Electrochromic properties of InN:Sn films deposited by reactive evaporation

doi:10.1016/j.tsf.2009.07.174

Thin Solid Films

Volume 518, Issue 3, 1 December 2009, Pages 1001-1005

https://doi.org/10.1016/j.tsf.2009.07.174 Get rights and content

Abstract

Indium–tin nitride (InN:Sn) films were deposited by vacuum evaporation assisted by active nitrogen irradiation. A glancing-angle deposition scheme was applied to form isolated nanocolumnar structures in order to expand surface area of the films. X-ray diffraction analysis revealed that the films consisted of crystallites of InN:Sn in a wurtzite structure and amorphous InN:Sn matrix. The doped tin atoms did not work as donor in the InN:Sn films but electrons-trapping sites. The electrochromic amplitude was reduced with increase in tin composition. Despite that the tin doping caused the decrease in carrier density, the color-change region of the InN:Sn films shifted slightly toward shorter wavelength.

Introduction

Electrochromic (EC) materials have a unique property of reversible color change with a burst of electrical charge. Much attention has been paid to the EC materials for application to optical switching devices, e.g., huge-size displays, anti-glare mirrors, smart windows, through which the light and heat can be controlled electrically, and so on. [1], [2], [3]. Indium nitride (InN), one of the group-III nitride semiconductor, has been reported to show an electrochromic behavior in aqueous solution of sodium sulfate [4]. We have investigated the EC properties of the InN films deposited by reactive plasma processes, and found that the EC phenomenon of InN is based on a mechanism similar to so-called Burstein–Moss shift induced by alternation of surface adsorbates [5]. The reaction can be summarized as follows (Fig. 1):

(i)
the InN films have a degenerated semiconducting property due to a lot of nitrogen vacancies, so that Fermi level locates in the conduction band,
(ii)
the surface-terminating atoms can become “active sites” on the InN grains which meet the solution,
(iii)
the active sites are terminated with –OH/–H in anodic/cathodic polarization,
(iv)
the –OH/–H terminations make the carrier density of the InN film lower/higher, then Fermi level shifts lower/upper,
(v)
the higher/lower Fermi level causes wider/narrower optical gap, so that the optical absorption edge shifts toward shorter/longer wavelength, respectively.

Based on this model, the original carrier density becomes one of the key factors which determine the wavelength region of the EC phenomenon. The higher carrier density makes the optical gap wider, which causes blue-shift of the wavelength region. Therefore, we can expect that doping a IV-group element in InN films shows a color change in a shorter wavelength region, if the carrier density of InN increases and its optical gap widen by doping. In this study, we deposited indium–tin nitride (InN:Sn) films by using a vacuum evaporation system assisted with an active nitrogen source and investigated the effect of Sn mixing on the EC properties of the films.

Section snippets

Experimental details

The InN:Sn films were prepared by evaporation of indium and tin through a downstream of an active nitrogen flow. The deposition system consists of a vacuum chamber (SK-KP090, Shink) and an active nitrogen source (RNS-20S, ULVAC) as illustrated in Fig. 2. Two alumina-coated crucibles of tungsten wire (BR-3, Furuuchi Chemical) were used for evaporation of metallic indium and tin shots (6N purity, Furuuchi Chemical), respectively. After evacuation of the system by using a turbo-molecular pump

Crystallinity

The InN:Sn films were found to have a wurtzite crystal structure from the XRD results shown in Fig.3. Increasing Sn composition makes the following evolutions in the XRD profiles of the films;

(1)
no additional diffraction peak,
(2)
the 100 diffraction peak shifts toward higher angle,
(3)
the diffraction peaks become broader and weaker.

These results can be interpreted as follows;

(1)
the Sn atoms occupy randomly the substitutional positions of indium atoms in the intrinsic wurtzite structure of pure InN,
(2)
the

Conclusions

We prepared Sn-doped InN films by vacuum evaporation assisted with an active nitrogen source. The GLAD technique succeeded in this process to deposit microvillus-like nanocolumnar structure to enhance the EC amplitude. We aimed in this study to blue-shift the EC region of InN by doping Sn as donor, but found that small amount of Sn mixing in InN causes a decrease in the native carrier density. This result can be explained by presuming that the Sn atoms work as carrier trapping sites because the

Acknowledgement

This work was supported in part by “Grant-in-Aid for Young Scientist (A)” (No. 16686039, 2004–2006) and “Grant-in-Aid for Scientific Research (B)” (No. 19360287, 2007–2010), the Ministry of Education, Science, Sports and Culture, Japan.

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Electrochromic properties of InN:Sn films deposited by reactive evaporation

Abstract

Introduction

Section snippets

Experimental details

Crystallinity

Conclusions

Acknowledgement

Thin Solid Films

Thin Solid Films

Appl. Surf. Sci.

Large-area chromogenics materials and devices for transmittance control

Sol. Energy Mater.

J. Electrochem. Soc.

Proc. S.I.D.

J. Electrochem. Soc.