Manufacture of silicon-based devices having disordered sulfur-doped surface layers

• US 7,354,792 B2
• Filed: 09/24/2004
• Issued: 04/08/2008
• Est. Priority Date: 05/25/2001
• Status: Expired due to Term

• Current Assignee: President and Fellows of Harvard College (Harvard Corporation)
• Original Assignee: President and Fellows of Harvard College (Harvard Corporation)
• Inventors: Mazur, Eric, Carey, James Edward III
• Primary Examiner(s): Pham; Long
• Application Number:
  US10/950,248
• Publication Number:
  US 20080044943A1
• Time in Patent Office: 1,292 Days
• Field of Search: 438/95, 438/48, 438/57
• US Class Current: 438/95
• CPC Class Codes: H01L 21/02686 Pulsed laser beam
• CPC Class Codes: H01L 21/268 using electromagnetic radiation, e.g. laser radiation
• CPC Class Codes: H01L 31/02363 of the semiconductor body itself, e.g. textured active layers
• CPC Class Codes: H01L 31/0288 characterised by the doping material
• CPC Class Codes: H01L 31/1804 comprising only elements of Group IV of the Periodic Table
• CPC Class Codes: H01L 31/1864 Annealing
• CPC Class Codes: Y02E 10/547 Monocrystalline silicon PV cells
• CPC Class Codes: Y02P 70/50 Manufacturing or production processes characterised by the final manufactured product

Abstract

The present invention provides methods of fabricating a radiation-absorbing semiconductor wafer by irradiating at least one surface location of a silicon substrate, e.g., an n-doped crystalline silicon, by a plurality of temporally short laser pulses, e.g., femtosecond pulses, while exposing that location to a substance, e.g., SF₆, having an electron-donating constituent so as to generate a substantially disordered surface layer (i.e., a microstructured layer) that incorporates a concentration of that electron-donating constituent, e.g., sulfur. The substrate is also annealed at an elevated temperature and for a duration selected to enhance the charge carrier density in the surface layer. For example, the substrate can be annealed at a temperature in a range of about 700 K to about 900 K.

Claims

1. A method of fabricating a radiation-absorbing semiconductor structure, comprising:

irradiating each of a plurality of locations on a surface of a silicon substrate with one or more femtosecond laser pulses while exposing said surface to a sulfur-containing substance so as to generate a plurality of sulfur inclusions in a surface layer of said substrate, and

annealing said substrate at a temperature in a range of about 500 K to about 1100 K for a time duration in a range of about a few seconds to about a few hours.

2. The method of claim 1, further comprising selecting a duration of each laser pulse to be in a range of about 50 femtoseconds to about 500 femtoseconds.

3. The method of claim 1, further comprising selecting a fluence of each pulse to be in a range of about 1 kJ/m² to about 15 kJ/m².

4. The method of claim 1, further comprising selecting a central wavelength of said laser pulses to be in a range of about 200 nm to about 800 nm.

5. The method of claim 1, wherein said irradiating step comprises exposing each location to a number of laser pulses ranging from 2 to about 500.

6. The method of claim 1, further comprising subjecting each of said surface locations to successive laser pulses separated from one another by a time interval in a range of about 1 microsecond to about 1 millisecond.

7. The method of claim 1, further comprising performing said annealing step in any of an inert atmosphere or a low-pressure atmosphere.

8. The method of claim 1, further comprising selecting said sulfur-containing substance to be any of SF₆ and H₂S.

9. The method of claim 1, wherein said semiconductor structure absorbs radiation having wavelength components in a range of about 0.25 to about 10 microns.

10. The method of claim 1, wherein said semiconductor structure absorbs radiation having wavelength components in a range of about 0.35 to about 2.5 microns.

11. The method of claim 1, wherein said semiconductor structure exhibits a substantially uniform absorption for wavelength components in a range of about 1.2 to about 2.5 microns.

12. A method of fabricating a photodetector, comprising:

forming a semiconductor structure absorbing radiation in a wavelength range of about 0.25 to about 3.5 microns and exhibiting a diodic current-voltage characteristic in said wavelength range, said forming step comprising;

irradiating a surface of a silicon substrate at one or more locations thereof with one or more laser pulses having short pulse widths while exposing said surface to a sulfur-containing gas to generate a micro-structured layer having sulfur inclusions,

annealing said substrate at a temperature in a range of about 500 K to about 1000 K for a time duration in a range of about a few seconds to about a few hours, and

depositing a plurality of metallic contacts on selected portions of said semiconductor structure to allow applying a reverse bias voltage thereto.

13. The method of claim 12, wherein said pulse widths are selected to be in a range of about tens of femtoseconds to about tens of nanoseconds.

14. The method of claim 12, further comprising selecting said sulfur-containing substance to be any of SF₆ and H₂S.

15. The method of claim 12, further comprising performing said annealing step in an inert atmosphere.

16. The method of claim 12, wherein said depositing step comprises thermally evaporating one or more selected metals on said selected portions of the semiconductor structure.

17. The method of claim 12, further comprising selecting a thickness of said metal contacts to be in a range of about 5 nm to about 100 nm.

18. The method of claim 16, wherein said depositing step comprises thermally evaporating a chromium layer followed by thermally evaporating a gold layer on a selected portion of said semiconductor structure to generate at least one of said metal contacts.

19. The method of claim 18, further comprising selecting a thickness of said chromium layer to be in a range of about 1 nm to about 5 nm.

20. The method of claim 19, further comprising selecting a thickness of said gold layer to be in a range of about 4 nm to about 100 nm.

21. The method of claim 12, further comprising configuring said metal contacts so as to ensure that at least a portion of said micro-structured layer remains capable of receiving an external radiation.

22. A method of fabricating a semiconductor structure, comprising

irradiating a plurality of locations on a surface of a silicon substrate with one or more laser pulses having pulse widths in a range of about 50 to about 500 femtoseconds while exposing said surface to a substance having an electron-donating constituent so as to generate a micro-structured surface layer comprising inclusions incorporating said electron-donating constituent, and

annealing said substrate at an elevated temperature in a range of about 500 K to about 1100 K for a selected time period.

23. The method of claim 22, further comprising selecting said electron-donating constituent to be any of chlorine (Cl₂), nitrogen (N₂) and air.

24. The method of claim 22, wherein said irradiating step further comprises directing said laser pulses at said surface along a direction substantially perpendicular to said surface.

25. The method of claim 22, wherein said selected annealing period is in a range of about a few seconds to about a few hours.

26. The method of claim 22, wherein said annealing step is performed in an inert atmosphere.

27. A method of fabricating a radiation-absorbing semiconductor wafer, comprising:

irradiating at least one surface location of a silicon substrate with a plurality of temporally short laser pulses while exposing said location to a substance having an electron-donating constituent so as to generate a substantially disordered surface layer incorporating a concentration of said electron-donating constituent, and

annealing said substrate at an elevated temperature and for a duration selected to enhance charge carrier density in said substantially disordered surface layer.

28. The method of claim 27, wherein said elevated temperature and duration are selected to cause re-arrangement of atomic bonds in said surface layer so as to increase concentration of carrier electrons in surface layer.

29. The method of claim 27, wherein said elevated temperature and duration are selected to enhance electron carrier density in the surface layer by a factor of about 10% to about 200% relative to a pre-annealing level.

30. The method of claim 27, wherein said elevated temperature and duration are selected to cause said surface layer to form a diode junction with underlying silicon substrate.

31. The method of claim 27, wherein said elevated temperature and duration are selected such that said annealed substrate exhibits a responsivity in a range of about 1 A/W to about 200 A/W upon exposure of said surface layer to radiation having wavelengths in a range of about 250 nm to about 1100 nm and application of a reverse bias in a range of about 0.1 V to about 15 V to said wafer.

32. The method of claim 27, wherein said elevated temperature is selected to be in a range of about 500 K to about 1100 K.

33. The method of claim 32, wherein said duration is selected to be in a range of about a few seconds to about few hours.

34. The method of claim 27, further comprising selecting said short laser pulses to have pulse widths in a range of about tens of femtoseconds to about tens of nanoseconds.

35. The method of claim 27, further comprising selecting a fluence of said pulses and a partial pressure of said substance having an electron-donating constituent such that the concentration of said electron-donating constituent incorporated in said surface layer ranges from about 0.5 to about 5 atom percent.

36. A method of enhancing responsivity of a radiation-absorbing semiconductor wafer to incident radiation, comprising

irradiating at least one surface location of a silicon substrate with temporally short laser pulses while exposing the substrate to a substance having an electron-donating constituent so as to generate a microstructured surface layer having inclusions containing the electron-donating constituent, and

annealing the substrate at an elevated temperature and for a duration selected to enhance a responsivity of the microstructured substrate to radiation having at least one wavelength in a range of about 250 nm to about 1100 nm incident on the microstructured surface layer by at least a factor of about 10.

37. The method of claim 36, wherein the annealing temperature lies in a range of about 700 K to about 900 K.