Nov 28, 2008
Atomic layer deposition to create quantum-confined nanostructures
Unique quantum-level optical effects have been demonstrated using various deposition techniques that can produce low crystallite-size films. It is well known that quantum confinement (QC) can occur in semiconducting nanostructures, which results in a blue shift in bandgap. Atomic layer deposition (ALD) is an ideal technique to achieve such low-dimensional films since it can deposit films with Angstrom-level precision. Many studies have described the surface chemistry required for ideal growth and a wide variety of materials have been created using this technique. ALD research has picked up more steam recently due to the drive of the semiconductor industry to go to smaller and smaller dimensions.
It is clear that low-dimensional films can have significantly different properties than those of their bulk counterparts. We felt that the ALD body of literature would be significantly advanced if QC were experimentally demonstrated, as it would then offer an alternative process to the more standard molecular beam epitaxy (MBE) to fabricate such structures. ALD offers the added advantage of not being line-of-sight dependent, as the gas-phase molecules only chemisorb to surfaces (and surfaces of any geometry) that have proper functional groups. So developing ALD techniques that deposit quantum-confined layers can allow QC attributes to be added to innumerable flat, porous or even particle-based substrates and enhance a variety of applications.
In this study, recently published in Nanotechnology, amorphous TiO2 ALD films of 4–15 nm in total thickness were deposited on wafers and the absorbance band edge for each was measured using spectroscopic ellipsometry (figure 1). We used sound statistical methods to effectively design the experiments and were subsequently able to model the parameters that best described the change in bandgap. Using a regression line based on significant factors, we obtained values for the static dielectric constant and the effective mass of the electrons and holes as described in the Brus model shift. We explicitly showed that even in nanoscale amorphous TiO2 films, QC can occur and that the effective mass is approximately an order of magnitude lower than literature values. ALD may become a commercially viable technique to fabricate quantum-confined nanostructures.
About the author
This work was performed at the University of Colorado at Boulder. King will be completing his PhD this semester and has started a company (Boulder Surface Dynamics, Inc) to continue to work with quantum-confined nanostructures using ALD technology for various commercial applications. His thesis work pertains to studies of the optical properties of ALD films both on flat surfaces and on particle substrates. His advisor, Prof. Alan Weimer, continues to offer projects involving the functionalization of particles using ALD techniques. Prof. Weimer and Prof. Steven George started a company (ALD NanoSolutions, Inc) based on the particle coating technology platform developed in their labs.