Highly conductive and transparent laser ablated nanostructured Al: ZnO thin films

Applied Surface Science 257 (2010) 708–716

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Highly conductive and transparent laser ablated nanostructured Al: ZnO thin films R. Vinodkumar a , I. Navas a , S.R. Chalana a , K.G. Gopchandran a , V. Ganesan b , Reji Philip c , S.K. Sudheer a , V.P. Mahadevan Pillai a,∗ a b c

Department of Optoelectronics, University of Kerala, Kariavattom Thiruvananthapuram, Kerala 695581, India UGC-DAE Consortium for Scientific Research, Khandwa Road, Indore 452017, India Light & Matter Physics Group, Raman Research Institute, Sadashivanagar, Bangalore 560080, India

a r t i c l e

i n f o

Article history: Received 4 November 2009 Received in revised form 13 July 2010 Accepted 15 July 2010 Available online 22 July 2010 Keywords: Pulsed laser ablation Nanostructured zinc oxide films Aluminum doped ZnO Transparent conducting oxide Solar cell materials Luminescent materials Optical limiter

a b s t r a c t Al doped ZnO thin films are prepared by pulsed laser deposition on quartz substrate at substrate temperature 873 K under a background oxygen pressure of 0.02 mbar. The films are systematically analyzed using X-ray diffraction, atomic force microscopy, micro-Raman spectroscopy, UV–vis spectroscopy, photoluminescence spectroscopy, z-scan and temperature-dependent electrical resistivity measurements in the temperature range 70–300 K. XRD patterns show that all the films are well crystallized with hexagonal wurtzite structure with preferred orientation along (0 0 2) plane. Particle size calculations based on XRD analysis show that all the films are nanocrystalline in nature with the size of the quantum dots ranging from 8 to 17 nm. The presence of high frequency E2 mode and longitudinal optical A1 (LO) modes in the Raman spectra suggest a hexagonal wurtzite structure for the films. AFM analysis reveals the agglomerated growth mode in the doped films and it reduces the nucleation barrier of ZnO by Al doping. The 1% Al doped ZnO film presents high transmittance of ∼75% in the visible and near infrared region and low dc electrical resistivity of 5.94 × 10−6  m. PL spectra show emissions corresponding to the near band edge (NBE) ultra violet emission and deep level emission in the visible region. Nonlinear optical measurements using the z-scan technique shows optical limiting behavior for the 5% Al doped ZnO film. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Zinc oxide (ZnO), an n-type II-VI semiconductor with unique properties like wide band gap (∼3.37 eV) and large exciton binding energy (60 meV) is suitable for fabrication of short wave length optoelectronic devices such as UV light emitting diodes, diode lasers, transparent conducting oxide for window layer in hetero junction solar cells etc. [1,2]. It offers numerous possibilities for application in the micro-optoelectronic industry as it can be grown epitaxially and exhibits nano-scale geometrical structure such as nanowire, nanorods and nanoring [3]. Doped ZnO films can be advantageously used in thin film solar cells. The extrinsic doping in ZnO is realized by appropriate incorporation either by cationic or anionic substitution, through which high carrier concentration can be achieved and are expected to exhibit stable electrical and optical properties. ZnO can be doped with a wide variety of ions to meet the demands of several applications. Typical dopant elements like F, B, Al, Ga, In, Sn, etc. have been used to produce conducting ZnO

films [4–6]. Doping in ZnO can be achieved by replacing Zn2+ with atoms of elements of higher valancies such as In3+, Al3+ , Sn4+ , and Pb4+ . Al doped ZnO films are prominent and low-cost substitute for indium tin oxide (ITO) films as transparent conducting film in thin film photovoltaic. There are several deposition techniques used to grow ZnO thin films such as molecular beam epitaxy (MBE) [7], metal organic chemical vapor deposition (MOVCD) [8], magnetron controlled sputtering [9] and pulsed laser deposition (PLD) [10]. Compared to other vacuum deposition techniques, PLD meets almost all the criteria for the best deposition characteristics, such as substrate temperature, the energy of the atom flux, the relative and absolute arrival rates of atoms for compound films, and the pressure in the chamber. In this paper aluminum doped zinc oxide nanostructured films are prepared using pulsed laser ablation at different aluminum doping concentrations and their structural, morphological, optical and electrical properties are investigated systematically. 2. Experimental

∗ Corresponding author. Tel.: +91 471 2308167; fax: +91 471 2307158. E-mail addresses: [email protected], [email protected] (V.P. Mahadevan Pillai). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.07.044

ZnO targets are prepared at different doping concentration of Al2 O3 (weight percentage) viz. (1, 3, 5, 7 and 9 wt%). Al doped ZnO pellets (diameter 11 mm and thickness 3 mm) are prepared from

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3. Results and discussion

3.22 3.31 3.31 3.33 3.34 3.37

Band gap (eV)

7.6 21.1 69.5 18.7 29.6 26.2

rms surface roughness (nm)

1.61 1.99 1.84 1.84 1.83 1.80

Refractive index

47 1.2 20 20 22 24

Porosity (%)

88 75 75 73 51 38

Ta (Å)

– 0.555 0.451 0.382 0.232 0.132

Figure of merit (−1 )

– 5.94×10−6 1.38×10−4 3.15×10−4 4.38×10−5 1.03×10−4

dc resistivity at 300 K ( m)

– 33.84 94.99 237.96 132.00 98.76

Activation energy (×10−4 eV)

ZnO (99.99% purity, Aldrich) and Al2 O3 powders (99.99% purity, Aldrich) and well-sintered pellets (sintered at 1000 ◦ C for 5 h) are used for laser ablation. The deposition of the films are carried out inside a vacuum chamber using a Q-switched Nd:YAG laser (Quanta Ray INDI-series, Spectra Physics) with frequency-doubled 532 nm laser radiation with pulse width of 7 ns and repetition frequency of 10 Hz. Before irradiations, the deposition chamber is evacuated down to a base pressure of ∼10−6 mbar. The films are deposited for 30 min on quartz substrates (target to substrate distance–6 cm) at a substrate temperature of 873 K using a laser energy 160 mJ under a background oxygen pressure of 0.02 mbar. The Al doped ZnO films prepared for Al doping concentrations viz. 0, 1, 3, 5, 7 and 9 wt% are abbreviated as ZO, 1AZO, 3AZO, 5AZO, 7AZO and 9AZO respectively. The crystalline structure of the films are investigated by grazing incidence X-ray diffraction (GIXRD) (Siemens D5000 diffractometer) measurements using Cu K␣ radiation at 1.5406 A˚ wavelength. The surface morphology of the films is investigated by AFM (Digital Instruments Nanoscope III) measurements in contact mode. Micro-Raman spectra of ZnO films are recorded using Labram-HR 800 spectrometer equipped with an argon ion laser and the excitation is done by laser radiation of wavelength of 488 nm. Optical transmittance spectra of the films are recorded using a UV–vis double beam spectrophotometer (JASCO V-550) in the spectral range of 200–900 nm. The thicknesses of the films are determined by Dektak 6 M stylus profiler. DC electrical resistivity of the films are studied in the temperature range 70–300 K using 617-programmable electrometer in association with Lakeshore temperature controller and PRT sensor to measure the temperature of the samples. Photoluminescence spectra of the samples are recorded by Horiba Jobin Yvon Flourolog III modular spectroflourometer. The open aperture z-scan measurements are done using a frequency-doubled laser radiation at wave length 532 nm using 7 ns laser pulses from a Nd:YAG laser (Quanta Ray).

709

(2)

where D is the actual particle size, k is the correction factor which can be taken as unity and  is the strain. The strain in the films can be calculated from slope of the plot and the actual particle size in the film can be estimated from the y-intercept of the plot. The size of the particles corrected for strain effects is shown in Table 1. The size of the crystallites calculated by Williamson–Hall plot does not

c

5.2281 −1.8730 5.2054 −0.0383 5.2039 0.1777 5.2038 0.1916 5.1979 0.6987 5.1922 1.1971 – 3.2415 3.2299 3.2426 3.2433 3.2398 – 17 18 9 13 15

Stress (GPa) Lattice constant (Å)

Bulk values of lattice constants; a = b = 3.249 A˚ a Average transmittance in the wavelength region 400–900 nm.

k + 2 sin  D

14 14 17 8 12 14

ˇ cos  =

Debye Sherrer equation From W–H plot a = b

where  is the X-ray wavelength,  h k l is the Bragg diffraction angle and ˇh k l is the full width at half-maximum (FWHM) in radian of the main peak in the X-ray diffraction pattern. The calculated values of average size of the crystallites are in the range 8–17 nm (Table 1). The effect of crystallite size induced broadening and strain induced broadening in the full width at half maximum (FWHM) of XRD peak can be studied using Williamson–Hall plot [12]:

180 170 155 175 150 200

(1)

ZO 1AZO 3AZO 5AZO 7AZO 9AZO

0.9 ˇh k l cos(h k l )

Particle size (nm)

Dh k l =

Films Thickness (nm)

Fig. 1 shows the XRD patterns of undoped and Al doped ZnO thin films. The XRD patterns show that all the films are well crystallized with hexagonal wurtzite structure with preferred orientation along (0 0 2) plane. The XRD patterns suggest single crystalline-like structure for undoped ZnO (ZO) film and polycrystalline nature for the doped films. The absence of Al peaks in the XRD patterns suggests that Al is well dissolved in the ZnO lattice. The average size of the crystallites Dh k l in the films can be estimated by the Debye Scherer Eq. [11]

Table 1 Structural, optical and electrical parameters of laser ablated undoped and aluminium doped ZnO.

3.1. XRD studies

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3.2. Micro-Raman analysis ZnO has a hexagonal wurtzite structure and belongs to the space 4 ) with two formula units in the primitive cell. All group P63 mc (C6v the atoms occupy the C3v sites. The group theoretical calculation based on the correlation method suggested by Fateley et al. [15] predicts nine optical modes which are distributed as follows: optical

ZnO

Fig. 1. XRD spectra of laser ablated undoped and aluminum doped ZnO thin films deposited at different Al2 O3 doping concentrations.

vary extensively from the values of crystallites size obtained from the Debye Scherer equation indicating that the strain in the films is less. Table 1 shows the calculated values of lattice constants a and c for the films. The value of lattice parameter c in a film is important as it gives an indication of strain in the film. The undoped ZnO film and 1 wt% Al doped films show slightly higher c value and all other doped films show lower c value compared to the bulk value [JCPDS Ref. Code 00-005-0664]. The films with values of c greater than ˚ have a positive or extensive strain in them the bulk value (5.205 A) whereas those with lower values have a negative or compressive strain [13]. The bi-axial stress of the ZnO thin films is calculated using the following equation [14]  = −453.6

cf − cb cb

(3)

where cb = 5.205 A˚ is the c-axis lattice constant of bulk ZnO and cf is the c-axis lattice constant calculated from the XRD data. The negative sign for the calculated stress for the undoped and 1 wt% Al doped films indicate that the crystallites are in a state of compressive stress. For all the other films, the value of the bi-axial stress is positive indicating that the films are under tensile stress. Also, the value of the stress is found to increase with increase in Al doping concentration. There are several reasons for the stress in the films. They are (1) lattice mismatch between the amorphous substrates and the film, (2) difference in thermal expansion coefficient between the substrate and film material, (3) deposition under non-equilibrium conditions and (4) doping and composition gradient across the film surface. During the deposition of a material on to a substrate surface, its periodicity is lost at the interfaces or at the film surface and can lead to surface stress.

= A1 (IR,R) + 2B2 + E1 (IR,R) + 2E2 (R)

(4)

where A1 and E1 modes are active in both Raman and IR spectra, E2 modes are active only in Raman whereas B2 mode is inactive in both Raman and IR spectra. Based on earlier Raman investigations, the different Raman modes on the ZnO films can be expected as follows: A1 -TO modes ∼ 382 cm−1 , E1 -TO modes ∼ 407 cm−1 , A1 -LO modes ∼ 576 cm−1 , E1 -LO modes ∼ 587 cm−1 , E2 -low modes ∼ 102 cm−1 and E2 -high modes ∼ 438 cm−1 [16]. The shift in band position with deposition parameters can be attributed to residual stress, structural disorder and crystal defect in the films. The low frequency E2 mode is associated with the vibration of the heavy Zn sub-lattice, while the high frequency E2 mode involves only the oxygen atoms [16]. Fig. 2 shows the Raman spectra of the undoped and Al doped ZnO films. In the case of undoped ZnO film (ZO) the E2 (high) mode appears as two bands ∼ 440 and 492 cm−1 with large intensity and degeneracy lifted. The band ∼ 492 cm−1 may have contribution to its intensity from the Si–Si stretching mode of the quartz substrate. The A1 (LO) mode is observed ∼596 cm−1 with medium intensity. The shifting of this mode to higher wavenumber can be attributed to the distortion of ZnO6 octahedra in the film. In the Raman spectrum of ZO film A1 -TO and E1 -TO modes are not resolved. The band at 790 cm−1 can be a combination band of A1 -TO and E1 -TO modes. The Raman spectrum of the 1AZO film, the bands are well resolved and A1 -LO mode appears with the highest intensity at 579 cm−1 . In this film E2 -high mode appears as two medium intense bands at ∼456 and 509 cm−1 due to the lifting of degeneracy. E1 -TO modes is observed as a weak band ∼402 cm−1 . The bands at 274, 640 and 885 cm−1 can have origin from the dopants. As in ZO films the spectrum of 3AZO is not well resolved. The Raman spectra of the 5AZO and 7AZO films show better resolved bands compared to 3AZO film. As in 1AZO film, Raman spectrum of 9AZO film presents well-resolved bands compared to other films [17]. Regarding shape difference in the Raman spectra, the shape and the position of the Raman bands depend mainly on the crystallization, stress/strain, structural disorder/defects and the presence of dopants. Here, as the doping concentration is increased, the substitution of dopant into the Zn lattice can cause strain and the lattice oxygen can be shared by both Zn and Al. This will lead to neighboring disorder and local geometric disorientation, affecting the Raman bending and stretching mode vibrations as the molecular theory of vibration suggests. Zhang et al. [18] have pointed out that the decrease in the crystallite dimension to the nanometer scale can cause the frequency shift and broadening of Raman bands as a result of phonon confinement. In the case of nanocrystalline materials the phonons are spatially confined and the phonons all over the Brillouin zone will contribute to the first order Raman scattering due to breakdown of the phonon momentum selection rule q = 0. The phonon dispersion can cause the asymmetric broadening and the shift in the positions of the Raman bands. 3.3. AFM analysis Fig. 3 shows 3D AFM pictures of the undoped and Al doped ZnO thin films. The AFM images of all the films except 9AZO, present uniform distribution of grains. The ZO film presents a

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711

Fig. 2. Raman spectra of laser ablated undoped and aluminum doped ZnO films as a function of Al2 O3 doping concentration.

dense distribution of bigger grains with well-defined boundaries. 1AZO film presents distribution of grains of varying sizes, without well-defined grain boundaries. The AFM images of 3AZO and 7AZO films shows distribution of smaller grains with well-defined boundaries for large areas whereas the AFM picture of 5AZO present well-defined grains of different sizes with lot of porosity. The AFM picture of 9AZO presents agglomeration of smaller grains into bigger structures. The variations of surface roughness with Al doping concentration of the films are shown in Table 1. The undoped films has an rms surface roughness of 7.6 nm whereas the doped films shows high values of rms surface roughness ranging from 18.7 to 69.5 nm. The agglomerated growth mode in the doped films reveals that the nucleation barrier of ZnO is reduced by the doping of Al. 3.4. UV–vis spectra analysis Fig. 4 shows the transmittance spectra of undoped and Al doped ZnO thin films. The sharp fall of transmittance near the absorption edge (∼350 nm) observed for all the films indicates their good crystalline and direct band gap nature. The average transmittances in the wavelength region 400–900 nm of the films are shown in

Table 1. The undoped ZnO film has an average transmittance of 88%, whereas the 1AZO, 3AZO and 5AZO films show an average transmittance above 72%. As the Al doping concentration increases beyond this, the average transmittance decreases. The optical band gap E0 can be estimated from the Tauc plot: (˛h) = A(h − Eg )

n

(5)

where Eg is the band gap corresponding to a particular transition occurring in the film, A is a band edge constant,  is the transition frequency and the exponent n characterizes the nature of band transition. n = 1/2 and 3/2 corresponds to direct allowed and direct forbidden transitions and n = 2 and 3 corresponds to indirect allowed and indirect forbidden transitions, respectively [19,20]. The optical band gap Eg can then be obtained from the intercept of (˛h)2 vs. h for direct transitions. It is observed that for all the films, the best straight line is obtained for n = 1/2, which is expected for direct allowed transition. Though the pure ZnO film consists of ZnO nanoparticles (size – 14 nm) the optical band gap observed by this film is less than that of the bulk value. In nanostructured films, one can expect a blue shift in the optical band gap energy due to quantum confinement effect. The observed red shift in the band gap

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Fig. 3. 3D AFM images of laser ablated undoped and aluminum doped ZnO thin films at different Al2 O3 doping concentrations.

for the pure ZnO film can be attributed to the band bending effect due to the smaller size of the crystallites [21,22]. Stress or strain can affect the optical properties, as this will directly influence the band gap of the materials. Compressive strain leads to increase in band gap and the tensile strain leads to decrease in band gap. Stress/strain will affect the lattice parameter which in turn determines the band gap. Strain/stress can lead to valence band splitting also. The films with values of c greater than the bulk ˚ have a positive or extensive strain in them whereas value (5.205 A) those with lower values have a negative or compressive strain [13]. Here, all the films except undoped film, posses’ compressive strain and this will lead to increase in band gap. As the Al doping concentration increases, optical band gap increases in aluminum doped films. In doped semiconductors, there is a possibility of widening of optical band gap due to Burstein Moss (BM) effect. The observed variation of optical band gap in the Al2 O3 doped ZnO films can be due to the combined effect of band bending and Burstein Moss (BM) effect [23]. The knowledge of optical constants of materials is frequently of great interest in the design and analysis of materials to be used in optoelectronics. The extinction coefficient k is calculated using the relation: k=

˛ 4

(6)

where  is the wave length and ˛ is the absorption coefficient. The low values of the extinction coefficient in the visible and near infrared region for the undoped and doped films with lower Al doping concentrations suggest their surface smoothness and high transmittance of the films (Fig. 4(d)). Higher values of extinction coefficient in the visible and near infrared region are observed for films with higher Al doping concentration. It is well known that the optical reflection of a thin film is directly dependent on the refractive index of the film through the following relation [24]

n=

(1 + R) +



4R − (1 − R)2 k2 1−R

(7)

where R and k are the reflectance and extinction coefficients respectively. The refractive indices of all the doped ZnO films are calculated and are shown in Table 1. It is observed that the refractive indices of the Al doped ZnO thin films are higher than the undoped ZnO film in the measured wavelength range and decreases with increase in Al doping concentration. The decrease of refractive index with the increase of the Al doping concentration can be attributed to the increase of the carrier concentration in the ZnO: Al films [25].

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713

Fig. 4. (a–d) Transmittance, reflectance spectra, band gap energy and extinction coefficient of laser ablated undoped and Al2 O3 doped ZnO films.

The porosity of the films is calculated (Table 1) using the following equation [26].



P = 1−

n2 − 1 n2T − 1



× 100(%)

(8)

where n is the refractive index of the film and nT is the refractive index of bulk ZnO (nT ∼ 2.0). The undoped ZnO film has an average porosity of 47% whereas the doped films the porosity is less compared to that of undoped ZnO film. The porosity of the doped ZnO films increases with the increase in Al doping concentration. The refractive indices and porosity are strongly depends on the Al doping concentrations. 3.5. Photoluminescence studies PL spectrum of the undoped films presents PL emission in the UV and blue region whereas the Al doped films give green and yellow emission in addition to the emission in the UV and blue region (Fig. 5). The room temperature UV emission observed is attributed to free excitonic emission because of the high exciton binding energy of 60 meV. The emission in the blue–green region can be due to oxygen vacancies and porosity [26]. It might be related to a transition with in a self-activated center formed by a doubly ionized zinc vacancy (VZn −2 ) and the ionized interstitial Zni or due to the electronic transition from the bottom of the conduction band to the antisite defect OZn level [27–29]. 3.6. Electrical studies The dc resistance of the films in the temperature range 70–300 K is measured using four-probe method. The conduction characteristics of ZnO are primarily dominated by electrons generated by the O2− vacancy and Zn interstitial atoms [30,31]. Hong et al. have observed that ZnO film shows about 600 M/square of sheet resistance [32]. The high dc electrical resistance, in the order of M, is reported for ZnO films at 300 K by Sahay et al., and they attributed

this high resistance to the large density of extrinsic traps at the grain boundaries due to oxygen chemisorptions and the traps can deplete the grains and result in a charge carrier barrier at the grain boundaries [33,34]. It is known that Al2 O3 acts as an effective donor when it is doped in ZnO [35,36]. Al atoms in the films produce not only conduction electrons but also ionized impurity scattering centers. The resistivity of the Al doped ZnO films is related to the Al doping concentration, O vacancies, Al and Zn concentrations at interstitial sites, grain boundaries and ionized impurity scattering [37]. High Al concentration in the film does not always increase the freeelectron concentration proportionately since all the Al atoms may not replace the Zn atoms to contribute conduction electrons. The extra Al atoms might not occupy the correct places inside the zinc oxide crystallites because of the limited solubility of Al inside ZnO. The ionic radius of Al is smaller than that of Zn, and the excess Al may occupy interstitial positions and deform the crystal structure. Some Al may also form aluminum oxide between small zinc oxide crystallites. At low Al concentration, the free-electron density in the film increases. The deposition temperature strongly influences both the gas-phase reaction and the movement of dopant atoms to the positions in which they are electrically active. Hiramatsu et al., suggests that the internal stress suppresses the electronic conduction in the film and its removal enhances the mobility of electrons [38]. When the stress of the films increases, the defect also increases. Further, the increase of concentration of defects causes the increase of carrier concentration, and thereby, the resistivity decreases [39]. The room temperature electrical resistivity is calculated and is shown in Table 1 and Fig. 6(a). The 1AZO film shows the lowest dc electrical resistivity of 5.94 × 10−6  m and this value is comparable to the lowest range of reported values. The 3AZO, 5AZO, 7AZO and 9AZO films show the electrical resistivity values 1.38 × 10−4 , 3.32 × 10−4  m, 4.38 × 10−5  m and 1.03 × 10−4  m respectively. Thus, the aluminum doped ZnO films have lower dc electrical resistivity compared to that of reported values of pure ZnO thin film. The electrical conductivity in Al doped ZnO films is higher than that in pure ZnO films due to the contribution from Al3+

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Fig. 5. PL spectra of laser ablated aluminum doped ZnO thin films deposited at different Al2 O3 doping concentration.

ions on substitutional sites of Zn2+ ions and Al interstitial atoms as well as from oxygen vacancies and Zn interstitial atoms. The lowest value of dc electric resistivity observed in the present investigation is 5.94 × 10−6  m for 1 wt% Al2 O3 doped ZnO film (prepared at a substrate temperature 873 K on quartz substrate under oxygen ambience of 0.02 mbar). Under such preparation conditions, many of the oxygen vacancies in the films may get filled and films may show higher value of dc electric resistivity. However, the value of dc electrical resistivity obtained in the present case is less than the reported value of the dc electrical resistivity (8.01 × 10−6  m) of the as-deposited laser ablated aluminum doped ZnO films by Noh et al. [40]. Kim et al., have reported a dc electrical resistivity of 4.7 × 10−6  m for 3 wt% Al2 O3 doped zinc oxide films prepared at substrate temperature of 423 K by rf magnetron sputtering technique [1]. But, at a substrate temperature 573 K they have observed an electric resistivity in the order of 10−1  m only. Minami et al., have reported a value 3 × 10−6  m for dc electric resistivity in Al2 O3 doped zinc oxide films deposited on glass substrates kept at a substrate temperature of 423 K by dc + rf magnetron sputtering technique in hydrogen ambience (1.5%) [41]. Thus, in the present case, 1AZO film prepared at higher substrate temperature and under oxygen ambience shows comparable/better value of electric conductivity than that reported for Al doped films.

The electrical conductivity  can be expressed as an exponential function of temperature, T,



(T ) = 0 exp −

Ea KB T



(9)

where  0 is the pre-exponential factor including the charge carrier mobility and density of states, Ea is the activation energy of dc conductivity and KB the Boltzmann’s constant. The conductivity () of the films as a function of 1/T is displayed in ln  vs. 1000/T plot and is given in Fig. 5(b). The straight line nature of the Arrhenius plot indicates that the conduction is thermally activated, as often found in semiconductors. The activation energy of conduction for oxide semiconductors, which is the thermal energy required to hop the charges from one site to another, is less for Al doped films compared to pure ZnO samples. It is very significant to calculate the figure of merit ZnO since it belongs to the family of transparent conducting oxides. The figure of merit is defined as [42]  = −R ln T ˛

(10)

where  is the electrical conductivity, ˛ the absorption coefficient, R the resistance in  and T is the average transmittance. Most of the transparent conducting oxide thin films have a figure of merit between 0.5 and 2.5 −1 [41]. The figure of merit dependence on

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715

Fig. 6. (a) Resistivity and (b) activation energy of laser-ablated aluminum doped ZnO films.

For these studies, open aperture z-scan measurements [47] were done at 532 nm using 7 ns laser pulses from a frequency-doubled Nd:YAG laser (Quanta Ray). An optical limiting behavior (reduced transmission at higher laser intensities) was observed, and the optical limiting curve extracted from the z-scan data for the 5AZO film is shown in Fig. 7. The nonlinear transmission is found to occur due to an effective three-photon type absorption, which numerically fits to the corresponding transmission equation given by [48]

 T =−

(1 − R)2

exp(−˛0 1) √ p0

+ p0 exp(−t 2 )] dt

Fig. 7. Optical limiting behavior of the 5AZO film when excited by 7 ns laser pulses at 532 nm. The open aperture z-scan curve is shown in the inset. Solid curves are numerical fits to the experimental data obtained using Eq. (11).



−∞

ln[

∞

1 + p20 exp(−2t 2 ) (11)

where T is the light transmission through the sample, ‘R’ is the surface reflectivity, and p0 is given by 2 (1 − R)2 I02 L, where is the effective three-photon absorption coefficient and I0 is the onaxis peak intensity. ˛0 is the linear absorption coefficient. The effective three-photon absorption coefficient is calculated to be 9.9 × 10−20 m3 /W2 . These measurements show that the present material is a potential candidate for optical limiting applications. 4. Conclusions

Al doping level is shown in Table 1 and the best figure of merit is for 1 wt% Al doped ZnO. This high figure of merit is attributed to the combination of high transmittance and low sheet resistance for the 1AZO film. 3.7. Nonlinear optical studies There are a number of reports on the nonlinear light transmission behavior of ZnO nanoparticles in the nanosecond and ultrafast laser pulse excitation regimes [43–46]. Owing to this widespread interest in the nonlinear light transmission of ZnO based nanomaterials, we carried out open aperture z-scan measurements in the present samples using 7 ns laser pulses at 532 nm. Results show the occurrence of effective three-photon absorption, which is a behavior similar to that reported recently in Na doped ZnO nanoparticle dispersions [43].

Al incorporated ZnO films are synthesized on quartz substrates by pulsed laser ablation technique. All the films exhibit hexagonal wurtzite structure with (0 0 2) preferred orientation of grain growth. The observation of the high-frequency E2 mode and the longitudinal optical A1 (LO) mode in the Raman spectra of the films supports the XRD findings that the films are of hexagonal wurtzite structure. The AFM image of all the films shows a uniform distribution of grains. Aluminum doping strongly influences the optical properties of the ZnO thin films. The red shift in the band gap for the pure ZnO film compared to the bulk value can be attributed to the band bending effect due to the smaller size of the crystallites, and the observed increase in optical band gap due to Al2 O3 doping compared to undoped film can be due to the combined effect of band bending and the Burstein Moss (BM) effect. The co-existence of PL emission in the UV–vis region, high optical transmittance,

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