Abstract
We report observations and origins of Ga-rich GaGe droplets and the localized etching of Ge-rich GaGe thin films grown on GaAs (100) substrates by metalorganic chemical vapor deposition. Micron and sub-micron dots, dot-in-holes, and holes have been fabricated by controlling the partial pressures of the Ga and Ge precursors as well as the substrate temperatures. The dot-in-hole features can also be converted to empty holes via post-growth sonication in hot deionized water due to the low melting point of the Ga-rich dots. Enhanced Raman scattering (ERS) of the Ge–Ge lattice vibrational mode has been observed at the wall of the concentric dot-in-hole structures as well as in the empty holes. To interpret the ERS mechanism, we have carried out finite-difference time-domain (FDTD) simulations, which reveal an enhanced electrical field for the obtained structures as a result of interference between the incident and reflected waves at the surface of the thin films that, in turn, results in the observed ERS. These findings greatly enrich the conventional droplet epitaxy and etching techniques and widen their applications.
Highlights
A careful literature survey shows that droplet epitaxy (DEPI) and droplet etching (DETC) studies are dominated by molecular beam epitaxy (MBE), explorations of DEPI by using metalorganic chemical vapor deposition (MOCVD) have started soon after MBE;[22,23] these studies are generally limited to III–V materials.[2,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22]
A careful observation of the atomic force microscopy (AFM) images reveals that the opening of the holes is aligned with their edges parallel to the [110] and [1−10] axes of the GaAs (100) substrate, indicating that the GaGe thin films are epitaxially grown on the 4500 | CrystEngComm, 2016, 18, 4499–4507
The small full-width at half-maximum (FWHM) values (≤18 arcsec) of the high-resolution X-ray diffraction (HRXRD) Ge (004) rocking curves as well as the clear fringes of the θ– 2θ scans are indicative of the high crystal quality of the GaGe thin films and a smooth interface of GaGe-on-GaAs
Summary
The epitaxial growth of III–V arsenide/antimonide quantum dots (QDs) via converting pre-deposited metal droplets, e.g., Ga droplets on a ZnSe substrate,[1] under arsenic and/or antimony flux, so-called droplet epitaxy (DEPI), was discovered two decades ago.[1,2] This method, being different from the conventional Stranski–Krastanov growth mechanism where a lattice mismatch between the growing materials and the template/substrate is generally required and the grown structuresA careful literature survey shows that DEPI and DETC studies are dominated by molecular beam epitaxy (MBE), explorations of DEPI by using metalorganic chemical vapor deposition (MOCVD) have started soon after MBE;[22,23] these studies are generally limited to III–V materials.[2,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22] In comparison, DEPI and DETC studies on hetero-substrates are much fewer in the literature heterogeneous material integrations,[1,23] e.g., III–V-on-Si/Ge, are highly desired for electrical and optoelectronic applications such as bipolar transistors,[24] tandem solar cells,[25,26,27] broadband photodetectors,[28] etc.[23,29,30,31,32] Recent developments in surface plasmonics and metamaterials have seen many new applications of combining metallic nanoparticles and semiconductors in various configurations to enhance light absorption and/or scattering.[33,34,35,36] For this purpose, both unintentional surface V-shaped defects (i.e., V-pits) and intentionally patterned V-shaped holes in semiconductors have been employed for locating metal dots to enhance their surrounding optical near-field so as to couple into the active regions embedded in the semiconductor structures.[35,37]Here, we demonstrate a novel mechanism for creating Ga droplets on GaGe thin films epitaxially grown by MOCVD on lattice-matched GaAs (100) substrates; by tuning the Ga composition (i.e., via adjusting the flow rate ratio of Ga/Ge precursors) and the growth temperature, dot, dot-in-hole, and Communication hole structures are well created in the GaGe thin films due to the finite solubility of Ga atoms in the crystalline Ge matrix and vice versa.[38,39,40,41] Enhanced Raman scattering (ERS) of the Ge–Ge lattice vibrational mode has been observed at the ringlike walls around the droplet-structured areas as well as in the holes. The grown structures are of great consequence in studies and application of the electrical and optoelectronic properties of GaGe thin films
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