ashkhen yesayan, stepan petrosyan, jean-michel...

Ashkhen Yesayan*, Stepan Petrosyan*, Jean-Michel Sallese**

* Institute of Radiophysics and Electronics (IRPhE), NAS Armenia **Swiss Federal Institute of Technology in Lausanne (EPFL),

Switzerland

� Effect of Surface Traps on NW Photoconductivity

� NW conduction type p-n-switching due to surface traps � The effect of interface traps on electrical

characteristics of NW junctionless (JL) FETs

MOS-AK, September 2017, Leuven 2

Outlines

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Ø DuetohighsurfacetovolumeratioNanowires(NWs)arethemostaffectedbysurfacetrapstopologies

TheeffectofsurfacetrapsondeviceperformanceespeciallyispronouncedwhileconsideringNWsasphotoconductors

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Generalconsidera-ononsurfacetraps

Cross-sectionalscanningelectronmicroscopyimageMOS-AK, September 2017, Leuven

Surfacetrapsresultfromtheinterruptionoftheperiodiclatticestructureatthesurfaceofacrystal.

Thesurfacerecombina5onofnonequilibriumelectron-holepairsthroughthesurfacestatesistheprevailingmechanisminNWPC

4MOS-AK, September 2017, Leuven

electrons and holes generated in conduction and valence band, respectively.

Minority carriers recombination with the trapped majority carriers.

bulk recombination through middle band-gap recombination centers: (t~ps)

surface recombination through the surface states: (t~minutes)

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Current-over-time graph for CdS NW (experiment, R=250 nm).

J. Milam, L. Lauhon, J. Allen. Nanoscape, 2, 43 (2005).

Y.Dan,Appl.Phys.Lett.106,053117(2015)

Thecontrastin-medomainwasusedtoobtaininforma-onabouttheenergydensityoftrapstates.

MOS-AK, September 2017, Leuven ESSDERC

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The model of photoconductivity When the light is on the dynamics of PC is determined by rate equation written for the surface density of electrons ns:

( ) kTE

sCholescapts

sSs

S

etnvsNdtdn

tnNRsvndtdn −

−⎟⎠

⎞⎜⎝

⎛−−= )()()( .

MOS-AK, September 2017, Leuven

S.Petrosyan,A.Yesayan,S.Nersesyan,“Photocurrentrelaxa.onsandgaininsemiconductornanowires,”DOI:10.1117/12.2180885Proc.SPIE,v.9440,2014.

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Photocurrent over time graph illustrated in the scale of milliseconds.

Comparison of analytical model and experimental data:

N.A. Stanford, P. T. Blanchard, K.A. Bertness et al. J. Appl. Phys. 107, 034318 (2010).


The PC decay after turning the light off. GaNNW

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Conclusion: q  Thedevelopedmodelexplainboththedarkconductivityanddynamics

ofPCtransients

q  Thekineticsofthephotoconductivityischaracterizedbylong-periodrelaxationprocesses.ThepersistentPCcanbeobservedevenatroomtemperatures

q  Atsmallintensitieswecalculateveryhighphotoconductivegain(108-109).GainfollowsaninversepowerlawGn~P-k,wherek=0.9.Alsoonemayexpectthatsuchaultra-highgaincouldallowreachingevensingle-photondetectioninaNWbasedphotoconductor

q  SurfacetrapsplaycrucialroleinNWPC!

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� NW conduction type p-n-switching due to surface traps

� The effect of interface traps on electrical characteristics of NW junctionless (JL) FETs


Outlines

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Ø Par-allydepleted(onlypartoftheelectronsaretrappedonthesurface)

Ø Fullydepleted(alltheelectronsareremovedfromthevolume)

Ø  Inverted(electronsfromvalancebandarecapturedbysurfacetrapsandholesbecomemajority)

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TheeffectofsurfacetrapsonasingleNWLet’sconsidern-types/cNWandacceptortypesurfacetrapsUpondependingonNWparameters:radius(R),thedopingdensity(ND)andthedensityofsurfacestates(Ns)theNWcanbe:

Ø  InversionduetothesurfacetrapswhendecreasingtheNWradiusExperimental

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SurfacetrapsultimateeffectonNW:theconductiontypeswitching

N.Han,F.Wang,etal.“Controllablep-nswitchingbehaviorsofGaAsnanowiresviaaninterfaceeffect,”ACSNano,vol.6,no.5,pp.4428–4433,2012.

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Wherewecanignoretheinversionandwherenot?

Numerical“exact”solution

ü  WhentheNWishighlydopedtheinvertedchargeisnegligibleevenatverysmallradii(5nm)andhighdensityofsurfacetraps.ForlightlydopedNWstheinversiontakesplacealreadyat40nmradius MOS-AK, September 2017, Leuven

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Ananalyticalpotential-basedmodeltocalculatetheinvertedchargeinmoderateorlightlydopedNWs

ü A regional approach is applied. 3 electrostatic regions alongtheNWradiusare considered and applydifferentsimplifyingassumptionsforeachregion:-  inversionregionclosetothe

surface-  nextfollowstheregionoffull

depletion-  theelectro-neutralregioncloseto

thecenterofNWü  These regional approximations allow us to get the continuous potential

distribution in radial direction throughout the whole NW. 0x

1x



Valida5onofanaly5calpoten5almodelwith3DSentaurusTCAD

Potential and mobile charge radial distribution depicted respectively on left and right axes in NWs with 20 nm radius: a), and 10 nm radius: b).

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The dependences of floating (xo, x1) points on NW radii at normalized radial axis: left axis, and at non-normalized axis: the inset

Calcula5onsforNWswithdifferentradii

CentralandsurfacepotentialsforNWswithdifferentradii.Lines:analyticalmodel,circles:numericalcalculations(Wolfram)


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Invertedchargecalcula5on

Charge neutrality equation:0

1

2

0

2 2 ( ) d n( ) dxR

S Dx

Rn R N p r r r r r rπ π π⎛ ⎞

= + −⎜ ⎟⎜ ⎟⎝ ⎠∫ ∫

2(2 )mob S DQ q Rn R Nπ π= − Total mobile charge in NW:

( )1 exp (1)C S

D

SS N E

N kT

Nn

u=

+ − −( ) ( ) ( )

( )1 1

1 1D 2 22

i D 0 1

Ie e

I

u ui

S

AnNn N

A nA

−⎛ ⎞+ = ⋅⎜ ⎟⎜ ⎟

⎝ ⎠

* 2/m mobQ q Rρ π=


Conclusion: q  We clarified the conditions when the surface traps

effect can cause inversion in NW. q  The potential model was developed including both

type of mobile charges in Poisson equation. q  The derived potential model provides a simple way

to calculate the inverted charge density raised due to surface traps effect.

q  The model accurately predicts the radius

dependent conductivity type switching in semiconductor NWs which is important for optimal design of nanowire devices.

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� The ultimate effect of surface traps on NW conduction type, causing p-n-switching, analytical model

� The effect of interface traps on electrical

characteristics of NW junctionless (JL) FETs


Outlines

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q  For sensing applications junctionless (JL) FETs are the most used, as there are no junctions along the NW: conducting channel, source and drain have the same doping type and density, which makes easy the fabrication process.

q  TheeffectofinterfacetrapsonJLFETcharacteristicswaspresented

HowstrongistheimpactofsurfacetrapsonNWbasedFETs?

A.  Yesayan,F.JazaeriandJ.M.Sallese,IEEETransactionsonElectronDevices,vol.63,no.3,pp.1368-1374,March2016.


�  J.-M. Sallese, N. Chevillon, C. Lallement, B. Iniguez, and F. Pregaldiny, “Charge-based modeling of junctionless double-gate field-effect transistors,” IEEE Trans. Electron Devices, vol. 58, no. 8, pp. 2628–2637, Aug. 2011.

�  J.-M. Sallese, F. Jazaeri, L. Barbut, N. Chevillon, and C. Lallement, “A common core model for junctionless nanowires and symmetric double-gate FETs,” IEEE Trans. Electron Devices, vol. 60, no. 12, pp. 4277–4280, Dec. 2013.


Chargeneutralityrequiresabalancebetweenchargesinthesemiconductorandchargesattheinterfaces

Modelingself-depletioninducedbytrapsinungatedhighlydopedNWs

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TheeffectofinterfacetrapsonelectricalcharacteristicsofhighlydopedSiNWs

Markers–TCADSynopsysSentarus3DsimulationsLines–physics-basedcompactanalyticalmodel


-Almostfulltrappingoccursinmid-gapstates-Theexponentialdependenceisverywellcapturedwhichwouldnotbepossiblerelyingonfulldepletionapproximation.

NWchanneldepletionin% NWnormalizedmobilechargeinlinearandlogscale

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ChargebasedmodelingofNWjunctionlessFETswithinterfacetraps

A.  Yesayan,F.Pregaldiny,andJ.-M.Sallese,SolidStateElectron,vol.89,pp.134-138,Nov.2013.

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(a)

(b)

TheeffectofinterfacetrapsonelectricalcharacteristicsofNWjunctionless(JL)FETs

Anexponentialdistributionforthetrapdensitywithrespecttotheenergyisconsidered

Lines:analyticalmodelCircles:SynopsysSentarus3DTCAD


To make more visible the effect of surface traps, the drain current is also obtained for zero density of interface traps at VD = 1V and is depicted with black dashed-line.

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Ø Inclusion of surface traps in nanowire topologies has been done on top of a charge-based model developed previously.

Ø Different trap energies and densities were analyzed in detail.

Ø The proposed model reproduces correctly the results obtained from TCAD simulations.

Ø This research represents an interesting development which can be useful for the design of nanowire based biosensors where the free surface is used to sense biological solutions and chemicals.

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Conclusion


Armenia

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TechnologiesinArmenia

Mic

ro-a

nd n

ano-

elec

tron

ics

Pho

toni

cs

Adv

ance

d M

ater

ials

Nan

otec

hnol

ogie

s

Bio

tech

nolo

gy

Advanced Manufacturing

Well developed*

Less developed

*) Approximate picture


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ITservicesexportIf we look to IT services export chart we see that with this index Armenia is placed at the top of CIS countries.

The chart is provided from World Bank publications 2012.


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Na5onalandinterna5onalcollabora5on

Thechartbelowillustratesthedistribu-onofR&Dcompaniesbylocalandforeignownerships.

Accordingtodata(sourceEIF),151foreigncompaniesopera-nginArmeniahavethefollowingdistribu-onofownership.

Thereare360R&Dcompaniesopera-nginArmenia.

ThesechartsarefromEnterpriseIncubatorFounda-on(EIF)“ArmenianICTSector2012StateofIndustryReport.”


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Usefulllinkstofindmoreaboutthecurrentsitua5oninscienceandac5vi5esofresearchgroups:

Research & Development

• Armenian Journal of Physics: http://ajp.asj-oa.am/ • Journal of Contemporary Physics, Publisher Springer Verlag http://www.springer.com/physics/particle+and+nuclear+physics/journal/11958

• Center of Semiconductor Devices & Nanotechnologies: http://www.semicond.ysu.am/ • State Committee of Science: http://scs.am/?hl=en_US • Armenian Academy of Sciences: http://www.sci.am/index.php?langid=1

Commercial-Technology oriented organizations

• Enterprise Incubator Foundation (EIF): http://www.eif.am/eng • Viasphere Technopark: http://www.viasphere.com/technopark/tenants.htm

• Union of Information Technology Enterprises: http://uite.org/en/projects


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• Armenian universities have close cooperation with private businesses. Example of such cooperation is: •  Interdepartmental Chairs of “Microelectronic Circuits and Systems” established by Synopsys at SEUA, YSU, RAU since 2004.

Synopsys has the heavy investment in local microelectronics education. Established a presence in Armenia in 2004 Synopsys currently is one of the largest IT employers in Armenia with more than 570 employees.

ü These cooperation of business and high education provides Armenia with highly qualified and industry-ready IT specialists.

Coopera-onofHighEduca-onwithPrivateBusinesses


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IRPhE / Solid State Physics Laboratory

Research activities: [email protected] q  Thin film solar cells research

ü Thin-film CIGS solar cells on non-conducting perlite

q  Theoretical studies of nanowires and nanospheres

q  Electronic Device Compact Modeling [email protected]

ESSDERC MOS-AK, September 2017, Leuven

ashkhen yesayan*, stepan petrosyan*, jean-michel...

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ashkhen yesayan, stepan petrosyan, jean-michel...