arXiv subjects

Filippo Giannazzo

Publications and source records attributed to Filippo Giannazzo.

14 recordsLinked to original sources

arXiv2024

Impact of the NO annealing duration on the SiO2/4H-SiC interface properties in lateral MOSFETs: the energetic profile of the near-interface-oxide traps

In this work, the effects of the duration of the post deposition annealing (PDA) in nitric oxide (NO) on the properties of SiO2/4H-SiC interfaces in n-channel lateral MOSFETs are investigated, with a special focus on the modifications of the energy profile of near-interface-oxide traps (NIOTs). For this purpose, the electrical characteristics of lateral MOSFETs were studied in strong inversion conditions, monitoring the threshold voltage variations due to charge trapping effects. To determine the energetic position of the NIOTs with respect of the SiO2 conduction band edge, the Fermi level position in the insulating layer was evaluated by TCAD simulations of the band diagrams. PDAs of the gate oxide of different duration resulted into similar shape of the energetic profile of the traps inside the insulator with respect of the SiO2 conduction band edge, but with different magnitude. Finally, the effective decrease of the insulator traps is demonstrated despite a saturation of the interface state density under prolonged PDAs and in particular the charge trapped at the NIOTs is reduced from 1-2 x 1011 cm-2 down to 3 x 1011 cm-2 varying the PDA duration from 10 up to 120 min in NO at 1175 degree C.

cond-mat.mtrl-sci↗

arXiv2024

Complementary two dimensional carrier profiles of 4H-SiC MOSFETs by Scanning Spreading Resistance Microscopy and Scanning Capacitance Microscopy

This paper reports the results presented in an invited poster during the International Conference on Silicon Carbide and Related Materials (ICSCRM) 2023 held in Sorrento, Italy. The suitability of scanning probe methods based on atomic force microscopy (AFM) measurements is explored to investigate with high spatial resolution the elementary cell of 4H-SiC power MOSFETs. The two-dimensional (2D) cross-sectional maps demonstrated a high spatial resolution of about 5 nm using the SSRM capabilities. Furthermore, the SCM capabilities enabled visualizing the fluctuations of charge carrier concentration across the different parts of the MOSFETs elementary cell.

physics.app-ph↗

arXiv2023

Current transport in Ni Schottky barrier on GaN epilayer grown on free standing substrates

In this paper, the Ni Schottky barrier on GaN epilayer grown on free standing substrates has been characterized. First, transmission electrical microscopy (TEM) images and nanoscale electrical analysis by conductive atomic force microscopy (C-AFM) of the bare material allowed visualizing structural defects in the crystal, as well as local inhomogeneities of the current conduction. The forward current-voltage (I-V) characteristics of Ni/GaN vertical Schottky diodes fabricated on the epilayer gave average values of the Schottky barrier height of 0.79 eV and ideality factor of 1.14. A statistical analysis over a set of diodes, combined with temperature dependence measurements, confirmed the formation of an inhomogeneous Schottky barrier in this material. From a plot of FB versus n, an ideal homogeneous barrier close to 0.9 eV was estimated, similar to that extrapolated by capacitance-voltage (C-V) analysis. Local I-V curves, acquired by means of C-AFM, displayed the inhomogeneous distribution of the onset of current conduction, which in turn resembles the one observed in the macroscopic Schottky diodes. Finally, the reverse characteristic of the diodes fabricated in the defects-free region have been acquired at different temperature and its behaviour has been described by the thermionic field emission (TFE) model.

cond-mat.mtrl-sci↗

arXiv2023

Threshold voltage instability by charge trapping effects in the gate region of p-GaN HEMTs

In this work, the threshold voltage instability of normally-off p-GaN high electron mobility transistors (HEMTs) has been investigated by monitoring the gate current density during device on-state. The origin of the gate current variations under stress has been ascribed to charge trapping occurring at the different interfaces in the metal/p-GaN/AlGaN/GaN system. In particular, depending on the stress bias level, electrons (VG < 6 V) or holes (VG > 6 V) are trapped, causing a positive or negative threshold voltage shift {DVTH, respectively. By monitoring the gate current variations at different temperatures, the activation energies associated to the electrons and holes trapping could be determined and correlated with the presence of nitrogen (electron traps) or gallium (hole traps) vacancies. Moreover, the electrical measurements suggested the generation of a new electron-trap upon long-time bias stress, associated to the creation of crystallographic dislocation-like defects extending across the different interfaces (p-GaN/AlGaN/GaN) of the gate stack.

physics.app-ph↗

arXiv2022

2D graphitic-like gallium nitride and structural selectivity in confinement at graphene/SiC interface

Predictive first-principles calculations suggest graphitic-like GaN to be theoretically possible. Thus far, it has not been experimentally reported. We report on GaN monolayer in a buckled geometry obtained in confinement at graphene/SiC interface by metalorganic chemical vapor deposition (MOCVD). Conductive atomic force microscopy (C-AFM) has been employed to probe vertical current injection through the graphene/SiC interface and to establish the uniformity of the intercalated areas. Scanning transmission electron microscopy (S/TEM) has been employed for atomic resolution imaging and spectroscopy. Discontinuity in the anticipated stacking sequence of graphitic-like GaN monolayers has been exposed and reasoned as a case of simultaneous formation of Ga-N and Ga-O bonds. The formation of Ga-O bonds acquires importance in instigating chemical-species-specific structural selectivity in confinement at atom-size scale.

cond-mat.mtrl-sci↗

arXiv2021

Direct atomic layer deposition of ultrathin aluminium oxide on monolayer $MoS_2$ exfoliated on gold: the role of the substrate

In this paper we demonstrated the thermal Atomic Layer Deposition (ALD) growth at 250 °C of highly homogeneous and ultra-thin ($\approx$ 3.6 nm) $Al_2O_3$ films with excellent insulating properties directly onto a monolayer (1L) $MoS_2$ membrane exfoliated on gold. Differently than in the case of 1L $MoS_2$ supported by a common insulating substrate ($Al_2O_3/Si$), a better nucleation process of the high-k film was observed on the 1L $MoS_2/Au$ system since the ALD early stages. Atomic force microscopy analyses showed a $\approx 50\%$ $Al_2O_3$ surface coverage just after 10 ALD cycles, its increasing up to $>90\%$ (after 40 cycles), and an uniform $\approx$ 3.6 nm film, after 80 cycles. The coverage percentage was found to be significantly reduced in the case of 2L $MoS_2/Au$, indicating a crucial role of the interfacial interaction between the aluminum precursor and $MoS_2/Au$ surface. Finally, Raman spectroscopy and PL analyses provided an insight about the role played by the tensile strain and p-type doping of 1L $MoS_2$ induced by the gold substrate on the enhanced high-k nucleation of $Al_2O_3$ thin films. The presently shown high quality ALD growth of high-k $Al_2O_3$ dielectrics on large area 1L $MoS_2$ induced by the Au underlayer can be considered of wide interest for potential device applications based on this material system.

physics.app-ph↗

arXiv2021

Interfacial electrical and chemical properties of deposited SiO2 layers in lateral implanted 4H-SiC MOSFETs subjected to different nitridations

In this paper, SiO2 layers deposited on 4H-SiC and subjected to different post deposition annealing (PDA) in NO and N2O were studied to identify the key factors influencing the channel mobility and threshold voltage stability in 4H-SiC MOSFETs. In particular, PDA in NO gave a higher channel mobility (55 cm2V-1s-1) than PDA in N2O (20 cm2V-1s-1), and the subthreshold behavior of the devices confirmed a lower total amount of interface states for the NO case. This latter could be also deduced from the behavior of the capacitance-voltage characteristics of 4H-SiC MOSFETs measured in gate controlled diode configuration. On the other hand, cyclic gate bias stress measurements allowed to separate the contributions of interface states (Nit) both on the upper and bottom parts of the 4H-SiC band gap and near interface oxide traps (NIOTs) in the two oxides. In particular, it was found that NO annealing reduced the total density of charges trapped at the interface states down to 3 x 1011 cm- 2 and those trapped inside the oxide down to 1 x 1011 cm-2. Electron energy loss spectroscopy demonstrated that the reduction of these traps in the NO annealed sample is due to the lower amounts of sub-stoichiometric silicon oxide (~ 1nm) and carbon-related defects (< 1nm) at the interface, respectively. This correlation represents a further step in the comprehension of the physics of the SiO2/4H-SiC interface explaining the mobility and threshold voltage behavior of 4H-SiC MOSFETs.

cond-mat.mtrl-sci↗

arXiv2021

Ohmic contacts on n-type and p-type cubic silicon carbide (3C-SiC) grown on silicon

This paper is a report on Ohmic contacts on n-type and p-type type cubic silicon carbide (3C-SiC) layers grown on silicon substrates. In particular, the morphological, electrical and structural properties of annealed Ni and Ti/Al/Ni contacts has been studied employing several characterization techniques. Ni films annealed at 950°C form Ohmic contacts on moderately n-type doped 3C-SiC (ND ~ 1x1017cm-3), with a specific contact resistance of 3.7x10-3 Ωcm2. The main phase formed upon annealing in this contact was nickel silicide (Ni2Si), with randomly dispersed carbon in the reacted layer. In the case of a p-type 3C-SiC with a high doping level (NA ~ 5x1019cm-3), Ti/Al/Ni contacts were preferable to Ni ones, as they gave much lower values of the specific contact resistance (1.8x10-5 Ωcm2). Here, an Al3Ni2 layer was formed in the uppermost part of the contact, while TiC was detected at the interface. For this system, a temperature dependent electrical characterization allowed to establish that the thermionic field emission rules the current transport at the interface. All these results can be useful for the further development of a devices technology based on the 3C-SiC polytype.

physics.app-ph↗

arXiv2021

Effect of high temperature annealing (T > 1650°C) on the morphological and electrical properties of p-type implanted 4H-SiC layers

This work reports on the effect of high temperature annealing on the electrical properties of p-type implanted 4H-SiC. Ion implantations of Aluminium (Al) at different energies (30 - 200 keV) were carried out to achieve 300 nm thick acceptor box profiles with a concentration of about 1020 at/cm3. The implanted samples were annealed at high temperatures (1675-1825 °C). Morphological analyses of the annealed samples revealed only a slight increase of the surface roughness RMS up to 1775°C, while this increase becomes more significant at 1825°C (RMS=1.2nm). Room temperature Hall measurements resulted in a hole concentration in the range 0.65-1.34x1018/cm3 and mobility values in the order of 21-27 cm2V-1s-1. The temperature dependent electrical measurements allowed to estimate an activation energy of the Al-implanted specie of about 110 meV (for the post-implantation annealing at 1675°C) and a fraction of active p-type Al-dopant ranging between 39% and 56%. The results give useful indications for the fabrication of 4H-SiC JBS and MOSFETs.

physics.app-ph↗

arXiv2021

Temperature-dependent Fowler-Nordheim electron barrier height in SiO2/4H-SiC MOS capacitors

This paper reports on the physical and temperature-dependent electrical characterizations of the oxide/semiconductor interface in MOS capacitors with a SiO2 layer deposited on 4H-SiC using dichlorosilane and nitrogen-based vapor precursors. The capacitors, subjected to a standard post deposition annealing process in N2O, exhibited an interface state density Dit = 9.0 x 1011cm-2eV-1 below the conduction band edge. At room temperature, a barrier height (conduction band offset) of 2.8 eV was observed, along with the presence of negative charges in the insulator. The SiO2 insulating properties were evaluated by studying the experimental temperature-dependence of the gate current. In particular, the temperature-dependent electrical measurements showed a negative temperature coefficient of the Fowler-Nordheim electron barrier height (dFB/dT = - 0.98 meV/°C), which was very close to the expected value for an ideal SiO2/4H-SiC system and much lower compared to the values reported for thermally grown SiO2. This smaller dependence of FB on the temperature and the increase of the current level with temperature in the transcharacteristics measured in the relative fabricated MOSFETs represents a clear advantage of our deposited SiO2 for the operation of MOSFET devices at high temperatures.

physics.app-ph↗

arXiv2021

Ni Schottky barrier on heavily doped phosphorous implanted 4H-SiC

The electrical behavior of Ni Schottky barrier formed onto heavily doped (ND>1019 cm-3) n-type phosphorous implanted silicon carbide (4H-SiC) was investigated, with a focus on the current transport mechanisms in both forward and reverse bias. The forward current-voltage characterization of Schottky diodes showed that the predominant current transport is a thermionic-field emission mechanism. On the other hand, the reverse bias characteristics could not be described by a unique mechanism. In fact, under moderate reverse bias, implantation-induced damage is responsible for the temperature increase of the leakage current, while a pure field emission mechanism is approached with bias increasing. The potential application of metal/4H-SiC contacts on heavily doped layers in real devices are discussed.

cond-mat.mtrl-sci↗

arXiv2020

Thermal Annealing Effect on Electrical and Structural Properties of Tungsten Carbide Schottky Contacts on AlGaN/GaN heterostructures

Tungsten carbide (WC) contacts have been investigated as a novel gold-free Schottky metallization for AlGaN/GaN heterostructures. The evolution of the electrical and structural/compositional properties of the WC/AlGaN contact has been monitored as a function of the annealing temperature in the range from 400 to 800°C. The Schottky barrier height ($Φ$B) at WC/AlGaN interface, extracted from the forward current-voltage characteristics of the diode, decreased from 0.8 eV in the as-deposited and 400°C annealed sample, to 0.56 eV after annealing at 800 °C. This large reduction of $Φ$B was accompanied by a corresponding increase of the reverse bias leakage current. Transmission electron microscopy coupled to electron energy loss spectroscopy analyses revealed the presence of oxygen (O) uniformly distributed in the WC layer, both in the as-deposited and 400°C annealed sample. Conversely, oxygen accumulation in a 2-3 nm thin W-O-C layer at the interface with AlGaN was observed after the annealing at 800 °C, as well as the formation of W2C grains within the film (confirmed by X-ray diffraction analyses). The formation of this interfacial W-O-C layer is plausibly the main origin of the decreased $Φ$B and the increased leakage current in the 800°C annealed Schottky diode, whereas the decreased O content inside the WC film can explain the reduced resistivity of the metal layer. The results provide an assessment of the processing conditions for the application of WC as Schottky contact for AlGaN/GaN heterostructures.

physics.app-ph↗

arXiv2020

Identification of two trapping mechanisms responsible of the threshold voltage variation in SiO$_2$/4H-SiC MOSFETs

A non-relaxing method based on cyclic gate bias stress is used to probe the interface or near-interface traps in the SiO$_2$/4H-SiC system over the whole 4H-SiC band gap. The temperature dependent instability of the threshold voltage in lateral MOSFETs is investigated and two separated trapping mechanisms were found. One mechanism is nearly temperature independent and it is correlated to the presence of near interface oxide traps that are trapped via tunneling from the semiconductor. The second mechanism, having an activation energy of 0.1 eV, has been correlated to the presence of intrinsic defects at the SiO$_2$/4H-SiC interface.

physics.app-ph↗

arXiv2020

Electron trapping at SiO2/4H-SiC interface probed by transient capacitance measurements and atomic resolution chemical analysis

Studying the electrical and structural properties of the interface of the gate oxide (SiO2) with silicon carbide (4H-SiC) is a fundamental topic, with important implications for understanding and optimizing the performances of metal-oxide-semiconductor field effect transistor (MOSFETs). In this paper, near interface oxide traps (NIOTs) in lateral 4H-SiC MOSFETs were investigated combining transient gate capacitance measurements (C-t) and state of the art scanning transmission electron microscopy in electron energy loss spectroscopy (STEM-EELS) with sub-nm resolution. The C-t measurements as a function of temperature indicated that the effective NIOTs discharge time is temperature independent and electrons from NIOTs are emitted toward the semiconductor via-tunnelling. The NIOTs discharge time was modelled taking into account also the interface state density in a tunnelling relaxation model and it allowed to locate traps within a tunnelling distance up to 1.3nm from the SiO2/4H-SiC interface. On the other hand, sub-nm resolution STEM-EELS revealed the presence of a Non-Abrupt (NA) SiO2/4H-SiC interface. The NA interface shows the re-arrangement of the carbon atoms in a sub-stoichiometric SiOx matrix. A mixed sp2/sp3 carbon hybridization in the NA interface region suggests that the interfacial carbon atoms have lost their tetrahedral SiC coordination.

physics.app-ph↗