Surface Structure-Controlled Monolithic Multiple Color Semipolar GaN-based Light-Emitting Diodes
Gun-Woo Lee, Jae-Hyeok Oh, and Sung-Nam Lee
Abstract
In this study, monolithic red, green, and blue emission semipolar (11-22) GaN-based light-emitting diodes (LEDs) were developed using the SiO2 hexagonal pattern epitaxial lateral overgrowth (HPELO) technique. We found that the semipolar HPELO GaN film can significantly increase the arrowhead-like surface (ALS) structure, which can distribute the indium incorporations of the InGaN active layer. Because of the relatively thick lateral overgrowth technique used in the HPELO semipolar GaN template, its ALS structure is found to be much larger than that of the conventional semipolar GaN/m-sapphire template. The indium composition of the InGaN active layer grown on the semipolar HPELO GaN template is higher than that of the conventional GaN template because the crystallographic planes of the ALS structure have a higher indium incorporation rate than the semipolar (11-22) plane, resulting in a longer emission wavelength for the semipolar HPELO GaN-LEDs. Therefore, an emission wavelength from semipolar HPELO GaN-LEDs can be significantly blue-shifted because of the strong band-filling effect at different indium incorporations within the ALS structure. Thus, semipolar HPELO GaN-LEDs can be applied to achieve amber to blue emission for monolithic multi-color emitters.
1. Introduction
III-nitrides have been of great interest for next-generation full-color lighting sources, such as light-emitting diodes (LEDs), where colors range from violet to red in the visible light spectrum. However, it has been difficult to achieve high-efficiency green and red emissions from conventional polar c-plane InGaN-based LEDs because of indium segregation and piezoelectric polarization effects [16]. Using nonpolar and semipolar GaN films to significantly reduce the piezoelectric polarization and increase the indium incorporation in the InGaN active region has been actively researched in recent years [711]. In general, the rate of indium incorporation into GaN films is influenced by the crystallographic planes, where particularly semipolar (11-22) GaN films have shown a higher indium incorporation rate than nonpolar (10-10) and (11-20) GaN films, indicating that semipolar (11-22) GaN films are an alternative candidate for achieving high-efficiency green-and red-emission LEDs [9]. Consequently, semi-polar GaNLEDs have been studied for their potential use in highly efficient optical devices [10,11]. However, there are limitations in achieving highperformance longer-wavelength semipolar LEDs using semi-polar GaNLEDs due to an abundance of crystalline defects, such as high treading dislocations (~109 cm−2), basal stacking faults (~105 cm−1), and arrowhead-like surface (ALS) structures [1114]. Many of these crystal defects can form a significant band-filling effect in the emission spectrum due to strong inhomogeneous indium incorporations [13, 14]. Specifically, the ALS structure consists of a few crystal planes, such as (20-21), (10-11), and (11-22), which can exhibit various emission wavelengths because of the variations in indium incorporations during the growth of an InGaN active region [1519].
The epitaxial lateral overgrowth (ELO) method is one of the most extensively utilized defect reduction strategies for GaN films [2023]. For instance, it has been reported that the SiO2 hexagonal pattern (HP) can replace the conventional stripe pattern to improve the light extraction efficiency of the films [19,20]. Additionally, ALS structures can be applied to various indium incorporated crystal surfaces to produce differing red to blue emission wavelengths that are formed due to the strong band-filling effect in those regions [19]. However, the growth of semipolar HPELO GaN templates exhibits several challenges when developing these large surfaces of ALS structures primarily owing to the varying growth rates at each corner of the hexagonal pattern [19,20]. Therefore, in this study, we use the ALS structure of a semipolar HPELO GaN-LED to achieve monolithic multiple-color emissions by maximizing the indium localization states and further increasing the band-filling effect.
2. Experimental details
We prepared conventional 2.0 µm-thick semipolar (11-22) GaN templates grown on m-plane sapphire substrates using a high-temperature one-step growth process via metalorganic chemical vapor deposition (MOCVD) [10]. Subsequently, a 100 nm-thick SiO2 hexagonal pattern was grown using a plasma-enhanced chemical vapor deposition system. Conventional photolithography and chemical wet etching procedures were used to fabricate a 15 µm-wide SiO2 hexagonal pattern and a 4.0 µm-wide opening area. A three-step growth procedure was used to perform ELO of semipolar GaN films using these templates [19,23]. Then, using conventional 2.0 µm-thick semipolar GaN and 8.0 µm-thick HPELO-GaN templates, a basic LED epitaxial structure was grown simultaneously. The LED epitaxial structure consisted of a 3.0 µm-thick n-type GaN, In0.2Ga0.8N/GaN with five multiple quantum wells (MQWs), and 0.1 µm-thick p-type GaN. The five InGaN/GaN MQWs were used as active layers having 3.0 nm-thick In0.2Ga0.8N wells and 8.0 nm-thick GaN barriers. After the growth of the LED epitaxial wafer, LED dies of size 500 × 500 µm2 were formed using the general mesa-structured LED fabrication technique with a top-top electrode structure, as shown in Fig. 1.
Optical microscopy and atomic force microscopy (AFM) were used to observe the influence of the SiO2 pattern on the surface structural properties of the conventional semipolar GaN-LED and HPELO GaNLED grown simultaneously by MOCVD. The crystal planes of the ALS structure were defined by measuring the geometric structure of the sidewall facets in the AFM line scan. In addition, electroluminescence (EL) measurements were used to evaluate the optical emission properties of the conventional semipolar GaN-LED and HPELO GaN-LED. Lastly, micro-EL photographic images of the HPELO GaN-LED were measured using a 1,000 × magnification objective.
3. Results and discussion
3.1. ALS structures of conventional semipolar GaNLEDs and HPELO GaN-LEDs
Figures 2(a) and 2(b) show macroscopic surface images of the conventional semipolar GaN-LED and HPELO GaN-LED using optical microscopy, respectively. Both samples presented typical ALS structures seen in semipolar GaN films. However, where the ALS structure of the conventional semipolar GaN-LED is very small and dense, the HPELO GaN-LED showed a very large size and low density generated from the hexagonal SiO2 pattern [18]. In general, it is known that ALS structures are repeatedly arranged on hexagonal patterns because anisotropic lateral growth occurs over the SiO2 hexagonal pattern, as shown by the white dotted line in Fig. 2(b) [19]. Thus, visibly huge ALS structures were formed over the out-of-focus dark regions, which are the SiO2 hexagonal patterns of HPELO GaN-LED. It is believed that indium incorporation in the ALS structure can be drastically affected by the HPELO process compared to conventional semipolar GaN templates because of the formation of large ALS structures with different crystal planes [19,20]. AFM measurements were performed for the conventional semipolar GaN-LED and HPELO GaN-LED to measure the microscopic surface structure, as shown in Figs. 2(c) and 2(d), respectively. Similarly, for the optical microscopic images, the ALS structure of the HPELO GaN-LED is approximately five times larger than that of the conventional semipolar GaN-LED. In particular, the indium incorporation has been reported to decrease in the order of the front (20-21) and side (10-11) edge planes and the upper (11-22) in the crystal plane of the ALS structure [1517]. This means that the conventional semipolar GaN-LED and HPELO GaN-LED can exhibit various indium incorporation-related multiple wavelength emissions.
In addition, it can be inferred that the HPELO GaN-LED can exhibit a multiple color range with high performance owing to the ALS structure and lateral growth-induced low crystal defects.
3.2. Crystal properties of conventional semipolar GaNLEDs and HPELO GaN-LEDs
Figure 3(a) shows the ω-rocking curves of the conventional semipolar GaN and HPELO GaN-LEDs. The full width at half maximum (FHWM ~ 812 arcsec) of the ω-rocking curve in the HPELO GaNLED was much lower than the conventional semipolar FWHM (~1424 arcsec). This means that the HPELO GaN films contain some crystal defects as a result of GaN and SiO2 hexagonal pattern friction during the lateral growth process, but the crystal defects of the HPELO GaNLED are much less than those of the conventional semipolar GaNLED [20]. Implying that the HPELO process is very effective in improving the crystallinity of the semipolar GaN films. Moreover, the low-angle shoulder of the conventional semipolar GaN-LED can be attributed to the anisotropic growth rates, such as the higher growth rate of [0001] than [000-1] [10]. As shown in Fig. 3(b), ω/θ scans of the conventional semipolar GaN-LED and HPELO GaN-LED were performed to analyze the structural properties of the InGaN/GaN MQWs in both LEDs. The InGaN 0th peak of the conventional semipolar GaN-LED and HPELO GaN-LED are located at 68.43 and 68.07°, respectively. This indicates that even though both LEDs were grown simultaneously, the indium composition of the HPELO GaN-LED was higher than that of the conventional semipolar GaN-LED. Generally, because the HPELO GaN film is grown using a three-step growth technique to planarize the surface structure on the SiO2 pattern, the thickness of the HPELO GaN film is greater than the conventional semipolar GaN template [23]. Because the bowing temperature of the HPELO GaN-LED is higher than that of the conventional semipolar GaN-LED, the growth temperature of InGaN MQWs is subsequently decreased to avoid the rise in the bowing-induced noncontact region, increasing the indium composition. In addition, the front edge of the ALS structure of the HPELO GaN-LED is incorporated with indium more than that in the conventional semipolar GaN-LED, as shown in Fig. 2. Therefore, the indium composition of the InGaN MQWs in the HPELO GaN-LED is higher than that in the conventional semipolar GaN-LED, which is consistent with the InGaN 0th peak of the ω/θ scans. This means the EL wavelength of the HPELO GaN-LED may be longer than that of the conventional semipolar GaN-LED.
3.3. Electroluminescence behaviors of conventional semipolar GaN-LEDs and HPELO GaN-LEDs
Figures 4(a) and 4(b) show the EL spectra of the conventional semipolar GaN-LED and HPELO GaN-LED when operation current increases from 0.1 to 100 mA, respectively. At a low operation current of 0.1 mA, the emission wavelength of conventional semipolar GaN-LED is 538 nm, whereas the wavelength for HPELO GaN-LED is 600 nm. As shown in Fig. 3(b), this is consistent with the results seen for the InGaN 0th peak of the ω/θ scans. As the operation current increases from 0.1 to 100 mA, the EL wavelength of the semipolar GaN-LED was blue-shifted from 538 to 460 nm, whereas the EL wavelength of the HPELO GaN-LED was significantly blue-shifted from 600 to 470 nm, as shown in Figs. 4(a)–(c). When a high injection current density of 30 A/cm2 was introduced into both samples, the blue-shifted emission wavelength saturated at 465 nm, as shown in Fig. 4(d). This implies that the indium-localized states of the HPELO GaN-LED are much deeper than those of the conventional semipolar GaN-LED. Furthermore, the ALS structure is much larger for the HPELO GaN-LED when compared to the conventional semipolar GaN-LED, as seen in Fig. 2. In general, the rate of indium incorporation is considerably affected by the surface crystal planes of the ALS structure in semipolar GaN-LEDs [1517]. From Fig. 4(c), we speculate that the indium incorporation of the HPELO GaN-LED may be higher than that of the conventional semipolar GaN-LED which can be attributed to the size difference of the ALS structure and higher warpage phenomenon of the HPELO GaN-LED. Moreover, the film thickness must be greater in HPELO GaN-LED to planarize the lateral growth surface over SiO2 mask patterns during epitaxial lateral growth; thus, the strain-induced wafer warpage of HPELO GaN-LED is greater than that of conventional semipolar GaN-LED. This means that the growth temperature of the HPELO GaN-LED is slightly lower when compared to that of the conventional semipolar GaN-LED because of the reduced heat transfer from the susceptor to the substrate. As a result of the lower growth temperature of the HPELO GaN-LED, the indium composition of the HPELO GaN-LED is believed to be higher than that of the conventional semipolar GaN-LED, which is consistent with the X-ray diffraction (XRD) results shown in Fig. 3(b). High indium localization states were generated using the HPELO GaN-LED with different and high indium compositions, leading to a significant band-filling effect and large blueshift phenomena when injection current increases. Furthermore, as shown in the inset of Fig. 4(c), the front edge region of the ALS structure exhibited red emission at a very low injection current as a result of the highest indium incorporation, and then it changed from amber to blue emission via green emission as the injection current increased. Thus, it is clear that the ALS structure can increase the indium localization states, which can form various emission wavelengths from red to blue as injection current increases owing to different indium incorporation rates into the crystal facets of the ALS structure [24]. This implies that the HPELO GaN-LED can be applied to represent multiple color emissions (amber to blue) from a single HPELO GaNLED by controlling the injection current density. Figures 5(a) and 5(b) show the macro-EL photographs of a conventional semipolar GaNLED and an HPELO GaN-LED wafer when the operation current increases from 0.1 to 50 mA, respectively. This demonstrates that conventional semipolar GaN-LEDs and HPELO GaN-LEDs exhibit yellow and amber emissions at a low current (~0.1 mA), respectively. When the operation current was increased to 50 mA, the conventional semipolar GaN-LED emitted yellow to blue emission, whereas the HPELO GaN-LED emitted amber to blue emission. From these results, the semipolar HPELO GaN-LEDs can produce amber to blue emissions by controlling the band filling effect at strongly indiumlocalized states in the ALS structure. Thus, semipolar HPELO GaNLEDs can be applied to achieve amber to blue emission for monolithic multi-color emitters
4. Conclusions
The ALS structures of the semipolar GaN-LEDs can be controlled by the HPELO process due to the anisotropic growth rate on the SiO2 hexagonal pattern region. The ALS structure of the HPELO GaNLED is much larger than that of the conventional semipolar GaN-LED, which may affect indium incorporation in the ALS structure. In addition, higher indium incorporation may occur in HPELO GaN-LEDs because they have greater warpage than conventional semipolar GaNLEDs. Consequently, at a low injection current (~0.1 mA), the EL wavelength (~600 nm) of the HPELO GaN-LED is much longer than the conventional semipolar GaN-LED wavelength (~540 nm). Moreover, as the operation current increased from 0.1 to 100 mA, the emission wavelength of the HPELO GaN-LED blue-shifted from 600 to 460 nm, whereas the wavelength of the conventional semipolar GaN-LED blue-shifted from 538 to 460 nm. Thus, semipolar HPELO GaN-LEDs can be applied to achieve amber to blue emission for monolithic multicolor emitters.
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