High mobility In₂O₃:H transparent conductive oxides prepared by atomic layer deposition and solid phase crystallization

Abstract

The preparation of high‐quality In2O3:H, as transparent conductive oxide (TCO), is demonstrated at low temperatures. Amorphous In2O3:H films were deposited by atomic layer deposition at 100 °C, after which they underwent solid phase crystallization by a short anneal at 200 °C. TEM analysis has shown that this approach can yield films with a lateral grain size of a few hundred nm, resulting in electron mobility values as high as 138 cm2/V s at a device‐relevant carrier density of 1.8 × 1020 cm–3. Due to the extremely high electron mobility, the crystallized films simultaneously exhibit a very low resistivity (0.27 mΩ cm) and a negligible free carrier absorption. In conjunction with the low temperature processing, this renders these films ideal candidates for front TCO layers in for example silicon heterojunction solar cells and other sensitive optoelectronic applications. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Correlating the silicon surface passivation to the nanostructure of low-temperature a-Si:H after rapid thermal annealing

“…The image shows a sample prepared at 50 C which was capped with molybdenum oxide (MoO x ) and hydrogendoped indium oxide (In 2 O 3 :H), both prepared by atomic layer deposition (ALD). 39,40 Although the surface was not fully atomically flat, no or a very minor epitaxial growth can be observed. The occurrence of this minor epitaxy at a deposition temperature of 50 C can be understood by considering that (nano)crystalline growth is observed at a slightly higher temperature of 75 C, as is illustrated by the Raman spectra shown in Fig.…”

On the solid phase crystallization of In2O3:H transparent conductive oxide films prepared by atomic layer deposition

“…As reported in earlier work, the crystallization process improves the optical properties by leading to a strong reduction in the Drude contribution at low photon energies and to an increase in the optical bandgap. 7 For a sample prepared at 130 o C the electrical properties improve to a lesser extent after annealing, whereas expectedly annealing a sample at its deposition temperature of 200 o C does not significantly improve its electrical properties. In order to explain why the best optoelectronic properties are obtained for films prepared at the lowest deposition temperature, top-view SEM and cross-sectional TEM imaging are employed to study the crystal morphology of both as-deposited and postcrystallized samples.…”

“…Due to a lower required Ne, optical losses in the infrared (IR) due to free carrier effects are reduced. In recent years, new In2O3-based TCOs with a higher carrier mobility of >50 cm 2 /Vs have gained significant interest, examples include Zn-doped indium oxide (IZO) 2 , W-doped indium oxide (IWO) 3 , Modoped indium oxide (IMO) 4 and H-doped indium oxide (In2O3:H, also referred to as IO:H or IOH) [5][6][7] . Due to the greatly-reduced IR-losses in such TCOs, they have found direct application in various solar cell devices, mainly leading to increased short-circuit current densities Jsc when compared to conventional ITO.…”

“…13,14,15 Among these high-mobility TCOs, In2O3:H is capable of achieving the highest mobility with values even up to 138 cm 2 /Vs. 7 This material was originally developed by Koida et al, using reactive magnetron sputtering at room temperature from an In2O3 target with the addition of H2O vapour, which resulted in mostly amorphous In2O3:H films. The crucial step for achieving high μ values >100 cm 2 /Vs is a solid phase crystallization (SPC) step by thermal annealing at 160-400 o C. 5 Recently we have demonstrated a similar approach to prepare highmobility In2O3:H using an atomic layer deposition (ALD) process instead of sputtering.…”

“…Also here, a crystallization step at 150-200 o C is key to get μ values up to 138 cm 2 /Vs. 7,16 Besides achieving such high mobility values, the main advantages of the ALD process are the low temperature processing (post-crystallization can take place at temperatures as low as 150 o C), the ability to conformally deposit on non-planar surfaces, and the fact that plasma-induced damage, typically encountered during sputtering ("sputter damage") 17 , is absent during ALD. 18,19 Despite the importance of the crystallization step in realizing high-mobility In2O3:H, little is known about the crystallization process itself.…”