Impacts of reduction of deep levels and surface passivation on carrier lifetimes in p-type 4H-SiC epilayers
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Impacts of reduction of deep levels and surface passivation on carrier lifetimes in p-type 4H-SiC epilayers are investigated. The authors reported that the carrier lifetime in n-type epilayers increased by reduction of deep levels through thermal oxidation and thermal annealing. However, the carrier lifetimes in p-type epilayers were not significantly enhanced. In this study, in order to investigate the influence of surface passivation on the carrier lifetimes, the epilayer surface was passivated by different oxidation techniques. While the improvement of the carrier lifetime in n-type epilayers was small, the carrier lifetime in p-type epilayers were remarkably improved by appropriate surface passivation. For instance, the carrier lifetime was improved from 1.4 μs to 2.6 μs by passivation with deposited SiO2 annealed in NO. From these results, it was revealed that surface recombination is a limiting factor of carrier lifetimes in p-type 4H-SiC epilayers.Keywords:
Passivation
Carrier lifetime
Wide-bandgap semiconductor
Surface States
The traditional passivation processes, standard RCA cleaning and 1% HF aqueous solution immersion, were introduced to passivate the surfaces of p-type Si(100) wafers for heterojunction with intrinsic thin-layer (HIT) solar cell application. The passivation stability was checked by monitoring the time decay of the effective minority carrier lifetime of the Si wafer in air via the microwave photoconductive decay (μPCD) method. The results show that the passivation effect is greatly dependent on the initial surface morphology of
the Si wafer. During the subsequent exposure in air, the obtained effective minority carrier lifetime decays rapidly, corresponding to that the Si surface states increase greatly. Such decay occurs more severely on the textured surface than on the polished one, which gives a time limitation to the subsequent processes in the HIT solar cell fabrication. So, such passivation processes need further improvement. The results also prove that the effective
minority carrier lifetime measurement can be adopted as an efficient and convenient method to check the passivation stability of the Si surface treated by wet-chemical methods.
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Stability of the passive 18-8 steel in acid solution is found to depend on the potential during the etching treatment as well as the passivation treatment. The potential of the steel during the passivation treatment is controlled by changing the concentration of nitric acid solution or by using a potentiostat. Also a constant potential of the steel during the etching treatment is attained in a dilute nitric acid solution or in a concentrated sulphuric acid solution with or without applying the external polarization. Stability which is decided by measuring the self-activation time in oxygen-free sulphuric acid solution increases with increasing the potential of the passivation treatment, and a critical potential, at which the stability changes abruptly, is admitted irrespective of the method of the passivation treatment, i. e., chemical passivation or potentiostatically controlled passivation. Structural changes of the passive film are concluded to take place at this critical potential of 0.4 volt (vs. SCE). Etching potential, also, changes the self-activation time of the passive steel treated at a constant passivation condition. The maximum stability is obtained at -0.32V of the etching potential. This fact is explained by assuming the selective enrichment of chromium on the surface bofore passivation.
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Impacts of reduction of deep levels and surface passivation on carrier lifetimes in p-type 4H-SiC epilayers are investigated. The authors reported that the carrier lifetime in n-type epilayers increased by reduction of deep levels through thermal oxidation and thermal annealing. However, the carrier lifetimes in p-type epilayers were not significantly enhanced. In this study, in order to investigate the influence of surface passivation on the carrier lifetimes, the epilayer surface was passivated by different oxidation techniques. While the improvement of the carrier lifetime in n-type epilayers was small, the carrier lifetime in p-type epilayers were remarkably improved by appropriate surface passivation. For instance, the carrier lifetime was improved from 1.4 μs to 2.6 μs by passivation with deposited SiO2 annealed in NO. From these results, it was revealed that surface recombination is a limiting factor of carrier lifetimes in p-type 4H-SiC epilayers.
Passivation
Carrier lifetime
Wide-bandgap semiconductor
Surface States
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Floating junction (FJ) passivation is a relatively recent passivation scheme which has both experimentally and theoretically demonstrated superior passivation than either oxide or back surface field passivation. In addition, it is suited not only to high efficiency laboratory cells, but also to commercial solar cells. The improvement in surface passivation for commercial cells is an especially critical issue in achieving lower cost solar cells through reducing substrate thickness while simultaneously increasing efficiency. Despite the many advantages of FJ passivation, its applicability has been limited by the apparent inability to translate the excellent modelling results into actual solar cells. The objective of this paper is to present a complete analysis of FJ passivation and to demonstrate a method by which the problems with FJ passivation can be eliminated. Experimental evidence as well as theoretical modelling demonstrates that a solar cell with an optimized rear FJ is insensitive to parasitic effects.
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The electrochemical performance of 317L stainless steel used in medicine under different conditions of passivation (different contents of HNO3 solution, different passivation time and different passivation temperatures) was studied. The results show that the pitting potential of 317L stainless steel used in medicine can reach about 1.0 V (SCE) when electrochemically tested in 0.9% NaCl solution after the steel was passivated in 30% HNO3 solution at 35℃ for 6 h, which indicates that the passivation film has a relatively strong resistance to corrosion. The results also show that the corrosion resistance of the passivation film on the surface of 317SS can be increased after suitable amount of K2Cr2O7 is added into HNO3 passivation solution.
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An environmentally acceptable Cr-and P-free passivation agent SF-563 suitable for coating of Al wares was developed.The optimized passivation technology was established,and the properties of as-obtained passivation coating were evaluated.As the results,the optimized passivation parameters are suggested as bath concentration of 2%(mass fraction),pH value of 2.5～3.5,passivation time of 60～90 s,and passivation temperature of 10～35 ℃.Upon completion of passivation,the treated Al alloy surface does not need to be washed with water and can be directly dried at a maximum temperature of 120 ℃.Besides,the high-performance chemical conversion coating obtained under the optimized passivation parameters possesses excellent performance comparable to that of hexavalent Cr passivation coating,showing promising application in pre-treatment of Al alloy wares before coating.
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