Transmission Photocathode Research Articles

Nature, origin and effect of dislocations in epitaxial semiconductor layers

The paper is concerned with the manner in which dislocations in Group 3–5 compound epitaxial layer structures are generated, propagate and interact with one another. The different types of epitaxial layer and dislocation behaviour are initially reviewed. The examination methods used are based on electron microscopy. The TEM is used in conjunction with plan-view, cross-section, and angle-lap specimens to determine the detailed nature and three-dimensional distribution of the dislocations. The SEM EBIC and CL methods are used with bulk specimens to obtain electrical and luminescent information. In particular, the latter methods give micrograph-type images showing dark spots and lines corresponding to individual dislocations, the contrast arising because of electrical carrier recombination taking place at the dislocations. These methods are used to investigate the dislocation behaviours occurring in a wide range of specimens including homo- and hetero-epitaxial layers, and embracing small, medium and large mismatches. An attempt is made to obtain quantitative data concerning the processes occurring. The work has provided a better understanding in many instances, and sometimes allowed the occurrence of the dislocations to be better controlled. An example is given of the application of the results to the improvement in the quality of a GaAs transmission photocathode device-type structure.

Broadband GaAs transmission photocathode

The operational range of the glass-sealed negative-electron-affinity GaAs photocathode has been extended to 3.0 eV by increasing the Al concentration of the GaAlAs passivation layer and decreasing its thickness to approximately 0.3 μm.

Large-area electrochemical planing of p-type gallium arsenide for photocathodes

A large-area precision-thinning technique has been developed for p-type GaAs and its use in fabricating III/V transmission photocathodes has led to a marked improvement in the thickness uniformity of the active layers. The technique employs a raster-scanned, feedback-controlled, electrochemical planning method which eliminates the need to grow a 'stopping layer'. Samples having both (100) and (111)B oriented output surfaces have been processed, a shiny reflecting finish being produced in both cases provided that the appropriate method is used for regulating the etching current.

TEM examination of dislocations generated in GaAS photocathode structures

Basic studies have been performed on GaAs epitaxial multilayer structures of the type used for transmission photocathodes. The substrates were horizontal Bridgman (100)GaAs, and the layers either GaAs or GaAlAs grown by the LPE method and Zn-doped. TEM examinations were made to determine the nature and distribution of dislocations present in the structures, deduce how they were formed and suggest how they might be eliminated. Special attention was given to those dislocations formed by surface stresses generating 60° dislocations with at the specimen surface and moving down on inclined {1ll} slip planes to form a network at an underlying interface. One feature of the work was the TEM examination of ∽ 1° angle-lap and 90° cross-section thinned specimens to determine the three-dimensional distribution of the dislocations. Results are reported for three different aspects of the work.First, when GaAs layers are grown by the LPE method.

Interpretation of the surface boundary conditions in the diffusion model for NEA photoemission

The recombination velocity parameter S in the boundary conditions for diffusion of photoexcited electrons is expressed in terms of the quantum-mechanical reflectance of the interface and transport parameters for the interfacial region. The recombination velocity at the photoemitter-substrate interface in a transmission photocathode is expressed in terms of material parameters. Recent experimental data are analysed and factors favouring low recombination velocities are discussed. The ratio of the empirical surface escape probability P to the theoretical surface transmittance T may vary above or below unity. Physically, P/T>1 arises because an electron reflected back from the surface into the bulk has a significant probability of re-entering the bandbending region.

Interfacial lattice mismatch effects in III–V compounds

The effects of lattice mismatch upon the microstructure, macrostructure, and device performance of III–V heteroepitaxial structures have been evaluated. Lattice mismatched VPE heteroepitaxial materials are shown to exhibit misorientation, tetragonal distortion, cracking and cross-hatching, all of which can be related to the relief (or non-relief) of lattice mismatch. Materials which are grown under tension are shown to crack much more readily than those in compression, and require a significantly thicker compositionally graded region in order to completely eliminate cracking. The electro-optical properties of transmission photocathodes and transmission secondary electron multipliers are found to be very sensitive functions of lattice mismatch and correlate well with transmission electron microscope evidence from these devices.

Glass−sealed GaAs−AlGaAs transmission photocathode

A GaAs/GaAlAs/GaAs/GaAlAs heterostructure has been prepared on a GaAs susbtrate, bonded to 7056 Corning glass, and the substrate and first AlGaAs removed chemically, utilizing the differential etching characteristics of GaAs and AlGaAs in NH4OH−H2O2 and HF solutions. The resulting structure of GaAs/AlGaAs/glass has excellent layer morphology, uniform thickness, and good transmission photocathode performance.

Nucleation and growth of Ga 1− xAl xAs on (111) GaP

The nucleation and initial growth of heteroepitaxial layers of Ga1−xAlxAs on (111) A and B GaP substrates have been investigated. By studying the effect of surface treatments on the nucleation density on (111) B GaP, growth methods have been developed which result in smooth, continuous epitaxial layers ideally suited for the preparation of transmission photocathodes.

GaAs transmission photocathode grown by hybrid epitaxy

Transmission GaAs/GaAlAs/GaP photocathodes have been grown by a combination of LPE and VPE (hybrid) epitaxy. The quality of the VPE GaAs material is comparable to that grown by LPE, and the surface uniformity and quality is comparable to that generally obtained by VPE. The GaAs/GaAlAs heterojunction is characterized by low interface recombination velocity.

Erratum: Liquid-phase epitaxial growth of (AlGa)As on polished and roughened GaP substrates for transmission photocathodes

It is shown that growth of smooth layers of (AlGa)As on GaP by liquid phase epitaxy (LPE) can be achieved if the GaP substrate surface possesses a certain degree of roughness when growth starts. This roughness can be introduced by various methods, including preferential chemical etching or in situ melt‐back, but the best surface morphology of the epitaxial layer is obtained after mechanical lapping of the substrate. This striking phenomenon is attributed to an increase of the density of nuclei which results in a more uniform distribution of the grains characteristic for this system with large lattice mismatch.

Liquid-phase epitaxial growth of (AlGa)As on polished and roughened GaP substrates for transmission photocathodes

It is shown that growth of smooth layers of (AlGa)As on GaP by liquid phase epitaxy (LPE) can be achieved if the GaP substrate surface possesses a certain degree of roughness when growth starts. This roughness can be introduced by various methods, including preferential chemical etching or in situ melt-back, but the best surface morphology of the epitaxial layer is obtained after mechanical lapping of the substrate. This striking phenomenon is attributed to an increase of the density of nuclei which results in a more uniform distribution of the grains characteristic for this system with large lattice mismatch.

LPE GaAs/(Ga, Al)As/GaAs transmission photocathodes and a simplified formula for transmission quantum yield

GaAs/(Ga,Al)As/GaAs transmission photocathodes were fabricated by a multiple layer liquid epitaxy technique using GaAs substrates. The GaAs substrate was subsequently removed by bubble etching. Transmission mode white light sensitivity of 340 μA/lm and an electron diffusion length of 5 μm or more can be routinely achieved. The reflection sensitivity was found to be lower than that of a similarly prepared layer grown directly on the GaAs substrate. The spectral response, when analyzed with the yield formula, indicated negligible recombination at the GaAs / (Ga,Al)As interface. Also, despite the almost perfect lattice match between GaAs and AlAs, some bowing of the GaAs / (Ga,Al)As samples was observed, suggesting the presence of internal stresses. A simplified formula is derived for the transmission quantum yield which enables graphic solution of surface recombination velocity at the photocathode input surface and the ratio of photocathode thickness to the electron diffusion length without tedious computer calculations.

Thin Film GaAs Photocathodes Deposited on Single Crystal Sapphire by a Modified rf Sputtering Technique

Thin films of GaAs were grown epitaxially on single crystal sapphire substrates by a modified sputtering technique. The (0001), (0112¯) and (0221¯) substrate orientations were used at various temperatures from 150 ° to 500 °C, and the carrier concentration of about 1018/cm3 was achieved in GaAs films by the diffusion of zinc. The spectral response of the photoelectrons was measured in the incidence and transmission mode after activation by cesium and oxygen. The photoelectric yield at 2.8-eV photon energy was within an order of magnitude to that from a bulk crystal for a 0.88-μ-thick reflection photocathode. An electron diffusion length of 0.32 μ and an escape probability of 0.005 was measured for photon energies near the band edge. The photoelectric yield from transmission photocathodes was within an order of magnitude to that from the reflection photocathodes only at wavelengths longer than 0.8 μ.

A GaAs-Cs-O transmission photocathode

Polycrystalline GaAs layers have been deposited on sapphire substrates by the AsCl3-Ga-H2 vapour transport method, and treated with caesium and oxygen. A layer 0·6 μm thick when operated as a photocathode has shown a luminous sensitivity of 70 μA lm−1, in transmission. Thicker layers have shown lower sensitivities. Spectral response curves are given for layers 0·6 and 0·8 μm thick and show yields out to 1·4 ev.An attempt has been made to relate the results to an elementary theory for transmission photocathodes.

Microwave phototubes with transmission photocathodes

Microwave phototubes with transmission photocathodes