Study Of GaN MIS-HEMT With High-K Dielectric Gate And Its Key Mechanisms

Firstly,the growth process of atomic layer deposition is optimized in this dissertation.The high-quality Al2O3 and HfO2 are grown based on this.For the 500?-Al2O3 layer,the standard error of five test points is only 1.5,which indicates a good homogeneity.For Al2O3 with the different thicknesses,we find that the density of interface state decreases with the increasing thickness.The reason is that the thicker Al2O3 can offer better effect of passivation.For HfO2,the standard error of the 253.8?-HfO2 is about 2.3.The Ar～+Bombarding XPS test shows the rates of Hf5d and O1s are 34%and 63%,respectively.Deviate from Al2O3,N1s always exists in HfO2,which indicates N atoms exist in HfO2 with the interstitial or substitutional form.The growth temperature of HfO2 is optimized in this dissertation.We find that HfO2 with the growth temperature of 270℃ has a higher dielectric constant and lower density of interface state.MIS-HEMT with the PECVD Si3N4 dielectric are fabricated and studied in this dissertation.Although the gate leakage current decreases to 1×10-8A,the gate capacitance is only 30pF,which is about 51.6%lower than HEMT,indicating low-k dielectric will weaken the control capacity of gate.Because the dielectric constant of Al2O3 is larger than the Si3N4 dielectric,we fabricated MOS-HEMTs using the ALD-Al2O3 with different thicknesses.The gate leakage currents of MOS-HEMTs with 3.5nm,7.5nm and 10nm Al2O3 are 2×10-8A,4×10-8A,1×10-7 A respectively.The results show that the thicker dielectric layer thickness,the lower the gate leakage current.The gate capacitances for the three kinds of devices are 46.7pF,39.8pF,36.2pF,reflecting the thicker dielectric layer also not conducive to the control capacity of gate.In addition,the saturation current increases with the increasing film thickness,which is analyzed from the aspect of passivation.NbAlO MIS-HEMT with the ultrathin ALD-Al2O3 dielectric as transition layer are fabricated and studied in this dissertation.NbAlO dielectric has high dielectric constant and fine temperature stability,thus the corresponding MIS-HEMTs also present good characteristics.The C-V hysteresis of MIS-HEMT is only 20mV,and the threshold voltage shift is less than 80mV between 10KHz to 1 MHz.The corresponding density of interface state is only(0.38～1.1)×1012cm-2eV-1,which proves the superiority of the use of ultrathin ALD-Al2O3 dielectric as transition layer.The transconductance of MIS device is 146mS/mm,only reduces 21%compared to HEMT(185mS/mm),which is better than MIS devices with low-k dielectrics.In addition,the GVS of MIS device is 3.96V,but the HEMT is only 1.36V.The output current of MIS device is 920mA/mm and is about 260mA/mm larger than HEMT when the gate bias is 0V.The reason for the increase of output current of MIS device can be explained by calculation of carrier concentration.The oxygen vacancy and it’s passivation of high-k dielectric are investigated by density functional theory(DFT)in this dissertation.The results of calculation of first-principles show that there are no differences of density of states(DOS)of bulk O and O near the oxygen vacancy,but gap states are produced for Hf atoms near oxygen vacancy.The vacancy level is located nether the conduction band and the level is 0.55eV lower than HfO2 conduction band.If the MOS system is positive bias(substrate electron injection),the gap state,whose wave functions are mainly from the Hf 5d orbital,can act as the trap level of trap-assited tunneling,which can aggravate positive gate leakage current obviously.This result can explain the lager positive gate leakage current of HfO2 MIS-HEMT perfectly.N ions passivation is proposed to solve this problem and the corresponding mechanism is explained in terms of first-principles.This dissertation studys the withdraw-method of density of interface state in all the layers of MIS-HEMT systematically.The corresponding models of density of interface are established.The density of interface state DT of NbAlO/Al2O3 MIS-HEMT can be calculated by this method.The DT of insulator/AlGaN is(0.38～1.1)×1012cm-2eV-1,lower than HEMT((2.2～6.2)×1012cm-2eV-1).The distributions of interface traps in forbidden band are calculated.The results show the interface traps near the bottom of the conduction band can be passivated by NbAlO effectively.The current collapses of NbAlO MIS-HEMT and HEMT are investigated using the model.When the gate and drain quiescent biases are(0V,0V),the MIS-HEMT and HEMT present the same level of collapse.However,when the quiescent biases are(-8V,1V),the collapses of MIS-HEMT and HEMT are 15%and 58%respectively.We can obtain that the DT and the τT for MIS-HEMT and HEMT by trap models.The processes of electron capture of traps in the pulse model are studied by calculation results.Furthermore,the advantage of NbAlO MIS-HEMT in restrain of current collapse is explained by these processes.1D Possion simulation tool is used to analyze the change of 2DEG and its subband distributions as the work-temperature rises in terms of theory.The change of carrier mobility as the work-temperature rise is calculated.The carrier mobility reduces from 1600cm/Vs to 300cm2/Vs as the temperature from 300K to 700K.This result shows that the degradation of mobility is the root of electrical properties degradation of HEMT.On this condition,the temperature dependent properties of MIS-HEMT are investigated.The output current and the series conductance decrease gradually as temperature rises,which is similar with HEMT.This result attributes to the degradation of mobility.The gate leakage current of MIS-HEMT increases as temperature rises before 200℃,but it is tending towards stability(1×10-5A)after 200℃.This representation is much better than HEMT.In addition,the transconductance reduces gently at high temperature,which also attributes to the low gate leakage current of MIS-HEMT at high temperature.Radiation tolerance of MIS-HEMT is studied in this dissertation.There are many corpuscular radiations in the application areas of MIS-HEMT.Proton is a common radiation in these corpuscular radiations.In order to check the performance of radiation tolerance for MIS-HEMT,it is subjected to 3MeV protons at fluences of 1×1015p/cm2.After proton irradiation,there is a forward flat-band voltage shift of about 0.94 V.The main reason for this shift is explained.The peak densities of NCV for MIS-HEMT before and after proton irradiation are 1.45×1020/cm 3 and 1.23×1020/cm 3,respectively.After proton irradiation,the peak density decreases about 15.2%.This value is better than HEMT.In addition,the DC degradation of MIS-HEMT is smaller than HEMT after radiation obviously.The reason of the good performance of radiation tolerance for NbAlO MIS-HEMT can be explained by the variations of DT and τT located in interface of AlGaN and the channel before and after radiation.