Hydrothermally Grown Single-Crystalline Zinc Oxide; Characterization and Modification

Abstract

An overview of our recent results on characterization and modification of high-resistivity n-type bulk zinc oxide samples, grown by hydrothermal techniques, is given. Three specific topics are addressed; (i) the role of lithium (Li) as an electrically compensating impurity, (ii) extrinsic n-type doping by hydrogen implantation, and (iii) influence of annealing conditions on deep band emission. In (i), furnace annealing of as-grown samples at temperatures above ~800 °C is shown to cause out-diffusion of residual Li impurities and concurrently, the resistivity decreases. After annealing at 1400 °C, a resistivity close to 10 -1 Ω cm is obtained and the Li content is reduced from above 10 17 cm -3 to the mid 10 15 cm -3 range, providing evidence for the crucial role of Li as an electrically compensating impurity. For ion-implanted samples, vacancy clusters evolve during post-implant flash lamp annealing (20 ms duration) and these clusters appear to trap and deactivate Li with a resulting improvement of the n-type conductivity. However, these clusters have a limited stability and start to dissociate already after 1h at 900 °C, accompanied by a decrease in the conductivity. For topic (ii), n-type doping by hydrogen implantation is shown to enhance the conductivity by about 5 orders of magnitude already in the as-implanted state. Despite substantial loss of hydrogen, the conductivity remains stable, or even increases, after annealing up to ≥ 600 °C, and necessary conditions for doping by hydrogen are discussed. In (iii), the origin of the commonly observed deep band emission from monocrystalline zinc oxide is investigated using a concept of annealing as-grown samples in different atmospheres. A strong influence by the atmosphere and temperature is observed and the results can be interpreted in terms of dominant effects on the emission by vacancy-related defects.

Lithium and electrical properties of ZnO

“…Li is, indeed, one of the main candidates, and it has been proposed that its amphoteric behavior results in self-compensation and an increase in the resistivity [5]. Thus, Li Zn is considered a major contributor to [E A ] [21], which is strongly substantiated by the correlation between the present TAS/TDH data ( Fig. 2 and Table II) and the SIMS data in Fig.…”

Defect evolution and impurity migration in Na-implanted ZnO

“…Wafers C and D were heat treated before ion implantation (preannealed) at 1500 • C to reduce the amount of Li by at least two orders of magnitude. 19 Note that the preannealing causes an increase in the Na background concentration from <10 15 cm −3 (wafers A and B) to the 10 16 cm −3 range (wafers C and D) due to furnace contamination. A standard mechanical polishing process using diamond slurry with grain size from 5 μm down to 0.25 μm on rotating nylon suspension disks followed the preanneals of the wafers C and D in order to restore the surface quality.…”

“…If the concentration of Na incorporated beyond the implanted region would be determined by the Na Zn formation energy, one would expect similar concentrations in diffusion tails for wafers C and D as for wafers A and B provided that the E F position is the same. However, E F is high in the bulk of wafers C and D due to the lower Li concentration remaining after preannealing at 1500 • C. 19 Hence the concentration of Na i , assumed to be the mobile species, is highly suppressed in the bulk of wafers C and D and out-diffusion via the surface prevails. However, it is likely that a low diffusion flux of Na i toward the bulk still exists, as corroborated by the depletion of Li from the mid-10 15 to the mid-10 14 cm −3 range in depth interval ∼0.5-1.5 μm for sample D 3, Fig.…”

“…17,18 Thus, in n-type hydrothermally grown (HT) ZnO, which contains 10 17 Li/cm 3 , the predominant configuration is Li Zn , often resulting in highly resistive material. 19,20 In this study, the interaction between Li and Na has been investigated by implanting Na into (i) HT samples with a Li content of ∼4 × 10 17 cm −3 and (ii) HT samples subjected to postgrowth anneals reducing the Li content to the 10 15 cm −3 range. Especially, secondary ion mass spectrometry (SIMS) and positron annihilation spectroscopy (PAS) have been combined to reveal the role of zinc vacancies (V Zn ) in the interaction between Li and Na.…”

Interaction between Na and Li in ZnO

“…This excludes unambiguously that implantation-induced ͑intrinsic͒ defects only are responsible for the Li redistribution observed in Fig. 1 The high resistivity of the HT ZnO wafers used indicate that Li acts as acceptor 19 predominantly residing on zinc site. On the other hand, the implanted Na is probably randomly distributed existing in different configurations, e.g., clusters and precipitates, of limited temperature stability.…”