Preparation Of Cobalt/Chromium-Doped Germanium Based Diluted Magnetic Semiconductors

Spintronics has attracted considerable attention in current research fields of condensed physics, new materials and information science. Diluted magnetic semiconductors (DMSs) are formed by partial substitution of atoms of host semiconductors with some magnetic transition-metal (TM) ions such as V, Cr, Mn, Fe, Co and Ni. DMS takes advantage of both charge and spin properties of electrons,and will play an important role in the integrated circuit and other material with memory. Very recently, significant attention has been given to the theoretical study and fabrication procedure of Ge-based DMS due to their possible full compatibility with mainstream silicon technology.In this thsesis, germanate ion solution was used to prepare Co doped Ge samples. Based on lots of experiments, a wet chemical-hydrogen reduction process and a wet chemistry-physical evaporation method were developed for preapartion of the doped Ge samples. The fabrication conditions, morphology, structure and properties of the as-obtained samples were studied. The observed ferromagnetic behaviors and their origions are discussed according to structural and electrical features of the samples.The experiments and main contributions are as follows:1) Firstly, a few of fabrication procedures for Co doped Ge were investiguated. Ge_1-xCo_x(X=0.75, 3.2 and 11.5%) diluted magnetic semiconductor films were fabricated using a wet chemical-hydrogen reduction method. XRD, HRTEM and Raman analyses disclose that Ge_1-xCo_x sample with x<3.2% has a single cubic crystal Ge phase. Some ultra-small secondary phases or dopant-rich regions which can not be detected by XRD analysis might have been formed in the 11.5% Co-doped sample. The Ge_1-xCo_x film samples are all p type and have high hole concentrations at room temperature. The samples exhibit typical semiconducting characteristic and the the 3.2% and 11.5% films have the lowest and highest electric resistances, respectively. The fields (H)-dependent magnetizations (M) of the Ge_1-xCo_x films were analyzed at room temperature. The 0.75% and 11.5% samples show superparamagnetism and blocked superparamagnetism, respectively. The hysteresis loop measured at 300K shows that the Ge96.8Co3.2 sample is ferromagnetic. The observed ferromagnetism is on only a local scale, and most likely arises from DMS materials.2) Both sol-gel-hydrogen reduction and hydrothermal-hydrogen reduction approaches have been used to synthetize Co doped Ge dilute magnetic semiconductor samples. The samples synthesized by sol-gel-hydrogen reduction have irregular morphology. XRD analyses disclose that Ge_1-xCo_x sample with x<1% does not have a secondary phase. Some CoGe secondary phases have been formed in the 2.5% Co-doped sample. Magnetic testing results indicated that ferromagnetic ordering is established in 1% samples at room temperature. XRD and Raman analyses disclose that Ge_1-xCo_x sample synthesized by hydrothermal-hydrogen reduction has a single cubic crystal Ge phase and cobalt atoms have been incorporated into Ge matrix.3) Ge_1-xCo_x film samples with normal Co content of 1%, 3% and 6% were fabricated using cobaltous nitrate as well cobaltous acetate as Co sources. The films were grown via a wet chemical-physical evaporation process. Influences of Co concentration on the morphology, crystal structure, optical properties and magnetic behaviors were investiguated. Better preparation conditions were obtained. All Ge_1-xCo_x samples have single cubic crystal Ge phases. Raman and HRTEM analyses also imply that cobalt atoms have been incorporated into Ge matrix. With increase of dopant concentration, a small amount of doping atoms locate at interstitial sites of germanium lattice. Visible photoluminescence (PL) emission has been detected in the as-prepared Ge_1-xCo_x film samples. The position and intensity of the PL peak do not remarkably change with increasing Co concentration. The Ge_1-xCo_x samples are all p type and has high hole concentrations. The M-H curves of the Ge_1-xCo_x films show hysteresis loops, indicating that ferromagnetic ordering are established in these samples at room temperature.