Nonequilibrium Charge Carriers Research Articles

Defects responsible for charge carrier removal and correlation with deep level introduction in irradiated β-Ga2O3

Carrier removal rates and electron and hole trap densities in β-Ga2O3 films grown by hydride vapor phase epitaxy (HVPE) and irradiated with 18 MeV α-particles and 20 MeV protons were measured and compared to the results of modeling. The electron removal rates for proton and α-radiation were found to be close to the theoretical production rates of vacancies, whereas the concentrations of major electron and hole traps were much lower, suggesting that the main process responsible for carrier removal is the formation of neutral complexes between vacancies and shallow donors. There is a concurrent decrease in the diffusion length of nonequilibrium charge carriers after irradiation, which correlates with the increase in density of the main electron traps E2* at Ec − (0.75–0.78) eV, E3 at Ec − (0.95–1.05) eV, and E4 at Ec − 1.2 eV. The introduction rates of these traps are similar for the 18 MeV α-particles and 20 MeV protons and are much lower than the carrier removal rates.

Influence of structural rigidity of Cu(II)/Mo(VI) complexes on the photoconductivity of film polymeric composites

Novel film polymeric composites based on the non-photoconducting polyvinyl butyral with heterometallic complexes (NH4)4n{[Cu(en)(H2O)][P2Mo5O23]}n·3.5nH2O (1) and (NH4)2n{[Cu(en)2][Cu(en)(H2O)][P2Mo5O23]}n·3nH2O (2) [en = ethylenediamine] have been prepared. Both compounds are similar and contain Strandberg anions [P2Mo5O23]6– linked to Cu-diamine species. The main structural difference between complexes is the presence of additional bridging [Cu(en)2] fragments in 2, which provide an extra connectivity between the neighboring polyoxometalates. However, the different saturations and relaxation times of the photoresponse, as well as different activation energies, are shown. The variability of the photoconducting properties can be explained by the influence of a chain rigidity on the transport of non-equilibrium charge carriers.

Dipolar Biexcitons in Lateral Traps in Si/SiGe/Si Heterostructures

Si/Si1–xGex/Si heterostructures with large-scale (micrometer-size) lateral potential fluctuations at the upper SiGe/Si-cap heterointerface are grown. These potential fluctuations are caused by partial strain relaxation in the SiGe layer. Low-temperature photoluminescence (PL) spectra show that these fluctuations form lateral traps where photoexcited nonequilibrium charge carriers are accumulated and bind into dipolar excitons, which ultimately recombine. At temperatures below 6 K, a new narrow line with a width considerably less than that of the dipolar exciton PL line emerges in the spectra as the level of excitation increases. It is shown that this line is associated with the recombination of dipolar biexcitons in large-scale traps.

Electro- and Photophysical Properties of Polymeric Film Composites with Monomers of Pyrido[2,1-b]benzothiazole Derivatives

It was found that film composites of the copolymer of styrene with octyl methacrylate with additions of the monomers of pyrido[2,1-b]benzothiazole derivatives have hole-type photoconductivity while the internal photoelectric effect in this medium is determined by the photogeneration of charge carriers from the monomers and transfer of nonequilibrium charge carriers through these monomers. In transition to a monomer with a more efficient conjugation system the photoconductivity and photovoltaic response are increased.

Photoconductivity of Macroporous and Nonporous Silicon with Ultrathin Oxide Layers

The photoconductivity of macroporous silicon with ultrathin oxide layers of 2.7–30 nm in thickness at short-wave optical excitation was studied. The following feature was revealed: a nonmonotonic change in the photoconductivity as a function of the oxide thickness. At a minimum thickness, the photoconductivity is negative; in the interval 6.8–15 nm, it is very much suppressed; at 15–30 nm, it is positive. Suppression of photoconductivity over a wide thickness range indicates an abnormally high concentration of traps and capture centers for charge carriers of both signs. Such a change in the photoconductivity corresponds to the known results on the continuous morphological rearrangement of the oxide in the thickness range from 6–7 nm to 12–15 nm from the coesite-like (4-membered SiO4 tetrahedra rings) to the tridymite-like (6-membered SiO4 tetrahedra rings). The suppression of photoconductivity in the intermediate range probably demonstrates the collective, antisynergetic action of these coexisting oxide forms on the nonequilibrium charge carriers. These coexisting oxide forms manifest themselves as an unusual collective defect.

SEPARATE DETERMINATION OF THE PHOTOELECTRIC PARAMETERS OF THE BASE REGION OF N+−P(N)−P+ SILICON STRUCTURE BY NONCONTACT METHOD BASED ON QUANTUM EFfi CIENCY RELATION MEASUREMENTS AT TWO WAVELENGTHS

A noncontact method for the determination of recombination parameters of p(n) layer local ranges of silicon n+−p(n)−p+ structures is considered. The method is based on local illumination of the investigated structure by two different absorbed light beams. Initially both beams illuminate simultaneously one side of the local range of this structure and then another side. The intensities оf the light beams are modulated at one frequency so the alternative photo−voltage becomes equal to zero. In this case the short current regime is established for its alternating component. As a consequence the nonilluminated parts of the structure do not shunt its illuminated part. The ratios of the light beam intensities are measured under these conditions. In this work we calculated nomograms for separate determination of the nonequilibrium charge carrier lifetime of the illuminated p(n) local space and its surface recombination velocity using the measured intensity ratios. The calculations were performed at low injection level for one dimensional case. The nomograms were calculated at wave lengths of 1064 and 808 nm for various thicknesses of the n+−p(n)−p+ structures and various modulation frequencies. It was found that the nomograms almost do not depend on the modulation frequency if the live time of the nonequilibrium charge carriers is less than the modulation period. Furthermore, we observed that the nomograms shift substantially and change their shape for thin structures if the diffusion time of nonquilibrium charge carriers from the rear side of the structure to its face side becomes less than their lifetime. In this case the nomograms may be only used for the determination of the surface recombination velocity of the nonquilibrium charge carriers at the rear side of the structure.

Diffusion length of non-equilibrium minority charge carriers in β-Ga2O3 measured by electron beam induced current

The spatial distribution of electron-hole pair generation in β-Ga2O3 as a function of scanning electron microscope (SEM) beam energy has been calculated by a Monte Carlo method. This spatial distribution is then used to obtain the diffusion length of charge carriers in high-quality epitaxial Ga2O3 films from the dependence of the electron beam induced current (EBIC) collection efficiency on the accelerating voltage of a SEM. The experimental results show, contrary to earlier theory, that holes are mobile in β-Ga2O3 and to a large extent determine the diffusion length of charge carriers. Diffusion lengths in the range 350–400 nm are determined for the as-grown Ga2O3, while processes like exposing the samples to proton irradiation essentially halve this value, showing the role of point defects in controlling minority carrier transport. The pitfalls related to using other popular EBIC-based methods assuming a point-like excitation function are demonstrated. Since the point defect type and the concentration in currently available Ga2O3 are dependent on the growth method and the doping concentration, accurate methods of diffusion length determination are critical to obtain quantitative comparisons of material quality.

State-Resolved Metal Nanoparticle Dynamics Viewed through the Combined Lenses of Ultrafast and Magneto-optical Spectroscopies.

Electronic carrier dynamics play pivotal roles in the functional properties of nanomaterials. For colloidal metals, the mechanisms and influences of these dynamics are structure dependent. The coherent carrier dynamics of collective plasmon modes for nanoparticles (approximately 2 nm and larger) determine optical amplification factors that are important to applied spectroscopy techniques. In the nanocluster domain (sub-2 nm), carrier coupling to vibrational modes affects photoluminescence yields. The performance of photocatalytic materials featuring both nanoparticles and nanoclusters also depends on the relaxation dynamics of nonequilibrium charge carriers. The challenges for developing comprehensive descriptions of carrier dynamics spanning both domains are multifold. Plasmon coherences are short-lived, persisting for only tens of femtoseconds. Nanoclusters exhibit discrete carrier dynamics that can persist for microseconds in some cases. On this time scale, many state-dependent processes, including vibrational relaxation, charge transfer, and spin conversion, affect carrier dynamics in ways that are nonscalable but, rather, structure specific. Hence, state-resolved spectroscopy methods are needed for understanding carrier dynamics in the nanocluster domain. Based on these considerations, a detailed understanding of structure-dependent carrier dynamics across length scales requires an appropriate combination of spectroscopic methods. Plasmon mode-specific dynamics can be obtained through ultrafast correlated light and electron microscopy (UCLEM), which pairs interferometric nonlinear optical (INLO) with electron imaging methods. INLO yields nanostructure spectral resonance responses, which capture the system's homogeneous line width and coherence dynamics. State-resolved nanocluster dynamics can be obtained by pairing ultrafast with magnetic-optical spectroscopy methods. In particular, variable-temperature variable-field (VTVH) spectroscopies allow quantification of transient, excited states, providing quantification of important parameters such as spin and orbital angular momenta as well as the energy gaps that separate electronic fine structure states. Ultrafast two-dimensional electronic spectroscopy (2DES) can be used to understand how these details influence state-to-state carrier dynamics. In combination, VTVH and 2DES methods can provide chemists with detailed information regarding the structure-dependent and state-specific flow of energy through metal nanoclusters. In this Account, we highlight recent advances toward understanding structure-dependent carrier dynamics for metals spanning the sub-nanometer to tens of nanometers length scale. We demonstrate the use of UCLEM methods for arresting interband scattering effects. For sub-nanometer thiol-protected nanoclusters, we discuss the effectiveness of VTVH for distinguishing state-specific radiative recombination originating from a gold core versus organometallic protecting layers. This state specificity is refined further using femtosecond 2DES and two-color methods to isolate so-called superatom state dynamics and vibrationally mediated spin-conversion and emission processes. Finally, we discuss prospects for merging VTVH and 2DES methods into a single platform.

Intraexciton and Intracenter Terahertz Radiation from Doped Silicon under Interband Photoexcitation

Terahertz photoluminescence of boron- and phosphorus-doped silicon at low temperatures under interband photoexcitation is investigated. The lines of radiative transitions between free-exciton levels and between the levels of shallow impurity centers are observed. The intensities of these lines exhibit different dependences on temperature and excitation intensity. At temperatures near the temperature of liquid helium (T ~ 5 K), the terahertz radiation spectrum features a broad band (about 18–20 meV wide) with a peak at an energy of about 20–22 meV. This band is apparently associated with radiative transitions of nonequilibrium charge carriers from the states of the continuum to the state of an electron–hole liquid.

Effects of light on ferroelectric polarization and leakage current

We report the enhancement of polarization in polycrystalline ferroelectric thin films under illumination of light. The (Pb0.6Li0.2Bi0.2) (Zr0.2Ti0.8)O3 (PLBZT) thin films were fabricated on a Pt/TiO2/SiO2/Si (100) substrate by pulsed-laser deposition (PLD) technique. The illumination of weak monochromatic light having a wavelength comparable to the bandgap of PLBZT showed the development of non-equilibrium charge carriers which enhance switchable polarization and displacement current. A Positive-up Negative-down (PUND) analysis also supports the enhancement of switchable polarization under illumination. The fatigue test indicates nearly 20–30% decrease in polarization after a long time write and read cycles. These results may provide an extra degree of freedom to create electrical WRITE and optical READ logic states.

Generation Kinetics of Nonequilibrium Charge Carriers in Crystals with Deep Impurities Involving Two-Center Transitions between Band and Impurity States

A new and efficient mechanism of nonlinear photoexcitation of a transparent crystal with deep impurity centers is proposed. It is hypothesized that the energy of a quantum of light is smaller than the energy gap between the bottom of the conduction band and the impurity level, but is larger than the gap between the impurity level and the top of the valence band. The kinetics of nonlinear cascade generation of nonequilibrium charge carriers is considered taking into account two-center processes in which the energy transfer and the photon absorption occur in one and the same elementary event. The dependences of the quasi-equilibrium concentrations of nonequilibrium charge carriers in the bands and of the occupancy of impurity states on the laser-radiation intensity are obtained. It is shown that the generation process of nonequilibrium electron–hole pairs is of a threshold nature. Depending on the concentration of impurity centers, the threshold intensities can be ~105–107 W/cm2, while the setting time of the quasi-equilibrium occupancies of electronic states is ~10–0.1 ns.

Theoretical and Experimental Research of Luminescent Properties of Nanoparticles

Theoretical research is performed on isolated nanoparticles (NPs) and NP-containing composite films with establishing analytically the concentration of nonequilibrium charge carriers and luminescence intensity as functions of surface recombination rate s, radius, diffusion length, lifetime of minority charge carriers, and other parameters. A hyperbolic dependence of the photoluminescence intensity (PL) on the parameter s is found. It is shown theoretically and experimentally that the photostimulated rise of s of NPs brings about quenching, while its decline favors amplification of PL. The microphotoluminescence PL intensity is established as a function of exciting laser exposure time for powdered carbon nanoparticles (CNPs), solutions of CNPs, and composite films based on PVOH polymers and CNPs with average particle diameters of 1.3 and 1.7 nm. Furthermore, the PL signal intensity of films processed at temperatures of 100–200°C decreases upon their exposure to a high-power-density 532-nm excitation radiation and rises in the case of films treated at 220–340°C. Almost always, the PL intensity of exposed dry CNP powders abruptly drops. The study allows putting forward a new method for contactless and rapid measurement of the parameters of luminescent NPs.

Noncontact determination of the rate of surface recombination of nonequilibrium charge carriers at the p–p+ (n–n+) boundaries of n+–p(n)–p+ silicon structures by means of compensation

Calculations show that the rate of the surface recombination of nonequilibrium charge carriers at the back of a p(n) region with known lifetime values can be determined for n+–p(n)–p+ structures of silicon by non-contact measurement. This allows us to determine the photosensitivity contrast along a surface structure upon its local illumination.

Recombination properties of dislocations in GaN

The recombination activity of threading dislocations in n-GaN with different dislocation densities and different doping levels was studied using electron beam induced current (EBIC). The recombination velocity on a dislocation, also known as the dislocation recombination strength, was calculated. The results suggest that dislocations in n-GaN giving contrast in EBIC are charged and surrounded by a space charge region, as evidenced by the observed dependence of dislocation recombination strength on dopant concentration. For moderate (below ∼108 cm−2) dislocation densities, these defects do not primarily determine the average diffusion length of nonequilibrium charge carriers, although locally, dislocations are efficient recombination sites. In general, it is observed that the effect of the growth method [standard metalorganic chemical vapor deposition (MOCVD), epitaxial lateral overgrowth versions of MOCVD, and hydride vapor phase epitaxy] on the recombination activity of dislocations is not very pronounced, although the average diffusion lengths can widely differ for various samples. The glide of basal plane dislocations at room temperature promoted by low energy electron irradiation does not significantly change the recombination properties of dislocations.

Monte-Carlo study of drift mobility in ultrathin organic layers: Miller-Abrahams and Marcus models

It is shown by the means of Monte-Carlo modeling that the drift mobility decreases considerably along with the increasing of the layer thickness from 10 to 100 nm, if the hopping rates are calculated in the framework of the Marcus model, according to approximately the same low, as in the case of the Miller-Abrahams model. The diffusion current can contribute approximately the same contribution, as an energetic relaxation of the generated non-equilibrium charge carriers, for the case of moderate disorder.

On the Problem of Determining the Bulk Lifetime by Photoconductivity Decay on the Unpassivated Samples of Monocrystalline Silicon

In the indirect band gap semiconductors, and in particular, in silicon, the lifetime of nonequilibrium charge carriers is determined by recombination through impurity centers and it is inversely proportional to their concentration. Therefore, this is the most important parameter for determining the quality of the material. The noncontact methods of its measurement, in particular, the noncontact measurements of the decay of the photoconductivity (PC) constant, are the most important ones. The surface recombination strongly influences the shape of the curve. The calculation of the lifetime in the bulk by the decay constant remains relevant since there is no single-valued analytic solution of the continuity equation for this case. In the samples of monocrystalline silicon with unpassivated surfaces, the relaxation of PC is analyzed by numerical methods. The applicability of the known formulas for estimating the contribution of surface recombination to the effective PC relaxation time is discussed. It is shown that the period of time for which the fast exponents disappear depends on the relative thickness of the sample to be measured. The effective decay time is determined by the maximum value of the surface component of the relaxation time and is described by wellknown formulas only in this piece of the relaxation curve. The effective relaxation time attains saturation by the time when the signal intensity reaches 45% of the maximum value (the origin of the effective decay time according to the recommendation of the SEMI MF 1535 standard) only for the samples with a thickness of less than 3–5 diffusion lengths. For larger thicknesses, the contribution of the fast exponents to the effective PC relaxation time is observed right up to 5% of the maximum signal (i.e., until reaching the noise level of the signal to be measured). In this case using the recommended by SEMI MF 1535 standard formulas leads to a sufficiently large systematic error (up to 20%) in evaluating the free charge carrier’s lifetime.

Eu modified Cu2O thin films: Significant enhancement in efficiency of photoelectrochemical processes through suppression of charge carrier recombination

We propose a simple way to increase incident photon-to-current conversion efficiency (IPCE, Y) for electrodeposited p-type Cu2O films through addition of Eu(III) to the electrodeposition bath. This is the first reported enhancement of photocurrent for Cu2O modified with a rare-earth element. Our study is based on hypothesis that a large ionic radius of Eu(III) promotes its precipitation in form of inclusions of another phase, which act as getter centers leading to purification of host material from detrimental impurities and, correspondingly, to increase in lifetime of non-equilibrium charge carriers. SEM, EDX and XRD analyses indicate that addition of Eu(III) results in some increase of Cu2O crystallite size and growth of a secondary Eu containing phase without changing the Cu2O lattice parameters. Electrochemical impedance spectroscopy indicates invariance of acceptor concentration and flat band potential for Eu modified films. Remarkable increase of charge carriers’ lifetime, which manifests in the growth of Cu2O exciton photoluminescence (PL) intensity and PL decay constant, leads to increased IPCE for the Eu modified films. Optimization of Eu(III) concentration in the electrodeposition bath allows attaining the cathodic photocurrent as high as 3.2 mA cm−2 (35 mW cm−2 Xe lamp illumination), which is as ≈40% greater than that for the pure Cu2O film and corresponds to Y ≈ 100% at wavelength λ < 450 nm.

Kinetics of photoconductivity of c-Si with amorphous heterogeneities

The influence of amorphous inclusions of a cylindrical shape on the photoconductivity of crystalline silicon under the action of radiation from the photosensitivity region of the structure under study was theoretically investigated. The photoconductivity of the structure is analyzed depending on the geometric dimensions of the inclusions, their location in the structure, and the rate of surface recombination of nonequilibrium charge carriers. It is established that when the fraction of inclusions in the structure increases, the processes of generation of nonequilibrium charge carriers are determined mainly by an amorphous matrix. The appearance of the effect of negative photoconductivity under certain conditions has been revealed.

Effects of local photoexcitation of high-concentration charge carriers in silicon

The phenomena occurring at the local pulsed photoexcitation of intrinsic nonequilibrium highconcentration charge carriers in silicon are experimentally investigated. The effect of a substantial increase in the lifetime of photoexcited charge carriers is found. It is shown that the effect of a substantial increase in the lifetime of charge carriers is caused by a change in the degree of degeneracy and displacement of the impurityrecombination level towards the Fermi level due to local thermoelastic deformation of the crystal and the corresponding distribution of the concentration of nonequilibrium charge carriers.

Green light activated hydrogen sensing of nanocrystalline composite ZnO-In2O3 films at room temperature

The possibility of reducing the operating temperature of H2 gas sensor based on ZnO-In2O3 down to room temperature under green illumination is shown. It is found that sensitivity of ZnO-In2O3 composite to H2 nonmonotonically depends on the oxides’ content. The optimal ratio between the components is chosen. The new mechanism of nanocrystalline ZnO-In2O3 sensor sensitivity to H2 under illumination by green light is proposed. The mechanism considers the illumination turns the composite into nonequilibrium state and the photoconductivity change in the H2 atmosphere is linked with alteration of nonequilibrium charge carriers recombination rate.