Ab Initio Supercell Calculations Research Articles

Theoretical study of small silicon clusters in4H−SiC

We have studied the small clusters of silicon and carbon interstitials by ab initio supercell calculations in 4H-SiC. We found that silicon interstitials can form stable and electrically active complexes with each other or with a carbon interstitial. Local vibration modes and ionization energies were also calculated in order to help the identification of the defects. We propose that silicon interstitials can emit from these clusters at relatively high temperatures, which may play an important role in the formation of the DI center. © 2007 The American Physical Society.

A Theoretical Study on Aluminium-Related Defects in SiC

We have investigated several aluminum-related complexes in 4H-SiC by ab initio supercell calculations. The binding energies of the defects predict high thermal stability and complex formation between aluminum and carbon interstitials in SiC. We show that the carbon vacancy can be attached to a shallow the aluminum acceptor and form a very stable defect. We also found that aluminum interstitial forms stable and metastable complexes with one or two carbon interstitials. The possible relation of these defects to the recently found aluminum- related DLTS centers is also discussed.

Ab initiotheoretical study of hydrogen and its interaction with boron acceptors and nitrogen donors in single-wall silicon carbide nanotubes

Silicon carbide nanotubes have a great potential for biological applications. It is of interest to explore the electronic properties of these nanotubes, and how those are modified in the presence of impurities. Hydrogen is a common impurity that can appear during the growth of silicon carbide nanotubes or in the environment. In this paper we studied the properties of one and two hydrogen atoms in armchair and zigzag silicon carbide nanotubes by ab initio supercell calculations beyond the standard density functional theory. We found that a single hydrogen atom is an amphoteric defect: it can act as a donor as well as an acceptor, depending on the adsorption site. However, both sites are nearly equally stable; therefore these defects compensate each other in SiC nanotube semiconductors. We found that hydrogen can absorb onto SiC nanotubes when atomic hydrogen is present in the environment. In addition, we investigated the incorporation of hydrogen by applying BH and NH molecules in the environment of SiC nanotubes. The simulations predict that boron dissolves into SiC nanotubes together with hydrogen; thus boron can be used to raise the concentration of hydrogen in SiC nanotubes. Nitrogen is also incorporated with hydrogen. We predict that the shallow boron acceptor and the nitrogen donor may be activated in these processes.

Ab initiosupercell calculations on aluminum-related defects in SiC

Ab initio supercell calculations of the binding energies predict complex formation between aluminum and carbon interstitials in SiC. In high-energy implanted SiC aluminum acceptor can form very stable complexes with two carbon interstitials. We also show that carbon vacancy can be attached to shallow aluminum acceptor. All of these defects produce deep levels in the band gap. The possible relation of these defects to the recently found aluminum-related deep-level transient spectroscopy centers is discussed. © 2007 The American Physical Society.

Divacancy Model for P6/P7 Centers in 4H- and 6H-SiC

Electron paramagnetic resonance (EPR) studies of the P6/P7 centers in 4H- and 6H-SiC are reported. The obtained principal values of the hyperfine tensors of C and Si neighbors are in good agreement with the values of the neutral divacancy (VCVSi 0) calculated by ab initio supercell calculations. The results suggest that the P6/P7 centers, which were previously assigned to the photo-excited triplet states of the carbon vacancy-carbon antisite pairs in the double positive charge state (VCCSi 2+), are related to the triplet ground states of the C3v/C1h configurations of VCVSi 0.

A Theoretical Study on Doping of Phosphorus in Chemical Vapor Deposited SiC Layers

Ab initio supercell calculations have been carried out to investigate the doping of phosphorus in chemical vapor deposited (CVD) SiC layers. CVD conditions have been simulated by using the appropriate chemical potentials for hydrogen and phosphorus (P). We found that the most abundant defect is P at Si-site followed by P at C-site. The calculated concentrations of the P-donors and free carriers in CVD grown SiC agree with the experimental results.

Ab initiostudy of nitrogen and boron substitutional impurities in single-wall SiC nanotubes

Silicon carbide nanotubes have a great potential for application in chemical sensors in harsh environment or in biological sensors. It is of interest to explore the electronic properties of these nanotubes, and how those are modified in the presence of impurities. It is well known that nitrogen and boron atoms are common contaminations in bulk silicon carbide (SiC). Nitrogen preferentially substitutes the carbon site making $n$-type conductivity in bulk SiC. Boron substitutes both carbon and silicon sites forming a deep and a shallow acceptor in bulk SiC, respectively. In this paper we have studied these defects in armchair and zig-zag SiC nanotubes by ab initio supercell calculations. We found that nitrogen forms relatively shallow or deep donor state depending on the width of the band gap of the SiC nanotube. Boron is a relatively deep or shallow acceptor at carbon and silicon sites, respectively, like in bulk SiC polytypes. The site preference of boron depends on the stoichiometry of the SiC nanotubes. We have found no significant difference in the properties of boron substitutional defect between armchair and zig-zag nanotubes.

Divacancy in 4H-SiC

Electron paramagnetic resonance and ab initio supercell calculations suggest that the P6/P7 centers, which were previously assigned to the photoexcited triplet states of the carbon vacancy-antisite pairs in the double positive charge state, are related to the triplet ground states of the neutral divacancy. The spin density is found to be located mainly on three nearest C neighbors of the silicon vacancy, whereas it is negligible on the nearest Si neighbors of the carbon vacancy.

Identification of divacancies in 4H-SiC

The P6/P7 centers in 4H-SiC were studied by electron paramagnetic resonance (EPR) and ab initio supercell calculations. The hyperfine coupling constants of C and Si neighbors obtained by EPR are in good agreement with the calculated values for the neutral divacancy, V C V Si 0 . Our results suggest that the P6/P7 centers, which were previously assigned to the photo-excited triplet states of the carbon vacancy–carbon antisite pairs in the double positive charge state ( V C C Si 2 + ) , are related to the triplet ground states of the C 3 v / C 1 h configurations of V C V Si 0 .

Doping of phosphorus in chemical-vapor-deposited silicon carbide layers: A theoretical study

Ab initio supercell calculations have been carried out to investigate the doping of phosphorus in chemical-vapor-deposited (CVD) silicon carbide (SiC) layers. We simulated the CVD conditions by using the appropriate chemical potentials for hydrogen and phosphorus (P). We find that the site selection of P is not affected by the presence of hydrogen, and hydrogen does not practically passivate the P donors. We find that the most abundant defect is P at the Si-site followed by P at the C-site. The calculated concentrations of the P donors and free carriers in CVD grown SiC agree with the experimental findings.

Activation of shallow boron acceptor in C∕B coimplanted silicon carbide: A theoretical study

Ab initio supercell calculations have been carried out to investigate the complexes of boron acceptors with carbon self-interstitials in cubic silicon carbide. Based on the calculated binding energies, the complex formation of carbon interstitials with shallow boron acceptor and boron interstitial is energetically favored in silicon carbide. These bistable boron defects possess deep, negative-U occupation levels in the band gap. The theoretical results can explain the observed activation rates in carbon-boron coimplantation experiments.

Possibility for the electrical activation of the carbon antisite by hydrogen inSiC

Calculations predict the carbon antisite to be the most abundant intrinsic defect in silicon carbide in a wide range of doping. The isolated carbon antisite is, however, optically and electronically inactive, therefore, difficult to observe by usual experimental techniques. However, ${\mathrm{C}}_{\mathrm{Si}}$ can trap mobile impurities forming electrically active complexes. We will show by ab initio supercell calculations that the hydrogen interstitial is trapped by the carbon antisite forming an electrically active defect which might be detectable by different spectroscopic techniques. The key to activate ${\mathrm{C}}_{\mathrm{Si}}$ by hydrogen is to introduce sufficient amount of hydrogen in the $\mathrm{SiC}$ samples and to avoid formation of vacancies or boron-hydrogen complexes. We have found that the concentration of ${\mathrm{C}}_{\mathrm{Si}}+\mathrm{H}$ complex is above ${10}^{13}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ in highly doped $p$-type chemical vapor deposited (CVD) layers as well as in highly doped $p$-type and $n$-type $\mathrm{SiC}$ samples annealed in high temperature high pressure (HTHP) ${\mathrm{H}}_{2}$ gas. The concentration of ${\mathrm{C}}_{\mathrm{Si}}+\mathrm{H}$ complex can be enhanced in $\mathrm{Al}$-doped CVD and HTHP $\mathrm{SiC}$ samples by applying the appropriate post-annealing temperature. The ${\mathrm{C}}_{\mathrm{Si}}+\mathrm{H}$ complex might be also detected in $\mathrm{Al}$-doped $\mathrm{SiC}$ samples irradiated at room temperature by low energy ${{\mathrm{H}}_{2}}^{+}$ ions.

Ab initio study of 3C inclusions and stacking fault–stacking fault interactions in 6H-SiC

Ab initio supercell calculations of cubic inclusions in 6H-SiC are performed. The cubic inclusions can be created in perfect 6H-SiC by the propagation of successive partial dislocations having the same Burgers vector in neighboring basal planes, i.e., multiple stacking faults. We have studied the electronic structures and the total energies of 6H-SiC single crystals that contain one, two, three, and four stacking faults, based on density functional theory in local density approximation. Our total energy calculations have revealed that the second stacking fault energy in 6H-SiC is about six to seven times larger than that of an isolated stacking fault, which is actually opposite the trend recently obtained for 4H-SiC [J. Appl. Phys. 93, 1577 (2003)]. The effects of spontaneous polarization on the electronic properties of stacking disorders are examined in detail. The calculated positions of the quantum-well-like localized bands and stacking fault energies of 3C inclusions in 6H-SiC are compared with those previously determined in 4H-SiC, and the possibility of local hexagonal to cubic polytypic transformations is discussed in light of the formation energy and quantum-well action.

Aggregation of carbon interstitials in silicon carbide: A theoretical study

Ab initio supercell calculations have been carried out to investigate clusters of carbon interstitials in 3C- and 4H-SiC. Based on the calculated formation energies, the complex formation of carbon interstitials or their aggregation to carbon antisites is energetically favored in SiC. The electronic and vibronic properties of the carbon interstitials and their aggregates depends strongly on the polytype. Using the calculated hyperfine constants and local vibrational modes of carbon clusters the possible relation to known carbon-related centers will be discussed.

Calculation of Hyperfine Constants of Defects in 4H-SiC

Knowledge about the creation and diffusion of intrinsic point defects is crucial for devising annealing strategies after irradiation steps as, e.g., implantation. Experimental information can be obtained by observing the appearance and/or disappearance of characteristic electrical, optical or magnetic spectra, however, these have to be first assigned to a given defect. In case of silicon carbide even this very first task has not been accomplished yet in case of the carbon vacancy, with which two different electron spin resonance (ESR) centers (anneling out at very different temperatures) have been identified. Ab initio all-electron supercell calculations have been carried out to determine the hyperfine constants of several defects in 4H-SiC in order to justify the models of the measured ESR signals. The quality of the results were tested on the well-documented case of interstitial hydrogen in silicon.

Stability and clusterization of hydrogen-vacancy complexes inα−Fe:Anab initiostudy

By means of ab initio supercell calculations based on the density-functional theory, we have investigated stability of hydrogen-monovacancy complexes $({\mathrm{VH}}_{n})$ and their binding preferences in $\ensuremath{\alpha}\ensuremath{-}\mathrm{Fe}.$ We have found that ${\mathrm{VH}}_{2}$ is the major complex at ambient condition of hydrogen pressure, which corrects the conventional model implying the ${\mathrm{VH}}_{6}$ predominance. It is also demonstrated that monovacancies are not hindered from binding by the hydrogen trapping in the case of ${\mathrm{VH}}_{2}$ predominance. Besides, the presence of hydrogen is found to facilitate formations of line-shaped and tabular vacancy clusters without the improbable accumulation. These anisotropic clusters can be closely associated with the fracture planes observed in experiments on hydrogen embrittlement in Fe-rich structural materials such as steel. The present results should suggest implications of hydrogen-enhanced vacancy activities to microscopic mechanism of hydrogen embrittlement in those materials.

Oscillator strengths and excitation energies of Ge and Si nanocrystals from ab initio supercell calculations

We demonstrate that strained germanium nanocrystals embedded in hexagonal SiC constitute a type-I heterostructure with localization for both electrons and holes. Nonetheless, the oscillator strengths for the lowest transitions are very small. We relate these results to the pressure dependence of the oscillator strengths and radiative lifetimes in free Ge crystallites. The pressure dependence of the gap energies of the free crystallites as well as the strong reduction of the transition probabilities is interpreted in terms of recent results of the pressure dependence of the respective bulk materials.

Impurity-Controlled Dopant Activation - The Role of Hydrogen in p-Type Doping of SiC

Hydrogen is a natural contaminant of SiC growth processes, and may influence the doping efficiency. Hydrogen incorporation proportional to that of boron was observed during CVD growth while the amount of hydrogen was two orders of magnitude less than the aluminum concentration. Passivation by complex formation with hydrogen has been proven both for Al and B. The experimentally observed reactivation energy of these complexes differ by 0.9 eV. Our ab initio supercell calculations in 4H-SiC indicate, that in the absence of hydrogen, boron is incorporated as isolated substitutional and prefers the carbon site, while under typical CVD conditions boron is incorporated together with hydrogen (in equal amounts), favoring the silicon site. Therefore, the presence of H is advantageous for the activation of B as a shallow acceptor. In contrast to boron, aluminum is incorporated independently of the presence of hydrogen as isolated substitutional at the silicon site. The calculated difference between the dissociation of the stable dopant plus hydrogen complexes agrees very well with experiments. Vibration frequencies for the dopant complexes have been also calculated.

Modification of SiC Properties by Insertion of Ge and Si Nanocrystals - Description by ab initio Supercell Calculations

Modification of SiC Properties by Insertion of Ge and Si Nanocrystals - Description by ab initio Supercell Calculations

Passivation of p-type dopants in 4H-SiC by hydrogen

Experimental investigations showed passivation of the p-type dopants B and Al in 4H–SiC by the formation of B+H and Al+H complexes. The dissociation energies of these complexes differed by 0.9 eV . Ab initio supercell calculations have been performed to investigate the interaction of H with B and Al in hexagonal 4H–SiC. The total energy, geometry and electronic structure of the possible complexes have been determined. Site dependencies have also been investigated. The most stable configurations were found with H at a bond center site next to B at the Si site, and with H at the antibonding site of a carbon atom which is first neighbor to Al at a Si site. Both the B Si+H BC and the Al Si+H AB C complexes turned out to be electrically inactive. The different structure of the passivated complexes explains the observed difference in their dissociation energy: the calculated difference of the binding energies of these complexes is 0.9 eV, which agrees well with the experimental finding.