It is known since long ago that “birth defects” of crystals appearing during their nucleation and growth may hardly be eliminated by further “education,” like heat treatment. Even less hope, if any, exists for fabricating fancy device structures unless we use detailed techniques of crystal growth, techniques based on interfacial processes, and determine the real defect structure of the crystal or film.

These practical issues encourage those who are excited by the goal to penetrate into processes at solid surfaces and should encourage those who make decisions on funding this research.

In what follows I emphasize selected general issues that seem to be hot today and that have been addressed at Symposium B of the 1992 Fall MRS Meeting. Of course, the selection is a matter of taste.

This paper gives some of the highlights of a panel discussion on surface diffusion held Monday, November 30, 1992 at the Fall MRS Meeting in Boston, Massachusetts. Four invited speakers discussed computer modeling techniques and scanning tunneling microscopy experiments that have been used to provide new understanding of the atomistic processes that occur at surfaces. We present a summary of each of the invited talks, indicate other presentations on surface diffusion in this proceedings, and provide a transcript of the two discussion sessions.

First-principles calculations and corresponding experimental results underline the importance of basic chemical concepts in understanding bonding and diffusion of atoms at and on metal surfaces. Examples discussed include the outward relaxation of clean Be(0001), the separation energies of metal-adatom dimers, concerted substitutional self-diffusion on fcc(001) and (110) surfaces, and adsorption and diffusion barrier sites for adatoms near steps.

We present the use of an STM to make quantitative observations of time-dependent mass flow associated with the decay of two-dimensional clusters on the Au(111) surface. When formed and observed in air, layered islands with well-defined edges located on larger terraces are generally found to decay in such a way that their areas decrease linearly in time over periods ranging from minutes to several hours depending on the island size. This is in contrast to the behavior of similar features formed and observed under ultra high vacuum conditions, which do not appear to decay over experimental periods of several days. The linear decay is consistent with models that have been used previously to describe growth of 2-dimensional clusters on surfaces. We discuss possible decay mechanisms, and the role that adsorbates may play in influencing the decay.

The analysis of surface diffusion in interactive overlayers deviates from the simple picture of diffusion as a random walk. Non-equilibrium processes (the growth of ordered or the disordering of already ordered islands) under constant coverage or deposition rate, can be easily used to measure surface diffusion. For the 0/W(110) system, such experiments measured an ordering activation energy ED=0.6±0.05 eV which is different from the one measured in disordering experiments ED=1 ± 0.05 eV. The difference can be accounted for by the interactions. Ordering experiments in the Ag/Si(111)-√3x√3R30 system under constant deposition, suggest the presence of non-thermal mobility.

Crystal growth and surface diffusion have been studied in the Ag/Ge(lll) system using UHV-SEM based techniques, biassed secondary electron imaging (b-SEI), micro-AES and RHEED. Ag was deposited through and past a mask of holes held close to the substrate at 300<Td< 775K. Under certain conditions, the Ag patches were observed to split into two regions corresponding to the √3×√3R30° (hereafter √3) and a lower coverage 4×4 structure, each of which were easily observable using b-SEI. These patch widths were measured as a function of Td, and of annealing times at temperatures Ta, and effective diffusion coefficents extracted. The diffusion length of adatoms over the 4×4 structure is larger than that over the √3 structure. These observations are modelled using kinetic rate equations, and the results are compared with previous studies of Ag/Si(111). We find that energies characterising processes on top of the √3 layers of both systems are very similar, but that processes involved in the formation of the layers are quite different. The coverage of the √3 Ag/Ge(111) layer is close to 1 ML for all Td studied, unlike √3 Ag/Si(111). where it depends on deposition and annealing conditions.

The surface diffusion coefficient of sulfur on the (110) surface of nickel was determined at temperatures of 623, 673 and 723 K by measurement of surface diffusion profiles using an Auger Electron Spectrometer. A half-plane monolayer of segregated sulfur was used as a source and the analysis was done by a Boltzmann-Matano method. The activation energy for surface diffusion of sulfur was determined to be about 106 kJ/mol and pre-exponential Do to be about 6×10-3 m2/s from the Arrhenius plot of D versus 1/T. The presence of carbon on the surface of nickel was shown to decrease the surface diffusion of sulfur. Also, the mechanism of spreading from the half-plane source changed from a classic surface diffusion on clean surfaces to a linear mobility of the sulfur-carbon interface on surfaces covered with carbon.

Dodson-type potentials have been constructed for As-As, Si-As, Sb-Sb, and Si-Sb interactions. The parameters were determined by fitting to the energies obtained in first-principles calculations for As-Si and Sb-Si clusters with a variety of geometries. These potentials have been applied in molecular dynamics simulations of As4 and Sb4 chemisorbing on Si(100) at high temperature (900K). We find that both molecules tend to chemisorb in stringlike configurations, and that dissociation into two dimers can occur within only a few picoseconds.

Using embedded atom potentials fit to both bulk and surface properties, we explore the submonolayer vapor deposition of Au and Pt onto reconstructed (100) surfaces with the MD code DAMSEL. The surface geometries are determined by computational annealing over .6 ns. Surface reconstruction has a strong influence on the effects of adatoms. In Au and Pt the deposited atoms are absorbed into the surface and surface structures form by atomic replacement sequences and collective motion over the surface and substrate. On Au the reconstructed corrugated surface evolves into one characterized by two-dimensional mounds as coverage is increased. On reconstructed Pt, which exhibits alternately quasihexagonal and bulk-like regions, adatoms initially form strings above the bulk-like regions parallel to the (110] corrugation rows. At about .5 monolayer coverage the quasihexagonal structure of the top substrate layer transitions to a bulk-like structure in both Au and Pt

Desorption Mass Spectrometry (DMS) is used to monitor In desorption kinetics at the InAs/GaAs (001) heterointerface. In incorporation is determined to be significantly affected by temperature, surface reconstruction, and the V/III flux ratio. It is shown that the onset of the As-stabilized (2×4)-to-In-stabilized (4×2) surface phase transition is surface stoichiometry dependent, and that both the As-stabilized and the In-stabilized surface exhibit first order desorption behavior. Desorption activation energy of In from the (2×4) and the (4×2) surface is measured. In addition to an In-stabilized regime, an In-accumulated regime is identified by its deviation from first order desorption behavior.

Knowledge about atomic scale motions is essential to the understanding of dynamical phenomena on surfaces, such as diffusion, phase transitions, and epitaxial growth. We demonstrate that the addition of a very small number of Pb atoms to a Ge(111) surface reduces the energy barrier for activated processes, thus allowing one to observe concerted atomic motions and metastable structures on this surface near room temperature using a tunneling microscope. The activation energy for surface diffusion of isolated substitutional Pb atoms in Ge(111)-c(2×8) was measured by observing individual atomic interchanges from 24°C to 79°C. We also observed the formation and annihilation of metastable structural surface excitations, which are associated with large numbers of germanium surface atoms in one row of the c(2×8) reconstruction shifting along that row like beads on an abacus. The effect provides a new mechanism for atomic transport on semiconductor surfaces and can explain a number of other observed phenomena associated with Ge(111) surfaces, including the surface diffusion of Pb atoms.

Atom-probe field ion microscopy is capable of imaging solid surfaces with atomic resolution, and at the same time chemically analyzing atoms selected by the observer from the atomic image. The samples are restricted to those having a tip shape, but in many cases this is no longer a drawback since structures in high-tech materials are reducing in size to that comparable to or smaller than the field ion emitter tip. This technique is finding many applications in different areas. Our recent applications of this technique to the study of the dynamical behavior of surfaces and surface atoms and their mechanisms and energetics, and the atomic scale chemical and composition analysis will be briefly described.

Scanning tunneling microscopy (STM) is used to study the structure of Al-√3×√3 domains on the Si(lll)-7×7 surface and the atomic arrangement near the domain boundary. Al-√3×√3 domains grow from the lower side of the <112> step and extend over the Si-7×7 terrace. The phase transition is observed to occur in units of the 7×7 size. Detailed investigation at around the boundary reveals that faulted halves of the 7×7 unit are adjacent to the boundary on the Si-7×7 side, while on the Al-√3×√3 side, Al adatoms occupy the T4 sites except for the rows adjacent to the phase boundaries where Al atoms occupy the Si adatom sites. The latter Al atoms play an important role to retain the dimer structure at the boundary.

The growth and surface morphology of thin LPCVD silicon films were investigated with an atomic force microscope (AFM). Silicon films of 30 nm thicknesses were deposited on SiO2 using SiH4 at four different temperatures between 550 °C and 625 °C. These AFM results permitted visualization of silicon surface granularity, roughness, and the transition with temperature from amorphous to crystalline structure between 550 °C and 580 °C. The surface of the amorphous film deposited at 550 °C is very smooth and the film is continuous physically, while the film formed at 580 °C appears crystalline, rough and porous. At 600 °C and 625 °C the films are fully crystalline. For these higher temperature films surface roughness and the average grain size decreased significantly compared to 580 °C film. Crystallinity and film continuity were further examined by x-ray diffraction (XRD) and cross-sectional TEM measurements.

Structure changes of Si(111)-Pb surfaces due to deposition and heat treatment are studied by REM-RHEED. Surface structures observed are summarized as a phase diagram. Formation of an incommensurate layer(α) and a phase transition between incommensurate structures of α and α' is described.

Recent experimental results aimed at the measurement of surface diffusion constants by use of scanning tunnelling microscopy and diffraction techniques have reawakened interest in the statistical properties of two-dimensional island nucleation and growth in the submonolayer regime. Classical homogeneous rate equation studies published over twenty years ago established a simple relationship among the number density of stable islands, the deposition flux, and the adatom surface diffusion constant. Recent Monte Carlo simulation studies confirm this prediction and considerably extend the scope of such a scaling description of submonolayer epitaxial growth. In this article, I review the current status of theory and experiment in this area and suggest some areas for future research.

Concentration profiles of surface atoms have been obtained from Monte Carlo simulations of epitaxial growth with anisotropic nearest-neighbor bonding on two different vicinal GaAs(001) surfaces which are misoriented toward [110] (denoted as A) and [110] (denoted as B). These profiles are compared with predictions based upon the original Burton-Cabrera-Frank (BCF) theory and with a modified BCF theory which accounts for the fact that all atoms, not simply free adatoms, can participate in surface diffusion. On both surfaces the modified BCF equation provides a more accurate description of the adatom concentration profiles than that obtained using only the free adatom concentration. This is due to step detachment processes which leads to an effective source of free adatoms that is not taken into account in the original BCF approach. Above the temperature where the growth becomes dominated by step advancement, the curvature of the concentration profile of potentially active atoms on both surfaces are similar but the concentrations differ due to the difference of the capture efficiency of the two types of step edges.

We have calculated the height-height correlation function by means of Monte Carlo simulations of a solid-on-solid model. These simulations reveal that the position and sizes of the islands formed during the first half-monolayer are highly correlated, a feature which has also been noted in recent x-ray studies of growth by OMCVD. This morphology results in the emergence of a characteristic length, ℓ, in the correlation function, which is found to obey the scaling law ℓ = (D)/J)γ, where D is the diffusion length, and J is the deposition flux. The value of the exponent γ varies with coverage and depends on the growth conditions.

As design rules shrink to conform with ULSI device dimensions, gate dielectrics for MOSFET structures are required to be scaled to even thinner proportions. Upon scaling the gate oxides below ∼60Å some properties of the device, such as interface roughness, that are negligible for thicker films become critical and must be evaluated. Microroughness at the interface of ultrathin MOS capacitors has been shown to degrade these devices.

We are studying the interfacial region of ∼50Å SiO2 on Si using the quantum oscillations in Fowler-Nordheim tunneling currents. The oscillations are sensitive to the electron potential and abruptness of the film and its interfaces. In particular, inelastic scattering and/or thickness inhomogeneities in the film will reduce the amplitude of the oscillations. We are using the amplitude of the oscillations to examine the degree of microroughness at the interface that results from a pre-oxidation high temperature anneal in an inert ambient containing various amounts of H2O. Preliminary AFM imaging has shown correlations supporting our microroughness interpretation of the quantum oscillation amplitudes.

GaAs films grown on exact (111) substrates in the reconstruction regime always show facets. We have studied the surfaces of these films with an atomic force microscope. The facets are composed of vicinal surfaces which are tilted by small angles (∼ 2°) from the exact (111) plane. From the geometry of the facet, average displacement of adatoms is calculated. The results indicate that the diffusion length under the growth condition is more than 0.2 μm.