For more than three decades, SiC has been investigated as a wide band gap semiconductor. This paper reviews ohmic and rectifying metal contacts on n- and p-type α- and β-SiC reported throughout that time period. Electrical characteristics, Schottky barrier heights (SBHs), thermal stability, and chemical reactions are discussed. Most metals formed very good rectifying contacts in the as-deposited condition on both n- and p-type 6H-SiC with Schottky barrier heights ⪖ 1 eV. Low ideality factors (n 1100 V) have been displayed in this material. The electrical properties of contacts on 3C-SiC have been more dependent on the quality of the 3C-SiC films, which have been plagued by higher defect densities than 6H-SiC. In general, a partial pinning of the Fermi level has been evidenced by positive correlations, which are less than 1 (≈ 0.2–0.6), between the SBHs and the metal work functions. Ohmic contacts with low contact resistivities (⪕ 10−5 Ω cm2), especially important for high power applications, on any of the SiC polytypes have been exceptionally difficult to achieve. Most of the ohmic contacts have relied on high doping concentrations in combination with annealing at temperatures between 800 and 1300 °C. Annealed Ni and Al have primarily been used in ohmic contact metallizations for n- and p-type SiC, respectively. The tendency of SiC to react with metals to form carbides and/or silicides at potential device operating temperatures ( ∼ 600 °C) can be a problem for potential long term applications at high temperature. These critical issues are discussed along with future perspectives for research approaches.

A critical review of ohmic and rectifying contacts for silicon carbide

An overview of ohmic contacts on solar cells is presented The fundamentals of metal-semiconductor contacts are reviewed, including the Schottky approach, Fermi level pinning by surface states, and the mechanisms of thermionic emission, thermionic/field emission, and tunneling for current transport The concept of contact resistance is developed and contact resistance data for several different contact materials on both silicon and gallium arsenide over a range of doping densities are summarized Finally, the requirements imposed by solar cells on contact resistance are detailed

Solar cell contact resistance&#8212;A review

Fowler–Nordheim tunneling of electrons through “crested” energy barriers (with the height peak in the middle) is much more sensitive to applied voltage than that through barriers of uniform height Calculations for trilayer barriers, with layer parameters typical for wide-band-gap semiconductors, have shown that by merely doubling the voltage, the tunnel current may be changed by more than 16 orders of magnitude It is argued that this effect may be used for the implementation of nonvolatile random-access memories combining a few ns cycle time with a few years retention time and for ultradense electrostatic data storage

Layered tunnel barriers for nonvolatile memory devices

An algorithm to include the Pauli exclusion principle in the Ensemble Monte Carlo method is presented. The results indicate that significant changes in the transport properties of GaAs have to be expected when degenerate conditions are reached. Important repercussions should be found in the modeling of microwave devices, where one often deals with highly doped regions.

Degeneracy in the ensemble Monte Carlo method for high-field transport in semiconductors

The properties of hot electrons in systems where electrons and phonons experience quantum confinement are reviewed. The modifications to the behaviour of electrons and phonons brought about by confinement are described, particularly with reference to the principal scattering mechanism. The latter include the interaction with longitudinal optical phonons and plasmons, along with carrier-carrier effects. Some conflict in the literature concerning Fuchs-Kliewer polaritons is discussed. Low-temperature interactions with acoustic phonons are described. A central topic is that of energy relaxation, and the experimental and theoretical data relating to this form a large part of the review. Energy relaxation mechanisms in the femtosecond to nanosecond regimes, including intersubband and well-capture processes, are eventually summarized. An equally large section deals with hot-electron transport; in which negative differential resistance and other instabilities associated with parallel transport are discussed before turning to ballistic transport and impact ionization.

https://iopscience.iop.org/article/10.1088/0034-4885/54/2/001/pdf

Hot electrons in low-dimensional structures

A new majority carrier rectifying device is demonstrated which is made in gallium arsenide by molecular beam epitaxy. It exhibits thermionic transport with positive and negative bias and the barrier height and degree of asymmetry in the I/V characteristic can be continuously varied. The device also shows a constant capacitance with bias.

Planar-doped barriers in GaAs by molecular beam epitaxy

Nonalloyed ohmic contacts to n-GaAs with contact resistances (ρc) below 1·0×10−7 Ω cm2 have been obtained using a Ge/GaAs heterojunction system. Metals are evaporated on heavily arsenic-doped germanium (Ge) layers grown on GaAs. Low values of ρc result from the low Schottky barrier height (≈ 0·50 eV) and the high doping levels obtainable for n-Ge(≈ 1020 cm−3).

Ultra low resistance ohmic contacts to n-GaAs

A new form of two-state switching device is proposed which is formed entirely within the semiconductor bulk during epitaxial growth. The predicted switching action is demonstrated using MBE-grown gallium arsenide and the device is shown to have optical sensitivity. The device is amenable to simple design rules and is free of the ‘forming’ state often associated with the tunnel MIS switch.

Regenerative switching device using MBE-grown gallium arsenide

A new majority carrier rectifying barrier device designated as a "planar-doped barrier" (PDB) has been fabricated in GaAs by MBE. The key feature of the PDB structure is that the barrier height and degree of asymmetry in the I-V characteristics can be continuously and independently controlled. In addition, the barrier can be designed to yield a constant capacitance. Experimental I-V characteristics agree well with a thermionic emission model. This planar-doped-barrier concept offers significant advantages over Schottky barriers and may, in fact, evolve into a new generic class of barrier devices.

K. Board

Papers

Planar-doped barriers in GaAs by molecular beam epitaxy

Ultra low resistance ohmic contacts to n-GaAs

Regenerative switching device using MBE-grown gallium arsenide

GaAs planar doped barriers by molecular beam epitaxy

Erratum: Ultra low resistance ohmic contacts to n-GaAs