Andreas Fissel

Epitaxial growth of SiC thin films on Si-stabilized α-SiC(0001) at low temperatures by solid-source molecular beam epitaxy

Abstract Epitaxial growth of SiC on α-SiC(0001) substrates was carried out at relatively low temperatures (900–1000°C) and high growth rates (about 1 nm/min), by means of solid-source molecular beam epitaxy controlled by a quadrupole mass spectrometry based flux meter. The films were obtained on silicon-stabilized surfaces showing (3 × 3) and (2 × 2) superstructures. The reflection high-energy electron diffraction (RHEED) and transmission electron microscopy (TEM) investigations show, independent of the surface orientation, that the film growth can be quantified in two steps: In the initial stage of growth, at first the Si-determined superstructure will be formed. Despite the low temperature, growth of SiC then proceeds in a layer-by-layer mode leading to flat film-substrate interfaces. This result demonstrates the significance of surface reconstruction for the growth process, corresponding to results recently obtained using gas source molecular beam epitaxy. The films grow by stacking of laminae of α-and β-SiC, respectively, which may be attributed to fluctuations in the Si adlayer thickness. Films grown on off-oriented substrates contain many defects, likely double-positioning boundaries, directly associated with surface steps. For increasing film thickness during the film growth this boundaries may be eliminated. Films grown on well-oriented substrates show only a few of these defects. In case of increasing Si excess flux during the growth, the TEM investigations, RHEED patterns and damped RHEED-oscillations indicate an abrupt change in the growth process. This is due to the formation of Si islands on the film surface and the SiC island growth by carbonization of these islands. Only β-SiC was found in this case.

Advances in the molecular-beam epitaxial growth of artificially layered heteropolytypic structures of SiC

The controlled growth of SiC heteropolytypic structures consisting of hexagonal and cubic polytypes has been performed by solid-source molecular-beam epitaxy. On on-axis substrates, 4H/3C/4H–SiC(0001) and 6H/3C/6H–SiC(0001) structures were obtained by first growing the 3C–SiC layer some nanometer thick at lower substrate temperatures (T=1550 K) and Si-rich conditions and a subsequent growth of α-SiC on top of the 3C–SiC layer at higher T (1600 K) under more C-rich conditions. On off-axis substrates, multiheterostructures consisting of 4H/3C- or 6H/3C-stacking sequences were also obtained by first nucleating selectively one-dimensional wire-like 3C–SiC on the terraces of well-prepared off-axis α-SiC(0001) substrates at low T(<1500 K). Next, SiC was grown further in a step-flow growth mode at higher T and Si-rich conditions. After the growth, many wire-like regions consisting of 3C–SiC were found also within the hexagonal layer material matrix indicating a simultaneous step-flow growth of both the cubic and...

Investigation of two-dimensional growth of AlN(0 0 0 1) on Si(1 1 1) by plasma-assisted molecular beam epitaxy

The conditions for the growth of single-crystalline wurtzite aluminum nitride (AlN) films on Si(1 1 1) have been established by plasma-assisted molecular beam epitaxy. The yield of atomic nitrogen from the RF-source has been investigated by optical and by mass-spectrometry. AlN films grown under RF-source conditions with the highest yield of atomic nitrogen, as determined by mass-spectrometry, generally show narrowest AlN(0 0 0 2) rocking curve linewidths in X-ray diffraction. By supplying nitrogen and aluminum close to unity, AlN has been grown two-dimensionally with atomically smooth surfaces. Surface reconstructions have been investigated by reflection of high-energy electron diffraction as a function of the substrate temperature and the Al-overpressure. High-resolution transmission electron micrographs taken at the heterointerface show a coincidence between both materials with a ratio of 5·dAlN(2110) to 4·dSi(110). The misfit is accomodated within the very first monolayers on either side.

Growth of atomically smooth AlN films with a 5:4 coincidence interface on Si(111) by MBE

Epitaxial aluminum nitride AlN(0001) thin films have been grown by plasma-assisted molecular beam epitaxy (PA-MBE) on Si(111). The influence of the composition of the nitrogen plasma on the crystal quality, as judged by X-ray diffractometry (XRD) and atomic force microscopy (AFM), has been investigated. Under an Al/N vapor phase ratio close to unity atomically smooth AIN films have been grown at 850°C substrate temperature with maximum growth rates of 2.5 nm min - 1 . A √3 × √3 and a more Al-rich 2 × 6-surface reconstruction have been observed. Transmission electron microscopy (TEM) investigations show that these films are homogeneous 2H-AIN single crystals. Their defect structure consists of threading dislocations mostly. The hetero-interface is abrupt and flat. Processed high-resolution (HR) TEM images demonstrate a 4×d Si(T10) to 5 × d AlN(2110) coincidence between substrate and epilayer. The XRD FWHM of the (0002)-diffraction peak of 0.5 μm AIN is 0.06° in the ω/2 scan and 0.32° in the ω scan. Phonon modes of AIN have been detected by Raman and infra-red spectroscopy.

Growth of columnar aluminum nitride layers on Si(111) by molecular beam epitaxy

Abstract Single crystalline aluminum nitride (AlN) thin films are deposited by molecular beam epitaxy (MBE) using thermally evaporated aluminum and RF-plasma excited nitrogen gas. In this paper we report on films grown on Si(111) at substrate temperatures of 800° with growth rates between 65 and 350 nm h −1 . All layers consist of hexagonal and exactly c -axis oriented AlN crystals with column-like structure. For the smoothest layers surface roughness (rms) around 1 nm is obtained. In the XRD-spectra ( ω -scan) we have achieved a minimum FWHM of 0.4° (=25′) for the AlN(00.2) reflex. At maximum growth rates (350 nm h −1 ) for AlN a transition zone of about 200 nm is formed with high defect density compared to the subsequent growth. For lower growth rates (65 nm h −1 ) no transition zone exists. Application of a substrate nitridation leads to a partial loss of epitaxial relation between AlN layer and Si(111)-substrate.

Growth of 6H–SiC on 6H–SiC(0001) by migration enhanced epitaxy controlled to an atomic level using surface superstructures

Epitaxial growth of 6H–SiC on 6H–SiC(0001) via two‐dimensional nucleation was realized at 930 °C by solid‐source molecular beam epitaxy using the alternate supply of Si and C. The deposition was controlled to an atomic level by surface superstructures. The growth was started on the (√3×√3)R30° surface which turns into the (1×1) phase upon deposition of about 1 monolayer silicon and recurs after subsequent deposition of about 1 monolayer carbon. Deviations from the monolayer deposition and, moreover, growth around substrate related defects result in the deposition of 3C–SiC.

Stranski–Krastanov growth of Si on SiC(0001)

Abstract The molecular beam epitaxial growth of Si on SiC(0001), exhibiting a Stranski–Krastanov mode, was investigated by reflection high-energy electron diffraction. Several surface superstructures were observed in the initial stage of growth. After exceeding a critical coverage, Si island formation sets in. Under near-equilibrium conditions, the critical coverage was 1.4 monolayers and corresponds to the occurrence of a 3×3 superstructure remaining as a wetting layer after the island formation. Island formation at high deposition rates (R) and low temperatures (T) is kinetically delayed, which can be described as function of R and the diffusivity D by a relationship t c ∝ R/D . Si islands, which were relatively uniform size of several nm with a density of 1011 cm−2, were obtained under these conditions. At lower R values the critical thickness is only a function of T, indicating that the incorporation time of adatoms is the relevant time scale for surface diffusion. Ordered arrays of small dots were also grown on vicinal surfaces at higher T and lower R values, which can be attributed to a lower diffusivity at step edges, acting as perfect sinks for the Si adatoms. Furthermore, two different kinds of islands were found with a (111)/(0001) and (110)/(0001) epitaxial relationship.

Investigations of Stranski-Krastanov growth kinetics of Si-dots on 6H-SiC(0001)

The growth kinetics of Si dots grown on 6H-SiC(0001) by molecular beam epitaxy were studied in real time by reflection high-energy electron diffraction. The critical thickness for the Stranski–Krastanov growth mode transition was found to be kinetically delayed leading to a gradual decrease of this thickness with increasing temperature (T). At T<625 °C and coverages below the critical thickness, a post-deposition evolution of dots is clearly established. The dot growth process is, under these conditions, mainly determined by the mass transfer out of the two-dimensional layer towards the Si dots. The dots grown on top of a 1 monolayer (ML) thick wetting layer are quantum sized with typical dimensions of 5–6 nm in height and 20–30 nm in diameter after a long post-deposition evolution times at 2–3 ML coverages. Above 625 °C and coverages above the critical thickness, the dot growth is only determined by surface-diffusion kinetics resulting in the growth of larger dots.

Si/Ge-nanocrystals on SiC(0001)

Abstract The growth and structure of Si- and Ge-nanocrystals was investigated using high resolution X-ray diffraction (HRXRD) and atomic force microscopy (AFM). AFM-images were used to determine the lateral and vertical dimensions of the nanocrystal. HRXRD measurements show clearly that Si- and Ge-nanocrystals grow on 6 H –SiC(0001) preferentially in two different orientations — 〈111〉 and 〈110〉 — with respect to the surface normal. The growth of Ge-nanocrystals on Si-rich 6 H –SiC(0001) surfaces leads to the formation of Si/Ge-alloy nanocrystals. Both types of nanocrystals grow coherently with respect to the substrate. Hence, due to the respective lattice mismatch, the degree of coherence was found to be much better for Si-nanocrystals.

High-quality SiC epitaxial layers and low-dimensional heteropolytypic SiC structures grown by solid-source MBE

The growth of SiC layers on hexagonal (or α-) SiC(0001) has been performed by solid-source MBE between 1300 and 1600 K. The α-SiC layers have been grown homoepitaxial via step-flow on off-axis substrates, whereas pseudomorphic cubic (or 3C-) SiC layers were obtained on α-SiC via nucleation and subsequent step-flow. Under more equilibrium-like conditions, 3C-layers nearly free of twin-boundaries were obtained. The SiC layers were of high quality and without unintentional doping, as revealed by photoluminescence investigations. The controlled growth of SiC heteropolytypic structures consisting of hexagonal and cubic polytypes, such as 4H/3C/4H-SiC(0001) and 6H/3C/6H SiC(0001), has also been demonstrated. Such structures were obtained by changing the growth conditions from lower temperatures (1550 K) and Si-rich Si/C ratio (3C-SiC) to higher temperatures (1600 K) and more C-rich Si/C ratio. On off-axis substrates, such heterostructures were also obtained by first nucleating selectively wire-like 3C-SiC nuclei on the terraces of well-prepared α-SiC(0001) substrates at low T (<1500 K) and a subsequent step-flow of both the 3C wires and the surrounding α-SiC material.