Moldavian Journal of the Physical Sciences, Vol.3, N1, 2004
SOME ASPECTS OF DEVELOPMENT OF PHYSICS OF NON-CRYSTALLINE SEMICONDUCTORS IN MOLDOVA Maria IOVU Center of Optoelectronics of the Institute of Applied Physics, Academy of Sciences of Moldova Academiei Str 1, MD-2028 Chisinau, The Republic of Moldova
[email protected] Prof. B.T.Kolomiets and N.A.Goryunova (Sanct-Petersburg, Russia) have discovered the chalcogenide glassy semiconductors (ChG) for the first time at the beginning of 1950 years. Academician A.M.Andriesh initiated the investigations in the field of ChG in R.Moldova in 1962 in the Institute of Applied Physics (IAP) of the Academy of Sciences of Moldova. In order to extend these investigations, on March 24, 1970 in the IAP there was created the scientific laboratory “Photoelectrical Properties of Semiconductors”, and on March 16, 1993 – the Center of Optoelectronics. The non-crystalline semiconductors present a group of materials with physical properties different from crystalline semiconductors due to disordered structure of atomic network in these materials. Chalcogenide glasses As2S3, As2Se3, As2Te3 etc.) and some semiconductors in the state of thin films (silicon, germanium, selenium et al.) belong to this group. Physical properties of the non-crystalline semiconductors may be studied more deeply in comparison with those of materials of the same composition, but existing in crystalline state. This fact has been considered as very important for several prestigious experimental and theoretical laboratories all over the world, which concentrated attention on investigation of the ChG. The interest to the investigation of these materials was also proved by the fact that they obtained world-wide appreciation as materials perspective for utilisation as optical waveguides and registration media in opto-electronics, integrated optics and fibre-optics devices. In the case of light propagation in optical waveguides the light action produces pronounced photoinduced optical phenomena, including non-linear ones. The scientific group of the Center of Optoelectronics implemented a systematic investigation of the photo-electric and optical properties of ChG, in particular in the case of wide-gap chalcogenide glasses with low electrical conductivity, that changed drastically under action of light. These investigations are of fundamental and applied character, which provided both evaluation of the energy spectrum and mechanisms of generation, transport and recombination of charge carriers in non-crystalline semiconductors and structures on their base and elaboration of recommendations for its applications in practice. At the Centre of Optoelectronics the basic physical relations of electro-physical and optical properties of the semiconductor systems arsenic sulphide-antimony sulphide (M.S. Iovu, S. Şutov, M.A. Iovu) and arsenic sulphide-germanium (D. Ţiuleanu, E. Colomeico) have been studied. It was found that these properties were characteristic of the specific energy structure of the band gap with a wide spectrum of localised states. It was demonstrated that this state energy distribution is possible to modify through variation of chalcogenide glass composition. It was effectuated a complex experimental study of the transient processes of the dispersive transport, of photoconduction and of photoinduced optical absorption, the specific character of which is determined by the non-equilibrium occupation conditions of the localised centres present and quasi-continuously distributed in rather wide energy spectrum. With the aim of detailing of the structure of the localised state spectrum both photoelectric 17
Moldavian Journal of the Physical Sciences, Vol.3, N1, 2004
phenomena stimulated by the thermal treatment and characteristics of the injection currents were studied. For the first time, in dispersive transport regime, the drift mobility values in thin films of chalcogenide glasses have been determined, with inclusion the films prepared at various technology conditions and doped with various metal impurities. The peculiarities of transient currents in thin chalcogenide glassy films were studied under action of supplementary illumination, which changed the localised state occupation; the basic parameters of such materials were determined (M.S. Iovu, S. Şutov, M.A. Iovu). In collaboration with Doctors Hability V. Arkhipov and A. Rudenko (Moscow, Russia) in the case of dispersive transport a theoretical model of multiple trapping of charge carriers was developed, which allowed explaining the experimental results and features of the drift in chalcogenide glasses, photoinduced absorption, photoconductivity kinetics and specific negative transient current observation. Physical phenomena in heterostructures with thin layers and at the contact interface between vitreous semiconductors and various metals were investigated. Several novel methods for studying of localised state spectroscopy were used: electrography spectroscopy of deep centres (S. Malcov, V. Verlan) and a spectroscopy based on the junction capacitance measurements (S. Şutov, A. Simaşchevici, I. Vasiliev), the results of which helped to establish the shape of the distribution and the energy position of the localised states in arsenic sulphide and arsenic selenide. The phenomena of electrostimulated chemical transformations and surface deformations in thin-film structures of the metal-vitreous semiconductor type have been detected and studied (D.I.Ţsiuleanu, G.M.Triduh, M.S.Iovu, V.V.Bivol, E.A.Achimov). On its base novel media for optical information registration with high resolution and sensibility monitored by the electric field were developed. In order to increase the integral as well as spectral photosensitivity of the optical information recording layers based on the chalcogenide glasses heterojunctions and heterostructures were constructed with the effect of sensitisation due to injection. On the base of amorphous heterostructures there were elaborated a series of photosensitive elements with expanded spectral region monitored by the electric field, photoelectric device for measuring of the plant leaf area, the X-ray detecting elements, humidity sensors and others. Starting from the results obtained in course of the study of photosensitive registration media there were put forward the recommendations for application of chalcogenide glasses in xerography (S.D.Şutov, A.I.Buzdugan, M.S.Iovu) and photothermoplastic recording, as well as for optical image registration with use of the process controlled by the electric field, in form of photo- and electronresist, sensors and photoelectric converters. The main advantage of these devices in comparison with the existing optical information registration media is high sensitivity in the wide spectral region with high optical resolution (up to 3000 lines per mm). The recent years new investigations in searching and development of novel methods of recording of optical and holographic information as well as of effective media for optical registration have been done. On the base of detailed study of electro-physical and optical properties of chalcogenide glassy semiconductors and heterostructures composed of them some new physical processes were investigated as suitable for recording of optical image and holograms. Electrically stimulated chemical transformations at the metal-chalcogenide glass boundary, photo-contact recording, electrostimulated microdeformation of chalcogenide film surface have been indicated as a ground for storage media of a new type. At present optical layout is designed, complex experimental sets are assembled, the holograms of various types (rainbow holograms, focused image holograms, Fourier holograms, volume holograms) are formed with high diffraction efficiency (up to 40%), large scene depth (up to 15 сm) and 18
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dimensions from 0.5 mm2 to 140 cm2. A technology process of obtaining of metal matrix for multiplication of patterns from a hologram origin was developed. The study of the structural transformations in thin films of vitreous semiconductors stimulated by low-energy electron irradiation presents interest from the practical point of view in forming with the aid of this effect of phase and relief diffraction elements with submicron period for various optoelectronic applications. These investigations were extended and developed in frame of the BGP between the Civilian Research and Development Foundation, US and Moldavian Research and Development Association, R.Moldova (Grant award number ME2-3020). The project deals with novel technological approach in manufacturing of security holograms for protecting documents and anti-counterfeiting. To increase the degree of security a combined technique was elaborated, in which over the optical hologram some special extra diffraction structures produced by electron-beam recording are added. The project was elaborated at the Purdue University, West Lafayette, IN, the USA, and at the Center of Optoelectronics of the Academy of Sciences of Moldova, Chisinau, the Republic of Moldova. The part of the project at the Purdue University involved both experimental and theoretical work. In the experimental part, the project utilizes advanced technologies available. The attempts were concentrated on optimizing the use of the e-beam system as well as the reactive ion etching system for generating diffractive optical elements with phase modulation so that high diffraction efficiency can be achieved. The work was focused on the synthesis of discrete, binary computer-generated holograms (CGH’s). In the Center of Optoelectronics, Chisinau, two types of materials were studied: chalcogenide glass (CG) As2S3 from the set of inorganic resists and carbazol containing polymers poly-N-epoxypropilcarbazol (PEPC) and carbazolilalkylmethacrylat (CAM) with octylmethacrylat (OMA) from the group of organic materials. The recording regimes and conditions for both holographic and e-beam techniques were examined and set to computer monitoring. The results of the project will serve as a basis for an improved technology, which has potential commercial value in the market in Moldova. The investigations in the area of optical fibres effectuated at the Centre of Optoelectronics by Acad. A.M.Andriesh and dr. I. Culeac are of pioneer character. The last years a technology for optical fibres pulling from chalcogenide glasses without crucible was elaborated. The experimental equipment for optical fibre characterisation by various techniques was mounted: the method of nearby field, the method of remote field and others. Experimental apparatus for studying of the absorption spectra in bulk, thin-film and fibre samples was installed, and optical fibres were obtained of the chalcogenide glasses. For the first time the effect of photoinduced absorption in optical fibres made from chalcogenide glasses was observed and the steady-state and transient characteristics of photoinduced absorption were experimentally studied. The photoinduced absorption technique was suggested as a method for spectroscopy of localised states in chalcogenide glasses. Some approaches were initiated to elaborate sensors with optical fibres for measurement of various physical quantities. In frame of the scientific co-operation programme with the Institute of Optoelectronics (Romania) a optical fibre sensor was elaborated for registration of microdeformation with high performance. This work was awarded with 1st grade Diploma at International Symposium SIOEL-95 (1995, Bucharest, Romania). In frame of the INTAS 99-01229 Project the thermal synthesis of As2S3, As8.3Ge25Ga1.7S65, As17Ge17Ga1Se65, and As8Ge25Ga2Se65 glasses doped with small amounts of luminescent additives (Mn, Nd, Sm, Dy, Er, Ho and Pr) was made using common vacuum 19
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melting technique. GLS glasses were prepared by melting from standard sulphides and oxides (supplied by Merck LTD, UK) at 1150 oC inside vitreous carbon crucibles and annealed at 540 oC for 1 hour and cooled to ambient at 1 oC/min. Bulk polished samples and thin films on glass substrates were prepared from the obtained ingots for optical, photoelectrical, and electron transport measurements. IR transmission spectra of As2S3 and metal-doped As2S3 glasses are characterised by several weak absorption bands associated with presence of hydrogen and oxygen contaminations. The observed changes in the IR spectra are most likely related to interactions of a portion of the introduced metal dopants with the inherent impurities of the host glass, such as H and O atoms. The optical reflectance spectra of bulk As2S3:Me (Me: Sm, Dy, Mn) glasses were measured on polished surfaces in the photon energy interval from 1.8 eV to 6.2 eV. A broad double reflectivity peak is observed in this region in accordance with earlier studies reported in the literature. The observed prominent influence of impurity is unusual for the studied region of fundamental absorption and may be tentatively associated with the intermolecular interactions, which are important in chalcogenide glasses and are able to induce significant changes in the valence band density-of-states distribution. More sensitive to the introduction of impurity appeared to be the region of exponential absorption tail (Urbach edge region), where the absorption significantly increases in doped glass, especially for Mn impurity. Photoconductivity spectra measurements are sometimes effective in revealing of weak absorption bands at photon energies slightly lower than the absorption edge, especially in thin-film samples. This tool was successful for As2Se3:0.5 at.%Dy sample, for which a wide impurity band located around 1.05 eV appeared in the photoconductivity spectrum. The band obviously had complex structure, which could be associated with optical absorption in Dy3+ ion due to transitions from the basic level 6H15/2 to the upper terms 6F7/2, 6F9/2, 6F11/2, 6H7/2 and 6 H9/2. This fact indicates the transfer of energy absorbed in a rare-earth ion to localised electrons in the host glass providing additional photoexcited conduction. The luminescence spectra of dysprosium-doped As2S3 glasses with the level of Dy3+ concentration of 0.05, 0.15, 0.25 and 0.5 at.% were investigated. Increasing of Dy3+ ion concentration increases the luminescence intensity, the fact which confirms the high solubility of rare-earth ions in the host glass. The above outlined spectroscopic results concerning the effect of metal impurity on optical properties of As2Se3 have been summarised in the report presented by Prof. M.S. Iovu at the 1st International Workshop on Amorphous and Nanostructured Chalcogenides – Fundamentals and Applications, (Bucharest, Romania, June 25-28, 2001) and to the XIIIth International Symposium on Non-oxide glasses and New Optical Glasses (Pardubice, September 9-13, 2002). It is of special interest to study propagation of light and its interaction with a solid, the physical phenomena, which occur in transparent materials from the optical point of view under conditions when the dimensions of the solid are of the order of wavelength. In these conditions the light might be directed by forming of a special profile of the refraction index realised in optical circuit similar to that in microelectronics, but the electric current is substituted by a controlled beam of photons, thus incarnating the conception of the integrated optics. It was demonstrated that chalcogenide glasses are very promising materials to solve this problem. An experimental basis was set up and a technology was elaborated for obtaining such kind of light waveguides together with specific investigation techniques (massspectroscopy, spectroscopy of planar optical waveguides). Application of a large set of lasers allowed realisation of experimental techniques for characterisation of optical losses, of diffraction gratings in planar waveguides, coupling of light in guide with optical fibre. 20
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On the base of these studies several practical applications have been realised: • planar chalcogenide glass waveguides ; • spectral demultiplexer cu diffraction grating in planar guide (laboratory model); • spectral demultiplexer for communication to optical fibre (industrial prototype); • all-optical modulator with control power of 10 mW; • light waveguide monitored by treatment of the As-S film with a CO2 laser. The programme of investigations involves international scientific co-operation, first of all with the scientific centres in Rome, Bucharest and Moscow. In frame of the Centre of Optoelectronics there were studied nonlinear optical processes in chalcogenide glasses and physical mechanisms of interaction of non-crystalline semiconductors with optical radiation of strong electromagnetic field in a laser pulse were investigated. It was demonstrated that under excitation of chalcogenide glass by laser light pulse the phenomenon of pulse profile distortion is observed, i.e. the optical hysteresis, that is the increase of the absorption coefficient. The peculiarities of the optical hysteresis and nonlinear transmittance of laser pulses in thin chalcogenide glass films were studied under conditions when the duration of laser pulses varied from 10-6s down to 10-12 s. It was found that in strong electromagnetic field of a laser pulse, when a threshold value of intensity is overpassed, the light absorption is increased leading to the dependence of the type of hysteresis of the light intensity at the output of the sample in dependence on the light intensity at the input. The magnitude of the absorption variation depends on the amplitude of the laser pulse at the input. The registered effect has a reversible character. A mechanism of formation of optical nonlinearity was proposed, in which the participation of nonequilibrium phonons plays the important role. In the Center of Optoelectronics certain experience in preparation and study of chalcogenide glasses doped with metal impurities is accumulated (Dr.hab. M.S.Iovu). Recently the effect of suppressing of photodarkening by metal impurities, such as Sn, Mn and rare-earths Sm and Dy in chalcogenide glasses was revealed. In other words the effect of glass structure stabilization by metal dopants has been observed. The metal impurities were found to induce changes in the intermediate order of amorphous matrix dependent on the type of impurity. These investigations have been served as a base for the CRDF-MRDA BGP Grant (Grant award number ME2-3028). The systematical study of the behavior of metal dopants (rare earth metals and tin) as a factor influencing photostructural transformations in chalcogenide glasses (As2Se3) was the main goal of the project. The choice of rare earths (Nd, Sm, Dy, Ho, Er, Pr) was determined by the fact that these dopants are actually used in industrial applications in photonics. The tin as a dopant was included as a model probe atom to permit Mössbauer spectroscopy local structural characterization of the impurity in a chalcogenide glass. A set of samples of bulk glasses and thin-films doped with above listed impurities were prepared and characterized with respect to the effect of impurity on the structure and photodarkening phenomenon, particularly on the relaxation under light exposure in doped amorphous films in dependence on the type and concentration of the impurity, and thermal treatment. X-ray diffraction, temperature-modulated scanning calorimetry, Raman scattering and 119Sn Mössbauer spectroscopy were used as the tools effective for glass structure investigation. Doping of amorphous chalcogenide films by metals assists in stabilizing the glassy matrix with respect to light exposure and thermal treatment. On the basis of the structure-to-property correlation, the role of metal impurity in reformation of molecular cluster structure was studied with particular attention to retardation of relative slip motion of
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clusters by the impurity. Recommendations concerning the stabilization of the glass structure by impurity against the photostructural transformations have been suggested. The studies executed in the Centre of Optoelectronics present interest from the point of view of development of elements of optoelectronics and integrated optics. A part of the results has been obtained in co-operation with authors from Rome (M. Bertollotti, E. Fazio, F. Michelotti, C Sibilia et al.), Romania (T. Necşoiu, M. Piopescu), Belgium (G.J.Adriaenssens, V.I.Arkhipov), Great Britain (A.B.Seddon, D.Furniss), Greece (E.I.Kamitsos, C.P.E.Varsamis), Russia (Yu.S.Tveryanovich), Ukraine (O.Spotyuk), USA (P.Boolchand). In the field of physics of non-crystalline semiconductors 8 books, 1000 articles, obtained more 100 inventions were published. The scientific results and elaborations were presented at different international conferences and exhibitions, awarded with Diplomas, Gold and Silver Medals. Under supervision of the academician A.Andriesh during his scientific activity a large group of young scientists defend the Doctor Thesis (S.D.Sutov, M.S.Iovu, D.I.Tsiuleanu, V.N.Ciumas, N.A.Enachi, A.I.Buzdugan, A.A.Popescu, M.A.Iovu, V.V.Bivol, V.G.Abashkin, M.R.Cernii, E.A.Achimov, I.A.Cojocaru, V.N.Dolghieru, S.A.Malkov, V.I.Verlan, E.G.Hancevschi, N.A.Gumeniuc, E.P.Colomeico, I.P.Culeac, etc.). A new scientific school in the Republic of Moldova in the field of physics of non-crystalline semiconductors was created, which is recognized worldwide. References [1] VITREOUS ARSENIC SULPHIDE AND ITS ALLOYS. Authors: A.M.Andriesh, M.S.Iovu, D.I.Tsiulenu, S.D.Shutov, Ed. Stiintsa, Chisinau, 1981 (in Russian). [2] NON-STATIONARY INJECTION CURRENTS IN NON-CRYSTALLINE SOLIDS.Authors: V.I.Arkhipov, A.I.Rudenko, A.M.Andriesh, M.S.Iovu, S.D.Shutov, Ed. Stiintsa, Chisinau, 1983 (in Russian). [3] VITREOUS SEMICONDUCTORS FOR PHOTOELECTRICAL RECORDING SYSTEMS. Authors: A.M.Andriesh, V.V.Bivol, A.I.Buzdugan, M.S.Iovu, L.M.Panasyuk, G.M.Triduh, V.I.Fulga, D.I.Tsiuleanu, S.D.Shutov, Ed. Stiintsa, Chisinau, 1988 (in Russian). [4] PHYSICS OF CHALCOGENIDE GLASSES. Authors: M.Popescu, A.Andriesh, V.Ciumas, M.Iovu, S.Shutov, D.Tsiuleanu, Ed. Stiintifica Bucharest-I.E.P. Stiinta, Chisinau, 1996 (in Romanian). [5] PHYSICS AND APPLICATIONS OF NON-CRYSTALLINE SEMICONDUCTORS IN OPTOELECTRONICS. Eds. A.Andriesh & M.Bertolotti, 1977, Kluwer Academic Publishers, NATO Series. 3. High Technology, V.36. [6] PHOTOINDUCED PHENOMENA AND ELEMENTS FOR INTEGRATED OPTICS ON THE BASE OF NON-CRYSTALLINE SEMICONDUCTORS, Author: A.Popescu, Ştiinţa, Chişinău, 2003 (in Romanian). [7] CONTRIBUTIONS TO NON-CRYSTALLINE SEMICONDUCTOR PHYSICS AND TO OPTOELECTRONICS. Homage Book dedicated to the Academician Andrei Andriesh and Professor Serghei Shutov with their 70-th Anniversaries, Eds: Artur Bzdugan and Mihail Iovu, Ştiinţa, Chişinău, 2003. [8] BIO-BIBLIOGRAPHY OF THE ACADEMICIAN ANDREI ANDRIEŞ. Eds: Artur Buzdugan & Maria Iovu, Ştiinţa, Chişinău, 2003, 136 pag.
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