595 research outputs found
Silicon Surface Passivation by Mixed Aluminum Precursors in Al2O3 Atomic Layer Deposition
No full textDimethylaluminum chloride (DMACl) is a cost-effective aluminium precursor alternative to conventional trimethylaluminium (TMA) for Atomic Layer Deposited (ALD) Al2O3. The DMACl water process shows better passivation after high temperature firing when compared with conventional TMA water process. However, after low-temperature post-anneal its passivation quality is slightly worse than with TMA. Here we show that a mixed use of TMA and DMACl precursors in the ALD process results in better surface passivation both after 400 °C post-anneal and after an 800 °C firing step. The high-quality passivation results from the low interface defect density and high negative charge at the surface. Specifically, we investigate the role of chlorine in the ALD Al2O3 passivation by varying the TMA and DMACl pulse proportions.Peer reviewe
Effect of Different ALD Al2O3 Oxidants on the Surface Passivation of Black Silicon
No full textAbstractWe study how different oxidants in atomic layer deposition of aluminium oxide (ALD Al2O3) affect the surface passivation of black silicon. Here we show that processes using ozone cause higher fixed charge but surprisingly lead to lower lifetimes in black silicon samples as compared to water-based samples. In planar samples however, the best surface passivation is reached with O3-based processes. In case of water as oxidant, the planar wafers suffer from severe blistering and poorer surface passivation, while this seems to be the best process for black silicon. To find a reason for the lifetime differences we also study different Al2O3 stacks where both H2O and O3 are used as oxidants. In conclusion, surface texture seems to affect the optimal oxidant in the ALD process
Detection of Microcracks in Cz-Si Wafer Manufacturing by Photoluminescence Imaging
No full textPhotoluminescence imaging (PLI) is a widely accepted, fast, and contactless method for detecting crystal defects in crystalline silicon solar cells and solar-grade silicon wafers. However, it is less known by semiconductor wafer manufacturers despite the similarities between photovoltaic (PV) and semiconductor wafers. This study focuses on the detection of microcracks by PLI during high-quality Czochralski silicon (Cz-Si) wafer manufacturing. The results show that in case of low resistivity (<25 mΩ cm) wafers, microcracks can be detected at any stage of the processing—even after diamond-wire slicing. When resistivity increases, visibility of microcracks reduces in process steps that produce uneven surfaces. Nevertheless, they can still be detected after slurry-wire slicing, lapping, alkaline etching, and polishing. According to the results, unlike resistivity, other material parameters such as dopant species, crystal orientation, and wafer thickness have no similar impact on visibility of microcracks in PLI. Furthermore, all wafers produce a decent photoluminescence (PL) signal without a need for separate sample preparation. Based on these results, general recommendations for the in-line detection of microcracks for Cz-Si wafer manufactures are provided. While this study focuses on microcracks, the results and discussion include broader perspectives on the defect characterization in Cz-Si wafer manufacturing via PLI.Peer reviewe
Surface passivation of black silicon phosphorus emitters with atomic layer deposited SiO2/Al2O3 stacks
No full textBlack silicon (b-Si) is a promising surface structure for solar cells due to its low reflectance and excellent light trapping properties. While atomic layer deposited (ALD) Al2O3 has been shown to passivate efficiently lightly-doped b-Si surfaces and boron emitters, the negative fixed charge characteristic of Al2O3 thin films makes it unfavorable for the passivation of more commonly used n+ emitters. This work studies the potential of ALD SiO2/Al2O3 stacks for the passivation of b-Si phosphorus emitters fabricated by an industrially viable POCl3 gas phase diffusion process. The stacks have positive charge density (Qtot = 5.5·1011 cm-2) combined with high quality interface (Dit = 2.0·1011 cm-2eV-1) which is favorable for such heavily-doped n-type surfaces. Indeed, a clear improvement in emitter saturation current density, J0e, is achieved with the stacks compared to bare Al2O3 in both b-Si and planar emitters. However, although the positive charge density in the case of black silicon is even higher (Qtot = 2.0·1012 cm-2), the measured J0e is limited by the recombination in the emitter due to heavy doping of the nanostructures. The results thus imply that in order to obtain lower saturation current density on b-Si, careful optimization of the black silicon emitter profile is needed.Peer reviewe
Recombination activity of light-activated copper defects in p-type silicon studied by injection- and temperature-dependent lifetime spectroscopy
No full textThe presence of copper contamination is known to cause strong light-induced degradation (Cu-LID) in silicon. In this paper, we parametrize the recombination activity of light-activated copper defects in terms of Shockley—Read—Hall recombination statistics through injection- and temperature dependent lifetime spectroscopy (TDLS) performed on deliberately contaminated float zone silicon wafers. We obtain an accurate fit of the experimental data via two non-interacting energy levels, i.e., a deep recombination center featuring an energy level at Ec−Et=0.48−0.62 eVEc−Et=0.48−0.62 eV with a moderate donor-like capture asymmetry (k=1.7−2.6) k=1.7−2.6) and an additional shallow energy state located at Ec−Et=0.1−0.2 eVEc−Et=0.1−0.2 eV, which mostly affects the carrier lifetime only at high-injection conditions. Besides confirming these defect parameters, TDLS measurements also indicate a power-law temperature dependence of the capture cross sections associated with the deep energy state. Eventually, we compare these results with the available literature data, and we find that the formation of copper precipitates is the probable root cause behind Cu-LID.Peer reviewe
Formation kinetics of copper-related light-induced degradation in crystalline silicon
No full textLight-induced degradation (LID) is a deleterious effect in crystalline silicon, which is considered to originate from recombination-active boron-oxygen complexes and/or copper-related defects. Although LID in both cases appears as a fast initial decay followed by a second slower degradation, we show that the time constant of copper-related degradation increases with increasing boron concentration in contrast to boron-oxygen LID. Temperature-dependent analysis reveals that the defect formation is limited by copper diffusion. Finally, interface defect density measurements confirm that copper-related LID is dominated by recombination in the wafer bulk.Peer reviewe
Modeling Field-effect in Black Silicon and its Impact on Device Performance
No full textBlack silicon (b-Si) has improved the performance of solar cells and photodetectors due to the excellent optics and surface passivation achieved with atomic layer deposition (ALD) dielectric films. One major reason for the success is the strong field effect caused by the high density of fixed charges present in the dielectric. Depending on the device, the field effect can be utilized also in a more active role than for mere surface passivation, including the formation of floating and/or induced junctions in silicon devices. However, in order to utilize the field effect efficiently, a deeper understanding of the thin-film charge-induced electric field and its effects on charge carriers in b-Si is required. Here, we investigate the field effect in b-Si using the Silvaco Atlas semiconductor device simulator. By studying the electric field and charge-carrier profiles, we develop a model where the electrical properties of b-Si can be approximated with a planar surface, which significantly simplifies the device-level simulations. We validate the model by simulating the spectral response of a b-Si -induced junction photodiode achieving less than 1% difference compared with experimental device performance in a wide range of wavelengths. Finally, we apply the model to study how variation in surface recombination velocity affects the short-wavelength sensitivity and dynamic range in a b-Si photodiode.Peer reviewe
Fs-laser significantly enhances both above- and below-bandgap absorption in germanium
No full textFs-laser irradiation is a promising fabrication method for future broadband optoelectronic applications as it creates antireflective micro- and nanoscale structures on semiconductor surfaces and introduces below-bandgap absorption; however, its application has mainly been limited to silicon. This paper demonstrates that fs-laser technology enables high optical absorption both above and below the bandgap in germanium (Ge). With optimized laser parameters, we achieve a maximum above-bandgap absorptance of 95% and over 70% below-bandgap absorptance, due to the creation of surface microstructures and structural defects, respectively. Raman spectroscopy reveals that under intense laser irradiation, Ge may undergo a phase transition to structures with a narrower bandgap extending the absorption to the mid-infrared region. Furthermore, we develop a hyperdoping process using Ti coating pre-laser processing followed by rapid thermal annealing, which results in 90% above-bandgap absorption and a 12% relative increase in below-bandgap absorption along with a high degree of crystallinity. The increased below-bandgap absorption is attributed to Ti doping and is twice as high as reported earlier. Our findings should have significant implications for the future Ge-based infrared applications.Peer reviewe
Cu gettering by phosphorus-doped emitters in p-type silicon: Effect on light-induced degradation
No full textThe presence of copper (Cu) contamination is known to cause relevant light-induced degradation (Cu-LID) effects in p-type silicon. Due to its high diffusivity, Cu is generally regarded as a relatively benign impurity, which can be readily relocated during device fabrication from the wafer bulk, i.e. the region affected by Cu-LID, to the surface phosphorus-doped emitter. This contribution examines in detail the impact of gettering by industrially relevant phosphorus layers on the strength of Cu-LID effects. We find that phosphorus gettering does not always prevent the occurrence of Cu-LID. Specifically, air-cooling after an isothermal anneal at 800°C results in only weak impurity segregation to the phosphorus-doped layer, which turns out to be insufficient for effectively mitigating Cu-LID effects. Furthermore, we show that the gettering efficiency can be enhanced through the addition of a slow cooling ramp (-4°C/min) between 800°C and 600°C, resulting in the nearly complete disappearance of Cu-LID effects
Charged thin film enables dopant free ohmic metal–semiconductor contact formation
No full textOhmic contacts are conventionally achieved by externally doping the surface of a semiconductor substrate underneath a metal contact. To avoid the inconveniences that come with heavy doping, we propose an alternative way of achieving an ohmic Al-Si contact utilizing a highly charged atomic layer deposited (ALD) Al2O3 thin film. The idea is to utilize the negative charge of ALD Al2O3 to attract holes towards the surface of the Si substrate and thereby induce a p + region and consequently an Al/p + Si contact. The results show that the Al2O3 induced contacts are not only ohmic, but also have a low contact resistivity of 0.24 mΩ⋅cm2. This matches the requirements of various electron devices such as photodiodes indicating potential for the proposed contact formation method.Peer reviewe
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