D. Nesheva

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.

Thin and superthin photoconductive CdSe films deposited at room substrate temperature

Low conductive and photosensitive CdSe films were deposited on substrates at room temperature. X-ray diffraction spectra showed a microcrystalline cubic structure for film thicknesses greater than 20 nm and an amorphous structure for film thicknesses below 10 nm. An optical band gap, Eg0, of 1.6 eV was determined using the Tauc-dependence usually employed for amorphous semiconductors. The dark conductivity, σ, of 10−9 Ω−1 cm−1 was measured in the as-deposited state, but an increase of five orders of magnitude was observed after heating the layer above 450 K. High photosensitivity was observed under illumination with white light as well as with monochromatic light over a wide spectral region (400–750 nm). A conclusion is reached concerning the existence of compensated donor and acceptor defects in the as-deposited state.

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.

Stepwise optical absorption in amorphous SeTe/CdSe superlattices

Abstract Optical absorption and photoconductivity of new chalcogenide amorphous SeTe/CdSe superlattices of pnpn type are investigated. The optical absorption coefficient α changes stepwise with the variation of the intensity and duration of the illumination. The analysis of the experimental results indicates the formation of subbands in the conduction and valence band. The essential role of the indirect transitions at sublayer thickness below 100 A is discussed.

Small-angle X-ray diffraction studies on interface sharpness of amorphous Se/CdSe superlattices

Abstract X-ray diffraction measurements have been performed to evaluate the interface sharpness of Se/CdSe amorphous multilayers as well as their thermal stability. A value of about 1.6 nm has been measured as the effective interface thickness, di. It decreased to di ≈ 1.3 nm after the samples were annealed at an optimum temperature, Ta ≈ 360 K. A negative interdiffusion coefficient has been obtained in this case. More complicated temperature dependences of the X-ray diffraction peaks have been observed at higher annealing temperatures. Two different thermally dependent processes have been considered.

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)