One of the most remarkable phenomena in the early days of silicon technology was the “emitterpush” effect which causes an enhanced diffusion of dopants below regions with high phosphorus concentrations (see Section 5.8.3). To explain it, it was soon concluded that some sort of farranging “interaction particle” is needed. Since other impurities as well as extended defects could be excluded, this role was equally soon attributed to the intrinsic point defects introduced in Section 1.1.3. Indeed, intrinsic point defects play important roles in nearly all theories of impurity diffusion in silicon. Therefore, before proceeding with the latter in the next chapters, the properties of intrinsic point defects will be reviewed in the following.

Intrinsic Point Defects

Deep level transient spectroscopy measurements of electron traps in MeV proton‐ and alpha‐irradiated n‐type silicon have been performed. Six deep levels are found in proton‐irradiated samples, while only three appear after alpha irradiation. The influence of the irradiation dose on the defect production is investigated together with the depth concentration profiles. The profiles scale with the nuclear energy deposition, but in the case of the doubly negative charged state of the divacancy at EC −0.24 eV, the peak concentration at the end of the track is less pronounced relative to the tail region towards the surface. It is proposed that the singly negative charged state at EC −0.42 is more probable in a highly distorted lattice and it is shown that the formation of the singly negative charged state of the divacancy dominates the defect production for higher doses.

Deep level transient spectroscopy analysis of fast ion tracks in silicon

Silicon samples doped with tin and/or phosphorus have been irradiated at room temperature by 2.0 MeV electrons in order to study the generation of divacancy centers (V2). The samples were analyzed using low‐temperature infrared absorption measurements and deep level transient spectroscopy. The production of V2 is found to be very small in the tin‐doped samples, which suggests that the generation process hinges strongly on the existence of mobile monovacancies. Substitutional Sn atoms act as efficient traps for migrating vacancies and form stable vacancy‐tin centers with a corresponding reduction in the formation of ‘‘ordinary’’ vacancy‐related defects. Indeed, at temperatures above ∼150 °C where the vacancy‐tin centers dissociate, a pronounced increase in the V2 concentration occurs. On the other hand, measurements of the V2 concentration in the non tin‐doped samples as a function of electron dose using different dose rates reveal that the steady‐state concentration of monovacancies during irradiation has...

Generation of divacancies in silicon by MeV electrons: Dose rate dependence and influence of Sn and P

Deep-Level Transient Spectroscopy (DLTS) has been used to investigate the defects in high-resistivity silicon detectors after 1×10 11  proton cm −2 irradiation at room temperature. The ( E c −0.43±0.02) eV peak is seen in all materials after irradiation by neutrons or protons and has been investigated carefully. Annealing experiments show that it consists of four individual defects, one of which is the single negative divacancy. Two defects ( E c −0.35±0.02) eV and ( E c −0.45±0.03) eV anneal out at 70°C. A level ( E v +0.20±0.02) eV was also observed to anneal out at about 60°C in a p-type epitaxial diode after neutron irradiation. We propose that these levels can be identified as to be V 3 + ( E v +0.20±0.01) eV, V 3 − ( E c −0.45±0.03) eV and V 3 −− ( E c −0.35±0.02) eV charge states of the trivacancy, V 3 . A defect at ( E c −0.37±0.02) eV annealed out at 170°C and may be the four vacancy, V 4 .

Deep-level transient spectroscopy studies of silicon detectors after 24-GeV proton irradiation and 1 MeV neutron irradiation.

Defects produced in 8 MeV proton irradiated silicon were studied using Fourier transform infrared spectroscopy (FTIR) and positron annihilation measurements (PAS). Isothermal annealing of the divacancy absorption band monitored using FTIR, has been compared with PAS on similar samples. The two methods agree perfectly during isothermal annealing at 150 °C, but at 250 °C the 1.8 μm absorption band disappears after annealing for 60 min, whereas positron lifetime and trapping rate remain constant, and annealing to 500 °C is required to remove the divacancy response. Since divacancies are not mobile at 150 °C their annealing can be ascribed to recombination with mobile interstitials. The discrepancy observed during annealing at 250 °C is suggested to be a consequence of some sort of divacancy agglomeration.

Divacancies in proton irradiated silicon: Comparison of annealing mechanisms studied with infrared spectroscopy and positron annihilation

The infrared optical absorption peak at 0.34 eV in silicon, which is usually associated with the singly negative charge state of the divacancy, has been investigated in electron-irradiated samples with use of different optical excitation conditions. It is proposed that a strong Jahn-Teller distortion makes it possible to populate this charge state, when the defect initially is in the neutral charge state, either by the capture of a photoexcited free electron from the conduction band, or by the direct photoexcitation of an electron from the valence band to a defect orbital. Experimental evidence for the existence of these reactions is presented. A defect level at ${E}_{c}$-0.54 eV, frequently associated with the singly negative charge state of the divacancy, is identified as one of the levels from which these photoexcited free electrons originate. The 0.34-eV peak is attributed to an internal transition in the singly negative charge state of the divacancy center, implying the existence of a shallow defect state at approximately ${E}_{c}$-0.07 eV for this charge state. Experimental support is given for the existence of this shallow state. A tentative explanation, based on the strong Jahn-Teller distortion of the singly negative charge state, is suggested for the fact that the doubly negative charge state is not observed at temperatures below 90 K.

B. G. Svensson

Papers

Infrared absorption studies of the divacancy in silicon: New properties of the singly negative charge state.