North American Molecular Beam Epitaxy Conference NAMBE 8

North American Molecular Beam Epitaxy Conference (NAMBE), 8 -11 -2009 Improved Regrowth of Self-Aligned Ohmic Contacts for III-V FETs Mark A. Wistey Now at University of Notre Dame mwistey@nd. edu A. K. Baraskar, U. Singisetti, G. J. Burek, M. J. W. Rodwell, A. C. Gossard University of California Santa Barbara P. Mc. Intyre, B. Shin, E. Kim Stanford University Funding: SRC mwistey@nd. edu

Outline: Regrown III-V FET Contacts • Motivation for Self-Aligned Regrowth • Facets, Gaps, Arsenic Flux and MEE • MOSFET Results • Conclusion Wistey, NAMBE 2009 2

Motivation for Regrowth: Scalable III-V FETs Classic III-V FET (details vary): { Source Large Area Contacts Gap Drain Gate Large Rc { Top Barrier or Oxide Low doping Channel Bottom Barrier In. Al. As Barrier Implant: straggle, short channel effects III-V FET with Self-Aligned Regrowth: High Velocity Channel Small Raccess Small Rc High mobility access regions High doping: 1013 cm-2 avoids source exhaustion Wistey, NAMBE 2009 • Disadvantages of III-V’s High barrier Self-aligned, no gaps Gate High-k n+ Regrowth • Advantages of III-V’s Channel In(Ga)P Etch Stop Bottom Barrier 2 D injection avoids source starvation 5 nm } Ultrathin doping layer Dopants active as-grown 3

MBE Regrowth: Bad at any Temperature? 200 nm Gap • Low growth temperature (<400°C): –Smooth in far field –Gap near gate (“shadowing”) –No contact to channel (bad) Gate Source-Drain Regrowth Si. O 2 Metals high-k Channel • High growth temperature (>490°C): – Selective/preferential epi on In. Ga. As – No gaps near gate – Rough far field – High resistance Wistey, NAMBE 2009 Gate Source-Drain Regrowth: 50 nm In. Ga. As: Si, 5 nm In. As: Si. Si=8 E 19/cm 3, 20 nm Mo, V/III=35, 0. 5 µm/hr. SEMs: Uttam Singisetti 4

High Temperature MEE: Smooth & No Gaps 460 C 490 C Gap 540 C 560 C Si. O 2 dummy gate In=9. 7 E-8, Ga=5. 1 E-8 Torr Wistey, NAMBE 2009 Smooth regrowt h No gaps, but faceting next to gates Note faceting: surface kinetics, not 5

Si. O 2 Shadowing and Facet Competition Fast surface diffusion = slow facet growth Slow diffusion = rapid facet growth Shen & Nishinaga, JCG 1995 [11 1] [10 ter fas ter Si. O 2 [1 11 ] [100] 0] fas Slow diffusion = fast growth Fast surface diffusion = slow facet growth [1 11 ] [100] • Shen JCG 1995 says: Increased As favors [111] growth Good fill next to gate. Wistey NAMBE 2009 • But gap persists 6

Gate Changes Local Kinetics 1. Excess In & Ga don’t stick to Si. O 2 Gate sidewall Si. O 2 or Si. Nx 2. Local enrichment of III/V ratio 4. Low-angle planes grow instead [100] 3. Increased surface mobility • Diffusion of Group III’s away from gate Wistey NAMBE 2009 7

Change of Faceting by Arsenic Flux • In. Ga. As layers with increasing As fluxes, separated by In. Al. As. Si. O 2 In. Al. As In. Ga. As markers Cr W Original Interface Increasing As flux 5 x 10 -6 2 x 10 -6 1 x 10 -6 0. 5 x 10 -6 (Torr) • Lowest arsenic flux → “rising tide fill” • No gaps near gate or Si. O 2/Si. Nx • Tunable facet competition Wistey, NAMBE 2009 Growth conditions: MEE, 540*C, Ga+In BEP=1. 5 x 10 -7 Torr, In. Al. As 500 -540°C MBE. 8

Control of Facets by Arsenic Flux • In. Ga. As: Si layers with increasing As fluxes, separated by In. Al. As. Faceting 11 [1 [100] [1 1 1] W Original Interface Conformal Si. O 2 Cr 5 x 10 -6 2 x 10 -6 1 x 10 -6 0. 5 x 10 -6 (Torr) ] Increasing In. Ga. As As flux Si. O 2 [100] Si. O 2 In. Al. As markers Wistey, NAMBE 2009 ] [100] [1 11 Growth conditions: MEE, 540*C, Ga+In BEP=1. 5 x 10 7 Torr, In. Al. As 500 -540°C MBE. Si. O 2 • Lowest arsenic flux → “rising tide fill” • No gaps near gate or Si. O 2/Si. Nx • Tunable facet competition Filling 9

Low-As Regrowth of In. Ga. As and In. As In. Ga. As In. As regrowth In. Ga. As regrowth (top view) • No faceting near gate • Smooth far-field too • Low As flux good for In. As too. • In. As native defects are donors. Bhargava et al , APL 1997 • Reduces surface depletion. 4. 7 nm Al 203, 5× 1012 cm-2 pulse doping In=9. 7 E-8, Ga=5. 1 E-8 Torr Wistey, NAMBE 2009 SEMs: Uttam Singisetti 10

In. As Source-Drain Access Resistance 4. 7 nm Al 203, In. As S/D E-FET. 740 Ω-µm • Upper limit: Rs, max = Rd, max = 370 Ω−μm. • Intrinsic gmi = 0. 53 m. S/μm • gm << 1/Rs ~ 3. 3 m. S/μm (source-limited case) ➡ Ohmic contacts no longer limit MOSFET performance. Wistey, NAMBE 2009 11

Conclusions • Reducing As flux improves filling near gate • Self-aligned regrowth: a roadmap for scalable III-V FETs –Provides III-V’s with a salicide equivalent • In. Ga. As and relaxed In. As regrown contacts –Not limited by source resistance @ 1 m. A/µm –Results comparable to other III-V FETs. . . but now scalable Wistey, NAMBE 2009 12

Acknowledgements • Rodwell & Gossard Groups (UCSB): Uttam Singisetti, Greg Burek, Ashish Baraskar, Vibhor Jain. . . • Mc. Intyre Group (Stanford): Eunji Kim, Byungha Shin, Paul Mc. Intyre • Stemmer Group (UCSB): Joël Cagnon, Susanne Stemmer • Palmstrøm Group (UCSB): Erdem Arkun, Chris Palmstrøm • SRC/GRC funding • UCSB Nanofab: Brian Thibeault, NSF Wistey, NAMBE 2009 13