Papers by Author: Anna Cavallini

Abstract: Capacitance versus Voltage (C-V) and Deep Level Transient Spectroscopy (DLTS) measurements of Al+ implanted p+n diodes with Al+ implanted Junction Termination Extension are here studied. These diodes present C-V characteristics like graded junction for low forward bias values, i.e. > 0.4 V , or like abrupt junctions for large reverse bias, i.e. between 0.4V and -10V. The depth range of the graded junction, computed by the capacitance values, is much larger than the simulated tail of the ion implanted Al+ profile. DLTS spectra have been measured both in injection and standard configuration and always show minority carrier traps in the temperature range 0-300K. Three are the minority carrier related peaks, one attributed to the Al acceptor and the others to the D and D1 defects. The depth distribution of these hole traps will be discussed with respect to the apparent carrier concentration, obtained by C-V analysis.

Abstract: Hydrogenated nanocrystalline silicon for photovoltaic applications has been investigated. Morphological properties, as well as electrical properties, have been investigated with high spatial resolution by scanning force microscopy analyses (AFM, Atomic Force Microscopy and C-AFM conductive AFM). A major problem regarding the electronic properties is to understand where the current flows. The present contribution aims to clarify which of the material phases mainly contributes to the conduction mechanism.

Abstract: This paper reports on the defects created in a 6H-SiC p-type substrate by a process of ion implantation and a quite low temperature annealing (1300 °C), suitable for the realization of the source/drain regions of a MOSFET because it does not give rise to step bunching phenomena. Current voltage measurements showed the presence of a group of diodes featured by excess current. The effects of defects under the implanted layer on the transport properties of the diodes were investigated by DLTS: four hole traps were detected in all the measured diodes; besides, a broadened peak around 550 K was detected in the diodes that show excess current.

Abstract: Dislocations and impurities in silicon have been widely investigated since many years, nevertheless many questions on this subject remain still unsolved. As an example, theory, models and experimental phenomena provide evidence of the existence of shallow bands in silicon induced by the dislocation strain field. Nevertheless, only deep bands, likely associated with contamination at dislocations, have been detected up to now by junction spectroscopy. The present contribution reviews several results, obtained by the authors, on dislocation impurity interactions and their effects on the electronic properties of defect states in silicon. Point and extended defects introduced in p-type Cz Si by oxygen precipitation and plastic deformation have been investigated with electrical methods. Different materials (oxygen precipitated and deformed Cz Si and Fz Si) were examined in order to separate the role of oxygen precipitation, plastic deformation and metallic contamination on non-radiative electronic transitions at defect centers. A deep hole trap, named T1, has been associated to dislocation-related impurity centers, while additional deep traps have been related to contamination by grown-in transition metals and to clusters involving oxygen atoms. Moreover, experimental results obtained by junction spectroscopy assessed the existence of dislocation related shallow states. These were found to be located at 70 and 60 meV from the valence and conduction band edge, respectively.

Abstract: High performance SiC detectors for ionising radiation have been designed, manufactured and tested. Schottky junctions on low-doped epitaxial 4H-SiC with leakage current densities of few pA/cm2 at room temperature has been realised at this purpose. The epitaxial layer has been characterised at different dose of radiations in order to investigate the SiC radiation hardness. The response of the detectors to alpha and beta particle and to soft X-ray have been measured. High energy resolution and full charge collection efficiency have been successfully demonstrated.

Abstract: We report on the investigation of electrical properties of polycrystalline 3C-SiC thin films deposited on oxidized Si by low pressure chemical vapor deposition (LPCVD) to obtain bi-layer structures [Si(100)/SiO2/poly 3C-SiC] for pressure sensors and micro-electromechanical system (MEMS) applications. Polycrystalline 3C-SiC films have been preliminary characterized in their compositional, structural, morphological and electrical properties. Moreover, metal contact definition has been carefully optimized by transmission line method (TLM) analyses performed at different temperatures. We focuses the attention on the evaluation of the bulk resistivity (ρ), the specific contact resistivity (ρc) and their behavior dependence on the temperature because these are the characteristics of major importance for the fabrication of pressure sensors or MEMS.

Abstract: n+/p diodes have been realized by 300°C phosphorus ion implantation and subsequent annealing at 1300°C. An electrical characterization of the devices as well as a study of the defects introduced by the implantation process has been made. I-V measurements pointed out that the diodes maintain a good rectifying behavior up to 737K. DLTS analyses detected the presence of three traps, T2, T3 and T4, which are not due to the implantation process, and a high energy trap, T5, that could be related to the surface states at the Ni/SiC interface.

Abstract: The effects of irradiation with protons and electrons on 4H-silicon carbide epilayers were investigated. The particle energy was 6.5 and 8.2 MeV. The electronic levels associated with the irradiation-induced defects were analyzed by current-voltage characteristics and deep level transient spectroscopy measurements up to 700 K. In the same temperature range the apparent free carrier concentration was measured by capacitance-voltage characteristics to monitor possible compensation effects due to the deep level associated to the induced defects. Introduction rate, enthalpy and capture cross-section of such deep levels were compared and some conclusions about the nature of the defects were drawn.