Band alignment at amorphous/crystalline silicon hetero-interfaces
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Band alignment at amorphous/crystalline silicon hetero-interfaces L. Korte, T. F. Schulze, C. Leendertz, M. Schmidt and B. Rech Helmholtz-Zentrum Berlin für Materialien und Energie, Institut Silizium Photovoltaik, Kekuléstr. 5, D-12489 Berlin, Germany ABSTRACT We present an investigation of the band offsets in amorphous/crystalline silicon heterojunctions (a-Si:H/c-Si) using low energy photoelectron spectroscopy, ellipsometry and surface photovoltage data. For a variation of deposition conditions that lead to changes in hydrogen content and the thereby the a-Si:H band gap by ~180 meV, we find that mainly the conduction band offset 'EV varies, while 'EC stays constant within experimental error. This result can be understood in the framework of charge neutrality (CNL) band lineup theory. INTRODUCTION Due to their high power conversion efficiency potential [1], amorphous/crystalline silicon (a-Si:H/c-Si) solar cells are currently in the focus of many research activities. The essential feature of these cells is the use of a-Si:H/c-Si heterojunctions for charge carrier extraction and wafer surface passivation. In two recent papers, we have investigated the band line-up at the a-Si:H/c-Si heterointerface. The main findings were that the valence band offset 'EV is independent on the doping of both c-Si substrate and a-Si:H thin film [2], and that the widening of the a-Si:H band gap with increasing hydrogen content in the film leads primarily to an increase in 'EV, while the conduction band offset 'EC is only slightly changed (compatible with zero change within the error margin) [3]. In the present paper, we elaborate on those results, making use of additional information on the work function, ionization energy and electron affinity in the a-Si:H layers. Furthermore, we use the charge neutrality level (CNL) concept [4] to discuss the physical origins of the measured band offsets. In order to obtain the CNL in a-Si:H, we suggest to use calculations based on the defect pool model [5]. EXPERIMENT A set of ~10 nm thin (i)a-Si:H layers were deposited by RF-PECVD (fRF = 13.56 MHz) on 3 :cm (n)c-Si wafers (FZ, {111}, mirror-polished). The hydrogen content in the film was varied using different deposition temperatures (Tdepo = 130-210°C), pressures (p = 0.5, 1 and 4 mbar: “low pressure” (LP), “medium pressure” (MP) and “high pressure” (HP)) and hydrogen dilution ratios ([H2]/[SiH4] = 0 or 10). As shown in [3], this results in a variation of the band gap Eg and structural disorder in the film. Eg was obtained from fitting spectral ellipsometry (SE) data using a Tauc-Lorentz approach and the program rig-vm [6]. Using surface photovoltage (SPV), we measure the band bending eMs in the c-Si wafer and calculate the band offsets 'EV = EF,aSi – (EF,c-Si,bulk-eMs), 'EC = Eg,aSi – Eg,cSi – 'EV (cf. Fig. 1 and [3]). Photoelectron spectroscopy in the constant final state yield (CFSYS) mode [7] was carried out in UHV at photon energies of 3 – 7 eV. The measured quantity is the internal photoelectron yield Yint, i.e. the ratio between the spectr
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