(Invited) Enhanced Efficiency of AlInN Nanowire Ultraviolet Light-Emitting Diodes Using Photonic Crystal Structures

Abstract

High efficiency ultraviolet (UV) light-emitting diodes (LEDs) are in high demand for a wide range of applications [1-3]. Due to its wide bandgap, covering from deep UV to near visible, AlGaN semiconductor has been intensively studied for high efficiency UV LEDs. However, the performance of AlGaN UV LEDs is still limited, especially in the deep UV wavelength region, due to large dislocation density and extremely inefficient p-type doping in AlGaN film. Therefore, AlGaN UV LEDs have been suffering from low internal quantum efficiency (IQE), poor light extraction efficiency (LEE), and low external quantum efficiency (EQE). Alternatively, AlInN alloy is relatively unexplored for light-emitters even though it holds great potential application in UV and visible light-emitting devices. Recent studies have reported that InAlN has a large optical gain for deep UV LEDs. In this context, we have successfully developed AlInN nanowire LEDs with strong emission in the UV wavelength regime. The AlInN nanowires achieved a high IQE of >50% and significantly strong Transverse Magnetic (TM) polarized emission at 299nm.The AlInN nanowire LED structures were spontaneously grown on n-Si (111) substrates under nitrogen rich conditions by a Veeco GEN II molecular beam epitaxy (MBE) system. The AlInN nanowire was grown on GaN nanowire template. The device active region consists of ~ 100 nm n-Al0.78In0.22N, 40 nm undoped Al0.75In0.25N well and 100 nm p-Al0.78In0.22N layers. The GaN nanowire segments were grown at ~ 800 °C, nitrogen flow rate of 0.5-1.0 sccm, and forward plasma power of ~ 400 W. However, the growth temperature of the active region was at ~ 670 ̶ 700 °C to increase the In incorporation in AlInN segments and the nitrogen flow rate was kept at 2.5 sccm. The MBE grown AlInN under nitrogen rich condition offers an effective approach to eliminate the composition inhomogeneity in the AlInN. The nanowires are vertically aligned to the Si substrate with quite uniform dimension, as illustrated in Figure 1(a). Figure 1(b) presents the structure of a single AlInN nanowire LED. The photoluminescence (PL) spectra of AlInN nanowires were measured using a 266 nm diode-pumped solid-state laser as the excitation source. The peak emissions vary from 280nm to 365nm by varying the Al/In composition in the AlInN layers. The peak emission is shifted to shorter wavelength when the substrate temperature gets increase which is attributed to the increased In adatom desorption at higher growth temperature, resulted in the reduced In composition in the AlInN segment. The AlInN/GaN nanowire exhibits relatively high IQE which is estimated of ~ 52% at room temperature [3, 4]. Illustrated in Figure 1(c), the UV nanowire LED holds strong emission spectra at ~299 nm with a negligible shift in the peak wavelength, attributing to the high crystalline quality and reduced quantum-confined Stark effect in the AlInN nanowires. The LED has a low turn-on voltage of ~5V and low reverse leakage current less, i.e., ~0.5 µA at -8.6V. Moreover, as shown in Figure 1(d), we have demonstrated that the LEE of the AlInN nanowire LEDs could reach ~ 63% using hexagonal photonic crystal nanowire structures which is significantly higher compared to that of the random nanowire arrays [5]. This study paves the way for the development of high-power ultraviolet light-emitters using AlInN nanowire arrays.