Low-temperature sol-gel synthesis of high mobility and thermal stability ZnCdO alloy thin films via native defects control

Cheuk Kai Gary Kwok^1*, Kin Man Yu², Ye-Bean Cho³, Malkesh K. Patel³, Joondong Kim³, Dong-Wook Kim¹

¹Physics, Ewha Womans University, Seoul, Korea
²Physics, National Sun Yat Sen University, Kaohsiung, Taiwan
³Electrical Engineering, Incheon National University, Incheon, Korea

* Presenter:Cheuk Kai Gary Kwok, email:garykwok@ewha.ac.kr

Wide-gap transparent conducting oxides (TCOs) exhibiting a high electrical conductivity (σ>10⁴ S/cm) with a high mobility (μ>10 cm²/Vs) as well as a high optical transparency (>85%) within a wide spectral range (λ>2000 nm) in have garnered significant attention for their applications in thin film photovoltaics and optoelectronic devices, such as low-e windows, flat panel displays, thin film transistors (TFTs) and UV photodetectors. Currently commercialized and extensively used TCOs such as Sn doped In₂O₃ or Al doped ZnO can achieve a high visible transparency and a low resistivity (ρ<10^-⁴ Ω-cm), but their infrared transparency (λ<1000 nm) is significantly limited by free carrier effects due to their heavily doped degenerate nature. To overcome these limitations, novel TCOs with unique desirable properties for specific device applications have been developed by alloying two or more materials with different electronic and crystal structures. ZnO has long been recognized as a technologically promising material, especially for optoelectronic devices, UV photodetectors and transparent film transistors (TFTs), owing to its earth abundance and large exciton binding energy of 60 meV. However, nominally undoped ZnO is rather insulating with a low electron concentration and a low mobility, which hampers its applications especially in fast-switching electronics. On the other hand, undoped CdO exhibits a high electron concentration (~2×10²⁰/cm³) with a high μ of ~100 cm²/Vs. Alloying ZnO with CdO is expected to enhance the electrical properties of ZnO. However, the room temperature stable phases for ZnO and CdO are wurtzite (WZ) and rocksalt (RS), respectively, hence they may not form stable random alloy at high composition. Previous works using non-equilibrium growth techniques, namely sputtering and pulsed filter cathodic arc deposition, have successfully synthesized Zn_1-xCd_xO alloy thin films and reported thermally stable WZ-phase Zn_1-xCd_xO up to x<0.7 in an O-rich environment due to oxygen interstitials (O_i) native defects. In this work, we explore the low-cost and non-vacuum compatible sol-gel method to synthesize highly conducting and transparent Zn_1-xCd_xO alloy thin films. We demonstrated the high phase stability of WZ-alloys in the Zn-rich region (x<0.15), with a reliable n-type conductivity due to effective removal of O_x compensating acceptors by annealing. On the other hand, we achieve RS-phase Cd-rich alloys (x>0.85) with high μ of ~110 cm²/Vs which is one of the best reported for solution processes synthesized TCOs and comparable to Zn_1-xCd_xO thin films grown by high vacuum techniques. Meanwhile, absorption edge redshifts in both WZ and RS Zn_1-xCd_xO alloys with decreasing x, and a wide bandgap tunability is illustrated. This work gives implications on the formation of pure-phase WZ-ZnCdO alloy with enhanced mobility by a low-temperature solution means followed by optimal annealing. Properties of mixed-phase Zn_1-xCd_xO alloys at the mid composition range will be also discussed.

Keywords: Transparent conducting oxide, optoelectronic properties, pure phase, thermal stability