Abstract
This paper describes a new method to grow thin germanium (Ge) epilayers (40 nm) on c-Si substrates at a low growth temperature of 180°C using electron cyclotron resonance chemical vapor deposition (ECR-CVD) process. The full width at half maximum (FWHM) of the Ge (004) in X-ray diffraction pattern and the compressive stain in a Ge epilayer of 683 arcsec and 0.12% can be achieved. Moreover, the Ge/Si interface is observed by transmission electron microscopy to demonstrate the epitaxial growth of Ge on Si and the surface roughness is 0.342 nm. The thin-thickness and smooth surface of Ge epilayer grown on Si in this study is suitable to be a virtual substrate for developing the low cost and high efficiency III-V/Si tandem solar cells in our opinion. Furthermore, the low temperature process can not only decrease costs but can also reduce the restriction of high temperature processes on device manufacturing.
Highlights
High conversion efficiency is very important for solar cells, and the multijunction solar cell is a very promising approach from the point of view of high performance
Different energy band gap (Eg) materials can be applied as absorber layers, so that high conversion efficiency can be achieved for multijunction solar cell as a consequence of better utilization of the solar spectrum [1]
The results presented above demonstrate that, with the increase of working pressure, the structural quality of the Ge (004) epilayer can be better improved because of the higher dissociation identified by Ge∗/GeH∗ ratio measured by optical emission spectroscopy (OES)
Summary
High conversion efficiency is very important for solar cells, and the multijunction solar cell is a very promising approach from the point of view of high performance. Colace et al developed an inexpensive low temperature process using the thermal evaporation method to grow Ge films at a lower growth temperature They successfully obtained an epitaxial Ge structure on a Si substrate at a temperature of about 300–400∘C [23, 24], which has been used for near infrared photodetection [24]. We attempt to grow the Ge epilayer on Si at a low growth temperature Such a Ge epilayer can used to be buffer layer to solve the lattice mismatched between III-V materials and Si in order to combine these two materials to develop the low cost and high efficiency III-V/Si tandem solar cell which has been reported [29]. Because the bandgap of Ge is 0.66 eV, Ge epilayer thickness for the III-V/Si tandem solar
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