Gallium nitride (GaN) has great potential applications in high-power and high-frequency electrical devices due to its superior physical properties.High dislocation density of GaN grown on a foreign substrate leads to poor crystal quality and device reliability.The homo-epitaxial growth of GaN material has low dislocation density,which is the foundation of high performance of AlGaN/GaN highelectronic mobility transistor.However,it is difficult to prepare flat surface of GaN template or GaN substrate in thermal treatment process under the metal-organic chemical vapor deposition (MOCVD) ambient condition in which hydrogen (H2) is commonly used to clean the substrate surface,i.e.,to remove impurities from the substrate surface,since H2 would greatly enhance GaN decomposition in MOCVD high-temperature condition and etch GaN into roughness surfaceIn this work,an alternation gas model of ammonia/hydrogen (NH3/H2) mixed gas and H2 gas is designed.This technique is used in a thermal treatment process of GaN template and substrate by MOCVD.Then,we in-situ grow AlGaN/GaN HEMTs (high electron mobility transistors) on GaN template and GaN substrate,respectively.A series of alternation gas samples with various H2 treatment times is investigated.Optical microscope and atomic force microscope are used to observe the morphologies of GaN template and AlGaN/GaN HEMTs and two-dimensional electron gas (2DEG) mobility and density of AlGaN/GaN HEMTs are measured by contactless Hall measurement.Optical properties of AlGaN/GaN HEMTs are analyzed by photoluminescence at room temperature.The residual impurities of C and O in the GaN epilayer and the interfacial region between GaN epilayer and GaN substrate are analyzed by secondary ion mass spectrometry.The study results show that H2 enhances GaN decomposition in MOCVD at high temperature,and GaN decomposition greatly strengthens with H2 treatment time increasing leading to rough surface and the decrease of 2DEG mobility.The NH3/2 mixed gas could suppress GaN decomposition and avoid roughn surface,but go against cleaning out the purity from GaN surface,and the relativive intensity of the yellow band is higher.The NH3/2 mixed gas and 2 gas alternate thermal treatment model with proper 2 treatment time on GaN template or GaN substrate,not only obtains atomically flat surface of GaN template and HEMT structure,but also cleans out the purity from GaN surface,which is conducive to the increase of the electric properties of HEMT material.The highest 2DEG mobility reaches to 2136 cm2/V·s with 1 min 2 treatment in the alternate gas thermal treatment process grown on GaN templates and the electrical properties of HEMT material turn excellent.Finally,an alternate model with 5 min NH3/2 mixed gas followed by 1 min 2 and then 4 min mixed gas of thermal treatment process is used,the surface morphology of HEMT grown on GaN substrate shows highly uniform atomically steps and the root-mean-square value is 0.126 nm for 2 μm×2 μm scan area;the HEMT 2DEG mobility 2113 cm2/V·s grown on GaN substrate shows good electric properties,the residual impurities of C and O in the interfacial region between GaN epilayer and GaN substrates are below 1×1017 cm-3,showing clean interfacial.