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We investigated metal-organic vapor phase epitaxy grown droplet epitaxy (DE) and Stranski–Krastanov (SK) InAs/InP quantum dots (QDs) by cross-sectional scanning tunneling microscopy (X-STM). We present an atomic-scale comparison of structural characteristics of QDs grown by both growth methods proving that the DE yields more uniform and shape symmetric QDs. Both DE and SKQDs are found to be truncated pyramid-shaped with a large and sharp top facet. We report the formation of localized etch pits for the first time in InAs/InP DEQDs with atomic resolution. We discuss the droplet etching mechanism in detail to understand the formation of etch pits underneath the DEQDs. A summary of the effect of etch pit size and position on fine structure splitting (FSS) is provided via the k · p theory. Finite element (FE) simulations are performed to fit the experimental outward relaxation and lattice constant profiles of the cleaved QDs. The composition of QDs is estimated to be pure InAs obtained by combining both FE simulations and X-STM results. The preferential formation of {136} and {122} side facets was observed for the DEQDs. The formation of a DE wetting layer from As-P surface exchange is compared with the standard SKQDs wetting layer. The detailed structural characterization performed in this work provides valuable feedback for further growth optimization to obtain QDs with even lower FSS for applications in quantum technology.

We present a detailed atomic-resolution study of morphology and substrate etching mechanism in InAs/InP droplet epitaxy quantum dots (QDs) grown by metal–organic vapor phase epitaxy via cross-sectional scanning tunneling microscopy (X-STM). Two different etching processes are observed depending on the crystallization temperature: local drilling and long-range etching. In local drilling occurring at temperatures of ≤500 °C, the In droplet locally liquefies the InP underneath and the P atoms can easily diffuse out of the droplet to the edges. During crystallization, the As atoms diffuse into the droplet and crystallize at the solid–liquid interface, forming an InAs etch pit underneath the QD. In long-range etching, occurring at higher temperatures of >500 °C, the InP layer is destabilized and the In atoms from the surroundings migrate toward the droplet. The P atoms can easily escape from the surface into the vacuum, forming trenches around the QD. We show for the first time the formation of trenches and long-range etching in InAs/InP QDs with atomic resolution. Both etching processes can be suppressed by growing a thin layer of InGaAs prior to the droplet deposition. The QD composition is estimated by finite element modeling in combination with X-STM. The change in the morphology of QDs due to etching can strongly influence the fine structure splitting. Therefore, the current atomic-resolution study sheds light on the morphology and etching behavior as a function of crystallization temperature and provides a valuable insight into the formation of InAs/InP droplet epitaxy QDs which have potential applications in quantum information technologies.

We report a detailed structural characterization of single and double layers of InAs and InAlAs quantum dots (QDs) and their wetting layers (WLs) by atomic force microscopy (AFM) and cross-sectional scanning tunneling microscopy (X-STM). The X-STM analysis with atomic resolution showed that the InAlAs WL consists of two distinct layers: a bottom part where all the Al atoms of the InAlAs alloy settled, and a top part containing exclusively In and Ga atoms. The QDs formed from the InAlAs layer contains no Al atoms at all and lie on top of the Al-rich WL. In the double layers of QDs, the InAlAs QDs were used as a seed to influence the nucleation of the InAs QDs grown on top. A gradual decrease in the density of the top InAs QDs was observed in the AFM images with increasing thickness of the GaAs spacer. The X-STM images showed that both QDs layers were completely intermixed for a 2-nm-thick spacer, while effective strain-induced stacking of both types of QDs was observed for a GaAs spacer thickness of 4 nm. However, both QD layers were completely decoupled for a GaAs spacer thickness of 8 nm and could thus be treated as individual layers.

The excellent optoelectronic performance of lead halide perovskites has generated great interest in their fundamental properties. The polar nature of the perovskite lattice means that electron-lattice coupling is governed by the Fröhlich interaction. Still, considerable ambiguity exists regarding the phonon modes that participate in this crucial mechanism. Here, we use multiphonon Raman scattering and THz time-domain spectroscopy to investigate Fröhlich coupling in CsPbBr3. We identify a longitudinal optical phonon mode that dominates the interaction, and surmise that this mode effectively defines exciton-phonon scattering in CsPbBr3, and possibly similar materials. It is additionally revealed that the observed strength of the Fröhlich interaction is significantly higher than the expected intrinsic value for CsPbBr3, and is likely enhanced by carrier localization in the colloidal perovskite nanocrystals. Our experiments also unearthed a dipole-related dielectric relaxation mechanism which may impact transport properties.

Highly tensile-strained Ge quantum dots (TS-Ge-QDs) emitting structures with different size were successfully grown on InP substrates by molecular beam epitaxy. Dislocation-free TS-Ge-QDs were observed by transmission electron microscopy. Finite element modeling indicates a maximum tensile strain of 4.5% in the Ge QDs, which is much larger than the required strain to achieve direct band gap conversion of Ge based on theoretical prediction. Photoluminescence (PL) from a direct band-gap-like transition of TS-Ge-QDs with a peak energy of 0.796 eV was achieved and confirmed by the etch depth-dependent PL, temperature-dependent PL, and excitation-power-dependent PL. In addition, a strong defect-related peak of 1 eV was observed at room temperature. The band structure of the TS-Ge-QDs emitting structures was calculated to support the experimental results of PL spectra. Achieving PL from direct band-gap-like transitions of TS-Ge-QDs provides encouraging evidence of this promising highly tensile strained semiconductor-nanostructure-based platform for future photonics applications such as integrated light sources.

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 ∼4×10^11 cm^−2. APT experiments revealed that the QDs are GaAs rich with smaller amounts of In and Sb. Finite element 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 finite element 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 quantum dot 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.

The fundamental understanding of quantum dot (QD) growth mechanism is essential to improve QD based optoelectronic devices. The size, shape, composition, and density of the QDs strongly influence the optoelectronic properties of the QDs. In this article, we present a detailed review on atomic-scale characterization of droplet epitaxy quantum dots by cross-sectional scanning tunneling microscopy (X-STM) and atom probe tomography (APT).We will discuss both strain-free GaAs/AlGaAs QDs and strained InAs/InP QDs grown by droplet epitaxy. The effects of various growth conditions on morphology and composition are presented. The efficiency of methods such as flushing technique is shown by comparing with conventional droplet epitaxy QDs to further gain control over QD height. A detailed characterization of etch pits in both QD systems is provided by X-STM and APT. This review presents an overview of detailed structural and compositional analysis that have assisted in improving the fabrication of QD based optoelectronic devices grown by droplet epitaxy.

Cross-sectional scanning tunneling microscopy (X-STM) was employed to characterize the InAs submono-layer quantum dots (SMLQDs) grown on top of a Si-doped GaAs(001) substrate in the presence of (2×4) andc(4×4) surface reconstructions. Multiple layers were grown under different conditions to study their effectson the formation, morphology, and local composition of the SMLQDs. The morphological and compositionalvariations in the SMLQDs were observed by both filled and empty-state imaging. A detailed analysis of indiumsegregation in the SMLQD layers was described by fitting the local indium-concentration profile with a standardsegregation model. We observed a strong influence of the arsenic flux over the indium incorporation andformation of the SMLQDs. We investigated the well-width fluctuations of the InGaAs quantum well in which theSMLQDs were formed. The well-width fluctuations were small compared to the more pronounced compositionfluctuations in all the layers. The lateral compositional variations lead to the formation of indium-rich clusterswhich can act as quantum dots and yield charge-carrier confinement.

Hydrogenation of nitrogen (N) doped GaAs allows for reversible tuning of the band gap and the creation of site controlled quantum dots through the manipulation of N−nH complexes, N−nH complexes, wherein a nitrogen atom is surrounded by n hydrogen (H) atoms. Here we employ cross-sectional scanning tunneling microscopy (X-STM) to study these complexes in the GaAs (110) surface at the atomic scale. In addition to that we performed density functional theory (DFT) calculations to determine the atomic properties of the N−nH complexes. We argue that at or near the (110) GaAs surface two H atoms from N−nH complexes dissociate as an H2 molecule. We observe multiple features related to the hydrogenation process, of which a subset is classified as N-1H complexes. These N-1H related features show an apparent reduction of the local density of states (LDOS), characteristic to N atoms in the GaAs (110) surface with an additional apparent localized enhancement of the LDOS located in one of three crystal directions. N−nH features can be manipulated with the STM tip. Showing in one case a switching behavior between two mirror-symmetric states and in another case a removal of the localized enhancement of the LDOS. The disappearance of the bright contrast is most likely a signature of the removal of an H atom from the N−nH complex.

While GeSn alloys with high Sn content constitute direct group-IV semiconductors, their growth on Si remains challenging. The deposition of a few monolayers of pure Sn on Ge and their overgrowth with Ge using molecular beam epitaxy can be a means of obtaining Sn-rich quantum wells with very high Sn content while maintaining high crystal quality. Here, we provide structural and compositional information on such structures with very high accuracy. Based on our characterization results we theoretically predict transition energies and compare them with experimental results from photoluminescence measurements. Our results constitute the groundwork for tuning the molecular beam epitaxy based growth of Sn-rich quantum wells and dots for applications in electronic and optoelectronic devices.

Dopant atoms can be incorporated into nanowires either via the vapor-liquid-solid mechanism through the catalyst droplet or by the vapor-solid growth on the sidewalls. Si is a typical n-type dopant for GaAs, but in nanowires it often suffers from a strongly amphoteric nature in the vapor-liquid-solid process. This issue can be avoided by using Te, which is a promising but less common alternative for n-type doping of GaAs nanowires. Here, we present a detailed investigation of Te-doped self-catalyzed GaAs nanowires. We use several complementary experimental techniques, such as atom probe tomography, off-axis electron holography, micro-Raman spectroscopy, and single-nanowire transport characterization, to assess the Te concentration, the free-electron concentration, and the built-in potential in Te-doped GaAs nanowires. By combing the experimental results with a theoretical model, we show that Te atoms are mainly incorporated by the vapor-liquid-solid process through the Ga droplet, which leads to both axial and radial dopant gradients due to Te diffusion inside the nanowires and competition between axial elongation and radial growth of nanowires. Furthermore, by comparing the free-electron concentration from Raman spectroscopy and the Te-atom concentrations from atom probe tomography, we show that the activation of Te donor atoms is 100% at a doping level of 4×1018cm-3, which is a significant result in terms of future device applications.

Dopant atoms can be incorporated into nanowires either via the vapor-liquid-solid mechanism through the catalyst droplet or by the vapor-solid growth on the sidewalls. Si is a typical n-type dopant for GaAs, but in nanowires it often suffers from a strongly amphoteric nature in the vapor-liquid-solid process. This issue can be avoided by using Te, which is a promising but less common alternative for n-type doping of GaAs nanowires. Here, we present a detailed investigation of Te-doped self-catalyzed GaAs nanowires. We use several complementary experimental techniques, such as atom probe tomography, off-axis electron holography, micro-Raman spectroscopy, and single-nanowire transport characterization, to assess the Te concentration, the free-electron concentration, and the built-in potential in Te-doped GaAs nanowires. By combing the experimental results with a theoretical model, we show that Te atoms are mainly incorporated by the vapor-liquid-solid process through the Ga droplet, which leads to both axial and radial dopant gradients due to Te diffusion inside the nanowires and competition between axial elongation and radial growth of nanowires. Furthermore, by comparing the free-electron concentration from Raman spectroscopy and the Te-atom concentrations from atom probe tomography, we show that the activation of Te donor atoms is 100% at a doping level of 4 x 10(18) cm(-3), which is a significant result in terms of future device applications.

The practical realization of epitaxial quantum dot (QD) nanocrystals led before long to impressive experimental advances in optoelectronic devices, as well as to the emergence of new technological fields. However, the necessary capping process is well-known to hinder a precise control of the QD morphology and therefore of the possible electronic structure required for certain applications. A straightforward approach is shown to tune the structural and optical properties of InAs/GaAs QDs without the need for any capping material different from GaAs or annealing process. The mere adjust of the capping rate allows controlling kinetically the QD dissolution process induced by the surface In-Ga intermixing taking place during overgrowth, determining the final metastable structure. While low capping rates make QDs evolve into more thermodynamically favorable quantum ring structures, increasing capping rates help preserve the QD height and shape, simultaneously improving the luminescence properties. Indeed, a linear relationship between capping rate and QD height is found, resulting in a complete preservation of the original QD geometry for rates above ∼2.0 ML s−1. In addition, the inhibition of In diffusion from the QDs top to the areas in between them yields thinner WLs, what could improve the performance of several QD-based optoelectronic devices.