Growth of novel wide bandgap room temperature ferromagnetic semiconductor for spintronic applications

This work presents the development of a GaN-based dilute magnetic semiconductor (DMS) by metal organic chemical vapor deposition (MOCVD) that is ferromagnetic at room temperature (RT), electrically conductive, and whose magnetic properties can be tuned by doping the semiconductor n- or p-type. GaN is an excellent candidate for RT spintronic applications, as it has been theoretically predicted to have a Curie temperature (TC) greater than RT. Additionally, the development of a GaN-based DMS is significant as it can be incorporated into optoelectronic devices such as light emitting diodes (LEDs) to create spin-LEDs, which provide a good platform for analyzing spin transport in heterostructures. This work employed a series of transition metals by a single epitaxial growth technique at the thin film and the nanostructure level with the aim of determining the magnetic mechanism in these materials. Additionally, this work presents the first report on gadolinium doping of GaN by MOCVD and also provides the first report on p-Ga1-xGdxN.;The transition metal (TM) series, namely chromium, manganese, and iron, was investigated in this work in order to analyze the effect of the TM acceptor level and the number of unpaired spins on the magnetic and material properties of the semiconductor. This research presents a series of TM-dopants by a single growth technique, which eliminates the ambiguity present when comparing results from different growth techniques. Structural characterization revealed that single phase and strain free Ga1-xTMxN films were obtained. Optical measurements revealed that Mn is a deep acceptor in GaN, while Hall measurements showed that these Ga1-xTMxN films were semi-insulating, making carrier mediated exchange unlikely. Hysteresis curves were obtained at RT for all the Ga1-xTMxN films with the maximum saturation magnetization of 16 emu/cm3 obtained for 1.5% Mn doping in GaN. Furthermore, the effect of n- and p-doping on the magnetic properties was analyzed and in-depth magnetic measurements revealed that there is a high tendency for TMs to cluster in GaN and the observed magnetization is due to magnetic clusters rather than the double exchange mechanism previously proposed by experimentalists and theorists.;The TM elements were incorporated into GaN nanostructures to obtain a better understanding of the impact of size on the magnetic properties of the semiconductor. It was determined that TMs enhance nucleation and provide additional support for the TM clustering seen in thin films. Additionally, a clear superparamagnetic behavior is observed for the Ga1-xTMxN nanostructures, which is consistent with the thin film observations.;Gadolinium is the only rare earth element with both unfilled d and f orbitals and thus has a very high atomic magnetic moment and provides an alternate route for creating a RT DMS. Molecular beam epitaxy growth studies have shown that Gd-doped GaN results in an extremely large magnetic moment at room temperature. However, the mechanism behind the large magnetic moment is not clear. Despite the observation of this large magnetic moment, no published reports exist on Gd doping of GaN by MOCVD. This work presents the first report of Gd (0--4%) doping of GaN by MOCVD and presents the impact of n- and p-dopants on its magnetic and material properties. Gadolinium incorporation in GaN resulted in a large magnetization strength of 20 emu/cm3 and the material displays n-type conductivity. Silicon was introduced to Ga0.98Gd 0.02N as an n-type dopant, which greatly enhanced the magnetization to 110 emu/cm3 upon doping with 1018 cm -3 silicon atoms. Doping the Gadolinium-based DMS with 10 19 cm-3 magnesium atoms (p-type dopant) increased the saturation magnetization to 500 emu/cm3. All the Ga 1-xGdxN films including the n- and p-type films were conductive and are suitable for integration into device structures. The ferromagnetic properties of Ga1-xGdxN are attributed to Ruderman-Kittel-Kasuya-Yosida (RKKY) indirect exchange interaction where the interactions between the magnetic ions are facilitated by the intrinsic donors (nitrogen vacancies) that are present in the material. Furthermore, the addition of electrons and holes results in spin-splitting of the conduction band and valence band respectively and the ferromagnetic phase is stabilized through s-f and p-d coupling. It has been experimentally shown that holes are more effective in stabilizing the ferromagnetic phase in Ga1-xGdxN providing an experimental answer to the theoretical debate over which carrier better facilitates ferromagnetic exchange interactions. (Abstract shortened by UMI.)...