Electrical properties of tellurium thin films

Semiconducting properties of evaporated tellurium thin films, in the thickness range of 100 to 400 Å, are studied and correlated with observed structural properties. It is found that less-than-monolayer gold films can act as nucleation sites and stimulate the growth of large crystallites in deposited Te films. The Au-nucleated Te films are preferentially oriented with the c axis in the substrate plane and have crystallite diameters ranging from 2 to 5 µm. Hall mobilities as high as 250 cm
²
/V ċ s are observed in 400-Å Au-nucleated films with 5-µm crystallites. These large-grain films exhibit a temperature dependence for mobility of the form µ ∼ T
^3/2
between 85°K and 250°K, while the carrier concentrations in the films do not change appreciably with temperature. Transconductances greater than 1000 µmhos are achieved for Au-nucleated Te thin-film transistors with 3-mil channels (operating with a saturated drain current of 1 mA). Several devices exhibit field-effect mobilities greater than 100 cm
²
/V ċ s, a value consistent with the observed Hall mobilities for similar films. Transconductance measurements indicate that Te thin-film transistor (TFT) instabilities result primarily from hole trapping at the Te-insulator interface. It is possible to alter the threshold voltage of Te TFTs by applying a gate bias at room temperature. Improved stability (changes in V
0
less than 50 mV in 1 h) is observed at 77°K. From the observed changes in threshold, a lower limit of the trapping-state density at the surface is inferred to be 5×10
¹²
traps/cm
²
. The surface-state density at the Te-SiO interface is estimated to be less than 6×10
¹²
surface states/cm
²
ċ eV as determined from capacitance and conductance measurements.