Lecture 22 –Thin Film DepositionEECS 598-002 Winter 2006Nanophotonics and Nano-scale FabricationP.C.Ku2EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.KuOverview MBE (molecular beam epitaxy) MOCVD (metal-organic chemical vapor deposition) ALE (atomic layer epitaxy) All of the above techniques provide single crystalline epitaxy with atomic layer precision.3EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.KuMBE schematicsourceContinuous Azimuthal RotationFrom UCSB course materials: http://www.ece.ucsb.edu/courses/ECE594/594F_F05Gossard/September 27b.pdf4EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.KuMBE operation MBE operates under an ultra-high vacuum (< 10-10torr) environment. That means during growth, the background residual gas such as H2O, CO2, and etc have negligible partial pressures compared to the sources. The source either evaporates at ~ 600。C (solid-source MBE) or cracks into elemental form (gas-source MBE) and deposits onto a heated substrate (~ 400。C).5EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.KuMBE system6EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.KuVacuum pumps MBE is mostly about vacuum. It is crucial to maintain a good vacuum environment to ensure the quality of the epitaxial layers. But it is also the UHV environment that makes a lot of insitu monitoring techniques applicable (e.g. RHEED). Cryo pump~ 10-6torrIon pump~ 10-9torrCryo shroud< 10-10torr7EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.KuRGA (residual gas analyzer)1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77 81 85 89 93 97-131.0x10-121.0x10-111.0x10-101.0x10-91.0x10Atomic Mass UnitsTorrRGA Analog ScanMay 26, 2004 10:16:44 AMPGC< 9E-11 (gauge limit)All cells @stand-by temp.LN2chilling8EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.KuEffusion cell (Knudsen cell) The mean free path of the evaporated source element is long enough (if pressure is < 10-5torr during growth) to allow the element to travel to the substrate surface along a straight path.PBN = pyrolytic boron nitride9EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku10EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.KuRHEED RHEED = reflection high energy (5-100 keV) electron diffraction One RHEED oscillation corresponds to one monolayer growthFrom http://www-opto.e-technik.uni-ulm.de/forschung/jahresbericht/2002/ar2002_fr.pdf11EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.KuSurface reconstruction RHEED can also be used to monitor the surface reconstruction because of its shallow incidence angle (< 5。)From http://www.courses.vcu.edu/PHYS550/presentations2000/rheed.pdf12EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.KuFree MBE simulator http://uberfast.ece.ucsb.edu/~mgrund/kmcinteractive/kmcinteractive.html Runs only on Mac OS13EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.KuMOCVD14EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.KuHydrogen purification Using palladium cell, the hydrogen can be purified to > 99.9999999% purity.15EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.KuMOCVD vs MBE Advantages Faster growth rate (favored by industry) Wide temperature control range. Better film quality. Shorter system downtime Disadvantages Toxic sources  Huge set of parameters. Hard to control. Not UHV environment. Some insitu monitoring techniques are not applicable.16EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.KuMOCVD operation Precursors (e.g. TEGa for Ga and TBA for As) carried by hydrogen gas to the reactor. Precursors transported from reactor top to the heated substrate surface with a flux rate determined by flow pattern. Precursors crack due to high temperature on top of the wafer surface. Ga+As (gas phase) Æ GaAs (stable solid compound)AsCCCCTBAsubstrateAsGaCCTEGaCCCCGaCCC17EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku18EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.KuGrowth regimes Reaction limited regime: At low temperature (e.g. < 500。C for GaAs growth), the diffusion process is faster than the surface process. The growth rate increases with temperature.  Diffusion limited regime: At higher temperature (e.g. 600。C for GaAs growth), the diffusion process is slower than the surface process. The growth rate is limited by the diffusion process and therefore controlled only by the mass flow. It is the normal growth regime.19EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.KuBubbler theoryMFCH2/N2PCFcFrrrrc ciorPPFF FPPP==−Pr = vapor pressure of the MOsource = f(T)Water bath20EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.KuRotating disk reactor Rotating-disk reactor schematics and fluid flow pattern as follows* Metal-organic precursors decompose into reactant adatoms near the wafer surface in the boundary layer.* W. G. Breiland et al, Material Science and Engineering, R24 (1999) p.241T=15oCT=640oCBoundary layer21EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.KuAtomic layer epitaxy Similar to MOCVD but instead of flowing all source species at the same time, each source species of the compound reaches the substrate surface in an alternating fashion.e.g. AlAs growthCarrier gasAl sourceAs sourcetime22EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.KuALE operation Since the ALE growth is a self-limiting process, one monolayer is deposited during each cycle.From Nanotechnology 10 (1999)