N.V. Abrosimov

Czochralski growth of Si- and Ge-rich SiGe single crystals

Si 1-x Ge x single crystals (0 < x < 0.15) with diameter up to 2 in were grown by conventional Czochralski technique. The method for the growth of Ge 1-x Si x crystals using continuous feeding the melt with a number of Si rods was developed and the Ge 1-x Si x crystals (0 < x < 0.3) were grown. The growth process both Si 1-x Ge x and Ge 1-x Si x crystals was carried out using the automated control system based on the crystal weighing. The peculiarities of weight control at the stage of conical part formation are considered.

Single crystal growth of Si1 − xGex by the Czochralski technique

(111)- and (100)-Si 1-x Ge x single crystals (0 < x < 0.1) with diameters of up to 48 mm were grown by Czochralski technique using a crystal pullet with resistance heating. An automated control system based on crystal weighing has been developed. Software provides feedback control as well as the program changes of some process parameters to optimize crystal growth conditions. Ge distribution and crystal quality were investigated by infrared transmission measurements at the absorption edge and X-ray Lang topography, respectively.

Experimental and theoretical study of the diffraction properties of various crystals for the realization of a soft gamma-ray Laue lens

Crystals are the elementary constituents of Laue lenses, an emerging technology which could allow the realization of a space borne telescope 10 to 100 times more sensitive than existing ones in the 100 keV - 1.5 MeV energy range. This study addresses the current endeavor to the development of efficient crystals for the realization of a Laue lens. In the theoretical part 35 candidate-crystals both pure and two-components are considered. Their peak reflectivity at 100 keV, 500 keV and 1 MeV is calculated assuming they are mosaic crystals. It results that a careful selection of crystals can allow a reflectivity above 30% over the whole energy range, and even reaching 40% in its lower part. Experimentally, we concentrated on three different materials (Si_{1-x}Ge_x with gradient of composition, mosaic Cu and Au) that have been measured both at ESRF and ILL using highly-monochromatic beams ranging from 300 keV up to 816 keV. The aim was to check their homogeneity, quality and angular spread (mosaicity). These crystals have shown outstanding performance such as reflectivity up to 31% at ~600 keV (Au) or 60% at 300 keV (SiGe) and angular spread as low as 15 arcsec for Cu, fulfilling very well the requirements for a Laue lens application. Unexpectedly, we also noticed important discrepancies with Darwins model when a crystal is measured using various energies.

Lateral photovoltage scanning (LPS) method for the visualization of the solid–liquid interface of Si1−xGex single crystals

Besides the application of Si 1-x Ge x as thin layers in electronic devices, well-defined mixed bulk crystals are successfully used for X ray, γ- and neutron-diffraction optics. For developing growth methods for such crystals, the shape of the growth interface and the striation pattern of the distribution of the components and the dopants must be known. Applying for the first time, the lateral photovoltage scanning (LAS) method to different Si 1-x Ge x mixed crystals, their striation patterns and shapes of the growth interface have been determined even for highly dislocated crystals where X-ray topography is not able to show the Ge-striations. However, also dislocation slip lines can be observed clearly with LPS when using high optical excitation.

Neutron transmutation doping of silicon 30Si monoisotope with phosphorus

Phosphorus-doped silicon 30Si monoisotope samples with a highly homogeneous impurity distribution at a concentration of 5 × 1016 cm−3 were obtained for the first time by means of neutron transmutation doping.

X-ray imaging of structural defects in Si1−xGex single crystals using a white synchrotron beam

Nonmonochromatic (white) synchrotron radiation with a high spatial coherence makes it possible to use different types of interaction of X-rays with matter simultaneously: diffraction, refraction, absorption, and fluorescence. In this case, the structure of materials is studied by the real-time recording of high-resolution images of different types under the same conditions. The use of X-ray images for studying the structural quality is demonstrated by the example of Czochralski-grown Si1 −xGex single crystals. The effect that the germanium content has on the formation and evolution of the defect structure is analyzed and the relationship between the structure and properties is investigated. The experiments were performed on the Pohang Light Source (Pohang, Republic of Korea).

Current-voltage characteristics of Si/Si1 − xGex heterodiodes fabricated by direct bonding

We have studied the current-voltage (I–U) characteristics of Si/Si1 − xGex (0.02 < x < 0.15) heterodiodes fabricated by direct bonding of (111)-oriented n-type single crystal silicon wafers with p-type Si1 − xGex wafers of the same orientation containing 2–15 at % Ge. An increase in the germanium concentration NGe in Si1 − xGex crystals is accompanied by a growth in the density of crystal lattice defects, which leads to a decrease in the minority carrier lifetime in the base of the heterodiode and an increase in the recombination component of the forward current and in the differential resistance (slope) of the I–U curve. However, for all samples with NGe ≤ 15 at %, the I–U curves of Si/Si1 − xGex heterodiodes are satisfactory in the entire range of current densities (1 mA/cm2–200 A/cm2). This result shows good prospects for using direct bonding technology in the fabrication of Si/Si1 − xGex heterostructures.

Bistability of hydrogen donors in proton-implanted GeSi alloy

Shallow hydrogen donors (H-donors) were formed in a Ge1 − xSix (x = 0.012) alloy by the implantation of low-energy protons followed by heat treatment at 275°C The electrical properties of these donors have been studied using the method of capacitance-voltage characteristics. It is established that a certain fraction of the H-donors exhibit bistability, whereby their concentration changes reversibly when the sample temperature is cycled within 100–200°C. The properties of reversible H-donors in germanium are analogous to those of the bistable H-donors in silicon.

Examination of Si1-xGex Laterally Graded Crystals for Use in High Brilliance Synchrotron Beams

We report about the feasibility of employing laterally graded Si 1-x Ge x crystals as x-ray monochromators for synchrotron radiation beams. This type of crystals can be used in a divergent beam with nearly the same energy resolution and spectral flux as a regular crystal in a parallel beam. The effect is achieved due to continous changes in the lattice parameter with varying Ge concentration in the Si crystal. Theoretical calculations for BESSY II dipole radiation show, that the optimum gradient of Ge along the surface should be about 1.0 at. % per cm. The properties of the crystals have been investigated by x-ray diffraction and fluorescence analysis with radiation from a conventional x-ray tube. We found nearly perfect crystal quality for Ge concentrations up to 7.0 at. % and gradients of Ge up to 1.1 at. % Ge per cm which would already be sufficient for applications in the x-ray monochromator.

Formation of donors in germanium–silicon alloys implanted with hydrogen ions with different energies

The distributions of hydrogen-containing donors in Ge1–xSix (0 ≤ x ≤ 0.06) alloys implanted with hydrogen ions with an energy of 200 and 300 keV and a dose of 1 × 1015 cm–2 are studied. It is established that, at the higher ion energy, the limiting donor concentration after postimplantation heat treatment (275°C) is attained within ~30 min and, at the lower energy, within ~320 min. In contrast to donors formed near the surface, a portion of hydrogen-containing donors formed upon the implantation of ions with the higher energy possess the property of bistability. The limiting donor concentration is independent of the ion energy, but decreases from 1.3 × 1016 to 1.5 × 1015 cm–3, as the Si impurity content in the alloy is increased from x = 0.008 to x = 0.062. It is inferred that the observed differences arise from the participation of the surface in the donor formation process, since the surface significantly influences defect-formation processes involving radiation-induced defects, whose generation accompanies implantation.