Ioan Costina

Residual Metallic Contamination of Transferred Chemical Vapor Deposited Graphene

Integration of graphene with Si microelectronics is very appealing by offering a potentially broad range of new functionalities. New materials to be integrated with the Si platform must conform to stringent purity standards. Here, we investigate graphene layers grown on copper foils by chemical vapor deposition and transferred to silicon wafers by wet etching and electrochemical delamination methods with respect to residual submonolayer metallic contaminations. Regardless of the transfer method and associated cleaning scheme, time-of-flight secondary ion mass spectrometry and total reflection X-ray fluorescence measurements indicate that the graphene sheets are contaminated with residual metals (copper, iron) with a concentration exceeding 10(13) atoms/cm(2). These metal impurities appear to be partially mobile upon thermal treatment, as shown by depth profiling and reduction of the minority charge carrier diffusion length in the silicon substrate. As residual metallic impurities can significantly alter electronic and electrochemical properties of graphene and can severely impede the process of integration with silicon microelectronics, these results reveal that further progress in synthesis, handling, and cleaning of graphene is required to advance electronic and optoelectronic applications.

Imaging of strain and lattice orientation by quick scanning X-ray microscopy combined with three-dimensional reciprocal space mapping

Numerous imaging methods have been developed over recent years in order to study materials at the nanoscale. Within this context, scanning X-ray diffraction microscopy has become a routine technique, giving access to structural properties with sub-micrometre resolution. This article presents an optimized technique and an associated software package which have been implemented at the ID01 beamline (ESRF, Grenoble). A structural scanning probe microscope with intriguing imaging qualities is obtained. The technique consists in a two-dimensional quick continuous mapping with sub-micrometre resolution of a sample at a given reciprocal space position. These real space maps are made by continuously moving the sample while recording scattering images with a fast two-dimensional detector for every point along a rocking curve. Five-dimensional data sets are then produced, consisting of millions of detector images. The images are processed by the user-friendly X-ray strain orientation calculation software (XSOCS), which has been developed at ID01 for automatic analysis. It separates tilt and strain and generates two-dimensional maps of these parameters. At spatial resolutions of typically 200–800 nm, this quick imaging technique achieves strain sensitivity below Δa/a = 10−5 and a resolution of tilt variations down to 10−3° over a field of view of 100 × 100 µm.

Oxygen Vacancy Induced Room Temperature Ferromagnetism in Pr-Doped CeO2 Thin Films on Silicon

Integration of functional oxides on Si substrates could open a pathway to integrate diverse devices on Si-based technology. Oxygen vacancies (Vo(··)) can strongly affect solid state properties of oxides, including the room temperature ferromagnetism (RTFM) in diluted magnetic oxides. Here, we report a systematical study on the RTFM of oxygen vacancy engineered (by Pr(3+) doping) CeO2 epitaxial thin films on Si substrates. High quality, mixed single crystalline Ce1-xPrxO2-δ (x = 0-1) solid solution films were obtained. The Ce ions in CeO2 with a fluorite structure show a Ce(4+)-dominant valence state in all films. The local crystal structures of the films were analyzed in detail. Pr doping creates both Vo(··) and PrO8-complex defects in CeO2 and their relative concentrations vary with the Pr-doping level. The RTFM properties of the films reveal a strong dependence on the relative Vo(··) concentration. The RTFM in the films initially increases with higher Pr-doping levels due to the increase of the F(+) center (Vo(··) with one occupied electron) concentration and completely disappears when x > 0.2, where the magnetic polaron concentration is considered to decline below the percolation threshold, thus long-range FM order can no longer be established. We thus demonstrate the possibility to directly grow RTFM Pr-doped CeO2 films on Si substrates, which can be an interesting candidate for potential magneto-optic or spintronic device applications.

Atomic Vapor Deposition of Strontium Tantalate Films for MIM Applications

Sr-Ta-O thin films were deposited as high-k dielectrics for metal-insulator-metal applications on 200-mm TiN/Si(100) substrates from a single-source Sr[Ta(OEt)₅(methoxyethoxide)]₂ precursor using atomic vapor deposition technique. The variation of process pressure affects the Sr/Ta ratio in the films. Dielectric layers with optimized composition of Sr₂Ta₂O_7-delta possess a capacitance density of 5.5 fF/mum² in combination with a voltage linearity coefficient of 80 ppm/V² and a quality factor of 52 at 10 kHz. The optimized films with thickness of 30 nm exhibit a leakage current density of 7 ldr 10^-9 A/cm² at 2 V and a breakdown strength of 3.2 MV/cm, and, therefore, meet the requirements of the current International Roadmap for Semiconductors.

Material insights of HfO2-based integrated 1-transistor-1-resistor resistive random access memory devices processed by batch atomic layer deposition.

With the continuous scaling of resistive random access memory (RRAM) devices, in-depth understanding of the physical mechanism and the material issues, particularly by directly studying integrated cells, become more and more important to further improve the device performances. In this work, HfO2-based integrated 1-transistor-1-resistor (1T1R) RRAM devices were processed in a standard 0.25 μm complementary-metal-oxide-semiconductor (CMOS) process line, using a batch atomic layer deposition (ALD) tool, which is particularly designed for mass production. We demonstrate a systematic study on TiN/Ti/HfO2/TiN/Si RRAM devices to correlate key material factors (nano-crystallites and carbon impurities) with the filament type resistive switching (RS) behaviours. The augmentation of the nano-crystallites density in the film increases the forming voltage of devices and its variation. Carbon residues in HfO2 films turn out to be an even more significant factor strongly impacting the RS behaviour. A relatively higher deposition temperature of 300 °C dramatically reduces the residual carbon concentration, thus leading to enhanced RS performances of devices, including lower power consumption, better endurance and higher reliability. Such thorough understanding on physical mechanism of RS and the correlation between material and device performances will facilitate the realization of high density and reliable embedded RRAM devices with low power consumption.

Non-uniform depth distributions of Sn concentration induced by Sn migration and desorption during GeSnSi layer formation

The distributions of Sn concentration in GeSnSi layers formed on Ge substrate at various temperatures were investigated. High deposition temperature (Td) induces significant Sn migration and desorption, which have activation energies of 0.75 eV and 0.27 eV, respectively. A model quantitatively clarified the Sn migration fluxes during the deposition, which increase not only with increasing Td but also with the layer thickness. A non-negligible Sn flux compared with the supplied flux was found at 350 °C at the surface of the 200-nm-thick layer. Consequently, designs of layer thickness and Td taking into account the appropriate Sn flux are important to form a GeSnSi layer with uniform Sn content for future optoelectronics.

Impact of the precursor chemistry and process conditions on the cell-to-cell variability in 1T-1R based HfO 2 RRAM devices

The Resistive RAM (RRAM) technology is currently in a level of maturity that calls for its integration into CMOS compatible memory arrays. This CMOS integration requires a perfect understanding of the cells performance and reliability in relation to the deposition processes used for their manufacturing. In this paper, the impact of the precursor chemistries and process conditions on the performance of HfO2 based memristive cells is studied. An extensive characterization of HfO2 based 1T1R cells, a comparison of the cell-to-cell variability, and reliability study is performed. The cells’ behaviors during forming, set, and reset operations are monitored in order to relate their features to conductive filament properties and process-induced variability of the switching parameters. The modeling of the high resistance state (HRS) is performed by applying the Quantum-Point Contact model to assess the link between the deposition condition and the precursor chemistry with the resulting physical cells characteristics.

Atomically controlled processing for Ge CVD epitaxial growth

The concept of atomically controlled processing for group IV semiconductors is based on atomic-order surface reaction control. This approach is especially important for the epitaxial deposition of very thin (nm) layers. Here, the existences of Ge oxide in the CVD reactor resulting from former Ge deposition and hydrogen termination of the wafer surface is impacting the epitaxial growth essentially. Therefore the evaporation of Ge oxide is suppressed by Si coating the reactor before wafer loading and/or Si capping after Ge growth and/or very low temperature SiH4 treatment after wafer loading. By the use of Si0.5Ge0.5 buffer layer, hydrogen termination of the surface is reduced. As a result, nm-order thick Ge epitaxial growth with very short incubation period and the suppression of surface roughness generation is realized.

Strain Control of Si and Si1-yCy Layers in Si/Si1-yCy/Si(100) Heterostructures

This paper demonstrated that lattice constant and Raman shifts of Si-Si and Si-C modes of Si_1-yC_y are normalized by C fraction obtained from XPS C_1s peak at 283.3 eV that is substitutional C fraction. By stripe-shape patterning of the Si(10 nm)/Si_0.98C_0.02(20-60 nm)/Si(100) heterostructure, compressive-strained Si cap layer was realized.

Phosphorus Atomic Layer Doping in Ge Using RPCVD

P atomic layer doping (P-ALD) of Ge is investigated at temperatures between 100oC and 300oC using a single wafer reduced pressure CVD system. Hydrogen-terminated and hydrogen-free Ge (100) surface are exposed to PH3 followed by Ge deposition at 300oC. P adsorption is suppressed by hydrogen-termination of Ge surface. On hydrogen-free Ge surface, incorporated P dose is increased with increasing PH3 exposure time and saturation behavior is observed. The saturation value of P by P-ALD at 300oC is ~1.5e14 cm-2, which is close to a quarter of monolayer of Ge surface. The saturation value is not depending on PH3 partial pressure. The incorporated P dose is able to be described by Langmuir type kinetic. The electrical active P concentration of ~6e19cm-3, which is ~6 times higher active P concentration compared to solid solubility is obtained.