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Research Papers: Conduction

Modeling of Temperature Cycles Induced by Pico- and Nanosecond Laser Pulses in Zinc Oxide and Molybdenum Thin Films

[+] Author and Article Information
D. Scorticati

Chair of Applied Laser Technology,
Faculty of Engineering Technology,
University of Twente,
Enschede 7500AE, The Netherlands
e-mail: d.scorticati@alumnus.utwente.nl

G. R. B. E. Römer, A. J. Huis in't Veld

Chair of Applied Laser Technology,
Faculty of Engineering Technology,
University of Twente,
Enschede 7500AE, The Netherlands

D. F. de Lange

Facultad de Ingeniería,
Universidad Autónoma de San Luis Potosí,
San Luis Potosí C. P. 78290, Mexico

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received March 9, 2015; final manuscript received September 28, 2015; published online November 3, 2015. Assoc. Editor: Alan McGaughey.

J. Heat Transfer 138(3), 031301 (Nov 03, 2015) (9 pages) Paper No: HT-15-1183; doi: 10.1115/1.4031733 History: Received March 09, 2015; Revised September 28, 2015

The aim of this paper is to study the benefits of applying ultrashort pulsed lasers over nanosecond pulsed lasers for selective (i.e., superficial) heat treatment of materials in general and for selective heat treatment of thin films in particular. To this end, a background of the physics that govern the absorption of light and subsequent diffusion of heat in semiconductor and metallic materials is provided, when exposed to picosecond or nanosecond laser pulses, with a fluence below the ablation threshold. A numerical model was implemented using a commercial finite-element modeling package, to simulate the temperature fields in thin films induced by laser pulses. The results of the simulations provide insight in the temperature cycles and corresponding timescales, as function of the processing parameters, such as fluence, pulse duration, pulse repetition frequency, and laser wavelength. Numerical simulations were run for thin films of molybdenum (Mo) and zinc oxide (ZnO) on a glass substrate, which are materials commonly adopted as (back and front) electrodes in thin film solar cells.

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Figures

Grahic Jump Location
Fig. 1

Sketch of the simulated geometry in the cylindrical coordinates r and z. The slab of material rotates around the z axis (optical axis of the laser beam). The dimensions in r and z of the slab are taken large enough to avoid errors in the computational results. The laser beam impinges the film (thickness tf) from the top.

Grahic Jump Location
Fig. 2

Simulated electron and lattice temperatures of ZnO thin film on glass, irradiated at 10 ps and F0 = 0.03 J cm−2 and 15 μm focus diameter. Te is probed in the center of the laser spot (r = 0) at the film surface (z = 0), while Tl is also probed in the center of the laser spot, but at three positions, i.e., z = 0 (surface), z = −100 nm, and z = −tf.

Grahic Jump Location
Fig. 3

Simulated electron and lattice temperatures of ZnO thin film on glass, irradiated at 10 ns and F0 = 0.106 J cm−2 and 15 μm focus diameter. Te is probed in the center of the laser spot (r = 0) at the film surface (z = 0), while Tl is also probed in the center of the laser spot, but at three positions, i.e., z = 0 (surface), z = −100 nm, and z = −tf.

Grahic Jump Location
Fig. 4

Simulated electron and lattice temperatures of ZnO thin film on glass, irradiated at 10 ps and F0 = 0.03 J cm−2 and 50 μm focus diameter. Te is probed in the center of the laser spot (r = 0) at the film surface (z = 0), while Tl is also probed in the center of the laser spot, but at three positions, i.e., z = 0 (surface), z = −100 nm, and z = −tf.

Grahic Jump Location
Fig. 5

Simulated electron and lattice temperatures of Mo thin film on glass, irradiated at 10 ps and F0 = 0.135 J cm−2 and 15 μm focus diameter. Te is probed in the center of the laser spot (r = 0) at the film surface (z = 0), while Tl is also probed in the center of the laser spot, but at three positions, i.e., z = 0 (surface), z = −100 nm, and z = −tf.

Grahic Jump Location
Fig. 6

Simulated electron and lattice temperatures of Mo thin film on glass, irradiated at 10 ns and F0 = 0.84 J cm−2 and 15 μm focus diameter. Te is probed in the center of the laser spot (r = 0) at the film surface (z = 0), while Tl is also probed in the center of the laser spot, but at three positions, i.e., z = 0 (surface), z = −100 nm, and z = −tf.

Grahic Jump Location
Fig. 7

Simulated electron and lattice temperatures of Mo thin film on glass, irradiated at 10 ps and F0 = 0.135 J cm−2 and 50 μm focus diameter. Te is probed in the center of the laser spot (r = 0) at the film surface (z = 0), while Tl is also probed in the center of the laser spot, but at three positions, i.e., z = 0 (surface), z = −100 nm, and z = −tf.

Grahic Jump Location
Fig. 8

Simulated F0 (a), d′ (b), and t′ (c) of ZnO and Mo bulk, as function of the laser pulse duration τp. The inset (d) shows the detail of the highlighted part in plot (a).

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