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Interfacial atomic site characterization by photoelectron diffraction for 4H-AlN/4H-SiC( )

heterojunction

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2016 Jpn. J. Appl. Phys. 55 085701

(http://iopscience.iop.org/1347-4065/55/8/085701)

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Interfacial atomic site characterization by photoelectron diffraction

for 4H-AlN/4H-SiC(11�20) heterojunction

Naoyuki Maejima1*†, Masahiro Horita1, Hirosuke Matsui1,2, Tomohiro Matsushita3, Hiroshi Daimon1, and Fumihiko Matsui1*

1Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), Ikoma, Nara 630-0192, Japan2Research Center for Materials Science (RCMS), Nagoya University, Nagoya 464-8602, Japan3Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, Sayo, Hyogo 679-5198, Japan

*E-mail: maejima.naoyuki.mg3@ms.naist.jp; matui@ms.naist.jp†Present address: National Institute for Quantum and Radiological Science and Technology (QST), Sayo, Hyogo 679-5148, Japan

Received March 2, 2016; accepted May 6, 2016; published online July 6, 2016

The interfacial atomic structure of an AlN thin film on a nonpolar 4H-SiC(11�20) substrate grown by atomic Al and N plasma deposition was studiedby photoelectron diffraction and spectroscopy. The epitaxial growth of the thin film was confirmed by the comparison of element-specificphotoelectron intensity angular distributions (PIADs). Depth profiles were analyzed by angle-resolved constant-final-state-mode X-rayphotoelectron spectroscopy (AR-XPS). No polar angular dependence was observed in Al 2p spectra, while an additional intermixing componentwas found in interface-sensitive N 1s spectra. The site-specific N 1s PIADs for the AlN film and an intermixing component were derived from twoN 1s PIADs with different binding energies. We attributed the intermixing component to SiN interfacial layer sites. In order to prevent SiN growth atthe interface, we deposited Al on the SiC(11�20) substrate prior to the AlN growth. A significant reduction in the amount of intermixing componentsat the AlN/SiC interface was confirmed by AR-XPS. © 2016 The Japan Society of Applied Physics

1. Introduction

Silicon carbide (SiC) has a great potential for power deviceapplications owing to its wide bandgap and high breakdownfield.1) Since most of the SiC devices are fabricated on the(0001) face, the channel is formed on the nonpolar (11�20)face in trench-structure metal–oxide–semiconductor field-effect transistors (MOSFETs). A SiC MOSFET fabricated onthe nonpolar plane (11�20) face with a SiO2 gate dielectricshows a higher channel mobility than that fabricated on the(0001) face.2) Aluminium nitride (AlN) is a candidate for analternative gate dielectric because of its large bandgap energyand high dielectric constant. Horita et al. successfully grewa high-crystal-quality 4H-AlN thin film on a 4H-SiC(11�20)substrate using an atomically flat SiC surface and controlledthe N=Al deposition ratio.3) They confirmed by high-resolution transmission electron microscopy (HRTEM) thatthe AlN thin film has low densities of stacking faults andthreading dislocations. Surface roughness was examinedby atomic force microscopy (AFM); however, the AlN=SiCinterface atomic structure was unclear. It is essential tocharacterize the interface atomic structure for the betterunderstanding and improvement of the device characteristics.

Photoelectron diffraction is a powerful element-selectiveand nondestructive atomic structure analysis method forburied subsurfaces and interfaces. Atomic scale analysesof semiconductor interfaces prepared by homoepitaxial orheteroepitaxial growth were carried out by this method.4,5)

Interface structures between an insulator thin film and asemiconductor substrate were also determined by two-dimensional photoelectron diffraction pattern analysis withmultiple scattering simulation.6,7) Atom intermixing atvarious semiconductor heterojunctions could be clarified byphotoelectron diffraction analysis.8–11) Previously, we meas-ured 2π steradian photoelectron intensity angular distribu-tions (PIADs) from a vicinal 6H-SiC(0001) substrate coveredby an epitaxial SiON thin film and investigated the inter-face structure.12,13) The anisotropic step bunching along the[11�20] direction results in a preferential appearance ofterraces with one type of stacking orientation.14) Therefore,

threefold symmetric PIADs predominantly originating fromthe top layer were obtained. We directly visualized the localatomic arrangement around N atoms passivating danglingbonds at the SiO=SiC interface and clarified that they occupystacking fault sites.

Furthermore, a site-specific electronic structure can bedetermined by combining photoelectron diffraction analysiswith X-ray photoelectron spectroscopy or X-ray absorptionspectroscopy (XAS). For example, the electronic and mag-netic structures of a Ni thin film on the Cu(001) surface foreach atomic layer were determined using atomic site-specificforward focusing peaks (FFPs) for resolving layer-specificXAS spectra.15) In the present study, we applied this site-specific method to intermixing component state analysis. Asa result, we succeeded in improving the atomic structure ofthe interface.

2. Experimental methods

We prepared two AlN=SiC samples, A and B, in a molecularbeam epitaxy chamber. Sample A was grown by atomic Aland N plasma deposition at a calibrated rate.3) On the basis ofthe results of the interface study of sample A, the improvedpreparation method for sample B was applied. In the caseof sample B preparation, one monolayer Al deposition onSiC(11�20) before the codeposition of Al and N was carriedout. The thickness of the AlN thin film for both samples wasestimated to be 7Å from the deposition rate determined byRHEED oscillation observation. The dosage of this film wasequivalent to five atomic layers.

The experiments were performed at the circularly polarizedsoft-X-ray beamline BL25SU of SPring-8, Japan.16) The twosamples were transferred to a measurement chamber by usinga portable high-vacuum chamber to prevent the oxidationof the thin film and the interface. PIADs from the samplesurface were measured using a display-type spherical mirroranalyzer (DIANA).17–19) The acceptance angle of the analyz-er is ±60°. Circularly polarized light was incident along thenormal direction of the sample surface for all measurements.The emission angle (θ) dependence of 45 ± 60° relative tothe sample surface normal was measured simultaneously. By

Japanese Journal of Applied Physics 55, 085701 (2016)

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085701-1 © 2016 The Japan Society of Applied Physics

scanning the sample azimuth over 360°, we collected 2π-steradian PIAD data. A set of 2π-steradian PIADs excitedby σ+ and σ− helicity light was measured by switching thepath of storage ring electrons in twin helical undulatorsat 0.1Hz.20) Angle-resolved constant-final-state-mode X-rayphotoelectron spectra were obtained by scanning photonenergy at a fixed photoelectron kinetic energy. Angle-resolved spectra were obtained using integrated photoelec-tron signals. The energy window of the analyzer was set to2 eV. All experiments were performed at room temperature.

3. Results and discussion

We measured the angle-resolved Al 2p and N 1s constant-final-state-mode photoelectron spectra at a kinetic energy

of 600 eV for sample A, as shown in Figs. 1(a) and 1(b),respectively. There was no significant difference betweensurface and interface-sensitive Al 2p spectra. Thus, the Alatoms at the interface and in the thin film have the samebinding energy. On the other hand, we found that theinterface-sensitive N 1s peak was shifted to the larger-binding-energy side, suggesting the existence of chemicallyshifted components in the interface region other than the idealAlN site.

The N 1s angle-resolved constant-final-state-mode photo-electron spectra at a kinetic energy of 600 eV of sample Bare shown in Fig. 1(c). There was no significant differencebetween the two spectra. These results indicate that sample Bhas an abrupt interface.

The chemical shift of the N 1s peak corresponding tosample A indicates that there are several N components atthe interface. The N 1s binding energies of 397.5 and 396.4eV for the SiN and AlN components, respectively, reportedelsewhere were used as the reference values.21,22) The resultsof curve fitting for N 1s spectra by AlN and SiN componentsfor sample A are shown in Fig. 2. We observed that twochemical components, SiN and AlN, exist in the depositedfilm.

In order to confirm the interface structure, componentratios at various detection angles were measured. Figure 3(a)shows the N 1s peak intensity ratio of AlN to SiN derivedfrom experimental data and the ratios corresponding tothree models. Each data point was the average of the data at apolar angle of approximately ±5°. As shown in Fig. 3(b),model α has the SiN component at the surface, model βat the interface, and model γ distributed uniformly in thefilm. Each model is composed of five AlN layers with a SiNcomponent. We adjust the amount of the SiN componentof each model such that it fits with the experimental data. Theinelastic mean free path of AlN and SiC is 1.5 nm at a kineticenergy of 600 eV.23) However, we estimated from experi-mental results that the inelastic mean free path lengthdecreased to 1.0 nm. Models α and γ did not fit to experi-mental data at high emission angles, while model β fit toexperimental data at all angles. These results indicate thatthe SiN component was not segregated at the surface butat the interface. The numbers of AlN and SiN layers inmodel β were 4.6 and 3.4, respectively.

N 1s

(a)

(b)

Al 2p

(c)

Sample A

Sample

Sample

5º

65º

5º

65º

Sample A

Sample BSample

5º

65º

N 1s

Fig. 1. (Color online) (a) Al 2p and (b) N 1s angle-resolvedphotoelectron spectra corresponding to the sample A surface. The spectrawere normalized by the area of the peak. The surface- and interface-sensitivespectra were indicated by open squares and solid circle dots, respectively.(c) N 1s photoelectron spectra corresponding to the sample B surface. Theinterface component was greatly suppressed.

Sample A

Fig. 2. (Color online) Results of curve fitting for N 1s spectra by AlN thinfilm and SiN interface components. The binding energy of the interfacecomponent was deeper than that of the surface.

Jpn. J. Appl. Phys. 55, 085701 (2016) N. Maejima et al.

085701-2 © 2016 The Japan Society of Applied Physics

We now focus on the intermixing N components formedduring the growth of sample A. The substrate surface waspre-exposed to nitrogen gas as the source of N radical speciesbefore Al deposition and N plasma deposition. Thus, weconcluded that the SiN intermixing layer was formed at theinterface.

The chemical shift of the SiN component is explained bythe higher electron negativity of the Si atom than of the Alatom. In order to prevent chemical reaction between nitrogengas and the SiC surface, we performed one monolayer Aldeposition on the SiC surface in sample B preparation priorto the codeposition of Al and N.

As a result, we succeeded in improving the AlN=SiCinterface. The N and Al atom binding energies were the samein the AlN thin film and at the abrupt interface. There areequal numbers of Si–N and Al–C bonds at the abrupt AlN=SiC interface. In this case, the interface electrical charge isneutral. The result of sample B suggests the formation of theabrupt AlN=SiC interface.

We examined the interface structure of sample A byphotoelectron diffraction analysis. Si 2p and C 1s PIADswith a photoelectron kinetic energy of 600 eV were measuredat excitation photon energies of 708 and 889 eV, as shownin Figs. 4(a) and 4(b), respectively. They are the substrate-specific components. Al 2p and N 1s PIADs with a photo-electron kinetic energy of 600 eV were measured at excitationphoton energies of 698 and 1002.8 eV as shown in Figs. 4(c)and 4(d), respectively. They are the thin-film-specific com-ponents. The FFPs marked by crosses and solid circles corre-spond to the directions from the emitter atom site to the Nand Al atom directions, respectively, as shown in Fig. 4(e).The Al2 and N2 atom sites are the second- and third-nearestneighboring atoms for the Al0 emitter atom, respectively. TheN2 and Al2 atom sites are the second- and third-nearestneighboring atoms for the N0 emitter atom, respectively.Kikuchi-band-like patterns overlapped with FFPs corre-

sponding to the N1 and Al1 atoms in Al 2p and N 1s PIADs,respectively. Al 2p and N 1s PIADs have similar struc-tures to Si 2p and C 1s PIADs, respectively. Therefore,we confirmed that the AlN thin film was grown epitaxiallyon SiC(11�20). N 1s binding-energy-selective PIADs with aphotoelectron kinetic energy of 600 eV were measured atthe binding energies of 396.3 and 397.3 eV. The differencebetween 396.3 and 397.3 eV binding-energy-selective PIADsindicates dissimilar patterns from these two. This differencealso indicates that the chemically shifted component has anatomic structure different from the ideal AlN atomic struc-ture. The binding-energy-selective patterns show linear com-binations of two site-selective patterns. We performed theleast square fitting for N 1s spectra using these two com-ponents to estimate the ratio of the two components. As aresult, the site-selective PIADs, PAlN and PSiN, were obtainedusing the following equations:

a b

c d

� �PAlN

PSiN

� �¼ P396:3

P397:3

� �; ð1Þ

PAlN

PSiN

� �¼ a b

c d

� ��1 P396:3

P397:3

� �; ð2Þ

(b)

(a)

SiN component at surface

uniformly distributedSiN component

α β γ

SiC

SiN

AlN1.5 Å

SiC

SiN

AlN

SiC

AlN&

SiN

SiN component at interface

Sample A

Fig. 3. (Color online) (a) N 1s peak intensity ratio of AlN to SiNcomponents at various emission angles. (b) Three calculated models of AlN=SiC with SiN intermixing component.

Fig. 4. (Color online) (a) Si 2p and (b) C 1s 2π-steradian PIADs from theSiC(11�20) clean surface with photon energies of 708 and 889 eV,respectively. (c) Al 2p and (d) N 1s PIADs from the AlN thin film withphoton energies of 680 and 1002.8 eV, respectively. The photoelectronkinetic energy was set at 600 eV. (e) Side and local atomic views of 4H-AlN=4H-SiN=4H-SiC(11�20) structure. N0 and N2 exist behind the Al0 and Al2atoms along the [0001] direction from the side view, respectively.

Jpn. J. Appl. Phys. 55, 085701 (2016) N. Maejima et al.

085701-3 © 2016 The Japan Society of Applied Physics

where a and b indicate the ratio of AlN and SiN com-ponents in 396.3 eV binding-energy-selective patterns, andc and d indicate the ratio of the AlN and SiN componentsin 397.3 eV binding-energy-selective patterns. These param-eters are dependent on emission angle and were estimatedfrom the intensities of the AlN and SiN components at 396.3and 397.3 eV. P396.3 and P397.3 are the binding-energy-selective PIADs measured at the binding energies of 396.3and 397.3 eV, respectively.

The site-specific PIADs, PAlN and PSiN, were derived bysolving an inverse matrix of Eq. (2) as shown in Figs. 5(a)and 5(b). FFPs corresponding to the second- and third-nearestneighboring atoms are indicated by crosses and solid circles,respectively. The (10�10) Kikuchi-band-like structure in C 1sPIAD is also observed in PSiN but not in PAlN. This is becausethe SiN components are located at the interface, as shownin Fig. 3(b) β. Thus, the (10�10) Kikuchi-band-like structurein the SiN-site-selective PIAD resembles the bulk PIADstructure. The intensity maxima of FFPs corresponding tothe second- and third-nearest neighboring atoms in PAlN

appeared in the same direction as those in the C 1s PIAD.On the other hand, the intensity maxima of FFPs correspond-ing to the second- and third-nearest neighboring atomsin PSiN appeared in the 1.9 and 3.8° smaller polar angledirections, respectively, than those in the C 1s PIAD. Theseresults indicate that the SiN atomic structure is strained fromthe 4H-SiC structure. We succeeded in the direct observationof the SiN intermixing layer at the AlN=SiC(11�20) interfaceby site-specific photoelectron diffraction.

4. Conclusions

We succeeded in reducing the density of intermixing atomicsites at the interface by changing the growth conditions byreferring to intermixing component information. We ob-served the interface atomic structures between five AlNlayers and the 4H-SiC(11�20) substrate. Al 2p and N 1sPIADs have the same structures as Si 2p and C 1s PIADs,respectively. We confirmed the epitaxial growth of 4H-AlN.Furthermore, we observed the strained interface structure byphotoelectron diffraction. This analysis method contributes to

the improvement of the atomic structure of the semiconductorheterojunction interface.

Acknowledgments

We would like to thank Professors T. Kimoto and J. Suda ofKyoto University for supplying the 4H-AlN=4H-SiC(11�20)samples and valuable discussions. We appreciate the gratefulsupport of Dr. T. Nakamura and Dr. T. Muro in synchrotronradiation experiments. The synchrotron radiation experimentswere performed at the BL25SU of SPring-8 with the approvalof the Japan Synchrotron Radiation Research Institute(JASRI) (Proposal Nos. 2012A1548, 2012B1487, and2013A1624). One of the authors (N.M.) acknowledges aJSPS Grant-in-Aid for Scientific Research on InnovativeAreas “3D Active-Site Science” (No. 26105007 2604).

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(a) (b)

Fig. 5. (Color online) Site-selective N 1s PIADs from (a) AlN thin filmand (b) interface SiN layers. The intensity maxima of forward focusing peaksindicated by crosses and solid circles correspond to the second- and third-nearest neighboring atoms, respectively. The dashed lines indicate the ð10�10ÞKikuchi-band-like structures.

Jpn. J. Appl. Phys. 55, 085701 (2016) N. Maejima et al.

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