Direction dependent electrical and optical properties of gallium nitride nanowires

Gallium Nitride (GaN), due to its anisotropic and polar nature, exhibits direction dependent properties. Thus controlling the growth direction of GaN nanowires and studying its properties is important for practical applications. A direct nitridation scheme was used for the controlled synthesis of GaN nanowires in two distinct directions, and direction using amorphous substrate. A simple procedure based on homoepitaxy was used to determine the growth direction of the resulting nanowires. It was observed that homoepitaxy onto the wires grown in direction resulted in microprismatic island growth at the ends while homoepitaxy onto wires grown in the direction resulted in the formation of two dimensional microbelts. This work on synthesis was previously done and reported.;In this thesis, individual GaN nanowire (NW) field effect transistors (FETs) were fabricated using the as-synthesized nanowires in two distinct directions and their direction dependent electrical and optical properties were studied. Gate dependent electrical transport measurements performed on the devices fabricated using the a-axis nanowires have shown an increase in conductivity with the applied gate voltage, while no gate dependent conductivity is observed in the devices fabricated using the 'c' axis nanowires. The electron carrier density and electron mobility for the a-direction GaN nanowire FETs is estimated to be 2*1018cm-3 and 170cm 2/V-s.;Resistance measurements performed on these devices have shown that the 'a' axis nanowires are usually low resistive (usually in the order of hundreds of KΩ's) when compared to 'c' axis nanowires which are highly resistive (order of GΩ's) Resistances of the devices were also measured by varying the length between the contacts. Observation showed that the resistance dropped down tremendously in the case of c-axis nanowires from GΩ's to KΩ's, but in the case of a-axis nanowires such a tremendous drop in the resistance was not observed. It was observed to decrease linearly in the case of a-axis nanowires. The non gate dependent conductivity and high resistance observed in c-axis nanowires is attributed to the stacking faults present in these nanowires. A theoretical model has been used to support this assumption.;This thesis also presents optical characterization of the as-synthesized nanowires. Raman spectroscopy, photoluminescence, photoconductivity and ultra violet (UV) visible absorption measurements on the samples containing a- and c-axis nanowires have shown that bandgap of the nanowires grown along the a-axis blue shifts by about 45-70meV compared to the nanowires grown along c-axis.