High Density Electron-hole Plasma Research Articles

Carrier transport mechanism of highly-sensitive niobium doped titanium dioxide/p-Si heterojunction photodiode under illuminations by solar simulated light

Nano-crystalline Nb doped anatase TiO2 (NTO) thin film was deposited on p-type Si substrate for fabrication of n-NTO/p-Si heterojunction photodiode using RF magnetron sputtering technique. Rutherford backscattering spectrometry and Raman spectroscopy results suggest the substitutional incorporation of Nb5+ ions in anatase TiO2 lattice, which is evidenced from large stiffening of Eg(1) and softening of B1g Raman modes. The current density-voltage (J-V) characteristics are measured and important diode parameters of n-NTO/p-Si heterojunction diode are determined, such as ideality factor, barrier height, and series resistance. Diode exhibits excellent behavior under dark condition as rectification ratio is found to be ∼7 × 102 with high forward current density ∼1.54 A/cm2 at 5 V. The n-NTO/p-Si heterojunction works as an efficient photodiode in reverse bias under simulated solar light illumination with high contrast ratio ∼225 at −2 V and very high photo responsivity ∼2.7 A/W at −5 V. The high photo responsivity of photodiode is mainly due to the generation of high density electron-hole plasma in NTO depletion region by the absorption of incident UV range photons. Thus n-NTO/p-Si heterojunction diode is suitable device for highly sensitive Ultra-Violet photodiode applications.

Communication: The formation of rarefaction waves in semiconductors after ultrashort excitation probed by grazing incidence ultrafast time-resolved x-ray diffraction.

We explore the InSb-semiconductor lattice dynamics after excitation of high density electron-hole plasma with an ultrashort and intense laser pulse. By using time resolved x-ray diffraction, a sub-mÅ and sub-ps resolution was achieved. Thus, a strain of 4% was measured in a 3 nm thin surface layer 2 ps after excitation. The lattice strain was observed for the first 5 ps as exponentially decaying, changing rapidly by time and by depth. The observed phenomena can only be understood assuming nonlinear time dependent laser absorption where the absorption depth decreases by a factor of twenty compared to linear absorption.

Silicon as a virtual plasmonic material: Acquisition of its transient optical constants and the ultrafast surface plasmon-polariton excitation

Ultrafast intense photoexcitation of a silicon surface is complementarily studied experimentally and theoretically, with its prompt optical dielectric function obtained by means of time-resolved optical reflection microscopy and the underlying electron-hole plasma dynamics modeled numerically, using a quantum kinetic approach. The corresponding transient surface plasmon-polariton (SPP) dispersion curves of the photo-excited material were simulated as a function of the electron-hole plasma density, using the derived optical dielectric function model, and directly mapped at several laser photon energies, measuring spatial periods of the corresponding SPP-mediated surface relief nanogratings. The unusual spectral dynamics of the surface plasmon resonance, initially increasing with the increase in the electron-hole plasma density but damped at high interband absorption losses induced by the high-density electron-hole plasma through instantaneous bandgap renormalization, was envisioned through the multi-color mapping.

Ultrafast spin-polarized lasing in a highly photoexcited semiconductor microcavity at room temperature

We demonstrate room-temperature spin-polarized ultrafast ($\sim$10 ps) lasing in a highly optically excited GaAs microcavity. This microcavity is embedded with InGaAs multiple quantum wells in which the spin relaxation time is less than 10 ps. The laser radiation remains highly circularly polarized even when excited by nonresonant elliptically polarized light. The lasing energy is not locked to the bare cavity resonance, and shifts $\sim$10 meV as a function of the photoexcited density. Such spin-polarized lasing is attributed to a spin-dependent stimulated process of correlated electron-hole pairs. These pairs are formed near the Fermi edge in a high-density electron-hole plasma coupled to the cavity light field.

Exciton-Mott Physics in a Quasi-One-Dimensional Electron-Hole System

We investigate the correlation effect in the quasi-one-dimensional electron-hole (e-h) system under thermal equilibrium. A self-consistent screened T-matrix approximation developed here enables the description of an e-h pair under any ionization ratio, the portion of quasielectrons or quasiholes moving almost freely. Our phase diagram on the ionization ratio provides a unified description of exciton Mott physics from the low-density exciton gas towards the high-density electron-hole plasma, and predicts a first order transition at low temperature. The interband optical absorption-gain spectra are also evaluated, which succeeded in explaining semiquantitatively all aspects of the recent experimental observations in the strongly photoexcited quantum wires.

Numerical investigation of the nanosecond opening-mechanism of drift step recovery diodes

A circuit-fluid coupled model is proposed for describing the dynamics of the electron-hole plasma and the evolution of the electric field in the drift step recovery diodes (DSRDs). The model takes account of the real doping profile of the p+-p-n+ type silicon DSRD and the elementary processes such as the impact ionization, Shockley-Read-Hall recombination and Auger recombination. The current opening process of the DSRD is simulated with the proposed model and then the nanosecond opening-mechanism is investigated. The opening process is found to appear at the base region of the DSRD, due to the high density electron-hole plasma formed at the forward pumping period. According to the opening rate, the opening process can be divided into two stages: a slow stage near the p+-p boundary and an abrupt one at p-n junction. Moreover, simulation results show that the p+-p-n+ type DSRD with a lower base tends to generate pulses with shorter rise time and higher peak voltage.

Fluorescence of laser-created electron-hole plasma in graphene

We present an experimental observation of non-linear up- and down-converted optical luminescence of graphene and thin graphite subject to picosecond infrared laser pulses. We show that the excitation yields to a high density electron-hole plasma in graphene. It is further shown that the excited charge carries can efficiently exchange energy due to scattering in momentum space. The recombination of the resulting non-equilibrium electron-hole pairs yields to the observed white light luminescence. Due to the scattering mechanism the power dependence of the luminescence is quadratic until it saturates for higher laser power. Studying the luminescence intensity as a function of layer thickness gives further insight into its nature and provides a new tool for substrate independent thickness determination of multilayer flakes.

Dynamics of Strongly Degenerate Electron−Hole Plasmas and Excitons in Single InP Nanowires

Low-temperature time-resolved photoluminescence spectroscopy is used to probe the dynamics of photoexcited carriers in single InP nanowires. At early times after pulsed excitation, the photoluminescence line shape displays a characteristic broadening, consistent with emission from a degenerate, high-density electron-hole plasma. As the electron-hole plasma cools and the carrier density decreases, the emission rapidly converges toward a relatively narrow band consistent with free exciton emission from the InP nanowire. The free excitons in these single InP nanowires exhibit recombination lifetimes closely approaching that measured in a high-quality epilayer, suggesting that in these InP nanowires, electrons and holes are relatively insensitive to surface states. This results in higher quantum efficiencies than other single-nanowire systems as well as significant state-filling and band gap renormalization, which is observed at high electron-hole carrier densities.

Superfluorescence from dense electron-hole plasmas in high magnetic fields

Cooperative spontaneous recombination (superfluorescence) of electron-hole plasmas in semiconductors has been a challenge to observe due to ultrafast decoherence. We argue that superfluorescence can be achieved in quantum-confined semiconductor systems and present experimental evidence for superfluorescence from high-density photoexcited electronhole plasmas in magnetized quantum wells. At a critical magnetic field strength and excitation fluence, we observe a clear transition in the band-edge photoluminescence from omnidirectional output to a randomly directed but highly collimated beam. Changes in the linewidth, carrier density, and magnetic field scaling of the emission spectra correlate precisely with the onset of random directionality and are consistent with cooperative recombination.

Cooperative Recombination of a Quantized High-Density Electron-Hole Plasma in Semiconductor Quantum Wells

We investigate photoluminescence from a high-density electron-hole plasma in semiconductor quantum wells created via intense femtosecond excitation in a strong perpendicular magnetic field, a fully quantized and tunable system. At a critical magnetic field strength and excitation fluence, we observe a clear transition in the band-edge photoluminescence from omnidirectional output to a randomly directed but highly collimated beam. In addition, changes in the linewidth, carrier density, and magnetic field scaling of the photoluminescence spectral features correlate precisely with the onset of random directionality, indicative of cooperative recombination from a high-density population of free carriers in a semiconductor environment.

Transient strain driven by a dense electron-hole plasma.

We measure transient strain in ultrafast laser-excited Ge by time-resolved x-ray anomalous transmission. The development of the coherent strain pulse is dominated by rapid ambipolar diffusion. This pulse extends considerably longer than the laser penetration depth because the plasma initially propagates faster than the acoustic modes. X-ray diffraction simulations are in agreement with the observed dynamics.

Laterally doped heterostructures for III–N lasing devices

To achieve a high-density electron-hole plasma in group-III nitrides for efficient light emission, we propose a planar two-dimensional (2D) p-i-n structure that can be formed in the quantum well layers due to efficient activation of donors and acceptors in the laterally, selectively doped barriers. We show that strongly nonequilibrium 2D electron-hole plasma with density above 1012 cm−2 can be realized in the i region of the laterally biased p-i-n structure, enabling the formation of interband population inversion and stimulated emission from such a lateral current pumped emitter (LACE). We suggest that implementation of the lateral p-i-n structures provides an efficient way of utilizing potential-profile-enhanced doping of superlattices and quantum wells for electric pumping of nitride-based lasers.

EVOLUTION OF ELECTRON-HOLE PLASMA AND EXCITONS IN ZnO EPITAXIAL THIN FILMS STUDIED BY FEMTOSECOND SPECTROSCOPY

Time evolution of a high density electron-hole plasma (EHP) and excitons in ZnO epitaxial thin films was studied under resonant excitation of the excitonic state using pump-probe absorption and optical Kerr gate luminescence techniques with sub-picosecond time resolution in order to elucidate dynamics of the photo-excited high density carriers. The bleaching of the exciton absorption occurs without any measurable delay time, suggesting that the excitonic state instantaneously becomes unstable because of the dissociation of the high density excitons into the EHP due to the screening of Coulomb interaction. This leads to an instantaneous bandgap reduction. Then the recovery of the renormalized bandgap as well as that of the exciton bleaching takes place with decreasing the carrier density due to radiative recombination of electrons with holes. From the time-resolved absorption and luminescence experiments, the recovery time is determined to be 25 v ps.

Condensation of electron-hole pairs in bulk GaAs at room temperature under conditions of femtosecond cooperative radiation

The results are given of an experimental investigation of the spectral characteristics of cooperative recombination of a high-density electron-hole plasma in GaAs. It is demonstrated that, under conditions of generation of high-power femtosecond pulses of superradiation, the properties of electrons and holes differ considerably from their properties under conditions of lasing or regular spontaneous recombination. The peak of the cooperative radiation line (ℏω=1.405 to 1.407 eV) is shifted inward into a (nonrenormalized) forbidden band. It is located 20 meV lower on the energy scale than the lasing peak and more than 40 meV below the center of the line of spontaneous recombination at the same pumping level. This corresponds to electron-hole condensation to the bottom of the bands. The properties of cooperative recombination may be defined by the pairing of electrons and holes and by the formation of a short-lived coherent electron-hole BCS state. The estimated value of the order parameter Δ is approximately 2 meV.

Structural changes in GaAs induced by ultrafast (fs) laser pulses

Ultrafast changes in the crystal structure of GaAs induced by intense femtosecond laser pulses are detected and investigated. Atomic force microscopy and Raman microprobe analysis of the laser-treated area show centrosymmetric (disordered) features which are different from the original zinc-blend structure of the GaAs lattice. The frozen-in structure shows evidence for a special heat transfer from the laser-induced crater to the boundary, namely the heat has been transferred ballistically by a high-density electron-hole plasma.

Second nonequilibrium-phonon bottleneck for carrier cooling in highly excited polar semiconductors

Cooling of high-density electron-hole plasma excited in direct-gap polar semiconductors by a picosecond pulse of an extreme intensity is considered. Convenient models of carrier coupling with LO phonons via screened Fr\"ohlich interaction, nonequilibrium LO-phonon creation by free-carrier intraband relaxation, and fermion recombination-induced heating were combined with a description of second-generation nonequilibrium phonons produced by anharmonic decay of the nonequilibrium LO phonons. An additional source of nonequilibrium LO phonons due to nonradiative capture by deep centers via multiphonon emission was also introduced. A numerical simulation for a CdS crystal was performed. Even at room temperature, the carrier cooling is shown to exhibit a noticeable slow component for plasma densities above ${10}^{19}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}.$ At the late stage of the relaxation, the relevant time constant $(\ensuremath{\sim}100\mathrm{ps})$ is found to be comparable with the carrier lifetime, while the magnitude of the excess effective temperature is determined by nonequilibrium population of the second-generation phonons. The ``width'' of the second nonequilibrium-phonon bottleneck is demonstrated to be determined by peculiarities of the lattice vibration spectrum.

Many body effects in transient luminescence spectra of a homogeneous electron-hole plasma in CdTe/CdMnTe quantum wells

Using time resolved photoluminescence spectroscopy, we have investigated many body effects and the dynamics of a high density electron-hole plasma in strained CdTe/CdMnTe quantum wells. Great care has been taken to achieve a spacially homogeneous plasma by mesa etching of single quantum well structures. Our experimental data show strong band filling effects up to the barrier energy due to the Pauli principle within a few tenths of ps. A lineshape analysis yields plasma densities up to 2 × 10 12 cm -2 and carrier temperatures up to 335 K (bath temperature 2 K). A band gap renormalization of the emission spectrum up to around 23 meV as compared to a low excitation reference spectrum is observed. The reduction of the carrier density due to recombination causes an increase of the emission intensity at the 1e-1hh exciton transition, indicating an enhancement of the oscillator strength below the Mott density.

Femtosecond carrier dynamics in the presence of a cold plasma in GaAs and AlGaAs

The femtosecond dynamics of hot carriers interacting with a cold high density electron-hole plasma are investigated in GaAs and Al0.2Ga0.8As. Studies are performed using a three pulse pump-probe technique where a cold plasma is first generated by a femtosecond pulse, then pump-probe transmission measurements are performed after a few hundred picoseconds delay. The results indicate only a small increase of the hot carrier thermalization rate even for plasma densities as high as 1018 cm−3.

Femtosecond relaxation dynamics of hot carriers in CdSe

Abstract Temporal behavior of hot carriers in CdSe is investigated with subpicosecond accuracy, by using a novel pump-probe technique, in which the self-phase modulation effect in an optical fiber is utilized to extend the spectral width of the probe pulse. Pump-pulse-induced changes in the spectrum of the probe pulse are compared with a calculation that takes into account the formation of a high-density electron-hole plasma, and a succeeding thermal relaxation of the plasma. Excellent agreement between theory and experiment is obtained, when the carrier-LO phonon interaction is included.

Otpically illuminated dielectric interfaces as high-speed far-infrared modulators

A new device for the gigahertz modulation of far-infrared radiation is analytically and numerically analyzed. It consists of a thin layer of a high-mobility, direct-bandgap semiconductor, such as GaAs, in which a high-density electron-hole plasma is rapidly created and destroyed, thereby rapidly changing the free-carrier reflectivity of the active layer. Illumination by a high-power, near-infrared laser diode array generates the plasma through intrinsic photoconduction. It is shown that this device acis primarily as an amplitude modulator, and that its efficiency increases sharply with increasing far-IR frequency, in contrast to a Schottky diode, which acts primarily as a phase modulator, and whose efficiency falls off sharply with far-IR frequency. The breakeven frequency lies at about 1.5 THz, depending slightly on the assumed device parameters. The relative advantage of the new device increases rapidly with increasing far-infrared frequency. At an operating frequency of 2.5 THz (119 μm), for example, a 1 GHz modulation bandwidth may be achieved with a single-sideband conversion loss of only-21 db, versus a Schottky's loss of-39 db, assuming a laser diode power of 1 W, which is readily available from recently developed laser diode arrays.