Stages Of Epitaxial Growth Research Articles

Kinetic Monte Carlo simulation study of the early stages of epitaxial SiC (0001) growth

A kinetic Monte Carlo (kMC) simulation with a cluster-multiple labeling technique was carried out to study the dynamic behavior of Si-C clusters during the early stage of epitaxial growth. In the model, a crystal lattice was established to fix the physical location of atoms and interatomic bonding. Events, such as adatoms adsorption, diffusion on the planar surface, and attachment to and detachment from clusters of adatoms were considered in the model. The Si and C atoms were treated individually to achieve more elaborate information for dynamic behavior of the clusters in the crystal surface. Moreover, a cluster multiple labeling technique, also known as the Hoshen - Kopelman algorithm was used to identify Si-C clusters. Then the properties of the clusters were calculated and analyzed by the algorithm. The simulation results showed that the number of Si-C clusters gradually decreases with increasing substrate temperature and decreasing deposition rate, and that the average size of Si-C clusters at high temperatures is much larger than that at low temperatures. Furthermore, the Si:C ratio on the growing surface has a significant effect on the surface morphology. At Si-rich conditions, the cluster density is relatively high, and the cluster shape is not homogeneously distributed. While with the increase of C mole fraction, the cluster density gradually decreases and the cluster shape becomes more uniform. Therefore, a higher concentration of C in deposition flux is beneficial to the early stage of nucleation. These findings are consistent with predictions for the case of the epitaxial growth of SiC.

Stabilization of thick, rhombohedral Hf0.5Zr0.5O2 epilayer on c-plane ZnO

Metastable rhombohedral hafnia-based ferroelectric films are emerging as a promising candidate in ferroelectric nonvolatile memory technologies, but the limited critical thickness impedes their applications. Herein, a 35-nm-thick rhombohedral Hf0.5Zr0.5O2 epilayer was stabilized on ZnO(0001) under an oxygen-deficient condition. Domain matching epitaxy, which facilitates the accommodation of misfit strain, allows the epitaxial growth of the (111)-oriented rhombohedral Hf0.5Zr0.5O2 film. We propose that a strong symmetry constraint is imposed on the epilayer at the initial epitaxial growth stage, i.e., the plane adjacent to ZnO(0001) should have a threefold symmetry. Although the bulk monoclinic phase is much more stable than the rhombohedral phase, our first principles calculations reveal that these two phases are energetically comparable with each other when this symmetry constraint is considered. Moreover, our results show that the incorporation of doubly charged oxygen vacancies is also powerful in shifting the energy balance between competing phases, making the metastable rhombohedral phase more stable.

Removal behavior of micropipe in 4H-SiC during micromachining

Silicon carbide is a sound semiconductor material because of its good performance in high-voltage and high-frequency fields. However, micropipe — a special 3 dimensions defect commonly is introduced at the epitaxial growth stage — is easy to be passed to the machining stage, and deteriorates the final device performance. In the current work, we employ micromachining atomistic simulations with a single abrasive particle to evaluate whether these defects are eliminated by varying the penetration depth. The simulation results are analyzed from the aspects of structure and energy, atomistic flow field, stress distribution, and temperature; the behavior models of the finished groove are proposed. By these analyses, it is found that it seems to be impossible to completely eliminate the effect of the micropipe by adjusting the penetration depth during micromachining, based on our MD simulation cases; but 4H-SiC shows different dynamic behaviors under different penetration depths, which may be a strategy to detect or control the effect of micropipe. • 4H-SiC micromachining atomistic model with the micropipe defect was constructed. • Completely eliminating the effect of micropipe by adjusting penetration depth was proven to be impossible by MD simulation. • Behavior models of the finished groove were summarized. • Strategy to detect or control the effect of micropipe was proposed.

Surface morphology evolution and underlying defects in homoepitaxial growth of GaAs (110)

The evolution of the surface morphology and underlying pyramidal defects in homoepitaxial GaAs (110) layers was investigated with respect to the layer thickness up to 1 µm. Ga and As atoms coexisting in a one-to-one atomic ratio on the (110)-oriented GaAs surface exhibit different incorporation rates and adatom migration rates, giving rise to local As atomic disordering in the epitaxial layer. Accordingly, this disordering triggers distinctive shapes of three-dimensional surface islands composed of specific facets and underlying microtwins. Single triangular-shaped islands initially appear at an early stage of epitaxial growth. With increasing layer thickness, all of the single triangular islands suddenly become paired triangular islands by forming secondary single triangular islands nearby, like decalcomania. A further increase of the layer thickness induces a complete transformation into four-legged starfish shapes containing directional side edges. At this stage, the underlying pyramidal defects of the surface islands are filled with {111} microtwins. Surprisingly, the evolutions of island shapes and underlying twin formation are closely associated with the strain relaxation in the thick GaAs (110) layers, which is unexpected in any homoepitaxial system. We found that a higher growth temperature retards the shape evolution of surface islands, leading to suppression of compressive strain.

Initial stages of the epitaxial growth of AlN on GaN (1 1 1)-(2 × 2) surface: Ab-initio studies

Initial stages of the aluminium nitride epitaxial growth on the gallium nitride (1 1 1)-(2 × 2) surface have been investigated by first-principles total-energy calculations. Studies have been performed within the periodic density functional theory as implemented in the PWscf code of the quantum espresso package. The electron-ion interactions are treated according to the pseudopotential approach. The exchange-correlation energies are treated within the generalized gradient approximation according to Perdew-Burke-Ernzerhof parameterization. First we investigate the Al adsorption on the surface at high symmetry sites with results showing the T4 site as the most stable geometry. The aluminium (Al) coverage varies from ¼ to 1 monolayer. The Al incorporation into the GaN atomic structures occurs by displacing the first layer Ga atoms, with Ga becoming the new adatom to be adsorbed on the surface at high symmetry sites having the T4-2 as the most favorable structure. Surface formation energies (SFE) are calculated; according to results the AlN bilayer formation on top of the surface is the most favorable structure. Moreover the Ga chain on the Al terminated surface may be formed with SFE energy value similar to the Al layer under a Ga layer. Electronic properties have been investigated by calculating the density of states (DOS) and projected density of states (PDOS) of the most favorable structures. At the Fermi level the density of states displays no energy gap indicating that surfaces are metallic.

Phase field modeling of solidification microstructure evolution during welding

The misorientation angle between the preferred crystalline orientation and the temperature gradient influences both the incubation time and the average wavelength of any initial instability during the planar growth stage, as well as the dendrite growth direction and the primary dendrite arm spacing during the subsequent epitaxial growth stage. The solidification microstructure gradually changes from normal or tilted dendrites to a seaweed-like structure as the misorientation angle increases. The simulation of the initial wavelength, primary dendrite arm spacing and interface morphology are in general agreement with the experimental observations.

Scanning tunneling microscopy study of the early stages of epitaxial growth of CoSi2 and CoSi films on Si(111) substrate: Surface and interface analysis

We report scanning tunneling microscopy observations of solid-phase epitaxial growth of ultra-thin CoSi2 and CoSi films at heating temperatures in the range of 350–600°C using a disordered ‘(1×1)’ phase of Co/Si(111) as a pre-existing template. It is found that depending on the growth conditions two types of epitaxial cobalt silicide films can be formed. At coverages below ~2 ML (1 ML=7.83×1014atoms/cm2) of Co, CoSi2 phase grows in the form of flat two-layer patches spreading over the surface at elevation of the temperature. It consists of precisely two the completely filled Co layers and four Si layers above the interface. At coverages higher than ~2 ML of Co, the layer-by-layer growth of CoSi phase with the CsCl-type crystal structure also takes place. The CoSi phase consists of one or more Co-Si double layers, which are located on top of the second Si-Co-Si triple layer of CoSi2, i.e., it has а coherent double interface CoSi/CoSi2/Si(111). According to experimental findings and ab initio total-energy calculations, the structures with the eightfold coordination of interfacial Co atoms and lateral unit cells of the silicide layer and substrate mutually rotated by 180° prove to be the most stable ones.

Selective epitaxial growth of stepwise SiGe:B at the recessed sources and drains: A growth kinetics and strain distribution study

The selective epitaxial growth of Si1-xGex and the related strain properties were studied. Epitaxial Si1-xGex films were deposited on (100) and (110) orientation wafers and on patterned Si wafers with recessed source and drain structures via ultrahigh vacuum chemical vapor deposition using different growing steps and Ge concentrations. The stepwise process was split into more than 6 growing steps that ranged in thicknesses from a few to 120 nm in order to cover the wide stages of epitaxial growth. The growth rates of SiGe on the plane and patterned wafers were examined and a dependence on the surface orientation was identified. As the germanium concentration increased, defects were generated with thinner Si1-xGex growth. The defect generation was the result of the strain evolution which was examined for channel regions with a Si1-xGex source/drain (S/D) structure.

In situ observation of low temperature growth of Ge on Si(1 1 1) by reflection high energy electron diffraction

In this paper we investigate the initial stages of epitaxial growth of Ge on Si(111) and the impact of growth temperature on strain evolution in situ by reflection high energy electron diffraction (RHEED) for temperatures between 200°C and 400°C. The change in surface morphology from a flat wetting layer to subsequent islanding that is characteristic for Stranski–Krastanov growth is monitored by spot intensity analysis. The corresponding critical layer thickness is determined to 3.1<dc<3.4ML. In situ monitoring of the strain relaxation process reveals a contribution of the Si(111) 7×7-surface reconstruction to the strain relaxation process. High resolution transmission electron microscopy confirms that the Ge islands exhibit a high degree of structural perfection and an ordered interfacial misfit dislocation network already at a growth temperature of 200°C is established. The temperature dependency of island shape, density and height is characterized by atomic force microscopy and compared to the RHEED investigations.

Evolution of film curvature profile during light-emitting diode epitaxial growth

ABSTRACTWafer bowing control is critical to improve epitaxial growth quality, especially for large wafers inside a metal–organic chemical vapor deposition reactor. An in-situ monitoring system was developed for real-time characterization of curvature mapping and epitaxial layer reflectance and temperature. Using deflectometry with parallel laser beams and position detector arrays, simultaneous curvature scans were conducted in two perpendicular directions over 4-in wafers. The developed system eliminated the effect of vibrations. Growth transients and curvature mappings are presented for the stages of epitaxial growth. Thermal and thickness gradient effects were analyzed to explain aspherical bowing. This technique appears suitable for closed-loop process optimization during light-emitting diode growth and other epitaxial growth processes.

Growth of High-Quality GaAs on Ge by Controlling the Thickness and Growth Temperature of Buffer Layer

High-quality GaAs thin films grown on miscut Ge substrates are crucial for GaAs-based devices on silicon. We investigate the effect of different thicknesses and temperatures of GaAs buffer layers on the crystal quality and surface morphology of GaAs on Ge by metal-organic chemical vapor deposition. Through high resolution x-ray diffraction measurements, it is demonstrated that the full width at half maximum for the GaAs epilayer (Ge substrate) peak could achieve 19.3 (11.0) arcsec. The value of etch pit density could be 4×104 cm−2. At the same time, GaAs surfaces with no pyramid-shaped pits are obtained when the buffer layer growth temperature is lower than 360°C, due to effective inhibition of initial nucleation at terraces of the Ge surface. In addition, it is shown that large island formation at the initial stage of epitaxial growth is a significant factor for the final rough surface and that this initial stage should be carefully controlled when a device quality GaAs surface is desired.

Unintentionally doped semi-insulating GaN with a low dislocation density grown by metalorganic chemical vapor deposition

Unintentionally doped semi-insulating GaN films possessing a low dislocation density are grown via metalorganic vapor phase epitaxy on a sapphire substrate. The obtained electrical resistivity reaches 4.8 × .a8 Ω·cm, and the full width at half maximum of the x-ray diffraction rocking curves of the (002) and (102) planes is as low as 201.6 and 378.0 arcsec, respectively. The GaN films are grown at low pressure (LP) to introduce residual carbon and to compensate background donors. The dislocation density in these samples is significantly reduced by using a low V/III ratio in the initial epitaxial growth stage. It is also found that the resistivity of the LP-grown samples decrease when the residual carbon concentration is excessive, which is attributed to the self-compensation of carbon impurities. Therefore, the carbon concentration of LP-grown GaN should be carefully modulated to maintain its high resistivity.

Phase Transformation of Molecular Beam Epitaxy-Grown Nanometer-Thick Gd2O3 and Y2O3 on GaN

High quality nanometer-thick Gd₂O₃ and Y₂O₃ (rare-earth oxide, R₂O₃) films have been epitaxially grown on GaN (0001) substrate by molecular beam epitaxy (MBE). The R₂O₃ epi-layers exhibit remarkable thermal stability at 1100 °C, uniformity, and highly structural perfection. Structural investigation was carried out by in situ reflection high energy electron diffraction (RHEED) and ex-situ X-ray diffraction (XRD) with synchrotron radiation. In the initial stage of epitaxial growth, the R₂O₃ layers have a hexagonal phase with the epitaxial relationship of R₂O₃ (0001)(H)<1120>(H)//GaN(0001)(H)<1120>(H). With the increase in R₂O₃ film thickness, the structure of the R₂O₃ films changes from single domain hexagonal phase to monoclinic phase with six different rotational domains, following the R₂O₃ (201)(M)[020](M)//GaN(0001)(H)<1120>(H) orientational relationship. The structural details and fingerprints of hexagonal and monoclinic phase Gd₂O₃ films have also been examined by using electron energy loss spectroscopy (EELS). Approximate 3-4 nm is the critical thickness for the structural phase transition depending on the composing rare earth element.

Basal Plane Dislocation Mitigation in SiC Epitaxial Growth by Nondestructive Substrate Treatment

An optimized molten KOH–NaOH–MgO eutectic mixture is used to pretreat the 8° off-axis 4H-SiC substrate prior to chemical vapor deposition epitaxial growth. Superior to the conventional pure KOH etching method, by way of the eutectic method, the conversion of basal plane dislocation (BPD) to threading edge dislocation in the subsequent epitaxial growth is independent of the etch pit size pregenerated on the substrate. Even with a very mild treatment using the eutectic mixture, which does not generate discriminable etch pits on the substrate, an epilayer with low BPD density <22 cm–2 (<0.1% of BPDs on the substrate) is still achieved, and further, there is no degradation of morphology on the entire sample surface. All of the BPD conversions occur at the initial stage of epitaxial growth.

Influence of substrates on epitaxial growth of B-site-ordered perovskite La2NiMnO6 thin films

Epitaxial La2NiMnO6 films were grown on (100)-oriented SrTiO3 and (LaAlO3)0.3-(SrAl0.5Ta0.5O3)0.7 (LSAT) substrates under various growth conditions by pulsed laser deposition. Ferromagnetism and transport properties of B-site-ordered perovskite are greatly influenced by growth conditions. Optimum growth conditions of epitaxial La2NiMnO6 films, which exhibit the ferromagnetism similar to bulk properties, are different for SrTiO3 and LSAT substrates probably due to the difference in surface migration. Since the initial stage of epitaxial growth might vary with the type of substrates, the ordering of Ni and Mn ions is different. Therefore, the ferromagnetism of La2NiMnO6 depends on the epitaxial growth conditions and type of substrates.

Thulium induced reconstructions of the Si(001) surface

The initial stages of epitaxial growth of Tm on Si(001) are studied by using scanning tunneling microscopy. Tm induces two kinds of surface reconstructions: 2×4 and a striped pseudo 2×7 phase. The morphology of the 2×4 is similar to the 2×4 phase induced by other rare earth metals. The pseudo 2×7 phase has some common structural elements with the conventional 2×7 phase but shows a broad width distribution. Growth of silicide islands and nanowires is seen at higher metal coverages after post deposition annealing. Differences in behavior with respect to other rare earth metals can be attributed to differences in the stable bulk silicides in this system.

Superstructure phase transitions in Ge on Si (111) at the initial stage of epitaxial growth

The surface phase transitions 7 × 7 → 5 × 5 for two-dimensional and c2 × 8 →7 × 7 and 7 × 7 → c2 × 8 for three-dimensional Ge islands epitaxially grown on Si(111) were found experimentally using scan- ning tunneling microscopy. The first two transitions are associated with an increase in the level of misfit strains, while the last is related to a decrease in the stress level during plastic relaxation.

Communication: Stable carbon nanoarches in the initial stages of epitaxial growth of graphene on Cu(111)

To fully exploit the device potential of graphene, reliable production of large-area, high-quality samples is required. Epitaxial growth on metal substrates have shown promise in this regard, but further improvement would be facilitated by a more complete understanding of the atomistic processes involved in the early growth stages. Using first-principles calculations within density functional theory, we have investigated the energetics and kinetics of graphene nucleation and growth on a Cu(111) surface. Our calculations have revealed an energetic preference for the formation of stable one-dimensional carbon nanoarches consisting of 3-13 atoms when compared to two-dimensional compact islands of equal sizes. We also estimate the critical cluster size that marks the transition from nanoarch dominance to island dominance in the growth sequence. Our findings may provide the structural link between nucleated carbon dimers and larger carbon nanodomes, and are expected to stimulate future experimental efforts.

Heteroepitaxial Growth of Isoreticular Metal−Organic Frameworks and Their Hybrid Films

Here we report the heteroepitaxial growth of isoreticular metal−organic frameworks (IRMOFs), that is, growing one IRMOF on another at the single crystal level. The presence of an amine compound in the growth solution of shell crystals is found to be critical for the heteroepitaxial growth of the IRMOFs by preventing IRMOF core (or seed) crystals from dissolving during the epitaxial growth stage. Single-crystal X-ray diffraction proved the epitaxy of the shell crystals grown on the surface of the core crystals. IRMOF hybrid films were also prepared on porous supports by heteroepitaxially growing IRMOF-3 crystals on the surface of IRMOF-1 seed crystal layers. These results are expected to enable the preparation of a series of novel hybrid IRMOFs and their thin films and membranes with multifunctionalities and improved properties for advanced applications.

Thermal Analysis Study of the Growth Kinetics of Carbon Nanotubes and Epitaxial Graphene Layers on Them

With the aim of improving the yield and purity of carbon nanotubes (CNTs), we have studied the chemical vapor deposition (CVD) growth of CNTs with Fe−Mo powder catalyst and acetylene precursor by using the technique of in situ simultaneous thermal analysis (STA). We could distinguish this way the rapid catalytic growth stage of CNTs and the slow epitaxial growth stage of graphitic impurities on the nanotubes, which were discovered to be graphene layers. Both reactions were first order, and the activation energies were determined to be 177 and 189 kJ/mol, respectively. In the rapid-growth stage, the growth of CNTs could be further tuned from kinetically controlled regime to gas-diffusion-controlled regime, where the CNTs exhibit a longer growth time and a higher yield, using appropriately shaped crucibles. These results are helpful for both elucidating the growth mechanism of CNTs and improving the quality of mass-produced CNTs.