Emanuela Schilirò

Negative charge trapping effects in Al2O3 films grown by atomic layer deposition onto thermally oxidized 4H-SiC

This letter reports on the negative charge trapping in Al2O3 thin films grown by atomic layer deposition onto oxidized silicon carbide (4H-SiC). The films exhibited a permittivity of 8.4, a breakdown field of 9.2 MV/cm and small hysteresis under moderate bias cycles. However, severe electron trapping inside the Al2O3 film (1 × 1012 cm−2) occurs upon high positive bias stress (>10V). Capacitance-voltage measurements at different temperatures and stress conditions have been used to determine an activation energy of 0.1eV. The results provide indications on the possible nature of the trapping defects and, hence, on the strategies to improve this technology for 4H-SiC devices.

Interface Electrical Properties of Al2O3 Thin Films on Graphene Obtained by Atomic Layer Deposition with an in Situ Seedlike Layer

High-quality thin insulating films on graphene (Gr) are essential for field-effect transistors (FETs) and other electronics applications of this material. Atomic layer deposition (ALD) is the method of choice to deposit high-κ dielectrics with excellent thickness uniformity and conformal coverage. However, to start the growth on the sp2 Gr surface, a chemical prefunctionalization or the physical deposition of a seed layer are required, which can effect, to some extent, the electrical properties of Gr. In this paper, we report a detailed morphological, structural, and electrical investigation of Al2O3 thin films grown by a two-steps ALD process on a large area Gr membrane residing on an Al2O3-Si substrate. This process consists of the H2O-activated deposition of a Al2O3 seed layer a few nanometers in thickness, performed in situ at 100 °C, followed by ALD thermal growth of Al2O3 at 250 °C. The optimization of the low-temperature seed layer allowed us to obtain a uniform, conformal, and pinhole-free Al2O3 film on Gr by the second ALD step. Nanoscale-resolution mapping of the current through the dielectric by conductive atomic force microscopy (CAFM) demonstrated an excellent laterally uniformity of the film. Raman spectroscopy measurements indicated that the ALD process does not introduce defects in Gr, whereas it produces a partial compensation of Gr unintentional p-type doping, as confirmed by the increase of Gr sheet resistance (from ∼300 Ω/sq in pristine Gr to ∼1100 Ω/sq after Al2O3 deposition). Analysis of the transfer characteristics of Gr field-effect transistors (GFETs) allowed us to evaluate the relative dielectric permittivity (ε = 7.45) and the breakdown electric field (EBD = 7.4 MV/cm) of the Al2O3 film as well as the transconductance and the holes field-effect mobility (∼1200 cm2 V-1 s-1). A special focus has been given to the electrical characterization of the Al2O3-Gr interface by the analysis of high frequency capacitance-voltage measurements, which allowed us to elucidate the charge trapping and detrapping phenomena due to near-interface and interface oxide traps.

Advances in the fabrication of graphene transistors on flexible substrates

Graphene is an ideal candidate for next generation applications as a transparent electrode for electronics on plastic due to its flexibility and the conservation of electrical properties upon deformation. More importantly, its field-effect tunable carrier density, high mobility and saturation velocity make it an appealing choice as a channel material for field-effect transistors (FETs) for several potential applications. As an example, properly designed and scaled graphene FETs (Gr-FETs) can be used for flexible high frequency (RF) electronics or for high sensitivity chemical sensors. Miniaturized and flexible Gr-FET sensors would be highly advantageous for current sensors technology for in vivo and in situ applications. In this paper, we report a wafer-scale processing strategy to fabricate arrays of back-gated Gr-FETs on poly(ethylene naphthalate) (PEN) substrates. These devices present a large-area graphene channel fully exposed to the external environment, in order to be suitable for sensing applications, and the channel conductivity is efficiently modulated by a buried gate contact under a thin Al2O3 insulating film. In order to be compatible with the use of the PEN substrate, optimized deposition conditions of the Al2O3 film by plasma-enhanced atomic layer deposition (PE-ALD) at a low temperature (100 °C) have been developed without any relevant degradation of the final dielectric performance.

Metal-Organic Chemical Vapor Deposition (MOCVD) Synthesis of Heteroepitaxial Pr0.7Ca0.3MnO3 Films: Effects of Processing Conditions on Structural/Morphological and Functional Properties

Calcium-doped praseodymium manganite films (Pr0.7Ca0.3MnO3, PCMO) were prepared by metal-organic chemical vapor deposition (MOCVD) on SrTiO3 (001) and SrTiO3 (110) single-crystal substrates. Structural characterization through X-ray diffraction (XRD) measurements and transmission electron microscopy (TEM) analyses confirmed the formation of epitaxial PCMO phase films. Energy dispersive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS) characterization was used to confirm lateral and vertical composition and the purity of the deposited films. Magnetic measurements, obtained in zero-field-cooling (ZFC) and field-cooling (FC) modes, provided evidence of the presence of a ferromagnetic (FM) transition temperature, which was correlated to the transport properties of the film. The functional properties of the deposited films, combined with the structural and chemical characterization collected data, indicate that the MOCVD approach represents a suitable route for the growth of pure, good quality PCMO for the fabrication of novel spintronic devices.

Atomic Layer Deposition of Al2O3 Thin Films for Metal Insulator Semiconductor Applications on 4H-SiC

This work reports on the growth and characterization of Al2O3 films on 4H-SiC, by Plasma Enhanced-Atomic Layer Deposition (PE-ALD). Different techniques were used to investigate the morphological, structural and electrical features of the Al2O3 films, both with and without the presence of a thin SiO2 layer, thermally grown on the 4H-SiC before ALD. Capacitance-voltage measurements on MOS structures resulted in a higher dielectric constant (ε~8.4) for the Al2O3/SiO2/SiC stack, with respect to that of the Al2O3/SiC sample (ε~ 6.7). Moreover, Current density-Electric Field measurements demonstrated a reduction of the leakage current and an improvement of the breakdown behaviour in the presence of the interfacial thermally grown SiO2. Basing on these preliminary results, possible applications of ALD-Al2O3 as gate insulator in 4H-SiC MOSFETs can be envisaged.

Plasma enhanced atomic layer deposition of Al2O3 gate dielectric thin films on AlGaN/GaN substrates: The role of surface predeposition treatments

Al2O3 thin films were deposited by plasma enhanced atomic layer deposition (PEALD) from trimethylaluminum precursor and oxygen plasma at 250 °C on AlGaN/GaN heterostructures. Before deposition, the sample surfaces were treated with the following solutions: (A) H2O2:H2SO4 (piranha), (B) fluoride acid (HF) + HCl, and (C) piranha + HF for 10 min each. Transmission electron microscopy analysis revealed that, independently from the surface preparation, all the films are adherent and uniform with thicknesses of about 27–28 nm. However, a different structural evolution has been observed under electron beam effect. In particular, while all the as-deposited films were found to be amorphous, the formation of polycrystalline grains was observed on the sample deposited after the A treatment. On the other hand, oriented layers were formed on the samples deposited after B and C treatments. This result is an indication that in the case of HF-based treatments, the PEALD occurred on a very clean AlGaN surface, which can a...

Thermal and plasma-enhanced atomic layer deposition of hafnium oxide on semiconductor substrates

Hafnium oxide thin films have been deposited on Si(001) substrates by atomic layer deposition from the tetrakis-dimethylamino hafnium precursor using both conventional thermal and plasma-enhanced methods. The structural, compositional and morphological film characterization has been carried out by transmission electron microscopy, X-ray photoelectron spectroscopy and atomic force microscopy. All the data indicate that some reactive phenomena occur at the film/substrate interface forming a hafnium silicate layer. The electrical characterization of the two deposited layers has been carried out in order to evaluate its potential implementation as an alternative dielectric. Their dielectric constant values have been evaluated to be 7.5 and 5.5 for films deposited by the plasma-enhanced and thermal ALD processes, respectively.

Nanoscale electrical mapping of two-dimensional materials by conductive atomic force microscopy for transistors applications

Two-dimensional (2D) materials are currently object of many interests both from a basic and a technological standpoint. In particular, graphene (Gr) and the semiconducting transition metal dichalcogenides (including MoS2, WS2, MoSe2, WSe2) have been widely investigated for transistors applications. As a matter of fact, 2D materials present peculiar nanoscale structural and electrical inhomogeneities, related to the specific synthesis mechanisms and to the interaction with the substrate, which are ultimately reflected in the macroscopic electrical behaviour of electronic devices based on these systems. In this context, conductive atomic force microscopy (CAFM) is the method of choice to investigate the mechanisms of current injection between contacts and 2D materials and/or the lateral homogeneity of 2D materials electrical properties. This paper will discuss some case studies of CAFM applications to Gr and MoS2, to illustrate the potentiality of this characterization method for 2D materials investigation....

Advances in the Fabrication of Large-Area Back-Gated Graphene Field-Effect Transistors on Plastics: Platform for Flexible Electronics and Sensing

Graphene (Gr) is currently one of the most appealing materials as conductive transparent electrode for flexible electronics, thanks to its bendability/stretchability accompanied by small variations of the electrical properties after mechanical deformations. In addition, the field-effect tunable carrier density combined to a high mobility and saturation velocity make it an excellent channel material for field-effect transistors (FETs) even on flexible substrates. By proper design of the device structure (channel length, top- or back-gate configuration), Gr-FETs can be used for high-frequency (RF) electronics or for high-sensitivity chemical, biological, and environmental sensors exploiting transconductance variations in response to the chemi/physisorption of molecular species on Gr channel. In particular, miniaturized and flexible Gr-FET sensors can represent a strong advance with respect to current sensors technology and will be extremely useful for “in situ” applications. Here we report a wafer scale and semiconductor fab compatible processing strategy to fabricate arrays of Gr-FETs on a PEN substrate, adopting a local back-gate configuration, with a thin Al2O3 gate dielectric film deposited at low temperature (100 °C) by plasma-assisted Atomic Layer Deposition (ALD) and transfer of large-area Gr grown by chemical vapor deposition on copper foils. Electrical characterization of the fabricated devices is presented and their suitability for solid ion sensing FET (IS-FET) applications is discussed.