Passivation of Silicon Surfaces Using Atomic Layer Deposited Metal Oxides

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Passivation of Silicon Surfaces Using Atomic Layer Deposited Metal Oxides Jun Wang1, Mahdi Farrokh-Baroughi1, Mariyappan Shanmugam1, Roohollah Samadzadeh-Tarighat2, Siva Sivoththaman2, and Sanjoy Paul1 1 Department of Electrical Engineering and Computer Science, South Dakota State University, Brookings, SD-57007, USA 2 Electrical Engineering and Computer Science Department, University of Waterloo, Waterloo, N2L3G1, Canada ABSTRACT Surface passivation of silicon substrates using atomic layer deposited Al2O3 and HfO2 thin films are assessed. Al2O3 and HfO2 dielectric layers with various thicknesses were deposited on both sides of n-type (100) FZ-Si substrates (resistivity 4 – 6 Ω-cm) at 200°C by atomic layer deposition (ALD) system. The effective excess carrier lifetime of as-deposited oxide/Si/oxide structure was measured by microwave-photoconductivity-decay (MWPCD) measurement technique and it was observed that the thicker ALD dielectrics lead to higher effective excess carrier lifetime and better surface passivation. The measurements showed average excess carrier lifetime values of 302 µs and 347 µs for as-deposited Al2O3 and HfO2 passivated Si substrates with 150 ALD cycles, respectively. MWPCD and capacitance-voltage (C-V) measurements suggest that as-deposited ALD HfO2 layer leads to a better surface passivation compared to as-deposited ALD Al2O3 layer. Further, the results suggest that there exist fixed negative charges in the bulk of the ALD dielectrics and this contributes to the field effect passivation of the silicon surfaces. INTRODUCTION The need for lower cost silicon solar cells combined with rather high cost of pure Si material requires using thinner Si substrates (less than 150 µm). Surface passivation of advanced Si solar cells is becoming more important since the surface/volume ratio in thin Si substrates and the contribution of the surfaces in the overall performance of these solar cells is increasing. A well-passivated surface significantly reduces recombination of photogenerated carriers at front and back surfaces of Si substrate and enhance VOC, ISC, and even fill factor of solar cells. Recently, atomic layer deposited Al2O3 layer has been used to passivate surface of Si wafers [1]. ALD technique for surface passivation purpose is very appealing because it provides high level of control on the thickness of dielectrics, low temperature growth conditions, high quality films, and applicability over large areas. Hoex et al. studied the passivation effect of the annealed ALD Al2O3 layer on p-type Si substrates [2-4]. They showed that as-deposited Al2O3 does not show passivation effect while a medium temperature post-deposition anneal at 425 °C on ALD Al2O3 films leads to outstanding passivation of Si surfaces. They analyzed the passivation mechanism of Al2O3 and concluded that the negative fixed charge present in Al2O3 films was especially beneficial for the surface passivation of p-type c-Si wafers [5]. J. Schmidt et al. have used ALD Al2O3 film to passivate p+ emitter on n type Si sola