Adsorption and decomposition of triethylphosphine (TEP) and tertiarybutylphosphine (TBP) on GaP(001) studied by HREELS and TPD

ap~ieO surface science ELSEVIER

Applied Surface Science 121/ 122 (1997) 245-248

Adsorption and decomposition of triethylphosphine (TEP) and tertiarybutylphosphine (TBP)on GAP(001) studied by HREELS and TPD G. Kaneda, T. Takeuchi, J. Murata, N. Sanada, Y. Fukuda * Research Institute of Electronics, Shizuoka Unit.'ersity, Hamamatsu 432, Japan

Received 30 October 1996; accepted 21 February 1997

Abstract We find that TEP is adsorbed molecularly and TBP is partially dissociated at RT. Some of the TEP molecules on the surface are desorbed and the others are decomposed into CzH 4 and H 2, evolving from the surface. The desorption peak of the TEP molecule is increased in intensity under hydrogen ambience. For TBP, the TPD peaks for C4H 9 radical and C4H 8 are found. The peaks of C a l l 9 radical and C4H s are increased and decreased in intensity, respectively, under hydrogen ambience. The decomposition mechanisms of TEP and TBP on the GAP(001) surface are discussed. © 1997 Elsevier Science B.V. PACS: 61.16.- d; 82.40.- g; 82.65.My; 81.15.Gh Keywords: Gallium phosphide; Low index single crystal surfaces; High-resolution electron energy loss spectroscopy; Temperature-programmed desorption; Triethylphosphine; Tertiarybutylphosphine;Chemisorption; Surface chemical reaction

1. Introduction Phosphine has been used as a precursor for growth of I I I - V compound semiconductor films by various techniques (see, for example, Refs. [1-4]). Recently, much attention has been paid to alternatives [5] such as alkyl-phosphine for the growth, because phosphine is toxic, hazardous, and pyrolyzed at high temperature. Triethylphosphine (TEP) and tertiarybutylphosphine (TBP) are attractive and promising precursors as an alternative to phosphine. However,

* Corresponding author. Fax: +81-53-4740630; e-mail: royfuku @rie.shizuoka.ac.jp.

there is a possibility of carbon contamination in the films when using precursors with alkyl groups, which would influence the electrical characteristics of the films. Decomposition of TEP and TBP on a clean Si(001) surface was studied [6,7]. It was found that TEP and TBP are decomposed into ethylene and isobutylene through /3-hydride elimination, respectively. However, no study on adsorption and decomposition of the alkyl-preeursors of phosphorus on GaP has been reported. Therefore, we have studied adsorption and decomposition of TEP and TBP on the clean GAP(001)-(4 X 2) surface using HREELS and TPD

0169-4332/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0169-4332(97)00298-5

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G. Kaneda et al./Applied Surface Science 121 / 122 (19971 245-248

to elucidate the decomposition mechanism of the precursors.

2. Experimental A GAP(001) sample (n-type, carrier concentration: 3) was introduced into an ultrahigh vacuum chamber• The clean GAP(001)-(4 × 2) surface, which was confirmed by reflection high-energy electron diffraction (RHEED), was obtained by several cycles of Ar ion bombardment and annealing at 450°C. TPD spectra for TEP and TBP were obtained by heating the sample with and without hydrogen gas ambience (1 X 10 -~ Torr) up to 400°C with a heating rate of about 4°C/s. The details were described elsewhere [8]. The resolution of HREELS was typically about l0 meV at an incident energy of 5 eV for this experiment. All spectra were measured in specular direction with a incident angle of 60 ° to the surface normal.

118 amu

x 5

TEP ÷

3 0 amu

x 2

02H6 ÷

x 1

C2H4 +

27 amu

1017 c m

,t

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0

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.

.

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.

200

100

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"

500

(°C)

x 5

TEP ÷

x 2

C2H6 ÷

(b)

: -" ", a

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400

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3. Results and discussion

E

27 amu

HREELS spectra of the clean, TEP (10 L)- and TBP (10 L)-adsorbed surfaces at RT were measured. Peaks at 389 and 773 cm ] for the clean surface are ascribed to the phonon of the sample [9]. Upon adsorption of TEP, the peaks at 2954, 1418, 1258 (shoulder), 1026, and 754 cm 1 appear. They can be assigned to v(CH 2) and u(CH3), 6(CH3), ~o(CH 3) and ~-(CH2), v(CC) and y(CH 3) [10,11], and v(PC) [12,13], where v, 6, w, r, and y represent stretching, deformation, wagging, twisting, and rocking vibrations, respectively. The wave numbers found here are close to the result of TEP on Si(001) [7]. These assignments, especially for v(PC), indicate that TEP is adsorbed molecularly on the surface at RT. We found the vibrational peaks at 2912, 2360, 2280, 1904, 1424, 1184, 1000, and 800 cm -~ for the surface exposed to TBP at RT. They can be ascribed to u(CH3), v(PH) for PH, v(PH) for PH 2, v(GaH), ~(CH) for C - C H 3, T(CH3), ~ ( P H 2) + 6(PH), and v(PC) [12,14,15]. Since the v(GaH) mode at 1904

k

0

i

100

C2H4 +

x 1

i

I

200

i

J

300

Substrate temperature

i

I

400

i

500

(°C)

Fig. 1. TPD spectra for TEP on the GAP(001) surface (a) without and (b) with (1 x 10 v Tort) hydrogen ambience.

cm i [16] is observed, some of the TBP molecules are partially dissociated, leaving hydrogen on gallium atoms. The tertiary butyl group is not moved to the substrate at RT because the v(PC) mode at 800 c m - ] was observed. Fig. 1 shows TPD spectra for TEP on GAP(001) without (a) and with (b) hydrogen ambience. Desorption peaks of TEP, C2H 4, and H 2 can be seen in Fig. l(a) and no other gases can be detected. Taking into account the fragmentation of TEP (at about

247

G. Kaneda et al. /Applied Surface Seience 121 / 122 (1997) 245-248

120°C) to C 2 H 4 and H2, the peaks of C2H 4 and H 2 are divided into two components at about 120 and 220°C. The peaks at about 220°C are due to decomposition of TEP into C 2 H 4 and H 2. C ~ H 4 is also detected in Fig. l(b) under hydrogen ambience but the hydrogen intensity is too strong to obtain the TPD spectra. The peak of C 2 H 4 is also divided into two components at about 120 and 220°C in (b). It is found that the peak intensity ratio of C z H 4 at about 120°C to that at 220°C is increased under hydrogen ambience. The evolution of C 2 H 6 , Ga atoms, and ethyl-Ga species from the surface is not observed in TPD spectra for TEP on GAP(001) although they were detected for triethylgallium (TEG) on GaAs(001) (see, for example, Ref. [17]). We found no carbon on the surface after the TPD experiment. We discuss the decomposition mechanism of TEP on the GAP(001) surface based on the results of HREELS and TPD. TEP chemisorbed as a molecule is desorbed between 50 and 200°C. TEP is dissociated into ethylene which is evolved between 100 and 300°C, leaving phosphorus atoms on the surface. Since the desorption peak of C 2 H 4 at about 220°C is lower in temperature than that at about 300°C for triethylgallium on GaAs(001) [17], the ethyl group would not move on Ga atoms. In general, decomposition of precursors with alkyl groups occurs through B-hydride elimination [7]. Therefore, hydrogen in the ethyl-group would be pulled out by surface gallium atoms. The hydrogen is evolved as a molecule between 100 and 300°C although the contribution from the fragment of ethylene is also included in the peak. The decrease in the intensity ratio of the peak at about 120°C to that at about 220°C under hydrogen ambience suggests the increase in the amount of TEP desorbed. For this reason we speculate that the /3-hydride elimination is hindered by adsorption of hydrogen on surface gallium atoms, leading to evolution of TEP without decomposition into ethylene. On the other hand, since hydrogen on Ga atoms is evolved at about 200°C [18], some of the TEP molecules on the surface can be decomposed through /3-hydride elimination. Fig. 2 shows TPD spectra of TBP on the GAP(001) surface without (a) and with (b) hydrogen ambience. We find the peak at about 100°C for the C 4 H 9 radical which is not due to the fragment of C 4 H I 0 and TBP because both of them are not evolved from

90 amu

x 2

TBP +

(a)

,.'....'.-.....~'. _ . , . . . . ..~ ..-~,... ~ . . . . . . . ~ . . ~ . .

58 amu

x 2

C4Hm ÷

57 amu

x 1

O4H~+

¢)

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2 amu

x 1/30

H2÷

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200

Substrate

90 amu

500

(0(3)

TBP*

x 2

t.,o:

400

temperature

x 2

58 amu ..;...

300

(b)

04Hlo +

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;.'"t"..".'~'..

57 ainu

x I

56 amu

x 1

~ ~ .~'..~:. •

04HQ

c

i

0

I

100

i

I

200

Substrate

i

04H8+

i

300

temperature

i

i

400

i

500

(°C)

Fig. 2. TPD spectra for TBP on the GAP(001) surface (a) without and (b) with (1 × 10 7 Torr) hydrogen ambience.

the surface. Since the spectra for C4H s and H 2 have some peaks in (a), we divided them into two at about 180 and 320°C for the former and three at about 100, 180, and 320°C for the latter. The peaks at about 100°C for C s H 9 and at about 180°C for C4H s in (b) are increased and decreased in intensity, respectively. Since some of the TBP molecules are partially dissociated, C 4 H 9 radicals would be easily evolved from the surface at low temperature. The hydrogen

248

G. Kaneda et al. /Applied Surface Science 121 / 122 (1997) 245 248

peak at about 100°C is due to the fragmentation of a C4H 9 radical but there is little contribution of C 4 H 8 due to the fragmentation of the radical j u d g i n g from the results in (a) and (b). W e speculate that the peaks at about 180 and 320°C for C 4 H 8 are due to dissociation of T B P (or partially dissociated TBP) and a tertiary butyl group on Ga atoms through B-hydride elimination, respectively, which leads to formation of hydrogen at the same temperature. U n d e r hydrogen ambience, hydrogen would be adsorbed on some of the Ga atoms, leading to increase and decrease in intensities of the peaks for C 4 H 9 radical at about 100°C and for C 4 H s at 180°C, respectively, because the adsorbed hydrogen would hinder B-hydride elimination. No carbon on the surface is also detected after the T P D experiment. A n ethyl radical is formed at higher temperatures than the formation temperature of ethylene in decomposition of triethylgallium on G a A s [19]. However, it was reported that C 4 H s is formed through a C 4 H 9 radical in decomposition of T B P [20]. The difference in ethyl and tertiary butyl group would be due to the fact that the electron-repelling effect in the latter is stronger than that in the former

Acknowledgements The authors gratefully acknowledge partial support from the Ministry of Education, Science and Culture, Japan and the Tokai F o u n d a t i o n for Technology.

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