Large contact resistance due to Fermi level pinning effect at the interface between metal and n-type Ge strongly restricts the performance of Ge device on Si substrate. In this paper, the contacts of metal Al and Ni with n-type Ge and p-type Ge epitaxial layers grown by UHV/CVD are comparatively studied. It is found that the contact of NiGe/n-Ge is better than that of Al/n-Ge at the same dopant concentration. When the concentration of P is 21019 cm-3, the ohmic contact of NiGe/n-Ge with c down to 1.4310-5 cm2 is demomstrated, which is about one order of magnitude lower than that of Al/n-Ge contact. The specific contact resistance of NiGe/p-Ge is 1.6810-5 cm2 when the B concentration is 4.21018 cm-3, corresponding to that of Al/p-Ge. Compared with Al/n-Ge contact, P segregation at the interface between NiGe and Ge, rather than lowering Schottky barrier height, is the main reseaon for achieving the low specific contact resistance. NiGe/Ge contact should be a good choice for contact electrode for Ge devices at present.

Silicon-based light emitting materials and devices with high efficiency are inarguably the most challenging elements in silicon (Si) photonics. Band-gap engineering approaches, including tensile strain and n-type doping, utilized for tuning germanium (Ge) to an optical gain medium have the potential for realizing monolithic optoelectronic integrated circuit. While previous experimental research has greatly contributed to optical gain and lasing of Ge direct-gap, many efforts were made to reduce lasing threshold, including the understanding of high efficiency luminescence mechanism with tensile strain and n-type doping in Ge. This paper focuses on the theoretical analysis of lattice scattering in n-type Ge-on-Si material based on its unique dual-valley transition for further improving the efficiency luminescence of Ge direct-gap laser. Lattice scattering of carriers, including inter-valley and intra-valley scattering, influence the electron distribution between the direct valley and indirect L valleys in the conduction of n-type Ge-on-Si material. This behavior can be described by theoretical model of quantum mechanics such as perturbation theory. In this paper, the lattice scatterings of intra-valley scattering in valley and L valleys, and of inter-valley scattering between the direct valley and L valleys in the n-type Ge-on-Si materials are exhibited based on its unique dual-valley transition by perturbation theory. The calculated average scattering times for phonon scattering in the cases of valley and L valleys, and for inter-valley optical phonon scattering between valley and L valleys are in agreement with experimental results, which are of significance for understanding the lattice scattering mechanism in the n-type Ge-on-Si material. The numerical calculations show that the disadvantaged inter-valley scattering of electrons from the direct valley to indirect L valleys reduces the electrons dwelling in the direct valley slightly with n-type doping concentration, while the strong inter-valley scattering from the indirect L valleys to indirect valleys increases electrons dwelling in the direct valley with n-type doping concentration. The competition between the two factors leads to an increasing electrons dwelling in the direct valley with n-type doping in a range from 1017 cm-3 to 1019 cm-3. That the electrons in the indirect L valleys are transited into the direct valley by absorbing inter-valley optical phonon modes is one of the effective ways to enhance the efficiency luminescence of Ge direct-gap laser. The results indicate that a low-threshold Ge-on-Si laser can be further improved by engineering the inter-valley scattering for enhancing the electrons dwelling in the valley.

Thick Ge epitaxial layers are grown on Si(001) substrates in the ultra-high vacuum chemical vapor deposition system by using the method of low temperature buffer layer combining strained layer superlattices. The microstructure and the optical properties of the Ge layers are characterized by double crystal X-ray diffraction, HRTEM, AFM and photoluminescence spectroscopy. The root-mean-square surface roughness of the Ge epilayer with a thickness of 880nm is about 0.24 nm and the full-width-at-half maximum of the Ge peak of the XRD profile is about 273. The etch pit density related to threading dislocations is less than 1.5106 cm-2. The direct band transition photoluminescence is observed at room temperature and the photoluminescence peak is located at 1540 nm. It is indicated that the Ge epitaxial layer is of good quality and will be a promising material for Si-based optoelectronic devices