Decoupling the refractive index from the electrical properties of transparent conducting oxides via periodic superlattices

Abstract

We demonstrate an alternative approach to tuning the refractive index of materials. Current methodologies for tuning the refractive index of a material often result in undesirable changes to the structural or optoelectronic properties. By artificially layering a transparent conducting oxide with a lower refractive index material the overall film retains a desirable conductivity and mobility while acting optically as an effective medium with a modified refractive index. Calculations indicate that, with our refractive index change of 0.2, a significant reduction of reflective losses could be obtained by the utilisation of these structures in optoelectronic devices. Beyond this, periodic superlattice structures present a solution to decouple physical properties where the underlying electronic interaction is governed by different length scales.

Increasing the refractive index of materials via nanolamination: a-IGZO/TiO2 nanolaminates

“…The inclusion of thicker TiO 2 layers, up to 10 nm, results in progressively smaller deviations from the original conductivity up to 10 nm, beyond which it slowly decreases. The initial decrease is likely a result of under-stoichiometric ultrathin TiO 2 similar to that observed in corresponding [a-IGZO/SiO 2 ] i nanolaminates [32], although the scale of the effect is substantially smaller. Nonstoichiometric TiO 2 could equally reduce the carrier concentration of the a-IGZO layers, affecting the conductivity of the films.…”

“…All samples were deposited using radio-frequency (RF) magnetron sputtering. The details of the system and deposition methodology employed to form the superlattices are described elsewhere [32]. Samples were deposited on glass substrates using 2-in ceramic targets [a-IGZO (1:1:1 In:Ga:Zn) and TiO 2 ].…”

“…By inclusion of periodic 15-nm layers of TiO 2 the refractive index of the laminate TCO can be increased by n ≈ 0.39 with respect to bulk a-IGZO. Considering the narrow band of refractive indices of many of the best-performing n-type TCOs such as ITO [41], ZnO:Al [16], fluorine-doped tin oxide [42], indium zinc oxide [43], and IGZO [32], the demonstrated capacity to change the refractive index by such a margin could enable improvements not readily achievable otherwise. In addition to the altered real refractive index, the extinction coefficient (k) shows a small increase in the 1.5-2 eV range upon increase of the TiO 2 thickness due to the defective nature of the amorphous TiO 2 .…”

“…39. Figure 3 shows the conductivity σ and the Hall mobility μ H as a function of the refractive index (at 2 eV) (see also [32]. Clearly the electron mobility and the conductivity are well preserved, with a [10/10 nm] i nanolaminate exhibiting a change of n ≈ 0.25 while retaining 75% of the conductivity of unaltered a-IGZO and another [10/15] i nanolaminate showing similar conservation with a change of n ≈ 0.4 while maintaining 71% of the initial TCO conductivity.…”

“…To date such periodic multilayers have been used to control ferromagnetic properties [17], oxidization states [18], environmental stability [19,20], resistive switching [21], superconductivity [22], dielectric properties [23], optical properties [24], photoluminescence [25,26], magnetoelectric effects [27], thermoelectric effects [28][29][30][31], and gas barriers [20]. Previously we reported on a methodology for tuning the refractive index of materials via the use of [TCO/dielectric] i superlattice/nanolaminate structures [32]. These laminate TCOs enabled conservation of the conductivity and mobility of the component TCO material while reducing the composite refractive index of the structure in the visible range.…”