Structural, optical and non-linear optics properties of ...

J Mater Sci: Mater Electron DOI 10.1007/s10854-013-1214-2

Structural, optical and non-linear optics properties of highly doped molybdenum indium oxide thin film A. M. Al-Saie • F. Z. Henari • T. Souier M. Bououdina

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Received: 30 January 2013 / Accepted: 26 March 2013 Ó Springer Science+Business Media New York 2013

Abstract Molybdenum (Mo)-doped In2O3 thin film with 10 wt% was successfully prepared by evaporation method. After annealing at 600 °C the film changes it colour from very dark to a clear transparent film. SEM and AFM analysis reveal that the film is continuous with high metallic coverage [98 % and exhibits a granular structure with typical grain size of 50 nm. More interestingly, the film shows more than 90 % transparency from visible to near infrared region and with wide optical band gap of 4.26 eV. The widening of the band gap is due to the Burstein–Mo¨ss (BM) effect as Mo will occupy In sites within the structure of the film thus increasing the carrier concentration thus enhancing its electrical properties. The nonlinear optical properties of Mo-doped In2O3 film with glass substrate were investigated using z-scan technique. Under cw excitation the film exhibits large reverse saturation absorption and negative nonlinearities. The real and imaginary parts of third order susceptibility of the film

A. M. Al-Saie M. Bououdina Nanotechnology Centre, University of Bahrain, P.O. Box 32038, Sakhir, Kingdom of Bahrain A. M. Al-Saie M. Bououdina Department of Physics, College of Science, University of Bahrain, P.O. Box 32038, Sakhir, Kingdom of Bahrain F. Z. Henari Department of Basic Medical Sciences, Royal College of Surgeons in Ireland, Medical University of Bahrain, PO Box 15503, Al Sayh, Kingdom of Bahrain T. Souier (&) Masdar Institute of Science and Technology, PO Box 54224, Abu Dhabi, United Arab Emirates e-mail: [email protected]

were measured and found that the imaginary part which arises from the change in absorption is dominant.

1 Introduction Composite materials such as thin film transparent conducting oxide (TCO) have increasingly attracted great attention because of their electrical conductivity and optical transparency in the visible wavelength range and high infrared reflectance with a wide band gap of 3.68 eV [1]. These excellent properties have found numerous applications in the field of optoelectronics devices [2–4]. Among many TCOs, indium tin oxide (In2O3), Zinc oxide (ZnO), Cadmium oxide (CdO) are the most investigated due to their large values of optical nonlinearities [5–7], fast response as well as their potential use in areas such as image processing [8], optical switching [9, 10], etc. Improving the conductivity of thin film TCO is an important task, however, it can be achieved mainly by two methods: (1) increasing the carrier concentration or/and (2) improving carrier mobility. An increase in the carrier concentration, however, will cause a reduction in transparency of the thin film due to the increase in the photon capturing by the carriers. On the contrary, increasing the mobility will enhance the conductivity while maintaining the transparency of the thin film. It was reported that the resistivity of In2O3 oxide film is greatly decreased with no changes in the spectral transmittance when molybdenum is doped in it [11]. Compared to un-doped indium oxide (In2O3) films, molybdenum (Mo) doped indium oxide films demonstrated higher electron mobility and higher free carrier concentration which make this material a potential candidate for nonlinear properties investigations.

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However, these exceptional properties depend not only on the doping level but also on the deposition conditions and methods. Transparent conducting IMO films were grown by various methods including reactive thermal evaporation [11], pulse laser deposition [12], spray pyrolysis [13, 14] and sputtering [15–22]. Yamada et al. [15] found that the electrical conductivity of IMO films deposited by RF sputtering increases for wide doping concentration 0–6 %. In the contrary, Warmsingh et al. [12] found that optimal electrical properties is obtained for 2 wt% IMO deposited by pulse laser; increasing further the doping concentration leads to a decreases in the electrical conductivity. Seo et al. [14] show that the electrical conductivity of IMO films, deposited by spray pyrolysis, decreases by several orders of magnitude when the doping concentration is higher than 8 %. Simultaneously, the transmittance decreases up to 20 % in the visible regions. In this work we present the structure/morphology dependent optical properties of thermally evaporated 10 % molybdenum doped indium oxide (In2O3:Mo) films on a Corning glass substrate. The films were prepared from nanoparticles of (Mo, In2O3) mixture synthesized by mechanical milling. The crystal structure and morphology was studied by X-ray diffraction, Scanning Electron and Atomic Force Microscopes (SEM/AFM). We report on nonlinear measurements of In2O3:Mo under cw excitation using z scan technique at wavelengths 488 and 514 nm with output power of 35 mW. The nonlinear absorption b and nonlinear refractive index n2 and the third order nonð3Þ

linear susceptibility vI were measured. The origins of nonlinear effects were discussed.

2 Materials and methods Fritsch Pulverisette P6 was used for the preparation of the powder that will be used as target for the film deposition. The mechanical milling of In2O3 and Mo mixture powder was carried out under air using the following conditions: balls/powder ratio of 20, speed of 300 rpm, and milling time of 20 h. The as-milled powder was then pressed with hydraulic press to form a pellet; the pellet was then thermally heated with tungsten filament under vacuum using Edward A 360 coating system. The film grown on a Corning glass substrate was kept at room temperature in a vacuum of 10-5 torr. The thin film resulted from evaporation was dark brown in colour. It was then annealed in air at 600 °C for 1 h and then was cooled to room temperature. The resulting thin film became transparent with a very light violet shadow in colour.

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The as milled power used for the film growth was examined by Philips 1710 powder X-ray diffractometer ˚ ) in order to with a Cu K-a radiation (k = 1.5418 A reveals its crystalline structure. Our main concern is to ensure that the Molybdenum is completely dissolved in solid solution of In2O3. The crystallite size (CS) and the microstrain (MS) were estimated using peak profile analysis with a software provided with the diffractometer, where the Full Width at Half Maximum (FWHM) is determined, then used for the calculation by introducing a standard value for instrument contribution to the peak broadening. The structural properties of In2O3-Mo film were characterized by means of scanning probe and electron microscopy. QuantaTM 250 FEG scanning electron microscope (SEM) from FIE company was employed for high resolution imaging. Both Secondary Electron (SE) detector and Backscatter Electron detector (BSE) were used for structural and chemical contrast imaging. Moreover, SEM is equipped by Genesis Apex Energy-dispersive X-ray (EDX) spectroscopy system used for chemical content microanalysis. A complementary structural description of the film was obtained by means of atomic force microscopy (AFM). The experiments were performed on MFP-3D Stand Alone AFM microscope from Asylum Research. AC 240TS silicon tips have been used for tapping mode AFM imaging. The resonance frequency of the tip is 78 kHz and the spring constant is 2 N m-1 measured by thermal tuning. The AFM was used for both morphology analysis and film thickness measurements. Optical spectroscopy (Shimadzu UV-3600 UV–VISNIR) from UV to near-infrared wavelength range was used for transmittance and absorption measurements of Mo-doped In2O3 film in order to evaluate its optical-band gap. The nonlinear absorption of Mo-doped In2O3 film was investigated using well-known open aperture z-scan technique [23]. This technique relies on the fact that the intensity varies along the axis of the convex lens and it is maximum at the focus. Hence, by shifting the film through the focus, the intensity dependence can be measured as a change in transmission. The transmission for the film was measured with and without an aperture in the far-field of the lens, as the film moved through the focal point. This enables the nonlinear refractive index (closed aperture) to be separated from that of the nonlinear absorption (open aperture). The z-scan was performed with cw Argon laser at wavelengths 488 and 514 nm. The laser beam is focused onto film using a lens of 5 cm focal length to a beam waist of 20 mm, yielding a typical power density of 5.6 9 107 W m-2.


3 Results and discussion 3.1 Structural characterisation The chemical content of the thin films deposited on glass substrate was determined by Energy dispersive X-ray spectroscopy analyser which is controlled by Genesis software. The EDAX spectrum (Fig. 1) obtained under 15 keV reveals the presence of In and Mo elements in the thin film and only Mo-L and In-L are identified, whereas both Mo-K (20 keV) and In-K (28 keV) cannot be identified in the analysis since they have a high energy. Oxygen is also found to be present in the film as well as in the glass. The others chemical elements shown in the analysis are constituent of the glass substrate. The thin film exhibits a high degree of purity and is entirely composed of molybdenum and indium oxide. Figure 2a illustrates the X-ray diffraction (XRD) pattern of the as received In2O3 compound and Mo metal for comparison purpose. Figure 2b represents the XRD pattern of the as milled (Mo, In2O3) mixture powder (red marker)

Fig. 1 EDAX spectrum of chemical content of 10 % Mo doped In2O3 thin film deposited on glass. The peaks of other elements (Na, Si, Mg and Ca) are attributed to constituents of glass substrate

and after annealing at 600 °C for 1 h in air (green marker). It is clear from comparing the reference starting materials and the as-milled mixture, the presence of both peaks belonging to the two starting materials (Mo and In2O3) but with smaller intensity and broadening of their peaks, which is an indication of the formation of nanostructured materials. By annealing, the peaks belonging to Mo totally disappeared which an indication that Mo atoms dissolve into In2O3 compound, it is supposed that Mo will occupy In sites within In2O3 crystal lattice. Thus annealing the film at only 600 °C and for a shorter time of 1 h only, would be enough to form pure and single (Mo,In)2O3 phase. Accordingly, we can qualify the IMO film under study as a doped molybdenum idiom oxide film. Since the optical properties of thin film are intimately related to its surface morphology and defects content, nanostructural characterizations have been conducted by means of SEM, EDX and AFM techniques. Figure 3a shows SEM image of Mo-doped In2O3 thin film obtained with a secondary electron detector. The deposited thin film exhibits a granular structure with a presence of nanosize grains or particles. The distribution of grain size was determined by image analysis using Image Tools software and represented in Fig. 3b. The particle size lies between 10 up to 300 nm and the grain size distribution is centered at 50 nm. Figure 3c shows an SEM image of the film obtained by using a backscattered detector and reveals that the film exhibits the same chemistry without evidence of the presence of isolated metallic Mo particles at least within the analyzed regions. This result is in good agreement with XRD finding since all Mo was dissolved into Indium oxide matrix (all Mo peaks disappeared). However, the presence of dark spots (that occupied only 2 % of the area) was attributed to the existence of few discontinuities (holes) in the film structure. The metallic content in the film is found to be higher than 98 % which prove that the film is highly continuous and thus exhibits a higher electrical conductivity close to the bulk value by minimizing the electron grain boundaries scattering.

Fig. 2 a X-ray diffraction patterns of the as-received In2O3 and Mo reference samples, b X-ray diffraction patterns for as milled nanoparticles compound (red-line) and after annealing for 1 h at 600 °C (green-line)

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J Mater Sci: Mater Electron Fig. 3 a SEM image of thin film obtained with a secondary electron detector revealing the nano-grain morphology, b The distribution of nano-sized grain, and d SEM image of the film obtained by using a backscattered detector that reveals the chemical contrast

The granular structures are also confirmed by taking AFM image (in tapping mode) of the thin film sample as can be seen from the topography (height) image (Fig. 4a) and phase contrast image (Fig. 4b). The AFM height image shows that the surface is smooth with a root mean square (rms) of 5 nm, the morphology of the film is granular with the presence of nanosized grains or particles, the size of which lies between 15 and 120 nm. As can be seen, the phase image contrast image reveals more the nanostructure of the film and shows that the large particles (around 100 nm size) are in fact agglomeration of much smaller particles. The typical nanoparticle size is also about 50 nm. It is clear that the surface morphology of the IMO film is directly related to the high transparency of the film.

3.2 Optical band gap measurement The spectrum reported in Fig. 5a was recorded in the range of wavelengths from 250 to 3,000 nm. It is clear that the film is transparent of about 80 % in the range between 400–500 and 90 % for a wide range of spectra (i.e. from 500 to 3,000 nm) except in the range between 900 and 1,700 nm it is just below 90 %. Thus, it covers the range from the visible to near infrared. This result reveals a significant improvement of the transmittance of highly doped and thermally evaporated films over the low

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transmittance reported of films deposited by spray pyrolysis [14]. It is also worthy to note that according to AFM analysis, IMO film surface is rather smooth surface with a root mean square (rms) of 5 nm. It is clear that the surface morphology contributes to the observed high transparency of the IMO film. Absorption coefficients a were calculated on the basis of the transparency data (T) and the thickness d of bulk film, using the approximate relation [24, 25]: a¼

lnð1=T Þ d

ð1Þ

The average thickness of the film is found to be 170 nm from the AFM profile measurement. In order to determine the optical band gap (Eg)opt on the basis of the recorded transparency spectra for bulk film of the glasses from the system In2O3–Mo 10 % an analysis was carried out of the absorption coefficient a2 as a function of the energy of incident photons hm (Fig. 5b). In the region of high absorption (the absorption coefficient a 9 104 cm-1) [24] optical transitions take place between the valence and conduction band. The plot displayed a linear region, which could be extrapolated to a2 ? 0 to yield (Eg)opt at 4.26 eV [25]. Measurements on In2O3 single crystals showed that the onset of strong optical absorption is found to be 3.75 eV [26] and for thin film 3.8 eV [27]. We observe a widening of the band gap with increasing the doping level,


Fig. 4 AFM images of thin film obtained in tapping mode, a the height image and b the cantilever phase shift images. The two AFM scans reveal the nano-size grain structure of thin film

Fig. 5 a Spectrum transmission of the thin film for the range from 250 to 3,000 nm, b optical band gap determination by plotting a2 versus hm curve

while Seo et al. [14] observed a narrowing of the band gap while increasing the doping level of Mo:In2O3 films. The widening and narrowing of the band gap by is the results of two contributions: as the doping level increases, the band gap narrowing is a consequence of many body effects on the conduction and valence bands. The shrinkage is counteracted by Burstein-Mo¨ss effect, which gives a band gap widening as a result of the blocking of lowest states in the conduction band. In our cases, the observed widening is due to BM effect. It is a process by which the apparent bandgap of a semiconductor is increased as the absorption edge is pushed to higher energies as a result of all states close to the conduction band being populated. This is observed for a degenerate electron distribution such as that found in some degenerate semiconductors. Sn-doped In2O3 (ITO) is an n-type degenerated semiconductor with conduction band minimum (CBM) and valence band maximum (VBM) correspond to In 5 s and O 2p orbital, respectively [4]. The Mo6? ions (5 s1 4d5) with ionic radius of 6.2 9 10-11 m can possibly substitute In3? (4d10 5 s2 5p2 and of ionic radius 8.1 9 10-11 m) sites within In2O3 crystal lattice thereby creating some oxygen vacancies,

allowing free electrons present in the conduction band and to show a small effective mass. Sato et al. [27] reported that doping In2O3 with 0–10 % Sn resulting in widening the optical gap due to BM effect from 3.8 to 4.3 eV and transmittance over 80 % in the visible region (380–780 nm) for all measurements. However, beyond 1,000 nm, the transmittance decreased with increasing Sn concentration. 3.3 Nonlinear optical properties The absorbance spectrum of molybdenum doped indium oxide film was measured in the range of 300–1,400 nm and is shown in Fig. 6. It is observed that the film is highly transparent at the wavelengths (488 and 514 nm) used to in the z-scan experiment. Figure 7a shows the normalized transmittance without an aperture as a function of the distance along the lens axis z, for Mo-doped In2O3 film at wavelength 488 nm. The transmission is symmetric with respect to the focus (z = 0), where it has a minimum transmission. This demonstrates that the film exhibits reverse saturation

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lower by one order than the reported value at 633 nm, cw excitation for C60 thin films [28] and higher by one order than reported value ZnO:Al [29] ð3Þ

The imaginary part of the third order nonlinearities vI is related to nonlinear absorption coefficient by ð3Þ

vI ¼

n2 e0 ckb 2p

ð3Þ

where n is the linear refractive index (n = 1.6), k is the wavelength, eo is the permittivity of free space and c is the velocity of light. The experimentally determined values of ð3Þ

vI

Fig. 6 Linear absorption spectrum of In2O3: Mo thin film

absorption, RSA (optical limiting). In order to establish that the effect seen is due to Mo-doped In2O3 film, the z-scan (open aperture) was performed also with glass substrate, where no nonlinear absorption was observed with the intensity range used in the experiment, therefore we conclude that the above effect is due to presence of Mo-doped In2O3 film. The normalized transmission for the open z scan is given by [23]: Top ðzÞ ¼ 1 Duð1 þ xÞ1

ð2Þ

where x = z/z0 (with zo ¼ w 20 =k) is the diffraction length of the Gaussian beam and w0 is the beam’s waist, Du is the pffiffiffi nonlinear Phase change Du ¼ b Io l= 2 2 and l ¼ ð1 expðao dÞÞ=aO with aO is the linear absorption coefficient, d is the film thickness, l is the effective thickness of the sample. Io is the intensity of the laser beam at the focus; and b is the nonlinear absorption coefficient. The values of nonlinear absorption coefficient, calculated by fitting Eq. 1 to experimental data at 488 and 514 nm for the film are given in Table 1. These values are

Fig. 7 a Open aperture Z-scan response for molybdenum— embedded indium oxides under 488 nm. The points are representing the measurement data; the line corresponds to a fit to Eq. 1, b Closed aperture z-scan response for molybdenum—embedded indium oxides under 514 nm. The points are representing the measurement data; the line corresponds to a fit to Eq. 3

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for the sample are shown in Table 1. The normalized transmittance through closed aperture for Mo-doped In2O3 film at 514 nm is shown in Fig. 7b. The peak valley configuration is a signature of negative refractive nonlinearity. The non-symmetric height of peak and valley indicates that the nonlinear absorption given rise to refractive index changes through Kramers–Kronig relation. The normalized transmission is given by [30]: 4Du 2DUð3 þ x2 Þ T ðzÞ ¼ 1 ð1 þ x2 Þ ð9 þ x2 Þ ð1 þ x2 Þ ð9 þ x2 Þ ð4Þ Where DU is the nonlinear phase change and is related to nonlinear refractive index c through DU = kc I l, and k = 2p/k. The a nonlinear refractive index is given by c = DUk p I l. The values of nonlinear refractive index are calculated by fitting the experimental data with Eq. 4, see Table 1. The fitting was performed by keeping the same value of Du as that derived from Eq. 2. The nonlinear refractive index values reported here are higher by one orders than the value reported for In2O3 nanoparticle at 632.8 nm and for Fe2O3 at 514 nm [31]. These results quite encouraging for nonlinear application of Mo-doped In2O3 film with low power cw lasers. ð3Þ

The real part of the third order nonlinearities vR given by:

is

J Mater Sci: Mater Electron Table 1 Nonlinear- optical measurements k (nm)

a (m-1)

488 514

ð3Þ

vR

b (m W-1)

c (m2 W-1)

vI

9.1.5 9 104

6.49 9 10-3

7.05 9 10-10

1.70 9 10-4

4.50 9 10-4

4

-3

-10

-4

3.10 9 10-4

7.5 9 10

5.81 9 10

¼ 2n2 e0 c c

4.79 9 10

ð5Þ ð3Þ

The experimentally determined values of vR are shown in Table 1. By comparing the values of the imaginary and ð3Þ

ð3Þ

real nonlinearities one can conclude that vI [ vR , i.e., ð3Þ

vI which gives rise to the absorption change is dominant, and this can be seen from Fig. 5, where the valley is much larger than the peak. One can note that there is an increasing trend for the values of b, c and v (3) for 488 nm compared with the values measured at 514 nm and this may be due to higher linear absorption for 488 nm. From physical point of view, the mechanism responsible for nonlinear absorption in the spectral range used in the experiment is reverse saturation absorption as mentioned above. The reverse saturation absorption of Mo-doped In2O3 film arises from the interband transition across the band gap states via two photon absorption. The excitation wavelengths used in the experiment were 488 nm and 514 nm corresponding to the photon energies of 2.6, 2.4 eV respectively, which are smaller than the direct bandgap energy of 4.26 eV reported in this work. In this case, the only possible transition would be through a twophoton absorption that leads to direct pumping of electrons to higher state. The nonlinear refractive index may arise from the thermal effect, which may be originated from the use of cw laser beam. Thermal effect leads to thermalizing of hot electrons and subsequent dissipation of their energy leads to increase the surrounding temperature which results in the refractive index change.

4 Conclusion Mo-doped In2O3 film was prepared by vacuum evaporation method. EDX analysis reveals the presence of both In and Mo elements in the thin film. SEM shows a granular structure with the presence of nano-sized grains with a typical size of 50 nm, this granular structure was confirmed by AFM imaging. Moreover, the film is found to be highly continuous with a metallic content over 98 % which leads to high electrical conductivity close to the bulk by minimizing the grains boundaries scattering of the electrons. Optical transmission revealed that the transparency of the thin film is more than 90 % for most of the spectra with optical gap of about 4.25 eV. Nonlinear absorption

ð3Þ

ð3Þ

vR (esu)

(esu)

1.40 9 10

coefficients and nonlinear refractive indices in molybdenum embedded indium oxide were measured using z-san technique, under cw excitation. Two photon absorption mechanisms was used to explain the observed reverse saturation. The negative sign of nonlinear refractive index data is indicative of the dominance of thermal lensing. The result of the present work is important from physical point of view and technical applications in optoelectronic devices.

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