Formation of titanium and cobalt germanides on Si (100) using rapid thermal processing

Titanium and cobalt germanides have been formed on Si (100) substrates using rapid thermal processing. Germanium was deposited by rapid thermal chemical vapor deposition prior to metal evaporation. Solid phase reactions were then performed using rapid thermal annealing in either Ar or N₂ ambients. Germanide formation has been found to occur in a manner similar to the formation of corresponding silicides. The sheet resistance was found to be dependent on annealing ambient (Ar or N₂) for titanium germanide formation, but not for cobalt germanide formation. The resistivities of titanium and cobalt germanides were found to be 20 μΩ-cm and 35.3μΩ-cm, corresponding to TiGe₂ and Co₂Ge, respectively. During solid phase reactions of Ti with Ge, we have found that the Ti₆Ge₅ phase forms prior to TiGe₂. The TiGe₂ phase was found to form approximately at 800° C. Cobalt germanide formation was found to occur at relatively low temperatures (425° C); however, the stability of the material is poor at elevated temperatures.     

The Crystal Structures of Eu5Ge3 and EuIrGe2

Eu₅Ge₃ and EuIrGe₂ were prepared from the elements in tantalum tubes, and their crystal structures were determined from single crystal X-ray data. Eu₅Ge₃ adopts the structure of Cr₅B₃: I4/mcm, a = 799.0(1)pm, c = 1 536.7(1)pm, Z = 4, wR2 = 0.0421 for 669 F² values and 16 variables. The structure of Eu₅Ge₃ contains isolated germanium atoms and germanium atom pairs with a Ge? Ge distance of 256.0 pm. Eu₅Ge₃ may be described as a Zintl phase with the formulation [5 Eu²⁺]¹⁰⁺[Ge]^4?[Ge₂]^6?. Magnetic investigations of Eu₅Ge₃ show Curie-Weiss behaviour above 50 K with a magnetic moment of μ_exp = 7.6(1) μ_B which is close to the free ion value of μ_eff = 7.94 μ_B for Eu²⁺. EuIrGe₂ is isotypic with CeNiSi₂: Cmcm, a = 445.5(2) pm, b = 1 737.4(4) pm, c = 426.6(1) pm, Z = 4, wR2 = 0.0507 for 295 F² values and 18 variables. The structure of EuIrGe₂ is an intergrowth of ThCr₂Si₂-like slabs with composition EuIr₂Ge₂ and AlB₂-like slabs with composition EuGe₂ in an AB stacking sequence. Both slabs are distorted when compared to the symmetry of the prototypes. The Ge? Ge distance of 256.6 pm in the AlB₂-like fragment is comparable to that in Eu₅Ge₃.     

Bridgman crystal growth of Yb2Ru3Ge4—A ternary germanide with a three-dimensional network of condensed distorted RuGe5 and RuGe6 units

The germanide Yb₂Ru₃Ge₄ was synthesized from the elements using the Bridgman crystal growth technique. The monoclinic Hf₂Ru₃Si₄ type structure was investigated by X-ray powder and single crystal diffraction: C2/c, Z=8, a=1993.0(3) pm, b=550.69(8) pm, c=1388.0(2) pm, β=128.383(9)°, wR₂=0.0569, 2047 F² values, and 84 variables. Yb₂Ru₃Ge₄ contains two crystallographically independent ytterbium sites with coordination numbers of 18 and 17 for Yb1 and Yb2, respectively. Each ytterbium atom has three ytterbium neighbors at Yb-Yb distances ranging from 345 to 368 pm. The shortest interatomic distances occur for the Ru-Ge contacts. The three crystallographically independent ruthenium sites have between five and six germanium neighbors in distorted trigonal bipyramidal (Ru1Ge₅) or octahedral (Ru2Ge₆ and Ru3Ge₆) coordination at Ru-Ge distances ranging from 245 to 279 pm. The Ru2 atoms form zig-zag chains running parallel to the b-axis at Ru2-Ru2 of 284 pm. The RuGe₅ and RuGe₆ units are condensed via common edges and faces leading to a complex three-dimensional [Ru₃Ge₄] network.     

Syntheses and Structures of the Germanides CaNiGe and MgCoGe as well as Chemical Bonding in CaNiGe and CaNi2Ge2

The intermetallic germanides CaNiGe and MgCoGe have been synthesized from the elements in sealed tantalum tubes in a high‐frequency furnace. The compounds were investigated by X‐ray diffractions both on powders and single crystals: CeFeSi structure type, P4/nmm, a = 4.19341(3), c = 6.6264(1) Å, wR2 = 0.030, 124 F² values, 10 variable parameters for CaNiGe and a = 3.8960(4), b = 6.1929(11) Å, wR2 = 0.048, 104 F² values, 10 variable parameters for MgCoGe. In CaNiGe and MgCoGe the transition metal and germanium atoms build [TGe] layers (T = Ni, Co), which are separated by the calcium and magnesium atoms, respectively. The crystal structures of CaNiGe and MgCoGe as well as chemical bonding in CaNiGe and CaNi₂Ge₂ are discussed in terms of LMTO bond structure calculation and analysis using the Electron Localization Function (ELF). The Ge–Ge bond formation in polyanionic network of CaNi₂Ge₂ can formally be regarded as oxidative coupling product of layers of CaNiGe.     

Structure and magnetic properties of rare-earth chromium germanides RECrxGe2 (RE=Sm, Gd-Er)

The ternary rare-earth chromium germanides RECr_xGe₂ (RE=Sm, Gd-Er) have been obtained by reactions of the elements, either in the presence of tin or indium flux, or through arc-melting followed by annealing at 800 °C. The homogeneity range is limited to 0.25?x?0.50 for DyCr_xGe₂. Single-crystal and powder X-ray diffraction studies on the RECr_0.3Ge₂ members revealed that they adopt the CeNiSi₂-type structure (space group Cmcm, Z=4, a=4.1939(5)-4.016(2) Å, b=16.291(2)-15.6579(6) Å, c=4.0598(5)-3.9876(2) Å in the progression for RE=Sm to Er), which can be considered to be built up by stuffing transition-metal atoms into the square pyramidal sites of a “REGe₂” host with the ZrSi₂-type structure. (The existence of YbCr_0.3Ge₂ is also implicated.) Only the average structure was determined here, because unusually short Cr-Ge distances imply the development of a superstructure involving distortions of the square Ge net. Magnetic measurements on RECr_0.3Ge₂ (RE=Gd-Er) indicated that antiferromagnetic ordering sets in below T_N (ranging from 3 to 17 K), with additional transitions observed at lower temperatures for the Tb and Dy members.     

Influence of the pre-treatment anneal on Co–germanide Schottky contacts

A thin cobalt layer is deposited by electron beam evaporation onto a germanium substrate after an in situ cleaning annealing at 400 or 700 °C. The effect of these pre-treatments on the Co/Ge Schottky barrier properties and on the germanide formation is investigated by using different techniques. A strong influence of the pre-treatment is observed. The pre-treatment at 700 °C removes the native oxide but enhances the diffusion of contaminants. After post-metal deposition annealing, the sample pre-treated at 700 °C shows a double layer structure due to interdiffusion, whereas some large isolated islands are present in the sample pre-treated at 400 °C.     

Formation of Ni(Ge1−xSnx) layers with solid-phase reaction in Ni/Ge1−xSnx/Ge systems

We have demonstrated the formation of Ni(Ge_1−ySn_y) layers on Ge_1−xSn_x layers by using solid-phase reaction for samples with Sn contents ranging from 2.0% to 6.5%. We have also investigated solid-phase reaction products in Ni/Ge_1−xSn_x/Ge samples after annealing and the crystalline properties of nickel-tin-germanide layer/Ge_1−xSn_x contact structures. After annealing at temperatures ranging from 350 to 550 °C, the formation of polycrystalline Ni(Ge_1−ySn_y) layers has been observed on epitaxial Ge_1−xSn_x layers with Sn contents ranging from 2.0% to 6.5%. We also observed anisotropic crystal deformation of NiGe with the incorporation of Sn atoms into substitutional sites in NiGe. In the case of the Ni/Ge_1−xSn_x/Ge sample with a Sn content of 3.6%, the formation of an epitaxial Ni₂(Ge_1−zSn_z) layer on the Ge_1−xSn_x layer was found. The formation of β-Sn crystallites was observed after annealing at above 450 °C in samples with a high Sn content of 6.5%. This β-Sn formation is due to the precipitation of Sn atoms. In all samples annealed at 350 °C, the morphology of Ni-Ge-Sn layers is smooth and uniform. However, the surface roughness and interface roughness increase for an annealing temperature of 550 °C. In particular, in the sample with a Sn content of 6.5%, the temperature at which agglomeration noticeably occurs is as low as 450 °C.     

Quaternary Germanides RE3TRh4Ge4 (RE = Ce,Pr, Nd; T = Nb,Ta) – A New Coloring Variant of the Aristotype AlB2

The quaternary germanides RE₃TRh₄Ge₄ (RE = Ce, Pr, Nd; T = Nb, Ta) were synthesized from the elements by arc‐melting and subsequent annealing in a muffle furnace. The structure of Ce₃TaRh₄Ge₄ was refined from single‐crystal X‐ray diffractometer data: new type, Pbam, a = 719.9(2), b = 1495.0(3), c = 431.61(8), wR₂ = 0.0678, 1004 F² values, and 40 variables. Isotypy of the remaining phases was evident from X‐ray powder patterns. Ce₃TaRh₄Ge₄ is a new superstructure variant of the aristotype AlB₂ with an ordering of cerium and tantalum on the aluminum site, whereas the honey‐comb network is built up by a 1:1 ordering of rhodium and germanium. This crystal‐chemical relationship is discussed based on a group‐subgroup scheme. The distinctly different size of tantalum and cerium leads to a pronounced puckering of the [Rh₄Ge₄] network, which shows the shortest interatomic distances (253–271 pm Rh–Ge) within the Ce₃TaRh₄Ge₄ structure. Another remarkable structural feature concerns the tantalum coordination with six shorter Ta–Rh bonds (265–266 pm) and six longer Ta–Ge bonds (294–295 pm). The [Rh₄Ge₄] network fully separates the tantalum and cerium atoms (Ce–Ce > 387 pm, Ta–Ta > 431 pm, and Ce–Ta > 359 pm). The electronic density of states DOS from DFT calculations show metallic behavior with large contributions of localized Ce 4f as well as itinerant ones from all constituents at the Fermi level but no significant magnetic polarization on Ce could be identified. The bonding characteristics described based on overlap populations illustrate further the crystal chemistry observations of the different coordination of Ce1 and Ce2 in Ce₃TaRh₄Ge₄. The Rh–Ge interactions within the network are highlighted as dominant. The bonding magnitudes follow the interatomic distances and identify differences of Ta bonding vs. Ce1/Ce2 bonding with the Rh and Ge substructures.