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Scientific reports 8(1), 7514 (2018). doi:10.1038/s41598-018-25841-7, The understanding of the correlation between structural and photoluminescence (PL) properties of self-assembled semiconductor quantum dots (QDs), particularly InGaAs QDs grown on (001) GaAs substrates, is crucial for both fundamental research and optoelectronic device applications. So far structural and PL properties have been probed from two different epitaxial layers, namely top-capped and buried layers respectively. Here, we report for the first time both structural and PL measurements from an uncapped layer of InGaAs QDs to correlate directly composition, strain and shape of QDs with the optical properties. Synchrotron X-ray scattering measurements show migration of In atom from the apex of QDs giving systematic reduction of height and enlargement of QDs base in the capping process. The optical transitions show systematic reduction in the energy of ground state and the first excited state transition lines with increase in capping but the energy of the second excited state line remain unchanged. We also found that the excitons are confined at the base region of these elliptically shaped QDs showing an interesting volume-dependent confinement energy scaling of 0.3 instead of 0.67 expected for spherical dots. The presented method will help us tuning the growth of QDs to achieve desired optical properties., Published by Springer Nature, Cham

The growth of GeTe thin films on a Si(111)-(√3 × √3)R30°-Sb surface is reported. At growth onset, the rapid formation of fully relaxed crystalline GeTe(0001)-(1 × 1) is observed. During growth, a GeTe(0001)-(√3 × √3)R30° surface reconstruction is also detected. Indeed, density functional theory (DFT) simulations indicate that the reconstructed GeTe(0001)-(√3 × √3)R30° structure is energetically competing with the GeTe(0001)-(1 × 1) reconstruction. The out-of-plane α-GeTe ||Si and in-plane α-GeTe ||Si epitaxial relationships are confirmed by X-ray diffraction (XRD). Suppression of rotational twist and reduction of twinned domains are achieved. The formation of rotational domains in GeTe grown on Si(111)-(7 × 7) is explained by domain matched coincidence lattice formation with the Si(111)-(1 × 1) surface. Atomic force microscopy (AFM) images show the coalescence of well-oriented islands with subnanometer roughness on their top part. van der Pauw measurements are performed to verify the...

The growth by molecular beam epitaxy of GeTe thin films on highly lattice-mismatched Si(111) substrates is reported. In situ reflection high-energy electron diffraction and quadrupole mass spectrometry were employed to monitor the growth process in real time and tune the deposition conditions. Epitaxy was achieved in a window of substrate temperatures between 220 and 270 °C, using a Ge/Te flux ratio of ∼0.4. Extensive ex situ X-ray diffraction characterization showed that the epitaxial layers crystallize in the rhombohedrally distorted rocksalt structure α-GeTe, with orientation relationships to the substrate α-GeTe[0001] ‖ Si[111] and α-GeTe〈100〉 ‖ Si〈〉. ω-scans of the α-GeTe(000n) reflections (n = 3, 6, 9) exhibit a full width at half maximum between 10 and 20 arcsec, indicating small mosaicity. However, a large twist is observed (∼14°), pointing to the presence of rotational domains. In addition, the layer matrix is affected by twinning. The epitaxial films exhibit a slight tensile in-plane strain, which might be due to the difference in thermal expansion coefficients between α-GeTe and Si, and/or small deviation from stoichiometric composition, i.e., vacancies in the Ge sub-lattice.

GeTe is an end-point of the GeTe-Sb2Te3 quasibinary alloys often referred to as phase-change memory materials. The polycrystalline nature of the crystalline films used in devices and the concomitant presence of grain boundaries complicate detailed structural studies of the local structure. Recent progress in the epitaxial growth of phase-change materials offers unique possibilities for precise structural investigations. In this work, we report on results of x-ray absorption near-edge structure (XANES) studies of GeTe and Ge2Sb2Te5 epitaxial films grown on Si and InAs substrates with (100) and (111) orientations. The results show a strong dependence of the local structure on the substrate material and especially orientation and are discussed in conjunction with polycrystalline samples and ab-initio XANES simulations.

Epitaxial thin layers were successfully grown in the metastable cubic phase on both slightly lattice-mismatched (GaSb) and highly lattice-mismatched (Si) templates. The higher quality of the films grown on (111)-oriented substrates is attributed to the tendency to form layered structures in the stable bulk phase as well as to the nature of distortion in the metastable cubic phase. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Sb2Te3 films are used for studying the epitaxial registry between two-dimensionally bonded (2D) materials and three-dimensional bonded (3D) substrates. In contrast to the growth of 3D materials, it is found that the formation of coincidence lattices between Sb2Te3 and Si(111) depends on the geometry and dangling bonds of the reconstructed substrate surface. Furthermore, we show that the epitaxial registry can be influenced by controlling the Si(111) surface reconstruction and confirm the results for ultrathin films.

A narrow growth window combined with highly temperature dependent compositional variations poses a serious problem for the growth of epitaxial GeTe–Sb2Te3 (GST) thin films. The problems are further aggravated by the weak coupling of the radiatively heated non-contact thermocouples to the substrate. An increase in surface temperature during growth as inferred from the increase in desorption of GeTe heteromolecules and the resulting change in alloy composition are studied. Using the desorption signal as a feedback to control the surface temperature, the thermocouple temperature was varied over the duration of the growth to maintain a constant desorption and hence constant surface temperature. Interestingly, the composition of the grown films varies along the GeTe–Sb2Te3 pseudobinary line just by varying the desorption without changing the supplied flux. The out-of-plane lattice constant of the epitaxially grown GST thin film increases with an increase in Ge concentration.

Weak antilocalization and electron-electron interaction effects are investigated in Sb${}_{2}$Te${}_{3}$ layers. We accomplish smooth top and bottom surfaces for the layer using molecular-beam epitaxy, as revealed by the Kiessig oscillations in the x-ray reflectivity. The two helical surface states of the layer are found to contribute identically to the weak antilocalization effect. They are left intact in spite of low mobility and high concentration of unintentionally doped holes. The magnitude of the electron-electron interaction effect is consistent with the indication that both of the surface states survive in the layer. The robustness of the surface states demonstrates superiority of Sb${}_{2}$Te${}_{3}$ over Bi${}_{2}$Se${}_{3}$ and Bi${}_{2}$Te${}_{3}$. We also show that the phase-change property of Sb${}_{2}$Te${}_{3}$ provides controllability to switch the existence of the surface states.

Phase change materials along the GeTe-Sb2Te3 pseudobinary line (GST) are grown by molecular beam epitaxy (MBE) on Si(111). The growth on (111) oriented substrates leads to greatly increased crystal quality compared to (001) oriented substrates, even for a high lattice mismatch. This holds true even for Si substrates which have a lattice mismatch of around 10% with respect to GST. The growth is controlled in situ via line of sight quadrupole mass spectrometer (QMS). Structural characterization is performed in situ by X-ray diffraction (XRD), which reveals a clear cubic symmetry of the film and a lattice slightly rhombohedrally distorted along the [111] direction.

Epitaxial Ge2Sb2Te5 films grown on Si(111) by molecular beam epitaxy were reversibly switched between crystalline and amorphous states over a large area using femtosecond laser pulses. The structural and spatial homogeneity of the as-grown epitaxial and laser-switched areas on the sample were investigated by synchrotron nanofocus high resolution x-ray diffraction. The investigation clearly demonstrated that the single crystalline metastable cubic phase of Ge2Sb2Te5 is restored after switching. No polycrystalline features, not only on the average but even on the nanometer scale of the x-ray beam, were observed.