Y. Panayiotatos

The role of La surface chemistry in the passivation of Ge

The oxidation of a Ge surface by molecular oxygen in the presence of ultrathin La, Al, and Hf layers was examined by in situ x-ray photoelectron spectroscopy. Upon exposure to O2, clean bare Ge and Hf-covered or Al-covered Ge surfaces show no Ge–O bond formation. On the contrary, a La-covered Ge surface strongly reacts with O2 forming a stable germanate LaGeOx compound. This has a beneficial side effect for the interface because the formation of volatile GeO is suppressed, resulting in the good passivating properties of LaGeOx. The photoemission results are correlated with the oxygen density differences in the corresponding oxides.

Comprehensive study on the properties of multilayered amorphous carbon films

Abstract Amorphous carbon (a-C) multilayered films consisting of sequential layers rich in sp2 (A) and sp3 (B) content have been developed by magnetron sputtering. We study here the effect of thickness d of the A layer in developing stable thick films with controllable stress and elastic properties. In situ spectroscopic ellipsometry is used to calculate the thickness and the composition of the individual layers. The latter were compared with those obtained by depth profiling X-ray photoelectron spectroscopy, which also provides the different chemical bonding of the multilayers in depth. The stress and hardness of the deposited a-C films were found to be related to the thickness of the Aj layers and the relative ratio dAj/dB of thicknesses. The possible mechanisms for the stress control, stability and enhancement of elastic properties of multilayered a-C films are discussed.

Mechanical performance and growth characteristics of boron nitride films with respect to their optical, compositional properties and density

Abstract We present a study of the growth processes of sputtered BN films, deposited on c-Si (001) substrates, at room temperature (RT) by rf magnetron sputtering. Nanoindentation (hardness ∼21 GPa), density (∼2.6 g/cm 3 ) and roughness by X-ray reflectivity and stress measurements indicated the existence of sp 3 -bonded BN with properties similar to those of crystalline BN being superior for optical applications due to their homogeneity offering a single refractive index and smooth surfaces. The growth mechanism of sputtered BN films is sensitive to the Ar partial pressure and the bias voltage applied to the substrate during deposition inducing bombardment of the film with Ar + ions. There is a narrow ion energy window where sp 3 -bonded BN is predominant providing dense, hard- and wear-resistant films. The fact that they are produced at RT and they are homogeneous with no gradient refractive index and smooth surfaces are correct for processes, such as optical applications on polymers or soft substrates.

A study on the bonding structure and mechanical properties of magnetron sputtered CNx thin films

Abstract Carbon nitride (CN x ) films have been deposited by reactive (RF) magnetron sputtering, in order to investigate the effect of the energetic ion bombardment during deposition (IBD), in terms of applied V b , on their bonding structure. Fourier Transform IR Ellipsometry (FTIRE) and X-ray photoelectron spectroscopy (XPS) were used for the investigation of the films bonding structure, while their mechanical properties were evaluated by nanoindentation measurements. At films grown with low negative V b , (low energy IBD) the N atoms are distributed homogeneously in substitutional sites in graphitic rings through both sp 2 and sp 3 bonds and in linear chains, through sp 2 bonds. In contrast, the high negative V b (high energy IBD) has been suggested to promote the non-homogeneous N distribution at localized regions in the films where the formation of sp 3 CN bonds is favored. This behavior was also evidenced by the C1s and N1s XPS peak components, assigned to the sp 3 and sp 2 carbon–nitrogen bonds. Also, high energy IBD films revealed increased values of hardness and elasticity, while hardness values up to 45 GPa were measured at localized regions.

Chemical stability of lanthanum germanate passivating layer on Ge upon high-k deposition: A photoemission study on the role of La in the interface chemistry

The chemical stability of La-containing passivating layers /high-k gate stacks on Ge substrates is investigated by x-ray photoelectron spectroscopy. It is found that upon exposure to oxygen, a La-covered Ge surface spontaneously converts to a stable LaGeOx passivating layer. However, in the presence of overlying metal layers such as Al and Hf or upon deposition of high-k metal oxide cap layers such as Al2O3 and HfO2, the LaGeOx layer strongly reacts with the overlayers forming LaAlOz and LaHfOz compounds. In the case of Al or Al2O3, the LaGeOx is completely dissociated, while in the case of Hf or HfO2, LaGeOx remains although with a change in composition. The results are interpreted in terms of oxygen densification according to which the strongly electropositive La drives the reaction toward the formation of oxides with the highest oxygen density. The results suggest that HfO2 has a better chemical compatibility with LaGeOx making it more suitable for the gate stack.

Microstructure and its effect on the conductivity of magnetron sputtered carbon thin films

Carbon thin films were grown by magnetron sputtering at room temperature on silicon substrates, with the substrate bias voltage varying from +10 to −200 V. Transmission electron microscopy analysis has shown that films deposited at Vb=+10 and −40 V are amorphous (α-C), while films deposited at Vb=−200 V are nanocrystalline (nc-C). Temperature dependent conductivity measurements were carried out in the temperature range 300–77 K. With respect to conductivity, the results indicate that the investigated carbon films are classified in three groups: (i) In α-C films deposited at Vb=+10 V (sp2 rich bonds), the variable range hopping (VRH) conduction dominates below 300 K. (ii) In α-C films deposited at negative Vb up to −100 V (sp3 rich bonds), VRH conduction dominates at low temperatures (T 150 K). (iii) In nc-C film deposited at Vb=−200 V, the conductivity is explained by a heteroquantum-dots model based on a t...

Post deposition annealing studies of lanthanum aluminate and ceria high-k dielectrics on germanium.

Abstract Germanium MOS transistors with high- k gates are good alternatives for the replacement of SiO 2 in order to improve the performance of modern devices. Especially rare-earth oxides on germanium deposited by molecular beam deposition (MBD) have shown improved electrical properties compared to previous used HfO 2 with a germanium oxynitride (GeON) interfacial layer. In this work we report on the influence of ex situ post-annealing treatment with forming gas on the electrical characteristics of LaAlO 3 /Al 2 O 3 /nGe and CeO 2 /nGe MIS capacitors. We have observed an improvement of the electrical characteristics after forming gas anneal (FGA) for LaAlO 3 /Al 2 O 3 /nGe in contrast to CeO 2 /nGe which shows no clear trend regarding the influence of FGA.

Variation of nitrogen incorporation and bonding configuration of carbon nitride films studied by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectroscopic ellipsometry

Abstract A detailed study of the bonding structure and chemical composition of carbon nitride (CN x ) thin films prepared on c-Si substrates by RF reactive magnetron sputtering is presented. The nitrogen content in 19×10 −3 and 4×10 −3 Ar/N 2 mixtures was varied between 0 and 100% while the bias voltage ( V b ) between +10 and −200 V. The films’ elemental composition and chemical bonding were determined by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopic ellipsometry (FTIRSE). The N/C ratio decreased substantially for films grown with high negative V b . The N1s spectral region was fitted with multiple peaks corresponding to various carbon–nitrogen bonding configurations (sp 3 , sp 2 and sp 1 ), and oxygen-bonded N. At films grown with V b =+10 V, the N atoms distribution among the possible bonding structures with C was unaffected by the P N 2 changes. These films exhibit a mixed sp 1 /sp 2 /sp 3 character with an enhanced contribution of sp 2 CN and sp 1 CN and NC bonds, as evidenced by FTIRSE. A substantial increase of the sp 3 CN bond contribution in films grown with −40≤ V b ≤−200 V was evidenced by both XPS and FTIRSE.

Structural, electrical, and low-frequency-noise properties of amorphous-carbon–silicon heterojunctions

The structural, electrical, and low-frequency-noise properties of heterojunctions of amor- phous-carbon (a-C) films grown on either n- or p-type single-crystal silicon are investigated. The a-C films were deposited by rf magnetron sputtering at room temperature with varying the substrate bias Vb, from +10 to −200 V. The study includes measurements of x-ray reflectivity (XRR), low-frequency noise at room temperature, and dark current–voltage (I–V) and capacitance–voltage (C–V) characteristics over a wide temperature range. Analysis of the XRR data indicates the presence of a thin SiC layer between a-C and Si, with thickness increasing up to about 1.8 nm for Vb=−200 V. The results show that the noise properties of the devices are independent of the SiC interlayer and the a-C film deposition conditions, while the noise of the a-C/n-Si heterojunctions is about four orders of magnitude lower than that of the a-C/p-Si heterojunctions. Analysis of the I–V and C–V data shows that the rectification properties of t...

Raman and photoluminescence study of magnetron sputtered amorphous carbon films

Abstract The structural and optical properties of amorphous carbon films, grown by r.f.-magnetron sputtering on silicon substrates, were studied by Raman and photoluminescence spectroscopy in dependence of the substrate bias voltage V b . The intensity ratio I (D)/ I (G) of the Raman bands of disordered graphite ( D -band) and graphite (G-band) decreased significantly (a) by reversing bias from positive (+10 V) to negative (−20 V) and (b) by reducing the negative bias from −200 to −120 V. The intensity ratio I (D)/ I (G) exhibited an almost flat minimum in the bias-region from −120 to −20 V which is indicative of an increase of the fraction of sp 3 -bonded material. In the same bias-range, photoluminescence emission at 2.2–2.5 eV appeared blue-shifted and more efficient. Changes in photoluminescence energy are attributed to increase of sp 3 content of the films becoming more transparent when deposited at substrate-bias between −20 and −120 V. Increase of photoluminescence intensity is probably related to increased number of defects due to increasing structural disorder.