Nitrogen-doped Float-zone Silicon Research Articles

Measurements of Dislocation Locking by Near-Surface Ion-Implanted Nitrogen in Czochralski Silicon

A modified dislocation unlocking technique is used to measure dislocation locking due to nitrogen implanted into Czochralski silicon. The results show that near-surface dislocations can be locked by implanted nitrogen. The magnitude of the locking measured suggests that nitrogen transport proceeds by a dissociative mechanism, where transport occurs by the splitting of immobile dimers into fast monomers, rather than movement of nitrogen dimers. In other experiments, nitrogen-doped float-zone silicon is investigated using the standard dislocation unlocking technique. The results give an activation energy for effective nitrogen diffusion in silicon of at . Using the assumption that the dislocation locking strength per nitrogen atom is the same as that of oxygen, a value of can be inferred for the effective diffusivity prefactor. If analyzed using the dissociative model, an activation energy of 1.1–1.4 eV is found for nitrogen monomer diffusion, with a diffusivity prefactor of .

Nitrogen diffusion and interaction with dislocations in single-crystal silicon

The results of dislocation unlocking experiments are reported. The stress required to unpin a dislocation from nitrogen impurities in nitrogen-doped float-zone silicon (NFZ-Si) and from oxygen impurities in Czochralski silicon (Cz-Si) is measured, as a function of the unlocking duration. It is found that unlocking stress drops with increasing unlocking time in all materials tested. Analysis of these results indicates that dislocation locking by nitrogen in NFZ-Si is by an atomic species, with a similar locking strength per atom to that previously deduced for oxygen atoms in Cz-Si. Other experiments measure dislocation unlocking stress at 550 °C in NFZ-Si annealed at 500–1050 °C. The results allow an effective diffusivity of nitrogen in silicon at 500–750 °C to be inferred, with an activation energy of 3.24 eV and a diffusivity prefactor of approximately 200 000 cm2 s−1. This effective diffusivity is consistent with previous measurements made at higher temperatures using secondary ion mass spectrometry. When the results are analyzed in terms of a monomer-dimer dissociative mechanism, a nitrogen monomer diffusivity with an activation energy in the range of 1.1–1.4 eV is inferred. The data also show that the saturation dislocation unlocking stress measured at 550 °C in NFZ-Si is dependent on the anneal temperature, peaking at 600–700 °C and falling toward zero at 1000 °C.

Measurements of Dislocation Locking by Near-Surface Ion-Implanted Nitrogen in Czochralski Silicon

A modified dislocation unlocking technique is used to measure dislocation locking due to nitrogen implanted into Czochralski silicon. The results show that near-surface dislocations can be locked by implanted nitrogen. The magnitude of the locking measured suggests that nitrogen transport proceeds by a dissociative mechanism, where transport occurs by the splitting of immobile dimers into fast monomers, rather than movement of nitrogen dimers. In other experiments, nitrogen-doped float-zone silicon is investigated using the standard dislocation unlocking technique. The results give an activation energy for effective nitrogen diffusion in silicon of 3.24{plus minus}0.25eV at 500 to 750{degree sign}C. Using the assumption that the dislocation locking strength per nitrogen atom is the same as that of oxygen, a value of 200,000cm2s-1 can be inferred for the effective diffusivity pre-factor. If analysed using the dissociative model, an activation energy of 1.1 to 1.4eV is found for nitrogen monomer diffusion, with a diffusivity pre-factor of 30cm2s-1.

Nitrogen in silicon: Diffusion at 500–750 °C and interaction with dislocations

The results of dislocation unlocking experiments using nitrogen-doped float-zone silicon are reported. Dislocation unlocking stress is measured in specimens subjected to anneals for a range of durations and temperatures. Analysis of the rate of the initial rise in unlocking stress with annealing time gives an activation energy for nitrogen diffusion of 3.24 eV in the 500–750 °C temperature range. Numerical simulations of nitrogen diffusion to the dislocation core allow an approximate value of 200,000 cm 2 s −1 to be estimated for the diffusivity pre-factor. These diffusion measurements are consistent with the results of higher temperature secondary ion mass spectrometry out-diffusion experiments in the literature. Other measurements made at up to 1050 °C followed by fast quenching indicate that nitrogen’s ability to lock dislocations is substantially reduced at high temperatures.

Out‐diffusion of nitrogen from float‐zone silicon measured by dislocation locking

AbstractA measurement of nitrogen out‐diffusion from nitrogen‐doped float‐zone silicon made using a dislocation locking technique is presented. Specimens containing a well‐defined array of dislocation half‐loops are subjected to identical anneals at 750 °C, during which nitrogen diffuses both to the surface and to the dislocations. The specimens are then chemically etched so as to remove different thicknesses of material from the surface. The stress required to move the dislocations away from the nitrogen is then measured. The variation in this unlocking stress with thickness of material removed allows some measure of nitrogen diffusivity to be deduced. The result obtained is consistent with an extrapolation of SIMS out‐diffusion measurements previously performed at higher temperatures, but indicates a different activation energy for out‐diffusion to that associated with dislocation locking by nitrogen. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of high pressure annealing on electrical properties of nitrogen and germanium doped silicon

The effect of annealing at 720–920K under enhanced pressure (up to 1.4GPa) in argon ambient on electrical properties of the surface layer of the Czochralski and Float zone grown nitrogen doped silicon (doping level 1×1014–5×1015cm−3) and Czochralski grown germanium doped silicon (doping level 7×1017cm−3) was investigated by electrical C–V, I–V and admittance methods. The decrease in electron concentration was observed in nitrogen doped Czochralski grown silicon annealed under normal pressure at 720K. The stress induced increase in electron concentration was observed in nitrogen doped Czochralski grown silicon independently on nitrogen doping level in the range 1×1014–5×1014cm−3. No dependence of electron concentration on hydrostatic pressure was observed in nitrogen doped Float zone silicon even for high doping level of 5×1015cm−3. Germanium doped silicon indicated the strongly enhanced creation of thermal donors if processed at 1.1GPa. The observed effects can be explained as a result of the oxygen–nitrogen complexes in nitrogen doped silicon and the pressure stimulated creation of thermal donors both in nitrogen and germanium doped silicon.

Nitrogen in silicon: Transport and mechanical properties

A novel dislocation locking technique is applied to nitrogen-doped float-zone silicon. Specimens containing well-defined arrays of dislocation half-loops are subjected to isothermal anneals of controlled duration, during which nitrogen diffuses to the dislocations. The stress required to bring about dislocation motion is then measured, generally at 550°C. The segregation of nitrogen to dislocations is found to be stable to at least 1200°C, and the dislocation unlocking stress measured at 550°C is of similar magnitude to that for oxygen in Czochralski silicon. At all annealing temperatures studied, the measured unlocking stress as a function of annealing time initially rises linearly before taking a constant value. The rate of the initial rise is found to be strongly dependent on temperature, with an activation energy of 1.5eV. In a separate set of experiments, the magnitude of the dislocation unlocking stress measured is also found to depend upon the temperature at which the unlocking process takes place in the 500–700°C temperature range. The dislocation locking technique is also used to give some measure of nitrogen out-diffusion, by measuring the unlocking stress as a function of material etched away after annealing. For specimens annealed at 750°C for 15h, the dislocation unlocking stress is found to rise with increasing depth away from the specimen’s surface, before it takes an approximately constant value. By fitting an error function to these data, the diffusion coefficient of nitrogen in silicon is deduced to be approximately 4×10−11cm2s−1 at 750°C. This value is consistent with a SIMS investigation by Itoh and Abe.

Deep level generation in nitrogen-doped float-zoned silicon

In nitrogen-doped Float-Zone (FZ) silicon, nitrogen does not show any electrical activity in as-grown state. However, deep centers are known to emerge after annealing at relatively high temperatures like 900 or 1000°C. In the present work it was found that in FZ samples of relatively high initial resistivity (greater than 1000Ωcm, p- and n- type) the generation of deep centers is well pronounced after annealing at a lower temperature (680°C). In p-type samples, deep donors with an energy level above the midgap are produced and the material is converted into n-type of an extremely high-resistivity. In n-type samples, deep acceptors (with an energy level also above the midgap) are generated. The deep center concentration is on the order of 1013cm−3. A possible origin of the deep centers is interaction of a fast-diffusing interstitial nitrogen species with other impurity centers.

Effects of Annealing on the Electrical Properties of Nitrogen-Doped Float-Zoned Silicon

The nitrogen impurity, in nitrogen-doped Float-Zone (FZ) silicon, is known to show no electrical activity in as-grown state, but to produce deep centers after annealing at relatively high temperatures like 900 or 1000{degree sign}C. In the present work it was found that in FZ samples of relatively high initial resistivity (greater than 1000 Ohm.cm, p- and n- type) the generation of deep centers is well pronounced after annealing at a lower temperature (680{degree sign}C). In p-type samples, deep donors are produced, and the material is converted into n-type of an extremely high-resistivity. Another striking feature of this material is an extremely low apparent Hall mobility indicating to a well-pronounced non-uniformity in the carrier distribution. In n-type samples, deep acceptors are generated. The deep center concentration is on the order of 2x1013 cm-3. A possible identification of the deep centers is substitutional nitrogen.

Oxygen and Nitrogen Transport in Silicon Investigated by Dislocation Locking Experiments

The behavior of oxygen and nitrogen impurities in silicon has been investigated using a novel dislocation locking technique. The locking effect of oxygen in Czochralski silicon (CZ-Si) was investigated in the temperature range and was found to display five well-defined regimes as a function of annealing time. Results indicate that enhanced transport of oxygen to dislocations takes place for temperatures below . Numerical simulations of the enhanced oxygen transport indicate that the effective diffusivity becomes dependent on oxygen concentration with an activation energy of approximately . The same technique has been used to investigate nitrogen transport in nitrogen-doped float-zone silicon in the temperature range and shows nitrogen to have a comparable locking effect to oxygen in CZ-Si, despite being present in a concentration that is 2 orders of magnitude lower. The stress required to unlock dislocations at which have previously been immobilized by nitrogen during an annealing step, initially increases approximately linearly with the duration of the anneal before saturating to a steady-state value of approximately for all anneal temperatures investigated. An expression for the transport of nitrogen to the dislocations was deduced, which has an activation energy of .

Nitrogen transport in float‐zone and Czochralski silicon investigated by dislocation locking experiments

AbstractDislocation locking experiments have been used to investigate nitrogen‐doped float‐zone silicon (NFZ‐Si). Experiments on NFZ‐Si with a nitrogen concentration of 2.2 × 1015 cm–3 were carried out at different annealing temperatures (550–830 °C) for different annealing times (0–1500 hours) and experiments on NFZ‐Si with a nitrogen concentration of 3 × 1014 cm–3 were carried out at 600 °C for 0–1200 hours. After an initial rise, the unlocking stress was found to saturate for all combinations of conditions investigated. The rate of the initial rise was found to be consistent with diffusion by a dimeric nitrogen species. The saturation value of the unlocking stress was found to be dependent on the nitrogen concentration. Experiments were also carried out on nitrogen‐doped Czochralski silicon (NCz‐Si) with an oxygen concentration of 5.74 × 1017 cm–3 and a nitrogen concentration of 2.10 × 1015 cm–3 at 600 °C from 0–5 hours. The dislocation locking due to oxygen appeared to be unaffected by the presence of nitrogen for the conditions investigated. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dislocation luminescence in nitrogen-doped Czochralski and float zonesilicon

This paper reports the results of a study of the effect of nitrogen on the opticalproperties of dislocations in nitrogen-doped Czochralski and nitrogen-doped floatzone silicon samples where the nitrogen doping was carried out by adding Si3N4in the molten silicon charge or by nitrogen gas dissolution. Dislocations wereintroduced by plastic deformation at 650°C. In nitrogen-doped plasticallydeformed samples, emissions in the range of the D1–D4 bands of dislocations arepresent with a significant shifting from the energies and intensities of thecorresponding bands in nitrogen-free samples. It has been shown that the maineffect of nitrogen could be the enhancement of the oxygen precipitation. Theresults confirm the suggestion of some of the present authors that luminescence at0.830 eV is associated with some intrinsic properties of oxygen precipitates.