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ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTDark I-V curves.Figure 4.5.36Local ideality factor.Figure 4.5.37Some example outputs, using the default parametersettings, follow:Figure 4.5.36 shows that a reasonable diodecharacteristic is shown, with good turn-on andrectification.Figure 4.5.37 shows the increase in the idealityfactor with current is as expected with adiode transiting from domination by radiativerecombination (n=1) to being dominated byShockley Read Hall recombination (n=2). But thelarge regime in which n=3 and then region whereit goes to much higher values, are indicative ofa non-physical aspect to the model. The mostlikely explanation is that rather than two diodesin series there are actually several, probably avariable number in different parts of the device, thecombination of which lead to a composite idealityfactor.The demonstrator is useful as an iterative tool usedto simulate the real measured data from a device.The parameters used for such a fit then define thevalues of the circuit elements in the EC model.Attachment of physical meaning to these elementswill then require further development and interfacewith the EMA/quantum mechanical model.4.5.2.5 Doping in Si QD nanostructuresFabrication of a PV device from the Si QD materialsrequires a control of work function such as to allowseparation of photogenerated electron-hole pairs.Methods by which this can be achieved includefabrication of a grown or diffused p–n junctionor p–i–n junction with the Si QD multilayers asthe i-region. These require careful control ofthe work functions of the p and n-regions. P-njunction devices have been fabricated and dopingdemonstrated. Progress on the theory explainingthese doping effects using conventional dopantsand the possibility of other modulation dopingapproaches has been made.As discussed in Section 4.5.2.4, rectifying p-n andp-i-n structures can be fabricated with ‘conventional’doping by incorporation of B and P duringsputtering growth [4.5.9, 4.5.10]. Also, formation ofp- and n-type materials is clearly demonstrated inthe Si QD nanostructure materials with Si nitrideinterlayers doped with either P or B discussed inSection 4.5.2.3.2; in the Sb doped n-type Si QDs inSi 3N 4in Section 4.5.2.3.1; in the p-type Ge QD inSiO 2material in section 4.5.2.3.4; and in MOS typedevices doped with either P or B by a diffusionanneal discussed in [4.5.43].However, the doping mechanisms taking placein these structures are not well understood.Theoretical work has shown that direct doping ofthe QDs is prevented by segregation of impurityatoms from the perfectly crystalline QDs [4.5.44,4.5.23]. Experimentally this is supported by dataon the free electron density in Si nanocrystalsusing Electron Paramagnetic Resonance (EPR),which show that 95% of P atoms are segregatedto the surface of the nanocrystals and that theircontribution to doping is at least an order ofmagnitude lower than the atomic concentration[4.5.45]. Further evidence that doping causeselectronic changes comes from the quenching ofluminescence on the incorporation of P (or Au) inSi nanostructures [4.5.46] and also from a smallenhancement of luminescence observed at lowP doping levels followed by quenching at higherlevels [4.5.47]. This latter being explained by thepassivation of QD surface states by low levels of Pincreasing luminescence and then the saturation ofthis mechanism by excess P. This also explains thenon-monotonic behaviour of activation energy seenin P doped nanostructures [4.5.10].If direct doping of the QDs is not occurring,then another possibility is modulation dopingof the matrix SiO 2, as is commonly used in III-Vnanostructures; but this is ruled out because ofthe very high ionisation energies of about 5eV thatwould be required for the resultant deep defectsin a dielectric matrix [4.5.48]. These clearly willnot occur at room temperature, or even at highertemperatures.78
ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTStoichiometricdielectric, 2-5nm a) b)Two models for doping of Si QD nanostructures. (a)An extended sub-oxide region surrounding the Si QDsallows doping by P or B at relatively shallow levels. Thesethen can provide carriers to be captured by the QDsthrough a modulation doping mechanism. (b) QD sizeenhancement (suppression) by P (B) doping results in atype I heterojunction between P and B doped material.This facilitates carrier separation due to the much greatermobility of electrons – the Dember effect [4.5.49].Figure 4.5.404.5.2.5.1 Doping mechanisms inSi QD nanostructuresResearchers:Dawei Di, Xiaoming Hao, Ivan Perez‐Wurfl,Gavin ConibeerAs there is clear evidence for doping by P and B,a third possibility is that it is the sub-oxide QD/matrix interface region, with its shallower dopingdefect levels, that is doped by the P or B atoms, thusproviding free carriers to be captured by the QDs[4.5.49]. As shown in Fig. 4.5.38, ab-initio calculationof the levels associated with B atoms associateddirectly with the interface region on the surface ofa Si QD indicate that there are no shallow or deeplevels introduced by the B within the effective bandgap (HOMO-LUMO gap). The reason is that thestrong affinity of B for O results in very strong B-Obonds, effectively splitting any available OMOs andUMOs a long way apart and well inside the alreadyexisting density of states.However, this approach considers the interfacebetween Si QD and SiO 2matrix to be very abrupt.In fact there is highly likely to be an extendedtransition region of a sub-oxide around the QD. Ifthis region is a sufficiently extended silicon suboxide(SiO x) it could provide flat Bloch bands ableto be doped in a pseudo bulk-like regime. In orderthat B and P doping levels are shallow enough tobe ionised in such a region, x would need to be lessthan about 0.5. Such a region would be amorphousand hence not able to be modelled with ab-initiomethods. But the presence of such regions can bedetermined from absorption measurements.There is some support for this theory in theabsorption data for Si QDs in SiO 2[4.5.5] shown inFig. 4.5.39. This shows a region of strong absorptionwith an absorption edge which blue shifts asthe O to Si ratio increases, which translates to anincrease in QD size. But there is also an additionalweakly absorbing tail which extends well into theeffective band gap region. This tail is likely to bedue to an amorphous region around the Si QDs. Anexplanation for the band gap of this being largerthan bulk Si is if this region consists of sub-oxidematerial. If this region were doped with B or P, itcould provide free carriers to be captured by theQDs in a modulation doping mechanism, thusgiving rise to the p- or n-type behaviour observed.(This is illustrated schematically in Fig. 4.5.40 (a).)An alternative explanation for the rectifyingcharacter of the junctions obtained canbe based on the modification to the Si QDcrystalisation discussed in Section 4.5.2.3.2,for P and B doping of Si QDs in oxide withnitride interlayers [4.5.49]. The fact that P (B)doped material produces larger (smaller)QDs means that its confined energy levelsand hence effective band gap will be smaller(larger) than undoped material underotherwise similar conditions. Thus a junctionbetween a P and a B doped region willactually be a type-I heterojunction betweensmall and large band gap materials,respectively, as illustrated in Fig. 4.5.40 (b).It is also very likely that the mobility of electronsin these materials, whilst not big, will still be muchgreater than that of holes. Hence photogeneratedelectrons near the junction would experience adrift field sweeping them into the P doped materialwith its lower conduction band edge, whilst holeswith their very limited mobility would be immobile.This would result in electrons accumulating in the PAbsorption (10 6 cm -1 )252015105Ab-initio calculation usingGAUSSIAN: (a) the density of statesfor a Si 83/84OH 62oxide terminated QDwith and without a B atom at theinterface; (b) the optimised Si QD,consisting of Si 83B-OH 62, with the Batom at top centre (in pink).Figure 4.5.3805.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5Energy (eV)Room temperature absorptionco-efficient of annealed SiO x/SiO 2multilayer films with variousx (1100 o C, 1hour). The inset isan estimate of the approximateoptical band gap (filled squares:strong absorption; empty circles:absorption tail) [4.5.5].Figure 4.5.39Wavelength (nm)300 400 500 600 700 800Energy (eV)4.84.44.03.62.42.22.01.8m=2m=1/20.8 0.9 1.0 1.1 1.2 1.3O/Si ratioSiO 0.86/SiO 2SiO 1.00/SiO 2SiO 1.30/SiO 279
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