Study On Interface Treatment Technique And The Internal Mechanism For AlGaN/GaN MIS-HEMT

GaN-based metal-insulator-semiconductor high electron mobility transistor?MIS-HEMT?is an ideal choice for high voltage power switches and microwave power devices due to its lower gate leakage current and larger gate voltage swing compared with traditional Schottky-gate HEMT.However,the introduction of gate dielectrics in MIS-HEMT results in a large amount of interface states and fixed charges,which lead to reliability problems such as V_thh drift,thus seriously affect the stability and security of circuit systems.Therefore,based on interface pretreatment and anneal technique,the interface characteristics and mechanism of GaN-based MIS-HEMT were studied in this thesis.Finally,AlGaN/GaN MIS-HEMT devices with high interface quality and low leakage current were achieved.A novel plasma-assisted diffusion controlled interface oxidation?DCIO?pretreatment method was proposed in this thesis,which effectively improved the interface quality of GaN-based MIS-HEMT.DCIO pretreatment could lead to about 2 nm crystalline-like interfacial layer between Al₂O₃ insulator and nitride layers,thus in DCIO pretreatment sample low amount of net positive charges was found,of approximately 9.5×10¹²cm^-2.DCIO led to a decrease in interface charges by 7.6×10¹²cm^-22 and 3.8×10¹²cm^-2 compared with the case with reaction-controlled surface oxidation?RCSO?and none oxidation,respectively.The results shows that for interfacial characteristics of MIS devices it's not the existence of interfacial oxide layer that matters,but the quality of interfacial layer.Implementing interfacial oxide layer with high quality can actually improve the interfacial characteristics of MIS-HEMT.The effects and mechanism of post-deposition anneal?PDA?on the interface characteristics of GaN-based MIS-HEMT devices in different atmosphere and temperature were systematically studied in this thesis,and finally optimized interface treatment scheme was obtained,which significantly decreased fixed interface charges and off-state leakage of MIS-HEMT.PDA applyed in O₂ showed that fixed interface charges of GaN-based MIS-HEMT devices not only include fixed charges within oxide dielectric layer,so it didn't meet with reversible heat treatment effect that fixed oxide charges complied with.PDA treatment in O₂atmosphere were able to reduce fixed interface charges and off-state leakage effectively,but density of interface traps increased slightly after treatment.Compared with PDA at 450?,PDA at 550?and 650?reduced the fixed interface charges of MIS devices by2.34×10¹²cm^-2 and 4.56×10¹²cm^-2,increased the interface traps by 0.94×10¹¹cm^-2 and0.23×10¹²cm^-2 respectively,meanwhile off-state leakage decreased by more than one order of magnitude successively,what's more,off-state leakage decrease was found related to the decrease of mesa isolation leakage and fixed interface charges,besides,PDA at 750?could destroy electrode.PDA in N₂ showed the same trend of decreasing the fixed interface charges and increasing the interface traps slightly with the increase of annealing temperature.Under the same anneal temperature,the interface traps of N₂ PDA samples and O₂ PDA samples were basically the same,but the reduction of fixed charges and off-state leakage in N₂ PDA samples was not as obvious as that in O₂ PDA samples.The fixed interface charges in N₂PDA samples at 750?are at the same level with those within O₂ PDA samples at 550?,relatively poor Al₂O₃/GaN interface and higher fixed interface charges made N₂ PDA samples presented relatively high off-state leakage.Considering both interface quality and device performance of MIS-HEMT,it is determined that the most desirable PDA treatment condition in this thesis is annealing in O₂ atmosphere under 650?for 15 minutes.Post-metalization anneal?PMA?technique was study and optimized in this thesis,which significantly reduced the interface traps and the off-state leakage of MIS-HEMT.Unlike PDA,PMA at 550?could destroy the contact performance of gate electrodes,thus causing failure of devices.After PMA,the number of fixed charges did not change significantly or only exhibit slight decrease,but the density of interface traps decreased,what's more,off-state leakage was decreased significantly as the temperature go higher,the off-state leakage of the device with PMA at 500?decreased by more than four orders of magnitude,we attributed this phenomenon to the improvement of gate contact performance after annealing.Finally,we considered that the ideal PMA treatment condition to improve device characteristics is:annealing at 500?for 2 minutes in N₂ atmosphere.Finally,DCIO interface pretreatment and the ideal PDA and PMA treatment scheme were applied to Al₂O₃/AlGaN/GaN MIS-HEMT with partially recessed-gate,and MIS-HEMT with excellent interface performance were achieved with V_th=2.75 V,I_ON/I_OFF over 10⁹,large output current of 865 mA/mm,g_max of 271 mS/mm,which is 54 mS/mm larger than MIS-HEMT without recessed-gate.