Shoji Ichimura

Electrical and Optical Properties of GaSe-SnO2 Heterojunctions

The capacitance-voltage (C-V) and forward biased current-voltage (I-V) characteristics, electroluminescence (E.L) and photovoltaic effect of GaSe–SnO2 heterojunction diodes are measured. SnO2 layer is deposited on the c-plane of GaSe by using the spray method. The C-V and I-V characteristics of these diodes reveal the existence of a high resistivity layer, probably due to the diffusion of Sn into GaSe. The width of this layer is about 2.6 µm. And the current transport mechanism at low voltage is space-charge-limited. The trap density and the energy level of the trap from the valence band estimated by Lampert theory are about 5×1013~1×1014 cm-3 and 0.4~0.6 eV, respectively. The electroluminescence spectra at 275 K show one emission band due to free exciton recombination.

XPS and AES Studies on the Oxidation of Layered Semiconductor GaSe

We have studied the oxidation of layered compound GaSe with no dangling bonds on the cleaved surface using XPS and AES techniques. At room temperature, the cleaved surface is not oxidized in an oxygen atmosphere. When subjected to Ar ion sputtering, the surface starts to exhibit the behavior of metallic Ga owing to dissipation of the first sublayer of Se in the primitive layer, Se–Ga–Ga–Se. The thin layer of metallic Ga thus exposed is easily oxidized. In the thermal oxidation of cleaved GaSe in air, the oxygen diffuses into the primitive layer and combines with Ga, severing the intralayer bonding between the Se and Ga atoms. At temperature higher than 450°C, the oxygen is also intercalated between the primitive layers from the sides perpendicular to the layers. No Se oxides are observed under any of the oxidation conditions.

Raman scattering of adsorbed molecules

The excitation energy dependence of the surface enhanced Raman scattering of adsorbed molecules is calculated based on the effective resonance model for a molecule–metal system. As a consequence of the effective resonant Raman process of the adsorbed molecule, a remarkable departure from an ω4 law is obtained near the excitation energies which satisfy the resonant condition. The significance of the substrate metal on the degree of the enhancement is discussed with the use of a result that the position of the effective resonant level is closely connected with that of the Fermi level. A comparison with the recent experimental results on the excitation profiles is also made.

Theory of Surface Excitons in Molecular Crystals

A theory of surface excitons in molecular crystals is presented using the localized perturbation method. Conditions for the existence of surface excitons and the criterion to determine whether surface exciton states lie above or below bulk exciton states are given in terms of the enviromental shift term and the exciton transfer term within the nearest neighbour approximation. It is shown that localization energies of surface excitons are not sensitive to the crystal thickness. Densities of states for bulk and surface exciton states are calculated. The Davydov splitting of surface excitons is also evaluated.

Observation of GaSe?SnO2 Heterostructure by XPS and AES

The depth profile of the elemental composition of the GaSe–SnO2 heterostructure has been studied by XPS and AES. The SnO2 layer was prepared by spraying a solution of SnCl4 and SbCl3 in ethyl alcohol on to the the cleaved surface of GaSe heated to ~400°C in air. After the solution had been sprayed on for about 5 secs., an SnO2 layer of thickness ~460 A formed, and a Ga2O3 layer of thickness ~120 A formed under the SnO2 layer. The Ga2O3 layer is a likely origin of the high-resistivity layer observed in the GaSe–SnO2 heterostructure.

Surface excitons in mixed molecular crystals

A theory of surface excitons in mixed molecular crystals is presented by extending the coherent potential approximation developed in the study of an infinite bulk crystal to a suitable one for a semi‐infinite crystal. The effect of the semi‐infinite nature of the crystal is taken into account by a localized perturbation method. The surface coherent potential is introduced to calculate the concent ration dependence of the energy of the surface excitons. The results give a qualitative explanation of the recent observation of surface excitons in the mixed crystal system antheracene–perdeuteroanthracene.

Molecule Adsorbed on Metal Surface–Role of Electron-Hole Excitation in Metal–

Fanos treatment for the interaction between a discrete level and a continuum state is used to calculate the eigenstates of molecules adsorbed on a metal surface, where both surface plasmon and electron-hole pair excitations due to the Coulomb field of the molecular dipole are taken into account. Spectral density of states of adsorbed molecules shows more broadened structure than that calculated by a simple image dipole model. The role of electron-hole excitations on the Raman scattering of adsorbed molecules is then investigated.

Davydov Splitting of Surface Excitons

Using the experimental results of the a - and b -polarized reflection spectra of anthracene, Davydov splitting of surface excitons is discussed. The exciton transfer terms and the environmental shift are calculated by comparing our theoretical calculations with the experimental data.

Electrical and optical properties of intercalated GaSe compound

Abstract We present the first studies on the formation of an intercalated GaSe compound with iodine by the anode reaction in the electrolyte solution of p-bromoaniline and mercury di-iodine in ethylene glycol. The X-ray diffraction patterns show a continuous decrease in the intensity of the GaSe (0, 0, 10) line at 57.7° and that of the (0, 0, 08) line at 45.5° in the early stage of the anode reaction. The appearance of the new peaks at 45.8 and 58.0° suggested the formation of the intercalated compound. The thickness along the crystal c -axis expanded by two or three times. The absorption edge shifted about 10 meV to the higher energy side and the electrical resistance in the crystal ab -plane for the intercalated compound was found to be the same as that of pure GaSe, but the resistance along the c -axis decreased by about three orders of magnitude.