Showing papers by "David K. Ferry published in 2020"

Exploiting intervalley scattering to harness hot carriers in III–V solar cells

Abstract: Hot carrier solar cells offer the potential to exceed the Shockley–Queisser limit. So far, however, there has been no clear route to achieve this result. Recently, the exploitation of the satellite valleys of the solar absorber material has been proposed as a feasible approach to harness hot carriers. Here, we show that, upon photoinduced and field-aided intervalley scattering to upper L-valleys, hot carriers can be harnessed in InGaAs/AlInAs heterojunctions at voltages defined by the upper valley (~1.25 V in the ideal case) rather than the bandgap of the InGaAs absorber (0.75 eV) under practical operational conditions. The efficiency of the present system does not exceed the single bandgap limit due to a mismatch in the valley degeneracy across the n+-AlInAs/n-InGaAs interface. However, we suggest that this is not a fundamental limitation to the realization of a hot carrier solar cell. Hot carrier solar cells offer greater conversion efficiency than single junction cells but they have yet to be demonstrated in real devices. Esmaielpour et al. show that hot carriers are harnessed from metastable valleys in III–V heterojunction devices at voltages greater than the absorber bandgap.

Challenges, myths, and opportunities in hot carrier solar cells

Abstract: Hot carrier solar cells hold the promise of efficiency significantly greater than that predicted by the Shockley–Queisser limit. Consequently, there has been considerable effort to create cells that achieve this goal, but so far, this has not been realized. There are many reasons for this. Here, the principles of the concept will be discussed along with some myths that hinder the future development of such devices. Also, a new approach to the hot carrier solar cell is described along with some recent experimental results that support such an approach.

Transport in Semiconductor Mesoscopic Devices

Abstract: Modern electronics is being transformed as device size decreases to a size where the dimensions are significantly smaller than the constituent electron's mean free path. In such systems the electron motion is strongly confined resulting in dramatic changes of behaviour compared to the bulk. This book introduces the physics and applications of transport in such mesoscopic and nanoscale electronic systems and devices. The behaviour of these novel devices is influenced by numerous effects not seen in bulk semiconductors, such as the Aharonov–Bohm Effect, disorder and localization, energy quantization, electron wave interference, spin splitting, tunnelling and the quantum Hall effect to name a few. Including coverage of recent developments, and with a chapter on carbon‐based nanoelectronics, this book will provide a good course text for advanced students or as a handy reference for researchers or those entering this interdisciplinary area.

Complex Systems in Phase Space.

Abstract: The continued reduction of semiconductor device feature sizes towards the single-digit nanometer regime involves a variety of quantum effects. Modeling quantum effects in phase space in terms of the Wigner transport equation has evolved to be a very effective approach to describe such scaled down complex systems, accounting from full quantum processes to dissipation dominated transport regimes including transients. Here, we discuss the challanges, myths, and opportunities that arise in the study of these complex systems, and particularly the advantages of using phase space notions. The development of particle-based techniques for solving the transport equation and obtaining the Wigner function has led to efficient simulation approaches that couple well to the corresponding classical dynamics. One particular advantage is the ability to clearly illuminate the entanglement that can arise in the quantum system, thus allowing the direct observation of many quantum phenomena.

Valley Photovoltaics and the Search for the Hot Carrier Solar Cell

Abstract: In order to achieve a hot carrier solar cell, one must: (a) prevent significant energy loss to phonons; and (b) extract primarily the hot carriers. We have recently suggested a new approach to these problems by using metastable upper energy levels to store the photogenerated carriers. A proof of concept cell based upon an InGaAs/InAlAs structure has yielded encouraging results that suggest that valley storage is occurring and yielding a $V_{OC} > E_{G}$ . Current output, however, is restricted apparently due to a barrier between the L valleys in the two materials. New materials may provide the solution to this problem and lead to a new HCSC with greatly enhanced efficiency.

A Comprehensive Study of the Bridge Site and Substrate Relaxation Asymmetry for Methanethiol Adsorption on Au(111) at Low Coverage.

Abstract: We use dispersion-corrected density functional theory to explore the bridge-site asymmetry for methanethiol adsorbed on Au(111) with two different S-C bond orientations. We attribute the asymmetry to the intrinsic character of the Au(111) surface rather than the adsorbate. The preference for bridge-fcc versus bridge-hcp SCH3 adsorption sites is controlled by the S-C bond orientation. The system energy difference favors the bridge-fcc site by 8.1 meV on the unrelaxed Au(111) surface. Relaxing the Au substrate increased this energy difference to 26.1 meV. This asymmetry is also reflected in the atomic displacement of the relaxed Au surface. Although in both cases, the bridge-site Au atoms shift away from the fcc 3-fold hollow site, the motion is greater for the bridge-fcc allowing a more favorable geometry for the sulfur atom to bond to the bridging atoms. We confirm that the adsorption energy is strongly dependent on the S-C bond orientation and position, which can be understood in terms of a simple coordination geometry model. This work has important implications for alkanethiol surface diffusion and the structure of their self-assembled monolayers.

Wigner functions in optoelectronics: Wave-packet phase-space Monte Carlo solver for waveguide-ring coupling

Abstract: Quantum metrology, computing, and sensing are areas generating great interest in photonic devices. Quantum effects in photonic semiconductor devices are, therefore, an area of increasing interest. For short time scales, transient behaviors, and localized interactions, quantum transport solvers are needed to accurately model such behavior. Here, we employ a novel approach to solving the photon quantum Boltzmann equation utilizing a technique similar to particle Monte Carlo for acquiring the photon Wigner function, demonstrating its effectiveness at modeling quantum effects such as entanglement arising from the coupling of a waveguide to a ring resonator.