Germanium (Ge) is an interesting material for high efficiency solar cell applications. Ge cells are also employed in thermophotovoltaic devices and photovoltaic multiple junction solar cells which is the current technology used to obtain Ge p/n junction by dopant diffusion. Ge is a low bandgap semiconductor      (E_g= 0.67 eV) that also finds application in infrared detection. The study of mechanical properties of the germanium thin films has gained much attention in recent days and is required to use them as structural and functional elements in device fabrication. Heteroepitaxial Ge layers were deposited by means of metal organic vapor phase epitaxy (MOVPE) on Si (001) substrates using iso-butyl germane (iBuGe) as organic precursor and hydrogen as carrier gas. Germanium layers were grown at low pressure (60 mbar) and in the temperature range of 550-675°C at a growth time of 15 minutes. The removal of native oxide from the Ge substrate prior to the growth was done with HCl wet chemical etching and by performing a thermal annealing of the substrate at 650 ºC for 5 minutes.

The deposited epilayers were characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), high resolution X-ray diffraction (HRXRD) and transmission electron microscopy (TEM) and SPM coupled Hysitron TI950 Triboindenter. The AFM analysis shows that the surface was rough in general and with 3D islands, regardless of growth conditions such as temperature, pressure and iBuGe partial pressure. The HRXRD profile of the epilayers confirmed good crystallographic quality of the Ge epilayers. The growth rate was influenced by the growth temperature and partial pressure. The TEM results have been confirmed good interfaces and many defects were found. The mechanical deformation behaviour has been investigated by nanoindentation technique using Berkovich and Vickers indenters to study the properties such as hardness and elastic modulus. The occurrence of elbow phenomenon and their elasto-plastic properties appears to be related to sudden propagation of threading dislocation by pre-existing defects in the materials with the indentation load.

References

1.      D.J. Oliver, J.E. Bradby, J.S. Williams, Journal of Applied Physics, 101, p. 043524, (2007)

2.      G. Timò, C. Flores, R. Campesato, Cryst. Res. Technol. 40, (2005),pp. 1043-1047.

3.      V.M. Andreev, V.P. Khovostikov, N.A. Kalyuzhnyi, S.S. Titkov, O.A. Khvostikova, M.Z. Shvarts,  Semiconductors 38, (2004), pp. 355-359.

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