Nanooptics and photonics

Photoconductivity kinetics analysis of InSe layered 
crystals in the frequency domain
Fliunt O. Ye.
Faculty of Electronics and Computer Technologies, Ivan Franko National University of Lviv. Dragomanov str., 50, Lviv-79005, Ukraine. 
E-mail: ofliunt@gmail.com

Indium selenide is perspective material for photovoltaic devices that could be explained by its value of the band gap of about 1.2 eV and possibility to create optical quality surface by usual cleavage of the crystal along natural layers plane. Now a few-layered InSe with higher band gap of about 1.4 eV also becomes an object of intensive studies as promising 2D photovoltaic material. 
The photoconductivity kinetics of materials may be presented in the temporal or in the frequency domains. Till now the total majority of photoconductivity measurements are carried out in the time domain giving temporal decay of photocurrent after abrupt termination of light illumination of the sample under study. Often it doesn’t allow to determine the true shape of even the largest decay component of photoconductivity, that usually is considered as exponential time dependence corresponding to Debye form. Photoconductivity kinetics in the frequency domain gives higher resolution within the wide range up to about four orders of frequency, that usually inaccessible by usual temporal techniques. 
In this work the kinetic of photoconductivity in InSe crystals have been measured in the form of complex light-beam intensity modulated spectra in the frequency range from 10 Hz to 100 kHz at different temperatures. Since the spectra of real and imaginary components of modulated photoconductivity are not independent functions of the frequency, the validity of complex modulated photoconductivity spectra may be checked using Kramer-Kroning integral transforms that gives some advantage of using modulated techniques. The temporal kinetic of photoconductivity has been obtained from the spectra of as real so imaginary part of modulated photoconductivity, that has been used for checking the validity of the InSe spectra. 
The photoconductivity decay in the time domain allows to describe the form of photoconductivity kinetics of InSe crystals more precisely in comparison with usual time domain photoconductivity decay technique and propose the corresponding model of excess charge carriers recombination via localized levels in the forbidden gap.