Confined Ge nanocrystals in an oxide matrix for multiple bandgap solar cells

Confined Ge nanocrystals in an oxide matrix for multiple bandgap solar cells

Zschintzsch, M.; Jeutter, N.; von Borany, J.; Mücklich, A.; Sahle, C.

Si and Ge nanocrystals are believed to be a promising material for high efficient 3rd generation multiple bandgap thin film solar cells. Photoluminescence studies of the quantum confinement effect in Si and Ge nanocrystals showed the feasibility of this approach [1, 2].
Our design is sketched in Fig. 1 which uses a stack of well separated, mono-dispersed Ge nanoclusters of different size on an absorbing Ge layer which allows bandgap tuning for light absorption from the blue to the infrared region. In this conference contribution investigations on Ge nanocrystals formation in GeOx-SiO2 multilayer structures will be reported. The goal is to achieve well separated, equally sized and dense nanocrystal superlattices only by the variation of the layer thicknesses and the GeOx composition. The GeOx-SiO2 multilayers were deposited via reactive DC magnetron sputtering. A process window for the oxygen partial pressure in the O2/Ar sputtering gas mixture allows both, SiO2 formation for the separation layers as well as GeOx films with tuneable stoichiometry in the range of x = 0.2 - 2 [4]. The films stoichiometry were always controlled with RBS. The nanocrystals growth during annealing is vertically limited by the SiO2 separation layer while the phase separation of the GeOx layer during annealing leads to laterally GeO2 separated Ge nanocrystals.
Very smooth interfaces of the multilayers with roughness’s below 1 nm and a SiO2 separation layer thickness < 2 nm could be revealed with XRR and TEM – fig. 2 shows an annealed sample with Ge nanocrystals of 3 nm size. XANES synchrotron measurements (fig. 3) show that the phase separation of GeOx~1 -> Ge + GeO2 is already completed at 400°C. GIXRD and Raman scattering confirme a crystallisation temperature of about 550°C. Ge nanocrystals with well defined sizes between 2 and 10 nm have be fabricated by the variation of the GeOx sublayer thickness. As derived from RAMAN measure¬ments, the degree of crystallinity decreases with reduced layer thickness. The energy dependent optical parameters were determined by means of photoluminescence, ellipsometry and absorbance mea¬surements. The very thin and smooth SiO2 separation layer enables interesting possibilities for charge transport via direct tunnelling.
[1] G. Conibeer et al. Thin Solid Films 511-512, 654 (2006)
[2] Y. M. Niquet et al. Applied Physics Letters 77, 1182 (2000)
[3] F. Dimroth et al. Materials Research Society Bulletin 32, 230 (2007)
[4] M. Zschintzsch et al. Journal Applied Physics 107, 0343061-8 (2010

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