Substrate Effects on Transport and Dispersion in Delta-...Substrate Effects on Transport and...

Substrate Effects on Transport and Dispersion in Delta-Doped β-Ga2O3 Field Effect Transistors

Chandan Joishi, Zhanbo Xia, Joe McGlone, Yuewei Zhang, Aaron R. Arehart, Steven A. Ringel, Siddharth Rajan

Electrical and Computer Engineering,Materials Science and Engineering,

The Ohio State University, Columbus, OH, USA

Chandan Joishi, Saurabh LodhaEE Department, IIT Bombay, Mumbai, India

Funding: DTRA, IMR IITB-OSU Alliance, ONR EXEDE MURI

EMC 2018 [email protected] [email protected]

Outline

2

• Introduction and motivation

• Semi-insulating substrates

• Si delta-doped β-Ga2O3 MESFETs

• Results

• Conclusions


Outline

3




• Results

• Conclusions


Wide bandgap semiconductors: applications

4 EMC 2018 [email protected] [email protected]

Military

Satellite Comm.

Wireless broadband

5G mmWave

Cell phones

RFelectronics

5G sub-6GHz

Base stations

Radar

Automotive

Railways

Inverters Powerelectronics

Turbines

Powergrid

Power supplies

Ships and vessels

Audio amplifiers

Wide bandgap semiconductors

• Smaller system size • Harsh environments• Lower loss

β-Ga2O3 bulk substrates


• Melt-based growth techniques, availability of high

quality native substrates

• High device yield due to low defect density

• Controlled n-type doping (Si, Sn, Ge)/ insulating

(Fe) films – first wide bandgap semiconductor

ρ = 10-3 – 1012 Ω.cm

n = 1015 – 1020 cm-3

Band gap = 4.6 eV

Electron effective mass = 0.28 m0

GalliumOxygen

β- phase: Monoclinic crystal structure

Other polymorphs : α, γ, δ and ε

S. J. Pearton et al. APL, 5, 011301 (2018)

β-Ga2O3: where it stands


Properties 4H-SiC GaN Ga2O3

Bandgap (eV) 3.3 3.4 4.6Breakdown Field (MV/cm) 2.5 3.3 8

Relative dielectric constant (εr) 9.7 9 10

Electron mobility (cm2/Vs) 1000 2000 (2DEG)

200-300 (Bulk)

Saturation Velocity (x107 cm/s) 2 2.5 ~2

Thermal Conductivity (Wcm-1K-1)

4.9 2.1 0.27 [010]; 0.11[001]


𝑃𝑃𝑜𝑜𝑜𝑜𝑜𝑜 =𝑉𝑉𝐵𝐵𝐵𝐵𝐼𝐼𝑀𝑀𝑀𝑀𝑀𝑀

8

≈𝑰𝑰𝑴𝑴𝑴𝑴𝑴𝑴𝑭𝑭𝑩𝑩𝑩𝑩𝒗𝒗𝒔𝒔𝒔𝒔𝒔𝒔

𝟏𝟏𝟏𝟏𝟏𝟏𝒇𝒇𝝉𝝉

High frequency applications: power density not limited by low thermal conductivity







200-300 (Bulk)


BFOM/BFOM(Silicon) (εμEC3) 317 1450 3214

JFOM/JFOM(Silicon) ( (vsat Ec )2 ) 278 1022 2844Substrate cost High High LowHeterojunction No Yes Yes


4.9 2.1 0.27 [010]; 0.11[001]



8










200-300 (Bulk)


BFOM/BFOM(Silicon) (εμEC3) 317 1450 3214

JFOM/JFOM(Silicon) ( (vsat Ec )2 ) 278 1022 2844Substrate cost High High LowHeterojunction No Yes Yes


4.9 2.1 0.27 [010]; 0.11[001]

•



8




• High reliabilityLow cost substrate

Previous device reports (selected)


Field-plated MOSFET VBR = 755 V (MBE)

NICT (Japan) – Wong et.al. IEEE EDL 37, 2 (2016)

Schottky Barrier Diode VBR > 1000 V (HVPE)

NICT (Japan) – Konishi et.al.DRC 2016

MOSFET withfT = 3GHz (MBE)AFRL – AJ Green et.al.IEEE EDL 38,6 (2017)

Why delta doped FETs?


High Mobility in high charge regime

Enables scaling of Gate to Channel distance

• High concentration of 2DEG

• High gate breakdown voltage

(Lowest Elec. Field between

gate and channel)

• High trans-conductance

• Higher mobility than uniformly

doped film (for similar doping)

Enables exploration of Ga2O3- based high frequency devices

Outline

11




• Results

• Conclusions


Semi insulating substrates


• Substrate requirements: RF and power

• Low conduction and switching loss

• Minimization of parasitic capacitance

Substrate

Buffer

Epilayer

S DG







• Fermi level pinning near mid-gap

• Compensation of residual donors and acceptors

Substrate

Buffer

Epilayer

S DG







• Fermi level pinning near mid-gap

• Compensation of residual donors and acceptors

• Fe as a deep level acceptor

• The most common transition metal to realize semi-insulating substrates

Substrate

Buffer

Epilayer

S DG

T. Nishioka et al., JAP, 51, 5789 (1980)

Fe doped semi-insulating substrates


InP, GaAs, SiC

• Efficient SRH centers

• Fe diffusion during epi growth

GaN

• Current collapse in HEMTs

• Non radiative center

GaN literature

Fe doped substrates in β-Ga2O3


• Native n-type substrates from melt

growth techniques

• Ec-0.78 eV a deep level acceptor

• Diffusion during growth and implantation

anneal.

Tamura (010) substrates (＞1010Ω.cm)

No report of effect of Fe diffusion on β-Ga2O3 based device performance

Wong et al. APL106, 032105 (2015)

DFT and DLTS- Fe doped Ga2O3

Outline

17




• Results

• Conclusions


MBE growth of β-Ga2O3


Growth rate along the (010) orientation faster than (-201)

(010) : 240 nm/hr(-201): 50 nm/hr

• Substrate: Bulk (010) β- Ga2O3

• Substrate Temperature: 700o C

• O2 plasma power : 300 W

• Growth Rate: 155 – 240 nm/ hour

• Ga flux: 4x10-8 Torr – 8x10-8 Torr (O-rich conditions)

1.0x10-7 2.0x10-7140

160

180

200

220

240

Gro

wth

Rate

(nm

/hr)

Ga Flux (Torr)

Growth phase diagramOkumura et. al. (Speck Group- UCSB)

Excess Ga on Ga2O3 results in the formation of volatile Ga2O (resulting in etching of Ga2O3)

Riber M7, O-plasma MBE

β- Ga2O3 δ- Doped MESFET


Fe- doped (010) β-Ga2O3substrate

UID β-Ga2O3 (buffer)

UID Ga2O3 (cap)

Si δ- doping

S G Dn+

Ga2O3

-3.5 nm

3 nm

1 μm

1 μm0 nm

5 nm

100 nm buffer

600 nm buffer

• As grown surface

• tRMS ~ 0.6 nm for 100 nm buffer

• tRMS ~ 0.7 nm for 600 nm buffer• S/D contact using regrowth process flow

Contact regrowth process flow


δ doped MESFET

SiO2 Growth Mask

δ doped MESFET δ doped MESFET

δ doped MESFET

1. SiO2 DepositionLithography with thick PR

2. ICP/RIE SiO2 Etch 3. SiO2 wet etching for the remaining SiO2

5. SiO2 wet etching for regrowth overlap

Photoresist

2DEG

δ doped MESFET

4. Ohmic recess

δ doped MESFET

6. n++ Ga2O3 regrowth

Contact regrowth process flow


7. BOE lift off

SEM image of channel and regrown contact region

• Regrown Ga2O3 on SiO2 lift off in BOE

• Regrown Ga2O3 completely covers crevice plane

• Contact resistance ~ 0.35 Ohm-mm

δ doped MESFET

n ++ regrown Ga2O3

n ++ regrown Ga2O3

Channel

Source Drain

Outline

22




• Results

• Conclusions


SIMS profiling



UID β-Ga2O3

0 200 400 600 8001013

1014

1015

1016

1017

1018

Fe c

once

ntra

tion

(ato

ms/

cm3 )

Depth (nm)

Detection limit

• Fe diffusion in buffer layer

• 200-250 nm before detection limit

SIMS profiling



UID β-Ga2O3

0 200 400 600 8001013

1014

1015

1016

1017

1018

Fe c

once

ntra

tion

(ato

ms/

cm3 )

Depth (nm)

Detection limit

• Fe diffusion in buffer layer

• 200-250 nm before detection limit

• 100 nm and 600 nm buffer thickness studied

100 nm

600 nm

Two terminal IVs


0 100 200

10-5

10-4

10-3

10-2

600 nm buffer 100 nm buffer

Curre

nt (µ

A/m

m)

Voltage (V)

DD

0 200 400 60040

60

80

100

120 Electron mobility Charge density

Buffer thickness (nm)

Hal

l mob

ility

(cm

2 /Vs)

1.4

1.6

1.8

Cha

rge

dens

ity (x

1013

cm

-2)

• Buffer leakage < 40 nA/mm at 200V

• Increase in Hall mobility for thick buffer.

• Increase in charge density with thickness

• Back-depletion from Fe

0 25 50 75 100 125

0.0

0.5

1.0

1.5

Ener

gy (e

V)

Depth (nm)

EC (600 nm buffer) EC (100 nm buffer) EF

Towards substrate

Mesa

2-DEG profile and IDS-VGS characteristics


20 40 60 800

2

4 600 nm 100 nm

Dop

ing

conc

entra

tion

(×10

19 c

m-3)

Position (nm)

• Similar position of the 2-DEG from charge profile

• Three terminal transfer characteristics

• ION/IOFF ~ 105

• IOFF ~ 2 μA/mm, limited by Ni/Au/Ni gate

• VP = -8 V (100 nm), -11 V (600 nm)

-12 -8 -4 010-6

10-5

10-4

10-3

10-2

10-1

gm (m

S/m

m)

VGS (V)

IDSVGS

gm

I DS (

A/m

m)

0

7

14

21

28

35VDS = 10V

-16 -12 -8 -4 010-6

10-5

10-4

10-3

10-2

10-1

IDSVGS

gm

VGS (V)

I DS (

A/m

m)

gm (m

S/m

m)

0

4

8

12

16

20VDS = 10V

100 nm buffer

600 nm buffer

-10 -8 -6 -4 -2 0

0.0

0.1

0.2

0.3

CG (µ

F/cm

2 )

VGS (V)

f = 1 MHz

100 nm buffer

DC-RF dispersion


100nm buffer 600nm buffer

• For DC and pulsed IVs,

• 0.1% duty cycle

• Pulsed IV

• Current collapse for 100 nm buffer

• Reduced knee-walkout dispersion for 600 nm buffer

• Better characteristics with 600 nm buffer

0 5 10 150.0

0.1

0.2

I DS (A

/mm

)

VDS (V)

Line : DC Symbols : 5 µs

VGS = 2V, ∆VGS = -2V

(VGSQ, VDSQ) = (-12, 15)

0 5 10 150.0

0.1

0.2VGS = 2 V, ∆VGS = -2 V


I DS (A

/mm

)

VDS (V)

(VGSQ, VDSQ) = (-15, 15)

10 15 20 250.0

0.5

1.0

1.5

2.0 600 nm buffer 100 nm buffer

(RO

N -

RO

N, (

0, 0

)) /R

ON

, (0,

0)

VDSQ (V)

VGSQ= -12 V

VGSQ= -15 V

5 µs

5 ms

time

Volta

ge

VDSQ

VGSQ

VGS

VDS

Conclusions


• δ-doping a promising approach for scaled RF devices based onβ-Ga2O3

• High current density and transconductance• Fe diffusion into the buffer exhibits current collapse• Hall mobility seen to increase with buffer thickness• Buffer thickness greater than 600 nm can enable better

transport and dispersion propertiesThank you!

0 200 400 600 8001013

1014

1015

1016

1017

1018

Fe c

once

ntra

tion

(ato

ms/

cm3 )

Depth (nm)

Detection limit0 200 400 600

40

60

80

100

120 Electron mobility Charge density

Buffer thickness (nm)

Hal

l mob

ility

(cm

2 /Vs)

1.4

1.6

1.8

Cha

rge

dens

ity (x

1013

cm

-2)

0 5 10 150.0

0.1

0.2VGS = 2 V, ∆VGS = -2 V


I DS (A

/mm

)VDS (V)

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