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Introduction to thin film growth and molecular beam epitaxy

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Introduction to thin film growth and molecular beam epitaxy

  1. 1. Introduction to Thin Film Growth and Molecular Beam Epitaxy Oleg Maksimov maksimov@netzero.net
  2. 2. Slides outline Survey of physical vapor deposition techniques Pulsed laser deposition Sputtering Molecular beam epitaxy RHEED Oxide Growth TiO2 - anatase SrTiO3 or [(TiO2)m/(SrO)n], with m = n Novel layered complex oxides [(TiO2)m/(SrO)n], with m ≠ n
  3. 3. Survey of vacuum deposition techniques Physical Vapor Deposition Chemical Vapor Deposition Pulsed Laser Deposition Metal-organic Sputtering Atomic layer Molecular Beam Epitaxy Etc… Uses thermodynamical / Uses chemical processes to produce mechanical processes to produce thin film. thin film. The substrate is exposed to more The source material is placed in volatile precursors, which react an energetic environment, so its and/or decompose on the substrate particles can escape and surface. condense on the substrate.
  4. 4. Pulsed laser deposition •A high-power pulsed laser is focused on the target. The target is ablated to form a plume of atoms, molecules, and particulates directed towards the substrate. •The advantages of PLD are the high deposition rates and possibility to produce multi component thin films with preserved composition under the high partial oxygen pressure. complex oxides •The challenges include minimizing particulate formation and obtaining uniform wafer coverage.
  5. 5. Sputtering metals •The sputtering target is bombarded with gaseous ions under high voltage acceleration. As the ions collide with the target, atoms of the target material are ejected against the substrate, where they condense. •The advantage of sputtering is that a wide variety of materials can be sputtered in a reactive atmosphere. •The disadvantages are the absence of in-situ monitoring tools, poor control of the charged plasma, and re-sputtering from the substrate.
  6. 6. Molecular beam epitaxy (MBE) The advantages of MBE •Growth is preformed in UHV environment minimizing impurity incorporation; •In-situ growth monitoring is possible; •Each material is vaporized independently from its own effusion cell; •Multiple sources are used to grow alloy films and hetero compound structures; semiconductors •Deposition is controlled at sub- monolayer level. Extremely flexible technique since growth parameters are varied independently. Invented in late 1960’s at Bell Laboratories by J. R. Arthur and A. Y. Cho.
  7. 7. Disadvantages of MBE The disadvantages of MBE Effect of Base Pressure Pressure Mean Free Path • Growth is performed under low oxygen partial pressure; (Torr) (m) • Very low deposition rates: 1 µm 1 7 x 10-5 – 100 nm per hour are used; • High equipment cost and long 10-3 7 x 10-2 set up time; • Extreme flexibility 10-4 0.7 (uncontrolled flexibility = 10-5 7 chaos!) • The other meanings of MBE: 10-6 70 Many Boring Evenings 10-7 700 Mostly Broken Equipment Mega-Buck Evaporation 10-9 70 x 103 Make-Belief Experiments source – substrate distance ~ 0.3 m
  8. 8. MBE growth system
  9. 9. Types of MBE Solid-Source MBE (SS-MBE) Group-III and -V molecular beams for III-V semiconductors (InxGa1-xAs); Group-II and -VI molecular beams for II-VI semiconductors (HgxCd1-xTe); Other for IV-VI semiconductors, Heusler alloys, silicides, metals… Plasma-assisted MBE (PA-MBE) Group-III molecular beams and nitrogen plasma source for nitrides (AlxGa1-xN); Oxygen plasma or atomic oxygen source for oxides(MgxZn1-xO, TiO2); Reactive-MBE (R-MBE) Group-III molecular beams and ammonia gas injector for nitrides (AlxGa1-xN); Ozone gas injector for oxides;
  10. 10. Effusion cells Heating System Radiation heating, tantalum wires with PBN insulators Thermal insulation Shield made out of refractory metal and water cooling coil 100 °C ...1000 °C low temperature cells Temperature range 800 °C ...1400 °C high temperature cells up to 2000 °C based on custom design Temperature stability <= 0.1 K depending on the PID controller
  11. 11. Single and dual filament cells
  12. 12. Types of crucibles - do not decompose, react with the charge material, or outgas impurities under operating conditions; - made of Ta, Mo, BeO, graphite, and pyrolytical boron nitride. Cylindrical crucible offers good charge material capacity and long term flux stability. However, uniformity of the deposited film is reduced. Conical crucible offers excellent uniformity in the expense of charge material capacity. The long-term flux stability is poor and geometry permits large shutter flux transients.
  13. 13. Beam flux monitoring Z-travel Bayard-Alpert ionization gauge or quartz crystal monitor
  14. 14. Epitaxial growth Atoms / molecules arriving to the substrate surface may undergo: • absorption to the surface, depend on • surface migration and dissociation, substrate • incorporation into the crystal lattice, temperature • thermal desorption. Therefore, epitaxial growth is ensured by: • very low rate of impinging atoms, • long migration path on the surface, • high possibility of the subsequent surface reactions.
  15. 15. Growth modes in epitaxy Columnar Step-Flow The mode by which epitaxial film grows depends on: •the interface energy, •the lattice mismatch between substrate and film, •the growth temperature, •the flux of the incoming atoms. The process can be complicated by surface segregation and alloying.
  16. 16. Frank-van der Merwe growth mode Columnar Step-Flow - Low interface energy and small lattice mismatch are necessary. - Low rate of incoming atoms and long migration path also promote layer-by-layer growth. -(AlxGa1-xAs/GaAs, ZnSe/GaAs, TiO2/LaAlO3, BaO/SrTiO3).
  17. 17. Volmer-Weber growth mode Columnar Step-Flow - Island growth is possible in the hetero epitaxial systems with high interface energy and large lattice mismatch (Al/Ge).
  18. 18. Stranski-Krastanov growth mode Columnar Step-Flow - Layer + island growth is possible in the systems with low interface energy and large lattice mismatch (InAs/GaAs, CdSe/ZnSe, SrO/LaAlO3). - High rate of incoming atoms and short migration path also promote layer + island growth.
  19. 19. Columnar growth mode Columnar Step-Flow -Columnar growth occurs in the case of extremely low surface mobility of incoming atoms and growth anisotropy – preferential growth direction (GaN/Si or GaN/GaAs). - Film has a fiber structure. Columns have well defined boundaries and facets.
  20. 20. MBE-grown GaN on GaAs (TEM) On-zone-axis bright-field image showing the High-resolution image collected near the GaN GaN/GaAs. The film has a columnar structure. film surface along GaN [11-20] zone axis, Insert is a SAD pattern collected from the top part showing two neighboring columns. The boundary of the film. between columns appears amorphous.
  21. 21. Step-flow growth mode Columnar Step-Flow - To promote step-flow growth substrate is slightly mis-oriented (∼ 10 - 20 ) from a low-index plane. Annealing (H2/Ar, O2) results in a high density of well-oriented terraces (steps) of monatomic height (SiC, MgO). Arriving atoms migrate to the step boundaries that are preferential binding sites.
  22. 22. Surface of SiC (0001) AFM image of a commercial (0001) Photograph of the hydrogen 6H-SiC wafer. The surface exhibits etcher assembly. randomly oriented scratches induced by the vendor’s mechanical polish.
  23. 23. Hydrogen etching of SiC (0001) AFM image of the same (0001) 6H-SiC wafer after hydrogen etching at 1650°C, 650 Torr, 10% H2 in 90% Ar at ~1100 sccm flow for 1 hour.
  24. 24. In-situ growth monitoring Reflective high energy electron diffraction (RHEED) RHEED is sensitive to surface structures and reconstructions and is used to: 1. Observe removal of contaminants from the substrate surface - surface reconstruction; 2. Calibrate growth rates – RHEED intensity oscillations; 3. Estimate the substrate temperature - surface reconstruction; 4. Determine the stoichiometry - surface reconstruction; 5. Analyze surface morphology – RHEED pattern; 6. Study growth kinetics – RHEED intensity oscillations.
  25. 25. RHEED geometry A high energy (~10 - 30 keV) electron beam is directed to the sample surface at a grazing angle (~1- 30). The diffracted beam is detected by fluorescence on the phosphorus screen. Surface unit cell size - distance between streaks / spots; Atomically flat surface – diffraction streaks; Rougher surface – transmission spots; Layer-by-layer growth mode - intensity oscillations.
  26. 26. Interpretation of RHEED patterns (1) Diffraction pattern from nearly ideal smooth surface; (2) Diffraction pattern from smooth surface with 1 2 a high density of atomic steps; (3) Transmission diffraction through 3D 3 4 clusters; (4) Diffraction from polycrystalline or textured surface.
  27. 27. RHEED intensity oscillations Different stages of layer-by-layer growth by nucleation of 2D islands and the corresponding intensity of the diffracted RHEED beam. - Direct measure of growth rates in MBE since oscillation frequency corresponds to the monolayer growth rate. - Magnitude of the RHEED oscillations damps because as the growth progresses, islands nucleate before the previous layer is finished.

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