Synthesis of Wide Bandgap β‐Ga2O3 Rods on 3C-SiC-on-Si

This paper presents the synthesis of single crystalline, selfcatalytic β-Ga2O3 rods by a low pressure chemical vapor deposition technique. The effects of oxygen concentration and growth temperature on the morphology and growth rate of β-Ga2O3 rods were studied. The β-Ga2O3 rods were synthesized on a 3C-SiC film deposited on a silicon substrate utilizing high purity gallium (Ga) metal and oxygen (O2) as source materials, and argon (Ar) as a carrier gas. X-ray diffraction, high resolution transmission electron microscopy, and Raman spectroscopy measurements were performed for the as-grown β-Ga2O3 rods, which revealed a monoclinic phase of β-Ga2O3 with single crystalline microstructure. The selected area electron diffraction pattern recorded on a single β-Ga2O3 rod further verified their single crystalline nature. Because of their high crystalline quality and large surface area to volume ratio, these β-Ga2O3 rods have a great potential for surface related applications such as photocatalysis, chemical sensing, and deep-ultraviolet photodetection. ■ INTRODUCTION Ga2O3 is a wide bandgap semiconductor (Eg ≈ 4.9 eV at room temperature) with excellent chemical and thermal stability up to 1400 °C. Unlike other transparent oxides, it exhibits high transparency in the deep-UV and visible wavelength region because of its very large bandgap. It acts as an insulator under perfectly stoichiometric conditions and becomes n-type when an adequate amount of oxygen vacancies are present. In order to achieve controllable carrier conductivity and carrier mobility demanded by practical device applications, intentional doping is necessary. Several groups have explored n-type and ptype doping of Ga2O3. However, major challenges still exist for successful p-type doping of Ga2O3 due to low dopant solubility, deep acceptor levels, and self-compensation processes. The synthesis of high material quality Ga2O3 with low defects will undoubtedly advance the development of pdoping control for the material. Because of the attractive properties of Ga2O3, considerable efforts have been made to synthesize Ga2O3 as bulk materials, thin films, and nanostructures. Growth methods such as molecular beam epitaxy (MBE), metal organic chemical vapor deposition (MOCVD), pulsed laser deposition (PLD), and atmospheric pressure chemical vapor deposition (APCVD) have been investigated for the synthesis of Ga2O3 thin films. However, the reported growth rates are generally slow (0.09−2.3 μm/h). On the other hand, synthesis of Ga2O3based nanostructures have attracted much attention recently, producing geometric structures which enable investigations relating optical and electrical properties to the quantum confinement effect and surface morphology of the nanostructures. Syntheses of Ga2O3-based nanostructures such as nanowires, nanoribbons, nanosheets, nanobelts, and nanocolumns have been reported. However, most of these nanostructures are synthesized using catalysts which can potentially introduce impurities into the material. A few catalyst-free synthesis techniques of Ga2O3 nanomaterials have been developed using chemical vapor deposition (CVD). These nanostructures can serve as building blocks for novel nanodevices such as deep UV photodetectors, nanophotonics switches, sensors,and field effect transistors. SiC is a well-known wide bandgap semiconductor material widely used in high power electronic devices. It can form many crystal structures by stacking each tetrahedrally bonded Si−C bilayer on top of each other in different ways which are known as polytypes. Among them, 3C-SiC (also known as β-SiC) is the only polytype with cubic (or zinc blende) crystal structure. In 3C-SiC, each SiC bilayer can be oriented into only three possible positions with respect to the lattice, while the tetrahedral bonding between the C atom and Si atoms is maintained. The stacking sequence is ABC if these three layers are denoted as A, B, C. On the other hand, 6H-SiC (also known as α-SiC) has a hexagonal crystal structure. It is composed of two-thirds cubic bonds and one-third hexagonal bonds with a stacking sequence of ABCACB of the bilayer. Because of its small bandgap (∼2.4 eV), 3C-SiC has advantages compared to the other polytypes of SiC that permit inversion at lower electric field strength. Also it has isotropic and higher electron Hall mobility compared to that of 6H-SiC. Synthesis of Ga2O3 thin layers on 6H-SiC substrates by gallium evaporation in oxygen plasma and the sol−gel technique has been reported previously. Gas sensors based on Ga2O3/ Received: November 4, 2015 Revised: December 2, 2015 Published: December 10, 2015 Article pubs.acs.org/crystal © 2015 American Chemical Society 511 DOI: 10.1021/acs.cgd.5b01562 Cryst. Growth Des. 2016, 16, 511−517 SiC heterojunction have been fabricated previously. In addition, because of the current difficulty associated with p-type doping of Ga2O3, a heterojunction photodetector device comprised of n-type Ga2O3 and p-type SiC has been reported. 48 Furthermore, by using semiconducting SiC as the growth substrate instead of insulating materials such as sapphire, vertical electrical-injection devices can be directly fabricated. In this paper, we report the synthesis of β-Ga2O3 rods on 3CSiC-on-Si substrates by low pressure chemical vapor deposition (LPCVD). The effects of oxygen concentration and growth temperature on the growth rate and morphology of the rods were systematically studied. Our studies indicate that both parameters play a crucial role in controlling the synthesis of the β-Ga2O3 rods. This study provides insights for the synthesis of β-Ga2O3 materials on the 3C-SiC-on-Si substrates which have not been investigated previously. ■ EXPERIMENTAL SECTION A single zone tube furnace with programmable temperature controller and precise pressure controller was used for the synthesis of the βGa2O3 rods. 3C-SiC-on-Si was used as the substrate for the growth. The 3C-SiC films were deposited on Si (100) substrates by an APCVD technique reported elsewhere. Prior to the growth, the substrates were cleaned with acetone and isopropanol, rinsed by deionized water, and dried under nitrogen flow. High purity gallium pellets (Alfa Aesar, 99.99999%) were used as the source material. The source crucible was placed at the center of the furnace. The substrates were placed horizontally at the downstream of the quartz tube. Before purging the system, the chamber was pumped down to a base pressure of ∼1 mTorr. Prior to heating up the chamber to the desired growth temperature, the chamber was purged with argon for 30 min. The chamber was then heated up to the growth temperature at a rate of 20 °C/min under a flow of argon and kept at that temperature for 40 min. The samples were taken out after cooling down to room temperature under argon flow. To investigate the effects of the growth temperature and oxygen concentration on the material morphology and growth rate of the Ga2O3 rods, several experiments were carried out by varying the growth temperature (780−950 °C) and O2 volume percentage (1.2−9.1%). The oxygen volume percentage was varied by keeping the oxygen flow rate fixed at 5 sccm (standard cubic centimeters per minute) and controlling the Ar flow rate from 50 to 400 sccm. The growth pressure was maintained at around 2 Torr. The crystal structure, chemical composition, and morphology of the Ga2O3 rods were characterized by field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), transmission electron microscopy (TEM), and Raman spectroscopy. XRD spectrum was collected on a Rigaku D/ Max 2200 with Cu Kα radiation (1.54 Å). EDS spectra and FESEM images were taken by Helios 650. High resolution transmission electron microscopy (HRTEM) images and selected-area electron diffraction (SAED) were taken using a FEI Tecnai F30 at 300 kV. To prepare the TEM sample, the Ga2O3 rods were first dispersed in ethanol and then transferred to a Cu grid covered with carbon film. The Raman spectrum was taken at room temperature using a linearly polarized laser beam of 532 nm. The beam was focused on the sample by a 100× objective. The laser power and the beam diameter were ∼200 μW and ∼1 μm, respectively. ■ RESULTS AND DISCUSSION Growth temperature and oxygen volume percentage are the two growth parameters that were found to be critical for the synthesis of Ga2O3 rod structures on the 3C-SiC-on-Si substrates. To investigate the effects of growth temperature on the synthesis of Ga2O3 rods, five growth experiments were carried out at different growth temperatures with fixed oxygen percentage. For these growth experiments, the oxygen volume percentage was kept fixed at 4.8%, and the growth temperature was varied between 780 and 950 °C. Figure 1 shows FESEM images of the Ga2O3 rods grown at 780 °C, 870 °C, and 900 °C. As observed from the images, the rods became longer as the Figure 1. Top view FESEM images of Ga2O3 rods grown at oxygen volume percentage of 4.8% with different growth temperatures. (a) 780 °C, (b) 870 °C, and (c) 900 °C. Inset in (a) shows the high magnification top view FESEM image of the Ga2O3 rods. Crystal Growth & Design Article DOI: 10.1021/acs.cgd.5b01562 Cryst. Growth Des. 2016, 16, 511−517 512 growth temperature increased from 780 to 870 °C. As the growth temperature further increased to 900 °C, the growth rate of the Ga2O3 rod decreased. Polyangular rods with faceted sidewalls were formed under all growth conditions. Most of the rods overlapped and impinged on neighboring rods due to the random orientations. Some of them protruded from the surface of the substrate. The rods have diameters in the range of ∼200−700 nm and lengths of ∼3−12 μm. The effects of oxygen concentration on the synthesis of the Ga2O3 rods were investigated for a fixed growth temperature. Five growth experiments were conducted on 3C-SiC-on-Si substrates by varying the oxygen volume percentage at a fixed temperature of 900 °C. The oxygen volume percentage was varied by keeping the oxygen flow rate fixed at 5 sccm and varying the Ar flow rate between 50 and 400 sccm. Figure 2 shows the top view FESEM of the Ga2O3 rods grown with oxygen volume percentages of 1.6%, 4.8%, and 9.1%. As observed by comparing the images, the height of the rods decreases as the oxygen volume percentage increases from 1.6% to 9.1%. The variation in the oxygen concentration did not seem to have an impact on the morphology of the rods. We observe polyangular rods grown under different oxygen volume percentages. We found both growth temperature and oxygen volume percentage have a significant impact on the growth rate of the Ga2O3 grown via LPCVD. To quantitatively illustrate these effects, Figure 3 plots the growth rate of the as-grown Ga2O3 rods as a function of the growth temperature (Figure 3a) and oxygen volume percentage (Figure 3b). As shown in Figure 3a, the growth rate is 2.12 μm/h at 780 °C and increases to a maximum of 9.67 μm/h at 870 °C. A further increase of the growth temperature results in a decrease in the growth rate. This can be understood by examining the amount of Ga vapor present as a function of the growth temperature. As the growth Figure 2. Top view FESEM images of Ga2O3 rods grown at 900 °C with different oxygen volume percentage; (a) 1.6%, (b) 4.8%, and (c) 9.1%. Insets show the high magnification top view FESEM images of the rods. Figure 3. Estimated growth rates of Ga2O3 rods as a function of (a) temperature and (b) oxygen volume percentage. Crystal Growth & Design Article DOI: 10.1021/acs.cgd.5b01562 Cryst. Growth Des. 2016, 16, 511−517 513 temperature increases from 780 to 870 °C, the amount of Ga vapor concurrently increases, which leads to an increase in the growth rate of Ga2O3. However, for a fixed oxygen volume percentage of 4.8%, a further increase of the growth temperature might lead to insufficiency of oxygen for reaction of Ga vapor. This could prevent the excess Ga to react with oxygen. Another possibility for the reduced growth rate at higher temperatures is the formation and desorption of Ga2O which forms at a higher temperature due to the reaction of excess Ga with Ga2O3 surface oxides. Such a dependence of growth rate on the deposition temperature has also been observed for the synthesis of β-Ga2O3 thin films by plasma assisted molecular beam epitaxy (MBE) and metal organic vapor phase epitaxy. Figure 3b shows the growth rate as a function of the oxygen volume percentage for the β-Ga2O3 rods. The growth rate increases with increasing oxygen volume percentage, reaching a maximum of 12.5 μm/h at ∼1.6%. Further increases in the oxygen volume percentage results in a decrease in the growth rate. This is because although Ga has a moderate vapor pressure at high growth temperature, only a small amount of Ga reaches the substrate when no or a small amount of oxygen is introduced in the growth system. When a sufficient amount of oxygen is introduced in the growth chamber, Ga easily oxidizes to Ga2O. Ga2O has a much higher vapor pressure than metallic Ga. Thus, a large amount can be easily evaporated and transported to the substrate by the carrier gas resulting in a higher growth rate. On the other hand, excessive concentrations of oxygen might prevent the Ga vapor from reaching the substrate, which in turn reduces the growth rate. Thus, proper control of the oxygen concentration is critical for achieving a higher growth rate of the Ga2O3 rods. The elemental composition of the Ga2O3 rods was characterized by EDS analysis. A spectrum taken along the body of a rod is shown in Figure 4. Analysis reveals that the rod is primarily composed of gallium and oxygen. No other element peaks were detected during the analysis. Quantitative analysis reveals that the atomic ratio of Ga and O in the rod is ∼2:3, which is in good agreement with the stoichiometry and composition of bulk Ga2O3. The crystallinity of the as-grown Ga2O3 rods was confirmed by XRD. A representative spectrum is shown in Figure 5. The diffraction peaks match perfectly with monoclinic β-Ga2O3, which suggests the growth of high quality Ga2O3 rods. On the basis of the 2θ data of the (002) peak located at 31.8°, the lattice parameter is c = 5.78 Å, which is in good agreement with the lattice parameter of bulk Ga2O3 (c = 5.8 Å). 52 No other Ga2O3 phases (α, γ, δ, or ε) or impurities were observed in the diffraction spectrum, indicating high purity β-Ga2O3 rods were synthesized through this process. The sharp diffraction peaks also revealed that the prepared samples are of high crystalline quality. Besides the diffraction peaks of β-Ga2O3, two peaks corresponding to the (200) and (220) planes of the 3C-SiC film and one peak corresponding to the (400) plane of the Si substrate were identified in the spectrum. The detailed morphology and crystal structure of the asgrown Ga2O3 rods were further investigated with TEM, HRTEM, and SAED. The analysis of the rod growth direction and assignment of Miller indexes were based on both SAED and HRTEM images. Figure 6a shows a TEM image of a single rod having a uniform lateral dimension of ∼250 nm along its entire length. The HRTEM image shown in Figure 6b reveals that the rod is structurally perfect. The inset of Figure 6b shows the SAED pattern taken along the [111] zone axis. The SAED pattern confirms that the synthesized materials are single crystalline monoclinic β-Ga2O3. Note that additional spots and streaks were observed in the electron diffraction pattern shown in an inset of Figure 6b, which indicates that some stacking faults or twins are formed in the rods. Similar phenomena have been reported previously. Lattice fringes are clearly visible in the HRTEM images of Figure 6b,c. From Figure 6c, the resolved lattice fringes with marked interplanar spacings of 0.29 and 0.28 nm correspond to monoclinic β-Ga2O3 (11 ̅0) and (202 ̅) planes, respectively. The rod growth direction is shown by an arrow in Figure 6c, which is parallel to the (202 ̅) crystallographic plane. A room temperature Raman spectrum of the β-Ga2O3 rods is shown in Figure 7. Nine Raman active modes at 144.4 (Bg), 169.6 (Ag), 200.8 (Ag), 346.4 (Ag), 417.6 (Ag), 479 (Bg), 629.7 (Ag), 653.4 (Bg), and 767.5 (Ag) cm −1 are visible in the spectrum. The Raman active modes in Ga2O3 can be classified into three categories, namely, low (200 cm−1), mid (∼480−310 cm−1), and high (∼770−500 cm−1) frequency modes. The low frequency modes 144.4, 169.6, and 200.8 cm−1 are assigned to the translation and libration of tetrahedra-octahedra chains. The mid frequency modes 346.4, 417.6, and 479 cm−1 are associated with deformation of Ga2O6 octahedra. The high frequency modes 629.7, 653.4, and 767.5 cm−1 are assigned to bending and stretching of GaO4 tetrahedra. The modes are ∼0.4−7 cm−1 blue-shifted compared to the bulk Raman modes. The estimated Raman shift values from the as-synthesized βGa2O3 rods are relatively low as compared to those reported Figure 4. EDS spectrum of the as-grown Ga2O3 rods grown at 900 °C with oxygen volume percentage of 4.8%. Figure 5. XRD spectrum of Ga2O3 rods grown at 900 °C and oxygen volume percentage of 4.8%. The corresponding indices are marked above the respective diffraction peaks. Crystal Growth & Design Article DOI: 10.1021/acs.cgd.5b01562 Cryst. Growth Des. 2016, 16, 511−517 514 previously. Gao et al. reported a redshift of 4−23 cm−1 for Ga2O3 nanorods as compared to pure Ga2O3 powder. 56 They concluded that the presence of twin and edge dislocations in the nanorods is responsible for the red-shift of the Raman peaks. On the other hand, Rao et al. found a blue-shift of 10− 40 cm−1 for Ga2O3 nanowires growing along the [110] crystallographic direction. They attributed this blue-shift to growth direction induced internal strains in the nanowires. Therefore, the small blue-shift values indicate that the assynthesized β-Ga2O3 rods are under less strain as compared to similar nanostructures reported previously. ■ CONCLUSION In summary, a large quantity of β-Ga2O3 rods was synthesized on 3C-SiC-on-Si substrate by a catalyst-free, wafer-scale LPCVD technique. EDS spectra show that the as-grown Ga2O3 rods are composed of Ga and O and correspond to the stoichiometry of bulk Ga2O3. SAED and HRTEM data show that the synthesized rods are of high quality and have a single crystalline monoclinic structure. Our studies revealed that both the growth temperature and oxygen concentration have significant impact on the control of the Ga2O3 rod structural morphology and growth rate. The self-catalyzed growth mechanism produced defect-free Ga2O3 rods, which can potentially serve as building blocks for solar blind deep-UV photodetectors and chemical sensing technologies. ■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Author Contributions ‡(S.R. and L.H.) Equally contributed first authorship. Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTSThe authors acknowledge financial support through start-upfunds from Case Western Reserve University. 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