Heteroepitaxy of high-quality Ge on Si by nanoscale seed pads grown through a SiO_2 interlayer

摘要

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 SiO_2 film is produced on Si in a H_2O_2 and H_2SO_4 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 SiO_2, 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-SiO_2 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 10~6 cm~(-2) and threading dislocations (TD) far below 10~6 cm~(-2). The SF and TD propagating to the surface form etch pits, when immersed in a solution containing HF, HNO_3, glacial acetic acid, and I_2. The total EPD, as a statistically more reliable estimate on SF and TD than the plan-view TEM, is consistently less than 2xl0~6 cm~(-2), where SFs constitute 99 %, and TDs constitute 1 %. That is, the TD density is ～10~5 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 (～10~(11) cm~(-2)) of nanoscale Ge seed pads interspaced by 2- to 12-nm-wide SiO_2 patches and (2) the SiO_2 patches serving as artificially introduced dislocation centers. Burgers circuit around each SiO_2 patch results in b = (1/2)[210]. We have also determined that the surface mechanism responsible for the selective growth of Ge on Si over SiO_2 is the high desorption rate of Ge adspecies based on their low desorption activation energy of 42 ± 3 kJ/mol.