Magnetism and Transport in the Group-IV Dilute Magnetic Semiconductor Ge-x-Mnx

Dolph, Melissa Ann Commisso, Department of Physics, University of Virginia
Wolf, Stuart, Department of Materials Science and Engineering, University of Virginia
Fowler, Michael, Department of Physics, University of Virginia
Lu, Jiwei, Department of Materials Science and Engineering, University of Virginia
Poon, Joe, Department of Physics, University of Virginia

Dilute magnetic semiconductors (DMS) have gained interest over the past decade because of their potential applications in spintronics. DMS systems exhibit carriermediated ferromagnetism, a property which enables electric - field control over the magnetization. In this thesis, the DMS Ge1,xMnx was studied. Germanium (Ge) is a desirable semiconductor because of its high hole mobility and its compatibility with silicon. Manganese (Mn) was chosen for its exhibition of indirect ferromagnetic coupling. These properties are deemed necessary for the realization of a magnetic quantum - dot based device such as a magnetic spin switch. Ion implantation of Mn into Ge - on - insulator and molecular beam expitaxy (MBE) were two methods utilized to synthesize the studied Ge1,xMnx thin films. Several of the Ge1,xMnx films were treated with rapid thermal annealing (RTA) or pulsed laser melting (PLM) in an attempt to improve the crystallinity and make more of the Mn ferromagnetically active. Of all the Ge1,xMnx systems studied, those that exhibited correlated magnetic and magneto - transport properties were those with hole concentrations on the order of 1019 - 1020 holes/cm3. Although these systems exhibited robust magneto - transport properties, electric - field biasing proved ineffective at modulating the carrier concentration and ultimately the magnetization as monitored by the ordinary and anomalous Hall effects and the resistivity. The inefficiency of the electric gating was attributed to localized trap states which reside in the energy band gap of the defect - plagued Ge semiconductor. iii If crystallinity is a prerequisite for gatability, PLM may prove useful as it was found to restore the single crystal structure damaged during Mn implantation. However, PLM resulted in an undesirable segregation of Mn toward the surface of the film (as observed by secondary ion mass spectrometry (SIMS)); a decrease in the saturation magnetization after PLM was attributed to this segregation. The information obtained through SHVIS was useful in obtaining a better understanding of the magnetism exhibited by the laser melted samples. Polarized neutron reflectometry (PNR) was found to be a non - invasive alternative to SHVIS which provided information on the nuclear and magnetic depth profiles in lightly Mn doped and weakly magnetic Ge1,xMnx films. The magnetization profiles obtained through PNR were correlated to direct magnetometry results.

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PHD (Doctor of Philosophy)
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