Alfonso Ruggiero

Schottky--ohmic transition in nickel silicide/SiC-4H system: is it really a solved problem?

The transition from Schottky to ohmic contact in the nickel silicide/SiC system during annealing from 600 to 950 °C was investigated by measuring the electrical properties of the contact and by analyzing the microstructure of the silicide/SiC interface. The graphite clusters formed by carbon atoms during silicidation are uniformly distributed into the silicide layer after annealing at 600 °C and they agglomerate into a thin layer far from the silicide/SiC interface after annealing at 950 °C. At this temperature an increase of the Schottky barrier height was measured, while deep level transient spectroscopy evidences the absence of the 0.5 eV peak related to the carbon vacancies.

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.

Epitaxial Layers Grown with HCl Addition: A Comparison with the Standard Process

The growth rate of 4H-SiC epi layers has been increased by a factor 3 (up to 18μm/h) with respect to the standard process with the introduction of HCl in the deposition chamber. The epitaxial layers grown with the addition of HCl have been characterized by electrical, optical and structural characterization methods. An optimized process without the addition of HCl is reported for comparison. The Schottky diodes, manufactured on the epitaxial layer grown with the addition of HCl at 1600 °C, have electrical characteristics comparable with the standard epitaxial process with the advantage of an epitaxial growth rate three times higher.

Schottky-Ohmic Transition in Nickel Silicide/SiC-4H System: the Effect of Non Uniform Schottky Barrier

The transition from Schottky to ohmic contact in the nickel silicide/SiC system during annealing from 600 to 950 °C was investigated by measuring the electrical proprieties of the contact and by analyzing the microstructure of the silicide/SiC interface. After annealing at 950 °C the I-V characterisation shows the presence of a non uniform Schottky barrier with respect to the almost ideal diodes annealed at 600°C, which can be responsible for the rectifying-ohmic transition in the electrical behaviour.

Optimisation of epitaxial layer growth by schottky diodes electrical characterization

The influence of the epitaxial layer growth parameters on the electrical characteristics of Schottky diodes has been studied in detail. Several diodes were manufactured on different epitaxial layers grown with different Si/H2 ratio and hence with different growth rates. From the electrical characterization a maximum silicon dilution ratio can be fixed at 0.04 %. This limit fixes also a maximum growth rate that can be obtained in the epitaxial growth, with this process, at about 8 μm/h. Several epitaxial layers have been grown, using this dilution ratio, with different temperatures (1550÷1650 °C). At 1600 °C the best compromise between the direct and the reverse characteristics has been found. With this process the yield decreases from 90% for a Schottky diode area of 0.25 mm2 to 61% for the 2 mm2 diodes. Optimizing the deposition process to reduce the defects introduced by the epitaxial process, yield of the order of 80% can be reached on 1 mm2 diodes.

Correlation between Leakage Current and Ion-Irradiation Induced Defects in 4H-SiC Schottky Diodes

The defects formation in ion-irradiated 4H-SiC was investigated and correlated with the electrical properties of Schottky diodes. The diodes were irradiated with 1 MeV Si+-ions, at fluences ranging between 1×109cm-2 and 1.8×1013cm-2. After irradiation, the current-voltage characteristics of the diodes showed an increase of the leakage current with increasing ion fluence. The reverse I-V characteristics of the irradiated diodes monitored as a function of the temperature showed an Arrhenius dependence of the leakage, with an activation energy of 0.64 eV. Deep level transient spectroscopy (DLTS) allowed to demonstrate that the Z1/Z2 center of 4H-SiC is the dominant defect in the increase of the leakage current in the irradiated material.

Effects of Epitaxial Layer Growth Parameters on the Defect Density and on the Electrical Characteristics of Schottky Diodes

The effects of the Si/H2 ratio on the growth of the epitaxial layer and on the epitaxial defects was studied in detail. A large increase of the growth rate has been observed with the increase of the silicon flux in the CVD reactor. Close to a Si/H2 ratio of 0.05 % silicon nucleation in the gas phase occurs producing a great amount of silicon particles that precipitate on the wafers. The epitaxial layers grown with a Si/H2 ratio of 0.03% show a low defect density and a low leakage current of the Schottky diodes realized on these wafers. For these diodes the DLTS spectra show thepresence of several peaks at 0.14, 0.75, 1.36 and 1.43 eV. For epitaxial layers grown with higher values of the Si/H2 ratio and then with an higher growth rate, the leakage current of the Schottky diodes increases considerably.

Defect Evolution in Ion Irradiated 6H-SiC Epitaxial Layers

Deep-Level Transient Spectroscopy and room temperature photoluminescence were used to characterise a 6H-SiC epitaxial layer irradiated with 10 MeV C+ and to follow the defect annealing in the temperature range 300-1400 °C. The intensity of luminescence peak at 423 nm, related to band to band transitions, decreases after irradiation and it is slowly recovered after annealing in the temperature range 1000-1400 °C. The DLTS spectra of low temperature annealed samples show the presence of several overlapping traps, which anneal and evolve at high temperatures. After 1200 °C a main level at Ec-0.43 eV (E1/E2) is detected. The comparison between luminescence and DLTS results indicates that the defect associated with the E1/E2 level is mainly responsible for the luminescence quenching after irradiation.

Defects in He+ Irradiated 6H-SiC Probed by DLTS and LTPL Measurements

Deep-Level Transient Spectroscopy (DLTS) and Low Temperature Photholuminescence (LTPL) were used to characterize the defects introduced in 6H-SiC epitaxial layer by 2.5 MeV He + irradiation and their behaviour with thermal annealing in the temperature range 200 - 900 °C. The DLTS spectra of as irradiated sample show different levels at an energy of 0.40 eV (E 1/E 2), 0.51 eV (RD 5) and 0.71 eV (Z 1/Z 2) from the bottom of the conduction band. The concentration of the E 1/E 2 level increases with annealing temperature, while the concentration of the remaining peaks decreases. The LTPL of the irradiated sample shows a series of sharp peaks and broad bands in the region 470-500 nm. A set of peaks (473-479 nm) related to carbon vacancies anneals at 600 °C, a second set (470 nm) is stable up to 900 °C and a third set (L 1-473 nm and L 2 - 480 nm) anneals starting from 400 °C. The comparison of both measurements suggests that the Z 1/Z 2 (0.71 eV) DLTS level and the L 1 (L 2) luminescence peaks can be related to the same defect center.

Correlation Study of Morphology, Electrical Activation and Contact formation of Ion Implanted 4H-SiC

This paper reports a detailed study of the electrical activation and the surface morphology of 4H-SiC implanted with different doping ions (P for n-type doping and Al for p-type doping) and annealed at high temperature (1650–1700 °C) under different surface conditions (with or without a graphite capping layer). The combined use of atomic force microscopy (AFM), transmission electron microscopy (TEM), and scanning capacitance microscopy (SCM) allowed to clarify the crucial role played by the implant damage both in evolution of 4H-SiC surface roughness and in the electrical activation of dopants after annealing. The high density of broken bonds by the implant makes surface atoms highly mobile and a peculiar step bunching on the surface is formed during high temperature annealing. This roughness can be minimized by using a capping layer. Furthermore, residual lattice defects or precipitates were found in high dose implanted layers even after high temperature annealing. Those defects adversely affect the electrical activation, especially in the case of Al implantation. Finally, the electrical properties of Ni and Ti/Al alloy contacts on n-type and p-type implanted regions of 4H-SiC were studied. Ohmic behavior was observed for contacts on the P implanted area, whilst high resistivity was obtained in the Al implanted layer. Results showed a correlation of the electrical behavior of contacts with surface morphology, electrical activation and structural defects in ion-implanted, particularly, Al doped layer of 4H-SiC.