Longitudinal Optical Phonon Interaction Research Articles

Effects of electron–phonon coupling and Rashba spin–orbit interaction on thermodynamic and magnetic properties of quantum dots

The thermodynamic and magnetic properties of quantum-dot structures subjected to an applied magnetic field were studied, including the longitudinal optical–phonon interaction and the Rashba spin–orbit effect. The Schrödinger equation was solved to determine the energy levels. The partition function was evaluated by summing the accessible energy levels and was then utilized to calculate the thermomagnetic functions. In this paper, we present magnetic properties by considering three interacting polarons. Our results indicated that at B=0T, the susceptibility exhibits diamagnetic behavior for all values of the Rashba spin–orbit parameter. However, the magnetic susceptibility increases with an applied magnetic field, and the system exhibits paramagnetic behavior under moderate magnetic fields. However, in situations with and without the polaron effect, the susceptibility is saturated at 0(meV/T2) under large magnetic fields. In this study, we showed that the Rashba spin–orbit interaction (SOI) strengthens the cutoff magnetic field Bc (the B value at which the magnetic nature of the dot changes from diamagnetic to paramagnetic). Rashba SOIs reduce the mean energy of the system, including polaronic interactions. Under the polaron effect, the heat capacity curve shifts to lower temperatures. A quantitative description of the magnetocaloric effect (ΔS) as a function of the Wigner and Rashba spin–orbit parameters is presented.

Optical absorption of conduction electrons in semiconductor nanowires in the presence of the electron–phonon interaction

We calculate the energy states and the optical absorption coefficient for electrons in a nanowire in the presence of the Rashba and the longitudinal-optical phonon interactions. The interplay of those interactions results in a splitting in the electron dispersion relation at zero wavevector that grows linearly for the ground state as the strength of the lateral quantum confinement increases. For higher states, the energy splitting increases more rapidly till the state of resonant polaron is reached, then it increases slowly due to the pinning effect. The frequency separation between the well-resolved absorption peaks and their number are greatly influenced by the state of the polarons.

Temperature- and size-dependent photoluminescence in colloidal CdTe and Cd x Zn1−x Te quantum dots

Semiconductor colloidal quantum dots (QDs) of CdTe and alloyed Cd x Zn1−x Te QDs with N-acetyl-L-cysteine capping ligands are synthesized by a reflux method in aqueous solution. Alloying provides a new degree of freedom to tune the optical and electronic properties of the nanocrystals. The photoluminescence (PL) of Cd x Zn1−x Te QDs is sharper and displays a highly enhanced quantum yield (QY) of 65% relative to the 16% of CdTe QDs. The fluorescence of Cd x Zn1−x Te QDs is observed to be highly stable for over 12 months without degradation, while that of CdTe QDs begins to mildly flocculate around 8 months of storage. To characterise the material structure and composition, UV-Vis absorption spectroscopy, x-ray powder diffraction, transmission electron microscopy, and inductively coupled plasma mass spectrometry measurements are carried out. To understand the fundamental processes that play in the luminescence behaviour, temperature- and size-dependent PL spectra are investigated in the range 80–300 K. The Varshni and O’Donnell equations fit well on the PL peak emission energies and the Huang–Rhys parameter indicates the strengthening of exciton–phonon coupling in the nanocrystals upon alloying and with decreasing nanocrystal sizes. PL linewidth analysis reveals that the inhomogeneous broadening is considerably reduced in Cd x Zn1−x Te QDs relative to CdTe. Moreover, the quantum confinement effect of the nanocrystals leads to an increase in exciton–acoustic phonon interactions with the coefficients ranging between 26.9 and 95.6 µeV K−1 compared to the bulk CdTe value of 0.72 µeV K−1. Exciton–longitudinal optical phonon interactions are made stronger by the ZnTe alloying with the coefficients lying in the range between 24.8 and 41.7 meV and also with the effect of increasing crystal size. An Arrhenius plot of PL integrated area is used to calculate the thermal activation energy value E a of the non-radiative recombination channel, which is 132 meV for CdTe QDs and a higher value of 185 meV for Cd x Zn1−x Te QDs. This is consistent with the observed QY enhancement in Cd x Zn1−x Te QDs as a higher E a value indicates reduced generation of non-radiative recombination centres and a decrease in defect densities upon alloying. Cd x Zn1−x Te QDs with enhanced fluorescence properties serve both as a medium for studying fundamental effects of alloying and its properties, and for practical applications such as biomedical labelling and optoelectronics.

Optical characteristics of GaAsSb alloy after rapid thermal annealing

GaAsSb ternary alloy is a promising material for application in infrared optoelectronic devices. In this letter, the investigation of carrier recombination in the as-grown and rapid thermal annealing (RTA) treated GaAsSb samples has been carried out. It was found that after thermal treatment the emission of the GaAsSb material was enhanced and could be maintained up to room temperature. These phenomena can be ascribed to the decrease of non-radiative combination defects in the GaAsSb sample, which implies an improved crystal quality. Moreover, the localized exciton–longitudinal optical phonon interaction is slightly increased after RTA treatment. It is suggested that the interaction depends strongly on the localized states, and the photoluminescence emission intensity can be significantly increased after suitable RTA treatment. Promoting better optical emission in GaAsSb is very useful for its practical application.

Effect of impurity bound polaron on optical absorption in a GaAs modified Gaussian quantum dot

In the present work, the linear, third-order nonlinear and total optical absorption coefficients of a GaAs modified Gaussian quantum dot is investigated theoretically with considering electron longitudinal optical (LO) phonon and Coulomb impurity. We have employed the Lee–Low–Pines–Huybrecht variational method to calculate the ground and first excited states energy. The absorption coefficients calculations have also been performed by using the compact density matrix approach. Our calculations have been performed for different quantum dot sizes and various confinement potentials. The absorption coefficient resonance peaks shift toward higher energies when the LO phonon and the Coulomb potential are considered. The LO phonon interaction and the Coulomb potential have play key important in optical absorption coefficients.

Influence of magnetic field and Rashba spin–orbit coupling on strong-coupling magnetopolarons in quantum disks

On the basis of Lee–Low–Pines (LLP) unitary transformation, the influence of external magnetic field, Rashba spin–orbit coupling and quantum size effect on the ground-state interaction energy of strong-coupling magnetopolarons in quantum disks (QDs) is studied by using the Tokuda improved linear combine operator method. The results show that the ground-state interaction energy of magnetopolarons consists of four parts: the energy caused by the confinement potential of QDs, interaction energy between the electron and external magnetic field, electron and longitudinal-optical (LO) phonon interaction energy and additional term of Rashba effect originating from phonons. The electron–LO phonon interaction energy Ee- ph and additional term of Rashba effect are always negative; the absolute value |Ee- ph | increases with increasing transverse confinement strength ω0, cyclotron frequency of external magnetic field ωc and electron–LO phonon coupling strength α, but decreases with increasing the thickness of QDs L; the state properties of magnetopolarons are closely linked with the sign of the ground-state interaction energy of magnetopolarons E int and change of E int with ωc, ω0, α and L. In addition, the vibration frequency of magnetopolarons λ increases with increasing ωc, ω0 and α, but decreases with increasing L. For the ground state of magnetopolarons in QDs, the electron–LO phonon interaction plays a significant role, meanwhile, the influence of Rashba spin–orbit coupling effect cannot be ignored.

Properties of a Strong Coupling Bipolaron in an Asymmetric Quantum Dot

We study a strong coupling bipolaron’s vibrational frequency, self-trapping energy and potential induced by the electron–longitudinal optical (LO) phonon interaction in an asymmetric quantum dot (AQD). The effects of the electron–phonon coupling strength, the transverse and longitudinal effective confinement lengths are taken into account by using linear combination operator and unitary transformation methods. It is found that the vibrational frequency is an increasing function of the electron–phonon coupling strength, whereas it is a decreasing one of the transverse and longitudinal effective confinement lengths. The absolute values of the self-trapping energy and the potential induced by the electron–LO phonon interaction will increase with increasing coupling strength or decreasing effective confinement lengths.

The role of electron-electron scattering in gain modulation of a mid-infrared quantum cascade laser in strong magnetic field

We present a theoretical study of electron relaxation processes and the optical gain of the mid-infrared quantum cascade laser subjected to a strong magnetic field along the growth direction. The intensity of emitted radiation shows strong oscillations as a function of the magnetic field, due to modulation of the upper laser state lifetime. A particular goal of this analysis is to estimate the combined contribution of the electron scattering due to longitudinal optical phonon interactions and electron–electron interactions on the laser output properties. The structure under consideration comprises a triple quantum well GaAs/Al0.45Ga0.55 active region and is intended for operation at ∼9.3 µm. The electron distribution over the states of the system is found by solving the full set of rate equations which describe the transitions between energy levels, and is subsequently used to determine the optical gain.

Exciton–phonon interaction in cylindrical semiconductor quantum dots

The exciton–longitudinal optical phonon interaction is theoretically investigated for the case of polar semiconductor cylindrical quantum dots embedded in semiconductor matrix. The theory is developed within the dielectric continuum model considering the Fröhlich interaction between electrons and confined bulk longitudinal optical phonons for a configurational interaction model of quantum dot. Representative longitudinal optical phonon mode for the exciton–phonon interaction is predicted for cylindrical InAs/GaAs quantum dots.

Effects of exciton–longitudinal optical phonon interaction and quantum confinement on exiton binding energies in ZnS1-xSex/ZnS single quantum wells

Effects of exciton–longitudinal optical phonon interaction and quantum confinement on exiton binding energies in ZnS_1-<i>x</i>Se_<i>x</i>/ZnS single quantum wells

EFFECTS OF CONFINED LO AND SO PHONON MODES ON POLARON IN FREESTANDING CYLINDRICAL QUANTUM WIRE WITH PARABOLIC CONFINEMENT

The electron self-energy and correction to the electron effective mass in a freestanding quantum wire with parabolic confining potential was investigated by the perturbation approach. Both the electron-confined longitudinal optical (LO) phonon and surface optical (SO) phonon interactions were considered. Results shows that, for small wire radius, the contributions of electron–LO phonon interaction to the electron self-energy and the correction to the electron effective mass are relatively small in compare with those of the electron–SO phonon interaction.

Contactless electroreflectance and photoluminescence characterization of Zn0.68Be0.06Mg0.26Se crystalline alloys

Abstract In this report, results of optical characterization of Bridgman-grown Zn 0.68 Be 0.06 Mg 0.26 Se crystalline alloys in the near-band-edge interband transitions region using temperature-dependent contactless electroreflectance (CER) and photoluminescence (PL) in the temperature range 10–300 K and time-resolved PL measurements at 10 K are presented. Two features with separation energy of 40 meV are observed in the near-band-edge CER spectra. The origins of these two features are attributed to the fundamental band-edge excitonic transitions of the coexisted different crystal phases in accordance with the X-ray diffraction and transmission electron microscope results. The increase of the broadening parameter of the excitonic feature of the PL spectra with increasing temperature has been attributed to the electron–longitudinal optical phonon interactions. In addition, the transient PL spectrum at 10 K in the excitonic emission region exhibits the asymmetric lineshape and can be deconvoluted into two Gaussian features with one order of magnitude difference in the decay time. These two components are assigned to the recombination of free excitons and localized excitons in this quaternary compound.

Theory of spontaneous emission of quantum dots in the linear regime

We develop a fully quantum-mechanical theory for the interaction of light andelectron–hole excitations in semiconductor quantum dots. Our theoretical analysis resultsin an expression for the photoluminescence intensity of quantum dots in the linear regime.Taking into account the single-particle Hamiltonian, the free-photon Hamiltonian, theelectron–hole interaction Hamiltonian, and the interaction of carriers with light, andapplying the Heisenberg equation of motion to the photon number expectation values, tothe carrier distribution functions and to the correlation term between the photongeneration (destruction) and electron–hole pair, we obtain a set of luminescence equations.Under quasi-equilibrium conditions, these equations become a closed-set of equations. Wesolve them analytically, in the linear regime, and we find an approximate solution of theincoherent photoluminescence intensity. The validity of the theoretical analysisis tested by investigating the emission spectra in the high-temperature regime,interpreting the experimental findings for the emission spectra of a lens-shapedIn0.5Ga0.5As self-assembled quantum dot. Our theoretical predictions for the interlevel spacing as well asfor the dephasing time caused by electron–longitudinal optical phonon interactions are ingood agreement with the experimental results.

Subpicosecond time-resolved Raman studies of electron–longitudinal optical phonon interactions in InN

Electron–longitudinal optical phonon scattering rate in InN has been directly measured by subpicosecond time-resolved Raman spectroscopy. The experimental results show that for a thick layer of InN grown on GaN, the average total electron–longitudinal optical phonon scattering rate is (5.1±1.0)×1013s−1. This enormous electron–longitudinal optical phonon scattering rate, which is comparable to that observed in GaN, has been attributed to the extremely polar nature of InN.

Hot-phonon temperature and lifetime in biased boron-implanted SiO2/Si/SiO2 channels

Microwave noise temperature is measured as a function of supplied electric power in 100 nm thick silicon on insulator layers. At 293 K, the estimated hot-hole energy-relaxation time decreases from ∼9 ps at power below 0.05 nW/hole down to ∼1.55 ps in the power range (2–10) nW/hole. The results are interpreted in terms of hot-hole interaction with longitudinal optical (LO) phonons. A comparison of the experimental data with those calculated in the hot-hole-temperature approximation indicates accumulation of non-equilibrium optical phonons (termed hot phonons). In the power range of the dominant hole–LO phonon interaction, the increase in equivalent hot-phonon temperature is proportional to the increase in hot-hole temperature. The estimated value for the hot-phonon lifetime, (1.75 ± 0.4) ps, is comparable with the hot-hole energy-relaxation time at the high bias, ∼1.55 ps.

On the binding energies of excitons in polar quantum well structures in a weak electric field

The binding energies of excitons in quantum well structures subjected to an applied uniform electric field by taking into account the exciton longitudinal optical phonon interaction is calculated. The binding energies and corresponding Stark shifts for III–V and II–VI compound semiconductor quantum well structures have been numerically computed. The results for GaAs/AlGaAs and ZnCdSe/ZnSe quantum wells are given and discussed. Theoretical results show that the exciton–phonon coupling reduces both the exciton binding energies and the Stark shifts by screening the Coulomb interaction. This effect is observable experimentally and cannot be neglected.

Optical Characteristics of InAs/GaAs Double Quantum Dots Grown by MBE with the Indium-Flush Method

We fabricated InAs/GaAs double quantum dot (QD) structures by molecular beam epitaxy (MBE) with the Indium-Flush method, where the energy separation between the electron levels of two QDs was less than the longitudinal optical (LO) phonon energy with a different barrier thickness. We confirm the peak energy shift between the double QDs in the photoluminescence (PL) spectra and assign this shift to the wave function coupling effect between the double dots. We also measured the time resolved PL spectra and observed the carrier transfer from smaller QDs to larger ones in the time domain. By estimating the tunneling time between double QDs, we obtain a tunneling time that is longer than the exciton decay time in single QD. Additionally, we mention the fade-out of the electron LO phonon interaction with the electron wave function coupling between double QDs based on the result of photoluminescence excitation measurements. These results suggest that our structures are attractive for quantum information processing.

Electron–longitudinal optical phonon interaction between Landau levels in semiconductor heterostructures

Calculations of electron--longitudinal optical phonon scattering rates between Landau levels in semiconductor heterostructures with a magnetic field applied parallel to the direction of confinement are reported. We have found that the scattering rate shows strong oscillations as a function of the applied field, depending on the configuration of the energy states in the structure. In the limit $\stackrel{\ensuremath{\rightarrow}}{B}0,$ the expressions for the intersubband scattering rate between Landau levels reduce to those obtained in the case of two-dimensional subbands at $B=0\mathrm{T}.$

Effect of electron-phonon interaction on surface states in zinc-blende GaN, AlN, and InN under pressure

A variational approach is used to study the surface states of electrons in a semi-infinite polar semiconductor under hydrostatic pressure. The effective Hamiltonian and the surface-state levels are derived including the effects of electron-optical phonon interaction and pressure. The numerical computation has been performed for the surface-state energies versus pressure for zinc-blende GaN, AlN, and InN. The results show that the effect of electron-optical phonon interaction lowers the surface-state energy. It is also found that the effect of electron-surface optical phonon interaction is much bigger than the effect of electron-half space longitudinal optical phonon interaction for surface-state levels. It indicates that the surface-state energies and the influence of electron-phonon interaction increase with pressure obviously.

Effects of confined longitudinal optical phonons on the exciton binding energy in a cubic quantum dot

The binding energy of an exciton in a cubic quantum dot has been calculated with and without including the electron (hole)-confined longitudinal optical phonon interactions, using a variational approach and within the effective mass approximation. The quantum confinement effect is described by an infinitely deep well in the envelope-function approximation. The charge carrier phonon coupling is treated within the adiabatic approximation. As the dot size increases, the results show that the contribution of the confined longitudinal optical phonon on the binding energy decreases and the correction to the excitonic state increases.