The Influence Of GaN Nucleation Layer Process On GaN-based Epitaxial Films Grown By MOCVD

The research of group III nitride alloys and their application have become avery hot research fieldin the semiconductor industry，As a representative of thethird generation of semiconductor materials, GaN-based semiconductormaterials has a great development in the high temperature, high frequency, highpower and microwave photonics field，a large number of studies have shownthat direct and large band gap semiconductor materials，such as GaN, still have abroad space and prospects for development. However, due to GaN high meltingpoint, great vapor pressure equilibrium, the productionof GaN material isextremely difficult and very costly, which also makes the GaN epitaxial growthmainly in heterogeneous substrate (e.g. sapphire) carried on, But large latticemismatch and thermal heterogeneity between the substrate and the GaNmismatch lead to the GaN epitaxial layer dislocation density is large (about108-1010cm-2), relatively poor crystal quality.These high density of dislocations serious impact on the crystal quality and the optical and electrical properties ofsemiconductor materials.In order to improve the quality of the GaN epitaxialfilm growth, In this paper, the nucleation layer growth process as a startingpoint,Through optimized the temperature and ammonia flow two processparameters during nucleation layer growth to improve the crystal quality of theGaN epitaxial film，the growth mechanism of nucleation layer and the influencemechanism to upper layer of undoped GaN epitaxial films was furtherdiscussion, the main results are as follows:Firstly, this paper use the metal-organic chemical vapor deposition(MOCVD) devices grown five groups of GaN epitaxial film samples withdifferent nucleation layer growth temperature (respectively610℃,630℃,650℃,670℃,690℃) anddetected them by in-situ monitoring, AFM, HRXRD, PLand HALL techniques.Combined with experimental data shows that the growthtemperature of the crystal nucleation layer of GaN films have a majorimpact.Only at a suitable temperature, GaN nucleation layer will formthree-dimensional shape of the nuclear island with a uniform size and moderatedensity after annealing, and then successfully achieve the growth patternstransformation from3D to2D growth mode in the subsequent growth process,finally get high quality GaN epitaxial films.As the nucleation layer growthtemperature of650℃, we get the most compact and smooth GaN epitaxial film,with the lowest dislocation density, the best optical performance, the lowestconcentration carrier concentration and the highest carrier mobility. In addition to temperature, this paper also explores the ammonia flow rateto the impact of low-temperature GaN nucleation layer growth,And the impacton the subsequent high-temperature non-doped GaN crystal quality of epitaxialthin films.In this study,we still use Aixtron MOCVD equipment,by two-stepgrowth method on sapphire substrate grown GaN epitaxial thin filmswith GaNnucleation layer with different ammonia flow rate(respectively A sample400sccm, B sample800sccm, C sample1200sccm, D sample1600sccm, Esample2400sccm).The samples were experimented by HRXRD, PL, AFM andHALL,then we get the results that when the ammonia flow rate duringnucleation layer growth is800sccm,the dislocation density of samples were thelowest, and withthe best the photoelectric properties.