Pulse laser depostion of thin film

Abdalla Darwish

Dillard University, Physics and Engineering Department, New Orleans, LA 70122

Laser Matter Research Lab , Kenner, LA 70065University of New Orleans, New Orleans, LA 70122

A nanosecond pulsed laser deposition at room temperature was used to fabricate a waveguide of SiC:Ge:Fe. The waveguide was used as an optical sensor to detect the sound wave disturbance under water. It was observed that the HeNe laser drives the optical sensor to produce multiple diffraction rings, which are affected by the sound wave disturbance to produce unique clusters of rings with elongated shape pointing away from the source of the acoustic waves..

1. Fabrication of a waveguide using the pulsed laser deposition technique.

2. Testing the stability of the waveguide under laser application and under harsh environment.

3.Testing the nonlinear optical response of the waveguide as optical sensor to measure the acoustic waves in deep water .

The process of ablation preserves the stoichiometry of the target material well, many different target materials can be used. Bulk off-cuts make very suitable targets.

Substrate heating allows epitaxial growth and hence films with high crystalline quality. CO2 laser heating can heat substrates to temperatures up to 2000 °C.

The plume and film stoichiometry can be altered by a background gas.

Relatively high growth rates up to 10 µm per hour can be achieved.

PLD is well suited to experimental work because it is relatively simple to change the setup and deposition conditions.

1.The target material is ablated by a KrF excimer laser or Nd:YAG laser.2. The ejected material forms a plasma plume and expands across to the substrate where it is deposited as a film.3. The substrate is heated by a raster-scanned CO2 laser.4. A background gas is used to control the plume stoichiometry and dynamics.

The target (crystal, sintered ceramic..) The substrate temperature Background gas used and pressure Target-substrate distance Energy density (laser fluence) on target Repetition rate of the laser Stability of all these with time (hours..)

Particulates can form scattering centres and contribute significantly to the overall loss.

Solutions: Burial

Thick films are less affected. Capping layers allow burial without increasing the core size.

Target reconditioningRestricting growth runs to < 2 hours prevents the target surface degrading by a significant amount.

PLD: A laser beam of Nd:YAG 2nd harmonic 532 nm of about 150 mJ pulse energy was tightly focused onto the target surface with an f = 25 cm focal lens.

The beam impinged the rotating target at 45 degree angle respect to the target surface normal.

The resulting film thickness of about 400 nm In the waveguides Ge/Fe an optical fiber was

impeded underneath the Ge layer to be used for special transition of multi-wavelength process.

1. The surface roughness and the homogeneity of the deposited thin film surfaces were analyzed using SEM before and after using the sensor

2. The roughness of the surface was measured as 10 nm and in some cases as high as 15 nm.

3. Study the effect of the acoustic waves on the morphology of the thin film after long exposure to the temperature variation in deep water, and the degradation of some spots due to mechanical/thermal stress which caused a crack on the thin film.

Some defects on the thin film due to PLD process

SEM of the thin film with 100 mJ and 150 mJ energy pulse before using the thin film

SEM of the thin film after 10 hours and 30 hours of usages

X-section of the thin film

Laser in The optical sensor

Beam expander

Acoustic wave

Beam collector

Beam expander

Diffraction pattern without acoustic wave, T=85 F Diffraction pattern with acoustic wave, T= 85F

Diffraction pattern with acoustic wave T=65 FDiffraction pattern with acoustic wave , T=40 F

Diffraction pattern with acoustic wave , T=105 F

Diffraction pattern with acoustic wave after 65 hours of usages and exposure to many different temperature over the period of one year

The phenomenon of self-phase modulation is commonly observed in strongly absorptive organic solutions and thin films. Radiation-less relaxation of these species produces a temperature rise which modulates their refractive index as

n = no + (dn/dT) ∆T

The special part of the modified plane wave after crossing the medium can then be written as: E(r,z) = 1/2 Eo(r,z) exp[I (kz - Φ)]

The total number of rings N N = Φo /2π 5

From the mathematical treatment, the nonlinear phase shift can be expressed. Φ = kz(n2 E 2 /no) 6

and for a film of thickness L, equation (6) can be rewritten as Φo = kL (n2 E 2 /no) = 2 π L (n2 E 2 /noλ) 7

the nonlinear refractive coefficient(n2) can be estimated from the number of fringes as:

n2 = N no λ / L E o 2

But, due to the difference in the thermal linear absorption of the different layers of the waveguide, Then the individual stresses, are,

σGe = - {(tSiC EGe ESiC ) / (tGe EGe + tSiC ESiC )}(λGe – λSiC) ΔT and

σSiC = {(tGe EGe ESiC ) / (tGe EGe + tSiC ESiC )}(λGe – λSiC) ΔT,

Which will increase with increasing the temperature gradient, and will lead to more number of diffraction rings and at the threshold of the waveguide molecular stability, the layers will start either collapsing or the thin film start cracking .

0 100 200 300 400 500 600 700 8000

20

40

60

80

100

Ga

σ (M

Pa)

Ge

∆T Co

The optical waveguide to detect acoustic wave was fabricated using laser pulse deposition technique.

Diffraction rings were observed with HeNe and Ar lasers at different water temperature and depth of 15 feet of water.

The thermal effect and acoustic waves were responsible for multi-clusters diffraction rings and elongated shape.

More measurements will be made with different thickness of the thin film to extend the life time of the optical sensor and determine the optimum operating conditions.

Now we are fabricating compact crystal accelerators to generate pulsed Electron , X-ray beams and potentially neutrons

The fabrication and processing techniques have been proven successful for these two basic first steps towards more advanced structures.

Stoichiometry control needs to be improved to avoid cracking problems due to film stress.

With a carefully chosen optics setup, high power levels from diode stacks can be coupled efficiently into PLD waveguides.

Improved loss, output power and slope efficiency should come with improved crystal quality.

PLD films can tolerate a high level of pumping power without cracking.

This project was supported in part by NSF, AFOSR and the Navy .