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ARTICLE IN PRESS

Nuclear Instruments and Methods in Physics Research B xxx (2005) xxx­xxx www.elsevier.com/locate/nimb

Smoothing of ZnO films by gas cluster ion beam
H. Chen *, S.W. Liu, X.M. Wang, M.N. Iliev, C.L. Chen, X.K. Yu, J.R. Liu, K. Ma, W.K. Chu
Department of Physics and Texas Center for Superconductivity at University of Houston, University of Houston, Houston, TX 77204, USA Available online

Abstract Planarization of wide-band-gap semiconductor ZnO surface is crucial for thin-film device performance. In this study, the rough initial surfaces of ZnO films deposited by r.f. magnetron sputtering on Si substrates were smoothed by gas cluster ion beams. AFM measurements show that the average surface roughness (Ra) of the ZnO films could be reduced considerably from 16.1 nm to 0.9 nm. Raman spectroscopy was used to monitor the structure of both the as-grown and the smoothed ZnO films. Rutherford back-scattering in combination with channeling effect was used to study the damage production induced by the cluster bombardment. ñ 2005 Elsevier B.V. All rights reserved.
PACS: 81.65.þb; 36.40.þc; 61.82.Pv; 85.40.Xx Keywords: Gas cluster ion beam; ZnO; Planarization

1. Introduction Zinc oxide is an II­VI direct-band-gap wurtzitestructured semiconductor with an energy bandgap of 3.37 eV at room temperature. The technological significance of the large energy band-gap in ZnO has been an effective driving force for research in this material. Indeed, ZnO is a promising candidate material for conventional hybrid optoCorresponding author. Tel.: +1 713 743 8259; fax: +1 713 743 8201. E-mail address: chui@mail.uh.edu (H. Chen).
*

electronic integrated circuits (OEICs) because it is transparent to the light of wavelength in the range of 0.4­2 lm, exhibits intrinsic semiconductor properties, is piezoelectric and magnetic, has sizable non-linear polarizability coefficients and shows excellent conducting behavior with suitable doping [1­7]. The general concept of photonic integration is based on the transmission of light through passive waveguides interconnecting various active optical devices on ZnO:X semiconductor substrates. Therefore, low-loss ZnO optical waveguides are key components crucial for monolithic OEICs

0168-583X/$ - see front matter ñ 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.07.100


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[8­10]. However, it was found both theoretically and experimentally that the optical propagation loss in ZnO thin films is significantly affected by film surface morphology due to scattering at rough surfaces [11,12]. Therefore, it is important to fabricate ZnO-based optical wavegiudes by following a certain film growth technique providing low defect density and surface polishing. Both physical and chemical methods have been employed to smooth ZnO film surfaces [13,14]. However, all these existing methods have problems. It is necessary to develop a new method to planarize the surface of ZnO for OEIC application. Over the past decade, a technique using gas cluster ion beam (GCIB) was developed as a material modification method for surface cleaning, smoothing of the surface topography and lowdamage sputtering [15­17]. GCIB is an ion beam of cluster of gaseous atoms with cluster size as big as a few thousand atoms or molecules bonded by van der Waals force. As each constituent atom or molecule in a cluster has partition energy of only several eVós (For example, the constituent atoms of a 30 keV cluster ion with a mean cluster size of 3000 has only 10 eV/atom), the bombarding effects of cluster ions on solid surfaces are different from those induced by monomer ions [18]. In monomer ion sputtering at normal incidence, surface atoms are removed layer by layer and the surface morphology is mostly preserved with some blurring of surface features. In cluster ion beam sputtering, the simultaneous arrival of constituent atoms at the same location produces multiple-collisions between incident atoms and the target surface atoms, which results in the lateral sputtering of many target atoms. Those lateral-sputtered atoms redeposit on depressions of the surface, thus, leading to smoothing of the solid surface. Once the surface is smoothed, additional sputtering will remove the new surface layer and maintain the smoothness. Such a unique smoothing effect from a cluster ion beam has been applied to smooth the surfaces of many materials, including metals [15], high Tc-superconducting YBCO films [16] and diamond films [17]. In this study, ZnO films grown by conventional on-axis r.f. magnetron sputtering are smoothed

with CO2 cluster ion beams. The surface topography was studied by Atomic Force Microscopy (AFM). Raman spectroscopy was used to characterize the changes of structure of both the asgrown and the smoothed ZnO films. Rutherford back-scattering (RBS) in combination with channeling was performed on a commercial ZnO single crystal (Princeton Scientific) to study the damage production induced by cluster bombardment.

2. Experimental Zinc oxide (ZnO) films were deposited on Si(0 0 1) substrates by r.f. magnetron sputtering. The details of the deposition will be described elsewhere [19]. XRD h­2h spectrum shows only (0 0 1) reflections and the rocking curve measurement on the ZnO(0 0 0 2) reflection has a FWHM of about 0.5°. High-resolution TEM shows excellent epitaxial growth of the ZnO films on the Si(0 0 1) substrate [19]. A schematic view of the gas cluster ion beam experimental setup used for the ZnO smoothing has been presented elsewhere [15]. The clusters are generated by adiabatic expansion of CO2 gases through a lava nozzle. The clusters mean size is around 1000. This is inferred from published data by choosing appropriate source pressure [20]. Epitaxial ZnO films deposited on Si by r.f. sputtering were irradiated with 30 keV CO2 clusters at a normal incident angle with several different dosages ranging from 5 · 1014 clusters/cm2 to 1 · 1016 clusters/cm2. Sputtering yields were obtained from the sputtered depth as measured by RBS. The surface morphology and roughness were studied by AFM. The Raman spectra were measured using a Jobin Yvon HR640 spectrometer equipped with microscope, notch filters and a liquid-nitrogen-cooled CCD detector. The 514.5nm line from an Ar ion laser was used for excitation. A 100· objective was used for micro-Raman with a typical laser input power of 1 mW. The diameter of the focal point of the beam was approximately 2 lm. To quantify the damage induced by the smoothing process, ZnO single crystals were bombarded together with the asdeposited ZnO films. Damage production was


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studied by RBS/channeling using an 1.7 MV tandem accelerator (NEC, 5SDH) with a 2.0 MeV alpha particle beam. The beam was focused and collimated to a 0.7 mm spot. Back-scattered alpha particles were collected in a silicon barrier detector positioned at 165° relative to the direction of the incident beam.

3. Results and discussion ZnO surface morphologies as observed by AFM before and after irradiation by accelerated CO2 GCIB are shown and compared in Fig. 1. The as-deposited ZnO film shows columnar morphology with a rough surface (see Fig. 1(a)) since the crystalline growth rate varies along different crystalline directions. The typical average surface

roughness (Ra) of the as-deposited ZnO film is around 16.1 nm. Fig. 1(b)­(d) show the surface morphology changing as a function of the incident CO2 GCIB dose at normal incidence with 30 keV energy. It was found that the columnar protrusions were removed during GCIB bombardment, but moderate undulations still remain over the surface when the incident cluster ion dose reaches 5 · 1015 clusters/cm2 (see Fig. 1(c)). For higher dose bombardment, such as 1 · 1016 clusters/cm2, the AFM image measured that the average surface roughness of the ZnO film can be improved gradually from 16.1 nm to 0.9 nm (see Fig. 1(d)). The same improvements were obtained on six samples with different initial surface roughness ranging from 10.0 nm to 16.1 nm. The total removed layer was around 65.2 nm after 30 keV CO2 cluster irradiation at dose of 1 · 1016 clusters/cm2. We mea-

Fig. 1. AFM images of the ZnO films before and after CO2 gas cluster ion beam irradiation. (a) Unirradiated, cluster ion irradiated ZnO surface at (b) 5 · 1014 clusters/cm2, (c) 5 · 1015 clusters/cm2 and (d) 1 · 1016 clusters/cm2.


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sured the thickness of the removed layer using RBS before and after cluster bombardment. According to molecular dynamic simulations [21,22], the cluster ion beam smoothing result comes not only from the lateral sputtering effect but also from differing sputtering rates at irradiation sites with different slopes. The sputtered atoms migrate to or redeposit in valleys. As a result, cluster ion beams preferentially remove the surface protrusions, leading to a smooth solid surface. After smoothing with GCIB, we studied the possible crystalline structure modification induced by the cluster ion bombardment. Fig. 2 shows the room temperature Raman spectrum of the ZnO film irradiated with CO2 gas cluster at a dose of 1 · 1016 clusters/cm2 in the Z ÏXY ÷Z scattering configuration. The Raman spectrum of the as-deposited ZnO film is also shown for comparison. The wurtzite-structured ZnO belongs to the space group C 4 with two formula units in the primitive 6v cell. The optical phonon modes can be classified as 1A1 + 2B1 + 1E1 + 2E2 [23]. Among these, both the A1 and the E1 modes are Raman and infrared active, the E2 modes are Raman active only and the B1 modes are inactive. For the as-deposited ZnO film, Raman spectrum in the Z ÏXY ÷Z scattering configuration shows only one sharp peak corresponding to the E2 mode at 437.3 cmþ1. The line at 521 cmþ1 belongs to the silicon substrate.

ZnO/Si
E
2

z(xy)z

as grown E2 smoothed

Si substrate

400 420 440 460 480 500 520 540 560 580 600

RAMAN SHIFT [cm-1]
Fig. 2. Raman spectra of unirradiated and CO2 gas cluster irradiated ZnO films.

For the ZnO film smoothed with CO2 cluster ions, neither additional lines nor shift of the line from the E2 mode was observed. The intensity of the line from the E2 mode for the smoothed ZnO film is much weaker than the initial one because the film became much thinner. These results indicated that the smoothed ZnO film retain excellent crystalline symmetry. Although GCIB improved the surface roughness without changing the crystalline symmetry, we still have to consider the undesirable effects due to surface damage to the crystalline structure during irradiation. To quantify the damage formation induced by the cluster ion bombardment, we perform RBS/channeling measurements on a high-quality commercially available ZnO single crystal substrate before and after CO2 cluster bombardment. The cluster bombardment of ZnO single crystal was done simultaneously with the smoothing of the ZnO films. RBS/channeling spectra from both the initial and cluster irradiated ZnO single crystal substrate shown in Fig. 3 indicate that the initial single crystal had a vmin % 2.5% and the surface peak in the channeled spectrum corresponded to a layer of 1.1­2.0 nm. After the 1 · 1016 clusters/cm2 cluster ion bombardment, the vmin increased to 6.4% with the damaged layer thickness being 6.8­11.1 nm. It is interesting to mention that both the vmin and the damage layer thickness are independent of the incident cluster ion beam dosage within the range of the current study (from 1 · 1015 to 1 · 1016 clusters/cm2). This can be understood to follow from the large cluster size of the incident beam. The damaged surface layer has to be removed before further device processing, or else, it will degrade device performance. Either chemical etching or physical methods such as post-smoothing annealing can be used to remove or re-grow the damage surface layer. In the case of the chemical etching method, the damage layer was completely removed by a 0.1% HCl solution in 2 s as shown in Fig. 3. The channeled spectrum of the chemically etched specimen is the same as that of the initial single crystal. However, the surface roughness of the smoothed ZnO film increased from 0.9 nm to 2.3 nm. As an alternative method, thermal annealing re-grew the crystal as verified by chan-

SCATTERING INTENSITY [arb. units]


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40000 35000 30000
Before irradiation 16 2 30keV, 1x10 clusters/cm After chemical etching Random

Relative Yield

25000 20000 15000 10000 5000 0 0 100 200 300 400

Channel number
Fig. 3. RBS/channeling analysis of the surface damage in ZnO single crystal induced by gas cluster irradiation before and after irradiation and smoothed specimen with chemical etching.

neling. However, the average surface smoothness worsens to 5.3 nm due to the non-uniform regrowth. Better methods of damaged layer removal while maintaining the surface smoothness are under development.

References
[1] A. Onodera, K. Yoshio, H. Satoh, H. Yamashita, N. Sakagami, Jpn. J. Appl. Phys., Part 1 37 (1998) 5315. [2] M. Joseph, H. Tabata, T. Kawai, Appl. Phys. Lett. 74 (1999) 2534. [3] T. Nagata, A. Ashida, N. Fujimura, T. Ito, J. Cryst. Growth 237­239 (2002) 533. [4] T. Wakano, N. Fujimura, Y. Morinaga, N. Abe, A. Ashida, T. Ito, Physica E 10 (2001) 260. [5] K. Ando, H. Saito, Z. Jin, T. Fukumura, M. Kawasaki, Y. Matsumoto, H. Koinuma, Appl. Phys. Lett. 78 (2001) 2700. [6] A. Suzuki, T. Matsushita, Y. Sakamoto, N. Wada, T. Fukuda, H. Fujiwara, M. Okuda, Jpn. J. Appl. Phys., Part 1 35 (1996) 5457. [7] M. Hiramatsu, K. Imaeda, N. Horio, M. Nawata, J. Vac. Sci. Technol. A 16 (1998) 669. [8] M.S. Wu, A. Azuma, T. Shiosaki, A. Kawabata, J. Appl. Phys. 62 (1987) 2482. [9] N. Mais, J.P. Reithmaier, A. Forchel, M. Kohls, L. Spanhel, G. Muller, Appl. Phys. Lett. 75 (1999) 2005. ¨ [10] S.F. Yu, C. Yuen, S.P. Lau, Y.G. Wang, H.W. Lee, B.K. Tay, Appl. Phys. Lett. 83 (2003) 4288. [11] D.J. Channin, J.M. Hammer, M.T. Duffy, Appl. Opt. 14 (1975) 923. [12] A. Ashida, H. Ohta, T. Nagata, Y. Nakano, N. Fujimura, T. Ito, J. Appl. Phys. 95 (2004) 1673. [13] M. Kadota, M. Minakata, T. Kasanami, Jpn. J. Appl. Phys. 31 (Suppl. 31-1) (1992) 225.

4. Conclusion We have studied the planarization of the rough surfaces of r.f. sputtering deposited ZnO films by using an accelerated gas cluster ion beam. Atomic level smoothing of the ZnO films was achieved. The damaged layer induced by the cluster ion bombardment can be removed by chemical etching.

Acknowledgement This work was supported by the State of Texas through the Texas Center for Superconductivity and Advanced Materials at the University of Houston and in part by Robert A. Welch Foundation.


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H. Chen et al. / Nucl. Instr. and Meth. in Phys. Res. B xxx (2005) xxx­xxx [18] I. Yamada, G.H. Takaoka, M.I. Current, Y. Yamashita, M. Ishii, Nucl. Instr. and Meth. B 74 (1993) 341. [19] S.W. Liu, C.L. Chen, in press. [20] O.F. Hagena, Surf. Sci. 106 (1981) 101. [21] N. Toyoda, N. Hagiwara, J. Matsuo, I. Yamada, Nucl. Instr. and Meth. B 161­163 (2000) 980. [22] M. Moseler, O. Rattunde, J. Nordiek, H. Haberland, Nucl. Instr. and Meth. B 164­165 (2000) 522. [23] R. Loudon, Adv. Phys. 13 (1964) 423.

[14] A.Y. Polyakov, N.B. Smirnov, E.A. Kozhukhova, V.I. Vdovin, K. Ip, Y.W. Heo, D.P. Norton, S.J. Pearton, Appl. Phys. Lett. 83 (2003) 1575. [15] I. Yamada, J. Matsuo, N. Toyoda, A. Kirkpatrick, Mater. Sci. Eng. R 34 (2001) 231. [16] W.K. Chu, Y.P. Li, J.R. Liu, J.Z. Wu, S.C. Tidrow, N. Toyoda, J. Matsuo, I. Yamada, Appl. Phys. Lett. 72 (1998) 246. [17] N. Toyoda, N. Hagiwara, J. Matsuo, I. Yamada, Nucl. Instr. and Meth. B 148 (1999) 639.