A Wigner crystal, a regular electron lattice arising from strong correlation effects 1-6 , is one of the earliest predicted collective electronic states. This many-body state exhibits quantum and classical phase transitions 7 and has been proposed as a basis for quantum information processing applications 8, 9 . In semiconductor platforms, two-dimensional Wigner crystals have been observed under magnetic field 10-17 or moiré-based lattice potential 18-21 where the electron kinetic energy is strongly suppressed. Here, we report bilayer Wigner crystal formation without a magnetic or confinement field in atomically thin MoSe2 bilayers separated by hexagonal boron nitride. We observe optical signatures of robust correlated insulating states formed at symmetric (1:1) and asymmetric (4:1 and 7:1) electron doping of the two MoSe2 layers at cryogenic temperatures. We attribute these features to the bilayer Wigner crystals formed from two commensurate triangular electron lattices in each layer, stabilized via inter-layer interaction 22, 23 . These bilayer Wigner crystal phases are remarkably stable and undergo quantum and thermal melting transitions above a critical electron density of up to 6 ´ 10 12 cm -2 and at temperatures of ~40 K. Our results demonstrate that atomically thin semiconductors provide a promising new platform for realizing strongly correlated electronic states, probing their electronic and magnetic phase transitions, and developing novel applications in quantum electronics and optoelectronics 24-28 .Atomically thin heterostructures made of graphene and transition metal dichalcogenide (TMD) monolayers can host a variety of correlated electronic states [29][30][31][32][33] . Recent advances in materials growth and heterostructure fabrication have enabled the preparation of high-quality heterostructures with minimal disorder [34][35][36][37][38] . The large effective masses of charge carriers 39, 40 and the weak Coulomb screening in TMDs suppress the Fermi energy and enhance electron

Single layers of two-dimensional (2D) materials hold the promise for further miniaturization of semiconductor electronic devices. However, the metal–semiconductor contact resistance limits device performance. To mitigate this problem, we propose modulation doping, specifically a doping layer placed on the opposite side of a metal–semiconductor interface. Using first-principles calculations to obtain the band alignment, we show that the Schottky barrier height and, consequently, the contact resistance at the metal–semiconductor interface can be reduced by modulation doping. We demonstrate the feasibility of this approach for a single-layer tungsten diselenide (WSe2) channel and 2D MXene modulation doping layers, interfaced with several different metal contacts. Our results indicate that the Fermi level of the metal can be shifted across the entire band gap. This approach can be straight-forwardly generalized for other 2D semiconductors and a wide variety of doping layers.