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Germanium Photodetector Technologies for Optical Communication Applications

Semiconductor Technologiesismatch of-42% between the two heterostructure materials This enables a high qualityGe epitaxy film withheading dislocsections of this chapter deal with the state-of-the-art Ge photodetector technologies Wegin with the discussion on the designs of evanescent-coupled Ge p-i-n photodetectoraccountable for the leakage generin such devicedependthe applelucidated Factors limiting the detector speed performance andso discussedThis chapter also aims to discuss the demonstration of Schottky barrier engineered Geechanism responsible for the generation of high leakage current in such a detectordealt with Novel concepts adopted to address thisthrough Schottky barriermodulation are presented The approaches are based on bandgap engineering as well asFermi level de-pinning by segregating valenceding adsorbateetal/germanium interface, The recent technological breakthrough in employing all Group-(APD) is presented next The fabricprocess and the design of GeSi APD featuringon ciude the chapter with a summary providing the readers with the core discussed Wethe performance metrics of the various Ge-based photodetector scheme2 Hetero-Epitaxy of Germanium on SilicoThe key chalgh quality germanium(Ge)epitaxy growth onSi) rests withhatch between the two heterostructure materialstch strain has been shown to give rise to two majorthreading dislocations and (2) rough surface morphology due to 3D Stranski-Krastanov(SKromise the efficiency of a photodetector Strategies proposedature tome these challenges vary to a large extent

Oneintuitiveproach is to grow a silicon-germanium(SiGe)layer by compositionally grading its Gencentration up to 100% Using low-energy plasma enhanced chemical vapour depositionOh et al(2002)showed that for every 10% increase in the Ge mole fraction, a lineawhichintegrationIn an effort to further reduce the thickness of these active layers, huang et al(2004osed another approach based on the optimization of two thin SiGe buffer layers witharying Ge concentration In such approach, a 0 6um thick Sio4sGeo ss buffer was first grownnd then followed by an intermediate Si3 Geo 6s buffer with a thickness of 04um An in-situdensity before the growth5um thick Ge epilayof400°CThrough this approach,ws the threading dislocations to be trapped at the hetero-ntechopen

Germanium Photodetector Technologies for Optical Communication Applications0LateralWHMTime(ps6 Impulse responses of the VPD and LPD detectors melength of1550nm A smaller FWHM pulse width of -244achieved in a vpd asLPD, which corresponds to a-3db bandwidth3GHZdetector's bandwidth can be further evidenced by the eye patterns mDCA Figs that higphoto-detection upbit-rate85Gb/s can be achieved by the vPD detector The clean eye patterns clearly illustrate thelow noise property of the detector

Higher speed measurements are possible through furtheraling of the detector geometry to reduce the device capacitance5 Gb/s625Gb/s425Gb85 Gb/spatterns(PRBS 2-1ts of the vpd at a bias of -10v The detectordemonstrated high sensitivity and low-noise photodetection up to a bit-rate of 85Gb/s Thelow noise property of the detector can be clearly illustrated by the clean eye patternsntechopen

Semiconductor TechnologiesFig 8(a) shows the total device capacitance measured as a function of the applied reverseoltages for different detector geometry Obviously, elongating the detector lengththe capacitance due to a larger effective detectorMoreover, in thance drops drasticallynearly plateaus off at high voltage regime This is attributed to the widening of the depllayer width as the applied bias is raised Further increase in the bias across the alternatingn+ junctions would lead to a total depletion oThe theoretical modelling results of the RC-time constant and thetime bandwidthsignificantly, but it leads to a degraded RC-time bandwidth Tmitation, one could scale the detector length to achieve lower capacitance for bandwidthL255rR aL=75uma L=50um0004060

810121,416pplied voltage V, (v)p+ to n+ devarious detector lengths (b) Downscaling of detector length results in bandwidthenhancement due to a reduced device capacitance35 Impact of Band-Traps-Band Tunneling on Dark Current Generationn order to gain insight into the leakage mechanism for the Ge detectors,the gy analysis of the darkIDark was performed (Ang et al, 2009) In this analysis,modelled using the following functional formwhere t denotes theesponsible for the leakage generation Fig, 9(a) plots the bias dependence of darkGe p-i-n photodetector measured at increasing temperature range from 303Kntechopen

Germanium Photodetector Technologies for Optical Communication Applicationsbe observed from this figure, temperature has a significaimpact on IDark Increahe operating temperature and the applied bias are foundlog plot of I9(b) A straight line fitting to this plot yields a gradient which corresponds to the activationenergy Ea At a fixed reversed bias of -0 5V, the extracted Eg is observed to be nearly half ofhe Ge bandgap energy Ex, which eltes that the dark current mechanism is dominatedr1952)ThislI unexpected as the large lattictch between theheterostructure materials could result in a Ge epitaxial film with high threading dislocationdensity, The existence of such defects has been shown to affect the effective carrier lifetiman increase in the leakage current, as discussed earlier in Section 32strength, as illustrated in Fig 10(a) Increasing the field intensitythe depletion regionshown to reg Eu responsible for the leakage genxponential increase in the dark current trend The mechanism responsiblefor this is that electric field enlarges the band-bending which leads to an enhanced electronselling frgradation

For instance, anthe electric field strength from 17kv/cm to 25kv/cm enhances the darkfrom 0 27uA to 044uA, showing more than 60% IDork degradation(b)373KTemperature: 296-373KE-037evlkT(ev)g 9(a) Plot of dark current characteristics as a function of applied bias for a Ge p-i-notodetector with increasing temperature range from 303 K to 373 K(b)An extraction ofthe activation energy for leakage generation as a function of applied biasntechopen

SemiconductorGe Bandgap Energy -066\045040903%035030Temperature: 296 K0,2000020Electric Field E (kv/Depletion Width W (um)Er for leakahe applied electric field, giving rise to a decreasing Ea trenintensiA reduced Eg at high field regime leadincreased dark current generation (b)Plotlark current dependence on depletion width Wo of a Ge p-i-n photodetector Scaling Wpds to significantly higher darkSuch band-traps-band tunneling effect is observed to demonstrate a strong dependencethe depletion width WD which separates the p+ and n+ metallurgical junctions

In thisf the ge detector is kfrom 06-18um To avoid a difference in the contact area due to a variation of intrinsic Gedth, the metal geometry is also altered such that the total metal contactcomparable for all designs Note that a reduced Wp is often desirable from the perspectiveof enhancing the detector's bandwidth performance Fig 10(bnificant dark current degrawhe dark current density by -29%, which is furtheraggravated to-90% when Wo reaches 0, 6umThe underlying mechanism responsible for such phenomenon can be explained using theFig 11 When operated in the high field regime, enlarged banddetector with wide Wp [Fig 11(b)) It is also noteworthy to highlight that the dark currentdensity begins to plateau for WD>13um, whiches that the influence of band-trapsband tunnelling on the leakage generation becomes relatively less piThis finding suggests that a design trade-off needs to be considered in the course of scalingWD for enabling bandwid th enhancement as it would lead to a more pronounced darkcurrent degradativentechopen

Germanium Photodetector Technologies for Optical Communication Applicationsa)Enhanced btE-FieldE-FieldNarrow l中Wide Wobending resultsng of Ge bandhiches electrons and holes tunnellingct centres Such phenomenon is observed to become increasinglyprominent for devices with(a)WD as compared to that with(b) wide WDHowever, high dark current issue experienced in these detectors imposes much ntegralo"citance and easefothe achievement of poor signal-to-noise (SNR) ratio This drawback would be furtheraggravated when a narrow bandgap material such as Ge is employed, where high darkcurrent is predominantly attributed to the low hole Schottky barrier height as a result ofpinning near the valence band edgrimental demonstrationshowed that Ge msm photodetector withgrated soI ribexhibited highdark cuchievoet aL, 2007)Such darkt level is way too high to be acceptable for high speed receiveresign which typically tolerates a leakage current below 1OuAaims to deal with this problem through the application ofuppress the leakage current in Ge MSM photodetector

The concepts are based uporSchottky barrier modulatugh bandgap engineeringell as Fermi level de-gby segregating valence mending adsorbate at the metal/germanium interface4on using Large Bandgap Materialbandgap material for Schottky barrier modulation has been widelypression in Ge MsM photodetector Oh et al(2004)reported the fabrication of metal-Ge-metal photodetector featuring thin amorphous -Ge layerndwiched between the metal and germanium interface to increase the Schottky barrierreduction of dark currentorders of magnitude was achieved Laih et al (1998), on the other hand, adoptedamorphous-Si layer in a U-grooved metal-semiconductor-metal photodetector to enablentechopen

Semiconductor Technologiesdark current suppression by more than three orders of magnitude In this work, a notculating the Schottky barrierheight in a Ge MSM photodetector with an integratedbegins with an 8-inch silicon-on-insulator(SOl) substrate with(100)urface orientation The SoI substrate features a silicon body thickness of -250 nm andburied oxide thickness of -1 um Si micro-waveguide was first formed by using anisotropicdry etching to achieve straigdewall profile forw propagation loss Afterthe gewere then patterned bycleaned with standard piranha solution (ie a mixture of sulfuric acid(H SO4)withmoval Thewith standard SCl (NH, OH: H O2: H2O) and then subjected to a HF-last wet cleaning foror deposition(UHVCVD) system ThebakingN2 ambient at80o°Cforoxide removal and followed by the deposition of a-5 nin Si buffer at 530C A thin SiGe buffer layer was then deposited to have a gradualtransition from pure Si to pure Ge at the hetero-interface

A Ge seed layer with a thickness ofthen grown using low temperature at 370 C before the growth of a -30Onmeased temperature, Precursor gasesdisilane sighs andSiGeGe lavers The defects dhin the Ge epilrevealed a uniform distribution oflual tensile strain in the as-grown Ge film on Sisubstrate, which was attribuo the difference in the thermal expansion coefficienbetween Ge and Si during coolingMetalbarrier and the Ge epitaxialfilm(b) SEMed Ge-on-SOI MSMdetectoguide The Ge detector features an effective device width w anlength L of 2 6um and 52um, respectively The metal contacts spacing S is-lunntechopen

Germanium Photodetector Technologies for Optical Communication Applicationsm如After contact hen crystalline silicon-carbon (Si: C) epilayer of -18nm wasusing disilane(Si2H6) and dilutedmethylsilane(SiH3 CH3) precursor gases Stptimum Si: C thickness was chosen basedthe considerations for actingopress leakage current whilehieving low defects density at the heterojunction Chlorine (Cl) precursor gaslective epitaxial growth The mole fractionted in the si c film was measured to be -1% basedMeanwhile, the totaobtained from SIMS analysis was found to beual to -1 %, which means that around 03% of carbon was incorporated in the interstitialtes Despite a substantial latticetch, the si: Cbi-dimensional and appears tobe of good crystalline quality, as confirmed by the fast Fourier transform (FFT)diffracmple Metallization consisting of TaN/Al(250A/6000A)nd patterneddelete the db) shows themicroscopy(SEM) image of the evanescent coupled Ge-on-SOI MSM photodetecteintegrated seguide The detector features an effective deW of 2

6 um and 52 um, respectively The spacing S between the metal electrodes of thephotodetector was lithographically defined to be -1 umGE師h-01evSi WaveguidFig 13, Cross-sectional schematic of Ge photodetector featuring metal-semiconductor-metalfiguration Strong Fepinning resultshole Schottky barrier heightwhich forms the root cause for the generation of high leakage currentFig 13 depicts the cross-sectional schematic of a MSM configured Ge photodetectore lowering effect, the Ge detection region between the metalelectrodes will bedepleted under high applied bias The total dark current /rotalflowing through the photodetector can then be described by the following expressionbbh/kTTotal"Jpwhere Jp On)is the heected from the anode(cathode), and Ap "(An")the Richardson s constant for hole (electron) Both the hole current and electron current arntechopen

Germanium Photodetector Technologies for Optical Communication Applicationsinterfaces This enables a significant reduction in the dislocation density of the as-grown Geimproving the detectors dark current performancen yet another approach, Colace et al(1999)proposed a direct hetero-epitaxy growth of Gen Si through theinsertion of such thin bufferds theof 3D SK growth, and allows the misfitdislocations to be concentrated at the hetero-interfaces However such approach r

equires acyclic annealing process to be carried out at both high and low temperature(900C/780C)thermal annealing approach, Lial(1999)had alsostratedimprovement in both the surface roughness and the dislocation density When combinedh the selective area growth,erage threading dislocation density as low as 23x106as achieved

However, the needs for a high temperature post-epitaxy Ge anneal withng cycle time present a majIn this work, selective epitaxial growth of Ge on silicon-on-insulator (son) was performing an ultra-highchemicalntional approaches, a thin pseudo-graded SiGe buffer with a thickness of -20nm isproposed in this study to relieve the large lattice mismatch stress between theheterostructure materials (Fig 1) The Ge mole fraction within the SiGe buffer iscompositionally graded from 10% to-50% The precursor gases used for the SiGe grovmprise of diluted germane(GeHa) and pure disilane(Si2H6) A relatively thin Ge semperature of370°Cuse of a low temperature growth is intended to suppress adatoms migration on Si andus prevents the formation of 3D SK growth, which allows a flat Ge surface morphology tiUpon obtaining a smooth Ge seed layer, the epitaxy prodthen increased toC to facilitate faster epitaxy growth to obtain the desired Gethickness Using this approach, high quality Ge epilayer with a thickness of up to-2um hasbeen demonstrated, along with the achient of threading dislocation density as10 cm 2 without undergoing any high temperature cyclical thermal annealing stepDOnmGeOnmFig 1(a)Schematic view of the layer stack for the direct hetero-epitaxy grow(b) High resolution TEM micrograph showing the effectiveness of a pseudo-graded SiGeuffer in reducing the threading dislocation density within the Ge epilayntechopen

Semiconductor Technologiesa)bFieldRMS -028 nmOxid2(a)Scanning electron microscopy (SEM) image showing the achievof excellentGe epitaxy rowth and selectivity on SOl substrate(b) Excellent Ge surfaceachieved, as determined using atomic force microscopy (AFMhasdeposition and etch back approach In each deposition cycle, the ge growth time is carefully廣avoid exceeding the incubation time needed for Ge seeds to nucleate on thefilm After every Ge deposition cycle, a short etch back process using chlorine(Clgas will then be introduced to remove possible Ge nucleation sites on this allows a highly selective Ge epitaxy process to be developed, along wit3 High Performance Germanium p-i-n PhotodetectorDue to its poor absorption coefficient as inherited by the large bandgap energy, silicon(Si)has been known to be prohibitive for the realization of photodetector that is capable ofming efficient optical detections at wavelengths commonly used in optical fibersmaller bandgap energy such as germanium(Ge) to provide favorable optical absorptionoperty at these wavelengths

Recent research progressthe photodetectortechnology development has clearly shown that Ge is attracting growing interest as theal due tobsorption coefficient2002) In additioCMOS fabrication technmakes it an attracterial to enable the demonstratioe near-infrared photodetector(Soref, 2006)However, the long absorption length in Ge at 155udifficult to meetthe high quantum efficiency requirement for a surface illuminated photodetector Despitety of merely 0 2A/wconstraint requires the growth of a thick Ge epilayenable full absorption at thisngth Ulately, hetero-epitaxy of Ge with such thicknesstegration challenge such as high threading dislocation densities that would lead tocreased leakage currentthiiver sensitivity, An alternative approarelax this requirement makes use ofntechopen

Germanium Photodetector Technologies for Optical Communication Applicationsleveraging on the detector length, one would be able to achieve enhanced photo-responsivity improvementperformance of the photodetector can also be simultaneously optimized by tweaking thethickness to retransit time dIn this section, the different designs of waveguidGe photodetector featuring pn configuration are discussed The performance mch as dark current, responsivit3 1 Ge-on-soI Photodetector Designs and Fabricationled Ge-on-soI photodetector designPhotodetectors featuring vertical p-i-nt(VPD) and lateral p-i-n(LPD) configurations wereh thesic region thickness (ti-ce)co-defined by the Ge thickness (tce)and theimplant region [ Fig 3(a) The width W and length L of this VPD designis 8um and 100um, respectively For the LPD design, both the p+ and nformed in the gethe intrinsic regiolineddefining the spacing of these alternating contacts [ Fig 3(b)

Note that thelength L of this LPD design is 20um and 100um, respectivelyVertical p-i-n PhotodetGNDSi WaveguideSi WaveguideSi wGSi wp+ Sip+ SiFig 3 (a)SEM micrograph showing the design ofded gehotodetector featuring vertical p-il-lI configuration, (b)Ge photodetector design with antechopen

Semiconductor Technologiesment for thick Ge epilthe optical absorption efficiency As a result of the difference inthe refractidex between si and ge thewaveguide will be up-coupled into the Ge absorbing layer to allow optical signal to beencoded into its elevalent efficiently The insertion of a thick buried oxide(BOX)fabricatiguide integrated Ge photodetector begins with theOI substrate with a starting overlying Si thickness of -220nm and a buried oxide(BOX)thickness of -2um Channel waveguide with a nano-taper featuring a width of -200nm wafirst formed by anisotropic dry etching to obtain a smooth sidewall profile for enabling lowdone to form the Si anodes in a VPD detector A moderately high p-tyrcarefully chosen for the anode formation towhile not impact the quality of the as-grown Ge epitaxy film High dose p+ contaas subsequently performed and dopants are activated using rapid thermal1030 C for 5s to obtain good Si ohmic contacts

After depositing a 600A thick field oxide asadopted to preserve the top Si surface quality from possible damage by thechemical vapor deposition(UHVCVD) epitaxy reactor The selective Ge epitaxy processommenced with the deposition of a low temperature pseudo-graded silicon-germaniumby a Ge seed layto the achievement of low defects level within the Ge film, the high temperature post-pitaxy Ge anneal typically used for defects annihilaipped to reduce the overallthermal budget, High dosctive phosphorous and boron implants were then performeontacts, respectively After the deposition of inter-layer dielectric (ILD), contact andfabrication Fig, 3(a) and 3(bshow the top-view scanning electron microscopy (SEM) images of the VPD and LPDaffecting the shot noise(ls) in a photodetector according tethe following expressionemental charge, B the bandwidth, ID the dark current of the detector,iation Under a carefully controlled situatieerefore be neglected However, thermal generation andling current due to strong electric field give rise to considerable darkthe shot noise and thus affects the signal-tontechopen

Germanium Photodetector Technologies for Optical Communication Applicationsertical PInLateral Pin pd元=1550Applied Voltage V, (V)Fig 4 Theharacteristics of the vpd and lpd detectors measured undervoltage characteristics of the vpd and lpd detectors under darkthe detectors, showing a forward-to-reverse current ratio of -4 orders of magnitude Fcplied bias of-10V, the dark current (lder)in a VPD was measured to be -050

7nA/um2), which is below the typical 1, Oua generally considered to be the upper limfor hiin a LPdnowed a much higheralue of-3 8uA (or -19nA/um2)er to better understangthe factors which affect the dark current density (Dar), let us review the expression thatmiconductor diodDarkwhere g denotes the elemental charge, n; the intrinsic carriwidth, and tor the effective carrier lifetime Clearly, an increase in the depletion layer widthon the dark current perfd possiblyslain the higher Idork experienced in a LPD detector In addition, it is also important to notethat Iark exhibits a strong dependence on the effective carrier lifentrolledboth the lifetime associateh the Shockley-Read-Hall recombination (TsRH)and therrier drift time across the space charge region(tanin)as follosrH driftntechopen

Semiconductor TechnologiesSRHand e the electric field strength It is obvious that the reverse darkdefects density within the Ge epilayer andepitaxy quality would be important to reduce the leakage ciin the applied reverse bias has also resulted in an aggravated dark current degradation,which elucidates that Idark has a strong dependence on the electric field strength A furthersis on this phenomenon will be covered in a later discussion33 Responsivity Characteristicsresponsivity (R)of a photodetector can be described using the following expression9R-IPhoto/ Popt"nq/huenotes the photocurrent, Powt the incident optical power, n the quantumarge, h the Planck constant, and u the frephotons which are absorbed in germanium generate electron-hole pairs which willbe collected as photocurrent under applied electric field This photocurrent is linearldependent on the incident optical power before saturation is reached Moreover, alike theantum effiethe responsivity of the detector shouldavelength dependenHence, the responsivity of a detector will be significantly higher at wavelength where thephoton energy enables electron-hole pair generation through direct transitiare the responsivity perfoce between the vpd and lpd550nm into the soguide

The typical optical propagation lossSOISi waveguide and the indled throughdirectly into the Si nano-taper For an incident light power of -300uw, opticalmeasurements showed that both the vpd and lpd detectors achieved aphotocurrent level at high applied biases beyond -10V, Fig 5 compares the responsivity oftagesteresting to note that the verticPin detector demonstrated a lower responsivity as compared to the lateral Pin detector forbiases below -o5 v thisd possibly be due to an enhancedrecombinationprocess at the high density of defect centres near the Ge-Si heterojunction This is setent valueHth an increased electrostatic potential across the depletion layer, thephoto-generated carrierthe space charge region with enhancbility before they can recombine at these recombination centresntechopen

Germanium Photodetector Technologies for Optical Communication ApplicationsFor an applied bias larger than -10 V, a comparable responsivity was measured for both thertical and lateral Pin detectors Despite that the metallurgical junction is separated byerely 0 8um, a lateral Pin detector showed a high absolute responsivity of -09 A/w, Thessible mechanisms accountable for such highlowing reasons Firstly, under high reversed bias, the intrinsic Ge region (ie

between andneath the metallurgical junction) was simulated to be totally depleted, as confirmed usingMEDICI device simulator When photon is absorbed to produce electron and hole pairs, thecollectedby the electrode as photocurrent Secondly, optical simulation shows that more than 80% ofin the SOi waveguide is absorbed within the first 25 um of theng on the long absorption length design, neincidencephotonsntribute to the achievement of high responsivityLateral pin detectorVertical PIn DetectorApplied Voltage V, (Vity as a function of applied voltages for both the VPD and LPD detectorseasured at a wavelength of 1550nm3 4 Impulse Response CharacteristicsThe impulse response of a photodetector is limited by both the carrier transit time (rnd the RC time constant (Rc) which can be modelled using the following expressionssat1enotes theturation velocity d thewidth, and rc theresistances and capacitances associated with the detector and its peripheral circuitryntechopen

Semiconductor TechnologiesAs described in these equations, the factors governing the fundamental response time limdrift time across the space charge region, and(2)thedevice junction capacitance Drift of carriers is influenced by the electric field applied acrosethe space chargeexpressed usingwhere u denotes the carrierand E the electric field Clearly, increthe electricvelocity is reached Sze(1981on the order of 107 cm/sthe higher carrty in ge asared to thatof Si makes it a material of choice to enable the realiRC time constant The junction capacitance(Ci) which arises from the ionized donors(Np)nd acceptors(NA) is expresse2Nhere e denotes the material permittivity, A the cross-sectional area of the detector, and wthe depletion layer width Intuitively, rethe deviceidth are both beneficial to reduce the junction capacitance However, adopting theformer approach could lead to ain the responsivity perfofor optical absorption is decreased

The latter approach in enlarging thethe space charge region Therefore, an optimization of the Retime constant and the carrier transit time will be crucial in determining the overabandwidth performance of the detector, as dictated using the following expressionBdB(11)the factors affecting the speed performance of the VPD and LPDpulse response measurements were performedlength ofmeasurements Both the detectorscharacterizede probes and theere captured with a high speed sampling oscillFig 6 shows thata vpd detector achieved a smaller futh-at-half-maximum(FWHM) pwidth of24 4ps as compared to that of LPd detector with a slightly larger FWHM of -28 9ps Thiseduces the carrier transit timeused as a metric to gauge the speed performance of the detectors By performing a fast3dB bandwidth of 113 and -101 GHhieved in the vpd and lpd detectors, resntechopen