Characterization of Polycrystalline Silicon Grown by Gas-Assisted-Solidification
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CHARACTERIZATION OF POLYCRYSTALLINE SILICON GROWN BY GAS-ASSISTED-SOLIDIFICATION H.L. HWANG, M.H. YANG, T.H. HSEU and C.H. National Tsing Hua University Hsin-chu, Taiwan, R.O.C.
CHU
Polycrystalline Si crystals grown by gas-assistedsolidification have been characterized by chemical and physical techniques with particular emphasis on carbon determination. The life time of this material appears to be not correlated with [01 or [C]. Gas-assisted-solidification (GAS) is a method which can grow Si poly-crystals for low-cost solar cells. In this method, Si is cast in a crucible by forced freezing that is initiated by means of a cold finger placed beneath the bottom center of the crucible. The detailed system design and crystal growth was reported in reference [1]. Analytical techniques with detection limits down to the ppb level for B, P, As and metallic contents in Si have been developed [2,31. As to the determination of carbon in silicon, a wide variety of methods have been reported which include Coulometry [4], Gas Chromatography [5], IR Spectrometry [6], Mass Spectrometry [7], Charged-particle Activation Analysis [8] and Conductimetry [9,10]. In this study, a conductimetric method based principally on Lanza et al. [91 was established, in which carbon was oxidized and its content was determined by monitoring the conductivity change of an NaOH solution. The method needs only simple and low-cost set-up and is suitable for routine analysis with high accuracy. Fig.l shows the complete set-up for the carbon determination, in which part A is for carrier gas purification, part B is the electrolysis cell for calibration, part C is the sample reaction cell, and part D is for absorption-conductivity measurements.
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Part Bt
(1) (3) (1)
flow meter, (2) gas bubbler ( filled with conc. H2 S0 4 ). molecular sieves. D.C. power supply, (2) calibration cell, (3) desiccant (silica
Part Ct
(1)
syringe,
Part Di
(1) water bath, (2) sintered glass disk, (3) absorptionconductivity cell, (4) conductivity meter.
Part A:
Fig.i
(2)
sample
reaction cell,
Flow diagram for the determinatin
(3)
of Carbon in
desiccant.
Silicon
Mat. Res. Soc. Symp. Proc. Vol. 59. 0 1986 Materials Research Society
gel).
452
calibrated by introducing exactly The system is first known amounts of CO 2 which is generated from an oxalic solution such as C 2 H2 04-2CO2 +2e-+2H+. With a current of 2.0 mA, the generation rate of 0.912 Pg C02/s was estimated. For the absorption of C0 2 , dilute NaOH solution was shown to be the best absorbent [10] and a 2.5xl0M solution was chosen in this study. The result of calibration indicates a linear change in conductance with the absorption of CO2 in the range of 0-20pg C0 2 /ml. The minimum CO 2 concentration which can be determined with reasonable certainty is about 0.2 pg C/ml, which is equivalent to about 6vg carbon in an absorption solution of 30 ml. The weighed sample (0.1-0.5 g) was dissolved in a sol
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