INDUSTRY CPV
Figure 7. Transmission electron microscopy image of the 2 µm-thick InGaAs layer grown by MOCVD on Translucent virtual-germanium template
Figure 6. Growth of 5 µm of germanium ‘on-silicon’ does not exceed a wafer bow of 30 µm, allowing this meet the specification for silicon lines
1–2 percent indium concentration (see Figure 7). Secondary ion mass spectrometry analysis confirms the stability of the virtual germanium/silicon substrate, and the absence of diffusion into III-V material. Simple, trial solar cell devices have been produced with this platform, and they exhibit acceptable fill factors and open-circuit voltages (see figure 8).
When these vertically integrated devices are formed that involve direct growth on silicon, the germanium layer acts as one of the junctions, while the silicon wafer is used as the bottom contact layer. In this type of device, p-type doping must be added during the epitaxial process. This is possible: Initial trial runs show no adverse interactions between boron, germanium and tin, and template wafers have been formed with doping levels of 5 x 1017 3 x 1018
cm-3 cm-3
in the structure. Goals for the future
Our challenging journey has focused on designing a 1eV layer into a sub cell structure that is silicon-
References 1. M. Lebby et. al. Compound Semiconductor July 2012, p 37 2. S. Ringel et. al. MRS Proceedings 836 “III-V multijunction materials and solar cells on engineered SiGe/Si substrates (2004) 3. R. Hinige et. al. Semiconductor Science and Technology 21 775 (2006) 4. S. Thomas et. al. IEEE Electron Device Letters 26 428 (2005) 5. NREL reference (
http://sunlab.site.uottawa.ca/pdf/whitepapers/ HiEfficMjSc-CurrStatus&FuturePotential.pdf
6. PRN Newswire, 15th October 2012 “Solar Junction Breaks it’s own world record” 7. R. Soref et. al.
Mat.Res.Soc.Symp. Bol 958 (2007) 0958-L01-08 “Advances in SiGeSn/Ge Technology”
54
www.compoundsemiconductor.net July 2013
Figure 8. Current-voltage characteristics of a single junction solar cell grown on both germanium templates and bulk germanium
to , which is an ideal range for base layers
© 2013 Angel Business Communications. Permission required.
based, and includes group IV materials and III-Vs. Significant strides in this direction have already been made – such as forming templates based on thick Ge(Sn) layers lattice-matched to silicon, and the growth of III-Vs on top of templates to form photovoltaic structures – but there is much more to do. This includes incorporating doped junctions, multi-junction structures, and SiGeSn films for the 1eV layer to access ultra-high efficiency devices.
While much of the focus so far has been on interface engineering, lattice engineering, and bandgap engineering, this must now be allied to continual optimization of crystal growth and device structural design, as this multi-pronged effort will hold the key to improving results very quickly in the coming months.
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