Atom Probe tomography를 이용한 ZnO 나노구조 내 원소 및 성장 기구 분석

Abstract

ZnO nanostructures have been studied because of their great potential for future applications such as electronic, optoelectronic, and magnetoelectronic devices. These structures include not only 1-D nanostructure such as nanowires, nanobelts, and nanotubes, but also 2-D nanostructure such as nanowalls. ZnO nanowalls have a large surface ratio, and they can provide many advantages such as a wide detectable range of sensitivity for sensors and low contact resistance for electronic devices. However, no research has demonstrated nanowall formation using any other catalyst. In this study, we investigated the structural, optical, electrical and compositional properties and the differences between Ni and Au as a catalyst for ZnO nanowall growth. ZnO nanostructure were formed on a-plane (110) sapphire substrate by a VLS technique under various condition. ZnO nanowires with specific plane (103) only grew as time goes on. It confirms significantly that the primary growth plane of the nanostructure was switched to (103) plane by introducing Ni into the ZnO lattice. ZnO nanowalls with about 100 nm thickness and 1 µm height were thus formed under specific condition (15 Torr, 950℃ for 10 min). Grown ZnO nanowires using Ni catalyst and Au catalyst were single crystalline with defects such as many dislocations. Au catalyst even though both two ZnO nanowires had similar lattice. PL emission spectra of the as-prepared ZnO nanowalls exhibited an ultraviolet emission peak (about 380 nm) at room temperature. The Raman spectrum showed comparable values reported for high-quality ZnO bulk crystals. The resistances obtained were on the order of 2.86×106 Ω for the nanowires grown using Ni catalyst and 1.98×106 Ω for the nanowires grown using Au catalyst. Zn diffused and became alloyed with Al2O3 in the sapphire substrate. Zn diffusion formed interlayers between the ZnO nanowall structure and the substrate. In particular, the interlayers below the network and the ZnO nanowalls had distinctive Zn, Al, and O compositions. Also the present study demonstrates that APT performed to find out the accurate elemental distribution of ZnO nanowalls by using a Ni catalyst instead of the Au catalyst and growth mechanism of ZnO nanowalls using APT. As getting the similar ratio of Al2+ and Al3+ from two samples of A and B, it can analyze accurately without consideration of variable. The 3D atom map revealed the distribution of Ni, Al and other element in the analyzed volume of the ZnO-NiO-interlayer-substrate. To avoid the wrong composition profile of Zn, Zn2+ was calculated by using the quantity of other isotope except 64Zn2+. Ni was distributed with about 1.09 atomic % and 1.33 atomic % in the interlayer of sample A and B, respectively. This meant that nickel diffused simultaneously with Zn during the formation of the interlayer by the reaction of sapphire and ZnO. However, the concentration of Ni in the interlayer of both sample A and B is low. Also few Ni in sapphire substrate of both samples remained because typical non-reactive system of Ni-Al2O3 disturbed Ni diffusion. Al atoms diffused out of the interfacial region or the buffer layer, as reported previously. Nucleation begins at the NiO network surface and then the Ni network was completely oxidized as NiO. Thus, Ni can act as an effective catalyst similarly to Au in the VLS method and could also be associated with the ZnO nanowall network. The sequence of the nanowall formation is as follows: the Ni networks were formed by thermal heating up to 950 ºC. The Zn vapor dissolved the Ni networks by evaporation. Zn diffused into the sapphire substrate and formed a ZnAl2O4 interlayer with a very rough interface. This interlayer is believed to facilitate the growth of ZnO nanowalls. STEM-EDS compositional analysis showed that the stoichiometry of NiO in the sample was 1:1 (Ni:O = 50:50 at.%). In addition, APT confirmd that the interlayer was ZnAl2O4 because the actual measured stoichiometry was about 1.1:2:3.9 (Zn:Al:O). APT showed that the atomic ratio of Ni:Zn:O:Al determined by at composition analysis of the NiO network region was about 17:28:52.5:2.5.Finally we tried to investigate the ZnO nanowire as a function of laser energy by femtosecond laser assisted APT. It is known that charge ratio of each element was significantly changed by laser energy. A single nanowire was attached on an electropolished tungsten tip by using Pt welding for observing APT analysis by experimental condition. Laser energy was set to 40 nJ and applied voltage was changed from 3 kV to 12 kV at 30 K. Sample diameter was measured from FE-SEM image. APT detected not only elements such as Zn and O ion but also oxide molecular ions, such as O2+, ZnO3+, Zn2O2+, Zn2O+, Zn3O22+, Zn2O2+ and ZnO+ from mass spectrum. The ratio of Zn+ / Zn2+ and flux increased as a function of applied voltage. These results could be predicted considering the probability of post-ionization as a function of field strength for all elements of interest in field evaporation. The increase of applied voltage leads to higher electrical field to evaporate atoms and post-ionization. The charge ratio variation or flux was not significantly by temperature increase at ZnO nanowire sample. It is predicted that the surface temperature of ZnO momentarily reach at 800K as per computed results. This momentary temperature would rapidly reduce the effect of base temperature of ZnO nanowire sample. Because ZnO nanowire was oxide material with poor conductivity and the effect of laser pulse was maximized, the plot of the peak ratio between Zn+ and Zn2+ and flux as a function of diameter was disputed. Laser energy increases the evaporation field and it leads evaporated length increase. However, detecting efficiency was saturated near 15 nJ. Previous reported, the higher field in regions by laser incidence promotes the field dissociation of cluster ions. In low laser energy, dissociated cluster ions may be divided with ion and neutral atoms or molecules. In high laser energy, amount of cluster was not neutralized by dissociation. Especially high density was displayed from specific sharp region of asymmetric apex of nanowire. This is the reason high evaporation field was produced by small radius locally in nanowire tip apex.