1. University of California Santa Barbara Yingda Dong 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. University of California Santa Barbara Yingda Dong Motivations Base resistance (R B ) is a key factors limiting HBT’s high frequency performance. Sub-collector Substrate E C B RB RB  f max 

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

4. University of California Santa Barbara Yingda Dong Base contact on n-type material Is it possible to make the base contact on n-type material? S.I. substrate N+ subcollector SiO 2 N- collector P+ base SiO 2 P+ N+ Base metal P+ N+ Base metal Emitter Emitter contact metal Collector Metal Collector Metal  Base metal contact on n- type extrinsic base  R B could be reduced  Metal to base contact over field oxide  C BC can be reduced  Large emitter contact area  R E can be reduced High f t, f max, ECL logic speed…

5. University of California Santa Barbara Yingda Dong S.I. substrate Polycrystalline Base Contact in InP HBTs 1) Epitaxial growth2) Collector pedestal etch, SiO 2 planarization N+ subcollector N- collector P+ base S.I. substrate N+ subcollector SiO 2 subcollector P+ base SiO 2

6. University of California Santa Barbara Yingda Dong Polycrystalline Base Contact in InP HBTs 3) Extrinsic-base regrowth 4) Deposit base metal, encapsulate with SiN, pattern base and form SiN sidewalls S.I. substrate N+ subcollector SiO 2 subcollector P+ base SiO 2 P+ extrinsic base N+ extrinsic base S.I. substrate N+ subcollector SiO 2 subcollector P+ base P+ N+ Base metal SiO 2 P+ N+ Base metal

7. University of California Santa Barbara Yingda Dong Polycrystalline Base Contact in InP HBTs 5) Regrow emitter S.I. substrate N+ subcollector SiO 2 N- collector P+ base SiO 2 P+ N+ Base metal P+ N+ Base metal Emitter Emitter contact metal Collector Metal Collector Metal n+/p+ interface  Is it rectifying or ohmic?  If ohmic, is the interfacial contact resistivity low enough?

8. University of California Santa Barbara Yingda Dong P+ GaSb / N+ InAs Heterostructure We propose to use p+ GaSb capped with n+ InAs as the extrinsic base. E C EVEV P+ GaSb N+ InAs E C EVEV EfEf  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. University of California Santa Barbara Yingda Dong Early Interests in InAs(n)/GaSb(p) Material System InAs(n)/GaSb(p) heterostructure has been studied in 1990s with focuses on: Applied Bias Current Density  Negative differential resistance (NDR)  Application in high frequency tunneling diodes 1x10 5 A/cm 2

10. University of California Santa Barbara Yingda Dong Focus of This Work  The contact resistivity across the InAs(n)/GaSb(p) interface at relatively low current density (<10 4 A/cm 2 ). (No NDR at low current density)  The dependence of contact resistivity on the doping concentration in InAs and GaSb layers.

11. University of California Santa Barbara Yingda Dong MBE Growth of Test Structures S.I. InP 400Å p+ GaAs 0.51 Sb 0.49 500Å p+ Grading from GaAs 0.51 As 0.49 100Å p+ GaSb 1000Å n+ InAs Carbon doped Silicon doped  Samples grown in a Gen II system  Sb source valved and cracked  CBr 4 delivered through high vacuum leak valve  Layer structure designed for InP HBT’s extrinsic base  for processing reasons, total thickness constrained

12. University of California Santa Barbara Yingda Dong Measurement of Interfacial Contact Resistivity S.I. InP 400Å p+ GaAs 0.51 Sb 0.49 500Å p+ Grading from GaAs 0.51 As 0.49 100Å p+ GaSb 1000Å n+ InAs 1)Transmission line patterns defined, Ti/Pt/Au contact metal deposited and lifted-off.

13. University of California Santa Barbara Yingda Dong S.I. InP 400Å p+ GaAs 0.51 Sb 0.49 500Å p+ Grading from GaAs 0.51 As 0.49 100Å p+ GaSb 1000Å n+ InAs 2) Mesa defined to limit the current flow. Measurement of Interfacial Contact Resistivity

14. University of California Santa Barbara Yingda Dong S.I. InP 400Å p+ GaAs 0.51 Sb 0.49 500Å p+ Grading from GaAs 0.51 As 0.49 100Å p+ GaSb 1000Å n+ InAs 3) Contact resistivity between metal and n+ InAs layer measured. Measurement of Interfacial Contact Resistivity

15. University of California Santa Barbara Yingda Dong S.I. InP 400Å p+ GaAs 0.51 Sb 0.49 500Å p+ Grading from GaAs 0.51 As 0.49 100Å p+ GaSb 1000Å n+ InAs Y Axis intercept = Contact resistance between metal and InAs Measurement of Interfacial Contact Resistivity

16. University of California Santa Barbara Yingda Dong S.I. InP 400Å p+ GaAs 0.51 Sb 0.49 500Å p+ Grading from GaAs 0.51 As 0.49 100Å p+ GaSb n+ InAs n+ InAs n+ InAs n+ InAs 4) Top InGaAs layer selectively etched Measurement of Interfacial Contact Resistivity

17. University of California Santa Barbara Yingda Dong S.I. InP 400Å p+ GaAs 0.51 Sb 0.49 500Å p+ Grading from GaAs 0.51 As 0.49 100Å p+ GaSb n+ InAs n+ InAs n+ InAs n+ InAs Y Axis intercept = Contact resistance between metal and InAs + contact resistance between InAs and GaSb Measurement of Interfacial Contact Resistivity

18. University of California Santa Barbara Yingda Dong Contact Resistivity’s dependence on p-type GaSb layer’s doping S.I. InP 400Å p+ GaAs 0.51 Sb 0.49 500Å p+ Grading from GaAs 0.51 As 0.49 100Å p+ GaSb n+ InAs n+ InAs n+ InAs n+ InAs  Silicon doping in n-type InAs layer fixed at 1x10 17 cm -3  Carbon doping in p-type GaSb varied

19. University of California Santa Barbara Yingda Dong Contact Resistivity’s dependence on n-type InAs layer’s doping S.I. InP 400Å p+ GaAs 0.51 Sb 0.49 500Å p+ Grading from GaAs 0.51 As 0.49 100Å p+ GaSb n+ InAs n+ InAs n+ InAs n+ InAs  Carbon doping in p-type GaSb layer fixed at 4x10 19 cm -3 and 7x10 19 cm -3.  Silicon doping in p-type GaSb varied.

20. University of California Santa Barbara Yingda Dong Resonant Enhancement of Current Density E C EVEV EVEV InAs/GaSb E C EVEV EVEV InAs/GaSb/AlSb/GaSb Formation of a quantum well layer between the InAs/GaSb interface and an AlSb barrier  resonant enhancement of the current density For the single InAs/GaSb interface, reflection occurs due to imperfect coupling of InAs conduction-band states and GaSb valence-band states

21. University of California Santa Barbara Yingda Dong Experiment Result E C EVEV EVEV InAs/GaSb E C EVEV EVEV InAs/GaSb/AlSb/GaSb Si: 1x10 17 cm -3 C: 7x10 19 cm -3 Si: 1x10 17 cm -3 C: 7x10 19 cm -3 12Å AlSb Contact resistivity: 6.0x10 -7  -cm 2 Contact resistivity: 5.4x10 -7  - cm 2

22. University of California Santa Barbara Yingda Dong Comparison with metal on p+ InGaAs Doping Density of p-GaSb (cm-3) Doping Density of n-InAs (cm-3) Contact Resistivity (Ω-cm2) 2x10 19 1x10 17 2.8x10 -6 2x10 19 6x10 17 3.0x10 -6 4x10 19 1x10 17 1.3x10 -6 4x10 19 1x10 19 1.6x10 -6 4x10 19 5x10 19 9.0x10 -7 7x10 19 1x10 17 6.0x10 -7 7x10 19 1x10 19 8.2x10 -7 7x10 19 5x10 19 4.2x10 -7 Lowest interfacial contact resistivity obtained: ~ 4x10 -7  -cm 2 Contact resistivity of metal on p+ InGaAs: ~1x10 -6  -cm 2

23. University of California Santa Barbara Yingda Dong Questions Answered S.I. substrate N+ subcollector SiO 2 N- collector P+ base SiO 2 P+ N+ Base metal P+ N+ Base metal Emitter Emitter contact metal Collector Metal Collector Metal n+/p+ interface  Is it rectifying or ohmic? -- YES  If ohmic, is the interfacial contact resistivity low enough? -- YES

24. University of California Santa Barbara Yingda Dong 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. University of California Santa Barbara Yingda Dong Acknowledgement This work was supported by the DARPA—TFAST program