| AVS 62nd International Symposium & Exhibition | |
| Thin Film | Wednesday Sessions |
| Session TF+SS-WeM |
| Session: | ALD Surface Reactions and Precursors |
| Presenter: | Bart Macco, Eindhoven University of Technology, Netherlands |
| Authors: | B. Macco, Eindhoven University of Technology, Netherlands H.C.M. Knoops, Oxford Instruments Plasma Technology, UK M.A. Verheijen, Eindhoven University of Technology, Netherlands W.M.M. Kessels, Eindhoven University of Technology, Netherlands |
| Correspondent: | Click to Email |
Recently, we have reported on an atomic layer deposition (ALD) process to prepare H-doped indium oxide (In2O3:H) transparent conductive oxides (TCOs) with an extremely high carrier mobility (138 cm2/Vs) and low resistivity (0.27 mΩcm) at low processing temperatures (<200 oC) [1]. This high carrier mobility is especially promising for silicon heterojunction solar cell applications, as it allows for a low resistivity at low carrier density, thereby nullifying parasitic free carrier absorption in the infrared. Here we focus on new insights into the physical mechanisms during the preparation process and explain how such high mobility can be obtained through analysis of the electron scattering and doping mechanisms.
The preparation starts with ALD of In2O3:H at 100 oC using InCp, H2O and O2 as growth precursors. The films are amorphous, although a low density of very small crystallites is present. Subsequently the films are crystallized by annealing at 150-200 oC. Electron microscopy reveals that crystallization proceeds from grain growth from the pre-existing crystallites without additional nucleation, which makes the final grain size and optoelectronic properties independent of annealing temperature. The resulting crystals extend over the film thickness of 75 nm and have a typical lateral size of a few hundred nm. Analysis of the grain growth by electron microscopy shows a thermally activated behavior, with an activation energy of 1.4 eV.
A combination of temperature-dependent Hall measurements and spectroscopic ellipsometry has been employed to distinguish between the various scattering mechanisms and dopants in crystallized In2O3:H films. Key results include the fact that carrier mobility is only limited by ionized impurity and phonon scattering processes and that other extrinsic defect scattering such as neutral impurity and grain boundary scattering can be neglected. Since only unavoidable scattering processes play a role, this means that this TCO has the highest possible mobility at this carrier density. The analysis also excludes a significant contribution from doubly charged donors (i.e. oxygen vacancies), and the source of doping is confirmed to be singly charged H. Furthermore, by comparison of the absolute H-concentration and the carrier density in crystallized films, it is deduced that <4 % of the incorporated H is an active dopant in crystallized films. Therefore, it can be concluded that inactive H atoms do not contribute to defect scattering, which explains why In2O3:H films are capable of achieving a much higher carrier mobility than conventional In2O3:Sn films.
[1] Macco et al., P.S.S. - Rapid Res. Lett., DOI: 10.1002/pssr.201409426