Epitaxial Growth of ZnGa2O4: A New, Deep Ultraviolet Semiconductor

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Epitaxial Growth of ZnGa2O4: A New Deep Ultraviolet Semiconductor Candidate Ray Hua Horng, Chiung-Yi Huang, Sin-Liang Ou, Tzu-Kuang Juang, and Po-Liang Liu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01159 • Publication Date (Web): 05 Oct 2017 Downloaded from http://pubs.acs.org on October 6, 2017

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Crystal Growth & Design

Epitaxial Growth of ZnGa2O4: A New Deep Ultraviolet Semiconductor Candidate Ray-Hua Horng,*,† Chiung-Yi Huang,† Sin-Liang Ou,‡ Tzu-Kuang Juang,§ and Po-Liang Liu§

†

Institute of Electronics, National Chiao Tung University, Hsinchu 30010, Taiwan, R.O.C.

‡

Department of Materials Science and Engineering, Da-Yeh University, Changhua 51591,

Taiwan, R.O.C. §

Graduate Institute of Precision Engineering, National Chung Hsing University, Taichung 40227,

Taiwan, R.O.C. KEYWORDS: ZnGa2O4, DEZn flow rates, metalorganic chemical vapor deposition, single crystalline, electrical properties.

ACS Paragon Plus Environment

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ABSTRACT ZnGaO films were grown on c-plane sapphire substrates by metalorganic chemical vapor deposition using diethylzinc (DEZn), triethylgallium (TEGa), and oxygen. The flow rate of DEZn was 10–60 sccm and those of TEGa and oxygen were held constant. The ZnGaO film prepared at a DEZn flow rate of 10 sccm adopted a (-201)-oriented single-crystalline β-Ga2O3 phase, whereas those prepared at 30–60 sccm exhibited a (111)-oriented single-crystalline ZnGa2O4 phase. Based on Hall measurements, ZnGaO films (10 sccm DEZn) possessed very poor electrical properties, which were similar to those of β-Ga2O3. On the other hand, the carrier concentration in ZnGaO films increased from 1.94 × 1014 to 6.72 × 1016 cm-3 and the resistivity decreased from 5730 to 67.9 Ω-cm when increasing the DEZn flow rate from 30 to 60 sccm. According to compositional analyses, the improved electrical properties of ZnGaO films upon increasing DEZn flow rate from 30 to 40 sccm are due to the increasing Zn content, and the enhancement from 50 to 60 sccm could be due to increased C content. Cathodoluminescence results also confirm the ZnGa2O4 structure for ZnGaO films prepared at DEZn flow rates of 30– 60 sccm and reveal their use for ultraviolet applications.

INTRODUCTION Semiconducting oxides have become important materials for novel devices with new functionality due to wide bandgap, chemically and thermally stable characteristics. Recently, zinc gallate (ZnGa2O4) materials with a spinel structure have attracted much attention in their characteristic investigation and device fabrication. Normally, a spinel is a cubic close-packed oxide with two tetrahedral sites and one octahedral site per unit cell. ZnGa2O4 possesses an energy bandgap of approximately 5.2 eV and can emit blue light through the transitions of self-


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activation centers.1 Due to its transparency in the near-ultraviolet region, good conductivity, high chemical and thermal stabilities, and excellent optical characteristics, ZnGa2O4 has been used for several optoelectronic applications such as vacuum fluorescence displays, low-voltage fieldemission displays, and ultraviolet photodetectors.2–4 Up to now, many 1D ZnGa2O4 nanostructures including nanocrystals, nanowires, and nanorods have been proposed.3–6 Although several optoelectronic devices prepared with 1D ZnGa2O4 nanostructures can obtain high performance, the stability of devices is still an important issue for their practical applications. Thus, the development of thin-film ZnGa2O4 materials and related devices are urgently required. On the other hand, ZnGa2O4 films prepared by various techniques including sol-gel method,7 chemical vapor deposition,8 sputtering,9 and pulsed laser deposition10,11 always had polycrystalline structures. This indicates that single-crystalline ZnGa2O4 films are relatively difficult to prepare. In fact, the single-crystalline growth of ZnGa2O4 films is easily impeded due to the high congruent melting temperature and perfect immiscibility of the ZnO–Ga2O3 material system. As mentioned above, various methods have been proposed to prepare ZnGa2O4 films. However, almost all research on ZnGa2O4 films has focused on phosphor applications and involved investigation of their luminescent characteristics; very few studies have been conducted on the optical properties (transparency and cathodoluminescence) of ZnGa2O4 films.8 Among various growth techniques, metalorganic chemical vapor deposition (MOCVD) is a very beneficial technology for obtaining high-quality single-crystalline films. Additionally, MOCVD possesses other advantages such as high deposition rate, conformal mapping over complex structure, and large area uniformity, which are helpful to improve the characteristics of the deposited films. In our previous research, ZnGaO films have been prepared by MOCVD and potentially used for metal-oxide-semiconductor field-effect transistor applications.12 The


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previous results presented the high feasibility of ZnGaO films for power device applications; however, the material characteristics of these films were not detail analyzed and discussed. In this study, the structural, microstructural, morphological, compositional, chemical bonding, electrical, and cathodoluminescence (CL) properties of the MOCVD grown ZnGaO films prepared at various DEZn flow rates were investigated in detail. Additionally, the phase stability of ZnGaO films was systematically analyzed via first-principles calculations. EXPERIMENTAL SECTION In this study, the ZnGaO thin films were prepared on c-plane (002) sapphire substrates using a modified Emcore-D180 MOCVD system. Diethylzinc(DEZn), triethylgallium (TEGa), and oxygen (99.999%) were used as the precursors. Moreover, Ar (99.999%) was employed as the carrier gas, which was passed through bubblers to deliver DEZn and TEGa vapors to the reactor. The growth pressure, substrate temperature, and growth time of the ZnGaO films were maintained at and 90 minutes, respectively. The flow rates of TEGa and oxygen were maintained at 50 and 200 sccm, respectively. Various flow rates for the DEZn source were used for the deposition of ZnGaO films: 10, 30, 40, 50, and 60 sccm. The crystal structures of ZnGaO films were measured by double-crystal X-ray diffraction (XRD, ANalytical, X’Pert Pro MRD). A Cu Kα line (λ = 1.541874 Å) was adopted as the radiation source, and a Ge (220) crystal was used as the monochromator. The surface morphologies of ZnGaO films were observed by SEM. The microstructure and orientation relationship of ZnGaO films was characterized by transmission electron microscopy (TEM, JEOL JEM-2100F). The chemical bonding states and compositions of these films were analyzed by X-ray photoelectron spectroscopy (XPS, ULVAC-PHIPHI 5000). Electrical characteristics were determined from van der Pauw Hall measurements (ACCENT, HL-5500PC) at a magnetic field strength of 3200 Gauss. The optical properties and energy


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bandgap (Eg) of the ZnGaO films were investigated by CL measurements using 20 kV (integration time 1 sec) by monoCL3 of JEOL JSM-7000F with grating 2400 l/mm and blazed 240 nm.

RESULTS AND DISCUSSION Figure 1a displays the results of XRD θ–2θ scan for the ZnGaO films prepared at DEZn flow rates of 10, 30, 40, 50, and 60 sccm, in which 2θ was increased from 10 to 70. JCPDS data of the normal spinel ZnGa2O4 (card no. 38-1240) is given for reference. The XRD patterns of these films are similar. It can be found that these films all possess three diffraction peaks located near 2θ positions of 18.5–18.9°, 37.7–38.1°, and 58.1–58.8°, respectively. We focused on the second diffraction peaks of these films by limiting the 2θ range from 36.9° to 38.7°, as shown in Figure 1b. When the DEZn flow rate was 10 sccm, the diffraction peak of this film was located at 2θ = 38.17 and approached that of Ga2O3(-402). As increasing the DEZn flow rate to 30, 40, 50, and 60 sccm, the peak positions of these four films were located at 2θ = 37.93, 37.89, 37.77, and 37.73, respectively. Obviously, as the DEZn flow rate was increased, the diffraction peak of the film shifted to a lower 2θ value. Note that the diffraction peak of ZnGa2O4(222) is located at 2θ = 37.34. This reveals that the crystal structure of the ZnGaO film was transformed from β-Ga2O3 to ZnGa2O4 with increasing DEZn flow rate. The shift in peak position can be ascribed to the inclusion of Zn2+ in the Ga2O3 structure, where the atomic radii of Zn2+ and Ga3+ were 0.74 and 0.63 Å, respectively. Thus, at a lower DEZn flow rate (i.e., 10 sccm), the film possesses a single-crystalline β-Ga2O3 structure, and the three diffraction peaks shown in Figure 1a are indexed to (-201), (-402), and (-603) planes. At higher DEZn flow rates


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(30–60 sccm), the films exhibited a single-crystalline ZnGa2O4 structure, and the three diffraction peaks are indexed to (111), (222), and (333) planes. Figure 2a–e shows the high-resolution transmission electron microscopy (TEM) bright field images of ZnGaO films deposited at DEZn flow rates of 10, 30, 40, 50, and 60 sccm, respectively. As shown in Figure 2a, the d-spacing of the film is 4.706 Å, which is very close to the d-spacing of Ga2O3(-201), i.e., 4.69 Å. When the DEZn flow rate was increased to 30, 40, 50, and 60 sccm, the d-spacing values of the films shown in Figure 2b–e are 4.731, 4.747, 4.769, and 4.786 Å, respectively. These d-spacing values are comparable with the typical d-spacing of ZnGa2O4(111), i.e., 4.808 Å. It is apparent that the TEM observations agree well with the XRD results, as displayed in Figure 1. Figure 2f displays the selected area electron diffraction (SAED) pattern of the ZnGaO film grown at a DEZn flow rate of 60 sccm (Figure 2e). The regularly arranged diffraction dots suggest that the ZnGaO film had a single-crystalline structure, which is in good agreement with the XRD result. In addition, this diffraction pattern indicates that the ZnGa2O4 phase of the ZnGaO film was formed along the [111] direction with a [1-21] zone axis. With the exception of the analyses of d-spacing values and SAED, the lattice distortion phenomenon was also found in the ZnGaO films. Based on our observations, the lattice arrangements shown in the ZnGaO films prepared at DEZn flow rates of 10 and 30 sccm were more regular. However, as the DEZn flow rate was increased to 40–60 sccm, obvious lattice distortions can be observed. Figure 3a–e displays the cross-sectional scanning electron microscopy (SEM) images of ZnGaO films deposited at DEZn flow rates of 10, 30, 40, 50, and 60 sccm, respectively. Meanwhile, the plan-view SEM images of these five films are shown in Figure 3f–j, respectively, which show that the surface morphologies of these ZnGaO films are very similar to each other.


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However, as shown in Figure 3a–e, we can find differences in the surface morphology between the ZnGaO films deposited at DEZn flow rates of 10 and 30–60 sccm. The ZnGaO film grown at 10 sccm exhibited a column structure with a thickness of 210 nm. As the DEZn flow rate was higher than 30 sccm, the ZnGaO films showed a film-type structure, and the thicknesses of ZnGaO films prepared at DEZn flow rates of 30, 40, 50, and 60 sccm were 118, 115, 90, and 90 nm, respectively. According to our observations, the column structure is loose (Figure 3a), and the film-type structure is dense (Figure 3b–e). In addition, β-Ga2O3 and ZnGa2O4 adopt monoclinic and cubic close-packed structures, respectively, and the cubic close-packed ZnGa2O4 possesses a larger cell density than the monoclinic Ga2O3. As in the case mentioned above, the film prepared at a DEZn flow rate of 10 sccm exhibited a β-Ga2O3 structure with a higher growth rate, whereas the films grown at 30–60 sccm had a ZnGa2O4 structure with a lower growth rate. The growth rate of the ZnGaO films prepared at DEZn flow rates of 50–60 sccm is slightly lower than that at 30–40 sccm. This is probably why the film prepared at DEZn flow rates of 50–60 sccm is more close to the ZnGa2O4 structure, as indicated by Figure 1b. Based on the XRD, TEM, and SEM results, we can confirm that the Zn addition into the ZnGaO film prepared at a DEZn flow rate of 10 sccm had a doping effect on the Ga2O3 structure. Moreover, the Zn added into the ZnGaO films prepared at DEZn flow rates of 30–60 sccm reacted with Ga and O atoms to form the ZnGa2O4 structure. X-ray photoelectron spectroscopy (XPS) measurements can be used to quantitatively and qualitatively characterize the chemical properties of these ZnGaO films. Figure 4a–c shows the O1s, Ga2p3, and Zn2p3 core level XPS spectra of these ZnGaO films prepared at various DEZn flow rates, respectively. These XPS spectra were all fitted well with Gaussian functions. As shown in Figure 4a, when the ZnGaO films were grown at DEZn flow rates of 10, 30, 40, 50,


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and 60 sccm, the O1s peaks are located at 530.8, 530.8, 531.0, 531.0, and 531.2 eV, respectively. In general, because of the formation of O2- ions in oxygen-deficient regions, the O1s peak would appear at around 531 eV,13 resulting in non-stoichiometric oxides. In Figure 4b, the Ga2p3 peaks of these five films are located at 1118.0, 1118.0, 1118.2, 1118.4, and 1118.4 eV, respectively. Compared with the elemental Ga with a binding energy of 1116.6 eV,14 the Ga2p3 peaks of ZnGaO films exhibit a positive shift, revealing that the Ga atoms in these films are in compounds, not in the elemental state. Besides, the Zn2p3 peaks of these ZnGaO films appear at 1021.8, 1022.0, 1021.6, 1021.8, and 1021.6 eV, respectively, as shown in Figure 4c. The binding energy of the Zn2p3 peak for pure Zn metal is about 1021.0 eV.15 This indicates that the Zn2+ added to the films forms part of a compound. The Zn, Ga, O, and C contents of the ZnGaO films determined by XPS measurements are summarized in Table 1. It is worthy to mention that the thickness of ZnGaO is very thin ( 90-128 nm). The chemical compositions are almost the same using the profile(no shown data) of XPS analysis. It can be seen that the Ga content of the ZnGaO film gradually decreased from 46.4 to 26.9 at.%. Meanwhile, the Zn contents of ZnGaO films deposited at DEZn flow rates of 10, 30, 40, 50, and 60 sccm are 2.0, 8.1, 10.9, 10.5, and 10.0 at.%, respectively. This implies that the Zn addition into the film is saturated as the DEZn flow rate is increased to 40 sccm. Moreover, the C contents of the films grown at 10–40 sccm were very low (0.9–1.4 at.%), but increased to 3.7 and 18.9 at.% at 50 and 60 sccm, respectively. The chemical formula of DEZn is (C2H5)2Zn, and the addition of C into these films could originate from the DEZn source. During epilayer growth, C2H5 is easily decomposed to C and H, and most of the C atoms can be pumped out of the chamber. However, some of the C atoms would take part in the reaction during epilayer growth. In this study, most of the C atoms were pumped out at DEZn flow rates of 10–40 sccm. Upon increasing DEZn flow rate to 50–60 sccm,


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more C atoms cannot immediately be pumped out of the chamber and, thus, react during the ZnGaO deposition. The ratios of Ga/Zn and O/(Zn+Ga) for these five ZnGaO films are also shown in Table 1. When ZnGaO films were prepared at DEZn flow rates of 10, 30, 40, 50, and 60 sccm, their Ga/Zn ratios were evaluated to be 23.2, 4.84, 3.55, 3, and 2.69, respectively. The extremely high Ga/Zn ratio of 23.2 can prove again that the Zn atoms only play the role of dopant in the ZnGaO film prepared at 10 sccm. The decreased Ga/Zn ratio with increasing DEZn flow rate implies that the composition of the ZnGaO films gradually approaches that of perfect ZnGa2O4 (Ga/Zn: 2). This result can also explain why the crystal structure of the ZnGaO film becomes more and more close to that of perfect ZnGa2O4 as the DEZn flow rate is increased from 30 to 60 sccm, as shown in Figures 1b and 2. Besides, the O/(Zn+Ga) ratios of these ZnGaO films were determined to be 1.05, 1.08, 0.99, 1.29, and 1.20 for DEZn flow rates of 10, 30, 40, 50, and 60 sccm, respectively. For pure Ga2O3 and ZnGa2O4 materials, the O/Ga and O/(Zn+Ga) ratios are 1.5 and 1.33, respectively. Thus, for the ZnGaO film grown at a DEZn flow rate of 10 sccm, the amount of oxygen vacancies in the film is very large compared with pure Ga2O3. Compared with pure ZnGa2O4, the amounts of oxygen vacancies in the ZnGaO films prepared at 30 and 40 sccm are still large. Nevertheless, the two ZnGaO films prepared at 50 and 60 sccm possess fewer oxygen vacancies than the other films. This reveals that fewer point defects (net vacancies) were formed in these two ZnGaO films. In general, a well-known mechanism in n-type oxide semiconductor is that O vacancies and Ga interstitial can generate free electrons in the conductor band and works as shallow donors. It means that O vacancies and Ga interstitial were positive ions.16-18 These effective positive ions would affect the electrical properties of ZnGaO epilayers.


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Figure 5 shows the variations in carrier concentration, mobility, and resistivity of ZnGaO epilayers as a function of DEZn flow rate from 30 to 60 sccm. It should be mentioned that the electrical properties of the ZnGaO epilayers prepared at a DEZn flow rate of 10 sccm are either too low or too high to be detected. Based on our knowledge, the electrical properties of β-Ga2O3 are very poor. Although less Zn2+ ions were doped into the Ga2O3 structure as the ZnGaO film was grown at a DEZn flow rate of 10 sccm, the extremely low Zn content (2.0 at.%) cannot be beneficial for improving the electrical properties of the film significantly. With increasing DEZn flow rate from 30 to 60 sccm, the carrier concentration of ZnGaO epilayers increased from 1.94 × 1014 to 6.72 × 1016 cm-3, while the resistivity reduced from 5730 to 67.9 Ω-cm. This indicates that increasing the DEZn flow rate is helpful to enhance the electrical properties of ZnGaO films. The electrical resistivities of ZnO-based films are mainly governed by the amount of oxygen vacancies.19 In other words, more oxygen vacancies formed in ZnO-based films may result in lower resistivities. As shown in Table 1, oxygen vacancies were formed in all ZnGaO films. Especially for the ZnGaO films deposited at DEZn flow rates of 10, 30, and 40 sccm, large amounts of oxygen vacancies could be produced in these three films. Apparently, the trend in the electrical resistivities of these ZnGaO films does not agree with that in the amounts of oxygen vacancies. This implies that the electrical resistivity of Ga2O3 or ZnGa2O4 is not dominated by the formation of oxygen vacancies. On the other hand, the saturation of Zn content in ZnGaO films occurred when the DEZn flow rate was higher than 40 sccm (Table 1). Therefore, according to our observations, the continuous reduction in the electrical resistivity of ZnGaO films prepared at 50 and 60 sccm could be attributed to the increment of C content. Generally, the residual C atoms would lead to the formation of defects in the film. However, in our study, the electrical properties of ZnGaO films can be improved by the residual C atoms. Additionally,


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the mobilities of ZnGaO films grown at DEZn flow rates of 30, 40, 50, and 60 sccm were 5.6, 2.8, 2.3, and 1.4 cm2/Vs, respectively. In other words, a higher mobility can be achieved by the ZnGaO film prepared at a DEZn flow rate of 30 sccm, and the mobility decreased gradually with increasing DEZn flow rate. As shown in Figure 2, the microstructural deformation shown in the ZnGaO films prepared at DEZn flow rates of 40–60 sccm was due to the formation of lattice distortion, resulting from there being relatively more C added to the crystal structure owing to the small size of C atoms. Moreover, the lattice arrangement of the ZnGaO film grown at 30 sccm was more regular than that of these three films. Figure 6a and b shows the high-resolution TEM images of ZnGaO films prepared at DEZn flow rates of 30 and 60 sccm. The TEM magnification taken for Figure 6 is higher than that for Figure 2. It is clear that the ZnGaO film grown at 30 sccm indeed possesses less lattice distortions (Figure 6a). With increasing the DEZn flow rate to 60 sccm, the lattice distortion phenomenon in the ZnGaO film becomes more obvious (Figure 6b). Moreover, the lattice distortion in the film would lead to a reduction in mobility.19,20 This can explain why the mobility of ZnGaO films reduced gradually when the DEZn flow rate was higher than 40 sccm. Strangely, as shown in Table 1, the C contents of ZnGaO films grown at DEZn flow rates of 40, 50, and 60 sccm are 0.9, 3.7, and 18.9 at.%, respectively. As mentioned above, the ZnGaO film prepared at 60 sccm had the lowest Ga/Zn ratio of 2.69 compared with the other films, indicating this film possessed the largest relative amount of the Zn in the crystal structure and was close to the ZnGa2O4 composition. Nevertheless, this film possessed the largest C amount which resulted in the lattice distortion shown in Figures 2 and 6b. As a result, the lowest mobility was obtained for this ZnGaO film.


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Figure 7a and b shows the high-resolution TEM images taken from the interfaces between sapphire and ZnGaO films that prepared at the DEZn flow rates of 10 and 60 sccm, respectively. Additionally, various atoms such as Zn, Ga, O, and Al were analyzed and marked in these figures. According to the atomic arrangements, the crystal structures of ZnGaO films deposited at the DEZn flow rates of 10 and 60 sccm belonged to Ga2O3 and ZnGa2O4, respectively, which agreed well with the XRD results again. On the other hand, the atomic arrangement of the ZnGaO film grown at the DEZn flow rate of 10 sccm is much more unclear in comparison to that prepared at the DEZn flow rate of 60 sccm. This indicates that the crystal quality of the ZnGaO film deposited at the DEZn flow rate of 10 sccm is lower than that of the ZnGaO film deposited at the DEZn flow rate of 60 sccm. The obtained results are very well consistent with the above data measured by XRD. Finally, the optical properties of ZnGaO films were measured by CL at room temperature. Figure 8a shows the CL spectra of ZnGaO films prepared at DEZn flow rates of 30–60 sccm. There existed an obvious luminescence peak at a wavelength of 332 nm in all epilayers, and its corresponding Eg value was evaluated to be 3.73 eV. The strong emission band located at 332 nm is commonly due to a radiative carrier transition from the donor level to the valence band. To further investigate the CL characteristics of these films, we reduced the range of the y-axis of Figure 8a, as shown in Figure 8b. Besides the strong emission band at 332 nm, two weak luminescence peaks at wavelengths of 236 and 499 nm can be also found in all films, and their Eg values are 5.25 and 2.48 eV, respectively. By incorporating Zn into Ga2O3 to form ZnGa2O4, it would result in a donor–acceptor pair transition. Therefore, the intrinsic green emission band (499 nm) can be efficiently suppressed. The luminescence peak at 236 nm could be attributed to the transition of electrons from the conduction band to the valence band, and its Eg value (5.25


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eV) is very close to the theoretical Eg value of ZnGa2O4 (5.2 eV).1 This result can prove again that the ZnGaO films prepared at DEZn flow rates of 30–60 sccm indeed exhibit the ZnGa2O4 structure, and the two distinct luminescence peaks centered at 236 and 332 nm reveal that these ZnGaO films are highly promising for short-wavelength applications. To expand on the structural trends observed with high-resolution TEM, we undertook a series of first-principles calculations to study the phase stability of Ga2O3 and ZnGa2O4. In all calculations, we adopted the generalized gradient approximation PW91 (GGA-PW91) methodology21,22 as implemented in the Vienna ab initio simulation package (VASP).23–25 Ga8O12 (Ga2O3, space group: C2/m, monoclinic) and Zn8Ga16O32 (ZnGa2O4, space group: Fd-3m, cubic) were optimized by ab initio GGA-PW91 calculations. A plane wave cutoff energy of 500 eV and all atomic positions are optimized by full relaxation to zero force positions. The convergence of electronic properties was achieved with a kinetic energy cutoff of 500 eV and 30 (Ga8O12) and 4 (Zn8Ga16O32) irreducible k-points generated within the first Brillouin zone. The optimized lattice parameters a, b, and c of Ga8O12 (Zn8Ga16O32) are 12.19 Å, 3.04 Å, and 5.81 Å (8.29 Å, 8.29 Å, and 8.29 Å), respectively. The elastic properties of Ga8O12 and Zn8Ga16O32 were also calculated and are shown in Table 2. Zn8Ga16O32 has higher longitudinal compression (C11), transverse expansion (C12), and shear modulus (C44) compared with Ga8O12. The bulk modulus B, from which B = (C11+2C12)/3, shows that Zn8Ga16O32 has higher resistance under hydrostatic pressure compared with Ga8O12. The Young’s modulus E (i.e., stress divided by strain) can be expressed as E = (C11-C12)(C11+2C12)/(C11+C12). We show that Ga8O12 (E = 122.12 GPa) is more elastically soft than Zn8Ga16O32 (E = 165.56 GPa). The elastic coefficient 2C12/C11 is the ratio of out-of-plane to in-plane strain. Interestingly, 2C12/C11 of both Ga8O12 and Zn8Ga16O32 have nearly the same value of 1.15. Our results suggest that Ga8O12 could relax strain in a lattice-


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mismatched epilayer. However, it should be noted that tensile stress is generated in the Zn-doped Ga8O12 upon increasing Zn concentration, in accordance with the above experimental observations (see Figure 2). This implies that Zn-doped Ga8O12 is no longer a stable phase upon increasing the Zn concentration, but Zn8Ga16O32 is. Figure 9 reveals bulk energies per unit cell with different interplanar spacings of Ga8O12(-201) and Zn8Ga16O32(111). The calculated equilibrium interplanar spacings (deq) of Ga8O12(-201) and Zn8Ga16O32(111) are 4.68 Å and 4.78 Å, respectively, in agreement with the experimental results showing 4.69 Å for Ga8O12(-201) and 4.731–4.786 Å for ZnGa2O4(111). Our results suggest that sufficient tensile strain in Zn-doped Ga8O12 provides a driving force for Zn8Ga16O32 formation. Tensile strain energies in Zn-doped Ga8O12 are generated under the tensile biaxial strain due to the Zn incorporation into the crystal structure. The tensile region of Ga8O12 (d > deq) is denoted by the shaded region in Figure 9. Upon increasing the Zn concentration and sufficient expansion, Zn-doped Ga8O12 is no longer a stable phase but Zn8Ga16O32 is a stable minimum corresponding to deq = 4.78 Å. Finally, we are interested in epitaxial stabilization of Zn8Ga16O32. Using harmonic elasticity theory, the epitaxial qharm(Gˆ )  1 

stabilization is expressed in the form of an equation:

B C11  2 harm , which depends

ˆ on the growth direction G .26 Here, Δ = C44－0.5(C11－C12), and the bulk modulus B and elastic stiffnesses C11, C12, and C44 are listed in Table 2. For cubic lattice, the geometry factor

 harm is

2 4 2 given by:  harm sin (2 )  sin ( ) sin (2 ) , where  and  are the polar and azimuth angle in

the spherical polar coordinate, respectively. In Figure 10, it can be clearly seen that the

qharm () is the highest and the qharm () is much softer than or , where d eq (4.78 Å) is the equilibrium interplanar spacing of Zn8Ga16O32(111).


14

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In this study, we have thoroughly characterized MOCVD-grown ZnGa2O4 films. Most importantly, the correlations between crystal, microstructural, morphological, compositional, electrical, and CL properties of these ZnGaO films were also analyzed in detail, with emphasis on their electrical properties (carrier concentration, mobility, and resistivity). As far as we know, the electrical properties of ZnGa2O4 films are similar to those of Ga2O3 films and are hard to improve. This could be the reason why almost no research into ZnGa2O4 films has focused on the electrical properties until now. Our study has demonstrated MOCVD-grown singlecrystalline ZnGa2O4 films with wide bandgaps, mobilities of 1.4–5.6 cm2/Vs, and a lowest electrical resistivity of 67.9 Ω-cm. This reveals that these ZnGaO films have great potential for optoelectronic and microelectronic applications such as photodetectors, light-emitting diodes, and next-generation semiconductor power devices. CONCLUSIONS In summary, ZnGaO films were prepared by MOCVD at DEZn flow rates of 10–60 sccm, and their structural, morphological, compositional, electrical, and CL characteristics were investigated in detail. At a lower DEZn flow rate of 10 sccm, the crystal structure of ZnGaO films was (-201)-oriented single-crystalline β-Ga2O3, and a (111)-oriented single-crystalline ZnGa2O4 structure can be obtained at DEZn flow rates of 30–60 sccm. Since the ZnGaO film prepared at a DEZn flow rate of 10 sccm was similar in nature to β-Ga2O3, its electrical properties were very poor. When the DEZn flow rate was varied from 30 to 60 sccm, the carrier concentration of ZnGaO films increased from 1.94 × 1014 to 6.72 × 1016 cm-3, and the resistivity reduced from 5730 to 67.9 Ω-cm. The XPS results indicate that the Zn content in ZnGaO films saturates as the DEZn flow rate is higher than 40 sccm, whereas the C content increases significantly when increasing the DEZn flow rate to 50–60 sccm. The increment of C content can


15


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Page 16 of 31

improve the electrical properties of ZnGaO films. The ZnGa2O4 structure obtained in the ZnGaO films (DEZn flow rates: 30–60 sccm) can be also proved by CL results, which show that the luminescence peak centered at 236 nm (5.25 eV) occurred in all films. Additionally, the CL results show that these ZnGaO films are highly promising for ultraviolet applications.

Acknowledgements This work was financially supported by the Ministry of Science and Technology (Taiwan, R.O.C.) under the Contract Nos. MOST 104-2221-E-005-031-MY3 and 105-2221-E-009-183MY3.


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(a) (311)

JCPDS: 38-1240

(511)/(333) (220)

(400)

ZnGa O (222)

ZnGa O (111) 2

(422)

(222)

(111)

2

4

ZnGa O (333)

4

2

4

DEZn= 60 sccm

Intensity (a.u.)

DEZn= 50 sccm

DEZn= 40 sccm

DEZn= 30 sccm Ga O (-402) 2

Ga O (-201) 2

3

20

3

DEZn= 10 sccm

30

40

50

Ga O (-603) 2

3

60

Two theta (degree)

(b)

ZnGa2O4(2 2 2)

Ga2O3(-4 0 2)

37.344 37.73

38.394

DEZn: 60 sccm 37.77

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


DEZn: 50 sccm 37.89 DEZn: 40 sccm 37.93 DEZn: 30 sccm 38.17 DEZn: 10 sccm

37.0

37.5

38.0

38.5

Two theta (degree) Figure 1. (a) XRD patterns of ZnGaO films prepared at various DEZn flow rates of 10-60 sccm and (b) XRD patterns shown in Fig. 1(a) with the limited 2θ range from 36.9 to 38.7. JCPDS data of the normal spinel ZnGa2O4 (card no. 38-1240) is given for reference.


17


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Page 18 of 31

Figure 2. High-resolution TEM bright field images of ZnGaO films deposited at various DEZn flow rates of (a) 10, (b) 30, (c) 40, (d) 50, and (e) 60 sccm. (f) SAED pattern of the ZnGaO film shown in Fig. 2(e)


18

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


. Figure 3. Cross-sectional and corresponding plan-view images of SEM for ZnGaO films deposited at the DEZn flow rates of 10, 30, 40, 50, and 60 sccm.


19


(a)

O1s DEZn: 10 sccm

Intensity (a.u.)

DEZn: 30 sccm DEZn: 40 sccm DEZn: 50 sccm DEZn: 60 sccm

526

528

530

532

534

536

538

540

Binding energy (eV)

(b)

3

Ga2p

Intensity (a.u.)

DEZn: 10 sccm DEZn: 30 sccm DEZn: 40 sccm DEZn: 50 sccm DEZn: 60 sccm

1110

1115

1120

1125

1130

1135

Binding energy (eV)

(c)

Zn2p

1015

3

DEZn: 10 sccm DEZn: 30 sccm DEZn: 40 sccm DEZn: 50 sccm DEZn: 60 sccm

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 31

1020

1025

1030

1035

1040

Binding energy(eV)

Figure 4. (a) O1s, (b) Ga2p3, and (c) Zn2p3 core level XPS spectra of ZnGaO films prepared at various DEZn flow rates. Dotted lines showed the binding energies of elemental O1s, Ga2p3 and Zn2p3, respectively.


20

-3

2 (cm /Vs) C (cm )  10

800

C: concentration

600 400 200 0 6

: mobility

4 2 0

 (-cm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


14

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10

4

10

3

10

2

10

1

: resistivity

30

40

50

60

DEZn flow rate (sccm)

Figure 5. Variations in carrier concentration, mobility, and resistivity of ZnGaO films as a function of DEZn flow rate from 30 to 60 sccm.


21


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Page 22 of 31

Figure 6. High-resolution TEM bright field images of ZnGaO films prepared at the DEZn flow rates of (a) 30 and (b) 60 sccm. The TEM magnification taken for Fig. 6 is higher than that for Fig. 2.


22

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. High-resolution TEM bright field images taken from the interfaces between sapphire and ZnGaO films that prepared at the DEZn flow rates of (a) 10 and (b) 60 sccm. The atoms of various elements are analyzed and marked in these figures.


23


Intensity (a.u.)

(a)

3.73eV

CL spectra DEZn: 30 sccm DEZn: 40 sccm DEZn: 50 sccm DEZn: 60 sccm

5.25eV

200

300

400

500

600

Wavelength (nm)

(b)

CL spectra DEZn: 30 sccm DEZn: 40 sccm DEZn: 50 sccm DEZn: 60 sccm

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 31

200

300

400

500

600

Wavelength (nm)

Figure 8. (a) CL spectra of ZnGaO films prepared at the DEZn flow rates of 30-60 sccm and (b) CL spectra shown in Fig. 6(a) with the lowered scale range of y-axis.


24

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Zn8Ga16O32(111)

Bulk energies per unit cell (eV/Å3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ga8O12(-201)

Ga2O3 Ĝ[-201]

ZnGaO Tensile region

Ĝ[111]

Zn8Ga16O32

Interplanar spacing (Å)

Figure 9. Bulk energies per unit cell of Ga8O12(-201) and Zn8Ga16O32(111) as a function of interplanar spacings. The shade region indicates the tensile region leads more unfavorable Ga8O12 and more favorable Zn8Ga16O32 structures. The atoms are represented by spheres: Ga (Brown, large), Zn (purple, middle), and O (red, small).


25


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Page 26 of 31

Zn8Ga16O32 (deq = 4.78 Å) [001]

[111]

[010] [100]

Figure 10. Epitaxial softening function for Zn8Ga16O32 at deq = 4.78 Å. The hollow along and bulge along or represent the epitaxial softening and hardening behavior, respectively.


26

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Elemental compositions (Ga, Zn, O, and C), the ratios of Ga/Zn and O/(Zn+Ga) for ZnGaO films prepared at various DEZn flow rates of 10-60 sccm.

DEZn flow rate (sccm)

Ga (at.%)

Zn (at.%)

O (at.%)

C (at.%)

Ga/Zn

O/(Zn+Ga)

10

46.4

2.0

50.7

0.9

23.2

1.05

30

39.2

8.1

51.3

1.4

4.84

1.08

40

38.7

10.9

49.5

0.9

3.55

0.99

50

31.5

10.5

54.3

3.7

3

1.29

60

26.9

10.0

44.2

18.9

2.69

1.20

Table 2. Summary of calculated elastic constants and coefficients for Ga8O12 and Zn8Ga16O32.

Structure

Space

C11

C12

C44

B

E

group

(GPa)

(GPa)

(GPa)

(GPa)

(GPa)

2C12/C11

Ga8O12

C2/m

210.55

121.09

99.03

150.91

122.12

1.15

Zn8Ga16O32

Fd-3m

285.94

164.69

116.69

205.11

165.56

1.15


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Page 28 of 31

AUTHOR INFORMATION Corresponding Author *Prof. Ray-Hua Horng, Tel: +886-3-5712121 ext. 54138, E-mail: [email protected] Author Contributions The paper was written through contributions of all authors. All authors have given approval to the final version of the paper. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the Ministry of Science and Technology (Taiwan, R.O.C.) under the Contract Nos. MOST 104-2221-E-005-031-MY3 and 105-2221-E-009-183-MY3. REFERENCES (1) Omata, T.; Ueda, N.; Ueda, K.; Kawazoe, H. Appl. Phys. Lett. 1994, 64, 1077–1078. (2) Kim, J. S.; Kang, H. I.; Kim, W. N.; Kim, J. I.; Choi, J. C.; Park, H. L.; Kim, G. C.; Kim, T. W.; Hwang, Y. H.; Mho, S. I.; Jung, M. C.; Han, M. Appl. Phys. Lett. 2003, 82, 2029–2031. (3) Li, Y. J.; Lu, M. Y.; Wang, C. W.; Li, K. M.; Chen, L. J. Appl. Phys. Lett. 2006, 88, 143102-1–143102-3. (4) Lou, Z.; Li, L.; Shen, G. Nano Res. 2015, 8, 2162–2169. (5) Hu, J. Q.; Bando, Y.; Liu, Z. W. Adv. Mater. 2003, 15, 1000–1003. (6) Chen, L.; Jiang, D.; Liu, X.; Qiu, G. Chemphyschem 2014, 15, 1624–1631. (7) Park, K. W.; Yun, Y. H.; Choi, S. C. J. Electroceram. 2006, 17, 263–266.


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(8) Oshima, T.; Niwa, M.; Mukai, A.; Nagami, T.; Suyama, T.; Ohtomo, A. J. Cryst. Growth 2014, 386, 190–193. (9) Kim, Y. J.; Jeong, Y. H.; Kim, K. D.; Kang, S. G.; Lee, K. G.; Han, J. I.; Park, Y. K.; Cho, K. I. J. Vac. Sci. Technol. B 1998, 16, 1239–1243. (10) Reshmi, R.; Krishna, K. M.; Manoj, R.; Jayaraj, M. K. Surf. Coat. Tech. 2005, 198, 345– 349. (11) Yi, S. S.; Kim, I. W.; Bae, J. S.; Moon, B. K.; Kim, S. B.; Jeong, J. H. Mater. Lett. 2002, 57, 904–909. (12) Shen, Y. S.; Wang, W. K.; Horng, R. H. IEEE J. Electron Devices Soc. 2017, 5, 112–116. (13) Xiao, Z.; Liu, Y.; Zhang, J.; Zhao, D.; Lu, Y.; Shen, D.; Fan, X. Semicond. Sci. Technol. 2005, 20, 796–800. (14) Briggs, D.; Seah, M. P. Practical Surface Analysis, Wiley, New York, USA, 1979. (15) Lee, J. K.; Tewell, C. R.; Schulze, R. K.; Nastasi, M.; Hamby, D. W.; Lucca, D. A.; Jung, H. S.; Hong, K. S. Appl. Phys. Lett. 2005, 86, 183111-1–183111-3. (16) Wang, Z. L.; Yin, J. S.; Jiang, Y. D. Micron 2000, 31, 571–580. (17) Yu, F. P.; Ou, S. L.; Wuu, D. S. Opt. Mater. Express 2015, 5, 1240–1249. (18) Yao, J.; Xu, N.; Deng, S. Chen, J; She, J; Shieh, H. P. D.; Liu, P. T. Huang, Y. P; IEEE Trans. Electron Devices, 2011, 58, 1121–1126. (19) Ou, S. L.; Liu, H. R.; Wang, S. Y.; Wuu, D. S. J. Alloys Compd. 2016, 663, 107–115. (20) Heo, Y. W.; Norton, D. P.; Pearton, S. J. J. Appl. Phys. 2005, 98, 073502-1–073502-6. (21) Kresse, G.; Joubert, D. Phys. Rev. B 1999, 59, 1758–1775. (22) Perdew, J. P.; Chevary, J. A.; Vosko. S. H.; Jackson, K. A.; Petersen, M. R.; Fiolhais, C. Phys. Rev. B 1992, 46, 6671–6687.


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(23) Kresse, G.; Furthmüller, J. Comp. Mater. Sci. 1996, 6, 15–50. (24) Kresse, G.; Furthmüller, J. Phys. Rev. B 1996, 54, 11169–11186. (25) Kresse, G.; Hafner, J. J. Phys. Condens. Matter. 1994, 6, 8245–8257. (26) Ozoliņš, V.; Wolverton, C.; Zunger, A. Phys. Rev. B 1998, 57, 4816–4828.


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For Table of Contents Use Only Epitaxial Growth of ZnGa2O4: A New Deep Ultraviolet Semiconductor Candidate Ray-Hua Horng, Chiung-Yi Huang, Sin-Liang Ou, Tzu-Kuang Juang, and Po-Liang Liu

Zn8Ga16O32(111)

Bulk energies per unit cell (eV/Å3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ga8O12(-201)

Ga2O3 Ĝ[-201]

ZnGaO Tensile region

Ĝ[111]

Interplanar spacing (Å) ZnGa2O4 is a new deep ultraviolet semiconductor candidate. The related epilayers and phase stability are grown by MOCVD and calculated by first-principles, respectively. Not only the theoretically prediction, the measured data of XRD, TEM, XPS and CL also demonstrat that the sigle crystal ZnGa2O4 can be epitaxial growth on sapphire substrate.


31

Epitaxial Growth of ZnGa2O4: A New, Deep Ultraviolet Semiconductor

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