Thermal Donor Formation In Silicon Research Articles

Thermal donor formation in silicon enhanced by high-energy helium irradiation

The enhanced thermal donor (TD) formation was investigated in the float-zone n-type silicon irradiated with 7MeV helium ions at fluences from 5×109 to 1×1012cm−2 and subsequently annealed up to 500°C. Results show that radiation damage produced by helium ions remarkably enhances TD formation when annealing temperature exceeds 375°C, i.e. when the majority of vacancy-related recombination centers anneals out. At low fluences (below 1×1011cm−2), the profile of radiation enhanced TD follows well the distribution of primary damage–vacancies. The excess concentration of TD is proportional to helium fluence and peaks at 1×1014cm−3 if annealing temperature reaches 475°C. At higher fluences of helium, annealing temperature must be increased to stimulate formation of excess TDs. In this case, the concentration profile of TDs is more complex. Its growth starts from the irradiated surface and, with increasing temperature, it gradually extends up to the end-of-range of helium ions. Again, the excess TDs reach its maximum concentration of about 1.2×1014cm−3 at 475°C.

The lower boundary of the hydrogen concentration required for enhancing oxygen diffusion and thermal donor formation in Czochralski silicon

Hydrogen-enhanced thermal donor formation is achieved in p-type Czochralski silicon after exposure to hydrogen plasma and posthydrogenation annealing. Hydrogen diffusivities for the temperatures between 350 and 450 °C are determined based on spreading resistance probe measurement. The hydrogen diffusion is found to be trap limited. Two relationships (for different temperature ranges) are established to describe the lower boundary of the hydrogen concentration required for enhancing oxygen diffusion and thermal donor formation in silicon. The result reveals that hydrogen atoms both in free and in trapped states can enhance the oxygen diffusion.

Nature of the nuclei for thermal donor formation in silicon (or another variant of accelerated oxygen diffusion)

The influence of preliminary heat treatment at 800 °C on the accumulation and annealing kinetics of thermal donors formed at 450 °C in silicon single crystals is investigated by performing four-point measurements of the electrical resistivity. The activation energies for the generation and annealing of thermal donors were found to increase appreciably after preliminary heat treatment at 800 °C. The experimental results are discussed with consideration of the role of the various phase states of oxygen in as-grown Si. An interpretation is proposed with allowance for the effect of the internal elastic stresses created by microfluctuations of the oxygen and thermal donor concentrations in the Si lattice on the diffusion coefficient of oxygen atoms. A quantitative estimation of the dimensions of the microfluctuations gives a value on the order of hundreds of angstroms. The generation and annealing kinetics of thermal donors are described within the Kaizer-Frisch-Reiss model with allowance for the locally accelerated (within the microfluctuations) diffusion of oxygen. The microfluctuations contain roughly 1–3% of the total number of oxygen atoms in the crystal. This low percentage complicates the observation of the accelerated diffusion of oxygen by direct methods, but is sufficient for thermal donor generation.

The Nature of Precursors for the Thermal Donor Formation in Silicon

The Nature of Precursors for the Thermal Donor Formation in Silicon

Ultrashallow thermal donor formation in silicon by annealing in ambient oxygen

Czochralski-grown silicon wafers doped with phosphorus (∼1014 cm−3) have been annealed in nitrogen, wet nitrogen, oxygen, argon, and vacuum ambients at 470 °C for times up to 500 h. Sample characterization was made using predominantly electrical techniques such as admittance spectroscopy and thermally stimulated capacitance measurements but also secondary ion mass spectrometry (SIMS) and Fourier transform infrared spectroscopy were employed. In all samples, an increasing concentration of free carrier electrons is observed with increasing annealing time, reaching a maximum of ∼1016 cm−3 at 100 h. For durations in excess of 100 h gradual decrease of the free electron concentration takes place except for the samples treated in wet nitrogen and oxygen atmospheres, which display donors stable even after 200 h. These stable centers are found to have shallower donor level positions in the energy band gap (∼25 meV below the conduction band edge Ec) than those of the centers formed in vacuum, argon, and nitrogen atmospheres (∼35 meV below Ec). The latter centers are associated with the well-established shallow thermal donors (STDs) while the origin of the former ones, which are labeled ultrashallow thermal donors (USTDs) is less known. However, on the basis of a wealth of experimental results we show that the USTDs are most likely perturbated STDs modified through interaction with fast-in diffusing oxygen species, possibly oxygen dimers. Further, comparison between the electrical data and the SIMS measurements reveals unambiguously that neither the STD nor the USTD centers involve nitrogen, in contrast to recent suggestions in the literature.

Kinetic study of oxygen dimer and thermal donor formation in silicon

Computer simulations of the generation kinetics of thermal double donors (TDD's) in Czochralski-grown silicon have been performed and compared with experimental data for samples heat treated at temperatures between 350 and 420 \ifmmode^\circ\else\textdegree\fi{}C for durations up to 500 h. The experimental data were obtained by Fourier-transform infrared spectroscopy exploring the recent finding that local vibrational modes can be associated with the individual TDD's. A model assuming sequential generation of the TDD's and a fast diffusing oxygen dimer has been found to quantitatively reproduce the experimental data. The diffusivity of the oxygen dimer was estimated to be $\ensuremath{\sim}{10}^{6}$ times the value of interstitial oxygen at 400 \ifmmode^\circ\else\textdegree\fi{}C, with an activation energy of \ensuremath{\sim}1.3 eV and a preexponential factor of $\ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}{\mathrm{cm}}^{2}{\mathrm{}\mathrm{s}}^{\mathrm{\ensuremath{-}}1}.$ The transformation from TDD1 to TDD2 is well described by a first-order reaction having an activation energy of \ensuremath{\sim}2.5 eV, strongly indicating that the process involves motion of interstitial oxygen atoms $({\mathrm{O}}_{i}).$ This conclusion is further supported by the deduced value for the preexponential factor, being very close to that for the jump frequency of ${\mathrm{O}}_{i}.$

Models of Oxygen Loss and Thermal Donor Formation in Silicon by the Clustering of Rapidly Diffusing Oxygen Dimers

Models of Oxygen Loss and Thermal Donor Formation in Silicon by the Clustering of Rapidly Diffusing Oxygen Dimers

Depth Profiles for Hydrogen‐Enhanced Thermal Donor Formation in Silicon: Spreading Resistance Probe Measurements

Hydrogen enhancement of formation rates for oxygen‐related thermal donors in Si has been investigated for dependence on: the source of hydrogen, hydrogen isotope, and exposure time and temperature. Hydrogen injection efficiency is an important variable and depth profiles are dependent upon the surface preparation of samples exposed in an electron cyclotron resonance plasma where ion energies are ≤35 eV. Formation rates up to at 400°C have been observed. A sublinear dependence of the donor formation rate on beam current under 50 keV ion implantation is interpreted as a competition between oxygen‐hydrogen and hydrogen‐hydrogen interactions. Dependence on isotope mass and on exposure time in the plasma indicates hydrogen is the diffusing species that determines the penetration depth for the enhanced donor formation. Peculiar box‐like depth profiles and high formation rates near the advancing front produced in RF plasma exposures are suggestive of hydrogen accumulation near the advancing front. The temperature dependence for the penetration depth gives an activation energy of . This energy is attributed to trap‐limited diffusion wherein hydrogen lowers the energy barrier for the oxygen motion necessary to form thermal donors.

Vibrational infrared-absorption bands related to the thermal donors in silicon

Thermal donor formation in silicon at 390, 420, and 450 °C has been investigated and correlated with IR vibrational absorption bands observable at room temperature. It is suggested that oxygen is clustering in different structures giving rise to absorption bands at 975, 988, 1000, 1006, and 1012 cm−1. The bands at 975, 988, and 1000 cm−1 are related to the three first appearing double donors as studied by low-temperature infrared spectroscopy. The 1012 cm−1 band is related to the most frequently appearing type of structure and is suggested to correspond to a different type of donor, possibly the shallow thermal donors or the new thermal donors. The development of the new bands with heat treatment time shows a complex behavior. The early stages of thermal donor formation as studied at 390 °C are not governed by ordinary long-range oxygen diffusion but by a transformation process of pre-existing clusters corresponding to the 1012 cm−1 band. At 450 °C the cluster transformation process appears in parallel with an aggregation of oxygen atoms diffusing to the clusters. Another group of vibrational bands appears in the range 724–745 cm−1. These bands develop simultaneously with the 975–1012 bands and are suggested to be related to a different vibrational mode of the oxygen atoms in the clusters.

Hydrogen introduction and hydrogen-enhanced thermal donor formation in silicon

Hydrogen has been introduced from a rf plasma into Czochralski Si at 275 °C. Most of the hydrogen is trapped near the surface where it forms Si—H bonds, but a small fraction diffuses into the Si. This fraction enhances oxygen-related thermal donor (TD) formation rates in a diffusionlike profile during subsequent furnace anneals between 350 and 400 °C. A hydrogen concentration that is only a few percent of the oxygen concentration is sufficient to enhance the TD formation rate, indicative of a hydrogen-catalyzed process. Maximum concentrations for TDs after annealing at 400 °C exceed that for retained hydrogen. A mechanism of hydrogen diffusion through oxygen traps and correlated hydrogen-promoted oxygen diffusion is proposed to explain the enhanced TD formation rates.

Effect of Oxygen Concentration on the Kinetics of Oxygen Loss and Thermal Donor Formation in Silicon at Temperatures between 350° C and 500° C

Effect of Oxygen Concentration on the Kinetics of Oxygen Loss and Thermal Donor Formation in Silicon at Temperatures between 350° C and 500° C

The Kinetics of Oxygen Loss and Thermal Donor Formation in Silicon at Temperatures between 350°C and 500°C

The Kinetics of Oxygen Loss and Thermal Donor Formation in Silicon at Temperatures between 350°C and 500°C

Effect of oxygen concentration on the kinetics of thermal donor formation in silicon at temperatures between 350 and 500 °C

Early measurements of the initial rate of thermal donor formation in Czochralski silicon at 450 °C revealed a dependency on the fourth power of the oxygen concentration. This result has led to the view that the core of the defects contained four oxygen atoms. We now show that this dependency is observed only for anneals close to 450 °C. Our results indicate that oxygen dimerization controls the thermal donor formation kinetics but that the dimers become increasingly unstable above 400 °C.

Depth Profiles for Hydrogen-Assisted Thermal Donor Formation in Silicon

Concentration and depth profiles for thermal donor formation in Si under hydrogen plasma exposure have been investigated by infrared, IR, absorption and spreading resistance probe, SEP measurements. Donors measured by SRP are shown to correlate with oxygen-related thermal donors, TD's detected by IR. Hydrogen-independent precursor nuclei formation at the hydrogenation temperature, and continued hydrogen activity in TD formation after hydrogenation are inferred from effects of extended pre and post heating on TD concentrations and profiles. A model wherein diffusing hydrogen activates precursor sites is presented to explain concentrations and profiles for TD's formed in a hydrogen plasma. 7 refs., 6 figs.

Thermal donor formation in silicon: A new kinetic model based on self-interstitial aggregation

A new kinetic model is presented for the formation of thermal donors near 450 °C in oxygen-rich silicon. This model is based on the aggregation of the self-interstitials generated by the early stage of oxygen precipitation. Good agreement is obtained with published experimental kinetics, and the model is able to account for several other observations, such as the influence of carbon.

Some new features of thermal donor formation in silicon at T < 800 K

Some new features of thermal donor formation in silicon at T < 800 K

On the Question of Oxygen Diffusion During Oxygen Related Thermal Donor Formation In Silicon.

In an attempt to decide the question whether enhanced oxygen diffusion is important for heat-treatments of silicon at ∼450ºC where thermal donors are formed we have conducted two types of experiments aimed at providing a measure of the “effective” oxygen diffusivity. First, we have extensively measured the temperature dependence of the thermal donor introduction rate for very short heat treatment times (20min). This measurement provides the thermal activation energy of TD formation. Since effects of long range diffusion and formation of large oxygen clusters are negligible for suchtimes and temperatures and, presumably, thermal donor formation at the lowest heat treatment temperatures is oxygen diffusion limited, it should be possible to interprete the obtained activation energy in terms of oxygen diffusivity. The change of the interstitialoxygen content is immeasureable for 20min heat treatment times. Therefore, the decay of the interstitial oxygen content was measured for longer heat treatments at 450ºC (up to 500hours). The two experiments are complementary in several ways: In one experiment the oxygen diffusion activation energy is extracted, while the other measurement provides the value of the diffusion coefficient at a given temperature. In one case thermal donors are monitored for short heat treatment times while in the other experiment the interstitial oxygen content is measured for long heat treatment times. The present measurements are different from other diffusion experiments in this temperature range where theatomic jump of isolated oxygen is monitored [1]. Here we attempt to extract an effective oxygen diffusivity under conditions of thermal donor formation since the thermal donor formation process itself might be the cause of an enhanced oxygen diffusivity.

Effects of heat treatments of GaAs on the near-surface distribution of EL2 defects

A cluster model is proposed for the midgap state EL2 in GaAs based on a defect reaction similar to that of the thermal donor formation in silicon. Anomalous depth profiles of the EL2 defects in bulk GaAs created by heat treatment have been quantitatively explained, assuming that the EL2 defect is a four point defect cluster. Possible candidates of the point defects are discussed.

Self-interstitials and thermal donor formation in silicon: new measurements and a model for the defects

Silicon containing oxygen (1018 cm-3) and carbon (7*1017 cm-3) has been heated at 450 degrees C for times up to 600 h to generate thermal donors. Measurements indicate that for every two oxygen atoms that are precipitated there is generation of one self-interstitial. The latter defects are detected by means of their selective trapping by carbon. These results lead to a proposed model of a thermal donor consisting of a small oxygen agglomerate with a bonded self-interstitial. All relevant data appears to be explicable on this basis.

Oxygen diffusion and thermal donor formation in silicon

The information available on the diffusion of oxygen and on the formation of thermal donors in silicon is critically reviewed. In this context the effects of intrinsic point defects on the diffusion-controlled growth of oxygen precipitates is investigated in some detail. Seemingly contradictory experimental results on the diffusivity of oxygen in silicon at temperatures around 400° C are explained in terms offast-diffusing gas-like molecular oxygen in silicon. The concept of molecular oxygen is also invoked in a newly suggested model of thermal donor formation in silicon. The diffusivity of molecular oxygen in silicon is estimated to be around 10−9cm2s−1 at 450° C, almost nine orders of magnitude higher than the diffusivity of atomic oxygen in interstitial position.