Abstract
Doping is generally understood as a strategy for including additional positive or negative charge carriers in a semiconductor, thereby tuning the Fermi level and changing its electronic properties in the equilibrium limit. However, because dopants also couple to all of the microscopic degrees of freedom in the host, they may also alter the nonequilibrium dynamical properties of the parent material, especially at large dopant concentrations. Here, we show how substitutional doping by tungsten at the 1 at. % level modifies the complex electronic and lattice dynamics of the phase-change material vanadium dioxide. Using femtosecond broadband spectroscopy, we compare dynamics in epitaxial thin films of pristine and tungsten-doped VO2 over the broadest wavelength and temporal ranges yet reported. We demonstrate that coupling of tungsten atoms to the host lattice modifies the early electron-phonon dynamics on a femtosecond timescale, altering in a counterintuitive way the ps-to-ns optical signatures of the phase transition. Density functional theory correctly captures the enthalpy difference between pristine and W-doped VO2 and shows how the dopant softens critical V-V phonon modes while introducing new phononic modes due to W-V bonds. While substitutional doping provides a powerful method to control the switching threshold and contrast of phase-change materials, determining how the dopant dynamically changes the broadband optical response is equally important for optoelectronics.
| Original language | English |
|---|---|
| Article number | 115148 |
| Journal | Physical Review B |
| Volume | 102 |
| Issue number | 11 |
| DOIs | |
| State | Published - Sep 2020 |
Funding
Materials preparation and laser experiments were supported by the National Science Foundation (J.N. and R.F.H.: ECS-0801985; K.A.: OIA-1832898) and the Defense Threat Reduction Agency (K.A. and R.F.H.: HDTRA1-01-1-0047). Density functional theory (DFT) calculations were partly sponsored by the Department of Energy (Grant No. DE-FG02-09ER46554: B.W., W.L., and S.T.P.), by the National Science Foundation (Grant No. DMR-1207241: B.W., S.T.P.), and by the McMinn Endowment at Vanderbilt University (S.T.P.). This research used resources of the Center for Functional Nanomaterials, which is a US DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704. Computations were performed at the National Energy Research Scientific Computing Center. Portions of this work were performed at the Vanderbilt Institute of Nanoscale Science and Engineering, using facilities renovated under NSF ARI-R2 DMR-0963361. The high-temperature XRD work was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. We thank Prof. Simon Wall and Prof. Raanan Tobey for helpful discussions.