(poster) New Method for Ohmic Metal to Si Contact Formation Utilizing Highly Charged ALD Dielectric.

Ohmic contacts are typically achieved by increasing the charge carrier concentration under the metal by heavily doping the surface layer of the semiconductor substrate. However, such a heavy doping can cause damage to the semiconductor crystal structure and increase electrical losses via charge carrier recombination. Moreover, the doping processes require the use of high temperatures, which adds process complexity and restricts the use of certain material combinations. Thus, it would be highly beneficial for the entire semiconductor industry if the ohmic contact could be formed without external doping.
Here we propose a novel concept for ohmic metal to semiconductor contact formation. We utilize a specific atomic layer deposited (ALD) dielectric that is known to have a high charge density when deposited on top of semiconductor. A schematic of the concept is depicted in Figure 1. The charge present in the dielectric induces an electric field to the underlying substrate. Depending on the polarity of the charge, electric field pushes one type of carrier to the bulk and attracts the other towards the surface, leading to the formation of either electron or hole rich surface layer similarly to conventional external doping via phosphorus or boron, respectively. We propose that by etching suitable openings into the dielectric and by depositing a metal layer on top, an ohmic metal to silicon contact could be achieved. The dielectric-attracted carriers should drift underneath the openings and enable current flow in the metal-Si interface.
For experimental testing of the proposed concept, we selected Al2O3 as the ALD dielectric, Si as the substrate and Al as the metal. To characterize the quality of the formed Al to Si contact, we fabricated transfer length method test structures and carried out I-V measurements on them. An example of the obtained I-V curves is presented in Figure 2. The results reveal that an ohmic contact can be achieved with the proposed structure, and that the contact resistivities can get as low as 1.10 mΩcm2, which is comparable to that of conventional heavily doped contacts (1 mΩcm2). As the contact resistivity can be supposedly lowered by increasing the Al2O3 charge with e.g. different ALD precursors, higher post-deposition anneal temperatures and nanostructuring the substrate surface, the proposed concept might have potential to dethrone heavy doping as the industry standard for achieving low-resistivity ohmic contact.