(Invited) Spatially–Resolved Luminescence Properties of Etched Quantum Well Microstructures
Authors: Landesman Jean-Pierre , Diak Ethan , LaPierre Ray , Levallois Christophe , Ghanad-Tavakoli Shahram 
Published: August 2024

Fabrication processes for photonic devices, especially micro- or nano-photonic devices, call for some kind of control of the materials’ optical properties at the stage of the final device. Our study focusses on measuring the optical quality of quantum well (QW) containing materials after device processing, using steps such as etching which allows fabrication of optical waveguides for example. We first designed and realized specific material structures based on an InP substrate, with a series of InAsxP1-x quantum wells such as illustrated on the figure. The samples were grown by gas-phase molecular beam epitaxy. The samples were characterized by spatially-resolved photoluminescence (PL) at low temperature (10 K). The top right part shows a typical spectrum measured from the sample we have grown: sharp lines associated with each QW can be clearly identified thanks to a careful grading of the As/P composition in each QW, while keeping its thickness to a constant value (typically 7 to 8 nm). Then we etched the samples (in the form of rectangular ridges as shown) using different etching processes, based either on wet etching or reactive ion etching (RIE), and recorded the PL spectra with high spatial resolution across the obtained structures. In parallel, the samples after etching were also characterized by cross-sectional scanning electron microscopy. Data processing from the series of PL spectra was then performed in order the characterize the effects of etching on the QW materials.

We show PL intensity profiles across a series of etched stripes of different widths resulting from a RIE process. These intensity profiles were obtained from the lines associated with the different QWs in the structure. One can observe, first of all, that the PL intensity gradually increases from the edges to the center of the stripes, over a range of up to 10 µm. One can also observe that for “wide” stripes, i.e. 30 or 50 µm, the PL intensity for each line saturates at the same level in the central part. However, for the narrower stripes (10 or 20 µm), the PL signal does not reach the same level. This is the signature of some PL-limitation phenomenon, which extends its effects far from the etched sidewalls. A “standard” phenomenon resulting solely from non-radiative recombination at the etched sidewalls is excluded. Non-radiative recombination can only affect regions which are less than one charge carrier diffusion length from the sidewalls. Diffusion length in our materials is less than 1 µm. Therefore, some kind of “long range” interaction seems to affect our samples. We propose that this long range interaction is due to some stray static electric field resulting from the presence of residual ions left by the etching process at the sidewalls. The electric field affects the PL intensity through dissociation of excitons. Moreover, we observed that different conditions for the etching process (wet etching versus RIE, crystallographic orientation of the stripes) impact the magnitude of this PL intensity quenching.

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Figure 1