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MBE Application to key materials - T.Vijaykumar 1 Outline Introduction Technical data • Effusion cells • Growth mechanism • RHEED Application • Quantum dots • HEMT • High Tc Superconductors • GMR Summary Invented in late 1960’s at Bell Laboratories by J. R. Arthur and 2 A. Y. Cho. 3 4 Diagram of a typical MBE system growth chamber 5 Molecular beam epitaxy (MBE) is performed with different types of semiconducting materials like: i) Group IV elemental semiconductors like Si, Ge, and C ii) III-V-semiconductors: arsenides (GaAs, AlAs, InAs), antimonides like GaSb and phosphides like InP iii) II-VI- semiconductors: ZnSe, CdS, and HgTe Electrons move through GaAs five times faster than through silicon. 6 Features of MBE 7 1. very low deposition rates typically 1um/hr or 1A/sec 2. typically in ultra-high vacuum 3. Uses high purity elemental charge materials. 4. very well controlled growth 5. films with good crystalline structure 6. often use multiple sources to grow alloy films 7. deposition rate is so low that substrate temperature does not need to be as high . Epitaxy: Growth of film with a crystallographic relationship with the substrate Types: Homoepitaxy & Heteroepitaxy. For good epitaxy: deposition rate - 8 Types of MBE The Gas-Source MBE (GS-MBE) III-V semiconductors, group-V materials are hydrides such as arsine (AsH3) or phosphine (PH3) Metalorganic MBE (MO-MBE) group-III materials are metalorganic compounds. e.g., TEGa and TMIn Solid-Source MBE (SS-MBE) group-III and -V molecular beams. Effusion cells 9 Cylindrical crucible offers good charge material capacity, but uniformity decreases as charge material is depleted. It offers excellent long-term flux stability, but permits large shutter flux transients. Conical crucible offers reduced charge material capacity, excellent uniformity, and poor long-term flux stability, and permits large shutter flux transients. SUMO crucible offers excellent charge material capacity, excellent uniformity, excellent long-term flux stability, and minimal shutter-related flux transients. - made of Ta, Mo, and pyrolytic boron nitride (PBN) 10 - do not decompose or outgas impurities even when heated to 1400ºC. Valved cracker Effusion cell 11 - combines the evaporation and cracking of elements like P,S, As, Se, Te etc. Technical Data Heating System Radiation heating, tantalum wires with PBN insulators Temperature range 100 °C ...800 °C bulk zone 100 °C ...1000 °C cracker Temperature stability <= 0.1 K depending on the PID controller Bake out temperature 250 °C MBE growth mechanism. 12 Atoms arriving at the substrate surface may undergo • absorption to the surface, • surface migration, • incorporation into the crystal lattice, • thermal desorption. depends strongly on the temperature of the substrate.. Growth modes: 13 At very high temperature of substrate, there are many different possible surface diffusion mechanisms: Ehrlich-Schwoebel barrier epitaxial growth is ensured by- • very low rates of impinging atoms, • migration on the surface and • subsequent surface reactions Depending on the migration rates, different growth modes can result: 14 - high migration rate (IBAD is followed) Frank vander merwe growth mode. - high rate of incoming atoms and high Ehrlich-Schwoebel barrier, island growth will occur. Volmer-Weber or Stranski-Krastanov growth modes. - Stranski-Krastanov growth is possible in a heteroepitaxial system. 15 Reflection high-energy electron diffraction (RHEED). 16 sensitive to surface changes, either due to structural changes or due to adsorption. 1. to calibrate growth rates, 2. observe removal of oxides from the surface, 3. calibrate the substrate temperature, 4. monitor the arrangement of the surface atoms, 5. determine the proper arsenic overpressure, 6. give feedback on surface morphology, 7. provide information about growth kinetics A high energy beam (3-100 keV) is directed at the sample surface at a grazing angle. - distance between the streaks - surface lattice unit cell size. - atomically flat surface - sharp RHEED patterns. - rougher surface - diffused RHEED pattern. layer-by-layer growth mode - RHEED oscillations. 17 RHEED is sensitive for surface structures and reconstructions. 18 a) GaAs(100) - 1x1 b) GaAs(100) - 2x1 electron beam is incident in the (110) with 8.6 keV Interpretation of some RHEED patterns 19 20 Different stages of layer-by-layer growth by nucleation of 2D islands and the corresponding intensity of the diffracted RHEED beam. 21 (a) Transmission through high and wide crystal; (b) Transmission through high and narrow crystal; (c) Transmission through short and wide crystal; (d) Diffraction from nearly flat asperities. RHEED intensity oscillations: 22 - direct measure of growth rates in MBE. - oscillation frequency corresponds to the monolayer growth rate. - incident angle dependence of the oscillations suggest that interference between electrons scattering from the underlying layer and the partially grown layer contribute to these oscillations. - magnitude of the RHEED oscillations damps because as the growth progresses, islands nucleate before the previous layer is finished. 23 Beam Equivalent Pressure - To measure the growth rate. - Proportional to the flux at the sample surface and hence the growth rate. - The biggest difficulty is, the gauge gets coated. Epitaxial Growth of AlxGa1-xAs 24 Substrate temperatures - 580ºC-650ºC. Requires an As overpressure to prevent the surface from becoming Ga rich. GaAs, there is a large window for which there is both unity sticking and sufficient mobility. Ternary or quaternary compounds - the window becomes smaller - differences in the relative bond strengths of the different group III adatoms. RHEED can be used to determine the minimum amount of As required to maintain the proper stoichiometry by measuring the incorporation ratio. Values of the incorporation ratio for GaAs is 1.3 to 1.8. (100) is the predominant substrate orientation for MBE growth of compound 25 semiconductors. Growth of InGaAlAs on InP: Incorporating In into AlxGa1-xAs will decrease the bandgap. In.52Al.48As - 1.49 eV 0.74 eV - In.53Ga.47As - small enough bandgap to detect light at the important wavelengths of 1.3 mm and 1.55 mm. Material Lattice Mismatch Constant : (Å) GaAs 5.653 -3.7% on InP AlAs 5.661 -3.5% on InP InP 5.869 +3.7% on GaAs Lattice constants and mismatch for GaAs, AlAs and InP. Quantum dots 26 structures based on highly lattice mismatched materials. mean free path and the de Broglie wavelength of free carriers exceed the critical sizes of structures. carriers experience a three-dimensional quantum confinement. InAs grows layer-by-layer till critical coverage - wetting layer (WL). After θc=1.6 ML (w~0.5 nm), Stranski-Krastanow 3D growth occurs. relaxation of the elastic energy which builds up as the thickness of mismatched epilayers increases. 27 AFM image of InAs/GaAs Quantumdots. 28 Mismatched heteroepitaxial systems for quantum dots: III-V compounds arsenides (InGaAs/AlGaAs ,InAs/InGaAs, InAlGaAs /AlGaAs ) phosphides (InAs/InP, InP/InGaP ), antimonides and nitrides (GaN/AlN), IV-IV compounds Ge/Si and SiGe/Si II-VI compounds CdSe/ZnSe and Mixed-group compounds InAs/Si. Bennett, Magno, and Shanabrook Appl. Phys. Lett. 68 (4), 22 (1996) 29 E = Eg + Ee + Eh E - emission energy Eg - quantumdot bandgap energy Ee - electron confinement energy Eh - hole confinement energy Nt: Exciton binding energy is neglected. Tuning QD emission 30 C. K. Chia et al., J. Crystal. Growth, 288, 57-60 (2006) High Tc Superconductivity: 31 increases in the Tc in thin films of copper oxide superconductors hydrostatic pressures compressive epitaxial strain. Under compressive epitaxial strain a much larger increase in Tc is possible. Requires the choice of a suitable system and substrate combination The two essential structural features in copper oxide superconductors – 1. CuO2 planes 2. charge reservoirs (CR) The possible control knobs are: 1. the distance between the magnetic (black arrows) Cu atoms dab, 2. the corrugation of the CuO2 planes alpha 3. the thickness of CR dCR 4. the interlayer distance between two consecutive CuO2 planes dIL 5. the distance between the charge reservoir and the CuO2 plane dCT. the lattice deformations associated with the strain fundamentally modify the energy scales, leading to the formation and condensation of the superconducting pairs. 32 "214" film on a SrTiO3 substrate, the Cu and O atoms of the CuO2 planes are expanded. "214" thin film on a SrLaAlO4 substrate in-plane compression and an out-of-plane expansion. - redox reaction at interface is a serious problem fabricating tunnel junctions using high-Tc Cuprates, J. P. Locquet et al., Nature 394, 453 (1998) MgB2 a novel material for high Tc superconductor. 33 The graphite-like array of boron (shown in black) -has great promise for superconducting electronics operated at T ~ 20 K. Josephson tunnel junctions (MgB2/AlOx/MgB2) fabricated on sapphire -C substrates below 300° C. K. Ueda, S. Saito, K. Semba, T. Makimoto and M. Naito, APL 86 , 172502 (2005) Gallium Nitride (GaN) HEMT 34 Dr. Takashi Mimura - inventor of HEMT ( - FET with a junction between two materials with different band gaps as the channel instead of an n-doped region. Features of GaN HEMT high frequency power transistors for RF transmission applications covering the 1-50 GHz band. inherently higher transconductance, good thermal management and higher cutoff frequencies. prime candidate for high power microwave applications. M. Aslf Khan, J. N. Kuznia, et al, Appl. Phys. Lett., 65(9):1121 extremely low noise device in terrestrial and space telecommunications 35 systems, radio telescopes in the area of astronomy, DBS receivers and a car navigation receivers. In 1985, HEMT was announced as a unique microwave semiconductor device with the lowest noise characteristics in the world. - can receive TV programs from satellites 36,000Km above the ground. 36 - The antenna is less than 30 centimeters in diameter, GMR heterostructures: 37 - prepared in 1986, using MBE - magnetoresistance is much larger than that of the intrinsic magnetoresistance of the Fe layers themselves Oscillatory coupling - a very general property of almost all transition-metal magnetic multilayered systems the nonferromagnetic layer comprises one of the 3d, 4d, or 5d transition metals or one of the noble metals The oscillation period was found to be just a few atomic layers, typically about 10 Å, but varying up to ~20 Å. - ferromagnetic cobalt layers separated by thin copper layers, was found to exhibit very large GMR effects even at room temperature GMR sandwiches can achieve sensitivities to such fields of perhaps as much as five hundred times greater than conventional materials. For copper thickness (typically 1.9 nm) - anti-parallel magnetic coupling between 38 successive cobalt layers in zero applied field. the magnitude of the interlayer exchange coupling decreases much more rapidly with increasing Cu thickness. "spin valve" effect: GMR read heads allows the reading of extremely small magnetic bits at an areal density of 2.69 gigabits per square inch. One minute please………….. IBM J . RES. DEVELOP. VOL 42 NO. 1 JANUARY 1998 Summary 39 - MBE is a versatile technique for device fabrication. - A good lab for studying Molecular dynamics and surface mechanism. - The primary application for MBE - grown layers is the fabrication of electronic devices . www.research.ibm.com/research/demos/gmr/1.swf www.research.ibm.com/research/demos/gmr/2.swf Appropriate other meanings of MBE 40 • Mind Boggling Experiment • Mostly Broken Equipment • Mega-Buck Evaporator • Medieval Brain Extractor • Money Buys Everything • Management Bullshits Everyone
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