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The investigation of the physical properties of the GaS(Cu)/GaAs, GaSe(Cu)/GaAs AND GaS(Cu)/GaSe(Cu) junctions


Author: Elmira Cuculescu
Degree:doctor of physics and mathematics
Speciality: 01.04.10 - Semiconductors physics and engineering
Year:2008
Scientific adviser: Mihail Caraman
doctor habilitat, professor, Moldova State University
Institution:

Status

The thesis was presented on the 18 April, 2008
Approved by NCAA on the 19 June, 2008

Abstract

Adobe PDF document – 0.50 Mb / in romanian

Thesis

CZU 621.315.592

Adobe PDF document 6.20 Mb / in romanian
192 pages

Keywords

Gallium sulphide, gallium selenide, oxides, single crystal, thin films, heterojunction, optical absorption, reflection, photoluminescence, electroluminescence, electroconductivity, mobility, spectra, photocurrent, thermal annealing, current flow mechanism, concentration, localized states, excitons, mean free path, effective mass, dispersion, refractance

Summary

The investigation of the optical, photoelectrical and luminescent properties of the GaS and GaSe single crystals, undoped and doped with Cu (0.05 and 0.1 at. %) has been carried out. The structures of oxide semiconductor type (In2O3, SnO2, Bi2O3, ZnO, Cu2O)/p-GaSe(Cu) have been manufactured. Thin film heterojunctions and heterojunctions with optical contact of GaSe(Cu)/GaAs, GaS(Cu)/GaAs, GaS(Cu)/GaSe(Cu) type have been fabricated. Their photoelectrical, photo- and electroluminescent characteristics in the visible and close IR spectral range have been investigated. The intrinsic structural defects presence in GaSe crystals has been established by photoluminescent analysis. Their concentration can be controlled by low concentration copper doping and by annealing in vacuum or in an inert gas (Ar) ambient at 690÷700 K. The electrical conductivity of the undoped and Cu doped GaSe crystals for concentrations up to 0.1 at. % is of p-type and it is a function of temperature and Cu atoms concentration. The holes’ concentration in undoped GaSe crystals is increasing from 7•1011 cm-3 at T=230 K to 9•1012 cm-3 at 350 K and has an activational temperature behavior. The technology of thin films GaS and GaSe deposition on amorphous and crystalline (glass, GaAs) and their doping with Cu has been developed. The Cu doping of the non-crystalline GaSe causes the electrical conductivity increase from 1•10-5 -1cm-1 to 4.5•10-2 -1cm-1. The nature of the optical transitions has been established and the optical band gap Egopt determined for these films. The optical band gap Egopt depends on the evaporator temperature tev, and decreases at room temperature from 1.92 eV for tev=2320°C to 1.27 eV (tev=1470°C). The optical band gap of the non-crystalline GaS(Cu) layers at room temperature equals to 2.425 eV. This value is in good agreement with the same parameter known for the single crystals.

From the analysis of the absorption spectra and photoelectrical characteristics, the electrons mean free path and the ratio of the surface recombination rate to the ambipolar diffusion coefficient for GaSe (0.1 at. % Cu) crystals have been determined, these being equal to 0.05 and 0.60 m respectively.

The analysis of current-voltage dependencies with and without illumination shows that low-lifetime localized states are formed at the interface of the metal oxide / p-GaSe(Cu) heterojunctions, and their concentration decreases in the following order of structures: Bi2O3, Cu2O, ZnO, SnO2, In2O3/p-GaSe(Cu). The non-equilibrium charge carriers transport mechanism through the studied heterojunctions has been established.

The non-equilibrium charge carriers generation in heterojunctions with optical contact of p-GaSe(Cu)/n-GaAs type takes place in GaSe(Cu) layer, while in the case of p-GaSe(Cu)/n-GaS(Cu) heterojunction - in GaSe(Cu) and GaS(Cu) thin films. The mean free path of the non-equilibrium charge carriers in the GaSe layer of the p-GaSe(Cu)/n-GaS(Cu) heterojunction interface is 1.6 m.

The investigation of the absorption and photoluminescence spectra of the p-GaSe(Cu)/n-GaAs heterojunction (GaSe – thin film) has shown that an intermediary Ga2Se3 layer is formed at the interface of the heterojunction as a result of annealing at 700-710 K.

At forward biases, the p-GaSe(Cu)/n-GaAs and n-GaS(Cu)/p-GaSe(Cu) heterojunctions with optical contact, exhibit radiative properties in the yellow-red spectral region. The nature of the electroluminescence for these heterojunctions has been established.

The main results of the Doctoral Thesis were published in 30 scientific papers.

The Thesis is written in Romanian and consists of 120 text pages, 99 figures, 7 tables and 236 references.