Characterization of single-crystal SiC polytypes using X-ray and Auger photoelectron spectroscopy

Applied Surface Science 184 (2001) 167–172

Characterization of single-crystal SiC polytypes using X-ray and Auger photoelectron spectroscopy J.T. Wolana,*, B.A. Graysona, G. Akshoya, S.E. Saddowb a

Center for Microelectronics Research, University of South Florida, Tampa, FL 33620, USA Department of ECE, Emerging Materials Research Laboratory, Mississippi State, MS 39762, USA

b

Abstract Carbon core-valance-valence (KVV) Auger electron spectroscopy (AES) and core-level X-ray photoelectron spectroscopy (XPS) features are shown to be conspicuously different for cubic, hexagonal and rhombohedral phases of SiC. This was also observed for p- and n-type 6H–SiC epilayers. The origin and assignments of these features are discussed. Application of these results to the identification of post-growth SiC phases is presented. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Silicon carbide; AES; XPS; Polytypes

1. Introduction Silicon carbide is considered a promising compound semiconductor material for high-temperature, high-power, and high-frequency electronic devices due to its high breakdown field ð2:5  106 V=cmÞ, high thermal conductivity (4.9 W/(cm K)), high saturated electron drift velocity ð2  107 cm=sÞ, relatively large energy band gap, and chemical stability. From a viewpoint of crystallography, SiC is very well known as a material with polytypism. Polytypism is the phenomenon of taking different crystal structures in one-dimensional variation with the same chemical composition. The variation of occupation sites along the c-axis brings about different crystal structures named polytypes. In Ramsdell’s notation, polytypes are represented by the number of layers in the unit cell and the crystal system (C for cubic, H for hexagonal, and R for rhombohedral) [1–5]. Illustrations of several *

Corresponding author. Tel.: þ1-813-974-6250; fax: þ1-813-974-3651. E-mail address: [email protected] (J.T. Wolan).

common polytypes are shown in Fig. 1 [6]. The sequential stacking and order of three recognized distinct layers gives rise to a cubic zinc blende structure or 3C–SiC with a stacking order A, B and C (where A, B and C denote the three distinct layers), a hexagonal or wurtzite structure (the 4H–SiC and 6H– SiC used in this study), and a rhombohedral structure or 15R–SiC with a possible stacking order of ABCBACABACBCACB. The band gap energy and, consequently, the electrical and optical properties differ from one polytype to another, rendering polytype identification a critical parameter in SiC crystal growth [6]. Under certain growth conditions, multiple polytypes are possible on the same substrate and have been reported [7]. Characterization methods that identify the polytype of as-grown SiC epilayers are essential. The purpose of this study is to investigate the differences in KVV AES line shapes and core-level XPS binding energies and loss features between different polytypes and charge carriers in SiC for C spectra in order to identify a fingerprinting method. Studies based on X-ray diffraction (XRD) and infrared reflectivity techniques

0169-4332/01/$ – see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 4 9 7 - 4

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Fig. 1. Stacking order illustration of several common SiC polytypes [6].

characterizing cubic 3C–, hexagonal 4H– and 6H–, and rhombohedral 15R–SiC polytypes have been published [6,8]. In addition, a similar study using AES and XPS to fingerprint hexagonal and cubic BN has been reported [9].

2. Theory Core-valence-valence (KVV) AES can provide extensive details regarding local chemical bonding and electronic structure at solid surfaces and in free molecules. Since the core-hole wave function is localized, only the valence charge immediately surrounding the atomic site contributes to the Auger decay. The KVV spectral line shape obtained provides the orbital symmetry and chemical environment about the corehole site. It is well known that KVV Auger spectra of carbon exhibit line shapes characteristic of the bonding state of carbon in many solids [10–15]. Similarly, the core-level 1s XPS spectra of carbon exhibits a bulk plasmon-loss feature that contributes to the Auger spectral line shape. These localized KVV spectral features are well manifested in the overall energy distribution of these electrons. In this study, AES and XPS analysis was investigated as a means to identify epitaxially grown 4H–SiC, 6H–SiC and 3C– SiC polytypes and 15R–SiC bulk crystals. In addition, p- and n-type 6H–SiC epilayers were compared.

3. Epitaxial growth Using a horizontal cold-wall chemical vapor deposition (CVD) system, 3–4 mm thick SiC epitaxial

layers were grown on 4H–SiC(0 0 0 1) 8.08 and 6H– SiC(0 0 0 1) 3.58 off-axis SiC substrates to promote 4H–SiC and 6H–SiC epilayers, respectively. For the cubic phase, a 1–2 mm thick epitaxial 3C–SiC layer was grown on a Si(1 1 1) using a well-know growth process that first coats the substrate with a layer of carbon to provide a buffer layer (i.i., ‘carbonization’) for the epitaxial 3C–SiC [16]. All SiC epitaxial growth was conducted using a standard dual-precursor CVD growth technique described elsewhere [17]. The 15R–SiC bulk crystal was obtained from Sterling Semiconductor and is not an epitaxially grown material.

4. Characterization The air-exposed SiC samples were analyzed using a Physical Electronics (PHI) Model 1600 surface analysis system (base pressure 1010 Torr). The samples were ultrasonically cleaned in methanol, acetone, and ethanol and allowed to air dry, then mounted and inserted into the ultrahigh vacuum system. The XPS/AES instrumentation is based on the Physical Electronics Model 10-360 spherical capacitor energy analyzer (SCA) equipped with an Omni Focus III small area lens (800 mm diameter analysis area) and multichannel detection (MCD) technology. The XPS data was acquired with the SCA operating in the fixed analyzer transition (FAT) mode using a non-monochromatic Mg Ka (1253.6 eV) source operated at 300 W. The survey spectra were collected over a range of 0–1100 eV (Mg) using a pass-energy of 46.95 eV, and high-resolution spectra were acquired using a pass-energy of 23.50 eV. The samples were analyzed at an electron take-off angle of 308. Complementary Auger electron survey spectra were obtained at 4.0 kV, 10 mA beam conditions and an electron take-off angle of 308. Auger spectra were collected in pulse counting mode. Binding energies were referenced to the adventitious C 1s peak at 284.6 eV. All experiments were conducted at ambient temperature. Following an initial set of XPS and AES survey and highresolution spectra to serve as a base-line; each sample was sputter cleaned using 5 kV Arþ for XPS and AES survey followed by C 1s high-resolution spectra being taken in 15–30 min intervals. This was done until the oxygen signature was reduced to approximately the noise level (75 min) and consistent spectra where

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Fig. 2. XPS survey spectra obtained from an n-type 3C–SiC epilayer: (a) as-entered; (b) after 75 min; 5 kV Arþ sputter cleaning. Note the reduction of the O feature and the presence of the N peak after the ion cleaning.

obtained. No alterations other than reduction in the native oxide signature, which include oxygen as well as adventitious carbon, were performed. Since nitrogen was used as the n-type dopant, a large increase in the N 1s peak was seen after removal of the native oxide layer as shown in Fig. 2. Note that no Ar has been incorporated into the near-surface region after sputter cleaning. All spectra were consistent and repeatable.

5. Results and discussion The kinetic energy (KE) of an Auger electron involving three energy levels 1, 2 and 3 is given by EAuger ¼ Eð3Þ  Eð1Þ  Eð2Þ  D

(1)

where E(3), E(1), and E(2) are single electron binding energies of levels 3, 1, and 2, respectively; for C and Si these are one core and two valence levels. D is the interaction energy of the two final-state holes and is negligible for large band gap materials [16–18]. One can predict the approximate energies of various C KVV Auger transitions using Eq. (1) neglecting D

and calculated densities of states in the valence bands [19]. Several Auger transitions are presented for 3C– SiC and 4H–SiC based on this procedure and are in good agreement with experimental values summarized in Table 1. Carbon KVV Auger spectra from 15R–SiC, 4H– SiC, and 3C–SiC are shown in Fig. 3. The C Auger spectrum from 3C exhibits at least five distinct peaks, the main peak A1 (265.0 eV) and four smaller peaks A2 (263.0 eV), A3 (256.0 eV), A4 (252.5 eV) and A5 (243.5 eV) on the low KE side of the main peak and A6 (272.6 eV) on the high KE side. All AES peak positions are taken as the midpoint of the maximum and minimum deviation of the derivative curve, i.e., at the second derivative. Although each Si atom is surrounded by four carbon atoms and sp3 type bonding is exhibited in each of the polytypes, we observe distinct identifiable features. This is in contrast to an earlier study [18] of cubic and hexagonal SiC. The samples in that study were identified as industrial SiC(0 0 0 1) on ion implanted Si(1 1 1) substrates (not epitaxial) and the Auger data appears not to have been differentiated which assists with small feature contributions.

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Table 1 Peak positions and their possible assignments for C KVV Auger spectra of SiC

3C Peak positions (eV) Position relative to A1 Calculated Possible assignment 4H Peak positions (eV) Position relative to A1 Calculated Possible assignment a

A1

A2

A3

A4

A5

A6

265 0 264.9 KCV3V4

263 2

256 9

252.5 12.5

243.5

272.6

BPa

KCV1V3

264 0

256 8

KCV3V5 252.2 11.5

Bulk plasmon-loss.

The nonparabolic energy dispersions of the valence bands have been shown to strongly influence the densities of states in these polytypes [21]. The fine features observed in this study might be attributed to secondary Auger affects and related to these nonparabolicities; a more detailed analysis is currently underway. Of the ploytypes investigated in this study, the densities of states for C in the valence band could only be found for the 3C and 4H polytypes in the

literature [19,21]. Those for 3C being represented in Fig. 4. The valence band density-of-states of 3C–SiC from Ref. [21] shows six peak energies V1 (3.2 eV), V2 (7.9 eV), V3 (7.8 eV), V4 (10.3 eV), and V5 (11.8 eV) and V6 (15.4 eV), relative to the Fermi level. This can give rise to several KCViVj transition assignments. Of these, KCV3V4 (265.1 eV), KCV3V5 (263.6 eV), and KCV1V3 (272.2 eV) may be assigned (with good agreement) to A1, A2 and A6, respectively. The core-level C 1s XPS spectra of 3C–SiC shown in Fig. 5 indicates a board bulk plasmon-loss peak at 21 eV higher binding energy from the core peak. The Auger peak at A5 may be attributed to the bulk plasmon-loss shown in the figure by subtracting this value (21 eV) from A1 resulting in 244 eV. The broad Auger peak A5 can be assigned as a bulk plasmon-loss arising from A1 for both the 3C and 15R polytypes. Several shoulders and smaller features can be seen in the high-resolution Auger spectra, however,

Fig. 3. High-resolution C Auger spectra obtained from: (a) 15R– SiC; (b) 4H–SiC; (c) 3C–SiC polytypes after native oxide removal.

Fig. 4. Theoretical density-of-states for C atom in 3C–SiC from Ref. [20].

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Fig. 5. C 1s XPS spectra of 3C–SiC epilayer. Note the bulk plasmonloss peak that occurs at 21 eV from the 1s peak. The small peaks at 10 eV lower BE from the main peak are due to X-ray satellites.

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no attempt is made for transition assignments in this present study. This is due to the complexity of these spectral shapes and the use of a relatively high primary electron current (10 mA) which has been shown to mask fine AES structure in other crystal systems [9]. In an attempt to develop an Auger fingerprint for nand p-type SiC material, high-resolution Auger C spectra was obtained from n- and p-type 6H–SiC is presented as in Fig. 6. Nitrogen, in the form of N2, and Al, from trimethyl-aluminum, is used during CVD growth for n- and p-type doping, respectively. Several conspicuously different spectral shape features can be seen and KEs have been assigned and presented in Table 1. Due to the unavailability of density-of-states information for this system, no attempt is made to identify the specific transitions associated with these features. However, distinctions can be made based spectral shape differences.

Fig. 6. High-resolution C KVV Auger spectra obtained from: (a) n-type 6H–SiC; (b) p-type 6H–SiC epilayers.

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6. Conclusions There are significant differences in the Auger line shapes between 3C–SiC, 4H–SiC and 15R–SiC singlecrystal polytypes for C core-valance-valance (KVV) spectra. In addition, several conspicuously different Auger C KVV spectral features are observed between n- and p-type 6H–SiC single-crystal epilayers. These features may be adequately different for the AES spectra to be used as ‘‘fingerprints’’ of these polytypes and charge carriers in SiC for C KVV spectra. Further, investigations are currently underway.

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