The Effects Of Gate Dielectrics On Performances Of AlGaN/GaN Metal-insulator-semiconductor High-electron-mobility Transistors

Due to the spontaneous polarization and piezoelectric polarization, a highconcentration of two-dimensional electron gas （2DEG） exists at the AlGaN/GaN hetero-junction interface. Because of the superior properties of the AlGaN/GaN hetero-structure field-effect transistors （HFETs）, AlGaN/GaN hererojunction is considered tobe an important candidate material for high-frequency, high-temperature andhigh-power semiconductor device. The AlGaN/GaN HFETs is also called as high-electron-mobility transistors （HEMTs）.For the excellent frequency characteristics, the high-frequency AlGaN/GaNHEMTs often used to be the metal-semiconductor high-electron-mobility transistors（MESHEMTs） structure. However, the practical application of AlGaN/GaNMESHEMTs was limited by the serious current collapse and large gate leakage current.The gate dielectrics can not only effectively suppress the current collapse, but alsogreatly reduce the gate leakage. The structure of device with gate dielectrics is called asmetal-insulator-semiconductor high-electron-mobility transistors （MISHEMTs）.However, the study of AlGaN/GaN MISHEMTs mainly focused on the non-polar gatedielectrics. Little attention was paid to the effect of the polarization of gate dielectricson the performance of AlGaN/GaN MISHEMTs.This dissertation compares the impacts of non-polar gate dielectrics and polar gatedielectrics on the performance of the AlGaN/GaN MISHEMTs. The Al₂O₃thin filmswere used as the non-polar gate dielectrics. The impacts of the polarizationcharacteristics on the performance of the AlGaN/GaN MISHEMTs were selected tworepresentative typical polar dielectric materials. One was the beta alumina dopedsodium （β-Al₂O₃, sodium-beta-aluminium, abbreviated as SBA）. The SBA has aunusual crystal structure that is distinctly two dimensional, with an about1.13nmaluminium ixude layer and a sodium ions. The sodium ions can move freely only indirections parallel to the lattice planes, but restricted to a few angstroms in thedirectiona normal to the planes. Therefore, it’s not only an excellent ionic conductor, butalso an outstanding dielectric. For the polarization of sodium ions, the SBA is also an electrolyte. The other polar dielectric material was the ferroelectric. Considering thecompatibility, a new ferroelectric material—HfTiO ferroelectric thin film was chose as agate dielectric.The first phase of this work involved the non-polar dielectric—Al₂O₃thin film.The Al₂O₃thin films were deposited between the source and drain of the AlGaN/GaNHFETs by molecular beam epitaxy （MBE）. The AlGaN/GaN MISHEMTs structureswere prepared. The performance of the MISHEMTs, as well as the concentration andmobility of2DEG were measured. The results show: the2DEG concentration andmobility were increased by the Al₂O₃thin films. The maximum increase of2DEGconcentration and mobility were30%and23%respectively. The comparative study ofthe device performance show that the DC output characteristics of the AlGaN/GaNMISHEMTs have been significantly improved attributing to the Al₂O₃films. Comparedwith the conventional MESHEMTs, the gate leakage current is reduced by about twoorders of magnitude. The maximum output drain current density was increased from590to790mA/mm, which is about34%improvement. When the gate bias is0V, thesaturation output current densities of the AlGaN/GaN MESHEMTs and MISHEMTswere360and700mA/mm respectively, which is increased by about94%. Themaximum transconductance was increased from120to170mS/mm, which is about42%improvement. The comparative analysis of high-resolution X-ray diffraction（HRXRD） shows that the tensile stress was introduced into the AlGaN barrier layer afterthe deposition of Al₂O₃thin films, resulting in the improvement of2DEG concentrationand mobility. The effect factor was explained by the theoretical results of empiricalformula method and the First Principles.The second phase of this work involved the SBA electrolyte thin film. The sodiumions were doped into Al₂O₃thin film by the introduction of Na molecular during theMBE growth process. The microstructure analysis of the prepared thin films shows thatthe grown thin films were amorphous. The X-ray photoelectron spectroscopy （XPS）analysis result was consistent with the characteristic peak of SBA. The measurements ofdielectric properties show that the dielectric constant of the film was about50at1MHzfrequency, which is lager than that of Al₂O₃thin film. The result suggests that thesodium ions exhibit the polar property. The result also shows that the SBA thin filmexhibits good insulation characteristic. Therefore, it’s suitable for gate dielectric material. Then, the AlGaN/GaN MISHEMTs with SBA gate dielectrics were prepared.The results show: when it forms the AlOx/AlGaN interface between SBA thin films andAlGaN barrier layer, the impacts on the device performance were the same as Al₂O₃gate dielectric, specially the threshold voltage shifted more negatively. However, whenit forms the Na+/AlGaN interface between SBA thin films and AlGaN barrier layer, theeffect of the gate dielectric on the threshold voltage of MISHEMTs was completelydifferent. The threshold voltage of MISHEMTs shifted positively about2V by theinfluence of the interface positive charge. The results suggest the important influence ofthe interface characteristics of polar dielectrics and semiconductor on the deviceperformance.The final phase of this work involved the HfTiO ferroelectric thin film. The furtherstudy of the effects of another typical polar dielectric—ferroelectric on thecharacteristics of AlGaN/GaN MISHEMTs was carried out. The HfTiO thin films wereprepared by MBE. The material characteristics studies show that the HfTiO thin filmexhibits ferroelectric properties, which belongs to the class of the HfO₂-basedferroelectrics. The ferroelectric properties of HfTiO thin film were improved byadjusting the ratio of titanium and hafnium in the film, optimizing the film depositiontemperature and regulating the oxygen content of the film. The device performancesstudy results show that the device threshold voltage is also shifted positively from-4to-0.6V. The positively shift of the threshold voltage related to the channel2DEG wereinjected into and accumulation at the interface of HfTiO thin film and AlGaN barrierlayer.