Journal of Babylon University/Pure and Applied Sciences/ No.(2)/ Vol.(22): 2014
PreparationofSchottkydevices
(Al-GaAsNi-GaAs)andstudyofsome photoelectronicproperties
BurakYahyaKadem
BabylonUniversity/CollegeofScience/PhysicsDept
Abstract
Foursamplesofmetal(n-type)semiconductorcontacthadbeenpreparedasaformofSchottkycontact, AluminumandNickelmetalsandsemiconductorsubstrateGaAs(donor) where used.TheOhmiccontacthasbeen firstlymadewith thickness(500 nm)usingAluminumfortwosamplesandNickelforothertwo,foursamples werecollected. Thesesampleswereannealedunderthetemperatureof(450K)andpressure(10-4Torr)for (30min.)toavoid the interfaciallayers.ThenSchottkycontact where made usingAluminumtwiceandNickeltwice with (120nm)thicknessand then weannealedthesamplesundertemperature(450K)andpressure(10-4Torr), thesamplesareasfollows:(Al/GaAs/AlOhmic,Al/GaAs/NiOhmic,Ni/GaAs/AlOhmic, Ni/GaAs/Ni Ohmic). Thephotocurrentasafunctionof wavelength was calculated andit wasfoundthatthe maximumvalueforthesample(Al/GaAs/AlOhmic)inthewavelength(800nm), thedarkcurrentis (1.9x10-9Ampere). The detectorcoefficients ofthesamples where calculated, themaximumResponse at thewavelength(800nm)was(0.157Ampere/Watt)andmaximumSpecificDirectivityatthesamewavelength is(63.7x1011Hz½Watt-1),themaximumNoiseEquivalentPoweris(0.157WattHz -½)andthemaximum Efficiencyis(24.4%)atthesamewavelength,thephotocurrentandResponse valuesdependedonthe absorptioncoefficientsofmetalsandworkfunctionsthesamplesoperatedwithintheareaunderthenear- infrared.
Keywords:Thinfilms;Schottkydevices;GaAs;optoelectronicsproperties.
الخلاصة
تم تحضير اربع نماذج شوتكي باتصال معدن شبه موصل (مانح) على شكل تماس شوتكي حيث تم باستخدام معدني الالمنيوم والنيكل وارضيه شبه موصل ارسنيد الكاليوم (مانح), تم اولاً اجراء الاتصال الاومي بسمك (500 nm) باستخدام معدن الالمنيوم لنموذجين ومعدن النيكل لنموذجين فكانت المحصله اربع نماذج وتمت عمليه التلدين لهذه النماذج تحت درجه حرارة (450 كلفن) وضغط (10-4تور) لمدة نصف ساعه لتفادي حالات السطح البينيه ومن ثم اجراء اتصال شوتكي باستخدام معدن الالمنيوم مرتين والنيكل مرتين وبسمك (120 nm) ومن ثم اجراء عمليه التلدين الحراري تحت درجه (450 كلفن) وضغط (10-4تور) فكانت النماذج كالاتي: ( المنيوم- ارسنيد الكاليوم وبتماس اومي المنيوم, المنيوم- ارسنيد الكاليوم وبتماس اومي نيكل, نيكل - ارسنيد الكاليوم وبتماس اومي المنيوم , نيكل - ارسنيد الكاليوم وبتماس اومي نيكل) ومن ثم تم حساب قيم التيار الضوئي كداله للطول الموجي حيث كانت اعلى قيمه للتيار الضوئي عند الطول الموجي (800 nm) وكانت قيمه تيار الظلام ( 1.9 x 10-9امبير) ومن ثم حساب معاملات الكاشف للنماذج الاربعه, حيث تم حساب اعلى قيمه للاستجابه عند الطول الموجي (800 nm) وكانت (0.157 Amp./ Watt) واعلى قيمه للكشفيه النوعيه كانت عند نفس الطول الموجي (63.7 X1011 Hz1/2 Watt -1) في حين كانت اقل قدرة مكافئة للضوضاء هي (0.157 Watt Hz-1/2) واعلى كفاءه كميه مسجله هي ( 24.4%) وسجلت قيم التيار الضوئي والاستجابه اعتمادا على معامل امتصاص المعدن وداله شغله وكانت النماذج تعمل ضمن منطقه تحت الحمراء القريبه.
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Journal of Babylon University/Pure and Applied Sciences/ No.(2)/ Vol.(22): 2014
Introduction:
Infrareddetectorsenteringtheirthirdgenerationofdevelopmentwithgreaterdemandon their performanceandcapabilities,nolongeristhegoalofjustachievinginfrared imagesbutnowit isrequiredtohavegreater performancewithbetteruniformityoverlargerarea,lowercost,and multispectral detection (Binh-Minh Nguyen, DarinHoffman, EdwardKwei-wei Huang, SimeonBogdanov,Pierre-Yves Delaunay,ManijehRazeghi,andMeimeiZ.Tidrow,2009). Photodetectors basedondifferentabsorptionmaterialswereusedforacorrespondingspectral range,forinstance,inthevisiblewavelength regionSi-basedphotodetectorsarepreferred, whileintheultraviolet(UV)wavelengths theIII–Vnitridesarethepromisingmaterials.Itis alsowellknownthattheGaAshassuperior performance fordetectioninthe600–900-nm wavelength range.(Meng-Chyi Wu,Yun-HsunHuangandChong-LongHo, 2007).It was reported thatonthecharacterization ofZnSe-basedSchottkybarrier photodetectors grown on semi-insulatingGaAsby molecular-beamepitaxy, the spectralresponseofthedevicesshowsshort wavelengthresponsivitiesof0.10A/Wanddetectivities as high as 1.4x1012cm Hz1/2W-1 ( MonroyE.,VigueF.,CalleF.,IzpuraJ.I.,Mun˜ozE.andFaurieJ.P., 2000). Schottkyphotodiodeswithindium–tin–oxide (ITO) were fabricated and tested, it wasutilizedfordetectionin theultraviolet spectra (λ400 nm),near-IR spectra (λ~850 nm)andIR spectra (λ~1550nm). Thematerial propertiesofthinITOfilmswere characterized usingresonant-cavity-enhanced(RCE)detectorstructures,improvedefficiency performancewasachieved(NecmiBiyikli and IbrahimKimukin, 2004). It wasreported that thegrowthandcharacterization oftype-IIInAs/GaSbsuperlatticephotodiodesgrownonaGaAssubstrate,thedetector exhibitedadifferentialresistanceatzerobias andaquantum efficiencyof36.4%at77K, providingaspecificdetectivity of 6x1011cm.Hz½.W-1(Binh-Minh Nguyen,DarinHoffman,EdwardKwei-weiHuang,SimeonBogdanov, Pierre-YvesDelaunay, ManijehRazeghiandMeimeiZ.Tidrow, 2009). On the other hand, it wasreported that the growth and characterizationoflongwavelengthinfrared type-II InAs/GaSbsuperlatticephotodiodes,the quantumefficiencyattainstheexpectedvalueof 20%atzerobias,resultinginaJohnson limited detectivityof 1.1x 1011Jones (Abdollahi PourS.,Nguyen B-M., BogdanovS. ,Huang E. K. andM. Razeghi, 2009). I. H. Campbell, 2010) demonstrated that the organic photodiodeswitha transparencyof ~80%throughoutthe visiblespectrumand with up to~80%externalquantumefficiency(EQE)inthe nearinfraredunderreversebias.
Theoretical part:
In Figure (1) n-type semiconductor brought into contact with a metal, Due to the positive work function difference between the metal and semiconductor, electrons are able to lower their energy by moving from the semiconductor into the metal forming a depletion region. Schottky barrier of a specific height will form and makes it difficult to inject electrons into the semiconductor. In opposite direction the semiconductor surface potential is ΦS at zero bias and it changes with the applied bias. Thus, the resistance of the Schottky contact depends on the direction of the current flow. Schottky contacts are difficult to describe mathematically as they involve complicated transport mechanisms like thermionic emission and quantum tunneling. However, in case they are not essential for the device performance, Schottky contacts are often treated in a strongly simplified way, the carrier concentrations at the contact depend on the current densities. (Boer K.W.1990)
Figure(1)Metal-semiconductor(a)beforecontact(b)aftercontact
A photoconductor is a semiconductor device which exhibits a change in conductance (resistance) when photon energy is incident on it. Photon energy incident on the detector produces an electron-hole pair which lowers the detector resistance by producing more carriers; the change in the photoconductor resistance produces a change in the voltage. On the other hand, with photo emissive detectors, the photon energy must be greater than the band gap (Eg) at wavelengths less than (Ronald W. Waynant and Marwood N, 2000):
λ max = hc / Eg …… (1)
λ max isthecriticalwavelength,hisPlankconstant(6.63x10-34 J.s),cisspeedoflightin vacuum(3x1010cm/s). Whentheradiantenergyislessthanthebandgap(Eg)(theband gapenergy fortheGaAsis 1.42)(SzeS.M.1990),thethermionicemissionisdominated because itisgenerated inside the metal only, this region is inside the (near infrared) wavelengths (BurakY.K, 2001).TheSpectral Response(R)isafunctiontothePhotocurrent densityandequalto:(BuddeW, 1983)
R =J ph / PN …… (2)
Jph isPhotocurrentdensity(Ampere/cm2);PN ispowerperunitarea(Watt/cm2). The generationofelectron–holepairsrequiresthe interactionwithother particlesthatcanbe detectedaselectricsignals(Joachim piprek2003)thesignalsisthephotocurrentthenwe calculatedtheNoiseEquivalent Power(NEP)asafunctiontotheSpectralResponse:(Jones R.C., 1954)
NEP=In/R……(3)
Inisthenoisecurrentand{ In=(2qID∆F) ½}ID isDarkcurrent(BurakY.K,2001). The Detectivity(D)is calculatedby equation(4):(BuddeW, 1983)
D = 1 / NEP …… (4)
Thenumber oftheelectron-holegeneratedforeachfallenphotonisknown asquantum efficiency(η)andgiveninthefollowingequation:(SzeS.M., 1990,BuddeW, 1983)
η =Rhc/ λq ……(5)
(λ)isthewavelengthin(nm),(q) istheElementarycharge(1.60218x10-19C)(SzeS.M., 1990). For chargedparticles,ionizationmay occuralongthepathoflightbymanylow-recoil collisionswiththeelectrons.Photonshavefirsttoundergo aninteractionwithatargetelectron orwiththesemiconductor nucleus. Inanycasepartoftheenergy absorbedinthe semiconductor willbeconvertedintoionization(thecreation ofelectron–hole pairs)therest intophonons(latticevibrations),whichmeansfinallyintothermal energy.Thepartofenergy convertedintoelectron–hole paircreationisapropertyofthedetectormaterial.Itisonly weakly dependentonthetypeandenergyoftheradiationexceptatverylowenergiesthatare comparablewiththe bandgap. (GerhardLutz 1999)
Experimental Result and discussion:
Thesampleshavebeenprepared byusing n-typeGaAsasasemiconductor substratewith resistivity (2x10-6 Ohm.cm)andtwotypesofmetalsNi and AlasSchottkycontact andOhmiccontact fordifferentsamples. Firstly, the GaAssubstrates were cleaned byusing NH4OH:H2O(1:2)for2min(GerhardLutz ,1999) andthenbywaterfor5min. Al were employed as Ohmiccontactfortwosamplesand Niforothertwowith(500 nm) thickness. Thesamples were annealed under vacuumpressure(10-4 Torr) and(450K)for30mintoobtain a goodOhmiccontactandsmallcontactresistance (ChenC.P., 1994).AfterthattheSchottkycontact were fabricatedwith NifortwosamplesandAlforother twowith(120nm)thicknessthen they were annealedunder(450K)temperatureand(10-4 Torr)vacuumpressurefor30mintoavoid theinterfacial layereffectontheSchottky barrierheight(SBH)becauseit ispresumedthatoxygen playanimportantroleinforming SchottkybarrierandtheOhmiccontacts (OtsuboM., 2004).Thesamplesare:{Al/GaAs/Al Ohmic, Al/GaAs/NiOhmic),Ni/GaAs/AlOhmic,Ni/GaAs/NiOhmic}. ThePhotocurrentwasmeasuredbyusingtungsten light with wavelengthsrange (400-1100nm). Figure(2)showsthephotocurrent asafunctiontothewavelength,it is noticeably thatthesample(Al/GaAs/AlOhmic)givesthebestresult ofphotocurrent whiletheothersampleshadlessthanthisresultespecially thesampleswith(Ni)asSchottky contact,theabsorptioncoefficientdependsonthematerialandalsoonthewavelengthoflight which isbeingabsorbed.
Figure(2)Photocurrentforthefoursamplesasafunctiontothewavelength
Figure(2) could be divided intothreeregions asafunctiontothewavelength(Raheem G.K.,2007):
- Wavelengths lessthan(800nm): Thephotocurrentincreasedwithwavelengthabsorption ofshortwavelengthshappenedintheregionnearthesurfaceduetopossession ofalarge absorption coefficient,whichmeanslessdepthabsorption, thisphenomenoniscausesa gradualincreaseintheconcentrationofcarriersgeneratedbyrecombination atthesurface areawhichmakestheincreaseinthevalueoftheresponseisasubject tothepossibilityof collectedchargecarriers
- Wavelengthsbetween(800-900nm): Thisregionhasthehighestvalueofthephotocurrent whereitisassumedforthelight absorptionwithin the depletionregionandthis meanshigh efficiencyintheseparationofelectron–holepairs generatedbytheelectricfield and thelackofrecombinationcomparewith1stregion.
- Wavelengthsmorethan(900nm):Wherethereisadecreaseinthevalueofthephoto-currentcanbeinterpretedtothelongwavelength which hadlessabsorption becausethephoton energydoesnothave enoughpowertogenerate theelectron –holepairsthenlowratioof generatedcarriersinthedepletionregion.
Figure(3)showsthephotocurrentasafunctionofthephotonenergy, it could be dividedinto threeregions:(SzeS.M., 1990):
- WhenPhotonenergy is lessthanthesemiconductorbandgapenergy, it could be said thatthe absorptionhappenedinsidethesemiconductorbandgapbecauseoftheenergylevels, this levels created from the semiconductor impurities, then the electron transition becameextrinsic.
Figure(3)Photocurrentforthefoursamplesasafunctiontothephotonenergy
- WhenthePhotonenergy is morethanthesemiconductorbandgapenergy, the differentbetweenthetwoenergiesandthetransitionbecameintrinsic. The absorption increaseswhenthephotonenergyincreasesandthentheabsorptioncoefficients (α) increases asin equation(6)andtheabsorptionhappened atthesurfaceofthe semiconductor(MartinA.green,1989):
α= B*(hν–Eg)……(6) WhenB*is constantequalto (2x104)
- Whenthe Photonenergyis equalto the semiconductorbandgap energythen the absorption is the maximum in this energy (due to equation 1, the maximum wavelengthwe hadis875nm)andthis happenedinwavelengthsbetween(800-900nm).
Fromfigure(4)it could be said thatthemaximumresponsewas(0.157Ampere/Watt)forthesample(Al/GaAs/AlOhmic)in the wavelengthof(800nm), thedetectorswere respondedintheIRregion. Fromfigure(5),thespecific detectivityversusthewavelength was dependedonthewavelengthandthesamplespreparation. The detectivity increased in thesample(Al/GaAs/Al Ohmic) to hit a peak with (63.7x1011Hz½Watt-1) atthewavelength(800nm),thisbecauseitisafunctionofthe responsesensitivityasequations(3,4) showed. TheminimumNEPwas(0.157x10-12(WattHz-½) forthesamesample andwavelength. From figure(7)thequantumefficiencyversuswavelength were showed thehighestresult(24.40%)at(800nm) forthesample(Al/GaAs/Al Ohmic), this could be attributed to itsrelationwiththe response(the responseis the numberofthe generatedelectroninsidethedetectionforthe incidentlight)thus it isa functiontothe responseasequation(5).
Figure(4)Responseforthefoursamplesasafunctionofthe wavelength
Thetables(1, 2)showtheresults wegetfor thefour samples:
Table(1)showstheresultsofthecurrentsperwavelengths(800nm)forthefoursamplesinthiswork
Samples / Iph(Ampere) / In(Ampere) / ID(Ampere)Al-GaAs(AlOhmic) / 157 x10-9 / 2.47 x10-14 / 1.9 x10-9
Ni-GaAs(AlOhmic) / 118 x10-9 / 2.5 x10-14 / 2x10-9
Al-GaAs(NiOhmic) / 141 x10-9 / 2.5 x10-14 / 2x10-9
Ni-GaAs(NiOhmic) / 102 x10-9 / 2.5 x10-14 / 2x10-9
Table(2)showstheDetectorParameterperwavelengths(800nm)forthefoursamplesinthiswork
Samples / R (Ampere/watt) / D(Hz ½ Watt -1) / NEP(Watt Hz -½) / η %Al-GaAs(AlOhmic) / 0.157 / 63.7 x1011 / 0.157 x10-12 / 24.40
Ni-GaAs(AlOhmic) / 0.118 / 47.2 x1011 / 0.212 x10-12 / 18.34
Al-GaAs(NiOhmic) / 0.141 / 56.5 x1011 / 0.177 x10-12 / 21.91
Ni-GaAs(NiOhmic) / 0.102 / 40.8 x1011 / 0.245 x10-12 / 15.90
Conclusions:
1-Photocurrentandresponsedependedonthemetaltypeas absorptioncoefficientsand workfunctionthenthe depletionregionthatthe carriergenerationhappenedinsideit.
2-Theresultdependedon the sourcetype(photonenergy)
3-AneffectofOhmiccontactbetweenthesamples.
4-Themaximumresponseandefficiencythatwegetwasinwavelengthof(800 nm)this meansthedetectorswhichwemadeworks intheIRregion
Figure(5)specificdetectivityforthefoursamplesasafunctiontothewavelength
Figure(6)Noiseequivalentpowerforthefoursamplesasafunctiontothewavelength
Figure(7)Quantumefficiencyforthefoursamplesasafunctiontothewavelength
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