Investigations On N-related Doping Of ZnO Thin Films And Ohmic Contacts To P-ZnO

ZnO is a promising semiconductor with a wide band gap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature.It is considered to be a good material choice for short-wavelength optoelectronic devices,such as violet-blue and blue light-emitting diodes and ultra-violet lasers.In addition,ZnO had many other advantages,including the availability of large-area single crystal,the eco-friendly nature,the high radiation resistance,the possibility to be wet chemical etched,and the availability of alloys system(ZnMgO and ZnCdO) for bandgap engineering,etc. Furthermore,ZnO can be fabricated to be numerous nano-structures,such as nanodots, nanowires,nanotubes,nanobelts,and nanorings.With the expected unique properties in low-dimensional system,ZnO is a potential candidate for nano-sacle optoelectronic devices and biomedical applications too.In order to realize these device applications,the fabrication of both high-quality p-type and n-type ZnO must be solved first.However,due to the asymmetric doping limitations,p-type ZnO is hard to realize and has been thought to be a bottleneck in the development of ZnO-based devices.Therefore,in this work,we concentrated on growth and characterization of p-type ZnO thin films,and also attempted to understand the p-type doping mechanism.Moreover,we investigated ohmic contacts to p-ZnO and designed a ZnO homo-junction LED device,because designing a proper device structure and fabricating ohmic electrodes with low contact resistivity are also important issues to be settled.The work included:1.Li-N-H codoped p-type ZnO thin films were prepared by dc reactive magnetron sputtering.The effects of substrate temperature and various dopants on the properties if Li-N-H codoped ZnO were discussed.An optimal substrate temperature of 550℃was found to get a best p-type conduction with the resistivity of 25.2Ωcm, the Hall mobility of 0.5 cm²/(Vs),and the hole concentration of 4.92×10¹⁷ cm³.The Li dopant was found to accelerate the grain growth and help to stabilize the p-type conduction in ZnO thin films.The N dopant mainly contributed to decreasing the resistivity of the thin films.And the H dopant would passivate the acceptors during the film growth and helped to enhance the doping concentration of acceptors.And H dopant was driven out by post annealing,resulting in acceptors activated and good p-type conduction.2.N-doped p-type ZnO thin films were grown by plasma-assisted metalorganic chemical vapor deposition(MOCVD) at a low substrate temperature of 250℃.By comparative study on using NO and N₂O as the N dopant source respectively,the author confirmed that NO was a better N dopant source in a plasma-assisted condition. The rf power showed a repugnant effect on the structural and electrical properties of the ZnO:N thin films.A moderate radio frequency(RF) power of about 150 W was demonstrated to get a balance between the crystal quality and the p-type conduction of the ZnO:N films.3.The specific condition for p-type transition in ZnO:N films grown by plasma-free MOCVD method was demonstrated.A proper N/O ratio in the gas mixture played a key role in realizing p-type ZnO under the quasi-equilibrium doping condition.And the effects of substrate temperature on the structural and electrical properties were discussed too.4.Ni/Au and Ni/Pt Ohmic contacts to p-ZnO were fabricated using electron beam evaporation and circular transmission line model(CTLM) method.A lowest specific contact resistivity of 3.81×10^-6Ωcm² was achieved for Ni/Pt ohmic contact to p-ZnO:N after annealing at 450℃in nitrogen for 1 min.The activation of acceptors, diffusions and interface reactions induced by rapid thermal annealing were ascribed to the formation mechanism of both Ohmic contacts.A Pt-Ni alloy layer formed under the metal surface and protected the acceptors from out diffusion,which resulted in a better contact property for the Ni/Pt electrode.A Pt-Ni-Zn / p⁺-ZnO/p-ZnO model was proposed to describe the carrier distributions in the interface between Ni/Pt contact and p-ZnO:Li thin film.5.A ZnO homo-junction LED was prepared adopting standard device fabrication process.The typical rectifyingâ… -â…¤characteristic was observed.The ZnO dry etching parameters were discussed at the same time.A better etching effect was obtained using Cl₂/Ar gas mixture as the dry etching ambience.