Masahiro Miyao

Influence of Exposure to CO, CO2 and H2O on the Stability of GaAs Photocathodes

The influence of CO, CO2 and H2O gases on the stability of negative-electron-affinity (NEA) GaAs photocathodes during operation is investigated in the present work. We have found that exposure both to H2O and CO2 decreases the photocurrent of the photocathode. However, exposure to CO, which is known as a harmful gas to various photocathodes, has little effect on the photocathode stability. Furthermore, the effects of these gases on the restoration of the photocurrent by additional cesium deposition are investigated. These results are discussed with regard to the Cs/O activation layer which plays an important role in NEA GaAs photocathodes.

MBE growth mechanisms of ZnSe: Flux ratio and substrate temperature

Abstract The purpose of this paper is to present a new growth model to clarify the MBE growth processes of ZnSe. It is found that the relation between the growth rate and the substrate temperature calculated from our model is in quite good agreement with the measured one. Some thermodynamic values relevant for the growth kinetics are estimated by comparison of the calculation with the experiment. It is also shown that the deadsorption-rate constant of Se atoms from ZnSe(100) surface is obtained when the model is applied to explain the result reported by Yao et al., and it is in agreement with the one deduced from our experimental result.

Composition control of GaAsP grown by molecular beam epitaxy

Abstract This paper describes composition control of GaAsP alloy grown by molecular beam epitaxy. The relationship between the growth condition and the film composition is explained applying a precursor state growth model to GaAsP growth. The activation energies and the parameters appearing in the relationship are determined by fitting the calculated compositions to the experimental ones. The effect of flux intensities, substrate temperature and growth rate on the composition is discussed.

MBE growth mechanism of ZnSe: growth rate and surface coverage

Abstract The growth rate and surface coverage as functions of growth temperature and molecular beam intensity for MBE growth of (100)ZnSe are examined by the model accounting for the precursor states and surface coverage. The calculated results on the growth rate are presented and the growth temperature and flux ratio dependence of growth rate is described. The calculated results on the surface coverage are also presented. It is found that the growth temperature should be chosen at around 360°C in order to achieve surface stoichiometry during MBE growth. The correlation between surface coverage and experimental RHEED patterns is also reported.

Local Tunneling Barrier Height Imaging on Si(111) Surface

We measured the spatial distribution of a local tunneling barrier height by means of the AC height modulation method. We performed this measurement on clean Si(111)7 x 7 surfaces and the surfaces exposed to O 2 gas. On clean surfaces, 7 x 7 structures, which are the same as those in topographic images, were observed in the tunneling barrier height mode. It was found that the spatial distribution of local tunneling barrier height was modified along a step. It was also found that the topography and the spatial distribution of local tunneling barrier height were modified by exposure to O 2 gas.

Cleaning of gap surface by forming oxide film

Abstract Forming an oxide film on GaP crystal was found effective for removing carbon contaminants from its surface by heating in an ultra high vacuum. AES signal from the carbon on the GaP surface with the oxide film disappeared by heating at 630° C. The clean surface thus obtained showed clear LEED patterns which were composed of the superposition of diffraction from facets of {110} and from a native surface.

Structure of GaAs heteroepitaxial layer grown on GaP(001) by molecular beam epitaxy

Abstract This paper describes a growth mechanism in the early stage of GaAs heteroepitaxy on GaP (001). The GaAs film is grown on a pre-grown GaP layer by molecular beam epitaxy (MBE). The GaAs layer grows two-dimensionally on a GaP substrate up to 2 ML. The two-dimensional growth mode changes to the three-dimensional one over 2 ML in the layer thickness. The transformation from the two-dimensional layer into the islands caused by the migration of the GaAs is investigated by using the intensity profile of the RHEED pattern and the angle-resolved XPS. The islands formed in the initial growth stage are also observed by scanning tunneling microscopy (STM) in real space. The STM images revealed the island formation with an anisotropic structure.

The state of Cs on negative electron affinity surfaces

Abstract We examine the flash desorption of Cs from NEA surfaces, and measure the workfunction change during NEA activation. The lifetime changes on NEA photocathodes under the operation of decay and reactivation cycles are also measured. The thickness and structure of the Cs/O layer are discussed. We discard the heterojunction model. The workfunction is changed not by the state of the Cs/O layer but by the amount of ionic Cs adsorbed on the top surface.

Photoemission of Cs Valence Electrons from a Cs Monolayer on Si(111) 2×1

The adsorption of Cs atoms on the Si(111) 2×1 surface has been studied by photoelectron spectroscopy using synchrotron radiation. A photoemission peak due to Cs 6s valence electrons is clearly observed at full monolayer coverage by tuning photon energy at around 50 eV over an available energy range from 30 to 130 eV. The peak appears at almost the same initial energy (-1 eV) as that of the surface state of the clean Si(111) 2×1 surface. It is shown that the insulator-metal transition in the Cs monolayer takes place with increasing coverage.

LASERTRON, a new type of RF source using photoemission triggered by a pulsed laser beam

Abstract A new type of rf source, LASERTRON, is studied for a future high-energy electron-positron linear collider. In LASERTRON, a pulsed electron beam is extracted from a photocathode irradiated by a pulsed laser beam. The electron beam is then accelerated into a cavity from which rf power is extracted. Three prototypes of LASERTRON are fabricated and their fundamental characteristics are studied. The maximum rf power of 1.6 kW is extracted successfully at a rf frequency of 2884 MHz.