I. Tarasov

Minority-carrier lifetime enhancement in edge-defined film-fed grown Si through rapid thermal processing-assisted reduction of hydrogen-defect dissociation

This paper demonstrates that a very short, 1‐s, simultaneous firing of screen-printed Al at the back and SiNx antireflection (AR) coating at the front can significantly enhance the minority-carrier lifetime in edge-defined film-fed grown (EFG) ribbon Si via SiNx-induced hydrogen passivation of defects. It was found that 1‐s firing in a rapid thermal processing system at an optimum temperature improved the average minority-carrier lifetime from 3to>80μs, resulting in ∼16% efficient 4‐cm2 screen-printed EFG Si cells. It is proposed that rapid thermal firing enhances the retention of hydrogen at defect sites by minimizing the hydrogen-defect dissociation. A combination of simulations and experiments reveals that the dissociation of hydrogen is extremely rapid at conventional firing temperatures of ∼700°C. An activation energy of 2.4–2.6eV was determined for the hydrogen-defect dissociation in EFG Si. This activation energy, in conjunction with the room-temperature photoluminescence data, suggests that the im...

Defect passivation in multicrystalline silicon for solar cells

We report on the effect of hydrogen passivation in ribbon multicrystalline silicon (mc-Si) wafers from SiNx:H anti-reflecting layer using simultaneous rapid thermal annealing of Al back-contact and SiNx anti-reflection coating on the front (RTP-Al∕SiNx). Scanning room-temperature photoluminescence spectroscopy revealed a strong inhomogeneity in the increase of minority carrier lifetime caused by the hydrogen defect passivation in mc-Si. We present experimental evidence that RTP-Al∕SiNx processing leads to strong lifetime enhancement caused by hydrogen defect passivation in low-lifetime regions of mc-Si wafers. Additional details on the hydrogenation mechanism are revealed in a course of the dehydrogenation study. Hydrogen out-diffusion shows a different rate or activation energy between high and low lifetime regions of the wafers.

Investigation of spatially non-uniform defect passivation in EFG Si by scanning photoluminescence technique

This paper shows that both hydrogenation of defects from SiN/sub x/ coating and thermally-induced dehydrogenation of defects are rapid and occur simultaneously in EFG Si during cell processing. Room-temperature scanning photoluminescence mappings, before and after the SiNx-induced hydrogenation, revealed that hydrogenation of defective regions is effective and pronounced, with more than an order of magnitude increase in lifetime, compared to the rest of the bulk. In addition, FTIR measurements showed the concentration of bonded hydrogen in the SiN/sub x/ film decreases with the increase in annealing temperature and time. However, the rate of release of hydrogen from the SiN/sub x/ film decreases sharply after the first few seconds. Based on this understanding, a process was developed for a co-firing of SiN/sub x/ film and screen-printed Al and Ag in RTP unit, which produced 4 cm/sup 2/ EFG Si cell with highest efficiency of 16.1%.

Effect of handling stress on resonance ultrasonic vibrations in thin silicon wafers

Resonance Ultrasonic Vibration (RUV) metrology offers a sensitive non-destructive real-time solution to silicon wafer crack detection. The stresses generated in the wafers by the handling device used in the RUV method may have a significant influence on the effectiveness of this method, particularly for thinner wafers. The handling stresses produced by different designs of the vacuum wafer holders and their effects on the resonance properties of the ultrasonically excited wafer are studied using Finite Element Analysis (FEA) and confirmed by RUV tests. FEA results and RUV experiments show that optimization of the wafer handling stress obtained by redesigning the wafer holder does not alter the resonance frequencies and mode shapes of the wafer significantly compared to the free vibration case. Therefore, it is possible to use RUV approach for crack detection in thin silicon wafers without significant modification.

Spatially Resolved Photoluminescence and Thermally Stimulated Luminescence in Semi-Insulating SiC Wafers

We report on non-contact and non-destructive spatially resolved characterization of traps and luminescence centers in vanadium-free semi-insulating 6H-SiC. Two optical techniques were employed: photoluminescence (PL) mapping and thermally stimulated luminescence (TSL) imaging on SiC wafers. PL and TSL topography reveal inhomogeneity at the periphery regions of the wafers. Low-temperature PL spectra show broad bands with the maxima at 1.75eV and 1.2eV, including a sharp zero-phonon line at 1.344eV. The TSL glow curves at T>80K show different peaks in the visible and infrared bands. The luminescence spectrum of the 105K TSL peak replicates 1.75eV band, while the 120K peak corresponds to the 1.2eV band. Additionally, the high temperature TSL peak at 210K shows an excellent match with 1.344eV zero phonon line. The trap energies of different peaks are calculated. We discuss a model of complex defects composed of closely spaced electron (hole) trap and UD3 defect.

Photoluminescence and Thermally Stimulated Luminescence in Semi-Insulating SiC

We performed non-contact and non-destructive spatially resolved characterization of traps and recombination centers in semi-insulat i g SiC wafers using thermally stimulated luminescence (TSL) and scanning photoluminesce ce (PL). In Vfree samples, we observe a broad PL band with a maximum at 1.18eV ac companied with a sharp zero-phonon line at 1.34 eV. Intense TSL in the visible and IR spectral regions yield a glow curve with the maximum at 110K attributed to nitrogen traps. The TSL spectrum closely resembles the PL spectrum of the 1.18 eV PL ba nd. Comparison of TSL in V-doped and V-free samples is performed. TSL images are c or lated with PL maps.

Stress diagnostics in multicrystalline silicon wafers using an acoustic technique

Residual stress is generated in silicon crystals during growth of material for use as substrates for solar cells. This stress affects yield in processing the wafers into cells and modules. We report here on the application of a resonance acoustic method, used previously to measure stress in CZ silicon wafers, to characterize multicrystalline EFG silicon ribbon wafers.

Non-contact defect diagnostics in Cz-Si wafers using resonance ultrasonic vibrations

A new resonance effect of generation of sub-harmonic acoustic vibrations was applied to characterize defects in as-grown and processed Cz-Si wafers. Ultrasonic vibrations were generated into standard 8″ wafers using an external ultrasonic transducer and their amplitude recorded in a non-contact mode using a scanning acoustic probe. By tuning the frequency, f, of the transducer we observed generation of intense sub-harmonic acoustic mode (“whistle” or w-mode) with f/2 frequency. The characteristics of the w-mode-amplitude dependence, frequency scans, spatial distribution allow a clear distinction versus harmonic vibrations of the same wafer. The origin of sub-harmonic vibrations observed on 8″ Cz-Si wafers is attributed to a parametric resonance of flexural vibrations in thin silicon circular plates. We present evidence that “whistle” effect shows a strong dependence on the wafer’s growth and processing history and can be used for quality assurance purposes.