Fabrizio Roccaforte

Richardson's constant in inhomogeneous silicon carbide Schottky contacts

The electrical characterization of nickel silicide Schottky contacts on silicon carbide (4H–SiC) is reported in this article. In spite of the nearly ideal behavior of the contact at room temperature (n=1.05), the electrical behavior monitored in a wide temperature range exhibited a deviation from the ideality at lower temperatures, thus suggesting that an inhomogeneous barrier has actually formed. A description of the experimental results by the Tung’s model, i.e., considering an effective area of the inhomogeneous contact, provided a procedure for a correct determination of the Richardson’s constant A**. An effective area lower than the geometric area of the diode is responsible for the commonly observed discrepancy in the experimental values of A** from its theoretical value in silicon carbide. The same method was applied to Ti/4H–SiC contacts.

Barrier inhomogeneity and electrical properties of Pt∕GaN Schottky contacts

The temperature dependence of the electrical properties of Pt∕GaN Schottky barrier was studied. In particular, a Schottky barrier height of 0.96eV and an ideality factor of 1.16 were found after a postdeposition annealing at 400°C. Nanoscale electrical characterization was carried out by the conductive biased tip of an atomic force microscope both on the bare GaN surface and on the Pt∕GaN contacts. The presence of a lateral inhomogeneity of the Schottky barrier, with a Gaussian distribution of the barrier height values, was demonstrated. Moreover, GaN surface defects were demonstrated to act as local preferential paths for the current conduction. The temperature dependent electrical characteristics of the diodes were discussed in terms of the existing models on inhomogeneous barriers and correlated to the nanoscale electrical characterization of the barrier. In this way, the anomalous electrical behavior of the ideality factor and of the Schottky barrier and the low experimental value of the Richardson’s ...

High responsivity 4H-SiC Schottky UV photodiodes based on the pinch-off surface effect

In this letter, high responsivity 4H-SiC vertical Schottky UV photodiodes based on the pinch-off surface effect, obtained by means of self-aligned Ni2Si interdigit contacts, are demonstrated. The diode area was 1mm2, with a 37% directly exposed to the radiation. The dark current was about 200pA at −50V. Under a 256nm UV illumination, a current increase of more than two orders of magnitude is observed, resulting in a 78% internal quantum efficiency. The vertical photodiodes showed an ultraviolet-visible rejection ratio >7×103 and a responsivity a factor of about 1.8 higher than a conventional planar metal-semiconductor-metal structure.

Temperature behavior of inhomogeneous Pt∕GaN Schottky contacts

In this letter, a correlation between the nanoscale localized electrical properties of the Pt∕GaN Schottky barrier and the temperature behavior of macroscopic Schottky diodes is demonstrated. Although a significant improvement of the ideality factor of the diodes is achieved after annealing at 400°C, local current-voltage measurements, performed with a biased tip of a conductive atomic force microscope, revealed the inhomogeneous nature of the barrier. Its nanoscale degree of homogeneity was quantitatively described by means of Tung’s model [Phys. Rev. B 45, 13509 (1992)], allowing the authors to explain the temperature dependence of the electrical characteristics of the macroscopic diodes.

Highly Efficient Low Reverse Biased 4H-SiC Schottky Photodiodes for UV-Light Detection

Ultraviolet light detection has a wide range of scientific and industrial applications. In particular, SiC photodiodes have been proposed because of their robustness even in harsh environments, high quantum efficiency but excellent visible blindness, very low dark current, and high speed. Here, we report on the electrical and optical performances of high efficient large area 4 H-SiC Schottky photodiodes working in the photovoltaic regime. We demonstrate that the high signal-to-noise ratio along with the low operating reverse voltage in spite of the large sensitive area makes them suitable in low power consumption applications requiring high sensitivity down to 250 nm.

OHMIC CONTACTS TO SIC

In this chapter, the most significant results obtained in the last decade in the field of ohmic contacts to SiC are reviewed. First, the basic concepts related to the physics of ohmic contacts and to the contact resistance measurement techniques are briefly reported. Then, some aspects concerning the formation of low resistance (10-5-10-6 Ωcm2) ohmic contacts on n-type and for p-type SiC are discussed, focusing on Ni-based and Al/Ti-based contacts. Examples of innovative applications on practical devices are also reported, as the simultaneous formation of ohmic contacts on n- and p-type SiC for vertical power MOS devices, obtained adding Al to a standard Ni contact, and a single-metal technology for ohmic and rectifying contacts in MESFETs, using Ti or Ni without post-deposition annealing.

New Achievements on CVD Based Methods for SiC Epitaxial Growth

The results of a new epitaxial process using an industrial 6x2” wafer reactor with the introduction of HCl during the growth have been reported. A complete reduction of silicon nucleation in the gas phase has been observed even for high silicon dilution parameters (Si/H2>0.05) and an increase of the growth rate until about 20 µm/h has been measured. No difference has been observed in terms of defects, doping uniformity (average maximum variation 8%) and thickness uniformity (average maximum variation 1.2 %) with respect to the standard process without HCl.

Correlation between microstructure and temperature dependent electrical behavior of annealed Ti/Al/Ni/Au Ohmic contacts to AlGaN/GaN heterostructures

This letter reports on the temperature behavior of the structural and electrical properties of Ti/Al/Ni/Au contacts to AlGaN/GaN heterostructures. While Ohmic contacts formed at 750 °C showed a decreasing temperature behavior of the specific contact resistance ρC, which was explained by a thermionic field emission mechanism, an increasing trend is observed in the contacts formed at 850 °C. In this case, ρC exhibits a “metal-like” behavior, i.e., describable by a T1.8 dependence. The microstructural analysis of the interfacial region allowed to explain the results with the formation of metallic intrusions contacting directly the two dimensional electron gas.

Influence of high-temperature GaN annealed surface on the electrical properties of Ni/GaN Schottky contacts

In this work, the electrical properties of Ni/GaN Schottky contacts formed on high-temperature annealed (1100–1200 °C) GaN surfaces were studied. Although the morphology of the GaN surface was not changing after annealing, a worsening of the electrical behavior of the Schottky contact occurred, with a reduction in the barrier height and an increase in the leakage current. Moreover, a different temperature dependence of the reverse electrical characteristics of the Schottky diodes was observed. In particular, for the sample annealed at 1150 °C for 5 min, one-dimensional variable-range-hopping conduction was one of the dominant carrier transport mechanisms. The presence of a high density of interface states was indicated as a possible reason of this electrical behavior.

Epitaxial NiO gate dielectric on AlGaN/GaN heterostructures

This letter reports on epitaxial nickel oxide (NiO) films grown by metal-organic chemichal vapor deposition on AlGaN/GaN heterostructures. The grown material was epitaxial, free from voids and exhibited a permittivity of 11.7, close to bulk NiO. This approach is advantageous with respect other methods such as the thermal oxidation of Ni films due to a better reproducibility and film quality. A reduction of the leakage current in Schottky diodes with an interfacial NiO layer has been observed and described using the metal-insulator-semiconductor Schottky model. The results indicate that these films are promising as gate dielectric for AlGaN/GaN transistors technology.