Modeling of Porous Silicon Nanowires PhD Day, December 3rd, 2014 Aleandro Antidormi 31284 2nd year 1 Tutors: G. Piccinini , L. Boarino2 1 M. Graziano Cooperator: D. Chiabrando1,2 1 Department of Electronics and Telecommunications, Politecnico di Torino, Italy 2 INRIM - Istituto di Ricerca Metrologica, Torino, Italy Attended Classes • Electrochemical Bio/Nano/CMOS interfaces, 4 CFU, • • • • • • • • 10/10/2013 Fisica dei sistemi mesoscopici, 4 CFU, 24/07/2013 Introduzione alla formulazione hamiltoniana di sistemi classici e quantistici, 4 CFU, 30/10/2014 Metodi statistici avanzati, 5 CFU, 13/10/2014 Multi-scale and multi-physics numerical modelling techniques, 4 CFU, 29/06/2013 Nanomagnetismo e spintronica, 4 CFU, 26/03/2014 Sistemi quantistici dissipativi a stati finiti, 6 CFU, 11/07/2014 Il metodo Monte Carlo, 6 CFU, 20/10/2014 Nanocomputing: dispositivi, circuiti e architetture, 8 CFU, attending A. Antidormi PhD Day, Dec. 3rd, 2014 Modeling of Porous Silicon Nanowires 2 / 15 Research context - Porous Silicon (PS) • PS is a very interesting material • Structure: I Network of Silicon Nanocrystals I Size: few nanometers I All around pores • Unique transport properties I NOT wide band-gap semiconductor I NOT insulator • Electrical transport in PS largely depends on I I pores concentration pores dimensions * images reproduced by courtesy of NanoFacility Piemonte, INRIM A. Antidormi PhD Day, Dec. 3rd, 2014 Modeling of Porous Silicon Nanowires 3 / 15 Research Context - PS Nanowires • PS-NWs have many versatile properties I Self-assembly fabrication techniques I No lithography I Integrability with standard Silicon-based technologies • Many possibilities of application I Focus on sensing • Large ratio surface area to volume • High chemical reactivity at room temperature • Gas molecules fill pores =⇒ change of the conductivity of PS • Measure of the change of conductivity =⇒ estimation of gas concentration A. Antidormi PhD Day, Dec. 3rd, 2014 Modeling of Porous Silicon Nanowires 4 / 15 PS-NWs Fabrication • Fabrication process: Metal-Assisted Etching (MaE) • p-doped Si substrate covered by Polystyrene nanospheres (1) I diameter=210nm ±5% • spin coating -> self-assembled hexagonal-packed monolayer • Oxigen Plasma etching used to reduce spheres dimensions (2) • Au film deposited using e-gun evaporator • Spheres removed in ultrasonic bath (3) • MaE (4) performed with sample in (H2 O : H2 O2 : HF 1 : 1 : 3)) A. Antidormi PhD Day, Dec. 3rd, 2014 Modeling of Porous Silicon Nanowires 5 / 15 Addressed Research Problems • Irregular shape of PS-NWs makes modelling and simulation difficult • Analytical solution for the conduction impractical: I Simulative approach is the most promising way • At present no models available for porous materials in physics-based simulators I Electrical properties depend on size and shape of pores A. Antidormi PhD Day, Dec. 3rd, 2014 Modeling of Porous Silicon Nanowires 6 / 15 Novel contribution • We developed a simplified model suitable for simulator TCAD Atlas • Simulation of many PS-NWs accomplished I Pore distribution and concentration taken from OUR devices • Statistical treatment of the problem given the unknown pore distribution • Analysis of results leads to a better comprehension of electron transport in porous structures A. Antidormi PhD Day, Dec. 3rd, 2014 Modeling of Porous Silicon Nanowires 7 / 15 Adopted Methodologies - Porous Distribution TEM • TEM analysis has been performed after scratching wires on a grid • Lighter dots = silicon nanocrystals in a porous structure • Statistical data about position and size of pores have been deduced I Average area of pores A = 4.1nm2 A. Antidormi PhD Day, Dec. 3rd, 2014 Modeling of Porous Silicon Nanowires 8 / 15 Adopted Methodologies - PS-NW Model • We modelled PS-NW as a 3D-wire with square section • Material: p-doped crystalline silicon, NA = 1015 atoms/cm3 • Non-rectifying contacts to avoid influence of Schottky barriers • Pores modelled with pore segments with squared sections • Number, position and size of pores randomly chosen in accordance with data from fabricated devices • Pores geometry is rather arbitrary: pores can intersect creating complicated structures A. Antidormi PhD Day, Dec. 3rd, 2014 Modeling of Porous Silicon Nanowires 9 / 15 Adopted Methodologies - Simulation • Simulated device characteristics taken from fabricated devices: • Channel length 100nm, Squared section 900nm2 • Uniform Distribution of pores along the channel • Pore depth normally distributed µd = 19.2nm, σd = 11.4nm • Pore side normally distributed µs = 6.3nm, σs = 3.2nm A. Antidormi PhD Day, Dec. 3rd, 2014 Modeling of Porous Silicon Nanowires 10 / 15 Adopted Methodologies - Results −8 x 10 a) Current [A] 1 b) 0.5 0 0 2 4 Applied Voltage [V] 6 8 • Current-Voltage characteristic for one PS-NW; • a) Va = 0.2V , b) Va = 2.5V , c) Va = 8.0V • Field- Dependent Mobility leads to current saturation A. Antidormi PhD Day, Dec. 3rd, 2014 Modeling of Porous Silicon Nanowires 11 / 15 c) Adopted Methodologies • Conduction in PS-NWs largely influenced by the concentration and distribution of pores • Field-dependent mobility limits current flow • Higher porous concentrations =⇒ stronger spatial confinement of carriers =⇒ quantum mechanical effects arise • Energy quantization =⇒ reduced states density =⇒ increased resistivity • Moreover, interaction with gas molecules changes the surface properties of the wire • Ab-initio atomistic study becomes necessary (ATK VNL simulator) A. Antidormi PhD Day, Dec. 3rd, 2014 Modeling of Porous Silicon Nanowires 12 / 15 Adopted Methodologies • From microscopic to macroscopic description of conduction: • In very short Si-NWs, electron transport shows coherent properties; (Wire length phase relaxation length) • Current calculation possible through Landauer formalism (via transmission spectrum) • Pores inserted in the wire modify the transmission spectrum =⇒ change in current and conductance • From the effect of one pore to the transmission spectrum, average effect of a pore distribution obtainable. A. Antidormi PhD Day, Dec. 3rd, 2014 Modeling of Porous Silicon Nanowires 13 / 15 Published and Submitted Journal Papers • A. Antidormi, D. Chiabrando, M. Graziano, L. Boarino, G. Piccinini, "Methodology modeling of MaE-fabricated Porous Silicon Nanowires", Microelectronics and Electronics (PRIME), 2014 10th Conference on Ph.D. Research in Grenoble, June 2014 A. Antidormi PhD Day, Dec. 3rd, 2014 Modeling of Porous Silicon Nanowires 14 / 15 Future Work • Electrical characterization of experimental PS-NWs and comparison data vs. simulation • Development of electric model for circuit simulator based on elementary resistive blocks • Understand effects of pores on transmission spectrum of Si-NWs via atomistic simulation • Predict transmission spectrum for a given pore distribution and evaluate current • Understand effects of gas molecules on electron transport and changes in conductance (gas sensing application) • Validate the model with simulation and experimental data A. Antidormi PhD Day, Dec. 3rd, 2014 Modeling of Porous Silicon Nanowires 15 / 15
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