E. S. Alves

Observation of distinct electron-phonon couplings in gated bilayer graphene.

A Raman study of a back gated bilayer graphene sample is presented. The changes in the Fermi level induced by charge transfer splits the Raman G band, hardening its higher component and softening the lower one. These two components are associated with the symmetric (S) and antisymmetric vibration (AS) of the atoms in the two layers, the later one becoming Raman active due to inversion symmetry breaking. The phonon hardening and softening are explained by considering the selective coupling of the S and AS phonons with interband and intraband electron-hole pairs.

Hysteresis in the resistance of a graphene device induced by charge modulation in the substrate

We have fabricated graphene devices on lightly doped Si substrates and show that pronounced changes in resistance versus gate voltage, R(Vg), characteristics of these devices at 77 K are induced by the variation in the charge distribution in substrate with both gate voltage and illumination. The R(Vg) of the graphene devices in the dark shows remarkable changes as the carriers in the underlying substrate go through accumulation, depletion, and inversion regimes. We demonstrate the possibility of using a graphene device as an optical-latch.

Self-Induced Persistent Photoconductivity in Resonant Tunneling Devices

We report on the observation of a novel effect in resonant tunneling devices (RTD): a self‐induced persistent photoconductivity (SIPPC). The SIPPC manifests itself as a permanent shift of the resonant peak position to lower voltages, which is induced by the RTD itself and not by external sources. The SIPPC is due to the presence of DX centers in the device, which are ionized by light generated in the device itself through the recombination of electron‐hole pairs created by impact ionization of hot electrons in the depletion layer of the device.

Migration of silicon atoms in planar-doped GaAsAlGaAs modulation doped fluid effect transistor heterostructures grown by molecular beam epitaxy

Abstract Photo-Hall and Shubnikov-de Hass measurements were performed on four silicon planar-doped Al 0.3 Ga 0.7 As GaAs MODFET heterostructures. We varied the nominal silicon concentration (4.0 × 1012 and 6.2 × 1012 cm−2), the growth temperature (500 °C and 620 °C) and the distance between the planar-doped layer and the heterojunction (40 A and 100 A). We obtained the carrier mobility against carrier density curves by changing the carrier density using the persistent photoconductivity effect. An IR photo-diode was used to illuminate the samples. Parallel conduction in two channels, the heterojunction channel and the planar-doped layer, was observed for three samples. It was not observed for the sample grown at 500 °C with the 100 A spacer. The effects were explained taking account of the silicon profile. Our results also indicate that, for the samples grown at 500 °C, the interface roughness is the dominant scattering mechanism limiting the mobility of the two-dimensional electron gas formed at the heterojunction, while ionized impurities are dominant for samples grown at 620 °C.