THIRTY YEARS LABORATORY “PHOTOELECTRICAL AND OPTICAL PHENOMENA IN WIDE GAP SEMICONDUCTORS”

History

At the establishment of the Institute of Solid State Physics (ISSP) in 1972 the section “II-VI Semiconductors and Devices” of the Institute of Physics was transformed in Department “Physical Problems of Optoelectronics and Functional Elements”. Head of the Department was Assoc. Prof. Stefan Kanev, who was also a vice-Director of ISSP. In 1973 the Laboratory of the Assist. Prof. Elena Vateva, a vice-Head of this Department, was established as a Research Group “Photoelectrical and Optical Phenomena in Wide Gap Semiconductors”. In 1978 Prof. Dr. Sci. S. Kanev left the Institute, with a part of Members of the Department and founded Central Laboratory of Solar Energy and New Energy Sources. The Research Group of Assoc. Prof. E. Vateva was expanded with the main part of the members of the Department “Amorphous Semiconductors” of Prof. Razum Andreichin , who had just retired at that time. The Research Group became a part of the Department “Physical Problems of the Microelectronics” with Head Prof. Dr. Sci. J. Kassabov. In 1989 the Research Group got statute of a self-dependent Laboratory “Photoelectrical and Optical Phenomena in Wide Gap Semiconductors” headed Prof. Dr. Sci. Elena Farhi – Vateva. In 1999 she retired, and became Associated Member of ISSP. Since 2000 Assoc. Prof. Dr. Diana Nesheva is Head of the Laboratory.

Main fields of research and results

1.1 Solar energy conversion

At the time of establishment of ISSP electrical and photoelectrical properties of CdS monocrystals were carring out, which had been set up by Prof. M. Borisov after the middle of the 20^th century. After 1971 in the section “II-VI Semiconductors and Devices”  S. Kanev, E. Vateva, I. Georgieva, N. Koparanova, V. Stoianov etc. worked in this field.
        Syntered CdS, suitable for effective photocells and high quality photoresistors, has been prepared by S. Kanev, V. Shekerdjiski, V. Stoianov and M. Lakova for the first time in the world practice. Detailed investigations on the influence of Cu and Cd impurities on the reversible photodegradation of the high-sensitive CdS lead to phenomenological models based on so-called photochemical reactions. The found possibilities for lowering of the degradation allowed development of new heterojunction structures with wide spectral sensitivity and stability in time. Possibilities for holographic recording were also shown. Essential results on enhancement of stability and speed of photoresistor response were obtained by inclusion of Cr₂O₃ in the CdS, which resulted in production of devices, thus confirming that II-VI compounds are suitable for applications. The physical problems of solar energy conversion in Bulgaria were further developed in the Central Laboratory of Solar Energy and New Energy Sources.

1.2 Recombination centers and photoinduced changes in crystal defect structure

The early investigations of CdS crystals showed two types of photoinduced phenomena: isothermal decrease (fatigue, ageing or photodegradation) and increase (sensitization) of photoconductivity. These phenomena were explained with reconstruction of structural defects, which act as recombination centers. New approaches for determination of some parameters of the centers were developed based on a generalized analysis of the intensity dependence of photoconductivity, as well as of double injection peculiarities in CdS diodes. A new model for explaining fatigue was proposed, based on the presence of traps with Coulomb repulsive barriers. An effect of high temperature sensitization was discovered and studied, which was the first phenomenon in II-VI compounds explained with creation of new defects.
         Photoconductivity and photoinduced phenomena in amorphous CdS films (deposited at substrate temperature <150K) were another field of investigation. Some parameter of recombination centers, geminate recombination and transport dispersion were explored.

1.3 Electrophotografic elements and investigations

The high sensitivity of CdS and the well understood photoinduced phenomena made possible the development of two types of electrophotographic photoreceptors. The first one used heterogeneous films of powdered CdS in suitable organic binder. Correlation between photoconductivity and xerographic discharge in these layers was obtained. Using suitable CdS material electrophotographic elements with high quality of the image recording in xerographic or photodielectric regimes were patented. The second type electrophotographic structures were prepared by vacuum deposition of a-CdS and polymers (polyethilen etc.) playing role of adhesive, transport and protective electrophotographic layers. The idea to separate these layers in many alternating deposited sublayers allowed to optimize the xerographic parameters of those structures. Xerographic structures with different transport and generation layers on the base of As-based chalcogenide layers have also been designed and investigated.
        The experience in preparation of ML allowed fabrication of a new class of multilayers – amorphous chalcogenides superlattices(initiated by Prof. Vateva) - whose properties were related to both classical and quantum size effect.

1.4 High-ohmic chalcogenide glasses

In the initial years of ISSP the investigations of Prof. Nadjakov were carried on by his disciples. In the department of one of the latter, Prof. Andreichin, the existence of the photoelectret state in chalcogenide glassy semiconductors was found. This phenomenon was studied in As₂S₃ and related glasses in detail. Studies of chalcogenide glasses were carried on in Bulgaria for the first time just after their discovery by the Russian scientists Prof. Kolomiets and Prof. Goryunova. At one time or another in the investigations took part M. Nikiforova, P. Simidchieva, P. Ivanova, G. Getov, E. Skordeva, L. Yourukova, M. Baeva, D. Arsova and other scientists belonging to the section of Prof. Andreichin. It was pointed out that the photoelectret state (PES) in amorphous chalcogenides comprises dark and light-induced high field polarization (connected with filling of not too deep traps) as well as photoelectret polarization due to filling of deep traps. The investigations showed that the polarization and depolarization in dark displayed a multi-exponential kinetics, which was further reduced to a sum of rapid and slow components. Additives of Ag or Cu changed the number of components and their relaxation times, as well as the polarization amplitudes. It was found that the PES of As-based glasses had all common features of the crystalline photoelectrets. A conclusion was made that a non-uniform trap distribution exists in the band gap of glasses which has been confirmed with thermo-depolarization experiments. Ternary As-based glasses were also prepared and their physicochemical and electrical properties have been studied. Joint spectroscopic and structural studies made in collaboration with Grigorovici et al. proved the existence of the thermodynamically postulated structural units in As₂S₃Ge_x system. An agreement was achieved between measured values of various physical quantities and those calculated with molecular additivity laws.
        The high-ohmic materials may show high electrostatic electrification. Studies on the process connected with the maximum quantity of electric charges on the surface of solids as well as in light fuels have been carried out since 1976. Various electric safety methods were developed.

1.5 Photoinduced structural changes and medium range order of As- and Ge-based glasses and thin films

After 1978, investigations of the properties of Ge- and As-based amorphous thin films were started in order to develop systems with well expressed photoinduced changes (photobleaching and photodarkening), suitable for optoelectronic applications. A new method of modification of the defect structure of As- and Ge-based chalcogenides was developed using co-evaporation of chalcogenides and impurities at low substrate temperatures. Electrically active defects were introduced which influenced their photoelectric properties. Changes in the solubility were found and an improvement of some parameters of the films as negative photoresists was achieved.
        A new class of materials such as Ge_xAs_40-xS₆₀ system, containing stoichiometric (As₂S₃) and non-stoichiometric (Ge₂S₃) components was selected prospective for high sensitive photoresists. Optical properties of Ge_xAs_40-xS₆₀ films and their photoinduced changes were studied. Parallel studies of the selective solubility of those films lead to discovery of a new solvent with surface active substance of emulgator type SAS-E (patented and widely used in the laboratory practice). Highest sensitivity for silverless inorganic positive photoresist was achieved (~1.10^-1J/cm²). Correlation between the compositional dependencies of the selective solubility and photoinduced changes of the optical bandgap was found. It was explained in terms of possible changes in the bond arrangement and taking into account the bond energies. Because of their suitable optical and mechanical properties as well as own photolithography, Ge_xAs_40-xS₆₀ films were probed as antireflecting coatings of GaAs LED matrices.
        Special attention was given to the role of Ga and In on the photoinduced phenomena in GeS_x glasses under exposure to band gap light. Introduction of these elements resulted in stronger photobleaching effect, which was accompanied by more pronounced photostimulated etching effect. These glasses are of negative photoresist type but can be converted into positive one by an appropriate previous annealing. The observed photoinduced phenomena were associated with a certain degree of ordering in the local structure, involving enhanced orientation of existing bonds and/or extension of the short range order. Modification of the GeS_x structure by the introduction of Ga (In) through a low-temperature co-evaporation improved the selective etching parameters and the sensitivity (confirmed by a Bulgarian patent). Plasma etching can also be applied to form a negative type of photoresists based on Ge-S glasses.
        Complex investigations of the medium range order were carried out and peculiarities (related to the 2D-3D transition) in the compositional dependencies of many optical, physicochemical and structural properties were found in films from Ge_xAs_40-xS(Se)₆₀. It was shown for the first time that the 2D-3D Tanaka’s transition, proposed for glasses, can be observed in thin chalcogenide films from suitable systems.
        Studies on basic physicochemical and thermodynamical parameters of Ge_xSb_40-xSe₆₀ glasses by the methods of DTA, DSC, TMA, X-ray diffraction and Raman scattering were carried out and the observed peculiarities with the composition variation were explained assuming changes in the medium range order.