JOURNAL OF APPLIED PHYSICS
VOLUME 87, NUMBER 7
1 APRIL 2000
The grain size effects on the photoluminescence of ZnOÕ␣-Al2O3 grown by radio-frequency magnetron sputtering Kyoung-Kook Kim,a) Jae-Hoon Song, Hyung-Jin Jung, and Won-Kook Choib) Thin Film Technology Research Center, Korea Institute of Science and Technology, Cheongryang P.O. Box 131, Seoul 130–650, Korea
Seong-Ju Park Department of Materials Science and Engineering, Center for Electronic Materials Research, Kwangju Institute of Science and Technology, Kwangju 500–712, Korea
Jong-Han Song Advanced Analysis Center, Korea Institute of Science and Technology, Cheongryang P.O. Box 131, Seoul 130–650, Korea
共Received 20 May 1999; accepted for publication 25 October 1999兲 ZnO thin films were epitaxially grown on ␣-Al2O3 共0001兲 substrate by radio-frequency 共rf兲 magnetron sputtering. Among the ZnO films deposited at 550 °C, the film deposited at 80 W has the narrowest full width half maximum 共FWHM兲 of x-ray diffraction 共XRD兲 -rocking curve, 0.16°, indicating a highly c-axis oriented columnar structure. The FWHM of XRD -rocking curve of the ZnO film deposited at 120 W and 600 °C was 0.13° with a minimum channeling yield, 4%–5%. In photoluminescence 共PL兲 measurement, only the sharp near band edge emission was observed at room temperature 共RT兲. The FWHM of PL peak was decreased from 133 to 89 meV as rf power increased from 80 to 120 W at 550 °C, and that of film deposited at 120 W and 600 °C showed 76 meV which is lower value than any other ever reported. These PL results were somewhat opposite to that of XRD. From transmission electron microscopy analysis, grain size and defects were found to affect the PL properties. In this study, the PL property of undoped ZnO thin films is discussed in terms of the crystalline structure and the size of grain. © 2000 American Institute of Physics. 关S0021-8979共00兲06303-9兴
luminescence was not still entirely investigated. In this study the high quality ZnO thin film is grown on sapphire 共0001兲 substrate using rf magnetron sputtering technique and the characteristics of PL at RT is studied. The ZnO thin films were grown on sapphire single crystal substrates 共0001兲 using horizontal-axis type rf magnetron sputtering. The sintered ZnO 共99.999%兲 and a mixed plasma of Ar and O2 were used for sputtering. The ZnO films with the thickness of 1–3 m were deposited at a substrate temperature of 550 and 600 °C and rf power of 60–120 W. The working pressure was 10 mTorr. Epitaxial relations between ZnO films and the sapphire substrate were measured by XRD scan. PL of the as-grown ZnO films were taken using He-Cd laser (⫽325 nm兲 at RT. The microstructure of ZnO grain and the interface of ZnO/␣-Al2O3 共0001兲 were analyzed by transmission electron microscopy 共TEM兲. Figure 1共a兲 shows XRD rocking for the ZnO thin films grown on an ␣-Al2O3 共0001兲 substrate. Among the ZnO films deposited at 550 °C with various of powers of 60–120 W, the one grown at 80 W show the narrowest -rocking curve FWHM, 0.16°, indicating a highly c-axis oriented columnar structure. As power was increased or decreased the FWHMs value increased to 0.30°–0.44°, indicating that the ZnO films became polycrystalline due to increases in the mosaic structure. Degradation of crystallinity can be explained by the presumption that the power lower than 80 W was insufficient for sputtered atoms to efficiently migrate on
For the last few years, wide-band gap semiconductors such as GaN and its alloys have been intensively studied to develop a light emitting diode, a short-wavelength diode laser, and ultraviolet 共UV兲 detector. Commercialization of green-blue light-emitting diode 共LED兲 as well as blue laser deposition 共LD兲 marked an epoch for the application of optoelectronic devices.1 ZnO has rapidly emerged as a promising analogue to GaN because of a large band gap of 3.37 eV, low power threshold for optical pumping at RT, and highly efficient UV emission resulted from a large exciton binding energy of 60 meV at RT. Stimulated excitonic emission and lasing from bulk ZnO,2 and more recently similar band edge and deepcenter photoluminescence between bulk ZnO and GaN were reported.3 Recently, room temperature optically pumped lasing in the ZnO films on sapphire were reported.4 To date the heteroepitaxial ZnO thin films have been grown by organometallic chemical vapor deposition 共OMCVD兲,5 laser molecular beam epitaxy 共MBE兲,4 plasmaassisted MBE, and pulsed laser deposition 共PLD兲 for fabrication of hybrid optoelectronic devices, but rf magnetron sputtering is hardly ever adopted for this purpose. However, the influence of crystallinity of the ZnO thin films on photoa兲
Also with Department of Materials Science and Engineering, Center for Electronic Materials Research, Kwangju Institute of Science and Technology, Kwangju 500–712, Korea; electronic mail:
[email protected] b兲 Electronic mail:
[email protected] 0021-8979/2000/87(7)/3573/3/$17.00
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FIG. 2. BS/channeling spectra for ZnO films deposited at 共a兲 550 °C, 60 and 80 W, and at 共b兲 600 °C, 120 W.
FIG. 1. 共a兲The FWHM variation of XRD -rocking curve, 共b兲 XRD scans of ZnO film grown on ␣-Al2O3 共0001兲 substrate at 550 °C, 80 W.
the surface and that higher than 80 W was too high to give enough time necessary for finding stable site. And then such a fast growth can not support stable grain growth in the ZnO thin films and may degrade the crystallinity of ZnO thin films. For the ZnO film deposited at 600 °C and 120 W, the FWHM of x-ray -rocking curve of the ZnO film was 0.13°, showing better crystallinity as predicted than those of films deposited at 550 °C. The in-plane alignment of ZnO films grown on sapphire 共0001兲 substrate at 550 °C and 80 W was measured by using -scans of four-circle XRD 关Fig. 1共b兲兴. As shown at the ¯ 2兴 plane of ZnO films grew sixfold bottom panel, the 关011 ¯ 2兴 symmetry on sapphire 共0001兲 substrate.6 Also, the 关011 plane of sapphire 共0001兲 substrate has threefold symmetry. These results showed that the unit cell of ZnO films rotated to 30° with respect to the unit cell of sapphire 共0001兲 substrate to accommodate large lattice mismatch between ZnO and sapphire substrate 共0001兲. That is, the in-plane epitaxial ¯ 0兴储Al O 关112 ¯ 0兴. Thus, relationship was found to be ZnO关101 2 3 the epitaxial growth of ZnO film is controlled by domain matching epitaxy where integral multiples of planes or lattice constants match across the interface. Figure 2共a兲 shows the aligned and random backscattering spectra of ZnO films deposited at 550 °C, 60, and 80 W. The minimum channeling yields for both films are almost the same as much as m ⫽50%. The channeling yield exhibits as steep increase with a depth. Meanwhile, as both power and substrate temperature increased to 120 W and 600 °C, the minimum yield is sharply decreased 4%–5%, indicating high crystal quality the ZnO layer. This means that defect density of the ZnO film grown at 600 °C and 120 W is much smaller than those deposited at 550 °C. According to Vispute et al.,7 the minimum channeling yield for the ZnO film on sapphire
showing m ⫽⬃2% deposited by PLD smoothly increased up to 10% near the interface because intrinsic dislocation due to the large lattice misfit should be exited. The film deposited at 600 °C and 120 W is believed to be high c-axis oriented film with large grain and small amount of defect. In TEM analysis, for the ZnO films grown at 550 °C 关Fig. 3共a兲 and 3共b兲兴, it is found that columns of the film at 80 W were highly c-axis oriented but those at 120 W are not. The grain size of ZnO film grown at 120 W is larger than that at 80 W. But, for the ZnO film grown at 600 °C and 120 W, grain size is the largest and c-axis orientation is excellent 关Fig. 3共c兲兴. From TEM images, the increase of growth temperature improved the crystallinity of ZnO film because of sufficient thermal energy due to high growth temperature. Thus when growth temperature is low, the improvement of crystallinity needs lower growth rate due to decrease of rf power. In this study, very prominent only near band edge 共NBE兲 emission without deep-level emission around 2.5 eV is sharply observed except for those deposited at 60 W and 550 °C 关Fig. 4兴. The peak position of NBE was varied from 3.3 eV 关共a兲 in Fig. 4兴 to 3.36 eV 关共f兲 in Fig. 4兴. Among the films deposited at 550 °C, the largest FWHM of PL spectra was 113–133 meV at 80 W and the lowest value of 89–91 meV was measured at 120 W. So the optical properties of ZnO films are improved with the increase of rf power above 80 W. From these results, the PL properties of the ZnO films seem to be improved with increases in rf power. For the ZnO films grown at 550 °C, PL spectra show an opposite trend compared to the XRD results. The crystallinity of ZnO films grown at 80 W was better than those at 120 W, while the PL properties of ZnO films deposited at 120 W was conversely better than those at 80 W. This can be reasoned that the increase in mosaic structure had an influence on the formation of the defect such as dislocation, vacancy, and interstitial defect and also increased the diffusion of defects during growing process of ZnO films. Thus, the density of defects would be reduced inside the columns. In case of ZnO films grown at 600 °C, the FWHM value of PL spectrum curve was 76–89 meV, and these values are believed smaller value than have ever reported. Considered the -rocking FWHM value of 0.13°, both crystallinity and the optical property were simultaneously improved. This suggests that due to enough thermal energy supplied by increase of the growth temperature atoms move to stable sites and that impurities moved to grain boundary. Therefore the defect density of
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FIG. 4. PL spectra of ZnO films at RT.
FIG. 3. Cross section TEM images of ZnO films deposited at 共a兲 550 °C, 80 W and at 共b兲 550 °C, 120 W, and at 共c兲 600 °C, 120 W.
and 600 °C with the mixture of Ar:O2 ⫽4:1 shows slight uprising of deep-level emission even though it has almost the same XRD -rocking curve FWHM of 0.13° and minimum yield of ⬃5%. There have been so many investigations about the origin of deep-level emission in ZnO such as impurity and oxygen deficiency, etc. At this moment, the occurrence of deep-level emission intuitively relates to oxygen deficiency and the accurate reason will be reported later. In conclusion, ZnO films with high crystalline and optical quality could have been grown on ␣-Al2O3 共0001兲 by rf magnetron sputtering. The ZnO film deposited at 120 W and 600 °C exhibits the optimum values of XRD -rocking FWHM and channeling minimum yield of 0.13° and 4%– 5%, and corresponding FWHM of PL at RT is 76–89 meV, which is a narrower value than any other ever observed. Through TEM study, PL properties of the ZnO films are found to closely relate to microstructure like grain size and defects. Our results manifestly suggests that heteroepitaxial ZnO films with high quality can be grown on sapphire substrate for optoelectronic device by rf magnetron sputtering. This work was supported in part by Basic Researches for the Future 共2N18600兲.
inside column is diminished and PL properties of ZnO films are improved. Also from TEM study, it is found that the PL property of thin films with large grain size having lower density of defects was improved. If growth rate of film has not enough due to decrease of rf power, the optical properties of films would be degraded because of small grain size. Luminescence of GaN/sapphire was reported to be inhomogeneous and thus closely correlated with microstructure.8 NBE emission originated in the bulk of GaN hexagonal crystallite and deep-level emission are associated with the crystal boundaries and dislocation. In the present work, a similar phenomenon could be observed in the structure of ZnO/ sapphire. On the other hand, the film 共f兲 deposited at 120 W
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