Hi,
Use thinner Ni (if possible) and/or slower deposition rate.
Ni has a lot of internal stress, and often doesn't still well
to GaN.  For your reference, we deposit up to 1000A Ni on GaN
at about 3-5 A/sec and do not have any problems.

Bill

>I am using e-beam to deposit Ni(1.5kA) on GaN. My Chamber
pressure is below
>1e-6 and my deposition rate is below 10A/s. I have done the
tape test on the
>sample and found out that my metal is not sticking very well
to the sample.