Passivation of Surface Recombination at the Si-Face of 4H-SiC by Acidic Solutions

Abstract

We carried out carrier lifetime measurements for 4H-SiC single crystals in aqueous solutions with various pH by the microwave photoconductivity decay method. For both n-and p-type 4H-SiC, carrier lifetimes measured by Si-face excitation were longer in acidic aqueous solutions compared with carrier lifetimes measured in other solutions. On the other hand, for C-face excitation, carrier lifetimes did not depend on immersion solutions. These results indicate that carrier recombination centers at the surface of the Si-face was passivated by hydrogen ions. We also estimated surface recombination velocities Ss by a numerical analysis. S of the Si-face was reduced from 700 cm/s (in Na Silicon carbide (SiC) has attracted attention for high-power, highfrequency and high-temperature electronic devices due to its superior electrical and physical properties.1 In particular, bipolar devices based on 4H-SiC single crystals are operable in higher voltage ranges than conventional Si devices and thus development of bipolar devices is highly expected. Furthermore, SiC is applicable as photocathode for solar-to-hydrogen energy conversion. 2,3 In these applications, a carrier lifetime is an essential parameter for optimum design of the devices.1,2 So far, a point defect called the Z 1/2 center in n-type 4H-SiC single crystals has been identified as a dominant carrier recombination center.4-6 On the other hand, the carrier lifetime depends on surface recombination as well as bulk recombination induced by point defects and dislocations. Therefore, understanding of surface recombination is important for controlling the carrier lifetime and the surface recombination velocities Ss have been estimated for the Si-and C-faces of 4H-SiC.6-10 However, passivation methods for surface recombination on SiC have been rarely reported. In the case of Si, formation of surface oxide layers or immersion into HF suppress surface recombination and they have been utilized in practical applications. 11-13For 4H-SiC, although formation of high quality oxide layers is applicable to suppress surface recombination, 14,15 processes to form such layers are relatively complicated. In addition, SiC with insulating oxide layers cannot operate as photocathodes. Therefore, other methods to passivate SiC surfaces are required and, to establish passivation methods, we need to understand physics and chemistry for surface recombination phenomenon. In this report, we focus on passivation of surfaces of 4H-SiC single crystals by aqueous solutions. We measured carrier lifetime for 4H-SiC in various aqueous solution and discussed effects of the solutions on the surface condition. ExperimentalSamples used in this work were freestanding n-and p-type 4H-SiC epilayers which were originally grown with ∼150 μm thick on the (0001) Si-face with 4• woff angle toward <11 20> of bulk single crystal 4H-SiC substrates and then the substrates were completely polished. The n-type samples have a nitrogen donor concentration of 1 × 10 15 cm −3 , while the p-type samples have an aluminum ac...

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