Ge Planes Research Articles

Brittle-ductile transition and nano-surface generation in diamond turning of single-crystal germanium

Single-crystal germanium (Ge) has been widely used in infrared optics and ultra-precision components, however, the appearance of alternating brittle and ductile fan-shaped patterns on machined surfaces seriously affects the machined surface quality. The present research focused on the brittle-ductile transition and its effect on the nano-surface generation in single-point diamond turning (SPDT) of (100), (110) and (111) crystal planes of Ge. Face cutting and plunge cutting experiments were performed, the surface anisotropy and brittle-ductile transition regime during cutting were analyzed, and the evolution of crystalline structure was discussed with molecular dynamics (MD) simulation. Theoretical and experimental results show: (1) The anisotropy of the machined surface is closely related to the atomic arrangement order and lattice structure. The anisotropy of the (111) crystal plane is stable while the (110) is unstable. (2) The critical length and depth of both the elastic-ductile and brittle-ductile transition are anisotropic; the depth and length of brittle regions are shallower and shorter than those of ductile regions and proportional to the tool edge radius. (3) The critical depth and thrust force of the elastic-ductile transition obtained in MD simulation can be employed to predict the changing trend of the brittle-ductile transition. (4) The thickness of amorphous layer produced on the machined surface is proportional to the ratio of cut depth to tool edge radius. The 1¯10 crystal orientation of the (111) crystal plane shows the best cutting performance. This paper provides deep insight into the brittle-ductile transition and nano-surface generation in SPDT of Ge.

Brittle-Ductile Transition and Nano-Surface Generation in Single-Point Diamond Turning of Single-Crystal Germanium

Single-crystal germanium (Ge) has been widely used in infrared optics and ultra-precision components, however, the appearance of alternating brittle and ductile fan-shaped patterns on machined surfaces seriously affects the machined surface quality. The present research focused on the brittle-ductile transition and its effect on the nanosurface generation in single-point diamond turning (SPDT) of (100), (110) and (111) crystal planes of Ge. Face cutting and plunge cutting experiments were performed, the surface anisotropy and brittle-ductile transition regime during cutting were analyzed, and the evolution of crystalline structure were discussed with molecular dynamics (MD) simulation. Theoretical and experimental results show: (1) The anisotropy of the machined surface is closely related to the atomic arrangement order and lattice structure. The anisotropy of the (111) crystal plane is stable while the (110) is unstable. (2) The critical length and depth of both the elastic-ductile and brittle-ductile transition are anisotropic; the depth and length of brittle regions are shallower and shorter than those of ductile regions and proportional to the tool edge radius. (3) The critical depth and thrust force of the elastic-ductile transition obtained in MD simulation can be employed to predict the changing trend of the brittle-ductile transition. (4) The thickness of amorphous layer produced on the machined surface is proportional to the ratio of cut depth to tool edge radius. The crystal orientation of the (111) crystal plane shows the best cutting performance. This paper provides deep insight into the brittle-ductile transition and nanosurface generation in SPDT of Ge.

Influence of Annealing Condition on Threading Dislocation Density of Ge Grown by RPCVD

The influence of annealing conditions on the crystallinity of Ge deposited on Si (001) is investigated. Ge deposited with postannealing at 800°C or 850°C, five cycles of postannealing at 750°C and 850°C and several cycles of annealing at 800°C or 850°C during the Ge growth by interrupting the deposition step (cyclic annealing) are compared. To check the threading dislocation density (TDD) of the deeper part of the Ge layer, thinning by HCl vapor phase etching (VPE) followed by Secco defect etching is performed for 5 µm thick Ge of all annealing variants. By comparing TDD of the same Ge thickness without / with HCl VPE, TDD reduction after VPE is observed for the samples with cyclic annealing process only. In the case of the cyclic annealing process, 5 µm thick Ge shows the same TDD level compared to that with five cycles of postannealing at 750°C and 850°C with lower thermal budget. A lower amount of tilted Ge planes at the interface is confirmed for the sample with cyclic annealing indicating higher crystal quality also in the deeper part of the Ge layer.

Influence of annealing conditions on threading dislocation density in Ge deposited on Si by reduced pressure chemical vapor deposition

The influence of annealing conditions on the crystallinity of Ge deposited on Si(001) is investigated. Ge deposited with postannealing at 800 °C and 850 °C, five cycles of postannealing at 750 °C and 850 °C (temperature-swing postannealing) and several cycles of annealing at 800 °C or 850 °C during the Ge growth by interrupting the deposition step (cyclic annealing) are compared. To check the threading dislocation density (TDD) of the deeper part of the Ge, thinning by HCl vapor phase etching (VPE) followed by Secco defect etching is performed for 5 μm thick Ge of all three annealing variants. By comparing the TDD of the same Ge thickness with and without HCl VPE, TDD reduction by VPE is observed for the sample using the cyclic annealing process only. Lower TDD is observed at higher postannealing temperature. By the five cycles of temperature swinging, TDD becomes around a half compared to conventional postannealing at 850 °C. In the case of the cyclic annealing process significant improvement of TDD is observed with increasing Ge thickness. Even at a maximum temperature of 800 °C, the same or lower TDD levels were observed for higher than 2 μm thick Ge compared to that with five cycles of temperature-swing postannealing. For the sample with cyclic annealing at 800 °C, a lower Si diffusion length into Ge is also observed for the cyclic annealing process indicating a lower thermal budget. A lower amount of tilted Ge planes at the interface is confirmed showing higher crystal quality also in the deeper part of the Ge layer.

Electrical switching properties of nanostructured multilayer superlattice Ge–Si…Cu…Si–Ge prepared by electron beam evaporation on glass

In this study, the fabrication of nanostructured multilayer superlattice Ge–Si…Cu…Si–Ge using electron beam evaporation on glass substrates at a temperature of 200 °C has been reported. The structural, optical and electrical characteristics of such films were then studied by means of DC polarization, powder X-ray diffraction (XRD), scanning electron microscopy, atomic force microscopy (AFM), Rutherford backscattering (RBS) and UV–visible spectroscopic techniques. The XRD pattern obtained showed an amorphous state along with some reflected XRD signals from the Ge planes while there were no reflected signals from the Si and Cu layers. The average estimated grain size diameter ( D a v . ) is 30.13 nm as found from the XRD experiment using the Debye–Scherrer equation. The grain size as estimated from the AFM images was found to be within 28–31 nm, which is in good agreement with the XRD average value. RBS analysis confirmed the multilayer structure of the superlattice. The optical allowed indirect band gap of the superlattice obtained from the Tauc plot was found to be 1.19 eV, which is greater than that of bulk Ge semiconductor (0.66 eV). This fact indicates that the incorporation of a Cu layer instead of the Ge layer enables us to change the indirect to direct transitions in Ge/Si devices. Current density–voltage ( J – V ) characteristics of the nanodevice showed an electrical switching effect (VCNR) with the largest ON/OFF current ratio of the order of 10 6 at 15 V.

Germanium network connecting precipitates in an Mg-rich Al-Mg-Ge alloy

Precipitates in an Al-0.87Mg-0.43Ge (at.%) alloy, heat-treated for 16 h at 200 degrees C, were investigated by transmission electron microscopy and annular dark-field scanning transmission electron microscopy (ADF-STEM). Earlier studies of Al-Mg-Si-(Cu) have shown that an Si network exists within all precipitates. Here, it was investigated whether the heavier, more easily detectable germanium atom would behave similarly. The precipitates were more similar to those found in Al-Mg-Si-Cu alloys with a high fraction of disordered phases than to ternary Al-Mg-Si. All precipitate cross-sections along [001]Al imaged by ADF-STEM showed that Ge atoms arrange in triangular columns separated by approximately 0.4 nm. Along these columns, the precipitate's 0.405-nm periodicity and coherency (along its needle axis) imply a Ge plane periodicity of 0.405 nm. A germanium network, therefore, exists in all precipitates in this alloy, with a hexagonal sub-cell (SC) a = b approximately 0.4 nm, c = 0.405 nm, which is very similar to the Si network in Al-Mg-Si-(Cu). The network always appears as ordered. Disorder in a precipitate must, therefore, be caused by the other atoms in the structure between Ge atoms. One difference between precipitates of the ternary systems Al-Mg-Ge and Al-Mg-Si is the orientation of the diamond element network (SC) base in {001}Al. In Al-Mg-Ge, a <100>SC edge falls along <100>Al. This coincides with the orientation in some precipitates in quaternary Al-Mg-Si-Cu. In ternary Al-Mg-Si, one SC base is parallel with a <510>Al direction.

Cluster expansion and optimization of thermal conductivity in SiGe nanowires

We investigate the parametrization and optimization of thermal conductivity in silicon-germanium alloy nanowires by the cluster-expansion technique. ${\text{Si}}_{1\ensuremath{-}x}{\text{Ge}}_{x}$ nanowires are of interest for thermoelectric applications and the reduction in lattice thermal conductivity $({\ensuremath{\kappa}}_{L})$ is desired for enhancing the thermoelectric figure of merit. We seek the minimization of ${\ensuremath{\kappa}}_{L}$ with respect to arrangements of Si and Ge atoms in 1.5 nm diameter [111] ${\text{Si}}_{1\ensuremath{-}x}{\text{Ge}}_{x}$ nanowires, by obtaining ${\ensuremath{\kappa}}_{L}$ from equilibrium classical molecular-dynamics (MD) simulations via the Green-Kubo formalism, and parametrizing the results with a coarse-grained cluster expansion. Using genetic algorithm optimization with the coarse-grained cluster expansion, we are able to predict configurations that significantly decrease ${\ensuremath{\kappa}}_{L}$ as verified by subsequent MD simulations. Our results indicate that superlatticelike configurations with planes of Ge show drastically lowered ${\ensuremath{\kappa}}_{L}$.

Structural properties of the formation of yttrium germanides thin films on the Si(111) surface

We have performed first principles total energy calculations to investigate the deposit of yttrium digermanide on the Si(111) surface. We have used the periodic density functional theory as implemented in the Quantum-ESPRESSO package. For the adsorption of a monolayer of yttrium digermanide on the Si(111)-(1 × 1) surface, we have found that the most stable geometry corresponds to a configuration with Y atoms occupying the T4 site above a second layer Si atom, and with a Ge bilayer on top of the structure. The atomic structure of the Ge bilayer is similar to that of Si (Ge) in the bulk but rotated 180° with respect to the crystal. For the three dimensional growth of a few layers of yttrium digermanide on Si(111) we have considered a hexagonal structure with (√3 × √3) periodicity, similar to the one found in the growth of few layers of YSi 2 on Si(111): graphite-like Ge planes (with vacancies) intercalated with yttrium planes. As in the case of a single layer of YGe 2, there is a formation of a Ge bilayer on top of the structure. In this case, the Ge down atoms of the bilayer, which are on top the vacancies, move down towards the vacancy, while Ge atoms in the graphitic layer, which are below the Ge up atoms of the bilayer, are displaced towards the vacancy.

First principles total energy calculations of the structural and electronic properties of YGe 2 in AlB 2 type structures

We have performed first principles total energy calculations to investigate the structural and electronic properties of bulk YGe 2, in AlB 2 type configurations. Although the experimental stoichiometry is Y 2Ge 3, we have also considered the ideal YGe 2, and the Y 3Ge 5 configuration found for rare earth disilicides. Our studies using a unit cell with the ideal AlB 2 structure result in a lattice parameter a larger and a c/ a ratio smaller than the experimental values. On the other hand, the calculated structural parameters for the configurations with vacancies in the graphitic-like Ge planes are in excellent agreement with experimental values. The introduction of vacancies results in more favorable Y–Y and Ge–Y distances. The densities of states display p (of Ge) and d (of Y) orbital hybridizations, and they show that vacancies play an important role in the stability of the compound.

Epitaxy relationships between Ge-islands and SiC(0 0 0 1)

Reflection high-energy electron diffraction (RHEED) has been used to determine epitaxy relationships and in-plane orientations between Ge and SiC(0001). Three monolayers of Ge have been deposited at 500°C on a graphitized SiC (6√3 × 6√3)R30° reconstructed surface, this surface supporting epitaxial Ge island growth in a Volmer–Weber mode. Nucleation of relaxed Ge-islands gives rise to transmission electron diffraction patterns allowing to deduce that pure Ge grows according to only one epitaxy relationship Ge{111}//SiC(0001). These {111}-Ge-islands have two in-plane orientations, a preferential one, Ge〈-1-12〉//SiC〈1-100〉 and a minority one, Ge〈-1-12〉//SiC〈10-10〉, deduced one from the other by a 30° rotation around the 〈111〉-Ge (or [0001]-SiC) growth axis. Due to the three-fold symmetry of the {111}-Ge plane, each in-plane orientation is degenerated into two twin orientations, differing by a 180° angle around Ge〈111〉.

RE3Ga 9Ge ( RE=Y, Ce, Sm, Gd and Yb): compounds with an open three-dimensional polygallide framework synthesized from liquid gallium

The RE 3Ga 9Ge compounds ( RE=Y, Ce, Sm, Gd and Yb) were synthesized at 850°C in quantitative yield from reactions containing excess liquid Ga. The orthorhombic crystal structure is characterized by a unique three-dimensional open Ga framework with parallel straight tunnels. In the tunnels, inserted are arrays of the RE atoms together with interpenetrated monoatomic RE–Ga–Ge planes. A complex disordered arrangement of the RE and Ga atoms is observed in the monoatomic plane. Depending on the extent of disorder, the crystal structure could be presented either in a sub-cell (no ordering) or in a super-cell (partial ordering). Single-crystal X-ray data for Ce 3Ga 9Ge sub-structure: space group Immm, Z=2, cell parameters a=4.3400(12) Å; b=10.836(3) Å; and c=11.545(3) Å; super-structure: space group Cmma, Z=8, cell parameters a=8.680(3) Å; b=23.090(7) Å; and c=10.836(3) Å. The refinement based on the full-matrix least squares on F o 2[ I>2 σ( I)] converged to final residuals R 1/w R 2=0.0226/0.0528 and 0.0729/0.1569 for the sub- and super-structures, respectively. The relationship between the disordered sub-structure and partially ordered super-structure is discussed. Magnetic susceptibility measurements show Curie–Weiss behavior at the temperatures above 30 K with the negative Weiss constants Θ=−49(1) and−7.7 K for Gd and Ce analogs, respectively. An antiferromagnetic transition is observed in the Gd analog at T N=26.1 K. The μ eff obtained for both analogs is close to the RE 3+ free-ion value.

Bulk Homogeneous Uniaxial Stress Arising from Surface Damage of Crystalline Silicon and Germanium

Abrasion of two opposing surfaces of either crystalline Si or Ge produces a compressive, homogeneous, uniaxial stress extending throughout the bulk perpendicular to the surfaces. This is concluded by analyzing the splittings, intensities, and polarizations of the sharp Lyman absorption lines of bulk shallow impurities in the abraded materials. This effect so far has been observed for samples of thickness, t, from 0.4 to 5 mm for Si ground with water slurries of SiC or alumina with optical faces coplanar with {100} and {111} planes while {100}, {110}, {111}, and {112} planes of Ge have been abraded with SiC in water and examined over the range 0.8 ≤ t ≤ 3.0 mm. For both materials, the internal stress is found to be inversely proportional to t. Controlled etching indicates the damaged layers producing the bulk stress to be &lt; 0.5 μm thick.

Formation of a single interface-near, δ-like Ge nanocluster band in thin SiO2 films using ion-beam synthesis

The possibility to create a δ-like, interface-near Ge nanocluster band in a 20 nm thin SiO2 layer by ion-beam synthesis is demonstrated. The role of the post-implantation annealing conditions for the formation of Ge nanoclusters in the center of the layer, near the interface, or in both regions is discussed. The presence of hydrogen in the annealing atmosphere accelerates the redistribution of Ge in SiO2. By applying a two-step annealing process, preannealing in hydrogen containing atmosphere at low temperature followed by a rapid thermal annealing at high temperature, the controlled fabrication of a single δ-like, interface-near Ge nanocluster band was achieved. In some clusters 〈100〉 lattice planes of Ge were observed. From this and the similar contrast situation for amorphous clusters it is concluded that the interface-near clusters consist of elementary germanium.

Nucleation and growth of germanium on silicon (111)

The nucleation and growth of Ge on Si(111) substrates were investigated by transmission electron microscopy (TEM). It was found that growth is initiated by the formation of three-dimensional islands. At a very early stage of growth a polycrystalline layer was obtained. The orientation of this Ge film, however, improved as growth proceeded, resulting in an epitaxial film when the islands were fully coalesced. It was found that the major defects present in the Ge epilayer were microtwins (primary and secondary twins) occurring on the inclined {111} planes of Ge.

Growth processes in the initial stages of deposition of Ge films on (100) Si surfaces by GeH 4 source molecular beam epitaxy

Growth process in the initial stages of deposition of Ge films on (100)Si surfaces by GeH 4 source MBE has been studied by in-situ reflection high energy electron diffraction (RHEED) observation. The growth mode is of the Stranski-Krastanov type, and the strained monolayer overgrowth on the substrate surfaces takes place over the observed temperature range of 300–600 C. The growth behavior in the nucleation and island growth stages is found to change strikingly near 500 ° C. The activation energy of the growth rate above 500 ° C (0.24 eV) is 10 times as large as than that below 500 ° C (0.023 eV). In spite of these marked differences, the predominant facets of the growing islands is {8111} Ge planes initially and this changes to {311} Ge eventually with growth in the two temperature ranges. Half-order streaks associated with a (2×1) structure are observed to incline to 〈811〉 Ge in the diffraction patterns. This analysis gives information on the atomic arrangement of {811} Ge.

Schottky barriers and band lineups at interfaces

The principle concepts underlying our understanding of Schottky barriers for well-characterized interfaces are reviewed. The Tersoff ‘‘neutral point’’ picture based on metal-induced gap states seems promising but does not explain submonolayer results. The Tersoff band lineup model for semiconductors is tested by a Si–Ge heterojunction calculation. The difference between Si and Ge potentials is small enough to allow the junction potential to be decomposed into contributions from individual Ge planes in a Si matrix. Planes away from the junction lead to an ‘‘intrinsic’’ offset while those near the junction give a much smaller ‘‘junction sensitive’’ offset, justifying Tersoff’s assumption of an intrinsic process. A small change in the Schottky offset is strongly screened by a dielectric constant of order 10 as predicted by Tersoff. The offset is shown to be best understood using the principle of strict charge neutrality, emphasizing the irrelevance of interface dipoles as causing the offset. The offset is caused by the need to line up the hybrid energies (or neutral points) plus a correction of order 0.14 eV which compensates for the difference in bond strength between silicon and germanium.

Space-charge layers on Ge surfaces. I. dc conductivity and Shubnikov—de Haas effect

We have prepared metal-insulator-semiconductor ($M\ensuremath{-}I\ensuremath{-}S$) structure for major symmetry planes of Ge. The samples are examined and characterized systematically by capacitance-voltage ($C\ensuremath{-}V$) and conductivity measurements [$\ensuremath{\sigma}(V)$ and $\frac{d\ensuremath{\sigma}}{d{V}_{g}}$]. Lacquer-coated specimens of (111) surfaces prepared in this work give a new period in the magneto-oscillation spectrum and thus resolve the question of missing electrons raised in the experiments of Weber et al. We determine the energy splitting of the two lowest electron subbands on (111) Ge and predict the occupancy of the lowest level of the threefold degenerate valleys for ${N}_{s}$ above \ensuremath{\sim}1.0 \ifmmode\times\else\texttimes\fi{} ${10}^{12}$ ${\mathrm{cm}}^{\ensuremath{-}2}$.

Surface Energy of Germanium and Silicon

Surface energy is one of the most fundamental parameters of a solid since it depends directly on the binding forces of the material. Indeed, it is a measure of the work necessary to separate a material into two parts along a plane. Very few measurements have been made of this quantity and these by indirect means, because of difficulties of measurement and interpretation. This report deals with a direct measurement of specific surface energy of Ge and Si made by the cleavage technique. By measuring the force just necessary to move a crack “in reversible fashion” the surface energy can be obtained. The measured value for the {111} planes of Ge and Si are 1060 ergs/cm2 and 1230 ergs/cm2. From these measured values the energy of the planes {100} and {110}can be estimated. They are: Using the measured value of specific surface energy, assuming this is due to the appropriate number of broken , the bond strength of Ge and Si was obtained as ; .

Atom Ejection in Low Energy Sputtering of Single Crystals of fcc Metals and of Ge and Si

The spatial distribution of atoms ejected from low index planes of Cu, Ni, Au, Al, Ge, and Si have been studied for bombardment by Hg+, Ne+, and Ar+ ions of energies up to 800 ev. In addition, the atom ejection for Hg+-ion bombardment of various Ge planes (12° apart) belonging to the [110] zone has been investigated in detail. For the fcc crystals, ejection was observed in 〈110〉 directions at all energies, in〈100〉 directions at higher energies, and in 〈411〉 directions under certain conditions. The ejection in the 〈110〉 and 〈100〉 directions are interpreted to be the result of focusons intercepting the surface with sufficient energy to cause a surface atom to be ejected. It is shown that a reasonable interpretation of the atom ejection in 〈411〉 directions is that a 〈110〉 focuson may intercept and eject a surface atom which is in a twin position rather than a normal position. For ion bombardment of Ge and Si atom ejection is observed in 〈111〉 and 〈100〉 directions. A possible interpretation is that many of the natural interstitial positions are filled near the surface as a result of the bombardment and that the structure resembles more that of a bcc structure. The systematic deviation in ejection directions from the associated crystallographic direction observed for the Hg+-ion bombardment of various Ge surfaces suggests that the atomic collisions resulting in atom ejection for Ge and Si occur in the outer few surface layers.