Enhanced Photoelectric and Photothermal Responses on Silicon Platform by Plasmonic Absorber and Omni‐Schottky Junction

Recent progresses in plasmon‐induced hot electrons open up the possibility to achieve photon harvesting beyond the fundamental limit imposed by band‐to‐band transitions in semiconductors. To obtain high efficiency, both the optical absorption and electron emission/collection are crucial factors that need to be addressed in the design of hot electron devices. Here, we demonstrate a photoresponse as high as 3.3mA/W at 1500nm on a silicon platform by plasmonic absorber (PA) and omni‐Schottky junction integrated photodetector, reverse biased at 5V and illuminated with 10mW. The PA fabricated on silicon consists of a monolayer of random Au nanoparticles (NPs), a wide‐band gap semiconductor (TiO₂) and an optically thick Au electrode, resulting in broadband near‐infrared (NIR) absorption and efficient hot‐electron transfer via an all‐around Schottky emission path. Meanwhile, time and spectral‐resolved photoresponse measurements reveal that embedded NPs with superior absorption resembling plasmonic local heating sources can transfer their energy to electricity via the photothermal mechanism, which until now has not been adequately assessed or rigorously differentiated from the photoelectric process in plasmon‐mediated photon harvesting nano‐systems.     

Theoretical assessment of electronic transport in InN

Among the group-III nitrides, InN displays markedly unusual electronic transport characteristics due to its smaller effective mass, high peak velocity and high background electron concentration. First, a non-local empirical pseudopotential band structure of InN is obtained in the light of recent experimental and first-principles results. This is utilized within an ensemble Monte Carlo framework to illuminate the interesting transport properties. It is observed that InN has a peak velocity which is about 75% higher than that of GaN while at higher fields its saturation velocity is lower than that of GaN. Because of the strongly degenerate regime brought about by the high background electron concentration, the electron–electron interaction is also investigated, but its effect on the steady-state and transient velocity–field characteristics is shown to be negligible. Finally, hot phonon generation due to excessive polar optical phonon production in the electron scattering and relaxation processes is accounted for. The main findings are the appreciable reduction in the saturation drift velocity and the slower recovery from the velocity overshoot regime. The time evolution of the hot phonon distribution is analysed in detail and it is observed to be extremely anisotropic, predominantly along the electric force direction.     

Electron drift velocity and mobility in graphene

We present a theoretical study of the electric transport properties of graphene-substrate systems. The drift velocity, mobility, and temperature of the electrons are self-consistently determined using the Boltzmann equilibrium equations. It is revealed that the electronic transport exhibits a distinctly nonlinear behavior. A very high mobility is achieved with the increase of the electric fields increase. The electron velocity is not completely saturated with the increase of the electric field. The temperature of the hot electrons depends quasi-linearly on the electric field. In addition, we show that the electron velocity, mobility, and electron temperature are sensitive to the electron density. These findings could be employed for the application of graphene for high-field nano-electronic devices.     

Hot‐electron drift velocity and hot‐phonon decay in AlInN/AlN/GaN

A nanosecond‐pulsed current–voltage technique was applied to study hot‐electron transport along the two‐dimensional electron gas channel confined at a nominally undoped AlInN/AlN/GaN heterointerface. Hot‐electron drift velocity was deduced under the assumptions of uniform longitudinal electric field and field‐independent electron sheet density. At a fixed electric field strength, a resonance‐type non‐monotonous dependence of the velocity on the electron density was found in the investigated range from 1 to When the electric field increased from 20 kV/cm to 80 kV/cm, the peak velocity increased from ～1.1 to cm/s, and the position of the resonance shifted from ～1.1 to ～1.2 respectively. The resonance position correlates with that for the fastest decay of hot phonons known from independent experiment. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Two- and three-photon spectroscopy of solids under high pressure

Abstract

The present article reviews measurements under hydrostatic pressure on the tetrahedrally-bonded semiconductors CuCI, CuBr, ZnO, ZnS, ZnSe, ZnTe, Cds, and AgGaS₂, by two-photon spectroscopy and on the alkali halides NaC1, KBr, KI, and RbI by two- and three-photon spectroscopy. It is shown that these nonlinear techniques yield a much higher precision than linear spectroscopy in the determination of the pressure dependence of the electronic band gap. Additionally, it is often possible to determine the pressure dependence of other parameters of the electronic band structure like exciton binding energy, spin-orbit coupling, crystal-field interaction, and Luttinger parameters. In the case of the alkali halides with their rather large band gaps there is a further advantage of three-photon spectroscopy, namely that pressure-cell windows need only be transparent for photons of one third of the band gap energy.     

Role of hot electron transport in scintillators: A theoretical study

Despite recent intensive study on scintillators, several fundamental questions on scintillator properties are still unknown. In this work, we use ab‐initio calculations to determine the energy dependent group velocity of the hot electrons from the electronic structures of several typical scintillators. Based on the calculated group velocities and optical phonon frequencies, a Monte‐Carlo simulation of hot electron transport in scintillators is carried out to calculate the thermalization time and diffusion range in selected scintillators. Our simulations provide physical insights on a recent trend of improved proportionality and light yield from mixed halide scintillators. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Inverted structure perovskite solar cells: A theoretical study

We analysed perovskite CH₃NH₃PbI_3-xCl_x inverted planer structure solar cell with nickel oxide (NiO) and spiro-MeOTAD as hole conductors. This structure is free from electron transport layer. The thickness is optimized for NiO and spiro-MeOTAD hole conducting materials and the devices do not exhibit any significant variation for both hole transport materials. The back metal contact work function is varied for NiO hole conductor and observed that Ni and Co metals may be suitable back contacts for efficient carrier dynamics. The solar photovoltaic response showed a linear decrease in efficiency with increasing temperature. The electron affinity and band gap of transparent conducting oxide and NiO layers are varied to understand their impact on conduction and valence band offsets. A range of suitable band gap and electron affinity values are found essential for efficient device performance.     

Plasmon‐Enhanced Second‐ and Third‐Order Harmonic Generation in Spherical Free‐Electron Nanoclusters with Diffuse Surface

The nonlinear scattering of a laser pulse off spherical nanoclusters with free electrons and with a diffuse surface is examined in the collisionless hydrodynamics approximation in the framework of perturbation theory with respect to the laser pulse intensity, as well as of the steady‐state approximation. In a previous publication [S.V. Fomichev and W. Becker, Phys. Rev. A 81 , 063201 (2010)] we reported the full nonlinear hydrodynamic model of forced collective electron motion confined to a cluster with diffuse surface and introduced two different perturbation theories corresponding to different laser intensity regimes. In the current paper, in the framework of this hydrodynamic model we focus on the properties of plasmon resonance‐enhanced third‐harmonic generation in a spherical cluster and its dependence on the shape of its diffuse surface whose role increases for nonlinear processes. At the same time, the quadrupole second‐harmonic generation in a spherical cluster is also inspected as a necessary intermediate step. Both cold metal clusters in vacuum or in a dielectric surrounding and hot laser‐heated and laser‐ionized clusters are considered within the same approach for a wide range of the fundamental laser frequency. Nonlinear laser excitation of the dipole plasmon Mie resonance in spherical clusters, as well as of other respective multipole plasmon resonances is investigated analytically and numerically in detail (position, width, and strength) versus the cluster‐surface diffuseness, the outer ionization degree in charged clusters, the electron‐density diffuseness, and their interplay. Under certain conditions, depending on the various cluster parameters, different secondary nonlinear resonances are found. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electronic structure and covalency in superionic conductors

The electronic states of noble metal halides and alkali halides are calculated by the DVXα cluster method to get more microscopic evidence for the p — d hybridization and the covalency in noble metal halides. It is found that both components of anti-bonding and bonding exist in the diagram of overlap population (DOP) for AgX (X=halogen) and these two components are made up of the 4d band of Ag ion and the p band of halogen ion, which form the p — d hybridization. The covalency of noble metal halides is in the border between that of the fourfold coordinated compounds such as AgI and that of the sixfold coordinated compounds such as AgCl. These calculation results on the covalency are compared with the Phillips's ionicity. Paper presented at the Patras Conference on Solid State Ionics, Patras, Greece, Sept. 14 – 18, 2004.     

Complexation of bis‐crown stilbene with alkali and alkaline‐earth metal cations. Ultrafast excited state dynamics of the stilbene‐viologen analogue charge transfer complex

The complex formation of bis(18‐crown‐6)stilbene ( 1 ) and its supramolecular donor‐acceptor complex with N,N′‐bis(ammonioethyl) 1,2‐di(4‐pyridyl)ethylene derivative ( 2 ) with alkali and alkaline‐earth metal perchlorates has been studied using absorption, steady‐state fluorescence, and femtosecond transient absorption spectroscopy. The formation of 1 ?Mⁿ⁺ and 1 ?(Mⁿ⁺)₂ complexes in acetonitrile was demonstrated. The weak long‐wavelength charge‐transfer absorption band of 1 · 2 completely vanishes upon complexation with metal cations because of disruption of the pseudocyclic structure. The spectroscopic and luminescence parameters, stability constants, and 2‐stage dissociation constants were calculated. The initial stage of a recoordination process was found in the excited complexes 1 ?M⁺ and 1 ?(M⁺)₂ (M = Li, Na). The pronounced fluorescence quenching of 1 · 2 is explained by very fast back electron transfer (τ_et = 0.397 ps). The structure of complex 1 · 2 was studied by X‐ray diffraction; stacked ( 1 · 2 )_m polymer in which the components were connected by hydrogen bonding and stacking was found in the crystal. These compounds can be considered as novel optical molecular sensors for alkali and alkaline‐earth metal cations.     

Interstitial-exciton interaction in KBr:Na

The intensity of luminescence of the alkali halides submitted to an ionizing radiation decreases when color centers are created. The action of the interstitial as a quenching center is directly shown in KBr:Na. At LNT, the crystal has an emission band (430 nm), due to the exciton localized close to a sodium, which disappears according to a kinetics anticorrelated with that of the formation of the H_A (interstitials stabilized by Na) unde ionizing radiation.     

Influence of electronic structures of doped TiO2 on their photocatalysis

The modification of bandgap of TiO₂ was intensively studied for decades to improve its visible light absorbance efficiency. The practical application potential of TiO₂ as photocatalysts for water splitting and water purification has motivated enduring experimental and theoretical research of the doping effects in bulk and nanosized TiO₂ using transition metals, rear earths, p‐block metals and metalloids, and non‐metal elments as dopants to decrease the bandgap of TiO₂. This review summarized the typical theoretical results of the dopant induced variation in electronic structure, bandgap, and density of states of TiO₂. The codoping effects of metal/metal, metal/non‐metal combinations were also introduced briefly to display the modification of electronic structures. Some results were accompanied by experimental results to demonstrate the influence of improved light absorbance efficiency on the photocatalytic performance. The doping effects on the density of states of surface were also summarized briefly. The metal dopants show clear influences on the 3d electrons of titanium to elevate or depress the minimum of conduction band, while the non‐mental dopants mainly interact with the 2p electrons of oxygen to change the position of the maximum of the valence band. The review also noticed the theoretical development of the doping effect with the establishment of novel models, such as the water–TiO₂surface interaction. It should be noted that the theoretical models rarely consider the doping induced variation of defect types and concentration, Fermi level position, surface active sites, and charge transport due to the ground state simulation and shortcoming of density functional theory (DFT). The phenomenological explanations of the experimental results are arbitrary in most of the reports. A universal model is required to explain the complex dependence of the process of photocatalysis on the semiconducting properties, such as bandgap, Fermi level, charge transport, and surface states. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Band structure and optical properties of alkali metal halides

Optical spectra, photoemission spectra, and photoconductivity spectra of alkali metal halides are analyzed using energy band structure calculations and selection rules for direct and indirect transitions. The main feature of the band structure of MeN₃ (Me: Na, K, Rb, Cs) as proposed by the authors is the presence of two conductivity bands, one anionic and the other cationic in nature. A weak dispersion of the valence subband indicates that phonons may yield a significant contribution to the observed spectra. All of the optical and photoemission spectra so far reported for the metal azides may be explained on the basis of the proposed band model.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 3–7, January, 1995.     

电致发光加速层二氧化硅的电子高场迁移率

根据非晶态半导体的能带理论，讨论了分层优化薄膜电致发光方案中非晶二氧化硅加速层中的电子在高电场中的输运行为.研究结果表明：在高电场下，由于电场的存在降低了陷阱之间的平均势垒高度.在费密能级附近处的杂质及缺陷定域态和导带尾定域态中，电子的输运主要表现为电场增强的热辅助式跳跃传导；而在导带扩展态中，电子的输运仍像晶态半导体那样表现为共有化运动.此外，以实验数据为基础，计算出了非晶二氧化硅中电子的迁移率、最小金属电导率、导带迁移率边界状态密度及费密能级处的状态密度. 关键词：     

Rotational analysis of weak (4, 6) hot band in the comet-tail (AΠi-XΣ) system of the CO

The ro-vibrational spectrum of the weak (4, 6) hot band in the comet-tail (A²Π_i-X²Σ⁺) system of CO⁺ is observed by employing optical heterodyne and magnetic rotation enhanced velocity modulation spectroscopy. One hundred and twenty-four spectral lines are assigned to this band; thus, precise molecular constants of the levels involved are obtained from a weighted nonlinear least-squares fitting procedure combining with our previous spectrum of the (3, 6) band.     

Influence of light waves on the thermoelectric power under large magnetic field in III‐V,ternary and quaternary materials

We study theoretically the influence of light waves on the thermoelectric power under large magnetic field (TPM) for III‐V, ternary and quaternary materials, whose unperturbed energy‐band structures, are defined by the three‐band model of Kane. The solution of the Boltzmann transport equation on the basis of this newly formulated electron dispersion law will introduce new physical ideas and experimental findings in the presence of external photoexcitation. It has been found by taking n‐InAs, n‐InSb, n‐Hg_1‐xCd_xTe and n‐In_1‐xGa_xAs_yP_1‐y lattice matched to InP as examples that the TPM decreases with increase in electron concentration, and increases with increase in intensity and wavelength, respectively in various manners. The strong dependence of the TPM on both light intensity and wavelength reflects the direct signature of light waves that is in direct contrast as compared with the corresponding bulk specimens of the said materials in the absence of external photoexcitation. The rate of change is totally band‐structure dependent and is significantly influenced by the presence of the different energy‐band constants. The well‐known result for the TPM for nondegenerate wide‐gap materials in the absence of light waves has been obtained as a special case of the present analysis under certain limiting conditions and this compatibility is the indirect test of our generalized formalism. Besides, we have also suggested the experimental methods of determining the Einstein relation for the diffusivity:mobility ratio, the Debye screening length and the electronic contribution to the elastic constants for materials having arbitrary dispersion laws.     

Fluid Model of a Sheath Formed in Front of an Electron Emitting Electrode Immersed in a Plasma with Two Electron Temperatures

The formation of a sheath in front of a negatively biased electrode (collector) that emits electrons is studied by a one‐dimensional fluid model. Electron and ion emission coefficients are introduced in the model. It is assumed that the electrode is immersed in a plasma that contains energetic electrons. The electron velocity distribution function is assumed to be a sum of two Maxwellian distributions with two different temperatures, while the ions and the emitted electrons are assumed to be monoenergetic. The condition for zero electric field at the collector is derived. Using this equation the dependence of electron and ion critical emission coefficients on various parameters ‐ like the ratio between the hot and cool electron density, the ratio between hot and cool electron temperature and the initial velocity of secondary electrons ‐ is calculated for a floating collector. A modification of the Bohm criterion due to the presence of hot and emitted electrons is also given. The transition between space charge limited and temperature limited electron emission for a current‐carrying collector is also analyzed. The critical potential, where this transition occurs, is calculated as a function of several parameters like the Richardson emission current, the ratio between the hot and cool electron density, the ratio between hot and cool electron temperature and the initial velocity of secondary electrons. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Changes in the vibrational modes of carbon nanotubes induced by electron‐beam irradiation: resonance Raman spectroscopy

Influence of electron‐beam (e‐beam) irradiation on multi‐walled (MW) and single‐walled (SW) carbon nanotube films grown by microwave chemical vapor deposition technique is investigated. These films were subjected to an e‐beam energy of 50 keV from a scanning electron microscope for 2.5, 5.5, 8.0, and 15 h, and to 100 and 200 keV from a transmission electron microscope for a few minutes to ∼2 h continuously. Such conditions resemble an increased temperature and pressure regime enabling a degree of structural fluidity. To assess structural modifications, they were analyzed prior to and after irradiation using resonance Raman spectroscopy (RRS) in addition to in situ monitoring by electron microscopy. The experiments showed that with extended exposures, both types of nanotubes displayed various local structural instabilities including pinching, graphitization/amorphization, and formation of an intramolecular junction (IMJ) within the area of electron beam focus possibly through amorphous carbon aggregates. RRS revealed that irradiation generated defects in the lattice as quantified through (1) variation of the intensity of radial breathing mode (RBM), (2) intensity ratio of D to G band (I_D/I_G), and (3) positions of the D and G bands and their harmonics (D* and G*) and combination bands (D + G). The increase in the defect‐induced D band intensity, quenching of RBM intensity, and only a slight increase in G band intensity are some of the implications. The MW nanotubes tend to reach a state of saturation for prolonged exposures, while the SW ones transform from a semiconducting to a quasi‐metallic character. Softening of the q = 0 selection rule is suggested as a possible reason to explain these results. Furthermore, these studies provide a contrasting comparison between MW and SW nanotubes. Copyright © 2006 John Wiley & Sons, Ltd.     

Luminescence of AgHal phosphors containing a silver sulfide impurity

In a study of the luminescence spectra of AgHal single crystals containing various levels of a silver sulfide impurity, the mechanism for the luminescence is established. The thermal quenching of the steady-state luminescence and of the light sum of the luminescence of AgHal (Ag₂S) phosphors is due to ionic processes, as in pure silver halides. The activation energy for the thermal quenching of the light sum is found. The ratio of the cross sections for electron recombination with holes localized at the luminescence centers responsible for the various luminescence bands is found in a kinetic study of the decay of the optical emission stimulated in these bands.