Thermal Donors Research Articles

On the defect responsible for the carrier injection-induced degradation of uncompensated n-type Czochralski silicon

It is generally taken for granted that the bulk minority carrier lifetime in uncompensated phosphorus-doped silicon (Si) wafers is stable under illumination. In contradiction with this long-time belief, we report on a fast and significant lifetime degradation under light soaking for Czochralski wafers sampled from the top ingot end. We demonstrate that defect formation also proceeds during carrier injection by forward biasing Si heterojunction solar cells made from such wafers, suggesting a carrier-induced degradation rather than merely light-induced. The involved bulk defect is shown to preferentially form at the wafer center, where double Thermal Donors are in high concentration. The role of Thermal Donors is investigated further and the obtained preliminary results support their involvement in the reported carrier-induced degradation.

Photoluminescence due to early stage of oxygen precipitation in multicrystalline Si for solar cells

To analyze the early stage of oxygen precipitation in n-type multicrytalline Si, the spectral change of photoluminescence (PL) induced by thermal treatment at 450–650 °C was investigated in relation to the changes in excess donor and interstitial oxygen concentrations. We observed the characteristic PL bands in the near-band-edge region and sharp lines in the deep-level region in correspondence with the generation of thermal donors and new donors. The observed PL spectral variation is essentially the same as that in Czochralski-grown Si annealed at 450–650 °C.

Effect of proton irradiation on the breakdown voltage of a high-voltage p–n junction

The effect of hydrogen-related shallow thermal donors and acceptor-like defects arising under proton irradiation of silicon on the breakdown voltage of a high-voltage p–n junction is considered. The relations making it possible to compute the breakdown voltage of the irradiated p–n junction taking into account the increase in critical field intensity during the ingress of hydrogen-related shallow thermal donors into the region of collisional ionization of the p–n junction are obtained. A technique for determination of the layer position and the minimum dose of hydrogen-related shallow thermal donors making it possible to decrease the breakdown voltage by a specified amount is proposed.

Ground-state splitting of ultrashallow thermal donors with negative central-cell corrections in silicon

Ultrashallow thermal donors (USTDs), which consist of light element impurities such as carbon, hydrogen, and oxygen, have been found in Czochralski silicon (CZ Si) crystals. To the best of our knowledge, these are the shallowest hydrogen-like donors with negative central-cell corrections in Si. We observed the ground-state splitting of USTDs by far-infrared optical absorption at different temperatures. The upper ground-state levels are approximately 4 meV higher than the ground-state levels. This energy level splitting is also consistent with that obtained by thermal excitation from the ground state to the upper ground state. This is direct evidence that the wave function of the USTD ground state is made up of a linear combination of conduction band minimums.

Bulk Lifetimes up to 20 ms Measured on Unpassivated Silicon Discs Using Photoluminescence Imaging

With high-efficiency silicon solar cells approaching 25% efficiency in mass production, the requirements on the bulk lifetime and its uniformity across the wafer and the ingot increase dramatically. Since some cell architectures require these high lifetimes on starting material, the need arises for characterization methods to measure very high bulk lifetimes that are spatially resolved at an early stage before cell processing. A method based on the spectral ratio of two photoluminescence images is applied here on two unpassivated silicon discs from different positions within a Czochralski-grown phosphorous-doped n-type silicon ingot. The method allows the determination of spatially resolved bulk lifetime images on samples with adequate thickness and can be done within seconds and without the need to passivate surfaces. As-grown bulk lifetimes up to 20 ms are measured on the ingot's central disc, indicating recent improvements in crystallization technology, but are strongly reduced closer to the crown. Evidence suggesting the impact of thermal donors on the lifetime and effective doping concentration near the crown is found from combining spectral photoluminescence and infrared spectroscopy analyses. The technique could find applications in research and development activities, particularly in the optimization of Czochralski silicon crystal growth conditions.

Effect Of Annealing On The Behavior Of Oxygen Dissolved In Germanium And Optical Properties Of Single Crystals

<p>The annealing effect in the temperature range from 350 to 450 °С on the behavior of interstitial oxygen О<sub>i</sub> dissolved in germanium and on optical properties of single crystals has been investigated by Fourier transformed infrared spectrometry. It was found that oxygen band maximum of 843 cm<sup>–1</sup> shifted toward the 856 cm<sup>−1</sup> with oxygen concentration increasing after annealing at the oxygen partial pressure ranged from 1 to 10<sup>3</sup> Pа. The annealing at lower values led to the decrease of 843 cm<sup>–1</sup> band intensity due to the formation of thermal donors (TD) on the basis of dissolved oxygen atoms О<sub>i</sub>. It was established that TD formation and the decrease of unbound oxygen concentration affected optical properties of the crystals.</p>

Electrically active light-element complexes in silicon crystals grown by cast method

Electrically active light-element complexes called thermal donors and shallow thermal donors in silicon crystals grown by the cast method were studied by low-temperature far-infrared absorption spectroscopy. The relationship between these complexes and either crystal defects or light-element impurities was investigated by comparing different types of silicon crystals, that is, conventional cast-grown multicrystalline Si, seed-cast monolike-Si, and Czochralski-grown Si. The dependence of thermal and the shallow thermal donors on the light-element impurity concentration and their annealing behaviors were examined to compare the crystals. It was found that crystal defects such as dislocations and grain boundaries did not affect the formation of thermal or shallow thermal donors. The formation of these complexes was dominantly affected by the concentration of light-element impurities, O and C, independent of the existence of crystal defects.

Identification of Lifetime-limiting Defects in As-received and Heat Treated Seed-end Czochralski Wafers

Major impurity-induced defects in Czochralski silicon are known to be related to oxygen and metallic elements. In this study we focus on seed-end wafers, and submit them to different solar cell process-related annealings. We find that in the as-received state, these seed-end wafers contain a central low carrier lifetime core. We demonstrate that the effective carrier lifetime in the as-received state is simultaneously governed by several defects including oxygen-related thermal donors and a defect deactivated at low temperature. After high temperature steps, a strong recombination center is formed in the central core. Preliminary results of effective carrier lifetime versus temperature measurements directly support oxygen precipitates to be limiting the carrier lifetime. In the light of all results, we propose a qualitative model for the high temperature behavior of these seed-end wafers.

Decrease of Thermal Donors in Si Single Crystal by Czochralski Method

Decrease of Thermal Donors in Si Single Crystal by Czochralski Method

Recombination activity associated with thermal donor generation in monocrystalline silicon and effect on the conversion efficiency of heterojunction solar cells

The recombination properties of the carrier lifetime-limiting center formed during the generation of oxygen-related thermal donors (so called “old” thermal donors) in n-type Czochralski silicon were determined over a wide range of thermal donors' concentrations. The procedure involved (1) determining the various energy levels associated with dopants with the help of temperature Hall effect measurements, (2) clarifying which energy level limits the carrier lifetime by temperature lifetime spectroscopy, and (3) determining the recombination parameters of the involved defect from room-temperature carrier lifetime curves. Our results support the fact that a deep energy level in the range of 0.2–0.3 eV below the conduction band limits the carrier lifetime. The second family of thermal donors, featuring bistable properties, was tentatively identified as the corresponding defect. From the obtained experimental data, the influence of the defect on the amorphous/crystalline silicon heterojunction solar cell conversion efficiency was simulated. It is observed that for extended donor generation, the carrier lifetime is reduced by orders-of-magnitude, leading to unacceptable losses in photovoltaic conversion efficiency. A key result is that even for samples with thermal donor concentrations of 1015 cm−3—often met in seed portions of commercial ingots—simulations reveal efficiency losses greater than 1% absolute for state-of-the-art cells, in agreement with recent experimental studies from our group. This result indicates to crystal growers the importance to mitigate the formation of thermal donors or to develop cost-effective processes to suppress them at the ingot/wafer scale. This is even more critical as ingot cool-down is likely to be slower for future larger ingots, thus promoting the formation of thermal donors.

Mechanism of formation of ultrashallow thermal donors in carbon-doped oxygen-rich monocrystalline silicon preannealed to introduce hydrogen

We previously reported on ultrashallow thermal donors (USTDs) in carbon-doped oxygen-containing monocrystalline silicon (Czochralski-grown, CZ-Si) crystals that were preannealed to introduce hydrogen at 1300 °C, and then annealed at 480 °C. In this study, the formation mechanism of the USTDs was evaluated. It was observed that an increase in the intensity of UTSDs leads to a reduction in that of hydrogen-related shallow thermal donors [STD(H)s], and the sum of the area intensities of the lines in the transmission spectra of USTDs and STD(H)s is nearly constant when the silicon crystals are annealed for longer than 10 h at 480 °C. We also found some thermally activated processes linked to the formation of USTDs. We thus conclude that the mechanism is composed of the high-speed formation of STD(H)s in the first stage and carbon modulation of the electronic structure of STD(H)s in the second stage.

An effective nucleation rate for oxide precipitation in Czochralski silicon and a simple framework for multi-step anneal calculations

A simple and physically consistent quantitative model of oxide precipitate nucleation in Czochralski-Silicon (Cz-Si) can significantly improve predictions of wafer characteristics during fabrications of semiconductor devices or solar cells. Towards that end, a classical nucleation theory based nucleation rate expression is developed for oxide precipitation in Cz-Si wafers in which vacancy supersaturation is not high. The precipitate morphology is assumed to be thin flat plates. The estimates of critical cluster sizes from new model corroborate the theory that thermal donors or some other embryonic form of oxygen clusters can be precursors of oxide precipitates. The new model predicts effective nucleation rates which are in agreement with previously reported data. The nucleation rates predictions from the new model and diffusion limited growth of precipitate plates of constant thickness are used in a simple framework to calculate precipitate densities after multi-step anneals reported previously in literature. The modeled precipitate densities are found to be in reasonable agreement with corresponding experimental values.

Single-crystal p–i–n-Si thin-film solar cells grown on Si substrate by sputter epitaxy

An intrinsic sputter-epitaxial (SE) Si film with a thickness of 1000 nm and a 50-nm-thick n+ SE-Si film were successfully grown as the light-absorbing layer and emitter layer, respectively, on a heavily doped p-Si(100) wafer to form the p–i–n junction of a solar cell (SC). Heavily doped n+ SE-Si with an electron concentration n of 3 × 1020 cm−3 was grown by cosputtering of Sb with Si. The characteristics of SE-Si grown at 310 °C was investigated in relation to annealing temperature. The oxygen concentration in SE-Si was ∼1018 cm−3, which was found to originate from the gas released in the chamber. Oxygen-induced thermal donors then became the source of n in the film, and n was reduced to 1 × 1016 cm−3 after forming-gas annealing at 700 °C because the thermal donors were neutralized by hydrogen. The SC exhibited a maximum internal quantum efficiency of 73.7%.

Oxygen aggregation kinetics, thermal donors and carbon-oxygen defect formation in silicon containing carbon and tin

Localized vibrational mode spectroscopy measurements on Czochralski silicon (Cz-Si) samples subjected to isothermal annealing at 450 °C are reported. First, we studied the effect of carbon (C) and tin (Sn) isovalent dopants on the aggregation kinetics of oxygen. It is determined that the reduction rate of oxygen is described by the Johnson-Mehl-Avrami equation in accordance with previous reports. The activation energy related with the reaction rate constant of the process is calculated to increase from Cz-Si, to C-doped Cz-Si (CCz-Si), to Sn-doped Cz-Si contained C (SnCz-Si). This is attributed to the presence of the isovalent dopants that may impact both the kinetics of the oxygen atoms and also may lead to the formation of other oxygen-related clusters. Second, we studied the effect of Sn on the formation and evolution of carbon-oxygen (C-O) defects. It was determined that the presence of Sn suppresses the formation of the C-O defects as indicated by the reduction in the strength of the 683, 626, and 58...

On the shape of n-type Czochralski silicon top ingots

Industrial scale n-type monocrystalline silicon ingots with different crown shape and shouldering area have been grown and characterized in terms of minority carrier lifetime, resistivity and concentration and distribution of interstitial oxygen (Oi), voids and thermal donors (TD). The properties have been correlated with the process parameters. The results indicate that there is a relationship between the top ingot shape and the corresponding minority carrier lifetime quality of the first part of the ingot body. Oxygen incorporation is directly correlated to the time that the melt free surface is in contact with the purged gas atmosphere as well as the temperature of the melt. The relative P-band position is dependent on the process parameters at the early part of the ingot and the corresponding oxide particles could be included on the first wafers obtained from the ingot. The results also show that the early body quality is not always improved after TD annealing. This suggests that not all types and sizes of TD are dissolved during standard annealing processes. Other types of oxygen-related defects are most likely present and contribute to the lowered minority carrier lifetime found at this region. Both higher oxygen and vacancy concentration contribute to the formation of these defects.

Identification of a fast diffusing oxygen species in silicon based on the generation rate of thermal donors

Oxygen out‐diffusion profiles reported at low temperature (400–500 °C) show a long‐range oxygen migration. By the present analysis, the near‐surface oxygen loss observed in these experiments is found to be well consistent with the known generation rate of thermal donors (TDs). Accordingly, the fast‐diffusing oxygen species (FDS) – responsible for the long‐distance oxygen diffusion – is a relatively large oxygen cluster (most likely, O6) belonging to the TD family. The model developed here accounts for a persistent value of the out‐diffusion distance – close to 2 μm for all temperatures and durations – and well reproduces the observed concentration profiles.

A New Method for the Determination of the Dopant-Related Base Resistivity Despite the Presence of Thermal Donors

During cooling of Czochralski grown silicon ingots, thermal donors can be formed. These oxygen clusters influence the measurement of the base resistivity in the as-cut state. Thermal donors distort the calculation of the emitter sheet resistance during inline control because two measurements-one before and one after emitter diffusion-are needed. In this paper, we quantify the negative influence of the thermal donors on the base resistivity measurements, as well as on the calculation of the emitter sheet resistance, and present a new photoluminescence-based method for the determination of the dopant-related base resistivity, despite the presence of thermal donors in the as-cut state of Czochralski grown silicon wafers. With the photoluminescence base resistivity method, the dopant-related base resistivity and, therefore, the emitter sheet resistivity can be calculated with significantly higher accuracy than standard base resistivity measurements.

Structural and electrical properties of oxygen complexes in Cz and FZ silicon crystals implanted with carbon ions.

PACS34.50.Dy; 61.10.-i; 68.35.DvElectronic supplementary materialThe online version of this article (doi:10.1186/1556-276X-9-693) contains supplementary material, which is available to authorized users.

The formation of shallow-donor distribution profiles in proton irradiation of silicon

A study of the formation of shallow hydrogen-containing donors (hydrogen-related shallow thermal donors, STD(H)) in silicon under proton irradiation followed by annealing in a temperature range of 300–500°C is reported. The effect of postimplantation annealing regimes on the concentration distribution of shallow donors at various proton energies and fluences is examined. It is shown that the shape of the concentration profiles strongly varies with temperature and annealing duration when a fixed concentration of radiation defects is introduced and equally with energy and dose at a given annealing temperature. It is also shown that the process in which hydrogen-containing shallow donors are formed is accompanied by the appearance in n-type silicon of H-induced buried n′-layers, the formation of which near the pn junction in the high-resistivity n-base of diode structures allows the breakdown voltage of high-voltage pn junctions to be controlled. In the general case, this makes it possible to improve the characteristics of power silicon devices of various purposes.

Processing‐induced near‐interfacial thermal donor generation in (100)Si/Si‐oxycarbide insulator structures revealed by electron spin resonance

AbstractLow temperature electron spin resonance (ESR) study of Cz‐(100)Si/insulator structures with organosilicate films of low dielectric constant κ grown at 300 °C using plasma‐enhanced chemical vapor deposition (PECVD) reveals, after subjection to UV‐irradiation assisted thermal curing at 430 °C to remove organics, the observation of the NL8 ESR spectrum. This indicates the generation in the c‐Si substrate of singly ionized thermal double donor (TDD) defects with a core contained of oxygen atoms. The generation is found to be highly non uniform, which is concluded to be the result of interfacial stress acting as the major driving component in the enhancement of TDD formation during thermal treatment. This suggests substantial stress being involved with PECVD organosilicate low‐κ glasses. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)