Deactivation of Li by vacancy clusters in ion-implanted and flash-annealed ZnO

Abstract

Li is present in hydrothermally grown ZnO at high concentrations and is known to compensate both n-and p-type doping due to its amphoteric nature. However, Li can be manipulated by annealing and ion implantation in ZnO. Fast, 20 ms flash anneals in the 900-1400°C range result in vacancy cluster formation and, simultaneously, a low-resistive layer in the implanted part of the He-and Li-implanted ZnO. The vacancy clusters, involving 3-4 Zn vacancies, trap and deactivate Li, leaving other in-grown donors to determine the electrical properties. Such clusters are not present in sufficient concentrations after longer ͑1 h͒ anneals because of a relatively low dissociation barrier ϳ2.6± 0.3 eV, so ZnO remains compensated until Li diffuses out after 1250°C anneals.ZnO has great potential as a material for optoelectronic applications. 1,2 Hydrothermally ͑HT͒ grown ZnO material is of particular interest, as this growth method is scalable. 3 However, electronic doping issues in ZnO in general, and in HT ZnO in particular, are not fully controlled or understood. For example, the role of lithium needs to be addressed. HT ZnO is synthesized in a solution containing LiOH and is therefore abundant with Li. Lithium's lattice position decides whether it exhibits donor-or acceptorlike character in ZnO; occupying zinc sites ͑Li Zn ͒ it is an acceptor, occupying interstitial sites ͑Li i ͒ it is a donor. 4-6 This amphoteric behavior 7 explains why Li doping produces highly resistive or even semi-insulating 8 material. Interestingly, it has recently been reported that in sputtered thin ZnO films Li may act as a dominating p-type dopant. 9 However, the atomistic doping mechanism is not well understood, and the doping efficiency depends strongly on the sputtering and annealing conditions. 9 It is thus important to investigate if and how Li can be ͑i͒ stabilized as Li Zn or Li i , ͑ii͒ deactivated or gettered, or ͑iii͒ removed from the HT ZnO material. Either scenario would facilitate electronic doping, minimizing compensation by amphoteric Li.Ion implantation introduces intrinsic defects, and in some cases electronic states associated with the implanted impurity. However, activation of the implanted impurities by annealing results in limited modifications in the conductivity of the highly resistive HT ZnO, 10 presumably because of the amphoteric role of Li. The measurements in Ref. 10 were performed on highly Li-contaminated samples employing conventional anneals at temperatures Շ1000°C; these conditions were probably insufficient to remove Li from the samples, but sufficient to promote Li amphoteric behavior. In this work we report on how an extremely fast heat treatment ͑ϳ20 ms͒, so called flash annealing, influences the interaction between Li and the implantation-induced defects and how it affects the electrical properties of the ion-implanted ZnO. We have used two types of ions, He + and Li + . The former generates intrinsic defects only, whereas the latter, in addition, alters the Li concentration in the sample. Thus, we are able to ...

Defect evolution and impurity migration in Na-implanted ZnO

“…35 The defects in the as-implanted samples (e.g., A 1) are clearly larger in size than a single V Zn , and after annealing (A 2), they evolve into even bigger vacancy clusters with a size of probably at least 3-5 V Zn 's (and a corresponding amount of V O 's). 36 The results from sample A 3 appear to be closer to the V Zn line than the rest of the data, indicating a smaller contribution from Li Zn , correlating with the Li redistribution in Fig. 1(a).…”

Nitrogen and vacancy clusters in ZnO

“…This is seen in both undoped and Al/Ga-doped thin films 21,[26][27][28] as well as in studies in bulk crystals. [10][11][12]22,25 Clusters of vacancies in ZnO have been observed mainly in implantation studies 4,23,[29][30][31][32][33][34][35][36][37][38] and also after heavy irradiation. 21 These vacancy clusters have been shown to disappear (become unstable) in thermal treatments at temperatures of about 800°C, with a single exception: the N-implantation damage is stable up to at least 1000°C in ZnO.…”

Interaction between Na and Li in ZnO

“…It is known that intrinsic defects and defect complexes may be responsible for Li redistribution in ZnO during postimplantation annealing too, typically reported to occur at temperatures Ն600°C. 18 Hence, the evolution observed in Fig. 1 may be alternatively explained in terms of interaction of group-Ia dopants with implantation-induced defects and a "control" measurement is decisive.…”