Study On Ultra-wide Bandgap AlN/AlGaN Heterostructures And HEMT Devices

Over the past two decades,III-nitride semiconductors represented by GaN have obvious competitive advantages in the power electronic devices application of high-frequency,high-voltage and high-power due to their advantages such as wide bandgap,high polarization intensity,high breakdown field strength and high electron saturation velocity.In III-nitride semiconductor systems,the breakdown field strength of AlN is approximately four times that of GaN.As a ternary alloy compound composed of AlN and GaN,AlGaN's bandgap can be modulated from 3.45 eV to 6.2 eV depending on the composition of Al.The breakdown field strength is up to 3 MV/cm¹2 MV/cm.Therefore,the adoption of AlGaN instead of conventional GaN as the channel layer with heterostructure can significantly improve the application potential of the device in high-voltage and high-power.At present,most of the international studies on the epitaxial growth of AlGaN channel heterojunction and its HEMT devices are based on AlGaN/AlGaN heterostructure with high Al component AlGaN as the barrier.However,for AlGaN/AlGaN heterostructures,a relatively thick barrier is required to ensure a high concentration of 2DEG in the channel,which will adversely affect the high-frequency characteristics of devices.In addition,the channel layer and barrier layer of AlGaN/AlGaN heterostructure are ternary alloy compounds,and the carrier in the channel is badly scattered by the alloy,resulting in poor electrical transport characteristics.Based on the above background,this paper proposes to use Ultra-wide bandgap AlN as the barrier layer to realize the preparation of AlGaN channel heterostructure,which can not only ensure the high concentration of 2DEG in the channel,but also reduce the thickness of the barrier layer,so as to improve the frequency characteristics of the device.In addition,the use of binary AlN barrier layer can effectively alleviate the alloy disordered scattering influence of AlGaN channel,so as to improve the transport characteristics of carriers.This paper focuses on the research of AlN/AlGaN heterostructure design,material epitaxial growth and HEMT device preparation.The main work is as follows:1.By changing the thickness of AlN barrier layer,the AlN/Al_0.18Ga_0.82N heterostructure grown on sapphire substrate with gradually increasing Al composition of the AlGaN buffer layer.The influence of the thickness of AlN barrier layer on the transport of carrier and the material morphology of AlN/Al_0.18Ga_0.82N heterostructure was studied in detail.The results show that,for AlGaN channel layer with 18%Al component,when the thickness of AlN barrier layer is 2 nm,the epitaxial material shows good atomic step flow morphology,and the crystal quality and electrical properties of heterogeneous junction material achieve the optimal with Square resistance achieving 1065?/?.2.Based on the successful preparation of high-quality AlN/AlGaN heterostructure materials,AlN/Al_0.18Ga_0.82N HEMT devices were successfully developed,and the device process was optimized according to the characteristics of the materials.Optimized device of ohmic contact resistance is 4.03?mm,schottky reverse leakage at 2.6 x 10-4 mA/mm.At the same time,the saturation output current density of the device is up to 421 mA/mm,and the leakage current is 0.22 mA/mm when the device is switched off.When the gap between gate and drain is 7?m,11?m and 21?m,the breakdown voltage of the device is up to 439 V,520 V and 900 V,respectively.When the gap between gate and drain is 51?m,the breakdown voltage of HEMT device exceeds 2000 V,which is much better than that of conventional GaN channel HEMT devices.3.The pulse AlN insertion layer method and GaN insertion layer method are proposed to achieve high quality AlN heteroepitaxy on sapphire substrate.AFM test results show that pulsed AlN insertion layer and GaN insertion layer are conducive to promoting the two-dimensional growth mode of AlN epitaxial materials and can significantly improve the surface morphology of the materials.In the scanning range of 5×5?m²,the RMS of the surface of AlN epitaxial thick film samples using GaN insertion layer method is as low as0.155 nm,and the difference between peak and valley is 1.7 nm.4.Based on the heterostructure requirements of the altra-wide bandgap channel,the study on the epitaxial growth of AlN/Al_0.75Ga_0.25N heterostructure was carried out on the AlN baseplate by abrupt growth and reverse-graded growth respectively.AFM test results show that the introduction of Al_xGa_1-xN?x>0.75?buffer layer can effectively alleviate the lattice mismatch between Al_0.75Ga_0.25N channel layer and AlN buffer layer,and significantly improve the crystal quality and surface morphology of the material.This result is of guiding significance for the preparation of Ultra-wide bandgap channel heterostructure.