Intrinsic stress of ultrathin epitaxial films

The present article focuses on the stress developing during the deposition of ultrathin epitaxial films in the thickness range of a few atomic layers. The studied systems exhibit the three well-known modes of film growth: Stranski–Krastanow mode [Ge/Si(001), Ge/Si(111), Ag/Si(111)], Frank–Van der Merwe mode [Fe/MgO(001)] and Volmer–Weber mode [Ag/mica(001), Cu/mica(001)]. The experimental results demonstrate the important role of the misfit strain as well as the contribution of surface stress effects as mechanisms for the stress in single atomic layers. Received: 26 April 1999 / Accepted: 25 June 1999 / Published online: 6 October 1999     

c-Axis orientation control of YBa2Cu3O7−x films grown on inclined-substrate-deposited MgO-buffered metallic substrates

Biaxially textured YBa₂Cu₃O_7−x (YBCO) films were grown on non-textured metal substrates with inclined-substrate-deposited (ISD) MgO as template. The biaxial texture feature of the films was examined by X-ray pole-figure analysis, φ-scan, and 2θ-scan. A tilt angle of 32° of the MgO[001] with respect to the substrate normal was observed. Epitaxial growth of YBCO films with c-axis tilt angle of 32° with respect to the substrate normal was obtained on these substrates with SrTiO₃(STO) as buffer layer. Whereas, by choosing yttria-stabilized ZrO₂ and CeO₂ instead of STO as buffer layer, a c-axis untilted YBCO film was obtained. Higher values of T_c=91 K and J_c=5.5×10⁵ A/cm² were obtained on the c-axis untilted YBCO films with 0.46 μm thickness at 77 K in zero field. Comparative studies revealed a unique role of CeO₂ in controlling the orientation of the YBCO films grown on ISD-MgO buffered metal substrates.     

Preparation and photocatalytic activity of CeO2/TiO2 interface composite film

The CeO₂/TiO₂ and TiO₂/CeO₂ interface composite films were prepared on glass substrates by the sol-gel process via dip-coating and calcining technique. The scanning electron microscopy (SEM) revealed that the TiO₂ layer has a compact and uniformity glasslike surface with 200 nm in thickness, and the CeO₂ layer has a coarse surface with 240 nm in thickness. The X-ray diffractometer (XRD) analysis showed that the TiO₂ layer is made up of anatase phase, and the CeO₂ layer is structured by cubic fluorite phase. Through a series of photo-degradation experiments, the relationship of the photocatalytic activity with the constituents of the films was studied. In virtue of the efficient interfacial charge separation via the process of electron transfer from TiO₂ to CeO₂, the photocatalytic activity of the CeO₂/TiO₂ composite film is high. Contrarily, the photocatalytic activity of the TiO₂/CeO₂ composite film is low, due to its inert surface made up of CeO₂ with broad bandwidth. Apart from the effect of the film structure, the effect of film thickness on photocatalytic activity was also discussed.     

Effect of Al Doping on Properties of SiC Films

Undoped and Al-doped 3C-SiC films are deposited on Si（100） substrates by low-pressure chemical vapour deposition. Effects of aluminium incorporation on crystallinity, strain stress, surface morphology and growth rate of SiC films have been investigated. The x-ray diffraction patterns and rocking curves indicate that the crystallinity is improved with aluminium doping. Raman scatting patterns also demonstrate that the strain stress in SiC films is released due to the incorporation of Al ions and the increase of film thickness. Furthermore, due to the catalysis of surface reaction which is induced by trimethylaluminium, the growth rate is increased greatly and the growth process varies from three-dimensional island-growth mode to step-flow growth mode.     

Effect of microstructure of MgO buffer layer on BaTiO3 grown on silicon substrates

MgO ultrathin films were grown on Si(1 0 0) substrates as buffer layers for the growth of ferroelectric BaTiO₃ thin films by laser molecular beam epitaxy (L-MBE). The deposition process of MgO buffer layers grown on silicon was in situ monitored by reflection high-energy electron diffraction (RHEED). The structure of BaTiO₃ films fabricated on MgO buffers was investigated by X-ray diffraction. Biaxially textured MgO was obtained at high laser energy density, but when the laser energy was lowered, MgO buffer was transformed to the form of texture with angular dispersion with the increase of the film thickness. BaTiO₃ films grown on the former buffer were completely (0 0 1) textured, while those on the latter were (0 0 1) preferred orientated. Furthermore, the fabricated MgO buffers and BaTiO₃ films had atomically smooth surface and interface. All these can reveal that the quality of textured MgO buffer is a key factor for the growth of BaTiO₃ films on silicon.     

New design of nozzle structures and its effect on the surface and crystal qualities of thick GaN using a horizontal HVPE reactor

High-quality thick GaN films without cracks were achieved by using a new nozzle structure in the reactor grown by the hydride vapor phase epitaxy on sapphire substrates. Optical contrast microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray double diffraction (XRD) and cathodoluminescence (CL) were carried out to reveal the surface, crystal and optical properties of the GaN epilayer. It was found that the nozzle structure in the reactor has a large effect on the growth rate, surface flat, crystal quality, and the uniformity of the growth. Compared with the old one, the new nozzle structure (denoted as multi-layers nozzle) can improve dramatically the properties of thick GaN. Mirror, colorless and flat GaN thick film was obtained and its (0 0 0 2) FWHM results were reduced from 1000 to 300 arcsec when the new nozzle was used. AFM result revealed a step flow growth mode for GaN layer with the new nozzle. Room-temperature CL spectra on the GaN films showed a strong near-band-edge peak for the new nozzle, but there is only weak emitting peak for the old nozzle. New nozzle structure can improve the uniform of flow field near the surface of substrates compared with the old one, which leads to the improvement of properties of GaN thick film by hydride vapor phase epitaxy (HVPE).     

Influence of Different Interlayers on Growth Mode and Properties of InN by MOVPE

We grow InN epilayers on different interlayers by metal organic vapour phase epitaxy （MOVPE） method, and investigate the effect of interlayer on the properties and growth mode of InN films. Three InN samples were deposited on nitrided sapphire, low-temperature InN （LT-InN） and high-temperature GaN （HT-GaN）, respectively. The InN layer grown directly on nitrided sapphire owns the narrowest x-ray diffraction rocking curve （XRC） width of 300 aresee among the three samples, and demonstrates a two-dimensional （2D） step-flow-like lateral growth mode, which is much different from the three-dimensional （3D） pillar-like growth mode of LT-InN and HT-GaN buffered samples. It seems that mismatch tensile strain is helpful for the lateral epitaxy of InN film, whereas compressive strain promotes the vertical growth of InN films.     

Stability of MgO(1 1 1) films grown on 6H-SiC(0 0 0 1) by molecular beam epitaxy for two-step integration of functional oxides

Crystalline magnesium oxide (MgO) (1 1 1), 20 Å thick, was grown by molecular beam epitaxy (MBE) on hydrogen cleaned hexagonal silicon carbide (6H-SiC). The films were further heated to 740 °C and 650 °C under different oxygen environments in order to simulate processing conditions for subsequent functional oxide growth. The purpose of this study was to determine the effectiveness and stability of crystalline MgO films and the MgO/6H-SiC interface for subsequent heteroepitaxial deposition of multi-component, functional oxides by MBE or pulsed laser deposition processes. The stability of the MgO films and the MgO/6H-SiC interface was found to be dependent on substrate temperature and the presence of atomic oxygen. The MgO films and the MgO/6H-SiC interface are stable at temperatures up to 740 °C at 1.0 × 10⁻⁹ Torr for extended periods of time. While at temperatures below 400 °C exposure to the presence of active oxygen for extended periods of time has negligible impact, exposure to the presence of active oxygen for more than 5 min at 650 °C will degrade the MgO/6H-SiC interface. Concurrent etching and interface breakdown mechanisms are hypothesized to explain the observed effects. Further, barium titanate was deposited by MBE on bare 6H-SiC(0 0 0 1) and MgO(1 1 1)/6H-SiC(0 0 0 1) in order to evaluate the effectiveness of the MgO as a heteroepitaxial template layer for perovskite ferroelectrics.     

Effects of the sputtering time of ZnO buffer layer on the quality of GaN thin films

ZnO thin films with different thickness (the sputtering time of ZnO buffer layers was 10 min, 15 min, 20 min, and 25 min, respectively) were first prepared on Si substrates using radio frequency magnetron sputtering system and then the samples were annealed at 900 °C in oxygen ambient. Subsequently, a GaN epilayer about 500 nm thick was deposited on ZnO buffer layer. The GaN/ZnO films were annealed in NH₃ ambient at 950 °C. X-ray diffraction (XRD), atom force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) were used to analyze the structure, morphology, composition and optical properties of GaN films. The results show that their properties are investigated particularly as a function of the sputtering time of ZnO layers. For the better growth of GaN films, the optimal sputtering time is 15 min.     

Amorphous film thickness dependence for epitaxy of perovskite oxide films under excimer laser irradiation

We have studied the epitaxial growth of perovskite manganite LaMnO₃ (LMO) on SrTiO₃(1 0 0) in the excimer laser assisted metal organic deposition process. The LMO was preferentially grown from the substrate surface by the KrF laser irradiation. The study of amorphous LMO film thickness dependence on epitaxial growth under the excimer laser irradiation revealed that the photo-thermal heating effect strongly depended on the amorphous film thickness due to a low thermal conductivity of amorphous LMO: the ion-migration for chemical bond-forming at the reaction interface would be strongly enhanced in the amorphous LMO film with the large film thickness about 210 nm. On the other hand, the photo-chemical effect occurred efficiently for the amorphous film thickness in the range of 35-210 nm. These results indicate that the epitaxial growing rate was dominated by the photo-thermal heating after the photo-chemical activation at the growth interface.     

Strain relaxation in SiGe layer during wet oxidation process

The strain relaxation in SiGe layer on silicon substrate during wet oxidation at 1000 °C was investigated. It was proposed that the competition between Ge accumulation and diffusion led to different strain-relaxation behaviors. At the very beginning, Ge atoms at the oxidizing interface were quickly accumulated due to the high oxidation rate resulting in the additional nucleation of misfit dislocations (therefore a lot of threading dislocations) to relieve stress after the thickness of the Ge condensed layer was larger than the critical value. And then, when the Ge accumulation rate was less than the diffusion rate, Ge content started to decrease from a maximum value and the strain in the SiGe layer was mainly relieved through surface roughing and the degree of strain relaxation reached a maximum. When the samples were further oxidized, Ge accumulation could be neglected because of the self-limiting oxidation and the Ge diffusion dominated the consequent processes. As a result, Ge content at the interface was reduced, with the contribution of the strain relaxation in SiO₂ viscously, leading to the decrease of degree of strain relaxation in the SiGe layers slowly.     

Optical parameters in SnO2 nanocrystalline textured films grown on p-InSb (111) substrates

Spectroscopic ellipsometry and photoluminescence (PL) measurements on SnO₂ nanocrystalline textured films grown on p-InSb (111) substrates by using radio-frequency magnetron sputtering at low temperature were carried out to investigate the dependence of the optical parameters on the SnO₂ thin film thickness. As the SnO₂ film thickness increases, while the energy gap of the SnO₂ film decreases, its refractive index increases. The PL spectra show that the broad peaks corresponding to the donor-acceptor pair transitions are dominant and that the peak positions change with the SnO₂ film thickness. These results can help improve understanding for the application of SnO₂ nanocrystalline thin films grown on p-InSb (111) substrates in potential optoelectronic devices based on InSb substrates.     

Preparation and structural properties of MgO films grown on GaAs substrate

Epitaxial MgO thin films have been grown on semiinsulating GaAs (0 0 1) substrates using electron beam (e-beam) evaporation. X-ray diffraction indicates c-axis oriented MgO with (0 0 2) reflection only and rocking curve widths ∼2.2-3°. Transmission electron microscopy (TEM) analyses confirm an epitaxial growth of the MgO films. We study the microstructure and the defects at the interface between the MgO film and the GaAs substrate. Auger electron Spectroscopy (AES) concentration depth profiles reveal no contamination of the MgO films by As and Ga at different temperatures of the deposition process.     

Surface morphology of highly mismatched InSb films grown on GaAs substrates by molecular beam epitaxy

InSb films on GaAs(001) substrates with and without GaAs buffer layer have been grown by molecular beam epitaxy. Rather than surface undulations, aligned ripples and pyramidal hillocks along the orthogonal 〈110〉 directions were observed on the surface of InSb films. Both the preferential growth and the termination of ripples were proved to be related to strain‐driven mass transport. A model was proposed to elucidate the formation of the hillocks, which are more efficient to relax strain than ripples. Due to the strain relaxation through hillocks with small bases predominantly, the surfaces of the InSb films grown without a GaAs buffer layer are smoother than those of films grown with a GaAs buffer layer. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of completely (110)-oriented Pt film on Si (100) by using MgO as a buffer by pulsed laser deposition

(110)-textured MgO films were grown on Si (100) with etching and without etching by pulsed laser deposition. The deposited MgO films were shown to be droplets-free. The MgO film was used as a buffer layer to further grow Pt film on Si (100). A completely (110)-oriented Pt film was obtained on such a buffer layer and its surface is very smooth with a roughness of about 7.5 nm over 5×5 μm. This can be used as a new oriented Pt electrode on silicon for devices. Received: 23 January 2001 / Accepted: 27 April 2001 / Published online: 27 June 2001     

Synthesis and characterization of (h 0 h)-oriented silicalite-1 films on α-Al2O3 substrates

A simple and well-designed synthesis procedure is proposed to fabricate silicalite-1 films on porous α-Al₂O₃ substrates on purpose of preventing the aluminum leaching. The continuous and 2 μm thick seed layer of silicalite-1 crystals is fabricated by using a spin coater. The first-time seeded growth is performed to synthesize a thin layer of intergrown ZSM-5 crystals on the silicalite-1 seed layer, where the use of low alkalinity and short synthesis time is to reduce the aluminum leaching. The intergrown layer of ZSM-5 crystals serves as a barrier to block the aluminum leaching from porous α-Al₂O₃ substrates in the second-time seeded growth, leading to the formation of ca. 11 μm thick intergrown and oriented silicalite-1 films with an extremely high Si/Al ratio. According to SEM images and XRD measurements, the as-synthesized silicalite-1 film is dense, continuous, and (1 0 1)-oriented. The electron probe microanalysis (EPMA) of the resulting film demonstrates that there is no aluminum leaching in the second-time seeded growth. The leaking tests confirm that non-zeolitic pores in the silicalite-1 film are negligible.     

Influence of strain on the atomic and electronic structure of manganite films

A study of the long-range, local and electronic structure of Nd_0.5Sr_0.5MnO₃ films of varying thickness between 500 and 2000 Å has been performed. Local structure measurements at the Sr K-edge reveal a reduction of the Mn-O-Mn bond angles in films below 1000 Å. Spin-polarized measurements reveal splitting of the Mn 3d e_g state in the strained region of the films and are consistent with a two-layer model for thick films with a relaxed undistorted layer on top of a strained structurally distorted layer near the substrate.     

Thickness dependence of electrical properties of ITO film deposited on a plastic substrate by RF magnetron sputtering

Indium tin oxide (ITO) films with various thicknesses in range of 40-280 nm were prepared onto a plastic substrate (PMMA). Deposition was carried out with RF magnetron sputtering method and the substrate temperature was held at ∼70 °C, in lack of the thermal damage to the polymer substrate. Changes in microstructure and electrical properties of ITO films according to their thicknesses were investigated. It was found that amorphous layer with thickness of 80 nm was formed at the interface on the polymer substrate and polycrystalline ITO could be obtained above the thickness. Conductivity of ITO films was found to be strongly dependent on the crystallinity. Consequently, it is suggested that crystallinity of the deposited films should be enhanced at the initial stage of deposition and the thickness of amorphous region be reduced in order to prepare high quality ITO thin films on polymer substrates.     

Crystallization of amorphous SiC and superhardness effect in TiN/SiC nanomultilayers

TiN/SiC nanomultilayers with various constituent layer thicknesses were prepared by magnetron sputtering using TiN and SiC ceramic targets. X-ray diffractometer, scanning electron microscope, energy dispersive spectrometer, high-resolution transmission electron microscope, atomic force microscope and nanoindenter were employed to study the growth, microstructure and mechanical properties of these films. Experimental results revealed that amorphous SiC, which is more favorable under normal sputtering conditions, was forced to crystallize and grew epitaxially with TiN layers at thicknesses of less than 0.8 nm. The resultant films were found to form strong columnar structures, accompanied with a remarkable hardness increment. Maximal nanoindentation hardness as high as 60.6 GPa was achieved when SiC thickness was ∼0.6 nm. A further increase of SiC thickness caused the formation of amorphous SiC, which blocked the epitaxial growth of the multilayers, resulting in the decline of film's hardness. Additionally, investigations on multilayers different in TiN layer thicknesses showed that they are insensitive in both microstructure and hardness to the fluctuation of TiN layer thickness. The formation of epitaxially grown structure between crystalline SiC and TiN layers was found to be responsible for the obtained superhardness in multilayers.     

Angular and NiFe thickness dependence of exchange bias in IrMn/NiFe/IrMn thin film

Exchange biased IrMn/NiFe/IrMn thin films were studied as a function of NiFe thickness. In plane angular dependence of a resonance field distribution which is measured by FMR was analyzed as a combined effect of an unidirectional anisotropy and an uniaxial anisotropy. The unidirectional anisotropic field and the uniaxial anisotropic field were linearly varied with NiFe thickness while the films with a thicker NiFe layer do not follow the linear variation. Resonance field and linewidth variations were also analysed with NiFe thickness.