Chalcogenide glasses and thin films, thermal and photoinduced phenomena

E. Skordeva, Z. Ivanova , D. Arsova, V. Pamukchieva, E. Vateva
 

Investigations on disordered materials, namely chalcogenide glasses and thin films, are traditional in the Laboratory. This tradition is descending to the 50^th years of the 20^th Century. The studies were the first carried on in Bulgaria and began only a few years after the Russian scientists Prof. N.A. Goryunova and Prof. B.T. Kolomiets had discovered the semiconductor behaviour of a wide group of inorganic glasses. Prof. Razum Andreichin, chief of the laboratory at that time, was initiator and founder of these investigations. He found the existence of the photoelectret state in chalcogenide glassy semiconductors. This phenomenon (discovered in sulfur by Prof. G. Nadjakov, the founder of the Institute) was studied in As₂S₃ and related glasses in detail. Between 1976 and 1999 Prof. Elena Vateva was head of the Laboratory. She initiated and developed the investigation of chalcogenide thin films. She is founder of the studies of the medium-range orders influence on the photo-induced changes in chalcogenide systems. The study of disordered materials – glasses and thin films – is still one of the topics in the investigations carried on in the laboratory.
 

Equipments for preparation of chalcogenide glasses and thin films are available in the laboratory. Annealing as well as band-gap illumination in non-containing oxygen atmosphere can be performed. Various optical, photoelectrical, electrical, xerographical and physico-chemical properties can be investigated.
 

The main interest is concentrated on Ge-As(Sb)-chalcogenides. Glasses and thin films are investigated. Some of the results, previously obtained in the investigated ternary systems, have supported the idea about the existence of a topological phase transition, related to the medium-range order. The topological transition reflects in a series of peculiarities observed in the compositional dependencies of: atomic volume and compactness; heat capacity; microhardness; thermal diffusivity; selective solubility; optical band gap and refractive index; first sharp X-ray diffraction peak and pre-FSDP features, etc.

The possibility for application of chalcogenide films and glasses in optical recording stimulated studies of photoinduced structural changes. At the beginning special attention has been paid to the irreversible ones. Now both irreversible and reversible photoinduced (PI) changes are studied. Different lines from the glass-forming regions are investigated. Compositions from the Ge-As-S system with giant PI changes have been found. Investigations of the structure and structure-related properties of amorphous chalcogenide glasses and thin films are still in progress. Various spectroscopy techniques (IR, Raman, X-ray diffraction, XPS, Mössbauer, EXAFS) as well as experiments on the optical transmission and thermal properties, especially at low temperatures, have been used. Thermo- and photo-crystallization are also investigated.
 

A great part of the studies is carried out in collaboration with colleagues from other institutes and departments. During the last decade a number of international collaboration studies have been accomplished with scientists from France, Greece, Ukraine, Russia, Israel, Turkey, Poland.
New proposals for collaboration are welcome.
 

Recently, rare earth doped chalcogenide glasses have intensively been studied for application in integrated optoelectronic devices. Especially, Er-doped Ge-Ga-S glasses have played an important role in telecommunications because of their Er³⁺ intra-4f emission at 1.54 mm. We have prepared a wide range of glassy hosts with different ratio of GeS₂/Ga₂S₃ at increasing Er₂S₃ content up to quenching compositions. The influence of excitation on the photoluminescence (PL) emission and corresponded effects of narrowing and broadening have been investigated. The observed features and related transitions in Er³⁺ state have been specified by spectra deconvolution. The PL efficiency has been improved by temperature decreasing down to 4.2 K. The obtained PL lifetime of 3.2 ms proves potential usability for optical devices in third telecommunication window at around 1540 nm. PL results of Er-implanted amorphous layers have also been evaluated. The distribution and changes of the basic structural units at higher Er contents have been determined from Raman scattering. These studies have been made in collaboration with colleagues from Czech Republic, Canada and India.

A list of the works published the last 5 years are given in the part “References" of the site of the laboratory.