Can I build a deck close to the ground?
Joist hangers would be fine. You can bury concrete post piers to act as your actual piers as well. Do try to leave at least a few inches of clearance though th facilitate air flow so it doesn't get too soggy underneath. 

But when building a deck that low, I'd suggest you also consider a patio, instead. A stone, concrete or tile patio will outlive any wooden deck.