Ge1-x Mnx/Si (001) Heteroepitaxy: A Study of How Mn Incorporates During Molecular Beam Epitaxial Growth of Self Assembled Quantum Dots

The development of spintronics promises to yield a novel set of spin based devices that offer less power consumption and heat dissipation compared to charge driven devices. The underlying technology exploits the spin degree of freedom of electrons in solid-state devices and seeks to control and manipulate the magnetism in semiconductors through carrier concentration. Group IV dilute magnetic semiconductors (DMS) with room temperature ferromagnetism could be functionalized as a type of fundamental building block for spintronics devices.
Motivated by recent reports of above-room temperature dilute ferromagnetism in Mn-doped Ge quantum dots (QDs) grown by molecular beam epitaxial (MBE) co-deposition on Si (001), we examine the morphology, structure and chemistry of this system in detail. The goal of this work is to correlate the heteroepitaxial growth and the resulting magnetic properties of the MnxGe1-x/Si (001) self-assembled QDs as a potential quantum-confined DMS system. DMS strain-induced quantum dots pose a particular challenge to synthesize, since far-above-chemical-equilibrium Mn incorporation in Ge requires low growth temperatures and low surface diffusivity to minimize formation of unwanted phases, while quantum dot self-assembly inherently requires elevated growth temperatures and high surface diffusivity. We systematically explore the effect of the relative Mn:Ge flux during MBE growth. It is a critical focus of this work to clearly ascertain where Mn resides in our Ge-QD/Si (001) films, and in so doing to contribute to our understanding of the basic origins of magnetic ordering in this system.
We synthesized heteroepitaxial self-assembled heteroepitaxial QD’s by MBE co-deposition of Ge1-xMnx with x = 0 – 0.16. Depending on the doping level, Mn is observed to partition into solution in the Ge QD layer, into arrays of Mn-rich Si solutions buried directly below Ge islands, and into epitaxial and endotaxial Mn silicide phases. In all our Mn-containing films, only low temperature magnetism is observed, with Curie temperatures less than 220 K and saturation moments 4 – 7 µB/Mn. The magnetic signal is extremely weak, given that even our most Mn-rich sample contains only 4.7x1014 Mn/cm2 total, required careful analysis and interpretation. Nonetheless, it is clear that we do not observe indications of room temperature ferromagnetism, contrary to recently published results.