Germanium Template Assisted Integration of Gallium Arsenide Nanocrystals on Silicon: A Versatile Platform for Modern Optoelectronic Materials

Abstract

at visible wavelengths. [1][2][3] However, since the band-to-band transition in Si does not simultaneously obey the laws of energy and momentum conservation, spontaneous emission rates are low as compared with those of direct semiconductors and efficient Si-based solid state lightning remains challenging. [4,5] Numerous options to improve the efficiency of Si-based light emission have been investigated, most of which rely on quantum confinement in integrated nanocrystals, quantum wells or superlattices and the introduction of efficient (quasi-direct) transitions by sophisticated strain-and materials-engineering methods. [6][7][8][9][10][11][12][13][14] Direct band gap III-V semiconductor devices on Si substrates have generated remarkable results, nevertheless their hybrid integration to the present complementary metal-oxide-semiconductor (CMOS) technologies is still tedious, in particular due to the high lattice mismatch between Si and most III-V materials. [15][16][17][18] Because of its compatibility with the CMOS process flow and a low lattice mismatch of germanium (Ge) and gallium arsenide (GaAs), Ge heteroepitaxy on Si has received awareness as a template (or so-called virtual substrate) for III-V integration, i.e., for the growth and fabrication of high efficiency solar cells. [19,20] Metal organic vapor phase epitaxy is used to grow gallium arsenide (GaAs) nanocrystals (NCs) on germanium (Ge) templates on nanoscopic silicon (Si) threads prepared by reactive ion etching. Scanning transmission electron microscopy with energy dispersive X-ray measurements shows an epitaxial growth of the GaAs on the Ge template that is supported by the Si thread, and that Ge doping is induced to the GaAs by the template. On Ge templates of about 60 nm diameter, as-grown GaAs NCs show a very regular rhombicdodecahedral outer shape that can be explained by a preferential growth along the <110> plane. Photoluminescence measurements of the Ge/GaAs structures reveal radiative emission peaks on top of the GaAs band-to-band emission and at sub-band gap energies. While high energy peaks are originating from Ge acceptor levels in GaAs, sub-band gap peaks can be explained by radiation from Ge donor and acceptor bands that are amplified by photonic modes hosted in the rhombicdodecahedral GaAs NCs. This study shows that a template-assisted crystal growth at the nanoscale opens up routes for a versatile integration of strongly emitting nanomaterials for a use in on-chip solid state lighting and photonics.