Salient features of the fabrication of Si(Li) detectors with a large-volume working region

Abstract

Semiconductor detectors with a large working volume are needed to solve certain paroblems in nuclear spectroscopy. The development of such detectors became possible with advances in growing large-diameter silicon single crystals with the required properties, satisfying the requirements for obtaining detectors based on them. One important requirement for obtaining detectors with a large working volume is that its resistance must be high. This is achieved by using the lithium ion drift process in the volume of the semiconductor detector.However, even though the mobility of lithium ions in silicon is anomalously high, a long time is required for diffisuin-drift compensation of lithium in a large volume (W ≥ 2 mm, S ≈ 20 -80 cm 2 ). In addition, the usual method of compensation, which includes drift of lithium ions from a distributed source, produced from one end of the crystal by preliminary diffusion of lithium, does not give a uniform drift front beause of the nonuniformity of the temperature field in a large-volume crystal. The diffusion region can also become very diffuse during prolonged drift, which will result in very thick entry and exit windows and will decrease the effectiveness of the spectromeric characteristics of the detector.A new method for producing p-i-n structures was developed to decrease substantially the time required for compensation of a large volume of silicon by lithium ions and to eliminate at the same time the negative consequences of holding the crystal at a high temperature and under a high voltage. The method consists of the following. Drift of lithium ions from two ends of prepared samples is conducted to a depth sufficient for the required compensation of the initial acceptor impurity in silicon. Figure 1a shows the typical diffusion profile obtained for the distribution of the lithium concentration by the proposed method in p-Si with ρ = 10 Ω·cm and N 0 ≈ 2·10 17 cm -3 . The negligible deviation of the lithium penetration depth at the two opposite end surfaces of the crystal is due to the fact that lithium did not enter simultaneously at both ends.Before the drift voltage is applied, the crystal is shaped in the form shown schematically in Fig. 1b. After the standard required technical operations have been performed, the crystals are placed into a setup where lithium ions drift into the volume of the crystal, as shown in Fig. 2a. The drift regime was chosen taking account of the low resistance of the initial silicon [1]. It is easy to show that in the proposed method the compensation time for a prescribed volume of a p-Si crystal is decreased by a factor of 4. Indeed, according to the empirical formula for the dependence of the depth of the compensation region for the case of unilateral drift where µ is the mobility of lithium ions and U and t 1 are the drift voltage and drift time, respectively.For bilateral drift, the path traversed by the lithium ions is two times shorter, i.e., Hence for the compensation time for bilateral drift and t 2 = 1/4t 1 . t t 1 2 2 / = W U t i / . 2 2 2...

Laboratory nuclear-physics methods for determining the characteristics of charged particle silicon detectors

“…For comparison, all spectra have been normalized to the maximum in the left part of the spectrum (point А in Fig. 5), which can be attributed to the decrease in the detector efficiency at low energies [1,5,6].…”

Special features of formation of high-performance semiconductor detectors based on αSi-Si(Li) heterostructures

“…However, up to date require ments imposed on these detectors lead to the necessity of increasing their sizes up to d > 50-100 mm and W > 2 mm. In this case, physical, technological, and struc tural decisions should take the properties of the initial large size crystal into account in an attempt to meet optimum electrical, radiometric, and spectrometric characteristics of the detectors [6][7][8][9].…”