Nonequilibrium Carriers Research Articles

Influence of Energetic Particles and Electron Injection on Minority Carrier Transport Properties in Gallium Oxide

The influence of various energetic particles and electron injection on the transport of minority carriers and non-equilibrium carrier recombination in Ga2O3 is summarized in this review. In Ga2O3 semiconductors, if robust p-type material and bipolar structures become available, the diffusion lengths of minority carriers will be of critical significance. The diffusion length of minority carriers dictates the functionality of electronic devices such as diodes, transistors, and detectors. One of the problems in ultrawide-bandgap materials technology is the short carrier diffusion length caused by the scattering on extended defects. Electron injection in n- and p-type gallium oxide results in a significant increase in the diffusion length, even after its deterioration, due to exposure to alpha and proton irradiation. Furthermore, post electron injection, the diffusion length of an irradiated material exceeds that of Ga2O3 prior to irradiation and injection. The root cause of the electron injection-induced effect is attributed to the increase in the minority carrier lifetime in the material due to the trapping of non-equilibrium electrons on native point defects. It is therefore concluded that electron injection is capable of “healing” the adverse impact of radiation in Ga2O3 and can be used for the control of minority carrier transport and, therefore, device performance.

Lateral ambipolar drift of the excess charge carriers in the GaAs-based heterostructures with quantum wells and impurity δ-layers in the adjacent barriers

The low-temperature ambipolar photoconduction in the InGaAs/GaAs and GaAs/AlGaAs heterostructures containing the compositional quantum well (QW) tunnel-coupled with the impurity δ-well have been studied under a wide electric field range. The “stretched” bipolar drift length in the InGaAs/GaAs structure is shown to achieve a few mm at the fields of ∼70 V/cm and exceed that in the GaAs/AlGaAs structure at the comparable field strength by half an order of magnitude. The photoconductivity decay is up to several tens μs long and its behavior is fitted by the single- and “stretched-" exponent functions. The obtained results are shown to be caused by spatial separation of the non-equilibrium charge carriers in the potential profile engineered by such QW configuration and the dispersion law of holes in them.

Trap States and Carrier Diffusion in Czochralski (100) Single Crystal β-Ga2O3

Deep trap spectra and carrier diffusion lengths were measured for unintentionally doped β-Ga2O3 bulk crystals with (100) orientation. The 20-mm diameter, 15-mm length boule was pulled by the Czochralski method from gallium oxide in (010) direction. It is found that the net density of shallow donors in (100) plates cleaved from the crystal was 2.6 × 1017 cm−3, with ionization energies of 0.05 eV measured from admittance spectra. Three deep electron traps with respective ionization energies of 0.6 eV (concentration 1.1 × 1014 cm−3), 0.8 eV (concentration 3.9 × 1016 cm−3) and 1.1 eV (concentration 8.9 × 1015 cm−3) were detected by Deep Level Transient Spectroscopy. The dominant 0.8 eV trap is associated with the E2 centers due to Fe acceptors, the two other traps are the well documented E1 and E3 centers. The major deep acceptors in the lower half of the bandgap have optical ionization threshold of 2.3 eV and concentration of 4 × 1015 cm−3 and are believed to be due to the split Ga vacancies acceptors. The diffusion length of non-equilibrium charge carriers was 90 nm. The electrical properties of these (100) oriented crystals grown by Czochralski are quite similar to those synthesized by the undoped Edge-defined Film-Fed Growth technique.

High performance and recyclable Ag/ZnO/PM substrate for the detection of organic pollutants.

A sensitive and recyclable substrate was fabricated through in situ reduction of silver nanoparticles (NPs) on zinc oxide nanorods (NRs). The prepared silver nanoparticles/zinc oxide nanorods/polyamide mesh (Ag/ZnO/PM) substrate exhibited not only excellent surface-enhanced Raman scattering (SERS) performance to R6G with a limit of detection (LOD) of 10-12 M, mainly attributed to the synergistic effect of the suitable size and the nanoscale gaps of the Ag NPs to produce local surface plasmon resonance (LSPR), but also outstanding self-cleaning property via UV irradiation due to its significant photocatalytic property based on the non-equilibrium carriers generated by ZnO and the presence of Schottky junctions between Ag and ZnO. The substrate showed good recycling stability even after five cycles. Furthermore, the successful recyclable application of Ag/ZnO/PM for tetracycline hydrochloride (TC) detection with high sensitivity further suggested that it is a promising candidate for constructing a portable SERS platform to detect organic pollutants.

EFFICIENCY OF ACTIVATED NANO-HETEROJUNCTIONS ON SILICON AND SILICON CARBIDE SUBSTRATES

The concept, model, and examples of activated nanoscale heterojunctions on special silicon carbide and silicon substrates to ensure maximum power with combined geometric and quantitative scaling of semiconductor energy converter chips are considered. The issue of maximum efficiency of energy conversion and efficiency of separation of electron-hole pairs is investigated. A variant of optimizing the scaling solution is implemented by heterojunctions with variations in the sequences of layers with an increase in the concentration and direction of movement of nonequilibrium carriers for further increasing voltage conversion with charge pumping. Numerical simulation was performed to test the model with thin layers of GaN, GaP on SiC, SiC/Si. For the first time, definitions of activation of the heterojunction and activated nanoheterojunction as a fundamental structure are proposed. The accuracy of the proposed models is compared with the accuracy of known models, and it is shown that the results obtained are better than some solutions of these models known in the literature.

Subnanosecond switching of GaAs diode due to impact ionization in collapsing bipolar Gunn domains

Subnanosecond switching of high-voltage GaAs diodes initiated by a steep reverse voltage ramp (≥1 kV/ns) is investigated experimentally and numerically. The triggering occurs at the voltage Um which exceeds stationary breakdown voltage Ub, similar to delayed impact ionization breakdown in Si picosecond-range high-voltage diodes known as silicon avalanche sharpening (SAS) diodes. Despite this similarity, we argue that the triggering mechanism of GaAs diodes is qualitatively different and is based on negative differential electron mobility in GaAs. Spatial patterns of recombination emission indicate formation of narrow conducting channels. Numerical simulations reveal the spontaneous appearance of high-field (∼400 kV/cm) bipolar Gunn domains in these channels. The agreement of experiments and simulations demonstrates that impact ionization within collapsing Gunn domains is responsible for a rapid increase of nonequilibrium carrier concentration and subnanosecond switching of the reversely biased GaAs diode to the conducting state with low (<100 V) residual voltage. In particular, the proposed mechanism explains why GaAs diodes exhibit subnanosecond switching at moderate overvoltage ratio Um/Ub ≈ 1.5 smaller than typical overvoltage Um/Ub ≈ 2 required for switching Si diodes, and the origin of anomalous delay of several nanoseconds that is experimentally observed in such regimes.

Leveraging plasmonic hot electrons to quench defect emission in metal-semiconductor nanostructured hybrids.

Modeling light-matter interactions in hybrid plasmonic materials is vital to their widening relevance from optoelectronics to photocatalysis. Here, we explore photoluminescence (PL) from ZnO nanorods (ZNRs) embedded with gold nanoparticles (Au NPs). A progressive increase in Au NP concentration introduces significant structural disorder and defects in ZNRs, which paradoxically quenches defect related visible PL while intensifying the near band edge (NBE) emission. Under UV excitation, the simulated semi-classical model realizes PL from ZnO with sub-bandgap defect states, eliciting visible emissions that are absorbed by Au NPs to generate a non-equilibrium hot carrier distribution. The photo-stimulated hot carriers, transferred to ZnO, substantially modify its steady-state luminescence, reducing NBE emission lifetime and altering the abundance of ionized defect states, finally reducing visible emission. The simulations show that the change in the interfacial band bending at the Au-ZnO interface under optical illumination facilitates charge transfer between the components. This work provides a general foundation to observe and model the hot carrier dynamics and strong light-matter interactions in hybrid plasmonic systems.

Impact of Solid-State Charge Injection on Spectral Photoresponse of NiO/Ga2O3 p–n Heterojunction

Forward bias hole injection from 10-nm-thick p-type nickel oxide layers into 10-μm-thick n-type gallium oxide in a vertical NiO/Ga2O3 p–n heterojunction leads to enhancement of photoresponse of more than a factor of 2 when measured from this junction. While it takes only 600 s to obtain such a pronounced increase in photoresponse, it persists for hours, indicating the feasibility of photovoltaic device performance control. The effect is ascribed to a charge injection-induced increase in minority carrier (hole) diffusion length (resulting in improved collection of photogenerated non-equilibrium carriers) in n-type β-Ga2O3 epitaxial layers due to trapping of injected charge (holes) on deep meta-stable levels in the material and the subsequent blocking of non-equilibrium carrier recombination through these levels. Suppressed recombination leads to increased non-equilibrium carrier lifetime, in turn determining a longer diffusion length and being the root-cause of the effect of charge injection.

Measurement of the Lifetime and of the Spin-Relaxation Time of Electrons in Semiconductors by the Optical-Orientation Method

It was shown recently [1, 2] that in semiconducting crystals it is possible to attain an appreciable orientation of the spins of the non-equilibrium carriers as a result of interband transitions caused by absorption of polarized light. Observation of optical orientation makes it possible to extend to semiconductors the research methods widely used in atomic spectroscopy [3], and particularly to investigate relaxation processes in a crystal under stationary conditions.

Observation of Optical Orientation of Equilibrium Electrons in N-Type Semiconductors

The first experiments on optical orientation of free carriers in semiconductors were performed recently [1– 5]. Since holes usually do not become oriented in the valence band, owing to rapid relaxation of their spins [6], most experiments were performed on p-type materials and the oriented carriers were the electrons propelled into the conduction band by circularly polarized light. Thus, only the spins of non-equilibrium carriers were oriented, a phenomenon analogous to the optical orientation of excited gas atoms [7]. However, we have previously observed [3] optical orientation also in n-type material. We show in the present paper that this phenomenon is due to optical orientation of equilibrium electrons, i.e., the orientation is realized in the ground state, in analogy with “optical pumping” of atoms [7].

Spectral (in)dependence of nonequilibrium charge carriers lifetime and density of states distribution in the vicinity of the band gap edge in F8BT polymer

Photoelectric properties of the F8BT polymer are investigated by the Constant photocurrent method (CPM) based on the photocurrent spectra Δjph(hν) and absorption spectra αCPM(hν). The negligibly weak dependence of charge carriers lifetime on the photon energy is found for hν = 2.2–2.7 eV. The peculiarities of the absorption spectra and the photoconductivity spectra are revealed to be determined mainly by the Gaussian distribution of the density of states (DOS). The density of trap states in the F8BT polymer thin films in the exponential DOS is estimated to be 103 times higher than in the Gaussian DOS.

OPTICALLY PUMPED BIPOLAR TRANSISTOR

The properties of a bipolar npn transistor were studied when exposed to unmodulated incoherent radiation created by a “white” LED. The static and dynamic characteristics of the transistor were measured at various exposure intensities. It is shown that the change in the characteristics of the transistor under optical influence is due to an increase in the lifetime of nonequilibrium charge carriers and the photovoltaic effect in p-n junctions. For these reasons, the gain increases, the switching threshold decreases, and the transistor speed increases. The results obtained are applicable both to the creation of high-speed transistors and integrated circuits of a fundamentally new type.

Probing the photocurrent in two-dimensional titanium disulfide

Generating photocurrent in a condensed matter system involves the excitation, relaxation, and transportation of charge carriers. As such, it is viewed a potent method for probing the dynamics of non-equilibrium carriers and the electronic band structure of solid state materials. In this research, we analyze the photoresponse of the mechanically exfoliated titanium disulfide (TiS2), a transition metal dichalcogenide whose classification as either a semimetal or a semiconductor has been the subject of debate for years. The scanning photocurrent microscopy and the temperature-dependent photoresponse characterization expose the appearance of a photovoltaic current primarily from the metal/TiS2 junction in an unbiased sample, while negative photoconductivity due to the bolometric effect is observed in the conductive TiS2 channel. The optoelectronic experimental results, combined with electrical transport characterization and angle-resolved photoemission spectroscopy measurements, indicate that the TiS2 employed in this study is likely a heavily-doped semiconductor. Our findings unveil the photocurrent generation mechanism of two dimensional TiS2, highlighting its prospective optoelectronic applications in the future.

Eco-Friendly Zn-Ag-In-Ga-S Quantum Dots: Amorphous Indium Sulfide Passivated Silver/Sulfur Vacancies Achieving Efficient Red Light-Emitting Diodes.

I-III-VI quantum dots (QDs) and derivatives (I, III, and VI are Ag+/Cu+, Ga3+/In3+, and S2-/Se2-, respectively) are the ideal candidates to replace II-VI (e.g., CdSe) and perovskite QDs due to their nontoxicity, pure color, high photoluminescence quantum yield (PLQY), and full visible coverage. However, the chaotic cation alignment in multielement systems can easily lead to the formation of multiple surface vacancies, highlighted as VI and VVI, leading to nonradiative recombination and nonequilibrium carrier distribution, which severely limit the performance improvement of materials and devices. Here, based on Zn-Ag-In-Ga-S QDs, we construct an ultrathin indium sulfide shell that can passivate electron vacancies and convert donor/acceptor level concentrations. The optimized In-rich 2-layer indium sulfide structure not only enhances the radiative recombination rate by preventing further VS formation but also achieves the typical DAP emission enhancement, achieving a significant increase in PLQY to 86.2% at 628 nm. Moreover, the optimized structure can mitigate the lattice distortion and make the carrier distribution in the interior of the QDs more balanced. On this basis, red QD light-emitting diodes (QLEDs) with the highest external quantum efficiency (EQE; 5.32%) to date were obtained, providing a novel scheme for improving I-III-VI QD-based QLED efficiency.

Evaluation of ambipolar diffusion coefficient in AlxGa1−xN semiconductor

Numerical approach to account for the influence of randomly fluctuating potential on carrier mobility in compound semiconductors with compositional disorder is developed and exploited for calculating the heavy hole-defined ambipolar diffusion coefficient in AlxGa1-xN layers. The rates of inelastic heavy hole-phonon scattering were obtained by integrating the quasi-classical hole-phonon scattering rates for the virtual crystal over the semiclassical confining potential obtained by using the local landscape method. The calculated dependence of the heavy-hole diffusion coefficient on the aluminum content in AlxGa1-xN crystal in a wide compositional range (x = 0 – 1) evidences a substantial influence of localization on the diffusivity of nonequilibrium carriers and is in a good quantitative correspondence with the experimental data obtained in this work by using the light-induced transient grating technique. The adopted Monte Carlo simulation technique also enables diffusivity calculations in other multicomponent compounds (e.g., high-In-content InGaN) and their structures, in particular, those for LEDs and µ-LEDs.

Terahertz photoconductive antennas based on silicon-doped GaAs (111)A

In this study we investigate a relatively new material for terahertz (THz) photoconductive antennas (PCAs): GaAs epitaxial films grown on (111)A-oriented semi-insulated GaAs substrates and doped with Si. GaAs:Si (111)A films with the required high resistance without postgrowth annealing have been grown by molecular-beam epitaxy. The Si doping of (111)A-oriented GaAs was expected to lead to the formation of acceptors that facilitate the activation of As[Formula: see text] traps. The relaxation times of nonequilibrium charge carriers in the films were measured in pump-probe experiment, and, finally, the electron mobility was estimated from the carrier lifetime and current–voltage characteristics measured under pulsed pumping by Ti:sapphire femtosecond laser. Dipole and bow-tie PCAs were fabricated, and the THz emission spectra and the total THz emission power were measured with varying applied voltage and optical excitation power. The properties of GaAs:Si (111)A films and the PCAs based on them are compared with LTG-GaAs (100) and (111)A-oriented films.

Transport and Interfacial Injection of d-Band Hot Holes Control Plasmonic Chemistry.

Harnessing nonequilibrium hot carriers from plasmonic metal nanostructures constitutes a vibrant research field with the potential to control photochemical reactions, particularly for solar fuel generation. However, a comprehensive understanding of the interplay of plasmonic hot-carrier-driven processes in metal/semiconducting heterostructures has remained elusive. In this work, we reveal the complex interdependence among plasmon excitation, hot-carrier generation, transport, and interfacial collection in plasmonic photocatalytic devices, uniquely determining the charge injection efficiency at the solid/liquid interface. Measuring the internal quantum efficiency of ultrathin (14-33 nm) single-crystalline plasmonic gold (Au) nanoantenna arrays on titanium dioxide substrates, we find that the performance of the device is limited by hot hole collection at the metal/electrolyte interface. Our solid- and liquid-state experimental approach, combined with ab initio simulations, demonstrates more efficient collection of high-energy d-band holes traveling in the [111] orientation, enhancing oxidation reactions on {111} surfaces. These findings establish new guidelines for optimizing plasmonic photocatalytic systems and optoelectronic devices.

Coupling of Pyro-Piezo-Phototronic Effects in a GaN Nanowire.

In this paper, we systematically investigate the synergistic regulation of ultraviolet and mechanical loading on the electromechanical behavior of a GaN nanowire. The distributions of polarization charge, potential, carriers, and electric field in the GaN nanowire are analytically represented by using a one-dimensional model that combines pyro-phototronic and piezo-phototronic properties, and then, the electrical transmission characteristics are analyzed. The results suggest that, due to the pyro-phototronic effect and ultraviolet photoexcited non-equilibrium carriers, the electrical behavior of a nano-Schottky junction can be modulate by ultraviolet light. This provides a new method for the function improvement and performance regulation of intelligent optoelectronic nano-Schottky devices.

The effect of micro-deformations created by illumination on the temperature dependence of the mobility of nonequilibrium non-thermalized charge carriers in media without a center of symmetry

It is shown that the energy of nonequilibrium nonthermalized holes in ZnS crystals with a cubic unit cell does not depend on temperature. The nature of the temperature dependence of the nonequilibrium nonthermalized holes mobility in ZnS crystals depends on the average relaxation time upon scattering by ions and neutral impurity atoms, dislocations, and microstrains.

Photoexcitation dominated electrical behaviors in a nano GaN PN junction

In this paper, we investigate the regulation behaviors of photoexcited nonequilibrium carriers in a GaN piezoelectric semiconductor PN junctions. It is shown that the electromechanical field of the PN junction has a fast response to the illumination intensity. This is attributed to the ultraviolet photoexcited non-equilibrium carriers, which can shield the piezoelectric charges and significantly weaken the built-in barrier height. Furthermore, the electrical transmission characteristics are also regulated. It implies that the electrical behaviors of a nano GaN PN junctions are highly sensitive to ultraviolet light, and such a contactless approach can be applied to control the nano GaN devices.