Study Of Electrical And Optical Properties On GaN-Based HEMT And LED

Since the 1950s,the first generation of semiconductor(elemental semiconductor)materials represented by silicon(Si)have emerged and been widely used.It replaced bulky tubes,leading to the development of integrated circuits,the microelectronics industry and the IT industry,so that opening the "silicon age".Since the 90s of the last century,with the development of the semiconductor industry,the second generation semiconductor(compound semiconductor)materials represented by gallium arsenide(GaAs)and indium phosphide(InP)have started to emerge.Compared with the first generation semiconductor materials,the second generation semiconductor materials have higher electron mobility and large forbidden band width,making it widely used in the fields of high frequency and wireless communication.Subsequently,the third-generation semiconductor(wide bandgap semiconductor)materials represented by gallium nitride(GaN)and silicon carbide(SiC)have attracted widespread attention and have been widely used in microelectronic devices and optoelectronic devices,due to the advantages of high thermal conductivity,high breakdown field strength,high saturated electron drift rate and high bonding energy.In particular,the rapid development of GaN-based semiconductor materials has become a hot topic in the research and development of semiconductors:(i)GaN-based semiconductor materials can be used as high-electron-mobility transistor(HEMT)due to its excellent properties such as high thermal conductivity,high breakdown field strength,and high saturation electron drift rate,having a wide applications in the fields of national economy and national defense construction,such as wireless communication base stations,satellites,radars,automotive electronics,aerospace,nuclear industry and military electronics;(2)Group ? nitride material has direct bandgap across the entire visible spectrum,covering a wavelength range from the infrared(InN,?0.7 eV)to the ultraviolet(AlN,?6.2 eV).Therefore,it can be used as light-emitting diode(LED),widely used in solid state lighting,backlight,display and many other fields.With the development of material growth technology,especially the development of metal-organic chemical vapor deposition(MOCVD)epitaxy technology,it is possible to prepare high-quality ?-nitride semiconductors.Despite this,there are still many problems with the performance and stability of GaN-based semiconductor materials and related devices.For example,GaN-based HEMT devices have defects such as excessive defects,excessive buffer leakage current(BLC),and GaN-based LED devices have the problems of low efficiency at high current and difficult growth of yellow-green LED.Therefore,for further optimize the material growth conditions and improve device performance and stability,it is very important to further study and discuss the origin and distribution of impurities and defects in the above structures or devices,and clarify the generation(injection),transport,and compound mechanisms of carriers.The spectroscopy measurement is one of the high sensitivity and non-destructive tools to represent the composition of the alloy,the type of impurity and structure defect,and carrier recombination and transport mechanism of semiconductor material.In this dissertation,to study the optoelectronic characteristic of HEMT and LED devices,the test method is mainly based on spectral characterization,combined with other measurements,such as Atomic Force Microscope(AFM),Transmission Electron Microscopy(TEM),Hall measurement and ?-? measurement.The main work is summarized as follows:(1)Photoluminescence(PL)characteristics of GaN epilayersThe GaN epitaxial layer was fabricated on a sapphire substrate by MOCVD,and the PL optical test was used to analyze the optical properties of the GaN epitaxial layer.We explored the origin of the luminescence peaks,including the luminescence peaks associated with the near band eage(NBE).For example,the ground state peak of the free exciton(FXA),the first excited state peak(FXB),and the radiation peak related to the defect,such as ultraviolet luminescence(UVL),blue luminescence(BL),Yellow luminescence(YL).(2)Influence of AIN barrier thickness on AlN/GaN heterostructure optical and transport propertiesThe AIN barrier thickness's influence on the surface morphologies,material qualities,and optical and transport properties,of an AlN/GaN heterostructure was investigated by AFM,HRTEM,PL and Hall measurements.The measurement results showed that compared with the sample with a thin AIN barrier(3 nm),the sample with a thicker barrier(6 nm)had a higher density of Pit or cracks with greater penetration depths,and showed defect-related UVL emission enhancement,weakened NBE emission accompanied by its peak redshift,and significantly decreased electron mobility.The results can be explained by the fact that,with increased AIN barrier thickness,the strain-induced cracking effect resulted in strain relaxation and increased AlN/GaN heterostructure defects.This caused the deterioration in heterostructure quality,and introduced additional scattering.(3)Intrinsic relationship between photoluminescence and electrical characteristics in modulation Fe-doped AlGaN/GaN HEMTs.First,AlGaN/GaN structures with different Fe-doped concentrations(0,1×1018 and 2×1020 cm-3)were epitaxially grown on a sapphire substrate by MOCVD.Second,the photo-electric properties of Fe-doped AlGaN/GaN HEMTs were studied by characterization methods such as PL,Hall and ?-? measurements.The measurement results showed that,compared with the as-grown sample,the slightly-doped sample showed a downward shift of the Fermi level in the GaN buffer layer due to the introduction of an FeGa3+/2+ acceptor level.This resulted in a decrease in the yellow luminescence(YL)emission intensity accompanied by the appearance of an IR emission,and a decrease in the BLC.In contrast,the heavilydoped sample showed an upward shift of the Fermi level compared with the slightly-doped sample due to the increase of O donors incorporated into the GaN buffer layer under the large flow rate of the Fe source.This resulted in an increased YL emission intensity accompanied by a decrease in the IR emission intensity,and an increase in the BLC,compared with the slightly-doped sample.The intrinsic relationship between the PL and BLC characteristics is expected to provide a simple and effective method to understand the variation of the electrical characteristic in the HEMT structure by optical measurements.(4)Influence of low temperature p-GaN layer on the optical properties of the GaN-based blue LEDsTwo different InGaN/GaN multiple quantum well(MQWs)LED samples,without,and with,a low-temperature(LT)p-GaN layer,have been grown,and the optical properties of the samples investigated by EL and PL methods over the temperature range of 6 to 300 K and a injection current range of 0.01 to 200 mA.The measurement results show that there is no difference in PL characteristics between the two samples,but the EL characteristics are quite different.Compared to GaN-based blue LED samples that did not grow LT p-GaN layers,the EL peaks of the GaN-based blue LED samples grown with the LT p-GaN layer were significantly red-shifted accompanied by an increase in intensity and line-width.In addition,the sample grown with the LT p-GaN layer also increases the external quantum efficiency(EQE),especially improving the efficiency droop by 6%at large injection currents.Through the analysis of the above experimental results,the following physical model was established:(i)The growth of the LT p-GaN layer can effectively prevent the high temperature effect on the MQWs when growing the p-AlGaN electron blocking layer(mainly acting on the last well layer),protect the In fluctuation in the InGaN well layer,and finally improve the The local effect in InGaN layer;(ii)The LT p-GaN layer can reduce the stress in the MQWs and improve the QCSE,thereby increasing the electron-hole wavefunction overlap;(iii)The LT p-GaN layer can block the upward propagation of structural defects from the underlying layer,improving the lattice quality of the subsequently grown p-AlGaN electron blocking layer and the p-GaN layer,thereby increasing the holes in the concentration p-type layer.These improvements in performance will help improve the EQE of GaN-based blue LEDs.(5)The photoelectric properties of the GaN-based green LEDsInjection current,and temperature,dependences of the electroluminescence(EL)spectrum from green InGaN/GaN MQWs based light-emitting diodes(LED)grown on a Si substrate,are investigated over a wide range of injection currents(0.5?A-350 mA)and temperatures(6-350 K).The results show that an increasing temperature can result in the change of injection current-dependent behavior of the EL spectrum in initial current range.That is,with increasing the injection current in the low current range,the emission process of the MQWs is dominated by filling effect of low-energetic localized states at the low temperature range of around 6 K,and by Coulomb screening of the quantum confinement Stark effect followed by a filling effect of the higher levels of the low-energetic localized states at the intermediate temperature range of around 160 K.However,when the temperature is further raised to the higher temperature range of around 350 K,the emission process of the MQWs in the low current range is dominated by carrier-scattering effect followed by non-radiative recombination process.The aforementioned current-dependent behaviors of the EL spectrum are mainly attributed to the strong localized effect of the green LED,as confirmed by the anomalous temperature dependence of the EL spectrum measured at the low injection current of 5 ?A.In addition,the injection current dependence of external quantum efficiency at different temperatures shows that,with increasing temperature from 6 to 350 K,in addition to the enhanced non-radiative recombination,electron overflow becomes more significant,especially in the higher temperature range above 300 K.