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Epitaxial ScN(001) thin films were grown on MgO(001) substrates by dc reactive magnetron sputtering. The deposition was performed in an Ar/N2 atmosphere at 2 × 10-3 Torr at a substrate temperature of 850 °C in a high vacuum chamber with a base pressure of 10-8 Torr. In spite of oxygen contamination of 1.6 ± 1 at. %, the electrical resistivity, electron mobility, and carrier concentration obtained from a typical film grown under these conditions by room temperature Hall measurements are 0.22 mΩ cm, 106 cm2 V-1 s-1, and 2.5 × 1020 cm-3, respectively. These films exhibit remarkable thermoelectric power factors of 3.3-3.5 × 10-3 W/mK2 in the temperature range of 600 K to 840 K. The cross-plane thermal conductivity is 8.3 W/mK at 800 K yielding an estimated ZT of 0.3. Theoretical modeling of the thermoelectric properties of ScN calculated using a mean-free-path of 23 nm at 300 K is in very good agreement with the experiment. These results also demonstrate that further optimization of the power factor of ScN is possible. First-principles density functional theory combined with the site occupancy disorder technique was used to investigate the effect of oxygen contamination on the electronic structure and thermoelectric properties of ScN. The computational results suggest that oxygen atoms in ScN mix uniformly on the N site forming a homogeneous solid solution alloy. Behaving as an n-type donor, oxygen causes a shift of the Fermi level in ScN into the conduction band without altering the band structure and the density of states. [ABSTRACT FROM AUTHOR]

Possibilities to improve the Seebeck coefficient S versus electrical conductance G trade-off of diffusive composite nano-structures are explored using an electro-thermal simulation framework based on the non-equilibrium Green's function method for quantum electron transport and the lattice heat diffusion equation. We examine the role of the grain size d, potential barrier height [uppercase_phi_synonym]B, grain doping, and the lattice thermal conductivity κL using a one-dimensional model structure. For a uniform κL, simulation results show that the power factor of a composite structure may be improved over bulk with the optimum [uppercase_phi_synonym]B being about kBT, where kB and T are the Boltzmann constant and the temperature, respectively. An optimum [uppercase_phi_synonym]B occurs because the current flow near the Fermi level is not obstructed too much while S still improves due to barriers. The optimum grain size dopt is significantly longer than the momentum relaxation length λp so that G is not seriously degraded due to the barriers, and dopt is comparable to or somewhat larger than the energy relaxation length λE so that the carrier energy is not fully relaxed within the grain and |S| remains high. Simulation results also show that if κL in the barrier region is smaller than in the grain, S and power factor are further improved. In such cases, the optimum [uppercase_phi_synonym]B and dopt increase, and the power factor may improve even for [uppercase_phi_synonym]B (d) significantly higher (longer) than kBT (λE). We find that the results from this quantum mechanical approach are readily understood using a simple, semi-classical model. [ABSTRACT FROM AUTHOR]

The Seebeck coefficient (S) of composite nano-structures is theoretically explored within a self-consistent electro-thermal transport simulation framework using the non-equilibrium Green's function method and a heat diffusion equation. Seebeck coefficients are determined using numerical techniques that mimic experimental measurements. Simulation results show that, without energy relaxing scattering, the overall S of a composite structure is determined by the highest barrier within the device. For a diffusive, composite structure with energy relaxation due to electron-phonon scattering, however, the measured S is an average of the position-dependent values with the weighting factor being the lattice temperature gradient. The results stress the importance of self-consistent solutions of phonon heat transport and the resulting lattice temperature distribution in understanding the thermoelectric properties of a composite structure. It is also clarified that the measured S of a composite structure reflects its power generation performance rather than its cooling performance. The results suggest that the lattice thermal conductivity within the composite structure might be engineered to improve the power factor over the bulk by avoiding the conventional trade-off between S and the electrical conductivity. [ABSTRACT FROM AUTHOR]

The possibility of increasing the performance of thermionic cooling devices by relaxing lateral momentum conservation is examined. Upper limits for the ballistic emission current are established. It is then shown that for most cases, nonconserved lateral momentum model produces a current that exceeds this upper limit. For the case of heterojunctions with a much heavier effective mass in the barrier and with a low barrier height, however, relaxing lateral momentum may increase the current. These results can be simply understood from the general principle that the current is limited by the location, well or barrier, with the smallest number of conducting channels. They also show that within a thermionic emission framework, relaxing lateral momentum conservation does not increase the upper limit performance in most cases, and when it does, the increase is modest. More generally, however, especially when the connection to the carrier reservoir is poor and performance is well below the upper limit, relaxing lateral momentum conservation could prove beneficial. [ABSTRACT FROM AUTHOR]

The role of dimensionality on the electronic performance of thermoelectric devices is clarified using the Landauer formalism, which shows that the thermoelectric coefficients are related to the transmission, T(E), and how the conducting channels, M(E), are distributed in energy. The Landauer formalism applies from the ballistic to diffusive limits and provides a clear way to compare performance in different dimensions. It also provides a physical interpretation of the “transport distribution,” a quantity that arises in the Boltzmann transport equation approach. Quantitative comparison of thermoelectric coefficients in one, two, and three dimensions shows that the channels are utilized more effectively in lower dimensions. To realize the advantage of lower dimensionality, however, the packing density must be very high, so the thicknesses of the quantum wells or wires must be small. The potential benefits of engineering M(E) into a delta function are also investigated. When compared with a bulk semiconductor, we find the potential for ∼50% improvement in performance. The shape of M(E) improves as dimensionality decreases, but lower dimensionality itself does not guarantee better performance because it is controlled by both the shape and the magnitude of M(E). The benefits of engineering the shape of M(E) appear to be modest, but approaches to increase the magnitude of M(E) could pay large dividends. [ABSTRACT FROM AUTHOR]

A spin metal-oxide-semiconductor field-effect transistor (spin MOSFET), which combines a Schottky-barrier MOSFET with ferromagnetic source and drain contacts, is a promising device for spintronic logic. Previous simulation studies predict that this device should display a very high magnetoresistance (MR) ratio (between the cases of parallel and antiparallel magnetizations) for the case of half-metal ferromagnets (HMF). We use the nonequilibrium Green’s function formalism to describe tunneling and carrier transport in this device and to incorporate spin relaxation at the HMF-semiconductor interfaces. Spin relaxation at interfaces results in nonideal spin injection. Minority spin currents arise and dominate the leakage current for antiparallel magnetizations. This reduces the MR ratio and sets a practical limit for spin MOSFET performance. We found that MR saturates at a lower value for smaller source-to-drain bias. In addition, spin relaxation at the detector side is found to be more detrimental to MR than that at the injector side, for drain bias less than the energy difference of the minority spin edge and the Fermi level. [ABSTRACT FROM AUTHOR]

A real-space quantum transport simulator for graphene nanoribbon (GNR) metal-oxide-semiconductor field-effect transistors (MOSFETs) has been developed and used to examine the ballistic performance of GNR MOSFETs. This study focuses on the impact of quantum effects on these devices and on the effect of different type of contacts. We found that two-dimensional (2D) semi-infinite graphene contacts produce metal-induced-gap states (MIGS) in the GNR channel. These states enhance quantum tunneling, particularly in short channel devices, they cause Fermi level pinning and degrade the device performance in both the ON-state and OFF-state. Devices with infinitely long contacts having the same width as the channel do not indicate MIGS. Even without MIGS quantum tunneling effects such as band-to-band tunneling still play an important role in the device characteristics and dominate the OFF-state current. This is accurately captured in our nonequilibrium Greens’ function quantum simulations. We show that both narrow (1.4 nm width) and wider (1.8 nm width) GNRs with 12.5 nm channel length have the potential to outperform ultrascaled Si devices in terms of drive current capabilities and electrostatic control. Although their subthreshold swings under forward bias are better than in Si transistors, tunneling currents are important and prevent the achievement of the theoretical limit of 60 mV/dec. [ABSTRACT FROM AUTHOR]

The general theory for quantum simulation of cubic semiconductor n-type metal-oxide-semiconductor field-effect transistors is presented within the effective-mass equation approach. The full three-dimensional transport problem is described in terms of coupled transverse subband modes which arise due to quantum confinement along the body thickness direction. Couplings among the subbands are generated for two reasons: due to spatial variations of the confinement potential along the transport direction and due to nonalignment of the device coordinate system with the principal axes of the constant energy conduction-band ellipsoids. The problem simplifies considerably if the electrostatic potential is separable along the transport and confinement directions, and further if the potential variations along the transport direction are slow enough to prevent dipolar coupling (Zener tunneling) between subbands. In this limit, the transport problem can be solved by employing two unitary operators to transform an arbitrarily oriented constant energy ellipsoid into a regular ellipsoid with principal axes along the transport, width, and confinement directions of the device. The effective masses for several technologically important wafer orientations for silicon and germanium are calculated in this paper. [ABSTRACT FROM AUTHOR]

We develop a drift-diffusion equation that describes ballistic transport in a nanoscale metal-oxide-semiconductor field effect transistor (MOSFET). We treat injection from different contacts separately, and describe each injection with a set of extended McKelvey one-flux equations [Phys. Rev. 123, 51 (1961); 125, 1570 (1962)] that include hierarchy closure approximations appropriate for high-field ballistic transport and degenerate carrier statistics. We then reexpress the extended one-flux equations in a drift-diffusion form with a properly defined Einstein relationship. The results obtained for a nanoscale MOSFET show excellent agreement with the solution of the ballistic Boltzmann transport equation with no fitting parameters. These results show that a macroscopic transport model based on the moments of the Boltzmann transport equation can describe ballistic transport. [ABSTRACT FROM AUTHOR]

Cu2ZnSn(S,Se)4 (CZTSSe) solar cells typically exhibit high short-circuit current density (Jsc), but have reduced cell efficiencies relative to other thin film technologies due to a deficit in the open-circuit voltage (Voc), which prevent these devices from becoming commercially competitive. Recent research has attributed the low Voc in CZTSSe devices to small scale disorder that creates band tail states within the absorber band gap, but the physical processes responsible for this Voc reduction have not been elucidated. In this paper, we show that carrier recombination through nonmobile band tail states has a strong voltage dependence and is a significant performance-limiting factor, and including these effects in simulation allows us to simultaneously explain the Voc deficit, reduced fill factor, and voltage-dependent quantum efficiency with a self-consistent set of material parameters. Comparisons of numerical simulations to measured data show that reasonable values for the band tail parameters (characteristic energy, capture rate) can account for the observed low Voc, high Jsc, and voltage dependent collection efficiency. These results provide additional evidence that the presence of band tail states accounts for the low efficiencies of CZTSSe solar cells and further demonstrates that recombination through non-mobile band tail states is the dominant efficiency limiting mechanism. [ABSTRACT FROM AUTHOR]

One-dimensional (1D) and two-dimensional (2D) metal-oxide-semiconductor field-effect transistors are compared using an approach based on the top-of-the-barrier ballistic transport model. The results for model devices show that 1D and 2D transistors behave quite similarly if the electrostatics is assumed to be perfect. Distinctive features of 1D transport are difficult to observe at room temperature. The effects of band structure on I-V and C-V characteristics of Si and InAs nanowire transistors are also examined using the sp3d5s* tight-binding model. It is found that band structure effects in 1D transistors are most distinctively reflected in the drain current versus gate bias or transconductance versus gate bias for low drain bias at low temperatures. Some effects may also be observed in nanowire C-V characteristics. [ABSTRACT FROM AUTHOR]

While the effect of interfacial morphology on the short circuit current (ISC) of organic photovoltaic devices (OPVs) is well known, its impact on open circuit voltage (VOC) and fill-factor (FF) are less clear. Since the output power of a solar cell Pout=ISCVOCFF, such understanding is critical for designing high-performance, morphology-engineered OPVs. In this letter, we provide an explicit analytical proof that any effort to radically improve VOC by tailoring bulk heterojunction morphology is futile, because any increase in ISC due to larger interface area is counterbalanced by corresponding increase in recombination current, so that the upper limit of VOCBHJ cannot exceed that of the corresponding planar heterojunction devices, i.e., VOCBHJ≤VOCPHJ. We discuss the implication of this VOC-constraint on the efficiency optimization of organic solar cells. [ABSTRACT FROM AUTHOR]

Power dissipation has become a major obstacle in performance scaling of modern integrated circuits and has spurred the search for devices operating at lower voltage swing. In this letter, we study p-i-n band-to-band tunneling field effect transistors taking semiconducting carbon nanotubes as the channel material. The on current of these devices is mainly limited by the tunneling barrier properties, and phonon-scattering has only a moderate effect. We show, however, that the off current is limited by phonon absorption assisted tunneling, and thus is strongly temperature dependent. Subthreshold swings below the 60 mV/decade conventional limit can be readily achieved even at room temperature. Interestingly, although subthreshold swing degrades due to the effects of phonon scattering, it remains low under practical biasing conditions. [ABSTRACT FROM AUTHOR]

We investigate the role of electron-phonon scattering and gate bias in degrading the drive current of nanotube field-effect transistors (FETs). Optical phonon scattering significantly decreases the drive current only when gate voltage is higher than a well-defined threshold. For comparable electron-phonon coupling, a lower phonon energy leads to a larger degradation of drive current. Thus in semiconductor nanowire FETs, the drive current will be more sensitive than in carbon nanotube FETs because of the smaller phonon energies in semiconductors. Acoustic phonons and other elastic scattering mechanisms are most detrimental to nanotube FETs irrespective of biasing conditions. [ABSTRACT FROM AUTHOR]

Electronic transport in a carbon nanotube metal-oxide-semiconductor field effect transistor (MOSFET) is simulated using the nonequilibrium Green’s functions method with the account of electron-phonon scattering. For MOSFETs, ambipolar conduction is explained via phonon-assisted band-to-band (Landau–Zener) tunneling. In comparison to the ballistic case, we show that the phonon scattering shifts the onset of ambipolar conduction to more positive gate voltage (thereby increasing the off current). It is found that the subthreshold swing in ambipolar conduction can be made as steep as 40 mV/decade despite the effect of phonon scattering. [ABSTRACT FROM AUTHOR]

The question of what bandstructure produces the best thermoelectric device performance is revisited from a Landauer perspective. We find that a delta-function transport distribution function (TDF) results in operation at the Mahan-Sofo upper limit for the thermoelectric figure-of-merit, ZT. We show, however, the Mahan-Sofo upper limit itself depends on the bandwidth (BW) of the dispersion, and therefore, a finite BW dispersion produces a higher ZT when the lattice thermal conductivity is finite. Including a realistic model for scattering profoundly changes the results. Instead of a narrow band, we find that a broad BW is best. The prospects of increasing ZT through high valley degeneracy or by distorting the density-of-states are discussed from a Landauer perspective. We conclude that while there is no simple answer to the question of what bandstructure produces the best thermoelectric performance, the important considerations can be expressed in terms of three parameters derived from the bandstructure-the density-of-states, D(E), the number of channels, M(E), and the mean-free-path, λ(E). [ABSTRACT FROM AUTHOR]

The spin field effect transistor (spinFET), consisting of two ferromagnetic source/drain contacts and a Si channel, is predicted to have outstanding device and circuit performance. We carry out a rigorous numerical simulation of the spinFET based on the nonequilibrium Green's function formalism self-consistently coupled with a Poisson solver to produce the device I-V characteristics. Good agreement with the recent experiments in terms of spin injection, spin transport, and the magnetoresistance ratio (MR) is obtained. We include factors crucial for realistic devices: tunneling through a dielectric barrier, and spin relaxation at the interface and in the channel. Using these simulations, we suggest ways of optimizing the device. We propose that by choosing the right contact material and inserting tunnel oxide barriers between the source/drain and channel to filter different spins, the MR can be restored to ∼2000%, which would be beneficial to the reconfigurable logic circuit application. [ABSTRACT FROM AUTHOR]

Presents the scattering matrix calculation of the distribution function in semiconductor devices. Use of the technique in silicon devices exhibiting strong nonlocal effects; Simulation of a hot-electron, n-i-n diode; Convergence of the technique with the Poisson's equation.

Presents a transition matrix approach for simulating coupled electron/phonon/photon dynamics. Use of direct Monte Carlo simulation to calculate the transition matrix; Utilization of simple matrix multiplication to determine particle populations; Application of the technique to quantum well laser devices.