AlGaAs superlattice

AlGaAs superlattice

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AlGaAs: Optical constants and superlattice reflectivity

Configuration file
../demo/scout/algaas_superlattice/algaas_superlattice.sc2

Main window

What do you see?
In the upper left corner you see a sketch of the system: 10 AlGaAs double layers (layer 1 with thickness d1 and Al content x1, layer 2 with d2 and x2, respectively) are deposited on a GaAs substrate. The dielectric functions of the two AlGaAs versions are shown in the lower left corner. The dashed blue and red curves represent the real and imaginary part of the dielectric function of layer 1, the solid lines those of layer 2. The upper right corner displays the reflectance spectrum between 1 and 3 eV. In the section 'Parameters' you see the current values of the four parameters d1, d2, x1 and x2. Finally, there are two sliders for a graphical variation of d1 and x1.

What can you do?
Move the slider for d1 and watch how the reflectance spectrum changes. The superlattice can be considered as a Bragg reflector. It features a band of high reflectivity due to constructive interference of waves reflected at the many internal interfaces. The frequency position of the peak depends on d1, of course.

·	Move the slider to d1=0: Now the many layer system is collapsed to a single layer and you see a simple 'sine-like' interference pattern.

·	Slowly increasing d1 you can watch the Bragg peak moving in from the right.

·	Around 60 nm thickness a second peak appears at 3 eV, also moving to the left with increasing d1.

·	Besides the large peaks there are, especially at the left side, narrow 'regular' interference patterns. These are due to interference of waves reflected at the top of the layer stack and at the bottom, i.e. they represent the total thickness of the layer stack.

Now move d1 to 200 nm and start to investigate the influence of x1:

Slowly increase x1 and watch how the dielectric function of material 1 responds. Note the shift of the fundamental gap from 1.3 eV (GaAs) to higher values. A large number of parameters (33) are used to model the complex interband transition structure of this material. All these parameters are controlled as slave parameters by the Al content as the only master parameter.

·	Approaching the value 0.9 of x2, the dielectric functions get closer and closer.

·	Now look at the reflectance spectrum. While x1 gets close to x2, the Bragg peaks disappear and only the interfence pattern of a single thick layer is seen.

·	If you are an experienced SCOUT user you can open the menu and the list of fit parameters. Create sliders for d2 and x2 and generate rather complex reflectance spectra by varying d1, x1, d2 and x2.