Structures And Properties Of Aluminum Doped Zinc Oxide Films By Pulsed Laser Deposition

ZnO is wide bandgap semiconductor which is used for various applications. The most unique property of ZnO is its large exciton binding energy of 60meV, which is much larger than those of GaN (28meV), ZnSe (20meV) and ZnS (39meV). Because of this large binding energy, the exciton is stable at room temperature even in bulk crystals. Owing to these properties, ZnO is considered as a promising material for light-emitting devices and semiconductor laser with low thresholds in the UV region, such as light-emitting diodes and laser diodes. However due to its low conductivity compared to metal, the introduction of metal dopants have been investigated. Among the various dopants for n-type ZnO thin films, Al, Ga, and In have been determined to be the most suitable materials. These elements are known to enhance the electrical and optical properties of ZnO film, Almost all transparent conducting oxide films (TCO) films now being put to practical use are made of indium tin oxide (ITO), a compound likely to become unavailable due to the growing scarcity of its component metal indium. To cope with the depletion of indium, a strong interest has recently turned to TCO films based on Al doped ZnO thin films, doped ZnO thin films have also been widely used in transparent conducting layers because of their higher thermal stability and good resistance against hydrogen plasma processing damage compared with ITO. Specially, ZnO:Al thin films with high c-axis orientated crystalline structure along(002) plane are extensively studied for practical applications such as solar cells, electroluminescence displays, etc. Pure and doped ZnO thin films have been deposited by using several growth techniques, such as chemical vapor deposition (CVD), molecular beam epitaxy (MBE), sol-gel deposition, RF magnetron sputtering, oxidation of metal zinc film, pulsed laser deposition (PLD), etc. Among them, the PLD technique is more useful in obtaining multi component thin films compared with other techniques, because of its advantage of simple hardware, atomic-layer control. The composition of film is quite close to that of the target, films may crystallize at lower substrate temperature due to the high excitation energy. In this paper, the high c-axis orientation Al-doped ZnO thin films were obtained on glass substrate by PLD method, the effects of aluminum concentration on the microstructure, electrical and optical properties of Al-doped ZnO thin films were investigated.In consideration of low-cost preparation of Al-doped ZnO thin films, we applied a very common Nd-YAG Laser of 1064nm(which is much cheaper than the excimer Laser) and alloy targets (which are much cheaper than ceramic targets) to decrease the price of Al-doped ZnO thin films. Many reports thought the surface of Al-doped ZnO thin films by Nd-YAG Laser have a lot of big droplets. However, it is possible to obtain relatively high quality Al-doped ZnO films through control the process parameters including laser energy density, oxygen pressure, substrate temperature and substrate species. Polycarbonate is an interesting substrate material due to its reduced weight, transparency and high shock resistance. The melting point of polycarbonate is between 140 and 145°C. The extremities of the working temperatures for short periods are 130°C to maintain its properties. Polycarbonate resists weak acids and bases as well as alcohols and organic oils. Deposition of ZnO:Al thin films on polycarbonate substrates is particularly significative. Researches indicate that n-type ZnO films can be well prepared, this is due to the high self-compensating process on doping derived from the intrinsic donor defects such as oxygen vacancy (Vo) and zinc interstitial (Zni) atoms, so n-type ZnO films through doping is the key step for application in the fields of ZnO-based opoelectrical devices, in which great progress have been made now. Their photoluminescence and electrical properties have been studied in this paper. It is, therefore, of great value to study the effects of the deposition parameters in order to obtain ZnO:Al thin films with optimum properties, several conclusions through theory and experiment investigation are made:(1) Al-doped ZnO films were prepared by PLD with the alloy target containing different amounts of Al. The amounts of Al in ZnO:Al thin films deduced from EDS are proportional to the Al amount in the target. The crystal structure of ZnO:Al thin films is hexagonal wurtzite, XRD revealed that the grain size of the ZnO:Al thin films decreases with increasing the Al content in the targets. From the spectrometer transmittance data, the band gap energies of Al-doped ZnO films were calculated by a linear fitting method. The band gap is found to be large with the increasing dopant concentration. It is found from the photoluminescence measurement that near band edge (NBE) emission and deep-level emission are observed in pure ZnO thin films. However, when Al is doped into thin films, the deep-level emission of the thin films is depressed. As the concentration of Al increases, the peak of NBE emission has a blue-shift to region of higher photon energy, which is coincident with those values calculated by the linear fitting from the transmittance data.(2) Samples with the lowest resistivity, 8.78×10-4?cm, and transmittance over 80% at the visible region were prepared by using the 2.0wt. % Al target at 200°C. concentration increases from 5.00×1018 /cm3 for undoped ZnO to 2.50×1020/cm3 for the ZnO:Al thin film prepared with the 2.0wt. % Al target, but decreases to 1.00×1020/cm3 for sample prepared with the 8.0wt. % Al target. This indicates that not all Al atoms in the film are activated.(3) It was observed that 2.0wt.% of Al is the optimum doping amount in the target to achieve the minimum film resistivity and the highest UV emission without deep-level emission. Furthermore, the intensity of UV emission peak of ZnO:Al thin films in our study is strongly dependent on the contents of Al.(4) ZnO:Al thin films were successfully prepared on quartz glass substrates by PLD in oxygen at room temperature, using the ablation of alloy target (2.0wt.% A1). XRD revealed that aluminum atoms replace the zinc atoms in the hexagonal lattice and ZnO:Al thin films exhibited a preferential (101) orientation. Furthermore, the film deposited at 1.0 J/cm2 shows high optical transmission (85%) in the visible range.(5) ZnO: Al films have been prepared on polycarbonate (PC) substrates by pulsed laser deposition technique at low substrate temperature (room-1000C). The films were analysed with AFM, PL and XRD techniques, Results show that polycrystalline ZnO: Al films having a preferred orientation with the c-axis perpendicular to the substrate and strongly ultraviolet photoluminescence, the AFM images of ZnO: Al films indicate surface roughness and grain size are different due to substrate temperature variation. (6) N-Al codoped p-type ZnO thin films have been realized by PLD in N2O ambient. N2O acts both oxygen and nitrogen doping source. The films were characterized with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Hall measurements. Results showed that the codoping with N and Al induces the formation N-Al bands in ZnO thin films. We found that the existence of Al increases the concentration of N impurity, which acts as acceptors and which may form the Al-N bonds. The codoped film obtained at 5000C possesses good p-type conduction at room temperature with a carrier density of 2.49×1017 cm- 3 ,a resistivity of 68.3?cm and a Hall mobility of 0.54cm2/ Vs, as well as good crystallinity with (002) orientation and high transmittance in the visible region, which is required to provide a transparent p-n junction.