The cooling effect on structural, electrical, and optical properties of epitaxial a-plane
ZnO:Al on r-plane sapphire grown by pulsed laser deposition
Chun-Yen Peng, Yuan-An Liu, Wei-Lin Wang, Jr-Sheng Tian, and Li Chang
Citation: Applied Physics Letters 101, 151907 (2012); doi: 10.1063/1.4759032 View online: http://dx.doi.org/10.1063/1.4759032
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/101/15?ver=pdfcov Published by the AIP Publishing
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The cooling effect on structural, electrical, and optical properties of epitaxial
a-plane ZnO:Al on r-plane sapphire grown by pulsed laser deposition
Chun-Yen Peng, Yuan-An Liu, Wei-Lin Wang, Jr-Sheng Tian, and Li Changa)
Department of Materials Science and Engineering, National Chiao Tung University, 30010 Hsinchu, Taiwan
(Received 13 July 2012; accepted 1 October 2012; published online 11 October 2012)
Here, the unambiguous effect of cooling rate on structural, electrical, and optical properties of a-plane ZnO:Al on r-plane sapphire grown by pulsed laser deposition at 700C is reported. A high cooling rate (100C/min) can result in stripe morphology along m-direction and significant
deformation on the epitaxial films of a-plane ZnO:Al with deteriorated crystallinity and significantly lowered resistivity. Also, photoluminescence spectra exhibit high intensities of excess violet and green emissions with low intensity of near band edge luminescence. Comparison with purea-plane ZnO films is also presented.VC 2012 American Institute of Physics.
[http://dx.doi.org/10.1063/1.4759032]
As a wide direct bandgap wurtzite semiconductor, zinc oxide (ZnO) is an attractive material for potential applica-tions in light emitting devices, transparent thin-film transis-tors, and transparent electrodes. Especially, it exhibits an intense near-band-edge excitonic emission at room tempera-ture due to its large exciton binding energy (60 meV), and low electrical resistivity with high transparency can be achieved by doping with Al, Ga, and In. These advantages of ZnO-based materials have attracted much attention in the last few years.
Because of the low cost, abundant resource availability and low electrical resistivity compared with other IIIA impurities, Al-doped ZnO (AZO) is a potential transparent conducting oxide (TCO) to replace for indium tin oxide.1 Though deposition of polycrystalline AZO as TCO has been intensively studied for practical applications, fundamental understanding is still lacking. Epitaxial ZnO films doped with Al (ZnO:Al) may be beneficial for studying the proper-ties of TCO due to better crystallinity.
Recently, the growth of nonpolar epitaxial films has attracted a lot of interest as nonpolar ZnO can have better light emitting performance without the quantum confined Stark effect often observed inc-plane ZnO.2–5R-plane sap-phire is a commonly used substrate for growth of a-plane ZnO films. However, there are large anisotropic structural properties along two in-plane directions between nonpolar ZnO and substrate. For example, the lattice mismatch between ZnO ½0001 and sapphire ½1011 is 1.5% and the thermal mismatch is 40.8%, while the lattice mismatch between ZnO½1 100 and sapphire ½1210 is 18.3% and the thermal mismatch is 7%.6–8Such large anisotropic structural properties may strongly affect the crystallinity of epitaxial film.
For actual light-emitting devices, a good and stable n-type layer ofa-plane ZnO is required as well. Among vari-ous doped ZnO, ZnO:Al is a promising n-type candidate.
Only a few reports have shown growth of epitaxial a-plane ZnO:Al onr-sapphire with properties,9–11though the effects of large anisotropic lattice mismatch between ZnO and sap-phire on structural properties have been reported by many studies.12–16 In this paper, the effect of cooling rate on the crystal, electrical, and optical properties for epitaxiala-plane ZnO:Al on r-plane sapphire is reported, which are signifi-cantly different from those of ZnO deposited under identical conditions.
ZnO:Al epitaxial films were deposited by pulsed laser deposition (PLD) in a home-made system and a Pascal laser molecular beam epitaxy (laser-MBE) system. For dedicated cooling experiments, the film deposition was carried out in the Pascal laser-MBE system, which was equipped with a fiber optic pyrometer, a proportional integral derivative con-troller, and an infrared diode laser on the substrate coupled with a 10 10 mm2back plate to provide accurate
closed-loop temperature control for heating and cooling. The sub-strates of r-plane sapphire were in a size of 8 8 mm2.
Before deposition, the substrate was heated to 850C for thermal cleaning in vacuum of about 106torr for 30 min in the chamber. The growth conditions were the substrate tem-perature at 700C with 10 mtorr oxygen partial pressure. Ablation was done by applying a pulsed KrF excimer laser (3 J/cm2, 5 Hz) on a sintered target fabricated from the
mixture of 99.0 wt. % ZnO and 1.0 wt. % Al2O3 powders
with 99.9% purity. For controlled cooling experiments, 200 nm thick films were grown, followed by direct cooling with different rate in oxygen ambient of 10 mTorr. For com-parison, pure ZnO films were also grown under the same condition with controlled cooling. To evaluate crystallinities, x-ray diffraction (XRD) with x-ray rocking curves (XRCs) and reciprocal space maps (RSM) were performed in a PAN-alytical X’Pert Pro (MRD) high-resolution x-ray diffraction system employing a Ge (220) monochromator and Ge (220) channel cut analyzer. Surface morphologies of the deposited films were examined in a JEOL JSM-6700F field emission
0.3 T at room temperature (ACCENT HL5500 Hall measure-ment system). The photoluminescence (PL) spectra were acquired at 12 K by using a 325 nm Hd-Cd laser with power of about 15 mW as the excitation light source.
A typical x-ray 2h/h diffraction pattern of ZnO:Al films on r-plane sapphire is shown in Fig. 1(a), illustrating that only the peaks of sapphirer-plane and ZnO:Al a-plane can be seen at 52.55 and 56.68 without other peaks. Also, the phi-scan pattern in Fig. 1(b) shows the positions of ZnO f1100g peaks in coincidence with sapphire ð1213Þ and ð2113Þ with spacing angle of 180, indicating that the
epi-taxial relationship betweena-plane ZnO:Al and sapphire is ð1120ÞZnO:Al//ð1102Þsapphire and½1100ZnO//½1210sapphire as ZnO grown onr-plane sapphire. Figure1(c)shows a typical SEM image of the200 nm thick ZnO:Al epitaxial films. At first sight, it looks like a stripy morphology commonly observed ona-plane ZnO film. Interestingly, after determina-tion of the in-plane direcdetermina-tions, the stripes shown in Fig.1(c)
are actually normal to its c-direction different from stripes parallel to its c-direction usually seen on a-plane ZnO.12 The average width of these stripes is about 20–30 nm. The m-direction stripe characteristics have been observed on a-plane GaN by Craven et al.17 Also, it is known that the stress may play a significant role on surface morphology due to strain relaxation of a film,18–21 which can arise from the large thermal mismatch along½0001ZnO direction with r-plane sapphire. Hence, the features of stripe-like morphol-ogy with grooves observed in the ZnO:Al films might be related to its growth evolution and thermal stress. To verify the effect of thermal stress on such stripe-like morphology of a-plane ZnO:Al epitaxial film, we therefore carried out controlled cooling experiments after film growth of ZnO
and ZnO:Al under the same deposition conditions for comparison.
Figure2shows AFM surface morphologies of ZnO and ZnO:Al epitaxial films with 30 and 100C/min cooling rates. For pure ZnO grown on r-plane sapphire with 30 and 100C/min cooling rates, smooth surfaces with stripes along ½0001ZnO are observed in Figs. 2(a) and 2(b). Generally, both cooling rates do not have significant effects on surface roughness and morphology of the ZnO films. However, there are a few grooves along½1100ZnO, which can be clearly seen in three dimensional perspective view as shown in Fig.2(c)
fora-plane ZnO epitaxial film with 100C/min cooling rate. Such grooves have0.5 nm depth with 20–80 nm spacing. In contrast, relatively rougher surfaces are observed on ZnO:Al epitaxial films as shown in Figs.2(d)and2(e). How-ever, large and coarse stripy islands elongated along ½0001ZnOcan be recognized in Fig.2(e), which actually con-sist of many fine stripes with grooves along ½1100ZnO as shown in Figs.2(e)and2(f)similar to the above SEM obser-vation. For the 30C/min cooled ZnO:Al, while the island-like morphology is clearly seen in Fig.2(d), the fine stripes with shorter length can also be observed along ½1100ZnO. It is noticed that there exist a higher density of fine stripes with grooves on the ZnO:Al film with higher cooling rate. These fine stripes width along ZnOc-direction are about 20–30 nm width with 0.5–1 nm deep grooves for both ZnO:Al films with different cooling rate.
In addition to the appearance of a high density of fine stripes with grooves along m-direction with high cooling rate, the cooling effect on crystallinities has been also inves-tigated with XRCs and RSMs. Figures 3(a) and 3(b) show XRCs of ZnO epitaxial films with 30 and 100C/min
FIG. 1. (a) X-ray 2h/h diffraction pattern, (b) phi-scan patterns, and (c) SEM image ofa-plane ZnO:Al film on sapphire.
FIG. 2. AFM images of (a)-(c) ZnO and (d)-(f) ZnO:Al films with (a) and (e) 30C/min and (b), (c), (e), and (f)
100C/min cooling rates.
cooling, respectively. The full width at half maximum (FWHM) of XRC //cZnOis narrower than that of?cZnOfor
both pure-ZnO films as reported in the literatures due to large lattice mismatch present in ZnOm-direction makes c-mosaic greater than m-mosaic for a-plane ZnO grown on r-plane sapphire.22Also, it is illustrated that the higher cooling rate can result in degradation of crystallinity as the FWHMs of both XRCs //c and?c are broadened from 0.48to 0.60and
0.74 to 0.97, respectively. However, crystallinities of ZnO:Al epitaxial films grown on r-plane sapphire along //cZnOand?cZnOexhibit different characteristics as XRCs in
Figs.3(c) and3(d) show the larger FWHM in //c than?c. The FWHMs of XRCs?c for both films are in the range of 1.5–1.6, while the FWHM of XRC //c is 2.36 for 30C/ min and 3.96 for 100C/min. Also, the broad XRCs//c for ZnO:Al exhibit extended asymmetric distribution, which has been further examined with RSMs as shown in Figs.3(e)and
3(f) for the reflections of ð1120ÞZnO:Al and ð2204Þsapphire
from the ZnO:Al film with 100C/min cooling rate. A long extended asymmetrical tail is seen in Fig. 3(e) for ð1120ÞZnO:Al consistent with the asymmetric distribution
observed in XRC. Also, an asymmetrical intensity distribu-tion in the opposite direcdistribu-tion can be seen in Fig. 3(f) for ð2204Þsapphire. Hence, it may indicate that both the ZnO:Al
epitaxial film and sapphire substrate are tilted around the rotation axis of ½1120sapphire towards opposite directions
away from each other. As such opposite tilting of sapphire reflection away from ZnO:Al one, they might have compen-sated with bending strains or deformation from each other,
probably due to thermal stresses caused by cooling. For the pure ZnO films, however, no such phenomena can be observed in RSMs (not shown). Therefore, it is reasonably believed that Al dopants play a critical role on the deforma-tion in the films due to the cooling effect. Further investiga-tion may need to understand the exact role of Al dopants in thea-plane ZnO:Al film with cooling.
Hall measurements for ZnO:Al epitaxial films with 30 and 100C/min cooling show that the carrier concentrations are 3.5 1020 and 6.7 1020cm3, respectively.
Interest-ingly, even though the cooling rate has a significant effect on the crystallinities, a lowered resistivity of 6.37 104X-cm has been obtained for the 100C/min cooled ZnO:Al film than 1.62 103X-cm for the 30C/min cooled one. As the electron concentration may be compensated with oxygen,9,23 higher cooling rate with oxygen ambient could reduce the exposure time to oxygen during the cooling process. As a result, the lower resistivity with higher carrier concentration can be obtained from the sample with higher cooling rate. For the pure ZnO films, the cooling rate does not have a sig-nificant effect on the electrical properties, which show the resistivities in the order of 102X-cm with carrier concentra-tions of 1018cm3. In comparison with ZnO, ZnO:Al can have better electrically conducting properties with worse crystallinity.
Figure4shows low-temperature PL spectra of ZnO and ZnO:Al. In Fig.4(a), it is observed that luminescences of the pure ZnO epitaxial films are simply dominated by near band edge emissions with deep level emissions being hardly seen.
FIG. 3. XRCs of (a) and (b) ZnO and (c) and (d) ZnO:Al films with (a) and (c) 30C/min and (b) and
(d) 100C/min cooling rates. (e) and (f)ð1120Þ ZnO:Al
andð2204ÞsapphireRSMs for the ZnO:Al film on
For near band edge emissions in both 30 and 100C/min cooled ZnO films, typical bound exciton recombination (D0, X) appears at3.35 eV (20 meV FWHM) and defect-related emission (A0, X) at3.31 eV (30 meV FWHM).24–26 Also, it is noticed that the difference in cooling rate makes significant variation on PL intensity. The intensity of (D0, X) emission for 30C/min cooled ZnO is one order of magni-tude higher than that of 100C/min one under the same ex-perimental condition for PL measurements. Also, the luminescence intensity ratio of (D0, X):(A0, X) is decreased from 2:1 (30C/min) to 1:1 (100C/min). For the ZnO:Al epitaxial films, the variation of PL characteristics with cool-ing rate is significantly different from ZnO films. Not only broader near band edge emissions appear at different posi-tions with dissimilar intensity but also deep level emissions exhibit varied distribution as shown in Fig.4(b). It shows the near band edge emission peak at 3.37 eV with 186 meV FWHM and typical VO-related deep level green emission
27
at2.74 eV for the 30C/min cooled ZnO:Al epitaxial film.
With 100C/min cooling rate, the ZnO:Al epitaxial film shows about two and half times lower intensity for the near band edge emission peak compared with the 30C/min sam-ple. Also, deep level green emission at2.77 eV and excess Zni related deep level violet emission
27
at 3.01 eV are observed in Fig.4(d)for the 100C/min cooled ZnO:Al epi-taxial film. It is known that higher carrier concentration may result in broader near band edge emission due to impurity band broadening.28,29However, such broader near band edge emissions are not observed in thec-plane ZnO:Al epitaxial film, which has 1.7 1020cm3 carrier concentration.30 Also, the VO-related deep level and excess Znirelated
emis-sions are only appeared in our ZnO:Al epitaxial films with intensity increasing with cooling rate. Accordingly, the broader near band edge emissions with excess deep level emissions observed in the a-plane ZnO:Al epitaxial films may be resulted from Al-related defects in ZnO, which are generated from cooling.
In summary, crystallinity deterioration has been observed in Al-dopeda-plane ZnO epitaxial films grown on r-plane sapphires with cooling rate of 30 and 100C/min in comparison with ZnO. Asymmetrical deformation of the a-plane ZnO:Al films in c-direction and fine stripes along ½1100ZnO:Al are only observed in Al-doped ZnO films with
fast cooling rate. Also, the ZnO:Al film with higher cooling rate shows lower resistivity but with worse crystallinity. Finally, the difference in cooling rate may result in lower lu-minescence intensity and significant change on photolumi-nescence properties ofa-plane ZnO:Al films.
The authors gratefully acknowledge the National Sci-ence Council (Taiwan, ROC) for supporting this work (NSC Contract No. NSC98-2221-E-009-042-MY3).
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