1. Introduction

Visible light communication (VLC), an advanced wireless communication technology that can combine lighting and optical communications together, has attracted extensive attention in recent years due to the advantages of energy efficiency, high data transmission rates, enhanced security, no radio frequency (RF) interference and so on [1–4]. VLC is increasingly used to realize indoor white-lighting and optical wireless communication (OWC), long-distance underwater wireless optical communication (UWOC), as well as vehicle-to-vehicle sensing and communications. The OWC based on white-lighting is a new communication technology which combines the advantages of high speed, flexibility and green environmental protection. It uses visible light as carrier, as well as takes indoor free space as channel, integrating the characteristics of optical communication and wireless communication. And it also conforms to the development trend of white light emitting diode (LED) devices as the next generation of solid lighting source. Therefore, it has great significance and research value. In addition, the Wireless Fidelity (Wi-Fi) has own inherent properties, namely frequency and bandwidth sharing, resulting in the phenomenon of the co-channel interference and the limited total bandwidth access. Hence, with the increasing number of mobile access terminal users, the communication rate of wireless users decreases, even leading to wireless users not access to it. The VLC technology can easily realize space multiplexing, and provide users with high bandwidth communication links in densely populated areas. Compared with traditional Wi-Fi, VLC technology has the advantages of high speed and large capacity.

The best-known method to realize commercial white LED in solid state lighting (SSL) is the combination of InGaN-based blue LED and yellow-emitting phosphor materials, such as Y_3-xAl₅O₁₂:xCe³⁺ (YAG:Ce) [5,6]. If it is used as the light source of VLC, its lower modulation bandwidth becomes an important bottleneck of the transmission rate of VLC. Because the response speed of phosphor material is very slow, the modulation bandwidth is only a few Megahertz (MHz) [7,8]. Even if the light with the corresponding wavelength of phosphor is filtered in the receiving terminal, its modulation bandwidth is also limited by the resistance-capacitance (RC) time delay of the broad area white LED, the quantum-confined Stark effect (QCSE), and the In_xGa_1-xN/GaN multiple quantum wells (MQWs) from intrinsic piezoelectric polarization filed. Therefore, the modulation bandwidth is usually less than 20 MHz [9–13]. Besides, the “blue LED + yellow phosphors” white LED suffers from the problems of the phosphors efficiency droop and the low color rendering index (CRI) caused by the emission deficiency in the visible spectrum. In addition, multi-chip LEDs, for example, by combining red-green-blue (RGB) chips or red-green-blue-yellow (RGBY) chips, can achieve high CRI and a higher modulation bandwidth, but the multi-chip modulation is complex for communication.

Micro-LEDs can overcome the above limits of the modulation bandwidth due to the reduction of the RC time delay and the release of the polarization field. Recent works have demonstrated that Micro-LEDs with a typical size of tens of µm acquire the bandwidth of hundreds of MHz, higher than broad area LEDs [14,15]. However, there is great requirement of higher CRI LEDs. For the traditional Micro-LED, it usually has a fixed epitaxial layer and MQWs structure, which largely determines that the Micro-LED can only emit monochromatic light and limit its multi-color applications [16]. In order to emit multi-color in the Micro-LEDs, the structure of MQWs has to be changed. To date, for the phosphor-free multi-wavelength structure, the blue light based on short-wavelength region has a higher luminous efficiency, and the green light has a lower luminous efficiency. The yellow/red light based on long-wavelength region is even more difficult to achieve. If realizing the growth of multi-wavelength MQWs of Micro-LEDs together, it will lay the foundation for monolithic, high CRI, high-efficiency and cost-effective white LEDs. To the best of our knowledge, there are few reports of dual-color Micro-LED used in VLC.

In this work, we succeeded in fabricating four dual-wavelength In_xGa_1-xN/GaN Micro-LEDs with the diameters of 120, 100, 80 and 60 µm, respectively. The electroluminescence (EL) spectra and the light output power density (LOPD) characteristics are investigated in detail. It is found that compared with the broad area LED, the Micro-LEDs can sustain very high current density, deliver high density of light output power and achieve very high optical modulation bandwidth. Besides, the smaller diameter LED shows higher LOPD. More importantly, the droop rate in the blue quantum well (QW) is faster than that of the green QW. The maximum value of LOPD is determined by the blue QW for the broad area LED, while the maximum value of LOPD is determined by the green QW for the Micro-LED. Moreover, the -3dB modulation bandwidth of the dual-wavelength Micro-LEDs increases as the diameter decreases. Obviously, the Micro-LEDs with dual-wavelength In_xGa_1-xN/GaN MQWs show great potential applications in meeting both lighting and fast VLC.

2. Experimental

A dual-wavelength-based structure used in this study was initially grown on c-plane (0001) sapphire substrate in a metal organic chemical vapor deposition (MOCVD) system. After the deposition of a low-temperature nucleation layer on the sapphire substrate, it was followed to grow epitaxial layers of the dual-wavelength-based LED structure. The epitaxial layers included approximately 2 µm Si-doped n-GaN layer, a six-period In_xGa_1-xN/GaN MQWs layer, and approximately 437 nm Mg-doped p-GaN contact layer. The In_xGa_1-xN/GaN LED structure was shown in Fig. 1(c). The In_xGa_1-xN/GaN MQWs layer from top to bottom was composed of one period of blue QW (In_0.14Ga_0.86N), one period of green QW (In_0.23Ga_0.77N) and four periods of blue MQWs. The blue MQWs and green QW were grown at 740 °C and 690 °C, respectively, with the reactor pressure of 150 Torr, while the barriers were also grown at 810 °C under the 150 Torr. It was worth emphasizing that a thin GaN barrier had been inserted between the blue QW and the green QW, which was closer to the p-GaN contact layer, and the thickness was about one-half of the normal GaN barrier. The target of adopting this MQWs structure was to emit dual-wavelength light. The first QW close to the p-GaN contact layer was designed as blue QW, because blue QW close to the p-GaN contact layer can contribute to reduce the height of the barrier and enable more holes to reach the green QW. The design of the thin GaN barrier between the blue QW and the green QW may help holes to easily inject into the bottom QW. After that, four groups of the dual-wavelength In_xGa_1-xN/GaN Micro-LEDs based on 4×4 circular mesa arrays with the diameters of 120, 100, 80, and 60 µm were fabricated by the photolithographic mask and inductively coupled plasma (ICP) techniques, respectively. Finally, we used the wet etching method with 5% KOH to treat surface of the Micro-LEDs.

 

Fig. 1. The schematic diagram of the dual-wavelength In_xGa_1-xN/GaN Micro-LED. (b) The section diagram of a single micro-column. (c) The detailed drawing of the dual-wavelength In_xGa_1-xN/GaN structure.

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The dual-wavelength-based Micro-LEDs were processed into chips using standard processing techniques as follows: first of all, the Ti/Al/Ti/Au (20/60/30/100 nm) stacks were deposited around the mesa as n-electrode. The stacks were annealed utilizing a rapid thermal process (RTP) at 1000 °C for 30 s in N₂ ambient. The p-electrode was made up of Ni/Au (20/20 nm) metal stacks and was annealed at 700 °C for 1 min in N₂ ambient. Afterwards, a SiO₂ passivation layer with the thickness of 1100 nm was deposited by plasma-enhanced chemical vapor deposition (PECVD). In order to get the SiO₂ holes, we adopted the combination of dry and wet etching technologies. Compared with the dry etching method, the wet one had less cost and was more convenient. However, wet etching had the characteristics of isotropy. If the corrosion time was too long, the SiO₂ holes would become larger, even resulting in the disappearance of passivation effect. Despite of the accuracy of dry etching technology, it may bring some damages to metal electrode and thus influence the device performance. Thus, we first etched SiO₂ by dry etching technology. The dry etching was based on CHF₃/Ar gas chemistry with a RF power of 100 W. We performed two dry etching processes, and each etching time was 16 minutes. When it was about 50 nm away from the p-type electrode, we etched SiO₂ with BOE for 25 s. Then an Al/Ti/Au (1700/50/300 nm) metal layer was deposited on the exposed n-electrode and p-electrode. Figures 1(a)–1(b) showed the schematic diagram of the fabricated Micro-LEDs. Finally, the dices were bonded on an AlN ceramic submount by solder paste interconnect to form LED arrays with different diameters. In this step, we adopted reflow soldering technology to adhere the dice on AlN ceramic submount and then heated it for a few seconds with a temperature of 260 °C by eutectic mounter. A broad area LED based on dual-wavelength structure with the active region area of approximately 153200 µm² was also fabricated as a reference sample.

3. Results and discussion

The scanning electron microscopy (SEM) image of the dual-wavelength In_xGa_1-xN/GaN Micro-LED with the diameter of 100 µm is shown in Fig. 2(a). It can be seen that the morphology of the sixteen micro-columns is almost the same, as highly uniform array. Figure 2(b) shows height measurement result of the dual-wavelength In_xGa_1-xN/GaN Micro-LED with the diameter of 100 µm by step profiler. The height of the micro-column is approximately 900 nm. Figure 2(c) provides a direct observation of the image from the Micro-LED top emission under the same current density of 20 A/cm², which shows a very uniform luminescence.

 

Fig. 2. (a) The SEM image of the dual-wavelength In_xGa_1-xN/GaN Micro-LED with the diameter of 100 µm. (b) The height measurement result of the dual-wavelength In_xGa_1-xN/GaN Micro-LED with the diameter of 100 µm by step profiler. (c) The optical microscope images of the dual-wavelength In_xGa_1-xN/GaN Micro-LEDs (top view) emission patterns with different sizes under the same current density of 20 A/cm².

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Figures 3(a)–3(e) show the changes of the peak intensity values as a function of current densities for the broad area LED and the Micro-LEDs with different diameters. In the Micro-LEDs, the strain of both the blue and green QWs is released. In comparison, the green QW presents a larger strain relaxation than the blue QW due to high In composition. Thus, the intensity of the green light rises faster than that of the intensity of the blue light with the increase of current densities. It can also be seen that blue and green peak intensity values decrease dramatically at high current density. The carrier localization in nanoscale In-rich regions plays a crucial role in light emission area of the blue and green MQWs. With the increase of current density injection, electrons will be released to the conduction band owing to the filling up of the localized state band. Thus, these electrons can move to the defects nearby and form non-radiative recombination with injected holes, intensifying self-heating effect and resulting in the decline of blue and green peak intensity values [17]. Obviously, the blue peak intensity value reaches the maximum at 200 A/cm², while the green peak intensity value has been increasing for the broad area LED. However, the blue peak intensity values reach the maximum at 100 A/cm², 200 A/cm², 300 A/cm², and 400 A/cm², and the green peak intensity values reach the maximum at 300 A/cm², 400 A/cm², 600 A/cm², and 700 A/cm² for the Micro-LEDs with the diameters of 120, 100, 80, and 60 µm, respectively. That is to say, the droop effect in the blue QW is faster than that of green QW, which may be caused by the positions of the blue and green QW. Under the injection of high current density, the blue QW is more prone to carrier leakage, leading to the peak intensity value to decrease easily. In addition, the smaller diameter Micro-LED can withstand higher current density. This phenomenon is explained by analyzing the difference between the electron and hole concentrations on the edge and in the center for Micro-LEDs with different diameters. The electron overflow is highest on the edge and lowest in the center for Micro-LEDs with the diameter of 120 µm, but it is relatively uniform for Micro-LED with the diameter of 60 µm. Besides, with the increase of current density injection, the carrier concentration distribution becomes increasingly non-uniform especially for the Micro-LED with the diameter of 120 µm, following obvious current crowding. The relatively high local carrier concentration at the edge of larger Micro-LED enhances Auger recombination and other non-radiative recombination, resulting in local overheating and then the catastrophic device degradation [18,19]. The smaller Micro-LED exhibits more uniform in-plane hole and electron concentration distributions, which explain why the smaller Micro-LED can sustain a higher injection current density [20]. Therefore, smaller Micro-LED favors in a more uniform current spreading, which leads to the improved electrical characteristics [21]. In addition, when dual-wavelength Micro-LED is used for lighting, the blue-green ratio is different under different current densities. Thus, the current density of Micro-LED can be fixed at the highest CRI value by power supply and circuit design. If need to adjust the brightness during lighting, it may be realized by controlling the number of Micro-LEDs.

 

Fig. 3. (a) The changes of the peak intensity values as a function of current densities for (a) the broad area LED, and the Micro-LEDs with the diameters of (b) 120 µm, (c) 100 µm, (d) 80 µm, and (e) 60 µm, respectively. (f) The LOPDs as a function of current densities for the broad area LED and the Micro-LEDs with different diameters.

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Figure 3(f) shows the LOPDs as a function of current densities for the broad area LED and the Micro-LEDs with different diameters. The active region area of all micro-columns is used when calculating LOPD of the Micro-LED arrays. It can be observed that the maximum value of LOPD is determined by the blue QW for the broad area LED. In comparison, when the green peak reaches the maximum, the LOPD of the Micro-LED reaches the maximum. This is because green peak intensity value in the Micro-LED gets greatly increased compared with the broad area LED. The reasons may be stated as followed. Firstly, the high In component in the green QW has bigger lattice mismatch between the green QW and the barrier layer, resulting in lower luminous efficiency caused by the introduction of larger piezoelectric polarization field. After etching, the QCSE is greatly relieved owing to the strain relaxation, thereby the green light emission intensity is significantly enhanced. Secondly, the Micro-LED has waveguide effect and high LEE, which enables more light generated from the active region and results in the stronger green peak intensity value. Thirdly, due to the waveguide effect, blue light is easier to be absorbed by green QW, which makes the secondary excitation of green QW from blue light in the vertical direction, thereby enabling the enhanced peak intensity of green QW. Fourthly, compared with broad area LED, the Micro-LED can effectively alleviate the crowding effect of current and make the current distribution more uniform, which alleviates the self-heating effect and increases the radiation recombination probability in the green QW. Therefore, the enhancement of the green peak intensity value for the Micro-LED is considered as a consequence the combined effects of the reduction in the QCSE and the crowding effect, high LEE as well as geometric shape. Even more interesting is that the smaller diameter Micro-LED can provide higher LOPD at the same current densities. The increase in LOPD in the smaller device is mainly owing to the better uniformity of the current spreading [21]. It is also noteworthy that the Micro-LED with the diameter of 60 µm provides the highest maximum LOPD of up to 13.44 W/cm², which is over 2.2 times higher than the value in the Micro-LED with the diameter of 120 µm with a maximum LOPD of 5.98 W/cm², as well as is over 4 times higher than the value in the broad area LED with a maximum LOPD of 3.34 W/cm².

Further analysis of the electrical and optical characteristics in those LED devices are conducted, as shown in Figs. 4(a)–4(b). The EL spectra under a current of 20 mA are presented. As shown in Fig. 4(a), the spectra contain the positions of the blue and green peaks in the broad area LED and Micro-LEDs with different diameters, respectively. It shows that the positions of the blue and green peak in the broad area LED are located at 452.3 nm and 537 nm, respectively. While the positions of the blue and green peak in the Micro-LEDs with the diameters of 120 µm, 100 µm, 80 µm, 60 µm are located at 441.9 nm and 513 nm, 441.2 nm and 507 nm, 440.3 nm and 506 nm, 439.2 nm and 505 nm, respectively. Obviously, the positions of the blue and green peak in the Micro-LEDs all show distinct blue-shift compared with the broad area LED. The EL peak wavelength positions of the Micro-LEDs demonstrate a blue-shift due to both the band-filling effect and the QCSE, which is dominated depending on the change of full-width at half-maximum (FWHM). The FWHM decreases owing to the weakening of QCSE, while the FWHM increases due to the band filling effect [22,23]. In our test result, we can observe a narrow FWHM in blue peak on Micro-LEDs arrays, which reduces about 4∼6 nm compared to broad area LED. It implies the localized strain relaxation in the In_xGa_1-xN/GaN MQWs region is dominated [24]. And the green peak presents a larger blue-shift than the blue peak, due to the stronger influence of the strain relaxation on the light emission for high In composition [25]. Besides, via comparing the broad area LED and Micro-LEDs injected with the same level of current of 20 mA, the smaller diameter Micro-LED shows higher brightness, owing to much higher injected current density.

 

Fig. 4. (a) The EL spectra of the broad area LED and Micro-LEDs with different diameters under a current of 20 mA. (b) The far-field beam profile of the broad area LED and the Micro-LEDs with different diameters at a driving current of 50 A/cm².

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Next, we carry out the measurement of the light output angular distribution for the broad area LED and Micro-LEDs with different diameters at 50 A/cm², as shown in Fig. 4(b). Here, the intensity has been normalized. The viewing angles (where light emission intensity is 50% of the maximum) for the broad area LED, the Micro-LEDs with the diameter of 120 µm, 100 µm, 80 µm, 60 µm are ∼133.31°, ∼112.03°, ∼116.04°, ∼120.17° and ∼124.14°, respectively. The Micro-LEDs with different diameters exhibit a smaller divergence angle compared with the broad area LED, indicating more photons are redirected to the vertical direction of the chips [26]. It is also noted that the viewing angle tends to decrease, as the diameter of the micro-column increases, illustrating increased directionality of light propagation from the bigger diameter Micro-LED. This is because the bigger diameter Micro-LED has bigger area of inclined sidewall, resulting in more reflection from micro-column sidewall and vertical extraction of photons. Here, Micro-LEDs with the different diameters are based on 4×4 circular mesa arrays and the column amount does not change with the size.

The dual-wavelength In_xGa_1-xN/GaN Micro-LEDs above mentioned not only lay the foundation for SSL but also are used for VLC. Figures 5(a)–5(c) demonstrate the frequency response characteristics and the extracted -3dB modulation bandwidths of the broad area LED and the four Micro-LEDs with the different diameters at different injection current densities from 30 A/cm² to 300 A/cm². Figure 5(a) shows the -3dB modulation bandwidths in the Micro-LED with the diameter of 60 µm gradually increases from 60.71 MHz to 117.80 MHz. More specifically, we can observe from Fig. 5(a) that the bandwidth increases with the increase of the injection current density, which may be attributed to the decrease of the effective carrier lifetime at higher current density. Considering the bi-molecular recombination mechanism, the modulation bandwidth can be expressed as follow [27,28]:

 

Fig. 5. (a) The frequency responses of the Micro-LEDs with the diameter of 60 µm as a function of the injection current densities. (b) A comparison of the frequency response of the broad area LED and the four Micro-LEDs with different diameters at the same injected current of 300 A/cm². (c) Comparison of extracted -3 dB modulation bandwidths of the broad area LED and the four Micro-LEDs with different diameters as a function of the injection current densities.

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As shown in the Fig. 5(b), the -3dB modulation bandwidths of the broad area LED and the four Micro-LEDs with the diameters of 120 µm, 100 µm, 80 µm, 60 µm reach ∼48.41 MHz, ∼70.16 MHz, ∼80.33 MHz, ∼92.08 MHz, ∼117.80 MHz at 300 A/cm², respectively. Figure 5(c) further summarizes the dependence of the -3dB bandwidth in the different diameters as well as injection current dentisties. It's easy to see that the -3dB bandwidth of the four Micro-LEDs with the different diameters are much higher than that of the broad area LED, either at the low current density or at the high current density, indicating the superiority of the high bandwidth of our dual-wavelength Micro-LED devices. It can learn from Fig. 5(c) that the higher the injected current density of the Micro-LED is, the higher the modulation bandwidth is. Moreover, at the same injected current density the observed bandwidth is higher for smaller Micro-LED. This result is different from a previous report with 450 nm devices [29]. This difference may be attributed to improve current spreading across the active area of the smaller diameter Micro-LED as well as an associated reduction in the junction temperature [15,20]. However, the -3dB modulation bandwidth of our dual-wavelength Micro-LED needs further improvement. Under the driving of positive voltage, the holes provided by p-GaN and the electrons provided by n-GaN can recombine in the QW. Because of the large effective mass and low mobility of the holes, the recombination usually occurs in the QW near the p-GaN side. In our dual-wavelength MQWs structure, the first pair of the QW near p-GaN is the blue QW, followed by a pair of green QW and four pairs of blue QWs. Compared with the single green QW structure, holes need to pass through the blue QW before they can recombine with the electrons in the green QW. Secondly, due to the secondary excitation of green QW from blue light, holes and electrons can recombine again. Both the processes prolong the carrier life, thereby affecting the modulation bandwidth to a certain extent. But dual-wavelength Micro-LEDs can save the cost and further reduce integrated device size compared with traditional monochrome Micro-LEDs realized by massive transfer. This is preliminary research results, and our next work will focus on optimizing the technical processes to get wider dual-wavelength and high modulation bandwidth Micro-LEDs.

4. Conclusion

In summary, we have performed a systematic investigation on the size dependence of LOPD, spectra characteristics, the far field emission patterns and -3dB modulation bandwidths in the dual-wavelength In_xGa_1-xN/GaN Micro-LEDs. Our experimental results confirm that the smaller diameter Micro-LED devices can deliver higher LOPD as well as sustain much higher current density. And the enhancement of the green peak intensity value for the Micro-LEDs are integrated results of multi-factor effects, namely, the reduction in the QCSE and the crowding effect, high LEE as well as geometric shape. We have also demonstrated that the -3dB modulation bandwidth increases with the increase of the injection current density. Besides, we observe that a high -3dB modulation bandwidth may be obtained when the diameter decreases from 120 µm to 60 µm, and the -3dB modulation bandwidth of 60 µm Micro-LED shows a 1.4-fold increase compared with board area LED when the current density is 300 mA/cm². This study provides the potential direction of developing system for the SSL and VLC based on white LEDs.