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Influence of size effect and sputtering conditions on the crystallinity and optical properties of ZnO thin films
Optics Communications 269 (2007) 346–350 www.elsevier.com/locate/optcom
Influence of size effect and sputtering conditions on the crystallinity and optical properties of ZnO thin films V. Kapustianyk a, B. Turko a, A. Kostruba b,c, Z. Sofiani d, B. Derkowska d, S. Dabos-Seignon d, B. Barwin´ski e, Yu. Eliyashevskyi e, B. Sahraoui d,* a
Scientific Technical and Educational Center of Low Temperature Studies, Ivan Franko National University of Lviv, Dragomanova Street 50, UA-79005 Lviv, Ukraine b Lviv Academy of Commerce, Golovatskogo Street 10, UA-79005 Lviv, Ukraine c Institute for Physical Optics, Dragomanova Street 23, UA-79005 Lviv, Ukraine d Laboratory POMA, UMR CNRS 6136, University of Angers, 2 Boulevard Lavoisier, 49045 Angers, France e Institute of Experimental Physics, University of Wrocław, PL-50-204 Wrocław, Poland Received 2 March 2006; received in revised form 4 August 2006; accepted 10 August 2006
Abstract The effects of the thickness variation, substrate type and annealing on the crystallinity parameters, luminescent and optical properties of the zinc oxide (ZnO) thin films were reported. The thin films were deposited on the glass and the amorphous quartz substrates by the standard RF-magnetron sputtering method using ZnO targets in the argon atmosphere. It has been found that the films deposited on the glass substrate manifest a clear size effect. Both the structural and the optical parameters show clearly minima on their thickness dependences. It has been shown that annealing of the comparatively thick ZnO films leads to increase of the crystallite sizes that are followed by a considerable rise of the cathodoluminescence intensity. The corresponding model of the crystallite growth is proposed. Ó 2006 Elsevier B.V. All rights reserved. PACS: 78.20. e; 78.20.Ci; 78.40.Fy; 78.60.Hk Keywords: ZnO thin films; Refractive index; Cathodoluminescence
1. Introduction ZnO has been widely used for many applications, for example, surface acoustic wave (SAW) devices and transparent electrodes due to its excellent piezoelectric and transparent properties [1,2]. Moreover, ZnO has recently attracted great attention to application of optoelectronic devices because of the better excitonic properties compared to those of GaN. It is known that ZnO is characterized by the large bond strength and the extreme stability of excitons, indicated by the stronger exciton binding energy (60 meV), which is larger than that of GaN (24 meV) *
Corresponding author. Tel.: +33 2 41 73 54 22; fax: +33 2 41 73 52 16. E-mail address: [email protected] (B. Sahraoui).
0030-4018/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2006.08.034
[3,4]. The stability of the exciton makes ZnO a promising material for the realization of efficient excitonic lasing at room temperature [5]. ZnO has been extensively studied considering their potential applications in optoelectronics and photonic devices in the ultraviolet (UV) region [5–7] because of this strong exciton binding energy. Many groups of scientist have used glass, quartz and c-plane sapphire as a substrate for growth of ZnO thin films because of their low cost [8–10]. The effects of metal-ion doping on the optical band gap (Eg), refractive index (n) and extinction coefficient (k) of such nanocrystalline ZnO films have been also studied [8,9]. Nevertheless, in some cases the obtained samples show a rough or porous surface morphology due to the large lattice mismatch (if any) and large difference in the thermal expansion
V. Kapustianyk et al. / Optics Communications 269 (2007) 346–350
coefficient between the ZnO layer and substrate [10]. In order to improve the quality of the ZnO films, several groups have used a thin buffer layer such as GaN, MgO, ZnS, etc. [11,12]. Recently, the paper concerning influence of the low temperature-grown effect of ZnO buffer layer on the properties of ZnO thin films was published [13]. The present work can be considered as a continuation of such kind of investigations. Firstly, this paper concerns the influence of ZnO film thickness on its structural and optical properties. We assumed that this layer could be also used as a buffer in the ZnO/ZnO or in more complex systems. Moreover, it is important to consider the influence of other important buffer layer, such as ITO transparent electrode and other technological factors on the crystallinity and physical properties of the ZnO-based thin films. 2. Experiment ZnO thin films were deposited on the glass and the amorphous quartz substrates by the standard RF-magnetron sputtering method using ZnO targets in the argon atmosphere at the gas pressure of 10 3 Torr. The substrate temperature and RF-power were fixed at 300 °C and 100 W, respectively. The distance from target to substrate was 60 mm and the magnet field strength was 0.1 T. In order to remove any contamination the target was presputtered for 10 min before deposition. The surface morphology of ZnO films was monitored by atomic force microscopy (AFM) (Digital Instruments – USA with Nanoscope E controller) working in the contact mode and equipped with OTR8 probe (Veeco NanoProbeTM). The length and the spring constant of the applied V-shaped cantilever were 200 lm and 0.15 N/m, respectively. The used constant forces were about 10 nN. All measurements were performed in air. The ex situ ellipsometry measurements were performed with a serial null ellipsometer LEF-3 M in PCSA (polarizer–compensator–sample–analyzer) arrangement. The light source was He–Ne laser (k = 632.8 nm). The cathodoluminescence of ZnO film was examined under excitation with 4 keV electrons at the electron beam current 4 nA. 3. Results and discussion The surface morphology of ZnO films with different thicknesses sputtered on the various substrates was monitored by AFM. The dependence of crystallinity and crystallite sizes on the film thickness was revealed by their micrographs (see Fig. 1). The root mean square (RMS) roughness and grain size were measured on the different parts of the thin film surface. The RMS roughness and the average grain size as a function of layer thickness, as well as the type of a substrate and conditions of the film thermal treating are presented in Table 1. The results obtained from the AFM images were found to be in a good agreement with data of our previous work [15], where the
347
real crystal structure was analyzed considering the X-ray diffraction (XRD) pattern profile. The first part of Table 1 presents the data concerning ZnO thin films deposited under the stable conditions on the glass substrates. The time of the magnetron sputtering was varied in the framework of 0.5–60 min and determined the thickness of the specimens. One can note that the minimal values of the RMS roughness and the average grain size for the films deposited on the glass substrates were observed for the layer thickness of 18 nm. In this case, the average grain size is close to the Bohr radius of the exciton for ZnO (5 nm) [14] and that would imply the manifestation of the quantum confinement effect. These assumptions are confirmed by results of transmission spectra presented in our previous paper [15], where the energy gap increases with decrease in the crystallite grain size. At the same time such behaviour of the RMS roughness differs from those observed for the ZnO films deposited on the sapphire [1]. In the last case, one could observe that the maximum of this parameter is around d = 10 nm. This implies the different mechanisms of thin film growth on the amorphous glass and on the oriented c-plane sapphire crystalline substrates, respectively. As shown in Table 1, replacing the glass substrate on the amorphous quartz is followed by increase in both of the surface roughness and the crystallite sizes. The latter was found to be much larger in comparison with the Bohr exciton radius. Increasing of the crystallite sizes follows annealing of such films. The film with the maximal crystallinity is obtained after annealing at T = 850 °C in air. At the same time, the best quality (practically mirror-like surface) was obtained on the basis of the ITO buffer layer. So we can conclude, that the ZnO films deposited on the buffer layer usually exhibit good structural and optical properties. The thickness dependences of the optical parameters of ZnO thin films determined using the variable angle ellipsometry technique are presented in Fig. 2. The extinction coefficient (curve 2) manifests a smooth dependence on the film thickness with a clear step above 75 nm and saturation at the thicknesses higher than 90 nm. The dependence of the refractive indices versus the film thickness is also shown in Fig. 2 (curve 1). On the basis of the AFM study, one can conclude that the thinnest film possesses the island structure and at the same time is characterized by the minimal effective values of the refractive index as well as of the extinction coefficient. At higher values of the sputtering time, the islands coalesce and one can obtain already a smooth surface. Nevertheless, the dependence of the refractive indices on the film thickness show the clear minimum around d = 44 nm that also reflects some kind of size effect. At the same time, it is necessary to take into account that this minimum is shifted considerably to the higher thicknesses in comparison with the corresponding minima of the structural parameters (Table 1). It is clear that such a complex thickness dependence of the refractive index correlates with the grain sizes and the texture parameters of the thin
348
V. Kapustianyk et al. / Optics Communications 269 (2007) 346–350
Fig. 1. Three-dimensional AFM micrographs for the ZnO films of different thicknesses: (a) ZnO/glass, thickness d = 10 nm; (b) ZnO/glass, d = 18 nm; (c) ZnO/glass, d = 120 nm; (d) ZnO/glass, d = 245 nm; (e) ZnO/glass, d = 600 nm; (f) ZnO/ITO/glass, d = 600 nm; (g) ZnO/quartz, d = 600 nm; (h) ZnO/ quartz, d = 600 nm (the sample was annealed at T = 850 °C in Zn vapor); (i) ZnO/quartz, d = 600 nm (the sample was annealed at T = 850 °C in air).
film (although this correlation is not trivial). The higher the sizes of the crystallites, the smaller the fraction of the quasi-amorphous materials. This regularity seems to
be correct at least for the comparatively thick films (with d > 44 nm). Under such circumstances, one can suppose that the complex thickness dependence of the refractive
V. Kapustianyk et al. / Optics Communications 269 (2007) 346–350
349
Table 1 The RMS roughness and average grain size of the ZnO thin films as a function of the layer thickness, type of a substrate and conditions of the experiment Sample
Structure
Film thickness (nm)
Annealing
RMS roughness (nm)
Average grain size (nm)
(a) (b) (c) (d) (e) (f) (g) (h) (i)
ZnO/glass ZnO/glass ZnO/glass ZnO/glass ZnO/glass ZnO/ITO/glass ZnO/quartz ZnO/quartz ZnO/quartz
10 18 120 245 600 600 600 600 600
– – – – – – – At T = 850 °C in Zn vapor At T = 850 °C in air
1.12 0.99 8.00 12.2 10 0.89 21 22 28
40 ± 5 10 ± 2 93 ± 14 83 ± 12 100 ± 16 16 ± 1 65 ± 10 170 300
0.8 2.04
0.6
Refractive index
2.02
n 0.5 2.00 0.4
k
1.98
0.3 0.2
Extinction coefficient
0.7
1.96 0.1 0.0 250
1.94 0
50
100
150
200
Thickness [nm]
Fig. 2. The dependence of the refractive index n (curve 1) and the extinction coefficient k (curve 2) on the film thickness for the samples deposited on the glass substrate.
index (Fig. 2) reflects the ratio of ZnO in the crystalline and amorphous state, respectively. Moreover, it was found that the spectral parameters considerably depend on the film crystallinity. In particular, the intensity of the cathodoluminescence for three different ZnO thin films deposited on the quartz substrate increases with growth of the crystallite size (see Fig. 3). This concerns both excitonic and two deep level luminescence bands observed in such materials [15] around 370–380 nm, 430– 440 nm and 500–560 nm, respectively. The green cathodo-
100
No 3
Intensity [a.u.]
80
luminescence band with a maximum above k = 500 nm is associated with oxygen vacancies Vo in the form of F+ centers. The blue band in the 430–440 nm region is due to acceptors, i.e. zinc vacancies VZn that are formed in presence of excess oxygen in the ZnO crystal lattice. The UV emission at 372–380 nm is due to radiative recombination of bound excitons. The expected quantum confinement effect would manifests itself in the UV cathodoluminescence for the thinnest ZnO films. Unfortunately, it was impossible to observe this effect due to the high absorption of the UV light by the glass substrate. In the case of the ZnO/quartz films the crystallites are too large in comparison with the Bohr radius of the exciton. It is interesting to note that, the influence of the fraction of the quasi-amorphous materials in the polycrystalline ZnO films is connected with the static disorder. This is reflected in the fact that the absorption edge follows the so-called, modified or glass-like, Urbach’s rule [16]. It is necessary to note that the single crystal ZnO is characterized by fulfillment of usual Urbach’s rule [17]. To explain the structural evolution of ZnO thin film on the glass substrates during sputtering, we present a schematic diagram illustrated in Fig. 4. The films thinner than 10 nm possess an isle structure. With increase of the film thickness the islands grow, coalesce and form a continuous film. Within the framework from 10 to 50 nm the film should be considered as ‘‘amorphous’’ since it consists of a comparatively small nanocrystallites (with an average size nearly 10 nm and a small value of the RMS roughness around 1 nm). It is necessary to note that the refractive index decreases with the thickness increase from 10 nm to
60
40
No 2
20
No 1
0 360
400
440
480
520
560
600
640
680
720
Wavelength [nm]
Fig. 3. Cathodoluminescence spectra of the ZnO thin films deposited on the quartz substrate: No. 1 – sample (g); No. 2 – the film with a thickness d = 600 nm annealed in air at T = 600 °C during 3 h; No. 3 – sample (i). Notations as shown in Table 1.
Fig. 4. Schematic diagram of the structural evolution of ZnO thin film deposited on the glass substrates. The ordinate axis presents the film thickness in nm.
350
V. Kapustianyk et al. / Optics Communications 269 (2007) 346–350
50 nm (Fig. 2). In thickness ranges from 50 to 90 nm the sizes of the crystallites increase reaching saturation at the thickness close to 90 nm that is consistent with corresponding behaviour of the refractive index (Fig. 2). Further film’s growth reveals no significant change of the average crystallite sizes. 4. Conclusion The effects of the thickness variation, substrate type and annealing on the crystallinity parameters, optical and luminescent properties of the ZnO thin films were reported. The films deposited on the glass substrate manifest a clear size effect: the minima of the RMS roughness and crystallite sizes are observed around the d = 18 nm thickness. In this case, the average grain size is close to the Bohr radius of the exciton for ZnO. This would imply the manifestation of the quantum confinement effect. Unfortunately, it was impossible to observe this effect in the cathodoluminescent spectra due to the high absorption of the UV light by the glass substrate. The minimum,on the corresponding dependence of the refractive index, was found to be a shifted toward larger values of the thickness (around d = 44 nm). It is clear that, the refractive index reflects the complex dependence of the crystal structure on the deposition time and consequently on the film thickness. Such dependence first of all is determined by the changing ratio of ZnO in the crystalline and amorphous state, respectively. Annealing of the comparatively thick ZnO films leads to increasing of the crystallite sizes, which is followed by a considerable rise of the cathodoluminescence intensity. The film with the maximal crystallinity is obtained after annealing at T = 850 °C in air. It has been also found that the most flat films, with a mirror-like surface, are obtained on the ITO buffer layer. It is very important since the transparent ITO electrodes are very often used in the UV light emitting devices based on the ZnO layers.
Acknowledgements The Ministry of Education and Science of Ukraine had supported this work. The authors express their gratitude to T. Riabchuk for the help in the preparation of paper. References [1] C.R. Gorla, N.W. Emanetoglu, S. Liang, W.E. Mayo, Y. Lu, M. Wraback, H. Shen, J. Appl. Phys. 85 (1999) 2595. [2] T. Minami, T. Yamamoto, T. Miyata, Thin Solid Films 63 (2000) 366. [3] P. Zu, Z.K. Tang, G.K.L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Solid State Commun. 103 (1997) 459. [4] D.C. Reynolds, D.C. Look, B. Jogai, H. Morko, Solid State Commun. 103 (1997) 643. [5] D.M. Bagnall, Y.F. Chen, M.Y. Shen, Z. Zhu, T. Goto, T. Yao, J. Cryst. Growth 184 (1998) 605. [6] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, S. Koyama, M.Y. Shen, T. Goto, Appl. Phys. Lett. 70 (1997) 2230. [7] X.L. Guo, J.H. Choi, H. Tabata, T. Kawai, Jpn. J. Appl. Phys. 40 (2001) 177. [8] J. Lee, W. Gao, Z. Li, M. Hodgson, J. Metson, H. Gong, U. Pal, Appl. Phys. A 80 (2005) 1641. [9] A. Mendoza-Galva´n, C. Trejo-Cruz, J. Lee, D. Bhattacharyya, J. Metson, P.J. Evans, U. Pal, J. Appl. Phys. 99 (2006) 014306. [10] A. Ohtomo, K. Tamura, K. Saikusa, K. Takahashi, T. Makimo, Y. Segawa, H. Koinuma, M. Kawasaki, Appl. Phys. Lett. 75 (1999) 2635. [11] A. Nahhas, N.K. Kim, J. Blachere, Appl. Phys. Lett. 78 (2001) 1511. [12] Y.F. Chen, S. Hong, H. Ko, M. Nakajiama, T. Yao, Appl. Phys. Lett. 76 (2000) 559. [13] Kyu-Hyun Bang, Deuk-Kyu Hwang, Jae-Min Myoung, Appl. Surf. Sci. 207 (2003) 359. [14] S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, Superlattice Microst. 34 (2003) 3. [15] M.R. Panasiuk, B.I. Turko, V.B. Kapustyanyk, G.A. Lubochkova, V.P. Rudyk, A.P. Vaskiv, V.M. Davydov, Funct. Mater. 12 (2005) 3. [16] B.I. Turko, V.B. Kapustianyk, V.P. Rudyk, G.A. Lubochkova, B.A. Simkiv, Zh. Prikl. Spektosk. 73 (2006) 200. [17] I.P. Euzmina, Kristallografiya 13 (1968) 920.
Influence of size effect and sputtering conditions on the crystallinity and optical properties of ZnO thin films V. Kapustianyk a, B. Turko a, A. Kostruba b,c, Z. Sofiani d, B. Derkowska d, S. Dabos-Seignon d, B. Barwin´ski e, Yu. Eliyashevskyi e, B. Sahraoui d,* a
Scientific Technical and Educational Center of Low Temperature Studies, Ivan Franko National University of Lviv, Dragomanova Street 50, UA-79005 Lviv, Ukraine b Lviv Academy of Commerce, Golovatskogo Street 10, UA-79005 Lviv, Ukraine c Institute for Physical Optics, Dragomanova Street 23, UA-79005 Lviv, Ukraine d Laboratory POMA, UMR CNRS 6136, University of Angers, 2 Boulevard Lavoisier, 49045 Angers, France e Institute of Experimental Physics, University of Wrocław, PL-50-204 Wrocław, Poland Received 2 March 2006; received in revised form 4 August 2006; accepted 10 August 2006
Abstract The effects of the thickness variation, substrate type and annealing on the crystallinity parameters, luminescent and optical properties of the zinc oxide (ZnO) thin films were reported. The thin films were deposited on the glass and the amorphous quartz substrates by the standard RF-magnetron sputtering method using ZnO targets in the argon atmosphere. It has been found that the films deposited on the glass substrate manifest a clear size effect. Both the structural and the optical parameters show clearly minima on their thickness dependences. It has been shown that annealing of the comparatively thick ZnO films leads to increase of the crystallite sizes that are followed by a considerable rise of the cathodoluminescence intensity. The corresponding model of the crystallite growth is proposed. Ó 2006 Elsevier B.V. All rights reserved. PACS: 78.20. e; 78.20.Ci; 78.40.Fy; 78.60.Hk Keywords: ZnO thin films; Refractive index; Cathodoluminescence
1. Introduction ZnO has been widely used for many applications, for example, surface acoustic wave (SAW) devices and transparent electrodes due to its excellent piezoelectric and transparent properties [1,2]. Moreover, ZnO has recently attracted great attention to application of optoelectronic devices because of the better excitonic properties compared to those of GaN. It is known that ZnO is characterized by the large bond strength and the extreme stability of excitons, indicated by the stronger exciton binding energy (60 meV), which is larger than that of GaN (24 meV) *
Corresponding author. Tel.: +33 2 41 73 54 22; fax: +33 2 41 73 52 16. E-mail address: [email protected] (B. Sahraoui).
0030-4018/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2006.08.034
[3,4]. The stability of the exciton makes ZnO a promising material for the realization of efficient excitonic lasing at room temperature [5]. ZnO has been extensively studied considering their potential applications in optoelectronics and photonic devices in the ultraviolet (UV) region [5–7] because of this strong exciton binding energy. Many groups of scientist have used glass, quartz and c-plane sapphire as a substrate for growth of ZnO thin films because of their low cost [8–10]. The effects of metal-ion doping on the optical band gap (Eg), refractive index (n) and extinction coefficient (k) of such nanocrystalline ZnO films have been also studied [8,9]. Nevertheless, in some cases the obtained samples show a rough or porous surface morphology due to the large lattice mismatch (if any) and large difference in the thermal expansion
V. Kapustianyk et al. / Optics Communications 269 (2007) 346–350
coefficient between the ZnO layer and substrate [10]. In order to improve the quality of the ZnO films, several groups have used a thin buffer layer such as GaN, MgO, ZnS, etc. [11,12]. Recently, the paper concerning influence of the low temperature-grown effect of ZnO buffer layer on the properties of ZnO thin films was published [13]. The present work can be considered as a continuation of such kind of investigations. Firstly, this paper concerns the influence of ZnO film thickness on its structural and optical properties. We assumed that this layer could be also used as a buffer in the ZnO/ZnO or in more complex systems. Moreover, it is important to consider the influence of other important buffer layer, such as ITO transparent electrode and other technological factors on the crystallinity and physical properties of the ZnO-based thin films. 2. Experiment ZnO thin films were deposited on the glass and the amorphous quartz substrates by the standard RF-magnetron sputtering method using ZnO targets in the argon atmosphere at the gas pressure of 10 3 Torr. The substrate temperature and RF-power were fixed at 300 °C and 100 W, respectively. The distance from target to substrate was 60 mm and the magnet field strength was 0.1 T. In order to remove any contamination the target was presputtered for 10 min before deposition. The surface morphology of ZnO films was monitored by atomic force microscopy (AFM) (Digital Instruments – USA with Nanoscope E controller) working in the contact mode and equipped with OTR8 probe (Veeco NanoProbeTM). The length and the spring constant of the applied V-shaped cantilever were 200 lm and 0.15 N/m, respectively. The used constant forces were about 10 nN. All measurements were performed in air. The ex situ ellipsometry measurements were performed with a serial null ellipsometer LEF-3 M in PCSA (polarizer–compensator–sample–analyzer) arrangement. The light source was He–Ne laser (k = 632.8 nm). The cathodoluminescence of ZnO film was examined under excitation with 4 keV electrons at the electron beam current 4 nA. 3. Results and discussion The surface morphology of ZnO films with different thicknesses sputtered on the various substrates was monitored by AFM. The dependence of crystallinity and crystallite sizes on the film thickness was revealed by their micrographs (see Fig. 1). The root mean square (RMS) roughness and grain size were measured on the different parts of the thin film surface. The RMS roughness and the average grain size as a function of layer thickness, as well as the type of a substrate and conditions of the film thermal treating are presented in Table 1. The results obtained from the AFM images were found to be in a good agreement with data of our previous work [15], where the
347
real crystal structure was analyzed considering the X-ray diffraction (XRD) pattern profile. The first part of Table 1 presents the data concerning ZnO thin films deposited under the stable conditions on the glass substrates. The time of the magnetron sputtering was varied in the framework of 0.5–60 min and determined the thickness of the specimens. One can note that the minimal values of the RMS roughness and the average grain size for the films deposited on the glass substrates were observed for the layer thickness of 18 nm. In this case, the average grain size is close to the Bohr radius of the exciton for ZnO (5 nm) [14] and that would imply the manifestation of the quantum confinement effect. These assumptions are confirmed by results of transmission spectra presented in our previous paper [15], where the energy gap increases with decrease in the crystallite grain size. At the same time such behaviour of the RMS roughness differs from those observed for the ZnO films deposited on the sapphire [1]. In the last case, one could observe that the maximum of this parameter is around d = 10 nm. This implies the different mechanisms of thin film growth on the amorphous glass and on the oriented c-plane sapphire crystalline substrates, respectively. As shown in Table 1, replacing the glass substrate on the amorphous quartz is followed by increase in both of the surface roughness and the crystallite sizes. The latter was found to be much larger in comparison with the Bohr exciton radius. Increasing of the crystallite sizes follows annealing of such films. The film with the maximal crystallinity is obtained after annealing at T = 850 °C in air. At the same time, the best quality (practically mirror-like surface) was obtained on the basis of the ITO buffer layer. So we can conclude, that the ZnO films deposited on the buffer layer usually exhibit good structural and optical properties. The thickness dependences of the optical parameters of ZnO thin films determined using the variable angle ellipsometry technique are presented in Fig. 2. The extinction coefficient (curve 2) manifests a smooth dependence on the film thickness with a clear step above 75 nm and saturation at the thicknesses higher than 90 nm. The dependence of the refractive indices versus the film thickness is also shown in Fig. 2 (curve 1). On the basis of the AFM study, one can conclude that the thinnest film possesses the island structure and at the same time is characterized by the minimal effective values of the refractive index as well as of the extinction coefficient. At higher values of the sputtering time, the islands coalesce and one can obtain already a smooth surface. Nevertheless, the dependence of the refractive indices on the film thickness show the clear minimum around d = 44 nm that also reflects some kind of size effect. At the same time, it is necessary to take into account that this minimum is shifted considerably to the higher thicknesses in comparison with the corresponding minima of the structural parameters (Table 1). It is clear that such a complex thickness dependence of the refractive index correlates with the grain sizes and the texture parameters of the thin
348
V. Kapustianyk et al. / Optics Communications 269 (2007) 346–350
Fig. 1. Three-dimensional AFM micrographs for the ZnO films of different thicknesses: (a) ZnO/glass, thickness d = 10 nm; (b) ZnO/glass, d = 18 nm; (c) ZnO/glass, d = 120 nm; (d) ZnO/glass, d = 245 nm; (e) ZnO/glass, d = 600 nm; (f) ZnO/ITO/glass, d = 600 nm; (g) ZnO/quartz, d = 600 nm; (h) ZnO/ quartz, d = 600 nm (the sample was annealed at T = 850 °C in Zn vapor); (i) ZnO/quartz, d = 600 nm (the sample was annealed at T = 850 °C in air).
film (although this correlation is not trivial). The higher the sizes of the crystallites, the smaller the fraction of the quasi-amorphous materials. This regularity seems to
be correct at least for the comparatively thick films (with d > 44 nm). Under such circumstances, one can suppose that the complex thickness dependence of the refractive
V. Kapustianyk et al. / Optics Communications 269 (2007) 346–350
349
Table 1 The RMS roughness and average grain size of the ZnO thin films as a function of the layer thickness, type of a substrate and conditions of the experiment Sample
Structure
Film thickness (nm)
Annealing
RMS roughness (nm)
Average grain size (nm)
(a) (b) (c) (d) (e) (f) (g) (h) (i)
ZnO/glass ZnO/glass ZnO/glass ZnO/glass ZnO/glass ZnO/ITO/glass ZnO/quartz ZnO/quartz ZnO/quartz
10 18 120 245 600 600 600 600 600
– – – – – – – At T = 850 °C in Zn vapor At T = 850 °C in air
1.12 0.99 8.00 12.2 10 0.89 21 22 28
40 ± 5 10 ± 2 93 ± 14 83 ± 12 100 ± 16 16 ± 1 65 ± 10 170 300
0.8 2.04
0.6
Refractive index
2.02
n 0.5 2.00 0.4
k
1.98
0.3 0.2
Extinction coefficient
0.7
1.96 0.1 0.0 250
1.94 0
50
100
150
200
Thickness [nm]
Fig. 2. The dependence of the refractive index n (curve 1) and the extinction coefficient k (curve 2) on the film thickness for the samples deposited on the glass substrate.
index (Fig. 2) reflects the ratio of ZnO in the crystalline and amorphous state, respectively. Moreover, it was found that the spectral parameters considerably depend on the film crystallinity. In particular, the intensity of the cathodoluminescence for three different ZnO thin films deposited on the quartz substrate increases with growth of the crystallite size (see Fig. 3). This concerns both excitonic and two deep level luminescence bands observed in such materials [15] around 370–380 nm, 430– 440 nm and 500–560 nm, respectively. The green cathodo-
100
No 3
Intensity [a.u.]
80
luminescence band with a maximum above k = 500 nm is associated with oxygen vacancies Vo in the form of F+ centers. The blue band in the 430–440 nm region is due to acceptors, i.e. zinc vacancies VZn that are formed in presence of excess oxygen in the ZnO crystal lattice. The UV emission at 372–380 nm is due to radiative recombination of bound excitons. The expected quantum confinement effect would manifests itself in the UV cathodoluminescence for the thinnest ZnO films. Unfortunately, it was impossible to observe this effect due to the high absorption of the UV light by the glass substrate. In the case of the ZnO/quartz films the crystallites are too large in comparison with the Bohr radius of the exciton. It is interesting to note that, the influence of the fraction of the quasi-amorphous materials in the polycrystalline ZnO films is connected with the static disorder. This is reflected in the fact that the absorption edge follows the so-called, modified or glass-like, Urbach’s rule [16]. It is necessary to note that the single crystal ZnO is characterized by fulfillment of usual Urbach’s rule [17]. To explain the structural evolution of ZnO thin film on the glass substrates during sputtering, we present a schematic diagram illustrated in Fig. 4. The films thinner than 10 nm possess an isle structure. With increase of the film thickness the islands grow, coalesce and form a continuous film. Within the framework from 10 to 50 nm the film should be considered as ‘‘amorphous’’ since it consists of a comparatively small nanocrystallites (with an average size nearly 10 nm and a small value of the RMS roughness around 1 nm). It is necessary to note that the refractive index decreases with the thickness increase from 10 nm to
60
40
No 2
20
No 1
0 360
400
440
480
520
560
600
640
680
720
Wavelength [nm]
Fig. 3. Cathodoluminescence spectra of the ZnO thin films deposited on the quartz substrate: No. 1 – sample (g); No. 2 – the film with a thickness d = 600 nm annealed in air at T = 600 °C during 3 h; No. 3 – sample (i). Notations as shown in Table 1.
Fig. 4. Schematic diagram of the structural evolution of ZnO thin film deposited on the glass substrates. The ordinate axis presents the film thickness in nm.
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50 nm (Fig. 2). In thickness ranges from 50 to 90 nm the sizes of the crystallites increase reaching saturation at the thickness close to 90 nm that is consistent with corresponding behaviour of the refractive index (Fig. 2). Further film’s growth reveals no significant change of the average crystallite sizes. 4. Conclusion The effects of the thickness variation, substrate type and annealing on the crystallinity parameters, optical and luminescent properties of the ZnO thin films were reported. The films deposited on the glass substrate manifest a clear size effect: the minima of the RMS roughness and crystallite sizes are observed around the d = 18 nm thickness. In this case, the average grain size is close to the Bohr radius of the exciton for ZnO. This would imply the manifestation of the quantum confinement effect. Unfortunately, it was impossible to observe this effect in the cathodoluminescent spectra due to the high absorption of the UV light by the glass substrate. The minimum,on the corresponding dependence of the refractive index, was found to be a shifted toward larger values of the thickness (around d = 44 nm). It is clear that, the refractive index reflects the complex dependence of the crystal structure on the deposition time and consequently on the film thickness. Such dependence first of all is determined by the changing ratio of ZnO in the crystalline and amorphous state, respectively. Annealing of the comparatively thick ZnO films leads to increasing of the crystallite sizes, which is followed by a considerable rise of the cathodoluminescence intensity. The film with the maximal crystallinity is obtained after annealing at T = 850 °C in air. It has been also found that the most flat films, with a mirror-like surface, are obtained on the ITO buffer layer. It is very important since the transparent ITO electrodes are very often used in the UV light emitting devices based on the ZnO layers.
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