Wurtzite Zinc-Oxide (w-ZnO) is a wide bandgap semiconductor that holds promise in power electronics applications, where heat dissipation is of critical importance. However, large discrepancies exist in the literature on the thermal conductivity of w-ZnO. In this paper, we determine the thermal conductivity of w-ZnO using first-principles lattice dynamics and compare it to that of wurtzite Gallium-Nitride (w-GaN)--another important wide bandgap semiconductor with the same crystal structure and similar atomic masses as w-ZnO. However, the thermal conductivity values show large differences (400 W/mK of w-GaN vs. 50 W/mK of w-ZnO at room temperature). It is found that the much lower thermal conductivity of ZnO originates from the smaller phonon group velocities, larger three-phonon scattering phase space and larger anharmonicity. Compared to w-GaN, w-ZnO has a smaller frequency gap in phonon dispersion, which is responsible for the stronger anharmonic phonon scattering, and the weaker interatomic bonds in w-ZnO leads to smaller phonon group velocities. The thermal conductivity of w-ZnO also shows strong size effect with nano-sized grains or structures. The results from this work help identify the cause of large discrepancies in w-ZnO thermal conductivity and will provide in-depth understanding of phonon dynamics for the design of w-ZnO-based electronics.

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Thermal Conductivity of Wurtzite Zinc-Oxide from First-Principles Lattice Dynamics--a Comparative Study with Gallium Nitride

The LO-phonon—plasmon coupling in n-ZnO single crystals is investigated by Raman spectroscopy at room and liquid nitrogen temperatures. The energy shift and halfwidth broadening of the LO-phonons relative to the uncoupled modes are analyzed to get the concentration and mobility of the charge carriers. These results are compared with those from electrical measurements. The uncoupled phonon modes are measured in high-ohmic crystals in which the interaction with the charge carriers can be neglected. For the low energy E2 mode an extreme small halfwidth of 0.3 cm−1 is found. At 77 K with increasing intensity of the exciting continuous laser light (hω = = 2.54 eV < Eg = 3.3 eV, P = 800 mW) nonequilibrium carriers are generated not connected with an increase of the lattice temperature.



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Raman study of the phonon Halfwidths and the phonon-plasmon coupling in ZnO

The consistent parameters of hexagonal zinc oxide single crystals are obtained in the frame of the one-oscillator model by analyzing the dispersion spectra of IR light reflection. The anisotropy coefficients of the electron effective masses and the plasmon damping Coefficients are measured. The calculated values of electron mobility and conductivity for zinc oxide are in good agreement with the reference data available.



[Russian Text Ignored]

Anisotropy of the ZnO single crystal reflectivity in the region of residual rays

Static and dynamic responses of excitons in state-of-the-art bulk and epitaxial ZnO are reviewed to support the possible realization of polariton lasers, which are coherent and monochromatic light sources due to Bose condensation of exciton–polaritons in semiconductor microcavities (MCs) To grasp the current problems and to pave the way for obtaining ZnO epilayers of improved quality, the following four principal subjects are treated: (i) polarized optical reflectance (OR), photoreflectance (PR) and photoluminescence (PL) spectra of the bulk and epitaxial ZnO were recorded at 8 K Energies of PR resonances corresponded to those of upper and lower exciton–polariton branches, where A-, B- and C-excitons couple simultaneously to an electromagnetic wave PL peaks due to the corresponding polariton branches were observed Longitudinal–transverse splittings (ωLT) of the corresponding excitons were 15, 111 and 131 meV, respectively The latter two values are more than two orders of magnitude greater than that of GaAs being 008 meV (ii) Using these values and material parameters, corresponding vacuum-field Rabi splitting of exciton–polaritons coupled to a model MC mode was calculated to be 191 meV, which is the highest value ever reported for semiconductor MCs and satisfies the requirements to observe the strong exciton–light coupling regime necessary for polariton lasing above room temperature (iii) Polarized OR and PR spectra of an out-plane nonpolar ZnO epilayer grown by laser-assisted molecular beam epitaxy (L-MBE) were measured, since ZnO quantum wells (QWs) grown in nonpolar orientations are expected to show higher emission efficiencies due to the elimination of spontaneous and piezoelectric polarization fields normal to the QW plane They exhibited in-plane anisotropic exciton resonances according to the polarization selection rules for anisotropically-strained wurzite material (iv) Impacts of point defects on the nonradiative processes in L-MBE ZnO were studied using time-resolved PL making a connection with the results of positron annihilation measurement Free excitonic PL intensity at room temperature naturally increased with the increase in nonradiative lifetime (τnr) The value of τnr increased and density or size of Zn vacancies (VZn) decreased with increasing growth temperature (Tg) in heteroepitaxial films grown on a ScAlMgO4 substrate, and the use of homoepitaxial substrates further reduced VZn density The value of τnr was shown to increase with the decrease in gross density of positively and negatively charged and neutral point defects including complexes rather than with the decrease in VZn density The results indicate that the nonradiative recombination process is governed not by single point defects, but by certain defects introduced with the incorporation of VZn, such as VZn-defect complexes As a result of defect elimination by growing the films at high Tg followed by subsequent post-growth in situ annealing, combined with the use of high-temperature-annealed ZnO self-buffer layer, a record long τnr for spontaneous emission of 38 ns was obtained at room temperature By using progressively improving epitaxial growth methods, the polariton laser effect is expected to be observed at room temperature in the near future

https://iopscience.iop.org/article/10.1088/0268-1242/20/4/009/pdf

Exciton-polariton spectra and limiting factors for the room-temperature photoluminescence efficiency in ZnO

Photoreflectance spectra of a high-quality ZnO epilayer on the ScAlMgO4 (0001) substrate grown by laser molecular-beam epitaxy exhibited clear excitonic resonances due to three excitons associated with uppermost valence bands (A, B, and C excitons). The oscillator strengths of the B and C excitons are larger than that of the A exciton. Their broadening was interpreted to be due to the contribution by exciton–polaritons in terms of large longitudinal–transverse splitting of respective excitons. Dependence of the exciton energy on temperature was well fitted assuming the Bose–Einstein statistics giving the Einstein characteristic temperature ΘE of 380 K (33 meV).

K. Thoma

Papers

Lattice dynamics of ZnO