Razvan Pascu

Microstructures and growth characteristics of polyelectrolytes on silicon using layer-by-layer assembly

AbstractGrowth processes of nanocomposite layers obtained by polyelectrolytes, poly(sodium 4-styrenesulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDADMAC), self-assembled on silicon surface using layer-by-layer (LbL) technique were investigated, and theoretical and experimental data are herein reported. Complementary microstructural and compositional analyses techniques (scanning electron microscopy, ellipsometry, X-ray reflectivity, zeta (ξ) potential measurements and attenuated total reflection infrared spectroscopy) were used for deep characterization of the multilayer structure formation. Electrophoretic zeta (ξ) potential measurements indicated that the surface charge was either positive or negative, depending on the polyelectrolyte used (PDADMAC or PSS). ATR-IR spectra confirmed the successfully silanization process and then, the building up of the nanocomposite layer. Morphological investigation and X-ray reflectivity demonstrated the growth process and cross-section size of the bilayers. Ellipsometric measurements were in very good agreement with SEM and XRR, showing once again the successful deposition of polyelectrolyte multilayers.

Two Packaging Solutions for High Temperature SiC Diode Sensors

A partially electrically isolated package with a gold wire and fully isolated solution with a metallic piston, respectively, are designed and tested for high temperature sensors (400°C) based on SiC Schottky barrier diodes (SBD). Electrical behavior and sensor performance are very close for both packaging solutions. The stress due to contact pressure and higher cost are some disadvantages for pressure contact technology.

High temperature sensor based on SiC Schottky diodes with undoped oxide ramp termination

A simple technology of 4H-SiC Schottky diode with oxide ramp termination is presented. The ramp is controlled by two undoped layers: an annealed (compact) oxide layer and as deposited layer, respectively.

Temperature behavior of 4H-SIC MOS capacitor used as a gas sensor

Temperature behavior of 4H-SiC MOS capacitor is investigated. A new layout with several active MOS structures is proposed. Measured C-V characteristics show good thermal stability up to 300 C. The main electrical parameters of the MOS structure have been extracted by measurements on wafer and encapsulated samples.

SiO2/4H-SiC interface states reduction by POCl3 post-oxidation annealing

Interface state density (D_it) at SiO₂/4H-SiC interfaces is investigated using capacitance-voltage (C-V) characterization. Two different measurement methods for D_it determination (both C-V at different temperatures in the range of 80-300K and high frequency (HF) vs quasi static (QS) characteristics) have been used. A significant reduction of D_it is observed, almost one order of magnitude, after a post oxidation annealing (POA) in POCl₃ ambient, compared to as-grown dry oxidized sample.

Electrical Characterization of Ni-Silicide Schottky Contacts on SiC for High Performance Temperature Sensor

The electrical behavior and stability of a temperature sensor based on 4H-SiC Schottky diodes using Ni2Si as Schottky contact, are investigated. The ideality factor and the barrier height were found to be strongly dependent on the post-annealing temperature of the Ni contact (which lead to the formation of Ni2Si). A nearly ideal Schottky device, with the barrier height approaching the high value of 1.7eV, and a slight temperature dependence, was obtained after an annealing at TA=800°C. This high barrier height proves that Ni2Si is suitable as Schottky contact for temperature sensors, able to reliably operate up to 450°C. Sensor sensitivity levels between 1.00mV/°C and 2.70 mV/°C have been achieved.

A promising technology of Schottky diode based on 4H-SiC for high temperature application

A Schottky diode technology based on silicon carbide (SiC) with ramp oxide termination is presented. The improvement of the Schottky, respectively ohmic contact is conditioned by a rapid thermal processing to form Ni-silicide. The Schottky diodes (SD) have been electrical characterized in forward bias in range of temperature 300-573 K to verify their capacity to operate like temperature sensors. The results show an improvement of the electrical parameters with increasing of the temperature.

Engineering Graphene Quantum Dots for Enhanced Ultraviolet and Visible Light p-Si Nanowire-Based Photodetector

In this work, a significant improvement of the classical silicon nanowire (SiNW)-based photodetector was achieved through the realization of core-shell structures using newly designed GQDPEIs via simple solution processing. The poly(ethyleneimine) (PEI)-assisted synthesis successfully tuned both optical and electrical properties of graphene quantum dots (GQDs) to fulfill the requirements for strong yellow photoluminescence emission along with large band gap formation and the introduction of electronic states inside the band gap. The fabrication of a GQDPEI-based device was followed by systematic structural and photoelectronic investigation. Thus, the GQDPEI/SiNW photodetector exhibited a large photocurrent to dark current ratio (Iph/Idark up to ∼0.9 × 102 under 4 V bias) and a remarkable improvement of the external quantum efficiency values that far exceed 100%. In this frame, GQDPEIs demonstrate the ability to arbitrate both charge-carrier photogeneration and transport inside a heterojunction, leading to simultaneous attendance of various mechanisms: (i) efficient suppression of the dark current governed by the type I alignment in energy levels, (ii) charge photomultiplication determined by the presence of the PEI-induced electron trap levels, and (iii) broadband ultraviolet-to-visible downconversion effects.

An Investigation of SiC Schottky Contact Barrier Inhomogeneity for Temperature Sensing Applications

This paper proposes a method of characterizing silicon carbide Schottky diodes with inhomogeneous contacts in temperature sensing applications. Using the energy activation technique, temperature intervals where the effective barrier height is constant are determined. Unlike the conventional barrier which increases with temperature for inhomogeneous diodes, the effective barrier has physical meaning and can be used for sensor performance evaluation. The utility of effective barrier analysis is confirmed on fabricated Ni/4H-SiC Schottky diodes with different annealing conditions and different degrees of barrier non-uniformity. The good agreement between calculated and experimental data proves the suitable behavior of inhomogeneous diodes as sensors for different temperature ranges.

The effect of the post-metallization annealing of Ni/n-type 4H-SiC SCHOTTKY contact

The Schottky diode for temperature sensor based on 4H-SiC is presented. This paper is focused on the improvement of the Schottky contact and interface stabilization using an annealing in Ar atmosphere. The diodes have been measured in range of temperature 50-150°C.