1. Introduction

Nitride-based micro-LEDs with a typical size of tens of microns have been used to demonstrate the potential application in VLC with the data rate of several Gbps [1,2]. Compared with conventionally high power broad-area LEDs, InGaN-based micro-LEDs can operate at extremely high injection current densities of several kA cm⁻² to achieve high density of light output power and high modulation bandwidth [3]. Therefore, micro-LEDs are also very suitable light source for VLC, which have great potential to supplement the existing radio frequency communication technology [4,5]. However, for VLC application, InGaN micro-LEDs need to be operated at large current densities to attain high modulation bandwidth, which makes reliability issues one of the biggest obstacles to market competitiveness. Nowadays, the studies on InGaN micro-LEDs for VLC are mainly focused on the improvement of optical power and modulation bandwidth [6,7], the degradation mechanisms of micro-LED photoelectric characteristics are still relatively limited, including carrier transport and recombination mechanisms.

Although micro-LEDs with smaller size have better heat dissipation capacity and less light output power degradation during operation under lower injection current [8–10], the high current density would still generate some defects within or around the active region, which can increase non-radiative recombination rate, resulting in the decrease of internal quantum efficiency (IQE) [11]. Furthermore, the optical power and frequency response degradation behavior of GaN-based micro-LEDs have been investigated under different modes [12]. The electrical stress-related defects not only reduce the effective carrier concentration injected into QWs, but also increase the carrier lifetime and decrease the modulation bandwidth. However, for the measurements of micro-LEDs’ optoelectronic properties under the stress tests, previous works mainly focused on the optical power by using an integrating sphere under room temperature and studied the degradation behaviors [13,14]. In addition, the temperature and injection current dependence of electroluminescence just have been utilized to study the optical properties of InGaN quantum-well structure [15,16]. But till now, using the injection current- and temperature-dependent EL spectra of the InGaN/GaN MQWs in aging study to investigate the degradation mechanisms have been less reported. The temperature- and current-dependent optical properties and the carrier transport and recombination mechanisms of the current-stressed InGaN micro-LEDs have been less studied.

In this study, InGaN-based blue micro-LEDs were stressed for 300 hours at an extremely high injection current density 15 kA/cm², which corresponds to the maximum optical power of the micro-LED chip. The injection current and temperature dependence of EL properties of the InGaN/GaN MQWs were measured. The peak wavelength, peak energy and the full width at half maximum (FWHM) of the EL spectra were discussed. And the carrier transport and recombination process in the InGaN/GaN MQWs of micro-LED devices before and after stress have been analyzed systematically. These findings would provide in-depth understanding of the current VLC technology at the device level and the theoretical basis for further improving the reliability of micro-LEDs, promote the application of nitride optoelectronic devices in optical communication system and the development of wireless optical communication technology.

2. Experiments and results

The epitaxial structure of the nitride micro-LED chip is composed of sapphire substrate, GaN buffer layer, n-type GaN layer, InGaN/GaN MQWs, electron blocking layer (EBL) and p-type GaN layer, as shown in Fig. 1(a). After the epitaxial growth, micro-LED chip is fabricated by using standard processing technology. The mesa diameter of the micro-LED chip is 30µm. The n-electrode and p-electrode composed of Cr/Al/Ti/Au were deposited on the exposed n-GaN and mesa to form the ohmic contact, respectively. Figure 1(b) is the structure diagram of the micro-LED device. The EL spectra and optical power were measured using integrating sphere, the dominant wavelength is 470 nm and the maximum optical power of the micro-LED chip is 6.2 mW, as shown in Fig. 1(c) and 1(d). The current-voltage (I-V) characteristics, Keithley 2400 source meter, respectively.

 

Fig. 1. The schematic diagram of the epitaxial structure of InGaN/GaN MQWs (a) and device structure (b) for InGaN micro-LED. The EL spectrum (c) and injection current dependence of optical power of micro-LED.

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For the injection current and temperature dependent EL spectra measurements, the micro-LEDs were installed in a high-precision cryogenic probe station, which allows the temperature changing from 5 to 300 K. The semiconductor characteristic parameter meter 4200-SCS provides the driving current, then the EL spectra signals were collected by the Ocean Optics spectrometer. For aging study, a group of InGaN based micro-LEDs have been fabricated and an array of micro-LEDs with good and similar photoelectric performance were selected for the injection current and temperature dependent EL spectra measurements.

Figure 2(a) and 2(b) shows the EL spectra at different current density for the initial and stressed devices. For the fresh sample, with the increase of the current density, the peak wavelength shifts to shorter wavelength from 494 nm to 465 nm (blue shift Δλ=29 nm) because of the Coulomb screening of the piezoelectric-field induced quantum confined Stark effect (QCSE) [17]. Nevertheless, the blue shift of peak wavelength for the stressed sample from 1 mA to 100 mA is about 18 nm. The blue shift is 11 nm smaller compared with the initial EL spectra because of the increase of defect-assisted carrier tunneling and the decrease of the Coulomb screening effect [12]. In addition, there is a shoulder at lower wavelength of the EL spectra after stress, which is more obvious at low injection current density, as shown in the violet shaded rectangle in Fig. 2(b). The result indicates that this is not due to the higher junction temperature caused by large injection current density, but may be because of the change of carrier transport and recombination mechanisms after stress [18,19].

 

Fig. 2. The current dependent EL spectra for the initial (a) and stressed (b) micro-LEDs, measured at 300 K. (c) Light output-current (L–I) and (d) I-V characteristics of micro-LEDs before and after current stress.

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Figure 2(c) shows the optical power as a function of current density for the initial and stressed micro-LEDs, the insert is in a logarithmic scale. The optical power of the micro-LED decreases at every current density after stress. In addition, the optical power under lower current density degrades more significantly than higher current density, indicating that the SRH nonradiative recombination increased for the stressed device. In addition, the slopes of log L-log I curves shown in insert change from 1.0 to 2.1 at low current density, revealing that there is an increase of the defects in the active layer after stress [20,21]. Figure 2(d) shows the I-V characteristics of micro-LED, the reverse-bias current of the micro-LED increases from 10⁻⁸ to 1.31 × 10⁻⁶ A at the certain voltage -3 V. When under forward bias, the semi-log scale I-V curve for stressed micro-LED can be divided into two main regions: the low-forward bias region (from 0 to 3 V) and the bias region above the turn-on voltage (U > 3 V). In low-forward bias region, I-V characteristic is governed by trap assisted tunneling (TAT) [22]. For stressed micro-LED, the increase of leakage current is due to the generation of defects within active region, leading to an increase in TAT [23]. In addition, the rapid increase of leakage current in 0-1 V is because the carrier transport mechanism is dominated by TAT. When the voltage increases, the proportion of TAT gradually decreases with the increase of carrier diffusion-recombination, and the leakage current increases slowly in 1-3 V [24]. The ideal factors calculated from the I-V data in the region of 0-1 V, which increases from 1.9 to 3.4 after stress, indicating the generation of defect within active region, leading to an increase in TAT [25,26], which is consistent with the discussion of L-I curves. The real character of these defects will be discussed below.

Figure 3 shows the current dependent EL spectra at low temperature 5 K and high temperature 300 K, respectively. It can be seen that although the EL spectrum corresponding to each current is dominated by a Gaussian EL peak arisen from the multi-quantum well region of the LED chip, the EL spectra of the LED before and after stress have changed. The dominant peak at 5 K under 10 mA shows a 18 nm redshift after stress. As discussed in I-V curve of stressed micro-LED, the increase of leakage current is due to the generation of defects within active region, leading to the trap-assisted tunneling (TAT) of carriers. The TAT decrease the effective carrier concentration injected into the QWs significantly, and reduce the Coulomb screening effect, causing the redshift of the dominant EL peak.

 

Fig. 3. Temperature dependent EL spectra for InGaN based micro-LED under injection current 10 mA and 80 mA, respectively.

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In order to study the type of the defects caused by current stressed, temperature dependent EL spectra of the micro-LED were measured, as shown in Fig. 4(a). Figure 4(b) shows the integrated intensities of EL emission with the increase of temperature. The activation energy (E) of defects is extracted from the temperature-dependent EL spectra using the calculated method of Shuhei Ichikawa et al. and the value of Ea is about 200 meV [27], which may be related to the N-vacancy defects and consistent with the data reported from other studies [28–30]. In addition, in the high temperature region, because of the emission process of the MQWs is affected by the carrier scattering effect and the non-radiative recombination process, causing the deviation between fitting data and experiment results. Therefore, N-vacancy defects significantly decrease the effective carrier concentration injected into the quantum wells and reduce the Coulomb screening effect.

 

Fig. 4. (a) Temperature dependent EL spectra of the micro-LED; (b) Integrated intensities of EL emission with the increase of temperature.

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Figure 5(a) is the schematic diagram of the energy band of the carrier distribution and recombination mechanism in the InGaN/GaN MQWs region before current stress, most of the injected carriers are in the localized state. After stress, the increase of N-vacancy defects causes an increase in non-radiative recombination centers (PD-NRC) in MQWs, which leads to an increase in the non-radiative recombination rate [18,31]. When the electron-hole pair combines in the form of nonradiative recombination, the electron energy is converted to vibrational energy of lattice atoms, i.e. phonons. Thus, the electron energy is converted to heat and stimulate part of the electrons in the localized state transition to a higher energy level, causing some carriers redistribute and have a state-filling effect in higher-energy states in MQWs, as shown in Fig. 5(b) and 5(c). The radiative recombination of higher-energy-level electrons and the holes in the valence band would release higher energy photon, causing the appearance of the shorter-wavelength emission peak 1 in the EL spectra, as shown in Fig. 3(a) and 3(c). In addition, the defects formed by current stress in quantum barriers would capture a part of the carriers injected into the active region, but these defects can be frozen at low temperature and lose the binding effect on the carrier [16]. The radiative recombination of the carriers in quantum barrier under low temperature and injection current can result in another emission peak at 390 nm (peak 2) in the EL spectra.

 

Fig. 5. The energy band diagram of carrier distribution and recombination mechanism in InGaN/GaN MQWs of micro-LED before (a) and after (b) current stress, (c) Carrier thermal excitation process assisted by nonradiative recombination.

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To investigate the influence of temperature on the EL spectra of the InGaN/GaN MQWs, the EL peak energy and FWHM of micro-LED before and after stress were extracted with the injection current under different temperatures (5 K, 100 K, 300 K), as shown in Fig. 6. After the stress, the EL peak energy is reduced in the entire current range. Under low injection current, the leakage current assisted by N-vacancy defects would decrease the effective carrier concentration injected into the QWs and reduces the Coulomb screening effect of QCSE. However, with the increase of injection current, the coulomb screening effect gradually becomes significant, resulting the increase of the peak energy. When the micro-LED under high injection current, the increase of junction temperature will cause the shrinkage of the energy band gap according to the Varshni empirical equation ${E_\textrm{g}}(T) = {E_0} - {{\alpha {T^2}} / {(\beta + T)}}$, which will lead to the redshift of the emission peak with the increase of current density [19]. Figure 6(a), 6(c) and 6e show the EL peak energy declines slightly (redshift of emission wavelength) with the increase of injection current density. However, the value of the decline is very small, which may be attributed to the lower junction temperature (T_j) of micro-LEDs compared with broad-LED, the T_j under 15 kA/cm³ corresponding to the maximum optical power of the micro-LED chip is 60°C, which is about less than half of that of broad- LEDs [32,33]. The maximum ΔE of peak energy is the difference between 2.577 eV and 2.569 eV for the fresh sample under 300 K, just 0.008 eV. In addition, the emission peak 1 for the stressed sample shows a blueshift with the increase of injection current shown in Fig. 6(a) and 6(c), which indicates the peak 1 also affected by the QCSE.

 

Fig. 6. The dominant EL peak and FWHM of micro-LED at different temperatures and different currents before and after current stress.

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In addition, the current dependent FWHM of EL spectra at different temperatures before and after stress are shown in Fig. 6(b), 6(d) and 6(f). Under low injection current, the carrier band filling effect would increase with the increase of injection current density. The filling carriers of the low-energy localized states in quantum well dominate the radiative recombination process of the MQWs, leading to an initial steep broadening of FWHM, especially at low temperature [16]. Under temperature 5 K, the recombination process of electron-hole pairs in the QW region is dominated by low-energy state filling. With the temperature increasing, the filling effect of high-energy localized states occurs at temperature ∼100 K. When the temperature is further increased to about 300 K, the emission process of the MQWs is affected by the carrier scattering effect and the non-radiative recombination process. When the injection current further increases, the increase of junction temperature stimulates the excess carriers to occupy the higher energy level states, causing the following slow rise of FWHM. As for the stressed micro-LED, due to the increase of defect-assisted carrier tunneling [23,34], the carrier band filling effect is reduced, which causes the slower broadening of FWHM with the increase of injection current compared with fresh micro-LED.

3. Conclusion

In this study, the optoelectronic properties of InGaN blue micro-LEDs were investigated before and after the current stress. The injection current and temperature dependence of EL spectra of the InGaN/GaN MQWs were measured. The peak energy and FWHM of EL spectra before and after stress with the injection current under different temperatures were analyzed systematically. The results show that the optical power of the micro-LEDs under every injection current degrades after stress, especially at lower current density. In addition, the slope of log L-log I curves change from 1.0 to 2.1 at low current density, the ideality factor extracted from the I-V curves change from 1.9 to 3.4 after current stress, indicating there is an increase of the defects in the active layer after stress. In addition, the activation energy of defects evaluated from the temperature-dependent EL spectra is about 200 meV, which could be related to the N-vacancy related defects. Besides, the peak wavelength, peak energy and the FWHM of the injection current and temperature dependence of EL spectra were discussed. The results show that the increase of SRH nonradiative recombination causes some carriers redistribute and a state-filling effect in higher-energy states in MQWs, resulting in the appearance of the shorter-wavelength luminescence in the EL spectra. These findings can help to further understand of degradation mechanisms of InGaN micro-LEDs operated under high current density.