Hot Carriers In Semiconductors Research Articles

Quantization effects in semiconductor nanostructures and singlet fission in molecular chromophores for photovoltaics and solar fuels

Hot carriers in semiconductors are electrons and/or holes that have energies greater than carriers that reside at the top and bottom of the conduction and valence bands, respectively; the latter carriers are in equilibrium with the lattice and have a temperature equal to the lattice (ambient) temperature. Hot carriers are created in semiconductors upon the absorption of photons with energies greater than the bandgap. The excess energy above the bandgap energy is in the form of kinetic energy. The hot carriers can cool to the lattice temperature via electron–phonon scattering and establish separate Boltzmann distributions for electrons and holes at the band edges. Thus, upon cooling, the excess kinetic energy of the hot carriers is transformed into heat and is unavailable to be converted into electrical or chemical free energy in a solar photon-converting cell. This hot-carrier cooling process significantly limits the maximum possible power conversion efficiency (PCE) of the free energy of solar photons into electrical free energy or chemical free energy in chemical bonds of fuels. However, if hot-carrier cooling can be slowed such that the hot carriers can be extracted before cooling to create higher photovoltages, or utilized to create additional electron-hole pairs through carrier multiplication, then beneficial use can be made of the excess energy of hot carriers and the maximum thermodynamic PCE can be dramatically increased. Quantization effects in semiconductor nanostructures, unique properties of some bulk semiconductors, and exciton multiplication in both semiconductors and molecular chromophores that undergo singlet fission have shown a path forward for potential dramatic increases in the PCEs of solar photons into electricity and solar fuels. The status, history, and future promise of the science and technology of these future approaches for different types of photovoltaic cells and cells for solar fuels will be discussed.

Giant photothermoelectric effect in silicon nanoribbon photodetectors

The photothermoelectric (PTE) effect enables efficient harvesting of the energy of photogenerated hot carriers and is a promising choice for high-efficiency photoelectric energy conversion and photodetection. Recently, the PTE effect was reported in low-dimensional nanomaterials, suggesting the possibility of optimizing their energy conversion efficiency. Unfortunately, the PTE effect becomes extremely inefficient in low-dimensional nanomaterials, owing to intrinsic disadvantages, such as low optical absorption and immature fabrication methods. In this study, a giant PTE effect was observed in lightly doped p-type silicon nanoribbons caused by photogenerated hot carriers. The open-circuit photovoltage responsivity of the device was 3-4 orders of magnitude higher than those of previously reported PTE devices. The measured photovoltage responses fit very well with the proposed photothermoelectric multiphysics models. This research proposes an application of the PTE effect and a possible method for utilizing hot carriers in semiconductors to significantly improve their photoelectric conversion efficiency.

Hot-carrier separation in heterostructure nanowires observed by electron-beam induced current

The separation of hot carriers in semiconductors is of interest for applications such as thermovoltaic photodetection and third-generation photovoltaics. Semiconductor nanowires offer several potential advantages for effective hot-carrier separation such as: a high degree of control and flexibility in heterostructure-based band engineering, increased hot-carrier temperatures compared to bulk, and a geometry well suited for local control of light absorption. Indeed, InAs nanowires with a short InP energy barrier have been observed to produce electric power under global illumination, with an open-circuit voltage exceeding the Shockley-Queisser limit. To understand this behaviour in more detail, it is necessary to establish control over the precise location of electron-hole pair-generation in the nanowire. In this work we perform electron-beam induced current measurements with high spatial resolution, and demonstrate the role of the InP barrier in extracting energetic electrons.We interprete the results in terms of hot-carrier separation, and extract estimates of the hot carriers’ mean free path.

Nanosecond mid-infrared pulse generation via modulated thermal emissivity

We demonstrate the generation of nanosecond mid-infrared pulses via fast modulation of thermal emissivity enabled by the absorption of visible pump pulses in unpatterned silicon and gallium arsenide. The free-carrier dynamics in these materials result in nanosecond-scale modulation of thermal emissivity, which leads to nanosecond pulsed thermal emission. To our knowledge, the nanosecond thermal-emissivity modulation in this work is three orders of magnitude faster than what has been previously demonstrated. We also indirectly observed subnanosecond thermal pulses from hot carriers in semiconductors. The experiments are well described by our multiphysics model. Our method of converting visible pulses into the mid infrared using modulated emissivity obeys different scaling laws and can have significant wavelength tunability compared to approaches based on conventional nonlinearities.

20th International Conference on Electron Dynamics in Semiconductors, Optoelectronics and Nanostructures (EDISON 20)

The 20th International Conference on Electron Dynamics in Semiconductors, Optoelectronics and Nanostructures (EDISON 20) was held at the Hyatt Regency Hotel and Conference Center in downtown Buffalo, New York, USA from July 17–21, 2017. The meeting serves as the premier forum for researchers working on the study of nonequilibrium phenomena in semiconductors, from all parts of the globe, and was organized locally by faculty at the University at Buffalo and the University of Rochester.EDISON is an international conference series with biennial meetings that cover recent progress in the field of electron dynamics in solid-state materials and devices. Initially established with the title Hot Carriers in Semiconductors (HCIS), the first meeting took place in Modena, Italy in 1973. The name was changed to International Conference on Nonequilibrium Carrier Dynamics in Semiconductors at the 1997 Berlin meeting, and again at the 2009 meeting in Montpellier to the current form. Since then, EDISON conferences have been held in Santa Barbara, USA (2011); Matsue, Japan (2013); Salamanca, Spain (2015); and Buffalo, USA (2017).The EDISON 20 program included 12 invited talks, 50 oral contributions, 116 accepted abstracts, and one special seminar. In total, 152 participants from 22 countries gathered to share and exchange ideas and recent results. Approximately 45% of the participants were from North America, 30% from Europe, and 25% from the Asia Pacific region. Fully 30% of the participants were students, and there was a very good balance between experimental and theoretical approaches. Topics with the most contributions were: Electronic and optical properties of graphene and other 2D materials; Nonequilibrium carrier transport; and Terahertz phenomena in semiconductor materials and devices.I thank all the members of the Program Committee and the International Advisory Committee for their valuable input, as well as Professor Jonathan Bird and Alex Desha for their many hours devoted to organizing a successful EDISON conference. Also critical to the success of the conference is financial support, so I thank the School of Engineering and Applied Sciences, the Vice President for Research and Economic Development, the Department of Physics, and the Department of Electrical Engineering at the University at Buffalo, as well as the U.S. Department of Energy, Kurt J. Lesker Company, and the Hajim School of Engineering & Applied Sciences at the University of Rochester for their sponsorship of EDISON 20. Finally, I thank all the scientists who contributed to the preparation and peer-review of these proceedings.Erik Einarsson, EditorEDISON 20 Publications ChairUniversity at Buffalo

Hot Carrier Extraction from Multilayer Graphene.

Hot carriers in semiconductor or metal nanostructures are relevant, for instance, to enhance the activity of oxide-supported metal catalysts or to achieve efficient photodetection using ultrathin semiconductor layers. Moreover, rapid collection of photoexcited hot carriers can improve the efficiency of solar cells, with a theoretical maximum of 85%. Because of the long lifetime of secondary excited electrons, graphene is an especially promising two-dimensional material to harness hot carriers for solar-to-electricity conversion. However, the photoresponse of thus far realized graphene photoelectric devices is mainly governed by thermal effects, which yield only a very small photovoltage. Here, we report a Gr-TiOx-Ti heterostructure wherein the photovoltaic effect is predominant. By doping the graphene, the open circuit voltage reaches values up to 0.30 V, 2 orders of magnitude larger than for devices relying upon the thermoelectric effect. The photocurrent turned out to be limited by trap states in the few-nanometer-thick TiOx layer. Our findings represent a first valuable step toward the integration of graphene into third-generation solar cells based upon hot carrier extraction.

Tracking Primary Thermalization Events in Graphene with Photoemission at Extreme Time Scales.

Direct and inverse Auger scattering are amongst the primary processes that mediate the thermalization of hot carriers in semiconductors. These two processes involve the annihilation or generation of an electron-hole pair by exchanging energy with a third carrier, which is either accelerated or decelerated. Inverse Auger scattering is generally suppressed, as the decelerated carriers must have excess energies higher than the band gap itself. In graphene, which is gapless, inverse Auger scattering is, instead, predicted to be dominant at the earliest time delays. Here, <8 fs extreme-ultraviolet pulses are used to detect this imbalance, tracking both the number of excited electrons and their kinetic energy with time-and angle-resolved photoemission spectroscopy. Over a time window of approximately 25fs after absorption of the pump pulse, we observe an increase in conduction band carrier density and a simultaneous decrease of the average carrier kinetic energy, revealing that relaxation is in fact dominated by inverse Auger scattering. Measurements of carrier scattering at extreme time scales by photoemission will serve as a guide to ultrafast control of electronic properties in solids for petahertz electronics.

Injection deep level transient spectroscopy: An improved method for measuring capture rates of hot carriers in semiconductors

An improved method for measuring the cross sections for carrier trapping at defects in semiconductors is described. This method, a variation of deep level transient spectroscopy (DLTS) used with bipolar transistors, is applied to hot carrier trapping at vacancy-oxygen, carbon-oxygen, and three charge states of divacancy centers (V2) in n- and p-type silicon. Unlike standard DLTS, we fill traps by injecting carriers into the depletion region of a bipolar transistor diode using a pulse of forward bias current applied to the adjacent diode. We show that this technique is capable of accurately measuring a wide range of capture cross sections at varying electric fields due to the control of the carrier density it provides. Because this technique can be applied to a variety of carrier energy distributions, it should be valuable in modeling the effect of radiation-induced generation-recombination currents in bipolar devices.

Ab initio study of hot carriers in the first picosecond after sunlight absorption in silicon.

Hot carrier thermalization is a major source of efficiency loss in solar cells. Because of the subpicosecond time scale and complex physics involved, a microscopic characterization of hot carriers is challenging even for the simplest materials. We develop and apply an abinitio approach based on density functional theory and many-body perturbation theory to investigate hot carriers in semiconductors. Our calculations include electron-electron and electron-phonon interactions, and require no experimental input other than the structure of the material. We apply our approach to study the relaxation time and mean free path of hot carriers in Si, and map the band and k dependence of these quantities. We demonstrate that a hot carrier distribution characteristic of Si under solar illumination thermalizes within 350fs, in excellent agreement with pump-probe experiments. Our work sheds light on the subpicosecond time scale after sunlight absorption in Si, and constitutes a first step towards abinitio quantification of hot carrier dynamics in materials.

Coulomb scattering of excited carriers within RPA; calculation in real time domain

We have carried out a large-scale simulation of transient behavior of hot carriers in semiconductors excited by ultrashort laser pulses treating Coulomb interaction to high orders within the framework of random phase approximation (RPA). We have used the non-equilibrium Green's function method with the explicit dependence on two time variables and integrated the Dyson's equation numerically in the time domain on the large-scale parallel computer. We examine the dynamics of Coulomb screening by comparing the time development of polarization and the photon echo signal under two types of approximation to Coulomb interaction effects, i.e., RPA and HF (Hartree–Fock approximation).

Transport and fluctuations in nonlinear-dissipative systems: role of interparticle collisions

The goal of the paper is to overview contemporary theoretical and experimental research of the microwave electric noise and fluctuations of hot carriers in semiconductors, revealing sensitivity of the noise spectra to non-linearity in the applied electric field strength and, especially, in the carrier density. During the last years, investigation of electronic noise and electron diffusion phenomena in doped semiconductors was in a rapid progress. By combining analytic and Monte Carlo methods as well as the available experimental results on noise, it became possible to obtain the electron diffusion coefficients in the range of electric fields where inter-electron collisions are important and Price’s relation is not necessarily valid. Correspondingly, a special attention to the role of inter-electron collisions and of the non-linearity in the carrier density while shaping electric noise and diffusion phenomena in the non-equilibrium states will be paid. The basic and up-to-date information will be presented on methods and advances in this contemporary field - the field in which methods of non-linear analytic and computational analysis are indispensable while seeking coherent understanding and interpretation of experimental results.

Transient carrier dynamics calculation in time domain; Coulomb scattering within RPA

We have carried out a large scale simulation of transient behavior of hot carriers in semiconductors excited by ultrashort laser pulses treating Coulomb interaction to high orders within the framework of the random phase approximation. We have used the non-equilibrium Green's function method and integrated the Dyson's equation numerically in the time domain on a large scale parallel computer. We have investigated cases where carriers are excited by double-pulsed laser light. The polarization at certain momentum was found not to diminish after the second pulse has passed in spite of the presence of the Coulomb scattering which normally causes both transverse and longitudinal polarization decay.

Broadly Tunable Femtosecond Sources in the Mid-Infrared and Two-Wavelength Ultrafast Spectroscopy of Holes in GaAs

The recent development of periodically poled lithium niobate (PPLN) has led to a variety of highly efficient and compact all-solid-state nonlinear optical devices. Of particular interest are the optical parametric oscillators. We have most recently demonstrated for the first time a highly efficient broadly tunable femtosecond optical parametric oscillator (fs OPO) using PPLN. The threshold for oscillation of the PPLN fs OPO is remarkably low, on the order of 50 mW. As a result, it is now possible to use a diode-pumped and frequency-doubled Nd:YVO 4 laser, rather than an Ar-ion laser, as the pump source for the mode-locked Ti:sapphire laser for pumping the fs OPO. Such an OPO can operate from approximately 1 μm to 5.4 μm. Femtosecond sources have been used in a variety of experiments to study the dynamics of hot carriers in semiconductors. Because of the limited available wavelength range of earlier femtosecond sources, most such studies have been limited to electron dynamics in the conduction band of III–V compounds such as GaAs. With the extended tuning range of the recently developed fs OPO's, it is now possible to study a much wider variety of materials and transitions. We have recently obtained preliminary results on the ultrafast dynamics of holes in GaAs using two-wavelength tunable femtosecond spectroscopy, by pumping the valence-to-conduction band transition at approximately 800 nm and simultaneously probing the transitions between the split-off band and the perturbed heavy-hole band at approximately 3 μm. These recent results are summarized and discussed briefly.

Mechanism of impact ionization enhancement in GaAs p-i-n diodes under high illumination conditions

Recent experimental work on AlGaAs p-i-n photodiodes [Dudley R A and Vickers A J 1995 Proc. Hot Carriers in Semiconductors IX (New York: Plenum)] has generated evidence that the transient photocurrent produced by high-intensity pulsed laser illumination is larger than expected for any given value of the bias. It has been postulated that this enhancement is due to the modification of the internal electric field by the photogenerated charge. A similar effect has been seen in calculations for metal-semiconductor-metal semiconductors [Dunn G M, Rees G J and David J P R 1997 Semicond. Sci. Technol. 12 692-7], but we present here details of the first calculation of the effect in a p-i-n diode. A drift-diffusion model is used to calculate photocurrent versus reverse bias for several values of illumination intensity, illustrating the enhancement effect. The origin of the enhancement is then highlighted and explained.

Indirect intraband radiative transitions of hot carriers in semiconductors

Indirect intraband radiative transitions of hot carriers in semiconductors

Screening and energy loss by hot carriers in semiconductors.

Screening and energy loss by hot carriers in semiconductors.

Radiative efficiency limit of terrestrial solar cells with internal carrier multiplication

Charge multiplication by hot carriers in semiconductors opens a new perspective for improved photovoltaic energy conversion. Here, we estimate the theoretical limit for the efficiency of solar cells that make use of the best possible carrier multiplication and have the lowest possible recombination loss, i.e., radiative recombination only. For a terrestrial cell under 100 mW/cm2 air mass 1.5 global illumination, the uppermost efficiency amounts to 44.2%. Cells with internal carrier multiplication require a modified description of their saturation current density.

Erratum: "Energy-loss rate of hot carriers in semiconductors with nonequilibrium phonon distribution in the extreme quantum limit at low temperatures"

A theoretical model has been developed to investigate the effect of nonequilibrium phonons or hot phonons on the energy-loss rate of hot carriers in semiconductors in the extreme quantum limit at low temperatures. The acoustic-phonon scattering via the deformation potential and piezoelectric scattering are assumed to be the dominant scattering mechanisms at low temperatures. The model includes band nonparabolicity, energy nonequipartition of phonons, Landau-level broadening, and classical free-carrier screening. The energy-loss rates of hot electrons with nonequilibrium phonons as well as the thermal phonon distribution have been calculated using the above-mentioned model for n-type InSb. These theoretical results have also been compared with experimental results for n-type InSb at B=3 T and ${\mathit{T}}_{\mathit{L}}$=4.2 K. The energy-loss rate with the thermal phonon distribution is found to be much higher than the experimental result. The energy-loss rate of hot electrons calculated with use of the nonequilibrium phonon distribution with phonon boundary scattering is reduced compared with the values of the energy-loss rate obtained with use of the equilibrium phonon distribution. The incorporation of nonequilibrium phonons brings the theoretical results into agreement with the experimental data, giving a reasonable value for the phonon lifetime. However, the phonon lifetime required to fit the experimental data is found to be much higher than the values obtained from phonon boundary scattering. This discrepancy may be attributed to the acoustic-phonon mismatch factor.

Energy-loss rate of hot carriers in semiconductors with nonequilibrium phonon distribution in the extreme quantum limit at low temperatures.

A theoretical model has been developed to investigate the effect of nonequilibrium phonons or hot phonons on the energy-loss rate of hot carriers in semiconductors in the extreme quantum limit at low temperatures. The acoustic-phonon scattering via the deformation potential and piezoelectric scattering are assumed to be the dominant scattering mechanisms at low temperatures. The model includes band nonparabolicity, energy nonequipartition of phonons, Landau-level broadening, and classical free-carrier screening. The energy-loss rates of hot electrons with nonequilibrium phonons as well as the thermal phonon distribution have been calculated using the above-mentioned model for n-type InSb. These theoretical results have also been compared with experimental results for n-type InSb at B=3 T and ${\mathit{T}}_{\mathit{L}}$=4.2 K. The energy-loss rate with the thermal phonon distribution is found to be much higher than the experimental result. The energy-loss rate of hot electrons calculated with use of the nonequilibrium phonon distribution with phonon boundary scattering is reduced compared with the values of the energy-loss rate obtained with use of the equilibrium phonon distribution. The incorporation of nonequilibrium phonons brings the theoretical results into agreement with the experimental data, giving a reasonable value for the phonon lifetime. However, the phonon lifetime required to fit the experimental data is found to be much higher than the values obtained from phonon boundary scattering. This discrepancy may be attributed to the acoustic-phonon mismatch factor.

Noise and correlation functions of hot carriers in semiconductors

We present a unifying theory of electronic noise appropriate to semiconductor materials in the presence of electric fields of arbitrary strength. In addition to thermal noise, a classification scheme for excess noise indicating different microscopic sources of fluctuations responsible for number and mobility fluctuations is provided. On the basis of simple two-level models, numerical calculations using a Monte Carlo technique are performed for the case of p-type Si at 77 K. The primary quantity which is evaluated by the theory is the auto-correlation function of current fluctuations which, subsequently, is analyzed in terms of correlation functions of the relevant physical variables. Accordingly, the corresponding current spectral-densities are determined and then compared with direct experimental results and/or analytical expressions. Important subjects which have been investigated are: (i) the effect of field assisted ionization on generation-recombination noise from shallow impurity levels; (ii) the contribution to the total noise spectrum of cross-correlation terms coupling fluctuations in velocity with those in energy and number; (iii) the current random telegraph signal and the corresponding spectral density associated with a mobility fluctuator. In all cases the numerical calculations are found to be in satisfactory agreement with experiments and/or analytical expressions thus fully supporting the physical reliability of the theoretical approach here proposed.