Graduate School at MC2 2016 course: Micro- and Nanoprocessing Technologies lecture: Pulsed deposition of thin films of functional materials

Lecturer: Andrei Vorobiev

Course responsible: Ulf Södervall 1 Objectives

You will learn about:

• concept of deposition PLD of thin films

• functional materials

• features of PLD of functional materials

Literature:

1) H. M. Christen and G. Eres, Recent advances in pulsed-laser deposition of complex oxides, J. Phys.: Condens. Matter 20 (2008) 264005 (16pp)

2) of thin films: applications-led growth of functional materials / edited by Robert Eason, N.J., Wiley, cop. 2007

3) Pulsed laser deposition of thin films / edited by Douglas B. Chrisey and Graham K. Hubler, New York, Wiley, cop. 1994

2 Outline

• History and fundamentals of PLD • PLD of functional materials • Equipment • Advantages and limitations • Mechanisms of pulsed Laser • Film nucleation and growth in PLD • Splashing and forward peaking • PLD of HTS and ferroelectric thin films • Summary

3 Definitions

Laser is an electronic-optical device that produces coherent radiation. The term is acronym for Light Amplification by Stimulated Emission of Radiation.

Pulsed laser deposition (PLD) is a physical vapor deposition technique where a high power pulsed laser beam is focused to strike a target of the desired composition. Material is then vaporized and deposited as a on a substrate facing the target. This process can occur in ultra high or in the presence of a background , such as oxygen when depositing films of oxides.

Functional materials are materials having physical properties sensitive to the external effects (temperature, electric and magnetic fields, pressure etc.).

4 Evolution of laser technology and its applications

Chrisey, D. B., and G. K. Hubler (1994), Pulsed Laser Deposition of Thin Films, Wiley, New York, p. 2. 5 Historical Development of PLD

1917 - Albert Einstein postulated the stimulated emission process.

1960 - Maiman constructed the first ruby laser.

1962 - Breech and Cross used ruby laser to vaporize and excite from a solid.

1965 - Smith and Turner used ruby laser to deposit thin films.

1970s - i) reliable electronic Q-switches for generating very short pulses; ii) high-efficiency second harmonic generators for shorter wavelength.

1987 - PLD in Bellcore group used successfully to grow HTS YBCO.

1990 - PLD growth of ferroelectric Bi based oxide films in Ramesh group.

1990s - PLD production related issues concerning reproducibility and large-area scale up have begun to be addressed.

2000s - Numerous device applications based on PLD films of functional materials (YBCO, BSTO etc.) are being explored.

6 Functional materials

function of Functional material External parameters: properties: • temperature • permittivity • pressure • permeability • electric field • resistivity • magnetic field • optical wavelength • refractivity • absorbed gas • sound velocity • pH value • … • …

• Utilizing a functional material offers higher functionality of a system.

• Science and technology rely heavily on the development of functional materials.

7 Mobile Convergence

Portable Media MP3 PDA

Cellular Smart Phone Phones Convergent devices

DSC Gaming Mobile Video Imaging 8 Adapted from Philips Functional materials

• Ferroelectric………………………….. BaxSr1-xTiO3

• High temperature superconductor….YBa2Cu3O7

• Magnetic field sensor……………….. La1-xCaxMnO3

• Surface acoustic wave sensor………LiNbO3

• Liquid petroleum gas sensor………...Pd-doped SnO2

• High temperature piezoelectric………Ta2O5

• Fast- conductor…………………….Y2(SnyTi1-y)2O7 • …

• A wide range of functional materials are complex oxides. • A key requirement in preparations is to control compositional evolution. • A unique feature of PLD is stoichiometric preservation of composition.

9 PLD of Functional materials

x

Pulsed laser deposition of thin films / edited by Douglas B. Chrisey and Graham K. Hubler, New York, Wiley, 1994

• PLD and S are the most appropriate techniques for deposition of complex oxides.

• PLD reproduces target stoichiometry in an oxidizing ambient.

10 Concept of PLD

PLD stages: • laser of target • dynamic of • film nucleation and growth

Chrisey, D. B., and G. K. Hubler (1994), Pulsed Laser Deposition of Thin Films, Wiley, New York, p. 3.

• The laser-target interaction: electromagnetic energy is converted into electronic excitation and then into thermal/mechanical energy to cause ablation. • A plume: atoms, , , , clusters, particles, and molten globules. • The plume expands with hydrodynamic flow characteristics. 11 Growth of BSTO films by PLD

Vacuum chamber

0.4 mbar O2 Laser

BSTO target

Calas PLD System - MC2ProcessLab, Chalmers 1 Hz Heater at 650C

12 Advantages-Disadvantages of PLD

Advantages Disadvantages

• versatile method (any material) • splashing of micron-sized particulates • congruent evaporation • small area of uniformity (1 cm2) • high deposition rates (10s nm/min) • non-conformal coverage • clean process • extremely complex models hinder • plume at high energy theory based improvements • reactive (oxygen) • broad range of gas pressures

• To a large extent the two first problems have been solved. • Congruent (stoichiometric) evaporation – main advantage for PLD of films of functional materials.

13 Laser basics

Light Amplification by Stimulated Emission of Radiation

• When perturbed by a photon matter may create another photon.

• The second photon has the same direction, frequency, phase and polarization.

• The first photon is not destroyed (no absorption) - light amplification.

14 Laser basics (cont.) laser major parts

Pump source

Gain medium

1) The pump source provides energy to the medium.

2) The gain medium transfers energy into the laser beam amplified by stimulated emission.

3) The optical resonator provides a narrow, low-divergence beam.

15 Wavelengths of common

Laser Type Wavelength (mm) fluoride (Excimer-UV) 0.193 Helium neon (yellow) 0.594 Krypton chloride (Excimer-UV) 0.222 Helium neon (orange) 0.610 Krypton fluoride (Excimer-UV) 0.248 vapor (red) 0.627 Xenon chloride (Excimer-UV) 0.308 Helium neon (red) 0.633 Xenon fluoride (Excimer-UV) 0.351 Krypton (red) 0.647 Helium cadmium (UV) 0.325 Rohodamine 6G dye (tunable) 0.570-0.650

Nitrogen (UV) 0.337 Ruby (CrAlO3) (red) 0.694 Helium cadmium (violet) 0.441 Gallium arsenide (diode-NIR) 0.840 Krypton (blue) 0.476 Nd:YAG (NIR) 1.064 Argon (blue) 0.488 Helium neon (NIR) 1.15 vapor (green) 0.510 Erbium (NIR) 1.504 Argon (green) 0.514 Helium neon (NIR) 3.39 Krypton (green) 0.528 Hydrogen fluoride (NIR) 2.70 Frequency doubled 0.532 Carbon dioxide (FIR) 9.6 Nd YAG (green) Carbon dioxide (FIR) 10.6 Helium neon (green) 0.543 Krypton (yellow) 0.568 Copper vapor (yellow) 0.570

• The PLD range is 200 - 400 nm (absorption by matter is strong enough).

• uses “excited dimer” pseudo-molecules for the gain media.

16 Excimer laser basics KrF electronic potential

excimers formation reactions

+ + Ar + 2Ar Ar2 + Ar + + charge-transfer Ar2 + Kr Kr + 2Ar

Kr* + eˉ Kr + eˉ excitation Kr+ + 2 eˉ

Kr* + F2 (KrF)* + F harpooning Kr+ + Fˉ + He (KrF)* + He recombination

• Excimers are only stable in excited states. • If excimers are generated, the medium is automatically in with the unstable ground state.

• Technical implementation: gas mixtures in high-voltage gas discharge 17 Commercial excimer laser design cross sectional view of excimer laser tube

• Pulsed mode provides non-equilibrium vaporization (10000 K in 10 nm surface layer).

2 • KrF Lambda Physik, Inc. typical parameters: =248 nm, =30 ns, r=10 Hz, E0=1.5 J/cm 18 Advantages of PLD

Advantages Versatile method

• versatile method (any material) 1/2 E = (2Φ/c0n) • congruent evaporation • high deposition rates (10s nm/min) Φ – power density (109 W/cm2) • clean process c – velocity of light

• plume at high energy 0 – permittivity of vacuum • reactive gases (oxygen) n – refractive index (1.5) 6 • broad range of gas pressures E – electric field (10 V/cm)

• The electric field inside the material (106 V/cm) is sufficiently high to cause dielectric breakdown. • Thus, any material will be transformed to form a plasma.

19 Philosophy of multicomponent film deposition

location 1 location 2

source of adequate transport coated elements of elements substrate (target)

energy a pure film of the correct composition

“…This process transports elements from one location to another by supplying energy to elements in a source, causing them to be transported to a surface to be coated. Ideally, such a process coats the surface with a pure film of the correct composition.” T. Venkatesan and Steven M. Green, The Industrial Physicist, p. 22 (1996)

20 Decomposition by equilibrium

Ba

Ti

1700 C BaTiO source x Ti -2 Pe =110 torr Ba 2 equilibrium heater Pe =510 torr

2 Ba Ba Pe J Pe 4 J  Ti  Ti  510 2kTm J Pe

• Equilibrium heaters: resistive, e-beam, inductive systems. • The vapour and deposited films initially are almost pure Ba.

• The composition of deposited films would slowly drift enriching by Ti. 21 Congruent evaporation by PLD

Criterion for congruent evaporation laser beam L ≤ dev 1 L  2(D ) 2 - heated thickness evaporants dev ls F dev  ln - evaporated thickness 2 Fth L nonequilibrium heating SrTiO3 ablation by pulse =30 ns: L ≈ 0.3 µm

target dev ≈ 0.2 µm E. G. Gamaly et al., Physics of Plasmas, 9 (2002) 949

Nonequilibrium heating by pulsed laser beam produces a flash of evaporants that deposit on a substrate as a film with composition identical to that of the target. 22 Experimental data

Bi2Sr2Ca1Cu2Ox HTS grown by PLD Energy (meV) 1.8 2.0 2.2 2.4 2.6 2.8 3.0 50

40

30

20 Normalized yield Normalized 10

0 300 350 400 450 500 550 Channel

T. Venkatesan and Steven M. Green, in The Industrial Physicist, 1996, p. 23 • Rutherford back scattering: solid line – expected yield, dots – measured composition.

• PLD replicates the composition of the source in the film. 23 Mechanisms of Pulsed Laser Sputtering

primary mechanisms secondary mechanisms

• Emitted particles with sufficiently high density interact, lose memory of primary mechanism and therefore described by secondary mechanisms.

• Collisional sputtering cannot occur with laser pulses because photons transfer energy (0.004%) less than displacement threshold E ≈25 eV. d 24 Thermal Sputtering temperatures for vaporization vaporization rate

Al2O3 TOF temperature < 1900 K

• Thermal sputtering, in the sense of vaporization from a transiently heated target, may require temperatures well above the melting or boiling points.

• In the case of Al2O3 the particle emission by thermal sputtering is not possible at such low temperatures. 25 Mechanisms of Pulsed Laser Sputtering

primary mechanisms secondary mechanisms

• Emitted particles with sufficiently high density interact, lose memory of primary mechanism and therefore described by secondary mechanisms.

• Collisional sputtering cannot occur with laser pulses because photons transfer energy (0.004%) less than displacement threshold E ≈25 eV. d 26 Electronic Sputtering

high laser-pulse energies low laser-pulse energies

E n excited electrons energy: E  g el N

SiO2 sputtering:

N ≈ 51022 atoms/cm3

n ≈1022 atoms/cm3

Eg = 1 eV

-1 Eel ≈ 210 eV Teff ≈ 3000 K

Tm ≈ 1687 K

• High laser-pulse energies: dense excitation increases the total energy of atoms and the vapor pressure by orders of magnitude. • Low laser-pulse energies: defects form in and near the target surface, migrate to the surface which leads to the energetic expulsion of individual atoms. 27 Mechanisms of Pulsed Laser Sputtering

primary mechanisms secondary mechanisms

• Emitted particles with sufficiently high density interact, lose memory of primary mechanism and therefore described by secondary mechanisms.

• Collisional sputtering cannot occur with laser pulses because photons transfer energy (0.004%) less than displacement threshold E ≈25 eV. d 28 Exfoliation Sputtering

exfoliation of W target thermal stress evaluation

convenient measure of thermal shock:

• The thermal shocks occurred repeatedly and if not released by melting result in cracking and exfoliation.

• Exfoliation Sputtering does not contribute to film growth but creates defects. 29 Mechanisms of Pulsed Laser Sputtering

primary mechanisms secondary mechanisms

• Emitted particles with sufficiently high density interact, lose memory of primary mechanism and therefore described by secondary mechanisms.

• Collisional sputtering cannot occur with laser pulses because photons transfer energy (0.004%) less than displacement threshold E ≈25 eV. d 30 Hydrodynamic Sputtering asperities formation in Au target thermal expansion model

minimum droplet size

• The asperities develop on the target surface due to thermal expansion and accelerated away during cooling period. • Hydrodynamic Sputtering does not contribute to film growth but creates defects. 31 Film Nucleation and Growth

by Dietrich R. T. Zahn

• Layer-by-layer – potentially high quality epitaxial films

• 3D islanding – potentially polycrystalline films. 32 Expected effects of PLD conditions

small clusters (high ) critical cluster radius:  2(a   a   a  ) r*  1 cv 2 sc 2 sv 3a3GV volume free energy: kT  P  kT   large clusters (low ) GV   ln    ln()   Pe   P – pressure of the arriving atoms

Pe – vapor pressure of the film atoms

typical deposition rates: cluster dissociation/nucleation (low R) 1 nm/min - sputtering; 100 nm/min - PLD

range of PLD repetition rate R: 1-100 Hz • Small cluster size (PLD) promotes Layer-by-layer growth, since adatoms on small clusters will be more likely to add to the edges. • Repetition rate may control the nucleation and growth mode. 33 Splashing

Drawbacks of PLD SEM image of YBCO film

• splashing of micron-sized particulates • small area of uniformity (1 cm2) • non-conformal coverage • extremely complex models hinder theory based improvements

• Splashing is a major drawback of PLD.

• Splashing is an intrinsic problem, therefore it is difficult to overcome.

• In an electronic device the particulates can induce the formation of defects and scattering centers that lower carriers mobility, shorten the minority lifetime, and downgrade the damage thresholds. 34 Mechanisms of Splashing

Subsurface Boiling Shock Wave Recoil Exfoliation Pressure Expulsion

randomly molten molten shaped globules globules particulates

by Jonathan Dickinson • Subsurface boiling: Subsurface layer is superheated before surface reaches evaporation point. • Shock Wave Recoil Pressure Expulsion: Expansion of plume causes drop in pressure/shock wave just above surface. • Exfoliation: Repetitive forms microdendrits carried toward by plume. 35 Reducing of Splashing effects of processing parameters

SEM of YBCO films maximum laser power density without splashing:

Dmax  LHev tr =533 nm 252 Subsurface Boiling L  1 2 (  f  Km )

L- the range of surface penetration of the light  - mass density =1064 nm  - electrical conductivity f – frequency of the radiation

Chrisey, D. B., and G. K. Hubler (1994), Pulsed Laser Deposition of Thin Films, Wiley, New York, p. 184. The splashing decreases with: • the laser power at the expense of decreasing in deposition rate.

• the frequency of radiation at the expense of non-congruent evaporation. 36 Reducing of Splashing (cont.) mechanical particle filter plume manipulation

Vc  nfl

• Mechanical particle filter: Larger particulates move slower (20 m/s) and are caught by rotating vanes.

• Plume manipulation: Heavier particulates travel away from the substrate. Scattered species travel along a bisecting trajectory. 37 Reducing of Splashing (cont.) off-axis PLD target surface improvement

Dmax  LHev tr no splashing < Dmax  - mass density smooth Ge film deposited from molten target

H. Sakur et al., J. Appl. Pjys 65 (1989) 2475

• off-axis PLD: The light species undergo scattering by ambient gas and deposit on substrate facing 180 to the plume direction. • Target surface improvement: High density and smooth surfaces are desirable (polish target before each run). 38 Small Area of Uniformity

YBCO

cos11 cos

t  cos11 - sharp angular dependence

50% over 20 mm T. Venkatesan et al., Appl. Phys. Lett. 52 (1988) 1193.

• The flux is strongly forward peaked resulting in small area uniformity (1cm2). • Application/commercialization requires large area films (4 inch or large) for cost effective production. 39 Models of Angular Distribution calculated deposit profiles exact solution

approximations polynomial

strongly supersonic Kelly

cosine-power f()=cos @ u=0

• The forward peaking phenomenon arises from collisions between plume species.

• The collision effect is important when a monolayer is removed in tens of ns. 40 Angular Dependence of Composition schematic of experimental geometry composition ratios

Y1Ba2Cu3Ox

Z. Trajanovic et al., Appl. Phys. Lett. 66 (1995) 2418

MY=89 MBa=137 MCu=64

RY=180 pm RBu=215 pm RCu=135 pm

• The Cu is lighter than Ba and Y and scatters more readily off the straight path. • The Ba has large cross section for oxigen scattering than those of Y and Cu. 41 Large-Area PLD Approaches schematic of a large-area PLD system off-axis and rotational/translational PLD

Chrisey, D. B., and G. K. Hubler (1994), Pulsed Laser Deposition of Thin Films, Wiley, New York, pp. 294-295.

Basic approaches to scaling-up PLD: • rastering laser beam • off-axis positioning • rotating/translating substrate 42 Large-Area PLD Films

thickness distribution of Y2O3 films YBCO film composition uniformity

Pulsed laser deposition of thin films: applications-led growth of functional materials / ed. by Robert Eason, N.J., Wiley, 2007 pp. 193-194.

• The films are obtained by the laser rastering technique on rotating substrates. • The uniformity of thickness is ±4% over 8-inch area. • The uniformity of composition is ±1.5% over 6-inch area.

43 Ferroelectric Films Made by PLD

permittivity and loss tangent vs field 1400 0.05 Nonlinear polarization P 1200 Ba0.75Sr0.25TiO3 0.04 1000 m=qΔ 0.03 800  P=Nm E

tan 600 0.02

permittivity 400 Field dependent permittivity 0.01 200 ε 0 0 P=0E -400 -200 0 200 400 P=0(-1)E E (kV/cm) E

• Polarization due to ionic displacement Δ  0 A • Field dependent permittivity – voltage tunable capacitor C  (varactor) d 44 Ferroelectric Varactors

200 GaAs-Schottky Shown are also Q-factors of: BST/Pt/Au (PLD) Si abrupt junction varactor 100 (Metelics, MSV34,060-C12, Q=6500 @ 50 MHz, V=-4V)

BST/Pt (PLD) GaAs HBV (Darmstadt University of

Q-factor Technology, fcut-off=370 GHz)

Si GaAs dual Schottky diode (United Monolithic Simiconductors, 10 Q=1/tan GaAs-HBV DBES105a, 1 10 fcut-off=2.4 THz). Frequency (GHz)

A. Vorobiev, P. Rundqvist, K. Khamchane, and S. Gevorgian, Appl. Phys. Lett. 83, 3144 (2003)

Ferroelectric varactors may compete with the semiconductor analogous. 45 Ferroelectric Microwave Devices Tunable Delay Line

100 µm delay time vs frequency

0 V D. Kuylenstierna, A. Vorobiev, P. Linnér, and S. Gevorgian, IEEE Trans. Microwave Theory and Techn., 53 (2005) 2164.

20 V

Applying dc voltage between 2 ports the delay time can be tuned. 46 HTS films made by PLD Design for bicrystal junction DC SQUID

Chrisey, D. B., and G. K. Hubler (1994), Pulsed Laser Deposition of Thin Films, Wiley, New York, pp. 359. • Bicrystal grain-boundary junction exploits the weak-link behavior induced by high-angle boundaries at bicrystal interface.

• Polycrystalline YBCO films are not good due to many grain boundaries throughout the SQUID loop itself. 47 Grain-boundary YBCO junctions The bolometer response of grain boundary XRD /2-scan of a PLD YBCO film YBCO Josephson oscillator

YBCO/YSZ/sapphire 14 bicrystal substrate

1.7 THz

E. Stepansov et al., J. Appl. Phys. 96, 3357 (2004) • /2- and -scan reveal no additional peaks due to CuO and -particles or other orientations of YBCO in the a-b plane.

• High characteristic frequency and low microwave loss allows terahertz applications (direct Josephson detectors, oscillators, spectrometers etc.) 48 Summary

You have learned:

• basic principles of PLD of thin films

• functional materials - offer higher functionality of a system - multicomponent oxides (stoichiometry required)

• features of PLD of functional materials - advantages/disadvantages of PLD of functional materials - laser sputtering mechanisms (non-equilibrium heating preserves stoichiometry) - effects on film growth (layer-by-layer growth is promoted) - splashing (reducing approaches) - plume forward peaking (large area films approaches are demonstrated) - state-of-the-art YBCO and BSTO films/devices made by PLD at MC2

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