|IGZO; TFT; laser
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Amorphous In-Ga-Zn-O (a-IGZO) thin film transistors (TFTs) hold great potential for large area and flexible electronics. In addition to amorphous IGZO, a partially crystallized form of IGZO, referred to as c-axis aligned crystalline (CAAC) IGZO has been demonstrated, retaining the excellent uniformity of the amorphous phase while improving stability and exhibiting extremely low leakage current. Current research is focused on improving both the mobility and stability of IGZO TFTs. In this work, we seek to improve the electrical performance using controlled deposition and annealing techniques, and to understand the mechanisms for observed mobility improvements. We demonstrate a dual layer IGZO TFTs that addresses both stability and performance issues when compared to single layer structures. Devices consisted of a 310 °C deposited CAAC 20 nm thick channel layer capped by a second, 30 nm thick, 260 °C deposited amorphous IGZO layer. The TFT exhibits a saturation field-effect mobility of ~20 cm2/Vs; exceeding by >30% the mobility of 50 nm thick single layer reference TFTs fabricated with either a-IGZO or CAAC IGZO. The deposition temperature of the amorphous layer influences the mobility of the underlying CAAC transport layer. Deposited at room temperature (RT), the mobility in the 310 °C deposited CAAC layer is initially low (6.7 cm2/Vs), but rises continuously with time over 58 days to 20.5 cm2/Vs, comparable to the value observed when the amorphous layer was deposited at 260 °C. The findings are consistent with a hypothesis that the top amorphous layer has a higher solubility for impurities and/or structural defects than the underlying nanocrystalline iii transport layer and that these defects can equilibrate even at room temperature. The rate of equilibration is consistent with the known diffusivity of hydrogen. We demonstrate properties of IGZO TFTs fabricated using laser spike annealing (LSA) with a scanned continuous wave CO2 laser. For peak annealing temperatures near 430 °C and a 1ms dwell, TFTs exhibit saturation field-effect mobilities above 70 cm2/Vs with Von near -3V. This value is over 4 times higher than furnace-annealed control samples (~16 cm2/Vs). This behavior is not due to large-scale structural change of the IGZO, as X-ray data shows no change for LSA anneals up to 800 °C for 1-2 ms. However, the mobility of those devices decay from ~50 to ~10 cm2/Vs in 10 and 3 days when storing at room temperature and 50 °C, respectively. The activation energy of mobility decay constant is estimated to be ~0.6 eV, which is consistent with the activation energy of hydrogen diffusion in a-IGZO. A model linking hydrogen donors which migrate to passivate and deactivate trap states associated with oxygen vacancies after the optimized LSA anneal is proposed to explain the high mobility. This mobility is also shown to be comparable to the estimated trap-free mobility in oxide semiconductors and suggests that shallow traps can be deactivated by transient thermal annealing under optimized conditions. The observed 75 cm2/Vs is… Advisors/Committee Members: Van Dover,Robert B. (committeeMember), Kan,Edwin Chihchuan (committeeMember).