In Al AsIn Ga AsIn P DHBTs with

In. Al. As/In. Ga. As/In. P DHBTs with Polycrystalline In. As Extrinsic Emitter Regrowth D. Scott, H. Xing, S. Krishnan, M. Urteaga, N. Parthasarathy and M. Rodwell University of California, Santa Barbara dennis@umail. ucsb. edu 805 -893 -8044, 805 -893 -3262 fax

Advantages of In. P vs. Si. Ge HBTs In. P HBT Material Properties: Si/Si. Ge HBT Material Properties: • Available lattice-matched materials allows for emitter bandgap wider than base, allowing for higher base doping and lower base sheet resistance • Allowable lattice mismatch limits Ge: Si alloy ratio resulting in smaller emitter-base bandgap difference and higher base sheet resistance • Electron velocities reported as high as 4 107 cm/s • 4: 1 lower electron velocity is seen in silicon In. P HBT Processing Technology : Si/Si. Ge Processing Technology : • High topography mesa structure allows for small-scale integration • Planar process using silicon CMOS technology allows for VLSI • Base-emitter junctions defined by etching and depositing a self-aligned base metal results in low yield and limits emitter scaling • Self-aligned base-emitter junctions are diffused, extrinsic base and emitter wider than the active junction allows for high degree of scaling

Evolution Cbc Reduction in III-V HBTs Emitter Collector Base Subcollector S. I. Substrate Mesa HBT Cbc Reduction HBT Emitter Base Collector Subcollector Collector Transferred Substrate HBT S. I. Substrate Highly Scaled HBT

UCSB Highly Scaled HBT UCSB has demonstrated laterally scaled HBTs with emitters written by e-beam lithography. These HBTs show problems with: • High emitter resistance, Rex • Low yield These devices demonstrated lower than predicted values of f despite aggressive thinning of the epitaxial layers.

Si/Si. Ge HBT Process Advantages • 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 Published Si/Si. Ge HBT f as high as 210 GHz In. P-based HBT f as high as 341 GHz • LSI and VLSI capabilities

Polycrystalline n+ In. As Polycrystalline In. As grown on Si. Nx Hall measurements as high as: • Doping = 1. 3 1019 cm-3, Mobility = 620 cm 2/V • s • Results in doping-mobility product of 8 1021 (V • s • cm)-1 Compare these numbers to In. Ga. As lattice matched to In. P: • Doping = 1. 0 1019 cm-3, Mobility = 2200 cm 2/V • s • Results in doping-mobility product of 22 1021 (V • s • cm)-1 Polycrystalline In. As has potential as an extrinsic emitter contact!

Base-Collector Template for Regrown Emitter HBT Base-collector template as-grown Base-collector template prior to regrowth

Regrown Emitter Fabrication Process Regrowth Emitter/cap etch Base/collector etch Metalization

Large-area Small-emitter HBTs

First Attempt Results Regrown area Si. Nx Regrown area very rough Transistor action!!

Growth and Process Improvements Regrown area Si. Nx First attempt at the baseemitter junction without RHEED or pyrometer Regrown area Si. Nx Second attempt with improved pre-regrowth processing and RHEED/pyrometer features added to the wafer

Growth and Process Improvements First attempt at the baseemitter junction without RHEED or pyrometer Second attempt with improved pre-regrowth processing and RHEED/pyrometer features added to the wafer

Base-emitter Regrowth SEM Detail

Base-emitter Regrowth SEM 2 μm emitter regrowth 30 K magnification 1 μm emitter regrowth 55 K magnification

Second Attempt DC Results Unintended In. Al. As Layer (>50Å) Common-emitter gain, β > 15 • wide-bandgap layer acts as a current block from emitter to base • reduces common-emitter gain • may account for the dip in common-emitter curves

Base-emitter Current Leakage Evidence of resistance seen in the base-emitter diode Evidence of base-emitter leakage seen in Gummel

Third Attempt DC Results Common-emitter gain, β > 20

Base-collector Grade Design Error In. P collector In. Ga. As base Base-collector band diagram with the incorrect base-collector grade. This mistake may account for the oscillations seen in the HBT I-V curve. In. P collector In. Ga. As base Base-collector band diagram with the corrected base-collector grade. A thin, heavily-doped layer was inserted between the grade and collector to pull the conduction band down at the grade-collector junction.

Regrowth with Buried Base Contact

In. P HBTs with polycrystalline In. As extrinsic emitter regrowth Objective: • Emulate high-yield 0. 2 um Si. Ge emitter process • Polycrystalline extrinsic emitter wide contact for low resistance Future Work: • RF devices need to be designed and demonstrated • Ga. As. Sb based DHBTs should be demonstrated • Higher scaling in the regrown emitters needs to be examined Growth Related Work: • A low-resistance p-type polycrystalline contact needs to be verified • Regrowth of the base will need to be explored to obtain a fully planar HBT completely analogous to the Si/Si. Ge HBT

In. P HBTs with polycrystalline In. As extrinsic emitter regrowth Objective: Emulate high-yield 0. 2 um Si. Ge emitter process Polycrystalline extrinsic emitter wide contact for low resistance Future Work (short-term): Improve DC characteristics. Improve base capping layer to lower extrinsic base resistance Ga. As. Sb base layers for higher carbon incorporation Deep submicron scaling of regrown emitter. RF device demonstration Future work (long-term): full Si. Ge-like process flow for submicron In. P HBT regrown emitter, regrown extrinsic base over buried dielectric spacer for Ccb reduction

Future Work DC Device Work: • DC characteristics should be demonstrated without the design errors • Improvements will be made to the base capping layer to lower extrinsic base resistance • Ga. As. Sb based DHBTs should be demonstrated • Higher scaling in the regrown emitters needs to be examined RF Device Work: • RF devices need to be designed and demonstrated Growth Related Work: • A low-resistance p-type polycrystalline contact needs to be verified • Regrowth of the base will need to be explored to obtain a fully planar HBT completely analogous to the Si/Si. Ge HBT