The effect of annealing in forming gas on the a-IGZO thin film transistor performance and valence band cut-off of IGZO on SiNx

• The electrical properties of a-IGZO TFTs annealed in forming gas at 350 °C are studied.
• A shift in valence band cut-off relative to as-deposited sample is observed.
• The concentration of different cations varied for as-deposited and annealed sample.
• The electrical conductivity of TFTs annealed in forming gas increased by 105 times.

In this investigation, the carrier concentration gradient between channel and contact region is achieved to improve the Thin film Transistors (TFT) performance by employing annealing at 350 °C in forming gas (N2 + 5% H2). The contact region is covered with Mo metal and the channel region is only exposed to forming gas to facilitate the diffusion controlled reaction. The TFT using a-IGZO active layer is fabricated in ambient of Ar:O2 in ratio 60:40 and the conductivity of the order of 10−3 S/cm is measured for as-deposited sample. The electrical conductivity of an annealed sample is of the order of 102 S/cm. The device performance is determined by measuring merit factors of TFT. The saturation mobility of magnitude 18.5 cm2V−1 s−1 has been determined for W/L (20/10) device at 15 V drain bias. The extrapolated field effect mobility for a device with channel width (W) 10 μm is 19.3 cm2V−1 s−1. The on/off current ratio is 109 and threshold voltage is in the range between 2 and 3 V. The role of annealing on the electronic property of a-IGZO is carried out using X-ray photoelectron spectroscopy (XPS). The valance band cut-off has been approximately shifted to higher binding energy by 1 eV relative to as-deposited sample.