Initial Stage Hydrogen Movement and IR Absorption Proportionality Constants In Hot-Wire Deposited SiN 1.2 :H During High
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Initial Stage Hydrogen Movement and IR Absorption Proportionality Constants In Hot-Wire Deposited SiN1.2:H During High-Temperature Annealing H.D. Goldbach1, V. Verlaan1, C.H.M. van der Werf1, W.M. Arnoldbik1, H.C. Rieffe2, I.G. Romijn2, A.W. Weeber2 and R.E.I. Schropp1 1 Utrecht University, SID – Physics of Devices, PO. BOX 80 000, 3508 TA Utrecht, The Netherlands 2 ECN Solar Energy, PO. Box 1, NL 1755 ZG Petten, The Netherlands
ABSTRACT Silicon nitride (SiNx:H) layers were deposited using a high deposition rate Hot-Wire CVD technique (up to 5 nm/s) and their application as passivating antireflection coating on multicrystalline silicon solar cells was investigated. An important aspect is the hydrogen release and diffusion during a short annealing treatment at temperatures well above the deposition temperature. Such annealing treatments (firing) are used during contact formation of multicrystalline solar cells. A series of SiNx:H layers with Si/N ratio in the range of 0.7-1.45 was first characterized using optical R/T measurements, Fourier Transformed Infrared Spectroscopy (FTIR) and Elastic Recoil Detection (ERD) to determine optical constants n and k, Si-H, N-H and Si-N bond densities, Si-H peak position and H content. To get more insight in the passivation process during firing, which takes place at roughly 800 °C, samples with a N/Si ratio of 1.2 were annealed at this temperature for different times (15 s – 10 min). The total bonded H content, measured with FTIR, using conventional proportionality constants as proposed by Lanford and Rand [1], seems to show an increase within the first 60 s. At longer annealing times, the bonded H content decreases, as expected. The behavior of the H content under annealing measured by FTIR and ERD can be explained by using different matrix elements for calculating the H content from the FTIR data. The matrix element for the N-H stretching mode (3343 cm-1) was determined to be (4.7±0.2)x1020 cm-2 while the Si-H stretching (2193 cm-1) was found to be (0.55±0.04)x1020 cm-2. Such high matrix elements for the N-H stretching mode have not been reported earlier. The value of the Si-H stretching mode matrix element is within the reported values for different back bonding scenarios, but low for a peak position of 2193 cm-1. The ratio between the matrix elements of 8.5 differs strongly from the ratio of 2 observed by Lanford [1] or Bohne [2].
INTRODUCTION SiNx:H layers are generally used in multicrystalline silicon solar cells [3]. Their function is to passivate the surface as well as the bulk of the wafer and to form an antireflection coating. This SiNx:H coating has to posses a high refractive index (n = 2.1) [4] for optimal antireflective conditions and at the same time a low extinction coefficient k for low absorption. Screen-printed solar cells with Hot-Wire deposited SiNx:H already have efficiencies in excess of 15.5 %, close to standard reference cells with SiNx:H deposited by PECVD [5].
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Passivation of the bulk of multicrystalline Si occurs during the firing pro
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