Thesis (M.Sc.)--City University of Hong Kong, 2005. / At head of title: City University of Hong Kong, Department of Physics and Materials Science, MSc in materials technology and management AP6306 dissertation. Title from title screen (viewed on Sept. 4, 2006) Includes bibliographical references.

Made available in DSpace on 2014-06-11T19:35:08Z (GMT). No. of bitstreams: 0 Previous issue date: 2010-10-19Bitstream added on 2014-06-13T19:05:19Z : No. of bitstreams: 1 xavier_cs_dr_araiq.pdf: 7304148 bytes, checksum: 9e9cbe33cb65b8a93adedf0a25b98dd1 (MD5) / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Neste trabalho, foi estudado a formação dos pós de hexahidroxodizincato de cálcio dihidratado, CaZn2(OH)6.2H2O (CZO) sobre condições hidrotermais-microondas (HM) e partindo de diferentes reagentes e surfactantes. Estes pós foram analisados por difração de raios X (DRX), microscopia eletrônica de varredura com canhão de elétrons por emissão de campo (FEG-MEV), espectroscopia de absorção na região do ultravioleta visível (UV-vis), espectroscopia na região do infravermelho (IV) e medidas de fotoluminescência (FL). Os padrões de DRX confirmaram que a fase CZO pura foi obtida após o processamento HM realizado a 130 ºC por 2 h com a síntese partindo de nitratos, enquanto que as demais apresentaram fases secundárias. As micrografias FEG-MEV indicaram que as modificações bem como o crescimento das micropartículas de CZO foram orientadas pelo mecanismo de coalescência e Ostwald-ripening. O espectro UV-vis ilustrou que este material tem um band gap óptico indireto. Os pós de CZO puros exibiram uma emissão amarela quando excitados por um comprimento de onda de 350 nm em temperatura ambiente, enquanto que os demais exibiram uma emissão na região do laranja / In this work, it was studied the formation of calcium hexahydroxodizincate dehydrate, CaZn2(OH)6.2H2O (CZO) powders under microwave-hydrothermal (MH) conditions and using the different reagents and surfactants. These powders were analyzed by X-ray diffraction (XRD), Field-emission gum scanning electron microscopy (FEG-SEM), ultraviolet-visible (UV-vis) absorption spectroscopy, infra red spectroscopy (FTIR) and photoluminescence (PL) measurements. XRD patterns confirmed that the pure CZO phase was obtained after MH processing performed at 130 ºC for 2 h. FEG-SEM micrographs indicated that the morphological modifications as well as the growth of CZO microparticles are governed by Ostwal-ripening and coalescence mechanisms. UV-vis spectra showed that this material have an indirect optical band gap. The pure CZO powders exhibited an yellow PL emission when excited by 350 nm wavelength at room temperature, whereas the other powders exhibited the orange emission

Made available in DSpace on 2014-06-11T19:35:08Z (GMT). No. of bitstreams: 0 Previous issue date: 2001Bitstream added on 2014-06-13T19:05:24Z : No. of bitstreams: 1 scarpari_sl_dr_araiq.pdf: 858274 bytes, checksum: 1ca8f028f0b8fbc920a78b7c9fb00781 (MD5) / Os objetivos deste trabalho foram a preparação, a caracterização e o estudo da luminescência de alguns complexos de difenilfosfinatos de lantanídeos trivalentes. Os seguintes compostos foram preparados: i) Ln [O2P(Ph)2]3 ou Ln (DFF)3,em que Ln= La, Nd, Eu, Gd e Tb e ii) compostos binários Gdn:Eu1-n (DFF)3.xH2O, em que n varia de 0.99 à 0.5 e x não é determinado . Os difenilfosfinatos de lantanídeos formam pós incolores, cristalinos, insolúveis, química e térmicamente estáveis. Algumas diferenças entre ácido difenilfosfínico e os compostos formados puderam ser reveladas através da espectroscopia vibracional: i) nos espectros de IR, deslocamentos da absorção das ligações P-O para energias menores atestaram a complexação e a comparação entre espectros dos complexos permitiu a atribuição das absorções por volta de 120 cm-1 como sendo um modo de vibração da ligação Ln-O; ii) a diminuição do número de linhas das vibrações P-O observadas nos espectros Raman indicou aumento da simetria em torno do fósforo. Pela análise dos difratogramas de raios-x foi identificado o sistema cristalino triclínico e determinados os parâmetros de cela unitária. Através da técnica de EXAFS foram determinadas as distâncias de ligação Ln-O (R, em Å) e o números de coordenação do metal (NC) dos difenilfosfinatos de Nd, Eu e Tm que são respectivamente : (R /NC) 2,34/7,9 ; 2,34/6,5 e 2,20/7,1. O ácido difenilfosfínico e os difenilfosfinatos de La e Gd apresentam fracas emissões no visível enquanto o composto de Nd emite fracamente na região do infravermelho. Os complexos de Eu e Tb apresentam fortes emissões características quando excitados de 273 nm à 200 nm . Eu(DFF)3 e alguns compostos da série Gd:Eu foram investigados até 100 nm sob radiação de fonte Síncroton... / This work reports thepreparation, characterization and of photoluminescence studies of some lanthanide (III) diphenylphosphinate complexes. The following compounds were prepared: i) Ln [O2P(Ph)2]3 or Ln(DPP)3, where Ln= La, Nd, Eu, Gd and Tb ii) binary compounds Gdn:Eu1-n (DPP)3.xH2O, where n ranges from 0.99 to 0.5 and x is uncertain. The lanthanide diphenylphosphinates are colorless, crystalline and insoluble powders, showing thermal and chemical stabilities. Some differences between the diphenylphosphinic acid and the complexes could be revealed through the vibrational spectroscopy: i) in the IR spectra, the shifts to lower energy of the absorption of P-O bond attested the complexation and the comparison among spectra of the complexes allowed the attribution of the absorptions around 120 cm- 1 as a mode of the Ln-O vibration; ii) the decrease of the number of lines P-O vibrations in the Raman spectra indicated increase of the symmetry around the phosphorus. By analysis of the powder X-ray diffractometry was identified the triclinic crystalline system and the unitary cell parameters. Through the technique of EXAFS were determined the Ln-O length (R, in Å) and coordination numbers of the metal (CN) of the diphenylphosphinates of Nd, Eu and Tm that are respectively: (R /NC) 2,34/6,5; 2,34/7,9 and 2,20/7,1. The diphenylphosphinic acid and the La and Gd diphenylphosphinates present weak emissions in the visible range whereas the Nd compound emits weakly in the infrared region. The Eu and Tb complexes exhibit strong characteristic emissions when excited at 273-200 nm range. Eu(DPP)3 and some compounds of the Gd:Eu series exhibited the same behavior when investigation was extended up to 100 nm radiation of Syncroton source. In the binary compounds, the emission...(Complete abstract, click electronic access below)

In this thesis, the effects of various chemical treatments on the surface modification of bulk InAs are investigated. The study focuses on the chemical processes that occur upon the exposure of the surface to sulphur-, chlorine- and bromine-containing solutions and oxygen, and the resulting changes to the electronic structure of the surface, as deduced from photoluminescence (PL) measurements, X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), Raman scattering and scanning electron microscopy (SEM). Three processing treatments were evaluated: i) treatment with sulphur-based solutions (Na2S:9H2O, (NH4)2S + S, [(NH4)2S / (NH4)2SO4] + S); ii) etching in halogen-based solutions (bromine-methanol and HCl: H2O); and iii) thermal oxidation. A significant overall enhancement in PL response was observed after chemical treatment or thermal oxidation, which is associated with a reduction in surface band bending. These changes correlate with the removal of the native oxide, in addition to the formation of well-ordered layers of In-S (or In-As)O as a passivating layer, indicating that electronic passivation occurs at the surface. The passivating effect on sulphide treated surfaces is unstable, however, with an increase in band bending, due to reoxidation, observed over periods of a few days. The lowest re-oxidation rate was observed for ([(NH4)2S / (NH4)2SO4] + S). Etching in HCl:H2O and Br-methanol solutions of appropriate concentrations and for moderate times (1 min) resulted in smooth and defect-free InAs surfaces. Etching completely removed the native oxides from the surface and enhanced the PL response. The adsorption of bromine and chlorine onto the InAs surface led to the formation of As-Brx , In-Brx, As-Clx and In-Clxcompounds (x = 1, 2, 3), as inferred from changes in the In 3d3/2; 5/2 and As 3d core level binding energies. The etch rate was found to decrease because of strong anisotropic effects. The improvements in surface properties were reversed, however, if the concentrations of the etchants increased or the etch time was too long. In the worst cases, pit formation and inverted pyramids with {111} side facets were observed. Surface treatments or thermal oxidisation significantly enhanced the PL intensity relative to that of the as-received samples. This was due to a reduction in the surface state density upon de-oxidation, or in some cases, to the formation of a well ordered oxide layer on the surface. The overall increase in PL intensity after treatment is ascribed to a reduction in band bending near the surface. This allows several welldefined peaks not observed or reported previously for bulk InAs (with a carrier concentration n~2x1016 cm-3), to be studied. A combination of PL and XPS measurements before and after the various treatments was used to identify the chemical nature of the impurities giving rise to bound exciton recombination in InAs (111).

In this study the growth and characterisation of undoped and Si-doped AlxGa1-xN has been performed. The layers were grown using low-pressure metalorganic vapour phase deposition (MOCVD) on sapphire substrates. The optical and electrical properties of the AlxGa1-xN layers were studied using variable temperature Hall effect and photoluminescence measurements. AlxGa1-xN layers were grown over the entire composition range. Room temperature ultraviolet (UV) transmission measurements showed that the material quality was very good for layers with an Al content, x, of 0 _ x _ 0.5. However, the quality of layers of higher composition was seen to rapidly decrease with increasing x. The electrical and optical properties of AlxGa1-xN with x < 0.5 were also good, comparable to those reported on in literature. The study of the Si-doping of AlxGa1-xN was performed in two parts; firstly a series of Al0.23Ga0.77N samples was grown in which the doping level was increased from zero to n _ 3 × 1018 cm-3. A similar, albeit a less rigorous, study was performed for Al0.41Ga0.59N and Al0.5Ga0.5N. A second series of samples was then grown in which the doping level was kept constant, while the Al content was incrementally increased. Room temperature Hall effect measurements performed on Si-doped Al0.23Ga0.77N showed that the electron concentration did not scale linearly with the silane flow, as was the case in GaN. It was also seen that the electron mobility of the layers increased with slight Si-doping, possibly due to an improvement in the crystalline quality and/or a change in the conduction mechanism. It was also found that at higher compositions (x = 0.41 and 0.50) an increase in the doping level resulted in an increase in the mobility. Variable temperature Hall effect and photoluminescence measurements, performed on the Al0.23Ga0.77N samples, revealed a good correlation between the first PL activation energy E1 and the donor activation energy ED, prompting the conclusion that the first PL recombination channel in AlxGa1-xN is due to the delocalisation of excitons bound at neutral Si donors. Furthermore, E1 and ED were seen to decrease with n1/3, as is the case for GaN and other semiconductor materials. It was also observed that strong exciton localisation occurs in slightly Si-doped material, with the amount of localization becoming less at higher doping levels. Possible mechanisms responsible for the second PL recombination channel of activation energy E2 were also proposed. The electrical and optical properties of the second set of AlxGa1-xN samples was then studied. The PL properties of undoped AlxGa1-xN were typical of a homogeneous alloy system, with the increase in the PL FWHM and exciton localisation energies with x following the trend predicted by alloy disorder theory. The variation of the band gap energy with the Al content could not, however, be fitted over the entire composition range using a single bowing parameter. It was proposed that this was due either to an effect of the 9 7 valence band crossover, or due to exciton localisation at alloy disorder and/or impurities. As was the case for GaN and Al0.23Ga0.77N, all undoped material was highly resistive. As was mentioned earlier, the exciton localisation energies increased according to alloy disorder theory in undoped AlxGa1-xN. In the doped samples, however, a large increase in the donor localisation energy was measured for x > 0.3. The possibility that Si could become a DX-centre in AlxGa1-xN was then investigated. However, Hall effect measurements showed that the Si activation energy increased in good agreement with the model of a shallow effective mass state donor, with no sudden increase in ED being observed up to x = 0.4. It was then suggested that the increase in the E1 and E2 activation energies, as well as the exciton localisation energies, could be due to the 9 7 valence band crossover, which occurs at roughly the same composition. However, due to the scarcity of reports on the valence band structure in AlxGa1-xN no conclusions could be made at this stage as to the effect of the 9 7 valence band crossover on the PL properties of AlxGa1-xN.

In this work, the optical properties of ZnO doped with arsenic and nitrogen were studied. The ZnO samples were grown by Metalorganic Chemical Vapor Deposition (MOCVD). The solubility of nitrogen in the ZnO films, as well as its activation upon annealing, was also investigated. Hydrogen is known as a major source for passivation of the acceptors in ZnO:N. Therefore, it is crucial to dissociate the complex(es) formed by nitrogen and hydrogen and diffuse out the hydrogen in order to prevent the reformation of such complexes. High temperatures (≥ 600 C) are required for these purposes. In order to effectively remove the hydrogen impurities from the sample, it is important to know the optical fingerprints of hydrogen and its thermal stability. Therefore, the effects of annealing and hydrogen plasma treatment on bulk ZnO (hydrothermally grown) were first studied. The use of bulk material for this purpose was motivated by the well-resolved photoluminescence (PL) lines observed for bulk ZnO, which allow the identification of the different lines related to hydrogen after plasma treatment. Annealing at 850 C was effective for the removal of most of the hydrogen related transitions in the near-band-edge emission. Also, additional transitions at ~3.364 eV and ~3.361 eV were observed after hydrogen plasma treatment, which were ascribed to hydrogen-Zn vacancy complexes. In this work, a comparative study of the annealing ambient and temperature on ZnO films grown on GaAs substrate, using diethyl zinc (DEZn) and tertiary butanol (TBOH), showed that arsenic diffuses in the ZnO films and gives a shallow level in the band gap, which is involved in an acceptor-bound exciton line at 3.35 eV. This shallow level is visible when annealing is performed in oxygen, but not when annealing is performed in nitrogen, and indeed only for annealing temperatures around 550 C. However, annealing in either ambient also causes zinc to diffuse from the ZnO films into the GaAs substrate, rendering the electrical properties deduced from Hall measurements ambiguous. For ZnO:N, NO was used as both oxygen and nitrogen sources. Monitoring the concentration of nitrogen, carbon and hydrogen in the ZnO films, the formation of different complexes from these impurities were deduced. Furthermore, an investigation of the effect of annealing on the concentrations of impurities showed that their out- diffusion was strongly dependent on the crystalline quality of the ZnO films. For porous ZnO films, obtained at low growth temperatures (≤310 C), the out-diffusion of impurities was efficient, whereas for films grown at higher temperatures, which have improved crystalline quality, the out-diffusion was practically nonexistent. The out-diffusion of unwanted impurities may activate the nitrogen dopant in the ZnO films, as was confirmed by the PL measurements on the different samples grown at different temperatures. PL transitions at ~3.24 eV and ~3.17 eV were related to substitutional NO. These transitions were more dominant in the spectra of samples grown at low temperatures. An additional transition at ~3.1 eV was assigned to a donor-acceptor pair transition involving VZn, instead of NO, as previously reported.

The modification of the band edge or emission energy of semiconductor quantum well light emitters due to image charge induced phenomenon is an emerging field of study. This effect observed in quantum well light emitters is critical for all metal-optics based light emitters including plasmonics, or nanometallic electrode based light emitters. This dissertation presents, for the first time, a systematic study of the image charge effect on semiconductor–metal systems. the necessity of introducing the image charge interactions is demonstrated by experiments and mathematical methods for semiconductor-metal image charge interactions are introduced and developed.

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

published_or_final_version / Physics / Master / Master of Philosophy

published_or_final_version / Physics / Master / Master of Philosophy