Ultra-thin LPCVD SiNx passivated contacts: A possibility?
Gurleen Kaur1,2, Tanmay Dutta1,3, Zhi Peng Ling2, Marvic John Naval2, Mohammad S. M. Saifullah3, Rolf Stangl2, Aaron Danner1
1Spin and Energy Lab, Department of Electrical and Computer Engineering, National University of Singapore, Singapore, Singapore
/2Solar Energy Research Institute of Singapore, National Unversity of Singapore, Singapore, Singapore
/3Institute of Materials Research and Engineering, A*STAR (Agency for Science and Technology Research), Singapore, Singapore

Metal induced recombination losses at the contact are a major contributor to the losses in a solar cell. The use of passivated contacts i.e. deploying passivating tunnel layers with highly doped capping layers between Si and metal have resulted in improved efficiencies. They successfully passivate the defects at the Si surface and enhance the selectivity of the contact. The SiOx/poly-Si passivated contacts with moderate fixed charge has led to cell efficiencies > 25%. This creates a case for exploring the possibility of using ultrathin silicon nitride (SiNx) films exhibiting a higher positive fixed charge in a SiNx/poly-Si passivating contact.
 
In this work, we optimized the factors strongly influencing the passivation performance of ultrathin SiNx layer, deposited by industrial, LPCVD, including (i) film thickness, (ii) annealing conditions (temperature, time and ambient), and (iii) surface pre-treatment, for application in a passivating contact. Using methodology developed by Brendel et al., we quantify the efficiency potential of these contacts by measuring the recombination current density, Jo and the tunnelling resistance, ρc.
 
Our preliminary experiments report excellent surface passivation with high effective minority carrier lifetime (461 ms) and low Jo/side (45 fA.cm-2) by ~1.5 nm thick LPCVD SiNx films when subjected to air ambient anneal at 800°C for 30 mins. This is due to formation of high positive fixed charge density (1.5×1012 cm-2). Air ambient annealed samples have higher lifetime as compared to the forming gas annealed (208 μs) samples but also result in an increased overall film thickness (5 nm), which might be detrimental to tunneling of charge carriers. Furthermore, OH terminated passivation was found to be better than H terminated ones. FTIR and XPS have also been employed to study the material properties in more detail. Corresponding contact resistance measurements for integrating these ultra-thin SiNx films into devices with passivated contacts are currently on-going.

Area: Sub-Area 4.4: Passivated Contacts, Carrier Selective Contacts and Hetero-Junction Structures