Electron-hole Pair Generation Rate Research Articles

Prebreakdown excitation of crystals at double multiphoton resonance: III. Forbidden transitions

Within the three-band model of a crystal, we calculated the probabilities of forbidden-allowed four-photon transitions between the valence band and the conduction band under conditions of two-photon resonance on the adjacent transition between two conduction bands. We show that the transformation of the electronic band spectrum under the action of high-power light of a prebreakdown intensity leads to a nonmonotonic dependence of the rate of photogeneration electron-hole pairs on the radiation intensity. We determined conditions under which a small change in the intensity in the range 1011–1013 W/cm2 leads to an increase in the generation rate of electron-hole pairs by more than an order of magnitude, which can cause an increase in the number of nonequilibrium carriers to values sufficient to trigger processes of destruction of the material.

Plasmon Resonant Enhancement of Photocatalytic Water Splitting Under Visible Illumination

We demonstrate plasmonic enhancement of photocatalytic water splitting under visible illumination by integrating strongly plasmonic Au nanoparticles with strongly catalytic TiO2. Under visible illumination, we observe enhancements of up to 66× in the photocatalytic splitting of water in TiO2 with the addition of Au nanoparticles. Above the plasmon resonance, under ultraviolet radiation we observe a 4-fold reduction in the photocatalytic activity. Electromagnetic simulations indicate that the improvement of photocatalytic activity in the visible range is caused by the local electric field enhancement near the TiO2 surface, rather than by the direct transfer of charge between the two materials. Here, the near-field optical enhancement increases the electron-hole pair generation rate at the surface of the TiO2, thus increasing the amount of photogenerated charge contributing to catalysis. This mechanism of enhancement is particularly effective because of the relatively short exciton diffusion length (or minority carrier diffusion length), which otherwise limits the photocatalytic performance. Our results suggest that enhancement factors many times larger than this are possible if this mechanism can be optimized.

Plasmonic enhancement of photocatalytic decomposition of methyl orange under visible light

By integrating strongly plasmonic Au nanoparticles with strongly catalytic TiO 2, we observe enhanced photocatalytic decomposition of methyl orange under visible illumination. Irradiating Au nanoparticles at their plasmon resonance frequency creates intense electric fields, which can be used to increase electron–hole pair generation rate in semiconductors. As a result, the photocatalytic activity of large bandgap semiconductors, like TiO 2, can be extended into the visible region of the electromagnetic spectrum. Here, we report a 9-fold improvement in the photocatalytic decomposition rate of methyl orange driven by a photocatalyst consisting of strongly plasmonic Au nanoparticles deposited on top of strongly catalytic TiO 2. Finite-difference time-domain (FDTD) simulations indicate that the improvement in photocatalytic activity in the visible range can be attributed to the electric field enhancement near the metal nanoparticles. The intense local fields produced by the surface plasmons couple light efficiently to the surface of the TiO 2. This enhancement mechanism is particularly effective because of TiO 2’s short exciton diffusion length, which would otherwise limit its photocatalytic efficiency. Our electromagnetic simulations of this process suggest that enhancement factors many times larger than this are possible if this mechanism can be optimized.

Prebreakdown excitation of crystals at double multiphoton resonance: II. Analysis of the effects of transformation of electronic band spectrum

The probabilities of multiphoton transitions between the valence and conduction bands at two-photon resonance on the adjacent transition between two conduction bands are calculated within the three-band model of a crystal. It is shown that, due to the effects of transformation of electronic band spectrum in the field of a strong light wave, the dependence of the multiphoton generation rate of electron-hole pairs on the light intensity j contains ranges where a small change in j leads to an increase in the generation rate of electron-hole pairs by several orders of magnitude, which may increase the number of nonequilibrium carriers to the level sufficient for initiating destruction of the material.

Tip-enhanced secondary emission of a semiconductor quantum dot

We present a theoretical study of the luminescence enhancement in a semiconductor quantum dot in the near field of a metallic tip. The electric near field induced by a plane electromagnetic wave near the apex of the perfectly conducting conical tip has been calculated analytically. The results obtained have been used to evaluate the interband matrix element which determines the electron-hole pair generation rate and luminescence intensity of a semiconductor quantum dot. Secondary emission calculations have been carried out using a reduced density matrix approach assuming a radiation frequency close to resonance with the quantum dot fundamental transition. In order to establish the most favorable experimental conditions, we have analyzed the dependence of the interband matrix element and luminescence spectra intensity on the quantum dot size and the distance between the quantum dot and the tip apex. It has been demonstrated that the presence of the metallic tip results in a significant enhancement of the luminescence intensity and leads to a partial violation of the standard selection rules.

Current–voltage curve asymmetry in a-Si:H solar cells

The J–V curve asymmetry of an a-Si:H solar cell, that is exhibited under carrier mobility interchange, has been investigated computationally. The electron- and hole mobilities ( μ n and μ p) considered here are 13.2 and 2.2 cm 2V −1 s −1. We use the symbol ( n, p) to denote the case μ n =13.2 cm 2 V −1 s −1, μ p =2.2 cm 2 V −1 s −1, and the symbol ( p, n) denotes the case μ p=13.2 cm 2 V −1 s −1, μ n =2.2 cm 2 V −1 s −1. The maximum power output P M for the case ( n, p) is some 153% greater than that for the case ( p, n). We have investigated the possibility that this asymmetry is due to the asymmetrical character of the material and structural properties of the device. For example, the p-layer is thinner than the n-layer (we regard this as a positional asymmetry) and the dangling-bond defects in the p-layer are closer to midgap than the dangling-bond defects in the n-layer (we regard this as an energetic asymmetry). Other asymmetries include the dopant parameters, intrinsic layer defects, tail states and optical electron–hole pair generation rate profile. We eliminate the various asymmetries to construct a symmetrical cell which we find exhibits no difference in P M for the two cases. We then set about introducing the various asymmetries into the symmetrical cell. We find that doped layer thickness asymmetry favours the case ( p, n), but with only a slight difference in P M of about 0.5%. Dopant parameter asymmetry does not cause P M to differ for either case. Asymmetries pertaining to defects in the doped layers favour the case ( p, n) with a P M difference of about 3%. Intrinsic layer defect asymmetry has a moderate effect (20%) and favours ( n, p). An effect of similar magnitude (34%) arises from tail state asymmetries, although in favour of ( p, n). However, the positional asymmetry of the optical generation rate profile causes P M for the case ( n, p) to be some 227% greater than for the case ( p, n). It is clear that the asymmetry in the actual cell (153%) must be dominated by the asymmetry in the optical generation rate profile, since this was the only asymmetry capable of producing an effect large enough to be consistent with that observed in the actual cell. We conclude that the asymmetry in the actual cell performance is due to the nett effect of the various material and structural asymmetries, but that the positional asymmetry of the optical generation rate profile is the dominant cause.

The bulk damaged effects of clustered defects in irradiated silicon detectors

In this paper a new semiconductor device model based on deep acceptor states and clustered defects are used to explain the significant fraction of discrepancies in effective doping data from neutron-irradiated devices. It is postulated that the main difference between neutron and gamma damage is in the formation of dense cluster during neutron irradiation. So V2, E70, E170 and P6 defects that are estimated to be present inside the dense cluster will be responsible for leakage current annealing by exchange charge reaction. This mechanism can lead to an increase in electron–hole pair generation rate around two-orders of magnitude. Therefore by using the annealing behavior of leakage current, the concentration and spatial distribution of these defects are calculated and can be used to explain the effective doping concentration versus fluence truly.

Optical generation rate of electron-hole pairs in multilayer thin-film photovoltaic cells

A technique for calculating the optical generation rate of electron-hole pairs (EHPs) in the absorber layers of a multilayer photovoltaic cell is described, taking into account the multiple internal reflections that typically occur in such multilayer cells. The technique is based on the assumption that all multiply reflected and transmitted light, within the cell, combines incoherently and enables the determination of the EHP generation rate at every position within the absorber layers. This, in turn, yields the optical generation rate profile - a quantity that is needed in one-dimensional computer simulations of solar cells. Calculations pertaining to a typical a-Si:H-based solar cell indicate that ignoring the effect of front layers, such as glass and transparent conducting oxides (TCOs), has a significant effect on the calculated optical generation rate profile. Also, calculations suggest that the TCO in a typical glass/TCO/a-Si:H solar cell should have a real refractive index of ~2.0, in good agreement with the observed fact that ZnO is a good TCO for these types of cells.

Two-dimensional numerical simulation of edge-generated currents in type-inverted, p/sup +/-n single-sided silicon microstrip detectors

A theoretical study is presented showing that the reverse leakage current thermally generated at the cutting edge of type-inverted p/sup +/-n single-sided silicon microstrip detectors is limited. Such behaviour is shown to be related to a self-limiting mechanism acting on the edge surface generation, which prevents the net generation rate of electron-hole pairs at the detector edge from exceeding a saturation value, as the local hole density approaches its equilibrium value.

Hot-electron transport and impact ionization process inHg0.8Cd0.2Te

We present an expression for the electron impact ionization rate under moderate electric field within the framework of the Lei-Ting balance equation approach. We consider the case where the hole gas is cold and in thermal equilibrium, and the final electrons are located near the conduction-band edge. Using the Kane band model to describe the conduction band, we calculate the conduction\char21{}conduction\char21{}heavy-hole\char21{}conduction electron-hole pair generation rate and current-voltage characteristics of n-type ${\mathrm{Hg}}_{0.8}{\mathrm{Cd}}_{0.2}\mathrm{Te}.$

Analysis of switching dynamics of asymmetric Fabry–Perot symmetric self-electro-optic effect devices with extremely shallow quantum wells

This article investigates the effects of asymmetric Fabry–Perot (AFP) cavity structures on the switching dynamics of symmetric self-electro-optic effect devices (S-SEEDs) made of AlxGa1−xAs/GaAs (x⩽0.05) extremely shallow quantum wells (ESQWs). We analyze the switching dynamics of AFP ESQW S-SEEDs (AE-SEEDs) by means of an impulse photocurrent response function and corresponding voltage transients of the two AFP p-i(ESQWs)-n diodes connected in series. The photocurrent response function is obtained by using an appropriate electron-hole pair generation rate and the Green’s function method. The response function includes the LO phonon scattering from bound to continuum states and the carrier transit in the continuum states. Large internal optical fields and thin intrinsic region thicknesses of AFP SEEDs reduce the required incident switching energy and operating voltage, respectively. Our analyses show that the switching energy of an impedance-matched AE-SEED is about 3.0 fJ/μm2, while that of a conventional antireflection coated ESQW S-SEED (E-SEED) is about 4.1 fJ/μm2. Considering practical RC time constants and device sizes, the switching time of an impedance-matched AE-SEED is found to be as low as 10 ps while that of an E-SEED is about 18 ps for the same incident energy of 4.1 fJ/μm2.

Optically detected ESR studies of recombination processes in a-Si:H

A study of recombination processes in high-quality (intrinsic) and light-soaked hydrogenated amorphous silicon (a-Si:H) using the technique of optically detected electron spin resonance (ODESR) is reported. A time-domain detection scheme is employed, instead of the conventional lock-in technique, to record the ODESR lineshapes, which effectively minimizes the well-known interference between the ‘enhancing’ and ‘quenching’ signals. Monitoring the photoluminescence (PL), below 1.1 eV, in the intrinsic sample, a nearly symmetric enhancing signal ( g = 2.008) related to radiative recombination of spin pairs in shallow band-tail states and an asymmetric quenching signal ( g = 2.005) related non-radiative recombination of spin pairs in deep band tail states are observed. A quenching signal at half-field ( g ≌ 4) is also observed. The light-soaked sample exhibits an additional signal ( g = 2.006) which is related to the dangling bonds. The PL excitation energy, E x, is varied from above to below the optical gap to study possible changes in the recombination processes. It is found that the ODESR lineshapes are essentially the same for E x > 1.5 eV and some subtle changes occur for E x < 1.5 eV. The signal intensity, however, depends strongly on the excitation energy for a constant electron-hole pair generation rate.

The Growth, Steady State and Decay of the Photocarrier Population at low Temperatures

ABSTRACTWe used the transient infrared photoconductivity to measure the rise and fall of the non-equilibrium photocarrier concentration n at helium temperatures and the conditions for recombination. The growth rate of n was found to be only a few percent of the electron-hole pair generation rate G in agreement with LESR data of Street and Biegelsen. The saturation value of n is about 1017cm−3 and nearly independent of G. After turning off the generation light, the residual n requires an hour to decrease by a factor 2 but it can be made to recombine in seconds by infrared light.

Defect-related generation-recombination region in heterojunction diodes

A numerical analysis is presented of semiconductor heterojunctions containing deep defect levels, in order to study the mechanism of generation or recombination of electron-hole pairs induced by thermal or optical transitions involving deep defect energy levels. The final aim is to determine the width of the effective generation or recombination region Wgr and compare to the width of the space-charge region W. Numerical solution of Poisson’s equation and the current-transport equations yield the electrical potential and the quasi-Fermi levels for electrons, holes, and occupied deep defects. The resulting transition rates yield the net generation rate of electron-hole pairs as function of position x. An application is made to an InGaAs/InP heterojunction with a midgap level on the InGaAs side. Results for a Si homojunction with a deep acceptor level at Ec−0.54 eV are given for comparison. The applications show that in all cases of reverse bias Wgr is smaller than W, reaching for some parameters describing the heterojunction system a value of 1/3 for the ratio Wgr/W. Usually the two widths are assumed to be equal. The physical origin of this reduction is the existence of competing recombination processes due to free-carrier tails extending from the neutral regions into the space-charge region. In heterojunction structures, this effect is even enhanced in case of accumulation of free carriers due to conduction- or valence-band discontinuities at the junction plane. It is shown that if one considers defects states only in a finite region close to the junction, this may yield marked differences in the value of the resulting current as compared to the homojunction case.

Kinetics of Light-Induced Degradation of a-Si:H Solar Cells Compared to I-Layer Films

ABSTRACTWe report the CPM defect density of a-Si:H material and the performance characteristics of pin solar cells during high-intensity light-soaking. In one group of experiments we compared the effects of soaking with monochromatic light from a Kr+ laser to white light from a Xe arc lamp. The effects are identical for the same electron-hole pair generation rate. In a second group of experiments we light-soaked at the two different temperatures of 50°C and 90°C. At 90°C the defect density saturates at a lower value than at 50°C, and correspondingly the cell performance parameters saturate at higher values.

Electron-hole pair generation rate of a monolithic integrated waveguide/photodetector: application to the modeling of monolithic integrated waveguide/p-i-n photodiodes

A report on the theoretical study of an integrated waveguide/photodetector is presented. The optical modeling is presented of an integrated waveguide/photodetector based on the absorption of the evanescent optical field in an absorbing layer which is deposited on the waveguiding layer. From this modeling, the expression of the electron-hole pair generation rate for such a device is established. This result is applied to the calculation of the dynamic quantum efficiency of an integrated waveguide/p-i-n photodiode. The static and dynamic behaviors of GaInAs p-i-n photodiodes monolithically integrated on a classical N/sup -//n/sup +/ InP homostructure waveguide or on a GaInAsP/InP heterostructure waveguide are discussed, and optimized structures are pointed out. >

Universal behaviour of the normalized photoconductivity at low temperatures in amorphous semiconductors

We have measured the photoconductivity ϱ p between 4K and 300K for a variety of amorphous semiconductors including Si with and without hydrogen, Ge, Se, and several chalcogenide alloys. Below a certain temperature ϱ p of each sample becomes asymptotically constant and ϱ p ∞ G γ with 0.93 <γ<1.0. where G is the electron-hole pair generation rate. At 4.2 K the value of ϱ p / eG lies between 4×10 −14 and 3×10 −11 cm 2/V. At higher temperatures ϱ p increases rapidly and γ falls to around 0.5. The data test recently proposed theories of σ p at low temperature.

Theory of electrical transport and recombination in polycrystalline semiconductors under optical illumination

A method for describing electrical transport in polycrystalline material has been developed by solving a current continuity equation that involves drift, diffusion and nonuniform generation rate of electron-hole pairs. By taking the recombination current at the grain boundary interface, at the grain boundary space-charge region, and at the bulk into consideration a theoretical method for grain boundary recombination has also been developed. While attempting to develop the theory we have taken care to include the effects of Shockley-Read-Hall recombination statistics, of trap-assisted recombination statistics, and of the heavy dopings of the semiconductor region(s). On the basis of the present theory the dependence of the grain-boundary potential-barrier height V on the grain size, doping concentrations, and carrier generation rate has been studied in detail. It is found that the effective recombination velocity at the edge of the grain boundary space-charge region is a function of V, and that it increases with grain size until it attains a peak value.

Theoretical characteristics of a solar cell with a specific thin-film design

Expressions for the net generation rate of electron-hole pairs under illumination and the ideal Current-voltage ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J-V</tex> ) characteristics of a specific type of thin-film solar cell are derived. The thin-film configuration consists of an n- on p-type semiconductor with a rough upper surface (which is approximated by a distribution of smooth microscopic surfaces or facets randomly oriented) and a diffusive, perfectly reflecting lower surface. This configuration leads to a significant enhancement of the radiation flux in the film and theoretically to a higher collection probability and conversion efficiency compared to the conventional solar cell based on a much thicker crystalline film. The <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J-V</tex> characteristics are finally expressed in terms of a set of double integrals in a form suitable for numerical analysis.

Variable-gap cells with several junction energy gaps for a high efficiency solar energy system

j system for solar energy conversion based on graded-band-gap cells which match the solar spectrum. If a compound semiconductor is grown with a variable composition along the thickness, a particular section of the semiconductor can produce a surface on which the energy gap varies according to the spectral dispersion given by a suitable optical system. The following aims are therefore achieved: (a) a high utilization factor of the solar spectrum; (b) a drastic reduction in the surface recombination; (c) an increase in the diffusion length of the minority carriers. The electron-hole pair generation rate is derived, the continuity equation is solved and the photocurrents are determined, as the previous results of other workers are not suitable in this case. The spectral response of the system reaches a very high value for almost the whole solar spectrum.