Study on the Degradation Performance of AlGaN-Based Deep Ultraviolet LEDs under Thermal and Electrical Stress
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe authors investigated the degradation performance of AlGaN-based deep UV LEDs under thermal and electrical stress. This work appears to overlap with some previously published work that was also not cited, such as: DOIs: 10.1016/j.sse.2019.01.004; 10.1364/OE.486393; 10.1364/OE.457740 to name a few. Thus, this paper does not offer any significant improvement to what has already been reported. It is therefore not recommended for publication.
Author Response
Comments 1: [The authors investigated the degradation performance of AlGaN-based deep UV LEDs under thermal and electrical stress. This work appears to overlap with some previously published work that was also not cited, such as: DOIs:10.1016/j.sse.2019.01.004; 10.1364/OE.486393; 10.1364/OE.457740 to name a few. Thus, this paper does not offer any significant improvement to what has already been reported. It is therefore not recommended for publication.]
Response 1: [Thanks for your review.We have carefully considered your comments and explain them as follows. The paper mentioned above is valuable for the reliability study of AlGaN-based DUV LEDs. However, AlGaN-based DUVs with p-GaN films as the p-type contact layer have a strong absorption for the DUV, resulting in the decrease of the wall plug efficiency. Therefore, p-AlGaN films are now used as p-type contact layers to improve the wall plug efficiency. With the improvement of the epitaxial growth process, the preparation, growth, and electrode process of epitaxial materials have also changed. As a result, the failure mechanism of AlGaN-based DUV LEDs with p-AlGaN film as the p-type contact layer is different from that of p-GaN film as the p-type contact layer. Therefore, it is very necessary to investigate the degradation of AlGaN-based DUV LEDs with p-AlGaN film as p-type contact layer.]
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe article on the degradation mechanism of AlGaN-based deep ultraviolet LEDs with a p-AlGaN contact layer under different stress conditions. Overall, the study is original and significant, providing valuable insights into the reliability and performance of these LEDs. The scientific soundness is high, and the findings are of interest to the readers.
The presentation of the results is clear, especially in describing the effects of thermal, electrical, and thermoelectric stresses on optical power, leakage current, ideality factor, series resistance, and junction temperature of the LEDs. The conclusions drawn from the results are well-supported and contribute to the existing knowledge in the field.
However, I recommend improving the quality of presentation by providing a more detailed description of the research methods in the full article for better clarity. Additionally, ensuring that the article is easily understood by readers, especially in explaining the specific mechanisms behind the degradation of the LED properties, can enhance the overall merit of the study.
In conclusion, the article addresses an important research gap and offers valuable insights for improving the reliability of AlGaN-based DUV LEDs. With some enhancements in the presentation of methods and ensuring reader comprehension, the article can further contribute to the field of optoelectronics.
Author Response
Comments 1: [The article on the degradation mechanism of AlGaN-based deep ultraviolet LEDs with a p-AlGaN contact layer under different stress conditions. Overall, the study is original and significant, providing valuable insights into the reliability and performance of these LEDs. The scientific soundness is high, and the findings are of interest to the readers. The presentation of the results is clear, especially in describing the effects of thermal, electrical, and thermoelectric stresses on optical power, leakage current, ideality factor, series resistance, and junction temperature of the LEDs. The conclusions drawn from the results are well-supported and contribute to the existing knowledge in the field. However, I recommend improving the quality of presentation by providing a more detailed description of the research methods in the full article for better clarity. Additionally, ensuring that the article is easily understood by readers, especially in explaining the specific mechanisms behind the degradation of the LED properties, can enhance the overall merit of the study. In conclusion, the article addresses an important research gap and offers valuable insights for improving the reliability of AlGaN-based DUV LEDs. With some enhancements in the presentation of methods and ensuring reader comprehension, the article can further contribute to the field of optoelectronics.]
Response 1: [Thanks for your decision and comments on my manuscript. We have carefully considered the suggestion of reviewer and make some changes. We have tried our best to improve and made some changes in the manuscript. The blue part has been revised according to your comments.
The chips with the size of 250×450(µm2) are based on flip-chip processing technology. Single chip is flip-chip mounted on AlN submount using gold tin alloy soldering. Finally, a batch of 10 DUV LEDs were soldered onto a PCB. To investigate the failure mechanisms under thermal and electrical stresses AlGaN-based DUV LEDs, a series of aging tests were conducted, including single thermal stress aging tests (A), single electrical stress with air cooling aging tests (B), single electrical stress aging tests (C), and thermoelectric complex stress aging tests (D). The aging tests are shown in Table 1. A constant current was applied to the DUV LED aging samples on the PCB board with the LED-4005 current source. The temperature of different aging tests was provided by high and low temperature test chambers. Cooling methods for different aging test samples include natural cooling and air cooling. Natural cooling is the natural heat exchange between the heat generated by the DUV LEDs and the environment. Air cooling is the convective heat transfer between the heat generated by the DUV LEDs and the environment under the operation of the fan. To investigate the associated failure mechanisms, the optical, electrical and thermal properties of AlGaN-based DUV LEDs before and after the aging tests were measured by different analysis technologies. The electroluminescence, optical power and forward voltage were collected automatically by a UV LED/Module radiation test system. Current-voltage(I-V) characteristics was measured by semiconductor parameter analyzer. The case temperature of the DUV LEDs was measured by a thermocouple. The thermal resistance of the DUV LEDs was measured by a T3ster with heat current 40 mA and sense current 1 mA. All the aging tests were carried out for 1000 hours. The photoelectric parameters of the DUV LED were measured at a current of 40 mA when natural cooled to room temperature.]
Reviewer 3 Report
Comments and Suggestions for AuthorsThis manuscript reports a study of the degradation performance of AlGaN-based deep ultraviolet LEDs under thermal and electrical stress. The heterostructures of AlGaN-based DUV LEDs are obtained via MOCVD on the sapphire substrate. The authors employ ageing tests comparing electroluminescence spectra. Optical and electric properties are investigated through the perspectives of thermal and electrical stress thus providing systematic overlook of their influence on properties with high practical value for applications.
This manuscript provides a convincing report of impactful results. However, a minor revision is still needed. The following aspects are advisable for being addressed before acceptance for publication:
1: Title could be shortened. “… under the thermal and electrical stress” is stylistically incorrect. The article “the” should be removed.
2: The investigated ranges of thermal stress, electrical stress, and thermoelectric complex stress should be better motivated (are wider ranges worth investigating and if not, why). The explicit emphasis of these ranges should be more obvious throughout the text.
3: Heterostructures are schematically discussed. Thicknesses of the layered structure (Fig. 1) should be made evident and discussed.
4: Conclusions should be mentioning 2-3 of the most important characterization results explicitly.
5: The authors should be mentioning that similar group IIIA nitride hetero and nano-structures can be approached by atomistic modelling methodology (e.g., G. Sfuncia, et al., CrystEngComm 25 (41), 5810-5817 (2023); and Manoel Alves Machado Filho, et al., Cryst. Growth Des. 2024, 24, 11, 4717–4727) bringing direct understanding of structural and electronic properties.
Comments on the Quality of English LanguageThe manuscript exhibits some misspellings as well as long and sometimes unclear sentences.
Author Response
Comments 1: [Title could be shortened. “… under the thermal and electrical stress” is stylistically incorrect. The article “the” should be removed.]
Response 1: [The title of the manuscript has been changed to "The degradation performance of AlGaN-based deep ultraviolet LEDs under thermal and electrical stress". Mention exactly where in the revised manuscript this change can be found – page 1, paragraph 1, and line 2.]
Comments 2: [The investigated ranges of thermal stress, electrical stress, and thermoelectric complex stress should be better motivated (are wider ranges worth investigating and if not, why). The explicit emphasis of these ranges should be more obvious throughout the text.]
Response 2: [This study mainly systematically analyses and compares the effects of high thermal, high electrical, and high thermoelectric complex stress on the optoelectronic degradation of AlGaN-based DUV LEDs. This study focuses on that photoelectric degradation under thermoelectric composite stress is higher than the sum of single thermal stress and single electrical stress. And current experimental data can support this idea. Meanwhile, other wider range research work is also being done, which contains different electrical stress aging tests, different thermal stress aging tests, and different thermoelectric complex stress aging tests. Once the tests finish and some other findings are found, our team will launch a secondary report immediately.]
Comments 3: [Heterostructures are schematically discussed. Thicknesses of the layered structure (Fig. 1) should be made evident and discussed. ]
Response 3: [This part has been added to the revised manuscript as follows.
In this study, we have designed an AlGaN-based deep ultraviolet (DUV) LED structure with a p-AlGaN film serving as the p-type contact layer. Figure 1 shows the 278nm LED growth heterostructure. The structures were prepared by the metalorganic chemical vapor deposition (MOCVD) on the sapphire substrate. The basic (0001)-oriented LED structure consists of, top to bottom, a 15 nm p-Al0.5Ga0.5N layer, a 10 nm n-Al0.8Ga0.2N layer, a 2 nm Al0.6Ga0.4N layer, a 22 nm n-Al0.8Ga0.2N layer, a five-period Al0.7Ga0.3N/Al0.5Ga0.5N(17nm/3nm) multiple quantum wells (MQW), a 1500 nm n-Al0.6Ga0.4N layer, a 1500 nm n-Al0.7Ga0.3N layer, a 2300 nm AlN layer. Trimethyl-gallium(TMGa), trimethyl-aluminum(TMAl) and NH3 were used as the sources of Ga, Al and N, respectively. For the n- and p-type dopants, we were used silane and cyclopentadienyl magnesium, respectively. Finally, the Cr/Al/Ni/Au (30/80/40/80 nm) stacks were deposited around the mesa as n-electrodes. The stacks were annealed utilizing a rapid thermal process at 630℃ for 7 min in N2 ambient. The p-electrode comprises Ni/Au (20/50 nm) metal stacks and was annealed at 580℃ for 7 min in N2 ambient. Mention exactly where in the revised manuscript this change can be found – page 2, paragraph 4, and line 94.]
Comments 4: [Conclusions should be mentioning 2-3 of the most important characterization results explicitly.]
Response 4: [Conclusion have been changed in the revised manuscript as follow. In this study, the degradation mechanism of AlGaN-based DUV LEDs with a p-AlGaN contact layer under thermal, electrical and thermoelectric complex stresses is investigated. A series of aging tests are conducted, including single thermal stress aging tests, single electrical stress with air cooling aging tests , single electrical stress aging tests, and thermoelectric complex stress aging tests. The results indicate that both temperature and current are important factors affecting the photoelectric properties of the AlGaN-based DUV LED. After 1000 hours aging tests, the peak wavelength of AlGaN-based LED remains unchanged, while the turn-on voltage and series resistance increase. The degradation of optical and electrical properties under the thermal and electrical stress could be not only attributed to the degradation of the device’s ohmic contacts, but also due to the metal electrode elements entering the p-AlGaN layer through thermal diffusion, leading to the generation of tunnelling current and the generation of defects within or around the active region. It can be obtained that higher junction temperature of the LEDs leading to the more defects, the larger of leakage current and the faster the degradation of optical power. Therefore, the optoelectronic performance and reliability of AlGaN-based DUV LEDs with p-AlGaN contact layer can be improved in the future by preparing p-AlGaN layers with gradient aluminium fractions and controlling the junction temperature of the LEDs during operation. This work will provide a solid theoretical foundation for the preparation of AlGaN-based DUV LEDs with high optical power output. Mention exactly where in the revised manuscript this change can be found – page 9, paragraph 4, and line 317.]
Comments 5: [The authors should be mentioning that similar group IIIA nitride hetero and nano-structures can be approached by atomistic modelling methodology (e.g., G. Sfuncia, et al., CrystEngComm 25 (41), 5810-5817 (2023); and Manoel Alves Machado Filho, et al., Cryst. Growth Des. 2024, 24, 11, 4717–4727) bringing direct understanding of structural and electronic properties.]
Response 5: [In the first reference mentioned above, except that the predictions routinely achievable by first-principles calculations using metal–organic chemical vapor deposition (MOCVD), it also reported a GaN monolayer in a buckled geometry obtained in confinement at the graphene/SiC interface. In another paper, the self-induced formation of core−shell InAlN nanorods (NRs) is addressed at the mesoscopic scale by density functional theory (DFT)-resulting parameters to develop phase field modeling (PFM). In this study, the degradation mechanism of AlGaN-based DUV LEDs with a p-AlGaN contact layer under thermal, electrical and thermoelectric complex stresses was investigated. The research method mentioned above is not very appropriate to the content of this paper study. The atomic modelling is often used to understand the impurities, nanostructures and electronic properties of group III-nitrides. When we carry out research about epitaxial wafer growth mechanism, model calculation mentioned above will be adopted to deepen the research depth.]
Reviewer 4 Report
Comments and Suggestions for Authorssee the attachment
Comments for author File: Comments.pdf
see the attachment
Author Response
Comments 1: [correction of writing errors, 20mA; 85℃respectively technologyof; Zheng[12]conducted; Differentialstructure etc.]
Response 1: [Some of the writing errors have been corrected in the revised manuscript.]
Comments 2: [Chapter 2, in which the sample preparation is described, is recommended to be developed in such a way as to express much more clearly the method/stages of making the samples, including the electrodes.]
Response 2: [This part has been added to the revised manuscript as follows. In this study, we have designed an AlGaN-based deep ultraviolet (DUV) LED structure with a p-AlGaN film serving as the p-type contact layer. Figure 1 shows the 278nm LED growth heterostructure. The structures were prepared by the metalorganic chemical vapor deposition (MOCVD) on the sapphire substrate. The basic (0001)-oriented LED structure consists of, top to bottom, a 15 nm p-Al0.5Ga0.5N layer, a 10 nm n-Al0.8Ga0.2N layer, a 2 nm Al0.6Ga0.4N layer, a 22 nm n-Al0.8Ga0.2N layer, a five-period Al0.7Ga0.3N/Al0.5Ga0.5N (17nm/3nm) multiple quantum wells (MQW), a 1500 nm n-Al0.6Ga0.4N layer, a 1500 nm n-Al0.7Ga0.3N layer, a 2300 nm AlN layer. Trimethyl-gallium(TMGa), trimethyl-aluminum(TMAl) and NH3 were used as the sources of Ga, Al and N, respectively. For the n- and p-type dopants, we were used silane and cyclopentadienyl magnesium, respectively. Finally, the Cr/Al/Ni/Au (30/80/40/80 nm) stacks were deposited around the mesa as n-electrodes. The stacks were annealed utilizing a rapid thermal process at 630℃ for 7 min in N2 ambient. The p-electrode comprises Ni/Au (20/50 nm) metal stacks and was annealed at 580℃ for 7 min in N2 ambient. Mention exactly where in the revised manuscript this change can be found – page 2, paragraph 4, and line 94.]
Comments 3: [In figures 2-6, the units of measurement of the sizes on the axes are recommended to be given with a space.]
Response 3: [This part has been acorrected in the revised manuscript as shown in Figure 2-6.]
Comments 4: [For figure 7, it is recommended to describe exactly the representation (k=f(Rth)...), to present the notation of each quantity, The specific thermal resistance of each package layer was plotted according to the differentialstructure function, as shown in Figure 7.]
Response 4: [Differential structure function formulations were added as follows:
The differential structure function can be calculated by the following formula.
where is the volume heat capacity, is the thermal conductivity, and is the cross-sectional area on the heat transfer path. Mention exactly where in the revised manuscript this change can be found – page 8, paragraph 3, and line 275. ]
Comments 5: [Give the citations for relations 1 and 2.]
Response 5: [Relations 1 and 2 have been cited in the original article. The formula for calculating the ideality factor[22] is represented asï¼›
The formula for calculating the junction temperature is represented as[26]ï¼›
[22]Masui H. Diode ideality factor in modern light-emitting diodes. Semiconductor Science and Technology. 2011;26 (7):075011-075011. doi:10.1088/0268-1242/ 26/7/075011.
[26] Liu, D.; Ru, Z.;Liu, F.; Zhang, C.; Huang, J. Research on test method of thermal resistance and junction temperature for LED modules. null. 2016;0 (0):0-0. doi:10.1109/eurosime.2016.7463318.]
Comments 6: [Chapter 5 conclusions must be developed in such a way as to present the purpose of the study, the method used and the most important results.]
Response 6: [Conclusion have been changed in the revised manuscript as follow.
In this study, the degradation mechanism of AlGaN-based DUV LEDs with a p-AlGaN contact layer under thermal, electrical and thermoelectric complex stresses is investigated. A series of aging tests are conducted, including single thermal stress aging tests, single electrical stress with air cooling aging tests , single electrical stress aging tests, and thermoelectric complex stress aging tests. The results indicate that both temperature and current are important factors affecting the photoelectric properties of the AlGaN-based DUV LED. After 1000 hours aging tests, the peak wavelength of AlGaN-based LED remains unchanged, while the turn-on voltage and series resistance increase. The degradation of optical and electrical properties under the thermal and electrical stress could be not only attributed to the degradation of the device’s ohmic contacts, but also due to the metal electrode elements entering the p-AlGaN layer through thermal diffusion, leading to the generation of tunnelling current and the generation of defects within or around the active region. It can be obtained that higher junction temperature of the LEDs leading to the more defects, the larger of leakage current and the faster the degradation of optical power. Therefore, the optoelectronic performance and reliability of AlGaN-based DUV LEDs with p-AlGaN contact layer can be improved in the future by preparing p-AlGaN layers with gradient aluminium fractions and controlling the junction temperature of the LEDs during operation. This work will provide a solid theoretical foundation for the preparation of AlGaN-based DUV LEDs with high optical power output. Mention exactly where in the revised manuscript this change can be found – page 9, paragraph 4, and line 317.]
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsThe authors have conducted studies on the degradation of p-AlGaN as a contact layer. However, given the extensive existing literature on this topic, the additional information provided in this study appears to be limited. Thus, is not recommended for publication.
Author Response
Comments 1:The authors have conducted studies on the degradation of p-AlGaN as a contact layer. However, given the extensive existing literature on this topic, the additional information provided in this study appears to be limited. Thus, is not recommended for publication.
Response 1:
Thanks for your review.We have carefully considered your comments and explain them as follows.
With the development growth technologyof AlGaN-based DUV wafers, p-AlGaN layer is seleced as the p-type contact layer, insteading of the traditional p-GaN layers aimming to higher uv-transmission. Previously, the device lifetime was only about a few hundred hours. With the improvement of epitaxial layer growth process and chip preparation process, the lifetime of DUV LEDs of our team has been improved to about 10000 hours. The previous reports of degradation mechanism are no longer applicable to the newest failure mechanism analysis. However, fewer degradation studies have been put forward for DUV LEDs with p-AlGaN contact layer. Besides that, the degradation mechanism of DUV LEDs is mainly investigated by constant electric stress aging tests. However, research of systematic analysis and comparison the degradation mechanism of AlGaN-based DUV LEDs under single thermal, single electrical, and thermoelectric composite stress conditions are also relatively scarce. Therefore, it is very necessary to investigate the degradation of AlGaN-based DUV LEDs with p-AlGaN film as p-type contact layer.
