Factors controlling electron transport properties in transparent amorphous oxide semiconductors

Metal cation species and post-annealing treatments were extracted as the primary factors affecting the electron transport properties in transparent amorphous oxide semiconductors (TAOS). Amorphous (a-) In–Ga–Zn–O (a-IGZO) thin films were taken as an example to examine the effects of deposition conditions and thermal annealing. Two groups of a-In–GZO films (deposited at the ‘optimized’ and ‘unoptimized’ conditions) were deposited by changing laser pulse energy used for pulsed laser deposition. The electron transport properties of the as-deposited films fabricated at the unoptimalized condition were improved by thermal annealing to a level which is almost the same as that in the films deposited at the optimized condition. The temperature range effective for the improvement is ⩾300 °C, which is fairly lower than the on-set temperature (∼500 °C) for crystallization. To extract the effects of constituent metal cations, transport properties of a-Sn–Ga–Zn–O (a-SGZO) films and their TFTs were compared with those of the a-IGZO. Comparison of the TFT performances between the a-IGZO and a-SGZO channels revealed that the In-based system has much better performances when the device is fabricated without intentional annealing. Much stronger absorption tail was seen for the as-deposited a-SGZO thin films, implying the formation of a low valence state, Sn2+, and relevant defect states locating near the conduction band minimum deteriorate the TFT performances.