  In an ultrathin topological insulator (TI) film, a hybridization gap opens in
the TI surface states, and the system is expected to become either a trivial
insulator or a quantum spin Hall insulator when the chemical potential is
within the hybridization gap. Here we show, however, that these insulating
states are destroyed by the presence of a large and long-range-correlated
disorder potential, which converts the expected insulator into a metal. We
perform transport measurements in ultrathin, dual-gated topological insulator
films as a function of temperature, gate voltage, and magnetic field, and we
observe a metallic-like, non-quantized conductivity, which exhibits a weak
antilocalization-like cusp at the low magnetic field and gives way to a
nonsaturating linear magnetoresistance at large field. We explain these results
by considering the disordered network of electron- and hole-type puddles
induced by charged impurities. We argue theoretically that such disorder can
produce an insulator-to-metal transition as a function of increasing disorder
strength, and we derive a condition on the band gap and the impurity
concentration necessary to observe the insulating state. We also explain the
linear magnetoresistance in terms of strong spatial fluctuations of the local
conductivity, using both numerical simulations and a theoretical scaling
argument.
