Abstract
Highly phosphorous doped epitaxial emitters grown using low-temperature PECVD (LT-PECVD) process have been investigated after subjecting them to different rapid thermal processing (RTP) treatments. Cross-sectional High Resolution Transmission Electron Microscopy (HRTEM) was used to analyze the interface quality and the changes in the atomic arrangement due to the different RTP treatments. Three different RTP temperature profiles were used for this study: (i) ?Pulsed Profile? with high heating and cooling rates of 70 and 35?C/s and with a peak temperature of 750?C applied in 4 cycles of 25 s each, (ii) ?Two-Step Profile?, where the temperature is ramped to 400?C at a rate of 37.5?C/s, stabilized for 10 minutes, ramped again go 750?C where it is annealed for 60s, and (iii) ?Multi-step Profile?, in which a peak annealing temperature of 850?C is reached after undergoing several intermediate temperatures and different ramp rates. Hydrogen effusion induced by the RTP treatments, its influence on the quality of the epitaxial emitter at both interface and the bulk are investigated. Secondary Ion Mass Spectroscopy (SIMS) was used to monitor the phosphorous concentration and to estimate the hydrogen content. Cross-sectional HRTEM confirmed that a very high quality single crystal epitaxial emitter is results from our LT-PECVD proccess. The developed epitaxial emitter is very promising for LT Si solar cells as confirmed by spectral response on the prepared test structures. SIMS estimates indicate that the hydrogen concentration gets reduced by more than one order of magnitude after the RTP treatment with a pulsed profile. The strong hydrogen effusion with RTP treatments having high heating and cooling rates (the pulsed and 2-step profiles) affects both the interface and the bulk quality of the n + p junction test structures. The defect density seems to increase resulting in more recombination centers. RTP treatment with lower ramps (the multi-step profile) is found to be beneficial.