Stranski-Krastanov Islands Research Articles

Advanced preparation of plan-view specimens on a MEMS chip for in situ TEM heating experiments

In situ transmission electron microscopy (TEM) is a powerful tool for advanced material characterization. It allows real-time observation of structural evolution at the atomic level while applying different stimuli such as heat. However, the validity of analysis strongly depends on the quality of the specimen, which has to be prepared by thinning the bulk material to electron transparency while maintaining the pristine properties. To address this challenge, a novel method of TEM samples preparation in plan-view geometry was elaborated based on the combination of the wedge polishing technique and an enhanced focused ion beam (FIB) workflow. It involves primary mechanical thinning of a broad sample area from the backside followed by FIB-assisted installation on the MEMS-based sample carrier. The complete step-by-step guide is provided, and the method’s concept is discussed in detail making it easy to follow and adapt for diverse equipment. The presented approach opens the world of in situ TEM heating experiments for a vast variety of fragile materials. The principle and significant advantage of the proposed method are demonstrated by new insights into the stability and thermal-induced strain relaxation of Ge Stranski–Krastanov islands on Si during in situ TEM heating.Graphical abstract

Stability of heteroepitaxial coherent growth modes on nanowire radial surfaces

The thermodynamic stability of the coherent shell and Stranski-Krastanov heteroepitaxial growth modes on cylindrical nanowires is analyzed theoretically. In contrast to previous studies, the exact geometrical shape of pyramidal Stranski-Krastanov islands is considered for the calculation of surface- and elastic-energy contributions. In particular, the impact of the pyramid base angle on the relaxation energy is included. Moreover, the dependence of island facet surface-energy density on the distance of the surface to the nanowire substrate is taken into account. Maps of the growth-mode stability are derived for the Si-core/Ge-shell structure, which display the favored mode as a function of deposited volume, wetting-layer thickness, and nanowire radius. We show that for nanowire radii in the range between 15 and 100 nm, the Stranski-Krastanov mode becomes stable only for larger pyramid angles. In addition, it is found that the exact value of the surface-energy density significantly influences the transition between the two growth modes, leading to a stabilization of islands bound by low-energy crystal facets, as observed in experiments.

Surfactant-mediated Stranski-Krastanov islands

A criterion for the growth of Stranski–Krastanov islands mediated by surfactants is obtained. It involves that surfactants decrease the island nucleation barrier. Thereby, at a low coverage (θ ≪ 1 mono layer) surfactants facilitate islands formation. At a high coverage (θ ≈ 1 mono layer) the islands growth might be still facilitated, but also hampered under special conditions. The latter is caused by the reduction of the ad-atom surface diffusion. We report on a non-monotonic dependence of Ge/Si(100) island array density on the Sb coverage and discuss it in the frame of the obtained criterion.

Phase separation in strained epitaxial InGaN islands

Phase separation (PS) produces InN composition gradients in InGaN islands, which may be important for light emitting diodes, solar cells, and lasers. Thus, the control of PS is critical, and the kinetic growth process, which is suggested to be important for controlling PS in Stranski-Krastanov islands, becomes a key factor in producing materials for optoelectronic devices. We present atomistic-strain-model Monte Carlo simulations for PS in strained epitaxial InGaN islands. Our simulations illustrate how the PS in InGaN islands depends on the kinetic growth mode and subsurface diffusion, and thus suggest ideas for controlling the microstructure of alloy islands formed during epitaxial growth.

Kinetics of the evolution of InAs/GaAs quantum dots to quantum rings: A combined x-ray, atomic force microscopy, and photoluminescence study

We present an experimental study of the evolution of InAs/GaAs quantum dots partially capped with GaAs, as an annealing process transforms them first into quantum rings and later into holes penetrating the whole cap layer. Shape, composition, and optical emission were monitored as a function of annealing time by means of atomic force microscopy, x-ray photoemission microscopy, and photoluminescence, respectively. Our results show a progressive dissolution of the original dot, with the outdiffused material forming a second wetting layer on the planar region surrounding the dot. For the longest annealing time, in a situation close to thermodynamic equilibrium, no residual dot material is left in the holes, and an In-rich layer covers uniformly the surface. Our findings were examined by taking into account the surface chemical potential previously written for Stranski-Krastanov islands partially covered with a cap layer. We show that the energy gain due to the formation of the second wetting layer is the driving force for the shape and composition evolution, but its analytical expression should be modified, with respect to previous formulations, by taking into account the observed material interdiffusion.

Mass transport and alloying during InN growth on GaN by molecular-beam epitaxy

During Stranski-Krastanov (SK) growth of InN on GaN by molecular-beam epitaxy, a mass transport is noted from the two-dimensional wetting layer and/or the surface excess metal adlayers to the SK islands when the excess nitrogen flux is used for deposition. The extent of mass transport depends on the material coverage. For growth under the excess indium flux condition, no such mass transport is observed.

In situ monitoring of internal strain and height of InAs nanoislands grown on GaAs(001)

A monitoring technique for molecular beam epitaxial growth of InAs∕GaAs(001) nanoislands is presented. With the help of synchrotron radiation, x-ray diffraction intensity mapping in reciprocal space has been measured during growth. The internal strain distribution and height of the Stranski-Krastanov islands were monitored at a temporal resolution of 9.6s. The relaxation process of internal strain inside the Stranski-Krastanov islands displayed significant dependence on the growth temperature.

On the various impact of chemical composition and elastic strain in SiGe nanoscale islands to the diffuse X-ray scattering

High-resolution X-ray diffraction in combination with kinematical scattering simulations based on finite element calculations for the elastic strain field has been utilized to investigate the chemical composition of SiGe/Si(0 0 1) Stranski–Krastanov islands. Therefore, the intensity distribution in the vicinity of the symmetric (0 0 4) reflection has been three dimensionally measured and compared with respective simulations. Around an asymmetrical reflection, e.g. Si(1 1 3), the impact of lateral and vertical strain tensor components on the diffraction pattern decouples. This allows to distinguish between tetragonally distorted regions at the island bottom and elastically relaxed unit cells of cubic shape at the island apex in accordance with finite element calculations. Artificially switching off and on the various scattering abilities of silicon and germanium in otherwise equivalent simulations yields practically no difference in the resulting scattering. Thus, at X-ray energies far from absorption edges, the diffuse scattering is dominated by elastic strain which is caused by the chemical profile.

Size, shape, and ordering of SiGe/Si(001) islands grown by means of liquid phase epitaxy under far-nonequilibrium growth conditions

Applying scanning electron microscopy, we have studied the evolution of shape and lateral positional correlation of Si1−xGex/Si(001) Stranski–Krastanov islands grown by means of liquid phase epitaxy (LPE). However, in contrast to conventional near-equilibrium LPE, a distinctly higher cooling rate of 10 K/min ensures extremly nonequilibrium growth conditions. The facet inclination of subsequent island stages decreases from nearly vertical sidewalls toward {111}- and {101}-type facets. Energy dispersive x-ray microanalysis yields a size-independent germanium content of 8.9% within islands between 760 and 1700 nm base width which is—by more than a factor of 2—smaller than islands of the same concentration grown in a near-equilibrium LPE process. Square-like formations of subsequently smaller islands around a large central island indicate only next to island interactions during the lateral self-assembling.

High-density self-assembled GaN nanoislands on SiC (0001) by molecular-beam epitaxy

We report the fabrication of self-assembled coherent GaN nanoislands on Ga-adsorbed √3×√3R30° 6H–SiC(0001) surfaces via Stranski–Krastanov (SK) growth mode by molecular-beam epitaxy at 760°C. The initial GaN deposition reveals a stable two-dimensional growth (2D) mode under Ga-rich growth conditions (Ga∕N⪢1), as evidenced by the variation of in-plane misfit strain (Δa∕a0). We show that above ∼2.2 monolayer (ML) surface coverage, the 2D layer spontaneously transformed into SK islands by the accommodation of elastic strain in the epitaxial lattice misfit under a vacuum. The SK islands density is found to be ∼4×1011cm−2 and the mean height and diameter is 2±0.5nm and 20±5nm for 3.2 ML surface coverage, respectively.

Dynamically stable gallium-induced 3×3-SiC (0001) surface for two-dimensional GaN nucleation by molecular-beam epitaxy

Two-dimensional (2D) GaN nucleation on a gallium (Ga) induced 3×3-SiC (0001) surface was achieved by molecular-beam epitaxy. The Ga adsorbate on SiC (0001) revealed well-ordered ∛×∛−R30° and 3×3 surface reconstructions with respect to Ga adatom coverage. The 3×3 surface was found to be highly reactive toward 2D GaN nuclei. This enhances the surface mobility of impinging adatoms during GaN growth, which promotes a step-flow growth mode. On the ∛×∛−R30° surface, polygonal Stranski–Krastanov islands formed during the initial stages, coalescing as thickness increased. The GaN growth kinetics was found to depend on the Ga atomistic process of the reconstructed SiC (0001) surfaces.

GaN quantum dot density control by rf-plasma molecular beam epitaxy

We report on the growth of GaN quantum dots and the control of their density in the Stranski–Krastanov mode on AlN (0001) by rf-plasma molecular beam epitaxy at 750 °C. After depositing the equivalent of 2–3 ML GaN coverage, as limited by N fluence under Ga-droplet growth conditions, excess Ga was desorbed and Stranski–Krastanov islands formed under vacuum. We present the dependence of island density as a function of GaN coverage (for two growth rates: 0.10 and 0.23 ML/s), as estimated from atomic force microscopy and cross-sectional transmission electron microscopy. With a GaN growth rate of 0.23 ML/s, the island density was found to vary from less than 3.0×108–9.2×1010 cm−2 as the GaN coverage was varied from 2.2 (critical thickness) to 3.0 ML. For a GaN growth rate of 0.10 ML/s, the island density varied from 2.0×1010 to 7.0×1010 cm−2 over a GaN coverage range of 2.0–3.0 ML. For each growth rate, the GaN islands were found to be of nearly uniform size, independent of the quantum dot density.

Equilibrium Morphology in Two-Dimensional Heteroepitaxy on an Elastic Substrate

The equilibrium morphology of a two-dimensional (2D) heteroepitaxial adsorbate on a semi-infinite elastic substrate is studied by means of a hybridized method of stress relaxation dynamics in the epitaxial layer and the elastic Green's function in the substrate. With a small misfit f and a spontaneous surface stress σ 0 , the epitaxial adsorbate shows a Stranski–Krastanov (SK) configuration with a wetting first layer and islands on further layers. By comparing the results with a rigid substrate, we found that the softness of the substrate turns out to weaken the self-limiting effect of the island size, and the SK islands are found to coarsen indefinitely in the present 2D system.

Energetics in the growth mechanism of semiconductor heteroepitaxy

We phenomenologically investigate the growth behavior in lattice-mismatched heteroepitaxy. We focus on the critical thickness of misfit dislocations (MDs) in Volmer–Weber (VW) and Stranski–Krastanov (SK) modes. Calculated results show that critical thickness in VW mode is much smaller than that in SK mode. This is because energy loss in MD formation in VW islands is smaller than that in SK islands due to the absence of wetting layers during VW growth.

First principles and macroscopic theories of semiconductor epitaxial growth

We discuss the relation between microscopic mechanism and macroscopic growth behavior. First, we introduce a macroscopic theory of growth behavior in semiconductor heteroepitaxy that includes the effects of the formation of Stranski–Krastanov islands and the misfit dislocations. This theory can reproduce the various types of growth behavior observed in heteroepitaxial growth. Next, we have formulated a procedure for determining the phenomenological parameters that includes atomistic informations. The critical thickness of InAs/GaAs(1 1 0) heteroepitaxy obtained by this procedure is in good agreement with the scanning tunneling microscopy (STM) observations.

Evolution of strained Ge islands grown on Si(111): a scanning probe microscopy study

We have followed the evolution of strained Ge/Si(111) Stranski–Krastanov islands by atomic force and scanning tunneling microscopies. Following the morphological evolution during the annealing of the samples we were able to recognize the key features of the relaxation process in these structures. The introduction of edge misfit dislocations after a critical thickness, and the inhomogeneous strain field inside the islands, lead to an intra-island ripening mechanism. We show that this mechanism changes the island shape from truncated tetrahedron to “atoll-like”.

Atomistics of the epitaxial growth of Cu on W(110)

Time-resolved in situ STM has been used to study the epitaxial growth of Cu on W(110) at temperatures ranging from room temperature up to 300°C. Sequences of STM images directly show the atomistics of the growth process on the surface. Additionally, SPA-LEED has been used to obtain precise information about the lateral geometrical dimensions of the different developing Cu structures. A sequence of three different structures is needed to transfer the Cu film to its intrinsic fcc-like behavior. The first layer of Cu on W(110) grows pseudomorphically in a fractal geometry. According to this growth behavior strain effects are considered to play a predominant role. With the second Cu layer a strained structure originates which is divided into small domains by a periodic trench network. A structure model for this double layer is suggested. Depending on the deposition conditions the trench network is decorated by a complex chain structure of third layer islands. Beginning with the completion of the third layer a periodic dislocation network appears. This structure can be attributed to an fcc Cu(111) layer with a slightly expanded surface unit mesh. Beyond the fourth layer a Cu on Cu(111) growth is established, which is only slightly affected by the underlying transition layer. Higher temperature deposits show an incomplete triple layer with a nearly perfect chain structure indicating that a relaxed third layer is metastable. Cu in excess grows in wedge-shaped 3D Stranski–Krastanov islands with atomically smooth (111) surfaces even on a misoriented substrate.

Growth of direct bandgap GalnP quantum dots on GaP substrates

GaInP has a direct bandgap for In concentrations higher than approximately 30%, and the band-lineup between GaInP and GaP is type-II for In concentrations less than 60%. Therefore, in order to use GaInP as the active light-emitting layer in an optoelectronic device grown on GaP, the strain induced by the lattice mismatch between GaInP and GaP has to be somehow managed such that formation of crystal defects is suppressed. One method is to grow the layer thinner than the critical thickness. Another method that recently received much attention is to grow strain-induced Stranski-Krastanov islands (sometimes referred to as self-assembled quantum dots). Small droplets of highly latticmismatched materials have been embedded into single crystals without generating defects such as threading dislocations and stacking faults using this method. We have grown a series of GaInP/GaP layers by metalorganic chemical vapor deposition and have studied the light emission from them. Ordered GaInP islands were found to be responsible for the light emission. We present the light emission characteristics of these ordered GaInP/GaP islands, and their dependence on various growth parameters.

Self-assembled growth of Sn on Ge (001)

The growth of Sn layers on Ge (001) surfaces was studied with respect to the formation of Stranski–Krastanov islands due to the large lattice mismatch. Atomic force microscopy shows the transition to Stranski–Krastanov growth at an Sn coverage of 2.33 atomic monolayers on the surface. The high Sn segregation causes crystalline defects by overgrowing the Sn islands. Using Sb as a surfactant, it is possible to suppress the Stranski–Krastanov growth.

Photoluminescence and transmission electron microscopy investigation of SiGe quantum wires grown on patterned Si substrates

Growth of SiGe by low pressure chemical vapor deposition on nonplanar Si substrates is studied for nominal Ge concentrations of 0.4 ≤ x Ge nom.≤1. Self-organized growth leads to the formation of approx. 30 nm wide SiGe quantum wires at convex corners of the substrate. In photoluminescence (PL) spectra of samples with x Ge nom. = 0.4 we identify transitions from quantum wells on the flat parts of the substrate and from quantum wires. The energetic positions of the quantum wire transitions are in good agreement with Ge concentrations measured by spatially resolved energy dispersive X-ray spectroscopy, using a scanning transmission electron microscope (TEM). We find that the Ge concentration inside the wire is considerably lower than the nominal value for growth on planar parts of the substrate. Even for wires grown with x Ge nom. = 1, where only GeH 4 and H 2 are present during growth, PL and TEM indicate a Ge concentration as low as 32% for the wires. In such growth experiments we observe different regimes of strain relaxation. While quantum wires and wells are heavily decorated with Stranski-Krastanov islands in larger structures, smaller structures (≤5 μm) exhibit homogeneous thickness.