Transcript MBE Growth of ErAs/In(Ga)As Epitaxial Ultra-Low Resistance Ohmic Contacts Seth Bank*, U.

MBE Growth of ErAs/In(Ga)As Epitaxial
Ultra-Low Resistance Ohmic Contacts
Seth Bank*, U. Singisetti, A.M. Crook, J.D. Zimmerman, J.M.O. Zide,
M.J.W. Rodwell, and A.C. Gossard
ECE and Materials Departments
University of California, Santa Barbara, CA
2006 North American Molecular Beam Epitaxy Conference
Durham, North Carolina
*[email protected]
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Outline
• Motivation
• Previous Work
• Approach
• Results
• Conclusion
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Heterojunction Bipolar Transistor Scaling*
1
kT
kT
 t base  t collector  C je
 Cbc
 RexCbc  RcollCbc
2ft
qIE
qIE
• Mission:
– 2:1 increase in ft with R’s, G’s, I’s, and V’s constant.
• Decrease t’s 2:1 by vertical scaling
t base  Tb2 2Dn
t collector  Tc 2veff
• Doubles capacitances! (Ci=eA/Ti)
– Want C’s reduced 2:1
*M.J.W. Rodwell, IEEE Trans. Electron. Dev., 2001
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HBT Scaling Laws* (Cont.)
1
kT
kT
 t base  t collector  C je
 Cbc
 RexCbc  RcollCbc
2ft
qIE
qIE
• Doubles capacitances! (Ci=eA/Ti)
– Reduce Area 4:1 (lateral scaling)
• Require R’s stay constant
– Rex =
SRi,emitter= (……) + Rcontact, emitter
Rc ,eC 
r c eA
A T

er c
T
– Rcontact, emitter = rc/A  rc must reduce 4:1
• Similar for other contacts
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*M.J.W. Rodwell, IEEE Trans. Electron. Dev., 2001
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Current Approaches to Reduce rc
• Historically employ empiricism:
– Careful oxide removal & passivation
– Reactive metallization/annealing
• Advances required for future scaling
– Progress is uncertain!
Au
Pt
Reacted region
InGaAs
Pt/Au Contact after 4hr 260C Anneal
TEM : Lysczek, Robinson, & Mohney, Penn State
Sample: Urteaga, RSC
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Current State-of-the-Art
• Current reports:
– Emitter contact limits devices
• ~5 W-mm2 in state-of-art HBTs1
– Emitter, base, and collector
– ~10% of total delay due to emitter
• Serious impediment to THz HBTs
– 1 W-mm2 required
• Solution: Epitaxial metals?
– Rare-earth pnicitides2
•
•
•
•
Thermodynamically stability
Continuous As-sublattice
No interfacial contaminants
Compatible with III-V MBE
Approximate Schottky barrier potential
– Emitter as test case
1E.
Lind et al., Dev. Res. Conf., 2006, Late News
Palmstrøm et al., J. Appl. Phys., 1988.
TEM: D. O. Klenov, Appl. Phys. Lett., 2005
2C.J.
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Previous Work & Extrapolations
Schottky Barrier (eV)
0.6
0.4
0.2
Strained
0.0
Zimmerman,
J. Vac. Sci. Technol. B, 2005
Vurgaftman,
J. Appl. Phys., 2001
Chuang, Physics of
Optoelectronic Devices, 1995
Unstrained
-0.2
0.0
0.1
0.2
0.3
0.4
0.2 0.4 0.6 0.8 1.0
Gallium Content
(InGaAs)
InAlAs Content
(InAlGaAs)
• Tunable SBH with III-V composition1
– Expect “perfect” n-Ohmic for Ga < 20%
1J.D.
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Zimmerman et al., J. Vac. Sci. Technol. B, 2005
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Molecular Beam Epitaxy Growth Details
• Layer structure:
Al 1500 Å
– Transfer length method (TLM)
• 1-D approximation
– Lt/L = 3 sufficient
ErAs 75 Å
n+ InAs (3.5x1019 cm-3) 100 Å
n+ InGaAs (3.5x1019 cm-3) 950 Å
UID-InAlAs 1000 Å
• MBE Growth
– In(Ga)As:Si ~ 0.5 mm/hr
SI-InP
• 450oC
• 3.5x1019 cm-3 active
– ErAs ~ 0.2 ML/s
• 450oC
Lt
– Cap with Al* to protect ErAs
• Remove AlOx with
Ar+
L
clean
Lt/L >> 1
*A.Y. Cho and P. Dernier, J. Appl. Phys., 1978
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A Few TLM Sources of Error
• 1-D approximation
– Vertical
• Large Lt/L
– Horizontal
• Wide mesa
• Mesa isolation
• Errors: (±0.2 W-mm2)
– Pad spacing (SEM)
– Resistance
• Proper probing
– 4 probes, close to gap
• Long contacts (many Lt)
– Minimize end resistance
• Metal/metal interface (AlOx)
– Serious issue
• Thick aluminum cap
• Ar+ sputter
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Films – “Perfect” Ohmic Contacts
-0.3
Resistance (W)
Energy (eV)
0.0
20
EF
ErAs
In0.53Ga0.47As
InAs
-0.6
EC
-0.9
15
10
5
0.7 ± 0.2 W-mm2
EV
-1.2
0
0
10
Position (nm)
0
20
5
10
15
20
Spacing (mm)
25
30
• Measuring extremely low contact resistances
– Sufficient for THz HBTs1 (litmus test)
• Studying layer structure effects
– Remaining errors should overestimate rc
1M.J.W.
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Rodwell, IEEE Trans. Electron. Dev., 2001
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Conclusions and Future Directions
• Epitaxial Ohmic contacts
– ~1 W-mm2 measured
• At the limits of TLM technique
– Litmus test:
• Demonstrate HBT w/ErAs
• Other future work:
– Generalize to collector and base
• p-type contact: Er-V/Ga(As)Sb
• Epitaxial regrowth
Resistance (W)
20
15
10
5
0.7 ± 0.2 W-mm2
0
0
5
10
15
20
Spacing (mm)
25
30
– Study band alignment
– Superior metallization (Mo)
– Apply to other devices:
• e.g. HEMTs
• Acknowledge:
– Office of Naval Research (ONR)
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The End
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