Low temperature growth of zinc oxide on insulator utilizing graphene buffer layer for transferable electronics

Abstract

Intelligent system-on-chip (SoC), which is the heterogeneous integration of devices on insulator/silicon (Si) platform and other arbitrary substrates, is considered as the most promising next-generation technology. Since the insulator and those arbitrary substrates are generally amorphous, the direct growth of crystalline semiconductor materials is extremely difficult. Hence, a breakthrough of clever growth technology is demanded. Zinc oxide (ZnO) is one of the promising metal-oxide materials for many device applications like sensors, optoelectronic devices, etc. Buffer or template layer has been widely utilized to reduce the large lattice mismatch between the grown materials and insulators or arbitrary substrates. In this study, graphene, which is flexible, transparent and possesses a similar hexagonal atomic arrangement structure to ZnO, was chosen as a buffer or template layer. Since most of the arbitrary substrates possess low- melting temperatures, the growth of ZnO had to be performed at low temperatures. Three low- temperature techniques were used; combination of thermal evaporation and oxidation, hydrothermal deposition and hot-water-beam chemical vapour deposition (CVD). For thermal evaporation, first, ZnO film with a thickness of ~350 nm was deposited, followed by oxidation treatment at 450°C in oxygen ambient. The oxidation times varied between 30 to 120 minutes. Oxidation of physically deposited ZnO was to minimize the oxygen vacancies or to increase the crystallinity of ZnO with the appearance of diffraction peaks corresponded to (0002), (10-10) and (10-11), and these peaks increased with the oxidation time up to 60 min. However, the peak intensity showed a decrease with broad FWHM of (0002) after 60 min of oxidation which was speculated to be caused by the intermixing of ZnO and graphene. For the hydrothermal process, which was carried at 90°C for 3 hours, a graphene/glass and a ZnO/ glass were used as the substrates. No growth of ZnO was obtained on graphene/glass. It was speculated that graphene with low defects might not promote the nucleation of ZnO. However, the growth of ZnO nanorods on ZnO- seeded was obtained with a considerable small FWHM of 0.2892° for the (0002) peak. However, the intensity ratios of the ultraviolet emission (Iuv) and visible emission (Ivis) for both ZnO grown by thermal evaporation combined with oxidation, and hydrothermal process were around 1.05. This suggested that defects or oxygen vacancies were still high. Finally, ZnO was grown on graphene/SiO2/Si by hot water beam CVD with a growth time ranging from 20-60 min at a fixed substrate temperature of 500°C. As expected, a ZnO layer with high crystallinity (small FWHM of 0.0743°- 0.1955° for the (0002)) was obtained. Since the location of the 2 of ZnO (0002) was close to the bulk value, this seemed to suggest less residual tensile stress compared to the other two methods. Extremely low defect of CVD-grown ZnO layer was also confirmed from Iuv/Ivis measurement, suggesting the potential for the device fabrication. The use of graphene as the buffer or template layer provides the potential for transferable electronics since the adhesion of graphene and substrate is extremely weak.