Influence Of GaN Nucleation Layer Growth Onditions On GaN-based Epitaxial Films Rows By Metal Orgainc Chemical Apor Deposition

GaN-based semiconductor materials have potential applications in highbrightness LEDs, short wavelength lasers, ultraviolet detectors, high temperatureand high power electronic devices, due to their wide direct bandgap, highsaturated electron drift velocity, small dielectric constant, good thermalperformance and high temperature resistance, anti high electric field, acid alkalicorrosion resistance and excellent chemical stability etc. In this paper, we growhigher quality GaN using a two-step growth method on MOCVD, evaluate boththe electronic and material properties of the crystal, and propose physicalmechanism. At the same time, the influence of NH3flow when low temperatureNL anneal on the crystal quality and optical properties of GaN film isinvestigated. The main results are as follows:In this paper, the influence of the low-temperature (LT) GaN nucleation layergrowth time on the crystal quality and optical properties of GaN film isinvestigated. It finds that the optimal LT nucleation layer growth time caneffectively reduce the crystal defects and is favorable to forming the annihilationof dislocations. GaN films are grown on c-plane sapphire substrates byMetal-organic Chemical Vapor Deposition (MOCVD). Crystal quality andoptical properties are characterized by atomic force microscopy (AFM),scanning electron microscopy (SEM), high-resolution X-ray Diffraction(HRXRD) and photoluminescence (PL) spectra, respectively. In AFM images,the island density decreases as the growth time increases, while the size of islands becomes larger and the uniformity of island size deteriorates as thegrowth time increases, leading to the phenomenon that the number ofinterfaces, which form during the nucleation island coalescence, first increaseand then decrease as detected by SEM. The changes to the interfaces result inthe same variation trends of screw and edge dislocation densities as measured byHRXRD. As the nucleation time increases, the ratio of the band edge emissionpeak intensity and the yellow luminescence peak intensity tested byphotoluminescence has the same change trend which is confirmed by HRXRD.It is show that GaN islands with different size and density could lead todifferent mechanisms of dislocation evolution，thereby forming a GaN epitaxiallayers with different dislocation density and optical properties. Throughcontrolling the nucleation time, GaN films with the smallest dislocation densitycould be obtained.In this paper, the influence of NH3flow when low temperature NL annealon the crystal quality and optical properties of GaN film is investigated. As NH3flow increases, dislocation density decreases first and then increases, which ischaracterized by AFM，because the GaN NL decomposition rates decrease withincreasing NH3flow. Hall measurements show that the carrier concentrationdecreases first and then increases.The ratio of the yellow luminescence peakintensity and the band edge emission peak intensity tested by photoluminescencehas the same change trend which is confirmed by Hall. It is show that differentNH3flows when low temperature NL anneal could lead to different NLdecomposition rates， thereby forming a GaN epitaxial layers with opticalproperties and photoelectric properties.