phys. stat. sol. (c) 2, No. 7, 2293 – 2296 (2005) / DOI 10.1002/pssc.200461560 Revisiting electrochromism in InN K. Scott1, A. Butcher*1, Marie Wintrebert-Fouquet1, Patrick P.-T. Chen1, Richard Wuhrer2 and Matthew R. Phillips2 1 2 Physics Department, Macquarie University, Sydney NSW 2109, Australia Microstructural Analysis Unit, Faculty of Science, University of Technology, Sydney, Broadway NSW 2007, Australia Received 13 July 2004, revised 28 July 2004, accepted 20 February 2005 Published online 1 April 2005 PACS 68.37.Hk, 68.49.Sf, 82.45.Vp We confirm changes to the band-gap of InN thin films treated in an electrochemical cell in which water electrolysis is evident. Electrical properties of the films were also affected. It is suggested that the change in the film resistivity results from hydrogen incorporation or removal during the electrolysis (dependent on sample polarity). The presence of grain boundaries is believed to enhance the penetration of chemical species into the InN resulting in a greater net change in the observed properties. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction The older literature on InN includes several papers by Takai et al. [1–3] reporting on observations of electrochromism. In the “as grown” state the samples of Takai et al. were reddish brown, in accord with a ~ 1.9 eV band-gap, however the optical transmission was changed radically towards the yellow when the samples were pulsed to cathodic polarization in an electrochromic cell, and towards the green/dark grey when pulsed to anodic polarization. The films would maintain their coloration for approximately 10 days before reverting to their original colour, the samples could also be cycled through these changes. In light of recent observations of the variation in the band-gap of InN we decided to reinvestigate the effects observed by Takai et al. Surface electron accumulation has also been identified as a potential problem for InN [4], electrochemical treatments may offer a means of controlling the surface. Samples grown by RF sputtering and remote plasma enhanced chemical vapour deposition (RPECVD) were treated in an electrochromic cell. Variations in the absorption spectra and electrical properties of the films are reported. Some secondary ion mass spectroscopy (SIMS) results are also provided. 2 Experimental procedure The RF sputtered sample was grown as previously described [5]. These samples are highly c-axis oriented with sharp (0002) and (0004) X-ray diffraction peaks evident. Figure 1a shows a scanning electron microscope (SEM) secondary electron cross-sectional image of a typical sample. The material is polycrystalline with columinar structure but with small grain width along the aaxis (< 100 nm in diameter), high amounts of excess nitrogen can be present [6,7] and adventitious oxygen, resulting from atmospheric exposure, is typically found at 5-15 % atomic concentration as an amorphous oxynitride along the grain boundaries [8]. Apparent band-gaps typically vary in the range from 1.9-2.3 eV [5]. RPECVD samples were also grown as previously described [9]. These samples are also highly c-axis oriented, again with sharp X-ray diffraction peaks being evident. Figures 1b –1d show typical SEM secondary electron cross-sectional images. The material studied here is also polycrystalline (though single crystal material can be grown) with columinar structure but with large grains of the order of 100-200 nm in width. Oxygen levels have been found by SIMS to be typically less than 1%, with the * Corresponding author: e-mail: sbutcher@physics.mq.edu.au, Phone: +61 2 9850 8916, Fax: +61 2 9850 8115 © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2294 K. Scott et al.: Revisiting electrochromism in InN residual oxygen again probably being the result of atmospheric oxidation along the grain boundaries. Apparent band-gaps for the as grown material can vary from 0.9 to as high as 1.8 eV and appear to be largely dependent on the stoichiometry of the nitrogen and indium [10] though a Moss-Burstein effect is also expected. The electrical properties of the RPECVD samples studied here are better than those of the RF sputtered sample. Table 1 lists some of the film properties. The RF sputtered sample was grown on glass, while the RPECVD samples were grown on glass or sapphire. The samples were subjected to a high anodic or cathodic field in a electrochemical cell. The cell contained water purified with a Milli-Ro system, a gold electrode, and the highly conductive InN (see table 1) forming the other electrode. The Au and InN electrodes were maintained at a 2-3 mm separation. Relatively high voltages (either +20 V or –20 V relative to the other electrode) were applied to the samples. These were much higher than the values applied by Takai et al., however no salt based ions were introduced into the solution as these could be a source of contamination that would complicate the experimental analysis. Deionised 18 MΩ·cm water was initially used as the solvent, however low ion currents resulted in reduced modification to the samples so that the Milli-Ro purified water was used with the low level of residual ions providing the electrolysis current required. Under these conditions the ion current was sufficient for electrolysis, with the visible formation of oxygen bubbles at the positive electrode and hydrogen at the negative. Changes related to the formation of oxygen and hydrogen could of course be expected during these experiments. The optical band-gaps of the samples were determined, before and after treatment, using optical absorption measurements carried out with a Cary UV-Vis-IR transmission spectrometer. Hall Van der Pauw measurements were also made before and after. A Cameca 5F dynamic SIMS system was used to qualitatively analyse the hydrogen content of the films. The results are provided below. (a) (c) (b) (d) Fig. 1 SEM micrographs of InN film cross-sections taken with a LEO Supra 55VP SEM using secondary electron in-lens imaging. (a) RF sputtered film on glass (glass at lower right of film). (b) RPECVD 18 Sep 2003 film on c-plane sapphire (sapphire on lower right). (c) RPECVD 15-16 May 2004 film on glass (glass substrate is on upper left, with charging evident). (d) RPECVD 15-16 May 2004 film on sapphire (sapphire on upper left). 3 Results Table 1 summarises the results of the electrolysis experiments. The apparent band-gaps were determined from absorption square plots (an approximation for high carrier concentration material that is known to overestimate the band-gap by approximately the amount of band-tailing present [11]). The apparent band-gap values in the table are given as Eg + EF, where Eg is the band-gap and EF is the value of the Fermi level when in the conduction band. The treatment applied to each sample is also described briefly in the table. It is apparent that the application of a negative bias results in a reduction in the sample resistivity, and that the application of a subsequent positive bias returns the sample to near its original resistivity or to a slightly higher value. There could be several reasons for the observed electrical variation related to changes in surface chemistry. However, the formation of hydrogen ions in the solvent © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim phys. stat. sol. (c) 2, No. 7 (2005) / www.pss-c.com 2295 during the electrochemical treatment suggests that there is another possibility. H+ ions will migrate to the InN surface when the film is under negative bias, and hydrogen introduction is known to occur very easily for some semiconductors, by simply boiling in water for instance [12], while the drifting of Table 1 Treatment results for the InN samples studied. Sample and growth temperature substrate 26 Sep 2003 350oC 26 Sep 2003 350oC sapphire sapphire o 18 Sep 2003 400 C o 18 Sep 2003 400 C sapphire sapphire 15-16 May 2004 500 oC 15-16 May 2004 500 oC sapphire sapphire 15-16 May 2004 500 oC sapphire 15-16 May 2004 500 oC sapphire 15-16 May 2004 500 oC 15-16 May 2004 500 oC glass glass 15-16 May 2004 500 oC glass 4-9 April 2003 40 oC 4-9 April 2003 40 oC glass glass Treatment EG + EF (eV) RPECVD Samples As grown 1.68 –20 V for 5 1.60 min As grown 1.40 –20 V for 5 1.38 min As grown 1.25 –20 V for 5 1.25 min +20 V for 3 1.25 min +20 V for 8 1.25 min As grown 1.12 -20 V for 3 1.08 min +20 V for 3 1.10 min RF sputtered sample As grown 2.10 -20 V for 5 min 2.02 Carrier –3 Density (cm ) Mobility (cm2/V⋅s) Resistivity (Ω·cm) 1.4x1020 - 8 - 5.43x10–3 5.43x10–3 5.9x10 19 6.8x10 66 58 1.593x10 –3 1.589 x10 9.5x1019 1.2x1020 280 226 2.46x10–4 2.30x10–4 1.2x1020 211 2.44x10–4 1.1 x1020 202 2.69x10–4 9.2x1019 - 100 - 6.74x10–4 5.41x10–4 9.3x1019 98 6.82x10–4 2.7 x1020 10 2.35x10–3 2.8x1020 34 6.6x10–4 19 –3 hydrogen ions inside a semiconductor can be enhanced by the application of an electric field [13]. H+ ions are believed to act as donors in InN and to be the only stable form of hydrogen that will exist in that material [14]. The reduction of resistivity under negative bias is therefore consistent with the introduction of hydrogen ions into the InN. While the increase of resistivity with positive bias is consistent with their removal. The change in carrier concentrations for the samples do not seem to support this theory, however the Hall measurements are much more sensitive to noise (producing as much as approximately a +/- 20% error here) than the resistivity measurements, which show a high level of reproducibility to the value of the significant figures shown in the table. Figure 2 shows the hydrogen levels resident in “as grown” RF sputtered and RPECVD films. The high hydrogen level in the RF sputtered sample is presumed to be the result of sample hydrolysis along the grain boundaries on exposure to air. The hydrogen present in the as-grown material indicates why an increase in film resistivity, above the original value, can be achieved under positive bias. If the treatment is aggressive enough some of the hydrogen originally present in the films can be removed. Other possibilities for the observed changes in sample resistivity cannot be excluded. The treatment used may cause surface modifications that may also result in the changes observed. Other measurements are required to examine this possibility. Regardless of the exact mechanisms of the changes seen, for the RF sputtered sample a greater change in sample resistivity is evident, probably as the result of the greater surface area made available by the sample’s crystal grains. As shown in Fig. 1a the grains are much smaller for the RF sputtered sample than for the other samples grown by RPECVD. Commensurate with this, the RF sputtered sample also © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2296 K. Scott et al.: Revisiting electrochromism in InN shows a greater change in the apparent band-gap. The band-gap changes are undoubtedly of chemical origin, however the exact nature is not understood and may be related to the modification of sample nonhomogeneities, such as indium inclusions, or the removal of oxide layers in the case of negative bias application. Under positive bias a black layer was noted to form on the sample surfaces, which was poorly adherent and easily removed. This layer may have been an oxide or hydroxide surface layer formed from the negative oxygen ions that migrate to the surface when under positive polarity. RF Sputtered o RPECVD 350 C RPECVD 420o C o RPECVD 500 C 1 + + H /N ion yield 10 0.1 Fig. 2 SIMS hydrogen ion signal for as grown films, presented qualitatively as a ratio of the nitrogen ion yield. The higher value observed for the RF sample is believed to be the result of hydroxide formation at grain boundaries upon exposure of the sample to atmosphere. The hydrogen levels in the films were 20 times that seen in Si. 4 Conclusions Changes in the band-gap of InN were confirmed when the material was present in an electro0.01 0 200 400 600 800 chemical cell in which electrolysis of hydrogen and oxyDepth from sample surface (nm) gen was present. It was suggested that electric field induced introduction and drifting of hydrogen may influence the resistivity of the as grown material. It is also suggested that grain boundaries allow the transport of active ions into the film affecting a greater volume of sample. Acknowledgements We would like to acknowledge the support of a Macquarie University Research Development Grant, and an award for SIMS time provided by the Australian Institute of Nuclear Science and Engineering. K. S. A. Butcher would also like to acknowledge the support of an Australian Postdoctoral Fellowship and the travel support provided by the AOARD and AFOSR. My thanks also to Lt Col. Todd Steiner for his invitation to me under the Window on Science program. References [1] O. Takai, J. Ebisawa, and Y. Hisamatsu, Preparation of Indium Nitride by R. F. Reactive Ion Plating, in Proceedings of the 7th International Conference on Vacuum Metallurgy, the Iron and Steel Institute of Japan, Tokyo (1982) p. 129. [2] O. Takai, J. Ebisawa, and Y. Hisamatsu, Properties of Indium Nitride Films Prepared by R. F. Reactive Ion Plating, in Proceedings of the 7th International Conference on Vacuum Metallurgy, the Iron and Steel Institute of Japan, Tokyo (1982) p. 137. [3] O.Takai, J.Soc. Information Display 25, 305 (1984). [4] H. Lu, W. J. Schaff, L. F. Eastman, and C. E. Stutz, Appl. Phys. Lett. 82, 1736 (2003). [5] K. S. A. Butcher, M. Wintrebert-Fouquet, P. P.-T. Chen, T. L. Tansley, and S. Srikeaw, Mat. Res. Soc. Symp. Proc. 693, 341 (2002). [6] K. S. A. Butcher, M. Wintrebert-Fouquet, Motlan, S. K. Shrestha, H. Timmers, K. E. Prince, and T. L. Tansley, Mat. Res. Soc. Symp. Proc. 743, 707 (2003). [7] K. S. A. Butcher, M. Wintrebert-Fouquet, P. P.–T. Chen, T. L. Tansley, H. Dou, S. K. Shrestha, H. Timmers, M. Kuball, K. E. Prince, and J. E. Bradby, Nitrogen Rich Indium Nitride, accepted for publication J. Appl. Phys. [8] S. Kumar, L. Mo, Motlan and and T. L. Tansley, Jpn. J. Appl. Phys. 35, 2261 (1996). [9] M. Wintrebert-Fouquet, K. S. A. Butcher, and P. P.-T Chen, Inn Grown By Remote Plasma Enhanced Chemical Vapor Deposition, accepted J. Cryst. Growth. [10] K. S. A. Butcher, M. Wintrebert-Fouquet, P. P.-T. Chen, K. E. Prince, H. Timmers, S. K. Shrestha, T. V. Shubina, S. V. Ivanov, R. Wuhrer, M. R. Phillips, and B. Monemar: Non-stoichiometry and Non-homogeneity in InN, this proceedings. [11] I. Hamberg, C. G. Granqvist, K.-F. Berrgren, B. E. Sernelius, and L. Engstrom, Phys. Rev. B 30, 3240 (1984). [12] A. J. Tavendale, A. A. Williams, and S. J. Pearton, Appl. Phys. Lett. 48, 590 (1986). [13] A. J. Tavendale, A. A. Williams, and D. Alexiev, Appl. Phys. Lett. 47, 316 (1985). [14] S. Limpijumnong and C. G. Van de Walle, phys. stat. sol. (b) 228, 303 (2001). © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim