Electric-field-controlled ferromagnetism in high-Curie-temperature Mn0.05Ge0.95 quantum dots

Abstract

Electric-field manipulation of ferromagnetism has the potential for developing a new generation of electric devices to resolve the power consumption and variability issues in today's microelectronics industry. Among various dilute magnetic semiconductors (DMSs), group IV elements such as Si and Ge are the ideal material candidates because of their excellent compatibility with the conventional complementary metal-oxide-semiconductor (MOS) technology. Here we report, for the first time, the successful synthesis of self-assembled dilute magnetic Mn(0.05)Ge(0.95) quantum dots with ferromagnetic order above room temperature, and the demonstration of electric-field control of ferromagnetism in MOS ferromagnetic capacitors up to 100 K. We found that by applying electric fields to a MOS gate structure, the ferromagnetism of the channel layer can be effectively modulated through the change of hole concentration inside the quantum dots. Our results are fundamentally important in the understanding and to the realization of high-efficiency Ge-based spin field-effect transistors.

Magnetic MnxGe1-x Dots for Spintronics Applications

“…Both these materials are reported to be ferromagnetic (Cho et al, 2002, Jamet et al, 2006, Pramanik et al, 2003. The reported experimental results also clearly showed the presence of carrier mediated ferromagnetism in DMS Ge (Chen et al, 2007a, Xiu et al, 2010. A recent study predicted that the T c could be increased by enhancing the substitutional doping of Mn in Ge and Si, via co-doping -by adding conventional electronic dopants such as As or P during the Mn doping process (Maekawa, 2004).…”

“…The low T c of III-V DMS, however, presents a challenge to further realize room-temperature controlled ferromagnetism (Weisheit et al, 2007). Alternatively, the recent experiments on Mn 0.05 Ge 0.95 QDs show a high T c above room temperature and gate modulated ferromagnetism over 100 K (Xiu et al, 2010). While the leakage current suppressed the gate modulation, room-temperature controlled ferromagnetism would not be impossible because of the high T c of this system.…”

“…It is understood that this can be achieved only by growing cluster free and uniformly doped Mn x Ge 1-x samples. Experimental reports show that T c as high as 400 K is obtained for cluster free Mn x Ge 1-x samples grown (1) using sub-surfactant epitaxial method (Kimura and Otani, 2008) (2) epitaxial QDs (Xiu et al, 2010) and (3) nanowire structures , Majumdar et al, 2009b, van der Meulen et al, 2008. However, there are only very few theoretical studies depicting the relation between the quantum confinement and its influence on the origin of ferromagnetism in low dimensional DMS structures Silsbee, 1985, Patibandla et al, 2006), (Kelly et al, 2003).…”

“…Our experiments showed that the Mn doping behavior and magnetic properties are dramatically different when the material dimension becomes smaller. (Chen et al, 2007a, Xiu et al, 2010 Bulk Mn x Ge 1-x films always tend to form clusters no matter what growth conditions are. When it comes to one and zero dimensions, however, the cluster formation may be possibly limited because the Mn concentration cannot accumulate enough and the strain can be easily accommodated to minimize the metallic clusters (Wang et al, 2008a.…”

“…The EDS analysis over many QDs reveals a Mn/Ge atomic ratio of about 0.144:1. Since approximately 1/3 volume fraction of Mn is distributed in the Mn x Ge 1-x QD (Xiu et al, 2010), the average Mn concentration can be estimated to be 4.8 ± 0.5 %. Note that the deviation was determined by a thorough study of many Mn 0.05 Ge 0.95 QDs.…”