Theoretical models of Schottky-barrier height formation are reviewed. A particular emphasis is placed on the examination of how these models agree with general physical principles. New concepts on the relationship between interface dipole and chemical bond formation are analyzed, and shown to offer a coherent explanation of a wide range of experimental data.

Recent advances in Schottky barrier concepts

This review presents an overview of the thermal properties of mesoscopic structures. The discussion is based on the concept of electron energy distribution, and, in particular, on controlling and probing it. The temperature of an electron gas is determined by this distribution: refrigeration is equivalent to narrowing it, and thermometry is probing its convolution with a function characterizing the measuring device. Temperature exists, strictly speaking, only in quasiequilibrium in which the distribution follows the Fermi-Dirac form. Interesting nonequilibrium deviations can occur due to slow relaxation rates of the electrons, e.g., among themselves or with lattice phonons. Observation and applications of nonequilibrium phenomena are also discussed. The focus in this paper is at low temperatures, primarily below $4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, where physical phenomena on mesoscopic scales and hybrid combinations of various types of materials, e.g., superconductors, normal metals, insulators, and doped semiconductors, open up a rich variety of device concepts. This review starts with an introduction to theoretical concepts and experimental results on thermal properties of mesoscopic structures. Then thermometry and refrigeration are examined with an emphasis on experiments. An immediate application of solid-state refrigeration and thermometry is in ultrasensitive radiation detection, which is discussed in depth. This review concludes with a summary of pertinent fabrication methods of presented devices.

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Opportunities for mesoscopics in thermometry and refrigeration: Physics and applications

The formation of the Schottky barrier height (SBH) is a complex problem because of the dependence of the SBH on the atomic structure of the metal-semiconductor (MS) interface. Existing models of the SBH are too simple to realistically treat the chemistry exhibited at MS interfaces. This article points out, through examination of available experimental and theoretical results, that a comprehensive, quantum-mechanics-based picture of SBH formation can already be constructed, although no simple equations can emerge, which are applicable for all MS interfaces. Important concepts and principles in physics and chemistry that govern the formation of the SBH are described in detail, from which the experimental and theoretical results for individual MS interfaces can be understood. Strategies used and results obtained from recent investigations to systematically modify the SBH are also examined from the perspective of the physical and chemical principles of the MS interface.

The physics and chemistry of the Schottky barrier height

The structure and properties of metal-semiconductor interfaces

Memristors are two-terminal passive circuit elements that have been developed for use in non-volatile resistive random-access memory and may also be useful in neuromorphic computing. Memristors have higher endurance and faster read/write times than flash memory and can provide multi-bit data storage. However, although two-terminal memristors have demonstrated capacity for basic neural functions, synapses in the human brain outnumber neurons by more than a thousandfold, which implies that multi-terminal memristors are needed to perform complex functions such as heterosynaptic plasticity. Previous attempts to move beyond two-terminal memristors, such as the three-terminal Widrow-Hoff memristor and field-effect transistors with nanoionic gates or floating gates, did not achieve memristive switching in the transistor. Here we report the experimental realization of a multi-terminal hybrid memristor and transistor (that is, a memtransistor) using polycrystalline monolayer molybdenum disulfide (MoS2) in a scalable fabrication process. The two-dimensional MoS2 memtransistors show gate tunability in individual resistance states by four orders of magnitude, as well as large switching ratios, high cycling endurance and long-term retention of states. In addition to conventional neural learning behaviour of long-term potentiation/depression, six-terminal MoS2 memtransistors have gate-tunable heterosynaptic functionality, which is not achievable using two-terminal memristors. For example, the conductance between a pair of floating electrodes (pre- and post-synaptic neurons) is varied by a factor of about ten by applying voltage pulses to modulatory terminals. In situ scanning probe microscopy, cryogenic charge transport measurements and device modelling reveal that the bias-induced motion of MoS2 defects drives resistive switching by dynamically varying Schottky barrier heights. Overall, the seamless integration of a memristor and transistor into one multi-terminal device could enable complex neuromorphic learning and the study of the physics of defect kinetics in two-dimensional materials.

Multi-terminal memtransistors from polycrystalline monolayer molybdenum disulfide

Using simplified models it is shown that the effective height of a Schottky barrier can be controlled over a wide range using highly doped surface layers. Experimental results on silicon having surface layers formed by low energy ion implantation verify the main features of the models and show that the use of surface layers brings flexibility to the Schottky barrier system.

J.M. Shannon

Papers

Control of Schottky barrier height using highly doped surface layers