Green Luminescence Band in ZnO:  Fine Structures, Electron−Phonon Coupling, and Temperature Effect

Abstract

The green emission band of ZnO has been investigated by both experimental and theoretical means. Two sets of equally separated fine structures with the same periodicity (close to the longitudinal optical (LO) phonon energy of ZnO) are well resolved in the low-temperature broad green emission spectra. As the temperature increases, the fine structures gradually fade out and the whole green emission band becomes smooth at room temperature. An attempt to quantitatively reproduce the variable-temperature green emission spectra using the underdamped multimode Brownian oscillator model taking into account the quantum dissipation effect of the phonon bath is done. Results show that the two electronic transitions strongly coupled to lattice vibrations of ZnO lead to the observed broad emission band with fine structures. Excellent agreement between theory and experiment for the entire temperature range enables us to determine the dimensionless Huang-Rhys factor characterizing the strength of electron-LO phonon coupling and the coupling coefficient of the LO and bath modes.Historically, zinc oxide (ZnO) is a technologically important material thanks to its piezoelectric characteristics and other unique properties such as its transparency up to the near ultraviolet (UV). It is also known that ZnO is a semiconductor with a wide band gap (∼3.37 eV) and an extremely large exciton binding energy (as high as 60 meV). 1 Recently, it has attracted renewed research interest due to its newly-found application potential in exciton-type short-wavelength optoelectronic devices that are functional at room temperature or above. 1-3 Despite a long history of industrial applications, a clear understanding of some fundamental properties of ZnO still remains elusive. 1,2,4-7 For example, contention still surrounds the microstructural origin. 4,8 To date, very different defect origins, such as the substitutional Cu 2+ on the zinc site, 9 oxygen vacancy (V O ), 10 zinc vacancy (V Zn ), 11 and interstitial zinc (Zn i ), 12 have been suggested to be responsible for the green band of ZnO. Among them, the substitutional Cu 2+ model proposed first by Dingle 9 has received much attention due to the distinct spectral features of a sharp zero-phonon line (ZPL) and a broad longitudinal optical (LO) phonon sideband at low temperature. [13][14][15] Taking into account only the coupling between one LO phonon mode and one electronic transition, Kuhnert and Helbig 13 employed a Poission distribution, I n ) S n e -S /n!, to fit the line shape of the green emission band and then obtained a Huang-Rhys factor of S ) 6.5. It is well-known that the Poission distribution simply gives only a backbone of the absorption or luminescence line shape of the electron-LO phonon coupling system. Broadening due to acoustic-phonon-bath dissipation and the temperature effect cannot be accounted for in the model. Moreover, in addition to the first set of structures, the second set of structures with the same periodicity was also observed but its origin is not yet understood. 9,...

Origins of green band emission in high-temperature annealed N-doped ZnO

“…3. Being similar to the results in the literature [3,9,19], each LO replica exhibits two series of fine structures with a fixed splitting energy of $ 30 meV. This observation postulates that there are two electronic transitions coupled with the lattice vibration.…”

Structural and optical properties of Zn0.9Mn0.1O/ZnO core-shell nanowires designed by pulsed laser deposition

“…The emission band is vibronic assisted by a ϳ71 meV LO phonon, and our data are in accordance with those published by Dingle 21 who previously associated the emission to the presence of Cu 2+ in the samples. Also, the two sets of fine structures equally separated by ϳ31 meV were identified in accordance with the data recently reported by Shi et al 22 Particle induced x-ray emission ͑PIXE͒ analysis performed in the samples under study revealed that Cu is present in the nanorods ͑Table I͒.…”

A fast sonochemical approach for the synthesis of solution processable ZnO rods

“…3, neither the sonochemical nor the hydrothermal ZnO rod spectra present a well-defined green peak and this is somehow to be expected at room temperature. 23 Instead, the subband gap is spread over a relative large range ͑from 450 to 700 nm approximately͒. The peak itself of this band is shifted toward the yellow region at 587 nm for the sonochemical ZnO rods and 569 nm for the hydrothermal sample.…”