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
EMC 2018 [email protected] [email protected]
Outline
3
• Introduction and motivation
• Semi-insulating substrates
• Si delta-doped β-Ga2O3 MESFETs
• Results
• Conclusions
EMC 2018 [email protected] [email protected]
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
5 EMC 2018 [email protected] [email protected]
• 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
6 EMC 2018 [email protected] [email protected]
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]
S. J. Pearton et al. APL, 5, 011301 (2018)
𝑃𝑃𝑜𝑜𝑜𝑜𝑜𝑜 =𝑉𝑉𝐵𝐵𝐵𝐵𝐼𝐼𝑀𝑀𝑀𝑀𝑀𝑀
8
≈𝑰𝑰𝑴𝑴𝑴𝑴𝑴𝑴𝑭𝑭𝑩𝑩𝑩𝑩𝒗𝒗𝒔𝒔𝒔𝒔𝒔𝒔
𝟏𝟏𝟏𝟏𝟏𝟏𝒇𝒇𝝉𝝉
High frequency applications: power density not limited by low thermal conductivity
β-Ga2O3: where it stands
7 EMC 2018 [email protected] [email protected]
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
BFOM/BFOM(Silicon) (εμEC3) 317 1450 3214
JFOM/JFOM(Silicon) ( (vsat Ec )2 ) 278 1022 2844Substrate cost High High LowHeterojunction No Yes Yes
Thermal Conductivity (Wcm-1K-1)
4.9 2.1 0.27 [010]; 0.11[001]
S. J. Pearton et al. APL, 5, 011301 (2018)
𝑃𝑃𝑜𝑜𝑜𝑜𝑜𝑜 =𝑉𝑉𝐵𝐵𝐵𝐵𝐼𝐼𝑀𝑀𝑀𝑀𝑀𝑀
8
≈𝑰𝑰𝑴𝑴𝑴𝑴𝑴𝑴𝑭𝑭𝑩𝑩𝑩𝑩𝒗𝒗𝒔𝒔𝒔𝒔𝒔𝒔
𝟏𝟏𝟏𝟏𝟏𝟏𝒇𝒇𝝉𝝉
High frequency applications: power density not limited by low thermal conductivity
β-Ga2O3: where it stands
8 EMC 2018 [email protected] [email protected]
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
BFOM/BFOM(Silicon) (εμEC3) 317 1450 3214
JFOM/JFOM(Silicon) ( (vsat Ec )2 ) 278 1022 2844Substrate cost High High LowHeterojunction No Yes Yes
Thermal Conductivity (Wcm-1K-1)
4.9 2.1 0.27 [010]; 0.11[001]
•
S. J. Pearton et al. APL, 5, 011301 (2018)
𝑃𝑃𝑜𝑜𝑜𝑜𝑜𝑜 =𝑉𝑉𝐵𝐵𝐵𝐵𝐼𝐼𝑀𝑀𝑀𝑀𝑀𝑀
8
≈𝑰𝑰𝑴𝑴𝑴𝑴𝑴𝑴𝑭𝑭𝑩𝑩𝑩𝑩𝒗𝒗𝒔𝒔𝒔𝒔𝒔𝒔
𝟏𝟏𝟏𝟏𝟏𝟏𝒇𝒇𝝉𝝉
High frequency applications: power density not limited by low thermal conductivity
• High reliabilityLow cost substrate
Previous device reports (selected)
9 EMC 2018 [email protected] [email protected]
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?
10 EMC 2018 [email protected] [email protected]
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
• Introduction and motivation
• Semi-insulating substrates
• Si delta-doped β-Ga2O3 MESFETs
• Results
• Conclusions
EMC 2018 [email protected] [email protected]
Semi insulating substrates
12 EMC 2018 [email protected] [email protected]
• Substrate requirements: RF and power
• Low conduction and switching loss
• Minimization of parasitic capacitance
Substrate
Buffer
Epilayer
S DG
Semi insulating substrates
13 EMC 2018 [email protected] [email protected]
• Substrate requirements: RF and power
• Low conduction and switching loss
• Minimization of parasitic capacitance
• Semi-insulating substrates
• Fermi level pinning near mid-gap
• Compensation of residual donors and acceptors
Substrate
Buffer
Epilayer
S DG
Semi insulating substrates
14 EMC 2018 [email protected] [email protected]
• Substrate requirements: RF and power
• Low conduction and switching loss
• Minimization of parasitic capacitance
• Semi-insulating substrates
• 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
15 EMC 2018 [email protected] [email protected]
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
16 EMC 2018 [email protected] [email protected]
• 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
• Introduction and motivation
• Semi-insulating substrates
• Si delta-doped β-Ga2O3 MESFETs
• Results
• Conclusions
EMC 2018 [email protected] [email protected]
MBE growth of β-Ga2O3
18 EMC 2018 [email protected] [email protected]
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
19 EMC 2018 [email protected] [email protected]
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
20 EMC 2018 [email protected] [email protected]
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
21 EMC 2018 [email protected] [email protected]
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
• Introduction and motivation
• Semi-insulating substrates
• Si delta-doped β-Ga2O3 MESFETs
• Results
• Conclusions
EMC 2018 [email protected] [email protected]
SIMS profiling
23 EMC 2018 [email protected] [email protected]
Fe- doped (010) β-Ga2O3substrate
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
24 EMC 2018 [email protected] [email protected]
Fe- doped (010) β-Ga2O3substrate
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
25 EMC 2018 [email protected] [email protected]
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
26 EMC 2018 [email protected] [email protected]
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
27 EMC 2018 [email protected] [email protected]
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
Line : DC Symbols : 5 µs
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
28 EMC 2018 [email protected] [email protected]
• 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
Line : DC Symbols : 5 µs
I DS (A
/mm
)VDS (V)
Top Related