Elsevier

Journal of Crystal Growth

Volume 311, Issue 1, 15 December 2008, Pages 200-204
Journal of Crystal Growth

Growth of c-oriented ZnO films on (0 0 1) SrTiO3 substrates by MOCVD

https://doi.org/10.1016/j.jcrysgro.2008.10.017Get rights and content

Abstract

The growth direction of ZnO thin films grown by metal-organic chemical vapor deposition (MOCVD) is modulated by pretreatment of (0 0 1) SrTiO3 (STO) substrates. ZnO films show a-oriented smooth surface with epitaxial relationship of 001ZnO//110 STO on as-received STO, and c-axis columnar growth with 010ZnO//110 STO on etched STO, respectively. The orientation alteration of ZnO films is supposed to be caused by the change of STO surface polarity. In addition, the c-ZnO films exhibit an enhanced photoluminescence (PL) intensity due to the improved crystal quality, while the blueshift of PL peak is attributed to the smaller tensile strain. These results show that high quality c-ZnO, which is essential for electronic and optoelectronic device applications, can be grown on (0 0 1) STO by MOCVD.

Introduction

ZnO thin films have increasing applications in light-emitting diodes (LED), thin film transistor (TFT) and high electron mobility transistor (HEMT) due to its large exciton binding energy and mobility. The growing direction is a key element to determine the electrical and optical properties, and the most favorable crystal orientation for these preceding applications is the polar direction [1], [2], [3], [4]. Although the nonpolar ZnO films are expected to show higher emission efficiency due to elimination of spontaneous and piezoelectric polarization fields, the poor crystal quality results in a lower emission intensity. As a result, the LED employs the high quality c-oriented ZnO films, so does TFT. The HEMT structure depends on the realization of a high density two-dimensional electron gas (2DEG) using electric polarization effects, which demands the fabrication of c-ZnO films. Furthermore, the polar surface of ZnO is more stable than nonpolar (1 1 0) surface. Therefore, c-ZnO films are highly required for practical applications.

SrTiO3 (STO) single crystal can become a superconductor and ferroelectric by substitution or strain, although it remains paraelectric down to 0 K without stress [5]. On the other hand, it has been widely used as substrates 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]. Due to carrier limitation and coupling effect, the heterojunction of ZnO/STO can potentially create new material properties to apply in new multifunctional devices. The c-ZnO TFT has been fabricated on (0 1 1) STO [6], but usually (1 1 0) oriented ZnO (referred as a-ZnO) films is obtained on (0 0 1) STO [7], [8], [9], which is in general the favorite substrate for many functional oxides. So it makes sense to achieve c-ZnO films on (0 0 1) STO.

It is found that the pretreatment method of (0 0 1) 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 (0 0 1) STO substrates, whereas SrRuO3 [12] and BaTiO3 [13] films exhibit different initial morphology and crystallinity on the as-received and etched (0 0 1) STO substrates, respectively. However, there is little research about the growth behavior of ZnO films on (0 0 1) STO substrates with different pretreatments. In addition, ZnO films have been prepared on STO by pulsed laser deposition (PLD) and laser molecular beam epitaxy (LMBE) [7], [8], [9], but no previous report is about metal organic chemical vapor deposition (MOCVD), which can lead to high-quality films and applicable to industrial mass-production.

In this paper, high quality c-ZnO films are prepared on etched (0 0 1) STO substrates by MOCVD. The structural, morphological and optical properties are performed by X-ray diffraction (XRD), field-emission scanning electron microscope (SEM) and photoluminescence (PL) spectroscopy. For comparison, the results from a-ZnO films on as-received STO are also presented.

Section snippets

Experiments

The (0 0 1) STO single crystal wafers (Kejing Materials Technology Co., Ltd) with sizes of 10×5×0.5mm3 are used as substrates. Chemically etching is conducted in a HF solution at room temperature. ZnO films are 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 are the reactants and nitrogen gas is used as the carrier gas. The samples are grown at 600C for 60 min with the same bubbled DEZn

Results and discussion

Fig. 1 shows XRD θ2θ patterns of ZnO thin films deposited on as-received and etched (0 0 1) STO substrates. ZnO films exhibit only a single hexagonal wurtzite phase, and evidently different growing directions, with nonpolar (1 1 0) orientation (a-ZnO) on as-received STO and polar (0 0 2) orientation (c-ZnO) on etched STO, respectively. The highly oriented ZnO films suggest that the nucleation and crystal growth is initiated near the substrate surface. The average grain size is estimated to be 24 and

Conclusion

In summary, epitaxial ZnO thin films have been obtained by MOCVD. It is interesting and useful that ZnO films exhibit a-oriented smooth surface with epitaxial relationship of 001ZnO//110 STO on as-received STO, and c-axis columnar growth with 010ZnO//110 STO on etched STO substrates, respectively. The orientation alteration of ZnO films is supposed to be caused by the change of STO surface polarity. PL spectra exhibit a strong NBE emission without any deep-level emission. The NBE

Acknowledgments

This work was supported by the 973 program (2006CB604908, 2006CB921607), and the National Natural Science Foundation of China (60625402).

References (28)

  • Y.T. Zhang et al.

    Opt. Mater.

    (2004)
  • H. Tampo et al.

    J. Crystal Growth

    (2007)
  • O. Dulub et al.

    Surf. Sci.

    (2002)
  • H. Fujisawa et al.

    J. Crystal Growth

    (2002)
  • T. Yoshimura et al.

    J. Crystal Growth

    (1997)
  • M. Kasuga et al.

    J. Crystal Growth

    (1981)
  • J.B. Lee et al.

    Thin Solid Films

    (2003)
  • J.Z. Perez et al.

    Appl. Phys. Lett.

    (2006)
  • Y.L. Li

    Phys. Rev. B

    (2006)
  • E. Bellingeri et al.

    Appl. Phys. Lett.

    (2005)
  • X.H. Wei et al.

    Appl. Phys. Lett.

    (2007)
  • Y.L. Wu et al.

    Appl. Phys. Lett.

    (2008)
  • M. Karger et al.

    Phys. Rev. B

    (2005)
  • A. Infortuna et al.

    J. Appl. Phys.

    (2006)
  • Cited by (21)

    • On the growth of conductive aluminum doped zinc oxide on 001 strontium titanate single crystals

      2016, Applied Surface Science
      Citation Excerpt :

      SrTiO3 is a high dielectric constant insulator (ϵ = 300 at room temperature) which can be used as substrate for fabrication of field effect transistors (transparent and epitaxial multistructures). Reviewing the published works on ZnO thin films deposited on (001) STO, two growth orientations were reported with questionable discussions: along a-axis (zinc oxide grows with (110) plane parallel with (001) face of the substrate) [9,17–21] and along c-axis (with 001 texture of the thin film) [22,23]. Bellingeri et al. [9] are extremely evasive concerning the structure of ZnO on (001) STO, only mentioning without any proof that ZnO grows 110, their results and discussions are mostly focused on films deposited on (011) STO.

    View all citing articles on Scopus
    View full text