IR Tomography of the Lifetime and Diffusion Length of Charge Carriers in Semiconductor Silicon Ingots
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ABSTRACT A nondestructive method for estimating quality of single-crystal Si ingots is proposed. The method provides a three-dimensional pattern of the lifetime and diffusion length of charge carriers
inside Si ingots up to 300 mm indiameter and I m in length. The method employs optical probing of ingots with laser-emitted radiation and includes laser-induced photoinjection of charge carriers
followed by laser-assisted monitoring of their spatial distributions and time evolution inany part of the ingot about 1cm in size. INTRODUCTION The lifetime -cand the diffusion length L of non-equilibrium carriers are the main parameters
that characterize the quality of Si as a semiconductor material. Measurements of - and L are widely used to analyze both silicon wafers and device structures fabricated on them, and numerous methods for measuring these parameters are progressing rapidly. However, the traditional approach to characterization of silicon ingots as a whole has remained unchanged since early 50ths: usually, on the both end faces of ingot, two wafers are cut whose surfaces are properly treated to reduce the detrimental influence of surface recombination. Afterwards, a parameter of interest is measured using one of currently available methods specially devised for semiconductor wafers (see, e.g., ASTM F 1535-94). However, nondestructive measurements of the above parameters inmonolithic ingots are also of obvious interest both from the viewpoint of checking the quality of Si ingots as a final product by manufacturers (without any uncertainties in
"-brought about by surface recombination)
and for gaining information about the spatial
distribution of x inside ingot when choosing optimal growth conditions or giving the ingot additional thermal or radiation treatments, e.g., in the course of its neutron transmutation doping. The aim of the present paper is to develop a new technique for elucidating the threedimensional pattern of both c and L inCz-grown and float-zone Si ingots on the up-to-date level of modern experimental facilities. The physical essence of the proposed method is a two-beam optical probing of the ingot under study with the help of pulsed photoinjection of excess electrons and holes, and their subsequent monitoring via observation of light absorption by free carriers, which was previously used for measuring "r in semiconductor wafers [1] and rectangular bars of millimeter cross section [2]. In [1,2], to generate excess carriers, a pulsed YAG laser with X=1.06 Aim was used. The radiation emitted by a YAG laser is absorbed over a length of about 1mm inside silicon. In order to trace the decay in time of the concentration of excess carriers An, the radiation emitted by a He-Ne laser with X=3.39 Am was used, which is only weakly absorbed in moderately doped silicon (in this case, the cross section of light absorption is of the order of 10-17 cm"2 [1,2]). The beams were directed on to the object under study aligned so that their axes crossed at an angle at
a point that lied on the surface of the sample
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