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The feasibility of achieving carrier inversion of a properly doped crystal via optical excitation is studied. This process involves a host substrate doped with deep donors for n-type light characteristic and compensated by a shallow acceptor for p-type characteristic in the dark. This substrate is analyzed using well-known semiconductor equations. In addition conditions which must exist for carrier inversion are also specified. The solutions found are applied to a realistic set of dopants for illustrative purposes as well as indication of feasibility range. This inversion technique may possibly be used to generate bipolar junctions and thus devices. Other forms of photoconductivity are also qualitatively considered to supplement and extend the range of the inversion techniques applications. The processing of circuits using the developed concept offers possible interesting and useful advantages over existing techniques. The motivation for further research thus becomes obvious and is indeed the purpose of the thesis. / Master of Science

Electron spin relaxation and carrier recombination were investigated in gallium indium nitride arsenide (GaInNAs) multiple quantum wells, using picosecond optical pulses. Pump-probe experiments were carried out at room temperature, using pulses produced by a Ti:sapphire pumped optical parametric oscillator. The peak wavelengths of the excitonic resonances for the quantum well samples were identified using linear absorption measurements, and were found to be in the range 1.25µm-1.29µm. Carrier recombination times were measured for three samples of varying nitrogen content, and were observed to decrease from 548 to 180ps as nitrogen molar fractions were increased in the range 0.45-1.24%. Carrier recombination times were also measured for samples which had undergone a post-growth annealing process, and were found to be signicantly shorter compared to times measured for as-grown samples. Electron spin relaxation time was investigated for samples with quantum well widths in the range 5.8-8nm, and was found to increase with increasing well width, (i.e. decreasing quantum confinement energy), a trend predicted by both D'Yakonov-Kachorovskii and Elliott-Yafet models of spin relaxation in quantum wells. In a further study, longer spin relaxation times were exhibited by samples containing higher molar fractions of nitrogen, but having nominally constant quantum well width. Spin relaxation times increased from 47ps to 115ps for samples containing nitrogen concentrations in the range 0.45-1.24%. Decreases in spin relaxation time were observed in the case of those samples which had been annealed post-growth, compared to as-grown samples. Finally, all-optical polarisation switching based on spin relaxation of optically generated carriers in GaInNAs multiple quantum wells was demonstrated.

Orientador: Fernando Iikawa / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-06T18:02:06Z (GMT). No. of bitstreams: 1 Godoy_MarcioPeronFrancode_D.pdf: 4057709 bytes, checksum: 0df1e56082150d4109dcf891f05d4da6 (MD5) Previous issue date: 2006 / Resumo: Neste trabalho estudamos as propriedade estruturais e ópticas de pontos quânticos auto-organizados de InP crescidos sobre o substrato de GaAs. Esta estrutura apresenta o alinhamento de bandas tipo-II na interface, confinando o elétron no ponto quântico, enquanto o buraco mantém-se na barreira, próximo à interface devido à interação coulombiana atrativa. As amostras foram crescidas por epitaxia de feixe químico (CBE) no modo Stranskii-Krastanov. Os pontos quânticos apresentam raio médio de 25 nm e grande dispersão de altura (1-5 nm) e ocorre a relaxação parcial do parâmetro de rede, chegando a 2 %, em pontos quânticos superficiais. Do ponto de vista de propriedades ópticas, a fotoluminescência de pontos quânticos superficiais exibe uma eficiente emissão óptica, devido a baixa velocidade de recombinação dos estados superficiais do InP, e reflete a densidade e distribuição bimodal de tamanhos. Além disso, sua emissão óptica em função da intensidade de excitação exibe comportamento diverso em comparação com pontos quânticos cobertos com uma camada de GaAs. Em pontos quânticos cobertos, determinamos a energia de ativação térmica, que varia de 6 a 8 meV, e é associada à energia de ligação do éxciton ou energia de ionização do buraco. O decaimento temporal da luminescência de pontos quânticos é de 1,2 ns, um tempo relativamente curto para um ponto quântico tipo-II. A análise das propriedades magneto-ópticas em pontos quânticos individuais, inédita em QDs tipo-II, permitiu verificar que o fator-g do éxciton é praticamente constante, independentemente do tamanho dos QDs, devido ao fato dos buracos estarem levemente ligados. Por fim, mostramos a versatilidade do sistema acoplando-o a um poço quântico de InGaAs. Este acoplamento introduz mudanças na superposição das funções de onda do par elétron-buraco que permitem a manipulação do tempo de decaimento da luminescência e da energia de ligação excitônica / Abstract: We have investigated structural and optical properties of InP self-assembled quantum dots grown on GaAs substrate. This system presents a type-II band lineup where only electrons are confined in the InP quantum dots. The InP/GaAs quantum dots were grown by chemical beam epitaxy in the Stranskii-Krastanov mode. Our quantum dots present a mean radius of 25 nm and large height dispersion, 1-5 nm, and a partial relieve of the strain up to 2 % is observed. The photoluminescence spectra of surface quantum dots show an efficient optical emission, which is attributed to the low surface recombination velocity in InP. We observed a bimodal dispersion of the dots size distribution, giving rise to two distinct emission bands. A remarkable result is the relatively large blue shift of the emission band from uncapped samples as compared to those for capped dots. In capped quantum dots, we obtained the thermal activation energy, from 6 to 8 meV, which is associated to the exciton binding energy or hole ionization energy. The observed luminescence decay time is about 1.2 ns, relatively short decay time for type II system. We investigated magneto-optical properties using single-dot spectroscopy. The values of the exciton g factor obtained for a large number of single InP/GaAs dots are mainly constant independent of the emission energy and, therefore, of the quantum dot size. The result is attributed to the weak confinement of the holes in InP/GaAs QDs. We have also investigated structures where InP quantum dots are coupled to a InGaAs quantum well. This system permits the manipulation of the wave function overlap between electron-hole in order to control the optical emission decay time and exciton binding energy / Doutorado / Física / Doutor em Ciências