1. Introduction
Gallium nitride (GaN) has attracted much attention due to its excellent optoelectronic property with a direct band gap of 3.4 eV, high mobility and excellent thermal stability[1]. It is widely accepted that GaN nanostructures are highly promising materials for producing devices with excellent performance, such as LDs[2], LEDs[3], solar cells[4], sensors[5], and piezo-electric nanogenerators[6]. GaN nanostructures have many superior properties due to the possibility of quantum confinement, coaxial heterostructures, correlated photon emission, and photonic crystal effects, which make it a potential contender for nanolasers and nanoLEDs[7, 8]. In particular, these GaN nanostructures could provide additional advantages for light emission, quality improvement of regrowth, etc[9, 10]. GaN nanostructures such as nanorods (NRs) with high aspect-ratio and large surface-to-volume ratio can dramatically reduce the dislocation density in the upper part of the NRs[11]. Besides, the nanopillars with small footprints help to relieve the strain induced by thermal expansion mismatch and avoid crack generation. The light-extraction efficiency is expected to increase owing to the non-planar geometry of nanowires. Compared to thin films, heterostructures on GaN nanostructure arrays with low defect density are much easier to fabricate and lead to high-performance devices[12]. Also, GaN nanostructures can provide a unique opportunity for understanding the electronic, optical, and mechanical properties of the material.
Nano-GaN has been successfully synthesized via different methods, such as molecular beam epitaxy[13], selective growth on patterned substrates by metalorganic chemical vapor deposition (MOCVD)[14], and hydride vapor-phase epitaxy (HVPE)[15]. These above-mentioned methods are so-called "bottom-up" techniques, where nanostructures are grown from atoms or molecules to clusters. However, they often show a broad distribution of size, length, and orientation.
Contrary to bottom-up synthesis, a top-down etching process is an alternate way to achieve nanostructures[16, 17]. Top-down methods, such as inductively coupled plasma (ICP) etching, is more economic and extensively controlled and could be applied in the fabrication of a large-area nano-GaN in the future. In this work, we fabricate uniform GaN nanopillar arrays by inductively coupled plasma etching using Ni nano-islands as the masks. A high density of
2. Experiment
Figure 1 shows a schematic diagram of the formation of GaN nanopillars on sapphire. A 2
3. Results and discussion
Figure 2 shows the SEM images of 35 nm thick Ni film annealed using ammonia at 850 ℃ for different times. Ni film was transformed to nano-islands after the NH
Figure 4 shows the SEM images of Ni film with different initial thickness annealed using ammonia at 850 ℃ for 12 min. It can be seen that the size of the Ni islands increased with increasing the Ni film thickness, but the uniformity became worse. As can be seen above, we think that uniformly distributed Ni nano-islands can be obtained by controlling the NH
Figure 5 shows the SEM image of GaN with 35 nm Ni film by NH
Because GaN nano-pillars have a homogeneous vertical shape, we think that some Ni particles remain on the top of the GaN nanopillars. SEM with energy dispersive X-ray (EDX) spectrometry has been carried out for the presence of Ni and the residual Ni can be seen only on the top part of the pillars (Fig. 6).
The results of the Raman scattering measurements are shown in Fig. 7, where the E
PL spectra of the GaN film and GaN nanopillars array are measured at room temperature (RT). The PL spectrum of GaN nanopillars has a sharper and stronger near-band-edge emission as compared with GaN film, where a 61 meV redshift can be observed. ICP damage, a Stokes shift, or defect impurity states[22, 23] may induce the redshift of the PL band. A biaxial stress relaxation of about 2.8 GPa is estimated by using the proportionality factor
4. Conclusions
High-density GaN nanopillar arrays have been successfully fabricated using Ni nano-islands as the inert masks. The thickness and the NH