Yu. Kudriavtsev

Work function change caused by alkali ion sputtering

Abstract In the presented work we performed an experimental study of the work function decrease for a silicon sample caused by Cs+ ion bombardment. We varied the energy of primary Cs+ ions as well as the angle of incidence in order to reach a different concentration of implanted cesium ions and to find a dependence of the work function change on the cesium surface concentration. A “surface dipoles” model was developed. The model based on the electronegativity concept considers formation of Si–Cs dipoles with corresponding dipole moment. An electric field of these dipoles results in a decrease of the work function, whereas a partly ionic character of Si–Cs bond increases the surface binding energy of cesium and, as a consequence, its surface concentration. A good correlation between the model and the experimental data was found.

Comparative secondary ion mass spectroscopy analysis of solar cell structures grown by pulsed laser ablation and ion sputtering

We performed a complex secondary ion mass spectroscopy (SIMS) 3D analysis of solar cell structures based on II–VI semiconductors. The chemical composition analysis, as well as the depth distribution of the main elements and contamination were done for AuCu/CdTe/CdS/conducting glass structures. A structure where the II–VI compounds were grown by pulsed laser ablation (PLA) was compared with another structure grown by ion sputtering deposition (ISD). In both cases contamination due to O, C and Hw as found at high concentrations, particularly at the boundaries between crystallites. In addition to the SIMS depth profiling, the surface roughness (SR) was analysed by atomic force microscopy (AFM). Poor SIMS depth resolution was correlated to high surface roughness. The root-mean-square of the surface roughness (Rrms )w as found to be higher for ISD than for PLA structures. In addition, the lateral distribution of the main components and contamination were observed in the microscope mode with a resolution of about 1 µm. A larger lateral contamination was correlated to a larger Rrms of the analysed surface. Experimental ‘diffusion’ tails of Cu and Au from the ohmic contacts on the CdTe layer are also explained by a high Rrms for this layer.

Sputtering of III–V semiconductors under argon atom and ion bombardment

Abstract III–V semiconductors sputtering under argon neutral and ion projectiles with energies from 150 to 600 eV was investigated. It is shown that the dependency on energy of sputtering yield is well described by the Sigmund-Falcone and Haff-Switkowski models in Yudins approximation. It analyses the relation of the surface binding, atomization and amorphization energies.

Maximum concentration of implanted projectiles during ion sputtering

Abstract Implantation of primary ions during surface sputtering was considered. A simple equation for maximum concentration of implanted projectiles was developed. Several practical applications were performed with use of developed equation.

Characterization of Silicon Doped with Sodium upon High-Voltage Implantation

The depth distribution profiles of sodium atoms in silicon upon high-voltage implantation (ion energy, 300 keV; implantation dose, 5 × 1014 and 3 × 1015 cm −2) are investigated before and after annealing at temperatures in the range Tann = 300–900°C (tann = 30 min). Ion implantation is performed with the use of a high-resistivity p-Si (ρ= 3–5 kΩ cm) grown by floating-zone melting. After implantation, the depth distribution profiles are characterized by an intense tail attributed to the incorporation of sodium atoms into channels upon their scattering from displaced silicon atoms. At an implantation dose of 3 × 1015 ions/cm2, which is higher than the amorphization threshold of silicon, a segregation peak is observed on the left slope of the diffusion profile in the vicinity of the maximum after annealing at a temperature Tann = 600°C. At an implantation dose of 5 × 1014 ions/cm2, which is insufficient for silicon amorphization, no similar peak is observed. Annealing at a temperature Tann = 700°C leads to a shift of the profile toward the surface of the sample. Annealing performed at temperatures Tann ≥ 800°C results in a considerable loss of sodium atoms due to their diffusion toward the surface of the sample and subsequent evaporation. After annealing, only a small number of implanted atoms that are located far from the region of the most severe damages remain electrically active. It is demonstrated that, owing to the larger distance between the diffusion source and the surface of the sample, the superficial density of electrically active atoms in the diffusion layer upon high-voltage implantation of sodium ions is almost one order of magnitude higher than the corresponding density observed upon low-voltage implantation (50–70 keV). In this case, the volume concentration of donors near the surface of the sample increases by a factor of 5–10. The measured values of the effective diffusion parameters of sodium at annealing temperatures in the range Tann = 525–900°C are as follows: D0 = 0.018 cm2/s and Ea = 1.29 eV/kT. These parameters are almost identical to those previously obtained in the case of low-voltage implantation.

Depth-profile analysis of nanostructures by SIMS: Depth resolution function

A new model of the depth-resolution function for secondary-ion mass spectrometry, which takes into account recoil implantation, ion mixing, and surface roughness formation under ion irradiation, is considered. A simple three-parameter equation is proposed to describe the depth-resolution function. Analytical expressions are obtained for two parameters.

Implantation of sodium ions into germanium

The donor properties of Na atoms introduced by ion implantation into p-Ge with the resistivity 20–40 Ω cm are established for the first time. Na profiles implanted into Ge (the energies 70 and 77 keV and the doses (0.8, 3, 30) × 1014 cm−2) are studied. The doses and annealing temperatures at which the thermoprobe detects n-type conductivity on the sample surface are established. After implantation, the profiles exhibit an extended tail. The depth of the concentration maximum is in good agreement with the calculated mean projected range of Na ions Rp. Annealing for 30 min at temperatures of 250–700°C brings about a redistribution of Na atoms with the formation of segregation peaks at a depth, which is dependent on the ion dose, and is accompanied by the diffusion of Na atoms to the surface with subsequent evaporation. After annealing at 700°C less than 7% of the implanted ions remain in the matrix. The shape of the profile tail portions measured after annealing at temperatures 300–400°C is indicative of the diffusion of a small fraction of Na atoms into the depth of the sample.

Interaction of water vapor with silicate glass surfaces: Mass-spectrometric investigations

The secondary ion mass-spectroscopy technique was used to study the results of hydration of borosilicate, aluminosilicate, and soda-lime silicate glasses in 1H218O water vapor containing 97% of the isotope 18O. It is shown that hydration of the surface of the soda-lime silicate glass occurs as a result of the ion-exchange reaction with alkali metals. In the case of borosilicate and aluminosilicate glasses, water molecules decompose on the glass surface, with the observed formation of hydrogenated layer in the glass being the result of a solid-state chemical reaction—presumably, with the formation of hydroxides from aluminum and boron oxides.

On the delta-type doping of GaAs-based heterostructures with manganese compounds

Heterostructures, which incorporate GaAs/InGaAs/GaAs quantum wells and are doped with spatially remote monatomic Mn layers, are formed on GaAs(001) substrate under conditions of multilayer buildup by the method of molecular-beam epitaxy. Combined studies of the obtained samples were performed by the method of secondary-ion mass spectrometry, by measurements of X-ray diffraction, and using a transmission electron microscope. The heterostructures under study with a doping impurity concentration amounting to 0.5 single layers are elastically stressed and demonstrate planar clearly defined interfaces without visible extended or point defects. A method for visualization of the distribution of the manganese concentration in the three-dimensional GaAs matrix in the vicinity of a quantum well is suggested. According to experimental results, there is a probability for manganese diffusion into the GaAs/InGaAs/GaAs quantum well when the critical thickness of the GaAs buffer layer is decreased to a value smaller than 3 nm.

Sputtering of the target surface by Cs+ ions: Steady-state concentration of implanted cesium and emission of CsM+ cluster ions

Experimental data for the variation of the work function on the Si and GaAs semiconductor surfaces irradiated by cesium ions are presented. The formation mechanism of CsM+ cluster ions (M is the analyte) is considered. Ionization potentials for some CsM molecules are calculated, and a simple experimental technique to determine the concentration of cesium penetrating into the subsurface region of various materials during cesium ion sputtering is suggested. This technique uses a preimplanted potassium as an “internal standard.”