Microstructures of GaN films grown by low pressure metal-organic vapor phase epitaxy on sapphire substrates (original) (raw)
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Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 1995
Transmission electron microscopy was used for the characterization of GaN epitaxial layers grown by molecular-beam epitaxy on two different substrates: sapphire ͑Al 2 O 3 ͒ and 6H-SiC. GaN layers grown on both substrates crystallize with the wurtzite structure. Despite the very different lattice mismatch associated with their two substrates, similar types of defects were formed in the GaN layer; only their density differed. In addition to small-angle subgrain boundaries two other types of defects were seen in cross-sectioned samples: defects parallel to the growth surface and microtwins with a width of about 8 -10 nm perpendicular to the growth surface. The parallel defects were identified as stacking faults leading to a local fcc atom arrangement in the layer and are believed to be growth defects. The density of these faults decreased with layer thickness. However, the density of the vertical microtwins remained constant through the layer. Slight local lattice twists between the microtwins and surrounding areas or differences of stoichiometry are suggested as an explanation for the observed contrast of the high-resolution images.
Surface and Coatings Technology, 2006
A comparative study of GaN grown on sapphire by metalorganic chemical vapor deposition (MOCVD) is performed by transmission electron microscopy (TEM), atomic force microscopy (AFM) and infrared reflectance. A correlation between the microstructure features revealed by TEM/AFM and optical characteristics obtained from infrared reflectance is explored. TEM observations reveal the GaN epilayers with high densities of threading dislocations. Dislocations in the undoped GaN tend to form open core structures, while dislocation lines in the Si-doped GaN are sharper and the strain contrast is much more discrete. Also the GaN buffer layer grown at low temperature is found to transform into the thermodynamically stable wurtzite structure during high temperature post-buffer GaN growth. The infrared reflectance shows the corresponding behavior. The interference fringes of the Si doped sample were observed with the reflectance reduction and contrast damping, which can be interpreted by the presence of a transition/defect layer near the interface of GaN/ sapphire.
physica status solidi (c), 2007
In this study, we grew non-polar a-plane GaN thin films on r-plane sapphire using a series of growth conditions by metal-organic chemical vapor deposition. The results showed that high temperature and lowpressure conditions benefited two-dimension growth could lead to a fully coalesced a-plane GaN layer with a very smooth surface. The best surface morphology with an excellent mean roughness of 10.5 Å was obtained. The different thickness AlN as a nucleation layer and the different δ/β ratio were also considered. The results revealed that the surface morphology would get worse when the thickness of nucleation layer and δ/β ratio were away from the values of optimal condition. The observation of transmission electronic microscopy shown the lowest density of threading dislocations was 1.85×10 10 /cm 2 .
High resolution X-ray diffraction and X-ray topography study of GaN on sapphire
Materials Science and Engineering: B, 1999
High resolution X-ray diffraction and X-ray topography study of GaN thin films, grown on sapphire (11.0) substrate by reduced pressure metalorganic vapor phase epitaxy (MOVPE) under various conditions, were performed. The strained lattice parameters, stress, misorientation and dislocation density of GaN films were estimated. The experimental stress compares well with the theoretical stress obtained from the difference in the thermal expansion coefficient between the film and substrate. The dislocation density was found to be highest in the thinner GaN film. It was also higher in the film without any buffer layer. For the same carrier concentration, the mobility of one of the film was lower than the other which could be due to the presence of higher dislocation density. Slip lines associated with dislocations, stacking faults, cellular structure of dislocations and double positioning boundaries were found in the X-ray topography from the GaN films.
Journal of Crystal Growth, 2008
This study reports on the reduction of dislocations in the GaN grown on the stripe-patterned r-plane sapphire substrates via metalorganic chemical vapor deposition (MOCVD). The stripes oriented along the sapphire [1 1 2 0] direction with asymmetrical side faces were fabricated by lithographic and wet-etching processes. The two etching sides of sapphire-striped mesa are {0 0 0 1} and {1 1 0 1} faces. GaN grown on both etching facets exhibits different epitaxial relationships with the sapphire substrate. The GaN grown from the {0 0 0 1} side face of the sapphire mesa contains a low-dislocation density in the order of 10 7 cm À2 . The interfacial regions between the GaN and patterned sapphire substrate are also studied to clarify the behavior of GaN epitaxial growth on the inclined sapphire faces and defect-reduction mechanism. r
Journal of Crystal Growth, 2010
The present study focused on the effect of an intermediate-temperature (IT; $ 900 1C) buffer layer on GaN films, grown on an AlN/sapphire template by hydride vapor phase epitaxy (HVPE). In this paper, the surface morphology, structural quality, residual strain, and luminescence properties are discussed in terms of the effect of the buffer layer. The GaN film with an IT-buffer revealed a relatively lower screw-dislocation density (3.29 Â 10 7 cm À 2) and a higher edge-dislocation density (8.157 Â 10 9 cm À 2) than the GaN film without an IT-buffer. Moreover, the IT-buffer reduced the residual strain and improved the luminescence. We found that the IT-buffer played an important role in the reduction of residual strain and screw-dislocation density in the overgrown layer through the generation of edge-type dislocations and the spontaneous treatment of the threading dislocation by interrupting the growth and increasing the temperature.