The chemisorption of ethylene (CzH4) on SiWOlXZ X 1) at 300 K has been studied by scanning tunneling microscopy @TM) and spectroscopy @I’S). l%posure of the surface to C,H, does not cause large scale rearrangement of the original Si surface atoms. The adsorption of individual molecules and changes in the local structure can, however, be observed. At low coverage, the &II, molecules prefer to adsorb on alternate dimer sites creating either a local (2 x 2) or d2 x 4) structure. The individual domains are relatively small (< 50 ii> and the change in reconstruction cannot be detected by any diffraction technique. STS recorded from regions of the surface covered by adsorbates show the removal of the relatively weak ?r-bo& formed by the dangling bonds and the participation of the dangling bond electrons in stronger sp3-like bonds to the carbon atoms of the molecule after chelation. The (2 X 1) dimer surface characteristic of clean Si@Ol) is restored upon heating the adsorbate covered surface. The pictures are consistent with a model in which the desorbed C2H4 molecules leave restored dimers behind and the molecules which are left remain undissociated on the same type of dimer sites as before.

Molecular beam epitaxy (MBE) is an ideal vehicle for the study of initial growth processes in thin film growth, both for the degree of control available in the amount of material deposited and for its compatibility with in situ diagnostic probes. In this paper we discuss nucleation mechanisms involved in the growth of GaAs on GaAs and InAs on GaAs from elemental sources, in each case on the three low index substrate orientations, i.e. (001), (110) and (111)A. We have used a combination of reflection high energy electron diffraction (RHEED) and scanning tunnelling microscopy (STM) to measure both dynamic changes and nucleation events over the range 0.05 to 5.0 monolayers (MLs). These data have been analysed using scaling theory, dynamic Monte Carlo simulation and self-consistent rate equations. Perhaps the most important and striking feature of the results is the uniqueness of behaviour of the (001) orientation in both material systems.

The initial stages of homoepitaxy on GaAs(001) are studied with atomic-resolution scanning tunneling microscopy and Monte Carlo simulations that include the zinc blende structure of GaAs, the (2 x 4) reconstruction of the (001) surface, and the kinetics of As-2 incorporation. The reconstruction is found to favor nucleation on the top-layer arsenic dimers and to cause small islands to be unstable until they adopt the local (2 x 4) structure.

Scanning tunneling microscopy (STM) data are presented for all three phases of the GaAs(111)B surface prepared in situ by molecular beam epitaxy (MBE). The nature of the transitional phase is revealed for the first time by atomic resolution STM and shown to consist of structural units derived from both of the well known (2 × 2) and (√19 × √19)R23.4° surfaces. Individual ringlike units of the Ga-rich surface are stable structures on the (2 × 2) reconstructed surface. By varying the population of these features, the surface has access to a continuous stoichiometric pathway between the (2 × 2) and (√19 × √19)R23.4° reconstructions.

Submonolayer island-size distributions an obtained with scanning tunneling microscopy and used to infer the nucleation and growth kinetics of islands on the three low-index surfaces of GaAs. Comparison with Monte Carlo simulations reveals that on the (110) and (111)A surfaces, random nucleation is followed by the attachment and detachment of single atoms at island edges. But on the (001) surface (using As-4), nucleation is initiated in the trenches of the 2 x 4 reconstruction by pairs of Ga atoms. Growth then proceeds over locally filled trenches, also by the capture of pairs of Ga atoms.

Scanning tunnelling microscopy (STM) and reflection high energy electron diffraction (RHEED) have been used to study the deposition of Si in the presence of an arsenic flux at 560 degrees C on vicinal GaAs(001)-(2 x 4) surfaces grown in situ by molecular beam epitaxy (MBE). RHEED studies showed that the surface structure changed from (2 x 4) to asymmetric (3 x 1) at a Si coverage of similar to 0.1 ML, before a symmetric (3 x 1) structure was formed at a coverage of similar to 0.4 ML. Deposition of low coverages of Si (< 0.1 ML) resulted in a disordered surface with STM images showing a high degree of kinking of the (2 x 4) surface. The disorder is caused by Si atoms occupying the vacant Ga sites in the missing dimer trenches of the clean (2 x 4) surface. With increasing Si coverage, neighbouring Si atoms in the trenches are eventually covered with As, bridging the missing dimer trenches and forming elongated rectangular units. The number of these units becomes greater as the coverage of Si is increased and there is a change in periodicity from 8 to 12 Angstrom along [<(1)over bar 10>] consistent with the appearance of the symmetric (3 X I) RHEED pattern. Detailed studies carried out on vicinal surfaces, offcut by 1 degrees towards both (111)A and (111)B planes, showed that for coverages up to 0.5 ML of Si, the distribution of the Si is uniform across the surface and there is no preferential interaction with the step edges.

An atomistic model is presented based on scanning tunneling microscopy results and tight binding calculations which explains the observation of disorder on the GaAs(001)-(2 X 4) surface grown by molecular beam epitaxy. Calculations show that occupation by As of vacant Ga sites in the missing dimer trenches of the (2 X 4) unit cell is responsible for the surface disorder in the form of kinks in the dimer rows. The disordered surface is energetically favorable for a range of additional As coverage up to 0.25 monolayer.

Scanning tunnelling microscopy (STM) and reflection high energy electron diffraction (RHEED) have been used to study the deposition of Si below 400‡C onto GaAs (001) surfaces grown in situ by molecular beam epitaxy (MBE). The emphasis is on the island formation and growth, as well as surface ordering, for submonolayer quantities of Si (up to 0.2 ML) deposited on two different As-rich reconstructions of GaAs (0 0 1) (2 × 4) and c(4 × 4). For deposition on the c(4 × 4) surface, an asymmetric (3 × 1) RHEED pattern is formed, a consequence of anisotropic “needle-like” islands, which grow adjacent to each other along the [1 1 0] direction and produce a three-fold periodic superstructure. Individual islands grow by a site exchange process in which the additional As layer of the c(4 × 4) structure acts as a surfactant and enables the Si atoms to occupy Ga sites in the GaAs lattice. In contrast, Si deposition on the (2 × 4) surface does not lead to any new surface periodicities as monitored by RHEED. The Si atoms form poorly ordered clusters distributed randomly across the surface. The site exchange process cannot occur in this case as the (2 × 4) surface is terminated with only one layer of arsenic. Instead, the Si atoms occupy sites on top of the outer arsenic layer.

Scanning tunnelling microscopy (STM) and reflection high energy electron diffraction (RHEED) have been used to study the deposition of Si at 560 degrees C in the presence of an arsenic flux on GaAs(001)-(2 X 4) surfaces grown in situ by molecular beam epitaxy (MBE). RHEED studies indicate that for the low Si deposition rates used, the surface structure undergoes a series of changes as the Si coverage is increased up to 2.5 monolayers; (2 X 4) --> asymmetric (3 X 1) --> symmetric (3 X 1) --> (3 X 2) --> (4 X 2). At low coverages, the Si preferentially occupies vacant second layer Ga sites in the missing dimer trenches of the (2 x 4) reconstructed clean surface and a disordered, kinked surface is formed. Adjacent Si atoms in the trenches are eventually covered with As, bridging the missing dimer trenches and forming elongated rectangular units with a periodicity along [<(1)over bar 10>] of Angstrom. The number of these units becomes greater as the Si coverage is increased and there is a change in periodicity from 8 to 12 Angstrom along [<(1)over bar 10>] consistent with the appearance of the symmetric (3 X 1) structure. Detailed studies on vicinal surfaces misoriented by 1 degrees towards both (111)A and (111)B indicate no preference for the Si to align itself along the step edges. As the Si coverage is increased, the Si displaces Ga atoms from the second layer of the structure and forms a series of two dimensional islands which appear as ''holes'' in the surface. These islands eventually dominate the surface and result in a terminating Si layer and the observation of the (3 x 2) structure. Subsequent Si growth occurs by dimer chain formation on top of this layer and the strings of Si dimers that are formed at high coverages are characteristic of STM observations of the homoepitaxial growth of Si on Si(001).

Atomic resolution scanning tunnelling microscopy (STM) has been used to study the As-terminated reconstructions formed by GaAs(001) surfaces grown in situ by molecular beam epitaxy (MBE). Specific emphasis has been placed on the transition from a (2 × 4) to c(4 × 4) surface with increasing amounts of As. STM images of the initial (2 × 4) surface, corresponding to the β phase, showed an ordered structure with unit cells containing two As dimers. With increasing amounts of As, the intensity of the 24 streak in the RHEED pattern weakened considerably. Although STM images of this (2 × 4) phase again only showed two As dimers per unit cell, the surface was characterized by a considerable degree of disorder and a large number of kinks. The results are consistent with the (2 × 4) β phase having a structure with unit cells based on two As dimers and Gn absent from the missing dimer trenches. The kinks formed on the more As-rich (2 × 4) structure are then caused by the additional As occupying these vacant Ga sites producing an electron rich site. Quenching to lower temperatures in the presence of As leadsto the c(4 × 4) structure. STM images of this surface indicate that the top layer of the structure is based on rectangular units, which when complete, consist of a total of six As atoms. The wide coverage range for which this reconstruction can be maintained is explained by a varying number of missing As atoms from the basic six atom structural unit. A new structural model is proposed for the c(4 × 4) structure based on its formation from the starting (2 × 4) surface and involves a mixed third layer containing both Ga and As.

Scanning tunneling microscopy (STM) has been used to study the deposition of Si at 400 °C onto the c(4×4) surface reconstruction of GaAs(001) surfaces grown in situ by molecular beam epitaxy (MBE). Emphasis is placed on the nucleation, island formation, and surface ordering, as a consequence of adsorption onto the c(4×4) surface. With increasing Si coverage, a series of anisotropic ‘‘needlelike’’ islands is formed. A site exchange model is proposed in which the deposited Si atoms displace the top layer As atoms of the c(4×4) structure, with the displaced As atoms forming dimers in a new top layer.

Atomic resolution scanning tunnelling microscopy (STM) has been used to study the adsorption of Si on GaAs(001) surfaces, grown in situ by molecular beam epitaxy (MBE), with a view to understanding the incorporation of Si in δ-doped GaAs structures. Under the low-temperature deposition conditions chosen, the clean GaAs surface is characterized by a well-defined c(4 × 4) reflection high-energy electron diffraction (RHEED) pattern, a structure involving termination with two layers of As. Filled states STM images of this surface indicate that the basic structural unit, when complete, consists of rectangular blocks of six As atoms with the AsAs bond in the surface layer aligned along the [110] direction. Deposition of < 0.05 ML of Si at 400°C onto this surface shows significant disruption of the underlying structure. A series of dimer rows are formed on the surface which, with increasing coverage, form anisotropic “needle-like” islands which show no tendency to coalesce even at relatively high coverages (∼ 0.5 ML). The formation of these islands accompanies the splitting of the 12 order rods in the RHEED pattern along [110]. As the Si is known to occupy only Ga sites, the Si atoms displace the top layer As atoms of the c(4 × 4) structure, with the displaced As atoms forming dimers in a new top layer. The results are consistent with a recently proposed site exchange model and subsequent island formation for surfactant mediated epitaxial growth.

Atomic resolution scanning tunneling microscopy (STM) has been used to study the Asterminated (2×4) and c(4×4) reconstructions formed on GaAs(001) surfaces grown in situ by molecular beam epitaxy (MBE). Filled states STM images of the (2×4) surface always showed unit cells consisting of two As dimers in the top layer. Cooling this surface under an As flux led initially to a highly kinked (2×4) phase before the transition to the c(4×4) structure. At no point were three As dimers observed in the top layer for the (2×4) unit cell. The c(4×4) structure involves the chemisorption of a second layer of As onto an already As-terminated surface. STM images of this surface showed a series of bright rectangular blocks consisting, when complete, of three pairs of As atoms.

Scanning tunneling microscopy (STM) has been used to study GaAs(001)-(2X4) surfaces grown by molecular-beam epitaxy. Although reflection high-energy electron diffraction always showed a characteristic (2x4) pattern, STM images indicated significant differences in the composition of the surfaces depending on the nature of the quenching conditions. A previously unreported, locally disordered (1X2) structure was observed under As-deficient conditions. This was found extending from step edges and in missing-dimer holes, where the top layer of As had been removed to expose the second layer. The atoms in this exposed layer are identified as Ga and they form short rows in the [110] direction. The twofold periodicity is due to a vacancy structure, with Ga atoms located at alternate sites along the [110] direction. These results may rationalize some of the recent controversies regarding the composition and structure of the As-terminated (2X4) surface prepared by decapping methods.

The chemisorption of ethylene (C2H4) on Si(001)(2 × 1) at 300 K has been studied by scanning tunneling microscopy (STM) and spectroscopy (STS). Exposure of the surface to C2H4 does not cause large scale rearrangement of the original Si surface atoms. The adsorption of individual molecules and changes in the local structure can, however, be observed. At low coverage, the C2H4 molecules prefer to adsorb on alternate dimer sites creating either a local (2 × 2) or c(2 × 4) structure. The individual domains are relatively small (<50 Å) and the change in reconstruction cannot be detected by any diffraction technique. STS recorded from regions of the surface covered by adsorbates show the removal of the relatively weak π-bond formed by the dangling bonds and the participation of the dangling bond electrons in stronger sp3-like bonds to the carbon atoms of the molecule after chemisorption. The (2 × 1) dimer surface characteristic of clean Si(001) is restored upon heating the adsorbate covered surface. The pictures are consistent with a model in which the desorbed C2H4 molecules leave restored dimers behind and the molecules which are left remain undissociated on the same type of dimer sites as before.

Scanning tunnelling microscopy (STM) has been used to image the adsorption of trimethylgallium (TMGa) on GaAs(001)-(2×4) surfaces prepared in situ by molecular beam epitaxy (MBE). Filled states images of the clean surface are dominated by (2×4) unit cells containing only two As dimers. Upon exposure of this surface to TMGa at room temperature, bright oval-shaped features are observed which are centred on the arsenic dimers of the unit cell. These arise from tunnelling from Ga-C bonds of the adsorbed molecules. At low coverages, preferential adsorption on unit cells adjacent to occupied sites along the [ % % MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqr1ngB % PrgifHhDYfgasaacH8srps0lbbf9q8WrFfeuY-Hhbbf9v8qqaqFr0x % c9pk0xbba9q8WqFfea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qqQ8fr % Fve9Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaeaaeaaakeaacaGGBb % GabGymayaaraGaaGymaiaaicdacaGGDbaaaa!3BB3! $$[\bar 110]$$ 110] direction is observed. A detailed statistical analysis of a large number of adsorption sites shows that there is an increased probability of about 24% for adsorption next to a (2×4) unit cell which is occupied relative to an unoccupied one.

The authors present some observations of Si(001)-(2*1) and GaAs(001)-(2*4) surfaces. They show what is seen when the surface is exposed to ethyne and to ethene and provide a foundation for investigating adsorbates on semiconductor surfaces.

The scanning tunnelling microscope (STM) has been used to study two quite different systems. First, the unsaturated hydrocarbons ethene and ethyne, chemisorbed on a silicon substrate, and secondly, a monolayer of 4,4'-bipyridyl disulfide (PySSPy), chemisorbed on a gold single-crystal sphere, in pure water. The images of the hydrocarbons on silicon showed, at low coverages, random clusters of adsorbates, but at higher coverages, approaching saturation the molecules prefer to be adsorbed on alternate dimer sites. The in situ STM study of the PySSPy on gold, revealed that the monolayer exhibits a marked molecular alignment with specific sites on the gold surface. The periodic unidirectional array showed a separation of 46+/-3 angstrom with a corrugation of ca. 1.5 angstrom.