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

Abstract

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 (ω LT ) of the corresponding excitons were 1.5, 11.1 and 13.1 meV, respectively. The latter two values are more than two orders of magnitude greater than that of GaAs being 0.08 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 (1 1 2 0) 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

Observation of strong exciton–photon coupling at temperatures up to 410 K

“…We attribute the observed collapse of the strong coupling to the strong exciton broadening and the very small exciton oscillator strength at these temperatures. In order to maintain the strong coupling up to the predicted maximum temperature of 610 K [27], which we consider to be a realistic aim, the exciton oscillator strength has to be increased by improving the ZnO cavity material. Furthermore, ZnO quantum wells (QW) can be used as active medium.…”

Valence-band-ordering of a strain-free bulk ZnO single crystal identified by four-wave-mixing spectroscopy technique

“…The excitons involved in the respective transitions are termed A-, B-, and C-excitons and abbreviated as X A , X B , and X C . Numerous reports have emerged over the past 50 years postulating either C 9 -symmetry [9][10][11][12][13][14][15][16][17][18] or C 7 -symmetry 1, [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] for the uppermost E 1 VB, as shown schematically on the right and left ends in Fig. 1(a), respectively.…”

Study of exciton–polariton modes in nanocrystalline thin films of ZnO using reflectance spectroscopy

“…Thin films of significantly better quality, e.g. grown on substrates with better lattice matching, are expected to demonstrate such effects [23,24]. However, the damping length is significantly larger than the grain size (~ 80 nm) and thus the film may be expected to show random lasing behaviour due to multiple scattering at grain boundaries, under high level excitation.…”