High　quality　GaN　films　on　SiC　with　low　thermal　boundary　resistance　(TBR)　are　achieved　by　employing　an　ultrathin　low　Al　content　AlGaN　buffer　layer.　Compared　with　the　conventional　thick　AlN　buffer　layer,　the　ultrathin　buffer　layer　can　not　only　improve　the　crystal　quality　of　the　subsequent　GaN　layer　but　also　reduce　the　TBR　at　the　GaN/SiC　interface　simultaneously.　The　ultrathin　AlGaN　buffer　layer　is　introduced　by　performing　a　pretreatment　of　the　SiC　substrate　with　trimethylaluminum　followed　by　the　growth　of　GaN　with　an　enhanced　lateral　growth　rate.　The　enhanced　lateral　growth　rate　contributes　to　the　formation　of　basal　plane　stacking　faults　(BSFs)　in　the　GaN　layer,　where　the　BSFs　can　significantly　reduce　the　threading　dislocation　density.　We　reveal　underling　mechanisms　of　reducing　TBR　and　dislocation　density　by　the　ultrathin　buffer　layer.　We　propose　this　work　is　of　great　importance　toward　the　performance　improvement　and　cost　reduction　of　higher　power　GaN-on-SiC　electronics.