Your squats aren't bad, but you do need to go a bit deeper. I do see a potential cause for your back pain, however. You are going into overextension, especially on your ascent. Watch how your back moves as you squat. On the way down, you arch a little more and then on the way up, you arch more yet. The cure will be to keep your back still. This will mean your chest will be pointed towards the floor more than it is now. See if that helps. Also, definitely get yourself some lifting shoes. You will like em.