Nonlinear absorption properties of ZnO and Al doped ZnO thin films under continuous and pulsed modes of operations

Optics and Laser Technology 102 (2018) 147–152

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Nonlinear absorption properties of ZnO and Al doped ZnO thin films under continuous and pulsed modes of operations K.M. Sandeep ⇑, Shreesha Bhat, S.M. Dharmaprakash Department of Physics, Mangalore University, Mangalagangotri 574199, India

a r t i c l e

i n f o

Article history: Received 1 September 2017 Received in revised form 11 December 2017 Accepted 16 December 2017

Keywords: Thin films Pulsed mode laser Continuous wavelength regime Reverse saturable absorption Saturable absorption

a b s t r a c t In the present investigation, we present the variations in nonlinear optical (NLO) properties of undoped and Al doped ZnO (AZO) films under two different off-resonant regimes using continuous and pulsed mode lasers. Z-scan open aperture experiment is performed to quantify nonlinear absorption constant and imaginary component of third order susceptibility. Reverse saturable absorption (RSA) and saturable absorption (SA) behaviors are noticed in both undoped and AZO films under pulsed mode and continuous wavelength (CW) regime respectively. The RSA and SA behavior observed in the films are attributed to two photon absorption (TPA) and thermal lensing properties respectively. The thermal lensing is assisted by the thermo-optic effects within the films due to the continuous illumination of the laser. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Nonlinear optical materials use photons instead of electrons in the applications such as data storage, telecommunication, and data processing. Thus, nonlinear optical materials are different in their working mechanism with respect to other optical materials [1–3]. The field of nonlinear optics involves the variations of optical parameters (nonlinear absorption coefficient, nonlinear refractive index, third order nonlinear susceptibility) associated with the materials when they interact with the intense coherent source of light. The intense coherent light can be achieved adapting lasers. However, the interaction of high intense radiation may deteriorate the materials properties forever and thus; it is expected to choose a suitable light source to understand the nonlinear properties of a material [4]. Optical materials such as two dimensional nanostructures (thin films) are very sensitive due to their smaller thickness values and their large surface to volume ratio. Semiconducting oxide materials are promising in nonlinear applications due to their perfect crystalline nature and higher stability at elevated pressure and temperature [5]. ZnO is one among the wide band gap semiconducting oxide materials having potential of being used in different optoelectronic and photonic applications [6,7]. ZnO shows strong third order nonlinearity when it is exposed to a high intense coherent beam focused by a convex lens [8–10]. In fact, ZnO nanostructures have shown higher order ⇑ Corresponding author. E-mail address: [email protected] (K.M. Sandeep). https://doi.org/10.1016/j.optlastec.2017.12.031 0030-3992/Ó 2017 Elsevier Ltd. All rights reserved.

nonlinearity compared to some organic materials [11]. Thus, ZnO is one among the promising nonlinear optical materials. However, ZnO nanostructures are affected by the type laser source used to study nonlinear properties. Laser source with short pulse width can damage the material itself. Deterioration of structural and nonlinear properties cannot be precluded in this regard. Thus, proper choice of laser source must be a primary focus prior to nonlinear studies. Ponnusamy et al. [12] studied the nonlinear absorption process in Co:ZnO nanoparticles under different laser modes of operations. They observed reverse saturable absorption mechanism in nanoparticles under the nanosecond pulse width laser and saturable absorption phenomenon under the continuous wavelength regime. A similar observation was also made by C. Torres-Torres et al. [2] They observed RSA mechanism in ZnO film under picoseconds open aperture Z-scan at 1064 nm and SA mechanism under femtosecond open aperture Z-scan at 825 nm. In the present discussion, we will show that the open aperture Z-scan curves are dependent on the different modes of laser operations. Also, the dominance of thermo-optic effects and their influence on the observed nonlinear properties of ZnO and AZO thin films fabricated by sol-gel spin coating technique are discussed. 2. Experimental 2.1. Film preparation Standard sol-gel spin coating technique was employed for the preparation of ZnO and Al doped ZnO thin films. The film

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preparation is followed according to Sandeep et al. [13] Therefore we do not discuss the preparation method in detail. The molar concentration of the sol maintained was 0.3 M. The concentration of Al dopant was fixed at 2 at. wt.% and 4 at. wt.%. 2.2. Characterization tools To understand the crystal structure of ZnO and AZO films, powder X-ray diffraction spectrometer were used (Rigaku Miniflex). The XRD spectra were recorded by scanning samples at 2°/min. The nonlinear properties of films were studied by Z-scan technique. The Z-scan technique is a single beam characterization tool used to quantify the nonlinear absorption constant and nonlinear refractive index of nonlinear optical materials. The Z-scan measurement involves two divisions; open aperture and closed aperture techniques. Open aperture technique is used to quantify the nonlinear absorption coefficient (b) and closed aperture for nonlinear refractive index (n2). We used CW diode pumped solid state laser (DPSS) operating at 532 nm, emitting a linearly polarized wave in a TEM00 mode with an output power of 200 mW. A plano-convex lens of focal length 28.6 cm was used to focus the beam on the sample. Also, we have used second harmonic of a quanta Nd:YAG laser with 6.8 GW cm
2, 7 ns operating at 532 nm and 1 Hz repetition rate. A plano-convex lens of focal length 20.5 cm is used to focus the coherent light on the sample. The room temperature luminescence studies were performed to understand the electronic transitions involved by exciting the films at a wavelength of 275 nm using Edinburgh luminescence spectrometer. 3. Results and analysis 3.1. XRD studies The XRD spectra of ZnO and AZO films (Fig. 1(a)) show strong reflections along (0 0 2) crystal growth orientation of hexagonal wurtzite structure. This suggests that large number of crystallites orient along (0 0 2) plane or c-axis. In a hexagonal wurtzite structure, c-axis possesses minimum surface energy (1.6 J/m2) compared to other crystalline planes. Due to the lower surface energy of c-axis, crystallites orient along this axis [14]. The diffraction intensity of the c-axis is reduced in AZO films with increasing Al doping concentrations along with the presence of new reflection along (1 0 0) plane. The intensity of (1 0 0) peak is enhanced in AZO films when the doping concentration is increased to 4 at. wt %. The crystallites orientation along (1 0 0) plane reduces the caxis orientation in AZO films compared to ZnO. The presence of reflections along (0 0 2) and (1 0 0) planes confirms the polycrystalline nature of the AZO films. The polycrystalline nature arises when crystallites orient along different planes leading to different crystallite size along different crystalline planes. The absence of Al or Al based compounds reflection peaks confirm the purity of AZO films and absence of phase segregation as well. The orientation of crystallites along (1 0 0) plane in AZO films leads to lattice disorder. The diffraction angles of (0 0 2) plane for ZnO, 2 at. wt.% AZO, and 4 at. wt.% AZO films are 34.37°, 34.39°, and 34.41° respectively. The shift in diffraction peak towards higher diffraction angle side upon Al doping is due to the decrease in the values of lattice parameters [15]. The lattice parameters are calculated considering wurtzite structure of the films using the expression a ¼ pffiffi3 ksin h and c ¼ sink h respectively. The decreasing trend in lattice parameters (Table 1) with increasing Al doping concentration is attributed to the difference in ionic radii of Zn2+ (0.074 nm) and Al3+ (0.053 nm). The lattice parameters in AZO films decrease when Al3+ ions substitute Zn2+ ions in the wurtzite crystal lattice.

Fig. 1. XRD spectra of ZnO and AZO films (a) showing strong c-axis orientation (b) variations in intensities of diffraction.

The variations in lattice parameters induce stress in the films. The stress induced along c-axis in ZnO and AZO films with respect to standard bulk ZnO is calculated using the expression [16]:



e ¼
4:5

 c
c0  1012 dyne=cm2 c0

where c is the calculated lattice parameter of the samples, and c0 is the unstrained lattice parameter of the bulk ZnO (c0 = 5.205 Å, JCPDS#36-1451). From Table 1, the calculated stress values along c-axis show negative sign. The negative sign indicates the presence of compressive stress in the films. The compressive stress in the films is due to the interstitial sites occupied by the Zn2+ and O2
in the ZnO lattice [17]. However, the compressive stress is reduced as the Al doping concentration is increased. This confirms that Al3+ ions occupy the substitutional positions in the ZnO lattice during doping process; leading to reduced compressive stress in AZO films. In the present study, with increasing Al doping concentrations, the diffraction angles of (0 0 2) peaks approaches to the standard bulk ZnO (34.43°) leading to perfect crystallinity in AZO films. However, the identification of reflection peaks along the (1 0 0) plane in AZO films confirms the lattice disorder in the AZO lattice. The lattice disorder can contribute to various defect states in the AZO films [18]. The crystallite size (D) of the films is calculated using DebyeScherrer formula [19]. From Table 1, the crystallite size values

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K.M. Sandeep et al. / Optics and Laser Technology 102 (2018) 147–152 Table 1 Structural and optical parameters of ZnO and AZO thin films. Sample ZnO 2%AZO 4%AZO

a (Å) 3.009 3.007 3.005

c (Å) 5.210 5.208 5.206

e (dyne/m2) 9


4.30  10
2.61  109
0.86  109

D (nm)

Eg (eV)

INBE/IDLE

25.8 21.4 19.7

3.27 3.24 3.22

5.19 3.68 2.63

showed a decreasing trend upon Al doping. The reduced crystallite size in AZO leads to the formation of large number of grain boundary defects along with other structural defects arise during film deposition. Thus, we speculate that the observed variations in nonlinear properties of ZnO and AZO films are the result of lattice disorder due to polycrystalline nature of the films, grain boundary defects, and point defects. 3.2. UV–VIS spectroscopy studies To determine the band gap of the prepared films, Tauc’s plot is considered (Fig. 2). It is observed from Fig. 2 that the absorption edge of AZO films shifted towards lower energy values with increasing Al doping concentrations. Thus, the band gap values of AZO films decrease compared to ZnO thin film. The redshift in band gap values of AZO films is attributed to the lattice disorder observed in AZO films. Due to lattice disorder in AZO films along (1 0 0) plane, large number of defect states form below the conduction band edge. The formation of defect states below the conduction band edge results in bandgap shrinkage (Table 1). 3.3. Room Temperature Luminescence (RTPL) studies The RTPL spectra of ZnO and AZO films were recorded at an excitation wavelength of 275 nm (Fig. 3(a)). The spectra show a dominant emission peak around 375–388 nm, and it is termed as near band edge (NBE) emission peak [20]. NBE emission peak is attributed to band to band carrier recombination. It is observed from the spectra that the NBE emission peak has shifted towards longer wavelength region with increasing Al doping concentrations. The redshift of NBE peak is attributed to deceased band gap in AZO films due to the formation of some defect energy states below the conduction band edge, arising out of the lattice disorder in AZO films [18]. These defect states act as non-radiative recombination centers caused by Auger process [19]. According to Auger process, the energy released by recombination electron will be

Fig. 3. (a) PL spectra of prepared films showing the red shift in NBE emission intensity (b) magnified PL spectra showing DLE peaks.

Fig. 2. Energy gap determination of prepared films using Tauc’s plot.

converted as phonons. Auger process involves in the generation of phonons during the electron recombination [21]. From PL spectra, three minute emission peaks are observed in the visible region (465–495 nm). The emission peaks in the visible region are regarded as defect level emission (DLE) peaks [22]. The DLE peaks are due to the presence of various defect states located in the band structure of ZnO and AZO films. In the present study, we observed DLE peaks at 468 nm, 482 nm, and 494 nm (Fig. 3 (b)) and they are attributed to various defect states such as oxygen vacancies (VO), zinc vacancies (VZn), oxygen antisites (OZn) and zinc antisites (ZnO) [23]. The presence of DLE peak at 468 nm is attributed to neutral charge oxygen vacancy (VO) defect states in undoped and AZO

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films [24]. Oxygen vacancies (singly charged, doubly charged and neutral charge) are predominant in ZnO and AZO thin films due to the fact that these films were annealed at 500 °C in air environment after the deposition. Neutral charge oxygen vacancies actively take part in the defect related luminescence in ZnO. K.M. Wong et al. have attributed neutral charge oxygen vacancies as the origin of green emission in ZnO nanowire [24]. Further, VO may act as charge compensating center for one additional negative charge released into the AZO lattice by Al3+. In the present study, we observed from XRD studies that Al3+ substituted Zn2+ in the ZnO lattice. Thus, the extra negative charge released by Al is compensated by the VO states which lead to charge neutrality in AZO thin films. Also, VO is stable in the neutral charge state in n-type ZnO. Furthermore, according to A. Jannoti et al., the chances of contribution from charged VO defects to the charge neutrality are neglected because of their higher formation energies in ZnO and AZO films [25]. To gain further insight into the observed DLE peaks enhancement, we calculated the ratio of emission intensities of NBE peaks (INBE) to DLE peaks (IDLE at 468 nm). The calculated INBE/IDLE values show a decreasing trend. This confirms AZO films are influenced by surface and grain boundary defects. Thus, from XRD, UV and RTPL studies it can extracted that, the enhanced defect states in AZO films depends on the disorder of the AZO lattice and crystallite size instead of stress components in the present study. Thus, we ascribe the enhanced defect states in AZO films to reduced crystallite size and AZO lattice disorder. 3.4. Nonlinear optical studies The open aperture Z-scan curves of the prepared films under pulsed mode and continuous laser modes are shown in Fig. 4(a-c) and 5(a-c). Two different natures of the curves under different laser sources are observed. The presence of normalized valley in both undoped and Al doped ZnO films confirm the RSA behavior of the samples under the pulsed laser mode. The exhibition of RSA behavior under pulsed laser mode is attributed to TPA process involved in the films. Although RSA behavior can arise due to various factors such as two/three photon absorption, free carrier absorption, excited state absorption, and nonlinear scattering; TPA is a dominant mechanism in semiconductors. TPA dominates other mechanisms when the energy (E) of Nd:YAG laser having pulse width 7 ns satisfies the condition, Eg > E > Eg/2 [6]. Where Eg represents energy gap values of ZnO and AZO films. The energy gap values of ZnO and AZO films were calculated using Tauc’s plot and the values are given in Table 1. Thus, the observed RSA mechanism is majorly contributed by TPA process. TPA process involves in the absorption of a pair of photons by a material [4,26]. The rate of absorption purely depends on the intrinsic properties of a material. Thus, TPA can be effectively used to probe the material properties. In the present scenario, the intermediate states present in the ZnO and AZO aid to TPA process because of the fact that the energy of pulsed laser (2.21 eV) is insufficient to excite the carriers in ZnO and AZO (>2.21 eV) to the excited states and thus two photon absorption is assisted by intermediate states. In a pure TPA process, two photons are absorbed only when the lifetime of intermediate states is very much less than the pulse width of the excitation source used. From Fig. 4(b, c), the enhanced valley with increased Al doping concentration suggests that the presence of virtual intermediate states between the band structure of the films. These virtual states (grain boundary defects) have very short lifetime (in terms of picoseconds) supporting the TPA mechanism [27]. However, it is known that the lifetime of defect states present at the defect levels varies from nanoseconds to several microseconds and these defect level defect states would act as trapping centers which reduce the

Fig. 4. Open aperture Z-scan curves using pulsed mode lasers of (a) ZnO, (b) 2%AZO, and (c) 4% AZO.

strength of TPA. In the present case, the enhanced grain boundary and surface defects in AZO films result in the formation of localized states just below the conduction band. This, in turn, leads to band gap shrinkage. The decrease in band gap is confirmed by the red shift in luminescence emission peaks (Fig. 3).

K.M. Sandeep et al. / Optics and Laser Technology 102 (2018) 147–152

Under the continuous wavelength regime, the open aperture curves of all the films showed a reverse trend i.e. the presence of normalized peaks (Fig. 5(a-c)) compared to open aperture curves under the pulsed mode laser. The presence of normalized peaks in all the films is attributed to the saturable absorption (SA) process. The SA process involves in the maximum transmittance of 1.020

a Norm. Transmittance

1.015

1.010

ZnO 1.005

1.000 -10

-5

0

5

10

z (mm) 1.025

b

Norm. Transmittance

1.020 1.015

2% AZO 1.010 1.005 1.000 -15

-10

-5

0

5

10

15

z (mm)

1.07

Norm. Transmittance

1.05 1.04

the electromagnetic radiation as the sample moves towards the focus. The SA process in ZnO nanostructures is observed by various researchers [2,28,29]. Irimpan et al. [29] observed the similar saturable absorption in self assembled ZnO films and they attributed it to the saturation of linear absorption of electromagnetic radiation due to the presence of defect states in the films. Similar observations were also made by Sandeep et al. [28] in Al and Li doped ZnO thin films prepared by sol-gel spin coating method. In the present study, the observed SA behavior in films is attributed to thermo-optic effects due to the continuous illumination laser source used [2]. The thermo-optic effects lead to thermal lensing of the films. This, in turn, results in bleaching of carriers in the ground state. The accumulation of carriers in the excited states hinders two photon absorption and hence the transmittance of the films increases when the sample is moved towards the focal point. Sometimes, the continuous illumination may lead to excessive heating of the films which results in the creation of localized plasma in the films. The creation of localized plasma (also known as laser plasma) ionizes the entire sample to an extent that the carriers are no longer able to absorb photons from the laser source, leading to a reduction in the absorption rate and an increased transmission. However, the generation of laser plasma in ZnO and AZO films in the present study is highly unexpected because of the fact that the output power obtained from the continuous wavelength laser (200 mW) which is not sufficient to generate laser plasma. Thus, the observed SA behavior in ZnO and AZO films is solely attributed to thermo-optic effects in the prepared films. Nagaraja et al. [6] have studied the nonlinear properties of Mn doped ZnO films under continuous wavelength regime. However, they observed RSA behavior of the samples than SA behavior observed in our case. This is due to the variations in the thickness of the samples. In their studies, the thickness of the samples was 125 nm, 350 nm for ZnO and Mn doped ZnO films respectively. In our case, the thickness of the samples was in the range of 75– 85 nm. We assume that the observed SA mechanism may come due to lower film thickness. ZnO and AZO films with such low thickness values are more prone to thermal effects generated by intense light from the laser. Therefore we ascribe the observed SA behavior of the ZnO and AZO samples under the continuous wavelength regime to thermo-optical effects than the saturation of linear absorption due to defect states in the films. The XRD results also proved that single c-axis orientation in all the films leading to a perfect crystallinity in the films which avoids the possible creation of point defects and other native defects to a greater extent. 3.4.1. Quantification of nonlinear parameters To extract the third order nonlinear absorption coefficient values associated with the ZnO and AZO samples, open aperture Zscan is performed. The normalized transmittance of the open aperture curve is represented using the equation

c

1.06

4% AZO

TðzÞ ¼ 1


1.03

bI0 Leff

ð1Þ

2 3

2 ð1 þ x2 Þ

where I0 is the on-axis irradiance at the focus and Leff is the effective length of the sample represented as

1.02 1.01

Leff ¼

1.00 0.99

151

-10

-5

0

5

10

15

z (mm) Fig. 5. Open aperture Z-scan curves using continuous wavelength mode lasers of (a) ZnO, (b) 2%AZO, and (c) 4% AZO.

ð1
e
aL Þ

a

where a represents the absorption coefficient. A theoretical fit to Eq. (1) gives the value of b. The value of b can be effectively used to quantify another important nonlinear parameter imaginary component of third order susceptibility (vi ), specifies the radiation loss in the sample at a given frequency and it is given as ð3Þ

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Table 2 Nonlinear parameters of ZnO and AZO films under two modes of laser operation. b (m/W) (pulsed mode)

Sample


7

4.63  10 5.8  10
7 9.81  10
7

ZnO 2% AZO 4% AZO

við3Þ ðesuÞ ¼

e0 c2 n20 k b ðm=WÞ 4p2

b (m/W) (continuous mode)
7

v(3) (esu) (pulsed mode) i
8


0.20  10
0.38  10
7
1.20  10
7

1.56  10 2.40  10
8 4.21  10
8

ð2Þ

where e0 is the vacuum permittivity and c is the velocity of light in vacuum. The values of b and vi are given in Table 2. From Table 2, it is noted that the b values are one order smaller while showing SA behavior compared to RSA behavior. As from the above discussion, the open aperture curves under continuous wavelength regime are influenced by thermo-optic effects. Thermo-optic effects always hinder the NLO properties exhibited by optical materials. The variation in b and v parameters as estimated using two different modes of laser operation is due to two different mechanisms involved in the films. The b and v parameters obtained using pulsed mode laser is due to RSA mechanism and the nonlinear parameters obtained using continuous wavelength mode is due to saturable absorption mechanism. The values nonlinear absorption and imaginary component of third order susceptibility are in accordance with the previously reported values [27]. Thus, ZnO and AZO films can be effectively used in NLO applications such as nonlinear optical switching, optical memory managements, and saturable absorbers. ð3Þ

4. Conclusions We have tested the nonlinear properties of ZnO and AZO films under two different modes of laser operations and observed two completely different behaviors of the samples. Pulsed mode regime resulted in RSA behavior and it was attributed to TPA mechanism whereas continuous wavelength regime resulted in thermal effects in the samples. The continuous illumination with high intensity leads to the thermo-optic effects, which results in SA mechanism in all the films. The luminescence spectra showed the redshift in NBE emission with increasing Al doping concentration confirming the presence of localized defect states below the conduction band having shorter lifetimes compared to pulse width of the Nd:YAG laser. Acknowledgments The authors thank the Coordinator, DST-FIST and UGC-SAP, Department of Physics, Mangalore University, for providing facilities for the characterization of thin films and technical support to carry out the work. KMS acknowledges the financial assistance received from the UGC – India.

v(3) (esu) (continuous mode) i
0.10  10
8
0.16  10
8
0.51  10
8

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