Since the study of T. F. Deutsch et al. in 1981 [1], many studies on laser doping have been carried out. In particular, Dr. Aravinda Kar's laboratory at the University of Central Florida has published many research reports on the adaptation of semiconductor devices as doping systems [2-4] . However, it still does not seem that laser doping system is accepted in the market as a mass production equipment. It is presumed that one of the reasons is that the mechanism of laser doping is still unknown, and that the condition setting is complicated accordingly. Therefore, we have understood the mechanism of laser doping so that process conditions can be set even with TCAD (macro model) [5] , but we felt that it is necessary to further understand the structural changes of SiC due to laser irradiation. So, in this work, we tried to understand the state of change during laser irradiation on the 4H-SiC structure. In particular, there are few reports on the laser irradiation phenomenon that occurs even below the peritectic temperature, so it’s necessary to understand the phenomenon through experiments.

To form a diffusion layer by laser doping, it is necessary to set various parameters (laser fluence, laser pulse width, wavelength, etc.). However, if the laser fluence is increased, the phenomenon shifts from the doping process to the ablation process, so it is necessary to carry out the doping process with a limited fluence. Therefore, it is premised that the 4H structure of SiC has not changed when laser irradiation is performed. In this study, a dopant material was applied to 4H-SiC in advance, and the laser of each fluence was irradiated with different irradiation times, and the state change of 4H-SiC was observed and investigated.

The outline of the experiment model is shown in Figure 1. To form an n-type layer on the SiC substrate, a SiN thin film layer as a dopant is applied in advance. A SiN film with a film thickness of 200 nm was applied by plasma CVD.

Figure 1: The outline of the experiment model.

The SiC is heated by a laser beam that has passed through the SiN film. The pulse width was fixed at 82 ns, and the structural changes of 4H-SiC when irradiated with the KrF excimer laser were investigated using the fluence and the number of irradiation shots as parameters. These experiments were performed in the doping-capable region (Not the region where SiC melts and recrystallizes and changes from 4H structure to 3C structure) where SiC was less than 3100 K of the peritectic temperature. It has been confirmed that the maximum temperature (around 3000K, depending on the irradiation conditions) is reached within several tens of ns after one irradiation shot, and the substrate has sufficiently returned to room temperature by the start of the next irradiation shot (because the cycle is 10 msec). Therefore, each irradiation shot is thermally repeated in the same state. If the laser irradiation is purely a photothermal phenomenon, the temperature of the substrate will rise and fall without affecting the structure of SiC.

Figure 2 shows the results of observing the SiC surface condition (after removing SiN layer) after irradiation under each condition by an optical microscope. Also, as a reference, the same evaluation was performed on the SiC bare substrate.

Figure 2: The results of observing the SiC surface condition after removing SiN layer.

Under the condition of one irradiation shot, no surface change (no surface damage) was confirmed at any fluence. It is thermally independent for each irradiation shot, and the same state is obtained each time, but surface damage is confirmed as the number of irradiations increases. In addition, it can be seen that the surface structure changes faster when the SiC surface is coated with SiN layer than the bare SiC substrate. As can be seen from the enlarged photograph, the surface damage is dotted with black supraorbital ridges. Next, Figure 3 shows the result of observing this surface with a microscopic Raman spectroscopy and the result of observing its cross section by TEM. Raman spectroscopy reveals that the SiC surface is separated into Si and C layers. In addition, a separation layer was confirmed by cross-sectional TEM with a thickness of a few nanometers on the top surface. However, the internal structure below it maintains the 4HSiC structure, and it was reconfirmed that there is no transition to amorphous or 3C-SiC unless the conditions exceed the peritectic temperature.

Figure 3: The results of observing the SiC surface by Raman and cross-sectional TEM.

In addition, it is confirmed that as the number of irradiations increases, the separation layer Si on the surface shows a unique shape with protruding protrusions up to a height of 20 nm.

In this experiment, irradiation was basically performed in a temperature range lower than the threshold value, which is the peritectic temperature. Therefore, it is not a region where structural changes occur photothermally. However, in reality, surface damage is confirmed when the number of irradiations is increased. It can be understood that photochemical reactions other than photothermal reactions occur on the surface. As suggested by A. F. Mohammed et al. [6], it is necessary to consider the combination of photothermal, photochemical, photomechanical, and photophysical for the structural change of the SiC surface due to laser irradiation. This time, we were able to suggest from experiments that the photochemical reaction causes changes in the surface structure even in regions that do not change with the photothermal reaction.

About The Number of Irradiation Shots

Since the substrate temperature has returned to room temperature after each shot until the next shot, the temperature conditions for each irradiation are independent and thermally the same.

Figure 3: change in surface roughness.

Figure 3 shows the change in surface roughness due to surface damage when the number of shots is increased. Occurrence of surface roughness differs depending on each fluence, but the same tendency is confirmed. Roughness increases as the number of shots increases, but when the number of shots is further increased, surface flattening is confirmed. It is thought that the SiC surface changes as a photochemical reaction with each laser irradiation even below the peritectic temperature. It is considered that the SiC separation layer is formed on the surface by the photochemical reaction and the height changes, but the separation layer is flattened or ablated by the subsequent irradiation and decreased. The photon energy of the KrF excimer laser, 5.0 ev, is lower than the total SiC of 6.12 eV (= 4H-SiC bandgap: 3.26 eV + Si–C binding energy: 2.86 eV), so the photochemical reaction does not occur immediately. However, a photochemical reaction involving multiphoton absorption.

Figure 4: Surface damage fluence.

Figure 4 shows the number of irradiation shots that cause surface damage for each fluence. Obviously, the number of shots that damage the surface depends on the irradiation shots, and it can be seen that even at low fluence, a Si-C separation layer is formed on the surface by a photochemical reaction. In other words, even if the fluence is low, it depends on the number of shots = irradiation time = total number of photons. The relationship between the fluence (FD) in which a damage layer is formed on the surface and the number of shots (n) is defined by equation (1) from the experimental results. F_D??1/3?Log(n) + 2.2(threshold fluence for peritectic temperature) [J/cm3] ………(1) The reason why the damage fluence is proportional to the cube root of the irradiation time is unknown, but it is inferred that it is related to the number of photons and molecules in the volume.

About Surface Damage Area

Further detailed examination was carried out on the surface damage area. Separation of Si and C has been confirmed in the nanometer region from the surface, but when irradiation is continued, a protruding ridge (about20nm) is confirmed from the experimental results (Figure 2). Although the microscopic Raman spectroscopy does not have high resolution in the depth direction, the material change of the surface damage structure was confirmed by measuring the measurement interval in the depth direction at 100 nm pitch. The results are shown in Figure 5.

Figure 5: SiC surface details by microscopic Raman spectroscopy.

The separation of Si layer and C layer (graphene layer) can be confirmed. It can also be seen that the Si on the surface shows a protruding ridge. The characteristic shape is explained by self-focusing by the Kerr effect and by plasma generation [7,8]. A shape equivalent to the TEM observation result in Figure 2 was confirmed. In TEM observation result, it was confirmed that the C layer on the raised Si surface was thinly present, and it can be understood that the Si layer and the C layer were alternately present [9].

About The Internal Structure

The internal structure was also measured in detail by microscopic Raman spectroscopy. The results are shown in Figure 6.

Figure 6: SiC internal details by microscopic Raman spectroscopy.

From the results shown in Figure 3, the 4H-SiC structure is maintained under the extremely thin damaged area (several nanometers) on the surface. However, Raman analysis confirms tensile stress in that area. The relational expression between the LO and TO peak position shift amount of SiC and the tensile stress is given below equation (2) [10].

Where  and  were the Raman shift of the TO mode with and without residual stress, respectively,  and  were the Raman shift of LO mode with and without residual stress, respectively,  was the measured residual tensile strees in GPa

From the Raman measurement results, the estimated tensile stress was estimated to be 1.1 to 1.2 GPa in the vertical direction and 110 to 120 MPa in the horizontal direction. This distortion is thought to be due to the generation of shock waves by laser irradiation and the photomechanical reaction.

In the doping-capable region, that is, the region where the 4H-SiC structure is not changed by laser irradiation (temperature lower than the peritectic temperature of 3100K), the effects of laser irradiation and changes in the SiC substrate were studied. A summarizing diagram of the experimental results is shown in Figure 7.

Figure 7: The SiC structural changes of 4H-SiC by excimer laser.

As a result, a separation layer of Si and C was formed on the extremely thin surface of the substrate by the photochemical reaction. It was confirmed that this damage depends on the fluence, and the number of photons irradiated. It was also found that although the internal structure below it maintains the 4H-SiC structure, the strain of tensile stress due to the photomechanical reaction is widely present. Therefore, when considering the diffusion of the dopant (nitrogen N in this case),

• Si-C separation layer generated by photochemical reaction (region of several nm)
• Heat-affected zone (HAZ 1) of optical penetration length (region of ~ 70 nm)
• Heat-affected zone (HAZ 2) of strain region generated by photomechanical reaction

It turns out that it is necessary to calculate under the diffusion condition of at least three layers.

Acknowledgement

The authors would like to thank Mr. Masaharu Edo and Dr. Shinya Takashima of Fuji Electric Co., Ltd. for the discussion about data. This work was done for Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Photonics and Quantum Technology for Society 5.0”(Funding agency :QST).

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