Surface recombination evaluation of a silicon wafer by using the photoluminescence intensity ratio method for solar cell applications

Photoluminescence (PL) was one of the useful characterization techniques for direct-band-gap and high-quantum-efficiency materials in the past. PL is now used to characterize silicon epitaxial layers or wafers in a nondestructive and contactless way at room temperature. In this research, we use a Silicon Photoenhanced Recombination (SiPHER) PL system with a 532 nm or a 827 nm wavelength. The high-intensity laser source of 1 μm or 1 mm in diameter was able to map the entire wafer at modest resolution or scan a smaller area at higher resolution. This PL system detects two signals, the PL intensity and the surface reflectance (SR) intensity, from the test sample. The PL intensity strongly depends on the surface recombination velocity, bulk lifetime, and trap density. The SR intensity depends on surface condition, layer thickness and optical properties. We have determined the carrier distribution in semiconductors following light excitation and use these distributions to calculate the PL signals to extract various surface recombination velocities and other recombination components. We focus on how the surface recombination varies widely even in high-lifetime wafers or layers because it depends on the state of the surface, i.e., bare, passivated, contaminated, oxidized, etc. The surface recombination velocity (Sr) has a major effect on the lifetime. If the number of bulk traps is low; hence, the Sr can be extracted from the PL measurement. To determine the Sr, we measured the PL intensity ratio (RPL) and optical reflection at the two exciting wavelengths. We prepared various treated surface samples and measured the PL, SR, and RPL; then, we quantized surface recombination velocity for each sample. The values of the Sr were 2 x 10⁴, 6 x 10⁴, and 8 x 10⁴ cm/s for the control, the soft scrubbed sample, and the hard-scratched sample, respectively.