Correlation between Optical and Structural Characteristics in Coaxial GaInN/GaN Multiple Quantum Shell Nanowires with AlGaN Spacers

Abstract

High crystalline quality coaxial GaInN/GaN multiple quantum shells (MQSs) grown on dislocation-free nanowires are highly in demand for efficient white-/micro-light-emitting diodes (LEDs). Here, we propose an effective approach to improve the MQS quality during the selective growth by metal–organic chemical vapor deposition. By increasing the growth temperature of GaN barriers, the cathodoluminescent intensity yielded enhancements of 0.7 and 3.9 times in the samples with GaN and AlGaN spacers, respectively. Using an AlGaN spacer before increasing the barrier temperature, the decomposition of GaInN quantum wells was suppressed on all planes, resulting in a high internal quantum efficiency up to 69%. As revealed by scanning transmission electron microscopy (STEM) characterization, the high barrier growth temperature allowed to achieve a clear interface between GaInN quantum wells and GaN quantum barriers on the c-, r-, and m-planes of the nanowires. Moreover, the correlation between the In incorporation and structure features in MQS was quantitatively assessed based on the STEM energy-dispersive X-ray spectroscopy mapping and line-scan profiles of In and Al fractions. Ultimately, it was demonstrated that the unintentional In incorporation in GaN barriers was induced by the evaporation of predeposited In-rich particles during low-temperature growth of GaInN wells. Such residual In contamination was sufficiently inhibited by inserting low Al fraction (∼6%) AlGaN spacers after each GaInN well. During the growth of AlGaN spacers, AlN polycrystalline particles were deposited on the surrounding dummy substrate, which suppressed the evaporation of the predeposited In-rich particles. Thus, the presence of AlGaN spacers certainly improved the uniformity of In fraction through five GaInN quantum wells and reduced the diffusion of point defects from n-core to MQS active structures. The superior coaxial GaInN/GaN MQS structures with the AlGaN spacer are supposed to improve the emission efficiency in white-/micro-LEDs.

Identification of multi-color emission from coaxial GaInN/GaN multiple-quantum-shell nanowire LEDs

“…Compared with the EL spectra, CL emission peaks of the m-and r-planes are corresponding to the EL emission peak at 440 nm, as marked by the rectangle in Figures 2(a) and (c). On the other hand, the CL spectra of the r-and cplane indicates that the EL peak at 540 nm is contributed by the emission from both the r-and c-planes, since the In incorporation rate is higher in the apex region as confirmed through energy-dispersive X-ray spectroscopy line-scan profiles on nanowires [30]. However, a deviation was confirmed for the peaks at 440 and 405-430 nm in the EL and CL spectra, respectively.…”

“…Meanwhile, the Mg-doping level in the p-GaN shell is approximately 3  10 19 cm −3 , as confirmed through three-dimensional atom probe tomography. Detailed processes about the growth of the n-GaN core and MQS active layer can be found in our previous work [30]. To investigate the optical and electrical properties in nanowire-based LEDs, six samples were prepared under different growth conditions of the MQS layer, as listed in Table 1.…”

“…In samples c, d, and e, AlGaN spacers with an Al composition of ~6% were inserted after each GaInN well growth. In this study, AlGaN spacers were used to improve the crystalline quality and suppress In decomposition of GaInN wells when ramping up the growth temperature for GaN barriers [30]. To further confirm the emission features in nanowire-based LEDs, sample e was prepared at a lower growth temperature (740°C) for GaInN wells.…”

“…In such core-shell nanowires, a high internal quantum efficiency (IQE) of approximately 60% was demonstrated at room temperature (300 K), which is higher than that of c-plane LEDs in a similar visible wavelength range [29]. Besides, IQE in MQS nanowires has also been improved to 69% by adopting an AlGaN spacer after each GaInN quantum well growth and a high growth temperature for GaN quantum barriers [30].…”

Growth Defects in InGaN‐Based Multiple‐Quantum‐Shell Nanowires with Si‐Doped GaN Cap Layers and Tunnel Junctions

“…These V-pits may be due to the TDs generated from the c-plane active layers. [41][42][43] Figure 3a1, a2 shows the SEM images of the NW-MQS structure grown up to the TJ layers. The height and width of the NW-MQS structure are %1300 and 740 nm, respectively.…”