Molecular Beam Epitaxy of Low Resistance Polycrystalline P-Type GaSb

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Molecular Beam Epitaxy of Low Resistance
Polycrystalline P-Type GaSb
Y. Dong, D. Scott, Y. Wei, A.C. Gossard and M.
Rodwell. Department of Electrical and Computer
Engineering, University of California, Santa
Barbara
yingda_at_ece.ucsb.edu 1-805-893-3812
15th IPRM 2003 Santa Barbara, CA
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Outline

• Motivations
• Polycrystalline material for InP HBTs extrinsic
  base
• Why choose GaSb
• MBE growth of Poly-GaSb
• Electrical Properties of Poly-GaSb
• Conclusions

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InP Vs SiGe HBTs
Advantages of InP HBTs over SiGe HBTs201 lower
base sheet resistance, 51 higher base
electron diffusivity 31 higher collector
electron velocity, 41 higher breakdown-at
same ft.
However, InP HBTs have not provided decisive
advantages over SiGe HBTs in mixed-signal ICs.
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Strong Features of Si/SiGe HBT Process

• Highly scaled
• Very narrow active junction areas
• Very low device parasitics
• High speed
• Low emitter resistance using wide n polysilicon
  contact
• Low base resistance using large extrinsic
  polysilicon contact
• High-yield, planar processing
• High levels of integration
• LSI and VLSI capabilities

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Polycrystalline Base Contact
SiGe HBT process extensive use of poly-Si for
base contact

• The Advantages of Polycrystalline Base Contact
• Reduce the B-C capacitance by allowing
  metal-to-base contact over the field oxide
• Reduce the base resistance by highly doping the
  polycrystalline extrinsic base

High Maximum Oscillation Frequency (Fmax), ECL
logic speed
Low CBC, RBB
Can a similar technology be developed for InP
HBTs ?
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Polycrystalline Base Contact in InP HBTs
2) Collector pedestal etch, isolation, SiO2
planarization
1) Epitaxial growth
SiO2
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Polycrystalline Base Contact in InP HBTs
4) Deposit base metal, encapsulate with SiN,
pattern base and form SiN Sidewalls
3) Base Regrowth
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Polycrystalline Base Contact in InP HBTs
5) Regrow InAlAS/InGaAs emitter
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Properties of Polycrystalline Material

• Small crystallites join together at grain
  boundaries
• Inside each crystallite single crystal
• At grain boundaries a large number of traps
  ? Fermi level pinned

Polycrystalline InAs
Polycrystalline GaSb
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Material Choices for Polycrystalline Base

• Polycrysalline material choices
• GaAs
• Wide bandgap ? low hole mobility
• Fermi level pinned in mid-bandgap
• ? large band-bending barrier
• GaSb
• Narrow bandgap ? high hole mobiliy
• Fermi-level pinned on valence band
• InSb
• Narrow bandgap
• low melting point (520 ?C)
• ? Can not withstand emitter regrowth

Schematic diagram of suggested energy band
structure near grain boundary in p-type of GaAs
and GaSb
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MBE Growth of Polycrystalline GaSb
GaAs
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Influence of V/III Beam Flux Ratio

• Hole mobility changes little with V/III ratio
• Hole concentration increases with decreasing
  V/III ratio
• (Reason Carbon must displace antimony to be
  effective p-type dopant)

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Influence of Growth Temperature

• Hole concentration changes little with growth
  temperature
• Hole mobility decreases with growth temperature

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Grain Sizes Temperature Dependence
SEM pictures of poly-GaSb samples
Polycrystalline GaSb Grown at 520 ?C Gain size
350nm
Polycrystalline GaSb Grown at 475 ?C Grain size
100nm
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Poly-GaSbs Grain Size and Resistivity

• Grain size increases steadily with growth
  temperature
• Resistivity increases rapidly when grain size
  exceeds the film thickness

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Small Grain Vs. Large Grain

• Small grain
• More grain boundaries for carriers to cross
• Larger total boundary areas connecting
  crystallites

• Large grain
• Fewer grain boundaries for carriers to cross
• Smaller total boundary areas connecting
  crystallites

Small band bending barrier ? Total connecting
boundary area more important
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Grain Size Vs Film Thickness
SiO2
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Grain Size Vs Film Thickness
SiO2
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Grain Size Vs Film Thickness
SiO2
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Grain Size Vs Film Thickness
When the film thickness approaches the grain
size, the total connecting boundary area will be
significantly reduced
Rapid resistivity increase
SiO2
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Thickness Dependence
Bulk resistivity has strong dependence on film
thickness
Poly GaSb Thickness (?) Hole ConcentrationNs (cm-3) Mobility ? (cm2/Vs) Bulk Resistivity ? (?cm)) Sheet resistivity ?S (?/)
3000 8.2e19 10.2 7.5e-3 240
2000 8.0e19 8.6 9.1e-3 450
1500 8.1e19 5.8 1.3e-2 900
1000 7.8e19 5.1 1.6e-2 1550
Sheet resistivity increases very fast with
decreasing thickness
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Comparison Between Poly-GaSb and Poly-GaAs
Poly-GaSb by MBE (This work) Poly-GaAs by GSMBE (N.Y. Li et al, 1998)
Carbon doping density (cm-3) 8x1019 8x1019
Grain Size (Å) 700 4002000
Film Thickness (Å) 3000 4000
Bulk Resistivity (?-cm) 7.5x10-3 1x10-1
With similar carbon doping level, grain size and
film thickness, the resistivity of poly-GaSbs
resistivity is more than one order of magnitude
lower than that of poly-GaAs.
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Conclusions

• Poly-GaSb proposed to be used as extrinsic base
  material for InP HBTs
• Low resistance poly-GaSb films can be achieved by
  MBE growth using CBr4 doping
• The resistivity of poly-GaSb has strong
  dependence on films thickness and grain size,
  particularly when the film thickness is
  comparable with the grain size.

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Acknowledgement
This work was supported by the DARPATFAST program