Study On The Multiphonon Structure In The Photoluminescence Spectra Of ZnO

ZnO is a wide direct band-gap II-VI semiconductor with appealing optoelectronic properties.The availability of large-scale single crystal substrates with relatively low cost makes ZnO a promising candidate for blue and ultraviolet optoelectronic devices.However,high-quality and reproducible p-type doping has not been achieved for decades,which hinders the applications of ZnO in the field of semiconductor devices.Besides,many fundamental issues of ZnO are still unsettled,such as origins of green luminescence(GL),yellow/orange luminescence(YL/OL)and red luminescence(RL),and mechanism of multiple longitudinal optical(LO)phonon emission in the luminescence processes.The current knowledge about these issues is far from complete,and even some controversial conclusions were drawn.The research on these physical problems is beneficial to the understanding of the fundamentals in ZnO as well as the development of ZnO-based optoelectronic devices.To control the surface status of ZnO,ZnO single-and poly-crystal samples were fabricated with different surface and defect conditions,by methods such as O2/H2 annealing and surface passivation.Photoluminescence(PL)and photoluminescence excitation(PLE)spectroscopy were employed to systematically study the properties of the near-band-edge(NBE)and visible emissions.Some luminescent processes in ZnO with the assistance of multiple LO phonons were investigated in detail.The main contents and conclusions are summarized as follows:(1)The excitonic PL of ZnO were usually identified only from the PL spectra of single-crystals,such as the emissions from donor-bound excitons(DX),free excitons(FX),and the two-electron satellites(TES)of neutral donor-bound excitons(D0X)at low temperature.However,it was found that some emission processes were closely connected to the crystallinity of ZnO.In this study,the low-temperature PL of single-and poly-crystal ZnO samples were investigated in detail and some excitonic emissions were demonstrated related to the propagation properties of light in the samples.The emission lines at?3.324 eV(P2)and?3.310 eV(P1)were usually attributed to TES and first-order LO phonon assisted FX recombination,respectively.However,P2 were visible only in the PL spectra of single-crystals,while P1 were more prominent in poly-crystals.Based on the analysis of literature and the difference between single-and poly-crystals,it was confirmed that the TES emission accounts for only a small portion in P2.Similar to P1,the majority of P2 can be attributed to LO phonon assisted PL of exciton polaritons.By linearly fitting the peak positions of P1,P2 and the phonon replicas,the zero phonon lines(ZPLs)of P1 and P2 were determined to be at?3.383 eV and?3.393 eV,indicating they originated from the LO phonon assisted emissions from the mixed mode of A and B excitons(PA,B)and upper polariton branch of B exciton(UPBB),respectively.Furthermore,a PL mechanism was proposed based on the scattering of exciton polariton.The interband polariton scattering by grain boundaries were proposed responsible for the enhanced P1 and suppressed P2 in poly-crystals.(2)As the excitation photon energy(hvex)is approaching the interband transition energy of ZnO,multiple resonant Raman scattering(RRS)peaks can be observed in addition to the excitonic luminescence,with intervals of LO phonon energy.The understanding of multiphonon RRS falls in two categories:multiple scattering of light with LO phonons and cascade emission of LO phonons by hot excitons.Using the excitation-emission matrix(EEM)method,multiphonon RRS was observed in a wide range of the NBE region from various ZnO samples,and its relationship with temperature and surface non-radiative recombination were investigated.It was indicated in the EEMs that the luminescent multiphonon structure below the FX and D0X peaks did not shift with hvex,while the multiphonon RRS was observed in the whole excitonic PL region shifting with hvex.The intensity of multiphonon RRS was found exponentially increasing with decreased temperature,but not affected by surface non-radiative recombination.Based on the calculated phonon spectrum of ZnO,the multiphonon RRS was suggested as cascade scattering of exciton polariton by LO phonons,and the narrow energetic distribution of LO phonon was proposed to be important for the appearance of the multiphonon structure.Moreover,an additional multiphonon process with varied intervals was observed in the PLE spectra of the D0X peak,which was attributed to the resonant excitation of D0X by second-order Raman process,which reflects the capture of exciton polaritons by the defects via the multiphonon scattering.(3)The origins of visible PL bands in ZnO have been unsettled for a long time.In particular,the GL bands with and without LO phonon structure was found switchable by annealing atmosphere,but the mechanism remains unclear.In this study,the type and density of defects and excitons in single-and poly-crystal samples were controlled by different polishing,annealing,and surface passivation processes,and the behavior of visible bands in ZnO were investigated in detail.The GL-S and structureless GL were found enhanced after annealing in O2 and H2,respectively,and both GL bands exhibited unambiguous excitonic resonant excitation.The x-ray photoelectron spectroscopy revealed the deficiency of oxygen in ZnO samples after annealing in both O2 and H2.As such,the temperature evolution of structureless GL was successfully fitted with a band-tail excitonic PL model,and the GL-S band was resolved considering the degenerate excitonic PL model.In addtion,the OL band was found enhanced by excitation below the D0X energy and by surface passivation.The OL band was therefore suggested related to defects deeply beneath the surface and the lower polariton branch(LPB).