| Recently,a great deal of interest has been fueled in the development of polycrystalline or amorphous transparent conducting oxide(TCO) semiconductors used for practical thin-film transparent electrode applications,due to their excellent electrical and optical properties. Aluminum-doped zinc oxide(AZO) thin films,which are the most important TCO,with low resistivity and high transparency in visible range are promising as alternatives to ITO for thin-film transparent electrode applications,since AZO source materials are inexpensive and non-toxic.Using a zinc oxide target doped with Al2O3(2 wt%.),transparent conductive Al-doped zinc oxide(AZO) films with highly(002)-preferred orientation were deposited on quartz substrates at room temperature by RF magnetron sputtering in this paper.Optimization of deposition parameters were based on RF power,Ar pressure in the vacuum chamber,film thickness,and distance between the target and substrate.The structural,electrical,and optical properties of the AZO thin films with and without annealing were investigated by XRD,AFM, SEM,XPS,AES,Hall measurement,and optical transmission spectroscopy etc.Moreover,a series of p-GaN Ohmic contact and GaN-based light emitting diodes with AZO and AZO/Ni, Pt,Ag transparent electrodes have been fabricated.The electrical and optical properties of the devices have been investigated.The important results were obtained as follows:1.The crystal structure of the AZO films is hexagonal wurtzite and shows the typical c-axis crystallographic orientation.The films are loose,due to deposition at low temperature. All samples are almost stoichiometric with the little O atoms chemisorbed at the grain boundary.The 500 nm-thickness AZO films with an electrical resistivity as low as 2.88×10-4Ω·cm and an average optical transmission of 90.4%in the visible range were obtained at RF power of 300 W,Ar flow rate of 30 sccm,and target distance of 7 cm.2.After conventional thermal annealing in N2 atmosphere for 20min at 400℃～700℃,the resistivity of the film gets worse with increase of annealing temperature and reached 2.09×10-2Ω·cm at 500℃,while the Hall mobility and the carrier concentration decreased to 3.71 cm2/Vs and 8.04×1019cm-3 respectively.As annealing at 700℃,the film became an insulator,then the B-M effect disappeared and optical band-gap reduced.That implies Al in the films was combined by O after annealing,leading to the decrease of Hall mobility and carrier concentration. 3.After conventional thermal annealing in N2/H2 atmosphere for 20min at 500℃,the resistivity of AZO film slightly increased to 8.65×10-4Ω·cm,while the Hall mobility and the carrier concentration also slightly decreased to 12.6 cm2/Vs and 5.73×1020 cm-3 respectively. The main reason for non-influence after annealing is that O atoms in the film were deoxidized by H,and the Al ions were avoided being oxidation by O.However,as annealing at 600℃,as a result of more O atoms was combined by H,the AZO films became thin.As annealing at 700℃,all O atoms in the AZO films were reacted by H,and remains Zn in the substrate, while Zn evaporates at more than 500℃temperature ambient.4.A layer of metal(Ni,Ag,or Pt) was deposited by e-beam evaporation techniques onto the surface of quartz substrates,coated with AZO(Al-doped ZnO),to form metal/AZO double film structures,using RF manetron sputtering.The electrical and optical properties of Ni,Pt,Ag/AZO double film structures were studied.The deposition of AZO films on the surface of a metal layer resulted in the limitation of crystal quality with a slight reduction of their optical transmittance.However,the sheet resistivity and the average transmittance in the visible range of 2nm Ni/250nm AZO double films is 21.0Ω/å£and 76.5%respectively,which is still suitable as transparent electrode for GaN-based LEDs.5.AZO transparent contacts to p-GaN have been fabricated in this paper.The contact with single AZO layer shows non-line I-V characteristics,due to the damage of p-GaN layer by high energy sputtering ion.However,it was found that deposition of thin metal films(Ni, Ag,and Pt) by evaporation before sputtering AZO films will improve the I-V characteristics. The specific contact resistance without any annealing was determined to be 5.01×10-3, 5.74×10-3,and 3.90×10-3Ω·cm2 for 5nm Ni/AZO,5nm Ag/AZO,and 5nm Pt/AZO layer contacts respectively.In additional,a series of GaN based LEDs with transparent AZO as ohmic p-contact have been designed and fabricated.The forward voltages(Vf) measured at 20 mA were equal to 5.12,3.20,3.40,and 3.05 V for the 500nm AZO,5nm Ni/250nm AZO,5nm Ag/250nm AZO,and 5nm Pt/250nm AZO blue LED,respectively.And the light intensity of these LEDs was 40.4,32.3,28.7,and 29.4 mcd respectively.
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