Minority-carrier lifetime and defect content of n-type silicon grown by the noncontact crucible method

• Published in: Journal of crystal growth 2014-12, Vol.407, p.31-36
• Main Authors: KIVAMBE, Maulid, POWELL, Douglas M, CASTELLANOS, Sergio, JENSEN, Mallory Ann, MORISHIGE, Ashley E, NAKAJIMA, Kazuo, MORISHITA, Kohei, MURAI, Ryota, BUONASSISI, Tonio
• Format: Article
• Language: English
• Subjects: Applied sciences; Chromium; Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Crucibles; Crystal defects; Defects and impurities in crystals; microstructure; Discharges for spectral sources (including inductively coupled plasmas); Electric discharges; Energy; Exact sciences and technology; Gettering; Growth from melts; zone melting and refining; Impurities; Ingots; Materials science; Methods of crystal growth; physics of crystal growth; Natural energy; Photovoltaic conversion; Physics; Physics of gases, plasmas and electric discharges; Physics of plasmas and electric discharges; Point defects (vacancies, interstitials, color centers, etc.) and defect clusters; Silicon; Solar cells; Solar cells. Photoelectrochemical cells; Solar energy; Structure of solids and liquids; crystallography
• ISSN: 0022-0248; 1873-5002
• DOI: 10.1016/j.jcrysgro.2014.08.021

We evaluate minority-carrier lifetime and defect content of n-type photovoltaic silicon grown by the noncontact crucible method (NOC-Si). Although bulk impurity concentrations are measured by inductively coupled plasma mass spectroscopy to be less than one part per million, homogeneously throughout the as-grown material we observe lifetimes in the ~150 mu s range, well below the theoretical entitlement of single-crystalline silicon. These observations suggest the presence of homogeneously distributed recombination-active point defects. We compare an industry-standard gettering profile to an extended gettering profile tailored for chromium extraction, to elucidate potential gains and limitations of impurity gettering. Near the ingot top, gettering improves lifetimes to 750 and > 1800 mu s for standard and extended profiles, respectively. Relatively lower gettered lifetimes are observed in wafers extracted from the ingot middle and bottom. In these regions, concentric-swirl patterns of low lifetime are revealed after gettering. We hypothesize that gettering removes a large fraction of fast-diffusing recombination-active impurities, while swirl microdefect regions reminiscent of Czochralski silicon can locally limit gettering efficiency and lifetime. Apart from these swirl microdefects, a low dislocation density of < 10 super(3) cm super(-2) is observed. The millisecond lifetimes and low dislocation density suggest that, by applying appropriate bulk microdefect and impurity control during growth and/or gettering, n-type NOC-Si can readily support solar cells with efficiencies > 23%.