J. C. Tsang

Photocurrent Imaging and Efficient Photon Detection in a Graphene Transistor

We measure the channel potential of a graphene transistor using a scanning photocurrent imaging technique. We show that at a certain gate bias, the impact of the metal on the channel potential profile extends into the channel for more than one-third of the total channel length from both source and drain sides; hence, most of the channel is affected by the metal. The potential barrier between the metal-controlled graphene and bulk graphene channel is also measured at various gate biases. As the gate bias exceeds the Dirac point voltage, VDirac, the original p-type graphene channel turns into a p-n-p channel. When light is focused on the p-n junctions, an impressive external responsivity of 0.001 A/W is achieved, given that only a single layer of atoms are involved in photon detection.

Measurements of alloy composition and strain in thin GexSi1−x layers

The utility of Raman spectroscopy for the simultaneous determination of composition and strain in thin GexSi1−x layers has been investigated. Using data from the literature and new data for the strain shift of the Si‐Si phonon mode presented here, we show how Raman spectra provide several different means of measuring composition and strain in samples as thin as 200 A. We demonstrate that for largely relaxed layers with compositions near x=0.30, Raman scattering can measure the composition, x, with an accuracy of ±0.015 and the strain, e, with an accuracy ±0.0025. The accuracy of the alloy composition obtained from Raman spectra is comparable or, in the case of very thin layers, superior to that measured by other techniques such as x‐ray diffraction, electron microprobe, and Auger electron spectroscopy.

Luminescence degradation in porous silicon

We have studied the stability of the luminescence from porous Si in the presence of a variety of ambient gases (e.g., N2, H2, forming gas, and O2). Although the optical properties are fairly stable under most conditions, illumination in the presence of O2 causes a substantial decrease in luminescence efficiency. Infrared measurements show that the surfaces of degraded samples are oxidized. The luminescence lifetime of the degraded material is found to be substantially reduced, and the density of Si dangling bonds increases by more than two orders of magnitude, which suggests that oxidation of the surface introduces nonradiative recombination channels. These observations indicate that the electronic properties at the surface of the porous Si play a key role in obtaining efficient luminescence from this material.

Chemical doping and electron-hole conduction asymmetry in graphene devices.

We investigate poly(ethylene imine) and diazonium salts as stable, complementary dopants on graphene. Transport in graphene devices doped with these molecules exhibits asymmetry in electron and hole conductance. The conductance of one carrier is preserved, while the conductance of the other carrier decreases. Simulations based on nonequilibrium Greens function formalism suggest that the origin of this asymmetry is imbalanced carrier injection from the graphene electrodes caused by misalignment of the electrode and channel neutrality points.

Growth and strain compensation effects in the ternary Si1−x−yGexCy alloy system

Strain compensation is an important aspect of heterostructure engineering. In this letter, we discuss the synthesis of pseudomorphic Si1−yCy and Si1−x−yGexCy alloy layers on a silicon (100) substrate by molecular beam epitaxy using solid sources and the controlled strain compensation that results from the introduction of the ternary system. The introduction of C into substitutional sites in the crystal lattice is kinetically stabilized by low‐temperature growth conditions (400–550 °C) against thermodynamically favored silicon‐carbide phases. The lattice constant in Ge is about 4% larger than in Si, whereas in diamond it is 52% smaller. Consequently, the compressive strain caused by 10.8% Ge in a pseudomorphic Si1−xGex alloy can be compensated by adding about 1% carbon into substitutional lattice sites of the film assuming Vegard’s law of linear change of the lattice constant in the alloy as a function of the composition. Using x‐ray diffraction, we observe a partial strain compensation in Si0.75−yGe0.25Cy...

Role of contacts in graphene transistors: A scanning photocurrent study

A near-field scanning optical microscope is used to locally induce photocurrent in a graphene transistor with high spatial resolution. By analyzing the spatially resolved photoresponse, we find that in the

Bright Infrared Emission from Electrically Induced Excitons in Carbon Nanotubes

n

Energy Dissipation in Graphene Field-Effect Transistors

-type conduction regime a

Enhanced raman scattering from carbon layers on silver

p\text{\ensuremath{-}}n\text{\ensuremath{-}}p

Synthesis of Si1−yCy alloys by molecular beam epitaxy

structure forms along the graphene device due to the doping of the graphene by the metal contacts. The modification of the electronic structure is not limited only underneath the metal electrodes but extends