Study On The Homoepitaxial Growth Of Nitrogen-polar GaN Thin Films And Preparation Of Quantum Wells On GaN Substrates

GaN semiconductor is a direct bandgap semiconductor material,and has the characteristics of wide band gap,large breakdown voltage and high saturation electron drift velocity,which is very suitable for the preparation of semiconductor light-emitting devices and high-frequency high-power electronic devices.GaN grown along the c-axis has two polarities,gallium polarity(also called metallic polarity)and nitrogen polarity.The GaN epitaxially grown along the+c axis[0001]direction is called gallium polar GaN,and the GaN epitaxially grown along the-c axis[0001 ]direction is called nitrogen polar GaN.Due to the high crystal quality of gallium-polar GaN,most of the current GaN-based devices are metal polar.However,compared with metal polar nitride materials,nitrogen polar nitride materials have unique advantages in the application of light-emitting devices and electronic devices.For example:Nitrogen-polar In GaN has a higher In incorporation rate;the opposite polarized electric field direction in Nitrogen-polar LEDs can improve carrier injection efficiency;Nitrogen-polar GaN/Al GaN-based high electron mobility Transistor(High Electron Mobility Transistor,HEMT)has stronger gate control capability and lower ohmic contact resistance.However,researchers found it difficult to obtain nitrogen-polar GaN films with smooth surfaces.The low crystal quality has long restricted the development of nitrogen polar materials and devices.At present,GaN-based materials and devices are mostly prepared based on heterogeneous substrates such as sapphire(Al₂O₃),silicon(Si),and silicon carbide(Si C).However,heteroepitaxy will introduce large residual stress and high density of defects in the epitaxial layer,which will seriously reduce the efficiency and working life of the device.Comprehensive consideration,this thesis chooses GaN as the substrate,and carries out the research on the homoepitaxial growth of nitrogen polar GaN thin films.At the same time,the In GaN/GaN quantum well structure was prepared on the nitrogen polar GaN thin film obtained by homoepitaxy.The specific research contents of this paper are as follows:1.Research on homoepitaxial growth of nitrogen polar GaN films on GaN substrates.The effects of growth parameters(V/III ratio,growth temperature)and pulse growth method on the surface morphology and crystal quality of nitrogen-polar GaN thin films were investigated.The research shows that when the growth temperature is1050?and the V/III is 1400,the nitrogen polar GaN film grown on the GaN substrate has a large number of hexagonal hillocks on the surface.By increasing the growth temperature and decreasing the V/III ratio,the density of hexagonal hillocks on the surface of nitrogen-polar GaN is reduced,but the surface is still relatively rough.To this end,we propose a pulse growth method to improve the surface morphology and crystalline quality of epitaxial GaN.In this method,the nitrogen source NH₃ is kept continuously supplied,and the high-flow and low-flow gallium source TMGa are periodically supplied alternately.Nitrogen polar GaN films with smooth surface were obtained based on pulse growth method.The surface morphology and crystal quality of nitrogen-polar GaN were further improved by optimizing the source flow rate and pulse time during the pulse growth process.Under optimal conditions,the root mean square roughness of the GaN surface in an area of 20×20?m~2 is only 0.72 nm.Meanwhile,the crystalline quality of this sample is high,and its(0002)and(101 2)plane X-ray diffraction rocking curve half-widths are 26 arcsec and 24 arcsec,respectively.2.Study of Preparation and luminescence properties of nitrogen-polar In GaN/GaN quantum wells on GaN substrates.The effects of the growth temperature of the barrier layer,the V/III ratio of the potential well layer and the growth of the barrier layer by the pulse method on the characteristics of the nitrogen-polar In GaN/GaN quantum wells during the quantum well growth were studied.The research shows that increasing the growth temperature of the quantum well barrier layer can reduce the density of the triangular hillocks on the surface of the quantum well,and improve the surface morphology and crystal quality of the quantum well.In addition,by increasing the V/III ratio of the potential well layer from 20,000 to 50,000,the surface morphology and crystal quality of the quantum well were further improved,and the density of hillocks on the surface of the quantum well was significantly reduced.At the same time,the light-emitting properties of the quantum wells have also been significantly improved.Finally,the quantum well barrier layer is grown by the pulse method,the surface of the prepared quantum well is smooth and flat,and the hillock completely disappears.In addition,the luminescence intensity of the quantum well was further improved by optimizing the on and off time(t_on/t_off)of the trimethylgallium source.When t_on/t_off=3s/9 s,the quantum well has the strongest luminescence intensity and the best surface flatness,and its surface root mean square roughness in an area of 20×20?m~2 is only0.6 nm.