Effect of working pressure and temperature on ZnO film deposited on free-standing diamond substrates
ZHAO Ping(赵 平)1, XIA Yi-ben(夏义本)1, WANG Lin-jun(王林军)1, LIU Jian-min(刘健敏)1, XU Run(徐 闰)1, PENG Hong-yan(彭鸿雁)2, SHI Wei-min(史伟民)1
1. School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China;
2. Department of Physics, Mudanjiang Normal College, Mudanjiang 157012, China
Received 10 April 2006; accepted 25 April 2006
Abstract: The structure characteristic and electric performance of ZnO film deposited on nucleation side of free-standing diamond substrates under different heating temperatures (Th) of substrate and working pressures (p) were studied. The structure of the ZnO films tested by X-ray diffraction shows that ZnO film of high c-axis orientation is deposited on the nucleation side of free-standing diamond substrate which is extremely smooth when Th=250 ℃ and p=0.4 Pa. After annealing at 480 ℃ in N2 atmosphere, the SEM and the AFM analyses demonstrate that the c-axis orientation of ZnO film is obviously enhanced. The resistivity of ZnO films also increases up to 8?105 W?cm which is observed by I-V test.
Key words: ZnO film; c-axis orientation; free-standing diamond film; surface acoustic wave device; radio-frequency magnetron sputtering
1 Introduction
Diamond has the highest acoustic wave velocity among all materials and thus it can be expected to be a candidate of the substrate for high frequency surface acoustic wave (SAW) devices. Utilizing the high SAW velocity of a diamond substrate, approximately 10 km/s phase velocity can be expected by a ZnO/diamond structure, which provides great advantages in the manufacture of high frequency SAW devices[1]. Conventional chemical vapor deposition (CVD) diamond substrates used in SAW devices exhibit high surface roughness which will cause the degradation of device characteristics and difficulty with device fabrication, such as distortion of frequency responses, large insertion loss, poor interdigital transducer (IDT) patterns and low fabrication yield[2]. It has been demonstrated that unpolished nucleation side of free-standing diamond has lower surface roughness and higher surface resistivity than growth side, which meets the requirement for SAW device substrates[3].
Among recently developed piezoelectric thin film materials, ZnO has been considered to be a very promising film for fabricating small-size high-frequency SAW devices. The use of ZnO film for SAW devices requires several important properties as below: 1) excellent c-axis orientation to obtain high electro-mechanical coefficient; 2) nanocrystalline grain size to decrease scattering of SAW and insertion loss; 3) resistivity higher than 105W×cm to reduce operation loss.
In this paper, the influence of substrate heating temperature and working pressure on crystalline structure and quality of ZnO film deposited on the nucleation side of free-standing diamond by radio-freqency(RF) magnetron sputtering was investigated, and the optimal conditions for ZnO film with high quality and c-axis orientation were obtained.
2 Experimental
The 100 μm thick polycrystalline diamond layers were deposited on silicon substrates (1 mm×1 mm) by hot filament chemical vapour deposition (HFCVD) method[4]. After deposition, the silicon substrates were chemically etched and then the nucleation side of free-standing diamond layers with a surface roughness less than 5 nm was obtained. ZnO films were deposited on the nucleation side of free-standing diamond substrates by radio-frequency magnetron sputtering method with a ZnO target with purity of 99.99% and 120 mm in diameter. The sputtering chamber was evacuated to 3×10-3 Pa before sputtering. During the film deposition, the ambient gas was argon (99.999%). The sputtering power was kept to be 180 W in this experiment. The samples were divided into two groups in order to investigate the influence of substrate heating temperature (Th) and working pressure (p) on c-axis orientation and crystal quality of ZnO film. The deposition parameters are listed in Table 1. The thickness of ZnO film was 600-700 nm.
Table1 Deposition parameters of ZnO films
After settling down the best condition of sputtering, annealing treatment was introduced to improve the quality of ZnO films. The ZnO films were annealed at 480 ℃ in N2 atmosphere for 1 h, the temperature rised and declined at about 1 ℃/min.
The c-axis orientation and crystalline structure were identified from X-ray diffraction (XRD) (Cu Kα, λ=
0.154 056 nm) measurements. The surface morphology was observed by atomic force microscope (AFM) (AP-0190). The I-V curve was measured by semiconductor analysis system (4200-SCS) after depositing electrode (1 mm diameter) on ZnO film at room temperature.
3 Results and discussion
3.1 Dependence of ZnO films on substrate tempera-ture
Fig.1 shows the variation of the preferred c-axis orientation of ZnO film at different substrate temperatures. When Th=0 ℃, the (002) ZnO peak near 34.4? is the weakest, however the (001) peak (31.7?) and (101) peak (36.2?) are stronger. When Th=250 ℃, the (002) peak is the strongest. According to the XRD spectra, the (002) orientation level can be estimated by Tc(002)=I(002)/ΣIZnO[5], where I(002) represents the (002) peak intensity, ΣIZnO represents the sum intensity of all the ZnO peaks. From Fig.2, when Th=0, Tc(002)=42% means that the ZnO film is in the polycrystalline mix growth condition. With the increase of substrate temperature, Tc(002) upgrades first and then declines. At Th=250 ℃, Tc(002) reaches 76.5%, which shows the best c-axis orientation of ZnO film.
Fig.1 XRD patterns of samples in group A at 180 W, 0.6 Pa and different heating temperatures
According to Bragg formula d=λ/(2sinθ), when Th=0, 150, 200, 250, 300 ℃, the interplanar spacing d=0.269, 0.261, 0.259, 0.262 and 0.261 nm, as to standard powder sample, d=0.260 nm. When Th=0 ℃, the interplanar spacing is much larger than the standard one. The reason is that when the temperature is too low, the nucleation particles with little energy are quickly “cooled down” so as not to diffuse to lattice sites of lowest energy. When the substrates are heated, the interplanar spacings are closer to the standard value which means the the inner stress of (002)-planar is decreased. Fig.2 also shows the full width half maximum (FWHM) of (002)-peak which reflects the level of c-axis orientation. When the substrate temperature is less than 250 ℃, FWHM declines with the rise of temperature, the ZnO crystallite size increases, the crystal boundary decreases and crystal quality are improved; when the substrate temperature is higher than 250 ℃, the crystal quality oppositely drops due to a large number of defects
Fig.2 Tc(002) and FWHM as function of Th
introduced by part of the surface rich of Zn atoms[6]. So 250 ℃ was chosen as substrate temperature to deposit high quality ZnO films.
3.2 Dependence of ZnO films on working pressure
Fig.3 shows the variation of the preferred c-axis orientation of ZnO film at different working pressures when the substrate temperature is fixed at 250 ℃. When p=0.4 Pa, the (002)-peak is higher than that of any other sample in group B, and reaches the maximum value of 87.9%, which demonstrates excellent c-axis orientation, but when p deviates from 0.4 Pa, Tc(002) declines to some extent and the orientation becomes worse.
Fig.3 XRD patterns of samples in group B at 180 W, 250 ℃and different working pressures
According to Bragg formula, when the pressure p=0.2, 0.4, 0.6 and 0.8 Pa, the interplanar spacing d=0.260, 0.260, 0.261 and 0.260 nm. The samples under each pressure have (002)-interplanar spacing close to the standard value. Fig.4 also shows the tendency of FWHM values. The FWHM of sample at working pressure of 0.4 Pa is 0.5?, better than those of other samples, which shows the sample has the best crystal quality when p=0.4 Pa.
Under lower pressure, the collision chances between
Fig.4 Tc(002) and FWHM as function of p
depositing particles and arsenic atoms decrease, which leads to the increase of particle average free path. So when the working pressure is too low, the high sputtering rate makes ZnO molecules quickly accumulate on the substrates and some molecules deposited earlier could not diffuse to lattice sites of minimum energy. This will worsen the (002) orientation and produce a great amount of lattice defects and declines the crystal quality. When the working pressure increases, the sputtering rate decreases and the thickness of ZnO film in same deposit time decreases as result. When the ZnO layer is quite thin, the lattice mismatches and growth competition among the neighboring crystals are not finished and there still exist many defects which make the (002) orientation and crystal quality become worse[7].
3.3 Optimization of ZnO films
High annealing temperature leads to high migration of the atoms, since the (002)-orientation is originally dominated, the orientation of other orientation will be restrained[8]. Then after annealing treatment, there is an extremely high (002) peak as shown in Fig.5, as a result, highly oriented crystalline ZnO is formed. Fig.6 shows the I-V curve of ZnO after annealing. The resistivity could be roughly calculated as 8×105 W?cm, which is larger than that of ZnO films as-grown. As the resistivity of ZnO is directly related to the number of electrons, the raised resistivity after annealing is because the oxygen vacancies formed by oxygen decrease from the ZnO film via the annealing process in a reducing atmosphere[9]. In addition, when the annealing treatment is performed in a reducing atmosphere, the carrier’s concentration may increase by desorption of oxygen at the grain boundaries which act as traps for carriers. Fig.7 shows the AFM image of ZnO thin film, indicating that the c-axis orientation and crystal quality of ZnO film are enhanced after annealing and the grain size is nanocrystalline. The
Fig.5 XRD patterns of ZnO thin films as-grown and after annealing at 250 ℃ and 0.4 Pa
Fig.6 I-V curve of ZnO thin film after annealing at 480 ℃ in N2 at mosphere
Fig.7 AFM image of ZnO thin film after annealing
surface roughness of ZnO film is about 0.51 nm.
4 Conclusions
ZnO films are deposited on the nucleation side of free-standing diamond by RF magnetron sputtering at different substrate temperatures and working pressures. The c-axis orientation and some parameters of lattice, such as Tc(002), d, FWHM, are researched and the best conditions of sputtering, Th=250 ℃, p=0.4 Pa, are chosen to deposit ZnO film of high c-axis orientation. The results of XRD patterns, AFM image and I-V analysis demonstrate that after annealing treatment, the crystal quality, c-axis orientation and resistivity increase so as to meet the requirement of substrate for ZnO/diamond SAW devices.
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(Edited by CHEN Wei-ping)
Foundation item: Project (60577040) supported by the National Natural Science Foundation of China; Project (0404) supported by the Shanghai Foundation of Applied Materials Research and Development; Projects(0452nm051, 05nm05046) supported by the Nano-technology Project of Shanghai; Project (T0101) supported by the Shanghai Leading Academic Disciplines
Corresponding author: WANG Lin-jun; Tel: +86-21-56333514; Fax: +86-21-56332694; E-mail: ljwang@staff.shu.edu.cn
