Japanese Journal of Applied Physics (original) (raw)
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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.
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.
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.
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.
Efficiency droop in AlGaN crystal-based UVB LEDs in the context of electron blocking mechanism
Journal of Crystal Growth , 2022
Aluminum gallium nitride (AlGaN) based UVB LEDs are delivering an increase in efficiency under low current drive, however, the devices are confronted with high efficiency droop and high operating voltages during the measurement on bare-wafer under high current drive. Such issues in UVB LEDs are attributed to using conventional p-type multi-quantum-barrier electron-blocking layer (p-MQB EBL) and fixed Al-contents in the p-AlGaN hole-source layer (HSL). When the conventional p-MQB EBL and bulk p-AlGaN HSL was replaced by Algraded ud-AlGaN EBL and Al-graded p-AlGaN HSL, respectively, the efficiency droop has been remarkably suppressed and almost uniform external-quantum efficiency (EQE) under high injection current was confirmed. Also, the operating voltages under 150 mA drive were significantly reduced from 64 V (conventional p-MQB EBL) to 24 V (new polarized ud-AlGaN EBL) in UVB LEDs. These improvements are attributed to the smooth valance band edge without any potential barrier in both Al-graded ud-AlGaN EBL and Al-graded p-AlGaN HSL for efficient injection efficiency. At the same time the new structure was considered useful for both blocking of high energy electron overshooting toward the p-side as well as Mg-diffusion toward the multi-quantum wells (MQWs) under high current drive. These experimental results were also confirmed by simulations.
Suppressing the efficiency droop in AlGaNbased UVB LEDs
Nanotechnology , 2021
The optoelectronic properties of semiconducting aluminum gallium nitride (AlGaN)-based ultraviolet-B (UVB) light-emitting diodes (LEDs) are crucial for real-world medical applications such as cancer therapy and immunotherapy. However, the performance of AlGaN-based UVB LED devices is still poor due to the low hole injection efficiency. Therefore, we have numerically investigated the performance of AlGaN-based UVB LEDs for the suppression of efficiency droop as well as for the enhancement of hole injection in the multiquantum wells (MQWs). The influence of the undoped (ud)-AlGaN final quantum barrier (FQB), as well as the Mg-doped multiquantum barrier electron blocking layer (p-MQB EBL), on the efficiency droop has been focused on specifically. To evaluate the performance of the proposed device, we have compared its internal quantum efficiency (IQE), carrier concentration, energy band diagram, and radiative recombination rate with the conventional device structure. Furthermore, the influence of Al composition in the Al-graded p-AlGaN hole source layer (HSL) on the operating voltages of the proposed UVB LEDs was considered. The simulation results suggest that our proposed structure has a high peak efficiency and much lower efficiency droop as compared to the reference structure (conventional). Ultimately, the radiative recombination rate in the MQWs of the proposed UVB LED-N structure has increased up to ∼73%, which is attributed to the enhanced level of electron and hole concentrations by ∼64% and 13%, respectively, in the active region. Finally, a high efficiency droop of up to ∼42% in RLED has been successfully suppressed, to ∼7%, by using the optimized ud-AlGaN FQB and the p-MQB EBL, as well as introducing Al-graded p-AlGaN HSL in the proposed UVB LED-N structure.
Smart, high-power ultraviolet (UV)-B light-emitting diode (LED) light sources are demanded for both medical and agricultural applications, including vitamin D3 production in human skin (294-304 nm), immunotherapy (310 nm), cancer therapy (295-310 nm) and enriching phytochemicals in plants (310 nm). To achieve this, we have combined graded stacks of AlGaN buffer layer (BL), AlGaN multi quantum wells (MQWs) with high internal quantum efficiency (IQE), a transparent p-AlGaN contact layer, and a highly-reflective p-type electrode for the fabrication of a UV-B LED. By optimizing the growth conditions, we demonstrated an output power of 7.1 mW for a 310 nm UV-B LED under bare-wafer conditions using a highly reflective Ni/Mg p-electrode. We also demonstrated a high IQE of 47% for UV-B emission from UV-B LED at 295 nm, by using a graded n-AlGaN BL. The light-extraction efficiency (LEE) was increased by introducing both a highly-transparent p-AlGaN and a highly reflective Ni/Mg p-electrode. As a result, we achieved an EQE of 4.4% at a dc drive current of 20 mA under CW-operation at RT and a maximum output power of 13 mW for a 295 nm UV-B LED for medical applications.