Highly Transparent p-AlGaN-Based (326-341 nm)-Band Ultraviolet-A Light-Emitting Diodes on AlN Templates: Recent Advances and Perspectives (original) (raw)
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ACS Appl. Electron. Mater. , 2020
As per the Minamata Convention on Mercury, regulation on mercury use will be stricter from the year of 2020, and safe AlGaN-based ultraviolet (UV) light sources are urgently needed for killing SARS-CoV-2 (corona virus). AlGaN-based ultraviolet-B (UVB) light-emitting diodes (LEDs) and UVB laser diodes (LDs) have the potential to replace toxic mercury UV lamps. Previously, the internal-quantum-efficiency (η int) was enhanced from 47 to 54% in AlGaN UVB multiquantum wells (MQWs). However, some nonlinear behaviors in both light output power (L) and external quantum efficiency (η ext) in the 310 nm band UVB LEDs were observed, and later on, such nonlinearities were overcome by reducing the thicknesses of quantum well barriers (T QWB) in MQWs. After relaxing the n-AlGaN electron injection layer up to 50% underneath the MQWs and using a highly reflective Ni/Al p-electrode, L and η ext of the 310 nm band UVB LED were greatly improved from 12 mW and 2.3% to record values of 29 mW and 4.7%, respectively. Similarly, for the 294 nm band UVB LED, η ext and L values, respectively, were also remarkably improved up to 6.5% and 32 mW at room temperature under bare wafer conditions using a better carrier confinement scheme in the MQWs as well as using a moderately Mg-doped p-type multiquantum-barrier electron-blocking layer (p-MQB EBL). The moderately doped p-MQB EBL was used to achieve better hole transport to enhance the hole injection toward the MQWs as well as to block the high-energy electron from overshooting. Possible explanations and recommendations for the improvements in the performances of 294−310 nm UVB LEDs are broadly discussed. Most importantly, such controllable multi-UVB-wavelength emitters may extend nitride-based LEDs to previously inaccessible areas, for example, electrically pumped AlGaN-based UVB LDs.
AlGaN-based ultraviolet-A (UVA) light-emitting-diodes (LEDs) at emission under 330 nm are of great importance for numerous applications, including medicine and photo-chemical technologies. In this Letter, a highly relaxed n-AlGaN electron injection layer (EIL) underneath the multi-quantum wells (MQWs) for the suppression of both threading dislocation densities and piezoelectric effect was attempted. When the Ga-rich n-AlGaN EIL in the UVA LED was relaxed up to 75%, the full width at half-maximum values of the X-ray rocking curves for the (10-12) planes were reduced from our previous value of approximately 793 to 564 arcsec. Subsequently, a maximum light power of 3.1 mW was achieved in the 326 nm band UVA LED. However, carrier confinement and transport issues in the MQWs were observed. To resolve these issues of carrier confinement and transport, we provide a short roadmap for experimental efforts to realize an internal quantum efficiency (IQE) beyond 53% in AlGaN UVA-MQWs. A key feature of wide bandgap Al x Ga 1−x N materials is the direct bandgap (3.4 ∼ 6.2 eV) which can be tuned for different narrow-band UV emission and can have a large saturation drift velocity [1,2]. Due to the improved performances, lower cost, eco-friendliness, and smartness in size, the AlGaN-based UVA LEDs offer significant advantages over the conventional UV technologies, including toxic mercury UV lamps (Minamata Convention of 2020) [2,3]. AlGaN-based lateral-type (Lat-UVA LED) devices in the wavelength range of 320-340 nm have many applications in the area of sensing, data storage devices, medical, photochemical, UV curing, counterfeit detection, forensics, materials processing, biomedical, and DNA analysis [2]. However, the performance of the ultraviolet-B (UVB) and UVA LED device remains low and quite challenging when compared to the deep ultraviolet (DUV) LED [2,4-8]. The most challenging issue is the existence of a high level of threading dis-location densities (TDDs) in the Al x Ga 1−x N electron injection layer (EIL) underneath the multi-quantum wells (MQWs), which are caused by the lattice mismatch of 2.8% between the AlN template and AlGaN buffer layer (BL) [2,8-11]. A second challenge is the existence of strong spontaneous and piezoelec-tric polarization charges induced at the interfaces of the active layers when III-nitride materials are grown on c-sapphire [2,12]. So far, very little work has been done on these two issues to improve the performance of 326 nm band AlGaN UVA LEDs grown on AlN templates on sapphire substrate [2,13]. In our previous work on AlGaN-based UVA LED (sample-I: grown at 1130 • C), the full width at half-maximum (FWHM) values of the XRCs along the (10-12) plane were found to be as high as 793 arcsec (high level of edge-type TDDs) in the slightly relaxed n-AlGaN EIL (relaxation ratio ∼ 30%) [13]. We choose two strategies to address these issues: introducing a reasonable number of AlGaN-based superlattices (SLs) as a buffer layer between the AlN template and n-AlGaN EIL [13] and growing a thick and highly relaxed n-AlGaN EIL (Ga-rich condition) on the over-layer of the n-AlGaN BL with respect to the fully relaxed AlN template at a relatively high growth temperature (without SLs: this Letter). However, AlGaN UVA LEDs grown on a AlN template on sapphire substrate without using AlGaN SLs as a BL underneath the MQWs remain relatively unexplored [2,13]. In this Letter, we employ an approach different from the SL-based BL. Here we engineer a Ga-rich (III/V: from 1652.0 to 1594.0) 2 µm thick and a highly relaxed n-AlGaN BL, including n-AlGaN EIL to enhance the relaxation condition (reduction of TDDs) and reduce the piezoelectric field in the UVA-MQWs. However, we are confronted with carrier confinement issues in the MQWs due to the low Al-alloy difference between the quantum well barrier (QWB) and quantum well (QW). Similarly, we are also confronted with the carrier transport issues due to the unoptimized thicknesses of the QWB and QW in the MQWs. To address these issues, a revised structure of AlGaN BL-based UVA-MQWs at 326 nm emission with 0146-9592/20/020495-04 Journal
Progress and Outlook of 10% Efficient AlGaN-Based (290-310 nm) Band UVB LEDs
physica status solidi (a) – applications and materials science (pss a), 2023
Eco-friendly and low-cost aluminum gallium nitride (AlGaN) for the epitaxial growth of ultraviolet-B (UVB) light-emitting diodes (LEDs) on c-Sapphire has the possibility of high external-quantum efficiency (EQE). In this review paper a special growth techniques for 50% relaxed and 4 μm thick AlGaN buffer layer underneath the multi-quantum wells (MQWs) are challenged to achieve a maximum internal-quantum efficiency of 50–57% in 310–290 nm band UVB LEDs. The influence of a thin “Valley” layer in p-type multi-quantum barrier electron-blocking layer on 2D hole generation and injection via intraband tunneling was attempted. Finally, the influence of soft polarized Mg-doped p-type Al-graded AlGaN hole injection layer assisted by excimer laser annealing for better hole injection toward the MQWs was investigated and quite high hole concentration of 2 × 1016 cm−3 and resistivity of 24 Ω-cm at room temperature was achieved. Consequently, the EQE of transparent 310 and 304 nm UVB LEDs, respectively, reached to a confirmed world record values of ≈5% and ≈10% with light powers of 29 and 40 mW on wafer. This EQE value can surpass 21% if flip-chip, nanoPSS, photonic crystal, and lens with highly reflective p-electrodes are incorporated in LED.
Influence of Undoped-AlGaN Final Barrier of MQWs on the Performance of Lateral-Type UVB LEDs
Aluminium gallium nitride (AlGaN)-based ultraviolet-B light-emitting diodes (UVB LEDs) are expected to offer smart size, wider choice of UVB light emission in the wavelengths range of 280 nm > λ > 320 nm, and low cost as well as low power consumption compared with other UV light sources including toxic mercury UV-lamps. The hole-tunneling from p-AlGaN side of UVB LED into the multi-quantum-wells (MQWs) is strongly dependent on the thickness (T FB) and Al-contents of undoped (ud)-AlGaN final barrier (FB). Herein, the photolumi-nescence (PL) efficiency from MQWs of the UVB LED devices is investigated and compared with the electroluminescence (EL) spectra as a function of T FB. Subsequently, the dependence of PL efficiency, external quantum efficiency (EQE), and light output power on the T FB of UVB LEDs is attempted, using the same growth condition for all samples except variation in T FB. When T FB is set to 6-7 nm, improvements in the EQE and light output power, respectively, from 4.3% and 7 mW to the high values of 5.6% and 17 mW in emission band of 295-300 nm under continuous-wave (cw) at room temperature (RT) are achieved.
Improved external quantum efficiency of 293 nm AlGaN UVB LED grown on an AlN template
Smart, low cost and environmentally safe AlGaN-based UVB LEDs are promising in many real world applications including medical as well as agricultural sciences. The main purpose of this work is to develop a crystal growth technique for an n-AlGaN buffer layer (BL) including an n-AlGaN current spreading layer (CSL) for obtaining a high internal quantum efficiency (IQE) from UVB-emitting multi quantum wells (MQWs). By the reduction of the edge type threading dislocation densities in the n-AlGaN CSL, as well as the optimization of the quantum well (QW) thickness, the IQE of about 42% was improved for UVB MQWs, with an emission wavelength of 294 nm. Subsequently, the external quantum efficiency improved from 2.7% to 3.3% at 20 mA under the continuous wave (CW) operation and the maximum output power also improved from 10.8 mW to 12.5 mW at 126 mA, respectively. 293 nm UVB LED light sources are very useful for the application of vitamin D3 production in the human body.
Far-ultraviolet-C (Far-UVC) light-emitting-diodes (LEDs) offer a promising technology for the disinfection of surface, air, water, food and airborne disease transmission in occupied spaces, including COVID-19 (SARS-CoV-2) and other viral diseases, when it is meticulously designed, engineered, and applied. Research should continue on both the safety and efficacy of AlGaN-based Far-UVC LEDs, as well as the material choices and device designs to develop highly efficient solid-state UV germicidal irradiation (UVGI) at 222 nm emission to replace toxic low-pressure mercury lamps emitting at 253.7 nm. However, the key issue of hole concentration inside the multi-quantum-wells (MQWs) of AlGaN-based Far-UVC LEDs with high Al-contents is quite critical. Therefore, theoretical studies of AlGaN-based Far-UVC LEDs may suggest sufficient evidence for immediate consideration and implementation for the epitaxial growth of 222 nm-band Far-UVC LED technology during this worldwide health crisis. In this paper, the initial design of the Al-graded p-AlGaN hole source layer (HSL) on the performances of Far-UVC LED was compared with conventional bulk p-AlGaN HSL (non-graded)based LED devices. For the evaluation of the device's performances, the energy band diagram, internal quantum efficiency (IQE), electrons and holes concentration, radiative recombination rate, and current density vs voltage characteristic were compared. It was found that LEDs at 222 nm emission without using the undoped (ud)-AlGaN final-quantum-barrier (FQB) and only keeping the Al-graded Mg-doped p-AlGaN HSL showed high carrier injection into the MQWs. The variation in the energy band diagram around the p-AlGaN electron-blocking layer (EBL)/p-AlGaN HSL region and p-AlGaN HSL/p-GaN contact-layer (CL) indicates that the introduction of the Al-graded p-AlGaN HSL, as well as the special choice of Al composition at the interfaces, are quite promising for the enhancement of hole injection toward MQWs. The simulation results suggest that the proposed structure of the Al-graded p-AlGaN HSL after omitting the ud-AlGaN FQB structure in the Far-UVC LED is quite useful for achieving high peak efficiency, as well as for suppressing the efficiency droop when compared to the conventional bulk Far-UVC LED. After introducing a new design of 40 nm-thick p-AlGaN HSL in the Far-UVC LED, the radiative recombination rate in the first two quantum-wells of MQWs has been improved up to B50%. The enhanced radiative recombination rate is attributed to the enhanced level of electron and hole concentrations by B26% and 53%, respectively, in the MQWs. Ultimately, after removing the ud-AlGaN FQB and using 40 nm-thick Al-graded (Al: 100% to 20%) p-AlGaN HSL, the efficiency droop has been remarkably reduced from B39% (Bulk-LED) to B19% in the new design of Far-UVC LED structure.
AlGaN-based ultraviolet-B (UVB) LEDs at 310 nm emissions are expected to offer safe and smart size UVB-light sources compared to the toxic mercury UV-lamp. Previously, the issue of nonlinearity in the emitted light output power (L) as well as in the external quantum efficiency (EQE) of 310 nm band UVB LEDs were observed. First, the influence of both the number of n-AlGaN buffer layers (BLs) and the type of p-electrodes on the recovery of linear behavior in the L and EQE were investigated. It was found that the nonlinearity in the L and EQE of UVB LED is independent of the number of BLs as well as type of p-electrodes. Therefore, finally the dependence of nonlinearity in the L and EQE on the thickness of quantum-well-barrier (T QWB) of multi-quantum-wells (MQWs) were also considered. Subsequently, the issue of nonlinear behavior in the L and EQE was resolved by the thickness reduction of T QWB from 25 to 10 nm in the MQWs. Similarly, a reasonable value of improvement in both L and EQE, respectively, up to 12 mW and 2.2% of 310nm band UVB LED were realized.
Scientific Reports, 2022
Crystal growth of eco-friendly, ultrawide bandgap aluminium gallium nitride (AlGaN) semiconductor-based ultraviolet-B (UVB) light-emitting diodes (LEDs) hold the potential to replace toxic mercury-based ultraviolet lamps. One of the major drawbacks in the utilisation of AlGaN-based UVB LEDs is their low efficiency of about 6.5%. The study investigates the influence of Al-graded p-type multi-quantum-barrier electron-blocking-layer (Al-grad p-MQB EBL) and Al-graded p-AlGaN hole source layer (HSL) on the generation and injection of 3D holes in the active region. Using the new UVB LED design, a significant improvement in the experimental efficiency and light output power of about 8.2% and 36 mW is noticed. This is accomplished by the transparent nature of Al-graded Mg-doped p-AlGaN HSL for 3D holes generation and p-MQB EBL structure for holes transport toward multi-quantum-wells via intra-band tunnelling. Based on both the numerical and experimental studies, the influence of sub-nanomet...