Epitaxial growth of III-V compounds on (111) silicon for solar cells

A multi-junction device comprising:

• a Si (111) substrate having a (111) orientation for a top surface, the top surface having atomically-flat terraces separated by steps, wherein steps have a step height of a lattice constant of silicon divided by a square root of 3;
• a GaAs epi film grown over the terraces of the top surface of the Si (111) substrate at a first temperature that causes the GaAs epi film to grow horizontally from the steps along the terraces wherein the GaAs epi film is a coincidence-site lattice (CSL);
• a diffusion layer formed within the Si (111) substrate from As diffusing from the GaAs epi film when the GaAs epi film is grown;
• wherein the diffusion layer and the Si (111) substrate form a lower cell that is a p-n junction having a silicon bandgap;
• a first tunnel diode having a first n+ layer grown over the GaAs epi film and a first p+ layer grown over the first n+ layer, the first tunnel diode allowing reverse-biased current to flow during operation of the multi-junction device;
• a second cell having a second p-type layer grown over the first p+ layer of the first tunnel diode, and a second n-type layer grown over the second p-type layer, the second cell formed from a second III-V compound material and having a p-n junction with a second bandgap;
• wherein the GaAs epi film has fewer than 5×105 defects per square cm of surface area;
• wherein the second III-V compound material is AlXGa1-XAs, where X is between 0.1 and 0.6, or from a AlYGa0.5-YIn0.5P, where Y is between 0.1 and 0.25;
• wherein the first tunnel diode and the second cell have layers that are grown at temperatures higher than the first temperature,
• whereby the multi-junction device has two junctions connected by the first tunnel diode and the GaAs epi film creates a low-defect initial film over the Si (111) substrate.

A multiple junction solar cell comprising:

• Si (111) substrate means for supporting junctions formed in layers grown over the Si (111) substrate means by epitaxial growth, the Si (111) substrate means having a (111) orientation for a top surface, the top surface having terraces separated by steps, wherein steps have a step height of a lattice constant of silicon divided by a square root of 3, wherein the terraces are atomically-flat;
• coincidence-site lattice (CSL) layer means for providing a low-defect layer of III-V material grown over silicon in the Si (111) substrate means, the CSL layer means grown over the terraces of the top surface of the Si (111) substrate means at a first temperature that causes the CSL layer means to grow horizontally from the steps along the terraces to match a lattice of the Si (111) substrate means;
• first cell means for generating current when sunlight creates electron-hole pairs in a first p-n junction with a first bandgap, the first p-n junction formed by a n-type first-cell layer and a p-type first-cell layer;
• second CSL layer means for adjusting a lattice constant of the first cell means, the second CSL layer means grown over the first cell means;
• first tunnel diode means for conducting tunneling current in a reverse direction, the first tunnel diode means having a first n+ layer and a first p+ layer, the first p+ layer being grown over the second CSL layer means;
• second cell means for generating current when sunlight creates electron-hole pairs in a second p-n junction with a second bandgap, the second p-n junction formed by a n-type second-cell layer and a p-type second-cell layer, the n-type second-cell layer being grown over the first n+ layer of the first tunnel diode means;
• second tunnel diode means for conducting tunneling current in the reverse direction, the second tunnel diode means having a second n+ layer and a second p+ layer, the second p+ layer being grown over the p-type second-cell layer;
• third cell means for generating current when sunlight creates electron-hole pairs in a third p-n junction with a third bandgap, the third p-n junction formed by a n-type third-cell layer and a p-type third-cell layer, the n-type third-cell layer being grown over the second n+ layer of the second tunnel diode means;
• wherein the third bandgap has a higher energy than the second bandgap, and the second bandgap has a higher energy than the first bandgap.