1. Introduction

Nanostructures such as nanowires, nanorods and nanoparticles are considered to be promising building blocks for next generation highly-efficient electronic, optoelectronic and sensor devices [1–4]. In addition, the optical / electrical properties of those nanostructures in mesoscopic systems are of fundamental interest. To date, there are a few major methods for obtaining nanowires and nanorods [4, 5]. Firstly, catalytic growth of nanowires by the Vapor-Liquid-Solid (VLS) mechanism has been widely studied, leading to demonstrations of nanowire-based light-emitting diodes (LEDs), transistors and solar cells [2–6]. The nanowire geometry is effective in mitigating the biaxial strains when heteroepitaxy is unavoidable, and can result in high quality crystals with low dislocation densities in the nanostructures [7]. However, the uniformity and reproducibility of the nanostructures grown by bottom-up approach such as VLS still have to be improved for commercial applications because they are difficult to control in a sufficiently precise manner [4, 8]. Also, it is challenging to grow and fabricate devices containing quantum-wells because of the nature of the core-shell structures in VLS growth [9]. By contrast, nanowires or nanorods can also be fabricated by (chemical or dry) etch of established thin film layers containing lateral quantum-wells. Since the starting materials are well-understood thin-film layers, the nanostructures with very high uniformity can be reproducibly fabricated. This approach is compatible with current semiconductor manufacturing processes.

Nanosphere lithography (NSL) has been widely employed to create nano-sized patterns because it is very simple, scalable to large wafers and reproducible with high throughput, compared with electron-beam lithography and nano-imprint methods [10–14]. Cheung et al. reported that Si nanopillars with aspect ratios up to 10 could be fabricated by deep reactive-ion-etch of Si wafers coated with polystyrene nanobeads [15]. Kim et al. demonstrated uniform GaN nanorods by the combination of Chlorine-based inductively-coupled etch and NSL [16]. However, the electrical and optoelectronic properties of GaN-based nanorod LEDs with MQWs fabricated by NSL and subsequent etch have not been reported although this information is critical to optimize the performance of nano-optoelectronic devices. In this paper, we demonstrate a facile and reproducible method to fabricate uniform GaN-based nanorod LED structures with MQWs by combining NSL and ICP etching.

2. Experimental details

Nanospheres were synthesized by the Stöber process, which enabled us to obtain SiO₂ particles by mixing NH₃ (3.57 g), water, ethanol, methanol and diluted TEOS (2.605 g), followed by centrifugation at 3500 rpm for 10 min and sonication to obtain mono-dispersed nanospheres [17]. In our experiment, the diameter of the SiO₂ nanospheres was around 400 nm. Figure 1 shows the detailed fabrication processes to obtain GaN-based nanorod-LED structures containing InGaN/GaN MQWs. Schematic images are shown at the left, while SEM (FE-SEM S-4700, Hitachi) images are at right of the Fig. 1. Firstly, the synthesized SiO₂ nanospheres were coated on top of a commercial GaN-based blue MQW LED by the conventional spin-coating method at 2000 rpm. The SEM image of Fig. 1(a) confirms the presence of a mono-layer of SiO₂ nanospheres with good long-range ordering. Li et al. reported that point defects and line dislocations in natural lithography are unavoidable [18]. Chlorine-based inductively-coupled plasma etch (Multiplex ICP (STS)) was employed with the SiO₂ nanospheres masking patterns at a pressure of 5 mTorr with a mixture of BCl₃ (5 sccm) and Cl₂ (30 sccm). The SEM image in Fig. 1(b) shows the nanorods, whose diameter was defined by that of the SiO₂ nanospheres. The length of the nanorods was controlled by the duration of the ICP etch. In our condition, the dry-etch rate was approximately 6 nm/s, and the length of our InGaN/GaN MQW nanorod LED structures was ~1.2 μm. The SEM image of Fig. 1(c) was obtained after subsequent buffered oxide etchant (BOE) treatment. By comparing Fig. 1(b) with Fig. 1(c), it is evident that the residual SiO₂ was removed by the BOE (HF:H₂O = 6:1, J. T. Baker). Since a high density plasma can damage the crystal structures during ICP etching, KOH-based wet-etch (1 mol/L) was performed to remove the damaged surface to avoid any degradation of the performance of our nanorod-LEDs. The top-view SEM image [Fig. 1(d)] shows that the hexagonal facet was exposed after KOH treatment.

 

Fig. 1 Fabrication processes to obtain uniform GaN-based nanorod LED structures (left: schematic, right: SEM images) (a) LED wafer coated with SiO₂ nanospheres by natural lithography (b) nano-pillars after ICP etch (c) after BOE etch to remove residual SiO₂ nanospheres (d) after KOH etch to remove the plasma damaged surface.

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A pre-patterned SiO₂/p-Si substrate was prepared by conventional photolithography and electron-beam evaporation processes. The metallization was Ti/Au (20 nm / 80 nm), which was rapid thermal annealed at 300°C for 30 sec. A dilute suspension of nanorods in iso-propyl alcohol solution was dispersed on the SiO₂/Si wafer with pre-defined Ti/Au layers. After the MQWs nanorods were transferred to the pre-patterned SiO₂/p-Si substrate, 12 nanorods were chosen to make focused-ion beam (FIB)-deposited Pt (FEI, NOVA 200) contacts to both ends of the nanorods. The beam acceleration voltage was 30 kV with operating currents of 50 pA using a metal-organic precursor, where the width and height of Pt metallization was 500 nm and 100 nm, respectively. Electrical properties of the resultant structures were obtained using a semiconductor parameter analyzer (Agilent 4155C) connected to the probe station. Electroluminescence (EL) images were obtained by a DSLR camera (EF 100mm f/2.8 Macro USM lens, Canon, 600D) and optical microscope. Movie files showing the on/off operation of nanorod LED devices were recorded by using a CCD camera.

3. Results and discussion

Figure 2(a) confirms that the nanorod LED structures are very uniform in size. The bright spots in Fig. 2(b) are from the MQWs in nanorod LEDs because they are located just below p-GaN layer. Among them, twelve GaN-based nanorod LEDs were bridged to the pre-patterned Ti/Au pads by directly writing FIB-deposited Pt. Eight out of twelve GaN nanorod LEDs emitted bright EL under forward bias conditions, which indicates that our method to obtain GaN-based nanorod LEDs with MQWs by using a combination of NSL and dry-etch is reproducible and reliable. Figures 3(a) and 3(b) are the microscope images before and when applying a forward bias to the nanorod LEDs. Figure 3(d) is the high magnification image of Fig. 3(c), which shows bright EL from the nanorod LEDs connected to the pre-patterned Ti/Au. Movie file (Media 1) shows the on/off operations of our nanorod LED devices.

 

Fig. 2 (A) SEM and (b) Cathodoluminescence image of uniform GaN-based nanorod LED structures

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Fig. 3 Microscope images (a) before and (b) when applying a forward bias to the GaN-based nanorod LED with InGaN/GaN MQWs (Media 1) (c, d) DSLR camera images under forward bias condition.

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SEM images were obtained to investigate the nanorod LED devices in detail. Figure 4 shows the microscope and SEM images in which the GaN nanorod LED with MQWs was bridged to the pre-patterned SiO₂/p-Si substrate by FIB-deposited Pt. The width and height of FIB-deposited Pt were 500 nm and 100 nm, respectively in Fig. 4(b). Figures 4(c) and 4(d) are two different nanorod LED devices processed by FIB, in which we observed formation of nanoblisters between the FIB-deposited Pt and nanorods. There was swelling underneath FIB-deposited Pt, which can be attributed to the generated heat and Ga + -ion damage during FIB deposition [19]. The I-V characteristics from the nanorod LEDs bridged by the FIB-deposited Pt showed Ohmic behavior, which is interesting because Schottky behavior has been commonly observed from Pt metalliztion contacts to GaN. Similar results from FIB-deposited Pt on GaN nanowires were reported [19, 20]. This can be explained by the local high temperature during Ga⁺-ion irradiation and the surface defects induced by plasma etching that can result in high local n-type carrier concentrations, facilitating formation of Ohmic contacts.

 

Fig. 4 (a) Optical microscope image (b, c, d) SEM images after FIB metal deposition.

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The turn-on voltage of the nanorod LED device in Fig. 5(a) was about 13 V. To find out the origin of this high turn-on voltage, Pt line patterns with same length, height and width as Fig. 5(a) were directly written by FIB technique, as shown in Fig. 5(b). Note that there was no nanorod in Fig. 5(b). The total resistance of our GaN-based nanorod LED device with FIB-deposited Pt can be expressed as R(total) = R(contact) + R(metal pads) + R(LED diode) + R(FIB-Pt line), where R(contact) represents the total contact resistances between FIB-deposited Pt and the nanorod LED (p-GaN to Pt and n-GaN to Pt). R(FIB-Pt line) is not significant according to Figs. 5(a) and 5(b). Since the turn-on voltage of commercial thin-film blue LED devices made from same wafer was around 3 V, the high turn-on voltage in the nanorod devices can be explained by the high contact resistances at both contacts between the FIB-deposited Pt and GaN nanorods since R(metal pads) can be ignored.

 

Fig. 5 I-V characteristics from (a) InGaN/GaN MQWs nanorod LED and (b) FIB-deposited Pt line pattern without nanorod.

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4. Conclusion

A facile method for GaN-based nanorod LEDs with high uniformity in size and device performance was demonstrated by combining a simple spin-coating of nanospheres and a subsequent dry-etch. After transfer to a pre-defined SiO₂/p-Si wafer, FIB metal deposition technique was employed to form a bridge between GaN nanorod LEDs and pre-defined Ti/Au layers. Ohmic behavior was observed from the metal contacts to these structures. The higher turn-on voltage relative to companion thin film LEDs can be attributed to the high contact resistances between the GaN nanorods and FIB-deposited Pt.