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LABORATORY
PHOTOELECTRICAL AND OPTICAL PHENOMENA IN WIDE BAND GAP SEMICONDUCTORS

HEAD: Assoc.Prof. Diana Nesheva, Ph.D.
tel: 7144-226; e-mail: nesheva@issp.bas.bg

TOTAL STAFF: 11
RESEARCH SCIENTISTS: 8

Assoc.Prof. E.Skordeva, Ph.D.
Assoc.Prof. Z.Ivanova, Ph.D.
Assoc.Prof. D.Arsova, Ph.D.
Assoc.Prof. V.Pamukchieva, Ph.D.
Assoc.Prof. Z.Aneva, Ph.D.
Asist.Prof. L.Yurukova
Asist.Prof. Z.Levi
A.Rachkova, chemist
I.Bineva, chemist
E.Zaharincheva, technologist
G.Tschauschew, physicist


RESEARCH ACTIVITIES:

1. DISORDERED MATERIALS - CHALCOGENIDE GLASSES AND THIN FILMS

    Compositional dependences and photoinduced changes of the optical properties of GexSb20-xÒe80 thin films are investigated. It is found that the optical band gap of the films increases with x. Compositional dependences of the mechanical strain in GexSb40-xS60 films deposited on c-Si substrates are studied and two extrema are observed related to the existence of the "strain free phase" and topological phase transition. Illumination and annealing of the films result in decrease and increase of the mechanical strain, respectively. The influence of the light and thermal treatments on the local electronic structure of GexSb40-xS60 glasses and thin films is explored by 121Sb Mossbauer spectroscopy. A relation is observed between the Mossbauer parameters and the level of structural disorder of the materials studied. The kinetics of crystallization and phase transition of bulk GexSb40-xSe60 glasses are investigated by differential-thermal and x-ray diffraction analyses. It is shown that at x≤27 these glasses are very stable since they do not undergo any crystallization.

    EXAFS and XANES investigations of the local structure of glasses and thin films from the As2S3-Ge2S3 and As2S3-GeS2 systems are performed. The light and thermally induced changes of the structure are also studied, the results being still under processing.

    Low-constrained glasses from the AsxS1-x system are synthesized having average coordination number between 2.16 and 2.4. Their density is determined and some physico-chemical parameters are calculated. Raman scattering spectra are measured on three different compositions in the temperature range 293-23 Ê and a well-pronounced Boson peak is observed in the low-frequency region. Thin AsxS1-x films are also deposited and compositional dependences of some their optical properties are explored.

    Basic physicochemical, structural and luminescence properties of Er-doped Ga2S3-GeS2 glasses are investigated. They show high thermal and mechanical stability (Òg=360-4000C, Tc-Òg~150 0C, HV=207-250 kg/mm2, d=2.9-3.4 g/cm3). It is shown that Er3+ ions play role as charge compensator of the non-bridging sulfurs due to local transformation of the main tetrahedral structural units. The strong emission band at 1.54 μm under excitation with λ=1.06 μm is assigned to transitions between sub-levels of the 4I13/2- and 4I15/2.levels. It is found that the photoluminescence intensity increases by about 30 times with the addition of 25 mol % Ga2S3 into GeS2, therefore the presence of Ga causes the rather enhanced Er solubility.

2. LOW-DIMENSIONAL SYSTEMS

    Carrier transport mechanism is studied in 1 μm thick Si-SiO2 thin films containing Si nanocrystals (NCs) with average size of 3.2-3.7 nm. The films are deposited on "conductive" (≤5 Ω.cm) and "resistive" (≤50 Ω.cm) silicon substrates. The samples on the resistive substrates show linear (Ohmic) current-voltage (I-V) characteristics at applied fields as high as 2.106 V/cm while the samples on the conductive substrates exhibit one Ohmic (at lowest fields) and two super linear parts. It is concluded that in the Ohmic regions current is due to thermally generated carriers in the SiO2 matrix. At fields >3.104 V/cm space charge limited currents are suggested. Spectral photocurrent measurements performed show that carriers photogenerated in both silicon substrates and Si nanocrystals have contribution to the total current measured. It is observed that photocarriers in Si NCs are excited by light whose energy is higher than the optical band gap of bulk silicon, which proves the quantum size increase of the optical band gap of nanocrystals.

    Transient photocurrent and thermally stimulated currents are measured in polycrystalline CdSe single layers as well as in SiOx-CdSe superlattices and composite films. The density of defect state distribution is calculated in the upper half of the band gap of the CdSe NCs. Two kinds of defect states are observed: that disposed at ~ 0.55 åV below the conduction band is assigned to bilk defect; the other one situated ~ 0.7 åV is related to defects at CdSe-CdSe interface.

3. AC ELECTROLUMINESCENCE. ELECTROLUMINESCENT STRUCTURES AND DISPLAYS

    Basic colorimetric characteristics are studied of electroluminescence structures having protective layers from TiO2 and As2S5. The relief of the layers is varied and their surface roughness is studied. A relation between the maximum height and mean-statistic deviation of the roughness and structure characteristics is established.

    New electroluminescence structures comprising a silicon-polymer layer are designed that have both protective and packing functions. As-prepared structures show pretty good brightness and time stability. A thermal treatment procedure is developed which increases the brightness of these structures between 6 and 10 times. This result opens the door for fabrication of bright displays with a high stability.

    Films from various conjugated polymers are produced and the possibility for application of those films is explored in development of new kinds of electroluminescence structures.
    Layers from ZnS powder in an epoxy oligomer are prepared. Various concentrations of fullerens and highly dispersive carbon are added to the films and their effect of the film properties is investigated. In order to clarify the process of electoluminescence activation in the films, measurements of diffusion scattering, dielectric losses, electrical resistance and ZnS electroluminescence are carried out. Investigations of electrical, optical and luminescence properties of gamma-modified ion-implanted carbon and selenium in various polymers are also in progress.


PUBLICATIONS:
  1. D. Arsova, V. Pamukchieva, E. Vateva, E. Skordeva, "Photo- and thermo-induced bandgap and volume changes in Ge-based chalcogenide films", J. Mat. Sci.: Materials in Electronics, 14, 835-836 (2003).
  2. D. Platikanova, E. Skordeva, D. Arsova, "Short- and medium-range order in thin As2S1.5Se1.5-Gex films: irreversible photostructural changes", J. Mat. Sci.: Materials in Electronics, 14, 833-834 (2003).
  3. E. Vateva, D. Arsova, E. Skordeva, V. Pamukchieva, "Irreversible and reversible changes in band gap and volume of chalcogenide films", J. Non-Cryst. Sol., 326&327, 243-247 (2003).
  4. V.Pamukchieva, A.Szekeres, K.Todorova, "Optical study of GexSb20-xTe80 chalcogenide films", J. Mat. Sci.: Materials in Electronics, 14, 837-838 (2003).
  5. K.Christova, A.Manov, V.Pamukchieva, A.G.Fitzerald, L.Jiang, "Mechanical stress study of amorphous GexSb40-xS60 films", J. Non-Cryst. Sol., 325, 142-149 (2003).
  6. Z.G. Ivanova, E. Cernoskova, V.S. Vassilev, S.V. Boycheva, "Thermomechanical and structural characterization of GeSe2-Sb2Se3-ZnSe glasses", Mater. Lett., 57, 1025-1028 (2003).
  7. Z.G. Ivanova, V.S. Vassilev, E. Cernoskova, Z. Cernosek, "Physicochemical, structural and luminescence properties of Er-doped Ge-S-Ga glasses", J. Phys. Chem. Solids, 64, 107-110 (2003).
  8. Z.G. Ivanova, Z. Aneva, Z. Cernosek, E. Cernoskova, V.S. Vassilev, "Influence of Ga on the luminescence efficiency of Er-doped Ge-S-Ga glasses", J. Mat. Sci.: Materials in Electronics, 14, 761-762 (2003).
  9. V. Balitska, R. Golovchak, A. Kovalskiy, E. Skordeva and O. I. Shpotyuk, "Effect of Co60 γ-irradiation on the optical properties of As-Ge-S glasses", J. Non-Cryst. Sol., 326-327, 130-134 (2003).
  10. S.Balabanov, K.Velitchkova, K. Krezhov, M. Sendova-Vassileva, "Photo-luminescence of carbon implanted polyethylene and its modification by gamma irradiation", Vacuum, 69, 107-112 (2003).
  11. K. Krezhov, K. Velitchkova, S. Balabanov, "Transport properties of selenium implanted polymer composites", Vacuum, 69, 113-118 (2003).
  12. T. Tsvetkova, S. Balabanov, E. Skordeva, S. Kitova, J. Sielanko, D. Maszka, J. Zuk, "Ion implantation induced surface morphology changes in As3Se2 films", Vacuum, 69, 471-475 (2003).
  13. Bineva, D. Nesheva, Z. Aneva, Z. Levi, C. Raptis, H. Hofmeister, S. Stavrev, "Effects of annealing atmosphere and substrate on the photoluminescence and Raman scattering from Si nanocrystals in SiO2 matrix", J. Material Sci.:Materials for Electronics, 14, 799 (2003).
  14. S. Reynolds, C. Main, I. Zrinscak, Z. Aneva and D. Nesheva, "A study of electronic defects in hydrogenated amorphous silicon prepared by the expanding thermal plasma technique", Mat. Res. Soc. Symp. Proc. 762, A19.14.1 (2003).
  15. V.S. Vassilev, Z.G. Ivanova, "Reversible α↔β phase transition in the narrow-gap semiconducting Ag4SSe compound", Bulletin of the Chemists and Technologists of Macedonia, Skopje (Macedonia), 22, No 1, pp. 21-24 (2003).
  16. V.S. Vassilev, L.N. Alzhihmany, P. Petkov, Z.G. Ivanova, "Glass-formation region, properties and structure of Ag2Se3-GeSe2-Ag4SSe chalcogenide system", Intern. Scientific Conference, 20-21 November, Gabrovo, Vol. 2, 349-352 (2003), in Bulgarian.
  17. Ê. Kolentsov, L. Yurukova, Ò. Êåhlibarov, À. Zheliazkova, "Îrganic color displays and possibilities for their application in textile industry", Tekstil i obleklo, 8, 11-12 (2003), in Bulgarian.
  18. Bineva, D. Nesheva, M. Sendova-Vassileva, Z. Aneva and Z. Levi, "Annealing behaviour of photoluminescence from a-SiOx thin films", Nanoscience and Nanotechnology, Eds. E. Balabanova and I. Dragieva, Heron press, 3, 91 (2003).
  19. R. Kotsilkova, E. Krusteva, D. Nesheva, T. Djunova, S. Stavrev, "Carbon nanoparticles in polymer: rheological properties and conductivity", Nanoscience and Nanotechnology, Eds. E. Balabanova and I. Dragieva, Heron press, 3, 169 (2003).
  20. E. Skordeva, "Photoinduced changes in amorphous GexAs(Sb)40-xS60 films", in the Homage Book dedicated to the Academician, Professor Andrei Andriesh and Prof. Serghei Shutov with their 70's anniversaries "Contributions to Non-Crystalline Semiconductor Physics and to Optoelectronics", eds. Arthur Buzdugan&Mihai Iovu, Int. Association of Academies of Sciences, Chisinau, pp. 40-49 ( 2003) .
  21. L. Pramatarova, E. Pecheva, D. Nesheva, Z. Levi, Z. Aneva, R. Pramatarova, U. Bismayer, T. Petrov, "Study of modified solid surfaces by nanostructured CdSe in SiOx thin films", Physica Status Solidi C, 0, No3, 1070-1074 (2003).

ONGOING RESEARCH PROJECTS:

Financed by the Bulgarian Academy of Sciences:
  1. Investigation of amorphous low-coordinated As-S materials
  2. Semiconductor nanoparticles in SiOx thin film matrix: formation, structure and properties
  3. Fabrication of electroluminescence structures based on new materials and media and investigation of their electroluminescence characteristics.
  4. Luminescence of chalcogenide glasses doped with rare earth elements.
Financed by the Bulgarian Ministry of Education and Sciences:
  1. Photoinduced structural changes in Ge-As(Sb)-S glasses and films
  2. Defect states in photoconductors of various dimensionality.

TEACHING ACTIVITIES:

Technical University, Sofia: "Semiconductor electroluminescence displays", 20 hours lectures and 4 diploma works prepared by students from the Technical University.


COLLABORATION:
  1. Electronic properties of nanocrystalline II-VI semiconductors, University Abertay Dundee, Scotland.
  2. Nano and micro scale structural transformations in thin films from glassy semiconductors: physical problems and possible applications, Physico-Technical Institute of the Russian Academy of Sciences, St. Petersburg, Russia.
  3. Thermodynamic and optical investigations on chalcogenide glasses, Joint Laboratory of Solid State Chemistry, Pardubice, Czech Republic.
  4. Defects and transport parameters of nanocrystalline CdSe and amorphous Si, University Abertay Dundee, Scotland.
  5. Àmorphous semiconductors:structure, properties and possible applications, Indian Institute of Technology, Kanpur, India.

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