Browsing by Author "Peng, Zhangbao"

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Interplay of carriers and deep-level recombination centers of 275-nm light-emitting diodes - Analysis on the parasitic peaks over wide ranges of temperature and injection density
(2019-08-05)
Wu, Tingzhu
;
Lin, Yue
;
Peng, Zhangbao
;
Chen, Huashan
;
Shangguan, Zhibin
The low luminance efficiency, poor reliability and parasitic peaks have greatly limited the commercialization of deep ultraviolet (DUV) light-emitting diodes. Tasks of identifying the culprits of these deficits are of paramount importance but remains unaccomplished. We employ the full-range temperature (20 K - 300 K) measurement on 275-nm DUV devices that subjected to a 15-hour current-stress aging. The results suggest that the primary culprit of fast luminous decay is the proliferation of non-radiative centers. The origins of two main parasitic peaks are identified. The 310-nm peak is considered to solely come from deep-level radiative centers (DLRCs) that only dwell in the active region. Whereas, the 400-nm peak is proven to be dual-sources. One is related to the DLRCs in the active region, which only can be observed at very low currents; the other emerging at higher currents are associated with similar kinds of DLRCs located in the p-region, which only are excited when electrons overflow. This new discovery also demonstrates that a thorough investigation on the interplay among carriers and various types of defects should be conducted on the basis of the measurement that is taken under a wide temperature range, as well as under a proper forward voltage. This is to let the quasi-Fermi level shift across deep defect levels, the band-edge, and to over-band, whereby these recombination sites are exposed to deficit, moderate and saturated electron environment so that their natures can be well tested. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Multi-Azimuth Failure Mechanisms in Phosphor-Coated White LEDs by Current Aging Stresses
(2018-04-01)
Peng, Zhangbao
;
Guo, Ziquan
;
Wu, Tingzhu
;
Zhuang, Peng
;
Ye, Zhicheng
We have experimentally analyzed multi-azimuth degradation mechanisms that govern failures of commercially-available high-power (1 Watt) phosphor-coated white (hppc-W) light-emitting diodes (LEDs) covered with peanut-shaped lenses under three current-stress aging (CSA) conditions. Comprehensive analyses focus on photometric, chromatic, electrical, thermal and packaging characteristics. At the packaging level, (a) the decrease of the phosphor-conversion efficiency; (b) the yellow-browning of the optical lens; and (c) the darkening of the silver-coated reflective layer deposited with extraneous chemical elements (e. g., C, O, Si, Mg, and Cu, respectively) contribute collectively to the integral degradation of the optical power. By contrast, Ohmic contacts, thermal properties, and angles of maximum intensity remain unchanged after 3840 h aging in three cases. Particularly at the chip level, the formation of point defects increases the number of non-radiative recombination centers, and thus decreases the optical power during aging stages. Nevertheless, in view of the change of the ideality factor, the Mg dopant activation and the annealing effect facilitate the increase of the optical power in two specific aging stages (192 h similar to 384 h and 768 h similar to 1536 h). This work offers a systematic guidance for the development of reliable LED-based light sources in general-lighting areas.
The Impact of Luminous Properties of Red, Green, and Blue Mini-LEDs on the Color Gamut
(2019-05-01)
Guo, Weijie
;
Chen, Nan
;
Lu, Hao
;
Su, Changwen
;
Lin, Yue
Color gamut is of the paramount importance in the display. Light-current-voltage(L-I-V) characteristics of red, green, and blue flip-chip mini-light-emitting diodes (LEDs) (100 mu m x 200 mu m) are investigated at temperatures similar to the operational temperatures. Both the ideality factor and the temperature dependence of external quantum efficiency (EQE) suggest that the nonradiative loss in red mini-LED is higher. We also illustrate the intensive lateral luminous intensity fluctuation for red mini-LED under the over-driving current by capturing the spatial emission mapping. The influence of temperature and driving current on the chromaticity coordinates of mini-LEDs is determined. Furthermore, we propose a drive-current algorithm to maximize the color gamut of the trichromatic mixed light at different temperatures. The results indicate that, for the application of self-emitting mini-LED displays, the maximum color gamut is more sensitive to green emission, whereas enhancing the radiative recombination in red mini-LED could contribute a lot to the improvement on net performance.