Isochronal and isothermal diffusion experiments of gallium (Ga) in zinc oxide (ZnO) have been performed in the temperature range of 900-1050 C. The samples used consisted of a sputterdeposited and highly Ga-doped ZnO film at the surface of a single-crystal bulk material. We use a novel reaction diffusion (RD) approach to demonstrate that the diffusion behavior of Ga in ZnO is consistent with zinc vacancy (V Zn) mediation via the formation and dissociation of Ga Zn V Zn complexes. In the RD modeling, experimental diffusion data are fitted utilizing recent density-functional-theory estimates of the V Zn formation energy and the binding energy of Ga Zn V Zn. From the RD modeling, a migration energy of 2.3 eV is deduced for Ga Zn V Zn , and a total/effective activation energy of 3.0 eV is obtained for the Ga diffusion. Furthermore, and for comparison, employing the so-called Fair model, a total/effective activation energy of 2.7 eV is obtained for the Ga diffusion, reasonably close to the total value extracted from the RD-modeling.
Cuprous oxide (Cu<sub>2</sub>O) is a promising material for large scale photovoltaic applications. The efficiencies of thin film structures are, however, currently lower than those for structures based on Cu<sub>2</sub>O sheets, possibly due to their poorer transport properties. This study shows that post-deposition rapid thermal annealing (RTA) of Cu<sub>2</sub>O films is an effective approach for improving carrier transport in films prepared by reactive magnetron sputtering. The as-deposited Cu<sub>2</sub>O films were poly-crystalline, p-type, with weak near band edge (NBE) emission in photoluminescence spectra, a grain size of ~100 nm and a hole mobility of 2 - 18 cm<sup>2</sup>/Vs. Subsequent RTA (3 min) at a pressure of 50 Pa and temperatures of 600 - 1000 °C enhanced the NBE by 2-3 orders of magnitude, evidencing improved crystalline quality and reduction of non-radiative carrier recombination. Both grain size and hole mobility were increased considerably upon RTA, reaching values above 1µm and up to 58 cm<sup>2</sup>/Vs, respectively, for films annealed at 900 - 1000 °C. These films also exhibited a resistivity of ~50 - 200 Ω cm, a hole concentration of ~ 10<sup>15</sup> cm<sup>-3</sup> at room temperature, and a transmittance above 80.
Epitaxial Cu2O films grown by reactive and ceramic radio frequency magnetron sputtering on single crystalline ZnO (0001) substrates are investigated. The films are grown on both O- and Zn-polar surface of the ZnO substrates. The Cu2O films exhibit a columnar growth manner apart from a ∼5 nm thick CuO interfacial layer. In comparison to the reactively sputtered Cu2O, the ceramic-sputtered films are less strained and appear to contain nanovoids. Irrespective of polarity, the Cu2O grown by reactive sputtering is observed to have (111)Cu2O||(0001)ZnO epitaxial relationship, but in the case of ceramic sputtering the films are found to show additional (110)Cu2O reflections when grown on O-polar surface. The observed CuO interfacial layer can be detrimental for the performance of Cu2O/ZnO heterojunction solar cells reported in the literature.
Dopant diffusion of indium (In) in single crystal zinc oxide is studied by secondary ion mass spectrometry and is interpreted using a reaction-diffusion model that invokes predictions from density functional theory (DFT). An apparent activation energy of 2.2 eV is obtained for the diffusion of In, when the local Fermi-level position is about 0.2 eV below the conduction band edge. The diffusion of In is found to be significantly faster that that reported for the other group III donors, aluminum and gallium, with several orders of magnitude higher effective diffusivities, that can be assigned to a lower migration barrier for the diffusion of In. Furthermore, our results reveal self-consistency in previous DFT results of defect formation-and migration energies. From this, the diffusion of In is suggested to occur through mobile charged zinc vacancies -V Zn 2 that form intermediate mobile ( V In Zn Zn ) − pairs. The pairs in turn dissociate rather readily at the studied temperatures (850 °C-1150 °C), which results in distinct and abrupt diffusion fronts for the In depth distribution profiles.
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