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Antisites as possible origin of irradiation induced photoluminescence centers in SiC : A theoretical study on clusters of antisites and carbon interstitials in 4H-SiC

The D I low-temperature photoluminescence center is a well-known defect stable up to 1700°C annealing in SiC, still its structure is not known after decades of study. Combining experimental and theoretical studies in this paper we will show that the properties of an anti-site pair can reproduce the measured one-electron level position and local vibration modes of the D I center and the model is consistent with other experimental findings as well. We give theoretical values of the hyperfine constants of the anti-site pair in its paramagnetic state as a means to confirm our model.

We show that in the SiC/SiO2 system the interface states in the lower half of the gap are the consequence of the behavior of oxygen in SiC. Investigating the elemental steps of oxidation on a simple model by means of ab initio density functional calculations we find that, in course of the oxidation, carbon-vacancy (V-C) - oxygen complexes constantly arise. The V-C+O complexes have donor states around E-V+0.8 eV. Their presence gives rise to a thin transition layer which is not SiO2 but an oxygen contaminated Si-rich interface layer producing the aforementioned gap states.

Aluminum nitride films were grown by molecular beam epitaxy on sapphire (0001) substrates. In some samples, alternating GaN/AlN thin layers were incorporated prior to the deposition of the main AlN layer in an effort to possibly reduce defects. The cathodoluminescence spectra show a sharp bandedge peak, which we tentatively assign to the optical recombination of a donor bound exciton (D 0 ,X), sometimes accompanied by weak one- and two-longitudinal optical phonon replicas. There is also a broad band with maxima in the range 3200-3700 A. A large value for the ratio (D 0 ,X)/(broad band) tends to be associated with a narrow (D 0 ,X) linewidth.

Implants of Er, Al and N with energies ranging from 120 keV to 2 MeV were carried out in n- and p-type 6H-SiC. The aim is to identify electrically active defects in the bandgap which can be associated with the 1.54 μm Er3+ luminescence in 6H-SiC. Measurements are conducted by means of deep level transient spectroscopy (DLTS) as well as low temperature photoluminescence (LTPL). In n-type 6H-SiC implanted with Er, we find well known DLTS damage levels whereas strong characteristic lines of Er3+, near 1.54 μm, are seen in the photoluminescence. In p-type 6H-SiC implanted with Er two DLTS peaks can be uniquely associated with the Er3+, on the other hand, no luminescence is observed. Conversion of such p-type 6H-SiC into n-type, by nitrogen implantation, recovers the photoluminescence. We conclude that the Er-associated defect levels in the lower half of the gap are likely not involved in the energy transfer mechanism which is responsible for the 1.54 μm luminescence.

Adsorption of acetylene on the Si (1 0 0) –(2×1) surface was studied at room temperature by scanning tunneling microscopy (STM) and theoretical modeling. Acetylene molecules adsorb nondissociatively in three distinctive bonding configurations, not one, as reported previously. In Configuration I, C 2 H 2 molecules adsorb in di-σ form over a single silicon dimer with a relative population of 0.33 at low coverage. Configuration II has an initial formation probability of 0.51 and involves adsorption across the ends of two adjacent dimers in the same row. In Configuration III (r-bridge) C 2 H 2 forms four bonds as it symmetrically bridges two neighboring dimers. Relative populations depend strongly on coverage. Above a coverage of ∼0.13 an acceleration in the rate of formation of Configuration III (r-bridge) is observed.

The structural, electrical, and optical properties of GaN grown on 6H-SiC(0001) substrates by molecular beam epitaxy are studied. Suitable substrate preparation and growth conditions are found to greatly improve the structural quality of the films. Threading dislocation densities of about 3×109 cm−2 for edge dislocations and

STM-assisted nanostructure formation using precursor molecules at negative sample bias is controlled by the applied electric field. Sharp nanostructures are produced from the Cu I (hfac) (vtms) molecule on the Si(111)-(7×7) surface at 300 K. The growth process involves the field-induced surface diffusion from regions outside the growing nanostructure, which acts to supply molecules to the nanostructure growth region under the tip. Molecular decomposition occurs above a critical electric field under the tip. The growth process starts when the electric field at the sample surface exceeds a critical level of ∼0.2 V A −1 . Simulations of the growth mechanism, which involves both the radial diffusion of the precursor species from outer sample surface regions and their activation underneath the tip, give quantitative agreement with the experimental data.

Epitaxial silicon carbide is likely to contain hydrogen and vacancies ( $V$); therefore, $V+n\mathrm{H}$ complexes are likely to influence its electronic properties. Using ab initio calculations we show that neutral and positive H atoms are trapped by carbon vacancies ( ${V}_{\mathrm{C}}$) in three-center bonds with two Si neighbors. The double positive charge state of ${V}_{\mathrm{C}}+\mathrm{H}$ is not stable in equilibrium and in the triply positive state H binds only to one of the Si neighbors. At most two H atoms can be accommodated by a single ${V}_{\mathrm{C}}$. The ${V}_{\mathrm{C}}+n\mathrm{H}$ complexes have donor character and exhibit rather atypical vibration modes for Si-H bonds. Occupation levels and spin distributions were calculated and compared for ${V}_{\mathrm{C}}+\mathrm{H}$ and ${V}_{\mathrm{C}}$.

The scanning tunneling microscope in a near-field emission mode has been employed to create nanostructures using the hexafluoroacetylacetonate Cu (I) vinyltrimethylsilane precursor molecule on the Si(111) surface at 300 K. Two distinctive mechanisms controlling the nanostructure formation have been delineated. The first process involves excitation of the molecule by the applied electric field, and the field induced surface diffusion acts to supply molecules to the nanostructure growth region under the tip. The second mechanism involves the dissociation of the molecule by an electron attachment process. The generated nanostructure topology is quite different for each excitation mechanism. Narrow cone-like structures are produced by the electric field while broad structures of lower height are produced by the electron attachment process. Both mechanisms operate simultaneously in the low bias voltage regime (V

First principle calculations have been carried out to investigate the position of a boron atom in a divacancy of cubic silicon carbide. The perfect lattice was modeled by a large molecular cluster. The total energy of the cluster was calculated within the local density approximation of the density functional theory and the wave function was expanded by linear combination of Gaussian type atomic orbitals. The results of the calculations on the boron-vacancy system resolve the contradiction between magnetic resonance and photoluminescence experiments regarding the deep boron center, establishing the ${\mathrm{B}}_{\mathrm{Si}}{+V}_{C}$ configuration as its origin.

The electrical and optical properties of oxygen (O)-implanted 6H-silicon carbide (SiC) chemical vapor deposition (CVD) epilayers are investigated by Hall effect, admittance spectroscopy, deep level transient spectroscopy (DLTS) and low temperature photoluminescence (LTPL). Two types of O-related centers are found: shallow donors in the energy range of (129–360) meV below the conduction band edge as well as deep acceptor-like defects at EC−480 meV, EC−560 meV and EC−610 meV. For the shallow donors, a certain sensitivity to heat treatments is demonstrated in terms of a decrease of their ionization energies when exposing the O+-implanted epilayers to temperatures at 1650–1800°C. In addition, evidence has been found for the incorporation of O in as-grown 4H-SiC CVD epilayers indicated by the observation of deep O-related defect centers in DLTS spectra.

Optical studies provide a great amount of information about acceptors in SiC. The optical recombination of neutral acceptor four particle (bound exciton) complexes has been observed in 4H, 6H, 15R and 3C SiC for Al; 4H, 6H and 3C SiC for Ga; and in 4H SiC for the shallow B acceptor. A model for the electronic structure of neutral acceptor four particle complexes based on symmetry provides a description of the no-phonon spectra for Al and Ga, when the possibility of lattice strain is included. This model is consistent with the numbers of no-phonon lines observed for Al and Ga in 3C, 4H and 6H SiC, but there are too many no-phonon lines for Al in 15R SiC. The model is further developed and applied to Zeeman spectra of Al and Ga four particle complexes in 6H SiC, providing further insight into their electronic structure.

We report the values of the absorption coefficient of 4H, 6H and 3C SiC at room temperature, in the range 3900–2968 A. By using the known shift in the bandgap with temperature, we also present estimates of the absorption coefficient of 4H, 6H and 3C SiC at 2 K. A table is given for penetration depths at 300 and 2 K for seven common lasers used to pump SiC in this region.

Scanning tunneling microscopy has been employed to create nanostructures at both polarities of the applied bias voltage, using the hexafluoroacetylacetonate Cu (I) vinyltrimethylsilane precursor molecule on the Si(111) surface at 300 K. Two distinctive mechanisms controlling the formation of nanostructures have been delineated. The first mechanism causes the dissociation of the molecule by an electron attachment process. The second process involves excitation of the molecule by the applied electric field; here field induced surface diffusion acts to supply molecules to the nanostructure growth region under the tip. Both mechanisms have a threshold in the range of 2.5–4.5 V, and while the electron current induced process operates only at positive sample bias, the field induced mechanism works at both bias polarities. Model calculations of the field assisted growth mechanism, which involves both the radial diffusion of the precursor species from outer surface regions and their activation underneath the tip, show a very reasonable quantitative agreement with the experimental data.

The effect of a low energy electron beam on the decomposition of hexafluoroacetylacetonate Cu(I) vinyltrimethylsilane (Cu(I) precursor) on the Si(111)-(7×7) surface at room temperature has been studied using x-ray photoelectron spectroscopy and Auger electron spectroscopy. It was found that electron bombardment causes the decomposition of the Cu(I) precursor and an electron energy threshold for decomposition was found to be 4±0.5 eV. This result demonstrates that the decomposition of the Cu(I) precursor in the threshold region occurs through a dissociative electron attachment mechanism. Elemental composition studies of the grown films show that they are contaminated with carbon, and that the copper fraction is increased in the film compared to the precursor composition.

AlN single crystals have been investigated using photoluminescence (PL) and optical detection of electron paramagnetic resonance in the PL (ODEPR). All crystals were found to exhibit intense PL extending from the visible into the near infrared. Several $S=1$ centers, each with its own distinct emission spectrum, and distant $S=1/2$ pair recombination centers have been observed via ODEPR. In all except one center, $D5,$ no hyperfine structure was observed preventing chemical identification of the impurity involved. In the case of $D5$ the partially resolved hyperfine structure suggests interaction with a 100% abundant nucleus of $I\ensuremath{\sim}5/2.$ We present arguments to associate it with a displaced host aluminum atom.

A statistical analysis of the dissociative chemisorption of ${\mathrm{Cl}}_{2}$ on the $\mathrm{Si}(100)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}1)$ surface at 300 K has been carried out in the Cl coverage range up to 0.17 ML using scanning tunneling microscopy. The adsorption of two Cl atoms on Si dimer sites in adjacent silicon dimer rows was found to be kinetically favored (probability $P$ is 0.52). ${\mathrm{Cl}}_{2}$ dissociation also occurs with Cl atoms bonded to neighboring Si dimers in the same dimer rows $(P=0.33).$ ${\mathrm{Cl}}_{2}$ dissociation on a single Si dimer is least likely $(P=0.15).$ The least probable Cl pair configuration on a single Si dimer at room temperature is most favored thermodynamically, indicating that kinetic factors control the site selection process at 300 K. The observed selectivity of the Cl pair configurations, produced at 300 K, is consistent with ${\mathrm{Cl}}_{2}$ dissociative chemisorption occurring through a mobile precursor-mediated channel. At increasing ${\mathrm{Cl}}_{2}$ exposure, the density of the fully $\mathrm{Cl}$-terminated dimers increases, and at saturation the surface consists of only these chlorine configurations. Tunneling spectroscopy has been employed to characterize the Cl-saturated Si(100) surface, and it has been found that filled-state images mainly probe the bonding ${\ensuremath{\sigma}}_{\mathrm{d}}$ states of the Si-Si dimers. Empty-state images probe the antibonding ${\ensuremath{\sigma}}_{\mathrm{S}\mathrm{i}\ensuremath{-}\mathrm{C}\mathrm{l}}^{*}$ states in the overlayer.

Two distinct boron-related centers are known in silicon carbide polytypes, one shallow (ionization energy ∼300 meV) and the other deep (∼650 meV). In this work, 4H SiC homoepitaxial films are intentionally doped with the shallow boron center by controlling the silicon to carbon source gas ratio during chemical vapor deposition, based on site competition epitaxy. The dominance of the shallow boron center for samples grown with a low Si/C ratio, favoring the incorporation of boron onto the silicon sublattice, is verified by the temperature dependent Hall effect, admittance spectroscopy and deep level transient spectroscopy. In these samples a peak near 3838 A appears in the low temperature photoluminescence spectrum. Further experiments support the identification of this peak with the recombination of a four particle (bound exciton) complex associated with the neutral shallow boron acceptor as follows: (1) The intensity of the 3838 A peak grows with added boron. (2) Momentum conserving phonon replicas are observed, with energies consistent with other four particle complexes in SiC. (3) With increasing temperature excited states are observed, as for the neutral aluminum and gallium acceptor four particle complexes. However, the intensity of the shallow boron spectrum is quenched at lower temperatures than the corresponding spectra for Al and Ga, and the lineshapes are strongly sample dependent. These results may be related to the unusual configurational and electronic structure of this center inferred from recent spin resonance experiments by other groups. When the Si/C ratio is high, the optical signatures of the deep boron center, nitrogen-boron donor-acceptor pairs and conduction band to neutral acceptor free-to-bound transitions, are observed in the photoluminescence. At T=2 K well resolved, detailed nitrogen-boron pair line spectra are observed in addition to the peak due to distant pairs. As the temperature is raised, the donor-acceptor pair spectrum decreases in intensity while the free-to-bound no-phonon peak appears. Extrapolation of the temperature dependence of the free-to-bound peak to T=0 K, after correction for the temperature dependence of the exciton energy gap, leads to the value EA(B)−EX=628±1 meV, where EA(B) is the ionization energy of the deep boron center and EX is the binding energy of the free exciton which, for 4H SiC, can only be estimated at this time.

We report ballistic-electron emission microscopy (BEEM) investigation of Pd, Pt Schottky contacts on 6H-, 4H-SiC, and Pd/15R-SiC. Measured Schottky barrier heights of 6H- and 4H-SiC samples appear spatially uniform up to the fitting error due to noise (0.03–0.04 eV and 0.1–0.2 eV for 6H- and 4H-SiC, respectively). In 4H-SiC, we observed an additional conduction band minimum (CBM) ∼0.14 eV above the lowest CBM, which provide direct experimental verification of band theoretical calculation results. Additionally, we sometimes observed enhancement in ballistic transmittance over regions intentionally stressed by hot electron injection using BEEM. We also report recent results on Pd/15R-SiC sample indicating a higher CBM ∼0.5 eV above the lowest CBM. In Pd/15R-SiC, interesting large variations in BEEM spectra at different locations were observed, possibly suggesting an inhomogeneous metal/semiconductor interface.

We performed ballistic electron emission microscopy (BEEM) measurements on Pt and $\mathrm{Pd}/4H$- and $6H\ensuremath{-}\mathrm{SiC}$ Schottky contacts, obtained Schottky barrier heights, and observed qualitatively different behavior of the BEEM spectra for $4H$- and $6H\ensuremath{-}\mathrm{SiC}.$ In particular, in $4H\ensuremath{-}\mathrm{SiC}$ we observed an additional minimum $\ensuremath{\sim}0.14\mathrm{eV}$ above the lowest conduction-band minimum. We calculated the electronic band structure for both polytypes and found it in good quantitative agreement with our BEEM observations as well as good qualitative agreement with previous SiC band-structure calculations.

We have studied the emission related to multiple bound exciton complexes (MBEC) in 3C SiC, using time resolved photoluminescence spectroscopy. In addition to the normal exciton bound to the neutral nitrogen donor, we observe at low temperatures four peaks in the no-phonon spectral region due to the transition of MBEC{sup m} complexes with up to five (m=5) electron-hole pairs, according to the shell model. The observed emission is due to the recombination of one electron-hole pair in the complex. The time decay of these emissions decreases with increasing m, ranging from 160 ns to 21 ns. For phonon assisted emissions we observe additional splitting of the MBEC due to excited electron states. All emissions within each shell have similar decay times, due to fast relaxation of the electrons inside each shell. (orig.) 12 refs.

The effect of electron beam impact on the oxidation of Si (100) by oxygen has been studied using x-ray photoelectron spectroscopy and Auger electron spectroscopy. It was found that electron beam impact can enhance the oxidation of Si (100) by oxygen at low temperatures, resulting in silicon dioxide formation. Furthermore, electron energy-dependent film growth experiments were carried out on O2(a) and an electron attachment resonance energy of ∼10.1 eV was found. A possible electron-induced oxidation mechanism is proposed which involves dissociative electron attachment for adsorbed O2 species and the formation of O and O− species from adsorbed O2−.The effect of electron beam impact on the oxidation of Si (100) by oxygen has been studied using x-ray photoelectron spectroscopy and Auger electron spectroscopy. It was found that electron beam impact can enhance the oxidation of Si (100) by oxygen at low temperatures, resulting in silicon dioxide formation. Furthermore, electron energy-dependent film growth experiments were carried out on O2(a) and an electron attachment resonance energy of ∼10.1 eV was found. A possible electron-induced oxidation mechanism is proposed which involves dissociative electron attachment for adsorbed O2 species and the formation of O and O− species from adsorbed O2−.