Heteroepitaxy of Ge on Si(001) with pits and windows transferred from free-standing porous alumina mask

Abstract

This paper reports the use of ultrathin free-standing porous alumina membrane (PAM) in pattern transferring for selective epitaxial growth (SEG) of Ge dots and films on Si. PAM, as a large-scale, controllable and lithography-free mask, can transfer nanopatterns onto Si without introducing any contaminants. High-density Ge dots are achievable with Ge adatoms confined in Si pits transferred from PAM. High-quality Ge films can also be grown on Si substrates through SiO2 nano-windows. In this work, 80 and 60 nm pore sizes of PAM were transferred to 70, 50 and 20 nm windows for comparison. For the former two sizes, over-etching of Si beneath every SiO2 window forms epi-seeds to improve intermixing of Ge-Si. No threading dislocations can be observed emanating from the epi-seeds due to the decreased lattice mismatch. An innovative shadow-etching technique utilizing the aspect ratio of PAM further decreased the lateral dimension of patterns from 60 to 20 nm. Cross-sectional transmission electron microscopy images show that the selective epitaxial Ge films grown from a 20 nm-width interface are defect free, which is attributed to the exponential decay of strain energy as well as Ge-Si intermixing.

Germanium epitaxy on silicon

“…In the author's lab, a defect density comparison has been made by growing a Ge film on bare Si substrates and on a 70 nm-diameter Si pit with SiO 2 windows; the result is shown in the transmission electron microscopy (TEM) image of figure 4 [28]. Ge grown on planar Si substrates with the typical two-step process has a threading density larger than 10 8 cm −2 , as shown in figure 4(a).…”

“…The Ge selectively grew on the seed pads rather than on SiO 2 , and the seeds coalesced to form the epilayer. Another lithography-free substrate pattering method uses an ultrathin free-standing porous alumina membrane (PAM) as an etching mask [28]. PAM is fabricated by a typical two-step electrochemical procedure using high-purity aluminum foil as raw material; it has uniform ordered hexagonal-aligned patterns, with controllable pore size and AR.…”

“…(b) High-resolution cross-sectional TEM image shows the details of the Ge/Si interface. The strain contrast below the interface indicates the diffusion of Ge in the Si substrate [28]. …”

Design and Fabrication of Wafer-Level Microlens Array with Moth-Eye Antireflective Nanostructures

“…In order to generate a uniform layer of nanostructures on the lens array mold, we adopted the AAO process, which has been widely examined on aluminum sheet samples for nanopore templating applications [40,41,42]. However, the suitability of this chemical process for curved array surfaces, and its capacity to maintain the surface accuracy and minimal roughness of a mirror-quality precision mold was unknown.…”

Fabrication of Multiscale-Structure Wafer-Level Microlens Array Mold

“…To generate a uniform layer of nanostructures on the MLA mold, we adopted the two-step AAO process, which is widely used for nanoporous template applications [28,29]. In the AAO experiments, we investigated characteristics, such as pore size, interpore distance, and nanolayer thickness, by varying the applied voltage, electrolyte concentration, and reaction time.…”