(Invited) Effect of the Plasma Etching on InAsP/InP Quantum Well Structures Measured through Low Temperature Micro-Photoluminescence and Cathodoluminescence
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Plasma etching is widely used for the fabrication of photonic devices with InP, GaAs, and their ternary or quaternary compounds. Generally speaking, these processes are rather well controlled in terms of the geometry and morphology of the fabricated features. However, the microscopic interaction between the etching ions / radicals and the semiconductors have not been paid much attention [1]. In order to investigate this issue, we design and grow dedicated quantum well (QW) samples. The samples are grown on InP substrates and include a series of strained InAsP QWs with different As/P compositions [1]. The luminescence from these samples is very intense and the lines associated with each QW are very sharp at low temperature, as illustrated in figure 1. Due to the different As/P compositions, each line can be related to a specific QW. The QWs are grown at different depths below the surface (between 300 nm and 1 µm). Thus, by examining the changes in shape and position of each line after plasma etching, it is possible to probe the interactions at different depths. In addition, we can also record micro-photoluminescence (µPL) spectra at very low temperature using an optical cryostat [2]. By choosing the excitation wavelength in the near infra-red (1064 nm), we avoid the presence of the InP-related luminescence lines on the spectra. This allows us to also investigate the lateral effects that plasma etching can produce e.g. on narrow stripes. Our etching experiments are performed using an inductively coupled plasma reactor, and different kinds of etching chemistries. As an example, we illustrate in figure 2 the different interactions taking place when etching a stripe (width: 50 µm) with CH4/H2 and Cl2/CH4/Ar. Figure 2 shows that the CH4/H2 etching broadens the PL lines for all QWs, whereas the Cl2/CH4/Ar in contrast yields much sharper lines than on the reference un-etched sample (figure 1). We have also investigated the intensity variation for each line across the ridges, and observed quantitative differences for the different etching processes. In another set of experiments, similar samples were measured using low temperature cathodoluminescence, while simultaneously biasing the samples at different positive and negative voltages (bias applied between surface and bulk). Strong peak shifts and broadening were observed, related to a quantum confined Stark effect [3], indicating the presence of charged species after some of the etching processes. These charged species interact with the built-in electric field in our samples. A mechanism describing the interaction between the plasma and the semiconductor material will be discussed based on the different luminescence measurements and mappings performed.[1] J.P. Landesman, J. Jiménez, C. Levallois, F. Pommereau, C. Frigeri, A. Torres, Y. Léger, A. Beck and A. Rhallabi, J. Vac. Sci. Technol. A, 34, 041304-1 (2016).[2] C. M. Haapamaki, PhD dissertation, McMaster University (2012).[3] L. Vina, E. E. Mendez, W. I. Wang, L. L. Chang, and L. Esaki, J. Phys. C: Solid State 20, 2803 (1987).Figure 1
