Deep Ultraviolet Light-emitting Diodes Research Articles

Metal-insulator-semiconductor structure for deep-ultraviolet light-emitting diodes to increase the electron injection in the cathode region

Abstract In this work, we propose using metal-insulator-semiconductor (MIS) structure on the n-AlGaN layer to reduce the contact resistance and improve the wall-plug efficiency (WPE) for AlGaN based deep ultraviolet light-emitting diodes (DUV LEDs). The thin insulator between the cathode metal and the n-AlGaN layer can share the bias, which makes the n-AlGaN layer surface less depleted and thus the MIS-type cathode structure favors the intraband tunneling effect for electrons. Moreover, the adoption of the MIS-structure aligns the electron affinity for the metal above the conduction band for the n-AlGaN layer, and this avoids the electron transport by thermionic emission. Thanks to the MIS-type cathode, we find that excellent electron transport can be obtained even if the metal with big electron affinity is used. This work also conducts an in-depth investigation into the impact of the relative dielectric constant, the bandgap, the affinity, the length and the thickness for the insulator on the electron transport and the WPE of DUV LEDs. We find that an insulator with a small dielectric constant is required for promoting the electron tunneling in the MIS-type cathode region.

Inactivation of Listeria and E. coli by Deep-UV LED: effect of substrate conditions on inactivation kinetics

Irradiation with deep-ultraviolet light-emitting diodes (DUV LEDs) is emerging as a low energy, chemical-free approach to mitigate microbial contamination, but the effect of surface conditions on treatment effectiveness is not well understood. Here, inactivation of L. innocua and E. coli ATCC25922, as examples of Gram-positive and Gram-negative bacteria, respectively, by DUV LED of 280 nm wavelength was studied. Surface scenarios commonly encountered in environmental, clinical or food processing environments were used: nutrient rich surfaces, thin liquid films (TLF), and stainless steel surfaces (SS). DUV LED exposure achieved 5-log reduction for both strains within 10 min in most scenarios, except for TLF thicker than 0.6 mm. Inactivation kinetics in TLF and on dry SS followed the Weibull model (0.96 ≤ R2 ≤ 0.99), but the model overestimated inactivation by small-dose DUV on wet SS. Confocal microscopy revealed in situ that bacteria formed a dense outer layer at the liquid-air interface of the liquid droplet, protecting the cells inside the droplet from the bactericidal DUV. This resulted in lower than anticipated inactivation on wet SS at small DUV doses, and deviation from the Weibull model. These findings can be used to design effective DUV LED disinfection strategies for various surface conditions and applications.

Near vacuum-ultraviolet aperiodic oscillation emission of AlN films

An accurate measurement of the refractive index is necessary for the optical design of both deep ultraviolet laser diodes and light-emitting diodes. Generally, the refractive indices along different crystallographic axes of anisotropic thin films are measured using variable angle spectroscopic ellipsometry. However, there are still some limitations concerning this method. Here we proposed a potential method to measure the band edge refractive index of wide bandgap semiconductor. An aperiodic oscillation emission phenomenon due to the Fabry-Perot effect was observed in the fluorescence spectrum of an AlN film with a thickness of 1500 nm. Based on the characteristics of the fluorescence spectrum and the definition of Fabry-Perot effect, we obtained the ordinary refractive index of the AlN thin film near the band edge directly. This refractive index measurement method is a supplement to the variable angle ellipsometry, and it is a more direct and effective method for transferred film and thinner samples to measure the fluorescence spectrum.

Suppression of dislocation-induced spiral hillocks in MOVPE-grown AlGaN on face-to-face annealed sputter-deposited AlN template

AlGaN films were grown on face-to-face annealed sputter-deposited AlN/sapphire (FFA Sp-AlN) templates via metalorganic vapor phase epitaxy (MOVPE), and the growth behavior of the AlGaN films was investigated. The sapphire substrates with small off-cut exhibited poor surface flatness of AlGaN grown on the FFA Sp-AlN templates owing to the formation of large hillock structures. To understand the origin of these hillock structures, the crystallinity and surface morphology of conventional fully MOVPE-grown AlN/sapphire (MOVPE-AlN) templates and the FFA Sp-AlN template were comprehensively studied. The screw- and mixed-type threading dislocation density of the FFA Sp-AlN template was estimated to be approximately 1.8 × 106 cm−2, which was two orders of magnitude lower than that of the MOVPE-AlN template. Consequently, the uniquely observed growth of the hillock structures in the FFA Sp-AlN templates was attributed to their low density of screw- and mixed-type threading dislocations. The large surface off-cut sapphire substrates suppressed the hillock structures on the FFA Sp-AlN templates. The improvement in surface flatness resulted in better optical properties of multiple quantum wells grown on the AlGaN layer. These results demonstrate a promising method for achieving highly efficient and cost effective AlGaN based deep ultraviolet light-emitting diodes.

Electronic structure of AlFeN films exhibiting crystallographic orientation change from c- to a-axis with Fe concentrations and annealing effect

Wurtzite AlN film is a promising material for deep ultraviolet light-emitting diodes. However, some properties that attribute to its crystal orientation, i.e., c-axis orientation, are obstacles in realizing high efficiency devices. Constructing devices with non-c-axis oriented films is a solution to this problem; however, achieving it with conventional growth techniques is difficult. Recently, we succeeded in growing a-axis oriented wurtzite heavily Fe-doped AlN (AlFeN) films via sputtering. In this article, we report the electronic structures of AlFeN films investigated using soft X-ray spectroscopies. As-grown films were found to have conduction and valence band structures for a film with c-axis in film planes. Simultaneously, it was found that large gap states were formed via N-p and Fe-d hybridization. To remove the gap states, the films were annealed, thereby resulting in a drastic decrease of the gap states while maintaining a-axis orientation. We offer heavy Fe-doping and post annealing as a new technique to obtain non-polar AlN films.

AlGaN Deep-Ultraviolet Light-Emitting Diodes Grown on SiC Substrates

The disinfection industry would greatly benefit from efficient, robust, high-power deep-ultraviolet light-emitting diodes (UV–C LEDs). However, the performance of UV–C AlGaN LEDs is limited by poor...

Numerical investigation on the device performance of electron blocking layer free AlInN nanowire deep ultraviolet light-emitting diodes

We report on the illustration of the first electron blocking layer (EBL) free AlInN nanowire light-emitting diodes (LEDs) operating in the deep ultraviolet (DUV) wavelength region (sub-250 nm). We have systematically analyzed the results using APSYS software and compared with simulated AlGaN nanowire DUV LEDs. From the simulation results, significant efficiency droop was observed in AlGaN based devices, attributed to the significant electron leakage. However, compared to AlGaN nanowire DUV LEDs at similar emission wavelength, the proposed single quantum well (SQW) AlInN based light-emitters offer higher internal quantum efficiency without droop up to current density of 1500 A/cm2 and high output optical power. Moreover, we find that transverse magnetic polarized emission is ∼ 5 orders stronger than transverse electric polarized emission at 238 nm wavelength. Further research shows that the performance of the AlInN DUV nanowire LEDs decreases with multiple QWs in the active region due to the presence of the non-uniform carrier distribution in the active region. This study provides important insights on the design of new type of high performance AlInN nanowire DUV LEDs, by replacing currently used AlGaN semiconductors.

Temperature-Dependent Carrier Recombination and Efficiency Droop of AlGaN Deep Ultraviolet Light-Emitting Diodes

We investigate temperature-dependent carrier transfer and efficiency droop on AlGaN-based deep ultraviolet light-emitting diodes. The Shockley-Read-Hall (SRH) recombination and carrier leakage are highly associated with the poor thermal stability. The existence of Auger recombination and carrier leakage is identified by the m-power method. A modified ABC model with an additional term f(n) related to carrier leakage is employed to analyze the evolution of multiple recombination mechanisms. The SRH process strongly suppresses both Auger recombination and carrier leakage at low currents. At high currents, the latter two processes are responsible for the efficiency droop and exhibit an anti-correlation upon temperature.

Light Extraction Efficiency Optimization of AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes

Using finite-difference time-domain method, the light extraction efficiency (LEE) of AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) is investigated. Simulation results show that compared to flat sapphire substrate, the nano-patterned sapphire substrate (NPSS) expands the extraction angles of top surface and sidewalls. As a result, the LEE of transverse-magnetic (TM) polarized light is improved significantly. Roughening on the backside of n-AlGaN surface significantly enhances the LEE of top surface of thin-film flip-chip DUV LEDs. However, the LEE of sidewalls of thin-film flip-chip DUV LEDs is greatly weakened. For bare DUV LED, the LEE of flip-chip LED on NPSS is estimated to be about 15%, which is around 50% higher than that of thin-film flip-chip DUV LED with roughening on the backside of n-AlGaN surface.

Monolayer GaN excitonic deep ultraviolet light emitting diodes

We report on the molecular beam epitaxy and characterization of monolayer GaN embedded in N-polar AlN nanowire structures. Deep ultraviolet emission from 4.85 to 5.25 eV is measured by varying the AlN barrier thickness. Detailed optical measurements and direct correlation with first-principles calculations based on density functional and many-body perturbation theory suggest that charge carrier recombination occurs predominantly via excitons in the extremely confined monolayer GaN/AlN heterostructures, with exciton binding energy exceeding 200 meV. We have further demonstrated deep ultraviolet light-emitting diodes (LEDs) with the incorporation of single and double monolayer GaN, which operate at 238 and 270 nm, respectively. These unique deep ultraviolet LEDs exhibit highly stable emission and a small turn-on voltage around 5 V.

Enhanced wall plug efficiency of AlGaN-based deep-UV LEDs using Mo/Al as p-contact

P-type contacts with a high reflectivity in the ultraviolet spectral region made of molybdenum/aluminum (Mo/Al) on AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) emitting at 265 nm have been investigated. Optimized Mo/Al contacts are shown to have a high optical reflectivity above 75% at 265 nm. DUV LEDs with an absorbing p-AlGaN heterostructure operated at 20 mA show a 15% higher light output power and a 1 V lower voltage when Mo/Al instead of Pt is used as p-contact. The effect on the voltage of DUV LEDs with a UV-transparent p-side heterostructure is similar. Moreover, DUV LEDs with a Mo/Al contact show a lower operation voltage compared to LEDs with an indium tin oxide/aluminum (ITO/Al) p-contact where the ITO is intended to form a semitransparent low-resistance contact and the Al serves as a reflector.

Interplay between various active regions and the interband transition for AlGaN-based deep-ultraviolet light-emitting diodes to enable a reduced TM-polarized emission

Al-rich AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) have a low light extraction efficiency, especially when the emission wavelength is shorter than 280 nm, and this is partially because of the dominant transverse-magnetic polarized light. Our results show that the transverse-electric (TE) polarized light can be obtained even if the emission wavelength becomes even shorter by reducing the quantum well thickness. The ultrathin quantum well enables the enhanced TE-polarized emission that arises from the redistributed subbands for holes. On the contrary to the common belief, we observe a blueshift for the emission wavelength when the AlN composition in the quantum barrier increases. The internal quantum efficiency (IQE) for DUV LEDs with ultrathin quantum wells is no longer determined by the quantum-confined Stark effect, while quantum barrier with high AlN composition is vitally important to improve the electron injection efficiency and thus enhance the IQE.

Surface Plasmon Coupling with Radiating Dipole for Enhancing the Emission Efficiency and Light Extraction of a Deep Ultraviolet Light Emitting Diode

In this paper, we numerically investigated the emission characteristics of surface plasmon (SP)-enhanced deep ultraviolet light emitting diode (DUV-LED) by employing Al/Al2O3 core-shell nanoparticle(NP) structure on the p-GaN contact layer by utilizing finite-difference time-domain (FDTD) method. The results suggest that normalized dipole power of TE (TM) polarization is enhanced (inhibited) in the DUV range by coupling with in-plane substrate localized surface plasmon (LSP) mode from Al/Al2O3 core-shell nanoparticle (NP). It is found that normalized upward extraction power for both polarizations can also be significantly increased by scattering effect when photons and excitons coupling with in-plane air LSP mode induced on the NP; thus, the light extraction efficiency (LEE) can be substantially enhanced. The depth d between quantum well (QW) and NP and NP size have a remarkable influence on LSP resonance wavelength. Through careful regulation of NP size and depth d, the emission characteristics in DUV range (240–280 nm) exhibit a considerable amelioration.

Design and Optimization of Electron-Blocking Layer in Deep Ultraviolet Light-Emitting Diodes

Optical and electrical performance and the relevant physical properties of deep ultraviolet (DUV) AlGaN-based light-emitting diodes (LEDs) are numerically investigated. The influence of bulk AlGaN electron-blocking layer (EBL) with various Al compositions and Mg-doping concentrations on the output performance of DUV LEDs is systemically explored. Simulation results show that the DUV LED with bulk Al0.85Ga0.15N EBL with a relatively low Mg-doping concentration of $2\times 10^{19}$ cm−3 may have high wall-plug efficiency. The LED structure with Al0.55Ga0.45N/Al0.75Ga0.25N superlattice (SL) EBL is proposed in this study, which is shown to have high wall-plug efficiency similar to the LED with bulk Al0.85Ga0.15N EBL while the Al composition of AlGaN EBL can be effectively reduced. The proposed LED structure with SL EBL provides a practical way to fabricate high-crystalline-quality and high-efficiency DUV LEDs.

Performance Improvement of AlGaN-Based Deep Ultraviolet Light-Emitting Diodes With Step-Like Quantum Barriers

AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) still confront many challenges, which is partially limited by the poor carrier injection in the active region. Although incorporating a high Al-composition quantum barrier (QB) may boost carrier confinement capability, it will aggravate the quantum confined Stark effect (QCSE) and thus deteriorate the optical performance. In this article, a DUV LED structure with step-like QBs has been proposed and carefully investigated. This unique QB structure suppresses the electron overflow into the p-side of the device and benefits the hole injection efficiency simultaneously, thereby promoting the radiative recombination rate in the active region. As a result, the internal quantum efficiency (IQE) and light output power (LOP) of the DUV LED with step-like QBs are significantly improved with an enhancement factor of 40% under 60 mA current injection. Therefore, our step-like QB design provides a feasible approach to the enhancement of the optical performance of DUV LEDs.

Two-dimensional analysis of the nonuniform quantum yields of multiple quantum wells for AlGaN-based deep-ultraviolet LEDs grown on AlN templates with dense macrosteps using cathodoluminescence spectroscopy

AlGaN-based deep-ultraviolet light-emitting diodes (LEDs) incorporating uneven multiple quantum wells (MQWs) with inclined and terrace zones, which were fabricated on an AlN template with dense macrosteps, have exhibited a high internal quantum efficiency (IQE). To investigate the microscopic structure of uneven MQWs, cathodoluminescence (CL) mapping characterization was carried out, and the maps of the CL intensity at 300 K relative to that at 38 K were obtained for uneven MQWs that targeted 265 and 285 nm LEDs. At an electron beam current of less than 1.0 nA, the signals from inclined and terrace zones of the uneven MQWs were confirmed to satisfy the nonsaturated excitation condition at 300 K. Nonradiative recombination (NR) was insufficiently frozen even at 38 K, specifically on the terraces in the 265 nm MQW, suggesting high concentrations of NR centers due to point defects (PDs). In contrast, NR in the 285 nm MQW at 38 K was closer to freeze-out. The concentration of PDs in the 285 nm MQW was likely to be lower than that in the 265 nm MQW. Finally, the ratios of the CL intensity at 300 K to those at 38 K were mapped, demonstrating an approach to creating an approximate map of IQE. The values in the CL intensity ratio maps are discussed by considering the analytical error factors. The results support the model of localized current injection through Ga-rich stripe zones in the n-AlGaN cladding layer.

Enhanced Optical Performance of AlGaN-Based Deep Ultraviolet Light-Emitting Diodes by Electrode Patterns Design

Low external quantum efficiency of deep ultraviolet light-emitting diodes (DUV LEDs) and current crowding can result in considerable heat generation, which has a great negative impact on device performance. In this paper, we investigate the influence of different electrode patterns on the photoelectric and thermal performance of DUV LEDs. We find that different electrode designs can achieve drastically different optical powers, with the superior design being the n-type electrode surrounding the active region. Moreover, compared with the counterpart, the superior design does not affect the electrical performance. The main reason is that the N-surrounding electrode pattern can provide enough current paths for carrier transport, thus realizing a more uniform current injection and can further improve the external quantum efficiency for DUV LEDs.

Influence of Silicon-Doping in n-AlGaN Layer on the Optical and Electrical Performance of Deep Ultraviolet Light-Emitting Diodes

The influence of two different methods of silicon doping in AlGaN layer, that is, modulation-doping (MD) and delta-doping (DD), on the optical and electrical performance of deep ultraviolet light-emitting diodes (DUV-LEDs) has been investigated. Both the photoluminescence and electroluminescence intensities in the Si-DD structure are stronger than those obtained by the Si-MD method, while the forward voltage and reverse leakage current are slightly smaller in the DD structure than that in the MD structure. Compared with the MD structure, the DD structure shows higher capacitance-voltage characteristics. This study suggests that the DD method can improve the optical and electrical performance of DUV-LEDs.

Heteroepitaxial Growth of High-Quality and Crack-Free AlN Film on Sapphire Substrate with Nanometer-Scale-Thick AlN Nucleation Layer for AlGaN-Based Deep Ultraviolet Light-Emitting Diodes.

High-quality and crack-free aluminum nitride (AlN) film on sapphire substrate is the foundation for high-efficiency aluminum gallium nitride (AlGaN)-based deep ultraviolet light-emitting diodes (DUV LEDs). We reported the growth of high-quality and crack-free AlN film on sapphire substrate with a nanometer-scale-thick AlN nucleation layer (NL). Three kinds of nanometer-scale-thick AlN NLs, including in situ low-temperature AlN (LT-AlN) NL, oxygen-undoped ex situ sputtered AlN NL, and oxygen-doped ex situ sputtered AlN NL, were prepared for epitaxial growth of AlN films on sapphire substrates. The influence of nanoscale AlN NL thickness on the optical transmittance, strain state, surface morphology, and threading dislocation (TD) density of the grown AlN film on sapphire substrate were carefully investigated. The average optical transmittance of AlN film on sapphire substrate with oxygen-doped sputtered AlN NL was higher than that of AlN films on sapphire substrates with LT-AlN NL and oxygen-undoped sputtered AlN NL in the 200–270 nm wavelength region. However, the AlN film on sapphire substrate with oxygen-undoped sputtered AlN NL had the lowest TD density among AlN films on sapphire substrates. The AlN film on sapphire substrate with the optimum thickness of sputtered AlN NL showed weak tensile stress, a crack-free surface, and low TD density. Furthermore, a 270-nm AlGaN-based DUV LED was grown on the high-quality and crack-free AlN film. We believe that our results offer a promising and practical route for obtaining high-quality and crack-free AlN film for DUV LED.

Mg and In Codoped p-type AlN Nanowires for pn Junction Realization.

Efficient, mercury-free deep ultraviolet (DUV) light-emitting diodes (LEDs) are becoming a crucial challenge for many applications such as water purification. For decades, the poor p-type doping and difficult current injection of Al-rich AlGaN-based DUV LEDs have limited their efficiency and therefore their use. We present here the significant increase in AlN p-doping thanks to Mg/In codoping, which leads to an order of magnitude higher Mg solubility limit in AlN nanowires (NWs). Optimal electrical activation of acceptor impurities has been further achieved by electron irradiation, resulting in tunnel conduction through the AlN NW p-n junction. The proposed theoretical scenario to account for enhanced Mg incorporation involves an easy ionization of In-vacancy complex associated with a negative charging of Mg in In vicinity. This leads to favored incorporation of negatively charged Mg into the AlN matrix, opening the path to the realization of highly efficient NW-based LEDs in the DUV range.