MBE Growth of ErAs/In(Ga)As Epitaxial Ultra-Low Resistance Ohmic Contacts

1. 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 *sbank@ece.ucsb.edu

2. Outline • Motivation • Previous Work • Approach • Results • Conclusion

3. Heterojunction Bipolar Transistor Scaling* • 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 • Doubles capacitances! (Ci=eA/Ti) • Want C’s reduced 2:1 *M.J.W. Rodwell, IEEE Trans. Electron. Dev., 2001

4. HBT Scaling Laws* (Cont.) • Doubles capacitances! (Ci=eA/Ti) • Reduce Area 4:1 (lateralscaling) • Require R’s stay constant • Rex = SRi,emitter= (……) + Rcontact, emitter • Rcontact, emitter = rc/A  rc must reduce 4:1 • Similar for other contacts *M.J.W. Rodwell, IEEE Trans. Electron. Dev., 2001

5. 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

6. Approximate Schottky barrier potential 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 • Emitter as test case 1E. Lind et al., Dev. Res. Conf., 2006, Late News 2C.J. Palmstrøm et al., J. Appl. Phys., 1988. TEM: D. O. Klenov, Appl. Phys. Lett., 2005

7. Previous Work & Extrapolations • Tunable SBH with III-V composition1 • Expect “perfect” n-Ohmic for Ga < 20% 1J.D. Zimmerman et al., J. Vac. Sci. Technol. B, 2005

8. Molecular Beam Epitaxy Growth Details Al 1500 Å • Layer structure: • Transfer length method (TLM) • 1-D approximation • Lt/L = 3 sufficient • MBE Growth • In(Ga)As:Si ~ 0.5 mm/hr • 450oC • 3.5x1019 cm-3 active • ErAs ~ 0.2 ML/s • 450oC • Cap with Al* to protect ErAs • Remove AlOx with Ar+ clean ErAs 75 Å n+ InAs (3.5x1019 cm-3) 100 Å n+ InGaAs (3.5x1019 cm-3) 950 Å UID-InAlAs 1000 Å SI-InP Lt L Lt/L >> 1 *A.Y. Cho and P. Dernier, J. Appl. Phys., 1978

9. 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

10. Films – “Perfect” Ohmic Contacts • Measuring extremely low contact resistances • Sufficient for THz HBTs1 (litmus test) • Studying layer structure effects • Remaining errors should overestimate rc 0.7 ± 0.2 W-mm2 1M.J.W. Rodwell, IEEE Trans. Electron. Dev., 2001

11. 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 • Study band alignment • Superior metallization (Mo) • Apply to other devices: • e.g. HEMTs • Acknowledge: • Office of Naval Research (ONR) 0.7 ± 0.2 W-mm2

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