A nitride-based light-emitting structure composed of a GaN nanowire core and a GaInN/GaN multi-quantum shell (MQS) is promising for high performance optoelectronic devices. By growing high crystalline quality MQS on the nonpolar (m-plane) sidewall of the nanowires, an improvement of luminous efficiency is expected. In this work, we induced the sputtering growth of n++-GaN shell on the tunnel junction/p-GaN/MQS/nanowire structures. By performing sputtering by the optimized condition, we were able to demonstrate a device with an operating voltage of about 1.0 V lower than that of the sample without sputtering.
Suppressing the emission from (0001) plane region with low luminescence efficiency that exists on the top of nanowires (NWs) is necessary to realize highly efficient GaInN/GaN multi quantum shell (MQS) light-emitting diodes (LEDs). In this study, we attempted to improve the crystal growth by introducing electron blocking layers (EBLs) and to suppress the current leak in NW LEDs by using SiO2 insulating film on top of NWs. The EBL structure features different thicknesses at each crystalline plane to reduce the current injection into (0001) plane, suppress red emission with low luminescence efficiency, and improve the light output by 2.4 times. Given that the luminescence from (0001) plane region remains, further optimization of EBL growth conditions, such as V/III ratio and Al composition, is essential. In addition, the luminescence from (0001) plane region and the current leakage can be reduced by forming SiO2 insulating film on top of the NWs. Although even if an insulating film was formed on the top of NWs on which EBLs were grown, the SiO2 adhering to (1-101) plane resulted in a decrease in light output and destruction due to high resistance. The results indicate the possibility of realizing highly efficient GaInN/GaN MQS-NW LEDs by inserting EBL structures between MQS and p- GaN shell.
Current injection through tunnel junctions (TJs) can enhance the external quantum efficiency of nanowire (NW) and multi-quantum-shell-based optical devices, compared. However, control of the impurity concentration profile is difficult in such tiny structure. In this study, we show a simple evaluation method of impurities in TJs growing flatly on m-plane GaN substrates, which have the same crystalline orientation as the luminescent surface of MQS/NWs. It was found to decrease the differential resistance by increase the concentration of Mg in p^(++)-GaN in the TJ.
GaInN/GaN multi-quantum shells nanowires (NWs) are coaxially grown in non-polar m-plane or semi-polar r-plane surface, which is expected to improve the luminous efficiency. The emission wavelengths usually redshift from the sidewall to top c-plane region. However, the emission from c-plane has low luminous efficiency. In this research, the c-plane area of NWs in one sample was removed by dry etching prior to the fabrication process, while the other one without c-plane etching was prepared to investigate the effect of c-plane region on the luminescence intensity. The sample with etching shows 12 times higher output power than the sample without etching.
A nitride-based light-emitting structure composed of a GaN nanowire core and GaInN/GaN multi-quantum shells (MQSs) is promising for high performance optoelectronic devices. By growing high crystalline quality MQS on the nonpolar (m-plane) sidewall of the nanowires, an improvement of luminous efficiency is expected. For Mg activation in p-GaN under the tunnel junction is a big challenge, in this work, we carried out the sputtering growth of n-GaN capping layer on the tunnel junction/p-GaN/MQS/nanowire structures for the first time. Single crystalline n-GaN was successfully grown mainly on the tip of the nanostructures.
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