LO-like Phonon Research Articles

Impact of material-dependent radiation – longitudinal optical phonon interaction on thermal electric-dipole radiation from surface metal − semiconductor grating structures

Infrared thermal radiation emission in the 8.5 – 28 THz frequency region is obtained using surface metal–semiconductor grating structures on undoped (u-) GaAs, u-GaP, u-ZnO, u-GaN, and n-type SiC in a temperature range of 430 – 630 K. These emissions resonate with longitudinal optical (LO) phonon or LO-like lattice vibration energies determined by the zero points of the real parts of the dielectric functions in the surface structures. The emissions of materials with Reststrahlen bandwidths of a few tens of cm−1 show the emissions resonating with their LO phonon modes, while materials with bandwidth of more than 170 cm−1 show peak energies significantly lower than the LO phonon: LO-like phonon resonance. The emission intensity is found to be dominated by the balance of radiative and nonradiative LO or LO-like phonon annihilation rates. The radiative rate is dominated by the LO-phonon–radiation interaction Hamiltonian and the Bose-Einstein factor. High emission intensity is obtained for the structure on u-ZnO with intense LO-like phonon–radiation interaction. The dependence of the emission intensity on temperature and emission window width for various materials shows the effect of material-dependent metal/semiconductor interface conditions on the emission efficiency.

Mid-infrared thermal radiation resonating with longitudinal-optical like phonon from n++-doped GaN–semi-insulating GaN grating structure

The mid-infrared emission mechanism of line-and-space structures of metallic plates on dielectric materials is substantiated using high conductive n-doped (n++-) GaN–semi-insulating (SI-) GaN microstripe structures on an SI-GaN epitaxial layer, which was veiled when using line-and-space structures of Au plates. The present structure exhibits a few thermal emission lines originating from electric dipoles resonating with the coherent longitudinal optical (LO) phonon-like lattice vibration, which are formed by the local depolarization electric field in the surface n++-GaN/SI-GaN/n++-GaN regions. The energies of the LO-phonon-like modes shift from the original LO-phonon energy of GaN to the lower energy region, which contrasts with the LO-phonon resonant emission from the microstructures on GaAs. These emission lines have another notable feature, i.e. the observed peak energies are independent of the polar emission angle for both s- and p-polarizations, unlike the emissions by surface phonon polaritons showing a significant directive nature of peak energies. The results show that each peak energy of the present emission lines is positioned at the zero-point of the real part of the electric permittivity comprising the components of the transverse optical phonon and other electric dipoles induced by the LO-like modes, excluding the target mode. The significant peak-energy shift of the LO-like phonons is applicable to materials with wide Reststrahlen bands, which contrasts with that of the nearly LO-phonon resonating feature of materials with narrow Reststrahlen bands, such as GaAs. The peak energy shift depending on the emission direction is observed for Au–GaN stripe structures. This property is ascribed to the imperfect Au/GaN interface with surface states through the theoretical analysis of the modified electric permittivity in the surface region, numerical simulation of the local electric field via finite-difference time-domain calculation, and experimental studies on a Ti–GaN structure and emission peaks originating from an LO-like phonon of the α-Al2O3 substrate.

Overcoming the Miscibility Gap of GaN/InN in MBE Growth of Cubic InxGa1-xN.

The lack of internal polarization fields in cubic group-III nitrides makes them promising arsenic-free contenders for next-generation high-performance electronic and optoelectronic applications. In particular, cubic InxGa1-xN semiconductor alloys promise band gap tuning across and beyond the visible spectrum, from the near-ultraviolet to the near-infrared. However, realization across the complete composition range has been deemed impossible due to a miscibility gap corresponding to the amber spectral range. In this study, we use plasma-assisted molecular beam epitaxy (PAMBE) to fabricate cubic InxGa1-xN films on c-GaN/AlN/3C-SiC/Si template substrates that overcome this challenge by careful adjustment of the growth conditions, conclusively closing the miscibility gap. X-ray diffraction reveals the composition, phase purity, and strain properties of the InxGa1-xN films. Scanning transmission electron microscopy reveals a CuPt-type ordering on the atomistic scale in highly alloyed films with x(In) ≈ 0.5. Layers with much lower and much higher indium content exhibit statistical distributions of the cations Ga and In. Notably, this CuPt-type ordering results in a spectrally narrower emission compared to that of statistically disordered zincblende materials. The emission energies of the films range from 3.24 to 0.69 eV and feature a quadratic bowing parameter of b = 2.4 eV. In contrast, the LO-like phonon modes that are observed by Raman spectroscopy exhibit a one-mode behavior and shift linearly from c-GaN to c-InN.

Raman shifts and photoluminescence of the InSb nanocrystals ion beam-synthesized in buried SiO2 layers

The InSb nanocrystals embedded in buried SiO2 layers were obtained by the In+ and Sb+ ion implantation into the SiO2 layers thermally grown on Si wafers followed by the hydrogen transfer of a Si film and high-temperature annealing at 800–1100 °C. Transmission electron microscopy, Raman spectroscopy and photoluminescence were used to study the sample properties. The spherical-shaped InSb nanocrystals distributed close to the implanted atom profiles were obtained in buried SiO2. The InSb TO- and LO-like phonon modes at 187 cm−1 and 195 cm−1 were observed in the Raman spectra. The effect of both phonon quantum confinement and stresses on the phonon frequency shift was calculated. The TO-LO splitting obtained from the ion-beam synthesized InSb nanocrystals was 3 cm−1 less than that from the unstressed bulk monocrystalline InSb. The obtained effect is discussed in the frames of decreasing the transverse effective charge, as well as that of the surface phonon influence. The photoluminescence peak at 1524 nm (0.81 eV) was seen in the low-temperature PL spectra from the samples annealed at 900–1000 °C. Its energy position corresponds to the localized charge carrier energy in the InSb nanocrystals.

Effects of electric field and shape on the bound polaron in a wurtzite ellipsoidal finite-potential quantum dot

The ground state of bound polaron in a weakly prolate ellipsoidal quantum dot in the presence of an external electric field has been calculated within the effective-mass approximation by using a variational method in the framework of perturbation theory. A Fröhlich-like electron-phonon interaction Hamiltonian which accounts for the longitudinal optical (LO) and transverse optical (TO) polarizations mixing due to the anisotropy is used in the calculation. The binding energy of the bound polaron is calculated by taking the electron couples with both branches of LO-like and TO-like phonons. The interaction between impurity and phonons has also been considered. The results show that the influence of phonons is dependent strongly upon the dot shape and the anisotropy effects of wurtzite crystals, and the influence of electric field is obvious only for the large quantum dot size. The TO-like phonon contribution to the binding energy is positive, but the contribution of LO-like phonon is negative and more important than TO-like phonon.

Bound polaron in a strained wurtzite GaN/AlxGa1−xN cylindrical quantum dot

Within the effective-mass approximation, a variational method is adopted to investigate the polaron effect in a strained GaN/AlxGa1−xN cylindrical quantum dot. The electron couples with both branches of longitudinal optical-like (LO-like) and transverse optical-like (TO-like) phonons and the built-in electric field are taken into account. The numerical results show that the binding energy of the bound polaron is reduced obviously by the polaron effect on the impurity states. Furthermore, the contribution of LO-like phonons to the binding energy is dominant, and the anisotropic angle and Al content influence on the binding energy are small.

Bound polaron in a wurtzite GaN/Al xGa 1 − xN ellipsoidal finite-potential quantum dot

A variational method is used to study the ground state of a bound polaron in a weakly oblate wurtzite GaN/Al x Ga 1 − x N ellipsoidal quantum dot. The binding energy of the bound polaron is calculated by taking the electron couples with both branches of LO-like and TO-like phonons due to the anisotropic effect into account. The interaction between impurity and phonons has also been considered to obtain the binding energy of a bound polaron. The results show that the binding energy of bound polaron reaches a peak value as the quantum dot radius increases and then diminishes for the finite potential well. We found that the binding energy of bound polaron is reduced by the phonons effect on the impurity states, the contribution of LO-like phonon to the binding energy is dominant, the anisotropic angle and ellipticity influence on the binding energy are small.

Bound polaron in a wurtzite nitride semiconductor ellipsoidal quantum dot

The binding energy of a hydrogenic donor impurity in weakly oblate ellipsoidal quantum dot is investigated with a variational method by taking the electron couples with both branches of the mixing properties of the LO and TO polarizations (LO-like and TO-like phonons) due to the anisotropic effect into account. The interaction between impurity and phonons has also been considered to obtain the binding energy of bound polaron. The numerical computation has been performed for wurtzite nitride semiconductor GaN. The results show that the binding energy of bound polaron is reduced by the phonons effect on the impurity states. It indicates that the contribution of LO-like phonon to the binding energy is dominant, and the anisotropic angle and ellipticity influence on the binding energy are small.

SELF-TRAPPING ENERGY AND EFFECTIVE MASS OF POLARON IN WURTZITE NITRIDE SEMICONDUCTORS

A variational approach is used to study the intermediate-coupling polaron in bulk III-V nitride semiconductors with wurtzite crystal structure within the macroscopic dielectric continuum model and the uniaxial model. The polaronic self-trapping energy and effective mass are theoretically derived for the LO and TO polarizations mixing due to the anisotropy effect. The numerical computation has been performed for the polaronic self-trapping energy and effective mass for wurtzite nitrides GaN , AlN and InN . The results show that the self-trapping energies of the wurtzite nitrides are bigger than the zinc-blende structures above calculated materials. It is also found that the structure anisotropy increases the electron–phonon interaction in wurtizte nitride semiconductors. It indicates that the LO-like phonon influence on the polaronic effective mass and self-trapping energy are dominant and anisotropy effect is obvious.

Polaron properties in ternary group-III nitride mixed crystals

Polaron properties are studied in bulk wurtzite nitride ternary mixed crystals AxB1-xN (A, B = Al, Ga, In) with the use of a dielectric continuum Frohlich-like electron-phonon interaction Hamiltonian. The polaronic self-trapping energy and effective mass are analytically derived by taking the mixing properties of the LO and TO polarizations due to the anisotropy effect into account in the mono-phonon approximation. The numerical computation has been performed for the wurtzite ternary mixed crystal materials InxGa1-xN, AlxGa1-xN, and AlxIn1-xN as functions of the composition x. The results show that the polaronic self-trapping energies in the wurtzite structures are bigger than that in zinc-blende structures for the materials calculated. It is also found that the structure anisotropy increases the electron-phonon interaction in wurtizte nitride semiconductors. The results indicate that the LO-like phonon influence on the polaronic self-trapping energy and effective mass is dominant, and the anisotropy effect is obvious.

Intermediate-coupling polaron properties in wurtzite nitride semiconductors

Intermediate-coupling polarons in bulk III–V nitride semiconductors with wurtzite crystal structure are studied within the macroscopic dielectric continuum model and the uniaxial model. The polaronic self-trapping energy and effective mass are analytically derived by taking the mixing properties of the LO and TO polarizations due to the anisotropic effect into account. The numerical computation has been performed for wurtzite nitrides GaN, AlN, and InN. The results show that the polaronic self-trapping energies in the wurtzite nitrides are bigger than that in zinc-blende structures for the materials calculated. It is also found that the structure anisotropy increases the electron–phonon interaction in wurtzite nitride semiconductors. It indicates that the LO-like phonon influences on the polaronic effective mass and self-trapping energy are dominant, and the anisotropic effect is obvious.

Transmission of longitudinal optical phonons through a barrier in uniaxial crystals

Transmission of longitudinal optical (LO) phonons in double heterostructures with uniaxial anisotropy is investigated within a dielectric continuum model. When LO-like phonons are generated in the left semi-infinite layer, their transmission probability through a barrier to the right semi-infinite layer is calculated. ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}/\mathrm{GaN}/{\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ and $\mathrm{GaN}/{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}/\mathrm{GaN}$ systems are considered for numerical calculations. In contrast to the isotropic case, where there is no transmission, it is shown that the propagation probability could be significant with only about 1% anisotropy. The propagation probability of phonons is found to increase with increasing wavelength of the LO-like phonon for a given barrier width.

The polycarbosilane-to-Si xC 1− x conversion studied by inelastic neutron scattering and infrared absorption

Polycarbosilane (PC) is a precursor which is converted to Si x C 1− x fibers by pyrolysis in an inert gas atmosphere. The changes in the atomic vibrational spectrum during the conversion process from PC to Si x C 1− x have been examined by means of inelastic neutron scattering (INS) and infrared absorption (IR). The existence of transverse-optical(TO)- and longitudinal-optical(LO)-like phonon modes due to amorphous SiC clusters was established in the original and pyrolyzed PC by INS measurements. After pyrolyzing at 700–800°C, these modes appear distinctly around 730 and 930 cm −1, respectively, in the INS spectra. Pyrolysis at 1000°C makes these TO- and LO-like phonon modes sharper.