Phonons in 3C-, 4H-, and 6H-SiC

~," ~:~,':~+~'-'~i~i'~"~ i ~ :

~,:,~.~.........

surface ~cience ELSEVIER

Surface Science 324 (1995) L328-L332

Surface Science Letters

P h o n o n s in 3 C - , 4 H - , and 6 H - S i C H. Nienhaus *, T.U. Kampen, W. M/Snch Laboratorium ]fir FestkiJrperphysik, Gerhard-Mercator-Universit?ttDuisburg, D-47048 Duisburg, Germany

Received 21 October 1994; accepted for publication 10 November 1994

Abstract

Silicon carbide epilayers of cubic (3C) and hexagonal (4H and 6H) polytypes were investigated by Auger electron spectroscopy, high-resolution electron energy-loss spectroscopy and Raman spectroscopy to determine the excitation energies of the optical Fuchs-Kliewer surface phonons and their relation to bulk phonon frequencies. The surfaces were treated in a buffered hydrofluoric acid solution. Loss structures attributed to excitation of Fuchs-Kliewer phonons were clearly resolved. Their energies were found at 115.9 ___1 meV irrespective of the SiC polytype. The experimental data agree with values calculated from the experimental btflk phonon frequencies and tabulated dielectric constants. Keywords: Auger electron spectroscopy;Electron energy loss spectroscopy;Low index single crystal surfaces; Phonons; Raman scattering

spectroscopy; Silicon carbide; Vibrations of adsorbed molecules

Since considerable progress in monocrystalline growth and high-quality epitaxially grown films of silicon carbide (SIC) has been achieved, interest in SiC for semiconductor device applications has increased rapidly [1,2]. A variety of experimental methods was applied to study surface and bulk properties of cubic (3C) and hexagonal (4H, 6H) SiC

[1,31. Auger electron spectroscopy (AES) [4-13] to investigate the chemical surface composition after different treatments as well as Raman scattering [14,15] to study bulk phonons have been widely applied to SiC. However, high-resolution electron energy-loss spectroscopy (HREELS) was used only in a few experimental investigations with 3C-SiC(001) surfaces [13,16]. The optical Fuchs-Kliewer (FK) sur-

* Corresponding author. Fax: +49 203 379 3163.

face phonon which is a well-characterized property of polar semiconductors [17,18] was found at an excitation energy of 116 + 2 meV. The present study investigate the excitation energies of FK and bulk phonons in 3C-, 4H- and 6H-SiC. For isotropic media the energy h ogvK of FK surface phonons is related to the frequency O~TO of the transverse optical (TO) bulk phonon and the static and electronic dielectric constants % and ~ , respectively, by [18] 1

hoFK = v

(1)

Measurements at polar semiconductors revealed that wrx is rather insensitive to surface orientation and reconstruction as well as near surface stochiometry due to a large HREELS probing depth of several hundred ~ngstrSms and a long decay length of the FK phonons [18]. For this reason, no attempt was

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ho,TO.

H. Nienhaus et aL / Surface Science 324 (1995) L328-L332

made to optimize the surface preparation in the course of the present investigation. With Raman scattering different TO branches were observed at 4H- and 6H-SiC crystals due to backfolding of phonon bands into smaller Brillouin zones [14]. Actually, Raman scattering is an appropriate tool to distinguish the different polytypes [15]. The SiC samples used in the present study were epilayers of about 1 /~m thickness grown on S i ( l l l ) surfaces in case of cubic SiC and on 4H- and 6H-SiC substrates for the hexagonal polytypes. The 4H- and 6H-SiC epilayers were doped p-type with a free carrier density in the range of 1018-1019 cm -3. The surfaces were cleaned by etching in a 50% hydrofluoric acid (HF) solution for one minute and successive dipping in a buffered solution of H F / N H a F / NH4OH (pH = 9) for further two minutes. Such wet chemical treatment passivates S i ( l l l ) surfaces by a layer of H-atoms [19]. However, in contrast to Si the SiC surfaces are not hydrophobic after this etching procedure. After removal from the buffered solution, the samples were transferred into the ultrahigh vacuum chamber within a few minutes. Low-energy electron diffraction revealed that the hexagonal surfaces were oriented along the (0001) axis whereas the 3C-SiC surface had a {111} orientation, i.e., the same as the Si substrate underneath. In Fig. 1 first-derivative AES spectra of etched SiC surfaces are displayed. The primary electron energy was adjusted to 3 keV. Obviously, all three surfaces are contaminated as O(KLL) lines at 510 eV and, in case of 3C- and 4H-SiC, even N(KLL) peaks at 381 eV are detected. The carbon Auger structure at about 271 eV exhibits mainly a carbidic character with its typical two small structures on the low-energy side of the strong AES peak [12]. In all cases no step in the main C(KVV) AES line is observed which is characteristic for graphite [8,12]. The Si(LVV) signal at about 88 eV shows the typical 3-eV shift towards lower kinetic energies in SiC relative to silicon [7]. But no change in structure or further energy shift is observed which might be caused by S i - O bonds [20]. Thus, the oxygen detected by AES seems to be predominantly bound in adsorbed molecules. The AES intensity ratios of Si(LVV) to C(KVV) signals are determined as 0.4, 0.19, and 0.26 for 3C-, 4H-, and 6H-SiC, respectively. Even for clean C-

Si'C

'

'

'

Ep=3keV

t-

C

W Z

4H

e-

t.lJ

<

6H i 0

I 200

I

I 400

, eV

600

Electron Energy Fig. 1. Auger electron spectra of SiC epilayers etched in HF and

rinsed in a buffered solution of HF/NH4F/NH4OH with pH = 9. The energy of the primary electrons was adjusted to 3 keV.

terminated surfaces these ratios are much to small [7], i.e., carbon-containing contaminations are present on the surface. In HREELS experiments the primary energy and the angle of incidence of the incoming electrons were adjusted to 5 eV and 55 °, respectively. The instrumental energy resolution amounted to 40 cm-1 = 5 meV. HREEL spectra recorded with the three SiC polytypes 3C, 4H and 6H are presented in Fig. 2. They all show the same structures, namely the single and double excitations of FK phonons, FK1 and FK2, respectively, two broad features, 6 and v, and combination losses of FK with 6 and v. Surface roughness and contaminations result in poor loss intensities and a high background. Although both effects made inelastic electron scattering experiments difficult FK phonons could be resolved on all three SiC surfaces. Irrespective of the SiC polytype, the excitation energy of the FK phonon is found as h t o ~ = 935 + 8 cm -1 = 115.9 + 1 meV. This value is almost identical with the 1 1 6 _ 2 meV reported for 3C-SiC [13,16]. The full width at half maximum (FWHM) of the FK1 loss measures 95 cm-1 = 12 meV for 3Cand 4H- and 130 cm -1 = 16 meV for 6H-SiC material and equals the one of the elastic peak. This

1-1. Nienhaus et a L / Surface Science 324 (1995) L 3 2 8 - L 3 3 2 i

1

i

FK1

i

SiC

fi×5

Ep=5eV

v

3C

×5

4H ql e-

c

u

/ 0

i 0

I

1000

I

i

2000

3000

I ~4000 cm"1

Energy Loss

Fig. 2. High-resolution electron energy-lossspectra of SiC epilayers. The samples were the same as used in the AES studies of Fig. 1. The instrumental resolution amounted to 40 cm- 1 = 5 meV. Loss structures FK1 and FK2 are due to single and double excitations of Fuchs-Kliewer phonons; 6 and ~, losses are attibuted to hydrocarbon bending modes and valence vibrations, respectively. F W H M exceeds the instrumental resolution. This finding is explained by free carrier surface plasmon excitations [21]. Both the v and the 6 structures are much broader than the single FK1 loss. At the 3Cand 4H-SiC samples, the F W H M of the ~, loss amounts to 175 cm -1 = 21.7 meV and at 6H-SiC to 220 cm -1--- 27.3 meV. The v and 6 features are assigned to the excitation of C - H x valence and bending vibrations, respectively. Broad loss structures centered at 2920 cm - 1 = 362.1 meV were also observed in H R E E L spectra recorded with hydrogenated diamond surfaces [22,23] and with hydrocarbon molecules adsorbed on metal surfaces [24]. These energy losses originate from the

.

excitation of valence vibrations in aliphatic - C H 3 and - C H 2 groups. On the other hand, C - H and S i - H vibrations would give rise to sharp loss peaks at 2839 cm - 1 = 3 5 2 meV and 2085 cm - 1 = 2 5 9 meV, respectively [22,23,25]. Such lines were never observed in the present study. Another signature of C-H2, 3 groups are their bending modes in the energy range between 1250 cm - 1 = 155 meV and 1460 cm -1 = 181 meV [24]. Valence vibrations of O - H bonds have excitation energies of 3650 cm -1 = 453 meV [26]. However, combination losses of u and FK will appear in the same energy range and may superimpose the O - H vibration line. The energies of the FK surface phonons and the bulk TO phonons are related by Eq. (1). Bulk optical phonons may be observed by using Raman scattering. The Raman spectra displayed in Fig. 3 were recorded at normal incidence of the unpolarized primary light beam. The 514.5-nm line of an Ar+-ion laser was used. The SiC samples were the same as in the HREELS experiments. At 3C-SiC, the longitudinal optical (LO) phonon of the Si substrate is observed in addition to the LO and TO phonons of the epilayer. The two hexagonal polytypes exhibit a pronounced doublet structure of their TO phonons. It is due to the enlarged elementary cell of 4H- and 6H-SiC crystals relative to wurtzite-structure 2H-SiC [14]. Table 1 lists the phonon energies as measured in the present study. The TO and LO phonon energies agree well with values published by other groups [14,15]. The bulk optical phonon frequencies WLO and O)To are connected with the dielectric constants by the Lyddane-Sachs-Teller (LST) relation

( O)L___~O )2 = es WTo

e~

(2)

For cubic SiC, the LST relation is satisfied since the dielectric constants were determined as e= = 9.72 and ~ = 6.52 [27]. From the present TO data and by using these values of the dielectric constants, Eq. (1) gives a FK phonon energy of 948 cm - 1 = 117.6 meV which agrees well the experimental value within the limits of experimental error. For the hexagonal polytypes, their anisotropy has to be taken into account. The dielectric constants

H. Nienhaus et aL / Surface Science 324 (1995) L328-L332 [

sic

i

I

[

I

= 6.52 and ELLa= 6.70 [26]. For (0001) oriented surfaces an effective dielectric constant Eelf = has to be used in Eq. (1) ([28]). The high energy TO mode given in Table 1 satisfies the LST relation. Therefore, this frequency is inserted into Eq. (1) and a FK phonon energy of 941 cm -1 = 116.7 meV is obtained for 6H-SiC. Provided 4H- and 6H-SiC have identical dielectric constants, the FK phonon energy of 4H-SiC results as 950 cm -1 --- 117.8 meV. Considering the limits of experimental error and the simplifying assumptions of the model leading to Eq. (1), the agreement between experiment and theory is excellent. In conclusion, the excitation energies of TO and LO bulk phonons and of Fuchs-Kliewer surface phonons in 3C-, 4H-, and 6H-SiC epilayers were measured with Raman and inelastic electron scattering, respectively. The samples were dipped in hydrofluoric acid and rinsed in a buffered solution of H F / N H 4 F / N H 4 O H . Irrespective of the SiC polytype, the energies of the FK phonons were found at 115.9 _ 1 meV. These data agree well with values calculated from the experimental frequencies of bulk TO phonons and tabulated static and electronic dielectric constants.

X = 51/,.5nm

3C LO{Si)

4H TO

tO

E o

6H TO

0 "I""'- ~=X.... J 400

600

---~'~ 800

1000 crfi 1 1200

Wavenumber Fig. 3. Raman scattering spectra of SiC epilayers. The samples were the same as used in the AES and HREELS studies of Figs. 1 and 2, respectively. At 3C-SiC the LO phonon of the Si substrate is excited as well.

differ parallel and perpendicular to the (0001) axis. Experimental values are only available for 6H-SiC and were reported as • . s = 9.60, ell s = 10.03, • ±

SiC polytype

h ¢/)FK

• O)TO

h (.OLo

h O)FK (theory)

3C

935 (115.9) 935 (115.9) 935 (115.9)

794.2 (98.5) 775.7 (96.2) 765,6 (94.9)

972.7 (120.6) 967.3 (119.9) 969.5 (]20.2)

948 (117.6) 950 (117.8) 941 (116.7)

6H

795.6 (98.7) 787.9 (97.7)

This study would have been impossible without the kind support by Professor Helbig, Universit~it Niirnberg-Erlangen, and the company CS GmbH, Munich. They provided us with the hexagonal and the cubic SiC-epilayer samples, respectively.

References

Table 1 Phonon excitation energies in different SiC polytypes

4H

Acknowledgements

Values are measured in c m - 1 or - in brackets - in meV. The error of htOFK is --+8 cm -1 = -+1 meV. The errors of all other values are -+ 1 c m - 1 = -+ 0.1 meV.

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