Based On Pretreatment Of The Substrate And Gan Epitaxial Growth And Device Research

As already known, the conventional epitaxial lateral overgrowth (ELO)technology requires a relatively complicated procedure, and the mask is used in itgenerally, moreover, interaction of the wings with the mask underneath is commonlyobserved in ELO structures. This phenomenon leads to a downward tilt of the ELOwings and to creation of low angle boundaries over the mask edges as well as causingproblems for defect-free coalescence of tilted wings grown from adjacent seeds. In thepresent dissertation, high brightness GaN-based blue light-emitting diodes (LEDs)have been successfully grown on the c-plane specially treated surface of the sapphiresubstrate by metal-organic chemical vapor-phase deposition (MOCVD) in order toovercome those problems existing in the conventional ELO technique. It is found thatthe crystallographic tilt in the conventional ELO is caused by the plastic deformationresulting from the bending of threading dislocation and the elasticity deformationrelated with the GaN/SiNx interfacial forces. In the present technology, there is noinsertion of mask materials for selective growth such as SiN_x masks, the elasticitydeformation can be avoided. Compared with the conventional ELO technique, thetechnology is much simpler, the crystallographic tilt can also be greatly decreased. Thecrystal and optical quality of GaN epilayer and the performance of LED have beeninvestigated intensively. The main contents of the dissertation are as followings.1. The theory and technology related with the dissertation have been set forth.Especially, the optical principle of laser reflectance real-time monitoring system wasexpounded. 2. The effect factors of surface treated to the sapphire substrate have been discussed.Comparing the results which were obtained when the etching temperature and etchingtime varied, the dislocation could be displayed very clearly and accurately when theetch solution with KOH solvent at 280℃lasting for 30min-60min. The surface treatedto the sapphire substrate had less effect on the crystal and optical quality of it.3. The optical quality of GaN epilayer has been characterized by spectroscopicellipsometry (SE), both the thickness and refractive index dispersion relationship ofthe film were obtained by fitting the experimental data. Based on the refractive indexdispersion relationship and the transmission spectra of GaN film, the thickness of GaNepilayer was also calculated. The two values of the thickness obtained by these twodifferent methods were in good agreement.4. The mechanism of low dislocation density and high quality of GaN epilayer grownon the treated surface of the sapphire substrate has been dissertated. The vapor phaseepitaxy starts from the regions where there is not etching pits, the two wings coalesceon the etch pits to grow a uniform and even film laterally by varying the epitaxyconditions. The etch pits can slacken part lattice mismatch between GaN epilayer andsapphire substrate, and part of dislocation bending in the consequent process of lateralgrowth renders it not to reach GaN film surface, so the dislocation density can bedecreased drastically. The hollow structure of etch pits can partly release stress of GaNfilms to improve the quality of GaN films.5. The characters of the GaN film were analyzed by double crystal X-ray diffraction(DC-XRD), scanning electron microscope (SEM), atomic force microscope (AFM), SE,Photoluminescence (PL) and transmission spectra in order to investigate the effect ofsurface treated to the sapphire substrate on the crystal and optical quality of GaNepilayer. These results indicated that the epilayer grown on treated surface of sapphiresubstrate exhibited excellent crystal and optical quality, the DC-XRD FWHMs of theGaN film in (0002) plane and (10-12) plane were as low as 208.80arcsec and320.76acrsec, respectively, the dislocation density was reduced down 5.5×10⁷ cm^-2, the yellow luminescence (YL) was nearly invisible in the PL spectrum, and the FWHM ofPL was also only 8.2nm, the higher transmission ratio and the greater modulation depthcan be shown in the transmission spectrum.6. The effect of surface treated time for the substrate on the GaN epilayer has beenstudied. With the increase of etch time, the quality of GaN epilayer improved, butwhen the etch time was prolonged to some extent. The epilayer quality deterioratedinstead. The quality of GaN films grown on sapphire substrate that was chemicallyetched for 50 minutes at 280℃was the best, the DC-XRD FWHMs of the GaN film in(0002) plane and (10-12) plane were as low as 202.68arcsec and 300.24acrsec,respectively, the dislocation density was reduced down 3.1×10⁷ cm^-2, and the theFWHM of PL was decreased to 6.7nm.Based on the above studies, high performance blue LEDs were prepared on thesurface treated sapphire substrate. At the forward injection current of 20mA, the outputoptical power was higher than 6roW, with peak wavelength of 455-460nm. The reverseleakage current was less than 0.05μA at the reverse voltage of 5V.