You must fed enough oxygen into the chamber.

I agree with Zhu, may be a higher substrate temp. will help.

I also agree with Zhu. Also you can make post annealing at around 500 degree celsius

The deposition temperature for the NIO is very high so e-beam deposition is the best option for NiO thin film deposition. For stoichiometric composition one can try different option like.

 1. You can add desired amount of oxygen during thin film deposition or

2. post deposition annealing treatment at different temperature (300 -500 degree Celsius) in air.