Electrical Properties of Low-Temperature Processed Sn-Doped In2O3 Thin Films: The Role of Microstructure and Oxygen Content and the Potential of Defect Modulation Doping

Andreas Klein,Joachim Brötz,Jonas Hunka,Daniel Bellet,Hervé Roussel,Getnet Kacha Deyu

doi:10.3390/ma12142232

Abstract

Low-temperature-processed ITO thin films offer the potential of overcoming the doping limit by suppressing the equilibrium of compensating oxygen interstitial defects. To elucidate this potential, electrical properties of Sn-doped InO (ITO) thin films are studied in dependence on film thickness. In-operando conductivity and Hall effect measurements during annealing of room-temperature-deposited films, together with different film thickness in different environments, allow to discriminate between the effects of crystallization, grain growth, donor activation and oxygen diffusion on carrier concentrations and mobilities. At , a control of carrier concentration by oxygen incorporation or extraction is only dominant for very thin films. The electrical properties of thicker films deposited at room temperature are mostly affected by the grain size. The remaining diffusivity of compensating oxygen defects at is sufficient to screen the high Fermi level induced by deposition of AlO using atomic layer deposition (ALD), which disables the use of defect modulation doping at this temperature. The results indicate that achieving higher carrier concentrations in ITO thin films requires a control of the oxygen pressure during deposition in combination with seed layers to enhance crystallinity or the use of near room temperature ALD.

Highlights

Transparent conductive oxides (TCOs) are key materials for electrodes in display and solar cell technologies [1,2,3,4,5,6]
Low-temperature-processed ITO thin films offer the potential of overcoming the doping limit by suppressing the equilibrium of compensating oxygen interstitial defects
The results indicate that achieving higher carrier concentrations in ITO thin films requires a control of the oxygen pressure during deposition in combination with seed layers to enhance crystallinity or the use of near room temperature atomic layer deposition (ALD)

Summary

Introduction

Transparent conductive oxides (TCOs) are key materials for electrodes in display and solar cell technologies [1,2,3,4,5,6]. The most prominent TCO materials are Sn-doped In2 O3 (ITO), Al-doped ZnO (AZO). Highest electrical conductivities of ∼104 S/cm are obtained with ITO, having carrier concentrations of 1–2 × 1021 cm−3 and mobilities of ∼40 cm2 /Vs [7]. Even higher conductivities are desirable, for example to reduce optical losses in solar cells by using thinner TCOs or wider cells in thin film modules. The conductivity can be increased either by a higher carrier concentration or a higher carrier mobility. If thermodynamic equilibrium of defect concentrations can be established, the concentration of free electrons in TCOs is limited by the formation of self-compensating intrinsic defects [6,8,9,10]

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Conclusion