Epitaxial Growth And Device Fabrication Of Si-based AlInGaN/AIN/GaN HEMT

Gallium Nitride(GaN)has superior properties,such as large bandgap and high breakdown field.Thanks to these advantages,GaN-based high-electron-mobility transistors(HEMTs)have emerged as excellent candidates for power conversion and high frequency applications in recent years.Among which,quaternary AlInGaN barrier based HEMT plays an important role in both high power and high frequency area owing to its low channel conductance as well as suppressed short channel effect.Therefore,this dissertation focuses on the AlInGaN/GaN HEMT,including its epitaxial growth,device fabrication and gate leakage mechanism.The main achievements are as follows:1.MOCVD epitaxial growth of AlInGaN/AlN/GaN heterostructure was systematically studied,including the high quality growth of GaN-on-Si and quaternary AlInGaN barrier.The influence of growth temperature,growth rate,chamber pressure and the Ga composition of the AlInGaN barrier on the surface morphology and the 2DEG characteristics were analyzed by using X-ray diffraction(XRD),transmission electron microscope(TEM),Hall and some other test methods.It was found that as the growth temperature of the barrier increased,the In composition decreased and the pits' density on the surface of AlInGaN/GaN heterostructure decreased significantly.If the growth rate was faster,the In composition in the barrier would increase,while the surface morphology would degenerate.With the decrement of Ga component,the 2DEG concentration would be enhanced.Based on the above results,high crystalline quality AlInGaN quaternary alloy has been obtained under the growth condition of higher temperature,lower growth rate,and medium Ga content.2.The unintentional incorporation of Ga in the AIN spacer in AlInGaN/AlN/GaN heterostructure has been experimentally verified and carefully suppressed.If the growth of the heterostructure was terminated at the AIN spacer and ramped to the room temperature under the protection of the ammonia,it was found that the surface is full of pits.It is confirmed that 1-nm-thick AIN spacer could not cover the surface of the GaN channel layer uniformly.As a result,the exposed GaN channel will decompose during the temperature ramping.A cross-section of the specifically designed AlN/GaN multilayer structures was characterized by TEM,and it was found that Ga was unintentionally incorporated into the "AlN" spacer.It can be speculated that the main source of Ga is the decomposition of the underlying GaN.The decomposition can be effectively suppressed by modulating the growth condition of the "AlN" spacer,reducing the unintentionally incorporated Ga significantly.Thus a high-quality heterostructure interface can be obtained.The interface roughness scattering and the remote alloy scattering has been reduced well,improving the 2DEG mobility.3.The HEMTs based on AlInGaN/AlN/GaN heterostructure were fabricated.Compared with the traditional AlGaN/AlN/GaN HEMTs,the on-resistance of AlInGaN/AlN/GaN HEMTs was almost reduced by half,indicating the higher 2DEG density.However,the gate leakage of AlInGaN/GaN HEMT was still two orders of magnitude higher than that of AlGaN/GaN HEMT.By adopting SiNx deposited by low pressure chemical vapor deposition(LPCVD)as the gate dielectric,the gate leakage of the AlInGaN/AlN/GaN HEMT was successfullyreduced by three orders of magnitude with ON/OFF ratio of 10,which was comparable to the AlGaN/AlN/GaN HEMT.4.The gate leakage mechanism of AlInGaN/AlN/GaN HEMT devices was investigated by the temperature dependent I-V measurement.The leakage current of different heterostructure could be modeled by Poole-Frenkel(PF)emission@low electric field and Fowler-Nordheim(FN)tunneling @high electric field respectively.Based on the leakage analysis,a novel heterostructure with composite barrier,AlInGaN/AlGaN/AlN/GaN,was proposed.The AlInGaN is used to induce the high density of 2DEG,while the AlGaN can suppress the gate leakage effectively.The gate leakage of AlInGaN/AlGaN/GaN has been successfully reduced by two orders of magnitude.5.Ultra-thin barrier layer of GaN-based HEMT was required to avoid short channel effect in RF and microwave applications,such as Al-rich AlGaN.However,Al-rich AlGaN barrier grown on GaN is usually under a large tensile stress,which often leads to surface grooves or even micro-cracks,affecting device performance and reliability.To circumvent this problem,a trace amount of indium was added during the growth of Al-rich AlGaN barrier layer to increase the migration ability of Al atoms on the growth surface.Therefore,the uniformity of Al content was improved and the local stress was effectively reduced.As a result,a pit-free high-quality AlGa(In)N/GaN heterostructure with a step-flow surface morphology was obtained together with a well-confined 2DEG of high mobility.A 55-nm T-gate high-electron-mobility transistor was fabricated with a quaternary barrier AlGa(In)N/AlN/GaN heterostructure.The device showed a low on-resistance of 1.6 ?-mm,a maximum current density of 1.25 A/mm,a peak extrinsic dc transconductance of 420 mS/mm,and the cutoff frequencies fT/fmax of 145/220 GHz,which gives a record high value of(fT · fmax)1/2 = 178 GHz among the reported GaN-on-Si RF devices.