FUNDAMENTALS OF SEMICONDIJC'IOK PHYSICS AND DEVICES

Bloch electron conductivity perpendicular to the layers of a superlattice (period d) is evaluated using an extension of the balance-equation approach [X.L. Lei and C. S. Ting, Phys. Rev. B 32, 1112] to narrow-band transport. The perpendicular peak drift velocity v p and the critical field E c , at which the drift velocity peaks, are analyzed as functions of miniband width. Our theoretical prediction that E c d increases with decreasing miniband width agrees well with the data of Sibille et aL [Phys. Rev. Lett. 64, 52 (1990)], even for the samples of narrowest miniband width in their experiment.

We present a theoretical analysis of the effect of inelastic electron scattering on current and its fluctuations in a mesoscopic quantum dot (QD) connected to two leads, based on a recently developed nonperturbative technique involving the approximate mapping of the many-body electron-phonon coupling problem onto a multichannel single-electron scattering problem. In this, we apply the B\"uttiker scattering theory of shot noise for a two-terminal mesoscopic device to the multichannel case with differing weight factors and examine zero-frequency shot noise for two special cases: (i) a single-molecule QD and (ii) coupled semiconductor QDs. The nonequilibrium Green's function method facilitates calculation of single-electron transmission and reflection amplitudes for inelastic processes under nonequilibrium conditions in the mapping model. For the single-molecule QD we find that, in the presence of the electron-phonon interaction, both differential conductance and differential shot noise display additional peaks as bias-voltage increases due to phonon-assisted processes. In the case of coupled QDs, our nonperturbative calculations account for the electron-phonon interaction on an equal footing with couplings to the leads, as well as the coupling between the two dots. Our results exhibit oscillations in both the current and shot noise as functions of the energy difference between the two QDs, resulting from the spontaneous emission of phonons in the nonlinear transport process. In the "zero-phonon" resonant tunneling regime, the shot noise exhibits a double peak, while in the "one-phonon" region, only a single peak appears.Comment: 10 pages, 6 figures, some minor changes, accepted by Phys. Rev.

Graphene based field effect transistor for the detection of ammonia J. Appl. Phys. 112, 064304 (2012) Unipolar behavior of asymmetrically doped strained Si0.5Ge0.5 tunneling field-effect transistors Appl. Phys. Lett. 101, 123501 (2012) Efficient physical-thermal model for thermal effects in AlGaN/GaN high electron mobility transistors Appl. Phys. Lett. 101, 122101 (2012) Light/negative bias stress instabilities in indium gallium zinc oxide thin film transistors explained by creation of a double donorThe terahertz absorption spectrum of plasmon modes in a grid-gated double-quantum-well ͑DQW͒ field-effect transistor structure is analyzed theoretically and numerically using a first principles electromagnetic approach and is shown to faithfully reproduce important physical features of recent experimental observations. We find that the essential character of the response-multiple resonances corresponding to spatial harmonics of standing plasmons under the metal grating-is caused by the static spatial modulation of electron density in the channel. Higher order plasmon modes become more optically active as the depth of the electron density modulation in the DQW tends towards unity. The maximum absorbance, at plasma resonance, is shown to be 1/2. Furthermore, the strongest absorption also occurs when the standing plasmon resonance coincides with the fundamental dipole mode of the ungated portion of the channel.

Motivated by a recent experiment on nonlinear tunneling in a suspended carbon nanotube connected to two normal electrodes ͓S. Sapmaz et al., Phys. Rev. Lett. 96, 26801 ͑2006͔͒, we investigate nonequilibrium vibration-mediated sequential tunneling through a molecular quantum dot with two electronic orbitals asymmetrically coupled to two electrodes and strongly interacting with an internal vibrational mode, which is itself weakly coupled to a dissipative phonon bath. For this purpose, we establish rate equations using a generic quantum Langevin equation approach. Based on these equations, we study in detail the current-voltage characteristics and zero-frequency shot noise, paying special attention to the advancement or postponement of the appearance of negative differential conductance and super-Poissonian current noise resulting from electronphonon-coupling induced selective unidirectional cascades of single-electron transitions.

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Self-consistent field theory is used to obtain the non-local plasmon dispersion relation of monolayer graphene which is Coulomb-coupled to a thick conductor. We calculate numerically the undamped plasmon excitation spectrum for arbitrary wave number. For gapped graphene, both the lowfrequency (acoustic) and high frequency (surface) plasmons may lie within an undamped opening in the particle-hole region. Furthermore, we obtain plasmon excitations in a region of frequency-wave vector space which do not exist for free-standing gapped graphene.

We present a theoretical analysis of several aspects of nonequilibirum cotunneling through a strong Coulomb-blockaded quantum dot (QD) subject to a finite magnetic field in the weak coupling limit. We carry this out by developing a generic quantum Heisenberg-Langevin equation approach leading to a set of Bloch dynamical equations which describe the nonequilibrium cotunneling in a convenient and compact way. These equations describe the time evolution of the spin variables of the QD explicitly in terms of the response and correlation functions of the free reservoir variables. This scheme not only provides analytical expressions for the relaxation and decoherence of the localized spin induced by cotunneling, but it also facilitates evaluations of the nonequilibrium magnetization, the charge current, and the spin current at arbitrary bias-voltage, magnetic field, and temperature. We find that all cotunneling events produce decoherence, but relaxation stems only from inelastic spin-flip cotunneling processes. Moreover, our specific calculations show that cotunneling processes involving electron transfer (both spin-flip and non-spin-flip) contribute to charge current, while spinflip cotunneling processes are required to produce a net spin current in the asymmetric coupling case. We also point out that under the influence of a nonzero magnetic field, spin-flip cotunneling is an energy-consuming process requiring a sufficiently strong external bias-voltage for activation, explaining the behavior of differential conductance at low temperature: in particular, the splitting of the zero-bias anomaly in the charge current and a broad zero-magnitude "window" of differential conductance for the spin current near zero-bias-voltage.