Complete Report - University of New South Wales

ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTSome images of the structure as grown and modelled.(Left) the near-perfect stacking of the structure isshown. (Centre) in-plane InAs QD arrangement.(Right) simple hexagonal superlattice structureshowing stacking. The QDs are similar to flatteneddisks with in plane dimensions of 4050 nm andheights of 7 nm.Figure 4.5.57Carrier relaxation curves for InN.The photoexcited carrier densityis 1.5 x 10 19 cm -3 . The blue dashedcurve is a single exponential fit.Figure 4.5.55Schematic of the simple hexagonalsuperlattice of InAs QD disks,showing their relative dimensions.The red square indicatesorthorhombic repeat unit for thesimple hexagonal system.Figure 4.5.56case, and k Bis the Boltzmann constant.T Cis the fitted parameter and representsthe hot carrier temperature. Figure 4.5.55shows the carrier temperature transient,which seems to follow an exponentialbehaviour quite well (blue line – fit).The fitting of the calculated temperaturedata has been performed using asingle exponential.Here τ THrepresents the carriers thermalisationtime constant, whereas C and K are two constantparameters. The fitted value for τ THis 7 ps. Thislong cooling constant can be attributed to the hotphonon effect due to the long lifetime of the A1(LO)phonon [4.5.64,4.5.66]. This hot carrier relaxationvelocity is still too high to achieve a considerableefficiency gain in an InN hot carrier solar cell.However, it has been demonstrated that, for InN,the carrier cooling velocity is strictly related to thequality of the material and slower carrier coolingconstants compared to the ones calculated in thischapter have been reported in the literature [4.5.67].4.5.3.3.3 Nanostructures for absorbersNanostructures offer the possibility of modificationof the phonon dispersion of a composite material.III-V compounds or indeed most of the cubic andhexagonal compounds can be considered as veryfine nanostructures consisting of ‘quantum dots’of only one atom (say In) in a matrix (say N) withonly one atom separating each ‘QD’ and arrangedin two interpenetrating fcc lattices. Modelling ofthe 1D phonon dispersion in this way gives a closeagreement with the phonon dispersion for zincblendeInN extracted from real measured data forwurtzite material.Similar ‘phonon band gaps’ should appear ingood quality nanostructure superlattices, throughcoherent Bragg reflection of modes such that gapsin the superlattice dispersion open up [4.5.61]. Thereis a close analogy with photonic structures in whichmodulation of the refractive index in a periodicsystem opens up gaps of disallowed photonenergies. Here modulation of the ease with whichphonons are transmitted (the acoustic impedance)opens up gaps of disallowed phonon energies.4.5.3.3.3.1 Force constant modelling ofIII-V QD materials by SK growthResearchers:R. Patterson, Y. Kamikawa, G. ConibeerCollaboration with:Yoshitaka Okada – University of Tokyo3D force constant modelling, using the reasonableassumption of simple harmonic motion of atomsin a matrix around their rest or lowest energyposition, reveals such phononic gaps [4.5.68].The model calculates longitudinal and transversemodes and can be used to calculate dispersions ina variety of symmetry directions and for differentcombinations of QD sub-lattice structure and superlatticestructure.III-V Stranski-Krastinov grown QD arrays of InAs inInGaAs and InGaAlAs matrices are fabricated at theUniversity of Tokyo using MBE. We are investigatingthese for evidence of phonon dispersionmodulation and potential slowed carrier cooling.In order to understand the expected phonondispersions these are being modeeled using the 3Dforce constant technique.Lattice matched and strain compensated materialpairs that may produce large phonon bandgaps areof interest. Previous iterations in the design of thesestructures indicated the importance of separating“light” and “heavy” atoms to different parts of thenanostructure. Initially, the lightest atom in thesystem, As, was present in both the QD and thematrix. This meant that the reduced mass of bothregions (proportional to the sum of the inverses88