Function Of Carrier Density Research Articles

Magnetotransport characterization of AlGaN/GaN interfaces

AbstractMagnetotransport studies of high quality AlGaN/GaN heterojunctions are reported. The transport and quantum lifetimes were respectively deduced from low temperature mobility and Shubnikov de Haas measurements as a function of carrier density in metal organic vapor phase epitaxy grown AlGaN/GaN/sapphire heterostructures. It is found that contrary to theoretical predictions both quantum and transport lifetimes have the same bell shape behavior versus carrier density. We determined experimental ratio of quantum to transport lifetime varying from 9 to 16 for carrier densities in 1–9 × 1012 cm–2 range. We show that the quantum lifetime is very sensitive to interface scattering at high carrier density. Therefore, our work shows how Shubnikov de Haas experiments can be used to characterize the quality of AlGaN/GaN interfaces used in modern light emitting devices. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth and subband structure determination of high mobility hole gases on (0 0 1) and (1 1 0) GaAs

We introduce high mobility two-dimensional hole systems (2DHSs) in AlGaAs/GaAs heterosystems grown by molecular beam epitaxy (MBE). Utilizing a carbon filament source, two-dimensional hole gases (2DHGs) were fabricated on the (0 0 1) and (1 1 0) crystal plan. These samples exhibit low-temperature mobilities beyond 10 6 cm 2/V s at densities varying from 0.6 to 2.3×10 11 cm −2. In order to explore the subband structure as a function of carrier density, aluminum top gates were deposited on the samples. The carrier density was found to show a hysteretic behavior when tuned with an external electric field. The origin of this effect will be discussed. It opens a way to adjust the hole density in a wide range within a single sample in the absence of an externally applied electric field.

Exciton Recombination Dynamics in CdSe Nanowires: Bimolecular to Three-Carrier Auger Kinetics

Ultrafast relaxation dynamics of charge carriers in CdSe quantum wires with diameters between 6 and 8 nm are studied as a function of carrier density. At high electron-hole pair densities above 10(19) cm(-3) the dominant process for carrier cooling is the "bimolecular" Auger recombination of one-dimensional (1D) excitons. However, below this excitation level an unexpected transition from a bimolecular (exciton-exciton) to a three-carrier Auger relaxation mechanism occurs. Thus, depending on excitation intensity, electron-hole pair relaxation dynamics in the nanowires exhibit either 1D or 0D (quantum dot) character. This dual nature of the recovery kinetics defines an optimal intensity for achieving optical gain in solution-grown nanowires given the different carrier-density-dependent scaling of relaxation rates in either regime.

Why RKKY exchange integrals are inappropriate to describe ferromagnetism in diluted magnetic semiconductors

We calculate Curie temperatures and study the stability of ferromagnetism in diluted magnetic materials, taking as a model for the exchange between magnetic impurities a damped Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction and a shor t range term representing the effects of superexchange. To properly include effects of spin and thermal fluctuations as well as geometric disorder, we solve the effective Heisenberg Hamiltonian by means of a recently developed semi-analytical approach. This approach, ``self-consistent local Random Phase Approximation (SC-L RPA)'', is explained. We show that previous mean-field treatments, which have been widely used in the literature, largely overestimate both the Curie temperatures and the stability of ferromagnetism as a function of carrier density. The discr epancy when compared to the current approach was that effects of frustration in RKKY oscillations had been strongly underestimated by such simple mea n-field theories. We argue that the use, as is frequent, of a weakly-disordered RKKY exchange to model ferromagnetism in diluted III-V systems is inconsistent with the observation of ferromagnetism over a wide region of itinerant carrier densities. This may be puzzling when compared to the apparent success of calculations based on {\it ab-initio} estimates of the coupling; we propose a resolution to this issue by taking RKKY-like interactions between resonant states close to the Fermi level.

Theoretical and experimental results of MMW plasma gratings in silicon quartz image guide technology

The propagation characteristics of an optoelectronically induced plasma grating on a single moded silicon–quartz image guide are experimentally investigated at W-band frequencies and compared with theoretical results based on a transversal homogeneous plasma profile. Because of the long diffusion lengths of the silicon semiconductor considered, an optimum carrier density and, hence, an optimum optical power density is found which results in a maximum magnitude of plasma-induced grating reflection coefficient. To understand the behavior of the reflection coefficient versus charge carrier density, the variation of phase and attenuation constants as a function of carrier density in the dark and excited waveguide sections are investigated. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 1928–1932, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21835

Quantum and transport lifetimes of two-dimensional electrons gas in AlGaN∕GaN heterostructures

The transport and quantum lifetimes were respectively deduced from low-temperature mobility and Shubnikov–de Haas measurements as a function of carrier density in metal organic vapor phase epitaxy-grown AlGaN∕GaN∕sapphire heterostructures. We show experimentally that the lifetime ratio varies as a bell curve, qualitatively confirming a recent theoretical prediction. However the experimental ratio varied much less than was theoretically predicted: From 9 to 19 for carrier densities in 1–9×1012cm−2 range. Moreover, we show the variation of quantum time with carrier density presents some discrepancy with the theoretical study. We also show that transport to quantum lifetime ratio cannot be used alone as a clear figure of merit from AlGaN∕GaN heterojunctions.

Temperature and magnetization-dependent band-gap renormalization and optical many-body effects in diluted magnetic semiconductors

We calculate the Coulomb interaction induced density, temperature and magnetization-dependent many-body band-gap renormalization in a typical diluted magnetic semiconductor ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ in the optimally doped metallic regime as a function of carrier density and temperature. We find a large $(\ensuremath{\sim}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ band-gap renormalization which is enhanced by the ferromagnetic transition. We also calculate the impurity scattering effect on the gap narrowing. We suggest that the temperature, magnetization, and density dependent band-gap renormalization could be used as an experimental probe to determine the valence band or the impurity band nature of carrier ferromagnetism.

Electron density dependence of the electronic structure of InN epitaxial layers grown on sapphire (0001)

The temperature dependence of the resistivity of InN was investigated as a function of carrier density. The carrier density was changed from ${n}_{e}=1.8\ifmmode\times\else\texttimes\fi{}{10}^{18}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}1.5\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ by Si doping. The InN investigated showed metallic conduction above $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. At lower temperatures there was a resistivity anomaly originating from carrier localization in the $a\text{\ensuremath{-}}b$ plane, which was confirmed by the magnetoresistance at $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The Shubnikov--de Haas oscillation showed that InN had a spherical Fermi surface and its radius increased according to the increase of ${n}_{e}$ when ${n}_{e}<5\ifmmode\times\else\texttimes\fi{}{10}^{18}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. In addition, an oscillation corresponding to the constant carrier density of $4.5\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ was observed in the field applied perpendicular to the $a\text{\ensuremath{-}}b$ plane. This oscillation showed an anomalous angle dependence on the magnetic field. Taking into account this density, we determined the critical carrier density of the Mott transition to be $2\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. Anisotropy of localization was observed within the $a\text{\ensuremath{-}}b$ plane, which indicates that the distribution of the electrons was not uniform in the $a\text{\ensuremath{-}}b$ plane. The ${n}_{e}$ dependence of the magnetoresistance revealed an electronic structure change around $5\ifmmode\times\else\texttimes\fi{}{10}^{18}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. From these results, an electronic structure at the fundamental absorption edge of InN grown on sapphire (0001) was presented.

Density-dependent spin susceptibility and effective mass in interacting quasi-two-dimensional electron systems

Motivated by recent experimental reports, we carry out a Fermi liquid many-body calculation of the interaction induced renormalization of the spin susceptibility and effective mass in realistic two dimensional (2D) electron systems as a function of carrier density using the leading-order `ladder-bubble' expansion in the dynamically screened Coulomb interaction. Using realistic material parameters for various semiconductor-based 2D systems, we find reasonable quantitative agreement with recent experimental susceptibility and effective mass measurements. We point out a number of open questions regarding quantitative aspects of the comparison between theory and experiment in low-density 2D electron systems.

Hanle Effect Measurements of Spin Lifetime in Zn0.4Cd0.6Se Epilayers Grown on InP

We use the Hanle effect to study spin relaxation in ZnxCd1−xSe epilayers grown on lattice-matched InP substrates. We study three samples with a fixed composition (x = 0.4) and with varying levels of n-doping, as well as an undoped sample with x = 0.5. Our measurements show that the spin relaxation time changes non-monotonically as a function of carrier density, with a maximum transverse spin lifetime of ~10.5 ns at low temperatures for a sample doped near the metal–insulator transition.

Spin wave theory of ferrimagnetic double perovskites

We present a theoretical study of magnetic properties of metallic double perovskite ferrimagnets such as Sr 2FeMoO 6 and Sr 2FeReO 6. The analysis is based on the Kondo-type Hamiltonian in which charge carriers are constrained to be antiparallel to Fe local moments with spin S. The spectrum of spin wave excitations is derived based on the model Hamiltonian within the 1/ S expansion. The ground state phase diagram as a function of carrier density is also discussed.

Ferromagnetic III–V and II–VI Semiconductors

AbstractRecent years have witnessed extensive research aimed at developing functional, tetrahedrally coordinated ferromagnetic semiconductors that could combine the resources of semiconductor quantum structures and ferromagnetic materials systems and thus lay the foundation for semiconductor spintronics. Spin-injection capabilities and tunability of magnetization by light and electric field in Mn-based III–V and II–VI diluted magnetic semiconductors are examples of noteworthy accomplishments. This article reviews the present understanding of carrier-controlled ferromagnetism in these compounds with a focus on mechanisms determining Curie temperatures and accounting for magnetic anisotropy and spin stiffness as a function of carrier density, strain, and confinement. Materials issues encountered in the search for semiconductors with a Curie point above room temperature are addressed, emphasizing the question of solubility limits and self-compensation that can lead to precipitates and point defects. Prospects associated with compounds containing magnetic ions other than Mn are presented.

Effects of tensile strain in barrier on optical gain spectra of GaInNAs/GaAsN quantum wells

The band structures, optical gain spectra, and transparency radiative current densities of compressive-strained GaInNAs quantum wells (QWs) with different tensile-strained GaAsN (N composition from 0 to 3%) barriers are systematically investigated using a modified 6×6 k⋅p Hamiltonian including the heavy hole, light hole, and spin-orbit splitting bands. We found that the transition energy decreases when increasing the N composition in the barrier. The optical gain spectra and maximum optical gain as a function of carrier density and radiative current density are obtained for the GaInNAs/GaAsN QWs with well width of 5 nm, InW=28%, and NW=2.66% emitting around 1.55 μm. The transparency carrier density increases with the nitrogen composition in the GaAsN barrier. The transparency radiative current density decreases with more nitrogen being added into the barrier, which is in agreement with the recent experimental observation.

Linewidth enhancement factor of lattice-matched InGaNAs/GaAs quantum wells

The linewidth enhancement factors of lattice-matched 1.5 μm wavelength InGaNAs/GaAs and InGaAs/InP single-quantum-well structures are calculated using microscopic theory and 10×10 effective-mass Hamiltonian. InGaNAs/GaAs quantum wells have a lower threshold carrier density and higher differential gain resulting in a lower linewidth enhancement factor compared with InGaAs/InP quantum wells. For applications which require high gain and carrier densities, InGaNAs/GaAs quantum wells have a much lower linewidth enhancement factor over a temperature range 300–400 K. This lower value originates from the large electron effective mass caused by the nitrogen incorporation. The linewidth enhancement factor of InGaNAs is almost clamped as a function of carrier density and temperature compared with InGaAs. This effect is due to the enhanced match between conduction-valence band density of states and the improved electron confinement caused by the large conduction band offset and deep quantum wells.

Carrier recombination processes in MOVPE and MBE grown 1.3 μm GaInNAs edge emitting lasers

By measuring the spontaneous emission (SE) from metal organic vapour phase epitaxy (MOVPE) grown ∼1.3 μm GaInNAs/GaAs-based lasers during normal operation, we have quantitatively determined the variation of each of the current paths present in the devices as a function of temperature from 130 to 370 K and compared these with results previously obtained for molecular beam epitaxy (MBE) grown GaInNAs lasers. From the SE measurements we determine how the current, I, close to threshold, varies as a function of carrier density, n, which enables us to separate out the main current paths corresponding to monomolecular (defect-related), radiative or Auger recombination respectively. We find that at room temperature (RT), defect-related recombination contributes ∼360 A/cm 2 (MBE) and ∼565 A/cm 2 (MOVPE) to the total current density at threshold. Radiative recombination accounts for ∼110 A/cm 2 (MBE) and 195 A/cm 2 (MOVPE) of J th with the remaining ∼180 A/cm 2 (MBE) and 760 A/cm 2 (MOVPE) are due to non-radiative Auger recombination. Our results suggest that a larger threshold carrier density in the MOVPE grown device in comparison to the MBE lasers, can reasonably explain the larger current densities of the different recombination processes at RT. We tentatively associate this with higher optical loss processes in the MOVPE grown material.

A quantitative study of radiative, Auger, and defect related recombination processes in 1.3-μm GaInNAs-based quantum-well lasers

By measuring the spontaneous emission (SE) from normally operating /spl sim/1.3-/spl mu/m GaInNAs-GaAs-based lasers we have quantitatively determined the variation of each of the current paths present in the devices as a function of temperature from 130 K to 370 K. From the SE measurements we determine how the current I close to threshold, varies as a function of carrier density n, which enables us to separate out the main current paths corresponding to monomolecular (defect-related), radiative or Auger recombination. We find that defect-related recombination forms /spl sim/55% of the threshold current at room temperature (RT). At RT, radiative recombination accounts for /spl sim/20% of I/sub th/ with the remaining /spl sim/25% being due to nonradiative Auger recombination. Theoretical calculations of the threshold carrier, density as a function of temperature were also performed, using a ten-band k /spl middot/ p Hamiltonian. Together with the experimentally determined defect-related, radiative, and Auger currents we deduce the temperature variation of the respective recombination coefficients (A, B, and C). These are compared with theoretical calculations of the coefficients and good agreement is obtained. Our results suggest that by eliminating the dominant defect-related current path, the threshold current density of these GaInNAs-GaAs-based devices would be approximately halved at RT. Such devices could then have threshold current densities comparable with the best InGaAsP/InP-based lasers with the added advantages provided by the GaAs system that are important for vertical integration.

Electron-Hole Dynamics in MOCVD-Grown InGaAs/GaAs Quantum Dots Emitting at 1.3 ?m

A study of the electron-hole relaxation dynamics in metalorganic chemical vapour deposition (MOCVD)-grown InGaAs/GaAs quantum dots (QDs) emitting at 1.3 μm is presented. The photoluminescence (PL) rise and decay times are measured as functions of carrier density and temperature, showing that the electron-hole relaxation into the QD ground state occurs within a few picoseconds. We find that the emission of two longitudinal optical (LO) phonons is the dominant capture process at room temperature, whereas carrier-carrier scattering dominates the relaxation process at low temperatures and high carrier densities. Finally, the MOCVD-grown QD structures show relatively small PL quenching; the quenching is caused by thermal carrier escape from the QD ground state via absorption of two LO phonons.

Crossover Dynamics with Low Energy Excitations in Jahn–Teller Co Perovskites

La1 − x Sr x CoO3 serves as a prototype for studying crossover characteristics from static to dynamic lattice effects associated with Jahn–Teller (JT) excitations. With the use of the pair density function analysis as a function of carrier density, evidence for localized lattice distortions associated with a JT active mode is provided. Energy resolved measurements of the dynamic structure factor show that the lattice dynamics change with doping, concomitant to the local distortions that are correlated to the magnetic and transport transitions of this system. The activation of these effects might be in response to a dynamic JT mode that is invoked by fluctuations to the intermediate-spin configuration. It is suggested that such intermediate-spin clusters might be present in the metallic state of these oxides.

Many-body effects on excitonic optical properties of photoexcited semiconductor quantum wire structures

We study carrier-interaction-induced many-body effects on the excitonic optical properties of highly photoexcited one-dimensional semiconductor quantum wire systems by solving the dynamically screened Bethe-Salpeter equation using realistic Coulomb interaction between carriers. Including dynamical screening effects in the electron-hole self-energy and in the electron-hole interaction vertex function, we find that the excitonic absorption is essentially peaked at a constant energy for a large range of photoexcitation density $(n=0--6\ifmmode\times\else\texttimes\fi{}{10}^{5} {\mathrm{cm}}^{\ensuremath{-}1}),$ above which the absorption peak disappears without appreciable gain; i.e., no exciton to free electron-hole plasma Mott transition is observed, in contrast to previous theoretical results but in agreement with recent experimental findings. This absence of gain (or the nonexistence of a Mott transition) arises from the strong inelastic scattering by one-dimensional plasmons or charge density excitations, closely related to the non-Fermi-liquid nature of one-dimensional systems. Our theoretical work demonstrates a transition or a crossover in one-dimensional photoexcited electron-hole systems from an effective Fermi liquid behavior associated with a dilute gas of noninteracting excitons in the low-density region $(n<{10}^{5} {\mathrm{cm}}^{\ensuremath{-}1})$ to a non-Fermi liquid in the high-density region $(n>{10}^{5} {\mathrm{cm}}^{\ensuremath{-}1}).$ The conventional quasistatic approximation for this problem is also carried out to compare with the full dynamical results. Numerical results for exciton binding energy and absorption spectra are given as functions of carrier density and temperature.

Exciton formation assisted by LO phonons in quantum wells

Kinetics of exciton formation involving LO phonons is investigated in quantum wells. Considering the formation of an exciton from a free excited electron-hole pair due to LO-phonon emission, an expression is derived for the rate of formation of an exciton as a function of carrier densities, temperature, and wave vector ${\mathbf{K}}_{\ensuremath{\Vert}}$ of the center of mass of excitons in quantum wells, and the formation time of an exciton is also calculated. The theory is applied to GaAs quantum wells, in which it is found that the exciton formation dominantly occurs at ${\mathbf{K}}_{\ensuremath{\Vert}}\ensuremath{\ne}0.$