Effects of oxygen flow ratio and thermal annealing on defect evolution of aluminum doped zinc oxide thin films by reactive DC magnetron sputtering

Abstract

Al doped ZnO (AZO) is a promising transparent conducting oxide to replace the expensive Sn doped In2O3 (ITO). Understanding the formation and evolution of defects in AZO is essential for its further improvement. Here, we synthesize transparent conducting AZO thin films by reactive DC magnetron sputtering. The effects of oxygen flow ratio as well as the rapid thermal annealing (RTA) in different conditions on their structural and optoelectrical properties were investigated by a variety of analytical techniques. We find that AZO thin films grown in O-rich conditions exhibit inferior optoelectrical performance as compared with those grown in Zn-rich conditions, possibly due to the formation of excessive native acceptor defects and/or secondary phases (e.g. Al2O3). Temperature-dependent Hall measurements indicate that mobilities of these highly degenerate AZO films with N > 1020 cm−3 are primarily limited by ionized and neutral impurities, while films with relatively low N ∼ 1019 cm−3 exhibit a temperature-activated mobility owing to the grain-barrier scattering. As N increases, the optical band gap of AZO thin film increases as a result of Burstein–Moss shift and band gap narrowing. RTA treatments under appropriate conditions (i.e. at 500 °C for 60 s in Ar) can further improve the electrical properties of AZO thin film, with low resistivity of ∼6.2 × 10−4 Ω cm achieved, while RTA at high temperature with longer time can lead to the formation of substantial sub-gap defect states and thus lowers the electron mobility. X-ray photoelectron spectroscopy provides further evidence on the variation of Al (Zn) content at the surface of AZO thin films with different processing conditions.

Comparative Study of Aluminum-Doped Zinc Oxide, Gallium-Doped Zinc Oxide and Indium-Doped Tin Oxide Thin Films Deposited by Radio Frequency Magnetron Sputtering

“…Post annealing is often used to improve the optoelectronic properties of TCOs even for thin films deposited with substrate heating [42][43][44][45]. Moreover, additional thermal processes are common for solar cells and other devices such as smart windows [46].…”

Preferential zinc sputtering during the growth of aluminum doped zinc oxide thin films by radio frequency magnetron sputtering

“…[33,34] Very recently it was demonstrated that gallium doped zinc oxide and ITO exhibit the same dependence of the selfbias with pressure as reported for AZO, however, only AZO exhibited the most noticeable correlation of the optoelectronic properties with the erosion track. [35] The optoelectronic and analytical properties of AZO thin films by sputtering have been intensively studied including X-ray photoelectron spectroscopy (XPS), [36][37][38][39] , X-ray diffraction (XRD) [36,[39][40][41][42] and Raman [37,40] for various parameters (pressure [43][44][45] , power [46][47][48] , target to substrate distance [32,49] , substrate material [50] , oxygen gas fraction [51,52] , substrate temperature [39] , plasma excitation mode [18,53,54] , annealing temperature, [55,56] target erosion [16] , Al doping fraction [57] ). However, so far, no conclusive correlation has been reported between the role of negative oxygen ions and the physical and chemical properties of AZO thin films as to be able to obtain resistivities below 10 -3 cm for a deposited area comparable with that of the target.…”

“…34 The optoelectronic and analytical properties of AZO thin films by sputtering have been intensively studied by X-ray photoelectron spectroscopy (XPS), 35–38 X-ray diffraction (XRD) 35,38–41 and Raman spectroscopy 36,39 for various parameters ( e.g. , pressure, 42–44 power, 45–47 target to substrate distance, 31,48 substrate material, 49 oxygen gas fraction, 50,51 substrate temperature, 38 plasma excitation mode, 18,52,53 annealing temperature, 54,55 target erosion, 16 and Al doping fraction 56 ). However, so far, no conclusive correlation has been reported between the role of negative oxygen ions and the physical and chemical properties of AZO thin films as to be able to obtain resistivities below 10 −3 Ω cm for a deposited area comparable with that of the target.…”

Selective Surface Treatment Using Atmospheric Ar Plasma Jet for Aluminum‐doped Zinc Oxide Based Transparent and Flexible Electronics

“…[16] AZO electrodes still have the disadvantage of lower conductivity compared to ITO, so various methods are needed, such as thermal annealing and plasma treatment, to improve their performance. [17][18][19][20] However, the polymer substrate used to fabricate transparent and flexible electrodes can be easily deformed by thermal stress, which makes it difficult to use the thermal annealing method. [21] Plasma treatment, which can be used at relatively lower temperatures, is considered to be a good alternative to improve sheet resistance.…”