M. Vivona

SiO2/4H-SiC interface doping during post-deposition-annealing of the oxide in N2O or POCl3

In this Letter, we report a quantitative analysis of the n-type doping occurring at SiO2/4H-SiC interfaces during post-deposition-annealing (PDA) in N2O or POCl3 of a 45 nm thick oxide. In particular, a nanoscale characterization using scanning capacitance microscopy on the cross section of metal-oxide-semiconductor capacitors allowed to determine the electrically active nitrogen and phosphorous concentration under the SiO2 layer after PDA in N2O and POCl3, i.e., 5 × 1017 cm−3 and 4.5 × 1018 cm−3, respectively. The technological implications have been discussed considering the possible impact of a PDA-induced “counter doping” of the p-type body region of a n-channel metal-oxide-semiconductor-field-effect-transistor on the device threshold voltage.

Thermal stability of the current transport mechanisms in Ni-based Ohmic contacts on n- and p-implanted 4H-SiC

Studying the temperature dependence of the electrical properties of Ohmic contacts formed on ion-implanted SiC layers is fundamental to understand and to predict the behaviour of practical devices. This paper reports the electrical characterization, as a function of temperature, of Ni-based Ohmic contacts, simultaneously formed on both n- or p-type implanted 4H-SiC. A structural analysis showed the formation of the Ni2Si phase after an annealing leading to Ohmic behaviour. The temperature-dependence of the specific contact resistance indicated that a thermionic field emission mechanism (TFE) dominates the current transport for contacts formed on p-type material, while a field emission (FE) is likely occurring in the contacts formed on n-type implanted SiC. The values of the barrier height were 0.75 eV on p-type material and 0.45 eV on n-type material. The thermal stability of the current transport mechanisms and related physical parameters has been demonstrated upon a long-term (up to 95 h) cycling in the temperature range 200–400 °C.

Ti/Al/W Ohmic contacts to p-type implanted 4H-SiC

In this work, the morphological, structural, and electrical properties of Ti/Al/W contacts to p-type implanted silicon carbide (4H-SiC) have been monitored as a function of the annealing temperature (800–1100 °C). The increase of the annealing temperature induces a transition from a rectifying to an Ohmic behavior, with a specific contact resistance of 5.8 × 10−4 Ωcm2. The electrical behavior has been correlated with the morphological and structural analyses. In particular, the transition to an Ohmic behavior was accompanied by a gradual increase of the surface roughness and by the occurrence of a reaction leading to the formation of new phases in the stack and at the interface (TiAl3, W(SiAl)2, and TiC). The presence of Al-rich protrusions penetrating in the SiC substrate was also observed. From the temperature dependence of the electrical parameters, a barrier height of 0.69 eV for this system was determined. The thermal stability of the contacts has been demonstrated for long-term (up to 100 h) thermal cycling at 400 °C.

Electrical Characteristics of Schottky Contacts on Ge-Doped 4H-SiC

We report on the electrical characteristics of Ni/4H-SiC Schottky contacts fabricated on a Ge-doped 4H-SiC epilayer. The morphology and the current mapping carried out by conductive atomic force microscopy on the epilayer allowed observing nanoscale preferential conductive paths on the sample surface. The electrical characteristics of Ni contacts have been studied before and after a rapid thermal annealing process. A highly inhomogeneous Schottky barrier was observed in as-deposited diodes, probably related to the surface electrical inhomogeneities of the 4H-SiC epilayer. A significant improvement of the Schottky diodes characteristics was achieved after annealing at 700°C, leading to the consumption of the near surface epilayer region by Ni/4H-SiC reaction. After this treatment, the temperature behavior of the ideality factor and Schottky barrier height was comparable to that observed on commercial 4H-SiC material.

Effect of germanium doping on electrical properties of n-type 4H-SiC homoepitaxial layers grown by chemical vapor deposition

The effect of germanium (Ge) on n-type 4H-SiC is experimentally studied by electrical characterization of homoepitaxial layers grown by chemical vapor deposition (CVD). Measurements show that electrical properties of epitaxial layers can be changed by intentional incorporation of germane (GeH4) gas during the deposition process. On the nanoscale, two-dimensional mappings acquired with conductive atomic force microscopy show preferential conductive paths on the surface of Ge-doped samples, which are related to the presence of this isoelectronic impurity. Hall effect measurements confirm that also macroscopic electrical properties of n-type 4H-SiC are improved due to incorporation of Ge into SiC during CVD growth. In particular, despite equal free electron concentration, enhanced mobility in a wide temperature range is measured in Ge-doped samples as compared to a pure 4H-SiC layer. Based on our results from Hall effect measurements as well as admittance spectroscopy and deep level transient spectroscopy, i...

Comparative Study of the Current Transport Mechanisms in Ni2Si Ohmic Contacts on n- and p-Type Implanted 4H-SiC

The knowledge of the temperature behavior of Ohmic contacts is an important issue to understand the device operation. This work reports an electrical characterization as a function of the temperature carried out on nickel silicide (Ni2Si) Ohmic contacts, used both for n-type and p-type implanted 4H-SiC layers. The temperature dependence of the specific contact resistance suggested that a thermionic field emission mechanism dominates the current transport for contacts on p-type material, whereas a current transport by tunneling is likely occurring in the contacts on n-type implanted SiC. Furthermore, from the temperature dependence of the electrical characteristics, the activation energies for Al and P dopants were determined, resulting of 145 meV and 35 meV, respectively. The thermal stability of the electrical parameters has been demonstrated upon a long-term (up to ~100 hours) cycling in the temperature range 200-400°C.

Characterization of SiO2/SiC Interfaces Annealed in N2O or POCl3

This paper reports a comparative characterization of SiO2/SiC interfaces subjected to post-oxide-deposition annealing in N2O or POCl3. Annealing process of the gate oxide in POCl3 allowed to achieve a notable increase of the MOSFET channel mobility (up to 108 cm2V-1s-1) with respect to the N2O annealing (about 20 cm2V-1s-1), accompanied by a different temperature behaviour of the electrical parameters in the two cases. Structural and compositional analyses revealed a different surface morphology of the oxide treated in POCl3, as a consequence of the strong incorporation of phosphorous inside the SiO2 matrix during annealing. This latter explained the instability of the electrical behaviour of MOS capacitors annealed in POCl3.

Anomalous Fowler-Nordheim tunneling through SiO 2 /4H-SiC barrier investigated by temperature and time dependent gate current measurements

This paper reports on the conduction mechanisms and trapping effects in SiO2/4H-SiC MOS-based devices subjected to post deposition annealing in N2O. In particular, the anomalous Fowler-Nordheim (FN) tunnelling through the SiO2/4H-SiC barrier observed under consecutive reverse bias sweeps was studied by temperature and time dependent gate current measurements. The excess of gate current with respect to the theoretical FN predictions was explained by a charge-discharge mechanism of Near Interface Traps (NITs) in the oxide. The gate current transient was described with a semi-empirical analytical model, modifying the standard FN model with a time-dependent electric field to account for the neutralization of trapped charges at NITs.

Preliminary Study on the Effect of Micrometric Ge-Droplets on the Characteristics of Ni/4H-SiC Schottky Contacts

This work reports on the morphological and electrical characteristics of Ni/4H-SiC Schottky contacts, fabricated on epitaxial layers intentionally covered by micrometric size Ge-droplets. Specifically, the Ge-droplets behave as preferential paths for the vertical current conduction, as observed at nanometric scale by conductive atomic force microscopy. As a consequence, the electrical I-V characteristics of these Ni contacts revealed the presence of a double-barrier, thus indicating an inhomogeneity in the interface. This behavior was associated to the local Schottky barrier lowering contribution due to the Ge-presence. These results can be useful to explore the possibility of controlling the contact (Schottky or Ohmic) properties by changing the size and the distribution of the surface impurities.

Evolution of the Electrical and Structural Properties of Ti/Al/W Contacts to p-Type Implanted 4H-SiC upon Thermal Annealing

The mechanism of Ohmic contacts formation to p-type SiC is a fundamental and technological concern continuously under debate. Typically, Ti/Al-based contacts are a good choice for Ohmic contacts to p-type SiC, even though some aspects strictly related to the specific nature of Al (susceptibility to oxidation, low melting temperature, etc.) remain to be optimized. In this work, the evolution of the electrical properties of a Ti/Al/W multiple-layer contact has been studied by TLM characterization and correlated to the changes in the morphology and microstructure upon thermal annealing. The formation of an Ohmic contact has been observed after a thermal annealing at 1100°C, discussing the possible reasons determining the transition to an Ohmic behavior.