Polaron absorption in amorphous tungsten oxide films

Berggren, Lars; Azens, A.; Niklasson, Gunnar A.

doi:10.1063/1.1384853

Cited by 83 publications

(53 citation statements)

References 22 publications

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“…Such coupling was previously found to produce large phonon-drag [36,37] and polaronic effects in both LAO/STO interfaces and amorphous WO 3 thin films [38,39]. Another possibility is that the modified defect profile with respect to conventional LAO/STO interfaces determines a mass enhancement of the 2DES bands [40].…”

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confidence: 93%

Insulator-to-Metal Transition at Oxide Interfaces Induced by WO₃ Overlayers

Mattoni

Baek²,

Manca

et al. 2017

ACS Appl. Mater. Interfaces

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Interfaces between complex oxides constitute a unique playground for 2D electron systems (2DES), where superconductivity and magnetism can arise from combinations of bulk insulators. The 2DES at the LaAlO 3 /SrTiO 3 interface is one of the most studied in this regard, and its origin is determined by both the presence of a polar field in LaAlO 3 and the insurgence of point defects, such as oxygen vacancies and intermixed cations. These defects usually reside in the conduction channel and are responsible for a decreased electronic mobility. In this work, we use an amorphous WO 3 overlayer to control the defect formation and obtain an increased electron mobility in WO 3 /LaAlO 3 /SrTiO 3 heterostructures. The studied system shows a sharp insulator-to-metal transition as a function of both LaAlO 3 and WO 3 layer thickness. Low-temperature magnetotransport reveals a strong magnetoresistance reaching 900% at 10 T and 1.5 K, the presence of multiple conduction channels with carrier mobility up to 80 000 cm 2 V −1 s −1 and an unusually high effective mass of 5.6 me. The amorphous character of the WO 3 overlayer makes this a versatile approach for defect control at oxide interfaces, which could be applied to other heterestrostures disregarding the constraints imposed by crystal symmetry.The formation of a two-dimensional electron system (2DES) at the interface between band insulators SrTiO 3 (STO) and LaAlO 3 (LAO) is among the most intriguing effects studied in oxide electronics [1]. Gate tunable superconductivity [2,3], strong spin-orbit coupling [4,5] and magnetism [6,7] are some of the many phenomena observed. The origin of this 2DES is a long standing question in the solid state community and recent results indicate that a consistent picture should take into account both the built-in polar field and the presence of point defects [8][9][10]. Among these, oxygen vacancies and cation off-stoichiometry in STO are capable of inducing a 2DES [11,12]. However, defects residing in the conductive channel are usually responsible for a decreased electronic mobility [13]. In order to promote high electron mobility, it is crucial to confine donor sites away from the conducting plane, without preventing the 2DES formation in the STO top layers. Previous attempts to control the defect concentration profile and thus enhance the mobility involved the use of crystalline insulating overlayers [14,15] [18,19]. A promising material to control defect formation is tungsten oxide WO 3 . The several possible oxidations states of tungsten make WO 3 particularly active in undergoing redox reactions. For this reason this material is often utilized in electrochemical applications and electrochromic devices [20][21][22]. Also, both crystalline and amorphous WO 3 can host vacancies and interstitial atoms, thus allowing cation accommodation and diffusion, with a tendency to form compounds such as tungsten bronzes [23,24]. Recent progress demonstrated the high-quality growth of WO 3 thin films on perovskite materials [25][26][27].In this work we co...

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confidence: 93%

Insulator-to-Metal Transition at Oxide Interfaces Induced by WO₃ Overlayers

Mattoni

Baek²,

Manca

et al. 2017

ACS Appl. Mater. Interfaces

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“…In the abstract of the paper the authors comment: "more recent 'polaron' spectra in WO 3 films may also be artifacts," with reference to a recent paper of ours. 1 However, this claim was not substantiated in the paper of Scott et al, our measurements on WO 3 were not discussed any further.…”

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confidence: 42%

“…Interference effects are well known in reflectance (R) and transmittance (T) measurements on thin films; they are also present for the WO 3 films considered in Ref. 1. Interference effects are especially serious if the thin film is deposited on a reflecting substrate; a configuration that Scott et al appear to have used.…”

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Comment on “Infrared spectra and second-harmonic generation in barium strontium titanate and lead zirconate titanate thin films: ‘Polaron’ artifacts” [J. Appl. Phys. 94, 3333 (2003)]

Niklasson

Berggren

Azens

2004

Journal of Applied Physics

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Scott et al. [Scott et al., J. Appl. Phys. 94, 3333 (2003)] have questioned our measurements of the optical absorption of amorphous tungsten oxide, and have suggested that the polaron absorption might be an artifact due to interference effects. In this comment we point out that we obtained the absorption coefficient from measurements of both the transmittance and the reflectance of thin films deposited on transparent substrates. By this method the “true” absorption is easily obtained.

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“…This material, which shows electrochromism, gasochromism, and photochromism, has applications in smart windows, rear view mirrors in automobiles and gas sensors. Electrochromism in WO 3 , which involves ion/electron insertion and extraction, has been studied [1][2][3] widely since its discovery in 1973 by Deb [4]. Intercalation of metal ions (M + ) into tungsten oxide results in a tungsten bronze M x WO 3 .…”

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confidence: 99%

Electrochromic switching of WO3 nanostructures and thin films

2006

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We present transmission measurements through tungsten tri-oxide nanostructures and thin films prepared by sol-gel process on micro-contact printed substrates. Identical electrochromic switching times are found for both the nanostructures and the bulk films with equal thicknesses upon intercalation of H + ions. We attribute the large change in the transmission through nanostructures at 632 nm, which can not be solely explained by absorption, to diffraction effects.

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Polaron absorption in amorphous tungsten oxide films

Cited by 83 publications

References 22 publications

Insulator-to-Metal Transition at Oxide Interfaces Induced by WO₃ Overlayers

Insulator-to-Metal Transition at Oxide Interfaces Induced by WO₃ Overlayers

Comment on “Infrared spectra and second-harmonic generation in barium strontium titanate and lead zirconate titanate thin films: ‘Polaron’ artifacts” [J. Appl. Phys. 94, 3333 (2003)]

Electrochromic switching of WO3 nanostructures and thin films

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Polaron absorption in amorphous tungsten oxide films

Cited by 83 publications

References 22 publications

Insulator-to-Metal Transition at Oxide Interfaces Induced by WO3 Overlayers

Insulator-to-Metal Transition at Oxide Interfaces Induced by WO3 Overlayers

Comment on “Infrared spectra and second-harmonic generation in barium strontium titanate and lead zirconate titanate thin films: ‘Polaron’ artifacts” [J. Appl. Phys. 94, 3333 (2003)]

Electrochromic switching of WO3 nanostructures and thin films

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Insulator-to-Metal Transition at Oxide Interfaces Induced by WO₃ Overlayers

Insulator-to-Metal Transition at Oxide Interfaces Induced by WO₃ Overlayers