Study On Photoluminescence Spectra And Threading Dislocation Properties Of GaN

Gallium Nitride (GaN) is a direct and wide-band-gap semiconductor material with good optical and electrical properties, which presents extensive potential application in realizing blue to ultraviolet light-emitting optoelectronic devices and high power microwave devices. Deep level emission induced by defects and impurities in GaN based optoelectronic devices can reduce the efficiency of radiation recombination transition from valence to conduction band. Most previous studies on Photoluminescence (PL) properties of GaN thin films have been performed. However the PL spectra of different samples are dissimilar and the explanations of the origins of deep level emission in GaN are still disputed. It is necessary to study more on PL properties of GaN.This thesis presents an experimental investigation of the selected GaN samples grown by metal organic vapor phase epitaxy (MOVPE). The PL spectra excited separately by four different excitation light sources. The correlation of the PL spectra and excitation light sources are investigated. It has been found that the yellow luminescence band (YLB) appears when continuous wave Xe lamp and He-Cd laser sources are used. The central wavelength of wider YLB is located at near 550 nm. The YLB tends to disappear when He-Cd and YAG pulse wave sources are employed, and the main peak of the band edge emission is observed at 365nm. This phenomenon is attributed to impurity band saturation when the samples were excited by the pulse wave light sources with the high power density. It is concluded that the yellow luminescence evidently depends on the excited light source. This finding may throw some new light on the origins of the deep level emission in GaN.The deep level luminescence in GaN, especially yellow band is directly related to the intrinsic dislocation defects in the materials. The GaN heteroepitaxial layers are commonly grown on a sapphire substrate. However, since sapphire and GaN have poor matching in the lattice parameter, a high density (108-1010 cm-2) of dislocations exist in the GaN epitaxial layers, and such dislocations threading to the surface of GaN act as nonradiative centers and light scattering centers to reducing radiation efficiency of the material. It has been reported that only screw and mixed threading dislocations (TDs) act as nonradiative centers, while further researches have showed that part of edge TDs also have nonradiative recombination effects on the nonequilibrium minority carrier. In a word the correlation between types of TDs and nonradiative centers has not been identified yet. Therefore, it is merit to understand the mechanism of YLB by investigating the properties of threading dislocations in GaN.In this thesis, atomic force microscopy (AFM) is applied to obtain a map of TDs in marked region in GaN growth by metal-organic chemical vapor deposition (MOCVD). Radius and depth of every etch pits are measured respectively before and after wet-chemical etching. The location of pits on the as-grown surface, the structure of the etch pits and the etching rate are analyzed to identify the type of the TDs.It is found that hot phosphoric acid (H3PO4) etching of as-grown surface resultes in formation of etch pits. And there is a corresponding relationship between the radius and the depth of the etch pits. The etch pits with larger radiuses also have larger depth, and the smaller radiuses of etch pits, the smaller depth of them too. There are etch pits in two distinct size grades, the percentage of the smaller etch pits and the larger etch pits are 52.7%, and 47.3%, respectively. The smaller etch pits located within the terraces between steps on the as-grown surface are associated with edge-type TDs, and the larger etch pits are positioned at surface-step termination before etched, corresponding to the surface terminations of mixed or pure-screw TDs. Structure of the same etch pits are measured respectively before and after wet-chemical etching. And it is found that the structure of the etch pit changes from one form into another with etching time from 10min to 20min, which implies the relations of the types of TDs and the etch pits structures are unconvincing . Moreover,the experimental data show that the radius etching rate of two types of TDs are 10 times faster than their depth etching rate, which validated the reports of that the rate of the horizontal movement of steps is greatly exceed the vertical etching rate. This well interprets the phenomenon of the sweeping of small pits formed on edge TDs after long time etching. Because the depthes of the edge TDs after long time etching are too small to be measured by AFM. Subsequently, mappings of TDs and nonradiative centers are given by scanning electron microscopy (SEM) and Cathodoluminescence (CL) panchromatic microscopy. Contrasted the SEM image with the CL image, it is found that the etch pits density of the sample etched 80min determined from SEM is in close agreement with the density measured by AFM, while the nonradiative centers density is 2 times more than this etch pits density. This phenomenon indicated that after long time etching the edge-type TDs (half part of the TDs) are almost invisible by AFM and SEM. But as TDs, they exist in GaN epitaxial layers all the same, which appear in the CL image. It was demonstrated that the edge and the mixed/screw TDs are all act as nonradiative centers. The results lay a foundation for intensive study of the optical properties in a signle region of different type dislocation.