Milena Beshkova

Two-Dimensional Nucleation of Cubic and 6H Silicon Carbide

The initial stage of heteroepitaxial growth of 3C-SiC and homoepitaxial growth of 6H-SiC on nominal 6H-SiC on-axis substrates has been studied. Before 3C-SiC starts to nucleate, 6H-SiC grows in a step-flow growth mode due to a slight off-orientation of the substrate surface already at about 1500oC. In the 1650-1700oC temperature interval 3C-SiC nucleates as 2D islands. A distance away from the 3C-SiC island 6H-SiC grows in step-flow mechanism. In the vicinity of the 3C-SiC islands the 6H-SiC growth steps start to change direction and even split into two steps with the equal height of 0.5 nm, which is approaching the unit cell size of cubic SiC. When the supersaturation is lower in comparison with the conditions for 3C-SiC growth, there is only formation of 6H-SiC, i.e. homoepitaxial growth. The growth mode of 6H-SiC is dependent on temperature. At the lowest temperature there is spiral growth while at higher temperature 2D nucleation is preferred.

Properties of 3C-SiC Grown by Sublimation Epitaxy on Different Type of Substrates

3C-SiC layers have been grown by using sublimation epitaxy at a temperature of 2000°C, on different types of on-axis 6H-SiC(0001) substrates. The influence of the type of substrate on the morphology of the layers investigated by Atomic Force Microscopy (AFM) is discussed. Stacking faults are studied by reciprocal space map (RSM) which shows that double positions domains exists.

Structural Properties of 3C-SiC Grown by Sublimation Epitaxy

The present paper deals with morphological and structural investigation of 3C-SiC layers grown by sublimation epitaxy on on axis 6H-SiC(0001) at source temperature 2000 °C, under vacuum conditions (<10-5 mbar) and different temperature gradients in the range of 5-8 °C/mm. The layer grown at a temperature gradient 6 °C/mm has the largest average domain size of 0.4 mm2 assessed by optical microscope in transmission mode. The rocking curve full width at half maximum (FWHM) of (111) reflection is 43 arcsec which suggests good crystalline quality. The AFM image of the same layer shows steps with height 0.25 nm and 0.75 nm which are characteristic of a stacking fault free 3C-SiC surface and c-axis repeat height, respectively.

Sublimation epitaxy of AlN layers on 4H-SiC depending on the type of crucible

Epitaxial layers of aluminum nitride ≤335 μm thick have been grown at temperatures of 1900 and 2100 °C on 10×10 mm2 (0 0 0 1)-oriented α(4H) silicon carbide (SiC), with growth times of 1 and 4 h, via sublimation-recondensation in a RF-heated graphite furnace. The source material was polycrystalline AlN. The sublimation process was performed in three types of graphite (C) crucible: C1, C2 with inner diameters of 35 and 51 mm, respectively, and C3 with the same inner diameter as C1, but coated with a layer of TaC. The surface morphology reflects the hexagonal symmetry of the substrate, suggesting an epitaxial growth for samples grown in C1 and C3 crucibles for all growth conditions. The same symmetry is observed for AlN layers grown in the C2 crucible, but only at 2100 °C. X-ray diffraction analyses confirm the epitaxial growth of AlN samples with the expected hexagonal symmetry. A high-resolution X-ray diffractometer was used to assess the quality of the single crystals. A full width at half maximum of 242 arcsec was achieved for an AlN layer grown in the crucible coated with TaC.

TEM Investigation of the 3C/6H-SiC Transformation Interface in Layers Grown by Sublimation Epitaxy

In the present work Conventional and High Resolution Transmission Electron Microscopy has been used to examine the structure and types of interfaces between 3C-SiC and 6H-SiC for samples grown by Sublimation Epitaxy. The layers were grown on on-axis 6H-SiC substrates at different temperature gradients. The changed growth conditions influence on the nucleation of 3C-SiC on the 6H-SiC substrates and their competition with nucleation of 6H-SiC islands. Three specific types of 3C/6H-SiC interfaces were observed and the implications of these observations are discussed.

Properties of AlN layers grown by sublimation epitaxy

Epitaxial layers of aluminum nitride (AlN)less than or equal to 80 mum thick have been grown at the temperatures 1900 and 2100 degreesC on 10x10mm(2) 4H-SiC substrates via sublimation recondensation in a RF heated graphite furnace. The source material was polyerystalline sintered AlN. A maximum growth rate of 80 mum/h was achieved at 2100degreesC and seed to source separation of I mm. The surface morphology reflects the hexagonal symmetry of the seed that suggesting an epitaxial growth. All crystals show strong and well defined single crystalline XRD patterns. Only the (002) reflection positioned at around 36.04 was observed in symmetric Theta-2Theta scan. The rocking curves FWHM (full width half maximum) and peak positions arc reported.

On Applicability of Time-Resolved Optical Techniques for Characterization of Differently Grown 3C-SiC Crystals and Heterostructures

We applied a number of time-resolved optical techniques for investigation of optical and photoelectrical properties of cubic SiC grown by different technologies on different substrates. The excess carriers were injected by a short laser pulse and their dynamics was monitored by free-carrier absorption, light-induced transient grating, and photoluminescence techniques in a wide excitation range. Combining an optical and electrical probe beam delay, we found that free carrier lifetimes in differently grown layers vary from few ns up to 20 μs. Temperature dependences of carrier diffusivity and lifetime revealed a pronounced carrier trapping in thin sublimation grown layers. In free-standing layers and thick sublimation layers, the ambipolar mobility was found the highest (120 cm2/Vs at room temperature). A linear correlation between the room-temperature band edge emission and carrier lifetime in differently grown layers was attributed to defect density, strongly dependent on the used growth conditions.

Properties of thin silicon carbide films prepared by rapid thermal annealing

We report experimental results related to the structural and electrical properties of thin SiC films. Thin carbon films with thicknesses 50 A and 300 A were deposited by R.F. sputtering and processed by rapid thermal annealing (RTA) for 3 min at temperatures of 800 °C and 1400 °C in a vacuum chamber at 2×10−5 Torr. The thin films properties were studied by Raman spectroscopy and electrical cross-conductance.

Carrier Dynamics in Hetero- and Homo-Epitaxially Sublimation Grown 3C-SiC Layers

We investigated non-equilibrium carrier dynamics in ~20μm thick 3C-SiC layers, grown by sublimation epitaxy directly on 6H-SiC substrate or buffered by a 3C seed layer. Differential transmission and light-induced transient grating techniques were applied to determine the ambipolar diffusion coefficient, carrier lifetime, and thermal activation energy of defects. The temperature dependences of ambipolar mobility and lifetime in 80-700 K range revealed the carrier scattering processes as well the impact of defects on the recombination rate, thus indicating slightly improved photoelectrical parameters of the homoepitaxially grown 3C layer. The determined thermal activation energies of 35 and 57 meV were attributed to the nitrogen impurity.

The Influence of the Temperature Gradient on the Defect Structure of 3C-SiC Grown Heteroepitaxially on 6H-SiC by Sublimation Epitaxy

In the present work the structural quality of 3C-SiC layers grown by sublimation epitaxy is studied by means of conventional and high resolution transmission electron microscopy. The layers were grown on Si-face 6H-SiC nominally on-axis substrates at a temperature of 2000°C and different temperature gradients, ranging from 5 to 8 °C /mm. The influence of the temperature gradient on the structural quality of the layers is discussed. The formation of specific twin complexes and conditions for lower stacking fault density are investigated.