The characteristics of high-sensitivity photodetectors suitable for wide bandwidth optical communication systems are summarized. Photodiodes, photomultipliers, and photoconductive detectors for wavelengths from 0.3 µm to 10.6 µm are covered. The use of internal current gain by means of avalanche and electron multiplication and by means of optical heterodyne detection to increase sensitivity of high speed photodetectors is discussed. The application to visible and infrared laser communication systems is reviewed.

Photodetectors for optical communication systems

Ga1−xInxAs alloys in the composition range 0≤x≥0.52 and band‐gap (Eg) range of 1.38 to 0.74 eV were activated with Cs and O2. Samples of different carrier concentrations were investigated. For band gaps down to about 0.8 eV, the photothreshold was equal to the band gap. The longest wavelength threshold determined was 1.58 μm. To the best of our knowledge, this represents the longest wavelength response yet achieved for photoemission into vacuum from a III‐V compound. The surface escape probability, B, was derived from the quantum yield data for each sample. The B‐vs‐Eg data were analyzed according to a surface escape model which includes the effects of (i) a finite‐width initial energy distribution of photoexcited carriers, (ii) the bent‐band region and (iii) various types of surface potential barriers. Surface escape probability data pertaining to a single doping density could be explained by a model that includes only a work‐function barrier or simple step potential. However, in order to explain the dat...

Photoelectron surface escape probability of (Ga,In)As : Cs–O in the 0.9 to [inverted lazy s] 1.6 μm range

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

Recently developed 3-5 compound semiconductor photocathodes show promise of substantial advantages over conventional photocathodes in increased yield and longer wavelength response. This paper reviews the physical principles of operation, preparation, and activation of these new types of photocathode. Also discussed are current fabrication techniques, problem areas, practical difficulties, and operating characteristics of experimental cathodes.

3-5 compound photocathodes: A new family of photoemitters with greatly improved performance

Electron energy loss in the band‐bending region of the p‐type III–V semiconductor in a III–V photocathode is an important factor in determining the escape probability and the optimum doping. From measurements of photoelectric yield near threshold from Cs2O‐activated n‐type GaAs, the position of the Fermi level at the GaAs–Cs2O interface was determined for {110}, {100}, {111A}, and {111B} surfaces. Assuming the Fermi‐level position at the GaAs surface to be independent of doping, the band bending for p‐type GaAs is greatest for the {111A} face and least for the {111B} face. The measured escape probabilities of photoexcited electrons from different crystalline faces of optimally activated 5 × 1018/cm3 Zn‐doped liquid epitaxial GaAs correlate well with the band‐bending measurements. The {111B} sample has an escape probability of 0.489 and a luminous sensitivity of 1837 μA/lm.

Dependence on Crystalline Face of the Band Bending in Cs2 O‐Activated GaAs

High-yield photoemission, already reported for GaAs-Cs, is also observed for GaAs-Cs-O. Spectral distributions of yields from both GaAs and tungsten with surface coatings of Cs-O indicate that the work function of such coatings is at least as low as that of GaAs-Cs. Photoemissive yields from GaAs-Cs-O have approximated to the highest observed from GaAs-Cs and have exhibited greater stability. It is concluded that electrons, photoexcited in the GaAs, can diffuse through the coating of Cs-O without a significant loss in number.

https://iopscience.iop.org/article/10.1088/0022-3727/1/2/303/pdf

A A Turnbull

Papers

Photoemission from GaAs-Cs-O