Influence of deep levels on space charge density at different temperatures in γ-irradiated silicon

https://doi.org/10.1016/j.nima.2004.05.062Get rights and content

Abstract

In this work, it is shown that the analysis of thermally stimulated currents (TSC) and current transients (CT) at constant temperature can be a suitable tool to study the influence of deep levels on space charge density N(T) in irradiated silicon diodes. In particular, the occurrence of space charge sign inversion (SCSI) can be related to signal discontinuities in TSC and CT measurements. This approach has been adopted in this work to study devices made of standard Float Zone (FZ) and Diffusion Oxygenated Float Zone silicon, irradiated by γ-rays up to a dose of 300 Mrad. Our study shows that all the samples are inverted at 50 K after a low-temperature excitation. Several space charge sign inversions, from positive to negative and vice versa, have been observed between cryogenic and room temperature, and have been related to carriers emission from dominating deep traps. Only standard FZ silicon remains inverted at room temperature after a dose of 300 Mrad.

Introduction

The application of silicon detectors in modern high-energy physics experiments needs to face the constraints imposed by an extremely severe radiation environment where these devices will be operated. This applies in particular to the instrumentation at the Large Hadron Collider (LHC) at CERN [1], [2], where the luminosity will reach values as high as 1034 cm−2 s−1. A possible upgrade of LHC (“SuperLHC”) will increase this value by one order of magnitude, leading to fast hadron fluences at the innermost radius up to ∼1016 cm−2, expected for 5 years of operation [3]. The present silicon detector technology is not able to withstand such levels of irradiation. Typically, silicon position-sensitive particle detectors are now produced from n-type high resistivity (1–6 kΩ cm) phosphorous-doped float zone material, with segmented p+ boron implants in the form of microstrips or pixels [4] and a uniform n+ implant on the rear side to obtain an ohmic contact. Detectors are required to work above full depletion, to maximize the collected charge signal. The microscopic effect of radiation is to produce defects, which behave as traps of charged carriers or generation–recombination centers. Point defects, for e.g. vacancy (V) or double vacancy (V2), impurity complexes, and clusters, which are extended aggregates of defects, are created. Radiation-induced defects produce macroscopic damage to the device, affecting operative parameters as full depletion voltage Vdep, leakage current Idep and charge collection efficiency CCE (see Refs. [5], [6] for two reviews). In order to improve the radiation hardness of silicon detectors, a thorough understanding of the correlation between irradiation, radiation-induced microscopic lattice disorder and detriment of operative characteristics is then necessary.

One of the main microscopic effect of radiation on silicon detectors is the change of the effective doping concentration Neff of the space charge region measured at room temperature. This is due to the initial shallow donor removal or compensation, and to the generation of radiation-induced charged deep levels [5], [6]. The changes in Neff after irradiation have been widely investigated. It is well known that, by increasing the fluence of fast hadrons or the irradiation dose, the sign of Neff in standard float zone n-type silicon changes. This effect is known as space sign charge inversion (SCSI). Correspondingly, a shift of the junction from the p+ side to the n+ side is observed [7]. When the irradiation dose is raised beyond SCSI, the growth of the space charge density produces an increase of the full depletion voltage. It has been proven that the deliberate addition of oxygen (with a concentration [O]>1017 cm−3) in the bulk material increases the radiation hardness of the device with respect to proton and γ-ray exposure [8], [9]. In the case of high resistivity Standard Float Zone (STFZ) silicon, irradiated by γ-rays from a 60Co source, the inversion dose is about 230 Mrad. On the contrary, in Diffusion Oxygenated Float Zone Silicon (DOFZ) no SCSI has been observed up to 1.6 Grad [10]. The relationship between Neff at room temperature and the distribution of radiation-induced deep levels has been investigated in Ref. [11]: the SCSI occurring in STFZ Si has been attributed to deep defects lying around midgap, which are suppressed in DOFZ material.

It has been recently shown that discontinuities in Thermally Stimulated Currents (TSC) spectra [12] and Current Transient (CT) measured at constant temperature [13] are related to SCSI. This work presents a first qualitative correlation between radiation-induced deep level population and the changes in space charge density N(T) due to temperature in STFZ and DOFZ silicon. It has been carried out by means of the TSC and CT techniques in the range 30–300 K using silicon detectors γ-irradiated with a 60Co source up to the dose of 300 Mrad.

Note that, in general, the space charge density depends on sample history, especially on bias and excitation, because different fractions of deep levels may be charged depending on experimental conditions. At room temperature the equilibrium between deep levels and carriers population is reached within a fraction of millisecond, and N equals the effective doping concentration Neff which can be measured by capacitance–voltage profiles. However, the situation may be different at a lower temperature. The space charge density at different temperatures N(T) is measured in this work using, as a standard condition, low-temperature carriers injection.

Section snippets

Experimental methods

The general approach followed by the high-energy particle community in radiation-damage studies is to investigate primarily the radiation effects in silicon detectors using the simplified geometry of a single pad detector, i.e. a single-pad p+ n n+ diode (SPD). Usually the electrode area on the p+ side (Ap), cover only a portion of the whole area of the device, due to the presence of a structure of surrounding guard-rings. On the contrary, the electrode on the n+ side covers the whole device

Samples and experimental setup

Two silicon p+n n+ diodes, produced using Wacker n-type silicon with 〈1 1 1〉 orientation, are considered. One of the samples is made with STFZ silicon, the other with DOFZ Si obtained by a 72 h oxygen diffusion at 1150°C. Samples thickness is w=285 μm. The electrode on the p+ side has an area Ap=25 mm2 smaller than the area An=100 mm2 of the electrode on the n+ side, which covers the entire device. Irradiation was performed by the 60Co source of the Brookhaven National Laboratory, Upton, NY, with 1.25

Experimental results

The TSC spectrum of SFTZ sample below 80 K is shown in Fig. 1. Several peaks are revealed below 55 K. They are currently under study and will not be considered here. The VO−/0 (generated by vacancy-oxygen defect) and CiCs−/0 (from interstitial carbon-substitutional carbon complex) deep levels are responsible for the feature between 55 and 75 K. VO−/0 corresponds to a deep level at 0.17 V below conduction band [19] with a capture cross section σ in the range 10−14–10−15 cm2 [20], [21]. The CiCs−/0

Evaluation of N(T)

An approximate description of N(T) can be obtained from the observation of SCSI temperatures in TSC spectra. The case of the DOFZ sample is shown in the upper plot of Fig. 4. No relevant trap emissions take place between 300 K and the temperature of the V2 discharge, near 170 K. Three SCSI occur during V2, CiOi and VO discharge. It must be noted that the sample is inverted at 50 K, even if the bulk had a starting n-type character. A SCSI+/− subsequent to phosphorus ionization must occur, and it

Conclusions

This work demonstrates that by low voltage (Vrev<Vdep) TSC and CT analysis it is possible to study qualitatively the space charge density N(T) as a function of the temperature. With these methods space charge sign inversion can be detected and accounted in terms of discharge of the main radiation-induced defects. The temperature dependence of N has been studied in STFZ and DOFZ silicon, irradiated with γ-rays up to the fluence of 300 Mrad, and significant differences are revealed in the

Acknowledgements

The authors wish to thank Mr. Andrea Baldi from DEF for his help in preparing the experimental setup, and Prof. Gunnar Lindstroem, from University of Hamburg, for his helpful suggestions.

References (32)

  • J. Varela

    Nucl. Phys. B

    (1995)
  • Z. Li et al.

    Nucl. Instr. and Meth. A

    (1997)
  • Z. Li et al.

    Nucl. Instr. and Meth. A

    (2001)
  • E. Fretwurst et al.

    Nucl. Instr. and Meth. A

    (2003)
  • Z. Li et al.

    Nucl. Instr. and Meth. A

    (2003)
  • I. Pintilie et al.

    Nucl. Instr. and Meth. A

    (2003)
  • M. Moll

    Nucl. Instr. and Meth. A

    (1997)
  • D.C. Schmidt

    Nucl. Instr. and Meth. B

    (1997)
  • L.F. Makarenko

    Physica B

    (2001)
  • J. Stahl et al.

    Nucl. Instr. and Meth. A

    (2003)
  • The Large Hadron Collider, conceptual design, The LHC study group, CERN/AC/95-05 (LHC), 20 October...
  • CERN Internal Report, no....
  • H.F.W. Sadrozinski

    IEEE Trans. Nucl. Sci.

    (2001)
  • M. Bruzzi

    IEEE Trans. Nucl. Sci

    (2001)
  • G. Lindstroem

    Nucl. Instr. and Meth. A

    (2003)
  • I. Pintilie et al.

    Appl. Phys. Lett.

    (2001)
  • Cited by (0)

    View full text