Abstract
We investigated metal-organic vapor phase epitaxy grown (InGa)(AsSb)/GaAs/GaP Stranski–Krastanov quantum dots (QDs) with potential applications in QD-Flash memories by cross-sectional scanning tunneling microscopy (X-STM) and atom probe tomography (APT). The combination of X-STM and APT is a very powerful approach to study semiconductor heterostructures with atomic resolution, which provides detailed structural and compositional information on the system. The rather small QDs are found to be of truncated pyramid shape with a very small top facet and occur in our sample with a very high density of ∼4 × 1011 cm−2. APT experiments revealed that the QDs are GaAs rich with smaller amounts of In and Sb. Finite element (FE) simulations are performed using structural data from X-STM to calculate the lattice constant and the outward relaxation of the cleaved surface. The composition of the QDs is estimated by combining the results from X-STM and the FE simulations, yielding ∼InxGa1 − xAs1 − ySby, where x = 0.25–0.30 and y = 0.10–0.15. Noticeably, the reported composition is in good agreement with the experimental results obtained by APT, previous optical, electrical, and theoretical analysis carried out on this material system. This confirms that the InGaSb and GaAs layers involved in the QD formation have strongly intermixed. A detailed analysis of the QD capping layer shows the segregation of Sb and In from the QD layer, where both APT and X-STM show that the Sb mainly resides outside the QDs proving that Sb has mainly acted as a surfactant during the dot formation. Our structural and compositional analysis provides a valuable insight into this novel QD system and a path for further growth optimization to improve the storage time of the QD-Flash memory devices.
Highlights
The compositional analysis of quantum dots (QDs) by atom probe tomography (APT) is given in “QDs: composition” section, which is further supported by the Finite element (FE) simulations performed to fit the local lattice constant and outward relaxation of the cleaved QD
QDs: size and shape The growth started with a GaP buffer layer followed by nm of AlP barrier layer to increase the hole localization energy
QDs is important as the QD-Flash storage time strongly depends on localization potential and capture crosssection[25], which in turn depends on the size, shape, and composition of the QDs
Summary
Optoelectronic devices with self-assembled quantum dots (QDs) as an active medium have shown superior properties in many applications, such as semiconductor lasers[1,2], single and entangled photon emitters[3,4,5,6,7,8,9,10,11,12], solar cells[13], and quantum information technology[14,15,16,17,18,19]. The write time of the QD-Flash device is limited only by the thermal capture of charge carriers into the QDs, which occurs on the order of picoseconds at room temperature. The erase time strongly depends on the localization energy and the applied bias. The storage time “τ” mainly depends on localization energy (depth of the localization potential) and the capture cross-section (σ∞-scattering probability of holes)[30,31,32]. High localization energies with low-capture cross-section are preferred to obtain long storage time. Both parameters are strongly influenced by the size, shape, and composition of the QDs, as the bigger dots increase the localization potential at the expense of increasing capture
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