GB1526937A - Iii-v semiconductor photocathodes - Google Patents

Abstract

1526937 Photocathodes STANDARD TELEPHONES & CABLES Ltd 9 Dec 1975 [17 Dec 1974] 54385/74 Heading H1D In a method of making a transmission type, III-V semi-conductor photocathode, the semiconductor body containing a layer which is to form the active layer of the photocathode is placed on a surface of a billet of expansion matched glass on which it is to be mounted, and pressure is applied between the body and the billet while the assembly is heated by an induction heater which preferentially heats the semi-conductor body to a temperature above the softening point of the glass while leaving the bulk of the billet at a temperature beneath the softening point, the temperature of the assembly being reduced once the supporting surface of the billet has softened and the glass has yielded a preset amount and the assembly then being annealed. The surface of the semi-conductor body in contact with the billet may be the active layer which may be covered with a recombination inhibiting larger band gap layer and/or a glassy passivation layer. As shown, initially the semi-conductor body comprises a P-type GaAs substrate 2 on which is grown a thin N-type GaAs blocking layer 1, a 6 Ám. thick P-type GaAs layer 3 which forms the active layer of the photocathode and a 10 Ám. thick GaAlAs layer 4 which reduces recombination at the surface. The body also has a surface coating 6. The glass billet 7, suitably formed of a dense barium crown glass for example, is outgassed and annealed if the subsequent induction heating takes place under vacuum, however it need not be so treated if the induction heating takes place in argon. The photocathode and billet are mounted in a susceptor jig, Fig. 3 (not shown), and disposed in a bonding apparatus, Fig. 2 (not shown), which is provided with a ram (34) for applying, typically, a pressure of 1 kg. to the assembly. The induction heating of the semi-conductor body and local softening of the surface of the glass billet is continued until the glass has yielded a set amount, e.g. 50 Ám. After the assembly has been annealed the optical input face of the glass billet is polished and provided with an antireflection magnesium fluoride coating, which also protects the surface during the removal by electrochemical etching (details of which are given) of the substrate and blocking layers 1 and 2. Subsequently the photocathode is subjected to a conventional caesiating activating treatment.