Pulsed sputtering epitaxial growth of m-plane InGaN lattice-matched to ZnO

m-Plane GaN and InGaN films were grown on m-plane ZnO substrates at ~350 °C by pulsed sputtering deposition. It was found that the critical thickness of the m-plane GaN films grown on ZnO lies between 25 and 62 nm, whereas 180-nm-thick m-plane In0.12Ga0.88N can be coherently grown on ZnO substrates, which is explained well by theoretical calculations based on an energy-balance model. The coherently grown m-plane InGaN on ZnO exhibited narrow X-ray rocking curves compared with the m-plane GaN grown on ZnO. These results demonstrate the benefit of lattice-matched ZnO substrates for epitaxy of high-quality nonpolar InGaN films.

Over the last decade or so, the growth, physics, and device applications of nonpolar and semipolar nitrides have made significant progress 1,2 . The first planar nonpolar m-plane GaN was grown on LiAlO 2 substrates by Waltereit et al. 3 , which motivated the researchers to grow nonpolar GaN films on other substrates such as sapphire and SiC for light-emitting diodes (LEDs) 4,5 . Subsequently, the use of nonpolar and semipolar bulk substrates prepared by hydride vapor phase epitaxy (HVPE) increased, enabling the fabrication of bright LEDs and green laser diodes (LDs) [6][7][8][9] . Throughout history, it was observed that high-performance devices need to be fabricated on high-quality substrates. Therefore, bulk GaN substrates are now widely used for nonpolar light-emitting devices [10][11][12] ; however, the use of GaN substrates presents the problem of lattice mismatch with InGaN 13,14 . This lattice mismatch between GaN and InGaN could be detrimental, particularly for long-wavelength LEDs and LDs. To overcome the lattice mismatch problem, the use of ZnO substrates has been proposed for the growth of high-quality nonpolar InGaN 15,16 . As shown in Fig. 1, compared with GaN, ZnO exhibits a smaller lattice mismatch along both the a-and c-axes with In x Ga 1−x N (x > 0.09). Another advantage is that high-quality ZnO substrates can be prepared using hydrothermal methods 17 , and ZnO substrates are larger than the state-of-the-art bulk GaN substrates. It can be noted that the use of ZnO substrates is the best way to fabricate nonpolar InGaN-based devices because nonpolar ZnO substrates are considerably cheaper than nonpolar GaN bulk substrates. It is natural to try to grow nitride films on ZnO because it is the only wurtzite material that is lattice-matched to group III nitrides. HVPE 18,19 , metalorganic vapor phase epitaxy 20 , and molecular beam epitaxy 21 of GaN and InGaN on ZnO have been reported, and a reasonable epitaxial relationship of nitride and ZnO has been confirmed 20 . From these results, it can be expected that high-quality nonpolar nitrides could also grow on ZnO substrates. However, it is difficult to grow nitride films on chemically vulnerable ZnO substrates by chemical vapor deposition. To take full advantage of the lattice-matching nature of ZnO and grow high-quality nitride films, the chemical/interfacial reactions between nitrides and ZnO need to be completely suppressed.
A decrease in the growth temperature is important to suppress the interfacial reactions between nitrides and ZnO. Recently, a low-temperature epitaxial growth technique was developed called pulsed sputtering deposition (PSD) 22 . This technique enables the growth of nitride films with high control over the film thickness and allows deposition over a large area. PSD is particularly suitable for InGaN growth because its growth temperature is lower than those of MBE, MOVPE, and HVPE. In PSD growth, a pulsed supply of indium and gallium atoms with high kinetic energy enhances the migration of adatoms on the surface, leading to a dramatic reduction in the growth temperature. Using this technique, high-quality n-type 23 and p-type 24 GaN films were successfully grown with room temperature carrier mobilities of 1008 and 34 cm 2 V −1 s −1 , respectively. Moreover, LEDs were also fabricated at temperatures lower than 500 °C 25 . These results indicate the potential of PSD for the fabrication of nitride-based devices on ZnO substrates. In this article, the structural properties of nonpolar m-plane GaN and InGaN films grown on ZnO substrates by PSD are reported.

Results and Discussion
First, the structural properties of the GaN films grown on m-plane ZnO were investigated. Figure    wurtzite GaN. The polycrystalline growth of GaN was also observed on the m-plane ZnO substrates when the growth was performed at high temperatures (above 500 °C) using pulsed laser deposition 26  Next, the structural characterization of a single crystalline m-plane GaN film on ZnO was performed using X-ray diffraction (XRD). The full widths at half maximum (FWHM) of the X-ray rocking curves (XRCs) for m-plane GaN (25 nm thick) grown at 340 °C were 155 and 130 arcsec with the X-ray incident angle perpendicular to [1120] and [0001], respectively. In order to experimentally determine the critical thickness for the lattice relaxation of m-plane GaN grown on ZnO by PSD, four samples with different film thicknesses were prepared (25,62,110, and 160 nm) and the lattice constants of GaN films were measured using the X-ray reciprocal space map (RSM) technique. Figure 3 shows the lattice constants of GaN films with different thicknesses. While the lattice constant along the c-axis was almost unchanged for all film thicknesses, the lattice constant along the a-axis relaxed at a film thickness between 25 and 62 nm and gradually approached the lattice constant of the completely relaxed GaN. The predominant lattice relaxation along the a-axis can be explained by the larger lattice mismatch of the a-axis (1.9%) compared with that of the c-axis (0.4%).
The theoretical critical thicknesses of m-plane GaN (and InGaN) heteroepitaxially grown on ZnO were calculated by employing the energy-balance model proposed by People and Bean 27 . In this model, the critical thickness is equal to the film thickness at which the energy density of a misfit dislocation coincides with the strain energy density stored in the film. According to Huang et al. 28 , the critical thickness h crit of nonpolar wurtzite crystals can be expressed as follows: where G is the shear modulus, b the magnitude of the Burgers vector of the misfit dislocation, a the lattice constant of GaN (or InGaN), ν the Poisson's ratio of GaN, r c the core radius of the misfit dislocation, U the strain energy per unit volume, S the elastic compliance coefficient, and ε the strain. Figure 4 depicts the dependence of the calculated critical thickness of m-plane In 1−x Ga x N/ZnO on In composition, x. The critical thickness of GaN (x = 0) is calculated to be 45 nm, which well explained the experimental result shown in Fig. 3. The critical thickness reaches a maximum value of 360 nm at the In composition of 11.5%. The lattice mismatches between In 0.115 Ga 0.885 N and ZnO are −0.65% (a-axis) and + 0.72% (c-axis). For an In 0.115 Ga 0.885 N film on ZnO, both the strains are balanced, resulting in the maximum critical thickness. Finally, the structural properties of the 180-nm-thick m-plane In 0.12 Ga 0.88 N grown on ZnO substrates were investigated. The film was grown at a temperature as low as 350 °C to prevent interfacial reactions between InGaN and ZnO. The RHEED and AFM analyses revealed that the m-plane InGaN grown on ZnO possesses atomically flat surfaces similar to those at of the m-plane GaN on ZnO. RSMs shown in Fig. 5 revealed that the growth of 180-nm-thick m-plane InGaN is coherent to the ZnO substrate, which supports the calculation shown in Fig. 4. The strains in In 0.12 Ga 0.88 N on ZnO were found to be +0.6% (a-axis), −0.8% (c-axis), and 0.0% (m-axis: normal to the surface). In this film, the strains along the a-and c-axes cancelled each other out, and no strains were applied along the m-axis. Figure 6 compares the XRCs for m-plane GaN and In 0.12 Ga 0.88 N grown at 350 °C. The thickness of both films was the same (180 nm). Since the 180-nm-thick GaN film released lattice strains by introducing structural defects, the FWHM of the rocking curves was as large as 288 arcsec. On the other hand, coherently grown m-plane InGaN exhibits a sharp rocking curve with an FWHM of 79 arcsec. These results indicate that the control of the lattice constant of InGaN by tuning the In composition is quite effective for obtaining high-quality nitride films on ZnO substrates. Electrical and optical properties of the InGaN films grown on ZnO are helpful information to assess the quality of the films. We tried to perform Hall-effect measurements of the InGaN films, but a reliable data was not obtained because of the conductive n-type ZnO substrate. In addition, for photoluminescence (PL) measurements using 325 nm excitation laser, ZnO emitted a strong UV luminescence (~380 nm), making it difficult to detect the PL coming from the thin InGaN film. If the InGaN film is peeled off from the substrate and transferred onto another substrate, the measurements would be successful. Unfortunately, we do not have a technique that enables the transfer of the thin InGaN film. The electrical and optical characterization of the film will be a next work.
The structural properties of m-plane GaN and InGaN films grown on lattice-matched ZnO substrates by PSD were investigated. It was found that m-plane GaN and InGaN films can be coherently grown at ~350 °C. Theoretical calculations have indicated that the critical thickness of m-plane In 0.12 Ga 0.88 N grown on ZnO is larger than that of m-plane GaN grown on ZnO, in agreement with the experimental results. The XRCs of m-plane In 0.12 Ga 0.88 N on ZnO were narrower than those of m-plane GaN on ZnO. This work is the first comprehensive study of the strains of nonpolar InGaN films on ZnO and it demonstrated the feasibility of ZnO as a substrate for  growing nonpolar InGaN films. These nonpolar materials could be useful for fabricating long-wavelength (yellow and red) light-emitting devices.

Methods
GaN and InGaN films were grown by PSD. High-purity Ga and In targets were separately sputtered in a Ar-N 2 mixture gas. Before growing the film, the m-plane ZnO substrates were annealed in a ceramic ZnO box at 1025 °C 29 so that the surface of substrate attained atomically flat steps. The growth temperature ranged from 300 °C to 540 °C. Structural characterization of the films was performed using RHEED, AFM, and XRD.