Method And Characteristic Of Nitride Epitaxial Growth On N-polar And Nonpolar GaN Substrates

GaN is one of the third-generation semiconductor materials.It has a wide bandgap,high electron saturation speed and good stability,it has shown a great advantage in the field of optoelectronic devices and electronic devices.GaN-based semiconductor materials have the characteristics of direct bandgap,which can be used to make efficient semiconductor optoelectronic devices.At the same time,the bandgap of GaN,In N,and AlN alloys can continuously cover the range of 0.7e V to 6.2e V,and their corresponding emission wavelengths also cover the entire visible light,GaN-based semiconductor materials have become irreplaceable for optoelectronic device applications.For electronic devices,GaN-based materials have a strong polarization effect.The strong polarization effect in conventional GaN materials generates high-density two-dimensional electron gas at the interface of Al GaN/GaN heterojunction.The two-dimensional electron gas has extremely high electron mobility which can greatly increase the frequency of high electronic devices such as electron mobility transistors.However,the strong polarization effect is harmful to GaN-based optoelectronic devices.It bends the energy band and causes holes and electrons to be separated in space,which will affect the recombination of holes and electrons.Then it leads to an efficiency decrease of LED and an increase of light emission wavelength.In order to eliminate the negative effect of the polarization effect in GaN-based photovoltaic devices,N-polar GaN materials and nonpolar GaN materials can be used.At present,whether it is an N-polar GaN material or a nonpolar GaN material,there are some difficulties in the application process.First,the quality of the common N-polar GaN material or nonpolar GaN material is poor,which is much worse than the existing Ga-plane GaN.It is difficult to meet the requirements of the application.In addition,there is still a problem of high background carrier concentration in the N-polar GaN material.In order to solve these problems,this paper uses bulk GaN materials as the substrate to perform epitaxial growth processes for N-polar GaN and nonpolar GaN materials.The optimization and the optical and electrical of the obtained materials were studied.The main research results of this paper are as follows:1.N-polar GaN materials were prepared on ordinary sapphire substrates and vicinal sapphire substrates using the Metal-Organic Chemical Vapor Deposition(MOCVD)process.The N-polar GaN samples using vicinal sapphire substrates were compared to the samples of ordinary sapphire substrates.The dislocation density has decreased by about an order of magnitude.Next,the flow rate of the Ga source in the MOCVD process was changed and the V/III was adjusted.A high-quality N-polar GaN homoepitaxial sample was obtained on the N-polar GaN bulk material substrate.The dislocation density is only 3.93×10⁶ cm^-2,which is about three orders of magnitude lower than that on sapphire substrate.The Raman,Photoluminescence(PL)and Hall test have further proved that the optimization of the Ga source flow rate improves the material quality significantly.2.The decrease of the flow rate of the Ga source during the growth process leads to a decrease in the C impurity content in the sample and an increase in the density of Ga vacancies.Combined with the edge type dislocation density calculated by the rocking curve of high resolution X-ray diffraction(HRXRD)and the PL results,it is proved that C related impurities are the main cause of yellow bands in N-polar GaN materials.3.By optimizing the V/III ratio in the MOCVD process,the background carrier concentration of the N-polar GaN sample was reduced.After the Ga source flow rate was reduced to 25%,the background carrier concentration of the sample was reduced by about95%.At the same time,the carrier mobility of the sample is affected by both the crystal quality of the sample and the background carrier concentration.The sample with the best crystal quality and the lowest background carrier concentration has the highest carrier mobility.4.High-quality m-plane GaN and AlN/GaN heterojunctions were obtained on a nonpolar m-plane GaN bulk substrate.At the same time,m-plane AlN was obtained on the m-plane GaN bulk material.Scanning Electron Microscope(SEM)test results showed that the surface of the sample with GaN and AlN/GaN heterojunction is relatively flat,while the surface of the AlN material with a thickness of 250 nm is rough and the crystallinity is poor.The HRXRD test results show that the material quality of the nonpolar m-plane GaN samples is higher,while the quality of the GaN materials deteriorates on the 250 nm AlN samples,and the crystal quality of the AlN materials is poor.5.The crystal quality of the nonpolar m-plane samples was further studied by the rocking curve test of HRXRD,the energy dispersive spectrometer analysis results of the SEM test,the Raman test,the PL test,and the X-ray photoelectron spectroscopy test.Among them,the m-plane GaN sample had the best crystal quality.The surface of the AlN material sample is poor and the AlN grain is unevenly distributed on the sample surface...