Search

The metal-semiconductor interface in self-aligned contact formation can determine the overall performance of nanoscale devices. This interfacial morphology is predicted and well researched in homogenous semiconductor nanowires (NWs) but was not pursued in heterostructured core/shell nanowires. We found here that the solid-state reactions between Ni and Ge/Si core/shell nanowires resulted in a protruded and a leading NiSiy segment into the channel. A single Ni2Ge/NiSiy to Ge/Si core/shell interface was achieved by the selective shell removal near the Ni source/drain contact areas. Using in situ transmission electron microscopy, we measured the growth rate and anisotropic strain evolution in ultra-short channels. We found elevated compressive strains near the interface between the compound contact and the NW and relatively lower strains near the center of the channel which increased exponentially below the 10 nm channel length to exceed 10% strain at ∼3 nm lengths. These compressive strains are expected to ...

We have deposited thin films of Cu3Ge on (100) Si by the sequential e‐beam deposition of an amorphous Ge layer followed by a polycrystalline Cu layer. The films were then annealed in flowing N2 for 30 min over a temperature range of 150–600 °C to induce complete recrystallization of the Cu–Ge films. Films of Cu3Ge form a smooth, atomically sharp interface with (100) Si over a wide range of anneal temperatures, as determined by transmission electron microscopy, indicating the lack of any chemical reactions, i.e., compound formation, at the film/substrate interface. We have observed by secondary ion mass spectrometry, that a substantial amount of Si has diffused into the Cu–Ge in varying amounts that exhibit strong dependence on the anneal temperature. Despite the large amount of Si that has diffused into the films, the Cu3Ge maintains its low resistivity (ρ=10–15 μΩ cm) up to an anneal temperature of 600 °C, at which point the film completely loses its structural integrity, i.e., it becomes discontinuous w...

We report the first study of interfacial reactions of a metal with Si1−x−yGexCy epitaxially grown on Si. The Ti/Si1−x−yGexCy/Si (0

The partially ionized beam (PIB) deposition technique was used to deposit CoGe2(001) thin films heteroepitaxially on GaAs(100) substrates. The epitaxial arrangement is CoGe2(001)/GaAs(100). It is found that the best epitaxy was obtained with an ion energy around 1100 eV and with a substrate temperature of 280 °C. The wafers were treated only by immersion in HF:H2O 1:10 immediately prior to deposition. Contacts grown at the optimal epitaxial formation conditions displayed Ohmic behavior, while contacts grown at higher or lower substrate temperatures had rectifying behavior. Epitaxial formation of CoGe2, a high melting point, low resistivity cobalt germanide phase, offers the possibility of forming a stable contact to n‐GaAs.

The morphological instability of copper germanide (Cu3Ge) films in contact with amorphous germanium is reported. Through secondary ion mass spectrometry, x‐ray diffraction, transmission electron microscopy, and electrical measurements, the breakdown of the continuous layer has been monitored. On the contrary, with Cu3Ge in contact with single crystal germanium, no instability is observed at the same temperature. The crystallization of the amorphous germanium appears to be the mechanism responsible for the instability.

This letter reports room‐temperature synthesis by ion beam mixing of the e1‐Cu3Ge phase which is a promising candidate for interconnect and contact material in very large scale integrated circuit technology. The resistivity of the mixed sample was found to be nearly the same as the one obtained from thermally prepared films. We briefly discuss the likely mechanisms of phase formation and conclude that reaction kinetics dominates over thermodynamic forces during phase formation. The sequence of phase formation is explained by effective heat of formation rule.

Modulation of the effective electron Schottky barrier height (eSBH) of NiGe/Ge contacts induced by phosphorous ion implantation after germanide formation was investigated by considering local inhomogeneity in the eSBH. Systematic studies of NiGe/Ge contact devices having various germanide thicknesses and ion implantation areas indicated the threshold dopant concentration at the NiGe/Ge interface required for eSBH modulation and negligible dopant diffusion even at NiGe/Ge interface during drive-in annealing, leading to variation in the eSBH between the bottom and sidewall portions of the NiGe regions. Consequently, this method makes it possible to design source/drain contacts with low-resistivity Ohmic and ideal rectifying characteristics for future Ge-based transistors.

We report a first observation of the remarkably low electrical resistivity of copper germanide thin films formed at temperatures below 200 °C. At these low temperatures, the e‐Cu3Ge phase with a monoclinic crystal structure is formed, with room‐temperature resistivity which can be as low as 5.5 μΩ cm. The films are electrically stable up to at least 600 °C, and, unlike pure copper, are also stable against oxygen and air exposure.

In this Letter, NiGe/SiO2/n-Ge ohmic contacts were demonstrated with Ge, rather than Ni, diffusion through the ion-implanted SiO2 films to form NiGe. The equivalent Schottky barrier height reduced from 0.58 eV for NiGe/n-Ge to ohmic contact. The anomalous diffusion behavior and accumulation of Ge in the SiO2 near the NiGe/SiO2 interface can be explained by vacancy-enhanced Ge diffusion. It is proposed that the presence of vacancies and Ge atoms embedded in the SiO2 layer play a significant role in the current enhancement by generation of multiple levels in the SiO2 band gap.

Herein the synthesis of Cu3Ge films by exposing Cu-Ge alloy films to microwave radiation is reported. It is shown that microwave radiation led to the formation of copper germanide at temperatures ca. 80 °C. The electrical properties of the Cu3Ge films are presented and compared for various annealing times. X-ray diffraction shows that the Cu3Ge films formed after microwave annealing is crystalline in the orthorhombic phase. Rutherford backscattering and X-ray photoelectron spectroscopy confirms the formation of copper oxide encapsulation layer. Despite the slight oxidation of Cu during the microwave anneal the lowest resistivity of Cu3Ge films obtained is 14 μΩ-cm.

Electrodeposition of copper germanide (Cu3Ge) thin films from an alkaline tartrate-complexed electrolyte is described. Current pulsing is shown to enable co-deposition of copper and germanium in the stoichiometric ratio Cu:Ge = 3:1, while providing smooth and compact electrodeposits. The presence of e-Cu3Ge phase with a monoclinic crystal structure is confirmed by X-ray diffraction of the as-deposited films. After annealing, Cu3Ge films exhibit an electrical resistivity of 45 μΩ cm; a value higher than previously reported resistivity of e-beam evaporated Cu3Ge films (∼10 μΩ cm). The higher electrical resistivity of the electrodeposited Cu3Ge films is attributed to smaller grain size and elevated impurity levels.

Poor ohmic contact by Fermi-level pinning to valence band (EV) edge is one of the major challenges for germanium (Ge) n-type metal–oxide–semiconductor field-effect transistor (nMOSFET). Using low work-function rare-earth ytterbium (Yb), good ohmic contact on n-type Ge with alleviated Fermi-level pinning was demonstrated. Such ohmic behavior depends strongly on the germanide formation condition, where much degraded ohmic contact at 600 °C rapid thermal annealing is due to the lower Yb/Ge composition found by energy-dispersive x-ray spectroscopy. The ohmic behavior of Yb-germanide/n-type-Ge has high potential for future high-performance Ge nMOSFET application.

In this work, nickel germanide Schottky contacts have been fabricated on n-type germanium (n-Ge) with an optimum barrier height of 0.63 eV. For rapid thermal annealing (RTA) temperatures above 300 °C, all phases of nickel and germanium convert to nickel mono-germanide (NiGe). However, higher RTA temperatures are also found to cause agglomeration of the NiGe phase and higher leakage current. So, the optimum temperature for Schottky-based source/drain contact formation on n-Ge is ∼300 °C, where the nickel mono-germanide phase is formed but without phase agglomeration.

Nickel germanide contacts are expected to play an important role in Ge-based electronics similar to that of their nickel silicide counterparts in Si devices. Here we have studied the solid state reaction between Ni contact pads and Ge nanowires. We observe the formation of axial nickel germanide segments after annealing at temperatures as low as 300 °C for 2 min. The nickel germanide segments are polycrystalline, without an epitaxial relationship to the Ge nanowire, in contrast to observations of epitaxial nickel silicide formation from Si nanowires. The crystal structure of the nickel germanide phase is consistent with the Ni2In prototype structure. Annealing above 400 °C results in fracture in the nickel germanide segment; however, nickel germanide segments as long as 1.7 μm can be formed by annealing at 400 °C for 5 min.

The thermal stability of NiGe films formed on epitaxial Ge on Si substrates was improved from 450to550°C by simply adding an ultrathin (∼1nm) Ti layer during Ni deposition, either as an intermediate layer between Ni and Ge or as a capping layer on Ni. The improvement was attributed to the formation of ternary Ni1−xTixGe phase near the NiGe surface, which acts as a capping layer to suppress agglomeration of the underlying NiGe film at an elevated temperature, as well as modification of the NiGe grain boundaries. The resistivity of NiGe is also slightly reduced by the Ti incorporation, making this method very promising for the germanium technology.

Electron backscattered diffraction (EBSD) was used to characterize the texture of NiSi films. The various texture components (i.e., the types of preferred orientation, e.g., axiotaxy or epitaxy) could be identified, and their volume fraction could be quantified. Moreover, the spatial distribution of texture components could be studied in blanket films. Orientation imaging microscopy using EBSD is a promising technique to study the texture of silicide or germanide films both on blanket and patterned substrates.

A strong influence of substrate crystallinity is observed for thin-film Co∕Ge reactions. For the detected phases (CoGe, Co5Ge7, and CoGe2), the formation temperatures on amorphous Ge (a-Ge) are found to be the lowest, while the highest are on single-crystalline Ge(100). Moreover, while the phase sequence on Ge(100) and polycrystalline Ge (poly-Ge) was unaltered, the formation of intermediate Co5Ge7 was not observed on a-Ge. It is likely that this is due to a promoted CoGe2 formation on a-Ge, resulting in a ∼200°C decrease in formation temperature (depending on the ramp rate). These observations suggest a strong competition among the formation of these Ge-rich phases.

Schottky contacts of Pt germanide films formed on n-Ge(001) through solid-state reaction between Pt and Ge(001) via rapid thermal annealing were investigated. Almost identical effective barrier heights of ∼0.619–0.626eV were obtained for PtGe∕n-Ge(001), Pt2Ge3∕n−Ge(001), and PtGe2∕n-Ge(001) Schottky contacts from current-voltage measurements. From the effective barrier height values, actual barrier heights of ∼0.653–0.663eV were determined by taking into account the image force induced barrier lowering in the presence of strong inversion layers at the interfaces. The actual barrier height values obtained were further validated by the good agreement between experimental and simulation results for capacitance-voltage characterization.

The reaction of the rare earth metal Ho with the Ge(001) surface at 440°C has been studied by scanning tunneling microscopy (STM). The self-assembly of ultrafine nanolines growing along substrate ⟨110⟩ directions has been observed, and based on atomic resolution STM images, the authors propose a model of the nanolines and comment on their relationship to the very initial stages of growth of a hexagonal germanide structure. The authors further report the presence of nanoscale trenches associated with well-ordered lines of missing dimer defects and discuss the relationship of these to the nanolines. Their results have possible applications involving interconnects or templating in nanoscale devices, and additionally, may provide insight into the nucleation mechanism of coarser nanowires.

Defect formation during Pd and Pt germanidation of n-type germanium, using rapid thermal annealing in the range of 300–500°C, is investigated by deep level transient spectroscopy. Small concentrations of an electron trap at ∼EC−0.385eV are found, which is believed to be associated with vacancy-related sputtering damage. This is supported by the observation of much higher concentrations of vacancy-related deep levels on as-deposited Pt and Pd Schottky barriers. A steep concentration profile rising towards the surface confirms the assignment to Ar ion-related damage. The applied thermal budget points out that the defects observed in the germanide samples are more stable than simple vacancy-related point defects and could correspond to vacancy clusters formed during the deposition and/or the annealing step.

Nickel-germanide Schottky barriers have been made on n-type germanium and evaluated by deep level transient spectroscopy in order to detect possible metal indiffusion during the 30s rapid thermal annealing (RTA) employed for the germanidation. It is shown that while no electron traps have been found for the 300 and 350°C RTA step, the double acceptor level at EC−0.3eV of substitutional nickel was observed for the 400 and 450°C samples. The corresponding concentration profile increases exponentially towards the surface from which an effective diffusion coefficient of ∼5×10−10cm2∕s at 450°C has been derived.

A thin metallic cobalt (Co) layer was deposited on a single-crystal Ge (001) surface at room temperature by the electron-beam evaporation of a pure Co metal source in an ultrahigh-vacuum transmission electron microscope. The formation and epitaxial growth of a cobalt germanide Co5Ge7 phase on the Ge (001) surface was studied in situ by gradually heating the sample from room temperature to ∼350°C. The occurrence of an epitaxial hexagonal-close-packed Co and the reaction between Co and Ge were observed at ∼225°C. After annealing at ∼300°C for 26.5 h, a continuous epitaxial Co5Ge7 film formed on the Ge (001) substrate. With further annealing at a higher temperature, the continuous Co5Ge7 layer broke up and formed three-dimensional islands in order to relieve the strain energy in the epitaxial Co5Ge7 layer. Two epitaxial relationships between Co5Ge7 and Ge, i.e., Co5Ge7⟨110⟩(001)∕∕Ge⟨100⟩(001) and Co5Ge7⟨001⟩(110)∕∕Ge⟨100⟩(001) were found by electron diffraction.

Cobalt was deposited on single-crystal Ge(001) surface at ∼350°C by electron-beam evaporation in an ultrahigh-vacuum transmission electron microscope. The deposited Co reacts with Ge to form nanosized islands with the cobalt germanide Co5Ge7 phase. The Co5Ge7 islands show square and rectangular shapes. Two epitaxial orientation relationships between Co5Ge7 and Ge were observed: Co5Ge7 ⟨110⟩(001)‖Ge⟨100⟩(001) and Co5Ge7⟨001⟩(110)‖Ge⟨100⟩(001).

Rectifying metal‐semiconductor contacts, produced by heat treatment of a thin evaporated layer of Pd/Ge on n‐type GaAs, have been studied using Auger electron spectroscopy, x‐ray diffraction, and current‐voltage measurements. During sintering at temperatures below 350 °C, the Pd and Ge were found to interdiffuse on the nonreacting GaAs substrate to form PdGe and Pd2Ge. The formation of the germanide phases altered the Ge LMM Auger spectra and produced an increase in the Schottky barrier energy from 0.67 to 0.85 eV.