Thermoelectric bismuth telluride ($Bi_2Te_3$) films were deposited on $4^{circ}$ off oriented (001) GaAs substrates using a modified metal organic chemical vapor deposition (MOCVD) system. The effects of substrate temperature on surface morphologies, crystallinity, electrical properties and thermoelctric properties were investigated. Two dimensional growth mode (2D) was observed at substrate temperature lower than $400^{circ}C$. However, three dimensional growth mode (3D) was observed at substrate temperature higher than $400^{circ}C$. Change of growth mechanism from 2D to 3D was confirmed with environmental scanning electron microscope (E-SEM) and X-ray diffraction analysis. Seebeck coefficients of all samples have negative values. This result indicates that $Bi_2Te_3$ films grown by modified MOCVD are n-type. The maximum value of Seebeck coefficient was -225 ${mu}V/K$ and the power factor was $1.86{ imes}10^{-3};W/mK^2$ at the substrate temperature of $400^{circ}C$. $Bi_2Te_3$ films deposited using modified MOCVD can be used to fabricate high-performance thermoelectric devices.