Investigation of optical properties of ZnO:In nanostructures grown using vapor cooling condensation technology at low temperature

• Main Authors: Ming-Kai Wang, 王明楷
• Other Authors: Tzung-Fang Guo
• Format: Others
• Language: en_US
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/18059673078394381818

碩士 === 國立成功大學 === 光電科學與工程研究所 === 95 === This study investigates the growth of ZnO nanorods using novel vapor cooling condensation method at low temperature. With assistance of anode aluminum oxide membrane (AAM), the isotropy and uniform ZnO nanorods array can be fabricated. Moreover, to improve the electrical conductivity of ZnO nanorods, Indium-doped ZnO (ZnO:In) is a promising structure. Indium is used to be n-dopant to increase the carrier concentration. Meanwhile, because of the raising carrier concentration, the difference of optical property between ZnO and ZnO:In nanorods can be observed depending on the Photoluminescence(PL) measurement,. ZnO nanorods are grown in thermal coater at quite low temperature due to the designed vapor cooling system. The growth of nano nucleuses can be suppressed due to the lower temperature which causes vapor ZnO quenched to a smaller particle rapidly and deposited into the AAM pores easily. In addition, the vacuum pumping system provides a driving force to make ZnO molecule through the same direction, and it is helpful for ZnO molecule to do the physical vapor condensation (PVD) through the pores of AAM template. The unremovable catalyst on the top of nanorods and the exceeding high processing temperature caused from the general chemical vapor deposition (CVD) method could be avoided by using vapor cooling condensation method, and it is helpful for the further application. In general, ZnO nanorods structure grown at low temperature do not have good electrical conductivity, and it will restrict the further device fabrication. In order to fabricate the ZnO nanorods structure device, doping Indium in ZnO is a proper way to increase the carrier concentration and improve the electrical conductivity. The doping atomic proportion of ZnO:In is 10:1. ZnO powder and Indium tablet are put on the same tungsten boat and heated. After maintaining high-enough temperature for 30 minutes, ZnO:In molecule would be brought out by diffusion mechanism. Later, ZnO:In molecule start to evaporated following the increasing temperature of tungsten boat. Through the observation of SEM, It can be seen that the grown ZnO nanorods are hexagonal close-packed wurtize structure. The diameter and length of the ZnO:In nanorods are about 200 nm and 100 nm, respectively. The C-axis preferred orientation and mutually vertical array can be found. From PL measurement, stronger green emission can be observed in ZnO:In nanorods indicating more defeat in ZnO:In nanorods in comparison with ZnO nanorods. In further, the even stronger green emission observed in annealed ZnO:In nanorods indicates the even more defects existing to increase the carrier concentration. Basing on the result of low temperature PL measurement, the UV emission of annealed ZnO:In nanorods become red-shift and broadening indicating that Indium element is successfully doping into ZnO nanorods to increase the carrier concentration. On the other hand, growing n-ZnO:In nanorods on the surface of p-GaN to form the n-ZnO:In nanorods / p-GaN heterojunction LEDs is the other important topic in this study. Photoresist would be used to be the isolating layer between ITO electrode and p-GaN substrate. The optical property of this nanostructure light emitting device would be discussed. By Electroluminescence(EL) measurement, a broad blue band at 435nm is observed in ITO / n-ZnO:In / p-GaN nanostructure heterojucntion, which is attributed to the excessively high carrier concentration of ZnO:In nanorods causing the depletion region locate in p-GaN layer mostly. In order to fabricate ZnO nanorods light-emitting diode, undoped ZnO nanorods are used to substitute for ZnO:In nanorods to form ITO / i-ZnO / p-GaN nanostructure heterojunction. Wet-oxidation is employed to reduce carrier concentration of ZnO nanorods. However, a similar broad band peak as p-n nanostructure diode is observed in p-i-n nanostructure diode by EL measurement. The luminescence layer not in i-ZnO nanorods might result from the insufficient reduction of carrier concentration in undoped ZnO nanorods treated by wet-oxidation.