Start: Lecture #4, 1/21/2010High Electron Mobility Transistor (HEMT)Modulation doping is used to create a high mobility g ychannel underneath a gateBand structures of various common heterostructuresThe concept of modulation doping: We can build a “Quantum Well” in which carriers prefer to stay, and provide these carriers by using “donor” layers which are heavily ddbtd t tib t t l t f tt idoped, but do not contribute to large amounts of scatteringTwo approaches to epitaxial growth: CVD and MBE.In CVD a gas is flowed over a crystalline “seed” substrate,whereas in MBE an evaporation is used.Discrete BJTs were made in bulk wafers, but but the resulting excessive collector resistance limited high-frequency response.Solved by epitaxial growth of high-R layer on low-R substrate.Two fundamental advantages of epitaxial over bulk wafers:1. One or more epi layers allows control over doping profile.2Physical properties of epi layer are different from bulk2.Physical properties of epi layer are different from bulk (generally O2and C- free.)I. CVD deposition processes: Si epitaxyCVD: Chemical Vapor DepositionEit f G ki()ti(d d)Epitaxy: from Greek epi(upon) taxis(ordered)Boundary layer formation in a horizontal Si CVD reactorCVD reaction to deposit Si from silicon tetrachloride/hydrogenHowever, the overall reaction is more complicated,pIt does not pay to be too impatient…Maximum growth rate for which monocrystalline silicon can be obtained as a function of temperatureMBE allows integration of analytical techniques into theanalytical techniques into the system for in‐situ monitoring.Invented in late 1960’s at Bell Laboratories by J. R. Arthur andA. Y. Cho.2233Diagram of a typical MBE system growth chamber 4455Molecular beam epitaxy(MBE)isperformedwithdifferent typespxy()pfffypof semiconducting materials like:i) Group IV elemental semiconductors like Si, Ge, and Cii)III‐V‐semiconductors:arsenides(GaAsAlAsInAs)antimonidesii)IIIVsemiconductors:arsenides(GaAs,AlAs,InAs),antimonideslike GaSb and phosphides like InPiii) II‐VI‐ semiconductors: ZnSe, CdS, and HgTeElectrons move through GaAs five times faster than through silicon. 66Features of MBE1. very low deposition rates typically 1um/hr or 1A/sec2. typically in ultra‐high vacuum 77yp yg3. Uses high purity elemental charge materials.4. very well controlled growth5. 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 .For good epitaxy:Epitaxy: Growth of film with a crystallographic relationship with the substrate Types: Homoepitaxy & Heteroepitaxy.For good epitaxy:deposition rate ‐Types of MBE88The Gas‐Source MBE (GS‐MBE)III‐V semiconductors,IIIV 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.MBE growth mechanism.1212Atoms arriving at the substrate surface may undergo• absorption to the surface,•surfacemigration,g• incorporation into the cr ystal lattice,• t hermal desorption.depends strongly on the temperature of the substrate..Growth modes:At very high temperature of substrate, there are many different possible surface1313At very high temperature of substrate, there are many different possible surface diffusion mechanisms:Ehrlich-Schwoebel barrier•very low rates of impinging atomsepitaxial growth is ensured by‐•very low rates of impinging atoms, •migration on the surface and • subsequent surface reactionsDepending on the migration rates, different growth modes can result:1414‐ high migration rate (IBAD is followed) Frank vander merwe growth mode.highrateofincomingatomsandhighEhrlichSchwoebelbarrierisland‐highrateofincomingatomsandhighEhrlich‐Schwoebelbarrier,islandgrowth will occur. Volmer-Weber or Stranski-Krastanov growth modes.‐Stranski-Krastanovgrowthispossibleinahetero epitaxialsystemStranskiKrastanovgrowthispossibleinahetero epitaxialsystem.Epitaxial Growth of AlxGa1-xAsSbt t tt580ºC650ºC2424Substrate 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 sufficientGaAs, there is a large window for which there is both unity sticking and sufficient mobility.Ternary or quaternary compounds‐the window becomes smallerTernary 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 orientat ion for MBE growth of compound semiconductors.2525Growth of InGaAlAs on InP:Incorporating In into AlGaAs will decrease the bandgapIncorporating 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. Mismatch Lattice ConstantMaterial Mismatch Lattice ConstantMaterial-3.7% on InP5.653GaAs: Constant (Å)-3.7% on InP5.653GaAs: Constant (Å)+3.7% on GA5.869InP-3.5% on InP5.661AlAs+3.7% on GA5.869InP-3.5% on InP5.661AlAsLattice constants and mismatch for GaAs, AlAs and InP. GaAsGaAsQuantum dotsstructures based on highly lattice mismatched materials2626structures 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.2727AFM image of InAs/GaAsQuantumdots.Mi h d h i i l f d2828Mismatched heteroepitaxial systems for quantum dots:III‐V compoundsid (I G A /AlG A IA/IGA IAlGA/AlG A )arsenides (InGaAs/AlGaAs ,InAs/InGaAs, InAlGaAs /AlGaAs )phosphides (InAs/InP, InP/InGaP ), antimonides and nitrides (GaN/AlN),IV‐IV compoundsGe/Si and SiGe/Si II‐VI compoundsCdSe/ZnSe and Mixed‐group compoundsInAs/Si.Bennett, Magno,