In this work, AlGaN layers were grown on sapphire by metal-organic vapor phase epitaxy (MOVPE) on (0001)-oriented sapphire substrates, with the intention to investigate the effect of varying Al/MO and V/III ratios on the Al incorporation into the AlGaN layers. The parameters Al/MO and V/III describe the proportions of source material inside the reactor. With the help of optical transmission measurements, characteristic cut-off wavelengths of the AlGaN layers were determined. These wavelengths were used to calculate the Al content x of the layers, leading to values between 26.6% and 52.1%. Using the two process parameters Al/MO and V/III as input and the Al content of the AlGaN layers as a response variable, the experimental results were further investigated with the help of the software STATGRAPHICS. An estimated response surface for the variable x was generated. It was found that the Al incorporation is only tunable within a wide range for high V/III ratios of about 900. For constant Al/MO ratios and varying V/III ratios, two different growth characteristics were observed at high and low Al/MO values. This behavior is ascribed to the superposition of two oppositional effects.
GaN-based optoelectronics allow covering the spectral range from green to UV. Silicon (Si) is an alternative substrate to the commonly used sapphire and silicon carbide (SiC) but requires sophisticated buffer structures. In this work, two high-temperature (HT) layer stacks and two low-temperature (LT) AlN layers were used for the growth of GaN buffers for optoelectronic devices on (111)-oriented Si substrates using AIXTRON metalorganic vapor phase epitaxy (MOVPE) reactors. AlN, AlGaN and GaN were grown as HT layer stack to form stress reduction layers. GaInN MQW (multi quantum wells), electroluminescence test structures (ELT) and AlN/GaN DBR (distributed Bragg reflectors) were deposited on these buffer structures on Si. The growth process was monitored by in-situ reflectivity measurements. Photoluminescence (PL), electroluminescence and the luminescence under high optical excitation of the samples on Si have been studied. Laser action at optical excitation was obtained in the MQW with a room temperature (RT) laser threshold of Ithr = 40 - 80 kW/cm2. Laser action was achieved up to 350°C. Electroluminescence emission from the ELT InGaN/GaN heterostructures was observed and measured under a minimal DC voltage of about 4 V. AlN/GaN DBR with ten periods showed reflectivities of 60% for wavelengths of 436 nm and 537 nm, respectively.
In this letter a number of latest results from the process development on AIXTRON production scale MOVPE reactors will be reported. Growth of GaN on alternative substrates has been examined. Up to 900 nm crack free GaN layer were deposited on Si using a double nucleation interfacing technique. Low yellow band vs. band-edge related photoluminescence emission ratios have been observed and sheet resistances of up to 3500 Ω have been achieved on 2" Si substrates. Also, first results are reported of the up-scaling of the Planetary Reactor to 24 x 2". First results from fully loaded runs show an average 2" on wafer peak wavelength standard deviation of 3.8 nm, average wafer to wafer standard deviation across all wafers of 2.0 nm and average 3 x 2" disk to disk standard deviation of 1.6 nm at an average wavelength of 477.9 nm across all wafers (evaluated with peak integration and 2 mm edge exclusion for the 2" wafers). Photoluminescence peak intensity (area) varied with 13.6% standard deviation wafer to wafer and 12.8% disk to disk. On wafer intensity deviation was 11.3%.