Nanooptics and photonics
Features of photoconductivity in Macroporous Silicon in the
Ultraviolet Range of the Optical Spectrum
Karas’ M.I., Karachevtseva L.A., Onyshchenko V.F.
Laboratory of Photonic Semiconductor Structures, V. Lashkaryov Institute of Semiconductor Physics, NASU, Prsp. Nauki, 41, Kiev-03028, Ukraine. E-mail:nikar@isp.kiev.ua.

The aim of this work was to study surface negative photoconductivity and its relaxation in the ultraviolet region of the optical spectrum, in which the maximum surface absorption of radiation in the visible region of the optical spectrum is achieved (K= 3∙105 см-1), and, accordingly, the minimum depth of radiation absorption 0.03 μm. The “slow” relaxation of negative monopolar photoconductivity in the structures of macroporous silicon in the ultraviolet wavelength range of 360 - 410 nm was investigated. It was found, that the relaxation time consists of two components: fast 1-2 s, during which the negative photoconductivity reaches about 70% of its maximum value, and slow - about 5 minutes, during which the photoconductivity reaches its maximum value. It was shown, that of the three components of photoconductivity [1]: bipolar photoconductivity of space-charge region (SCR) σb1, monopolar photoconductivity of SCR σm1 and bipolar photoconductivity quasineutral volume σb2 under conditions of strong surface absorption (K = 3∙105 cm-1), surface monopolar negative photoconductivity σm1 dominates and determines the negative sign of the photoconductivity of the structure of macroporous silicon as a whole.
 	From a comparison of our data with the literature [2], we assume,  that the fast component of  relaxation time of  the surface monopolar negative photoconductivity occurs due to “slow” surface levels, localized within 1 nm from the surface in the SiOx transition layer, and the the second component of the relaxation time, equal to 5 minutes, is due to “slow”  states,  localized in SiO2.
The relaxation time, associated with “slow”  surface levels, localized in the transition layer SiOx, decreased with an increase in the illumination intensity due to an increased surface bending of  zones Ys as τф.м.~ exp(-Ys)/Nt.м  [3].  . 

1.	Karas M.I. Negative photoconductivity in macroporous silicon structures // New Technologie.- 2010.-27,-№ 1.-P. 118-123. [In Russian].
2.	Belous A.I., Soloduha V.A., Shvedov S.V. Space electronics.-2015.-V.2.-P.724, (Moscow: Technosphere). [In Russian].
3.	Litovchenko V.G., Gorban A.P. and Kovbasyuk V.P. Investigation of the sticking effect of current photocarriers on a silicon surface // Physics of the solid state.- 1965.- 7, № 2.- P. 565-572. [In Russian].