Yu. A. Goldberg

Semiconductor near-ultraviolet photoelectronics

After a brief review of classification, application and sources of near-ultraviolet (UV) radiation the methods for fabricating UV photodetectors and characteristics of the photoconductive cells, p-n junction structure and Schottky barrier photodiodes are discussed. Characteristics of some light filters used in photodetectors and measuring devices are also reported. Now Si p-n structures are commonly used but Schottky diodes based on wide-gap (GaAsP, GaP, GaN, AlGaN, SiC) semiconductors are very attractive. They are insensitive to the infrared radiation and if necessary simple glass filters can be used for correcting the spectrum in such way that it covers just the near-UV region.

Temperature dependence of the photoelectric conversion quantum efficiency of 4H–SiC Schottky UV photodetectors

Ultraviolet Schottky photodetectors based on n-4H–SiC (Nd − Na = 4 × 1015 cm−3) epitaxial layers of high purity have been fabricated. Their spectral sensitivity range is 3.2–5.3 eV peaking at 4.9 eV (quantum efficiency is about ~0.3 electron/photon), which is close to the bactericidal ultraviolet radiation spectrum. The temperature dependence of the quantum efficiency of 4H–SiC Schottky structure has been investigated to determine the temperature stability and the mechanism of the photoelectric conversion process. At low temperatures (78–175 K) the quantum efficiency increases with increasing temperature for all photon energy values and then tends to saturate. We suppose that some imperfections in the space-charge region act as traps that capture both photoelectrons and photoholes. After some time the trapped electron–hole pairs recombine due to the tunnelling effect. At high temperatures (more than 300 K), the second enhancement region of the quantum efficiency is observed in the photon energy range of 3.2–4.5 eV. It is connected with a phonon contribution to indirect optical transitions between the valence band and the M-point of the conduction band. When the photon energy is close to a direct optical transition threshold this enhancement region disappears. This threshold is estimated to be 4.9 eV. At photon energies more than 5 eV a drastic fall of the quantum efficiency has been observed throughout the temperature interval. We propose that in this case the photoelectrons and photoholes are bound to form hot excitons in the space-charge region due to the Brillouin zone singularity, and do not contribute to the following photoelectroconversion process.

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.

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.

Mechanism of the current flow in Pd-(heavily doped p-AlxGa1−xN) ohmic contact

The physical mechanism of the current flow in Pd-(heavily doped p-AlxGa1−xN) ohmic contact is studied. Chloride-hydride epitaxy was used to grow the p-Al0.06Ga0.94N solid solution with uncompensated acceptor concentration Na–Nd ranging from 3×1018 up to 1019 cm−3. Thermal vacuum deposition and subsequent thermal treatment were used to form an ohmic Pd contact. It is shown that, after the thermal treatment, the Pd-p-Al0.06Ga0.94N barrier contact with a potential barrier height of about 2.3 V becomes ohmic and the barrier height decreases to approximately 0.05 V. For uncompensated acceptor concentration Na–Nd=3×1018 cm−3, thermionic emission is found to be the main mechanism of the current through the Pd-p-Al0.06Ga0.94N ohmic contact. An increase in Na–Nd to approximately 1019 cm−3 in the solid solution leads to a transition from thermionic emission (at high temperatures) to tunneling (at low temperatures).

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.

Current flow by metallic shunts in alloyed ohmic contacts to wide-gap semiconductors

A mechanism of current flow across alloyed ohmic contacts in lightly doped wide-gap A3B5 semiconductors has been investigated experimentally. Changes in the current–voltage and capacity–voltage characteristics of semiconductor–metal structures have been retraced on continuous heating, the semiconductors being GaAs and GaP and the metals In and Au. Furthermore, the temperature dependence of specific resistance has been studied in the 77–450 K range for In–GaN–In, In–GaP–In and In–GaAs–In ohmic contacts. A new mechanism of current flow across ohmic contacts has been proposed that lies in the suggestion that ohmic contacts in the In–GaN and In–GaP structures containing high-density dislocations should be formed by conducting metallic shunts that are connected across the space-charge layer. The shunts originate due to indium-atom precipitation on dislocations or other imperfections, the specific contact resistance being increased with temperature. In contrast, the shunts are of no importance at low dislocation density, for instance in In–GaAs structures, and the current flow mechanism is typical of Schottky diodes (thermionic or field emission at a certain carrier concentration in semiconductors).

Thermal-Field Forward Current in GaN-Based Surface-Barrier Structures

The voltage and temperature dependences of the capacitance and forward current in surface-barrier Ni-n-GaN structures are experimentally studied. The results are compared with the Padovani-Stratton thermofield emission theory. It is established that, in a temperature range of 250–410 K, the forward current of the Ni-n-GaN surface-barrier structures (the electron density in GaN is ∼1017 cm−3) is caused by a thermofield emission of electrons, whose energy is ∼0.1 eV below the potential-barrier top.