| GaN has excellent properties such as high electron mobility and high critical breakdown field,making it widely used in military,satellite communications,and electronic warfare.Therefore,they are currently the technology that countries are competing to develop ahead of one another.Currently,research on GaN HEMT device for high-frequency bands is still in its infancy.A further development is to reduce the parasitic capacitance and resistance of devices.In the current study by using n+-GaN as source and drain electrodes could reduce the resistance.One of these methods is the selective-area regrown(SAG)GaN.In this experiment,the SAG n+-GaN film was grown on HEMT epitaxial wafer by MOCVD.The main research content includes:1、Under the condition of using SiO2 as a mask,GaN cannot nucleate and grow on the SiO2 mask,which leads to the migration of precursors into the epitaxial window.This causes the epitaxial rate to be uncontrollable and surface morphology degradation.The experiment shows that as the thickness of the mask decreases,the surface morphology deteriorates and the epitaxial rate increases.In addition,the design of the epitaxial window also affects the epitaxy.The window-to-space ratio has the greatest impact on the window effect.When the ratio is small,there are enough precursors on the mask to migrate into the window,which increases the precursor concentration and makes the epitaxial rate uncontrollable2、The effects of epitaxial parameters such as Ⅴ/Ⅲ ratio,reaction chamber pressure,epitaxial temperature,and carrier gas on the epitaxial quality of SAG GaN films were explored.It is found that the factors that affect the epitaxial quality the most are the reaction chamber pressure and the carrier gas.When the pressure increases,the diffusion length of precursors becomes shorter,which suppresses the coalescence of atomic clusters on the surface of GaN film,leading to surface degradation.When N2 is used as a carrier gas,GaN can nucleate and grow on the mask surface,successfully consuming precursors and suppressing the window effect because the decomposition rate of GaN in N2 atmosphere is much slower than that in H2 atmosphere.3、To reduce the precursor migration on the mask,an AlN-SiO2 mask was proposed.By sputtering a layer of AlN on the SiO2 surface,GaN was successfully nucleated on the mask surface,which reduced the migration of the precursor.The surface roughness and epitaxial growth rate of GaN films were optimized,so that SAG GaN showed twodimensional growth with a surface roughness of only 0.35 nm.Samples using N2 as the carrier gas had a surface roughness of 0.55 nm.The epitaxial growth rates between different epitaxial windows were also relatively close.The growth rate ratio of the 30μm × 50 μm window to the 100 μm ×100 μm window was 1.12,effectively weakening the "window effect". |
