S. Kokenyesi

Fully-screened polarization-induced electric fields in blue∕violet InGaN/GaN light-emitting devices grown on bulk GaN

Photocurrent spectroscopy and hydrostatic-pressure-dependent electroluminescence are used to show that heavy 1×1019cm−3 Si doping of quantum barriers is sufficient to achieve full screening of polarization-induced electric fields (PIEFs) in nitride light emitting diodes (LEDs) and laser diodes (LDs) with InGaN quantum wells. Furthermore, it is shown that at currents close to lasing threshold in nitride LDs injected charge alone is sufficient to achieve full screening of PIEFs. In contrast, full screening at low currents can only be accomplished via Si doping of quantum barriers.

Laser-induced optical changes in amorphous multilayers

It is shown that the well-known blueshift of the fundamental absorption edge in as-deposited compositionally modulated amorphous Si/Ge and As6Se94/Se80Te20 multilayers (with periods of 4–8 nm) is further enhanced due to the thermal or laser-induced intermixing of adjacent layers. The laser-induced intermixing process, as supported by experiments and model calculations, can be attributed to both the local heating and photo-effects in As6Se94/Se80Te20 multilayers, while only the thermal effects were observed for Si/Ge multilayers. Structural transformations, based on this enhanced interdiffusion, provide good capability for spatially patterning optoelectronic devices and digital information recording.

Photoinduced mass-transport based holographic recording of surface relief gratings in amorphous selenium films

Surface relief gratings formation in amorphous selenium thin films in two recording configurations with light intensity modulation were studied in situ by real-time atomic force microscopy and diffraction efficiency measurements. We report observation of mass transport effect in films induced by band-gap irradiation when the light polarization of the recording beams has a component along the light intensity gradient (“p-p” scheme of recording) that allows obtaining giant stable gratings in this versatile chalcogenide material. On the contrary, only a pure scalar weak grating caused by photoinduced volume shrinkage is obtained in the “s-s” recording configuration, even for long-term irradiation.

Inversion of the direction of photo-induced mass transport in As20Se80 films: Experiment and theory

Diffusion mass transfer in thin chalcogenide films under illumination by a focused Gaussian beam have been studied both experimentally and theoretically. It is demonstrated that depending on the light intensity, waist of the beam, and the film thickness, one can obtain formation of either hillocks or dips in the illuminated regions. By comparison of the kinetics of hillock or dip formation on a surface of As20Se80 glass films with the results of our theoretical analysis, we have estimated the photo-induced diffusion coefficients, D, at various light intensities, I, and found D to be proportional to I (D = βI), with β ≈ 1.5 × 10−18 m4/J.

Kinetics of photoinduced surface patterning in chalcogenide thin films

The kinetics of photoinduced variations in surface profile in chalcogenide glass films under illumination is described. It is demonstrated that the competition between the stress-induced atomic flux (toward irradiated regions of the film) and the diffusion flux induced by an increase in the bulk energy due to broken bonds (and directed from irradiated to dark regions) can result in either a positive or negative net mass transfer in the irradiated region. Depending on the light intensity, one can obtain either formation of bumps or depressions in the illuminated regions.

Electron beam-induced mass transport in As?Se thin films: compositional dependence and glass network topological effects

Electron beam (e-beam)-induced changes of surface profile morphology in AscSe1−c (0.2 <c< 0.5) thin films are investigated as a function of the film composition. It is shown that the extent and value of local surface alterations follow the composition-related changes of glass parameters such as softening temperature and glass network connectivity. The giant e-beam-induced surface relief changes detected in the films As0.2Se0.8 are connected with lateral mass transport, which increases drastically near rigidity transition, i.e. at a coordination number r ∼ 2.2 of the glass structures when the rigidity starts to percolate through the structure. The model of the process, which reflects the compositional dependence of the stimulated mass transport, is presented. (Some figures may appear in colour only in the online journal)

Kinetics and chemical analysis of photoinduced interdiffusion in nanolayered Se/As2S3 films

We have studied the kinetics of photoinduced effects in nanolayered Se/As2S3 film by in situ optical absorption measurements, which reveal that photodarkening in these films is followed by photoinduced diffusion. An increase in disorder during photodarkening and its subsequent decrease during photoinduced diffusion were also observed. The observation of photodarkening of Se at room temperature when confined between As2S3 layers suggests that the glass transition temperature of Se shifts to higher energy. The analysis shows that the atoms which take part in photodarkening play a vital role in photoinduced diffusion. The x-ray photoelectron spectroscopy measurements show the atomic movements during photoinduced diffusion. It also shows that some of the As–S bonds are converted into As–Se bonds. Since it is energetically difficult to break an As–S bond to form an As–Se bond, we assume that the new bond formations are taking place by the bond rearrangement mechanism.

Observation of three-photon absorption and saturation of two-photon absorption in amorphous nanolayered Se/As2S3 thin film structures

We have studied the nonlinear optical properties of nanolayered Se/As2S3 film with a modulation period of 10 nm and a total thickness of 1.15 mu m at two [1064 nm (8 ns) and 800 nm (20 ps)] wavelengths using the standard Z-scan technique. Three-photon absorption was observed at off-resonant excitation and saturation of two-photon absorption at quasiresonant excitation. The observation of the saturation of two-photon absorption is because the pulse duration is shorter than the thermalization time of the photocreated carriers in their bands and three-photon absorption is due to high excitation irradiance. (c) 2007 American Institute of Physics.

Photoinduced interdiffusion in nanolayered Se∕As2S3 films: Optical and x-ray photoelectron spectroscopic studies

Photoinduced interdiffusion was observed with above band gap light in nanolayered Se/As2S3 films. It is discussed in terms of the optical parameters such as band gap, Urbach edge (E-e) [F. Urbach, Phys. Rev. 92, 1324 (1953)], and B-1/2 (Taucs parameter) [J. Tauc , Phys. Status Solidi 15, 627 (1966)]. Experimental data of B-1/2 and E-e for as-prepared samples do not show clear correlation implied by the Mott-Davis model [N. F. Mott and E. A. Davis, Electronic Process in Non-crystalline Materials (Clarendon, Oxford 1979), p. 287]. It is also shown that the optical parameters can be changed with a change in the Se sublayer thickness. Variations of these optical parameters as a function of modulation period and photoinduced interdiffusion were discussed in terms of the quantum confinement effect and changes in the valence and conduction bands. We proposed a model to explain the mechanism of Se diffusion in As2S3, which suggests that diffusion takes place through the wrong bonds. X-ray photoelectron spectroscopy (XPS) is used to investigate the chemical alternations in the bonding. The proposed model was supported by the XPS data.

Enhancement of photoluminescence intensity by photoinduced interdiffusion in nanolayered a-Se∕As2S3 films

Optical parameters of chalcogenide glass multilayers with 12–15 nm modulation lengths prepared by thermal evaporation can be changed by laser irradiation. Photoluminescence sPLd studies were carried out on such nonirradiated and irradiated multilayered samples of