Electron-hole Pair Creation Research Articles

Radiative Processes in Graphene and Similar Nanostructures in Strong Electric Fields

Low-energy single-electron dynamics in graphene monolayers and similar nanostructures is described by the Dirac model, being a 2+1 dimensional version of massless QED with the speed of light replaced by the Fermi velocity vF ≃ c/300. Methods of strong-field QFT are relevant for the Dirac model, since any low-frequency electric field requires a nonperturbative treatment of massless carriers in the case it remains unchanged for a sufficiently long time interval. In this case, the effects of creation and annihilation of electron-hole pairs produced from vacuum by a slowly varying and small-gradient electric field are relevant, thereby substantially affecting the radiation pattern. For this reason, the standard QED text-book theory of photon emission cannot be of help. We construct the Fock-space representation of the Dirac model, which takes exact accounts of the effects of vacuum instability caused by external electric fields, and in which the interaction between electrons and photons is taken into account perturbatively, following the general theory (the generalized Furry representation). We consider the effective theory of photon emission in the first-order approximation and construct the corresponding total probabilities, taking into account the unitarity relation.

CdTe layer structures for X-ray and gamma-ray detection directly grown on the Medipix readout-chip by MBE

This work investigates the potential of CdTe semiconducting layers used for radiation detection directly deposited on the Medipix readout-chip by MBE. Due to the high Z-number of CdTe and the low electron-hole pair creation energy a thin layer suffices for satisfying photon absorption. The deposition takes place in a modified MBE system enabling growth rates up to 10µm/h while the UHV conditions allow the required high purity for detector applications. CdTe sensor layers deposited on silicon substrates show resistivities up to 5.8×108Ωcm and a preferred (111) orientation. However, the resistivity increases with higher growth temperature and the orientation gets more random. Additionally, the deposition of a back contact layer sequence in one process simplifies the complex production of an efficient contact on CdTe with aligned work functions. UPS measurements verify a decrease of the work function of 0.62eV induced by Te doping of the CdTe.

Modeling of columnar recombination for high-energy photon generated electrons and holes in amorphous selenium

An analytical model is developed to study the mechanisms of X-ray generated free Electron–hole pair (EHP) creation energy in amorphous selenium (a-Se) at high electric fields. The model is presented to show the electric field and temperature dependence of the charge extraction yield limited by the columnar recombination for the materials that have widely unequal drift mobility for electrons and holes, such as a-Se. The model is compared with Jaffe’s columnar recombination model with widely varying field and temperature. In addition, the free EHP creation energy is calculated by incorporating the initial charge extraction yield and the charge collection efficacy of the free carriers. Also, the results of this model are compared with the recently published experimental results on EHP creation energy. The analysis of the results confirm that the proposed model gives the best possible explanation to the columnar recombination mechanisms in a-Se and the free EHP creation mechanisms at diagnostic X-ray exposures can be described by the columnar recombination.

Enigmatic HCl + Au(111) Reaction: A Puzzle for Theory and Experiment

The reactive scattering of HCl on Au(111) is currently one of the most peculiar reactions in the field of surface chemistry, as it so far eludes an accurate theoretical description. Possible reasons for the observed mismatch between theory and experiment that were not yet all considered in the computations are (i) dissipative effects due to the creation of electron hole pairs and excitations of surface atom motion that might inhibit reaction and (ii) use of an inappropriate density functional theory method or even its failure due to the occurrence of a charge transfer at the transition state. In this work, we address all of these possibilities and perform quasiclassical molecular dynamics simulations employing different methodologies. We use ab initio molecular dynamics simulations to account for surface atom motion and surface temperature effects, employing the PBE and the RPBE functionals. We also construct an accurate potential energy surface incorporating the six adsorbate degrees of freedom of HCl on...

Probing quantum systems from the inside while producing the world’s shortest optical pulses

In 1964, Professor L.V. Keldysh, with whom I share winning the Lomonosov Gold Medal, published what was to become a very influential paper. In his paper, he developed the theory of multiphoton ionization for atoms and the creation of electron-hole pairs for solids. Fifty years later, we generate the world’s shortest light pulses using electron wave packets that are extracted from rare gas atoms by an intense infrared pulse much as Professor Keldysh described. The ultrashort bursts of XUV radiation from many atoms add coherently to produce intense pulses as short as 65 attoseconds—the current world record. Similar highly nonlinear processes occur in other atoms, molecules and solids. In addition to its importance as a new source of soft X-ray radiation and ultrashort pulses, the radiation generated from ionizing material encodes information on the quantum system from which it was made. By analyzing the XUV radiation, not only can we image molecular orbitals but also determine the band structure of solids.

Surface plasmon enhanced photodetectors based on internal photoemission

Surface plasmon photodetectors are of broad interest. They are promising for several applications including telecommunications, photovoltaic solar cells, photocatalysis, color-sensitive detection, and sensing, as they can provide highly enhanced fields and strong confinement (to subwavelength scales). Such photodetectors typically combine a nanometallic structure that supports surface plasmons with a photodetection structure based on internal photoemission or electron–hole pair creation. Photodetector architectures are highly varied, including waveguides, gratings, nanoparticles, nanoislands, or nanoantennas. We review the operating principles behind surface plasmon photodetectors based on the internal photoelectric effect, and we survey and compare the most recent and leading edge concepts reported in the literature.

A Model for Neutron Radiation Damage in Metal Oxide Semiconductor (MOS) Structures

Neutron bombardment on semiconductor material causes defects, one such primary physical effect is the formation of displacement defects within the crystal lattice structure, and such defects effectively decrease the mean free path and thus shorten the recombination time. Ionizing radiation causes creation of electron-hole pair in the gate oxide and in parasitic insulating layers of the MOS devices. Calculations show increase of the dark current in depletion region caused by a single neutron. Determination of energy and angular distribution of primary knock on atoms, with 14 MeV neutron irradiation in silicon are presented.

Extremely long-lived magnetic excitations in supported Fe chains

We report on a theoretical study of the lifetime of the first excited state of spin chains made of an odd number of Fe atoms on Cu2N/Cu(100). Yan et al (Nat. Nanotech. 10, 40 (2015)) recently observed very long lifetimes in the case of Fe3 chains. We consider the decay of the first excited state induced by electron-hole pair creation in the substrate. For a finite magnetic field, the two lowest-lying states in the chain have a quasi-N\'eel state structure. Decay from one state to the other strongly depends on the degree of entanglement of the local spins in the chain. The entanglement in the chain accounts for the long lifetimes that increase exponentially with chain length. Despite their apparently very different properties, the behaviour of odd and even chains is governed by the same kind of phenomena, in particular entanglement effects. The present results account quite well for the lifetimes recently measured by Yan et al on Fe3

Fano factor evaluation of diamond detectors for alpha particles

This report is the first describing experimental evaluation of Fano factor for diamond detectors. High‐quality self‐standing chemical vapor deposited diamond samples were produced using lift‐off method. Alpha‐particle induced charge measurements were taken for three samples. A 13.1 ±0.07 eV of the average electron–hole pair creation energy and excellent energy resolution of approximately 0.3% were found for 5.486 MeV alpha particles from an 241Am radioactive source. The best Fano factor for 5.486 MeV alpha particles, calculated from experimentally obtained epsilon values and the detector intrinsic energy resolution, was 0.382 ± 0.007.

Driven quantum tunneling and pair creation with graphene Landau levels

Driven tunneling between graphene Landau levels is theoretically linked to the process of pair creation from vacuum, a prediction of quantum electrodynamics (QED). Landau levels are created by the presence of a strong, constant, quantizing magnetic field perpendicular to a graphene mono-layer. Following the formal analogy between QED and the description of low-energy excitations in graphene, solutions of the fully interacting Dirac equation are used to compute electron-hole pair creation driven by a circularly or linearly polarized field. This is achieved via the coupled channel method, a numerical scheme for the solution of the time-dependent Dirac equation in the presence of bound states. The case of a monochromatic driving field is first considered, followed by the more realistic case of a pulsed excitation. We show that the pulse duration yields an experimental control parameter over the maximal pair yield. Orders of magnitude of the pair yield are given for experimentally achievable magnetic fields and laser intensities weak enough to preserve the Landau level structure.

Columnar recombination for X-ray generated electron-holes in amorphous selenium and its significance in a-Se x-ray detectors

Although amorphous selenium (a-Se) has a long and successful history of application in optical and X-ray imaging, some of its fundamental properties are still puzzling. In particularly, the mechanism of carrier recombination following x-ray excitation and electric field and temperature dependences of the electron-hole pair creation energy (Wehp) remain unclear. Using the combination of X-ray photocurrent and pulse height spectroscopy measurements, we measure Wehp in a wide range of temperatures (218–320 K) and electric fields (10–100 V/µm) and show that the conventional columnar recombination model which assumes Langevin recombination within a column (a primary electron track) fails to explain experimental results in a wide range of electric fields and temperatures. The reason for the failure of the conventional model is revealed in this work, and the theory of the columnar recombination is modified to include the saturation of the recombination rate at high electric field in order to account for the experimental results in the entire range of fields and temperatures.

Strong hole-doping and robust resistance-decrease in proton-irradiated graphene.

Great effort has been devoted in recent years to improve the electrical conductivity of graphene for use in practical applications. Here, we demonstrate the hole carrier density of CVD graphene on a SiO2/Si substrate increases by more than one order of magnitude to n = 3 × 1013 cm−2 after irradiation with a high energy 5 MeV proton beam. As a result, the dc-resistance (R) of graphene is reduced significantly by 60%. Only a negligible amount of defect is created by the irradiation. Also the hole-doped low resistance state of graphene remains robust against external perturbations. This carrier doping is achieved without requiring the bias-gate voltage as is the case for other field effect devices. We make two important observations, (i) occurrence of the doping after the irradiation is turned off (ii) indispensability of the SiO2-layer in the substrate, which leads to a purely electronic mechanism for the doping where electron-hole pair creation and interlayer Coulomb attraction play a major role. A flux-dependent study predicts that an ultrahigh doping may be obtained by longer irradiation. We expect the irradiation doping method could be applied to other atomically thin solids, facilitating the fundamental study and application of the 2d materials.

Chemical energy dissipation at surfaces under UHV and high pressure conditions studied using metal-insulator-metal and similar devices.

Metal heterostructures have been used in recent years to gain insights into the relevance of energy dissipation into electronic degrees of freedom in surface chemistry. Non-adiabaticity in the surface chemistry results in the creation of electron-hole pairs, the number and energetic distribution of which need to be studied in detail. Several types of devices, such as metal-insulator-metal, metal-semiconductor and metal-semiconductor oxide-semiconductor, have been used. These devices operate by spatially separating the electrons from the holes, as an internal barrier allows only - or at least favours - transport from the top to the back electrode for one kind of carrier. An introduction into the matter, a survey of the literature and a critical discussion of the state of research is attempted.

Charge relaxation resistance in the cotunneling regime of multichannel Coulomb blockade: Violation of Korringa-Shiba relation

We study the low frequency admittance of a small metallic island coupled to a gate electrode and to a massive reservoir via a \emph{multi channel} tunnel junction. The ac current is caused by a slowly oscillating gate voltage. We focus on the regime of inelastic cotunneling in which the dissipation of energy (the real part of the admittance) is determined by two-electron tunneling with creation of electron-hole pairs on the island. We demonstrate that at finite temperatures but low frequencies the energy dissipation is ohmic whereas at zero temperature it is super-ohmic. We find that (i) the charge relaxation resistance (extracted from the real part of the admittance) is strongly temperature dependent, (ii) the imaginary and real parts of the admittance do not satisfy the Korringa-Shiba relation. At zero temperature the charge relaxation resistance vanishes in agreement with the recent zero temperature analysis [M. Filippone and C. Mora, Phys. Rev. B {\bf 86}, 125311 (2012) and P. Dutt, T. L. Schmidt, C. Mora, and K. Le Hur, Phys. Rev. B {\bf 87}, 155134 (2013)].

BiI3 single crystal for room-temperature gamma ray detectors

BiI3 single crystals were grown by the physical vapor transport method. The repeated sublimation of the starting material reduced impurities in the BiI3 single crystal to sub-ppm levels. The detector was fabricated by depositing Au electrodes on both surfaces of the 100-μm-thick BiI3 single crystal platelet. The resistivity of the BiI3 single crystal was increased by post-annealing in an iodine atmosphere (ρ=1.6×1011Ωcm). Pulse height spectroscopy measurements showed clear peaks in the energy spectrum of alpha particles or gamma rays. It was estimated that the mobility-lifetime product was μeτe=3.4–8.5×10−6cm2/V and the electron–hole pair creation energy was 5.8eV. Our results show that BiI3 single crystals are promising candidates for detectors used in radiographic imaging or gamma ray spectroscopy.

Coupled rate and transport equations modeling proportionality of light yield in high-energy electron tracks: CsI at 295 K and 100 K; CsI:Tl at 295 K

A high-energy electron in condensed matter deposits energy by creation of electron-hole pairs whose density generally increases as the electron slows, reaching the order of ${10}^{20}$ eh/${\mathrm{cm}}^{3}$ near the end of its track. The subsequent interactions of the electrons and holes include nonlinear rate terms and transport as first hot and then thermalized carriers in the nanometer-scale radial dimension of the track. Charge separation and strong radial electric fields occur in a material such as CsI with contrasting diffusion rates of self-trapped holes and hot electrons. Eventual radiative recombination has a nonlinear relation to the primary electron energy because of these interactions. This so-called intrinsic nonproportionality of electron response limits the achievable energy resolution of a given scintillation radiation detector material. We use a system of coupled transport and rate equations to describe a pure host (three equations) and one dopant (four more equations per dopant). Applying it first to the experimentally well-characterized system of CsI and CsI:Tl in this work, we use results of picosecond absorption spectroscopy, interband $Z$-scan measurements of nonlinear rate constants, and other experiments and calculations to determine most of the more than 20 rate and transport coefficients required for modeling. The model is solved in a track environment approximated as cylindrical and is compared to the proportionality curve and total light yield of undoped CsI at temperatures of 295 and 100 K, as well as thallium dopant in CsI:Tl at 295 K. With this degree of validation, the space and time distributions of carriers and excitons, both untrapped and trapped, are examined within the model to gain an understanding of the main competitions controlling the nonproportionality of response.

Model of Ni-63 battery with realistic PIN structure

GaN, with its wide bandgap of 3.4 eV, has emerged as an efficient material for designing high-efficiency betavoltaic batteries. An important part of designing efficient betavoltaic batteries involves a good understanding of the full process, from the behavior of the nuclear material and the creation of electron-hole pairs all the way through the collection of photo-generated carriers. This paper presents a detailed model based on Monte Carlo and Silvaco for a GaN-based betavoltaic battery device, modeled after Ni-63 as an energy source. The accuracy of the model is verified by comparing it with experimental values obtained for a GaN-based p-i-n structure under scanning electron microscope illumination.

Highly responsive MoS2 photodetectors enhanced by graphene quantum dots.

Molybdenum disulphide (MoS2), which is a typical semiconductor from the family of layered transition metal dichalcogenides (TMDs), is an attractive material for optoelectronic and photodetection applications because of its tunable bandgap and high quantum luminescence efficiency. Although a high photoresponsivity of 880–2000 AW−1 and photogain up to 5000 have been demonstrated in MoS2-based photodetectors, the light absorption and gain mechanisms are two fundamental issues preventing these materials from further improvement. In addition, it is still debated whether monolayer or multilayer MoS2 could deliver better performance. Here, we demonstrate a photoresponsivity of approximately 104 AW−1 and a photogain of approximately 107 electrons per photon in an n-n heterostructure photodetector that consists of a multilayer MoS2 thin film covered with a thin layer of graphene quantum dots (GQDs). The enhanced light-matter interaction results from effective charge transfer and the re-absorption of photons, leading to enhanced light absorption and the creation of electron-hole pairs. It is feasible to scale up the device and obtain a fast response, thus making it one step closer to practical applications.

A ternary g-C3N4/Pt/ZnO photoanode for efficient photoelectrochemical water splitting

In this work, graphite-like C3N4 (g-C3N4) is deposited on the surface of ZnO nanowires (NWs) and a nanocomposite g-C3N4/ZnO photoanode is made for the photoelectrochemical (PEC) water splitting under air mass 1.5 (AM 1.5) simulated sunlight illumination. Pt nanoclusters are further incorporated at the g-C3N4/ZnO interface and a tricomponent C3N4/Pt/ZnO photoanode is adapted. Both g-C3N4 and ZnO act as the light absorber to create electron hole pairs, while the Pt nanoclusters serve as the cocatalyst to facilitate the transfer of the photogenerated electrons. The g-C3N4/Pt/ZnO photoanode generates a photocurrent density of 120 μA cm−2 at 0.5 V vs. Ag/AgCl in a 0.5 M Na2SO4 solution, which is 9 times higher than that of the anode made of ZnO and 4.5 times higher than that of the g-C3N4/ZnO anode. Furthermore, with the visible light absorption of g-C3N4, the g-C3N4/Pt/ZnO photoanode gives a photocurrent density of 32 μA cm−2 at 0.5 V vs. Ag/AgCl under visible light (λ > 420 nm) illumination.

Photochemical transformation of graphene oxide in sunlight.

Graphene oxide (GO) is promising in scalable production and has useful properties that include semiconducting behavior, catalytic reactivity, and aqueous dispersibility. In this study, we investigated the photochemical fate of GO under environmentally relevant sunlight conditions. The results indicate that GO readily photoreacts under simulated sunlight with the potential involvement of electron-hole pair creation. GO was shown to photodisproportionate to CO2, reduced materials similar to reduced GO (rGO) that are fragmented compared to the starting material, and low molecular-weight (LMW) species. Kinetic studies show that the rate of the initially rapid photoreaction of GO is insensitive to the dissolved oxygen content. In contrast, at longer time points (>10 h), the presence of dissolved oxygen led to a greater production of CO2 than the same GO material under N2-saturated conditions. Regardless, the rGO species themselves persist after extended irradiation equivalent to 2 months in natural sunlight, even in the presence of dissolved oxygen. Overall, our findings indicate that GO phototransforms rapidly under sunlight exposure, resulting in chemically reduced and persistent photoproducts that are likely to exhibit transport and toxic properties unique from parent GO.