Q: Why ITO films have very high sheet resistance ?

I have some r.f sputtered ITO samples, but the sheet resistance are very high.(about mega Ohms/sq)

Here's the deposition parameters and conditions:

1.substrate-target distance: 90 mm

2.base pressure: 2*10^-6 torr

3.working pressure: 2.1*10^-3 torr

4.RF powers: 50W

5.Ar/O2 flow ratio: 20/0 sccm

6.substrate rotation speed: 30 rpm

7.film thickness : 180~200 nm

The dark yellow or green color in films.

A1: Adding oxygen to the sputtering atmosphere may help. We use 40/1 Ar/O2 flow rate. 

A2: Oxygen content in the ITO determines whether it will behave as a ceramic or as a metal. I would also suggest to add oxygen and play with the

ratio Ar/O2 untill you have good resistance. Since pumping speeds and therefore residence times are different for different vacuum systems you need

to optimize this for your particular system. 

A3: Suggest to introduce O2 during the deposition. The ratio you can try in the first place could be 20:1 (Ar:O2). You have to play with it. Adding O2

greatly help as far as transparency is concerned. 

To improve the sheet resistance, you have anneal the sample afterward. You can anneal it ambient conditions or in vacuum or inside the glove box although annealing in ambient conditions is preferred because of the presence of O2 in the atmosphere. Temperature should be around 275-300 C.

ITO film with thickness close to 100nm should give you a sheet resistance of ~30 Ohm/sq with >80% average transparency. If not, there is room for

further optimization of the deposition conditions. Although commercial ITO films are better than this. 

Comment: Instead of using RF sputtering you can also try DC sputtering. In my case, DC gave better results (reasons not sure)