Insights into Striations in N-type Czochralsky wafers investigated via low-temperature hyperspectral and temperature-dependent spectral photoluminescence

Striations in n-type Czochralski (Cz) wafers have been correlated with up to 1% absolute efficiency losses in completed solar cells [1]. Previous studies of the low-temperature photoluminescence (PL) spectra on Cz wafers correlated the striations with a defect PL band centered between 0.8 and 0.9 eV, the origin of which was suspected to be oxide nanoprecipitates [2,3]. In this study, we perform similar PL measurements on a sister wafer and reveal further insights regarding the spatial distribution and the temperature dependence of the defect. The sample was a 4 Ω.cm alkaline textured n-type Cz wafer, 170 μm thick and passivated with a stack of a-Si and SiNx. A 660 nm steady-state light-emitting diode (LED), focused via an objective lens, was used for excitation, resulting in a uniform intensity of ~2 Suns over a circular spot with a diameter of ~3 mm. The sample was cooled using a cryogenic temperature-controlled stage and positioned via a motorized XYZ stage. The PL was detected in confocal fashion via a fiber-coupled InGaAs spectrometer; PL was therefore only detected at the center of the excitation spot. Hyperspectral mappings were performed at 80 K, with a step size of 100 μm in each lateral direction and temperature-dependent measurements were performed at a single spot for each scan from 350 K to 80 K. The hyperspectral mapping was used to extract the band-to-band PL (PLBB) and defect PL (PL defect) maps. Figure 1(a) shows that the dark striations are visible in the 80 K PLBB spectra. The defect band at 80 K was relatively broad and centered at 0.85 eV [Fig. 1(d)]. The PL defect increased by an order of magnitude from 350 K to 80 K while PLBB decreased by two orders of magnitude in the same range. Both PLBB and PL defect yielded similar temperature dependencies inside and outside the dark striations. An Arrhenius fit to PLBB and PLdefect provides two activation energies, implying the defect band is actually the convolution of at least two separate defect bands [3] . The PL spectra were also collected at 12 K to have further insight about the defects related to them.. Figure 1: (a) PLBB map, (b) PLdefect map, (c) PLdefect/PLBB map, (d) PL spectra at selected locations in PLBB Our results suggest that the defects are distributed throughout the wafer, but at higher concentration in the dark striations. This is a new observation that has not been reported previously. The temperature-dependence of both PL bands indicates the presence of at least two defects. [1] G. Coletti et al., Sol. Energy Mater. Sol. Cells, vol. 130, pp. 647–651, 2014. [2] A. Le Donne et al., Appl. Phys. Lett., vol. 109, no. 3, 2016. [3] S. Binetti et al., J. Appl. Phys., vol. 92, no. 5, pp. 2437–2445, 2002.