Little known MBE facts: Growth rate (2): Super-lattices

Faebian Bastiman

Superlattices (SLs) were first discovered by their rather unique X-ray diffraction properties, and so it should come as no surprise that SL creation and XRD characterisation continue to go hand in hand. Rather than get into a semantic debate about where a SL starts and a multi-quantum-well MQW structure ends let’s concentrate on designing a useful structure to determine our growth rate. We want the XRD plot to have well defined satellite peaks so we will want >15 periods and we probably want to average over at least 4 satellite peaks (2 negative and 2 positive) so we will want a period of >10nm. A useful test structure is shown in figure 1 (below) which has the SL nomenclature [GaAs10AlAs10]15.

The 004 ω-2θ scan of this structure is shown in figure 2. A simple peak splitting and periodicity analysis can tell us our growth rates. So what we have just done with 300nm of deposited material is to calibrate both the Ga and Al growth rate. You can do that with 30ML of material and RHEED oscillations. So why bother?

 Well the fact is the SL is telling you more. Firstly, you can vary the thickness of the two layers, whilst even maintaining the same period if you wish, and that will allow you to establish exactly what the shutter transients are doing. Secondly the quality of the XRD can give indications on the interfacial roughness and structural quality of the SL. Thirdly, you can also do RT-PL on the structure which can quickly tell you the width of the GaAs layer from the peak wavelength (just ensure the AlAs is thick enough to avoid wave-function over lap!) and that can allow you to gauge the opto-electronic quality of the GaAs:  a useful little test structure indeed.

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