Amina Solano Lopes Ribeiro


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Solano Lopes Ribeiro

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Amina

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0009-0004-2739-4537

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2024 - 20252
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Publications 1 - 2 of 2
  • Magnetocapacitance investigations in InAs/GaSb quantum wells separated by an AlSb barrier
    Item type: Doctoral Thesis
    Solano Lopes Ribeiro, Amina (2024)
    In this thesis we investigate the magnetocapacitance response in the Indium Arsenide (InAs) and Gallium Antimonide (GaSb) quantum wells (QWs) separated by an Aluminium Antimonide (AlSb) insulating barrier. The conduction band (CB) edge for InAs lies below the valence band (VB) edge for GaSb, forming an inverted band alignment regime. Confined electrons in the InAs and holes in the GaSb QWs form a two-dimensional electron gas ($2$DEG) and a two-dimensional hole gas ($2$DHG), respectively. Due to the band structure, these charge carrier systems are spatially separated, enabling the formation of an exciton between the two QWs, extended in the growth direction. We grew a series of InAs/AlSb/GaSb heterostructures using the technique of molecular beam epitaxy (MBE). The widths of the InAs and GaSb QWs was fixed to $50$\,nm, to assure an inverted band structure. As a result of the confined electron and hole $2$D gases spatially apart from each other, we can associate it with a parallel plate capacitor consisting of electrons (n-type) corresponding to one plate and holes (p-type) as the other plate. We vary the thickness of the AlSb middle barrier between ($5-200$)\,nm, aiming to tune the coupling between electrons and holes and consequently the excitonic interaction. To study the electrical properties of such a system, we investigated the capacitance, modulated by a DC bias applied between the layers, as a function of magnetic field. To control the etch process for the mesa pattern distinguishing both layers, we implemented a method based on the characteristic reflectance of the different semiconductor materials in our heterostructure. Their distinct color spectrum allowed for precise and reliable etching. Furthermore, by developing a technique to fabricate spatially independent electrical contacts to the InAs and GaSb layers individually, we were able to establish a capacitance measurement technique that is novel to this material system, to study the charge carrier response, as a function of magnetic field. Efforts in searching the suitable metals for the individual ohmic contacts to InAs and GaSb respectively were taken. Based on the current to voltage (IV) traces, the electrical characterization from p- to n-type layers performed on a Hall bar (HB) geometry and a capacitance pattern showed that titanium/gold (Ti/Au) for the p-type and titanium/platinum (Ti/Pt) for the n-type contact were the best options to minimize leakage current between the layers. A diode-like behavior, predominantly in the capacitance patterns, allowed for high-quality magnetocapacitance characterizations. It turned out that the HB structures had a larger capacitive area for the n-type layer compared to the p-type one. Also, the HB geometry usually showed an ohmic behavior, influencing the signal to noise ratio in the main magnetocapacitance experiment. We award attention to the $20$\,nm AlSb barrier. Rooted in the capacitive measurement geometry we developed, we found for the first time in the InAs/GaSb system, an out-of-plane oscillatory response in capacitance as a function of magnetic field, representing the density of states (DOS) of the system. This is in good similarity to the in-plane longitudinal resistance of a 2D-layer, which is governed by the Shubnikov-deHaas (SdH) effect. We were able to tune the charge carrier densities by applying a DC voltage bias, and demonstrated the formation of beating signatures for forward biases. The coexistence of two distinguishable two-dimensional charge carrier systems of unequal densities was verified. The corresponding Landau phase diagram presents distinct features originating from the two observed densities. A giant Rashba coefficient ranging from $430-612$\,meV$\text{\AA}$ and large \textit{g}-factor covering values of $23-45$ for the non conductive DC bias region underlines the influence of spin orbit interaction. Although a diode-like behavior was observed for most of the AlSb barrier widths, the fabrication methodology proved to be a key aspect to observe magnetocapacitance oscillations in the long run, being predominantly measured for the $20$ and $100$\,nm of AlSb barrier. Misfit dislocations of the AlSb-containing interfaces were investigated by producing a lamella film made with focused ion beam (FIB) technique. The periodicity of the crystal lattice is clear with trivial interface intermixing of atoms grounded on the corresponding scanning electron microscopy (SEM) imaging and fast Fourier analysis in the reciprocal space. No room for leakage effects were observed in the pictured lamella spots. The band alignments for the InAs/AlSb/GaSb heterostructure were simulated, evaluating the equilibrium (i.e. zero DC bias applied) and non equilibrium scenarios for different AlSb barrier thicknesses. Considering the simulations for the $40$\,nm barrier width as a reference, we define two cases: AlSb layer thicknesses wider than the reference covering from $40$ to $120$\,nm barrier width and narrower ones for $5$ up to $27$\,nm. Independent of the barrier width, there is a built-in electric field already for zero DC bias voltage predicted. For the wider AlSb thicknesses, the larger it is the smaller the electric field becomes, as expected. Surprisingly, the simulations predict a transition from inverted to non inverted band structure for very wide barriers. Additionally, for all simulated structures the quasi-fermi energies for electrons ($\varepsilon_{F}^{electron}$) and holes ($\varepsilon_{F}^{hole}$) are distinguishable in the non equilibrium regime and clearly addressed for the narrower case, increasing the separation with narrow barrier widths.
  • Magnetocapacitance signatures of electrons and holes in InAs/GaSb quantum wells separated by an AlSb barrier
    Item type: Journal Article
    Solano Lopes Ribeiro, Amina; Schott, Rüdiger; Reichl, Christian; et al. (2025)
    Physical Review Research
    We observed magnetocapacitance oscillations in InAs/GaSb quantum wells separated by a 20 nm AlSb middle barrier. By realizing independent ohmic contacts for electrons in InAs and holes in the GaSb layer, we found an out-of-plane oscillatory response in capacitance representing the density of states of this system. We were able to tune the charge carrier densities by applying a DC bias voltage, identifying the formation of beating signatures for forward bias. The coexistence of two distinguishable two-dimensional (2D) charge carrier systems of unequal densities was verified. By developing a theory to describe the magnetocapacitance response of the system, we were able to validate that we have an electron-hole bilayer system. The Landau phase diagram presents distinct features originating from the bilayer channel with the additional presence of a 2D electron system at the surface of the InAs layer.
Publications 1 - 2 of 2