Mapping the Electrical Properties of ZnO‐Based Transparent Conductive Oxides Grown at Room Temperature and Improved by Controlled Postdeposition Annealing

Abstract

Indium tin oxide (ITO) is the current standard state‐of‐the‐art transparent conductive oxide (TCO), given its remarkable optical and electrical properties. However, the scarcity of indium carries an important drawback for the long‐term application due to its intensive use in many optoelectronic devices such as displays, solar cells, and interactive systems. Zinc oxide‐based TCOs can be a cost‐effective and viable alternative, but the limitations imposed by their transmittance versus resistivity tradeoff still keep them behind ITO. In this work, an in‐depth study of the structural and compositional material changes induced by specific postannealing treatments is presented, based on aluminum zinc oxide (AZO) and hydrogenated AZO (AZO:H) thin films grown by rf‐magnetron sputtering at room temperature that allows an extensive understanding of the films' electrical/structural changes and the ability to tune their physical parameters to yield increasingly better performances, which put them in line with the best ITO quality standards. The present investigation comprises results of thermal annealing at atmospheric pressure, vacuum, forming gas, H2 and Ar atmospheres and plasmas. Overall the study being performed leads to a decrease in resistivity above 40%, reaching ρ ≈ 3 × 10−4 Ω cm, with an average optical transmittance in the visible region around 88%. Such results are equivalent to the properties of state‐of‐the‐art ITO.

Transparent Electrodes for Efficient Optoelectronics

“…Postthermal treatments are known to improve the resistivity of AZO by increasing its free carrier density. [147] The electron mobility of sputtered AZO has been successfully increased up to 40-67 cm 2 V −1 s −1 by incorporating a capping layer and applying a high-temperature (650°C) annealing step. [148] Considering the temperature restriction of several devices, low R sh values are therefore mainly achieved by increasing N e , at the cost of an inflated FCA.…”

Deposition of ZnO based thin films by atmospheric pressure spatial atomic layer deposition for application in solar cells

“…5 In particular, aluminium-doped zinc oxide (AZO) is a promising candidate for the replacement of ITO, thanks to its earth abundance, low cost, and non-toxicity, and especially to its equivalent properties with ITO. Recent research shows values of resistivity as low as 3.0 Â 10 À4 X cm for AZO deposited by RF magnetron sputtering at room temperature 6 and even resistivity values in the order of 10 À4 X cm by Pulsed Laser Deposition (PLD) at 300 C. 7 The combination of this low resistivity with a fairly high transmittance (88%) makes AZO one of the most promising substitute to ITO. Concerning the fabrication methods, and, in particular, in the context of new generation solar cells, the ability to design low-cost, low-temperature, roll-to-roll compatible fabrication methods is a key factor to render such photovoltaic (PV) alternatives more competitive.…”

Optical Reflectivity of Spin-Coated Multilayered ZnO and Al:ZnO Thin Films

“…(1) For AZO, the composition serves as an indium-free alternative which lowers expense due to the higher relative abundance in the Earth's crust (75 ppm for Zn opposed to 0.16 ppm for In). (13) Altering the carrier concentration of a metal oxide such as ZnO with the addition of an interstitial dopant will also affect the optical properties. (16) The refractive index of the material, dielectric constant and plasma frequency are all modulated by the charge carrier concentration while the band-gap can also be widened by the stresses produced in the crystal lattice from the inclusion of a dopant atom.…”