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

Abstract

Depolarization effect on optical control of exciton states confined in GaAs thin films J. Appl. Phys. 110, 043514 (2011) Theoretical and experimental studies of surface plasmons excited at metal-uniaxial dielectric interface Appl. Phys. Lett. 98, 021113 (2011) Lateral spatial switching of excitons using vertical electric fields in semiconductor quantum rings Appl. Phys. Lett. 97, 173101 (2010) Enhanced exciton photoluminescence in the selectively Si-doped GaAs/AlxGa1−xAs heterostructures J. Appl. Phys. 108, 063522 (2010) Effect of correlation of local fluctuations on exciton coherence J. Chem. Phys. 132, 204503 (2010) Additional information on J. Appl. Phys. Spectroscopic and temporal four-wave-mixing (FWM) measurements are carried out on a strain-free bulk ZnO single crystal, in order to clarify the valence-band-ordering. Under the collinearly polarized lights with the electric-field component parallel to the c-axis, which can excite dipole-allowed C 1 -excitons, the FWM signal appears only in the energies corresponding to the B-exciton. Under the cross-linear polarization configuration exciting both C 5 -and C 1 -excitons, the FWM signal arising from the two-photon-coherence is absent in the energies corresponding to A-exciton. Both the results indicate that C 1 -exciton state belongs exclusively to B-exciton, meaning that the valence-band ordering is C 9 À C 7 À C 7 in order of decreasing electron energy for the present strain-free ZnO single crystal. V C 2012 American Institute of Physics.[http://dx

Verification of $\boldsymbol{\Gamma}_{7}$ symmetry assignment for the top valence band of ZnO by magneto-optical studies of the free A exciton state

“…Nevertheless, some authors [14][15][16][17][18][19][20][21][22] have rejected this interpretation in favor of the conventional 9 − 7 − 7 ordering (i.e. so > 0).…”

“…As such, we believe it is worth getting this work [28] formally published, which is mainly motivated by the very recent claim of the conventional 9 − 7 − 7 ordering by [22]. It is a matter of fact that even after the convincing experiments of [12], there are still other voices.…”

Radiative coupling of A and B excitons in ZnO

“…After that, however, Reynolds et al 11 investigated optically unallowed excitons by the mageto-optical responses, and identified the Γ 9 symmetry for the topmost valence band as well as the other wurtzite compounds. Since their work, studies identifying both Γ 7 [12][13][14] and Γ 9 [15][16][17] symmetry have been reported. From the group theory, the 1s-exciton groundstate symmetry in wurtzite-ZnO is represented as 18…”

“…17 ) without the view point of the radiation-induced component-mixing. On the other hand, the present results indicate that the coupled modes contributing the signals include both A and B exciton components comparably, which would change the conventional interpretation of the observed spectra.…”

“…The results will provide an alternative way of attributions of the optical signals in ZnO, which is in contrast to the conventional one where the A and B excitons are independently assigned to their respective peak structures. This would be also important on the controversial problems such as the symmetry ordering of ZnO valence bands because the discussions have been mainly made based on the conventional way [9][10][11][12][13][14][15][16][17] . The coupling between different excitonic states via radiation should be noted, particularly in a nano-to-bulk crossover size regime.…”