Quantum mechanical modeling of nanoscale light emitting diodes
Update: 2017-03-02 14:34:39      Author: yangjuan@csrc.ac.cn

Understanding the electroluminescence (EL) mechanism in nanoscale light emitting diodes (LED) devices is crucial to further advance the technology for more efficient lighting and enhanced communications. From the theoretical perspective, accurate description of the electrical-to-optical conversion processes is a challenging task, since the system is in the non-equilibrium state driven by an optical and electric field. The prevailing studies evaluate the performance of LED devices based on classical models, relying on parameters obtained either from experiments or first-principles calculations. However, these models fail to capture quantum phenomena and break down at the nanoscale. Chi-Yung Yam's group in CSRC formulate a quantum mechanical approach for modeling the EL processes in nanoscale LED. Based on non-equilibrium Green’s function quantum transport equations, interactions with the electromagnetic vacuum environment are included to describe electrically driven light emission in the devices. Taking into account the atomistic details and non-equilibrium statistics, EL spectra of nanoscale devices under different bias conditions can be simulated. Furthermore, the method offers the possibility of determining the radiation pattern and polarization of emitted light. This provides useful information for further improvement of the device performance as well as probing structural details in the junctions. The presented framework is illustrated by numerical simulations of a silicon nanowire LED device. EL spectra and emission characteristics of the nanowire device under different bias voltages are plotted in Fig.1.  

1.png

Figure 1 Electroluminescence spectrum of the silicon nanowire LED device for various forware bias voltages.

 

A single broad emission peak is observed corresponding to transitions from the conduction band to the valence band. They note that the intensity of photon emission in general increases with an applied bias voltage. For a low bias voltage, no light emission is observed. As the forward bias approaches the flat band position, electrons and holes are injected simultaneously from electrodes. For carriers injected into the device, they can either flow directly from source to drain electrodes or undergo inelastic scattering through electron–photon interactions. The latter results in radiative recombination and gives rise to photon emission. The emission intensity therefore increases substantially when the applied bias exceeds the built-in potential of the system. The frequency of emitted photons is mainly determined by DOS and the electron distribution in the conduction and valence bands.

 

For more information, please see the paper: “Quantum mechanical modeling the emission pattern and polarization of nanoscale light emitting diodes”, Wang, Rulin; Zhang, Yu; Bi, Fuzhen; Frauenheim, Thomas; Chen, GuanHua; Yam, ChiYung; Nanoscale 8, 13168-13173 (2016).

This research was supported by Development Fund of China Academy of Engineering Physics, NSFC and MOST.


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