PhD Orals for Ammar Nayfeh (TODAY)

Ammar Munir Nayfeh anayfeh at stanford.edu
Fri Feb 17 09:55:18 PST 2006



 				       PhD ORALS

                          Department of Electrical Engineering

                  "Heteroepitaxial Growth of Relaxed Germanium on Silicon"

                                      Ammar Nayfeh

                              Advisor: Prof. Krishna Saraswat

                               Friday, February 17th 2006
                         3:45pm (Refreshments served at 3:30pm),
                                    CIS-X Auditorium



ABSTRACT:

Ge is now emerging as a viable candidate to augment Si for CMOS device and
optoelectronic applications, making it essential to develop new methods
for heteroepitaxial growth of Ge on Si. This is not straightforward due to
the large lattice mismatch (4.2%) between Ge and Si, which limits the
quality of the heteroepitaxial growth. First, above the critical
thickness, misfit dislocations form to relive the stress and subsequently
thread to the surface making it unsuitable for any device application.
Second, growth of Ge on Si results in island morphology which leads to
large surface roughness.

We have developed a novel technique to achieve high quality
heteroepitaxial Ge layers on Si. The technique involves CVD growth of Ge
on Si, followed by in-situ hydrogen annealing with subsequent growth and
anneal steps and hence the name Multiple Hydrogen Annealing for
Heteroepitaxy (MHAH). Following the first Ge growth and hydrogen
annealing, the Ge surface roughness is reduced by 90% to 2.5nm Rrms.  An
additional CVD Ge layer is grown followed by the final hydrogen annealing.
Extensive cross section and plan-view transmission electron microscopy
(TEM) imaging was carried out showing that defects are concentrated near
the Ge/Si interface. Thus, using the MHAH method we achieved
heteroepitaxial-Ge on Si layers with dislocation density as low as
1x10^7cm^-2.  An experimentally based theoretical model was developed to
explain both the surface roughness reduction and the reduction in
dislocation density.  As a result, a complete model was formed for the
MHAH-Ge growth method.

The efficacy of this method is demonstrated by fabricating MOS devices and
optical detectors in Ge grown by MHAH.  MOS capacitors were fabricated on
MHAH Ge layers with negligible hystersis and low Dit, comparable to bulk
Ge, using GeOxNy gate dielectrics.   Dit was extracted to be asymmetric
indicating a possible reason why Ge NMOS mobility is severely degraded.
We have also successfully fabricated high-performance Ge p-MOSFETs.  The
Ge based devices exhibit a peak hole mobility of 250 cm^2/Vs which is 2X
larger than Si based PMOS universal mobility.  Furthermore, we have
demonstrated high-k/metal-gate compatibility using ALD grown Al2O3/Al gate
stack.  In addition, we have demonstrated the quality of the MHAH-Ge layer
by fabricating extremely efficient photo-detectors with minimal dark
current.

We conclude that MHAH-Ge can be used in achieving heterogeneous
integration of a high mobility pure Ge channel transistors and optical
devices directly on Si for future technology nodes.  In addition, it could
be used to fabricate GOI substrates, or for the eventual integration of
GaAs/Ge/Si for optoelectronics.











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