Site-controlled droplet epitaxy of GaAs quantum dots by deposition through shadow masks

Abstract

Droplet epitaxy is a versatile method for fabricating nanostructures, but the standard process relies on spatially random droplet formation resulting in poor control of the position of the final nanostructure. The authors demonstrate site-controlled droplet formation utilizing local Ga deposition combined with self-assembled droplet formation. The local Ga deposition is realized by employing a shadow mask with small apertures. Our experiments show that out-diffusion of Ga from the aperture area is important and can be minimized by creating a Ga saturated surface before depositing through the mask. The effects of Ga precoverage, Ga amount for droplet formation, and droplet formation temperature on size, height, and density of droplets have been investigated. With optimized parameters, the authors were able to generate a single droplet per aperture with over 90% probability.

Modeling of Masked Droplet Deposition for Site-Controlled Ga Droplets

“…The present experimental approach for the generation of site-controlled GaAs QDs by area-selective droplet deposition is described in earlier publications [ 23 , 24 ]. Therefore, we give here only a brief summary of the major points.…”

“…The samples were fabricated in a multi-chamber solid-source MBE system using semi-insulating (100) GaAs wafers as substrates [ 23 ]. In a first step, MBE growth was performed without a mask, and a 100 nm thick Al Ga As layer was deposited at usual MBE process parameters.…”

“…After that, the substrate was cooled down to 100 °C and transferred into the preparation chamber. There, a mask–substrate sandwich was created under ultrahigh vacuum (UHV) conditions employing a special manipulator in the preparation chamber [ 23 ]. The mask is a nano-patterned 100 nm thick Si N -membrane on a Si(100) support wafer [ 24 ].…”

“…First, modifications of the substrate surface before MBE growth cause a rearrangement of the planarly deposited material for site control [ 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 ]. Second, the material beam fluxes to the substrate surface are modified for a area-selective deposition [ 21 , 22 , 23 ]. We consider the second approach to be more flexible, since a patterned substrate often only allows geometries where the distances are close to the natural diffusion length of the unpatterned surface.…”

“…[ 24 ] and the process in ref. [ 23 ]. For QD generation, the Ga droplets can be crystallized into GaAs quantum dots using a crystallization step under As flux (droplet epitaxy [ 25 ]).…”