Nikolaos Tsavdaris

Superconducting properties of very high quality NbN thin films grown by high temperature chemical vapor deposition

Niobium nitride (NbN) is widely used in high-frequency superconducting electronics circuits because it has one of the highest superconducting transition temperatures (T-c similar to 16.5 K) and largest gap among conventional superconductors. In its thin-film form, the T-c of NbN is very sensitive to growth conditions and it still remains a challenge to grow NbN thin films (below 50 nm) with high T-c. Here, we report on the superconducting properties of NbN thin films grown by high-temperature chemical vapor deposition (HTCVD). Transport measurements reveal significantly lower disorder than previously reported, characterized by a Ioffe-Regel parameter (k(F)l) similar to 12. Accordingly we observe T-c similar to 17.06 K (point of 50% of normal state resistance), the highest value reported so far for films of thickness 50 nm or less, indicating that HTCVD could be particularly useful for growing high quality NbN thin films.

On Photoelectrical Properties of 6H-SiC Bulk Crystals PVT-Grown on 6H- and 4H-SiC Substrates

Free carrier lifetimes and diffusion coefficients were determined in 6H-SiC bulk crystals grown by PVT on 6H-and 4H-SiC seeds varying the temperature from 300 K to 650 K and at excess carrier densities ΔN0 from 1017 cm-3 to 1019 cm-3. Carrier generation was achieved by using a single or two-photon absorption of picosecond pulses at 351 and 532 nm, respectively. Fast and slow recombination transients revealed the decay time of free carriers and the presence of deep acceptor traps. The thermal trap activation energy Ea = 0.33 eV was determined in the 6H/4H sample and ascribed to the boron, while the presence of deeper traps is suggested in the 6H/6H sample. At room temperature and reaching conditions of trap saturation regime (ΔN0 1019 cm3), both crystals revealed the bipolar diffusion coefficient Da 4 cm2/s. For comparison, we also determined the photoelectrical parameters in commercial 6H-SiC crystals grown by the Lely and PVT techniques.

Open Issues in SiC Bulk Growth

In this paper, an overview of the SiC bulk growth processes is given with a special focus on the most recent results related to growth and modeling. In addition, even if SiC growth is a very old topic and that it is now considered as an « industrial development problem », we will show that there are still many open questions of both fundamental and technological importance related to its crystal growth. Process chemistry and surface mechanisms will be more specifically discussed.

Nitrogen Incorporation during Seeded Sublimation Growth of 4H-SiC and 6H-SiC

We address the problem of nitrogen incorporation during bulk crystal growth of 4H-SiC and 6H-SiC by seeded sublimation method. The partial pressure of nitrogen and temperature dependence were considered in bulk SiC crystals. Free carrier concentration and incorporated nitrogen were determined using Raman spectroscopy and Secondary Ion Mass Spectrometry, respectively. The incorporated nitrogen at the (000-1) C-face of 4H-SiC and 6H-SiC is found to be independent of the polytype of the crystal. Higher desorption rate at Si-face compared to C-face is found, using a Langmuir equation, which is attributed to the difference in bond density between the two polar faces. The increased nitrogen desorption when growth temperature increases is believed to be the most contributing factor, based on the temperature dependent trends.

Photoelectrical Parameters of a PVT Grown Bulk 15R-SiC Crystal at Different Stages of Growth

The electronic quality of a Physical Vapour Transport (PVT) grown 15R-SiC crystal at different stages of growth was assessed by time-resolved optical pump-probe techniques. The measured differential transmittivity (DT) kinetics for the layers corresponding to the initial, middle and final stages of growth revealed clear differences in the decay of the DT signal, indicating a decreasing concentration of traps at the later stages of the crystal growth. The estimated trap concentration in the initial layer was NT ≈ 1019 cm-3, while it decreased down to less than 2×1018 cm-3 in the top layer. The injection dependence of the diffusion coefficient at room temperature confirmed the gradual decrease of NT in the layers corresponding to later stages of growth. Accordingly, the bipolar diffusion coefficient in the middle and the top layer was Da ≈ 2 cm2/s, while Da = 0.9 cm2/s was measured in the layer closest to the seed.

Interface Shape: A Possible Cause of Polytypes Destabilization during Seeded Sublimation Growth of 15R-SiC

We report on the stabilization of 15R Silicon Carbide (15R-SiC) grown by seeded sublimation method. It was found that polytype transitions are directly related to the occurrence of facets on the grown crystals. Once a foreign polytype is formed, its propagation is governed mainly by the interface shape of the crystal and its evolution during growth. A concave crystal shape enhances the expansion of foreign polytypes, usually formed at the periphery of the crystal. Then, foreign polytypes can either overlap the original polytypes (constant concave crystal shape) or form inclusions (change to convex crystal shape). On the contrary, an initially slightly convex interface repels foreign polytypes towards the edge of the crystal. The optimization of the growth interface shape can be a key issue towards the stabilization of bulk 15R-SiC crystals.

Effect of Facet Occurrence on Polytype Destabilization during Bulk Crystal Growth of SiC by Seeded Sublimation

We report on polytype destabilization during bulk crystal growth of Silicon Carbide by seeded sublimation method. Polytype transitions are experimentally obtained and a thermodynamic analysis using classical 2D nucleation theory is used towards the understanding of the experimental results. Whether it is a thin lamella or an inclusion, it is found that the polytype transitions systematically occur on the (0001) facet. This suggests that the polytype switch takes place through classical 2D nucleation at the facet. We will show that two criteria must be fulfilled for the occurrence of a foreign polytype: i) minimization of nucleation energy and ii) presence of a facet. This is directly depending on the crystal shape (convex or concave) and its evolution with growth time.

Nondestructive Evaluation of Photoelectrical Properties of a PVT Grown Bulk 15R-SiC Crystal

Investigation of excess carrier dynamics in a 15R-SiC bulk layer grown by physical vapour transport (PVT) on 15R-SiC substrate has been carried out using pump-probe techniques: an interband carrier injection by a picosecond laser pulse and measuring the induced absorption and diffraction of a probe beam. For this task, differential transmittivity (DT) and light induced transient grating (LITG) techniques were used. Room temperature carrier lifetime varied in the 3 ns 8 ns range at excess carrier densities above ΔN0 = 7×1017 cm-3 and was ascribed to the recovery time of optically recharged carrier traps, and their activation energy of Ea = 75 meV was determined. The presence of recharged traps caused the injection-dependence of the diffusion coefficient D, whereby its value dropped below 0.1 cm2/s at ΔN0 < 1×1018 cm-3 and gradually increased up to 0.7 cm2/s at higher injections. At elevated temperatures (300 K < T < 700 K), when the traps are thermally activated, the diffusivity increased up to ~ 1.5 cm2/s and was independent on ΔN0. The overgrown layer parameters were comparable to those of the used 15R PVT seed.

Interaction between Vapor Species and Graphite Crucible during the Growth of SiC by PVT

Graphite crucible in seeded sublimation growth of Silicon Carbide (SiC) single crystal does not only act as a container but also as an additional carbon source. The modeling of the growth process integrated with the etching phenomenon caused by the interaction between vapor species and the graphite crucible is shown to be able to predict the shape of the crystal front during the growth. The additional fluxes produced at the graphite part are delivered to the growing crystal mainly at the crystal periphery. The results obtained from the modeling are in good agreement with the experimental ones.