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Posted: May 03, 2017

Thermodynamic disorder in GaN-based nanowires

(Nanowerk News) While GaN-based nanowires have the potential for realizing integrated optoelectronic systems because of their high quantum efficiencies, they are also expected to pave the way for new photodetector device architectures and improve photosensitivity.
GaN-based p-i-n power devices based on nanowires are suitable for attenuators, high-frequency switches, as well as photodetector applications. Nevertheless, non-radiative recombination affects the performance of devices based on such nanowires.
This issue necessitates a more in-depth understanding of the opto-electrothermal properties of group-III-nitride nanowires through a new thermodynamic perspective is necessary to further understand their applicability and operational and thermal stability in multifunctional applications.
Recently, researchers from the King Abdullah University of Science and Technology (KAUST), led by Xiaohang Li, Boon S. Ooi, and Iman S. Roqan, studied the photoinduced entropy of InGaN/GaN p-i-n double-heterostructure nanowires (shown below) using temperature-dependent photoluminescence (Applied Physics Letters, "Photoinduced entropy of InGaN/GaN p-i-n double-heterostructure nanowires").
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 hypothesized that the amount of generated randomness in the InGaN layers as the nanowire eventually increases as the temperature approaches room temperature.
InGaN/GaN p-i-n nanowires
(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. (Image: KAUST)
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 250 K, while below 250 K, they observed an oscillatory trend in the generated entropy of the system that stabilizes between 200 and 250 K.
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 centers, and hypothesized that non-radiative recombination due to the activation of non-radiative recombination channels contributes to the overall increasing trend in the entropy above 250 K.
“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” says Nasir Alfaraj, a PhD student in Li’s group and the paper’s first author. “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. Specifically, entropy generation in AlGaN and ZnO nanowires are of interest. They also plan to present comparisons between samples with varied nanowire diameters and thin films of different materials.
Source: King Abdullah University of Science and Technology
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