High-performance UV detector based on Ga-doped zinc oxide thin films

High-quality ultraviolet photoconductive detectors have been fabricated using Ga-doped zinc oxide layers grown by spray pyrolysis on glass substrates. The performance of the photoconductivity has been tested by the measurements of the current-voltage (I-V) characteristics under forward and reverse bias. The devices have been characterized to investigate the effect of buffer layer on the detector performances. The behaviour of photocurrent with respect to optical power density, wavelength and chopping time has been investigated. We achieved the highest responsivity of about 1125 A/W at 5 V bias at 365 nm peak wavelength. Our approach provides a simple and cost-effective way to fabricate high-performance ‘visible-blind’ UV detectors.     

Investigation of the intrinsic absorption edge in nanostructured polycrystalline zinc oxide thin films

We have studied the temperature variation of the intrinsic absorption edge of thin polycrystalline films of zinc oxide, obtained by high-frequency magnetron reactive sputtering. We ha ve observed that the intrinsic absorption edge in such films is described by a modified Urbach’s rule. We have calculated the effective frequency of phonons taking part in formation of the absorption edge. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 2, pp. 200–203, March–April, 2006.     

Deep UV laser induced luminescence in oxide thin films

Time-resolved luminescence experiments have been set up in order to study the interaction of 193-nm laser radiation with dielectric thin films. At room temperature, Al₂O₃ coatings show photoluminescence upon ArF excimer laser irradiation, with significant intensity contributions besides the known substrate emission. Time- and energy-resolved measurements indicate the presence of oxygen-defect centers in Al₂O₃ coatings, which suggests a strong single-photon interaction at 193 nm by F⁺ and F center absorption. Measurements on highly reflective thin-film stacks, consisting of quarter-wave Al₂O₃ and SiO₂ layers, indicate similar UV excitations, mainly from color centers of Al₂O₃. Received: 20 February 2002 / Accepted: 11 April 2002 / Published online: 5 July 2002     

Properties of sol-gel dip-coated zinc oxide thin films

Zinc oxide (ZnO) films were deposited on glass substrates by the sol-gel dip coating method using acrylamide route. The films were characterized by X-ray diffraction studies which indicated wurtzite structure. Optical absorption measurements indicated band gap in the range 3.17-3.32 eV. XPS studies indicated the formation of ZnO. The resistivity of the films were in the range 1000-10,000 ohm cm.     

The effect of UV irradiation on nanocrysatlline zinc oxide thin films related to gas sensing characteristics

The effect of ultraviolet (UV) light irradiation on the nanocrystalline ZnO thin films was investigated. The degree of crystallinity, electrical conductivity, optical properties and surface properties of ZnO thin films were measured as a function of UV irradiation time. It was found that the degree of crystallinity and electrical properties of ZnO films were affected by UV irradiation, however, no noticeable change in the surface morphology was observed. The gas sensing properties of as-deposited and UV irradiated films were also measured. It was observed that the gas sensing properties were affected by the UV irradiation. The irradiation time less than 5 min has improved the sensor while the irradiation time more than 5 min degraded the sensor characteristics for a UV power density of 2.45 W cm⁻².     

Reactive pulsed laser deposition of zinc oxide thin films

Zinc oxide thin films have been obtained by reactive pulsed laser ablation of a Zn target in O₂ atmosphere (gas pressure 2 Pa) using a doubled frequency Nd:YAG laser (532 nm) which was also assisted by a 13.56 MHz radiofrequency (rf) plasma. The gaseous species have been deposited on Si(100) substrates positioned in on-axis configuration and heated from RT up to 500 °C. The obtained thin films have been compared to those produced in the same conditions by ablation of a ZnO target. The deposited thin films have been characterized by scanning electron microscopy, X-ray diffraction, Raman and infrared spectroscopy techniques. The influence of the rf plasma on the morphological and structural characteristics of these thin films is also briefly discussed. PACS 81.15.Fg; 68.55.Jk; 78.30.j     

Cathodoluminescence properties of silicon nanocrystallites embedded in silicon oxide thin films

Cathodoluminescence (CL) spectra for the Si nanocrystallites embedded in a matrix of silicon oxide films are measured at room temperature. The CL spectra consist of two principal bands whose peak energies are in a near-infrared (NIR) region (<1.6 eV) and in a blue region (2.6 eV), respectively. The spectral feature of the NIR CL band is similar to the corresponding PL spectra. The strong correlation between the presence of Si nanocrystallites and the formation of the NIR CL band are found as well as the PL spectrum. The peak energy of the blue CL band is slightly lower than that of the luminescence band originating from oxygen vacancies (≡Si–Si≡) in SiO₂. Therefore, the blue CL band is considered to come from Si_n clusters with n3 in the oxide matrix. Under irradiation of electron beams, degradation of the intensity is observed for both the CL bands but the decay characteristics are different.     

Optical characterization of poly (ethylene oxide)/zinc oxide thin films

The optical characterization of poly (ethylene oxide)/zinc oxide thin films has been done by analyzing the absorption spectra in the spectral wavelength region 380–800 nm using a ultraviolet-spectrophotometer at room temperature. Thin film polymer composites made of poly (ethylene oxide) (PEO) containing zinc oxide (ZnO) filler concentrations (0%, 2%, 6%, 10%, and 14%) by weight were used in this study. The optical results obtained were analyzed in terms of the absorption formula for non-crystalline materials. The optical energy gap and other basic optical constants such as dielectric constants and optical conductivity were investigated and showed a clear dependence on the ZnO filler concentration. It was found that the optical energy gap for the composite films is less than that for the neat PEO, and that it decreases as the ZnO concentration increases. Enhancement of the optical conductivity was observed with increase in the ZnO concentration. Dispersion of refractive index was analyzed using the Wemple–DiDomenico single oscillator model. The refractive index (n), extinction coefficient (k), and dispersion parameters (E_o, E_d) were calculated for the investigated films.     

Investigation of chemical mechanical polishing of zinc oxide thin films

Zinc oxide has become an important material for various applications. Commercially available zinc oxide single crystals and as-grown zinc oxide thin films have high surface roughness which has detrimental effects on the growth of subsequent layers and device performance. A chemical mechanical polishing (CMP) process was developed for the polishing of zinc oxide polycrystalline thin films. Highly smooth surfaces with RMS roughness <6 Å (as compared to the initial roughness of 26 ± 6 Å) were obtained under optimized conditions with removal rates as high as 670 Å/min. Effects of various CMP parameters on removal rate and surface roughness were evaluated. The role of pH on the polishing characteristics was investigated in detail.     

Deposition of zinc oxide thin films by reactive pulsed laser ablation

Thin films of zinc oxide have been deposited by reactive pulsed laser ablation of Zn and ZnO targets in presence of a radio frequency (RF) generated oxygen plasma. The gaseous species have been deposited at several substrate temperatures, using the on-axis configuration, on Si (1 0 0). Thin films have been characterized by scanning electron microscopy, atomic force microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and infrared spectroscopy. A comparison among conventional PLD and reactive RF plasma-assisted PLD has been performed.     

Manifestation of a size effect in the behavior of the intrinsic absorption edge of nanostructured polycrystalline zinc oxide thin films

We have studied the behavior of the intrinsic absorption edge in zinc oxide thin films in the temperature range 80–300 K. We have observed that the intrinsic absorption edge in films with crystal sizes of ≈45 nm or larger is described by the empirical Urbach’s rule, while in films with crystallite sizes of ≈15 nm, it is described by a modified Urbach’s rule. We have calculated the effective frequency of phonons taking part in formation of the absorption edge. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 2, pp. 275–277, March–April, 2007.     

Anomalous temperature dependence of the electrical conductivity of zinc oxide thin films

The temperature dependence of the conductivity of Ga-doped ZnO thin films was examined in humid and dry atmospheres. The conductivity initially increases to the maximum (range 1) and then decreases to the minimum (range 2) and again increases (range 3) in the heating run under humid air atmosphere. The subsequent cooling reduces the conductivity to less than that for the heating run. The extent of the reduction becomes large as the cooling rate increases. The extrema character in the conductivity versus temperature curves does not disappear under nitrogen gas atmosphere provided it is humid. On the other hand, the conductivity in the ranges 1 and 2 decreases so significantly that the extrema character disappears in the driest air as possible. These behaviors are explained by assuming that the dissociatively chemisorbed states of water vapor act as electron donors and desorb at the higher temperature, and the desorption as well as the chemisorption require a time for attaining equilibrium. With this in view, a theoretical analysis is given of semiconducting films thinner than the depletion boundary layer thickness, and numerical curves fitting to the experimental results are obtained. The heat of chemisorption of nonionized water q° is 1.35 eV and the energy levels of water chemisorption donors and oxygen chemisorption acceptors are at 0.66 and 0.59 eV below the conduction band edge, respectively.     

Laser scribing of gallium doped zinc oxide thin films using picosecond laser

We report on a comprehensive study of picosecond laser scribing of gallium doped zinc oxide (GZO) thin films deposited on glass substrates using 355 nm, 532 nm and 1064 nm radiation, respectively. In this study, we investigated the influence of front side and rear side irradiation and determined single pulse ablation thresholds for all three wavelengths. Good ablation quality with full electrical isolation, steep groove walls and a smooth groove bottom was achieved by 355 nm rear side processing with a scanning speed of 224 mm/s. Ridges at the groove rims were found to be between 15 nm and 45 nm high. At similar scanning speed, laser scribing using 532 nm and 1064 nm radiation resulted in a lower ablation quality due to a higher roughness of the groove bottoms or higher ridges at the groove rims.     

Surface roughness characterization of Al-doped zinc oxide thin films using rapid optical measurement

Transparent conductive oxide thin films have been widely investigated in photoelectric devices such as flat panel display (FPD) and solar cells. Al-doped zinc oxide (AZO) thin films have been widely employed in FPD. Measuring the surface roughness of AZO thin films is important before the manufacturing of photoelectric device using AZO thin films because surface roughness of AZO thin films will significantly affect the performance of photoelectric device. Traditional methods to measure surface roughness of AZO thin films are scanning electron microscopy and atomic force microscopy. The disadvantages of these approaches include long lead time and slow measurement speed. To solve this problem, an optical inspection system for rapid measurement of the surface roughness of AZO thin films is developed in this study. It is found that the incident angle of 60° is a good candidate to measure the surface roughness of AZO thin films. Based on the trend equation y=−3.6483x+2.1409, the surface roughness of AZO thin films (y) can be directly deduced from the peak power density (x) using the optical inspection system developed. The maximum measurement-error rate of the optical inspection system developed is less than 8.7%.The saving in inspection time of the surface roughness of AZO thin films is up to 83%.     

Nonlinear optical properties of erbium doped zinc oxide (EZO) thin films

Undoped and Erbium (Er) doped zinc oxide (EZO) thin films were deposited on glass substrate by sol–gel method using spin coating technique with different doping concentration. EZO films were characterized using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), UV–VIS-NIR transmission and single beam z scan method under illumination of frequency doubled Nd:YAG laser. The deposited films were found to be well crystallized with hexagonal wurtzite structure having a preferential growth orientation along the ZnO (002) plane. A blue-shift was observed in the band gap of EZO films with increasing Er concentration. All the films exhibited a negative value of nonlinear refractive index (n₂) at 532 nm which is attributed to the two photon absorption and weak free carrier absorption. Third order nonlinear optical susceptibility, χ⁽³⁾ values of EZO films were observed in the remarkable range of 10^? 6 esu. EZO (0.4 at.%) sample was found to be the best optical limiter with limiting threshold of 1.95 KJ/cm².     

Refractive index and lattice constant of zinc oxide films modified in a low-temperature plasma

The crystallographic and optical properties of ZnO films obtained in the recombination burning zone of a low-temperature plasma are investigated. The refractive index is determined, and its correlation with the lattice constant along the c axis is found. A planar homogeneous structure consisting of two ZnO films with different refractive indices is fabricated to demonstrate areas of application of the technique suggested.     

Dielectric anomaly in Li-doped zinc oxide thin films grown by sol–gel route

Sol–gel route was employed to grow polycrystalline thin films of Li-doped ZnO thin films (Zn_1-xLi_xO, x=0.15). Polycrystalline films were obtained at a growth temperature of 400–500 °C. Ferroelectricity in Zn_0.85Li_0.15O was verified by examining the temperature variation of the real and imaginary parts of dielectric constant, and from the C–V measurements. The phase transition temperature was found to be 330 K. The room-temperature dielectric constant and dissipation factor were 15.5 and 0.09 respectively, at a frequency of 100 kHz. The films exhibited well-defined hysteresis loop, and the values of spontaneous polarization (P_s) and coercive field were 0.15 μC/cm² and 20 kV/cm, respectively, confirming the presence of ferroelectricity. PACS 77.22Ch; 77.22Ej; 77.80Bh     

The influence of oxygen on the optical properties of RF-sputtered zinc oxide thin films

In this article, we investigate the effects of oxygen partial pressure in the deposition chamber on the optical properties of zinc oxide (ZnO) thin films; in particular, we examine the variation of the refractive index with oxygen flux.ZnO thin films were deposited by radio-frequency (RF) magnetron sputtering and studied by means of X-ray diffraction (XRD) and spectroscopic ellipsometry (SE). We have found a preferential c-axis growth of ZnO films, with slightly variable deposition rates from 2.6 to 3.8 Å/s. Conversely, the refractive index exhibits, from ultraviolet (UV) to near infrared (IR), a considerable and almost linear variation when the oxygen flux value in the deposition chamber varies from 0 to 10 sccm.     

Morphological,optical, and nonlinear optical properties of fluorine-indium-doped zinc oxide thin films

Chemically sprayed fluorine-indium-doped zinc oxide thin films (ZnO:F:In) were deposited on glass substrates. A mixture of zinc pentanedionate, indium sulfate, and fluoride acid was used in the starting solution. The influence of both the dopant concentration in the starting solution and the substrate temperature on the transport, morphological, linear, and nonlinear optical (NLO) properties were fully characterized with atomic force microscopy (AFM), scanning-electron microscopy (SEM), UV-VIS, and photoluminescence (PL) spectroscopies, and the second-harmonic generation (SHG) technique, respectively. A decrease in the resistivity was observed for increasing substrate temperatures, reaching a minimum value of 1.2 × 10^?2 Ω cm for samples deposited at 500°C. The surface morphology was also dependent on the dopant concentration in the starting solution and on the substrate temperature. The X-ray diffraction (XRD) patterns revealed that the ZnO:F:In thin solid films are polycrystalline in nature fitting with a hexagonal wurtize type and showing (002) preferential growth for all of the studied samples. The optical transmittance of these films was found to be higher than 80%, from which the optical band gap of these samples was determined. Finally, a clear dependence on the quadratic NLO properties of the developed semiconducting ZnO:F:In thin films with the substrate temperatures was established, where huge x ⁽²⁾-NLO coefficients on the order of x ₃₃ ⁽²⁾ = 37 pm V^?1 were measured for high substrate temperatures.