Textured ZnO Research Articles

InGaN gallium nitride light-emitting diodes with reflective electrode pads and textured gallium-doped ZnO contact layer

We demonstrate a GaN-based light-emitting diode (LED) with nonalloyed metal contacts and textured Ga-doped ZnO (GZO) contact layer to serve as the n- and p-type electrode pads, respectively. Compared with the conventional LEDs with flat surface and Cr/Au metal contacts, the nonalloyed Ag/Cr/Au contacts used in the present experimental LEDs play the role of reflector to prevent the emitted light from absorption by the opaque electrode pads. Enhancement of light output power observed from the experimental LEDs is also due to the textured GZO layer that can disperse the angular distribution of photons at the GZO/air interface. With an injection current of 20 mA, the output power of experimental LEDs can be improved markedly by a magnitude of 30% compared with conventional GaN-based LEDs.

Texture ZnO Thin-Films and their Application as Front Electrode in Solar Cells

In this paper, three kinds of textured ZnO thin-films (the first kind has the textured structure with both columnar and polygon, the second posses pyramid-like textured structure only, and the third has the textured structure with both crater-like and pyramid-like), were prepared by three kinds of methods, and the application of these ZnO thin-films as a front electrode in solar cell was studied, respectively. In the first method with negative bias voltage and appropriate sputtering parameters, the textured structure with columnar and polygon on the surface of ZnO thin-film are both existence for the sample prepared by direct magnetron sputtering. Using as a front electrode in solar cell, the photoelectric conversion efficiency Eff of 7.00% was obtained. The second method is that by sputtering on the ZnO:Al self-supporting substrate, and the distribution of pyramid-like was gained. Moreover, the higher (8.25%) photoelectric conversion efficiency of solar cell was got. The last method is that by acid-etching the as-deposited ZnO thin-film which possesses mainly both columnar and polygon structure, and the textured ZnO thin-film with both crater-like and pyramid-like structure was obtained, and the photoelectric conversion efficiency of solar cell is 7.10% when using it as front electrode. These results show that the textured ZnO thin-film prepared on self-supporting substrate is more suitable for using as a front electrode in amorphous silicon cells.

3P-32 Fabrication of ultrasonic focusing ejector by using C-axis textured ZnO films(Poster Session)

3P-32 Fabrication of ultrasonic focusing ejector by using C-axis textured ZnO films(Poster Session)

The Effect of the Texture and the Density of ZnO Seed Layer on the Orientation of ZnO Nanorod Arrays

The effect of the texture and the density of ZnO seed layer on the orientation of ZnO nanorod arrays (ZNAs) was systematically investigated, and the vertical growth mechanism of ZNAs during hydrothermal deposition was also explained. It is demonstrated that the textured ZnO seed layer determines the orientation of ZNAs which grow on it. The interaction among nanorods, which derives from the high crystal seed density and the incomplete c-axis texture of ZnO seed layer, obviously reinforces the c-axis texture of ZNAs. According to the seed density behavior of ZnO seed layer, the dominate factor which influences the orientation of ZNAs has been studied. And the results classify the growth situations into three categories: the high density seed layer initiated growth, the middle density seed layer initiated growth and the sparsely separated seeds initiated growth. We also establish the correlation between the density and the orientation of ZNAs, this quasi-quantitative correlation can be used to fabricate the ZNAs with suitable density and orientation.

Structural, defect and optical properties of ZnO films grown under various O 2/Ar gas ratios

Thin wurtzite (0 0 2) textured ZnO thin films were deposited on glass substrates by radio frequency magnetron sputtering under O 2/Ar ratios R varying from 0.05 to 1.0 at room temperature. The structure of, and defects in, the films were investigated by XRD, SEM and slow positron beam techniques. The XRD spectra showed that ZnO thin films were polycrystalline with hexagonal structure and a good c-axis orientation perpendicular to the substrate. The thickness, grain size and the crystalline quality of the films strongly depended on R; the larger grain size and thicker ZnO films were grown when R was lower. Positron beam Doppler broadening measurements showed that in low R films additional vacancy-type defects (e.g. Zn-related vacancy complexes or clusters) were formed. Photoluminescence spectra found that the film with R = 0.4 had the highest luminescence efficiency, in good agreement with the best c-axis preferential orientation. The transmittance spectra of the films decreased with decreasing R, due to the thickness effect. Correlations between microstructure, defect and optical properties are discussed.

Characterization of Surface Morphology and Light Scattering of Transparent Conducting ZnO:Al Films as Front Electrode for Silicon Thin Film Solar Cells

Changes in the surface morphology and light scattering of textured Al doped ZnO thin films on glasssubstrates prepared by rf magnetron sputtering were investigated. As-deposited ZnO:Al films show a hightransmittance of above 80% in the visible range and a low electrical resistivity of 4.5×10-4Ω·cm. The surfacemorphology of textured ZnO:Al films are closely dependent on the deposition parameters of heater temperature,working pressure, and etching time in the etching process. The optimized surface morphology with a cratershape is obtained at a heater temperature of 350oC, working pressure of 0.5 mtorr, and etching time of 45seconds. The optical properties of light transmittance, haze, and angular distribution function (ADF) aresignificantly affected by the resulting surface morphologies of textured films. The film surfaces, havinguniformly size-distributed craters, represent good light scattering properties of high haze and ADF values.Compared with commercial Asahi U (SnO2:F) substrates, the suitability of textured ZnO:Al films as frontelectrode material for amorphous silicon thin film solar cells is also estimated with respect to electrical andoptical properties.

Influence of Deposition Parameters on Surface Texturing of ZnO:Al Films Prepared by In-line RF Magnetron Sputtering

AbstractA systematic study on the effect of sputtering deposition parameters on material properties of Al doped ZnO (ZnO:Al) films prepared by an in-line rf magnetron sputtering and on surface morphology of the films after wet etching process was carried out. For application to silicon thin film solar cells as a front electrode, the as-deposited films were surface-textured by a dilute HCl solution to improve the light scattering properties such as haze and angle resolved distribution of scattered light on the film surfaces. The microstructure of as-deposited films is affected significantly by the working pressure and film compactness decreases with increasing working pressure from 1.5 mTorr to 10 mTorr. High quality ZnO:Al films with electrical resistivity of 4.25 × 10-4 Ω cm and optical transmittance of 80% in a visible range are obtained at low working pressure of 1.5 mTorr and substrate temperature of 100℃. Crater-like surface morphologies are observed on the textured ZnO:Al films after wet etching. The size and shape of craters are closely dependent on the microstructure and film compactness of as-deposited films. Haze values of the textured ZnO:Al films are improved in a whole wavelength of 300 – 1100 nm compared to commercial SnO2:F films (Asahi U type) and incident light on the textured films is scattered effectively with 30° angle.

Photonic Crystal Back Reflector in Thin-film Silicon Solar Cells

AbstractOne-dimensional photonic crystals having desired broad region of high reflectance (R) were fabricated by alternating the deposition of amorphous silicon and amorphous silicon nitride layers. The effect of the deposition temperature and angle of incidence on the optical properties of photonic crystals deposited on glass substrate was determined and an excellent matching was found with the simulated results. The broad region of high R of photonic crystals deposited on flat and textured ZnO:Al substrates decreases when compared to the R of photonic crystals de-posited on glass. The performance of amorphous silicon solar cells with 1-D photonic crystals integrated as the back reflector was evaluated. The external quantum efficiency measurement demonstrated that the solar cells with the photonic crystals back reflector had an enhanced re-sponse in the long wavelength region (above 550 nm) compared to the cells with the Ag reflector.

Light trapping in hydrogenated amorphous and nano-crystalline silicon thin film solar cells

AbstractLight trapping effect in hydrogenated amorphous silicon-germanium alloy (a-SiGe:H) and nano-crystalline silicon (nc-Si:H) thin film solar cells deposited on stainless steel substrates with various back reflectors is reviewed. Structural and optical properties of the Ag/ZnO back reflectors are systematically characterized and correlated to solar cell performance, especially the enhancement in photocurrent. The light trapping method used in our current production lines employing an a-Si:H/a-SiGe:H/a-SiGe:H triple-junction structure consists of a bi-layer of Al/ZnO back reflector with relatively thin Al and ZnO layers. Such Al/ZnO back reflectors enhance the short-circuit current density, Jsc, by ˜20% compared to bare stainless steel. In the laboratory, we use Ag/ZnO back reflector for higher Jsc and efficiency. The gain in Jsc is about ˜30% for an a-SiGe:H single-junction cell used in the bottom cell of a multi-junction structure. In recent years, we have also worked on the optimization of Ag/ZnO back reflectors for nano-crystalline silicon (nc-Si:H) solar cells. We have carried out a systematic study on the effect of texture for Ag and ZnO. We found that for a thin ZnO layer, a textured Ag layer is necessary to increase Jsc, even though the parasitic loss is higher at the Ag and ZnO interface due to the textured Ag. However, a flat Ag can be used for a thick ZnO to reduce the parasitic loss, while the light scattering is provided by the textured ZnO. The gain in Jsc for nc-Si:H solar cells on Ag/ZnO back reflectors is in the range of ˜60-75% compared to cells deposited on bare stainless steel, which is much larger than the enhancement observed for a-SiGe:H cells. The highest total current density achieved in an a-Si:H/a-SiGe:H/nc-Si:H triple-junction structure on Ag/ZnO back reflector is 28.6 mA/cm2, while it is 26.9 mA/cm2 for a high efficiency a-Si:H/a-SiGe:H/a-SiGe:H triple-junction cell.

ZnO films obtained by photoelectrothermal oxidation, their properties, and possibilities of practical application

Zinc oxide films of optical quality are obtained on single-crystal ZnSe substrates by the developed method of photoelectrothermal oxidation. Comparative analysis of the structure and mechanical and electrical characteristics of the films, obtained by the methods of photothermal and photoelectrothermal oxidation, is performed. It is shown that photoelectrothermal oxidation makes it possible to fabricate highly textured ZnO films of large thickness at much lower temperatures. Such films are characterized by high adhesion with the surface of the crystalline substrate and a high mechanical strength. On the basis of the thermostable ZnO films and oxide-semiconductor heterostructures prepared by photoelectrothermal and photothermal oxidation, varistors, gas analyzers, light converters, and multifunctional IR optical elements are designed.

Chemical bath deposition of textured ZnO thin films in aqueous/ethanolic solution

ZnO thin films have been prepared by chemical bath deposition in aqueous/ethanolic solution. The film texture was successfully controlled by varying the volume ratio of water to ethanol. Films consisting of densely oriented nanorod arrays with the c-axis perpendicular to the substrate were fabricated in aqueous solution. The crystals became increasingly tilted as more ethanol was introduced to the solution, resulting in the cracked nanocolumns and the smoothed crystals. The crystal size was decreased with increasing ethanol content, and granular morphology was obtained in films deposited in ethanolic solution. A gradual evolution of the film texture is possibly due to the inhibited crystal growth in solution with higher ethanol content.

Oxidative annealing of ZnSe/GaAs heterostructures

We annealed ZnSe/GaAs heterostructures in the oxygen atmosphere and investigated structural and optical properties of the forming films using X-ray diffraction and photoluminescence. While highly textured ZnO films were synthesized via low-temperature processing (~ 500 °C), high temperature processing (~ 800 °C) promoted reaction at the film/substrate interface and Zn loss from the film surface resulting in the polycrystalline ZnGa 2O 4 and ZnO 2.

Structural analysis of ZnO thin films obtained by d.c. sputtering and electron beam evaporation

ZnO thin films were produced by argon plasma assisted electron beam vacuum evaporation and d.c. magnetron sputtering deposition techniques. ZnO films are used in solar cells as transparent contact in heterojunction cells, and can be deposited on a variety of substrates by different techniques, including electron beam deposition and sputtering and laser ablation. ZnO thin films were prepared for photovoltaic applications and the structural properties were studied. The results showed that the sputtering and the vacuum evaporation techniques resulted, respectively, in a textured ZnO and ZnO plus Zn mixed phases. The annealing of the vacuum evaporation ZnO films resulted in films with high crystallinity.

Transparent and conductive ZnO:Al prepared by RF diode sputtering

High transparent and conductive, aluminium - doped zinc oxide thin films (ZnO:Al), were prepared by radio - frequency (RF) diode sputtering from ZnO+2 wt. % Al2O3 target on Corning glass 7059 substrates. The RF power and the substrate temperature modified their structure and surface morphology, electrical and optical parameters. The XRD patterns reveal only one sharp (002) diffraction line, providing a clear evidence for highly textured ZnO films, with a preferential c-axis orientation. Maximum intensity and the lowest full-width at half-maximum (FWHM) (3.26°) of the ΩM-scan were obtained for ZnO:Al deposited at 800 W RF power and 200°C substrate temperature. The developed ZnO:Al have a low electrical resistivity (2 × 10−3 Ωcm) and high optical transmittance (> 82 %, including Corning glass substrate) in the visible region. The direct optical band gap (~ 3.3 eV) increases with the increasing carrier concentration.

Characteristics of ZnO thin film surface acoustic wave devices fabricated using nanocrystalline diamond film on silicon substrates

The propagation characteristics of surface acoustic wave for the ZnO piezoelectric films and nanocrystalline diamond (NCD) films multilayer SAW devices on Si substrates were investigated. High surface acoustic wave velocity is achieved for a ZnO/NCD/Si multilayer structure excited by a bottom-electroded device configuration. The NCD films deposited on Si substrates (NCD/Si) not only possess smooth surface, but also show good compatibility with ZnO materials, such that [002] textured ZnO films can be directly grown on NCD/Si substrates without the necessity of using buffer layer. The surface acoustic wave velocity increased with the thickness of the NCD films. For the thickness of the ZnO and NCD films investigated, the 0th mode surface acoustic velocity in IDT/ZnO/NCD/Si structure achieves 5100 m/s, whereas the 1st mode SAW in the ZnO/IDT/NCD/Si structure reaches 8500 m/s.

Improvement of photon extraction efficiency of InGaN LEDs utilizing textured ZnO layer deposited by electrospray deposition

Improvement of photon extraction efficiency of InGaN LEDs has been achieved by deposition of textured ZnO layers on InGaN LED bare chips utilizing electrospray deposition. The electrical and spectral properties of the InGaN LED remain unchanged after the deposition of ZnO, while the InGaN LED with textured ZnO layer exhibits a wider far-field angular distribution. The optical power of the InGaN LED capped with ZnO layer is ∼30% higher than that without ZnO, which is due to increased photon escape probability as a result of the increased surface roughness.

COMPARISON OF MAGNETIC PROPERTY OF Cu-, Al-, AND Li-DOPED ZnO DILUTE MAGNETIC SEMICONDUCTOR THIN FILMS

The properties of Cu -, Al -, and Li -doped ZnO dilute magnetic semiconductor (DMS) have been analyzed and compared. Zincite with wurtzite structures have been synthesized successfully on SiO 2 (101) and SiO 2 (110) substrates in both the Cu – ZnO and Li – ZnO DMS. The highly textured ZnO (002) peaks were able to form in the Cu – ZnO system at 400°C. However, it formed at even much lower temperature in the Li – ZnO system, that is only 25°C. ZnO (002) peaks in both systems were formed without any impurity phases. However, no crystalline structure is synthesized in the Al – ZnO system. The thin films formed are amorphous. The structural and related magnetic properties of the films were analyzed by XRD, AFM, and VSM. The films were found to be at their highest magnetism at the value of 3.1 emu/cm3 for Co – ZnO and 2.5 emu/cm3 for Li – ZnO , synthesized at 400°C, and under 1 × 10-4 Torr oxygen partial pressure.

Fabrication and performance of highly textured electrodeposited ZnO back reflector for nc-Si solar cells

AbstractTo improve the cell efficiency of thin film solar cells textured back reflectors (BR) are widely used. This is particularly important in a-Si:H based solar cells due to low absorption coefficient at longer wavelengths. In this work we present a cost effective way to fabricate uniformly textured ZnO by using electrochemical methods. Further it was observed that Quantum Efficiency (QE) of shorter wavelengths also improved for highly textured ZnO BR. Together this resulted in more than 2mA increment in short circuit current density (Jsc) and 19% relative improvement in solar cell efficiency over sputter deposited BR. A possible mechanism responsible for the improved blue QE is also discussed.

Synthesis of zinc oxide nanopowder and nanolayer via chemical processing

In the present research work, the newest economic method for synthesis of ZnO nanopowder has been carried out via solochemical processing from an aqueous solution of a zinc-containing complex. The results obtained from XRD and TEM show that the nanoparticles are single crystals and have elongated particulate shape with a narrow size distribution. Solochemical processing can thus be an attractive method for industrial production of the nanopowders. The highly textured ZnO thin film with a preferred (002) orientation was also prepared by Two-Stage Chemical Deposition (TSCD), using an aqueous solution containing zinc complex on a soda-lime glass substrate. The films were characterised by XRD, SEM, EDAX, optical spectrophotometer and FTIR to justify the suitability for commercial device quality.

Control of in-plane and out-of-plane texture in shear mode piezoelectric ZnO films by ion-beam irradiation

ZnO polycrystalline films have a strong tendency to grow their c-axis perpendicular to the film surface, even on an amorphous substrate. However, unusual (101¯0) preferred orientations in which the c-axis lies in the substrate plane are often observed when the film is exposed to ion irradiation during its growth. To investigate the effect of ion irradiation on the (101¯0) preferred orientation, ZnO films were fabricated using a 0–1 keV oxygen ion-beam-assisted electron-beam evaporation of zinc. The results clearly indicated that the tendency of (101¯0) preferred orientation was enhanced with increasing ion energy and amount of ion irradiation. This demonstrated that the ion bombardment induced the (0001) preferred orientation to change into a (101¯0) preferred orientation which corresponds to the ion channeling direction. An in-plane preferred orientation was also obtained, probably because of deviations in the incident ion-beam direction from 5° to the substrate surface normal. These in-plane textured (101¯0) ZnO films, fabricated under the ion-beam irradiation of 0.5–1 keV, excited a shear acoustic wave without any longitudinal wave. The highest shear mode electromechanical coupling coefficient was found to be k15=0.16 in the film with ion-beam irradiation of 1 keV. This k15 value corresponds to 60% of that in a ZnO single crystal (k15=0.26).