Complete Report - University of New South Wales

250 – 500 nm 500 – 550 nm 550 – 600 nmARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORT600 – 650 nm 650 – 700 nm 700 – 750 nmPrinciple of oa-IV, shown for electron injection into the sample(left). If free carriers obtain additional energy by a massivephoton flux significantly above the band gap, a smaller bias fieldis required to reach a resonant condition. Sample stage of oa-IV(right); wavelength range indicated. Every 50 nm spectral rangehas a photon flux of approximately 50 Suns.Figure 4.5.50structure with uniform QD size in the transportdirection thus ensuring sharp resonances at theresonant energies of the structure. Since theconductance of the structure is more in the case ofSiC barriers because of the lower barrier height ofSiC, thicker barriers can be used to give the sameconductance as SiO 2, for which the very thin barriersare difficult to fabricate.Figure 4.5.49 (right) shows the I-V characteristics forthe double barrier QD structure with SiO 2dielectricand SiC barriers at temperature 10K and 300K.As can be seen, the structures with SiC potentialbarriers have higher current density at both lowtemperature and at room temperature. As thetemperature increases, the current density as well asthe width of the resonant peak increases. Also, thenegative differential resistances appear at slightlylower voltage in structures with SiC barrier thanthat in structures with SiO 2barrier. This is due to theincreased leakage and thus the lower confinementenergy for the SiC.4.5.3.2.2 Characterization of EnergySelective Contacts using ‘Opticallyassisted I-V’Researchers:Dirk König, Stephan Michard (RWTH Aachen,Germany), Binesh Puthen-VeettilCollaboration with:Daniel Hiller (IMTEK, University ofFreiburg, Germany)Optically assisted IV (oa-IV) is a new characterisationmethod which investigates an energy selectivecontact (ESC) fabricated on a Hot Carrier Absorbermaterial with optical excitations in a pre-definedwave length range. If a very large optical generationrate G optexists adjacent to the ESC, it can probe thisHC population immediately next to the contact.Alternatively, if the HC population within theabsorber is known, the ESC can be tested for itsenergy selectivity. The basic principle is to providefree carriers with additional energyby optical means, see Fig. 4.5.50. Theresonant level of the ESC can thenbe reached by carriers at a lower biasfield, shifting the tunnelling resonanceof the ESC – here a QD array – to lowerbias voltages V bias.Samples based on Si were providedby the University of Freiburg, seeFig. 4.5.51. Si is very far from beingan ideal Hot Carrier Absorber due tothe complete lack of a gap betweenoptical and acoustic phonon modesand hence no propensity to slowcarrier cooling. Furthermore, Sihas an indirect band gap in thespectral range of optical excitationwhich renders the interpretationof measured data rather complexdue to the absorption coefficient a Sichanging over two orders of magnitude. However,for initial proof of concept it is adequate providingthe excitation of hot electron-hole pairs in thematerial is sufficiently large and collection can bemade from very close to the ESC.A Xenon (Xe) arc lamp is a continuous light sourcewith one of the most constant photon fluxes F hninthe spectral range 1000 to 500 nm. Nevertheless,there are significant deviations in particular in thelong wavelength region (> 800 nm). We thereforecompare oa-IV curves which have the same V biasfor the current transition from forward to reversedirection, V bias(j → 0), indicating a constant carrierproduct (constant quasi-Fermi level positions) atthe ESC. Under these conditions, the current densityas a function of quasi-Fermi level position can beassumed to be identical for these curves and thiscurrent density can be treated as a constant offsetfor each V biasvalue.For liquid nitrogen (LN 2, T = 77 K) measurements,resonant tunnelling behaviour (negative differentialresistance – NDR) was detected for all spectralranges. With the method mentioned above, onlythe oa-IV curves from 800 – 750 nm and 650 –Sample structure for oa-IV (upper).Contact area on QD array is 1mm 2 , with ≈ 10 10 QDs. Opticalgeneration rate G optwithinballistic transport rangeto the ESC and its relativechange for photon wavelength intervals used inoa-IV (middle). For these, theabsorption coefficient of c-Sia Siand the photon flux of theXe arc lamp F hnwere used(lower). Full green rectanglesshow range of measurementswhich can be compared.Dashed green rectangles showmeasurements which cannotbe compared, but approachthe high optical injection limit.Please note breaks on leftordinate in both graphs.Figure 4.5.5185