionic structures of the native defects have turned out to be very demanding. In this paper, results of recent pseudopotential-plane-wave calculations within the supercell approximation are reviewed. Vacancies, self-interstitials as well as common dopant impurities are dealt with.     

Growth and defects of GaAs and InGaAs films on porous GaAs substrates

We investigated the opportunities to increase the electric uniformity of GaAs and InGaAs films grown by molecular-beam epitaxy (MBE) technique on monocrystalline (single crystal) GaAs: both on porous and conventional so-called “monolithic” (without pores) GaAs (100) substrates. The basic attention was given to study the electrically active defects in films by using scanning electron microscope (SEM) with new technique which is called “Rau-detector” [E.I. Rau, A.N. Zhukov and E.B. Yakimov, Solid-State Phenomena, 1998, v. 53-54, 327.]. We compared the main properties of epitaxial GaAs and InGaAs films grown on above mentioned substrates. The films grown on porous substrates had higher structural perfection including the following advantages: (a) smoother surface due to lateral growth mechanism; (b) less density of structural defects (without dislocation walls), the density of pyramidal defects was ∼ 2 × 10⁵ cm^− 2 as compared with the density 2 × 10⁷ cm^− 2 in the films grown on monolithic substrates; (c) less electrical activity of various structural defects and increased electric uniformity of grown films. The electrical activity of defects in films grown on porous substrates was essentially lowered due to gettering properties of porous substrate.     

Hydrogen trapping by defects in semiconductors studied by anelastic spectroscopy

We review some results obtained by anelastic spectroscopy on H-related defects in III–V semiconductors. Anelastic measurements on InP lead to the formulation of a model explaining the conversion to the semi-insulating (SI) state. Moreover, in GaAs:Zn an extraordinarily fast relaxation rate has been measured and a possible explanation has been suggested. This results are reviewed and discussed in the light of new experiments on InP:Zn, whose spectrum shows a relaxation process similar to the one in GaAs:Zn.     

Formation of WSi-based ohmic contacts to n-type GaAs

An outcome of the indirect doping concept conceived recently in NiGe-based Ohmic contacts has led to the development of annealed WSi-based Ohmic contacts to n-type GaAs for the first time. It was concluded that simultaneous addition of a “direct doping element” of Si in WSi_2.7 and an “indirect doping element (M)” such as Au, Pd, Cu, or Ag, was essential. The M(5 nm)/WSi_2.7(20nm)/W(50 nm) contacts showed Ohmic behavior after annealing with the lowest contact resistances of 0.4 Ω mm (6×10⁻⁶ cm²). In addition, the WSi-based contacts with a small amount of Au showed good thermal stability at 400 °C after contact formation. Microstructural analysis of the WSi contacts with Au showed formation of β-AuGa and WSi₂ compounds, which indicates that the Ohmic behavior would be due to heavy doping of Si at the GaAs surface induced by Ga out-diffusion. The mechanism of Ohmic contact formation of the present contacts agreed very well with that of the NiGe-based Ohmic contacts.     

Charged point defects in semiconductors

Native point defects control many aspects of semiconductor behavior. Such defects can be electrically charged, both in the bulk and on the surface. This charging can affect numerous defect properties such as structure, thermal diffusion rates, trapping and recombination rates for electrons and holes, and luminescence quenching rates. Charging also introduces new phenomena such as nonthermally photostimulated diffusion, thereby offering distinctive mechanisms for defect engineering. The present work incorporates the first comprehensive account of semiconductor defect charging, identifying correspondences and contrasts between surfaces and the bulk as well as among semiconductor classes (group IV, groups III–V, and metal oxides). For example, small lattice parameters, close-packed unit cells, and basis atoms with large atomic radii all inhibit the formation of ionized interstitials and antisites. The charged defects that exist in III–V and oxide semiconductors can be predicted with surprising accuracy from the chemical potential and oxygen partial pressure of the ambient. The symmetry-lowering relaxations, formation energies, and diffusion mechanisms of bulk and surface defect structures often depend strongly on charge state with similar qualitative behavior, although for a given material surface defects do not typically take on the same configurations or range of stable charge states as their counterparts in the bulk.     

Dielectric properties and model of hopping conductivity of GaAs irradiated by H ions

Experimental results on frequency and temperature dependences of conductivity of GaAs layers compensated by polyenergetic H⁺ implantation are presented. A model of hopping conductivity related to amphoteric defects is proposed.     

Reliability problems in state-of-the-art GaAs devices and circuits

A review of the reliability status of GaAs discrete devices and integrated circuits is given. In the present survey of new devices and circuits it is shown that a significant number of reliability problems continue to persist.     

Characterization of defects in gallium arsenide

It is well-known that the properties of semiconductor materials including gallium arsenide are controlled by defects and impurities. The characterization of these defects is important not only for better understanding of the solid state phenomena but also for improved reliability and performance of electronic devices. We have been investigating the defects in gallium arsenide for several years using deep level transient spectroscopy, photoconductivity, transient photoconductivity, photoluminescence etc. Results drawn from our recent studies are presented here to illustrate some of the problems concerning transition metal impurities, process-induced defects, occurrence of intracentre transitions and metastability of deep levels in gallium arsenide.     

Temperature-dependent Hall analysis of carbon-doped GaAs

The temperature dependence of the electrical properties, such as hole concentration, Hall mobility and resistivity of carbon-doped GaAs epilayers over a wide range of doping levels has been investigated. The carbon-doped GaAs epilayers have been grown by low pressure metalorganic chemical vapor deposition. The electrical properties have been obtained by Hall measurements. Experimental data on the carrier mobility, Hall effect, and resistivity over a wide temperature range have been analyzed and possible scattering mechanisms have been explained. Our experimental data show that the ionized impurity scattering tend to be dominant at temperatures below 100 K, while the lattice scattering as well as the ionized impurity scattering plays an important role at temperatures above 100 K. The dependence of the electrical on the doping levels has also been studied. In the case of heavily C-doped GaAs, the mobility curves are nearly flat at temperatures below 100 K and the mobility decreases as temperature increases above 100 K. The reason is that the degenerate conduction occurs at high doping level. The degenerate conduction begins at the hole concentration of about 2 × 10¹⁸ cm⁻³ at 77 K and at room temperature.     

Structure and thermodynamic stability of GaAs(001) surfaces

The surface energies for a set of structural models for the reconstructions occuring on GaAs(001) were calculated with the first principles pseudopotential method and the local density approximation. On the basis of these calculations we are able to rule out the single-dimer-vacancy model for the 2 × 4 surface as a possible equilibrium structure. In the As-rich limit we find the c(4 × 4) As addimer model to be energetically favorable. As the Ga chemical potential increases the surface is predicted to transform into the β2(2 × 4) structure having two As dimers in the top layer and a third As dimer in the third layer. The (2 × 4) structure, with two As dimers above a complete layer of Ga is found to be stable in a very narrow range. The β(4 × 2) structure and the model recently proposed by Skala et al. for the (4 × 2) reconstruction were determined to be energetically unfavorable. In the Ga-rich limit the lowest energy structure we have obtained is the β2(4 × 2) model.     

Two-dimensional modelling of diffusion of low-energy implanted arsenic in silicon at rapid thermal annealing

The two-dimensional model of arsenic diffusion in silicon at rapid thermal annealing is presented. A method of solving the nonlinear differential equations system is specified. Model calculations were performed for 15 keV As⁺ implanted in Si and annealed for 10 min at 950 °C. The results are in reasonable agreement with the experimental data, including the presence of local maximum of arsenic atoms near the surface.     

Using lifetime of point defects for dislocation bias in bcc Fe

The interaction between dislocations and point defects is key to deformation processes and microstructural evolution of structural materials. In this work, we compute the lifetime of point defects to describe their interaction with dislocations. This approach can accurately account for the effects of the dislocation core and anisotropic defect dynamics to accumulatively determine the capture efficiency, sink strength, and dislocation bias at different temperatures and dislocation densities. Particularly, the absorption of point defects by straight screw and edge dislocations in a model bcc iron system is studied. The maximum swelling rates based on the obtained bias factors are in close agreement with a variety of experimental measurements, including both neutron and ion-irradiation data, especially when considering the survival fraction for point defects from displacement cascades. This approach applies to many other processes and sinks, such as dislocation loops and interfaces, providing a powerful means to develop fundamental insights critical for improving radiation resistance and mechanical properties of structural materials through controlling defect interaction and evolution.     

RBS-C and ellipsometric investigations of radiation damage in hot-implanted GaAs layers

Semi-insulating (1 0 0) GaAs single crystalline substrates have been doubly Al⁺-implanted using ion beams of the 250 keV energy and the fluence F = 3.5 × 10¹⁶ cm⁻², and 100 keV with F = 9.6 × 10¹⁵ cm⁻² at six target temperatures ranging from 250 to 500 °C. The radiation damage introduced by such “hot implantation” was subsequently investigated by Rutherford Backscattering Spectrometry with Channeling (RBS-C) and Variable Angle Spectroscopic Ellipsometry (VASE) techniques. Using these experimental methods we determined a degree of lattice disorder. With the increasing implantation temperature the degree of disorder substantially decreases. No evidence of full amorphization of the implanted GaAs layers has been found in the present studies. The results of non-destructive ellipsometric characterization are in good agreement with the RBS-C investigations.     

Structure and formation energy of steps on the GaAs(001)-2 × 4 surface

The energies of various steps on the As-terminated GaAs(001)-2 × 4 surface are evaluated using a novel, approximate method of “linear combination of structural motifs”. It is based on the observation that previous total energy minimizations of semiconductor surfaces produced invariably equilibrium structures made of the same recurring local structural motifs, e.g. tetrahedral fourfold Ga, pyramidal threefold As, etc. Furthermore, such surface structures were found to obey consistently the octet rules as applied to the local motifs. We thus express the total energy of a given semiconductor surface as a sum of (i) the energies _M of the local structural motifs appearing in the surface under consideration and (ii) an electrostatic term representing the Madelung energy of point charges resulting from application of the octet rule. The motif energies are derived from a set of pseudopotential total energy calculations for flat GaAs(001) surfaces and for point defects in bulk GaAs. This set of parameters suffices to reproduce the energies of other (001) surfaces, calculated using the same pseudopotential total energy approach. Application to GaAs(001)-2 × 4 surfaces with steps reveals the following. (i) “Primitive steps”, defined solely according to their geometries (i.e. step heights, widths and orientations) are often unstable. (ii) Additional, non-geometric factors beyond step geometries such as addition of surface adatoms, creation of vacancies and atomic rebonding at step edges are important to lower step energies. So is step-step interaction. (iii) The formation of steps is generally endothermic. (iv) The formation of steps with edges parallel to the direction of surface As dimers (A steps) is energetically favored over the formation of steps whose edges are perpendicular to the As dimers (B steps).     

A cluster approach to hydrogen chemisorption on the GaAs(1 0 0) surface

Chemisorption properties of atomic hydrogen on the Ga-rich GaAs(1 0 0), (2×1) and β(4×2) surfaces are investigated using ab initio self-consistent restricted open shell Hartree–Fock (ROHF) total energy calculations with Hay–Wadt (HW) effective core potentials. The effects of electron correlation have been included using many-body perturbation theory through second order with the exception of β(4×2) symmetry due to computational limitations. The semiconductor surface is modeled by finite sized hydrogen saturated clusters. The effects of surface reconstruction have been investigated in detail. We report on the energetics of chemisorption on the (1 0 0) surface layer, including adsorption beneath the surface layer at an interstitial site, and also report on the possible dimer bond breaking at the bridge site. Chemisorption energies, bond lengths, and charge population analysis are reported for all considered sites of chemisorption.     

Degradation and breakdown characteristics of Al/HfYOx/GaAs capacitors

Effects of surface passivation and the interfacial layer on the reliability characteristics of Al/HfYO_x/GaAs metal-oxide-semiconductor capacitor structures are reported. Stress-induced leakage current mechanism, critical for understanding the degradation and breakdown in Al/HfYO_x/GaAs capacitors, has been studied in detail. While the devices fabricated with (NH₄)₂S-passivated GaAs substrates show both the soft and hard breakdown failure modes, capacitors with ultrathin interfacial layer (Ge or Si) show only hard breakdown. It is shown that the degradation dynamics follows more closely the logistic power-law relationship rather than the conventional power-law model, frequently used to describe leakage current conduction in high-k gate dielectrics.     

Liquid phase electro epitaxy growth kinetics of GaAs—A three-dimensional numerical simulation study

A three-dimensional numerical simulation study for the liquid phase electro epitaxial growth kinetic of GaAs is presented. The kinetic model is constructed considering (i) the diffusive and convective mass transport, (ii) the heat transfer due to thermoelectric effects such as Peltier effect, Joule effect and Thomson effect, (iii) the electric current distribution with electromigration and (iv) the fluid flow coupled with concentration and temperature fields. The simulations are performed for two configurations namely (i) epitaxial growth from the arsenic saturated gallium rich growth solution, i.e., limited solution model and (ii) epitaxial growth from the arsenic saturated gallium rich growth solution with polycrystalline GaAs feed. The governing equations of liquid phase electro epitaxy are solved numerically with appropriate initial and boundary conditions using the central difference method. Simulations are performed to determine the following, a concentration profiles of solute atoms (As) in the Ga-rich growth solution, shape of the substrate evolution, the growth rate of the GaAs epitaxial film, the contributions of Peltier effect and electromigration of solute atoms to the growth with various experimental growth conditions. The growth rate is found to increase with increasing growth temperature and applied current density. The results are discussed in detail.