Simulation results for the design and fabrication of an electrically driven solid state modulator are presented. The design criteria has identified various trade-offs in the manufacturing of an electrically driven modulator using a p-i-n diode to be made from high purity Ge. The issues relating to the doping, layer thickness and contact material for the diode fabrication are discussed. A compromise between the high 'ON' state transmission and uniformity is required to achieve the optimum performance from the device. Using FEMLAB and ATLAS a p-i-n diode with different apertures has been simulated which clearly show the effects of non-uniformity and the requirement of a mesh-type electrode for a uniform absorption across the large device apertures

Rutt, Harvey N.

Uppal, Suresh

Southampton (e-Prints Soton)

2nd EMRS DTC Technical Conference – Edinburgh 2005 B11 An Electrically Driven Solid State ModulatorSuresh Uppal, Harvey N. RuttOptoelectronics Research Center, University of SouthamptonSouthampton, SO17 1BJAbstractSimulation results for the design and fabrication of an electrically driven solid statemodulator are presented. The design criteria has identified various trade-offs in themanufacturing of an electrically driven modulator using a p-i-n diode to be made from highpurity Ge. The issues relating to the doping, layer thickness and contact material for thediode fabrication are discussed. A compromise between the high ‘ON’ state transmission anduniformity is required to achieve the optimum performance from the device. Using FEMLABand ATLAS a p-i-n diode with different apertures has been simulated which clearly show theeffects of non-uniformity and the requirement of a mesh-type electrode for a uniformabsorption across the large device apertures.Keywords: infrared, p-i-n diode, solid-state modulator, High purity GeIntroductionInfrared imagers using pyroelectricdetectors require optical modulation whichis currently achieved using a rotatingchopper blade. This, however, introducesmechanical components in the systemmaking it bulky at the same time affectingthe image quality during camera motion.An electrically driven modulator howevercan overcome these problems. Such amodulator can also be used for built incalibration or ‘thermal referencing’, and forproviding variable IR transmission insituations where mechanical solutions areundesirable.A solid state IR modulator should becapable of using the entire 8-14 micronoptical band, with high “ON” and low“OFF”   state transmission over a largeaperture (typically 1cm2). In addition itshould be polarisation insensitive, compact,and capable of incorporation into low F-number systems along with low powerconsumption. Acousto-optic, electro-opticand reflection modulators using plasmafrequency effects in semiconductors andeven LCD devices have been investigatedas methods of modulating IR beams butcannot fulfil all the requirements. Using anovel method of introducing excess carriersin suitably prepared Ge, adequatemodulation has been achieved in the 8-14micron [1] region. Based on this method anoptical modulator has been successfullyfabricated [2]. However, the opticalmodulator uses expensive laser diodes andwastes a lot of laser power. Non-uniformillumination, high power dissipation andnon-availability of an antireflection coatingin both near and mid IR range are otherissues with optical based modulator. A lowpower electrical modulator, however, willuse applied bias to inject carriers avoidinginefficiencies of laser sources.Material requirementTo make an electrically driven solid statemodulator for the IR range we shall requirea semiconductor material which is IRtransparent in the 8-14 micron range,indirect band gap with strong sub-bandtransitions in the IR range, and has highcarrier mobility and long carrier life times.The requirement for inter sub-band2nd EMRS DTC Technical Conference – Edinburgh 2005 B11 transitions arises because classical freecarrier Drude-Zener absorption is too weakfor a practical device. The availability ofsuitable dopants/coatings and ease offabrication are additional constraints.Germanium possesses the properties ofbeing an indirect band gap material with IRtransparency and interband transitions inthe required spectral region of 8-14microns. It is readily available in highpurity for use in nuclear detectors andpossesses long carrier lifetimes and highcarrier mobilities.Figure 1 shows the band diagram of Ge andthe absorption spectra of intrinsic and p-type Ge. The transitions between the lightand heavy hole band in the valence band ofGe are the desired interband transitions inthe mid IR.Fig 1: The band diagram of Ge (left)showing a) heavy hole to spin orbit b) lighthole to spin orbit and c) heavy hole to lighthole transitions. The correspondingabsorption spectra for intrinsic (top) and p-type Ge is shown on the right.Design CriteriaA simple p-i-n diode based modulator isshown in Fig.2.Fig 2: A simple p-i-n diode modulatorThe modulator optically ON (ElectricallyOFF) state transmission is given by      Ton= T0 exp -(Np*xp*σh),where T0=1 for 100% transmission, σh isthe hole absorption cross-section(5.33x10-16cm2), Np is the doping densityfor holes (/cm3) of the p-region, and xp isthe p-region thickness. In the optically OFF(electrically ON) state the absorption occursin the p as well as i-regions and hence thetransmission is given by Toff= T0 exp -(Np*xp*σh) exp (- np*σh),where np is the injected hole density (/cm2)and can be given by np=0∫rcp(x)dx, where cpis the hole concentration in the i-region forthe Forward Biased (electrically ON) diodeand r its thickness.For a high optically ON state transmissionwe shall require the product of Np and xp tobe the low. On the other hand for a lowoptically OFF state transmission we shallrequire maximum np (absorption from thep-layer is fixed). In this state, for a uniformcurrent injection we also need a lowresistance ‘p’ layer which requires a highdoping density.Plotting Ton for zero np, optically ON, as afunction of doping density of p layer givesus the acceptable limit for the doping of thep-layer as shown in Fig.3. This graphsuggests that for a 95% transmission in theON state we shall require a ‘p’ area dopingdensity of ~1x1014 /cm2.Fig 3: Ton Vs doping density of p-layerUsing the theory of a p-i-n diode, the holearea carrier density in the intrinsic region isgiven by Ta*J/q where Ta is the ambipolarlife time of carriers, J is the forward currentWavelength (µm)20.010.06.75.04.03.32.92.52.22.01.81.7Transmission (%)01020304050heavy hole - spin orbitlight hole -spin orbitheavy hole -light holemulti phonon bandtransitionsInput IRradiationp           i                nElectrical contacts2dModulatedIR radiation10131014101510160102030405060708090100p-type doping area density (cm-2)Ton (on state transimission, %)2nd EMRS DTC Technical Conference – Edinburgh 2005 B11 density in A/cm2 and q is the electriccharge. Thus a high carrier life time isessential. From the theory, we can alsopredict the optimum range for the thicknessof the i-layer. The voltage drop as afunction of normalised intrinsic regionlength is shown in Fig. 4. A long i region(2d>La, La is the ambipolar diffusionlength) means a large voltage drop in the i-region, a short i-region (2d<La) meansrecombination occurs in the heavily dopedend regions. Therefore the thickness of theintrinsic region is selected such that 2d~La.Fig 4: Voltage drop against the i-regionlength decides that optimum length of the Iregion is where 2d~La.Device simulation resultsThe simulations for the p-i-n diodestructure have been performed usingFEMLAB as well as ATLAS, a devicesimulation software from SILVACO [3]. AFEMLAB simulation structure is shown inFig.4.Fig 4: The FEMLAB simulation structureFor this device, Ge material parameterswere taken. A Gaussian shaped diffusionprofile was assumed for both p and n-typedopants. The model was solved for variousvoltages. A cross-section of the holeconcentration at a depth of 0.5 mm isshown in Fig. 5.It can be seen from the hole concentrationprofile that a non-uniformity exists acrossthe diode depth as the device size increases.This non-uniformity becomes worse for a2.5mm and larger devices. Since the holeconcentration appears as exponential in theexpression for transmission, even a smallhole variation across the depth leads a veryhigh value of non-uniformity in thetransmission from the structure.Fig 5: Hole concentration Vs diode widthfor a diode of 1mm aperture.Using ATLAS, similar results wereobtained. However, in ATLAS, the mobilitywas also made a function of theconcentration. Using same parameters asused in FEMLAB simulations, butmodifying the simulation structure toincorporate additional electrode help reducethe non-uniformity problem.The above analysis demonstrates the needfor a highly optimised electrode mesh toavoid non-uniformities in the transmissionwithin the device structure, for large deviceapertures.Process simulation resultsAttention has been paid to recognising thepotential p and n-type dopants, thermalprocessing cycles and contact material for0.1 0.5 1 5 1010-310-210-1100101Normalised intrinsic region length (2d/La)Drop across intrinsic region, Vm (V)0 0.5 1 1.5 2 2.51.522.533.544.5x 1016Diode width (mm)Hole concentration (/cm3 )0.7 V1 mm aperture 2.5 mm aperture 2nd EMRS DTC Technical Conference – Edinburgh 2005 B11 fabricating the p-i-n device using HPGe.One of the main constraints for deviceprocessing is to keep the material awayfrom contamination to maintain a highcarrier lifetime of the high purity material.This requires careful handling of materialand least processing steps to avoidcontamination. Cu, Fe etc are the mainimpurities to avoid since they have a largediffusion coefficient even at roomtemperature and they introduce deep levelsin the band gap thereby drastically reducingthe carrier lifetimes. This means that weshould avoid unnecessary high temperaturesteps wherever possible.Since ion-implantation is a roomtemperature method to introduce dopantsand we can control the amount of dopant, itseems the best choice for doping. Also ion-implantation is regularly used in presentday Si technology and hence is readilyavailable. As noted above, an area dopingdensity (Np*xp) of ~1e14/cm2 for high Tonmeans that for a volume doping density(Np) of ~1e17 /cm3, one needs xp of theorder of 10 microns. A higher value of Npwould degrade the carrier mobility. Amongthe p-type dopants Boron seems to be goodchoice since it is a light element andtherefore can be implanted to large depthsat sensible implantation energies. Atheoretical 300 keV Boron implantationprofile is shown in Fig.7.Fig.7: Theoretical as-implanted (B@300keV with 1e14 /cm2) and diffused(700C, 60mins) profile. The diffused profileshows no movement as a result of the verylow diffusion coefficient.Also, Boron is known to self-activate in Geafter implantation, thus avoiding the needfor high temperature electrical activationstep. However, it has very slow diffusioncoefficient as compared to other p-typedopants [4] hence may require variable highenergy implantation in the MeV range.Other p-type dopants such as Ga and In arealso being considered and require furtherinvestigation.The diffusion coefficients for n-typedopants in Ge are much higher than p-typedopants. Usually activation annealing isalso required after implantation. Fig. 7shows that even moderate temperature butsmall time annealing is effective inachieving good electrical activation anddiffusion. Thus a low energy, high doseimplantation can be used to obtain requireddoping profiles for n-type dopants in Ge.Fig 8: Spreading resistance profiles for n-type dopants in Ge after Rapid ThermalAnnealing. Good electrical activation canbe achieved with brief annealing times atmoderate temperatures (from Ref. 5).It is very important to obtain low resistanceohmic contacts to the p-i-n device. To avoida rectifying contact it is essential to choosea metal that has higher work function thanp-type semiconductor and lower that n-typesemiconductor surface. It has beensuggested that amorphous Ge can be usedas contact material. However, for ourapplication there are no major advantagesof using amorphous Ge layer.  This process0 0.2 0.4 0.6 0.8 1 1.2 1.4x 10-410151016101710181019B implant 300 keVDepth (cm)B conc (/cm3 )as-implanteddiffused2nd EMRS DTC Technical Conference – Edinburgh 2005 B11 adds an extra step in diode fabrication.Ideally, direct soldering to Ge should bechosen if possible as this avoids extra (hightemperature) processing steps e.g. metalevaporation etc. Using the same ‘type’dopant for electrode formation as used inbulk is likely to cause a heavy same typedoping in the very near region of electrodethereby increasing the potential barrier tothe minority carrier. A doped solder say Sbdoped for n-type and In or Ga doped for p-type may be used. The literature alsosuggest metals like Ni, Cr, and Ti beingused for contact electrodes but one has tobe extra careful so as to not perform anyhigh temperature steps after their depositionas these metals diffuse quite fast and killcarrier lifetime by introducing deep levelsin the band gap.Future WorkIn the coming months the simulation willcontinue to predict device behavior with afocus on 3-d modeling. Process simulationis being carried out to predict precisedopant distribution after thermalprocessing. Suitable dopants/ layerthicknesses shall be available from thesimulations and will be incorporated in theprocess simulation. It is planned to fabricatefirst a small area p-i-n diode with suitableohmic contacts and characterize it formodulator performance.References1 Rutt, HN, Manning PA, Donohue, PP,1996, International Patent,PCT/GB96/022212 Fairley, PD, 2000, PhD thesis,University of Southampton3 ATLAS user manual, ver. 5.8.4.R,Silvaco International, Santa Clara, USA4 Uppal S, Willoughby, AFW, Bonar,JM, Cowern, NEB, Grasby, T, Morris,RJH; Dowsett, MG, 1996, J. Appl.Phys., 96, 1376-13805 Chui, CO, Gopalakrishnan, K, Griffin,PB, Plummer, JD, Saraswat, KC, 2003,J. Appl. Phys., 83, 3275-3277AcknowledgementsThe work reported in this paper was fundedby the Electro-Magnetic Remote Sensing(EMRS) Defence Technology Centre,established by the UK Ministry of Defence.

An electrically driven solid state modulator

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