⇦Chinese Physics Letters, 2019, Vol. 36, No. 2, Article code 028501 Carrier Dynamics Determined by Carrier-Phonon Coupling in InGaN/GaN Multiple Quantum Well Blue Light Emitting Diodes * Sheng Cao (曹盛), Xiao-Ming Wu (吴小明)**, Jun-Lin Liu (刘军林), Feng-Yi Jiang (江风益) Affiliations National Institute of LED on Si Substrate, Nanchang University, Nanchang 330096 Received 16 August 2018, online 22 January 2019 *Supported by the National Science Foundation for Young Scientists of China under Grant No 11604137, the Jiangxi Province Postdoctoral Science Foundation Funded Project under Grant No 2015KY32, and the State Key Program of Research and Development of China under Grant Nos 2016YFB040060 and 2016YFB0400601.
**Corresponding author. Email: xiaomingwu@ncu.edu.cn Citation Text: Cao S, Wu X M, Liu J L and Jiang F Y 2019 Chin. Phys. Lett. 36 028501    Abstract Phonon sidebands in the electrolumiescence (EL) spectra of InGaN/GaN multiple quantum well blue light emitting diodes are investigated. S-shaped injection current dependence of the energy spacing (ES) between the zero-phonon and first-order phonon-assisted luminescence lines is observed in a temperature range of 100–150 K. The S-shape is suppressed with increasing temperature from 100 to 150 K, and vanishes at temperature above 200 K. The S-shaped injection dependence of ES at low temperatures could be explained by the three stages of carrier dynamics related to localization states: (i) carrier relaxation from shallow into deep localization states, (ii) band filling of shallow and deep localization states, and (iii) carrier overflow from deep to shallow localization states and to higher energy states. The three stages show strong temperature dependence. It is proposed that the fast change of the carrier lifetime with temperature is responsible for the suppression of S-shaped feature. The proposed mechanisms reveal carrier recombination dynamics in the EL of InGaN/GaN MQWs at various injection current densities and temperatures. DOI:10.1088/0256-307X/36/2/028501 PACS:85.60.Dw, 78.60.Fi, 78.67.De, 71.38.-k © 2019 Chinese Physics Society Article Text InGaN/GaN multiple quantum wells have attracted much attention due to their excellent performance in high brightness blue,[1] green,[1] and yellow[2] light emitting diodes. The carrier localization induced by the In composition fluctuations is widely believed to be a key role of the high efficiency of the InGaN-based LEDs.[3-5] The radiative recombination of carriers localized in In-rich region dominates the spontaneous emission in InGaN/GaN quantum well structures instead of nonradiative recombination at threading dislocations. Interestingly, the carrier or exciton localization is also related to the phonon sidebands (PSBs) observed on the low energy side of main peak (zero-phonon line) in the luminescence spectra, resulting from the coupling of the carriers or excitons with longitudinal optical (LO) phonons.[6-10] The carrier-phonon or exciton-phonon coupling is strong for the localized carriers or excitons due to the relaxation of the wave-vector conservation rule.[11] So far, the spectral features of LO phonon sidebands in luminescence of GaN-based materials, including their energy spacing from the zero-phonon line[12-15] and coupling strength[16-20] obtained from photoluminescence (PL) spectra, have been investigated. However, the injection current dependence of the energy spacing between the zero-phonon and first-order phonon-assisted luminescence lines has rarely been studied. In this work, PSBs in the electroluminescence (EL) spectra of InGaN/GaN multiple quantum well (MQW) blue light emitting diodes have been investigated. S-shaped injection current dependence of the energy spacing (ES) between the zero-phonon and first phonon replica lines is observed in the temperature range of 100–150 K. It becomes less distinct with increasing temperature from 100 to 150 K, and vanishes at temperature above 200 K. These injection current dependences of the energy spacing (ES) with temperature are explained by the carrier dynamics related to localization states in the InGaN/GaN MQWs at various temperatures. The InGaN/GaN MQWs used in this study were grown on 1 mm $\times$ 1 mm patterned 2-in silicon (111) substrates by a Thomas Swan close-coupled showerhead metalorganic chemical vapor deposition system.[21] The structure consists of the following layers in sequence: 100-nm AlN buffer grown under high temperature, 2.8 µm Si-doped GaN, 30 periods of In$_{0.05}$Ga$_{0.95}$N/GaN (1.9 nm/1.9 nm) superlattice for strain relief, 8 periods of In$_{0.15}$Ga$_{0.85}$N/GaN (3 nm/10 nm) MQWs and 220 nm p-type layer including 20 nm Al$_{0.2}$Ga$_{0.8}$N electron blocking layer. Vertical structure LED chips of 1 mm $\times$ 1 mm were fabricated with n-GaN up and roughened. The detailed chip fabrication process has been reported previously.[22] Temperature and injection current dependent EL spectra were measured by the system described in our previous work.[23] Time-resolved PL (TRPL) measurements were carried out using an FLS920 Fluorescence Spectrometer. Figure 1 shows representative EL spectra of InGaN/GaN multiple quantum wells (MQWs) measured at 35 A/cm$^{2}$ under 100 and 300 K. The LO-phonon sidebands can be clearly observed on the long wavelength side of the main EL peak (zero-phonon line) at 100 K. At 300 K, the sidebands also reveal themselves at the long wavelength side although less distinct. The integrated intensity and peak energy of the zero-phonon and first phonon replica lines are estimated using multi-Gaussian peak fitting, as illustrated in Fig. 1. Both the zero-phonon and first phonon replica lines are strong enough to ensure an accurate fitting. All of the fitting is under the constraint that it takes the same FWHM for each line.[16,17]
cpl-36-2-028501-fig1.png
Fig. 1. Representative EL spectra of InGaN/GaN multiple quantum wells blue light emitting diodes measured at 35 A/cm$^{2}$ under (a) 100 K and (b) 300 K. The solid lines show the result of multi-Gaussian peak fitting described in the text.
cpl-36-2-028501-fig2.png
Fig. 2. Energy spacing between the zero-phonon and first phonon replica lines versus current density in the temperature range of (a) 100–150 K and (b) 200–300 K.
The ES between the zero-phonon and first phonon replica lines is plotted against the injection current density at various temperatures in Fig. 2. The ES mainly varies from 83 to 90 meV in the temperature range of 100–200 K, which is close to the published value of the LO-phonon characteristic energy of GaN (91 meV) and InN (86 meV).[9,16] Interestingly, as the current density increases, ES exhibits an S-shaped behavior in the temperature range of 100–150 K. For instance, at the temperature of 100 K, ES increases in the current density range of 0.002 to 0.07 A/cm$^{2}$, decreases for 0.07–10 A/cm$^{2}$, and increases again for 10–35 A/cm$^{2} $. As the temperature increases, S-shaped injection current dependence is gradually suppressed and finally vanishes at temperatures above 200 K. It is found that the ES between the zero and its phonon lines is strongly temperature dependent.[12-15] Likewise, in the case of current injection, the ES is also strongly affected by the injected carriers. According to Permogorov's theory,[24] the shift ${\it \Delta}$ of the first phonon replica line maximum from its low-energy threshold increases linearly with temperature: ${\it \Delta}=3/2k_{\rm B}T$. The shift of the first phonon replica line with temperature will lead to the reduction of the ES between the zero-phonon and first phonon replica lines.[12-15] For instance, previous work has reported that the ES between the zero-phonon and first phonon replica lines of GaN obtained from PL spectra decreases from 81 to 72 meV with increasing temperature from 100 to 200 K.[14] However, the ES of this work obtained from EL spectra varies from 83 to 90 meV in the temperature range of 100–200 K, and even reaches 105 meV at 300 K, which deviates from Permogorov's theory, indicating the existence of mechanism leading to the relative shift of the zero-phonon line compared to the first phonon replica line with injection current.
cpl-36-2-028501-fig3.png
Fig. 3. Peak energy of the zero-phonon and first phonon replica lines versus current density in the temperature range of [(a), (c)] 100–150 K and [(b), (d)] 200–300 K.
In general, the shift of the emission peak with increasing injection current is explained by lifetime limited carrier relaxation into localized energy states,[25] band filling of localized energy states and Coulomb screening of quantum confined Stark effect (QCSE) induced by the piezoelectric field.[3,4] As the piezoelectric field is across the quantum well, Coulomb screening of QCSE should result in the equivalent blueshift of the zero-phonon and first phonon replica lines, not contributing to the relative shift of the zero-phonon line compared to the first phonon replica line. Figure 3 shows the peak energy of the zero-phonon and first phonon replica lines as a function of injection current density. As is expected, the peak energy of the zero-phonon line increases with increasing current density at all temperatures. However, at 100 and 120 K, the peak energy of the first phonon replica line decreases slightly at initial low current density and then increases at higher current density. The unusual redshift of peak energy of the first phonon replica line occurs in spite of Coulomb screening of QCSE, indicating the carrier relaxation to the strongly localized energy states.
cpl-36-2-028501-fig4.png
Fig. 4. Schematic diagrams indicating the possible mechanism of current density dependent carrier dynamics associated with localization states. (a) Carriers are prone to recombine at shallow localization states while only few ones can relax into deep localization states at initial low current density. (b) More carriers can relax from shallow into deep localization states at a current density slightly higher. (c) Carriers gradually fill shallow and deep localization states at an even higher current density. (d) Carriers overflow from deep localization states and populate shallow localization states and higher energy states at very high current density.
Different levels of localized states have been proposed based on wavelength-dependent and temperature-varying time-resolved PL measurements.[26] In this study, a schematic diagram is given in Fig. 4 to understand the current density dependent carrier dynamics associated with localization states. In InGaN/GaN systems, due to spatial distribution of the localization states, carriers may transport among different localization states.[26] Since the carrier lifetime is not long enough at initial low current density, injected carriers are prone to recombine before relaxing down into lower energy states at 100 K.[25,27] Therefore, few carriers can relax from shallow localization states (SLs) down into deep localization states (DLs). As current density increases, more carriers can relax down into DLs with the enhanced transport process, as evidenced by redshift of the first phonon replica line. With the end of carrier relaxation, band filling of DLs becomes dominant. DLs are gradually filled by carriers. As current density further increases, DLs become saturated, and then carriers could overflow from DLs and populate SLs and higher energy states. Thus carrier dynamics with injection current could be broadly divided into three stages: (I) carrier relaxation from SLs down into DLs, (II) band filling of SLs and DLs, and (III) carrier overflow from DLs to SLs and to higher energy states, which are responsible for the relative shift of the zero-phonon line compared to the first phonon replica line. Previous studies have revealed that only strongly localized excitons or carriers, which correspond to those populating the deep localization states, contribute significantly to the phonon lines.[11,16,28] Therefore, the carrier dynamics related to DLs are considered to be the key role for S-shaped injection current dependence of the ES between the zero-phonon and first phonon replica lines at 100 K. At stage I, at low current density, enhanced carrier relaxation from SLs down into DLs takes place with the increase of current density, which leads to the redshift of the first phonon replica line. Thus a relative redshift of the first phonon replica line compared to the zero-phonon line occurs, resulting in the increase of ES. At stage II, with further increasing the current density, band filling of SLs and DLs both occur. Due to the fewer number of DLs, the band filling of DLs is more prominent, contributing to the relative blueshift of the first phonon replica line compared to the zero-phonon line. Thus, a decrease of ES takes place. At stage III, with continuous increasing the current density, more and more carriers overflow from DLs and populate SLs and higher energy states. Due to weak localization, the recombination of these carriers contributes less significantly to the phonon lines, which leads to the slower blueshift of the first phonon replica line compared to the zero-phonon line. Thus an increase of ES occurs again.
cpl-36-2-028501-fig5.png
Fig. 5. Carrier lifetime monitored at the peak energy as a function of temperature.
As temperature increases, the S-shaped feature becomes less distinct from 100 to 150 K, and vanishes at temperature above 200 K, showing the strong temperature dependence of three stages. This could be attributed to the fast change of carrier lifetime with increasing temperature. TRPL measurements were performed to elucidate the kinetics of the carrier recombination. Figure 5 shows the carrier lifetime monitored at the peak energy as a function of temperature. As temperature increases from 100 to 150 K, the carrier lifetime decreases from 24.4 to 16.3 ns due to the enhanced nonradiative processes, allowing fewer carriers to reach DLs and thus more carriers to populate SLs and higher energy states. This implies that carrier relaxation to DLs and band filling of DLs are both suppressed. Consequently, the increase at stage I and the decrease of ES at stage II both become less evident. Moreover, due to the decreased carrier lifetime, higher injection current density is required to complete the process of carrier relaxation. This leads to the right shift of turning point between stages I and II, as shown by the dotted line in Fig. 2(a). Due to the further decreased carrier lifetime from 11.8 to 7.5 ns in the temperature range of 200–300 K, the localization effects in the carrier capture and recombination become insignificant. Then the Coulomb screening of QCSE is dominant, which results in the equivalent blueshift of the zero-phonon and first phonon replica lines, as mentioned previously. Thus a constant ES occurs at stages I and II. At stage III, the decrease of carrier lifetime enables more carriers to populate SLs and higher energy states instead of DLs, resulting in the more obvious rise of ES from 100 to 300 K. In summary, phonon sidebands in the electroluminescence (EL) spectra of InGaN/GaN multiple quantum well (MQW) blue light emitting diodes have been investigated. S-shaped injection current dependence of the energy spacing between the zero-phonon and first phonon replica lines is observed in the temperature range of 100–150 K. The S-shape becomes less distinct with increasing temperature, and vanishes at temperature above 200 K. The S-shaped injection dependence of energy spacing can be explained by the three stages of carrier dynamics related to localization states: (I) carrier relaxation from shallow into deep localization states, (II) band filling of shallow and deep localization states, and (III) carrier overflow from deep to shallow localization states and to higher energy states. The suppression of S-shaped feature by the increasing temperature is related to the strong temperature dependence of the three stages due to the fast change of the carrier lifetime. The proposed mechanisms reveal carrier recombination behavior in the EL of InGaN/GaN MQWs at various current densities and temperatures. 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