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Light emitting diodes are everywhere in today's technology. Most of them are built from Zinc-blende Structures, which covers the longer wavelength emission regimes. We used our tools to simulate such simple optical devices, and investigate it's efficiency limiting properties.


In the simulation we used a simple p-i-n diode structure. The highly doped regions are fabricated from GaAs material, while the doping on the n-side is $N_d^{+} = 1E25\frac{1}{m^3}$ and the acceptor density was set to $N_a^{-} = 1E25\frac{1}{m^3}$ . The non-doped region 'i' is set to $In_{0.8}Ga_{0.2}As$ alloy, which serves as an active region in the device, due its lower bandgap. The structure with geometrical properties is schematically drawn in figure 1.


Figure 1. Structure of the LED

Bandstructure at zero bias

The bandstructure of the device is plotted in figure 2. It shows that due to high doping, the fermi level is above the conduction band on the n-doped side.


Figure 2. Bandstructure at zero bias.

Active region

The active of th device is built from InGaAs alloy, due its lower bandgap, which results higher carrier concentration in that region. It is a quantum-well region, which creates a confinement for both electron and hole state. The ground state eigenfunctions for this confinement is plotted in figure 3. for electrons, and the heavy-holes.


Figure 3. Active region of the device with plotted electron and hole eigenfunctions in the quantum well.

Voltage Characteristics

If we apply bias on the device, we inject carriers to the Quantum well, and those could recombine which process results a photon. The radiative recombination process can be included in the solution of the carrier transport equations, alongside with other non-radiative recombination processes, such as SRH and Auger recombination in our simulation.

We can calculate the internal quantum efficiency of the device defined as: \begin{equation} IQE = \frac{\sum Radiative recombinations}{\sum All recombinations}, \end{equation}

which means we should integrate the full recombination and radiative recombinations in the device. And the ratio is the efficiency factor. The internal quantum efficiency is plotted in figure 4. around its maximum.


Figure 4. Internal quantum efficiency maximum.

Gain spectrum

For various bias voltages we can calculate absorption, emission, and gain spectrum of the active region in the device. For various bias voltages it is plotted in figure 5. here we neglected the imperfections in the quantum well, which would round down the edge of the emission/absorption curves.


Figure 5. Gain spectrum of the device for various bias voltages.
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  • Last modified: 2019/04/09 06:54
  • by zoltan.jehn