Regulations of Nanotechnology

Biography

Biography: Zinetula (Zeke) Insepov

Abstract

Surface acoustic wave stimulated transport of charge carriers generated in semiconductors and dielectrics under the influence of light is of great interest for the increase of the solar cell efficiency. A prospective application of SAW in solar cells could provide a 90% increase of the cell efficiency. SAWs propagating in piezoelectric crystals (piezoelectric semiconductor GaN and GaAs crystals included) have opposite potential values in the SAW minima and maxima due to the piezoelectric effect. The charges are then transported by SAW to the solar cell exit at the sound speed. By applying SAW in solar cells, the area of charge “harvest” from the surface of a semiconductor structure or a piezoelectric crystal can be increased and, hence, the solar cell efficiency can be increased too. In this work, the excited charge carriers were transported to a few hundreds of microns using surface acoustic waves. To visualize the charge transport, the electron beam induced current method (EBIC) was used which permits the visualization of charge distribution on the crystal surface by measuring the current flowing through the sample. Charges introduced by the primary electron beam and those generated in the crystal subsurface area are distributed between SAW minima and maxima according to the potential sign and are then carried by SAW at the acoustic wave velocity to the current collector at the exit. The YZ-cut of a LiNbO3 crystal was used to visualize the acoustically stimulated charge transport. To register the charge transport a Graphene IDT (current collector) was fabricated on the LiNbO3 crystal surface. The Graphene film was formed on the crystal surface by the transfer technique and the initial Graphene was synthesized on the surface of a Cu foil or a Ni film by CVD. Charge distribution on the surface of the YZ-cut of the LiNbO3 crystal was simultaneously visualized by the EBIC method. An EBIC image on the crystal surface during SEM scanning was obtained by measuring the current on the Graphene collector. Fig.1 presents an image of the crystal surface obtained by the EBIC method. The period of the structure is 60 m, which corresponds to the SAW wavelength of λ = 60 m, i.e. charges are distributed between the minima and maxima of the SAW. A lighter contrast corresponds to positive charges and the darker – to negative charges. In the area of the Graphene current collector, current is distributed in accordance with the structure period. The period of the observed image is 120 m, which is a double period of the Graphene IDT. A periodic modulation of the EBIC contrast with the period of Λ = 60 m can be seen on the free crystal surface, which corresponds to the distribution of positive and negative charges between the SAW minima and maxima. In the area of the Graphene current collector, a periodic structure with the period of 120 m and of a very high contrast (ratio of minima and maxima) is observed, which conforms to the Graphene IDT registration of charges delivered by SAW. The coincidence of the distribution of periods of positive and negative charges with the maxima and minima SAW, and the immutability of the distribution around the SAW propagation tract indicate that the charges are transferred to the current collector output device with acoustic wave velocity SAW.