Research On Al-rich AlGaN Based High-Voltage Power Devices

By adjusting Al mole fraction,the bandgap（E_g）of Al_xGa_1-xN ranges from 3.4 e V of Ga N to6.2 e V of AlN.Therefore,the critical electric field（E_c）of AlGaN material lies between～3MV/cm and～16 MV/cm by considering the E_critical=1.73×10⁵×（E_g）^2.5 for direct-gap semiconductors.Baliga’s figure-of-merit for Al-rich AlGaN material is higher than most semiconductor materials.When power devices have the same breakdown voltage,Al-rich AlGaN power devices theoretically show lower on-resistance and higher energy conversion efficiency against other devices based on semiconductors with lower Baliga’s figure-of-merit.Al-rich AlGaN power device is especially suitable for high-voltage and high-power applications.In addition,the carrier transport properties in AlGaN material are stable in extremely high temperature environment,which enables the thermal stability of AlGaN device’s electrical property.As a result,Al_xGa_1-xN power device is very suitable for extreme temperature operation,which further expands the application field of Al_xGa_1-xN power devices.Recently,Al-rich AlGaN based materials and devices have been widely investigated due to their high thermal conductivity and ultra-wide bandgap properties.Al-rich AlGaN is a kind of ultra-wide bandgap semiconductor and it is still in the early stage of development.The development of AlGaN material lags behind other ultra-wide bandgap semiconductors such as Ga₂O₃.Currently,due to the immature epitaxial growth and doping technique,when Al mole fraction is higher than 0.8 in Al_xGa_1-xN,n-type donor impurity no longer exhibits low activation energy.The ionization ratio of donor impurity is quite low and the conductivity becomes unfavorable for AlGaN device.For above reasons,our dissertation focuses on Al_xGa_1-xN materials with Al mole fraction no higher than 0.8.We designed Al_xGa_1-xN wafers with two different Al mole fracion of 0.8 and 0.6 respectively.We fabricated multiple high-voltage power devices on the AlGaN wafers.These devices have been systematically studied.The specific research results are described as follows:1.SIMS and XRD were used to characterize Al-rich AlGaN wafers.The wafer consisted of sapphire substrate,AlN buffer layer,Al_xGa_1-xN channel layer and（Al）Ga N cap layer from below to top.It was observed with SIMS that the intensity of aluminum element was reversely graded from the surface of Al_xGa_1-xN channel layer to the surface of（Al）Ga N cap layer.The Silvaco TCAD simulation was also employed to show that the reverse graded（Al）Ga N cap layer could eliminate the bandgap barrier between Al rich Al_xGa_1-xN channel layer and heavily-doped（Al）Ga N cap layer.Ohmic contact of AlGaN device was made on heavily-doped（Al）Ga N cap layer instead of AlGaN channel layer,which overcame the difficulty of making ohmic contact on AlGaN material.The Al mole fraction of Al_xGa_1-xN channel layer was accurately calculated by XRD.It was confirmed that the nominal value of Al mole fraction is consistent with the actual measurement result.The characterization of Al-rich AlGaN wafer laid foundation for device analysis in following content.2.Lateral Al_xGa_1-xN Schottky barrier diode was fabricated and analysed.Lateral Al_0.8Ga_0.2N and Al_0.6Ga_0.4N schottky barrier diodes with field-plate structure were fabricated on Al_0.8Ga_0.2N and Al_0.6Ga_0.4N wafers respectively.And electrical characteristics were measured.According to Silvaco TCAD simulation,the field-plate structure effectively alleviated the peak electric field in AlGaN channel layer and improved the device breakdown voltage.The device breakdown measurement shows that anode field-plate can help to increase breakdown voltage by 1 k V.When anode-cathode spacing is 20μm,the breakdown voltage of schottky barrier diode exceeds 3 k V and the average electric field strength exceeds1.5 MV/cm.The Al_0.8Ga_0.2N and Al_0.6Ga_0.4N schottky barrier diodes achieve power figure-of-merit of 55 MW/cm² and 70 MW/cm²,respectively.The power figure-of-merit value is the highest value among all Al-rich AlGaN schottky barrier diodes.Al_0.8Ga_0.2N and Al_0.6Ga_0.4N schottky barrier diodes also exhibited stable forward characteristics at high temperature.The on/off ratio of Al_0.8Ga_0.2N schottky barrier diode was 10⁶ even at 275℃.3.Lateral Al_xGa_1-xN depletion mode MOSFET was fabricated and analysed.Al₂O₃/Hf O₂bilayer was used as gate dielectric in Al_xGa_1-xN MOSFET.The wide bandgap of Al₂O₃ensured the bandgap alignment between Al₂O₃ and Al_xGa_1-xN.The high dielectric constant of Hf O₂ improved gate control capability and supressed gate leakage.When gate-drain length is 25μm,the breakdown voltage of Al_0.8Ga_0.2N MOSFET exceeds 3 k V,and the breakdown voltage of Al_0.6Ga_0.4N MOSFET reaches 2.8 k V.When gate-drain length is 2μm,the average electric field strength of Al_0.8Ga_0.2N MOSFET and Al_0.6Ga_0.4N MOSFET reaches3.4 MV/cm and 2.8 MV/cm,respectively.The electric field strength of 3 MV/cm is one of the highest values among all Al-rich AlGaN transistors.The depletion-mode Al_0.8Ga_0.2N and Al_0.6Ga_0.4N MOSFETs achieve power figure-of-merit of 38 MW/cm² and 55 MW/cm²,respectively.The high-performance of Al_0.8Ga_0.2N MOSFET and Al_0.6Ga_0.4N MOSFET show great promise for future ultra-wide bandgap power electronics.4.Characteristics of Al₂O₃/Al_xGa_1-xN interface were analysed.MOS capacitors were fabricated on Al_0.8Ga_0.2N and Al_0.6Ga_0.4N wafers respectively and the interface characteristics of Al₂O₃/Al_xGa_1-xN with different Al mole fraction were studied.By measuring and analysing capacitance-voltage curves of MOS capacitor,the trap density of interface trap with shallow energy level and border trap with deep energy level was calculated.The interface trap density with shallow energy level in Al₂O₃/Al_0.8Ga_0.2N is 2-3×10¹² cm^-2,while the density of border trap with deep energy level reaches 5×10¹³ cm^-2.By contrast,the density of border trap with deep energy level in Al_0.6Ga_0.4N MOS capacitor is only 8.3×10¹⁰ cm^-2.The trap density of 8.3×10¹⁰ cm^-2 only comprises 1.1%of the total detectable trap density in Al_0.6Ga_0.4N MOS capacitor.Interface trap with short emission time is in the majority of total detectable trap.The analysis of Al₂O₃/Al_xGa_1-xN interface characteristics plays an important role in promoting development of AlGaN MOSFET with high Al mole fraction.5.Lateral Al_0.6Ga_0.4N enhancement mode MOSFET was fabricated and analysed.As the trap density at Al₂O₃/Al_0.6Ga_0.4N interface was relatively lower,Al_0.6Ga_0.4N wafer was used to fabricate enhancement mode AlGaN MOSFET for better electrical performance.Recessed-gate and Hf_0.5Zr_0.5O₂ charge storage gate was employed to achieve enhancement mode operation.Hf_0.5Zr_0.5O₂ film deposited by ALD shows ferroelectricity after being annealed at500℃.The remnant polarization value of Hf_0.5Zr_0.5O₂ film is 34μC/cm².A positive voltage pulse was applied to the gate electrode for device initialization.Polarization can be induced by the voltage pulse.The polarization induced trapped electrons in the gate dielectric which were regarded as fixed negative charges in the gate stack.Al_0.6Ga_0.4N channel was completely depleted by the fixed negative charges.The threshold voltage of the MOSFET reaches 3.3 V.The average breakdown electric field strength is 4 MV/cm,which is the highest value among all Al-rich AlGaN transistors.The Al_0.6Ga_0.4N enhancement mode MOSFET achieves power figure-of-merit higher than 179 MW/cm²,which shows a conspicuous superiority of high breakdown voltage and comparable state-of-the-art PFo M compared to other enhancement mode MOSFETs with ultra-wide bandgap materials.The gate field-plate structure in MOSFET effectively alleviated electric field strength near the gate area,which helped to avoid the failure of charge storage gate under high voltage stress.The gate field-plate improved threshold voltage stability of the enhancement mode MOSFET with Hf_0.5Zr_0.5O₂ charge storage gate.