Control of epitaxial relationships of ZnO/SrTiO3 heterointerfaces by etching the substrate surface

Caihong Jia 0 1 2 Yonghai Chen 0 2 Xianglin Liu 0 2 Shaoyan Yang 0 2 Weifeng Zhang 1 Zhanguo Wang 0 2 0 Key Laboratory of Semiconductor Material Science, Institute of Semiconductors, Chinese Academy of Science , P.O. Box 912, Beijing 100083, People's Republic of China 1 Key Laboratory of Photovoltaic Materials of Henan Province and School of Physics and Electronics, Henan University , Kaifeng 475004, People's Republic of China 2 Key Laboratory of Semiconductor Material Science, Institute of Semiconductors, Chinese Academy of Science , P.O. Box 912, Beijing 100083, People's Republic of China Wurtzite ZnO thin films with different epitaxial relationships are obtained on as-received and etched (001), (011), and (111) SrTiO3 (STO) by metal-organic chemical vapor deposition (MOCVD). ZnO films exhibit nonpolar (1120) orientation with in-plane orientation relationship of <0001>ZnO//<110>STO on as-received (001) STO, and polar c-axis growth with <1100>ZnO//<110>STO on etched (001) STO substrates. ZnO films change from polar (0001) to semipolar (1012) oriented on as-received and etched (011) STO. On as-received and etched (111) STO, ZnO films show the same growing direction of polar (0001), but different in-plane orientations with 30 rotation. The change of epitaxial relationship of ZnO films on as-received and etched (001), (011), and (111) STO substrates is accompanied with the increase of lattice mismatch, decrease of bond density, and increase of substrate surface roughness. In other words, the epitaxial relationships of ZnO/STO heterointerfaces can be controlled by etching the substrates. These results show that polar, nonpolar, and semipolar ZnO films for different applications can be grown epitaxially on STO substrates by MOCVD. - Background Growth direction is a key element to determine the electrical and optical properties of ZnO thin films, and different orientations are demanded for various applications [1,2]. Polar ZnO films with a c-axis perpendicular to the growth plane are required for the high electron mobility transistor structure, which depends on the realization of a high-density two-dimensional electron gas using electric polarization effects. The nonpolar and semipolar ZnO films with a horizontal and inclined c-axis are expected to show higher emission efficiency in light-emitting diodes by eliminating or reducing the spontaneous and piezoelectric polarization fields [3-5]. SrTiO3 (STO) single crystal substrates have been widely used to deposit functional oxide films with superconductivity, ferroelectricity, and ferromagnetism owing to lattice match. Compared with other common substrates for ZnO growth, the integration of wurtzite ZnO and perovskite STO combines the rich properties of perovskites together with the superior optical and electrical properties of wurtzites [6-9]. Thus, the ZnO/STO heterojunction is expected to be applied in new multifunctional devices due to carrier limitation and coupling effect. On the other hand, it is found that the pretreatment method of (001) STO single crystal substrates will significantly influence the growth behaviors of thin films. For example, Pb(Zr,Ti)O3 [10] and (Sr,Ba)Nb2O6 [11] films show different growth modes and orientations on the TiO2- and SrO-terminated surfaces of (001) STO substrates, whereas SrRuO3 [12] and BaTiO3 [13] films exhibit different initial morphology and crystallinity on the as-received and etched (001) STO substrates, respectively. However, there is little research about the growth behavior of ZnO films on as-received and etched (001), (011), and (111) STO substrates. Furthermore, the control of epitaxial relationships for ZnO on STO has not been investigated in detail. In this paper, polar, nonpolar, and semipolar ZnO films are obtained on as-received and etched (001), (011), and (111) STO substrates by metal-organic chemical vapor deposition (MOCVD). X-ray -2 and scannings are performed to determine the out-of-plane and in-plane epitaxial relationships between ZnO films and STO substrates. Methods The substrates used were (001), (011), and (111) STO single crystal wafers with sizes of 10 5 0.5 mm3. The as-received STO substrates were polished and cleaned by an organic solution, while the etched substrates were further conducted in buffered HF solutions at room temperature. ZnO films were grown on both as-received and etched STO substrates by a home-designed and made vertical low-pressure MOCVD reactor. Bubbled diethylzinc (DEZn) and pure oxygen were the reactants, and nitrogen gas was used as the carrier gas. The samples were grown at 600C for 30 min with the same bubbled diethylzinc flux and carrier gas flux of oxygen. The flow rate of the pure oxygen gas was set at 1 slpm, and the flow rate of DEZn was set at 16 sccm. The pressure of the chamber was kept at 76 Torr. The epitaxial relationships were determined by X-ray -2 (XPert Pro MPD, PANalytical, Almelo, The Netherlands) and scannings (TTR III, Rigaku, Tokyo, Japan) with CuK radiation. Results and discussion Figure 1 shows the surface images of as-received and etched STO substrates taken by an atomic force microscope (AFM). It can be clearly seen that the STO surface varies from smooth for as-received to rough for etched. The surface roughness of as-received STO substrates is about 1 nm, while the etched STO surface is full of pits or trenches with a surface roughness of around 20 nm. Although some reports show that the surface of HF-etched STO is atomically flat with Ti-terminated surface since Sr atom is much more sensitive to HF attack than Ti atom [14], the etched STO surface in the present case is full of pits or trenches. The STO used in this work may not be a perfect single crystal and is assumed to be made up of nanograins [15]. The HF solution permeates into the grain boundaries and dissolves Sr atoms on the lateral sides. As etching proceeds, the grains shrink and the grain boundaries widen in size, leading to the appearance of pits or trenches. The tilted angles of pits or trenches from the surface are estimated from AFM to be 56.4, 41.8, and 64.0 on etched (001), (011), and (111) STO substrates, respectively. The pits and/or trenches may serve as patterned substrates to control the growth direction of ZnO films, which is essentially important for practical applications. X-ray -2 and scans were performed to identify the out-of-plane and in-plane orientation relationships between the films and substrates. In a scan, the number of peaks corresponds to the number of planes for a particular family that possesses the same angle (0< < 90) with the crystal surface, while the separation between peaks correlates with the angular separation between the corresponding projections of the normals to the scanning family onto the crystal surface. The angles of the ZnO films are respectively corrected by the scan of the STO substrates. It can be seen from Figure 2a that ZnO films show nonpolar (112 0) and polar (0001 (...truncated)