Synthesis and Nano-Processing of ZnO Nano-Crystals for Controlled Laser Action
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Synthesis and Nano-Processing of ZnO Nano-Crystals for Controlled Laser Action K. Okazaki, T. Shimogaki, M. Higashihata, D. Nakamura, and T. Okada Graduate School of Information Science and Electrical Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ABSTRACT Lasing characteristics of a single Zinc oxide (ZnO) nanosheet and a single ZnO nanowire were investigated by an ultraviolet light excitation. ZnO nanocrystals were synthesized by chemical vapor deposition (CVD) method, and those ZnO nanocrystals were excited by a thirdharmonic Q-switched Nd:YAG laser beam (355 nm, 5 ns). The emission spectra from a single ZnO nanocrystal was collected by an objective lens with a magnification factor of 100 or 50, coupled with a spectrometer with a light fiber. The area observed by the spectrometer is about 10 μm in diameter, and therefore the emission spectra from a single ZnO nanocrystal can be observed. The emission spectra showed the obvious lasing characteristics having mode structure and a threshold for lasing. The lasing threshold power density of a ZnO nanosheet and a ZnO nanowire were measured to be about 60 kW/cm2 and 150 kW/cm2, respectively. ZnO nanosheet can be a superior laser medium due to the lower threshold for lasing compared to the threshold of the ZnO nanowire. However, since the lasing spectra had mode structure, a single-longitudinal mode lasing would be required for a practical application. The single longitudinal mode lasing can be realized by a nanomachining of a grating on the ZnO nanocrystal surface due to distributed bragg reflector (DBR) laser. The minimum DBR pitch was estimated to be about 81 nm, which can be machined by focused-ion beam (FIB) focused up to 7 nm at minimum, and therefore, we demonstrated the nanomachining on a single ZnO nanowire. However, the singlelongitudinal mode lasing was not observed so far, and thus optimization of experimental conditions such as the DBR pitch, ion dose amount and increasing the number of repetition of DBR would be required. INTRODUCTION ZnO is a compound II-VI semiconductor material with a wide band-gap of approximately 3.37 eV at room temperature, and therefore ZnO has a potential for applications in an ultraviolet (UV) region. ZnO has similar characteristics to Gallium nitride (GaN) which has already been used for UV devices such as light emitting diode (LED) and laser diode (LD). GaN has a wide band-gap of 3.42 eV at room temperature, and the both ZnO and GaN have hexagonal wurtzite crystal structure. However, ZnO has some advantages over GaN, which include a significantly larger exciton binding energy of ZnO (60 meV) than the exciton binding energy of GaN (24 meV) and the thermal energy at room temperature (26 meV), and therefore efficient exciton emissions from ZnO can be expected. In addition, since there is a great deal of resources of ZnO compared to the resources of GaN, ZnO is a promising material replacing GaN for UV applications. ZnO nanocrystals have been attracting a great interest due to their importance in both
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