A study of Al2O3:C films on Si(100) grown by low pressure MOCVD

We report the characterization of carbonaceous aluminium oxide, Al2O3:C, films grown on Si(100) by metalorganic chemical vapor deposition. The focus is on the study of the effects of carbon on the dielectric properties of aluminium oxide in a qualitative manner. The carbon present in the aluminium oxide film derives from aluminium acetylacetonate used as the source of aluminium. As-grown films comprise nanometer-sized grains of alumina (∼ 20–50 nm) in an amorphous carbonaceous matrix, as examined by X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The films are shiny; they are smooth as observed by scanning electron microscopy (SEM). An attempt has been made to explore the defects (viz., oxide charge density) in the aluminium oxide films using room temperature high frequency capacitance – voltage (C-V) and current–voltage (I-V) measurements. The hysteresis and stretch-out in the high frequency C-V plots is indicative of charge trapping. The role of heteroatoms, as characterized by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy, in the transport of charge in Al2O3:C films is discussed.

Edge-defined, film-fed growth (EFG) technique for the preparation of alpha-Al2O3 : C crystal with highly sensitive thermoluminescence response

In this work, alpha-Al2O3:C, a highly sensitive thermoluminescence dosimetry crystal, was grown by the EFG method in which a graphite heating unit and shield acted as the carbon source during the growth process. The optical, luminescent properties and dosimetric characteristics of the crystal were investigated. The as-grown crystal shows a single glow peak at 536 K, which is associated with Cr3+ ions. After annealing in H-2 at 1673 K for 80 h, the crystal shows a single glow peak at 460 K and a blue emission band at 415 nm. The thermoluminescent response of the annealed crystal shows linear-sublinear-saturation characteristics in the dose range from 5 x 10(-6) to 100 Gy.

Growth of highly sensitive thermoluminescent crystal alpha-Al2O3 : C by the temperature gradient technique

In this work, an alpha-Al2O3:C crystal with highly sensitive thermoluminescence was directly grown by the temperature gradient technique (TGT) using Al2O3 and graphite powders as raw materials. The optical and luminescent properties and the dosimetric characteristics of the crystal were investigated. An as-grown alpha-Al2O3:C crystal shows a single glow peak at 462 K and a blue emission peak at 415 nm. The thermoluminescence (TL) response of the crystal shows a linear-sublinear-saturation characteristic. In the dose range from 5 x 10(-6) to 10Gy, the alpha-Al2O3:C crystal shows excellent linearity, and saturation was observed at about 30Gy. The sensitivity of the crystal decreases as the heating rate increases. (c) 2008 Elsevier B.V. All rights reserved.

Growth of alpha-Al2O3:C crystal with highly sensitive optically stimulated luminescence

In this work. an alpha-Al2O3:C crystal was directly grown by the temperature gradient technique (TGT) using Al2O3 and graphite powders as the raw materials. The optical, optically stimulated luminescence (OSL) properties and dosimetric characteristics of as-grown crystal were investigated. As-grown alpha-Al2O3:C crystal shows strong absorption band at 205, 230 and 256 nm. Three-dimensional thermoluminescence (TL) emission spectrum of the crystal shows a single emission peak at similar to 415 nm. The OSL decay curve can be fitted to two exponentials, the faster component and the slower component. The OSL response of the crystal shows a linear-sublinear-saturation characteristic. As-grown alpha-Al2O3:C crystal shows excellent linearity in the dose range from 5 x 10(-6) to 50 Gy. For doses higher than the saturation dose (100 Gy). the OSL sensitivity decreases as the dose increases. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.

Influence of carbon on the thermoluminescence and optically stimulated luminescence of α -Al2O3:C crystals

alpha-Al2O3:C crystal shows excellent thermoluminescence (TL) and optically stimulated luminescence (OSL) properties but the real role carbon plays in this crystal is still not clearly understood so far. In this work, alpha-Al2O3:C crystal doping with different amounts of carbon were grown by the temperature gradient technique, and TL and OSL properties of as-grown crystals were investigated. Additionally, a mechanism was proposed to explain the role of carbon in forming the TL and OSL properties of alpha-Al2O3:C. TL and OSL intensities of as-grown crystals increase with the increasing amount of carbon doping in the crystal, but no shift is found in the glow peak location at 465 K. As the amount of carbon doping in the crystals decreases, OSL decay rate becomes faster. With the increase in heating rate, the integral TL response of as-grown crystals decreases and glow peak shifts to higher temperatures. TL response decrease rate increases with the increasing amount of carbon doping in the crystals. All the TL and OSL response curves of as-grown crystals show linear-sublinear-saturation characteristic, and OSL dose response exhibits higher sensitivity and wider linear dose range than that of TL. The crystal doping with 5000 ppm carbon shows the best dosimetric properties. Carbon plays the role of a dopant in alpha-Al2O3:C crystal and four-valent carbon anions replace the two-valent anions of oxygen during the crystal growth process, and large amounts of oxygen vacancies were formed, which corresponds to the high absorption coefficient of F and F+ centers in the crystals.

α-Al_2O_3:C晶体的热释光和光释光特性

以高纯α-Al2O3和石墨为原料,采用温梯法生长了α-Al2O3:C晶体,使用RisΦTL/OSL-DA-15型热释光和光释光仪研究了其热释光和光释光特性.α-Al2O3:C晶体在462K附近有单一热释光峰,发射波长位于410nm.随着辐照剂量的增加,热释光强度逐渐增强,462K的热释光特征峰位置保持不变.α-Al2O3:C晶体的光释光衰减曲线由快衰减和慢衰减两个部分组成,随着辐照剂量的增加,快衰减部分衰减速率变化不大,而慢衰减部分衰减速率加快.在5×10-6—10Gy剂量范围内,α-Al2O3:C晶体的热释光剂量响应呈现良好的线性关系,30Gy时达到饱和;光释光剂量响应在5×10-6—60Gy剂量范围内呈现良好的线性关系,100Gy时达到饱和.与热释光相比,光释光剂量响应具有更高的灵敏度和更宽的线性剂量响应范围.

Comparison between blue and green stimulated luminescence of Al(2)O(3):C

This paper presents a systematic comparison of OSL signals from Al(2)O(3):C when stimulated with blue and green light. Al(2)O(3):C detectors were irradiated with various doses and submitted to various bleaching regimes using yellow, green and blue light. Most of the investigations were carried out using Luxel (TM)-type detectors used in the commercial Luxet (TM) and InLight (TM) dosimetry systems (Landauer Inc.). Al(2)O(3):C single crystals and Al(2)O(3):C powder were also used to complement the investigations. The results show that, although blue stimulation provides faster readout times (OSL curves that decayed faster) and higher initial OSL intensity than green stimulation, blue stimulation introduced complicating factors. These include incomplete bleaching of the dosimetric trap when the Al(2)O(3):C detectors are bleached with yellow or green light and the OSL is recorded with blue light stimulation, and an increased residual level due to stimulation of charge carriers from deep traps. The results warrant caution when using blue stimulation to measure the OSL signal from Al(2)O(3):C detectors, particularly if the doses involved are low and the detectors have been previously exposed to high doses. (C) 2010 Elsevier Ltd. All rights reserved.

Obtenção de biodiesel através da transesterificação do óleo de farelo de arroz utilizando KI/Al2O3

This study proposes to find a biodiesel through transesterification of rice bran oil with KI/Al2O3 checking the influence of two types of alumina (Amorphous and Crystalline) for conversion into methyl esters. The catalyst was synthesized by the wet impregnation method. Adding 30 mL of 35% KI(aq.) in 10 g of alumina, under stirring at 80 °C for 3 hours. The reaction conditions used in this study were optimized, with a molar ratio methanol:oil of 15:1, 8 h of reaction time and reflux temperature. The catalyst amount was varied in the range of 1 to 5 % wt. The solid catalysts materials were analyzed by: x-ray diffraction (XRD), thermogravimetry (TG), N2 adsorption/desorption, scanning electron microscopy (SEM) and basicity, for the identification of its structure and composition, verifying the presence of basic sites. The results showed that Al2O3(A) presents an amorphous structure, high surface area and a better catalytic activity, in relation to the catalyst synthesized with Al2O3(C) support that proved to have a more crystalline structure, having as well, a lesser surface area, enabling difficulties for the incorporation of active sites. The obtained biodiesel with 5% wt. KI/Al2O3(A) presented physicochemical properties within the standards specified by the Resolution No 7/2008 ANP and obtained the best reaction yield with 95.2%, according to quantitative measurement from the TG, which showed 96.2% conversion into methyl esters. It was furthermore found that with the increasing amount of the quantity of the catalyst in the reaction, there was also an increase in the ester content obtained. The specific mass and the kinematic viscosity were reduced with the increase of the amount of quantity of the catalyst, indicating an increase in the conversion of triglycerides

Response variation of optically stimulated luminescence dosimeters

This study investigates the variability in response of optically stimulated luminescence dosimeters (OSLDs). Examining the source of sensitivity variations in these dosimeters allows for a more comprehensive understanding of the Landauer nanoDots and their potential for current and future applications. In this work, OSLDs were scanned with a MicroCT scanner to determine potential sources for the variation in relative sensitivity across a selection of Landauer nanoDot dosimeters. Specifically, the correlation between a dosimeters relative sensitivity and the loading density of Al2O3:C powder was determined. When extrapolating the sensitive volume's radiodensity from the CT data, it was shown that there is a non-uniform distribution in crystal growth. It was calculated that a 0.05% change in the nominal volume of the chip produces a 1% change in the overall response. Additionally, the ‘true’ volume of an OSLD's sensitive material is, on average, 18% less than that which has been reported in literature, mainly due to the presence of air cavities in the material's structure. This work demonstrated that the amount of sensitive material is approximately linked to the total correction factor.

Response variation of optically stimulated luminescence dosimeters

Introduction This study investigates uncertainties pertaining to the use of optically stimulated luminescence dosimeters (OSLDs) in radiotherapy dosimetry. The sensitivity of the luminescent material is related to the density of recombination centres [1], which is in the range of 1015–1016 cm-3. Because of this non-uniform distribution of traps in crystal growth the sensitivity varies substantially within a batch of dosimeters. However, a quantitative understanding of the relationship between the response of an OSLD and its sensitive volume has not yet been investigated or reported in literature. Methods In this work, OSLDs are scanned with a MicroCT scanner to determine potential sources for the variation in relative sensitivity across a selection of Landauer nanoDot dosimeters. Specifically, the correlation between a dosimeters relative sensitivity and the loading density of Al2O3:C powder was determined. Results When extrapolating the sensitive volume’s radiodensity from the CT data, it was shown that there is a non-uniform distribution incrystal growth as illustrated in Fig. 1. A plot of voxel count versus the element-specific correction factor is shown in Fig. 2 where each point represents a single OSLD. A line was fitted which has an R2-value of 0.69 and a P-value of 8.21 9 10-19. This data shows that the response of a dosimeter decreases proportionally with sensitive volume. Extrapolating from this data, a quantitative relationship between response and sensitive volume was roughly determined for this batch of dosimeters. A change in volume of 1.176 9 10-5 cm3 corresponds to a 1 % change in response. In other words, a 0.05 % change in the nominal volume of the chip would result in a 1 % change in response. Discussion and conclusions This work demonstrated that the amount of sensitive material is approximately linked to the total correction factor. Furthermore, the ‘true’ volume of an OSLD’s sensitive material is, on average, 17.90 % less than that which has been reported in literature, mainly due to the presence of air cavities in the material’s structure. Finally, the potential effects of the inaccuracy of Al2O3:C deposition increases with decreasing chip size. If a luminescent dosimeter were manufactured with a smaller volume than currently employed using the same manufacturing protocol, the variation in response from chip to chip would more than likely exceed the current 5 % range.

Influence of GaN underlayer thickness on structural, electrical and optical properties of InN films grown by PAMBE

We present an extensive study on the structural, electrical and optical properties of InN thin films grown on c-Al2O3, GaN(130 nm)/Al2O3, GaN(200 nm)/Al2O3 and GaN(4 mu m)/Al2O3 by using plasma-assisted molecular beam epitaxy. The high resolution X-ray diffraction study reveals better crystalline quality for the film grown on GaN(4 mu m)/Al2O3 as compared to others. The electronic and optical properties seem to be greatly influenced by the structural quality of the films, as can be evidenced from Hall measurement and optical absorption spectroscopy. Kane's k.p model was used to describe the dependence of optical absorption edge of InN films on carrier concentration by considering the non-parabolic dispersion relation for carrier in the conduction band. Room temperature Raman spectra for the InN films grown on GaN show the signature of residual tensile stress in contrast to the compressive stress observed for the films grown directly on c-Al2O3. (C) 2012 Elsevier B.V. All rights reserved.