A novel theoretical framework for predicting heteroepitaxial film growth modes

Thin low-temperature heteroepitaxial film growth of Si on sapphire (SOS) was investigated by using the reactive ion beam deposition method proposed recently. This method uses low-energy controlled ionized species produced from reactive SiH4 gas plasma. By accommodating the ion energy to two film growth stages, i.e., an initial heteroepitaxial growth stage followed by a homoepitaxial growth stage, thin SOS films about 200 nm in thickness with no superficial microtwins can be grown at the low temperature of 600 °C. The energy level is adjusted to 300 eV in the initial heteroepitaxial film growth stage (about 50 nm in thickness), and then to 100 eV until the total film thickness is about 200 nm.

Single-crystal diamond films grown over a large area may open up a new field of solid-states physics and industry, because diamond has several advantageous properties such as high thermal conductivity, hardness and wide bandgap semiconducting characteristics. These advantages are most effectively obtained in the single crystalline phase. The highly oriented or heteroepitaxial growth of diamond is one of the most promising ways of obtaining single crystalline films. Of various substrates, heteroepitaxial growth on silicon (Si) or silicon carbide (SiC) has received much attention [8.1–8], because perfect and large single crystals can be provided in Si, and the development of large single-crystal SiC and heteroepitaxial SiC films on Si has advanced rapidly in recent years. However, highly oriented or heteroepitaxial diamond growth has also been realized on other materials, such as near noble metals, to exhibit the unique qualities of diamond with respect to nucleation and growth.

The effect of lattice mismatch between the substrate and the heteroepitaxial film on the initial process of ultra-thin film growth, such as the nuclear density, the average size of cluster, the scale behavior and the growth mode, is studied by the kinetic Monte-Carlo method. It is found that the negative (positive) mismatch, corresponding to the compressive (tensile) strain, can induce the growth process transferring from the nucleation stage to the intermediate one earlier (later). The larger the mismatch, the more apparent the above behavior. Under the same deposition condition, the negative mismatch can bring about lower nuclear density and larger average cluster size than the positive mismatch does. Furthermore, the nuclear density is found to follow the power law of Ns≈(F/D)χ, andχdecreases from 0.37 to 0.33 while the mismatch changes from –0.04 to 0.02. This behavior indicates that the growth mode of the ultra-thin film changes from including dimer diffusion into excluding dimer diffusion. Additionally, the scale theory of the cluster size is found to be valid even for the heteroepitaxial growth.

The hydroxyl-radical-assisted growth of ZnO films by remote plasma-enhanced metal-organic chemical vapor deposition (RPE-MOCVD) was investigated. Plasma was generated by an RF discharge in O2. The change of the carrier gas from N2 to H2 resulted in a significant increase in deposition rate. It was found from the spectroscopic characterization of light emitted by the reactor gas in the vicinity of a substrate that the effect of the carrier gas on deposition rate is related to the occurrence of OH radicals. Oxygen radicals and OH radicals, which were observed by spectroscopic measurement, promoted film growth and suppressed deep-level emissions in photoluminescence spectra. To elucidate the effects of O and OH radicals, hydrogen and helium remote plasma techniques were also used in film growth for comparison. Finally, the ZnO films fabricated by the OH radical-assisted growth were investigated by photoluminescence and X-ray diffraction (XRD) analyses. In addition, the Zn-termination effects of the sapphire substrate surface on heteroepitaxial ZnO film growth weres investigated.

Asymmetric ferroelectricity and conduction of anomalous leakage current were observed in heteroepitaxial BaTiO3 thin films grown by rf magnetron sputtering at 600° C on three different electrode/substrate combinations: SrRuO3/SrTiO3, Pt/MgO and Nb doped SrTiO3. The voltage shift of hysteresis loops of the capacitance was a linear function of the thickness of the BaTiO3 films and became as large as 10 V in the film with a thickness of 410 nm. The asymmetry did not disappear even after a heat treatment carried out at 800° C in air. On the other hand, a steep increase in the leakage current was observed in the heteroepitaxial films when a positive voltage was applied. The origin of both the asymmetric hysteresis and the anomalous conduction is discussed in terms of asymmetric crystal structure caused by misfit dislocations introduced in heteroepitaxial growth.

Epitaxial Mn-doped BiFeO3 (MBFO) thin films were grown on GaAs (001) substrate with SrTiO3 (STO) buffer layer by pulsed laser deposition. X-ray diffraction results demonstrate that the films show pure (00l) orientation, and MBFO (100)//STO(100), whereas STO (100)//GaAs (110). Piezoresponse force microscopy images and polarization versus electric field loops indicate that the MBFO films grown on GaAs have an effective ferroelectric switching. The MBFO films exhibit good ferroelectric behavior (2Pr ∼ 92 μC/cm2 and 2EC ∼ 372 kV/cm). Ferromagnetic property with saturated magnetization of 6.5 emu/cm3 and coercive field of about 123 Oe is also found in the heterostructure at room temperature.

β-AlN thin films were heteroepitaxially grown on sapphire (0001) substrates with a smooth surface showing steps in nitrogen atmospheres by pulsed laser deposition using sintered AlN targets, and their films were structurally studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Horizontally oriented growth of β-AlN(111) was confirmed by XRD in 2θ–θ scan. The epitaxial relationship between β-AlN and sapphire was derived to be β-AlN(111) ∥ sapphire (0001) from the TEM measurement. The existence of dislocation defects was implied from the electron diffraction pattern of the film. The heteroepitaxial growth probably occurs in domain match epitaxy, accompanied by the generation of dislocations defects at the interface.

Angle- and time-resolved two-photon photoemission (2PPE) was used to study systems of organic semiconductors on Ag(111). The 2PPE studies focused on electronic behavior specific to interfaces and ultrathin films. Electron time dynamics and band dispersions were characterized for ultrathin films of a prototypical n-type planar aromatic hydrocarbon, PTCDA, and representatives from a family of p-type oligothiophenes.In PTCDA, electronic behavior was correlated with film morphology and growth modes. Within a fewmonolayers of the interface, image potential states and a LUMO+1 state were detected. The degree to which the LUMO+1 state exhibited a band mass less than a free electron mass depended on the crystallinity of the layer. Similarly, image potential states were measured to have free electron-like effective masses on ordered surfaces, and the effective masses increased with disorder within the thin film. Electron lifetimes were correlated with film growth modes, such that the lifetimes of electrons excited into systems created by layer-by-layer, amorphous film growth increased by orders of magnitude by only a few monolayers from the surface. Conversely, the decay dynamics of electrons in Stranski-Krastanov systems were limited by interaction with the exposed wetting layer, which limited the barrier to decay back into the metal.Oligothiophenes including monothiophene, quaterthiophene, and sexithiophene were deposited on Ag(111), and their electronic energy levels and effective masses were studied as a function of oligothiophene length. The energy gap between HOMO and LUMO decreased with increasing chain length, but effective mass was found to depend on domains from high- or low-temperature growth conditions rather than chain length. In addition, the geometry of the molecule on the surface, e.g., tilted or planar, substantially affected the electronic structure.

Summary form only given. It has been shown that even small concentrations of nitrogen have a substantial effect on the growth of CVD diamond films, while larger concentrations may result in degraded and rough growth. In the present study we show the effect of nitrogen concentration on the growth, texture, morphology, and crystalline quality of CVD diamond films. All films are heteroepitaxially grown on a 3 inch silicon substrate and are scratch-seeded with a seeding density of 10/sup 9//cm/sup 2/. A computer controlled 5 inch discharge 2.45GHz microwave plasma reactor with ultra-high vacuum gas handling systems combined with optical emission spectroscopy allows exact control of the gas phase concentrations and reactor input variables. Nitrogen gas phase concentration is varied from l0ppm to over 1000ppm and is controlled by the measurement of CN bands in the optical emission spectrum of the plasma. Pressure ranges from 38-50 Torr, methane/hydrogen from 0.5-2%, and flow rate from 50-200 sccm. Growth and morphology are carefully determined using SEM while texture is obtained using XRD. Crystalline quality is measured by determining the FWHM of the diamond Raman peak at 1332/cm using unpolarized micro-Raman spectroscopy. Detailed growth, texture, morphology, and crystalline quality maps are given to show the sequential effect of increasing nitrogen versus all other reactor input variables.

The effect of lattice-mismatch induced stress upon the crystallographic structure, strain, strain relaxation, and the generation of different types of defects in heteroepitaxially grown SrTiO3 films on CeO2 buffered sapphire is examined. Depending on the thickness of the SrTiO3 layer, characteristic changes in the structural perfection of the layers, their crystallographic orientation with respect to the substrate system, and their strain is observed. For thin films misfit dislocations partially compensate the stress in the SrTiO3 layer, whereas cracks develop in thicker SrTiO3 films. The cracks are orientated along two predominant crystallographic orientations of the sapphire. The structural modifications and the formation of misfit defects and cracks are explained in a model based on lattice misfit induced stress, on the one hand, and energy considerations taking into account the stress release due to crack formation and the energy necessary for the formation of new surfaces at the crack, on the other hand. The impact of lattice misfit is discussed in two steps, i.e., intrinsic and thermal induced misfits during heteroepitaxial film growth at a given temperature and the subsequent cooling of the sample, respectively. The comparison of the theoretical predictions and the experimental observations demonstrate that intrinsic mismatch and thermal mismatch have to be considered in order to explain strain dependent effects in complex heteroepitaxial layer systems such as induced ferroelectricity of SrTiO3 on sapphire.

Recently, Bi2O2Se was revealed as a promising two-dimensional (2D) semiconductor for next generation electronics, due to its moderate bandgap size, high electron mobility and pronounced ambient stability. Meanwhile, it has been predicted that high-quality Bi2O2Se-related heterostructures may possess exotic physical phenomena, such as piezoelectricity and topological superconductivity. Herein, we report the first successful heteroepitaxial growth of Bi2O2Se films on SrTiO3 substrates via pulsed laser deposition (PLD) method. Films obtained under optimal conditions show an epitaxial growth with the c axis perpendicular to the film surface and the a and b axes parallel to the substrate. The growth mode transition to three-dimensional (3D) island from quasi-2D layer of the heteroepitaxial Bi2O2Se films on SrTiO3 (001) substrates is observed as prolonging deposition time of films. The maximum value of electron mobility reaches 160 cm2 V−1 s−1 at room temperature in a 70 nm thick film. The thickness dependent mobility provides evidence that interface-scattering is likely to be the limiting factor for the relatively low electron mobility at low temperature, implying that the interface engineering as an effective method to tune the low temperature electron mobility. Our work suggests the epitaxial Bi2O2Se films grown by PLD are promising for both fundamental study and practical applications.

Growth studies of hetero-epitaxial thin films with x-rays

An atomistic study of growth mode and microstructure evolution of amorphous carbon films by different incident carbon atoms

The initial stages of molecular-beam-epitaxy heteroepitaxial growth of ZnTe on GaAs(001) substrates are studied by in situ grazing incidence x-ray diffraction performed under ultrahigh vacuum. Pseudomorphic fully strained layers are observed for deposits up to 4 molecular layers (ML), whereas plastic relaxation starts after a critical thickness of about 5 ML together with the onset of a three-dimensional growth mode. Evidence for a normal strain gradient is obtained in partially relaxed layers. The results are confirmed by ex situ high-resolution transmission electron microscopy.

Growth mode of Ge films on Si(100) substrate deposited by ion beam sputtering