Hi guys. As a test sample, i made the following:

1. P-type Si substrate.
2. Grow a layer of thermal oxide of 5nm thick.
3. Grow a layer of oxide using PECVD of 50nm thick.
4. Deposited a layer of Al using thermal evaporation on top of the PECVD oxide.
5. Did lithography and etching to form patterns in the Al.
6. Deposited a layer of Au using thermal evaporation at the back of the device (below P-type Si substrate).

Then i did C-V characterisation. However, it gave me the low frequency C-V curve even though th frequency used was 100kHz, which was high frequency. By low frequency, it means the graph first had Cmax, dipped to a small value, then increased back to Cmax.

Have any of you encountered such problems before? Any suggestions about what could have been the problem to cause this?