molecular beam epitaxy of low resistance polycrystalline p-type gasb

University of CaliforniaSanta 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

[email protected] 1-805-893-3812 15th IPRM 2003 Santa Barbara, CA


Outline

Motivations

Polycrystalline material for InP HBT’s extrinsic base Why choose GaSb

MBE growth of Poly-GaSb

Electrical Properties of Poly-GaSb

Conclusions


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 ft.

However, InP HBTs have not provided decisive advantages over SiGe HBTs in mixed-signal ICs.


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


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 CBC, RBB

High Maximum Oscillation Frequency (Fmax), ECL logic speed…

Can a similar technology be developed for InP HBTs ?

SiGe HBT process: extensive use of poly-Si for base contact


Polycrystalline Base Contact in InP HBTs

N - c o l l e c t o r

N + s u b c o l l e c t o r

S . I . s u b s t r a t e

1) Epitaxial growth

N- c ol l e ct o r

N + s u b c ol l e ct o r

S. I . s u b s t r at e

SiO2

2) Collector pedestal etch, isolation, SiO2 planarization



3) Base Regrowth

SiO2

N + s u b c oll e c t o r

S. I . s u b s t r a t e

4

Extrinsic base

Intrinsic base

N- collector

4) Deposit base metal, encapsulate with SiN, pattern base and form SiN Sidewalls

SiO2

N + s u b c oll e c t o r

S.I . s u b st r at e

SiNBase Metal

N- collector



N + s u b c oll e ct or

S.I. s u b str at e

InAlAs/InGaAs emitter

Emitter contact

Collector contact

SiO2

N- collector

P++ extrinsic baseBase metalSiN

5) Regrow InAlAS/InGaAs emitter


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


Material Choices for Polycrystalline Base

Polycrysalline material choices:

GaAsWide bandgap low hole mobility

Fermi level pinned in mid-bandgap

large band-bending barrier

GaSbNarrow bandgap high hole mobiliy

Fermi-level pinned on valence band

InSbNarrow bandgap

low melting point (~520 οC)

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


MBE Growth of Polycrystalline GaSb

GaAs

SiO2 3000Å

1) 3000Å SiO2 deposited on Semi-insulating GaAs by PECVD.

Poly-GaSb2) Poly-GaSb samples were

grown in a Varian Gen II system.

Sb source valved and cracked

CBr4 delivered through high vacuum leak vavle

Growth rate fixed at 0.2 μm/hr


Influence of V/III Beam Flux Ratio

0 2 4 6 8 10 12 14 16 18 20 22 242x1019

3x1019

4x1019

5x1019

6x1019

7x1019

8x1019

9x1019

Growth Temperature: 440°C

Hole M

obility (cm2/vs)

Hol

e C

once

ntra

tion

(cm

-3)

BEP(Sb)/BEP(Ga)

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0 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)


Influence of Growth Temperature

420 440 460 480 500 5204x1019

5x1019

6x1019

7x1019

8x1019

9x1019

Hole M

obility (cm2/vs)

Hol

e C

once

ntra

tion

(cm

-3)

Growth Temperature (°C)

1

2

3

4

5

6

7

Film thickness: 1000ÅBEP(Sb)/BEP(Ga)=5

Hole concentration changes little with growth temperature

Hole mobility decreases with growth temperature


Grain Size’s Temperature Dependence

Polycrystalline GaSb Grown at 520 οC

Gain size: ~350nm

Polycrystalline GaSb Grown at 475 οC

Grain size: ~100nm

SEM pictures of poly-GaSb samples


Poly-GaSb’s Grain Size and Resistivity

420 440 460 480 500 5205.0x10-3

1.0x10-2

1.5x10-2

2.0x10-2

2.5x10-2

3.0x10-2

3.5x10-2

4.0x10-2

4.5x10-2

Film thickness

Grain size (nm

)

Res

istiv

ity

-cm

Growth Temprature (°C)

0

50

100

150

200

250

300

350

400

Grain size increases steadily with growth temperature

Resistivity increases rapidly when grain size exceeds the film thickness


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


Grain Size Vs Film Thickness

SiO2


SiO2



SiO2


When the film thickness approaches the grain size, the total connecting boundary area will be significantly reduced

Rapid resistivity increase


Thickness Dependence

1000 1500 2000 2500 30006.0x10-3

8.0x10-3

1.0x10-2

1.2x10-2

1.4x10-2

1.6x10-2

Growth Temperature: 440ºC

Re

sis

itiv

ity

-c

m

Layer Thickness (Å)

Poly GaSb Thickness

(Ǻ)

Hole Concentration

Ns (cm-3)

Mobility

(cm2/Vs)

Bulk Resistivity

(cm))

Sheet resistivity

S (/)�

3000 8.2e19 10.2 7.5e-3 240

2000 8.0e19 8.6 9.1e-3 450

1500 8.1e19 5.8 1.3e-2 900

1000 7.8e19 5.1 1.6e-2 1550

Bulk resistivity has

strong dependence on

film thickness

Sheet resistivity increases very fast with decreasing thickness


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)

8x1019 8x1019

Grain Size (Å) ~700 400~2000

Film Thickness (Å)

3000 4000

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.


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 CBr4 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.


Acknowledgement

This work was supported by the DARPA—TFAST program

molecular beam epitaxy of low resistance polycrystalline p-type gasb

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