Search

Electrical properties of defects formed in n-Si(FZ) following 8 and 15 MeV proton irradiation are investigated by Hall effect measurements over the wide temperature range of T ≈ 25 to 300 K. Close attention is paid to the damaging factor of proton irradiation, leaving aside passivation effects by hydrogen. The concept of defect production and annealing processes being accepted in the literature so far needs to be reconsidered. Contrary to expectations the dominant impurity-related defects produced by MeV protons turn out to be electrically neutral in n-type material. Surprisingly, radiation acceptors appear to play a minor role. Annealing studies of irradiated samples of such complex defects as a divacancy tied to a phosphorus atom and a vacancy tied to two phosphorus atoms. The latter defect features high thermal stability. Identification of the dominant neutral donors, however, remains unclear and will require further, more detailed, studies. The electric properties of the material after proton irradiation can be completely restored at T = 800°C.

Hall effect and conductivity measurements are taken on Si1 − x Ge x of n- and p-type at x ≤ 0.05. Much attention is given to electrical measurements over a temperature interval of 25 to 40 K where the mobility of charged carriers is strongly affected by alloy scattering. The partial mobility of electrons and holes due to this scattering mechanism is estimated for n-Si1 − x Ge x and p-Si1 − x Ge x at small x. Together with this, an effect of the presence of Ge atoms upon the ionization energy of phosphorus and boron impurities is investigated. Some points related to an inhomogeneous distribution of Ge atoms in Si1 − x Ge x are discussed.

The present experiments using fast electron and gamma-irradiation are aimed at checking the model of defect formation in oxygen-lean n-Si(FZ) in a quantitative way. Electrical measurements are taken over a wide temperature range of 20 to 300 K. Analysis of equations of charge balance making use of the statistics of charge carriers in non-degenerate semiconductors demonstrates that group-V impurity atoms strongly interact with intrinsic point defects. As a result, the concentration of shallow donor states is markedly decreased. This loss of shallow donors, −δND, is accompanied with an increase in the concentration of radiation-produced deep acceptors, +δNArad, being equal in magnitude but opposite in sign. Such behavior correlates quantitatively with the formation model of donor-vacancy pairs put forward earlier by Watkins and Corbett, what has been proved on the basis of electrical data for the first time. The formation kinetics of these complexes is discussed. Defects of interstitial type in irradiated material appear to be electrically neutral in n-Si. However, their production in the course of electron- and gamma-irradiation is believed to be responsible for drastic changes in the mobility of charge carriers at cryogenic temperatures.

Rates of carrier removal from the conduction band in n-type FZ-Si and 4H-SiC irradiated with 8- and 15-MeV protons at room temperature are discussed. Calculated rates of formation of primary radiation defects (Frenkel pairs) in these materials are presented and compared with the corresponding experimental values. Protons create defects in collision cascades involving recoil atoms formed in the crystal lattice itself. The results are compared with similar data previously obtained in irradiation of n-type FZ-Si and 4H-SiC with 900-keV electrons, in which case isolated so-called close Frenkel pairs are absolutely dominant among primary radiation defects. It has been found that the E-center model adequately describing the decrease in the electrical conductivity of n-FZ-Si upon electron irradiation is inapplicable to interpretation of experimental data for proton-irradiated materials. It is suggested that a pronounced annealing of “simple” radiation defects of the type of close Frenkel pairs occurs during irradiation at room temperature.

Nuclear-particle detectors based on SiC with a structure composed of an n+-type substrate, a p-type epitaxial layer, and a Schottky barrier are studied. Structures with a ∼10-µm-thick 6H-SiC layer exhibit transistor properties, whereas those with a ∼30-µm-thick 4H-SiC layer exhibit diode properties. It is established that a more than tenfold amplification of the signal is observed in the transistor-type structure. The amplification is retained after irradiation with 8-MeV protons with a dose of at least 5×1013 cm−2; in this case, the resolution is ≤10%. Amplification of the signal was not observed in the structures of diode type. However, there were diode-type detectors with a resolution of ≈3%, which is acceptable for a number of applications, even after irradiation with the highest dose of 2×1014 cm−2.

It is shown that irradiation of Ni-SiC structures with protons at elevated temperatures promotes metallurgical reactions at the Ni-SiC interface owing to a diffusion mechanism stimulated by the generation of radiation defects. The most pronounced effect of mixing at the metal-semiconductor interface is observed if the thickness of the metal film is equal to the projected range of protons.

Buried nanoscale damaged layers formed in Si and SiC crystals via 50-and 100-keV proton implantation were studied. It is shown that the sensitivity of atomic-force microscopy is sufficiently high to detect the initial stages of the development of hydrogen-containing voids and microcracks in subsurface layers of irradiated crystals and to study exfoliation of the crystals in the plane of microcracks. As a result, quantitative criteria for the formation of buried damaged layers in the studied crystals were derived; also, the conditions for blistering and for implementation of the “Smart-Cut” technology were determined.

The p+-n-n+ structures based on 6H-SiC films grown by chemical vapor deposition on the n+ substrate were irradiated with 8-MeV protons with a dose of 8×1015 cm−2. In order to stabilize the material, it was annealed for 10 min at 450°C. As a result, the resistivity of the film was ρ=5×109 Ω cm. The effect of proton irradiation was studied by alpha spectrometry. The 5.77-MeV alpha particles were detected for both reverse and forward bias voltages applied to the structure. The results of the following two modes of detection were compared: (i) a particle traverses the structure without losing much of its energy, and (ii) a particle is stopped in the structure. It is shown that, in the former case and under a forward bias, a signal is formed by a mechanism involving a “through-conducting channel.” This makes it possible to determine the product of lifetime of electrons by their mobility. The situation in which the particle range R does not exceed the film thickness was analyzed; this situation is typical of spectrometry. It is noted that a decrease in R results in different behavior of the signal for the bias voltages of opposite polarity. Thus, for forward bias, the signal amplitude decreases more rapidly and for larger values of R.

Capacitance-related methods and electron spin resonance were used to study the deep-level centers formed in n-4H-SiC as a result of irradiation with 8-MeV protons. For the samples, Schottky diodes and p-n structures formed on the layers either obtained by sublimational epitaxy or produced commercially by CREE Inc. (United States) were used. It was found that the type of centers introduced by irradiation is independent of the technology of the material growth and the type of charged particles. On the basis of the results of annealing the defects and the data of electron spin resonance, the possible structure of the centers is suggested.

Capacitance methods and electron spin resonance (ESR) were applied to study deep centers in n-6H-SiC irradiated with 8 MeV protons. Schottky diodes and p-n structures grown by sublimation epitaxy or commercially produced by CREE Inc. (United States) were used. The type of the irradiation-induced centers is independent of the material fabrication technology and the kind of charged particles used. Irradiation results in an increase in the total concentration of donor centers. The possible structure of the centers is suggested on the basis of data on defect annealing and ESR.

Analysis is given of the progress in the modification of semiconductors by proton beams in fields such as proton-enhanced diffusion, ion-beam mixing, and formation of porous layers. This method of modification (doping) is shown to have high potential in monitoring the properties of semiconductor materials and designing devices of micro and nano electronics as compared to the conventional doping techniques such as thermal diffusion, epitaxy, and ion implantation.

The method of spin-dependent recombination was used to record electron spin resonance (ESR) spectra of recombination centers in a thin (∼1 µm) surface layer of p-type silicon grown by the Czochralski method and irradiated by protons with energies of ∼100 keV. Spectra of excited triplet states of the oxygen + vacancy complex (A-centers) were observed along with complexes consisting of two carbon atoms and an interstitial silicon atom (CS-SiI-CS complexes). The intensity of the ESR spectra of these radiation-induced defects was found to be largest at irradiation doses of ∼1013 cm−2, and decreased with increasing dose, which is probably attributable to passivation of the radiation-induced defects by hydrogen.

The distribution of hydrogen in Si and SiC following high-temperature proton irradiation (Tirr=20–700 °C) is studied by secondary-ion mass spectrometry. It is shown that the hydrogen concentration profile in SiC depends weakly on irradiation temperature. In Si appreciable alteration of the concentration profile is observed already at Tirr⋍300 °C, and the profile completely loses its concentration gradient at Tirr⋍700 °C.