Transparent Ohmic Contacts to N-polar n-type GaN M. A. Hopkins a, S. Thornley b, J. Dutson b, G. Christmann c, S. Nicolay c, I. Marozau c, O. Sereda c, J. Niemela d, M. Creatore d, J. Ellis e, D.W.E. Allsopp a 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 1
Why transparent contacts to N-face n-gan? Simplify processing and improve efficiency of vertical LEDs Substrate p-contact insulator MQW n-gan p-gan N-face n-gan TCO n-contact Carrier wafer Carrier wafer Typical structure of a highefficiency LED With transparent contact Ga-face (ion etched): Ti/Al Contact resistance (ρ c ) 5 x 10-5 Ωcm 2 N-face (ion etched): Ti/Al ρ c 2 6 x10-4 Ωcm 2 and deteriorates on annealing 2
Samples and Preparation Substrate: LED structure, bonded to carrier ICP etched to N-face n-gan Surface treatments: no treatment Hydrochloric acid in-situ Ar, H 2 and O 2 plasma TCO Carrier wafer n-gan TCO: Aluminium-doped Zinc Oxide (AZO) remote-plasma sputtering atomic layer deposition (ALD) 3
Resistance (Ω) Measurement of the contact resistance between TCOs and N-face n-gan Contact resistances were mostly measured with a linear transmission line structure Current (A) 0,4 0,3 0,2 0,1 0-1 -0,8-0,6-0,4-0,2-0,1 0 0,2 0,4 0,6 0,8 1-0,2-0,3-0,4 TCO n-gan Ti/Al/Ni/Au 4,0 3,0 2,0 1,0 0,0 0 5 10 15 20 Pad separation (μm) 4
Current (A) Results: effect of different surface treatments on the contact resistance 0,3 ALD AZO Treatment Contact resistance 0,2 ρ c (Ωcm 2 ) 0,1 No plasma N/A 0-1 -0,5 0 0,5 1-0,1-0,2-0,3 No plasma Ar plasma HCl; no plasma H-plasma O-plasma 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 (ρ c = 2-8 x 10-5 Ωcm 2 ) B:ZnO by PECVD textured (KOH roughened or ion etched) n-gan O-plasma didn t work for: AZO by remote plasma sputtering 5
Contact resistance (Ωcm 2 ) Contact resistance (Ωcm 2 ) Process window for H 2 plasma treatment: remote sputtering system 7,E-05 6,E-05 5,E-05 4,E-05 3,E-05 2,E-05 1,E-05 = planar X = KOH roughened Varied 1. Exposure time 2. Plasma power 0,E+00 0 2 4 6 8 10 12 Time (mins) Contact resistance is insensitive to exposure time and plasma power 7,E-05 6,E-05 5,E-05 4,E-05 3,E-05 2,E-05 1,E-05 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 0 1 2 3 Power (kw) 6
Current (A) H 2 plasma treatment: thermal stability of contacts 0,25 0,2 0,15 0,1 0,05 0-1 -0,5-0,05 0 0,5 1-0,1-0,15-0,2-0,25 RT 150 C 250 C IV graphs (TL) for H-plasma treated contacts: as grown and annealed Samples annealed in N 2 at 150ºC and 250ºC The contact resistance increased by a factor of 3 to 4 ρ c (RT) 8 x 10-5 Ωcm 2 ρ c (150ºC) 2 to 3 x 10-4 Ωcm 2 ρ c (250ºC) 3 to 4 x 10-4 Ωcm 2 Compared favourably to metal contacts Contact resistance is low enough to use in LEDs 7
Effect of H-plasma:TEM images of interface Protective C n-gan n-gan H-plasma, no AZO: Layer with high defect concentration at surface of GaN (up to 20nm thick) H-plasma + AZO: nano-crystalline AZO layer at the interface But GaN defect layer not seen No plasma + AZO: Neither high contrast layer seen 8
Current (A) Contact resistance (Ωcm 2 ) Temperature dependence of the IV characteristics and ρ c ALD H-plasma 0,1 299K 200K 0,05 100K ALD O-plasma 0,08 100K 0,06 200K 0,04 299K 0,02 AZO GaN 0-0,5-0,3-0,1 0,1 0,3 0,5-0,05-0,1 Current (A) 0-0,5-0,3-0,1-0,02 0,1 0,3 0,5-0,04-0,06-0,08 E F 1,E-03 1,E-04 1,E-05 ALD H plasma 0 100 200 300 400 ALD O-plasma H-plasma/higher ρ c : ρ c decreases with temperature O-plasma/lower ρ c : ρ c almost temperature independent consistent with tunnelling mechanism - defect assisted or narrow space charge region Temperature (K) 9
Summary HCl acid clean: Contact resistance between AZO and ICP-etched N-face n-gan of 5x10-3 Ωcm 2 too high for LEDs H-plasma reduced contact resistance to 2-8x10-5 Ωcm 2 suitable for LEDs Wide process window Contact resistance increased by x3 on annealing at 150ºC O-plasma With ALD contact resistance 3.5x10-5 Ωcm 2 contact resistance is very weakly temperature dependent consistent with tunnelling, possibly defect assisted or due to enhanced surface doping 10
Acknowledgements 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) The End thank you for listening 11
Current (A) H 2 plasma treatment: comparison between Ga-face and N-face Ga-face 0,4 N-face 0,3 Current (A) 0,2 0-0,5-0,3-0,1 0,1 0,3 0,5-0,2-0,4 No plasma H-plasma 0,2 0,1 0-1 -0,5 0 0,5 1-0,1-0,2-0,3 No plasma H-plasma ρ c = 8 x 10-5 Ωcm 2 ρ c = 2-8 x 10-5 Ωcm 2 Plasma clean unnecessary for contacts to the Ga-face 12
Light extraction efficiency 10 13
Current (A) Results 1: No surface treatment Contacts were non-ohmic for all TCOs and deposition techniques (fig. 1) 78.4K 101K 125K 150K 200K 250K 300K 325K 2,0E-01 1,5E-01 1,0E-01 5,0E-02 0,0E+00-2 -1,5-1 -0,5 0 0,5 1 1,5 2-5,0E-02 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 heterojunctions 1 1,0E-05 5,0E-06 0,0E+00-0,1-0,05 0 0,05 0,1 But what is the dominant conduction mechanism? -5,0E-06-1,0E-05 1 Kwok K. Ng, Complete guide to Semiconductor Devices (Wiley-IEEE Press, 2002) 6
Current (A) Current (A) No surface treatment: temperature dependence of IV characteristics 78.4K 101K 125K 150K 200K 250K 300K 2,0E-01 1,5E-01 1,0E-01 5,0E-02 0,0E+00-2 -1,5-1 -0,5 0 0,5 1 1,5 2-5,0E-02 Conduction is NOT due to thermionic or thermionic field emission Conduction likely due to tunnelling through a thin barrier AZO n-gan 1,0E-03 1,0E-04 1,0E-05 0,1 0,2 0,3 0,4 78.4K 101K 125K 150K 200K 250K 300K 325K E C1 Energy band diagram E C2 E F Semi-log IV characteristics for n-gan/azo deposited by ALD Gradient of the semi-log IV plot is temperature-independent 15