Surface studies of phase formation in Co–Ge system: Reactive deposition epitaxy versus solid-phase epitaxy

Abstract

Morphological evolution of cobalt germanide epilayers, CoxGey, was investigated in situ by scanning tunneling microscopy and spectroscopy and reflection high-energy electron diffraction, as a function of deposition method and, hence, the phase content of the epilayer. During reactive deposition epitaxy, in which Co atoms were evaporated onto a flat pseudomorphic Ge/Si(001) wetting layer at 773 K, the first phase formed was cobalt digermanide, CoGe2, in the form of elongated pyramidal islands. Each of these three-dimensional islands has locally exerted an additional strain on the Ge wetting layer already strained at the Ge/Si(001) interface, lifting the layer metastability and causing, in turn, the formation of three-dimensional Ge pyramids underneath every CoGe2 island. Solid-phase epitaxy of Co onto the same Ge/Si(001) epilayer resulted in the formation of more Co-rich germanide islands. Coupling of strain from these germanides to the epitaxial Ge/Si(001) strain has also facilitated a two-dimensional-to-three-dimensional transition of the Ge layer, however, with the germanide islands located at the Ge pyramid troughs, rather than crests. The difference in the relative location of germanide and germanium islands in these two cases is explained by accommodation of the large lattice-constant germanides at the more relaxed regions of the Ge pyramid crests and the smaller lattice-constant at the compressed Ge pyramid troughs.

Structural analysis of Co thin films grown on Ge(111) at room temperature by x‐ray photoelectron diffraction

“…2 -4 Compared with Co/Si, less is known about Co/Ge, partially because of its complexities and the amount of available data. 5 However, a consensus seems to have been reached that a reacted interfacial layer occurs below about three monolayers (ML) of Co films grown on Ge(111) using molecular beam epitaxy (MBE) at ambient temperatures, and then pure Co layers form on top of this reacted interfacial layer as the Co coverage increases. 6 -9 The Co/Ge(111) films below 3.8 ML deposited at room temperature were reported as exhibiting no magnetic properties, which was considered to result from the formation of non-magnetic Co-Ge compounds.…”

Formation and evolution of epitaxial Co5Ge7 on Ge(001) surface by reactive deposition inside an ultrahigh-vacuum transmission electron microscope

“…6 Recently, epitaxial growth of Co 5 Ge 7 and CoGe 2 islands on a strained Ge epilayer on Si͑001͒ was studied by scanning tunneling microscopy ͑STM͒ and reflection high-energy electron diffraction ͑RHEED͒. 7 In this letter, we report the formation and evolution of epitaxial Co 5 Ge 7 phase on a single crystal Ge͑001͒ surface by reactive deposition in an in situ ultrahigh vacuum transmission electron microscope ͑UHV TEM͒.…”

Morphological evolution of epitaxial cobalt–semiconductor compound layers during growth in a scanning tunneling microscope

“…Modifications to this surface due to exposure to Co atoms were monitored in situ, either by STM ͑in constant-current mode using electrochemically etched W tips, described elsewhere 5,7,[9][10][11] ͒, or by RHEED in ͗100͘ and ͗110͘ azimuths. Modifications to this surface due to exposure to Co atoms were monitored in situ, either by STM ͑in constant-current mode using electrochemically etched W tips, described elsewhere 5,7,[9][10][11] ͒, or by RHEED in ͗100͘ and ͗110͘ azimuths.…”

“…Yet, it does not grow as a monocrystalline two-dimensional layer on the Si͑001͒ surface, rather it forms misoriented threedimensional islands. 7,8 In this work we try to shed light on the factors that affect surface morphology of ultrathin heteroepitaxial cobaltsemiconductor compound layers in relation to the kinetics of phase formation in those layers. 6 CoGe 2 forms threedimensional islands on Ge/Si͑001͒ that are very similar to the CoSi 2 ones on Si͑001͒.…”