Thermal Double Donors 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.

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.

Separation of Individual Thermal Double‐Donor Levels by Laplace Deep‐Level Transient Spectroscopy

Thermal double donors (TDDs) are created by heat treatment of Czochralski‐grown Si at temperatures around 450 °C. A family of individual defects with very similar donor (TDDn0/+ with n = 0–16) and double‐donor (TDDn+/++) levels is known today. The close‐level positions of the individual TDDs result in broad and asymmetric deep‐level transient spectroscopy (DLTS) peaks. Herein, Laplace DLTS are used to study this asymmetry. Several well‐separated Laplace DLTS peaks are resolved in our samples. Metastable properties of the TDDs are taken as advantages to identify the double‐donor levels (++/+) of TDD0, TDD1, and TDD2.

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.

Sensitivity enhanced FTIR investigation of defects introduced by RTA pre-treatment in Czochralski silicon wafers

The investigation of vacancy oxygen complexes in silicon wafers by FTIR is not easy because their concentration is close to the detection limit. In order to enhance the sensitivity of the FTIR measurement we investigated stacked samples of about 1 cm thickness at temperature close to liquid helium temperature. This method was applied to study the absorption bands of defects in as-grown silicon wafers, rapid thermal annealing (RTA) pre-treated wafers, and in RTA pre-treated wafers with subsequent anneals at 800 °C for short periods. We found that the RTA pre-treatment at 1250 °C could not fully annihilate the thermal double donors which were present in the as-grown wafer. By RTA at 1100 °C annihilation was possible. In the wafer pre-treated by RTA at 1250 °C we found the absorption bands of VO4 at 985 cm−1 and 991 cm−1 in the measurements carried out at room temperature and at 6 K, respectively. In this wafer we also detected an unknown band at 1030 cm−1. The VO4 band and the unknown band at 1030 cm−1 disappeared immediately after annealing at 800 °C for 10 min. Instead, the bands at 1096 and 1099 cm−1, both assigned to VO5,6, appeared. These bands are already present in the as-grown sample but their absorption coefficient decreases during RTA at 1100 °C. In samples annealed at 800 °C for 30 min or longer a new absorption band at 1053 cm−1 appears which can be also assigned to VO5,6 complexes.

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)

Near-interfacial thermal donor generation during processing of (100)Si/low-κ Si-oxycarbide insulator structures revealed by electron spin resonance

A low-temperature multifrequency electron spin resonance (ESR) study has been carried out on Cz-(110)Si/insulator structures with organosilicate films of low dielectric constant κ grown at 300 °C using the plasma-enhanced chemical vapor deposition method (PECVD). After subjection to a short-term UV-irradiation-assisted thermal curing treatment at 430 °C to remove the organic component from the low-κ film and obtain optimal porosity, the NL8 ESR spectrum of C2v symmetry is observed, characterized by g1 (//[100] = 1.999 83(8), g2(//[011] = 1.992 74(8), g3 = (//[]) = 2.001 15(8). Based on previous insight, this reveals the generation in the c-Si substrate of singly ionized thermal double donor (TDD) defects with a core containing oxygen atoms. Remarkably, the generation is found to be highly nonuniform, and the defect density depth profile shows an exponential-like decay (decay length ∼3.8 μm) from the oxide/Si interface inward the Si substrate, thus exposing the defect formation as an interface-administered effect. Upon analysis, the strain induced by interfacial stress in the c-Si beneath the interface is suggested as the major driving component in the enhancement of TDD formation during thermal treatment, suggesting that substantial stress is involved with PECVD organosilicate low-κ glasses. The result represents a different and affirmative illustration of the influence of strain on TDD formation. Based on the principal g values, the observed TDD is closest to the NL81 type, the one formed first in bulk c-Si through oxygen agglomeration during short-term thermal treatment.

Interstitial oxygen behavior for thermal double donor formation in germanium: Infrared absorption studies

The kinetics of the reduction of interstitial oxygen (Oi) due to the formation of thermal double donors (TDDs) upon heat treatment in an oxygen-rich Ge crystal were investigated at various temperatures. Specimens were prepared from a Ge crystal with oxygen at a concentration of 4–5 × 1017 cm−3 grown by a new Czochralski method and were heat-treated in the temperature range 300–500 °C. Shrinkage of a dissolved oxygen absorption peak at 855 cm−1 and simultaneous development of a thermal double donor peak at 780 cm−1 were observed by infrared absorption spectroscopy at room temperature. The formation of TDDs was also detected electrically. Reduction of dissolved oxygen concentration upon the heat treatments was kinetically analyzed. The activation energy of the reduction of Oi concentration was evaluated to be 1.7 and 2.0 eV in the early and prolonged stages of the heat treatment, respectively, close to that of Oi diffusion. From the results, TDD development process was indicated to be the merge reaction of Oi-Oi to form oxygen dimers (Odimer) in the early stage of the heat treatment and further enlargement of Odimer to TDDs by absorbing Oi in the prolonged stage of the heat treatment in accordance with the On-2NN model.

Enhanced Formation of Thermal Donors in Germanium Doped Czochralski Silicon Pretreated by Rapid Thermal Annealing

The thermal donor formation at 425oC - 450oC in Ge doped Czochralski (GCZ) silicon having about 1016 cm-3 Ge content pretreated by rapid thermal annealing (RTA) and conventional furnace annealing (CFA) has been investigated using low-temperature infrared spectroscopy (LT-IR). The measurements prove that lightly Ge doping can enhance the formation of thermal double donors in the initial stage of the low temperature annealing after RTA process. Ge induced additional grown-in oxygen precipitates during silicon ingot growth and the abundant self-interstitials during RTA may be the reason for the enhancement. However, after extending the annealing time at the low temperatures, the thermal donor concentration in the GCZ silicon is lower than that in the conventional CZ silicon. In final, the mechanism is also discussed.

The Electrical and Optical Properties of Point and Extended Defects in Silicon Arising from Oxygen Precipitation

Oxygen precipitation in Si is a complex set of processes which has been studied over many years. Here we review theoretical work relating to the precipitation process. At temperatures around 450°C oxygen atoms become mobile and form a family of thermal double donors. The structure of these defects and the origin of their electrical activity is discussed. At temperature around 650°C these donors disappear and there is a growth of SiO2 precipitates along with rod like defects which are extended defects involving Si interstitials. At higher temperatures these collapse into dislocation loops. The structure and electrical properties of the rod like defect are described and compared with those of dislocations.

Diffusion Coefficient of Hydrogen in Silicon at an Intermediate Temperature

The hydrogen diffusion coefficient in silicon (Si) in an intermediate temperature region (429–800 °C) is obtained using an enhanced formation of thermal double donors (TDDs), which is a phenomenon caused by in-grown hydrogen. An as-grown Czochralski (CZ) Si ingot contains hydrogen as a low-concentration impurity. Although present in a very low quantity, it is possible to observe an enhanced TDD formation due to in-grown hydrogen as a variation of the resistance of the specimen using a high-resistance ingot (>500 Ω cm). The hydrogen diffusion coefficient obtained using the depth profile of TDDs, which are generated under the influence of the depth profile of the in-grown hydrogen, shows a value close to that given by Van Wieringen and Warmoltz (VWW) [Physica 22 (1956) 849]. Furthermore, it is found that the activation energy of hydrogen diffusion in the intermediate temperature region is almost the same as that obtained by VWW. This result indicates that the hydrogen diffusion coefficient can be expressed by an equation given by VWW at an intermediate temperature region.

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.

Thermal donors formation via isothermal annealing in magnetic Czochralski high resistivity silicon

A quantitative study about the thermal activation of oxygen related thermal donors in high resistivity p-type magnetic Czochralski silicon has been carried out. Thermal donor formation has been performed through isothermal annealing at 430°C up to a total time of 120min. Space charge density after each annealing step has been measured by transient current technique. The localized energy levels related to thermal double donors (TD) have been observed and studied in details by thermally stimulated currents (TSCs) in the range of 10–70K, and activation energies E and effective cross sections σ have been determined for both the emissions TD0∕+ (E=75±5meV, σ=4×10−14cm2) and TD+∕+ (E=170±5meV, σ=2×10−12cm2). The evolution of the space charge density caused by annealing has been unambiguously related to the activation of TDs by means of current deep level transient spectroscopy TSC, and current transients at constant temperature i(t,T). Our results show that TDs compensate the initial boron doping, eventually provoking the sign inversion of the space charge density. TD’s generation rate has been found to be linear with the annealing time and to depend critically on the initial interstitial oxygen concentration, in agreement with previous models developed on low resistivity silicon.

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.

"New Donors" in Czochralski Grown Silicon Annealed at T≥ 600°C under Compressive Stress

Effects of compressive stress on oxygen agglomeration processes in Czochralski grown silicon heat treated at T= 450OC, used as a reference temperature, and T= 600OC to 800OC are investigated in some detail. Compressive stresses of about P= 1 GPa lead to enhanced formation of Thermal Double Donors in materials annealed over a temperature range of T= 450OC – 600OC. It has been shown that the formation of thermal donors at T= 450OC under normal conditions and compressive stress is accompanied with loss of substitutional boron. In contrast, the concentration of the shallow acceptor states of substitutional boron in silicon annealed under stress at T≥ 600OC remains constant. An enhancement effect of thermal donor formation is gradually weakened at T≥ 700OC. The oxygen diffusivity sensitive to mechanical stress is believed to be responsible for the observed effects in heat-treated silicon.

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.

Stable radiation-induced donor generation and its influence on the radiation tolerance of silicon diodes

Four different kinds of silicon materials (standard float zone—FZ, oxygen enriched FZ—DOFZ, Czochralski—Cz and thin epitaxial layers grown on Cz substrates—Epi/Cz) have been investigated in this work. They have been irradiated with 60Co-γ-radiation and 24 GeV/ c protons. The differences in the changes observed in the effective doping concentration ( N eff) after proton irradiation of Cz and Epi silicon can be explained by the balance between the formation of two types of defects—a deep acceptor (the I center, referred to as I p in this paper in order to avoid any confusion with interstitials) and a shallow donor (the bistable donor (BD) complex). While the formation of the I p center depends on the concentration of interstitial oxygen, the BD is generated in materials with high concentration of oxygen dimers. Following the generation of the IO 2i defect—detected after low irradiation fluences only in Epi and Cz material—the average dimer concentration in 50 μm Epi/Cz was estimated to be of only 2.7 times lower than in Cz diodes. A value of ∼2.7×10 −12 cm 2 for the electron capture cross-section of the Thermal Double Donors (TDDs), present in the Cz material, was also measured for the first time. The positive space charge introduced by ionization of the BD centers in Epi diodes was directly determined from TSC experiments for two irradiation fluences. The determined values show a linear fluence dependence for the formation of BD centers. By taking into account the BD and the I p center generation as well as the donor removal, the change of the effective doping concentration N eff at 20 °C in Epi/Cz and Cz diodes after an annealing time of 120 min at 60 °C can be explained up to a negative space charge introduction rate of ∼2.2×10 −2 cm −1 thought to be determined by negatively charged clusters. Long-time annealing experiments at 80 °C have shown that the generation of BDs represents a stable damage.

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.

Radiation hardness of silicon—a challenge for defect engineering

Thermally stimulated current measurements of silicon particle detectors have been performed for defect characterization after high levels of γ- and proton-irradiation. Two defects closely correlated with the detector performance were monitored: a deep acceptor (I) and a bistable donor (BD). In oxygen rich silicon the deep acceptor is largely suppressed while the BD generation is strongly enhanced. The influence of the starting material (standard float zone (FZ), oxygen enriched FZ, Cz, epitaxial silicon) on the formation of these two defects is discussed. Identification of the I-defect with the V2O complex, and of BD with the thermal double donor TDD2 is suggested.