Electron-hole Density Research Articles

IMPLICIT-EXPLICIT MULTISTEP FINITE ELEMENT-MIXED FINITE ELEMENT METHODS FOR THE TRANSIENT BEHAVIOR OF A SEMICONDUCTOR DEVICE

The transient behavior of a semiconductor device consists of a Poisson equation for the electric potential and of two nonlinear parabolic equations for the electron density and hole density. The electric potential equation is discretized by a mixed finite element method. The electron and hole density equations are treated by implicit-explicit multistep finite element methods. The schemes are very efficient. The optimal order error estimates both in time and space are derived.

Electron-hole recombination density matrices obtained from large configuration-interaction expansions

An efficient computational method for constructing general transition density matrices such as the electron-hole recombination density matrix from large configuration interaction expansions is presented. The present string-based Slater-determinant approach is completely general and can be used to describe simultaneous annihilation or creation of one or several electron-hole pairs. The method can also be applied on electron-hole annihilation processes involving excitations of remaining particles. In this work, the method has been implemented and used for obtaining the radiative recombination rates of electrons and holes confined in a semiconductor ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ quantum dot sample. The qualitative features of an experimental single-dot photoluminescence (PL) spectrum are well explained by the calculated PL spectra including exciton, biexciton, and triexciton transitions. The obtained exciton-biexciton and biexciton-triexciton splittings agree well with the experiment. The present computational approach provides accurate and reliable interpretations of PL spectra for quantum dots.

Observation of transient current induced in silicon carbide diodes by ion irradiation

The high-speed current transients induced in SiC p–n diodes by 9 MeV Ni 2+ and 12 MeV Ni 3+ were measured using transient ion beam induced current (TIBIC). The amplitude of the TIBIC signals increases with increasing bias voltage. The transient current fall-time reduces with increasing bias voltage. These results are attributed to the increase in the electric field and the space-charge region with bias. However, the charge collection efficiency is below 100% even if the depletion width is greater than the Ni ion range. This can be attributed to the large nuclear stopping of Ni and recombination of the highly dense electron–hole pairs in the space-charge region. Our result indicates that pulse height defect (PHD) plays a significant role in the charge collection mechanism in these SiC p–n diodes for highly ionizing particles.

Near-band-edge photoluminescence of wurtzite-type AlN

Temperature-dependent photoluminescence (PL) measurements were performed for A-plane and C-plane bulk AlN single crystals and epitaxial layers on sapphire. A strong near-band-edge (NBE) emission and deep-level luminescence were observed. At low excitations, the emission spectra are dominated by free and bound excitonic transitions and their LO-phonon replicas. At high excitations, the broadening and redshift of the NBE band is attributed to dense electron–hole plasma formation. The PL spectra differences of bulk single crystals and epilayers is explained by the electron–hole plasma expansion peculiarities.

Superfast fronts of impact ionization in initially unbiased layered semiconductor structures

A mode of impact ionization breakdown of a p–n junction is suggested: We demonstrate that when a sufficiently sharp voltage ramp is applied in reverse direction to an initially unbiased equilibrium p+–n–n+ structure, after some delay the system will reach a high conductivity state via the propagation of a superfast impact ionization front. The front travels towards the anode with a velocity vf several times larger than the saturated drift velocity of electrons vs leaving a dense electron–hole plasma behind. The excitation of the superfast front corresponds to the transition from the common avalanche breakdown of a semiconductor structure to a collective mode of streamer-like breakdown. We propose that similar fronts can be excited not in layered structures but in plain bulk samples without p–n junctions. Our numerical simulations apply to a Si structure with typical thickness of W∼100 μm switched in series with a load R∼100 Ω, with a voltage ramp of A>1012 V/s applied to the whole system. Our simulations show that first there is a delay of about 1 ns during which the voltage reaches a value of several kilovolts. Then, as the front is triggered, the voltage abruptly breaks down to several hundreds of volts within ∼100 ps. This provides a voltage ramp of up to ∼2×1013 V/s hence up to 10 times sharper than the externally applied ramp. We unravel the source of initial carriers which trigger the front, explain the origin of the time delay in triggering the front, and we identify the mechanism of front propagation.

Ferromagnetism in the Kondo-lattice model

We propose a modified Rudermann, Kittel, Kasuya, Yosida (RKKY) technique to evaluate the magnetic properties of the ferromagnetic Kondo-lattice model. Together with a previously developed self-energy approach to the conduction-electron part of the model, we obtain a closed system of equations which can be solved self-consistently. The results allow us to study the conditions for ferromagnetism with respect to the band occupation n, the interband exchange coupling J, and the temperature T. Ferromagnetism appears for relatively low electron (hole) densities, while it is excluded around half-filling $(n=1).$ For small J the conventional RKKY theory $(\ensuremath{\sim}{J}^{2})$ is reproduced, but with strong deviations for very moderate exchange couplings. For not-too-small n a critical ${J}_{c}$ is needed to produce ferromagnetism with a finite Curie temperature ${T}_{C},$ which increases with J, then running into a kind of saturation, in order to fall off again and disappear above an upper critical exchange J. Spin waves show a uniform softening with rising temperature, and a nonuniform behavior as functions of n and J. The disappearance of ferromagnetism when varying T, J, and n is uniquely connected to the stiffness constant D becoming negative.

Fast electron–hole plasma luminescence from track-cores in heavy-ion irradiated wide-band-gap crystals

Measurements of fast luminescence decays and time-resolved spectra reveal novel ultra-short-lived luminescence (several tens ps) in heavy-ion irradiated single crystals of LiF, NaF, NaCl, KCl, KBr, KI, RbI, CsCl, CsBr, CsI, α-alumina and MgO. The luminescence is characterized by its super-linear increase to excitation density, non-tailing decay, sample dependence and temperature-insensitive decay and yield. The luminescence can not be attributed to known excited species, but vigorous interaction among dense electron–hole pairs, i.e. plasma.

Tuning of 2-D Silicon Photonic Crystals

AbstractWe demonstrate two ways in which the optical band-gap of a 2-D macroporous silicon photonic crystal can be tuned. In the first method the temperature dependence of the refractive index of an infiltrated nematic liquid crystal is used to tune the high frequency edge of the photonic band gap by up to 70 nm as the temperature is increased from 35 to 59°C. In a second technique we have optically pumped the silicon backbone using 150 fs, 800 nm pulses, injecting high density electron hole pairs. Through the induced changes to the dielectric constant via the Drude contribution we have observed shifts up to 30 nm of the high frequency edge of a band-gap.

Depolarization shift of the intersubband resonance in a quantum well with an electron-hole plasma

We study the depolarization shift of the electronic intersubband resonance at various electron-hole plasma densities. Using a time-resolved interband-pump--intersubband-probe technique, we measure the intersubband transitions in GaAs quantum wells. The pump pulse generates an electron-hole plasma, which then gradually decays by radiative recombination, while the probe pulse monitors the conduction band intersubband absorption spectrum as a function of the momentary plasma density. The measured time/density-dependent shift of the intersubband resonance energy is quantitatively explained by a density functional model accounting for the static and dynamic many-body effects in the electron-hole plasma. It turns out that the holes play an important role in minimizing the static Hartree potential. Their presence, however, does not significantly affect the measured dynamic depolarization shift of the electronic intersubband resonance.

Femtosecond buildup of Coulomb screening in a photoexcited electron–hole plasma

In an extreme nonequilibrium electron–hole plasma in GaAs, we observe how Coulomb screening builds up on a femtosecond time scale. To this end, the dynamics of the complex dielectric function throughout the mid-infrared is probed with uncertainty-limited temporal resolution by means of ultrabroadband THz spectroscopy. We show that the system becomes conducting instantaneously, whereas collective behavior such as Coulomb screening and plasmon scattering exhibits a delayed onset. Thus, the ultrafast formation of dressed quasiparticles is directly monitored for the first time. The time scale for these phenomena is of the order of the inverse plasma frequency. Our findings support recent quantum kinetic calculations of the temporal evolution of the Coulomb interaction after ultrafast excitation of a dense electron–hole plasma.

Metal–insulator transition of spatially separated electrons and holes in mixed type I–type II GaAs/AlAs quantum wells

Low temperature photo-induced far-infrared and microwave absorption studies of mixed type I–type II GaAs/AlAs multiple quantum wells have revealed features related to a metal–insulator transition. The far infrared experiments show an absorption feature at fields below electron cyclotron resonance (eCR) that shifts up with increasing electron–hole density and eventually pins to the eCR field position. In microwave experiments a broad, photoinduced absorption line is observed at fields above that of eCR; the position and strength of this feature depend on excitation intensity, temperature and microwave power. This disparate behavior can be understood qualitatively in terms of internal transitions of spatially separated electrons bound to holes localized laterally in short-range well-width fluctuations and clustered in long-range potential maxima in the narrow wells. These transitions evolve with increasing excitation intensity via a metal-insulator transition into dimensional magnetoplasma resonances of “puddles” of an electron liquid confined laterally by the potential of the clustered holes.

Coexistence of excitonic lasing with electron–hole plasma spontaneous emission in one-dimensional semiconductor structures

We report that excitonic lasing gain coexists with spontaneous optical emission characteristic of an electron–hole plasma in highly photoexcited one-dimensional semiconductors. The experiments probe quantum T-wire laser structures optimized for high photoexcitation. Evidence of dense electron–hole plasma is clearly seen in the spontaneous recombination measured when lasing emission displays distinct excitonic character. These findings differ strikingly from those in higher dimentional semiconductors, and offer insights on optical processes considered by recent theories of dense electron–hole plasmas.

Comment on “NMR in manganese perovskites: Detection of spatially varying electron states in domain walls”

Papavassiliou et al. [Phys. Rev. B 58, 12 237 (1998)] observed ``apparently strange'' ${}^{55}\mathrm{Mn}$ NMR line shape and ${T}_{2}$ versus frequency dependence in ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}.$ The authors believe that the NMR signal originates in the domain walls and that the electron hole density varies across the domain wall for $T<80\mathrm{K}.$ Their conclusion is that the charge variation in the domain walls is responsible for the observed strong magnetoresistivity at low temperatures. We suggest that the ``strange'' line shape is likely to result from the experimental conditions and not from the spatial variation of the charge density.

Coulomb effects in spatially separated electron and hole layers in coupled quantum wells

We report on the (magneto-) optical study of many-body effects in spatially separated electron and hole layers in GaAs/Al x Ga 1 - x As coupled quantum wells (CQWs) at low temperatures ( T = 1.4 K) for a broad range of electron-hole ( e-h ) densities. Coulomb effects were found to result in an enhancement of the indirect (interwell) photoluminescence (PL) energy with increasing the e-h density both for a zero magnetic field and at high fields for all Landau level transitions; this is in contrast to the electron-hole systems in single QWs where the main features are explained by the band-gap renormalization resulting in a reduction of the PL energy. The observed enhancement of the ground state energy of the system of the spatially separated electron and hole layers with increasing the e-h density indicates that the real space condensation to droplets is energet- ically unfavorable. At high densities of separated electrons and holes, a new direct (intrawell) PL line has been observed: its relative intensity increased both in PL and in absorption (measured by indirect PL excitation) with increasing density; its energy separation from the direct exciton line fits well to the X - and X + binding energies previously measured in single QWs. The line is therefore attributed to direct multiparticle complexes. © 2001 MAIK "Nauka/Interperiodica".

Electron–hole drops in synthetic diamond

Abstract In this paper we give the first report of a high-density phase of condensed electron–hole drops in diamond. Synthetic crystals grown by the high pressure–high temperature technique were optically highly excited by laser pulses with photon energies above the band-gap of diamond. At temperatures below ≈170 K we observe novel broad luminescence bands, in addition to the well-known free exciton spectra, with characteristic features identifying them as originating from electron–hole drops. Fits to the photoluminescence spectra yield an electron–hole density within the drops of n 0 =9.6×10 19 cm −3 , a reduced band-gap of E g ′=5.224 eV (instead of E g =5.49 eV) due to the attractive interaction of the particles in the condensed phase, and a work function of φ =52 meV with respect to the free exciton threshold E gx .

Band-gap renormalization in quantum wire systems: dynamical correlations and multi-subband effects

We study the band-gap renormalization in a model semiconductor quantum wire due to the exchange-correlation effects among the charge carriers. We construct a two-subband model for the quantum wire, and employ the GW-approximation to obtain the renormalized quasi-particle energies at the optical band edge. The renormalization is calculated as a function of electron-hole plasma density and the wire radius. Our results show that the very presence of the second subband affects the renormalization process even in the absence of occupation by the carriers. We compare the fully dynamical random-phase approximation results to the quasi-static case in order to emphasize the dynamical correlation effects. Effects of electron-phonon interaction within the two-subband model are also considered.

Condensation of a hot electron–hole plasma in tunneling silicon MOS structures

A recombination radiation line of free electrons and holes is observed in electroluminescence spectra of tunneling [100] silicon MOS diodes at T=300–700K. A very high quantum efficiency of the luminescence and unusual spectral features of the luminescence line are explained by the appearance of dense states of an electron–hole plasma. A dense surface plasma is created in silicon doped by phosphorus. Condensation of a hot electron–hole plasma into dense electron–hole flexes, representing a phase separation, occurs in a strong electrical field in silicon doped by boron. This is accompanied by appearance of a negative differential resistance of the diode.

Interband and intersubband absorption in HgCdTe multiple quantum wells

We present calculated and experimental interband and photoinduced intersubband absorption spectra for undoped HgCdTe multiple quantum wells. The results show that there is an enhancement of the interband absorption at energies close to the barrier energy for electrons. The calculated interband absorption, based on a full $8\ifmmode\times\else\texttimes\fi{}8 \mathbf{k}\ensuremath{\cdot}\mathbf{p}$ Kane model to obtain the electronic structure, predicts that this absorption enhancement is caused by an increase in the electron-hole joint density of states due to band mixing for confined and continuous electron states. The photoinduced electron intersubband absorption presents an asymmetric line shape due to the nonparabolic bands. The calculated spectra reproduce well the experimental results.

Ultrafast all-optical modulation of infrared radiation via metal-semiconductor waveguide structures

We present a novel optical-optical semiconductor switching technique for application to infrared laser beam modulation and ultrashort infrared laser pulse switching. This method relies on the ultrafast optical excitation, with femtosecond above-bandgap laser radiation, of an air-filled metal-clad semiconductor waveguide. Guided electromagnetic wave analysis combined with time-varying dielectric properties of the semiconductor layer are used to investigate the ultrafast switching speed of the structure. The device is capable of modulation at various infrared wavelengths. In particular, we investigate intensity modulation of the quasi-TE/sub 10/ mode for 10.6-/spl mu/m laser radiation. At an electron-hole photoinjection density of /spl sim/1.8/spl times/10/sup 18/ cm/sup -3/, an extinction ratio of 83 dB is demonstrated. This ratio is significantly higher than that exhibited by current optical-optical semiconductor switches. Potential applications to all-optical Mach-Zehnder metal-clad semiconductor modulators and self-limiting switches are also discussed.

High-temperature point defect equilibrium in CdTe modelling

High-temperature (673–1173 K) electrical property ( σ, R H) measurements in undoped and In-doped (up to 1×10 20 at/cm 3) CdTe were performed under different stoichiometric conditions. Cadmium (tellurium) vapor pressure or temperature electron (hole) density dependences were obtained. The results are described by a computed defect structure model, which optimizes the defect reaction constants. A new In incorporation defect reaction is proposed, explaining the dopant self-compensation mechanism.