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Investigation of O7+ swift heavy ion irradiation on molybdenum doped indium oxide thin films
Radiation Physics and Chemistry 81 (2012) 589–593
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Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Investigation of O7 þ swift heavy ion irradiation on molybdenum doped indium oxide thin films V. Gokulakrishnan a, S. Parthiban a,c, E. Elangovan c, K. Jeganathan b, D. Kanjilal d, K. Asokan d, R. Martins c, E. Fortunato c, K. Ramamurthi a,n a
Crystal Growth and Thin Film Laboratory, School of Physics, Bharathidasan University, Tiruchirappalli 620024, India Centre for Nanoscience and Nanotechnology, School of Physics, Bharathidasan University, Tiruchirappalli 620024, India c CENIMAT-I3N and CEMOP-UNINOVA, Materials Science Department, FCT-UNL, Caparica Campus, 2829-516 Caparica, Portugal d Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India b
a r t i c l e i n f o
abstract
Article history: Received 23 February 2011 Accepted 24 February 2012 Available online 3 March 2012
Molybdenum (0.5 at%) doped indium oxide thin films deposited by spray pyrolysis technique were irradiated by 100 MeV O7 þ ions with different fluences of 5 1011, 1 1012 and 1 1013 ions/cm2. Intensity of (222) peak of the pristine film was decreased with increase in the ion fluence. Films irradiated with the maximum ion fluence of 1 1013 ions/cm2 showed a fraction of amorphous nature. The surface microstructures on the surface of the film showed that increase in ion fluence decreases the grain size. Mobility of the pristine molybdenum doped indium oxide films was decreased from 122 to 48 cm2/V s with increasing ion fluence. Among the irradiated films the film irradiated with the ion fluence of 5 1011 ions/cm2 showed relatively low resistivity of 6.7 10 4 O cm with the mobility of 75 cm2/V s. The average transmittance of the as-deposited IMO film is decreased from 89% to 81% due to irradiation with the fluence of 5 1011 ions/cm2. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Transparent conducting oxide (TCO) Indium molybdenum oxide (IMO) Thin films Swift heavy ion (SHI) irradiation Structural Electrical and optical properties
1. Introduction Transparent conducting oxides (TCOs) constitute an unusual class of materials possessing two physical properties such as high optical transparency (T) and high electrical conductivity (s) that are generally considered to be mutually exclusive (Hartnagel et al., 1995). This peculiar combination of physical properties is only achievable if a material has sufficiently large energy band gap ( 43.1 eV) so that it is non-absorbing or transparent to the visible light. Among the different metal oxides, III–VI binary In2O3 (IO) is the potential material due to its wide band gap, low resistivity (r) and high environmental and thermal stability when exposed to different atmospheres at temperatures as high as 1000 1C (Minami et al. 1999). The reason for the low r has not been elucidated yet, presumably due to the complex structure of the unit cell of crystalline In2O3 that consists of 80 atoms and the complex nature of the transport mechanisms in polycrystalline films (Nakazawa et al., 2006). Mo doped indium oxide (IMO) belongs to the class of highly degenerate wide-gap semiconductors which is extensively used in optoelectronic applications (such as flat panel displays and solar cells) due to its combined
n
Corresponding author. Tel.: þ91 431 2407057; fax: þ91 431 2407045. E-mail address: [email protected] (K. Ramamurthi).
0969-806X/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2012.02.037
properties of high transmittance and low resistivity. IMO thin films have attracted many researchers owing to their high carrier mobility and high near infrared transparency (Meng et al. 2001; Warmsingh et al., 2004). The IMO thin films have also been identified as superior to indium tin oxide (ITO) thin films (Yamada et al., 2006). Energetic ion beam offers much broader range of possibilities compared to the ion implantation, to modify the structure and properties of material (Fink and Chadderton, 2005), caused by the very high local energy density deposited into the solid along the path of the ions. In fact, the energy loss of swift heavy ion irradiation (SHI) may be up to some tens of keV/nm, which results in a very short (some hundred of picoseconds), very local (a cylinder of approximately 10 nm in diameter) and very high (some 0.1 eV/ atom) excitation of solid (Avasthi, 2000). Under such extreme conditions, highly non-equilibrium process may be initiated. The characteristics of the ion beam and the properties of the materials also determine the response of the materials to SHI. Irradiations of the tin doped indium oxide thin films by energetic ions modified their structural, electrical and optical properties and evidenced that optimal irradiations improve the electrical and optical properties (Haynes et al., 1997; Shigesato et al., 1993a; Shigesato et al., 1994). Shigesato et al. (1993a) have reported that the implantation of O7 þ (1.7 1015 ions/cm2) and H þ (1.3 1015 ions/cm2) ions on ITO films decreases the carrier concentration and increases Hall mobility. Morgan et al., (1995) have studied the effect of 15 keV H þ
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V. Gokulakrishnan et al. / Radiation Physics and Chemistry 81 (2012) 589–593
irradiation of 1 1015 and 1 1017 ions/cm2 fluences on ITO thin films, which displayed radiation hardness up to a dose of 1016 cm2. In the present work, we report the effects of swift heavy ion beam of 100 MeV O7 þ on the structural, optical and electrical properties of spray deposited IMO thin films for the fluence of 5 1011, 1 1012 and 1 1013 ions/cm2.
2. Experimental details Indium (III) chloride (0.065 M) was dissolved in 2 ml of hydrochloric acid (HCl) at 90 1C and diluted with 73 ml of double distilled water and was used as the source of In. Molybdenum (V) chloride (0.5 at%) dissolved in 2 ml of HCl by heating at 90 1C and diluted with 23 ml of ethanol was used as the source of Mo. Mixture of both the solutions was used to deposit IMO films with a thickness of 300 nm on Corning 1737 glass substrates at 400 1C using a spray pyrolysis experimental setup. The spray time and interval was maintained as 1 s and 30 s, respectively. The asdeposited IMO films with 0.5 at% Mo possess better electrical and optical properties in comparison with other doping concentrations. Hence, the 0.5 at% Mo doped IMO thin films have been selected for swift heavy O7 þ ion irradiation. Three identical samples of the selected pristine IMO films were irradiated by 100 MeV swift heavy O7 þ ions with ion fluences of 5 1011, 1 1012 and 1 1013 ions/cm2, using the 15UD Pelletron tandem accelerators at Inter University Accelerator Centre (formerly known as Nuclear Science Centre), New Delhi, India. A vacuum of 10 6 Torr was maintained during the irradiation experiments and the ion beam current was maintained at around 1 pnA (denotes particle nano-ampere). To ensure uniformity of irradiation, the beam is scanned over an area of 1 1 cm2 of the sample with an electromagnetic scanner. The range of 100 MeV O7 þ ions in the films calculated using SRIM-2008 (Version 2008.4) software was about 46.35 mm, which is greater than the thickness of the film. Hence, the bombarding ions pass through the entire film and are stopped in the substrate. For 100 MeV O7 þ energetic ions the electronic and nuclear energy loss values calculated are ˚ respectively. 1.537 102 and 8.843 10 2 eV/A, The X-ray diffraction patterns were obtained using the computer controlled PANalytical X pert PRO X-ray diffraction system (Cu Ka ˚ in Bragg–Brentano geometry (y/2y with a wavelength of 1.5406 A) coupled). The nature of surface microstructures were obtained using atomic force microscopy (AFM) (Agilent 5500). The thickness of the films was measured by the reflection method using filmetrics F20. The electrical parameters were measured using Hall measurements setup (ECOPIA-HMS 3000) at room temperature with a permanent magnet of 0.57 T for fixed Hall current of 1 mA and delay time of 0.5 s. The optical transmittance was measured using a double beam spectrometer (Shimadzu UV-1700) with a bare substrate in the path of the reference. Hence, the transmittance spectra reported in this study refer only to film transmission.
3. Results and discussion 3.1. Structural properties The structural analyses of pristine and 100 MeV O7 þ ion irradiated films were performed using X-ray diffraction (XRD) technique. XRD peaks of the pristine and irradiated IMO films were identified by matching with the standard data (ICDD card no. 06-0416) as shown in Fig. 1 which confirmed the cubic bixbyite structure of indium oxide (IO). XRD pattern obtained from the pristine and irradiated IMO films shows a strong preferential orientation along (222) direction. The observed diffraction peaks demonstrate the polycrystalline
Fig. 1. XRD patterns of the IMO films and O7 þ irradiated ions.
Fig. 2. FWHM and Grain size of IMO and O7 þ irradiated films with various fluence range.
structure of the irradiated IMO films. X-ray diffraction intensity of the prominent (222) peak decreases with increase in ion fluence while (211) peak was completely destroyed at higher fluence. Their crystallite size was calculated from (222) peak using the Scherrer’s formula. The full width half maximum and crystalline size of the (222) peak for the irradiated films are plotted in Fig. 2 and FWHM value monotonically increases and grain size decreases with increase in the ion fluence. Increase in the fluence of O7 þ SHI irradiation reduces the intensity of diffraction peaks of IMO films, thus the results show a tendency to amorphisation in these films. This kind of amorphisation also observed in the O7þ irradiated IO films (Gokulakrishnan et al., 2011). 3.2. Surface morphological analysis The surface microstructures of the pristine and irradiated IMO films recorded by AFM are shown in Fig. 3. The surface morphology of the pristine IMO film shows regularly distributed granular shaped crystallites with an average grain size of 120 nm and
V. Gokulakrishnan et al. / Radiation Physics and Chemistry 81 (2012) 589–593
591
Fig. 3. AFM microstructures of pristine and O7 þ ions irradiated IMO films.
with a root mean square (RMS) roughness of 27 nm. At a higher fluence the surface pattern of the irradiated IMO films was drastically changed in the shape and distribution, as shown in the Fig. 3. Thus grainy character of the pristine film changes significantly after irradiation with various ion fluences. Such drastic change in the size and shape of the crystallites at higher fluence is attributed to the high density of electronic excitations induced by SHI irradiation under multiple ion impacts in the near surface region (Agarwal et al., 2008). The films irradiated with the ion fluence of 5 1011 ions/cm2 is comprised of grains with a few disordered granular shaped crystallites having average grain size of 80 nm with RMS roughness of 16 nm. The decrease in RMS roughness authenticates a relatively smooth surface. When the ion fluence is increased to 1 1012 ions/cm2, the size of the granular grains becomes smaller. Further, the RMS roughness is strikingly decreased to 12.1 nm. When the ion fluence is further increased to 1 1013 ions/cm2, the average grain size is decreased to 38 nm and RMS roughness is significantly reduced to 11 nm. Thus a marked change in the surface RMS roughness is observed after irradiation. At higher fluence of 1 1013 ions/ cm2 local heating provides additional energy for rearrangement of atoms which smoothens the film surface. AFM images evidently show that the grain size decreases with increase in the ion fluence. XRD patterns shown in Fig. 1 also corroborate with the results from AFM studies. The foregoing discussion concludes that the fluence of 1 1013 ions/cm2 has decreased the grain size and significantly increased the smoothness of the surface of the pristine films. Decrease of grain size and increase of surface smoothness in the O7 þ irradiated IO films were also observed by Gokulakrishnan et al., 2011. 3.3. Electrical properties The electrical properties of pristine and irradiated IMO films were estimated from the room temperature Hall measurements
in van der Pauw configuration. The negative sign of Hall coefficient of films confirmed the n-type conductivity. The variation in the electrical properties of pristine and ion irradiated IMO thin films is shown in Fig. 4. The pristine IMO (0.5 at% Mo) film shows the best possible combination of electrical properties as follows: bulk resistivity (r) of 5.2 10 4 O cm, carrier concentration (n) of 0.95 1020 cm 3 and mobility (m) of 122 cm2/V s. The highest r of 7.2 10 4 O cm is obtained for the IMO film irradiated with an ion fluence of 1 1013 ions/cm2. The resistivity of the IMO films due to irradiation of O7 þ ions decreases with increase in the film carrier density (n). The increase in n may be a result of oxygen vacancies created due to annealing by the ion irradiation under the vacuum of 10 6 Torr, which has contributed more free electrons increase to n. On the other hand, the m of ion irradiated films (1 1013 ions/cm2) is decreased from 122 to 48 cm2/ V s. The probable reason for decrease in m is that O7 þ ion irradiation lead to increase the scattering effect due to increase in carrier concentration. The electron scattering sources such as grain boundaries and acoustic phonons, neutral and ionised impurity centres have been found to affect the electrical and optical properties of the films (Gerlach et al., 1978; Lu et al., 2006; Koida and Kondo, 2006; Koida and Kondo, 2007; Adurodija et al., 2000). The experimental data obtained were used to derive the mean free path (l) to clarify the dominat scattering mechanism in pristine and ion irradiated IMO thin films. The mean free path was calculated using a sufficiently degenerate gas model (Kittel., 1985; Yamada et al., 2000) given by l ¼ ð3p2 Þ1=3 ð_=e2 Þr1 n2=3
ð1Þ
where r and n representes the resistivity and carrier concentration, respectively. Mean free path value of 11.5 nm obtained from the pristine films is decreased to 6.74 nm for the films irradiated with 1 1013 ions/cm2 fluence. In most TCOs, the grain boundaries and acoustic phonons played a secondary role, since the mean free path is usually much smaller than the average grain
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Fig. 5. Comparison of transmittance spectra obtained from the pristine and SHI irradiated IMO films.
Fig. 4. Comparison of electrical properties of the pristine and SHI irradiated IMO films.
size. In the present work, the calcualted l (ranging between 6.74 and 11.5 nm) is much smaller than the average grain size determined from XRD analysis of pristine and ion irradiated IMO thin films. Therefore, grain boundary scattering appears not to play an important role in the pristine and irradiated IMO films. (Shigesato et al., 1993b). 3.4. Optical properties The transmittance spectra of the as-deposited and ion irradiated IMO thin films in the wavelength range 300–1000 nm are shown in the Fig. 5 as a function of O7 þ ion fluences. The average visible transmittance (AVT) calculated in the wavelength range 400–800 nm is 89% for the as-deposited IMO films. The AVT has been decreased to 79, 74, and 66% for the ion fluences of 5 1011, 1 1012 and 1 1013 ions/cm2, respectively. The reduction in optical transmittance due to irradiation may result from the incorporation of lattice defects which may increase the absorption in optical region. It is also known that the transmittance of irradiated TCO films depends on the concentration of oxygen vacancies in the film (Tahar et al.,1998). In the present case, the transmittance of IMO film decreases with increase in the fluence. A similar behaviour of decrease in the transmittance of zinc oxide thin films following the irradiation by heavy Au ion of 100 MeV has been reported (Ratheesh Kumar et al., 2005). The optical band gap of the IMO film was estimated from the relation
ahn ¼ ðhnEg Þ1=2
ð2Þ
This equation gives the direct band gap energy Eg when the straight portion of (ahn)2 against hn plot is extrapolated to the point a ¼0. The variation of band gap as a function of ion fluence is shown in Fig. 6. The estimated value of energy gap for pristine sample is
Fig. 6. Plot of (ahn)2 versus hn for pristine and 100 MeV O7 þ ion irradiated IMO films.
3.71 eV and is reduced to 3.54, 3.44 and 3.38 eV for films irradiated with the fluence of 5 1011, 1 1012 and 1 1013 ions/cm2, respectively. This shows a consistent decrease of energy gap with increase in ion fluence. The shift in Eg upon irradiation can be attributed to several reasons. The decrease in the energy gap with increasing ion fluence is due to creation of intermediate energy levels (Chaudhary et al., 2004; Mohanta et al., 2004; Narayanan et al., 1997).
4. Conclusions 0.5 at% Mo doped indium oxide thin films with improved properties were deposited on Corning-F1737 glass substrates at 400 1C using economic spray pyrolysis technique and irradiated by swift heavy O7 þ ions of 100 MeV energy with different fluences. XRD studies confirmed the cubic bixbyite structure of polycrystalline IO for the both pristine and ion irradiated IMO films. X-ray diffraction intensity of the prominent (222) peak decreases with increase in ion fluence while (211) peak was completely destroyed at 1 1013 ions/cm2 fluence. The AFM
V. Gokulakrishnan et al. / Radiation Physics and Chemistry 81 (2012) 589–593
studies confirmed the evolution of surface microstructures as a function of ion fluence. The grain size of irradiated IMO films decreases to 38 nm from 128 nm (pristine) and RMS values decreases to 11 nm from 27 nm (pristine) for 1 1013 ions/cm2 fluence. The electrical resistivity of pristine IMO film is significantly increased from 5.2 10 4 to 7.2 10 4 O cm for the ion fluence of 1 1013 ions/cm2. Mobility of the pristine film is decreased from 122 to 48 cm2/V s for 1 1013 ions/cm2 fluence. The average transmittance of the as-deposited IMO film is decreased from 89 to 66% for O7þ SHI irradiation of 1 1013 ions/cm2 fluence.
Acknowledgement Authors thanks the Inter-University Accelerator Centre, New Delhi, India, for providing the financial support through the UFUP Project scheme (Project code—UFUP-41313). References Adurodija, F.O., et al., 2000. Effect of Sn doping on the electronic transport mechanism of indium–tin-oxide films grown by pulsed laser deposition coupled with substrate irradiation. J. Appl. Phys. 88, 4175–4180. Agarwal, D.C., et al., 2008. Thermal and ion induced annealing of nanocrystalline ZnO thin film deposited by atom beam sputtering. J. Phys. D: Appl. Phys. 41, 0453051–0453056. Avasthi, D.K., 2000. Some interesting aspects of swift heavy ions in materials science. Curr. Sci. 78 (11), 1297–1303. Chaudhary, Y.S., et al., 2004. A study on 170 MeV Au13 þ irradiation induced modifications in structural and photo electrochemical behavior of nanostructured CuO thin films. Nucl. Instrum. Methods B 225, 291–296. Fink, D., Chadderton, L.T., 2005. Ion–solid interaction: status and perspectives. Braz. J. Phys. 35, 735–740. Gerlach, E., et al., 1978. Ionized impurity scattering in semiconductors. Phys. Status Solidi B 86, 479–482. Gokulakrishnan, et al., 2011. Effects of O7 þ swift heavy ion irradiation on indium oxide thin films. Nucl. Instrum. Methods B 269, 1836–1840. Hartnagel, H.L., et al., 1995. Semiconducting Trasparent Thin Films. Institute of Physics Publishig, Bristol.
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Haynes, T.E., et al., 1997. Ion beam modification of transparent conducting indium–tin-oxide thin films. Nucl. Instrum. Methods Phys. Res. B 121, 221–225. Kittel, C., 1985. Introduction to Solid State Physics, 5th ed. Maruzen, Tokoyo. Koida, T., Kondo, M., 2006. High electron mobility of indium oxide grown on yttriastabilized zirconia. J. Appl. Phys. 99, 1237031–1237036. Koida, T., Kondo, M., 2007. Comparative studies of transparent conductive Ti, Zr, and Sn-doped In2O3 using a combinatorial approach. J. Appl. Phys. 101, 0637131–0637136. Lu, J.G., et al., 2006. Structural, optical, and electrical properties of (Zn, Al) O films over a wide range of compositions. J. Appl. Phys. 100, 0737141–07371411. Meng, Y., et al., 2001. A new transparent conductive thin film In2O3:Mo. Thin Solid Films 394, 219–223. Minami, T., et al., 1999. Stability of transparent conducting oxide films for use at high temperatures. J. Vac. Sci. Technol. A 17, 1822–1826. Mohanta, D., et al., 2004. SHI-induced grain growth and grain fragmentation effects in polymer-embedded CdS quantum dot systems. Mater. Lett. 58, 3694–3699. Morgan, D.V., et al., 1995. Radiation damage in indium tin oxide (ITO) layers. Thin Solid Films 258, 283–285. Nakazawa, H., et al., 2006. The electronic properties of amorphous and crystallized In2O3 films. J. Appl. Phys. 100, 0937061–0937068. Narayanan, K.L., et al., 1997. SHI-induced grain growth and grain fragmentation effects in polymer-embedded CdS quantum dot systems. Physica B 240, 8–12. Ratheesh Kumar, P.M., et al., 2005. Modifications of ZnO thin films under dense electronic excitation. J. Appl. Phys. 97, 0135091–0135096. Shigesato, Y., et al., 1993a. Study of the effect of ion implantation on the electrical and microstructural properties of tin-doped indium oxide thin films. J. Appl. Phys. 73, 3805–3811. Shigesato, Y., et al., 1993b. Study of the effect of Sn doping on the electronic transport properties of thin film indium oxide. Appl. Phys. Lett. 62, 1268–1270. Shigesato, Y., et al., 1994. Ion implantation and dynamic recovery of tin-doped indium oxide films. Trans. Mater. Res. Soc. Jpn. 17, 503. Tahar, R.B.H., et al., 1998. Tin doped indium oxide thin films: electrical properties. J. Appl. Phys. 83, 2631–2643. Warmsingh, C., et al., 2004. High-mobility transparent conducting Mo-doped In2O3 thin films by pulsed laser deposition. J. Appl. Phys. 95, 3831–3833. Yamada, N., et al., 2000. Donor compensation and carrier-transport mechanisms in tin-doped In2O3 films studied by means of conversion electron 119Sn ¨ mossbauer spectroscopy and hall effect measurements. Jpn. J. Appl. Phys. 39, 4158–4163. Yamada, N., et al., 2006. Effects of post deposition annealing on electrical properties of Mo-doped indium oxide (IMO) thin films deposited by RF magnetron co-sputtering. Jpn. J. Appl. Phys. 45, L1179–L1182.
Contents lists available at SciVerse ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Investigation of O7 þ swift heavy ion irradiation on molybdenum doped indium oxide thin films V. Gokulakrishnan a, S. Parthiban a,c, E. Elangovan c, K. Jeganathan b, D. Kanjilal d, K. Asokan d, R. Martins c, E. Fortunato c, K. Ramamurthi a,n a
Crystal Growth and Thin Film Laboratory, School of Physics, Bharathidasan University, Tiruchirappalli 620024, India Centre for Nanoscience and Nanotechnology, School of Physics, Bharathidasan University, Tiruchirappalli 620024, India c CENIMAT-I3N and CEMOP-UNINOVA, Materials Science Department, FCT-UNL, Caparica Campus, 2829-516 Caparica, Portugal d Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India b
a r t i c l e i n f o
abstract
Article history: Received 23 February 2011 Accepted 24 February 2012 Available online 3 March 2012
Molybdenum (0.5 at%) doped indium oxide thin films deposited by spray pyrolysis technique were irradiated by 100 MeV O7 þ ions with different fluences of 5 1011, 1 1012 and 1 1013 ions/cm2. Intensity of (222) peak of the pristine film was decreased with increase in the ion fluence. Films irradiated with the maximum ion fluence of 1 1013 ions/cm2 showed a fraction of amorphous nature. The surface microstructures on the surface of the film showed that increase in ion fluence decreases the grain size. Mobility of the pristine molybdenum doped indium oxide films was decreased from 122 to 48 cm2/V s with increasing ion fluence. Among the irradiated films the film irradiated with the ion fluence of 5 1011 ions/cm2 showed relatively low resistivity of 6.7 10 4 O cm with the mobility of 75 cm2/V s. The average transmittance of the as-deposited IMO film is decreased from 89% to 81% due to irradiation with the fluence of 5 1011 ions/cm2. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Transparent conducting oxide (TCO) Indium molybdenum oxide (IMO) Thin films Swift heavy ion (SHI) irradiation Structural Electrical and optical properties
1. Introduction Transparent conducting oxides (TCOs) constitute an unusual class of materials possessing two physical properties such as high optical transparency (T) and high electrical conductivity (s) that are generally considered to be mutually exclusive (Hartnagel et al., 1995). This peculiar combination of physical properties is only achievable if a material has sufficiently large energy band gap ( 43.1 eV) so that it is non-absorbing or transparent to the visible light. Among the different metal oxides, III–VI binary In2O3 (IO) is the potential material due to its wide band gap, low resistivity (r) and high environmental and thermal stability when exposed to different atmospheres at temperatures as high as 1000 1C (Minami et al. 1999). The reason for the low r has not been elucidated yet, presumably due to the complex structure of the unit cell of crystalline In2O3 that consists of 80 atoms and the complex nature of the transport mechanisms in polycrystalline films (Nakazawa et al., 2006). Mo doped indium oxide (IMO) belongs to the class of highly degenerate wide-gap semiconductors which is extensively used in optoelectronic applications (such as flat panel displays and solar cells) due to its combined
n
Corresponding author. Tel.: þ91 431 2407057; fax: þ91 431 2407045. E-mail address: [email protected] (K. Ramamurthi).
0969-806X/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2012.02.037
properties of high transmittance and low resistivity. IMO thin films have attracted many researchers owing to their high carrier mobility and high near infrared transparency (Meng et al. 2001; Warmsingh et al., 2004). The IMO thin films have also been identified as superior to indium tin oxide (ITO) thin films (Yamada et al., 2006). Energetic ion beam offers much broader range of possibilities compared to the ion implantation, to modify the structure and properties of material (Fink and Chadderton, 2005), caused by the very high local energy density deposited into the solid along the path of the ions. In fact, the energy loss of swift heavy ion irradiation (SHI) may be up to some tens of keV/nm, which results in a very short (some hundred of picoseconds), very local (a cylinder of approximately 10 nm in diameter) and very high (some 0.1 eV/ atom) excitation of solid (Avasthi, 2000). Under such extreme conditions, highly non-equilibrium process may be initiated. The characteristics of the ion beam and the properties of the materials also determine the response of the materials to SHI. Irradiations of the tin doped indium oxide thin films by energetic ions modified their structural, electrical and optical properties and evidenced that optimal irradiations improve the electrical and optical properties (Haynes et al., 1997; Shigesato et al., 1993a; Shigesato et al., 1994). Shigesato et al. (1993a) have reported that the implantation of O7 þ (1.7 1015 ions/cm2) and H þ (1.3 1015 ions/cm2) ions on ITO films decreases the carrier concentration and increases Hall mobility. Morgan et al., (1995) have studied the effect of 15 keV H þ
590
V. Gokulakrishnan et al. / Radiation Physics and Chemistry 81 (2012) 589–593
irradiation of 1 1015 and 1 1017 ions/cm2 fluences on ITO thin films, which displayed radiation hardness up to a dose of 1016 cm2. In the present work, we report the effects of swift heavy ion beam of 100 MeV O7 þ on the structural, optical and electrical properties of spray deposited IMO thin films for the fluence of 5 1011, 1 1012 and 1 1013 ions/cm2.
2. Experimental details Indium (III) chloride (0.065 M) was dissolved in 2 ml of hydrochloric acid (HCl) at 90 1C and diluted with 73 ml of double distilled water and was used as the source of In. Molybdenum (V) chloride (0.5 at%) dissolved in 2 ml of HCl by heating at 90 1C and diluted with 23 ml of ethanol was used as the source of Mo. Mixture of both the solutions was used to deposit IMO films with a thickness of 300 nm on Corning 1737 glass substrates at 400 1C using a spray pyrolysis experimental setup. The spray time and interval was maintained as 1 s and 30 s, respectively. The asdeposited IMO films with 0.5 at% Mo possess better electrical and optical properties in comparison with other doping concentrations. Hence, the 0.5 at% Mo doped IMO thin films have been selected for swift heavy O7 þ ion irradiation. Three identical samples of the selected pristine IMO films were irradiated by 100 MeV swift heavy O7 þ ions with ion fluences of 5 1011, 1 1012 and 1 1013 ions/cm2, using the 15UD Pelletron tandem accelerators at Inter University Accelerator Centre (formerly known as Nuclear Science Centre), New Delhi, India. A vacuum of 10 6 Torr was maintained during the irradiation experiments and the ion beam current was maintained at around 1 pnA (denotes particle nano-ampere). To ensure uniformity of irradiation, the beam is scanned over an area of 1 1 cm2 of the sample with an electromagnetic scanner. The range of 100 MeV O7 þ ions in the films calculated using SRIM-2008 (Version 2008.4) software was about 46.35 mm, which is greater than the thickness of the film. Hence, the bombarding ions pass through the entire film and are stopped in the substrate. For 100 MeV O7 þ energetic ions the electronic and nuclear energy loss values calculated are ˚ respectively. 1.537 102 and 8.843 10 2 eV/A, The X-ray diffraction patterns were obtained using the computer controlled PANalytical X pert PRO X-ray diffraction system (Cu Ka ˚ in Bragg–Brentano geometry (y/2y with a wavelength of 1.5406 A) coupled). The nature of surface microstructures were obtained using atomic force microscopy (AFM) (Agilent 5500). The thickness of the films was measured by the reflection method using filmetrics F20. The electrical parameters were measured using Hall measurements setup (ECOPIA-HMS 3000) at room temperature with a permanent magnet of 0.57 T for fixed Hall current of 1 mA and delay time of 0.5 s. The optical transmittance was measured using a double beam spectrometer (Shimadzu UV-1700) with a bare substrate in the path of the reference. Hence, the transmittance spectra reported in this study refer only to film transmission.
3. Results and discussion 3.1. Structural properties The structural analyses of pristine and 100 MeV O7 þ ion irradiated films were performed using X-ray diffraction (XRD) technique. XRD peaks of the pristine and irradiated IMO films were identified by matching with the standard data (ICDD card no. 06-0416) as shown in Fig. 1 which confirmed the cubic bixbyite structure of indium oxide (IO). XRD pattern obtained from the pristine and irradiated IMO films shows a strong preferential orientation along (222) direction. The observed diffraction peaks demonstrate the polycrystalline
Fig. 1. XRD patterns of the IMO films and O7 þ irradiated ions.
Fig. 2. FWHM and Grain size of IMO and O7 þ irradiated films with various fluence range.
structure of the irradiated IMO films. X-ray diffraction intensity of the prominent (222) peak decreases with increase in ion fluence while (211) peak was completely destroyed at higher fluence. Their crystallite size was calculated from (222) peak using the Scherrer’s formula. The full width half maximum and crystalline size of the (222) peak for the irradiated films are plotted in Fig. 2 and FWHM value monotonically increases and grain size decreases with increase in the ion fluence. Increase in the fluence of O7 þ SHI irradiation reduces the intensity of diffraction peaks of IMO films, thus the results show a tendency to amorphisation in these films. This kind of amorphisation also observed in the O7þ irradiated IO films (Gokulakrishnan et al., 2011). 3.2. Surface morphological analysis The surface microstructures of the pristine and irradiated IMO films recorded by AFM are shown in Fig. 3. The surface morphology of the pristine IMO film shows regularly distributed granular shaped crystallites with an average grain size of 120 nm and
V. Gokulakrishnan et al. / Radiation Physics and Chemistry 81 (2012) 589–593
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Fig. 3. AFM microstructures of pristine and O7 þ ions irradiated IMO films.
with a root mean square (RMS) roughness of 27 nm. At a higher fluence the surface pattern of the irradiated IMO films was drastically changed in the shape and distribution, as shown in the Fig. 3. Thus grainy character of the pristine film changes significantly after irradiation with various ion fluences. Such drastic change in the size and shape of the crystallites at higher fluence is attributed to the high density of electronic excitations induced by SHI irradiation under multiple ion impacts in the near surface region (Agarwal et al., 2008). The films irradiated with the ion fluence of 5 1011 ions/cm2 is comprised of grains with a few disordered granular shaped crystallites having average grain size of 80 nm with RMS roughness of 16 nm. The decrease in RMS roughness authenticates a relatively smooth surface. When the ion fluence is increased to 1 1012 ions/cm2, the size of the granular grains becomes smaller. Further, the RMS roughness is strikingly decreased to 12.1 nm. When the ion fluence is further increased to 1 1013 ions/cm2, the average grain size is decreased to 38 nm and RMS roughness is significantly reduced to 11 nm. Thus a marked change in the surface RMS roughness is observed after irradiation. At higher fluence of 1 1013 ions/ cm2 local heating provides additional energy for rearrangement of atoms which smoothens the film surface. AFM images evidently show that the grain size decreases with increase in the ion fluence. XRD patterns shown in Fig. 1 also corroborate with the results from AFM studies. The foregoing discussion concludes that the fluence of 1 1013 ions/cm2 has decreased the grain size and significantly increased the smoothness of the surface of the pristine films. Decrease of grain size and increase of surface smoothness in the O7 þ irradiated IO films were also observed by Gokulakrishnan et al., 2011. 3.3. Electrical properties The electrical properties of pristine and irradiated IMO films were estimated from the room temperature Hall measurements
in van der Pauw configuration. The negative sign of Hall coefficient of films confirmed the n-type conductivity. The variation in the electrical properties of pristine and ion irradiated IMO thin films is shown in Fig. 4. The pristine IMO (0.5 at% Mo) film shows the best possible combination of electrical properties as follows: bulk resistivity (r) of 5.2 10 4 O cm, carrier concentration (n) of 0.95 1020 cm 3 and mobility (m) of 122 cm2/V s. The highest r of 7.2 10 4 O cm is obtained for the IMO film irradiated with an ion fluence of 1 1013 ions/cm2. The resistivity of the IMO films due to irradiation of O7 þ ions decreases with increase in the film carrier density (n). The increase in n may be a result of oxygen vacancies created due to annealing by the ion irradiation under the vacuum of 10 6 Torr, which has contributed more free electrons increase to n. On the other hand, the m of ion irradiated films (1 1013 ions/cm2) is decreased from 122 to 48 cm2/ V s. The probable reason for decrease in m is that O7 þ ion irradiation lead to increase the scattering effect due to increase in carrier concentration. The electron scattering sources such as grain boundaries and acoustic phonons, neutral and ionised impurity centres have been found to affect the electrical and optical properties of the films (Gerlach et al., 1978; Lu et al., 2006; Koida and Kondo, 2006; Koida and Kondo, 2007; Adurodija et al., 2000). The experimental data obtained were used to derive the mean free path (l) to clarify the dominat scattering mechanism in pristine and ion irradiated IMO thin films. The mean free path was calculated using a sufficiently degenerate gas model (Kittel., 1985; Yamada et al., 2000) given by l ¼ ð3p2 Þ1=3 ð_=e2 Þr1 n2=3
ð1Þ
where r and n representes the resistivity and carrier concentration, respectively. Mean free path value of 11.5 nm obtained from the pristine films is decreased to 6.74 nm for the films irradiated with 1 1013 ions/cm2 fluence. In most TCOs, the grain boundaries and acoustic phonons played a secondary role, since the mean free path is usually much smaller than the average grain
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Fig. 5. Comparison of transmittance spectra obtained from the pristine and SHI irradiated IMO films.
Fig. 4. Comparison of electrical properties of the pristine and SHI irradiated IMO films.
size. In the present work, the calcualted l (ranging between 6.74 and 11.5 nm) is much smaller than the average grain size determined from XRD analysis of pristine and ion irradiated IMO thin films. Therefore, grain boundary scattering appears not to play an important role in the pristine and irradiated IMO films. (Shigesato et al., 1993b). 3.4. Optical properties The transmittance spectra of the as-deposited and ion irradiated IMO thin films in the wavelength range 300–1000 nm are shown in the Fig. 5 as a function of O7 þ ion fluences. The average visible transmittance (AVT) calculated in the wavelength range 400–800 nm is 89% for the as-deposited IMO films. The AVT has been decreased to 79, 74, and 66% for the ion fluences of 5 1011, 1 1012 and 1 1013 ions/cm2, respectively. The reduction in optical transmittance due to irradiation may result from the incorporation of lattice defects which may increase the absorption in optical region. It is also known that the transmittance of irradiated TCO films depends on the concentration of oxygen vacancies in the film (Tahar et al.,1998). In the present case, the transmittance of IMO film decreases with increase in the fluence. A similar behaviour of decrease in the transmittance of zinc oxide thin films following the irradiation by heavy Au ion of 100 MeV has been reported (Ratheesh Kumar et al., 2005). The optical band gap of the IMO film was estimated from the relation
ahn ¼ ðhnEg Þ1=2
ð2Þ
This equation gives the direct band gap energy Eg when the straight portion of (ahn)2 against hn plot is extrapolated to the point a ¼0. The variation of band gap as a function of ion fluence is shown in Fig. 6. The estimated value of energy gap for pristine sample is
Fig. 6. Plot of (ahn)2 versus hn for pristine and 100 MeV O7 þ ion irradiated IMO films.
3.71 eV and is reduced to 3.54, 3.44 and 3.38 eV for films irradiated with the fluence of 5 1011, 1 1012 and 1 1013 ions/cm2, respectively. This shows a consistent decrease of energy gap with increase in ion fluence. The shift in Eg upon irradiation can be attributed to several reasons. The decrease in the energy gap with increasing ion fluence is due to creation of intermediate energy levels (Chaudhary et al., 2004; Mohanta et al., 2004; Narayanan et al., 1997).
4. Conclusions 0.5 at% Mo doped indium oxide thin films with improved properties were deposited on Corning-F1737 glass substrates at 400 1C using economic spray pyrolysis technique and irradiated by swift heavy O7 þ ions of 100 MeV energy with different fluences. XRD studies confirmed the cubic bixbyite structure of polycrystalline IO for the both pristine and ion irradiated IMO films. X-ray diffraction intensity of the prominent (222) peak decreases with increase in ion fluence while (211) peak was completely destroyed at 1 1013 ions/cm2 fluence. The AFM
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studies confirmed the evolution of surface microstructures as a function of ion fluence. The grain size of irradiated IMO films decreases to 38 nm from 128 nm (pristine) and RMS values decreases to 11 nm from 27 nm (pristine) for 1 1013 ions/cm2 fluence. The electrical resistivity of pristine IMO film is significantly increased from 5.2 10 4 to 7.2 10 4 O cm for the ion fluence of 1 1013 ions/cm2. Mobility of the pristine film is decreased from 122 to 48 cm2/V s for 1 1013 ions/cm2 fluence. The average transmittance of the as-deposited IMO film is decreased from 89 to 66% for O7þ SHI irradiation of 1 1013 ions/cm2 fluence.
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