Transition metal dichalcogenides (TMDs) such as MoS 2 , MoSe 2 , and WSe 2 have emerged as promising two-dimensional semiconductors. Many anticipated applications of these materials require both p-type and n-type TMDs with long term doping stability. Here we report the synthesis of substitutionally doped WSe 2 crystals using Nb and Re as p-and n-type dopants, respectively. Hall coefficient and gate-dependent transport measurements reveal drastically different doping properties between nominally 0.5% Nb-and 0.5% Re-doped WSe 2 . While 0.5% Nb-doped WSe 2 (WSe 2 :Nb) is degenerately hole doped with a nearly temperature independent carrier density of ∼ 10 19 cm −3 , electrons in 0.5% Re-doped WSe 2 (WSe 2 :Re) are largely trapped in localized states below the mobility edge and exhibit thermally activated behavior. Charge transport in both WSe 2 :Nb and WSe 2 :Re is found to be limited by Coulomb scattering from ionized impurities. Furthermore, we have fabricated vertical van der Waals junction diodes consisting of multilayers of WSe 2 :Nb and WSe 2 :Re. Finally, we have demonstrated reverse rectifying behavior as a direct proof of band-to-band tunneling in our WSe 2 :Nb/WSe 2 :Re diodes.

The antiperovskite oxide Sr 3 SnO has attracted substantial interest due to its topologically non-trivial band structure. Sr-deficient Sr 3-x SnO can become superconducting, making it a candidate intrinsic topological superconductor. Here, we show that epitaxial, phase-pure Sr 3-x SnO films can be synthesized by molecular beam epitaxy (MBE) using solid Sr and SnO 2 sources. We show that Sn-rich growth conditions result in a large amount of a Sn-rich impurity phase, which is challenging to detect in x-ray diffraction. Carrier densities and the amount of the impurity phase change systematically with the growth conditions, indicating that MBE provides excellent control over the films' stoichiometry. We discuss the electrical properties, including quantum interference phenomena, which support the topological nature of the films.

BaSnO3, a high mobility perovskite oxide, is an attractive material for oxide-based electronic devices. However, in addition to low-field mobility, high-field transport properties such as the saturation velocity of carriers play a major role in determining device performance. We report on the experimental measurement of electron saturation velocity in La-doped BaSnO3 thin films for a range of doping densities. Predicted saturation velocities based on an LO-phonon emission model show good agreement with measurements indicating that optical phonon emission is the likely mechanism for velocity saturation in La-doped BaSnO3 films. Density-dependent saturation velocity in the range of 1.6x10 7 cm/s reducing to 2x10 6 cm/s is predicted for δ-doped BaSnO3 channels with carrier densities ranging from 10 13 cm -2 to 2x10 14 cm -2 respectively. These results are expected to aid the informed design of BaSnO3 as the active material for high-charge density electronic transistors.

We report an electron transport study of an La-doped perovskite oxide BaSnO3 thin film grown by molecular beam epitaxy on (110) DyScO3 as a function of electron concentration, by etching the film step-by-step with nanometer precision. Inductively coupled plasma-reactive ion etching with BCl3/Ar plasma is used for etching depth control. The local doping and electron density are experimentally determined after each etching step. The results show that the electron mobility is dominated by threading dislocations if the electron concentration is below 7.8 × 1019 cm−3, while ionized impurities and phonon scattering become more dominant at electron concentrations greater than 1.2 × 1020 cm−3. The charging state of thread dislocations is estimated to be 6.2. Furthermore, using the etch process to control the electron concentration and channel thickness, a gate-recessed ferroelectric field effect transistor is fabricated with 10 nm HfO2 as a gate dielectric. The device exhibits a saturation current of 29.9 mA/mm with a current on/off ratio of Ion/Ioff = 8.3 × 108 and a ferroelectric polarization charge density of 1.9 × 1013 cm−2. Under the forward gate bias sweep, the device operates in the enhancement mode with a threshold voltage of 3 V. Under the reverse gate sweeping bias, the device operates in the depletion mode with a threshold voltage of –1.5 V.

The possible connection between superconductivity and polar order in SrTiO3 has been discussed extensively in the recent literature. Here, the thickness dependence of the superconducting and ferroelectric transitions in strained, epitaxial films of SrTiO3 films is studied. Both superconductivity and ferroelectricity are absent in thinner films (25 nm and below). We discuss the possible origins of the thickness dependence of both phenomena.