Estimation of internal and external quantum efficiency in InGaN quantum wells by using the simultaneous photoacoustic and photoluminescence measurement method and integrating-sphere method

Abstract

To obtain a comprehensive understanding of the electron states and carrier dynamics of nitride semiconductors requires accurate measurement of internal quantum efficiency (IQE). We have estimated the IQE of GaN films and InGaN quantum wells (QW) by using the simultaneous photoacoustic (PA) and photoluminescence (PL) measurement method, with valid results so far. However, if the PL from a sample is reabsorbed inside that sample and the resulting excited carriers cause non-radiative recombination, the heat generated in this process is detected in the PA measurement, in which case what should be counted as "luminescence" is counted as "heat generation." This may result in IQE being estimated to be lower than it actually is. If this effect becomes dominant, researchers end up measuring external quantum efficiency (EQE) when they are supposed to measure IQE. In this study, in order to experimentally verify whether this phenomenon could occur, we measured IQE and EQE using the same InGaN-QW samples and compared the obtained values to examine the validity of the IQE estimation. We used the simultaneous PA and PL measurement method to measure IQE and the integrating-sphere method to measure EQE. The measured samples were three InGaN-QW samples (480-nm luminescence) on GaN substrates and three InGaN-QW samples (520-nm luminescence) on sapphire substrates. The wavelength of the excitation light was 405 nm, and only the QW layer was selectively excited. As Fig. 2 shows, IQE was estimated to be 30 to 40% for the QWs on the GaN substrates and 4 to 7% for the QWs on the sapphire substrates. (In the figure, all the results of the multiple measurements conducted on the same samples are plotted.) On the other hand, EQE was estimated to be 10 to 20% for the QWs on the GaN substrates and 1 to 2% for the QWs on the sapphire substrates. IQE was higher than EQE for all the samples, and the light-extraction efficiency calculated from these data was 20 to 40%, which can be considered a valid result. While the linear relationship between IQE and PL intensity does not necessarily hold, all the samples exhibited a linear relationship between EQE and PL intensity (Fig. 2). This suggests that the values of IQE estimated by the simultaneous PA and PL measurement method are valid.