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Phase Formation Behavior in Ultrathin Iron Oxide.
Indrek Jõgi1, T Jesper Jacobsson, Mattis Fondell
1Institute of Physics, University of Tartu , Riia 142, Tartu 51014, Estonia.
Langmuir : the ACS Journal of Surfaces and Colloids
|October 28, 2015
Summary
Hybrid atomic layer deposition (ALD) and pulsed chemical vapor deposition (pCVD) enables phase-pure hematite film growth. This method minimizes substrate sensitivity for applications like solar hydrogen production, even for ultrathin films.
Area of Science:
- Materials Science
- Nanotechnology
- Surface Chemistry
Background:
- Nanostructured iron oxides, particularly hematite, are crucial for applications like gas sensors and solar hydrogen production.
- Atomic layer deposition (ALD) is a key technique for depositing ultrathin films, but it is sensitive to substrate and temperature, affecting film properties.
- Island formation and crystallite growth during ALD can limit the deposition of phase-pure iron oxide films.
Purpose of the Study:
- To investigate the formation of iron oxide phases using a hybrid atomic layer deposition/pulsed chemical vapor deposition (ALD/pCVD) approach.
- To analyze the influence of substrate type (Si(100) and SiO2) and film thickness on iron oxide phase purity.
- To explore the potential for achieving phase-pure hematite films with reduced substrate sensitivity.
Main Methods:
- Deposition of iron oxide films (3-50 nm) using a hybrid ALD/pCVD technique at the edge of the ALD window.
- Analysis of film composition and structure using X-ray diffraction, high-resolution Raman spectroscopy, and optical spectroscopy.
- Evaluation of film properties on different substrates, including Si(100), SiO2, and SnO2:F.
Main Results:
- Ultrathin films (<10 nm) showed island formation and phase-dependent crystallization, limiting phase purity on non-lattice-matching substrates.
- Films on SiO2 (10-20 nm) could be recrystallized into hematite, while those on Si(100) did not recrystallize.
- Phase-pure hematite films were directly formed using ALD/pCVD for thicknesses >20 nm, with minimal substrate influence on Si or SiO2.
- On lattice-matched SnO2:F, hematite phase formation was observed for films as thin as 3 nm.
- Optical spectroscopy revealed a quantum-confined blue-shift in the absorption edge for the thinnest films.
Conclusions:
- Hybrid ALD/pCVD offers a robust method for depositing phase-pure hematite films, overcoming limitations of standard ALD.
- Film thickness and substrate choice significantly impact phase formation and recrystallization.
- This technique shows promise for fabricating high-quality hematite films for advanced applications, particularly on lattice-matched substrates.
