1. Introduction

III-nitride based tunnel junction (TJ) formed by Mg heavily doped p ⁺GaN and highly Si-doped n ⁺GaN, has become attractive for III-nitride light-emitting diodes (LEDs) and laser diodes (LDs). [1–7] III-nitride based TJ can improve the current spreading and reduce the light absorption, resulting in an enhanced external quantum efficiency (EQE). [1,2] AlGaN-based TJ can significantly increase the light extraction efficiency of AlGaN-based deep ultraviolet (DUV) LEDs with a high carrier injection efficiency since the common transparent conductive oxide layer such indium tin oxide (ITO) leads to a high absorption loss in the DUV wavelength range. [3,4] Another important application of TJ is the vertically integrated LEDs or micro-size LEDs (µLEDs). [5–7]

In the hybrid TJ, the p ⁺GaN is grown by metal organic chemical vapor deposition (MOCVD) and the Si-doped n ⁺GaN is grown by molecular-beam epitaxy (MBE). The hybrid TJ has been well developed and implemented in the III-nitride optoelectronics devices. Yonkee et al. reported InGaN blue LEDs with a very high peak wall-plug efficiency (WPE) of 72% using hybrid TJ contact. [2] GaN semipolar edge emitting LDs and vertical-cavity surface-emitting laser with hybrid TJ contact have been demonstrated. [8,9] Pandey et al. presented an AlGaN DUV LEDs with a maximum external quantum efficiency (EQE) of 11% using AlGaN/GaN/AlGaN TJ. [3] The advantage of hybrid TJ includes a sharp Si/Mg doping profile and there is no issue about the activation of the buried p-GaN. [1,2] Fully MOCVD-grown TJ face the issue associated with the passivation of the buried p-GaN by H⁺ in NH₃-rich MOCVD reactor, although a lot of attempts have been made to improve the fully MOCVD-grown TJ performance, such as selective area growth and sidewall chemical treatment by KOH solution. [10–13] III-nitride cascaded µLEDs are a promising approach to improve the output power of single devices and integrate µLEDs with multiple emission colors. III-nitride cascaded µLEDs MOCVD-grown TJ is very challenging because a lot of difficulties need to be addressed, such as the activation of the bottom p-GaN and the injection of carriers from one junction to another through TJ [14,15]. For the III-nitride cascaded µLEDs with MOCVD-grown TJ, sidewalls become the only visible path to drive out the H⁺ from the buried p-GaN, resulting in a nonuniform electrical injection and luminescence from the bottom µLEDs. Hybrid TJ is a promising technology to achieve high performance III-nitride cascaded µLEDs due to a low growth temperature of n ⁺GaN in the MBE reactor without H⁺, which can passivate the Mg in the p-GaN. The carrier transportation process can be easily understood. However, there are no reports about the application of hybrid TJ for the multiple color III-nitride cascaded µLEDs with independent junction control.

In this study, we demonstrated high performance InGaN-based cascaded blue/green µLEDs using hybrid TJ. Uniform blue, green and blue/green emissions can be controlled independently. The carrier transportation between different junctions through TJ was discussed.

2. Experiments

Fig. 1(a) shows the cross-sectional schematic epitaxy structure of the InGaN blue and green µLEDs stacks with hybrid TJ. The InGaN blue green LEDs were grown by MOCVD, which were consisted of 3 µm unintentional doped (uid) GaN, 3 µm Si-doped n-GaN, 20 pairs InGaN/GaN superlattices (SLs), 6 pairs InGaN/GaN blue multiple quantum wells (MQWs), 20 nm AlGaN electron blocking layer (EBL), 100 nm Mg-doped p-type GaN and 20 nm Mg heavily doped p ⁺GaN. The blue LEDs epitaxy wafer was treated with aqua regia for 10 min and then loaded into MBE chamber for the overgrowth of 20 nm n ⁺GaN and 380 nm n-GaN. The Si-doped concentration in the n ⁺GaN was as high as 2 × 10²¹cm^-3. The top InGaN green LEDs were grown using MOCVD. Similar epitaxy structure of the InGaN green LEDs using InGaN/AlGaN cap layer/GaN MQWs can be seen in our previous reports. [16,17] Blue/green µLEDs stacks with three contact pads were fabricated, enabling the independent junction control. 100-nm ITO was deposited on top of the green µLEDs. Reactive ion etching was used to expose the n-GaN layers of the InGaN blue and green µLEDs. The mesa size of the blue µLEDs and green µLEDs was 100 × 100 µm² and 80 × 80 µm², respectively. 5 pairs SiO₂ and Ta₂O₅ stacks were deposited by ion beam deposition (Veeco NEXUS). 30-nm SiO₂ layer was deposited by atomic layer deposition for sidewall passivation. Al/Ni/Au were deposited as the n-GaN ohmic contact pads. There are three contact pads, which were labeled as ①, ② and ③ in Fig. 1(a). Fig. 1(b) shows the scanning electron microscope (SEM) top-view image of the fabricated blue/green cascaded µLEDs, where the contact pads were labelled correspondingly. Independent junction control can be realized through injecting the current through different contact pads: blue emission, green emission and blue/green emission can be controlled by injecting current through pads ② and ③, ① and ②, ① and ③.

 

Fig. 1. (a) Cross-sectional schematic structure of the cascaded blue/green µLEDs with hybrid TJ; (b) SEM image of the fabricated cascaded µLEDs.

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3. Results and discussion

Electrical luminous images by microscope of the operating µLEDs at 2 mA are shown in Figs. 2(a) to 2(c). Uniform blue, green and blue/green emissions can be clearly seen. Figs. 2(d) to 2(f) are the emission spectra. The peak wavelength for the blue and green µLEDs is 431 nm and 517 nm, and the full-width half maximum is 19 nm and 30 nm, respectively. By injecting the current through pads ① and ③, both blue µLEDs and green µLEDs work simultaneously and the emission spectrum in Fig. 2(f) shows blue and green peaks. It is noted that no additional annealing was treated for the fabricated µLEDs. The uniform blue emission from the TJ blue µLEDs confirms that the buried p-GaN of the blue µLEDs is fully activated. The overgrowth of green LEDs by MOCVD on top of the hybrid TJ won’t affect the activation of the buried p-GaN in the blue µLEDs. For the cascaded blue/green µLEDs with MOCVD-grown TJ, additional annealing is required to active the buried p-GaN and the hydrogen can be only driven out through sidewalls. [5,6,11–13,18] Such issue associated with the full activation of p-GaN always results in a non-uniform electrical luminous image of the bottom blue µLEDs in the cascaded blue/green µLEDs with fully MOCVD-grown TJ. On contrast, the buried p-GaN in the cascaded µLEDs with hybrid TJ remains fully activated, followed by a more uniform current spreading and electrical luminous. The buried p-GaN remains fully activated after the overgrowth of the top n ⁺GaN by MBE. [1,2] Once the hybrid TJ was formed, the buried p-GaN won’t be passivated during the overgrowth of the green LEDs even under an NH₃-rich ambient at a high growth temperature.

 

Fig. 2. Electrical luminous images for the (a) TJ blue µLEDs, (b) green µLEDs, and (c) blue/green µLEDs at 1 mA. (d), (e) and (f) is the emission spectrum correspondingly.

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The current-forward voltage (I-V) curves of the blue TJ µLEDs, green µLEDs and blue/green µLEDs were shown in Fig. 3(a). The forward voltage at 20 A/cm² for the blue µLEDs and green µLEDs is 3.56 V and 3.48 V, respectively, followed by a turn on voltage of 1.9 V and 2.4 V. The differential resistance (R_d) of the TJ blue µLEDs and green LEDs with ITO contact were shown in Fig. 3(b). At 20 A/cm², the R_d is 4.3 × 10⁻² Ω cm² and 1.5 × 10⁻² Ω cm² for the TJ blue µLEDs and green µLEDs. As the current density increases, the difference of R_d between the TJ blue µLEDs and green µLEDs becomes less significant, proving the excellent contact of the hybrid TJ to the buried blue µLEDs. [19]

 

Fig. 3. (a) I-V curves of the TJ blue µLEDs, green µLEDs and cascaded blue/green µLEDs; (b) Differential resistance of TJ blue µLEDs and green µLEDs with ITO contact.

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Fig. 4(a) shows the emission spectra of the blue µLEDs + green µLEDs at various injection current. Both µLEDs were operating at the same time and there are two emission peaks form the spectra. As the injection current increases from 0.3 mA to 4 mA, the blue emission peak wavelength shifts from 432 nm to 427 nm, and the green emission peak wavelength shifts from 519 nm to 509 nm, which is caused by the carriers filling effect and the screening of the polarization electric fields in the MQWs. [20] To understand the carrier transportation between different junction diodes, the integrated intensity ratio (ρ) of the green peak to blue peak is plotted in Fig. 4(b). At a low current of 0.3 mA, ρ is about 1.3, which means that carriers were firstly captured at the green active ration, followed by the radiative recombination. The main reason for the higher green peak intensity is due to the lower energy bandgap of the InGaN green QWs. ρ decreases dramatically as the injection current increases and then stabilizes at a value of 0.34, which means that blue emission is much stronger than the green emission at high current. The main reason is that the EQE of the blue µLEDs is much higher than that of the green µLEDs. Another factor is the devices area of the blue µLEDs is larger than the green µLEDs. Such intensity ratio in the hybrid EL spectrum is tunable by controlling the EQE of the blue µLEDs and green µLEDs. [21]

 

Fig. 4. (a) Spectra at various injection current for blue + green µLEDs working at the same time; (b) The integrated intensity ratio of green/blue peak at various injection current.

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The output power-current curves of the blue µLEDs, green µLEDs, and blue/green µLEDs are shown in Fig. 5(a). The on-wafer output power increases linearly with the injection current for all the µLEDs. At 10 mA, the output power of the blue TJ µLEDs and green µLEDs is 2.96 mW and 0.97 mW, respectively. The EQE from the integrating sphere versus current density for the blue TJ µLEDs and green µLEDs is presented in Fig. 5(b). The peak EQE for the blue TJ µLEDs and green µLEDs is 30% and 12% respectively. The EQE of blue µLEDs is almost three times higher than the EQE of green µLEDs, which agrees well with the intensity ratio of green peak to blue peak with a value of 0.34. Therefore, the carrier transportation process can be interpreted in this way: the carriers are firstly injected into the green active region, resulting in green emission; as the current increases, the carriers start injecting through the green and blue active region, followed by green and blue emission; the carriers injected into the blue and green active regions seem to equal as suggested from the values of the EQE and the intensity ratio of the blue/green µLEDs. This means that carriers injection efficiency from one junction to another through the hybrid TJ is close to 100%. In our previous reports about the cascaded µLEDs, [6,7] the TJs were fully grown by MOCVD. The H⁺ in the buried p-GaN can be only driven out from the sidewalls of the devices, thus it is difficult to make the buried blue µLEDs working properly. We cannot well investigate the carrier transportation behavior in the cascaded µLEDs with fully MOCVD grown-TJs since the buried p-GaN is difficult to be fully activated. Hybrid TJs is a good solution to overcome this challenge in the cascaded µLEDs. To the best of our knowledge, this is the first demonstration of the cascaded µLEDs with independent junction control with hybrid TJs. These results show promising potential for the vertical integration of full color µLEDs to solve the mass transfer issue. [22]

 

Fig. 5. (a) Output power versus current for TJ blue µLEDs, green µLEDs, and blue/green µLEDs; (b) EQE at different current density of the blue µLEDs and green µLEDs.

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Finally, the simulated band diagram of the cascaded blue and green LEDs with a TJ was simulated using one-dimensional drift-diffusion solver as shown in Figure 6. The thickness and doping in each layer of the TJ were specified in the experimental sections. Under the forward bias of 7 V (applied between cascaded blue and green µLEDs), the TJ between two LEDs is reverse biased so that the valence band maximum of the p-GaN of TJ will be raised above the conduction band minimum of the n-GaN. Electrons can tunnel from the valence band of p-GaN to the conduction band of the n-GaN through band-to-band tunneling process. Meanwhile, holes created by the same process in the p-GaN are injected into the active region of the blue µLED under electric field and recombine with the electrons from the contact. Therefore, this design theoretically enables the efficient cascaded µLEDs with independent junction control, which agrees well with our experimental results.

 

Fig. 6. Schematic structure for the cascaded LEDs with TJs on top and band diagram at forward bias of 7 V. The band diagram shows the flow of electrons in the TJs.

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4. Conclusion

In summary, we demonstrate vertical integration of InGaN blue/green µLEDs using hybrid TJ. The hybrid TJ offers a simple approach to realize nitride-based cascaded µLEDs with independent junction control. Multiple color emissions can be tuned independently. The injection efficiency of the carriers from one junction to another is close to 100%. This design provides a promising approach to improve the output power of nitride-based LEDs with single chip and integrate µLEDs with multiple emission colors.