Controlling electrical and optical properties of wurtzite CdxZn1-xO with high Cd contents via native defects manipulation by low-temperature annealing

Bandgap energies in wurtzite (WZ) structured CdxZn1-xO alloys are known to decrease with increasing Cd content (x). Our previous work demonstrated that WZ-CdxZn1-xO alloys with a high Cd content of x similar to 0.6 and a low gap of 2eV can be stabilized by oxygen interstitials when grown in an O-rich environment. However, such O-rich WZ-CdxZn1-xO alloys have poor electrical properties due to compensating native defects. In this work, we synthesized pure WZ phase CdxZn1-xO thin films with different Cd contents by magnetron sputtering in an oxygen-rich environment. Changes in structural, electrical, and optical properties of these O-rich wurtzite CdxZn1-xO after rapid thermal annealing were investigated. While alloys with a low Cd composition of 0.2 can maintain a pure wurtzite structure up to 500 degrees C, phase separation occurs at a lower annealing temperature of similar to 400 degrees C for Cd-rich (x=0.6) films. Isochronal and isothermal annealing studies reveal the kinetics of native defects in these alloys. Highly mobile hydrogen interstitial donor defects, oxygen interstitials, and more stable cation vacancies outdiffuse sequentially as the annealing temperature increases from <300 to >400 degrees C. By exploiting the difference in the energy barrier between acceptor defects removal and phase separation, a pure wurtzite phase alloy with a low bandgap of 2eV and decent electrical properties was realized by annealing O-rich WZ-Cd0.6Zn0.4O at 300 degrees C with an extended annealing duration of >100s. These results demonstrate a practical way to obtain low-gap oxide semiconductors with strong optical absorption and controllable electrical conductivities. Published under an exclusive license by AIP Publishing.

Automated summary

Bandgap energies in wurtzite structured CdxZn1-xO alloys decrease with increasing Cd content. This study synthesized pure wurtzite phase CdxZn1-xO thin films with different Cd contents in an oxygen-rich environment. The changes in structural, electrical, and optical properties of these films after rapid thermal annealing were investigated. The results demonstrate a practical way to obtain low-gap oxide semiconductors with strong optical absorption and controllable electrical conductivities.