Effects Of Exciton Diffusion And Surface Recombination On The Photoluminescence Of GaN And ZnO

As wide direct band-gap semiconductor materials,ZnO and GaN have been widely used in the research and application of optoelectronic devices such as blue light emitting diode(LED),ultraviolet(UV)laser diodes(LD),semiconductor lighting etc.GaN has become a rather important semiconductor for industrially fabricating light emitting diodes and high-power electronic devices.However,some basic science problems remain in pressing for solution,such as the origin of ultraviolet luminescence(UVL),blue luminescence(BL)and yellow luminescence(YL)in GaN.As a wide-gap semiconductor having optoelectronic applications overlapping with GaN,ZnO has some advantages that the binding energy is relatively large(?60 meV)and the fairly high-quality bulk single crystals are available via relatively simple growth technology,thus resulting in a potentially lower cost for ZnO-based devices.But stable p-doping still obstructs the road towards ZnO-based optoelectronics,which might be related to a lack of comprehensive understanding of the defects,such as the origin of green luminescence(GL),YL and red luminescence(RL)in ZnO.Traditional viewpoint believed that semiconductor fluorescence is related to excitons and defects in the inside of samples.But people paid little attention to the effects of surface state on the luminescence of semiconductor.For example,surface recombination have not been taken into account in previous studies on the dynamics of excitonic luminescence,which might lead to the lack of clear physical meaning for the fitting parameters.In this thesis,surface passivations with SiOxNy and SiO2 were engineered to eliminate the influence of surface recombination.Based on this,we systematically study the effects of exciton diffusion and surface recombination on GaN and ZnO luminescence by the photoluminescence(PL).photoluminescence excitation(PLE)and time-resolved PL(TRPL)measurements.The research contents and main results in this thesis are summarized as follows:(1)Effects of exciton diffusion and surface recombination on the exciton emission of GaN and ZnO:GaN films with or without H3PO4 etching and ZnO single-crystal samples grown by a hydrothermal method were passivated by coating a layer of SiOx films using magnetron sputtering method or a layer of SiOxNy films using a specially designed plasma enhanced chemical vapor deposition system.The effects of surface passivation on the exciton emission were researched.Moreover,we established the excitonic luminescence theoretic model based on exciton diffusion and surface recombination and systematically analyzed the dynamic evolution of ZnO excitonic luminescence.Research results indicate that surface passivation produced a pronounced PL increase from UV to visible spectral range by a factor of ten more in GaN samples with and without H3PO4 etching.Besides,surface passivation also resulted in the intensity inversion of donor bound exciton(D0X)and free exciton(FXA)at low temperature.It proved that surface recombination is one of the major radiative recombination channels of FX in unpassivated GaN samples.Similarly,it is found that surface passivation leads to a significantly enhancement of band-edge(2-3 orders of magnitude)and visible room-temperature luminescence(several or dozens of times)in ZnO bulk single crystals.Moreover,surface passivation also decreases the relative intensity of FX in ZnO at low temperature.By solving the continuity equation of excitons under steady excitation,analytical formulas for the integral PL intensity of excitons with limits of zero and extremely large surface recombination rate were given and used for fitting the experimental data taken from the samples with and without surface passivation,respectively.Room temperature diffusion coefficient and diffusion length of ZnO were estimated by comparing the PL spectra of ZnO with and without surface passivation.The effects of surface polishing,crystal orientation and annealing treatment on exciton diffusion were discussed.(2)Effects of exciton diffusion and surface recombination on the visible emission of GaN:High resistance GaN films grown by metal organic chemical vapor deposition with and without ultrahigh vacuum annealing were passivated by coating a layer of SiOx or SiOxNy films.The temperature dependences of the visible PL emission were researched in the undoped GaN samples.We also analyzed the excitation characteristic and excitation-dependent PL spectra of UVL,BL and YL bands by the temperature-dependent PLE spectra.Research results indicate that the influence of surface states is less in visible PL spectra characteristic,while the influence is very clear in the fluorescence dynamic evolution.The YL emission is dominated in GaN visible spectra at room temperature.With the decrease in temperature,the YL band in passivated GaN was reduced slowly,much different from the temperature-dependent behavior of YL band in GaN samples without passivation.The BL band in passivated GaN appeared two saturation regions at 120-160 K and below 70 K,separately corresponding to the BL1 and BL2 bands.UVL band emission was observed blow 140 K.including UVL1 and UVL2.PLE spectra and excitation-dependent PL spectra showed that resonance excitation of BL2 and YL bands were observed at the longitudinal optical and transverse optical mode of A or B excitons,respectively.It indicated that the BL2 and YL bands were related to exciton-polaritons.Similarly,the UVL1 and UVL2 were considered to be related to D0X and ionized donor bound exciton,respectively.Time-dependent PL intensities of the UV,BL,and YL bands of GaN induced by intensive ultraviolet-light irradiation at low temperature was investigated in GaN samples.A general formula was deduced from rate equations for describing time-dependent PL intensities of the UV,BL,and YL bands,and then reasonable transition rates were obtained for different PL bands of the GaN sample under an intensive ultraviolet-light irradiation.(3)Effects of exciton diffusion and surface recombination on the visible emission of ZnO:ZnO single-crystal samples with different crystal orientations were passivated by coating a layer of SiOx or SiOxNy films.The temperature dependences of the visible PL emission were researched in the ZnO single-crystal samples.We also analyzed the excitation characteristic and possible origins of the YL and GL bands by the excitation-emission PL association analysis and TRPL spectra.Research results indicate that surface states obviously influenced the visible PL spectra characteristic.The shapes and energy positions of visible band for passivated samples were almost identical.The GL emission with central wavelength of?510 nm was dominated in GaN visible spectra at room temperature.And the intensity and energy position of the GL band are nearly independent of the temperature.As temperature decreases,the intensity for YL band increases gradually without a tendency to saturation and the central wavelength of YL band had a redshift.As a result,the YL band with central wavelength of?620 nm was dominated in GaN visible spectra at 12 K.Through excitation-emission association analysis for visible PL at low temperature,we found that the GL band was related to the energy above D0X and resonance excitated with D0X.The YL band was resonance excitated with exciton-polaritons and the strongest excitation under the energy lower than D0X were obtained in PLE spectra.On this basis,the D0X-related luminescence processes for GL and exciton-polariton-related luminescence processes for YL were further put forward.Furthermore,we report a new structured RL2 observed under the excitation light in resonance with excitons.Differing from the fine structure of RLl assigned to Fe3+ center,the new structured RL2 is visible only under the excitation light capable of generating excitons and is enhanced at the incident frequency in resonance with the DX as well as the DX excitation states(n?2).Based on the binding energy determined by the equation of donor-acceptor-pair(DAP)fluorescence,the excitons or electrons and holes trapped on the DAPs of O and Zn vacancies(VO-VZn)were suggested to be responsible for the new structured RL.