Photoconductivity Study of CrB and Cr I in Silicon
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PHOTOCONDUCTIVITY STUDY OF CrB AND CrI IN SILICON A. Schlette, R. Kienle, A. Ddrnen, W. Kirner, and K. Thonke; Universitdt Stuttgart, 4. Phys. Institut, Pfaffenwaldring 57, D 7000 Stuttgart 80
ABSTRACT We observe in photoconductivity (PC) measurements EMT-like excited the pseudodonor CrB, which are responsible for the capture of free due to their short lifetime. The energetic position of the ground be located at E + (292 ± 5) meV. A finestructure of the (CrB) 0 V
states of electrons state can transition
consisting of at least 15 components Is observed in PC and high-resolution Fourier transform photoluminescence. In acceptor-free silicon a step in photoconductivity at 205 ± 10 meV is ascribed to Interstitial Cr, whereas a second step at - 425 meV could be due to substitutional Cr. INTRODUCTION Chromium Is a transition metal which diffuses fast into silicon , tends to form complexes with different acceptors [1], and acts as efficient recombination center [2]. Especially the chromium-boron pair was studied in some detail by EPR [1], Photoluminescence (PL) [3,4], and DLTS (4,5].This CrB complex is made up of a boron atom residing on a substitutional site, being negatively charged In the ground state, and an Interstitial positively charged chromium atom with electron configuration 3d5 [1]. It acts as deep donor with binding energy E v +0.27 eV [4]. The process giving rise to PL spectra was Interpreted as recombination of a shallow bound electron (binding energy - 15 meV) according to thermal data, into the strongly localized donor state. It remained unclear if the charge state of Cr or B is altered during this step [3]. In both EPR and PL a [111] axis was found for the CrB-pair. Isolated chromium on interstitial sites was also detected first by EPR [1]. It gives rise to a donor state 0.23 eV below the conduction band [4,6].No luminescence signal was reported for this defect. Similar to iron, chromium seems to prefer interstitial sites and was found in a substitutional configuration only after codiffusion with copper [1] or irradiation [7]. From photoconductivity measurements a donor level at E -0.4 eV was ascribed to Cr [7]. C 5 EXPERIMENTAL The samples used in this study were prepared as described in Ref. 4. The chromium concentration is estimated to vary between 1013 and 2x1015 [4]. Photoconductivity spectra were recorded with a Bomem DA3 Fourier transform spectrometer equipped with a quartz beamsplitter and halogen light source for the near infrared wavelength region, and a KBR beamsplitter and globar source for the mid infrared. The samples were placed in a gas flow type cryostat, and the sample current was amplified by a transimpedance amplifier. RESULTS:
CHROMIUM BORON PAIRS
For boron concentrations of - 2xi0is cm-3 in the starting material and chromium diffusion at 1200 °C both atomic species are abundant in about the same concentration. So the concentration of CrB pairs should already be high and most boron acceptors are expected to be compensated. A typical spectrum obtained on such samples in the energy
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