Lin You

Influence of Metal–MoS2 Interface on MoS2 Transistor Performance: Comparison of Ag and Ti Contacts

In this work, we compare the electrical characteristics of MoS2 field-effect transistors (FETs) with Ag source/drain contacts with those with Ti and demonstrate that the metal-MoS2 interface is crucial to the device performance. MoS2 FETs with Ag contacts show more than 60 times higher ON-state current than those with Ti contacts. In order to better understand the mechanism of the better performance with Ag contacts, 5 nm Au/5 nm Ag (contact layer) or 5 nm Au/5 nm Ti film was deposited onto MoS2 monolayers and few layers, and the topography of metal films was characterized using scanning electron microscopy and atomic force microscopy. The surface morphology shows that, while there exist pinholes in Au/Ti film on MoS2, Au/Ag forms a smoother and denser film. Raman spectroscopy was carried out to investigate the metal-MoS2 interface. The Raman spectra from MoS2 covered with Au/Ag or Au/Ti film reveal that Ag or Ti is in direct contact with MoS2. Our findings show that the smoother and denser Au/Ag contacts lead to higher carrier transport efficiency.

SnTe field effect transistors and the anomalous electrical response of structural phase transition

SnTe is a conventional thermoelectric material and has been newly found to be a topological crystalline insulator. In this work, back-gate SnTe field-effect transistors have been fabricated and fully characterized. The devices exhibit n-type transistor behaviors with excellent current-voltage characteristics and large on/off ratio (>106). The device threshold voltage, conductance, mobility, and subthreshold swing have been studied and compared at different temperatures. It is found that the subthreshold swings as a function of temperature have an apparent response to the SnTe phase transition between cubic and rhombohedral structures at 110 K. The abnormal and rapid increase in subthreshold swing around the phase transition temperature may be due to the soft phonon/structure change which causes the large increase in SnTe dielectric constant. Such an interesting and remarkable electrical response to phase transition at different temperatures makes the small SnTe transistor attractive for various electronic...

Subsurface imaging of metal lines embedded in a dielectric with a scanning microwave microscope

We demonstrate the ability of the scanning microwave microscope (SMM) to detect subsurface metal lines embedded in a dielectric film with sub-micrometer resolution. The SMM was used to image 1.2 μm-wide Al–Si–Cu metal lines encapsulated with either 800 nm or 2300 nm of plasma deposited silicon dioxide. Both the reflected microwave (S 11) amplitude and phase shifted near resonance frequency while the tip scanned across these buried lines. The shallower line edge could be resolved within 900 nm ± 70 nm, while the deeper line was resolved within 1200 nm ± 260 nm. The spatial resolution obtained in this work is substantially better that the 50 μm previously reported in the literature. Our observations agree very well with the calculated change in peak frequency and phase using a simple lumped element model for an SMM with a resonant transmission line. By conducting experiments at various eigenmodes, different contrast levels and signal-to-noise ratios have been compared. With detailed sensitivity studies, centered around 9.3 GHz, it has been revealed that the highest amplitude contrast is obtained when the probe microwave frequency matches the exact resonance frequency of the experimental setup. By RLC equivalent circuit modeling of the tip-sample system, two competing effects have been identified to account for the positive and negative S 11 amplitude and phase contrasts, which can be leveraged to further improve the contrast and resolution.

Electromagnetic field test structure chip for back end of the line metrology

A test chip to produce known and controllable gradients of surface potential and magnetic field at the chip surface and suitable for imaging with various types of scanning probe microscopes is presented. The purpose of the test chip is to evaluate various SPMs as metrology tools to image electro-magnetic fields within nanoelectronic devices and multi-level interconnects, and as metrology tools to detect defects in back end of line (BEOL) metallization and packaging processes. Four different levels of metal are used to create different buried structures that, when biased, will produce varying electric field and magnetic field distributions. Contacts to the chip are made via wire bonds to a printed circuit board (PCB) that allows programed external biases and ground to be applied to specific metal levels while imaging with a SPM. DC and high frequency COMSOL simulations of the test structures were conducted to determine the expected field distributions. Electric field can be imaged via scanning Kelvin force microscopy (SKFM); magnetic field via scanning magnetic force microscopy (MFM); and the capacitance of buried metal lines via scanning microwave microscopy (SMM). The combination of precisely known structures and accurate simulations will allow the spatial resolution and accuracy of various SPMs sensitive to electric field (potential) or magnetic field to be determined and improved.