J. Phys. Chem. C 2008, 112, 9595–9599
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Relationship between Nanostructure and Optical Properties of ZnO Thin Films Graziella Malandrino,*,† Manuela Blandino,† Maria E. Fragala,† Maria Losurdo,‡ and Giovanni Bruno‡ Dipartimento di Scienze Chimiche, UniVersita` di Catania, ISTM-CNR and INSTM UdR Catania, Viale A. Doria 6, I-95125 Catania, Italy, and Institute of Inorganic Methodologies and of Plasmas, IMIP-CNR, and INSTM, UdR Bari, Via Orabona, 4-70126 bari, Italy ReceiVed: January 8, 2008; ReVised Manuscript ReceiVed: April 7, 2008
Nanostructured ZnO thin films have been grown on quartz substrates by metal organic chemical vapor deposition. Zn(tta)2•tmeda (H-tta ) 2-thenoyltrifluoroacetone, tmeda ) N,N,N′,N′-tetramethylethylenediamine) has been used as Zn precursor. The impact of deposition temperature, in the range 400-750 °C, on film nanostructure and optical properties has been investigated. The X-ray diffraction patterns show that film crystallinity improves upon increasing the substrate temperature. This is consistent with morphologies observed through scanning electron microscopy (SEM) and atomic force microscopy (AFM), which indicate that grain dimensions increase ranging from 80 nm rounded grains at 400 °C to barlike grains of about 200-400 nm at the highest investigated temperature of 750 °C. The AFM investigation shows that the roughness is also a function of the substrate temperatures and parallels the grains size of ZnO films. The UV-vis transmission spectrum shows that ZnO films grown at 600 °C on quartz are highly transparent in the visible region. Furthermore, spectroscopic ellipsometry is used for investigating the dependence of the ZnO dielectric function and optical gap on film nanostructure. 1. Introduction Zinc oxide (ZnO) is a wide band gap semiconductor (3.37 eV) with hexagonal wurtzite structure showing several outstanding physical properties for optical and electronic applications.1 ZnO has good photoconductivity and a high optical transparency in the visible and infrared spectral regions, and it has a large exciton binding energy that enables the use of ZnO thin films for organic light emitting diodes2 and efficient UV lasers.3 Stoichiometric ZnO films are highly resistive, but less resistive films can be made either by creating oxygen vacancies, which act as donors, or by doping with Al, Ga, or In.2,4 In fact, the electrical and optical properties of the films are critically dependent on their microstructure, texture, and surface morphology, on the presence of defects, and on the structure of the film-substrate interface.5 Thin films of ZnO can be used as a window layer as well as one of the electrodes in solar cells.6 Along with these applications, ZnO films have also been used in varistors,7 in gas sensors,8 and as the transducer of a biosensor, etc. In addition, ZnO films have interesting wetting properties, which are important for many industrial processes, such as cleaning, drying, painting, coating, adhesion, heat transfer, and pesticide applications. Many deposition techniques have been developed to prepare ZnO thin films on Si, sapphire, glass, GaAs (001), LiNbO3, etc. substrates. High quality ZnO thin films have been deposited by several techniques such as chemical bath deposition (CBD),9 spray pyrolysis,10 pulsed laser deposition (PLD),2a,4 electrodeposition,11 filtered cathodic vacuum arc technique,3c rf-sputtering,2b,c,12 atomic layer deposition (ALD),13 molecular beam epitaxy (MBE),14 chemical vapor deposition (CVD),15 and metal organic chemical vapor deposition (MOCVD).5b,16 Among them, MOCVD offers high growth rate and efficiency, large area uniformity, and conformal step * Corresponding author. E-mail:
[email protected]. † Universita ` di Catania. ‡ Institute of Inorganic Methodologies and of Plasmas.
coverage.17 It possesses other many advantages: excellent crystallinity, mechanical stability, good adhesion of films, and easy mass production of high-quality films. In addition, one important point of MOCVD with respect to other processes for, e.g., rf-sputtering is the ease of depositing doped films due to the easy availability of different doping agents and the possibility of easily changing the doping concentration. Nevertheless, the success of an MOCVD process depends critically on the availability of volatile, thermally stable precursors that exhibit high and constant vapor pressures, since poor performances affect the film properties. Dimethyl-18 and diethyl-Zn19 complexes have been widely applied to MOCVD processes of ZnO films, but these precursors have some drawbacks due to their pyrophoric nature.20 Solid acetate-,21 alkoxide-,22 and acetylacetonate-zinc23 complexes have other drawbacks due to effects of crystallite sizes on the precursor evaporation rate and, hence, on the film growth rate in the case of solid source.24 We recently reported on the synthesis of a diamine adduct of the Zn(tta)2 (H-tta ) 2-thenoyltrifluoroacetone) moiety, Zn(tta)2•tmeda (tmeda ) N,N,N′,N′-tetramethylethylenediamine).25 This precursor is water-free, thermally stable, and volatile and has been successfully applied to plasma-assisted MOCVD growth of ZnO films.26 This work reports the deposition of good quality ZnO films by MOCVD on fused silica (quartz) using the Zn(tta)2•tmeda precursor. Quartz substrates have been used in order to investigate the potential of this novel precursor to grow ZnO films on a nonepitaxial transparent substrate, i.e., a type of economic substrate comparable to those used to produce films suitable for applications in solar cells or in organic light emitting diodes. The impact of the deposition temperature on the nanostructure, surface morphology, and optical properties of ZnO films has been investigated. It has been found that crystallinity improves and grain size increases with the increase of deposition temperature.
10.1021/jp8001492 CCC: $40.75 2008 American Chemical Society Published on Web 06/05/2008
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2. Experimental Section ZnO films were deposited on quartz substrates 10 mm × 10 mm2 using a reduced pressure horizontal hot wall MOCVD reactor. The Zn(tta)2•tmeda precursor, applied for the deposition of ZnO films, was synthesized as previously reported.25 The precursor evaporation temperature was kept at 170 °C, while the substrate temperature was varied in the range 400-750 °C. Ar (150 sccm) and O2 (150 sccm) flows were used as carrier and reaction gases, respectively. The mass flows were controlled with a 1160 MKS flowmeter using an MKS147 electronic control unit. Depositions were carried out for 60 min. The total pressure in the reactor was about 3 torr. θ-2θ X-ray diffraction (XRD) patterns were recorded on a Bruker-AXS D5005 θ-θ X-ray diffractometer, using Cu KR radiation operating at 40 KV and 30 mA. The surface morphologies of ZnO films were examined by field emission scanning electron microscopy (FE-SEM) using a ZEISS VP 55 microscope. AFM images were obtained in high amplitude (tapping mode) through an NT-MTD instrument. The noise level before and after each measurement was 0.01 nm. The atomic composition of the films was analyzed by EDX using a windowless Oxford INCA Energy solid state detector. The transmission and absorption spectra were performed on a double-ray Jasco V-560 UV-vis spectrometer in the 200 and 800 nm range. Structural and optical characterizations of the ZnO thin films were also performed using spectroscopic ellipsometry (SE) that measured the pseudodielectric function, 〈ε〉 ) 〈ε1〉 + i〈ε2〉, in the range 0.75-6.5 eV using a phase modulated spectroscopic ellipsometer (UVISEL-Jobin Yvon) at an incidence angle of 70°.27 SE spectra were analyzed in terms of optical models based on the Bruggeman effective medium approximation (BEMA).28 In the models, the experimental dielectric function of quartz substrate measured before starting deposition was used. The optical functions of ZnO deposited thin films were parametrized using the model by Adachi.29 Two-layer models substrate/film/ surface roughness (e.g., see Figure 6) were used. Surface roughness was simulated by a BEMA mixture of 50% ZnO and 50% voids. 3. Results and Discussion 3.1. Structural/Compositional Characterization and Growth Regime. A systematic study on the effect of deposition temperature on structural nature, morphologies, and optical properties of ZnO films has been carried out. The compositional purity of the ZnO films has been confirmed by energy dispersive X-ray analysis (EDX). The EDX spectrum shows the presence of the Zn L peaks at about 1.010 KeV. The peak at 1.730 KeV is due to the Si KR peak of the quartz substrates. In addition, note that the use of the windowless EDX detector allowed the detection of the O KR peak at 0.560 KeV and the exclusion of any S and/or F contamination, thus ruling out the presence of any sulfide and/or fluoride phase. Figure 1 shows X-ray θ-2θ scans of ZnO films grown on quartz and deposited at different substrate temperature of 400, 500, 600, and 750 °C. At 400 °C, the XRD pattern of the ZnO film does not show any diffraction peak, but only a big bump at about 20° associated with the amorphous nature of the film. At 500 °C, small peaks are observed at 34.46° and 36.30° that may be associated with the 0002 and 101j1 reflections, respectively, of the ZnO wurtzite phase [ICDD no. 36-1451]. Nevertheless, the film shows a poor crystallinity confirmed by the presence of the broad bump around 20°. Films deposited at temperatures g600 °C exhibit sharp diffraction peaks charac-
Figure 1. XRD patterns of ZnO films grown on quartz at (a) 400 °C, (b) 500 °C, (c) 600 °C, and (d) 750 °C.
Figure 2. Arrhenius plot of deposition rate of ZnO film growth on quartz vs 1/T.
teristics of the ZnO wurtzite phase. The intensities of the X-ray diffraction pattern do not match those reported in the ICDD database. In fact, the 0002 reflection has a 100% intensity with respect to the 44% in the powder pattern, thus indicating a slight preferential orientation of the grains along the 〈0002〉 direction growth. In particular, the intensity of the 0002 ZnO reflection increases with increasing the substrate temperature, while the other peaks associated with reflections 101j0, 101j1, 101j2, 112j0 show a very small intensity. To investigate the growth regime of the ZnO MOCVD process, an Arrhenius plot (Figure 2) has been derived reporting the growth rate in function of the deposition temperature. The film thickness has been obtained through the ellipsometric measurements considering the model reported in Figure 6. The thickness of film deposited at 750 °C could not be evaluated through SE due to the highly scattering surface (vide infra). Therefore, only the films deposited in the 400-600 °C interval are considered in the Arrhenius plot. The linear relationship is a clear indication that the growth occurs under a kinetic regime in this temperature range. The apparent activation energy of the deposition process has been evaluated to be 55 ( 7 kJ/mol. 3.2. Morphological Characterization. The film surface has been investigated through FE-SEM images. The ZnO films exhibit a surface structure consisting of grains with different size and morphology depending on deposition temperatures. The SEM images of the samples deposited at 400 and 500 °C are reported in Figure 3a,b. The morphology of these two films consists of well connected and rounded grains with a diameter of about 80 nm. Films deposited at 600 °C (Figure 3c) present a uniform surface with about 100 nm grains, while SEM images of films deposited at 750 °C show (Figure 3d) a morphology which consists of barlike grains coming out as outgrowths from
Optical Properties of ZnO Thin Films
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Figure 3. SEM images of ZnO films grown on quartz at (a) 400 °C , (b) 500 °C, (c) 600 °C, and (d, e) 750 °C.
the uniform surface. These grains are about 200-300 nm in length and about 100 nm wide (Figure 3e). The results obtained by SEM investigation are also confirmed by the root-mean-square (rms) roughness of the film surface measured by AFM. Figure 4 shows the AFM images of the films grown at different deposition temperatures of 400 °C (Figure 4a), 500 °C (Figure 4b), 600 °C (Figure 4c), and 750 °C (Figure 4d). The AFM images also confirm that grain dimensions increase with increasing deposition temperature. The rms roughness of the film surface (measured on a 2 µm × 2 µm area) indicates that the surface roughness of ZnO films increases upon increasing the grain size (Figure 4e), and in particular, values of the films deposited at 400, 500, 600, and 750 °C are 1.097, 1.879, 6.752, and 38.658 nm, respectively. It is interesting to compare these data with the thickness of the rough surface (t2) estimated through ellipsometric measurements. These values are reported for comparison in Figure 4e. The very high surface roughness of films deposited at 750 °C is responsible for the light scattering, which hampers the ellipsometric measurement (vide infra). An efficient scattering of light obtained through a suitable surface texture is a prerequisite, besides the transparency, to achieve light trapping needed in solar cells.30 In fact, light trapping is achieved by combining textured transparent conductive oxide films as front contacts and highly reflective back contacts. It is worth noting that, to date, the suited surface texture is realized by postdeposition wet-chemical etching of smooth sputter-deposited ZnO films,30 while our simple MOCVD approach produces ZnO films potentially useful for light trapping applications through a single-step process.
3.3. Optical Properties. The transmission spectrum of a ZnO film grown on quartz at 600 °C is shown in Figure 5. This spectrum shows that ZnO films are highly transparent in the visible region. In fact, the transmission between 400 and 800 nm is nearly 90%, with a minimum value of 84% around 600 nm. A sharp absorption is observed in the UV region at 364 nm (3.4 eV) because of the band-to-band transition. A deeper insight into the relationship among deposition temperature, nanostructure, and optical properties has been obtained by the SE analysis. Figure 6 shows the experimental spectra of the pseudodielectric function acquired at the incidence angle of 70° for the ZnO film deposited on quartz at 600°. The spectra are characterized by interference fringes for photon energies below 3.4 eV (where ZnO is transparent), which are due to the multiple reflections at the substrate/ZnO interface; from fitting this region, the film thickness has been derived. At approximately 3.4 eV the onset of the ZnO fundamental absorption can be seen. At energies above the fundamental gap, the pseudodielectric function is affected by the surface morphology and roughness because of the high absorption coefficient resulting in a low light penetration depth. The experimental spectra have been fit to the two-layer BEMA model including a bulk ZnO layer and a surface roughness layer (simulated by a BEMA mixture of 50% bulk and 50% voids) sketched at the bottom of Figure 6. The agreement between experimental and calculated spectra according to the best-fit model is also shown in the figure. The same approach has been applied to ZnO films deposited at the various temperatures and the spectra of the dielectric function derived for the various films are shown in Figure 7. For comparison, spectra of single crystal ZnO have
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Figure 4. AFM images of ZnO films grown on quartz at (a) 400 °C, (b) 500 °C, (c) 600 °C, and (d) 750 °C. In graph e the rms roughness obtained through AFM and the roughness estimated through ellipsometry (ES t2) vs temperature are reported: the line is a guide to the eye.
Figure 5. UV-vis transmission spectrum of a ZnO film deposited at 600 °C.
also been reported. All films have an optical gap of 3.34 ( 0.2 eV but do not show the ZnO exciton due to their amorphous or polycrystalline structure, which is different from the c-axis oriented single crystal ZnO. Furthermore, the dispersion of the dielectric function depends on the crystallinity and on the grain size. In particular, the dielectric function of films is lower than that of the c-axis oriented single crystal because of the finite size of grains and the presence of grain boundaries. The amplitude of the dielectric function increases moving from amorphous to polycrystalline materials. However, among the present films, the highest density and absorption are observed for films deposited at 500 °C that present a dense nanostructure consisting of randomly oriented nanocrystallites embedded in an amorphous tissue. For films deposited at 600 °C, despite the better crystallinity and larger grains, the dielectric function and, hence, the refractive index (ε1 )n2 -k2, where n is the refractive index and k is the extinction coefficient) decrease because of the reduced amorphous tissue and the presence of grain boundaries causing lower polarizability per unit of volume.31
Figure 6. Measured (dots) and calculated (black lines) SE spectra of the real, 〈ε1〉, and imaginary, 〈ε2〉, parts of the pseudodielectric function of a ZnO film deposited at 600 °C on quartz. The BEMA model used to fit data is also reported.
An ellipsometric spectrum could not be recorded for the film deposited at 750 °C because of light scattering effects due to the highly structured surface with a consequent very high roughness. 4. Conclusions ZnO films have been grown on quartz substrates at temperatures ranging from 400 to 750 °C by the low-pressure MOCVD technique. The results of XRD show that the crystallinity improves with deposition temperature. In particular, films
Optical Properties of ZnO Thin Films
Figure 7. Spectra of the real, ε1, and of the imaginary, ε2, parts of the dielectric function derived using the BEMA model of Figure 6 for the ZnO films deposited at various temperatures in comparison with a c-axis oriented ZnO single crystal (c-ZnO).
become preferentially c-axis oriented with increasing substrate temperature, even on this amorphous substrate. The SEM and AFM images indicate that the morphology and in particular the grain size and the rms roughness of the ZnO films are also functions of growth temperature. Furthermore, optical data show that films are transparent for energies below 3.4 eV, and that the ZnO dielectric function depends on film crystallinity. The present study points to the use of this novel precursor as a powerful MOCVD precursor which yields ZnO films not only on single crystal substrates, but also on amorphous substrates such the nonepitaxial quartz. Finally, this simple synthetic procedure may be envisaged as a viable route to the preparation of ZnO films with characteristics suited for light trapping. Acknowledgment. This work has been supported by CNRINSTM within the PROMO “Nanostrutture organiche, organometalliche polimeriche ed ibride: ingegnerizzazione supramoleculare delle proprieta` fotoniche e dispositivistica innovative per optoelettronica” project. The European project NanoCharM (NMP3-CA-2007-218570) is also acknowledged. We thank the LaSTMA laboratory (POR 2000–2006: Misura 3.15 Sottoazione C) for allowing the use of the EDX detector. References and Notes ¨ zgu¨r, U ¨ .; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; (1) (a) O Doan, S.; Avrutin, V.; Cho, S.-J.; Morkoc¸, H. Appl. Phys. ReV. 2005, 98, 041301. Mater. Today 2004, June. Special issue on ZnO films and nanostructures. (2) (a) Kim, H.; Horwitz, J. S.; Kim, W. H.; Qadri, S. B.; Kafafi, Z. H. Appl. Phys. Lett. 2003, 83, 3809. (b) Kim, T. W.; Choo, D. C.; No, Y. N.; Choi, W. K.; Choi, E. H. Appl. Surf. Sci. 2006, 253, 1917. (c) Jiang, X.; Wong, F. L.; Fung, M. K.; Lee, S. T. Appl. Phys. Lett. 2003, 83, 1875. Yamauchi, H.; Iizuka, M.; Kudo, K. Jpn. J. Appl. Phys., Part 1 2007, 46, 2678. (3) (a) Ryu, Y. R.; Lubguban, J. A.; Lee, T. S.; White, H. W.; Jeong, T. S.; Youn, C. J.; Kim, B. J. Appl. Phys. Lett. 2007, 90, 131115/1–131115/ 3. (b) Li, H. D.; Yu, S. F.; Lau, S. P.; Leong, E. S. P. Appl. Phys. Lett.
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