Open Access
Add to BibSonomyAbstract
Zinc oxide thin films were deposited via radio frequency magnetron sputtering. Hydrogenation of the films was achieved by annealing them at 400 °C under a hydrogen flow rate of 100 sccm. The influence of annealing on the structural, morphological, and optical properties of the films were investigated. X-ray diffraction indicated that the films were polycrystalline and that their crystallinity improved upon annealing. Atomic force microscopy revealed the columnar structure of the films and indicated that the surface roughness increased with annealing. The optical constants of the films were derived from spectrophotometric measurements. The optical properties were improved upon annealing, as revealed by the increase of the refractive index, decrease of the extinction coefficient, and the widening of the band gap.
© 2014 Optical Society of America
1. Introduction
Zinc oxide (ZnO) is a II-VI semiconductor that has received vast interest during the last three decades. It has a direct and wide band gap (~3.3 eV), which makes it transparent to most of the visible spectrum. In addition, among wide band gap semiconductors, ZnO has the largest exciton binding energy (~60 meV), which is much larger than the thermal energy of 25 meV at room temperature. This allows lasing action based on excitonic transitions at room temperature [1,2]. Due to these unique properties, ZnO thin films have found important applications in several fields, including transparent conductors [3], solar cells [4], ultraviolet optoelectronics [1,2], gas sensors [5] and bio sensors [6], spintronics [7], thin film transistors [8], and surface acoustic wave devices [9]. Hydrogen plays a beneficial role in improving the properties of ZnO thin films. Hydrogenation of ZnO thin films provides the benefits of: (i) thermal and chemical stability, (ii) reduction of the intrinsic resistivity, and (iii) passivation of defects, which is of crucial importance to high-efficiency solar cells [10,11]. Several methods have been applied to obtain hydrogenated ZnO thin films, such as (i) incorporation of hydrogen during film deposition [10,12,13], (ii) post-deposition H+ ion implantation [14], (iii) post-deposition hydrogen plasma irradiation [15,16], and (iv) post-deposition thermal annealing in a hydrogen ambient. Our work in this paper is related to the last approach. Extensive research has been performed to investigate the influence of annealing in a hydrogen atmosphere on the properties of undoped ZnO thin films deposited by several techniques, including chemical vapor deposition [17,18], pulsed laser deposition [19,20], molecular beam epitaxy [21], sol-gel deposition [22], and radio-frequency (RF) magnetron sputtering [11,23–25]. The major emphasis in these investigations was on the influence of annealing on the electrical [11,17,18,21,22,24] and photo-luminescent [19–21,23] properties. Fewer studies investigated the influence of annealing on the band gap [11,22].
Optoelectronic applications of ZnO thin films require accurate determination of the values and dispersion of the optical constants, namely the refractive index, the extinction coefficient, and the band gap. These properties depend critically on the film-synthesis technique, the experimental parameters during deposition, and post-deposition treatment. Therefore, detailed studies have been performed on the determination of the optical constants of ZnO thin films deposited by various techniques, such as filtered cathodic vacuum arc deposition [26], sol-gel deposition [27,28], and pulsed laser deposition [29]. The influence on the optical constants of these films as a result of annealing in air [26,27] or argon [28] has been investigated. In particular, special emphasis has been devoted to the analysis of the optical constants of RF-sputtered ZnO thin films [30–33], and the influence of annealing in air [31], oxygen [32], and vacuum [33] on them.
Annealing in hydrogen has been found to drastically influence the electrical properties of ZnO thin films, reducing their intrinsic resistivity by several orders of magnitude [11]. However, the influence of hydrogen annealing on the optical constants of the films has yet to be determined. In this work, ZnO thin films were deposited by RF magnetron sputtering. Subsequently, the films were annealed in a hydrogen atmosphere. The influence of hydrogen annealing on the optical constants of ZnO thin films was investigated.
2. Experimental details
ZnO thin films were deposited using RF magnetron sputtering. Thin film deposition was performed using an Oerlikon Univex 350 sputtering system. The target was a 3-inch-diamter ZnO disk (99.9% purity). The RF sputtering power was 150 W, and the source-to-substrate distance was 10 cm. First, the system was evacuated to a base pressure of 5 × 10−4 Pa. Then, the sputtering gas (argon of 99.99% purity) was admitted into the system. The argon flow rate was 6 sccm (standard cubic centimetre per minute). Plasma was obtained through application of the RF power. The target was pre-sputtered for 5 minutes before a shutter was opened allowing the sputtered species to be deposited onto the substrates. The films were deposited onto fused silica substrates at a substrate temperature of 400 °C. For better film uniformity, the substrates were rotated during deposition at a speed of 6 rpm. Sputtering was performed for two hours. Post deposition annealing was performed in a tube furnace, which was first evacuated to a base pressure of 10−2 Pa. Then, high-purity hydrogen (99.99%) was admitted through a mass flow controller. Annealing was performed at a temperature of 400 °C, under a hydrogen flow rate of 100 sccm for four hours. These conditions were selected to maximize the influence of hydrogen annealing. Previous studies indicated that the properties of the films were modified up to an annealing time of 1 hour and then were saturated thereafter [17,34]. Moreover, most of the previous annealing studies were performed in a forming gas. Experiments performed in pure hydrogen were mostly done under a hydrogen flow rate less than 100 sccm [25,35]. Annealing the films at or below 300 °C did not result in significant changes in their properties. We also attempted to anneal the films at a temperature of 500 °C or 600 °C. However, the films were completely removed after such annealing (dry etching), as was also reported by other investigators [20,36]. The structural properties of the films were investigated using X-ray diffraction (XRD) and atomic force microscopy (AFM). XRD was performed using a Bruker D8 Advanced Diffractometer equipped with a Cu-Kα radiation source. The 2θ step and step acquisition time were 0.02° and 1.00 s, respectively. The surface morphology of the films was examined using contact mode AFM (Veeco Innova diSPM). The surface of the sample was probed with a silicon tip of 10 nm radius oscillating at its resonant frequency of 300 kHz. The scan area was 5 × 5 μm2, and the scan rate was 2 Hz. The optical properties of the films were determined by measuring the normal-incidence transmittance (T) and reflectance (R) of the films over the wavelength range λ of 300 to 1000 nm, using a Jasco V-570 double beam spectrophotometer. The electrical properties of the films were measured using the four-point probe method (Ecopia Hall effect measurement system, HMS-3000). The resistivity was found to be 1.2 × 101 and 9.0 × 10−3 Ω.cm for the as-deposited and annealed films, respectively. The carrier concentrations were found to be 2 × 1017 and 5 × 1019 cm−3 for the as-deposited and annealed films, respectively.
3. Results and discussion
3.1 Structural properties
Figure 1 shows the XRD patterns of the films. Identification of the peaks was based on the International Center for Diffraction Data (ICDD) file number 01-076-0704. The as-deposited films were polycrystalline, with enhanced growth along the (100) direction. The same polycrystalline structure was preserved by the annealed films, although with higher peak intensity. The crystallite size, as calculated using the Scherrer formula [37], was found to be 10.2 nm and 13.2 nm for the as-deposited and annealed films, respectively. Annealing provides thermal energy that induces coalescence among the small crystallites of the as-deposited films, resulting in enhanced crystalline growth [31].
AFM was used to characterize the morphology and film growth mode and also to estimate the surface root-mean-square roughness (Rrms). A three-dimensional AFM image of an as-deposited film is shown in Fig. 2. On the nano-scale, the films had non-uniform and rough surfaces, with a columnar microstructure. The values of Rrms were 12.8 nm and 16.0 nm, for the as-deposited and annealed films, respectively.
3.2 Optical properties
Figure 3 shows typical reflectance and transmittance spectra of an as-deposited ZnO thin film and the spectra of the same film after annealing. The films were transparent down to a wavelength (λ) of 450 nm. The transmittance spectra of the thin films were characterized by the appearance of maxima and minima that are characteristic of dielectric films, indicating the presence of sharp interfaces between the films and substrates.
In the transparency region of the films, the transmittance spectra were fitted using the formula for the transmittance of a thin film on a transparent substrate [38,39]:
withwhere n is the refractive index of the film, k is the extinction coefficient of the film, ns is the refractive index of the substrate, d is the thickness of the film, and β = exp (– 4πkd/λ). To fit the experimental transmittance spectra using Eq. (1), models for the dispersion of n and k must be implemented. The refractive index of the films was modeled by a Cauchy dispersion formula:where no, A1 and A2 are constants. The extinction coefficient of the films was modeled by Urbach law of exponential absorption below the band gap:where ko is a constant, E1 is an energy representing the onset of absorption, and δ is the Urbach band energy width. The least squares method was used to fit the experimental transmittance spectra using Eq. (1), with Eqs. (2) and (3) as the models for the optical constants. The fitting parameters were no, A1, A2, d, ko, E1, and δ. The best-fit parameters are shown in Table 1, and the correlation between the experimental spectra and the theoretical fits was 99.6%. These parameters were used to calculate the optical constants of the films.
Table 1. Best-fit parameters for the optical properties of the films
The dispersion curves of the refractive index are shown in Fig. 4(a), along with a comparison with the refractive index of RF-sputtered films reported in the literature [31] as well as those of bulk ZnO [40]. The increase of the refractive index due to annealing may be explained on the basis of the packing density of the films. The morphological results indicated that the films had columnar microstructure, which induces voids and reduces the packing density of the films [41]. An estimate of the density of a film may be obtained from the Lorentz-Lorenz relation [42]:
where ρf and ρb are the densities of the film and bulk material, respectively; and nb is the refractive index of the bulk material. The relative density (ρf/ρb) was calculated at λ = 550 nm. The resulting values were 0.926 and 0.933 for the as-deposited and annealed films, respectively. In these calculations, the value of nb was taken from reference 40, and the values of n were calculated using our dispersion relations. Thus, annealing provides thermal energy that increases the mobility of the atoms of the films, thereby increasing the packing density of the films [43].
Fig. 4 Dispersion curves of the refractive index (a) and extinction coefficient (b). Literature values are also included in (a) for comparison.
The variation of the extinction coefficient with wavelength is shown in Fig. 4(b). In general, the films exhibited low absorption, as revealed by the low values of k (< 5 × 10−3). The reduction of the extinction coefficient upon annealing could be attributed to the improved crystallinity of the annealed films. In this spectral region, absorption is caused by structural defects rather than by inter-band electronic transitions [33]. The density of structural defects is proportional to the degree of structural disorder and lack of crystallinity. The increase of crystallite size implies improved crystallinity and reduction of structural defects, which resulted in reduced optical absorption. The decrease of k with annealing indicates the passivation of defects with hydrogenation [15].
In the fundamental absorption region (λ ≤ 400 nm), the absorption coefficient (α) is related to the transmittance and reflectance of the films as:
Moreover, above the fundamental absorption edge, the dependence of the absorption coefficient on the incident photon energy (E) is given by:where αo is a constant, Eg is the band gap of the material, and the exponent η depends on the type of transitions involved. For a direct band gap material, η = ½. The band gap was calculated using the above Eqs. and was found to be 3.29 and 3.31 eV for the as-deposited and annealed films, respectively. These values are consistent with the results shown in Fig. 3. The band gap of the as-deposited films is identical to those reported for RF sputtered ZnO thin films [44]. The slight increase of the band gap upon annealing was observed in various hydrogenated ZnO thin films, and was attributed to the Burstein-Moss effect [15], which is due to the Fermi level moving into the conduction band as a result of the increase of the carrier concentration that increased by a factor of two upon annealing.4. Conclusions
In conclusion, we obtained hydrogenated RF-sputtered ZnO thin films through annealing in 100 sccm of hydrogen at 400 °C for four hours. As a result of annealing, there was a drastic increase of the electrical of conductivity of the films by four orders of magnitude. Annealing also lead to an enhancement of the crystallinity of the films. The optical properties were improved significantly by hydrogenation, as exhibited by the densification of the films, the reduced optical absorption, and the wider band gap. This result is contrary to the general perception that annealing of ZnO leads to increased roughness and thus deterioration of the optical properties. The resulting properties render hydrogenated ZnO thin films very important for optoelectronic applications, such as transparent conductors and high efficiency solar cells.
Acknowledgment
This work was supported by the Physics Department of King Fahd University of Petroleum and Minerals.
References and links
1. U. Özgür, Y. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Dogan, V. Avrutin, S. J. Cho, and H. Morkoc, “A comprehensive review of ZnO materials and devices,” J. Appl. Phys. 98(4), 041301 (2005). [CrossRef]
2. C. Klingshirn, ““ZnO: From basics towards applications,” Phys,” Status Solidi B-Basic Solid State Phys. 244(9), 3027–3073 (2007). [CrossRef]
3. J. H. Lee, K. H. Ko, and B. O. Park, “Electrical and optical properties of ZnO transparent conducting films by sol-gel method,” J. Cryst. Growth 247(1-2), 119–125 (2003). [CrossRef]
4. T. P. Chou, Q. F. Zhang, G. E. Fryxell, and G. Z. Cao, “Hierarchically structured ZnO film for dye-sensitized solar cells with enhanced energy conversion efficiency,” Adv. Mater. 19(18), 2588–2592 (2007). [CrossRef]
5. M. Suchea, S. Christoulakis, K. Moschovis, N. Katsarakis, and G. Kiriakidis, “ZnO transparent thin films for gas sensor applications,” Thin Solid Films 515(2), 551–554 (2006). [CrossRef]
6. Z. Yan, X. Y. Zhou, G. K. H. Pang, T. Zhang, W. L. Liu, J. G. Cheng, Z. T. Song, S. L. Feng, L. H. Lai, J. Z. Chen, and Y. Wang, “ZnO-based films bulk acoustic resonator for high sensitivity biosensor applications,” Appl. Phys. Lett. 90(14), 143503 (2007). [CrossRef]
7. F. Pan, C. Song, X. J. Liu, Y. C. Yang, and F. Zeng, ““Ferromagnetism and possible application in spintronics of transition-metal-doped ZnO films,” Mater. Sci. Eng,” R-Rep. 62(1), 1–35 (2008). [CrossRef]
8. P. K. Shin, Y. Aya, T. Ikegami, and K. Ebihara, “Application of pulsed laser deposited zinc oxide films to thin film transistor devices,” Thin Solid Films 516(12), 3767–3771 (2008). [CrossRef]
9. J. B. Lee, M. H. Lee, C. K. Park, and J. S. Park, “Effects of lattice mismatches in ZnO/substrate structures on the orientations of ZnO films and characteristics of SAW devices,” Thin Solid Films 447-448, 296–301 (2004). [CrossRef]
10. Y. H. Kim, S. Z. Karazhanov, and W. M. Kim, ““Influence of hydrogen on electrical and optical properties of ZnO films,” Phys,” Status Solidi B-Basic Solid Sate Phys. 248(7), 1702–1707 (2011). [CrossRef]
11. B. L. Zhu, J. Wang, S. J. Zhu, J. Wu, R. Wu, D. W. Zeng, and C. S. Xie, “Influence of hydrogen introduction on structure and properties of ZnO thin films during sputtering and post-annealing,” Thin Solid Films 519(11), 3809–3815 (2011). [CrossRef]
12. A. Singh, S. Chaudhary, and D. K. Pandya, “Hydrogen incorporation induced metal-semiconductor transition in ZnO:H thin films sputtered at room temperature,” Appl. Phys. Lett. 102(17), 172106 (2013). [CrossRef]
13. L. Y. Chen, W. H. Chen, J. J. Wang, F. C. Hong, and Y. K. Su, “Hydrogen-doped high conductivity ZnO films deposited by radio-frequency magnetron sputtering,” Appl. Phys. Lett. 85(23), 5628–5630 (2004). [CrossRef]
14. S. Kohiki, M. Nishitani, T. Wada, and T. Hirao, “Enhanced conductivity of zinc oxide thin films by ion implantation of hydrogen atoms,” Appl. Phys. Lett. 64(21), 2876–2878 (1994). [CrossRef]
15. P. F. Cai, J. B. You, X. W. Zhang, J. J. Dong, X. L. Yang, Z. G. Yin, and N. F. Chen, “Enhancement of conductivity and transmittance of ZnO films by post hydrogen plasma treatment,” J. Appl. Phys. 105(8), 083713 (2009). [CrossRef]
16. J. J. Dong, X. W. Zhang, J. B. You, P. F. Cai, Z. G. Yin, Q. An, X. B. Ma, P. Jin, Z. G. Wang, and P. K. Chu, “Effects of hydrogen plasma treatment on the electrical and optical properties of ZnO films: Identification of hydrogen donors in ZnO,” ACS Appl. Mater. Interfaces 2(6), 1780–1784 (2010). [CrossRef] [PubMed]
17. S. J. Baik, J. H. Jang, C. H. Lee, W. Y. Cho, and K. S. Lim, “Highly textured and conductive undoped ZnO film using hydrogen post-treatment,” Appl. Phys. Lett. 70(26), 3516–3518 (1997). [CrossRef]
18. X. L. Chen, X. H. Geng, J. M. Xue, and L. N. Li, “Two-step growth of ZnO films with high conductivity and high roughness,” J. Cryst. Growth 299(1), 77–81 (2007). [CrossRef]
19. J. S. Kang, H. S. Kang, S. S. Pang, E. S. Shim, and S. Y. Lee, “Investigation on the origin of green luminescence from laser-ablated ZnO thin films,” Thin Solid Films 443(1-2), 5–8 (2003). [CrossRef]
20. T. Li, T. S. Herng, H. K. Liang, N. N. Bao, T. P. Chen, J. I. Wong, J. M. Xue, and J. Ding, “Strong green emission in ZnO films after H2 surface treatment,” J. Phys. D Appl. Phys. 45(18), 185102 (2012). [CrossRef]
21. W. W. Liu, B. Yao, Y. F. Li, B. H. Li, Z. Z. Zhang, C. X. Shan, D. X. Zhao, J. Y. Zhang, D. Z. Shen, and X. W. Fan, “Structure, luminescence and electrical properties of ZnO thin films annealed in H2 and H2O ambient: A comparative study,” Thin Solid Films 518(14), 3923–3928 (2010). [CrossRef]
22. Y. Natsume and H. Sakata, “Electrical and optical properties of zinc oxide films post-annealed in H2 after fabrication by sol-gel process,” Mater. Chem. Phys. 78(1), 170–176 (2003). [CrossRef]
23. W. S. Shi, O. Agyeman, and C. N. Xu, “Enhancement of the light emissions from zinc oxide films by controlling the post-treatment ambient,” J. Appl. Phys. 91(9), 5640–5644 (2002). [CrossRef]
24. J. Kim, M. C. Kim, J. Yu, and K. Park, “H2/Ar and vacuum annealing effect of ZnO thin films deposited by RF magnetron sputtering system,” Curr. Appl. Phys. 10(3), S495–S498 (2010). [CrossRef]
25. H. W. Park, K. B. Chung, and J. S. Park, “A role of oxygen vacancy on annealed ZnO film in the hydrogen atmosphere,” Curr. Appl. Phys. 12, S164–S167 (2012). [CrossRef]
26. Y. C. Liu, S. K. Tung, and J. H. Hsieh, “Influence of annealing on optical properties and surface structure of ZnO thin films,” J. Cryst. Growth 287(1), 105–111 (2006). [CrossRef]
27. M. P. Bole and D. S. Patil, “Effect of temperature on the optical constants of zinc oxide films,” J. Phys. Chem. Solids 70(2), 466–471 (2009). [CrossRef]
28. S. W. Xue, X. T. Zu, W. L. Zhou, H. X. Deng, X. Xiang, L. Zhang, and H. Deng, “Effects of post-thermal annealing on the optical constants of ZnO thin film,” J. Alloy. Comp. 448(1-2), 21–26 (2008). [CrossRef]
29. X. W. Sun and H. S. Kwok, “Optical properties of epitaxially grown zinc oxide films on sapphire by pulsed laser deposition,” J. Appl. Phys. 86(1), 408–411 (1999). [CrossRef]
30. E. M. Bachari, G. Baud, S. Ben Amor, and M. Jacquet, “Structural and optical properties of sputtered ZnO films,” Thin Solid Films 348(1-2), 165–172 (1999). [CrossRef]
31. V. Gupta and A. Mansingh, “Influence of postdeposition annealing on the structural and optical properties of sputtered zinc oxide films,” J. Appl. Phys. 80(2), 1063–1073 (1996). [CrossRef]
32. N. A. Suvorova, I. O. Usov, L. Stan, R. F. DePaula, A. M. Dattelbaum, Q. X. Jia, and A. A. Suvorova, “Structural and optical properties of ZnO thin films by rf magnetron sputtering with rapid thermal annealing,” Appl. Phys. Lett. 92(14), 141911 (2008). [CrossRef]
33. M. F. Al-Kuhaili, S. M. A. Durrani, I. A. Bakhtiari, and M. Saleem, “Optical constants of vacuum annealed radio frequency (RF) magnetron sputtered zinc oxide thin films,” Opt. Commun. 285(21-22), 4405–4412 (2012). [CrossRef]
34. B. Y. Oh, M. C. Jeong, D. S. Kim, W. Lee, and J. M. Myoung, “Post-annealing of Al-doped ZnO films in hydrogen atmosphere,” J. Cryst. Growth 281(2-4), 475–480 (2005). [CrossRef]
35. C. Huang, M. Wang, Z. Deng, Y. Cao, Q. Liu, Z. Huang, Y. Liu, W. Guo, and Q. Huang, “Effects of hydrogen annealing on the structural, optical and electrical properties of indium-doped zinc oxide films,” J. Mater. Sci. Mater. Electron. 21(11), 1221–1227 (2010). [CrossRef]
36. H. Tong, Z. Deng, Z. Liu, C. Huang, J. Huang, H. Lan, C. Wang, and Y. Cao, “Effects of post-annealing on structural and electrical properties of Al-doped ZnO thin films,” Appl. Surf. Sci. 257(11), 4906–4911 (2011). [CrossRef]
37. B. D. Cullity and S. R. Stock, Elements of X-Ray Diffraction, 3rd Ed. (Prentice-Hall, 2001), pp 167–171.
38. O. S. Heavens, Optical Properties of Thin Solid Films (Dover, 1991), p.78.
39. J. C. Manifacier, J. Gasiot, and J. P. Fillard, “A simple method for the determination of the optical constants n, k and the thickness of a weakly absorbing thin film,” J. Phys. E 9(11), 1002–1004 (1976). [CrossRef]
40. H. Yoshikawa and S. Adachi, “Optical constants of ZnO,” Jpn. J. Appl. Phys. 36(1), 6237–6243 (1997). [CrossRef]
41. D. Mergel, “Modeling thin TiO2 films of various densities as an effective optical medium,” Thin Solid Films 397(1-2), 216–222 (2001). [CrossRef]
42. M. Harris, H. A. Macleod, S. Ogura, E. Pelletier, and B. Vidal, “The relationship between optical inhomogeneity and film structure,” Thin Solid Films 57(1), 173–178 (1979). [CrossRef]
43. T. Tan, Z. Liu, H. Lu, W. Liu, and H. Tian, “Structure and optical properties of HfO2 thin films on silicon after rapid thermal annealing,” Opt. Mater. 32(3), 432–435 (2010). [CrossRef]
44. B. E. Sernelius, K. F. Berggren, Z. C. Jin, I. Hamberg, and C. G. Granqvist, “Band-gap tailoring of ZnO by means of heavy Al doping,” Phys. Rev. B Condens. Matter 37(17), 10244–10248 (1988). [CrossRef] [PubMed]
