Nano engineering of ZnO; intrinsic defects and group-I impurities

Abstract

Within the last decade, zinc oxide has become the second most published semiconductor right after Si. The popularity of ZnO is explained by its optical and electrical properties. It is highly desired for optoelectronic applications, lasers and white-light LEDs, and as a transparent conducting oxide, it is suitable for a wide range of screen, display and window applications. However, its applicability is hindered by the lack of ptype conductivity which partly arises from our inability to understand ZnO’s intriguing defect interactions.

This thesis is dedicated to nano engineering of ZnO by ion implantation. That includes studying intrinsic-, extrinsic- and ion-implantation induced defects and their interactions. We begin by defining the model system as hydrothermally (HT) grown ZnO. HT growth is a scalable way to produce ZnO, but with a large Li-contamination (2 − 5 × 1017 cm−3). With a combined use of positron annihilation spectroscopy and secondary ion mass spectrometry we have been able to identify positron annihilation signature of Li in substitutional Zn-site. We have shown that the majority of Li in HT ZnO reside in the acceptor configuration on the substitutional Zn-site, rendering HT ZnO highly resistive. Li was also shown to be the reason behind the positron annihilation lifetime discrepancies in the literature, measured in ZnO grown with different methods.

Knowing that Li resides on Zn-site, we were able to push it to interstitial site by implanting Zn. In fact, a several μm wide Li-depletion region appears behind the Znimplantation peak after annealing, which coincides with low resistive region observed by scanning spreading resistance microscopy. However, Ne/Ar implantations create only minor redistribution of Li with increasing resistivity at the implantation peak. Interestingly, O-implantation created a build-up of Li at the implantation peak with a large increase in resistivity. Therefore, the depletion of Li was attributed to Zninterstitials diffusing into the bulk and pushing Li out of the Zn-site.

Using the Li as a tracer element, we were able to determine the localization of implanted impurities in the ZnO-lattice. Zn-sublattice elements (Na, K, Al, Cd and B) showed clear Li depletion regions while inert elements did not. In addition, N was shown to exhibit similar features with both O-implantation and inert element implantations. Therefore, it was concluded that it is possible to distinguish between Zn and O-sublattice elements, and furthermore, this method shows depletion of Li in Pand Sb-implanted samples, which is in accordance with the present day understanding of the localization of these elements.

Implantation of Na, belonging to the group-I with Li, was studied in the same HT ZnO environment. As a diffusing element that substitutes Zn, Na exhibits an interaction with Li, kicking Li out from the Zn-site. This results in similar depletion of Li as with Zn interstitials. In addition, Na was shown not to have similar signature in positron annihilation spectroscopy as Li, in accordance with the larger size of the Na ion. The presence of Na was also shown to reduce the concentration of open-volume defects upon post-implant anneals.

The diffusion of Na was shown to be trap-limited, where substitutional Li acts as the main trap with a contribution from Zn-vacancies. Activation energy of 2.4 eV and a high prefactor (D0 ∼ 104 cm2/s) was extracted for the diffusion of Na, suggesting that the diffusion process is limited by the release of Na from the implantation peak.