Study on Cubic GaN Growth on (001) Rutile TiO2 Substrates by ECR-MBE
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Study on Cubic GaN Growth on (001) Rutile TiO2 Substrates by ECR-MBE T.Araki, H.Mamiya, K.Kitamura and Y.Nanishi Dept. of Photonics, Ritsumeikan University, 1-1-1 Noji-higashi, Kusatsu, Shiga 525-8577, Japan
ABSTRACT GaN layers were grown on a (001) rutile TiO2 substrate by electron cyclotron resonance plasma-excited molecular beam epitaxy. For the first time, c-GaN with a preferential growth orientation was obtained. Based on the results from electron diffraction and X-ray diffraction analysis, we found that c-GaN with the growth direction of [110] was grown on the TiO2 substrate. The formation of c-GaN was also confirmed by cathodoluminescence, in which a luminescence peak was observed at 3.24eV.
INTRODUCTION Nitride semiconductors; GaN, AlN and InN crystallize in both hexagonal wurtzite and cubic zinc-blende crystal structure. Until now, most of all the GaN-based optical and electronic devices are of wurtzite hexagonal GaN (h-GaN) because of both metastable nature of zinc-blende cubic GaN (c-GaN) and a lack of suitable substrates for the c-GaN growth. However, c-GaN has been expected to have various advantages in the physical properties over those of h-GaN due to its higher crystallographic symmetry, resulting in smaller effective mass, lower phonon scattering, higher doping efficiency and so on [1]. Moreover, for the growth of c-GaN on cubic (001) substrates, the absence of piezoelectric polarization fields will help us to understand the emission mechanism of multiple quantum wells [2,3]. As for the device applications, c-GaN grown on appropriate substrates is also superior to h-GaN in the ease of cleaving and the formation of back-side electrode. Thus, a large number of studies to grow c-GaN on various substrates have been carried out [4]. Substrates used in these studies are mainly GaAs [5-7], 3C-SiC [8-10] and Si [11,12]. It has been difficult, however, to grow high quality c-GaN because of the large mismatch with the substrates and the lack of enough stability at growth temperatures, resulting in h-GaN phase mixing. Therefore, further study concerning even about substrate material is still necessary to realize growth of high quality c-GaN. TiO2 has rutile (body-centered tetragonal) crystal structure. Figure 1 shows the crystal structure of a TiO2 unit cell. Lattice constants along a-axis and c-axis are 4.593 Å and 2.959 Å, respectively. When (001) c-GaN is grown on (001) rutile TiO2, lattice mismatch between c-GaN (a = 4.531 Å) and the TiO2 substrate is -1.3%, which is much smaller than those of other substrates; -20 % for (001) GaAs, 4 % for (001) 3C-SiC and 17% for (001) Si. Therefore, TiO2 is expected as a lattice-matched substrate for the growth of (001) c-GaN. We should consider that TiO2 substrate is not stable at high temperature and a reducing atmosphere including hydrogen. It
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was found that TiO2 substrate was reduced during metal organic chemical vapor deposition growth of GaN at around 1000°C [13]. This result suggests that molecular beam epitaxy (MBE), by which a low growth t
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