Transparent Ohmic Contacts to N-polar n-type GaN

1

M. A. Hopkinsa, S. Thornleyb, J. Dutsonb, G. Christmannc,

S. Nicolayc, I. Marozauc, O. Seredac, J. Niemelad, M.

Creatored, J. Ellise, D.W.E. Allsoppa

a. Dept. of Elec. and Electron. Eng., University of Bath, BA2 7AY, UK

b. Plasma Quest Ltd, Osbourne Way, Hook, Hampshire, RG27 9UT, UK

c. CSEM, Rue Jaquet-Droz 1, 2002 Neuchatel, Switzerland

d. Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands

e. Plessey Semiconductors Ltd., Roborough, Plymouth, PL6 7BQ, UK

2

Why transparent contacts to N-face n-GaN?

• Simplify processing and improve efficiency of ‘vertical’ LEDs

Carrier wafer

insulator

Typical structure of a high-

efficiency LED

n-GaN

n-contact

p-contact p-GaNMQW

Substrate

TCO

n-GaN

Carrier wafer

With transparent contact

N-face

Ga-face (ion etched): Ti/Al → Contact resistance (ρc) ≤ 5 x 10-5 Ωcm2

N-face (ion etched): Ti/Al → ρc ≈ 2 – 6 x10-4 Ωcm2 and deteriorates on annealing

3

Samples and Preparation

TCO

Substrate: LED structure,

bonded to carrier

ICP etched to N-face n-GaN

TCO:

Aluminium-doped Zinc Oxide (AZO)

remote-plasma sputtering

atomic layer deposition (ALD)

Surface treatments:

no treatment

Hydrochloric acid

in-situ Ar, H2 and O2 plasma

Carrier wafer

n-GaN

4

Measurement of the contact resistance between TCOs and N-face n-GaN

Contact resistances were

mostly measured with a linear

transmission line structure

Ti/Al/Ni/AuTCO

n-GaN

0,0

1,0

2,0

3,0

4,0

0 5 10 15 20

Resis

tance (

Ω)

Pad separation (μm)

-0,4

-0,3

-0,2

-0,1

0

0,1

0,2

0,3

0,4

-1 -0,8 -0,6 -0,4 -0,2 0 0,2 0,4 0,6 0,8 1

Curr

ent

(A)

Voltage (V)

5

Results: effect of different surface treatments on the contact resistance

-0,3

-0,2

-0,1

0

0,1

0,2

0,3

-1 -0,5 0 0,5 1

Curr

ent

(A)

Voltage (V)

No plasma

Ar plasma

HCl; no plasma

H-plasma

O-plasma

ALD AZO Treatment Contact resistance

ρc (Ωcm2)

No plasma N/A

Ar plasma N/A

HCl/No plasma > 1 x 10-3

H plasma 8 x 10-5

O plasma 3.5 x 10-5

Ti/Al/Ni/Au 2 x 10-4

H-plasma also worked for: AZO by remote plasma sputtering (ρc = 2-8 x 10-5 Ωcm2)

B:ZnO by PECVD

textured (KOH roughened or ion etched) n-GaN

O-plasma didn’t work for: AZO by remote plasma sputtering

Process window for H2 plasma treatment: remote sputtering system

6

Varied

1. Exposure time

2. Plasma power

• Contact resistance is insensitive to exposure time and plasma power

• For both planar and KOH roughened surface

• But with an O-plasma the contacts were non-ohmic for all exposure times and plasma powers tried

0,E+00

1,E-05

2,E-05

3,E-05

4,E-05

5,E-05

6,E-05

7,E-05

0 2 4 6 8 10 12

Con

tact re

sis

tan

ce

(Ω

cm

2)

Time (mins)

0,E+00

1,E-05

2,E-05

3,E-05

4,E-05

5,E-05

6,E-05

7,E-05

0 1 2 3

Con

tact re

sis

tan

ce

(Ω

cm

2)

Power (kW)

= planar

X = KOH roughened

7

H2 plasma treatment:thermal stability of contacts

-0,25

-0,2

-0,15

-0,1

-0,05

0

0,05

0,1

0,15

0,2

0,25

-1 -0,5 0 0,5 1

Curr

ent

(A)

Voltage (V)

RT

150 C

250 C

• Samples annealed in N2 at 150ºC

and 250ºC

• The contact resistance increased

by a factor of 3 to 4

• ρc (RT) ≈ 8 x 10-5 Ωcm2

ρc (150ºC) ≈ 2 to 3 x 10-4 Ωcm2

ρc (250ºC) ≈ 3 to 4 x 10-4 Ωcm2

IV graphs (TL) for H-plasma treated

contacts: as grown and annealed

• Compared favourably to metal

contacts

• Contact resistance is low enough

to use in LEDs

Effect of H-plasma:TEM images of interface

8

H-plasma, no AZO:

Layer with high defect

concentration at surface of

GaN (up to 20nm thick)

No plasma + AZO:

Neither high contrast

layer seen

n-GaN

H-plasma + AZO:

nano-crystalline AZO layer

at the interface

But GaN defect layer not

seen

n-GaN

Protective C

9

Temperature dependence of the IV characteristics and ρc

-0,1

-0,05

0

0,05

0,1

-0,5 -0,3 -0,1 0,1 0,3 0,5

Cu

rre

nt (A

)

Voltage (V)

299K

200K

100K

1,E-05

1,E-04

1,E-03

0 100 200 300 400

Co

nta

ct re

sis

tan

ce

(Ω

cm

2)

Temperature (K)

ALD H plasma

ALD O-plasma

-0,08

-0,06

-0,04

-0,02

0

0,02

0,04

0,06

0,08

-0,5 -0,3 -0,1 0,1 0,3 0,5

Cu

rre

nt (A

)Voltage (V)

100K

200K

299K

H-plasma/higher ρc :

• ρc decreases with temperature

O-plasma/lower ρc:

• ρc almost temperature independent

• consistent with tunnelling mechanism

- defect assisted or narrow space

charge region

ALD H-plasma ALD O-plasma

EF

AZO GaN

Summary

HCl acid clean:

• Contact resistance between AZO and ICP-etched N-face n-GaN of 5x10-3 Ωcm2 → too high for LEDs

H-plasma

• reduced contact resistance to 2-8x10-5 Ωcm2 → suitable for LEDs

• Wide process window

• Contact resistance increased by x3 on annealing at 150ºC

O-plasma

• With ALD contact resistance 3.5x10-5 Ωcm2

• contact resistance is very weakly temperature dependent →consistent with tunnelling, possibly defect assisted or due to enhanced surface doping

10

Acknowledgements

11

The End – thank you for listening

Plasma Quest Ltd,

CSEM,

Eindhoven University of Technology,

Plessey Semiconductors Ltd.

This research has received funding from the European Union’s Horizon

2020 research and innovation programme under grant agreement No.

641864 (INREP - Towards Indium free TCOs)

H2 plasma treatment: comparison betweenGa-face and N-face

12

N-face

-0,3

-0,2

-0,1

0

0,1

0,2

0,3

-1 -0,5 0 0,5 1

Curr

ent

(A)

Voltage (V)

No plasma

H-plasma

ρc = 8 x 10-5 Ωcm2 ρc = 2 - 8 x 10-5 Ωcm2

Ga-face

-0,4

-0,2

0

0,2

0,4

-0,5 -0,3 -0,1 0,1 0,3 0,5

Curr

ent

(A)

Voltage (V)

No plasma

H-plasma

• Plasma clean unnecessary for contacts to the Ga-face

13

Light extraction efficiency

10

Contacts were non-ohmic for all TCOs and deposition techniques (fig. 1)

6

Results 1: No surface treatment

-1,0E-05

-5,0E-06

0,0E+00

5,0E-06

1,0E-05

-0,1 -0,05 0 0,05 0,1

-5,0E-02

0,0E+00

5,0E-02

1,0E-01

1,5E-01

2,0E-01

-2 -1,5 -1 -0,5 0 0,5 1 1,5 2Curr

ent

(A)

Voltage (V)

78.4K

101K

125K

150K

200K

250K

300K

325K

Figure 1: IV characteristics for

AZO deposited by ALD

IV graphs are:

•anti-symmetric at low bias,

•asymmetric at higher biases, current

increases exponentially with bias

•non-saturating in reverse bias

Behaviour often seen in n-n isotype

heterojunctions1

1 Kwok K. Ng, Complete guide to Semiconductor Devices (Wiley-IEEE Press, 2002)

But what is the dominant conduction

mechanism?

15

No surface treatment: temperature dependence of IV characteristics

Gradient of the semi-log IV plot is temperature-independent

1,0E-05

1,0E-04

1,0E-03

0,1 0,2 0,3 0,4

Curr

ent

(A)

Voltage (V)

78.4K

101K

125K

150K

200K

250K

300K

325K

Semi-log IV characteristics for n-GaN/AZO

deposited by ALD

Conduction is NOT due to thermionic

or thermionic field emission

Conduction likely due to tunnelling

through a thin barrier

AZO n-GaN

EC2EC1

EF

Energy band diagram

-5,0E-02

0,0E+00

5,0E-02

1,0E-01

1,5E-01

2,0E-01

-2 -1,5 -1 -0,5 0 0,5 1 1,5 2

Curr

ent

(A)

Voltage (V)

78.4K101K125K150K200K250K300K