Thermal Double Donors Research Articles

Comparison of electronic structure and properties of hydrogen-associated and thermal double donors in silicon

Infrared (IR) and electron paramagnetic resonance (EPR) studies of quenching-dependent hydrogen-related double donor (HDD) formed in proton-implanted n-Si and p-Si upon annealing above 300°C were carried out. IR data taken at liquid He and N2 reveal that quenching-dependent IR absorption lines is due to HDD electronic excitations. An analysis of Si–H local vibrational modes and EPR data on AA1 EPR center allow to conclude that HDD is an interstitial-type complex with C2v-symmetry. There are similarities in structures and properties of HDD and thermal double donors (TDD). However, uniaxial stress-induced alignment and recovery kinetics of HDD (AA1 EPR center) show strong difference in comparison with TDD (NL8 EPR center).

Thermal donor and antimony energy levels in relaxedSi1−xGexlayers

Shallow energy levels in molecular-beam epitaxially grown relaxed ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ layers with $x=0.05$ and 0.1 have been investigated by admittance spectroscopy and thermally stimulated capacitance. The shallow levels observed in as-grown layers have been identified as related to Sb and thermal donors (TD's). The Sb related donor level has an activation energy of 33 meV and 32 meV for the ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ layers with $x=0.05$ and 0.1, respectively. For $x=0.1,$ the TD's have a concentration of $\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{14}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ and the level exhibits a gradual shift in energy position from 32 to 28 meV below the conduction-band edge during annealing at 470 \ifmmode^\circ\else\textdegree\fi{}C for 36 h but no significant change in intensity occurs. The TD's appear to be single donors and no generation of the thermal double donors during the annealing at 470 \ifmmode^\circ\else\textdegree\fi{}C for 36 h has been observed.

Local vibrational mode spectroscopy of thermal donors in germanium

We report the observation of infrared (IR) vibrational bands related to individual thermal double donors (TDs) in Ge crystals enriched with either 16Oor18O isotopes of oxygen. The TDs were generated by heating the oxygen-doped Ge at 300°C and 350°C. IR absorption spectra were measured at room temperature (RT) and 10K (LT). In the RT spectra two broad bands at about 600 and 780cm−1 are found to develop upon the TD generation. In the LT spectra a splitting of the bands into series of rather sharp bands (up to 9 resolved lines) related to a double-donor (DD) configuration of individual TDs (TD1–TD9) is observed. A manifestation of bistability of the first four TD species (TD1–TD4) is observed in absorption spectra measured at 10K after different cooling conditions. The pairs of lines due to bistable TDs in the DD configuration are detected after cooling under the band-gap illumination. They are found to transform to the sets of three lines after cooling in the dark. These triplets are assigned to the local vibrational modes (LVMs) of a neutral (X) configuration of bistable TDs. Oxygen isotopic shifts of LVMs due to TDs are determined and compared with those for vibrational modes of interstitial oxygen in Ge.

Assignment of EPR spectrum for bistable thermal donors in silicon

It has been shown that bistability of thermal double donors (TDD) in silicon can be observed by EPR technique. The spectrum of a bistable TDD species (TDD2) has been isolated using heat-treatment of Czochralsky-grown n-type silicon crystals with initial resistivity 4.5Ωcm, at temperature 400°C and subsequent irradiation with electrons (E=3.5MeV). The principal values of the TDD2 g-tensor are determined as g1=1.9928, g2=2.0009, g3=1.9999. An interpretation of some EPR data is given in the framework of the two-center model of the TDD structure.

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 ${\mathrm{O}}_{i}.$

Stress-induced alignment and reorientation of hydrogen-associated donors in silicon

Electron paramagnetic resonance (EPR) studies have been made of the stress-induced alignment and the subsequent recovery of the double donor (AA1 EPR center) that is formed in float-zone silicon following hydrogen implantation and annealing for ∼20 min at a temperature above ∼300 °C. The obtained data compare with those of the thermal double donor (NL8 EPR center). The activation energy for atomic reorientation of the (HDD+) AA1 defect is (2.3±0.1) eV. The reorientation rate is greater than that of the (TDD+) NL8 defect formed in Czochralski Si by a factor of 104. Both centers have C2v symmetry and piezospectroscopic measurements reveal a large compressional strain along their [001] (three-axis) direction. However, in contrast to the NL8, the core of the AA1 defect produces also large compressional strain along its one-axis parallel to [110] direction. These data demonstrate unambiguously that the two centers have different molecular structures, in spite of their similar EPR spectra and electrical properties. It is suggested that the two centers have similar core structures (a 〈001〉-oriented self-interstitials complexes), while the outer shell structure incorporates hydrogen or oxygen atoms, respectively.

Thermal donors in silicon-rich SiGe

We have investigated thermal donor formation at 450 °C in oxygen-rich SiGe with more than 1% Ge content by Hall measurements and infrared spectroscopy. These measurements prove the presence of double donors in annealed Si-rich SiGe samples. The ionization energies of the donors correspond to those of the thermal double donors in Si, only the formation rate is reduced. Due to alloy effects, we observe only broadbands in the low-temperature infrared absorption spectra in SiGe instead of individual lines of neutral and singly ionized thermal donors in Si. The shift of these absorption bands to lower energies with longer heat treatment and a corresponding decrease of the ionization energy found from Hall measurements indicates the formation of shallower donor species.

Calculation of 2p levels for thermal double donors in silicon

A two-center model is developed to explain the electronic structure of thermal double donors (TDD) in silicon. Calculations of 2p levels of singly ionized TDD’s are performed in the effective mass approximation. From a comparison of the calculated results with the experimental data, the internuclear distance between the two electrically active atoms is evaluated as 0.75–0.95 nm for the TDD1–TDD3 and 1.35–1.75 nm for the next four species: TDD4–TDD7.

Stress-induced alignment of NL8 thermal donors in silicon: Energetics and kinetics

Preferential alignment is produced for the NL8 thermal double donors (TDDs) when uniaxial stress is applied during their formation at ∼450 °C. The recovery kinetics reveal that they can reorient readily on the time scale of their lifetime and their individual alignments therefore reflect the strain fields that each introduces into the lattice. Analysis reveals that each of the donor cores produces large compressional strain along its C2v [001]′ axis, the magnitude of which decreases monotonically with increase in the TDD species series, suggesting strain relief as the mechanism for nearby oxygen accumulation. The rate and activation energy for the reorientation suggests that the process is limited by the diffusion motion of the nearby interstitial oxygen atoms, with ∼5 jumps being required for TDD3, the third in the series, and progressively more for the subsequent ones.

Theoretical studies on nitrogen - oxygen complexes in silicon

Semi-empirical PM3 cluster calculations are used to show that stable, electrically active NO complexes may exist in silicon. Based on their relative stability with respect to oxygen and nitrogen pairs, the retardation of thermal double donor formation in the presence of nitrogen is explained, but an equilibrium concentration much less than that of NN pairs is predicted. It is also shown that interaction of NO with a single nitrogen atom creates a bistable NNO defect, while encounter with an oxygen or an NN pair preserves the electrical activity of the NO centre. The possible role of the NO complex in shallow thermal donor formation is discussed.

Infrared vibrational bands related to the thermal donors in silicon

Two groups of infrared (IR) localized vibrational bands in the regions 975–1015 and 724–748 cm−1 have been correlated with the well-known IR electronic bands due to the thermal double donors (TDs) and with the TD concentration from resistivity measurements. The two groups are suggested to be due to two different vibrational modes of oxygen atoms in a TD core. The vibrational bands at 975, 988, 999, and 1006 cm−1 are correlated to TD1, TD2, TD3, and TDs≥TD4, respectively, while the band at 1012 cm−1 correlates to the NL10 center. A calibration coefficient for the TD-related vibrational bands was determined. This calibration coefficient can be used to estimate the sizes of the TD-related centers, assuming that the calibration coefficient for interstitial oxygen is applicable on the oxygen atoms of these centers. This results in that all of the TD-related bands originates from centers of 1–2 oxygen atoms, suggesting these bands to be due to the vibrations of oxygen atoms in a TD core. The different positions of the TD-related bands could be explained by differently strained environments caused by different oxygen clusters. It is suggested that these clusters will develop into larger oxygen precipitates, which at the end of TD formation appear in the spectrum with a broad IR band at about 1060 cm−1. The early stages of the TD formation at temperatures below about 450 °C are closely related to a transformation process of preexisting clusters related to the 1012 cm−1 band. This explains the formation of the early TDs at low temperatures, when the interstitial oxygen concentration is nearly constant.

An explanation of electronic structure for thermal double donors in silicon

A model of two donor centers is suggested to explain electronic structure of thermal double donors in silicon. The model consistently explains the two valley structure of the ground state, the splitting of the 2 p ±-levels and other data obtained from infra-red absorption investigations of these defects.

Effect of Carbon on Thermal Double Donor Formation in Silicon

Effect of Carbon on Thermal Double Donor Formation in Silicon

Thermal donors in silicon: an investigation of their structure with electron nuclear double resonance

Singly ionized thermal double donors (TDD+), which have the so-called NL8 EPR spectrum, have been investigated with electron nuclear double resonance (ENDOR) in silicon doped with various acceptors (Al,B,In,Ga). No differences were detected in the ENDOR spectra and no signals due to any of the acceptors. TDDs appear to be heat treatment centres which do not involve any impurities except oxygen. The magnetic isotope 17O was diffused into B-doped oxygen-poor float-zone silicon and TDD+'s were formed with various durations of heat treatment at 470 degrees C up to 200 h. The total intensity of the 17O ENDOR lines increases overproportionally compared to the increase of the 29Si ENDOR lines with longer heat treatment, indicating that more oxygen is added upon TDD+ growth from earlier to later species. The 17O ENDOR lines of the latter have an almost vanishing superhyperfine interaction. Their quadrupole interactions are almost the same as those found in the earliest species detected with ENDOR. From its theoretical interpretation it is suggested that oxygen is always in an interstitial position between two Si atoms. The C2v symmetry appearing in the EPR and 29Si ENDOR spectra does not necessarily reflect the true defect symmetry, which may be lower since the addition of 17O does not noticeably influence the distribution of the unpaired spin density. Therefore the suggestion drawn from earlier measurements on the two early species observable with ENDOR, that four oxygen atoms may be in the defect core, should be revised. It seems that the model proposed by Deak et al. (1991-1992) is compatible with our ENDOR results.

Thermal Donor Generation in Boron- and Aluminium-Doped Czochralski Silicon

Generation of thermal donor centres in oxygen-rich silicon doped with boron and aluminium acceptors has been studied with the FTIR technique. It has been found that upon annealing 470°C two kinds of absorption series were generated. One of them belonged to the well-known first ionization 1eve1 of silicon thermal (double) donors (TD,s): TD° /TD+ . The second series was identified with the so-called shallow thermal donors (STD,s). The generation kinetics of the two series was followed for both kinds of acceptor doping and significant differences has been found. The results of the FTIR investigations were further compared with the magnetic resonance findings allowing for their mutual correlation and more general conclusions.