Perhaps it would help if we knew what you wanted to improve on what you can get from an iphone 6.

If you just want better sound, you might want to try a Zoom attachment such as the IQ6 with your iphone.

I've used a Zoom H5 for video shot on a Nexus 5 and it made a world of difference.  That setup's quite a bit more complicated than attaching an IQx to an iphone, but worth the trouble for me.