| Advanced dielectric materials play a crucial role in the further development of semiconductor integrated circuits.As an important part of GaN-based HEMTs devices,the quality of gate dielectric and passivation dielectric materials will directly affect the performance of the device.Since the "ideal dielectric material" suitable for GaN-based devices may prefer to have high dielectric constant,wide band gap,high potential barrier height,high temperature stability and low interface state density,etc.,current common dielectric materials are basically unable to meet these requirements simultaneously.This thesis focuses on the preparation of new ternary nitride AlBN dielectric films by pulsed laser deposition,high temperature stability characteristics,and the effect of surface potential on the electrical characteristics of HEMTs devices.The main research contents and results of the thesis are as follows.1.The high-quality AlBN amorphous dielectric films were successfully fabricated at room temperature by pulsed laser deposition(PLD).The effects of growth parameters,including growth pressure,substrate temperature and laser energy on the film quality of ternary nitride AlBN dielectric films were studied,and the surface morphology and phase structure of the grown AlBN films were measured by a series of testing techniques.The test and characterization of chemical composition and other characteristics show that the grown AlBN medium is an amorphous film with a smooth surface and uniform element distribution.In addition,the relative optical properties and electrical properties of AlBN films were tested and analyzed by XPS,CAFM and Ⅰ-Ⅴ.It was found that the forbidden band of amorphous AlBN dielectric films was 5.73 eV,and the conduction band order of amorphous AlBN/GaN was 2.67 eV,which may provide good electron confinement and blocking effect.2.Since many annealing treatments at around 800℃ are often experienced in the process of device fabrication,the stability and reliability of dielectric materials are particularly important.The AlBN amorphous dielectric films were annealed at 800℃,and the related properties of the annealed films were investigated.The results show that the AlBN dielectric films still have a stable amorphous structure after annealing at 800℃.In addition,the forbidden band gaps of the AlBN dielectric films after annealing is 5.78 eV.And,the AlBN/GaN interface maintains a conduction band offsets greater than 2.0 eV.In addition,the dielectric constant values of AlBN of both roomtemperature-deposited and annealed at 800℃ are higher than 10(1 MHz).The leakage current density of the annealed AlBN films is one order of magnitude lower than that of the unannealed AlBN films.The characteristics of high temperature stability,large conduction band offsets(>2.0 eV),high dielectric constant(>10)and low leakage current make the ternary amorphous AlBN dielectric material more suitable for devices operating at high temperature.3.The effects of AlBN,AlN and AlSiN on the 2DEG were investigated.The Hall test results show that the AlBN dielectric film with a thickness of 13 nm can reduce the sheet resistance of the heterojunction by more than 25%,and increase the density of 2DEG in the channel by nearly 40%,but the mobility of 2DEG is slightly reduced;however,the AlN dielectric layer with the same thickness can increase the density of 2DEG in the channel by about 26%,while the mobility of 2DEG remains basically unchanged;in contrast,the AlSiN layer with a thickness of 13 nm increases the density of 2DEG in the channel by 27%with a small increase in the mobility of 2DEG.The contact potential difference between the three dielectric materials and GaN was tested by SKPM,and the relative surface potentials of AlBN/GaN,AlN/GaN and AlSiN/GaN were obtained as 0.90 V,0.35 V and 1.10 V,respectively.Due to the different influences of the surface potential of different passivation layers on the AlGaN/GaN heterojunction,the changes in the triangular barrier(well)are also different,which leads to different changes in the density and mobility of 2DEG in the channel.4.AlGaN/GaN MIS-HEMT devices with AlBN gate dielectric layer and Schottky gate HEMT(S-HEMT)devices were prepared,and the DC characteristics and C-V characteristics of the devices were tested for comparison.The results show that the SHEMT has a threshold voltage of-1.6 V,a switching current ratio of about 103,and a peak transconductance of 5×10-5 S,while the MIS-HEMT with the same device size has a threshold voltage of-6.0 V and a switching current ratio higher than 105,the peak transconductance is still about 5×10-5 S.This indicates that the introducing of the AlBN gate dielectric layer has little effect on the peak transconductance of the MIS-HEMT.In addition,the 32 nm-thick AlBN gate insulating layer could significantly reduce the forward and reverse gate leakage currents by more than 2 orders of magnitude.The CV results show that the AlBN dielectric layer causes an increase in the density of 2DEG,which is consistent with the Hall test results.Moreover,the interface state density between the AlBN layer and GaN obtained from the C-V hysteresis curve is of the order of 1011 cm-2.5.AlGaN/GaN Schottky gate HEMT devices with an amorphous AlBN passivation dielectric were studied.The effects of the 74 nm-thick amorphous AlBN passivation layer on the transfer,output,gate leakage and breakdown characteristics of the AlGaN/GaN S-HEMTs were investigated by electrical measurements.The results show that the switching current ratios of the S-HEMT devices before and after passivation are about 103 and 104,respectively.The peak transconductance is found to be 51 mS/mm and 109 mS/mm,respectively.The saturation output current of S-HEMT devices after passivation is increased by 86%and the gate leakage current is reduced by nearly an order of magnitude compared with that before passivation.The thermal electron emission model(TE)was used to fit the temperature-variable gate leakage curve,and the results show that the Schottky barrier increase owing to the use of AlBN passivation,which changes the surface properties of the heterojunction.The results obtained above show that AlBN dielectric film is a very promising candidate as gate dielectric layer and surface passivation for AlGaN/GaN HEMTs. |
