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    Ptiyaica Wbrid Spt mb v

    19 7 4 7

    Arraysof quantum dots promise improved

    performancefor semiconductor lasersandhavethe

    added advantage that theycanassem ble themselves

    Quantum-dot lasers

    KARLEBERL

    bulk semiconductor material

    SEMICONDUCTOR lasers

    are key

    components

    in

    many

    of

    the appliances that wetakefor granted in everyday life.

    Fibre-optic communications and compact-disc players

    are

    perhaps the two best known examples. There is alsoa

    world-wide research

    and

    development effort

    to

    improve

    the performance of these devices by making them smaller,

    brighter, more efficient

    or

    capable

    of

    lasing

    at

    new wave-

    lengths. A new class

    of

    semicond uctor laser

    - a

    quantum-

    dot laser that self assembles

    -

    is showing great promise

    in

    many

    of

    these areas.

    Semicon ductor lasers emit light when

    an

    electron

    in the

    conduction band recombines with an empty electron

    state

    or

    hole

    in

    the valence b and. The wavelength

    of

    the

    lightisusually determined by thebandgap ofthe semi-

    conductor

    - the

    energy difference between

    the top of

    the

    valence band

    and the

    bottom

    of the

    conduction band.

    Gallium indium phosphide,

    for

    example,

    has a

    band

    gap

    of about

    1.9

    eV, which leads

    to the

    emission

    of

    red light.

    The band gap, whichisresponsibleformostofthe use-

    ful properties

    of

    sem iconductors,

    is due to the

    quantum

    nature of the electron waves as they travel throughthe

    crystal. Quantum mechanics

    forbids the electrons from

    having energies inside

    the

    band gap.

    In

    a

    bulk semiconductor,

    such as silicon or gallium

    arsenide,

    an

    electron

    in the

    conduction band behaves like

    a free electron and can occupy

    states with continuous values

    of momentum and energy.

    This picture changes dramat-

    ically, however, when the

    motion

    of the

    electron

    is

    restricted in one or more

    dimensions (figure la

    and

    b .

    Iftheelectrons are confinedin

    a small enough region, their

    energy levels will be quantized,

    similar

    to the

    energy levels

    in

    an atom.

    One way to

    make

    such

    an

    artificial atom

    or

    quantum dot

    is to

    surround

    a small regionofsemiconduc-

    tor with another semiconduc-

    tor thathas alarger bandgap

    (figure lc).

    The

    electrons

    are

    effectively confined

    in the

    region with

    the

    smaller band

    gap as they seek to reduce

    their energy.

    Ina simplified modeltheconfinement potentialcan be

    considered

    as a

    square well (figure Id .

    In

    this model

    the

    separationof the discrete energy levels depends inversely

    on the size ofthequantumdot.Th e dot must be 10

    nm or

    smaller

    for the

    energy levels

    of

    the quan tum well

    to be

    separated

    by

    more than

    the

    thermal energy

    at

    room

    tem-

    perature. Such quantum dots contain only a few thousand

    atoms

    and

    should

    be

    able

    to

    emit light

    at

    wavelengths

    determine d by the energy levels

    in

    die do t, rather tha n

    the

    band-gap energy.

    However, various processes conspiretostopasemicon-

    ductor emitting light. Impurity atoms

    can, for

    example,

    trap the charge carriers and prevent recombination.

    Electrons

    and

    holes

    can

    also emit phonons

    (i.e.

    lattice

    vibrations) rather than photon s when diey recombine. To

    overcome these problems, the surface

    of

    the qu antum

    dot

    mustbe passivated :inother words,thesmall-band-gap

    material must

    be

    completely embedded

    in the

    barrier

    material without any crystal defects

    and

    impu rities.

    Quantum dots are widely used for experiments that

    semiconductor quantum dot

    10

    nm

    band gap

    light

    emission

    1 a) The periodic crystal potential in a piece of bulk

    semiconductor material allows almost free motion for

    electrons, which results in energetic bands separated by a

    forbidden band gap. Within the conduction band, electrons

    have continuous kinetic energy and momentum, o) The

    situation is totally different for an electron confined in a very

    small ~ 10 nm in diameter) semiconductor cluster and the energy spectrum becomes discrete like that for atoms.

    c) A schematic illustration of a passivated quantum dot with the charge carriers purple) confined in the small-

    band-gap material red), which is completely embedded in a large-band-gap material blue) without any crystal

    defects and impurities present. The charge carriers are confined in all directions. The location of the electron is

    indicated schematically and is similar to the orbitals in atoms, d) A model square-well confinement potential

    showing the quantized energy states red lines).

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    4 8 Physics World September 1997

    elastic strain relaxation

    wetting layer

    2(a) Cluster formation during epitaxial growth of a semiconductor

    material (red) on top of another se miconductor with a smaller (by a few

    per cent) lattice constant (blue). For example, indium phosphide has a

    4%

    larger lattice constant than gallium indium phosphide, so InP forms

    clusters as shown in figure 3. The cluster formation is energetically

    favourable, because the lattice can elastically relax compressive strain

    and thus reduce strain energy, (>) In an alternative growth mode the

    epilayer is laterally com pressed to match the substrate lattice. This so

    called "pseudomorphic growth" is observed for heterostructures that

    have only a small lattice mismatch.

    probe the fundamental quantum behaviour of semicon-

    ductors. They also have many potential device appli-

    cations. Th e most promising of these are single-electron

    transistors (see Harmans in further reading) and quan-

    tum-dot lasers. The latter offer the possibility of improved

    device performance and increased flexibility to adjust the

    laser wavelength.

    Early days

    Semiconductor lasers have developed rapidly since their

    invention in

    1962.

    Th e first semicondu ctor laser consisted

    ofagallium arsenide p -n junction that was forward biased

    such that electron-hole recombination took place in the

    depletion region between the positively doped (p) and

    negatively doped (n) regions. Polished surfaces (called

    facets) perpendicular to the junction were used to form

    the reso nant cavity needed for laser action. The se facets

    fill the role played by mirrors in conventional lasers.

    The performance of the early semiconductor lasers was

    greatly improved by sandwiching the gallium arsenide

    (GaAs) active layer between aluminium gallium arsenide

    (AlGaAs) layers in a hetero structure device. This

    improved laser performance for two reasons. First, the

    AlGaAs layers have a larger

    band gap than the GaAs

    layers, which helps to confine

    the charge carriers in the

    active layer. Second, AlGaAs

    has a lower refractive index

    than GaAs, which confines the

    light within the active layer

    and thus acts as a waveguide.

    In 1970 this concept lead to

    the continuous room-tem-

    perature operation of GaAs/

    AlGaAs heterostructure lasers.

    A key figure-of-merit for a

    semiconductor laser is the

    current density needed for lasing to start. This figure

    should be as low as possible and by 1975 threshold cur-

    rent densities of about 50 0Acm

    2

    had been achieved with

    active layers abou t 0.1 |0.m thick. The practical use of

    semiconductor lasers in real applications such as fibre-

    optic data transmission began around this time.

    In 1974 Raymond Dingle ofBellLaboratories in the US

    demonstrated quantum confinement of charge carriers for

    the first time. And in 1979 Won -Tien Tsan g, also of Bell

    Labs, built the first semiconductor laser based on quan-

    tum confinement. The active region in Tsang's laser was

    only about 0.01 um (10 nm) thick, so the charge carriers

    (i.e. electrons and holes) were only free to move in just

    two dimensions. The se qua ntum-w ell lasers had three

    advantages over previous devices: considerably higher

    light amplification or optical gain ; m uch lower thresho ld

    current densities (less than 50A cm

    2

    ) and significantly

    improved tem perature stability. The se advantages are all

    linked to the redu ced energy spread of occupied states for

    electrons and holes in quantum-well devices compared

    with bulk semiconductors.

    The next step - from quantum-well lasers to quantum-

    wire lasers - came in the late 1980s. In quantum-wire

    lasers the charge carriers are free to move in only one

    direction, which further increases