John Kouvetakis

Boron-carbon-nitrogen materials of graphite-like structure

Abstract New structural relatives of graphite with the composition BxCyNx have been synthesized from the interaction of boron trichloride, acetylene and ammonia at 400–700°C. A hexagonal layer structure type is indicated by X-ray and electron diffraction data. Binding energies of 1s electrons (ESCA) for B, C and N indicate that each graphite-like sheet is a composite of all three elements. The material of approximate composition B0.35C0.30N0.35 is a semiconductor and is intercalated by both strong reducing and oxidizing agents.

Ge-Sn semiconductors for band-gap and lattice engineering

We describe a class of Si-based semiconductors in the Ge1−xSnx system. Deuterium-stabilized Sn hydrides provide a low-temperature route to a broad range of highly metastable compositions and structures. Perfectly epitaxial diamond-cubic Ge1−xSnx alloys are grown directly on Si(100) and exhibit high thermal stability, superior crystallinity, and crystallographic and optical properties, such as adjustable band gaps and lattice constants. These properties are completely characterized by Rutherford backscattering, low-energy secondary ion mass spectrometry, high-resolution transmission electron microscopy, x-ray diffraction (rocking curves), as well as infrared and Raman spectroscopies and spectroscopic ellipsometry. Ab initio density functional theory simulations are also used to elucidate the structural and spectroscopic behavior.

Novel aspects of graphite intercalation by fluorine and fluorides and new B/C, C/N and B/C/N materials based on the graphite network

Materials of composition BC3, C5N and BC2N have been prepared by reactions which are driven by favorable TΔS values associated with HC1 elimination. X-Ray and electron diffraction, electron microscopy, EELS and Auger spectroscopy all establish that these materials have their atoms in graphite-like networks. BC3 is a semi-metal, and BC2N is a small bandgap semiconductor. C5N is best formed at 680°C and is characterized by an interlayer spacing of 3.52 A. At higher reaction temperatures (CN)2 is lost and at 980°C the composition is ∼ C14N and the average interlayer spacing is 3.43 A. The C/N material becomes more graphite-like dimensionally and the electrical conductivity increases towards that of graphite as the nitrogen content falls. These observations suggest that the N atoms may not be in the same plane as the C atoms. Each of the B/C, C/N and B/C/N materials has a unique intercalation chemistry, which will be compared with that of graphite. The graphite network is also preserved in graphite fluorinated by F− carriers at ∼ 20°C. A remarkably inert material of composition C1.3F and its CXF relatives (in which the sp2 carbon of graphite is preserved) will be described. At composition C1.3F the interlayer spacing is 6.4 A but the graphite-like ao parameter (ao = 2.478 A) is only slightly larger than that in graphite itself, therefore most F atoms in C1.3F must have F atom neighbors at only 2.48 A.

Type-I Ge∕Ge1−x−ySixSny strained-layer heterostructures with a direct Ge bandgap

The electronic properties of Ge∕Ge1−x−ySixSny strained-layer heterostructures are predicted theoretically. It is found that a lattice-matched system with fully strained Ge layers and relaxed Ge1−x−ySixSny alloys can have a direct fundamental bandgap with spatial localization in the Ge layers (type I). The Si and Sn concentrations for which such a direct bandgap obtains are close to those that have already been experimentally demonstrated [M. Bauer, C. Ritter, P. A. Crozier, J. Ren, J. Menendez, G. Wolf, and J. Kouvetakis, Appl. Phys. Lett. 83, 2163 (2003)]. The required level of tensile strain in the Ge layers is compatible with Si–Ge technology. The predicted direct bandgap values are as high as 0.6eV.

A novel graphite-like material of composition BC3, and nitrogen–carbon graphites

Interaction of benzene with boron trichloride at 800 °C yields a graphite-like metallic solid of composition BC3, and chlorine–pyridine mixtures at 800 °C give a nitrogen–carbon also having a graphite-like structure.

Direct-gap photoluminescence with tunable emission wavelength in Ge1−ySny alloys on silicon

Direct-gap photoluminescence has been observed at room temperature in Ge1−ySny alloys grown on (001) Si substrates. The emission wavelength is tunable over a 90 meV (200 nm) range by increasing the Sn concentration from y=0 to y=0.03. A weaker feature at lower energy is assigned to the indirect gap transitions, and the separation between the direct and indirect emission peaks is found to decrease as a function of y, as expected for these alloys. These results suggest that Ge1−ySny alloys represent an attractive alternative to Ge for the fabrication of laser devices on Si.

Extended performance GeSn/Si(100) p-i-n photodetectors for full spectral range telecommunication applications

First-generation n-i-GeSn/p-Si(100) photodiode detectors with Ge0.98Sn0.02 active layers were fabricated under complementary metal oxide semiconductor compatible conditions. It is found that, even at this low Sn concentration, the detector quantum efficiencies are higher than those in comparable pure-Ge device designs processed at low temperature. Most significantly, the spectral range of the GeSn device responsivity is dramatically increased—to at least 1750 nm—well beyond the direct band gap of Ge (1550 nm). This allows coverage of all telecommunication bands using entirely group IV materials.

Synthesis of ternary SiGeSn semiconductors on Si(100) via SnxGe1−x buffer layers

Single-phase Si1−x−yGexSny alloys with random diamond cubic structures are created on Si(100) via ultrahigh vacuum chemical vapor deposition reactions of SnD4 with SiH3GeH3 at 350 °C. Commensurate heteroepitaxy is facilitated by Ge1−xSnx buffer layers, which act as templates that can conform structurally and absorb the differential strain imposed by the more rigid Si and Si–Ge–Sn materials. The crystal structure, elemental distribution and morphological properties of the Si1−x−yGexSny/Ge1−xSnx heterostructures are characterized by high-resolution electron microscopy, including electron energy loss nanospectroscopy, x-ray diffraction (rocking curves) and atomic force microscopy. These techniques demonstrate growth of perfectly epitaxial, uniform and highly aligned layers with atomically smooth surfaces and monocrystalline structures that have lattice constants close to that of Ge. Rutherford backscattering ion channeling shows that the constituent elements occupy random substitutional sites in the same ave...

Perfectly tetragonal, tensile-strained Ge on Ge1−ySny buffered Si(100)

High-quality, tensile-strained Ge layers with variable thickness (>30nm) have been deposited at low temperature (350–380°C) on Si(100) via fully relaxed Ge1−ySny buffers. The precise strain state of the epilayers is controlled by varying the Sn content of the buffer, yielding tunable tensile strains up to 0.25% for y=0.025. Combined Raman analysis and high resolution x-ray diffraction using multiple off-axis reflections reveal unequivocally that the symmetry of tensile Ge is perfectly tetragonal, while the strain state of the buffer (∼200nm thick) remains essentially unchanged. A downshift of the direct gap consistent with tensile strain has been observed.

Direct gap electroluminescence from Si/Ge1−ySny p-i-n heterostructure diodes

Electroluminescence spectra from Si/Ge1−ySny heterostructure diodes are reported. The observed emission is dominated by direct gap optical transitions and displays the expected compositional dependence of the peak energy. Weaker indirect gap emission is also observed, and their energies are consistent with a closing of the indirect-direct separation as the Sn concentration is increased. The intensity of the EL spectra shows a superlinear dependence on the injection current, which is modeled using a Van Roosbroeck–Shockley expression for the emission intensity. The model assumes quasiequilibrium conditions for the electrons populating the different valleys in the conduction band of Ge.