C. Coccorese

F. M. Gelardi

F. Migliore

G. Laurella

M. Boscaglia

M. Cannas

M. D. Pirnaci

S. Agnello

English

Archivio istituzionale della ricerca - Università di Palermo

Exciton lifetime of 4H-SiC modified bythermal annealing in high pressureO2G. Laurella1, F. Migliore1, M. Cannas1, F. M. Gelardi1, C. Coccorese 2M. Boscaglia2, M. D. Pirnaci2, S. Agnello1,31Department of Physics and Chemistry "Emilio Segrè", University of Palermo, 2STMicroelectronics, Catania,3AtenCenter, University of Palermoemail: gianmarco.laurella@unipa.it, simonpietro.agnello@unipa.it1. INTRODUCTIONSilicon Carbide (SiC) is an advanced material belonging to the family of Ultra Wide Bandgap semiconductors known for its excellent characteristics and a promising material forhigh power applications due to its potential to improve the energy efficiency and lowering energy consumptions. For these reasons, there is a rising interest from researchers and industryto synthesize good quality wafers with high carrier mobility and low defect density. For electronic applications a thin oxide gate is needed, which is usually produced by ChemicalVapour Deposition (CVD) of SiO2 . It is crucial that the defect density at the interface between SiC and SiO2 is as low as possible to mantain good electrical characteristics. Therefore,several Post Deposition Annealing (PDA) methods have been developed to passivate and reduce these defects located on the interface between oxide and semiconductor. However, thesemethods usually involve high temperature (1100- 1300 °C) thermal annealing in gases as NO, N2O, POCl3 at atmospheric pressure for a short time interval. Here we present a differentPDA conducted at low temperature (400 °C) in an high pressure (50 bar) pure O2 atmosphere. This method could provide valuable insight on the fundamental oxidation properties ofSiC and at the same time offer a cost-effective alternative to standard PDA.2. METHODSThe presence of defects in SiC crystal lattice inducesvariations in the vibrational modes, that can be highlightedusing Raman spectroscopy.It is also possible to evaluate the quality from theexciton lifetime τ . In our experimental setup (Fig. 1) wemeasure the PL intesity decay as a function of time afterthe excitation pulse. The PL intensity follows:I(t) = I(0) exp(−t/τ)We conducted thermal treatments in high pressure O2atmosphere (300− 800 PSI) in a reactor like in Fig. 2.Figure 1: Schematic of tunable lasersystem used for TRPLFigure 2: Parr reac-tor for thermal treat-ments3. SAMPLESSamples studied in this work are representative of threesteps in the production line of SiC wafers.• CIG1: Epitaxial layer of SiC on a highly-doped sub-strate• CIG6: a 50 nm SiO2 layer is deposited on the epitax-ial SiC layer• 11EA: subjected to thermal annealing (PDA) in NOin order to improve electrical characteristics4. CHARACTERIZATIONComparing samples’ Raman spectra, we can see (Fig. 3) that in the Longitudinal Optical Phonon Coupled (LOPC) mode(∼ 980 cm−1) a "shoulder" appears on the right in samples with oxide layer (CIG6, 11EA). Given that the LOPC mode ishighly sensitive to doping,[1] this feature could be attributed to an increased density of defects at the SiC/SiO2 interfaceand can give us information about the different doping levels between epitaxial layer and substrate.When excited at λ = 266 nm, samples show an emission peak centered at 390 nm (Fig. 4) due to excitonic re-combination. We want to follow the time evolution of this excitonic recombination process under the assumption that themean lifetime τ is a quality indicator of the material:[2,3] high τ means low defect density and vice versa.Figure 3: Raman spectra, comparison between samples. Figure 4: Steady state PL emission showing a single peak centred at390 nm.5. RESULTSA first characterization on as received samples shows that CIG1 and 11EA have higher lifetime τ = 20 ns compared toCIG6 which has τ = 10 ns (Fig. 5)For each sample, thermal annealing process is performed in O2 at T=400 °C for different pressures and durations.Figure 5: As received sample Figure 6: T=400 °C, P=800 PSI, t=48hFigure 7: T=400 °C, P=800 PSI, t=2h Figure 8: T=400 °C, P=300 PSI, t=2h6. CONCLUSION• Samples show two different initial exciton lifetime be-fore thermal treatments• Different results were obtained for pressure above orbelow Pcrit(O2) = 730 PSISAMPLE RESULTSCIG1 τ decreases for P > PcritCIG6 τ increases for P > Pcrit11EA τ not affected by thermal treatments• Thermal treatment at P = 300 PSI shows no changein lifetime• CIG6 undergoes a recovery process similar to that ob-tained with NO PDAFurther studies will involve a more in-depth analysis on thetime-dependence of this thermal annealing process, XPScharacterization and electrical parameters measurements.5. REFERENCES[1] Sun, G.S., et al., 2011. Determination of the transport properties in 4H-SiC wafers by Raman scattering measurement. Chinese Physics B[2] Migliore, F. et al, 2024, β-rays induced displacement damage on epitaxial 4H-SiC revealed by exciton recombination, Applied Physics Letters[3] Tawara, T. et al, 2004. Evaluation of Free Carrier Lifetime and Deep Levels of the Thick 4H-SiC Epilayers. MSF 457–460, 565–568.

Exciton lifetime of 4H-SiC modified by thermal annealing in high pressure O2

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Exciton lifetime of 4H-SiC modified by thermal annealing in high pressure O2

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