Impact of Carrier Profile and Rear-Side Reflection on Photoluminescence Spectra in Planar Crystalline Silicon Wafers at Different Temperatures

Abstract

Abstract-The increasing use of spectral photoluminescence as an advanced and accurate diagnostic tool motivates a comprehensive assessment of the effects of some important optical and electrical properties on the photoluminescence spectra from crystalline silicon wafers. In this paper, we present both modeling results and measurements to elucidate the effects of the internal reflectance at the planar wafer surfaces, as well as the carrier profile varying across the sample thickness due to an increased rear-surface recombination velocity, as a function of temperature. These results suggest that the accuracy of existing spectral PL techniques may be improved by using higher temperatures due to the increased effect of the carrier profile at higher temperatures. They also show that changes in the photoluminescence spectrum shape caused by the addition of a rear-side specular reflector offset those caused by changes in the carrier profile due to increased rear surface recombination, and therefore, considerable care needs to be taken when changing the rear-side optics. Finally, the possible impact of variations in the rear-side reflectance on the band-band absorption coefficient and radiative recombination coefficient, which have previously been determined using the spectral photoluminescence technique, is assessed and demonstrated to be insignificant in practice.

Micro‐photoluminescence spectroscopy on heavily‐doped layers of silicon solar cells

“…The spectra from the light and shallow profiles (samples 3 and 4) are closer to that of the undiffused sample, and thus the presence of their diffused layer is much more difficult to observe at room temperature. Note that this signature is opposite to the effects caused by non‐uniform excess carrier profile and surface reflectivity, in which the higher wavelength side of the normalized spectrum is not affected due to the very long absorption depth of photons in this wavelength region, whereas the low wavelength side is affected more significantly at higher temperatures .…”

“…Therefore, the dependence of the BB peak intensities on the excitation power is different for the two layers. Since the PL intensity ∼( N D + Δ n ) × Δ n where N D is the doping density and Δ n is the excess carrier density , the PL signal is expected to be approximately linearly proportional to Δ n in the diffused layer, but should be a quadratic function of Δ n in the silicon substrate. When the excitation power is increased, the PL signal of the diffused layer increases more slowly than that of the substrate, and thus the normalized diffused BB peak is less pronounced.…”

“…By capturing the band‐to‐band (BB) photoluminescence signal, fundamental parameters of silicon such as the band‐to‐band absorption coefficient , radiative recombination coefficient , and temperature and doping dependencies of the silicon band gap have been determined. In addition, spectral PL methods have also been employed as a characterization tool in photovoltaics, for example to extract the diffusion length of minority carriers in silicon wafers and bricks , to quantify the light trapping capability of plasmonic structures , to examine the impacts of surface reflectance and different carrier profiles on PL spectra, or to evaluate the laser‐doped layers of silicon solar cells by applying the band‐gap‐narrowing effect in heavily‐doped silicon .…”

Quantification of Sheet Resistance in Boron‐Diffused Silicon Using Micro‐Photoluminescence Spectroscopy at Room Temperature

“…At low temperatures, there is no photon reabsorption due to the very small values of the absorption coefficient in the wavelength range of interest . Thus, any change in the relative spectral shape reflects changes in electronic structures inside Si wafers.…”

“…The stronger correlation with the sheet resistance than with the total dopant concentration stems from the fact that sheet resistance is a depth‐wise integration of the product of the doping density N A and the majority carrier mobility μ . Meanwhile, PL intensity is a depth‐wise integration of Δn × (Δn + N A ), in which the excess carrier density Δn is also related to the mobilities of both majority and minority carriers . Therefore, among the three parameters (surface doping density, total dopant concentration, and sheet resistance), the intensity ratio between 1140–1250 nm and 1120–1140 nm reflects most clearly the sheet resistance of the diffused layers.…”