Incomplete oxidation in back channel of GaInZnO thin-film transistor grown by rf sputtering

Thin-film transistors (TFT's) may have a low-quality back channel in addition to the high-quality main channel. The presence of the back channel can deteriorate TFT turn-off characteristics. We studied properties of the back channel in bottom-gate GaInZnO TFT's grown by rf sputtering. X-ray photoelectron spectroscopy (XPS) results confirmed existence of low-quality oxide in the back channel. We observed that 200 °C annealing increased binding energies of metals and oxygen on the surface. This energy increase can be explained as a result of tighter bonding between metals and oxygen. When the top surface of GaInZnO was removed by Ar etching, XPS did not show such energy increase after the annealing. XPS also showed that the top surface has significantly higher In concentration compared to the bulk.