L. Palmetshofer

Damage and polymerization by ion bombardment of C60

Sublimated C60 fullerite films have been implanted with 160−300 keV H, He, C, and Ar ions with doses ranging from 1×1012 to 5×1016 cm−2. Raman scattering showed a structural transformation of the fullerite to amorphous carbon at certain doses depending on the projectile. This amorphization process is correlated to the energy loss by nuclear collisions. Electronic stopping leads to no detectable disruption of fullerene molecules but to a polymerization of C60. This new phase is evidenced by several changes in the Raman spectrum, of which a new line at about 1463 cm−1 is most prominent.

Defect levels in H+‐, D+‐, and He+‐bombarded silicon

Defect levels produced by H+, D+, and He+ bombardment of silicon with different phosphorus doping and oxygen content were investigated using transient capacitance spectroscopy. After He+ implantation only pure damage defect levels occur, whereas after H+ and D+ implantation additional hydrogen‐related defects are observed. For vacancy‐related defects both the peak concentration and the halfwidth of the profiles depend only on the Fermi energy. The profiles were found to be broader than the theoretical vacancy distribution. The broadening which increases with decreasing doping level is explained by a model based on electric‐field‐enhanced diffusion. The production behavior of the defects shows a linear dependence of the sheet concentration on the ion dose for vacancy‐related defects and a quadratic dependence for the hydrogen related defect E(0.30). This defect is tentatively identified as the (H‐V) pair.

Boron channeling implantations in silicon: Modeling of electronic stopping and damage accumulation

Channeling implantations of 20 keV boron into silicon have been performed with doses between 1013 and 1016 cm−2 in the [100], [110], and [211] direction, and parallel to a (111) plane. Simulations using an empirical electronic stopping model agree very well with the experimental results. The model has been obtained considering a large number of random and channeling implantations published in the literature. It contains a nonlocal and an impact parameter dependent part with the nonlocal fraction increasing with energy. Moreover, a computationally efficient damage accumulation model is presented which takes point defect recombination into account. It is found that due to interactions within a recoil cascade only 1/8 of the generated damage is stable, and that damage saturation takes place at a concentration of 4×1021 cm−3. Comparison of simulations and experiments indicates that displaced atoms reside on random positions rather than on tetrahedral interstitial sites in the silicon lattice.

On the local structure of optically active Er centers in Si

We report high resolution (<0.05 cm−1) photoluminescence (PL) spectra of erbium implanted float‐zone (FZ) and Czochralski grown (CZ) silicon. We show that the PL spectrum of cubic Er centers observed in CZ‐Si annealed at 900°C is the dominant emission in FZ‐Si for the same annealing conditions. We assign it to isolated, interstitial erbium. We observe also two other kinds of optically active Er centers with lower than cubic site symmetry: (i) O‐related (found only in CZ Si) and (ii) those related to radiation defects. We conclude that coimplantation with light elements does not lead to the formation of Er‐codopant complexes, but rather to Er forming complexes with implantation induced lattice defects.

Different Er centres in Si and their use for electroluminescent devices

Abstract At low temperatures, Er in Si produces a big variety of spectra in the 1.5xa0μm region which can be identified by high-resolution spectroscopy as being due to either interstitial Er or different complexes of Er with oxygen, intrinsic defects and other light impurities. Although the luminescence yield can be improved by codoping with light elements (C, N, O, F, etc.) all of these centres show strong thermal quenching of the luminescence above 150–200xa0K. There is, however, one type of rather broad spectrum in heavily Er- and O- doped Si, which is seen up to temperatures of 400xa0K and above. This spectrum can be excited in Si by hot electrons generated in a reverse biased diode. The same spectrum appears also in other Si related materials like porous Si and in silica with the same temperature dependence. In these materials, excitation spectroscopy is possible and it shows also close agreement of the excitation spectra. From these findings we infer that Er is incorporated in another surrounding and we propose Si–Er–O nano-precipitates since the spectra of other candidates, like Er 2 O 3 , are clearly different. We review recent work on the excitation and quenching mechanisms and we discuss consequences for technology.

Doping of fullerenes by ion implantation

Abstract C 60 films have been implanted with 30 keV K + ions with doses from 1×10 14 to 1×10 16 cm −2 at target temperatures up to 350°C. Conductivity measurements performed in situ show a decrease in sheet resistivity with increasing dose for fullerene samples implanted at elevated temperatures, whereas implantation at room temperature results in no change of resistivity. After exposing the samples to air the resistivity remains constant over weeks. Raman scattering showed that an amorphous surface layer is formed by the implantation process and that the fullerene molecules beyond this layer remain undestroyed. Secondary ion mass spectrometry and Rutherford backscattering indicate that at elevated implant temperatures K diffuses into deeper regions of the film whereas oxygen is present only in a surface layer about 10 nm thick.

Ion beam radiation damage of thin fullerene films

Abstract Previous work on the radiation damage of fullerene by energetic ions does not unambiguously reveal whether fullerene destruction is dominantly due to the transfer of nuclear, or electronic energy. New data is presented, using new systems of analysis, which show that in general it is collisionally induced destruction, due to nuclear transfers, which dominates. This even remains the case when the absolute amount of energy transferred by means of electronic excitation of he fullerene exceeds that from nuclear transfers by more than a hundredfold. Only in the specific case of high energy, heavy ion bombardments can major bulk destruction of fullerence be assigned to energy transfers which are specifically electronic in origin in addition, however, there are quite minor destructive influences due to electronic energy transfer during irradiations of thin film fullerene targets with low energy proton beams.

On the environment of optically active Er in Si-electroluminescence devices

We report sharp, atomlike electroluminescence spectra close to 1.54 μm from a low-dose (3.5×1018u2009cm−3) erbium-implanted silicon light-emitting diode operating under forward bias. The well-resolved Stark splitting identifies the isolated interstitial Er with cubic site symmetry as the source. The full width at half maximum of the most intense line is 0.5 nm. A comparison with a highly Er (5×1019u2009cm−3) and O (1×1020u2009cm−3) doped diode with a high doping gradient grown by molecular beam epitaxy and with Er-implanted silica is given with respect to fine structure and thermal quenching. The room-temperature emission of the highly Er and O doped diode is ascribed to Er containing silica precipitates within the c-Si matrix.

Optical Er-doping of Si during sublimational molecular beam epitaxy

We report the first application of sublimation molecular beam epitaxy to grow uniformly and selectively doped Si : Er layers with a high concentration of Er and O (up to ∼ 10 19 cm -3 ) and high quality of crystalline structure. All samples exhibit photoluminescence close to 1.54 μm up to 200 K. New optical active Er center with their main lines at 6502, 6443, 6393, 6342, 6337 and 6268 cm -1 was observed in the PL spectra.

Ion‐implantation‐induced damage and resonant levels inPb1−xSnxTe

The dependence of the carrier concentration on the implantation dose and on the temperature was investigated in ion‐implanted thin films of Pb1−xSnxTe (0⩽x<0.1). By assuming a twofold defect level in the conduction band we are able to fit the experimental results. With increasing tin content the energy of the defect level shifts towards the conduction‐band edge. By extending the results to SnTe a general model for the understanding of the electrical properties of ion‐implanted Pb1−xSnxTe (0⩽x⩽1) is suggested.