Complete Report - University of New South Wales

ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTPeak at 1.205eV of the conductance vs. energy plot fordifferent values of σ for configurational disorder (left).Conductance vs. energy plot with peak at 1.573 eV fordifferent values of σ for morphological disorder (right).Conductance given in units of 2e 2 /h.Figure 4.5.48Planar view of the double barrierstructure formed by Si QDs in SiO 2matrix, with SiC barriers of 2nmwidth (left). Mean diameter of QDsis 1.8nm. I-V characteristics for thedouble barrier structure with SiO 2and SiC barriers at temperature 10Kand 300K (right).Figure 4.5.494.5.3.2 Energy Selective ContactsThe requirement for a narrow range of contactenergies can be met by an energy selectivecontact (ESC) based on double barrier resonanttunnelling. Tunnelling to the confined energy levelsin a quantum dot layer embedded between twodielectric barrier layers, can give a conductancesharply peaked at the line up of the Fermi level onthe ‘hot’ absorber side of the contact with the QDconfined energy level. Conductance both belowthis energy and above it should be very significantlylower. This is the basis of the current work ondouble barrier resonant tunnelling ESCs.4.5.3.2.1 Modelling of QD structuresResearchersBinesh Puthen-Veettil, Dirk König,Gavin ConibeerWe developed a robust 2 dimensional modelfor describing the transport properties throughquantum dot structures and have used this modelto understand the filtering characteristics of EnergySelective Contacts (ESCs). In this way we are ableto compute the effective filtering in 2 dimensionsby running numerical simulations. The model isdeveloped from a discretized Schrödinger equationby considering the sample volume as a collectionof discrete points and using an effective massapproximation method over the entire volume.During fabrication of the quantum dots in adielectric matrix for selective energy contacts,different kinds of irregularities can be presentin the structure, the major disorders beingconfigurational (disorders in the position of thedots) and morphological (disorders in the size ofthe dots). The extent to which configurational andmorphological disorders determine the electricalproperties of the overall structure is investigatedusing simulation runs of the model. The disordersare assumed to follow a normal distribution fromthe mean position and size. The results show theoutcome of an average of 1000 simulation runs withdifferent standard deviation (σ) values.Figure 4.5.48 shows the simulation results forresonance in 2.6nm Si dot in SiO 2matrix underdifferent orders of configurational disorders. Asthe disorders increases from σ =0 to σ =1 theconductance decreases by 53%, but the resonantenergy remains the same at 1.205eV. This showsthe confined energy in the QDs does not changeas their size is fixed but the effective filteringreduces dramatically. Figure 4.5.49 shows thesimulation results for a 2.2 nm Si dot in a SiO 2matrix under different orders of morphologicaldisorders. As the disorder increases from σ =0 toσ = 1, the conductance decreases by 60% and theresonant peak remains the same at 1.573eV. Butthe morphological disorders cause major impactcompared to configurational disorders because ofthe widening of the energy selection window. Thisis due to the distribution in size of the QDs, sinceQDs with different sizes have different resonantenergies which are slightly different from the meanresonant energy, the average of them all increasethe spread of the resonant peak thus reducing theefficiency of the double barrier structure as energyselective contacts.A planar representation of the double barrierstructure is shown as in Fig. 4.5.49 (left). The barriersare usually high band gap dielectric materials likeSiO 2, Si 3N 4or SiC. SiC barriers have advantagesover SiO 2barriers that SiC barriers in double barrierstructure makes a very good diffusion barrierfor silicon during processing, which can yield a84