Z.L. Pei

X-ray photoelectron spectroscopy and auger electron spectroscopy studies of Al-doped ZnO films

The chemical state of oxygen, aluminum and zinc in Al-doped ZnO (ZAO) films was investigated by X-ray photoelectron spectroscopy (XPS), as well as the transition zone of the film-to-substrate, by auger electron spectroscopy (AES). The results show that zinc remains mostly in the formal valence states of Zn2+. A distinct asymmetry in Al 2p(3/2) photoelectron peaks has been resolved into two components, one is metallic Al and the other is oxidized Al. The depth profile of the two components revealed that metallic Al mainly exists in the thin surface layer. The close inspection of Ols shows that Ols is composed of three components, centered at 530.15 +/- 0.15, 531.25 +/- 0.20 and 532.40 +/- 0.15 eV, respectively. AES reveals an abrupt transition zone between the ZAO and quartz substrate

Optical and electrical properties of direct-current magnetron sputtered ZnO:Al films

In this study, high-quality ZnO:Al (ZAO) films were prepared by using dc reaction magnetron sputtering technology. The effect of Al doped in ZnO films on electrical and optical properties and its scattering mechanism were discussed in detail. The results showed that Al2O3 could be effectively removed by controlling oxygen flow and Al-doped concentration during deposition of ZnO:Al films. Zn, Al, and oxygen elements were well distributed through the films. For highly degenerated ZnO:Al semiconductor thin films, it was revealed that ionized impurity scattering dominated the Hall mobility of the films in the low-temperature range; while the lattice vibration became a major scattering mechanism in the high-temperature range. The grain-boundary scattering only played a major role in the ZAO films with small grain size (as compared to the electron mean-free path). The photoelectric properties of ZAO films showed that the lower resistivity (similar to 5x10(-4) Omega cm) was obtained, and transmittance in the visible range and reflectance in the IR region were above 80% and 60%, respectively

Formation of Al-doped ZnO films by dc magnetron reactive sputtering

Highly preferred (002) orientation transparent conductive Al-doped ZnO (ZAO) films were successfully prepared by de magnetron reactive sputtering from a Zn target mixed with Al of 2.0 wt.%. The film has a resistivity of 4.80 X 10(-4) n cm and a visible transmittance of as high as 90%. XPS analysis indicates Al-enrichment on the film surface. The asymmetry of Al 2p(3/2) XPS peak is resolved into two components: one centering at 72.14 eV attributed to metallic Al and the other having a binding energy of 74.17 eV due to oxidized Al

Investigation on the electrical properties and inhomogeneous distribution of ZnO:Al thin films prepared by dc magnetron sputtering at low deposition temperature

A study of the electrical properties and spatial distribution of the ZnO:Al (AZO) thin films prepared by dc magnetron sputtering at low deposition temperature was presented, with emphasis on the origin of the resistivity inhomogeneity across the substrate. Various growth conditions were obtained by manipulating the growth temperature T-S, total pressure P-T, and ion-to-neutral ratio J(i)/J(n). The plasma characteristics such as radial ion density and floating/plasma potential distribution over the substrate were measured by Langmuir probe, while the flux and energy distribution of energetic species were estimated through Monte Carlo simulations. The crystalline, stress and electrical properties of the films were found to be strongly dependent on T-S and J(i)/J(n). Under the low J(i)/J(n) (< 0.3) conditions, the T-S exerted a remarkable influence on film quality. The films prepared at 90 degrees C were highly compressed, exhibiting poor electrical properties and significant spatial distribution. High quality films with low stress and resistivity were produced at higher T-S (200 degrees C). Similarly, at lower T-S (90 degrees C), higher J(i)/J(n) (similar to 2) dramatically improved the film resistivity as well as its lateral distribution. Moreover, it indicated that the role of ion bombardment is dependent on the mechanism of dissipation of incident species. Ion bombardment is beneficial to the film growth if the energy of incident species E-i is below the penetration threshold E-pet (similar to 33 eV for ZnO); on the other hand, the energy subimplant mechanism would work, and the bombardment degrades the film quality when E-i is over the E-pet. The energetic bombardment of negative oxygen ions rather than the positives dominated the resistivity distribution of AZO films, while the nonuniform distribution of active oxygen played a secondary role which was otherwise more notable under conditions of lower T-S and J(i)/J(n). (c) 2007 American Institute of Physics.

Comparative investigation of fracture behaviour of aluminium-doped ZnO films on a flexible substrate

The differences in the fracture behaviour of aluminium-doped ZnO (AZO) films on a flexible substrate subjected to tensile and compressive strains were investigated by using a simple-support bending method. It is found that the crack density and the crack spacing of the AZO films become saturated both in the face-out (FO) and in the face-in (FI) bending tests when the applied strain reaches a certain value. The saturated crack density of the AZO film in the FO bending is higher than that of the film in the FI bending. Crack-initiation strain of the film in the FO bending is smaller than that of the film in the FI bending. The fracture energy of the AZO films is also analyszed. Failure behaviours of the films are examined and exhibit different failure mechanisms for the films subjected to tensile and compressive strains.

Properties of ZnO:Al films on polyester produced by dc magnetron reactive sputtering

High preferred (002) orientation Al-doped ZnO (ZAO) films have been prepared by de magnetron reactive sputtering of an alloy target (98.5 wt.% Zn-1.5 wt.% Al) onto polyester substrates. The structural, mechanical, electrical and optical properties were investigated. The ZAO film has a sheet resistance of as low as 47.5 Omega/square and an average visible transmittance of about 70%. An average IR reflectance of about 70% in the region of 2.5-25 mum also has been obtained

Dependence of structural, electrical, and optical properties of ZnO : Al films on substrate temperature

ZnO:Al (ZAO) films were deposited on fused silica substrates heated to 350 degreesC by de magnetron reactive sputtering from a Zn target mixed with 1.5 wt% Al. Films deposited on a substrate heated to a temperature between room temperature and 300 degreesC were (001)-oriented crystals, but those grown at 350 degreesC consisted of crystallites with (001) and (101) orientations. The dependence of electrical properties such as resistivity, carrier concentration, and Hall mobility on temperature was measured. The results indicate that the carrier concentration and Hall mobility increase with increasing temperature up to 250 degreesC, though the Al content remains unchanged in this temperature range. The probable mechanisms are discussed. The minimum resistivity of ZAO films is 4.23 x 10(-4) R cm, with a carrier concentration of 9.21 x 10(20) cm(-3) and a Hall mobility of 16.0 cm(2) v(-1) s(-1). The films show a visible transmittance of above 80%.

Properties of transparent conducting ZnO : Al oxide thin films and their application for molecular organic light-emitting diodes

ZnO : Al (ZAO) films were deposited on glass substrates by a reactive mid-frequency sputtering system. The microstructural, electrical, and optical properties of ZAO films were investigated. It was observed that the polycrystalline film was (0 0 2n) textured with columnar structure. The minimum resistivity was 1.39×10−4 Ω cm with a carrier concentration of 1.58×1021 cm−3 and a Hall mobility of 28.2 cm2 V−1 s−1, correspondingly with the c-axis nearly equal to the value of ZnO powder and the minimum mechanical stress therein. The average transmittance of 80.8% in the visible range and infrared reflectance of over 86% in the 1600–4400 cm−1 interval were obtained. The ZAO films were used as the transparent anodes to fabricate light-emitting diodes, and a luminance efficiency of 2.09 cd A−1 was measured at a current density of 5.38 A m−2.

Effect of Enhanced Plasma Density on the Properties of Aluminium Doped Zinc Oxide Thin Films Produced by DC Magnetron Sputtering

Aluminum doped zinc oxide (AZO) thin films were prepared by DC magnetron sputtering at low substrate temperature. A coaxial solenoid coil was placed near the magnetron target to enhance the plasma density (J(i)). The enhanced plasma density improved significantly the bulk resistivity (p) and its homogeneity in spatial distribution of AZO films. X-ray diffraction (XRD) analysis revealed that the increased J(i) had influenced the crystallinity, stress relaxation and other material properties. The AZO films deposited in low plasma density (LPD) mode showed marked variation in rho (ranging from similar to 6.5x10(-2) to 1.9x10(-3) Omega.cm), whereas those deposited in high plasma density (LPD) mode showed a better homogeneity of films resistivity (ranging from similar to 1.3x10(-3) to 3.3x10(-3) Omega.cm) at different substrate positions. The average visible transmittance in the wavelength range of 500-800 nm was over 80%, irrespective of the deposition conditions. The atomic force microscopy (AFM) surface morphology showed that AZO films deposited in HPD mode were smoother than that in LPD mode. The high plasma density produced by the coaxial solenoid coil improved the electrical property, surface morphology and the homogeneity in spatial distribution of AZO films deposited at low substrate temperature.

GROWTH CONTROL OF ZnO FILMS ON GLASS BY CHEMICAL BATH DEPOSITION

The effects of substrate-placing manner, deposition temperature and solution concentration on the preparation of ZnO films by chemical bath deposition were investigated in this work. The structures and the morphologies of as-deposited ZnO films were characterized by X-ray diffraction and scanning electron microscope, respectively. The results show that compared with vertically insertion, floating the substrates on the solutions could greatly improve the c-axis orientation of the films. The growth of ZnO films with orientation would also be favored at lower precursor concentrations and temperatures. When the concentration of Zn(CH3COO)2·2H2O is 0.033 M/L and the deposited temperature is 60°C, ZnO films which are well crystallized and highly c-axis orientated were obtained.