Presentation on theme: "Laser-diamond interaction – damage during laser graphitization"— Presentation transcript:
1Laser-diamond interaction – damage during laser graphitization Modelling the devicedamage during laser graphitizationTzveta Apostolova1, Stefano Lagomarsino2,3,Silvio Sciortino2,3, Chiara Corsi4,5, Marco Bellini61Institute for Nuclear Research and Nuclear Energy2Istituto Nazionale di Fisica Nucleare3 Dipartimento di Fisica, Università di Firenze4 Dipartimento di Fisica, Università di Firenze5LENS Florence6INO-CNR Florence
2MotivationLaser engineering of diamond for writing conductive paths is an important subject of research for its application in radiation detection (3D detectors)[1,2]. S. Lagomarsino et al Appl. Phys. Lett. 103, (2013) S. Lagomarsino , et al Diamond & Related Materials 43 (2014) 23–28A deep insight of the process of laser graphitization of diamond is critical to tune at best the laser parameters and obtain low resistivity channels with minimum damage of the surrounding diamond lattice.Simulate ultra-short laser-induced electronic excitation, absorption, and the subsequent relaxation processes in CVD monocrystalline diamond and compare to the results of experiment.
3Why a 3D architecture for diamond trackers? Since their very introduction (1997), 3D achitectures for silicon was intended to solve problems of radiation hardness in silicon detectors.Lowering charge trapping probability in the bulkThus: increasing collection efficiency++++++------(Nucl. Instr. and Meth. A 395 pp (1997) )
4How it is madeSince 2009, a simple 3D pulsed laser technique has been made avalilable for microfabrication of 3D graphitic structures in the bulk Diamond (for optical applications)Since 2009, a simple 3D pulsed laser technique has been made avalilable for microfabrication of 3D graphitic structures in the bulk Diamond (for optical applications)T.V. Kononenko et al., Femtosecond laser microstructuring in the bulk of diamond, Diamond and Relat. Mater. 18 (2009) 196–199This technique has been used by the collaborators to make conductive electrodes for 3D detectors.
5Our experimental approach: The transient current technique (TCT) is used to measure laser induced current transients.mAms500 V
6Our theoretical approach: Theoretical modeling (Quantum kinetic formalism based on a Boltzmann-type equation including photo-excitation, free-carrier absorption, impact ionization, Auger recombination of electron-hole plasma, thermal exchange with the lattice is performed.The transient conduction electron distribution functions, electron densities photo-generated and the average electron energies during the pumping fs-laser pulses are evaluated and damage criteria are given.
7Timescales of various electron and lattice processes in laser-excited solids. Inverse bremsstrahlungExciton formation/ non-radiative exciton decay1Timescales of variouselectron and lattice processes inlaser-excited solids (afterref. 10). Each green barrepresents an approximate rangeof characteristic times over arange of carrier densities from1017 to 1022cm–3.The triangles atthe top show the current state-ofthe-art in the generation of shortpulses of electromagneticradiation:1 5 fs (visible),2 120 fs(X-ray),3 0.5 fs (far ultraviolet).Original picture by S.K. Sundaram, Nature Materials 1 (4) (2002) and edited for additional relevant processes
8Mechanisms of absorption and deposition of energy and response of the material. PIIIIBE-EE-PHNXFXDAROriginal picture by S.K. Sundaram, Nature Materials 1 (4) (2002) eddited for the relevant processes
9Coupling to lattice QM – Power density PI Rate equations IB, II, E-EConduction bandAR, XF, XD,E-PHNelectronCoupling to latticeQM – Power densityRate equationsLaser radiationForbidden bandPIholeLaser -PI, MPIValence bandCVD diamond
10Boltzmann type scattering equation Huang, Apostolova… PRB 71, , 2005
11Photo-ionization-Keldysh approach L.V. Keldysh, JETP 20, 1965, Apostolova et al in press NIMA, 2014, Otobe et al, PHYSICAL REVIEW B 77, , 2008
12Exiton formation and decay J. Zeller, et al, in: G.J. Exarhos, A.H. Guenther, N. Kaiser, K.L. Lewis, M.J. Soileau, C.J. Stolz (Eds.), 2003: pp. 515–526.
13intravalley acoustic phonon intervalley phononHuang, Apostolova… PRB 71, , 2005,B. K. Ridley, Quantum Processes in Semiconductors (Clarendon, 1999)
14Electron-electron scattering Impact ionizationApostolova et al, in press, NIMA, 2014
15𝜕𝑛 𝜕𝑡 =𝛻∙ 𝐷 𝑎 𝛻𝑛− 𝑛− 𝑛 0 𝜏 𝐴 − 𝑛− 𝑛 0 𝜏 𝑟 𝜕𝑛 𝜕𝑡 =𝛻∙ 𝐷 𝑎 𝛻𝑛− 𝑛− 𝑛 0 𝜏 𝐴 − 𝑛− 𝑛 0 𝜏 𝑟𝜕𝐸 𝜕𝑡 =𝛻∙ 𝑘 𝑡ℎ,𝑒 𝛻 𝐸 3 𝐾 𝐵 𝑛 +𝛻∙ 𝐷 𝑎 𝐸 𝑛 𝛻𝑛 − 𝐸−3 𝑘 𝐵 𝑇 𝜏 𝑒−𝑝ℎ +𝐸 𝑔 𝑛− 𝑛 0 𝜏 𝐴A - auger recombination time (inversely proportional to n2)r- recombination time for processes in which energy is directly released to the latticee-ph - electron-phonon energy relaxation timekth,e - plasma thermal conductivityDa- ambipolar diffusivity, dependent both on the plasma temperature - E/(3kBn) and on the lattice temperature TDa - 2 𝑘 𝐵 𝜃 𝜇 𝑒 (𝑇) 𝜇 ℎ (𝑇) 𝜇 𝑒 (𝑇)+ 𝜇 ℎ (𝑇)
24Classification of laser damage to semiconductors and dielectrics Optical damageElectrical damageStructural damage
25ConclusionsA theoretical simulation accounting for the excitation processes in the bulk of diamond, induced by femtosecond laser irradiation has been carried out.The input parameters correspond to the experimental conditions of fabrication of graphitic conductive channels, from low field intensity to below about the threshold of laser graphitization.The model is in very good qualitative agreement with the experimental measurements of transient currents excited by the laser beam focused inside the diamond bulk.
26ConclusionsAn evaluation of the lattice temperature confirms the non-thermal nature of the graphitization process. A deeper understanding of the process will be useful to predict the outcome at different process parameters (wavelength, intensity, pulse width, repetition rate) and to plan useful improvements of the technology.
27OutlookMore processes will be added to the calculation such as electron-electron scattering, electron-phonon scattering, impact ionization as well as non-radiative recombination for indirect band-gap materials.The calculation will be extended to times after the end of the applied laser irradiation, i.e., tens and hundreds of picoseconds.