Oct 7, 2009
Nanostructuring boosts thermoelectric power in semiconductor layers
There are two types of free electrons inside a solid - transport electrons, having energies close to the Fermi energy, and core ones, having energies less than the Fermi energy. In general, research efforts have focused on transport electrons and paid less attention to core electrons as they did not participate in charge and heat transport. However, core electrons define important characteristics of solids and today’s nanotechnology allows fabrication of structures having dimensions comparable to the de Broglie wavelength of core electrons.
In a recent study, which was published in the journal Nanotechnology, scientists have used this new technological possibility to force some core electrons to occupy higher energy levels and increase the Fermi energy. Core electrons begin to participate in transport and boost thermoelectric power.
In the device, periodic nano protrusions fabricated on the layer surface forbid some quantum states for free electrons, owing to the wave nature of the electron. This effect was named as quantum state depression (QSD). Following the Pauli Exclusion Principle, QSD rejected electrons occupy quantum states with higher energies and the Fermi energy increases. The value of the increase depends on protrusion height and can turn an intrinsic semiconductor into n+ type. Subsequently, an additional p+ type layer is grown on the top of the protrusion. The charge depletion region, formed inside the protrusion, reduces its effective height and makes it temperature dependent. This increases the Seebeck coefficient and does not affect electrical and thermal conductivities. Consequently, the thermoelectric figure of merit increases by one order of magnitude with respect to the bulk material.
Besides thermoelectrics, QSD can be utilized in semiconductor electronics as an alternative to donor doping. QSD doping does not introduce scattering centres and thus offers a high electron mobility layer for ultra high frequency electronics.
The QSD effect has been studied theoretically both for metals and semiconductors. So far, experimental investigations have been carried out only for metals. Experiments with semiconductors are now in a preparation phase.
About the author
The work was performed at the Tbilisi State University, Georgia, and at the Physics Department of New York University, US. Avto Tavkhelidze is a senior scientist at Tbilisi State University. His research interests include quantum interference effects in nanostructures, energy conversion on the basis of electron tunnelling and electron emission, SET and ballistic MOS transistors, and high-temperature superconductivity.