Frank-van Der Merwe Growth Mode Research Articles

Low power silicon evaporation source—Construction, performances, and applications

The construction of a low power silicon evaporation source for thin film deposition applications is proposed in this article. A few differently shaped tungsten filaments were mounted inside a quartz glass crucible, which assured an effective heating of silicon wafer pieces. The sublimation process was monitored at filament powers in the range of 8–22 W, which corresponds to temperatures far below the melting point of Si. The operation of the evaporation source requires only the use of a low voltage power supply. All considered models of evaporation sources are characterized by an easy construction. The measurements carried out with the use of a quartz crystal microbalance sensor enabled to determine the deposition rate at different filament powers for all constructed Si sources and confirm the long-term stability of the silicon flux. The experimental data exhibit the Polany–Wigner dependence of the deposition rate as a function of inverse of power at higher filament powers. Auger electron spectroscopy was used to monitor the deposition of Si on Ag(100) under constant silicon flux. The Auger signal recorded from Si and Ag reflects the growth of subsequent silicon layers. This enabled the determination of the Frank–van der Merwe growth mode of Si on Ag(100) at early stages of growth, the formation time of one Si monolayer, and, thus, the deposition rate. The presented designs of the Si source exhibit long time stable evaporation fully controlled by the applied filament power, which is crucial in the precise adsorption of ultrathin Si layers.

Growing twisted bilayer graphene at small angles

Growing twisted bilayer graphene at small angles

Frank-van der Merwe growth in bilayer graphene

Frank-van der Merwe growth in bilayer graphene

Sputter-engineering a first-order magnetic phase transition in sub-15-nm-thick single-crystal FeRh films

Equiatomic FeRh alloys undergo a fascinating first-order metamagnetic phase transition (FOMPT) just above room temperature, which has attracted reinvigorated interest for applications in spintronics. Until now, all attempts to grow nanothin FeRh alloy films have consistently shown that FeRh layers tend to grow in the Volmer-Weber growth mode. Here we show that sputter-grown sub-15-nm-thick FeRh alloy films deposited at low sputter-gas pressure, typically $\ensuremath{\sim}0.1$ Pa, onto (001)-oriented MgO substrates, grow in a peening-induced Frank-van der Merwe growth mode for FeRh film thicknesses above 5 nm, circumventing this major drawback. The bombardment of high-energy sputtered atoms, the atom-peening effect, induces a rebalancing between adsorbate-surface and adsorbate-adsorbate interactions, leading to the formation of a smooth continuous nanothin FeRh film. Chemical order in the films increases with the FeRh thickness, ${t}_{\text{FeRh}}$, and varies monotonically from 0.75 up to 0.9. Specular x-ray diffraction scans around Bragg peaks show Pendell\"osung fringes for films with ${t}_{\text{FeRh}}\ensuremath{\ge}5.2$ nm, which reflects in smooth well-ordered densified single-crystal FeRh layers. The nanothin film's roughness varies from 0.6 down to about 0.1 nm as ${t}_{\text{FeRh}}$ increases, and scales linearly with the integral breadth of the rocking curve, proving its microstructured origin. Magnetometry shows that the FOMPT in the nanothin films is qualitatively similar to that of the bulk alloy, except for the thinnest film of 3.7 nm.

Compact protein (bovine serum albumin/human serum albumin) layer under Langmuir-Blodgett deposition on hydrophilic Si (001) surface

Langmuir monolayers of globular proteins (bovine serum albumin, i.e., BSA and human serum albumin, i.e., HSA) are formed on the water surface at pH ≈ 7.0, and compact protein layers are deposited on hydrophilic Si (001) surface using the Langmuir-Blodgett deposition method. Stability of these protein monolayers are studied from the normalized molecular area-time (A/A0-t) curves, which confirms that deposition over a long period of time is possible from both the protein monolayers. Compact bimolecular layered structure of the protein films are deposited on Si surface at higher surface pressure (17 mN/m) of the BSA and HSA monolayers, which is confirmed from the electron density profiles (EDPs) obtained from the analysis of the X-ray reflectivity data. Mostly the thickness of the molecular layers increases as the molecular tilting takes place and new molecules are deposited inside the vacant positions of the layer and as a result the electrons per unit area increases. EDPs obtained from the deposited BSA and HSA films also confirm that the increment in molecular tilting and electrons per unit area is protein specific. EDPs and surface morphology obtained from the atomic force microscopy images of the deposited films confirm that interrupted layer-by-layer or Frank-van der Merwe growth mode is followed in such protein multilayer deposition.

Simulation of the Inversion of the Aluminum Profile Gradient at Liquid Phase Epitaxy of GaAl(P, As, Sb) Heterostructures

The inversion of the aluminum profile gradient in the epitaxial Ga-Al-Nm (Nm=P, As, Sb) heterostructures grown by the forced cooling from a liquid phase were investigated.Simulation of the liquid phase epitaxy is carried out in the Pfann approximation in the following assumptions. The problem is one-dimensional, the epitaxial film grows in Frank–van der Merwe growth mode. The diffusion in the solid phase is frozen. The decrease in temperature is so slow that the diffusion in the liquid phase significantly exceeds the solid film growth rate. The degree of supersaturation of the liquid solution is so small that the composition of growing solid layers is determined by the equilibrium phase diagram. Given that in the liquid phase gallium predominates and the fraction of aluminum and phosphorus, arsenic or stibium do not exceed several percent, the liquid phase is considered as a dilute quasi-regular solution. The crystalline phase is considered as an ideal solid solution due to almost identical crystalline lattices in each pair GaP-AlP, GaAs-AlAs, GaSb-AlSb.Numerical calculations are carried out for growth processes in several different initial compositions of the growth solution: for Ga-Al-P – from 1500 K liquidus isoterm, for Ga-Al-As – from 1200 K and for Ga-Al -Sb – from 850 K. The results are presented in the graphs of the solid phase composition depended of the crystallization temperature and of the film layer depth, as well as on the diagram of moving the liquid phase figurative point during the growth process. Comparison of simulation results with experimental data confirms the correctness of the applied model.It is shown that at each temperature there is an inverse composition characterized by a certain content of aluminum in the solid phase. If, on cooling, the figurative point of the growth solution intersects the inverse curve then an inversion of the aluminum profile gradient occurs: the epitaxial film, which previously was growing with a decrease in the aluminum, increases further with its increase. The reason for such a phenomenon is the possible compensation of two interconnected competing factors when the growth solution cools: a decrease in the amount of nonmetal in the liquid phase and an increase in the aluminum segregation coefficient. The relationship between the inverse curve shape and the phase diagram parameters is established. The principal possibility of growing in a single technological process of Ga-Al-P and Ga-Al-As (but not Ga-Al-Sb) planar heterostructures suitable for the manufacturing of microelectronic waveguide devices is shown.Ref. 22, fig. 9, tabl. 1.

Dynamics of faceted thin films formation during vapor deposition

In this study, an anisotropic phase-field model was established to simulate the growth of crystalline thin films during vapor deposition. The formation and evolution of characteristic surface with faceted morphologies were demonstrated, in accordance with the regularly obtained microstructure in the actual experiments. In addition, the influences of deposition parameters, such as the deposition rate and the interfacial energy, on the formation mechanism of the characteristic morphology were also illustrated. While a relatively low surface energy of substrate was regarded, the faceted islands were formed, owing to the anisotropic interfacial energy of thin films. In the condition of a high surface energy of substrate, the layered structures of deposited films were produced, which was corresponding to the Frank–van der Merwe growth mode. As various deposition rates were utilized in the numerical simulations, diverse surface morphologies were developed on the basis of the dominant mechanisms, correlating with the adatom diffusion and the deposition kinetics. According to the calculation results, it was observed that a surface character with flattened morphology was generally driven by the adatom diffusion, while the factor of the deposition kinetics was inclined to roughen the surface of thin films. These numerical simulations enhanced the knowledge of thin film growth and facilitated the progress of the vapor deposition technology for advanced applications.

Versatile Solution-Processed Synthesis of Two-Dimensional Ultrathin Metal Chalcogenides Following Frank-van der Merwe Growth.

Two-dimensional (2D) ultrathin metal chalcogenides represent a class of promising materials for various applications thanks to attractive physicochemical properties. However, a reliable pathway for fabricating ultrathin metal chalcogenides nanosheets, regardless of the bulk crystals of their 3D counterparts, still remains a challenge. Herein, we present a versatile solution-processed template synthesis strategy, in which a single molecular-level precursor anneals to ultrathin single-crystal nanosheets with the aid of lattice-matching templates, following the Frank-van der Merwe growth mode and featuring high quality, low cost, scalability, and processability. Following this strategy, Sb2S3, MoS2, and ZnS nanosheets are successfully prepared as representatives for materials whose bulk counterparts possess 1D, 2D, and 3D crystal structures, respectively, and the growth mechanism is confirmed by crystal mode analysis. As a proof-of-concept application, MoS2 and Sb2S3 nanosheets are used for gas sensor and flexible photodetector applications, respectively, which exhibit excellent performance. The method can also be easily extended to other ultrathin nanosheets like single metals, metal oxide, metal nitride, and heterostructures.

Fe on W(001) from continuous films to nanoparticles: Growth and magnetic domain structure

The evolution of the structural and magnetic properties of Fe films during growth on the W(001) surface have been studied with low energy electron diffraction, real-time low energy electron microscopy, and quasi-real-time, spin-polarized low energy electron microscopy in the absence of a magnetic field (virgin state). Depending on the growth temperature, different growth modes are observed: growth of atomically rough and highly strained (10.4% tensile) pseudomorphic films at room temperature, kinetically limited layer-by-layer growth (quasi--Frank-van der Merwe growth mode) of smooth pseudomorphic films up to 4 monolayers at around 500 K and growth of fully relaxed three-dimensional Fe islands on top of a thermodynamically stable 2-monolayer-thick wetting layer (Stranski-Krastanov growth mode) at and above 700 K. Around 500 K, layered growth is terminated by partial (2 monolayers) dewetting of the metastable Fe film and formation of thin, partially relaxed, elongated islands on a thermodynamically stable 2 monolayer film. Ferromagnetic order is first detected during growth at room temperature at 2.35 monolayer Fe film thickness. The magnetization is in-plane with a thickness-dependent direction, rotating from the substrate $\ensuremath{\langle}110\ensuremath{\rangle}$ directions at 3 monolayers toward the $\ensuremath{\langle}100\ensuremath{\rangle}$ directions at 4 monolayers and back again toward the $\ensuremath{\langle}110\ensuremath{\rangle}$ directions at about 8 monolayers. The in-plane spin reorientation that occurs at room temperature is accompanied by significant changes of the magnetic domain structure. In the Frank-van der Merwe growth regime, large magnetic domains are observed in metastable 3 and 4 monolayer films. The isolated three-dimensional Fe islands that form in the Stranski-Krastanov regime have vortex, quasi-single domain ($C$ state), or single magnetic domain structures, depending on their size and shape. The detailed results that are obtained with high thickness, lateral and azimuthal angular resolution with spin-polarized low energy electron microscopy are compared with earlier laterally averaging and laterally resolving magnetic studies, and discrepancies are explained.

Controlling nucleation of monolayer WSe2 during metal-organic chemical vapor deposition growth

Tungsten diselenide (WSe2) is a semiconducting, two-dimensional (2D) material that has gained interest in the device community recently due to its electronic properties. The synthesis of atomically thin WSe2, however, is still in its infancy. In this work we elucidate the requirements for large selenium/tungsten precursor ratios and explain the effect of nucleation temperature on the synthesis of WSe2 via metal-organic chemical vapor deposition (MOCVD). The introduction of a nucleation-step prior to growth demonstrates that increasing nucleation temperature leads to a transition from a Volmer–Weber to Frank–van der Merwe growth mode. Additionally, the nucleation step prior to growth leads to an improvement of WSe2 layer coverage on the substrate. Finally, we note that the development of this two-step technique may allow for improved control and quality of 2D layers grown via CVD and MOCVD processes.

A step-by-step experiment of 3C-SiC hetero-epitaxial growth on 4H-SiC by CVD

To investigate the growth mechanism of hetero-epitaxial SiC, a step-by-step experiment of 3C-SiC epitaxial layers grown on 4H-SiC on-axis substrates by the CVD method are reported in this paper. Four step experiments with four one-quarter 4H-SiC wafers were performed. Optical microscopy and atomic force microscopy (AFM) were used to characterize the morphology of the epitaxial layers. It was previously found that the main factor affecting the epilayer morphology was double-positioning boundary (DPB) defects, which normally were in high density with shallow grooves. However, a protrusive regular “hill” morphology with a much lower density was shown in our experiment in high-temperature growth conditions. The anisotropic migration of adatoms is regarded as forming the morphology of DPB defects, and a new “DPB defects assist epitaxy” growth mode has been proposed based on the Frank-van der Merwe growth mode. Raman spectroscopy and X-ray diffraction were used to examine the polytypes and the quality of the epitaxial layers.

In situ AFM investigation of heterogeneous nucleation and growth of sodium oxalate on industrial gibbsite surfaces in concentrated alkaline solution

In situ atomic force microscopy (AFM) was used to study the nucleation behaviour of sodium oxalate on the gibbsite substrate in high ionic strength (5M NaCl) and alkaline solution (pH=11.5). The growth of oxalate three-dimensional (3D) islands was observed on gibbsite (001) faces at medium oxalate supersaturation (σ=(Cinitial−Cequlibrium)/Cequlibrium=1.2). The 3D islands can be preferentially oriented with their (200) twin planes parallel to the gibbsite substrate (001) faces confirmed by both the calculation of lattice mismatch and AFM images. The lattice mismatch calculation also indicated that the most energetically favourable epitaxial relationship for oxalate growth on gibbsite is the growth of oxalate (200) planes on gibbsite (100) faces. In agreement with the prediction, the formation of heteroepitaxial layers of sodium oxalate on the gibbsite (100) faces were observed at high oxalate supersaturation (σ=1.4). The total strain energy ( γtotal ) in this oxalate layer was 6.62mJ/m2 smaller than the interfacial tension of the gibbsite-solution interface ( γgs ) indicating that oxalate growth on gibbsite (100) faces is governed by Frank-van der Merwe growth mode. Meanwhile, oxalate 3D islands form on such heteroepitaxial layers, suggesting the Stranski–Krastanow mode for oxalate epitaxial growth.

Evolution of kinetically controlled In-induced surface structure on Si(5 5 7) surface

• Evolution of In induced superstructures on Si(5 5 7) surface during RT and HT adsorption/desorption process. • Kinetics is governed by substrate temperature which exhibits various growth modes (FM, SK, VB) under different conditions. • Strain relaxation play significant role in the commencement of desorption/rearrangement of atoms. • A consolidated phase diagram of In/Si(5 5 7) interface has been reported with new √3 × √3-R30̊ and 4 × 1 phases. This paper introduces issue of kinetically controlled and temperature driven superstructural phase transition of Indium (In) on atomically clean high index Si(5 5 7)-7 × 1 surface. Auger electron spectroscopy analysis reveals that at room-temperature (RT) with a controlled incident flux of 0.002 ML/s; In overlayers evolve through the Frank-van der Merwe growth mode and yield a (1 × 1) diffraction pattern for coverage ≥1 ML. For substrate temperature <500 °C, growth of In follows Stranski–Krastanov growth mode while for temperature >500 °C island growth is observed. On annealing the In/Si(5 5 7) interface in the temperature range 250–340 °C, clusters to two dimensional (2D) layer transformation on top of a stable monolayer is predominated. In-situ RT and HT adsorption and thermal desorption phenomena revealed the formation of coverage and temperature dependent thermally stable In induced superstructural phases such as (4 × 1) at 0.5 ML (520 °C), (√3 × √3-R30°) at 0.3 ML (560 °C) and (7 × 7) at 0.1 ML (580 °C). These indium induced superstructures could be utilized as potential substrate for the growth of various exotic 1D/2D structures.

Influence of temperature on the controlled growth kinetics and superstructural phase formation of indium on a reconstructed Si (113) 3 × 2 surface

The kinetics of growth, thermal stability and superstructural phase formation of the indium atom on a reconstructed Si (113) 3 × 2 surface at room temperature (RT), as well as at high substrate temperature (HT), is discussed. It was observed that at a very low flux rate of 0.08 ML min−1, In-adsorption at RT follows the Frank-van der Merwe (FM) growth mode, while for HT (>200 °C), In-islands (the Volmer–Weber-growth mode) were formed. The residual thermal desorption (RTD) analysis revealed the anomalous behaviour of temperature-driven layering to the clustering rearrangement of In atoms on the Si (113) surface for RT- and 200 °C-grown systems. The RTD study also demonstrates the effect of temperature on growth kinetics as well as on the multilayer/monolayer desorption pathway. The calculated bilayer desorption energy was found to be different for RT- (TB, 0.48 eV) and HT- (TB, 1.57 eV) grown In/Si(113) systems, while the monolayer desorption energy (TM, 2.56 eV) was the same in both the cases. Various coverage-dependent superstructural phases, such as Si(113) 3 × 2 + 3 × 1, 3 × 1, 3 × 2 + 1 × 3 and 1 × 1, have been observed during the RT- and HT-growth of In on the Si (113) surface. A complete phase diagram of In/Si(113) is deduced which depicts the evolution of novel phases as a function of substrate temperature and coverage.

High quality relaxed Ge layers grown directly on a Si(0 0 1) substrate

After a long period of developing integrated circuit technology through simple scaling of silicon devices, the semiconductor industry is now embracing technology boosters such as strain for higher mobility channel material. Germanium is the logical supplement to enhance existing technologies, as its material behaviour is very close to silicon, and to create new functional devices that cannot be fabricated from silicon alone (Hartmann et al. (2004) [1]). Germanium wafers are, however, both expensive and less durable than their silicon counterparts. Hence it is highly desirable to create a relaxed high quality Ge layer on a Si substrate, with the provision that this does not unduly compromise the planarity of the system. The two temperature method, proposed by Colace et al. (1997) [2], can give smooth (RMS surface roughness below 1 nm) and low threading dislocation density (TDD <10 8 cm −2) Ge layers directly on a Si(0 0 1) wafer (Halbwax et al. (2005) [3]), but these are currently of the order of 1–2 μm thick (Hartmann et al. (2009) [4]). We present an in depth study of two temperature Ge layers, grown by reduced pressure chemical vapour deposition (RP-CVD), in an effort to reduce the thickness. We report the effect of changing the thickness, of both the low temperature (LT) and the high temperature (HT) layers, emphasising the variation of TDD, surface morphology and relaxation. Within this study, the LT Ge layer is deposited directly on a Si(0 0 1) substrate at a low temperature of 400 °C. This low temperature is known to generate monolayer islands (Park et al. (2006) [5]), but is sufficiently high to maintain crystallinity whilst keeping the epitaxial surface as smooth as possible by suppressing further island growth and proceeding in a Frank-van der Merwe growth mode. This LT growth also generates a vast number of dislocations, of the order of 10 8–10 9 cm −2, that enable the next HT step to relax the maximum amount of strain possible. The effect of varying the HT layer thickness is studied by depositing on a LT layer of fixed thickness (100 nm) at a higher growth temperature of 670 °C. We find that the HT layer allows Ge-on-Ge adatom transport to minimise the surface energy and smooth the layer. The final step to the technique is annealing at a high temperature that allows the dislocations generated to glide, increasing the degree of relaxation, and annihilate. We find that annealing can reduce the TDD to the order of 10 7 cm −2, but at a cost of a significantly roughened surface.

Growth Modes of Ba on Mo(110) Substrate

The growth modes of Ba deposited on a Mo (110) substrate have been investigated by using reflection high-energy electron diffraction (RHEED) and scanning electron microscopy (SEM). In this study, the RHEED specular beam intensity oscillations apparatus is employed to confirm the surface flatness of thin Ba layers on Mo(110) substrate. Ba grows on the Mo(110) substrate via the Stranski-Krastanov growth mode in the temperature range between 300 and 550 °C, in which a monolayer with a (1.5×1.8) structure grows initially, followed by the growth of Ba islands with the (111)Ba plane. Between room temperature (R.T.) and 300 °C, Ba grows on the Mo(110) substrate via the Frank-van der Merwe growth mode, however only several periods of RHEED specular beam intensity oscillations could be observed. Such results suggest that several flat Ba layers grew on Mo(110) substrate via Frank-van der Merwe growth mode and Ba layers have much difficulty in keeping the surface flatness on a Mo(110) substrate. In the temperature range from 300 to 550 °C, only single oscillation was observed, suggesting that the Ba/Mo(110) system adopts the Stranski-Krastanov growth mode at such temperatures with the (1.5×1.8) monolayer.

Morphological, structural and electrical investigations on non-polar a-plane ZnO epilayers

We report on the growth of non-polar a-plane ZnO by CVD on r-plane-sapphire-wafers, a-plane GaN-templates and a-plane ZnO single-crystal substrates. Only the homoepitaxial growth approach leads to a Frank–van-der–Merwe growth mode, as shown by atomic force microscopy. The X-ray-diffraction spectra of the homoepitaxial thin films mirror the excellent crystalline quality of the ZnO substrate. The morphological and the structural quality of the homoepitaxial films is comparable to the best results for the growth on c-plane ZnO-substrates. The impurity incorporation, especially of group III elements, seems to be reduced when growing on the non-polar a-plane surface compared to the c-plane films as demonstrated by secondary ion mass spectrometry (SIMS). Optical properties have been investigated using low temperature photoluminescence measurements. We employed capacitance–voltage measurements ( C– V) to measure the background carrier density and its profile from substrate/film interface throughout the film to the surface. In thermal admittance spectroscopy (TAS) specific traps could be distinguished, and their thermal activation energies and capture cross sections could be determined.

Heteroepitaxial praseodymium sesquioxide films on Si(1 1 1): A new model catalyst system for praseodymium oxide based catalysts

The structure, growth and stoichiometry of heteroepitaxial Pr 2O 3 films on Si(1 1 1) were characterized by a combined RHEED, XRD, XPS and UPS study in view of future applications as a surface science model catalyst system. RHEED and XRD confirm the growth of a (0 0 0 1) oriented hexagonal Pr 2O 3 phase on Si(1 1 1), matching the surface symmetry by aligning the 〈 1 0 1 ¯ 0 〉 oxide in-plane direction along the 〈 0 1 1 ¯ 〉 Si azimuth. After an initial nucleation stage RHEED growth oscillation studies point to a Frank-van der Merwe growth mode up to a thickness of approximately 12 nm. XPS and UPS prove that the initial growth of the Pr 2O 3 layer on Si up to ∼1 nm thickness is characterized by an interface reaction with Si. Nevertheless stoichiometric Pr 2O 3 films of high crystalline quality form on top of these Pr–silicate containing interlayers.

Surface structures of Fe on Mo(1 1 0) surface investigated by RHEED

The surface structures of iron deposited on a Mo(110) surface have been investigated by reflection high-energy electron diffraction (RHEED), and three kinds of surface structures were observed. Up to 1.0ML of Fe deposition thickneeses, 1 × 1 structure appeared in all substrate temperature range from RT to 1200°C. At thicknesses ranging from 0.5 to 1.0ML, α structure which is the commensurate structure expressed by [31, 02] in matrix notation appeared at 300–550°C. More than 1.0ML, the β structure appeared at 400–800°C. The β structure is also the commensurate structure expressed by [120, 012], exhibiting the same atomic arrangement as bcc(110) surface. In the thickness range beyond 1.0ML, Fe film grew as the Frank–van der Merwe growth mode at room temperature to 400°C and Stranski–Krastanov growth mode from 400 to 800°C, in which the β structure was the first monolayer.

A structural analysis of Bi/Si(1 0 0) 2 × n surfaces by ICISS

We have investigated the structure of Bi-adsorbed Si(100) 2×n surfaces with various Bi coverages by low-energy electron diffraction (LEED) and impact-collision ion scattering spectroscopy (ICISS). Intensities of He+ scattered from Bi and Si atoms were measured as a function of ion incidence angle. Bi–Bi interatomic distances for the toplayer including Bi-dimers were estimated by the shadow-cone technique. Bi-signal peaks appeared even at considerably large angles of incidence, which are attributed to collisions with Bi atoms in sub-surface layers. Bi adsorption is not completed at a mono-layer level, but continues to form multiple layers, fitted in the Si lattice, in the Frank-van der Merwe growth mode.