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Abstract
In this study, deep-ultraviolet light-emitting diodes (DUV LEDs) with different chip sidewall geometries (CSGs) are investigated. The structure had two types of chip sidewall designs that combined DUV LEDs with the same p-GaN thickness. By comparing the differences of the characteristics such as the external quantum efficiency droops, light output power, light extraction efficiency (LEE), and junction temperature of these DUV LEDs, the self-heated effect and light-tracing simulation results have been clearly demonstrated to explain the inclined sidewalls that provide more possibility pathway for photons escape to increase the LEE of LEDs; thus, the DUV LEDs with the CSG presented improved performance. These results demonstrate the potential of CSG for DUV LED applications.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Recently, AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) have garnered significant attention for various applications such as water purification, phototherapy, disinfection, UV curing, biomedical instrumentation, and sterilization [1–3]. With the development of a high-crystal-quality AlGaN material growth method, the internal quantum efficiency (IQE) of DUV LEDs has increased significantly, owing to better hole injection efficiency and higher radiative recombination efficiency. However, despite rapid progress in improving the performance of DUV LEDs, the efficiency of AlGaN-based LEDs with wavelengths corresponding to the UV-C (200–280 nm) range remains extremely low to justify the replacement of current UV lamps. In comparison with traditional blue/green InGaN-based LEDs, the external quantum efficiency (EQE) of AlGaN-based DUV LED remains low for several reasons. First, heavily Mg-doped GaN for better ohmic p-contact and hole injection into the Mg-doped AlGaN region results in high absorption of DUV light and, thus, low light extraction efficiency (LEE) [4]. Second, a transparent current spreading layer is still under study to address the contradiction between ohmic contact with p-GaN and absorption of DUV light. Third, the transverse magnetic (TM) polarization of emitted DUV light from the AlGaN/GaN multiple quantum wells (MQWs)due to the reordering of the valence energy bands in the high-Al-content AlxGa(1-x)N material results in strong sidewall emission [5–9]. Although some UV LED structures have been introduced to reduce light absorption in GaN materials [10–12], the LEE of DUV LEDs is still significantly lower than that of visible LEDs.
LEE is one of the most important factors for the improvement of EQE in DUV LEDs. For the improvement of LEE, there are several methods that can be adopted: First, the LEE of DUV LEDs can be improved using a material with low absorption of UV light; Second, people can change the design stacks or materials to improve the reflectivity of distributed Bragg reflectors; Third, to achieve enhanced side lighting effect, the structures of DUV LEDs with different angles should be designed to enhance the side lighting efficiency. For an LED with traditional structures, it is difficult for the emitted light to escape from the active region, thus causing serious internal optical loss by absorption, total reflection, or transformation to heat [13–15]. To maximize the extraction efficiency of LEDs, techniques such as modifying the geometry of the LED chip should be valued and sought, thus reducing the photon path length for extraction, and enhancing the LEE of LEDs [16,17].
In this study, the sidewall effect on the performance of DUV LEDs with an almost vertical sidewall and chip sidewall geometry (CSG) was systematically studied. These results demonstrate that the light output power (LOP) was improved by 13.15% and the LEE was improved by 10.5% for the DUV LED with CSG. The junction temperature of the Ref DUV LED was slightly higher than that of the CSG DUV LED because of the differences in their self-heating effects. In addition, from the light pattern and light-tracing simulation results, the inclined sidewalls provide more possibilities for photon escape to increase the LEE of LED [18–20].
2. Experiments
DUV LEDs were grown on a c-plane sapphire substrate by metal-organic chemical vapor deposition. Trimethylgallium and trimethylaluminum were used as group-III element sources while NH3 served as the group-V element source. The structure of the DUV LEDs comprised a 2 µm-thick Si-doped Al0.45Ga0.55N layer, Si-doped Al0.4Ga0.6N/Al0.6Ga0.4N multiple quantum wells, 30 nm-thick p-Al0.75Ga0.25N layer (electron-blocking layer), Mg-doped p-GaN layer, and 5 nm-thick Mg-doped p + -GaN layer (ohmic contact layer). After epilayer growth, these samples were cleaned using H2SO2: H2O2: H2O = 5:1:1 for 5–10 min to remove organic residues. A mesa pattern was formed by photolithography, followed by inductively coupled plasma (ICP) dry-etching to produce n-Al0.45Ga0.55N as the n-electrode. Thereafter, two types of isolation patterns were formed by two types of photoresists (PRs), one type of PR (No-NR7) is from F-company, after photograph, which at exposure intensity developed time for 50 s and oven baking at 120 °C, form the Reference (Ref.) The PR morphology is shown in Fig. 1(a). The other PR (No-562) from E-company at exposure intensity developed time for 50 s and oven baking at 130 °C after the lithography, which formed another isolation of the LED device, shown in Fig. 1(b). Thereafter, we used the best combination of Cl2 and BCl3 gases by the ICP dry-etching process to obtain two different chip sidewalls. The corresponding scanning electronic microscopy (SEM) images are shown in Fig. 1(c) and (d). Subsequently, Ti/Al/Ni/Au (15/100/50/250 nm) layers were deposited by e-beam evaporation and subsequently annealed at 950 °C for 30 s to serve as the n-pad. ITO, as the p-contact electrode with p-GaN, was deposited by sputtering and subsequently annealed at 600 °C for 300 s. A metal stack pattern (Ni/Au:50/250 nm) was deposited on the ITO thin film for p-spreading layer deposition. For resolving this reliability problem, a 1 µm-thick SiO2 layer was deposited on the entire device by plasma-enhanced chemical vapor deposition (PECVD) at 240 °C. Notably, the high-quality, compactness, and thick SiO2 can be obtained by PECVD. Cr/Pt/Au/AuSn (5/50/250/6000 nm) layers were deposited on p- and n-contact bonding pads. After the conventional LED processes, the DUV LED wafer was subjected to laser scribing into chips, and the emissive region of each individual LED chip was 508 × 508 µm2. Finally, DUV LED chips were mounted on an AlN ceramic submount in a flip chip structure by soldering flux at 240 °C for 5 min for further electrical and optical measurements.
Fig. 1. (a) SEM image of photoresistor No-NR7, (b) SEM image of photoresistor No-562, (c) Side wall of Ref DUV LED using No-NR7 hard mask and (d) CSG of DUV LED using No-562 hard mask.
A schematic of a representative DUV LED is shown in Fig. 2(a). The thicknesses of the SiO2 and metal electrode layers were measured by KLA-Tencor profile meter P-10. The refraction of the passivation layer was measured using an n&k 1280 analyzer (n&k Technology, Inc.). The LOP of the packaged LEDs and the thermal behavior (junction temperature) were measured using a calibrated integrating sphere (INSTRUMENT SYSTEMS) and a TERALED& T3Ster combined system, respectively. The thin-film carrier concentration and mobility were measured using standard Hall effects with a four-probe configuration (ACCENT HL5500 PC). The current–voltage (I–V) characteristics of the LEDs were determined at 25 °C using an Agilent 4155B semiconductor parameter analyzer. To evaluate the effect of the CSG on LED performance, two types of LED samples were fabricated using the same epitaxial structure: Ref- and CSG-LEDs with an ITO thickness of 110 nm. Figure 2(b) shows a schematic of the Ref and CSG DUV LEDs.
Fig. 2. (a) Schematic of DUV LED with whole chip processed structure and (b) the schematic of the Ref and CSG DUV LEDs.
3. Results and discussion
Figure 1(c) and (d) present the differences of the sidewall shapes of Ref- and CSG-LEDs. The effects of the CSG on the performance of DUV LEDs are demonstrated as follows: Fig. 3(a) shows the forward I–V characteristics of the Ref- and CSG-LEDs. All LED structures exhibited normal p–n diode behavior at a forward bias. At an injection current of 100 mA, the forward voltages (VFs) of the Ref- and CSG-LEDs were 6.327 and 6.313 V, respectively. The I–V curves exhibited similar characteristics. The dynamic resistances of the CSG and Ref DUV LEDs as functions of the applied voltage are shown in Fig. 3(b). When the applied voltage was in the range of 4.0–7.0 V, the CSG and Ref DUV LEDs were not significantly different in dynamic resistances. The dynamic resistances of the CSG and Ref DUV LEDs were 20.91 and 20.33 Ω under 5.5 V, respectively. This indicates that the CSG does not affect the optoelectronic properties of the LED.
Moreover, the output power–current (L–I) and EQE were studied. EQE is the ratio of the number of photons per unit time emitted into space to the number of electrons injected into the luminescent layer, which can be defined as follows [21]:
Fig. 4. Output power and EQE as a function of injection current for Ref and CSG DUV LED UVC devices.
The modified emission wavelength is based on temperature variations, in the same way that the luminosity changes. Primarily, the semiconductor bandgap varies depending on the temperature, resulting in a change in the wavelength. Figure 5(a) shows the current-wavelength characteristics of the Ref and CSG LEDs at room temperature: the slightly blue-shift (both approximately 0.06 nm) were observed from 40 to 100 mA. Moreover, from 100 to 600 mA, the thermal-induced red-shift of the Ref and CSG LEDs were 1.21 and 1.12 nm, respectively [23–25]. Because the LEE of the CSG-LED is better than that of the Ref-LED, less heat is produced than in Ref. [22]. In addition, the surface of CSG DUV LED is smaller than that of Ref DUV LED, thus the current crowding effect would be a little more obvious in the CSG DUV LED, thus the wavelength of CSG DUV LED is always larger than that of Ref DUV LED. Figure 5(b) shows the butterfly-like light patterns of the Ref and CSG DUV LEDs at an injection current of 100 mA, that is because the DUV LEDs with the sidewall geometric for these two structures and packaged using the flip chip, the sidewall geometric could contribute to light escape from the sidewall of sapphire even with the small angle. For comparison, the output light patterns of the LEDs with the CSG structure were higher than those of the LEDs with the traditional structure, particularly in the range of 30–60 °C. This can be attributed to the oblique epitaxial sidewall of the CSG structure, which can capture more photons than the near-horizontal structure. When light escapes from 30° to 90°, the critical angles of sapphire/air and Al0.45Ga0.55N/SiO2 and the mesa edge angle are limited. Therefore, the unescaped light was trapped inside the LED as the guiding mode propagated and escaped from the CSG. The radiation pattern of the CSG DUV LED between 45° and 90° is larger than that of the Ref DUV LED, which is supported by the simulation results obtained using light-tracing software (Trace Pro). Figure 6 shows the candela maps and light tracing of these DUV LEDs, for the refractive indices of GaN and SiO2 in Ref DUV LED and CSG DUV LED are same, which lead to the same transmittance [26,27], thus the influence of SiO2 layer could be ignored, and the difference should be mainly attributed to the difference of chip sidewall geometric of these DUV LEDs. According to our experience, the DUV LED with the chip sidewall geometric of the smaller inclined angel usually has the better performance (LEE, EQE et al.). The relevant light-tracing images of the oblique sidewall have a large number of lights that can be extracted from the oblique wall because the total internal reflection is reduced, and the possibility of improvement of photons escaping from the semiconductor to the air.
Fig. 5. (a) Forward I–W from 40 to 360 mA, (b) Light patterns of Ref and CSG DUV LEDs at an injection current of 100 mA.
In general, the semiconductor components are closely related to the temperature, and a higher output power of LEDs is usually obtained at lower temperatures. Next, we discuss the Ref and CSG DUV LEDs for a constant injection current that varies as a function of junction temperature (Tj) and ambient temperature, where Tj denotes the temperature at the p-n junction where light is generated. Figure 7(a) shows the VF at the junction current of 100 mA, under ambient temperature from 20 to 120 °C, and the VFs at room temperature are 6.33 and 6.34 V of the Ref and CSG DUV LEDs, respectively. The temperature sensitive coefficients (dV/dT) of the Ref and CSG DUV LEDs were 9.3 and 8.2 mV/K, respectively. The slight differences in dV/dT should be attributed to the self-heat effect of the LEDs; for the CSG DUV LED with higher LEE and less heat to storage, the characteristic of low temperature-sensitive coefficients could be observed. Figure 7(b) shows the temperature-dependent LOP of the Ref and CSG DUV LEDs (dLOP/dT). The output power values of the Ref and CSG DUV LEDs are 9.6 and 10.21 mW, respectively, at the junction current of 100 mA and ambient temperature of 25 °C. The dLOP/dT values of the Ref and CSG DUV LEDs were 205 and 176 mW/K, respectively. The slope of the CSG DUV LED was slightly lower than that of the Ref DUV LED, which can be attributed to its smaller temperature sensitive coefficients; thus, the smaller change in the semiconductor bandgap and the weaker confinement of the barrier in the MQWs lead to the LOP decrease consequence [28,29]. Figure 7(c) shows the effects of wavelength on the ambient temperature increase (dWLP/dT). The values of the Ref and CSG DUV LEDs are 0.155 and 0.153 nm/K, respectively, which should be also attributed to the less current crowing effect in the CSG DUV LEDs. The table in Fig. 7(c) shows that the junction temperature varied with the current.
Fig. 7. (a) Forward voltage, (b) output power, and (c) wavelength of Ref and CSG DUV LEDs as functions of ambient temperature, at the injection current of 100 mA.
4. Conclusion
In this study, we fabricated DUV LEDs with different CSGs by changing the process of lithography, dry etching, and reflector deposition. The sidewall effect on the performance of DUV LEDs with an almost vertical sidewall and CSG was systematically studied. The results demonstrate that the LOP was improved by 13.15% and the LEE was improved by 10.5% for the DUV LED with CSG. The junction temperature of the Ref DUV LED was slightly higher than that of the CSG DUV LED because of the differences in their self-heating effects. In addition, from the light pattern and light-tracing simulation results, the inclined sidewalls provide more possibilities for photon escape to increase the LEE of the LED.
Funding
Fujian Province Central Guidance Local Science and Technology Development Fund Project In 2022 (2022L3058); National Natural Science Foundation of China (62274138); Major Science and Technology Project of Xiamen (3502Z20191015); Science and Technology Plan Project in Fujian Province of China (2021H0011); National Natural Science Foundation of China (11904302).
Acknowledgments
The authors would like to thank Prof. Nakamura of the UCSB, Prof. Hedeto Mayate of Mie University for their helpful discussions, and the Advanced Semicoductor Processing and Devices Lab (http://rhhlab.wixsite.com/astdl) of National Yang Ming Chiao Tung University for the measurement support.
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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