Complete Report - University of New South Wales

ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTIIA IIIA IB IIB IIIB IVB VB VIBBe B C N OMg Al Si P SCa Sc Cu Zn Ga Ge As SeSr Y Ag Cd In Sn Sb TeBa La Au Hg Tl Pb Bi PoErUse of the periodic table to analyse possible analogue compounds of InNbased on atomic mass combination and electro-negativity.Figure 4.5.65h=hole). This requires a reasonable DOS atall energies above E g, or at least only verynarrow gaps between energy levels, ≤kT 300K. This in turn requires a good overlapof wavefunction for carriers through thematerial, i.e. poor electronic confinementor a reasonable conductor. This condition ismet in all inorganic semiconducotrs quiteeasily, with e-e scattering rates of less than100fs for reasonable carrier concentrations.It may not be met in organicsemiconductors or in nanostructures withlarge barrier heights.7. Good carrier transport in order to allowtransport of hot carriers to the contacts.This is similar to 6, except that it onlyneed be in the direction of the contacts,probably the z growth direction. Areasonably low resistance is probably good.8. It should be possible to make good quality,highly ordered, low defect material.Preferably it should be easy and cheap todo this.9. Earth abundant and readily available andprocessable constituent elements andprocesses.10. No or low toxicity of elements, compoundsand processes.[(E LO, E O(min)& E LAare the maximum optical,minimum optical and maximum acousticphonon energies respectively.]InN has most of these properties, except 4, 8 &9, and is therefore a good model material for ahot carrier cell absorber.Analogues of InNAs InN is a model material, but has theproblems of abundance and bad materialquality, another approach is to use analoguesof InN to attempt to emulate its near idealproperties. These analogues can be II-IV-nitridecompounds, large mass anion III-Vs, group IVcompounds/alloys or nanostructures.II-IV-Vs: ZnSnN; ZnPbN; HgSnN; HgPbNWith reference to Fig 4.5.65, it can be seen thatreplacement of In on the III sub-lattice withII-IV compounds is analogous and is now quitewidely being investigated in the Cu 2ZnSnS 4analogue to CuInS 2[4.5.72].ZnGeN can be fabricated [4.5.73] and is mostdirectly analogous with Si and GaAs. However,its band gap is large at 1.9eV. It also has a smallcalculated phononic band gap [4.5.74]. ZnSnNhas a smaller electronic gap (1 eV) and largercalculated phononic gap [4.5.74]. It is howeverdifficult to fabricate, and also its phononic gapis not as large as the acoustic phonon energymaking it difficult to block Klemens decaycompletely. HgSnN or HgPbN should bothhave smaller E gand larger phononic gaps.These materials have not yet been fabricated[4.5.75].Large mass cation:The Bi and Sb compounds have largepredicted phononic gaps and Bi is a relativelyabundant material, with only low toxicity[4.5.75]. BiB has the largest phononic gapbut AlBi, Bi 2S 5, Bi 2O 3(bismuthine) are alsoattractive. Similarly SbB has a large predictedphononic gap. That for AlSb is the same sizeas the acoustic phonon energy and its bandgap is 1.5eV, making it marginal as an absorbermaterial and similar to InP.Group IIIA III-VsLaN and YN both have large phononic gapswhilst that for ScN is too small.The Lanthanides can also form III-Vs. ErN andother RE nitrides can be grown by MBE. Thephononic band gaps of the Er compounds arepredicted to be large, because of the heavyEr cation, but its discrete energy levels makeit not useful as an absorber, although thecombination of properties in a nanostructurecould be advantageous.Group IV alloy/compounds:All of the combinations Si/Sn, Ge/C or Sn/Clook attractive with large gaps predicted in1D models. However being all group IVs theyonly form weak compounds. UnfortunatelySiC, whether 3C, 4H or 6H, has too narrowa phononic gap. Nonetheless GeC doesform a compound and is of significantinterest [4.5.76].There are also several other inherentadvantages of group IV compounds/alloys allof which are associated with the four valenceelectrons of the group IVs which result inpredominantly covalent bonding:a) The elements form completely covalentlybonded crystals primarily in a diamondstructure (tetragonal is also possible as inβSn). However for group IV compoundsthe decreasing electronegativity down thegroup results in partially ionic bonding.This is not strong in SiC and whilst it tendsto give co-ordination numbers of 4, cannonetheless result in several allotropes ofdecreasing symmetry: 4c, 4h, 6h. However,as the difference in period increases for theas yet theoretical group IV compounds, sotoo does the difference in electronegativityand hence also the bond ionicity and thedegree of order. For a hot carrier absorberthis is ideal because it is just such a largedifference in the period which is neededto give the large mass difference andhence large phononic gaps. All of GeC,SnSi, SnC (and the Pb compounds) havecomputed phononic gaps large enoughto block Klemens decay, and should alsotend to form ordered diamond structurecompounds.b) Because of their covalent bonding, thegroup IV elements have relatively smallelectronic band gaps as compared to theirmore ionic III-V and much more ionic II-VIanalogues in the same period: e.g. Sn0.15eV, InSb 0.4eV, CdTe 1.5eV. In fact toachieve approximately the same electronicband gap one must go down one periodfrom group IV to III-V and down anotherperiod from III-V to II-VI: e.g. Si 1.1eV, GaAs1.45eV, CdTe 1.5eV. This means that forgroup IV compounds there is greater scopefor large mass difference compounds92