e-Journal of Surface Science and Nanotechnology 31 October 2017

e-J. Surf. Sci. Nanotech. Vol. 15 (2017) 109-114 Regular Paper

Analysis of Nitrogen State on MOS Interface of 4H-SiC m-Face after Nitric OxidePost Oxidation Annealing (NO-POA)

Kimimori Hamada∗

Power Electronics Development Division, Toyota Motor Corporation,543, Kirigahora, Nishihirose-cho, Toyota, Aichi 470-0309, Japan, and

Doctoral Program in Nano-Science and Nano-Technology,Graduate School of Pure and Applied Sciences, University of Tsukuba,

1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japan

Akira Mikami and Hideki NaruokaPower Electronics Development Division, Toyota Motor Corporation,543, Kirigahora, Nishihirose-cho, Toyota, Aichi 470-0309, Japan

Kikuo YamabeGraduate School of Pure and Applied Sciences, University of Tsukuba,

1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japan(Received 1 August 2017; Accepted 23 August 2017; Published 31 October 2017)

It is known that the interface nitrogen density of the 4H-SiC Si-face, C-face, and a-face increases as a result ofthe NO-POA process, that the electron mobility increases as the nitrogen density increases, and that each face hasa different interface nitrogen saturation density. In contrast, the anisotropy of the nitridation characteristics ofthe m-face, which is regarded as a promising channel for high-performance trench MOSFETs, is not well known.To identify the nitridation status of the m-face after NO-POA, the MOS interface structures with a SiO2 formedon m-face by CVD and treated by NO-POA was investigated by SIMS, HAXPES, XPS, and XAFS. In the sameway as the other faces, it was found that the nitrogen segregates on the SiO2/SiC MOS interface, that most of thenitrogen combines with Si, and that the interface nitrogen density has a unique saturation value. The nitrogendensity saturation value on the m-face measured by SIMS was 9.8 × 1014 cm−2. This value is approximately 1.5times the exposed carbon density on the top surface of the m-face. [DOI: 10.1380/ejssnt.2017.109]

Keywords: Silicon carbide; Silicon oxides; Nitrides; Nitrogen oxides; Semiconductor-insulator interfaces

I. INTRODUCTION

Silicon carbide (SiC) has excellent material character-istics for power semiconductor devices, such as high elec-tric field breakdown strength and high thermal conduc-tivity [1, 2]. However, one issue of SiC devices is the diffi-culty of obtaining the expected low on-resistance charac-teristics because of the low electron mobility of the MOSinterface due to its high interface trap density (Dit) [3].Recently, the nitric oxide post-oxidation annealing (NO-POA) process has been widely recognized as a very effec-tive approach to reduce Dit and improve on-resistance [4–8]. This process increases the MOS interface electronmobility by segregating nitrogen on the MOS interfaceand reducing the Dit near the conduction band, which isknown to depress mobility. Consequently, it is very impor-tant to know the nitridation status at the MOS interface.The adoption of a trench MOS gate structure is also re-garded as another promising way to reduce on-resistancebecause of its larger per-unit area gate width comparedto a normal planar gate structure [9–12]. When a trenchMOSFET is fabricated on a mass production commercialSi-faced SiC wafer, the MOS channel surface is formedat one of the a-face, m-face, or the face between them.Although the a-face and m-face reportedly show highermobility than the Si or C faces, less is known about thecharacteristics of the gate interface compared to the Si orC faces [13]. Especially, there are no reports describing

∗ Corresponding author: kimimori˙hamada@mail.toyota.co.jp

the anisotropy of the nitridation status of the m-face af-ter the NO-POA process, which has a direct impact onmobility [14].

The research described in this paper investigated thenitridation status of the MOS interface structures on them-face by SIMS, HAXPES, XPS, and XAFS. This wasaccomplished by carrying out NO-POA after forming aSiO2 film by low-pressure chemical vapor deposition(LP-CVD). This paper also considers the NO-POA nitridationmechanism of the m-face MOS interface.

II. EXPERIMENTAL DETAILS

1.5-inch m-faced heavy doped n-typed 4H-SiC waferswith 5 µm epitaxial layers doped to a concentration of 1×1016 cm−2 were used in this study. A Si-faced 4-inch waferwith the same doping concentrations and epitaxial layerthickness was used as a reference. After forming a 75 nmSiO2 film by LP-CVD on the surface of the wafers, NO-POA was carried out in NO (10%) gas at 1 atm at 1000,1150 and 1300◦C for 30 minutes on the m-face wafers, andunder the same gas conditions at 1300◦C for 30 minuteson the Si-face wafer.

The interface structure was analyzed by secondaryion mass spectroscopy (SIMS), hard X-ray photoelec-tron spectroscopy (HAXPES), X-ray photoelectron spec-troscopy (XPS), and X-ray absorption fine structure(XAFS).

SIMS was used to obtain the depth profile of N fromthe SiO2 film to the SiC substrate. This study defined theMOS interface nitrogen density as the total number of ni-

ISSN 1348-0391 c⃝ 2017 The Surface Science Society of Japan (http://www.sssj.org/ejssnt) 109

Volume 15 (2017) Hamada, et al.

trogen atoms between−6 and +6 nm of the MOS interfaceper unit area (cm−2). Although the detailed conditionsof the SIMS measurement have not been disclosed by theSIMS engineering service company, the MOS interface ni-trogen saturation density is reported to be 4.8×1014 cm−2

at 1 atm and 1150◦C after a 4-hour NO-POA process onthe Si-face of a 4H-SiC wafer. In addition, the error onSIMS data of the nitrogen density was reported to be20% in comparison to nuclear reaction analysis (NRA)results [15, 16]. This study references this saturation den-sity value and SIMS error when discussing the absolutevalue of nitrogen density.The HAXPES analysis utilized the SPring-8 BL47XU

beam line with the objective of analyzing the chemicalbonding state of the N at the interface. The incident X-ray energy was set to 7944 eV and the photoelectron take-off angle was set to 89◦. Due to the high kinetic energy ofthese photoelectrons(at least 6 keV), information can beobtained simultaneously from the MOS interface, SiO2

bulk, and SiC crystals when the SiO2 film is less than10 nm. Therefore, hydrofluoric acid (HF) etching wascarried out before the analysis to reduce the maximumSiO2 thickness to 10 nm.The XPS analysis utilized a PHI-5500 device and the

MgKα line X-ray source. The photoelectron take-off an-gle was set to 45◦. As XPS has the same measurementprinciple as HAXPES, the chemical bonding state of Ncan be analyzed. Because the incident X-ray energy ofXPS is smaller than that of HAXPES, all SiO2 was re-moved by HF wet etching. Therefore, the measurementdepth is around 1 to 2 nm from the surface of the SiCcrystal, and this research attempted to separate the in-formation obtained from the SiO2 bulk and SiC crystalsby subtracting the XPS data from the HAXPES data.The XAFS analysis utilized the Aichi SR BL-7U beam

line X-ray source and electron yield X-ray absorptionspectrum measurement method with the objective of an-alyzing the chemical bonding state of N by a differentanalysis principal from HAXPES and XPS. The measuredXAFS spectra were compared to XAFS spectra obtainedby first-principle calculation, and the bonding states ofthe N were estimated. All SiO2 was removed by HF wetetching and the interfaces were directly measured.

III. EXPERIMENTAL RESULTS

A. SIMS measurements

Figure 1(a) shows the N profile SIMS measurement re-sults of the samples annealed in NO at 1000, 1150 and1300◦C for 30 minutes. All the N concentration peaksare set at 0 nm. The measured data of Si, C, and O (notshown) confirmed that N segregates on the MOS interface.When the NO-POA process time was set to 30 minutes,the density and concentration of N in both the SiO2 filmand the interface increased in accordance with the an-nealing time. Figure 1(b) shows the N profile SIMS mea-surement results of the m-face samples annealed in NOat 1300◦C for 30 and 90 minutes, and the Si-face sampleannealed in NO at 1300◦C for 30 minutes as a reference.The measured MOS interface N densities are summarizedin Table I. When the m-face sample was annealed in NO

m-face

1300 C

1150 C

1000 C

(a)

1300 Cm-face

90 mins30 mins

Si-face 30 mins

(b)

FIG. 1. SIMS measurement results of samples treated underthe conditions of (a) NO-POA in NO (10%) gas at 1 atm at1000◦C (colored in red), 1150◦C (colored in blue) and 1300◦C(colored in green) for 30 minutes on m-face wafers after form-ing 75 nm SiO2 film by LP-CVD and (b) NO-POA in NO(10%) gas at 1 atm at 1300◦C for 30 minutes (colored in black)and 90 minutes (colored in orange) on m-face and 30 minutes(colored in blue) on Si-face wafer.

at 1300◦C, the N density became saturated at 30 min-utes and the saturation value was 9.8 × 1014 cm−2. Incontrast, when the Si-face sample was annealed under thesame conditions, the N density reached 4.8× 1014 cm−2,which is about half of the m-face saturation density. ThisSi-face N density is almost the same as the reference datamentioned in the SIMS description above. Therefore, theSIMS measurement error in this research is also expectedto be less than 20%. Dhar et al. reported that the Ndensities of a Si-face, C-face, and m-face annealed in NOat 1175◦C became saturated within 30 minutes and thateach face has a different saturation value [16]. This studyfound that the m-face has the same N density saturationbehavior as the other faces and that the m-face satura-

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e-Journal of Surface Science and Nanotechnology Volume 15 (2017)

TABLE I. MOS interface nitrogen density dependence on NO-POA conditions.

4H-SiC NO-POA N interface density

crystal face temperature and time (×1014atoms/cm2)

m-face (1100) 1300◦C, 30 mins 9.86

m-face (1100) 1150◦C, 30 mins 8.13

m-face (1100) 1000◦C, 30 mins 5.34

m-face (1100) 1300◦C, 90 mins 9.70

Si-face (0001) 1300◦C, 30 mins 4.80

tion value of 9.8 × 1014 cm−2 is unique. The experimen-tal results in Fig. 1(a) appear to show that the satura-tion value has temperature dependence. However, sinceit has been reported that N density saturation values arenot temperature-dependent at the Si-face, C-face, and m-face [16], it is likely that the N densities after 1000◦C and1150◦C annealing for 30 minutes had not yet reached sat-uration point. In addition, although the half value widthis almost 3.4 nm in the SIMS measurement results shownin Fig. 1(a) and (b), the N distribution area is expectedto be narrower than 1 nm because of SIMS resolutionlimitations and previously reported information [16].

B. HAXPES and XPS measurements

Figure 2 shows the Si 1s, O 1s, C 1s, and N 1s pho-toelectron spectra obtained by HAXPES. With respectto the binding energy axis, the SiC peak position of the1000◦C sample was determined to be 1840.6 eV by fittingthe SiO2 peak of the Si 1s spectrum to the value quoted inthe literature [17]. The SiC peaks of all the other sampleswere assumed to have the same binding energy. Conse-quently, the spectra of the other elements were shifted inaccordance with the difference between the above valueand the measured values. After carrying out this cali-bration, the SiC peak positions in the C 1s spectra ofevery sample matched, thereby confirming the accuracyof this analysis. The peak detected at 397.8 eV in theN 1s spectra was assumed to be Si–N bonding [8, 17, 18]and all samples had the same Si–N bonding status. Inthese graphs, each bonding energy peak value is set as1. The noise in the N 1s spectrum of the 1000◦C sam-ple is due to the greater thickness of the remaining SiO2

compared to the other samples, as evident from the SiO2

peak height of the Si 1s signal. In addition, the N den-sity itself is smaller than the other samples, as identifiedfrom the SIMS measurements. These results confirmedthat Si–N bonding is dominant for N. Figure 3 shows theN 1s HAXPES photoelectron spectra of the 1000, 1150,and 1300◦C samples. Because the 397.8 eV spectrum in-dicates that N forms three chemical bonds with Si, it isassumed that most of the N at the SiO2/SiC interface re-placed C. Furthermore, since all the N 1s HAXPES spec-tra sweep toward the higher energy side, the N 1s spectrawere fitted using a set of three Gaussian representations

FIG. 2. Si 1s, O 1s, C 1s, and N 1s photoelectron spectraobtained by HAXPES. Samples treated under the conditionsof NO-POA at 1000◦C (colored in red), 1150◦C (colored inblue) and 1300◦C (colored in green) for 30 minutes.

with different bonding configurations known as main Si–N bonding (397.8 eV), Si2-N-O bonding (398.3 eV), andSi-N-O2 or C–N bonding (399.4 eV) [19]. Samples at allthe NO annealing temperature showed a Si2-N-O bondingsignal strength of around 10% to 20% of the main Si–Nsignal value, and the existence of Si2-N-O bonding witha similar ratio may be assumed on the interface. Fig-ure 4 shows the N 1s XPS photoelectron spectra of the1000, 1150, and 1300◦C samples. These spectra were alsofitted using the same three peak values as adopted forthe HAXPES analysis. Regardless of the bonding states,most of the N is expected to exist at the SiC crystal sideon the SiO2/SiC interface because the XPS spectra aresimilar to the HAXPES spectra at each annealing tem-perature. Furthermore, the XPS spectra, which do notinclude information from the SiO2 film side, showed lesstemperature dependence. However, the HAXPES spectrashowed a slight temperature dependence at high energybonding states. This result indicates that the N bondingstates in the SiO2 bulk were more affected by temperaturethan the bonding states at the SiO2/SiC interface withina temperature range from 1000◦C to 1300◦C.

C. XAFS measurements

In Fig. 5, the lower three lines are the XAFS experimen-tal results and the upper six lines are the XAFS spectra ofseveral assumed N statuses obtained by first-principle cal-culation as references to identify the XAFS experimentalresults. The XAFS spectra of samples with a NO-POAtemperature of 1150 and 1300◦C closely resemble the cal-culated XAFS spectra of Si3N4 in which N replaces theC in SiC crystals. The XAFS spectrum of the 1000◦Cannealing sample has only a distant resemblance to thecalculated references. This is probably due to the lowerN interface density, which is not sufficient to absorb thecharacteristic X-ray energies of N bonding. In contrast,characteristics such as the replacement of Si in the SiC

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(a) 1300 C (b) 1150 C (c) 1000 C

FIG. 3. HAXPES N 1s spectra (colored in black) is divided into possible peak assignments of Si–N bonding (397.8 eV, coloredin green), Si2-N-O bonding (398.3 eV, colored in blue) and Si-N-O2 or C–N bonding (399.4 eV, colored in red). (a) 1300◦C, (b)1150◦C, and (c) 1000◦C

(a) 1300 C (b) 1150 C (c) 1000 C

FIG. 4. XPS N 1s spectra (colored in black) is divided into possible peak assignments of Si–N bonding (397.8 eV, colored ingreen), Si2-N-O bonding (398.3 eV, colored in blue) and Si-N-O2 or C–N bonding (399.4 eV, colored in red) for comparison toHAXPES spectra (a) 1300◦C, (b) 1150◦C, and (c) 1000◦C

FIG. 5. Calculated XAFS spectra (upper six lines) and exper-imental spectra (lower three lines).

crystal by N or interstitial N are not observed at all. In-cidentally, the XAFS N spectrum overlaps the XAFS Cspectrum and the peak at around 420–430 eV is in theXAFS spectrum, which does not appear in the obtainedXAFS spectra.

The XAFS analysis results show that NO-POA processconditions of 1000◦C and 30 minutes are not enough tocomplete nitridation of the SiO2/SiC interface. N on theinterface forms chemical bonds mainly with Si, and N islikely to replace C in the SiC crystal. These results agreeclosely with the results of SIMS, HAXPES, and XPS.

IV. DISCUSSION

This research investigated the N state of a SiO2 filmdeposited by LP-CVD on the 4H-SiC m-Face after NO-POA. It was found that most of the N segregates only onthe SiO2/SiC MOS interface, most of the N combines withSi, Si-N bonding is dominant for the N on the interface,N is likely to replace C in the SiC crystal, and the Ndensity saturates. The segregated N distribution area isvery thin, and is expected to be narrower than 1 nm.These m-face characteristics are similar to the Si-face, C-face, and a-face. Each face has a unique saturation value,the saturation value of the m-face was identified for thefirst time as 9.8× 1014 cm−2.

The N saturation densities of NO-POA at 1 atm werepreviously reported to be 4.8× 1014 cm−2 [15] and 3.5×1014 cm−2 on the Si-face and 1.06× 1015 cm−2 on the C-face [16], which can be compared to the amount of mono-layer C on each face. The calculated amount of mono-layer C on each face is shown in Table II. The ratio of Nto C on the Si-face is almost one-third and the ratio onthe C-face is almost one. In the case of the a-face, the sat-uration N density was reported to be 0.95×1015 cm−2 [16]and the ratio of N to C on the a-face is also almost one.Pennington et al. reported that the energy per surfacecell on the Si-face became smallest when the amount ofN near the surface is one-third of the amount of the Cmono-layer [20]. This report demonstrated the possibilitythat the saturation N density is decided by the existenceprobability of N atoms. In contrast, neither the satura-tion amount nor saturation mechanism of N on the m-face

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e-Journal of Surface Science and Nanotechnology Volume 15 (2017)

TABLE II. Measured N density, mono-layer C density, and the ratio of N to C on each face.

4H-SiC N density(cm−2) C density(cm−2) Ratio of N to C

crystal face experimental result on top of surface (N/C)

Si-face (0001) 4.8 / 3.5× 1014 1.22× 1015 0.39 / 0.29

C-face (0001) 1.06× 1015 1.22× 1015 0.87

a-face (1120) 9.5× 1014 9.98× 1014 0.95

m-face (1100) 9.8× 1014 6.47× 1014 1.51

FIG. 6. 4H SiC m-face ball and stick model with the 1st, 2nd,and 3rd internal planes within one unit cell. Yellow balls showcarbon atoms and gray balls show silicon atoms. The 1st planeis a usual m-face.

has been reported.

In this experiment, the nitrogen density saturationvalue on the m-face measured by SIMS was 9.8 ×1014 cm−2. Since the C density on the top surface ofthe m-face is 6.5 × 1014 cm−2, the ratio of N to C isapproximately 1.5. Therefore, it is likely that the satura-tion N density of the m-face is determined by a differentmechanism than the other faces. Figure 6 shows a 4H SiCm-face ball and stick model. Four C and Si atoms are in-cluded in one 4H SiC unit cell and, looking at the layoutof the C atoms on the m-face, C atoms repeatedly appearin a 2:1:1 rotation. For convenience, this paper defines

each plane that includes C atoms as the 1st, 2nd, and 3rdinternal plane from the top surface. Each plane is parallelwith the m-face and exists at equal intervals in the ver-tical direction of the m-face. Based on the experimentalresults, C atoms on both the 1st and 2nd planes are likelyto be replaced by N simultaneously during NO-POA ofthe m-face.

V. CONCLUSIONS

The N states on the SiO2/SiC interface after NO-POAwere investigated in detail by SIMS, HAXPES, XPS, andXAFS. It was found that most of the N combines onlywith Si at the SiC crystal side, which may be establishedby replacing C on the interface with N. The saturationN density of the m-face is around 9.8 × 1014 cm−2, 1.5times that of the C density on the m-face top surface.This is the first report to identify that the Si-face, C-face,a-face, and m-face have different saturation N densitiesand a different N to C ratio. Furthermore, this paperalso showed that the m-face has three internal planes thatappear at equal intervals in the vertical direction of them-face, which include a C ratio of 2:1:1. It was suggestedthat the unique saturation N density of the m-face maybe decided by the concurrent nitridation reaction of the1st and 2nd planes during NO-POA.

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