Effect of pack cementation temperatures on component, microstructure and anti-oxidation performance of Al-modified SiC coatings on C/C composites

The root mean square (rms) surface roughness extracted from atomic force microscopy is widely employed to complement the characterisation of ion implantation processes in 4H-SiC. It is known that the protection of a carbon film eliminates or mitigates roughening of the SiC surface during postimplantation annealing. This study, based on a rich original data collection of Al+ ion implanted 4H-SiC samples, allows for a quantitative description of the surface morphology as a function of the annealing temperature and time and of the Al implanted concentration. With increasing thermal budget, the evolution from flat, to blurred with ripples, granular, and finally jagged surface, results in a monotonous increase in the root mean square roughness. Additional information is given by the trends of the roughness exponent and of the correlation length, extracted from the height-height correlation function, which account for the surface evolution below 1700°C and for the effect of the Al implanted concentration on the ripple size, respectively. A combination of low roughness parameter and high correlation length identify the transition from ripples to jagged morphology. LAY DESCRIPTION: Selective area doping is a key step in the fabrication of hexagonal Silicon Carbide (4H-SiC) power electronic devices. It is achieved by ion implantation followed by a high temperature postimplantation annealing to restore the lattice and electrically activate the dopants. Aluminium, the preferred p-type dopant, is electrically activated at temperature ranging between 1500°C and 2000°C. The time required to complete the activation process is longer the lower the annealing temperature, spanning between some minutes and hundreds of hours. During annealing, 4H-SiC wafers are encapsulated by a temperature-resistant carbon layer (C-cap) in order to avoid step bunching and reduce surface roughening. Nevertheless, surface modifications can occur at high temperature. For this reason, the characterisations of 4H-SiC doping processes report not only the electrical activation of the dopants, but also the root mean square surface roughness obtained at the end of the process. However, rms values can be scattered because technological parameters such as the heating system and the way to deposit and remove the C-cap can affect the final result as well as the process parameters. Furthermore, the C-cap resistance to long annealing has been proven only by electrical measurements, but the surface morphology has never been observed. This work presents a quantitative characterisation of the surface morphology of Al implanted 4H-SiC as a function of the annealing temperature, time and of the Al implanted concentration, independent of the heating system and of the C-cap technology. The produced sample collection allowed to correlate characteristic surface features with the corresponding quantities extracted from image analysis that can be more sensitive to process parameters than the sole rms. These findings can be used to enrich process optimisation tools.

4H‐SiC surface morphology after Al ion implantation and annealing with C‐cap

The electrical characteristics of 4H-SiC p–i–n diodes with 8H-type in-grown stacking faults are investigated. The 4H-SiC p–i–n diodes have epilayers with a low Z1/2 center density formed by carbon implantation. The forward voltage drops of the 4H-SiC p–i–n diode with 8H-type in-grown stacking faults are larger than those of the 4H-SiC p–i–n diode without an 8H-type in-grown stacking fault. The differential on-resistance of the 4H-SiC p–i–n diode with 8H-type in-grown stacking faults is larger than the drift resistance of the drift layer calculated from the doping density and thickness of the drift layer. A large number of electrons are trapped at 8H-type in-grown stacking faults, and the effective carrier density decreases compared with the doping density.

Conductivity Degradation of 4H-SiC p-i-n Diode with In-Grown Stacking Faults (Special Issue : Solid State Devices and Materials)

1300°C Annealing of 1 × 1020 cm−3 Al+ Ion Implanted 3C-SiC/Si

The results of the first experiments for achieving the thermal equilibrium during 1300 °C annealing of 1×1020 cm-3 ion implanted Al+ in 3C-SiC are shown. X-ray diffraction, through reciprocal space maps and 2Θ scans, characterizes the 3C-SiC lattice recovery. The achievement of a ohmic behavior of Ni/Al/Ti alloy indicates the onset of a measurable electrical activation of the Al implanted layer. The Al electrical activation is qualified through the implanted layer sheet resistance.

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1300°C Annealing of 1×1020 Al+ Ion Implanted 3C-SiC

This study shows that a thin Ni film on Al/Ti/4H-SiC metal pads allows to preserve the pad form factor during a 1000 °C/2 min treatment, provided that the Al and Ti film thicknesses are sufficiently thin. Moreover, by reducing the Al to Ti thickness ratio, droplet formation in the contact area is avoided and a mirror-like appearance is obtained. This optimal contact morphology corresponds to a specific contact resistance of few 10-4 Ωcm2 at room temperature on p-type 4H-SiC with resistivity in the range 0.1 – 1 Ωcm.

Ni-Al-Ti Ohmic Contacts with Preserved Form Factor and Few 10- 4 Ωcm2 Specific Resistance on 0.1-1 Ωcm p-Type 4H-SiC

The activation energy for the electrical activation of 1x1019 cm-3 and of 1x1020 cm-3 ion implanted Al in 4H-SiC has been estimated. Ion implantation temperature and dose rate were in the range 430-500°C and around 1011 cm2s-1, respectively. Post implantation annealing temperatures varied between 1500 °C and 1950 °C. The annealing time per each annealing temperature was sufficiently long that the sheet resistance of the implanted layer could be equal to the stationary value at the applied annealing temperature. The Arrhenius plots of the room temperature sheet resistances with respect to the post implantation annealing temperatures featured an exponential trend for both the implanted Al concentrations. The activation energies of these plots are the activation energy for placing an implanted Al atom in a substitutional site, i.e. the electrical activation energy. Activation energies around 1 eV, equal within errors for the two implanted Al concentrations, were found.

Activation Energy for the Post Implantation Annealing of 1019 cm-3 and 1020 cm-3 Ion Implanted Al in 4H SiC

Thermal annealing plays a crucial role for healing the defectiveness in the ion implanted regions of DIMOSFETs (Double Implanted MOSFETs) devices. In this work, we have studied the effect of a double step annealing on the body (Al implanted) and the source (P implanted) regions of such devices. We found that a high temperature annealing (1750{\deg}C, 1h) followed by a lower temperature one (1500{\deg}C, 4h) is mandatory to achieve low defects concentration and good crystal quality in both the n- and p- type zones of the device.

Maria Concetta Canino

Papers

Ni-Al-Ti Ohmic Contacts with Preserved Form Factor and Few 10- 4 Ωcm2 Specific Resistance on 0.1-1 Ωcm p-Type 4H-SiC

1300°C Annealing of 1×1020 Al+ Ion Implanted 3C-SiC

Activation Energy for the Post Implantation Annealing of 1019 cm-3 and 1020 cm-3 Ion Implanted Al in 4H SiC

Double Step Annealing for the Recovering of Ion Implantation Defectiveness in 4H-SiC DIMOSFET

Double Step Annealing for the Recovering of Ion Implantation Defectiveness in 4H-SiC DIMOSFET