Effect of Changed Structure As Well As Composition on the Behaviour of Sn(Se,Te) Compound Semiconductor Thin Films and  Schottky Diodes for Solar Cell Applications

There has been a great motivation to achieve new material types and fabricate electronic devices which show unusual properties in the field of energy generation and transformation. The recent investigations in this area are directed towards the development of cost-effective, non-toxic and abundant materials, the chalcogeide materials have emerged as a convenient choice in the regard. IV-VI chalcogenide semiconductors have exhibited flexibility to possess varied bandgap energies (bandgap tailoring) which allowed the materials to tap different regions of the solar spectrum. These Devices have achieved highest energy conversion efficiencies among the thin film technologies (above 20%). Further, Sn(S,Se,Te) chalcogenide thin films have been reported to possess the optical and electrical properties suitable for applications for solar harvesting. These have also been reported to be used in infrared LEDs/Detectors, fast switches and optoelectronic Devices. Comparative analysis of some Sn(Se, Te) compound semiconductor thin films and schottky diodes have been made. SnSe used as primary material has transformed to SnSe2 on changing its structure from orthorhombic to hexagonal. It, however, attained SnTeSe (Orthorhombic) composition on replacement of some of the Se atoms with Te. Films of all the three types of materials have been deposited at different substrate temperatures and changed thickness. These were then characterized on the basis of their structural, morphological, electrical and optical properties. The grain size of SnSe films increased on its transformation to SnTeSe as well as SnSe2. The electrical as well as optical analysis of the films showed that the bandgap and resistivity decrease for SnSeTe, the same however increase for SnSe2. Further, the conductivity type remained p-type for SnSeTe but changed to n-type for SnSe2 films. The Schottky Barrier Diodes of the deposited films were formed and their characteristics studied for the temperature dependent current-voltage (I-V) and capacitance-voltage(C-V) behaviour. The current voltage characteristics of Al/n-SnSe2 Schottky diodes demonstrated better diode parameters (ideality factors, barrier heights and breakdown voltage) in comparison Ag/p-SnSe and Ag/p-SnSeTe Schottky Diodes. Further, the ideality factors decreased and barrier heights increased with increase in temperature in all types. The temperature dependence of the barrier heights and ideality factors has been explained on the basis of “barrier inhomogenities” existing over the metal-semiconductor interface. Further, there have been variations between the theoretically generated data (on the basis Gaussian Distribution Function) to that from the experimental results which indicated the possibility of the existence of tunneling current component existing in the current transport of the Schottky Diodes. The breakdown voltages of all the three types of undertaken diodes were increased with decrease in temperature and provided negative temperature coefficient and depicted soft breakdown due to ‘Defect Assisted Tunneling’ phenomenon existing over the Schottky interface.