Ex situ and in situ defect density measurements of a-Si:H by means of the cavity ring down absorption technique
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Ex situ and in situ defect density measurements of a-Si:H by means of the cavity ring down absorption technique A.H.M. Smets1, J.H. van Helden and M.C.M. van de Sanden2 Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands 1 [email protected], 2 [email protected] ABSTRACT For the first time it is demonstrated that the surface defect density can be measured using the cavity ring down (CRD) absorption technique and it is shown that CRD is more sensitive for surface defects than dual beam photoconductivity (DBP) technique. Ex situ measurements have shown that the surface defects of the oxidized a-Si:H surface are distributed over a surface region with thickness W(d). The obtained surface defect density is 1.0 × 1012 up to 1.4 × 1012 cm-2. During growth the a-Si:H surface defect density region has at least a thickness of 15 nm. During deposition of 15 nm a-Si:H the surface density increases up to a not yet saturated value of 1×1013 cm-2. INTRODUCTION The defect or dangling bond density in hydrogenated amorphous silicon (a-Si:H) films is an important parameter which determines the performance of a-Si:H thin film solar cells. Furthermore, it is commonly believed that during a-Si:H deposition the surface has a small surface coverage (ΘDB ≈ 10-3-10-6) of dangling bonds which acting as growth sites [1-4]. To improve the insight in the a-Si:H growth mechanism, an easily applicable technique is needed to monitor the dangling bond evolution during growth. The neutral dangling bond states are positioned in the middle of the band gap and are responsible for the small sub gap absorption (α = 0.1-100 cm-1) in the 0.8–1.3 eV range of aSi:H thin films [5]. The sub gap absorption can be measured indirectly using opto-electronic techniques such as constant photoconductivity measurements (CPM) [6], dual beam photoconductivity (DBP) [7] and opto-thermal techniques as photothermal deflection spectroscopy (PDS) [8,9]. The latter technique is reported to be the most sensitive for surface defects [8]. One main disadvantage of these techniques is the fact that they are indirect absorption measurements, i.e. the optical absorption is measured by detecting current or a temperature gradient. Moreover these techniques can not be applied in situ during a-Si:H growth. The pioneering work of Yamasaki et al. has revealed that dangling bond densities of 13 10 cm-2 in the surface region during growth can be measured using the in situ electron-spinresonance (ESR) technique [10]. Here we will show that a newly developed absorption technique, the surface cavity ring down (CRD) is suitable to detect the defects in the bulk and at the surface by means of sub gap absorption. EXPERIMENTAL DETAILS The surface CRD technique is a sensitive absorption technique, which enables us to measure the sub gap absorption directly [11]. The technique is based upon the measurement of the rate of absorption of a laser light pulse confined in an optical cavity. The optical cavity of length L is formed b
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