High resistivity silicon for detector applications

272

Nuclear Instruments and Methods in Physics Research A288 (1990) 272-277 North-Holland

Section I1. Industrial production

HiGH RESISTIVITY SILICON FOR DETECTOR

APPLICATIONS

P. D R E I E R Wacker Chemitronic GmbH, P.O. Box 1140, D 8263 Burghausen, FRG

After a short introduction to the production method for high resistivity silicon, important material parameters as lifetime and resistivity distribution are discussed.

High resistivity silicon for detector application is commercially produced by the ,qoating-zone (FZ) technique [1]. The starting materials are polycrystalline silicon rods. During growth a rod is continuously molten by electromagnetic induction and crystallized cruciblefree on top of the growing monocrystal [2]. An advantage of this method is that a contact of the melt with any foreign material ean be avoided. The crystal puller is water cooled, so contamination during the growth process is reduced to a minimum. The high purity of the polycrystalline starting material can be improved by the floating-zone method, as impurities are segregated in the melt and evaporate during the growtb process. Fig. 1 shows typical impurity levels of float-zone silicon, determined by neutron

activation analysis (left group), IR spectroscopy (C, O, N) and photoluminescence analysis (B, P). Electrically active impurities are boron and phosphorus which appear in typical concentrations in the range of 10 u a t . / c m 3. Due to an excess of phosphorus in the polycrystalli~le silicon the undoped FZ-silicon is always n-type. Fig. 2 shows the resistivity distribution of undoped FZ-silicon, as it is grown for the production of N T D (neutror transmutation doped) silicon. The majority of the silicon rods has a resistivity between 1000 and 7000 f~ cm, so for some detectors material from the normal production can be used. Minority carder lifetime is an important parameter, as it determines the magnitude of the leakage current.

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273

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Fig. 2. Resistivity distribution of undoped FZ-silicon. The carrier type is always n. silicon in general it is necessary to apply several purification passes. During growth in vacuum phosphorus and other impurities evaporate and are concentrated in the melt by segregation. As boron has a small evaporation rate and its distribution coefficient is nearly equal to 1, its concentration cannot be reduced by purification passes and one has to select polysilicon with a low

Fig. 3 shows the distribution of the lifetime in undoped FZ-silicon from normal production, measured by the PCD (photoconduction decay) method. Due to the high purity of the mater~al, typical values are between 2000 and 6000 I~s. For the production of n-type silicon with a resistivity of more than 5000 f~ cm and of high resistivity p-type

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boron content as the starting material. If necessary, resistivity can be adjusted by the introduction of a doping gas into the melt. Owing to the segregation effect impurities are not equally distributed in the crystal. Using two examples the dopant distribution of h-type and p-type high resistivity silicon in typical specifications will be discussed.

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Fig. 4 shows the (macroscopic) radial resistivity profile of n-type silicon, p = 3000-5000 fl cm, diameter = 2 in., (111)-orientation, measured by the four-point-probe (4-pp) technique. The crystal has been growlz without additional purification passes. Ten slices cut from the same ingot have been measured; all profiles are plotted together in fig. 4. According to the convection pattern

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Fig. 6. Radial resistivity variation of p-type high resistivity FZ-silicon slices, diameter = 2 in., p = 6000-9000 £ cm, (lll)-orientation, measured by 4-point-probe technique. in the melt during crystal growth a typical profile with a resistivity minimum in the center results. Whereas the 4-pp technique averages resistivity over a few millimeters, the spreading resistance (SR) method gives information about the microscopic distribution with a resolution of a few microns. Fig. 5 shows a SR-profile of a slice from the lot already measured in

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fig. 4. In the edge region sawtooth-like structures appear, the so-called striations. They result from rotation of the growing crystal in the inhomogeneous field of the induction coil. Macroscopic resistivity profile and striations can result in a total resistivity variation of more than 50~. Fortunately tight resistivity tolerances are usually not

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276

P. Dreier / High resistivity silicon

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Fig. 6 shows radial 4-pp profiles of slices from high resistivity p-type silicon, diameter 2 in., p-type, p = 6000-9000 £ cm. As the segregation coefficient of b o r o n is near one, the resistivity profile is flat. In fig. 7

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P. Dreier / High resistivity silicon

a radial SR-profile of one of the slices from fig. 6 is displayed. No striations can be detected. The resistivity increase in th,~ center is possibly caused by the compensating effect of the inhomogeneous phosphorus distribution. A photolumines,cence analysis (fig. 8) of one of the slices from the investigated p-type crystal revealed that the boron concentration was 3.3 × i012 a t . / c m 3, and the phosphorus concentration was 7 × 1011 a t . / c m 3, so the compensation level was 21%. In cases where high resistivity and doping homogeneity are required, the neutron transmutation doping method can be utilized. Fig. 9 shows as an example a SR-profile of 500 ~2 cm n-type N T D silicon. Starting material for the irradiation was high resisti~4ty silicon,

277

so only a small part of the phosphorus was inhomogeneously introduced during growth, The major.part comes from the neutron doping, so the phosphorus di,stribution is very homogeneous. Depending on the homogeneity requirements this method can be used up to 5000 f~ cm.

References [1] W. v. Ammon and H. Herzer, Nuci. Instr. and Metho 226 (1984) 94. [2] W. Keller and A. Miihlbauer, Floating-zone silicon, in: Preparation and Properties of Solid State Materials, ed. W.R. Wilcox (Dekker, New York, 1981).

V. INDUSTRIAL PRODUCTION