AlGaN Multiple Quantum Wells Research Articles

Novel 2D/3D Heterojunction for UV Light‐Emitting Diodes Using Hexagonal Boron Nitride as Hole Injection Layer

The AlGaN materials system has been extensively studied in order to improve the efficiency of UV‐B and UV‐C light‐emitting diodes (LEDs). While progress has been made, significant challenges remain at shorter wavelengths. Most notably, increased Al composition for shorter‐wavelength operation results in increased activation energy of Mg dopants, resulting in low p‐doping. Although p‐doped h‐BN, with a bandgap of 5.9 eV, has been proposed as a potential replacement of p‐doped AlGaN, there have not been demonstrations of LEDs fabricated from p‐doped h‐BN/AlGaN heterostructures. Such unique heterostructures combine 2D p‐doped h‐BN materials with 3D AlGaN materials. Herein, fabrication and characterization of p‐doped h‐BN/AlGaN multiple quantum wells (MQWs)/n‐AlGaN LEDs, demonstrating emission of light at 290 nm corresponding to the AlGaN MQWs, with weaker emission at 262 nm corresponding to the AlGaN barrier, are reported. These results conclusively show hole injection through p‐doped h‐BN into AlGaN and provide a proof of concept that p‐doped h‐BN can be an alternative hole injection layer for UV LEDs.

Unraveling carrier distribution in far-UVC LEDs by temperature-dependent electroluminescence measurements

The hole transport and the carrier distribution in AlGaN-based far-ultraviolett (UVC) light emitting diodes (LEDs) emitting around 233 nm was investigated. Temperature-dependent electroluminescence measurements on dual wavelength AlGaN multiple quantum well (MQW) LEDs show a strong shift in the spectral power distribution from 250 to 233 nm with decreasing temperature. Comparing experimental data with simulation shows that the hole mobility and the electron to hole mobility ratios have a significant influence on the carrier injection efficiency (CIE) and that the change in the spectral power distribution is originating from a change in the hole distribution in the MQWs. Poor hole injection and charge carrier confinement in the AlGaN MQW active region was identified as one of the main reasons for the low CIE in far-UVC LEDs.

Well Number Dependence of Internal Quantum Efficiency in AlGaN Quantum Wells on Low‐Dislocation Sputtered AlN Templates

The internal quantum efficiency (IQE) and cathodoluminescence intensity line profile of AlGaN multiple quantum well (MQW) structures on low‐dislocation face‐to‐face annealed sputtered AlN (FFA Sp‐AlN) and on conventional metalorganic vapor‐phase epitaxy‐grown AlN (MOVPE‐AlN) templates are evaluated and the effect of the number of quantum wells (QWs) on the IQE is discussed. The higher IQE in the FFA samples is probably due to the lower threading dislocation (TD) density; however, the IQE also increases with the number of QWs although the TD density remains constant. Effective diffusion length increases with the number of QWs, indicating that the AlGaN MQW layer helps to suppress point defect diffusion, resulting in IQE increase. Furthermore, the segregation of point defects into the TDs and point defect diffusion via the TDs may explain the difference in the IQE improvement rate between the MQWs on the FFA Sp‐AlN and MOVPE‐AlN templates.

Investigation of optical polarization characteristics of ultraviolet-C AlGaN multiple quantum wells by angle-resolved cathodoluminescence

AlGaN-based ultraviolet-C (UV-C) light-emitting diodes (LEDs) face challenges related to their extremely low external quantum efficiency, which is predominantly attributed to the remarkably inadequate transverse magnetic (TM) light extraction efficiency (LEE). In this study, we employ angle-resolved cathodoluminescence (ARCL) spectroscopy to assess the optical polarization of (0001)-oriented AlGaN multiple quantum well (MQW) structures in UV-C LEDs, in conjunction with a focused ion beam and scanning electron microscopy (FIB/SEM) system to etch samples with various inclination angles (θ) of sidewall. This technique effectively distinguishes the spatial distribution of TM- and transverse electric (TE)-polarized photons contributing to the luminescence of the MQW structure. CL spectroscopy confirms that UV-C LEDs with a θ of 35° exhibit the highest CL signal compared to samples with other θ. Furthermore, we establish a model using finite difference time domain (FDTD) simulation to validate the mechanism of the outcomes. The complementary contribution of TM and TE photons at different specific angles are distinguished by ARCL and confirmed by simulation. At angles near the sidewall, the CL is dominated by the TM photons, which mainly contribute to the increased LEE and the decreased degree of polarization (DOP) to make the spatial distribution of CL more uniform. Additionally, this method allows us to analyze the polarization of light without the need for polarizers, enabling the differentiation of TE and TM modes. This distinction provides flexibility for selecting different emission mode based on various application requirements. The presented approach not only opens up new opportunities for enhanced UV-C light extraction but also provides valuable insights for future endeavors in device fabrication and epitaxial film growth.

Deep ultraviolet AlGaN-multiple quantum wells with photoluminescence enhanced by topological corner state

The AlGaN-based deep ultraviolet light-emitting diode (DUV LED) has advantages of environmentally friendly materials, tunable emission wavelength, and easy miniaturization. However, an increase in Al composition leads to a decline in the lattice quality, thereby reducing the internal quantum efficiency (IQE). In addition, the light extraction efficiency (LEE) is limited due to the strong transverse magnetization polarization emission from the multiple quantum wells. Here, we designed the topological corner structure in AlGaN-MQWs, and the high electric field intensity in a tiny space at the corner results in an extremely high local density of optical states (LDOS), which could shorten the luminescence decay time of the emitter and increase the radiative rate by 26 times. Meanwhile, because the excited topological corner state resonance mode is a transverse-electric mode, enhancing only the transverse-electric luminescence without any gain for transverse-magnetic luminescence, thereby significantly improving the light extraction efficiency. Finally, according to theoretical calculations, the IQE could reach 68.75% at room temperature.

Deep ultraviolet random laser disinfection

Recently, the ineffectiveness of conventional antibiotic treatments against multidrug-resistant bacteria stimulated the development of novel effective disinfection technologies. Ultraviolet lasers, with several features, including high coherence, high directionality, monochromatic, and high intensity, were the ideal candidate light source for disinfection; however, conventional lasers are restricted by their inherent detriments such as high directionality, meticulous manufacturing, and the laser speckles. Random lasers have been considered powerful candidates for illumination sources due to their angle-free emission in recent years. However, their practical application remains scarce, and the result of using a deep ultraviolet random laser (DUV-RL) for disinfection has not ever been reported. Herein, the DUV-RL is realized by applying AlGaN multiple quantum wells (MQWs) and aluminum nanoparticles as the gain medium and plasmonic scattering centers. The proof of concept for the first DUV-RL disinfection in our study is examined by implementing this unique light source on model gram-negative bacteria (Escherichia coli). Additionally, the inherent angle-free emission characteristic of random lasers was confirmed and applied in treatment to achieve angle-free disinfection, which enables random lasers to overcome the major drawback of the high directionality of conventional lasers. Above all, the success of applying random laser systems in the disinfection arena fulfills the empty piece of current laser treatment. This research is expected to promote the development of DUV-RLs and enable them to compete with other light sources, such as mercury lamps and Ultraviolet-C (UVC) LEDs, in the near future.

Temperature-Dependent Optical Behaviors and Demonstration of Carrier Localization in Polar and Semipolar AlGaN Multiple Quantum Wells

Semipolar AlGaN multiple quantum wells (MQWs) have unique advantages in deep ultraviolet light emitters due to the weak Quantum-Confined Stark Effect. However, their applications are hampered by the poor crystalline quality of semipolar AlGaN thin films. Different treatments were developed to improve the crystal quality of semipolar AlGaN, including a multistep in situ thermal annealing technique proposed by our group. In this work, temperature-dependent and time-resolved photoluminescence characterizations were performed to reveal the carrier localization in the MQW region. The degree of carrier localization in semipolar AlGaN MQWs grown on top of the in situ-annealed AlN is similar to that of conventional ex situ face-to-face annealing, both of which are significantly stronger than that of the c-plane counterpart. Moreover, MQWs on in situ-annealed AlN show drastically reduced dislocation densities, demonstrating its great potential for the future development of high-efficiency optoelectronic devices.

Optical characterization of point defects on internal quantum efficiency in AlGaN quantum wells grown on face-to-face-annealed sputtered AlN templates

The correlation between the internal quantum efficiency (IQE) and the effective diffusion length estimated by the cathodoluminescence intensity line profile near the dark spots, including the effect of non-radiative recombination due to point defects, was experimentally clarified for AlGaN multiple quantum wells (MQWs) on face-to-face annealed (FFA) sputter-deposited AlN templates with different IQEs and similar dislocation densities. The IQEs, which were determined by temperature- and excitation-power-dependent photoluminescence measurements, were independent of the dark spot densities and increased with increasing effective diffusion length (Leff) estimated from the cathodoluminescence line profile analysis. These results suggested that the IQEs of the MQW/FFA samples were governed by the point defect density. The fitting results for the relationship between IQE and Leff and for that between IQE and Cmax explained the experimental results qualitatively.

High-quality AlGaN epitaxial structures and realization of UVC vertical-cavity surface-emitting lasers

AlGaN-based vertical-cavity surface-emitting lasers (VCSELs) have garnered recent interest due to their superior material properties and device benefits. Nevertheless, AlGaN-based VCSELs are extremely difficult to realize due to numerous technical limitations associated with both material epitaxial growth and chip fabrication. This study fabricated a high-quality AlGaN multiple quantum wells (MQWs) structure using epitaxial lateral overgrowth and analyzed it using X-ray diffraction (XRD) and photoluminescence (PL) measurements. With an edge dislocation density (DD) of 109 cm−2, XRD measurements reveal that the AlN template is nearly fully relaxed. The subsequent AlGaN/AlN superlattice (SL) layer is introduced to decrease the edge DD, and the edge DD in the MQWs is ∼108 cm−2. According to PL measurements, the internal quantum efficiency of the MQWs is as high as 62%, and radiative recombination dominated the emission of the MQWs at room temperature. Using these epitaxial wafers, ultraviolet radiation C (UVC) VCSELs were fabricated using various techniques, including laser lift-off (LLO) and chemical mechanical polishing (CMP). The crystallinity of the MQWs was unaffected by sapphire substrate removal using LLO. After removing the sapphire substrate using LLO and CMP, UVC surface-stimulated emission was observed in MQWs. AlGaN-based UVC VCSELs with lasing wavelengths of 275.91, 276.28, and 277.64 nm have been fabricated. The minimum threshold for UVC VCSELs is 0.79 MW cm−2, which is a record low.

High‐Efficiency E‐Beam Pumped Deep‐Ultraviolet Surface Emitter Based on AlGaN Ultra‐Thin Staggered Quantum Wells

AbstractA 2‐inch wafer‐scale electron‐beam (e‐beam) pumped deep‐ultraviolet surface emitter (DUVSE) with high efficiency and high output power at an emission wavelength of 248 nm is reported. This DUVSE benefits from ultra‐thin staggered AlN/AlGaN/GaN multiple quantum wells (MQWs), which compromise the electron–hole overlap and carrier confinement and thus significantly improve the emission efficiency. The wall‐plug‐efficiency (WPE) is increased by six times to 5.25% in comparison to that of conventional DUV light‐emitting devices (LEDs) based on AlGaN MQWs. This WPE is achieved under an anode voltage and current of 8 kV and 1 mA, where the output power is 420 mW. This output power can be further enhanced to 702 mW by increasing the anode current to 3 mA. The enhanced WPE and uniform electron beam distribution lighten the avenue to achieve a wafer‐scale high power dense DUV light source, which is a challenge for conventional DUV‐LEDs, in particular with an emission wavelength of less than 250 nm.

Role of Strain-Induced Microscale Compositional Pulling on Optical Properties of High Al Content AlGaN Quantum Wells for Deep-Ultraviolet LED

A systematic study was carried out for strain-induced microscale compositional pulling effect on the structural and optical properties of high Al content AlGaN multiple quantum wells (MQWs). Investigations reveal that a large tensile strain is introduced during the epitaxial growth of AlGaN MQWs, due to the grain boundary formation, coalescence and growth. The presence of this tensile strain results in the microscale inhomogeneous compositional pulling and Ga segregation, which is further confirmed by the lower formation enthalpy of Ga atom than Al atom on AlGaN slab using first principle simulations. The strain-induced microscale compositional pulling leads to an asymmetrical feature of emission spectra and local variation in emission energy of AlGaN MQWs. Because of a stronger three-dimensional carrier localization, the area of Ga segregation shows a higher emission efficiency compared with the intrinsic area of MQWs, which is benefit for fabricating efficient AlGaN-based deep-ultraviolet light-emitting diode.

Optical properties of quasi-ordered [formula omitted] AlGaN multi-quantum-well nanorod arrays

Quasi-ordered semi-polar 112̅2 plane AlGaN multiple-quantum-well (MQW) nanorod (NR) arrays with a period of 400 nm were successfully fabricated by a self-assembly technique using polystyrene spheres. The diameter and height of the NRs were about 360 and 600 nm, respectively. The photoluminescence (PL) peak wavelengths of both NR and planar MQWs were located at 266 nm at room temperature, demonstrating that the quantum-confined Stark effect was negligible in the semi-polar AlGaN MQWs. The PL integral intensity of the MQW NR arrays was enhanced by 1.5 times compared to that of planar MQWs. Moreover, the degree of polarization was increased from 15.9% to 26.2% when fabricating the NR arrays, demonstrating enhanced polarized emission. The finite-difference time-domain method was used to explain the enhancement of PL intensity, which was mainly due to the modified light extraction by the quasi-ordered NR arrays.

Anderson Localization Enabled Spectrally Stable Deep-Ultraviolet Laser Based on Metallic Nanoparticle Decorated AlGaN Multiple Quantum Wells.

Random lasers exhibit many exotic properties, including chaotic behavior, light localization, broad angular emission, and cost-effective fabrication, which enable them to attract both scientific and industrial interests. However, before the realization of their potential applications, several challenges still remain including the underlying mechanism and controllability due to their inherent multidirectional and chaotic fluctuations. Through more than two decades of collaborative efforts, the discovery of Anderson localization in random lasers provides a plausible route to resolve the difficulties, which enables one to tailor the number of lasing modes and stabilize the emission spectra. However, the related studies are rather rare and only restricted to limited wavelengths. In this study, based on enhanced Anderson localization assisted by surface plasmon resonance, spectrally stable deep-ultraviolet lasing action in AlGaN multiple quantum wells (MQWs) is demonstrated. Our work serves as firm evidence to demonstrate the underlying mechanism of stabilized deep-ultraviolet random laser action that multiple scattering of a light beam in a disordered medium can induce Anderson localization similar to electron behavior. This feature covers the whole spectral range, and it is a universal phenomenon of an electromagnetic wave. Notably, stabilized deep-ultraviolet random laser action has not been demonstrated in all previous studies, even though it has great academic interest and potential application in many areas from environmental protection to biomedical engineering.

Carrier dynamics near a crack in GaN microwires with AlGaN multiple quantum wells

Relaxation of tensile strain in AlGaN heterostructures grown on a GaN template can lead to the formation of cracks. These extended defects locally degrade the crystal quality, resulting in a local increase in non-radiative recombinations. The effect of such cracks on the optical and structural properties of core–shell AlGaN/AlGaN multiple quantum wells grown on GaN microwires is comprehensively characterized by means of spectrally and time-correlated cathodoluminescence (CL). We observe that the CL blueshifts near a crack. By performing 6 × 6 k.p simulations in combination with transmission electron microscopy analysis, we ascribe this shift to the strain relaxation by the free surface near cracks. By simultaneously recording the variations of both the CL lifetime and the CL intensity across the crack, we directly assess the carrier dynamics around the defect at T = 5 K. We observe that the CL lifetime is reduced typically from 500 ps to less than 300 ps and the CL intensity increases by about 40% near the crack. The effect of the crack on the optical properties is, therefore, of two natures. First, the presence of this defect locally increases non-radiative recombinations, while at the same time, it locally improves the extraction efficiency. These findings emphasize the need for time-resolved experiments to avoid experimental artifacts related to local changes of light collection.

Improved Optical Properties of Nonpolar AlGaN-Based Multiple Quantum Wells Emitting at 280 nm

The optical properties of nonpolar AlGaN multiple quantum wells (MQWs) emitting at 280 nm were investigated intensively using temperature-dependent and time-resolved photoluminescence spectra associated with the characterization of structural properties. The densities of superficial pits and basal-plane stacking faults (BSFs) were reduced by 33.8% and 35.9%, respectively, for nonpolar AlGaN MQWs due to the carefully optimized dual nitridation. It was found that the nonpolar MQWs emission can be significantly improved by reducing the BSFs density as the BSFs emission was the main competing channels. Moreover, an internal quantum efficiency of 39% for nonpolar Al0.43Ga0.57N MQWs at emission wavelength of 279 nm was achieved even the full width at half maximum values of X-ray rocking curves were 0.565° for c-direction and 0.797° for m-direction. This fact means that a highly efficient deep ultraviolet light sources can be expected by means of nonpolar AlGaN due to the elimination of quantum confined Stark effect induced the decrease in radiative lifetime.

Graphene-Assisted Quasi-van der Waals Epitaxy of AlN Film on Nano-Patterned Sapphire Substrate for Ultraviolet Light Emitting Diodes.

This protocol demonstrates a method for graphene-assisted quick growth and coalescence of AlN on nano-pattened sapphire substrate (NPSS). Graphene layers are directly grown on NPSS using catalyst-free atmospheric-pressure chemical vapor deposition (APCVD). By applying nitrogen reactive ion etching (RIE) plasma treatment, defects are introduced into the graphene film to enhance chemical reactivity. During metal-organic chemical vapor deposition (MOCVD) growth of AlN, this N-plasma treated graphene buffer enables AlN quick growth, and coalescence on NPSS is confirmed by cross-sectional scanning electron microscopy (SEM). The high quality of AlN on graphene-NPSS is then evaluated by X-ray rocking curves (XRCs) with narrow (0002) and (10-12) full width at half-maximum (FWHM) as 267.2 arcsec and 503.4 arcsec, respectively. Compared to bare NPSS, AlN growth on graphene-NPSS shows significant reduction of residual stress from 0.87 GPa to 0.25 Gpa, based on Raman measurements. Followed by AlGaN multiple quantum wells (MQWS) growth on graphene-NPSS, AlGaN-based deep ultraviolet light-emitting-diodes (DUV LEDs) are fabricated. The fabricated DUV-LEDs also demonstrate obvious, enhanced luminescence performance. This work provides a new solution for the growth of high quality AlN and fabrication of high performance DUV-LEDs using a shorter process and less costs.

Optically pumped room temperature low threshold deep UV lasers grown on native AlN substrates

We report here an optically pumped deep UV edge emitting laser with AlGaN multiple quantum wells (MQWs) active region grown on AlN substrate by low pressure organometallic vapor phase epitaxy (LP-OMVPE) in a high-temperature reactor. The 21 period Al_0.53Ga_0.47N/Al_0.7Ga_0.3N MQWs laser structure was optically pumped using 193 nm deep UV excimer laser source. A laser peak was achieved from the cleaved facets at 280.3 nm with linewidth of 0.08 nm at room temperature with threshold power density of 320 kW/cm². The emission is completely TE polarized and the side mode suppression ratio (SMSR) is measured to be around 14 dB at 450 kW/cm².

Impact of face-to-face annealed sputtered AlN on the optical properties of AlGaN multiple quantum wells

The impact of a face-to-face annealed sputtered AlN/sapphire (FFA Sp-AlN) template with threading-dislocation densities (TDDs) of 2 × 108 cm−2 and an n-type AlGaN (n-AlGaN) underlayer on optical properties of AlGaN multiple quantum wells (MQWs) with an ultraviolet C (UVC) emission is investigated comprehensively. For comparison of the FFA Sp-AlN template with low TDDs, a conventional MOVPE (metalorganic vapor phase epitaxially)-grown AlN/sapphire (MOVPE-AlN) template with TDDs of 1 × 109 cm−2 was prepared. Consequently, cathodoluminescence (CL), temperature-dependent photoluminescence (PL), and time-resolved PL (TR-PL) measurements verified that both the FFA Sp-AlN template and n-AlGaN underlayer are indispensable for obtaining MQWs with high internal quantum efficiencies, which decrease the TDDs and point defect (PD) densities. Our results revealed that 10-period quantum wells (10QWs)/n-AlGaN/AlN grown on the FFA Sp-AlN template exhibit a lower dark spot density in CL panchromatic intensity maps, a higher integrated emission intensity ratio from the temperature-dependent PL (from 15 to 300 K), and a longer nonradiative lifetime from the TR-PL measurements at 300 K compared with those grown on the MOVPE-AlN template. Moreover, we found that the optical properties of 10QWs/AlN in FFA Sp-AlN and MOVPE-AlN templates do not exhibit a significant difference because of the existence of numerous PDs. Our experimental results demonstrate the favorable impact of the FFA Sp-AlN template for low-TDDs and the n-AlGaN underlayer for low-PDs, which holds promise for highly efficient AlGaN deep-ultraviolet light-emitting devices.

AlGaN multiple quantum wells by PA-MBE for deep UV emission: Effect of growth interruptions

Growth of AlGaN multiple quantum wells by Plasma Assisted-Molecular Beam Epitaxy has been optimized for use in deep ultraviolet light emitting diodes. A series of samples were grown on c-plane sapphire using an AlN buffer layer, an ~1 μm AlGaN layer, followed by 40-pairs of wells and barriers. The III/V flux ratio was varied from near stoichiometric to excess group III and a plasma exposure step was introduced for some samples after the deposition of wells and barriers. The quality of the well-barrier interface was estimated by the number and intensity of X-Ray Diffraction (XRD) superlattice peaks. Our results indicate that interface abruptness is greatly enhanced by the addition of this process step. While the room temperature photoluminescence (PL) intensity increases significantly upon introduction of the interruption after the barrier layer, this effect is reversed with additional interruption after the well layers. Temperature dependent PL measurements show an anomalous variation with increasing temperature with a maxima around 110°-150°C. This is due to photogeneration of e-h pairs in the underlying AlGaN layer and its subsequent recombination in the quantum wells by diffusion through percolative barrier layers with in-plane potential fluctuations. This is arrested at lowest temperatures causing a reduction in PL intensity. The PL intensity ratio I300K/I4K is highest for samples grown under excess group III, but reduces with the addition of the growth interruption step after the barrier layer. Growth under stoichiometric conditions together with dual growth interruptions was found to be optimal, with improved PL properties while maintaining high interface quality.

Comparison of AlxGa1−xN multiple quantum wells designed for 265 and 285 nm deep-ultraviolet LEDs grown on AlN templates having macrosteps

AlGaN multiple quantum wells (MQWs) targeting 265 and 285 nm deep-ultraviolet LEDs grown on AlN templates with macrosteps were compared. In the cathodoluminescence images of the 285 nm MQW, high-intensity zones were observed along the edgelines of the macrosteps with wavelengths of 290–296 nm and on the terraces with wavelengths of 286–288 nm. Dark spots related to threading dislocations were seen in the entire 285 nm MQW. For 265 nm, high-intensity zones were limited along the edgelines and dark spots showed a low-contrast, which is likely to be caused by nonradiative recombination centers due to point defects.