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Characterization of Contact Resistivity on InAs/GaSb Interface

Characterization of Contact Resistivity on InAs/GaSb Interface. Y. Dong , D. Scott, A.C. Gossard and M.J. Rodwell. Department of Electrical and Computer Engineering, University of California, Santa Barbara.

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Characterization of Contact Resistivity on InAs/GaSb Interface

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  1. Characterization of Contact Resistivity on InAs/GaSb Interface Y. Dong, D. Scott, A.C. Gossard and M.J. Rodwell. Department of Electrical and Computer Engineering, University of California, Santa Barbara yingda@ece.ucsb.edu 1-805-893-3812 2003 Electronic Materials Conference

  2. E B C Sub-collector Substrate Motivations Base resistance (RB) is a key factors limiting HBT’s high frequency performance. fmax  RB

  3. Ec Ef + Metal Ev Tunneling Base Resistance A large contribution to base resistance: Contact resistance between metal and p-type base E B C Sub-collector Substrate Contact resistivity on p-type material is usually much higher than on n-type material. Reason: holes have larger effective mass than electrons.

  4. Emitter contact metal Emitter Base metal Base metal N+ N+ P+ P+ SiO2 P+ base SiO2 N- collector Collector Metal Collector Metal N+ subcollector S.I. substrate High ft , fmax , ECL logic speed… Base contact on n-type material Is it possible to make the base contact on n-type material? • Base metal contact on n-type extrinsic base  RB could be reduced • Metal to base contact over field oxide  CBC can be reduced • Large emitter contact area  RE can be reduced

  5. Polycrystalline Base Contact in InP HBTs 1) Epitaxial growth 2) Collector pedestal etch, SiO2 planarization P+ base SiO2 P+ base SiO2 N- collector subcollector N+ subcollector N+ subcollector S.I. substrate S.I. substrate

  6. Polycrystalline Base Contact in InP HBTs 4) Deposit base metal, encapsulate with SiN, pattern base and form SiN sidewalls 3) Extrinsic-base regrowth Base metal Base metal N+ extrinsic base N+ N+ P+ extrinsic base P+ P+ SiO2 P+ base SiO2 SiO2 P+ base SiO2 subcollector subcollector N+ subcollector N+ subcollector S.I. substrate S.I. substrate

  7. Emitter contact metal Emitter Base metal Base metal N+ N+ P+ P+ SiO2 P+ base SiO2 N- collector Collector Metal Collector Metal N+ subcollector S.I. substrate Polycrystalline Base Contact in InP HBTs 5) Regrow emitter • n+/p+ interface • Is it rectifying or ohmic? • If ohmic, is the interfacial contact resistivity low enough?

  8. EC P+ GaSb EV Ef EC N+ InAs EV P+ GaSb / N+ InAs Heterostructure We propose to use p+ GaSb capped with n+ InAs as the extrinsic base. • InAs-GaSb heterostructure forms a broken-gap band lineup • Mobile charge carriers tunnel between the p-type GaSb’s valence band and the neighboring n-type InAs’s conduction band  ohmic p-n junction

  9. Early Interests in InAs(n)/GaSb(p) Material System InAs(n)/GaSb(p) heterostructure has been studied in 1990s with focuses on: 1x105 A/cm2 • Negative differential resistance (NDR) • Application in high frequency tunneling diodes Current Density Applied Bias

  10. Focus of This Work • The contact resistivity across the InAs(n)/GaSb(p) interface at relatively low current density (<104 A/cm2). (No NDR at low current density) • The dependence of contact resistivity on the doping concentration in InAs and GaSb layers.

  11. 1000Å n+ InAs Silicon doped 100Å p+ GaSb 500Å p+ Grading from GaAs0.51As0.49 Carbon doped 400Å p+ GaAs0.51Sb0.49 S.I. InP MBE Growth of Test Structures • Samples grown in a Gen II system • Sb source valved and cracked • CBr4 delivered through high vacuum leak valve • Layer structure designed for InP HBT’s extrinsic base  for processing reasons, total thickness constrained

  12. Transmission line patterns defined, • Ti/Pt/Au contact metal deposited and lifted-off. 1000Å n+ InAs 100Å p+ GaSb 500Å p+ Grading from GaAs0.51As0.49 400Å p+ GaAs0.51Sb0.49 S.I. InP Measurement of Interfacial Contact Resistivity

  13. 1000Å n+ InAs 100Å p+ GaSb 500Å p+ Grading from GaAs0.51As0.49 400Å p+ GaAs0.51Sb0.49 Measurement of Interfacial Contact Resistivity 2) Mesa defined to limit the current flow. S.I. InP

  14. Measurement of Interfacial Contact Resistivity 3) Contact resistivity between metal and n+ InAs layer measured. 1000Å n+ InAs 100Å p+ GaSb 500Å p+ Grading from GaAs0.51As0.49 400Å p+ GaAs0.51Sb0.49 S.I. InP

  15. Measurement of Interfacial Contact Resistivity Y Axis intercept = Contact resistance between metal and InAs 1000Å n+ InAs 100Å p+ GaSb 500Å p+ Grading from GaAs0.51As0.49 400Å p+ GaAs0.51Sb0.49 S.I. InP

  16. n+ InAs n+ InAs n+ InAs n+ InAs 100Å p+ GaSb 500Å p+ Grading from GaAs0.51As0.49 400Å p+ GaAs0.51Sb0.49 S.I. InP Measurement of Interfacial Contact Resistivity 4) Top InGaAs layer selectively etched

  17. n+ InAs n+ InAs n+ InAs n+ InAs 100Å p+ GaSb 500Å p+ Grading from GaAs0.51As0.49 400Å p+ GaAs0.51Sb0.49 S.I. InP Measurement of Interfacial Contact Resistivity Y Axis intercept = Contact resistance between metal and InAs + contact resistance between InAs and GaSb

  18. n+ InAs n+ InAs n+ InAs n+ InAs 100Å p+ GaSb 500Å p+ Grading from GaAs0.51As0.49 400Å p+ GaAs0.51Sb0.49 S.I. InP Contact Resistivity’s dependence on p-type GaSb layer’s doping • Silicon doping in n-type InAs layer fixed at 1x1017cm-3 • Carbon doping in p-type GaSb varied

  19. n+ InAs n+ InAs n+ InAs n+ InAs 100Å p+ GaSb 500Å p+ Grading from GaAs0.51As0.49 400Å p+ GaAs0.51Sb0.49 S.I. InP Contact Resistivity’s dependence on n-type InAs layer’s doping • Carbon doping in p-type GaSb layer fixed at 4x1019cm-3 and 7x1019cm-3. • Silicon doping in p-type GaSb varied.

  20. Resonant Enhancement of Current Density InAs/GaSb EC For the single InAs/GaSb interface, reflection occurs due to imperfect coupling of InAs conduction-band states and GaSb valence-band states EV EC EV InAs/GaSb/AlSb/GaSb EC Formation of a quantum well layer between the InAs/GaSb interface and an AlSb barrier  resonant enhancement of the current density EV EC EV

  21. Experiment Result InAs/GaSb EC C: 7x1019 cm-3 Contact resistivity: 6.0x10-7 -cm2 Si: 1x1017 cm-3 EV EC EV 12Å AlSb InAs/GaSb/AlSb/GaSb EC C: 7x1019 cm-3 Si: 1x1017 cm-3 Contact resistivity: 5.4x10-7 -cm2 EV EC EV

  22. Comparison with metal on p+ InGaAs Lowest interfacial contact resistivity obtained: ~ 4x10-7-cm2 Contact resistivity of metal on p+ InGaAs: ~1x10-6-cm2

  23. Emitter contact metal Emitter Base metal Base metal N+ N+ P+ P+ SiO2 P+ base SiO2 N- collector Collector Metal Collector Metal N+ subcollector S.I. substrate Questions Answered • n+/p+ interface • Is it rectifying or ohmic? -- YES • If ohmic, is the interfacial contact resistivity low enough? -- YES

  24. Conclusions • Propose to use InAs(n)/GaSb(P) as extrinsic base of InP HBT • Investigate the contact resistivity between InAs(n)/GaSb(p) interface and its dependence on doping densities on both sides of the heterojunction. • Compare the InAs(n)/GaSb(p) interfacial contact resistivity with that of metal on p+ InGaAs.

  25. Acknowledgement This work was supported by the DARPA—TFAST program

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