Ultrascaled GaN HEMTs with thin AlN barriershxing/research/pdfs/201108... · Huili (Grace) Xing Electrical Engineering Department, University of Notre Dame Huili (Grace) Xing ([email protected])

Ultrascaled GaN HEMTs with thin AlN barriers

Huili (Grace) Xing

Electrical Engineering Department, University of Notre Dame

Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011 1

Ft – transport physics in GaN

Outline •  AlN/GaN HEMTs

–  Highest mobility with highest 2DEG (D. Jena) –  Ohmic contacts

•  InAlN/AlN/GaN HEMTs –  Very high mobility with very high 2DEG –  Ohmic contacts –  Passivation –  Emode –  Phonon-limited injection velocity (D. Jena)

•  Summary


Important for high power and high speed –  High ns, mobility and saturation velocity –  Thin barrier, short gate length, large barrier height

Barrier

Channel

Source Drain

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Gate

Ultra-scaled WBG semiconductor-based devices (e.g. Lg < 50 nm, Cg > 1.6 µF/cm2 – corresponding to ~ 5 nm thick AlN, ~ 1.6 S/mm assuming 1e7 cm/s)

0 100 200 300 400 500 600-5-4-3-2-101234

Ener

gy (e

V)

Thickness (A)

(3 nm) AlN/GaN

Ultra-thin AlN barrier GaN HEMTs

EOT = 1.4 nm

Huili (Grace) Xing ([email protected])

AlN/GaN by MBE

•  An AlN/GaN superlattice

Streak pattern due to SL

5nm

20nm

TEM By Kejia (Albert) Wang and Tom Kosel (U. of Notre Dame)

AlN (13 monolayers)

GaN

GaN


4 nm AlN/GaN (MBE)

Highest 2DEG and mobility in III-V nitrides

I. Smorchkova et al, 2000, APL Y. Cao et al, 2007, APL Y. Cao et al, 2008, APL


High conductivity window: 2-5 nm AlN Best metrics among all III-V heterostructures: ns = 2x1013 cm-2 with u = 1900 cm2/Vs Y. Cao et al, 2011, JCG

1.5 nm 6 nm AlN thickness

•  RT Hall measurement: –  ns ~ 2.75e13 cm-2

-  µ ~ 1367cm2/Vs -  Rsh ~ 166 ohm/sq

3.5 nm AlN

Sapphire substrate

GaN layers

S D

G S G & 3 nm Al2O3 gate dielectric

I. Ultrathin barrier AlN/GaN HEMT: structure Early studies

Zimmermann et al. IEEE EDL. 2008


S S

D

G

•  Gate oxide used to prevent leakage in 3.5 nm AlN/GaN HEMTs. •  Current densities & transconductance are high. •  Breakdown ~ 25 Volts. •  As measured ft/fmax ~ 52/60 GHz

Zimmermann et al. IEEE EDL. 2008 gm,int ~ 1 S/mm

I. Ultrathin barrier AlN/GaN HEMTs: performance Early Studies


Without de-embedding

I. Ultrathin barrier AlN/GaN HEMTs: performance Early studies

Zimmermann et al. IEEE EDL. 2008

The first reported AlN/GaN HEMT with mobility > 1000 cm2/Vs

Performance limited by poor ohmics, gate and buffer leakage

Also see Higashiwaki’s work in 2007 and HRL’s work in 2010/2011 Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011

Ohmic contact challenge

Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011

9

Solved by MBE regrown contacts

on metal-face GaN HEMTs, see HRL DRC 2010, UND ICNS 2011

On N-face GaN HEMTs, see UCSB APL 2009

T. Zimmermann et al,

PSS, 2008

D. Deen, PhD

dissertation, UND 2010

Outline

•  AlN/GaN HEMTs

–  Highest mobility with highest 2DEG –  Ohmic contacts

•  InAlN/AlN/GaN HEMTs –  Very high mobility & ns

–  Ohmic contacts –  Passivation –  Emode

•  Summary


D. Jena et al, PSS 2011

InAl(Ga)N/AlN/GaN HEMTs •  Attractive to cap the thin AlN barrier

–  GaN: substantially reduce ns (< 1.2 x 1013 cm-2)

–  InAlN: maintain high ns (> 1.5x1013 cm-2)


0 50 100 150 2000.00E+000

1.00E+013

2.00E+013

3.00E+013

4.00E+013

5.00E+013

6.00E+013

ns=2.5x1013 cm-2

AlGaN 30% AlInN 17% AlN

n s (cm

-2)

Barrier Thickness (A)

ns=1.4x1013 cm-2

d ~ 40 A

d = 180 A

IQE, IWN 2010

AlN/GaN

(Y. Cao

JCG’11)

InAlN

(IQE,

EPFL)

SiN passiva)on SiN passiva)on

In0.17Al0.83N (4.7 nm)

Alloyed ohmic contacts with Si and recess etch

In0.13Al0.83Ga0.04N 10 nm

GaN

SiC

AlN 1.0 nm

DS

2DEG GaN

SiC

AlN 1.0 nm DS

2DEG

ü  Mesa isola)on; ü  Alloyed Ohmics:

(ohmic recess) + (Si)/ Ti/Al/Ni/Au, RTP

•  Reliable Rc extrac)on necessitates passiva)on •  Smooth morphology

(Si)/Ti/Al/Ni/Au

0 5 10 15 20 25 300

20

40

60

80

R c = 0.23 Ω-‐mm,

R s h = 190 Ω/s q

As -‐proc es s ed S iN pas s ivated

R (Ω)

d (um)

R c = 0.17 Ω-‐mm,

R s h = 220 Ω/s q

TL M: InA lG aN, 860 C , 18 s

800 820 840 860 8800.0

0.3

0.6

0.9

1.2

1.5 InA lN (#B -‐1), w/o reces s

InA lN (#B -‐2), 3 nm reces s



InA lG aN (#C _311), w/o reces s

InA lG aN (#D _310), ~ 6 nm reces

~11 nm barrier, 18 s

Rc (Ω

-‐mm)

A nnealing Temperature (C )

5.7 nm barrier, 15 s ;

w/ 2 nm S iR. Wang et al., APEX 2011.


Alloyed Ohmics with Si and ohmic recess

InAlGaN

ü  No obvious Rc improvement for thin barriers; contact via sidewall is not efficient aIer etching away the en)re barrier;

ü  Rc improvement for thick barriers (Rc as low as 0.23 Ω•mm); no metal penetra)on into GaN;

ü  Ul)mate limit for tunneling: 1-‐nm AlN spacer.

InAlGaN, 860 C, 18 s

R. Wang et al., APEX 2011.


HEMTs with SiN passivation (I)

With increasing SiN thickness:

Ø  Mobility is nearly constant Ø  Carrier concentra)on increases and saturates (> 30 nm) Ø  Sheet resistance decreases and saturates (> 30 nm) Ø  gm increases. Why does gm increase with > 30 nm SiN? Ø  I increases, saturates (70 – 100 nm) and then decreases.

InAlN 4.7 nm

GaN

SiC

AlN 1.0 nm

DSG

2DEG

# A

Ni/Au

0 20 40 60 80 100500

550

600

650

700

f T (GHz)

Pea

k gm (mS/m

m)

S iN Thic knes s (nm)

30

40

50

60

70

80

90

100

(a)

0 20 40 60 800.0

0.5

1.0

1.5

2.0

2.5

3.0

µ (x 103

cm

2 /V.s)/ns (x10

-‐13 cm

-‐2)

Rsh (ohm/sq)

S iN Th ic knes s (nm)

As -‐g rwon

-‐400

-‐200

0

200

400

600 (c )

Lg = 160 nm


R. Wang et al., ISCS 2011. (presenta:on only)

14

HEMTs with SiN Passivation (II)

0 2 4 6 8 10 120.0

0.5

1.0

1.5

2.0

2.5

+3 V+2 V

0 V

A s -‐proc es s ed 30 nm S iN 70 nm S iN

I d (A/m

m)

V ds (V)

S tep = -‐1 V

-‐2 V

(a)

-‐4 -‐3 -‐2 -‐1 0 1 2 30.0

0.5

1.0

1.5

2.0 As -‐proc es s ed 30 nm S iN 70 nm S iN

I d (A/m

m)

gm (mS/m

m)

V g s (V)

Vds = 6 V

(b)

0

200

400

600

800

1000

-‐4 -‐3 -‐2 -‐1 0 1 2 310-‐8

10-‐6

10-‐4

10-‐2

100

10-‐8

10-‐6

10-‐4

10-‐2

100

A s -‐proc es s ed 30 nm S iN 70 nm S iN

Vds = 6 V I g (A/m

m)

V g s (V)

I d (A/m

m)

(c )

SiN 30 nm è 70 nm, both intrinsic delay and drain delay decreased – consistent with higher gm. Possible reason for higher gm: 1. Higher carrier velocity? 2. Shorter effective gate length Lg, eff thus higher carrier velocity?

✗

✔



15

Pulsed I-Vs

With 100 nm SiN passivation ~ 4% gate delay and ~ 7% drain delay

As-fabricated (without any passivation) ~ 30% gate delay and ~ 10% drain delay

Lg ~ 60 nm



16

HEMTs with Dielectric-Free Passivation (DFP)

InAl(Ga)N ~11 nm

GaN

SiC

AlN 1.0 nm

DS

G

2DEG

Plasma (DFP) O2

# B

Before DFP After DFP

Rsh (Ω/sq)

ns (x 1013 cm-2)

µ (cm2/V.s)

Rsh (Ω/sq)

ns (x 1013 cm-2)

µ (cm2/V.s)

InAlN_313 290 1.62 1330 257 1.86 1300

InAlGaN_310 227 1.45 1900 190 1.83 1790

With DFP in the access region only:

Ø  Mobility decreases slightly Ø  Carrier concentra)on increases Ø  Sheet resistance decreases Ø  gm increases slightly Ø  ft increases from 125 GHz to 210-‐220 GHz -‐8 -‐6 -‐4 -‐2 0 2

0.0

0.5

1.0

1.5

2.0

2.5 w/o DF P w/ DF P

I d (A/m

m)

gm (mS/m

m)

V g s (V)

Vds = 6 V

0

100

200

300

400

500

600 (a)

Lg ~ 60 nm


R. Wang et al., EDL 2011. 17

Dielectric-Free Passivation (DFP): I. InAlN

-‐50 -‐40 -‐30 -‐20 -‐10 010-‐13

10-‐11

10-‐9

10-‐7

10-‐5

10-‐3

10-‐1

Vbr = 50 V

w/o DF P w/ DF P

Vds = 0 V

Vg s (V)

I g (A/m

m)

V br = 12 V

108 109 1010 10110

10

20

30

40

50

60

/ |h21|2

/ U

w/o DF PVg s / Vds = -‐4.1 / 4.0 V

fT / fmax = 125 / 46 G Hz

w/ DF PVg s / Vds = -‐4.0 / 4.8 V

fT / fmax = 210 / 55 G Hz

Gain (dB)

F requenc y (Hz )

L g ~ 60 nm

-‐20 dB /dec

All delay components droped. Lg,eff > 120 nm prior to DFP

R. Wang et al. IEEE EDL, 2011


18

Dielectric-Free Passivation (DFP): II. InAlGaN

109 1010 10110

10

20

30

40

|h21|2

U

fT = 220 G Hz

fmax= 60 G Hz

Gain (dB)

F requenc y (Hz )

Vds = 4.7 V ,

Vg s = -‐3.7 V

L g ~ 66 nm

0 10 20 30 40 50 60 700

2

4

6

8

10

12

14

16

18

20

C alc ulations AlG aN, 2 nm S iN, T -‐g ate AlG aN, no pas s ivation, T -‐g ate AlN, 50 nm S iN, T -‐g ate InAlN, 50 nm S iN, T -‐g ate InAlN, 10 nm Al2O 3, I-‐g ate

InAlN, DF P , I-‐g ate InAlG aN, DF P , I-‐g ate

f T. L

g (GHz.µm

)

L g /tbar (un itles s )

Th is work 0 2 4 6 8 100.0

0.5

1.0

1.5

2.0

2.5

B ias po int: DC (Vg s , Vds ) = (0 V , 0 V )

(Vg s , Vds ) = (-‐8 V , 0 V )

(Vg s , Vds ) = (-‐8 V , 10 V )

I d (A/m

m)

V ds (V)

Vg s = 0 V

300 ns pu ls e width

R. Wang et al. EDL, 2011

Little dispersion observed in HEMTs with DFP Other attributes of DFP: 1. Air stable 2. Little parasitic capacitance

3. Large signal performance yet to be tested. ?

✔ ✔

W DFP Lg = 66 nm


19

R. Wang et al., IEEE EDL 2010


Characterization of TQT HEMTs at UND (2): Device Fabrication

Post Annealing Parameters: 400 C, 10 min, in forming gas (5% H2, balanced with Ar) * Annealing in N2 resulted in similar behaviors too.

RT Hall transport data: ns ~ 2.0 x 1013 cm-‐2

µ ~ 1160 cm2/Vs Rsh ~ 270 Ω/sq

Gate geometry Lg ~ 150 nm, Wg ~ 100 -‐ 600 µm Lsd ~ 2 µm, 3 µm Gate metal: Pt/Au

Ti based ohmic contact Rc ~ 0.6 Ω-‐mm

SiN InAlN 4.8 nm

GaN

SiC

AlN 1 nm S D

G

21 Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011

Characterization of TQT HEMTs at UND (3): E-mode

0 2 4 6 8 100.0

0.5

1.0

1.5

2.0Vg s = 4 V , S tep = -‐0.5 V

I d (A

/mm)

V ds (V )

L g = 150 nm, Wg = 3x50 um, L s d = 2 um

R. Wang, P. Saunier, et al., DRC, 2010; IEEE EDL, vol. 31 (12) 2010.

AlN InAlN

Pt Lg = 150 nm

SiN

GaN

0 1 2 3 40.0

0.5

1.0

1.5

2.0

g m (m

S/mm)

I d (A

/mm)

V g s (V)

Vds = 5 V

L g = 150 nm, Wg = 2x50 um

L s d = 2 um

0

200

400

600

800

ft/fmax ~ 70/105 GHz, without deembedding

Cross-‐sec)onal STEM


Characterization of TQT HEMTs at UND (4): Effect of Annealing

0 1 2 3 4 5 6 7 8 9 10

0.0

0.2

0.4

0.6

0.8

Wg = 8x50 um w/o annea ling w/ annea ling

I d (A/m

m)

V ds (V)

Vg s = 2 V , S tep = -‐0.5 V

-‐1 0 1 2 30.0

0.2

0.4

0.6

0.8

1.0

1.2

Vth = 0.6 V Vth = 1.2 V

Wg = 8x50 um

L s d = 3 um

w/o annealing w/ annealing

I d (A/m

m)

gm (mS/m

m)

V g s (V)

Vds = 6 V

0

100

200

300

400

500

600

InAlAs

K. Chen, et al., IEEE EDL, 1996. Pt/InAlAs HEMT

A. Fricke et al., APL, 1994.

Gate sink

aIer annealing In0.52Al0.48As

Pt

In0.52Al0.48As

Pt PtIn/PtAs2 AlAs


Characterization of TQT HEMTs at UND (5): pre-annealing Dit

•  High Dit (> 1x1013cm-2) observed in the as-fabricated E-mode devices

SS = (1+Cq + Cit

Cb

)kBTq

ln10



Gate diode breakdown

Ø  Forward effec)ve barrier height increases from 0.67 to 0.93 eV, akributed to an insula)ng layer formed aIer post annealing.

Characterization of TQT HEMTs at UND (6): Leakage Reduction

-‐1 0 1 2 310-‐15

10-‐13

10-‐11

10-‐9

10-‐7

10-‐5

10-‐3

10-‐1

101

10-‐15

10-‐13

10-‐11

10-‐9

10-‐7

10-‐5

10-‐3

10-‐1

101

S S ~ 62 mV/dec

Ion/Ioff = 1012

S S ~ 84 mV/dec

Ion/Ioff = 107

Vds = 6 V

w/o annealing w/ annealing

I g (A/m

m)

V g s (V)

I d (A/m

m)

(a)

Log-‐scale transfer curve

Ø  Dit decreases from 1.5 x 1013 to 1.1 x 1012 cm-‐2eV-‐1, extracted from subthreshold slopes, meaning post-‐annealing repairs the interface;

5

110

×



Characterization of TQT HEMTs at UND (7): Leakage Mechanism

Trap density decrease and barrier height increase are responsible for the gate leakage reduc)on. More work are being taken, such as STEM, EDX, modeling on leakage…

AlN GaN Pt

DT

Ef, s Ef, g

Interlayer (PtxIny, Al-‐O, …) Post-‐annealing

AlN GaN Pt

Direct tunneling (DT)/FN tunneling (FN)

Trap-‐assisted tunneling (TAT)

Ef, s Ef, g

φb

dieEv

20

exp( )exp( )dieTTAT die

qEqJ AEkT kT rε

πε= −

3/22 exp( )b

FN diedie

CJ BEEφ

= −

A depends on trap density.

B, C are constants. Pre-‐annealing



GaN

300 nm

300 nm

Thickness measured by a-step

metal

AFM

MBE

MBE regrown contact process flow

RMS=0.6 nm

MBE Regrown n+ GaN surface

Control regrowth (45 nm n+GaN): Sheet charge ~2e15/cm2

Mobility ~ 52 cm2/Vs Rsh~58 Ohm/sq

MB

E

crys

talli

ne

MBE amorphous

Barrier

GaN

Barrier

GaN

Barrier

Mask

MB

E

crys

talli

ne

MB

E

crys

talli

ne

GaN

Barrier

MB

E

crys

talli

ne

MB

E

crys

talli

ne

GaN

Barrier

MB

E

crys

talli

ne

•  Planar regrowth of high quality •  Lateral regrowth interface needs to be further characterized SEM

Regrown n+GaN on mask


G. Jia et al., ICNS 2011 (manuscript under preparation)

Regrown contacts with n+GaN/2DEG Rc ~ 0.05 ohm-mm


1.  Regrowth interface resistance ~ 0.05 ohm-mm

2.  Total Rc ~ 0.27 ohm-mm, dominated by metal/n+GaN resistance (~ 0.15 ohm-mm).

3.  We have also demonstrated metal/n+InGaN Rc < 0.02 ohm-mm.

G. Jia et al., ICNS 2011

Injection point phonon model and Peak injection velocity

29

•  Saturation currents from the phonon model fit experimental state-of-the-art •  The peak injection velocity is vp~1.3x107cm/s at room temperature •  The peak injection velocity occurs when ns~4x1012/cm2 @ source injection pt.

Op:cal phonon emission

Op:cal phonon Emission blocked

Peak injec:on velocity

Investigate here

T. Fang et al., DRC 2011


Peak fT with barrier and gate length scaling

30

•  Intrinsic peak fT is ‘fundamental’, limited by the onset of phonon emission •  Gate length scaling improves peak fT faster than vertical scaling •  Peak fT improvement is severely stalled by source/drain resistances

Lg=50 nm

fT peaks at phonon emission

T. Fang et al., DRC 2011

?


Summary

•  Ohmic contacts have been solved for GaN HEMTs –  Rc comparable to that of InAs and Si FETs, < 50 ohm-um

•  Low injection velocity in GaN limits the device speed –  Low mobility (phonon scattering, interface scattering) –  High effective mass (wide bandgap) –  Locked by optical phonons

•  Plasma treatments need to be better understood for reliable device operation

•  Novel ideas are necessary for GaN THz transistors


Acknowledgement

Students and postdocs Yu Cao David Deen Tian Fang Zongyang Hu Fazia Faria Jia Guo Guowang Li Chuanxin Lian Berardi Sensale-Rodriguez John Simon Ronghua Wang Tom Zimmermann Yuanzheng Yue Vladimir Protasenko


Collaborators (University of Notre Dame) Debdeep Jena Tom Kosel Gregory Snider Patrick Fay (Triquint Semiconductor) Paul Saunier (IQE RF LLC) Shiping Guo (Kopin) Wayne Johnson

GaN research Sponsored DARPA ONR AFOSR

Special thanks to

Umesh Mishra & Debdeep Jena

Documents

Ultrascaled GaN HEMTs with thin AlN barriershxing/research/pdfs/201108... · Huili (Grace) Xing Electrical Engineering Department, University of Notre Dame Huili (Grace) Xing ([email protected])