Ch. Wenger

Titanium-added praseodymium silicate high-k layers on Si(001)

Titanium-added praseodymium silicate layers on Si(001) are promising high-k insulators for silicon-based nanoelectronic devices. Synchrotron radiation x-ray photoelectron spectroscopy was applied to study the effect of titanium additives on the praseodymium silicate/Si system. Nondestructive depth profiling by variation of the photon energy shows that thermal annealing activates the diffusion of deposited titanium into the praseodymium silicate. A homogeneous praseodymium titanium silicate layer is formed that shows high-quality electrical properties.

Pulse-induced low-power resistive switching in HfO2 metal-insulator-metal diodes for nonvolatile memory applications

The conduction process as well as the unipolar resistive switching behavior of Au∕HfO2∕TiN metal-insulator-metal structures were investigated for future nonvolatile memory applications. With current-voltage measurements performed at different temperatures (200–400K), the Poole–Frenkel effect as conduction process was identified. In particular, we extracted a trap energy level at ϕt=0.35±0.05eV below the HfO2 conduction band to which a microscopic origin is tentatively assigned. From current-voltage measurements of Au∕HfO2∕TiN structures, low-power (as low as 120μW) resistive switching was observed. The required forming process is shown to be an energy-induced phenomenon. The characteristics include electric pulse-induced resistive switching by applying pulses up to 100μs and a retention time upon continuous nondestructive readout of more than 104s.

Thin BaHfO3 high-k dielectric layers on TiN for memory capacitor applications

Thin BaHfO3 dielectric films were investigated in view of future dynamic random access memory applications. The dielectric layers were prepared by physical vapor codeposition of BaO and HfO2 onto metallic TiN substrates. Films deposited at 400°C are amorphous, show low leakage [J(1V)<10−8A∕cm2] at capacitance equivalent thicknesses (CETs) down to ∼2nm and a dielectric constant of ∼23. Rapid thermal annealing of the amorphous BaHfO3 films induces crystallization in the cubic perovskite phase with a dielectric constant of ∼38. This k value was observed for films as thin as 8nm enabling CET value of ∼0.9nm.

Band alignment and electron traps in Y2O3 layers on (100)Si

Y2O3 films deposited by atomic vapor deposition on (100)Si with a 2 or 5 nm thick pregrown thermal SiO2 are investigated as possible charge trapping layers. Analysis of these structures using spectroscopic ellipsometry, photoconductivity, and internal photoemission reveals that Y2O3 has a 5.6 eV wide optical bandgap and a 2.0 eV conduction band offset with silicon. Photo(dis)charging experiments show that the optical energy depth of most of the traps exceeds 1.5 eV with respect to the Y2O3 conduction band, explaining the observed charge retention time of ∼108u2002s at room temperature, even in the absence of a blocking insulator.

Praseodymium silicate layers with atomically abrupt interface on Si(100)

Synchrotron radiation x-ray photoelectron spectroscopy was applied to study the solid state reaction between praseodymium and thin silicon dioxide layers on Si(100). Nondestructive depth profiling studies by variation of the incident photon energy indicate after praseodymium deposition at room temperature the reaction of the upper silicon dioxide to praseodymium oxide and silicide. High-temperature annealing of films with an appropriate praseodymium / silicon dioxide ratio results in homogeneous praseodymium silicate films with an atomically abrupt interface. Ab initio calculations corroborate the results of the photoemission study.

Si Segregation into Pr2O3 and La2O3 high-k gate oxides

Pr and La oxide thin films were investigated in the context of their application as high-k dielectrics in complementary metal oxide technology. The films were deposited by molecular beam epitaxy on bare and TiN-covered Si(001). The influence of growth and post-deposition annealing on the composition and electrical parameters was studied. We observed Si penetration from bare Si(001) into the growing film. Based on the results of capacitance–voltage measurements and ab initio calculations, we conclude that Si is a source of defects responsible for leakage currents.

Structure and thickness-dependent lattice parameters of ultrathin epitaxial Pr2O3 films on Si(001)

Pr2O3 grown heteroepitaxially on Si(001) is a promising candidate for applications as a high-k dielectric in future silicon-based microelectronics devices. The technologically important thickness range from 1to10nm has been investigated by synchrotron radiation-grazing incidence x-ray diffraction. The oxide film grows as cubic Pr2O3 phase with its (101) plane on the Si(001) substrate in form of two orthogonal rotation domains. Monitoring the evolution of the oxide unit-cell lattice parameters as a function of film thickness from 1to10nm, the transition from almost perfect pseudomorphism to bulk values is detected.

Thermal stability of Pr silicate high-k layers on Si(001)

Thermal stability of amorphous Pr silicate high-k layers on Si(001) was evaluated in view of complementary metal-oxide-semiconductor transistor processing requirements. Materials science techniques prove that no crystallization, no phase separation into SiO2 and Pr2O3, and no Pr silicide formation at the interface occur after 1min rapid thermal annealing treatment in N2 over the temperature range from 600to900°C. Electrical measurements confirm within this thermal budget well-behaved characteristics with k values between 11 and 13 and leakage currents about three orders of magnitude lower than in case of SiON reference layers.

Plasma-enhanced chemical vapor deposition of amorphous Si on graphene

Plasma-enhanced chemical vapor deposition of thin a-Si:H layers on transferred large area graphene is investigated. Radio frequency (RF, 13.56u2009MHz) and very high frequency (VHF, 140 MHz) plasma processes are compared. Both methods provide conformal coating of graphene with Si layers as thin as 20u2009nm without any additional seed layer. The RF plasma process results in amorphization of the graphene layer. In contrast, the VHF process keeps the high crystalline quality of the graphene layer almost intact. Correlation analysis of Raman 2D and G band positions indicates that Si deposition induces reduction of the initial doping in graphene and an increase of compressive strain. Upon rapid thermal annealing, the amorphous Si layer undergoes dehydrogenation and transformation into a polycrystalline film, whereby a high crystalline quality of graphene is preserved.

Resistive switching in TiN/HfO 2 /Ti/TiN MIM devices for future nonvolatile memory applications

Bipolar resistive switching in TiN/HfO₂/Ti/TiN devices using a CMOS technology process is demonstrated. The performance metrics include a retention time >10⁵ s and a cycling endurance in dc sweep mode >10². By controlling either the set current I_set or by setting an appropriate stop voltage V_stop, the devices hold the potential for multilevel operation. The results suggest that HfO₂-based MIM devices with Si CMOS compatible metal electrodes may be well suited for future embedded nonvolatile memory applications.