2.5 Behaviour of amorphous Ge contacts to monocrystalline silicon

The performance of evaporated amorphous Ge films (thickness $\text{～-500}\text{A}\text{?}$) contacts to etched n-and p-type silicon crystals of different resistivities are discussed. The Ge was 3 Ωcm n-type was also used but gave no difference), and as external contact to the Ge film an Au layer was evaporated. The behaviour of the aGeSi junction seems to be largely governed by interface effects (and thus depends on surface preparation), as is often the case with metal-Si junctions, but Ge gives more reproducible and less time-varying results.In the process of clarifying the function of the contact the following structures were investigated (1) aGe_pⁿSiIn (Hg), where the latter is an ohmic contact, (2) amSi_pⁿSi-metal where amSi is a surface region of the crystal which has been rendered amorphous by ion bombardment, (3) aGe_pⁿSi-metal. I-V and C-V measurements were performed. From the results we conclude that aGeSi junctions act as low-resistance contacts when fed by electron or hole currents from the crystal. The currents (holes and electrons) that are injected into the crystal from the film are limited by barriers to small current densities, usually in the range 10^?6 A cm^?2. It is suggested that the small hole currents are explained by an increase in the hole barrier, effected by positive charges at the interface or in the Ge film, which are built up when positive carriers (holes) are injected by the contact.