Thermal Donor Formation Research Articles

Development of the conductivity type inversion caused by thermal double donors’ formation in p-type high-resistivity silicon

The formation of thermal double donors (TDDs) during the iso-thermal annealing at 450 °C in low-oxygen and high-resistivity silicon was quantitatively investigated, and the oxygen related power parameter for low-oxygen and high-resistivity silicon was obtained. Excessive TDDs formed in the p-type high-resistivity silicon during the device manufacturing process will invert the conductivity to n-type. A quantitative relationship between the critical wafer start resistivity and oxygen concentration in p-type high-resistivity silicon was obtained. The p-type conductivity can be maintained after the device fabrication by controlling the resistivity below the critical resistivity corresponding to the oxygen concentration in p-type high-resistivity silicon.

The Effect of Pull Speed and Heat Treatment on Thermal Donors in Czhocralski Silicon

In silicon crystals manufactured by the Czochralski method, oxygen is incorporated as a contaminant originating primarily from dissolution of the quartz crucible used to contain the molten silicon feedstock. The oxygen can be found either as interstitials, agglomerates, or as oxide particles. Particular kinds of agglomerates are known to lead to the formation of thermal donors—electronic states in the bandgap of the silicon base material. These can act as sites for recombination of excited charge carriers and are called thermal donors due to their ability to inject two electrons into the conduction band of silicon by thermal excitation, leading to enhanced electrical conductivity. Neither of these features are desirable in Cz‐Si manufacture. Herein, the behavior of thermal donors as a function of the pull speed, and the position in the ingot, is investigated. Thermal treatment is provided, first for the formation of thermal donors and then for their removal. The aim is to investigate methods for minimalizing their incorporation in the crystal in the first place and then how they can be removed. Hyperspectral imaging and spectroscopy combined with Fourier‐transform infrared spectroscopy are used.

Effect of carbon, oxygen, and intrinsic defects on hydrogen-related donor concentration in proton irradiated n-type silicon

We investigated the effect of the concentration of carbon, oxygen, and irradiation-induced intrinsic defects on hydrogen-related donor (HD) concentration. Several n-type silicon wafers having different carbon and oxygen concentrations were irradiated with 2 MeV protons, subsequently annealed at 300–400 °C, and analyzed by spreading resistance profiling. The HD concentration had no correlation with carbon and oxygen concentration. Additionally, the HD concentration showed a strong increasing linear dependence with proton-irradiation dose at 350 and 400 °C and a square root dependence at 300 °C. In the decay process of HD concentration at 400 °C, fast- and slow-decay components were observed regardless of wafer type. Our results show that the HD formation is based on the interactive process of irradiation-induced intrinsic defects and hydrogen, rather than hydrogen-catalyzed thermal double donor formation. Magnetic-field-applied Czochralski (m:Cz) wafers with 300 mm diameter, which are critical for the production scaling of power devices, have a relatively higher oxygen concentration than conventional floating-zone wafers. Our results further suggest that controlling the intrinsic defect formation, rather than oxygen impurity concentration, is more important in realizing designed doping profiles with high accuracy and reproducibility for next-generation power devices using large-diameter m:Cz wafers as a standard starting material.

Ring‐Like Defect Formation in N‐Type Czochralski‐Grown Silicon Wafers during Thermal Donor Formation

This article presents experimental and simulation studies on the formation of recombination‐active ring‐like defects during thermal donor (TD) formation at 450 °C in n‐type Czochralski‐grown silicon wafers. With increasing anneal duration from 1 to 24 h, the resistivity and interstitial oxygen concentration of samples decrease, consistent with the formation of TDs. However, after a subsequent TD annihilation treatment, the resistivity recovers completely, while the interstitial oxygen concentration recovers only partially. Furthermore, the wafers exhibit ring‐like defects in photoluminescence images after the TD generation and remain persistent even after annihilation treatment. By applying a tabula rasa (TR) treatment prior to TD generation anneals, the net loss of interstitial oxygen is reduced, and the incubation time of the ring‐like defects is extended in comparison to as‐grown wafers. Finally, to investigate the possibility of low‐temperature precipitation, simulation of oxygen precipitation (OP) kinetics during the 450 °C annealing steps is performed using an enhanced effective diffusivity of oxygen. These simulations demonstrate that OP can occur simultaneously during TD formation, resulting in recombination‐active ring‐like defects.

Numerical method for thermal donors formation simulation during silicon Czochralski growth

Various upgrades of the Czochralski (Cz) growth process are currently being investigated in order to increase throughput and reduce production cost of high efficiency silicon-based solar cells. However, as-grown thermal donors (TD) in Cz silicon can significantly reduce the conversion efficiency of such solar cells. An accurate simulation tool is therefore required to investigate and optimize TD formation during crystal growth. A numerical method combining thermo-hydraulic simulations and a kinetic TD formation model was improved by the implementation of a more appropriate TD formation model, identified through a benchmark of the different models available in the literature. Three different Cz growth processes were investigated both numerically and experimentally. Numerical results are in remarkable agreement with TD concentrations measured along the three ingots by the OxyMap technique developed at CEA. The simulations were then used to detect when TD were formed during Cz processes. The reliability of the method was also assessed through sensitivity analyses, highlighting the critical importance of the input interstitial oxygen concentration. These results show that accurate estimates of axial TD concentration profiles can be obtained and so for very different Cz processes, supporting the robustness of the developed method and its relevance to process optimization and furnace design to reduce TD concentrations.

Formation of thermal donor enhanced by oxygen precipitation in silicon crystal

The formation of thermal donors in silicon is investigated using Czochralski silicon crystals grown with different grown-in defect regions, such as voids and nuclei of oxidation-induced stacking faults. It was found that the formation rate of thermal donors during annealing at 450 °C increases with an increase in the density of oxide precipitates in the regions containing different grown-in defects. The thermodynamic model for the formation of thermal donors shows that the electrically inactive oxygen trimers as nuclei of thermal donors in an as-grown crystal increase with an increase in the density of oxide precipitates, suggesting that the formation of the nuclei is enhanced due to silicon self-interstitials emitted by oxygen precipitations during the crystal growth.

Spectral Characteristics and Spatial Distribution of Thermal Donors in n‐Type Czochralski‐Silicon Wafers

In monocrystalline silicon rich in oxygen, thermal donors are formed at temperatures around 450 °C. These are widely accepted to be electrically active oxygen clusters acting as double donors to the conduction band. Exposure to higher temperatures (650 °C) reportedly eliminates them. Herein, a systematic study of the spatial distribution of thermal donor formation and elimination by heat treatment at 450 and 650 °C in commercial n‐type Czochralski‐silicon wafers with high and low content of interstitial oxygen atoms are reported. Hyperspectral imaging techniques with spectral and spatial resolution are used. Thermal donors form at 450 °C in a ring‐like pattern, significantly enhanced in oxygen‐rich material. The results indicate the formation of at least six different donor clusters, leading to a strong, characteristic spectral response upon photoexcitation. The emission related to direct band‐to‐band recombination (1.100 eV) become systematically stronger upon heat treatment at 450 °C. Subsequent treatment at 650 °C rearrange the spectral response into a single, homogenously distributed, broadband emission with peak energy of 0.767 eV. The emission related to band‐to‐band recombination is significantly reduced. A previously studied emission at 0.807 eV (D1) commonly related to impurities is found, providing evidence that this signal is related to the combination of defects and oxygen.

Kinetics of Generation of Thermal Donors in Silicon of Stabilization of States of Fast-Diffusing Impurities

The paper reports that the intensity of generation and the concentration of low-temperature thermal donors (LTD) at 450°C in silicon, where fast diffusing impurities (FDI) are stabilized by means of binding them into electrically neutral chemically bound complexes with sulfur, are significantly lower compared to their intensity and concentration in reference samples of silicon doped with sulfur and pure reference samples. The “Kaiser model” states that the initial rate of generation of low-temperature thermal donors is proportional to biquadrate, and their maximum concentration is to the third degree of oxygen concentration. According to the Kaiser model of thermal donor generation, LTD represent predominantly stable tetrahedrons (i.e. SiO₄ molecules). However, the Kaiser model does not take into account the possibility of interaction of oxygen with other impurities that might lead to the formation of electrically active centers. Based on the analysis of experimental results of Si<S> samples subjected to heat treatment the authors recommend a revision of the principles of LTD in silicon in view of behavior of FDI, as they play a key role in the process of the formation of LTD. Thus, in the present paper the authors effectively are attempting to shed light on the anticipated role of impurity centers with deep levels in the process of formation of thermal donors.

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.

Effect of oxygen dissolved in germanium on defect formation and optical properties of single crystals

Oxygen impurities and optical properties of Sb-doped germanium single crystals were investigated with Fourier transformed infrared spectrometry. The effect of oxygen on crystal structure was shown. The effects of annealing in the temperature range 623–723 K on the behavior of oxygen dissolved in germanium and on optical properties of single crystals have been investigated. It was found that annealing at lower oxygen partial pressure leads to the formation of thermal donors (TD) on the basis of dissolved oxygen atoms. It was established that TD formation and the decrease of unbound oxygen concentration is accompanied by the increase in temperature stability of the optical properties of crystals.

Formation of thermal defects in silicon grown by means of float zone melting

The factors of formation of thermal donors (TDs) and thermal acceptors (TAs) silicon with a low oxygen concentration grown by means of float-zone melting are discussed. The results of thermal treatment in a temperature range of 400–1150°C show that interstitial iron atoms produce the largest contribution to formation of TDs. Substitutional atoms of iron are probable drivers of TA formation in the process of hightemperature treatment (HTT). Iron precipitates formed during low-temperature annealing (400–600°C) also contribute to TA formation. The TD and TA densities after HTT depend on the type and the density of structural defects in the material and the conditions of thermal treatment: the cooling rate and the gas medium (oxygen or argon).

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.

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.

Formation of Shallow n-p Junctions in Cz-Si by Low-Energy Implantation of Carbon Ions

I ntroduction Heat treatments of Cz Si in the temperature range between 400 and 1200°С result in the aggregation of supersaturated interstitial oxygen leading to the formation of bulk defects. At low temperatures in the range between 450 and 650°C, the formation of small clusters (O3, O4) leads to shallow thermal donor (TD) centers. The presence of carbon has a considerable influence on thermal donor formation due to the small size of the substitutional carbon atom leading to a tensile stress field around it.It is possible to introduce into the subsurface layer of silicon the atoms of carbon in a wide range of concentrations, and to create a p-n junction in p-type Si. Taking into account the low diffusion coefficient of carbon in silicon and the low temperature of carbon-oxygen TD formation, it is possible to form shallow p-n junctions (<100 nm) by means of implantation of carbon ions.In this study, a low-temperature method is developed for the creation of shallow р-n junctions using carbon ion implantation controlled TD formation. The described approach can be used for different applications, in particular, for photo-converters manufacturing, as low temperatures do e.g. not lead to lifetime degradation of non-equilibrium charge carriers. Experimental (001) Cz Si wafers with oxygen concentration of about 8×1017 сm- 3 are implanted by carbon ions with an energy of 20 keV and doses between 6.0×1012 and 1.3×1015 cm- 2. After implantation, thermal annealing at temperatures between 550 and 750°С is performed during 20 to 300 minutes in argon ambient. Investigations of electro-physical properties of the structures are performed using the admittance method. This method allows to obtain information about the concentration and energy position of levels in the forbidden gap of the semiconductor that are responsible for the observed frequency dependence of capacitance in these structures. The type, size and concentration of structural defects in the investigated silicon samples are investigated using X-ray diffraction and diffuse scattering. Results and discussion It is shown that the concentration of TD's depends on annealing temperature and dose of carbon ions. The maximum concentration (3×1016 cm-3) of TD's is observed after annealing at 650°С and for a carbon dose of 6×1013 cm-2. Annealing at 750°С leads to an annihilation of the TD's.The temperature dependence of CVC's is shown in the figure for high-frequencies of the test signal (3 MHz). This dependency has a clear maximum of capacity in the voltage range from - 6 to 3 V. The existence of this maximum in the CVC indicates that in a certain part of the investigated structure there is an electric charge storage due to the creation of a considerable concentration of TD's near the surface of the p-type substrate. Analysis of the temperature dependence of the capacitance integral within the limits of the voltages, where this peak is observed, had been carried out. The value of the integral that is equal to the peak area under the CVC curve has been calculated, allowing to extract an activation energy for the TD's of 0.012 eV in the temperature interval between 25°С and 160°С.X-ray diffuse dispersion measurements are also performed on the samples containing TD's and reveal that the signal intensity increase correlates with an increase of TD concentration. After annealing at 650°С, the scattering intensity and the related concentration of point defects are at maximum. Annealing at 750°С leads to a decrease of the concentration of interstitial defects and of TD's. The nature of investigated TDs is related to interstitial and oxygen complexes. The implanted carbon is a stimulating factor for the gettering of oxygen, defect nucleation and growth of TD's. Conclusions It is shown that by implantation of carbon ions and subsequent annealing at temperatures between 600 and 700°C in Cz-silicon it is possible to form p-n junctions with breakdown voltages of 100-150 V. The highest breakdown voltages and lowest leakage currents are obtained using an implantation dose of 6×1013 cm-2 followed by annealing at 650°C.

Oxygen-Related Thermal Donor Formation in Dopant-Rich Compensated Czochralski Silicon

The thermal donor (TD) generation in dopant-rich compensated Czochralski silicon was studied by pulling an ingot from a feedstock containing large amounts of donors and acceptors. In a wafer located in the vicinity of the change of conductivity type, thermal donors were formed and the evolution of their concentration was similar to that in noncompensated lowly boron-doped silicon. Thus, simultaneous high densities of both boron and phosphorus do not have a significant impact on the TD formation. This brings the experimental evidence that for a given oxygen concentration and annealing temperature, the TD formation is controlled by the electron density.

Effect of dopant compensation on the temperature dependence of the transport properties in p-type monocrystalline silicon

In this paper, we investigate the temperature variations of the hole transport properties in initially uncompensated boron-doped Czochralski silicon progressively compensated through thermal donors activation. After each donor generation anneal, the boron and thermal donor concentrations in the samples are determined using (1) the change in carrier concentration at room temperature and (2) the analysis of the temperature variation of the carrier concentration in the range 77–350 K. By comparing both methods with theory, evidence is brought that down to 77 K the Hall factor is unaffected by compensation up to high compensation levels. This is of great interest for researchers working on new solar-grade materials since it nicely suggests that Hall factor models previously established for non-compensated silicon can be applied to compensated samples, for example, when extracting the individual dopant concentrations from the temperature variations of the hole concentration. At very high compensation levels, anomalous Hall data lead to erroneously low carrier mobility values. We showed that this artifact was due to the formation of a n-p-n transistor, arising from the preferential formation of thermal donors in the sample's subsurface. After rejecting these unphysical data from the analysis, we confirm that the hole mobility is greatly affected by compensation in the temperature range investigated. We eventually confront our experimental data to current mobility models and discuss the possible sources of discrepancy.

Deciphering mechanisms of enhanced-retarded oxygen diffusion in doped Si

We study enhanced/retarded diffusion of oxygen in doped silicon by means of first principle calculations. We evidence that the migration energy of oxygen dimers cannot be significantly affected by strain, doping type, or concentration. We attribute the enhanced oxygen diffusion in p-doped silicon to reduced monomer migration energy and the retarded oxygen diffusion in Sb-doped to monomer trapping close to a dopant site. These two mechanisms can appear simultaneously for a given dopant leading to contradictory experimental results. More generally, our findings cast a new light on phenomena involving oxygen diffusion: precipitation, thermal donors formation, and light induced degradation.

Diode Characteristics and Thermal Donor Formation in Germanium-Doped Silicon Substrates

Germanium-doped silicon is studied actively for application in microelectronics, in particular, as an improvement of the radiation hardness of the silicon substrate can be achieved. However, the impact of the introduced germanium on the electrical stability and properties of the silicon material and devices is also of special concern. In this contribution we report the results of a study on the effects of low temperature thermal anneals relevant for backend processing, on the electrical characteristics of p-on-n diodes fabricated on Czochralski-grown (CZ) Ge-doped-Si with two different germanium concentrations (1•1019 cm-3 and 1.2•1020 cm-3), as well as on control CZ Si wafers without Ge doping. The results show good diode yield and uniformity, with little differences between the electrical characteristics of the fabricated diodes on the different substrates. However, interestingly for device applications, a significantly lower thermal donor generation (by one order of magnitude) is found for the case of the higher Ge-doped material subjected to 450ºC thermal anneals.