Defect studies in as grown and irradiated zno for optoelectronic applications
Creators: name: Chan, Keng Siew
Publisher: name: The Australian National University
Year: 2015

description: ZnO is a direct wide bandgap ({u0303}3.37 eV) semiconductor that is promising for the fabrication of optoelectronic devices operating in the UV region. However, it is still difficult to reliably dope ZnO into p-type. As grown ZnO always shows a persistent n-type conductivity, which is attributed to native defects particularly O vacancies (Vo) and Zn interstitials (ZnI), and impurities such as H, Al, In and Ga. To improve the control over the conductivity in ZnO, a good understanding of the formation of defects and their interaction with impurities is required. This thesis aims to study this interaction by exploiting the large amount of defects generated from ion implantation damage. Additionally, implantation-induced voids are also well known to be used for ion cutting and impurities gettering in semiconductors. The first part of this thesis studies the formation of voids in single crystal ZnO substrates from high dose H implantation. After H implantation, a deformed layer with a uniaxial strain along the c-axis is produced. {u0303}0.12% of the implanted H remains after annealing at 950C. The implanted H also passivate the emission at {u0303}2.4 eV particularly after 600C annealing. Transmission electron microscopy (TEM) studies reveal that the H implanted region is decorated by nano-voids of diameter 2 - 40 nm. Annealing at different temperature induces noticeable changes to the shape, size and empty volume density of the nano-voids. At 800C, these nano-voids achieve thermal equilibrium shape. This allows the implementation of a reverse-Wulff construction to determine the formation energy and step energy of ZnO surfaces. In the presence of H, the O-terminated surface is found to be the more preferred surface polarity. This thesis also studies the interstitial mediated transient enhanced diffusion (TED) process in Li - containing ZnO samples after Zn implantation. During annealing at 700 - 800C, highly mobile ZnI is released as a burst from the implanted layer to displace substitutional Li in Zn site (LiZn) via a "kick-out" mechanism. This produces a significant Li depleted region in the samples. At 750C, the density of the injected ZnI is so substantial that Zn precipitates start to form. Secondary ion-mass spectrometry (SIMS) shows that the maximum concentration of LiZn getting displaced is significantly less than the implanted Zn. The Zn precipitates can therefore be accounted for a large fraction of the presumably mobile "missing" implanted ZnI that are not involved in the LiZn "kick-out" process. The later part of this thesis studies the fabrication of high quality Pd Schottky contacts to ZnO substrates. Deep level transient spectroscopy (DLTS) is subsequently implemented to investigate electrically active traps attributed to surface/subsurface defects, bulk defects, Li-related defect complexes and irradiation-induced defects. DLTS reveals surface/subsurface defects with an energy level {u0303}1.07 eV below the conduction band, which is tentatively related to Zn vacancy clusters produced by hydrogen peroxide treatment. Li implantation produces two levels at {u0303}0.55 eV and {u0303}1.25 eV below the conduction band. The {u0303}0.55 eV level is tentatively attributed to defect clusters that involved the mobile ZnI.