Ohmic contacts formation on AlGaN/GaN HEMTs by ...Trade off AlGaN thickness R C 0 3E+12 6E+12 9E+12...
Transcript of Ohmic contacts formation on AlGaN/GaN HEMTs by ...Trade off AlGaN thickness R C 0 3E+12 6E+12 9E+12...
Ohmic contacts formation on AlGaN/GaN HEMTs by introducing
uneven AlGaN layer structuresK. Tsutsui, M. Kamiya, Y. Takei,K. Kakushima,
H. Wakabayashi, Y. Kataoka, and H. Iwai
Tokyo Intitute of Technology
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Contact resistance: Rcon AlGaN/GaN HEMTs
AlGaN
GaN
RC
Rc= Rc2DEG+RcAlGaN+RcMetal/AlGaN
RcAlGaN
Rc2DEG
RC
metal
2DEG
Decrease in Rc on HEMTs are significant for low-loss operations.
Resistance of AlGaNlayer is an origin of Rc.
Thin AlGaN layer is better??
RcMetal/AlGaN
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2DEG concentration depending on AlGaN layer thickness
AlGaN
GaN
0
3E+12
6E+12
9E+12
1.2E+13
1.5E+13
0 10 20 30 40
2DEG
conc
. [10
12cm
-2]
AlGaN thickness [nm]
0
6
3
9
122DEG
Al0.3Ga0.7N
AlGaN layer:Thin AlGaN layer:Thick
2DEG
Met
al AlG
aN
GaN
Ev
Ec
EF
Met
al
AlG
aN
GaN
Ev
Ec2DEG
EF
PSP
PSP
PPE [000
1]
polarizedcharge
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Dependence of AlGaN thickness on Rc
AlGaN
GaN
AlGaN
GaN
RC
Trade off AlGaN thickness
RC
0
3E+12
6E+12
9E+12
1.2E+13
1.5E+13
0 10 20 30 40
2DEG
conc
. [10
12cm
-2]
AlGaN thickness [nm]
0
6
3
9
12
15
2DEG
Al0.3Ga0.7N
RC
Rc, RcA
lGaN , Rc
2DEGRc2DEG
RcAlGaN
Rc=Rc2DEG+RcAlGaN
RcAlGaN
Rc2DEG
AlGaN layer:Thin AlGaN layer:Thick
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Introduction of uneven structures on AlGaN layers
GaN
AlGaN
2DEG
metal
ThickThin
Increase in contact area Expansion of
2DEG under thin AlGaN regions
Metal edge close to high-density 2DEG
Expected effects
uneven structures under contact metals
Fringing effects
2DEG
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Purpose of this work
GaN
metal
AlGaN
2DEG
Effects of introducing uneven structures on AlGaN layers to reduce contact resistances are discussed.
A new approach to overcome the trade off relationship on AlGaN layer thickness.
- Simulation of horizontal distribution of 2DEG concentration under the uneven structure.
- Fabrication of the strucutres to indicate advantage of this method.
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Simulation of 2DEG concentration under uneven AlGaN layers
i-GaN
metal
500 nm
i-Al0.3Ga0.7N
2DEG
Model to calculate lateral distribution of 2DEG concentration
5 or 10 nm
25 nm
40 nm
Polarization charges are assumed to be determined by AlGaNthickness at each horizontal position.
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Lateral distribution of 2DEG concentration under uneven structures
2DEG
GaN
AlGaN
0.0E+00
2.0E+12
4.0E+12
6.0E+12
8.0E+12
1.0E+13
1.2E+13
180 190 200 210 220
2DEG
conc
entra
tion
[1012
cm-2
]
Position [nm]0 10 20-10-20
0
2
4
6
8
10
12
thickness10/25 nm
thickness5/25 nm
2DEGi-GaN
20 nm20 nm
5 or 10 nm25 nm
40 nmpitch
i-Al0.3Ga0.7N
flat AlGaN: 25 nm
flat AlGaN: 10 nm
flat AlGaN: 5 nm
Increase in 2DEG concentration at
pattern edges
metal
Metal closes to 2DEG with higher concentration at pattern edges.
Low contact resistance is expected.
rectangular structure
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2DEG distribution:Dependence of pit width
0.0E+00
2.0E+12
4.0E+12
6.0E+12
8.0E+12
1.0E+13
1.2E+13
160 170 180 190 200 210 220 230 240
2DE
G c
once
ntra
tion
[1012
cm-2
]
Position [nm]0 10 20-10-20 30 40-30-40
10 nm 5 nm
10 nm20 nm
pit widthflat AlGaN: 25 nm
flat AlGaN: 5 nm0
2
4
6
8
10
12
i-Al0.3Ga0.7N
2DEG
5/10/20 nm
25 nm
40 nmperiod
i-GaN5 nm
Higher concentration for narrower pit width
Lower contact resistance is expected.
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Various structures: triangular and domed
triangular
i-GaN
2DEG
metal
500 nm
i-Al0.3Ga0.7N
i-GaN
metal
500 nm
i-Al0.3Ga0.7N
2DEG
domed25 nm
5 or 10 nm
40 nm 40 nm
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2DEG distribution for various uneven structures
0.0E+00
2.0E+12
4.0E+12
6.0E+12
8.0E+12
1.0E+13
1.2E+13
160 170 180 190 200 210 220 230 240
2DE
G c
once
ntra
tion
[1012
cm-2
]
Position [nm]0 10 20-10-20 30-30 40-40
0
2
4
6
8
10
12 25 nm
5 nm
20 nm 20 nm25 nm 5 nm
20 nm5 nm25 nm 20 nm
5 nm25 nm
40 nmpitch
i-Al0.3Ga0.7N i-GaN
Domed
Triangular
Rectangular
AlGaN
2DEG
GaN
metal
Higher 2DEG concentrations were obtained for the triangular and doomed structures.
Advantage:Higher minimum 2DEG concentrations
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Experiments: Fabrication of contacts with uneven AlGaN layers
Fabrication of TLM test structures.
The uneven structures of AlGaN layers were formed by BCl3+Cl2 RIE using photo lithography.
TiN/TiSi2 non-alloy type ohmic contacts were employed.12
Patterns of pit formation
current flow 5 μm
5 μm
5 μm 10 μm
5 μm
10 μ
m
Region of thin AlGaN
5 μm 5 μm
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Flat (not uneven) contacts with various AlGaN thicknesses
GaN 2DEG
AlGaN 30 nmTiN/TiSi2 flat AlGaN layer with
5 ~ 30 nm in thickness
1.E-04
1.E-03
1.E-02
1.E-01
0 5 10 15 20 25 30
Con
tact
resi
stan
ce [Ω
cm2 ]
AlGaN thickness [nm]
950100010501100
10 nm even AlGaN@1000oC
Anneal temperature10-1
10-2
10-3
10-4
Trade-off relationship on AlGaN thickness was experimentally observed.
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Contact resistances of contacts with uneven AlGaN layers
GaN 2DEG
AlGaN30 nm 20 nm 5 nm
TiN/TiSi2
Lowering of contact resistance by introduction of the uneven AlGaN layer was experimentally observed.
0
2
4
6
25
30
Con
tact
resi
stan
ce [Ω
mm
]
Pattern
10nm-even uneven structures
@1000oC
TLM arrangement
Con
tact
resi
stan
ce [Ω
mm
]
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Conclusion
Effects of the uneven AlGaN layer structures on Rc were studied.
Fringing effects for lateral distribution of 2DEG concentration.
- Concentration of 2DEG under the regions where AlGaN thickness was thin was increased by this effect.
- Triangular and domed structures were more desirable than the rectangular structures.
Lowering of Rc by using the uneven AlGaN layer structures.
This new approach will be a promising technique for low resistive contact formation on AlGaN/GaN HEMTs by overcoming the inherent trade-off relation ship relating to the AlGaN barrier layers.
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