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Epitaxial thin film making

Moumita MistriCourse- CH4204Roll No – 13RS040

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Outline

1) Introduction about the Epitaxy,2) Uses,3) Main two categories : Homotaxy & Heterotaxy,4) Growth models,5) Validity of the growth models,6) Experimental Techniques,7) 3 main different kind of epitaxy : VPE, LPE & SPE,8) Molecular beam epitaxy,9) Case study 1 : Growth on ionic crystals with NaCl structure,10) Case Study 2 : Growth og Au and Ag on Cu.

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What is it ?

The term epitaxy comes from the Greek roots epi (ἐπί), meaning “above “, and taxis , meaning “in ordered manner”. It can be translated as “arranging upon” Epitaxy refers to the deposition of a crystalline over layer on a crystalline substrate, where there is registry between the over layer( the epitaxial layer) and substrate.Epitaxial films may be grown from gaseous or liquid precursors.

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Uses

For most thin film applications like hard and soft coatings, optical coatings, protective coatings.Extensively used in the growth of semiconductor materials.Forming layers and quantum wells in electronic and photonic devices—for example, in computer, video display, and telecommunications applications.

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Homotaxy

• A crystalline film is grown on a substrate or film of the same material.

• This technology is used to grow a film which is more pure than the substrate and to fabricate layers having different doping levels.

• Eg: Si on Si, f.c.c metals on f.c.c metals

Heterotaxy

• A crystalline film grows on a crystalline substrate or film of a different material.

• Often used to grow crystalline films of materials for which crystals cannot otherwise be obtained and to fabricate integrated crystalline layers of different materials.

• gallium nitride (GaN) on sapphire, aluminium gallium indium phosphide (AlGaInP) on gallium arsenide (GaAs) or diamond or iridium.

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The surface energy of the film material is larger than substrate. 3D islands nucleate and grow directly on the surface of the substrate.

The strain energy in the film is small compared with the surface energy of the film material.

After the completion of the 2D layer formation, growth continues, forming 3D islands.intermediate case between the layer-by-layer growth mode and the island growth mode.

The basic growth mechanisms(1) Frank-van der Merwe mechanism, (2) Volmer-Weber mechanism, (3) Stranski-Krastanov mechanism.

(1) (2) (3)

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Validity of the growth mechanism

If no chemical reactions, alloying or other changes of the surfaces-such as dissociation due to electron bombardment-occur during film growth.Proper systems and proper experimental techniques have to be chosen, while the experimental techniques were determined by applicability and availability.

Experimental techniques to study epitaxy

1)Transmission high energy electron diffraction(THEED), 2) Reflection high energy electron diffraction (RHEED),3)X-ray diffraction(XRD), 4) Transmission and replica electron microscopy (TEM and REM), 5)Field electron microscopy (FEM).

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Vapour-phase Epitaxy (VPE)• The growth occurs

at the interface between gaseous and solid phases of matter.

• Eg: Si deposition from Silicone terrachloride , silane, dichlorosilane or trichlorosilane as source gases.

• Hydride VPE , MOVPE

Liquid-phase Epitaxy(LPE)•It is a method to grow semiconductor crystal layers from the melt on solid substrates.•Temperature : below the melting point of the deposited semiconductor.•At equilibrium condition between dissolution and deposition layer formation is relatively fast and uniform.

Solid phase epitaxy(SPE)•It is usually done by first depositing a film of amorphous material on a crystalline substrate. The substrate is then heated to crystallize the film.•Solid-phase epitaxy is used primarily for crystal-damage healing.•Useful in incorporating low-solubility dopants in metals and Silicon.

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Molecular beam epitaxy (MBE)

It provides a pure stream of atomic vapour by thermally heating the constituent source materials.It was invented in the late 1960s at Bell Telephone Laboratories by J. R. Arthur and Alfred Y. Cho. MBE is widely used in the manufacture of semiconductor devices, including transistors for cellular phones and WiFi.Takes place in high vacuum or ultra-high vacuum (10−8 Pa)

A simple sketch showing the main components and rough layout and concept of the main chamber in a Molecular Beam Epitaxy system

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Case study 1GROWTH ON IONIC CRYSTALS WITH NaCI STRUCTURE

1) Au and Ag on LiF and MgO

Because of the small misfit (1.3 % and 1.5 % for Au and Ag on LiF, 3.0 % and 2.8 % for Au and Ag on MgO) a strong tendency to parallel epitaxy is expected.

Au on LiF :At a critical thickness which depends upon temperature and rate the epitaxial orientation disappears and "multiple twinned particles (MTP)" develop which disappear again in the later stages of film growth so that a continuous film has a < 111 > texture.

Ag on LiF : sticking coefficient of is so small and the tendency to form MTPsis so strong that the RHEED pattern shows MTPs from the very beginning.

Au and Ag on MgO: very high sticking coefficient and excellent parallel epitaxial orientation over the whole thickness range, despite the larger misfit compared with LiF

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2) Au and Ag on NaCI and KCI

Epitaxy in continuous films occurs only for Au on KCI, while continuous films of Au on NaCI and of Ag on NaC1 and KC1 have (111) fibre texture.The slope of the logarithm of the nucleation rate log I versus 1/T andversus R is the same independently of whether epitaxy occurs or not, and indicates that the "nuclei" are single atoms.

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Conclusions regarding the growth of Au and Ag on alkali halides

Au and Ag interact only weakly with alkali halide substrates, both as atoms and as crystallites. . The interaction increases in the sequence LiF, NaCl, KCl, KBr, KI, and with the influence of the interface on the film growth.The decisive factor for epitaxy is not lattice misfit but chemical (electronic) interaction between film and substrate.If continuous epitaxial films are to be grown in a weakly interacting system, then the surface energy of the metal has to be decreased and/or the interaction with the substrate increased. Probably both are happening in the various methods used to induce epitaxy in weakly interacting systems.The occurrence of epitaxy is not determined by geometrical factors but by chemical effects, it follows that these epitaxy-inducing processes are highly specific to each system.

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Case study 2

GROWTH OF Au AND Ag ON Cu Ag grows both on the {111} and the {100} plane with a { 111 } plane parallel to the substrate probably by the Frank-van der Merwe mechanism. Au does not produce ac(10 x2) pattern like Ag but a c(14 x2).chemical (electronic) factors play a very dominant role in the growth of metals on metals.

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Acknowledgement 1) A.K. Green, J. Dane), and E. Bauer, J. Vac. Sci. TechnoL, 7 (i970) 159.2) Thin Solid Films, Volume 12, Issue 1, September 1972, Pages 167–185.3) P.W. Palmberg and T. N. Rhodin, J. Chem. Phys., 49 (1968) 134.

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