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Title: Dissolution of Oxygen Precipitate Nuclei in n-Type CZ-Si Wafers to Improve Their Material Quality: Experimental Results

Abstract

We present experimental results which show that oxygen-related precipitate nuclei (OPN) present in p-doped, n-type, Czochralski wafers can be dissolved using a flash-annealing process, yielding very high quality wafers for high-efficiency solar cells. Flash annealing consists of heating a wafer in an optical furnace to temperature between 1150 and 1250 degrees C for a short time. This process produces a large increase in the minority carrier lifetime (MCLT) and homogenizes each wafer. We have tested wafers from different axial locations of two ingots. All wafers reach nearly the same high value of MCLT. The OPN dissolution is confirmed by oxygen analysis using Fourier transform infrared spectra and injection-level dependence of MCLT.

Authors:
; ; ; ; ; ; ;
Publication Date:
Research Org.:
National Renewable Energy Lab. (NREL), Golden, CO (United States)
Sponsoring Org.:
USDOE Office of Energy Efficiency and Renewable Energy (EERE), Solar Energy Technologies Office (EE-4S)
OSTI Identifier:
1343089
Report Number(s):
NREL/JA-5J00-66519
Journal ID: ISSN 2156-3381
DOE Contract Number:
AC36-08GO28308
Resource Type:
Journal Article
Resource Relation:
Journal Name: IEEE Journal of Photovoltaics; Journal Volume: 7; Journal Issue: 1
Country of Publication:
United States
Language:
English
Subject:
14 SOLAR ENERGY; 36 MATERIALS SCIENCE; spatial diversity; charge carrier lifetime; chemical etching; optical furnace; silicon solar cells

Citation Formats

Sopori, Bhushan, Basnyat, Prakash, Devayajanam, Srinivas, Tan, Teh, Upadhyaya, Ajay, Tate, Keith, Rohatgi, Ajeet, and Xu, Han. Dissolution of Oxygen Precipitate Nuclei in n-Type CZ-Si Wafers to Improve Their Material Quality: Experimental Results. United States: N. p., 2017. Web. doi:10.1109/JPHOTOV.2016.2621345.
Sopori, Bhushan, Basnyat, Prakash, Devayajanam, Srinivas, Tan, Teh, Upadhyaya, Ajay, Tate, Keith, Rohatgi, Ajeet, & Xu, Han. Dissolution of Oxygen Precipitate Nuclei in n-Type CZ-Si Wafers to Improve Their Material Quality: Experimental Results. United States. doi:10.1109/JPHOTOV.2016.2621345.
Sopori, Bhushan, Basnyat, Prakash, Devayajanam, Srinivas, Tan, Teh, Upadhyaya, Ajay, Tate, Keith, Rohatgi, Ajeet, and Xu, Han. Sun . "Dissolution of Oxygen Precipitate Nuclei in n-Type CZ-Si Wafers to Improve Their Material Quality: Experimental Results". United States. doi:10.1109/JPHOTOV.2016.2621345.
@article{osti_1343089,
title = {Dissolution of Oxygen Precipitate Nuclei in n-Type CZ-Si Wafers to Improve Their Material Quality: Experimental Results},
author = {Sopori, Bhushan and Basnyat, Prakash and Devayajanam, Srinivas and Tan, Teh and Upadhyaya, Ajay and Tate, Keith and Rohatgi, Ajeet and Xu, Han},
abstractNote = {We present experimental results which show that oxygen-related precipitate nuclei (OPN) present in p-doped, n-type, Czochralski wafers can be dissolved using a flash-annealing process, yielding very high quality wafers for high-efficiency solar cells. Flash annealing consists of heating a wafer in an optical furnace to temperature between 1150 and 1250 degrees C for a short time. This process produces a large increase in the minority carrier lifetime (MCLT) and homogenizes each wafer. We have tested wafers from different axial locations of two ingots. All wafers reach nearly the same high value of MCLT. The OPN dissolution is confirmed by oxygen analysis using Fourier transform infrared spectra and injection-level dependence of MCLT.},
doi = {10.1109/JPHOTOV.2016.2621345},
journal = {IEEE Journal of Photovoltaics},
number = 1,
volume = 7,
place = {United States},
year = {Sun Jan 01 00:00:00 EST 2017},
month = {Sun Jan 01 00:00:00 EST 2017}
}
  • Here, we form gallium-doped poly-Si:Ga/SiO 2 passivated contacts on n-type Czochralski (n-Cz) wafers using ion implantation of Ga and Ga-containing spin-on dopants. After annealing and passivation with Al 2O 3, the contacts exhibit i Voc values of >730 mV with corresponding Joe values of <5 fA/cm 2. These are among the best-reported values for p-type poly-Si/SiO 2 contacts. Secondary ion mass spectroscopic depth profile data show that, in contrast to B, Ga does not pileup at the SiO 2 interface in agreement with its known high diffusivity in SiO 2. This lack of Ga pileup may imply fewer dopant-related defectsmore » in the SiO 2, compared with B dopants, and account for the excellent passivation.« less
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  • The bonding at O (N) atom-terminated dangling bonds at steps in the 11[bar 2] direction is investigated by [ital ab] [ital initio] cluster calculations. We find that one Si atom and three O (N) atoms, i.e., a SiO[sub 3] (SiN[sub 3]) group, can bridge three dangling bonds such that the strain induced at the step by these bonding groups is negligible. The results help to explain the significant changes in the interface trap density [ital D][sub it] and the relative phase of the onefold and threefold contributions to the second-harmonic generation from oxide-terminated vicinal Si surfaces formed by thermal oxidationmore » at 850 [degree]C following annealing at temperatures [similar to]1000 [degree]C, where Si-O atom arrangements can readily occur.« less
  • In industrial silicon solar cells, oxygen-related defects lower device efficiencies by up to 20% (rel.). In order to mitigate these defects, a high-temperature homogenization anneal called tabula rasa (TR) that has been used in the electronics industry is now proposed for use in solar-grade wafers. This work addresses the kinetics of tabula rasa by elucidating the activation energy governing oxide precipitate dissolution, which is found to be 2.6 +/- 0.5 eV. This value is consistent within uncertainty to the migration enthalpy of oxygen interstitials in silicon, implying TR to be kinetically limited by oxygen point-defect diffusion. This large activation energymore » is observed to limit oxygen precipitate dissolution during standard TR conditions, suggesting that more aggressive annealing conditions than conventionally used are required for complete bulk microdefect mitigation.« less