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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