Convergence of optical spectroscopic system for characterization of InGaN/GaN multi-quantum well light-emitting diodes

We present a converged spectroscopic system design for performing photoreflectance (PR), electroreflectance (ER), electroluminescence (EL), photoluminescence (PL) and photovoltage (PV) measurements of semiconductors. The design of the experimental setup is described in detail. To test the performance of the system, measurements of a series of InxGa1-xN/GaN light emitting semiconductor with different indium composition of InGaN layer are carried out by use of this system. The experimental reflection and luminescence spectra are analyzed and discussed. The experimental results demonstrate the performance of this system. Optical and electrical properties of In0.15Ga0.85N/GaN multi-quantum well (MQW) light-emitting diodes (LEDs) with different quantum well (QW) thicknesses were investigated by electric-field dependent ER spectroscopy. From the ER measurements, we have observed the well-resolved transition peaks related to InGaN QW. Furthermore, the transitions related to yellow luminescence (YL) from Si-doped GaN and blue luminescence (BL) from Mg-doped GaN were observed in the ER spectra of In0.15Ga0.85N/GaN MQW LEDs. With increasing QW thickness, the additional transitions related to InGaN QW can be attributed to the recombination of excitons localized at the shallow potential states in InGaN QW, originating from the In-poor InGaN regions caused by indium phase separation in InGaN QW. By applying a reverse bias voltage, the ER features related to InGaN QW were shifted to higher energy, resulting from the reduction of quantum confined Stark effect in InGaN QW with increasing reverse bias voltage. On the other hand, the ER features from YL and BL band related to the deep and the shallow impurity state exhibit redshift and broaden with reverse bias voltage. These results can be attributed to the reduction of Coulomb interaction between donor and acceptor caused by the increase of depletion regions with increasing reverse bias voltage.