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

1

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

2

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

3

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

4

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

5

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.

6

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.

7

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

8

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.

9

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

10

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

11

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

13

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.

14

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

]

15

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.

16