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Molecular Beam Epitaxy Molecular Beam Epitaxy • In MBE, ‘molecular beams’ of semiconductor material are deposited onto a heated crystalline substrate to form thin epitaxial layers. • Deposited films are crystalline • Special thermal evaporation or sputter tool required • Growth rates = a few Å/s • Substrate temperatures can exceed 900oC • Surface measured using RHEED gun • Mass Spectrometer determines purity deposited material • Alternate types of MBE – metal-organic group III precursors (MOMBE), – gaseous group V hydride or organic precursors (gas-source MBE) – combination (chemical beam epitaxy or CBE). http://projects.ece.utexas.edu/ece/mrc/groups/str eet_mbe/mbechapter.html#EpionInP Effusion Cells • Thermal evaporation sources • Also called Knudsen or K-cells • Most important part of the MBE system • Ultra high purity material • Ultra high vacuum • Insulated walls to prevent heating of Crucible other sources • Highly accurate thermal probes • PID control on temperature and shutter • Shutter speed = fractions of a second UHV seal Shutter Not shown Substrate holder • To improve film uniformity substrates are spun azimuthally during the process. • GaN is typically grown at temperatures exceeding 700°C • For GaAs growth systems can reach temperatures of approximately 900°C • Molybdenum (Mo) substrate holders are used • Zn is used as a wetting material between the holder and the substrate. – Zinc’s partial pressure is low enough not to contaminate films during long depositions – Wets Mo and MgO sufficiently at T>700oC to be act as a seal between holder and substrate http://family-held.org/Rudy/publications/unpublished/dossier/dos83.htm Mass Spectrometer • Technique that determines chemical composition of a sample using it’s mass to charge ratio • Takes in evaporant from MBE • Ionizes species and accelerates it into a magnetic field • Mass determinates are based on the radius of curvature of the ion in the magnetic field • Use of multiple charge collectors (detectors) provides the ability to measure multiple materials and their % composition at one time RHEED • Reflection High Energy X-ray Diffraction (RHEED) • Used to characterize the surface of crystalline materials • Periodicity of the crystal produces diffracted orders that can be measured using a X-ray detector and used to determine the crystalline nature of the surface • Glancing angle of the X-ray source prevents electrons from escaping into the bulk of the material. • Thus atoms at the samples surface contribute to the diffraction pattern RHEED Oscillations • During crystal growth, the intensity of RHEED signal strength from fluxuates • The difference in signal strength between two peaks corresponds to the deposition of a single atomic layer • This allows us to measure the time constant for deposition and the uniformity of the film layer over time • This oscillation is highly dependent on the material system, electron energy, and scatter angle. Thus data is gathered and compared empirically using post process methods to validate a particular MBE deposition process • Amplitude and crystal orientation depend on material parameters http://projects.ece.utexas.edu/ece/mrc/groups/str eet_mbe/mbechapter.html#EpionInP RHEED Oscillation Damping • Damping occurs when islands nucleate during the growth process before the previous layer is completed • Recovery of signal after shutter closure in figure 1 is due Ga atoms breaking away from local nuclei and re-adsorbing on step edges of surface to form a more continuous layer • Al atoms in AlAs systems have considerably less mobility and remain in nucleated clusters after deposition. Thus the RHEED signal does not recover over time. • This damping process coupled by the required rotation of films during deposition in order to improve uniformity prevents RHEED measurements during commercial http://projects.ece.utexas.edu/ece/mrc/groups/str production eet_mbe/mbechapter.html#EpionInP Requirements for Temp and Pressure http://projects.ece.utexas.edu/ece/mrc/groups/street_mbe/mbechapter.html#EpionInP Lattice Strains in Epitaxial Growth • Different materials have different lattice constants • Growing 1 crystal on another strains the lattice restricting growth in thin films • As film grows, lattice strain is relieved in the form of vacancies and Strained Defect Density dislocations in the lattice Classical Deposition Lattice Defects Vs. thickness • Thus electrical and mechanical properties of thin films greatly depend on the previous film layer http://projects.ece.utexas.edu/ece/mrc/grou ps/street_mbe/mbechapter.html#EpionInP Limits in MBE growth • Lattice mismatches lead eventually lead to numerous dislocations in the deposited film • As the film becomes too thick, these dislocations become so dominant that crsytalinity Critical layer thickness of InXGa1-XAs on GaAs for cannot be maintained favorable formation of 60o threading dislocations as a function of InAs fraction, X. E. Fitzgerald, "Lattice Mismatch and Dislocations in InGaAs/GaAs Strained Heterostructures," Properties of Lattice-Matched and Strained Indium Gallium Arsenide, pp. 6-15, INSPEC, 1993. Solar Cells • II-IV and III-IV solar cells are generated using MBE for efficient energy conversion • Choice of material dictates band gap and operational wavelength • Deposition of antireflective coatings, using MOCVD, MBE, is also utilized in combination with these processes • Photonic crystals are now being investigated to improve internal reflection and overall efficiency http://stinet.dtic.mil/dticrev/PDFs/ada443798.pdf THz QCL Lasers • Quantum Cascade Lasers (QCL) – InGaAs/InALAs – GaAs/AlGaAs – Wavelength depends on film layer thickness – Light emitted parallel to the substrate Self-consistent calculation of a portion of the conduction band structure under an applied field of 3.5 kV cm1. Injectors and superlattice (SL) active regions are alternating with a layer sequence, starting from the injection barrier (in nm) 4.3/18.8/0.8/15.8/0.6/11.7/2.5/10.3/2.9/10.2/3.0/10.8/3.3/9.9, where the Al0.15Ga0.85As barriers are in bold face, the active region is in italic, and the 10.2nm GaAs well is silicon doped at 41016 cm3. The moduli squared of the wavefunctions are also shown, with the optical transition taking place across the 18meV minigap between the second H.E. Beere et al. / Journal of Crystal and first minibands (states 2 and 1). Growth 278 (2005) 756–764 VCSEL • Vertical Cavity Surface Emitting Lasers (VCSEL) • Multiple reflections within the cavity prior to emission significantly improve lasing efficiency • Out of plane lasing • 12o divergence • Single or narrow band wavelength • Secondary emission bands can be present in emitted light • Extremely sensitive to electrostatic discharge http://britneyspears.ac/physics/vcsels/vcsels.htm www.finisar.com/download_LvqFZ7VCSEL%20Proliferation.pdf.
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