JLMN-Journal of Laser Micro/Nanoengineering Vol. 12, No. 2, 2017

Femtosecond-Laser-Induced Defects on Silicon Carbide
Probed by Electrical Conductivity
Takuro Tomita*, Manato Deki*,***, Eizo Yanagita*, Yota Bando*, Yoshiki Naoi*,
Takahiro Makino** and Takeshi Ohshima***

Faculty of Engineering, Tokushima University,
2-1 Minamijosanjima, Tokushima, Japan
E-mail: tomita@tokusima-u.ac.jp**
National Institutes for Quantum and Radiological Science and Technology,
Watanukimachi, Gunma, Japan***
Present address: Center for Integrated Research of Future Electronics (CIRFE),
Institute of Materials Systems for Sustainability (IMaSS), Nagoya University,Chikusa-ku, Nagoya JapanThe temperature dependence of the electrical conductivity of a femtosecond-laser-modified regionin silicon carbide has been examined. The current-voltage characteristics of the modified region
showed ohmic characteristics. The specific resistivity exhibited a dramatic decrease of four ordersof magnitude in response to a temperature increase of 80 to 400 K. The temperature dependence ofthe specific resistance was successfully fit by a double exponential curve, indicating two defect levels. This result means that two types of levels were induced by the laser irradiation. The irradiation
fluence dependence of the energies of the two levels and the population ratios between them are discussed herein.
DOI: 10.2961/jlmn.2017.02.0004

Keywords: femtosecond laser, silicon carbide, electrical conductivity, temperature dependence,
modification

terial can be subjected to a single photon source [7–9],
which can be applicable to quantum spintronics and photonics. To realize quantum technology based on SiC substrate, the development of a femtosecond modification
technique, especially one facilitating local control of theelectrical conductivity is quite important. This aspect ispartially noteworthy because the transport of spin carriers
is one of the most fundamental issues influencing the development of quantum devices. Previously, we have reported on the electrical conductivity observed for femtosecondlaser-modified SiC [10, 11]. It was found that the electricalconductivity exhibits a dramatic increase above the threshold fluence [10] following laser modification, which indicates that the femtosecond laser modification technique canbe applied to control the local electrical conductivity of SiC.
However, the origin of the higher conductivity has not been
discussed. It has also been found that the electrical conductivity in the laser-modified region exhibits temperaturedependence, indicating the existence of trapping levels[11,12]. If an understanding of the nature of these trappinglevels is obtained, it is highly possible that the femtosecond
laser modification technique will become realized as a toolfor drawing electrical circuits both inside and on the surface of SiC. However, some issues require clarification. To
realize quantum devices using the femtosecond laser modification technique, one of the most important issues is themaximum operation temperature of the electrical circuits
fabricated using the femtosecond laser irradiation. There-1. Introduction
The potential applications of femtosecond laser irradiation are attracting considerable attention from a wide variety of industrial fields. For example, a femtosecond laser
can be used to cut metals, semiconductors, and glass with ahigh spatial precision of a few hundreds of nanometers.
Recently, femtosecond laser irradiation has also seen initialapplications in the biomedical field; for example, in laser
in-situ keratomileusis (LASIK) [1]. With a view to understanding the fundamental mechanism of femtosecond laser
processing, several studies have been conducted [2]. For
example, Tinen et al. have reported the results of ablation
dynamics investigations on semiconductors and metals
performed using the Linnik interferometer [3]. According
to their observations, the exfoliated films expand and form
a dome which continues to expand to approximately 1 ns.
We have also observed the initial process of femtosecond
laser ablation by applying a soft x-ray laser probe to several
irradiated metals [4,5]. The results show that the thin film
is exfoliated from the surface of the substrate after the laser
irradiation. Then, the exfoliated film expands toward thenormal relative to the sample surface, i.e. in the opposite
direction to the incident laser beam. Interestingly, the expanded shape of the exfoliated films corresponds to thelateral intensity profile of the incident laser beam [6]. Inaddition, the vacancies in the silicon carbide (SiC) substrate are attracting considerable interest, because this ma-72

JLMN-Journal of Laser Micro/Nanoengineering Vol. 12, No. 2, 2017from 80 to 400 K.

fore, in this study, we discuss the electrical conductivity ina modified region of an SiC specimen for a wide range ofambient temperatures in order to reveal the energy levels
and carrier population. This insight provides essential information on the carrier transport properties.

3. Results and Discussion
The I-V characteristics of the laser-modified areas in
the SiC are shown in Fig. 1. The irradiated fluence was 9.4
J/cm2, which is above the threshold fluence for the dramatic decrease in the resistance [10]. The I-V characteristics exhibited ohmic characteristics for all the examined
temperature, i.e. 80 to 400 K. It was clearly found that theslope increased significantly with increasing temperature.
Although the result is not shown, the I-V characteristics forthe sample prior to laser irradiation were consistently lessthan 1 pA up to an applied voltage of +100 mV, yielding atrendline that was almost colinear to the abscissa. This tendency was almost identical for all the examined temperatures. Therefore, it is obvious that the observed finite conductivity and the temperature dependence were induced by
the femtosecond laser modification.
The temperature dependence of the electrical conductivity for various fluence values is shown in Fig. 2. In theelectrical conductivity estimation, a space with dimensionsof 4.2-µm width and 50-nm depth was used as the modified
region; these dimensions were obtained from the optical
microscope observation and transmission electron microscopy (TEM) measurement, respectively [13]. Although thefemtosecond laser irradiated spot is ablated with the depthof submicron scale [13], the modification of the irradiatedmaterial occurs at the bottom of the crater [14]. Thus, it issupposed that the modification at the bottom of the crater isthe origin of the electrical conductivity. The electrical conductivity monotonically increased as the temperature increased from 80 to 400 K. This means that the increase in
the electrical conductivity was induced by the carriers,
which were excited by phonons. This results indicate thatthe laser-modified SiC has a semiconducting property. Inaddition, the electrical conductivity increased with increasing irradiation fluence.
To discuss the physics of the carrier conduction, we estimated the activation energies of each level in the modified SiC. In order to analyze these data, we employed theArrhenius conduction model. The data was fitted according

2. Experimental
We used semi-insulating 6H-SiC single crystal substrates (SiXON, Japan) with a specific resistance of ρ > 105
Ωcm. We conducted three-fold phosphorus (P) ion implantation to fabricate ohmic contact on the semi-insulating SiC.
The averaged phosphorous density beyond the depth ofapproximately 150 nm beneath the surface was approximately 5 × 1019 cm-3. For the metal contacts, aluminum(Al) was evaporated on the ion-implanted area in the vacuum chamber. The distance between the ion-implanted metal
contacts was approximately 1.2 mm. We used a Ti:sapphire
regenerative amplifier (Spitfire, Spectra Physics, USA) forthe light source. The wavelength was 800 nm and the pulse
duration, which was evaluated by a single-shot autocorrelator, was 130 fs. The pulse laser was operated at a repetitionrate of 1 kHz. To observe the irradiation on the SiC surface,
we used an inverted optical microscope (IX-70, Olympus,Japan) and a 10X objective lens with a numerical apertureof approximately 0.40. The radius of the beam focused on
the sample surface was 2.1 µm.
During the laser irradiation, the sample was constantly
translated at a rate of 100 µm/s by using a mechanical motorized stage (KS701-20LHD, Suruga Seiki, Japan). The
irradiation was performed between the Al contacts, and theelectric field of the incident laser beam was parallel to thescanning direction. The laser fluences were varied from 2.9
to 9.4 J/cm2 and the current-voltage (I-V) characteristics on
the laser-modified lines were measured. The applied voltage was from −100 to +100 mV with 10-mV resolution.
The temperature of the sample was reduced to 77 K using
liquid nitrogen, and the sample temperature was controlled

Fig. 1

Current-voltage (I-V) characteristics of femtosecond-modified region with irradiation fluence of
9.4 J/cm2. The temperature was varied from 80 to400 K. Current-voltage (I-V) characteristics offemtosecond-modified region with irradiation fluence of 9.4 J/cm2. The temperature was varied from80 to 400 K.

Fig. 2 Temperature dependence of specific resistivity of femtosecond-laser-modified region for various irradiation conditions. The fitting results obtained by assuming two defect levels are indicated by broken lines.

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JLMN-Journal of Laser Micro/Nanoengineering Vol. 12, No. 2, 2017

toσ = σ 1e−

E D1k BT+ σ 2e−

ED 2k BT[14]. The threshold fluence for the formation of the amorphous silicon carbide was approximately 0.5 J/cm2, andthat for the amorphous silicon and carbon was approximately 1.3 J/cm2. Although these values depend on the irradiation conditions, their order is roughly constant. Thus,
the fluence for which the significant electrical conductivity
occurs is far above the threshold fluence for the formationof amorphous silicon carbide, silicon, and carbon. Therefore, the formation of the amorphous phase material is notdirectly related to the change in the electrical conductivity.
Instead, we assume that piling and coalescence of thesematerials constitute the origin of the electrical conductivity.
If we assume that the average distance between each defectsite is 10 nm, the corresponding defect density is n = 1018
(1/cm3). On the other hand, n= 1015 (1/cm3) for a distanceof 100 nm. It is supposed that significant interaction orcoalescence between each site emerges for average distances ranging from 10 to 100 nm. It is also probable that theelectric conductivity significantly increases for carrier densities ranging from 1018 to 1015 (1/cm3). This explains why
the activation energies of E1 and E2 change for higher fluence.

(1)Here, σ is the electrical conductivity, ED1 and ED2 are theactivation energies of the defect levels, respectively, σ1 andσ2 are the electrical conductivities of each of these levels,respectively, kB is Boltzmann’s constant, and T is the sample temperature. It should be noted that the values of σ1 andσ2 are directly related to the impurity population. In thefitting procedure, the values of ED1, ED2, σ1, and σ2 wereused as the fitting constants. In Fig. 2, the fit result for eachirradiation fluence is indicated by a broken line. For all theexamined temperatures, the experimental results were well
fit by assuming that the constants σ1 and σ2 were non-zerocomponents, indicating that the two defect levels exist in
the laser-modified areas. In general, electrical conductionin amorphous semiconductors can be classified into threemechanisms: the conduction in the extended states, in theband tail state, and in the localized states at the Fermi level[15]. As regards the Arrhenius conduction, the carriers arethermally excited in the extended state. Therefore it is also
indicated that the temperature dependence of the femtosecond-laser-modified region exhibited Arrhenius behavior[16].
From the fitting results, the values of ED1 and ED2 weresuccessfully obtained for each irradiation fluence. Thesevalues are plotted in Fig. 3 using open symbols (left ordinate). The values of σ1 and σ2 are also plotted in this figureusing by closed symbols (right ordinate). The values of E1and E2 are almost constant at approximately 2 and 200 meV,
respectively, below 8 J/cm2. Above 8 J/cm2, the values ofE1 and E2 increase and decreases, respectively. Simultaneously, the values of σ1 and σ2 increase for fluence of morethan 8 J/cm2. The growth in the values of σ1 and σ2 indicates that the number of thermally excited carriers is increased, i.e. that the number of defect levels is increased.
Previously we reported the results of Raman spectroscopy and TEM observation for femtosecond-laser-modified
SiC, indicating the formation of a phase consisting ofamorphous silicon and amorphous carbon after laser irradiation. In addition, we observed an amorphous silicon carbide phase obtained under higher irradiation conditions

4. Conclusion
The temperature dependence of a femtosecond-lasermodified region on SiC was measured in this study. The I-Vcharacteristics showed ohmic properties. The temperaturedependence of the specific resistance was well fit by double exponential curves. The irradiation fluence dependenceof the trapping energy was also obtained. These defect levels were related to two phases, both of which form following femtosecond laser irradiation. One phase consists ofamorphous silicon carbide, whereas the other consists ofamorphous silicon and amorphous carbon. The effect of theinteractions between each site was also observed for higher
irradiation conditions.
Acknowledgments
This work was partially supported by a JSPS Grant-inAid for Challenging Exploratory Research (26630132), a
Grant-in-Aid for Scientific Research (B) (25286086), and a
Grant-in-Aid for Scientific Research on Innovative Areas(24110712). This work was also partially supported by theNippon Sheet Glass Foundation (NSG Foundation) for Materials Science and Engineering and the AMADA foundation.
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(Received: April 3, 2017, Accepted: June 3, 2017)75