KAUST team reveals thermodynamic disorder in GaN-based nanowires

25th April 2017

New study shows the thermodynamic entropy behaviour of InGaN/GaN nanowires

Figure 1: (a) Schematic and layer structure of the InGaN/GaN p-i-n nanowires, (b) plan-view, and (c) elevation-view SEM images of the nanowires.

While GaN-based nanowires have the potential for realising integrated optoelectronic systems because of their high quantum efficiencies, they are also expected to pave the way for new photodetector device architectures and improved photosensitivity.

GaN-based p-i-n power devices based on nanowires are suitable for attenuators, high-frequency switches, as well as photodetector applications. However, non-radiative recombination affects their performance. 

Recently, researchers from the King Abdullah University of Science and Technology (KAUST) have been looking at the operational and thermal stability of group-III-nitride nanowires to find out more about their applicability in multifunctional applications.

Led by Xiaohang Li, Iman S. Roqan, and Boon S. Ooi, the team studied the photoinduced entropy of InGaN/GaN p-i-n double-heterostructure nanowires (shown above) using temperature-dependent photoluminescence.

They defined the photoinduced entropy as a thermodynamic quantity that represents the unavailability of a system’s energy for conversion into useful work due to carrier recombination and photon emission. They have also related the change in entropy generation to the change in photocarrier dynamics in the InGaN active regions using results from time-resolved photoluminescence study. 

They hypothesised that the amount of generated randomness in the InGaN layers in the nanowires eventually increases as the temperature approaches room temperature.

To study the photoinduced entropy, the scientists have developed a mathematical model that considers the net energy exchange resulting from photoexcitation and photoluminescence. Using this approach, they observed an increasing trend in the amount of generated photoinduced entropy of the system above 250K, while below 250K, they observed an oscillatory trend in the generated entropy of the system that stabilises between 200 and 250K.

The decrease in the total recombination lifetimes with increasing temperatures reflects the fact that non-radiative recombination lifetime decreases, which the scientists attributed to the presence of surface defects on the nanowires.

They also attributed the increase in total recombination lifetime at low temperatures to the thermal annihilation of non-radiative recombination centres, and hypothesised that non-radiative recombination due to the activation of non-radiative recombination channels contributes to the overall increasing trend in the entropy above 250K.

“Since the entropy of a system sets an upper limit on the operational efficiency of a photoluminescent device, our study provides a qualitative description of the evolution in thermodynamic entropy generation in GaN-based nanowires,” Nasir Alfaraj, a PhD student in Li’s group and the paper’s first author, tells Compound Semiconductor. “Our findings will enable researchers working on developing and fabricating devices operating at various temperatures to better predict efficiency limitations.”

The researchers plan to further investigate the photoinduced entropy in other materials and types of structure. They also plan to present comparisons between samples with varied nanowire diameters and thin films of different materials. They stressed that they are open to collaborations with researchers who are interested in similar properties of their samples.

This collaborative work, which received financial support from KAUST and King Abdulaziz City for Science and Technology (KACST) is detailed in the paper: 'Photoinduced entropy of InGaN/GaN p-i-n double-heterostructure nanowires', from Applied Physics Letters, 110 (16) (2017).

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