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Abstract
This study demonstrates the performance improvements of InGaN-based red light-emitting diodes (LEDs) by fabricating micro-holes in the planar mesa. The peak wavelengths of the micro-hole LEDs (MHLEDs) exhibited a blue-shift of around 3 nm compared to the planar LEDs (PLEDs) at the same current density. The lowest full width at half maximum of MHLEDs was 59 nm, which is slightly less than that of the PLEDs. The light output power and external quantum efficiency of the MHLED with a wavelength of 634 nm at 20 mA were 0.6 mW and 1.5%, which are 8.5% higher than those of the PLED.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
InGaN-based blue/green light-emitting diodes (LEDs) have achieved great success in the areas of solid-state lighting, automotive headlights, and displays in the past decades [1–3]. The external quantum efficiency (EQE) could reach as high as 84.3% for InGaN-based blue LEDs [4]. Although the device performance declines rapidly with the longer emission wavelength, researchers have devoted efforts to realizing efficient InGaN-based yellow LEDs with an EQE of 26.4% on silicon substrates [5].
However, when the emission wavelength is extended further to the red region, InGaN-based LEDs suffer from a significant reduction in EQE to only 2.9% [6]. This critical reduction is mainly due to the high defect density in high-In-content InGaN quantum wells (QWs) and a strong quantum-confined Stark effect (QCSE). To overcome these problems, approaches have been proposed to reduce the lattice mismatch, such as the use of ScAlMgO4(0001) and β-Ga2O3 substrates [7,8]. Another useful method to realize high crystal quality of the InGaN layer is to use nanostructures like nanowires [9], quantum dots [10], platelets [11], or porous structures [12]. Our group has proposed several strain-compensation approaches to reduce defects, such as introducing AlN/AlGaN barriers [13] and hybrid QW structures [14], as well as modifying the thicknesses of a GaN template [15].
The performance of InGaN-based LEDs could also be improved by optimizing the device structures. Using sputtering instead of e-beam evaporation to obtain indium tin oxide (ITO) layers could enhance the light output power of amber and red LEDs after a two-step annealing process [16]. Patterns like micro-holes or nano-pillars on ITO [17,18], p-GaN [19–21], QWs [22–24], or n-GaN [5,25] layers can improve the light extraction efficiency (LEE) and result in a higher output power for lateral or vertical LEDs.
Osram developed a method called the UX:3 technique, where an n-contact layer is buried under the p-contact layer [26]. This method was developed to minimize the amount of metallization on top of the chip and improve the current spreading in the n-type GaN layer. Some nanostructure-based LEDs have also been investigated and demonstrated enhanced performance compared to planar LEDs [27,28]. However, these techniques require complicated fabrication processes, which might reduce the reliability and reproducibility of the LED chips.
We propose a device structure where a micro-hole array is fabricated in a planar mesa. All the fabrication processes are the same as those of standard planar LEDs. Current-voltage (I − V) curves were used to characterize the forward voltages and reverse leakage currents. We compared the electroluminescence (EL) characteristics between micro-hole LEDs (MHLEDs) and planar LEDs (PLEDs) at different currents, including the peak wavelength and full-width at half-maximum (FWHM). The spatial distribution over the whole device area and the angular distribution in the free space for the EL intensity were also examined. Finally, we measured the light output power and calculated the EQE for both MHLED and PLED die chips.
2. Experimental details
InGaN-based red LED epitaxial structures were grown on c-plane patterned sapphire substrates (PSS) by metalorganic vapor-phase epitaxy (MOVPE) in a single-wafer horizontal reactor at 100 kPa. In previous work, we reported on the epitaxial structures of InGaN-based red LEDs [15]. A diagram of a red MHLED is shown in Fig. 1(a). ITO films were first deposited on the LED structures as a transparent conductive layer and subjected to two-step annealing to form ohmic contacts with p-GaN [16]. Next, the device mesa with a micro-hole array was patterned by standard photolithography, followed by etching through the ITO and InGaN QWs using inductively coupled plasma to expose the n-type layer. Finally, electrodes based on Cr/Pt/Au (50 nm/200 nm/200 nm) were deposited on both the ITO and n-type layers as contact pads. We fabricated both PLEDs and MHLEDs on the same wafer to guarantee the same conditions for the performance comparison.
Fig. 1. (a) Diagram of InGaN red MHLEDs. (b) Microscope image of an MHLED. (c) High-resolution microscope image of the area in the white rectangle in (b).
Electrical pumping of the red LEDs was carried out using a probe station and a semiconductor parameter analyzer. The EL characteristics were measured under direct current operation at room temperature (RT). After fabricating the LED die chips, we measured the light output power in a calibrated integrating sphere.
3. Results and discussion
The microscope image of the rectangular MHLED is shown in Fig. 1(b). The PLED has the same rectangular shape as the MHLED. The difference between them is the micro-hole array that was made in the mesa area, as shown in the high-resolution microscope image in Fig. 1(c). The diameter of the micro-holes was 3 µm, and the pitch of the array was 6 µm. To avoid a short circuit between the n-type and p-type layers, we did not fabricate micro-holes for the p-electrode area. Thus, the active areas for the MHLEDs and PLEDs were estimated as 0.168 and 0.197 mm2, respectively. The active area is the area of InGaN QWs in both LEDs.
Figure 2 shows the typical I − V curves of a red MHLED and PLED at RT. In forward bias, the MHLED and PLED exhibit identical I − V characteristics. The curves display a sharp onset voltage below 3 V and then increase with the voltage, which is the standard behavior of a p-n junction diode. The forward voltage at 20 mA was determined to be around 3.6 V.
In reverse bias, the reverse current increased with the reverse voltage for both the MHLED and PLED, which was presumably caused by crystal defects. An MHLED has more sidewall area from the micro-hole array compared to a PLED. The induced damage at the sidewall surfaces by dry-etching possibly served as current leakage paths [29], which resulted in the increased leakage currents at -1 − 0 V in Fig. 2. But the reverse current of the MHLED decreases slightly at -5 V in comparison to the PLED. The decrease in reverse current at -5 V is reasonable for the MHLED because removing the active region could decrease the total number of defects and lead to less current leakage channels in the active region [30]. It also revealed that the negative effect due to the dry-etching of micro-holes was not significant and mainly influenced the device performance at low currents.
The EL spectra of MHLEDs and PLEDs were measured at 4 to 100 mA and RT. The peak wavelength of red LEDs strongly depends on the current density, so we plotted the peak wavelength with respect to the current density in Figs. 3(a), respectively. The average blue-shift of the peak wavelengths from 2 to 50 A/cm2 was approximately 44 and 45 nm for MHLEDs and PLEDs, respectively. This high amount of blue-shift of the red LEDs demonstrated that they suffer from significant QCSE. However, the blue-shift was a bit smaller for the MHLEDs than the PLEDs. This illustrates that the micro-holes helped to slightly reduce the blue-shift of the peak wavelengths when increasing the current densities.
Fig. 3. (a) EL peak wavelengths according to current density. (b) Raman spectra using 473-nm laser. (c) FWHMs according to current density. Each point in (a) and (c) is an average of five chips of MHLEDs or PLEDs.
At the same current density, the peak wavelengths of the MHLEDs had a blue-shift of approximately 3 nm compared to the PLEDs. However, our previous work demonstrated that red LEDs with different mesa areas show the same peak wavelength at the same current density [30]. Therefore, we hypothesized that this peak blue-shift of MHLEDs was not just due to the reduction of the mesa area. Instead, fabricating micro-holes in the InGaN active region played an important role in the reduction of QCSE. This phenomenon is similar to that in nanostructure-based LEDs [27,31], where the InGaN active region is partially relaxed.
To analyze the strain of the MHLED and PLED, we measured the micro-Raman spectra using 473-nm laser. The E2 and A1(LO) modes of GaN could be observed in Fig. 3(b). The A1(LO) mode of the InGaN QWs was difficult to distinguish because InGaN QWs were strained [32]. Both the E2 and A1(LO) modes for the MHLED exhibited a smaller Raman shift compared to those for the PLED in Fig. 3(b), implying the strain relaxation in the GaN layer [32,33]. It also illustrated that InGaN QWs would exhibit a slight strain relaxation as well as the GaN layer. Note that we measured the micro-Raman spectra using 532-nm and 633-nm laser (not shown here), but the E2 mode of GaN for the MHLED and PLED had the same Raman shift. This result was reasonable because the Raman spectra using longer-wavelength lasers reflected the residual strain from the bottom epi-layers under micro-holes. Therefore, we concluded that the blue-shift of the peak wavelength for the MHLED at the same current density in Fig. 3(a) originated from the slight strain relaxation of the active region.
The FWHMs of both MHLEDs and PLEDs dropped quickly to the lowest values at current densities below 20 A/cm2, as shown in Fig. 3(c). This quick decline in FWHM originated from the rapid saturation of the emission from deep localized states in the InGaN active region [34]. The lowest FWHM for MHLEDs was approximately 59 nm, which is slightly lower than that of PLEDs.
After the lowest values occurred, the FWHM of both MHLEDs and PLEDs increased with the current density. This continuous increase of FWHM after the lowest value has been reported to originate from the heat generation in the devices under continuous wave operation [30]. The FWHMs of InGaN LEDs are also strongly influenced by indium fluctuation in the InGaN active region. It is possible that removing the area of the InGaN active region decreases the total number of localized states and leads to less indium fluctuation. As a result, the MHLEDs always clearly exhibited a narrower FWHM than PLEDs at the same current density (Fig. 3(c)). Robin et al. reported that the FWHM is vital to achieving pure red emission [35]. Thus, lower FWHMs are expected to result from the micro-hole array and to be beneficial for pure red LEDs.
Figure 4(a) shows the EL spectra of a typical MHLED and PLED at 20 mA and RT. The main peak wavelengths of the MHLED and PLED were 634 and 640 nm, respectively. The 6-nm blue-shift of the peak wavelength for the MHLED was determined by two factors. The first factor was the higher current density injection for the MHLED, which could screen the QCSE in the InGaN active region and lead to the blue-shift of the emission peak. The other factor was the reduction of QCSE, as discussed above.
Fig. 4. (a) EL spectra, (b, c) EL intensity distribution, (d, e) Microscope images, and (f) Angular distribution of EL intensity of a typical MHLED and PLED at 20 mA, respectively.
Compared to PLEDs, the EL intensity of the MHLED was improved. This enhancement was confirmed by the spatial distribution of the EL intensity of the PLED and MHLED, as shown in Figs. 4(b) and 4(c). The red areas in Figs. 4(b) and 4(c) represent the device areas with high emission intensity. Clearly, the MHLED showed a larger red area, demonstrating higher EL intensity for the MHLED. One reason for this enhancement could be the reduction of QCSE in the MHLEDs because of the operation at higher current density, which helped to improve the internal quantum efficiency. Therefore, higher total intensity was achieved in the MHLED, even though its total emission area was 15% less than that of the PLED.
The most intense areas in the MHLEDs and PLEDs were close to the p-electrode (Figs. 4(b) and 4(c)). Furthermore, non-uniform EL intensity could be observed directly from the EL emission images in Figs. 4(d) and 4(e). All these emission images illustrated that the degree of the current spreading was similar for the MHLED and PLED, which was not good over the whole device mesa. This is reasonable because the current blocking technique was not used in our devices [36]. Injection currents were much easier to pass through the area around the p-electrode due to the short distance between n- and p-type layers for these areas. The higher current injection around the p-electrode eventually resulted in the non-uniform EL intensity distribution over the whole device mesa. Some groups have reported current blocking techniques such as introducing SiO2 insulating layers on p-GaN [37]. Thus, we expect that the intensity distribution could be largely improved by using such fabrication techniques.
There were some small dark spots that were randomly distributed in the PLED and MHLED. Some of those are pointed out by the black arrows in Figs. 4(d)-(e). These dark areas were mostly related to the defects in the InGaN active region. However, Fig. 4(e) shows that some of the micro-holes and bad areas have an overlap in the MHLED. This overlap means that some of the removed emission area in the MHLED was actually related to the defects in the active region. Therefore, removing these areas would contribute to reducing the non-radiative recombination in the active region, which was also demonstrated by the reverse current at -2 ∼ -5 V in Fig. 2. However, due to the induced non-radiative recombination centers at the sidewall surface by dry etching, the total number of non-radiative recombination centers might not significantly change. Therefore, we presumed that the reduced number of defects by fabricating micro-holes would not be the dominant factor for the enhancement of the EL intensity of the MHLED.
We also investigated the influence of the micro-hole array on the light extraction behavior for MHLEDs by angle-resolved EL measurements at RT. Figure 4(f) shows the angular distribution of the EL intensities for the MHLED and PLED. Both of their patterns were similar to a Lambertian source [38]. However, the MHLED had higher intensities at all angles in the free space. This pattern demonstrated that the micro-hole array could help to extract more light into the free space and increase the LEE of MHLEDs, which also explains the EL enhancement of the MHLED.
Finally, we measured the light output power of the MHLED and PLED die chips in an integrating sphere at RT. Figure 5(a) shows the light output power for the MHLED and PLED chips with respect to current. The corresponding EQE was also calculated and is plotted in Fig. 5(b). The light output power and EQE of the MHLED chip at 20 mA were 0.6 mW and 1.5%, respectively, which were 8.5% higher than those of the PLED.
Fig. 5. (a) Light output power and (b) EQE of a typical MHLED and PLED at different currents. The inset is an EL image of the MHLED at 20 mA.
As discussed above, we believed that the reduced number of defects was not the dominant factor for the emission enhancement. So here the EQE enhancement of the MHLED at 20 mA mainly resulted from two factors: the reduction of QCSE, and the increased LEE. If we carefully checked the emission wavelength in Fig. 3(a), we could observed that the peak wavelength of the MHLED at 20 mA (∼ 12 A/cm2) was the same as that of the PLED at 30 mA (∼ 15 A/cm2). If we supposed that the QCSE was similar for both MHLED and PLED in this situation because of the same peak wavelength, we could estimate that the EQE enhancement of the MHLED at 20 mA compared to that of PLED at 30 mA was due to the LEE enhancement. As a result, we could obtain that the LEE enhancement factor was 4.5%. Thus, the enhancement factor related to the reduction of QCSE could be estimated as 4%. Note that this reduction of QCSE was actually caused by two reasons: micro-hole structures and higher operation current density. The rough estimation illustrated that both the reduction of QCSE and increased LEE played an important role in the EQE enhancement of the MHLED.
For blue MHLEDs on planar sapphire substrates, the dominant factor for the EQE enhancement is the increased LEE [22,24]. This is reasonable because blue LEDs usually has much lower In contents and less QCSE in the active region. Also, the planar sapphire substrates cannot work well for the LEE. Our red MHLEDs were grown on PSS, which was able to significantly enhance the LEE. As a result, the increased LEE of our red MHLEDs was not as much as those blue MHLEDs on planar sapphire substrates [22,24]. Meanwhile, the QCSE in red InGaN QWs is much stronger and should be taken into consideration.
The EL emission picture of the MHLED die chip at 20 mA is shown in the inset in Fig. 5(b). The emission light looked red, although the FWHM was still larger than what is required for pure red [35]. The peak EQE of the MHLED chip was 1.6% at 30 mA, and its light output power exceeded 1 mW at 40 mA. At low injection currents, the EQE values of the MHLED were slightly lower than those of the PLED in Fig. 5(b). We presumed that the decrease in EQE at low currents was possibly attributed to the non-radiative recombination at the sidewalls of micro-holes, which was related to the current leakage in Fig. 2.
However, the MHLED exhibited a larger efficiency droop than the PLED, especially at high currents (there was a much larger droop at the current density scale). The current spreading is not good in Figs. 4(b)-(e), and we hypothesized that the heat generated at high currents could not be distributed effectively in the case of the MHLED chip, which has less active area. Therefore, the EQE drops rapidly at higher currents compared to the PLED.
4. Conclusion
In summary, we have improved the performance of InGaN-based red LEDs by introducing a micro-hole array. We observed less blue-shift of the peak wavelengths for MHLEDs than PLEDs as the current density increased. Also, the peak wavelengths of MHLEDs exhibited a blue-shift of approximately 3 nm compared to PLEDs at the same current density. We attributed these peak-shift behaviors to the slight strain relaxation and the reduction of QCSE in the active region with the micro-hole array.
The lowest FWHM of MHLEDs was obtained as 59 nm, which was slightly less than that of the PLEDs. The lower FWHM for MHLEDs could be explained by the reduction of the indium fluctuation after removing some of the emission area. The light output power and EQE of the bare MHLED die chip with a peak wavelength of 634 nm at 20 mA were 0.6 mW and 1.5%, which were around 8.5% higher than those of the PLED chip. The luminescence improvement at 20 mA could possibly be explained by the enhanced IQE due to the QCSE screening and the enhanced LEE by the micro-hole array.
Funding
King Abdullah University of Science and Technology (BAS/1/1676-01-01).
Acknowledgments
The fabrication processes in this work were supported by Nanofabrication Core Labs at KAUST.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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