Selective-Area Growth of Transferable InN Nanocolumns by Using Anodic Aluminum Oxide Nanotemplates
© The Author(s). 2017
Received: 29 December 2016
Accepted: 14 February 2017
Published: 23 February 2017
InN nanocolumn arrays were grown on c-plane sapphire with and without anodic aluminum oxide (AAO) nanotemplates. The crystalline quality of InN nanocolumns was significantly improved by selective-area growth (SAG) using AAO templates, as verified by X-ray diffraction measurements. Then, InN nanocolumns were transferred onto p-type silicon substrates after etching off the AAO templates. Current–voltage characteristic of the transferred n-InN/p-Si heterojunctions shows on/off ratio as high as 4.65 × 103 at 2 V. This work offers a potential way to grow transferable devices with improving performances.
III-Nitrides, with excellent optic and electronic properties, are widely used for solar cells, optical waveguides, high-speed electronics, and terahertz emitters . Among them, InN has the narrowest bandgap, lowest effective mass of electrons, and highest electron mobility and thus can be applied in high-speed electronics [2–4]. However, its low decomposition temperature, impurity-prone surface, and large lattice mismatch with common substrates hinder the further development of InN-based devices . In recent years, many studies focused on the growth of InN nanorods and nanocolumns on c-Al2O3, glass, Si (100), and Si (111) with or without buffer layers such as GaN, AlN, InGaN, and even low-temperature (LT) InN [5–10]. In general, sapphire is widely used as the substrate for growth of InN because of its availability, large area, high quality, high thermal stability, relatively low cost, and hexagonal symmetry, even though it has a large lattice mismatch with InN .
Selective-area growth (SAG) technique has been used to produce waveguides , facet lasers , and other nanostructures such as nanowires and nanocolumns [14–16]. It has been applied to achieve the epitaxial lateral overgrowth (ELOG) of GaN-based laser diodes for reducing the threading dislocation density. Until now, there are only a few reports on the SAG of InN, in which Mo-mask-patterned (0001) sapphire , Mo-mask-patterned (111) Si , nanohole-patterned GaN templates , and ultra-thin AlN masks were used . However, the AlN masks are not periodical, and the fabrication of the metal masks is relatively expensive and complex. Hence, more periodical and practical nano-sized masks should be developed for the SAG of InN-based devices.
Anodic aluminum oxide (AAO) templates are widely applied in nanostructure fabrication due to their high regularity, self-organized nanostructure, and low cost compared with electron beam lithography (EBL) . Furthermore, AAO is more easily etched compared to Pt, Mo, and AlN. Therefore, AAO templates were used in this study as both the SAG mask and the transfer template.
Results and Discussion
Figure 2 shows the cross-sectional SEM images. The AAO templates were found with an average pore diameter of 30 nm and interpore edge-to-edge distance of 70 nm as well as pore depth of 170 nm, as shown in Fig. 2a. The density of the pores was estimated to be 1.2 × 1010 cm−2. As shown in Fig. 2b, the diameters of the InN nanocolumns without AAO templates varied from 50 to 150 nm in top view. The density of the InN nanocolumns without the AAO templates was estimated to be 8 × 109 cm−2. As shown in Fig. 2c, the depth of the nanocolumns was 625 nm. As shown in Fig. 2d, the diameters of the InN nanocolumns with the AAO templates varied from 50 to 500 nm. The density of the InN nanocolumns with the AAO templates was estimated to be 1 × 109 cm−2. The decrease of the InN nanocolumns densities was mainly attributed to the selective lateral growth by adding the AAO templates. As shown in Fig. 2e, the InN nanocolumns can be either in growth direction or upside down after being transferred onto p-Si by etching AAO templates off using NaOH solution. The sizes of the transferred InN nanocolumns were in micrometer scales from 2 × 2 to 20 × 20 μm2. Fig. 2f shows the top view of the transferred upside down InN nanocolumns. The density of the transferred upside down InN nanocolumns with the AAO templates was estimated to be 1.1 × 1010 cm−2, which was in agreement with the density of the AAO template pores as shown in Fig. 2a. This indicates that the InN nanocolumns were selective-area grown from the pores of the AAO templates.
In this study, InN nanocolumn arrays were grown on c-plane sapphire with and without the AAO nanotemplates. The crystalline quality of the InN nanocolumns is significantly improved by the SAG using the AAO nanotemplates, as verified by XRD measurements. I–V characteristic of the transferred n-InN/p-Si heterojunctions shows on/off ratio as high as 4.65 × 103 at 2 V. This work offers a potential way to grow high-quality transferable devices with improving performances.
This work is supported by the NSFC under Grant No. 11574235 and the MOST China under Grant No. 2014GB109004. The authors would like to thank Dr. C. Chen for the technical support.
XW carried out the experiments and drafted the manuscript. GZZ and YX participated in the measurements. HW conceived the study and participated in its design. CL supervised the overall study and polished the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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