Electrical properties of HfO2 Al2O3InAs MOS capacitors

1
Electrical properties of HfO2/ Al2O3/InAs MOS
capacitors

• Dane Wheeler and A. Seabaugh
• University of Notre Dame
• L.-E. Wernersson
• Lund University

2
Motivation

• InAs has high electron mobility (33,000 cm2/V-s),
  allowing for high MOSFET drive currents at low
  operating voltages
• HfO2 and Al2O3 have high dielectric constants,
  allowing for low equivalent oxide thicknesses
  (EOTs) with low leakage currents
• Atomic layer deposition has been shown to reduce
  arsenic oxides at oxide-semiconductor interface,
  eliminating a source of Fermi level pinning

3
Matrix of 18 samples created to study various
film properties
Growth Temperature (C) 80 250 300 350
HfO2 Thickness (nm) 3.4 (30 cycles) 4 (40) 4.8
(50)
Post-metallization Treatment no anneal 200 C, 1
hr, N2 (probe station)
InAs Pre-treatment HCl BHF
Contact Metal PdAu TiAu
Interlayer Al2O3, 10 cycles, 300 C HfO2, 10
cycles, 300 C
4
TEM used to confirm oxide thickness

• TEM cross-section agrees with ellipsometry
  thickness measurements
• HfO2-InAs interlayer appears to exist

5
Growth rate determined by XPS and ellipsometry

• Data from XPS and ellipsometry used to calculate
  growth rate of 0.7 Å/cycle

6
XPS shows As-O removed byALD process
As-In
As-Hf-O
InAs with HfO2
As-O
InAs with no HfO2
XPS measurements show that native arsenic oxides
are removed by the HfO2 ALD process
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Direct tunneling is the dominate leakage mechanism

• Good agreement obtained between I-V data for
  different thickness films and direct tunneling
  model

8
Capacitance reduced with increased thickness

• As expected, capacitance is reduced with
  increased film thickness
• Films created with 30, 40, and 50 ALD cycles

9
Dielectric constant determined by plotting EOT
vs. physical thickness

• Plotting the accumulation regime capacitance
  (measured at 1 MHz with VG 1 V) versus
  physical film thickness, a relative dielectric
  constant of 28 is obtained
• Data also suggests an interlayer with an EOT of
  4.5 Å

10
Hysteresis is reduced with PMA

• Post-metallization anneal (PMA) reduces the
  bidirectional voltage sweep hysteresis
• PMA also decreases depletion capacitance

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Hysteresis is comparable to other III-V MOS

• Measured C-V hysteresis is comparable and often
  times lower than that of HfO2 on other III-Vs
• InGaAs/GaAs data from Goel et al., APL 89, 2006

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Al2O3 grown at InAs interface

• Al2O3 grown at InAs interface, followed by HfO2
  growth
• TMA known to remove native oxides
• Al2O3 provides large barrier to leakage
• HfO2 used as bulk oxide to increase e

Flat-band alignment with conduction and valence
band offsets to InAs
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Al2O3 reduces frequency dispersion and hysteresis
No interlayer
Al2O3 interlayer

• Frequency dispersion reduced from 7 to 2
• Hysteresis also reduced (insets)

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Al2O3 reduces leakage current

• Inclusion of thin (10 ALD cycles) Al2O3 layer
  reduces the leakage current two orders of
  magnitude while only adding 0.1 Å to the measured
  EOT

15
HfO2 growth temperature has significant influence
on leakage

• HfO2 films grown at temperatures ranging from 80
  to 350 C
• No. of growth cycles was held constant at 50, yet
  current differs by several orders of magnitude
• Growth temperature likely has significant effect
  on growth rate, thus influencing leakage current

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HfO2 growth temperature affects EOT

• Qualitative differences observed in C-V
  characteristics of different growth temperature
  films (left)
• Higher temperatures could yield denser films,
  resulting in the observed decrease in EOT (right)

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HfO2 growth temperature affects frequency
dispersion and hysteresis

• Sharp increase in bidirectional C-V sweep
  hysteresis at 350 C, indicating a greater
  presence of charge in the oxide

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Film strength determined by breakdown voltage and
equiv. breakdown field

• Absolute breakdown strength (left) is low
  compared to that of Si/SiO2 (10 MV/cm)
• However, equivalent breakdown (right) is quite
  high

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HfO2/Al2O3/InAs compares favorably with other
material systems

• Ultimately, the lowest EOT film with the lowest
  leakage is desired for MOSFET applications
• HfO2/Al2O3/InAs is promising in terms of this
  metric
• Work must be done to reduce bulk and interface
  charges

Si3N4/SiO2 from Yeo et al., EDL 21, 2000 ITRS
from 2007 edition high-performance, metal gate,
planar technology requirements
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Conclusions

• Low leakage hafnium and aluminum oxide capacitors
  created on InAs using ALD
• Hf precursor appears to remove native oxides
• Leakage is dominated by direct tunneling
• PMA reduces hysteresis
• Al2O3 interlayer shown to reduce leakage,
  hysteresis, and frequency dispersion while
  maintaining low EOT
• HfO2 growth temperature has great impact on
  device properties
• HfO2/Al2O3/InAs can meet ITRS leakage and EOT
  requirements for planar technology (through the
  36-nm node) and beyond

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Acknowledgements

• Collaborators
• Linus Fröberg, Anders Mikkelsen, Claes Thelander,
  and Kees-Jan Weststrate of Lund University
• John Suehle, NIST
• Work supported by NIST/SRC graduate fellowship