Comparison of Silicon Photoluminescence and Photoconductive Decay for Material Quality Characterization
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Comparison of Silicon Photoluminescence and Photoconductive Decay for Material Quality Characterization Steven Johnston1, Richard Ahrenkiel2, Pat Dippo1, Matt Page1, and Wyatt Metzger1 1 National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO, 80401 2 University of Denver, 2112 E. Wesley Ave., Denver, CO, 80208 ABSTRACT Minority-carrier lifetime in silicon directly relates to defect- and impurity-related recombination, and thus gives a measure of material quality. Lifetime measurements are useful in research laboratories and commercial production environments as an indicator for process development and quality control. While photoconductivity (PCD) techniques for measuring lifetime are commercially available, there has recently been interest in using photoluminescence (PL) to characterize lifetime in silicon because of the measurement speed to image an entire wafer and higher mapping resolution. The intensity of band-to-band PL is theoretically proportional to the effective bulk lifetime in low-injection conditions if carrier diffusion and reabsorption are neglected, surface recombination is small, and silicon properties, such as carrier concentration and the radiative recombination coefficient, are constant. We show data that compare lifetimes from PCD techniques to PL intensity for varying-resistivity, single-crystal silicon. Surface conditions are also varied (native oxide, thermal oxide, and HF etch/methyliodine solution), and the measured lifetimes are compared to corresponding PL intensity. INTRODUCTION Carrier recombination lifetime is an important parameter related to efficiency in silicon solar cells. Measurement techniques include transient photoconductive decay [1,2], transient free-carrier absorption [3], and quasi-static photoconductance [4]. Because these techniques typically measure a single point or a large spot, measuring lifetime uniformity on large samples often requires a series of individual measurements that can be time consuming, even for lowresolution maps. Recently, imaging techniques such as modulated free-carrier absorption [5,6] and photoluminescence (PL) mapping and imaging [7-15] can produce high-resolution maps in about one second. Here, we discuss PL imaging and how the PL intensity relates to lifetime measured by transient photoconductive decay techniques. Minority-carrier recombination in silicon is dominated by non-radiative transitions at room temperature. Siliconís intrinsic radiative recombination coefficient (B) is near 2x10-15 cm3/s [16], which results in lifetimes of many milliseconds for typical doping levels in solar cells of about 1016 cm-3. Being weak but measurable, the amount of PL from a sample is proportional to the radiative recombination efficiency, which is given by the radiative recombination rate divided by the total recombination rate, as shown in Eq. 1. 1 PL ∝
1
τ rad
τ rad
+
1
τ nr
≈
τ nr = BNτ nr τ rad
(1)
Here, τrad is the radiative recombination lifetime, and τnr is the non-radiative recombination lifetime. In the denominat
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