Photoluminescence and magneto-optical measurements are performed on a line peaking at 3.354 eV (labeled as NBX) in electron-irradiated ZnO. Even though the energy position of the NBX line is close to that for bound excitons in ZnO, it has distinctively different magneto-optical properties. Photoelectron paramagnetic resonance measurements reveal a connection and a charge-transfer process involving NBX and Fe and Al centers. The experimental results are explained within a model which assumes that the NBX is a neutral donor bound exciton at a defect center located near a Fe impurity and an Auger-type charge-transfer process occurs between NBX and Fe3+. While the NBX dissociates, its hole is captured by an excited state of Fe3+ and the released energy is transferred to the NBX electron, which is excited to the conduction band and subsequently trapped by a substitutional Al-zn shallow donor.

Magneto-optical and time-resolved photoluminescence (PL) spectroscopies are employed to evaluate electronic structure of a bound exciton (BX) responsible for the 3.364 eV line (labeled as I*(1)) in bulk ZnO. From time-resolved PL spectroscopy, I*(1) is concluded to originate from the exciton ground state. Based on performed magneto-PL studies, the g-factors of the involved electron and hole are determined as being g(e) = 1.98 and g(h)(//)(gh(perpendicular to)) = 1.2 (1.62), respectively. These values are nearly identical to the reported g-factors for the I* line in ZnO (Phys. Rev. B 86, 235205 (2012)), which proves that I*(1) should have a similar origin as I* and should arise from an exciton bound to an isoelectronic center with a hole-attractive potential.

Radiative carrier recombination processes in GaAs/GaNAs core/shell nanowires grown by molecular beam epitaxy on a Si substrate are systematically investigated by employing micro-photoluminescence (mu-PL) and mu-PL excitation (mu-PLE) measurements complemented by time-resolved PL spectroscopy. At low temperatures, alloy disorder is found to cause localization of photo-excited carriers leading to predominance of optical transitions from localized excitons (LE). Some of the local fluctuations in N composition are suggested to lead to strongly localized three-dimensional confining potential equivalent to that for quantum dots, based on the observation of sharp and discrete PL lines within the LE contour. The localization effects are found to have minor influence on PL spectra at room temperature due to thermal activation of the localized excitons to extended states. Under these conditions, photo-excited carrier lifetime is found to be governed by non-radiative recombination via surface states which is somewhat suppressed upon N incorporation. (C) 2014 AIP Publishing LLC.

Optically detected magnetic resonance (ODMR) complemented by photoluminescence measurements is used to evaluate optical and defect properties of ZnO nanowires (NWs) grown by rapid thermal chemical vapor deposition. By monitoring visible emissions, several grown-in defects are revealed and attributed to Zn vacancies, shallow (but not effective mass) donor and exchange-coupled pairs of Zn vacancies and Zn interstitials. It is also found that the intensity of the donor-related ODMR signals is substantially lower in the NWs compared with that in bulk ZnO. This may indicate that formation of native donors is suppressed in NWs, which is beneficial for achieving p-type conductivity.

We study the propagation of exciton-polaritons through bulk ZnO using time-resolved photoluminescence (PL) complemented by time-of-flight measurements of laser pulses. When the photon energy approaches donor bound exciton resonances, substantial time delays in PL light propagation are observed which reach up to 210?ps for a 0.55?mm thick crystal. By comparing results from time-of-flight measurements performed using PL light and laser pulses, the observed delay is shown to be due to the formation of exciton-polaritons and their spectral dispersion. It is also shown that the main contribution to the slow-down effect arises from free exciton-polaritons, whereas bound exciton-polaritons become important only in close vicinity to the corresponding resonances.

ZnO has in recent years been a subject of extensive research efforts driven by the prospect for a variety of potential applications. Recently a dramatic decrease in the group velocity of light propagating through bulk ZnO was demonstrated by time-of-flight studies of laser pulse propagation [Phys. Rev. B 83, 245212 (2011)]. By directly comparing the propagation of both photoluminescence (PL) and laser pulses, Chen et al. show that this effect is caused by the formation of free-exciton polaritons and is determined by their dispersion, whereas contributions of the bound-exciton polaritons become important only in close proximity of the corresponding resonances (see the article on pp. 1307–1311). The cover figure presents transient PL images measured from bulk ZnO within the spectral range of bound exciton recombination (I6 and I7 lines) in conventional back-scattering geometry (a) and in a forward-transmission configuration (b), i.e. during time-of-flight. The solid line in (b) represents time delays after propagation of light through the ZnO media, calculated by taking into account the formation of the exciton polaritons.

We show that coating ZnO nanowires (NWs) with a transition metal, such as Ni, can increase the efficiency of light emission at room temperature. Based on detailed structural and optical studies, this enhancement is attributed to energy transfer between near-band-edge emission in ZnO and surface plasmons in the Ni film which leads to an increased rate of the spontaneous emission. It is also shown that the Ni coating leads to an enhanced non-radiative recombination via surface states, which becomes increasingly important at low measurement temperatures and in annealed ZnO/Ni NWs.

Time-resolved and magneto-photoluminescence (PL) studies are performed for the so-called I6B and I7B excitonic transitions, previously attributed to neutral donor bound excitons involving a hole from the B valence band (VB), D0XB. It is shown that PL decays of these emissions at 2 K are faster than that of their I6 and I7 counterparts involving an A VB hole, which is interpreted as being due to energy relaxation of the hole assisted by acoustic phonons. From the magneto-PL measurements, values of effective Landé g factors for conduction electrons and B VB holes are determined as ge = 1.91, gh//=1.79, and gh⊥=0, respectively.

Abstract not Available.

Temperature dependent photoluminescence and cathodoluminescence (CL) spectroscopies are employed to investigate free exciton (FX) emissions in ZnO tetrapods. The intensity of the no-phonon line is found to be largely suppressed as compared with longitudinal optical phonon assisted transitions, in sharp contrast to bulk ZnO. From spatially resolved CL studies, this suppression is shown to strongly depend on structural morphology of the ZnO tetrapods and becomes most significant within areas with faceted surfaces. A model based on reabsorption due to multiple internal reflections in the vicinity of the FX resonance is suggested to account for the observed effect.