We　develop　in　this　paper　highly　efficient,　second　order　and　unconditionally　energy　stable　schemes　for　the　epitaxial　thin　film　growth　models　by　using　the　scalar　auxiliary　variable　(SAV)　approach.　A　main　difficulty　here　is　that　the　nonlinear　potential　for　the　model　without　slope　selection　is　not　bounded　from　below　so　the　SAV　approach　can　not　be　directly　applied.　We　overcome　this　difficulty　with　a　suitable　splitting　of　the　total　free　energy　density　into　two　parts　such　that　the　integral　of　the　part　involving　the　nonlinear　potential　becomes　bounded　from　below　so　that　the　SAV　approach　can　be　applied.　We　then　construct　a　set　of　linear,　second-order　and　unconditionally　energy　stable　schemes　for　the　reformulated　systems.　These　schemes　lead　to　decoupled　linear　equations　with　constant　coefficients　at　each　time　step　so　that　they　can　be　implemented　easily　and　very　efficiently.　We　present　ample　numerical　results　to　demonstrate　the　stability　and　accuracy　of　our　SAV　schemes.