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DRC2016 Sanyam Bajaj: bajaj.10@osu.edu Prof. Siddharth Rajan: rajan.21@osu.edu1
Ultra-Wide Bandgap AlGaN Channel
MISFET with Heterostructure
Engineered Ohmics
Sanyam Bajaj1, F. Akyol1, S. Krishnamoorthy1, Y. Zhang1, S. Rajan1
1Department of Electrical and Computer Engineering
The 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
DRC2016 Sanyam Bajaj: bajaj.10@osu.edu Prof. Siddharth Rajan: rajan.21@osu.edu
Outline
2
• Motivation
• Heterostructure engineered ohmics
• MISFET device operation
DRC2016 Sanyam Bajaj: bajaj.10@osu.edu Prof. Siddharth Rajan: rajan.21@osu.edu
Outline
3
• Motivation
• Heterostructure engineered ohmics
• MISFET device operation
DRC2016 Sanyam Bajaj: bajaj.10@osu.edu Prof. Siddharth Rajan: rajan.21@osu.edu
Applications – III-Nitrides
4
III-Nitrides
High composition AlGaN with large bandgap / breakdown field:- Next-generation High Power RF amplification / Switching
- Optoelectronics / Deep-UV emitters
Dissertation, Pil Sung Park,
OSU (2013)
DRC2016 Sanyam Bajaj: bajaj.10@osu.edu Prof. Siddharth Rajan: rajan.21@osu.edu
UWBG Material Systems
5
Properties 4H-SiC GaN Ga2O3 C AlN
Bandgap (eV) 3.3 3.4 4.9 5.5 6.2
Breakdown Field
(MV/cm)
2.5 3.3 8 10 12-16 [1]
Saturation velocity,
vsat (cm/s)
2x107 2x107 -- ~2x107 ~2.2x107 [2]
JFOM = Ecvsat /2π
(x107 MV/s)
0.8 1.1 2.5 3.2 3.8 - 5
Relative dielectric
constant (ε)
9.7 9 10 5.5 8.5
Electron mobility
(cm2/Vs)
1000 2000 300 2000 800 [3]
BFOM/BFOMSi
(εμEC3)
340 1450 3500 24660 > 26350
Polarization YES YES
UWBG AlGaN:- Extremely high (theoretical) critical breakdown field > 12 MV/cm
1Hudgins et al. IEEE Trans. on 18.3 (2003) 2Farahmand et al. IEEE Trans. on 48.3 (2001) 3Bajaj et al. APL 105.26 (2014)
DRC2016 Sanyam Bajaj: bajaj.10@osu.edu Prof. Siddharth Rajan: rajan.21@osu.edu
UWBG Material Systems
6
Properties 4H-SiC GaN Ga2O3 C AlN
Bandgap (eV) 3.3 3.4 4.9 5.5 6.2
Breakdown Field
(MV/cm)
2.5 3.3 8 10 12-16 [1]
Saturation velocity,
vsat (cm/s)
2x107 2x107 -- ~2x107 ~2.2x107 [2]
JFOM = Ecvsat /2π
(x107 MV/s)
0.8 1.1 2.5 3.2 3.8 - 5
Relative dielectric
constant (ε)
9.7 9 10 5.5 8.5
Electron mobility
(cm2/Vs)
1000 2000 300 2000 800 [3]
BFOM/BFOMSi
(εμEC3)
340 1450 3500 24660 > 26350
Polarization YES YES
UWBG AlGaN:- Extremely high (theoretical) critical breakdown field > 12 MV/cm
- High saturation velocity predicted (Monte Carlo calculations) – more research needed to
confirm experimentally
1Hudgins et al. IEEE Trans. on 18.3 (2003) 2Farahmand et al. IEEE Trans. on 48.3 (2001) 3Bajaj et al. APL 105.26 (2014)
DRC2016 Sanyam Bajaj: bajaj.10@osu.edu Prof. Siddharth Rajan: rajan.21@osu.edu
UWBG Material Systems
7
Properties 4H-SiC GaN Ga2O3 C AlN
Bandgap (eV) 3.3 3.4 4.9 5.5 6.2
Breakdown Field
(MV/cm)
2.5 3.3 8 10 12-16 [1]
Saturation velocity,
vsat (cm/s)
2x107 2x107 -- ~2x107 ~2.2x107 [2]
JFOM = Ecvsat /2π
(x107 MV/s)
0.8 1.1 2.5 3.2 3.8 - 5
Relative dielectric
constant (ε)
9.7 9 10 5.5 8.5
Electron mobility
(cm2/Vs)
1000 2000 300 2000 800 [3]
BFOM/BFOMSi
(εμEC3)
340 1450 3500 24660 > 26350
Polarization YES YES
UWBG AlGaN:- Extremely high (theoretical) critical breakdown field > 12 MV/cm
- High saturation velocity predicted (Monte Carlo calculations) – more research needed to
confirm experimentally
- Superior Johnson’s FOM and Baliga’s FOM – ideal for high power / high temperature / high
frequency applications
1Hudgins et al. IEEE Trans. on 18.3 (2003) 2Farahmand et al. IEEE Trans. on 48.3 (2001) 3Bajaj et al. APL 105.26 (2014)
DRC2016 Sanyam Bajaj: bajaj.10@osu.edu Prof. Siddharth Rajan: rajan.21@osu.edu
Outline
8
• Motivation
• Heterostructure engineered ohmics
• MISFET device operation
DRC2016 Sanyam Bajaj: bajaj.10@osu.edu Prof. Siddharth Rajan: rajan.21@osu.edu
Challenge: Ohmic Contact Formation
9
WBG semiconductors –
1. Low electron affinity
2. Low dopant ionization
• Result in large tunneling barrier
and width for electrons – low
tunneling probability, high RC
EF
ECΦB
EVAC
ΦM χS
Metal Semiconductor
e-
e
Wm b
eT 3
*24 2/1
DRC2016 Sanyam Bajaj: bajaj.10@osu.edu Prof. Siddharth Rajan: rajan.21@osu.edu
Challenge: Ohmic Contact Formation
10
EF
ECΦB
EVAC
ΦM χS
e-
EF
EC
EVAC
ΦM χS
ΔEC
AlGaN channel HEMT
structure: challenging to
achieve alloyed ohmics
• Electron affinity of AlGaN lower
than GaN
• Difficulty in spiking metal through
high Al composition in barrier
layer
WBG semiconductors –
1. Low electron affinity
2. Low dopant ionization
• Result in large tunneling barrier
and width for electrons – low
tunneling probability, high RC
DRC2016 Sanyam Bajaj: bajaj.10@osu.edu Prof. Siddharth Rajan: rajan.21@osu.edu
Previous work in Literature
11
0.3 0.4 0.5 0.6 0.7 0.81E-5
1E-4
1E-3
0.01
0.1
Yafune et al.
IEEE El.Lett.
(2014)
Yafune et al.
JJAP (2011)
Nanjo et al.
APL (2008)
Yafune et al.
JJAP (2011)
C (
.cm
2)
Al composition in AlGaN channel
Previous work on AlGaN channel HEMTs:1) Zr/Al/Mo/Au metal-based alloyed ohmic contacts up to 60% AlGaN channel [Yafune
et al., 2014]
EF
EC
EVAC
ΦM χS
ΔEC
DRC2016 Sanyam Bajaj: bajaj.10@osu.edu Prof. Siddharth Rajan: rajan.21@osu.edu
Previous work in Literature
12
0.3 0.4 0.5 0.6 0.7 0.81E-5
1E-4
1E-3
0.01
0.1
Yafune et al.
IEEE El.Lett.
(2014)
Yafune et al.
JJAP (2011)
Nanjo et al.
APL (2008)
Yafune et al.
JJAP (2011)
C (
.cm
2)
Al composition in AlGaN channel
Previous work on AlGaN channel HEMTs:1) Zr/Al/Mo/Au metal-based alloyed ohmic contacts up to 60% AlGaN channel [Yafune
et al., 2014]
2) Ion-implantation + alloying to achieve ohmic contacts up to 38% AlGaN channel
[Nanjo et al., 2008]
EF
EC
EVAC
ΦM χS
ΔEC
DRC2016 Sanyam Bajaj: bajaj.10@osu.edu Prof. Siddharth Rajan: rajan.21@osu.edu
Previous work in Literature
13
0.3 0.4 0.5 0.6 0.7 0.81E-5
1E-4
1E-3
0.01
0.1
Yafune et al.
IEEE El.Lett.
(2014)
Yafune et al.
JJAP (2011)
Nanjo et al.
APL (2008)
Yafune et al.
JJAP (2011)
C (
.cm
2)
Al composition in AlGaN channel
Previous work on AlGaN channel HEMTs:1) Zr/Al/Mo/Au metal-based alloyed ohmic contacts up to 60% AlGaN channel [Yafune
et al., 2014]
2) Ion-implantation + alloying to achieve ohmic contacts up to 38% AlGaN channel
[Nanjo et al., 2008]
- Contact resistance increased with higher Al composition in channel and barrier layers
EF
EC
EVAC
ΦM χS
ΔEC
DRC2016 Sanyam Bajaj: bajaj.10@osu.edu Prof. Siddharth Rajan: rajan.21@osu.edu
Heterostructure-Engineered Ohmics
14
A A’
High composition
n-AlGaN channel
S DA
A’
EF
EC
EV
AlGaNΦB
• Conduction band profile under the contacts – n-type doped wide
bandgap AlGaN with large Schottky barrier height
DRC2016 Sanyam Bajaj: bajaj.10@osu.edu Prof. Siddharth Rajan: rajan.21@osu.edu
Heterostructure-Engineered Ohmics
15
High composition
n-AlGaN channel
Reverse graded
n++ AlGaN -> GaN
S D
A’
A
EC
A A’
• Contact layer with reverse polarization-grading to GaN
• High doping concentration to compensate negative polarization
charges (reduce sheet resistance of contact layers)
EV
GaN
n++
Polarization grading
ΦB
EF
AlGaN
DRC2016 Sanyam Bajaj: bajaj.10@osu.edu Prof. Siddharth Rajan: rajan.21@osu.edu
Heterostructure-Engineered Ohmics
16
High composition
n-AlGaN channel
Reverse graded
n++ AlGaN -> GaN
S D
A’
A
EC
A A’
• Contact layer with reverse polarization-grading to GaN
• High doping concentration to compensate negative polarization
charges (reduce sheet resistance of contact layers)
• This approach does not require regrowth (challenging for high
composition AlGaN due to surface oxidation / GaN decomposition)
EV
GaN
n++
Polarization grading
ΦB
EF
AlGaN
DRC2016 Sanyam Bajaj: bajaj.10@osu.edu Prof. Siddharth Rajan: rajan.21@osu.edu
Experiment – n-type Al0.75Ga0.25N Channel
17
30nm Al0.75Ga0.25N (UID)
AlN on Sapphire
100nm Al0.75Ga0.25N
Si = 3x1019 cm-3
75%
6%50nm Graded
n++ 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
6
UID
AlGaN
AlNn-Al0.75
Ga0.25
N
Graded AlGaN
EF
EV
En
erg
y (
eV
)
Distance (nm)
AS GROWN:
Contact regionE
C
DRC2016 Sanyam Bajaj: bajaj.10@osu.edu Prof. Siddharth Rajan: rajan.21@osu.edu18
30nm Al0.75Ga0.25N (UID)
AlN on Sapphire
90nm Al0.75Ga0.25N
Si = 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
UID
AlGaN
En
erg
y (
eV
)
Distance (nm)
RECESSED:
Intrinsic region
n-Al0.75
Ga0.25
N AlN
EC
EF
EV
A A’
Experiment – n-type Al0.75Ga0.25N Channel
DRC2016 Sanyam Bajaj: bajaj.10@osu.edu Prof. Siddharth Rajan: rajan.21@osu.edu
XRD and AFM
19
- X-ray diffraction scan to confirm AlGaN channel / graded contact layer
- Atomic-Force Microscopy to confirm smooth surface morphology (as-
grown surface)
rms~1.1 nm
DRC2016 Sanyam Bajaj: bajaj.10@osu.edu Prof. Siddharth Rajan: rajan.21@osu.edu
Contact Resistance using TLM
20
Graded AlGaN
contact layer
30nm Al0.75Ga0.25N
(UID)
AlN on Sapphire
AlGaN channel
Non-alloyed ohmic contacts – Ti/Al/Ni/Au = 20/120/30/50 nm
S D
DRC2016 Sanyam Bajaj: bajaj.10@osu.edu Prof. Siddharth Rajan: rajan.21@osu.edu
Contact Resistance using TLM
21
30nm Al0.75Ga0.25N
(UID)
AlN on Sapphire
AlGaN channel
S D
RC1RC1
RSH
spacing
0 1 2 3 4 5 6 7
4
6
8
10
12
14
SP = 1.4x10-6 .cm2
RC1
= 0.15 .mm
RSH
= 158 /sq
Resis
tance (
ohm
)
Spacing (m)
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
DRC2016 Sanyam Bajaj: bajaj.10@osu.edu Prof. Siddharth Rajan: rajan.21@osu.edu
Contact Resistance using TLM
22
30nm Al0.75Ga0.25N
(UID)
AlN on Sapphire
AlGaN channel
S D
RC1RC1
RSH
spacing
0 1 2 3 4 5 6 7
4
6
8
10
12
14
SP = 1.4x10-6 .cm2
RC1
= 0.15 .mm
RSH
= 158 /sq
Resis
tance (
ohm
)
Spacing (m)
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
30nm Al0.75Ga0.25N
(UID)
AlN on Sapphire
75nm channel
S D
RC1RC1
RSH1 RSH2RSH1
spacing
2 4 6 8 10 12 1420
30
40
50
60
70
80
90
100
110
SP = 1.9x10-6 .cm2
RC1
+RSH1
= 0.32 .mm
RSH2
= 725 /sq
Resis
tance (
ohm
)
Spacing (m)
Cl2-based ICP-RIE etch to test contact to AlGaN channel
DRC2016 Sanyam Bajaj: bajaj.10@osu.edu Prof. Siddharth Rajan: rajan.21@osu.edu
Contact Resistance using TLM
23
30nm Al0.75Ga0.25N
(UID)
AlN on Sapphire
AlGaN channel
S D
RC1RC1
RSH
spacing
0 1 2 3 4 5 6 7
4
6
8
10
12
14
SP = 1.4x10-6 .cm2
RC1
= 0.15 .mm
RSH
= 158 /sq
Resis
tance (
ohm
)
Spacing (m)
• ρSP = 1.9x10-6
Ω.cm2
30nm Al0.75Ga0.25N
(UID)
AlN on Sapphire
75nm channel
S D
RC1RC1
RSH1 RSH2RSH1
spacing
2 4 6 8 10 12 1420
30
40
50
60
70
80
90
100
110
SP = 1.9x10-6 .cm2
RC1
+RSH1
= 0.32 .mm
RSH2
= 725 /sq
Resis
tance (
ohm
)
Spacing (m)
0.3 0.4 0.5 0.6 0.7 0.8
1E-6
1E-5
1E-4
1E-3
0.01
0.1
Yafune et al.
IEEE El.Lett.
(2014)
Yafune et al.
JJAP (2011)
Nanjo et al.
APL (2008)
This work
Yafune et al.
JJAP (2011)
C (
.cm
2)
Al composition in AlGaN channel
Record-low ρSP to Wide bandgap AlGaN > 5 eV (Non-alloyed)
DRC2016 Sanyam Bajaj: bajaj.10@osu.edu Prof. Siddharth Rajan: rajan.21@osu.edu
Outline
24
• Motivation
• Heterostructure engineered ohmics
• MISFET device operation
DRC2016 Sanyam Bajaj: bajaj.10@osu.edu Prof. Siddharth Rajan: rajan.21@osu.edu
Al0.75Ga0.25N Channel MIS-FET
25
30nm Al0.75Ga0.25N (UID)
AlN on Sapphire
7nm n-AlGaN channel
S D
20nm Al2O3
G
- Recessed structure with 7 nm
n-Al0.75Ga0.25N channel
- 20 nm ALD Al2O3 followed by
700°C PDA (30s)
- C-V / doping profile confirmed
donor concentration and channel
thickness – some depletion at 0
bias
25 30 35 400
1
2
3
4
ND (
x1
019cm
-3)
Depletion (nm)
-14 -12 -10 -8 -6 -4 -2 00.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
V (Volts)
C (F
/cm
2)
100 kHz
GS S
D
DRC2016 Sanyam Bajaj: bajaj.10@osu.edu Prof. Siddharth Rajan: rajan.21@osu.edu
Al0.75Ga0.25N Channel MIS-FET
26
0 2 4 6 8 100
10
20
30
40
50L
G = 1 m, L
GD = 2 m
VG = -1 V
I D (
mA
/mm
)
VDS
(V)
VG = 2 V
-6 -4 -2 0 20
2
4
6
8
10
VGS
(V)
gm (
mS
/mm
)
0
10
20
30
40
I D (
mA
/mm
)
VDS
= 10 V30nm Al0.75Ga0.25N (UID)
AlN on Sapphire
7nm n-AlGaN channel
S D
20nm Al2O3
G25 30 35 40
0
1
2
3
4
ND (
x1
019cm
-3)
Depletion (nm)
- IDS_MAX > 40 mA/mm ; gm_MAX = 10 mS/mm (limited by low channel
mobility of 15 cm2/Vs)
- High Si doping needed to overcome defect related compensation –
expect higher mobility with lower dopant concentration
-14 -12 -10 -8 -6 -4 -2 00.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
V (Volts)
C (F
/cm
2)
100 kHz
- Recessed structure with 7 nm
n-Al0.75Ga0.25N channel
- 20 nm ALD Al2O3 followed by
700°C PDA (30s)
- C-V / doping profile confirmed
donor concentration and channel
thickness – some depletion at 0
bias
DRC2016 Sanyam Bajaj: bajaj.10@osu.edu Prof. Siddharth Rajan: rajan.21@osu.edu
Al0.75Ga0.25N Channel MIS-FET
27
0 2 4 6 8 100
10
20
30
40
50L
G = 1 m, L
GD = 2 m
VG = -1 V
I D (
mA
/mm
)
VDS
(V)
VG = 2 V
-6 -4 -2 0 20
2
4
6
8
10
VGS
(V)
gm (
mS
/mm
)
0
10
20
30
40
I D (
mA
/mm
)
VDS
= 10 V30nm Al0.75Ga0.25N (UID)
AlN on Sapphire
7nm n-AlGaN channel
S D
20nm Al2O3
G25 30 35 40
0
1
2
3
4
ND (
x1
019cm
-3)
Depletion (nm)
- IDS_MAX > 40 mA/mm ; gm_MAX = 10 mS/mm (limited by low channel
mobility of 15 cm2/Vs)
- High Si doping needed to overcome defect related compensation –
expect higher mobility with lower dopant concentration
- Low 2-terminal diode breakdown due to high net charge in the channel
(~ 5x1013 cm-2)
-14 -12 -10 -8 -6 -4 -2 00.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
V (Volts)
C (F
/cm
2)
100 kHz
-14-12-10 -8 -6 -4 -2 0 21E-8
1E-7
1E-6
1E-5
1E-4
1E-3
0.01
0.1
1
10
I G (
A/c
m2)
VG (V)
Diode Radius
= 100 m- Recessed structure with 7 nm
n-Al0.75Ga0.25N channel
- 20 nm ALD Al2O3 followed by
700°C PDA (30s)
- C-V / doping profile confirmed
donor concentration and channel
thickness – some depletion at 0
bias
DRC2016 Sanyam Bajaj: bajaj.10@osu.edu Prof. Siddharth Rajan: rajan.21@osu.edu
SUMMARY
28
- Achieved record-low specific contact resistance to UWBG
Al0.75Ga0.25N channel (NON-ALLOYED)
- Heterostructure engineered ohmics to UWBG AlGaN –
polarization-graded + doped contact layers
- 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
0.3 0.4 0.5 0.6 0.7 0.8
1E-6
1E-5
1E-4
1E-3
0.01
0.1
Yafune et al.
IEEE El.Lett.
(2014)
Yafune et al.
JJAP (2011)
Nanjo et al.
APL (2008)
This work
Yafune et al.
JJAP (2011)
C (
.cm
2)
Al composition in AlGaN channel
30nm Al0.75Ga0.25N
(UID)
AlN on Sapphire
75nm channel
S D
RC1RC1
RSH1 RSH2RSH1
spacing
2 4 6 8 10 12 1420
30
40
50
60
70
80
90
100
110
SP = 1.9x10-6 .cm2
RC1
+RSH1
= 0.32 .mm
RSH2
= 725 /sq
Resis
tance (
ohm
)
Spacing (m)
0 2 4 6 8 100
10
20
30
40
50L
G = 1 m, L
GD = 2 m
VG = -1 V
I D (
mA
/mm
)
VDS
(V)
VG = 2 V
top related