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Reflectance anisotropy spectroscopy (RAS), which was originally invented to monitor
epitaxial growth, can—as we have previously shown—also be used to monitor the reactive ion
etching of III/V semiconductor samples in situ and in real time, as long as the etching rate is not
too high and the abrasion at the etch front is not totally chaotic. Moreover, we have proven that—
using RAS equipment and optical Fabry-Perot oscillations due to the ever-shrinking thickness of the
uppermost etched layer—the in situ etch-depth resolution can be as good as +/-0.8 nm, employing a
Vernier-scale type measurement and evaluation procedure. Nominally, this amounts to +/-1.3 lattice
constants in our exemplary material system, AlGaAsSb, on a GaAs or GaSb substrate. In this
contribution, we show that resolutions of about +/-5.6 nm can be reliably achieved without a Vernier
scale protocol by employing thin doped layers or sharp interfaces between differently doped layers
or quantum-dot (QD) layers as etch-stop indicators. These indicator layers can either be added
to the device layer design on purpose or be part of it incidentally due to the functionality of the
device. For typical etch rates in the range of 0.7 to 1.3 nm/s (that is, about 40 to 80 nm/min), the RAS
spectrum will show a distinct change even for very thin indicator layers, which allows for the precise
termination of the etch run.
We present a study of optoelectronically active Ga(As)As quantum dots (QDs) on Al-rich AlxGa1-xAs layers with Al concentrations
up to x=90%. So far, however, it has not been possible to grow optoelectronically active Ga(As)As QDs epitaxially
directly on and in between Al-rich barrier layers in the AlGaInAsSb material system. A QD morphology might appear on the
growth front, but the QD-like entities will not luminesce. Here, we use photoluminescence (PL) measurements to show that thin
Al-free capsule layers between Al-rich barrier layers and the QD layers can solve this problem; this way, the QDs become
optoelectronically active; that is, the dots become QDs. We consider antimonide QDs, that is, Ga(As)Sb QDs, either on GaAs for
comparison or on AlxGa1-xAs barriers (x >10%) with GaAs capsule layers in between. We also discuss the influence of QD
coupling both due to stress/strain from neighboring QDs and quantum-mechanically on the wavelength of the photoluminescence
peak. Due to their mere existence, the capsule layers alter the barriers by becoming part of them. Quantum dots
applications such as QD semiconductor lasers for spectroscopy or QDs as binary storage cells will profit from this additional
degree of design freedom.