Nanoscale physics

Electrostatics of the nanowire radial p-n junctions
Borblik V.L.
Institute of Semiconductor Physics of the NAS of Ukraine. Prospect Nauki 41, Kyiv-03680, Ukraine. 
E-mail: borblik@isp.kiev.ua

A great interest is attracted now to radial core-shell p-n junctions built into nanowires or nanorods. This is connected with fruitful idea to use them in solar cells [1]. In this case, light may fall in axial direction of the nanostructure that provides complete light absorption but separation of the photo-excited electron-hole pairs occurs in perpendicular, radial, direction. 
Circular geometry of the p-n junction introduces some peculiarities into its electrostatics. In particular, depletion width of the core increases when radius of the metallurgical boundary of the p-n junction decreases [2]. Our consideration has revealed that depletion width of the shell, on the contrary, meanwhile decreases and the total depletion width of the p-n junction can both increase and decrease depending on doping levels. 
If p- and n- layers are made of different materials (heterojunction) values of the depletion widths in such the heterostructure p-n junction prove to be intermediate between those in radial homostructure p-n junctions made of the constituent materials (at the same doping levels). Analogous situation takes place for a barrier capacitance of the radial heterostructure p-n junction.
Increase in the core depletion width with decreasing in radius of the core-shell interface and simultaneous decrease in the shell depletion width can be important in such devices as heterostructure solar cells because this results in change of the relative contributions to the performance characteristics from different constituent materials. Namely, the lesser radius of the heterostructure p-n junction, the larger is contribution from the core material (under equal other conditions).
Sequences of the layers in core-shell heterojunction is of importance as well. The total depletion width of the heterojunction is larger in the p-n heterodiode with the core made of more wide-bandgap semiconductor.

1. Tian B., Zheng X., Kempa T. J., Fang Y., Yu N., Yu G., Huang J., Lieber C. M. Coaxial silicon nanowires as solar cells and nanoelectronic power sources // Nature. -2007.-449.-P. 885-890.
2. Petrosyan S., Yesayan A., Nersesyan S. Theory of nanowire radial p-n -junction // World Acad Sci Eng Technol.-2012.-71.-P. 1065-1070.