  Correlated electron materials (CEMs) host a rich variety of condensed matter
phases. Vanadium dioxide (VO2) is a prototypical CEM with a
temperature-dependent metal-to-insulator (MIT) transition with a concomitant
crystal symmetry change. External control of MIT in VO2 - especially without
inducing structural changes - has been a long-standing challenge. In this work,
we design and synthesize modulation-doped VO2-based thin film heterostructures
that closely emulate a textbook example of filling control in a correlated
electron insulator. Using a combination of charge transport, hard x-ray
photoelectron spectroscopy, and structural characterization, we show that the
insulating state can be doped to achieve carrier densities greater than 5x10^21
cm^(-3) without inducing any measurable structural changes. We find that the
MIT temperature (T_MIT) continuously decreases with increasing carrier
concentration. Remarkably, the insulating state is robust even at doping
concentrations as high as ~0.2 e-/vanadium. Finally, our work reveals
modulation-doping as a viable method for electronic control of phase
transitions in correlated electron oxides with the potential for use in future
devices based on electric-field controlled phase transitions.
