A. Karoui

Structure, Energetics, and Thermal Stability of Nitrogen-Vacancy-Related Defects in Nitrogen Doped Silicon

The electronic structure, formation energy, and thermal stability of nitrogen-vacancy related complexes in silicon have been investigated using density functional theory and semi-empirical Hartree-Fock calculations. The calculated energies of formation in the ground state showed that VN 2 was not stable, whereas V 2 N 2 when formed from VN 2 was the most stable, followed by N 2 and V 2 N 2 formed from a divacancy. The calculated free energy changes of the considered chemical reactions confirmed the low stability of VN 2 compared to V 2 N 2 . The latter can form during crystal growth from VN 2 , whereas reactions between N 2 and divacancy can also occur upon wafer heating. At low nitrogen concentration (∼5 X 10 13 cm -3 ), only about 10% of vacancy concentration was converted into VN 2 , while at a high nitrogen concentration (∼10 16 cm -3 ) about 75% of vacancies are trapped by nitrogen. V 2 N 2 appeared to create a potential well of -2.4 eV for oxygen and about -0.3 eV for vacancies, suggesting that the stable V 2 N 2 is a nucleus for oxygen precipitation while it is a weak trapping center for vacancies.

Oxygen precipitation in nitrogen doped Czochralski silicon wafers. I. Formation mechanisms of near-surface and bulk defects

Defect size distributions in nitrogen-doped Czochralski (N-CZ) silicon wafers were obtained using an oxygen precipitate profiler and Wright-Jenkins etching. These showed unique depth dependence in low-high and high-low-high cycled N-CZ wafers. Unique phenomena observed include a high defect concentration at the subsurface that decreases within the top 2μm of the so-called denuded zone. In contrast to N-free CZ Si for which the first high step annealing dissolves the grown-in defects, these appeared to be stable in N-CZ Si. As a result, the defect size distribution in the bulk was found to be independent of the annealing cycle. It was also found that the depth dependent defect concentration correlates well with oxygen and strongly with nitrogen secondary ion mass spectroscopy profiles, suggesting that nitrogen is the leading impurity in the defect formation processes even though introduced at very low concentration. Nitrogen appeared to effectively modify the nucleation regime by a drastic increase of the ...

Oxygen precipitation in nitrogen doped Czochralski silicon wafers. II. Effects of nitrogen and oxygen coupling

Nitrogen segregation and coprecipitation with oxygen in N-doped Czochralski (N-CZ) silicon wafers are investigated as a function of depth based on extended defect structure and chemical composition. High resolution nitrogen and oxygen secondary ion mass spectroscopy imaging revealed strong coupling of oxygen with nitrogen in annealed as well as in “as-grown” N-CZ Si wafers. In both cases, the near-surface regions appeared highly supersaturated in N and O forming a continuum of defects initiated by N-O complexes. The N and O stoichiometry depth profiles were found to depend on the material thermal history. The spatial variation of the stoichiometry ratio was also determined for precipitates using a combination of scanning transmission electron microscope (STEM) in Z-contrast mode with electron energy loss spectroscopy. The precipitate atomic and microstructures, analyzed by high resolution TEM and STEM, clearly demonstrate that second phase precipitate is precursor to a third phase that is an outer oxynitr...

Nano-scale analysis of precipitates in nitrogen-doped Czochralski silicon

Nitrogen-doped Czochralski (CZ) silicon wafers were heat treated with Lo-Hi annealing in argon. Nanoscale defects were then examined by high resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM) in the Z-contrast mode, and electron energy loss spectroscopy (EELS) analyses using a field emission JEOL 2010 with a resolution below 2 A. The structures of precipitates, stacking faults and interstitial aggregates were found to depend on their location relative to the wafer surface. Precipitate composition, strain at the interface and interface roughness were obtained and are discussed in connection with the point defects generated during crystal growth and modified during wafer annealing. An excellent correlation was found between Z-contrast line scans across the precipitates and the N to O concentration ratio determined with EELS. In the precipitate central region that ratio is between 1 and 6%, whereas at precipitate boundaries it reaches 17%.

In Situ Point Defect Generation and Agglomeration during Electron-Beam Irradiation of Nitrogen-Doped Czochralski Silicon

Samples of Czochralski silicon were observed after irradiation by a convergent electron beam in a transmission electron microscope. In a nitrogen-doped sample, the 200 keV electrons induced a vacancy-rich region containing point-defect clusters, surrounded by a ring rich in self-interstitials. No comparable effect existed in nitrogen-free reference samples. It is proposed that Frenkel pairs, created by electron collisions, are separated and stabilized by nitrogen or related complexes. Some interstitials become free to diffuse while the nitrogen, vacancies and oxygen agglomerate. This study demonstrates that the initial formation of voids and precipitate nuclei from point defects can be observed at low temperatures.

Segregation and enhanced diffusion of nitrogen in silicon induced by low energy ion bombardment

A sample of nitrogen-doped, single crystal Czochralski silicon was subjected to several different surface preparations. Secondary ion mass spectrometry depth profiling has shown that prolonged glancing-angle bombardment by 3–5kV Ar+ ions significantly increases the nitrogen concentration in the near surface by up to an order of magnitude over the bulk value. Concentrations are observed to be elevated over the bulk value to a depth up to 25μm. Nitrogen-implanted samples and samples with a 1nm surface nitride did not exhibit nitrogen segregation under the same conditions, but a sample with 100nm of surface nitride did exhibit ion bombardment induced drive-in. In nitride-free samples, the source of the nitrogen is indicated to be a nitrogen-rich layer in the first micron of material. The diffusion behavior of nitrogen in silicon is discussed and the Crowdion mechanism for diffusion is suggested as the enabling mechanism for the enhanced low temperature diffusion.

FREQUENCY-RESOLVED MICROWAVE REFLECTION PHOTOCONDUCTANCE

The effect of the carrier recombination process in silicon on the microwave reflection coefficient is analyzed in the frequency domain. The process is described using a two level recombination/trapping model. Carrier recombination kinetics are characterized by four parameters, two of which are related to the recombination and the other to the trapping processes. These parameters are evaluated for Czochralski silicon wafers based on Nyquist plots. In the evaluation procedure, a nonlinear simplex method is used for fitting the experimental data to the model.

Effect of Oxygen and Nitrogen Doping on Mechanical Properties of Silicon Using Nanoindentation

The effects of oxygen and nitrogen on the mechanical properties of Czochralski (CZ) and float zone silicon have been studied using nano-indentation. Nitrogen free FZ Si exhibited low hardness of 6.49 GPa and elastic modulus of 104 GPa. When doped with 2×10 15 cm −3 nitrogen, FZ Si hardness and elastic modulus increased to 8.2 and 182 GPa, respectively. In the near-surface denuded zone of N-doped CZ Si (N-CZ) the hardness correlates well with the O and N profiles. Distinct high hardness points, found in the O- and N- rich subsurface region, were attributed to precipitates. Nano-scratch tests of N-CZ Si confirmed the existence of hard phases, mostly small precipitates, whose density, estimated to be 2×10 13 cm −3 , is in the range of previously suggested nuclei density in as-grown N-CZ silicon.

“Umbrella”-like precipitates in nitrogen-doped Czochralski silicon wafers

Nitrogen effect on nucleation of oxygen precipitates in Czochralski Si has been investigated by transmission electron microscopy, Z-contrast imaging, and electron energy loss spectrometry (EELS). We have examined unusual “umbrella” shape oxygen precipitates in bulk of ingot in depths of more than 40 μm. Two predominant orientations of “umbrella” have been found along [110] and [−1−10] directions. We have investigated the distribution of nitrogen, oxygen, and interstitial Si by EELS profile taken simultaneously with HR Z-contrast image. The mechanism of nitrogen-enriched oxygen precipitates nucleation has been discussed.

Contactless characterization of silicon using variable injection level frequency resolved microwave photoconductance

An effect of the carrier recombination process in silicon materials on the microwave reflection coefficient is analyzed in the frequency domain. The process is described using a Shockley-Hall-Read (SHR) single level recombination model. Carrier recombination kinetics is characterized with four parameters, surface recombination velocity, activation energy, minority carrier capture cross section and concentration of the recombination centers. These parameters are evaluated for p-type FZ and CZ silicon wafers contaminated with Cu, Ni, and Fe based on the Nyquist plots. In the evaluation procedure a nonlinear simplex method is used for fitting the experimental data to the model.