M. F. Beaufort

Swelling of SiC under helium implantation

Single crystals 4H-SiC were implanted with 50 keV helium ions at temperatures up to 600 °C and fluences in the range 1×1016–1×1017cm−2. The helium implantation-induced swelling was studied through the measurement of the step height. The different contributions of swelling were determined by combining simulations of x-ray diffraction curves and transmission electron microscopy observations. At room temperature, amorphization occurs between 1 and 2×1016cm−2, inducing the decrease in density of about 15%. For high-temperature implants, amorphization does not occur. The strain profiles show saturation in the near-surface region, indicating that a threshold concentration of defects is reached. All the additional point defects created during the implantation have been supposed to annihilate. In the region of high-energy deposition density, the value of strain increases with fluence up to values larger than 6%. The elastic contribution to swelling has been obtained by integration of the strain profile determined...

Effect of implant temperature on defects created using high fluence of helium in silicon

Extended defects formed by high-fluence helium implantation in silicon have been studied as a function of the implantation temperature, from room temperature up to 800 °C. Transmission electron microscopy results show that the formation of cavities created by a 50 keV He implantation with a fluence of 5×1016 cm−2 can be divided into three stages. For implantation temperature up to about 300 °C the bubble size is relatively constant but the bubble density decreases due to the increase in dynamic annealing. Above 300 °C, where divacancies are no longer stable and when the helium is mobile, both the density and size of the cavities stay relatively constant. In this stage, helium starts to diffuse out and the cavities become more and more faceted as the temperature increases. Concurrently interstitial-type defects appear: small elongated rod-like defects at relatively low temperatures and large ribbon-like defects at 600 °C. Finally, for implantation at 800 °C, no cavities are formed and only dislocation loops and {113}’s (ribbon-like defects and rod-like defects), are observed depending on the deposited energy profile. At this temperature the defect annealing during implantation becomes efficient to convert ribbon-like defects into dislocation loops.Extended defects formed by high-fluence helium implantation in silicon have been studied as a function of the implantation temperature, from room temperature up to 800 °C. Transmission electron microscopy results show that the formation of cavities created by a 50 keV He implantation with a fluence of 5×1016 cm−2 can be divided into three stages. For implantation temperature up to about 300 °C the bubble size is relatively constant but the bubble density decreases due to the increase in dynamic annealing. Above 300 °C, where divacancies are no longer stable and when the helium is mobile, both the density and size of the cavities stay relatively constant. In this stage, helium starts to diffuse out and the cavities become more and more faceted as the temperature increases. Concurrently interstitial-type defects appear: small elongated rod-like defects at relatively low temperatures and large ribbon-like defects at 600 °C. Finally, for implantation at 800 °C, no cavities are formed and only dislocation loop...

Evolution of defects upon annealing in He-implanted 4H-SiC

The strain induced by room temperature helium implantation into 4H-SiC below the threshold amorphization dose results from both point and He-related defects. When the helium concentration is lower than 0.5% the strain profile follows the point defect profile, whereas at higher concentrations the He ions have a dominant effect on the strain. Upon annealing, the near surface strain progressively relaxes up to 1500 °C while the maximum strain relaxation stops at a temperature where helium ions agglomerate into platelets. When the vacancies become mobile, the platelets evolve into bubble clusters that expel dislocation loops whose migration is enhanced by the strain.

Helium implantation defects in SiC: A thermal helium desorption spectrometry investigation

Thermal helium desorption spectrometry was used to characterized helium implantation-induced defects in SiC. 6H–SiC, 4H–SiC, and β–SiC samples were implanted with helium at energies ranging from 100 to 3 keV and doses ranging from 1×1013 to 1×1015 cm−2. They were then subjected to ramp annealing up to 1800 K, with a constant heating rate of 10 K/s. Two groups of peaks contribute to the desorption spectrum: A low-temperature group centered at 600 K and a high-temperature group centered at 1200 K. The evolution of these desorption peaks with implantation dose and energy was studied. The first group (at 600 K) might be attributed to interstitial He and clusters of interstitial He. The second group (at 1200 K) could be related to He release from He-vacancy clusters. A shift of the latter group toward higher temperatures with increasing dose is ascribed to He-vacancy clustering in an Ostwald ripening process. However, the intrinsic properties of the materials used in the study seem to play an important role si...

Formation of bubbles by high dose He implantation in 4H–SiC

1.6 MeV He+ ions were implanted at room temperature into (0001)Si n-type 4H–SiC at a dose of 1×1017 cm−2. Cross-section transmission electron microscopy (TEM) and x-ray diffraction (XRD) were used to characterize the induced defects and the strain-induced effects before and after annealing. Infrared reflectivity was also used to localize changes in the microstructure. In the as-implanted samples, the TEM observations show a three layer damaged region consisting of a continuous amorphous layer surrounded with two buried crystalline zones. Bubbles of small diameter are readily formed in the as-implanted state. Only a few changes are observed after a 800 °C-30 min annealing. Voids formation and recrystallization of the amorphous state in different polytypes are observed for a 1500 °C-30 min annealing. Moreover, XRD shows that the dilatation of the c axis of the lattice observed after implantation completely disappears after the high temperature annealing implying the structural recovery of all the crystallin...

Helium implantation defects in SiC studied by thermal helium desorption spectrometry

Abstract Thermal helium desorption spectrometry (THDS) was used to characterize helium implantation induced defects in SiC. 6H–SiC and 4H–SiC single crystals were implanted with helium at energies ranging from 100 eV to 3 keV and doses ranging from 1013 to 10 15 cm −2 . They were then subjected to ramp annealing up to 1800 K, with a constant heating rate of 10 K s−1. To the desorption spectrum contribute two groups of peaks: one group at low temperature centered at 600 K and another group at high temperature centered at 1200 K. The evolution of these desorption peaks with implantation dose and energy has been studied. The first group (at 600 K) might be attributed to interstitial He and clusters of interstitial He. The second group (at 1200 K) could be related to the de-trapping of He from He–vacancies clusters. A shift of the latter group towards higher temperatures observed for increasing dose is ascribed to He–vacancy clustering in an Ostwald ripening process.

Dislocations induced by bubble formation in high energy He implantation in silicon

He+ ions were implanted into a (111) epitaxial n-type silicon wafer at 1.6 MeV and at a dose of 2×1016 cm−2. After implantation the samples were subjected to thermal annealing at 800 °C for 30 min. Cross section transmission electron microscopy (TEM) was used to characterize microstructural features of the induced defects. Even in the as-implanted samples the TEM observations revealed the formation of a buried layer containing a dense array of bubbles. After annealing, a large band of defects (bubbles, Frank dislocation loops) was observed as well as rows of prismatic punching related dislocations which can extend over several micrometers away from the buried layer. Planar clusters of helium bubbles, lying on the {001} planes, were also observed and were supposed to be involved in the generation of dislocation loops in the matrix. Their nucleation is discussed in terms of the trap-mutation process.

Influence of stoichiometry and structure on the optical properties of AlNxOy films

The AlNxOy system offers the possibility to obtain a wide range of responses, by tailoring the properties between Al, AlN and Al2O3, opening a significant number of possible applications. The aim of this work is to correlate the optical properties of AlNxOy thin films with their composition and structural features, taking as reference the binary systems AlNx and AlOy. In the AlNx system, the increase in the nitrogen content induced a wide variation in the optical properties, ranging from the typical profile of a polycrystalline Al-type film towards nearly constant reflectance values as low as 5%, as well as a smooth increase in samples transparency as the ratio N/Al approached unit. In the case of the AlOy system, the reflectance also decreased as the oxygen content increased; however, the transition to transparent films (Al2O3-like) was more abrupt. The ternary system AlNxOy, revealed optical responses that ranged from a typical profile of a polycrystalline Al-type film towards low and constant reflectance values in a wide range of x and y coefficients, ending up as semi-transparent when Al2O3-like films were formed. The unusual low optical reflectance of some films reveals some potential applications in solar power systems and sensors.

Localized exfoliation versus delamination in H and He coimplanted (001) Si

X-ray diffraction measurements as well as electron (scanning and transmission), optical, and atomic force microscopies are used to study the thermally induced stress relief mechanisms in coimplanted H+ and He+ ions into (001) Si substrates at moderate energies, resulting in damage layers located at ≈1.5 μm underneath the surface. By changing the implantation fluence rate from 0.25 to 1.5 μA cm−2, two distinct phenomena take place: localized blistering/exfoliations or complete surface delamination, resulting into freestanding 1.5 μm thick single crystalline Si films. The results are discussed on the basis of linear fracture mechanics arguments. Localized exfoliation is explained by means of distinct coarsening processes linking the initially formed gas filled nanosized platelets to crack structures of several micrometers in diameter. The delamination behavior is explained in terms of unstable crack propagation process triggered at a single nucleation site.

Cavities and dislocations induced in silicon by MeV He implantation

We implanted n-type silicon with 1.6 MeV helium at fluences ranging from 1×1016 to 1×1017 He/cm2 while keeping a constant dose rate. These samples were then subjected to 800 °C annealing for 30 min. The results obtained by means of cross-sectional transmission electron microscopy indicate that the density of cavities is fluence dependent with homogeneous distribution of cavity sizes when fluences of 5×1016 and 1017 He/cm2 are used. The threshold fluence required to form cavities is found to be between 1 and 2×1016 He/cm2. For the 2×1016 He/cm2 dose, we observed loop punching induced by a concerted action of overpressurized bubbles, whereas He implants at doses of 5×1016 and 1×1017/cm2 lead to the formation of {311} defects. At the same time, non Rutherford elastic backscattering (NREBS) experiments using 2.5 MeV H+ provide the fraction of helium remaining in cavities after different annealing times at 800 °C. The NREBS data show a fast He release process for short annealing times (<2000 s). Then, the He a...