The electrical conductivity of pure KI and CdI₂-doped KI has been studied in the temperature range 200 to 23°C. Two regions A and B (corresponding to different activation energy of conductivity) are identified. The region A can be given a conventional interpretation in terms of migration of cation vacancies in the bulk, their concentration being determined by impurities. U (the energy for migration of cation vacancies) amounts to 0.96 ± 0.02 eV. Observed activation energies higher than this value are accounted for by association and precipitation effects. Association energy of cation vacancies with impurities (0.48 eV. for Cd⁺₂) and heat of solution (0.25 eV. for CdI₂) obtained here are comparable with known values for other alkali halides. Region B found in this work represents unusual behaviour and has not previously been observed in any alkali halides. The activation energy of conductivity is considerably less than the energy needed for the migration of cation vacancies in the bulk. The activation energy E(formula omitted) (for region B) is about 0.57 eV. in a single crystal and 0.38 eV. in a pure KI pellet. Such low activation energies cannot be given a similar interpretation as for region A. It is suggested that the cation vacancies are in regions of unusually high mobility such as dislocations and grain boundaries. This effect may arise partly from a lower activation energy for motion of vacancies in these regions and partly from a vacancy concentration in these regions which increases with decreasing temperature, under the control of "space- charge" effects. The value 0.57 eV. appears to refer to isolated dislocations or low angle boundaries, while the value of 0.38 eV. refers to large angle intercrystalline boundaries in a pellet. A strong piece of evidence for this suggestion comes from the conductivity runs on single crystals. In an untreated single crystal, just as in pure pellets, two regions A and B are identified but region B disappears in crystals annealed overnight and reappears in a mechanically strained crystal. Moreover, region A remains almost undisturbed in each case. This means that the conduction process in region B is governed by dislocations and grain boundaries whereas region A is governed by motion of cation vacancies in the bulk. / Science, Faculty of / Chemistry, Department of / Graduate

The temperature-dependence (20-300K) of the far-infrared spectrum (20-1000cm⁻¹) of Trimethylammonium Iodide Tetracyanoquinodimethane (TMA)(I)(TCNQ) in its ordered phase, has been measured for the first time. Our measurments confirmed the existence of two phase transitions near 150 and 100K. At least three absorption bands are activated below the 150K phase transition. We have assigned two of them to the totally symmetric mode Ag⁹ and Ag¹⁰ of the TCNQ molecule. We have also observed evidence of Brillouin zone splitting, where one band above 150K, split into three, below 150K. Since the conduction band of (TMA)(I)(TCNQ) is 1/3 filled, we have interpreted this result as suggesting that the 150K transition is a Peierls distortion. We also observe a sharp increased in infrared absorption at 365cm⁻¹ and a weaker one at 100cm⁻¹. Since the D.C. conductivity measurments indicates that a 1000cm⁻¹ band gap is present at all temperatures, we interpret these absorption "edges" in terms of transitions between localised states in the band gap. These localised states would be introduced by either chemical impurities or by some randomness in the potential at each TCNQ site. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

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Nos últimos anos, acentuou-se o interesse em materiais semicondutores com alto número atômico e alto gap de energia para aplicações na detecção de radiação ionizante à temperatura ambiente, usando o método direto de detecção. Este trabalho apresentara as características de dois materiais semicondutores na forma de filme para o iodeto de chumbo e de cristal milimétrico para o iodeto de mercúrio. Os filmes foram depositados a partir da evaporadora térmica construída no próprio departamento e caracterizados em função de três distâncias de deposição. O melhor filme obtido foi produzido a 5 cm de distância da fonte, apresentando valores de gap de energia de 2,39 eV e energia de ativação de 1,1 eV. Com o aumento dessa distância as propriedades estruturais, ópticas e elétricas se deterioram, inclusive com variação da composição do material. Os cristais de iodeto de mercúrio foram dissolvidos no solvente orgânico N-N Dimetilformamida e crescidos utilizando a técnica de deposição de solvente. Diferentes taxas de crescimento foram usadas para o crescimento dos cristais. Os melhores resultados foram obtidos para crescimento na estufa a 80ºC, produzindo filmes com gap de energia de 2,2 eV e resistividade da ordem de 108 ?.cm, indicando dopagem não intencional ou excesso de defeitos. A composição obtida é de HgI3, e a razão da fotocorrente pela corrente de escuro quando na faixa de raios-X mamográficos é da ordem de 25. Com as otimizações indicadas no texto e na conclusão do trabalho esses materiais seriam fortes candidatos para aplicações comerciais em imagens médicas para energias na faixa mamográfica. / In the last years there has been a growing interest in semiconductor materials with high atomic number and optical band gap for applications as ionizing radiation detectors at room temperature, using the direct method of detection. Some materials such as lead iodide (PbI2) and mercury iodide (HgI2) have an optical band gap above 2,0 eV, operate at room temperature with low noise and low leakage current, present a high carrier mobility and high stopping power for ionizing radiation. Alternative methods are investigated by several researchers for the fabrication of these materials on top of large areas, with low fabrication time and costs as desirable for applications in medical imaging. In this sense, we present two methods for the development of the detectors. Thermal evaporation, with the development of the deposition system and chamber, is used for the fabrication of lead iodide thin films. The properties of the films were investigated as a function of deposition height. On the other hand, isothermal evaporation was used for the fabrication of mercury iodide milimetric crystals, for the first time using the organic solvent N.N- dimetilformamide (DMF). The properties of the crystals are investigated as a function of concentration and growth temperature. The structural, morphological and compositional properties of the films and crystals were investigated. The optical and electrical properties were also investigated for both films and crystals. The activation energy for electric transport and the test of the materials as sensors (using X-rays in the mammographic region) were studied. This work presents the main results for both materials: films of lead iodide and milimetric crystals of mercury iodide. The crystalline planes of the films have a preferential orientation along the (110) direction, and a morphology of vertical leaves, not similar to other reported results. The obtained composition is PbI2,5. The combination of the morphology and the configuration of co-planar contacts for charge collection reduce the sensitivity to X-rays exposure. A current density ratio (illuminated to dark) of the order of 1.53 was obtained. The best film was deposited at a distance of 5 cm to the source, and it has an optical gap of 2,39 eV and activation energy of 1,1 eV. With increasing deposition distance a degradation of the structural, optical and electrical properties was observed, even with the variation of the composition of the films. For mercury iodide, for any growth rate the shape of the crystals is always cubic. The morphology of the surface depends on the evaporation rate (it can be smooth or present macroscopic holes). The best results were obtained for a growth at 80o C, what leads to a crystal with optical gap of 2.2 eV and electrical resistivity of the order of 108 Ohmcm, what suggests non-intentional doping or excess of defects. The obtained composition was HgI3, and the current ratio (illuminated by mammographic X-rays to dark) was about 25. According to the discussions in the text and in the conclusions, the suggested optimizations could lead to the development of materials that might be useful for technological applications in medical imaging for the mammographic energy range.

Nos últimos anos, acentuou-se o interesse em materiais semicondutores com alto número atômico e alto gap de energia para aplicações na detecção de radiação ionizante à temperatura ambiente, usando o método direto de detecção. Este trabalho apresentara as características de dois materiais semicondutores na forma de filme para o iodeto de chumbo e de cristal milimétrico para o iodeto de mercúrio. Os filmes foram depositados a partir da evaporadora térmica construída no próprio departamento e caracterizados em função de três distâncias de deposição. O melhor filme obtido foi produzido a 5 cm de distância da fonte, apresentando valores de gap de energia de 2,39 eV e energia de ativação de 1,1 eV. Com o aumento dessa distância as propriedades estruturais, ópticas e elétricas se deterioram, inclusive com variação da composição do material. Os cristais de iodeto de mercúrio foram dissolvidos no solvente orgânico N-N Dimetilformamida e crescidos utilizando a técnica de deposição de solvente. Diferentes taxas de crescimento foram usadas para o crescimento dos cristais. Os melhores resultados foram obtidos para crescimento na estufa a 80ºC, produzindo filmes com gap de energia de 2,2 eV e resistividade da ordem de 108 ?.cm, indicando dopagem não intencional ou excesso de defeitos. A composição obtida é de HgI3, e a razão da fotocorrente pela corrente de escuro quando na faixa de raios-X mamográficos é da ordem de 25. Com as otimizações indicadas no texto e na conclusão do trabalho esses materiais seriam fortes candidatos para aplicações comerciais em imagens médicas para energias na faixa mamográfica. / In the last years there has been a growing interest in semiconductor materials with high atomic number and optical band gap for applications as ionizing radiation detectors at room temperature, using the direct method of detection. Some materials such as lead iodide (PbI2) and mercury iodide (HgI2) have an optical band gap above 2,0 eV, operate at room temperature with low noise and low leakage current, present a high carrier mobility and high stopping power for ionizing radiation. Alternative methods are investigated by several researchers for the fabrication of these materials on top of large areas, with low fabrication time and costs as desirable for applications in medical imaging. In this sense, we present two methods for the development of the detectors. Thermal evaporation, with the development of the deposition system and chamber, is used for the fabrication of lead iodide thin films. The properties of the films were investigated as a function of deposition height. On the other hand, isothermal evaporation was used for the fabrication of mercury iodide milimetric crystals, for the first time using the organic solvent N.N- dimetilformamide (DMF). The properties of the crystals are investigated as a function of concentration and growth temperature. The structural, morphological and compositional properties of the films and crystals were investigated. The optical and electrical properties were also investigated for both films and crystals. The activation energy for electric transport and the test of the materials as sensors (using X-rays in the mammographic region) were studied. This work presents the main results for both materials: films of lead iodide and milimetric crystals of mercury iodide. The crystalline planes of the films have a preferential orientation along the (110) direction, and a morphology of vertical leaves, not similar to other reported results. The obtained composition is PbI2,5. The combination of the morphology and the configuration of co-planar contacts for charge collection reduce the sensitivity to X-rays exposure. A current density ratio (illuminated to dark) of the order of 1.53 was obtained. The best film was deposited at a distance of 5 cm to the source, and it has an optical gap of 2,39 eV and activation energy of 1,1 eV. With increasing deposition distance a degradation of the structural, optical and electrical properties was observed, even with the variation of the composition of the films. For mercury iodide, for any growth rate the shape of the crystals is always cubic. The morphology of the surface depends on the evaporation rate (it can be smooth or present macroscopic holes). The best results were obtained for a growth at 80o C, what leads to a crystal with optical gap of 2.2 eV and electrical resistivity of the order of 108 Ohmcm, what suggests non-intentional doping or excess of defects. The obtained composition was HgI3, and the current ratio (illuminated by mammographic X-rays to dark) was about 25. According to the discussions in the text and in the conclusions, the suggested optimizations could lead to the development of materials that might be useful for technological applications in medical imaging for the mammographic energy range.

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