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CNAA / Theses / 2010 / June /

Trapping and recombination processes of the non-equilibrium charge carriers in amorphousAs-Se and As(Sb)2Se3:Sn


Author: Harea Diana
Degree:doctor of physics and mathematics
Speciality: 01.04.10 - Semiconductors physics and engineering
Year:2010
Scientific adviser: Mihail Iovu
doctor habilitat, professor, Institute of Applied Physics of the ASM
Institution:
Scientific council:

Status

The thesis was presented on the 30 June, 2010
Approved by NCAA on the 6 October, 2010

Abstract

Adobe PDF document0.42 Mb / in romanian

Keywords

amorphous semiconductor, chalcogenides glases, thin films, refractive index, trapping processes and recombination processes, optical bias, photoconductivity, deep states, density of states, holographical registration, photosensitive element

Summary

The dissertation has been issued in Kishinev in 2010, written in the Romanian language, contains introduction, 4 chapters, conclusions, 140 references, 108 pages of the basic text, 57 figures.

The thesis is dedicated to the complex investigation of optica land photoelectrical properties of amorphous films of chalcogenide systems As100-xSex (x=40÷98), As2Se3:Snx and Sb2Se3:Snx with concentration 0÷10.0 at.% Sn. Experimentally was established that increasing of As concentration in As100-хSex.chalcogenic system and of Sn concentration in As2Se3:Sn and Sb2Se3:Sn lead to the shift of the optical absorption edge in the red region of the spectrum. From the optical absorption spectra were calculated the values of the refractive index, established it dependence on the composition of amorphous film, and the modification of the refractive index under light irradiation and heat treatment. In amorphous de As100-хSex.films the effect of photodarkening and the process of registration of holographic information was investigated, the experimental data were compared with those of modification of the refractive index. The experimental results of steady-state photoconductivity spectra and photocurrent relaxation were used for calculation of the function of distribution of the localized state (DOS) and energy parameters of amorphous As100-хSex şi As2Se3:Sn films. The experimental data show that the trapping characteristic times of the charged carriers by the deep traps in amorphous As2Se3 increase by doping of Sn impurities. The composition effect, Sn impurities and additional illumination influence the form of the relaxation curves of the photocurrent and the distribution function DOS.

The relaxation of the persistent photoconductivity with the typical time constant τ=2.2 - 25.31 s is described by the stretched exponential function. The effect of persistent photoconductivity appears duet o the existence in the investigated materials of the defect states deep localized. A broad energy distribution of the defect states centered at 1,4 eV with an excitation energy less than the band gap was observed in all amorphous As2Se3:Sn films. The basic parameters of these localized states were estimated, total concentration, the energy position of the maximum of distribution, width half, etc. The experimental results are interpreted in framework of the model of D+ and D- defects with negative correlation energy. In order to determine the DOS function also was used the experimental data regarding photocapacitance relaxation. It was established that for all amorphous films of As100-xSex system The energy distribution of the localized states g(E) is characterized by the broad maximum situated near the mid of the band gap. Estimation show that for amorphous As100-xSex films x>60 a non monotony variation of the trap concentration in respect with composition for x = 72÷80 was observed. In region of this maximum the trap concentration is estimated to be de Nt= 2,4x10-17 cm-3 . It was demonstrated that in this region the pair of coordinated defects like As4 +, Se1 - is dominated.

On the basis of the materials under study were formulated practical recommendations for utilization of the amorphous films of the chalcogenic system As100-xSex as registration media for optical and holographic information. The amorphous As2Se3:Sn films can be used as photosensitive element of photodetector (Patent MD No. 3548). The main results of the thesis were published in 45 scientific articles (5 articles and 40 theses).