K. Hirose, K. Tanahashi, H. Takato, Y. Cho
Keywords: active dopant density, silicon solar cell, scanning nonlinear dielectric microscopy
Summary:Ion implantation has been proposed as a new process technique for low cost and high efficiency solar cell . The conversion efficiency is affected by the implantation and annealing conditions. Evaluation of carrier distribution in emitter of silicon solar cell is important to understand the detailed physical phenomenon in device. Most common type of silicon solar cell has texture structure in the front surface. Therefore, direct application of secondary ion mass spectroscopy or spread resistance analysis for evaluation of carrier distribution in emitter is difficult. Cross-sectional measurement using microscope is very useful way to characterize the inside of device. Scanning probe microscopy (SPM) techniques are powerful tools for characterizing two-dimensional carrier distribution with high lateral resolution. Especially, scanning nonlinear dielectric microscopy (SNDM)  has high capacitance sensitivity (~10−22F/√Hz) and can visualize detailed carrier distribution. In addition, SNDM can quantify the carrier density using standard sample. In this study, we quantify the carrier density distribution and then, analyze the dopant diffusion by comparing the experimental results using SNDM with simulation result. Measured emitter was formed on a p-type (boron) wafer. Mass-analyzed and ionized P was implanted at the textured surface with an acceleration voltage 10 keV and a dose of 4×1015 atoms/cm2. Annealing was conducted at 900 °C for 10 minutes to recover the crystalline and to diffuse P. The 2D carrier distribution was observed using complementary SNDM and dC/dz-SNDM measurements. The distribution can be roughly described as two distinct regions, i.e., the surface and tail regions. Point defects and dopant ions form pairs and diffuse together. In the surface region, the point defect density is lower than the impurity density, whereas the point defect density is comparable with the dopant density in the tail region. Two Gaussian functions describe the P distribution well. The tail region occupies almost the entire emitter. Effective diffusivities of 9.4×10−15 cm2/s in the surface region and 1.3×10-13cm2/s in the tail region were obtained from the fitted Gaussian function in the surface and the tail region, respectively. In addition, the 3D P distribution was estimated using the superposition principle. The estimated 3D P distribution was in good agreement with the SNDM results. Therefore, it was concluded that SNDM is a useful method for evaluation of the active dopant distribution in Si solar cells.  H. Hieslmair, I. Latchford, L. Mandrell, M. Chun, and B. Adibi, Photovolt. Int., 18, 58 (2012). Y. Cho, A. Kirihara and T. Saeki, Rev. Sci. Instrum. 67, 2297 (1996).