Zinc Oxide/Copper Oxide Mixed Films Deposited by CVD

Abstract

Metal organic chemical vapor deposition (MOCVD) was used to prepare ZnO/Cu 2 O mixed films. The process condition studied were: deposition temperature from 360 o C to 440 o C, partial pressures of oxygen, 190-380 torr, precursor copper acetylacetonate (Cu(acac) 2 ), 0.21 torr, and zinc acetylacetonate(Zn(acac) 2 ), 0.45 torr. AES and XPS analysis showed average elemental content being Cu, 52-66%, Zn, 11-20%, O, 30-40%. Copper presented primarily as Cu(I) ions, and Zn as Zn(II) ions. SEM, SAM and XRD indicated that deposited films were polycrystalline with composite structure. Some films had a continuous ZnO thin layer with flat surface, equiax fine grained. On the top of ZnO layer were covered by discrete, irregular shape, coarse Cu 2 O grains. Primary Cu 2 O phases were (110), (111), and (200) orientation. ZnO phases were (002), and (103). SEM showed the impact of deposition temperature on Cu 2 O grain size. Such data were used to estimate the activation energy for grain growth. In fact, to our experiences from these studies, binary metal oxide CVD showed a wide spectrum of film microstructure and morphology. These phenomena reflect the result caused by both thermodynamics as film composition, and kinetics as mass transfer/ surface reaction rate. INTRODUCTIONSimultaneously depositing Zinc oxide/copper oxide mixed films is an attractive research topic. Zinc oxide and copper oxide are the basic ingredients for methanol synthesis catalysts. Recent research showed the phase boundary between ZnO domain and Cu 2 O domain were of great importance to catalyst surface activities [1]. For piezoelectric micro acuator use, Cu doped ZnO films with higher resistivity, which allow lower operation frequency(<1 MHz) for ZnO MEMS devices such as microlevers, can reduce the device driving voltage and save energy [3]. Lamb has used ion implantation to prepare Cu doped(10 16~1 0 17 ions/cm 2 ) ZnO films and after annealing at 500 o C, achieved a reisitivity of 3X10 12 Ωcm [2]. On the other hand, cuprite, or cuprous oxide Cu 2 O, is a metal deficient, p type semiconductor materials, with its hole density and conductivity vary with system oxygen pressure, and temperature. With antifluorite cubic unit lattice, Cu 2 O exhibits Schottky-type disorder, and has been used for rectifying devices [3]. In the energy band diagram of Cu 2 O, there are one localized energy level associated with the cation vacancy center and two localized energy levels resulting from the anion vacancy centers. R.F. sputtering has been used to deposit (Zn, Cu)O films with ZnO pellet target bonded on a larger copper plate [4]. Cu dpoed single crystal ZnO was also grown by chemical vapor transport [5]. A thin film deposition process, which delivered aerosol droplets containing dihydrate zinc acetate and copper acetate precursor solution onto heated substrate, has been claimed to produce Cu doped ZnO films with resistivity increased by a factor of 1000 [6].Recently, MOCVD has been employed to prepare oxide ceramics due to advantages over traditional process: In MOCVD,...