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Abstract
The electrical and optical improvements of AlGaInP micro-light-emitting diodes (µLEDs) using atomic-layer deposition (ALD) sidewall passivation were demonstrated. Due to the high surface recombination velocity and minority carrier diffusion length of the AlGaInP material system, devices without sidewall passivation suffered from high leakage and severe drop in external quantum efficiency (EQE). By employing ALD sidewall treatments, the 20×20 µm2 µLEDs resulted in greater light output power, size-independent leakage current density, and lower ideality factor. The forward current-voltage characteristic was enhanced by using surface pretreatment. Furthermore, ALD sidewall treatments recovered the EQE of the 20×20 µm2 devices more than 150%. This indicated that AlGaInP µLEDs with ALD sidewall treatments can be used as the red emitter for full-color µLED display applications.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Full-color micro-light-emitting diodes (µLEDs), of red, green, and blue emitters, are one of the most promising candidates for next-generation display applications, including near-eye and wearable displays [1,2]. Remarkable device performance has been demonstrated by highly energy-efficient blue and green III-nitride µLEDs; however, red emission remains a crucial challenge for III-nitride materials because of the large lattice mismatch between InN and GaN [3–7]. As a result, alternative approaches, such as III-nitride nanorods, quantum-dot color conversion, and Eu-doped GaN, have gained significant research attention recently, yet each method suffers from drawbacks of reliability, uniformity, or luminous efficiency [8,9]. The AlGaInP material system is a well-developed red emitter and has been employed in various applications with outstanding energy efficiency [10,11]. However, it has been shown computationally and empirically that AlGaInP has an order of magnitude higher surface recombination velocity than III-nitride materials, hence the external quantum efficiency (EQE) of AlGaInP LEDs drops dramatically as the device shrinks [12–15]. This makes AlGaInP based µLEDs inefficient and presents an issue that needs to be solved before employing for display applications. It has been reported that dielectric sidewall passivation using atomic-layer deposition (ALD) enhances the optical and electrical performances of InGaN µLEDs, and the decrease in peak EQE is reduced using sidewall treatments [16–19]. Nevertheless, the effectiveness of ALD sidewall passivation is unknown to AlGaInP µLEDs.
In this work, we demonstrated that the optoelectrical characteristics of AlGaInP µLEDs from 20×20 to 100×100 µm2 are improved by using ALD sidewall passivation. By employing ALD sidewall passivation, the devices resulted in better current-voltage characteristics, lower leakage current, and higher light output power. More importantly, the on-wafer EQE of 20×20 µm2 devices with ALD sidewall passivation was more than 150% higher than that without sidewall passivation. Furthermore, the use of trimethyl aluminum (TMA)/nitrogen plasma surface treatment further enhanced the forward current-voltage characteristics. This report revealed that the surface recombination issue of AlGaInP µLEDs can be mitigated by using proper sidewall treatments, thus highly efficient red AlGaInP µLEDs can be employed for display applications.
2. Experimental
The µLED devices were fabricated on a commercially available four-inch AlGaInP wafer grown on GaAs substrate using metalorganic chemical vapor deposition. The device design and geometry have been reported previously [4,16,19,20]. All µLEDs were fabricated in squares with edge length ranging from 20 µm, 40 µm, 60 µm, 80 µm, and 100 µm. All µLEDs were processed together from the same wafer to avoid variations in growth and fabrication. After organic solvent clean, 110 nm indium-tin oxide (ITO) was deposited via electron-beam evaporation as an ohmic and transparent p-contact. Figure 1(a) shows the ohmic p-contact current-voltage characteristics with different gap spacings of a circular transmission line measurement (CTLM) geometry. The ITO layer was removed using reactive-ion etching and the AlGaInP material was etched using chlorine-based inductively coupled plasma etching at 200°C. An omnidirectional reflector (ODR) that consisted of five pairs of alternating layers of silicon dioxide and tantalum pentoxide, followed by aluminum oxide (Al2O3) as a capping layer, was deposited using ion beam deposition. The ODR stack yielded 85% measured reflectance in the range of 630-650 nm to minimize light absorption and served as a metal isolation dielectric layer. For devices with sidewall treatments, TMA/nitrogen plasma was used for surface treatment before ALD where 50 nm of Al2O3 was deposited at 300°C. The precursors of the ALD Al2O3 layer were TMA and water. After ALD deposition, hydrofluoric acid was used to remove Al2O3 to open a window for metal deposition. The metal composed of 12/80/10/500 nm of Ge/Au/Ni/Au for common p- and n- metal contacts using electron-beam evaporation. Figure 1(b) shows a schematic of the fabricated device. The metal contacts were annealed at 430°C for 60 seconds under nitrogen to obtain ohmic n-contact. After device fabrication, on-wafer testing was performed to measure light output power-current-voltage characteristics, where a calibrated photodetector was placed vertically on top of the device to collect light emission.
Fig. 1. (a) current-voltage characteristics of ITO p-contact and (b) a schematic of the fabricated device. The inset is the CTLM structure, where the inner circle has a radius of 50 µm with different gap spacings range from 5 to 50 µm.
3. Results and discussion
Three sets of µLEDs with different sidewall treatments were fabricated: devices without sidewall treatments (reference), with ALD Al2O3 sidewall passivation (ALD), and with TMA/nitrogen plasma followed by ALD Al2O3 sidewall passivation (ALD + N). ALD Al2O3 was used due to its outstanding passivation feature [21]. The fabricated devices emitted at peak wavelength of 630 nm with full-width half maximum (FWHM) of 15 nm. The emission spectrum of a 100×100 µm2 device and the electroluminescence images of the five µLEDs at 1 A/cm2 with ALD sidewall passivation are shown in Fig. 2(a). The light output power characteristics of 100×100 and 20×20 µm2 devices are presented in Fig. 2(b). The 100×100 µm2 devices with sidewall passivation resulted in comparable light output power characteristic to those without sidewall passivation with 10% variation, whereas the 20×20 µm2 devices with sidewall treatments yielded 150% improvement in light output power. The 10% difference in light output power in the 100×100 µm2 devices could be caused by fabrication or measurement variations. Based upon the results from Monte Carlo ray tracing simulation using Synopsys LightTools software, the increase in light extraction efficiency by ALD Al2O3 sidewall passivation was 15% for all the five AlGaInP device dimensions, compared to devices without sidewall passivation. The boost in light extraction efficiency is caused by the difference in refractive index, where the dielectric material has a smaller refractive index than the semiconductor [16,22]. Moreover, it has been shown that ALD sidewall passivation reduces non-radiative recombination sites, such as surface defects and dangling bonds [17]. Therefore, the increase in light output power was attributed to the enhancement in light extraction efficiency and the decrease in non-radiative recombination at the surface.
Fig. 2. (a) Electroluminescence (EL) wavelength spectrum of a 100×100 µm2 AlGaInP µLED and (b) light output power characteristics of 100×100 and 20×20 µm2 µLEDs with different sidewall treatments. The insets in (a) are EL images of the five µLED sizes at 1 A/cm2.
Besides the optical performance, the advantages of ALD sidewall passivation can be distinguished from the electrical current-voltage characteristics. Figures 3(a) and 3(b) reveal the current-voltage properties from -4 to 4 V of 100×100 and 20×20 µm2 devices with different sidewall treatments. For the 100×100 µm2 devices, leakage current in forward-and reverse-bias conditions did not improve significantly, because the sidewall effects were not critical for devices with such low perimeter-to-area ratio. However, devices with sidewall treatments achieved better forward current-voltage characteristics than those without sidewall treatment. The voltage values at 20 A/cm2 were 3.0 V, 2.6 V, and 2.3 V for the 100×100 µm2 devices without sidewall passivation, with ALD sidewall passivation, and with the combination of TMA/nitrogen plasma and ALD sidewall passivation, respectively. The improvement in forward current-voltage characteristics could be due to a better surface interface or removal of parasitic resistance with ALD, and further investigations are required to confirm in the future work [23,24]. For the 20×20 µm2 devices, the benefits of ALD sidewall passivation were more pronounced. The device without sidewall passivation yielded high leakage current and sub-threshold turn-on voltage features, and ALD passivation remarkably improved both aspects. The use of TMA/nitrogen plasma further enhanced the forward current-voltage characteristics. The effects and mechanisms of TMA pretreatment and nitrogen plasma surface treatment on III-V semiconductor have been studied extensively, where TMA reacts with the native oxide to form Al2O3 and nitrogen removes carrier trapping states at the damaged surface [25–29].
Fig. 3. Current-voltage characteristics of (a) 100×100 and (b) 20×20 µm2 µLEDs with different sidewall treatments.
Because µLEDs have high perimeter-to-area ratios and the AlGaInP material has high surface recombination velocity, the use of surface treatment and ALD sidewall passivation is critical to suppress the Fermi-level pinning and reduced the leakage current significantly. Figure 4(a) shows the leakage current density at -3 V of the five device dimensions with different sidewall treatments. For µLEDs larger than 40×40 µm2, the leakage current density was comparable in the 3×10−6 A/cm2 range regardless of the sidewall passivation technique, and this was attributed to the relatively small perimeter-to-area ratio of the devices. However, the leakage current density of the 40×40 and 20×20 µm2 devices without sidewall treatment increased eight orders of magnitude higher than those with ALD and TMA/nitrogen sidewall treatments. The devices with sidewall treatments yielded size-independent leakage current density in the range of 3×10−6 A/cm2 from 20×20 to 100×100 µm2, thus ALD sidewall passivation is effective to suppress leakage current of AlGaInP µLEDs. The influence of sidewall treatments can be quantified by comparing the ideality factor, as shown in Fig. 4(b). The ideality factor was calculated using Eq. (1).
where n is the ideality factor, q is the elementary charge, k is the Boltzmann constant, T is the absolute temperature, I is the current, and V is the voltage. The ideality factor of devices without sidewall treatment increased as device size decreases, and this has been reported in the literature [15]. Because the µLEDs were processed from the same commercial wafer with minimal epitaxial growth variation, the difference in ideality factor was driven mostly by fabrication, such as plasma damage [15,30]. Moreover, there was an increase of 50% in ideality factor by shrinking the device dimensions from 100×100 to 20×20 µm2 for devices without sidewall treatment, showing that the size effect was presented [15]. On the other hand, the increase in ideality factor was about 6% for devices with TMA/nitrogen plasma followed by ALD sidewall passivation, indicating that the sidewall treatments improve the electrical performances of µLED and are effective for suppressing the size effect.Fig. 4. (a) The dependence of leakage current density at -3 V and (b) the ideality factor on AlGaInP µLED sizes with different sidewall treatments.
To study efficiency, on-wafer relative EQE measurements were performed. Figures 5(a) and 5(b) present the normalized EQE of 20×20 and 100×100 µm2 devices with different sidewall treatments and the decrease in EQE due to the size effect. From Fig. 5(a), the EQE of the 100×100 µm2 device was 10% higher for the standard than those with sidewall treatments. The small difference in EQE of the 100×100 µm2 devices could be due to fabrication and measurement variations, since the difference was within the error bars. Conversely, for 20×20 µm2 devices, ALD sidewall passivation boosted the relative EQE from 22% to 57% at 100 A/cm2, and the EQE enhancement was observed even at low current density range. The increase in EQE was a consequence of the improved light output power performance and the reduction of leakage current. As shown in Fig. 5(b), the EQE of devices without sidewall passivation decreased more than 50% for µLED sizes smaller than 40×40 µm2 compared to 30% with ALD sidewall passivation. This decrease in EQE was more severe in AlGaInP-based µLEDs than in InGaN-based µLEDs, because AlGaInP materials have much higher minority carrier diffusion length and surface recombination velocity than InGaN [13]. The drop in EQE was lessened by employing ALD sidewall passivation for devices smaller than 60×60 µm2, but the EQE decreased as the device dimensions shrink. More rigorous sidewall treatment optimizations, including the use of potassium hydroxide or sulfide-based wet chemicals, can be utilized with ALD sidewall passivation to achieve size-independent EQE performance on AlGaInP µLEDs [12,19,23].
Fig. 5. (a) Dependence of EQE on current density of 100×100 and 20×20 µm2 devices and (b) size-dependent EQE comparison with different sidewall treatments
4. Conclusion
We illustrated that the optoelectrical properties of AlGaInP red µLEDs can be greatly improved by employing ALD sidewall passivation. The use of ALD sidewall passivation resulted in higher light output power, size-independent leakage current, and lower ideality factor. Moreover, TMA/nitrogen plasma surface treatment further enhanced the forward current-voltage characteristics. Based upon the EQE performance, the 20×20 µm2 devices with ALD sidewall passivation demonstrated an improvement of more than 150% at 100 A/cm2, compared to the device without sidewall passivation. Nevertheless, the ideality factor and EQE remained dependent on device dimensions, suggesting additional sidewall treatments are required to eliminate the size effects in AlGaInP µLEDs. With more developments on sidewall passivation techniques, highly energy-efficient red AlGaInP µLEDs with mitigated size effects can be employed along with blue and green InGaN µLEDs for future full-color display applications.
Funding
Solid State Lighting and Energy Electronics Center, University of California Santa Barbara.
Acknowledgments
The authors would like to thank Mr. MJ Kennedy from Professor John Bowers group and Dr. Bei Shi from Professor Jonathan Klamkin group for their assistances in device fabrication. A portion of this work was performed in the UCSB nanofabrication facility.
Disclosures
The authors declare no conflicts of interest.
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