E. A. Posse

Peculiarities in the mechanism of current flow through an ohmic contact to gallium phosphide

The temperature dependence of the electric resistance of the In-GaP ohmic contact has been studied in the range from 77 to 420 K. The resistance was measured in GaP plates of various thickness with two In ohmic contacts. The measured ohmic contact resistance increases with temperature in the interval from 230–420 K. It is suggested that the In-GaP ohmic contact is formed by metallic shunts appearing upon deposition of In atoms on dislocations and other imperfections present (with a density evaluated at (4.5–8)×107 cm−2) in the subsurface region of the semiconductor.

The mechanism of current flow in an alloyed In-GaN ohmic contact

The resistance of alloyed In-GaN ohmic contact is studied experimentally. In the temperature range 180–320 K, the resistance per unit area increases with temperature, which is typical of metallic conduction and disagrees with current flow mechanisms associated with thermionic, field-effect, or thermal field emission. It is assumed that In-GaN ohmic contact is formed by conducting shunts arising due to precipitation of In atoms on dislocations. As determined from the temperature dependence of the contact resistance, the number of shunts per unit contact area is ∼(107–108) cm−2, which is close to the dislocation density of 108 cm−2 measured in the initial material.

Dependence of the mechanism of current flow in the in-n-GaN alloyed ohmic contact on the majority carrier concentration

Based on the study of the temperature dependence of resistance of the In-n-GaN alloyed ohmic contacts, it is found that the mechanism of current flow in them substantially depends on the concentration N of uncompensated donors in GaN. At N = 5 × 1016 − 1 × 1018 cm−3, current mainly flows along the metallic shunts, and at N ⩾ 8 × 1018 cm−3 it flows by tunneling.

Flow of the current along metallic shunts in ohmic contacts to wide-gap III–V semiconductors

It is established experimentally that the contact metal—wide-gap semiconductor (GaAs, GaP, GaN) with the Schottky barrier transforms into the ohmic contact either in the process of continuous heating or in the process of holding at an elevated temperature before the formation of any recrystallized layers. In this case, resistance of the contact reduced to the unit area increases as the temperature increases for semiconductors with a high dislocation density (GaP, GaN). It is assumed that in such contacts, the current flows along the metallic shunts, which shorten the layer of space charge and are formed by metal atoms diffused along the dislocation lines or other imperfections of the semiconductor. In semiconductors with a low dislocation density (GaAs), resistance of the ohmic contact per unit area decreases with increasing the temperature as it was expected for the thermionic mechanism of current flowing.

Field and temperature dependencies of the quantum efficiency of GaAs and GaP Schottky diodes

An experimental and theoretical study of GaAs and GaP Schottky photodiode quantum efficiency is reported. The quantum efficiency was investigated as a function of temperature in the 80-360 K interval and as a function of electric field in the space-charge region in the interval. The photocurrent is found to increase strongly with temperature, by a factor of three for GaP diodes and by a factor of six for GaAs diodes. We believe that this is evidence of a high concentration of imperfections in the space-charge region. These imperfections manifest themselves only in photoelectric properties. Such defects act as traps and capture both photoelectrons and photoholes. At low temperatures, most of the pairs recombine, but some fraction of them escape from the traps due to thermal excitation and give an electric current which rises with temperature. The time of the capture has to be of the order of the carrier drift time, . The electric field dependence of the quantum efficiency is also evidence of the high trap concentration. We believe that this is due to a field-induced shift of the carrier energy level in the trap. At high temperature, the photon energy and electric field dependencies of the photocurrent tend towards saturation.

Current flow mechanism in ohmic contact to n-4H-SiC

Current flow in an In-n-4H-SiC ohmic contact (n ≈ 3 × 1017 cm−3) has been studied by analyzing the temperature dependence of the per-unit-area contact resistance. It was found that the thermionic emission across an ∼0.1-eV barrier is the main current flow mechanism and the effective Richardson constant is ∼2 × 10−2 A cm−2 K−1.

Mechanism of current flow in alloyed ohmic In/GaAs contacts

A mechanism of current flow in an alloyed ohmic In contact to low-doped gallium arsenide (n = 4 × 1015 cm−3) is studied. From the temperature dependence of the contact resistance per unit surface area, it is found that the basic mechanism of current flow is thermionic emission through a potential barrier 0.03 eV in height.

Ultraviolet radiation photodetectors based on structures consisting of a metal and a wide-bandgap semiconductor

Recently, much attention has been given to measuring and monitoring ultraviolet radiation from the Sun and artificial sources. Detectors based on various wide-bandgap surface-barrier structures, which are characterized by a linear dependence of the photocurrent on incident power density in the range of 10−2–103 W/m2 and can record various kinds of ultraviolet radiation, are described. For example, GaP detectors with a UFS-6 filter have a spectral photosensitivity range corresponding to that of solar ultraviolet radiation on the Earth’s surface. The spectral sensitivity of 4H-SiC surface-barrier photodetectors corresponds to the spectral curve of the bactericidal effect produced by ultraviolet radiation. A model has been developed for explaining the process of short-wavelength photoelectric conversion. According to this model, photogenerated electrons and holes can unite into hot excitons, being thus excluded from the photoelectric conversion process. The rise in quantum efficiency with increasing temperature, which has been established experimentally for photodetectors based on Schottky barriers, is attributed to the capture of photogenerated carriers by traps arising from fluctuations of the conduction and valence band edges, with subsequent thermal release of these carriers. These fluctuations are related to imperfections in the surface layer of the semiconductor, which is confirmed by the temperature independence of the quantum efficiency of photodetectors based on p-n structures.

Ohmic contact formation during continuous heating of GaAs and GaP Shottky diodes

Changes in the current-voltage and capacitance-voltage characteristics of semiconductor-solid metal structures (GaAs-Ni and GaP-Au Schottky diodes) during continuous heating have been studied. It is shown that the rectifying contacts are transmuted into ohmic contacts at some temperature Tohm. This transition precedes the possible formation of a recrystallized layer that is peculiar to conventional ohmic contacts. The transition temperature Tohm is substantially lower than the melting point of the metal. The current-voltage characteristics of structures annealed at different temperatures Tann and cooled to room temperature have been studied. It is shown that at some temperature Tann lower than some critical temperature T0 the structural properties remain virtually constant, that at Tohm>Tann>T0 the structures remain rectifying but excess currents appear, and that at Tann>Tohm the structures become irreversibly ohmic. It is assumed that after chemical interaction between the metal and the surface layer of the semiconductor, the newly formed surface acquires properties that account for the ohmic characteristics of the metal-semiconductor contact.

Temperature dependence of the short-wave quantum efficiency of GaAs and GaP surface-barrier photosensors

Abstract A study has been made of the temperature dependence of the quantum efficiency of photoelectric conversion in GaP and GaAs surface-barrier structures in the region of direct optical transitions. The temperature interval is 120–360 K. Experiments show that the short-wave quantum efficiency γ of the structures increases with temperature T . For higher photon energies the temperature increase is weaker and at high temperatures the γ = f ( T ) dependence approaches saturation. It is concluded that the increase of the quantum efficiency stems from a temperature-induced increase in the internal quantum yield of the photoelectric effect, not from the effect of temperature on the loss of non-equilibrium charge carriers.