Diameter Tuning of \( \beta \)-Ga2O3 Nanowires Using Chemical Vapor Deposition Technique
© The Author(s). 2017
Received: 20 October 2016
Accepted: 11 February 2017
Published: 9 March 2017
Diameter tuning of \( \beta \)-Ga2O3 nanowires using chemical vapor deposition technique have been investigated under various experimental conditions. Diameter of root grown \( \beta \)-Ga2O3 nanowires having monoclinic crystal structure is tuned by varying separation distance between metal source and substrate. Effect of gas flow rate and mixer ratio on the morphology and diameter of nanowires has been studied. Nanowire diameter depends on growth temperature, and it is independent of catalyst nanoparticle size at higher growth temperature (850–900 °C) as compared to lower growth temperature (800 °C). These nanowires show changes in structural strain value with change in diameter. Band-gap of nanowires increases with decrease in the diameter.
In the past decade, wide band-gap semiconductor nanowires have received extensive research interest due to their potential device applications [1–10]. Recently, beta gallium oxide (β-Ga2O3) with its one dimensional morphology is emerging as one of the potential semiconductor oxide nanomaterial. It has shown promising device applications including high-temperature gas sensors, UV photodetectors, high power field effect transistors (FET), and photonic switches [2, 3, 9–15]. β-Ga2O3 exhibits advantageous properties including large band-gap with Eg ~ 4.7–4.9 eV at room temperature (RT), high breakdown field of 8 MVcm−1, and outstanding thermal and chemical stability at high temperatures [11, 16–19]. It has stable β-phase, which exhibits monoclinic crystal structure .
Several applications of \( \beta \)-Ga2O3 nanostructures have been explored [3, 9, 10]. To grow, thermal chemical vapor deposition (CVD) is the most accepted and widely used technique [14, 20, 21]. It is attractive due to high deposition rate, capability of producing highly dense and pure materials, reproducibility of synthesis, and ability to control the morphology of nanostructures by controlling process parameters. For nanowire growth, vapor-liquid-solid (VLS) or vapor-solid (VS) process are well established growth mechanisms . The β-Ga2O3 nanostructures including nanowires, nanosheets, nanobelts, and nanorods grown by CVD have been shown in number of reports [12, 20, 23, 24]. Nanowires as building blocks of nanodevices allow tuning of fundamental optical and electronic properties of devices by tuning nanowire diameter. Further, high aspect-ratio nanowires with different diameters are advantageous for catalytic functionalities and sensors due to large surface to volume ratio. Recently, Reddy et al.  have shown high photocatalytic activity of β-Ga2O3 nanorods under UV irradiation. Kumar et al.  have shown the catalytic activity of thermosensitive Ga2O3 nanorods. Li et al.  have demonstrated high performance of bridged Ga2O3 nanowires for solar-blind photodetection. Ling et al.  demonstrate pH sensor based on Ga2O3 nanowires and suggested possibilities for improvement in performance of pH sensors by nanowire dimensions related sensing capabilities. Conduction properties of nanowire depend on its diameter. Nanowires with diameter smaller than the depletion layer width existing due to surface state charges are completely depleted whereas nanowires with larger diameter have a conducting channel . Wie et al.  have studied diameter-dependent band-gap alteration in strained ZnO nanowires. Therefore, diameter tuning of Ga2O3 nanowire using experimental conditions is highly desirable to tune the nanodevice properties. In this work, diameter tuning of \( \beta \)-Ga2O3 nanowires by CVD technique using various experimental conditions have been investigated. The dependence of nanowire diameter on separation distances between metal source and substrate has been studied. Various growth temperatures with different Au nanoparticles were explored to tune the diameter of nanowires. The nanowires with different diameters have been further investigated using XRD, Raman, and UV-vis techniques.
\( \beta \)-Ga2O3 nanowires have been grown on Au nanoparticles coated sapphire substrate using CVD technique where Au nanoparticles serve as catalyst. The colloidal solutions of Au nanoparticles of sizes 50 nm (with concentrations ~3.4 × 1010 particles/mL) and 20 nm (with concentrations ~6.8 × 1011 particles/mL) were purchased from Ted Pella Inc. Au nanoparticles from colloidal solution were dispersed on sapphire substrate using two-step spin-coating method (3000 rpm for 60 s followed by 9000 rpm for 30 s) and then annealed at 200 °C in the presence of argon flow (50 mL/min). Gallium metal (purity 99.999% from Sigma Aldrich) and substrate are kept in same temperature zone. Oxygen and argon gases were used to grow \( \beta \)-Ga2O3 nanowires. Nanowire growth has been studied systematically using various growth temperatures, different Ar/O2 total flow rates, different flow rate ratios, and different separation distances between source metal and substrate under pressure of 2.5 Torr.
Field-emission scanning electron microscopy (FESEM) images were recorded at RT with electron beam energy of 5 and 10 keV using Raith e-line plus system. X-ray diffraction (XRD) measurements were performed using Rigaku having CuKα radiation. Raman analyses were performed at RT in backscattering configuration using an excitation wavelength of 514 nm using Horiba-LabRAM HR Evolution instrument. UV-vis spectrophotometer from Perkin Elmer (Model Lambda 1050) has been used for reflectance measurements.
Results and Discussion
where ρ, u, and μ are mass density, flow density, and viscosity, and ρu/μ is called the Reynolds number. For deposition process, reactant gases must diffuse through varying boundary layer thickness ∆ l shown by Eq. (2) toward downstream to reach the deposition surface (Fig. 3). It shows that the deposition process on wafer surface depends on S d value. It is well known that reactant species are consumed going downstream in CVD process [31, 34, 35]. Purushothaman and Jeganathan  have shown that gallium vapor pressure during growth decreases with increase in S d value. Menzel et al.  have also studied the decrease in metal vapor concentration with S d for nanowire growth using thermal CVD. Consequently, the concentration of reactant species is depleted with increase in S d value. Therefore, diameter of Ga2O3 nanowires is reduced due to increase in ∆ l and depletion of reactant species with increase in S d .
It is noted that catalyst droplets on tip of nanowires have not been present as shown by circles in inset of Fig. 2b, c. However, clusters were noticed in the root of nanowires. Dotted circle in inset of Fig. 2d shows the cluster (~600 nm) in nanowire root. This indicates the preferential nanowire nucleation follow root growth. Nanowire nucleation within catalyst depends on material-catalyst phase diagram, interfacial parameters, and contact angle of catalyst-nucleus and nucleus-substrate . The noted large size of root cluster is due to supersaturation in catalyst alloy induced by metal vapor pressure at high temperature . Adatom diffusion and direct nucleation of vapors play the crucial role in this root mediated Ga2O3 nanowires growth. These results show that diameter of root grown Ga2O3 nanowires can be tuned using S d and it decreases with increase in S d value under CVD process.
Modes with A u and B u symmetry are infrared active and modes with A g and B g symmetry are Raman active. Raman modes under non-resonant conditions depend on crystal orientation and polarization configurations specified by selection rules [41, 42]. On the basis of Raman study, unit cell of β-Ga2O3 consists of two formula unit cells as octahedral (Ga2O6) and tetrahedral (GaO4) [43, 44]. Low energy phonon modes (100–300 cm−1) correspond to the liberation and translation of tetrahedra-octahedra chains, moderate energy phonon modes (300–500 cm−1) correspond to deformation of Ga2O6 octahedra, and high energy phonon modes (600–800 cm−1) correspond to stretching and bending of GaO4 tetrahedra. In our previous report, Raman measurements from β-Ga2O3 nanowires and bulk single crystal have been reported . In the present case, the Raman peaks are in general agreement with the peaks reported for nanowires in , but we have observed further small shift as the nanowire diameter changes. For example, phonon mode at position 114.6 cm−1 for D1 shifts to the position 115.2 cm−1 for D2 and D3, while strongest phonon mode at position 202.3 cm−1 for D1 and D2 shifts to the position 201.2 cm−1 for D3. Similarly, phonon mode at position 652.9 cm−1 for D1 shifts to the position 654.3 cm−1 for D2 and 654.8 cm−1 for D3. While phonon mode at position 769.9 cm−1 for D1 shifts to the position 767.3 cm−1 for D3. It is reported that blue shift in the phonon modes is due to internal strain in the nanowires and red shift is due to the presence of defects in nanowires, such as O vacancies which cause abnormality in the Ga–O bond vibration [44, 46]. These small-shifted Raman positions in different nanowire diameters indicate variation in strain level as calculated from XRD measurements besides defects.
In conclusions, growth of \( \beta \)-Ga2O3 nanowires with different diameters has been explored under various experimental conditions using CVD technique. Diameter of \( \beta \)-Ga2O3 nanowire grown by catalyst alloy-mediated root growth can be tuned by adjusting growth conditions. The separation distance between metal source and substrate controls the nanowire diameter on the basis of boundary layer thickness and reactant vapor species. Nanowire diameter decreases with increase in the separation distance. Higher gas flow rate induces larger diameter; however, nanowire morphology gets deteriorated. Diameter of nanowires depends on growth temperature and catalyst nanoparticles size. Diameter of nanowire grown at higher temperature (850–900 °C) does not depend on catalyst nanoparticle size. The smaller diameter of nanowire (<50 nm) can be obtained using lower growth temperature (800 °C) and smaller catalyst nanoparticles size (20 nm). Ga2O3 nanowires exhibit \( \beta \)-phase with monoclinic crystal structure. XRD and Raman measurements indicate the changes in structural strain with nanowire diameter. Band-gap of these nanowires depends on its diameter, and it increases from 4.70 to 4.80 eV with decrease in nanowire diameter from several hundreds of nanometers to few hundreds of nanometers.
Authors acknowledge the Department of Electronics and Information Technology, Government of India and Nanomission: Department of Science and Technology (DST), India (Project No. Sr/NM/NS-1106/2012 (G)) for the partial financial support for this work. One of the authors (Mukesh Kumar) is thankful to Indian Institute of Technology Delhi for providing research fellowship.
This study is funded by the Department of Electronics and Information Technology, Ministry of Communications and Information Technology, India.
All authors (Mukesh Kumar, Vikram Kumar, R. Singh) give their efforts for the research outcomes. MK performed the experiments under the supervision of authors VK and RS. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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