Defect diagnostics using scanning photoluminescence in multicrystalline silicon

In the industrial production of multicrystalline silicon (mc-Si) wafers used for solar cells, linear wire-sawing defects are sometimes generated on the surfaces of mc-Si wafers. The presence of such wire-sawing defects makes the mc-Si wafers unsuitable for the fabrication of solar cells. In this work, we first studied the nature of the linear wire-sawing defects on the mc-Si wafers, and then investigated how these wire-sawing defects were generated during the wire-sawing process. It has been found that the linear wire-sawing defects are sawing ridges and ditches on the wafer surfaces, and direct evidence has suggested that they are generated due to the presence of SiC particles embedded within mc-Si. The SiC particles form an obstacle to the movement of the sawing wire during the wire-sawing process, and the sawing wire tends to climb over the SiC obstacle, resulting in the generation of the wire-sawing defects. A model for the generation of the wire-sawing defects has been proposed. This work will be of much practical interest to the commercial mc-Si wafer production communities for solar cells.

In a multicrystalline silicon (mc-Si) wafer, trapping effects frequently occur in the carrier lifetime measurement based on the quasi-steady-state photoconductance (QSSPC) technique. This affects the accurate measurement of the carrier lifetime of an mc-Si solar cell by causing distortions at a low injection level close to the Pmax point. Therefore, it is necessary to understand this effect and effectively minimize the trapping-center density. In this study, the variations in the minority carrier-trapping effect of hydrogen at different annealing temperatures in an mc-Si were observed using QSSPC, time-of-flight secondary ion mass spectroscopy, and atom probe tomography. A trapping effect was confirmed and occurred in the grain boundary area, and the effect was reduced by hydrogen. Thus, in an mc-Si wafer, effective hydrogen passivation on the grain area and grain boundary is crucial and was experimentally proven to minimize the distortion of the carrier lifetime.

Etching methods for texturing industrial multi-crystalline silicon wafers: A comprehensive review

Photoluminescence (PL) properties of the crystalline SiO2 coated with LiNbO3/LiTaO3 multilayers were studied using the 514.5 nm line excitation. The 2.25 eV PL band observed at room temperature can be Gaussian divided into two subbands at 2.19 and 2.30 eV, which can be clearly resolved in the 77 K PL spectrum. The 2.30 eV band is z polarized with a polarization degree of 0.25 and its intensity has a linear dependence on the excitation power. No similar behavior was observed for the 2.19 eV band. Spectral analysis indicates that the two PL bands arise from optical transitions of the self-trapped excitons and E′ defect pairs in the crystalline SiO2 surface, which are induced by the photorefractive effect of the coated multilayers.

Characterization of near-surface electrical properties of multi-crystalline silicon wafers

In this paper, we investigate the influence of different chemical treatments on the surface of multicrystalline Silicon (mc-Si) wafers, for both revealing grain boundaries and intra-grain defects. Electrical characterization by light beam-induced current (LBIC) was also carried out after the treatments. Several pieces of ~2 × 2 cm2, from mc-Si wafers, were mechanically polished and chemically etched and subsequently metallized with gold (on both surfaces) by sputtering, using optimized deposition times for doing transparent electrodes suitable for LBIC mapping. The surface treatments are discussed in terms of their capabilities to reveal the crystal defects and to provide the best conditions for efficient LBIC signals directly on silicon substrates, in the absence of a p–n junction. The best surface treatments allowing both revealing crystallographic defects and permitting the measurement of highly contrasted LBIC maps is the KOH etching. A large number of intra-grain defects are clearly revealed, being their electrical activity sensitively higher than that of the grain boundaries.

It was found that the addition of MnO2 particles into the HF/HNO3/H2O system could significantly improve the texturization etching performance on multi-crystalline silicon (mc-Si) wafer. For a wide component ratio range of HF/HNO3/H2O from HF-rich to HNO3-rich, by optimizing the MnO2 usage and the etching time, the addition of MnO2 particles always reduced the texture reflectance greatly. Low weighted average surface reflectance (Ra) for the AM1.5G sun spectrum in the wavelength range of 380–1100 nm was achieved on both the slurry wire sliced (SWS) mc-Si and the diamond wire sliced (DWS) mc-Si. Due to its excellent effect and simple processing, the MnO2/HF/HNO3/H2O etching system can be expected as a candidate for high-performance texturization on mc-Si wafer, especially on DWS mc-Si wafer.

Photoluminescence (PL) spectra induced by irradiation of ultraviolet photons are compared among low-density polyethylene (LDPE), crosslinked polyethylene (XLPE), and polypropylene (PP). Three PL bands appear around 4.2, 3.6, and 3.1 eV in LDPE and XLPE, while similar three PL bands are observed at similar energies in PP. The PL spectra and their decay profiles are independent of the presence of additives and are also independent of whether the samples were crosslinked or not. These results indicate that neither the additives nor the crosslinking has any significant effects on the respective three PLs in PE and PP. When the sample was pre-irradiated by the ultraviolet photons under different atmospheres (air, O2, and vacuum), all the PL intensities decrease with the progress of the pre-irradiation regardless of whether the sample is PE or PP. Therefore, all the PLs are considered to result from impurities. In all the pre-irradiated samples, a new PL band appears at 2.9 eV, of which intensity is stronger when the oxygen partial pressure during the pre-irradiation was lower. This PL is considered to be due to photo-induced conjugated double bonds. It has also been confirmed that water-tree degradation in LDPE or in XLPE does not contribute to PL.

Photoluminescence properties of thermal SiO 2 films implanted by silicon and nitrogen ions

We performed the heat treatment of CdTe films on GaAs substrates. The effect of heat treatment on a 1.47 eV band was characterized by photoluminescence (PL) spectra. As excitation density decreases, the emission peak shift of the 1.42 eV band is estimated to be 8.5 meV. As the excitation density decreases, the decrease in the intensity of the 1.47 eV band becomes faster than that of the 1.42 eV band. The 1.47 eV band decreases in intensity with heat treatment, while the 1.42 eV band increases in intensity. These results suggest that the origin of the 1.47 eV band is annealed by heat treatment. By comparing the changes of PL spectra with and without the 1.47 eV band of CdTe films before and after heat treatment, the increase in the intensity of the 1.42 eV band is considered to be related to the annealing of the origin of the 1.47 eV band.

We performed non-contact and non-destructive spatially resolved characterizationof traps and recombination centres in two-inch-diameter p-type 4H-SiCwafers using thermally stimulated luminescence (TSL) and scanning roomtemperature photoluminescence (PL). The TSL glow-curve maximum islocated at about 190 K for the Al-doped wafers and the TSL spectrum hasa maximum at 1.8 eV, which coincides with the spectrum of the ‘red’PL band in the same crystal. The TSL intensity exhibits a noticeableinhomogeneity across the wafers. The spatial distribution shows a negativecontrast compared to PL maps, indicating a variation of concentration ofthe TSL centres across the wafer. The origin of the centres is discussed.

This paper investigates deep convolutional neural networks (CNNs) for the assessment of defects in multicrystalline silicon (mc-Si) and high-performance mc-Si wafers for solar cell production based on photoluminescence (PL) images. We identify and train a CNN regression model to forecast the I-V parameters of passivated emitter and rear cells from given PL images of the as-cut wafers. The presented end-to-end model directly processes the PL image and does not rely on the human-designed image feature. Domain knowledge is replaced by a model based on a huge variety of empirical data. The comprehensive dataset allows for the evaluation of the generalizability of the model with test wafers from bricks and manufacturers not presented in the training set. We achieve mean absolute prediction errors as low as 0.11%abs in efficiency for test wafers from “unknown” bricks, which improves handcrafted feature-based methods by 35%rel at simultaneously lower computational costs for prediction. Samples with high prediction errors are investigated in detail showing an increased iron point defect concentration.

Deep level photoluminescence (PL) associated with the dominant midgap donor EL2 in semi-insulating (SI) GaAs crystals has been observed for the first time at room temperature. A broad emission band with a peak at 0.65 eV was observed always in commercial undoped SI GaAs wafers. The association of the 0.65 eV band with the EL2 level has been verified on the basis of the semiquantitative agreement of the distribution on wafers between the PL intensity and the EL2 absorption. Room-temperature PL spectroscopy can be used as a powerful tool to investigate the nonuniformity of the EL2 distribution on commercial wafers with a high spatial resolution.

The preparation and characterization of La doped TiO2 nanoparticles and their photocatalytic activity

Advanced metal-oxide-semiconductor (MOS) and metal-insulator-semiconductor (MIS) devices employ ultra thin dielectric gate layers. The physical dimensions are in the range of single digit to double digit nanometers. The effective oxide thickness (EOT) is significantly less than 10 nm. Pure SiO2 or combinations of SiO2and SiN layers are typically used as gate dielectrics. High dielectric constant materials (high-k dielectrics) and metal gates are also frequently used, depending on chip design.Physical dimensions of dielectric layer(s) are typically measured using ellipsometry and high resolution cross-section transmission microscopy (HRXTEM). Electrical characteristics are typically characterized using either contact or non-contact C-V and I-V measurements tools. Non-contact characterization techniques are generally preferred for in-line monitoring applications. However, spatial resolution of non-contact C-V and I-V measurement techniques is in the range of a few millimeters, which are 3 orders of magnitude larger than typical dimensions of actual devices. High spatial resolution, non-contact dielectric/Si interface quality characterization techniques must be developed.In this paper, ultra thin (~7 nm) SiO2/Si and ultra thin SiN/SiO2/Si wafers were prepared by low pressure thermal chemical vapor deposition (CVD) and thermal oxidation. Dielectric/Si interface quality was characterized by room temperature spectroscopic photoluminescence (PL) measurements. The PL intensity and spectra from Si were measured under different excitation wavelengths which have different probing depths. PL is an optical characterization technique which does not require any sample preparation and physical contact. Typical spatial resolution is in the range of a few microns. Small variations in dielectric/Si interface quality modulates electronic carrier behaviors significantly which results in large variations in PL intensity and spectra. Significant variations in PL intensity and spectra were measured from ultra thin (~7 nm) SiO2/Si wafers prepared by and low pressure thermal CVD and thermal oxidation. Si wafers with thermal oxide films generally showed significantly stronger PL intensity than Si wafers with thermal CVD grown oxide films (Fig. 1). Trends in PL intensity and spectra indicate the SiO2/Si interface quality of the thermally oxidized SiO2/Si is superior to that of the thermal CVD grown SiO2/Si (Fig. 2). PL characterization is found to be very sensitive to the integrity of dielectric/Si interface and thus, is promising as an in-line, non-contact interface quality monitoring technique.The excitation wavelength dependence of PL intensity and spectra of SiO2/Si and SiN/SiO2/Si will be discussed along with conventional physical and electrical characterization results.