Gallium diffusion in zinc oxide via the paired dopant-vacancy mechanism

Abstract

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.

Influence of Fermi level position on vacancy-assisted diffusion of aluminum in zinc oxide

“…Similar to that observed for Al, Ga doping has been shown to yield highly conductive ZnO samples [34][35][36]. In addition, the diffusion of Ga in ZnO has been demonstrated to exhibit characteristic boxlike depth profiles similar to those observed for Al [4,25,37], although holding a slightly higher diffusivity. Figure 4 shows the Al and Ga concentration vs depth distributions for the predoped sample (see Table I) Fig.…”

“…This doubleacceptor behavior has also been indicated experimentally by comparing positron annihilation spectroscopy results with Hall effect data [10,23,24]. Furthermore, recent diffusion studies of Al [4] and Ga [25] in ZnO have demonstrated a quadratic dependence of the apparent diffusion coefficient on the concentration of Al or Ga. In Refs.…”

“…However, in our case, with similar but not identical dopants, such an approach slightly overestimates the experimental profiles (not shown). The slightly more mobile Ga dopants [25] diffuse out of the bulk (to the film), and a net loss of total dopants occurs in the bulk (deep end of the Al profiles) after Al in-diffusion (compare the Ga concentration at depths beyond the Al tail before and after diffusion). This gives a contribution to the electrochemical potential gradient and a small (retarded) deviation from an erfc behavior.…”

“…In Refs. [4,25], it was further suggested that V 2− Zn is the dominant vehicle for the diffusion of Al/Ga through the formation of an intermittent substitutional dopant-vacancy complex.…”

The temperature-dependency of the optical band gap of ZnO measured by electron energy-loss spectroscopy in a scanning transmission electron microscope

“…14 Knowledge of temperature dependence of the band gap is also essential for controlling the desired electrical and optical properties through thermal processing, e.g., the point defect formation energy depends on the change in the band gap, which indirectly influences the resulting electrical properties and dopant diffusion behavior of the material. 2,[15][16][17] It is challenging to measure the optical band gap values at elevated temperatures with conventional methods, especially with emission-based techniques such as photoluminescence (PL) and cathodoluminescence (CL). [18][19][20] CL may in some cases also probe band gap variation with spatial resolution on the nanoscale; however, the non-radiative recombination of electron-hole pairs, and/or thermal quenching of radiative channels, effectively limit the emission of light, and thereby also the suitability of these techniques at elevated temperatures.…”