Ir/Au Ohmic Contacts on Bulk, Single-Crystal n-Type ZnO
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1000-L06-17
Ir/Au Ohmic Contacts on Bulk, Single-Crystal n-type ZnO J.S.
Wright1,
R. Khanna1, L. Stafford1, B.P. Gila1, D.P. Norton1, S.J. Pearton1, F. Ren2, I.I.Kravchenko3 of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611 2Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611 3Department of Physics, University of Florida, Gainesville, Florida 32611
1Department
Introduction
Device Characteristics
Purpose: Need for reliable, thermally stable Ohmic metallizations for ZnO. Because of difficulties with current common contact metals, there exists a heavy desire for metals with high melting temperatures and low reactivities.
A specific contact resistivity of ~5x10-5 Ω.cm2 was achieved for all contacts from as-deposited up to 900 ºC anneals. This relatively low specific contact resistivity suggests the Ir/ZnO interface is highly thermally stable. After annealing at 1000 ºC, the minimum specific contact resistivity drops to 3.6x10-6 Ω.cm2. After annealing at 1100 ºC the contact morphology degraded and electrical measurements were no longer consistent as agglomerates (likely Iridium) collected at the surface. The sheet resistance remained at ~5-7 Ω/ throughout all anneals.
400
800
2
1000
Annealing Temperature (C)
A comparison of the depth profiles for as-deposited (top left) versus annealed at 500 ºC (top right), 800 ºC (bottom left), or 1000 ºC (bottom right) Ir/Au contacts on ZnO is given in Figure 3. Ir appears to diffuse outward toward the surface while Au appears to diffuse inward toward the interface beginning after 500 ºC anneals. These results show better thermal stability as compared to typical Ti/Al/Pt/Au contacts which showed complete intermixing after annealing at just 600ºC. After the 1000 ºC anneal, both the Ir from the interface and the Au from the surface layer show heavy interdiffusion. In contrast to previous boride contacts, there is no evidence of a thin Iridium oxide layer or dissociation of ZnO at the interface, likely ruling out Ohmic conduction by the formation of oxygen vacancies or by a heavily doped near-surface region, respectively. Au
90
Au
90
60 Zn O
50 40 30 20 C
10 0
70 60 50
O
40 30 20 10
C
00
100 200 300 400 500 600 700 800
2
6. Depth profiles taken using AES
Zn
Au
Atomic Concentration (%)
70 Zn
50
O
40 30 20 10 0
Both specific contact resistivity and sheet resistance increase with annealing temperature under the O2 anneal by orders of magnitude while the N2 annealed contacts remain thermally stable up to 1000°C. An increase of barrier height on the n-type material might be a likely cause of the change in contact resistance. AES scans confirmed a higher percentage of Oxygen on the surface of contacts annealed in O2 as compared to those annealed in an N2 ambient, supporting the existence of an IrO2 layer after annealing in O2. 1
100 200 300 400 500 600 700 800
0.01 1E-3 1E-4 0
Sputter Depth (Å )
Fig 5. 800°C anneal
200
60
Zn
50 O
40
600
800
Sputter Depth (Å )
Fig 6. 1000°C
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