热释电红外探测器.pdf

Voltage-tunable two-colour quantum well infrared detector with Al-graded triangularconfinement barriersThis article has been downloaded from IOPscience.Please scroll down to see the full text article.2001 Semicond.Sci.Technol.16 285(iopscience.iop.org/0268-1242/16/5/301)Download details:IP Address:222.31.38.59The article was downloaded on 15/07/2011 at 02:59Please note that terms and conditions apply.View the table of contents for this issue,or go to the journal homepage for moreHomeSearchCollectionsJournalsAboutContact usMy IOPscienceINSTITUTE OFPHYSICSPUBLISHINGSEMICONDUCTORSCIENCE ANDTECHNOLOGYSemicond.Sci.Technol.16(2001)285288www.iop.org/Journals/ssPII:S0268-1242(01)15750-0Voltage-tunable two-colour quantum wellinfrared detector with Al-gradedtriangular confinement barriersA Guzm an1,J L S anchez-Rojas1,J M G Tijero1,2,J Hernando1,E Calleja1,E Mu noz1,G Vergara3,R Almaz an3,F J S anchez3,M Verd u3and M T Montojo31Departamento de Ingenier a Electr onica,ETSI Telecomunicaci on,Universidad Polit ecnicade Madrid,Ciudad Universitaria s/n,28040 Madrid,Spain2Departamento de F sica Aplicada,Escuela T ecnica Superior de Arquitectura,UniversidadPolit ecnica de Madrid,Avda.Juan de Herrera s/n,28040 Madrid,Spain3Centro de Investigaci on y Desarrollo de la Armada,Arturo Soria 289,28033 Madrid,SpainE-mail:guzmandie.upm.esReceived 24 July 2000,accepted for publication 7 February 2001AbstractWe report on a novel voltage-tunable stacked bound to quasi-continuumquantum well infrared detector designed to work in two of the atmosphericwindows(35 and 812 m).In order to improve the growth process bymolecular beam epitaxy,the abrupt interfaces between different Al-contentlayers were eliminated.A different choice based on Al-graded triangularconfinement barriers was used instead.This device can be voltagecontrolled due to the formation of field domains in the stacked structurewhen an external bias is applied.This results in a voltage drop across thedetectors proportional to their series resistance.Besides,a significantphotovoltaic response was achieved in the 35 m region and attributed toan unintentional asymmetry in the potential profile for the detector operatingin this range.1.IntroductionRecently,devices based on quantum well intersubbandtransitions have attracted much attention because of theiruseful applications.In this regard,some advantages that theGaAs technology provides compared to the state of the artCdHgTe detectors are the well known maturity,the low costand thus high yield,and the flexibility in band tailoring tooperate in different atmospheric windows.One major featureof the quantum well infrared photodetectors(QWIPs)is thepossibility to work in two or more spectral windows by properdesign of the energy bands.Substantial research effort hasbeen devoted to the optimization of these devices due tothe growing demand for multicolour IR detectors for remotetemperature sensing and imaging systems.They are alsohighlydesirablefortemperatureregistration,chemicalanalysisand target discrimination and identification.In addition,rapidprogress in this technology has made it possible to fabricatehigh-performance two-colour focal plane arrays,which havevery practical use in multicolour IR cameras 1.In a previous work,a bound to quasi-continuum devicedesigned to operate in the 812 m atmospheric window wasdescribed and characterized 2.The major advantage of thisdevice was the replacement of the rectangular confinementbarriers with Al-graded triangular ones,thus eliminating theabrupt interfaces between two different-Al-content AlGaAslayers.This leads to a more uniform growth,avoiding theuse of two Al cells or growth interruption.In this paper,theabove-mentioneddeviceisstackedwithaGaAs/AlAs/AlGaAsphotodetector to obtain a two-colour QWIP.The intersubbandtransition of the GaAs/AlAs/AlGaAs quantum well is alsofromboundstatetoquasi-continuumstate.Figure1showstheconductionbandpotentialprofileofthiskindofphotodetector.The use of AlAs inner barriers provides some advantages suchasenhancementoftheenergyandintensityoftheintersubbandtransition or the possibility to design the device to absorb inthe middle-infrared region(from 3 to 6 m).In this case,a transfer matrix method and a finite-element method wereused to estimate the position in energy of the bound andquasi-bound states in the conduction band,as well as thewavefunctions.The photovoltaic behaviour of the detector0268-1242/01/050285+04$30.00 2001 IOP Publishing LtdPrinted in the UK285A Guzm an et alAlAsAl0.3 Ga0.7 AsGaAsAl0.2 Ga0.8 AsFigure 1.Potential profile of the conduction band.The stackedstructure consists of a GaAs/AlAs/AlGaAs detector designed tooperate in the 35 m atmospheric window and a device withtriangular Al-graded confinement barriers tailored to absorb in the812 m window.in the 35 m region is often attributed to a strong asymmetrybetween GaAs/AlAs and AlAs/GaAs heterojunctions due tothe molecular beam epitaxy(MBE)growth process 3.It canbe also attributed to a different dopant diffusion in each ofthe AlAs confinement barriers on either side of the well 4.Any of these effects results in a built-in electric field acrossthe nominally undoped AlGaAs transport region,allowing theelectrons to escape and resulting in a net current flow with nobias applied.The two active regions are separated by a highlySi-doped(2 1018cm3)layer that can be used as a middleohmic contact to develop a three-terminal device.2.Sample detailsThe sample was grown by MBE using an MBE systemequipped with a valved cracker As cell.It consists of twostacks of multiple quantum wells as the active region with ahighly doped contact layer in between the two stacks.Thefirst stack,designed to operate in the range 35 m(MWIR),consists of 25 periods of a 300 Al0.3Ga0.7As barrier anda 55 GaAs Si-doped(2 1018cm3)well sandwichedbetween two 20 AlAs layers.The second stack is designedto operate in the range 812 m(LWIR),and consists of 25periods of a 300 Al0.2Ga0.8As barrier and a 65 GaAs Si-doped(2 1018cm3)well sandwiched between two 30 AlGaAs layers graded from 20 to 23%Al mole fraction.The whole active region is sandwiched between two Si-doped(2 1018cm3)GaAs contacts 5000 thick.The samplewas grown on semi-insulating GaAs(100)substrate using As4as the arsenic species.The growth temperature was 600Ccalibrated to the oxide desorption temperature at 580C.Inaddition,a constant III/V beam equivalent pressure ratio of25 was used.Since the group III element beam equivalentpressure changes for GaAs,AlGaAs and AlAs layers,precisemodifications in the As pressure must be made to obtain thedesired constant flux ratio.The triangular AlGaAs barrierswere obtained by gradually increasing the temperature of theAl cell during growth.Since the growth rate changes with Alcontent,an averaged value for this parameter was calculated,assumingalinearvariationinthetemperaturerangeconsideredhere(5C).Thesamplewasprocessedintophotodetectorswith200 m diameter using standard photolithography and wetchemical mesa etching.A ring metallization using AuGe/Auwasdepositedontopofthemesaandthenalloyedfortheohmiccontacts.2468101214160.00.20.40.60.81.01.21.41.6FTIR300 KAbsorbanceWavelength(m)Figure 2.Absorption spectrum measured at 300 K as a function ofwavelength.Two different peaks centred in each of the atmosphericwindows can be observed.3.ExperimentThe absorption of the device was measured at 300 K using aNicolet 750 Fourier transform infrared spectrometer(FTIR).The sample was processed into a waveguide of 5 mm longand 6 mm wide by polishing 45angle facets to enhancethe absorption 5.The normalization of the spectrumfor background effects was carried out using an identicalwaveguide made of GaAs without quantum wells.Theabsorbance spectrum of the above-described sample can beseen in figure 2.The peak at 4.77 m is due to the 55 wells,while the stronger absorption centred at 12.6 m is thecontribution of the other active region.We attribute the muchnarrower spectral width of the former peak to the presence ofthe two additional thin AlAs barriers.This leads to a spectralwidth intermediate between those of the bound to bound andthe bound to continuum cases.The rapid increase observedin the baseline of the spectrum is attributed to the free carrierabsorption in the doped contacts 6.The responsivity spectra were measured at 25 K on200mdiametermesadetectorsusingaglowbarasthesourceand a monochromator.A tilted substrate holder was usedto fix the sample with an angle of 45with respect to theIR beam.The absolute peak responsivities were calculatedreplacingthedetectorwithacalibratedpyroelectricradiometerand then dividing the photocurrent spectrum of the sampleby the system response obtained from the calibrated detector.Additionalconstantsmustbetakenintoaccounttocompensatethe different areas of the devices(radiometer and QWIP)andalso the reflection coefficient of the GaAs and the angle ofincidence as discussed in detail previously 2.In figure 3 we plot the responsivity of the sample as afunction of the bias voltage.The polarity is defined as positivewhen the higher potential is applied on the top of a mesa.Itcan be seen that at zero bias the device can operate only in the35 m window,and no photoresponse is observed in the 812 m window.This behaviour also occurs for bias voltagesless than 4 V in both forward and reverse bias.When voltageincreases in absolute value up to 6 V,the photoresponse at10 m becomes significant while the peak centred at 4.5 mtendstovanish,leadingtoabias-controlledtunablemechanismof two-colour QWIP.The origin of this effect can be explained286Voltage-tunable two-colour infrared photodetector246810121410.750.50.250 x10V=6 VV=5 VV=4 VV=3 VV=1 VV=0 VV=-1 VV=-3 VV=-4 VV=-5 VV=-6 VResponsivity(A/W)Wavelength(m)Figure 3.Responsivity measurements of the device as a function ofthe bias voltage.Strong photovoltaic behaviour can be observed at4.5 m.Selective photoresponse in each band is also possible.by considering the different Al contents of the confinementbarriers in each of the active regions.In the structure designedto absorb in the 35 m window,a higher Al mole fraction isused in both the separation and inner barriers(30 and 100%).This provides a more efficient confinement of the carriers,which results in a lower escape probability and dark currentwhen compared with the other structure.The lower the darkcurrent,the lower the conductance of the detector,so thevoltage drop in each device is inversely proportional to thatparameter 7.Therefore,only for bias voltages in absolutevalue greater than 4 V,the voltage drop across the 812 mdevice is high enough to allow the carriers to be swept outof the well.In a previous publication 2,we attributed theabsence of photovoltaic(0 V)response in the 812 m regionto a good uniformity in the potential profile of this activeregion as a result of the improved growth procedure.With thatsymmetric design,no intrinsic electric fields in the conductionband profile are expected,and,hence,photocurrent signalat 0 V(photovoltaic)should not be collected.In this case,because of the different conductance of the two structures,the1012 mstackisinalow-fielddomainevenforbiasvoltagesashighas4V.Thesymmetricincreaseinthesignalforpositiveand negative bias,and the absence of photocurrent in that low-field domain range,can be attributed to a symmetric potentialprofile on both sides of the wells,as mentioned above.In addition,significant photovoltaic response is observedat 4.5 m.This can be of great interest due to thesubstantial reduction in the noise signal which arises fromoperating the detector at zero bias.In this case,theasymmetric behaviour in the potential profile was attributed tounintentionalasymmetriesintheepitaxialstructures,probablyduetodifferentsegregationofSidopantineachAlAsbarrieroneithersideofthewell8.ItcanbealsoattributedtoadifferentoxygenincorporationintheGaAs/AlGaAsandAlGaAs/GaAsheterojunctions,resulting in an unintentional built-in electric-10-8-6-4-2024681010-1 110-1010-910-810-710-610-5Resistance(ohm)25KVoltage(V)Current(A)1051061071081091010101 1differentialresistanceFigure 4.IV characteristic and differential resistance measured atlow temperature(25 K).Field domain formation and photovoltaicbehaviour can be observed.field across the nominally undoped Al0.3Ga0.7As transportregion 3.The effect of this internal built-in electric fieldon the transport properties of the quantum well detectors hasbeenreportedin9.Byapplyinganexternalbiasitispossibleto compensate the existing built-in field.Under this condition,a photocurrent signal should not be observed.In our case,wecompensate the internal field when 1.3 V bias is applied.Atthis voltage the photosignal vanishes as expected.Theformationoffielddomains,aswellasthephotovoltaicbehaviour,can also be observed in the currentvoltage(IV)characteristic of the detector.Figure 4 shows thelow-temperature(25 K)IV and the differential resistancecurves for the two-colour QWIP measured using an HP4145parameter analyser.From this figure,four different featuresin the resistance can be identified at 6,4,4 and 6 V.Therefore,based on these curves we can see that,when thetwo structures are combined to form a two-colour detector,most of the voltage drop will be across the 35 m QWIPstack at low bias(|V|4 V).The voltage drop across theother stack becomes significant when the bias is greater than4 V in absolute value.At|V|=6 V the photosignal of the 35mQWIPvanishes,andthatofthe812mQWIPreachesamaximumvalue.Thephotovoltaicbehaviourofthedevicecanalso be seen from the IV characteristic,where a significantphotogenerated current at 0 V can be appreciated.It is notedthat the curve reaches its minimum current value at 1.4 V.Atthis voltage,the internal built-in field is compensated and thenthe photocurrent signal vanishes.This bias voltage is in goodagreement with that estimated from the photocurrent spectraof 1.3 V.4.ConclusionsAnoveltwo-colourQWIPstructurewasgrownandcharacterized.ThegrowthprocesswasimprovedbyusingthintriangularAl-gradedbarriers,whichavoidgrowthinterruption,and the use of two Al cells.The samples obtained wereprocessed into detectors and then absorption and responsivitymeasurements were performed onto them.At 300 K,thedevice shows two well resolved absorption peaks centredat 4.77 and 12.6 m.The different spectral width of thepeaks is attributed to the efficiency in the confinement of287A Guzm an et althe carriers due to the different Al mole fractions of theconfinement barriers.In addition,photocurrent measurementswere performed at 25 K in order to estimate the responsivityof these detectors.Selective photoresponse depending onthe bias voltage is observed and explained as a consequenceof the different resistance of each of the independent activeregions.Finally,significant photoresponse at zero bias wasmeasuredinthe35mregion(photovoltaic).Thisbehaviouris mainly attributed to asymmetries in the potential profileunintentionally introduced in the growth process,and can beof interest for low-noise app