Growing a lattice-mismatched, dislocation-free epitaxial film on Si has been a challenge for many years. Herein, we exploit nanoscale heterojunction engineering to grow high-quality Ge epilayer on Si. A 1.2-nm-thick chemical SiO2 film is produced on Si in a H2O2 and H2SO4 solution. When the chemically oxidized Si substrate is exposed to Ge molecular beam, relatively uniform-size nanoscale seed pads form in the oxide layer and "touchdown" on the underlying Si substrate. Although the touchdown location is random, the seed pad growth is self-limiting to 7 nm in size. Upon continued exposure, Ge selectively grows on the seed pads rather than on SiO2, and the seeds coalesce to form an epilayer. The Ge epilayer is characterized by high-resolution, cross-sectional as well as plan-view transmission electron microscopy, Raman spectroscopy, and etch-pit density (EPD). The cross-sectional TEM images reveal that the Ge epilayer is free of dislocation network and that the epilayer is fully relaxed at 2 nm from the heterojunction. The Raman shift of Ge optical phonon mode exactly matches that of relaxed bulk Ge, further supporting that the epilayer is fully relaxed. The cross-sectional TEM images, however, show that stacking faults exist near the Ge-SiO2 interface. A small fraction (~4x10-3%) of these stacking faults propagate to the epilayer surface. The plan-view TEM sampling provides an estimate on the density of stacking faults (SF) at approximately 106 cm-2 and threading dislocations (TD) far below 106 cm-2. The SF and TD propagating to the surface form etch pits, when immersed in a solution containing HF, HNO3, glacial acetic acid, and I2. The total EPD, as a statistically more reliable estimate on SF and TD than the plan-view TEM, is consistently less than 2x106 cm-2, where SFs constitute 99 %, and TDs constitute 1 %. That is, the TD density is ~105 cm-2 as a conservative upper bound. The reduction of strain density near the Ge-Si heterojunction, leading to high quality Ge epilayer, is attributed to (1) a high density (~1011 cm-2) of nanoscale Ge seed pads interspaced by 2- to 12-nm-wide SiO2 patches and (2) the SiO2 patches serving as artificially introduced dislocation centers. Burgers circuit around each SiO2 patch results in b = (1/2). We have also determined that the surface mechanism responsible for the selective growth of Ge on Si over SiO2 is the high desorption rate of Ge adspecies based on their low desorption activation energy of 42 ± 3 kJ/mol.