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This thesis documents a study of bonding stress and the reliability of GaAs-based lasers for high-power applications. GaAs-based lasers were bonded to oxygen-free high- conductivity (OFHC) copper heat sinks using a eutectic PbSn solder or a silver-filled conductive epoxy, and life tested. Epoxy-bonded devices were observed to have a larger failure rate on life test than solder-bonded devices. Bonding stress, as measured by the degree of polarization (DOP) of photoluminescence, was found to be the largest in epoxybonded devices. As well, the type of heat sink and bonding adhesive affected the stress in the laser material, with bonding stress increasing when there was a larger mismatch of coefficients of thermal expansion between the laser material, adhesive and heat sink. The reliability of the lasers was affected by the amount of force applied to unbonded laser chips. As the applied force increased on a chip centred on a groove, the rate of degradation in the output power increased. A limit in stress tolerance was observed in the lasers, which meant that larger amounts of stress would lead to increased rates of degradation in the output power. As well, the performance of lasers selected from a batch showing poor reliability degraded at an accelerated rate after several hours of operation under applied strain. / Thesis / Master of Engineering (ME)

<p> Periodic arrays of Au patterns (dots and lines) were produced via electron beam lithography (EBL). GaAs mesas were produced by using the Au structures as a mask and wet etching the GaAs (lll)B substrates, leaving Au resting above GaAs pillars. Annealing experiments at typical nanowire growth temperatures (550°C) were performed on both mesa-supported samples and a control sample without mesas, and were later characterized by scanning electron microscopy (SEM). From SEM images, a model is proposed to describe the evolution of the Au seed particle during exposure to typical growth conditions. The Au particle is subject to not only a melting process but is also modified by a volume increase due to incorporating Ga atoms and a subsequent crystal structure change. Palpable discrepancies between the mesa-supported and control samples were observed after annealing experiments, suggesting the mesas were effective in confining the migration of the Au. NW s were then grown via gas source molecular beam epitaxy (GS-MBE). Discemable variation amongst the results was evident when a comparison between annealed samples and the grown counterpart was made. The inconsistency is ascribed to the NW growth process beginning only after supersaturation at the growth interface. This saturation took place only after 2-D film growth on the substrate surpassed the height of the mesas rendering the structures less functional. </p> / Thesis / Master of Applied Science (MASc)

This thesis presents the study of x-ray emission from femtosecond laser micromachining and laser ablation of semiconductors. Prior to femtosecond machining experiments we investigated the nature of radiation emitted during the irradiation of solid targets with 120 femtosecond pulses with energies between 500 nJ and 0.3 mJ at a 1 kHz repetition rate. We have shown that the majority of the radiation was emitted below 10 keV with the high energy edge extending up to 25 keV. Under our experimental conditions K line emission was observed from materials with Z<32. We have also measured the x-ray dose rates during laser machining of various targets on the order of 10 mSv/h at a distance of 13 cm from the target. The implications for work pace safety, micromachining control, and potential for pulsed x-ray line sources for spectroscopic and imaging applications are discussed. In our studies of single shot femtosecond ablation of selected semiconductors: Si, GaAs, and InP, we have concentrated on the studies of microstructure and composition of the material after irradiation with 120 femtosecond pulses with energies between 2 nJ and 2 µJ. The resulting surface morphology, structure and composition of the micron scale ablation features on the semiconductors were studied by electron microscopy and atomic force microscopy. We found that no sharp threshold in the surface morphology was observed with increasing pulse power; however three ablation stages were identified based on the characteristic features of the ablation craters. TEM analysis revealed essentially no crystal damage beneath and in the vicinity of the ablation craters. In case of the binary semiconductors 5-30 nm polycrystalline grains were found over the ablated surfaces. The results were discussed in terms of the existing state of knowledge of ablation dynamics. The implications for practical micromachining applications are also discussed. / Thesis / Master of Engineering (ME)

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Neste trabalho foram processados e caracterizados dispositivos emissores de luz (LED’s) baseados em estruturas p-i-n a partir de filmes de GaAs dopados unicamente com Silício e crescidos através da técnica de Epitaxia de Feixes Moleculares sobre substratos de GaAs orientados nas superfícies (311)A e (211)A. Nas superfícies (311)A e (211)A, o Si tem comportamento anfótero, ou seja, pode ocupar tanto o sítio do Ga como o do As, o que resulta em filmes com portadores tipo n e p, respectivamente. As características elétricas dos filmes dependem das seguintes condições de crescimento: i) razão entre os fluxos de Ga e As; e ii) temperatura do substrato. As técnicas de caracterização utilizadas foram fundamentalmente: Fotoluminescência, IxV (corrente-tensão) e Efeito Hall. Os dispositivos p-i-n foram processados a partir de técnicas convencionais de fotolitografia e caracterizados a partir das técnicas de IxV e Eletrolumi-nescência. Os dispositivos estudados foram produzidos a partir de camadas de GaAs e AlGaAs crescidas nas direções [311]A e [211]A. Os dispositivos obtidos a partir de filmes dopados de GaAs apresentaram uma boa eficiência de emissão, porém foram observadas perdas ôhmicas ocasionadas pela natureza da junção obtida nestes planos, ocasionada por defeitos, principalmente nos filmes dopados tipo n que são crescidos em condições extremas de temperatura e pressão de Arsênio. Os dispositivos a base de AlGaAs apresentaram baixa eficiência e altas perdas, relacionadas com a alta compensação apresentada pelos filmes. Foram testados também dispositivos com heteroestruturas na camada intrínseca, apresentando resultados satisfatórios.

Submitted by Evandro Martin Lanzoni (evandrolanzoni@yahoo.com.br) on 2018-05-28T18:03:48Z No. of bitstreams: 1 dissertação mestrado evandro lanzoni_versão final.pdf: 4510657 bytes, checksum: dadfb13eef638b712a1e377fddcf85d2 (MD5) / Approved for entry into archive by Lucilene Cordeiro da Silva Messias null (lubiblio@bauru.unesp.br) on 2018-05-28T19:02:57Z (GMT) No. of bitstreams: 1 lanzoni_em_me_bauru.pdf: 4510657 bytes, checksum: dadfb13eef638b712a1e377fddcf85d2 (MD5) / Made available in DSpace on 2018-05-28T19:02:57Z (GMT). No. of bitstreams: 1 lanzoni_em_me_bauru.pdf: 4510657 bytes, checksum: dadfb13eef638b712a1e377fddcf85d2 (MD5) Previous issue date: 2018-03-29 / As técnicas de microscopia de sonda Kelvin (KPFM) e de microscopia de força eletrostática (EFM) são amplamente utilizadas para analisar a distribuição do potencial de superfície, porém com pouca aplicação em nanoestruturas semicondutoras auto-organizadas embutidas em um substrato. Neste trabalho, investigamos diretamente o acúmulo de carga dentro de estruturas mesoscópicas de GaAs (MGS) [1]. As estruturas são fabricadas através do crescimento sobreposto de um modelo de nano orifícios usando epitaxia de feixe molecular. Para tal, uma combinação de desoxidação assistida por Ga e ataque químico por gotículas localizadas foram utilizadas para criar orifícios iniciais com uma profundidade de ca. 10 a 15nm, que são posteriormente cobertos com 15nm de barreira AlxGax-1As e GaAs com 1nm, 2nm, 5nm, 10nm de espessura. Microscopia de força atômica e microscopia eletrônica de transmissão mostraram que a forma do orifício é preservada durante o crescimento de AlGaAs. Em seguida, esses orifícios são preenchidos com GaAs formando uma estrutura alongada sobre o buraco [1]. Investigamos o potencial de superfície local e a distribuição das cargas nestas estruturas com a técnica KPFM de passagem única. Portanto, uma voltagem AC de 5 V é aplicada a uma ponta metalizada e varremos a amostra no modo de contato intermitente. Observamos uma clara diferença de potencial na região central da estrutura, onde esperamos o furo preenchido. Então, um estudo sistemático com a técnica de KPFM mostrou a influência no acúmulo de carga quando a espessura de GaAs é alterada, bem como, quando modificamos a concentração de Al na barreira de AlGaAs. O cálculo simulando um poço de potencial com barreiras semi-finitas e finitas mostrou que não ocorre acúmulo de carga quando a espessura do GaAs é menor que 1,5 nm, corroborando com nossos resultados. Simulações do diagrama de banda e da densidade de elétron da estrutura permitem atribuir o acumulo de carga observado, aos diferentes níveis de energia da estrutura mesoscópica de GaAs em comparação com as camadas de GaAs circundantes. / Kelvin probe force microscopy and electric force microscopy techniques are widely used to analyze the distribution of the surface potential with little application to self-assembled semiconductor nanostructures embedded into a substrate. In this work, we directly investigate the charge accumulation inside mesoscopic GaAs structures [1]. The structures are fabricated by overgrowth of a nanohole template using molecular beam epitaxy. Therefore, a combination of Ga assisted deoxidation and local droplet etching is used to create initial holes with a depth of ca. 10 to 15nm, which are covered subsequently with 15nm of AlxGax-1As barrier and GaAs caps with 1nm, 2nm, 5nm, 10nm thicknesses. Atomic force microscopy and transmission electron microscopy results showed that the hole shape is preserved during the AlGaAs overgrowth. Then filled with GaAs forming an elongated mount over the hole [1]. We investigate the local potential and the charge distribution in these structures with a single pass Kelvin probe force microscopy technique. Therefore, an AC voltage of 5 V is applied to a metalized tip and scanned in tapping mode over the sample. We observed a clear potential difference in Kelvin probe force microscopy measurements in the middle of the structure, where we expect a filled hole. We systematically study by Kelvin probe force microscopy the influence on the charge accumulation when the GaAs thickness is changed, as well as the Al concentration in the AlGaAs barrier. Calculation of the particle in the box for semi-finite and finite barriers were done and show that no charge accumulation is observed for GaAs thickness lower than 1.5nm in the semi-finite barrier, corroborating with our results. Simulations of band gap and electron wavefunction of the structure allow us to ascribe the charge accumulation observed, to the different confinement of carriers inside of the unstrained mesoscopic GaAs structure compared to the surrounding GaAs layers.