Experimental Studies of Thin Tellurium Films

This thesis is concerned with the fabrication and characterisation of thin tellurium films on 7059 corning glass substrates under ultra high vacuum conditions deposited at 125 K and 300 K. Electrical measurements were performed on sandwich metal\Te\metal configurations in situ under ultra high vacuum conditions and in a separate vacuum system.
De conduction is found to be ohmic at low applied electric fields for current below about 3mA. This conduction regime changed to a space charge limited mechanism at high fields (typically above 106 V/cm). Space charge limited currents showed the existence of a fast decaying density of trapping states in the energy gap. De conductance vs. temperature measurements revealed this energy gap to be 0.315 eV for 100 nm thick tellurium films. Samples have also shown a T1/4 law behaviour for temperatures between 77 K and 290 K. The density of conduction states at the Fermi level is found to be 2x10-22 ev/cm3, This value decreased upon annealing of the tellurium samples.
Ac conductance measurements revealed the existence of transport by charge migration between potential wells associated with defect centres, giving an AwS relationship. Parameter s has been found to be temperature dependent with its gradient producing a value for the hopping barrier of 0.33 eV a value close to the bandgap of the tellurium. Ac conduction is due to bipolaron hopping. Optical absorption in 90 nm thick tellurium samples is due to direct transitions both allowed and forbidden, gives two absorption edges one at 4 eV and the other at about 0.5 eV for films deposited at 125 K and 300 K. Structural properties of Te films were analyzed using scanning electron microscopy. Room temperature scanning electron microscope photographs revealed the smooth fine grained Gontinuous surface structure of the deposited films, Transmission electron microscopy uncovered the polycrystalline nature of the evaporated films showing quite visible grain boundaries at room temperature. Electron diffraction patterns further verified the polycrystalline composition of the tellurium samples investigated.

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