Atomic Layer Deposition Reactor at the NTF Lab

Isvar A. Cordova

October 15, 2013

Atomic layer deposition (ALD) offers the ability to uniformly deposit conformal films over high aspect ratio nanostructure scaffolds with atomic precision. This makes ALD an extremely useful tool for the engineering of various nanomaterials. In our lab, we have constructed our own versatile ALD reactor to aid in the development of a wide range of nanodevices.

                                      Schematic of the ALD Reactor

One of the applications we are currently investigating is direct solar-to-hydrogen production by a water-splitting process that utilizes TiO2-based photoelectrochemical (PEC) anodes. Furthermore, thin TiO2 films coated by ALD over transparent conductive nanoparticle scaffolds can boost efficiency by decoupling the effective absorption length while minimizing the charge carriers’ path length to the interface with the electrolyte. However, this performance enhancement is still severely limited by the large bandgap intrinsic to TiO2, which restricts its sensitivity to photons in the UV and prevents such devices from harvesting photons in the visible range of the solar spectrum. A common method to achieve visible light absorption is to narrow the bandgap by extending the valence band edge through the substitutional doping of TiO2 with nitrogen (N:TiO2). However, the overall effect of such doping can sometimes be detrimental to the overall performance of TiO2 films and its mechanism is still not well understood, especially for thin ALD films.

To aid this knowledge, our group conducts material-property and electrochemical characterization of N:TiO2 films obtained through two mechanisms:

  1. the heat treatment of TiO2 films of various thicknesses under an NH3 atmosphere, and
  2. the direct incorporation of N into the ALD growth process in order to achieve atmic scale control and ensure uniform incorporation throughout the bulk of TiO2 films

Our goal is that this combination of approaches will elucidate the nature of surface vs. bulk effects of N incorporation will accelerate the development of more viable PEC water-splitting devices.