Study on the impurities segregation behaviour during zone refining of indium

Transcription

1 Study on the impurities segregation behaviour during zone refining of indium V. N. Mani* and K. Balaraju Centre for Materials for Electronics Technology (C-MET), Cherlapally, Hyderabad , India ABSTRACT In this study, we report the results pertaining to the segregation behavior of the selected metallic,,,,, and impurities during multi-pass zone refining of 4N+/5N pure indium metal. Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), and dynamic depth profiling were employed to measure and ascertain the impurities level in the starting material and the refined indium ingot. Segregation trend of said typical impurities has been studied for different regions of the refined ingot. It was observed that, majority of the selected impurities under study were traveled towards one end of the ingot, thus leading the other end in pure form. Key words: Ultra High Pure Indium, Zone Refining, Segregation, Impurities, Purity Analysis 1. INTRODUCTION InP and its related materials inherent and tailor-able properties, such as physico-chemical, electro-optical, high radiation and temperature tolerance, lattice match, band gap, and available superior doping, fabrication technologies make them as special candidates for smart micro-optoelectronic applications [1-2]. In view of this, InP based epi-structures, devices and gadgets are selectively used in the aerospace, defence, communication related instruments, control and navigation systems. Ultra high pure (7N) indium is one of the primary starting materials required for the preparation of InP bulk single crystals (substrates) as also for the epitaxy and fabrication of binary (InP/InP, InAs/InAs, InSb/ InSb), ternary (InAs/InP, InP/InP, InSb/InSb) and quaternary (InAsP/InP, InAsSb/InAs) device structures. The metallic impurities of most concern in 7N pure indium for above applications include,,,,, Bi, Si, Sn, Hg, Ca, Te and Pb. Presence of impurities in the starting materials results in the formation of chemical, electronic, energy related and other micro-level defects, which are very detrimental to end use application and overall device performance. Therefore ultra high pure materials such as indium and phosphorus are analyzed and certified very stringently, in the range of pbb levels. These ultra high purity In and P are used as starting materials to prepare device quality InP and related epitaxial structures. Zone refining (vacuum/inert), crystallization, gradient freezing and vapor refining techniques are widely used to purify 4N/5N indium to 7N /8N purity level and both Glow Discharge Mass Spectrometry (GD-MS) and Residual Resistivity Ratio (RRR) techniques are employed to analyze the ultra high pure (7N/8N) metal 3. Pre and post processing and packaging of material under class clean and inert environment are also the most crucial steps. Development of cost effective and environment friendly refining and purification processes suiting to the source and grade of input raw indium, yield improvement, packaging, recycling of impure metal from both the discarded and already processed metal ingots and InP scraps, design and development of fully automated and modular type refining and purification systems are the major current technological problems and challenges. In this direction, much progress has been made to prepare ultra high pure (8N) indium in the form of crystalline rods. 2. PURITY REQUIREMENT AND TESTING Optoelectronics device applications demand indium and phosphorus purity in the level of (7N), which is referred to as integrated circuit(ic) grade. For 7N pure metal, the total amount of the impurities in gallium must be less than 100 parts per billion (ppb). The metallic impurities of ut-most concern in IC

2 grade indium are,,,,, Bi, Si, Sn, Hg, Ca, Te and Pb. These elements along with carbon, oxygen and gaseous impurities should be present in concentrations ranging from 1 to 10 parts per billion( ppb), in both the indium and phosporus. The impurities contribute shallow and deep energy levels in the energy gap (InP), which are detrimental and eventually leads to common device failure, when used for bulk or epitaxial crystal growth for fabrication of As wafer and final device. Impurities are analyzed with atomic absorption spectrometry with graphite furnace (AASGF), inductively coupled plasma optical emission spectrometry (ICP-OES), inductively coupled plasma quadrupole mass spectrometry (ICPQMS), glow discharge mass spectrometry (GD-MS) and residual resistivity ratio(rrr) analysis. 3. ZONE REFINING AND SEGREGATION OF IMPURITIES IN INDIUM Materials can be purified (segregation) or given desired composition (doping) or variations of composition (leveling) by zone refining or melting. In zone-refining, a narrow liquid (or molten) zone travels through a relatively longer charge (or ingot) of solid carrying with it to a position of the solute impurities in the charge, since, the concentration of the impurities in the solid crystallizing from the molten zone is different from that of the remaining in the liquid, the impurities are swept to one end of the sample [3]. Factors that are affecting the refining process include (a) direction and rate of zone movement, (b) zone size and length of sample, (c) diameter, thickness, conductivity and compatibility to extreme temperature of the sample tube, (d) temperature gradient, (e) thermal conductivity of phases, (f) latent heat of fusion, (g) density differences between the solid and liquid, (h) speed of crystal formation, (i) surface tension of the liquid and (j) tendency of the liquid to super-cool. In practice, using a narrow zone and increasing number of passes leads to efficient refining effect. 4. EXPERIMENTAL DETAILS Upgraded version of the Basic Zone Refiner with nine narrow ring type resistive heaters and eight coolers has been used in the present study for zone refining of 3N+ pure indium. The instrumentation and other technical details of the system were discussed elsewhere [4]. 3N+/4N pure indium was chemically treated with supra pure acids (pickled, washed and cleaned). The cleaned sample was heated to C for three hours under gentle stirring using a hot plate in a clean environment for homogenization. Liquid indium was loaded into a cleaned special polymeric-plastic-glass tube of 10 mm i.d., 12 mm o.d. and 500 mm length. Further the same sample was super-cooled to (-) 30 0 C and kept for 48 hours. This super-cooled and solidified sample was again subjected to further homogenization by following a re-melting (200 0 C)-cooling (-10 0 C)-super-cooling (-30 0 C) scheme for duration of 6, 24 and 36 hours respectively using vertical type deep freezer. A pre-stage fifteen-pass cycle (each pass of three hours duration) zone refining experiment with circulation of coolant at (10 to 250C) on the homogenized sample has been carried out in order to sensitize the system parameters and establish the suitable experimental conditions and narrow zone(s). Final 65 pass zoning experiment on the zone-leveled sample was conducted by following the pre calibrated experimental conditions and parameters. Coolant with required flow rate and temperature range of (10 to 15 0 C) was maintained. Drive mechanism then started at a first cycle and the sample tube was allowed to traverse. The zone length of 3.5 to 4.5 mm and width of 25 to 30 mm was maintained and experiment continued to complete sixty-five refining cycles of duration of each three hours. Each completed cycle was recorded through a cycle counter. The ingot was cut into three portions every time analyzing the top (~150mm length), middle (~150mm) and bottom end (~150mm) portions separately for the trace impurities content. The sample tubes, PTFE beakers used for sample homogenization, preparation for analysis and sample storage boxes were cleaned with ultra pure acids. 5. RESULTS AND DISCUSSION Movement trend of selected impurities segregation along the zone-refined bar(s) has been mapped (Figs.1-3) using dynamic depth profiling analysis. From the qualitative results, it was observed that most of the impurities traveled along the melt-interface direction (towards ingot bottom portion), thus, leading to top portion in purest form. It was seen that, there was a significant improvement in purity of the refined sample (5N-top region) with respect to targeted impurities as compared to starting 3N+ /4Npure indium. Sampling and analysis were also repeated for bottom, middle and top portions for the three batches of the zone-

3 refined bar. The segregation trend and consistency of the purity test results respectively, were thus ascertained and confirmed. Effect of experimental conditions such as, number of passes, zone width and interdependence of heater and coolant temperature on zone refining of indium was studied. The inferences lead to suggest that these parameters and pre and post stage sample cutting, cleaning procedures and environmental conditions play role in reducing the impurity and contamination levels. In this study we have mainly concentrated on the segregation aspects of major metallic impurities present in 3N+/4N indium. Purity control and test aspects with specific reference to O, N, C elements and other gaseous impurities by oxidation, heating under vacuum and using exclusive carbon-nitrogen-oxygen (CHNOS) analyzer needs to be further researched. 6. CONCLUSION Segregation behavior of the metallic impurities present in Indian 3N+/4N grade indium has been studied through multi-pass zone refining. Movement trend of some of the selected impurities along the ingot show that, they are driven to and get concentrated at one end of the ingot leading other end in 5N pure form with reference to the targeted impurities, namely,,,,,, Pb, and B. The purity of the refined samples was confirmed by inductively coupled plasma optical emission spectrometry (ICP-OES) analysis. ACKNOWLEDGEMENTS One of the authors (VNM) thank Analysis Group for purity analysis and Smt.P.K.Girija for useful discussions on instrumentation aspects; C-MET authorities for extending the logistic support for implementation of the Project and Instrument Development Division, DST, Govt. of India, New Delhi for sponsoring the project on Automation and Upgradition of Zone-Refiner. REFERENCES 1. J.S. Jr.; Harris, Indium Phosphide and Related Materials, Vol.1, 333(2003) 2. K.Hashio, N. Hosaka, S. Fujiwara, T. Sakurada, R. Nakai, N.Hara, Y. Tsusaka and J. Matsui, Indium Phosphide and Related Materials, Vol.1, 542(2003) 3. W.G. PFANN in ``Zone Melting (New York: John Wiley &Sons, 2nd edn.1964). 4. V.N.Mani and K.Balaraju, Procs. Intl.Workshop on Preparation and Characterization of Technologically Important Single Crystals, Vol.1, 177(2001), Eds. S.K.Gupta et al., Delhi * vnm_crystal272001@yahoo.com; phone ; fax

4 Concentration(arb.units) Position from top(cm) Fig. 1. Bottom region sample - impurities segregation trend

5 Concentration(arb.units) Position from top(cm ) Fig. 2. Middle region sample - Impurities segregation trend C oncentration(arb.units) P osition from top(cm ) Fig. 3. Top region sample impurities segregation trend