Improvement of Fermi-Level Pinning and Contact Resistivity in Ti/Ge Contact Using Carbon Implantation

Effects of carbon implantation (C-imp) on the contact characteristics of Ti/Ge contact were investigated. The C-imp into Ti/Ge system was developed to reduce severe Fermi-level pinning (FLP) and to improve the thermal stability of Ti/Ge contact. The current density (J)-voltage (V) characteristics showed that the rectifying behavior of Ti/Ge contact into an Ohmic-like behavior with C-imp. The lowering of Schottky barrier height (SBH) indicated that the C-imp could mitigate FLP. In addition, it allows a lower specific contact resistivity (ρc) at the rapid thermal annealing (RTA) temperatures in a range of 450–600 °C. A secondary ion mass spectrometry (SIMS) showed that C-imp facilitates the dopant segregation at the interface. In addition, transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) mapping showed that after RTA at 600 °C, C-imp enhances the diffusion of Ge atoms into Ti layer at the interface of Ti/Ge. Thus, carbon implantation into Ge substrate can effectively reduce FLP and improve contact characteristics.

Imaging the electrostatic landscape of unstrained self-assemble GaAs quantum dots

Unstrained GaAs quantum dots are promising candidates for quantum information devices due to their optical properties, but their electronic properties have remained relatively unexplored until now. In this work, we systematically investigate the electronic structure and natural charging of GaAs quantum dots at room temperature using Kelvin probe force microscopy (KPFM). We observe a clear electrical signal from structures demonstrating a lower surface potential in the middle of the dot. We ascribe this to charge accumulation and confinement inside these structures. Our systematical investigation reveals that the change in surface potential is larger for a nominal dot filling of 2 nm and then starts to decrease for thicker GaAs layers. Using k . p calculation, we show that the confinement comes from the band banding due to the surface Fermi level pinning. Our results indicate that these self-assembled structures could be used to study physical phenomena connected to charged quantum dots like Coulomb blockade or Kondo effect.

Anomalously persistent p-type behavior of WSe2 field-effect transistors by oxidized edge-induced Fermi-level pinning

A technique to form the edge contact in two-dimensional (2D) based field-effect transistors (FETs) has been intensively studied for the purpose of achieving high mobility and also recently overcoming the...

High-specific-power flexible transition metal dichalcogenide solar cells

Semiconducting transition metal dichalcogenides (TMDs) are promising for flexible high-specific-power photovoltaics due to their ultrahigh optical absorption coefficients, desirable band gaps and self-passivated surfaces. However, challenges such as Fermi-level pinning at the metal contact–TMD interface and the inapplicability of traditional doping schemes have prevented most TMD solar cells from exceeding 2% power conversion efficiency (PCE). In addition, fabrication on flexible substrates tends to contaminate or damage TMD interfaces, further reducing performance. Here, we address these fundamental issues by employing: (1) transparent graphene contacts to mitigate Fermi-level pinning, (2) MoOx capping for doping, passivation and anti-reflection, and (3) a clean, non-damaging direct transfer method to realize devices on lightweight flexible polyimide substrates. These lead to record PCE of 5.1% and record specific power of 4.4 W g−1 for flexible TMD (WSe2) solar cells, the latter on par with prevailing thin-film solar technologies cadmium telluride, copper indium gallium selenide, amorphous silicon and III-Vs. We further project that TMD solar cells could achieve specific power up to 46 W g−1, creating unprecedented opportunities in a broad range of industries from aerospace to wearable and implantable electronics.

Probing electric properties of GaP nanowires with Kelvin probe force microscopy

Surface electronic properties of GaP nanowires were investigated using scanning probe force microscopy. I-V curves of individual free-standing NWs with different doping types were obtained. Surface Fermi level positions in the nanowires of different crystal phases and doping types were extracted using phase-modulated Kelvin probe force microscopy. The results indicate on weak Fermi level pinning in GaP nanowires. The difference between wurtzite and zinc blende GaP work function is observed.

Silicon Nitride Interface Engineering for Fermi Level Depinning and Realization of Dopant-Free MOSFETs

Problems with doping in nanoscale devices or low temperature applications are widely known. Our approach to replace the degenerate doping in source/drain (S/D)-contacts is silicon nitride interface engineering. We measured Schottky diodes and MOSFETs with very thin silicon nitride layers in between silicon and metal. Al/SiN/p-Si diodes show Fermi level depinning with increasing SiN thickness. The diode fabricated with rapid thermal nitridation at 900 ∘C reaches the theoretical value of the Schottky barrier to the conduction band ΦSB,n=0.2 eV. As a result, the contact resistivity decreases and the ambipolar behavior can be suppressed. Schottky barrier MOSFETs with depinned S/D-contacts consisting of a thin silicon nitride layer and contact metals with different work functions are fabricated to demonstrate unipolar behavior. We presented n-type behavior with Al and p-type behavior with Co on samples which only distinguish by the contact metal. Thus, the thermally grown SiN layers are a useful method suppress Fermi level pinning and enable reconfigurable contacts by choosing an appropriate metal.

Dipole-induced Ohmic contacts between monolayer Janus MoSSe and bulk metals

Utilizing a two-dimensional material in an electronic device as channel layer inevitably involves the formation of contacts with metallic electrodes. As these contacts can dramatically affect the behavior of the device, we study the electronic properties of monolayer Janus MoSSe in contact with different metallic electrodes by first-principles calculations, focusing on the differences in the characteristics of contacts with the two sides of MoSSe. In particular, we demonstrate that the Fermi level pinning is different for the two sides of MoSSe, with the magnitude resembling that of MoS2 or MoSe2, while both sides can form Ohmic contacts with common electrode materials without any further adaptation, which is an outstanding advantage over MoS2 and MoSe2.