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Improvement of Algan Homoepitaxial Tunnel Junction Deep-UV Light-Emitting Diodes by Controlling the Growth of N-Type Algan and Polycrystalline Mgzno/Al Electrodes

Submitted:

10 February 2023

Posted:

15 February 2023

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Abstract
Deep-ultraviolet (UV) light-emitting diodes (LEDs) based on AlGaN crystals have low light-emission efficiency; therefore, there is a need to improve this light-emission efficiency for a wide range of applications such as water and air sterilizations. UV-light-transparent device structures are considered one of the many solutions toward increasing light output power. To this end, the present study focused on developing a transparent AlGaN-based tunnel junction (TJ) as the anode of a deep-UV LED. Deep-UV LEDs composed of n+/p+-type AlGaN TJs were fabricated under the growth condition that reduced the carrier compensation in the n+-type AlGaN layers. The operating voltage was 10.8 V under the direct current (DC) operation of 63 A/cm2. In addition, magnesium zinc oxide (MgZnO)/Al reflective electrodes were fabricated to enhance the output power of the AlGaN homoepitaxial TJ LED. The output power was 57.3 mW under a DC operation of 63 A/cm2, and it was 1.7 times higher than that realized using the conventional Ti/Al electrodes. The combination of the AlGaN-based TJ and MgZnO/Al reflective contact allows further improvement of the light output power. This study confirms that the AlGaN TJ is a promising UV-transmittance structure that can obtain a high light-extraction efficiency.
Keywords: 
AlGaN
;  
tunnel junction
;  
light-emitting diode
;  
deep-ultraviolet
;  
MgZnO
Subject: 
Physical Sciences  -   Optics and Photonics

1. Introduction

Aluminum gallium nitride (AlGaN)-based light-emitting diodes (LEDs) emit deep-ultraviolet (UV) light and are utilized in several applications at different wavelengths such as in curing, sensing, and water and air sterilizations. These LEDs are considered replacements for the mercury lamps used in water and air sterilizations [1,2,3]. Deep-UV light with an emission wavelength below 290 nm can rapidly inactivate the deoxyribonucleic acid of virus and bacteria [4,5]; however, the light-emission efficiency (LEE) of deep-UV LEDs is considerably lower than that of low-pressure mercury lamps. The wall-plug efficiency of mass-produced deep-UV LEDs is a maximum of 10% because of the UV light absorption of the p-type gallium nitride (GaN) contact layer [6,7,8]. A p-type GaN contact layer is used in mass-produced deep-UV LEDs because a higher Al composition p-type AlGaN can lead to a higher ionization energy of magnesium (Mg) acceptors and a lower hole concentration [9,10,11,12,13,14]. AlGaN homoepixial tunnel junction (TJ) LEDs without a GaN or gallium indium nitride (GaInN) layer have been reported by several research groups for enhancing the light-extraction efficiency (LEE) of deep-UV LEDs; the LEE is high, and the device operates at a voltage of 13–50 V [15,16]. Further, it is necessary to increase the high Si-doping concentration at the TJ layer to increase tunnel probability. The resistivity of the high-Al-composition n-type AlGaN layer with a high Si-doping concentration (>6 × 1019 cm−3) is extremely high because of the self-compensation caused by the cation–vacancy–silicon (VIII–nSi) complexes [17,18,19,20,21,22]. Further, carbon atoms possibly cause carrier compensation by substituting nitrogen sites (CN), which can reduce conductivity in n-type GaN [23]. The growth condition of the high-Si-doped n+-type AlGaN needs to be controlled for suppressing the carrier compensation defects and reducing the operating voltage of AlGaN TJ LEDs.
In addition, UV light absorption occurs at electrodes, and we focus on high-Al composition AlGaN TJ LEDs and magnesium zinc oxide (MgZnO)/Al reflective electrode [24,25]. MgZnO is suitable for suppressing light absorption, and this can help widen the band gap from 3.34 to 7.8 eV by controlling the Mg composition in MgZnO. A previous study reported that sputtered MgZnO exhibited two crystalline structures after recrystallization by annealing; both structures exhibited high transmittance in the UV range and n-type conductivity [26].
In this study, we review key challenges of AlGaN TJ deep-UV LEDs. The growth conditions of the n+-type AlGaN of TJ are controlled such that carbon incorporation can be suppressed at high Si doping to reduce the operating voltage of the AlGaN TJ LED. Further, we demonstrate that MgZnO/Al reflective electrodes can fully exploit the transparent structure of TJs, which results in enhancing the LEE of deep-UV LEDs.

2. Materials and Methods

Deep-UV LEDs were grown using a metalorganic vapor phase epitaxy on c-plane sapphire substrates with a miscut angle of 0.35° toward the sapphire [11 2 ¯ 0] direction. Trimethylaluminium, trimethylgallium, triethylgallium, Bis(cyclopentadienyl)magnesium, monosilane gas, and ammonia gases were used as Al, Ga, Mg, Si, and N sources under hydrogen gas, respectively. The sapphire substrates were thermally cleaned in the H2 ambient, and then, a 3-μm-thick AlN was grown using a two-step growth technique [27]. Threading dislocation densities of screw and edge dislocations including mixed components in the AlN underlayer were estimated using an X-ray rocking curve at 9 × 107 cm−2 and 1 × 109 cm−2, respectively [28]. The 1.3-μm-thick n-type Al0.62Ga0.38N doped with a Si concentration of 3 × 1019 cm−3 was grown on an AlN template [21,22]. Multiple-quantum wells, an Al0.85Ga0.15N electron blocking layer (EBL), a p-type AlGaN, and a p+-type AlGaN were grown on the n-type AlGaN underlayer. The p+-type AlGaN was doped with Mg at a concentration of 1.7 × 1020 cm−3. Subsequently, n+-type and n-type AlGaN were grown under the same conditions as those of the n-type AlGaN underlayer, as indicated in Figure 1b. The mesa was formed by dry etching using HCl gas. Thereafter, we formed 20/150/50/100/240-nm-thick V/Al/Ti/Pt/Au electrodes as both n-type AlGaN electrodes, and they were simultaneously annealed under a nitrogen (N2) ambient at 720 °C for 30 s. Further, the annealing process contributes Mg activation under lateral hydrogen diffusion from the exposed mesa-parts of the p-type layers [29,30,31,32,33]. For comparison, we prepared a conventional pn-diode-based LED with a thin p-type GaN contact layer grown on a p-type AlGaN shown in Figure 1a. We adopted indium zinc oxide (IZO) for the anode. The emitted UV light was fully absorbed at the IZO electrode; the sizes of the LED and the anode, and the thickness of the sapphire substrate were 1 mm2, 0.56 mm2, and 200 μm, respectively. The light output power was measured using an integrating sphere. For the former, we prepared an AlGaN homoepitaxial TJ LED (TJ#1 to TJ#5) with various Si concentrations and C incorporations in the n+-type AlGaN layer, as summarized in Table 1. The carbon concentration was approximately 3.0 × 1018 cm−3 (TJ#1 and TJ#2), and it was reduced to 6.5 × 1017 cm−3 (TJ#3 to TJ#5) by changing the growth pressure from 50 mbar to 100 mbar. In case of the latter, we prepared MgZnO/Al electrodes for the TJ LED with TJ#5. We deposited a 50-nm-thick MgZnO electrode by RF magnetron sputtering at a substrate temperature of 200 °C, and a typical lift-off process was employed. The sputtering target for MgZnO was prepared as a 2-inch MgZnO sintered material of purity 4N, which is the MgO:ZnO mixing atomic ratio of 1:2. The RF power, sputtering gas, and gas pressure were 100 W, Ar, and approximately 3.4–3.5 × 10−1 Pa, respectively. After forming the MgZnO electrode, the conductivity was improved by annealing at 850 °C for 5 min under N2 ambient. In the cathode, Ti/Al electrodes were deposited by the electron beam (EB) method and alloyed at 450 °C under N2 ambient. Al/Ti/Pt/Au electrodes with 300/50/100/240 nm were formed on the MgZnO electrode via the EB method to obtain a high-reflective electrode. The reflectance of the electrodes for TJ LEDs was measured using a UV-visible spectrophotometer (UV-VIS). For comparison, Ti/Al electrodes for the TJ LED anode were prepared via the same process as the cathode.

3. Results and Discussions

3.1. AlGaN Homoepitaxial Tunnel-Junction Deep-UV LEDs with n-Type AlGaN based on Suppressed Complex Defect Formation

Forward voltage–current density characteristics for samples PN#1, TJ#1, TJ#2, TJ#3, and TJ#4 are presented in Figure 2 and measured by direct current (DC) operation at 300 K. The forward voltage (6.6 V) of the conventional PN LED (PN#1) was provided at 63 A/cm2. The characteristics were roughly the same as those reported previously [35,36,37]. The forward voltage of the TJ LEDs (TJ#1 and TJ#2) was extremely high and operated at approximately 16 V at 4 A/cm2. These TJ LEDs could not perform sufficient current injection; however, a slightly decreasing forward voltage trend was observed for TJ#2 relative to TJ#1. Forward voltages of TJ#3 and TJ#4 were 12.1 V and 10.3 V at 63 A/cm2, respectively, and this was significantly reduced by more than 6 V than those of TJ#1 and TJ#2. The high-doping Si concentration of the n+-type AlGaN was effective in reducing the forward voltage of the AlGaN TJ LEDs. Further, suppressing the C incorporation was more effective than the high Si-doping concentration of the n+-type AlGaN in reducing the forward voltage. The operating voltage of AlGaN TJ LEDs could be reduced because the carrier concentration of n+-type Al0.6Ga0.4N was increased by suppressing the C incorporation. The electrical characteristics of the n-type AlGaN at 300 K under the van der–Pauw Hall effect were measured. The carrier concentration and resistivity of the n+-type Al0.6Ga0.4N with a Si concentration of 1.2 × 1020 cm−3 based on TJ#2 was extremely low (< 1.0 × 1016 cm-3) and semi-insulating because of the compensation by CN, as shown in Figure 3a and ref. [34]. Those at a Si concentration of 1.2 × 1020 cm−3 based on TJ#4 were 3.5 × 1016 cm−3 and 23 Ωcm, respectively, because of the suppression of C incorporation in the n+-type AlGaN. This improvement contributes to the reduction in the forward voltage for TJ#4 compared to TJ#2.
The difference in the forward voltages between TJ#3 and TJ#4 suggests that the Si overdose above 6 × 1019 cm−3 is effective for improving TJ despite the reduction in the carrier concentration with an increase of Si concentration, as shown in Figure 3b. The reduction of the carrier concentration can be attributed to the self-compensation of VIII–nSi complexes [17,18,19,20,21,22]. However, the depletion layer width was found to be reduced to approximately 10 nm for the Si doping concentration of 1.2 × 1020 cm−3 [38]. Therefore, the Si overdose can contribute to a reduction in the depletion layer width, which results in an increase in the tunneling probability. Another possibility is trap-assisted tunneling through defects formed by the Si overdose although a further investigation is required. Therefore, we concluded that both the C reduction and high Si doping are keys to reduce the forward voltage of AlGaN-based TJs.
The present TJ structure has very thick TJ layer compared to the depletion layer width of approximately 10 nm shown in Figure 1b, and it can be a cause of the excess series resistance of the n+-type AlGaN layer. We further reduce the operation voltage to 8.8 V at a DC current of 63 A/cm2 by optimizing the TJ thickness [39].

3.2. Sputtered Polycrystalline MgZnO/Al Reflective Electrodes for Enhanced Light Emission in AlGaN-based Homoepitaxial Tunnel Junction DUV-LED

We evaluate MgZnO/Al reflective electrodes for an Al0.6Ga0.4N TJ LED (TJ#5) to enhance the LEE. The TJ LED of TJ#5 is performed under the optimized growth condition similar to that of TJ#4. These forward voltages are slightly increased by approximately 0.6 V when the Al composition of the p-type AlGaN increased from 50% (TJ#4) to 60% (TJ#5).
The current density–forward voltage characteristics of the AlGaN TJ LEDs using conventional Ti/Al and MgZnO electrodes are illustrated in Figure 4a. The forward voltages of the AlGaN TJ LEDs using Ti/Al and MgZnO/Al electrodes were 10.8 V and 10.3 V at a DC operation of 63 A/cm2, respectively. The forward voltage offset of approximately 1 V was observed for the AlGaN TJ LED using MgZnO/Al electrodes compared with that using the Ti/Al electrodes at a current density of 30–60 A/cm2. In addition, the forward voltages of the TJ LEDs of both Ti/Al and MgZnO/Al electrodes are comparable at a current density above 30 A/cm2. Therefore, we realized carrier injection into the TJ LED using MgZnO/Al electrodes. For more details, the contact resistivity and band alignment of the interface between the MgZnO electrode and n-type AlGaN contact layer are reported in ref. [40].
Figure 4b shows the current density–emission power characteristics and emission wavelength spectra of the AlGaN TJ LEDs. The emission wavelength is 284 nm at a DC operation of 63 A/cm2. The output powers of the AlGaN TJ LEDs with conventional Ti/Al electrodes and MgZnO/Al electrodes are 32.8 and 57.3 mW, respectively, at a DC operation of 63 A/cm2. The output power of the TJ LED using MgZnO/Al electrodes is enhanced to approximately 1.7 times using the Ti/Al electrodes. The external quantum efficiencies (EQEs) of the TJ LED using the Ti/Al electrodes and MgZnO/Al electrodes are 2.15% and 3.75%, respectively, at a DC operation of 63 A/cm2, as shown in Figure 4c. The highest output power is realized for AlGaN TJ LEDs. A maximum EQE of 3.78% is achieved for the AlGaN TJ LED using MgZnO/Al electrodes. The reflectance at an emission wavelength of 284 nm for the TJ LED with the Ti/Al electrodes and MgZnO/Al electrodes was 9.5% and 20.2%, respectively. The Ti/Al electrodes exhibited low reflectivity because of the alloyed metal. In addition, the MgZnO/Al electrodes exhibited high reflectivity because of the nonalloyed Al separated from the cathode annealing process. Therefore, it contributed to the high reflectance of the TJ LED with MgZnO/Al electrodes.
Figure 5a shows the absorption coefficient spectrum of only the n-type Al0.6Ga0.4N template and MgZnO (50 nm) on the n-type Al0.6Ga0.4N template. The absorption coefficients of both increased near 4.8 eV. The absorption coefficient of MgZnO on the n-type Al0.6Ga0.4N template increased near 4.0 eV. The band gap of wz-MgZnO is reported to be approximately 3.34–4.0 eV, which depends on the Mg composition [24]. The UV light absorption near 4.0 eV is attributed to wz-MgZnO. Figure 5(b) shows the thickness dependence of the transmittance of MgZnO based on the calculation from its absorption coefficient α = 1.6×105 cm−1 at the emission wavelength of 284 nm in the fabricated TJ LED. The transmittance of the MgZnO at a thickness of 50 nm is indicated as approximately 40%. We estimate a transmittance of more than 80% by reducing the MgZnO thickness to less than 10 nm for enhancing the output power of AlGaN LEDs. The TJ LED structure can be optimized by utilizing optical cavity effects toward the improvement of LEE with other enhancement approaches [41,42]. The thickness of the n-type AlGaN in contact with the AlGaN TJ should be optimized in the near-future to realize a higher output power.

4. Conclusions

We realized the improvement of the high-Al-composition AlGaN TJ deep-UV LEDs by controlling the growth of n-type AlGaN and polycrystalline MgZnO/Al electrodes. Two essential factors are considered to reduce the operating voltage of AlGaN TJ LEDs by changing the growth condition: suppression of C incorporation and doping of n+-type AlGaN at a high Si concentration. The operating voltage of AlGaN TJ LED was 10.8 V at a DC operation of 63 A/cm2. Highly reflective MgZnO/Al electrodes were fabricated as anodes for AlGaN TJ LEDs to enhance the output power of the AlGaN TJ LEDs. The output power of the 57.3 mW of TJ LED using MgZnO/Al electrodes was realized at an emission wavelength of 284 nm under the DC operation of 63 A/cm2; this was 1.7 times higher than that when using a conventional Ti/Al electrode. In the near future, the output power can be further improved by lessening the thickness of the MgZnO for AlGaN TJ LEDs.

Author Contributions

K. Nagata and T. Matsubara: writing–original draft preparation, K. Nagata, Y. Saito, M. Kushimoto, S. Tomai, S. Katsumata, Y. Honda, T. Takeuchi, and H. Amano; writing-review and editing, supervision, H. Amano. All authors have read and agreed to the published version of the manuscript.

Informed Consent Statement

Not applicable.

Acknowledgments

This work was supported by the MOE Program for the implementation of the innovative infection control and digital technologies with low CO2 emission.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Deep-UV LED structures for (a) PN junction and (b) TJ devices.
Figure 1. Deep-UV LED structures for (a) PN junction and (b) TJ devices.
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Figure 2. Forward voltage–current density characteristics measured by DC operation at room temperature for samples PN#1, TJ#1, TJ#2, TJ#3, and TJ#4. Copyright 2021 The Japan Society of Applied Physics. [34].
Figure 2. Forward voltage–current density characteristics measured by DC operation at room temperature for samples PN#1, TJ#1, TJ#2, TJ#3, and TJ#4. Copyright 2021 The Japan Society of Applied Physics. [34].
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Figure 3. Si concentration dependence of (a) resistivity, (b) carrier concentration, and (c) mobility of n-type Al0.62Ga0.38N. The red square (■) and black circle (●) represent the values of C concentrations of 1.8 × 1018 cm−3 and 6.5 × 1017 cm−3, as grown under pressures 50 mbar and 100 mbar, respectively.
Figure 3. Si concentration dependence of (a) resistivity, (b) carrier concentration, and (c) mobility of n-type Al0.62Ga0.38N. The red square (■) and black circle (●) represent the values of C concentrations of 1.8 × 1018 cm−3 and 6.5 × 1017 cm−3, as grown under pressures 50 mbar and 100 mbar, respectively.
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Figure 4. (a) Current density–forward voltage, (b) emission power–current density, and (c) EQE–current density characteristics of the fabricated TJ LEDs. Copyright 2022 The Japan Society of Applied Physics [40].
Figure 4. (a) Current density–forward voltage, (b) emission power–current density, and (c) EQE–current density characteristics of the fabricated TJ LEDs. Copyright 2022 The Japan Society of Applied Physics [40].
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Figure 5. Comparison of the absorption coefficient spectra of the n-type Al0.6Ga0.4N template and MgZnO deposited on the n-type Al0.6Ga0.4N. (b) Thickness dependence of the transmittance of MgZnO calculated from the absorption coefficient (α = 1.6 × 105 cm−1) at a photon energy of 4.3 eV. Copyright 2022 The Japan Society of Applied Physics [40].
Figure 5. Comparison of the absorption coefficient spectra of the n-type Al0.6Ga0.4N template and MgZnO deposited on the n-type Al0.6Ga0.4N. (b) Thickness dependence of the transmittance of MgZnO calculated from the absorption coefficient (α = 1.6 × 105 cm−1) at a photon energy of 4.3 eV. Copyright 2022 The Japan Society of Applied Physics [40].
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Table 1. Summary of the evaluated parameters for all LEDs. PN and TJ indicate the PN junction and TJ LEDs, respectively. [Si] and [C] are directly measured for the samples TJ#1, TJ#3, and TJ#4, whereas [Si] and [C] in samples TJ#2 and TJ#5 (labeled by*) are estimated from the dates for TJ#1, TJ#3, and TJ#4. Copyright 2021 The Japan Society of Applied Physics [34].
Table 1. Summary of the evaluated parameters for all LEDs. PN and TJ indicate the PN junction and TJ LEDs, respectively. [Si] and [C] are directly measured for the samples TJ#1, TJ#3, and TJ#4, whereas [Si] and [C] in samples TJ#2 and TJ#5 (labeled by*) are estimated from the dates for TJ#1, TJ#3, and TJ#4. Copyright 2021 The Japan Society of Applied Physics [34].
Sample p-AlGaN p+-AlGaN n+-AlGaN
Al composition [Si] (cm−3) [C] (cm−3)
PN #1 50% 50%
#2 50% 50%
TJ #1 50% 50% 6.3×1019 1.8×1018
#2 50% 50% 1.3×1020* 1.8×1018*
#3 50% 50% 6.3×1019 3.1×1017
#4 50% 50% 1.3×1020 3.1×1017
#5 60% 60% 1.3×1020* 3.1×1017*
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