Volume 28, Issue 4
July 2005, pages 303-382
pp 303-303 July 2005
pp 305-307 July 2005
The paper reviews the role of sintered tantalum as volumetric efficient electrical capacitor. Powder characteristics and sintering aspects are discussed. The role of impurities in influencing the electrical properties has been described. Today’s driving force behind the Ta market is the use of surface mounted versions known as chip types, for applications requiring a wide range of operational temperature, such as automotive electronics.
pp 309-311 July 2005
Tellurium (99.95 at.% purity) is purified using dry processes such as selective vapourization and zone melting in a thoroughly etched and cleaned quartz boat, under continuous flow of hydrogen (H2) gas. The tellurium ingot was quadruple zone refined (QZR) under continuous flow of H2 gas. Thus, the purified tellurium of ultra high purity (UHP) grade is analysed for 60 impurity elements in the periodic table using glow discharge mass spectrometer (GDMS). The sum of all elemental impurities indicate that the purity of tellurium as 7N (99.99999 at.%). The total content of gas and gas forming impurities like O, N and C are found to be within acceptable limits for opto-electronic applications.
pp 313-316 July 2005
Metal–organic vapour phase epitaxy/chemical vapour deposition (MOVPE/MOCVD) has emerged recently as the method of choice for large scale preparation of a variety of low dimension inorganic materials; particularly compound semiconductors, used in modern electronic and opto-electronic devices. The success of this process depends on the availability of suitable molecular precursors of desired purity. Group V hydrides (e.g. NH3, PH3, AsH3, SbH3) have been employed conventionally for deposition of III–V semiconductor materials. Inherent weakness of this hydride source, particularly heavier ones (for instance very low utilization (> 0.1%) of AsH3 in GaAs synthesis; ∼ 4 h half life of SbH3 at room temperature) has been a driving force to develop new molecular precursors with desirable properties. This talk will briefly review synthesis and purification of several precursors of groups III (Ga, In), IV, V (As, Sb) and VI (Se, Te).
pp 317-323 July 2005
Minority carrier lifeline, 𝜏, is one of the most important parameters which has a decisive effect on the performance of silicon devices based on excess carriers. The value of 𝜏 is greatly affected by the presence of impurities and defects in silicon and its value provides a fair indication of quality of the material. Photoconductivity decay (PCD) and photocurrent generation (PCG) methods are simple and low cost methods of measurement of minority carrier lifetime in silicon wafers. However, their application requires care. The PCD method can give quite misleading results in case of polycrystalline wafers if there exists potential barriers at the grain boundaries which may affect majority carrier mobility significantly. PCG needs creation of an induced 𝑝+–𝑝–𝑛+ structure of substantially good quality that should not degrade with time. For PCG method the 𝜏 measurement under vacuum conditions provides correct and consistent results.
pp 325-330 July 2005
Radionuclides have become powerful and indispensable tools in many endeavours of human activities, most importantly in medicine, industry, biology and agriculture, apart from R&D activities. Ready availability of radionuclides in suitable radiochemical form, its facile detection and elegant tracer concepts are responsible for their unprecedented use. Application of radioisotopes in medicine has given birth to a new branch, viz. nuclear medicine, wherein radioisotopes are used extensively in the diagnosis and treatment of variety of diseases including cancer. Artificial transmutation of an element employing thermal neutrons in a reactor or high energy particle accelerators (cyclotrons) are the routes of radioisotope production world over. Availability of high purity target materials, natural or enriched, are crucial for any successful radioisotope programme. Selection of stable nuclides in suitable chemical form as targets with desired isotopic and chemical purity are among the important considerations in radioisotope production. Mostly the oxide, carbonate or the metal itself are the preferred target forms for neutron activation in a research reactor. Chemical impurities, particularly from the elements of the same group, put a limitation on the purity of the final radioisotope product. Whereas the isotopic impurities result in the production of undesirable radionuclidic impurities, which affect their effective utilization. Isotope Group, BARC, is in the forefront of radioisotope production and supply in the country, meeting demands for gamut of radioisotope applications indigenously for over four decades now. Radioisotopes such as 131I, 99Mo, 32P, 51Cr, 153Sm, 82Br, 203Hg, 198Au etc are produced in TBq quantities every month and supplied to several users and to Board of Radiation and Isotope Technology (BRIT). Such a large production programme puts a huge demand on the reliable sources of availability of high purity target materials which are at present mostly met through import. Availability of suitable target materials, their purity considerations and our efforts in finding sources of raw materials for sustaining the radioisotope programme are discussed here.
pp 331-337 July 2005
Available analytical literature reveals that it is possible to identify a lot of procedures to carry out any determination using a plethora of analytical techniques. The fundamental analytical requirements for realizing the desired and acceptable information from a chemical analysis are representative nature of the sample, precision, accuracy, selectivity and sensitivity. These decide, to a larger extent, the selection of the most appropriate methodology in order to obtain chemical information from a system. A number of analytical methodologies are being used in the author’s laboratory for carrying out trace elemental analysis as a part of chemical quality control. Quite a good number of analytical challenges with specific reference to the characterization of high purity materials of relevance to nuclear technology were addressed and methodologies were developed for trace elemental analysis of both metallic and non-metallic constituents. A brief review of these analytical challenges and the analytical methodologies developed and also the future needs of analytical chemist are presented in this paper.
pp 339-343 July 2005
Facilities and some results of several spectroscopic methods which have potential applications in the field of analysis of solid high purity substances and which have been elaborated in Russia, will be discussed in this paper. Laser nondispersive atomic fluorescence method with glow discharge cathode sputtering atomiser, may be used for trace element determination as well as a tool for the investigation of technological processes, viz. deposition of thin films. Investigations on reduction of a background level in the new hollow cathode ion source for mass-spectrometry have been carried out. Laser mass spectrometry with tandem laser mass reflectron is successfully designed and applied for gaseous impurities determination in high pure silicon with limit of detection of 10-3–10-5 ppm wt. Several results of the layer-by-layer and bulk trace analysis of solids by high resolution mass spectrometry with radio frequency powered glow discharge ion source with the limits of detection at 10-1–10-3 ppm wt will be presented here. The traditional arc and spark emission technique still finds considerable use. One of the examples considered in the paper is the analysis of metalfullerenes. To overcome the calibration problem the fluorination process inside the electrode crater using zinc fluoride has been investigated.
pp 345-348 July 2005
With continual decrease of geometries used in modern IC devices, the trace metal impurities of process materials and chemicals used in their manufacture are moving to increasingly lower levels, i.e. ng/g and pg/g levels. An attempt is made to give a brief overview of the use of different analytical techniques in the analysis of trace metal impurities in ultrapure materials, such as, high-purity tellurium (7N), high purity quartz, high-purity copper (6N), and high purity water and mineral acids. In recent times mass spectrometric techniques such as ICP–MS, GD–MS and HR–ICP–MS with their characteristic high sensitivity and less interference effects were proved to be extremely useful in this field. A few examples of such application studies using these techniques are outlined. The usefulness of other analytical techniques such as F–AAS, GF–AAS, XRF, ICP–AES and INAA was also described. Specific advantages of ICP–MS and HR–ICP–MS such as high sensitivity, limited interference effects, element coverage and speed would make them powerful analytical tools for the characterization of ultrapure materials in future.
pp 349-353 July 2005
This paper briefly describes our work and the results on the growth of several III–V epitaxial semiconductor materials in high purity form by liquid phase epitaxy (LPE) technique. Various possible sources of impurities in such growth are listed and step-by-step procedures adopted to reduce them are discussed in particular reference to the growth of GaAs layers. The technique of growing very high purity layers by treating the melt with erbium is described for the growth of InGaAs and GaSb layers.
pp 355-360 July 2005
The growth of good quality layers of gallium nitride (GaN) as suitable for epitaxial growth is of great technological importance. Chloride vapour phase epitaxy (Cl–VPE) has been employed to grow good quality layers of GaN. The grown layers have been extensively characterized for their structural and optical properties. MOVPE grown GaN layers have been used to address process issues on device structuring and fabrication. GaN samples with different transition metal dopants have been synthesized and their usefulness as semi-magnetic materials, which are also identified as dilute magnetic semiconductors (DMS), have been evaluated. Better results have been obtained on the magnetic characteristics of GaN with ruthenium as the dopant. Nano dimensional structures of GaN have been obtained with excellent control of the growth parameters.
pp 361-366 July 2005
In the field of advanced ceramics two CRMs were developed in the last few years by the Federal Institute for Materials Research and Testing, one for silicon nitride and one for silicon carbide. Besides their application by industry they are appropriate to be used for the validation of special methods used for trace determination in accordance with high purity materials. This is demonstrated, for example, on ultrapure silicon carbide which was analysed by solid sampling electrothermal atomic absorption spectrometry (SS ET AAS).
BAM is also certifying primary pure reference materials used as the National Standards for inorganic analysis in Germany. The crucial point of this project is the certification of the total purity of high purity materials, each representing one element of the periodic table. A variety of different analytical methods was necessary to determine the trace contents of metallic and non-metallic impurities from almost the whole periodic table in the high purity materials. The primary CRMs of copper, iron and molybdenum are used as examples to demonstrate the modus operandi, analytical effects observed by using high resolution ICP mass spectrometry (HR ICP–MS) and the results.
pp 367-371 July 2005
Possibilities of measurements of low defect concentration in Si by the electrical methods are discussed. The problems arising in such measurements are illustrated by measurements of iron concentration in Si. It is demonstrated that gold diffusion experiments can be used for revealing and study of some electrically inactive defects. Possibility of nondestructive reconstruction of defect depth profiles by DLTS and using the profile obtained for understanding the defect nature is illustrated by the results of hydrogen and hydrogen related defect investigations. EBIC investigations of dislocations are shown to be a rather sensitive method for revealing recombination defects.
pp 373-378 July 2005
Measurement of trace elements is playing a vital role in industries and various sectors of science and technology including semiconductors, food, health and environmental sectors. In most of the cases a small error in measurement can vitiate all the measures taken for quality control and management. Many decisions regarding the suitability of material/products are based on the analysis. To reduce or eliminate the rejection rate of the products, accurate and reliable measurements are needed which can be achieved by the use of certified reference materials (CRMs). Their use in calibration of analytical equipments and validation of test methods ensures high quality in measurements and it provides traceability to the measurement data with national/international measurement systems (SI unit) also. In the present scenario of globalization of economy, use of certified reference materials (CRMs) in measurements is essential for global acceptance of products and test reports. Their use fulfil a mandatory requirement of international quality systems (ISO 9000, ISO/IEC standard 17025) including our national accreditation body, National Accreditation Board for Testing and Calibration Laboratories (NABL), World Trade Organization (WTO) etc. International manufacturers of CRMs are meeting most of the requirement of CRMs of the country. To meet the demand of CRMs indigenously, the National Physical Laboratory, India initiated a national programme on preparation and dissemination of certified reference materials.
pp 379-382 July 2005
The need for better gas turbine operating efficiency and reliability has resulted in tightening of specification and acceptance standards. It has been realized that some elements even at trace level, can have disastrous effect on high temperature properties. The present paper highlights the adverse effect of tramp elements and strategies that should be adopted to produce high purity superalloys.
Volume 42 | Issue 6
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