Craig G. Moe

Vacancy-oxygen complexes and their optical properties in AlN epitaxial films studied by positron annihilation

Vacancy-type defects in AlN grown by metal-organic vapor phase epitaxy (MOVPE) and lateral epitaxial overgrowth (LEO) using halide vapor phase epitaxy were probed by a monoenergetic positron beam. Doppler broadening spectra of the annihilation radiation were measured and compared to the spectra calculated using the projector augmented-wave method. For MOVPE-AlN, the concentration of vacancy-type defects was high near the interface between AlN and the GaN buffer layer, and the defect-rich region expanded from the interface toward the surface when the NH3 flow rate increased. For the sample grown on the AlN buffer layer, however, the introduction of such defects was suppressed. For LEO-AlN, distinct deep emission peaks at 3–6 eV were observed in cathodoluminescence spectra. From a comparison between Doppler broadening spectra measured for LEO-AlN and computer simulated ones, an origin of the peaks was identified as complexes of Al vacancy (VAl) and oxygen atoms substituting nitrogen sites such as VAl(ON)n (...

Epitaxial Lateral Overgrowth of High Al Composition AlGaN Alloys on Deep Grooved SiC Substrates

Fully coalesced Al0.93Ga0.07N films were demonstrated by metalorganic chemical vapor deposition on deep grooved SiC substrates. Lateral Al0.93Ga0.07N growth was achieved at low V/III ratios during growth. The deep grooves enabled coalescence despite of parasitic growth in the trenches. Dislocation reduction in the overgrown regions of the films was observed by transition electron microscopy and atomic force microscopy.

Milliwatt Power Deep Ultraviolet Light Emitting Diodes Grown on Silicon Carbide

Deep ultraviolet light emitting diode structures with a peak wavelength of 275 nm were grown by metalorganic chemical vapor deposition on (0001) silicon carbide. Despite its strong ultraviolet light absorption, silicon carbide was chosen as a substrate rather than sapphire for its improved thermal conductivity and the potential for vertically conducting devices. An output power of 0.11 mW was observed at 300 mA DC during single device on-wafer testing, and output powers of 2.09 mW at 1.3 A were obtained from a packaged, silicone encapsulated array of five devices. Forward voltages as low as 4.9 V at 20 mA were obtained. The injection profile of Cp2Mg during the p-AlGaN blocking layer was instrumental in the suppression of emission at undesired wavelengths and the realization of peak-to-defect level ratios greater than 100.

Internal efficiency analysis of 280-nm light emitting diodes

Compact ultraviolet light sources are currently of high interest for a range of applications, including solid-state lighting, short-range communication, and bio-chemical detection. We report on the design and analysis of AlGaN-based light-emitting diodes with an emission wavelength near 280 nm. Internal device physics is investigated by three-dimensional numerical simulation. The simulation incorporates a drift-diffusion model for the carrier transport, built-in polarization, the wurtzite energy band-structure of strained quantum wells, as well as radiative and nonradiative carrier recombination. Critical material parameters are identified and their impact on the simulation results is investigated. Limitations of the internal quantum efficiency by electron leakage and nonradiative recombination are analyzed. Increasing the stopper layer bandgap is predicted to improve the quantum efficiency and the light output of our LED substantially.

Metalorganic Chemical Vapor Deposition Conditions for Efficient Silicon Doping in High Al-Composition AlGaN Films

The influence of the metalorganic chemical vapor deposition conditions on the electrical and optical properties was investigated for silicon-doped AlxGa1-xN films with x>0.5 grown on sapphire and 6H–SiC substrates. At a constant silicon concentration in the layers, the electron carrier concentration was the highest in films deposited at reduced temperatures and high V/III ratios, despite the higher residual carbon and oxygen concentrations in the layers grown at lower temperatures. The electrical properties of the films were correlated with the intensity of the emission bands in the 410 to 440 nm and the 510 to 550 nm ranges observed in the 300 K photoluminescence spectra of the samples. The influence of impurities, native defects and threading dislocations on the film properties is discussed.

Electrical and structural characterization of Mg-doped p-type Al0.69Ga0.31N films on SiC substrate

We report on the electrical and structural characterization of Mg-doped Al-rich p-type Al0.69Ga0.31N alloys on SiC substrate grown by metal organic chemical vapor deposition. The impact of growth conditions and dopant activation annealing conditions on the electrical conductivity was investigated. The most efficient activation was achieved by annealing in N2 gas at 1000°C for 3min in the presence of a SiO2 cap. The impact of p‐GaN contact layer thickness was also studied. Higher growth temperature resulted in lower Mg incorporation, and for constant Mg concentration the resistivity increased with the increase in the growth temperature. Mg incorporation increased linearly with Mg effective flow. Transmission electron microscopy revealed the generation of vertical spike-like defects with increasing Mg doping. The resistivity also increased with the increase in Mg doping. A minimum resistivity of 10Ωcm was achieved at 670K.

Growth and fabrication of short-wavelength UV LEDs

Ultra-violet light emitting diodes with a peak wavelength of 293 nm were grown by MOCVD on AlN on sapphire. The maximum output power was 15 μW at 100 mA DC current injection for on wafer, room temperature testing. We have shown that by forming an interdigitated multi-fingered n-contact compared to a square geometry LED, the series resistance is reduced by ~ 8 - 15 Ω at 100 mA. This results in a 2 - 4 V reduction in drive voltage at 100 mA. The quantum wells exhibit a sharp electroluminescence peak at 293 nm with a 9 nm full-width at half maximum, but deep level related emission was observed at 2.56, 2.80, 3.52, and 3.82 eV. The high energy peaks, 3.52 and 3.82 eV, saturate with increasing drive current while the low energy peaks, 2.56 and 2.80 eV, increase with drive current proportional to the quantum well emission. This indicates the recombination mechanism for the low energy and high energy peaks is fundamentally different. We have also shown that forward bias leakage current in these devices is another factor limiting the quantum efficiency.

GaN based high brightness LEDs and UV LEDs

This talk summarizes the important materials and device results in gallium nitride based light emitter technology. GaN has emerged as the most promising material for high brightness LEDs with colors ranging from UV to blue, green, and white. Recent progress on ultra-violet (UV) emitting LEDs using AlGaN single quantum wells indicates wavelengths as short as 292 nm are achievable. UV LEDs are of great interest for solid state white lighting due to the high conversion efficiencies of typical phosphors in the UV spectrum. This paper focuses on recent progress in improving the properties of UV LEDs.