Low Temperature Aqueous Solution Route to Reliable p‑Type Doping in ZnO with K: Growth Chemistry, Doping Mechanism, and Thermal Stability

In this paper, we identified how the growth environment chemistry can critically influence the type and nature of the incorporated K defect in ZnO films grown using the aqueous solution route, which explains the switching between p- and n-type conductivities under different doping or thermal annealing conditions. This was achieved by relating the growth environment to the structural, optical, and electrical characteristics of the films. The thermal behavior of these defects up to 700 °C confirms the proposed doping mechanism. It is found that the best route to realizing p-type conductivity is through minimizing the amount of Ki and KZn–Ki complexes because films with high concentrations of Ki have a slow p-type recovery caused by the slow out-diffusion of Ki. The highest hole concentrations for as-grown films and those that were annealed at 700 °C for 30 min were 2.6 × 1016 and 3.2 × 1017 cm–3, respectively. The upper limit for p-type doping using this route appears to be about mid-1017 cm–3. Our results show that the low temperature aqueous solution synthesis route of ZnO:K is a promising solution toward reliable p-type conductivity for future device applications.