Superlattice-Induced Variations in Morphological and Emission Properties of GaInN/GaN Multiquantum Nanowire-Based Micro-LEDs

Abstract

Core−shell GaInN/GaN multiquantum shell (MQS) nanowires (NWs) are gaining great attention for high-efficiency microlight-emitting diodes (micro-LEDs) owing to the minimized etching region on their sidewall, nonpolar or semipolar emission planes, and ultralow density of dislocations. In this study, we evaluated the changes in NW morphologies and the corresponding device properties induced by GaInN/GaN superlattice (SL) structures. The cathodoluminescence intensities of the samples with 20 and 40 pairs of SLs were about 2.2 and 3.4 times higher, respectively, than that of the sample without SLs. The high-resolution scanning transmission electron microscopy (STEM) inspection confirmed that the high growth temperature of SLs prevented growth in the semipolar plane region close to the n-GaN core. A similar phenomenon was also observed for the GaN quantum barriers of the semipolar MQS region under a high growth temperature of 810 °C. This phenomenon was ascribed to the passivation of the semipolar plane surface by hydrogen atoms and the high probability of decomposition through NH 3 or N−H-related bonds. Although no clear SL grew on the semipolar plane near the n-core region, the top area of the nonpolar plane SL was expected to adequately suppress the point defects propagating from the n-GaN core to the semipolar plane MQS. The electroluminescence (EL) spectra and light output curves demonstrated a clear enhancement of more than 3-folds compared to the fabricated micro-LEDs without SL structures, which was associated with the improved crystalline quality of the MQS and enlarged area of the semipolar planes. Moreover, by increasing the growth time of GaN quantum barriers, the EL emission intensity of the micro-LED devices exhibited a 4-fold improvement owing to the reduced carrier overflow in the thickened GaN barriers on the semipolar (11̅ 01) planes. Thus, the results verified the possibility of realizing highly efficient NW-based micro-LEDs by optimizing the NW morphology using SL structures.

Investigation of emission plane control in GaInN/GaN multiple-quantum shells for efficient nanowire-based LEDs

“…This is attributable to the lower growth rate on the (11̄01)-plane than on the (0001) and (101̄0)-planes, since the outermost surface of the (11̄01)-plane is composed of N atoms and easily passivated by hydrogen atoms. 31,41 At a lower growth temperature, the thicker SL shell was grown on (11̄01)-planes, resulting in the larger (0001)-plane and smaller (11̄01)-plane. Consequently, the NW heights excluding p-GaN in samples a, b, and c were 1140, 1190, and 1050 nm, respectively.…”

“…Aerwards, 5 pairs of MQS structures were coaxially grown in all the samples by using optimized growth sequences, as reported in our previous work. 31,37 In samples j-p, the growth conditions of the SL were identical to that of sample a. The p-GaN shell on samples a-i and m-p was grown under the same conditions, including two stages at a constant growth temperature of 930 °C.…”

“… 30 For NW-LEDs, our research laboratory has also reported a 3.1-fold enhancement in electroluminescence (EL) intensity consequent to the introduction of SLs. 31 This was ascribed to the effect of the SL in suppressing the propagation of point defects, alongside alteration in the emitting plane due to the modifications in the shape of the NWs prompted by the incorporation of the SL. Additionally, there have been reports indicating that a higher InN mole fraction within GaInN/GaN SLs promotes the entrapment of point defects.…”

“…As the SL growth temperature decreased, the (0001)-plane of the NWs became larger and the (1 101)-plane shrunk. This is attributable to the lower growth rate on the (1 101)-plane than on the (0001) and (10 10)-planes, since the outermost surface of the (1101)-plane is composed of N atoms and easily passivated by hydrogen atoms 31,41. At a lower growth temperature, the thicker SL shell was grown on (1 101)-planes, resulting in the larger (0001)-plane and smaller (1 101)-plane.…”

Establishment of process to suppress (0001)-plane emission by introducing EBL in GaInN/GaN multi-quantum shells/nanowires for efficient 480 nm-LEDs

“…The crystal growth and device fabrication flow are shown in Figs 2(a)-(j). Si-doped n-GaN core NWs, 20 pairs of GaN/GaInN superlattice layers, and five pairs of GaInN/GaN MQS with AlGaN spacers were successively grown [36,37] . As shown in Table 1, samples including p-AlGaN EBLs with 23% Al composition were grown for 0 s (samples a and d), 30 s (sample b), and 60 s (samples c and e).…”