Optical properties of InGaN/GaN quantum wells on sapphire and bulk GaN substrate

Due to their wide band gap range of InGaN from 0.7 eV to 3.5 eV InGaN, heterostructures are used for optoelectronic applications covering the entire spectral range of visible light [1]. Pyroelectric fields influence the optical properties [2]. Additional strain-induced piezoelectric fields are typical for group III nitride heterostructures. Homoepitaxial growth on single GaN crystals is predicted to reduce the strain. We compare the luminescence properties of InGaN heterostructures on bulk GaN substrate and sapphire. We present the results of time-integrated and time-resolved photoluminescence (PL) spectroscopy, performed on InGaN single quantum wells (QW) with widths of 4.5 nm and 9.5 nm. The structures were grown by metal organic chemical vapor deposition on sapphire and high pressure bulk GaN substrates [3]. Samples deposited on both types of substrates were grown in the same run. This allows us to compare directly the population and recombination mechanisms in thin and wide QW’s on both substrate materials. Si-doped buffer/barrier layers were used to screen built-in electric fields. The PL was excited at 353 nm by the second harmonic wave of a mode-locked Ti:sapphire laser. The temporal width of the laser pulses was 2 ps at a repetition rate of 80 MHz. The PL measurements were performed in a helium-flow cryostat. The detection system consisted of two 0.35 m McPherson monochromators in subtractive mode and an ultra-fast photo detector (micro-channel plate) providing a spectral resolution of about 1 meV and a time resolution of better than 30 ps. Figure 1 shows the low temperature PL of the investigated samples. The QW structures on GaN emit at lower energies than their sapphire-grown counterparts. We suspect that the cause of this is a lower temperature of the bulk GaN substrate during growth (in spite of its better thermal conductivity). The 60 µm thick GaN substrate shows a stronger bowing and worse contact to the heating than the 300 nm thick sapphire substrate. This leads to a more efficient incorporation of indium into the structure grown on