Study On Composite Buffer Layer GaN-based Heterostructure Materials And Devices

For high breakdown voltage,high current density and excellent thermostability,GaN-based high electron mobility transistors?HEMTs?have attracted great attentions from both the academia and industy.As the invention of high brightness blue GaN LED won 2014 Nobel Prize in Physics,the wide band gap GaN material has been known to public.Although the thermostability of the GaN material is superiorly,the mobility and density of 2DEG in heterostructure interface is highly depend on temperature and the 2DEG is easily overflow from the quantum wells at high temperature and resulting in the degradation of the electrical characteristics of the device.For the double heterostructures with the back barrier inserted,2DEG confinement has been greatly enhanced due to the higher barrier below the channel and the buffer layer leakage and the degradation of the electrical characteristics have been reduced.Therefore,the design and growth of the back barrier and the buffer layers can directly affect the properties of the AlGaN/GaN/AlGaN/GaN double heterostructures.In this context,three kinds of GaN-based heterostructures with different composite buffer layers were designed and synthesized,and the theoretical and experimental studies were carried out from the material properties and the device characteristics.The main contents of this paper are showing below:1.The 1D Schrodinger-Poisson equation is used to simulate the band structure and 2DEG distribution of three kinds of GaN-based double heterostructures with different composite buffer layers.The principals and characters of_--?Al_xGa_1-xN/GaN/Al_yGa_1-yN?AlGaN/GaN/InGaN/GaN were also discussed.2.Three samples with different compsite buffer layers including sample A(Al_xGa_1-xN?x=0⁰.07?/GaN),sample B(Al_0.07Ga_0.93N/Al_xGa_1-x?x=0⁰.07?)and sample C(Al_xGa_1-xN?x=0.3⁰?/Al_xGa_1-xN?x=0.9,0.7,0.5,0.3?)are prepared by MOCVD expital growth,where Sample A and Sample B are gorwed on sapphire substrates and sample C gorwsed on silicon substrates.The high performance AlGaN/GaN/AlGaN/GaN double heterostructures can be fabricated by adopting an 300 nm Al_0.07Ga_0.93N/1000 nm Al_xGa_1-x?x=0⁰.07?compsite buffer layer.The optimized DH features a RSM of 0.19 nm,a FWHMs of 382 arcsec of?002?plane rocking curve by HRXRD,a carrier mobility of 1645 cm²/V·s,a 2DEG sheet carrier density of 1.02×10¹³ cm^-2.The Hall Effect measurements of three samples at different temperatures showed the degradation of carrier mobility of sample B is 75.28%when the temperature increased from 300 K to 573 K,which in sample A and sample C are 79.41%and 83.85%,respectively.The RSM of?004?plane of three samples shows that the GaN lattice of sample B has the smallest broadening in the q_z axis direction,indicating that the material quality of sample B is the best.Both the composite buffer layer of sample A and sample B were partially relaxed by the AlN nucleation layer,but the relaxation degree of sample B was significantly larger than that of sample A.This also explains the reason of the 2DEG density of sample B is higher than sample A.The actual Al composition and stress ineach AlGaN layers of sample C were calculated by the measurement of?104?plane RSM.At the same time,the mosaic structure of GaN layer epitaxial on Si was analyzed and calculated to obtain the size of the grain.3.The devices of sample B and sample C were prepared and the electrical characteristics were tested and analyzed at room temperature and high temperature.The saturated leakage current of sample B at room temperature is 774 mA/mm,which is reduced by 35%at temperature as high as 573 K.However,for sample C it is decreased by 44%.The DIBL of sample B was 16 mV/V,which was much less than 39 mV/V of sample C.Schottky gate leakage and saturation leakage current of sample B also shows strong stability at high temperature,as the introduction of the composite buffer layer.In a word,because of the better 2DEG confinement,the AlGaN/GaN/AlGaN double heterostructure with composite buffer layers has shown excellent performances both in material and device characteristics and is more suitable for the applications of high voltage,high frequency and high power devices.