Hot carrier solar cells were first proposed many decades ago. Over the intervening years, there has been a continuing quest to create these cells, which hold promise to shatter the Shockley–Queisser efficiency limit on single-junction solar cells. One approach considered is to use satellite valleys of the conduction band as metastable states for storing hot electrons until they can be extracted. Experimental efforts, however, have shown the presence of a barrier between the two materials, likely at the heterostructure interface between the absorber and extraction layer. Transfer across the interface is a real-space event rather than a momentum-space process. If the two bands from, and to, which the electron moves are not perfectly aligned, then tunneling must occur. The determination of the evanescent wave numbers that appear in tunneling coefficients are not the simple ones in textbooks but must be found from the full complex band structure of the two materials. Here, the nature of these evanescent states and their role in the tunneling of carriers across typical interfaces is examined using empirical pseudopotential methods.
KEYWORDS: Solar cells, Photovoltaics, Solar energy, Silicon, Perovskite, Manufacturing, Sustainability, Dye sensitized solar cells, Design, Energy efficiency
This report provides a snapshot of emerging photovoltaic (PV) technologies. It consists of concise contributions from experts in a wide range of fields including silicon, thin film, III-V, perovskite, organic, and dye-sensitized PVs. Strategies for exceeding the detailed balance limit and for light managing are presented, followed by a section detailing key applications and commercialization pathways. A section on sustainability then discusses the need for minimization of the environmental footprint in PV manufacturing and recycling. The report concludes with a perspective based on broad survey questions presented to the contributing authors regarding the needs and future evolution of PV.
KEYWORDS: Solar cells, Photovoltaics, Solar energy, Scattering, Virtual point source, Satellites, Quantum wells, Phonons, Indium gallium arsenide, Energy efficiency
Hot Carrier Solar Cells (HCSCs) are a proposed next-generation photovoltaic technology for overcoming the single-gap efficiency limit. Here, our latest work in developing protocols for effective hot carrier extraction and field aided scattering within the framework of valley photovoltaics (VP) will be presented. A study of various absorber/selective barrier material combinations provides insight into current bottlenecks towards the realization of a VP HCSC, and how these might be circumvented using several complementary experimental techniques.
Hot carrier solar cells (HCSCs) were first proposed many decades ago. Over the intervening years, there has been a continuing quest to create these cells that hold promise to shatter the Shockley–Queisser efficiency limit on single-junction solar cells. While there have been many positive and suggestive results in recent years, there remains no true operational HCSC. There are perhaps many reasons for this state. Here, many of the requirements for achieving such an HCSC will be discussed and some approaches will be modernized in terms of their science. Valley photovoltaics, in which carriers are transferred to higher-lying valleys of the conduction band will be described and the recent progress is discussed.
Time-resolved Raman spectroscopy on a subpicosecond time scale has been used to study the phonon dynamics
of both the A1(LO) and the E1(LO) phonons in InN. From the temperature-dependence of their lifetimes, we
demonstrate that both phonons decay primarily into a large wavevector TO phonon and a large wavevector TA/LA
phonon consistent with the accepted phonon dispersion relationship for wurtzite InN. Their lifetimes have been found to
decrease from 2.2 ps, at the low electron-hole pair density of 5×1017cm-3 to 0.25 ps, at the highest density
of 2×1019cm-3. Our experimental findings demonstrate that carrier-density dependence of LO phonon lifetime is a
universal phenomenon in polar semiconductors.
Subpicosecond time-resolved Raman spectroscopy has been used to measure the lifetime
of the LO phonon mode in GaN at T = 300K for photoexcited electron-hole pair density
ranging from 1016cm-3 to 2x1019cm-3. The lifetime has been found to decrease from 2.5 ps, at the lowest density to 0.35 ps, at the highest density. Our experimental findings should help resolve the recent controversy over the lifetime of LO phonon mode in GaN.
The lifetime of longitudinal optical phonon mode in GaN has been measured by
subpicosecond time-resolved Raman spectroscopy for photoexcited electron-hole pair
density ranging from 1016cm-3 to 2x1019cm-3 and at T = 300K. The lifetime has been
found to decrease from 2.5 ps, at the lowest density to 0.35 ps, at the highest density.
Possible mechanism for this observation has been discussed. Our experimental findings
help resolve the recent controversy over the lifetime of LO phonon mode in GaN.
Transient Raman spectroscopy has been used to study electron transport in a thick InN film grown on GaN at T = 300 K. Our experimental results demonstrate that under the subpicosecond laser excitation and probing, electron drift velocity in the Γ valley, which reaches as high as 7.5x107cm/sec, can exceed its steady state value by as much as 40%. Electron velocities have been found to cut off at around 2x108cm/s, significantly larger than those observed for other III-V semiconductors such as GaAs and InP. Our experimental results suggest that InN is potentially an excellent material for ultrafast electronic devices.
Non-equilibrium longitudinal optical phonons in a high quality, single crystal wurtzite structure InN sample have been studied by picosecond Raman spectroscopy. Our experimental results demonstrate that the bandgap of InN cannot be around 1.89 eV; but are consistent with a bandgap of around 0.8 eV. In addition, they disprove the idea that 0.8 eV-luminescence observed recently in InN is due to deep level radiative emission in InN.
We report experimental results on simultaneous measurement of electron as well as hole transient transport in an Al0.3Ga0.7As-based p-i-n semiconductor nanostructure by using picosecond/subpicosecond Raman spectroscopy. Electron and hole velocity overshoots are directly observed. These experimental results are discussed and explained.
Field-induced electron transport in an InxGa1-xN (x≅0.4) sample grown on GaN has been studied by subpicosecond Raman spectroscopy. Non-equilibrium electron distribution and electron drift velocity due to the presence of piezoelectric and spontaneous fields in the InxGa1-xN layer have been directly measured. The experimental results are compared with ensemble Monte Carlo calculations and reasonable agreements are obtained.
The concept of quantum computing has arisen as a methodology by which very rapid computations can be achieved. There also has been considerable discussion about physical implementations of the qubit. This has led, in recent years, to a situation in which quantum computing and quantum information theory are being rapidly developed. In general, the specific advantages offered by quantum computing have been somewhat nebulous. On the one hand, faster computing was promised, but we now know that no speedup of most algorithms exists relative the speed that can be obtained with massive parallel processing. Then, we are promised that the use of entanglement will make quantum computing possible with a much smaller use of resources. Yet, entanglement must be viewed as a hidden variable, which is not accessible in experiment. How does this provide the speedup? We have suggested that analog processing may provide a suitable alternative, and may be the basis which provides the speedup in quantum computing, but this is a controversial assertion. In this talk, we will discuss these particular viewpoints, along with several approaches to a wave basis for (quantum) computing.
Electric field-induced transient hole transport in an Al0.3Ga0.7As-based p-i-n nanostructure has been studied by picosecond Raman spectroscopy at T=300K. Our experimental results demonstrate that at T=300K, for a 5-ps excitation laser pulse and a hole density of nhapproximately equals 5x1017cm-3, transient hole drift velocity increases from zero to approximately equals (3+/- 0.7)x106cm/sec when the applied electric field intensity increases from E=0 to 15 kV/cm. The transient hole drift velocity then becomes saturated at approximately equals (8+/- 0.8)x106cm/sec for the applied electric field intensity of E>=25 kV/cm and up to 65 kV/cm.
Non-equilibrium electron distributions and energy loss rate in a metal-organic chemical vapor deposition-grown InxGa1-xAs1-yNy(x=0.03 and y=0.01) epilayer on GaAs substrate have been studied by picosecond Raman spectroscopy. It is demonstrated that for electron density napproximately equals 1018cm-3, electron distributions can be described very well by Fermi-Dirac distributions with electron temperatures substantially higher than the lattice temperature. From the measurement of electron temperature as a function of the pulse width of excitation laser, the energy loss rate in InxGa1-xAs1-yNy is estimated to be 64 meV/ps. These experimental results are compared with those of GaAs.
Decay of the longitudinal optical (LO) phonons in wurtzite GaN and AlxGa1-xN (x equals 0.1) has been studied by subpicosecond time-resolved Raman spectroscopy. In contrast to the usually-believed 2LA decay channel for LO phonons in other semiconductors, our experimental results show that, among the various possible decay channels, the LO phonons in wurtzite GaN and AlxGa1-xN (x equals 0.1) decay primarily into a large wavevector TO and a large wavevector LA or TA phonons. These experimental results are consistent with the recent theoretical calculations of the phonon dispersion curves.
KEYWORDS: Phonons, Raman spectroscopy, Aluminum, Picosecond phenomena, Monte Carlo methods, Scattering, Gallium, Raman scattering, Chemical species, Laser energy
Experimental Raman spectroscopy studies, performed by two different groups, to measure the relative strength of the polar modes in AlxGa1-xAs differed in that one showed a linear behavior with the Al concentration while the other showed a non-monotonic behavior. We have used an ensemble Monte Carlo code to study this system. It is found that the excitation energy and the electron concentration all contribute to the experimental discrepancies, while the excitation pulse width had little effect on the measured relative strength of the Al-mode. Apparently the relative strength of the Al-mode scattering is density dependent for x >= 0.25.
Electron ballistic transport and in a InP-based p-i-n nanostructure under the application of an electric field have been studied by time-resolved Raman spectroscopy at T equals 300 K. The time-evolution of electron distribution, electron drift velocity has been directly measured with subpicosecond time resolution. Our experimental results show that, for a photoexcited electron-hole pair density of n is congruent to 5 X 1016 cm-3, electrons travel quasi- ballistically--electron drift velocity increases linearly with time, during the first 150 fs. After 150 fs it increases sublinearly until reaching the peak value at about 300 fs. The electron drift velocity then decreases to its steady-state value.
Decay of the longitudinal optical (LO) phonons in wurtzite GaN has been studied by subpicosecond time-resolved Raman spectroscopy. Our experimental results show that, among the various possible decay channels, the LO phonons in wurtzite GaN decay primarily into a large wavevector TO and a large wavevector LA or TA phonons. These experimental results are consistent with the recent theoretical calculations of the phonon dispersion curves for wurtzite GaN.
Electron ballistic transport and in a InP-based p-i-n nanostructure under the application of an electric field have been studied by time-resolved Raman spectroscopy at T equals 300 K. The time-evolution of electron distribution, electron drift velocity has been directly measured with subpicosecond time resolution. Our experimental results show that, for a photoexcited electron-hole pair density of n equalsV 5 X 1016 cm-3, electrons travel quasi- ballistically--electron drift velocity increases linearly with time, during the first 150 fs. After 150 fs it increases sublinearly until reaching the peak value at about 300 fs. The electron drift velocity then decreases to its steady-state value.
The electron-LO phonon interaction in AlxGa1-xAs has been studied, for the alloy composition ranging from 0.1 to 0.4, by picosecond Raman spectroscopy. We have found that the relative strength of the Frohlich interaction of the two LO phonon modes is a nonlinear function, and increases non- monotonically with the Al composition. The possible origins of these novel results are discussed in term of nonlinear dielectric theory and ordering in the alloy.
We discuss the use of ensemble Monte Carlo techniques for the simulation of some opto-electronic devices. This approach has been reasonably successful for device simulation, and the Monte Carlo approach has worked very well for sub-picosecond time scales, but computational time becomes excessive for multi-picosecond simulations. Various nonlinear effects such as carrier-carrier scattering, non- equilibrium phonons, quantization in low-dimensional systems, and finite collision duration have all successfully been incorporated into the Monte Carlo method.
We have used electric-field-induced Raman scattering to quantitatively assess the effects of carrier screening on the average electric fields in a GaAs-based p-i-n nanostructure semiconductor under the subpicosecond laser photoexcitation. Our experimental results demonstrated that the effects of carrier screening on the average electric field were negligible for photoexcited electron-hole pair density of n less than or equal to 1015 cm-3. As the density of photoexcited carriers increased we observed a significant decrease of the average electric field. In particular, for n equals 1018 cm-3, a decrease of electric field of about 50% was found. All of these experimental results were explained by ensemble Monte Carlo simulations and very good agreement has been obtained.
We have studied transient electron transport in an InP p-i-n nanostructure semiconductor by subpicosecond Raman spectroscopy at T equals 300 K. Both the non-equilibrium electron distribution and electron drift velocity in the regime of electron velocity overshoot have been directly measured. It is demonstrated that electron drift velocity in an InP p-i-n nanostructure is significantly larger than that in a GaAs p-i-n nanostructure sample, as a result of the larger central to satellite valley energy separation in InP.
Monte Carlo simulation has been shown to be an effective approach to the study of ultrafast carrier relaxation in semiconductor bulk materials and in microstructures. We review the use of this methodology to study electron-electron and electron-hole interactions, non-equilibrium and confined phonons, and inter-subband relaxation in quantum wells. We also discuss the presence of the collision-duration on the short-time scale, and review the work of some other workers in the field. Finally, we discuss some of the limitations of the Monte Carlo technique.
We have studied non-equilibrium electron distributions and electron-longitudinal optical phonon scattering rates in wurtzite GaN by subpicosecond time-resolved Raman spectroscopy. Our experimental result show that for electron densities n greater than or equal to 5 X 1017 cm-3, the non-equilibrium electron distributions in wurtzite GaN can be very well described by Fermi-Dirac distribution functions with the effective electron temperature much higher than the lattice temperature. In addition, we find that the total electron-longitudinal optical phonon scattering rate in GaN is about one order of magnitude larger than that in GaAs. We attribute this enormous increase in the electron- longitudinal optical phonon scattering rate to the much larger ionicity in GaN.
Electron transport and phonon dynamics in a GaAs-based p-i-n nanostructure under the application of an electric field have been studied by time-resoled Raman spectroscopy at T equals 80 K. The time-evolution of electron density, electron distribution, electron drift velocity, and LO phonon population has been directly measured with subpicosecond time resolution. Our experimental results show that, for a photoexcited electron-hole pair density of n approximately equals 1017 cm-3, the effects of the drifting of electrons and electron intervalley scattering processes govern electron transport properties as well as the LO phonon dynamics. All of the experimental results are compared with ensemble Monte Carlo simulations and satisfactory agreement is obtained.
A picosecond laser excited GaAs p-i-n structure is studied using an ensemble Monte Carlo method to determine the temporal and spatial evolution of the hot electron distribution function. The experimental set-up we simulate is a novel method based on Raman scattering of light from the electrons to measure the drift velocity of electrons in GaAs at high electric fields. It is observed that the simulation agrees with the experimental results, however, the measured velocity is actually averaged over the time evolution of the spatial distribution of the Raman probe in the sample and underestimates the average velocity of electrons over the pulselength excited in the (Gamma) conduction band of a 1.909 eV laser pulse, which is calculated to be in the order of 8.5 X 107 cm/sec for fields of 25 kV/cm at a temperature of 77 K.
Femtosecond photoexcitation experiments have been used by many groups to investigate ultrafast scattering processes in semiconductors. Information on intervalley scattering rates can readily be deduced by monitoring valley populations in real time, and particularly, a number of groups have made measurements for (Gamma) -L and (Gamma) -X intervalley scattering in GaAs. However, due to the direct gap, the L-X scattering in GaAs can not be directly monitored. Recently, experiments to monitor the X valley population in indirect AlGaAs have been performed, and utilized to set up an upper bound for the L-X scattering lifetime. We have used an ensemble Monte Carlo (EMC) technique to calculate the evolution of valley populations in indirect AlGaAs illuminated by a femtosecond pulse laser. The time evolution of electron populations in the (Gamma) , L and X valleys is studied by varying the intervalley coupling constants. The L-X intervalley deformation potential is found to be DXL equals 1.5 +/- 0.5 X 108 eV/cm.
KEYWORDS: Scattering, Monte Carlo methods, Phonons, Plasmas, Semiconductors, Satellites, Particles, Computer simulations, Solid state electronics, Systems modeling
Realistic simulations of semiconductor plasmas require detailed, many-species descriptions of the phonon and electronic systems. Limited numerical power then usually requires simplifying approximations. One approximation is the use of a screened Coulomb interaction. When an accurate screening function is not available, or when a better electrostatics treatment is needed, one can perform ensemble Monte Carlo (EMC) simulations that use a phase-space- trajectories or 'molecular dynamics' (MD) evolution of the electronic ensemble. In these EMC/MD simulations, Coulomb scattering events are treated continuously in the MD evolution of electron trajectories rather than by instantaneous scattering in EMC. Dynamic scattering effects are then included accurately by the explicit correlated motion of the electron ensemble. The electron trajectories simulated by MD have until recently been completely classical. We describe extensions of EMC/MD into the semiclassical regime, thus incorporating quantum effects such as position-momentum uncertainty. The method takes account of the Fermi statistics of the many-electron ensemble, yielding spin-dependent exchange contributions to the forces and effective mass. We describe effects of these corrections on the velocity autocorrelation function and on thermalization of satellite-valley electrons.
With the current interest in ultrahigh speed electronic and optoelectronic devices, it is important to understand the
relaxation dynamics of nonequilibrium carriers in GaAs and other compounds. When coupled with detailed experimental
studies, this knowledge can provide significant insight into the physics of carrier transport in semiconductors
Metallization, and conductor systems in general, are a critical part of any VLSI chip, and as such can act to set limits on future down-scaling of such integrated circuits. Due to decreasing lateral and vertical dimensions, interconnections are rapidly becoming a problem in terms of device yield, reliability, signal delay time, and inter-device interactions. In this paper, we discuss how interconnection limitations will affect the scaling of advanced circuits. We will also cover a number of issues regarding the interconnection technologies that will be required in future ULSI circuits. The problems with conductor systems begin with the interconnection topology which provides constraints and limitations. The physical problems then begin with the deposition of the materials. For example, chemical vapor deposition of metal or metal-silicide interconnects causes several unique concerns due to surface chemistry, leading to undersirable reactions and compositional and structural nonuniformities. Similarly, factors such as control of step coverage are important for reduced geometries. Recent experiments and modeling techniques which address these problems will therefore be described. Lithographical aspects also pose problems in the scaling of metal lines and new pattern definition techniques will be discussed. Finally, isolation of information within dense crossing interconnects can become very difficult, with coupling causing degradation of information within localized devices.
Ultra-submicron GaAs MESFETs have been fabricated with gate lengths ranging from 25 nm to 80
nm, using an electron-beam lithography process. The MESFETs were fabricated on vapor-phase grown
wafers. The HEMT devices were fabricated on MBE grown wafers. Measurements of the
transconductances of these devices, as a function of the effective gate length, exhibit transconductance
degradation due to a diminishing aspect ratio. Velocity overshoot, saturation due to substrate current
(MESFETs), real space transfer (HEMTs) and/or source dependent minimum acceleration lengths(both).
In addition, the HEMTs with gate lengths less than 3Onm exhibit exponentially dependent current. This
suggests that tunneling is the dominant current mechanism and the final limit to scaling of conventional
FETs has been observed.
In the mesoscopic regime, where the characteristic length of a sample is comparable to the phase
coherence length of electrons, quantum interference leading to a magnetoconductance periodic in the
magnetic field coupled through a unit cell of a two-dimensional superlattice has been hypothesized for
many years. We report here on the observation of such periodic effects in quasi-two-dimensional
semiconductor structures with an additional two-dimensional periodic superlattice potential applied. In
LSSLs prepared on MODFET material, magnetoconductance measurements made at 4.2K show
Aharonov-Bohm type periodic oscillations with We periodicity in the flux coupled through each cell of the
superlattice. In LSSLs on MESFET material, we find that the conductance is also periodic in the magnetic
field, but with replicas of the negative magneto-resistance (signature of weak localization), with
separations in magnetic field corresponding to integer changes in the flux per superlattice cell. In addition,
the presence of a significant source-drain potential shifts the resonances in magnetic field. These effects
are observed for relatively low magnetic fields, B<1 Tesla. Furthermore, conductance fluctuations are
observed for samples whose dimensions are large compared to the inelastic mean free path.
We study theoretically the effect of few elastic scatterers on electron transport in the ballistic regime. In
laterally confined structures (quasi 1-d or 2-d), resonant transmission peaks occur when total electron
energy equals any miniband energy. Unit transmission probability is approached when the scattering defect
is small, and far from other scatterers, even if the scatterer is strong enough to decrease significantly the
conductance away from resonance. Both numerical and analytical methods are used. Resonances occur for
all shapes of confining potential.
Ultra-submicron gate GaAs MESFETs and AlGaAs/GaAs HEMTs have been fabricated in an electron- beam lithographic process with gate lengths varying from 25 to 80 nm. For gate length less than 100 nm, electrical characteristics deteriorate due to fringing capacitance at a low aspect ratio. Velocity overshoot is observed for gate length shorter than about 55 nm. The maximum effective electron saturation velocity
obtained is 3x107 cm/sec for a 30 nm HEMT. A maximum fT value of 167 GHz was obtained for a 37.5 nm MESFET.
Using ensemble Monte Carlo methods, coupled with a molecular dynamics (MD) approach for the
carrier-carrier interaction, we investigate the ultrafast relaxation of photoexcited carriers in GaAs. The
interaction of various scattering mechanisms and the dynamic screening of hot carriers in semiconductors
is studied. At a density for which the GaAs is degenerate (in equilibrium), scattering out of the excitation
volume is dominated in the initial tens of femtoseconds by electron-electron scattering, and the scattering
rate increases with increasing density. This rate increase agrees both in magnitude and in density
dependence with some recent experiments. The presence of electron-electron scattering modifies both the
population transition rates and carrier densities in the satellite valleys, primarily by reshaping the energy
distribution of carriers in the central valley. Intervalley processes also play a role in the initial decay and
the same processes play a modified role in the picosecond-scale luminescence decay. The intervalley
transition rates must be estimated carefully because the f-L population shift contains a significant fraction
of electrons that reach the L valleys by way of the X valleys. The exchange effect further modifies the
satellite valley populations and intervalley transition rates. It also reduces the rate for electrons to scatter
out of the excitation volume.
The transient expansion process of a photogenerated electron-hole plasma is examined using Ensemble
Monte Carlo techniques. The effect of nonequilibrium phonons on the expansion behaviour is included for the
first time. Both the spatial and temporal variations of the phonon population are taken into account. Numerical
results over the picosecond time scale demonstrate that the nonequilibrium phonons significantly enhance the
plasma expansion and alter the spatial carrier distribution profiles.
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