Preparation,Interface And Photoelectric Properties Of NiO/SiC Heterojunction

Silicon carbide(SiC),as the third generation semiconductor material,has the advantages of wider band gap,higher thermal conductivity,higher critical avalanche breakdown electric field and higher electron saturation rate,which makes it more suitable for being applied in high voltage power devices,high frequency devices,ultraviolet detectors(UVPD),short wave light emitting diodes(LEDs)and bipolar junction transistor(BJT)phototransistors.With the development of the research on SiC based device,because the inherent impurity incomplete ionization characteristic of p+-SiC restricts the hole-injection ratio in SiC p+n junction,p-NiO/n-SiC heterojunction is proposed to replace SiC homojunction to improve its performance.In order to expand the application potential of SiC based NiO/SiC heterojunction in optoelectronic devices,the preparation and properties of NiO films are studied.Then,based on the investigation of the interface properties,current transport mechanism,photoelectric properties of NiO/SiC heterojunction,the enhancement mechanism,dynamic characteristics and thermal stability of metal/p-NiO/n-SiC heterostructure with the high response are studied.In this paper,the main research contents and achievements are as follows:1.The preparation and properties of undoped and 5mol%Li-aopea NiO film by Sol-gel method are studied.The effect of 5mol%Li doping on the crystallinity,grain size,surface morphology,composition and photoelectric properties of the film is investigated.The results show that Li doping does not cause the lattice distortion of NiO structure,and the average grain size is about 20 nm.Secondly,Li doping changes the preferred orientation of the film grains,from the undoped<200>preferred orientation to the growth trend of<111>orientation.Thirdly,the optical band gap decreases from 3.75eV to 3.71eV due to Li doping.Finally,Li doping shortens the energy difference between Fermi level and valence band maximum,from 0.47 eV to 0.26 eV,and the carrier concentration increases from 1.32×1011cm-3 to 4.37×1014cm-3。2.Li-doped mechanism of NiO film is studied.XPS analysis shows that when Li impurities occupy Ni vacancies,the lattice Ni3+is reduced to Ni2+,resulting in the decrease of Ni3+concentration and the hole concentration.When Li impurities replace Ni2+ in the lattice,Ni2+ in the lattice is oxidized to Ni3+,resulting in the increase of Ni3+concentration and hole concentration.With the increase of Li doping from 0%,2mol%,5mol%,8mol%to 10mol%,the carrier concentration of NiO film first decreases,then increases,and finally tends to be basically unchanged.The undoped NiO carrier concentration of undoped NiO is 1.32×1011cm-3.The carrier concentration of 2mol%Li-doped NiO film is 1.3×10 cm-3.The carrier concentration of 5mol%Li-doped NiO film is 4.37×1014cm-3.The carrier concentration of 8mol%and 10mol%Li-doped NiO film is 1.39×1015 cm-3。3.The energy band type,band offset and interface state of p-NiO/n-SiC heterojunction is explored.A type-Ⅱ band alignment with the valence band offset of ΔEV为 1.937±0.023eV and the conduction band offset of ΔEc为2.437±0.007eV is obtained.The upward bending of the energy band at the interface of NiO/SiC heterojunction is caused by the existence of a negatively charged Ni vacancy at the interface,and its interface state density is 5×1012 eV-1·cm-2。4.The NiO/SiC heterostructures are prepared,and its optical and electrical properties are studied.The experimental results show that the p+-NiO/p-NiO/n-SiC heterojunction exhibits typical rectifying characteristics.However,due to the high interface state,the leakage current of the heterojunction is large and there is almost no photoelectric response.5.An improved Ag/p-NiO/n-SiC structure with the high response is proposed,and its enhancement mechanism,photoelectric characteristics and thermal stability is studied.The Ag/p-NiO/n-SiC structure with rectifying characteristics has a large photoresponse under UV irradiation with power of 0.5mW.When the reverse bias voltage is-5 V,and under 254 nm UV irradiation,the responsivity and quantum efficiency of the heterojunction are 597 mA/w and 291.7%,respectively.When the reverse bias voltage is-7V,and under 365 nm UV irradiation,the responsivity of the heterojunction reaches 295 mA/W,and the quantum efficiency reaches 100.3%.At the same time,the response speed is fast,and the rising edge time constant(τr)is 12.7ms and falling edge time constant(τf)is 35.6ms.In addition,when the forward current is 2mA,Ag/p-NiO/n-SiC structure has good thermal stability and negative temperature coefficient(-0.6mV/K).6.The forward carrier transport mechanism and reverse leakage current mechanism of p+-NiO/p-NiO/n-SiC heterostructure and Ag/p-NiO/n-SiC structure are investigated firstlly.By studying the temperature dependent J-V characteristics of p+-NiO/p-NiO/n-SiC heterojunction,it is found that the interface traps affect the forward carrier transport and reverse leakage current.When the heterostructure is in the forward bias,the trap assisted tunneling mechanism dominates the carrier transport in the low voltage region,and the trap assisted space charge limited current(SCLC)mechanism dominates the carrier transport in the high voltage region.When the heterojunction is in reverse bias,the SCLC mechanism controls the leakage current in the low temperature region below 200K,and the PF emission mechanism controls the leakage current in the temperature range of 250-300K.The room temperature J-V characteristics of Ag/p-NiO/n-SiC structures show that the forward carrier transport mechanism in the low voltage region is ohmic mechanism,and the carrier transport in the high voltage region is defect assisted SCLC.When the structure is in reverse bias,the leakage current is mainly ohmic mechanism.