Ronald L. Bell

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

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.

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

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.

Transferred electron photoemission from InP

A method of obtaining efficient photoemission in the wavelength region from 1 to 2 μm, using the transferred‐electron effect in a p ‐type semiconductor, is proposed. Experimental demonstration of emission from InP in this mode is reported.

Optimization of the InAsxP1−x–Cs2O Photocathode

Zinc‐doped InAsP liquid epitaxial layers with bandgaps between 0.4 and 1.34 eV were grown on InAs and InP substrates. The grown layers were 2–4‐μ thick with mirror‐smooth as‐grown surfaces. Preliminary phase diagram calculations based on Darkens quadratic formalism to describe the ternary liquid in equilibrium with the pseudobinary solid are in good agreement with the bandgaps of the grown layers determined by photoluminescence. The InAsxP1−x–Cs2O heterojunction barrier height as a function of composition has been measured using photoemission. For InAs the barrier is at 1.24 eV, and it decreases with decreasing arsenic concentration to a value of 1.16 eV for InAsP with a 1.27‐eV bandgap. For InAsxP1−x samples with bandgaps in the range 1.17–1.34 eV, high escape probabilities and efficient photoemission were observed. A typical cleaned (not cleaved) sample with a bandgap of 1.19 eV has a sensitivity of 600 μA/lm, 70 μA with a lumen source through a 2540 ir filter, a quantum efficiency of 1.5% at 1.06 μ, a...

Photoemission from cesium‐oxide‐activated InGaAsP

Zinc‐doped InGaAsP quaternary III‐V material of the proper composition range shows superior photoemission properties to either InGaAs or InAsP ternary material. The minority‐carrier diffusion length in the quaternary material is at least as long as that in InAsP and much longer than observed in InGaAs. The barrier height at the InGaAsP–Cs2O interface is lowered by cooling, giving increased electron escape probability and new highs in quantum efficiency over a wide wavelength range. For example, a 1.06‐μ quantum efficiency of 7.5%/incident photon was observed at − 90 °C.

PHOTOEMISSION FROM InP‐Cs‐O

Photoelectric measurements on cleaved p+ InP show that a process of cesiation and oxidation can produce a work function lower than the InP bandgap. Efficient photoemission results, with luminous efficiencies of 450 μA/lumen or better, and a threshold at 1.24 eV (1 μ).

Interfacial Barrier Effects in III‐V Photoemitters

Published data show strong evidence for the existence of an interfacial barrier on GaInAs photocathodes contrary to the claims made. Thermally assisted emission of photoelectrons over an interfacial barrier of height 1. 1 eV is shown here to explain the data. Energy distribution curves from a barrier‐limited InAsP sample show that thermally assisted emission dominates over tunneling emission. The barrier height is shown to increase when Rb is substituted for Cs in the activation layer.

CESIUM‐GaAs SCHOTTKY BARRIER HEIGHT

The Schottky barrier height at the interface of cesium metal and vacuum‐cleaved p‐type GaAs has been found to be 0.63 ± 0.03 eV. These measurements were made photovoltaically at 80°K and indicate that a heavy coverage of cesium leaves the surface roughly intrinsic.

Noise figure of the MCP image intensifier tube

Expressions are obtained for the noise figure of the photoelectric image intensifier type of tube using a microchannel plate in the linear gain mode, including effects of dark current and ionic or optical feedback in the MCP, and of the same device using the MCP in an electron-counting mode. Comparison shows that the latter should be a preferable mode of operation, if practical problems can be overcome.

Field-assisted semiconductor photoemitters for the 1—2-µm range

Photoemission data and model calculations are presented for a field-assisted semiconductor photoemitter which has achieved reflection-mode quantum efficiencies as high as 8.0 percent at 1.55 µm. The cathodes are p-p heterostructures employing lattice-matched InP-InGaAsP alloys. A thin electron semitransparent Schottky barrier provides the biasing contact for field-assisted electron emission. Parameters for optimal photoemission and sources of dark-current emission are discussed.