Confined Phonon Modes Research Articles

Intersubband absorption linewidth in a semi-parabolic quantum well: Comparison among three different phonon models

In this article, effects of different confined phonon models on intersubband absorption linewidth in semiparabolic quantum well (SPQW) is studied in detail through considering the optically detected magneto-confined phonon resonance (ODMCPR) effect. The projection operator and profile methods are utilized in order to calculate the linear conductivity tensor as well as magneto-phonon resonance absorption power (MPRAP) and measure the absorption linewidth or the full width at half maximum (FWHM) at the ODMCPR peak, respectively. We find that (i) the FWHM (absorption linewidth) as functions of the temperature, Landau level, and confining potential frequency; (ii) there is a different dependence in slab and guided phonon modes, as well as Huang–Zhu (HZ) phonon mode model on the FWHM; (iii) the confined phonon modes have great influence on the intersubband absorption linewidth in semiparabolic quantum well; (iv) the present calculations accord well with previous experimental studies.

Electron mobility modulated by optical phonons in Al Ga1-N/In Ga1-N/GaN/AlN heterostructures

Abstract The electron mobility influenced by optical phonons, the size of InGaN/GaN channels and In composition y in wurtzite AlxGa1-xN/InyGa1-yN/GaN/AlN heterostructures as basic structures of high electron mobility transistors (HEMTs) is investigated around room temperature. The electron states affected by the two-dimensional electron gas near the interfaces are obtained by self-consistently solving the Poisson and Schrodinger equations. Based on the dielectric continuum and Loudon's uniaxial crystal models combined with the transfer matrix methods, the dispersion relations and electrostatic potentials of interface (IF), confined (CO), half space, and propagating optical phonons are discussed in detail, as well as the corresponding transformations among different phonon modes. Furthermore, the mobility contributed by optical phonons is given by the Lei and Ting's force balance and energy balance equations. Our results show that the electron mobility in the heterostructures with composite InGaN/GaN channel is mainly dominated by IF and CO phonon modes, and is higher than that in the conventional heterostructures with GaN-channels. Meanwhile, the optimized size and In composition of the composite channel to improve the mobility are also clarified.

Influence of interface modes compared to confined modes on phonon-assisted cyclotron resonance effect in quantum wells

Abstract We study the influence of interface phonon modes on the phonon-assisted cyclotron resonance (PACR) effect in GaAs-AlAs quantum well (QW) structures by calculating the absorption power (AP) based on the operator projection technique in comparison with bulk and confined phonon modes in previous study [N. D. Hien, Superlattices Microstruct. 131 (2019) 86]. The full width at half maximum (FWHM) of the PACR absorption peak is obtained by utilizing the profile technique. On the other hand, the FWHM is described as functions of the QW width and temperature for different kinds of phonon modes (interface, bulk, confined modes) presented to compare. The calculations clearly demonstrate important roles of interface-mode as the QW width is small enough. Also, the result indicates that the interface optical modes also gives a considerable contribution to the electron–phonon scattering as compared with that of bulk and confined optical phonon modes.

Effect of quantum confinement on lifetime of anharmonic decay of optical phonons in semiconductor nanostructures

The anharmonic decay of phonons underlies many important effects in semiconductors, e.g. hotspot formation, phonon bottleneck and thermal resistivity. In this article, we evaluate the effect of quantum confinement on the anharmonic decay transition probability in a cubic isotropic material. This article focuses on the anharmonic decay of longitudinal optical phonons, generated from hot electrons, are directly related to formation of hotspots in the active region of semiconductor devices. The confinement effect has been realized in double interface heterostructure quantum well (DHSQW) (e.g. AlAs/GaAs/AlAs) and free-standing quantum well (FSQW) (e.g. GaAs) structures as the confined phonon modes have different properties inside the structures. The longitudinal–optical phonon decay rate is reduced for the case of a DHSQW compared to bulk material and for a FSQW the decay rate has a strong dependence on wavevector value of the three phonons involved.

Modification of the phonon spectrum of bulk Si through surface nanostructuring

In this paper, we present experimental evidence on the change of the phonon spectrum and vibrational properties of a bulk material through phonon hybridization mechanisms. The phonon spectrum in a finite material is strongly affected by the presence of free surfaces, which is the addition of a contribution from an essentially two-dimensional crystal. The phonon spectrum of a bulk material can hence be altered by a hybridization mechanism between confined phonon modes in nanostructures introduced on the surface of a bulk material and the underlying bulk phonon modes. We measured the heat capacities of bare and surface-structured silicon substrates originating from the same silicon wafer. Then, we deduced important features of the phonon spectra of the samples investigated through a rigorous analysis of the measured heat capacity curves. The results show that the shape and size of the nanostructures made on the surface of the bulk substrate have a strong effect on the phonon spectrum of the bulk material.

Effects of electron–optical phonon interactions on the polaron energy in a wurtzite ZnO/MgxZn1−xO quantum well**Project supported by the National Natural Science Foundation of China (Grant Nos. 11264027 and 11364030), the Project of Prairie Excellent Specialist of Inner Mongolia, China, and the “Thousand, Hundred and Ten” Talent Training Project Foundation of Inner Mongolia Normal University, China (Grant No. RCPY-2-2012-K-039).

We investigated the properties of polarons in a wurtzite ZnO/MgxZn1−xO quantum well by adopting a modified Lee–Low–Pines variational method, giving the ground state energy, transition energy, and phonon contributions from various optical-phonon modes to the ground state energy as functions of the well width and Mg composition. In our calculations, we considered the effects of confined optical phonon modes, interface-optical phonon modes, and half-space phonon modes, as well as the anisotropy of the electron effective band mass, phonon frequency, and dielectric constant. Our numerical results indicate that the electron–optical phonon interactions importantly affect the polaronic energies in the ZnO/MgxZn1−xO quantum well. The electron–optical phonon interactions decrease the polaron energies. For quantum wells with narrower wells, the interface optical phonon and half-space phonon modes contribute more to the polaronic energies than the confined phonon modes. However, for wider quantum wells, the total contribution to the polaronic energy mainly comes from the confined modes. The contributions of the various phonon modes to the transition energy change differently with increasing well width. The contribution of the half-space phonons decreases slowly as the QW width increases, whereas the contributions of the confined and interface phonons reach a maximum at d ≈ 5.0 nm and then decrease slowly. However, the total contribution of phonon modes to the transition energy is negative and increases gradually with the QW width of d. As the composition x increases, the total contribution of phonons to the ground state energies increases slowly, but the total contributions of phonons to the transition energies decrease gradually. We analyze the physical reasons for these behaviors in detail.

Unifying the low-temperature photoluminescence spectra of carbon nanotubes: the role of acoustic phonon confinement.

At low temperature the photoluminescence of single-wall carbon nanotubes show a large variety of spectral profiles ranging from ultranarrow lines in suspended nanotubes to broad and asymmetrical line shapes that puzzle the current interpretation in terms of exciton-phonon coupling. Here, we present a complete set of photoluminescence profiles in matrix embedded nanotubes including unprecedented narrow emission lines. We demonstrate that the diversity of the low-temperature luminescence profiles in nanotubes originates in tiny modifications of their low-energy acoustic phonon modes. When low-energy modes are locally suppressed, a sharp photoluminescence line as narrow as 0.7meV is restored. Furthermore, multipeak luminescence profiles with specific temperature dependence show the presence of confined phonon modes.

Observation of the full exciton and phonon fine structure in CdSe/CdS dot-in-rod heteronanocrystals.

Light emission of semiconductor nanocrystals is a complex process, depending on many factors, among which are the quantum mechanical size confinement of excitons (coupled electron-hole pairs) and the influence of confined phonon modes and the nanocrystal surface. Despite years of research, the nature of nanocrystal emission at low temperatures is still under debate. Here we unravel the different optical recombination pathways of CdSe/CdS dot-in-rod systems that show an unprecedented number of narrow emission lines upon resonant laser excitation. By using self-assembled, vertically aligned rods and application of crystallographically oriented high magnetic fields, the origin of all these peaks is established. We observe a clear signature of an acoustic-phonon assisted transition, separated from the zero-phonon emission and optical-phonon replica, proving that nanocrystal light emission results from an intricate interplay between bright (optically allowed) and dark (optically forbidden) exciton states, coupled to both acoustic and optical phonon modes.

Study of phonons in self-assembled InAs quantum dots embedded in an InGaAlAs matrix

We studied phonon modes propagating along the [311] direction in highly ordered, stacked InAs QDs embedded in an InGaAlAs matrix grown on InP(311) substrate by Raman spectroscopy. The Raman spectra showed clear doublets of folded quasi-longitudinal modes which have mixed transverse and longitudinal character. A broad feature observed in the Raman spectra was attributed to superposition of densely folded and/or confined phonon modes that is with agreement with our previous theoretical calculation. The phonon gap was observed and compared with our theoretical calculations.

Property comparison of polarons in zinc-blende and wurtzite GaN/AlN quantum wells

The properties of polarons in zinc-blende and wurtzite GaN/AlN quantum wells with Fröhlich interaction Hamiltonians are compared in detail. The energy shifts of polarons at ground state due to the interface (IF), confined (CO) and half-space phonon modes are calculated by a finite-difference computation combined with a modified LLP variational method. It is found that the two Fröhlich interaction Hamiltonians are consistent with each other when the anisotropic effect from the z-direction and the x-y plane is neglected. The influence of the anisotropy on the polaron energy shifts due to the IF phonon modes for a smaller well width or due to the CO phonon modes for a moderate well width is obvious. In addition, the built-in electric field has a remarkable effect on the polaron energy shifts contributed by the various phonon modes.

Polaronic Effect on the Electron Energy Spectrum in a Wurtzite InxGa1-xN/GaN Quantum Well

The properties of polarons in wurtzite and zinc-blende In x Ga 1- x N/GaN quantum wells are investigated by adopting an effective-mass approximation and a modified Lee–Low–Pines variational method. The energy shifts of polarons at ground state due to the confined, half-space and interface phonon modes (including symmetric and antisymmetric parts), as well as the ground state energy and transition energy, are obtained by a numerical computation. The results indicate that both ground state energy and transition energy of polarons decrease monotonically with increasing the well width. For a wider quantum well, the total contribution to the polaron energy mainly comes from the confined phonon modes, while for a narrower one, the total contribution is mainly from the interface and half-space phonon modes. The built-in electric field has a remarkable effect on the polaron energy shifts contributed by various phonon modes. It can be also found that the polaronic effect in In x Ga 1- x N/GaN quantum wells is much...

Probing Confined Phonon Modes by Transport through a Nanowire Double Quantum Dot

Strong radial confinement in semiconductor nanowires leads to modified electronic and phononic energy spectra. We analyze the current response to the interplay between quantum confinement effects of the electron and phonon systems in a gate-defined double quantum dot in a semiconductor nanowire. We show that current spectroscopy of inelastic transitions between the two quantum dots can be used as an experimental probe of the confined phonon environment. The resulting discrete peak structure in the measurements is explained by theoretical modeling of the confined phonon mode spectrum, where the piezoelectric coupling is of crucial importance.

Hole Mobility in Ultrathin Double-Gate SOI Devices: The Effect of Acoustic Phonon Confinement

We show that the effect of phonon confinement in ultrathin double-gate silicon-on-insulator (DGSOI) transistors on hole mobility is weaker than that predicted for electron mobility. To do so, confined phonon modes in SOI devices are computed, employing an elastic continuum model of acoustic phonons in a three-layer structure. A self-consistent k middot p-Poisson solver has been developed for the valence-band structure calculation, and Kubo-Greenwood formalism is used to compute the hole mobility in ultrathin DGSOI transistors under the combined effect of confined phonon and surface-roughness scattering.

Low-temperature lattice thermal conductivity in free-standing GaN thin films

Low-temperature (2 < T < 100 K) lattice thermal conductivity, κ p in a semiconducting free-standing thin film (FSTF) is studied using a modified Callaway model. The quantization of acoustic phonons in FSTFs is taken into account and explicit contributions to κ p from the shear, dilatational and flexural modes of the confined acoustic phonons are considered. The scattering of phonons is assumed to be by sample boundaries, impurities, dislocations and other phonons via both normal and umklapp processes. Numerical results are presented for the GaN system. The phonon confinement effect, the sample finite size effect and the relative importance of confined phonon modes are investigated. The role of dislocations in limiting κ p is also discussed. Results are compared with those based on bulk description of acoustic phonons; bulk phonons are found to underestimate (overestimate) κ p in the boundary scattering regime (at higher temperatures). The calculations demonstrate that low-temperature thermal conductivity studies can lead to a better understanding of phonon scattering in FSTFs.

Confined phonon scattering in multivalley Monte Carlo simulation of quantum cascade lasers

We present a detailed investigation of the effects that optical phonon confinement has on the electronic transport properties of GaAs-based multiple-quantum-well (MQW) quantum cascade laser (QCL) structures. Both confined and interface phonon modes are included based on the macroscopic dielectric continuum model. Interface phonon dispersions are obtained using the transfer matrix method with periodic boundary conditions. Scattering rates of both Γ- and X-valley electrons by all the interface and confined phonon modes are calculated and fully incorporated in the multivalley Monte Carlo simulation of a deep-active-well 6.7 μm GaAs-based MQW QCL. We find that the inclusion of phonon confinement enhances the electron-phonon scattering rates and output current to a relatively small extent with respect to the bulk phonon approximation.

Model of Raman scattering in self-assembledInAs∕GaAsquantum dots

Multiphonon resonant Raman scattering in self-assembled quantum disks is investigated using a nonadiabatic approach. The optical phonons and the electron-phonon interaction are considered within the multimode dielectric continuum model. The model exploits both electrostatic and mechanical boundary conditions for the relative ionic displacement vector, as well as the phonon spatial dispersion in bulk. The confined phonon modes in a quantum dot are hybrids of bulklike and interface vibrations. It is shown that nonadiabatic effects substantially increase the Raman scattering probabilities and the relative multiphonon integral intensities with respect to the one-phonon intensity. The calculated ratio of the two- and one-phonon integral intensities is close to the experimental value for self-organized $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ quantum dots.

Pressure effect on the electron mobility in AlAs/GaAs quantum wells

By taking the influence of optical phonon modes into account, this paper adopts the dielectric continuum phonon model and force balance equation to investigate the electronic mobility parallel to the interfaces for AlAs/GaAs semiconductor quantum wells (QWs) under hydrostatic pressure. The scattering from confined phonon modes, interface phonon modes and half-space phonon modes are analysed and the dominant scattering mechanisms in wide and narrow QWs are presented. The temperature dependence of the electronic mobility is also studied in the temperature range of optical phonon scattering being available. It is shown that the electronic mobility reduces obviously as pressure increases from 0 to 4GPa, the confined longitudinal optical (LO) phonon modes play an important role in wide QWs, whereas the interface optical phonon modes are dominant in narrow QWs, the half-space LO phonon modes hardly influence the electronic mobility expect for very narrow QWs.

Electron Raman scattering in asymmetrical multiple quantum wells system: Fröhlich interaction

The differential cross-section for an electron Raman scattering (ERS) process, taking into account or not the presence of one phonon in a semiconductor asymmetrical multiple quantum well, is calculated for T=0 K. We present a description of the confined phonon modes, considering the Fröhlich interaction in a GaAs/Al x Ga 1− x As system. We assume single parabolic conduction band with one electron. The emission spectra are discussed for different scattering configurations and the selection rules for the processes are also studied. Singularities in the spectra are found and interpreted. The ERS studied here can be used to provide direct information about the electron and phonon band structure of these systems, and give us information about the interband transitions.

Dynamics of coherent phonons in CsPbCl3 nanocrystals studied by femtosecond laser spectroscopy

AbstractWe report on the generation of coherent optical phonons in CsPbCl3 nanocrystals in CsCl by femtosecond laser‐pulse illumination. The oscillations of four confined phonon modes were identified and their properties (amplitude, frequency, and phase) in dependence on the excitation laser pulse wavelength were studied. The results are interpreted in terms of spectral dependence of the phonon generation mechanism. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Optical properties of IV–VI quantum dots embedded in glass: Size-effects

Measurements of the photoluminescence (PL), micro-PL, spatially-resolved PL, optical absorption and atomic force microscopy (AFM) of PbS and PbSe quantum dots (QDs) embedded in oxide glass matrix, were carried out. It was found that the energy gaps of the QDs showed pronounced anomalous temperature dependences. Their temperature coefficients depended strongly upon the size and shape of the QDs, and surface and/or confined phonon modes. In addition, the energy-dependent transfer rate of excitons from smaller to larger dots via electronic coupling was observed. It was predicted that further improvements in size selectivity, luminescence quantum yield, and well-controlled growth would enable highly efficient inter-dot energy transfer.