The investigation of electrical and optical properties of micro-scale AlGaN deep ultraviolet (DUV) light-emitting diodes (LEDs) emitting at ∼ 275 n m was carried out, with an emphasis on fabricated devices having a diameter of 300, 200, 100, 50, and 20 µm, respectively. It was revealed that the LED chips with smaller mesa areas deliver considerably higher light output power density; meanwhile, they can sustain a higher current density, which is mainly attributed to the enhanced current spreading uniformity in micro-scale chips. Importantly, when the diameter of LED chips decreases from 300 µm to 20 µm, the peak external quantum efficiency (EQE) increases by 20%, and the EQE peak current density can be boosted from 8.85 A / c m 2 and 99.52 A / c m 2 . Moreover, we observed a longer wavelength emission with enlarged full-width at half-maximum (FWHM) in the LEDs with smaller chip sizes because of the self-heating effect at high current injection. These experimental observations provide insights into the design and fabrication of high-efficiency micro-LEDs emitting in the DUV regime with different device geometries for various future applications.
AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) suffer from severe quantum confined Stark effect (QCSE) due to the strong polarization field in the quantum wells (QWs) grown on c-plane substrates. In this paper, we propose a novel DUV LED structure embedded with graded QWs in which the Al composition was linearly changed to screen the QCSE. A significant increase of the internal quantum efficiency and thus an enhancement of the light output power by nearly 67% can be achieved, attributing to the improvement of the electron-hole wave function overlap (Г e−hh) to 58.6% in the Increased-Al-composition graded QWs, as compared to the QW without grading (Г e−hh = 40.4%) and reverse grading (Г e−hh = 33.6%). Further investigations show that the grading profile of the Al composition in the QWs, including either linearly increases or decreases along the growth direction and the thickness of graded QWs, determine the polarization electrical field in the QWs and as a result, significantly affecting the performance of the devices. In the end, a careful optimization of the graded QWs is called. The proposed structure with such unique graded QWs provides us an effective solution to suppress the QCSE effect in the pursuit of high-performance DUV emitters.
In this Letter, we perform a comprehensive investigation on the optical characterization of micro-sized deep-ultraviolet (DUV) LEDs (micro-LEDs) emitting below 280 nm, highlighting the light extraction behavior in relation to the design of chip sidewall angle. We found that the micro-LEDs with a smaller inclined chip sidewall angle ( ∼ 33 ∘ ) have improved external quantum efficiency (EQE) performance 19% more than that of the micro-LEDs with a larger angle ( ∼ 75 ∘ ). Most importantly, the EQE improvement by adopting an inclined sidewall can be more outstanding as the diameter of the LED chip reduces from 40 to 20 μ m . The enhanced EQE of the micro-LEDs with smaller inclined chip sidewall angles can be attributed to the stronger reflection of the inclined sidewall, leading to enhanced light extraction efficiency (LEE). In the end, the numerical optical modeling further reveals and verifies the impact of the sidewall angles on the LEE of the micro-LEDs, corroborating our experiment results. This Letter provides a fundamental understanding of the light extraction behavior with optimized chip geometry to design and fabricate highly efficient micro-LEDs in a DUV spectrum of the future.
In this paper, a recently conducted measurement campaign for high-speed-train (HST) channels is introduced, where the downlink signals of an in-service Universal Mobile Terrestrial System (UMTS) deployed along an HST railway between Beijing and Shanghai were acquired. The channel impulse responses (CIRs) are extracted from the data received in the common pilot channels (CPICHs). Within 1318 km, 144 base stations (BSs) were detected. Multipath components (MPCs) estimated from the CIRs are clustered and associated across the time slots. The results show that, limited by the sounding bandwidth of 3.84 MHz, most of the channels contain a single line-of-sight (LoS) cluster, and the rest consists of several LoS clusters due to distributed antennas, leaking cable, or neighboring BSs sharing the same CPICH. A new geometry-based random-cluster model is established for the clusters' behavior in delay and Doppler domains. Different from conventional models, the time-evolving behaviors of clusters are characterized by random geometrical parameters, i.e., the relative position of BS to railway, and the train speed. The distributions of these parameters, and the per-cluster path loss, shadowing, delay, and Doppler spreads, are extracted from the measurement data. Index Terms-Geometry-based stochastic model, high speed train, random cluster model, time-variant channel, Universal Mobile Terrestrial System.
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