Deep Ultraviolet Light-emitting Devices Research Articles

Crystal orientation control of a-plane AlN films on r-plane sapphire fabricated by sputtering and high-temperature annealing

Face-to-face annealed and sputter-deposited aluminum nitride (FFA Sp-AlN) has potential in deep-ultraviolet light-emitting devices. Herein, the effects of the substrate off-cut angle (θsub) from an r-plane sapphire toward the c-axis projection direction and sputtering temperature (Tsp) on the crystallinity and surface morphology of a-plane AlN films are investigated. Increasing θsub in the minus-off direction, which occurs when the substrate surface approaches the sapphire c-plane, and lowering Tsp suppress the mixing of anomalous non-a-direction oriented domains. This reduced mixing enhances the surface flatness and crystallinity of a-plane FFA Sp-AlN. Moreover, the c-axis direction of the a-plane AlN film is inverted depending on the substrate off-cut angle. Ab initio calculations indicate that the interface stability between the r-plane sapphire substrate and the a-plane AlN film can explain the dependence of the crystallinity and c-axis orientation of FFA Sp-AlN on the surface off-cut of the r-plane sapphire.

Design of high-entropy films as ultra-violet light reflector

This study proposes the design and implementation of high-entropy alloy (HEA) thin films as materials for the reflector of deep ultraviolet light-emitting devices and marks the first application of HEA in optical field. In the material design phase, an Al-Co-Zn-Ni alloy is designed by considering the reflectivity, phase stability and binary formation energy. Reverse Monte Carlo calculation is employed to establish stable atomic structure. Frequency-dependent dielectric equations are derived from first principles calculations, with dielectric constants incorporating contributions from ions and electrons and combining classical Drude and Lorentz models. The dielectric equations yield optical constants that are used to calculate the reflectance. In the experimental phase, we systematically vary the aluminum content and prepare Al-Co-Zn-Ni thin films ranging from pure aluminum to equimolar compositions. In the deep ultraviolet region, the reflectance of thermally treated Al65Zn12Co12Ni11 and Al50Zn16Co17Ni17 thin films is found superior to that of gold by three times. Structural and crystalline changes with varying aluminum content are found, as well as intriguing phase separation and superlattice formation that persist until equimolar compositions. This unique nanoscale structural variation causes the Young's modulus and hardness of Al-Co-Zn-Ni thin films to initially increase and then decrease with decreasing aluminum content. There is a significant increase in the corrosion potential difference when reducing Al contents in Al-Co-Zn-Ni, indicating improved passivation effects as the equimolar composition is approached. By modulating the aluminum content, the nanoscale structure of Al-Co-Zn-Ni can be adjusted, thereby improving its optical, mechanical, corrosion resistance, and thermal stability properties. This demonstrates the high versatility of this material for practical optical applications.

Intrinsic excitation-dependent room-temperature internal quantum efficiency of AlGaN nanowires with varying Al contents

Aluminum gallium nitride (AlGaN) nanowires have become an emerging approach for semiconductor deep ultraviolet light-emitting devices. To further improve the device performance, it is critical to understand the optical quality of AlGaN nanowires. However, today, the room-temperature internal quantum efficiency (IQE) of AlGaN nanowires is predominantly analyzed by the temperature-dependent photoluminescence (PL) approach under one excitation power or taking the PL intensity ratio at the room temperature and low temperature with different excitation powers. In both cases, one needs to assume the low temperature IQE to be 100%, which is not always valid, in particular when the excitation power changes at the low temperature. In this work, we study the room-temperature IQE of AlGaN nanowires through the detailed excitation power-dependent PL experiments and theoretical analysis. This allows us to derive the intrinsic room-temperature IQE of AlGaN nanowires as a function of the excitation power. It is found that for an Al content in the range of 22%–54%, the IQE of all samples increases as the excitation increases, followed by an efficiency droop. Moreover, comparing different samples, the IQE at low excitations increases as the Al content increases, whereas the peak IQE reduces from 73% to 56% as the Al content increases. The underlying mechanisms are also discussed in this paper.

Controlled bunching approach for achieving high efficiency active region in AlGaN-based deep ultraviolet light-emitting devices with dual-band emission

Developing efficient active region structures with low sensitivity to threading dislocation density (TDD) is of great technical significance for AlGaN-based deep ultraviolet (DUV) light-emitting devices. Here, we propose an active region strategy by introducing bunching effect to form self-assembled sidewall quantum wells (SQWs) with much stronger carrier confinement, resulting in a significant enhancement of internal quantum efficiency (from 46% to 59%) compared to the commonly adopted multiple quantum wells (MQWs) due to the lower sensitivity to TDD. As a demo, an AlGaN-based DUV light-emitting diode (LED) with the proposed active region involving both SQWs and MQWs presents dual-band emission and a consequent 68% enhancement in light output power compared to the DUV-LED with only MQWs as the active region, suggesting that the proposed architecture is fully suitable for the development of high performance DUV light-emitting devices even based on poor or medium quality materials.

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.

The emergence and prospects of deep-ultraviolet light-emitting diode technologies

By alloying GaN with AlN the emission of AlGaN light-emitting diodes can be tuned to cover almost the entire ultraviolet spectral range (210–400 nm), making ultraviolet light-emitting diodes perfectly suited to applications across a wide number of fields, whether biological, environmental, industrial or medical. However, technical developments notwithstanding, deep-ultraviolet light-emitting diodes still exhibit relatively low external quantum efficiencies because of properties intrinsic to aluminium-rich group III nitride materials. Here, we review recent progress in the development of AlGaN-based deep-ultraviolet light-emitting devices. We also describe the key obstacles to enhancing their efficiency and how to improve their performance in terms of defect density, carrier-injection efficiency, light extraction efficiency and heat dissipation. This Review covers recent progress in AlGaN-based deep-ultraviolet light-emitting devices. The key technologies of how to improve their performance, carrier-injection efficiency, light extraction efficiency and heat dissipation are discussed.

Formation of conductive spontaneous via holes in AlN buffer layer on n+Si substrate by filling the vias with n-AlGaN by metal organic chemical vapor deposition and application to vertical deep ultraviolet photo-sensor

We have grown conductive aluminum nitride (AlN) layers using the spontaneous via holes formation technique on an n+-Si substrate for vertical-type device fabrication. The size and density of the via holes are controlled through the crystal growth conditions used for the layer, and this enables the conductance of the layer to be controlled. Using this technique, we demonstrate the fabrication of a vertical-type deep ultraviolet (DUV) photo-sensor. This technique opens up the possibility of fabrication of monolithically integrated on-chip DUV sensors and DUV light-emitting devices (LEDs), including amplifiers, controllers and other necessary functional circuits, on a Si substrate.

Stable configurations and electronic structures of hydrogenated graphyne

First-principles calculations are performed to investigate dynamical stability of hydrogenated graphyne and its electronic structures. We find that the zero point energy (ZPE) is important in evaluating the stability of hydrogenated graphyne. Based on the results of formation enthalpy, the hydrogenated configuration with only sp3 carbon atoms (eHH) is more stable than that with each carbon atom passivated by single hydrogen atom (eH). However, the Helmholtz free energy as functions of temperature indicates that eH is more favorable than eHH below 670K. Based on DFT-based phonon spectrum calculations, the dynamical stability of eHH and eH is confirmed. Of particular interest is that the band-gap feature of graphyne undergoes direct–indirect–direct transition with the increase in the concentration of hydrogen. The results indicate that eHH is favorable for the applications in the field of deep ultraviolet light-emitting devices.

High Mg-content wurtzite MgZnO alloys and their application in deep-ultraviolet light-emitters pumped by accelerated electrons

High Mg-content single-phase wurtzite MgZnO alloys with a bandgap of 4.35 eV have been obtained on sapphire substrate by introducing a composition-gradient MgxZn1−xO buffer layer. By employing the accelerated electrons obtained in a solid-state structure as an excitation source, an emission at around 285 nm, which is originated from the near-band-edge emission of the Mg0.51Zn0.49O active layer, has been observed. The results reported in this paper may provide a promising route to high performance deep-ultraviolet light-emitting devices by bypassing the challenging doping issues of wide bandgap semiconductors.

Deep-ultraviolet light-emitting device realized via a hole-multiplication process

By proper controlling the carrier generation and multiplication processes, an Au/MgO/Mg0.52Zn0.48O/MgxZn1−xO/n-ZnO structure has been designed and fabricated. In this structure, holes are multiplied via an impact ionization process in the MgO layer and injected into the Mg0.52Zn0.48O layer, and electrons are injected into the Mg0.52Zn0.48O layer from the n-ZnO layer through a composition-gradient MgxZn1−xO bridging layer. With the injection of electrons and holes, a deep ultraviolet emission at around 276 nm, coming from the Mg0.52Zn0.48O active layer, has been observed. The results reported in this letter may provide a promising route to high performance deep ultraviolet light-emitting devices.