Study On Depth-resolved Characterization Of Dislocations And The Relationship With Point Defects In GaN Material

III-nitride semiconductors including GaN, AlN and InN materials and devices haverecently been attracting great attention for their prominent performance. Because of thehigh melting temperature and low decomposition temperature, the main technique togrow single crystal III-nitride materials is hetero-epitaxy. However, due to the largelattice mismatch and thermal mismatch with the substrate, usually the epitaxial GaNfilms features a high dislocation density, which greatly degrades the performance andreliability of GaN-based devices. The traditional dislocation characterization methodssuch as defect pit-etching, decoration and electron microscopy are devastative to thesamples, while X-ray diffraction method can keep the samples intact. The conventionalX-ray ω Scan, however, can not show any information of the depth-dependent densityof dislocations of different types, which could be of great significance to the growthkinetics, defect formation as well as suppression mechanism of GaN crystal films andGaN devices’ performance and reliability. The current study of depth-dependentdislocations in GaN films with XRD can only get a mixture of mosaic tilt and twistresulted from edge, screw, and mixed dislocations and presents no method to separatethese defect effects efficiently. In this paper, research is carried out on thecharacterization method of depth-dependent dislocations including type and densityinformation in GaN films by XRD, and the relation between edge-type dislocation andpoint defects with the yellow luminescence (YL) as a bound. The main works andresults are listed as following:1. It was given the completely principle model, as well as the detailed measurementscheme and the data processing method, of depth resolved dislocation detection, whichtreated the GaN films as many slices in depth, and measured dislocations by ω-Scan ofXRD slice by slice from top to bottom.2. It was proposed that four principles should be met to realize the measurement ofscrew-type and edge-type dislocations respectively in consideration of the relationshipsbetween tilt/twist and screw/edge-type dislocations in mosaic structure GaN films. Thesuccess in measurement implies a breakthrough in separating tilt from twist with depthresolved XRD method. In the mean time we extrapolated this method to othersmaterials in a general way. It was found that the best atomic plane to investigate screwdislocation was (103) plane in mosaic GaN film, and for edge dislocation was (101) plane. Furthermore, the edge dislocation coefficient of (101) plane was deduced to be1.0961.3. It was analyzed the dislocation movement of GaN films with AlN interlayergrown on sapphire substrate by using depth resolved dislocation detection method. Itwas found that both screw dislocation and edge dislocation show highest density nearthe nuclear layer, and would experience a great declination within a range of about150nm above the nuclear layer. It was also demonstrated that although AlN interlayerwould introduce more edge dislocations in vicinity, it promoted the annihilation of edgedislocations efficiently on the whole, and was an efficient declination factor for screwdislocation.4. It was proposed a depth resolved strain analysis method based on2θ ωscanof XRD. Depth resolved strain measurement on GaN film grown on sapphire with AlNinterlayer indicated that the GaN film sustained compressive strain, and there was astrong correlation between the variation of strain and the decrease of dislocation, whichaccounted for that the dislocation generation and annihilation would change the latticeconstant to a certain extent. Finally we confirmed our analysis with Raman scatteringexperiment.5. It was given the information of intrinsic point defect in GaN films grown byMOCVD system by first principle calculation method. It was found that in GaN grownin (near) N rich atmosphere, the intrinsic point defects most easily to form are the Gavacancy or Ga vacancy related point defects, which may be a main factor that cause theyellow luminescence if exists.6. It was found that that there was a strong correlation between the ratio of YL toband edge emission peak (I YL/I BE) and edge-type dislocation density within top150nm of a series of GaN samples. The analysis concluded that the elastic strain field andthe coulomb potential field produced by edge-type dislocation could strongly attractpoint defects, so high density of edge-type dislocations corresponded to high I YL/I BE.