MOCVD growth of gallium nitride and fabrication of aluminum gallium nitride/gallium nitride double heterostructure LED
Metalorganic chemical vapor deposition of gallium nitride on sapphire at both atmospheric pressure (760 Torr) and low pressure (100 Torr) using ammonia (NH3), dimethylhydrazine (DMHz) and tertiarybutylamine (t-BNH2) was investigated. Growth quality was characterized using a scanning electron microscope (SEM), double crystal Xray diffraction (DCXRD), photoluminescence (PL), and Hall measurement. High structural and optical quality with uniform and smooth surface morphology could only be achieved using ammonia at low pressure with a typical DCXRD full width at half maximum (FWHM) of 300 arcseconds. Growth using ammonia at atmospheric pressure led to a rough surface with high-density hexagonal defects. A fully coalesced surface was observed after the growth parameter optimization in the GaN growth using dimethylhydrazine under low pressure. But PL and DCXRD indicate the crystal quality is far bellow expected. Tertiarybutylamine was also used as a nitrogen source in the low-pressure growth, however surface analysis shows only carbon deposition.
Silicon-doped and magnesium-doped GaN and AlGaN growths were performed at low pressure, and the doping level was calibrated by Hall measurement and secondary ion mass spectrometry (SIMS). The aluminum composition in AlGaN is about 15%, calibrated by PL, DCXRD and X-ray photoelectron spectroscopy (XPS). The highest n-type doping level in both GaN and AlGaN is up to 1 × 1019/cm3, while the highest p-type doping level in GaN and AlGaN are up to 1 × 1018/cm3 and 1 × 1017/cm3 respectively. P-type GaN was also achieved using a newly developed source - solution bis(cyclopentadienyl)magnesium (Cp2Mg). Comparison of PL spectra and DCXRD rocking curves shows similar results in the p-type GaN using solution and solid Cp2Mg source.
A simple p-n junction light emitting diode (LED) structure was grown under the optimized growth parameters. A mesa structure was fabricated using a reactive ion etching (IRE). PL measurement shows a broad strong emission at 430 nm, while electroluminescence (EL) measurement shows an additional emission at 570 nm with a higher intensity than that of the 430 nm emission. The turn-on voltage is about 6–7volts.
An AlGaN/GaN/AlGaN double heterojunction LED structure was developed. The active GaN region is either magnesium doped or silicon and magnesium co-doped. The effect of the magnesium-doped AlGaN cladding layer on the radiative emission from the active layer was investigated using PL measurement. It is found that p-type doping level in the AlGaN cladding layer must be high enough in order to avoid the luminescence absorption. After the growth optimization, PL spectrum shows the expected strong blue emission centered at 430nm.