Effects of morphology of buffer layers on ZnO/sapphire heteroepitaxial growth by RF magnetron sputtering
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Effects of morphology of buffer layers on ZnO/sapphire heteroepitaxial growth by RF magnetron sputtering Tomoaki Ide1, Koichi Matsushima1, Ryota Shimizu1, Daisuke Yamashita1, Hynwoong Seo1, Kazunori Koga1, Masaharu Shiratani1, and Naho Itagaki1, 2 1 Kyushu University, Motooka 744, Fukuoka, 819-0395, Japan 2 JST PRESTO, Gobancho, Chiyodaku, Tokyo, 102-0076, Japan ABSTRACT Effects of surface morphology of buffer layers on ZnO/sapphire heteroepitaxial growth have been investigated by means of “nitrogen mediated crystallization (NMC) method”, where the crystal nucleation and growth are controlled by absorbed nitrogen atoms. We found a strong correlation between the height distribution profile of NMC-ZnO buffer layers and the crystal quality of ZnO films. On the buffer layer with a sharp peak in height distribution, a single-crystalline ZnO film with atomically-flat surface was grown. Our results indicate that homogeneous and high-density nucleation at the initial growth stages is critical in heteroepitaxy of ZnO on lattice mismatched substrates. INTRODUCTION Zinc oxide (ZnO) has attracted attention as a potential alternative to gallium nitride (GaN) in light emitting diodes (LEDs) because of the wide and direct band gap (3.37 eV) and the large exciton binding energy of 60 meV, which is much larger than that of GaN (28 meV) [1-4]. Replacing GaN with ZnO will bring high efficient emission even at room temperature due to the large exciton binding energy. Such devices require single crystalline films with low defect density, however, the lattice matched epitaxial substrates for ZnO such as bulk ZnO and ScAlMgO4 substrates are expensive. Therefore, fabrication method of single crystalline ZnO films on cost-effective substrates is needed for replacement of GaN. C-plane sapphire has a great potential as an epitaxial substrate for ZnO because of its low cost and the availability in large area wafers. Due to the large lattice mismatch of 18%, however, ZnO films prepared on sapphire substrates have large number of crystal defects such as threshold dislocations and 30o rotate domains [5, 6]. Inserting a buffer layer is one of the promising means to reduce the crystal defects that related to lattice mismatch [7, 8]. The low temperature (LT) buffer layer is utilized to solve the problems coming from the lattice mismatch. The idea of LT buffer layers was first reported for the mean of fabrication of GaN on sapphire substrates [9]. The role of LT buffer layers is to release the strain energy due to the lattice mismatch, to provide high density of nucleation site and smooth surfaces, and to reduce the interfacial energy between films and substrates. Such buffer layers are obtained by depositing amorphous like layers at a lower temperature than the crystallization temperature along with subsequent annealing. However, because of the low crystallization temperature of ZnO, it is difficult to obtain amorphous like layers by just lowering the deposition temperature. Recently, we have developed a new type of buffer layers fabricated by “nitr
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