Thermal laser epitaxy

Last updated

Thermal laser epitaxy (TLE) is a physical vapor deposition technique that utilizes irradiation from continuous-wave lasers to heat sources locally for growing films on a substrate. [1] [2] This technique can be performed under ultra-high vacuum pressure or in the presence of a background atmosphere, such as ozone, to deposit oxide films. [3]

Contents

Diagram of a TLE chamber. Continuous-wave lasers are focused on sources inside a vacuum chamber. The localized heating induced by these lasers creates a flux of vapor from each source, which is then deposited onto a heated substrate. A gaseous atmosphere can be introduced via a gas inlet to grow compounds such as oxides. Tle-system-sketch.png
Diagram of a TLE chamber. Continuous-wave lasers are focused on sources inside a vacuum chamber. The localized heating induced by these lasers creates a flux of vapor from each source, which is then deposited onto a heated substrate. A gaseous atmosphere can be introduced via a gas inlet to grow compounds such as oxides.

TLE operates at power densities between 104 106 W/cm2, which results in evaporation or sublimation of the source material, with no plasma or high-energy particle species being produced. Despite operating at comparatively low power densities, TLE is capable of depositing many materials with low vapor pressures, including refractory metals, a process that is challenging to perform with molecular beam epitaxy. [4]

Physical process

Photograph of a freestanding silicon disc being heated locally by a laser in a TLE chamber. Silicon-TLE.jpg
Photograph of a freestanding silicon disc being heated locally by a laser in a TLE chamber.

TLE uses continuous-wave lasers (typically with a wavelength of around 1000 nm) located outside the vacuum chamber to heat sources of material in order to generate a flux of vapor via evaporation or sublimation. [1] Owing to the localized nature of the heat induced by the laser, a portion of the source may be transformed into a liquid state while the rest remains solid, such that the source acts as its own crucible. The strong absorption of light causes the laser-induced heat to be highly localized via the small diameter of the laser beam, which can also have the effect of confining the heat to the axis of the source. The resulting absorption corresponds to a typical photon penetration depth on the order of 2 nm due to the high absorption coefficients of α ~ 105 cm1 of many materials. Heat loss via conduction and radiation further localizes the high-temperature region close to the irradiated surface of the source. The localized character of the heating enables many materials to be grown by TLE from freestanding sources without a crucible. Owing to the direct transfer of energy from the laser to the source, TLE is more efficient than other evaporation techniques such as evaporation and molecular beam epitaxy, which typically rely on wire-based Joule heaters to reach high temperatures.

By heating the source, a flux of vapor is produced, the pressure of which frequently has an approximately exponential relation to temperature. The vapor is then deposited onto a laser-heated substrate. The very high substrate temperatures achievable by laser heating allow the use of adsorption-controlled growth modes, similar to molecular beam epitaxy, ensuring precise control of the stoichiometry and temperature of the deposited film. This precise control is valuable for growing thin-film heterostructures of complex materials, such as high-Tc superconductors. [5] [6] By positioning all lasers outside of the evaporation chamber, contamination can be reduced compared to using in situ heaters, resulting in highly pure deposited films.

The deposition rate of the vapor impinging upon the substrate is controlled by adjusting the power of the incident source laser. The deposition rate frequently increases exponentially with source temperature, which in turn increases linearly with incident laser power. [4]

The gas in the chamber can be incorporated in the deposition film. With the addition of an oxygen or ozone atmosphere, oxide films can readily be grown with TLE at pressures up to 102 hPa. [3] [7]

History

Shortly after the invention of the laser by Theodore Maiman in 1960, [8] it was quickly recognized that a laser could act as a point source to evaporate source material in a vacuum chamber for fabricating thin films. [9] [10] In 1965, Smith and Turner [10] succeeded in depositing thin films using a ruby laser, after which Groh deposited thin films using a continuous-wave CO2 laser in 1968. [11] Further work demonstrated that laser-induced evaporation is an effective way to deposit dielectric and semiconductor films. However, issues occurred with regard to stoichiometry and the uniformity of the deposited films, thus diminishing their quality compared to films deposited by other techniques. [12] [13] Experiments to investigate the deposition of thin films using a pulsed laser at high power densities laid the foundation for pulsed laser deposition, an extremely successful growth technique that is widely used today.

Experiments utilizing continuous-wave lasers continued to be performed throughout the latter half of the twentieth century, highlighting the many advantages of continuous-wave laser evaporation including low power densities, which can reduce surface damage to sensitive films. It proved challenging to achieve congruent evaporation from compound sources using continuous-wave lasers, and film deposition was typically limited to sources with high vapor pressures due to the low continuous wave power densities available. [14] [15] [16]

In 2019, the evaporation of sources using continuous-wave lasers was rediscovered at the Max Planck Institute for Solid State Research and dubbed "thermal laser epitaxy". This new technique uses elemental sources illuminated by high-power continuous-wave lasers (typically with peak powers around 1 kW at a wavelength of 1000 nm), thus allowing the deposition of low-vapor-pressure materials such as carbon and tungsten while avoiding issues with congruent evaporation from compound sources. [1] [2]

Related Research Articles

<span class="mw-page-title-main">Chemical vapor deposition</span> Method used to apply surface coatings

Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high-quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films.

<span class="mw-page-title-main">Epitaxy</span> Crystal growth process relative to the substrate

Epitaxy refers to a type of crystal growth or material deposition in which new crystalline layers are formed with one or more well-defined orientations with respect to the crystalline seed layer. The deposited crystalline film is called an epitaxial film or epitaxial layer. The relative orientation(s) of the epitaxial layer to the seed layer is defined in terms of the orientation of the crystal lattice of each material. For most epitaxial growths, the new layer is usually crystalline and each crystallographic domain of the overlayer must have a well-defined orientation relative to the substrate crystal structure. Epitaxy can involve single-crystal structures, although grain-to-grain epitaxy has been observed in granular films. For most technological applications, single-domain epitaxy, which is the growth of an overlayer crystal with one well-defined orientation with respect to the substrate crystal, is preferred. Epitaxy can also play an important role while growing superlattice structures.

<span class="mw-page-title-main">Molecular-beam epitaxy</span> Crystal growth process

Molecular-beam epitaxy (MBE) is an epitaxy method for thin-film deposition of single crystals. MBE is widely used in the manufacture of semiconductor devices, including transistors, and it is considered one of the fundamental tools for the development of nanotechnologies. MBE is used to fabricate diodes and MOSFETs at microwave frequencies, and to manufacture the lasers used to read optical discs.

<span class="mw-page-title-main">Pulsed laser deposition</span>

Pulsed laser deposition (PLD) is a physical vapor deposition (PVD) technique where a high-power pulsed laser beam is focused inside a vacuum chamber to strike a target of the material that is to be deposited. This material is vaporized from the target which deposits it as a thin film on a substrate. This process can occur in ultra high vacuum or in the presence of a background gas, such as oxygen which is commonly used when depositing oxides to fully oxygenate the deposited films.

A thin film is a layer of material ranging from fractions of a nanometer (monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films is a fundamental step in many applications. A familiar example is the household mirror, which typically has a thin metal coating on the back of a sheet of glass to form a reflective interface. The process of silvering was once commonly used to produce mirrors, while more recently the metal layer is deposited using techniques such as sputtering. Advances in thin film deposition techniques during the 20th century have enabled a wide range of technological breakthroughs in areas such as magnetic recording media, electronic semiconductor devices, integrated passive devices, light-emitting diodes, optical coatings, hard coatings on cutting tools, and for both energy generation and storage. It is also being applied to pharmaceuticals, via thin-film drug delivery. A stack of thin films is called a multilayer.

<span class="mw-page-title-main">Dielectric mirror</span> Mirror made of dielectric materials

A dielectric mirror, also known as a Bragg mirror, is a type of mirror composed of multiple thin layers of dielectric material, typically deposited on a substrate of glass or some other optical material. By careful choice of the type and thickness of the dielectric layers, one can design an optical coating with specified reflectivity at different wavelengths of light. Dielectric mirrors are also used to produce ultra-high reflectivity mirrors: values of 99.999% or better over a narrow range of wavelengths can be produced using special techniques. Alternatively, they can be made to reflect a broad spectrum of light, such as the entire visible range or the spectrum of the Ti-sapphire laser.

Chemical beam epitaxy (CBE) forms an important class of deposition techniques for semiconductor layer systems, especially III-V semiconductor systems. This form of epitaxial growth is performed in an ultrahigh vacuum system. The reactants are in the form of molecular beams of reactive gases, typically as the hydride or a metalorganic. The term CBE is often used interchangeably with metal-organic molecular beam epitaxy (MOMBE). The nomenclature does differentiate between the two processes, however. When used in the strictest sense, CBE refers to the technique in which both components are obtained from gaseous sources, while MOMBE refers to the technique in which the group III component is obtained from a gaseous source and the group V component from a solid source.

Atomic layer deposition (ALD) is a thin-film deposition technique based on the sequential use of a gas-phase chemical process; it is a subclass of chemical vapour deposition. The majority of ALD reactions use two chemicals called precursors. These precursors react with the surface of a material one at a time in a sequential, self-limiting, manner. A thin film is slowly deposited through repeated exposure to separate precursors. ALD is a key process in fabricating semiconductor devices, and part of the set of tools for synthesizing nanomaterials.

<span class="mw-page-title-main">Vacuum evaporation</span>

Vacuum evaporation is the process of causing the pressure in a liquid-filled container to be reduced below the vapor pressure of the liquid, causing the liquid to evaporate at a lower temperature than normal. Although the process can be applied to any type of liquid at any vapor pressure, it is generally used to describe the boiling of water by lowering the container's internal pressure below standard atmospheric pressure and causing the water to boil at room temperature.

<span class="mw-page-title-main">Parylene</span> Chemical compound

Parylene is the common name of a polymer whose backbone consists of para-benzenediyl rings −C
6
H
4
− connected by 1,2-ethanediyl bridges −CH
2
CH
2
−. It can be obtained by polymerization of para-xylyleneH
2
C
=C
6
H
4
=CH
2
.

Electron-beam physical vapor deposition, or EBPVD, is a form of physical vapor deposition in which a target anode is bombarded with an electron beam given off by a charged tungsten filament under high vacuum. The electron beam causes atoms from the target to transform into the gaseous phase. These atoms then precipitate into solid form, coating everything in the vacuum chamber with a thin layer of the anode material.

<span class="mw-page-title-main">Vacuum deposition</span> Method of coating solid surfaces

Vacuum deposition is a group of processes used to deposit layers of material atom-by-atom or molecule-by-molecule on a solid surface. These processes operate at pressures well below atmospheric pressure. The deposited layers can range from a thickness of one atom up to millimeters, forming freestanding structures. Multiple layers of different materials can be used, for example to form optical coatings. The process can be qualified based on the vapor source; physical vapor deposition uses a liquid or solid source and chemical vapor deposition uses a chemical vapor.

<span class="mw-page-title-main">Physical vapor deposition</span> Method of coating solid surfaces with thin films

Physical vapor deposition (PVD), sometimes called physical vapor transport (PVT), describes a variety of vacuum deposition methods which can be used to produce thin films and coatings on substrates including metals, ceramics, glass, and polymers. PVD is characterized by a process in which the material transitions from a condensed phase to a vapor phase and then back to a thin film condensed phase. The most common PVD processes are sputtering and evaporation. PVD is used in the manufacturing of items which require thin films for optical, mechanical, electrical, acoustic or chemical functions. Examples include semiconductor devices such as thin-film solar cells, microelectromechanical devices such as thin film bulk acoustic resonator, aluminized PET film for food packaging and balloons, and titanium nitride coated cutting tools for metalworking. Besides PVD tools for fabrication, special smaller tools used mainly for scientific purposes have been developed.

<span class="mw-page-title-main">Evaporation (deposition)</span> Common method of thin-film deposition

Evaporation is a common method of thin-film deposition. The source material is evaporated in a vacuum. The vacuum allows vapor particles to travel directly to the target object (substrate), where they condense back to a solid state. Evaporation is used in microfabrication, and to make macro-scale products such as metallized plastic film.

<span class="mw-page-title-main">Sputter deposition</span> Method of thin film application

Sputter deposition is a physical vapor deposition (PVD) method of thin film deposition by the phenomenon of sputtering. This involves ejecting material from a "target" that is a source onto a "substrate" such as a silicon wafer. Resputtering is re-emission of the deposited material during the deposition process by ion or atom bombardment. Sputtered atoms ejected from the target have a wide energy distribution, typically up to tens of eV. The sputtered ions can ballistically fly from the target in straight lines and impact energetically on the substrates or vacuum chamber. Alternatively, at higher gas pressures, the ions collide with the gas atoms that act as a moderator and move diffusively, reaching the substrates or vacuum chamber wall and condensing after undergoing a random walk. The entire range from high-energy ballistic impact to low-energy thermalized motion is accessible by changing the background gas pressure. The sputtering gas is often an inert gas such as argon. For efficient momentum transfer, the atomic weight of the sputtering gas should be close to the atomic weight of the target, so for sputtering light elements neon is preferable, while for heavy elements krypton or xenon are used. Reactive gases can also be used to sputter compounds. The compound can be formed on the target surface, in-flight or on the substrate depending on the process parameters. The availability of many parameters that control sputter deposition make it a complex process, but also allow experts a large degree of control over the growth and microstructure of the film.

<span class="mw-page-title-main">Vapor–liquid–solid method</span> Mechanism to grow nano wires

The vapor–liquid–solid method (VLS) is a mechanism for the growth of one-dimensional structures, such as nanowires, from chemical vapor deposition. The growth of a crystal through direct adsorption of a gas phase on to a solid surface is generally very slow. The VLS mechanism circumvents this by introducing a catalytic liquid alloy phase which can rapidly adsorb a vapor to supersaturation levels, and from which crystal growth can subsequently occur from nucleated seeds at the liquid–solid interface. The physical characteristics of nanowires grown in this manner depend, in a controllable way, upon the size and physical properties of the liquid alloy.

Stencil lithography is a novel method of fabricating nanometer scale patterns using nanostencils, stencils with nanometer size apertures. It is a resist-less, simple, parallel nanolithography process, and it does not involve any heat or chemical treatment of the substrates .

<span class="mw-page-title-main">Tantalum(V) ethoxide</span> Chemical compound

Tantalum(V) ethoxide is a metalorganic compound with formula Ta2(OC2H5)10, often abbreviated as Ta2(OEt)10. It is a colorless solid that dissolves in some organic solvents but hydrolyzes readily. It is used to prepare films of tantalum(V) oxide.

Jochen Mannhart is a German physicist.

Molecular layer deposition (MLD) is a vapour phase thin film deposition technique based on self-limiting surface reactions carried out in a sequential manner. Essentially, MLD resembles the well established technique of atomic layer deposition (ALD) but, whereas ALD is limited to exclusively inorganic coatings, the precursor chemistry in MLD can use small, bifunctional organic molecules as well. This enables, as well as the growth of organic layers in a process similar to polymerization, the linking of both types of building blocks together in a controlled way to build up organic-inorganic hybrid materials.

References

  1. 1 2 3 4 Braun, Wolfgang; Mannhart, Jochen (2019-08-14). "Film deposition by thermal laser evaporation". AIP Advances. 9 (8): 085310. Bibcode:2019AIPA....9h5310B. doi: 10.1063/1.5111678 . S2CID   202065503.
  2. 1 2 Braun, Wolfgang (2018). "Adsorption-controlled epitaxy of perovskites". arXiv: 2405.04075 [cond-mat.mtrl-sci].
  3. 1 2 Kim, Dong Yeong; Mannhart, Jochen; Braun, Wolfgang (2021-08-13). "Thermal laser evaporation for the growth of oxide films". APL Materials. 9 (8): 081105. Bibcode:2021APLM....9h1105K. doi:10.1063/5.0055237. S2CID   238646816 . Retrieved 2021-09-08.
  4. 1 2 3 Smart, Thomas J.; Mannhart, Jochen; Braun, Wolfgang (2021-03-09). "Thermal laser evaporation of elements from across the periodic table". Journal of Laser Applications. 33 (2): 022008. arXiv: 2103.12596 . Bibcode:2021JLasA..33b2008S. doi:10.2351/7.0000348. S2CID   232320531 . Retrieved 2021-09-08.
  5. Braun, Wolfgang; Jäger, Maren; Laskin, Gennadii; Ngabonziza, Prosper; Voesch, Wolfgang; Wittlich, Pascal; Mannhart, Jochen (2020-07-16). "In situ thermal preparation of oxide surfaces". APL Materials. 8 (7): 071112. Bibcode:2020APLM....8g1112B. doi: 10.1063/5.0008324 . S2CID   225595599.
  6. Kim, Dong Yeong; Mannhart, Jochen; Braun, Wolfgang (2021-08-04). "Epitaxial film growth by thermal laser evaporation". Journal of Vacuum Science & Technology A. 39 (5): 053406. Bibcode:2021JVSTA..39e3406K. doi: 10.1116/6.0001177 .
  7. Smart, Thomas J.; Hensling, Felix V. E.; Kim, Dong Yeong; Majer, Lena N.; Suyolcu, Y. Eren; Dereh, Dominik; Schlom, Darrell G.; Jena, Dubdeep; Mannhart, Jochen; Braun, Wolfgang (2023-05-08). "Why thermal laser epitaxy aluminum sources yield reproducible fluxes in oxidizing environments". Journal of Vacuum Science and Technology A. 41 (4): 042701. doi:10.1116/6.0002632. ISSN   0734-2101 . Retrieved 2024-06-03.{{cite journal}}: CS1 maint: date and year (link)
  8. Maiman, T. H. (1960). "Stimulated optical radiation in ruby". Nature. 187 (4736): 493–494. Bibcode:1960Natur.187..493M. doi:10.1038/187493a0. S2CID   4224209.
  9. Nichols, K. G. (1965). "Lasers and microelectronics". British Communications and Electronics. 12 (4): 368.
  10. 1 2 Smith, Howard M.; Turner, A. F. (1965). "Vacuum Deposited Thin Films Using a Ruby laser". Appl. Opt. 4 (1): 147–148. Bibcode:1965ApOpt...4..147S. doi:10.1364/AO.4.000147.
  11. Groh, G. (1968). "Vacuum Deposition of Thin Films by Means of a CO2 Laser". Journal of Applied Physics. 39 (12): 5804–5805. Bibcode:1968JAP....39.5804G. doi:10.1063/1.1656056.
  12. Hass, G.; Ramsey, J. B. (1969). "Vacuum Deposition of Dielectric and Semiconductor Films by Means of a CO2 Laser". Appl. Opt. 8 (6): 1115–1118. doi:10.1364/AO.8.001115. PMID   20072385.
  13. Ban, V.S.; Kramer, D. A. (1970). "Thin films of semiconductors and dielectrics produced by laser evaporation". Journal of Materials Science. 5 (11): 1573–4803. Bibcode:1970JMatS...5..978B. doi:10.1007/BF00558179. S2CID   137145469.
  14. Sankur, H.; Hall, R. (1985). "Thin-film deposition by laser-assisted evaporation". Appl. Opt. 24 (20): 3343–3347. Bibcode:1985ApOpt..24.3343S. doi:10.1364/AO.24.003343. PMID   18224054.
  15. Sankur, H.; Cheung, J. T. (1988). "Formation of dielectric and semiconductor thin films by laser-assisted evaporation". Appl. Phys. A. 47 (3): 271–284. Bibcode:1988ApPhA..47..271S. doi:10.1007/BF00615933. S2CID   98006904.
  16. Trujillo, O.; Moss, R.; Vuong, K.D.; Lee, D. H.; Noble, R.; Finnigan, D.; Orloff, S.; Tenpas, E.; Park, C.; Fagan, J.; Wang, X.W. (1996). "CdS thin film deposition by CW Nd:YAG laser". Thin Solid Films. 290–291: 13–17. Bibcode:1996TSF...290...13T. doi:10.1016/S0040-6090(96)09065-7.

Thermal Laser Epitaxy - Max Planck Institute for Solid State Research