Low-temperature micro-photoluminescence spectroscopy on laser-doped silicon with different surface conditions

Abstract

Low-temperature micro-photoluminescence spectroscopy (μ-PLS) is applied to investigate shallow layers of laser-processed silicon for solar cell applications. Micron-scale measurement (with spatial resolution down to 1 μm) enables investigation of the fundamental impact of laser processing on the electronic properties of silicon as a function of position within the laser-processed region, and in particular at specific positions such as at the boundary/ edge of processed and unprocessed regions. Low-temperature μ-PLS enables qualitative analysis of laser-processed regions by identifying PLS signals corresponding to both laser-induced doping and laser-induced damage. We show that the position of particular luminescence peaks can be attributed to band-gap narrowing corresponding to different levels of subsurface laser doping, which is achieved via multiple 248 nm nanosecond excimer laser pulses with fluences in the range 1.5-4 J/cm 2 and using commercially available boron-rich spin-on-dopant precursor films. We demonstrate that characteristic defect PL spectra can be observed subsequent to laser doping, providing evidence of laser-induced crystal damage. The impact of laser parameters such as fluence and number of repeat pulses on laserinduced damage is also analyzed by observing the relative level of defect PL spectra and absolute luminescence intensity. Luminescence owing to laser-induced damage is observed to be considerably larger at the boundaries of laser-doped regions than at the centers, highlighting the significant role of the edges of laser-doped region on laser doping quality. Furthermore, by comparing the damage signal observed after laser processing of two different substrate surface conditions (chemically-mechanically polished and tetramethylammonium hydroxide etched), we show that wafer preparation can be an important factor impacting the quality of laser-processed silicon and solar cells.

Sub-Bandgap Luminescence from Doped Polycrystalline and Amorphous Silicon Films and Its Application to Understanding Passivating-Contact Solar Cells

“…15 Moreover, sub-bandgap PL spectra have been employed to investigate the nature of various defects in c-Si wafers, such as dislocations, 16−20 metal precipitates, 21 oxygen precipitates, 22−25 or laser-induced defects. 26,27 In addition, some deposited thin films acting as both surface passivation and antireflection coating layers have also been demonstrated to emit strong PL signals at low temperatures. Their PL spectral shapes, peak locations, and intensities are very sensitive to the film composition and fabrication conditions, thus allowing the extraction of various film properties such as refractive indices and thicknesses.…”

“…Band-to-band photoluminescence (PL) spectra from c-Si have been used to extract fundamental parameters of the material such as the band-to-band absorption coefficient, − the radiative recombination coefficient, ,, temperature and doping dependencies , of the c-Si bandgap, dopant concentrations, , diffusion lengths of minority carriers in c-Si wafers , and bricks, as well as the light trapping capability of various plasmonic structures . Moreover, sub-bandgap PL spectra have been employed to investigate the nature of various defects in c-Si wafers, such as dislocations, − metal precipitates, oxygen precipitates, − or laser-induced defects. , In addition, some deposited thin films acting as both surface passivation and antireflection coating layers have also been demonstrated to emit strong PL signals at low temperatures. Their PL spectral shapes, peak locations, and intensities are very sensitive to the film composition and fabrication conditions, thus allowing the extraction of various film properties such as refractive indices and thicknesses. − …”

Review of Laser Doping and its Applications in Silicon Solar Cells

“…It is also clear from the literature that significant defect formation is only observed when laser irradiation surpasses the silicon melting threshold which can be regulated by the laser fluence, and that recrystallization velocity is a critical parameter for defect formation. In particular, dislocation density and oxygen introduction have been reported to have an impact on the surface electrical and optical properties of solar cells [94], [116]. Recently, Sun et al [78] determined that less than 20% degradation of the surface can be attained when restricting the laser-induced dislocation concentration to ∼10 6 cm −2 .…”

Performance of Large Area n-TOPCon Solar Cells with Selective Poly-Si Based Passivating Contacts Prepared by PECVD Method

“…Previous studies have demonstrated that noticeable defect formations occur only when laser irradiation surpasses the silicon melting threshold, with the recrystallization rate being a crucial parameter for defect generation. Specifically, the surface electrical and optical properties of solar cells are influenced by dislocation density and oxygen incorporation [34,35]. Generally, the types of defects commonly observed depend largely on two factors: (1) initial concentration of external impurities (carbon, oxygen, nitrogen, dopants); and (2) structural defects in the silicon crystal, which directly impact its melting threshold [36].…”