Charge Carrier Transport in c-Si/a-Si:H Heterojunctions
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CHARGE CARRIER TRANSPORT IN c-Si/a-Si:H HETEROJUNCTIONS A.SANDERS, H.-C. NEITZERT*,C.SWIATKOWSKI AND M.KUNST
Hahn-Meitner-lnstitut, Si: Solare Energetik, Glienicker Str. 100, 1000 Berlin 39, Germany *Present address:LPICM, Ecole Polytechnique, Palaiseau, France ABSTRACT
Contactless transient photoconductivity measurements in aSi:H/c-Si heterojunctions are presented. The experimental data are decomposed in the contribution of excess electrons in a-Si:H, the contribution of excess carriers in c-Si and the contribution of excess carriers injected from a-Si:H into c-Si. A value for the electron drift mobility of in a-Si:H of 1cm 2 V-1 s- 1 is determined. At low excitation densities a high injection efficiency is observed, which is quenched by bias illumination. EXPERIMENTAL
Films of a-Si:H were deposited from silane on n-doped crystalline silicon substrates (thickness
510ltm and p=502cm)
under standard
conditions for the production of state-of-art films (1),if not otherwise indicated. Transient photoconductivity measurements in the microwave frequency range (28.5-40 GHz) were performed with the Time Resolved
Microwave Conductivity (TRMC) method where TRMC-signals are induced by 10ns (FWHM) pulses of a Nd:YAG laser at 532nm. The TRMC signal (AP(t)/P) at time is proportional to the product of the number of excess carriers at time t and their respective mobilities (2). In aSi:H/c-Si heterojunctions only excess electrons in the a-Si:H part and excess electrons and holes in the c-Si part of the junction yield appreciable contributions to the TRMC signal: AP(t)/P=A(Anc(t)ýln+Apc(t)ltp+Ana(t)ýta)
(1)
where A is the sensitivity factor, Anc(t) and Apc(t) represent the total number of excess electrons and holes, respectively, characterized by their respective mobilities kn and ltp in the c-Si part of the junction and Ana(t) is the total number of excess electrons in the a-Si:H part characterized by the mobility .ta. The mobility 9'a is about three orders of magnitude smaller than I-t n or ltp.(1) Consequently an excess carrier injected from a-Si:H in c-Si will cause a drastic increase of the signal. This leads under certain conditions to an increase of the TRMC signal
Mat. Res. Soc. Symp. Proc. Vol. 297. (q'1993 Materials Research Society
1056
with time not connected to the duration of the production of charge carriers by the excitation pulse (10ns). This makes it convenient to distinguish three groups of excess carriers contributing to the TRMC signal: -Excess mobile electrons induced and present in a-Si:H -Excess mobile electrons and holes induced and present in c-Si -Excess mobile electrons and holes induced in a-Si and injected in c-Si The fourth group involving excess charge carriers induced in c-Si and injected in a-Si:H will not be taken into account, because the lifetime of excess carriers in a-Si:H is so short relative to lifetimes of charge carriers in c-Si, that their contribution can be neglected. However, this injection process can be important as a decay channel for carriers in c-Si. In that case this
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