Structural analysis of GaSb/GaAs quantum rings for Solar Cells

Abstract

A type‐II GaSb/GaAs quantum dot (QD)/quantum ring (QR) solar cell (SC) achieves higher photo‐response than its type‐I counterpart [1], as it supports an enhanced carrier recombination rate due to a larger separation between the electron and hole confinements [2]. This behavior leads to greater valence band offset [2] and, eventually, the solar cell is also able to function well into the near infrared (NIR) regions [3]. The stacking of several GaSb/GaAs QDs layers within the SC is essential to increase the photon absorption, however these heterostructures face a large lattice mismatch (7.8%) that causes a high local strain [4]. Because of this, sometimes QDs tend to relax by diffusing Sb from its center, followed by As/Sb exchange, giving place to nanostructures in the form of quantum rings (QR). [5]. In this communication, we analyze a GaSb/GaAs structure grown at 480ºC by molecular beam epitaxy (MBE) on a GaAs substrate. The GaSb layers (2.1 ML) are capped by two consecutive GaAs layers (10nm at 480ºC and 30nm at 580ºC), and this whole segment is repeated 10 times. The structural properties of this sample have been analyzed by diffraction contrast Transmission Electron Microscopy (TEM) in a JEOL 2100 LaB 6 microscope, working at 200 kV. 220 bright field (BF) images of the GaSb layers have shown that no dislocations or other type of defects appear in the structure, and only some strain contrasts due to the lattice mismatch are observed. Fig. 1 shows a 002 dark field (DF) image of the sample that clearly shows two‐lobe shaped nanostructures corresponding to the presence of QR. We have found that the average diameter of the QR is 14 ± 5 nm, with average diameter of the individual lobes of 4±2 nm and an average height of 3±2 nm. Also, it is worth highlighting that a vertical stacking of the QR is not observed which is a consequence of a reduced propagation of the strain to the subsequent QR layers because of the large thickness of the GaAs barrier layers. High angle annular dark field (HAADF) analyses using an aberration corrected electron microscope are in progress in order to obtain more detailed information about the composition and the strain in these heterostructures.

Electronic properties of type-II $$\hbox {GaAs}_{1-x} \hbox {Sb}_{x}$$/GaAs quantum rings for applications in intermediate band solar cells

“…Correspondingly, QR heights are generally found to be in the range h = 3 ± 2 nm. 35 Despite incomplete control over the precise morphology of individual QRs during epitaxial growth, we emphasise that GaAs 1−x Sb x /GaAs QRs offer additional benefits for IBSC applications due to the nature of carrier localisation within these structures. To elucidate these properties requires a more detailed, quantitative analysis of the electronic properties, to which we now turn our attention.…”

“…2(f), the results of our numerical calculations suggest that optimum IBSC sub-band gaps are obtained in short GaSb/GaAs QRs having h ≈ 2 nm, which lies well within the range of heights observed in real structures. 35 By reducing the QR Sb composition x at fixed h to form alloyed GaAs 1−x Sb x /GaAs QRs, we find that E L increases by ≈ 45 -50 meV for each 10% reduction in x. Therefore, in QRs having reduced Sb composition -due, e.g., to interfacial Sb-As intermixing 38,39 -the QR height should be slightly increased in order to restore E L to its optimum energy.…”

“…For our k•p calculations we begin by considering an exemplar GaSb/GaAs QR having inner radius a 1 = 5 nm, outer radius a 2 = 11.5 nm and height h = 3 nm (dimensions typical of epitaxially grown QRs 35 ). The solid blue and red lines in Fig.…”