University of California Santa Barbara Yingda Dong Molecular Beam Epitaxy of Low Resistance Polycrystalline P-Type GaSb Y. Dong, D. Scott, Y. Wei, A.C. Gossard and M. Rodwell. Department of Electrical and Computer Engineering, University of California, Santa Barbara th IPRM 2003 Santa Barbara, CA

University of California Santa Barbara Yingda Dong Outline  Motivations  Polycrystalline material for InP HBT’s extrinsic base  Why choose GaSb  MBE growth of Poly-GaSb  Electrical Properties of Poly-GaSb  Conclusions

University of California Santa Barbara Yingda Dong InP Vs SiGe HBTs Advantages of InP HBTs over SiGe HBTs ~20:1 lower base sheet resistance, ~ 5:1 higher base electron diffusivity ~ 3:1 higher collector electron velocity, ~ 4:1 higher breakdown-at same f t. However, InP HBTs have not provided decisive advantages over SiGe HBTs in mixed-signal ICs.

University of California Santa Barbara Yingda Dong Strong Features of Si/SiGe HBT Process  Highly scaled  Very narrow active junction areas  Very low device parasitics  High speed  Low emitter resistance using wide n+ polysilicon contact  Low base resistance using large extrinsic polysilicon contact  High-yield, planar processing  High levels of integration  LSI and VLSI capabilities

University of California Santa Barbara Yingda Dong Polycrystalline Base Contact The Advantages of Polycrystalline Base Contact :  Reduce the B-C capacitance by allowing metal-to-base contact over the field oxide  Reduce the base resistance by highly doping the polycrystalline extrinsic base Low C BC, R BB High Maximum Oscillation Frequency (F max ), ECL logic speed… Can a similar technology be developed for InP HBTs ? SiGe HBT process: extensive use of poly-Si for base contact

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

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

University of California Santa Barbara Yingda Dong Polycrystalline Base Contact in InP HBTs 5) Regrow InAlAS/InGaAs emitter

University of California Santa Barbara Yingda Dong Properties of Polycrystalline Material Polycrystalline InAs Polycrystalline GaSb  Small crystallites join together at grain boundaries  Inside each crystallite: single crystal  At grain boundaries: a large number of traps  Fermi level pinned

University of California Santa Barbara Yingda Dong Material Choices for Polycrystalline Base Polycrysalline material choices:  GaAs  Wide bandgap  low hole mobility  Fermi level pinned in mid-bandgap  large band-bending barrier  GaSb  Narrow bandgap  high hole mobiliy  Fermi-level pinned on valence band  InSb  Narrow bandgap  low melting point (~520 ο C)  Can not withstand emitter regrowth Grain boundary Ec Ev Ef Grain boundary Ec Ev Ef Schematic diagram of suggested energy band structure near grain boundary in p- type of GaAs and GaSb

University of California Santa Barbara Yingda Dong MBE Growth of Polycrystalline GaSb GaAs SiO Å 1) 3000Å SiO 2 deposited on Semi-insulating GaAs by PECVD. Poly-GaSb 2) Poly-GaSb samples were grown in a Varian Gen II system.  Sb source valved and cracked  CBr 4 delivered through high vacuum leak vavle  Growth rate fixed at 0.2 μm/hr

University of California Santa Barbara Yingda Dong Influence of V/III Beam Flux Ratio  Hole mobility changes little with V/III ratio  Hole concentration increases with decreasing V/III ratio (Reason: Carbon must displace antimony to be effective p-type dopant)

University of California Santa Barbara Yingda Dong Influence of Growth Temperature  Hole concentration changes little with growth temperature  Hole mobility decreases with growth temperature

University of California Santa Barbara Yingda Dong Grain Size’s Temperature Dependence Polycrystalline GaSb Grown at 520 ο C Gain size: ~350nm Polycrystalline GaSb Grown at 475 ο C Grain size: ~100nm SEM pictures of poly-GaSb samples

University of California Santa Barbara Yingda Dong Poly-GaSb’s Grain Size and Resistivity  Grain size increases steadily with growth temperature  Resistivity increases rapidly when grain size exceeds the film thickness

University of California Santa Barbara Yingda Dong Small Grain Vs. Large Grain Small grain:  More grain boundaries for carriers to cross  Larger total boundary areas connecting crystallites Large grain:  Fewer grain boundaries for carriers to cross  Smaller total boundary areas connecting crystallites Grain boundary Ec Ev Ef Small band bending barrier  Total connecting boundary area more important

University of California Santa Barbara Yingda Dong Grain Size Vs Film Thickness SiO 2

University of California Santa Barbara Yingda Dong SiO 2 Grain Size Vs Film Thickness

University of California Santa Barbara Yingda Dong SiO 2 Grain Size Vs Film Thickness

University of California Santa Barbara Yingda Dong SiO 2 Grain Size Vs Film Thickness When the film thickness approaches the grain size, the total connecting boundary area will be significantly reduced Rapid resistivity increase

University of California Santa Barbara Yingda Dong Thickness Dependence Poly GaSb Thickness (Ǻ) Hole Concentration N s (cm -3 ) Mobility  (cm 2 /Vs) Bulk Resistivity  (  cm)) Sheet resistivity  S (  /) e e e e e e e e Bulk resistivity has strong dependence on film thickness Sheet resistivity increases very fast with decreasing thickness

University of California Santa Barbara Yingda Dong Comparison Between Poly-GaSb and Poly-GaAs Poly-GaSb by MBE (This work) Poly-GaAs by GSMBE (N.Y. Li et al, 1998) Carbon doping density (cm -3 ) 8x10 19 Grain Size (Å)~700400~2000 Film Thickness (Å) Bulk Resistivity (  -cm) 7.5x10 -3 ~1x10 -1 With similar carbon doping level, grain size and film thickness, the resistivity of poly-GaSb’s resistivity is more than one order of magnitude lower than that of poly-GaAs.

University of California Santa Barbara Yingda Dong Conclusions  Poly-GaSb proposed to be used as extrinsic base material for InP HBTs  Low resistance poly-GaSb films can be achieved by MBE growth using CBr 4 doping  The resistivity of poly-GaSb has strong dependence on film’s thickness and grain size, particularly when the film thickness is comparable with the grain size.

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