1. Introduction

Zinc oxide (ZnO) nanostructures have been of great interest in optoelectronic and semiconductor devices, such as solar cells, ultraviolet (UV) light-emitting diodes (LEDs), gas sensors, and photo-detectors [1–7]. ZnO is a wide bandgap semiconductor (~3.37 eV) with a large exciton binding energy of 60 meV at room temperature. Controlling the shape and size of the nanostructures is required to facilitate their practical integration to nano-scale devices. This has led to extensive studies on the synthesis of ZnO nanostructures and corresponding physical and optical properties. Presently, there is extensive literature available on ZnO, mostly devoted to the synthesis and characterization of ZnO films and nanowires using various growth methods. These include pulsed laser deposition, chemical vapor deposition, molecular beam epitaxy, and aqueous solution epitaxy [8–14]. Among them, the hydrothermal growth or aqueous solution method to grow ZnO thin films or nanorod arrays has been shown to be advantageous due to its ease of processing, cost effectiveness, and low temperature requirements.

ZnO-based heterostructures like ZnO/ZnMgO, ZnO/ZnCdO, and ZnO/GaN are highly suitable for potential applications in UV LEDs and lasers [15–22]. Among them, ZnO/GaN heterostructures are particularly well-suited for high-efficiency near UV LEDs due to minimal differences in the lattice mismatch and the thermal expansion coefficient between ZnO and GaN. For this reason, the K-doping effects on ZnO nanostructures using nonpolar and semipolar GaN films are examined herein since achieving reproducible p-type ZnO is crucial for practical applications of ZnO films and nanostructures in optoelectronic devices. The guided growth of ZnO nanorods with controlled crystallographic orientations are considered as they may lead to large-scale integrations of nanostructures in practical ZnO/GaN heterostructure devices. This research explores the growth evolution and the optical properties of K-doped ZnO nanorods and pyramids on semipolar $\left(11\stackrel{-}{2}2\right)$ GaN films using a hydrothermal growth method.

2. Experimental details

2.6 μm-thick semipolar $\left(11\stackrel{-}{2}2\right)$ GaN epitaxial films were grown on m-plane sapphire substrates using a high temperature one-step growth process using an 11 × 2 inch AIX 2400 G3 metal-organic chemical vapor deposition system. Trimethylgallium and ammonia were used as gallium and nitrogen sources, respectively. Prior to the growth of the nucleation GaN layer, the sapphire substrate was thermally annealed at 1030°C in ambient H₂ and NH₃ to remove any surface contamination. A 100 nm-thick low-temperature GaN buffer at 550°C was deposited at a high V/III ratio of 2500, and semipolar GaN films were grown at 1050°C with a V/III ratio of 450 until full coalescence of the films occurred.

Prior to ZnO deposition, the semipolar GaN films were first cleaned with IPA, acetone, and deionized (DI) water under ultrasonic conditions for 5 min, before being blown dry with a stream of nitrogen. A simple aqueous solution growth method was employed to hydrothermally grow ZnO nanorods. 25mM zinc nitrate hydrate [Zn(NO₃)₂·6H₂O] solutions were dissolved in DI water, and equivolumetric 25mM of hexamethylenetetramine [C₆H₁₂N₄] solution was mixed together in a Teflon-lined autoclave. For K-doped ZnO growth, zinc acetate [Zn(CO₂CH₃)₂], potassium acetate [CH₃KO₂], and ammonium hydroxide [NH4OH] were mixed and stirred in deionized water. The semipolar GaN films were then loaded upside down in the aqueous solution, and placed in an oven at 90°C for 60 min. This solution growth step was repeated up to 15 times to produce dense ZnO nanorods. The crystal quality of the ZnO nanorods was characterized by a high resolution X-ray diffractometer (Bruker XRD System) with a Cu Kα₁ X-ray target source (λ = 1.5406 Å). The surface morphology of the ZnO nanorods were examined using a scanning electron microscope (SEM) and an atomic force microscopy. The optical properties of the ZnO nanorods were characterized at room temperature using a cathodoluminescence imaging and spectroscopy system (Gatan Mono CL3), installed on a JEOL SEM 4500. CL measurements were performed by scanning 80 × 80 μm areas with 18 nA electron beam irradiation at an accelerating voltage of 10 keV in the UV-visible spectral range. .

3. Results and discussion

Figures 1(a) and 1(b) show macro-views of SEM images of ZnO nanorods grown on semipolar $\left(11\stackrel{-}{2}2\right)$ GaN films using a hydrothermal growth method for 1hr (a) and 5hr (b), respectively. Most of the single-crystalline ZnO nanorods were highly inclined to the c-axis [0001] direction on semipolar $\left(11\stackrel{-}{2}2\right)$ GaN surface. Figure 1(c) shows the X-ray diffraction (XRD) pattern of the as-grown ZnO nanorods for 5hr. Two diffraction peaks at 2θ = 67.85° and 69.45° corresponded to semipolar $\left(11\stackrel{-}{2}2\right)$ ZnO and GaN, respectively. Note that the c-plane (0001) ZnO inclined to the normal $\left[11\stackrel{-}{2}2\right]$ direction (58°) could not be detected due to the asymmetric reflection geometry. The ZnO clusters were also observed on semipolar GaN surface, which could be indexed as wurtzite ZnO $\left(11\stackrel{-}{2}2\right)$ peak in XRD θ-2θ scan.

 

Fig. 1 (a) Macro view of an SEM image of ZnO nanorods for 1 hr and (b) 5hr grown on semipolar $\left(11\stackrel{-}{2}2\right)$ GaN film, and (c) the X-ray diffraction (XRD) pattern of the as-grown ZnO nanorods.

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Figure 2(a) shows an SEM image of the K-doped ZnO nanorods on semipolar GaN films, which was treated with potassium acetate for 1hr at 90°C. Interestingly enough, K-doped ZnO nanorods not only grew in the normal $\left[11\stackrel{-}{2}2\right]$ direction, but also along the c-axis [0001] and m-axis $\left[10\stackrel{-}{1}0\right]$ directions. The K-doped ZnO nanorods also showed bullet-shaped morphology, instead of hexagonal nanowires with the m- and c-facets, implying that the growth rate differences along the principal directions might be significantly reduced. Ionic liquids such as Kac and zinc acetate may enhance the growth rate of nonpolar $\left(11\stackrel{-}{2}0\right)$ and semipolar $\left(11\stackrel{-}{2}2\right)$ planes, which are energetically unfavorable as exposed surfaces. This includes semipolar $\left\{10\stackrel{-}{1}1\right\}$ planes [23]. Figure 2(b) shows an SEM image of the surface morphology of K-doped ZnO nanorods for 5hr. In this case, prolonged and denser nanorods with semipolar $\left[11\stackrel{-}{2}2\right]$, polar [0001], and nonpolar $\left[10\stackrel{-}{1}0\right]$ preferential orientations can be seen.

 

Fig. 2 SEM image of K-doped ZnO nanorods on semipolar GaN films for (a) 1hr and (b) 5hr, respectively.

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Figure 3(a) shows an SEM image of K-doped ZnO nanorods and pyramids for 15hr at 90°C. One of the most remarkable features is the micron-sized rectangular-based pyramids having multiple nonpolar and semipolar planes formed on the ZnO nanorods. As shown in Fig. 3(b), highly-inclined ZnO nanorods were observed along the c-axis and the m-axis directions. Figure 3(c) depicts a magnified SEM image of ZnO pyramids only a few microns wide with slowly growing planes faceted. Figure 4 shows XRD θ-2θ scans of the pure ZnO nanorods (a) and K-doped ZnO nanorods and pyramids for 1hr (b) and 15hr (c) respectively. Two sharp diffraction peaks at 2θ = 67.85° and 69.45° corresponded to $\left(11\stackrel{-}{2}2\right)$ ZnO and GaN, respectively. The full width at half maximum of the on-axis rocking curve of semipolar ZnO nanorods was measured to be ~47 arcsec. (not shown here) Other diffraction peaks can be indexed as the wurtzite ZnO $\left(10\stackrel{-}{1}0\right)$, (0002), $\left(11\stackrel{-}{2}0\right)$, and $\left(11\stackrel{-}{2}2\right)$. For sample (c), a strong peak at 2θ = 42.22° was observed, which could be assigned as the Zn $\left(10\stackrel{-}{1}1\right)$ peak. The asterisk denotes the monoclinic β-Ga₂O₃. Figure 4(c) indicates that multiple ZnO pyramids appeared on the ZnO nanorods, which consist of mostly nonpolar $\left(10\stackrel{-}{1}0\right)$, nonpolar $\left(11\stackrel{-}{2}0\right)$, and semipolar $\left(11\stackrel{-}{2}2\right)$ planes.

 

Fig. 3 (a) A top-view SEM image of K-doped ZnO nanorods and pyramids and magnified images of (b) ZnO nanorods and (c) ZnO pyramids grown on semipolar GaN film for 15hr at 90°C.

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Fig. 4 The XRD θ-2θ scans of (a) the pure ZnO nanorods, K-doped ZnO nanorods for (b) 1 hr and (c) 15hr, respectively.

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It is known that the optical properties of K-doped ZnO nanorods and films are significantly different from those of pure ZnO crystals. In this work, we examined the optical properties of K-doped ZnO nanorods and pyramids using cathodoluminescence (CL) measurements. Figure 5 shows CL spectra of the K-doped ZnO nanorods and pyramids as well as pure c-plane ZnO nanorods. The near band edge (NBE) peak was observed at 382 nm for the pure c-plane ZnO nanorods [24,25].

 

Fig. 5 The CL spectra of K-doped ZnO nanorods and pyramids as well as pure c-plane ZnO nanorods.

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In sharp contrast, a significant peak shift to 418 nm occurred for the K-doped ZnO nanorods with relatively broad emissions. For the K-doped ZnO pyramids, the peak position of the violet emissions was observed at 418 nm with 2.7 times enhanced intensity. The red-shift from 382 nm to 418 nm with K-doping may have originated from transitions based on Zn interstitial defect states and K-related energy states between the conduction and the valence bands [26–30]. In order to understand the features of the violet and blue emissions, CL were measured under different electron beam (e-beam) energies. Figure 6(a) represents the CL spectra of the K-doped ZnO pyramids collected at e-beam energies of 5 kV, 10 kV, 15 kV, and 20 kV, respectively, to study the effects of material homogeneity and electron penetration depth. Two shoulders were detected at 418 nm and 470 nm under e-beam energies of 10 kV and 15 kV. In the case of e-beam energy at 20 kV, two small shoulders could be also observed at 382 nm and 520 nm. The NBE peak position of K-doped ZnO was observed around 382 nm. The presence of green emissions at 520 nm are often reported in ZnO materials and commonly regarded as a deep-level luminescence dye to the oxygen vacancies [27–30].

 

Fig. 6 (a) CL spectra of the K-doped ZnO pyramids collected at e-beam energies of 5 kV, 10 kV, 15 kV, and 20 kV respectively, and (b) the multiple-peak Gaussian fittings of the CL spectra under 20 kV.

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Normally, as e-beam energy values increase, the penetration depth of the e-beam also increases. However, in this experiment the CL intensity did not increase significantly when the e-beam energy increased from 15 kV to 20 kV as shown in Fig. 6(a). To account for this, if the K concentration is higher at the surface of the ZnO pyramids, the violet and blue emissions might be more associated with extrinsic effects like K atoms, while the emissions at 382 nm and 520 nm could originate from intrinsic ZnO properties, such as the bandgap transition and the deep levels of interstitial oxygen ions, respectively. Figure 6(b) shows the multiple-peak Gaussian fittings of Fig. 6(a) under 20 kV, with 3 comparable emissions centered at 418, 470, and 520 nm. According to Zeng et al. [24], the violet emission around 415 nm could be due to Z_ni defect centers while the blue emissions at 440, 455, and 488 nm might originate from Zn extended defect states. We believe that the violet and blue emissions centered at 418 and 470 nm respectively are mainly the result of K interstitial and Zn interstitial defects. Furthermore, the electron transitions from K interstitial levels to the valence band can lead to significantly strong violet emissions at 418 nm for K-doped ZnO nanorods and pyramids.

4. Conclusion

In this work, we studied the growth evolution and optical properties of K-doped ZnO nanorods and pyramids on semipolar $\left(11\stackrel{-}{2}2\right)$ GaN films. The X-ray diffraction θ-2θ scan of the pure ZnO nanorods showed that most of the single-crystalline ZnO nanorods were highly inclined along the c-axis [0001] direction. In the case of K-doped ZnO nanorods, growth occurred not only in the normal $\left[11\stackrel{-}{2}2\right]$ direction, but also along the c-axis [0001] and m-axis $\left[10\stackrel{-}{1}0\right]$ directions. Micron-sized semipolar pyramids having nonpolar and semipolar planes were formed on the K-doped ZnO nanorods as the growth proceeded further. The cathodoluminescence measurements showed a significant peak shift to 418 nm for the K-doped ZnO nanorods, whereas the near band edge peak was centered at 382 nm for pure c-plane ZnO nanorods. The violet emissions centered at 418 nm are believed to be mainly due to K interstitial and Zn interstitial defects. The electron transitions from K interstitial levels to the valence band lead to relatively strong violet emissions at 418 nm for K-doped ZnO nanorods and pyramids.