Strain-enhanced silicon photon-to-electron conversion devices
Improved silicon solar cells, silicon image sensors and like photosensitive devices are made to include strained silicon at or sufficiently near the junctions or other active regions of the devices to provide increased sensitivity to longer wavelength light. Strained silicon has a lower band gap than conventional silicon. One method of making a solar cell that contains tensile strained silicon etches a set of parallel trenches into a silicon wafer and induces tensile strain in the silicon fins between the trenches. The method may induce tensile strain in the silicon fins by filling the trenches with compressively strained silicon nitride or silicon oxide. A deposited layer of compressively strained silicon nitride adheres to the walls of the trenches and generates biaxial tensile strain in the plane of adjacent silicon fins.
1. A photosensitive device, comprising:
a silicon substrate having a plurality of trenches on a surface, the trenches positioned to define at least one silicon fin; and
stressed material within at least two of the trenches, the stressed material inducing tensile strain within the at least one silicon fin to reduce the band gap of at least a portion of the silicon within the at least one silicon fin and to provide increased sensitivity of the tensile strained portion of the fin to longer wavelength light.
2. The device of claim 1 , wherein the photosensitive device is a silicon solar cell and wherein a p-n junction is formed in the at least one silicon fin.
3. The device of claim 1 , wherein the stressed material is compressively stressed silicon nitride adhered to at least one side surface of the at least one silicon fin.
4. The device of claim 1 , wherein the stressed material is tensile strained silicon oxide or silicon nitride adhered to walls of adjacent fins.
5. The device of claim 1 , wherein the at least one silicon fin is crystalline silicon and the trenches have a depth of between about 0.3 microns and twenty microns and the fins have a width of between about 0.2 microns and one micron and a length of between 0.3 microns and twenty microns.
6. The device of claim 1 , wherein the photosensitive device is a silicon solar cell and wherein a p-n junction is formed within a diffusion length of a reduced band gap and strained portion of the at least one silicon fin.
7. The device of claim 1 , wherein the photosensitive device is an image sensor cell and wherein a p-n junction is formed within a diffusion length of a reduced band gap and strained portion of the at least one silicon fin.
8. The device of claim 1 , wherein the photosensitive device is an infrared photodetector capable of detecting photons in a wavelength range between 800 nanometers and 1400 nanometers and wherein a p-n junction is formed within a diffusion length of a reduced band gap and strained portion of the at least one silicon fin.
9. The device of claim 1 , wherein the stressed material fills each of the two trenches.
10. The device of claim 1 , wherein the stressed material fills each of the two trenches and induces tensile strain in the one silicon fin.
11. The device of claim 10 , wherein the tensile strain is uniaxial in a direction normal to sidewalls of the one silicon fin.
12. The device of claim 10 , wherein the tensile strain in the one silicon fin is sufficient to reduce the bandgap of the silicon within the one silicon fin by an observable amount.