The effect of Al doping on linear and nonlinear optical properties of pulsed laser deposited (PLD) Zn1-xAlxO (AZO) thin films is reported. The increase in 002 peak in X-ray diffraction pattern up to x = 0.05 is indicative of improvement in the wurtzite phase and was also confirmed by enhancement in E2 (high) peak at 438 cm−1 of Raman spectra. The increase in the blue shift in the band gap up to x = 0.05 suggests the Burstein Moss effect. The red shift in the band gap for x>0.05 can be interpreted due to the band gap renormalization as a result of many body interaction effects. The two photon absorption coefficients (β) and nonlinear refractive indices (n2) measured via z-scan measurement were found to be in the range of 3.4-8.0 cm/W and (1.0-5.2) × 10−4 cm2/W for x = 0.00-0.10 respectively. An attempt is made to correlate the linear and nonlinear optical behavior and crystal structure as a function of level of Aluminum doping (x).
© 2016 Optical Society of America
Zinc oxide (ZnO) has emerged as an important material towards the fabrication of the optical and nano devices especially in ultraviolet region. ZnO is found in basically three forms (i) hexagonal wurtzite, (ii) zinc blende and (iii) rock salt structure. The most stable structure under the ambient condition as well as over a wide range of temperature is the wurtzite phase. This structure can be easily grown on the variety of substrate. The rock salt structure is highly unstable and it is observed only at high pressure. The zinc blende structure is stable at room temperature but it is difficult to grow on the substrate other than the cubic substrates. ZnO possesses direct band gap of ~3.37 eV and large excitonic binding energy (~60 meV) and these are the key factors facilitating its application in the UV device. Optical, nonlinear optical and electrical properties of ZnO can be tuned by suitable dopants. Metal doped ZnO thin films are used as a transparent conducting electrodes (TCE) in solar cell and optical waveguides . The doping of Aluminum in ZnO forming Zn1-xAlxO is relatively simple. The basic mechanism involved during the formation of the Al:ZnO (AZO) composite is well documented in the literature . Aluminum induces n-type impurity in the ZnO host material to reduce the electrical resistivity and improve the speed and durability of the devices. The incorporation of Al in the ZnO host material introduces an extra electron which remains as a free electron and moves throughout the crystal lattice and hence increases the carrier concentration. The Al3+ ion replaces Zn2+ ion into the ZnO crystal lattice producing stresses and defects in the system. This results in altering the structural, electrical, optical and nonlinear optical properties. The band gap widening in AZO films can be explained in terms of Burstein moss shift [3–6]. AZO films can be fabricated via RF magnetron sputtering, sol gel method, spray pyrolysis, chemical vapor deposition and pulsed laser deposition etc [7–11]. Pulsed Laser Deposition (PLD) is one of the versatile technique to grow the stoichiometric oxide (composites) films . In the earlier reports on the Al doped ZnO thin film using PLD technique, correlation between structural and linear and nonlinear optical properties are not documented [13–15]. Therefore, it necessitated to present a systematic study on the variation of various parameters and correlation among them as a function of Al concentration in the ZnO film using PLD technique. The present paper deals with the fabrication and characterization of Zn1-xAlxO (x = 0.00 to 0.10) thin films via PLD technique. The study of the structural property validates the c-axis oriented hexagonal growth of the films. Optical band gap energy and linear refractive index as a function of x was estimated using UV-VIS-NIR spectrum. Z-scan measurement [16,17] was performed to measure nonlinear absorption coefficient and nonlinear refractive index from which real and imaginary part of the third order susceptibility as a function of x is estimated.
2.1. Fabrication of thin films
Figure 1 shows the schematic diagram of the Pulsed Laser Deposition (PLD) setup. The PLD system consisted of a multi-port stainless steel vacuum chamber, programmable carrousel for target holder, substrate holder having provision to raise the substrate temperature in the range of room temperature (RT) to 750 °C during deposition. The PLD targets of Zn1-xAlxO (0≤x≤0.10) were prepared using solid state reaction method. Pure ZnO (99.9%) and Al2O3 powders were homogeneously mixed in appropriate proportion and crushed into a fine powder using pestle and mortar. A 5% solution of an organic binder, polyvinyl alcohol (PVA), was added as an adhesive. The mixed powder was pressurized up to 3 Tons to form dens pellets of diameter 13 mm. The pellets were sintered at 1150 °C for 6 hrs in an electric furnace.
The sintered pellete of AZO, was mounted on the carrousel which was continuously rotated in a programmable manner to maintain the uniform ablation of the target material during deposition of the thin film. The PLD chamber was first evacuated up-to a base pressure of 2.5 × 10−5 mbar using turbo molecular pump backed by the rotary pump and then maintained at a pressure of 0.1 mbar of oxygen for deposition of AZO films . The second harmonic of a Q-switched Nd:YAG laser (Model No. INDI-HG) was focused on to the target using 35 cm focal length lens through one of the view ports of the PLD chamber as shown in Fig. 1. The high laser fluence ~10J/cm2 at the target surface resulted into the laser induced plasma formation of the target material in the focal region. The plasma plume expanded in the presence of ambient oxygen gas, cooled down and deposited on the fused silica substrate placed parallel to the target and 3 cm apart.The films were deposited at 500 °C of substrate temperature for 30 minutes duration. Thickness of the film was measured using Stylus profilometer (Dektak) and found to be 500 ± 20 nm. Structural information of the deposited films was carried out using X-ray Diffractometer (Model No. Rigaku TTRAX III 18 kW) operating at 1.5046 Å of Cu-Kα line and Laser Micro Raman (Model No. LAB RAM 800) in the back scattering geometry using an Argon ion laser at 488 nm wavelength. Linear optical properties of the films were studied by recording UV-vis-NIR spectrum (Model No. SHIMADZU UV-3101 PC).
2.2. Nonlinear optical characterization via z-scan set-up
The nonlinear optical properties of the films were studied using the modified z-scan technique . This technique is based on the spatial beam broadening and narrowing of the Gaussian beam in far field as a result of the optical nonlinearity present in the sample. The schematic diagram of the z-scan experimental setup is shown in Fig. 2. A cw He:Ne laser is focused by a lens of 5cm focal length onto the sample placed on the motorized translational stage. The sample is translated longitudinally on either side of focal plane and the transmitted beam is recorded on the CCD camera. A neutral density filter (NDF) of 3.0 Optical Density (OD) was placed in front of CCD to avoid its saturation. An aperture is placed after the sample in order to prevent scattered light entering into CCD. The beam waist (ω0) of the focused laser beam is given as ω0 = fλ/d, where f is the focal length of the lens, λ is the wavelength of the laser (633nm) and d is the diameter of the direct (unfocussed) beam. The diameter of direct beam, d was measured by imaging it on a CCD before the lens and found to be ~1.5 mm. From this, the beam waist, ω0, was found to be ~21 microns. The Rayleigh length (z0 = πω02/λ) is estimated to be ~2.2 mm which is much higher than the sample thickness satisfying the thin sample approximation for the z-Scan. The peak intensity (I0) at the focal plane is evaluated to be 1.12 kW/cm2. Simultaneous information about the open as well as closed aperture z-scan signal is deduced from this single scanned images. The integrated intensity over the entire image (open z-scan, OA) as a function of distance w.r.t. focal point gives the information about the absorptive nonlinearity present in the sample. The information about the closed aperture (CA) z-scan is obtained by placing a suitable synthetic aperture through a MATLAB programe in the central region of the images of open z-scan and the integrated intensity of these partially masked images (closed z-scan) revealed the nonlinear refractive index.
The linear aperture transmittance, the ratio of closed aperture signal and open aperture signal (S = CA/OA) was kept at 0.4 which is within the optimum value in order to exclude the contribution of the absorptive nonlinearity for determination of refractive nonlinearity .
3. Results and discussion
3.1. Structural properties
Figure 3(a) shows the X-rays diffraction (XRD) pattern of the Zn1-xAlxO (0≤x≤0.10) thin films fabricated using PLD technique. From the Fig. 3(a) it is clearly observed that the intensity of the (002) diffraction peak is increased with Al doping concentration up to x = 0.05 and then it falls down slightly for x = 0.10.
The average crystallite size (D) was estimated by applying the Debye scherrer equation .
The crystallite size with error bar as a function of x is shown in Fig. 3(b). For pure ZnO, the crystallite size is 17 nm and that of 19 nm for x = 0.05 and thereafter it decreases further with the increase in x and reduces to 15 nm for x = 0.10. From XRD observation the lattice constants (a and c), Zn-O bond length (s) and inter-planer spacing (dhkl) are determined by the following equations .
Since the radius of Zn2+ ions (0.74 Å) is greater than that of Al3+ (0.54 Å), therefore film under goes additional stress and strain with Al-doping hence resulting into small modifications in the crystallite size and the lattice parameters . The marginal changes in the lattice parameters of 002 peak suggest the partial replacement of Zn2+ ions with the Al3+ ions in the crystal lattice. By comparing the crystallite size listed in Table 1, it is observed that the maximum value of the crystallite size is shown by the film with x = 0.05 indicating the best crystallinity in it. The minimum value of lattice constants (a and c) for x = 0.05 confirms that film is under compressive stress. The decrease in the crystallinity at 10 wt. % doping concentration may be due to the stress and defects present in the films. In fact, during formation of the Al-doped films, the replacement of Zn2+ ions by Al3+ ions takes place. The mismatch in ionic radii of the Zn2+ ions and Al3+ results in decreasing the crystallite size with Al-doping at x = 0.10. The stresses present in the thin films also affect the crystallinity. There are two types of stresses present in the thin films viz; intrinsic and extrinsic. The origin of intrinsic stress is the result of the defects in the films. The intrinsic stress causes the compression in the lattice parameters. The extrinsic stress in the present case arises due to the mismatch in the thermal coefficients of Al2O3 and ZnO.
For hexagonal crystal lattice, the linear stress components (σx, σy, σz) are written as .
The total stress in the film is written as .
The value of σ was calculated from Eqs. (7-9) and its variation with x is shown in Fig. 4. It exhibits that the stress in the films first increases and then starts decreasing, attains minimum value at x = 0.05 and then again starts increasing again with x. This suggests that the films possess both compressive as well as tensile stresses.
The increase in the stress in AZO film compared to that of the pure ZnO signifies the tensile stress which in turn is due to the increased lattice constants (a and c) for the films having x = 0.01 and x = 0.02 while the compressive stress in the films for x = 0.03 to x = 0.10 is observed as a result of decrease in the lattice constants as listed in Table 1. The Zn1-xAlxO film deposited for x = 0.05 exhibits the minimum stress suggesting complete replacement of the Zn2+ ions with the Al3+ ions in the zinc oxide crystal lattice and hence the crystal quality is improved as in accordance with the FWHM of 002 peak.
Figure 5 (a) shows the Raman spectra of un-doped and Al doped ZnO thin films. Zinc oxide (ZnO) possesses hexagonal wurtzite crystal structure belonging to C6v space group. According to the group theory, there are 12 phonons branches near the brillouin zone center, out of which 9 are optical phonon branches and 3 are acoustic phonon branches . The pure ZnO exhibits an A1 branch, a doubly degenerate E1 branch, two doubly degenerate E2 branches and two B branches. A1 and E1 braches are Raman and IR active, E2 branches are Raman active only and B branches are Raman inactive. The observed Raman shifts in case of pure ZnO crystal are E2(low)-101 cm−1, A1(TO)-380 cm−1, E1(TO)-407 cm−1, E2(high)-437 cm−1, A1(LO)-574 cm−1 and E1(LO)-583 cm−1 .
The observed dominating modes in the present studies are optical phonon modes E2 (high) and A1 (LO) at 438 cm−1 and 578 cm−1 respectively. Apart from these A1 (TO) mode at 378 cm−1 is observed in the Al doped films. A weak peak at 279.2 cm−1 is also observed in the Al doped ZnO films which may be due to the presence of Al2O3 . The dominating mode E2 (high) at 438 cm−1 is shown separately in Fig. 5(b) for all the values of x, indicating the increase in intensity up-to 5 wt% of Al content and there after it decreases with the further increase in x. This confirms that at 5 wt % the film possesses highly c-axis oriented wurtzite phase in confirmation with the XRD observation. A red shifts as well as blue shift for the E2 (high) mode in the Zn1-xAlxO (0≤x≤0.10) thin films are observed. It is well documented that an increase in the E2 phonon frequency is associated to compressive stress and a decrease in the E2 phonon frequency is ascribed to tensile stress . In the present case, both the tensile and compressive stress is also confirmed from XRD analysis, Fig. 3. Raman shift and associated intensity for E2 (high) peak in the Zn1-xAlxO thin films are listed in Table 2.
From the Table 2, the Raman shift is red shifted for 1wt% Al doped film compared to that of pure ZnO film. Further increase in the Al concentration leads to the blue shift in the spectra and at 5wt% concentration the shift exactly matches with that of pure ZnO film. Thus the optimum Al doping at which the mutual replacement of Zn2+ by the Al3+ takes place is 5wt% and is also supported by the XRD data as shown in Fig. 3 (a).
3.2. Optical properties
Optical properties of the Al doped ZnO thin films were carried out using UV-Vis-NIR spectra. From the absorption spectra of the films and using Tauc’s plot, the absorption coefficients and band gap energies of the films were determined. The UV-Visible-NIR transmittance spectra for for Zn1-xAlxO (0≤x≤0.10) thin films are shown in Fig. 6 (a) and the swanepoel envelope plot for pure ZnO thin film only is shown Fig. 6 (b). The maximum transmittance (>90%) in visible region is observed for pure ZnO film. From Fig. 6 (a) it is also observed that the transmission of the ZnO thin film decreases with increase the Al doping concentration. Ping Yang et.al  reported simulated and experimental results for Al doped ZnO thin film based on the Maxwell Garnett model for macroscopic mixing of dielectrics in the optical frequencies domain and concluded the inverse behavior of the transmitivity with increasing Al doping concentration. The results shown in Fig. 6 (a) are in good agreement with above. The reason behind the decrease in transmittance is mainly attributed to the increase in carrier concentration with the doping . Refractive index (n) in the wavelength range of 350-2000 nm were calculated from the Swanepoel envelop curve .
The linear refractive indices of the films were extracted from following equations .
The variation in the refractive indices with wavelength for various Al doping concentration is shown in Fig. 7.
The evaluated results, thickness of the film and linear refractive index, n (at a wavelength of 633 nm) with the aluminum doping (x) are listed in Table 3.
It can be observed from Table 3 that there is a decrease in the refractive index of ZnO film with the increase in Al concentration till 2% thereafter it increases. This decrease in the refractive index in the Al doped ZnO film is attributed to the increased carrier concentration. This behavior is nearly in agreement with that of reported in literatures [28–30].
The film thickness (L) was determined using the equation give below.
Where λ1 and λ2 are the wavelength of the two consecutive maxima or two consecutive minima in the transmission spectra and n1 and n2 are the respective refractive indices.
The film thickness obtained from the transmission spectra are in good agreement with the thickness measured using stylus profilometer. The deviation between these two measurement is <5%. The thickness listed in the Table 3, are used in all the proceeding calculations described below.
The optical band gap energy of the films was estimated using the Tauc’s plot. The Fig. 8 (a) shows the absorption edge of the films deposited on fused silica substrate for all the values of x and the Tauc’s plot for x = 0.05 only is shown in Fig. 8 (b).
The absorption coefficient and the band gap were determined from the following equations .Fig. 8 (a)), L is the thickness of the film, h is the Planck’s constant, B is a constant, ν is the frequency of photon, Eg is the band gap energy and m defines the type of transition involved. ZnO being a direct band gap semiconductor and therefore m = 1/2. The plot of Eg and α (at λ = 633nm) as a function of Al doping (x) is shown in Fig. 9.
It can be concluded from the Fig. 9 that the band gap energy (Eg) and the absorption coefficient (α) increases with the increase in x up to 0.05 and decreases at higher value (0.10) of x. Absorption coefficients and band gap energies as a function of x are listed in Table 4. This tunable behavior, the dependence of x on the band gap energy and the absorption coefficient in the Zn1-xAlxO thin films are described in the following paragraph.
The band gap energy in the Zn1-xAlxO thin films is enhanced from 3.25 eV to 3.64 eV with the increasing Al concentration up-to 5wt %. This blue shift in the band gap (BM shift) widening in the Al doped ZnO thin films can be explained using the Burstein Moss (BM) effect . According to the BM effect, the conduction band of the degenerate semiconductor is significantly filled with high carrier concentration and the lowest valence energy states are blocked, consequently the Fermi level gets shifted in the conduction band and hence the apparent band gap energy is increased. ZnO doped with Al element can be treated as degenerate semiconductor. The expression for the shift in band gap, ΔEgBM, of the degenerate semiconductor is given by the following expression [32,33].4, 34].
Equation (15) suggests the inverse relationship between BM shift and carrier effective masses. Therefore heavily doped semiconductors having light carrier masses (n type) exhibit the large BM shift. Hence the band gap is blue shifted with the increase in x in the AZO film. When the carrier concentration is increased beyond the Mott critical density in heavily doped n-type semiconductor, the carrier-carrier and carrier-impurity interactions start dominating and result into the band narrowing .
The interaction between the free charge carriers (Electrons in the conduction band and holes in the valence band) at high Al concentration results in the neutralizing the charge carriers and form a non-conducting layer of Aluminum Oxide (Al2O3) just below the conduction band . This non-conducting layer prevents the motion of the charge carriers resulting in the decreasing the mobility and hence the resistivity is increased. Thus there is a strong competition between the BM effect and many body interaction effect as the carrier concentration is increased. These interactions lead to the band edge shrinking effect and so the band gap energy starts decreasing as the Al doping is increased beyond 5%.
The carrier concentration is found to be in the range of 1018-1020 cm−3, enough to show the BM effect . It is found to be maximum at an optimum Al doping of 5 wt%. The further increase in the Al doping causes the lowering in the carrier density. This lowering in the carrier density leads to the decrease in the crystallinity in the film which is in good agreement with the XRD and Raman data.
3.3 Nonliear optical properties
Nonlinear optical properties of the Al doped ZnO thin films were undertaken using the z-scan technique. The open aperture z-scan spectra are shown in Fig. 11 for all the five values of x. All the films exhibit the transmittance minima at z = 0 in the spectra, suggesting the reverse saturation absorption (RSA) behavior. The normalize transmission for the open aperture is given by Eq. (16) .Table 3. Nonlinear absorption in the film is due to multi photon absorption (MPA), free carrier absorption (FCA), exited state absorption (ESA) and nonlinear scattering etc . Multiphoton absorption process is strongly dependent on laser intensity and nonlinear absorption process arries due to eihter interband transition or intraband absorption. Free carrier absorption is also a kind of nonlinear absorption process which depends on the excited state free carrier density. In the present case, the absorptive nonlinearity is mainly due to the free carrier absorption. However the contribution from the two photon induced absorption cannot be ruled out as the the excitation photon energy exceeds the half of the band gap energy .
The open aperture z-scan data was fitted to Eq. (16) and is also shown in Fig. 11. The error bars in the data points is the standard deviation calculated over 10 images for each position, z. The nonlinear absorption coefficient (β) was calculated from Eq. (16).
The closed aperture (CA) z-scan spectra for all the samples is shown in Fig. 12. The value of the nonlinear refractive index (n2) was obtained by fitting the normalized transmission Tclosed (z) for closed aperture data given by the Eq. (19) .
The peak followed by valley configuration of the CA curves in the Fig. 12 indicates a negative optical nonlinearity in the films. The separation between peak and valley position (Δzp-v) was estimated to be 1.69z0 confirming the third order processes in the thin films .
The real as well as imaginary part of the third order nonlinear susceptibility for the highly absorptive media can be evaluated by taking into account the real as well as imaginary part of the linear and nonlinear refractive index. The linear and nonlinear refractive indices (n0 and n2 respectively) can be expressed as.
Where, n0′ and n0′′ represent the real and imaginary parts of the linear refractive index respectively while n2′ and n2′′ represents real and imaginary part of the complex nonlinear refractive index.
The equation governing the complex nonlinear refractive index (n2,complex) and complex third order susceptibility (χ(3)complex) is represented as [40,41].
Taking χ(3)complex = χ(3)′ + iχ(3)′′ where χ(3) ′ and χ(3) ′′ are the real and imaginary part of χ(3), and using Eqs. (20-22), the expression for χ(3) ′ and χ(3) ′′ comes out as below.
In the present case, real part of the linear refractive index, n0′ = n as listed in Table 3, the linear extinction coefficient, n0′′ = (αλ/4π), the real part of the non linear refractive index n2′ = n2 and imaginary part of the nonlinear refractive index n2′′ = (βλ/4π). The nonlinear absorption coefficient (β), nonlinear refractive index (n2) and nonlinear susceptibilities (χ(3) ′ and χ(3) ′′) calculated from Eqs. (16), 19, 23-24) as a function of x are listed in Table 5. Figure 13 shows the variation of β and n2 as a function of Al doping, x.
From Table 5, the third order nonlinearities (χ(3) ′ and χ (3) ′′) are found to be increasing with increase in the Al doping up-to 5 wt. % and then decreases with further increase in doping level. The increasing behavior can be explained in terms of the improvement in crystallinity with the doping concentration. Also the increase in the carrier concentration at the higher Al concentration results in the increase in the electrical conductivity and hence the nonlinearity is increased . The decrease in optical nonlinearity at higher doping concentration, x = 0.10, is accounted due to the loss of carrier concentration as shown in Fig. 10.
Under cw laser the thermally induced optical nonlinearity plays a vital in the semiconductor. The response time (τ) of the thermally induced refractive index change for cw excitation source falls within the range 1sec<τ<100 μs and is given by the following equation .43] for pure ZnO and keeping into account the present geometrical parameter for z-scan experimental setup, comes out to be ~10−3 cm2/W indicating that the thermal nonlinerairty over rules over the other factors in the present case.
High quality c-axis oriented Zn1-xAlxO (0≤x≤0.10) thin films were fabricated using pulsed laser deposition technique. XRD and Raman spectra showed the enhancement in the crystal quality in the films with the increase in Al doping concentration upto 5wt % and reduction in the crystallinity for higher doping concentration. ZnO thin films doped with Al show very low transmission in IR and far IR region. This property can lead to the application in the field of IR filters. A blue shift in the optical band gap energy was observed in the films upto 5wt %. The blue shift in the observed band gap energy was due to Burstein Moss shift in the n-type degenerate semiconductor. The reduction in the band gap energy at higher Al doping was explained as a consequence of many body interaction effect resulting the band gap renormalization effect. The carrier density was estimated using the actual BM shift and was found to increase upto x = 0.05. Third order nonlinear optical coefficients (β and n2) estimated using z-scan technique were found to be increasing with increase in the Al concentration in the ZnO thin film till x = 0.05 and falls down for higher Al cocentration. The present observation suggests that NLO property of PLD AZO film can be tuned by controlling the Aluminum doping and can be used for NLO applications in UV spectral range.
Author would like to acknowledge the Central Instrument Facility (CIF), Indian Institute of Technology Guwahati for providing Laser Micro Raman System for sample analysis.
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