The synthesis of 3D InN architectures via chemical vapor deposition and their optical properties

Abstract

The indium nitride (InN) nanostructure is critical for the fabrication of optoelectronic and electronic devices. To date, great progress has been achieved in the controlled synthesis of low-dimensional nanostructures such as nanowires. However, the growth of the self-organized well-defined three-dimensional (3D) superstructures of uniform nanowires has rarely been reported. Herein, two types of 3D InN microstructures, which were synthesized by using chemical vapor deposition (CVD), were studied in detail to understand the growth mechanisms involved in the so-called “self-organization” schemes. The observations suggest that the formation of the InN microsphere and the split octahedron-like microstructure could be attributed to the different self-organization processes. However, the growth mechanisms of the 3D InN microstructures are substantially different from previous reports. The photoluminescence (PL) spectra of the InN microstructures recorded under room temperature conditions with a broad photoluminescence emission band can be clearly observed in the spectrum range of 540–800 nm, which strongly suggests that the obtained microstructures is a direct semiconductor that exhibits a band gap of ∼2.17 eV. Thus, it is anticipated that controlling the growth processes may be a potential route to tailor the morphology, microstructure, and even the properties of materials.

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Introduction

Semiconductors in III–V groups, especially the nitrides, have become extremely important technological materials over the past decade due to their large direct band gap range and high thermal conductivity as well as high hardness, which makes them suitable for the fabrication of optoelectronic and electronic devices.^1–5 In particular, among the III–nitride compounds, InN has drawn substantial attention for its structures and physical properties under different conditions due to its smallest electron effective mass and direct band gap, highest mobility, highest peak, and its saturation electron drift velocities.^6,7 However, the large-scale growth of InN nanostructures is still in a relative immature stage due to the difficulties associated with the extremely high equilibrium vapor pressure of indium and nitrogen and the low dissociation temperature of InN.^8–10

In this regard, many efforts have been made to fabricate different InN nanostructures and to investigate the crystal growth mechanisms. Great progress has been achieved in synthetic strategies, and in the controlled synthesis of the primary building blocks such as nanoparticles, nanowires,^11,12 nanobelts,¹³ and nanorods.^14,15 InN nanostructures have been prepared by several routes including atomic layer epitaxy (ALE), molecular beam epitaxy (MBE), chemical beam epitaxy (CBE), and chemical vapor deposition (CVD), etc.^15–20 Among these methods, CVD without catalyst assistance is simple, effective, and inexpensive for large-scale production. However, compared with other III–nitride compounds, the growth of 3D InN nanostructures, especially the self-organization of uniform primary building blocks into well-defined 3D superstructures, remains unclear. Recently, the self-organization of nanoparticles and the subsequent formation of well-defined nanostructures have attracted rapidly growing interest owing to the important applications in nanoelectronics, magnetics, optoelectronics, photonics, heterogeneous catalysis, and so forth.²¹ More importantly, this study provides the opportunity to explore the collective properties of assemblies of particles.^22,23

In this work, two well-defined 3D InN microstructures were synthesized via a simple CVD without catalyst assistance. Detailed characterizations of the morphology, structure, and composition of the sample were performed. The self-organization process and the potential growth mechanism are discussed. The optical properties of the as-obtained 3D architectures were investigated as well.

Experimental

InN microstructures were prepared via a CVD method.^24,25 Ultrasonically cleaned Si (100) wafers, without hydrofluoric acid etching, 1 cm long by 1 cm wide, were applied as substrates for InN microstructure deposition. The starting materials of 0.1 g In powders in high purity (99.99%) were placed uniformly in a quartz boat. The cleaned substrate was placed face down on the quartz boat. Then, the quartz boat was loaded at the center of a quartz tube, which was placed in a horizontal tube furnace. The quartz tube was degassed and then purged with high purity (99.999%) nitrogen several times. High purity (99.999%) ammonia was employed as the reactive nitrogen source. After removing O₂ and H₂O by evacuating and nitrogen flushing several times, the furnace was heated up under the protective flow of nitrogen. During the growth process, the flow rate of ammonia was set at 300 sccm and the furnace was maintained at 680 °C for 3 h. Finally, the tube and its contents were rapidly cooled down to room temperature with a cooling rate of 10 °C min⁻¹ under the protective flow of nitrogen. In order to determine the phase composition of the samples, a powder X-ray diffractometer (Shimadzu XRD-6000) with Cu Kα radiation (λ = 1.5418 Å) was used to record the X-ray diffraction (XRD) patterns at a scanning speed of 2° min⁻¹. The accelerating voltage and the applied current were 40 kV and 30 mA, respectively. A HITACHI S4800 field emission scanning electron microscope (FESEM) working at 18.0 kV, which was equipped with an energy dispersive spectrometer (EDS), was used to characterize the size, morphology, and composition of the samples. Photoluminescence spectra were examined using a fluorescence spectrophotometer (Edinburgh FLS920) with a Xe lamp excitation at room temperature.

Results and discussion

As shown in Fig. 1a, the diffraction peaks of the products deposited on the Si (100) substrate could be indexed according to the wurtzite InN with the cell parameters a = 3.535 Å and c = 5.684 Å, which are in good agreement with the values from the standard card (JCPDS no. 50-1239). In addition, the Si (400) peak and SiO₂ (101) peak are all from the substrate. The EDS spectrum also illustrates that the synthesized products consisted of only In and N elements (Fig. 1b). The morphology of the prepared InN samples were observed by SEM. As shown in the typical low-magnification SEM image (Fig. 2a and c), two kinds of microstructures, namely the microsphere and the split octahedron-like microstructure, were prepared in the upstream (the green area in Fig. 2e) and downstream regions (the red area in Fig. 2e) of the Si substrate, separately. The high-magnification SEM image (Fig. 2b) revealed the InN microsphere of ∼4 μm diameter contains a rough surface. In combination with the high-magnification SEM images in Fig. S1 (ESI†) and 2b, it can be clearly observed that the resulting InN spheres are caused by the self-organization of several nanowires. Also, from observing different angles of the split octahedron-like microstructure, as shown in Fig. 2d, it can be seen that each face of the split octahedron-like microstructure is a regular triangle with a ∼1.5 μm side length.

Fig. 1 (a) Typical XRD pattern of the as-prepared InN sample. (b) The corresponding EDS spectra of the InN sample.

Fig. 2 (a and b) Typical low- and high-magnification SEM images of InN microspheres in the upstream region. (c and d) Typical low- and high-magnification SEM images of the split octahedron-like microstructure in the downstream region. (e) Schematic representation showing two types of InN microstructure prepared in the upstream and downstream regions of the Si substrate separately.

It is well established that different growth conditions will lead to various morphologies of crystals. In order to reveal the formation process of the InN microstructures in more detail, a series of time-dependent experiments were carried out. Also, a corresponding XRD analysis and SEM observation were performed to investigate the time-dependent change of the morphology and composition of the samples. Fig. 3 shows the XRD patterns of the samples synthesized for 0.5, 1, 1.5, 2, 2.5, and 3 h, respectively. As can be seen from these patterns, at the initial time of 1 h, the obtained InN subunits showed an extremely low diffraction peak due to the presence of a poor crystallized phase. With further increases in the reaction time, the XRD patterns indicate that the characteristic peak of the InN phase becomes progressively sharper and stronger with a longer reaction time. The crystallization of InN was enhanced, as indicated by the higher diffraction intensity. Fig. 3 shows the typical XRD patterns of the InN microstructures with the reaction time extended to 3 h. All the diffraction peaks are indexed to the wurtzite structure of InN (JCPDS card no. 50-1239), except for the peaks from the Si substrate.

Fig. 3 XRD patterns of products obtained at 680 °C for different reaction times.

The transformation process of InN is demonstrated in a series of typical SEM images in Fig. 4. A possible schematic representation for the formation of the spherical structure is presented in Fig. 4g. The whole growth process could be divided into four stages. As shown in Fig. 4a, due to the high reaction temperature and NH₃ flow, the nucleation and growth of InN nanowires occurred rapidly in the primary stage (step I). The products composed of nanowires of ∼1.5 μm length were obtained in a short reaction time of 0.5 h. According to Wulff construction, the shape of a crystal is determined by the relative specific energy of the crystalline planes.^26,27 As is well known, the growth rate of low-index crystallographic planes is proportional to their surface energies. The fast growing planes with high energy disappear to leave behind the slowest growing planes with low energy as the exposed facets of the product. Thus, the growth rates of different crystallographic directions are varied and lead to the self-organization of low-dimensional products.^28,29 With the elongation of reaction time (step II), for the minimization of the overall energy of the system, several InN nanowires attached by one end until it became brush-like, as shown in Fig. 4b. With the aggregation process continuing, a large amount of imperfect InN pre-spheres are formed at a reaction time of 1.5 h (Fig. 4c). As the reaction proceeds, the dispersed nanowires could barely be observed, while the InN microspheres, resulting from the self-organization of the nanowires, were further formed (Fig. 4d, and inset). When the reaction time was prolonged to 2.5 h (Fig. 4e), the as-obtained primary InN microspheres became more complete and the surface of the microsphere became denser (step III). In this step, these primary InN spheres were metastable due to their high surface energy. In the subsequent crystal growth process (step IV), the reaction rate slowed down due to the depletion of the reactants. Then, in order to reduce the total surface energy, the endpoints of the nanowires at the surface region acted as new nucleation sites. The endpoints grew larger at the expense of reactants.³⁰ As a result, the surface of the microspheres became smoother and denser, and regular InN microspheres with a rough surface were obtained (Fig. 4f).

Fig. 4 SEM images of the InN microspheres formed at different reaction times, while all the other reaction parameters remain unchanged: (a) 0.5 h, (b) 1 h, (c) 1.5 h, (d) 2 h (the primary InN microsphere is shown clearly in the high-magnification image shown in the inset), (e) 2.5 h, and (f) 3 h; (g) schematic illustration for the formation of InN microspheres.

The transformation process of the split octahedron-like InN microstructures was also demonstrated, as can be seen in a series of typical SEM images shown in Fig. 5. Instead of the nanowires, the growth process began with the pure InN nanorods with lengths of around 300 nm (Fig. 5a, step I). As shown in Fig. 5b, after prolonging the reaction time to 1 h (step II), the frames of spiders were constituted by several nanorods, and with the elongation of reaction time (step III), they became aggregated into many irregular micrometer particles to reduce the total surface energy (Fig. 5c). In the subsequent crystal growth process (step IV), the split octahedron-like InN microstructures began taking shape among the micrometer particles after 2 h reaction (Fig. 5d), and then, the less-than-perfect split octahedron-like InN microstructures were further shaped (Fig. 5e). Similar with the primary InN spheres, the split octahedron-like InN microstructures were metastable due to the high surface energy. Subsequently, in order to further reduce the total surface energy, the defects at the surface region acted as new nucleation sites. At the expense of reactants, the irregular micrometer particles almost disappeared and the surfaces of the spit octahedron-like InN microstructures gradually became flattened. Eventually (step V), split octahedron-like InN microstructures with a smoother surface were obtained (Fig. 5f). The whole process can be clearly shown in the simple schematic representation in Fig. 5g. On the basis of the above XRD analysis and SEM observation, the formation of the two InN microstructures could be attributed to the self-organization process, which was also proven in multiple publications for other compounds.^21,30 Also, the formation of the split octahedron-like InN microstructures is different from the formation of the InN microspheres.

Fig. 5 The SEM images of the split octahedron-like InN microstructure formed at different reaction times while all other reaction parameters remain unchanged: (a) 0.5 h, (b) 1 h, (c) 1.5 h, (d) 2 h, (e) 2.5 h, and (f) 3 h; (g) schematic illustration for the formation of split octahedron-like InN microstructures.

Why is the self-organization process of the two different InN microstructures different?

For the growth of InN micro- or nanostructures via CVD, there are several impacting factors that influence the morphology of the sample. For instance, Naonori Sakamoto et al. reported that the substrate strongly affected the crystallization behavior of the InN flowers due to lattice matching between the substrate and InN.³¹ Also, the growth temperature has an important effect on the morphology of the InN materials. InN micro- and nanostructured morphologies can be interpreted as a dependence on the degree of supersaturation, whereas the morphology of the InN 1D nanostructures can be understood by the kinetics-limited and diffusion-limited processes.^32,33 In addition, NH₃ flux is considered as one of the most impacting parameters in growth progressing in a controlled manner. Liu et al. reported that NH₃ flux had a decisive influence on the growth direction of InN nanowires. Microstructure characterizations of a great number of InN nanowires demonstrated that the growth direction of InN nanowires changes with variable NH₃ flux.³⁴ In this work, the growth of ​ the two kinds of InN microstructures occurred in different regions of the Si substrate (Fig. 2e). EDS measurements performed on the microspheres and on the split octahedron-like microstructures also indicated that the molecular ratio of In : N of the two microstructures was different (Fig. S2, ESI†). We proposed that different self-organization mechanisms could be induced by the difference in nitrogen concentration. To investigate the role of nitrogen concentration in the formation of the InN microstructures, a series of experiments with different flow rates of the ammonia were carried out. On the basis of the SEM observation (Fig. S3, ESI†), it can be concluded that the nitrogen concentration has an important effect on morphology of InN microstructures.

As shown in Fig. 6, both the PL spectrum of the InN microspheres and the split octahedron-like InN microstructures at room temperature show a similar broad yellow-orange emission band. From the PL spectrum, both the PL emission bands of the InN microspheres and the split octahedron-like InN microstructures were centered at ∼2.17 eV. This is not similar to the InN nanostructures reported previously.³⁵ Yet InN nanostructures show PL emissions at different wavelengths, probably due to the existence of different amounts of defects or surface states.^36,37 It is suggested that the emission centered at 0.7–0.8 eV is the near band gap emission, while the emission centered over 1.7 eV is potentially ascribed to the oxygen impurities in InN system, the nitrogen-rich stoichiometry, or the Burstein–Moss shift induced by high electron concentrations of the InN microstructures. Yoshimoto et al. reported that oxygen incorporation in InN could result in the wide distribution of band gap ranging from 1.55 to 2.27 eV, since O contamination leads to a larger band-gap energy.³⁸ However, there is no direct evidence in our study that proves that O impurities are introduced into the microstructures, or that O impurities are concerned with the whole growth process of InN. It was found that the optical absorption edges of InN films and nanowires can range from 0.7 to 1.7 eV due to the Burstein–Moss shift.³⁹ Nevertheless, the experimental conditions indicate that the microstructures appear to be nitrogen rich, and therefore, nitrogen vacancies are unlikely to be responsible for the commonly observed high background carrier concentration.⁴⁰ The band gap of the InN was also found to increase as the nitrogen concentration increased.^40,41 The nitrogen defects in the InN microstructures arise from the nucleation and growth of InN subunits, namely, nanowires and nanorods, as well as the self-organization of the subunits.^37,42,43 Compared with other InN nanostructures,^8,37 in our case, the sample grown in a low temperature of 680 °C as well as the self-organization process should lead to a shorter wavelength emission due to the excess nitrogen. We therefore deduced that the nitrogen-rich stoichiometry may be a plausible reason for the ∼2.17 eV emission band of the InN microstructures.

Fig. 6 Room temperature PL spectrum of the InN microspheres and the split octahedron-like InN microstructures at the 510 nm excitation, respectively.

Conclusions

In summary, the InN microspheres and split octahedron-like InN microstructures were prepared via a simple CVD method without catalyst assistance. The as-synthesized InN microspheres had nearly monodisperse diameters that could be controlled to be around ∼4 μm. The split octahedron-like InN microstructures were also uniform, with diameters in the range of 2–3 μm. A detailed study on the growth process suggested different self-organization mechanisms for the growth of the microspheres and the split octahedron-like microstructures. The different organization process leading to different architectures could be induced by the difference in nitrogen concentrations. The nitrogen-rich stoichiometry also play a key role in the ∼2.17 eV emission band. Synthesis of the InN microsphere and of the split octahedron-like InN microstructure lays a foundation for further studies of the intrinsic properties of InN nanomaterials and their potential application in nanodevices and nanosystems.

Acknowledgements

This study was supported in part by the National Natural Science Foundation of China (Grant No. 51172087, 11304111, NSAF. No: U1330115), Graduate Innovation Fund of Jilin University (No: 2015140) and Specialized Research Fund for the Doctoral Program of Higher Education of China (20110061110011) Supported by Graduate Innovation Fund of Jilin University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra19865h

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