Complete Report - University of New South Wales

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