Defect Control And Photovoltaic Effects Of Silicon Based Gallium Nitride With Multi-interface Nanoheterojunctions

As a typical representative of the third generation semiconductor materials,direct wide bandgap gallium nitride?GaN?has the advantages of high carrier mobility,high breakdown electric field,high light absorption coefficient,strong chemical resistance and good thermal stability.It has been widely used in solar cells,light-emitting diodes,laser diodes,photodetectors,power microwave devices,etc.At present,GaN films and devices are mainly grown and prepared on sapphire substrates,which are mainly used as the supporting substrates.As we all known,using silicon material as the functional substrate has great technical advantages in the drive circuit design and device integration based on the mature silicon planar processing technology.However,there are serious lattice mismatch?16.9%?and thermal mismatch?54%?between GaN and crystalline silicon,resulting in defect effects and interface effects which will seriously affect the efficiency,stability and service life of silicon-based GaN devices.In order to explore the preparation methods of high-quality silicon-based GaN materials and devices,this paper used the silicon nanopillars array?Si-NPA?with a micrometer-nanometer triple hierarchical structure as the functional substrate,and through the three-dimensional stress release strategy to reduce the lattice mismatch and thermal mismatch between silicon and GaN.The defects control and photovoltaic effects of GaN/Si-NPA multi-interface nanoheterojunctions were studied by chemical vapor deposition?CVD?.The main results are as follows:?1?Controllable preparation and characterization of Si-NPA.Si-NPA with controlled morphology was prepared by hydrothermal method.It had typical micrometer-nanometer hierarchical structure.The height of the silicon pillars were?2.5?m,which were perpendicular to the sample surface.Each silicon pillar and the transition layers below the silicon pillars were composed of nanoporous structure,and the pore wall of silicon pillars were composed of sc-Siparticles with an average grain size of?3.4 nm wrapped by SiO_x.The PL spectrum shows a triple-band structure.The blue luminescence band at?460 nm resulted from the quantum confinement effect of different sizes of nc-Si,which originated from the band transition radiation recombination process.The two red luminescence bands at?610 nm and?670 nm originated from the radiation recombination process of different luminescence centers.?2?The CVD preparation and defect control of GaN/Si-NPA.In the absence of catalyst,GaN films were deposited on Si-NPA substrate,metal gallium as Gasource and ammonia as N source in tube furnace by chemical vapor deposition.The phase characterization?XRD?,morphology?SEM?,photoelectric properties?Abs?PL?J-V?of GaN/Si-NPA films were measured.A layer of ITO??200 nm?was sputtered on GaN layer by magnetron sputtering method and a layer of Ag??100 nm?was sputtered on sc-Silayer by vacuum evaporation method.They played the roles of electrodes to prepare solar cell devices.After annealing at high temperature,the GaN/Si-NPA films were more continuous and compact.The PL tests show that the quantity of defects in GaN film were significantly reduced.And the performance of solar cell devices after annealing were improved.?3?The controllable preparation,defect control and characterization of GaN/Si-NPA with the assist of metal catalyst platinum?Pt?.There were many defects in GaN films without the assist of catalyst from PL tests,and the photoelectric conversion efficiency of solar cell devices were low.In order to improve the controllability of GaN/Si-NPA films,a layer of?5 nm catalyst Pt was sputtered on Si-NPA by vacuum ion sputtering to assist the growth of GaN.With the help of catalyst Pt,the growth of GaN follows the Vapor-Liquid-Solid?VLS?growth mechanism.Compared with GaN/Si-NPA prepared without catalyst,the quantity of defects decreased.After annealing in NH₃ atmosphere,the absorption edges of GaN were blue-shifts,which were closer to the real bandgap of GaN?3.4 e V?.The PL tests show that the defect luminescence peaks were greatly weakened after annealed,which show that the defects in GaN were reduced and the film quality were improved.The types of defects reduced were gallium vacancy(V_Ga)and nitrogen interstitial?N_i?.The photovoltaic performance tests show that the maximum photoelectric conversion efficiency of annealed GaN/Si-NPA film was 0.26%.?4?The controllable preparation,condition optimization and performance characterization of GaN/ZnO/Si-NPA with ZnO as seed layers.Due to GaN and ZnO have the same crystal structure,both of them are hexagonal wurtzite structure,the difference of bandgap and lattice mismatch are 0.03 e V and 2%,respectively.So ZnO was chosen as seed layer to induce the growth of GaN.In this paper,ZnO was sputtered on Si-NPA with different time,and GaN was grown on ZnO layer.The physical properties,morphology and PL of ZnO layer and GaN grown on ZnO layer were tested to determine the optimal sputtering time of ZnO.The photovoltaic test shows that the open circuit voltage,the short current density,the filling factor and the photoelectric conversion efficiency of GaN film with the optimal sputtering time of ZnO were 1.32 V,0.18 m A/cm²,81.1%and 0.87%,respectively.