Bruce Ek

Graphene radio frequency devices on flexible substrate

Graphene is a very promising candidate for applications in flexible electronics due to its high carrier mobility and mechanical flexibility. In this paper, we present results on graphene RF devices fabricated on polyimide substrates with cutoff frequencies as high as 10 GHz. Excellent channel mobility and current saturation are observed in graphene long channel devices on polyimide. Graphene devices on polyimide also show very good temperature stability from 4.4 K to 400 K and excellent mechanical flexibility up to a bending radius of 1 mm. These demonstrated properties make graphene an excellent candidate for flexible wireless applications.

Development of graphene FETs for high frequency electronics

Recent advances in fabricating, measuring, and modeling of top-gated graphene FETs for high-frequency electronics are reviewed. By improving the oxide deposition process and reducing series resistance, an intrinsic cut-off frequency as high as 50 GHz is achieved in a 350-nm-gate graphene FET at a drain bias of 0.8 V. This fT value is the highest frequency reported to date for any graphene transistor, and it also exceeds that of Si MOSFETs at the same gate length, illustrating the potential of graphene for RF applications.

SiCGe Ternarv Allovs - Extending Si-based Heterostructures

We have synthesized Si1-y Cyand Si1-x-yCyGexalloys using Molecular Beam Epitaxy. When combined with the Si-Ge system. the new ternary system offers greater versatility and freedom in strain and bandgap engineering. Unlike the Si-Ge system the Si-C system has a high misfit (52%) and low solubility (≪ 10-6), with a propensity to compound formation, therefore. the structures are kinetically stabilized by low temperature growth. In this paper. we first describe bandgap engineering applied to this system. We then consider the growth methodology and critical thickness. Strain compensation and strain engineering using the ternary system is then described. Finally we show that thermal degradation of these films does not occur till ≫ 800°C first by interdiffusion and subsequently at higher temperatures by silicon carbide precipitation.

Atomic layer doping for Si

Abstract We report on initial results of the potential of self-limiting surface reactions for the doping of Si layers using molecular beam epitaxy (MBE) and atmospheric pressure chemical vapor deposition (APCVD). In MBE experiments using Sb as an n-type dopant a self-limiting process is obtained at a coverage of half a monolayer. No evidence of a self-limiting process has yet been found for p-type doping using B2H6 above 400 °C. In the case of MBE growth at temperatures below 400 °C the B is only partly activated (10%–20%). In APCVD grown samples B surface coverage leads to significant growth inhibition of the subsequent deposition of Si from SiCl2H2. Finally, preliminary results of atomic layer doping using AsH3 in APCVD indicate a self-limitation of chemisorption of AsH3 at about 0.1 monolayer at a temperature of 600 °C; however, subsequent growth of Si leads to a smearing out of the As due to segregation and to the residence time of As in the system.

Growth and strain symmetrization of Si/Ge/C/Sn quaternary alloys by molecular beam epitaxy

We have synthesized, via molecular beam epitaxy alloys of SixSnyC1−x−y with symmetric strain. In this work we report the growth of systems with varying compositions/band gaps including the first silicon‐based quaternary (Si/Ge/Sn/C) system, which offers an additional degree of freedom for strain and band gap engineering in Si‐based alloys. We report the growth of Si.955Sn.03C.015 alloys up to 4500 A in thickness and quaternaries of composition in the neighborhood of Si.835Ge.125Sn.03C.01. Infrared absorption spectroscopy and photoluminescence data have provided evidence of the potential for significant band gap modification in these alloys.

Effect of dual gate control on the alternating current performance of graphene radio frequency device

The excellent electrical properties of graphene, such as its high carrier mobility, gate tunability, and mechanical flexibility makes it a very promising material for radio frequency (RF) electronics. Here we study the impact of top and bottom gate control on the essential performance metrics of graphene RF transistors. We find that the maximum cut-off frequency improves as the bottom gate voltage is tuned towards the same polarity as the top gate bias voltage. These results can be explained by the bottom-gate tunable doping of the graphene underneath the metal contacts and in the under-lap region. These effects become more dramatic with device down-scaling. We also find that the minimum output conductance occurs, when the drain voltage roughly equals an effective gate voltage (Veff≈VTG+VBG⋅CBG/CTG, where VTG and VBG are top and bottom gate voltage, CTG and CBG are the respective gate capacitance). The minimum output conductance is reduced as the bottom gate bias increases, due to the stronger control of ...

Thermally Induced Precipitation of Silicon Carbide in a Semiconductor Matrix-Application to Nanoparticle Fabrication.

In this work the Si1-y Cy random alloy is used as a starting point for the creation of nano particles of β-SiC with the same lattice orientation as the Si lattice in which they are grown. These nano particles are between 3 and 8 nm in diameter and are randomly dispersed throughout the Si1-y Cy region, where 0.005<y<0.05. This ability to produce quantum antidots of wide bandgap material within the Si matrix should enable the exploration of mesoscopic phenomena.

Si1-yCy Alloys — Extending Si-Based Heterostructure Engineering

Si1− y C y alloy single layers and strained layer superlattices on Si (100) with C concentrations of up to a few percent have been synthesized using Solid Source Molecular Beam Epitaxy. While the presence of even small surface contaminants containing carbon disrupts Si epitaxy, we show that co-evaporated Si and C can yield defect-free epitaxial films with C concentrations up to 5 atomic %. Critical parameters include growth temperature, the total Si growth rate and the atom flux ratio. Outside the acceptable process window, the films are highly twinned, and in some cases amorphous. In addition, growth temperature also plays a significant role in preventing twinning and islanding. Low growth temperature also suppresses the precipitation of β-SiC and leads to the formation of pseudomorphic Si1− y C y random alloys. Secondary Ion Mass Spectroscopy, X-ray Diffraction, Raman Spectroscopy, and electron microscopy have been used to verify that the layers are indeed substitutional alloys without silicon carbide precipitation. We have also studied the thermal stability of these layers and find that the layers degrade by qualitatively different mechanisms: interdiffusion at low temperatures, and silicon carbide precipitation at high temperatures. The ability to synthesize and process these alloy layers in combination with Si and Si1− x Ge x layers, allows for exploitation of the ternary system and the possibility of more flexible bandgap engineering in a Si-based technology.