Yan-Qiang Cao

The effect of thermal treatment induced inter-diffusion at the interfaces on the charge trapping performance of HfO2/Al2O3 nanolaminate-based memory devices

The charge trapping memory devices based on different HfO2/Al2O3 nanolaminated charge trapping layers were prepared and investigated. The memory device with 6 interfaces HfO2/Al2O3 shows a memory window of 4.7 V in its capacitance-voltage curve and a better retention property. It was suggested that the thermal treatment would reduce the defects inside the bulk HfO2, but cause an inter-diffusion at the interface HfO2/Al2O3, which could create additional defects at HfO2/Al2O3 interface. Increasing the number of the interfaces could enhance the charge trapping capability of the devices. The band alignments were established to explain the variation trend of the memory window and the retention characteristics of the memory devices with different laminated structures.

Magnetic interactions in BiFe0.5Mn0.5O3 films and BiFeO3/BiMnO3 superlattices

The clear understanding of exchange interactions between magnetic ions in substituted BiFeO3 is the prerequisite for the comprehensive studies on magnetic properties. BiFe0.5Mn0.5O3 films and BiFeO3/BiMnO3 superlattices have been fabricated by pulsed laser deposition on (001) SrTiO3 substrates. Using piezoresponse force microscopy (PFM), the ferroelectricity at room temperature has been inferred from the observation of PFM hysteresis loops and electrical writing of ferroelectric domains for both samples. Spin glass behavior has been observed in both samples by temperature dependent magnetization curves and decay of thermo-remnant magnetization with time. The magnetic ordering has been studied by X-ray magnetic circular dichroism measurements, and Fe-O-Mn interaction has been confirmed to be antiferromagnetic (AF). The observed spin glass in BiFe0.5Mn0.5O3 films has been attributed to cluster spin glass due to Mn-rich ferromagnetic (FM) clusters in AF matrix, while spin glass in BiFeO3/BiMnO3 superlattices is due to competition between AF Fe-O-Fe, AF Fe-O-Mn and FM Mn-O-Mn interactions in the well ordered square lattice with two Fe ions in BiFeO3 layer and two Mn ions in BiMnO3 layer at interfaces.

Magnetic interactions in BiFe 0.5 Mn 0.5 O 3 films and BiFeO 3 /BiMnO 3 superlattices

The clear understanding of exchange interactions between magnetic ions in substituted BiFeO3 is the prerequisite for the comprehensive studies on magnetic properties. BiFe0.5Mn0.5O3 films and BiFeO3/BiMnO3 superlattices have been fabricated by pulsed laser deposition on (001) SrTiO3 substrates. Using piezoresponse force microscopy (PFM), the ferroelectricity at room temperature has been inferred from the observation of PFM hysteresis loops and electrical writing of ferroelectric domains for both samples. Spin glass behavior has been observed in both samples by temperature dependent magnetization curves and decay of thermo-remnant magnetization with time. The magnetic ordering has been studied by X-ray magnetic circular dichroism measurements, and Fe-O-Mn interaction has been confirmed to be antiferromagnetic (AF). The observed spin glass in BiFe0.5Mn0.5O3 films has been attributed to cluster spin glass due to Mn-rich ferromagnetic (FM) clusters in AF matrix, while spin glass in BiFeO3/BiMnO3 superlattices is due to competition between AF Fe-O-Fe, AF Fe-O-Mn and FM Mn-O-Mn interactions in the well ordered square lattice with two Fe ions in BiFeO3 layer and two Mn ions in BiMnO3 layer at interfaces.

Room temperature ferromagnetic Zn0.98Co0.02O powders with improved visible-light photocatalysis

The realization of ferromagnetic diluted magnetic semiconductors requires the fine tuning of the energy levels of the band structure, which might have important applications in photocatalysis. The magnetic properties and visible-light photocatalytic properties of Zn0.98Co0.02O nanoparticles post-annealed under different ambient conditions (vacuum, hydrogen, vacuum followed by hydrogen) have been studied. The X-ray diffraction patterns show that all samples have a hexagonal wurtzite structure without any impurity phase. However, the Raman spectra indicate a small amount of Co3O4 impurities, which is significantly suppressed during the hydrogenation process. The as-prepared and vacuum-annealed Zn0.98Co0.02O nanoparticles show dominant paramagnetic properties, while strong room temperature ferromagnetism is observed after the hydrogenation process. This is explained by the ferromagnetic mediation between doped Co ions by the interstitial H ions. The photocatalytic activity of the resulting samples was evaluated by the degradation of Rhodamine B under visible light irradiation. It was found that the Zn0.98Co0.02O annealed in vacuum exhibited the best photodegradation properties. The enhanced performance is mainly attributed to the existence of oxygen vacancies which might decrease the surface recombination centers and improve the charge separation efficiency. The photocatalysis of hydrogenated samples is deteriorated significantly due to the passivation of oxygen vacancies by hydrogen, which confirms the ferromagnetic mediation by H+ ions instead of oxygen vacancies. Interestingly, the vacuum-annealed Zn0.98Co0.02O samples further annealed under hydrogen ambient conditions exhibit both room temperature ferromagnetism and sizable photocatalytic ability.

Preparation and phase structures of Zn–Ti–O ternary compounds by atomic layer deposition

Zn–Ti–O (ZTO) ternary films with various Zn/Ti cycle ratios were deposited on Si substrates using diethylzinc and titanium isopropoxide as Zn and Ti sources by atomic layer deposition (ALD). The effect of Zn/Ti cycle ratio and postannealing temperature on the growth rate, composition, phase structure, and morphology of ZTO films were investigated by a series of analytical tools. It is found that for ALD ZTO films, the growth per cycle (GPC) of TiO2 deposited on ZnO-terminated surface is faster than that of pure TiO2, while the GPC of ZnO deposited on TiO2-terminated surface becomes slower than that of pure ZnO. This makes the Zn/Ti cycle ratio effect on the film composition become weak. The postannealing temperature and ALD sequence play important roles in facilitating the ZTO phase evolution. Pure Zn2TiO4 phase can be obtained in the (1:2)-ZTO films with 40.3 mol. % Ti content postannealed at 900 °C. For (2:5)-ZTO samples with 48.8 mol. % Ti content, pure h-ZnTiO3 phase can be formed at 700 °C. At 700 °C...

Atomic-Layer-Deposition Assisted Formation of Wafer-Scale Double-Layer Metal Nanoparticles with Tunable Nanogap for Surface-Enhanced Raman Scattering

A simple high-throughput approach is presented in this work to fabricate the Au nanoparticles (NPs)/nanogap/Au NPs structure for surface enhanced Raman scattering (SERS). This plasmonic nanostructure can be prepared feasibly by the combination of rapid thermal annealing (RTA), atomic layer deposition (ALD) and chemical etching process. The nanogap size between Au NPs can be easily and precisely tuned to nanometer scale by adjusting the thickness of sacrificial ALD Al2O3 layer. Finite-difference time-domain (FDTD) simulation data indicate that most of enhanced field locates at Au NPs nanogap area. Moreover, Au NPs/nanogap/Au NPs structure with smaller gap exhibits the larger electromagnetic field. Experimental results agree well with FDTD simulation data, the plasmonic structure with smaller nanogap size has a stronger Raman intensity. There is highly strong plasmonic coupling in the Au nanogap, so that a great SERS effect is obtained when detecting methylene blue (MB) molecules with an enhancement factor (EF) over 107. Furthermore, this plasmonic nanostructure can be designed on large area with high density and high intensity hot spots. This strategy of producing nanoscale metal gap on large area has significant implications for ultrasensitive Raman detection and practical SERS application.

Stepwise mechanism and H2O-assisted hydrolysis in atomic layer deposition of SiO2 without a catalyst

Atomic layer deposition (ALD) is a powerful deposition technique for constructing uniform, conformal, and ultrathin films in microelectronics, photovoltaics, catalysis, energy storage, and conversion. The possible pathways for silicon dioxide (SiO2) ALD using silicon tetrachloride (SiCl4) and water (H2O) without a catalyst have been investigated by means of density functional theory calculations. The results show that the SiCl4 half-reaction is a rate-determining step of SiO2 ALD. It may proceed through a stepwise pathway, first forming a Si-O bond and then breaking Si-Cl/O-H bonds and forming a H-Cl bond. The H2O half-reaction may undergo hydrolysis and condensation processes, which are similar to conventional SiO2 chemical vapor deposition (CVD). In the H2O half-reaction, there are massive H2O molecules adsorbed on the surface, which can result in H2O-assisted hydrolysis of the Cl-terminated surface and accelerate the H2O half-reaction. These findings may be used to improve methods for the preparation of SiO2 ALD and H2O-based ALD of other oxides, such as Al2O3, TiO2, ZrO2, and HfO2.

Atom probe microanalysis of an Elinvar type alloy

Summary The microstructure and chemistry of a Ti, Al doped Elinvar type alloy aged at 700°C for 24 hours were investigated with atom probe field ion microscopy. The morphology, size and composition of the γ-precipitates, as well as the composition of the γ-matrix were determined. The effects of the microstructure on the thermoelastic properties of the material were discussed on the basis of the theory about the ΔE-effect.

Improved thermal stability and electrical properties of atomic layer deposited HfO2/AlN high-k gate dielectric stacks on GaAs

The thermal stability and electrical properties of atomic layer deposited HfO2/AlN high-k gate dielectric stacks on GaAs were investigated. Compared to HfO2/Al2O3 gate dielectric, significant improvements in interfacial quality as well as electrical characteristics after postdeposition annealing are confirmed by constructing HfO2/AlN dielectric stacks. The chemical states were carefully explored by the x-ray photoelectron spectroscopy, which indicates the AlN layers effectively prevent from the formation of defective native oxides at elevated temperatures. In addition, it is found that NH3 plasma during AlN plasma-enhanced atomic layer deposition also has the self-cleaning effect as Al(CH3)3 in removing native oxides. The passivating AlN layers suppress the formation of interfacial oxide and trap charge, leading to the decrease of capacitance equivalent thickness after annealing. Moreover, HfO2/AlN/GaAs sample has a much lower leakage current density of 2.23 × 10−4 A/cm2 than HfO2/Al2O3/GaAs sample of 2.5...

Irreversible electrical manipulation of magnetization on BiFeO3-based heterostructures

We prepared several heterostructures, Co/Bi0.90La0.10FeO3 on surface oxidized Si or (111) SrTiO3 and NiFe/Bi0.90La0.10FeO3 on (001) SrTiO3 substrates using LaNiO3 as bottom electrode. With different strategies of voltage application, the exchange bias field HE decreased with increasing voltage irreversibly for all the heterostructures at room temperature. The chemical state at the NiFe/Bi0.90La0.10FeO3 interface was studied by X-ray photoelectron spectroscopy before and after the electrical manipulation. The oxidization of the metallic ferromagnetic layer at interface after the electrical manipulation has been confirmed, which might explain the irreversibility.