Polarity-Controlled GaN Epitaxial Films Achieved via Controlling the

Jun 26, 2018 - (4−6) GaN crystal, normally with a wurtzite structure, lacks an ... Researches have shown the successful epitaxial growth of c-plane ...
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Polarity-Controlled GaN Epitaxial Films Achieved via Controlling the Annealing Process of ScAlMgO4 Substrates and the Corresponding Thermodynamic Mechanisms Yulin Zheng,† Wenliang Wang,*,†,‡ Xiaochan Li,† Yuan Li,† Tao Yan,§ Ning Ye,§ and Guoqiang Li*,†,‡ †

State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China Guangdong Choicore Optoelectronics Co. Ltd., Heyuan 517003, China § Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China

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‡

S Supporting Information *

ABSTRACT: Polarity-controlled GaN epitaxial films with Ga- and N-polarity are intentionally obtained by controlling annealing processes of ScAlMgO4 substrates. It is proved by high-angle annular dark-field scanning transmission electron microscopy, in situ reflection high-energy electron diffraction, and scanning electron microscopy that the GaN epitaxial films grown on ScAlMgO4 substrates annealed in an hydrogen atmosphere reveal Ga-polarity, whereas those grown on ScAlMgO4 substrates annealed in an ambient atmosphere show N-polarity. The thermodynamic mechanism of determining the crystal polarity is systematically investigated by first-principles calculations. It is concluded that N atoms prefer to deposit on the top of Ga atoms adsorbed onto Al(Mg)-face ScAlMgO4 substrates, leading to Ga-polarity, whereas Ga atoms preferentially stay on the central of three N atoms stacked onto the O-face ScAlMgO4 substrates, resulting in N-polarity. This work demonstrates a promising approach for the fabrication of GaN-based devices with different polarities for various applications, such as the Ga-polarity GaN used for light-emitting diodes, whereas the N-polarity GaN adopted for enhanced high electron mobility transistors, etc. mismatch between GaN and sapphire,14,15 high-density dislocations and large stress are existed in epitaxial films. Therefore, the controlled growth of c-axis-oriented polarity GaN epitaxial films on sapphire substrates is relatively hard. Hence, ScAlMgO4 is expected to be one of the alternative substrates owing to the very small lattice and CTE mismatches between GaN and ScAlMgO4 of −1.8 and 9.7%, respectively.16,17 Therefore, it is scientifically feasible to realize highquality c-plane polarity-controlled GaN epitaxial films on ScAlMgO4 substrates. Researches have shown the successful epitaxial growth of c-plane GaN on ScAlMgO4 substrates.17−19 Despite these reports, intentionally controlling the polarity of GaN epitaxial films on ScAlMgO 4 substrates is not implemented yet, and the corresponding thermodynamic mechanisms are still unknown. Here, we present the polarity-controlled GaN epitaxial films, achieved via effectively controlling the annealing process of ScAlMgO4 substrates experimentally and the corresponding thermodynamic mechanisms theoretically. The ∼300-nm-thick

1. INTRODUCTION In recent years, we have witnessed the prevalence of GaN and its related compounds due to their surpassing properties,1−3 making them suitable for a broad application in solid-state lighting source, power electronic, microwave radio frequency, and other optoelectronic devices.4−6 GaN crystal, normally with a wurtzite structure, lacks an inversion plane along the caxis direction, causing a large spontaneous polarization field.7,8 Therefore, the crystal surfaces have inversed crystal polarity of either Ga-polarity (designated (0001)) or N-polarity (designated (0001̅)).8−10 On the one hand, Ga-polarity GaN epitaxial films are generally favorable for the fabrication of high-efficiency light-emitting diodes,4 laser diodes,5 and ultraviolet detectors,6 etc., due to their high crystalline quality and smooth surface morphology.11 On the other hand, Npolarity GaN epitaxial films are suitable for the manufacture of high-frequency normally off metal−insulator−semiconductor high electron mobility transistors and high-speed microwave transistors,12,13 etc., owing to their low Ohmic contact resistance and stronger suppression of short channel effects.13 To date, commercial GaN films-based devices are mostly based on sapphire substrates. However, due to the large lattice mismatch and coefficient of thermal expansion (CTE) © XXXX American Chemical Society

Received: May 10, 2018 Revised: June 26, 2018 Published: June 26, 2018 A

DOI: 10.1021/acs.jpcc.8b04410 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Schematic structures of (a) sample 1 and (b) sample 2; HAADF-STEM images and schematic illustrations of (c) Ga-polarity and (d) Npolarity GaN, where the bright spots in the HAADF images correspond to Ga atom and the dark ones correspond to N atom. The inset shows the simulated micrograph.

°C for 60 min, named sample 2. The schematic structures for samples 1 and 2 are shown in Figure 1a,b. After growth, samples 1 and 2 were both treated with molten KOH by wet chemical etching for 10 min to reveal the polarity of the GaN epitaxial films. 2.2. Characterization. The high-resolution TEM (HRTEM, JEM-2010HR, 200 kV), attached to energydispersive X-ray (EDX) spectrometry system and aberrationcorrected STEM (Titan Cubed Themis G2 300), was performed. We used a focused ion beam (FEI Helios 600i) and an Ar ion milling to prepare TEM specimens. In situ RHEED was used to monitor the whole growth. The as-grown GaN epitaxial films were analyzed by optical microscopy (OM, OLYMPUS, BX51M), high-resolution X-ray diffraction (Bruker D8 X-ray diffractometer with a Cu Kα1 X-ray source, λ = 1.5406 Å), field emission SEM (Hitachi S-4800), X-ray photoelectron spectroscopy (XPS, Thermal ESCALAB 250Xi), and micro-Raman spectroscopy (Renishaw inVia Raman spectrometer with a 532 nm laser as the excitation source). 2.3. Computational Method. The first-principles calculations, based on DFT with the electron exchange and correlation of the Perdew−Burke−Ernzerhof functional form of the generalized gradient approximation20 pseudopotentials, were performed. The ultrasoft pseudopotential plane-wave method was used, and the cutoff energy was set at 1380 eV. The reciprocal space integrations were chosen with the k-point mesh of 4 × 4 × 1 Monkhorst−Pack21 grids for both bulk and slab cases. The as-relaxed ScAlMgO4(0001) slabs contained 21 atomic layers with front six atomic layers fully relaxed by means of convergence test. All slabs had a thick vacuum region (25 Å) along the c-axis.

c-plane GaN epitaxial films were grown on ScAlMgO4(0001) substrates by pulsed laser deposition (PLD). It is proved that the polarity of GaN epitaxial films lies on the surface atomic state of ScAlMgO4 substrates, which are controlled by annealing processes of substrates. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), in situ reflection high-energy electron diffraction (RHEED), and scanning electron microscopy (SEM) measurements suggest that Ga-polarity GaN can be obtained on ScAlMgO4 substrates by annealing the substrates in hydrogen atmosphere, whereas the N-polarity GaN can be grown on ScAlMgO4 substrates by annealing the substrates in ambient atmosphere. Furthermore, the thermodynamic mechanisms for the formation of polarity-controlled GaN epitaxial films were investigated with density functional theory (DFT) calculations. This work provides an effective way to epitaxially grow polarity-controlled GaN films and reveals the corresponding thermodynamic mechanisms, which is of great significance in the fabrication of GaN-based optoelectronic devices with various applications.

2. EXPERIMENTAL AND COMPUTATIONAL METHODS 2.1. Sample Growth. Before growth, the precleaned ScAlMgO4(0001) substrates were annealed at 650 °C and a hydrogen pressure of 3.8 × 10−2 Torr for 60 min to remove the residual oxygen and contaminants on the surface, in an ultrahigh vacuum PLD growth chamber. The GaN epitaxial films were grown on as-prepared substrates by PLD, where the growth conditions are similar to our previous work.17 Accordingly, ∼300-nm-thick GaN epitaxial films were obtained and named sample 1. To investigate the effect of surface atomic state on the polarity of as-grown GaN epitaxial films, the other ∼300-m-thick GaN epitaxial films were grown under the same experimental conditions, except for the substrate annealing process, which was in an ambient atmosphere at 650

3. RESULTS AND DISCUSSION The polarity of as-grown GaN epitaxial films was determined by HAADF-STEM by observing the atomically resolved images. It has been reported that HAADF-STEM is a precise B

DOI: 10.1021/acs.jpcc.8b04410 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 2. (a) Typical OM image and photograph (inset) for as-grown GaN epitaxial films on ScAlMgO4 substrates; (b) XRD 2θ-ω scan for asgrown GaN epitaxial films on ScAlMgO4 substrates; (c) cross-sectional high- and (d) low-magnification TEM images of ∼300-nm-thick GaN epitaxial films on ScAlMgO4 substrates. Selected area electron diffraction (SAED) patterns for (e) GaN epitaxial films and (f) ScAlMgO4 substrates, where spots marked in red and white correspond to GaN and ScAlMgO4 planes, respectively.

Figure 3. RHEED patterns for as-grown GaN epitaxial films of (a) sample 1 and (b) sample 2. The SEM images for as-grown GaN epitaxial films of (c) sample 1 and (d) sample 2; (e) and (f) are the surfaces of GaN epitaxial films after molten KOH etching for (c) and (d), respectively.

software,24 where we choose JEOL 2100FCs (200 kV) as the microscope in a simulation setting. The image patterns were simulated under defocus parameters varying from 20 to 60 nm, with the electron beam incidence along GaN[112̅0]; the image patterns matched with the experimental ones. OM, XRD, XPS, and Raman were adopted to further study the properties of as-prepared samples. Figure 2a−f illustrates the characterization results for sample 1, which are very similar to sample 2. Hence, the repetitious results for sample 2 are not provided. The typical OM image and photograph (insert) confirm that the as-grown GaN epitaxial films are crack-free

structural analysis measurement, which provides a direct interpretation of an atomic configuration due to the Z (atomic number)-contrast nature.22 From the high spatial resolution, it can be identified that the Ga and N columns are apart and the polarity of GaN can be determined.2,23 The typical HAADFSTEM image of sample 1 exhibits Ga-polarity GaN23 with a wurtzite structure (Figure 1c), whereas that of sample 2 reveals wurtzite structure N-polarity GaN23 with a wurtzite structure (Figure 1d). The atom columns in these images are also confirmed by the inset images of simulated micrograph, which are obtained by image simulation embedded in the JEMS C

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Figure 4. (a) Schematic illustration of side views for ScAlMgO4 substrates; (b) seven types of stoichiometric terminations of ScAlMgO4 surface (marked from left to right as: Sc−O, O−MI, M−OI, O−O, O−MII, M−OII, and O−Sc).

migration of GaN precursors, which, of course, is beneficial for the formation of smooth GaN epitaxial films,13,28 whereas Npolarity GaN epitaxial films generally have a noticeable surface roughness due to the existence of hexagonal crystallites after etching.29 Here, the SEM images show that the surfaces of GaN epitaxial films for sample 1 nearly remain smooth after the wet chemical etching (Figure 3c,d), proving Ga-polarity, whereas those for sample 2 are rather rough after the wet chemical etching (Figure 3e,f), revealing N-polarity. By the combination of HAADF-STEM, RHEED, and SEM results, we can conclude that GaN epitaxial films of sample 1 are with Ga-polarity, whereas those of sample 2 are with Npolarity. However, the fundamental thermodynamic mechanisms of determining the crystal polarity are still unclear. Therefore, we performed first-principles calculations based on DFT to reveal this mechanism. By cleaving the ScAlMgO4 bulk along [0001], we find that atomic stacking sequence of the slab unit is −Sc−O−Mg/Al−O−O−Mg/Al−O− (Figure 4a), where the mixture atom Mg/Al contains Mg, with a composition of 0.5. Therefore, in total, seven types of stoichiometric terminations of ScAlMgO4 surfaces are obtained (Figure 4b). To determine the stabilities of different terminations of ScAlMgO4 surfaces, the surface energy Esurface of ScAlMgO4 surfaces was calculated by eq 1, which can be found in the Supporting Information and our previous work.30 According to eq 1, a stable surface model generally has a positive and small value of Esurface.30−32 The calculated Esurface values for seven terminations are summarized in Table S1, which exhibits that both the M−OI termination and O−Sc termination have the smallest average value of 0.038 J/cm2 among all terminations. These results are highly reliable due to the considered acceptable calculation deviation of ±0.0005 J/ cm2.31 The results suggest that the M−OI termination and O− Sc termination are identified to be the most stable terminations of ScAlMgO4 surface and would be employed as slab models in the next calculations. To understand the thermodynamic mechanism of adsorption and migration of GaN precursor atoms on ScAlMgO4 substrates at the initial growth stage, the adsorption energy calculated using DFT was implemented. The adsorption energy Eadsorption is calculated by eq 2 (see Supporting Information and our previous work30). After careful observation of the surfaces, four adsorption sites with high symmetry on M−OI and O−Sc ScAlMgO4 surfaces, labeled as “TO”,

(Figure 2a). Notably, the out-of-plane epitaxial relationship between GaN and ScAlMgO 4 is demonstrated to be GaN(0001)//ScAlMgO4(0001) by XRD 2θ-ω scan17 (Figure 2b). By combining the 2θ-ω scans of GaN(0002) and GaN(101̅2) (Figure S1a), the lattice mismatch between GaN and ScAlMgO4 is calculated to be −1.5%. Meanwhile, the typical XRD φ scan confirms the hexagonal GaN epitaxially grown on ScAlMgO4 substrates (Figure S1b). The clear evidence for GaN is also verified by XPS spectra, which show strong Ga−N bonds (Figure S2). Furthermore, the full-width at half-maximum (FWHM) of GaN(0002) and GaN(101̅2) Xray rocking curves (XRCs) for sample 1 are of 0.20 and 0.40°, respectively (Figure S1c,d). These FWHMs are smaller than those reported before,17,18 suggesting higher crystalline quality.25 In addition, the excellent crystalline quality of epitaxial films was also proved by Raman spectroscopy measurement, which proves the very small residual compressive stress in GaN epitaxial films (Figure S3). In addition, the interfacial properties for these GaN epitaxial films were investigated by HRTEM. The low-magnification cross-sectional TEM image confirms the thickness of GaN epitaxial films to be ∼300 nm (Figure 2d). Moreover, the highmagnification cross-sectional TEM image shows quite abrupt GaN/ScAlMgO4 heterointerfaces, illustrated by the white dashed line in Figure 2c. The results can be ascribed to the very small lattice and CTE mismatches between GaN and ScAlMgO416 and the effective suppression of the interfacial reactions by PLD.17 Moreover, EDX spectroscopy profiles confirm the elements for GaN/ScAlMgO4 heterointerfaces (Figure S4). Furthermore, selected area electron diffraction (SAED) for both GaN epitaxial films and ScAlMgO4 substrates demonstrates that the epitaxial relationships between GaN and ScAlMgO 4 are of GaN[0001]//ScAlMgO 4 [0001] and GaN[11̅00]//ScAlMgO4[11̅00], respectively17 (Figure 2e,f). To further confirm the polarity of GaN epitaxial films for samples 1 and 2, in situ RHEED was employed. Sharp and bright RHEED patterns and clear 1 × 1 reconstruction for sample 1 can be observed (Figure 3a), proving the Ga-polarity GaN epitaxial films with smooth surfaces.26,27 Nevertheless, the apparent 3 × 3 reconstructions for sample 2 can be noted (Figure 3b), confirming the N-polarity GaN epitaxial films.26,27 Moreover, the polarity of GaN epitaxial films can also be identified by etching the surface of GaN.27−29 It is emphasized that Ga-polarity GaN growth surface is favorable for the D

DOI: 10.1021/acs.jpcc.8b04410 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C “TM”, “TSc”, and “H” (Figure 5a,b), are considered. Additionally, the adsorption models are constructed with the coverage

site of ScAlMgO4 substrates annealed in an hydrogen atmosphere, whereas Ga atoms would stay on the TO adsorption site of ScAlMgO4 substrates annealed in an ambient atmosphere. To clarify the thermodynamic mechanism of polaritycontrolled GaN epitaxial films grown on ScAlMgO4 substrates with different annealing processes, the adsorption energies for Ga- and N-polarity GaN on M−OI and O−Sc ScAlMgO4 surfaces were determined. It is known that the polarity of the cplane GaN is determined by its structure.34 An N atom located in the top of a Ga atom results in Ga-polarity GaN (Figure 7a),

Figure 5. Schematic illustration of top views for (a) M−OI and (b) O−Sc ScAlMgO4 surfaces.

of 0.25, which has been widely accepted.30,33 According to eq 2, a stable adsorption model generally has a positive and large value of Eadsorption.34 The calculated Eadsorption values for M−OI and O−Sc ScAlMgO4 surfaces with each Ga and N atoms on high symmetric site are shown in Figure 6. For the M−OI ScAlMgO4 surface, the most stable adsorption sites for Ga and N adatoms are H and TM, respectively. Moreover, the average value of Eadsorption of Ga adatom on the H site of 6.74 eV is larger than that of N adatom on TM site of 5.72 eV, which means that Ga atoms would first adsorb on the M−OI ScAlMgO4 surface and stay on the most stable site (H), whereas on the O−Sc ScAlMgO4 surface, the most stable adsorption sites for Ga and N adatoms are both TO. Incidentally, the average value of Eadsorption for Ga adatom of 4.15 eV is larger than that of N adatom of 3.37 eV on both TO sites, indicating that the Ga atoms would first adsorb on O−Sc ScAlMgO4 surface and then stay on the most stable site of TO.30,34 These results can well explain the experimental annealing processes of substrates. The M−OI ScAlMgO4 surface (Al(Mg)-face) is related to ScAlMgO4 substrates annealed in an hydrogen atmosphere due to the removal of surface oxygen by hydrogen reduction,35,36 which exposes the Al(Mg)-face, whereas the O−Sc ScAlMgO4 surface (O-face) is connected to ScAlMgO4 substrates annealed in an ambient atmosphere owing to the atmospheric oxidation, which exposes the O-face.36 In view of the above discussion, it is safely concluded that Ga atoms would deposit on the H adsorption

Figure 7. (a) Schematic illustration of constructed Ga-polarity GaN on the M−OI ScAlMgO4 surface; (b) the calculated adsorption energies (where the labeled value is the average) for Ga- and Npolarity GaN on O−Sc and M−OI ScAlMgO4 surfaces.

whereas that in the center of the three Ga atoms brings about N-polarity GaN.8,34 The calculated adsorption energies are presented in Figure 7b. In view of the above discussion, on the M−OI ScAlMgO4 surface, the growth of Ga-polarity GaN is more stable than that of N-polarity GaN due to its larger adsorption energy of ∼7.04 eV, indicating that Ga-polarity GaN should predominantly grow on ScAlMgO4 substrates annealed in an hydrogen atmosphere, whereas on the O−Sc ScAlMgO4 surface, the formation of N-polarity GaN is more stable than that of Ga-polarity GaN owing to its larger adsorption energy of ∼7.69 eV, which demonstrates that it is N-polarity GaN that should predominantly form on ScAlMgO4 substrates annealed in an ambient atmosphere.

4. CONCLUSIONS In summary, we have effectively controlled the polarity of GaN epitaxial films grown on ScAlMgO4 substrates by controlling

Figure 6. Calculated adsorption energies for (a) O−Sc and (b) M−OI ScAlMgO4 surfaces with each Ga and N atoms on high symmetric site. E

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(3) Onen, A.; Kecik, D.; Durgun, E.; Ciraci, S. Lateral and vertical heterostructures of h-GaN/h-AlN: electron confinement, band lineup, and quantum structures. J. Phys. Chem. C 2017, 121, 27098−27110. (4) Wang, X.; Peng, W.; Yu, R.; Zou, H.; Dai, Y.; Zi, Y.; Wu, C.; Li, S.; Wang, Z. Simultaneously enhancing light emission and suppressing efficiency droop in GaN microwire-based ultraviolet light-emitting diode by the piezo-phototronic effect. Nano Lett. 2017, 17, 3718− 3724. (5) Sun, Y.; Zhou, K.; Sun, Q.; Liu, J.; Feng, M.; Li, Z.; Zhou, Y.; Zhang, L.; Li, D.; Zhang, S.; Ikeda, M.; Liu, S.; Yang, H. Roomtemperature continuous-wave electrically injected InGaN-based laser directly grown on Si. Nat. Photonics 2016, 10, 595−599. (6) Zhang, Y.; Chen, X.; He, M.; Liu, C.; Yin, Y.; Zou, X.; Li, S.; Wang, X. Ultrafast, superhigh gain visible-blind UV detector and optical logic gates based on nonpolar a-axial GaN nanowire. Nanoscale 2014, 6, 12009−12017. (7) Gleize, J.; Frandon, J.; Renucci, M. A.; Bechstedt, F. Influence of spontaneous and piezoelectric polarizations on the phonon frequencies in strained GaN/AlN superlattices. Phys. Rev. B 2001, 63, No. 073308. (8) Hellman, E. S. The polarity of GaN: a critical review. MRS Internet J. Nitride Semicond. Res. 1998, 3, No. e11. (9) Weyher, J. L.; Zauner, A. R. A.; Brown, P. D.; Karouta, F.; Barcz, A.; Wojdak, M.; Porowski, S. Growth of high quality, MOCVD grown Ga-polar GaN layers on GaN substrates after novel reactive ion etching. Phys. Status Solidi A 1999, 176, 573−577. (10) Wang, X.; Li, S.; Fundling, S.; Wei, J.; Erenburg, M.; Wehmann, H.-H.; Waag, A.; Bergbauer, W.; Strassburg, M.; Jahn, U.; Riechert, H. Polarity control in 3D GaN structures grown by selective area MOVPE. Cryst. Growth Des. 2012, 12, 2552−2556. (11) Sheikhi, M.; Li, J.; Meng, F.; Li, H.; Guo, S.; Liang, L.; Cao, H.; Gao, P.; Ye, J.; Guo, W. Polarity control of GaN and realization of GaN schottky barrier diode based on lateral polarity structure. IEEE Trans. Electron Devices 2017, 64, 4424−4429. (12) Zheng, X.; Li, H.; Guidry, M.; Romanczyk, B.; Ahmadi, E.; Hestroffer, K.; Wienecke, S.; Keller, S.; Mishra, U. K. Analysis of MOCVD SiNx passivated N-polar GaN MIS-HEMTs on sapphire with high f max VDS, Q. IEEE Electron Device Lett. 2018, 39, 409−412. (13) Palacios, T. Beyond the AlGaN/GaN HEMT: new concepts for high-speed transistors. Phys. Status Solidi A 2010, 206, 1145−1148. (14) Hearne, S.; Chason, E.; Han, J.; Floro, J. A.; Figiel, J.; Hunter, J.; Amano, H.; Tsong, I. S. T. Stress evolution during metalorganic chemical vapor deposition of GaN. Appl. Phys. Lett. 1999, 74, 356− 358. (15) Namkoong, G.; Doolittle, W. A.; Brown, A. S.; Losurdo, M.; Capezzuto, P.; Bruno, G. Role of sapphire nitridation temperature on GaN growth by plasma assisted molecular beam epitaxy: Part I. Impact of the nitridation chemistry on material characteristics. J. Appl. Phys. 2002, 91, 2499−2507. (16) Errandonea, D.; Kumar, R. S.; Ruiz-Fuertes, J.; Segura, A.; Haussühl, E. High-pressure study of substrate material ScAlMgO4. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, No. 144104. (17) Wang, W.; Yan, T.; Yang, W.; Zhu, Y.; Wang, H.; Li, G.; Ye, N. Epitaxial growth of GaN films on lattice-matched ScAlMgO4 substrates. CrystEngComm 2016, 18, 4688−4694. (18) Hellman, E. S.; Brandle, C. D.; Schneemeyer, L. F.; Wiesmann, D.; Brener, I.; Siegrist, T.; Berkstresser, G. W.; Buchanan, D. N. E.; Hartford, E. H. ScAlMgO4: an oxide substrate for GaN epitaxy. MRS Internet J. Nitride Semicond. Res. 1996, 1, No. e1. (19) Iwabuchi, T.; Kuboya, S.; Tanikawa, T.; Hanada, T.; Katayama, R.; Fukuda, T.; Matsuoka, T. Ga-polar GaN film grown by MOVPE on cleaved ScAlMgO4(0001) substrate with millimeter-scale wide terraces. Phys. Status Solidi A 2016, 214, No. 1600754. (20) Lu, N.; Guo, H. Y.; Li, L.; Dai, J.; Wang, L.; Mei, W. N.; Wu, X. J.; Zeng, X. C. MoS2/MX2 heterobilayers: bandgap engineering via tensile strain or external electrical field. Nanoscale 2014, 6, 2879− 2886. (21) Moontragoon, P.; Pinitsoontorn, S.; Thongbai, P. Mn-doped ZnO nanoparticles: Preparation, characterization, and calculation of

the annealing processes of substrates and investigated the corresponding thermodynamic mechanisms theoretically. Experimentally, HAADF-STEM accurately exhibited the atomic arrangement and thus directly demonstrated the polarities of GaN epitaxial films grown on ScAlMgO 4 substrates, annealed in an hydrogen and ambient atmosphere. In addition, RHEED and SEM further determined the polarity of GaN epitaxial films. Meanwhile, the thermodynamic mechanisms of determining the crystal polarity are well explained by theoretical calculations within DFT. We have constructed M−OI termination (Al(Mg)-face) and O−Sc termination (O-face) ScAlMgO4 surfaces for calculation, which are related to experimental annealing processes of substrates. The calculated adsorption energies for Ga- and N-polarity GaN on M−OI and O−Sc ScAlMgO4 surfaces suggest that Gapolarity GaN should predominantly grow on ScAlMgO4 substrates annealed in an hydrogen atmosphere, whereas Npolarity GaN should predominantly form on ScAlMgO4 substrates annealed in an ambient atmosphere. This work of effectively growing polarity-controlled GaN epitaxial films is of significance for the manufacture of GaN-based devices of different polarities for various applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b04410.

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XRD 2θ-ω scan and φ scan; XRCs; XPS spectra; Raman spectrum; EDX spectroscopy profiles; Table S1; eqs 1 and 2 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.W.). *E-mail: [email protected] (G.L.). ORCID

Ning Ye: 0000-0002-3679-4047 Guoqiang Li: 0000-0002-1493-6657 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study is supported financially by the National Natural Science Foundation of China (Grant No. 51702102), the National Science Fund for Excellent Young Scholars of China (Grant No. 51422203), the Natural Science Foundation of Guangdong Province (Grant No. 2017A030310518), the China Postdoctoral Science Foundation (Grant Nos 2018T110867 and 2017M610520), and the National Defense Scientific and Technological Innovation Special Zone (Grant No. 17-163-13-ZT-008-029-04).

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REFERENCES

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DOI: 10.1021/acs.jpcc.8b04410 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.8b04410 J. Phys. Chem. C XXXX, XXX, XXX−XXX