Spin-On Organic Polymer Dopants for Silicon

We introduce a new class of spin-on dopants composed of organic, dopant-containing polymers. These new dopants offer a hybrid between conventional inorganic spin-on dopants and a recently developed organic monolayer doping technique that affords unprecedented control and uniformity of doping profiles. We demonstrate the ability of polymer film doping to achieve both p-type and n-type silicon by using boronand phosphorus-containing polymer films. Different doping mechanisms are observed for boron and phosphorus doping, which could be related to the specific chemistries of the polymers. Thus, there is an opportunity to further control doping in the future by tuning the polymer chemistry. SECTION: Energy Conversion and Storage; Energy and Charge Transport T semiconductor industry demands silicon-doping techniques that produce precisely controlled doping profiles, a requirement that presents many technical challenges. Dopant atoms must be incorporated into the silicon lattice without disrupting the lattice or damaging the substrate. Furthermore, it is desired that the dopant concentration be precisely controlled and demonstrate uniformity over large areas. One conventional doping method is ion implantation, which involves the bombardment of silicon with high-energy dopant ions that replace Si atoms in the lattice. Ion implantation results in excellent doping uniformity over large surface areas. However, the process also produces point defects and vacancies in the lattice, which interact with the dopants to broaden the junction profile, limiting the formation of sub-10 nm doping profiles. This depth limitation will become increasingly restrictive as semiconductor electronic devices are scaled to nanometer dimensions. Furthermore, ion implantation is incompatible with nonplanar, nanostructured materials because the energetic ions have significant probability of penetrating completely through the nanostructure without remaining in the lattice while causing significant crystal damage. A second conventional method is to use spin-on dopants. This method entails spinning a dopant-containing solution onto silicon substrates, which is followed by a thermal annealing step during which the dopants diffuse into the substrate. Often times, a prediffusion annealing step is required to “glassify” the spin-on dopant layer. The dopant-containing solution usually contains either a mixture of SiO2 and dopant or silicon-containing polymers with dopant atoms incorporated into the polymer (for example, phosphosilicates or borosilicates). Spin-on doping is a simple, low-cost, nondestructive technique, but it suffers from a lack of dose control and uniformity over large areas. Furthermore, spin-on dopants often leave behind undesirable residues. While pure SiO2 and silicates are easily removed with wet etchants, the presence of residual organics from the solvent during the annealing process results in chemically modified layers that are very difficult to remove. Another approach currently under investigation for obtaining doped nanomaterials is the direct incorporation of dopants during nanomaterial synthesis. This approach has the advantage of producing high-quality doped lattices on the nanoscale but introduces many synthetic, scale-up, and integration challenges that still must be overcome. Recently, a monolayer doping procedure has been developed that overcomes the difficulties of conventional technologies and achieves high-quality doping profiles with high areal uniformity. During this procedure, a covalently bound, selfassembled monolayer of dopant-containing organic molecules is formed on the surface of silicon substrates. In a subsequent thermal annealing step, the dopant atoms are diffused into the silicon lattice (see Figure 1a). Because of the inherent uniformity of the self-assembled monolayer, as well as the ability to tune the chemistry of the dopant-containing organic molecule, this approach affords unprecedented control and uniformity of doping profiles. Monolayer doping has also resulted in the demonstration of the shallowest junctions Received: September 5, 2013 Accepted: October 3, 2013 Letter