Nanoscale physics

Transient surface photovoltage in nano-ZnO films sputtered onto Si substates
Shmid V.1, Podolian A.1, Nadtochiy A.1, Korotchenkov O.1, Romanyuk B.2, Oberemok O.2, Sabov T.2, Dubikovskiy O.2
1 Faculty of Physics, Taras Shevchenko Kyiv National University. Prospect Nauki, 46, Kyiv- 01601, Ukraine. 
E-mail: shmdvi@gmail.com
2 Lashkaryov Institute of Semiconductor Physics. Prospect Nauki, 41, Kyiv-03028, Ukraine.

The integration of thin films with Si technology implies serious limitations of the minimum thickness of the film, allowing to create an isotype heterojunction with a high qualitaty. This is a particularly competitive task as numerous thermal treatment steps are usually involved. One of the most important problems in this respect is the need to avoid the formation of large quantities of defects at the film/substrate boundary.
One promising type of such structures suitable for photovoltaic applications employs undoped ZnO layers grown on Si wafers [1]. Here, we report the surface photovoltage (SPV) decays taken in nano-ZnO films of thicknesses ranging from 11 to 120 nm, which are deposited on Si(100) substrates by the magnetron sputtering using a ZnO target with 1% of Al. X-ray diffraction (XRD) and secondary-ion mass spectrometry (SIMS) were employed to characterize the quality of the ZnO films.
The results show that the ZnO film with thickness of less than 70nm has roughly 3 times greater SPV magnitude compared with that having greater thicknesses. The SPV decays slow down accordingly exhibiting decay times of the order 10 s at the small film thicknesses and 100 s at the ones exceeding 70nm.
These results are interpreted in terms of the recombination and carrier trapping centers in the film and at the ZnO/Si boundary, whose concentration varies with the film thickness.

1.	Bedia Z., Bedia A., Kherbouche D., Benyoucef B. Electrical properties of ZnO/p-Si heterojunction for solar cell application // Intern. J. Mater. Eng.-2013.-3.-P. 59-65.