Georgios Zoulis

Combined effects of Ga, N, and Al codoping in solution grown 3C–SiC

We report on Ga-doped 3C–SiC epitaxial layers grown on on-axis (0001) 6H–SiC substrates using the vapor-liquid-solid technique and different Si1−xGax melts. The resulting samples have been investigated using secondary ion mass spectroscopy (SIMS), micro-Raman spectroscopy (μ-R) and, finally, low temperature photoluminescence (LTPL) spectroscopy. From SIMS measurements we find Ga concentrations in the range of 1018 cm−3, systematically accompanied by high nitrogen content. In good agreement with these findings, the μ-R spectra show that the Ga-doped samples are n-type, with electron concentrations close to 2×1018 cm−3. As expected, the LTPL spectra are dominated by strong N–Ga donor-acceptor pair (DAP) transitions. In one sample, a weak additional N–Al DAP recombination spectrum is also observed, showing the possibility to have accidental codoping with Ga and Al simultaneously. This was confirmed on a non-intentionally doped 3C–SiC (witness) sample on which, apart of the usual N and Al bound exciton lines,...

Low Doped 3C-SiC Layers Deposited by the Vapour-Liquid-Solid Mechanism on 6H-SiC Substrates

We report the results of a systematic investigation performed to reduce the residual n-type doping level of the 3C-SiC layers grown by the VLS mechanism on 6H-SiC(0001) on-axis substrate. This new approach, termed “High purity VLS” leads to low doped and low compensated material, which was confirmed by Raman and Low Temperature Photoluminescence spectroscopy. The resultant 3C morphology remains typical of single-domain layers and the n-type doping level could be estimated around 6x1016 cm-3.

LTPL Investigation of N-Ga and N-Al Donor-Acceptor Pair Spectra in 3C-SiC Layers Grown by VLS on 6H-SiC Substrates

Ga-doped 3C-SiC layers have been grown on on-axis 6H-SiC (0001) substrates by the VLS technique and investigated by low temperature photoluminescence (LTPL) measurements. On these Ga-doped samples, all experimental spectra collected at 5K were found dominated by strong N-Ga donor-acceptor pair (DAP) transitions and phonon replicas. As expected, the N-Ga DAP zero-phonon line (ZPL) was located at lower energy (~ 86 meV) below the N-Al one. Fitting the transition energies for the N-Al close DAP lines gave 251 meV for the Al acceptor binding energy in 3C-SiC and, by comparison, 337 meV for the Ga acceptor one.

Influence of the C/Si Ratio on the Dopant Concentration and Defects in CVD Grown 3C‐SiC Homoepitaxial Layers

The study reports on structural and optical investigations of (111) 3C‐SiC layers grown homoepitaxially by Chemical Vapour Deposition (CVD) at different C/Si ratios. The seeds were 3C‐SiC layers grown on on‐axis Si‐face (0001) 6H‐SiC substrates by Vapour‐Liquid‐Solid (VLS) mechanism in Si‐Ge or Si‐Sn melt. Transmission Electron Microscopy (TEM) investigation showed that the main defects reaching the VLS seed surface are dislocations, stacking faults (SFs) and twin boundaries. In the CVD layer the defect density is reduced compared to the VLS layer at low C/Si ratio (in the range of 1–3). The low‐temperature Photoluminescence (LTPL) spectra of all the layers display a defect related peak at 1.98 eV attributed to DI defect, while G bands are observed in the range of 1.82–1.92 eV range at C/Si ratio of 7 and 10. Common defect in CVD layers was multiple twin complex, which appears as a rule at the vicinity of the twinned domains. This multiple twin complex consists of four twins bound by two fully symmetrical...

Structural and Optical Investigation of VLS Grown (111) 3C-SiC Layers on 6H-SiC Substrates in Sn-Based Melts

The current communication focuses on the investigation of 3C-SiC layers grown by the Vapour-Liquid-Solid mechanism on on-axis Si-face 6H-SiC substrates in SiSn melts with different compositions and at different growth temperatures. The layers are studied by Transmission Electron Microscopy and Low Temperature Photoluminescence. It was found that for melts with Sn concentration higher than 60 at% large Sn-related precipitates are formed. The depth distribution of the Sn precipitates strongly depends not only on the melt composition but also on the growth temperature. Their formation strongly influences the stacking fault density and the dopant incorporation in the layers. Lower Sn concentrations combined with higher growth temperatures should result in 3C-SiC layer with enhanced structural quality.

Heavily p-Type Doping of Bulk 6H-SiC and 3C-SiC Grown from Al-Si Melts

We report in this work, the solution growth of heavily p-type doped 3C-SiC and 6H-SiC. Description of the 3C and 6H-SiC crystals in terms of defects and resistivity are presented and discussed with respect to growth conditions such as temperature, Al content in the melt and seed polarity. Crystals and thick layers are investigated by means of TEM, NDIC microscopy and Raman.

Optical Characterization of VLS+CVD Grown 3C-SiC Films by Non-Linear and Photoluminescence Techniques

Thin 3C-SiC(111) epilayers grown on 6H-SiC(0001) substrate by VLS and CVD procedures were studied by low temperature photoluminescence (LTPL) and nonlinear optical techniques at room and low temperatures. Free carrier density ((0.3-7)×1017 cm-3) and nitrogen concentration (4×1016 cm-3) in the layers were determined from Raman and LTPL data. Investigation of non-equilibrium carrier dynamics by using transient grating and free carrier absorption techniques provided an ambipolar diffusion coefficient Da (~2.5 cm2/s) and carrier lifetime τR (2-4 ns) values at room temperature. The temperature dependences of Da and τR in 40-300 K range revealed the scattering processes in high density plasma as well the impact of defects.

TEM and LTPL Investigations of 3C-SiC Layers Grown by LPE on (100) and (111) 3C-SiC Seeds

In the present work the defects appearing in layers grown by liquid phase epitaxy on different substrates are compared. The used seeds were (i) 3C-SiC with (111) orientation, grown heteroepitaxially on (0001) 4H-SiC or 6H-SiC substrates by continuous feed physical vapour transport process and the vapour-liquid-solid mechanism, respectively, and (ii) 3C-SiC wafer with (100) orientation from HOYA. The structural and optical investigation showed that (i) on the (111) substrates, due to the appearance of silicon and 6H-SiC inclusions, a layer which consisted of a sequence of long period polytypes was formed. The dominant polytype formed was 21R-SiC, which after successive transformation to 39R- and 57R- SiC led to the formation of 6H-SiC on the top of the layer. (ii) On the (100) substrates, a 3C-SiC layer with comparatively uniform defect density was formed. The main defects were stacking faults and their density was reducing during the process.

Seeding Layer Influence on the Low Temperature Photoluminescence Intensity of 3C-SiC Grown on 6H-SiC by Sublimation Epitaxy

We report on n-type 3C-SiC samples grown by sublimation epitaxy. We focus on the low temperature photoluminescence intensity and show that the presence of a first conversion layer, grown at low temperature, is not only beneficial to improve the homogeneity of the polytype conversion but, also, to the LTPL signal intensity. From the use of a simple model, we show that this comes from a reduced density of non-radiative recombination centers.

Optical Investigation of Defect Filtering Effects in Bulk 3C-SiC Crystals Grown by the CF-PVT Method Using a Necking Technique

We present the results of an optical investigation performed using low temperature photomuminescence and Raman spectroscopy on bulk 3C-SiC samples grown with the Continuous-Feed Physical Vapor Transport technique, using a small diameter neck to filter the defects and improve the as-grown material.