Stefan De Gendt

Bandgap opening in oxygen plasma-treated graphene

We report a change in the semimetallic nature of single-layer graphene after exposure to oxygen plasma. The resulting transition from semimetallic to semiconducting behavior appears to depend on the duration of the exposure to the plasma treatment. The observation is confirmed by electrical, photoluminescence and Raman spectroscopy measurements. We explain the opening of a bandgap in graphene in terms of functionalization of its pristine lattice with oxygen atoms. Ab initio calculations show more details about the interaction between carbon and oxygen atoms and the consequences on the optoelectronic properties, that is, on the extent of the bandgap opening upon increased functionalisation density.

Polarity effect on the temperature dependence of leakage current through HfO2/SiO2 gate dielectric stacks

A strong polarity effect on the temperature dependence of the leakage current in TiN/HfO2/SiO2/Si capacitors is reported. A model is proposed to explain these experimental results that combines tunneling through the stack and Frenkel–Poole hopping in the HfO2 layer, depending on the value of the gate voltage. It is shown that the polarity effect most probably results from the anisotropy of the band diagram of the HfO2/SiO2 stack, as well as from the location of the shallow traps with respect to the conduction band of the HfO2 layer. Comparison of the model with the experimental results allows an estimate of the trap depth to be between 0.5 and 0.8 eV.

Island growth in the atomic layer deposition of zirconium oxide and aluminum oxide on hydrogen-terminated silicon: Growth mode modeling and transmission electron microscopy

Atomic layer deposition (ALD) is used in applications where inorganic material layers with uniform thickness down to the nanometer range are required. For such thicknesses, the growth mode, defining how the material is arranged on the surface during the growth, is of critical importance. In this work, the growth mode of the zirconium tetrachloride∕water and the trimethyl aluminum∕water ALD process on hydrogen-terminated silicon was investigated by combining information on the total amount of material deposited with information on the surface fraction of the material. The total amount of material deposited was measured by Rutherford backscattering, x-ray fluorescence, and inductively coupled plasma–optical emission spectroscopy, and the surface fractions by low-energy ion scattering. Growth mode modeling was made assuming two-dimensional growth or random deposition (RD), with a “shower model” of RD recently developed for ALD. Experimental surface fractions of the ALD-grown zirconium oxide and aluminum oxid...

Measuring the electrical resistivity and contact resistance of vertical carbon nanotube bundles for application as interconnects

Carbon nanotubes (CNT) are known to be materials with potential for manufacturing sub-20 nm high aspect ratio vertical interconnects in future microchips. In order to be successful with respect to contending against established tungsten or copper based interconnects, though, CNT must fulfil their promise of also providing low electrical resistance in integrated structures using scalable integration processes fully compatible with silicon technology. Hence, carefully engineered growth and integration solutions are required before we can fully exploit their potentialities. This work tackles the problem of optimizing a CNT integration process from the electrical perspective. The technique of measuring the CNT resistance as a function of the CNT length is here extended to CNT integrated in vertical contacts. This allows extracting the linear resistivity and the contact resistance of the CNT, two parameters to our knowledge never reported separately for vertical CNT contacts and which are of utmost importance, as they respectively measure the quality of the CNT and that of their metal contacts. The technique proposed allows electrically distinguishing the impact of each processing step individually on the CNT resistivity and the CNT contact resistance. Hence it constitutes a powerful technique for optimizing the process and developing CNT contacts of superior quality. This can be of relevant technological importance not only for interconnects but also for all those applications that rely on the electrical properties of CNT grown with a catalytic chemical vapor deposition method at low temperature.

Performance Enhancement in Multi Gate Tunneling Field Effect Transistors by Scaling the Fin-Width

This paper discusses the electrical characterization of complementary multiple-gate tunneling field effect transistors (MuGTFETs), implemented in a MuGFET technology compatible with standard complementary metal oxide semiconductor (CMOS) processing, emphasizing the dependence of the tunneling current on the fin-width. A linear dependence of the tunneling current for narrow fins with the square root of the fin width is experimentally reported for the first time. The comparison between narrow fins and planar-like fins offers additional insights about the fin-width dependence. The output characteristic shows a perfect saturation, very attractive for analog circuits. The temperature dependence is measured indicating a weak dependence as expected for tunneling devices. Measured devices with a point slope of 46 mV/dec at low biases and an Ion/Ioff ratio of 106 at a supply voltage of 1.2 V for 25 nm wide fins are reported as best performing devices with a MuGFET technology using a high-k dielectric and a metal gate inserted gate stack.

Constant voltage stress induced degradation in HfO2/SiO2 gate dielectric stacks

Defect generation in HfO2/SiO2 gate dielectric stacks under constant voltage stress is investigated. It is found that the stress induced electrical degradation in HfO2/SiO2 stacks is different than in the SiO2 layer. The variation of the gate leakage current with different polarities shows different degradation characteristics after stress. Positive charge generation is also observed under both negative and positive gate voltage polarities. These degradation phenomena are explained by the composite effect of three components: neutral trap generation, electron trapping, and positive charge generation in the gate stacks.

Optical detection and characterization of graphene by broadband spectrophotometry

The spectra of optical constants, index of refraction (n), and extinction coefficient (k) of graphene and graphite are obtained in the wavelength range of 190–1000 nm (6.53–1.24 eV) using broadband optical spectrophotometry in conjunction with the Forouhi–Bloomer dispersion relations for n and k. Measurement is made possible by the use of a multilayer substrate consisting of bulk Si and a 3000 A SiO2 film. The effect of multiple internal reflections between the Si/SiO2 and SiO2/graphene interfaces amplifies the attenuating effect of the graphene layer, thereby improving the sensitivity of the reflectance measurement by a factor of 27 in the deep ultraviolet region of the spectrum. Maximum sensitivity is observed in the deep ultraviolet region of the spectrum, where a strong peak in the spectrum of the extinction coefficient of graphene is identified. The proposed method enables fast nondestructive angstrom-level thickness measurements of graphene and graphite. In this work, layers ranging in thickness bet...

Scaling to Sub- 1 nm Equivalent Oxide Thickness with Hafnium Oxide Deposited by Atomic Layer Deposition

The implementation of HfO 2 gate dielectrics in sub-45 nm devices requires optimization of nanometer-thin HfO 2 layers, deposited, e.g., by atomic layer deposition (ALD). In this work, we optimize ALD conditions such as precursor pulse time and deposition temperature for HfO 2 layers with physical thicknesses below 2 nm. Additionally, we investigate intermediate treatments in the ALD reaction cycle, such as exposure to gas-phase moisture or remote plasma at low temperature and thermal anneals. Such intermediate treatments affect both growth-per-cycle (GPC) and Cl-impurity content of the HfO 2 layers. The analysis of the process modifications allows a better understanding of the reaction mechanisms. H 2 O pulse times of 10 s must be applied to achieve saturation in GPC and Cl content. Using saturated H 2 O pulses decreases the gate leakage current in the sub-1 nm equivalent oxide thickness (EOT) range. The GPC is enhanced from ∼1.8 Hf/nm 2 for conventional ALD to 4 Hf/nm 2 for intermediate plasma treatments at low temperature. Intermediate anneals reduce the Cl content by about two orders of magnitude. Sufficient hydroxylation of the HfO 2 surface is one important factor controlling electrical properties in the sub-1 nm EOT range. The reduction of the Cl content does not systematically improve the electrical properties.

Self-assembled air-stable supramolecular porous networks on graphene.

Functionalization and modification of graphene at the nanometer scale is desirable for many applications. Supramolecular assembly offers an attractive approach in this regard, as many organic molecules form well-defined patterns on surfaces such as graphite via physisorption. Here we show that ordered porous supramolecular networks with different pore sizes can be readily fabricated on different graphene substrates via self-assembly of dehydrobenzo[12]annulene (DBA) derivatives at the interface between graphene and an organic liquid. Molecular resolution scanning tunneling microscopy (STM) and atomic force microscopy (AFM) investigations reveal that the extended honeycomb networks are highly flexible and that they follow the topological features of the graphene surface without any discontinuity, irrespective of the step-edges present in the substrate underneath. We also demonstrate the stability of these networks under liquid as well as ambient air conditions. The robust yet flexible DBA network adsorbed on graphene surface is a unique platform for further functionalization and modification of graphene. Identical network formation irrespective of the substrate supporting the graphene layer and the level of surface roughness illustrates the versatility of these building blocks.

Silicide Engineering to Boost Si Tunnel Transistor Drive Current

10nm. The high on-current is believed to be due to an enhanced electric field caused by silicide encroachment and dopant segregation. Low variability of the device performance is reported for the different fin widths. Temperature measurements also show that the current is due to different transport mechanisms at different gate biases. Temperature measurements and TCAD simulations both confirm the presence of trap- assisted tunneling (TAT) as main responsible for the degradation of the subthreshold swing. # 2011 The Japan Society of Applied Physics