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

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 analysed in terms of effective minority carrier lifetime. In another embodiment the effective lifetime is measured by another technique, enabling PL measurements to be analysed in terms of background doping density. In 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.

What is claimed is: 
     
         1 . A method of conducting an analysis of a silicon ingot or brick, said method including the steps of:
 (a) exciting at least a portion of at least one side facet of said silicon ingot or brick to produce photoluminescence;   (b) obtaining at least one image of the photoluminescence emitted from said at least one portion of at least one side facet; and   (c) interpreting said at least one image in terms of variations in the density of dislocations in said silicon ingot or brick.   
     
     
         2 . A method according to  claim 1 , wherein photoluminescence images obtained from different side facets of said ingot or brick are analysed to obtain an estimated density of dislocations within wafers subsequently cut from said ingot or brick. 
     
     
         3 . A method according to  claim 1 , wherein said at least one photoluminescence image is used to highlight regions of said ingot or brick of insufficient quality for wafer production. 
     
     
         4 . A method according to  claim 1 , wherein the at least one photoluminescence image is used as a cutting guide in wafer production. 
     
     
         5 . A method according to  claim 1 , wherein the information obtained from said method is used to improve the manufacturing of silicon bricks or ingots. 
     
     
         6 . A method according to  claim 1 , wherein the information obtained from said method is used to determine the price of wafers derived from said silicon ingot or brick. 
     
     
         7 . A method according to  claim 1 , wherein the at least one photoluminescence image is used as a guide in wafer production to sort wafers into quality bins. 
     
     
         8 . 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. 
     
     
         9 . A method according to  claim 1 , wherein steps (a) and (b) are performed when scanning said ingot or brick and an illumination/detection system relative to each other. 
     
     
         10 . A system for conducting an analysis of a silicon ingot or brick, said system including:
 a photodetection unit for obtaining at least one image or line scan of photoluminescence generated from at least one side facet of said silicon ingot or brick; and   a processor for interpreting said at least one photoluminescence image or line scan in terms of variations in the density of dislocations in said silicon ingot or brick.   
     
     
         11 . A system according to  claim 10 , wherein said processor is adapted to analyse photoluminescence images or line scans obtained from different side facets of said ingot or brick to obtain an estimated density of dislocations within wafers subsequently cut from said ingot or brick. 
     
     
         12 . A system according to  claim 10 , wherein said processor is adapted to highlight regions of said ingot or brick of insufficient quality for wafer production. 
     
     
         13 . A system according to  claim 10 , wherein said processor is adapted to obtain information from said at least one photoluminescence image or line scan for use as a cutting guide in wafer production. 
     
     
         14 . A system according to  claim 10 , wherein said processor is adapted to obtain information from said at least one photoluminescence image or line scan for use in improving the manufacturing of silicon bricks or ingots. 
     
     
         15 . A system according to  claim 10 , wherein said processor is adapted to obtain information from said at least one photoluminescence image or line scan for use in determining the price of wafers derived from said silicon ingot or brick. 
     
     
         16 . A system according to  claim 10 , wherein said processor is adapted to obtain information from said at least one photoluminescence image or line scan for use as a guide in wafer production to sort wafers into quality bins. 
     
     
         17 . A system according to  claim 10 , wherein said processor is adapted to obtain information from said at least one photoluminescence image or line scan for use in obtaining feedback on feedstock quality in the production of silicon wafers. 
     
     
         18 . A system according to  claim 10 , further comprising a mechanism for scanning said photodetection unit and said ingot or brick relative to each other. 
     
     
         19 . A system according to  claim 10 , further comprising:
 an optical source emitting light with wavelength longer than the band-gap of silicon; and   a detector for measuring the transmission of said light through said silicon ingot or brick.   
     
     
         20 . A system when used to implement the method according to  claim 1 .