Zon-Huang Lan

On-Chip Fabrication of Well-Aligned and Contact-Barrier-Free GaN Nanobridge Devices with Ultrahigh Photocurrent Responsivity**

In the last two decades, a wide range of semiconductor nanowires have been synthesized and used as building blocks for the development of a new generation of electronic and optoelectronic devices. [1–8] The integration of these nanowires into the thin-film-based microchip has become a critical problem in the practical application of the nanomaterial properties and for industrial manufacture. Single-wire-based devices have been shown to possess novel properties and provide a major platform for fundamental research. [9–13] However, the conventional fabrication of single-wire devices by the ‘‘pick and place’’ method is rather complicated and uneconomic, which is unsuitable for large-scale manufacturing. In contrast, devices based on an ensemble of nanowires are much easier to fabricate, thus reducing the barriers to practical applications. A method based on the bridging concept and showing the potential to directly integrate an ensemble of nanowires onchip was first demonstrated by Haraguchi et al. [14] Different from the traditional ‘‘bottom-up growth and then top-down processing’’, the nanowires are laterally grown across a trench and suspended between two film posts as nanobridges (NBs). [14–26] As the main device architecture and electrodes can be designed and prepared prior to NB growth, nanomaterial deterioration due to the post treatment in the

Enhanced dynamic annealing in Ga ¿ ion-implanted GaN nanowires

Ga+ ion implantation of chemical-vapor-deposited GaN nanowires (NWs) is studied using a 50-keV Ga+ focused ion beam. The role of dynamic annealing (defect-annihilation) is discussed with an emphasis on the fluence-dependent defect structure. Unlike heavy-ion-irradiated epitaxial GaN film, large-scale amorphization is suppressed until a very high fluence of 2×1016 ions cm−2. In contrast to extended-defects as reported for heavy-ion-irradiated epitaxial GaN film, point-defect clusters are identified as major component in irradiated NWs. Enhanced dynamic annealing induced by high diffusivity of mobile point-defects in the confined geometry of NWs is identified as the probable reason for observed differences.

Blueshift of yellow luminescence band in self-ion-implanted n-GaN nanowire

Optical photoluminescence studies are performed in self-ion (Ga+)-implanted nominally doped n-GaN nanowires. A 50 keV Ga+ focused ion beam in the fluence range of 1×1014–2×1016 ions cm−2 is used for the irradiation process. A blueshift is observed for the yellow luminescence (YL) band with increasing fluence. Donor–acceptor pair model with emission involving shallow donor introduced by point-defect clusters related to nitrogen vacancies and probable deep acceptor created by gallium interstitial clusters is responsible for the shift. High-temperature annealing in nitrogen ambient restores the peak position of YL band by removing nitrogen vacancies.

Hexagonal-to-Cubic Phase Transformation in GaN Nanowires by Ga+ Implantation

Hexagonal to cubic phase transformation is studied in focused ion beam assisted Ga+-implanted GaN nanowires. Optical photoluminescence and cathodoluminescence studies along with high-resolution transmission electron microscopic structural studies are performed to confirm the phase transformation. In one possibility, sufficient accumulation of Ga from the implanted source might have reduced the surface energy and simultaneously stabilized the cubic phase. Another potential reason may be that the fluctuations in the short-range order induced by enhanced dynamic annealing (defect annihilation) with the irradiation process stabilize the cubic phase and cause the phase transformation.

Direct evidence of nanocluster-induced luminescence in InGaN epifilms

x-ray diffraction, scanning electron microscopy, energy dispersive x-ray spectrometry, and cathodoluminescence measurements have been employed to study the correlation between optical and structural properties in InGaN epitaxial films. In-rich quantum dots were found to be dispersed throughout the film. By the combination of these measurements, we clearly identify that brighter luminescence arises from In-rich regions while dimmer luminescence corresponds to the Ga-rich matrix regions.

Self-Regulating and Diameter-Selective Growth of GaN Nanowires

We report diameter-selective growth of GaN nanowires (NWs) by using mono-dispersed Au nanoparticles (NPs) on a ligand-modified Si substrate. The thiol-terminal silane was found to be effective in producing well-dispersed Au NPs in low density on Si substrates so that the agglomeration of Au NPs during growth could be avoided. The resultant GaN NWs exhibited a narrow diameter distribution and their mean diameter was always larger than, while keeping a deterministic relation with, the size of the Au NPs from which they were grown. A self-regulating steady growth model is proposed to account for the size-control process.