Ultra-Wide Bandgap AlGaN Channel MISFET with Graded ... · MISFET with Graded Heterostructure Ohmic Contacts ... Baliga figure of merit ... – superior Johnson’s figure of merit

IWN2016 Sanyam Bajaj: [email protected] Prof. Siddharth Rajan: [email protected]

Ultra-Wide Bandgap AlGaN Channel MISFET with Graded Heterostructure

Ohmic Contacts

Sanyam Bajaj1, F. Akyol1, S. Krishnamoorthy1, Y. Zhang1, S. Rajan1

1Department of Electrical and Computer EngineeringThe Ohio State University, Columbus, OH USA

A. Armstrong2, A. Allerman2

2Sandia National Laboratories, Albuquerque, NM USA

Acknowledgment:ONR (Dr. Paul Maki), NSF (ECCS-1408416), Raytheon IDS Microelectronics

mailto:[email protected]



Outline

2

• Motivation

• Heterostructure graded ohmic contacts

• Experimental results

• MISFET device operation




Outline

3

• Motivation







Ultra-wide bandgap material systems

4

3 4 5 62468

10121416

AlN

Diamondβ-Ga2O3

GaN

Fitting:V

br ~ 0.15*(E

g)2.5 MV/cm

Brea

kdow

n Fi

eld

(MV/

cm)

Energy Bandgap (eV)

4H-SiC

• GaN – wide bandgap (3.4 eV)• Ultrawide bandgap (UWBG) material systems with bandgap exceeding

4 eV• AlN with extremely high (theoretical) breakdown field ~ 5X of GaN• Results in high composition AlGaN with superior device figures of

merits – next-generation rf amplifiers? Power switches?

Hudgins et al. IEEE TED 18.3 (2003)




ns=1013 cm-2

Switching figure of merit

5

ns=1013 cm-2

2DEG mobility:• Limited by Alloy Scattering +Optical Phonon Scattering

Al mole fraction in AlGaN

Bajaj et al., APL 105.26 (2014)




ns=1013 cm-2

Switching figure of merit

6

ns=1013 cm-2

ns=1013 cm-2

2DEG mobility:• Limited by Alloy Scattering + Optical Phonon Scattering

Baliga figure of merit (εμEC3):

• Superior for larger Al compositions in channel than GaN

Bajaj et al., APL 105.26 (2014)






AlGaN for rf electronics

7

3 4 5 62468

10121416

John

son'

s FO

M ( x

107 M

V/s)AlN

Diamondβ-Ga2O3

GaN

Br

eakd

own

Fiel

d (M

V/cm

)

Energy Bandgap (eV)

4H-SiC1

2

3

4

5

• AlGaN channels with predicted electron velocities comparable to GaN

– superior Johnson’s figure of merit (theoretical)

Farahmand et al. IEEE TED 48.3 (2001) Anwar et al. IEEE TED 48.3 (2001)




AlGaN for rf electronics / optoelectronics

8

3 4 5 62468

10121416

John

son'

s FO

M ( x

107 M

V/s)AlN

Diamondβ-Ga2O3

GaN

Br

eakd

own

Fiel

d (M

V/cm

)

Energy Bandgap (eV)

4H-SiC1

2

3

4

5

• AlGaN channels with predicted electron velocities comparable to GaN

– superior Johnson’s figure of merit (theoretical)

• Also enables deep-UV emitters and detectors

Fig. by Crystal IS (http://www.cisuvc.com/)

Farahmand et al. IEEE TED 48.3 (2001) Anwar et al. IEEE TED 48.3 (2001)




Key Challenges

9

Material Challenges: Defects, Mobility

Device Challenges: High contact resistances to AlGaNChannels

Li et al., IEEE EDL 20.7 (1999)

Yue et al., IEEE EDL 33.7 (2012)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.91E-7

1E-6

1E-5

1E-4

1E-3

0.01

0.1

France et al.APL (2007)

Wang et al.El. Mat. (2004)

Srivastava et al.El. Mat. (2009)

Yun et al.EDL (2006)

Baca et al.APL (2016)

Yafune et al.El.Lett. (2014)

Yafune et al.JJAP (2011)

Nanjo et al.APL (2008)


ρ C

(Ω.c

m2 )

Al composition in AlGaN channel

GaN HEMTs [ref]




Outline

10

• Motivation







EF

EC

EVACχS

Metal Semiconductor

ΦM

e-

W

ND+ Charge

Requirements:1. High channel electron affinity /

matching metal work function2. High doping density

• Result in small tunneling barrier and width for electrons – high tunneling probability

eWm B

eT 3*24 2/1φ−

=

QM

ΦB

Ohmic Contact Formation




EF

EC

EVACχS

Metal Semiconductor

ΦM

e-

W




Conventional n-GaN channel:• Relatively high electron affinity (4.1 eV)• Metals with similar work function result in small tunneling barrier –RC below 10-6 Ω.cm2

ΦB





Li et al., IEEE EDL 20.7 (1999) Yue et al., IEEE EDL 33.7 (2012)

GaN channel

AlGaN barrierS D

2DEG




Conventional n-GaN channel:• Relatively high electron affinity (4.1 eV)• Metals with similar work function result in small tunneling barrier –RC below 10-6 Ω.cm2

• GaN HEMTs – alloyed / regrown contacts give low RC to 2DEG

EF

EC

EVACχS

Metal Semiconductor

ΦM

e-

W

ΦB





Ohmic Contacts to UWBG AlGaN

14

Challenges:1. Low electron affinity of AlN

(0.6 eV) – high Schottkybarrier

2. Low doping efficiency

• Result in low tunneling probability, high RC EF

ECΦB

EVAC

ΦM χS

Metal Semiconductor

e-

W




Heterostructure-engineered ohmic contacts

15

UWBGn-AlGaNchannel

S DA

A’ EF

EC

EV

ΦB

• Conventional ohmic contact to n-type UWBG AlGaN channel –large Schottky barrier

χS

EVAC

ND+

ChargeQM

AlGa

N




UWBGn-AlGaN channel

Reverse gradedAlGaN -> GaN

S D

A’

A

• Contact layer with reverse composition-grading from wider bandgap AlGaN to lower bandgap GaN – lower Schottky barrier

EF

EC

EV

EVAC

ND+

Charge

QM

χS

PPZ+PSP

ΦB

GaN

AlGa

N





UWBGn-AlGaN channel

Reverse gradedAlGaN -> GaN

S D

A’

A


EF

EC

EV

EVAC

ND+

Charge

QM

χS

PPZ+PSP

ΦB

GaN

AlGa

N

• Negative polarization charge (spontaneous + piezoelectric) raises EC (0001 direction) –large barrier for electrons!

Jena et al., APL 81.23 (2002) Rajan et al., APL 84.9 (2004)





UWBGn-AlGaN channel

Reverse gradedn++ AlGaN -> GaN

S D

A’

A


EF

EC

EV

EVAC

ND+

ChargeQM

χS

PPZ+PSP

ΦB

Electron slab

n++ gradedAlGaN

• High donor concentration compensates negative polarization charge – flat ECprofile, low RSH

Jena et al., APL 81.23 (2002) Rajan et al., APL 84.9 (2004)

Park et al., IEEE EDL 36.3 (2015)





Outline

19

• Motivation







Experiment – n-type Al0.75Ga0.25N Channel

20

30nm Al0.75Ga0.25N (UID)

AlN on Sapphire

100nm Al0.75Ga0.25NSi = 3x1019 cm-3

75%

6%50nm Gradedn++ AlGaN

Si = 1020 cm-3

- 100 nm 75% n-AlGaN channel with EG = 5.35 eV (MBE growth on AlN/Sapphire template)- Si donor concentration = 3x1019 cm-3

- 50 nm n++ reverse polarization-graded contact layer

- Conduction band profile under ohmic region (as-grown)

A’

A

A A’

0 50 100 150 200-6

-4

-2

0

2

4

6UIDAlGaN

AlNn-Al0.75Ga0.25N

Graded AlGaN

EF

EV

Ener

gy (e

V)Distance (nm)

AS GROWN:Contact region EC





AlN on Sapphire

90nm Al0.75Ga0.25NSi = 3x1019 cm-3

75%

6%

- 100 nm 75% n-AlGaN channel with EG = 5.35 eV (MBE growth on AlN/Sapphire template)- Si donor concentration = 3x1019 cm-3

- 50 nm n++ reverse polarization-graded contact layer

- Conduction band profile under gate region (recessed)

A

A’

100 150 200-6

-4

-2

0

2

4

6

UIDAlGaN

Ener

gy (e

V)Distance (nm)

RECESSED:Intrinsic region

n-Al0.75Ga0.25N AlN

EC

EF

EV

A A’

Experiment – n-type Al0.75Ga0.25N Channel




Non-Alloyed Ohmics Contacts

22

Graded AlGaNcontact layer


AlN on Sapphire

AlGaN channel

Non-alloyed ohmic contacts – Ti/Al/Ni/Au = 20/120/30/50 nm

S D




Contact Resistance using TLM

23


AlN on Sapphire

AlGaN channel

S DRC1 RC1

RSH

spacing

0 1 2 3 4 5 6 7

4

6

8

10

12

14

ρSP = 1.4x10-6 Ω.cm2

RC1 = 0.15 Ω.mmRSH = 158 Ω/sq

Resis

tanc

e (o

hm)

Spacing (µm)

Non-alloyed ohmic contacts – Ti/Al/Ni/Au = 20/120/30/50 nm

As-grown structure:• RC1 (Metal-

semiconductor interface resistance) = 0.15 Ω.mm

• ρSP = 1.4x10-6

Ω.cm2

Recessed structure:• Net RC to 75% AlGaN

channel = 0.32 Ω.mm

• ρSP = 1.9x10-6

Ω.cm2





24


AlN on Sapphire

AlGaN channel

S DRC1 RC1

RSH

spacing

0 1 2 3 4 5 6 7

4

6

8

10

12

14

ρSP = 1.4x10-6 Ω.cm2

RC1 = 0.15 Ω.mmRSH = 158 Ω/sq

Resis

tanc

e (o

hm)

Spacing (µm)

As-grown structure:• RC1 (Metal-

semiconductor interface resistance) = 0.15 Ω.mm

• ρSP = 1.4x10-6

Ω.cm2

Recessed structure:• Net RC to 75% AlGaN

channel = 0.32 Ω.mm

• ρSP = 1.9x10-6

Ω.cm2


AlN on Sapphire

90nm channel

S DRC1 RC1

RSH1 RSH2RSH1

spacing

2 4 6 8 10 12 142030405060708090

100110

ρSP = 1.9x10-6 Ω.cm2

RC1+RSH1 = 0.32 Ω.mmRSH2 = 725 Ω/sq

Resis

tanc

e (o

hm)

Spacing (µm)

Cl2-based ICP-RIE etch to test contact to AlGaN channel





25


AlN on Sapphire

AlGaN channel

S DRC1 RC1

RSH

spacing

0 1 2 3 4 5 6 7

4

6

8

10

12

14

ρSP = 1.4x10-6 Ω.cm2

RC1 = 0.15 Ω.mmRSH = 158 Ω/sq

Resis

tanc

e (o

hm)

Spacing (µm)

• ρSP = 1.9x10-6

Ω.cm2


AlN on Sapphire

90nm channel

S DRC1 RC1

RSH1 RSH2RSH1

spacing

2 4 6 8 10 12 142030405060708090

100110

ρSP = 1.9x10-6 Ω.cm2


Resis

tanc

e (o

hm)

Spacing (µm)

Low ρSP to UWBG AlGaN ~ 5.3 eV (Non-alloyed)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.91E-7

1E-6

1E-5

1E-4

1E-3

0.01

0.1





This work






ρ C (Ω

.cm

2 )


GaN HEMTs [ref]




Outline

26

• Motivation







Al0.75Ga0.25N Channel MIS-FET

27


AlN substrate

12nm n-AlGaN channel

S D

20nm Al2O3

G

GS S

D

- Recessed structure with 12 nm n-Al0.75Ga0.25N channel

- 20 nm ALD Al2O3 followed by 700°C PDA (30s)

- C-V profile resulted in pinch-off voltage = - 6 V ; accumulation region with MESFET-like behavior ; charge = 1.5x1013 cm-2

-8 -6 -4 -2 0 2 40.0

0.1

0.2

0.3

VGS (V)

C GS (µF

/cm

2 )

0246810121416

n (x

1012

cm

-2)

10 kHz








AlN substrate


S D

20nm Al2O3

G

- IDS_MAX ~ 60 mA/mm ; gm_MAX = 14 mS/mm- fT_PEAK of 0.6 GHz ; fMAX_PEAK of 1.4 GHz- Limited by low channel mobility of 16 cm2/Vs- Defect related compensation

0 5 10 15 200

10

20

30

40

50

60

∆VG = -2 V

I D (m

A/m

m)

VDS (V)

VG = 2 V

-8 -6 -4 -2 0 20

10

20

30

40

50

60

VGS (V)

I D (m

A/m

m)

02468101214

gm (m

S/mm

)

VDS = 20 V

0.01 0.1 1 100

10

20

30

40

fT = 0.6 GHz

VDS = 25 VVGS = 4 V

rf ga

in (d

B)

Frequency (Hz)

|h21| U MSG









AlN substrate


S D

20nm Al2O3

G

- Vbr = 224 V @ VGS = -9 V for LGD = 1.1 μm in Fluorinert

- no field plates- Average field > 2 MV/cm – higher

than GaN FETs

0 50 100 150 200 2500

20406080

100120140 VGS = -9 V

ISIG

ID

I D,I S,

I G (µ

A/m

m)

VDS (V)

LGD = 1.1 µm(compliance)





SUMMARY

30

- Heterostructure graded ohmic contacts to UWBG AlGaN –compositional grading + high doping

- Achieved low specific contact resistance to Al0.75Ga0.25N channels (NON-ALLOYED)

- Demonstrated the 1st UWBG Al0.75Ga0.25N channel MISFET with low-resistance ohmics (MBE)

- This work removes one of the principle challenges for UWBG AlGaN devices; applications in large range of electronic and photonic devices


AlN on Sapphire

90nm channel

S DRC1 RC1

RSH1 RSH2RSH1

spacing

2 4 6 8 10 12 142030405060708090

100110

ρSP = 1.9x10-6 Ω.cm2


Resis

tanc

e (o

hm)

Spacing (µm)

0 5 10 15 200

10

20

30

40

50

60

∆VG = -2 V

I D (m

A/m

m)

VDS (V)

VG = 2 V

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.91E-7

1E-6

1E-5

1E-4

1E-3

0.01

0.1





This work






ρ C (Ω

.cm

2 )


GaN HEMTs [ref]



Documents

Ultra-Wide Bandgap AlGaN Channel MISFET with Graded ... · MISFET with Graded Heterostructure Ohmic Contacts ... Baliga figure of merit ... – superior Johnson’s figure of merit