Ashish Baraskar 1, Mark A. Wistey 3, Evan Lobisser 1, Vibhor Jain 1, Uttam Singisetti 1, Greg Burek 1, Yong Ju Lee 4, Brian Thibeault 1, Arthur Gossard 1,2 and Mark Rodwell 1 1 ECE and 2 Materials Departments, University of California, Santa Barbara, CA 3 Electrical Engineering, University of Notre Dame, In 4 Intel Corporation, Technology Manufacturing Group, Santa Clara, CA Our Approach: Surface preparation by atomic hydrogen cleaning Increased doping Ex-situ Ohmic Contacts to n-InGaAs Prepared by Atomic Hydrogen Cleaning Issue: InP HBTs need contact resistivity (ρ c ) 1 THz MJW Rodwell, Int. J. High Speed Electronics and Systems. 11,159 (2007); MJW Rodwell, CSIC (2008) ~10 −8 Ω-cm 2 ρ c with in-situ contacts, but process flows often demand ex-situ contacts Unpredictable native oxides; High temperature processing *M.J.W. Rodwell, IEEE Trans. Electron. Dev., (2001); U. Singisetti, DRC (2007) Fundamental Scaling Laws: To double device bandwidth: Cut transit time 2x: – Reduce thickness 2:1 ☺ – Capacitance increases 2:1 ☹ Cut RC delay 2x – Scale all resistivities by 4:1 Ashish Baraskar: University of California Santa Barbara PCSI-37

H cleaned (MBE) contacts are comparable to in-situ contacts InP (semi-insulating) 100 nm In 0.52 Al 0.48 As (NID) 100 nm In 0.53 Ga 0.47 As (n-doped) 20 nm Mo Experimental Details: Highly doped n-In 0.53 Ga 0.47 As by solid source MBE Surface exposed to air Atomic hydrogen cleaning in MBE Surface observed under RHEED before metal deposition Transmission Line Model (TLM) for contact resistance measurement ProcessSurface Preparationρ c (Ω-µm 2 )ρ h (Ω-µm) In-situAs grown1.0±0.64.3±2.6 Ex-situ (air exposure) H clean (MBE)1.3 ± ± 3.0 Dil. HCl2.3 ± ± 3.4 Experimental Results with Mo Contact Metal Electron concentration, n = 5x10 19 cm -3 Mobility, µ = 770 cm 2 /Vs Sheet Resistance, R sh = 15 ohm/□ (100 nm thick film) Ex-situ Ohmic Contacts to n-InGaAs Prepared by Atomic Hydrogen Cleaning Contact metal: Molybdenum (Mo) Ashish Baraskar: University of California Santa Barbara PCSI-37

Thermal Stability: Contacts annealed under N 2 flow at 300 and 400 o C for 1 minute duration Low resistivity maintained after anneal: (1.3 ± 0.5) × Ω-cm 2 Ex-situ Ohmic Contacts to n-InGaAs Prepared by Atomic Hydrogen Cleaning Acknowledgements ONR, DARPA-TFAST, DARPA-FLARE Poster No: We1645 Contacts suitable for THz transistors Conclusions: Atomic hydrogen effectively cleans air-exposed semiconductor surfaces ρ c ~ (1.3 ± 0.5) Ω-µm 2 for atomic hydrogen cleaned Mo-nInGaAs contacts Contact resistivity comparable to in-situ Mo-nInGaAs contacts Mo contacts are thermally stable on InGaAs up to 400 o C Ashish Baraskar: University of California Santa Barbara PCSI-37