Growth and Characterization of M-Plane GaN Thin Films Grown on γ-LiAlO2 (100) Substrates

M-plane GaN thin films were grown on LiAlO2 substrates under different N/Ga flux ratios by plasma-assisted molecular beam epitaxy. An anisotropic growth of M-plane GaN was demonstrated against the N/Ga flux ratio. As the N/Ga flux ratio decreased by increasing Ga flux, the GaN surface trended to a flat morphology with stripes along [112-0]. According to high-resolution X-ray diffraction analysis, Li5GaO4 was observed on the interface between GaN and LiAlO2 substrate. The formation of Li5GaO4 would influence the surface morphology and crystal quality.


Introduction
The wurtzite GaN and its ternary compounds were investigated widely in the last few decades because of their outstanding physical stable properties and the potential for high performance optoelectronic devices [1,2]. Due to the strong piezoelectric and spontaneous polarization fields along theaxis of GaN which were attributed to the dipole and stress asymmetry, the polar -plane GaN-based quantum wells (QWs) suffered from quantum-confined stark effect (QCSE) in the growth direction. As a result, the band structure of -plane GaN would be modified that further reduced the overlap of electron-hole wave functions, which were disadvantages for its optoelectronic performance [3,4]. In order to eliminate the effect of QCSE in the growth direction, a possible solution is to grow nonpolar (e.g., -plane and aplane) GaN-based QWs [5].
The -LiAlO 2 (LAO) substrate, which is tetragonal crystal structure and belongs to the space group P4 1 2 1 2, is an ideal substrate for -plane GaN epitaxial growth. Its lattice constants are identified as a LAO = b LAO = 0.5169 nm and LAO = 0.6268 nm. The LAO substrate shows a greatly small lattice mismatch with -plane GaN. The lattice mismatches between them are −1.7% and −0.3% in [1120] and [0001] directions of GaN, respectively [6][7][8][9]. However, the LAO substrate is hydrolytic and thermally less stable [10]. It is difficult to grow high quality -plane GaN on LAO substrates by general growth methods such as metal-organic chemical vapor deposition (MOCVD) or hydride vapor phase epitaxy (HVPE), which were processing at more than 1000 ∘ C. In the previous studies, we have grown -plane GaN on LAO and misoriented LAO substrates at relatively low growth temperature growth by plasma-assisted molecular beam epitaxy (PAMBE) and have found a large anisotropic growth mechanism and strain within the -plane GaN films [11,12]. In this study, we grew a series of -plane GaN thin films (about 80 nm) by PAMBE and investigated their inplane anisotropic properties.

Materials and Methods
Five samples of -plane GaN thin film, labeled as samples A, B, C, D, and E, were grown on LAO substrates by PAMBE system with standard effusion cell for Ga evaporation (99.9995% purity) and ultra-high pure nitrogen gas (99.9999% purity) supplied in a radio-frequency plasma source (Veeco model GEN 930). The 1 × 1 cm 2 LAO substrates were cut from polished 2 inch wafer, and the LAO crystal ingot was fabricated by using the traditional Czochralski pulling technique. Before mounting on a holder, the LAO substrates were degreased with acetone, isopropanol, and phosphoric acid (H 3 PO 4 : H 2 O = 1 : 30) in an ultrasonic bath for five minutes sequentially and deionized water for a few seconds and then dried with nitrogen gas immediately. Before epitaxial growth, a thermal treatment, out-gassed at 850 ∘ C for 10 minutes, was introduced to the LAO substrates and then the Ga wetting layer was performed for 5 minutes at 800 ∘ C in the MBE growth chamber. The GaN were grown for 30 minutes at 800 ∘ C under different N/Ga flux ratios. The N/Ga flux ratios of samples A, B, C, D, and E were 60.0, 54.5, 52.2, 50.0, and 45.8 (with the Ga flux were 1.00 × 10 −7 , 1.10 × 10 −7 , 1.15 × 10 −7 , 1.20 × 10 −7 , and 1.30 × 10 −7 torr), respectively. The N/Ga flux ratios were evaluated from the beam equivalent pressure of N source against that of Ga source. The in situ reflection high-energy electron diffraction (RHEED) was used to characterize the growth of GaN thin films. The surface morphology was obtained by scanning electron microscope (SEM) (JEOL JSM-6330TF). The structural properties and crystalline preferred orientations were characterized by high-resolution X-ray diffraction (XRD, Bede D1) using a SIEMENS D5000 X-ray diffractometer with a Cu anode and field emission transmission electron microscope (FE-TEM) (Phillips, model Tecnai F-20) with an electron voltage of 200 kV. The cross-sectional TEM specimens were prepared by focus ion beam (FIB) (Seiko Inc., model SII-3050). The optical properties of the samples were analyzed by polardependence photoluminescence (PL) (Horiba, Lab RAM HR Evolution) spectra which was measured using a continuous wave He-Cd laser (325 nm). The anisotropic growth mechanism could be attributed to the anisotropic lattice mismatch and thermal expansion mismatch between -plane GaN and LAO substrates. Consequently, this anisotropic surface diffusion behavior will be the longer growth steps in the [1120] direction due to the lower diffusion barrier and the shorter steps bunching in the [0001] direction due to higher diffusion barrier [13]. According to the SEM image in Figure 2(a), sample A shows the morphology of polycrystalline with small grain size (about 100 nm). Sample A was grown under the highest N/Ga ratio (N/Ga = 60.0). Under such high N/Ga flux ratio (low Ga flux), the LAO substrate would be damaged by the N 2 plasma during growth so that GaN could not be epitaxially grown on the LAO substrate although we had grown Ga parallel to [1120] as shown in Figure 2(e), which was similar to the morphology of samples B and C. The pits and cracks were observed on the upper right side of SEM image, as well.

Results and Discussion
To determine the crystallographic orientation, the samples were characterized by XRD for different azimuth. The high-resolution cross-sectional TEM image of sample D was taken along [1120], as shown in Figure 5. A clear interlayer (about 2 nm) was observed between -plane GaN and LAO substrate. The interlayer could be attributed to the self-assemble Li 5 GaO 4 . If the -plane GaN samples were grown under N-rich condition, LAO substrates reacted under irradiation of N 2 plasma and decomposed into the binary compounds (i.e., Al 2 O 3 or Li 2 O). Generally, LiXO 2 would transform into Li 5 XO 4 (X = Al, Ga) under the hightemperature condition or irradiation damage [15,16]. In our case, those binary compounds reacted with Ga atoms, provided by Ga wetting layer and leading to the formation of Li 5 GaO 4 . As the -plane GaN was grown under Ga-rich growth condition by increasing the Ga flux (sample E), the irradiation damage was prevented resulting in the absence of Li 5 GaO 4 .
Due to the in-plane strain within the -plane GaN films, the maxima of valence bands at the Brillouin-zone center (Γpoint) are split into three energy levels, that is, heavy hole (HH), light hole (LH), and spin-orbital crystal-field split-off hole (SCH) [17]. Under the anisotropic compressive in-plane   strain condition, the energy levels of interband transition were modified in the order of HH, SCH, and LH versus conduction band, of which corresponding emission energies are 1 , 2 , and 3 , respectively [18]. The polar-dependence PL spectra taken at room temperature were shown in Figure 6. As the polarization angles decreased from 90 ∘ to 0 ∘ , the emission transformed from 1 to 2 . Because sample A was a polycrystalline structure, the PL spectra of sample A contain a defect-related emission (2.6∼2.8 eV) and the zinc-blende (about 3.2 eV) GaN, wurtzite GaN emission (including an -plane 1 emission peak at 3.4011 eV). The 1 emission of samples B, C, D, and E were 3.4387 eV, 3.4337 eV, 3.4366 eV, and 3.4368 eV, respectively. The slightly shift of 1 emission could be attributed to the in-plane compressive strain. The polar-dependence PL spectra support the analyses of XRD measurements and SEM observations.

Conclusion
We have grown five -plane GaN thin films on the LAO substrates under different N/Ga flux ratios by PAMBE. Because of the anisotropic diffusion mechanism of adatoms, RHEED patterns show a streaky patterns for [1120] azimuth. For [0001] azimuth, it shows streaky, spotty, and modulated streaky patterns for different samples. Under the extreme Nrich growth condition, LAO substrates would be damaged by N 2 plasma, leading to a polycrystalline structure. The crystal quality could be improved and the surface trended to a smoother morphology by decreasing the N/Ga flux ratio. As the N/Ga flux ratio kept decreasing, the crystal quality became poorer and the surface trended to a rough morphology. The rough morphology and poorer crystal quality could be attributed to the formation of Li 5 GaO 4 which was observed by the XRD 2 -scan diagram for [1120] azimuth. The self-assembled Li 5 GaO 4 was formed if the -plane GaN films were grown under the intermediate growth condition between N-rich and Ga-rich, and it could be suppressed by the growing of -plane GaN under Ga-rich condition.

Conflicts of Interest
The authors declare that they have no conflicts of interest.