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 Electronic Materials Conference

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 

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.

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…

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

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

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?

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

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

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.

University of California Santa Barbara Yingda Dong MBE Growth of Test Structures S.I. InP 400Å p+ GaAs 0.51 Sb Å p+ Grading from GaAs 0.51 As Å 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

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

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

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

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

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

University of California Santa Barbara Yingda Dong S.I. InP 400Å p+ GaAs 0.51 Sb Å p+ Grading from GaAs 0.51 As Å 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

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 Å p+ Grading from GaAs 0.51 As Å 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

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 Å p+ Grading from GaAs 0.51 As Å 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.

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

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

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) 2x x x x x x x x x x x x x x x x x x x x x x x x10 -7 Lowest interfacial contact resistivity obtained: ~ 4x10 -7  -cm 2 Contact resistivity of metal on p+ InGaAs: ~1x10 -6  -cm 2

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

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.

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