Papers by Author: Roberta Nipoti

Abstract: Activation and compensation ratios feature the electrical doping efficiency of a semiconductor material by ion implantation. The estimation of these ratios requires a quantitative evaluation of the density of the implanted dopant in substitutional position and of the density of the compensator centers after the mandatory post implantation annealing treatment. In the case of Al⁺ ion implanted 4H-SiC, it is a common habit to determine acceptor density, compensator density and acceptor thermal ionization energy by fitting the curve of the drift holes temperature dependence with the charge neutrality equation. However, this strategy could lead to ambiguous results. In fact, this study shows several cases of Al⁺ ion implanted 4H-SiC of interest for electronic device fabrication, where at least two sets of such fitting outputs can reproduce the same experimental curve within the uncertainty of the data. Provided that a model for the carrier transport could be set-up, the contemporaneous fits of the temperature dependence of drift hole density and of drift hole mobility is proposed to alleviate the uncertainty of the estimated acceptor density, compensator density and acceptor thermal ionization energy.

Abstract: Encapsulating SiC with a carbon layer (C-cap) is a widely used technique to avoid step bunching during post implantation annealing. In this work we propose a mechanism that explains the roughening that the surface unavoidably undergoes during annealing under the C-cap. We investigated the reactions occurring at the interface between 4H-SiC and the C-cap by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and atomic force microscopy carried out at different stages of the sample processing: just after annealing, after C-cap removal in dry Oxygen, and after cleaning in buffered oxide etch solution. Our observations show that, even though the C-cap roughens for increasing annealing temperature and time, it is not visibly damaged even after 1950 °C 30 min annealing. After the C-cap removal the 4H-SiC surface was covered by a network of clusters that are eventually removed by buffered oxide etch solution. This occurrence suggests that, during the post-implantation annealing, the 4H-SiC surface decomposes and the escaped Si and C atoms are trapped inside the C-cap or at the interface between 4H-SiC and the C-cap. While C clusters are etched off in the dry O₂ atmosphere, the Si clusters oxidize and form SiO₂ nanoparticles which are finally etched by buffered oxide etch.

Abstract: In this work, we study the impact of the dose rate on the electrical properties of aluminum (p-body, p⁺-body-contact) and phosphorous (n-source/drain) implanted 4H-SiC. We find no significant differences for dose rates ranging from 1×10¹¹ cm^-2s^-1 to 2−7×10¹² cm^-2s^-1. AFM scans across implanted and non-implanted regions after thermal oxidation and subsequent oxide etching reveal a clear dependence of the oxidation rate on the conduction type and doping concentration. In addition, we observe an increasing (decreasing) oxidation rate for increasing doping concentrations of the n-type (p-type) ion implanted areas.

Abstract: Van der Pauw devices have been fabricated by double ion implantation processes, namely P⁺ and Al⁺ co-implantation. Similarly to the source area in a SiC VD-MOSFET, a 5 × 10¹⁸ cm^-3 P plateau is formed on the top of a buried 3 × 10¹⁸ cm^-3 Al distribution for electrical isolation from the n- epilayer. The post implantation annealing temperature was 1600 °C. Annealing times equal to 30 min and 300 min have been compared. The increase of the annealing time produces both an increase of electron density as well as electron mobility. For comparison a HPSI 4H-SiC wafer, 1×10²⁰ cm^-3 P⁺ ion implanted and 1700 °C annealed for 30 min was also characterized.

Abstract: This work takes into account low Al implanted concentrations of 3 x 10¹⁸ cm^-3 and 1 x 10¹⁹ cm^-3 to compare the results of 1600°C and 1950°C post-implantation annealing treatments, done with two different annealing times per given implanted Al concentration and post implantation annealing temperature. Current-voltage and Hall effect measurements were performed to have the drift hole density and the drift hole mobility curves in the temperature range 100 - 650 K. The fitting of these curves in the frame of a carrier transport into the extended states of the valence band were performed to estimate the Al acceptor density, the donor compensator density, and the Al acceptor ionization energy. Peculiar feature of hole density and hole mobility curves is a contemporaneous increase of both carrier density and mobility values with increasing annealing time, which is congruent with the output parameters of the fitting procedure. The latter shows an almost stable Al electrical activation and a decrease of compensation with increasing annealing time for constant annealing temperature and given implanted Al concentration.

Abstract: Fundamental aspects of transport in Al ion implanted p-type 4H-SiC are briefly reviewed, in the light of recent literature. Particular attention is paid on (i) the Hall factor and (ii) the role of disorder in the onset of a variable range hopping mechanism (VRH) at high temperatures as doping level increases, up to a 2D-VRH induced by extended defects in the heaviest doped samples. The study allowed to understand the critical balance between implanted impurity density and annealing temperature that leads to the searched doping level, ensuring an efficient electrical activation of implanted impurities, on a side, and, on the other side, avoiding stacking faults that cause anisotropic hopping transport.

Abstract: 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.

Abstract: 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.

Abstract: The carbon vacancy (V_C) is a major limiting-defect of minority carrier lifetime in n-type 4H-SiC epitaxial layers and it is readily formed during high temperature processing. In this study, a kinetics model is put forward to address the thermodynamic equilibration of V_C, elucidating the possible atomistic mechanisms that control the V_C equilibration under C-rich conditions. Frenkel pair generation, injection of carbon interstitials (C_i’s) from the C-rich surface, followed by recombination with V_C’s, and diffusion of V_C’s towards the surface appear to be the major mechanisms involved. The modelling results show a close agreement with experimental deep-level transient spectroscopy (DLTS) depth profiles of V_C after annealing at different temperatures.

Abstract: 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°C, 1h) followed by a lower temperature one (1500°C, 4h) is mandatory to achieve low defects concentration and good crystal quality in both the n-and p-type zones of the device.