Exploring the Replacement of Toxic Mercury UV Lamp using AlGaN Tunnel Junction in 254 nm-Band DUV LED

doi:10.21203/rs.3.rs-1975197/v1

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Exploring the Replacement of Toxic Mercury UV Lamp using AlGaN Tunnel Junction in 254 nm-Band DUV LED

https://doi.org/10.21203/rs.3.rs-1975197/v1

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published 30 Jun, 2023

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In the past, low-pressure mercury (Hg) -based deep-ultraviolet (DUV) lamps have been widely used for DUV applications. The exposure of pathogen to DUV radiation at 254 nm emission has been proven to be an effective way of their inactivation in unmanned environment. However, the low-pressure mercury (Hg) DUV lamps are toxic in nature and it consumes a lot of power too. Therefore, in this work, smart and clean single tunnel junction (TJ) AlGaN-based DUV light-emitting diode (LED) at 254 nm peak wavelength has been proposed. The simulated conventional DUV LED device has maximum internal quantum efficiency (IQE) of 50 % with an efficiency droop of 18 % under 200 A/cm². The performance of the conventional LED can be improved by employing a 5 nm-thin single layer AlGaN TJ. The calculated result shows that AlGaN TJ-based LED has a maximum IQE of 62 % with an efficiency droop of 15 % under higher injection current. In addition, the TJ LED emitted power has been improved by 34 % compared to the conventional LED structure. Such efficient AlGaN DUV LED at 254 nm emission wavelength will be an effective replacement of Hg-DUV lamp in the future.

AlGaN Tunnel Junction

Deep-ultraviolet LED

Internal Quantum Efficiency

Efficiency droop

Toxic Hg-UV lamp

In the last few decades, conventional mercury (Hg) and xenon lamps-based UV light sources have been widely used for the applications of food preservation, water sterilization, medical equipment sterilization, and surfaces disinfections [1]. Mercury UV lamps are highly toxic and have many health hazards [2, 3]. According to United Nations (UN), 17 sustainable development goals (17-SDGs) and the Minamata Convention of 2020 the usage of toxic mercury can be completely eliminated for clean environment, wellbeing and clean water for everyone [4,5]. In this context, green AlGaN based semiconductors are one of the most promising candidates for the fabrication of LEDs that would meet the requirements for both the Minamata Convention of 2020 and the 17 SDGs of the UN [6,7]. Moreover, humankind has always coexisted with various infectious diseases and germs since the inception of life. The spread of the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) has cast a deep shadow not only on humankind but also on the world economy and people's lifestyles [8]. Therefore, controlling all infectious diseases is a global issue for human society now and in the far future. Recent work has shown that emission at 222 nm and 254 nm can be more effective against pathogen contaminations in both manned and unmanned environment [9, 10]. Clinical study suggests that emission of UV light at 254 nm with power density of 1640 μW/cm² can completely inactivate the viral stocks [11]. AlGaN-based DUV LEDs and LDs have a high capability to replace toxic materials with tuneable wavelength, significantly reduced power consumption, small in size, and easy to install. However, the realization of AlGaN DUV emitters are still lagging behind Hg-UV Lamp because of several challenges which include extremely low internal quantum efficiency (IQE) due to higher threading dislocation density (TDD) [12], poor light extraction efficiency (LEE) due to the dominant transverse magnetic (TM)-polarized light emission instead over the transverse electric (TE)-polarized light emission [13], poor hole injection due to the issue of p-type doping in AlGaN, and parasitic radiative recombination rate attributed to the asymmetric injection of electron and hole toward the MQWs [14].

AlGaN-based DUV LEDs exhibit extremely low external quantum efficiency (EQE) and wall-plug efficiency (WPE) due to the low IQE, low hole injection and inefficient LEE [15]. To the best of our knowledge at present the maximum EQE of 20 % has been reported at emission wavelength of 275 nm and 10 % at emission wavelength of 265 nm for AlGaN-based DUV LEDs [16, 17]. For LEDs emitting < 260 nm, the EQE is in the range of 1.2 % [18]. Al composition of 70 % is needed in AlGaN to realize DUV LEDs with < 260 nm wavelength and such high Al contents causes serious issues with p-doping as well as for n-doping. Consequently, it can reduce the hole injection and can also promote the electron overflow from the active region toward p-side of the LED [19, 20]. Many efforts have been made to encounter these issues which include the use of band engineering by utilizing compositionally graded AlGaN layers, introducing new nanostructures such as nanowire, nanocolumn, quantum dot. High performances of DUV LEDs are expected by utilizing compositionally graded AlGaN hole-source layer for the generation of three-dimensional hole gas (TDHG) and implementing mirror like reflective p-electrodes [21-25].

TJ-based UV LED devices gain much attention because they alleviate some critical issues related to the asymmetric movement of electrons and holes and their injection efficiencies toward MQW. In a TJ-based structure conductive, transparent n-doped layer can be introduced which enhances the light extraction efficiency, current spreading layer, and carrier’s injection. In recent studies, AlGaN/GaN/AlGaN TJ has been used to achieve the high EQE and WPE of AlGaN DUV LEDs [26-29]. Such structure provides a promising path for higher carrier transport, lower resistance, and better performances. In this work, we extend the idea of AlGaN/GaN/AlGaN TJ LED by introducing a single n-AlGaN layer between two p-doped AlGaN layers to avoid the GaN absorption and offer a reduced resistance for higher hole injection toward active region at 254 nm emission. By varying the thickness of the AlGaN TJ-layer the carrier injection efficiency was improved theoretically. As a result, the IQE is enhanced and parasitic recombination rate is also reduced, which leads to high EQE and WPE.

Proposed an Efficient n-AlGaN-based single TJ in 254nm-Band DUV LED

The conventional LED structure used in these simulations is shown in Figure. 1a. The conventional LED is considered on the c-sapphire substrate, which is followed by the AlN buffer layer. Si-doping of 5.0×10¹⁸cm^-3 level was set in the 3 μm-thick n-Al_0.80Ga_0.20N electron source layer (ESL). The active region comprises 5 pairs of 3 nm-thick Al_0.68Ga_0.32N quantum wells (QWs) and 6 nm Al_0.78Ga_0.22N quantum barriers (QBs). The Al composition of the active region was adjusted according to the target emission wavelength of 254 nm. Al-rich Mg-doped 20 nm-thick p-Al_0.90Ga_0.10N acts as an electron blocking layer (EBL) which is followed by Mg-doped 150 nm-thick p-Al_0.8Ga_0.2N HSL. The Mg-doping concentration of EBL and HSL are set to 3.0×10¹⁸cm^-3 and 1.2×10¹⁸cm^-3, respectively. The conventional LED structure is modified by incorporating a single Si-doped n-Al_0.80Ga_0.20N TJ layer between EBL and HSL as shown in Figure. 1b. The Si-doping concentration of the TJ was fixed to 1.2×10¹⁷cm^-3 while the thicknesses of the TJ are allowed to vary from 5 nm to 50 nm.

The numerical studies were performed by the Advance Physical Model of Semiconductor devices (APSYS) tool which solved self-consistently drift-diffusion model, Poisson equation, and photon rate equations [30]. The energy band alignments of the LEDs are calculated by using the 6×6 k.p model [31]. The bandgap of GaN and AlN are set to be 3.4 eV and 6.2 eV respectively [32] and transparent AlGaN layer was chosen in between the AlN and GaN [26]. The bowing parameter and band offset ratio of AlGaN MQW are assumed to be 0.94 eV and 0.3/0.7 [33]. Other simulation parameters including radiative recombination coefficient, Shockley-Read-Hall (SRH) recombination lifetime and Augur recombination coefficient are set to be 0.5×10^-16m³/s, 100 ns, and 1.0×10^-46m³/s, respectively [34, 35]. The Mg activation energy in p-AlGaN is scaled linearly from 170 meV (GaN) to 670 meV ( AlN) [36]. The back loss is assumed to be 2400 m^-1 for 300 μm × 300 μm size LED chip. As the n-AlGaN TJ layer plays a vital role in carrier (electrons, holes) tunnelling phenomena so this model is considered for both the tunnelling of carrier and also for highly conductive ohmic contact [37]. Build-in polarization charge density due to piezoelectric polarization and spontaneous are assumed 50 % of the theoretical value which can be calculated by using the method proposed by Fiorentini et al [38]. All simulation is carried out at room temperature (RT).

The comparative studies of a conventional AlGaN LED structure and single Si-doped n-Al_0.80Ga_0.20N TJ based LED structure at emission wavelength of 254 nm were investigated by using simulation models, shown in Figure. 1a-b. The TJ layer`s thickness is optimized by calculating the performance of TJ-based LED for various thicknesses. The optical emitted power is calculated for both conventional and proposed TJ LEDs at 200 A/cm², shown in Figure. 2a. The conventional LED structure has an emitted power of 39.7 W/cm² which is improved by 34% to 53 W/cm² by using a 5 nm-thin single TJ layer. The IQE of TJ-based LED is improved up to 62% by using 5 nm-thin TJ, when compared to the conventional LED, shown in Figure. 2b. The use of 5nm-thin n-Al_0.8Ga_0.2N TJ offers higher IQE, low efficiency droop and high emission power in DUV LED, when compared to the conventional DUV LED or DUV LED with thicker TJ, shown in Figure. 2a-b.

To understand the underlying physics behind the improved performance of TJ-based LED we calculate the band alignment (conduction band, valance band) for conventional LED and 5nm-thin TJ-based LED, shown in Figure. 3a-b [39]. From Figure. 3a we can see that the reference device has conduction band barrier height Φ_e of 862 meV and valance band barrier height Φ_h of 360 meV, while the hole gain energy of 169 meV by passing from HSL to last-QB (LQB). In the case of TJ-based LED the Φ_e is 889 meV, Φ_h is 361 meV and hole gain energy ΔΦ is 193 meV. These calculated energy band values conclude that the TJ layer does not much affect the values of Φ_e and Φ_h but increases the hole injection toward the MQW, as a result the hole concentration inside the active region of TJ-based LED was highly enhanced. The increment in hole energy is not much obvious for the reported improvement but there is a quantum tunnelling probability of hole which is the main contributor of hole improvement in the active region. Hence it shows that TJ have not higher impact on band alignment but the improved performance is mainly contributed by the quantum tunnelling phenomena using 5nm-thin TJ structure. It can be explained by employing the physical relation given in Eq. (1), when the width of TJ reduces then the tunnelling probability may increase, as shown in Figure. 3a-b. In the case of 5 nm-thin TJ the tunnelling width reduces from 71 nm to 24 nm. The narrow is the width of the TJ the higher is the tunnelling probability; which can be represented in the following Eq. (1) [40]:

$${P}_{b}\cong exp\left({\int }_{0}^{{w}_{d}}\sqrt{\frac{2{m}_{t}^{*}{E}_{g}x}{{h}^{2}{w}_{d}}}dx\right)$$

Where ${P}_{b}$ is the tunnelling probability, ${w}_{d}$ represent the width of tunnelling layer, ${E}_{g}$ is the energy bandgap and ${m}_{t}^{*}$ is the carrier effective mass. The 5 nm-thin TJ offer higher ${P}_{b}$ for hole flow, which increases the hole concentration in the active region. As the hole concentration increases within MQW, the parasitic recombination rate will decrease due to the depletion of holes outside the MQW [41]. Conversely, the reduced parasitic recombination rate degrades the electron leakage from MQW as of the reduced availability of holes. In both cases, the hole injection efficiency effect electron and hole concentration within the MQW. The band energy is calculated for various thicknesses of the TJ layer and listed in Table. I. Herein, it can be summarized that as the thickness of the AlGaN TJ layer decreases, the hole injection toward the active region increases by obtaining more energy and higher tunnelling probability. Consequently, it can be deduced that high emitted power and IQE are attributed to the 5nm-thin single AlGaN -based TJ in DUV LED.

Table I: Conduction band barrier height (Φ_e,) valance band barrier height (Φ_h) and hole gain energy (ΔΦ) for conventional LED, TJ-based LED with 5, 20 and 50 nm thickness.

Energy	Reference LED	50 nm TJ layer LED	20 nm TJ layer LED	5 nm TJ layer LED
Φ_e (meV)	862	867	872	889
Φ_h (meV)	360	361	361	361
ΔΦ (meV)	169	173	180	193

To further clarify the effectiveness of single AlGaN-TJ based DUV LED, we examine the electron and hole concentration in the active region and compared with the conventional LED, shown in Figure. 4a-b. The electron concentration is slightly decreased in the last QW of the TJ-based LED because of n-doped AlGaN which helps to further block the electron toward the EBL and reduce the electron overflow from the active region as can be seen in the hetero-interface of LQB and EBL in Figure. 4a. The hole concentration scenario is different for the studied LED structures because of variation in hole energy and higher tunnelling phenomena as speculated in Figure. 4b. The hole concentration is almost double in each QW of TJ-based LED, when compared to the conventional LED device, which reveals the superiority of the TJ LED structure. This can further be elaborated in the inset plot of Figure. 4b where we compare the hole concentration of both LEDs in the region of EBL, HSL, and TJ layer, respectively. It can be seen in the inset of Figure. 4b that the hole accumulation is increased at the interface of EBL/HSL because of higher tunnelling probability of hole by TJ layer. This will improve the overall tunnelling probability of holes toward the MQW and hence a higher hole concentration will be available in the active region for better radiative recombination phenomena [42]. However, in the case of conventional LED, the hole accumulation at the interface of EBL and p-AlGaN HSL is lower and it generates the hole depletion region, which adversely affects the hole concentration in the active region. Consequently, a low hole concentration will be available in the active region for relatively low radiative recombination phenomena and the performances of the conventional LED is deteriorated.

The higher hole concentration will strongly affect the radiative recombination rate in the MQWs of TJ-based LED, which can be seen in Figure. 5a. Although the studied LEDs structure has the same concentration of electrons in the MQW except slight variation in the last QW, however due to the effective hole tunnelling phenomena, the TJ LED has a significantly higher radiative recombination rate as compared to conventional LED, shown in Fig. 5(a). In addition, the spontaneous recombination rate for TJ-based LED is higher as compared to reference LED, shown in Figure. 5b, which clearly shows the effectiveness of the 5nm-thin Si-doped n-AlGaN TJ layer. Moreover, it can be depicted from the spontaneous recombination rate that the AlGaN TJ layer only improved the recombination rate by providing a higher hole concentration without affecting the main emission peak of LED. Based on the simulation results, it can be concluded that thin TJ layer improves the hole injection efficiency toward the MQW which leads to a higher radiative recombination rate and spontaneous rate, when compared to the conventional LED. On the other hand, if we use a thick n-AlGaN TJ layer the hole injection toward the MQW is not much obvious as one can find via thin TJ layer and as a result the radiative recombination rate and spontaneous rate are deteriorated. This is quite promising single TJ-based DUV LED structure for the epi-growers of UV emitters using LP-MOVPE and MBE [43].

Research and development of efficient AlGaN-based DUV LED at 254 nm emission wavelength is the safe and effective replacement of toxic Hg DUV lamp. Carrier confinement and transport issues in the MQWs are quite critical [44, 45]. To resolve these issues of carrier confinement and transport, we provided a short roadmap for experimental efforts to realize internal quantum efficiency (IQE) beyond 70% in the AlGaN-TJ base DUV LEDs. In this regard, the proposed n-AlGaN single TJ-based LED will open a better way for the realization of highly efficient AlGaN-based DUV LEDs for fast disinfection of surfaces, air, food and water. It is believed that this finding will help in the improvement of EQE and WPE of the experimental AlGaN-based DUV LEDs.

In summary, transparent n-AlGaN tunnel-junction-based DUV LED at emission wavelength of 254 nm has been proposed to replace the toxic mercury (Hg) DUV lamp for the applications of water sterilization, surface disinfection, and air purification. The performance of the conventional AlGaN DUV LED structure is improved by incorporating an n-Al_0.8Ga_0.2N TJ layer between the p-AlGaN HSL and EBL. By varying the thickness of TJ layer in the DUV LED, it was found that 5 nm-thin TJ has strong influence on hole injection efficiency and it can reduce the parasitic recombination rate which leads to a higher radiative recombination rate in the active region of LED. The simulated conventional DUV LED device has maximum internal quantum efficiency (IQE) of 50% with an efficiency droop of 18% under 200 A/cm². On the other hand, the performance of the conventional LED was improved by employing a 5 nm-thin single layer n-AlGaN TJ in the DUV LED. Consequently, AlGaN TJ-based LED delivered a maximum IQE of 62% with an efficiency droop of 15% under higher injection current. It is believed that this finding will help in the improvement of EQE and WPE of the experimental AlGaN-based DUV LEDs at 254nm emission wavelength.

Ethical Approval: Not Applicable

Competing Interests: The authors have no competing interest to disclose.

Author' Contributions: All authors contributed equally to this work.

Funding: This research was supported by grants from Ningbo 2025 Key Innovation Project (2019B10129), Zhengzhou 1125 Innovation Project (ZZ2018-45), National Nature Science Foundation of China (62174148), and National Key Research and Development Program (NKRDP 2016YFE0118400).

Availability of Data: Data available on request from authors.

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No competing interests reported.

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Exploring the Replacement of Toxic Mercury UV Lamp using AlGaN Tunnel Junction in 254 nm-Band DUV LED

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