Depth Profile Analysis of Deep Level Defects in 4H-Sic

Article Depth Proﬁle Analysis of Deep Level Defects in 4H-SiC Introduced by Radiation

Tomislav Brodar 1,*, Luka Bakraˇc 1, Ivana Capan 1 , Takeshi Ohshima 2 , Luka Snoj 3, Vladimir Radulovi´c 3 and Željko Pastuovi´c 4

1 Ruder¯ Boškovi´cInstitute, Bijeniˇcka54, 10000 Zagreb, Croatia; [email protected] (L.B.); [email protected] (I.C.) 2 Takasaki Advanced Radiation Research Institute, National Institutes for Quantum and Radiological Science and Technology, 1233 Watanuki, Takasaki, Gunma 370-1292, Japan; [email protected] 3 Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia; [email protected] (L.S.); [email protected] (V.R.) 4 Australian Nuclear Science and Technology Organisation, 1New Illawarra Rd, Lucas Heights, NSW 2234, Australia; [email protected] * Correspondence: [email protected]

Received: 1 September 2020; Accepted: 19 September 2020; Published: 22 September 2020

Abstract: Deep level defects created by implantation of light-helium and medium heavy carbon ions in the single ion regime and neutron irradiation in n-type 4H-SiC are characterized by the DLTS technique. Two deep levels with energies 0.4 eV (EH1) and 0.7 eV (EH3) below the conduction band minimum are created in either ion implanted and neutron irradiated material beside carbon vacancies (Z1/2). In our study, we analyze components of EH1 and EH3 deep levels based on their concentration depth profiles, in addition to ( 3/=) and (=/ ) transition levels of silicon vacancy. A higher EH3 − − deep level concentration compared to the EH1 deep level concentration and a slight shift of the EH3 concentration depth profile to larger depths indicate that an additional deep level contributes to the DLTS signal of the EH3 deep level, most probably the defect complex involving interstitials. We report on the introduction of metastable M-center by light/medium heavy ion implantation and neutron irradiation, previously reported in cases of proton and electron irradiation. Contribution of M-center to the EH1 concentration profile is presented.

Keywords: silicon carbide; defects; ion implantation; DLTS; neutron radiation

1. Introduction Silicon carbide (SiC) is a wide band gap semiconductor suitable for high temperature, high-frequency and high-power applications [1,2]. The 4H polytype of SiC is preferred as a material for electronic components due to the high and isotropic mobility of carriers. 4H-SiC was demonstrated as a promising material for atomic-scale spintronics and quantum applications [3–5]. Qubits and single-photon emitters are realized by defect engineering. A long spin-coherence time at room temperature was observed for isolated single-point defects in ion-implanted SiC such as silicon vacancy (VSi), di-vacancy, and a carbon anti-site defect [6–8]. Defect engineering is also used for local carrier lifetime control necessary for the optimization of the switching loss of 4H-SiC power bipolar junction transistors [9] and elimination of the bipolar degradation [10]. In recent years, 4H-SiC radiation detectors have attracted attention due to their high radiation hardness and high signal to noise ratio [11–13]. Electrically active defects induced by irradiation negatively aﬀect the performance of 4H-SiC devices during their working lifetime. The characterization of irradiation introduced defects is crucial for future improvement of radiation hardness and extending the lifetime of 4H-SiC detectors.

Crystals 2020, 10, 845; doi:10.3390/cryst10090845 www.mdpi.com/journal/crystals Crystals 2020, 10, 845 2 of 16

The carbon vacancy VC is a dominant recombination center in 4H-SiC affecting the minority carrier lifetime. Therefore, the control of the minority carrier lifetime can be realized by controlling the carbon vacancy concentration [14–16]. The carbon vacancy concentration can be increased by using low-energy electron irradiation, which displaces only C atoms in the 4H-SiC material [17], or ion implantation [15]. On the other hand, thermal oxidation [18] or C ion implantation with subsequent annealing [19–21] can be used to introduce carbon interstitials into 4H-SiC, which recombine with carbon vacancies and reduce the carbon vacancy concentration. Acceptor levels of the carbon vacancy exhibit negative-U ordering [22]. Two components of the Z1/2 center were recently resolved by Laplace deep level transient spectroscopy (Laplace DLTS) and assigned to the (=/0) acceptor level of carbon vacancies residing on lattice sites with local cubic and hexagonal symmetry [23,24]. Two deep levels with energies around EC—0.4 eV and EC—0.7 eV (usually labeled as S1 and S2 or EH1 and EH3, respectively [25–31]) can be observed in n-type 4H-SiC after irradiation with neutrons or electrons and ion implantation. Their simultaneous appearance and correlation of their concentrations indicate that they correspond to different charge state transitions of the same defect [27]. EH1 and EH3 deep levels introduced by ion implantation have been recently assigned to V ( 3/=) and V (=/ ) Si − Si − charge transitions of the silicon vacancy [32,33], respectively. Furthermore, a metastable defect labeled as the M-center was previously observed in 4H-SiC after proton [34–36] and electron [37,38] irradiation. Two deep levels M1 and M3 with activation energies of 0.42 and 0.75 eV, respectively, are observed when M-center is in configuration A, while only one deep level M2 with an activation energy of 0.7 eV is detected when M-center is in configuration B [34]. Transformation of the M-center to configuration A occurs at room temperature under applied bias, while transformation to B configuration occurs at 450 K without bias. Low-energy electron irradiation (<220 keV), which displaces only carbon atoms in the 4H-SiC crystal, introduces two deep levels with activation energies 0.45 eV and 0.72 eV [39]. According to a recent study [39], they are two different transitions of the same defect related to carbon interstitials, labeled as the EH-center. The activation energy for annealing of EH-center (1.1 eV [39]) is different from the activation energy of S-center (1.8 eV [27]) and M-center (2.0 eV [37]), which indicates different defect species. In this paper, we investigate deep level defects in n-type 4H-SiC introduced by ion implantation and neutron radiation by the DLTS and Laplace DLTS techniques. The depth profiles of the introduced deep levels are comprehensively studied, in respect to their attributions and their effect on free carrier concentration. Inspection of the depth profiles revealed an interstitially related deep level defect, which contributes to the DLTS signal at the temperature of the EH3 deep level. Furthermore, transitions between two different configurations of the M-center and its presence in ion implanted and irradiated 4H-SiC samples are observed.

2. Materials and Methods n-type Schottky barrier diodes (SBDs) were produced on nitrogen-doped (up to 4.5 1014 cm 3) × − 4H-SiC epitaxial layers, approximately 25 µm thick [40]. The epitaxial layer was grown on the silicon face (8◦ off) of 350 µm thick silicon carbide substrate without the buffer layer. The Schottky barrier was formed by thermal evaporation of nickel through a metal mask with patterned square apertures of 1 mm 1 mm, while Ohmic contacts were formed on the backside of the silicon carbide substrate by × nickel sintering at 950 ◦C in the Ar atmosphere. 8 9 2 The SBDs were pattern-implanted with 7.5 MeV C and 2 MeV He ions with fluences of 10 –10 cm− 9 10 2 and 10 –10 cm− , respectively. All implantations were performed through the front nickel Schottky contact, at room temperature and zero applied bias. The selected ion energies provide similar ion depth ranges, which are accessible by the DLTS technique. The pattern-implantation of ions was performed at the ANSTO (Australian Nuclear Science and Technology Organisation) nuclear microprobe facility [41] as described in the previous study [30]. An area of a Schottky contact (1 mm 1 mm) was divided × into 512 pixels 512 pixels. An ion microbeam raster-scanned multiple times over the contact area, × implanting one or two ions at each pixel location before moving to the next location. The specified Crystals 2020, 10, 845 3 of 16

fluences were calculated as the number of implanted ions per area of Schottky contact. The fluences were specified for each SBD used in this study. The primary displacements created in collision cascades were spatially well separated, and their interaction with defects created in subsequent cascades was negligible. The trajectory of incident ions was normal to the surface of the Schottky contact, not aligned with the crystal axis. Ion depth ranges and total vacancy concentration profiles were calculated using the SRIM (The Stopping and Range of Ions in Matter) code [42]. The SBDs were irradiated with epithermal and fast neutrons at the Jozef Stefan Institute (JSI) TRIGA reactor in Ljubljana, Slovenia. Thermal neutrons with energy less than 0.55 eV were filtered Crystalsby irradiating 2020, 10, x FOR the PEER Schottky REVIEW barrier diodes inside a cadmium box with a wall thickness of 13 of mm. 17 The neutron energy spectrum and irradiation settings are the same as in the previously reported Schottkystudy [43 contact.]. Figure The1 displays fluences the were neutron specified spectra fo inr each the pneumatic SBD used tube in this (PT) study. irradiation The channel,primary displacementslocated in the created F-24 core in position,collision cascades obtained were by Monte spatially Carlo well simulation separated, using and their the MCNPinteraction code with [44] defectsand subsequently created in subsequent characterized cascades on the was basis negligible. of experimental The trajectory neutron of activation incident ions measurements was normal [ 45to]. theElastic surface collisions of the of Schottky incident contact, neutrons not with aligned atoms wi ofth SiC the crystal crystal result axis. in Ion the depth introduction ranges ofand intrinsic total vacancy concentration profiles were calculated12 using2 the SRIM13 (The2 Stopping and Range of Ions in defects. The selected neutron fluences were 10 cm− and 10 cm− . Matter) code [42].

101 Bare Cd-covered (1 mm)

100 Neutron flux per unit lethargy unit per flux Neutron

10-1 10-3 10-2 10-1 100 101 102 103 104 105 106 107 Energy (eV)

FigureFigure 1. 1. NeutronNeutron spectraspectra in in the the Jozef Jozef Stefan Stefan Institute Institute (JSI) (JSI) TRIGA TRIGA pneumatic pneumatic tube (PT) tube irradiation (PT) irradiation channel, channel,corresponding corresponding to bare and to Cd-coveredbare and Cd-covered irradiations, irradiations, obtained by meansobtained of Monteby means Carlo of simulations Monte Carlo and simulationssubsequently and characterized subsequently on the characterized basis of experimental on the neutronbasis of activation experimental measurements. neutron activation measurements. The averaged primary displacement (vacancy) generation rate (VGR) distributions, created in collision cascades of the 2.0 MeV He and 7.5 MeV C ions being implanted in the 4H-SiC epitaxial The SBDs were irradiated with epithermal and fast neutrons at the Jozef Stefan Institute (JSI) layer at normal incident not aligned with the crystal axis, was simulated for a total of 10,000 ions TRIGA reactor in Ljubljana, Slovenia. Thermal neutrons with energy less than 0.55 eV were filtered using the SRIM code [42]. As shown in Figure2, the VGR distribution gradually increased from the by irradiating the Schottky barrier diodes inside a cadmium box with a wall thickness of 1 mm. The surface of the epitaxial layer underneath the Ni contact layer and reached a sharp maximum in the neutron energy spectrum and irradiation settings are the same as in the previously reported study 4–5 µm depth range, close to the end of an ion range. The average VGR value per 2.0 MeV He ion was [43]. Figure 1 displays the neutron spectra in the pneumatic tube (PT) irradiation channel, located in 2.5 10 3 Å 1. The maximum VGR value for C vacancy was 1.6 10 2 Å 1 and for Si vacancy was the ×F-24− core− position, obtained by Monte Carlo simulation using× − the− MCNP code [44] and 1.2 10 2 Å 1 at a depth equal to 4.7 µm. The difference in silicon and carbon vacancy density is a subsequently× − − characterized on the basis of experimental neutron activation measurements [45]. result of the differences in the displacement threshold values for Si (35 eV) and C atoms (22 eV) in Elastic collisions of incident neutrons with atoms of SiC crystal result in the introduction of intrinsic 4H-SiC material [46]. The average VGR value for a single 7.5 MeV C ion was 1.4 10 2 Å 1, while the defects. The selected neutron fluences were 1012 cm−2 and 1013 cm−2. × − − maximum VGR value for C vacancy was 8.7 10 2 Å 1 and for Si vacancy was 6.4 10 2 Å 1 at a The averaged primary displacement (vacancy)× − generation− rate (VGR) distributions,× −created− in µ collisiondepth equal cascades to 4.6 ofm. the Therefore, 2.0 MeV eachHe and 7.5 MeV7.5 MeV carbon C ionions produced being implanted approximately in the five4H-SiC and halfepitaxial times layermore at silicon normal or carbonincident vacancies not aligned than with each the 2.0 crys MeVtal helium axis, was ion (alphasimulated particle) for a used total in of this 10,000 study. ions usingThe the displacementSRIM code [42] damage. As shown introduced in Figure by 2, neutron the VGR irradiation distribution in thegradually 4H-SiC increased epitaxial layerfrom wasthe surfacesimulated of the by epitaxial Fluka software layer underneath [47]. The neutronthe Ni contact spectrum layer shown and reached in Figure a sharp1 (blue maximum line) was in used the to select the neutron energies used in the simulation (627 values in the range 2 10 4–2 107 eV) 4–5 µm depth range, close to the end of an ion range. The average VGR value per× 2.0− MeV× He ion was 2.5 × 10−3 Å−1. The maximum VGR value for C vacancy was 1.6 × 10−2 Å−1 and for Si vacancy was 1.2 × 10−2 Å−1 at a depth equal to 4.7 µm. The difference in silicon and carbon vacancy density is a result of the differences in the displacement threshold values for Si (35 eV) and C atoms (22 eV) in 4H-SiC material [46]. The average VGR value for a single 7.5 MeV C ion was 1.4 × 10−2 Å−1, while the maximum VGR value for C vacancy was 8.7 × 10−2 Å−1 and for Si vacancy was 6.4 × 10−2 Å−1 at a depth equal to 4.6 µm. Therefore, each 7.5 MeV carbon ion produced approximately five and half times more silicon or carbon vacancies than each 2.0 MeV helium ion (alpha particle) used in this study. The displacement damage introduced by neutron irradiation in the 4H-SiC epitaxial layer was simulated by Fluka software [47]. The neutron spectrum shown in Figure 1 (blue line) was used to select the neutron energies used in the simulation (627 values in the range 2 × 10−4–2 × 107 eV) and to calculate the average displacement generation rate. A total of 106 incident neutrons were simulated

Crystals 2020, 10, x FOR PEER REVIEW 4 of 17

for each energy. The calculated displacement profile was used as the estimate of the vacancy concentration profile. The temperature-dependent current–voltage (I–V) and capacitance–voltage (C–V) characteristics of SBDs were measured using a Keithley 4200 SCS (Keithley Instruments, Cleveland, OH, USA). Radiation induced electrically active defects were characterized by the DLTS technique. DLTS measurements were carried out in the temperature range from 100 to 380 K. The SBDs were cooled down from room temperature without an applied bias before DLTS measurements. Subsequently, low temperature annealing at a temperature of 450 K for 30 min and a series of DLTS measurements in the temperature range up to 450 K were carried out. Before DLTS measurements, the SBDs were cooled down from 340 K with applied bias (−30 V) or 450 K without applied bias to observe A and B configurations of the M-center, respectively. The temperature ramp rate was 2 CrystalsK/min.2020 Concentration, 10, 845 profiles of deep level defects were determined from their DLTS amplitude,4 of 16 where different depth ranges were probed by increasing reverse bias VR in a step of 0.5 V and keeping constant the difference between pulse bias and reverse bias VP − VR = 1 V. The lambda effect 6 and[48,49] to calculatewas taken the into average account. displacement Capacitance generation transients rate. were A total measured of 10 incidentusing a neutronsBoonton were7200 simulatedcapacitance for meter each energy.and the The following calculated acquisition displacement settings: profile number was used of samples as the estimate 3 × 104, ofsampling the vacancy rate concentration10–80 kHz, and profile. number of averaged scans 100–800. The FLOG numerical routine [50] was used for the calculationThe temperature-dependent of Laplace DLTS spectra. current–voltage (I–V) and capacitance–voltage (C–V) characteristics of SBDs were measured using a Keithley 4200 SCS (Keithley Instruments, Cleveland, OH, USA). Radiation3. Results and induced Discussion electrically active defects were characterized by the DLTS technique. DLTS measurements were carried out in the temperature range from 100 to 380 K. The SBDs were3.1. Depth cooled Profiling down of from EH1 roomand EH3 temperature Deep Level withoutDefects an applied bias before DLTS measurements. Subsequently, low temperature annealing at a temperature of 450 K for 30 min and a series of DLTS The quality of the prepared SBDs was checked by temperature dependent current–voltage measurements in the temperature range up to 450 K were carried out. Before DLTS measurements, (I–V–T) and capacitance–voltage (C–V–T) measurements and they showed excellent rectifying the SBDs were cooled down from 340 K with applied bias ( 30 V) or 450 K without applied bias characteristics (see Figure S1). − to observe A and B configurations of the M-center, respectively. The temperature ramp rate was Figure 2 shows the effect of ion implantation and neutron irradiation on the free carrier 2 K/min. Concentration profiles of deep level defects were determined from their DLTS amplitude, concentration profile of 4H-SiC SBDs. Introduced defects caused a reduction of free carrier where different depth ranges were probed by increasing reverse bias V in a step of 0.5 V and keeping concentration as the free carriers were captured by the introduced acceptorR deep levels. In as-grown constant the difference between pulse bias and reverse bias V V = 1 V. The lambda effect [48,49] and neutron irradiated SBDs, a homogenous free carrier concentrationP − R was observed. The free was taken into account. Capacitance transients were measured using a Boonton 7200 capacitance carrier concentration in 7.5 MeV C ion implanted SBDs had a minimum at a depth of 4.8–5.0 µm meter and the following acquisition settings: number of samples 3 104, sampling rate 10–80 kHz, close to the calculated ion depth range at 4.55 µm. Likewise, in 2 MeV× He ion implanted SBDs the and number of averaged scans 100–800. The FLOG numerical routine [50] was used for the calculation minimum of the free carrier concentration and ion depth range were at 4.5–4.9 µm and 4.67 µm, of Laplace DLTS spectra. respectively.

Figure 2. Free-carrier concentration profiles of as-grown, ion implanted, and neutron irradiated n-type 4H-SiCFigure Schottky2. Free-carrier barrier concentration diodes (SBDs; solidprofiles lines, of leftas-gro y axis)wn, andion theimplanted, calculated and vacancy neutron concentration irradiated profilesn-type (dashed4H-SiC lines,Schottky right barrier y axis). diodes The concentration (SBDs; solid profiles lines, areleft obtainedy axis) and from the the calculated C–V characteristics vacancy measuredconcentration at 200 profiles K [51]. (dashed lines, right y axis). The concentration profiles are obtained from the C–V characteristics measured at 200 K [51]. 3. Results and Discussion

3.1. Depth Proﬁling of EH1 and EH3 Deep Level Defects

The quality of the prepared SBDs was checked by temperature dependent current–voltage (I–V–T) and capacitance–voltage (C–V–T) measurements and they showed excellent rectifying characteristics (see Figure S1). Figure2 shows the e ﬀect of ion implantation and neutron irradiation on the free carrier concentration proﬁle of 4H-SiC SBDs. Introduced defects caused a reduction of free carrier concentration as the free carriers were captured by the introduced acceptor deep levels. In as-grown and neutron irradiated SBDs, a homogenous free carrier concentration was observed. The free carrier concentration in 7.5 MeV C ion implanted SBDs had a minimum at a depth of 4.8–5.0 µm close to the calculated ion depth range at 4.55 µm. Likewise, in 2 MeV He ion implanted SBDs the minimum of the free carrier concentration and ion depth range were at 4.5–4.9 µm and 4.67 µm, respectively. Crystals 2020, 10, 845 5 of 16

Crystals 2020, 10, x FOR PEER REVIEW 5 of 17 DLTS spectra of as-grown, ion implanted, and neutron irradiated 4H-SiC SBDs are shown in DLTS spectra of as-grown, ion implanted, and neutron irradiated 4H-SiC SBDs are shown in Figure3. Only one asymmetric peak, labeled as Z 1/2, was present in the DLTS spectrum for as-grown Figure 3. Only one asymmetric peak, labeled as Z1/2, was present in the DLTS spectrum for as-grown 4H-SiC. The Z1/2 is a well-known deep level and previously assigned to a transition between double 4H-SiC. The Z1/2 is a well-known deep level and previously assigned to a transition between double negative and neutral charge state of carbon vacancy VC (=/0) [52]. As recently reported [23,24], negative and neutral charge state of carbon vacancy VC (=/0) [52]. As recently reported [23,24], two two emission lines Z1 (=/0) and Z2 (=/0) are resolved by the Laplace DLTS technique and assigned to emission lines Z1 (=/0) and Z2 (=/0) are resolved by the Laplace DLTS technique and assigned to carbon vacancies residing on two diﬀerent lattice sites with local cubic and hexagonal symmetry. carbon vacancies residing on two different lattice sites with local cubic and hexagonal symmetry.

Figure 3. DLTS spectra of as-grown, C and He ion implanted, and neutron irradiated SBDs. The voltage Figure 3. DLTS spectra of as-grown, C and He ion implanted, and neutron irradiated SBDs. The settings for the ion implanted SBDs (b,c) reverse bias VR = 3 V and pulse bias VP = 0.1 V are chosen voltage settings for the ion implanted SBDs (b,c) reverse bias− VR = −3 V and pulse bias− VP = −0.1 V are to probe the depth region below the ion depth range. In the case of as-grown and neutron irradiated chosen to probe the depth region below the ion depth range. In the case of as-grown and neutron SBDs (a,d), the reverse bias and pulse bias are VR = 10 V and VP = 0.1 V. The pulse width is 10 ms. irradiated SBDs (a,d), the reverse bias and pulse bias− are VR = −10 V and− VP = −0.1 V. The pulse width The emission rate is 50 1/s. is 10 ms. The emission rate is 50 1/s.

The concentration of Z1/2 and introduced two additional deep levels, labeled as EH1 and EH3, The concentration of Z1/2 and introduced two additional deep levels, labeled as EH1 and EH3, was increased by ion implantation and neutron irradiation. Activation energies and apparent electron was increased by ion implantation and neutron irradiation. Activation energies and apparent captured cross-sections of observed deep levels are listed in Table1. A fact that the same types of electron captured cross-sections of observed deep levels are listed in Table 1. A fact that the same deep levels were produced by implantation of two diﬀerent types of ions (light and medium heavy) types of deep levels were produced by implantation of two different types of ions (light and medium and neutrons clearly indicate that they were intrinsic defects [53]. The energy of impinging ions and heavy) and neutrons clearly indicate that they were intrinsic defects [53]. The energy of impinging neutrons was high enough to displace silicon atoms in the 4H-SiC crystal lattice. The observed EH1 ions and neutrons was high enough to displace silicon atoms in the 4H-SiC crystal lattice. The and EH3 deep levels were recently assigned to VSi ( 3/=) and VSi (=/ ) transitions of silicon vacancy observed EH1 and EH3 deep levels were recently −assigned to VSi (−−3/=) and VSi (=/−) transitions of VSi [32], respectively. Two emission lines for the EH1 deep level defect were recently resolved by the silicon vacancy VSi [32], respectively. Two emission lines for the EH1 deep level defect were recently resolved by the Laplace DLTS technique [31,32], and assigned to VSi residing on two different sites in the crystal with local cubic and hexagonal symmetry (Figure 4).

Crystals 2020, 10, 845 6 of 16

Laplace DLTS technique [31,32], and assigned to VSi residing on two diﬀerent sites in the crystal with localCrystals cubic 2020, and10, x hexagonalFOR PEER REVIEW symmetry (Figure4). 6 of 17

Table 1. Activation energies Ea and apparent electron capture cross sections σ determined from Table 1. Activation energies Ea and apparent electron capture cross sections σ determined from DLTSDLTS measurementsmeasurements of of ion ion implanted implanted and neutronand neutro irradiatedn irradiated SBDs. StandardSBDs. Standard deviations deviations of activation of energiesactivation are energies included. are Capture included. cross Capture sections cross are within sections an order are within of magnitude an order of theof magnitude calculated values. of the calculated values. 8 2 9 2 13 2 Deep Level 7.5 MeV C (10 cm− ) 2 MeV He (10 cm− ) Epithermal and Fast Neutrons (10 cm− ) 8 2−2 9 −22 132 −2 Deep Level 7.5Ea (eV)MeV C (10σ (cm cm )) 2 MeVEa (eV) He (10σ cm(cm) ) EpithermalEa (eV) and Fast Neutronsσ (10(cm )cm ) 2 2 2 Ea (eV) σ (cm15) Ea (eV) σ (cm ) 15 Ea (eV) σ (cm 14) EH1 0.41 0.02 3 10− 0.42 0.01 4 10− 0.43 0.01 1 10− EH1 0.41± ± 0.02 3× × 10−1515 0.42 ± 0.01 4 ×× 10−1514 0.43 ±± 0.01 1 × 10−1415 Z1/2 0.65 0.01 4 10− 0.68 0.01 1 10− 0.65 0.02 3 10− Z1/2 0.65± ± 0.01 4× × 10−1515 0.68 ± 0.01 1 ×× 10−1415 0.65 ±± 0.02 3 × 10−1515 EH3 0.70 0.04 1 10− 0.71 0.04 2 10− 0.71 0.03 1 10− EH3 0.70± ± 0.04 1× × 10−15 0.71 ± 0.04 2 ×× 10−15 0.71 ±± 0.03 1 × 10−15

(a) (b) EC VSi(-3/=) (h) EH1 VSi(-3/=) (k) VSi VSi(=/-) EH3

Figure 4. Schematic illustration of (a) silicon vacancy and (b) energy positions of the attributed deepFigure levels. 4. Schematic illustration of (a) silicon vacancy and (b) energy positions of the attributed deep levels. 8 2 Figure5a shows the DLTS spectra of 7.5 MeV C implanted (10 cm− ) SBD with different bias settingsFigure for probing5a shows deep the levelsDLTS inspectra the adjacent of 7.5 MeV depth C ranges.implanted The (10 EH18 cm and−2) SBD EH3 with defects different located bias in thesettings depth for regions probing illustrated deep levels by in hatched the adjacent and solid depth rectangles ranges. The in FigureEH1 and5b EH3 contribute defects to located their DLTS in the signalsdepth regions at 210 K andillustrated 350 K shownby hatched in Figure and5a. solid Defects rectangles in di fferent in Fi probedgure 5b depth contribute regions wereto their mutually DLTS comparedsignals at in210 regard K and with 350 theK shown measured in Figure free carrier 5a. Defects concentration in different (black probed line), depth which regions exhibited were a minimummutually duecompared to carrier in captureregard inwith the introducedthe measured acceptor free carrier levels. Weconcentration assumed that (black the concentration line), which ofexhibited the introduced a minimum acceptor due levelsto carrier is the capture largest in approximately the introduced at acceptor the position levels. of We the minimumassumed that in free the carrierconcentration concentration. of the introduced The amplitudes acceptor of peaks levels assigned is the tolargest EH1 andapproximately EH3 defects at are the roughly position the of same the inminimum the DLTS in spectrum free carrier obtained concentration. probing theThe depth amplitudes region of closest peaks to assigned the surface to (redEH1 andand blueEH3 curves), defects whichare roughly is in agreement the same within the reports DLTS ofspectrum other authors obtained [27, 32probing]. The peakthe depth assigned region to theclosest EH3 to defect the surface shows a(red rapid and increase, blue curves), while which the peak is in assigned agreement to the with EH1 reports defect of disappeared other authors in the[27,32]. region The coinciding peak assigned with theto the minimum EH3 defect in free shows carrier a rapid concentration increase, while (green the curve peak in assigned Figure5a). to Thethe EH1 peak defect assigned disappeared to the EH3 in defectthe region had thecoinciding largest amplitudewith the minimum in the DLTS in free spectrum, carrier concentration which was showing (green deepcurve levels in Figure in the 5a). depth The rangepeak assigned beyond the to minimumthe EH3 defect of free carrierhad the concentration, largest amplitude while thein the peak DLTS assigned spectrum, to the EH1which defect was wasshowing not observed deep levels in that in the region depth (brown range curvebeyond in Figurethe minimum5a). DLTS of spectrumfree carrier probing concentration, the depth while region the peak assigned to the EH1 defect was not observed in that region (brown curve in Figure 5a). DLTS farthest from the ion implanted region (gray curve) contained only one small peak, the Z1/2 defect, spectrum probing the depth region farthest from the ion implanted region (gray curve)9 contained2 which was present in as-grown 4H-SiC. DLTS measurements on 2 MeV He implanted (10 cm− ) SBD only one small peak, the Z1/2 defect, which was present in as-grown 4H-SiC. DLTS measurements on show that concentrations of EH1, Z1/2, and EH3 deep levels varied with depth in the same way as in 2 MeV He implanted (109 cm−2) SBD8 show2 that concentrations of EH1, Z1/2, and EH3 deep levels the case of 7.5 MeV C implanted (10 cm− ) SBD (see Figure S2). variedThe with measured depth in concentration the same way profilesas in the for case EH1 of 7.5 and MeV EH3 C implanted deep levels (10 are8 cm shown−2) SBD in (see Figure Figure6. Lambda-correctedS2). depth concentration profiles were estimated from DLTS signal amplitude ∆C using the followingThe measured equation concentration [49]: profiles for EH1 and EH3 deep levels are shown in Figure 6. Δ Lambda-corrected depth concentration profiles2 NwereSCR( WestimatedR) ∆ fromC DLTS signal amplitude C N (x) = (1) T 2 2 using the following equation [49]: λR WP λP CR 1 − WR WR −2()NW− Δ = SCR R C NxT () 22 λλW − CR (1) 1−−RPP WWRR

Crystals 2020, 10, x FOR PEER REVIEW 7 of 17 Crystals 2020, 10, 845 7 of 16 λ λ where CR , WR (WP ), and R ( P ) are steady state capacitance, space charge region width, and wherelambdaC lengthR, WR (atW reverseP), and λbiasR (λ VP)R are(pulse steady biasstate VP), respectively. capacitance, spaceVoltage charge settings region were width, selected and to lambda probe length at reverse bias VR (pulse bias VP), respectively. Voltage=−+− settings()() wereλλ selected to probe deep deep levels in narrow depth range, whose midpoint is at xWRR W PP/2. The levels in narrow depth range, whose midpoint is at x = [(WR λR) + (WP λP)]/2. The charge charge concentration NW() at the edge of the space charge− depletion −region [48], W , and concentration NSCR(WR)SCRat the R edge of the space charge depletion region [48], WR, and WPR were determinedWP were determined from the C–V from characteristics the C–V characteristic at the measurements at the measurement temperature. temperature. The lambda lengthsThe lambda were lengthscalculated were using calculated the free using carrier the concentration free carrier conc proﬁleentration measured profile at the measured temperature at the of temperature 200 K (as seen of 200in Figure K (as2 ),seen which in Figure corresponds 2), which to the corresponds net donor concentrationto the net donor [ 51 ],concentration and known activation[51], and known energy (Tableactivation1). energy (Table 1).

8 2 Figure 5. (a) DLTS spectra of 7.5 MeV C (10 cm− ) ion implanted n-type 4H-SiC SBD. Different voltage Figure 5. (a) DLTS spectra of 7.5 MeV C (108 cm−2) ion implanted n-type 4H-SiC SBD. Different settings, reverse bias VR and pulse bias VP, are used to probe different depth ranges (as seen in (b)). voltage settings, reverse bias VR and pulse bias V1P, are used to probe different depth ranges (as seen The rate window and pulse width tP are 50 s− and 10 ms. The spectra are shifted vertically for in (b)). The rate window and pulse width tP are 50 s−1 and 10 ms. The spectra are shifted vertically for clarity. (b) Schematic illustration of the depth regions contributing to the DLTS signal of EH1 at 210 K (hatchedclarity. (b rectangles)) Schematic and illustration EH3 at 350 of Kthe (solid depth rectangles) regions cont forributing the used to voltage the DLTS in the signal DLTS of measurements EH1 at 210 K (as(hatched seen in rectangles) (a)). The lambda and EH3 effect at was 350 taken K (solid intoaccount. rectangles) The depthfor the ranges used arevoltage compared in the with DLTS free measurementscarrier concentration (as seen (black in (a line))). The determined lambda effect from thewasC–V taken characteristic into account. at The 200 Kdepth (as in ranges Figure are2), whichcompared exhibits with afree minimum carrier concentration due to introduced (black acceptor line) determined deep levels. from the C–V characteristic at 200 K (as in Figure 2), which exhibits a minimum due to introduced acceptor deep levels. The estimated ratio of EH1 and EH3 deep level concentrations in ion implanted SBDs was close to 1:2 at the depths below the ion depth range (2–3 µm), while around the ion depth range the [EH1]:[EH3] ratio varied. The maximum of EH3 concentration profile was about four times larger than the maximum 8 2 9 2 of EH1 concentration in C (10 cm− ) implanted SBD, while it was two times larger in He (10 cm− ) ion implanted SBD. At the depths below the ion depth range, the shape of EH1 and EH3 concentration profiles followed the shape of the calculated concentration profile of the total introduced vacancies. As previously mentioned, EH1 and EH3 deep levels with activation energies around 0.4 eV and 0.7 eV were assigned to V ( 3/=) and V (=/ ) transitions of the silicon vacancy. In those transitions, Si − Si − the emission of a single electron from the same defect occurred. Therefore, we should observe the same concentration of deep levels assigned to VSi. In previous studies [32,54], the one-to-one correlation between their concentrations enabled their assignment to the transitions of the same defect. An increased concentration of the EH3 deep level compared to EH1 in a region beyond the maximum of SRIM calculated that the distribution of atomic displacements could be explained by the contribution of deep levels other than V (=/ ) with similar activation energy to the total measured DLTS signal. Si − A slight shift of the maximum of EH3 deep level profile towards larger depths compared to the EH1 deep level profile could indicate that the EH3 deep level defect, which is not necessary VSi, contains interstitials. In their study, Pellegrino et al. [55] demonstrated that a forward momentum transfer from the impinging ions to the lattice atoms in silicon can result in displacement between maximal values of vacancy and self-interstitial depth distributions. The EH1 deep level concentration had a maximum at a depth of 4.3 0.2 µm in C (108 cm 2) ion implanted SBD, while the EH3 deep level ± − concentration had a maximum at 4.5 0.1 µm. Likewise, concentrations of EH1 and EH3 deep levels ±

Crystals 2020, 10, 845 8 of 16 in He (109 cm 2) ion implanted SBD had maximums at depths of 3.9 0.2 µm and 4.3 0.1 µm, − ± ± respectively. The concentration of the Z deep level was the largest at a depth of 4.4 0.1 µm in the C 1/2 ± (108 cm 2) ion implanted SBD and at a depth of 3.8 0.1 µm in the He (109 cm 2) ion implanted SBD, − ± − which was close to the maximum of EH1 defect concentration proﬁle. The Z1/2 defect distribution Crystalsdepth 2020 proﬁle, 10, x agrees FOR PEER with REVIEW previous studies [15]. 8 of 17

Figure 6. Depth concentration profiles for EH1 and EH3 deep levels and total introduced vacancies Figure 6. Depth concentration profiles for EH1 and EH3 deep levels and total introduced vacancies calculated by SRIM and Fluka software [42,47]. The lambda effect was taken into account [49]. calculated by SRIM and Fluka software [42,47]. The lambda effect was taken into account [49]. Depth Depth profiling measurements were carried out at the temperatures of 210 K and 350 K. profiling measurements were carried out at the temperatures of 210 K and 350 K. In the case of neutron irradiation, a higher concentration of the EH1 deep level was observed at depthsThe below estimated 5 µm. ratio The of region EH1 and with EH3 higher deep EH1 level concentration concentrations was in insideion implanted the space SBDs charge was region close toat 1:2 reverse at the voltage depths 10below V used the duringion depth DLTS range measurements (2–3 µm), while in the around temperature the ion range depth up range to 380 the K. − [EH1]:[EH3]As previously ratio reported varied. [ 27The,56 ],maximum EH1 and of EH3 EH3 deep concentration levels can beprofile observed was about after afour suffi timesciently larger long 8 −2 thantime the annealing maximum in the of temperatureEH1 concentration range 350–400in C (10 K cm without) implanted bias, while SBD, the while applied it was bias two enhances times 9 −2 largerprocess in relatedHe (10 to cm their) ion appearance. implanted Therefore,SBD. At the EH1 depths concentration below the outside ion depth the range, space chargethe shape region of EH1was and lower EH3 as theconcentration process related profiles to its followed appearance the shap wase incomplete of the calculated during concentration low temperature profile annealing of the totalin the introduced temperature vacancies. range 350–380 K. The concentration of the peak at 350 K resulting from multiple overlappingAs previously deep mentioned, levels was much EH1 and higher EH3 than deep EH1 levels deep with level activation concentration. energies Thus,around a decrease0.4 eV and in 0.7concentration eV were assigned at greater to V depthsSi (−3/=) was and not VSi pronounced. (=/−) transitions of the silicon vacancy. In those transitions, the emission of a single electron from the same defect occurred. Therefore, we should observe the same concentration of deep levels assigned to VSi. In previous studies [32,54], the one-to-one correlation between their concentrations enabled their assignment to the transitions of the same defect. An increased concentration of the EH3 deep level compared to EH1 in a region beyond the maximum of SRIM calculated that the distribution of atomic displacements could be explained by the contribution of deep levels other than VSi (=/−) with similar activation energy to the total measured DLTS signal. A slight shift of the maximum of EH3 deep level profile towards larger depths compared to the EH1 deep level profile could indicate that the EH3 deep level defect, which is not necessary VSi, contains interstitials. In their study, Pellegrino et al. [55] demonstrated that a

Crystals 2020, 10, x FOR PEER REVIEW 9 of 17 forward momentum transfer from the impinging ions to the lattice atoms in silicon can result in displacement between maximal values of vacancy and self-interstitial depth distributions. The EH1 deep level concentration had a maximum at a depth of 4.3 ± 0.2 µm in C (108 cm−2) ion implanted SBD, while the EH3 deep level concentration had a maximum at 4.5 ± 0.1 µm. Likewise, concentrations of EH1 and EH3 deep levels in He (109 cm−2) ion implanted SBD had maximums at depths of 3.9 ± 0.2 µm and 4.3 ± 0.1 µm, respectively. The concentration of the Z1/2 deep level was the largestCrystals at2020 a ,depth10, 845 of 4.4 ± 0.1 µm in the C (108 cm−2) ion implanted SBD and at a depth of 3.8 ± 0.19 µm of 16 in the He (109 cm−2) ion implanted SBD, which was close to the maximum of EH1 defect concentration profile. The Z1/2 defect distribution depth profile agrees with previous studies [15]. Laplace DLTS spectra for EH1 and EH3 deep levels are shown in Figure7. The observed EH3 peak In the case of neutron irradiation, a higher concentration of the EH1 deep level was observed at was broad and in the vicinity of other peaks, like Z (=/0), which affected the resolution. Contrary to depths below 5 µm. The region with higher EH1 concentration2 was inside the space charge region at EH1 peak, which was recently resolved into two emission lines [31], the EH3 peak could not be resolved reverse voltage −10 V used during DLTS measurements in the temperature range up to 380 K. As by the Laplace DLTS technique. Moreover, a presence of an unresolved deep level, which is indicated previously reported [27,56], EH1 and EH3 deep levels can be observed after a sufficiently long time by the measured concentration profiles (Figure6), could lead to the broadening of the EH3 peak in the annealing in the temperature range 350–400 K without bias, while the applied bias enhances process Laplace spectra. The used FLOG numerical routine calculated the Laplace DLTS spectra with the least related to their appearance. Therefore, EH1 concentration outside the space charge region was lower possible number of peaks, which described well the measured capacitance transient, consistent with as the process related to its appearance was incomplete during low temperature annealing in the the measured data points and signal to noise ratio [50]. It is challenging to resolve a higher number of temperature range 350–380 K. The concentration of the peak at 350 K resulting from multiple deep levels with closely spaced emissions. However, the matching of the Laplace DLTS spectra (for overlapping deep levels was much higher than EH1 deep level concentration. Thus, a decrease in EH1 and EH3 deep levels) confirmed the observation of the same type of defects. concentration at greater depths was not pronounced.

. Figure 7. Laplace DLTS spectra of ion implanted and neutron irradiated SBDs at selected temperatures (a) 210 K and (b) 350 K. Pulse bias V and pulse width t are 0.1 V and 10 ms, respectively. Figure 7. Laplace DLTS spectra of ionP implanted and neutronP − irradiated SBDs at selected Reverse voltage V for neutron irradiated SBD is 10 V, while for ion implanted SBDs is 5 V. temperatures (a) 210R K and (b) 350 K. Pulse bias V−P and pulse width tP are −0.1 V and 10 −ms, respectively.The spectra areReverse shifted voltage vertically VR for for neutron clarity. irradiated SBD is −10 V, while for ion implanted SBDs is 3.2.− Metastable5 V. The spectra Defects are shifted vertically for clarity.

LaplaceIn order DLTS to obtainspectra additionalfor EH1 and information EH3 deep levels on unresolved are shown in deep Figure levels, 7. The we observed applied EH3 the peaklow-temperature was broad annealingand in the and vicinity DLTS measurementsof other peaks, in thelike temperature Z2 (=/0), which range affected up to 450 the K. The resolution. influence of low temperature annealing on EH1 and EH3 deep levels has already been reported in several studies [27,30,34–38,56]. Pastuovic et al. [30] have reported on changes of EH1 and EH3 deep level concentrations (labeled ET1 and ET2) after low temperature annealing in the temperature range up to 700 K. EH1 and EH3 deep levels completely anneal out in the temperature range from 600 to 700 K [28,30,37], accompanied by free carrier concentration recovery. DLTS measurements that occur Crystals 2020, 10, 845 10 of 16 after the initial annealing at 450 K for 30 min and measurement in the temperature range up to 450 K are stable, the unintentional annealing during the measurement does not affect DLTS spectra. As seen in Figure8a, the additional deep level labeled as EH4 was observed with an activation energy of 0.83–1.05 eV. The EH4 deep level was previously assigned to a defect complex or cluster of first-order defects [17,28,30]. Recently, a model of the broad EH4 peak was proposed consisting of three deep levels attributed to (+/0) transition of the carbon antisite-carbon vacancy complex in three energetically inequivalent configurations [57]. However, a possibility is left that other deep level Crystalsdefects, 2020 such, 10, asx FOR di-vacancy, PEER REVIEW also contribute to a broad EH4 peak. 11 of 17

8 2 9 2 Figure 8. (a) DLTS spectra of C (10 cm− ) and He (10 cm− ) ion implanted, and neutron irradiated SBD Figure 8. (a) DLTS spectra of C (108 cm−2) and He (109 cm−2) ion implanted, and neutron irradiated in the temperature range up to 450 K with A and B configuration of the M-center. DLTS spectrum with SBD in the temperature range up to 450 K with A and B configuration of the M-center. DLTS configuration B is measured after cooling down the SBD from 450 K without bias, while configuration spectrum with configuration B is measured after cooling down the SBD from 450 K without bias, A is realized by cooling down the SBD from above room temperature (340 K) with applied bias 30 V. while configuration A is realized by cooling down the SBD from above room temperature (340− K) The reverse bias VR = 3 V is used in case of ion implanted SBD, while VR = 10 V is used in case with applied bias −30 V.− The reverse bias VR = −3 V is used in case of ion implanted− SBD, while VR = of neutron irradiated SBD. The pulse bias and pulse width VP = 0.1 V and tP = 10 ms, respectively. −10 V is used in case of neutron irradiated SBD. The pulse bias and− pulse width VP = −0.1 V and tP = 10 The emission rate is 2.5 1/s. (b) Difference between two DLTS spectra with the M-center in configuration ms, respectively. The emission rate is 2.5 1/s. (b) Difference between two DLTS spectra with the A and configuration B (shown in (a)). The x-axis is shared between the vertically stacked graphs. M-center in configuration A and configuration B (shown in (a)). The x-axis is shared between the verticallyAs previously stacked mentioned, graphs. the depth profiling measurements (Figure6) indicate that an interstitially related deep level defect contributes to the DLTS signal at the temperature of the EH3 deep level. In FigureAs previously8a and henceforward, mentioned, this the contribution depth profiling is separately measurements labeled as (Figure X. 6) indicate that an interstitiallyAdditionally, related the deep metastable level defect M-center contributes was detected to the DLTS in the neutronsignal at irradiated the temperature and ion of implanted the EH3 deep4H-SiC level. (Figure In Figure8a). In 8a previous and henceforward, studies [ 34 this–38], contribution the introduction is separately of the M-center labeled as was X. reported only in theAdditionally, cases of proton the andmetastable electron M-center irradiation. was Thedetect M-centered in the was neutron transformed irradiated to configurationand ion implanted B by 4H-SiC (Figure 8a). In previous studies [34–38], the introduction of the M-center was reported only in the cases of proton and electron irradiation. The M-center was transformed to configuration B by cooling down SBDs from 450 K without applied bias, while configuration A was realized by cooling down SBDs with applied bias (−30 V) from above room temperature (340 K). M1 and M3 deep levels of the M-center were observed in configuration A, while the M2 deep level was observed in configuration B. They were resolved by subtracting two DLTS spectra with the M-center in configuration A and configuration B, as shown in Figure 8b. In a first approximation, the difference in DLTS spectra was only due to M-center deep levels [35]. The approximation was the best in the case of neutron irradiated (1013 cm−2) SBD due to the higher reverse voltage used in the measurements (−10 V) and uniform deep level concentrations. The coordinate configuration diagram describing the observed transitions of the M-center is shown in Figure 9. The shown

Crystals 2020, 10, 845 11 of 16

cooling down SBDs from 450 K without applied bias, while configuration A was realized by cooling down SBDs with applied bias ( 30 V) from above room temperature (340 K). M1 and M3 deep levels of − the M-center were observed in configuration A, while the M2 deep level was observed in configuration B. They were resolved by subtracting two DLTS spectra with the M-center in configuration A and configuration B, as shown in Figure8b. In a first approximation, the di fference in DLTS spectra was only due to M-center deep levels [35]. The approximation was the best in the case of neutron irradiated (1013 cm 2) SBD due to the higher reverse voltage used in the measurements ( 10 V) and uniform − − Crystalsdeep 2020 level, 10 concentrations., x FOR PEER REVIEW The coordinate configuration diagram describing the observed transitions12 of 17 of the M-center is shown in Figure9. The shown transitions are consistent with previously reported transitionsfindings [ 36are]. consistent Activation with energies previously of M1 andreported M2 deep findings levels [36]. were Activation determined energies using of the M1 subtracted and M2 deepDLTS levels spectra, were while determined activation using energy the subtracted of the M3 DLTS deep levelspectra, was while estimated activation based energy on the of activation the M3 deepenergy level of was the EH3estimated+ M3 based+ X peak.on the The activation subtracted energy DLTS of the spectra EH3 provided+ M3 + X toopeak. few The points subtracted for the DLTSArrhenius spectra graph provided of the M3too deepfew points level due for tothe the A transitionrrhenius graph of the M-centerof the M3 to deep A configuration level due to during the transitionDLTS measurements of the M-center at temperatures to A configuration above du 300ring K. AsDLTS best measur seen inements the case at of temperatures neutron irradiation above 300 due K.to As uniform best seen concentration in the case of of deep neutron levels, irradiatio the concentrationsn due to uniform of M1 andconcentration M2 deep levelsof deep were levels, the same.the concentrationsA smaller peak of height M1 and (concentration) M2 deep levels of the were M3 the deep same. level A seen smaller in Figure peak8b height was due (concentration) to the previously of thementioned M3 deep partiallevel seen transformation in Figure 8b of was the due M-center to the to previously A configuration mentioned during partial DLTS transformation measurement. of the M-center to A configuration during DLTS measurement.

FigureFigure 9. 9. CoordinateCoordinate configuration conﬁguration diagram diagram for for the the M-ce M-centernter constructed constructed based based on on M1, M1, M2, M2, and and M3 M3 13 13−2 2 activationactivation energies energies determined determined from from DLTS DLTS measurements measurements on neutron on neutron irradiated irradiated (10 (10 cm )cm SBD− ) and SBD activationand activation energies energies for transitions for transitions between between A and A B andconfigurations B conﬁgurations of M-center of M-center taken takenfrom [34]. from All [34 ]. energyAll energy values values are in are the in eV the unit. eV unit.

8 2 The concentration profiles measured on the C (10 cm− ) ion implanted SBD after 450 K annealing are shown in Figure 10. The concentration profile of EH1 was measured with the M center in configuration B, while EH1 + M1 and EH3 + M3 + X concentration profiles were measured with the M-center in configuration A. The overlap of profiles measured at a temperature of 210 K before (dotted lines) and after 450 K annealing indicates that the M-center was present in ion implanted SBDs before 450 K annealing at depths below the ion depth range. Moreover, an increase in M-center concentration at an ion depth range and a slight decrease in the EH3 + M3 + X concentration in the region below the ion depth range was observed after 450 K annealing. A decrease in the EH3 + M3 + X concentration profile could be due to defect reactions involving carbon interstitials, which are mobile at temperatures above 450 K [58]. The concentration profile of the M-center did not show significant deviation from EH1 concentration profile. In neutron irradiated SBD, uniform concentrations of the M-center,Figure EH1,10. EH1, and EH1 EH3 and deep M1, levelsand EH3 were and observed M3 depth in co thencentration whole depth profiles range in 7.5 (see MeV Figure C (108 S5).cm−2) ionM1 implanted and M3 deepSBD. The levels shown were EH1 not + resolved M1 concentr by Laplaceation profile DLTS (Configuration measurements. A) was The measured EH12 and EH3 peaksafter seen cooling in Figure SBD from7 partially 340 K down contained to 210 theK under contributions −30 V bias, of while M1 andEH1 M3profile deep was levels, measured respectively. after cooling down SBD from 450 K without bias. Depth profiles measured before 450 K annealing (as seen in Figure 6) are shown with dotted lines.

The concentration profiles measured on the C (108 cm−2) ion implanted SBD after 450 K annealing are shown in Figure 10. The concentration profile of EH1 was measured with the M center in configuration B, while EH1+M1 and EH3 + M3 + X concentration profiles were measured with the M-center in configuration A. The overlap of profiles measured at a temperature of 210 K before (dotted lines) and after 450 K annealing indicates that the M-center was present in ion implanted SBDs before 450 K annealing at depths below the ion depth range. Moreover, an increase in M-center concentration at an ion depth range and a slight decrease in the EH3 + M3 + X concentration in the region below the ion depth range was observed after 450 K annealing. A decrease in the EH3 + M3 + X concentration profile could be due to defect reactions involving carbon interstitials, which are mobile at temperatures above 450 K [58]. The concentration profile of the M-center did not show

Crystals 2020, 10, x FOR PEER REVIEW 12 of 17 transitions are consistent with previously reported findings [36]. Activation energies of M1 and M2 deep levels were determined using the subtracted DLTS spectra, while activation energy of the M3 deep level was estimated based on the activation energy of the EH3 + M3 + X peak. The subtracted DLTS spectra provided too few points for the Arrhenius graph of the M3 deep level due to the transition of the M-center to A configuration during DLTS measurements at temperatures above 300 K. As best seen in the case of neutron irradiation due to uniform concentration of deep levels, the concentrations of M1 and M2 deep levels were the same. A smaller peak height (concentration) of the M3 deep level seen in Figure 8b was due to the previously mentioned partial transformation of the M-center to A configuration during DLTS measurement.

Figure 9. Coordinate configuration diagram for the M-center constructed based on M1, M2, and M3 activation energies determined from DLTS measurements on neutron irradiated (1013 cm−2) SBD and Crystalsactivation2020, 10, 845energies for transitions between A and B configurations of M-center taken from [34]. All12 of 16 energy values are in the eV unit.

8 2 Figure 10. EH1, EH1 and M1, and EH3 and M3 depth concentration profiles in 7.5 MeV C (10 cm− ) Figure 10. EH1, EH1 and M1, and EH3 and M3 depth concentration profiles in 7.5 MeV C (108 cm−2) ion implanted SBD. The shown EH1 + M1 concentration profile (Configuration A) was measured ion implanted SBD. The shown EH1 + M1 concentration profile (Configuration A) was measured after cooling SBD from 340 K down to 210 K under 30 V bias, while EH1 profile was measured after after cooling SBD from 340 K down to 210 K under −−30 V bias, while EH1 profile was measured after cooling down SBD from 450 K without bias. Depth profiles measured before 450 K annealing (as seen cooling down SBD from 450 K without bias. Depth profiles measured before 450 K annealing (as seen in Figure6) are shown with dotted lines. in Figure 6) are shown with dotted lines. In this work, we utilized a depth and energy resolution of DLTS measurements to obtain 8 −2 concentrationThe concentration profiles of profiles deep levels measured introduced on the by ionC (10 implantation cm ) ion andimplanted irradiation, SBD and after compared 450 K annealingthem relative are toshown the vacancy in Figure profile. 10. The The concentration transformation profile of the of M-center EH1 was between measured A and with B configurations the M center inwas configuration used to resolve B, while M1, EH1+M1 M2, and and M3 EH3 deep + levelsM3 + X in concentration the DLTS spectra profiles and were obtain measured information with the on M-centerthe M-center in configuration depth profile. A. The overlap identification of profiles of the measured M-center at is a still temperature uncertain of with 210 previously K before (dottedreported lines) studies. and after 450 K annealing indicates that the M-center was present in ion implanted SBDs before 450 K annealing at depths below the ion depth range. Moreover, an increase in M-center concentration4. Conclusions at an ion depth range and a slight decrease in the EH3 + M3 + X concentration in the region below the ion depth range was observed after 450 K annealing. A decrease in the EH3 + M3 + We reported on a comparative DLTS study of EH1 and EH3 deep level defects in 4H-SiC X concentration profile could be due to defect reactions involving carbon interstitials, which are introduced by C and He ion implantation and neutron irradiation. Their intrinsic origin was confirmed. mobile at temperatures above 450 K [58]. The concentration profile of the M-center did not show We analyzed components of EH1 and EH3 deep levels by depth profiling and annealing procedure. Our results show that the deep levels of silicon vacancy, the M-center, and the defect involving interstitials contributed simultaneously to the DLTS signal at the temperature of EH1 and EH3 deep levels. The shift of the EH3 concentration profile maximum towards greater depths and higher concentration relative to EH1 concentration profile was observed and explained by the presence of an additional defect involving interstitials. Introduction of the M-center by different types of ion implantation and irradiation was demonstrated, leading to a hypothesis that the M-center was also an intrinsic defect.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4352/10/9/845/s1, 8 2 9 2 Figure S1: Current-voltage characteristics of SBDs implanted with 7.5 MeV C (10 cm− ) and 2 MeV He (10 cm− ) 13 2 ions, and neutron irradiated (10 cm− ) SBD at temperatures of 200 K and 300 K, Figure S2: (a) DLTS spectra of 9 2 2 MeV He (10 cm− ) ion implanted n-type 4H-SiC SBD. Different voltage settings, reverse bias VR and pulse bias 1 VP, are used to probe different depth ranges (as seen in (b)). The rate window and pulse width tP are 50 s− and 10 ms. The spectra are shifted vertically for clarity. (b) Schematic illustration of the depth regions contributing to the DLTS signal of EH1 at 210 K (hatched rectangles) and EH3 at 350 K (solid rectangles) for the used voltage in the DLTS measurements (as seen in (a)). The lambda effect was taken into account. The depth ranges are compared with free carrier concentration (black line) determined from C–V characteristic at 200 K (as in Figure2), which exhibits a minimum due to introduced acceptor deep levels, Figure S3: Depth concentration profiles for Z1/2 deep level and total introduced vacancies calculated by SRIM and Fluka software [1,2]. The lambda effect was taken into account [3]. Depth profiling measurements were carried out at the temperature of 315 K, Figure S4: Increase in free carrier concentration after annealing at the temperature of 450 K. Solid and dashed lines show free carrier concentration profiles measured before and after annealing at the temperature of 450 K, respectively. The profiles are calculated using C–V measurements at a temperature of 200 K, Figure S5: EH1, EH1 and M1, 9 2 13 2 EH3 and M3 depth concentration profiles in (a) 2 MeV He (10 cm− ) ion implanted and (b) neutron (10 cm− ) irradiated SBD. The shown EH1 + M1 concentration profile (Configuration A) was measured after cooling SBD from 340 K down to 210 K under 30V bias, while EH1 profile was measured after cooling down SBD from 450 K without bias. The black curve− in (b) shows depth concentration profile measured after cooling SBD from Crystals 2020, 10, 845 13 of 16

340 K down to 210 K under 10V bias. Depth profiles measured before 450 K annealing (as seen in Figure6) are shown with dotted lines. The− contribution of an additional deep level defect involving interstitials is labeled as 13 2 X, Figure S6: Pulse width dependence of EH1 and EH1 + M1 DLTS signal amplitudes in neutron (10 cm− ) irradiated SBD. DLTS signal amplitude of M1 deep level is determined by subtracting EH1 + M1 and EH1 DLTS signals. Measurements are carried out at a temperature of 210 K using 50 1/s emission rate window. Reverse and pulse voltages were 10 V and 0.1 V, respectively. − − Author Contributions: Formal analysis, T.B., Ž.P.; investigation, T.B., L.B., I.C., L.S., V.R., Ž.P.; resources, T.O.; writing—original draft preparation, T.B., I.C.; writing—review and editing, T.B., I.C., L.S., V.R., Ž.P.; supervision, I.C. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by NATO SPS Program, grant number 985215. The authors acknowledge the financial support from the Slovenian Research Agency (research core funding No. P2-0073). This research was partially funded by the National Collaborative Research Infrastructure Strategy (NCRIS) funding provided by the Australian Government. Acknowledgments: We would like to acknowledge Hidekazu Tsuchida and Norihiro Hoshino of Central Research Institute of Electric Power Industry for the supply of SiC substrates with epitaxially grown 4H–SiC single-crystal layers. The authors acknowledge assistance by the TRIGA research reactor operators. Conflicts of Interest: The authors declare no conflict of interest.

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