ZINC OXIDE NANOSTRUCTURES GROWN BY LOW TEMPERATURE METHODS WANG MIAO (M.Eng, TIANJIN UINVERSITY; B.Eng, TIANJIN UINVERSITY) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN ADVANCED MATERIALS FOR MICRO- AND NANO- SYSTEMS (AMM&NS) SINGAPORE-MIT ALLIANCE NATIONAL UNIVERSITY OF SINGAPORE 2009 ACKNOWLEGEMENTS First of all, I would like to express my sincere appreciation to my supervisors, Prof. Chua Soo Jin and Prof. Carl V. Thompson for their continuous supports, invaluable guidance, and encouragement throughout this research work. They have offered me insightful ideas and suggestions and have led me the scientific way to research with their profound knowledge and rich research experience. Without their help, I would not have been able to achieve this research goal. I am also extremely grateful to Dr. Gao Han and his team members, Ms. Tan Lee Kheng, Ms. Maria Chong A.S., and Ms. Audreay Ho Yoke Yee. Their assistance on the experiments of this work was very helpful. What’s more, I wish to extend my thanks to the numerous individuals for their contributions, which made this dissertation possible. I would like to express my special gratitude to Dr. Tay Chuan Beng, who has always provided his suggestion and support to my work without reservation. A lot of gratitude should go to Dr. Zang Keyan, Dr. Liu Hongfei, Dr. Teng Jinghua, Dr. Karen Ke Lin and Mr. Wang Benzhong for their instructive discussion. Great acknowledgement are due to Mr. Lau Jun Yong for the patterned polystyrene preparation, Mr. Norman Ang Soo Seng for the assistance on photolithography, Mr. Lim Poh Chong for the XRD measurement support, Ms. Doreen Lai Mei Ying for the SIMS measurement, Ms. Tan Hui Ru for the TEM measurement and Mr. Wan Nianfeng for the AFM measurement. Singapore-MIT Alliance (SMA) and National University of Singapore (NUS) are gratefully acknowledged for financial support. Institute of Materials Research and Engineering and Center for Optoelectronics in NUS provided all the experiment and characterization equipments. And I own special thanks to Azimuth Technology, Singapore, for providing their prototype commercialized ALD machine for my experiment. I would like to thank Prof. Choi Wee Kiong, Ms. Vinodhini Prabakaran, Ms. Juliana Chai Chwee Yeong, Ms. Hong Yanling, and Ms. Jocelyn Sales for all the administrative support during my PhD. I am also greatly indebted to all my classmates, colleagues, and friends for their positive influence in my life. At last but not the least, I would like to give my special thanks to my parents. Their unconditional love, encouragement and support enable me to go through the hard times and complete this work. TABLE OF CONTENTS ACKNOWLEGEMENTS ………………………………………………………….1 TABLE OF CONTENTS ………………………………………………………….3 SUMMARY……………………………………………………………………….10 LIST OF TABLES……………………………………………………………… .12 LIST OF FIGURES……………………………………………………………….13 CHAPTER INTRODUCTION………………… .…………………………19 1.1 Background…………………………………….……………………… 19 1.1.1 Unique properties of ZnO …………… ……………………….19 1.1.2 Research status of ZnO…………………………………….… .21 1.1.3 Synthesis method for ZnO nanostructures…………… ………22 1.1.3.1 Vapor-Solid process……………………………… .…… .23 1.1.3.2 Vapor-Liquid-Solid process ………………………………24 1.1.3.3 Metal-Organic Chemical Vapor Deposition/Metal-Organic Vapor Phase Epitaxy .…… .……………………………………….25 1.1.3.4 Molecular Beam Epitaxy…………………………… .… .26 1.1.3.5 Atomic Layer Deposition………………… .…………… 27 1.1.3.6 Solution methods……………………………… .……… .29 1.1.3.7 Electrodeposition……………… .……………………… .30 1.2 Motivation and objectives ………………….……………………………33 1.3 Thesis organization……………………….…………………………… .35 CHAPTER A BRIEFING ON THE SYNTHESIS AND CHARACTERIZATION METHODS…………………………………………….36 2.1 Synthesis method – Electrochemical deposition ….…………………….36 2.1.1 Fundamental principles of ECD……………… ………………36 2.1.2 Mechanism of ZnO electrodeposition…………… ………… .42 2.1.3 Experimental set up for ECD……………… …………………43 2.2 Synthesis method - Atomic Layer Deposition ………………………… 45 2.2.1 Growth mechanism……………………….…………………….45 2.2.2 Equipment for ALD…………………….…………………… 48 2.3 Characterization techniques…………………………………………… 49 2.3.1 Microscopy…………………………………………………….49 2.3.1.1 Scanning Electron Microscope & Energy Dispersive Spectrometer…… . ……………………………………………… .49 2.3.1.2 Atomic Force Microscopy…………… .………………….51 2.3.1.3 Transmission Electron Microscopy……………… .…… .53 2.3.2 X-Ray Diffractometry……… ……………………………… .55 2.3.3 Spectroscopy………………………………………………… .59 2.3.3.1 Photoluminescence spectroscopy.…….………………….…59 2.3.3.2 X-ray Photoelectron Spectroscopy …….………………… .62 2.3.3.3 Secondary Ion Mass Spectroscopy .….…………………….63 2.3.4 Hall Effect measurement……………………………………….65 2.3.5 Cyclic voltammetry…….……………………………………… 68 CHAPTER ELECTRODEPOSITION OF ZINC OXIDE NANOSTRUCTURES ON GOLD SURFACES AND THEIR PROPETIES……70 3.1 Study of key parameters of ECD…………………………………….… 70 3.1.1 Overpotential………………………………………………… .71 3.1.2 pH value……………………………………… ………………75 3.1.3 Electrolyte concentration……………………………………….80 3.1.4 Temperature……………………………… ………………… .81 3.1.5 Time…………………………………………………………….83 3.2 Optimized ECD condition……………………………………………….84 3.3 Properties of ZnO nanostructures on different type of Au films……… .87 3.3.1 Au films preparation and characterization…………………… 87 3.3.2 Morphologies of ZnO - SEM result……………………………89 3.3.3 Structural properties……………………………………………91 3.3.3.1 Out-plane crystal orientation - XRD ω-2θ scan………… .91 3.3.3.2 In-plane crystal orientation - XRD Pole Figures………… 93 3.3.4 Optical properties - Room Temperature Photoluminescence… 94 3.4 Other properties………………………………………………………….96 3.4.1 Composition ………………………………… ……………… 96 3.4.1.1 Atomic ratio…………… .……………………………… .96 3.4.1.2 SIMS result……………………………………… .………97 3.4.2 Electrical properties…………………… …………………… 98 3.5 Summary…………………….………………………………………….99 CHAPTER MORPHOLOGY CONTROL OF ZINC OXIDE ELECTRODEPOSITED ON GOLD SURFACES…………………………… .101 4.1 The influence of bath constituents…………………………………… .101 4.2 ZnO films grown by two-step ECD……………………………………104 4.3 Nanotube arrays using a second step etching………………………… 106 4.4 Study on AAO template assisted ECD…………………………………108 4.5 Patterned photoresist as a template………………………………… 114 4.6 Patterned polystyrene (PS) arrays as a template……………………….118 4.7 Summary……………………………………………………………….120 CHAPTER PROPERTIES CONTROL ON ZINC OXIDE NANOSTRUCTURES …………………………………………………………122 5.1 Defect theory of ZnO………………………………………………… 122 5.2 In-situ control of ZnO properties……………………………………….125 5.2.1 Growth rate………………………………………………… 125 5.2.2 Temperature………………………………………………… .125 5.3 Post thermal annealing…………………………………………………126 5.3.1 Ambient… .………………………………………………… 126 5.3.2 Temperature………………………………………………… .127 5.3.3 Duration of annealing…………………………………………128 5.3.4 Discussion of the effect of annealing…………………………129 5.3.4.1 Composition – SIMS results….……………………… 129 5.3.4.2 Electrical property – Hall measurement……………….…131 5.3.4.3 PL emission……………………………………… .…….133 5.4 Surface modification………………………………………………… .134 5.5 Doping……………………………….……………………………… .137 5.6 Summary……………………………………………………………….139 CHAPTER ECD OF ZINC OXIDE ON TRANSPARENT SUBSTRATES ………………………………………………………………………………140 6.1 ZnO ECD on Au films on different substrates….…………………… .140 6.1.1 Morphologies.…………………………………………………… 141 6.1.2 Structural properties………………………….……………………142 6.1.3 Optical properties……………….…………………………………143 6.2 ZnO ECD on ITO/FTO coated glass substrates……………………… 144 6.2.1 Discussion on the morphologies.……………….………………….144 6.2.2 Second step etching…………………………………………….….146 6.2.3 Structural properties…….…………………………………………146 6.2.4 Optical properties……….…………………………………………147 6.3 Summary………………………………………….……………………148 CHAPTER 7.1 ATOMIC LAYER DEPOSITION OF ZINC OXIDE……… .149 ALD growth condition optimization………………………………… 149 7.1.1 Time sequence optimization………………………………………150 7.1.2 Temperature……………………………………………………….152 7.2 ZnO films grown by ALD and their properties……………………… 153 7.2.1 Growth rate……………………………………………………… 153 7.2.2 Structural properties………………………………………………154 7.2.3 Chemical composition and chemical bonding energy…………….158 7.2.4 Optical properties ………………….…………………………… 160 7.2.4.1 Photoluminescence………………….… ……………………… 160 7.2.4.2 Current-voltage characteristics of n-ZnO / p-GaN heterostructure…………………………………… … …………………162 7.3 ZnO nanotubes grown by ALD……………………………………… .164 7.3.1 Sample preparation……………………………………………… .164 7.3.2 Growth rate……………………………………………………… .165 7.3.3 Structural properties……………………………………………….166 7.3.4 Optical properties………………………………………………….169 7.4 GaN-ZnO core-shell structure………………………………………….172 7.4.1 Sample preparation……………………………………………………173 7.4.2 Structural properties………………………………………………… 174 7.4.3 Photoluminescence……………………………………………………176 7.5 Summary…………………………………………… ……………………177 CHAPTER CONCLUSIONS…………………………………………… .179 References……………………………………………………………………….183 APPENDIX CALCULATION ON ECD CURRENT EFFICENCY………….188 APPENDIX CALCULATION ON UTILIZATION EFFICIENCY OF ALD PRECURSORS………………………………………………………………….189 Table 3-1 Comparison of UV vs. visible emission intensity from samples obtained by varying the electrolyte concentration and ECD potential applied oncentration(M) potential(V) 0.001 0.002 0.002 0.002 0.02 0.02 -0.9 -1 -1.05 -1.1 -0.9 -1 current UV/Vis FWHM ratio (uA/cm2) 100 10.8 20.45 100 7.2 21.9 180 7.55 27.8 300 3.3 30.6 320 1.8 24.6 600 0.5 26.1 From Table 3-1 it can be concluded that at lower concentration (0.001 M) and lower potential (-0.9 V), the optical property of as-deposited ZnO is better. A dilute solution and no stirring during the electrodeposition is to make diffusion rather than any other transport mechanisms (such as electric migration and convection) the only significant transport process. The deposition is therefore controlled at a low and stable growth rate. The potential was further examined by a cyclic voltammetry (CV) for Au coated Si working electrode with a scan rate of 10 mV/s in a 0.001M Zn(NO3)2 solution at 65°C shows in Figure 3-12. On the negative-going scan (from 0.5 to -1.6 V), the onset of cathodic current is observed at the potential of V. This initial curve is assigned to the reduction of NO32- ions (formula 3-1). The formation of ZnO or Zn(OH)2 occurs concomitantly and continues till -1.2 V. In the plateau region between -0.7 V to -0.9 V, the current is limited by the diffusion of Zn2+ to the cathode. Within this regime, more negative potential cannot make the current any larger because the current is limited by the rate of diffusion of NO3- to the electrode. The increase in the current density at the 85 juncture of -0.9 V indicate the deposition of Zn occurs therefore Zn rich ZnO is obtained when the potential is more negative than -0.9 V. Figure 3-12 CV scan of 0.001M Zn(NO3)2 from 0.5 V to -1.6 V to 0.5 V at 65°C An optimized concentration of 0.001 M and a potential of -0.9 V was chosen, under which a good quality of ZnO can be electrodeposited with a moderate growth rate. 3.3 Properties of ZnO nanostructures on different type of Au films Three types of Au films on Si substrates were studied: (1) single crystalline, (2) highly (111) textured polycrystalline, and (3) randomly aligned polycrystalline. They were used as cathodes to electrodeposit ZnO respectively under the optimized conditions. The properties of ZnO were characterized and an investigation on the influence of the properties of Au films in ZnO is discussed in this section. 86 3.3.1 Au films preparation and characterization Epitaxial Au films were prepared by removal of the native oxide layer on Si first. The Si (111) wafer was dipped into 1:50 HF solutions and then immediately put into an electron beam evaporator with a 10-9 Torr base pressure using a load lock mechanism. One µm thick Au layers were deposited at a rate of 0.1 nm/s. Polycrystalline Au films with a thickness of 400 nm were deposited with a base pressure of 10-6 to 10-7 Torr on Si (100) substrates with native oxide layers. After that, a thermal treatment on poly-Au on a Si substrate was performed in vacuum for 20 at 250°C, which gave more randomly aligned polycrystalline Au. The energetics of grain structure evolution in thin films on substrates are affected by the surface energy, the interface energy and the strain energy density, all of which vary with the crystallographic orientation of a grain. Normal grain growth for thin films with time independent grain size distribution cannot occur until the subpopulation of grains favored by surface and interface energies minimization or strain energy minimization has consumed all grains with other orientations. For fcc metals on amorphous substrates, for example, surface and interface energies generally appear to be minimized for grains with (111) texture, whereas strain energies are minimized for grains with (100) texture.85 An investigation of the crystal structures and morphologies of Au films was conducted by acquiring Au(100) pole figures and AFM of Au surfaces. In Figure 3-13(a), pole figure of epi-Au film, there are only peaks at the tilt angle, χ, of 54.7°, which is the angle between the (100) and (111) planes in a cubic system. 87 This verifies the epitaxy of Au with good alignment not only out-of-plane but also in-plane. The situation is different in Figure 3-13(b) from poly-Au. There is a uniform ring at 54.7° in Au (100) pole figure, meaning that the in-plane orientations of Au (111) textured grains are random. After thermal annealing at 250°C, the ring from Au(111) diffraction even diminish as shown in Figure 3-13(c), which indicates a deterioration of Au (111) crystallinity of the films after annealing. (The four peaks remains in the pole figure are from Si (111) because of the angle between Si (100) and Si (111) is 56.1°, which is very close to the angle between Au (111) and Au (100), and therefore its diffraction peak was collected because the resolution of XRD detector is not high enough.). Figure 3-13 Au (100) X-ray pole Figure of (a) epi-Au, (b) (111) textured poly-Au, and (c) randomly aligned poly-Au after annealing 88 AFM of the Au surfaces of as-grown poly-Au and annealed poly-Au are shown in Figure 3-14. It can be observed that the grain size of Au after annealing was increased and the RMS of poly-Au surface roughness changes from 26.81 nm to 36.77 nm. This suggests that during the thermal annealing at 250°C, the Au undergoes grain growth and that this grain growth favored grains without (111) texture. Figure 3-14 AFM 3D image of sample A (on epi-Au) and B (on poly-Au) in 10×10µm2 range 3.3.2 Morphologies of ZnO - SEM result The electrodeposition of ZnO was carried out at 65°C with potential of -0.9 V for 90 minutes on the three types of Au surfaces mentioned above. The electrolyte contained an aqueous solution of 0.001M Zn(NO3)2 with 0.1M KCl as the supporting electrolyte. The samples, hereafter referred to as A, B, and C, are ZnO deposited on epi-Au on Si(111), poly-Au on Si(100), and annealed poly-Au on Si(100), respectively. Figure 3-15 (a) - (c) are SEM images of the as-deposited samples A, B, and 89 C, which are ZnO ECD on epi-Au, highly (111) textured poly-Au, and more randomly aligned poly-Au, respectively. The ZnO deposited on epi-Au shows well-aligned vertical rods with hexagonal end planes of diameter around 100~150 nm. Coalescence of adjacent rods is visible over large areas after a deposition time of 90 minutes. The morphology of sample B is slightly different from A with some rods tilted and less densely packed. The diameters of the nanorods end planes are around 150~300 nm. From Figure 3-15(c), it can be seen that much larger nanorods (Φ~400 nm) with various orientations were obtained for sample C. Since the only difference between samples A, B and C is the properties of Au films where ZnO begins its nucleation and growth, the morphology and structure differences should be caused by this difference. (a) (b) (c) Figure 3-15 SEM images of (a) sample A on epi-Au (b) sample B on textured poly-Au and (c) sample C on randomly aligned poly-Au AFM can provide a general idea of the surface smoothness of ZnO obtained by ECD under optimized conditions. A comparison between sample A and B of the RMS is 8.02 nm vs. 16.378 nm, with a scan range of µm× 4µm. 90 3.3.3 Structural properties It has been reported that the in-plane epitaxial relationship between Au and ZnO is (1 x 1)Au(111)[110]//(1 x 1)ZnO(0001)[1120] and the mismatch is 12.7% (Figure3-16).86,87,88,89 Heteroepitaxial growth of ZnO on Au (111) therefore can be obtained. Figure 3-16 schematic illustration on lattice mismatch between Zn(001) and Au(111) 3.3.3.1 Out-plane crystal orientation - XRD ω-2θ scan The X-ray diffraction spectra measured in ω-2θ geometry from samples A, B, and C are illustrated in Figure 3-17. For comparison purposes, the XRD peak positions for commercial ZnO powder (data from the Joint Committee on Powder Diffraction Standards card 36-1451) have also been plotted. The XRD pattern from Sample A is a typical XRD of ZnO electrodeposited on epi-Au film. Only two peaks are observed, which can be assigned to ZnO (002) and Au (111) (Si peaks were filtered). The dominant peak, ZnO (002), is located at 2θ = 34.4°, whilst the full width at half maximum (FWHM) of this peak is about 91 0.12°. The absence of any other peaks confirms the good alignment of these nanorods with the c-axis perpendicular to the substrate surface. The inset of Figure 3-17(a) shows the XRD ω rocking curve of the ZnO (002) peak, with a full width at half-maximum (FWHM) of 1.30 which compares favorably with 1.45° observed in other work and indicates a good degree of alignment along the normal direction 89 of the substrate surface. For sample B, ZnO electrodeposited on poly-Au, not only Au (111) but (110) and (100) textures are also detected in the XRD pattern. This suggests the strain energy minimization dominate in defining the texture instead of surface -energy minimization. The consequence is the existence of weak ZnO (101), (102) and (103) diffraction peaks in addition to ZnO (002), indicating that some of the nanorods are tilted away from the direction perpendicular to the substrate (the angle between ZnO (101) and (002) is 62.1°, between ZnO (102) and (002) is 43.4° and between (103) and (002) is 32.2°). From the ZnO powder diffraction standard, the intensity ratio between the (002) and (101) peaks is 0.44; between (002) and (102) is 1.93; and between (002) and (103) is 1.6. In sample B the corresponding ratio are 46, and 7.9, respectively. It can be inferred that the number of (002) grains in sample B are 100 times more than that of (101) grains, times more than that of (102) grains and times more than that of (103) grains in the ZnO powder. The much stronger (002) peak in sample B indicates that ZnO nanorods prefer to grow with their c-axis normal to the substrate plane, which can be verified from the SEM image in Figure 3-15(b). 92 The diffraction intensities of Au (110) and (100) peaks relative to Au (111) became stronger in the XRD pattern of sample C. When ZnO was electrodeposited on the Au with more randomly aligned grains on a Si substrate, more peaks were found in the XRD pattern of sample C, which can all be indexed to wurtzite hexagonal ZnO phases. The relative intensity between ZnO (101) and (002) is increased. The angle between the newly emerged peaks (110) and (002) is 90°. Therefore, some of the ZnO nanorods are tilted at larger angles or even lying down. This morphology can be observed in Figure 3-15(c) as well. Figure 3-17 XRD pattern of sample A (ZnO on epi-Au), sample B (ZnO on textured poly-Au), and sample C (ZnO on more randomly aligned poly-Au) 3.3.3.2 In-plane crystal orientation - XRD Pole Figures To further investigate the relationship between the ZnO and Au surfaces on 93 in-plane crystal orientation, XRD pole figures were obtained for samples A and B. The epitaxy of Au with good alignment not only out-of-plane but also in-plane results in well-aligned in-plane orientation of ZnO (002) grains grown on top are well aligned as well. As shown in the pole figure for ZnO (101) in Figure 3-18(a), there are isolated peaks at a tilt angle of 62°, corresponding to the angle difference between (002) and (101) planes of ZnO. Since the poly-Au film had a random in-plane orientation on the textured (111) plane, the resultant ZnO layer also had a random in-plane orientation on the (002) plane. Therefore, the pole figure for ZnO (101) in Figure 3-18(b) shows a ring at a tilt angle of 62°, corresponding to the angle difference between (002) and (101) planes of ZnO. Figure 3-18 ZnO (101) X-ray pole Figure of (a) sample A on epi-Au and (b) sample B on (111) textured poly-Au 3.3.4 Optical Properties - Room Temperature Photoluminescence Figure 3-19 shows the PL spectra of the three samples with strong UV emission peaks centered at 369 nm (3.36eV) and weak visible emission covered 94 the whole visible range. The UV peak can be further divided into peaks located at 361.7 nm (3.42 eV), 369 nm (3.36 eV) and 378.2 nm (3.28 eV) by Lorentz deconvolution. From our observations, the PL spectrum of ZnO electrodeposited in the electrolyte containing only Zn(NO3)2 normally shows a strong UV emission peak centered at 379 nm and a weak visible emission covered the whole visible range. The peaks at 369 nm are from free-exciton emission (FX) 90,91 ; and the peak located at 378 nm is the result of the first LO phonon replica of FX (FX-1LO). This is reasonable because the LO phonon energy for ZnO is ~72 meV, near to the energy difference of these two peaks (80 meV). It has been reported that both FX and phonon-assisted FX emission may contribute to room-temperature band edge emission. 92 , 93 FX-1LO emission dominates in the ZnO sample deposited in Zn(NO3)2 solution, while FX emission dominates in the sample deposited from the electrolyte containing extra KCl. However, after thermal annealing, the UV peak of the latter goes back to 378 nm and the peak at 361 nm disappears. This is postulated to be related to the incorporation of Cl in ZnO, which will be discussed in Chapter on the study of defect and thermal annealing. The intensity ratio between the UV and visible emission peaks at room temperature is a criterion to judge the property of ZnO. In this specific case, the number is around 19.8 for sample A, 23.3 for sample B and 25 for sample C. The properties of PL spectrum indicate that the ZnO nanorod arrays prepared by ECD on all kinds of Au surfaces are of good crystal quality. 95 Figure 3-19 Room temperature PL spectrum of sample A (ZnO on epi-Au), sample B (ZnO on textured poly-Au), and sample C (ZnO on randomly aligned poly-Au) The optical properties of all the samples are very similar since the PL is more related to the surface state of ZnO, which will be comparable under the same electrodepositing conditions (including electrolyte, temperature and potential applied). 3.4 Other properties 3.4.1 Composition 3.4.1.1 Atomic ratio The EDX results show good stoichiometry of the ZnO with a composition of 50±2.5at% Zn to 50±2.5at% O from electrolyte both with and without KCl being added. There is no apparent signal from other elements, which means the 96 concentrations of other impurities are very low. An acceleration voltage of 15 eV was applied for EDX detection and the penetration depth can be 2-5 µm. The ZnO films being measured are with thickness of 400 nm ~ 1000 nm, the results therefore show the bulk atomic ratio of ZnO. A typical EDX spectrum of ZnO by ECD is shown in Figure 3-19. Figure 3-20 EDX spectrum of ZnO from ECD The atomic ratio at the surface measured by XPS is O:Zn=42:58. The detection depth of XPS is around 10 nm. It is deduced that there is O vacancies defect on the surface of as-deposited ZnO. 3.4.1.2 SIMS result SIMS is a more powerful way to identify the composition of materials in determining the impurities. Small amount of K and Na coming from DI water and glass beaker are mostly likely to incorporate into the deposit. By comparing the SIMS spectrum between ZnO electrodeposited from electrolytes with and without KCl, it can be observed that by adding KCl, more K and Na are introduced with an increased concentration from bulk to the surface of ZnO sample. 97 (a) (b) Figure 3-21 SIMS result from ZnO by electrodeposition with Zn(NO3)2 (a) and Zn(NO3)2 and KCl (b) in the electrolyte 3.4.2 Electrical properties In order to measure the electrical properties of the electrodeposited ZnO 98 through a Hall measurement, a thin layer of ZnO (40nm) was deposited by 200 cycles of ALD on an insulating substrate (sapphire substrate); and a thicker layer of ZnO was electrodeposited on this conductive ZnO seeding layer. The carrier concentration in the as-deposited ZnO film was 3.15 x 1018 cm-3; and the carrier mobility was 7.13 cm2/V-s. 3.5 Summary The influence of ECD parameters on the growth mechanism and the properties of ZnO nanostructures were discussed in this chapter. By comparing the photoluminescence spectrum among the samples electrodeposited under different conditions, optimized ECD experimental conditions were found to be in 0.001 M Zn(NO3)2 solution at 65°C under a potential of -0.9 V. Several groups of ECD experiments were carried out on Au surfaces with different crystallographic properties by varying the Au deposition parameters. The influence of these properties was discussed. By further comparing the morphological, structural and optical properties of the ZnO nanostructures grown under optimized ECD conditions on different Au surfaces, we drew the conclusion that the morphology and structure are closely related to the cathode surface ZnO deposited, whereas the optical properties are more related to the ECD experimental conditions. All in all, the characterization methods carried out showed that the as-deposited ZnO were nanorod arrays with strong UV emission centered at 379 nm. It was n-type ZnO 99 material being deficient in oxygen and with carrier concentration at the order of 1018 cm-3. 100 [...]... or background impurities such as hydrogen Another possible cause of low doping concentration is the low solubility of the dopant in ZnO 1.1.3 Synthesis methods for ZnO nanostructures ZnO nanostructures have been fabricated by various methods Vapor phase synthesis is the most extensively explored method for the synthesis of 1D ZnO nanostructures, such as thermal evaporation1,2,6,7,8, metal-organic chemical... placed in a zone with a temperature lower than that of the source material Decomposition of ZnO is a direct and simple method For example, Zhang et al25 , Yao et al26 and Kong et al27 have synthesized ZnO nanorods (or nanowires) by evaporating ZnO commercial powder However, this technique is limited to very high temperatures (~1400°C) Another direct method at relatively lower temperatures (500~700°C)... O2 and a growth temperature of 300 to 500°C, growth of ZnO nanorods on the Si/SiO2 substrate can be achieved.36 Instead of internal oxidation of Zn by the oxygen diffusing into Ag/Zn alloy in VLS method, the formation of ZnO in this method is by the reaction of Zn with intermediate Ag2O formed at a much lower temperature Figure 1-3 SEM images of ZnO nanorwires / nanorods synthesized by (a) VLS (b) MOCVD... Figure 7-12 XRD spectra of ZnO nanotubes and films after 100 cycles ALD 171 Figure 7-13 Comparison of PL spectra between ZnO NT arrays and films after 100 cycles ALD under the same conditions, and ZnO NT arrays after plasma treatment 172 Figure 7-14 PL spectra of (a) ZnO nanotube arrays grown by ALD, (b) nanorod arrays grown by ECD, and (c) nanotube arrays grown by ECD and etching ... second-step etching on the ZnO nanorods grown by ECD The properties of nanotubes obtained by these two methods were compared In order to combine the advantage of both ALD and ECD, a trial on obtaining epitaxial ZnO tubular-structures was performed by ALD using single crystalline GaN nanodots as templates finally GaN -ZnO core-shell nanostructures were achieved with nearly single crystal ZnO shell structure... crystalline and optical properties of ZnO nanostructures by both in-situ and ex-situ methods Chapter 6 presents the results for ZnO ECD on transparent substrates Chapter 7 discusses the experimental parameter optimization of ZnO ALD and demonstrates the successful deposition of ZnO films and nanotube arrays and their properties Epitaxial ZnO film on GaN and GaN -ZnO core-shell nanostructures are obtained Lastly,... epitaxy substrates 37 High temperature processing requirements prohibit the growth of ZnO on substrates with low melting points such as plastic and glass substrates, or circuit board and bio-materials which are not high temperature compatible As such, increasing attention has been paid to low temperature processes, including low temperature gas phase processes and liquid solution methods Atomic layer deposition... of ZnO was carried out by post-treatment including thermal annealing and plasma treatment Our study shows that the enhancement of UV emission by thermal annealing is due to the activation of H in the material; and the reduction of defect emission by plasma treatment is due to the reduction of surface defects ALD as another low temperature synthesis method to obtain ZnO was studied as well Epitaxial ZnO. .. of ZnO Light color – Zn; dark color – O 124 Figure 5-2 Electronic energy levels of different defects in ZnO from different studies in the literature 125 Figure 5-3 PL spectra of as -grown ZnO annealed in different ambients 129 Figure 5-4 PL of ZnO after annealing at different temperatures and the changes of the FWHM with changing annealing temperatures 130 Figure 5-5 PL spectra of ZnO. .. substrates by MOVPE.34 In these methods, no catalyst is employed for ZnO nanorod formation, which leads to preparation of high purity ZnO nanorods The catalyst-free growth mechanism of ZnO nanorods has not been thoroughly investigated but the main reason for anisotropic growth is anisotropic surface energy in ZnO, which depends on the crystal faces of wurtzite ZnO In addition, high speed laminar gas flow . treatment. 172 Figure 7-14 PL spectra of (a) ZnO nanotube arrays grown by ALD, (b) nanorod arrays grown by ECD, and (c) nanotube arrays grown by ECD and etching 173 17 Figure 7-15 SEM. temperature synthesis method to obtain ZnO was studied as well. Epitaxial ZnO films with high crystal and optical qualities were obtained on GaN surface on sapphire substrates by ALD at a temperature. ZINC OXIDE NANOSTRUCTURES GROWN BY LOW TEMPERATURE METHODS WANG MIAO (M.Eng, TIANJIN UINVERSITY; B.Eng, TIANJIN UINVERSITY)