Ge channel structures with extremely high compressive strain up to 2.8% were fabricated and their magnetotransport properties were evaluated. It was found that at the same hole density the sample with the higher strain showed the lower hole effective mass and that the compressive strain effectively reduced the effective mass. The Dingle ratios obtained were very high (>5) for all samples, indicating that remote impurity scattering was a dominant scattering mechanism rather than high angle scatterings caused by degradation of the channel layers. This result strongly suggests that Ge channels with extremely high strain are very promising for high performance complementary-metal-oxide-semiconductor applications.
The effects of gate bias on hole effective mass (m*) and Hall mobility were studied in strained-Ge channel modulation-doped structures. Shubnikov–de Haas oscillations were analyzed with and without the bias and a significant m* increase from 0.15 to 0.22 m0 was observed with the increase in the carrier density due to the strong nonparabolicity of the valence band. This is a clear demonstration that modification of carrier density via gating considerably affects m*, which may have critical effects on device properties. The gate bias dependence of Hall mobility was also investigated and the dominant scattering mechanism was clarified in various temperature and carrier density regions.
The strain and hole-density dependence of hole mobility was studied in compressively strained Ge channel modulation-doped structures. Ge channels with large strain (grown on SiGe buffers with Ge compositions down to 43%) were fabricated without any relaxation by low-temperature molecular beam epitaxy growth. It was found that the mobility monotonically increased with increasing strain. Since this increase in mobility was accompanied by an increase in hole density, the enhanced screening effect of the impurity scattering was thought to be a dominant mechanism for mobility enhancement due to strain. This mechanism was confirmed by backgating measurements, where the mobility increased with increasing hole density with the power of 0.5-0.7. It was also found that the inverted structure contributed to mobility enhancement particularly for highly strained structures, and the highest mobilities of 20 000 and 2000 cm 2 (V s) −1 at 8 K and room temperature were obtained for Ge channels with a strain of around 2%, indicating that Ge channels with extremely high strain are very promising structures for high performance CMOS applications.
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