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

Yufeng Zhang,Nanying Lin,Marcus Bär,Yaping Li,Huiqiong Wang,Xiaodan Wang,Rui Mu,Regan Wilks,Junyong Kang

doi:10.1038/srep23106

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.

Highlights

Heterojunctions rectification with high breakdown voltage and low leakage current is reported[21]; 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 devices[22]
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 cells[23]
O-related (XPS and XAES) signals can be found on the cleaned SiC substrate, and clear C-related peaks can be observed on the ZnO/SiC samples even after 30 min ZnO deposition

Summary

Samples and Instruments

Intrinsic (i.e., prepared without dopants intentionally added) ZnO layers were deposited on n-type 6H-SiC single crystal substrates by MBE. The SiC substrates, purchased from Kejing Materials Technology Co., were cleaned using the RCA cleaning procedure[26] before transferring them into the MBE deposition chamber (base pressure < 5 × 10−9 mbar). The prepared ZnO/SiC samples were characterized using UPS, XPS, and XAES. These measurements were performed in ultra-high vacuum (base pressure of analysis chamber < 2 × 10−10 mbar) employing a UV gas discharge lamp (SPECS UVS 10/35) using the He I (21.22 eV) line and a non-monochromatized Mg Kα (1253.56 eV) x-ray tube (SPECS XR 50) as excitation sources, respectively. The cleaned SiC substrate and the prepared ZnO thickness sample series were stored and transported under N2 inert atmosphere between deposition at XMU and characterization at HZB so that air exposure was minimized

Results and Discussion

Conclusion

Author Contributions

Additional Information