Fabrication of Vertical Light-Emitting Diodes Using Wurtzite InP/Alinp Core-Multishell Nanowires Towards Yellow Wavelength
Authors: Zheng Ziye , Kimura Shun , Azuma Yuki , Tomioka Katsuhiro , Motohisa Junichi 
Published: December 2024

Light emitting-diodes (LEDs) using nanostructures has been attracted much attention in the field of next generation solid-state lighting technologies. At present, conventional LEDs are fabricated mainly using nitride-related materials in blue-green region and arsenide/phosphide-related materials in red region. However, both of those materials have issue in significant luminous efficiency drop in yellow wavelength region due to low crystal quality or indirect bandgap. Hence, developing of new candidate materials with high luminous efficiency in yellow region is urgent issue for high color rendering properties. Wurtzite (WZ) phosphide-related III-Vs nanowires (NWs) materials are expected to be the alternative candidate material because crystal phase transitioned WZ changes the band structure from indirect band gap to direct bandgap. Thus, WZ-AlInP alloy would possibly emit the bright yellow color light due to direct gap transition. All WZ InP/AlInP core-multishell (CMS) NWs have been reported in recent study [1]. However, the controllability of Al content for the WZ AlInP shell layer and the performance of WZ InP/AlInP CMS NWs-based LEDs were still uncleared. Here, we report on the selective-area growth (SAG) of WZ InP/AlInP CMS NWs with various Al content and demonstration of vertical NW-LEDs using the WZ InP/AlInP CMS NWs.

A 20-nm-thick SiO2 sputtered p-InP (111)A substrates were used for the selective-area growth. Periodical hexagonal openings were formed using electron beam lithography and wet chemical etching. The opening diameter was 30 – 100 nm and pitch was 400 – 2000 nm. Vertical core WZ InP NWs and AlInP shells were grown by low-pressure horizontal metalorganic vapor phase epitaxy (MOVPE). Trimethylindium (TMIn), tertiarybutylphosphine (TBP) and trimethylaluminum (TMAl) were used as source materials, and diethylzinc (DEZn) and monosilane (SiH4) were used for p- and n- type dopants. The carrier concentration of the p-AlInP and n-AlInP were 4×1018 cm-3 and 2×1015 cm-3. The growth temperature for InP NWs was 660ºC with V/III = 24 and growth time was for 400 sec, and AlInP shells were grown at 480 - 600 ºC for 5 min. The Al content in vapor phase varied from 20% - 40%. After the CMS NW growths, we fabricated the vertical NW-LEDs. The NWs were covered with benzocyclobutene (BCB) by spin-coating. The top portion of the NWs was exposed by reactive ion etching (RIE). Then, Ti (2nm)/ITO (200nm) and AuZn were evaporated on the top of NWs and backside of the substrates respectively. The samples were annealed at 420°C in N2 for 3 min.

In the NW-LED, we designed tubular AlInP-based double heterostructure with radial pn junction on the sidewalls of WZ InP core NWs. Also, AlInP shell was designed with Al-graded and without graded layer. The growth results of the InP/AlInP CMS NWs without/with Al-graded layers showed both NWs had hexagonal pillar structure surrounded with {-211} facets rotated 30° against to the cleavage {-110} planes, indicating the WZ crystal structure of the AlInP shell was transferred from WZ InP core successfully. NW-LEDs exhibited moderate rectification properties with small series resistance. The ideality factor (n) was 2.69 and 4.09 for the NW-LED without/with Al-graded layer, which were larger than the moderate ideality factor of conventional LED (n = 1 - 2). The large n indicates that the tunneling current through the thin AlInP shells layers. Both NW-LEDs showed EL at room temperature under various current injection. Peak deconvolution of the EL revealed that three peak origins were dominant in both NW-LEDs. The EL peaks were 1.41, 1.48 and 1.54 eV for NW-LEDs without Al-graded layer. And the EL peaks were 1.41, 1.50 and 1.58 eV for NW-LED with Al-graded layer. The main EL peaks at 1.48 eV and 1.50 eV were originated from WZ AlInP shell active layers. The EL intensity from WZ-AlInP shell with Al-graded layer was higher than that of the shell without Al-graded layer. Further investigation of the Al controllability for InP/AlInP CMS NWs and the NW-LED devices toward visible color emission will be discussed.

[1]. F. Ishizaka et al., Nano Lett. 17, 1350 (2017).