Francesco Moscatelli

Numerical Simulation of Radiation Damage Effects in p-Type and n-Type FZ Silicon Detectors

In the framework of the CERN-RD50 Collaboration, the adoption of p-type substrates has been proposed as a suitable mean to improve the radiation hardness of silicon detectors up to fluencies of 1times10 16 n/cm2. In this work two numerical simulation models will be presented for p-type and n-type silicon detectors, respectively. A comprehensive analysis of the variation of the effective doping concentration (Neff), the leakage current density and the charge collection efficiency as a function of the fluence has been performed using the Synopsys T-CAD device simulator. The simulated electrical characteristics of irradiated detectors have been compared with experimental measurements extracted from the literature, showing a very good agreement. The predicted behaviour of p-type silicon detectors after irradiation up to 1016 n/cm2 shows better results in terms of charge collection efficiency and full depletion voltage, with respect to n-type material, while comparable behaviour has been observed in terms of leakage current density

Radiation hardness after very high neutron irradiation of minimum ionizing particle detectors based on 4H-SiC p/sup +/n junctions

In this work we analyzed the radiation hardness of SiC p/sup +/ n diodes used as minimum ionizing particle (MIP) detectors after very high 1 MeV neutron fluences. The diode structure is based on ion implanted p/sup +/ emitter in an n-type epilayer with thickness equal to 55 /spl mu/m and donor doping N/sub D/=2/spl times/ 10/sup 14/cm/sup -3/. The diode breakdown voltages were above 1000 V. At 1000 V the leakage currents are of the order of 1 nA for all the measured diodes. The full depletion voltage is near 220-250 V. The charge collection efficiency to minimum ionizing particle has been investigated by a /sup 90/Sr /spl beta/ source. At 250 V the collected charge of the unirradiated diodes saturates near 3000 e/sup -/. At bias voltages over 100 V the energy spectrum of the collected charge was found to consist of a signal peak well separated from the noise. At around 250 V the signal saturates, in agreement with CV results. These devices have been irradiated at 6 different fluences, logarithmically distributed in the range 10/sup 14/-10/sup 16/ (1 MeV) neutrons/cm/sup 2/. The leakage current after irradiation decreases. The collected charges decrease for increasing fluences, remaining very high only until some 10/sup 14/ n/cm/sup 2/.

Carbon-Cap for Ohmic Contacts on Ion-Implanted 4H – SiC

A pyrolyzed photoresist film is commonly used as a protective cap of the surface of ion-implanted 4H-SiC wafers during the postimplantation annealing process with the aim to prevent Si sublimation and step bunching formation. Such a film that is called carbon-cap (C-cap) is always removed after postimplantation annealing and before any other processing step of the SiC wafer. Here, we show that this C-cap is a continuous, hard, black, mirrorlike, and planar thin film that can be patterned by a reactive ion etching O 2 -based plasma for the fabrication of ohmic contact pads on both Al + - and P + -implanted 4H-SiC. This C-cap material has an electrical resistivity of 1.5 × 10 -3 Ω cm and a good resistance against scratch. Al (1% Si) wires can be ultrasonically bonded on the C-cap pads. Such a bonding and the C-cap adhesion to the implanted 4H-SiC surface are stable for electrical characterizations in vacuum between room temperature and 450°C. The measured specific contact resistance of the C-cap on a 1 × 10 20 cm -3 p+-implanted 4H-SiC is 9 × 10- 5 Ω cm 2 at room temperature. Micro-Raman characterizations show that this C-cap is formed of a nanocrystalline graphitic phase.

Nitrogen Implantation to Improve Electron Channel Mobility in 4H-SiC MOSFET

Normally off 4H-SiC MOSFET devices have been fabricated on a p-type semiconductor and electrically characterized at different temperatures. A gate oxide obtained by nitrogen ion implantation performed before the thermal oxidation of SiC has been implemented in n-channel MOSFET technology. Two samples with a nitrogen concentration at the SiO2/SiC interface of 5 X 1018 and 1.5 X 1019 cm-3 and one unimplanted sample have been manufactured. The sample with the highest N concentration at the interface presents the highest channel mobility and the lowest threshold voltage. For increasing temperature, in all the samples, the threshold voltage decreases, and the electron channel mobility increases. The latter case attains a maximum value of about 40 cm2/V ldr s at 200degC for the sample with the highest N concentration. These trends are explained by the reduction of interface electron traps in the upper half of the band gap toward the conduction band edge. These results demonstrate that N implantation can be effectively used to improve the electrical performances of an n-type surface channel 4H-SiC MOSFET.

Comprehensive modeling of bulk-damage effects in silicon radiation detectors

In this paper, the issue of numerical modeling of radiation-damaged silicon devices is discussed, with reference to radiation detectors employed in high-energy physics experiments. Since the actual physical picture is far too complex to be accounted for at a first-principle (i.e., defect kinetics) level and not yet fully understood, a hierarchical approach has been followed looking for a suitable approximation of macroscopic changes of the electrical behavior of silicon device induced by radiation damage. In particular, a three deep-level trapping mechanism is accounted for by means of Shockley-Read-Hall theory, whereas the shallow-level sensitivity on the radiation is considered by means of a donor-removal model.

Analysis of electron traps at the 4H–SiC/SiO2 interface; influence by nitrogen implantation prior to wet oxidation

Electron states at the SiO2/4H–SiC interface have been investigated using capacitor structures and especially, the influence of excess nitrogen, introduced by ion implantation, at the interface is studied in detail. Implanted and nonimplanted n-type samples with an interfacial concentration of nitrogen of ∼1019 cm−3 and 1016 cm−3, respectively, were analyzed by capacitance-voltage (C-V) measurements, performed at different temperatures and probe frequencies, and thermal dielectric relaxation current (TDRC) measurements performed in the temperature range of 35–295 K. Three main categories of electron states are disclosed, true interface states (Dit), fast near interface states (NIToxfast) and slow near interface states (NIToxslow). The density versus energy distributions of Dit and NIToxfast have been deduced from the TDRC data and they are shown to give a close quantitative agreement with the shape and frequency dependence of the C-V curves. Further, the amount of NIToxslow extracted from TDRC is demonstr...

Improved electrical characterization of Al–Ti ohmic contacts on p-type ion implanted 6H-SiC

This paper deals with the electrical characterization of low resistance Al–Ti 72/28 wt% ohmic contacts to a p-type ion implanted 6H-SiC layer. Transmission line model (TLM) structures were realized on the top of MESA islands defined in this ion implanted layer. A metal scheme composed of Al-1%Si(350 nm)/Ti(80 nm) was deposited by sputtering, photolithography defined and annealed at 1000 °C in Ar for 2 min. TLM structures were measured as a function of the temperature in the range 25–290 °C. The TLM data were mainly analysed by a two-dimensional finite difference simulation tool that takes into account the current crowding effect at the contact periphery. Extracted contact resistivity values fall in the low range of data from the literature. The sheet resistance values computed from the TLM data agreed with those measured using Van der Pauw devices realized next to the TLM structures.

An enhanced approach to numerical modeling of heavily irradiated silicon devices

Abstract In this paper we discuss an enhanced approach to the analysis of radiation-damaged silicon devices, with reference to numerical modelling implemented in a general-purpose device simulator. In particular, the emission and capture mechanism of deep levels are accounted for by means of Shockley–Read–Hall theory and shallow-level sensitivity to radiation is considered by means of a donor removal model. The effects produced by regions containing very high defect concentrations (referred to as “clusters”) are considered by calculating the direct charge exchange between two deep levels. The resulting analysis technique has been validated and calibrated by means of comparison with experimental measurements carried out on irradiated samples. The model is shown to provide comprehensive and accurate results for several radiation damage phenomena.

Investigation on the Use of Nitrogen Implantation to Improve the Performance of N-Channel Enhancement 4H-SiC MOSFETs

A gate oxide obtained by wet oxidation of SiC preimplanted with nitrogen has been investigated on MOS capacitors and implemented in a n-channel MOSFET technology. Different implantation fluences and energies in the ranges 1.5 X 1013-1 X 1015 cm -2 and 2.5-10 keV, respectively, were used with the aims to study the effect of the nitrogen concentration at the SiO2/SiC interface on MOSFET performance. The highest dose, which is able to amorphize a surface SiC layer, was also employed to take advantage of the faster oxidation rate of amorphous phase with respect to crystalline one. The electron interface trap density near the conduction band has been evaluated with different techniques both on MOS capacitors and MOSFET devices; a good agreement among the measured values has been attained. A strong reduction of the electron interface traps density located near the conduction band has been obtained in the samples with a high nitrogen concentration at the SiO2/SiC interface. The MOSFETs with the highest nitrogen concentration at the interface (~1 X 1019 cm -3) present the highest channel mobility (21.9 cm2/V .s), the lowest threshold voltage (2.4 V), and the smallest subthreshold swing (310 mV/decade at drain current of 10 -11 A).

Device simulations of silicon detectors: a design perspective

Device simulation allows for accurate analysis of device behavior, accounting for several physical details that cannot easily be taken into account within compact, equivalent-circuit models. This is especially true for some issues typical of the design of silicon radiation detectors, where silicon properties are exploited in a non-conventional way and radiation damage raises severe reliability concerns. In this paper, a couple of significant applications of device simulation to the investigation and design of advanced solid-state radiation sensors are presented. More specifically, (i) radiation damage influence on detectors operating at cryogenic temperatures is successfully modeled and (ii) features of an innovative scheme for CMOS active pixel sensors are analyzed by means of mixed-mode simulation tools. From these examples, the usefulness and potentiality of advanced simulation techniques in the perspective of radiation detectors can be appreciated.