Separation of doping density and minority carrier lifetime in photoluminescence measurements on semiconductor materials

• US 8,742,372 B2
• Filed: 07/19/2010
• Issued: 06/03/2014
• Est. Priority Date: 07/20/2009
• Status: Expired due to Fees

• Current Assignee: BT Imaging Pty., Ltd. (Aurora Solar Technologies, Inc.)
• Original Assignee: BT Imaging Pty., Ltd. (Aurora Solar Technologies, Inc.)
• Inventors: Trupke, Thorsten
• Primary Examiner(s): Kim, Kiho
• Application Number:
  US13/384,970
• Publication Number:
  US 20120181452A1
• Time in Patent Office: 1,415 Days
• Field of Search: 250/459.1
• US Class Current: 250/459.1
• CPC Class Codes: G01N 21/6489 Photoluminescence of semiconductors
• CPC Class Codes: G01N 2201/06113 Coherent sources; lasers
• CPC Class Codes: G01N 2201/10 Scanning
• CPC Class Codes: H01L 22/12 for structural parameters, e.g. thickness, line width, refractive index, temperature, warp, bond strength, defects, optical inspection, electrical measurement of structural dimensions, metallurgic measurement of diffusions electrical measurement of diffusions H01L22/14

Abstract

Methods are presented for separating the effects of background doping density and effective minority carrier lifetime on photoluminescence (PL) generated from semiconductor materials. In one embodiment the background doping density is measured by another technique, enabling PL measurements to be analyzed in terms of effective minority carrier lifetime. In another embodiment the effective lifetime is measured by another technique, enabling PL measurements to be analyzed in terms of background doping density. In yet another embodiment, the effect of background doping density is removed by calculating intensity ratios of two PL measurements obtained in different spectral regions, or generated by different excitation wavelengths. The methods are particularly useful for bulk samples such as bricks or ingots of silicon, where information can be obtained over a much wider range of bulk lifetime values than is possible with thin, surface-limited samples such as silicon wafers. The methods may find application in solar cell manufacturing for improving the manufacture of silicon ingots and bricks, or for providing a cutting guide for wafering.

Claims

1. A method of conducting an analysis of a semiconductor material, said method including the steps of:

(a) exciting said material to produce photoluminescence;

(b) measuring the intensity of the photoluminescence emitted from said material;

(c) normalising the measured photoluminescence intensity with regard to variations in the background doping density of said material to obtain a normalised photoluminescence intensity; and

(d) analysing said normalised photoluminescence intensity in terms of one or more properties of said material.

2. A method according to claim 1, wherein a substantial area of said material is excited, and said measuring step images the photoluminescence emitted from said area.

3. A method according to claim 1, wherein said material is a silicon ingot or a silicon brick and wherein said method is applied to at least one side facet of said ingot or brick.

4. A method according to claim 1, wherein said background doping density is measured experimentally.

5. A method according to claim 1, wherein said background doping density is determined empirically or calculated using a theoretical relationship.

6. A method according to claim 1, wherein said normalised photoluminescence intensity is interpreted as a measure of the effective minority carrier lifetime of said material.

7. A method according to claim 1, wherein said normalised photoluminescence intensity is converted to a measure of the bulk minority carrier lifetime of said material using a predetermined theoretical relationship between bulk lifetime and normalised photoluminescence intensity.

8. A method according to claim 7, wherein one said theoretical relationship is applied to multiple samples of said material with similar surface preparation.

9. A method according to claim 1, wherein said property is the area or volume density of dislocations in said material.

10. A method according to claim 1, wherein the photoluminescence measurements are used as a cutting guide in wafer production.

11. A method according to claim 1, wherein the information obtained from said method is used to improve the manufacturing of silicon bricks or ingots.

12. A method according to claim 1, wherein the information obtained from said method is used to determine the price of wafers derived from said material.

13. A method according to claim 1, wherein the photoluminescence measurements are used as a guide in wafer production to sort wafers into quality bins.

14. A method according to claim 1, wherein the information obtained from said method is used to obtain feedback on feedstock quality in the production of silicon wafers.

15. A system for conducting an analysis of a semiconductor material, said system including:

a photodetection unit for obtaining at least one image or line scan of photoluminescence generated from a surface of said material; and

a processor for normalising the measured photoluminescence intensity with regard to variations in the background doping density across said surface, and for analysing the normalised photoluminescence intensity in terms of one or more properties of said material.

16. A system according to claim 15, further including:

an optical source emitting light with wavelength longer than the band-gap of silicon or of said semiconductor material; and

a detector for measuring the transmission of said light through said silicon or semiconductor material.

17. A system according to claim 15, wherein said photodetection unit includes an InGaAs camera.

18. A system according to claim 15, wherein said photodetection unit includes a silicon camera.

19. A system when used to implement the method according to claim 1.

20. A method of conducting an analysis of a semiconductor material, said method including the steps of:

(a) exciting a portion of said material to produce photoluminescence;

(b) measuring the distribution of the photoluminescence emitted from said portion;

(c) normalising the measured photoluminescence distribution with regard to variations in the effective minority carrier lifetime across said portion; and

(c) analysing the normalised photoluminescence distribution in terms of variations in the background doping density of said material across said portion.

21. A method according to claim 20, wherein said material is a silicon ingot or a silicon brick and wherein said method is applied to at least one side facet of said ingot or brick.

22. A method according to claim 20, wherein said portion is in the form of a line scan or a two-dimensional area.

23. A method according to claim 20, wherein said method is applied to an ingot, brick or wafer of upgraded metallurgical grade silicon.

24. A method according to claim 23, wherein the position of a minimum in the distribution of said photoluminescence is fitted to obtain the ratio of donor and acceptor concentrations in the feedstock of said upgraded metallurgical grade silicon.

25. A method according to claim 20, wherein the photoluminescence measurements are used as a cutting guide in wafer production.

26. A method according to claim 20, wherein the information obtained from said method is used to improve the manufacturing of silicon bricks or ingots.

27. A method according to claim 20, wherein the information obtained from said method is used to determine the price of wafers derived from said material.

28. A method according to claim 20, wherein the photoluminescence measurements are used as a guide in wafer production to sort wafers into quality bins.

29. A method according to claim 20, wherein the information obtained from said method is used to obtain feedback on feedstock quality in the production of silicon wafers.

30. A system when used to implement the method according to claim 20.

31. A system for conducting an analysis of a semiconductor material, said system including:

a photodetection unit for obtaining at least one image or line scan of photoluminescence generated from a surface of said material; and

a processor for normalising the measured photoluminescence intensity in each part of said image or line scan with regard to variations in the effective minority carrier lifetime across said surface, and for analysing the normalised photoluminescence image or line scan in terms of variations in the background doping density across said surface.

32. A system according to claim 31, further including:

an optical source emitting light with wavelength longer than the band-gap of silicon or of said semiconductor material; and

a detector for measuring the transmission of said light through said silicon or semiconductor material.

33. A system according to claim 31, wherein said photodetection unit includes an InGaAs camera.

34. A system according to claim 31, wherein said photodetection unit includes a silicon camera.

35. A method of conducting an analysis of a semiconductor material, said method including the steps of:

(a) exciting a portion of said material to produce photoluminescence;

(b) measuring the distribution of the photoluminescence emitted from said portion; and

(c) analysing the photoluminescence distribution in terms of variations in the background doping density of said material across said portion.

36. A method according to claim 35, wherein the position of a minimum in said photoluminescence distribution is used to identify the position of a transition region from p-type to n-type in upgraded metallurgical grade silicon.

37. A method according to claim 35, wherein the photoluminescence measurements are used as a cutting guide in wafer production.

38. A method according to claim 35, wherein the information obtained from said method is used to improve the manufacturing of silicon bricks or ingots.

39. A method according to claim 35, wherein the information obtained from said method is used to determine the price of wafers derived from said material.

40. A method according to claim 35, wherein the photoluminescence measurements are used as a guide in wafer production to sort wafers into quality bins.

41. A method according to claim 35, wherein the information obtained from said method is used to obtain feedback on feedstock quality in the production of silicon wafers.

42. A system when used to implement the method according to claim 35.