ESTABLISHING A HIGH PHOSPHORUS CONCENTRATION IN SOLAR CELLS
Methods of controlling the diffusion of a dopant in a solar cell are disclosed. A second species is used in conjunction with the dopant to modify the diffusion region. For example, phosphorus and boron both diffuse by pairing with interstitial silicon atoms. Thus, by controlling the creation and location of these interstitials, the diffusion rate of the dopant can be controlled. In one embodiment, a heavier element, such as germanium, argon or silicon, is used to create interstitials. Because of the presence of these heavier elements, the dopant diffuses deeper into the substrate. In another embodiment, carbon is implanted. Carbon reduces the number of interstitials, and thus can be used to limit the diffusion of the dopant. In another embodiment, a lighter element, such as helium is used to amorphize the substrate. The crystalline-amorphous interface created limits diffusion of the dopant into the substrate.
1 . A method of creating a high dopant concentration near the surface of a solar cell, comprising;
utilizing a silicon substrate;
introducing a dopant into said substrate; and
introducing carbon into said substrate, wherein said carbon limits the diffusion of said dopant.
2 . The method of claim 1 , wherein said dopant and said carbon are introduced simultaneously.
3 . The method of claim 3 , wherein said dopant and said carbon are introduced via ion implantation, wherein a molecule comprising said dopant and said carbon is ionized.
4 . The method of claim 3 , wherein said dopant and said carbon are introduced via ion implantation, wherein said ion implantation is performed without a mass analyzer.
5 . The method of claim 3 , wherein said dopant and said carbon are introduced via furnace diffusion, wherein a gas comprising both said dopant and said carbon is heated.
6 . The method of claim 1 , wherein said dopant and said carbon are introduced sequentially.
7 . The method of claim 6 , wherein said dopant and said carbon are introduced via ion implantation.
8 . The method of claim 1 , further comprising determining an energy level for said carbon implant, so as to achieve said high dopant concentration near the surface.
9 . The method of claim 8 , wherein said energy level is between 1 keV and 60 keV.
10 . The method of claim 1 , wherein said carbon is introduced via ion implantation, and said dopant is introduced via furnace diffusion.
11 . The method of claim 1 , wherein said dopant is selected from the group consisting of phosphorus and boron.
12 . The method of claim 1 , further comprising performing a thermal anneal subsequent to said introduction of said dopant and said carbon.
13 . A method of modifying a dopant concentration of a solar cell, comprising;
utilizing a silicon substrate;
introducing a dopant into said substrate; and
introducing a second species into said substrate, wherein said second species increases the number of silicon interstitials within said solar cell.
14 . The method of claim 13 , wherein said dopant is selected from the group consisting of phosphorus and boron.
15 . The method of claim 13 , further comprising performing a thermal anneal subsequent to said introduction of said dopant and said second species.
16 . The method of claim 13 , wherein said second species is selected from the group consisting of argon, germanium and silicon.
17 . A method of creating a high dopant concentration near the surface of a solar cell, comprising;
utilizing a silicon substrate;
introducing a dopant into said substrate; and
introducing helium into said substrate, wherein said helium limits the diffusion of said dopant.
18 . The method of claim 17 , wherein said helium is implanted with sufficient dose such that at least a portion of said substrate is amorphized.
19 . The method of claim 17 , further comprises performing an anneal subsequent to said two introducing steps.
20 . The method of claim 17 , where said dopant is selected from the group consisting of phosphorus or boron.