On the growth of rare earth doped LiYF4 thin film by pulsed laser ...

On the growth of rare earth doped LiYF 4thin films by pulsed laser depositionDr. M.Anwar-ul-HaqProf. Paola BicchiDr. Stefano BarsantiDepartment of Physics,University of Sargodha,PakistanDepartment of Physics,University of Siena, ItalyInternational Scientific Spring at NCP from March 01-06, 20101

Layout of the presentationIntroductionThin filmsPulsed Laser Deposition (PLD)Experimental setupResultsNd 3+ :LiYF 4 thin filmsConclusions2

IntroductionAim of the research workDevelopment of rare earth (RE) ions-doped LiYF 4(YLF) fluoride thin films with characteristics suitablefor their use as active medium in micro-lasers sourcesin the 1-2 μm, via Pulsed Laser Deposition (PLD)Materials usedTargetsSubstratesRE ions- doped mono-crystalline YLFNd 3+ or Tm 3+Pure mono-crystalline YLF3

IntroductionWhy thin film in micro-laser area? [1,2]The realization of active optical devices in film shapewould magnifies all advantages of the micro-laserssystems by favoring; The removal of the heat in excess from the active media Enhancement of the confinement of the radiationsMaximizes the interaction zone between pump and active mediaReduction of the lasing threshold[1] D. B. Chrisey and G. K. Hubler, (Eds.), Pulsed laser deposition of thin films, John Wiley, New York (1994)[2] C. L. Bonner, A. A. Anderson, R. W. Eason, D. P. Shepherd, D. S. Gill, C. Grivas and N. Vainos, Opt. Lett. 22 (1997) 9884

Thin Films [3]DimensionsRealizationfrom fractions of a nanometer to severalmicrometers in thicknesson a suitable substrate or on previously deposited layersGrowth Mechanism [4][3] K. Wasa, M. Kitabatake and H. Adachi, Thin film materials technology: sputtering of compound materials, Springer, Heidelberg (2004)5[4] A. Rockett, The materials science of semiconductors, Springer, New York (2007)

Thin filmsMajor ways of thin films growth [5]Volmer-Weber growth(Island growth)Frank-van de Merwe growth(layer by layer growth)Stranski-Krastinov growth(mixed growth)[5] J. A. Venables, G. D. T. Spiller and M. Hanbucken, Rep. Prog. Sci. 47 (1984) 3996

Thin filmsThin film growth techniques [1] Liquid Phase Epitaxy Thermal Evaporation Sputtering Molecular Beam Epitaxy Chemical Vapour Deposition Ion ImplantationPulsed laser depositionEjection of materials from the target by highlyenergetic laser pulses, with subsequentdeposition/condensation on a suitable substrate[1] D. B. Chrisey and G. K. Hubler, (Eds.), Pulsed laser deposition of thin films, John Wiley, New York (1994)7

Thin filmsPulsed Laser Deposition (PLD) [1,6]Interaction of the photons withthe target causes material ejectionvia a thermal and / or electronicprocessThe ablated plume is a mixture ofenergetic particles such as atoms,molecules, electrons, ions, submicronsor micron-sized solidparticles and molten globulesTypical PLD experimental setup[1] D. B. Chrisey and G. K. Hubler, (Eds.), Pulsed laser deposition of thin films, John Wiley, New York (1994)8[6] P. R. Willmott and J. R. Huber, Rev. Mod. Phys. 72 (2000) 315

Thin filmsPLDAdvantages of PLD [6,7]laser external to the ablation/deposition chamber,flexibility of the experimental set-up,possibility of getting thin films of almost any kind of material,deposition can be performed either in vacuum or in presenceof a controlled background atmospherestoichiometry in the film can be maintained,films growth rates can be controlled,multiple layer films can be grown,deposition on substrates kept at any temperature, is possible[6] P. R. Willmott and J. R. Huber, Rev. Mod. Phys. 72 (2000) 315[7] J. Schou, Appl. Surf. Sci. 255 (2009) 51919

Thin filmsPLDWhy fluorides?Crystals doped with rare earthLaser sources in IR Oxides FluoridesAdvantages of fluorides over oxides [8, 9]Stronger emission cross sectionsLower phonon energyDisadvantagesSensitive to thermal shocks even for slow thermal gradient and to OH¯ radicalcontamination during growth even a few ppmFilms of RE ions-doped fluoridesExcellent optical properties+ Benefits of thin film[8] A. A. Kaminskii, Laser Crystals, Springer-Verlag, New York (1981)10[9] C. Garapon, S. Guy, S. Skasasian, A. Bensalah, C. Champeaux and R. Brenier, Appl. Phys. A 91 (2008) 493

ExperimentalsetupSubstrateSubstrate materialmono-crystalline YLFProperties of YLF crystal [11]YLF CrystalRadius ~ 4.7 – 7.2 mmThickness ~ 2 mmChemical FormulaWeightCrystal StructureMelting PointExperimental setupHardnessTransparencyDensityRefractive IndexesCrystallographic axisUnit cell of YLF [10]LiYF 4171.8 amuTetragonal819°C5.07 Mohs0.12- 7.3 µm3.99 g/cm 3n o= 1.4485n e= 1.4708Monoaxis (a, c)[10] E. Garcia and R. R. Ryan, Acta Crystallogr., Sect C 49 (1993) 2053[11] R. L. Aggarwal, D. J. Ripin, J. R. Ochoa and T. Y. Fan, J. Appl. Phys. 98 (2005) 10351411

ExperimentalsetupTarget materialsTarget crystalsDopant Concentration (at. %) N (cm -3 )11.4 x10 20Nd 3+1.52.1 x10 20Nd 3+ :LiYF 4 (Nd:YLF)Radius ~ 4.7 – 7.2 mm, Thickness ~ 3 mmThe YLF and Nd:YLF mono-crystals used during our PLD experiments were eithergrown by the NEST growth facility in Pisa or provided by a commercial supplier(VLOC, USA).12

ExperimentalsetupFilm growth systemAblation setupDeposition setup13

ExperimentalsetupPhotos of the UHV chamberGas valveVacuum gaugesAuxiliary viewportsLaserPlumeLaser entrancewindowHeaterConnectionsRotatingtarget holderSubstrate heater/holderholderShutter14

ExperimentalsetupPLD processHeatedsubstrateAdiabaticexpansionKnudsen layer15

ExperimentalsetupFilm in-situ analysisRealization of the film growthTo verify the presence of the rare earth ions in the film16

ExperimentalsetupCCD camera photo of the Nd:YLF filmsThe film was deposited with laser fluency of 10 J/cm 2 in 1 Pa of Heatmosphere at a substrate temperature of 650 °C.17

ExperimentalsetupFilm ex-situ characterisationsDetermining the kind of film depositedTo determine if the orientation from the substrateto the film has been transferredConcentration of Nd³ + ions presentSurface quality18

ExperimentalsetupSimplified level scheme and absorption curve for Nd:YLF [12,13]Simplified level schemeAbsorption curve• The fluorescence profiles of Nd 3+ -doped YLF crystals depend on they being recorded with E ||or E ⊥ to the crystal c-axis [13].• The Nd 3+ 4 F 3/2 manifold lifetime is concentration dependent in such a way that lowerconcentrated samples manifest a higher lifetime and vice versa [13].[12] A. A. S. da Gama, G. F. de Sa, P. Porcher and P. Caro, J. Chem. Phys. 75 (1981) 2583[13] J. R. Ryan and R. Beach, J. Opt. Soc. Am. B:Opt. Phys. 9 (1992) 188319

ExperimentalsetupSetup for the ex-situ characterization20

ResultsPlume analysisAblation in vacuumAblation in 1 Pa of HeResults21

ResultsPlume analysis• The expansion velocities of the plume species were found reduced in presenceof 1 Pa of He.• Both in vacuum and in 1 Pa of He, the expansion velocity of most of the plumecomponents saturates beyond 8 J/cm 2 .• Ablation threshold in vacuum for all the species was found to be 1.7 ± 0.3J/cm 2 with exception of Li for which 0.7 ± 0.3 J/cm 2 .• In vacuum, all the plume species were focused along the target normal exceptlightest neutral Li, which was found to point preferentially at 16° to the targetnormal.• In presence of 1 Pa of He, Li and all other plume species were focused along thetarget normal.• The FWHM of the angular distribution curves of the plume components wasfound reduced in 1 Pa of He compared to the similar measurements done invacuum and gave indication of the confinement of the plume in the presence of1 Pa of He.22

ResultsPlume shape(a) Vacuum(b) 1 Pa of He23

ResultsNd:YLF thin filmsDeposition at T s = 650°CPLD in vacuumPLD in 1 Pa of HeLow (4 J/cm 2 )and high (10 J/cm 2 )laser fluency24

Results• Vacuum - T s =650°CHigh fluency (10 J/cm 2 )Deposition conditionsLaser wavelength355 nmRepetition rateLaser Pulse durationLaser fluency10 Hz13 ns10 J/cm²Ablation time 20’TargetNd:YLF 1.5% at.SubstrateYLFVacuum 1 ×10-4 PaTarget-substrate distance35 mmTemperature of the substrate 650°C25

Results• Vacuum - T s =650°CHeating/cooling cycle of the substrate for film deposition26

Results• Vacuum - T s =650°CFilms in-situ analysisRealization of the filmPresence of Nd 3+ ions in the filmExample of interference pattern producedby a film deposited in vacuumPortion of the LIF spectrum following 355 nm excitation, recordedin the Nd:YLF bulk crystal and from a film grown in vacuum with10J/cm 2 laser fluency.27

Films ex-situ characterizations28

Results• Vacuum - T s =650°CUnpolarized, normalized, fluorescence spectraλ exc = 806.6 nm4F 3/2 → 4 I 9/24F 3/2 → 4 I 11/2Crystalline Nd:YF filmFrom Ref. [14][14] S. Barsanti, F. Comacchia, A. Di Lieto, A. Toncelli, M. Tonelli and P. Bicchi, Thin Solid Films 516 (2008) 200929

Results• Vacuum - T s =650°CLife time measurement of the 4 F 3/2 manifoldFilm deposited at T s =650°Cin vacuum with F l = 10 J/cm 2Film deposited at T s =650°C invacuum with F l =4 J/cm 2 [14]τ Target = 464 ± 2 μsτ Film= 242 ± 5 μsNd:YF film[14] S. Barsanti, F. Comacchia, A. Di Lieto, A. Toncelli, M. Tonelli and P. Bicchi, Thin Solid Films 516 (2008) 200930

Results• 1 Pa of He - T s =650°CDeposition in presence of 1 Pa of HeLaser wavelengthDeposition parameters355 nmRepetition rateLaser Pulse durationLaser fluencyAblation timeTargetSubstrateBack ground atmosphereTarget-substrate distanceTemperature of the substrate10 Hz13 ns4J/cm²85’Nd:YLF 1.5% at.YLF1 Pa of He35 mm650°C31

Results• 1 Pa of He - T s =650°CUnpolarized, normalized, fluorescence spectraλ exc = 806.6 nm4F 3/2 → 4 I 9/24F 3/2 → 4 I 11/2InhomogeneousCrystalline Nd:YLF film32

Results• 1 Pa of He - T s =650°CConcentration of Nd 3+ ions in the filmλ exc = 806.6 nm Single exponential decay4F 3/2 manifold lifetimeτ Target = 464 ± 2 μsτ Film average= 437 μsVariation from point to point ~ ± 10%Average Nd 3+ ion concentration in thefilm greater than in the target33

Results• 1 Pa of He - T s =650°CMorphological analysisVolmer-Weber growth34

Results• 1 Pa of He - T s =650°CHigh fluency (10 J/cm 2 )Deposition conditionsLaser wavelengthRepetition rate355 nm10 HzLaser Pulse duration13 nsLaser fluency10 J/cm²Ablation time 20’TargetNd:YLF 1.5% at.SubstrateBack ground atmosphereTarget-substrate distanceYLF1 Pa of He35 mmTemperature of the substrate 650°C35

Results• 1 Pa of He - T s =650°CUnpolarized, normalized, fluorescence spectraλ exc = 806.6 nm4F 3/2 → 4 I 9/24F 3/2 → 4 I 11/2HomogeneousCrystalline Nd:YLF film36

Results• 1 Pa of He - T s =650°COptical analysis ⎪⎪ and ⊥ to c-axis4F 3/2 → 4 I 9/24F 3/2 → 4 I 11/24F 3/2 → 4 I 9/24F 3/2 → 4 I 11/2The transition of interest for the possible lasing action wasfavored in the film as much as in the bulk.37

Results• 1 Pa of He - T s =650°COptical analysis ⎪⎪ and ⊥ to c-axisTransition 4 F 3/2 → 4 I 9/2The spectral profile in the film and inthe bulk changes in the same way inshifting the polarization from E || to E ⊥to the c-axis.The global spectral intensity, in both theemissions is higher when E || to c-axiswith only exception of⎛⎜⎝II|| 863⊥863//II|| 867⊥867⎞⎟⎠T arget≈3⎛⎜⎝II|| 863⊥863//II|| 867⊥867⎞⎟⎠Film≈238

Results1 Pa of He - T s =650°COptical analysis ⎪⎪ and ⊥ to c-axisTransition 4 F 3/2 → 4 I 11/2The spectral profile in the film and inthe bulk remains the same in shifting thepolarization from E || to E ⊥ to the c-axis.Some features compatible The with only a mismatch considerable if notcomplete transfer of orientation from the substrateto the film( I I ) 1. 21047 1053≈Film( I I ) 1. 21053 1047≈T arget39

Results• 1 Pa of He - T s =650°CConcentration of Nd 3+ ions in the filmλ exc = 806.6 nm Single exponential decay4F 3/2 manifold lifetimeτ Target = 464 ± 2 μsτ Film = 468 ± 5 μsτ Target = τ FilmSame Nd 3+ ion concentration inthe film and the target40

Results• 1 Pa of He - T s =650°CMorphological analysisMixed growth41

ConclusionsWe succeeded to deposit crystalline YLF films doped with Nd 3+ .The best film produced which showed promising optical qualities to reachthe goal of this project was the Nd:YLF one obtained in 1 Pa of He with alaser fluency of 10 J/cm 2 , when T s was 650°C.In fact in this case:• The grown film was a crystalline Nd:YLF film.• It was homogeneous.• It showed some spectral features compatible with a consistent, even if notcomplete, transfer of the substrate orientation to the film.• The concentration of the dopant ions was transferred from the bulk to thefilm.• It had a rather good surface quality42

Publications1. P. Bicchi, M. Anwar-ul-Haq and S. Barsanti In: A. N. Camilleri (Ed.),Radiation Physics Research Progress, ISBN: 978-1-60021-988-8, NovaScience Publishers, Inc., Hauppauge, NY (2008), pg. 193-217.2. S. Barsanti, M. Anwar-Ul-Haq and R. Bicchi, Thin Solid Films 517 (2009) 2029-2034.3. M. Anwar-ul-Haq, S. Barsanti, A. Bogi and P. Bicchi, Opt. Mat. 31 (2009) 1860-1863.4. M. Anwar-ul-Haq, S. Barsanti and P. Bicchi, IEEE NANO 2009,ISBN:978-981-08-3694-8, (2009) 373-376.5. M. Anwar-ul-Haq, S. Barsanti and P. Bicchi, DGaO Proceedings 2009-Http://www.dgao-proceedings.de, ISSN:1614-8436, (2009) P35.6. A. Bogi, S. Barsanti, M. Anwar-ul-Haq, P. Bicchi, Appl. Phys. A 98 (2010) 153-15943