Yu. A. Gol’dberg

Mechanisms of current flow in metal-semiconductor ohmic contacts

Published data on the properties of metal-semiconductor ohmic contacts and mechanisms of current flow in these contacts (thermionic emission, field emission, thermal-field emission, and also current flow through metal shunts) are reviewed. Theoretical dependences of the resistance of an ohmic contact on temperature and the charge-carrier concentration in a semiconductor were compared with experimental data on ohmic contacts to II–VI semiconductors (ZnSe, ZnO), III–V semiconductors (GaN, AlN, InN, GaAs, GaP, InP), Group IV semiconductors (SiC, diamond), and alloys of these semiconductors. In ohmic contacts based on lightly doped semiconductors, the main mechanism of current flow is thermionic emission with the metal-semiconductor potential barrier height equal to 0.1–0.2 eV. In ohmic contacts based on heavily doped semiconductors, the current flow is effected owing to the field emission, while the metal-semiconductor potential barrier height is equal to 0.3–0.5 eV. In alloyed In contacts to GaP and GaN, a mechanism of current flow that is not characteristic of Schottky diodes (current flow through metal shunts formed by deposition of metal atoms onto dislocations or other imperfections in semiconductors) is observed.

Semiconductor photoelectric converters for the ultraviolet region of the spectrum

Recently, ultraviolet photoelectronics emerged in response to the needs of medicine, biology, military equipment, and the problem of the hole in the ozone layer. A specific feature of this field of photoelectronics is the need to detect weak (albeit appreciably affecting vital human functions) signals against a background of intense radiation in the visible and infrared regions of the spectrum. Ultraviolet electronics relies on Si-based p-n structures and GaP-based Schottky barriers, p-n structures and Schottky barriers based on GaN and AlGaN (“solar-blind,” i.e., solar-radiation-insensitive devices), SiC structures with potential barriers (high-temperature devices), and ZnO-and ZnS-based photoresistors and Schottky diodes. In this review, the parameters of starting wide-gap semiconductors are given, physical foundations for photoelectric conversion and the principles of formation of ohmic contacts are described, characteristics of corresponding devices are given, and the envisaged lines of further studies are outlined.

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.

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.

Temperature dependence of the quantum efficiency of 4H-SiC-based Schottky photodiodes

Using metal-semiconductor structures based on a pure epitaxial layer of n-4H-SiC (Nd− Na=4×1015 cm−3), UV photodetectors were created with a maximum photosensitivity at 4.9 eV and a quantum efficiency up to 0.3 el/ph. The photosensitivity spectrum of the base structure is close to the spectrum of bactericidal action of the UV radiation. For photon energies in the 3.4–4.7 eV range, the quantum efficiency of the photoelectric conversion exhibits rapid growth with the temperature above 300 K, which is explained by the participation of photons in indirect interband transitions. This growth is not manifested when the photon energy is close to the threshold energy of direct optical transitions in the nondirect-bandgap semiconductor, which allows the threshold energy to be evaluated (∼4.9 eV).

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.

Effect of the electric field in the space-charge layer on the efficiency of short-wave photoelectric conversion in GaAs Schottky diodes

A study is reported of the quantum efficiency of the short-wave photoelectric effect as function of the reverse bias applied to GaAs Schottky diodes when the light absorption length is much shorter than the width of the space charge region. The quantum efficiency of photoelectric conversion is found to depend strongly on the contact electric field and the photon energy. The field independence of the quantum efficiency is interpreted in terms of a fluctuational trap model. The model can also be used to determine the loss factor for hot photocarriers, which is found to increase in a stepwise manner with increasing photon energy. This effect is explained in terms of the formation of excitons in X-and L-valleys of the semiconductor.

Selective 4H-SiC UV Detectors

Carcinogenic (bactericidal) radiation (λ = 200–300 nm with a peak at 254 nm) is present in natural (Sun) and artificial (lamps) source of UV radiation. Its intensity is very low as compared to other types of radiation, but it strongly affects the health of human beings. To prevent oncological diseases, it is important to monitor the carcinogenic radiation level; i.e., selective photodetectors are required. A UV photodetectors based on n-4H-SiC Schottly barriers and p+-n junctions are proposed. The quantum efficiency spectrum of such detectors is very close to the spectrum of relative action of carcinogenic radiation on human beings due to the direct optical transition at 4.9 eV in 4H-SiC. The quantum efficiency (at the spectral peak 254 nm) amounts to about 0.3 electrons/photon for virtually zero sensitivity in other spectral regions. Quantum efficiency in the wavelength range 247–254 nm is practically independent of temperature in the range from −100 to +300°C.

An instrument for measuring the intensity and dose of cancerigenic ultraviolet radiation

An instrument for measuring the intensity and dose of cancerigenic radiation from the Sun and artificial sources has been developed. A 4H-SiC Schottky barrier photodetector is used as the sensor. The sensitivity spectrum of the photoelectric transducer lies in the region 240–300 nm and has a maximum at 260 nm. The quantum efficiency is ≈0.3 electrons/photon (at λ = 254 nm), and its temperature coefficient at T = 250–310 K is < 0.1%/°C. The dimensions of the device are 3 × 6 × 11 cm.

Transition processes occurring under continuous and stepwise heating of GaAs surface-barrier structures

Evolution of the capacitance-voltage (C-U) and current-voltage (If-U and Ir-U) characteristics of solid metal-semiconductor structures (Ni/GaAs) in the process of their continuous and stepwise heating are studied. Properties of the initial structures obey the theory of thermionic emission. It has been shown that as a result of continuous heating, the rectifying structures become ohmic at a temperature of TOhm=720 K, which is substantially lower than the melting points of the metal or the metal-semiconductor eutectic. For comparison, properties of the structures annealed at different temperatures Tann are measured after cooling to room temperature (stepwise heating). In this case, I-U characteristics are closer to the initial ones for annealing temperatures TannT0, the characteristics display excess currents; and, finally, for Tann exceeding T0 by 200–300 K, the characteristics become purely ohmic. It is suggested that these effects are due to a chemical interaction between Ni and GaAs, which changes the properties of the semiconductor surface.