NOVELTY - The method comprises depositing a phosphorus-containing nucleation layer over and directly in contact with a p-type germanium substrate, depositing a second layer containing an arsenic dopant over the nucleation layer, and depositing a sequence of layers of semiconductor material over the second layer forming first and second solar subcells. The sequence of layers includes an arsenic-containing buffer layer adjacent to the nucleation layer such that the nucleation layer is interposed between the buffer layer and the p-type substrate.    USE - The method is useful for fabricating a multijunction solar cell, which is useful in mobile and telephone communications and other space related applications.    ADVANTAGE - The method is capable of efficiently and economically fabricating the multijunction solar cell with high power and energy conversion efficiencies, enhanced current-voltage characteristic and improved performance and without lattice mismatch.    DETAILED DESCRIPTION - The method comprises depositing a phosphorus-containing nucleation layer over and directly in contact with a p-type germanium substrate, depositing a second layer containing an arsenic dopant over the nucleation layer, and depositing a sequence of layers of semiconductor material over the second layer forming first and second solar subcells. The sequence of layers includes an arsenic-containing buffer layer adjacent to the nucleation layer such that the nucleation layer is interposed between the buffer layer and the p-type substrate. The nucleation layer serves as a diffusion barrier to the arsenic dopant such that diffusion of the arsenic dopant into the substrate is limited in depth by the nucleation layer, comprises a material having a similar lattice parameter as the substrate, and has a thickness of less than or equal to 350 Angstrom . The material contained in the nucleation layer serves as a source of a dopant that forms an n-p junction in the substrate. A two-step diffusion profile is formed in the substrate resulting in a shallow n-p junction in the substrate. The n-p junction is located 0.3-0.7 mu m from a top surface of the substrate adjoining the nucleation layer. The diffusion of the arsenic dopant into the first layer primarily involves solid state diffusion. The substrate has a diffusion region doped with n-type dopants. An upper portion of the diffusion region has a higher concentration of phosphorus atoms than arsenic atoms. The substrate has a diffusion profile arranged to provide current and voltage characteristics that optimize power output of the multijunction solar cell. The method further comprises disposing a third solar subcell over the second solar subcell, and diffusing phosphorus atoms from the nucleation layer and arsenic atoms from the buffer layer into the p-type substrate during deposition of the sequence of layers of semiconductor material to form an n-p junction in the p-type substrate so that an additional solar subcell of the multijunction solar cell is formed. A depth of diffusion of the arsenic dopant in the diffusion region is greater than a depth of diffusion of the phosphorus dopant in the two-step diffusion region.    DESCRIPTION OF DRAWING(S) - The figure shows a block diagram of a triple junction solar cell.