Optical Dipole Structure and Orientation of GaN Defect Single-Photon Emitters

GaN has recently been shown to host bright, photostable, defect single-photon emitters in the 600–700 nm wavelength range that are promising for quantum applications. The nature and origin of these defect emitters remain elusive. In this work, we study the optical dipole structures and orientations of these defect emitters using the defocused imaging technique. In this technique, the far-field radiation pattern of an emitter in the Fourier plane is imaged to obtain information about the structure of the optical dipole moment and its orientation in 3D. Our experimental results, backed by numerical simulations, show that these defect emitters in GaN exhibit a single dipole moment that is oriented almost perpendicular to the wurtzite crystal c-axis. Data collected from many different emitters show that the angular orientation of the dipole moment in the plane perpendicular to the c-axis exhibits a distribution that shows peaks centered at the angles corresponding to the nearest Ga–N bonds and also at the angles corresponding to the nearest Ga–Ga (or N–N) directions. Moreover, the in-plane angular distribution shows little difference among defect emitters with different emission wavelengths in the 600–700 nm range. Our work sheds light on the nature and origin of these GaN defect emitters

Role of Metal Contacts in Designing High-Performance Monolayer n‑Type WSe₂ Field Effect Transistors

This work presents a systematic study toward the design and first demonstration of high-performance n-type monolayer tungsten diselenide (WSe₂) field effect transistors (FET) by selecting the contact metal based on understanding the physics of contact between metal and monolayer WSe₂. Device measurements supported by ab initio density functional theory (DFT) calculations indicate that the d-orbitals of the contact metal play a key role in forming low resistance ohmic contacts with monolayer WSe₂. On the basis of this understanding, indium (In) leads to small ohmic contact resistance with WSe₂ and consequently, back-gated In–WSe₂ FETs attained a record ON-current of 210 μA/μm, which is the highest value achieved in any monolayer transition-metal dichalcogenide- (TMD) based FET to date. An electron mobility of 142 cm²/V·s (with an ON/OFF current ratio exceeding 10⁶) is also achieved with In–WSe₂ FETs at room temperature. This is the highest electron mobility reported for any back gated monolayer TMD material till date. The performance of n-type monolayer WSe₂ FET was further improved by Al₂O₃ deposition on top of WSe₂ to suppress the Coulomb scattering. Under the high-κ dielectric environment, electron mobility of Ag–WSe₂ FET reached ∼202 cm²/V·s with an ON/OFF ratio of over 10⁶ and a high ON-current of 205 μA/μm. In tandem with a recent report of p-type monolayer WSe₂ FET (Fang, H. et al. Nano Lett. 2012, 12, (7), 3788−3792), this demonstration of a high-performance n-type monolayer WSe₂ FET corroborates the superb potential of WSe₂ for complementary digital logic applications

High-Performance, Highly Bendable MoS₂ Transistors with High‑K Dielectrics for Flexible Low-Power Systems

While there has been increasing studies of MoS₂ and other two-dimensional (2D) semiconducting dichalcogenides on hard conventional substrates, experimental or analytical studies on flexible substrates has been very limited so far, even though these 2D crystals are understood to have greater prospects for flexible smart systems. In this article, we report detailed studies of MoS₂ transistors on industrial plastic sheets. Transistor characteristics afford more than 100x improvement in the ON/OFF current ratio and 4x enhancement in mobility compared to previous flexible MoS₂ devices. Mechanical studies reveal robust electronic properties down to a bending radius of 1 mm which is comparable to previous reports for flexible graphene transistors. Experimental investigation identifies that crack formation in the dielectric is the responsible failure mechanism demonstrating that the mechanical properties of the dielectric layer is critical for realizing flexible electronics that can accommodate high strain. Our uniaxial tensile tests have revealed that atomic-layer-deposited HfO₂ and Al₂O₃ films have very similar crack onset strain. However, crack propagation is slower in HfO₂ dielectric compared to Al₂O₃ dielectric, suggesting a subcritical fracture mechanism in the thin oxide films. Rigorous mechanics modeling provides guidance for achieving flexible MoS₂ transistors that are reliable at sub-mm bending radius

Hot Electron Transistor with van der Waals Base-Collector Heterojunction and High-Performance GaN Emitter

Single layer graphene is an ideal material for the base layer of hot electron transistors (HETs) for potential terahertz (THz) applications. The ultrathin body and exceptionally long mean free path maximizes the probability for ballistic transport across the base of the HET. We demonstrate for the first time the operation of a high-performance HET using a graphene/WSe₂ van der Waals (vdW) heterostructure as a base-collector barrier. The resulting device with a GaN/AlN heterojunction as emitter, exhibits a current density of 50 A/cm², direct current gain above 3 and 75% injection efficiency, which are record values among graphene-base HETs. These results not only provide a scheme to overcome the limitations of graphene-base HETs toward THz operation but are also the first demonstration of a GaN/vdW heterostructure in HETs, revealing the potential for novel electronic and optoelectronic applications

Scanning Tunneling Microscopy and Spectroscopy of Air Exposure Effects on Molecular Beam Epitaxy Grown WSe₂ Monolayers and Bilayers

The effect of air exposure on 2H-WSe₂/HOPG is determined <i>via</i> scanning tunneling microscopy (STM). WSe₂ was grown by molecular beam epitaxy on highly oriented pyrolytic graphite (HOPG), and afterward, a Se adlayer was deposited <i>in situ</i> on WSe₂/HOPG to prevent unintentional oxidation during transferring from the growth chamber to the STM chamber. After annealing at 773 K to remove the Se adlayer, STM images show that WSe₂ layers nucleate at both step edges and terraces of the HOPG. Exposure to air for 1 week and 9 weeks caused air-induced adsorbates to be deposited on the WSe₂ surface; however, the band gap of the terraces remained unaffected and nearly identical to those on decapped WSe₂. The air-induced adsorbates can be removed by annealing at 523 K. In contrast to WSe₂ terraces, air exposure caused the edges of the WSe₂ to oxidize and form protrusions, resulting in a larger band gap in the scanning tunneling spectra compared to the terraces of air-exposed WSe₂ monolayers. The preferential oxidation at the WSe₂ edges compared to the terraces is likely the result of dangling edge bonds. In the absence of air exposure, the dangling edge bonds had a smaller band gap compared to the terraces and a shift of about 0.73 eV in the Fermi level toward the valence band. However, after air exposure, the band gap of the oxidized WSe₂ edges became about 1.08 eV larger than that of the WSe₂ terraces, resulting in the electronic passivation of the WSe₂