Oxygen-related Thermal Donors Research Articles

Influence of extended crystallographic defects on the formation kinetics of oxygen-related thermal donors in multicrystalline silicon

Oxygen-related thermal donors generated generally form less effectively in multicrystalline than in Czochralski grown monocrystalline silicon. In addition to a lower interstitial oxygen concentration, carbon and extended crystallographic defects are possible candidates that can account for this feature. To isolate the potential effect of extended defects, we investigate the formation of thermal donors in a carbon-lean Czochralski-grown multicrystalline ingot, resulting from a structure loss during growth. The grain orientations and the densities of dislocations are characterized and are shown to be representative of typical multicrystalline silicon. Eventually, we demonstrate that the macroscopic thermal donor formation kinetics are not affected either by grain boundaries or by dislocations, in contrast with previous findings from the literature. Our results indirectly support the role of carbon as the main thermal donor inhibitor in multicrystalline silicon.

Electron emission and capture by oxygen-related bistable thermal double donors in silicon studied with junction capacitance techniques

It has been recently suggested that oxygen-related bistable thermal double donors (BTDDs) are responsible for the reduction of minority carrier lifetime and conversion efficiency of novel amorphous-crystalline Si heterojunction solar cells with their base formed from n-type Czochralski-grown (Cz) silicon [M. Tomassini et al., J. Appl. Phys. 119, 084508 (2016)]. To test this hypothesis, we have studied processes associated with carrier emission and capture by BTDDs in p+-n and Schottky barrier diodes on n-type Cz-Si materials with the use of junction capacitance techniques. By means of deep level transient spectroscopy, we have detected electron emission signals from the deep donor state of the BTDD-0 and BTDD-1 centers. The values of activation energy for electron emission (Eem) have been determined as 1.01 ± 0.01 and 0.91 ± 0.01 eV for the BTDD-0 and BTDD-1 centers, respectively. Such high Eem values are very unusual for defects in Si. We have carried out measurements of electron capture kinetics and associated shallow donor–deep donor transformations for the BTDD-0 and BTDD-1 defects at different temperatures in the diodes with different doping levels. Energy barriers for the capture-transformation processes have been determined. It is argued that BTDDs are responsible for carrier trapping in n-type Cz-Si crystals but are not effective recombination centers.

Effects of oxygen related thermal donors on the performance of silicon heterojunction solar cells

Absract The influence of oxygen-related thermal donors (TDs) on the performance of silicon heterojunction (SHJ) solar cells was explored experimentally in this paper. It is found that a certain number of thermal donors could do much harm to the performance of SHJ solar cells. The efficiency of SHJ solar cells is usually reduced by a value of ~ 1% absolute in the case of thermal donors with a concentration of nearly 1015 cm−3. The microwave photoconductance decay and Hall Effect studies have proved that the TDs can significantly reduce the carrier lifetime of n-type silicon substrate, but have no influence on the carrier mobility. Deep level transient spectroscopy (DLTS) measurements have further demonstrated that the TDs cause an energy level at Ec-0.13 eV with carrier capture cross-section of 10−15 cm2, which is responsible for the reduction of carrier lifetime and solar cell efficiency.

Oxygen-related defects in n-type Czochralski silicon wafers studied by hyperspectral photoluminescence imaging

Oxygen-related Thermal Donors in n-type Czochralski silicon (Cz-Si) wafers have been investigated using hyperspectral photoluminescence imaging and OxyMap. Thermal Donors give rise to two photoluminescence emissions, one narrow peak at 0.767 eV, and one broad band with centre peak at 0.72 eV that is also measurable at room temperature. The spectral imaging was first carried out on the sample cooled to 90 K, then repeated at room temperature (300 K). The possibility to delimitate at room temperature defects-rich zones with hyperspectral photoluminescence imaging is evidenced.

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.

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.

Applications of Photoluminescence Imaging to Dopant and Carrier Concentration Measurements of Silicon Wafers

Photoluminescence-based imaging is most commonly used to measure the excess minority carrier density and its corresponding lifetime. By using appropriate surface treatments, this high-resolution imaging technique can also be used for majority carrier concentration determination. The mechanism involves effectively pinning the minority excess carrier density, resulting in a dependence of the photoluminescence intensity on only the majority carrier density. Three suitable surface preparation methods are introduced in this paper: aluminum sputtering, deionized water etching, and mechanical abrasion. Spatially resolved dopant density images determined using this technique are consistent with the images obtained by a well-established technique based on free carrier infrared emission. Three applications of the technique are also presented in this paper, which include imaging of oxygen-related thermal donors, radial dopant density analysis, and the study of donor-related recombination active defects. These applications demonstrate the usefulness of the technique in characterizing silicon materials for photovoltaics.

Windmill-like Structure in Cz-Si

Windmill-like structures in as-grown Cz-Si wafers were characterised by several electrical and optical methods, such as 4PP resistivity, FTIR, SIRD and microwave PCD techniques. All these methods revealed the similar shape and size of the observed structure. Additionally, those windmill-like structures could be reproduced in the temperature distribution at the solidification interface numerically simulated in a certain time window using parameters being typical for the Czochralski process. The dependence of the minority carrier lifetime on resistivity in the investigated slices can be well fitted in the frame of the extended SHR theory using trap energies of EV,C ± (200-300) meV and a large asymmetry in the capture cross sections. Such defects may be oxygen-related double thermal donors with trap energies higher than EC – 200 meV. The FTIR map approves an enhanced concentration of Oi needed for an enhanced thermal donor formation in the windmill cogs and is well-correlated to the shear stress distribution obtained by SIRD Thus, we conclude that a windmill-like structure is formed mainly due to a non-ring-like distribution of oxygen at the solid-liquid interface.

Oxygen in Ge crystals grown by the B2O3 encapsulated Czochralski method

Local vibrations of oxygen in Ge crystals grown by the Czochralski method adopting liquid-B2O3 encapsulation and GeO2 powder doping were investigated by Fourier-transform infrared spectroscopy. Strong absorption peaks at 855cm−1, originating in local vibration of interstitially dissolved oxygen Oi as Ge–Oi–Ge quasi-molecules, developed depending on the doped amount of GeO2. Similarly, an absorption peak related to the combined vibration of Ge–Oi–Ge was found at 1264cm−1 and the conversion factor from the peak intensity to the oxygen concentration was evaluated to be 1.15×1019cm−2. By prolonged annealing at 350°C an absorption peak developed at 780cm−1, indicating formation of oxygen-related thermal donors. From the variations of carrier density and oxygen concentration, one donor was found to possess about 15 Oi atoms.

Behaviour of oxygen-related thermal donors in Ge crystals Czochralski-grown from the melt covered fully by B2O3

Oxygen-related thermal donors (OTDs) in oxygen-enriched Czochralski Ge crystals grown from a melt fully covered by B2O3 liquid were investigated by infrared spectroscopy. Interstitially dissolved oxygen concentrations [Oi] and thermal donor concentrations NTD in Ge specimens annealed at 350°C for 64h and at 550°C for 1h, followed by subsequent fast cooling to room temperature, were measured in comparison with those in as-grown Ge. By annealing at 350°C, an absorption peak developed at 780 cm−1 and the peak height at 855 cm−1 related to [Oi], decreased. The absorption coefficient at 780 cm−1 showed the same correlation to the difference between the total concentration of oxygen atoms and the dissolved oxygen concentration in the annealed specimens. It was found that the number of oxygen atoms forming the OTD increases with increasing annealing time at 350°C.

On conductivity type conversion of p-type silicon exposed to a low-frequency inductively coupled plasma of Ar + H2

The treatment of an Ar + H2 plasma generated by a low-frequency inductively coupled plasma system at 500 °C introduces an n-type region (of average electron concentration ∼1015 cm−3) on a Czochralski p-type substrate, forming a deep p–n junction. Examination by an electron microscope shows that the plasma treatment produces uniform nanocones on the surface and some defects, such as dislocations and platelets, in the subsurface. All these observed results are hydrogen-related. The conductivity type conversion is due to the formation of hydrogen-enhanced oxygen-related thermal donors (OTDs) as well as hydrogen-incorporated shallow thermal donors. The OTD-related signals are directly observed in the infrared absorption spectra. Both donors are annihilated after annealing at 550 °C for 10 min, resulting in conductivity recovery from n-type to original p-type. The electrical properties of the as-formed junction are investigated using current versus voltage (I–V), capacitance versus voltage (C–V) and Hall effect measurements. On this basis, the junction depth, carrier profile and hydrogen diffusion behaviour are studied. Moreover, a clear photovoltaic effect of the junction has been observed through the Suns-Voc and illuminated I–V tests.

Minority Carrier Lifetime in As-Grown Germanium Doped Czochralski Silicon

The minority carrier lifetime of as-grown germanium-doped Czochralski (GCZ) silicon wafers doped with germanium concentrations [Ge]=1016−1018 cm−3 is investigated in comparison with conventional CZ silicon samples. It is found that the lifetime distribution along the ingot changes with the variation of [Ge]. There is a critical value of [Ge] = 1016 cm−3 beyond which Ge can obviously influence the lifetime of as-grown ingots. This phenomenon is considered to be associated with the competition or combination between the oxygen related thermal donors (TDs) and electrically active Ge-related complexes. The related formation mechanisms and distributions are also discussed.

Oxygen loss and thermal double donor formation in germanium

Kinetics of interstitial oxygen loss and oxygen-related thermal double donor (TDD) generation upon heat treatments of Ge:O crystals at 350 °C have been studied. The TDD concentration ( N TDD) was derived from Hall effect measurements in the temperature range 77–400 K. The bistability of the first TDD species was taken into account. The interstitial oxygen concentration ([O i ]) in the crystals was determined from measurements of the intensity of the infrared absorption band at 855 cm −1 at room temperature with the use of a recently obtained calibration coefficient C O=1.05×10 17 cm −2. From an analysis of the [O i ]( t) and N TDD( t) kinetics a confirmation of recent suggestions about faster diffusivity of small oxygen clusters compared to the diffusivity of single interstitial oxygen atoms was obtained. Average numbers ( N) of oxygen atoms lost per TDD species created, N=Δ[O i ]/ N TDD, were calculated at different stages of the TDD generation. The obtained values of N are consistent with those expected in accordance with the recent models of the TDD structure. In particular, an average number of oxygen atoms per TDD species was about 5 at initial stages of the heat treatment when the first members (the TDD2 and TDD3 species) of the TDD family were dominant. N was found to be about 10 after extended anneals at 350 °C when the TDD6 and TDD7 species were dominant.

Enhanced formation of oxygen-related thermal donors in Ge crystals exposed to hydrogen plasma

Enhanced thermal donor (TD) formation and oxygen diffusion in silicon exposed to hydrogen plasma are well known and have been widely studied. Similar phenomena are expected to occur in oxygen-rich Ge crystals as well, but they have not been studied in detail yet. In this work the effects of hydrogenation in H +-plasma on the TD formation in Ge are investigated. Both oxygen-rich and oxygen-lean Ge crystals of n- and p-type conductivity were used. The samples were treated in dc H +-plasma in the temperature range 200–350 °C for different durations. The TD depth profiles were obtained by spreading resistance probe measurements on the angle-lapped surfaces of the samples. In oxygen-rich Ge crystals an enhanced formation of TDs has been found to occur upon hydrogenation as well as upon subsequent annealing at 350 °C deeply in the bulk up to depth of about 1 mm. The depth distribution of the TDs formed in enhanced way depends on the hydrogenation procedure as well as on the conditions of the subsequent heat-treatments. A possibility of the deep p–n-junction formation by H +-plasma treatment of the oxygen-rich Ge crystals has been demonstrated. An enhancement factor of the TD formation is estimated and a comparison with similar phenomena in silicon has been carried out.

High-resolution magnetic-resonance spectroscopy of thermal donors in silicon

Oxygen-related thermal donors in silicon form a group of defects with very similar properties. In a magnetic resonance experiment carried out under the normally applied conditions the individual species, up to 16 have been reported, are not distinguished. Due to lack of resolution only an overall integrated signal covering the whole group is recorded. In this paper two independent ways of improving the resolution in magnetic resonance are reported and applied to the Si-NL8 spectrum of the thermal double donors and to the structurally related Si-NL10 center. Line widths of the resonances caused by unresolved hyperfine interactions with the magnetic nuclei of isotope 29Si, nuclear spin I = 1 / 2 and natural abundance 4.7%, are near 0.3 mT. Monoisotopic silicon with the 29Si concentration reduced to 0.09% has been used to observe the Si:P, Si-NL8 and Si-NL10 resonances. In this material a line width as low as 0.045 mT has been observed. The second improvement of resolution is achieved using high-frequency high-field EPR spectrometers at 92 GHz and at 140 GHz for the Si-NL8 spectrum and at 349 GHz for Si-NL10. At these higher frequencies an increase of line structure is achieved by, e.g., a factor 15 when comparing a D band (140 GHz) with a classical X band (9 GHz) experiment. Due to ‘ g strain’ an increase of line width may occur. Under the high-resolution conditions individual species of thermal donors are separated.

VO<sub>n</sub> (n≥3) Defects in Irradiated and Heat-Treated Silicon

Local vibrational mode (LVM) spectroscopy has been used to study the evolution of vacancy-oxygen-related defects (VOn) in the temperature range 300-700°C in carbon-lean Cz-Si samples irradiated with MeV electrons or neutrons. New experimental data confirming an attribution of the absorption bands at 910, 976 and 1105 cm-1 to the VO3 complex are obtained. In particular, a correlated generation of VO3 and the oxygen trimer is observed upon irradiation of Cz- Si crystals in the temperature range 300-400°C. Strong evidence for the assignment of the bands at 991 and 1014 cm-1 to a VO4 defect is presented. The lines are found to develop very efficiently in the VO2 containing materials enriched with the oxygen dimer. In such materials the formation of VO4 is enhanced due to occurrence of the reaction O2i+VO2 ⇒ VO4. Annealing of the VO3 and VO4 defects at T ≥ 550C °C is found to result in the appearance of new defects giving rise to a number of O-related LVM bands in the range 990-1110 cm-1. These bands are suggested to arise from VO5 and/or VO6 defects. Similar bands also appear upon the annihilation of oxygen-related thermal double donors at 650°C in Cz-Si crystals pre-annealed at 450°C.

Calibration Factor for Determination of Interstitial Oxygen Concentration in Germanium by Infrared Absorption

Intensities of infrared absorption due to asymmetric stretching vibrations of interstitial oxygen atoms in Ge crystals enriched with 16O and 18O isotopes have been compared with oxygen concentrations determined by means of secondary ion mass spectrometry (SIMS). For Ge samples with oxygen content less than 5⋅1017 cm-3 a good correlation has been found between the values of oxygen concentration and values of absorption coefficient in maximum of the absorption band at 855.6 cm-1 with a proportionality coefficient CO = 0.95.1017 сm-2. It is argued that kinetics of oxygen-related thermal double donor formation and oxygen loss upon heat-treatments of Ge crystals at 350 оС cannot be described properly with the application of calibration coefficient CO = 5.1016 cm-2, which is widely used for the determination of oxygen concentration in Ge crystals.

General Model of Diffusion of Interstitial Oxygen in Silicon, Germanium and Silicon - Germanium Crystals

A theoretical modelling of the oxygen diffusivity in silicon, germanium and Si1-xGex (O) crystals both at normal and high hydrostatic pressure has been carried out using molecular mechanics, semiempirical and ab initio methods. It was established that the diffusion process of an interstitial oxygen atom (Oi) is controlled by the optimum configuration of three silicon (germanium) atoms nearest to Oi. The calculated values of the activation energy Ea (Si) = 2.59 eV, Ea(Ge) = 2.05 eV and pre-exponential factor D0(Si) = 0.28 cm2 s−1, D0(Ge) = 0.39 cm2 s−1 are in good agreement with experimental ones and for the first time describe perfectly the experimental temperature dependence of the Oi diffusion constant in Si crystals (T = 350–1200 °C). Hydrostatic pressure (P ≤ 80 kbar) results in a linear decrease of the diffusion barrier (∂P Ea (P) = −4.38 × 10−3 eV kbar−1 for Si crystals). The calculated pressure dependence of Oi diffusivity in silicon crystals agrees well with the pressure-enhanced initial growth of oxygen-related thermal donors. The simulation (PM5) has revealed that in Si1-xGex crystals there are two mechanisms of variation of Oi diffusion barrier. The increase of lattice constant leads to the linear increase of the diffusion barrier. Strains around Ge atoms decrease the diffusion barrier. Formation of gradient of diffusion barrier in the volume of Si1-xGex may be responsible for the experimentally observed suppression of generation of TD in Si1-xGex (O) crystals.

Thermal double donor annihilation and oxygen precipitation at around650 °C in Czochralski-grown Si: local vibrational mode studies

We have used local vibrational mode (LVM) spectroscopy to monitorthe formation of oxygen-related thermal double donors (TDDs) at450 °C and theirannihilation at 650 °C in carbon-lean Czochralski-grown (Cz-) Si crystals. A few samples were treated at650 °C under high hydrostatic pressure. It is found that the annihilation of TDDs at650 °C results not only in a partial recovery of the interstitial oxygen, but alsoin the appearance of a number of new O-related LVM bands in the range990–1110 cm−1. The positions of these lines and their shapes are identical to thoseobserved for Cz-Si irradiated with electrons or neutrons and annealed at600–700 °C. Since the lines appear upon annealing out ofV O3 and V O4 defects in irradiated samples, they are suggested to arise fromV Om (m>4) complexes. In both kinds of samples, pre-annealed and pre-irradiated,the new LVM bands disappear upon prolonged annealing at650 °C while enhanced oxygen precipitation occurs. TheV Om defects are suggested to serve as nuclei for oxygen precipitates developing at around650 °C. Highhydrostatic pressure is found to enhance further (up to 4–5 times) the oxygen precipitation process at650 °C in the samplespre-annealed at 450 °C.

P-N Junction Diodes Fabricated Based on Donor Formation in Plasma Hydrogenated P-Type Czochralski Silicon

AbstractA rather large amount of shallow donors is created in p-type Czochralski silicon (Cz Si) wafers after a hydrogen plasma exposure at ∼270 °C (substrate temperature) and a subsequent annealing in the temperature range of 350-450 °C. This two-step process has been used for the fabrication p-n junction diodes at low temperatures. Current-voltage characteristics show that the breakdown voltages of these diodes are higher than 100 V. The diode leakage is found to be improved after slow ramp annealing at temperatures up to 250 °C. Deep level transient spectroscopy measurements reveal that the oxygen related thermal donor is not the dominant doping species as expected before.