Electrical and Optical Properties of Amorphous SnO2:Ta Films, Prepared by DC and RF Magnetron Sputtering: A Systematic Study of the Influence of the Type of the Reactive Gas

Abstract

By reactive magnetron sputtering from a ceramic SnO2:Ta target onto unheated substrates, X-ray amorphous SnO:Ta films were prepared in gas mixtures of Ar/O2(N2O, H2O). The process windows, where the films exhibit the lowest resistivity values, were investigated as a function of the partial pressure of the reactive gases O2, N2O and H2O. We found that all three gases lead to the same minimum resistivity, while the width of the process window is broadest for the reactive gas H2O. While the amorphous films were remarkably conductive (ρ ≈ 5 × 10−3 Ωcm), the films crystallized by annealing at 500 °C exhibit higher resistivities due to grain boundary limited conduction. For larger film thicknesses (d ≳ 150 nm), crystallization occurs already during the deposition, caused by the substrate temperature increase due to the energy influx from the condensing film species and from the plasma (ions, electrons), leading to higher resistivities of these films. The best amorphous SnO2:Ta films had a resistivity of lower than 4 × 10−3 Ωcm, with a carrier concentration of 1.1 × 1020 cm−3, and a Hall mobility of 16 cm2/Vs. The sheet resistance was about 400 Ω/□ for 100 nm films and 80 Ω/□ for 500 nm thick films. The average optical transmittance from 500 to 1000 nm is greater than 76% for 100 nm films, where the films, deposited with H2O as reactive gas, exhibit even a slightly higher transmittance of 80%. These X-ray amorpous SnO2:Ta films can be used as low-temperature prepared transparent and conductive protection layers, for instance, to protect semiconducting photoelectrodes for water splitting, and also, where appropriate, in combination with more conductive TCO films (ITO or ZnO).

Transparent conductive SnO2 thin films via resonant Ta doping

“…2a, the raise in carrier density contributes largely to the significant decrease of resistivity from 1.21 to 1.17 × 10 −3 Ω cm, corresponding to an increase of conductivity from 0.82 for SnO 2 to 855.0 S cm −1 for Sn 0.98 Ta 0.02 O 2 . Such conductivity relatively surpasses that of TTO films grown by spray pyrolysis [27], sputterings [33][34][35], and atomic layer deposition (ALD) [36]. Nevertheless, the carrier concentration gradually saturates with the Ta doping level change, and starts to decline slightly to 2.0 × 10 20 cm −3 at x = 0.03, probably due to the introduction of non-activated Ta (or compensating effect at higher Ta doping).…”

Exceptionally high-temperature in-air stability of transparent conductive oxide tantalum-doped tin dioxide

“…During the past two decades, SnO 2 :Ta (TTO) thin lms were deposited using many different techniques on different substrates. [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38] They showed charge carrier mobilities and resistivities competitive with those of ITO. The best values of up to 130 cm 2 (Vs) −1 and down to 1.1 × 10 −4 U cm, respectively, were obtained when pulsed laser deposition was used.…”

“…Preliminary optical data published in a recent book chapter were promising. 18 Moreover, while XRD is used as a standard technique for structure analysis of SnO 2 :Ta, [19][20][21][22][23][24][25][26][27][28][29][31][32][33][34][35][36] only two papers discussed Raman data of the material. 33,35 Mientus et al 33 stated that they observed weak, broad features in the Raman spectra of as-deposited SnO 2 :Ta lms and ascribed them to the glass substrate.…”

“…18 Moreover, while XRD is used as a standard technique for structure analysis of SnO 2 :Ta, [19][20][21][22][23][24][25][26][27][28][29][31][32][33][34][35][36] only two papers discussed Raman data of the material. 33,35 Mientus et al 33 stated that they observed weak, broad features in the Raman spectra of as-deposited SnO 2 :Ta lms and ascribed them to the glass substrate. 33 Ramarajan et al published a Raman spectrum with a broad band ranging from 300 cm −1 to 700 cm −1 with a weakly developed ne structure.…”

“…33,35 Mientus et al 33 stated that they observed weak, broad features in the Raman spectra of as-deposited SnO 2 :Ta lms and ascribed them to the glass substrate. 33 Ramarajan et al published a Raman spectrum with a broad band ranging from 300 cm −1 to 700 cm −1 with a weakly developed ne structure. 35 The lack of convincing Raman data for TTO contrasts with numerous Raman studies of doped ZnO.…”