1. Introduction

Tunnel junctions (TJs) were firstly reported by Esaki in a heavily doped narrow-bandgap Ge pn junction, where a very thin depletion layer was formed at the pn junction interface [1]. It has been attractive to grow TJ on top of the p-type layers for III-nitride optical devices [2–8]. By operating the TJs at a reverse bias condition, electrons can tunnel through the thin depletion layer from p-type region to the n-type layer, which means that holes can be generated in the p-type layers and injected into the active regions. TJs offer several advantages such as a reduction of optical loss, an improvement of current spreading, and a better ohmic contact for GaN-based light-emitting diodes (LEDs) and laser diodes [2–5]. Moreover, TJs enable a cascaded structure with two or more LEDs in one device, where each light emitting junction is connected in series by a TJ [6–13]. Cascaded LEDs with independent junction control can be realized by utilizing each TJ. When we grow LEDs with different emission colors in one stack, cascaded LEDs with full colors or arbitrary colors can be realized. Akyol et al. reported two cascaded LEDs and three cascaded LEDs using molecular beam epitaxy (MBE)-grown TJs, however, the output power of the cascaded LEDs remains low [6,7].

Metalorganic chemical vapor deposition (MOCVD)-grown TJs face the challenge of p-type GaN activation [14]. Mg atoms in the p-type GaN would be re-passivated by hydrogen, which originates from the NH₃ decomposition, during the n-type GaN growth to form TJs on top of the underlaying p-type GaN [15]. The overgrown n-type GaN also prevents the hydrogen escape during the activation process, resulting in a poor activation efficiency of p-type GaN. In our previous studies, MOCVD-grown TJs LEDs with a low forward voltage were achieved by utilizing selective area growth (SAG) [16]. We have also pursued InGaN layers within the TJ to reduce the excess voltage for the TJ compared with reference ITO contacts [17,18]. In GaN-based micro-size LEDs (µLEDs), hydrogen could be driven out through sidewalls in the small dimension device, leading to a better p-type GaN activation efficiency [16–19]. Recently, µLEDs have become very attractive for the applications of augmented reality, virtual reality, large-area display, and visible light communication system, which are causing huge attentions from both academies and industries [20–23]. Transfer and assemble of millions of blue, green, and red µLEDs for display application is the biggest challenge for the mass production of µLEDs [23]. Cascaded µLEDs provide a promising solution to overcome such challenge. However, there is only a few reports about the GaN-based cascaded LEDs with independent control until now. Siekacz et al. presented a vertical integration of blue and green LEDs by TJs using plasma-assisted molecular beam epitaxy (PAMBE) [8]. MOCVD-grown TJs cascaded GaN-based LEDs with independent control remain highly desired since MOCVD technology meets the industry standard.

In this study, we demonstrate fully transparent MOCVD-grown cascaded blue and green µLEDs with independent junction control. All the LEDs and TJs were grown by MOCVD. Top TJ was grown as a fully transparent current spreading layer, so that we do not need any conductive oxide layer such as indium tin oxide (ITO) or ZnO.

2. Simulation and experiments

First, the band diagram of the fully cascaded TJs InGaN LEDs was investigated. Figure 1 shows the simulated band diagram of the cascaded InGaN LEDs consisted by blue light emitting junction, a TJ, green light emitting junction, and a TJ under forward bias conditions using the 1D drift-diffusion formalism. Under a forward bias of 7 V, the TJs between blue and green LED and the top TJ will be reversed-biased. Since both junctions are highly doped, the valence band maximum of the respective p-GaN will be raised above the conduction band minimum of the respective n-GaN layer. Electrons can tunnel from the valence band of the p-GaN layer to the conduction band of the n-GaN layer through band-to-band tunneling for the blue LEDs and the green LEDs. Meanwhile, holes created by the same process are injected to the active region under electric field and recombine with electrons transport from the other TJ, which efficiently assist the radiative recombination process in both LEDs. Therefore, this design theoretically enables the cascaded InGaN LEDs with independent junction control.

 

Fig. 1. 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 and holes in the TJs.

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The cross-sectional schematic structure of the cascaded blue/green µLEDs with individual contacts is shown in Figure 2(a). Standard blue LEDs were cleaned with chemical treatment and loaded into MOCVD reactor for the overgrowth of TJs and green LEDs immediately. n-InGaN/n⁺GaN/n-GaN with a thickness of 2 nm/20 nm/380 nm were overgrown to form TJs. The Si doping concentration in the n-InGaN and n⁺GaN was 1.7×10²¹ cm⁻³. Details about TJs growth conditions can be found in our previous publications [16–19]. The green emitting junction was consisted of 1 µm n-type GaN, 30 pairs In_0.06Ga_0.94N/GaN (3 nm/6 nm) superlattice (SLs), 5 pairs of InGaN/Al_0.30Ga_0.70N/GaN (3 nm/2 nm/9 nm) multiple quantum wells (MQWs), a 20 nm p-type AlGaN electron blocking layer (EBL), a 120 nm p-type GaN and a 20 nm heavily Mg-doped p⁺GaN [24]. Cascaded LEDs were taken out of the MOCVD reactor, following by a similar chemical treatment. TJ was grown on top of the green LEDs. The n-type GaN of the green LEDs and blue LEDs was exposed by the reactive ion etching (RIE) with a depth of 1 µm and 1.8 µm, respectively. The p-type GaN of blue and green LEDs were activated by rapid thermal annealing (RTA) at 700 °C for 30 minutes to remove the hydrogen in the p-type region through sidewalls [16–19]. Thanks to the small mesa size, both p-GaN could be fully activated. Omnidirectional reflector (ODR) composited of 5 pairs of SiO₂ and Ta₂O₅ stack with 20 nm Al₂O₃ on top were deposited by ion beam deposition. A 25-nm SiO₂ layer was deposited as sidewall passivation layer by atomic layer deposition (ALD) [16–19,21–23]. Al/Ni/Au (500 nm /100 nm /500 nm) were evaporated as the ohmic contact to the three n-GaN layers. The three pads labelled as ①, ②, and ③ were the ohmic contact to the n-GaN of the top TJ, n-GaN of the middle TJ, and the n-GaN of the blue LEDs. Figure 2(b) shows the scanning electron microscope (SEM) of the fabricated cascaded µLEDs. The contact pads were well labeled, which correspond to the labels in Figure 2(a). The cross-finger metal pad ② works as ohmic contact layer for the n-type GaN layer of green µLEDs and the TJs of blue µLEDs. By injecting current through pads of ① and ②, ② and ③, and ② and ③, we can control green emission, blue emission, and blue/green emission, respectively. The cross-finger contact layer was embedded into the mesa of green µLEDs, which can enlarge the exposed sidewall areas of the green µLEDs and improve the p-GaN activation efficiency. Also, it can improve current spreading for the blue µLEDs with more uniform current injection. Although it would introduce more sidewall damage for the green µLEDs, chemical treatment such as KOH or H₃PO₄ could be carried out to recovery the sidewall damage as shown in Ref. 25 from our group.

 

Fig. 2. (a) Detailed schematic structure of the full cascaded TJs blue and green µLEDs and (b) SEM image of the fabricated cascaded µLEDs.

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3. Results and discussion

Figures 3(a) to 3(c) show the emission spectra of the blue µLEDs, green µLEDs and blue/green µLEDs at an injection current of ∼0.4 mA. The inset shows the individual electrical luminous images for the same device took from microscope, and we can see uniform luminous images in the TJ blue µLEDs, TJ green µLEDs, and blue/green µLEDs, proving that the TJs are efficient and p-GaN has been activated. The emission peak wavelength for the TJ blue µLEDs and TJ green µLEDs is 455 nm and 508 nm, respectively, followed by a full-width half maximum (FWHM) of 18 nm and 33 nm. A very weak green shallow emission appears in the spectrum of the TJ blue µLEDs, which is caused by the excitation of the green MQWs by the blue light. Since the green emission peak is so weak that it would not affect the blue emission from the bottom blue µLEDs. Another consideration for growing green µLEDs on top is that the growth temperature of green MQWs is much lower than that of blue MQWs. For the cascaded blue/green LEDs, two emission peaks located at 445 nm and 507 nm were observed. Therefore, we can realize TJ blue µLEDs, TJ green µLEDs and blue/green µLEDs in the same devices by injecting the current through different contact pads.

 

Fig. 3. Electrical luminous spectrum for (a) TJ blue µLEDs, (b) TJ green µLEDs, and (c) blue/green µLEDs. The insets show the electrical luminous microscope images of blue µLEDs, green µLEDs and blue/green µLEDs, respectively.

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The emission spectra of the cascaded TJ blue/green µLEDs at various injection current from 0.3 to 3 mA is shown in Figure 4(a). Two clear emission peaks located in blue and green region appear when the current is higher than 0.3 mA. The extracted blue and green peaks wavelength versus injection current are shown in Figure 4(b). Both emission peaks show blue shift. The blue peak decreases from 447 nm to 443 nm, and the green peak reduces form the 513 nm to 502 nm as the current increases from 0.3 to 1 mA. The wavelength blue-shift is typically observed in the c-plane InGaN LEDs, which is due to carriers filling higher energy localized states and screening of the polarization-related electric fields. The integrated intensity ratio of the blue and green peaks (I_blue/I_green) versus current are also shown in Figure 4(b), which increases from 0.03 to 0.5 with increasing current from 0.3 to 1 mA [25]. The injected carriers were firstly captured in the blue active region at a small current. As the current increases, more carriers were radiatively recombined in the green active region. Such carrier dynamic behavior in the cascaded TJ µLEDs maybe caused by the non-radiative recombination paths related to the more sidewall damage in TJ green µLEDs. Further studies would be carried out to understand the carrier recombination process, especially for the sidewall recovered treatment [26].

 

Fig. 4. (a) Electrical luminous spectra at various injection current for blue/green µLEDs and (b) The extracted blue peak, green peak, and intensity ratio of blue/green peak at various injection current.

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The current-forward voltage (I-V) curves of the blue µLEDs, green µLEDs, and blue/green µLEDs are shown in Figure 5(a). At 1 mA, the forward voltage (V_f) in TJ blue µLEDs and TJ green µLEDs is 4.20 V and 3.48 V, respectively. The V_f of the cascaded blue/green µLEDs is 7.62 V at 1 mA, which is slightly smaller than the total voltage in TJ blue µLEDs and TJ green µLEDs. At a current density of 20 A/cm², the V_f for the TJ blue µLEDs (60×60 µm²) and TJ green µLEDs (∼53×53 µm²) is 4.06 V and 3.13 V, respectively, indicating efficient TJs and a p-type GaN activation efficiency. It is worth noting that the V_f in the TJ green µLEDs is one of the lowest V_f reported in the GaN-based LEDs with TJs contact [3,4,14,16–18]. Such low V_f in the TJ green µLEDs can be understood by the enlarged sidewalls areas shown in Figure 2(b), which provide the visible paths for the out diffusion of hydrogen during RTA. Moreover, the narrow bandgap of green QWs would result in the lower V_f in the TJ green µLEDs as compared to that of TJ blue µLEDs. The output power versus injection current for the blue µLEDs, green µLEDs and blue/green µLEDs was shown in Figure 5(b). The output power increases linearly with the injection current for the three LEDs. At 4 mA, the output power of the blue µLEDs, green µLEDs, and blue/green µLEDs is 0.89 mW, 0.43 mW and 1.41 mW, respectively. Those optical and electrical properties of the cascaded TJ µLEDs indicate that the width of the depletion region in the TJs were thin enough and the p-GaN layers were fully re-activated. Therefore, we can control the injection of carriers into blue, green, and blue/green active regions independently, showing huge promising potentials for the integration of multiple µLEDs.

 

Fig. 5. (a) I-V characteristics and (b) Output power versus current curves for TJ blue µLEDs, TJ green µLEDs, and blue/green µLEDs.

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4. Conclusion

In summary, we present high performance cascaded blue/green µLEDs with independent junction control utilizing MOCVD-grown TJs. This demonstration provides promising application of TJs for cascaded µLEDs with multiple color emissions in the same device.