Investigating carrier localization and transfer in InGaN/GaN quantum wells with V-pits using near-field scanning optical microscopy and correlation analysis

www.nature.com/scientificreports OPEN received: 14 October 2016 accepted: 06 January 2017 Published: 13 February 2017 Investigating carrier localization and transfer in InGaN/GaN quantum wells with V-pits using near-field scanning optical microscopy and correlation analysis MinKwan Kim1, Sunghan Choi2, Joo-Hyung Lee2, ChungHyun Park2,3, Tae-Hoon Chung4, Jong Hyeob Baek4 & Yong-Hoon Cho2,3 The V-pits and potential fluctuations in InGaN/GaN multiple quantum wells (MQWs) are key factors for understanding the performance of InGaN/GaN-based light-emitting diodes (LEDs). However, photoluminescence (PL) measurements using conventional optical microscopy only provide ensemble information due to the spatial resolution limit, known as the diffraction barrier, which hinders the analysis of dislocations and potential fluctuations. Here, in order to investigate the influence of the V-pits and potential fluctuations on local optical properties, we performed nanoscopic luminescence mapping for standard and V-pit InGaN/GaN MQWs samples with different sized V-pits using nearfield scanning optical microscopy (NSOM) with illumination mode (I-mode) at various laser excitation powers. From the nanoscopic PL mapping data, we could clearly observe luminescence features associated with dislocations and potential fluctuations in the InGaN/GaN MQWs. We also employed correlation analysis to quantitatively analyze the nanoscopic PL mapping data for the different MQWs samples. Based on the results of NSOM PL with I-mode and correlation analysis, we could demonstrate that carrier transfer in the MQWs sample with large sized V-pits is suppressed by deeper potential fluctuations and higher energy barriers compared to the standard sample. InGaN/GaN based light-emitting diodes (LEDs) have attracted tremendous attention for a wide range of applications, including displays and general illumination1–3. Among other advantages, they offer low energy consumption and an optical band gap that can be tuned from the visible to the ultraviolet spectral range by adjusting the indium composition4–9. Despite their popularity, many questions still remain about the factors that influence their emission efficiency. For example, InGaN/GaN LEDs exhibit high emission efficiency despite having high density ( >  108 cm−2) of threading dislocation (TD)10–12. Such dislocations, which are induced by lattice mismatch between GaN and structurally dissimilar substrates, usually act as one of non-radiative recombination centers and carrier leakage channels, leading to low emission properties13–18. It has recently been proposed that the high efficiency of InGaN/GaN LEDs is caused by the formation of thin quantum wells (QWs) on the inclined facets around V-pits generated at the end of TDs, which prevents non-radiative recombination processes at the TDs19–28. Because these high energy QWs form around V-pits, some research have proposed methods for growing large sized V-pits to further enhance the efficiency of InGaN/GaN LEDs29–32. Meanwhile, it has been known that potential fluctuations owing to indium composition inhomogeneity and thickness variations can cause carrier localization and hence hinder non-radiative recombination processes in the InGaN/GaN multiple QWs (MQWs)33–35. According to recent studies, the potential fluctuations also seem to be affected by the formation of the V-pits at 1 Graduate School of Nanoscience and Technology, Korea Advanced Institute of Science and Technology, Daejeon 34141, Republic of Korea. 2Department of Physics, Korea Advanced Institute of Science and Technology, Daejeon 34141, Republic of Korea. 3KI for the NanoCentury, Korea Advanced Institute of Science and Technology, Daejeon 34141, Republic of Korea. 4LED Research and Business Division, Korea Photonics Technology Institute, Gwangju 61007, Republic of Korea. Correspondence and requests for materials should be addressed to Y.-H.C. (email: yhc@ kaist.ac.kr) Scientific Reports | 7:42221 | DOI: 10.1038/srep42221 1 www.nature.com/scientificreports/ Figure 1. Schematic of sample structure and PL spectrum with SEM image. (a) Schematic of an InGaN/ GaN blue MQWs sample grown on a double polished sapphire substrate. Two types of samples with different sized V-pit, called the standard and V-pit samples, were used. Schematic of cross section of a V-pit structure is provided in the inset image. (b,c) SEM images of the standard sample and the V-pit sample. As the SEM images show, the V-pit sample has larger sized V-pits compared to that of the standard sample. (d) PL spectra of the standard and the V-pit samples are measured at 300 K. TDs due to strain relaxation around the V-pits36–38. Therefore, to improve the efficiency of InGaN/GaN LEDs, it is important to understand how the V-pits and potential fluctuations influence the optical properties of the LEDs in detail. However, optical investigation of V-pits and potential fluctuations using conventional microscopic photoluminescence (PL) techniques is difficult due to their limited spatial resolution, as V-pits and potential fluctuations occur in a sub-diffraction limit scale. In this study, a nanoscopic PL technique using near-field scanning optical microscopy (NSOM PL) was employed to investigate the influence of V-pits and potential fluctuations on local optical properties. NSOM overcomes the spatial resolution limit by detecting the near field, including high spatial frequency components which exponentially decay near the sample surface, using a metal-coated tip with aperture and a scanning probe microscopy technique. NSOM PL measurements have been used for analysis of local fluctuation properties39–45 and for investigation of indium composition variations and relation between potential barriers around dislocations46 in InGaN QWs due to its ability to obtain mappings with sub-diffraction resolution. In our work, we used NSOM PL with the illumination mode (I-mode) and a systematic correlation analysis. In order to verify the influence of V-pits and potential fluctuations, two types of samples having different sized V-pits were used. By comparing the two samples, we observed influence of the V-pits and potential fluctuations, using power dependent NSOM PL mapping results. Correlation analysis, a statistical method, was employed as it can provide quantitative values from nanoscopically analyzed information39,43–46. Using correlation analysis, the two samples were found to exhibit quantitatively different correlation tendencies in terms of local carrier density, based on the low and high power NSOM PL results of each sample. Furthermore, we were able to estimate how carrier transfer can be affected by the V-pits and potential fluctuations in the InGaN/GaN MQWs of each sample, based on the results of NSOM PL and correlation analysis. Experiment Two types of blue InGaN/GaN MQWs with different sized V-pits, hereafter designated standard and V-pit samples, were used to investigate the influence of V- (...truncated)