Heteroepitaxial Growth Process Research Articles

Microstructure and reflectance of porous GaN distributed Bragg reflectors on silicon substrates

Distributed Bragg reflectors (DBRs) based on alternating layers of porous and non-porous GaN have previously been fabricated at the wafer-scale in heteroepitaxial GaN layers grown on sapphire substrates. Porosification is achieved via the electrochemical etching of highly Si-doped layers, and the etchant accesses the n+-GaN layers through nanoscale channels arising at threading dislocations that are ubiquitous in the heteroepitaxial growth process. Here, we show that the same process applies to GaN multilayer structures grown on silicon substrates. The reflectance of the resulting DBRs depends on the voltage at which the porosification process is carried out. Etching at higher voltages yields higher porosities. However, while an increase in porosity is theoretically expected to lead to peak reflectance, in practice, the highest reflectance is achieved at a moderate etching voltage because etching at higher voltages leads to pore formation in the nominally non-porous layers, pore coarsening in the porous layers, and in the worst cases layer collapse. We also find that at the high threading dislocation densities present in these samples, not all dislocations participate in the etching process at low and moderate etching voltages. However, the number of dislocations involved in the process increases with etching voltage.

Temperature Investigation on 3C-SiC Homo-Epitaxy on Four-Inch Wafers.

In this work, results related to the temperature influence on the homo-epitaxial growth process of 3C-SiC is presented. The seed for the epitaxial layer was obtained by an innovative technique based on silicon melting: after the first step of the hetero-epitaxial growth process of 3C-SiC on a Si substrate, Si melts, and the remaining freestanding SiC layer was used as a seed layer for the homo-epitaxial growth. Different morphological analyses indicate that the growth temperature and the growth rate play a fundamental role in the stacking faults density. In details, X-ray diffraction and micro-Raman analysis show the strict relationship between growth temperature, crystal quality, and doping incorporation in the homo-epitaxial chemical vapor deposition CVD growth process of a 3C-SiC wafer. Furthermore, photoluminescence spectra show a considerable reduction of point defects during homo-epitaxy at high temperatures.

Hierarchically tailorable double-array film hybrids with enhanced photocatalytic and photoelectrochemical performances

The basic mantra of “structure-dictates-function” in chemistry has inspired tremendous interests in designing various architectures toward specific applications. In this work, hierarchically tailorable double-array structures composed of TiO2 nanotube arrays (TNTAs) and axially grown ZnO nanorod arrays (ZNRAs) have been constructed through a two-step anodization method combined with a heteroepitaxial growth process regulated by electrodeposited reduced graphene oxide (RGO). The obtained TNTAs/RGO/ZNRAs film hybrids exhibit enhanced photocatalytic and photoelectrochemical performances along with favorable photostability as compared to the single component and binary counterparts. This is ascribed to the synergistic effect of the matchable band alignment of the components, the 1D-on-1D double-array architecture with vectorial pathways for charge carriers transport, and the electrically conductive RGO for bridging the directional electron flow to further promote the separation and transfer of charge carriers. This work is anticipated to provide instructive recipe for rational design and construction of composite films with optimized photo(electro)catalytic performances.

One-Dimensional Zinc Oxide Decorated Cobalt Oxide Nanospheres for Enhanced Gas-Sensing Properties

In this study, one-dimensional (1D) zinc oxide was loaded on the surface of cobalt oxide microspheres, which were assembled by single-crystalline porous nanosheets, via a simple heteroepitaxial growth process. This elaborate structure possessed an excellent transducer function from the single-crystalline feature of Co3O4 nanosheets and the receptor function from the zinc oxide nanorods. The structure of the as-prepared hybrid was confirmed via a Scanning Electron Microscope (SEM), X-ray diffraction (XRD), and a Transmission Electron Microscope (TEM). Gas-sensing tests showed that the gas-sensing properties of the as-designed hybrid were largely improved. The response was about 161 (Ra/Rg) to 100 ppm ethanol, which is 110 and 10 times higher than that of Co3O4 (Rg/Ra = 1.47) and ZnO (Ra/Rg = 15), respectively. And the as-designed ZnO/Co3O4 hybrid also showed a high selectivity to ethanol. The superior gas-sensing properties were mainly attributed to the as-designed nanostructures that contained a super transducer function and a super receptor function. The design strategy for gas-sensing materials in this work shed a new light on the exploration of high-performance gas sensors.

(Invited) Microscopy and Spectroscopy on Thin-Films for (Electro-)Catalysis

Porous materials are of high importance, a.o. in the field of catalysis. A typical example of relevant and well-known crystalline, microporous materials is zeolites. These stable aluminosilicates possess tuneable Lewis and Brønsted acidity, in combination with microporous channels and/or cages allowing for size and shape selectivity. As a result, these zeolites are used for many catalytic applications, including large industrial processes such as fluid catalytic cracking (FCC). Another class of highly crystalline porous solids, namely metal-organic frameworks (MOFs), have gained a lot of attention over the last few decades. These materials, built-up of metal nodes and organic linkers, are interesting due to their huge versatility: material properties, such as pore size and Lewis acidity, can be tuned to its specific application by simply tailoring its building blocks. Comparable to zeolites, MOFs can be used for a wide range of applications, such as catalysis, separation and in electronic (sensing) devices. Of both zeolites and MOFs, a large number of framework structures are known. Most of these compounds have been developed through the process of trial-and-error and/or educated guesses. Ultimately, one wishes to apply true rational design to the development of catalysts for novel applications. The required fundamental understanding of crystal growth needed for such purposes is unfortunately still lacking in literature. To study nucleation and growth behaviour, a model system apt for near-field characterisation is needed. As a result, 2D thin-film structures have been developed to facilitate high control over, and easy analysis of, crystal growth. Additionally, by converting these porous materials into thin-films, the gate-way to (membrane) specific applications is opened. To gain fundamental insights into catalytic thin-films, and to establish structure-performance correlations, we apply multiple (in-situ) characterization techniques. Initially, hetero-epitaxial growth processes were probed to uncover crystal nucleation and growth mechanisms. This real-time in-situ research was performed on HKUST-1 surface-mounted MOFs, or SURMOFs. During a one-pot synthesis between metal-ions and linkers, the surface topology was continually measured using atomic force microscopy (AFM). On a single (10 x 10 µm2) AFM spot we could follow the formation of MOF islands over multiple timeframes. Using a tailor-made script, these (growing) islands were labelled and tracked over all timeframes. From this, a (temperature-dependent) growth rate could be established. Next to structural information, the chemical nature of the formed MOF grains was uncovered using infra-red reflection absorption spectroscopy (IRRAS) at grazing angles, a highly sensitive technique providing data from monolayer coverage and up. Additionally, a novel photo-induced force microscopy (PiFM, or AFM-IR) technique was applied to these SURMOFs, which is able in distinguishing grain specific chemical nature of the nm-sized MOF islands. This knowledge of nucleation and growth behaviour can be applied to the production of high-quality SURMOFs. Whereas the direct synthesis provides a facile medium for in-situ AFM investigations, a layer-by-layer synthesis offers high control over SURMOF fabrication quality. This synthesis, in which the growth phases of the inorganic and organic compounds are separated, is a perfect model system for the systematic study of growth parameters and their effect of resulting SURMOF quality. Using IRRAS, we can fully identify peaks in the HKUST-1 SURMOF spectrum, including precursor species, which were previously unknown in literature. Additionally, in contrast to previously posed growth mechanisms, our (AFM-)IR studies show growth to occur through a solution-mediated mechanism. Moreover, AFM-IR can be used to directly probe defect formation on the nm-scale, allowing for catalytic structure-performance correlations. In accordance with the found mechanism, existing synthesis procedures can be optimized in terms of quality and speed. For zeolite ZSM-5, an innovative approach to enhance catalytic performance is to prepare homo-epitaxially grown zeolite ZSM-5 as continuous, oriented films with accessibility to only the straight zeolite channels. Applying advanced characterization techniques, such as tip-enhanced Raman spectroscopy (TERS), single-molecule fluorescence microscopy as well as grazing incidence characterization methods, we can explore the synthesis mechanisms, as well as study 3-D diffusion and related reactivity through a single pore orientation of zeolite ZSM-5 down to the level of a single molecule. Furthermore, we show the methanol-to-hydrocarbons reaction on ZSM-5 thin-films can be studied with AFM-IR. AFM-IR is highly sensitive to the surface chemical structure with regards to Al3+ framework incorporation as well as hydrocarbon formation, providing a more complete nanoscale picture of the structural and catalytic information. These combined case studies show the strength of microscopy and spectroscopy tools in understanding and improving catalytic materials. Figure 1

Intermediate Structures in the Lifting of a Stacking Fault on the Si(111)–(7 × 7) Surface Studied by Monte Carlo Simulations

The lifting of a stacking fault in the (7 × 7) dimer adatom stacking-fault (DAS) structure, which is pivotal to the homo- and heteroepitaxial growth processes on Si(111), is studied by Monte Carlo ...

Uniform "Patchy" Platelets by Seeded Heteroepitaxial Growth of Crystallizable Polymer Blends in Two Dimensions.

Rectangular platelets formed by the self-assembly of block copolymers in selective solvents are of interest for a range of applications. Recently, we showed that the seeded growth of crystallizable blends of a block copolymer and homopolymer yields well-defined, low area dispersity examples of these two-dimensional (2D) structures. The key feature was the use of the same crystallizable polymer segment in the seed and blend components to enable an efficient homoepitaxial growth process. Herein we demonstrate that this 2D crystallization-driven self-assembly approach can be extended to heteroepitaxial growth by the use of different crystallizable polymers with compatible crystal structures. This allows the formation of well-defined "patchy" rectangular platelets and platelet block comicelles with different core chemistries. The use of scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy provided key information on the spatial location of the components in the resulting assemblies and thereby valuable insight into the 2D heteroepitaxial growth process.

Fabrication of hierarchical core–shell Au@ZnO heteroarchitectures initiated by heteroseed assembly for photocatalytic applications

Three dimensional dandelion-like hierarchical core–shell Au@ZnO heteroarchitectures with ZnO nanorods grown radially on Au nanoparticle (NP) cores have been successfully prepared with a high yield via a simple solution method involving heteroseed-induced nucleation and subsequent heteroepitaxial growth processes. Briefly, mercaptopropionic acid (MA) modified Au NPs were synthesized beforehand and served as nucleation centers for primary ZnO seed generation and Au@ZnO heteroseed formation. Then an epitaxial growth of ZnO nanorods (ZnO NRs) on the Au@ZnO heteroseeds resulted in the formation of Au@ZnO dandelions. The photocatalytic properties of as-prepared Au@ZnO dandelions were evaluated through rhodamine B (RhB) photodegradation under UV irradiation. The result showed that the Au@ZnO dandelions had improved photocatalytic performance compared with pure ZnO NRs and hybrids of ZnO NRs/Au NPs, due likely to the synergistic effect of the metal–semiconductor heterojunction and the unique dandelion-like hierarchical core–shell structure.

Growth mechanism and microstructure of low defect density InN (0001) In-face thin films on Si (111) substrates

Transmission electron microscopy has been employed to analyze the direct nucleation and growth, by plasma-assisted molecular beam epitaxy, of high quality InN (0001) In-face thin films on (111) Si substrates. Critical steps of the heteroepitaxial growth process are InN nucleation at low substrate temperature under excessively high N-flux conditions and subsequent growth of the main InN epilayer at the optimum conditions, namely, substrate temperature 400–450 °C and In/N flux ratio close to 1. InN nucleation occurs in the form of a very high density of three dimensional (3D) islands, which coalesce very fast into a low surface roughness InN film. The reduced reactivity of Si at low temperature and its fast coverage by InN limit the amount of unintentional Si nitridation by the excessively high nitrogen flux and good bonding/adhesion of the InN film directly on the Si substrate is achieved. The subsequent overgrowth of the main InN epilayer, in a layer-by-layer growth mode that enhances the lateral growth of InN, reduces significantly the crystal mosaicity and the density of threading dislocations is about an order of magnitude less compared to InN films grown using an AlN/GaN intermediate nucleation/buffer layer on Si. The InN films exhibit the In-face polarity and very smooth atomically stepped surfaces.

Stress Evaluation on Hetero-Epitaxial 3C-SiC Film on (100) Si Substrates

SiC is a candidate material for micro- and nano-electromechanical systems (MEMS and NEMS). In order to understand the impact that the growth rate has on the residual stress of CVD-grown 3C-SiC hetero-epitaxial films on Si substrates, growth experiments were performed and the resulting stress was evaluated. Film growth was performed using a two-step growth process with propane and silane as the C and Si precursors in hydrogen carrier gas. The film thickness was held constant at ~2.5 µm independent of the growth rate so as to allow for direct films comparison as a function of the growth rate. Supported by profilometry, Raman and micro-machined free-standing structures, this study shows that the growth rate is a fundamental parameter for low-defect and low-stress hetero-epitaxial growth process of 3C-SiC on Si substrates. Stress analysis performed by modify Stoney’s equation trough optical curvature measurement, Raman shift analysis and micro-machining of free-standing structures that shows apparent disagreement about the nature of the stress. These odds between the experimental data can be explained assuming a strong stress field located in the substrate and related to defects generated in the silicon during the growth process.

Cooperative and competitive concurrency in scientific computing. A full open-source upgrade of the program for dynamical calculations of RHEED intensity oscillations

Cooperative and competitive concurrency in scientific computing. A full open-source upgrade of the program for dynamical calculations of RHEED intensity oscillations

Multimode photoacoustic method for the evaluation of mechanical properties of heteroepitaxial diamond layers

A multimode photoacoustic method was developed for evaluating acoustically thick anisotropic layers, using surface acoustic waves. Such layers support multiple acoustic modes. This complicates the reverse problem, but on the other hand, makes it possible to extract more materials properties. Several mechanical properties of a layer-substrate system, consisting of a 110 μm thick heteroepitaxial chemical vapor deposited diamond layer on Ir/YSZ (yttria-stabilized zirconia)/Si(001), were evaluated, based on two surface acoustic modes. A dispersive and a nondispersive mode measured in two different crystallographic directions were employed to evaluate the three elastic stiffness coefficients C11, C12, C44, and the mass density of the diamond layer. It is demonstrated that accurate elastic moduli can be determined without special sample preparation, employing the layered system as obtained from the heteroepitaxial diamond growth process.

Characterization of the deflection of a new epitaxial piezoelectric micro-mirror: Modeling and experiment

Abstract Advanced 3D finite element analysis of a newly designed piezoelectric micro-mirror device was conducted to assess a priori the attainable deflection of micro-mirror. The deflection of micro-mirror has shown to vary with the competitions and/or cooperative interactions among deposited layers. The device was successfully fabricated using a heteroepitaxial growth process with Pt/PZT(1 1 1)/Pt(1 1 1)/γ-Al 2 O 3 (1 1 1)/Si(1 1 1) structure. Material anisotropy and technologically induced residual stress have shown to play a significant role on the deflection of micro-mirror. A good correlation was found between simulation results and experimentally measured deflection of fabricated micro-mirror.

Growth Rate Effect on 3C-SiC Film Residual Stress on (100) Si Substrates

SiC is a candidate material for micro- and nano-electromechanical systems (MEMS and NEMS). In order to understand the impact that the growth rate has on the residual stress of CVD-grown 3C-SiC hetero-epitaxial films on Si substrates, growth experiments were performed and the resulting stress was evaluated. Film growth was performed using a two-step growth process with propane and silane as the C and Si precursors in hydrogen carrier gas. The film thickness was held constant at ~2.5 µm independent of the growth rate so as to allow for direct films comparison as a function of the growth rate. Supported by profilometry, Raman and XRD analysis, this study shows that the growth rate is a fundamental parameter for low-defect and low-stress hetero-epitaxial growth process of 3C-SiC on Si substrates. XRD (rocking curve analysis) and Raman spectroscopy show that the crystal quality of the films increases with decreasing growth rate. From curvature measurements, the average residual stress within the layer using the modified Stoney’s equation was calculated. The results show that the films are under compressive stress and the calculated residual stress also increases with growth rate, from -0.78 GPa to -1.11 GPa for 3C-SiC films grown at 2.45 and 4 µm/h, respectively.

X-ray investigation of the interface structure of free standing InAs nanowires grown on GaAs $[\bar{1}\bar {1}\bar{1}]_{\mathrm{B}}$

The heteroepitaxial growth process of InAs nanowires (NW) on GaAs $[\bar{1}\bar{1}\bar{1}]_{\mathrm{B}}$ substrate was investigated by X-ray grazing-incidence diffraction using synchrotron radiation. For crystal growth we applied the vapor–liquid–solid (VLS) growth mechanism via gold seeds. The general sample structure was extracted from various electron microscopic and X-ray diffraction experiments. We found a closed Ga x In1−x As graduated alloy layer at the substrate to NW interface which was formed in the initial stage of VLS growth with a Au–Ga–In liquid alloy. With ongoing growth time a transition from this VLS layer growth to the conventional VLS NW growth was observed. The structural properties of both VLS grown crystal types were examined. Furthermore, we discuss the VLS layer growth process.

3C-SiC Heteroepitaxy on (100), (111) and (110) Si Using Trichlorosilane (TCS) as the Silicon Precursor.

The aim of this work is to improve the heteroepitaxial growth process of 3C-SiC on Si substrates using Trichlorosilane (SiHCl3) as the silicon growth precursor. With this precursor it has been shown that it is possible to simultaneously increase the growth rate of the process and avoid the nucleation of silicon droplets in the gas phase. Growth experiments were conducted on three (3) Si substrate orientations in order to assess the impact of the Si substrate on the resulting 3C-SiC film. X-ray Diffraction (XRD), Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM) analysis show the important role of the substrate orientation for the growth process. The different orientation of the substrate modifies the morphology of the 3C-SiC crystalline structure, mostly by changing the density of micro-twins and stacking faults inside the film.

Real time observation of ultrathin epitaxial oxide growth during alloy oxidation**We write this paper in memorial to René Franchy [1

We have studied the thermal oxidation of the intermetallic alloy CoGa in situ, in real time on the atomic scale, during the growth of an ultrathin, epitaxial Ga oxide layer. On the basis of an extended set of surface x-ray diffraction data, density functional theory calculations and core level spectroscopy data, we find that the oxide film consists of an oxygen ion double layer, which contains the basic building block of bulk β-Ga2O3. The oxide formation takes place via the nucleation of two-dimensional, anisotropic oxide islands which laterally grow and coalesce. A dramatic increase of the oxide island size is observed for low O2 pressures in the 10−8 mbar regime, which we interpret as the onset of a step flow like growth mode. This allows us to conclude that thermal oxidation can be considered as a hetero-epitaxial growth process, that follows similar atomistic growth principles to molecular beam epitaxy. As a consequence, the structural perfection of the oxide layer can be tailored by the appropriate choice of oxygen pressure and temperature.

Growth of 3C-SiC on Si Molds for MEMS Applications

A hetero-epitaxial 3C-SiC growth process in a low-pressure hot-wall CVD reactor has been developed on planar Si (100) substrates. The growth rate achieved for this process was about 10 μm/h. The process consists of silane/propane/hydrogen chemistry with HCl used as a growth additive to increase the growth rate. 3C-SiC has also been grown on 22, 52 and 123 +m deep etched MEMS structures formed by DRIE of (100) Si at a rate of about 8 +m/h. Secondary electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD) were used to analyze the quality of the 3C-SiC films.

A self-assembled molecular rectifier: two-dimensional crystals of cytochrome c formed on a flavolipid monolayer

In order to fabricate a molecular rectifier composed of a donor–acceptor molecular pair, we studied two-dimensional (2D) crystallization of cytochrome c (cyt c) on a flavin monolayer. Since the fluidity of a supporting monolayer on which proteins are crystallized is an essential factor for self-assembly of proteins, a flavolipid, flavophosphatidylcholine, was designed and synthesized for an electro-active supporting monolayer with fluidity. First, it was confirmed that a stable flavolipid monolayer with 2D molecular array was formed as a Langmuir monolayer. Then, the adsorption of cyt c molecules onto a flavolipid monolayer was performed at the air/water interface in a Langmuir trough. Finally, it was found that cyt c molecules were self-assembled to form 2D crystals on a flavolipid monolayer. The 2D crystals of cyt c were revealed to grow by a hetero-epitaxial growth process regulated by the 2D molecular arrays of a flavolipid monolayer. It was also found that cyt c molecules were adsorbed onto a flavolipid monolayer with specific molecular orientation facing the heme-crevice side of the cyt c molecule toward a flavolipid monolayer. The orientational adsorption of cyt c onto a flavolipid monolayer made it possible to transfer electrons one-way from flavolipid to cyt c based on the free energy difference as a molecular rectifier.

Diamond heteroepitaxy: pattern formation and mechanisms

Pattern formation that occurs during the initial condensation of diamond can provide insights into the physical mechanisms responsible for crystal growth. From a practical perspective, it can be exploited to develop reliable heteroepitaxial growth processes. Using (001) Ir on sapphire (α-Al 2O 3) or strontium titanate (SrTiO 3) as substrate, and a methane–hydrogen plasma in the presence of d.c. bias, we reproducibly generate dense arrays, 10 12 cm −2, of orientationally-ordered (001) diamond nanocrystallites. They appear on the Ir substrate after the rapid quench brought about by abrupt bias termination. The crystallite sizes are highly monodisperse, with typical lateral size below 10 nm. Under the appropriate conditions, the nascent diamond crystals self-organize into a lattice with six-fold coordination and correlation lengths of several lattice parameters. The regularity of the patterns raises the possibility of a collective instability of the excited carbon condensate. Since the crystallite density is so high, subsequent growth leads to rapid coalescence within a few minutes, resulting in a smooth, continuous film that covers the substrate. When the growth step is extended in time, thick single crystal plates have been grown over areas approaching 1 cm 2. Particular emphasis is placed on the time evolution of emergent patterns during the bias stage, early coarsening of diamond grains, and the textures during extended growth. These results are placed in the context of dynamical systems driven far from equilibrium.