Layer Growth Mode Research Articles

Depth profiles of Ag/Si(111) thin‐film interface regions

AbstractThe metal/semiconductor interface region of in situ deposited Ag thin films prepared on a Si(111) substrate has been investigated using AES sputter depth profiling in conjunction with proton‐induced x‐ray emission (PIXE). Interface regions have been modeled with the interface analysis program LOGIT, developed at the National Institute of Standards and Technology (NIST). Values for the parameters obtained from this model indicate that the rate of ion beam sputtering through samples prepared without simultaneous low‐energy ion activation tended to be noticeably greater than the rate of sputtering through samples prepared with simultaneous ion activation. Further, we found that one effect of the ion beam is temporarily to shift the fractional Ag surface coverage to higher values. The likely mechanism for this shift is sputter redeposition by the ion beam of Ag atoms from large nucleated Ag islands, transporting them to open areas between islands. These results support conclusions from an earlier study, which found that the effect of low‐energy simultaneous ion beam activation during thin‐film deposition was to shift the mode of growth of Ag on an Si(111) substrate from an SK (layer‐plus‐island) to an FM (layer) growth mode, to a degree dependent on the conditions of simultaneous ion co‐bombardment.

Consequences of Strain and Growth Kinetics on Growth Modes and Dislocation Formation in Strained Layer Epitaxy

ABSTRACTWe examine the growth modes of compound semiconductors grown by molecular beam epitaxy in lattice matched and strained layers. While in lattice matched layers it is possible to grow in the layer by layer mode by choosing proper growth conditions, in strained layers we find that for large strain, it is not possible to do so. This is due to the competition between surface strain energy which favors 3D growth and surface chemical energy which favors a layer by layer growth mode. The growth mode is shown to have significant effects on the nature of the dislocations generated beyond critical thickness.

Thermal desorption spectroscopy of CO adsorption on epitaxial Cu(111)-Pd(111) thin film surfaces

Thermal desorption spectroscopy experiments of CO adsorbates on vacuum deposited, epitaxial (111)Pd and (111)Cu-(111)Pd bilayer films were carried out under uhv conditions. Auger electron spectroscopy and LEED measurements showed that Cu grows epitaxially on (111)Pd films formed on mica in layer growth mode. CO adsorption on (111)Cu-Pd bilayers occurs in decreasing amounts as the Cu overlayer thickens. Beyond approximately two monolayers (ML) Cu, no CO adsorbs. The saturation CO coverages decrease linearly by approximately 25% for every 1/2 ML Cu deposited. The desorption energy corresponding to the peak of the main desorption line decreased as the Cu and/or CO coverages increased. A simple relationship between Cu coverage and the temperature at the peak of the main CO desorption line for different CO coverages was derived from the data. The results show that thin Cu overlayers on Pd significantly affect the adsorption and desorption characteristics of CO on these thin film bilayers.

Atomic layer molecular beam epitaxy (Almbe) of III?V compounds: Growth modes and applications

A new development of molecular beam epitaxy (MBE) for III–V compounds is described, based on cyclic perturbation of the growth front at atomic layer level by periodic pulsing, alternating or interrupting the molecular beams. The modification of the growth mechanism caused by this perturbations is discussed and related to periodic changes of surface stoichiometry which induce 2D growth mechanism by enhanced layer nucleation. Under appropriate modulation conditions, an atomic layer by layer growth mode can be achieved. A practical implementention of this mode, that we denote atomic layer MBE (ALMBE), is considered in which only group V beams are pulsed in a specially designed effusion cell. A number of growth applications of ALMBE are presented, including growth of highly mismatched heterostructures and short period superlattices containing two different group V elements such as arsenic and phosphorous.

Auger electron spectroscopy study of low coverage palladium adsorption on Si(111)

Adsorption of palladium on an Si(111) surface was studied by Auger electron spectroscopy (AES). The palladium coverages used are in the 0–5 monolayers (ML) range and 25 ML. For room temperature palladium deposition in the low coverage range, a layer growth mode of Pd 2Si was found, while the 25 ML palladium deposit exhibited unreacted palladium layers above the interfacial Pd 2Si. On annealing, silicon segregates to the surface of the silicide for temperatures higher than about 400 K: the silicon surface concentration, which may be temperature dependent, reaches its final value (approximately 1 ML) at about 600 K. In the case of the thick deposit (25 ML), the same silicon segregation occurs as soon as the unreacted palladium has been converted to Pd 2Si by interdiffusion. The Pd 2Si-Si interface investigated at low coverage shows unstable features depending on the experimental conditions.

Growth mechanism for molecular-beam epitaxy of group-IV semiconductors.

The microscopic homoepitaxial growth dynamics of Si and Ge are probed by the application of a Monte Carlo computer simulation, with attention focused upon the substrate temperature dependence of reflection high-energy electron-diffraction (RHEED) specular beam intensity oscillations. A model that involves short-range site selectivity by the incident atoms, but without surface diffusion, is found to produce a low-temperature layer-growth mode. By incorporating diffusion on a stepped surface at higher substrate temperatures, simulated RHEED profiles are obtained that are in qualitative agreement with those measured.

Low energy electron diffraction, Auger electron spectroscopy and angle-resolved photoemission from silver films on Si(100) and Si(111)

We studied the growth of thin silver films on Si(100)2 × 1 and Si(111) 7 × 7 between room temperature and 300 °C. At room temperature and with a silver flux of 2.4 × 10 12 atoms s -1 cm -2 a nearly exponential dependence of the Si L 2,3 VV and Ag M 4 VV Auger intensities indicates layer-by-layer (Frank-van der Merwe) growth up to a film thickness of a few monolayers. With increasing coverage θ silver grows in form of three-dimensional islands even at room temperature. In spite of the existence of the well-known Si(111)−(√3 × √3)R30° Ag surface structure after annealing (which we observe for θ = 1) we did not find an ordered Si-Ag interface layer for silver on Si(100). For elevated temperatures the growth of this system may therefore be described by the Volmer-Weber (pure three-dimensional island growth) mechanism, whereas silver condenses on Si(111) according to the Stranski-Krastanov (interface layer plus three-dimensional islands) growth mode.

Contact reaction between Si and rare earth metals

Reactions between Si and thin films of rare-earth metals (Gd, Dy, Ho, Er, plus Y and La) in the temperature range of 275–900 °C have been studied by using x-ray diffraction and ion backscattering spectrometry. The disilicides of these metals are apparently the first phase to form, forming rapidly within a narrow temperature range (325–400 °C), and are stable up to 900 °C. The growth does not follow a layered growth mode.

Direct observation of the nucleation and growth modes of Ag/Si(111)

Direct observation in an ultrahigh vacuum scanning electron microscope (SEM) has revealed the growth modes of Ag on Si(111), previously deduced indirectly from analysis of Auger electron spectra, and the form and number of crystallites as a function of deposition time and temperature. At high temperatures, T s > 200°C, the silver grows definitively in a Stranski-Krastanov (layer plus island) growth mode, with a very strong dependence of the island density on the deposition temperature, varying from ≲ l0 6 to ∼ 10 10 cm −2 between T s = 500 and 200°C. The crystals have more or less regular hexagonal forms with (111) and {1 1 1} faces predominating with a height to width ratio which decreases with deposition time in the range 0.1–0.6. At temperatures below 200°C, the island density is too high to be observed directly by SEM, and silver appears to grow in a Frank-van der Merwe (layer by layer) mode. However, this uniform deposit is unstable on heating above 200°C, and annealed deposits recreate rather similar islands. An analysis of published Auger amplitude-time curves is made to estimate the initial island density produced by depositions below T s = 200° C, and it is shown that these densities constitute a very reasonable extrapolation of the higher temperature SEM values. A model of Stranski-Krastanov growth is given in outline and applied to Ag/Si(111). It is argued that the island density is determined largely by island instability at high temperatures, even though condensation is complete. At low temperatures it is argued that Stranski-Krastanov growth becomes essentially equivalent to Frank-van der Merwe growth for high enough island density, for kinetic rather than thermodynamic reasons, without any change in the basic Stranski-Krastanov mechanism.