Alexander A. Shklyaev

LEED investigation of germanium surfaces cleaned by sublimation of sulphide films; structural transitions on clean Ge(110) surface

Abstract Sublimation of deposited germanium sulphide films at the temperatures as low as 350°C results in the appearance of LEED patterns of clean surfaces of germanium. In the interface between Ge(111) or Ge(110) and germanium sulphide, ordered structures are observed, namely Ge(111)−(2 × 1)S and Ge(110)−(10 × 5)S. The conclusion about the structure of the Ge(100) germanium sulphide interface cannot be made unambiguously. The structures of clean Ge(110) surfaces are described. The annealing of clean surfaces of Ge(110) at different temperatures leads to the formation of one of two possible surface structures. After annealing at temperatures below 380°C and above 430°C the Ge(110)−c(8 × 10) clean superstructure is observed. After annealing at temperatures from 380 up to 430°C the surface (110) is rearranged in vicinal planes of the (17 15 1) type with the (2 × 1) superstructure. These structures undergo reversible transitions from one to another at temperatures of about 380 and 430°C.

Local structure of Ge nanoislands on Si(111) surfaces with a SiO2 coverage

We have investigated the local structure and photoluminescence properties of ultrasmall Ge islands grown on Si(111) covered with SiO2. Scanning electron microscopy and transmission electron microscopy measurements show that the islands have a hemispherical shape, and depending on the growth temperature, can be either epitaxial or nonepitaxial. X-ray absorption near-edge structure measurements demonstrate that the nonepitaxial islands have the local structure of bulk diamond Ge and are very stable towards oxidation. The epitaxial islands are found to be partly oxidized, but no alloying with the Si substrate is observed. The nonepitaxial islands exhibit a photoluminescence peaked at 2.3 eV, which is typical of Ge nanocrystals embedded in SiO2. Possible mechanisms for the stability of the nonepitaxial Ge islands towards oxidation are discussed.We have investigated the local structure and photoluminescence properties of ultrasmall Ge islands grown on Si(111) covered with SiO2. Scanning electron microscopy and transmission electron microscopy measurements show that the islands have a hemispherical shape, and depending on the growth temperature, can be either epitaxial or nonepitaxial. X-ray absorption near-edge structure measurements demonstrate that the nonepitaxial islands have the local structure of bulk diamond Ge and are very stable towards oxidation. The epitaxial islands are found to be partly oxidized, but no alloying with the Si substrate is observed. The nonepitaxial islands exhibit a photoluminescence peaked at 2.3 eV, which is typical of Ge nanocrystals embedded in SiO2. Possible mechanisms for the stability of the nonepitaxial Ge islands towards oxidation are discussed.

LEED studies of vicinal surfaces of germanium

Clean germanium surfaces inclined at small angles to (111), (100) and (110) planes were investigated by LEED. Surfaces with orientations close to (111) and (100) are stepped and regular steps are retained in the whole temperature range investigated. Steps with (111) terraces and edges towards [211] have a height of about one interplanar distance d111 at all temperatures, and steps with edges towards [211] have a height of about two interplanar distances below 500°C and of about one interplanar distance above 500°C. Steps with (100) terraces and edges in the [011] direction have a height about two interplanar distances d100. The surfaces with orientations close to (110) are facetted at room temperature. The (17 15 1) facets are present on the surfaces oriented in the [110] zone and the (10 9 2) facets on the surfaces oriented in the [001] zone. At high temperatures (about 480 and 770°C respectively) a reversible structural reconstruction of these surfaces into stepped ones takes place.

Visible photoluminescence of Ge dots embedded in Si/SiO2 matrices

Ge island formation on ultrathin SiO2 films enabled us to fabricate multilayer structures of Ge dots ∼6–7 nm in diameter and with an extremely high dot density of 2×1012 cm−2. Each dot had a boundary with the SiO2 film and a Si spacer layer. The multilayer structures exhibited photoluminescence (PL) with a maximum in the range of 2–3 eV depending on the excitation energy. The PL was associated with recombination between holes confined within Ge dots and electrons localized in the radiative defect centers at the Ge-dot/SiO2 interfaces. The results suggest that this recombination is much more effective than that at the Si/SiO2 interface and supported by the hole migration from the Si spacer layers to the Ge dots.

Nanometer-scale germanium islands on Si(111) surface windows formed in an ultrathin silicon dioxide film

Three-dimensional Ge islands between 15 and 200 nm in size were found to grow only on Si(111) surface windows in ultrathin SiO2 film after Ge deposition and subsequent SiO2 decomposition. The size of Ge islands gradually decreased as the Ge thickness decreased. Pseudomorphic two-dimensional Ge layers with the 5×5 structure formed in surrounding areas of the windows. The windows were produced by selective thermal SiO2 decomposition induced by focused electron beam irradiation. These results suggest a new technique for nanometer-scale Ge island fabrication at given points on Si surfaces.

Observation of oscillating behavior in the reflectance difference spectra of oxidized Si(001) surfaces

Layer-by-layer oxidation of Si(001)-(2×1) surfaces was observed using the reflectance difference (RD) spectroscopy. Distinctive features in the RD spectra appeared near the E1 (3.3 eV) and E2 (4.2 eV) transition energies of Si. The polarity of these features was repeatedly reversed as the oxide thickness was increased to 4 monolayers (MLs). Oscillation of the RD amplitude near the E1 transition energy was observed in real time during the oxidation process. A half period of the oscillation corresponds to the oxidation of 1 ML. These results demonstrate the possibility of in situ counting and control of the number of oxidized layers.

Formation of three-dimensional Si islands on Si(111) with a scanning tunneling microscope

Silicon islands up to 10 nm in base length and 3 nm in height were grown on a Si(111) surface at room temperature with a scanning tunneling microscope at constant tunneling currents. The islands grew with constant rates at earlier growth stages by accumulating Si atoms from the surface area around the islands. The growth rate decreased when the island height exceeded 3 nm. At negative tip biases above 7 V, the technique produced a highly reproducible formation of the islands whose growth rate increased as the bias voltage increased.

Formation of Ge nanoislands using a scanning tunneling microscope

Germanium islands were grown on a sample surface by accumulating atoms from the surrounding area through directional surface diffusion initiated by the electric field of a scanning tunneling microscope (STM). The Ge islands grew with a constant rate determined by the tip–sample bias voltage. The parameters of tip–sample interaction were estimated from the kinetic data for island growth by using a scaling relationship among the growth rate, the dipole moment of atoms on surfaces, and the tip–sample bias voltage. The results show that continuous atom transfer with a STM occurs with a rate significantly higher for Ge than for Si.

Monosilane adsorption and initial growth stages of silicon layers on the (100) and oxidized silicon surfaces: Ellipsometric investigation

Abstract Monosilane adsorption kinetics on (100) and oxidized silicon surfaces has been studied by means of a fast-response ellipsometer. Monosilane adsorption on the (100) silicon surface is irreversible and has no activation barrier in the temperature range 20–500°C. Condensation coefficients of SiH 4 for the closed and the flowing adsorption technique are σ = 7 × 10 − and 5 × 10 −2 , respectively. The thickness of the adsorption layer increases in the temperature range 125–350°C which is evidence for the formation of a surface supermonolayer coverage. At T > 350°C decomposition of adsorption complexes takes place, accompanied by hydrogen desorption. This process is characterized by an activation energy of E = 44 ± 2 kcal/mol. Monosilane is adsorbed reversibly on the oxidized silicon surface in the temperature range 350–490°C and at a pressure of P −2 Torr. At pressure P > 5 × 10 −2 Torr adsorption complexes decompose and amorphous silicon grows. The growth rate is directly proportional to the monosilane pressure in the gas phase and increases with increasing temperature; the activation energy of this process is E = 56 ± 6 kcal/mol. The analysis of the kinetic curves indicates that the growth of amorphous silicon is a chain process at the initial stage.

Stability of the (0001) surface of the Bi2Se3 topological insulator

The inertness of the cleaved (0001) surface of a Bi2Se3 single crystal to oxidation has been demonstrated using X-ray photoelectron spectroscopy, as well as atomic-force and scanning tunneling microscopy and spectroscopy. No intrinsic bismuth and selenium oxides are formed on the surface after a month of storage in air. Atomically flat surfaces with macroscopic sizes (∼1 cm2) and rms roughness less than 0.1 nm have been prepared, and (1 × 1)-(0001) Bi2Se3 atomic structure has been resolved. The tunneling conductance measurements have shown that the energy dependence of the surface density of states is quasilinear in the band gap of Bi2Se3.