Nanostructured surfaces

Optical properties of the nanostructured n-GaP substrates 
B.I. Tsykaniuk1, O.F. Kolomys1, Iu.M. Nasieka, V.V. Strelchuk1, V.P. Makhniy2, M.M. Berezovskiy2, O.V. Kinzerska2
1 V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, 41 prospect Nauky, 03028 Kyiv, Ukraine.
E-mail: btsykaniuk@gmail.com
2 Yu. Fedkovych Chernivtsi National University, 2, Kotsyubinskogo str., 58012 Chernivtsi, Ukraine.
E-mail: oksanakinzerska@gmail.com

Surface nanostructured semiconductors have attracted increasing attention due to simple growth methods, their great prospects in the fundamental physical sciences and modern nanotechnology which are caused by their unique properties correspondingly to 2D-3D structures and quantum confinement effects.
In the present work the optical and structural properties of the surface nanostructured n-GaP substrates were examined using confocal Raman, photoluminescence, FTIR and AFM techniques. The surface nanostructures were created by using chemical treatment of n-GaP monocrystalline substrates (orientation (111)) with the KOH: NaNO3: 1:50 solution. After this treatment, the surface of the substrate became matte and material properties changed considerably. The mean-squared roughness varied from 0.6 nm for a mirror (side A) until 150-300 nm for the matte surfaces (side B), and its value is determined by the polarity of the plane (gallium (A) or phosphor (B) terminated). The size of the surface formations define their contribution into optical properties of the studied material. In the photoluminescence spectra of the gallium-terminated side (side A) the broad band with maximum emission at 2.27 eV is attributed to optical transitions in the indirect band gap of GaP substrate. The strong peak at 2.8 eV is caused by quantum-dimensional effects in the surface nanostructures. 
In the Raman spectra, two bands at 363 and 401 cm-1, attributed to LO and TO phonons of GaP are observed. The difference in the peak position of the LO band in bulk and nanostructured GaP (∼2 cm-1) is caused by the strain and “shape” effect arising from the increasing in surface area due to appearing of nanoroughness. Besides, the Raman scattering studies of nanostructured n-GaP substrates reveal a weak additional peak in the first order spectrum that can be clearly assigned to the surface optical (SO) phonons. The role of surface morphology, as well as electronic and electron-phonon processes in the formation of surface nanostructures are discussed.