Eun Ji Kim's Ph.D. Oral Exam

Eun Ji Kim eunjik at stanford.edu
Mon Mar 2 02:20:08 PST 2009


Interface and Defect Study of High Permittivity Dielectrics on Si and III-V
semiconductors

Eun Ji Kim
Advisors: Prof. Paul C. McIntyre in Materials Science and Engineering
Prof. Krishna C. Saraswat in Electrical Engineering

 Date: Thursday, Mar. 19, 2009
Time: 2:00 PM (Refreshments served at 1:45 PM)
Location: Packard Bldg. Rm. 202



In an effort to decrease electronic device dimensions and improve device
performance, high permittivity dielectrics have been introduced to
metal-oxide-semiconductor field effect transistors (MOSFETs). Even though
replacing SiO2 with high permittivity dielectrics enabled aggressive device
scaling, however, the introduction of new materials gave rise to fundamental
problems that could lead to device performance degradation, such as
reduction of the effective carrier channel mobility and threshold voltage
instabilities. Unsatisfied dangling bonds at the interface of the high
permittivity dielectrics and Si, intrinsic and extrinsic point defects in
high permittivity dielectrics, and remote phonon scattering are believed to
cause degradation of device performance. To understand the limitations to
the performance of MOSFETs with high permittivity dielectrics, it is
critical to probe phonon modes and defect states directly in high-k
dielectrics.

Inelastic electron tunneling spectroscopy (IETS) is employed to study soft
phonon modes and defect states in HfO2 grown by atomic layer deposition
(ALD) on Si. Observed spectral features suggest that monoclinic- and
tetragonal- HfO2 vibrational modes exist in the annealed HfO2 while
crystalline HfO2 vibrational modes are not detected in the as-deposited
samples, consistent with selective area electron diffraction analysis. In
addition to soft phonon modes of HfO2, changes in amplitude and energy of
spectral features were observed as the bias condition changes. We attribute
these features to defect-related states in HfO2 and analyze them in terms of
electron energy states in the HfO2 bandgap and reported oxygen vacancy
states in HfO2.

For further device scaling, III-V compound semiconductors are receiving
increasing attention for channel replacement in the
metal-oxide-semiconductor (MOS) technology beyond 22 nm node because of
their high intrinsic electron mobility. Unlike SiO2 that exhibits excellent
passivating properties on Si with low interface state densities, there
typically exists a large density of defect states at the interface of III-V
semiconductors and their native oxides. Previous research on GaAs showed
that less than 1 % of a monolayer of chemisorbed O2 can pin the Fermi level
at the semiconductor surface. Therefore, suppressing oxidation of the III-V
semiconductors’ surface prior to and during gate dielectric deposition could
be essential to achieving device performance superior to that of silicon in
nanoscale devices. Several different approaches have been demonstrated to
prepare III-V semiconductor-based MOS devices. However, previously attempted
methods resulted in frequency-dependent flat band voltage (Vfb) shift,
charge trapping in the dielectrics and a relatively high density of
interface trap states, possibly from unintentional oxidation of the III-V
channel.

We used In0.53Ga0.47As (100) channels that were capped with an arsenic layer
after channel epitaxial growth to avoid III-V oxidation during exposure of
the samples to air. The As capping layer was thermally desorbed *in-situ *in
a load-locked ALD reactor prior to Al2O3 gate dielectric deposition. By
preventing subcutaneous oxidation of the channel surface, we obtained
unpinned Al2O3/In0.53Ga0.47As interfaces with a low density of interface
states. The C-V characteristics show a hysteresis of less than 40 mV and
relatively small frequency dispersion in accumulation. The surface potential
swing (0.44~1.2 eV) calculated using the Berglund integral suggests the
absence of a high density of midgap interface states. The observation of
near-ideal flat band voltage values for Pt- and Al-electroded MOS capacitors
indicates the absence of a significant interface dipole. The
temperature-independence of the frequency dispersion of the accumulation
capacitance and its scaling with measurement frequency are consistent with
tunneling of carriers into defects in the Al2O3 layer, border traps.  This
also indicates a low interface state density for these devices.
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