Investigations on structural,optical and electrical parameters of spray deposited ZnSe thin films with different substrate temperature

ZnSe thin films have been deposited on high cleaned glass substrate by spray pyrolysis technique within the glass substrate temperature range (400 ^○C to 450 ^○C). The structural properties of ZnSe thin films have been investigated by (XRD) X-ray diffraction techniques. The X-ray diffraction spectra showed that ZnSe thin films are polycrystalline and have a cubic (zinc blende) structure. The most preferential orientation is along the (111) direction for all spray deposited ZnSe films together with orientations in the (220) and (311) planes also being abundant. The film thickness was determined by an interferometric method. The lattice parameter, grain size, microstrain and dislocation densities were calculated and correlated with the substrate temperature (T_S). The optical properties of ZnSe thin films have been investigated by UV/VIS spectrometer and the direct band gap values were found to be in the region of 2.65 eV to 2.70 eV. The electrical properties of ZnSe thin films have been investigated using the Van der Pauw method and the high quality ZnSe thin films were observed to develop at 430 ^○C with a resistivity of 56,4×10⁵ ohm cm, a conductivity of 1.77×10^-7 (Ω cm)^-1 and a hall mobility of 0.53 cm²/Vsec.     

Saturable absorbers for passive Q-switching of erbium lasers emitting in the region of 3 μm

The phototropic properties of Fe:ZnSe, Co:ZnSe, and Co:ZnS single crystals have been investigated. It is shown that these crystals can be used to advantage as the saturable absorbers in solid-state erbium lasers emitting in the region of the 3-μm range. The absorption cross sections of the ground states of the Co²⁺ ion in the ZnSe (σ_GSA = 11·10⁻²⁰ cm²) and ZnS (σ_GSA = 5.6·10⁻²⁰ cm²) crystals and of the Fe²⁺ ion in the ZnSe (σ_GSA = 50·10⁻²⁰ cm²) crystal at λ = 2.79 μm were determined experimentally. It has been established that the above-indicated crystals in the excited state absorb light weakly. The use of these crystals as passive Q switches made it possible to realize, for the first time, the regime of Q-switching of a Cr,Er:YSGG laser emitting at a wavelength of 2.79 μm. Single pulses with an energy of 60 mJ and a duration of 170 nsec were obtained in the regime of passive Q-switching. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 6, pp. 747–751, November–December, 2005.     

Production and characterization of Nd,Cr:GSGG thin films on Si(001) grown by pulsed laser ablation

Nd,Cr:Gd₃Sc₂Ga₃O₁₂ (GSGG) thin films have been produced for the first time. They were grown on Si(001) substrates at 650 °C by pulsed laser ablation at 248 nm of a crystalline Nd,Cr:GSGG target rod. The laser plume was analyzed using time-of-flight quadrupole mass spectroscopy, and consisted of elemental and metal oxide fragments with kinetic energies typically in the range 10 to 40 eV, though extending up to 100 eV. Although films deposited in vacuum using laser fluences of 0.8±0.1 J cm⁻² reproduced the Nd,Cr:GSGG bulk stoichiometry, those deposited using fluences above ≈3 J cm⁻² resulted in noncongruent material transfer and were deficient in Ga and Cr. Attempts to grow films using synchronized oxygen or oxygen/argon pulses yielded mixed oxide phases. Under optimal growth conditions, the films were heteroepitaxial, with GSGG(001)[100]∥Si(001)[100], and exhibited Volmer–Weber-type growth. Room-temperature emission spectra of the films suggest efficient non-radiative energy transfer between Cr³⁺ and Nd³⁺ ions, similar to that of the bulk crystal. Received: 1 October 1999 / Accepted: 15 October 1999 / Published online: 23 February 2000     

Amorphous chalcogenide AgSbS2 films prepared by pulsed laser deposition

The pulsed laser deposition technique has been applied to prepare amorphous ternary AgSbS films. The films were prepared from AgSbS₂ bulk glass using a KrF excimer laser. The composition of prepared films according to the results of the energy dispersive X-ray analysis and the Rutherford backscattering was close to bulk one. Optical transmission and spectral dependence of the refractive index proved the good optical quality of the films. The Ar⁺ ion laser dot exposures of the films show a potential applicability of the films as a new type of optical recording material. PACS 78.66.Jg; 81.15.Fg; 81.40.Wx     

Synthesis and characterisation of co-evaporated tin sulphide thin films

Tin sulphide films were grown at different substrate temperatures by a thermal co-evaporation technique. The crystallinity of the films was evaluated from X-ray diffraction studies. Single-phase SnS films showed a strong (040) orientation with an orthorhombic crystal structure and a grain size of 0.12 μm. The films showed an electrical resistivity of 6.1 Ω cm with an activation energy of 0.26 eV. These films exhibited an optical band gap of 1.37 eV and had a high optical absorption coefficient (>10⁴ cm^-1) above the band-gap energy. The results obtained were analysed to evaluate the potentiality of the co-evaporated SnS films as an absorber layer in solar photovoltaic devices. PACS 78.40.Fy; 68.60.-p; 61.10.Nz; 68.55.-a; 78.66.-w     

Surface roughness-aided hard X-ray emission from carbon nanotubes

Efficient low debris hard X-ray source based on multiwalled carbon nanotubes (MWNT) irradiated by intense, femtosecond laser over an intensity range of 10¹⁵–10¹⁷ W cm⁻² μm² is reported. The MWNT targets yield two orders of magnitude higher X-rays (indicating significant enhancement of laser coupling) and three orders of magnitude lower debris compared to conventional metallic targets under identical experimental conditions. The simple analytical model explains the basic experimental observations and also serves as a guide to design efficient targets to achieve low-debris laser plasma-based hard X-ray sources at low laser intensities suitable for multi-kHz operation.     

Preparation and nonlinear characterization of zinc selenide nanoparticles embedded in polymer matrix

Nanocomposites of ZnSe nanoparticles embedded in polyvinyl alcohol (PVA) matrix have been prepared by in-situ synthesis. ZnSe/PVA nanocomposites are characterized by X-ray diffraction (XRD), Transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and UV/Vis spectra. The nanocomposite structure is confirmed by the blue-shift of the absorption edge. The nonlinear refractive index and two-photon absorption (TPA) coefficient are measured by the Z-scan technique using low power CW He–Ne laser light. The results show that the ZnSe nanocomposite films show large optical nonlinearity and the magnitude of the third-order nonlinear susceptibility χ⁽³⁾ is calculated to be 2.62×13^?11 m²/V².     

Optimization of ZnSe film growth conditions for p-type doping

Wide bandgap semiconductors such as ZnSe and ZnO have attracted great interest due to their applications in solar cells, light emitting diodes, and lasers. However, these wide bandgap semiconductors are frequently difficult to be doped to heavy concentrations, greatly limiting their application. A substrate holder with a natural temperature gradient was developed for batch growth of films at different deposition temperatures, in order to investigate ZnSe film growth and doping challenges. Thin ZnSe films were grown by pulsed laser deposition and characterized using X-ray diffraction, optical transmission and reflection, Raman spectroscopy, and Energy Dispersive X-ray analysis. Deposition temperature and film stoichiometry (Zn:Se) are shown to be significant factors affecting ZnSe growth and doping. ZnSe films with improved crystallinity have been obtained by enriching with selenium and depositing at an optimized temperature. Heavily p-type ZnSe films with hole concentrations of ~2.7 × 10¹⁹ cm^?3 and resistivities of ~0.099 Ohm cm have been obtained (compared with previous reports of ~1 × 10¹⁸ cm^?3 and ~0.75 Ohm cm). The results, which are consistent with previous theoretical prediction of compensating defects in ZnSe films, can help to optimize ZnSe growth conditions and understand doping challenges in wide bandgap semiconductors.     

Patterning of PLZT and PSZT thin films by excimer laser

The patterning of lanthanum-doped lead zirconate titanate (PLZT) and strontium-doped lead zirconate titanate (PSZT) thin films has been examined using a 5-ns pulsed excimer laser. Both types of film were deposited by rf magnetron sputtering with in situ heating and a controlled cooling rate in order to obtain the perovskite-structured films. The depth of laser ablation in both PSZT and PLZT films showed a logarithmic dependence on fluence. The ablation rate of PLZT films was slightly higher than that of PSZT films over the range of fluence (10–150 J/cm²) and increased linearly with number of pulses. The threshold fluence required to initiate ablation was ∼ 1.25 J/cm² for PLZT and ∼ 1.87 J/cm² for PSZT films. Individual squares were patterned with areas ranging from 10×10 μm² up to 30×30 μm² using single and multiple pulses. The morphology of the etched surfaces comprised globules which had diameters of 200–250 nm in PLZT and 1400 nm in PSZT films. The diameter of the globules has been shown to increase with fluence until reaching an approximately constant size at ≤ 20 J/cm² in both types of film. The composition of the films following ablation has been compared using X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy. PACS 79.20.Ds; 82.80.Pv; 82.80.Ej     

Effect of deposition parameters on structural, optical and electrical properties of nanocrystalline ZnSe thin films

This paper presents the chemical bath deposition of zinc selenide (n-ZnSe) nanocrystalline thin films on non-conducting glass substrates, in an aqueous alkaline medium using sodium selenosulphate as Se²⁻ ion source. The X-ray diffraction studies show that the deposited ZnSe material is nanocrystalline with a mixture of hexagonal and cubic phase. The direct optical band gap ‘E_g’ for the as-deposited n-ZnSe films is found to be 3.5 eV. TEM studies show that the ZnSe nanocrystals (NCs) are spherical in shape. Formation of ZnSe has been confirmed with the help of infrared (IR) spectroscopy by observing bands corresponding to the multiphonon absorption. We demonstrate the effect of the deposition temperature and reactant concentration on the structural, optical and electrical properties of ZnSe films.     

Photoluminescence spectra of doped GaAs films

We have studied the dependence of the photoluminescence (PL) spectrum on the doping level and the film thickness of n-GaAs thin films, both experimentally and theoretically. It has been shown theoretically that modification of the PL spectrum of p-type material by p-type doping is very small due to the large valence-band hole effective mass. The PL spectrum of n-type material is affected by two factors: (1) the electron concentration which determines the Fermi level in the material; (2) the thickness of the film due to re-absorption of the PL signal. For the n-type GaAs thin films under current investigation, the doping level as well as the film thickness can be very well calibrated by the PL spectrum when the doping level is less than 2×10¹⁸ cm^-3 and the film thickness is in the range of the penetration length of the PL excitation laser. PACS 78.20.-e; 78.55.Cr; 78.66.Fd     

Operation of femtosecond Kerr-lens mode-locked Cr:ZnSe lasers with different dispersion compensation methods

We employed various low-cost dispersion compensation methods to generate femtosecond pulses from a Kerr-lens mode-locked (KLM) Cr:ZnSe laser operating near 2400 nm. Prism pairs made of CaF₂ and MgF₂ and slabs of BK7 and YAG were tested. Pulses as short as 92 fs were obtained when a CaF₂ prism pair was used in the resonator with a 1% output coupler. With a 6% output coupler and CaF₂ prism pair, pulse energies as high as 1.8 nJ were obtained. The KLM operating point was further analyzed for different dispersion compensation scenarios by using the soliton area theorem to determine the nonlinear refractive index (n ₂) of Cr:ZnSe. Results gave an n ₂ value of (1.2±0.2)×10⁻¹⁸ m²/W in agreement with previous reports.     

Study of ZnO and Ni-doped ZnO synthesized by atom beam sputtering technique

Zinc oxide (ZnO) and Ni-doped zinc oxide (ZnO:Ni) films are prepared by atom beam sputtering with an intent of growing transparent conducting oxide (TCO) material and understanding its physical properties. The crystalline phases of the films are identified by the grazing angle X-ray diffraction (GAXRD) technique. Thicknesses of the films are measured by ellipsometry. Chemical states of the elements present in the films are investigated by X-ray photoelectron spectroscopy (XPS), which indicates the presence of Ni in the ZnO environment in a divalent state. Average transmission across the ZnO:Ni film was determined to be ∼83% in the visible region, which is less than that (∼90%) of undoped ZnO films. The resistivity measured by van der Pauw technique of the ZnO:Ni film (∼9×10^-3 Ω cm) is two orders of magnitude smaller as compared to its undoped counterpart (1 Ω cm). For ZnO:Ni film an average carrier concentration of ∼1.4×10¹⁹ cm^-3 was observed by Hall measurements. Two important mechanisms reported in the literature viz. influence of d–d transition bands and electron scattering from crystallites/grains are discussed as the possible causes for the increase in conductivity on Ni doping in ZnO. PACS 73.50.Bk; 78.66.Li; 79.60.Dp; 61.05.cp     

Spectral kinetic properties and lasing characteristics of diode-pumped Cr<Superscript>2+</Superscript>:ZnSe single crystals

The temperature and concentration quenching of the luminescence of Cr²⁺ ions in Cr:ZnSe crystals is experimentally investigated and the radiative lifetime of the upper lasing level is determined. The values of the cross sections of absorption from the ground state of Cr²⁺ ions in Cr:ZnSe crystals are refined. It is shown that the absorption from the excited state of Cr²⁺ ions is negligible near 1.5 μm. Lasing in Cr:ZnSe single crystals is implemented with an efficiency of 31% and an output power of 186 mW using laser diode pumping at a wavelength of 1.77 μm.     

ZnS:Cr and ZnSe:Cr thin‐film waveguide structures as electrically pumped laser media with an impact excitation mechanism

ZnS:Cr and ZnSe:Cr are the focus of studies as media for broadly tunable optically pumped lasers operating in the near‐ and mid‐IR regions. It is of great interest to obtain such lasers electrically pumped. The effective electrical excitation of Cr²⁺ ions was demonstrated only in the case of the impact mechanism for thin‐film structures with insulator layers between an electroluminescent film and electrodes, which prevent avalanche breakdown at high electric field (≥ 1 MV/cm). To obtain lasing, the waveguide electroluminescent structures were used. For the first time, the stimulated emission and the laser oscillation were demonstrated in the ZnS:Cr waveguide structures. Although the lasing is unstable as yet, these results lay the foundations of a new direction in laser physics aimed at creation of electrically pumped lasers by dint of impact excitation. Published results concerning various aspects of ZnS:Cr and ZnSe:Cr thin films and waveguide structures as promising electrically pumped laser media are reviewed. The causes of the instability of the laser oscillation and means of improving their characteristics are also considered.     

Nickel oxide thin films synthesized by reactive pulsed laser deposition: characterization and application to hydrogen sensing

Transparent nickel oxide thin films were grown by reactive pulsed laser deposition. An ArF* (λ=193 nm, τ=12 ns) excimer laser source was used to ablate the Ni targets in a controlled pressure of ambient oxygen. The substrates were either kept at room temperature or heated to a selected temperature within the 200–400 °C range. Post-deposition heat treatment, which was applied to further promote crystallization and overcome any oxygen deficiency, yielded transparent thin films. The surface morphology and crystalline status of the synthesized thin structures were analyzed in correlation with their optical properties. A significant response to several concentrations of hydrogen was demonstrated when heating the nickel oxide films at 185 °C. PACS 78.66.Hf; 81.15.Fg; 82.47.Rs     

Excimer laser deposited CuO and Cu<Subscript>2</Subscript>O films with third-order optical nonlinearities by femtosecond <Emphasis Type="Italic">z</Emphasis>-scan measurement

By controlling the oxygen pressure, single-phase CuO and Cu₂O thin films have been obtained on quartz substrates using a pulsed laser deposition technique. The structure properties and linear optical absorption of the films were characterized by X-ray diffraction and UV–VIS spectroscopy. By performing z-scan measurements using a femtosecond laser (800 nm, 50 fs), the real and imaginary parts of the third-order nonlinear susceptibility, Re χ ⁽³⁾ and Im χ ⁽³⁾, of the films were determined. Both CuO and Cu₂O films exhibited large optical nonlinearities, which is comparable to those in some representative semiconductor films such as ZnO and GaN films using femtosecond laser excitation. Compared with Cu₂O films, the CuO films showed larger third-order nonlinear optical effects under off-resonance excitation. Furthermore, the mechanisms of the optical nonlinearities in CuO and Cu₂O films are explained in the main text. It was suggested that the reasons of the difference in their nonlinear refractive effects may be related to the different electronic structure in CuO and Cu₂O materials.     

Real-time measurement of oxidation dynamics of sub-stoichiometric tungsten oxide films by pulsed laser deposition

In this study WO _x films were deposited by laser ablation of ultra-pure (5N) tungsten trioxide targets onto SiO₂ or silicon substrates at 250°C temperature, 100 mTorr oxygen partial pressure and 1×10⁻⁵ Torr vacuum. Surface chemical states and compositions of the deposits were determined by X-ray photoelectron spectroscopy. The results showed that deposits in oxygen partial pressure contain W⁶⁺ with x∼3.1, while vacuum-deposited films have different W states with various percentage distributions as W⁴⁺>W⁵⁺>W⁶⁺>W⁰, and x∼1. We used fast electrical resistance measurement as a probe to study the deposition process. Film resistance as a function of deposition time in vacuum revealed some microsecond fluctuations modulated on the time variation curve of electrical resistance. We attribute these data to surface absorption and desorption of oxygen during layer deposition. Finally, the effect of the laser beam on the target’s structure, surface morphology and chemical state was studied. Our results revealed that in spite of structural variation by laser irradiation, the O/W ratio remained about 3.     

Structural characterization and optoelectronic properties of GaN thin films on Si(111) substrates using pulsed laser deposition assisted by gas discharge

GaN films have been grown on Si(111) substrates with a thin AlN buffer layer using pulsed laser deposition (PLD) assisted by gas discharge. The crystalline quality, surface morphology and optoelectronic properties of the deposited films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence (PL) spectroscopy, and room-temperature Van der Pauw–Hall measurements. The influence of the deposition temperature in the range 637–1037 K on the crystallinity of GaN films, the laser incident energy in the range 150–250 mJ/pulse on the surface morphology and the optoelectronic properties were systematically studied. The XRD analysis shows that the crystalline quality of the GaN films improves with increasing deposition temperature to 937 K, but further increase of the deposition temperature to 1037 K leads to the degradation of the crystalline quality. AFM results show that the surface roughness of the GaN films can be decreased with increasing laser incident energy to 220 mJ/pulse. Further increase of the laser incident energy to 250 mJ/pulse leads to an increase in the surface roughness. The optoelectronic properties of GaN films were also improved by increasing the laser incident energy to 220 mJ/pulse. GaN films which have a n-type carrier concentration of 1.26×10¹⁷ cm^-3 and a mobility of 158.1 cm²/Vs can be deposited at a substrate temperature of 937 K, a deposition pressure of 20 Pa and a laser incident energy of 220 mJ/pulse. Their room-temperature PL spectra exhibit a strong band-edge emission at 365 nm. PACS 81.15.Fg; 81.05.Ea; 78.20.-e; 73.61.Ey; 78.66.Fd     

Effect of the excimer laser irradiation on sol–gel derived tungsten–titanium dioxide thin films

A novel technique based on the excimer laser induced crystallization and modification of TiO₂ thin films is being reported. W⁺⁶ ions loaded TiO₂ (WTO) precursor films were prepared by a modified sol–gel method and spin-coated onto microscopic glass slides. Pulsed KrF (248 nm, 13 ns) excimer laser was used to irradiate the WTO amorphous films at various laser parameters. Mesoporous and nanostructured films consisting of anatase and rutile were obtained after laser irradiation at room temperature. The effect of varying W⁺⁶ ions concentrations on structural and optical properties the WTO films was analyzed by X-ray diffraction, field-emission scanning electron microscope, UV-Vis spectrophotometer and transmission electron microscope before and after laser treatment. Films irradiated for 10 pulses at 65–75 mJ/cm² laser fluence, exhibited anatase whereas higher parameters promoted the formation of rutile. XPS results revealed WO₃ along with minor proportion of WO₂ compounds after laser irradiation. Photo-absorbance of the WTO films was increased with increase in W⁺⁶ ions concentration in the film. TEM results exhibited a crystallite size of 15 nm which was confirmed from SEM results as well.