Optical and Structural Properties of Ga-Doped ZnO ...

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Journal of Nanoscience and Nanotechnology Vol. 13, 1–5, 2013

Optical and Structural Properties of Ga-Doped ZnO Nanorods Chih-Hung Hsiao1 , Chien-Sheng Huang2 , Sheng-Joue Young3 ∗ , Jia-Jyun Guo2 , Chung-Wei Liu1 , and Shoou-Jinn Chang1 1

Institute of Microelectronics and Department of Electrical Engineering, Center for Micro/Nano Science and Technology, Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 701, Taiwan 2 Department of Optoelectronics Engineering, National Yunlin University of Science and Technology, Douliou 64002, Taiwan 3 Department of Electronic Engineering, National Formosa University, Yunlin 632, Taiwan The high-density single crystalline Ga-doped ZnO nanorods were grown on a glass substrate using the hydrothermal method. The average length and diameter of the nanorods were approximately 2.36 m and 90 nm, respectively. The Ga-doped ZnO nanorods had hexagonal wurtzite structure and a sharp morphology. The morphology and structure of nanorods were characterized by fieldemission scanning electron microscopy (FESEM), high-resolution transmission electron microscope (HRTEM), X-ray diffraction (XRD) and photoluminescence (PL) spectroscopy, when the growth temperature of the nanorods was 90 C, which ensured high crystalline quality.

Keywords: Ga-Doped ZnO Nanorods, Hydrothermal Method.

ZnO has several favorable properties, such as a wide band gap of 3.37 eV at room temperature and a high exciton binding energy of 60 mV. Forms of ZnO with wurtzite structures have various morphologies such as nanowires, nanorods, nanotubes, nanobelts and nanorings with different sizes, forms, defects and dopants.1–5 The processes for fabricating ZnO nanostructures include electrochemical deposition,6 the hydrothermal method,7 pulsed laser deposition8 and chemical vapor deposition.9 In the last few years, they have been broadly applied in, for example, light emitting diodes,10 solar cells,11 gas sensors,12 and field emission devices.13 ZnO nanorods have been doped with various elements to alter their structural, electrical and optical characteristics. Al, Ga, and In (group III) elements have been used as dopants in the fabrication of n-type ZnO nanorods. Al doping has been regarded as a potential means of fabricating indium-tin-oxide materials, because it increase conductivity without reducing optical transmission. However, Al has high reactivity, so tends to undergo oxidation. Accordingly, Ga is favored as another excellent dopant, with high conductivity, low reactivity, and resistance to oxidation. ∗

Author to whom correspondence should be addressed.

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Since the covalent bond length of Ga O, 1.92 Å, is very close to that of Zn O, 1.97 Å, Ga-doped ZnO exhibited increased electron mobility and reduced electrical resistivity.14 Additionally, because Ga has a similar ionic radius (0.62 Å)15 and atomic radius (1.26 Å) to those of Zn (0.74 Å and 1.31 Å), while those of Al (0.5 Å, 1.26 Å) are dissimilar from those of In (0.81 Å, 1.44 Å), Zn ions are easily substituted and Zn vacancies easily occupied.16 17 This work reports the growth of high-density single crystalline Ga-doped ZnO nanorods on glass substrate by the hydrothermal method. The structural, physical, electrical and optical properties of Ga-doped ZnO are studied.

2. EXPERIMENTAL DETAILS Prior to the growth of vertically aligned Ga–ZnO nanorods, a ZnO thin film was firstly deposited on a glass substrate as a seed layer by radio frequency (rf) magnetron sputtering using a ZnO target with 99.99% purity at room temperature. The ZnO seed layer on the glass substrate was annealed at 600 C by rapid thermal annealing (RTA) for 5 minutes. The un-doped and the Ga-doped ZnO nanorods were grown on the glass substrate by the hydrothermal method. The growth temperature of un-doped ZnO

1533-4880/2013/13/001/005

doi:10.1166/jnn.2013.7938

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1. INTRODUCTION

Optical and Structural Properties of Ga-Doped ZnO Nanorods

nanorods was 90 C and the growth temperatures of Gadoped ZnO nanorods were 70 C, 80 C and 90 C. The duration of the process was 8 hours. The synthesis solution was a mixture of 24.75 mM zinc nitrate hexahydrate (Zn(NO3 2 · 6H2 O), 0.25 mM gallium nitrate hydrate (Ga(NO3 3 · xH2 O) and 25 mM methenamine (C6 H12 N4 , HMTA). The morphology, crystalline structures and microstructural properties of the Ga-doped ZnO nanorods were observed and analyzed by field-emission scanning electron microscopy (FESEM, Hitachi S-4700), high-resolution transmission electron microscopy (HRTEM, Philips Tecnai F20 G2), transmittance spectrophotometry (HITACHI U3900), X-ray diffractometry (XRD, Bruker D8 Advance) and Raman spectroscopy. The photoluminescence spectra of Ga-doped ZnO nanorods were obtained using a 325 nm He–Cd laser with a power of 5 mW.

3. RESULTS AND DISCUSSION Figure 1 shows top-view FESEM images of undoped and Ga-doped ZnO nanorods grown at 70 C, 80 C, and 90 C. The inset in Figure 1 shows a top-view high-resolution FESEM image. The Ga–ZnO nanorods that were grown on the ZnO seed layer were well aligned in the vertical direction. According to a previous report, the growth involves the following chemical reaction.18

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CH2 6 N4 + 6H2 O → 6HCHO + 4NH3 NH3 + H2 O →

NH4+ + OH−

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Zn2+ + 2OH− → ZnOH2

ZnOH2 → ZnO + H2 O

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During the synthesis, Zn(NO3 2 is the source of Zn2+ ions and HMTA acts as a pH buffer, maintaining a constant pH. Therefore, the presence of more surface defects at a lower pH reflects the presence of more H+ ions, which absorb more OH groups and O2− ions. Figure 1(a) shows parts of Ga–ZnO nanowalls and nanorods that were grown at 70 C. Dopant ions, such as Ga ions, encourage growth in 2D, repress growth in 1D.19 Figures 1(c) and (d) show top-view FESEM images of un-doped and Ga-doped ZnO nanorods that were grown at 90 C. The nanotip-like morphology is clearly seen. The top surfaces of ZnO nanorods that grew along the c-axis had polar Zn-terminated (0001) top sur¯ bottom surfaces and bounded faces, O-terminated 0001 ¯ planes.20 The ions and faces that were non-polar 0110 polar surfaces interacted electrostatically, most strongly as the reaction temperature was increased, so the non-polar ¯ planes that were associated with higher energy dis0110 ¯ appeared, and the polar 1011 planes, associated with lower energy, became preferred, causing the nanorods to exhibit a needle-like morphology. Figure 2 shows the average length and average diameter of these ZnO nanorods. The average length and average diameter increased with the temperature from 70 C to 90 C. The growth rate in the [001] direction is more sensitive to temperature than that in the [100] direction, as clearly revealed by Figure 1. Consequently, changing the

Fig. 1. (a)–(c) Top-view FESEM images of Ga-doped ZnO nanorods grown at 70 C, 80 C, and 90 C, respectively. (d) Un-doped ZnO nanorods grown at 90 C. Inset in top-view show enlarged FESEM image.

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growth temperature is one of the most important methods for controlling the morphological shape of the nanorod arrays. Guo et al. explained this phenomenon in relation to ZnO nanorods.21 The highest of the growth temperatures (90 C) yielded long ZnO nanorods by ensuring a high rate of deposition in the direction of the c-axis.

Figure 3(a) shows the low-magnification bright-field (BF) TEM image of a single Ga-doped ZnO nanorod. The nanorod was needle-like with a tapered tip. The inset in Figure 3(a) shows the selected-area electron diffraction (SAED) pattern of a single Ga-doped ZnO nanorod. This diffraction pattern shows that the nanorods are wurtzite structures. Figure 3(b) shows the HRTEM image of the lattice spacing at the edge part of the single Ga-doped ZnO nanorod in Figure 3(a). The 0.254 nm and 0.274 nm lattice spacings, observed from this HRTEM image, are equivalent to that of Ga–ZnO nanorod crystal.22 Figure 3(c) shows the energy depersive X-ray (EDX) spectrum of the ZnO nanorods formed herein. The nanorods contained 53.834% zinc, 45.639% oxygen and 0.526% gallium. Their composition, Ga001 Zn099 O, is very close to the desired composition. The small copper, iron, cobalt and carbon signals originate in the TEM copper grids, and machines signal the iron, cobalt and carbon in the adhesive film. Figure 3(d) shows a high-angle annular dark field (HAADF) scanning TEM image of the top of a Ga–ZnO nanorod with a diameter of ∼ 100 nm. The inset shows the

RESEARCH ARTICLE Fig. 3. (a) Low-magnification BF TEM image of Ga–ZnO nanorods. Inset in Figure 3(a) shows SAED pattern. (b), (c) High-magnification BF TEM image and EDX spectrum of edge of nanorod in (a). (d) HAADF STEM image and EDX line-scan of Zn, O and Ga.


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(a) Full XRD spectra of all samples.

EDX line-scan of Zn, O and Ga. The curves clearly show an abrupt change in composition and a very high intensity of the signal from the Zn atom. The Ga–ZnO nanorods that grew in the c-axis direction had polar Zn-terminated (0001) top surfaces and bounded faces that were polar ¯ planes when the reaction temperature was high.20 1011 The results show that these nanorods had a needle-like morphology. Figure 4 shows XRD spectra of undoped and Ga-doped ZnO nanorods that were grown at different temperatures. Based on the Joint Committee on Powder Diffraction Standards (JCPDS Card: 36-1451), the peaks in Figure 4 were the (100), (002), (101), (103) and (004) diffraction peaks of the wurtzite structure of ZnO. The (002) peak indicated that the nanorods were preferentially oriented along the c-axis. The small linewidths of the diffraction peaks revealed a high crystalline quality. The (002) diffraction peak intensity increased and the (100), (101), (103) diffraction peak intensities decreased as the growth temperature increased. Therefore, the Ga-doped ZnO nanorods that were grown at 90 C had the best crystalline quality. 4 , with two ZnO wurtzite structures have space group C6v formula units in the primitive cell. Therefore, the optical phonons at the center of the zone satisfy the irreducible relation: opt = A1 + E1 + 2E2 + 2B1 . A1 and E1 represent the two polar modes, which separate into LO and TO components with different frequencies. However, the two E2 are nonpolar modes, while E2H is the high-frequency mode and E2L is the low-frequency mode. The A1 , E1 and E2 mode are all Raman and infrared-active. Nevertheless, the two B1 are silent modes, and are infrared and Raman-inactive. Figure 5(a) shows the Raman spectrum of the nanorods measured at room temperature. It includes four peaks at approximately 331, 376, 436, and 580 cm−1 . The peaks at 331, 376, 436, and 580 cm−1 are associated with the E2H –E2L , A1 (TO), E2H and A1 (LO) modes, respectively.23 A wide peak that is centered at approximately 485 cm−1 is also observed. 4

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Figure 5(b) shows the transmittance of the grown nanorods, measured at room temperature. Doping the ZnO nanorods with gallium reduced their transmittance. However, the transmittance of the Ga-doped ZnO nanorods increased with the growth temperature, reaching a maximum at 90 C. As shown in Figure 1, the nanowalls grown at 70 C and the nanorods grown at 80 C and 90 C. This finding is explained by the fact that the transmittance of the nanowalls was lower than nanorods. The nanorods were well aligned in the vertical direction, so the light source was easily transmittance. Figure 6(a) shows the photoluminescence spectroscopy of the nanorods measured at room temperature. The PL emission peak comprised two peaks at approximately 386 and 593 nm, which were associated with ultraviolet emission (UV) and orange emission, respectively. The UV emission agreed with near-band-edge (NBE) emission, and the visible emission was caused by deep-level defects, such as interstitials and vacancies of oxygen and zinc. The orange emission was attributed to interstitials of oxygen and zinc.24 The intensity of UV emission increased with the growth temperature. However, the intensity of visible emission decreased as the growth temperature increased. Figure 6(b) plots the peak ratio (IUV /IVis of J. Nanosci. Nanotechnol. 13, 1–5, 2013

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Acknowledgments: This work was supported by National Science Council of Taiwan under contract numbers NSC 101-2221-E-150-043 and NSC 100-2221-E-150057. National Formosa University Research and Services Headquarters and Common Laboratory for Micro and Nano Science and Technology that provided the partial equipment for measurement are also acknowledged.

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References and Notes

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4. CONCLUSIONS In summary, needle-like high-density Ga-doepd nanorods were successfully grown on a glass substrate using hydrothermal method. These nanorods were the wurtzite structure. TEM EDX results show that their composition was GaZnO very close to the desired composition of Ga001 Zn099 O. PL results show that best crystalline quality and vertically alignment were obtained at a growth temperature of 90 C.

Received: 16 January 2013; Revised/Accepted: 2 May 2013.


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Fig. 6. (a) Room-temperature PL spectra and (b) ratios of UV emission to visible emission from un-doped ZnO and Ga-doped ZnO nanorods.

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