The isotype ZnO/SiC heterojunction prepared by molecular beam epitaxy – A chemical inert interface with significant band discontinuities

Scientific Reports, Mar 2016

ZnO/SiC heterojunctions show great potential for various optoelectronic applications (e.g., ultraviolet light emitting diodes, photodetectors, and solar cells). However, the lack of a detailed understanding of the ZnO/SiC interface prevents an efficient and rapid optimization of these devices. Here, intrinsic (but inherently n-type) ZnO were deposited via molecular beam epitaxy on n–type 6H-SiC single crystalline substrates. The chemical and electronic structure of the ZnO/SiC interfaces were characterized by ultraviolet/x-ray photoelectron spectroscopy and x-ray excited Auger electron spectroscopy. In contrast to the ZnO/SiC interface prepared by radio frequency magnetron sputtering, no willemite-like zinc silicate interface species is present at the MBE-ZnO/SiC interface. Furthermore, the valence band offset at the abrupt ZnO/SiC interface is experimentally determined to be (1.2 ± 0.3) eV, suggesting a conduction band offset of approximately 0.8 eV, thus explaining the reported excellent rectifying characteristics of isotype ZnO/SiC heterojunctions. These insights lead to a better comprehension of the ZnO/SiC interface and show that the choice of deposition route might offer a powerful means to tailor the chemical and electronic structures of the ZnO/SiC interface, which can eventually be utilized to optimize related devices.

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The isotype ZnO/SiC heterojunction prepared by molecular beam epitaxy – A chemical inert interface with significant band discontinuities

Abstract ZnO/SiC heterojunctions show great potential for various optoelectronic applications (e.g., ultraviolet light emitting diodes, photodetectors, and solar cells). However, the lack of a detailed understanding of the ZnO/SiC interface prevents an efficient and rapid optimization of these devices. Here, intrinsic (but inherently n-type) ZnO were deposited via molecular beam epitaxy on n–type 6H-SiC single crystalline substrates. The chemical and electronic structure of the ZnO/SiC interfaces were characterized by ultraviolet/x-ray photoelectron spectroscopy and x-ray excited Auger electron spectroscopy. In contrast to the ZnO/SiC interface prepared by radio frequency magnetron sputtering, no willemite-like zinc silicate interface species is present at the MBE-ZnO/SiC interface. Furthermore, the valence band offset at the abrupt ZnO/SiC interface is experimentally determined to be (1.2 ± 0.3) eV, suggesting a conduction band offset of approximately 0.8 eV, thus explaining the reported excellent rectifying characteristics of isotype ZnO/SiC heterojunctions. These insights lead to a better comprehension of the ZnO/SiC interface and show that the choice of deposition route might offer a powerful means to tailor the chemical and electronic structures of the ZnO/SiC interface, which can eventually be utilized to optimize related devices. Introduction ZnO is a promising semiconductor material for optoelectronic applications in the ultraviolet (UV) range due to its wide and direct bandgap of 3.4 eV and large exciton binding energy of 60 meV at room temperature1,2. Although p/n-homojunctions based on ZnO have been fabricated showing promising electroluminescence in the UV wavelength range3, preparing p-type ZnO is still challenging (e.g., due to low reproducibility, instability, low conductivity and limited carrier concentration1,2,4,5). An alternative approach to make use of the excellent optical properties of ZnO is to employ p/n-heterojunctions that are easy to realize by growing n-type ZnO on suitable p-type semiconductors, such as Al2O36,7, GaN8, GaAs9, Si10, and SiC11,12. Among these materials, SiC attracts great attention because it is a naturally p-type semiconductor with a bandgap of 3.0 eV (for 6H-SiC)13, has a relatively small lattice mismatch to ZnO (4–5% compared to, e.g. 18% between ZnO and Al2O3)12,14,15, and possesses many additional unique merits (e.g., high thermal stability, excellent chemical stability, high thermal conductivity, and electron mobility)13,16. It has been shown that SiC and ZnO can form high quality interfaces17 promising this material combination to be applied in and beneficial for various optoelectronic devices, such as light emitting diodes, photodetectors, and solar cells. For instance, p-SiC/i-ZnO/n-ZnO heterojunction based diodes have shown random UV lasing18; n-ZnO/p-6H–SiC heterojunction based diodes have demonstrated high photoresponsivity to UV light19; n-ZnO/n-6H-SiC layer stacks show rectifying characteristics20; for ZnO/n-4H-SiC heterojunctions rectification with high breakdown voltage and low leakage current is reported21; and the photovoltaic power conversion efficiency of n-ZnO/n-SiC/p-Si heterojunction based solar cells is significantly higher than that of n-ZnO/p-Si based devices22. In order to further improve the performance of related devices, it is necessary to gain detailed information of the chemical and electronic structure of the ZnO/SiC interface. Böhmer et al. discovered that SiOx and elemental Zn developed at the ZnO/a-SiC interface when prepared by plasma-enhanced chemical vapor deposition (PECVD), which affects band bending and leads to fill factor losses in related solar cells23. Fan et al. studied the ZnO/4H-SiC heterojunction prepared by metal-organic chemical vapor deposition (MOCVD). They report an absence of Si-O bonds at the interface, and the valence (VBO) and conduction (CBO) band offsets to be 1.61 eV and 1.50 eV, respectively14. Ashrafi et al. investigated the ZnO/6H-SiC interface prepared by pulsed laser deposition (PLD), and found a different chemical and electronic interface structure (i.e., Si-O bond formation at the interface, and a VBO of 1.38 eV)15,24. Overall, these results indicate that the chemical and electronic structures of the ZnO/SiC interface crucially depend on the ZnO preparation route employed and/or the properties of the SiC substrate. Molecular Beam Epitaxy (MBE) is one of the most popular growth techniques, due to its precise control over deposition parameters and in situ diagnostic capabilities1. However, the chemical and electronic structures of the ZnO/6H–SiC interface prepared by MBE have not been studied in detail. In this paper, we characterize the MBE-ZnO/6H–SiC interface using UV photoelectron (UPS), x-ray photoelectron (XPS) and x-ray excited Auger electron (XAES) spectroscopy and compare our findings to our previous study of the ZnO/6H-SiC interface prepared by radio frequency (RF) magnetron sputt (...truncated)


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Yufeng Zhang, Nanying Lin, Yaping Li, Xiaodan Wang, Huiqiong Wang, Junyong Kang, Regan Wilks, Marcus Bär, Rui Mu. The isotype ZnO/SiC heterojunction prepared by molecular beam epitaxy – A chemical inert interface with significant band discontinuities, Scientific Reports, 2016, Issue: 6, DOI: 10.1038/srep23106