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

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][4][5].
SrTiO 3 (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][7][8][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)O 3 [10] and (Sr,Ba)Nb 2 O 6 [11] films show different growth modes and orientations on the TiO 2 -and SrO-terminated surfaces of (001) STO substrates, whereas SrRuO 3 [12] and BaTiO 3 [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 θ-2θ and Ф 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 mm 3 . 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 600°C 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 θ-2θ (X'Pert Pro MPD, PANalytical, Almelo, The Netherlands) and Ф scannings (TTR III, Rigaku, Tokyo, Japan) with CuKα 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 θ-2θ and Ф scans were performed to identify the out-of-plane and in-plane orientation relationships between the films and substrates. In a Ф scan, the number of peaks corresponds to the number of planes for a particular family that possesses the same angle χ (0°< χ < 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 Ф angles of the ZnO films are respectively corrected by the Ф scan of the STO substrates.
It can be seen from Figure    Similarly, the in-plane orientation relationships for (0001) ZnO films on etched (001) STO can also be achieved from X-ray Ф scanning. Figure 2b  whereas in the direction of <112 -0> ZnO , a higher order matching with a mismatch of −1.9% can also be found for seven ZnO over six STO unit cells. The higher order matching has been proposed for the epitaxial growth in large lattice mismatch system [18], but the lower order matching is regarded as the leading growth mechanism. Although the lattice mismatch of the (112   related to the fact that ZnO films tend to be oriented in the (0001) direction even on amorphous substrates [19], implying that the restriction of substrates decreases and the surface energy becomes dominant for the growth of ZnO films on etched (001) STO. As a result, the (0001) plane having the lowest surface energy, the close-packing plane tends to be oriented on etched (001) STO substrates. Figure 3a shows that ZnO films exhibit (0002)        [1120] [100] [011]  (0002) is calculated to be 42.77°, which corresponds to the tilted angle of the trench in etched (011) STO (41.8°, as shown in Figure 1d). This phenomenon is similar to that of GaN on patterned (001) Si substrates [20]. The ZnO films on as-received (011) STO show similar X-ray θ-2θ and Ф scanning patterns with other reports [6,7], and the atomic arrangements are shown in Figure 3c.

Conclusions
In summary, epitaxial ZnO thin films have been obtained on as-received and etched (001), (011), and (111) STO substrates by MOCVD, and the epitaxial relationships were determined. It is interesting that ZnO films exhibit nonpolar (112 -0) orientation with an inplane orientation relationship of <0001> ZnO //<110> STO on as-received (001) STO, and polar (0001) orientation with <11 -00> ZnO //<110> STO on etched (001) STO substrates, respectively. The surface energy is supposed to be dominant for c-axis growth on etched (001) STO. ZnO films change from polar (0001) orientation to semipolar (101 -2) orientation on as-received and etched (011) STO. On as-received and etched (111) STO, ZnO films show the same growth direction with polar (0001), but different in-plane orientation with 30°rotation and a large lattice mismatch induced by the extra interface chemical energy of etched (111) STO with more dangling bonds. The change of epitaxial relationship for ZnO films on as-received and etched STO substrates is accompanied with the increase of lattice mismatch, decrease of bond density, and increase of substrate surface roughness. This investigation presents a very simple way to control epitaxial relationship of ZnO films with STO substrates, which is of technological interest in optoelectronic and electronic devices.