A. P. Vyatkin

Defects in Schottky barrier structures

ConclusionThe progress achieved in recent years in study of the element and phase composition, as well as the energy spectrum of electron states on semiconductor boundaries with metals and dielectrics, has shown the complexity and multifaceted nature of the physicochemical reactions-occurring on these boundaries. In the present review we have attemped to consider the question of the physical interrelationship between the phenomena of interdiffusion of elements and formation of intermetallic compounds, and the electrophysical properties of metalsemiconductor contacts. Analysis of the results of theoretical and experimental studies permits the conclusion that an important role in both potential barrier formation and charge carrier transmission is played by structural defects developed during these reactions in the contact region of the semiconductor.After chemical processing and deposition of metallic coatings stoichiometric composition defects are formed. Vacancies, antistructural defects, and defect complexes form surface electron states which together with intrinsic states ensure stabilization of the Fermi level on semiconductor boundaries with metals.In many cases structural defects are the vehicle by which physicoehemical interactions in thin film metal-semiconductor contacts affect the electrical parameters of devices and their stability. The initial stage of degradation, as a rule, is the appearance of excess currents in the forward branch of the CVC at temperatures of 150-77°K. Study of the physical nature of excess currents shows that their appearance is related to generation of structural defects during thermal processing, or mechanical or electrical testing of specimens. The defects create a system of inhomogeneously distributed deep centers in the space-charge region. The most probable mechanism for charge carrier transmission with participation of deep levels is resonant tunneling.Thus, to solve problems related to increasing reliability of Schottky barrier devices special attention should be given to conditions under which defects develop in the active region of the semiconductor diode and methods for eliminating these defects. Creation of conditions during device preparation favorable to formation of stable intermetallic compounds in the transition layer, maintenance of stoichiometric composition in the semiconductor, and inhibition of diffusion of electrically active metals, together with selection of barrier producing materials and protective coatings providing minimum mechanical stress in the contact will insure development of Schottky barrier devices having electrical parameters which are stable over a wide temperature range.

Structure and properties of interphase boundaries of gallium arsenide-metal (dielectric)

To study the nature and properties of potential barriers in gallium arsenide devices, we have investigated structural phase transitions in GaAs contacts with multilayer films containing refractory transition-metal borides (TiB2, LaB6). We verified the important role in degrading Schottky barrier device performance played by local mechanical stresses introduced at the interface by lateral nonuniformities in interphase interactions. We examine the electrical properties of MIS gallium arsenide devices, taking into account the high density of electronic surface states (ESS). We show it is possible to control the density of ESS by selecting the dielectric, and we discuss its deposition and annealing with a pulsed laser. We discuss the nature of potential barriers in gallium arsenide devices, drawing upon our data and previously published data and modern theoretical models.

Interphase interactions in a Pd-GaAs system and their effect on electrical properties of schottky-barrier structures I. effect of heat treatment on characteristics of GaAs-Pd/Ni contacts

The electrical characteristics of surface-barrier GaAs-Pd/Ni structures and the physicochemical interaction processes at the metal-semiconductor boundary were comprehensively investigated in relation to heat treatment in various atmospheres. X-Ray structural analysis showed that in the investigated system metallurgical reactions begin at 300–350‡C: Unstable intermediate phases (presumably Pd2Ga) are formed. At 400–550‡C all the palladium is converted to the bound state and the intermetallic compound PdGa is formed. The phase changes have no significant effect on the properties of diode structures fired in a hydrogen atmosphere. Heating in vacuum leads to degradation of the contact parameters at 300‡C or more. This effect is attributed to penetration of oxygen to the interface during formation of the intermediate phases.

Anomalous tensoelectric effects in gallium arsenide tunnel diodes

Anomalous tensoelectric phenomena induced in a tunnel p-n junction by a concentrated load and by hydrostatic compression were studied. The anomalous tensoelectric effects are caused by the action of concentrators of mechanical stresses in the vicinity of the p-n junction, giving rise to local microplastic strain. Under the conditions of hydrostatic compression prolate inclusions ∿100–200 å long play the role of concentrators. Analysis of irreversible changes in the current-voltage characteristics of tunnel p-n junctions made it possible to separate the energy levels of the defects produced with plastic strain of gallium arsenide.

Thermal instability of peak current in tunnel diodes

The effect of temperature change on stability of electrical characteristics of GaAs and GaSb tunnel diodes is studied. It is shown that thermal stresses developed at the boundary between the electrode alloy and the semiconductor lead to deformations (plastic and creep), localized mainly in the electrode layer. These deformations are sensed by the p-n junction which lies close to the phase boundary, which leads to a relaxation of peak current, and its hysteresis-type temperature dependence.

Formation of p-n junctions in tellurium-doped AlxGa1−xSb(As) (x = 0.15–0.20) solid solution layers

We study the variation of electron density n in depth of tellurium-doped epitaxial AlxGa1−xSb(As) (x = 0.15–0.20) layers. It is established that n decreases in proportion to the growth of the layer and, under definite conditions, the formation of p-n junctions in layers grown from a single melt or the growth of layers having an electron density below 1016 cm−3 is possible. It is shown that the cause of such a decrease of electron density is the variation of the composition of the solid solution in depth of the layer and the accompanying increase in the concentration of residual acceptor defects in the material.

Effect of electron irradiation on excess currents of GaSb tunnel diodes

Forward biased currents in GaSb tunnel diodes are studied. In the excess current region four segments with differing current transfer mechanisms can be distinguished in the forward branch of the current-voltage curve (CVC) for unirradiated diodes. Irradiation by electrons with energies of 2.2 MeV produces an increase in excess current accompanied by a change in the current transfer mechanisms in the individual segments. The increase in excess current is related to formation and realignment of radiation defects which produce shallow and deep levels in the GaSb forbidden zone. Tunnel spectroscopy with irradiation was used to determine energy levels of Ec-0.060 and Ec −0.2 eV in the n-region of the p-n junction.

Investigation of the band spectrum of semiconductors by means of a tunnel diode under pressure

The influence of hydrostatic compression on gallium antimonide tunnel diodes with different levels of doping of the n-type region was investigated. Analysis of the experimental dependence of the peak current on the pressure in the range from 0 to 4 kbar yielded the magnitude and the baric coefficient of the energy gap between the main minimum and the L minimum. The ionisation energy of the tellurium levels associated with the L minima is found as a function of the electron density in the n-type region.

Behavior of excess currents in n-type gallium arsenide tunnel diodes

An investigation was made into the nature of the excess currents in n-type gallium arsenide tunnel diodes and the influence of irradiation with 2.0-MeV electrons on the current-voltage characteristics of the diodes.

Influence of copper impurity and structure defects on the properties of compound CdSnAs2

A study was made of the mechanisms of formation and interaction of impurities and intrinsic defects in CdSnAs2 crystals during diffusion doping of this compound with copper and during subsequent low-temperature (300°C) annealing. This was done by diffusing copper into samples with different “initial” defect structure under conditions of controlled arsenic vapor pressure and applying the method of quasichemical reactions to analyze the experimental data. It was found that the special features of the physical properties of n-type CdSnAs2:Cu crystals — the low density and high mobility of electrons-were due to the high reactivity of copper resulting in binding of the original crystal defects to form neutral complexes. It was found that the “anomalous” annealing of CdSnAs2: Cu samples, resulting in n-p conversion of the type of conduction, was associated with precipitation of a solid solution of an accidental donor impurity (most probably oxygen). The diffusion doping with copper was found to be one of the most effective methods for controlling the properties of CdSnAs2, particularly in the preparation of high-resistivity crystals.