Fabienne Michelini

Enhanced thermopower under a time-dependent gate voltage

We derive formal expressions of time-dependent energy and heat currents through a nanoscopic device using the Keldysh nonequilibrium Green function technique. Numerical results are reported for a metal/dot/metal junction where the dot level energy is abruptly changed by a step-shaped voltage pulse. Analytical linear responses are obtained for the time-dependent thermoelectric coefficients. We show that the Seebeck coefficient can be enhanced in the transient regime up to an amount (here rising 40%) controlled by both the dot energy and the height of the voltage step.

Modeling of nanoscale solar cells: The Green's function formalism

Solar cells incorporating nano-structures represent a promising solution to overtake the Schockley-Queisser limit. On the other hand, the non-equilibrium Greens function formalism provides a sound conceptual basis for the development of quantum simulators that are needed for nanoscale devices. While this approach has already been applied to solar cells, it remains unfamiliar to most photovoltaic physicists. In this paper we show the main concepts of this formalism and illustrate it with a simple 1D model of solar cell. This model is applied to a thin film GaAs solar cell. Our investigations permit to show and analyze current flowing in the solar cell at the nanometer scale.

Flexible Photodiodes Based on Nitride Core/Shell p–n Junction Nanowires

A flexible nitride p-n photodiode is demonstrated. The device consists of a composite nanowire/polymer membrane transferred onto a flexible substrate. The active element for light sensing is a vertical array of core/shell p–n junction nanowires containing InGaN/GaN quantum wells grown by MOVPE. Electron/hole generation and transport in core/shell nanowires are modeled within nonequilibrium Green function formalism showing a good agreement with experimental results. Fully flexible transparent contacts based on a silver nanowire network are used for device fabrication, which allows bending the detector to a few millimeter curvature radius without damage. The detector shows a photoresponse at wavelengths shorter than 430 nm with a peak responsivity of 0.096 A/W at 370 nm under zero bias. The operation speed for a 0.3 × 0.3 cm2 detector patch was tested between 4 Hz and 2 kHz. The −3 dB cutoff was found to be ∼35 Hz, which is faster than the operation speed for typical photoconductive detectors and which is compatible with UV monitoring applications.

Theoretical comparison of multiple quantum wells and thick-layer designs in InGaN/GaN solar cells

This theoretical work analyzes the photovoltaic effect in non-polar InGaN/GaN solar cells. Our electronic transport model considers quantum behaviors related to confinement, tunneling, electron-phonon, and electron-photon scatterings. Based on this model, we compare a multiple quantum wells cell with its thick-layer counterpart. We show that the structure of multiple quantum wells is a promising design providing better compromise between photon-absorption and electronic transport. This balance is necessary since these two phenomena are shown to be antagonist in nanostructure based solar cells. In these devices, we also show that phonon absorption increases the short-circuit current, while phonon emission reduces the open-circuit voltage.

Three-dimensional k · p real-space quantum transport simulations of p-type nanowire transistors: Influence of ionized impurities

We present a three-dimensional quantum transport simulator for p-type nanowire transistors. This self-consistent model expresses a six-band k · p Hamiltonian within the non-equilibrium Green’s function formalism. Transport properties are analyzed with and without the presence of ionized impurities in the channel. We observe that inter-subband coupling generates a rich structure of peaks in the transmission coefficients even in the intrinsic situation. A single donor leads to a current decrease whereas its acceptor counterpart induces complicated resonant and anti-resonant features. Unlike n-type devices, our conclusions pinpoint that the p-type nanowire transistors exhibit intricate transmission variations that can potentially generate larger variability and whose modeling requires a multi-band based simulator.

Photovoltaic response in a resonant tunneling wire-dot-wire junction

Using the Greens function technique, we investigated the nonequilibrium photovoltaic response in a double barrier wire-dot-wire junction for tunneling coupling stronger than optical coupling. In the narrow window of photon-gap energy resonance, the photocurrent increases when the voltage increases from zero, which means a negative shunt conductance in the generator equivalent circuit, and forces a fill factor above one. We then show a counterintuitive behavior of such resonant tunneling photovoltaic systems: the photocurrent increases when the tunneling rate through contact decreases. The negative shunt conductance we observed hence rises in the density of states of semi-infinite wires that vanishes at band edges.

Theoretical comparison of Si, Ge, and GaAs ultrathin p-type double-gate metal oxide semiconductor transistors

Based on a self-consistent multi-band quantum transport code including hole-phonon scattering, we compare current characteristics of Si, Ge, and GaAs p-type double-gate transistors. Electronic properties are analyzed as a function of (i) transport orientation, (ii) channel material, and (iii) gate length. We first show that ⟨100⟩-oriented devices offer better characteristics than their ⟨110⟩-counterparts independently of the material choice. Our results also point out that the weaker impact of scattering in Ge produces better electrical performances in long devices, while the moderate tunneling effect makes Si more advantageous in ultimately scaled transistors. Moreover, GaAs-based devices are less advantageous for shorter lengths and do not offer a high enough ON current for longer gate lengths. According to our simulations, the performance switching between Si and Ge occurs for a gate length of 12 nm. The conclusions of the study invite then to consider ⟨100⟩-oriented double-gate devices with Si for gat...

Interband optical properties of silicon [001] quantum wells using a two-conduction-band k · p model

Using analytical k · p calculations, we are able to describe the zone-center interband optical properties of Si [001] quantum wells in agreement with first principle calculations. Within the k · p band formalism, we understand how the sp*-like character of the conduction band minimum determines a total anisotropy of the polarization. Similarly, its indirect gap nature generates atomic-scale oscillations of the optical matrix elements, which suggests a giant variability of the absorption. Our results are also in agreement with photoluminescence experiments on ultrathin Si/SiO2 films.

Quantum photovoltaics in wire-dot-wire junctions

We developed an effective tight-binding modeling for photovoltaic junctions made of a finite quantum dot chain connected to two semi-infinite quantum wires. We simulated I-V responses under resonant monochromatic illumination in the case of a two-dot junction by means of the Greens function technique. We thus showed a striking property in these dot-wire architectures: the photocurrent increases under bias.

Hot-carrier solar cell NEGF-based simulations

Ultra thin absorbers for the hot carrier solar cell applications are promising. Indeed, in these ultimate absorbers electronphonon scattering are reduced and thickness can be lower than the electron mean-free-path. In this case carriers reach contact ballistically. However it is important that the contact permits to extract these carriers. This theoretical study is about the extraction of the photogenerated carriers and particularly the ballistic extraction without any scattering. We show that quantum interaction between the ultra-thin absorber and the contact can be used to enhance the extraction. Particularly, a contact composed of a quantum well into a double barrier permits to increase the current compared to a simple contact. This improvement is due to a quantum resonance. This result is interesting for the hot carrier solar cells but also for all the ultra-thin cells.