Evaluation of crystal quality and dopant activation of
Smart Cut™- transferred 4H-SiC thin film using Raman spectroscopy

G. Gelineau¹), J. Widiez¹), E. Rolland¹), K. Vladimirova¹), A. Moulin¹),
V. Prudkovskiy¹), N. Troutot¹), P. Gergaud¹), D. Mariolle¹), S. Barbet¹),
V. Amalbert¹), G. Lapertot²), K. Mony²), S. Rouchier³), R. Boulet³), G. Berre³),
W. Schwarzenbach³), Y. Bogumilowicz¹)

1) CEA-Leti, Univ. Grenoble Alpes, F-38000 Grenoble, France

2) Univ. Grenoble Alpes, CEA, Grenoble INP, IRIG, PHELIQS,
38000 Grenoble, France

3) Soitec SA, Chemin des Franques, 38190 Bernin, France
E-mail: guillaume.gelineau@cea.fr

Recent restrictions on combustion-engine vehicles sells for 2035 and beyond from the European Union [1] encouraged expert analysts to predict the market shares of electric vehicles, and thus of power SiC modules, to skyrocket in the next few years [2]. From this market boom, a global SiC wafer shortage is feared, as the production ramp up from 6" to 8" wafer size is still to be effective. In this extent, the Smart Cut™ process offers an advantageous opportunity to overcome this challenge. Significant yield optimization and performance improvements have been achieved by combining the benefits of a low resistivity polycrystalline handle and a high quality 4H-SiC transferred layer [3]. In this paper, we present for the first time studies over a broad range of temperature (from RT to 1900°C) allowing to analyze the entire crystal recovery: from the evolution of the transferred layer crystal quality to the dopant activation.

The H⁺ ion implantation used along the Smart Cut™ process leads to electrical deactivation of the implanted material [4]. Annealing is then mandatory to recover all the properties of the transferred layer (see SSRM cross section resistance mapping Fig. 1a and electrical measurements Fig. 1b). The absence of any buried thermal oxide layer in SmartSiC™ substrates, which was present in previous studies [4], allowed us to investigate the recovery of the transferred layer over high temperatures, using different characterization techniques. Particularly, the study of the Raman Longitudinal Optic Plasmon-Coupled (LOPC) phonon is of great interest, as the position of this peak can be directly linked to the carrier concentration of a polar semiconductor such as SiC [5].

Two series of samples were prepared as follows. Wafers of 4°-off axis 4.10¹⁸ cm⁻³ N-doped bulk 4H-SiC were H⁺-implanted as conventional SmartSiC™ samples [3] before annealing up to 800°C for 30 minutes. Independently, another series was prepared using transferred SiC substrates [3] with the same implantation process, and annealed at temperatures from 950°C to 1900°C. The Raman spectra were acquired in backscattering geometry using a 532 nm laser excitation. For the first sample series, the position of the LOPC peak was determined by normalizing the intensity of the Raman response and subtracting the reference spectrum (non-implanted bulk SiC). For the second series, the position of the peak of the transferred layer was directly derived from the bare spectra, taking advantage of the polySiC receiver (which does not show any Raman response in the 950-1050 cm⁻¹ range). The raw data was then converted into carrier density using the fitting proposed in [5].

Fig. shows the LOPC Raman shifts and corresponding carrier concentrations obtained on both sample series. For temperatures below 1300°C, the former appears at frequencies below 964 cm⁻¹: corresponding to undoped 4H-SiC. The n-type carriers initially contained in the n+ 4H-SiC substrate are thus trapped in a defective crystal. XRD RSM measurements confirm the strong impact of the ion implantation on the crystal quality of the transferred layer at 950°C (Fig. a) with a visible expansion of the SiC cell lattice parameter (the residual presence of H after a 950°C anneal was assessed with SIMS measurements – not shown here). For temperatures above 1300°C, we observe a healing effect through the alignment of the (0004) Bragg peak of the transferred layer with that of the substrate (Fig. b). Then, the steep rise of the LOPC mode shift suggests an explosive dopant activation, that interestingly happens in the same range of temperatures than that of implanted nitrogen in SiC [6]. Eventually, the initial bulk carrier concentration is reached after a 1700°C anneal, with no further evolution when annealing up to
1900°C. Vertical I-V characterizations across the substrate were carried out to probe the electrical performance of the transferred SiC layer after various thermal treatment (Fig. 1b). Such measurements allowed us to confirm the above-mentioned conclusions, as the layer was insulating after a 950°C anneal and electrically conductive post 1700°C.

We hereby report for the first time studies of healing and reactivation mechanisms of a transferred 4H-SiC thin film over a wide range of temperatures. After 950°C, the thin monoSiC layer is insulating. The crystal is fully reorganized after a 1300°C anneal, and electrically conductive after 1700°C. SiC thin film transfer process optimizations can now be carried on using this protocol, for instance with the investigation of ion implantation alternatives – such as the dose or the nature of the ions.

<figure><img src="image_1.png" /><figcaption>Fig 1: (a) Cross-section SSRM and (b) I-V characteristics on test structures of transferred SiC substrates after 950°C and 1700°C annealing.</figcaption></figure>

<figure><img src="image_2.png" /><figcaption>Fig. 2: (a) position of the LOPC peak from both samples series as a function of the anneal temperature
(b) shows the correspondence in terms of carrier concentration after the relation given in [5].</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Fig. 3: Reciprocal Space Mappings (RSM) of the (0004) 4H-SiC XRD peak obtained on samples of mono-4H-SiC layer transferred on mono-4H-SiC receiver after annealing at (a) 950°C; (b) 1300°C.</figcaption></figure>

[1] European Commission, *CO₂ emission performance standards for cars and vans*. 2019.

[2] S. Laferriere et al., "Power SiC: Materials, Devices and Applications 2020", Yole, 2020.

[3] S. Rouchier et al., *Trans. Tech. Publ.*, 1062, 131-135, 2021.

[4] E. Hugonnard-Bruyère et al., *Mat. Sci. Eng.: B*, 61-62, 382-388, 1999

[5] H. Harima, et al., *J. Appl. Phys.*, 78, 1996, 1995.

[6] V. Simonka, et al., presented at SISPAD 2017, Kamakura, Japan, 2017.