Charge carrier transport in a-Si:H/c-Si heterojunctions
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Charge carrier transport in a-Si:H/c-Si heterojunctions
Susanne von Aichberger, Frank Wünsch and Marinus Kunst Dept. Solare Energetik, Hahn-Meitner-Institut, Glienicker Str. 100, D-14109 Berlin, Germany ABSTRACT Contactless transient photoconductivity measurements of a-Si:H / c-Si heterojunctions are presented. It is shown that n a-Si:H / n c-Si junctions furnish an excellent passivation of the n c-Si surface. For i a-Si:H / n c-Si junctions the passivation is better for thicker films, whereas for very thin films (10 nm or less) a deviating behaviour probably due to inhomogeneities is observed. For the p a-Si:H / n c-Si junction separation of excess charge carriers in the space charge region is observed leading to a slowly decaying tail of the signal. This separation is also observed in thick i a-Si:H / n c-Si samples where electrons are injected from the a-Si:H film in the c-Si substrate. INTRODUCTION a-Si:H / c-Si heterojunctions are promising structures for several (opto)electronic devices as e.g. solar cells [1,2], X-ray detectors and bipolar transistors. For an optimal performance of these devices a detailed knowledge of excess charge carrier kinetics is necessary. In this work these heterojunctions are investigated by contactless transient photoconductivity measurements. EXPERIMENT Hydrogenated amorphous silicon (a-Si:H) films have been produced by Plasma Enhanced Chemical Vapor Deposition (PECVD) of silane on single crystalline wafers, n-type and p-type doped. The samples were cleaned by a wet oxidation in hot H2SO4 / H2O2 mixture for 20 minutes and subsequently etch in HF (2%) for 1 minute. 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 [3]. TRMC signals were induced by 10ns (FWHM) pulses at 1064 nm and 532 nm. The TRMC signal (∆P(t) / P) is proportional to the photoconductance ∆S induced by the excitation pulse [3]: ∆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 + ∆na(t)µ ane + ∆pa(t)µ ape
(2)
where ∆n(t) (∆p(t)) is the number (in cm-2) of excess electrons (holes) at the time t after start of the excitation characterized by the mobility µn (µp) in the single crystalline substrate and the A24.1.1
same symbols with the subscript “a” refer to the same properties in the a-Si:H film. The charge carrier mobilities in the crystalline substrate are at least three orders of magnitude larger than in the a-Si:H film. This implies that the TRMC signal induced by 1064 nm radiation (volume excitation) is only due to excess carriers in the substrate. Illumination of the heterojunction by 532 nm radiation (surface excitation) leads also to a TRMC signal only due to excess charge carriers in the c-Si substrate if the a-Si:H film is thinner than 500 nm. For thicker films also excess charge carriers in the a-Si:H film may contribute to the TRMC signal. RESULTS AND DISCUSSION The deposition
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