Marie-Paule Besland

Impedance and electric modulus study of amorphous TiTaO thin films: highlight of the interphase effect

The influence of phases and phase’s boundaries of TiO2 and Ta2O5 in the dielectric and electric response of TiTaO (100 nm thick) elaborated by RF magnetron sputtering was highlighted by complex impedance spectroscopy. Dielectric and electric modulus properties were studied over a wide frequency range (0.1–10 5 Hz) and at various temperatures (−160 to 120 ◦ C). The diagram of Argand (e �� versus e � ) shows the contribution of phases, phases’ boundaries and conductivity effect on the electric response of TiTaO thin films. Moreover, the resistance of the material decreases when the temperature increases, thus the material exhibits a negative temperature coefficient of resistance. The electric modulus plot indicates the presence of two peaks of relaxation. The first relaxation process appears at low temperature with activation energy of about 0.22 eV and it is related to the first ionization energy of oxygen vacancies. The second relaxation process appears at high temperature with activation energy of about 0.44 eV. This second peak is attributed to the Maxwell–Wagner–Sillars relaxation. The plots of the complex dielectric modulus and the impedance as a function of frequency allow concluding to a localized relaxation due to the long-range conductivity in the TiTaO film. (Some figures may appear in colour only in the online journal)

Impact of magnetron configuration on plasma and film properties of sputtered aluminum nitride thin films

We have investigated the growth of the c-axis oriented aluminum nitride (AlN) thin films on (100) silicon by reactive dc magnetron sputtering at low temperature, considering the effect of the magnet configuration on plasma and film properties. It appears that a magnet modification can significantly modify both the plasma characteristics and the film properties. Electrical and optical characterizations of the plasma phase highlight that depending on the magnet configuration, two very different types of deposition process can be involved in the same deposition chamber. On the one hand, with a balanced magnetron (type 1), the deposition process enhances the production of AlN dimers in the plasma phase and enables to synthesize AlN films with different preferential orientations (100, 002, and even 101). On the other hand, a strongly unbalanced magnetron (type 2) provides a limited production of AlN species in the plasma phase and a strong increase in the ratio of ions to metal atom flux on the growing films. ...

Effect of surface preparation and interfacial layer on the quality of SiO2/GaN interfaces

In this work, SiO2/GaN MOS structures have been fabricated using Electron Cyclotron Resonance Plasma Enhanced Chemical Vapor Deposition (ECR-PECVD) for deposition of silicon dioxide (SiO2) at low temperature (300 °C) on N-type, NID and P-type GaN epitaxial layers. Surface preparation involving chemical, UV-Ozone oxidation and oxygen plasma oxidation have been investigated by XPS analysis of the GaN surfaces prior to SiO2 deposition. The association of UV ozone and plasma oxidation allows a complete removal of carbon contamination and has a huge beneficial effect on the quality of the SiO2/GaN interface. Electrical C-V characterizations put into evidence the improved quality of the SiO2/GaN interface with a low interface trap density of 1010 cm−2 eV−1. The advantage of this soft interface treatment is thus specially observed for the N-type samples without annealing step, whereas improvements are still needed in the case of NID and P-type samples.

Epitaxial growth of aluminum nitride on AlGaN by reactive sputtering at low temperature

We report the synthesis of 1μm thick single crystalline aluminum nitride films by dc magnetron sputtering on AlGaN∕GaN layer grown on sapphire substrate at low temperature (substrate temperature <250°C). The microstructure of c-axis oriented AlN films deposited on Si (100) and AlGaN ⟨0001⟩ substrates was studied by x-ray diffraction, selected area electron diffraction, and transmission electron microscopy. The optimization of process parameters, involving low energetic ion bombardment on film surface (20–30eV) during the growth, leads to an increase in the surface mobility and thus promotes AlN epitaxial growth on AlGaN substrate at 250°C.

Deposition of GaV4S8 thin films by H2S/Ar reactive sputtering for ReRAM applications

The chalcogenide compound GaV4S8 is promising for applications as active materials in non-volatile memory applications. We report here a comprehensive study on the thin-film deposition of this compound using a stoichiometric GaV4S8 target and H2S/Ar reactive sputtering. We show that a fraction of 0.5% to 1% of H2S in the reactive plasma is sufficient to compensate the sulfur deficiency that appears in films deposited in pure Ar plasma. This reactive plasma method allows avoiding the addition of elemental sulfur during the annealing treatment required to obtain a crystallized and stoichiometric GaV4S8 layer. A simple Au/GaV4S8/Au structure presents a resistive switching behaviour well suited for non-volatile memory applications.

Dielectric relaxation study of amorphous TiTaO thin films in a large operating temperature range

Two relaxation processes have been identified in amorphous TiTaO thin films deposited by reactive magnetron sputtering. The parallel angle resolved x-ray photoelectron spectroscopy and field emission scanning electron microscopy analyses have shown that this material is composed of an agglomerates mixture of TiO2, Ta2O5, and Ti-Ta bonds. The first relaxation process appears at low temperature with activation energy of about 0.26 eV and is related to the first ionisation of oxygen vacancies and/or the reduction of Ti4+ to Ti3+. The second relaxation process occurs at high temperature with activation energy of 0.95 eV. This last peak is associated to the diffusion of the doubly ionized oxygen vacancies VO. The dispersion phenomena observed at high temperature can be attributed to the development of complex defect such as (VO − 2Ti3+).

Different threshold and bipolar resistive switching mechanisms in reactively sputtered amorphous undoped and Cr-doped vanadium oxide thin films

This study investigates resistive switching in amorphous undoped and Cr-doped vanadium oxide thin films synthesized by sputtering deposition at low oxygen partial pressure. Two different volatile threshold switching characteristics can occur as well as a non-volatile bipolar switching mechanism, depending on device stack symmetry and Cr-doping. The two threshold switching types are associated with different crystalline phases in the conduction filament created during an initial forming step. The first kind of threshold switching, observed for undoped vanadium oxide films, was, by its temperature dependence, proven to be associated with a thermally triggered insulator-to-metal transition in a crystalline VO2 phase, whereas the threshold switch observed in chromium doped films is stable up to 90 °C and shows characteristics of an electronically induced Mott transition. This different behaviour for undoped versus doped films has been attributed to an increased stability of V3+ due to the Cr3+ doping (as evid...

Thin Layers Obtained by Plasma Process for Emerging Non-Volatile Memory (RRAM) Applications

In that work, we show that RF magnetron sputtering can be used to obtain pure and crystallized thin layers of GaV 4 S 8 films with a well controlled composition, depending mainly on RF power and deposition pressure. The obtained layers exhibit similar structural and physical properties as GaV 4 S 8 polycrystal or crystal. In addition, we have successfully demonstrated that a reversible resistive switching can also be obtained on thin layers. Electrical measurements will be further investigated on thinner layers and for shorter pulse time. Nevertheless, these preliminary results obtained on Au/Si substrates are very promising for further RRAM applications.

Bi3.25La0.75Ti3O12 films on La2Ti2O7 thin films prepared by chemical solution deposition

We report the synthetic and physical procedures for obtaining the Lanthanum-substituted Bi4Ti3O12 (BLT) ferroelectric films, Bi3.25La0.75Ti3O12, with its polarisation axis (a) oriented perpendicular to the surface of the electrode by employing partially oriented (001) polycrystalline thin films of La2Ti2O7 (LTO) as a buffer onto Si-wafers. The LTO thin film was achieved by dip coating and annealing at high temperatures while the BLT film was deposited using RF magnetron sputtering and annealing at a temperature as low as 650°C. Furthermore, the dependence of the thickness, grain size and orientation of the LTO films on the withdrawal speed of the dip-coating and annealing temperature is reported.

Comparison of GaN-based MOS structures with different interfacial layer treatments

In this work, C-V characteristics of MOS capacitors fabricated by depositing SiO2 by plasma-enhanced chemical-vapor-deposition at low temperature (300 °C) on an N-type GaN epitaxial layer using different technological procedures have been performed to analyze the quality and reliability of the resultant SiO2/GaN interface in MOS structures. C-V measures at different temperatures have been also performed to analyze the pyroelectric effect in all the GaN samples.