The crossover of preferred orientation in heteroepitaxial ZnO/MgO(0 0 1) films
Introduction
ZnO has attracted considerable attention owing to its promising optical and electronic properties, such as a wide band gap of 3.37 eV and strong piezoelectric polarization [1], [2], [3]. Recently, it was reported that wurtzite ZnO films grown on cubic substrates enhanced their device performances, which are quite sensitive to the crystallographic orientation(polarization) [4], [5], [6]. This has led to development of new functional electronic devices, such as semiconductor–ferroelectric heterojunction and transparent field effect transistor. In addition, a thin MgO layer, which is used as a buffer layer on sapphire(0 0 0 1) or Si(1 1 1) substrates, plays a significant role in controlling the polarity of overgrown ZnO films [7], [8], [9]. In this case, MgO was grown with (1 1 1) planes, which induced the preferred growth of overgrown ZnO with the (0 0 0 2) plane along the surface normal direction. On the other hand, in the direct deposition of ZnO on cubic substrates, the crystallographic directional relationship of ZnO films would be dependent on the growth techniques. For example, it was reported that the (1 1 2̄ 0) planes of ZnO were epitaxial to the (0 0 1) planes of SrTiO3 using molecular beam epitaxy [10], whereas a-axis texturing was observed on MgO(0 0 1) grown by pulse laser deposition [11]. Understanding how the preferred orientation varies during film growth and the ability to control the non-polar a-axis epitaxy is essential for utilizing the ZnO/cubic substrates system in device applications.
This paper reports the orientational crossover of ZnO films on MgO(0 0 1) substrates deposited by radio frequency (rf) magnetron sputtering. Initially, ZnO grew epitaxially with (0 0 0 2) planes along the surface normal direction. As the film thickness increased, the growth direction changed gradually to dominantly (1 0 1̄ 0) planes. The formation of ZnO islands was also observed at the later stage of the growth.
Section snippets
Experimental details
ZnO thin films were deposited by rf magnetron sputtering method. MgO(0 0 1) single crystal substrates were used as the cubic substrates in this study. Prior to deposition, the substrates were cleaned sequentially with trichroethylene, acetone and methanol in an ultrasonic bath, and then rinsed with deionized water. The samples were maintained at a growth temperature of 645 °C and a rf input power of 150 W. A mixed gas of Ar and O2 at a pressure of 3×10−2 Torr was introduced into the chamber during
Results and discussion
We first discuss the initial stage of film growth. Fig. 1(a) shows the XRD profile of a 12 nm thick sample. With an intense peak at Qz=2.9835 Å−1 corresponding to MgO(0 0 2), the ZnO(0 0 0 2) Bragg reflection was detected only at Qz=2.4 Å−1, where Qz, which is defined as Qz=4π sin θ/λ, is the momentum transfer of X-rays along the surface normal direction. Interference fringes around the ZnO(0 0 0 2) peak were observed, which originated from interference between the X-rays reflected at the surface and at the
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