Green/blue light emission and chemical feature of nanocrystalline silicon embedded in silicon-oxide thin film

2 in the range of annealing temperatures used. The PL intensity, weight of the Si⁴⁺ states, and the volume fraction of Si nanocrystals exhibit a large increase as T_a>750 °C. The PL peak position is independent of the annealing temperature (T_a). From our observations, the green/blue light emission is related to the defects. Received: 10 July 1996/Accepted: 2 April 1997     

Thermal effects on optical properties of silver ruby glass

2 O·10 CaO·74 SiO₂ mol%) glass doped with 0.11 and 0.35 wt.% silver are investigated. Heating treatments are carried out in a temperature range between 400 and 575 °C for times ranging from 30 to 300 min in different atmospheres and cooling rates. The starting glasses show a colourless and transparent appearance, but after thermal treatments under a reducing atmosphere become coloured, confirming the presence of silver colloids related to the 410-nm absorption band. On the other hand, the main effects of thermal treatments on the PL spectra concern those from samples treated in a reducing atmosphere. Thus, the intensity of both excitation and emission spectra chiefly diminishes in the 220–230 nm and 325–350 nm ranges, respectively. In addition, time-resolved spectra show the main ultraviolet (UV) emission centred above 330 nm upon excitation with 228-nm light. In contrast to the starting glass, we notice no shift or even slight shifts of the peak position to longer wavelengths with increasing delay time after pulse excitation, even for delay times as short as 0.01 ms. The results are discussed on the basis of transitions in which Ag⁺ ions are involved. Received: 6 February 1998/Accepted: 9 March 1998     

Structure, refractive-index dispersion and the optical absorption edge of chemically deposited ZnxCd(1-x)S thin films

Zinc cadmium sulfide, Zn_xCd_(1-x)S, thin films have been deposited by a simple and inexpensive chemical bath deposition method from an aqueous medium using thiourea as a sulfide-ion source. The structure of the deposited films has been characterized by X-ray diffraction and transmission electron microscopy. It was observed from X-ray diffraction that the as-deposited films were amorphous in nature. However Zn_xCd_(1-x)S films annealed at 423 K for 1.5 h show a crystalline structure with a small scattering volume. The obtained results were confirmed throughout the transmission electron microscopy and the corresponding electron-diffraction patterns. The optical constants of Zn_xCd_(1-x)S films annealed at 423 K for 1.5 h in the compositional range 0≤x≤1 were estimated using transmission and reflection spectra in the wavelength range 300–2500 nm. The band gap varies non-linearly with the value of x. The dependence of the refractive index on the wavelength obeys the single-oscillation model, from which the dispersion parameters and the high-frequency dielectric constant were determined. A graphical representation of the surface and volume energy-loss functions was also given. Received: 23 February 2001 / Accepted: 26 February 2001 / Published online: 27 June 2001     

Preparation and characterisation of compositionally graded BaxSr1-xTiO3 thin films

Ba_xSr_1-xTiO₃ thin films with a compositional gradient of x=0.3 to 1 (in 0.1 mole fraction increments) were fabricated on Pt/Ti/SiO₂/Si substrates using a modified sol–gel technique. The graded film crystallised in a perovskite structure and consists of a uniform microstructure with comparatively larger grains. The room-temperature relative dielectric constant (ε_r) and dielectric loss (cosδ) at 100 kHz were found to be 305 and 0.03 respectively. Dielectric peaks were not observed in the temperature range from -20 °C to 120 °C. The dielectric constant and dielectric loss were almost independent of temperature. Polarisation–electric field measurements at room temperature revealed a saturated but slim hysteresis loop with a remanent polarisation (P_r) of 0.6 μC/cm² and a coercive field (E_c) of 2.4 kV/mm. The graded film behaves as a stack of Ba_xSr_1-xTiO₃ capacitors connected in series and hence the dielectric Curie peaks are removed. Received: 4 October 2001 / Accepted: 17 October 2001 / Published online: 23 January 2002     

Eu3+ luminescence and Gd3+-Eu3+ energy transfer in K5Li2GdF10:Eu3+

Polycrystalline samples of europium-doped K₅Li₂GdF₁₀ have been obtained by a slow cooling of melted compound and investigated using spectroscopy methods. Luminescence from the ⁵ D ₂ level of Eu³⁺ is found to be weak. Intense visible emission upon excitation into the ⁵ D ₂ or higher energy levels has been attributed to overlapping transitions from long-lived ⁵ D ₁ and ⁵ D ₀ levels. A strong increase of the ⁵ D ₀ emission at the expense of the ⁵ D ₁ emission occurs between 5 K and 25 K without significant change of the ⁵ D ₁ lifetime. To account for this, it is supposed that both the radiative and the nonradiative transition rates are temperature-dependent. Efficient energy transfer from the ⁶ G _J levels of Gd³⁺ to Eu³⁺ ions has been evidenced by excitation spectra in the VUV region and VUV-excited luminescence. It has been concluded that the cross relaxation contributes to the energy-transfer process. Received: 8 May 2001 / Accepted: 11 May 2001 / Published online: 25 July 2001     

Photoluminescence characteristics of neodymium oxide nanocrystal/titania/ormosil composite sol-gel thin films

Neodymium (III) oxide nanocrystal/titania/organically-modified silane (ormosil) composite thin films have been prepared using a chemical approach consisting of a combination of inverse microemulsion and sol-gel techniques at low temperature. Transmission electron microscopy shows that the neodymium (III) oxide nanoparticles have a needle-like nanocrystal structure. A strong room temperature emission at 1064 nm, corresponding to the ⁴ F _3/2?⁴ I _11/2 transition, has been observed as a function of the heat treatment temperature used for the production of the composite thin films. In addition to this emission, two other main emissions at 890 nm and at 1336 nm have also been observed. In particular, there was a clear shoulder peak at 1145 nm, probably be due to the host matrix, which was observed in all the measured samples and this shoulder peak gave a maximum intensity after heat treatment at 300 °C. Received: 6 September 2000 / Accepted: 15 November 2000 / Published online: 20 June 2001     

Influence of substrate in photothermal measurements of thin film absorption

In photothermal measurements of low optical absorption coefficients, as in the case of thin films of amorphous semiconductors, the substrate absorption can play a non-negligible role. In this paper, the substrate influence is discussed and generalized on the basis of a theoretical model of a two-layer absorbing sample, and the interaction of the thermal and optical parameters which control the photothermal process in the sample is shown. Some experimental results obtained on thin films of amorphous silicon carbide deposited on different substrates are presented and discussed.     

A study of some optical properties of hafnium dioxide (HfO2) thin films and their applications

2 ), a series of films ranging in thickness from 50 to 10000 nm was prepared by using an electron beam gun inside an evacuated coating chamber of pressure 1×10^-5 mbar. The films were obtained on optical glass substrate by using oxygen with a backfill pressure of 2.4×10^-4 mbar during the deposition processes. The optical constants of the films were computed in the spectral wavelength region (350–2000 nm) from the transmission, reflection and thickness measurements. A computer program was created to determine two optical parameters n and k of the films, and this was achieved by entering the practical results into the computer program, which solved a series of equations for each wavelength. The effects created by changing various evaporation conditions (thickness, substrate temperature and evaporation rate) were studied in the spectral wavelength range, and the optimum values of the various conditions were obtained while achieving the best optical performance. According to the investigations of the HfO₂ material, two applications of the anti-reflection (AR) multi-layer coatings were achieved in two different spectral wavelength ranges. The first application was measured in the visible and near infra-red (VIS/NIR) range from 500 nm to 850 nm deposited on the glass substrate. The second application was measured in the infrared (IR) range from 7500 nm to 11500 nm deposited on germanium substrate. Computer modelling for designing the optical multi-layer system has been presented. The theoretical formulation and experimental results with the same specification were achieved. The correlation between the theoretical and the experimental results reveals a close agreement that offers a convenient method for predicting and controlling the multi-layer coating. By continuous measurement of the optical and mechanical (durability) performances of the coating process, high-quality films were produced in the manufacture of various optical devices. Received: 16 April 1997/Accepted: 5 August 1997     

The effect of alkaline earth metallic ions on the photoluminescence properties of sol-gel silica glass

Mg²⁺-, Ca²⁺-, Sr²⁺- and Ba²⁺-doped silica glasses have been prepared using sol-gel processing by employing Si(OC₂H₅)₄, MgCl₂6H₂O, CaCl₂2H₂O, SrCl₂6H₂O and BaCl₂2H₂O as precursors, with HCl as a catalyst. The UV–visibleabsorption spectra of the doped samples are almost the same as those of the undoped sample. The absorption bands of alkaline earth metallic ions have not been observed in the doped samples. Strong visible light has been observed from sol- gel silica glasses doped with alkaline earth metallic ions. The relative fluorescence intensity of the Sr²⁺-doped (the impurity mole ratio of Sr²⁺ was 0.268%) and the Ba²⁺-doped (the impurity mole ratio of Ba²⁺ was 0.448%) samples was about 4 times that of the undoped sample. The relative fluorescence intensity of the Mg²⁺-doped (the impurity mole ratio of Mg²⁺ was 0.069%) sample was about 2.5 times that of the pure glass sample. The relative fluorescence intensity of the Ca²⁺- doped (the impurity mole ratio of Ca²⁺ was 0.179%) sample was about 3 times that of the pure glass sample. Alkaline earth metallic ions affect the formation and conversion of luminescent defects in sol-gel silica glass. Thus, the relative fluorescence intensity of the doped samples increases more than that of the undoped sample. Received: 17 April 2001 / Accepted: 6 June 2001 / Published online: 30 August 2001     

The effects of annealing on the structure and magnetic properties of CoNi patterned nanowire arrays

NiCo alloy nanowire arrays were fabricated by using an electrochemical method in a porous anodic aluminum oxide (PAO) template. X-ray-diffraction results showed that there was a preferred orientation in the CoNi nanowire arrays with Ni content ranging from 0.2 to 0.8 in the as-electro-deposited state, while a random orientation was observed after the sample was heat treated in an argon atmosphere. When the shape anisotropy is very high in the sample, it is found that the squareness (Mr/Ms) of the hysteresis loops for the samples is only about 0.6 before annealing, but increases to about 0.9 after annealing. All the results are discussed qualitatively and an explanation for the magnetic reversal mechanism in terms of a localized magnetization model is given. As the CoNi alloy nanowire arrays have high bit densities, they can be used as perpendicular magnetic recording media of high-density quantum magnetic disks. Received: 8 January 2001 / Accepted: 6 June 2001 / Published online: 30 August 2001     

Effect of the growth conditions on infrared upconversion efficiency of CaS: Eu, Sm thin films

High-quality electron-trapping thin films CaS: Eu, Sm with red light output have been successfully deposited on quartz and single-crystal silicon substrates by electron beam evaporation (EBE) and radio frequency (RF) magnetron sputtering in vacuum and H₂S partial pressures. Infrared upconversion efficiency of CaS: Eu, Sm thin films under different growth conditions has been investigated by using ultrashort infrared (IR) laser pulse with 20 ps (full width at half-maximum (FWHM)). The results show that upconversion efficiency of CaS: Eu, Sm thin films depends strongly on growth conditions in spite of the existence of “exhaustion” phenomena. Microstructures identified by X-ray diffraction (XRD) indicate that crystallinity of CaS films relies on both substrate materials and growth conditions. The stoichiometric composition of CaS films was measured by secondary-ion mass spectrometry (SIMS). The post-annealing process was found to promote grain growth and activate strong luminescence so that it could obviously improve upconversion efficiency of CaS: Eu, Sm films, even though it had negative influence on transmittance and spatial resolution of these films. Received: 5 June 2000 / Accepted: 7 June 2000 / Published online: 23 August 2000     

Motional narrowing of broadband absorption of solvatochromic indicator Betaine 30

Temperature dependence of the absorption spectra of Betaine 30 (2,6-diphenyl-4-(2,4,6-triphenylpyridinio)-phenolate, widely known as Reichardt's dye) is compared with that of the broadband spectra of other dyes. The double value of the half-width (2 hwhm) of Betaine 30 absorption band between 10 and 293 K in a polymer decreases by 130±20 cm⁻¹, from 4220 to 4090 cm⁻¹. This unprecedented narrowing with increasing temperature is ascribed to hydrogen bond symmetrization due to thermally activated proton tunneling in a pigment-solvent complex. The averaging of charge distribution along the proton coordinate causes motional narrowing of inhomogeneous width.     

Light-induced charge transfer and kinetics of the NIR absorption of Nb4+ polarons in SBN crystals at low temperatures

Sr_1-xBa_xNb₂O₆ (SBN) crystals with open tungsten-bronze structure show enhanced photorefractive properties with doping of impurities such as Ce, Cr, Rh etc. Under illumination with Kr⁺ laser (647 nm) or Ar⁺ laser light (488 nm or 514 nm) or UV light at low temperature, pure and doped SBN crystals show a broad polaron absorption band around 0.7 eV (6000 cm^-1). The first step of a theoretical model involves the excitation of electrons by illumination from Cr³⁺/Ce³⁺ to higher excited states or the conduction band. The excited electrons can then be trapped by Nb⁵⁺ to form Nb⁴⁺ polarons and further on can directly tunnel through or hop over the potential barrier (with a value Δ≈0.15±0.02 eV) to recombine with Cr⁴⁺/Ce⁴⁺ ions. The experimental intensity dependence, temperature dependence, and decay process of the light-induced Nb⁴⁺ polarons can be fitted with the help of this model. Small, but systematic, differences lead to the additional assumption of different recombination rates of polarons at distinct distances from the Cr⁴⁺/Ce⁴⁺ recombination centers and therefore many parallel decay channels are active where each decay channel obeys a monoexponential decay law. A stretched exponential decay function is employed to fit in this case the decay process of the Nb⁴⁺ polarons at different temperatures and under illumination with different intensities. Due to the high dielectric constant value (ε₃₃ and ε₁₁ have values in the 10²-10³ range) at low temperature, the long range Coulomb attraction (to Ce³⁺ _Sr/Ba) or repulsion (from Cr³⁺ _Nb) of the electronic polaron is suppressed. The leading role in the attraction and the following trapping of the electronic Jahn–Teller polaron, both on Cr³⁺ _Nb and Ce³⁺ _Sr/Ba centers, is played by the indirect dipole–dipole interaction via the soft TO-mode. Received: 27 November 1998 / Revised version: 22 January 1999 / Published online: 7 April 1999     

Photoluminescence effect of Ru dye on alumina membranes with ordered pore arrays

We have investigated the quenching luminescence and the luminescence mechanisms of self-assembled Ru(II)L₂X₂ (Ru(II)(4,4’-dicarboxyl-2,2’-bipyridyl)₂(SCN)₂) dye in porous anodic alumina. Porous anodic alumina can emit visible light due to numerous oxygen vacancies formed during anodic oxidation. Energy transfer from the porous anodic alumina to the dye molecules causes the quenching and red shift of the PL peak, which gives a fundamental understanding of the PL mechanism in porous anodic alumina. Received: 29 May 2001 / Accepted: 3 August 2001 / Published online: 30 October 2001     

Crystallization kinetics of amorphous aluminum–tungsten thin films

82 W₁₈ to Al₆₂W₃₈ compounds ranged from 800 K to 920 K. The activation energy for the crystallization and the Avrami exponent were determined. The results indicated that the crystallization mechanism in films with higher tungsten content was a diffusion-controlled process, whereas in films with the composition similar to the stoichiometric compound (Al₄W), the interface-controlled crystallization probably occurred. Received: 30 March 1998/Accepted: 9 September 1998     

Impurity spectroscopy in glasses and disordered crystals: Inhomogeneous broadening and electron phonon coupling

Parameters of optical impurity spectra in disordered solids were calculated using the potential energy distribution of the ground state and the guest-host interaction potentials of the lower and upper states. The model can yield inhomogeneous band shapes, pressure shift coefficients of zero phonon lines, (pseudo)local phonon frequencies, and linear and quadratic coupling constants to phonons. Results are compared to properties of Shpol'skii multiplets and zero phonon holes burned over the broad spectra in glasses. Crystal spectra contain discrete lines, but the overall width of multiplets and the bands in weakly polar solvent glasses is similar, and so are the pressure shift coefficients. A decrease of zero phonon transition probability (Debye-Waller factor) with increasing (negative) solvent shift was predicted and confirmed for crystal spectra. In general, no correlation exists between the strength of the first- and second-order couplings, and the vibrational modes involved can be different. Moreover, no relationship was established between the line shift and broadening in a temperature interval from 5 to 100 K.     

Preparation and characterisation of ultrafine amorphous alloy particles

Received: 3 July 1998     

Photoluminescence of a silver-doped glass

The absorption, emission and excitation spectra of Ag⁺ ions in a soda lime glass doped with two different concentration of silver are investigated. Absorption spectra exhibit a main broad band peaked at about 260 nm (4.77 eV) with a shoulder at about 227 nm (5.46 eV). The relative height of the shoulder depends on silver concentration in the glass. Emission spectra of Ag⁺ are dominated by an ultraviolet broad band at about 330 nm (3.76 eV). The excitation spectra for this emission show a preponderant broad band peaked at about 227 nm (5.46 eV) which coincides with peak position of the shoulder displayed in the optical absorption spectra. A weak broad featureless emission band centred at about 550 nm (2.25 eV) with an excitation peak at about 242 nm (5.12 eV) is tentatively related to an impurity from the host silica glass rather than originated in silver-type centres. Comparison of the luminescence decay curves for both emissions show substantial differences between them. Consequently, the emissions in the time-resolved spectra can easily be discriminated.     

Excimer laser ablation of thin gold films on a quartz crystal microbalance at various argon background pressures

Received: 21 June 1996/Accepted: 9 December 1996     

Vibrational spectroscopy of SiC thin films deposited by excimer laser ablation

A large number of thin SiC films, prepared at different conditions by KrF excimer laser ablation of solid SiC targets and deposition onto Si substrates (some onto quartz glass (QG) and yttrium-stabilized zirconia (YSZ)) were characterized by infrared and Raman spectroscopy. The films consisted of nano- and microcrystalline SiC and contained nanocrystalline carbon in the case of QG or YSZ substrates. Raman spectra of nanocrystalline SiC (grains <30 nm) reflect the phonon density-of-state function of SiC by broad scattering effects at 220–600 and 650–950 cm⁻¹. Medium-size crystallites are represented by a relatively narrow asymmetric band at 790 cm⁻¹ and crystallites >200 nm by an additional asymmetric band at 960 cm⁻¹. Small satellite bands at 760 and 940 cm⁻¹, attributed to SiC surface layers, were resolved in some well-ordered samples. Optical modelling was needed to interpret the IR spectra. SiC films could be represented by an effective medium model containing a SiC host phase and embedded particles with free charge carriers. The crystalline order of SiC films can be estimated from the parameters of the SiC oscillators. Received: 5 October 1998 / Accepted: 8 January 1999 / Published online: 5 May 1999