Study On AlGaA/GaN MOS-HEMT With High K Gate Dielectric

The first part of this work focus on AlxGa1-xN/GaN heterostructure material system and330-340nm Ultra-Violet (UV) AlxGa1-xN/AlyGa1-yN or AlxGa1-xN/GaN Muti-quantum Well (MQW) material systems. The origin of Two Dimensional Electron Gas (2DEG) in AlxGa1-xN/GaN heterostructure has been analysed and the high temperature performance of AlxGa1-xN/GaN heterostructure has been measured. The analysis of AlxGa1-xN/AlyGa1-yN MQW structure has been made by X-Ray Diffraction (XRD) measurement. The optical properties of the MQW structures using AlGaN or GaN as quantum well (QW) at330-340nm have been investigated by temperature-dependent photoluminescence (PL) measurements. At room temperature, the PL intensity of the AlGaN MQW is significantly enhanced by more than one order of magnitude, compared with the GaN MQW. The temperature-dependent PL spectra show a clear "S" shape in the AlGaN MQW, a fingerprint of exciton localization, but not in the GaN MQW, as that ternary alloy is expected to causes a stronger exciton localization effect than binary alloy. Furthermore, the internal quantum efficiency of the AlGaN MQW, estimated using temperature dependent integrated intensity, is also significantly higher than that of the GaN MQW, which indicates that AlGaN ternary as a QW for the330-340nm UV emitter should be a better option. Quantum-Confined Stark Effect (QCSE) is investigated by power-dependent PL in AlxGa1-xN/AlyGa1-yN and AlxGa1-xN/GaN MQW. There is a weak Quantum-Confined Stark Effect in AlxGa1-xN/AlyGa1-yN MQW due to its weak stress. Finally, the self-heating effect in AlxGa1-xN/AlyGa1-yN MQW has been investigated by Electroluminescence (EL) measurement. The EL wavelength do not change with the increasing pulse current because of the self-heating effect being avoided under pulse condition, while the EL linewidth slightly increases with the increasing pulse current, which is due to other effect.Next, the basic characteristics of AlGaN/GaN Metal Oxide Semiconductor (MOS) capacitor with high K dielectric have been analysed by Capacitance-Voltage (C-V) measurement. First, the carrier concentration and density in AlGaN/GaN MOS capacitor with high K dielectric have been calculated according to C-V measurement. Then. C-V hysteresis has been used to calculate the interface state density between high K dielectric and AlGaN. The interface state density between Al2O3/AlGaN has also been evaluated by light-dependent C-V measurement. Furthermore, another method called frequency-dependent C-V has been used to calculate the interface state density between the high K dielectric and AlGaN. The calculation of interface state density using three different methods show the consistent results. The MOS capacitor with HfO2(3nm)/Al2O3(2nm) stack gate dielectric has the lowest interface state density indicating the high quality interface between Al2O3and AlGaN. The interface state density is the highest in MOS capacitor with HfAlO(5nm) or10nm Al2O3gate dielectric. The interface state density in MOS capacitor with3.5nm Al2O3gate dielectric is almost the same as that in HfO2(3nm)/Al2O3(2nm) stack gate dielectric. Finally, the effect of heat treatment on the characteristics of AlGaN/GaN MOS capacitor with high K dielectric has been studied. The high temperature performance of HfAlO dielectric has been improved by the incorporation of Al in the HfO2according to heat treatment. The heat treatment has been optimized to have good effect on the saturation capacitor, hysteresis, flatband voltage, leakage current and sheet resistance.Then, we report on a AlGaN/GaN Metal Oxide Semiconductor High Electron Mobility Transistor (MOS-HEMT) using atomic-layer deposited (ALD) Al2O3as the gate dielectric. By decreasing the thickness of the gate oxide to3.5nm, a device with maximum transconductance of130mS/mm is produced. The drain current of this1μm gate-length MOS-HEMT can reach720mA/mm at+3.0V gate bias. The unity current gain cutoff frequency (fT) and maximum frequency of oscillation (fMAX) are obtained as10.1GHz and30.8GHz,respectively.The breakdown voltage of the MOS-HEMT has been studied by Drain Current Injection Technique (DCIT). The VDS breakdown voltage of the MOS-HEMT with3.5nm Al2O3and10nm Al2O3as gate dielectric are66V and100V respectively. Drain current collapse in AlGaN/GaN MOS-HEMT has been studied systematically by applying pulsed stress to the device. Low-temperature layer of Al2O3ultrathin film which was used as both gate dielectric and surface passivation layer was deposited by atomic layer deposition (ALD). For HEMT, gate turn-on pulses induced large current collapse. However, for MOS-HEMT, no significant current collapse was found in the gate turn-on pulsing mode with different pulse widths which indicates the good passivation effect of ALD Al2O3. A small increase in Id in the drain pulsing mode is due to the relieving of self-heating effect. The comparison of synchronously dynamic pulsed Id-Vds characteristics of HEMT and MOS-HEMT further demonstrated the good passivation effect of ALD Al2O3.Finally, we have developed a novel AlGaN/GaN MOS-HEMT using stack gate3structure grown by atomic layer deposition (ALD). The stack gate consists of a thin HfO2(3nm) gate dielectric and a thin Al2O3(2nm) interfacial passivation layer (IPL). For the5nm stack gate, no measurable C-V hysteresis and smaller threshold voltage shift were observed, indicating that a high quality interface can be achieved using a Al2O3IPL on AlGaN substrate. Good surface passivation effects of the Al2O3IPL have also been confirmed by pulsed gate measurements. Devices with1-μm gate lengths exhibit a cutoff frequency (fT) of12GHz and a maximum frequency of oscillation (fMAX) of34GHz, as well as a maximum drain current of800mA/mm and a peak transconductance of150mS/mm,while the gate leakage current is at least six orders of magnitude lower than that of the reference HEMT at positive gate bias. We have also developed a novel AlGaN/GaN MOS-HEMT using HfAlO(5nm) dielectric grown by atomic layer deposition (ALD) by the incorporation of Al in the HfO2to improve the crystallization temperature for high temperature application. Devices with1-μm gate lengths exhibit an cutoff frequency (fT) of7.8GHz and a maximum frequency of oscillation (fMAX) of20GHz.