Z. Levi

Photoluminescence of CdSe nanocrystals embedded in a SiOx thin film matrix

Abstract Room- and low-temperature photoluminescence studies are reported on CdSe nanocrystals embedded in an SiOx thin film matrix. The main spectral feature for all samples and both temperatures is a broad band whose position does not change considerably with nanocrystallite size. The band is assigned to recombination through defect states, whose energy depends on the nanocrystallite size in such a way that they counter-balance the similar dependence of the optical band gap on the nanocrystallite size. A noticeable asymmetry on the low-energy side of this band at low temperatures is attributed to the existence of a variety of surface defect states and/or a large number of volume defect states in these nanocrystals. There is evidence to suggest that the energy band diagram of CdSe nanocrystals in SiOx matrix is considerably different from that of SiOx/CdSe multi-quantum wells.

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.

Nanocrystals of CdSe in thin film matrix

Nanocrystals of CdSe have been produced in an thin film matrix by thermal vacuum evaporation of SiO and CdSe. A new way of forming CdSe nanoclusters in the matrix has been used. The average size of the CdSe nanocrystals, between 2.4 nm and 6.0 nm, has been estimated from the (110) maximum in the x-ray diffraction spectra. Quantum size increase of the optical bandgap of the CdSe nanocrystals has been observed. A good coincidence between the average size of the nanocrystals calculated from the observed bandgap increase and x-ray diffraction measurement has been obtained. The substructure observed in the nanocrystal absorption can be related to relatively small nanocrystal size fluctuations around the average size.

Absorption and transport properties of Si rich oxide layers annealed at various temperatures

Thin films of SiOx (x = 1.15, d = 1 and 2 µm), deposited by thermal vacuum evaporation of SiO on n- and p-type crystalline Si or quartz substrates, and then furnace annealed at 250, 700 and 1000 °C, are studied. Optical and infrared transmission measurements prove phase separation upon annealing at 700 and 1000 °C and growth of amorphous Si nanoparticles upon annealing at 700 °C, whose optical band gap is ~2.6 eV. High-resolution electron microscopy data confirm growth of Si nanocrystals with average size ~5 nm in the films annealed at 1000 °C. Both kinds of transmission data were used to estimate the nanoparticle volume fraction and values of 0.2–0.25 and 0.25–0.30 for the films annealed at 700 and 1000 °C, respectively, are determined. Current–voltage characteristics (at fields >5 × 104 V cm−1) are measured on metal/SiOx/c-Si/metal structures to explore carrier transport mechanisms in all kinds of samples. They are nearly symmetric, which indicates that in all samples carrier transport via structures is dominated by the transport in the SiOx layers. It is concluded that current transport is space-charge-limited for the layers annealed at 250 °C. In the films further annealed at 700 °C containing amorphous nanoparticles, Poole–Frenkel transport mechanism is reported while tunnelling is assumed for the films annealed at 1000 °C.

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.

DC electrical conductivity in glasses and thin films

DC electrical conductivity for bulk and thin-film samples of the Sb chalcogenide glass system was measured in the temperature range 293-430 K. The activation energy and the pre-exponential factor have been determined. Values of suggest different mechanisms of conductivity in bulk glasses and in thin films. For all compositional dependences a peculiarity was observed in the region with an average coordination number which can be associated with a topological phase transition from a 2D to a 3D structure.

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..

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.