Microstructure enhanced absorption photosensitive devices
Techniques for enhancing the absorption of photons in semiconductors with the use of microstructures are described. The microstructures, such as pillars and/or holes, effectively increase the effective absorption length resulting in a greater absorption of the photons. Using microstructures for absorption enhancement for silicon photodiodes and silicon avalanche photodiodes can result in bandwidths in excess of 10 Gb/s at photons with wavelengths of 850 nm, and with quantum efficiencies of approximately 90% or more.
1. A photodetector with microstructure-enhanced photoabsorption comprising:
a cathode region;
an anode region;
reverse biasing circuitry including a negative voltage contact coupled to said anode region and a positive voltage contact coupled to said cathode region to drive said cathode region to a more positive voltage than said anode region; and
an epitaxially grown, crystalline, microstructure-enhanced photon absorbing semiconductor region, 0.1-5 μm thick, between the cathode and anode regions and comprising one or both of Si and Ge;
a multiplicity of spaced apart holes extending through said photon absorbing semiconductor region and do not extend into one of said anode and cathode regions, wherein said holes are in an aperiodic arrangement;
said photodetector having an active area that includes said multiplicity of holes and is configured to receive an incident beam of light;
wherein one of said anode and cathode regions is between a light incident surface of said photodetector and said photon absorbing semiconductor region;
said reverse biasing circuitry comprising a conductive layer over said active area of the photodetector including over said holes and over areas between said holes; and
said photodetector having a quantum efficiency greater than 30% for selected wavelength ranges and bandwidth in the gigabits per second range.
2. A photodetector according to claim 1 wherein said holes have widths equal to or shorter than a longest wavelength in said incident beam of light.
3. A photodetector according to claim 1 wherein said holes are tapered.
4. A photodetector according to claim 1 further including a substrate material and wherein said holes have a major longitudinal axis normal to an upper surface of the substrate material.
5. A photodetector according to claim 1 wherein said holes have major longitudinal axes that are not parallel to one another.
6. A photodetector according to claim 1 wherein said holes are oriented to reduce cross-talk in an application selected from a group consisting of: free space applications and coupling to multiple sources in an array.
7. A photodetector according to claim 1 wherein said holes are oriented to reduce sensitivity to any single direction of said incident beam of light.
8. A photodetector according to claim 1 wherein said regions comprise layers and said absorbing region is a layer between the layers of said anode and cathode regions.
9. A photodetector according to claim 1 wherein said holes extend through the other one of said anode and cathode regions.
10. A photodetector according to claim 1 wherein said microstructure-enhanced photon absorbing semiconductor region is formed of silicon.
11. A photodetector according to claim 10 wherein said photodetector is a silicon photodiode.
12. A photodetector according to claim 11 wherein said photodetector has the property of operating at a data bandwidth of greater than 5 gigabits per second, at wavelengths of 850 nanometers of said incident beam of light, with a quantum efficiency of at least 60%.
13. A photodetector according to claim 12 wherein said photodetector has the property of operating at a data bandwidth of greater than 10 gigabits per second, at wavelengths of 850 nanometers of said incident beam of light, with a quantum efficiency of at least 60%.
14. A photodetector according to claim 11 wherein said photodetector has the property of operating at quantum efficiency of greater than 60%.
15. A photodetector according to claim 10 wherein said photodetector is an avalanche photodiode.
16. A photodetector according to claim 15 wherein said avalanche photodiode is configured to detect said incident beam of light at a data bandwidth of greater than 5 gigabits per second, at wavelengths of 850 nanometers, and having a gain of greater than 2.
17. A photodetector according to claim 15 further comprising a multiplication region that includes a second plurality of holes.
18. A photodetector according to claim 17 wherein said second plurality of holes extends into one or more multiplication layers selected from a group consisting of: P multiplication layer, N multiplication layer and charge layer.
19. A photodetector according to claim 10 wherein said photodetector has the property of operating at a data bandwidth greater than 1 gigabit per second, at wavelengths of 980 nanometers of said incident beam of light, with a quantum efficiency of at least 40%.
20. A photodetector according to claim 10 wherein said photodetector has the property of operating at a data bandwidth greater than 0.5 gigabits per second, at wavelengths of 1000 nanometers of said incident beam of light, with a quantum efficiency of at least 30%.
21. A photodetector according to claim 1 wherein said active area is circular.
22. A photodetector according to claim 1 wherein said microstructure-enhanced photon absorbing semiconductor region is formed of silicon and germanium.
23. A photodetector according to claim 22 wherein said holes are formed by etching away portions of silicon and using a selective area growth process to grow germanium in the etched away portions.
24. A photodetector according to claim 1 wherein said microstructure-enhanced photon absorbing semiconductor region is are formed of germanium.
25. A photodetector according to claim 24 further including a substrate and wherein said cathode, absorbing and anode regions are formed above the substrate and said photodetector is configured to receive said incident beam of light from a substrate-side of the photodetector.
26. A photodetector according to claim 25 wherein said anode region comprises a germanium P layer formed by an epitaxial lateral overgrowth process.
27. A photodetector according to claim 25 wherein the photodetector is configured for a portion of the incident beam of light to pass through the absorbing region a first time, reflect from a surface and thereafter pass through the absorbing region a second time.
28. A photodetector according to claim 24 further including a substrate and wherein said cathode, absorbing and anode regions are formed above the substrate and said photodetector is configured to receive said incident beam of light from directions facing a side of the photodetector opposite of said substrate.
29. A photodetector according to claim 28 wherein said anode region comprises a silicon P layer.
30. A photodetector according to claim 24 wherein said photodetector is an avalanche photodiode and further comprises a multiplication region formed of silicon.
31. A photodetector according to claim 30 wherein said anode region comprises a germanium P layer formed by an epitaxial lateral overgrowth process.
32. A photodetector according to claim 30 wherein said avalanche photodiode is configured to detect said incident beam of light at a data bandwidth of greater than 1 gigabit per second, at wavelengths of 1750 nanometers or shorter, and having a gain of greater than 2.
33. A photodetector according to claim 30 wherein at least one germanium layer is grown epitaxiallly on a silicon layer.
34. A photodetector according to claim 1 wherein said plurality of holes include voids buried within the absorbing semiconductor region.
35. A photodetector according to claim 34 wherein the absorbing semiconductor region is made of a material selected from a group consisting of silicon and germanium.
36. A photodetector according to claim 1 further comprising a plurality of voids buried in a layer of material effectively lowering a refractive index at said range of wavelengths and lowering capacitance of said layer of material compared to the material when free of intentional voids.
37. A photodetector according to claim 36 wherein said layer of material is an N layer formed on a substrate material.
38. A photodetector according to claim 36 wherein said photodetector is an avalanche photodiode and said layer of material forms a multiplication layer of the avalanche photo diode.
39. A photodetector according to claim 36 wherein said plurality of voids are filled with a material selected from a group consisting: nitrogen, argon, polymer, amorphous semiconductor material, and glass.
40. A photodetector according to claim 1 wherein said holes are non-circular.