Quantum Dots

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Quantum Dots. What is a quantum dot?. In two words, a semiconductor nanocrystal. Easily tunable by changing the size and composition of the nanocrystal. Gallium Arsenide Quantum Dots. Gallium arsenide is a III-V semiconductor - PowerPoint PPT Presentation

Transcript of Quantum Dots

  • Quantum Dots

  • What is a quantum dot?In two words, a semiconductor nanocrystal.Easily tunable by changing the size and composition of the nanocrystal

  • Gallium Arsenide Quantum DotsGallium arsenide is a III-V semiconductorHigher saturated electron velocity and higher electron mobility than siliconGallium arsenide can emit and absorb light, unlike siliconNo silicon laser is possible (or has been made yet)

  • Energy Band LevelsElectrons exist in discrete energy levels in bulk semiconductor material.There exists a forbidden range of energy levels in any material called the band gap.

  • By absorbing some sort of stimulus (in light or heat form), an electron can rise to the conduction band from the valence band.This action leaves behind a hole in the valence band. The hole and the electron together are called an exciton.

  • The average distance between an electron and a hole in a exciton is called the Excited Bohr Radius.When the size of the semiconductor falls below the Bohr Radius, the semiconductor is called a quantum dot.

  • Tuning Quantum DotsBy changing size, shape, and composition, quantum dots can change their absorptive and emissive properties dramatically

  • Manufacturing methodsElectron beam lithographyMolecular beam epitaxy

  • Electron Beam LithographyElectrons are accelerated out of an electron gun and sent through condenser lens optics directly onto a wafer = (12.3 / V)Advantages:generation of micron and submicron resist geometriesgreater depth of focus than optical lithographymasks are unnecessaryOptical diffraction limit is not a real concern

  • Electron Beam LithographyDisadvantage(s):The lithography is serial (masks arent used; instead the beam itself sweeps across the wafer) => Comparatively low throughput ~5 wafers per hour at less than 1 micrometer resolutionThe proximity effect: Electrons scatter because they are relatively low in mass, reducing the resolution.Heavy ion lithography has been proposed, but still is in development stages

  • Molecular Beam EpitaxyMolecular beam epitaxy (MBE) is the deposition of one or more pure materials onto a single crystal wafer one layer of atoms at a time in order to form a perfect crystalThis is done by evaporating each of the elements to combine, then condensing them on top of the wafer.The word beam means that the evaporated atoms only meet each other on the wafer

  • Artificial AtomDouble Barrier HeterostructureDot: In0.05Ga0.95AsSource &Drain : GaAs2D Electron GasConfine with gate biasD ~ Fermi wavelength Discrete energy levels

  • Adding Electrons, changing Vgate2D-Harmonic OscillatorShell structure as in atomsMagic Numbers: 2, 6, 12...To add even electron requires only additional Coulomb energy

  • Comparison with HydrogenArtificial Atom: Energy levels ~ 1meV Size ~ 10m Weak magnetic fields can affect energy levelsHydrogen: Energy levels ~ 1eV Size ~ 1 Only strong magnetic fields can perturb energy levels Factor 1000...

  • Tuning the Quantum DotTune so we have one valence electronInitial state can be set by applying homogeneous magnetic field |0>Low temperature: kT < E (state gap)Now we have defined our single qubitEnergypositionGate biasSpin up - electronUnoccupied state

  • The Physical System: Excitons Trapped in GaAs Quantum DotsExciton - a Coulomb correlated electron-hole pair in a semiconductor, a quasiparticle of a solid. Often formed when photons excite electrons from the valence band into the conduction band. Wavefunctions are hydrogen-like i.e. an exotic atom though the binding energy is much smaller and the extent much larger than hydrogen because of screening effects and the smaller effective massesDecay by radiating photons. Decay time ~50ps-1nsHence can define the computational basis as absence of an exciton |0>, or existence of an exciton |1>

  • Stufler et al.Large wafer containing InGaAs QD was placed between a bias voltage and exposed to ultrafast laser pulses.

    Cos(/2)|0>+Sin(/2)|1>

    |1> => electric charge

    =>Photocurrent (PC)PC~Sin2(/2)

    -pulse corresponds to a population inversion

  • Quantum Dots. Optical and Photoelectrical properties of QD of III-V Compounds.Alexander Senichev Physics FacultyDepartment of Solid State [email protected] State University

  • IntroductionIf the size of semiconductor crystal is reduced to tens or hundreds of inter-atomic spacing, all major properties of material change because of size quantization effects.

  • IntroductionQuantum WellThe extreme case of size quantization is realized in semiconductor structures with confinement of carriers in three directions they are Quantum Dots.

    Quantum Dots

  • IntroductionGenerally, electronic spectrum of the ideal quantum dots is a set of discrete levels.

    Qualitative behavior of Density of States in:a) Bulk semiconductorb) Quantum Wellsc) Quantum Wiresd) Quantum Dots

  • Device application of QDsLasers with active area based on QDsLight-Emitting Device (LED) based on QDsQuantum Dots Solar Cells

  • Technology of QDs FormationThe base of technologies of QDs formation is self-organizing phenomenon.There are three types of initial stage of epitaxial growth:2D growth of material A on surface of substrate B ; (Frank-van der Merve)3D growth of material A on surface of substrate B ( Volmer-Weber method);Intermediate mode of growth the Stranski-Krastanow mode.

    2D growth3D growthStranski-Krastanow

  • Technology of QDs FormationMolecular Beam Epitaxy (MBE) MBE may be defined as the deposition of epitaxial films onto single crystal substrates using atomic or molecular beams.

    MBE involves elementary processes:Adsorption of atoms and molecules;Thermal desorption;3) Diffusion of adatoms on surface of substrate;4) Nucleation;Solid substrate1234

  • Technology of QDs FormationMolecular Beam Epitaxy (MBE)MBE system consist of:a growth chambera vacuum pumpa effusion (Knudsen) cellsa manipulator and substrate heateran in-situ characterization tool RHEED (reflection high energy electron diffraction)The typical rate of MBE growth is about 1 ML/s.

  • Technology of QDs FormationMolecular Beam Epitaxy (MBE)The oscillation of the RHEED signal exactly corresponds to the time needed to grown a monolayer. The diffraction pattern on the RHEED windows gives direct indication of the state of the surface.

  • Technology of QDs FormationMetal organic chemical vapor deposition (MOCVD)Metal organic chemical vapor deposition is a technique used to deposit layers of materials by vapor deposition process.

    MOCVD system contains:the gas handling system to meter and mix reagentsthe reactorthe pressure control systemthe exhaust facilities

  • Technology of QDs FormationMetal organic chemical vapor deposition (MOCVD)The basic chemistry equation of this reaction is as follows:

    Group III sources are trimetilgallium (TMGa), TMAl, TMIn.Group V sources are typically hydride gases such as arsine, phosphine. Growth rate and composition is controlled by partial pressures of the species and by substrate temperature

  • Dependence of QDs morphology on growth conditionsThe basic control parameters in the case of MBE growth:

    the substrate temperature;the growth rate;the quantity InAs, ratios of III/V materials;Exposure time in As stream;

    As research shows, morphology of QDs ensembles strongly depends on temperature of substrate and growth rate.

  • Dependence of QDs morphology on growth conditions

  • Optical properties of QDsPhotoluminescence spectra of various ensembles of QDs:

  • Optical properties of QDsThe major processes which explain the temperature behavior of QDs PL-spectra:Thermal quenching of photoluminescenceThermal quenching is explained by thermal escape of carriers from QD into the barrier (or wetting layer) Red shifting As experiment shows, at the temperature, when thermal quenching begins, we can see a following change: the maximum of PL line is shifting in the red region. Such behavior of PL spectrum is explained by thermal quenching of carriers and their redistribution between small and large QDs.

  • Optical properties of QDsThermal broadening of PL-spectrum.The one of the major factors which defines PL-line width is size dispersion of QDs, i.e. statistic disregistry in ensembles of QDs. Other process which affects on PL-line width is the electron-phonon interaction. Tunnel processesTunneling of carriers between QDs competes with escape of carriers from QDs in all temperature range. Probability of tunneling increases with temperature growth. Tunneling processes can affect on high-temperature component of photoluminescence spectrum.

  • Photoelectrical properties of QDsPhotoluminescence spectra at 10 K as a function of bias excited at (a) 1.959 eV above the GaAs band gap, (b) 1.445 eV resonant with the wetting layer, and (c) 1.303 eV resonant with the second dot excited state. Schematic excitation, carrier loss, and recombination processes are indicated for the three cases.Photocurrent spectra as a function of bias at 10 K. Quantum-dot features are observed for biases between -3 and -6 V. The inset shows photocurrent from two-dimensional wetting-layer transition, observed to its full intensity at biases of only ~ -0.5 V.

  • Semiconductor Quantum DotsJustin Galloway2-26-07Department of Materials Science & Engineering

  • IntroductionEffective Mass ModelReaction TechniquesApplicationsConclusion

    Outline

  • How Quantum DotsSemiconductor nanoparticles that exhibit quantum confinement (typicall