3D Islanding Research Articles

Optimized InAlAs graded buffer and tensile-strained dislocation filter layer for high quality InAs photodetector grown on Si

We demonstrate a low threading dislocation density (TDD) and smooth surface InAs layer epitaxially grown on Si by suppressing phase separation of InxAl1−xAs (x = 0 to 1) graded buffer and by inserting a tensile-strained In0.95Al0.05As dislocation filter layer. While keeping the total III–V layer below 2.7 μm to avoid thermal cracks, we have achieved a sixfold reduction of TDD in InAs on Si compared to the unoptimized structure. We found a strong correlation between the metamorphic InAs surface roughness and TDD as a function of InxAl1−xAs buffer thickness. An optimal thickness of 175 nm was obtained where both phase separation and 3D islanding growth were suppressed. Moreover, a tensile-strained In0.95Al0.05As dislocation filter layer and high growth temperature of the InAs cap layer further assisted the dislocation reduction process, which led to a TDD to 1.37 × 108 cm−2. Finally, an InAs p-i-n photodetector grown on the optimized InAs/Si template confirmed its high quality by showing an improved responsivity from 0.16 to 0.32 A/W at a 2 μm wavelength.

Surfactant-Mediated Growth and Patterning of Atomically Thin Transition Metal Dichalcogenides.

The role of additives in facilitating the growth of conventional semiconducting thin films is well-established. Apparently, their presence is also decisive in the growth of two-dimensional transition metal dichalcogenides (TMDs), yet their role remains ambiguous. In this work, we show that the use of sodium bromide enables synthesis of TMD monolayers via a surfactant-mediated growth mechanism, without introducing liquefaction of metal oxide precursors. We discovered that sodium ions provided by sodium bromide chemically passivate edges of growing molybdenum disulfide crystals, relaxing in-plane strains to suppress 3D islanding and promote monolayer growth. To exploit this growth model, molybdenum disulfide monolayers were directly grown into desired patterns using predeposited sodium bromide as a removable template. The surfactant-mediated growth not only extends the families of metal oxide precursors but also offers a way for lithography-free patterning of TMD monolayers on various surfaces to facilitate fabrication of atomically thin electronic devices.

Effects of substrate vicinality on 3D islanding in Ge/Si epitaxy

Common features of Ge growth on vicinal Si(001) and Si(111) surfaces are discussed. We show that surface vicinality has a twofold effect on three-dimensional islanding in this system. As long as the atomic structure of the singular surface is preserved, epitaxial islands undergo a morphological change which is dictated by the misorientation from the singular surface. When also the surface reconstruction is so affected by miscut that surface and interfacial energies are drastically modified, the islanding pathway diverges from that occurring on flat surfaces.

In situ atomic-scale visualization of oxide islanding during oxidation of Cu surfaces

Oxidation of Cu occurs via Cu2O islanding on an oxide wetting layer at a critical thickness of two atomic layers. The transition from 2D wetting-layer growth to 3D oxide islanding is driven energetically arising from the Cu-Cu2O interfacial interaction.

Growth, Structure, and Stability of Ag on CeO2(111): Synchrotron Radiation Photoemission Studies

The growth and interfacial electronic properties of Ag on CeO2(111) thin films have been studied by synchrotron radiation photoemission spectroscopy (SRPES), low energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS). Stoichiometric CeO2(111) thin films were grown on a Ru(0001) substrate. Ag grows as three-dimensional (3D) particles on the well-ordered CeO2(111) surface at 300 K with a number density of ∼1 × 1012 particles/cm2. When the CeO2(111) surface has a high density of defects, Ag initially populates these defect sites, leading to a two-dimensional (2D) island growth at low coverages followed by 3D islanding at high coverages. The binding energy of Ag 3d increases when the Ag particle size decreases, which is mainly attributed to the final-state screening. No strong interaction between Ag and CeO2(111) is found. The CeO2(111) surface is slightly reduced upon Ag deposition, which can be ascribed to the reverse spillover of oxygen atoms from the Ag−CeO2 boundary to the Ag nanoparticles. The Ag particles on CeO2(111) experience significant sintering when the temperature increases before they desorb from the surface.

Reduced pressure chemical vapor deposition of Ge thick layers on Si(0 0 1), Si(0 1 1) and Si(1 1 1)

We have carried out an in-depth structural study of Ge thick layers grown using the so-called “LT/HT” approach on Si(0 0 1), Si(0 1 1) and Si(1 1 1) wafers. The low temperature (400 °C) adopted for the first Ge layer plastically relaxes the strain in the Ge film without 3D islanding. The high temperature (750 °C) used for the growth of the second, topmost, Ge layer lowers the dislocation density and reduces the overall deposition time. High temperature thermal cycling (in-between 750 and 890 °C) was called upon to further reduce the amount of defects in the layers. Ge growth rates on (0 0 1) were consistently higher than the ones on (1 1 1), which were themselves higher than on (0 1 1) (be it at 400 °C, 100 Torr or at 750 °C, 20 Torr). The surfaces of Ge (0 0 1) layers were rather flat, as attested by their small root mean square (rms) roughness and Z ranges (<1 and 10 nm, respectively). By contrast, Ge (0 1 1) (Ge (1 1 1)) thick layers were really rough, with rms roughness and Z ranges typically 30 (60) times higher than on (0 0 1). Almost all Ge layers were in a tensile-strain configuration, with macroscopic degrees of strain relaxation in-between 101% and 106% (slightly higher on (1 1 1) than on (0 0 1) and on (0 1 1), however). Misfit dislocations were found (by cross-sectional transmission electron microscopy) to be confined in the first few hundreds of nanometres of 2.5 μm thick Ge layers on Si(0 0 1). As far as 2.5 μm thick Ge layers on (0 1 1) and (1 1 1) are concerned, numerous {1 1 1} stacking faults were present at the Ge/Si interface and also threading all the way to the surface. The threading “defect” density, which is close to 10 7 cm −2 on (0 0 1), is around 8×10 8 cm −2 on (0 1 1) and roughly 2×10 9 cm −2 on (1 1 1). The higher defect density on (0 1 1) and (1 1 1) is likely due to 60° misfit dislocations dissociation (into 30° and 90° partial dislocations). For such surface orientations, the 90° partial dislocation is expected to lead with in its trail the 30° partial dislocation (hence the stacking faults multiplication).

Epitaxial silicon overgrowth of C 60 on the Si(1 0 0)-2 × 1 surface

Scanning tunneling microscopy has been used to investigate silicon overgrowth of C 60 on the Si(1 0 0)-2 × 1 surface. It can be shown that crystal morphology and quality is highly dependent on temperature and C 60 coverage. A C 60 coverage of 0.02 ML was used to show the initial stage of silicon overgrowth (1.5 ML of Si), medium stage with crater formation, crater diminution (10 ML of Si) and finally crater overgrowth (100 ML of Si). Crystal quality can be improved by annealing of the stack at 750 °C for several minutes, but SiC formation at 800 °C deteriorates the stack. Segregation of C 60 plays an important role already at low coverage. Nevertheless, C 60 can be enclosed into the crystal within a few monolayers. This has been proven by STM, TEM and SIMS analysis. Higher C 60 coverage leads to 3D islanding, which was proven by STM and LEED, and eventually to polysilicon, shown by STM and TEM. Finally we give a short comparison to the Si(1 1 1)-7 × 7 surface.

Self-Formation of High-Density and High-Uniformity InAs Quantum Dots on Sb/GaAs Layers by Molecular Beam Epitaxy

Self-assembled InAs quantum dots (QDs) grown on Sb/GaAs buffer layers by molecular beam epitaxy were investigated to achieve high-density and high-uniformity QDs. The density of InAs QDs on Sb/GaAs layers was markedly higher than that of InAs QDs on conventional GaAs layers. The high-density nucleation of three-dimensional (3D) InAs islands was attributed to the formation of two-dimensional (2D) wirelike structures prior to 3D islanding. QD density was controlled from 3×1010 to 1×1011 cm-2 by changing Sb supply amount. In addition, the density of very large dots due to coalescence increased with increasing Sb supply amount. On Sb (0.6 atomic layers)/GaAs buffer layers, we obtained high-density and high-quality InAs QDs, which revealed a narrow photoluminescence linewidth of about 30–35 meV.

Adsorption of Pb on NiAl(1 1 0): energetics and structure

The adsorption of Pb onto a NiAl(1 1 0) single crystal surface at 300 K has been studied by Auger electron spectroscopy (AES), Low energy electron diffraction (LEED), molecular beam/surface scattering and single crystal adsorption calorimetry (SCAC). AES indicates a Stranski–Krastanov growth mode, i.e., Pb initially grows on NiAl(1 1 0) two-dimensionally until the first layer completes at 0.89 ML, where a 1 3 3 0 superstructure is observed by LEED, followed by 3D islanding. Measurements of the Pb gas that does not stick indicate that Pb sticks on NiAl(1 1 0) with an initial probability of 0.99. The initial heat of adsorption of Pb on NiAl(1 1 0) is 249 ± 10 kJ/mol. Due to the repulsive interactions between Pb adatoms, the heat of adsorption decreases within the first layer to a value identical to the heat of sublimation of bulk Pb (195 kJ/mol), where it remains at higher coverages. This first application of adsorption calorimetry on such a thick sample (75 μm versus 0.2–8 μm previously) demonstrates that adsorption calorimetry can be extended to a wider range of surfaces, since this thickness can be achieved with nearly any single crystal material by simple mechanical thinning.

Comprehensive overview on elastic strain relaxation mechanisms in nitride heterostructures: Stranski–Krastanow versus Frank–Van der Merwe growth mode

Both plastic and elastic strain relaxation of GaN on AlN grown by plasma assisted molecular beam epitaxy have been studied by reflection high energy electron diffraction (RHEED) and high resolution X-ray diffraction (HRXRD). Special attention has been paid to plastic strain relaxation of thick layers which has been found to occur from the beginning of the growth. A mechanism of misfit dislocation introduction based on the coalescence of dynamically formed platelets is proposed. Due to the lack of proper gliding planes in the wurtzite structure, such dislocations are not mobile, leading to an inhomogeneity of the strain state along the growth axis which has been put in evidence by X-ray diffraction measurements. The growth conditions suitable to achieve elastic relaxation through 3D islanding are precised.

Control in the initial growth stage of heteroepitaxial Si1−x−yGexCy on Si(0 0 1) substrates

The initial growth process of Si1−x−yGexCy epitaxial thin films on Si(001) surfaces is investigated by scanning tunneling microscopy (STM). The effects of the substrate temperature, the film thickness, and the C fraction in Si1−x−yGexCy on the film morphology are systematically examined on an atomic scale. At low temperatures, multi-layered islands are formed and this islanding is closely related to C atoms segregating to the surface during the film growth. As the temperature increases, the growth mode turns into a step-flow process, although high temperature growth promotes C segregation. In the growth of Si1−x−yGexCy films with large C fractions, phase separation between Si–C and Si–Ge, concomitant with C condensation and three-dimensional (3D) islanding on the surface of the growing films, is dominant. We find that the formation of a thin Si1−xGex layer prior to Si1−x−yGexCy film growth drastically improves the film structure, leading to a planar morphology even with large C fractions present in the film.

Growth mode and strain evolution during InN growth on GaN(0001) by molecular-beam epitaxy

Epitaxial growth of InN on GaN(0001) by plasma-assisted molecular-beam epitaxy is investigated over a range of growth parameters including source flux and substrate temperature. Combining reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM), we establish a relationship between film growth mode and the deposition condition. Both two-dimensional (2D) and three-dimensional (3D) growth modes of the film are observed. For 2D growth, sustained RHEED intensity oscillations are recorded while STM reveals 2D nucleation islands. For 3D growth, less than three oscillation periods are observed indicating the Stranski–Krastanov (SK) growth mode of the film. Simultaneous measurements of (reciprocal) lattice constant by RHEED suggest a gradual relaxation of the strain in film, which commences during the first bilayer (BL) deposition and almost completes after 2–4 BLs. For SK growth, 3D islanding initiates after the strain has mostly been relieved, presumably by dislocations, so the islands are likely strain free.

Thermodynamic driving force of formation of coherent three-dimensional islands in Stranski-Krastanov growth

The formation of coherent three-dimensional (3D) islands in highly mismatched epitaxy is discussed in terms of the traditional concept of wetting. It is shown that the wetting layer and the 3D islands represent different phases which cannot be in equilibrium with each other. The transfer of matter from the stable wetting layer to the 3D islands is thermodynamically unfavored. The experimentally observed critical misfit for coherent 3D islanding to occur and the coexistence of pyramids with discrete heights of two, three, four,\dots{}, monolayers can be explained assuming that the thermodynamic driving force of formation of coherent 3D islands on the surface of the wetting layer of the same material is the reduced average adhesion of the islands to that layer and that the islands height is a discrete variable.

Diffusion-induced islanding in heteroepitaxial systems

Abstract A microscopic lattice gas model for ultra-thin film dynamics is developed and applied to the case of heteroepitaxial growth. A set of non-linear kinetic equations for average occupations of adsorption sites in 3D lattice is studied analytically in a continual limit. It is found that within a range of parameters of heteroepitaxial system space-uniform state becomes unstable in critical thickness range and system undergoes spontaneous islanding. Space-ordered quasistationary solutions to the model equations describe the dynamics of 3D islanding induced by the uphill diffusion in the field of deposit–deposit and deposit–substrate interactions. Lateral size of islands depends on material constants of the system, surface temperature and deposition rate. Initial discrete system of non-linear kinetic equations is studied numerically; results for surface morphology describe dense arrays of 3D nanoislands.

Coherent Stranski-Krastanov growth in 1+1 dimensions with anharmonic interactions: An equilibrium study

The formation of coherently strained three-dimensional islands on top of the wetting layer in Stranski-Krastanov mode of growth is considered in a model in 1+1 dimensions accounting for the anharmonicity and non-convexity of the real interatomic forces. It is shown that coherent 3D islands can be expected to form in compressed rather than in expanded overlayers beyond a critical lattice misfit. In the latter case the classical Stranski-Krastanov growth is expected to occur because the misfit dislocations can become energetically favored at smaller island sizes. The thermodynamic reason for coherent 3D islanding is the incomplete wetting owing to the weaker adhesion of the edge atoms. Monolayer height islands with a critical size appear as necessary precursors of the 3D islands. The latter explains the experimentally observed narrow size distribution of the 3D islands. The 2D-3D transformation takes place by consecutive rearrangements of mono- to bilayer, bi- to trilayer islands, etc., after exceeding the corresponding critical sizes. The rearrangements are initiated by nucleation events each next one requiring to overcome a lower energetic barrier. The model is in good qualitative agreement with available experimental observations.

Relaxation study of In xGa 1− xAs/GaAs quantum-well structures grown by MBE on (0 0 1) and (1 1 1)B GaAs for long wavelength applications

A comparative study of the relaxation mechanisms of thin, single In x Ga 1− x As/GaAs quantum wells (QWs) grown simultaneously on (0 0 1) and (1 1 1)B GaAs substrates is presented. Transmission electron microscopy (TEM) studies indicate that the primary relaxation mechanism for high-indium content (0 0 1) samples is the formation of 3D islands. For the growth conditions used, this occurs at an indium content above 0.24 in our samples. Although initially coherent with the substrate, above a certain critical thickness catastrophic relaxation occurs through a very high density of edge dislocations (>10 12 cm −2) as the islands coalesce. A similar relaxation mechanism is not observed in the (1 1 1) orientation. Here, 3D islanding is suppressed and instead a novel configuration of misfit dislocations (MD) appears at x>0.24. This new MD configuration presents an additional misfit-relieving component not taken into account in the previous theoretical analysis of the critical layer thickness (CLT) for (1 1 1)B. However, the mechanism is relatively inefficient when compared to the catastrophic relaxation in the (0 0 1) case, and therefore, the (1 1 1)B substrate still offers considerable advantages. This is illustrated by photoluminescence results, which show that (1 1 1)B In x Ga 1− x As QWs are able to reach wavelengths as long as 1.1 μm at room temperature.

Si in-diffusion during the 3D islanding of Ge/Si(001) at high temperatures

The 3D islands of the Stranski–Krastanow system Ge/Si(001) that form either during the annealing of previously flat and nearly strain-relieved Ge films at 1020 K or directly at the Ge deposition at 1020 K are found to be composed of a mixture of Ge and Si, thus pointing to considerable interdiffusion at 1020 K. Direct measurement of the elastic energy unambiguously reveals that neither the 3D islanding nor the Si in-diffusion are driven by the reduction of misfit strain; this strain being the result of increasing configurational entropy.

Growth-mode modification of Bi onCdTe(111)Ausing Te monolayer deposition

Bi deposited on the $\mathrm{CdTe}(111)A$ (Cd-terminated) surface grows by three-dimensional (3D) islanding, while Bi deposited on the $\mathrm{CdTe}(111)B$ (Te-terminated) grows layer-by-layer. However, introducing a Te monolayer (ML) on the $\mathrm{CdTe}(111)A$ surface reduces the interfacial energy, thereby changing the growth mode of Bi from 3D islandlike to layer-by-layer growth. The Te ML remains where it is deposited, which differs from the growth mode in which the surface-active agent floats on the growing surface. By incorporating appropriate Te ML's, Bi/CdTe superlattices with sharper interfaces were observed. These superlattices were characterized by x-ray diffraction and transmission electron microscopy.

Stress and Relief of Misfit Strain of Ge/Si(001)

The intrinsic stress of the Stranski-Krastanov system Ge/Si(001) was investigated in the range 700--1050 K. Characteristic stress features indicate that the relief of the misfit strain proceeds mainly in two steps: (i) by the formation of 3D islands on top of the Ge wetting layer and (ii) via misfit dislocations in larger 3D islands and upon their percolation. The temperature dependence of strain relief by 3D islands as well as their nucleation and growth behavior support a kinetic pathway for 3D islanding.

Incorporation and stability of carbon during low-temperature epitaxial growth of Ge1−xCx (x&amp;lt;0.1) alloys on Si(100): Microstructural and Raman studies

Low-temperature (∼200 °C) molecular beam epitaxy of Ge1−xCx alloys grown on Si(100) have been extensively investigated by in situ reflection high-energy electron diffraction, ex situ x-ray diffraction, transmission electron microscopy, and Raman spectroscopy. Carbon concentrations were nominally varied from 0 up to ∼10 at. %. Selected samples were annealed in an Ar ambient at 750 °C to evaluate the stability of the thin films. A few films were also grown on Ge substrates. With increasing C concentration, the epitaxial growth mode changes from two dimensional layer growth to three dimensional island growth. The surface has a tendency to facet along {311} planes under certain growth conditions. The microstructure shows an increase in planar defect density with increasing C concentration. The x-ray diffraction data show that the lattice parameter decreases with increasing C concentration and that a maximum of 1 at. % C is incorporated substitutionally in Ge. Raman spectroscopy shows no clear Ge–C signal though extra intensity is measured at the energies where Ge–C modes may be expected. Films with nominal C concentrations greater than 2 to 3 at. % show clear evidence for amorphous C. We propose that under our growth conditions, nominal C in excess of about 2 to 3 at. % remains on the surface as amorphous C and plays an important role in 3D islanding, defect formation, and {311} faceting during epitaxial growth.