Study of iron-catalysed growth of β-Ga₂O₃ nanowires and their detailed characterization using TEM, Raman and cathodoluminescence techniques

The general morphology of Fe-catalysed β-Ga₂O₃ nanostructures, which consist of mainly NWs and NBs at low and high magnification, is presented in figures 1(a)–(d). The statistical analysis including errors in the diameter of NWs is depicted via a diameter histogram in the inset of figure 1(a). The diameters of approximately 100 NWs are in a range of 30–80 nm with an average of 55  ±  3 nm, and the lengths of most of these NWs are tens of micrometers. In addition, the width of β-Ga₂O₃ NBs is in the range of 150–400 nm and their lengths up to 30 μm. At higher magnification (figure 1(d)), the nanoparticles at the tip of the NWs are clearly visible. This verifies that the growth of β-Ga₂O₃ NWs is based on the VLS mechanism and it is further confirmed by TEM measurements. Zhu et al [32] studied Au, Ni and Fe-catalysed growth of ZnO NWs. They observed that the diameter of NWs was insensitive to the thickness of Fe thin film, whereas the diameter of NWs was dependent on the thickness of Au and Ni thin films. Therefore, they suggested that, in the case of Fe catalyst, NWs did not grow via the VLS growth process. The binary phase diagram of Fe and Ga, which demonstrates the compositional zone responsible for alloying, nucleation and growth can be seen in [39].

Zoom In Zoom Out Reset image size

EDXS analysis was further performed to determine the concentration of Ga and O in the as-grown β-Ga₂O₃ NWs. Besides the strong Ga and O signals, some small peaks are apparent corresponding to the Fe catalyst and carbon coating during EDXS measurements.

The XRD investigation was used to obtain the basic information about the structure and phase purity of as-grown β-Ga₂O₃ NWs on sapphire substrate using Fe as a catalyst. Various closely spaced peaks indexed to the monoclinic phase of Ga₂O₃ are illustrated in figure 2, and are in good agreement with the reported values of β-Ga₂O₃. These peaks are recognized according to Joint Committee for Powder Diffraction Standards (JCPDS) Card No. 11-0370. Miller indices (hkl) of β-Ga₂O₃, including (004), $({\rm{\bar 2 0 2}}),$ (111), (104), $({\rm{\bar 1 1 3}}),$ $(\bar{2}\text{}\text{ 1}\text{ 1}),$ (003), $(\bar{3}\text{}\text{ 1}\text{ 3}),$ (311), (020), (017), (402) and $(\bar{2}\text{}\text{ 1}\text{ 7}),$ are assigned in this present pattern. This XRD pattern reveals that no other crystalline modifications or impurities are present within the 2θ range (20–80°). The strong intensities of β-Ga₂O₃ diffraction peaks indicate that the resulting product (as-grown NWs) is monoclinic Ga₂O₃.

Zoom In Zoom Out Reset image size

Furthermore, to elucidate details of the morphology, structure and chemical composition of as-grown Fe-catalysed β-Ga₂O₃ NWs, we carried out TEM, STEM and EDXS investigations. The TEM image in figure 3(a) shows an overview of as-grown NWs demonstrating their general morphology. In figure 3(b) a cutout of (a) is magnified. A single NW is characterized, which ends in a particle. The SAED pattern (figure 3(c)) from the NW corresponds to the $[1\text{}\text{ \bar{1}}\text{ 2}]$ zone axis pattern of β-Ga₂O₃ and indicates $[1\text{}\text{ \bar{3}}\text{ \bar{1}}\text{ \bar{8}}]$ as the growth direction, which corresponds to the normal of the $(1\text{}\text{ \bar{1}}\text{ \bar{1}})$ plane. In agreement with XRD analysis, SAED shows that the as-grown NWs are single crystalline and have no other impurities or phases present.

Zoom In Zoom Out Reset image size

The STEM investigations clearly demonstrate that the single NW consists of the monoclinic phase of Ga₂O₃. The wire shows a segmented shape indicated by slight constrictions of the surface in annular dark-field-STEM (ADF-STEM). The atomic structure of the Fe-catalysed NWs was further characterized by ADF-STEM (see figure 4). The atomic structure image shows the atomic arrangement in $[1\text{}\text{ \bar{1}}\text{ 2}]$ orientation without any structural defects.

Zoom In Zoom Out Reset image size

Further, the chemical composition of as-grown β-Ga₂O₃ NWs was investigated by STEM-EDXS. The EDXS elemental maps are shown in figures 5(b)–(d), illustrating the spatial distributions of Ga (blue), O (orange) and Fe (green). It is clearly noticed that Ga and O are roughly homogeneously distributed throughout the NW and nanoparticle, whereas Fe is agglomerated at the NW tip. Furthermore, the EDXS line profiles show that the NW consists of Ga and O, whereas in the particle Fe is also detected. In addition, EDXS profiles of the integrated intensities of the signals of O K, Ga K, and Fe K lines were measured at points along lines across the NW (see figures 6(a) and (b)), across the particle (figures 6(c) and (d)) and across the interface between the wire and the nanoparticle (figures 6(e) and (f)). Obviously, the distributions of Ga and Fe develop in reverse from the left to the right in the particle, which indicates an interdiffusion of Ga and Fe between wire and particle. In contrast to Ga and Fe, O appears to be roughly homogeneously distributed, as already shown in the EDXS maps in figure 5.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

Moreover, to verify the interdiffusion between NW and particle, detailed structural and chemical composition analyses were performed at the interface of the β-Ga₂O₃ NW and the particle at the wire top (see figure 7). The high resolution–annular dark field (HR-ADF) image of the interfacial region between wire and particle clearly shows no change in the structure: The HR-ADF image of the particle reveals the atomic structure of β-Ga₂O₃, which indicates Ga₂O₃ and Fe in the particle.

Zoom In Zoom Out Reset image size

We thus suppose that the growth of the Fe-catalysed β-Ga₂O₃ NWs is mostly governed via a VLS mechanism. This requires the formation of an Fe–Ga–O alloy with the original Fe particle at the wire tip as verified by EDXS, and results in an increase in the volume of the particle.

Some additional information of composition on the surface of as-grown NWs has been obtained utilizing XPS measurements. Figure 8 shows a typical XPS survey spectrum of β-Ga₂O₃ NWs grown on sapphire at 950 °C for 3 h. The spectrum displays both the photoelectron and Auger electron peaks for Ga and O with carbon contamination. The peak position around 284.5 eV corresponds to the elemental carbon [41]. The binding energies around ~1118 and ~1145 eV are attributed to Ga 2p_1/2 and Ga 2p_3/2 electrons, respectively, as shown in figure 8(b). These sharp peaks around ~1118 and ~1145 eV for Ga 2p_3/2 and Ga 2p_1/2 core levels are associated with the Ga–O bonding in Ga₂O₃ [40, 41]. Figure 8(c) shows the O 1s peak around ~531 eV which is due to the presence of oxygen species in Ga₂O₃ [40–42]. From this XPS spectrum, the binding energy position of Ga 2p and O core levels are consistent with previous XPS studies [40–43]. This suggests that stoichiometric Ga₂O₃ NWs are synthesized properly on the sapphire substrate.

Zoom In Zoom Out Reset image size

Figures 9(a) and (b) show typical micro-Raman spectra recorded at the centre of a single β-Ga₂O₃ NW and on a bulk Ga₂O₃ single crystal serving as reference. To the best of our knowledge, such a comparative Raman study is reported for the first time. We observed 11 clear Raman modes in the probed spectral range (124–800 cm⁻¹) that are in excellent agreement with those published in the first, thorough Raman paper on this material [44]. Based on this similarity, we can unambiguously state that our β-Ga₂O₃ NWs have a monoclinic crystal structure and belong to the C_2h group symmetry, implying a unit cell with two formula units, Ga₂O₆ octahedra and GaO₄ tetrahedra. The measured Raman-active modes can be distinguished as follows: the first three peaks below 200 cm⁻¹ correspond to the low-frequency libration and translation of tetrahedron–octahedron chains, the next four peaks between 300 and 500 cm⁻¹ relate to the mid-frequency deformation of Ga₂O₆ octahedra, and the last four peaks between 600 and 800 cm⁻¹ are attributed to the high-frequency stretching and bending of GaO₄ tetrahedra [38, 44]. Table 1 summarizes the position of Raman peaks in the present study with respect to previous works [37, 38, 44] as well as the frequency shifts between the reference crystal and the NWs. Interestingly, we obtained quite small blue and red shifts ranging from 0.3 to 1.4 cm⁻¹, in contrast to blue shifts of 10–40 cm⁻¹ and red shifts of 4–23 cm⁻¹ found with regard to pure Ga₂O₃ powder reference [37, 38]. These relatively small deviations indicate that the structural properties of our β-Ga₂O₃ NWs in terms of strain and defect levels are very close to those of a pure bulk Ga₂O₃ single crystal as shown in figures 9(a) and (b). The Raman results along with the other analyses demonstrate the growth of single crystalline β-Ga₂O₃ NWs using Fe as a new catalyst.

Zoom In Zoom Out Reset image size

The SEM and its corresponding CL image are shown in figures 10(a) and (b). It is clear from the CL image that the luminescence originated from the entire length of NWs and NBs, indicating the existence of radiative centres. On the other hand, the entire region under these NWs and NBs appears black and behaves like a non-radiative centre. Recently, Navarro et al [45] studied the CL imaging and spectrum analysis of β-Ga₂O₃ NWs. They observed some bends, and it was suggested that these bends in the grown NWs were stacking faults which behaved as non-radiative centres. In addition, some triple junction nodes (called nanoflags) are observed along with β-Ga₂O₃ NWs and NBs, as shown with a red circle in figure 10(b). Johnson et al [23] have also observed some triangular shape type nanostructures in their controlled VLS growth of In₂O₃, Ga₂O₃, and SnO₂ NWs via chemical vapour transport using Au catalyst. They suggested that these triangular shape nanostructures might be due to nucleation on twins or the presence of a small Au particle at the end of the NW.

Zoom In Zoom Out Reset image size

The RT-CL spectrum was measured for Fe-catalysed β-Ga₂O₃ NWs grown on sapphire in the UV–visible range (300–800 nm), as shown in figure 11. A very wide defect emission band, having a strong UV–blue and a weak red emission centred at ~398 nm (3.2 eV) and ~710 nm (1.75 eV), is observed in our CL spectrum, while no intrinsic emission (band to band emission) is visible in the above mentioned spectral range [17]. To have a better understanding of all defect emissions, the UV-CL spectrum was deconvoluted, and consists of mainly two Gaussian peaks at wavelengths (energies) positioned at 395 nm (3.13 eV) and 450 nm (2.76 eV). The first peak (3.13 eV) is related to UV emission, whereas the second (2.76 eV) peak is assigned to blue emission. The origin of all emission peaks is reported in many previous studies [46–52]. Harwig et al [49] suggested that the UV emission was attributed to recombination of self-trapped excitons. They proposed that this had an intrinsic nature and could not be changed with external factors such as temperature, growth time, pressure, and substrates. However, Villora et al [50] presented that UV emission (3.55 and 3.13 eV) is related to oxygen vacancies. The blue emission (~2.95 eV) is assigned to recombination of an electron on a donor and a hole on an acceptor or donor–acceptor pair (DAP). The donors are formed by oxygen vacancies (V_O), whereas acceptors are formed by gallium (V_Ga) or gallium–oxygen vacancy pairs (V_Ga–V_O) [48, 49]. The oxygen vacancies and gallium–oxygen vacancy pairs were produced at high growth temperature [46, 50, 51]. As we already mentioned that we have performed our experiment at a sufficient high temperature (950 °C). Thus, sufficient numbers of oxygen vacancies are present in as-grown NWs, which give UV as well as blue emission, in accordance with the study by Villora et al. In addition to UV–blue emission, we have also observed a weak red emission at 710 nm (1.75 eV) in our CL results. For the first time, Song et al [53] suggested that red emission was related to recombination of electrons trapped by oxygen vacancies and holes captured by deep level acceptor impurities such as nitrogen. In our previous study, we presented an energy band diagram of Ga₂O₃ revealing all emission bands. Furthermore, we compare CL spectra of as-grown β-Ga₂O₃ NWs with a bulk Ga₂O₃ single crystal, which consists of two bands. It is clearly visible that the two spectra are quite similar, and this might be caused by a higher degree of perfection of the structure of our as-grown NWs.

Zoom In Zoom Out Reset image size

To estimate the bandgap of Fe-catalysed β-Ga₂O₃ NWs, the reflectance curve (figure 12) using a UV–visible–near infrared (UV–VIS–NIR) spectrophotometer is acquired. Then, we transformed this reflectance to the Kubelka–Munk (K–M) function F(R) using the K–M equation as described elsewhere [54]. The inset of figure 12 shows a plot of [F(R)hν]² versus photon energy (hν) for β-Ga₂O₃ NWs on sapphire substrate. In the case of parabolic band structure, the bandgap E_g and absorption coefficient (α) of a semiconductor are related via the well known equation

$$\begin{eqnarray}&&(\alpha h\upsilon )=A{{\left(h\upsilon -{{E}_{\text{g}}}\right)}^{m}}\end{eqnarray} \tag{ 1 }$$

where the value of m is 2 and 0.5 for indirect and direct bandgap semiconductors, respectively, and A is a constant. In the same way, the bandgap E_g and K–M function F(R) are also related by an equation as given in [54]

$$\begin{eqnarray}&&{{[F\left(R\right)h\upsilon ]}^{2}}=B~\left(h\upsilon -{{E}_{\text{g}}}\right)\end{eqnarray} \tag{ 2 }$$

where B is another constant. The estimated bandgap from the experimental curve is ~4.94 eV for Fe-catalysed β-Ga₂O₃ NWs on sapphire substrate, which is in good agreement with the bandgap of β-Ga₂O₃ estimated in the previous studies [6, 32].

Zoom In Zoom Out Reset image size

For comparison of NWs grown using different catalysts, we have published papers related to Au catalytic growth of β-Ga₂O₃ NWs [34, 35]. In the present study we have obtained single crystalline Fe-catalysed β-Ga₂O₃ NWs, which are comparable to the NWs grown using Au catalyst. CL properties are similar for the two catalysts. The bandgap of Fe-catalysed β-Ga₂O₃ NWs is 4.94 eV, whereas the Au-catalysed β-Ga₂O₃ NW bandgap was 4.74 eV [35].