LO Phonon Energy Research Articles

Hot exciton relaxation in multiple layers CdSe/ZnSe self-assembled quantum dots separated by thick ZnSe barriers

We have studied PL and PLE spectra of two samples (A and B) of MBE grown CdSe/ZnSe asymmetric double quantum wells with different amount of deposited CdSe layers separated by 14 nm ZnSe barrier. It has been found that PLE spectra of the states forming short wavelength side of the PL spectra of both deep and shallow QWs of the sample A as well as that of deep QW of the sample B demonstrate oscillating structure in the spectral ranges corresponding to exciton states of self-assembled quantum dots only. Meanwhile PLE spectra of the short wavelength states of shallow QW the sample B revealed pronounced oscillating structure with energy period of ZnSe LO phonon under excitation with photons in a wide energy range both in the regions of quantum-dot states and in that of free states in the ZnSe barrier. In these spectra creating of excitons with kinetic energies more than 0.3 eV was observed which considerably exceed the exciton binding energy as well as LO phonon energy (both appr. 0.03 eV). It has been concluded that oscillating structure of the PLE spectra arises due to cascade relaxation of hot excitons. We discuss the model which explains these experimental findings.

Quantum dot quantum cascade infrared photodetector

We demonstrate an InAs quantum dot quantum cascade infrared photodetector operating at room temperature with a peak detection wavelength of 4.3 μm. The detector shows sensitive photoresponse for normal-incidence light, which is attributed to an intraband transition of the quantum dots and the following transfer of excited electrons on a cascade of quantum levels. The InAs quantum dots for the infrared absorption were formed by making use of self-assembled quantum dots in the Stranski–Krastanov growth mode and two-step strain-compensation design based on InAs/GaAs/InGaAs/InAlAs heterostructure, while the following extraction quantum stairs formed by LO-phonon energy are based on a strain-compensated InGaAs/InAlAs chirped superlattice. Johnson noise limited detectivities of 3.64 × 1011 and 4.83 × 106 Jones at zero bias were obtained at 80 K and room temperature, respectively. Due to the low dark current and distinct photoresponse up to room temperature, this device can form high temperature imaging.

Photoluminescence studies of ZnO films fabricated by using a combination of a hydrothermal method and plasma-assisted molecular beam epitaxy regrowth

ZnO films were deposited on Si (100) substrates by using a two-step growth process. In the first step, ZnO nanorods were grown by using the hydrothermal method at 140 °C for 5 min. In the second step, a ZnO amorphous layer was deposited on the ZnO nanorods by spin-coating. After completion of the growth process, the films were annealed at 800 °C for 10 min, and a ZnO active layer was deposited on top of the amorphous layer by using plasma-assisted molecular beam epitaxy. Further, temperature-dependent photoluminescence (PL) measurement were conducted to study the optical properties of the prepared films. In the low-temperature PL spectra, emission peaks in the near-band-edge region were observed at 3.370, 3.362, 3.347, 3.329, 3.317, 3.288, 3.263, 3.219, 3.191, and 3.116 eV; these peaks were attributed to free excitons, neutral donor bound excitons, neutral acceptor donor excitons, two electron satellites, and donor acceptor pairs, respectively. These peaks were red-shifted, and their intensity decreased with increasing temperature. The binding energy of the donor was calculated as 43.1 meV by using the Haynes rule. Further, the value α and β, factors in the equation for the energy of localized excitons of donors and acceptors were obtained as 0.73 meV and 750 K, respectively, by fitting the free exciton (FX) peak according to Varshni’s equation. The full width at half-maximum of PL for the films was about 95.1 meV at room temperature; moreover, the following values were obtained for the films by using theoretical equations: the background impurity broadening, Γ0 = 62 meV, the parameter describing exciton-LO phonon interaction, Γ LO = 80 meV, LO phonon energy, ħω LO = 72 meV, and, the coupling strength of an exciton-acoustic phonon interaction, γ ph = 0.087 meV/K Furthermore, the activation energy was about 60.1 meV.

Picosecond dynamics of internal exciton transitions in CdSe nanorods

The picosecond dynamics of excitons in colloidal CdSe nanorods are directly measured via their 1s to 2p-like internal transitions by ultra-broadband terahertz spectroscopy. Broadened absorption peaks from both the longitudinal and transverse states are observed at 8.5 and 11 THz, respectively. The onset of exciton-LO phonon coupling appears as a bleach in the optical conductivity spectra at the LO phonon energy for times > 1 ps after excitation. Simulations show a suppressed exciton temperature due to thermally excited hole states being rapidly captured onto ligands or unpassivated surface states. The relaxation kinetics are manipulated and the longitudinal transition is quenched by surface ligand exchange with hole capturing pyridine.

Temperature effects on exciton–phonon coupling and Auger recombination in CdTe/ZnTe quantum dots

We report the results of experimental studies on temperature-dependent thermal escape and Auger recombination coefficients in CdTe/ZnTe quantum dots. We show that at low temperatures, there is a thermally activated transition between two different states separated by a localization energy of about 15.8 meV, while the primary non-radiative process at high temperatures is thermal escape assisted multi-longitudinal optical (LO) phonons absorption with three phonons. The most striking result is a rapid increase in the Auger coefficient and a reduction in the decay time with increasing temperature above 35 K. These results show that the Auger process is assisted by the participation of phonons with an energy threshold of 34.4 meV and an LO phonon energy of around 19 meV.

Tuning hole mobility in InP nanowires

Transport properties of holes in InP nanowires (NWs) were calculated considering electron-phonon interaction via deformation potentials, the effect of temperature, and strain fields. Using molecular dynamics, we simulate NW structures, the longitudinal optical phonon (LO-phonon) energy renormalization, and lifetime. The valence band ground state changes between light- and heavy-hole character, as the strain fields and the NW size vary. Drastic changes in the mobility arise with the onset of resonance between the LO-phonons and the separation between valence subbands.

Size dependent effective band gap, optical absorption coefficients and refractive index changes in a wide ZnS/Zn1−xMgxS strained quantum well

Exciton binding energy of a confined heavy hole exciton is investigated in a Zn1−xMgxS/ZnS/Zn1−xMgxS single strained quantum well with the inclusion of size dependent dielectric function for various Mg content. The effects of interaction between the exciton and the longitudinal optical phonon are brought out. The effect of exciton is described by the effective potential between the electron and hole. The interband emission energy as a function of well width is calculated for various Mg concentration with and without the inclusion of dielectric confinement. Non-linear optical properties are carried out using the compact density matrix approach. The dependence of nonlinear optical processes on the well width is investigated for different Mg concentration. The linear, third order non-linear optical absorption coefficients values and the refractive index changes of the exciton are calculated for different concentration of magnesium content. The results show that the exciton binding energy is found to exceed LO phonon energy of ZnS for x>0.2 and the incorporation of magnesium ions and the effect of phonon have great influence on the optical properties of ZnS/Zn1−xMgxS quantum wells.

LO-phonon-assisted polariton lasing in a ZnO-based microcavity

Polariton relaxation mechanisms are analyzed experimentally and theoretically in a ZnO-based polariton laser. A minimum lasing threshold is obtained when the energy difference between the exciton reservoir and the bottom of the lower polariton branch is resonant with the LO phonon energy. Tuning off this resonance increases the threshold, and exciton-exciton scattering processes become involved in the polariton relaxation. These observations are qualitatively reproduced by simulations based on the numerical solution of the semiclassical Boltzmann equations.

Terahertz dual-wavelength quantum cascade laser based on GaN active region

In this paper a novel terahertz (THz) quantum cascade laser (QCL) based on GaN/AlGaN quantum wells has been proposed, which emits at two widely separated wavelengths 33 and 52 μm simultaneously in a single active region. The large LO-phonon energy (∼90 meV), the ultrafast resonant phonon depopulation of the lower radiative levels, suppression of the electrons that escape to the continuum states and selective carrier injection and extraction all together lead to a considerable enhancement in the operating temperature of the structure. All calculations have been done at a temperature of 265 K. Moreover, similar behavior of the output optical powers is another remarkable feature, which makes both wavelengths useful for special applications.

Exciton-phonon coupling in single quantum dots with different barriers

The coupling between longitudinal-optical (LO) phonons and neutral excitons in two different kinds of InGaAs pyramidal quantum dots (QDs) embedded in either AlGaAs or GaAs barriers is experimentally examined. We find a slightly weaker exciton-LO-phonon coupling and increased linewidth of the phonon replicas for the QDs with GaAs barriers compared to the ones with AlGaAs barriers. These results, combined with the fact that the LO-phonon energy of the exciton is the same for both kinds of dots, are taken as evidence that the excitons mainly couple to LO-phonons within the QDs.

Longitudinal polar optical phonons in InN/GaN single and double het‐ erostructures

AbstractLongitudinal optical phonon energy in InN epi‐layers has been determined independently from the Raman spectroscopy and temperature dependent Hall mobility measurements. Raman spectroscopy technique can be used to obtain directly the LO energy where LO phonon scattering dominates transport at high temperature. Moreover, the Hall mobility is determined by the scattering of electrons with LO phonons so the data for the temperature dependence of Hall mobility have been used to calculate the effective energy of longitudinal optical phonons.The samples investigated were (i) single heterojunction InN with thicknesses of 1.08, 2.07 and 4.7 μm grown onto a 40 nm GaN buffer and (ii) GaN/InN/AlN double heterojunction samples with InN thicknesses of 0.4, 0.6 and 0.8 μm. Hall Effect measurements were carried out as a function of temperature in the range between T = 1.7 and 275 K at fixed magnetic and electric fields. The Raman spectra were obtained at room temperature. In the experiments, the 532 nm line of a nitrogen laser was used as the excitation source and the light was incident onto the samples along of the growth direction (c‐axis). The results, obtained from the two independent techniques suggest the following: (1) LO phonon energies obtained from momentum relaxation experiments are generally slightly higher than those obtained from the Raman spectra. (2) LO phonon energy for the single heterojunctions does not depend on the InN thickness. (3) In double heterostructures, with smaller InN thicknesses and hence with increased strain, LO phonon energy increases by 3% (experimental accuracy is < 1%) when the InN layer thickness increases from 400 to 800 nm (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

One-dimensional ZnO exciton polaritons with negligible thermal broadening at room temperature

Single ZnO microwires are investigated by angle-resolved photoluminescence spectroscopy. We show that confined optical modes similar to whispering gallery modes can strongly interact with excitons to form one-dimensional exciton polaritons at room temperature, with normal mode splitting exceeding 200 meV. With such a splitting, which is much larger than LO phonon energy, a strong quenching of the polariton-phonon interaction is achieved, even at room temperature and for large excitonic fractions. Thus, a record figure of merit of 50 for the ratio of the Rabi splitting to the polariton full width at half maximum is achieved as a consequence of negligible thermal contribution to dephasing.

Interplay between coherent and incoherent phonons in optically excited biased quantum wells

We present numerical simulations of the generation of coherent LO phonons in an electrically biased quantum well. An ultrashort laser pulse is used to simultaneously excite two exciton levels, which leads to an oscillating dipole moment that couples to the polar lattice and drives the phonons. The generation of coherent phonons becomes resonantly enhanced if the splitting of the two exciton levels is tuned to the LO phonon energy. In this case relaxation by emission of incoherent phonons becomes also possible. Our calculation therefore takes into account incoherent phonons on a quantum kinetic level as well. The intersubband relaxation by excitation of incoherent phonons competes with the generation of coherent phonons and is dominant in terms of the energy transferred.

Effect of Exciton-Longitudinal Optical Phonon Interaction on Exciton Binding Energies in CdxZn1-xS/ZnS Single Quantum Wells

We study the effect of the exciton-longitudinal optical (LO) phonon interaction on the exciton binding energies in CdxZn1-xS/ZnS single quantum wells (SQWs) for the Cd alloy content (x) range 0.1—0.3. The heavy- and light-hole exciton binding energies increase with the exciton-LO phonon interaction. The increase in the maximum heavy-hole (light-hole) exciton binding energy for x = 0.3 is 9.1 meV (4.9 meV). In narrow CdxZn1-xS/ZnS SQWs, the heavy-hole exciton binding energy calculated by taking into account the exciton-LO phonon interaction for values of x in the range 0.1—0.3 exceed the LO phonon energy of CdxZn1-xS. [DOI: 10.1380/ejssnt.2010.340]

Enhanced terahertz emission from coherent longitudinal optical phonons in a quantum well structure under applied bias

We report on the characteristics of terahertz waves emitted from coherent longitudinal optical (LO) phonons in a GaAs/AlAs multiple quantum well structure under an applied electric field. It is found that the intensity of the terahertz wave from the coherent LO phonon is resonantly enhanced in comparison with the intensity in a low electric-field region, under the condition that an intersubband energy is tuned to the LO-phonon energy of GaAs. The pump-energy dependence of the terahertz-wave intensity indicates that the enhancement originates from the double resonances in the Raman scattering process.

Transfer Dynamics of Spin-Polarized Excitons in ZnCdMnSe/ZnCdSe Double Quantum Wells

We study the transfer dynamics of spin-polarized excitons in diluted magnetic semiconductor (DMS) double quantum wells (QWs) of ZnCdMnSe/ZnCdSe, where the spin-polarized excitons tunnel from the non-magnetic ZnCdSe to DMS-ZnCdMnSe wells and subsequently relax to the ground state in the DMS well with phonon emissions. The exciton-transfer time decreases when the exciton-energy difference between both the wells is equal to the one-LO-phonon energy, indicating the LO-phonon-assisted acceleration of the spin transfer. Moreover, the exciton transfer alters into the separate transfer of the electron and the heavy hole when the electron potential difference becomes larger than the LO-phonon energy. This observation demonstrates the importance of energy-relaxation dynamics in the spatial transport of spins in semiconductor quantum structures.

Effects of resonant LO phonon assisted excitation on the photoluminescence spectra of InGaN/GaN quantum wells

AbstractWe have observed oscillations in the low temperature (T < 50 K) photoluminescence excitation spectra of an InGaN/GaN single quantum well at photon energies above the GaN bandgap. We attribute the features in the spectra to excitation of electrons at multiples of the LO phonon energy above the GaN conduction band edge. The rapid cooling of these electrons to the GaN conduction band edge and their subsequent capture leads to a shift, in the photoluminescence spectrum and hence the oscillations in photoluminescence. We interpret the shift in the spectrum as being due to a modification of the occupation of the distribution of localization centres by electrons. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Comparison of Gain Properties with Electron–Electron and Electron—LO-Phonon Interactions in Quantum Cascade Structure

The gain properties of (AlN)m/(GaN)n superlattice-based quantum cascade structure are investigated by using a nonequilibrium Green's function (NGF) theory. In this theory, the electron-electron interaction and electron—LO-phonon interaction are both considered. The gain spectra of QCL are calculated from some current-driven items, which are derived from these two interactions. The results show that the effect of the electron-electron interaction is notable in the low-photon-energy range and the electron—LO-phonon interaction only takes effect in the high-photon-energy range, where photon energy is close to or larger than LO-phonon energy of GaN materials.

Enhancement of terahertz radiation from coherent optical phonons via impulsive interference of excitons in GaAs/AlAs multiple quantum wells

We have investigated the terahertz (THz) radiation from the coherent GaAs-like longitudinal optical (LO) phonons at room temperature in GaAs/AlAs multiple quantum wells utilizing an optical gating method with an ultrashort pulse laser. It has been found that the intensity of the THz radiation with a long dephasing time is markedly enhanced under the condition that the energy spacing between the fundamental heavy-hole and light-hole excitons is almost equal to the LO-phonon energy. This fact demonstrates that the impulsive excitonic interference, which produces no beating oscillation at room temperature owing to a very fast dephasing time, acts as a driving force for the coherent LO phonon emitting the intense THz radiation.

Excitation energy dependence of the photoluminescence spectrum of anInxGa1−xN∕GaNsingle quantum well structure

We have observed oscillations in the low temperature $(Tl50\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ photoluminescence excitation spectra of an ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}∕\mathrm{GaN}$ single quantum well at photon energies above the $\mathrm{GaN}$ band gap. We attribute the features in the spectra to excitation of electrons at multiples of the LO phonon energy above the $\mathrm{GaN}$ conduction band edge. The rapid cooling of these electrons to the $\mathrm{GaN}$ conduction band edge and their subsequent capture leads to a shift, and hence the oscillations, in the photoluminescence spectrum. We interpret the shift in the spectrum as being due to a modification of the occupation of the distribution of localization centers.