Causes of the Difference Between Hall Mobility and Field-Effect Mobility for p-Type RF Sputtered Cu₂O Thin-Film Transistors

The high Hall hole mobility (μ
_Hall
) of cuprous oxide (Cu
₂
O) has caused great interest in using this semiconductor for p-type devices in a future complementary metal-oxide-semiconductor (CMOS) thin-film transistor (TFT) technology. However, in most studies, the field-effect mobility (μ
_FE
) achieved was reported to be much lower than μ
_Hall
. To understand the large gap between μ
_Hall
and μFE, in this work, we correlate RF sputtered p-type copper oxide (CuO
_x
) material properties with electrical characteristics by varying the film thickness. After vacuum annealing, the different thicknesses cause differences in the oxidation state and phase. Our results show that films with a higher Cu(I) fraction that form in the Cu
₂
O phase give better TFT performance than CuO films. Nonetheless, the TFTs still exhibit a much lower μ
_FE
of 0.005-0.1 cm
²
/V-s than the measured μ
_Hall
value of 12.1 cm
²
/V-s. We explore the reasons for this mobility gap by conducting contact resistance (RC) and interface trap (D
_it
) analysis. We find that the mobility difference can be minimized by reducing the high contact resistance (~33% of the total resistance in the ON-state) and high density of interface traps (~3 × 10
¹³
cm
^-2
eV
^-1
).