48 research outputs found
Gettering of interstitial iron in silicon by plasma-enhanced chemical vapour deposited silicon nitride films
It is known that the interstitial iron concentration in silicon is reduced after annealing silicon wafers coated with plasma-enhanced chemical vapour deposited (PECVD) silicon nitride films. The underlying mechanism for the significant iron reduction has remained unclear and is investigated in this work. Secondary ion mass spectrometry (SIMS) depth profiling of iron is performed on annealed iron-contaminated single-crystalline silicon wafers passivated with PECVD silicon nitride films. SIMS measurements reveal a high concentration of iron uniformly distributed in the annealed silicon nitride films. This accumulation of iron in the silicon nitride film matches the interstitial iron loss in the silicon bulk. This finding conclusively shows that the interstitial iron is gettered by the silicon nitride films during annealing over a wide temperature range from 250â°C to 900â°C, via a segregation gettering effect. Further experimental evidence is presented to support this finding. Deep-level transient spectroscopy analysis shows that no new electrically active defects are formed in the silicon bulk after annealing iron-containing silicon with silicon nitride films, confirming that the interstitial iron loss is not due to a change in the chemical structure of iron related defects in the silicon bulk. In addition, once the annealed silicon nitride films are removed, subsequent high temperature processes do not result in any reappearance of iron. Finally, the experimentally measured iron decay kinetics are shown to agree with a model of iron diffusion to the surface gettering sites, indicating a diffusion-limited iron gettering process for temperatures below 700â°C. The gettering process is found to become reaction-limited at higher temperatures
Acceptor levels of the carbon vacancy in -SiC: combining Laplace deep level transient spectroscopy with density functional modeling
We provide direct evidence that the broad Z peak, commonly observed
by conventional DLTS in as-grown and at high concentrations in radiation
damaged -SiC, has two components, namely Z and Z, with
activation energies for electron emission of 0.59 and 0.67~eV, respectively. We
assign these components to
transition sequences from negative- ordered acceptor levels of carbon
vacancy (V) defects at hexagonal/pseudo-cubic sites,
respectively. By employing short filling pulses at lower temperatures, we were
able to characterize the first acceptor level of V on both
sub-lattice sites. Activation energies for electron emission of 0.48 and
0.41~eV were determined for and
transitions, respectively. Based on trap filling kinetics and capture barrier
calculations, we investigated the two-step transitions from neutral to doubly
negatively charged Z and Z. Positions of the first and second
acceptor levels of V at both lattice sites, as well as
occupancy levels were derived from the analysis of the emission and capture
data
Theory of reactions between hydrogen and group-III acceptors in silicon
The thermodynamics of several reactions involving atomic and molecular
hydrogen with group-III acceptors is investigated. The results provide a
first-principles-level account of thermally- and carrier-activated processes
involving these species. Acceptor-hydrogen pairing is revisited as well. We
present a refined physicochemical picture of long-range migration, compensation
effects, and short-range reactions, leading to fully passivated
structures, where is a group-III
acceptor element. The formation and dissociation of acceptor-H and
acceptor-H complexes is considered in the context of Light and elevated
Temperature Induced Degradation (LeTID) of silicon-based solar cells. Besides
explaining observed trends and answering several fundamental questions
regarding the properties of acceptor-hydrogen pairing, we find that the
BH complex is a by-product along the reaction of H molecules with
boron toward the formation of BH pairs (along with subtraction of free holes).
The calculated changes in Helmholtz free energies upon the considered defect
reactions, as well as activation barriers for BH formation/dissociation
(close to eV) are compatible with the experimentally determined
activation energies of degradation/recovery rates of Si:B-based cells during
LeTID. Dihydrogenated acceptors heavier than boron are anticipated to be
effective-mass-like shallow donors, and therefore, unlikely to show similar
non-radiative recombination activity
Electrical Characterization of Thermally Activated Defects in n-Type Float-Zone Silicon
Float-zone (FZ) silicon is usually assumed to be bulk defect-lean and stable. However, recent studies have revealed that detrimental defects can be thermally activated in FZ silicon wafers and lead to a reduction of carrier lifetime by up to two orders of magnitude. A robust methodology which combines different characterization techniques and passivation schemes is used to provide new insight into the origin of degradation of 1 Ω·cm n-type phosphorus doped FZ silicon (with nitrogen doping during growth) after annealing at 500 °C. Carrier lifetime and photoluminescence experiments are first performed with temporary room temperature surface passivation which minimizes lifetime changes which can occur during passivation processes involving thermal treatments. Temperature- and injection-dependent lifetime spectroscopy is then performed with a more stable passivation scheme, with the same samples finally being studied by deep level transient spectroscopy (DLTS). Although five defect levels are found with DLTS, detailed analysis of injection-dependent lifetime data reveals that the most detrimental defect levels could arise from just two independent single-level defects or from one two-level defect. The defect parameters for these two possible scenarios are extracted and discussed
Electronic Properties and Structure of BoronâHydrogen Complexes in Crystalline Silicon
From Wiley via Jisc Publications RouterHistory: received 2021-06-27, rev-recd 2021-09-04, pub-electronic 2021-09-17Article version: VoRPublication status: PublishedFunder: Department of Science and Technology (DOST), Government of the PhlippinesFunder: Fundação para a CiĂȘncia e a Tecnologia in Portugal; Grant(s): UIDB/50025/2020, UIDP/50025/2020The subject of hydrogenâboron interactions in crystalline silicon is revisited with reference to light and elevated temperatureâinduced degradation (LeTID) in boronâdoped solar silicon. Ab initio modeling of structure, binding energy, and electronic properties of complexes incorporating a substitutional boron and one or two hydrogen atoms is performed. From the calculations, it is confirmed that a BH pair is electrically inert. It is found that boron can bind two H atoms. The resulting BH2 complex is a donor with a transition level estimated at E câ0.24 eV. Experimentally, the electrically active defects in nâtype Czochralskiâgrown Si crystals coâdoped with phosphorus and boron, into which hydrogen is introduced by different methods, are investigated using junction capacitance techniques. In the deepâlevel transient spectroscopy (DLTS) spectra of hydrogenated Si:P + B crystals subjected to heatâtreatments at 100 °C under reverse bias, an electron emission signal with an activation energy of â0.175 eV is detected. The trap is a donor with electronic properties close to those predicted for boronâdihydrogen. The donor character of BH2 suggests that it can be a very efficient recombination center of minority carriers in Bâdoped pâtype Si crystals. A sequence of boronâhydrogen reactions, which can be related to the LeTID effect in Si:B is proposed
IndiumâDoped Silicon for Solar CellsâLightâInduced Degradation and DeepâLevel Traps
From Wiley via Jisc Publications RouterHistory: received 2021-02-28, rev-recd 2021-06-11, pub-electronic 2021-07-21Article version: VoRPublication status: PublishedFunder: EPSRC (UK); Grant(s): EP/TO25131/1Funder: Department of Science and Technology (DOST), Government of the PhlippinesFunder: Fundação para a CiĂȘncia e a Tecnologia; Id: http://dx.doi.org/10.13039/100008382; Grant(s): UIDB/50025/2020, UIDP/50025/2020Indiumâdoped silicon is considered a possible pâtype material for solar cells to avoid lightâinduced degradation (LID), which occurs in cells made from boronâdoped Czochralski (Cz) silicon. Herein, the defect reactions associated with indiumârelated LID are examined and a deep donor is detected, which is attributed to a negativeâU defect believed to be InsO2. In the presence of minority carriers or above bandgap light, the deep donor transforms to a shallow acceptor. An analogous transformation in boronâdoped material is related to the BsO2 defect that is a precursor of the center responsible for BO LID. The electronic properties of InsO2 are determined and compared to those of the BsO2 defect. Structures of the BsO2 and InsO2 defects in different charges states are found using firstâprinciples modeling. The results of the modeling can explain both the similarities and the differences between the BsO2 and InsO2 properties
Evidence for Molybdenum-Hydrogen Bonding in p-Type Silicon upon Annealing under Illumination
Dynamics of Hydrogen in Silicon at Finite Temperatures from First Principles
Hydrogen defects in silicon still hold unsolved problems, whose disclosure is fundamental for future advances in Si technologies. Among the open issues is the mechanism for the condensation of atomic hydrogen into molecules in Si quenched from above (Formula presented.) to room temperature. Based on first-principles calculations, the thermodynamics of hydrogen monomers and dimers is investigated at finite temperatures within the harmonic approximation. Free energies of formation indicate that the population of (Formula presented.) cannot be neglected when compared to that of (Formula presented.) at high temperatures. The results allow us to propose that molecular formation occurs during cooling processes, in the temperature window (Formula presented.), above which the molecules collide with SiâSi bonds and dissociate, and below which the fraction of (Formula presented.) becomes negligible. The formation of (Formula presented.) and most notably of a fast-diffusing neutral species can also provide an explanation for the apparent accelerated diffusivity of atomic hydrogen at elevated temperatures in comparison to the figures extrapolated from measurements carried out at cryogenic temperatures. Finally, it is shown that the observed diffusivity of molecules is better described upon the assumption that they are nearly free rotors, all along the migration path, including at the transition state.</p
Acceptor levels of the carbon vacancy in 4H-SiC: Combining Laplace deep level transient spectroscopy with density functional modeling
The broad Z1=2 peak, was commonly observed by conventional deep level transient spectroscopy in as-grown and at high concentrations in radiation damaged 4H-SiC. The peak has two components, namely, Z1 and Z2, of which activation energies for electron emissions are 0.59 and 0.67 eV, respectively. We assign that these components have negative-U ordered acceptor levels of carbon vacancy (Vc) defects at hexagonal/pseudo-cubic sites, respectively