Manipulating Eletrons in Nanostrutured Semiondutors
Jorg P. Kotthaus
Center forNanoSiene(CeNS) andSektionPhysik
LMU,Geshwister Sholl-Platz1,80539 M unhen,Germany
Reeivedon22April,2001
A brief overviewof eletronmanipulations innanostruturedsemiondutors is given. Suh
ma-nipulations go down to average transfer of less than a single eletron per ontrolled step. The
appliationstonanodeviesbeyondthesaleddownsilionmiroeletronisisalsodisussed.
I Introdution
Fabriationtehnologieswithnanometerresolution
en-able us now to realize nanostrutured semiondutor
devies in whih the artiial geometri onnement
dominates the laws of transport and storage of
ele-trons. Thisgivesrisetonoveleletronibehaviorquite
inontrasttothediusiveeletronmotionthatgoverns
eletrontransport evenin todayssmall semiondutor
devies. Three distint examplesof eletron
manipu-lation in suh nanostruturesare disussed in the
fol-lowingand serveto explorethe possibilities forfuture
storageandsensingdevies.
II Eletroni Shell Strutureand
Exitations in Self-Assembled
Quantum Dots
Severalnanofabriationtehniqueshavemadeit
possi-ble to reate semiondutor quantum dots with
age-like potentials in whih the number of onned
ele-trons an bevoltage-tuned. Oneroute usesmoleular
beamepitaxyofstrainedlayerstoreatequantumdots
viaself-organizedStranskyKrastanovgrowth.
Embed-ding suhself-assembledInAsquantum dotsinto
suit-able eld-eet devies allows to injet few eletrons
intothesedotsonebyone[1℄. Thefew-eletronground
statesin their dependene oneletronoupationand
magnetieldsanbestudiedwithhighresolution
a-paitanespetrosopywhih allowsusto samplealso
smallensemblesofquantumdots[2,3℄. Theevolutionof
theeletronigroundstateenergieswitheletron
num-berand magnetield revealssingleeletronharging
viaCoulombblokadeas wellasalearshellstruture
ausedbyspatialquantizationoftheeletronimotion.
Theeletronistrutureanbewelldesribedbya
sim-ple parabolimodel[3℄. Capaitanespetrosopyan
also beemployedto study self-assemblednanoringsin
observable hanges in the eletroni ground state [4℄.
Onlargerensemblesofabout10 8
quantumdotsthe
in-trabandeletroniabsorptionin thefar-infrared[1,4,5℄
aswellastheexitoniabsorptioninthenear infrared
[6℄anbeinvestigated withtransmissionexperiments.
In partiular we observe qualitative hanges in these
spetrathatreetthellingoftheatom-likeshellsby
singleeletronsandagainanbewelldesribedby
the-ory. Reentlumineseneexperimentson single
quan-tum rings reveal the detailed dependene of the
exi-toniexitationsonhargeandspinstatesofthestored
arriers[7℄.
III Eletromehanial
Nanode-vies
Nanofabriation employing high resolution eletron
beam lithography ombined with seletive ething of
sariiallayersmakesitpossibletomanufature
nano-beams and {leversout of Si or GaAs rystalline lms
grownonasariiallayer[8℄. Usingtheexitationvia
theLorentzfore thatisexperienedbyaurrent
ar-ryingwirein the preseneof amagneti eld one an
study the resonant mehanial and eletromehanial
behavior of onduting nano-beams. Beause of their
small size typial mehanial resonanefrequenies of
suh wires lie in theregime of radio frequeniesup to
several100MHz. Thesuessfulexitationof
mehani-almotionanthenbedetetedeitherbymeasuringthe
rfpowerabsorbedbythedevieorbyapaitively
de-tetingthemotionofthewirewithrespettoaloseby
eletrode. AtlowpowertypialSi-beamsexhibita
sim-pleharmonimotionandthemehanialqualityfator
aneasilybededuedfrom thewidthofthefrequeny
spetrum to be typially several thousand. At higher
exitation power the nano-beams behave inreasingly
non-linear until oneeventually anobserve hysteresis
and bistability at the highest power level /9/. This
suhresonantlyvibratingnano-beamsanbedeteted.
Using alosebyeletrodewehavedemonstrated that
suhvibratingbeamsane. g. atassensitiveharge
sensors that are able to detet hanges of frations of
eletronsifinuenedonaneighboringeletrode[10℄.
Nanomehanial devies fabriated on silion an
alsobeemployedasfasteletrialswithesoras
shut-tlesforsmallamountofharges. InRef.11asuspended
silionlapperatsasamehanialswithbetweentwo
eletrodes,soureanddrain. Theswithanbedriven
bytheeletriforeexertedviavoltagesappliedaross
twoadditionalgateeletrodes,whih ausea
mehan-ial motion of the lapper. Beause of its very small
mass the resonanefrequeny of the lapper is in the
regime of 10 th
of MHz. If the tip of the lapper is
fabriated as a small metalli island deposited on an
isolatinglapperthedevieanalsobeusedasa
shut-tleforsmall parelsof harges. Presentlywethus an
thustransferonaveragelessthanasingleeletronwith
eah motion of the lapper. We study suh a
"quan-tum bell" as a possible devie to transfer and ount
eletronsinquantized unitsbyoperatingthemetal
is-land onhe lapperas amehanially ontrolled single
eletrontransistor/8,9/.
IV Tunable Potential Landsape
as Photoni Conveyor Belts
and Memories
Another novel mode of ombing mehanial motion
with eletron manipulation allows to proess optial
signals in quantum wells. Usually photoluminesene
inquantumwellsisaveryeÆientproessbywhih
in-ominglightwithenergyabovetheeetivebandgapof
aquantumwellisabsorbedandgenerateseletron-hole
pairs, whih reombine within nanoseonds to reemit
photons at the band gap energy. We use the
piezo-eletri potential of suÆiently strong surfae
aous-tiwaves(SAW) propagatingon GaAsquantum wells
to spatiallyseparateeletronandholesandtrapthem
in adjaent extrema of the potential superlattie
re-atedbytheSAW. Withseparationsinthemirometer
regime this easily extendsthe reombination lifetimes
by orders of magnitude. Furthermore the separation
an be reversed and thus an be employed for
inter-mediate storage of optialsignals. The dramati
sup-pressionofreombinationallowsthetrapped
eletron-holepairstosurfarossthedevies. Loallyattening
the potential wavein whih the eletron and holeare
trapped,e. g.,byasreeningmetaleletrodeorbyan
arrayof quantum dotsagainindues eÆient
reombi-nationandemissionofmonohromatilight. Insuha
dynami optial memory devie photonisignals have
onlylimitedbythetimeittakesfortheSAWtotravel
arossthehip. Inarelatedquasi-statiphotoni
mem-orywehavereentlyahievedstoragetimesforphotoni
signalsuptoseonds.
Saws propagating aross piezoeletri
heterostru-turesanalsobeusedto transformatwo-dimensional
eletronsystemintoagridof eletronwires
propagat-ing with sound veloity /15/ or assensors for optial
signalsandeletrialhargesasthesemodifythephase
andamplitudeofthepropagatingSAW.Basedonsuh
onepts we have reently realized a very simple and
fastpositiondetetorforphotonisignals/16/.
Simi-lardevieanbeemployedasafastandheapamera.
V Conlusion
The above examples should have demonstrated that
eletronsinnanostruturesanbemanipulatedinways
quitedierentfromthoseusedinlassialharge
trans-port. Thenewformsofeletrontransportandstorage
anontributeto ndingsemiondutordeviesfor
in-formation proessing and sensing beyond the area of
saleddownsilionmiroeletronis.Iwantto
aknowl-edge the essential ontributions from many
ollabora-torsasidentiedinthereferenesandontinuous
sup-portfromtheGermanSieneFoundation(DFG).The
Germanministryofeduation(BMBF), andthe
Volk-wagenFoundation.
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