Li-doped ZnO nanorods was grown on n-Si(111) substrate by chemical vapor deposition. XRD pattern showed that the nanorods are pure wurtzite ZnO of hexagonal crystal structure without any other oxide, such as Li2O. Hall effect experiment under Van der Pauw configuration showed that Li-doped ZnO nanorods behave the p-type conductivity with hole concentration of 6.72Χ1016 cm-3 and a Hall mobility of 2.46cm2 V-1 s-1 . A neutral acceptorbound exciton emission(AX)was confirmed by the measurements of temperature-dependent photoluminescence(PL) spectra. The optical acceptor energy level is calculated to be about 142 meV.
One dimensional nanomaterials have attracted great interest because of their unique and fascinating optical , electrical , mechanical and thermoelectrical properties together with their wide uses in fundamental scientific research and potential technical applications. Among these materials, ZnO is one of the most promising materials since it has a high mechanical and thermal stability, a wide bandgap (3.37 eV), and a large exciton binding energy(60 meV). Up to now, a number of ZnO nanomaterials with different morphologies and interesting structures ,such as ananowires, nanorods, nanospheres, nanotowers, nanotetrapods and nanocombs, have been successfully synthesized with different methods.
In order to realize nanosized ZnO electronic and optoelectronic devices, it is necessary to prepare both high-quality n-type and p-type ZnO. However, the growth of stable and reproducible p-type ZnO with high conductivity and high mobility is difficult because of its self-compensating effect,deep acceptor level and low solubility of acceptor dopants. It has been reported that Li-doped thin ZnO films behaved good p-type conductivity with high hole concentration by our previous studies and also by Ye,etal.indicating that Li is an optimal acceptor dopant for p-type ZnO.
In this work, we reported the growth of Li-doped ZnO nanorods on the Si substrate by a chemical capor deposition technique. The ZnO nanorods without any dopant under the same condition have also been grown for comparison. The optical properties of the Li-doped ZnO nanorods were also studied in detail.
The n-type Si(111) wafer was used as substrate. It was cleaned in an ultrasonic bath with acetone and alcohol at room temperature, respectively. Following that it was etched in diluted HF(10%) solution for 5 min, and then rinsed in deionized water and blown dry using high-purity nitrogen gas. ZnO nanorods were synthesized by chemical vapor deposition method in a horizontal tube furnace in air. The Zn-Li alloy with 2% Li was placed in a ceramic boat placed at the center of the tube fuinace. The Si substrate was laid above the ceramic boat. The distance between the Zn surce and the substrate was about 3 mm. The tube furnace was evacuated by using a mechanical rotary pump to remove the residual oxygen. The substrate was heated to an appropriate temperature of 600 ℃ at a rate of 10 ℃/min with a continuous flow of 200 sccm argon gas. About 2 min later, the mixed-gas of argon and oxygen with a ratio of 2:1 was introduced into the system for growth about 20 min. The yellowish powder on the Si substrate was obtained after the furnace was cooled down to room temperature naturally(denoted as samples A). Under the same conditions. We conducted another experiment, the only difference was that the pure metal Zn without any dopant was used as Zn source(denoted as sample B).
The morphology and structure of ZnO nanorods were characterized using a field emission scanning electron microscope(FE-SEM; Philips XL30FEG) with an accelerating voltage of 5kV and XRD measurement was performed by using a RigakuO/maxRA X-ray diffractometer with Cu Kαl radiation(λ=0.15418 nm). The electrical properties of ZnO nanorods were obtained by Hall measurements under the Van der Pauw configuration at room temperature. The temperature-dependent PL spectra were conducted on a 325 nm He-Cd laser with a liquid helium cooling system Jobin Yvon LabRAM HR800UV.
Results and Discussion
Fig.1(a) and (b) show the FE-SEM images of ZnO nanorod with Sample A and Sample B, respectively. From fig.1 ,it can be found that the synthesized Li-doped ZnO nanorods in sample A are about 50nm in diameter and 1 μm in length, which are obviously smaller than the ZnO nanorods in sample B(about 300nm in diameter and 10μm in lenghth).This imlies that the ZnO nanorods growth behavior is associated with Li dopant. Further study of the mechanism is in progress.
Fig.1 SEM images of the Li-doped ZnO nanorods(a) and ZnO nanorods without any dopant(b)
The inset of Fig.2 shows X-ray diffraction (XRD) patterns of ZnO nanorods with Sample A and Sample B, respectively. The dominant diffraction peaks of two samples correspond to diffraction of wurtzite hexagonal ZnO. There is no diffraction peak of any other oxide, such as Li2O, in the XRD patterns. The (002) diffractive peak for sample A is located at 34.34°，which is larger than that of sample B at 34.27°，as shown in Fig.2. Since the covalent radius of Li(0.123nm) is a little smaller than that of Zn(0.125nm),the increase of(002) diffraction peak of sample A compared to sample B indicates that Li atom substitutes for Zn lattice site in the ZnO. The full width at half-maximum(FWHM) for two samples is about 0.36°，which implies good crystallinity of ZnO nanorods(FWHM of ZnO substrate films is about 2°).
Fig.2 XRD patterns of around the (002)peak of Sample A,and sample B,respectively. Inset is the corresponding XRD profile in diffraction angles(2θ) of 30°——65°.
The sample A is associated with Li dopant, which is due to the fact that Li atom can replace Zn site in the Li-doped ZnO nanorods to form acceptor dopants.
Fig.3 shows the PL spectra of the sample A and sample B in the ultraviolet regiom at 20 K, respectively. It can be seen that the emission bands located at 3.306.3.234, and 3.162 eV can be observed in the both samples. The obvious differences between two samples are the emission bands located at 3.351 and 3.364 eV. In Sample B, the emission band shows the temperature-dependent redshift, which is a result of a transition from acceptor-bound exciton emission to free exciton(FX) emission as temperature increases. Therefore, this emission band can be attributed to A○X . The temperature-dependence of the emission bands located at 3.306 eV position fits well in an equation for a radiative electron transition from conduction band to acceptor(FA) given by
ＥＦＡ（Ｔ） ＝Ｅｇ（Ｔ）－ＥＡ ＋ｋＢＴ／２，
Where EFA (T) is the temperature-dependent FA transition energy, EA is the acceptor energy level, and kB is the Boltzmann constant. The acceptor energy level is calculated to be 142 meV, as shown in Fig.4(b), the value is slightly smaller than that of Sample A, which may be due to the smaller diameter of ZnO nanorods.
Fig.3 The PL spectra at 20K for the sample A and sample B, respectively（λｅｘ=325nm）
Since the energy difference between 3.162,3.234 eV and 3.306 eV are 72 and 144 meV,which are equivalent to one and two longitudinal optical(LO)-phonon energy of ZnO, respectively. Therefore, these two emission bands are ascribed to FA-1 LO and FA-2LO.
The p-type ZnO nanorods with hole concentrations of 6.72*1016 cm-3 and mobility of 2.46 cm2·V-1。S-1 is fabricated on n-Si(111) substrate by chemical vapor deposition using Li as dopant. The Li-doped p-type ZnO nanorods are of pure wurtzite ZnO crystal structure, and no other oxide, such as Li2O. The measurements of temperature-dependent PL spectra show that a meutral acceptor-bound exciton emission is found. The optical acceptor energy level is calculated to be about 142 meV.
