The Al x Ga1−x As/GaAs heterostructure system is potentially useful material for high‐speed digital, high‐frequency microwave, and electro‐optic device applications Even though the basic Al x Ga1−x As/GaAs heterostructure concepts are understood at this time, some practical device parameters in this system have been hampered by a lack of definite knowledge of many material parameters Recently, Blakemore has presented numerical and graphical information about many of the physical and electronic properties of GaAs [J S Blakemore, J Appl Phys 5 3, R123 (1982)] The purpose of this review is (i) to obtain and clarify all the various material parameters of Al x Ga1−x As alloy from a systematic point of view, and (ii) to present key properties of the material parameters for a variety of research works and device applications A complete set of material parameters are considered in this review for GaAs, AlAs, and Al x Ga1−x As alloys The model used is based on an interpolation scheme and, therefore, necessitates known values of the parameters for the related binaries (GaAs and AlAs) The material parameters and properties considered in the present review can be classified into sixteen groups: (1) lattice constant and crystal density, (2) melting point, (3) thermal expansion coefficient, (4) lattice dynamic properties, (5) lattice thermal properties, (6) electronic‐band structure, (7) external perturbation effects on the band‐gap energy, (8) effective mass, (9) deformation potential, (10) static and high‐frequency dielectric constants, (11) magnetic susceptibility, (12) piezoelectric constant, (13) Frohlich coupling parameter, (14) electron transport properties, (15) optical properties, and (16) photoelastic properties Of particular interest is the deviation of material parameters from linearity with respect to the AlAs mole fraction x Some material parameters, such as lattice constant, crystal density, thermal expansion coefficient, dielectric constant, and elastic constant, obey Vegard’s rule well Other parameters, eg, electronic‐band energy, lattice vibration (phonon) energy, Debye temperature, and impurity ionization energy, exhibit quadratic dependence upon the AlAs mole fraction However, some kinds of the material parameters, eg, lattice thermal conductivity, exhibit very strong nonlinearity with respect to x, which arises from the effects of alloy disorder It is found that the present model provides generally acceptable parameters in good agreement with the existing experimental data A detailed discussion is also given of the acceptability of such interpolated parameters from an aspect of solid‐state physics Key properties of the material parameters for use in research work and a variety of Al x Ga1−x As/GaAs device applications are also discussed in detail

GaAs, AlAs, and AlxGa1−xAs: Material parameters for use in research and device applications

Negative electron affinity (NEA) photocathodes are defined by the relationship between the potential barrier at the surface and the bottom of the conduction band in the bulk of the material. If the bottom of the conduction band liesabove the potential barrier at the surface, the device is said to have a negative electron affinity. In practice this condition is obtained by heavyp-doping of the semiconductor (to encourage downward band bending at the surface) and by adding a thin film (several atomic layers) of cesium richcesium oxide on the clean semiconductor surface. The physics, development, fabrication, and applications of the NEA cathode are reviewed.

Negative affinity 3–5 photocathodes: Their physics and technology

Semiconductors with negative electron affinity (NEA) surfaces are used as photoemitters, secondary emitters, and cold-cathode emitters. A comprehensive review of the characteristics and applications of these materials is presented, the concept of NEA is described, and a comparison is made between NEA and conventional emitters. Electron generation, transport, and emission processes of NEA emitters are discussed. NEA III-V compound photocathodes, especially GaAs, are described with respect to their fabrication, performance, and applications to photomultipliers and image intensifier tubes. The structure and performance of NEA secondary emitters are presented. NEA GaP secondary-emission dynodes represent the most important device application. NEA cold cathodes, using GaAs, Ga(As, P), or Si, have been investigated, and their performance characteristics are summarized. NEA Si cold cathodes have been incorporated in developmental TV camera tubes. The characteristics of these tubes are reviewed.

The application of semiconductors with negative electron affinity surfaces to electron emission devices

The oxidation of cesium has been studied using ultraviolet photoemission spectroscopy. Upon exposure of a fresh cesium film to oxygen, a very narrow peak appears in the energy distribution curves (EDC’s) about 2.6 eV below the Fermi level Ef and grows with increasing exposure. This peak is associated with oxygen ions dissolved in the cesium metal below the surface. After 3×10−5 Torr sec of exposure, additional structure begins to appear. This is associated with the precipitation of cesium oxides. The structure associated with the oxides changes with increasing oxygen exposure indicating the appearance of different oxides. The oxide penetrates appreciably to the surface only after strong oxide buildup has taken place beneath the surface. A sharp minimum of 0.7 eV is found in the work function over a relatively small range of oxygen exposure.

The oxidation of Cs—uv photoemission studies

Ultraviolet photoelectron spectra of Cs evaporated onto Ag and oxidized under ultra-high-vacuum conditions have been measured. Time variation of the spectra indicates the formation of Cs suboxides (e.g., ${\mathrm{Cs}}_{7}$O, ${\mathrm{Cs}}_{11}$${\mathrm{O}}_{3}$). The high infrared photoelectric yield of Ag/O/Cs cathodes can be explained as due to the excitattion of surface plasmons in these compounds.

Photoemission from Metallic Cesium Oxide Films

Description of new (III-V) photocathodes which show improvements in sensitivity of as much as ten to a hundred times over conventional cathodes in the near infrared and useful improvements at shorter wavelengths. The development stems from a combination of basic knowledge of the photoemission process, gained in the 1950s, and the advancing understanding of the technology of III-V materials, in the 1960s. The superior performance of these cathodes is due to the fact that the vacuum level at the surface lies below the bottom of the conduction band in the bulk of the material. Consequently, the threshold of response is set by the III-V bandgap. The bandgap (and the threshold of response) can be varied by alloying different III-V materials together. A reduction in thermionic emission is realized with these cathodes. At present no semitransparent III-V cathodes with comparably interesting performance are available. The problems hindering further improvements, as well as the problems of placing these cathodes in practical multipliers and image tubes, are discussed briefly.

W. E. Spicer

Papers

The iii-v photocathode: a major detector development