Effect of temperature on lateral growth of ZnO grains grown by MOCVD
Introduction
ZnO is an attractive material potential for applications such as light-emitting diodes [1], [2], photodetectors [3], [4], varistors [5], gas sensors [6], [7], surface acoustic wave filters [8], transparent conducting oxide [9], [10], [11], and optical modulator waveguides [12]. Among these applications, the blue optoelectronic application receives much attention because ZnO has unique properties that fulfill the requirement. With direct bandgap of 3.3 eV and a high exciton binding energy of 60 meV, ZnO is a potential candidate for blue optoelectronics [13], [14], [15], [16], and is even considered competitor for the well recognized GaN in the field of blue LEDs and LDs [17]. However, the growth of the ZnO film for such applications still remains a problem [18]. Thin film of epitaxial quality is often required, but the crystallographic anisotropy of the ZnO's wurtzite structure limits the achievement of the goal. The growth rate anisotropy of the hexagonal symmetry results in the formation of hexagonal nanorods and nanowires [19], [20], [21], [22], and producing a porous ZnO film. How to prepare dense film without the formation of nanorod structure is urgent to realize ZnO's practical usage.
It is known that the low VI/II ratio promotes the lateral growth of the ZnO grains [23]. To enhance the lateral growth in hope that all nanorods are in close contact and become densely packed, CO2 is chosen in place of O2 as the oxygen source in this study for the growth. The CO2 provides much lower oxygen partial pressure than O2 in the MOCVD process [24], so it provides much lower VI/II ratio environment.
In this study, we would like to explore the effect of temperature on ZnO growth by MOCVD. When VI/II ratio is largely reduced using CO2 in place of O2, the temperature may have significant effect on film morphology. Thus for the development of dense and large area thin film growth, we focus on the study of the temperature effect on ZnO film growth with low VI/II ratio. Furthermore, we would like to study if there is a way to further enhance the crystal alignment by growth temperature maneuver. We would like to demonstrate the crystal alignment enhancement of the film by a two-step temperature variation growth method. It is known that the nucleation and coalescence of grains is sensitive to the growth temperature. Using a two-step temperature variation growth model, the effect of the temperature variation on the nucleation and coalescence, and hence the subsequent effect on crystal alignment, can be explained satisfactorily.
Section snippets
Experimental procedure
The experiment was performed using CO2 as the oxygen precursor. Dimethylzinc (DMZn) was used as zinc precursor. The CO2 flow rate was 1000 sccm while the dimethylzinc flow rate was 0.58 sccm. The precursors were mixed together and flow into the deposition chamber through a showerhead nozzle with showerhead diameter of 100 mm. The substrate was placed right beneath the showerhead. The chamber pressure was kept at 200 Torr with the help of balanced argon flow. The growth experiment was performed at
Results and discussion
The resultant film morphologies and corresponding X-ray diffraction patterns for the films grown at three different growth temperatures are shown in Fig. 1. In X-ray diffraction (XRD) patterns, () peak in addition to (0 0 0 2) were present [Fig. 1(d)]. Our previous study (not shown) [25] of ZnO growth using low DMZn flow rate of 0.09 sccm under the same CO2 environment shows only (0 0 0 2) peak in the XRD patterns. The presence of () peak of this experiment suggests that some ZnO
Conclusion
We have successfully prepared dense and continuous ZnO film by MOCVD using increased DMZn flow rate with CO2 as oxygen precursor. We found that the growth temperature suppresses the lateral growth of ZnO. We also found that higher growth temperature promotes the coalescence of grains but reduces the crystal alignment. Using a two-step growth method, employing the initial growth at lower temperature followed by the growth at higher temperature, a densely packed ZnO film with larger grains and
Acknowledgements
The authors would like to acknowledge J.M. Wu, Y.Z. Jhang, and Y.A. Su for X-ray and EM analysis and H.T. Wang of Eastern Sharp Ltd. for MOCVD system assembly. This work was mainly supported by the funding from the National Science Council of the Taiwan under the Project No. NSC-93-2215-E-259-003. The FE-SEM operation was supported by the National Science Council of Taiwan under the Project No. NSC-96-2120-M-259-001.
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