Next Article in Journal
Interterminal Truck Routing Optimization Using Cooperative Multiagent Deep Reinforcement Learning
Previous Article in Journal
A Systematic Literature Review on the Automatic Creation of Tactile Graphics for the Blind and Visually Impaired
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Processes
Volume 9
Issue 10
10.3390/pr9101727
processes-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Deep Ultraviolet AlGaN-Based Light-Emitting Diodes with p-AlGaN/AlGaN Superlattice Hole Injection Structures

by
Tien-Yu Wang
1,
Wei-Chih Lai
1,2,*,
Syuan-Yu Sie
1,
Sheng-Po Chang
1,2,
Cheng-Huang Kuo
3 and
Jinn-Kong Sheu
1,2
1
Department of Photonics, National Cheng Kung University, Tainan 70101, Taiwan
2
Advanced Optoelectronic Technology Center, Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 70101, Taiwan
3
Institute of Lighting and Energy Photonics, College of Photonics, National Yang Ming Chiao Tung University, Tainan 71150, Taiwan
*
Author to whom correspondence should be addressed.
Processes 2021, 9(10), 1727; https://doi.org/10.3390/pr9101727
Submission received: 20 August 2021 / Revised: 16 September 2021 / Accepted: 24 September 2021 / Published: 26 September 2021
(This article belongs to the Section Materials Processes)
Browse Figures
Versions Notes

Abstract

:
The p-AlGaN/AlGaN superlattice (SL) hole injection structure was introduced into deep ultraviolet (DUV) light-emitting diodes (LEDs) to enhance their performances. The period thicknesses of the p-Al0.8Ga0.2N/Al0.48Ga0.52N SLs affected the performances of the DUV LEDs. The appropriate period thickness of the p-Al0.8Ga0.2N/Al0.48Ga0.52N SL may enhance the hole injection of DUV LEDs. Therefore, compared with the reference LEDs, the DUV LEDs with the 10-pair Al0.8Ga0.2N (1 nm)/Al0.48Ga0.52N (1 nm) SL presented forward voltage reduction of 0.23 V and light output power improvement of 15% at a current of 350 mA. Furthermore, the 10-pair Al0.8Ga0.2N (1 nm)/Al0.48Ga0.52N (1 nm) SL could slightly suppress the Auger recombination and current overflow of the DUV LEDs in a high-current operation region. In addition to improved carrier injection, the DUV LEDs with the p-Al0.8Ga0.2N/Al0.48Ga0.52N SL hole injection structure showed reduced light absorption at their emission wavelength compared with the reference LEDs. Therefore, the DUV LEDs with p-Al0.8Ga0.2N/Al0.48Ga0.52N SL may exhibit better light extraction efficiency than the reference LEDs. The enhancement of p-Al0.8Ga0.2N (1 nm)/Al0.48Ga0.52N (1 nm) SL may contribute to improvements in light extraction and hole injection.

1. Introduction

AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) have attracted attention in many fields, such as ultraviolet photolithography, high-density optical data storage, water purification, and portable chemical/biological agent detection/analysis systems [1,2,3,4,5]. The quality of AlGaN material has been greatly improved because of the optimization of epitaxy techniques; this improvement has also greatly enhanced the light-emission efficiency of DUV LEDs [6,7,8,9,10,11]. Despite the dramatic improvements in DUV LED performance, AlGaN-based DUV LEDs continue to face the problems of high-defect density and low-hole concentrations of high Al-content AlGaN material. Since the initial study on GaN-based LEDs, the low-hole concentrations of p-type GaN-based material has been a major issue that limits the efficiency of GaN-based LEDs because it causes poor hole injection [12,13,14,15,16,17]. Moreover, the problem of low-hole concentration for p-type AlxGa1−xN with x > 0.5 is worsening [10,11,12,13]. The doping efficiency for AlGaN with high Al content is still far from satisfactory due to a combination of factors involving limited solubility, high activation energy, increased donor compensations, and increased hole scatterings [14,15]. The p-type AlGaN/GaN heterojunction [18,19,20] and the AlGaN/GaN superlattice (SL) [21,22,23,24,25,26,27,28] have been reported to increase the hole concentrations of p-type GaN-based materials. Since then, similar concepts have been applied in many studies on GaN-based LEDs; such concepts include AlGaN/GaN/AlGaN quantum wells [29,30], AlGaN/GaN SLs [31], and graded AlGaN/GaN SLs [32,33]. Considering that the p-type AlGaN/GaN SL structure has presented promising results in GaN-based LEDs, a similar idea has been adopted to improve the hole concentration of high-Al content AlGaN in DUV LEDs [34,35]. Besides, p-AlGaN/AlGaN SL could also improve light extraction efficiency (LEE) of DUV LEDs [36]. Although the p-AlGaN/AlGaN SL structure has been proposed in many studies on DUV LEDs, revealing its other effects on DUV LEDs remains worthy. In this work, we replaced the hole injection layer of the 20 nm thick Mg-doped Al0.48Ga0.52N layer in the structure of the DUV LEDs with Mg-doped Al0.8Ga0.2N/Al0.48Ga0.52N SLs. The total thickness of the Mg-doped Al0.8Ga0.2N/Al0.48Ga0.52N SLs was kept at 20 nm. We prepared Mg-doped Al0.8Ga0.2N/Al0.48Ga0.52N SLs with different period thicknesses to understand how period thickness affected the optoelectrical characteristic of DUV LEDs. The theoretical (simulation) and experimental results for the DUV LEDs with the Mg-doped Al0.8Ga0.2N/Al0.48Ga0.52N SL hole injection layer, the electrical and optical properties of AlGaN-based DUV LEDs, and the fabrication process, were discussed in this study.

2. Materials and Methods

AlGaN-based DUV LEDs were epitaxially grown on a 2-inch (0001) sapphire substrate by using a Thomas Swan close-coupled showerhead 31 × 2-inch metalorganic chemical vapor deposition (MOCVD) system. In MOCVD growth, trimethylgallium, trimethylaluminum, and ammonia were used as the source materials of Ga, Al, and N, respectively. Biscyclopentadienyl magnesium and silane were used as p-type and n-type doping sources, respectively. A 20 nm thick AlN nucleation layer was grown on the sapphire substrate at the reactor temperature of 500 °C. The growth temperature was raised to 1250 °C for the growth of the 3 µm thick undoped AlN epitaxial layer. The screw and edge dislocation density of our AlN samples were 1.03 × 108 cm−2 and 1.64 × 109 cm−2, respectively. The AlN epitaxy layer was followed by the epitaxy of a 1.5 µm thick Si-doped Al0.6Ga0.4N layer. The DUV light-emitting MQW structure with 5-pair Al0.6Ga0.4N:Si (12 nm)/Al0.4Ga0.6N (2 nm) was grown at the reactor temperature of 1050 °C. A 2 nm thick undoped Al0.65Ga0.35N last thin barrier was then grown on the 5-pair Al0.6Ga0.4N (12 nm)/Al0.4Ga0.6N (2 nm) DUV MQW. An electron blocking layer (EBL) with a 7 nm-thick undoped Al0.7Ga0.3N layer and Mg-doped 3-pair AlGaN (7.5 nm)/AlGaN (3.5 nm) was grown on the Al0.65Ga0.35N last thin barrier. A 20 nm thick Mg-doped Al0.48Ga0.52N hole injection layer was grown on the EBL for the standard DUV LEDs as a reference. The SL structures of 5-pair Al0.8Ga0.2N (2 nm)/Al0.48Ga0.52N (2 nm), 10-pair Al0.8Ga0.2N (1 nm)/Al0.48Ga0.52N (1 nm), and 15-pair Al0.8Ga0.2N (0.66 nm)/Al0.48Ga0.52N (0.66 nm) were separately grown on EBL instead of the Mg-doped Al0.48Ga0.52N hole injection layer for the studied DUV LEDs. Then, the full structure of DUV LEDs was ended with a 20 nm thick Mg-doped graded Al composition AlGaN layer and a 10 nm thick Mg-doped GaN layer. The Al composition in the 20 nm thick Mg-doped graded Al composition AlGaN layer was linearly decreased from 47% to 25%.
Subsequently, standard processing steps were performed to fabricate 1000 µm × 1000 µm flip-chip DUV LEDs. The details of the flip-chip DUV LEDs and mounting processes were reported in our previous study [37]. The scheme of the DUV LED structures, the schematic and image of the flip-chip DUV LEDs, and the transmission electron microscopy (TEM) images of standard DUV LEDs and DUV LEDs with 10-pair Al0.8Ga0.2N (1 nm)/Al0.48Ga0.52N (1 nm) are shown in Figure 1. The standard DUV LEDs are denoted as the reference LEDs. The DUV LEDs with the 5-pair Al0.8Ga0.2N (2 nm)/Al0.48Ga0.52N (2 nm), 10-pair Al0.8Ga0.2N (1 nm)/Al0.48Ga0.52N (1 nm), and 15-pair Al0.8Ga0.2N (0.66 nm)/Al0.48Ga0.52N (0.66 nm) SL hole injection layers are denoted as LED 2/2, LED 1/1, and LED 0.66/0.66, respectively. The current–voltage (I–V) characteristics of the fabricated flip-chip DUV LEDs were measured by using a Keysight B1500 and HP-4156B semiconductor parameter analyzer for high-current and low-current measurements, respectively. The output power and emission spectra of the LEDs were acquired at room temperature by using a calibrated integrating sphere and a spectrometer (Ocean Optics USB2000).

3. Results and Discussion

The I–V characteristics and dynamic resistance of all flip-chip DUV LEDs were investigated, as presented in Figure 2. The 350 mA forward voltages (Vf) of the reference LED, LED 2/2, LED 1/1, and LED 0.66/0.66 were 6.69, 6.73, 6.46, and 6.38 V, respectively. The average Al composition of p-Al0.8Ga0.2N/Al0.48Ga0.52N SLs was 0.64, and it was higher than that of p-Al0.48Ga0.52N bulk. The LEDs with p-Al0.8Ga0.2N/Al0.48Ga0.52N SLs were expected to possess higher Vf due to higher series resistance. However, the reference LED and LED 2/2 exhibited close Vf. Moreover, the Vf of the DUV LEDs continuously decreased as the period thickness of the p-Al0.8Ga0.2N/Al0.48Ga0.52N SL was decreased. Contrary to bulk, SLs possessed poor vertical conductivity and better lateral conductivity [25,26,27,28]. The Vf reduction of LEDs with p-Al0.8Ga0.2N/Al0.48Ga0.52N SL could mainly contribute to improved current spreading. Regarding the period thickness of SL, the vertical conductivity of SL with short period thickness was higher than that of SL with long period thickness, but the lateral conductivity of SL with short period thickness was lower than that of SL with long period thickness [25,26,27,28]. In our results, the Vf of the DUV LEDs decreased as the period thickness of SL was decreased, and it indicated that the extent of vertical conductivity change could be more than that of lateral conductivity change as the period thickness of SL was decreased. Figure 2a shows the forward voltage dependent dynamic resistances of all DUV LEDs. The dynamic resistances of all DUV LEDs individually converged to a value when the forward voltage was larger than 6.5 V. The dynamic resistance near the Vf of the reference LEDs, LED 2/2, LED 1/1, and LED 0.66/0.66 were 19.1, 19.2, 18.4, and 18.2 Ω, respectively. The reference LED and LED 2/2 had almost the same dynamic resistance. Thus, the reference LED and LED 2/2 had close Vf. In addition, the dynamic resistance of the DUV LEDs decreased as the period thickness of the p-Al0.8Ga0.2N/Al0.48Ga0.52N SLs was decreased. The hole energy state in the well of the Al0.8Ga0.2N/Al0.48Ga0.52N SLs would be increased by reduction of the thickness of well. Besides, reduction of the barrier thickness may increase the tunneling probability of the hole carrier. This phenomenon effectively reduced the potential barrier on the hole carrier at interface of the EBL and Al0.8Ga0.2N/Al0.48Ga0.52N SLs. Therefore, LED 1/1 and 0.66/0.66 exhibited lower dynamic resistance than the reference LEDs and LED 2/2.
In addition to their reduced Vf and dynamic resistance, the DUV LEDs with the proposed p-Al0.8Ga0.2N/Al0.48Ga0.52N SL hole injection layer exhibited reduced current at applied voltages less than 3 V compared with the reference LEDs. The reduction in current at the applied voltage of 3 V implied that the DUV LEDs with the p-Al0.8Ga0.2N/Al0.48Ga0.52N SL hole injection layer had a lower defect-assisted recombination current than the reference LED. Besides, since the average Al composition of SLs (x = 0.64) was higher than the Al composition of bulk (x = 0.48), SLs would possess larger potential barrier than bulk. Additionally, the potential barrier could be enhanced by the piezoelectric field of SLs. As a result, the reduction in current at applied voltages less than 3V could contribute to large potential of SLs. Moreover, the proposed p-Al0.8Ga0.2N/Al0.48Ga0.52N SL hole injection layer improved the reverse I–V characteristics of the DUV LEDs. In Figure 2b, the reverse current at −15 V of the reference LEDs could be significantly reduced from − 4.95 × 10 − 3 A to − 4.57 × 10 − 5 A by replacing the p-AlGaN hole injection layer with the p-Al0.8Ga0.2N/Al0.48Ga0.52N SLs. The p-Al0.8Ga0.2N/Al0.48Ga0.52N SLs exhibited clear and sharp high-quality Al0.8Ga0.2N/Al0.48Ga0.52N interfaces in the TEM image provided in Figure 1c. The multiple high-quality Al0.8Ga0.2N/Al0.48Ga0.52N interfaces might have covered the pits and defects at the bottom of the structure well. Sun et al. [38] reported that threading dislocations (TDs) of AlGaN could be improved by inserting AlN/AlGaN superlattice. The AlN/AlGaN superlattice could function as dislocation filter, and it could benefit from the AlN/AlGaN SL coherent growth. Therefore, this phenomenon could explain the I–V characteristics at low forward voltage and the improvement in the reverse I–V characteristics of the DUV LEDs.
The hole injection of the DUV LEDs may be improved by replacing the p-AlGaN hole injection layer with the p-Al0.8Ga0.2N/Al0.48Ga0.52N SLs. This effect could result in the enhancement of the light output power of the DUV LEDs. Mondal et al. [11] and Yang et al. [12] have reported simulation results for similar DUV LED structures. They concluded that after the EBL of DUV LEDs, SLs or multilayer p-AlGaN/AlGaN structures effectively increased hole injection in the MQW region to enhance the light output power of the DUV LEDs. Figure 3 shows the measured output powers and external quantum efficiencies (EQEs) of all DUV LEDs as a function of the injection current density. The light output powers of the DUV LEDs driven at a current density of 350 A/cm2 were 42.74, 40.81, 49.24, and 44.57 mW, which corresponded to EQEs of 2.68%, 2.58%, 3.15%, and 2.83% for the reference LEDs, LED 2/2, LED 1/1, and LED 0.66/0.66, respectively. The light output power of the DUV LEDs with 5-pair Al0.8Ga0.2N (2 nm)/Al0.48Ga0.52N (2 nm) was approximately 5% less than that of reference LEDs. Comparison with the reference LED revealed that the DUV LEDs with 10-pair Al0.8Ga0.2N (1 nm)/Al0.48Ga0.52N (1 nm) presented the light output power enhancement of 15%, which was the largest improvement observed in this study. However, the enhancement in light output power decreased to 4.3% when the pair thickness of the p-Al0.8Ga0.2N/Al0.48Ga0.52N SLs was reduced to 1.32 nm. Relative to that of the reference LEDs, the light output power enhancement of the DUV LEDs with the p-Al0.8Ga0.2N/Al0.48Ga0.52N SLs was strongly affected by the pair thickness of the hole injection layer. Furthermore, the proposed p-Al0.8Ga0.2N/Al0.48Ga0.52N SL hole injection layer could slightly change the behavior of the efficiency droops of the DUV LEDs. The efficiency droops of the reference LED, LED 2/2, LED 1/1, and LED 0.66/0.66 were 36%, 37%, 31%, 34%, respectively. (The efficiency droop was defined as the EQE degradation from the peak of EQE to the EQE at current of 800 mA, efficiency droop = 1 − EQE peak EQE 800 mA .)
The light output power of LED 2/2 was reduced compared with that of the reference LEDs. Given that LED 2/2 had a slightly higher Vf and dynamic resistance than the reference LEDs, these results implied that the 5-pair Al0.8Ga0.2N (2 nm)/Al0.48Ga0.52N (2 nm) SL did not improve hole injection. Although the lateral conductivity of SL could be improved, the poor vertical conductivity of SL could weaken the hole injection. The long period thickness of 5-pair Al0.8Ga0.2N (2 nm)/Al0.48Ga0.52N (2 nm) SL may reduce hole transport despite the improved lateral conductivity. As the period thickness of SL was reduced, the lateral conductivity of SL decreased, and the vertical conductivity of SL increased. The 10-pair Al0.8Ga0.2N (1 nm)/Al0.48Ga0.52N (1 nm) SL might suffer decreased lateral conductivity, but it benefited from better vertical conductivity. Therefore, the enhanced output power of DUV LED with 10-pair Al0.8Ga0.2N (1 nm)/Al0.48Ga0.52N (1 nm) SL could have contributed to better hole injection. When the period thickness was further reduced, the 15-pair Al0.8Ga0.2N (0.66 nm)/Al0.48Ga0.52N (0.66 nm) SL might have suffered more decreased lateral conductivity despite improved vertical conductivity. In this case, the current spreading of 15-pair SL declined, and the enhancement of output power of DUV LED with 15-pair SL was lowered. In the perspective of the efficiency droops of our DUV LEDs, the reference LED and LED 2/2 possessed similar efficiency droop, and it indicated that the hole injection of reference LED was similar to that of LED 2/2. However, the LED 1/1 possessed slightly improved efficiency droop, and it also implied that the 10-pair SL could also improve current spreading. In the high-current injection region, the effects of Auger recombination and current overflow of LEDs could be dominant. The improved current spreading could suppress the effects of Auger recombination and current overflow. The LED 0.66/0.66 had larger efficiency droop than that of LED 1/1. It may be caused by reduced current spreading due to decreased lateral conductivity. The weakened current spreading could augment the effects of Auger recombination and current overflow. Hence, the efficiency droop of LED 0.66/0.66 worsened. The 10-pair Al0.8Ga0.2N (1 nm)/Al0.48Ga0.52N (1 nm) SL may be the optimal SL structure in this study because the DUV LEDs with this SL structure had the best electrical properties and the highest light output power. The appropriate period thickness of Al0.8Ga0.2N/Al0.48Ga0.52N SL may enhance hole injection efficiency to improve light output power and suppress the effects of Auger recombination and current overflow under high-current operation.
In addition to enhancing hole injection, the p-Al0.8Ga0.2N/Al0.48Ga0.52N SLs would modify the transmittance spectra of the DUV LEDs. Figure 4 shows the transmittance spectra of the reference LED, LED 2/2, LED 1/1, and LED 0.66/0.66. All DUV LED samples presented high transmittance in the visible-light wavelength region. However, the values varied among the samples. All DUV LED samples showed stepwise decrements in transmittance. The initial reduction in the transmittance of all DUV LEDs occurred at a wavelength near 365 nm, which corresponded to the absorption of the p-GaN contact layer. The second decrement in the transmittance of all DUV LEDs occurred at a wavelength near 275 nm and should originate from the absorption of AlGaN/AlGaN MQWs and other AlGaN layers with band gaps near 4.52 eV. However, the reference LEDs showed the lowest transmittance values within the wavelength range of 250–280 nm. The strong absorption of the reference LEDs within the wavelength range of 250–280 nm should be attributed to the absorption of the Al0.6Ga0.4N (12 nm)/Al0.4Ga0.6N (2 nm) DUV MQW and p-Al0.48Ga0.52N hole injection layer. The p-Al0.8Ga0.2N/Al0.48Ga0.52N SL hole injection layer possessed a larger effective band gap than the p-Al0.48Ga0.52N hole injection layer. The effective band gap would be enlarged by reducing the pair thickness of the p-Al0.8Ga0.2N/Al0.48Ga0.52N SLs. Therefore, LED 1/1 and LED 0.66/0.66 presented the highest transmittance within the wavelength range of 250–280 nm. The transmittance of DUV LEDs could be qualitatively related to light extraction efficiency (LEE) of DUV LEDs [36]. Comparing the output power results in Figure 3 with the transmittance results in Figure 4, the transmittance of LED 2/2 was higher that of reference LED within the wavelength range of 250–280 nm, but the output power of LED 2/2 was lower than that of reference. It implied that internal quantum efficiency (IQE) of LED 2/2 was lower than that of reference. This result also indicated that the hole injection of 5-pair SL was not improved. Both LED 1/1 and LED 0.66/0.66 possessed the highest transmittance within the wavelength range of 250–280 nm, but the output power of LED 0.66/0.66 was lower than that of LED 1/1. It also implied that the IQE of LED 0.66/0.66 was worse than that of LED 1/1. This can contribute to weakened current spreading due to decreased lateral conductivity.

4. Conclusions

We replaced the p-Al0.48Ga0.52N hole injection layer with p-Al0.8Ga0.2N/Al0.48Ga0.52N SLs to enhance the light output power of DUV LEDs. The period thickness of the p-Al0.8Ga0.2N/Al0.48Ga0.52N SLs affected the performances of the DUV LEDs. We found that the DUV LED with the 5-pair Al0.8Ga0.2N (2 nm)/Al0.48Ga0.52N (2 nm) SL structure showed increased Vf and reduced light output power compared with the reference LEDs. The SL structure could improve the performances of the DUV LEDs when its period thickness was reduced to less than 4 nm. Therefore, in this study, the DUV LEDs with the 10-pair Al0.8Ga0.2N (1 nm)/Al0.48Ga0.52N (1 nm) SL presented Vf reduction of 0.23 V and light output power improvement of 15% at a current of 350 mA. The improved light output power of the DUV LEDs could be attributed to the enhancement in hole injection by the 10-pair Al0.8Ga0.2N (1 nm)/Al0.48Ga0.52N (1 nm) SL. Moreover, the 10-pair SL could slightly suppress the Auger recombination and current overflow of the DUV LEDs in the high-current operation region due to better current spreading. However, the enhancement in light output power was reduced to 4.3% when the period thickness of the p-Al0.8Ga0.2N/Al0.48Ga0.52N SL structure was reduced to 1.32 nm (15-pair Al0.8Ga0.2N [0.66 nm]/Al0.48Ga0.52N [0.66 nm] SL). In addition to improved carrier injection, the DUV LEDs with the p-Al0.8Ga0.2N/Al0.48Ga0.52N SL structure demonstrated reduced light absorption at the emission wavelength compared with the reference LEDs. Therefore, the DUV LEDs with p-Al0.8Ga0.2N/Al0.48Ga0.52N SLs should have better light extraction efficiency than the reference LEDs. This phenomenon may also be ascribed to the enhancement in the light output of the DUV LEDs with the p-Al0.8Ga0.2N/Al0.48Ga0.52N SL hole injection structure.

Author Contributions

Conceptualization, and methodology, T.-Y.W. and W.-C.L.; chip process and data collection, S.-Y.S.; formal analysis and investigation, T.-Y.W., W.-C.L., S.-P.C., C.-H.K. and J.-K.S.; writing—original draft preparation, T.-Y.W. and W.-C.L.; writing—review and editing, W.-C.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Ministry of Science and Technology (MOST), Taiwan, grant number MOST 107-2221-E-006-186-MY3, MOST 104-2221-E-006-069-MY3, NSC 101-2221-E-006-066-MY3, and NSC 102-3113-P-009-007-CC2.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data underlying the results presented in this paper can be obtained from the authors upon reasonable request.

Acknowledgments

We extend our gratitude to the Ministry of Science and Technology (MOST), Taiwan for their financial support under grant nos. MOST 107-2221-E-006-186-MY3, MOST 104-2221-E-006-069-MY3, NSC 101-2221-E-006-066-MY3, and NSC 102-3113-P-009-007-CC2.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

References

  1. Morison, W.L. Phototherapy and Photochemotherapy for Skin Disease; CRC Press: Boca Raton, FL, USA, 2005. [Google Scholar]
  2. Kneissl, M.; Rass, J. (Eds.) III-Nitride Ultraviolet Emitters: Technology and Applications; Springer: Cham, Switzerland, 2016; ISBN 9783319240985. [Google Scholar]
  3. Hirayama, H.; Yatabe, T.; Noguchi, N.; Kamata, N. Development of 230–270 nm AlGaN-Based Deep-UV LEDs. Electron. Commun. Jpn. 2010, 93, 24–33. [Google Scholar] [CrossRef]
  4. Mehnke, F.; Guttmann, M.; Enslin, J.; Kuhn, C.; Reich, C.; Jordan, J.; Kapanke, S.; Knauer, A.; Lapeyrade, M.; Zeimer, U.; et al. Gas Sensing of Nitrogen Oxide Utilizing Spectrally Pure Deep UV LEDs. IEEE J. Sel. Top. Quantum Electron. 2017, 23, 29–36. [Google Scholar] [CrossRef]
  5. Grandusky, J.R.; Gibb, S.R.; Mendrick, M.C.; Moe, C.; Wraback, M.; Schowalter, L.J. High Output Power from 260 nm Pseudomorphic Ultraviolet Light-Emitting Diodes with Improved Thermal Performance. Appl. Phys. Express 2011, 4, 082101. [Google Scholar] [CrossRef]
  6. Islam, S.M.; Protasenko, V.; Lee, K.; Rouvimov, S.; Verma, J.; Xing, H.; Jena, D. Deep-UV Emission at 219 nm from Ultrathin MBE GaN/AlN Quantum Heterostructures. Appl. Phys. Lett. 2017, 111, 091104. [Google Scholar] [CrossRef]
  7. Takano, T.; Mino, T.; Sakai, J.; Noguchi, N.; Tsubaki, K.; Hirayama, H. Deep-Ultraviolet Light-Emitting Diodes with External Quantum Efficiency Higher than 20% at 275 nm Achieved by Improving Light-Extraction Efficiency. Appl. Phys. Express 2017, 10, 031002. [Google Scholar] [CrossRef]
  8. Pernot, C.; Fukahori, S.; Inazu, T.; Fujita, T.; Kim, M.; Nagasawa, Y.; Hirano, A.; Ippommatsu, M.; Iwaya, M.; Kamiyama, S.; et al. Development of High Efficiency 255--355 nm AlGaN-Based Light-Emitting Diodes. Phys. Status Solidi 2011, 208, 1594–1596. [Google Scholar] [CrossRef]
  9. Shatalov, M.; Sun, W.; Jain, R.; Lunev, A.; Hu, X.; Dobrinsky, A.; Bilenko, Y.; Yang, J.; Garrett, G.A.; Rodak, L.E.; et al. High Power AlGaN Ultraviolet Light Emitters. Semicond. Sci. Technol. 2014, 29, 084007. [Google Scholar] [CrossRef]
  10. Liao, Y.; Thomidis, C.; Kao, C.-K.; Moustakas, T.D. AlGaN Based Deep Ultraviolet Light Emitting Diodes with High Internal Quantum Efficiency Grown by Molecular Beam Epitaxy. Appl. Phys. Lett. 2011, 98, 081110. [Google Scholar] [CrossRef] [Green Version]
  11. Mondal, R.K.; Chatterjee, V.; Pal, S. AlInGaN-Based Superlattice P-Region for Improvement of Performance of Deep UV LEDs. Opt. Mater. 2020, 104, 109846. [Google Scholar] [CrossRef]
  12. Yang, X.; Sun, H.; Fan, X.; Zhang, Z.; Sun, J.; Yi, X.; Guo, Z. Optimization on the Luminous Efficiency in AlGaN-Based Ultraviolet Light-Emitting Diodes by Amendment of a Superlattice Hole Reservoir Layer. Superlattices Microstruct. 2017, 101, 293–298. [Google Scholar] [CrossRef]
  13. Martens, M.; Mehnke, F.; Kuhn, C.; Reich, C.; Kueller, V.; Knauer, A.; Netzel, C.; Hartmann, C.; Wollweber, J.; Rass, J.; et al. Performance Characteristics of UV-C AlGaN-Based Lasers Grown on Sapphire and Bulk AlN Substrates. IEEE Photon. Technol. Lett. 2014, 26, 342–345. [Google Scholar] [CrossRef]
  14. Kim, M.-H.; Schubert, M.F.; Dai, Q.; Kim, J.K.; Schubert, E.F.; Piprek, J.; Park, Y. Origin of Efficiency Droop in GaN-Based Light-Emitting Diodes. Appl. Phys. Lett. 2007, 91, 183507. [Google Scholar] [CrossRef] [Green Version]
  15. Schubert, M.F.; Xu, J.; Kim, J.K.; Schubert, E.F.; Kim, M.H.; Yoon, S.; Lee, S.M.; Sone, C.; Sakong, T.; Park, Y. Polarization-Matched GaInN/AlGaInN Multi-Quantum-Well Light-Emitting Diodes with Reduced Efficiency Droop. Appl. Phys. Lett. 2008, 93, 041102. [Google Scholar] [CrossRef]
  16. Kuo, Y.-K.; Chang, J.-Y.; Tsai, M.-C.; Yen, S.-H. Advantages of Blue InGaN Multiple-Quantum Well Light-Emitting Diodes with InGaN Barriers. Appl. Phys. Lett. 2009, 95, 011116. [Google Scholar] [CrossRef]
  17. Chang, J.-Y.; Tsai, M.-C.; Kuo, Y.-K. Advantages of Blue InGaN Light-Emitting Diodes with AlGaN Barriers. Opt. Lett. 2010, 35, 1368–1370. [Google Scholar] [CrossRef]
  18. Wang, C.H.; Ke, C.C.; Lee, C.Y.; Chang, S.P.; Chang, W.T.; Li, J.C.; Li, Z.Y.; Yang, H.C.; Kuo, H.C.; Lu, T.C.; et al. Hole Injection and Efficiency Droop Improvement in InGaN/GaN Light-Emitting Diodes by Band-Engineered Electron Blocking Layer. Appl. Phys. Lett. 2010, 97, 261103. [Google Scholar] [CrossRef] [Green Version]
  19. Han, S.-H.; Cho, C.-Y.; Lee, S.-J.; Park, T.-Y.; Kim, T.-H.; Park, S.H.; Won Kang, S.; Won Kim, J.; Kim, Y.C.; Park, S.-J. Effect of Mg Doping in the Barrier of InGaN/GaN Multiple Quantum Well on Optical Power of Light-Emitting Diodes. Appl. Phys. Lett. 2010, 96, 051113. [Google Scholar] [CrossRef] [Green Version]
  20. Schubert, E.F.; Grieshaber, W.; Goepfert, I.D. Enhancement of Deep Acceptor Activation in Semiconductors by Superlattice Doping. Appl. Phys. Lett. 1996, 69, 3737–3739. [Google Scholar] [CrossRef] [Green Version]
  21. Hsu, L.; Walukiewicz, W. Theoretical Transport Studies of p-Type GaN/AlGaN Modulation-Doped Heterostructures. Appl. Phys. Lett. 1999, 74, 2405–2407. [Google Scholar] [CrossRef]
  22. Saxler, A.; Mitchel, W.C.; Kung, P.; Razeghi, M. Aluminum Gallium Nitride Short-Period Superlattices Doped with Magnesium. Appl. Phys. Lett. 1999, 74, 2023–2025. [Google Scholar] [CrossRef]
  23. Kozodoy, P.; Hansen, M.; DenBaars, S.P.; Mishra, U.K. Enhanced Mg Doping Efficiency in Al0.2Ga0.8N/GaN Superlattices. Appl. Phys. Lett. 1999, 74, 3681–3683. [Google Scholar] [CrossRef]
  24. Kozodoy, P.; Smorchkova, Y.P.; Hansen, M.; Xing, H.; DenBaars, S.P.; Mishra, U.K.; Saxler, A.W.; Perrin, R.; Mitchel, W.C. Polarization-Enhanced Mg Doping of AlGaN/GaN Superlattices. Appl. Phys. Lett. 1999, 75, 2444–2446. [Google Scholar] [CrossRef]
  25. Kauser, M.Z.; Osinsky, A.; Dabiran, A.M.; Chow, P.P. Enhanced Vertical Transport in p-Type AlGaN/GaN Superlattices. Appl. Phys. Lett. 2004, 85, 5275–5277. [Google Scholar] [CrossRef]
  26. Goepfert, I.D.; Schubert, E.F.; Osinsky, A.; Norris, P.E.; Faleev, N.N. Experimental and Theoretical Study of Acceptor Activation and Transport Properties in p-Type AlxGa1−xN/GaN Superlattices. J. Appl. Phys. 2000, 88, 2030–2038. [Google Scholar] [CrossRef]
  27. Hertkorn, J.; Thapa, S.B.; Wunderer, T.; Scholz, F.; Wu, Z.H.; Wei, Q.Y.; Ponce, F.A.; Moram, M.A.; Humphreys, C.J.; Vierheilig, C.; et al. Highly Conductive Modulation Doped Composition Graded P-AlGaN/(AlN)/GaN Multiheterostructures Grown by Metalorganic Vapor Phase Epitaxy. J. Appl. Phys. 2009, 106, 013720. [Google Scholar] [CrossRef]
  28. Duboz, J.-Y. GaN/AlGaN Superlattices for P Contacts in LEDs. Semicond. Sci. Technol. 2014, 29, 035017. [Google Scholar] [CrossRef]
  29. Ju, Z.G.; Liu, W.; Zhang, Z.-H.; Tan, S.T.; Ji, Y.; Kyaw, Z.; Zhang, X.L.; Lu, S.P.; Zhang, Y.P.; Zhu, B.B.; et al. Advantages of the Blue InGaN/GaN Light-Emitting Diodes with an AlGaN/GaN/AlGaN Quantum Well Structured Electron Blocking Layer. ACS Photon. 2014, 1, 377–381. [Google Scholar] [CrossRef] [Green Version]
  30. Xia, C.S.; Li, Z.M.S.; Lu, W.; Zhang, Z.H.; Sheng, Y.; Hu, W.D.; Cheng, L.W. Efficiency Enhancement of Blue InGaN/GaN Light-Emitting Diodes with an AlGaN-GaN-AlGaN Electron Blocking Layer. J. Appl. Phys. 2012, 111, 094503. [Google Scholar] [CrossRef]
  31. Yan Zhang, Y.; An Yin, Y. Performance Enhancement of Blue Light-Emitting Diodes with a Special Designed AlGaN/GaN Superlattice Electron-Blocking Layer. Appl. Phys. Lett. 2011, 99, 221103. [Google Scholar] [CrossRef]
  32. Kang, J.; Li, H.; Li, Z.; Liu, Z.; Ma, P.; Yi, X.; Wang, G. Enhancing the Performance of Green GaN-Based Light-Emitting Diodes with Graded Superlattice AlGaN/GaN Inserting Layer. Appl. Phys. Lett. 2013, 103, 102104. [Google Scholar] [CrossRef]
  33. Lin, B.-C.; Chen, K.-J.; Han, H.-V.; Lan, Y.-P.; Chiu, C.-H.; Lin, C.-C.; Shih, M.-H.; Lee, P.-T.; Kuo, H.-C. Advantages of Blue LEDs With Graded-Composition AlGaN/GaN Superlattice EBL. IEEE Photon. Technol. Lett. 2013, 25, 2062–2065. [Google Scholar] [CrossRef]
  34. Zhang, Y.-Y.; Zhu, X.-L.; Yin, Y.-A.; Ma, J. Performance Enhancement of Near-UV Light-Emitting Diodes with an InAlN/GaN Superlattice Electron-Blocking Layer. IEEE Electron. Device Lett. 2012, 33, 994–996. [Google Scholar] [CrossRef]
  35. Muhin, A.; Guttmann, M.; Kuhn, C.; Mickein, E.; Aparici, J.R.; Ziffer, E.; Susilo, N.; Sulmoni, L.; Wernicke, T.; Kneissl, M. Vertical Conductivity and Poole–Frenkel-Ionization of Mg Acceptors in AlGaN Short-Period Superlattices with High Al Mole Fraction. Appl. Phys. Lett. 2020, 117, 252101. [Google Scholar] [CrossRef]
  36. Guttmann, M.; Susilo, A.; Sulmoni, L.; Susilo, N.; Ziffer, E.; Wernicke, T.; Kneissl, M. Light Extraction Efficiency and Internal Quantum Efficiency of Fully UVC-Transparent AlGaN Based LEDs. J. Phys. D Appl. Phys. 2021, 54, 335101. [Google Scholar] [CrossRef]
  37. Wang, T.-Y.; Lai, W.-C.; Sie, S.-Y.; Chang, S.-P.; Wu, Y.-R.; Chiou, Y.-Z.; Kuo, C.-H.; Sheu, J.-K. AlGaN-Based Deep Ultraviolet Light Emitting Diodes with Magnesium Delta-Doped AlGaN Last Barrier. Appl. Phys. Lett. 2020, 117, 251101. [Google Scholar] [CrossRef]
  38. Sun, W.H.; Zhang, J.P.; Yang, J.W.; Maruska, H.P.; Khan, M.A.; Liu, R.; Ponce, F.A. Fine Structure of AlN/AlGaN Superlattice Grown by Pulsed Atomic-Layer Epitaxy for Dislocation Filtering. Appl. Phys. Lett. 2005, 87, 211915. [Google Scholar] [CrossRef]
Processes 09 01727 g001 550
Figure 1. (a) Schematic of DUV LEDs with different p-AlGaN/AlGaN SLs; (b) schematic and image of the flip-chip DUV LEDs; (c) TEM images of standard DUV LEDs and DUV LEDs with 10-pair Al0.8Ga0.2N (1 nm)/Al0.48Ga0.52N (1 nm).
Figure 1. (a) Schematic of DUV LEDs with different p-AlGaN/AlGaN SLs; (b) schematic and image of the flip-chip DUV LEDs; (c) TEM images of standard DUV LEDs and DUV LEDs with 10-pair Al0.8Ga0.2N (1 nm)/Al0.48Ga0.52N (1 nm).
Processes 09 01727 g001
Processes 09 01727 g002 550
Figure 2. (a) Semi-log forward I–V of DUV LEDs with different p-AlGaN/AlGaN SLs. The inset of Figure 2a presents linear forward I–V curves of DUV LED samples at voltage range of 6–7 V. (b) Linear reverse I–V of DUV LEDs with different p-AlGaN/AlGaN SLs. The inset of Figure 2b presents semi-log reverse I–V curves of DUV LED samples at voltage range of −10–15 V.
Figure 2. (a) Semi-log forward I–V of DUV LEDs with different p-AlGaN/AlGaN SLs. The inset of Figure 2a presents linear forward I–V curves of DUV LED samples at voltage range of 6–7 V. (b) Linear reverse I–V of DUV LEDs with different p-AlGaN/AlGaN SLs. The inset of Figure 2b presents semi-log reverse I–V curves of DUV LED samples at voltage range of −10–15 V.
Processes 09 01727 g002
Processes 09 01727 g003 550
Figure 3. Current-dependent output power and EQE of DUV LEDs with different p-AlGaN/AlGaN SLs.
Figure 3. Current-dependent output power and EQE of DUV LEDs with different p-AlGaN/AlGaN SLs.
Processes 09 01727 g003
Processes 09 01727 g004 550
Figure 4. Transmittance of DUV LEDs with different p-AlGaN/AlGaN SLs. The inset of Figure 4 presents transmittance of these DUV LEDs within the wavelength range of 220–300 nm.
Figure 4. Transmittance of DUV LEDs with different p-AlGaN/AlGaN SLs. The inset of Figure 4 presents transmittance of these DUV LEDs within the wavelength range of 220–300 nm.
Processes 09 01727 g004
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Share and Cite

MDPI and ACS Style

Wang, T.-Y.; Lai, W.-C.; Sie, S.-Y.; Chang, S.-P.; Kuo, C.-H.; Sheu, J.-K. Deep Ultraviolet AlGaN-Based Light-Emitting Diodes with p-AlGaN/AlGaN Superlattice Hole Injection Structures. Processes 2021, 9, 1727. https://doi.org/10.3390/pr9101727

AMA Style

Wang T-Y, Lai W-C, Sie S-Y, Chang S-P, Kuo C-H, Sheu J-K. Deep Ultraviolet AlGaN-Based Light-Emitting Diodes with p-AlGaN/AlGaN Superlattice Hole Injection Structures. Processes. 2021; 9(10):1727. https://doi.org/10.3390/pr9101727

Chicago/Turabian Style

Wang, Tien-Yu, Wei-Chih Lai, Syuan-Yu Sie, Sheng-Po Chang, Cheng-Huang Kuo, and Jinn-Kong Sheu. 2021. "Deep Ultraviolet AlGaN-Based Light-Emitting Diodes with p-AlGaN/AlGaN Superlattice Hole Injection Structures" Processes 9, no. 10: 1727. https://doi.org/10.3390/pr9101727

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop