Electroluminescence And Photodetection Properties Of ZnO-based Heterojunction Devices

With wide direct bandgap of 3.37 eV,large exciton binding energy of 60 meV,rich in resouces,non-toxic and pollution-free,and can be easily wet-chemically etched,ZnO as a third generation semiconductor has attracted considerable attention in recent years.Especially in the field of light-emitting diodes?LEDs?and ultraviolet?UV?photodetectors?PDs?,ZnO is considered to be a new generation of main materials that have great potential to replace GaN.However,stable and reliable p-type ZnO has not yet been achieved,the performance of ZnO based LEDs and PDs is still worse than GaN based devices.Because p-type ZnO often requires more expensive equipment such as metal organic chemical vapor deposition or molecular beam epitaxy in experimental preparation,the research threshold and cost are high,so many researchers are devoted to the research of ZnO heterojunction optoelectronic devices.ZnO heterojunction optoelectronic devices are structurally diverse and peculiar to performance,and many physical mechanisms need to be further explored.The further study of ZnO heterojunction devices can not only open up new ideas for the application of ZnO,but also deepen the understanding of material properties and pave the way for the development and application of ZnO homojunction optoelectronic devices.In this thesis,we designed and fabricated a variety of optoelectronic devices based on ZnO heteroj unctions,and made some interesting and meaningful research results:First of all,n-GaN/i-HfO₂/n-ZnO isotype heterojunctions were fabricated on n-GaN substrates by a plused laser deposition system.It was found that such isotype heterostructure LEDs not only show bicolor electroluminescence?EL?at DC modes but also work at AC sinusoidal modes.Near white-light emission was obtained by tuning the symmetric AC driving signals to the asymmetric signals.The HfO₂ layer is served as an electron blocking layer?EBL?in the heterojunction.From the cross-sectional HRTEM images of the device,it can be found that the polycrystalline HfO₂ film exists with a monoclinic phase.Furthermore,the n-ZnO film still maintains the same growth orientation as the n-GaN film and is of high crystalline quality even on a polycrystalline high-k HfO₂ film.The EL spectra are very different when the polarity of the DC bais is changed.The strong visible emission at forward bias is mainly originated from the GaN defects,while the ultraviolet emssion at reverse bias can be attributed to the ZnO near band edge emission.When a 50 Hz symmetric sinusoidal voltage is applied to the device,AC EL can be obtained and the spectrum shape is similar to the large forward bias EL spectrum,which is mainly due to the large difference light-emitting threshold voltage of forward bias and reverse bias.More importantly,near white-light can be obtained by tuning the symmetric driving AC sinusoidal signals to the asymmetric ones.This simple and facile method that only by applying AC asymmetric signals to achieve white light emission on one single chip may provide an easy route for the white-light solid-state lighting industry and pave the way for the devolpment of one-chip multicolor LEDs.Secondly,LEDs based on ZnO were fabricated on a p-Si substrate by using a pulsed laser deposition system.Significant EL improvement was demonstrated with the insertion of an HfO₂ EBL in p-Si/n-ZnO heterojunctions.With a 40 nm HfO₂,the device has the best performance amd distinct near-band-edge emission at around 392 nm accompanying by a broadly strong visible emission is achieved.Current-voltage and capacitance-voltage measurements confirm that a proper thickness of the HfO₂ EBL can effectively balance the injection of electrons and holes,resulting in an increase of radiative recombination in the ZnO active layer and thus enhancing the EL performance of the devices.Five independent emissions corresponding to five different transition processes are proposed to clarify the EL origination of the heterojunction LEDs by Gaussian deconvolutions.It is hoped that results in this work should be helpful for the development of ZnO-based LEDs that can integrate ZnO with the Si planar technology.Thirdly,p-NiO/n-ZnO heterojunction LEDs were fabricated based on ZnO nanowire and p-type NiO film.As the ZnO nanowire arrays have a certain ups and downs and unevenness,we sputtered a layer of HfO₂ flim before the depositon of NiO.The HfO₂ layer can partly fill the gaps between the ZnO nanowires and effectively prevent the up and botterm electrodes directly contact.Interestingly,the p-NiO/HfO₂/n-ZnO nanowire heterojunctions only have EL at reverse bias and the main EL peak is located at around 385 nm,corresponding to the near-band-edge emission of ZnO.The device still has EL phenomenon after removing thep-NiO layer and the reverse bias EL mechanism is detailedly discussed combining the energy band diagram.At last,all inorganic,self-powered,narrowband and rapid response UV PDs were fabricated and investigated based on p-NiO/n-ZnO nanowire?NW?heterojunction.The p-NiO/n-ZnO NW heterojunction has larger PN junction areas than common planar device due to the large surface to volume ratio of ZnO NWs,which is better for fabricating PDs than LEDs.However,the surface oxygen obsorption will bring about surface trap states on ZnO NWs,which is harmful for the collection of photogenerated carriers.Al2O3 films grown by atomic layer deposition can greatly suppress the surface defects on ZnO NWs and improve the p-NiO/n-ZnO NW interface.With the insertion of Al2O3 layer,the photo-response of the photodetector in the 430-500 nm wavelength range is greatly inhibited and the full-width at half-maximum of the response spectrum is less than 30 nm.A large responsivity of 1.4 mA/W is achieved under a 380 nm UV irradiation?0.36 mW/cm2?at zero bias and the response time of the device is less than 0.04 s.Such a simple interface modification method might promote the developing of ZnO NW based narrowband photodetectors.