IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 53, NO. 5, OCTOBER 2017 3300206 Interactive Study of Electroreflectance and Photocurrent Spectra in InGaN/GaN-Based Blue LEDs Abu Bashar Mohammad Hamidul Islam, Dong-Soo Shin, Member, IEEE, and Jong-In Shim, Member, IEEE Abstract—We experimentally and theoretically investigate the relationship between the electroreflectance (ER) and photocur- rent (PC) spectra, and how they can be utilized to esti- mate the flat-band voltage and the bandgap energy of the InGaN/GaN-based quantum-well (QW) structure in blue light- emitting diodes. With theoretical modeling of ER and PC spectra, we calculate the changes in both the refractive index (n) and optical absorption (α) spectra from experimental ER and PC data by using the Kramers–Kronig relation. Then, we compare n (or α) spectra obtained differently from the ER and PC data and try to comprehend their physical meanings interactively. From these combined studies, we propose an exact method of determining the flat-band voltage, the piezoelectric field, the emission energy, the effective bandgap energy, and the Stokes shift of a QW structure under the quantum-confined Stark effect (QCSE). Index Terms— Light-emitting diodes, electroreflectance, photocurrent, absorption, flat-band voltage, piezoelectric field. I. I NTRODUCTION U SUALLY, InGaN/GaN multiple-quantum-well (MQW) light-emitting diodes (LEDs) grown on the conventional c-plane sapphire substrates possess internal electric fields induced by large macroscopic polarizations, namely, the spon- taneous polarization (due to the lack of inversion symmetry of material at zero strain) and the piezoelectric polarization (due to application of strain) [1]. The polarization field in InGaN-based MQWs is dominated by the piezoelectric polarization [2]–[4] because the spontaneous polarizations in the GaN and InN are approximately the same [2]. This large piezoelectric field is canceled if we externally apply a high reverse-bias voltage across the MQW LED [4], [5]. The voltage at which the piezoelectric field completely compensated is known as the flat-band voltage and is a key parameter for estimating the polarization-induced elec- tric field exactly. Thus, it is very important to interpret the proper flat-band condition for the InGaN-based LEDs from experimentally obtained spectroscopic data. The inter- nal electric field of the group III-N-based LEDs can be Manuscript received May 4, 2017; revised July 6, 2017 and August 4, 2017; accepted August 8, 2017. Date of publication August 16, 2017; date of current version August 30, 2017. (Corresponding author: Jong-In Shim.) A. B. M. H. Islam and J.-I. Shim are with the Department of Electron- ics and Communication Engineering, Hanyang University, Ansan 426-791, South Korea (e-mail: abmhis@kth.se; jishim@hanyang.ac.kr). D.-S. Shin is with the Department of Applied Physics and the Department of Bionanotechnology, Hanyang University, Ansan 426-791, South Korea (e-mail: dshin@hanyang.ac.kr). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JQE.2017.2740425 determined by various spectroscopic techniques [4] such as ER [5], PC [4], [6], photoluminescence (PL) [4], [7]–[9] and electrotransmission [10] with procedures of measur- ing the flat-band voltage and then calculating the electric field. Among these spectroscopic techniques, the ER method has a relatively good accuracy in measuring the flat-band voltage because it utilizes the phase change rather than the amplitude change in spectroscopic data. On the other hand, the PC and PL methods have some advantages of understanding the absorption and emission properties of an LED more directly over the ER method, but their signal intensities limit their usages. In fact, both the ER and PC spectra can be obtained simultaneously at the same reverse-bias condition and they are not independent of each other. In GaN-based LEDs, however, the relationship between them has not yet been systematically and thoroughly investigated even though there have been many reports focusing on the measurement of the flat-band voltage and the piezoelectric field [4]–[9]. Therefore, it is necessary to study the bias-dependent relationship between the ER and PC spectra, using separately calculated α and n from each spectroscopy, which will remove the limitation of the bias- dependent PL signal to comprehend the QCSE or absorption state of the quantum wells (QWs). Thus, by understanding the QWs under the internal electric field via the QCSE, it is expected to accurately find important physical properties such as the flat-band voltage, the emission energy, the effective bandgap energy, and the Stokes shift of the InGaN-based LEDs. In this research work, we interactively investigate the ER and PC spectra as a function of reverse bias voltage in InGaN/GaN-based blue LEDs by calculating n and α spectra based on the Kramers-Kronig (K-K) relation. The purpose of this work is to present a method of more clearly measuring the physical quantities related to the optoelectronic performances of InGaN-based blue LEDs. This paper is organized as follows. Section II describes the theoretical formalism, the device structure of the LED sample, and the measurement setup used in our study. Experimental data and their analyses are presented in Section III. Section IV summarizes the obtained results and concludes the paper. II. THEORETICAL ANALYSIS,DEVICE STRUCTURE, AND MEASUREMENT SETUP The relationship between the reflectance ( R) and the refrac- tive index (n) is given as R =[(n - 1)/(n + 1)] 2 when the 0018-9197 © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. 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