Hello Pawan,

to get an Ohmic contact, the contact potential difference between the 2 materials must be low. Normally, in technological processes, Au-wires are used for contacting an element. The direct contact with Au gives a Schottky-barrier. To avoid this, a step of materials with increasing contact potentials is used. Titanium has the best differnce and gives low contact differences. Al with Ti gives an Ohmic contact too. Both materials are highly affin to oxygen, so an additional layer must be used. Furthermore, you want to realize a mechanical stable contact like Ni. Therefore in the next step Ni is used (if you want to investigate your structure in an magnetic field, you must avoid magnetic contacts but normally Ni is a usual material for the must purposes.) The upper layer is Au . It is chemically stable.

We use in our Lab the combination Ti/Al and bond with Al/Si wires. This gives sufficiently well Ohmic contacts, but not if you use Au-wires.

With Regard

R. Mitdank

Thank You, Dear Rudiger

Role of every metal in ohmic contact:

You can divide these ohmic contacts into four layers.

1. Ti = Contact layer

Role: Limit the upper metal diffusion, Reaction with GaN interface (TiN/TaN) are thermodynamically favored, Out-diffusion of N-atoms from GaN leads to the formation of nitrogen vacancies which act as donors and increase the net carriers' concentrations below interface, Upon annealing, Ti based contact has the advantage to reduce the possible native oxide (Gallium Oxide) formed at the GaN interface.

2. Al = Over layer

Role: Form low work function compounds with the other materials.

3. Ni = Barrier layer

Role: Limits the inter-diffusion of the first two layers and also upper forth layers, High melting point (> 1400 ° C) , stabilize the multilayer during annealing.

4. Au = Cap layer

Role: Protective layer to minimize or prevent the oxidation of the underlying metals.

At 750°C ,intermixing of the original metal layer occurred. Forming Al-Ni & Al-Au. A thin layer of (AlAu4) observed. On the other hand, at 850°C, a non-uniform TiN layer at the interface observed. Some regions, this layer penetrate directly to the AlGaN barrier layer and directly contacting the 2DEG at the AlGaN interface. Therefore contact resistance improved at 850°C because it is better to metal-2DEG formation rather than tunneling.

Thank you so much, Surjit