Processing of Silicon Wafers Followed by Microwave Photoconductivity Measurements
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PROCESSING OF SILICON WAFERS FOLLOWED BY MICROWAVE PHOTOCONDUCTIVITY MEASUREMENTS A. SANDERS, H. WETZEL AND M. KUNST Hahn-Meitner Institut, Solare Energetik, D - 1000 Berlin 39, West Germany.
ABSTRACT The characterization of single crystalline silicon wafers for application in (opto)electronic devices by transient photoconductivity measurements is investigated. To this aim is the transient photoconductivity in Si wafers after different treatments determined by the Time Resolved Microwave Conductivity ( TRMC ) method. This technique is non-evasive and contactless and so in-situ measurements are possible. Application of TRMC measurements for process control and quality control of relevant process steps in the production of (opto)electronic devices is discussed in view of the experimental results presented.
INTRODUCTION The transient photoconductivity in a semiconductor reflects the interaction of excess charge carriers with the lattice, in particular lattice defects. It offers the possibility to get an idea about the influence and the distribution of electrically active defects in a material. If it is realized that the performance of most (opto)electronic devices is determined by charge carrier kinetics, it is obvious that transient photoconductivity measurements can be used for the characterization of semiconductors for application in (opto)electronic devices.
EXPERIMENTAL Time Resolved Microwave Conductivity (TRMC) measurement were performed in a Ka-band equipment as described before [1]. The time resolution of the measurements was determined by the width of 10 ns ( FWHM ) of the exciting laser pulse from a Nd:YAG laser at 1064 nm and 532 nm.
THEORY Excitation of Si wafers by light of 1064 nm or 532 nm induces equal concentrations of initially hot mobile electrons and holes, thermalizing to the corresponding band edges. As thermalization at room temperature is very fast and the mobility of other excess species is very small, it can be assumed that the photoconductivity is carried by electrons at the bottom of the conduction band and holes at the top of the valence band: Mat. Res. Soc. Symp. Proc. Vol. 189. 01991 Materials Research Society
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Aa(t)
=
An(t) ge
+
Ap(t) Ph
(1)
where An(t) and Ap(t) represent the concentration of excess electrons and holes, respectively, and ge and Ph the corresponding mobilities. The decay of the transient photoconductivity reflects the decay of mobile electrons and holes. In Si wafers excess charge carrier decay by recombination via lattice defects under the conditions considered in this work. In extrinsic Si under the present experimental conditions a homogeneous distribution of lattice defects generates a first order decay process i.e. characterized by a decay time independent of time and of excess carrier concentration. An inhomogeneous distribution of lattice defects, on the contrary, induces diffusion currents of excess carriers and so decay processes with a decay time changing with time after pulsed excitation until a stationary distribution has been established [2
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