Z. Aneva

Experimental studies on the defect states at the interface between nanocrystalline CdSe and amorphous SiOx

Superlattices of a-SiOx /nc-CdSe and thin composite films of SiOx doped with CdSe nanocrystals have been investigated. The CdSe nanocrystals size in both kinds of samples was determined by x-ray diffraction and HREM measurements. A significant difference has been found in the size values determined by both methods, which has been ascribed to appreciable nanocrystal lattice deformations. Subband absorption, room-temperature photoluminescence and thermally stimulated currents have been measured. It has been observed that in the superlattices the absorption in the tail region increases as sublayer thickness decreases. A new photoluminescence band has also appeared in the superlattices having thinnest (2.5 nm) CdSe sublayers. Two new maximums at about 220 K and 240 K, not existing in the CdSe single layers studied, have been found in thermally stimulated current spectra of the composite films. Both maximums are less expressed in the superlattices. The described results have been connected with a size-induced increase in the concentration of interface defect states in CdSe nanocrystals. It has been estimated that these defects are disposed at about 0.35 eV above the highest occupied molecular orbit in CdSe.

B12SiO20 monocrystals doped with transition metals

Abstract Bi 12 SiO 20 (BSO) crystals doped with Co, Ni, Cr, Mn, V and Fe have been studied. Classical transmission and reflection measurements and the constant photocurrent method have been employed to obtain the absorption coefficient in the 1.3–3.25 eV range. Temperature dependences of the photocurrent and thermostimulated currents have also been measured. From the changes induced in the absorption spectrum, the doping metals can be divided into three groups. The first group (Mn, Cr) causes a large increase in the total absorption coefficient. The second group (Co, Ni) leads to relatively weak changes in the whole spectral region studied. The third group (V, Fe) induces a strong “bleaching” effect in the “shoulder” region ( hv ≈ 2.5–2.8 eV) at high doping concentrations (> 1 mol% in the melt) accompanied by an absorption increase at the lower energies ( hv 2+ , V 5+ , Fe 3+ , Co 3+ and Ni 2+ ions.

Traps and recombination centers in pure and Co-doped Bi12SiO20 crystals

Abstract Pure and Co-doped (0.36-1.8 mol% Co) Bi 12 SiO 20 crystals have been studied. The temperature dependence of the dark and photoconductivity, thermostimulated currents and the spectral dependence of the photoconductivity have been measured. Exponentially distributed traps situated close to the conduction band bottom have been found in pure BSO crystals. It was found that Co atoms act as donors in BSO crystals, and two new traps at about 0.24 and 0.45 eV appear. Based on the observed thermal quenching of the photocurrent an identical energy position of the “slow” recombination centers has been determined in both type of crystals. The defects responsible for the “shoulder” absorption (at about 2.7 eV below the conduction band) have been found to play the role of these “slow“ recombination centers.

Crystal structure and spectral photosensitivity of thermally evaporated ZnxCd1−xSe thin films

Single layers of ZnxCd1−xSe with various compositions (x = 0.39, 0.52, 0.59, 0.69 and 0.8) were prepared by thermal vacuum evaporation. Consecutive deposition of films from ZnSe and CdSe with equivalent thickness of 0.12, 0.25 or 0.37nm was applied and the composition was varied by alloying ZnSe and CdSe films with different equivalent thicknesses. Single layers from ZnSe were prepared for comparison. X-ray diffraction, Raman scattering and spectral photocurrent measurements were carried out to get information on the film crystallinity and spectral photosensitivity. It was observed that both binary and ternary films are of cubic structure and nanocrystalline. No pure CdSe or ZnSe phases were found in the ZnxCd1−xSe films but the Raman data indicated that Cd-enriched nanosized regions can exist. The Raman scattering and spectral photocurrent results showed that the optical band gap of the ternary films gradually increased with increasing x and achieved a value of around 2.4eV for the films with x = 0.8. (Some figures in this article are in colour only in the electronic version)

Composition and structure of ZnxCd1?xSe single layers prepared by thermal evaporation of ZnSe and CdSe

Single layers of ZnxCd1?xSe with five different compositions and thickness of 400 nm have been prepared by thermal vacuum evaporation, through alloying of ultra thin ZnSe and CdSe films with equivalent thickness of 0.12, 0.25 or 0.37 nm. The deposition was carried out on rotating substrates kept at room temperature. The layer composition was varied by alloying ZnSe and CdSe films with different equivalent thicknesses. The film composition x = 0.39, 0.52, 0.59, 0.69 and 0.8 has been determined by Energy-Dispersive Spectroscopy and confirmed with Raman scattering data. The microstructure of ZnxCd1?xSe has been investigated by Atomic Force Microscopy and Raman scattering measurement. The Atomic Force Microscopy results have revealed that the layers are nanocrystalline and the grain size is ? 20 nm. The Raman scattering data have shown four replicas of the longitudinal optical phonons, thus confirming the conclusion for the layer crystallinity. The obtained results have shown that the applied deposition technique makes possible preparation of ternary nanocrystalline ZnxCd1-xSe layers with desired compositions..

Influence of Ga on the luminescence efficiency of Er-doped Ge–S–Ga glasses

Photoluminescence (PL) spectroscopy has been applied to a series of Er-doped (GeS2)100−x(Ga2S3)x glasses with varying x (0, 10, 20, and 25 mol %) and at a constant content of 0.6 mol % Er2S3. The PL band at 1534 nm, excited with 1064-nm light, has been characterized. The corresponding transitions in the Er3+ energy-level diagram have been specified by deconvolution of the spectra. It has been found that the PL intensity of the glasses becomes more definitive at x>10, and increases by about 30 times with the addition of 25 mol % Ga2S3.

Some properties of Bi12SiO20:Fe doped crystals

Abstract Pure and Fe-doped Bi 12 SiO 20 monocrystals have been studied. Five different Feoncentrations (0.36, 1.8, 3.6, 7.2 and 10.8 mol%) were incorporated. An increase of the dark conductivity by 4 orders of magnitude and a parallel decrease of the photoconductivity with increasing Fe-concentration were measured. A new trap with a concentration level of about 3 × 10 18 cm −3 , situated at about 0.42 eV below the conduction band bottom was observed. Contrasting changes in the absorption coefficient were found as a function of Fe-concentration. Three mechanisms are suggested to explain the observed changes in the electrical, photoelectrical and optical properties of the Bi 12 SiO 20 crystals with Fe content.

Effects of annealing atmosphere and substrate on the photoluminescence and Raman scattering from Si nanocrystals in a SiO2 matrix

Thin films of SiOx have been prepared on quartz or c-Si substrates by thermal evaporation of SiO in vacuum and post-annealed at 1373 K in an argon or hydrogen atmosphere. High-resolution electron microscopy has shown the existence of silicon nanocrystals in the annealed films, and this result has been confirmed by Raman scattering. Photoluminescence has been observed from annealed films and attributed to radiative recombination in Si nanocrystals. Its intensity is appreciably higher upon annealing in Ar than in H2. It is shown that substrates strongly affect the Raman scattering from Si nanocrystals in nc-Si–SiO2 thin films with low filling factors.

Annealing induced changes in ternary nanostructured ZnxCd1-xSe thin films: structure and morphology

Single layers of ZnxCd1-xSe with various compositions (x = 0.39, 0.59 and 0.8) were prepared by thermal vacuum evaporation at room substrate temperature. Consecutive deposition of small portions of ZnSe and CdSe with equivalent thickness of 0.12 or 0.37 nm was applied. X-ray diffraction and atomic force microscopy measurements were applied to explore the evolution of the crystal structure, microstructure, composition and surface morphology upon furnace annealing at 200 °C and 400 °C in an inert atmosphere. It has been found that as-deposited films were nanocrystalline with a grain size of around 10 nm and cubic structure. Upon annealing the size increased approximately three times and the cubic structure was preserved; no appearance of wurtzite phase was observed. It has been also ascertained that annealing caused significant reduction of the film surface roughness. Atomic force microscopy phase images revealed existence of a second phase on the surface of as-deposited films which disappeared after annealing. The effect of the preparation conditions on the film properties and annealing induced changes is discussed.

Optical Properties of ZnxCd1−xSe Single Layers Prepared by Thermal Vacuum Evaporation

Single layers from ZnxCd1−xSe with four different compositions and thickness of 400 nm were prepared by thermal vacuum evaporation through alloying of ZnSe and CdSe layers with an equivalent thickness of 0.12, 0.25 and 0.37 nm. The deposition was carried on rotating substrates kept at room temperature. The variation of the layer composition was achieved by changing the ratio of the deposition rates of ZnSe and CdSe. Optical transmission measurements have been carried out in the 400–2.500 nm spectral range and the Swanepoel’s approach has been applied in order to obtain the dispersion curves of the absorption coefficient and refractive index. The values of the refractive index determined are equal or a bit higher than those reported by other author for epitaxial layers. Values between 2.0 and 2.31 eV have been determined for the optical gaps which correspond to x‐values in the range 0.3–0.6. The obtained results indicate that preparation of films with desired compositions is possible at a precise control o...