Strained-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 process for making a photosensitive device, comprising:
providing a silicon substrate;
forming a mask on the silicon substrate, the mask having a mask pattern;
etching into the silicon substrate according to the mask pattern to form a plurality of trenches, pairs of trenches defining a plurality of silicon fins;
inducing tensile strain within at least a portion of two or more of the silicon fins, the inducing tensile strain comprising providing a material in contact with at least portions of sides of the two or more silicon fins; and
forming electrodes to provide electrical connections to the photosensitive device, wherein the tensile strain within the two or more silicon fins reduces the band gap of the at least a portion of two or more silicon fins and increases the sensitivity of the photosensitive device for photons having energies less than a band gap of bulk silicon.
2. The process of claim 1 , wherein the inducing strain includes inducing a tensile strain by depositing compressively stressed material to fill the grooves and removing excess compressively stressed material from above at least a portion of the two or more silicon fins.
3. The process of claim 1 , wherein the inducing strain includes inducing a tensile strain by growing a silicon oxide inside the trench by a thermal oxidation process.
4. The process of claim 1 , wherein the inducing strain includes depositing tensile stressed silicon nitride or silicon oxide to adhere to sides of the two or more silicon fins.
5. The process of claim 1 , wherein the providing a material comprises depositing compressively strained silicon nitride and depositing unstressed silicon nitride within the trenches to adhere to the compressively strained silicon nitride and further comprising removing excess unstressed silicon nitride and excess compressively stressed silicon nitride from above at least a portion of the two or more silicon fins.
6. The process of claim 2 , wherein the photosensitive device is a silicon solar cell and further comprising doping the two or more silicon fins to form two or more p-n junctions.
7. The process of claim 2 , wherein the photosensitive device is an image sensor cell and further comprising doping the two or more silicon fins to form two or more p-n junctions.
8. The process of claim 2 , wherein the photosensitive device is an infrared photodetector capable of detecting photons in a wavelength range extending from 800 nanometers to 1400 nanometers and further comprising doping the two or more silicon fins to form two or more p-n junctions.
9. The process of claim 2 , wherein the trenches are etched to a depth of between about three and ten microns and have a width of between about 0.2 and one micron.
10. The process of claim 2 , further comprising doping the two or more silicon fins to form two or more p-n junctions within a diffusion length of respective reduced band gap and strained portions of the two or more silicon fins.
11. The process of claim 1 , wherein the two or more silicon fins have a height of between about three and ten microns.
12. The process of claim 1 , wherein the two or more silicon fins have a length of between about 0.2 and 20 microns.
13. The process of claim 1 , wherein the two or more silicon fins have a width of between about 0.2 to 2 microns.
14. The process of claim 1 , wherein the two or more silicon fins have a height of between about three and ten microns, a length of between about 0.2 and 20 microns and a width of between about 0.2 to 2 microns.
15. The process of claim 8 , wherein the two or more silicon fins have a height of between about three and ten microns, a length of between about 0.2 and 20 microns and a width of between about 0.2 to 2 microns.