The Determination of Optoelectronic Properties of Microcrystalline andAmorphous Silicon Films
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The Determination of Optoelectronic Properties of Microcrystalline and Amorphous Silicon Films
Marinus Kunst, Susanne von Aichberger, Wilhelm Thom and Frank Wünsch Dept. Solare Energetik, Hahn-Meitner-Institut, Glienicker Str. 100, D-14109 Berlin, Germany ABSTRACT The study and characterization of the (opto)electronic properties of a-Si:H and µ c−Si films by contactless transient photoconductivity measurements is presented. The importance of minority carrier trapping is shown for the example of a-Si:H films prepared with different doping levels. It is shown that the microwave mobility determined by these measurements is a versatile tool for the characterization of the films. Examples are given by the study of µ c−Si films produced by laser crystallization of a-Si:H films and the optimization of the substrate temperature for the Hot Wire deposition of µ c−Si films. INTRODUCTION The determination of the (opto)electronic properties of thin films is often complicated by the use of contacts. Completely optical measurements do not yield properties that can be directly related to the electronic behaviour. In the present work it will be shown that contactless and nonperturbative photoconductivity measurements in the microwave frequency range can yield detailed and important information. The TRMC signal (= the ratio of the change in reflected microwave power ∆P(t) and the the incident DC microwave power P: ∆P(t)/P) is proportional to the photoconductance ∆S(t) induced by the excitation pulse: ∆P(t) = A ∆S(t) P
(1)
with A a proportionality factor (the sensitivity factor) and ∆S(t) given by: ∆S(t) = ∆n(t)µne + ∆p(t)µpe
(2)
where ∆n(t) (∆p(t)) is the number (in cm-2) of excess electrons (holes) at time t after the start of the excitation characterized by the mobility µn (µp). e is the elementary charge. It is convenient to define the microwave mobility µµ as: µµ =
∆Send ∆nend ∆pend = µn + µ e∆n 0 ∆n0 ∆n 0 p
(3)
where ∆Send is the end of Laser pulse conductance (at about 10ns) , ∆nend(∆pend) is the number of excess electrons (holes) at the end of the excitation pulse and, ∆n0 is the number of excess charge carrier pairs induced by the pulse. If no charge carrier decay occurs during the excitation pulse Eq. 3 yields: A23.2.1
µµ = µn + µp
(4)
This case is valid in single crystalline Si wafers. In the general case the main contribution to the signal is due to the charge carriers with the highest mobility and the lowest decay rate. EXPERIMENT Hydrogenated amorphous silicon (a-Si:H) films have been produced by Plasma Enhanced Chemical Vapour Deposition (PECVD) of silane on Corning 7053 substrates. Microcrystalline silicon (µ c−Si) films have been produced by PECVD, Hot Wire (HW) deposition and by laser induced cristallization of a-Si:H films. Contactless transient photoconductivity measurements in the microwave frequency range at 10 GHz have been performed with the Time Resolved Microwave Conductivity (TRMC) method as described previously [1]. TRMC signals were induced by 10 ns (FWHM) pulses at 1064 nm and 532 nm. RESULTS
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