Surface roughness plays a key role on power factor of sub-50 nm thermoelectric devices

The effect of surface roughness becomes dominant in sub-50 nm scale ultra thin devices and play a crucial role in determining their transport properties. In this study, we theoretically investigate the influence of surface roughness effects on the thermopower of the device subjected to a temperature gradient by using non equilibrium green function formalism. For systematic investigations the variations in thickness due to surface roughness have been modeled with a square wave profile characterized with three parameters: amplitude A₀, crest width λ_b and trough width λ_w. The investigations show that fluctuations in the thickness due to surface roughness result in alternate regions of low and high quantum confinement which cause the electrons to flow in higher energy levels. Thus higher voltages are required to nullify the current flowing due to temperature gradient resulting in Seebeck coefficient to increase. It is observed that conductance of the channel due to surface roughness effects decreases more than the increase in Seebeck coefficient and hence the power factor determined by the product of conductance and Seebeck coefficienent decreases.