Indium-free Transparent Ohmic Contacts to N-polar n-type GaN

Transcription

1 Indium-free 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, 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

2 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 Structure with TCO 2

3 Background: Metal contacts to n-gan Ga-face Ohmic contacts easy to make by depositing low work function metals (Ti/Al/Ni/Au) onto plasma etched the surface (or annealing in N 2 ) ρ c 8 x 10-5 Ωcm 2 (without any surface cleaning) Attributed to formation of a heavily doped n+ surface layer N-face Contact resistance typically x10 higher and non-ohmic We measured ρ c 2 6 x10-4 Ωcm 2 for Ti/Al/Ni/Au contacts Unstable: deteriorates on annealing Stutzmann et. al. Phys.stat.sol.(b) 228, No. 2, (2001) 3

4 Samples and Preparation TCO Substrate: LED structure, bonded to carrier ICP etched to N-face n-gan Carrier wafer n-gan Surface treatments: no treatment Hydrochloric acid in-situ Ar, H 2 and O 2 plasma H 2 plasma + vacuum break TCOs: Aluminium-doped Zinc Oxide (AZO) remote-plasma sputtering atomic layer deposition (ALD) Boron-doped Zinc Oxide (ZnO:B) LPCVD 4

5 Resistance (Ω) Measurement of the contact resistance between TCOs and N-face n-gan Contact resistances were measured with a linear transmission line structure Current (A) 0,4 0,3 0,2 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 Voltage (V) TCO n-gan Ti/Al/Ni/Au 4,0 3,0 2,0 1,0 0, Pad separation (μm) 5

6 Current (A) Results: effect of different surface treatments on the contact resistance 0,3 0,2 0,1 Treatment No plasma Contact resistance ρ c (Ωcm 2 ) N/A ,5 0 0,5 1-0,1-0,2-0,3 Voltage (V) No plasma Ar plasma HCl; no plasma H-plasma O-plasma Ar plasma N/A HCl/No plasma 5 x 10-3 O plasma 3.5 x 10-5 H plasma 8 x 10-5 Ti/Al/Ni/Au 2-6 x 10-4 IV graphs (TL) for ALD AZO to N-face n- GaN contacts with different surface treatments H-plasma also worked for: AZO by remote plasma sputtering (ρ c = 2-8 x 10-5 Ωcm 2 ) B:ZnO by LPCVD O-plasma didn t work for: AZO by remote plasma sputtering 6

7 Contact resistance (Ωcm 2 ) Contact resistance (Ωcm 2 ) Process window for H 2 plasma treatment: remote sputtering system 3,E-05 2,E-05 2,E-05 1,E-05 5,E-06 0,E Power (kw) Contact resistance vs plasma power 3,E-05 Varied: 1. Exposure time 2. Plasma power 3,E-05 2,E-05 2,E-05 Contact resistance is insensitive to exposure time and plasma power However, with an O-plasma the contacts were non-ohmic for all exposure time and plasma powers tried 1,E-05 5,E-06 0,E Time (mins) Contact resistance vs exposure time 7

8 Current (A) H 2 plasma treatment: thermal stability of contacts 0,25 0,2 0,15 0,1 0, ,5-0,05 0 0,5 1-0,1-0,15-0,2-0,25 Voltage (V) RT 150 C 250 C IV graphs (TL) for H-plasma treated contacts: as grown and annealed Samples were annealed in N 2 for 5 minutes at 150ºC and 250ºC The contact resistance increased by a factor of 3 ρ c (RT) 8 x 10-5 Ωcm 2 ρ c (250ºC) 2.5 x 10-4 Ωcm 2 Compared favourably to metal contacts Contact resistance is low enough to use in LEDs TCO layer could be used under metal contacts 8

9 Contact resistance (Ωcm 2 ) Current (A) What are the conduction mechanisms in these contacts? Temperature dependence of IV characteristics and ρ c 1,E-03 ALD H plasma ALD O-plasma 299K 1,E K 100K 5,E-02 1,E-04 0,E+00-0,5-0,3-0,1 0,1 0,3 0,5-5,E-02 1,E Temperature (K) Temperature dependence of ρ c for different pre-treatments -1,E-01 Voltage (V) IV graphs for AZO deposited by ALD with a H-plasma treatment E C1 qv f E C2 E F Results for O-plasma are inconsistent with thermionic (field) emission theory 10 9

10 Summary Contact resistance between AZO and ICP-etched N-face n-gan of 5x10-3 Ωcm 2 is too high for use in LEDs Exposure to an in-situ H-plasma prior to deposition of the AZO reduces the contact resistance 2-8x10-5 Ωcm 2 suitable for use in LEDs Effective for deposition by ALD, sputtering and LPCVD Contact resistance decreases with increasing temperature, consistent with thermionic (field) emission theory An in-situ O-plasma can reduce the contact resistance even further With an O-plasma treatment the contact resistance is only very weakly temperature dependant. The End thank you for listening July