2008/4/6 EE 235/NSE203 Nanoscale Fabrication; Lec. 11; Prof. Chang-Hasnain 1Surface Growth ModesHigh surface energy thin wetting layerHigh film energy 3D clusters{101} pyramids relax~ 50% strain energyvan der Merwe Volmer-Weber Stranski-Krastanow2008/4/6 EE 235/NSE203 Nanoscale Fabrication; Lec. 11; Prof. Chang-Hasnain 2Energy minimization as a driving force2008/4/6 EE 235/NSE203 Nanoscale Fabrication; Lec. 11; Prof. Chang-Hasnain 3Quantum-Dot Semiconductor Optical Amplifiers MOCVD ALE QDs100nmTEM photographInput signalsOutput signalsCurrentpnAFM image1.3-um CW lasing QD-SOA Device structure Optical gain spectrumK. Mukai et al., CLEO ‘99Ground stateExcitedstateInhomogeneousbroadeningWavelengthOptical gain2008/4/6 EE 235/NSE203 Nanoscale Fabrication; Lec. 11; Prof. Chang-Hasnain 4Alternative Growth Method: MOCVDMOCVD stands for Metal-Organic Chemical Vapor Deposition.Also named as OMVPE, MOVPE, …2008/4/6 EE 235/NSE203 Nanoscale Fabrication; Lec. 11; Prof. Chang-Hasnain 5MOCVD Basics• MOCVD is an epitaxial technology used for growing compound semiconductor-based epitaxial wafers and devices.– GaAs, InP, GaN, …– InGaAs, InGaAsP, …– Other N-, As-, P-, Sb- based materials.• Applications– Compound semiconductor based devices• Optoelectronic devices such as semiconductor lasers or LEDs• High speed electronic devices, e.g. HBT•Solar cells• OEIC (Optoelectronic Integrated Circuit)– Artificial structures for basic research• Low-dimensional structures (QW, QWW, QD)• Nonplanar structures•MEMS2008/4/6 EE 235/NSE203 Nanoscale Fabrication; Lec. 11; Prof. Chang-Hasnain 6Precursors we are using are …• As TBA (tertiary-butyl-arsine)• P TBP (tertiary-butyl-phosphine)• Ga TEGa (Tri-ethyl-gallium)• Al TMAl (Tri-methyl-aluminum)• In TMIn (Tri-methyl-indium)• Zn DEZn (Di-ethyl-zinc)•Si Si2H6 (gas source)•H2 Carrier gas2008/4/6 EE 235/NSE203 Nanoscale Fabrication; Lec. 11; Prof. Chang-Hasnain 7Source BubblersCarrier gasprecursorPoPr (vapor pressure)PiFcFrrrrc ciorPPFF FPPP==−Flow rate can be controlled by either pressure or bubbler temperature.2008/4/6 EE 235/NSE203 Nanoscale Fabrication; Lec. 11; Prof. Chang-Hasnain 8System ConfigurationAs Ga Al ZnInjection block (V)Injection block (III)H2reactor2008/4/6 EE 235/NSE203 Nanoscale Fabrication; Lec. 11; Prof. Chang-Hasnain 9Principles of MOCVD growth (cont.)Rotating Wafer Susceptor (~ 1000-2000 RPMT ~ 650oC)Reactor Top (T ~ 50oC)2008/4/6 EE 235/NSE203 Nanoscale Fabrication; Lec. 11; Prof. Chang-Hasnain 10Low Reynolds Number Requirement2008/4/6 EE 235/NSE203 Nanoscale Fabrication; Lec. 11; Prof. Chang-Hasnain 11Chamber Gas Flow Design2008/4/6 EE 235/NSE203 Nanoscale Fabrication; Lec. 11; Prof. Chang-Hasnain 12Stability vs. Spin Rate in Vertical-Type MOCVD2008/4/6 EE 235/NSE203 Nanoscale Fabrication; Lec. 11; Prof. Chang-Hasnain 13CVD MechanismsFrom EE143 Lecture Note2008/4/6 EE 235/NSE203 Nanoscale Fabrication; Lec. 11; Prof. Chang-Hasnain 14Chemical Reaction• Precursors crack due to high temperature on top of the wafer surface.• Ga+As (gas phase) Æ GaAs (stable solid compound)AsCCCCTBAsubstrateAsGaCCTEGaCCCCGaCCC2008/4/6 EE 235/NSE203 Nanoscale Fabrication; Lec. 11; Prof. Chang-Hasnain 15MOCVD vs. MBE• Advantages– Faster growth rate (3-10 um/hr vs. 1 um/hr)– Scalable to many wafers (100 X 2”)– Wide temperature control range. Better film uniformity.– Shorter system downtime (1-2 days vs. 1-2 weeks)– Possible to grow many compositions (by varying MFC flow rates)– Excellent morphology and thickness control• Disadvantages– Toxic sources Æ safety issues– Consumes large quantity of hydrogen Æ also a safety concern– Some memory effects– Huge set of parameters. Hard to optimize.2008/4/6 EE 235/NSE203 Nanoscale Fabrication; Lec. 11; Prof. Chang-Hasnain 16Arsine (AsH3) – As Source in Industry• Less expensive (comparing to TBA)• Very pure• Gas phase at room temperature• Highly Toxic2008/4/6 EE 235/NSE203 Nanoscale Fabrication; Lec. 11; Prof. Chang-Hasnain 17Arsine (AsH3) vs. Silane (SiH4)2008/4/6 EE 235/NSE203 Nanoscale Fabrication; Lec. 11; Prof. Chang-Hasnain 18Precursors we are using are …• As TBA (tertiary-butyl-arsine)• P TBP (tertiary-butyl-phosphine)• Ga TEGa (Tri-ethyl-gallium)• Al TMAl (Tri-methyl-aluminum)• In TMIn (Tri-methyl-indium)• Zn DEZn (Di-ethyl-zinc)•Si Si2H6 (gas source)•H2 Carrier gas2008/4/6 EE 235/NSE203 Nanoscale Fabrication; Lec. 11; Prof. Chang-Hasnain 19MOCVD2008/4/6 EE 235/NSE203 Nanoscale Fabrication; Lec. 11; Prof. Chang-Hasnain 20Colloidal QDs• Grown in solution phase• Au colloids by Michael Faraday 18472008/4/6 EE 235/NSE203 Nanoscale Fabrication; Lec. 11; Prof. Chang-Hasnain 21Fabrication – colloidal synthesis• Advantage– Cheapest, large volume, benchtop conditions– Least toxic• Self assembly of QD using electrochemical method– Template created using ionic reaction at electrolyte-metal interface– Results in spontaneous assembly of QDs2008/4/6 EE 235/NSE203 Nanoscale Fabrication; Lec. 11; Prof. Chang-Hasnain 22Basic Synthesis Mechanism• Temporary and discrete nucleation event, followed with slower and controlled growth on existing nuclei– Nucleation is created by rapidly increasing reagents into a vigorously stirred flask containing a hot coordinating solvent– Nucleation forms to relieve supersaturation– Control precursor addition to limit addition nuclei formation Æ focusing of the size distribution• Second Growth Phase: Ostwald ripening– Smaller nanocrystals (NCs) dissolve due to high surface energy.– Material is re-deposited onto larger NCsThermocoupleOrganometallicPrecursorsArHeating MantleGrowth temperature: 200-400 deg C2008/4/6 EE 235/NSE203 Nanoscale Fabrication; Lec. 11; Prof. Chang-Hasnain 232008/4/6 EE 235/NSE203 Nanoscale Fabrication; Lec. 11; Prof. Chang-Hasnain 24Ostwald ripening • A spontaneous process occurs because larger particles are more energetically favored than smaller particles. • The formation of many small particles is kinetically favored, (i.e. they nucleate more easily), large particles are thermodynamically favored. This is because small particles have a larger surface area to volume ratio than large particles and are consequently easier to produce. • Molecules on the surface are energetically less stable than the ones already well ordered and packed in the interior. • Large particles, with their greater volume to
View Full Document
Unlocking...