Roberta Nipoti

Growth and structure of titanium silicide phases formed by thin Ti films on Si crystals

The silicide formation with a titanium film deposited on 〈100〉 single‐crystal silicon, has been studied by using nuclear microanalysis, x‐ray diffraction, and transmission electron microscopy. The presence of interfacial defects and their possible role in the early stages of the reaction has been evidenced. The phase composition was dependent on the annealing temperature and time: at 550 °C only TiSi2 is observed; at higher temperatures (>600 °C), a thin TiSi2 layer at the interface is again observed, but Ti‐rich silicides grow on top of this layer by increasing the annealing time. For longer annealing times, all the reacted layer progressively transforms into TiSi2. The amount of reacted silicon grows with a (time)1/2 law; the activation energy of 1.8 eV reported for the growth of TiSi2 onto amorphous Si may be appropriate even in this case. The reaction proceeds at a rate of one order of magnitude higher than previously reported for reaction between silicon and an oxygen saturated titanium film. The kin...

Different methods for the determination of damage profiles in Si from RBS-channeling spectra: a comparison

Abstract RBS-channeling spectra of deep ion implants in silicon were analyzed to extract the displaced atoms depth profiles by different methods. In all cases it was assumed that defects in as-implanted samples can be described as atoms randomly displaced from the lattice sites. In the first method, based on the two beam model, the dechanneling induced by defects was calculated either linearly or following a recently developed semi-empirical formula. In the second method the analyzing beam was divided into a greater number of components to follow the transverse energy distribution of the ions. Finally a three-dimensional Monte Carlo code containing a detailed description of each ion path was used to identify the limits of the previous approaches. It is shown that when high amounts of damage are considered all the methods produce essentially the same profiles. On the contrary they considerably disagree in other cases. Moreover, Monte Carlo calculations indicate that to obtain reliable results it is necessary to take into account a correct description of the channeling energy loss process during ion penetration into a disordered crystal.

Ion implantation induced swelling in 6H-SiC

Ion implantation induced surface expansion (swelling) of 6H-SiC was investigated through the measurement of the step height between implanted and unimplanted areas. The samples were irradiated at room temperature with 500 keV Al+ ions in the dose range 1.25×1014–3×1015 ions cm−2. Swelling was related to dose and the area density of ion-induced damage measured by Rutherford backscattering channeling technique. The observed trend is consistent with the hypothesis that the volume expansion of the ion damaged crystal is proportional to the area density of displaced atoms, plus an additional relaxation occurring at the onset of the crystalline to amorphous transition.

Determination of He electronic energy loss in crystalline Si by Monte-Carlo simulation of Rutherford backscattering–channeling spectra

Spectra of He ions backscattered from thin (001) Si membranes and bulk Si/SiO2 wafers are analyzed with the aid of simulation based on the binary collision approximation. Due to precise control of experimental parameters and detailed fitting of random spectra in thin specimens, the curve of random electronic energy loss per unit length in the energy interval 1.5–3 MeV can be given with an estimated accuracy better than 2%. The electronic energy loss in 〈001〉 alignment is determined through the fit of channeling spectra measured in bulk Si/SiO2 samples. This is performed by the adjustment of the parameters of the semi-empirical model used in the calculation to describe the dependence of He electronic energy loss on the impact parameter of nuclear collisions. The model reproduces data reported in the literature for the 〈001〉 direction, but overestimates 〈011〉 electronic energy loss at energies below 1.5 MeV. The different ability to simulate energy loss in the two orientations is attributed to the limitations of the model to account for the non-uniform distribution of valence electrons in the Si lattice.

Carbon-Cap for Ohmic Contacts on Ion-Implanted 4H – SiC

A pyrolyzed photoresist film is commonly used as a protective cap of the surface of ion-implanted 4H-SiC wafers during the postimplantation annealing process with the aim to prevent Si sublimation and step bunching formation. Such a film that is called carbon-cap (C-cap) is always removed after postimplantation annealing and before any other processing step of the SiC wafer. Here, we show that this C-cap is a continuous, hard, black, mirrorlike, and planar thin film that can be patterned by a reactive ion etching O 2 -based plasma for the fabrication of ohmic contact pads on both Al + - and P + -implanted 4H-SiC. This C-cap material has an electrical resistivity of 1.5 × 10 -3 Ω cm and a good resistance against scratch. Al (1% Si) wires can be ultrasonically bonded on the C-cap pads. Such a bonding and the C-cap adhesion to the implanted 4H-SiC surface are stable for electrical characterizations in vacuum between room temperature and 450°C. The measured specific contact resistance of the C-cap on a 1 × 10 20 cm -3 p+-implanted 4H-SiC is 9 × 10- 5 Ω cm 2 at room temperature. Micro-Raman characterizations show that this C-cap is formed of a nanocrystalline graphitic phase.

Stopping and damage parameters for Monte Carlo simulation of MeV implants in crystalline Si

Semiempirical models of electronic energy loss and damage formation for MeV ions (B, P, As) implanted in silicon at room temperature were investigated through the comparison of measurements with Monte Carlo simulations of both impurity and damage depth distributions. Accurate prediction of dopant profiles in an amorphous target and in a low-dose implanted crystal is achieved by a proper parametrization of well known analytic stopping models. Moreover, to accurately describe the dynamic effects of damage accumulation in medium dose implants, a dependence on ion energy of the efficiency parameter used in the Kinchin–Pease (KP) model must be introduced in the simulation. Such a factor, determined by the fit of the measured integral of defect profiles, is found to decrease for P and As ions with increasing the nuclear energy released to primary recoil atoms, apparently reaching a saturation value of about 0.25. Full cascade simulations show that the increasing fraction of the primary recoils energy spent in e...

Titanium and nickel silicide formation after Q‐switched laser and multiscanning electron beam irradiation

The use of Q‐switched ruby laser and multiscanning electron‐beam annealing to produce the reaction of thin Ti and Ni films deposited onto silicon single crystals has been studied. Rutherford Backscattering (RBS), 16O(d, p)17O* nuclear reaction, scanning electron microscopy (SEM) observation, and x‐ray diffraction were used to characterize the reacted layers. It was found that laser annealing produces a reaction only at the metal‐semiconductor interface: the reacted layers are not uniform in composition and more similar to a mixture than to a well‐defined phase. On the contrary, the silicide layers produced by multiscanning e beam result from the solid‐state reaction of the whole metal film and have a layered structure with well‐defined phase composition and sharp interfaces both between the silicide phases and the underlying semiconductor in Ti/Si system. It was observed that the TiSi2 growth mechanism during e irradiation cannot be explained with the parabolic ’’diffusion controlled’’ mechanism operating...

Nitrogen Implantation to Improve Electron Channel Mobility in 4H-SiC MOSFET

Normally off 4H-SiC MOSFET devices have been fabricated on a p-type semiconductor and electrically characterized at different temperatures. A gate oxide obtained by nitrogen ion implantation performed before the thermal oxidation of SiC has been implemented in n-channel MOSFET technology. Two samples with a nitrogen concentration at the SiO2/SiC interface of 5 X 1018 and 1.5 X 1019 cm-3 and one unimplanted sample have been manufactured. The sample with the highest N concentration at the interface presents the highest channel mobility and the lowest threshold voltage. For increasing temperature, in all the samples, the threshold voltage decreases, and the electron channel mobility increases. The latter case attains a maximum value of about 40 cm2/V ldr s at 200degC for the sample with the highest N concentration. These trends are explained by the reduction of interface electron traps in the upper half of the band gap toward the conduction band edge. These results demonstrate that N implantation can be effectively used to improve the electrical performances of an n-type surface channel 4H-SiC MOSFET.

Formation of carbon vacancy in 4H silicon carbide during high-temperature processing

As-grown and pre-oxidized silicon carbide (SiC) samples of polytype 4H have been annealed at temperatures up to 1950 °C for 10 min duration using inductive heating, or at 2000 °C for 30 s using microwave heating. The samples consisted of a n-type high-purity epitaxial layer grown on 4° off-axis ⟨0001⟩ n+-substrate and the evolution of the carbon vacancy (VC) concentration in the epitaxial layer was monitored by deep level transient spectroscopy via the characteristic Z1/2 peak. Z1/2 appears at ∼0.7 eV below the conduction band edge and arises from the doubly negative charge state of VC. The concentration of VC increases strongly after treatment at temperatures ≥ 1600 °C and it reaches almost 1015 cm−3 after the inductive heating at 1950 °C. A formation enthalpy of ∼5.0 eV is deduced for VC, in close agreement with recent theoretical predictions in the literature, and the entropy factor is found to be ∼5 k (k denotes Boltzmanns constant). The latter value indicates substantial lattice relaxation around VC...

Electrical properties of Al2O3∕4H‐SiC structures grown by atomic layer chemical vapor deposition

Al2O3 films have been deposited on n-type and p-type 4H‐SiC by atomic layer chemical vapor deposition using trimethylaluminum as a precursor for aluminum and both H2O and O3 as an oxidant. After oxide deposition, annealing at different temperatures (800, 900, 1000°C) in argon atmosphere for different durations (1, 2, 3h) was performed. Bulk and interface properties of the oxide films were studied by capacitance-voltage, current-voltage, deep level transient spectroscopy, and thermally dielectric relaxation current (TDRC) measurements. The results reveal a decreasing flatband voltage with increasing annealing time, suggesting decrease of oxide charges and deep interface traps. After 3h annealing at 1000°C of the n-type samples, the flatband voltage is reduced to 6V compared to a value in excess of 40V for as-deposited samples. The TDRC measurements on annealed Al2O3∕SiC (n-type) capacitors showed substantially different spectra relative to conventional SiO2∕4H‐SiC control samples; in the former ones no sig...