Hsin-Hung Huang

Nonpolar resistive switching in the Pt/MgO/Pt nonvolatile memory device

Nonpolar resistive switching (RS), which is the coexistence of unipolar and bipolar RS characteristics, in the Pt/MgO/Pt memory device with the nonforming nature is demonstrated. The nonforming nature is ascribed to the relatively high defect density of the MgO film deposited by using the ion beam sputtering in Ar atmosphere. The results of Auger electron spectroscopy and x-ray photoelectron spectroscopy analyses combing with the temperature dependence of resistance suggest that metallic Mg filaments are formed in the low resistance state. The voltage-polarity-independent RESET process implies that filaments may be ruptured by local Joule heating, leading to nonpolar characteristics.

Mechanism of enhanced formation of C54–TiSi2 in high-temperature deposited Ti thin films on preamorphized (001)Si

Enhanced formation of C54-TiSi2 in high-temperature deposited Ti thin films on preamorphized (001)Si has been investigated by high-resolution transmission electron microscopy in conjunction with autocorrelation function analysis. The increase in the thickness of the amorphous TiSix layer is due to the preamorphization implantation for the most part. The dominant effect of high-temperature sputtering is to increase the density of crystallites in the amorphous TiSix layer. The enhanced formation of C54–TiSi2 in high-temperature deposited samples is attributed to the more extensive presence of silicide crystallites, which serve as nucleation sites, in the amorphous TiSix layer than that in samples deposited at room temperature.

Room-temperature fabricated ZnCoO diluted magnetic semiconductors

(0002) textured and epitaxial ZnCo0.07O films were fabricated at room temperature by ion beam deposition on Si substrates. Hall measurement revealed that ZnCo0.07O films were n-type semiconductors with carrier concentrations higher than 1019∕cm3. The carrier concentration of ZnCo0.07O can be manipulated by controlling the oxygen flow rate during deposition or by postannealing. The saturation magnetization and magnetoresistance ratios strongly depended on the carrier concentration. In addition, epitaxial (0002) ZnCo0.07O films, grown on Cu underlayers, showed room-temperature ferromagnetism, which may be potentially used for spintronic devices.

Coexistence of exchange-bias fields and vertical magnetization shifts in ZnCoO∕NiO system

Exchange fields accompanying vertical magnetization shifts were observed in the epitaxial ZnCo0.07O (dilute magnetic semiconductor)/NiO system after field cooling. Transitions of exchange fields and magnetization shifts were observed at 50K, above which the magnetization shift disappeared and the exchange field was significantly reduced. Both the exchange field and the magnetization shift increased with increasing cooling-field strength at temperatures below 50K, which might be attributed to the existence of “frozen” spins in ZnCoO. The observed linear dependence of the exchange field on the magnetization shift may directly elucidate the role of pinned spins on the exchange fields.

EFFECTS OF STRESS ON THE GROWTH OF TISI2 THIN FILMS ON (001)SI

Tensile stress induced by backside CoSi2 films on a silicon substrate has been found to enhance the growth of C54–TiSi2 on (001)Si. In contrast, compressive stress induced by backside oxide films on the silicon substrate was found to retard significantly the growth of C54–TiSi2 on (001)Si. For Ti on stressed (001)Si after rapid thermal annealing at 800u200a°C for 30 s, the thickness of the C54– TiSi2 films was found to increase and decrease with the tensile and compressive stress levels, respectively. The retarded growth is attributed to the hindrance of the migration of Si through the Ti/Si interface by the compressive stress. On the other hand, the presence of tensile stress promotes the Si diffusion to facilitate the formation of Ti silicide thin films.

Asymmetric double-shifted characteristics in epitaxial (002) exchange-biased IrMn/CoFe bilayers

Symmetric and asymmetric double-shifted loops were observed at room temperature in epitaxial (002) Ir20Mn80/Co50Fe50 samples placed in a setting field applied along collinear and noncollinear directions with the deposition-field direction, respectively. A coherent rotation model with extra unidirectional exchange and uniaxial anisotropies induced by the noncollinear setting field is proposed to explain the asymmetric double-shifted loops. Due to the existence of the biaxial magnetocrystalline anisotropy, the noncollinear setting field results in two interfacial Ir20Mn80 spin orientations, where the major spins determine the direction to observe double-shifted loops and the minor spins determine the field-shift and the asymmetry of double-shifted loops.

Formation of TiSi2 thin films on stressed (001)Si substrates

The effects of stress on the formation of TiSi 2 thin films have been investigated. Compressive stress present in the silicon substrate was found to retard significantly the transformation of high-resistivity C49-TiSi 2 to low-resistivity C54-TiSi 2 . On the other hand, the tensile stress present in the silicon substrate was found to promote the formation of C54-TiSi 2 . For Ti on stressed (001)Si substrates after rapid thermal annealing (RTA), the thickness of TiSi 2 films was found to decrease and increase with the compressive and tensile stress level, respectively. In addition, the thickness of amorphous interlayers (a-interlayers) between Ti films and silicon substrates was found to be thicker and thinner in the compressively and tensile-stressed samples, respectively. The results indicated that the compressive stress hinders the migration of Si through the Ti/Si interface, so that the transformation of C49- to C54-TiSi 2 is retarded. In contrast, the tensile stress promotes the Si diffusion to facilitate the formation of C54-TiSi 2 .

Reorientation of exchange anisotropy in epitaxial (002) IrMn/CoFe system

We investigated the reorientation of exchange anisotropy in the epitaxial (002) IrMn/CoFe bilayers. Transitions of exchange-bias field from a negative field to a positive field with increasing time scale were observed at room temperature. The negatively magnetized state combining with the strong magnetocrystalline anisotropy of CoFe may raise the probability that the interfacial spins of thin IrMn layers were thermally relaxed to the direction of CoFe magnetization; therefore, after a period of time, the exchange-bias direction was reversed. This process can be further enhanced when samples were placed under the setting field. The double-shifted magnetization curve observed in this system was used as a good indicator for the relaxation of interfacial spins of IrMn.

Exchange Bias Between ZnCoO and IrMn

High-quality epitaxial ZnCo0.07O films deposited at room temperature were obtained by using reactive ion beam sputtering. Room-temperature ferromagnetic behavior of ZnCoO was observed with a coercivity of 70 Oe. We observed the loop shift at 5 K in the field-cooled samples of ZnCoO/Cu/IrMn and an enhanced coercivity in the zero-field-cooled samples, which indicated the existence of exchange coupling between ZnCoO and IrMn through a thin Cu layer (0.15 nm). Furthermore, the exchange field of ZnCoO was increased from 55 to 113 Oe by increasing the cooling field from 3 to 15 kOe

Silicide Contacts for Sub-0.25 μm Devices

Low resistivity TiSi 2 , CoSi 2 and NiSi are the three primary candidates for metal contacts in sub-0.25 μ m devices. In the present paper, we review recent progress in the investigations of lowresistivity contacts, which include enhanced formation of C54-TiSi 2 on (001)Si by tensile stress, high temperature sputtering, and interposing Mo or TiN layer, improved thermal stability of C54-TiSi 2 by the addition of N 2 during Ti sputtering or N implantation in (001)Si, self-aligned formation of CoSi 2 on the selective epitaxial growth silicon layer on (001)Si, effects of stress on the epitaxial growth of CoSi 2 on (001 )Si, improvement of thermal stability of CoSi 2 by nitrogen ion implantation or high temperature sputtering, and improvement of thermal stability of NiSi by nitrogen ion implantation or compressive stress.