Abstract
In this work, flip-chip ultraviolet light-emitting diodes (FCUV-LEDs) on patterned sapphire substrate (PSS) at 375 nm were grown by an atmospheric pressure metal organic chemical vapor deposition (AP-MOCVD). A specialized reactive plasma deposited (RPD) AlN nucleation layer was utilized on the PSS to enhance the quality of the epitaxial layer. By using high-resolution X-ray diffraction, the full-width at half-maximum of the rocking curve shows that the FCUV-LEDs with RPD AlN nucleation layer had better crystalline quality when compared to conventional GaN nucleation samples. From the transmission electron microscopy (TEM) image, it can be observed that the tip and incline portion of the pattern was smooth using the RPD AlN nucleation layer. The threading dislocation densities (TDDs) are reduced from 7 × 107 cm−2 to 2.5 × 107 cm−2 at the interface between the u-GaN layers for conventional and AlN PSS devices, respectively. As a result, a much higher light output power was achieved. The improvement of light output power at an injection current of 20 mA was enhanced by 30%. Further photoluminescence measurement and numerical simulation confirm such increase of output power can be attributed to the improvement of material quality and light extraction.
Highlights
The III-nitride GaN-based light-emitting diodes (LEDs) have been applied in many commercial products due to the wide emission spectra from green to ultraviolet (UV) [1,2,3]
Flip-chip ultraviolet light-emitting diodes (FCUVLEDs) on patterned sapphire substrate (PSS) at 375 nm were grown by an atmospheric pressure metal organic chemical vapor deposition (APMOCVD)
A specialized reactive plasma deposited (RPD) AlN nucleation layer was utilized on the PSS to enhance the quality of the epitaxial layer
Summary
The III-nitride GaN-based light-emitting diodes (LEDs) have been applied in many commercial products due to the wide emission spectra from green to ultraviolet (UV) [1,2,3]. The multiple quantum well (MQW) active regions in the UV LED usually contain less indium and fewer localized states for carriers to by-pass the defects This situation makes UV LEDs more sensitive to TD in the material than regular blue LEDs [9,10]. In order to increase the output power, reducing the TDDs is one of the critical issues, especially in the short wavelength UV-LEDs. Several suggestions for the reduction of TDs have been reported, including epitaxial lateral overgrowth (ELOG) [11,12], cantilever epitaxy (CE) [13], defect selective passivation [14], microscale SiNx or SiOx patterned mask [15,16], and the use of patterned sapphire substrate (PSS) [17,18,19]. These methods require additional complex photolithography and etching process
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