| GaN based Ⅲ-Ⅴ nitrides have attracted great attention recently, as driven by the commercial success in their application in optoelectronic devices, such as blue light-emitting-diode (LED), ultraviolet (UV) photo-detector and short wavelength laser diode. The Eg=3.4eV for GaN at room temperature. Light emission from infrared light to ultraviolet light and the full color panel display of red, yellow, and blue light can be actualized; therefore, GaN has important applications foreground in optoelectronics and microelectronics. In addition, GaN has been attracted much attention as a candidate for fabrication of high temperature, high frequency and high power devices.To study the GaN and GaN-nanostructures’fabrications or performance, we can not only understand the new physical characteristics such as quantum size effect, surface effect and so on, but also can make preparations of nano-devices in the future. The performance of the blue or green light devices and UV optoelectronic devices can be largely improved by the GaN nano-materials. As GaN nano-materials have unique properties which are different from the bulk materials as well as the broad applications in electronics, photonics and ultra high density storage, et al, GaN nano-structures have attracted more and more attention in recent years.In this letter, a uniform GaN nano-pillars arrays have been successfully fabricated by inductively coupled plasma etching using self-organized Nickel nano-islands as the masks on GaN/sapphire. Ni nano-islands have been formed through the annealing under the ammonia. The size and density of GaN nano-pillars can be controlled by the thickness of Ni film, the NH3annealing time and the temperature. The Scanning electron microcopy (SEM) images, photoluminescence (PL) measurements, energy dispersive x-ray (EDS) and the Raman scattering measurements have been used to characterize the morphologies, optical and structural properties. The method was of relatively low cost, simple operation. The results obtained are as follows:1. In the thermal ammonia pretreated stage. Firstly, Ni film was transformed to nano-islands after the NH3annealing, which was attributed to the heating effect and partially etched off by NH3plasma at the same time. The ammonia has the etching effect on the nickel films and it makes the Ni films separate, smaller. Secondly, the average size of the Ni nano-particles reached the minimum value at about12min later and then increased until all the Ni islands were fully discrete. Lastly, it reached a balance stage between the NH3plasma’s etching effect and the Ni nanoparticles’mutual absorption. The average size of the Ni particles tends to be stable.2. When the other parameters were constant, the size of the Ni islands also depends on the original thickness of the nickel films which increased with the Ni film thickness increasing. The uniformity of the Ni nano-particles became worse when the thickness of Ni films reached a limited value.3. We think that the uniform distributed Ni nano-islands can be obtained by controlling the NH3annealing time and initial thickness of Ni films. In our research, the optimal parameters for the NH3annealing are as follows:etching duration of about12min, temperature of850℃and initial film thickness of35nm.4. After the ICP stage, the SEM images showed that there was a lateral etching in Cl2plasma which made the final diameter of the pillars smaller than that of Ni islands. The Raman spectra suggested that comparing with GaN bulk materials, the strain was transformed from the press stress in GaN film to the tensile stress in GaN nano-pillars due to the ICP etching and an enhancement of the near-band-edge PL intensity value indicated good optical quality of the GaN nano-pillars.5. The stress in materials could be effective relaxed through annealing the GaN nano-pillars at high temperature which could improve the light extraction and reduce the effects of ICP injury. |
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