Multiple bandgap solar cells based on confined Ge nanocrystals in an oxide matrix

Si and Ge nanocrystals are supposed 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]. We propose the design sketched in Fig. 1 using a stack of well separated, mono-dispersed Ge nanoclusters of different size which allows bandgap tuning for light absorption from the blue to the infrared region.
In this contribution investigations on Ge nanocrystals formation in GeOx-SiO2 multilayer structures will be presented. 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. The goal is to achieve well confined, graded, equally sized and dense nano¬crystal superlattices only by the variation of the layer thicknesses and the GeOx composition. The GeOx-SiO2 stacks 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].
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 with a size of 2 nm. XANES synchrotron measurements (fig. 3) show that the phase separation of GeOx~1 is already finished at 400°C. The crystallisation occurs at 550°C which was monitored by GIXRD and Raman scattering. The Ge nanocrystal size was determined to be 2, 3 or 6 nm for different samples by GIXRD, TEM and Raman. Ellipsometry and absorbance measurements showed additional information’s about the structural and optical parameters of the multilayers. The stochiometry of the stack was always controlled with RBS.
Ge nanoclusters of 2 … 6 nm in size could be formed by phase separation at a temperature of about 400°C and crystallised at 550°C into nanocrystals which allows a broad variety of substrate materials. The very thin and smooth SiO2 separation layer enable interesting possibilities for charge transport via tunnelling.
[1] G. Conibeer et al. TSF 511-512, 654 (2006)
[2] Y. M. Niquet et al. APL 77, 1182 (2000)
[3] F. Dimroth et al. MRS Bulletin 32,230 (2007)
[4] M. Zschintzsch et al. JAP, in print (2010)