B.T. Liu

High electron mobility and low sheet resistance in lattice-matched AlInN/AlN/GaN/AlN/GaN double-channel heterostructure

High electron mobility and low sheet resistance were achieved in lattice-matched AlInN/AlN/GaN/AlN/GaN double-channel (DC) heterostructure. Two-dimensional electron gas (2DEG) of the DC heterostructure was divided into the double channels and the room-temperature mobility was increased to 1430 cm2/V s by reducing the 2DEG density in each channel, compared with low electron mobility (1090 cm2/V s) for lattice-matched AlInN/AlN/GaN single-channel heterostructure. It was found that the 2DEG mobility was limited by thickness of the AlN interlayer between the double channels. After the structure optimization, the room temperature electron mobility of the DC heterostructure reached 1570 cm2/V s with sheet resistance of 222 Ω/◻.

Ni–Al diffusion barrier layer for integrating ferroelectric capacitors on Si

We report on the use of amorphous Ni–Al film (a-Ni–Al) as conductive diffusion barrier layer to integrate La0.5Sr0.5CoO3∕PbZr0.4Ti0.6O3∕La0.5Sr0.5CoO3 capacitors on silicon. Cross-sectional observation by transmission electron microscope reveals clean and sharp interfaces without any discernible interdiffusion/reaction in the sample. The physical properties of the capacitors are vertically characterized as the parameters of memory elements. Excellent ferroelectric properties, e.g., large remnant polarization of ∼22μC∕cm2, small coercive voltage of ∼1.15V, being fatigue-free, good retention characteristic, imply that amorphous Ni–Al is an ideal candidate for diffusion barrier for the high-density ferroelectric random access memories integrated with silicon transistor technology.

Atomic ordering kinetics of FePt thin films: Nucleation and growth of L10 ordered domains

To lower the L10 ordering temperature of FePt thin films, a detailed understanding of the nucleation and growth processes of L10 ordered domains is of particular importance. In the present study, a nucleation activation energy En=(0.5−0.6)±0.1 eV and a growth activation energy Eg=0.9±0.1 eV for the L10 ordered domains in the FePt thin films are determined by studying the individual annealing-temperature dependence of the nucleation and growth parameters of the L10 ordered domains. These data make a contribution to the understanding of the high L10 ordering temperature of FePt thin films for manufacturing magnetic recording media.

Bipolar resistive switching behavior of La0.5Sr0.5CoO3−σ films for nonvolatile memory applications

Polycrystalline La0.5Sr0.5CoO3 (LSCO) film is prepared on Pt/Ti/SiO2/Si substrates by pulsed laser deposition in order to explore the resistive switching behavior of the Ag/LSCO/Pt structure. It is found that the oxygen stoichiometric LSCO (LSCO-2) structure does not possess distinct rectifying behavior, while the oxygen deficient La0.5Sr0.5CoO3−σ (LSCO-1) structure exhibits very stable bipolar resistive switching behavior, which is attributed to the oxygen vacancies in the LSCO film. The resistance ratio between high resistance state and low resistance state is about 100, which can be maintained up to 200 cycles and 25 h with no observable degradation, indicating that the Ag/LSCO-1/Pt device possesses very good endurance and retention characteristics. Moreover, the current-voltage characteristic of Ag/LSCO-1/Pt heterostructure can be well explained by the space-charge-limited conduction mechanism. The present results provide essential evidence for the analysis of the switching mechanism and evaluation of...

Enhanced dielectric constant and fatigue-resistance of PbZr0.4Ti0.6O3 capacitor with magnetic intermetallic FePt top electrode

Both FePt/PbZr0.4Ti0.6O3(PZT)/Pt and Pt/PZT/Pt ferroelectric capacitors have been fabricated on Si substrates. It is found that up to 109 switching cycles, the FePt/PZT/Pt capacitor, measured at 50 kHz, with polarization decreased by 57%, is superior to the Pt/PZT/Pt capacitor by 82%, indicating that an intermetallic FePt top electrode can also improve the fatigue-resistance of a PZT capacitor. Maximum dielectric constants are 980 and 770 for PZT capacitors with FePt and Pt, respectively. This is attributed to the interface effect between PZT film and the top electrode since the interfacial capacitance of FePt/PZT is 3.5 times as large as that of Pt/PZT interface.

Diameter- and current-density-dependent growth orientation of hexagonal CdSe nanowire arrays via electrodeposition.

Controlling the growth orientation of semiconductor nanowire arrays is of vital importance for their applications in the fields of nanodevices. In the present work, hexagonal CdSe nanowire arrays with various preferential growth orientations have been successfully yielded by employing the electrodeposition technique using porous alumina as templates (PATs). We demonstrate by experimental and theoretical efforts that the growth orientation of the CdSe nanowires can be effectively manipulated by varying either the nanopore diameter of the PATs or the deposited current density, which has significant effects on the optical properties of the CdSe nanowires. The present study provides an alternative approach to tuning the growth direction of electrodeposited nanowires and thus is of importance for the fabrication of nanodevices with controlled functional properties.

Barrier performance of ultrathin Ni–Ti film for integrating ferroelectric capacitors on Si

Ultrathin amorphous Ni–Ti film is investigated as conductive diffusion barrier layer to integrate La0.5Sr0.5CoO3∕PbZr0.4Ti0.6O3∕La0.5Sr0.5CoO3 (LSCO/PZT/LSCO) capacitors on silicon. X-ray photoelectron spectroscopy results demonstrate that Ni in LSCO∕Ni–Ti∕Si heterostructure is not oxidized after 550°C annealing in oxygen. The structural properties of LSCO∕PZT∕LSCO∕Ni–Ti∕Si are characterized by x-ray diffraction and transmission electron microscopy. It is found that Ni–Ti film is still amorphous and that there are no discernible reactions at the interfaces of the sample. LSCO/PZT/LSCO capacitor, measured at 5V, possesses very good ferroelectric properties, such as low coercive field (∼1.28V), high remnant polarization (∼27.9μC∕cm2), and good fatigue-free characteristic, implying that ultrathin amorphous Ni–Ti film can be used as barrier layer for fabricating high-density ferroelectric random access memories.

Low-temperature synthesis of wurtzite ZnS single-crystal nanowire arrays

The synthesis of highly aligned and ordered wurtzite ZnS single-crystal nanowires at low temperatures is crucial for their applications in electronic and optoelectronic nanodevices. In the present study, wurtzite ZnS single-crystal nanowire arrays with a preferred growth along the [110] direction have been successfully prepared at a low temperature of T = 120 °C by employing electrochemical deposition techniques using the alumina template with 40 nm diameter pores. The microstructure of the ZnS nanowires was characterized by x-ray diffraction and transmission electron microscopy studies. A small deposition current is found to be favourable for the growth of wurtzite ZnS single-crystal nanowires and the cause has been discussed.

Effects of Annealing Pressures on the Ordering and Microstructures of FePt:Ag Nanocomposite Films

FePt:Ag (100 nm) nanocomposite thin films were prepared on naturally-oxidized Si substrates by dc magnetron sputtering at room temperature. X-ray diffraction (XRD) and transmission electron microscopy (TEM) are used to investigate the effects of annealing pressures on the ordering processes and microstructures of these films. The average sizes for the L1 0 ordered domains and the FePt grains are reduced to d = 9 nm and D = 13 nm from d = 19 nm and D = 34 nm, respectively, when the annealing pressure is enhanced to 0.6 GPa from room pressure at 873 K. Furthermore, the size distribution is improved into a narrow range. With increasing pressure, the coercivity of L1 0 -FePt:Ag thin films decreases from 15.1 to 7.6 kOe. In the present study, the effects of high pressure on the L1 0 ordering processes and microstructures of FePt:Ag nanocomposite films were discussed.

Microstructure and magnetic properties of L10-FePt thin films prepared under high pressures

The ultrafine grain size with a narrow size distribution is a critical issue for the development of L10-FePt thin films as ultrahigh density magnetic recording media. In this study, we succeeded in reducing both the grain size and the ordered domain size of L10-FePt thin films and simultaneously improving the size uniformity by annealing disordered FePt thin films at a high pressure of 1 GPa. This is attributed to the fact that high pressure promotes the nucleation of the L10 ordered domains and inhibits the growth of both the grains and the ordered domains during the L10 ordering transition.