Imec has shown that the
presence of hydrogen and/or inert species during Ge deposition significantly
improves the quality of the Ge layers grown on Si by solid phase epitaxy (SPE).
The resulting layers have excellent crystalline quality and low surface roughness,
making SPE a valuable alternative for conventional heteroepitaxy which is performed
typically at much higher temperatures. High-quality Ge layers on Si are needed
to explore the potential of Ge MOS devices for high-performance applications,
or for extending conventional Si electronics.
Imec has demonstrated that the presence of atomic hydrogen during Ge deposition
at low temperatures favors the formation of smooth and high-quality Ge layers
on Si by SPE. A similar observation is made when molecular hydrogen, molecular
nitrogen or chemical inert atoms or molecules are added during deposition. This
results in high-quality single-crystalline Ge layers with surface roughness
of only 0.4nm root mean square. In absence of these species, Ge layers grown
by SPE exhibit low crystalline quality. The availability of high-quality thin
Ge layers on Si is indispensible for the research on Ge and Ge/III-V devices.
Ge on Si can potentially replace Si CMOS for high-performance applications and
extends conventional Si electronics for e.g. optoelectronic applications.
(Color online) Comparison of Raman measurements of Ge layers deposited in vacuum (red, solid line) and deposited under N2 flux (blue, solid line with squares). For the vacuum deposited Ge layer, Ge–Ge stretch is observed, indicating the presence of structural ordering in the film. For the layer deposited under N2 flux no Ge–Ge stretch is visible, indicating complete disordering.
In case of SPE, an amorphous layer is deposited on a crystalline substrate
using methods such as (plasma enhanced) chemical vapor deposition ((PE)CVD)
or ultrahigh vacuum (UHV) deposition. Subsequent annealing of the structure
initiates crystallization at the interface, which continues towards the surface.
In this way, an epitaxial layer can be formed on the substrate. SPE allows straightforward
deposition of Ge on Si. Conventional heteroepitaxial growth on the contrary
requires additional steps in order to reduce surface roughness.
Typically, PECVD using germane (GeH4) molecules is used to deposit the initial
amorphous Ge layer. In this case, atomic hydrogen is inherently present and
can influence the crystallization process in many ways. Imec's research
shows that atomic hydrogen plays an important role during Ge deposition as it
lowers the surface mobility of adsorbed Ge atoms and consequently increases
the disorder of the deposited layer. Such a disordered layer is highly beneficial
for SPE where crystallization has to start at the interface before it starts
in the bulk. Atomic hydrogen is also incorporated into the growing layer, but
it does not affect the crystallization process. A similar explanation can be
given when fluxes of H2, N2 or chemical inert species are added during deposition
by UHV. They also reduce the surface mobility and thereby the structural ordering
of the Ge layers. In contrast to atomic hydrogen, these atoms are not incorporated
into the growing film. The Ge deposition is performed at low temperatures (typically
150°C), subsequent crystallization is done by thermal annealing at 600°C
in an N2 atmosphere for one minute. Annealing temperatures as low as 400°C
can be applied. The low temperatures present an important advantage with respect
to conventional heteroepitaxy, which is typically performed at much higher temperatures.
Detailed results of this study have been published by R.R. Lieten et al in
Applied Physics Letters 94, 2009, 'Solid phase epitaxy of amorphous Ge
on Si in N2 atmosphere' and in Applied Physics Letters 96, 2010, 'Hydrogen
and inert species in solid phase epitaxy'.