Energy Levels and Capture Cross-Sections of Thermal Donors in Silicon
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ENLH(.JY LEVELS AND CAPTURE CROSS-SECTIONS OF THERMAL DONORS IN SILICON A. HENRY, J.L. PAUTRAT, N. MAGNEA and K. SAMINADAYAR Centre d'Etudes Nucl6aires de Grenoble, DRF/SPh/PSC/ - 85 X 38041 GRENOBLE CEDEX
ABSTRACT lhe electrical properties of thermal donors centers, generated after annealing CZ-silicon at 450o C are carefully evaluated. The energy levels are corrected for the Poole-Frenkel lowering are compared to the ionization energy of the dominant centers revealed by infrared absorption experiments. The capture cross sections are found to be ;t 2 x 10-12 cm 2 for TDI and - 2 x 10- 1 3 cm 2 for TD2. Detailed examination of their temperature dependence reveals that they follow a common law cr a T -5 INTRODUCTION On annealing of Czochralsky Silicon at 4500 C donors centers are generated. the so-called Thermal Donors (TD). Infrared absorption experiments at low temperature reveal a set of 8-9 double donors whose ionization energies are very close each to other (1). The detailed structure of these centers is not known although a C2v symmetry has been deduced from EPR spectra (2) and from DLTS or IR spectra under stress 3,.4). Several models have been proposed for the structure of these centers. They all involve the formation of a small oxygen cluster since TD centers are clearly related to the early stages of oxygen precipitation (5,61. The electrical characterization has suggested that these TD centers are really double donors [7] but a precise determination of the concentrations, energy levels and capture cross sections using the same sample has not been performed up to now. EXPERIMENTAL TECHNIQUES In order to determine the electrical properties of these centers we have used the admittance spectroscopy technique either in the harmonic mode (81 or In the pulsed mode [9]. The working frequency ranges between 100 Hz and 100 KHz and the minimum duration of the pulses is 20 ns. The analysed samples can be cooled down to = 10 K and admittance spectroscopy spectra are usually recorded during slow heating. The test devices are Schottky diodes obtained by evaporating Gold on Silicon. Various Silicon crystal were analysed. The main parameters of the materials are listed in Table I. before and after the annealing treatment of 96 h. 4500 C under vacuum.
Infra-Red absorption spectra are recorded between 6 and 120K with a resolution better than 2 cm- 1 . EXPERIMENTAL RESULTS 1. Concentration of centers A typical admittance spectroscopy spectrum obtained under harmonic excitation is shown in Fig. 1. It has been obtained on sample = 413 at I KHz : Mat. Res. Soc. Symp. Proc. Vol. 59. -1986 Materials Research Society
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C
C
05Co
U
2
.
~~~
20
6
8
0
40
Temperature (K)
=413 annealed. Fig. 1. Admittance spectroscopy curves obtained at I KHz for The background phosphorus doping controls the freezing of carriers (34. 8 meV) while the two ionization states of TD are observed with 51.2 and 116.8 meV activation energies. the capacitance curve (a) decreases in 3 steps. associated to 3 peaks on the conductance curve (b). W
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