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Electron spin resonance study on heat-treated germanium surfaces
SURFACE SCIENCE 32 (1972) 694-702 © North-Holland Publishing Co.
ELECTRON SPIN RESONANCE STUDY ON HEAT-TREATED GERMANIUM SURFACES
ICHIRO WATANABE, NOBUKATSU TANAKA and TATSUO SH1MIZU
Department of Electronics, Faculty of Technology, Kanazawa University, Kanazawa, Japan
Received 26 October 1971 ; revised manuscript received 21 March 1972 Ge powder heated in air has shown two kinds of electron spin resonance signals at g -- 2.0034 (300-475 °C heating) and 2.0027 (475-700 °C). In the case of heating in vacuum of ~ 10-~ Torr, two kinds of signals at g = 2.0029 (400-650°C) and 2.0026 (400-650°C) have been observed in addition to the above two signals. The g = 2.0026 center disappears upon exposure to air. The signal intensity of the latter center increases and decreases by heating in CH4 and He atmosphere, respectively, consistent with the suggestion by Miller and Haneman that the center is associated with carbon contamination. From the measurement after the removal of the surface layer by etching, it is supposed that the disappearing center is situated on or near the surface, and that the g - 2.0029 center is situated at some depth below the surface.
I. Introduction N a r r o w electron spin resonance signals from surfaces of various semic o n d u c t o r powders have been reported by m a n y authors 1-6). Each of the signals appears by v a c u u m heat t r e a t m e n t in the range 400-600°C, a n d has similar properties; 9=2.0027_+0.0003 a n d line-width of a b o u t one gauss. The signals are reversibly b r o a d e n e d beyond detection by exposure to air or oxygen 1-3). Recent mass spectroscopic study 7) o n silicon heated in ultrahigh v a c u u m has shown the existence of various b a c k g r o u n d carbide c o n t a m i n a t i o n . C a r b o n c o n t a m i n a t i o n associated with silicon is also observed by Miller a n d H a n e m a n 6) a n d they conclude that the origin of the 9 = 2.0027 resonance is due to the c a r b o n c o n t a m i n a t i o n , especially methane, introduced from the v a c u u m system. D u r i n g heating, the m e t h a n e can decompose leaving c a r b o n rings which stabilize u n p a i r e d electrons. I n the present paper, detailed studies o n the electron spin resonance (ESR) signals which appeared from heat-treated g e r m a n i u m surfaces in various atmospheres are performed, a n d some new features a b o u t the signals are found. 694
ESR STUDY ON HEAT-TREATED Ge SURFACES
695
2. Experimental Three types of Ge samples have been used; (a) intrinsic polycrystal (50f~ cm), (b) n-type single crystal (10 ~ cm), (c) p-type single crystal (10 f~ cm). Also Si and GaAs have been used. The crystals were etched in a solution made up from H F and HNO3, and then were crushed in air into fine powder of an average particle size of about 1 ~tm by an agate mortar*. The surface area of the samples is estimated to be 1.1 × 104 cm2/g, if the particles are regarded as spheres of average size. Measurements were made by X-band ESR spectrometer of a detection sensitivity of about 5 × 1012 spins/gauss with 100 kHz field modulation. The signal intensity was determined from the numerical double integration of the first derivative with reference to the calibration signal of D P P H and Mn 2 ÷ in MgO. Accurate 9 values and line widths were measured by the use of a proton N M R marker. Isochronal heating (10min) was performed by inserting the quartz tube fixed in a vacuum system, with the 100 nag samples in it, into a preheated furnace. Measurements were made in vacuum of ,,~ 10 -2 Torr and in 1 atm, after cooling the sample to room temperature. All the three types of Ge powder showed similar resonance spectra, so that the (a) type Ge was mainly used in the experiment. For this sample, no ESR signal was observed before heat treatment in another more sensitive spectrometer of 5 × 51° spins/gauss sensitivity.
3. Results 3.1.
HEAT TREATMENT IN VACUUM
When Ge powder was heated in vacuum of ,-~ 10 - 2 Torr, the ESR spectra showed the behavior as shown in fig. 1, where the line width AH (peak-topeak value of the first derivative) and spin density are plotted as a function of isochronal heating temperature. Measurements were carried out at both pressure of ~ 1 0 -2 Torr and 1 atm. Similar behavior was observed for other samples heated in the range 80-10 -4 Torr. For the sample heated in 10-2 Torr at 550 °C and sealed off in ! 0-2 Torr, the line width was nearly independent of the temperature between 300 and 77°K. In this temperature range, the height of the signal was inversely proportional to temperature, obeying Curie's law. The narrow line has been known to be broadened beyond detection upon admission of air or oxygen in a reversible wayX-3). Our result, however, indicates that there is the remaining signal which does not disappear by * On the p o w d e r o f the a g a t e m o r t a r , an E S R line at g = 2.0077 can be obs e rve d after heat t r e a t m e n t , but its influence is h a r d l y detected in the present experiment.
696
I. WATANABE, N. T A N A K A AND T. SHIMIZU
admission of air, in addition to a disappearing one. The latter signal reappears by pumping. The 9 values of the spectra as a function of heating temperature are shown in fig. 2. Since open symbols below 400°C in this figure are considered to be the remaining signal as is evident from fig. l b, the g value of the disappearing signal is concluded as 2.0026_+ 0.0002, which agrees with the well-known value of the narrow line within the experimental error. On the other hand, the 9 value for the remaining signal gradually decreases from 2.0034_+ 0.0002 to 2.0026 + 0.0002 as the heating temperature increases. Such a change of 9 value can be explained if the remaining signal is made up from various centers with different 9 values, as will be discussed later. The remaining signal was detected for all three types of Ge samples and
{a) ~8
~
o
6 o
~lOZTorr ofmosphere
o
• • •
~6 (.9
o ".~ o~E,,,, \',
4
~
0
•
5
-- ~c__ ~ . ~
15
-
I
I
J
,~oo
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0 300
I
40O
,
,
500 600 Hea'l'ing Temperature (°C)
"~ 700
Fig. 1. ESR line-width (a) and spin density (b) versus isochronal (10min) heating temperature for Ge powder heated in ~ 1 0 2 Torr. Measurements were made at 10 -2 Torr and 1 atm after cooling to room temperature. The three types of symbol correspond to the three series of measurements, respectively.
ESR S T U D Y O N H E A T - T R E A T E D
Ge
697
SURFACES
also for GaAs, but for Si it was not detected. Both signal intensities, and hence the relative intensities between them depend on the ways of sample preparation, such as heating temperature, degree of vacuum or duration time in air after heating. The line shape for the resonance line changes remarkably with heating temperature. A typical spectrum heated at 625 °C and the change in the line
2.0036 o ,5 o n
:~
34
-~Z-C - ~ . . . . . . . . . . .
32
:
~lO'2Torr
-~
• • l atmosphere
30
I
~.............
26
o 2 . 0 0 2 4
ill
500
A
Q.... -~........... o--e....... A 0 0 O
~
Ii
400
500
l
'
600
I l
700
Heating Temperature (°C) Fig. 2. g value of E S R signals versus isochronal (10 min) heating temperature for Ge p o w d e r heated in ~ 1 0 -2 Torr. Measurements were made at N 10-2 T o r r and 1 atm. The three types of symbols correspond to the three series of measurements, respectively.
shape is shown in fig. 3, where a parameter S i S L indicates a ratio of measured signal intensity S to the Lorentzian intensity SL defined by S L = (rt/V"3)
A (AH) 2 ,
where A is the peak-to-peak height of a signal. If S is Gaussian, the ratio S/SL is about 0.29, therefore, the shape of the lines is intermediate between Gaussian and Lorentzian in the low temperature region. But in the high temperature region, the spectra show a longer tailing compared with the Lorentzian. 3.2.
HEAT TREATMENT IN AIR
The isochronal heating curve for the sample heated in air is somewhat different from that for the vacuum heated one as is shown in fig. 4. The result clearly indicates that there are two heating stages for a paramagnetic
698
[. WATANABE, N. TANAKA AND T. SHIMIZU
center. The center at the first stage (300-475°C) has a 9 value of 2.0034+ 0.0002, A H = 5 _ + I gauss and a maximum intensity at 350-400°C, and is annealed out at near 475°C. The line width for this center is rather narrow, compared with that for the 9 =2.0034 line observed in the case of vacuum heating in a similar temperature region. At the second stage (475-700°C),
o ~10"=Tort • • atmosphere
S/$L
5!,
300
4 0
500
600
HeatinTemperat g u(°C) re
700
(a)
t!=,j~/('Pr°t°n Piarker~j=t
i
i
L(
5G
(b) Fig. 3. (a) Line s h a p e versus isochronal (10 min) heating temperature for Ge powder heated in ~ 10 2 Torr. M e a s u r e m e n t s were m a d e at ~ 10 -2 T o r r a n d 1 a t m . S is the m e a sured intensity a n d SL is the Lorentzian intensity. T h e two types of s y m b o l c o r r e s p o n d to the two series o f m e a s u r e m e n t s , respectively. (b) Typical E S R s p e c t r u m m e a s u r e d in 1 arm for the sample heated at 625°C.
ESR STUDY
ON HEAT-TREATED
Ge
699
SURFACES
a resonance line appears at 9=2.0027_+0.0002 with a similar temperature dependent line width as is observed for the vacuum heated case. The signal intensity was almost identical in the measurement between vacuum and atmosphere for both centers.
30
25
CH,
b × 20
~15 c
c
H2
c~ir *
300
400
.
v
i
i
500 600 H e a t i n g Temperature ( ° c )
h.,,
700
Fig. 4. ESR spin density versus isochronal (10 min) heating temperature for Ge powder heated in air, H2, 10_2 Torr and CH4 atmosphere. Measurements were made at ~ 10-2 Torr after heating.
3.3. HEAT TREATMENTS IN METHANE AND IN HYDROGEN As for the origin of the narrow line, Miller and Haneman 6) have proposed CH 4 contamination among other gases as stated in the introduction. In order to examine their proposal, we tried to detect the effect of gaseous CH4 and H2. Gaseous CH 4 was prepared by heating a mixture of sodium acetate with caustic soda which was fixed in the vacuum system. The ESR intensities in the presence of the ambient of CH 4 and of H2 during isochronal heating are shown in fig.4 together with other data. The measurements were carried out after pumping the CH4 or H2 gas to ,,, 10- 2 Torr. The behavior of the g value and line width for both samples was similar to that for the vacuum heated sample. Upon exposure to air, the signal intensities were
700
i. WATANABE, N. TANAKA AND T. SHIMIZU
decreased except for low temperature heating but the remaining signals were also observed for both samples. 4. Discussion
From the experimental results described in the above section, the paramagnetic centers on the Ge surface heated in ~ 10- 2 Torr and their properties are summarized as follows. At low temperature heating (300-475°C), a g = 2.0034 center is produced. This center is considered to be not affected by the presence of oxygen during heating because it can be also observed in the case of air heating with similar intensity. At intermediate and high temperature heating above 400°C, there are two types of paramagnetic center. One is responsible for the disappearing signal (g=2.0026), which can be detected only in vacuum during the measurement. The other is responsible for the remaining signal, which is still present after introduction of air. The g value of this signal is considered to be 2.0029_+0.0002, since it has the maximum intensity at around 475°C, where its g value is 2.0029 (see figs. l b and 2), and furthermore, it is not produced in the presence of air during heat treatment. The slight increase of the line width at around 425°C in fig. la can be understood by the appearance of this g =2.0029 center. The line shape of the remaining signal has a longer tailing in comparison with the Lorentzian curve around 600°C, where the ratios S/SL are greater than I (fig. 3). Such an extreme tailing can not be expected from a single resonance line. Thus we propose that the 9=2.0027 line detected in the case of the air heating between 470 and 700°C is still present in addition to the g = 2.0029 line. The observed g shift in fig. 2 is consistent with the above statement. The production rate of the disappearing signal is largely affected by the atmospheric condition during the heat treatment. When the Ge powder is heated in a CH 4 atmosphere, the intensity of the ESR signal increases to about twice, compared with the vacuum heating. On the other hand, heating in a HE atmosphere also gives rise to similar results as the vacuum heating, but the signal intensity is reduced to about half, compared with the vacuum heating. The decrease by H 2 is explained as follows; if the disappearing signal is due to unpaired electrons on carbon rings, as suggested by Miller and Haneman 6), hydrogen in the CH4 molecule may have to be separated from the carbon during heating. Accordingly, the formation of the carbon ring is considered to be rather difficult, if the partial pressure of hydrogen during heating is high. Thus the disappearing signal is explained well on the basis of the suggestion by Miller et al.6). The presence of air during heating may place restriction on the formation of carbon rings as a result of the oxidation of carbons. The hardly detectable intensity of the dis-
ESR STUDY ON HEAT-TREATED
Ge
SURFACES
701
appearing signal tbund for the air heating will correspond to this remark. As for the remaining signal, a similar result has been found in ESR from coke deposits on silica aluminaS), where the intensity was reduced by 2 by the admission of air. In order to obtain further information about the remaining signal, here, one test was made. When the powder heated at 475°C in vacuum was rinsed with an etching solution made up from HF, HNO3 and H 2 0 in the ratio 1 : 1 : 1, the ratio of the intensity of the total signal to the remaining signal was decreased as a function of etching time (fig. 5). The decrease of the disappearing signal was also observed, if the powder was
Z%o 3
0
I I
~
I 3
i 4
i $
Etching Time (rain) Fig. 5.
Ratio of the spin density of total signal Sv to remaining signal Sa as a function of etching time for Ge powder heated at 475°C for 10 rain.
brought into contact with distilled water or acetone. As a result of these experiments, we propose that the disappearing center by exposure to air is attributed to unpaired electrons associated with carbons on or near the surface. On the other hand, the 9=2.0029 center is situated at some depth below the surface. It is well known that the carbon resonance is exchange narrowed up to about 550°C after which a broadening takes over. In our case, the spectra show such a feature. The maximum surface density of the disappearing center is ( l _ 0 . 5 ) x 1012 spins/cm 2, which correspond to a (1_+0.25)x 102A separation between the two centers if the paramagnetic center is distributed uniformly on the surface. The distance of 102A seems to be too large for causing the exchange narrowing. Therefore, we suppose that the paramagnetic center does not spread over the surface but may be localized at special contaminated positions.
702
1. W A T A N A B E , N . T A N A K A A N D T. S H I M I Z U
Acknowledgements The authors wish to acknowledge Messrs. S. Y a m a k o s h i a n d N. T o k u d a for their c o l l a b o r a t i o n at the early stage. O u r t h a n k s are also due to Messrs. M. K a n o h a n d H. H a r a for their experimental assistance.
References 1) H. Kusumoto and M. Shoji, J. Phys. Soc. Japan 17 (1962) 1678. 2) K. A. Miiller, P. Chan, R. Kleiner, D. W. Ovenall and M. J. Sparnaay, J. Appl. Phys. 35 (1964) 2254. 3) P. Chan and A. Steinemann, Surface Sci. 5 (1966) 267. 4) T. Arizumi, T. Mizutani and K. Shimakawa, Japan. J. Appl. Phys. 8 (1969) 1411. 5) T. Mizutani, K. Shimakawa and T. Arizumi, Japan. J. Appl. Phys. 9 (1970) 1478. 6) D. J. Miller and O. Haneman, Surface Sci. 19 (1970) 45; 24 (1971) 639. 7) R. C. Henderson, R. B. Marcus and W. J. Polito, J. Appl. Phys. 42 (1971) 1208. 8) C. P. Poole, Jr., E. N. Dicarlo, C. S. Noble, J. F. Itzel, Jr. and H. H. Tobin, J. Catalysis 4 (1965) 518.
ELECTRON SPIN RESONANCE STUDY ON HEAT-TREATED GERMANIUM SURFACES
ICHIRO WATANABE, NOBUKATSU TANAKA and TATSUO SH1MIZU
Department of Electronics, Faculty of Technology, Kanazawa University, Kanazawa, Japan
Received 26 October 1971 ; revised manuscript received 21 March 1972 Ge powder heated in air has shown two kinds of electron spin resonance signals at g -- 2.0034 (300-475 °C heating) and 2.0027 (475-700 °C). In the case of heating in vacuum of ~ 10-~ Torr, two kinds of signals at g = 2.0029 (400-650°C) and 2.0026 (400-650°C) have been observed in addition to the above two signals. The g = 2.0026 center disappears upon exposure to air. The signal intensity of the latter center increases and decreases by heating in CH4 and He atmosphere, respectively, consistent with the suggestion by Miller and Haneman that the center is associated with carbon contamination. From the measurement after the removal of the surface layer by etching, it is supposed that the disappearing center is situated on or near the surface, and that the g - 2.0029 center is situated at some depth below the surface.
I. Introduction N a r r o w electron spin resonance signals from surfaces of various semic o n d u c t o r powders have been reported by m a n y authors 1-6). Each of the signals appears by v a c u u m heat t r e a t m e n t in the range 400-600°C, a n d has similar properties; 9=2.0027_+0.0003 a n d line-width of a b o u t one gauss. The signals are reversibly b r o a d e n e d beyond detection by exposure to air or oxygen 1-3). Recent mass spectroscopic study 7) o n silicon heated in ultrahigh v a c u u m has shown the existence of various b a c k g r o u n d carbide c o n t a m i n a t i o n . C a r b o n c o n t a m i n a t i o n associated with silicon is also observed by Miller a n d H a n e m a n 6) a n d they conclude that the origin of the 9 = 2.0027 resonance is due to the c a r b o n c o n t a m i n a t i o n , especially methane, introduced from the v a c u u m system. D u r i n g heating, the m e t h a n e can decompose leaving c a r b o n rings which stabilize u n p a i r e d electrons. I n the present paper, detailed studies o n the electron spin resonance (ESR) signals which appeared from heat-treated g e r m a n i u m surfaces in various atmospheres are performed, a n d some new features a b o u t the signals are found. 694
ESR STUDY ON HEAT-TREATED Ge SURFACES
695
2. Experimental Three types of Ge samples have been used; (a) intrinsic polycrystal (50f~ cm), (b) n-type single crystal (10 ~ cm), (c) p-type single crystal (10 f~ cm). Also Si and GaAs have been used. The crystals were etched in a solution made up from H F and HNO3, and then were crushed in air into fine powder of an average particle size of about 1 ~tm by an agate mortar*. The surface area of the samples is estimated to be 1.1 × 104 cm2/g, if the particles are regarded as spheres of average size. Measurements were made by X-band ESR spectrometer of a detection sensitivity of about 5 × 1012 spins/gauss with 100 kHz field modulation. The signal intensity was determined from the numerical double integration of the first derivative with reference to the calibration signal of D P P H and Mn 2 ÷ in MgO. Accurate 9 values and line widths were measured by the use of a proton N M R marker. Isochronal heating (10min) was performed by inserting the quartz tube fixed in a vacuum system, with the 100 nag samples in it, into a preheated furnace. Measurements were made in vacuum of ,,~ 10 -2 Torr and in 1 atm, after cooling the sample to room temperature. All the three types of Ge powder showed similar resonance spectra, so that the (a) type Ge was mainly used in the experiment. For this sample, no ESR signal was observed before heat treatment in another more sensitive spectrometer of 5 × 51° spins/gauss sensitivity.
3. Results 3.1.
HEAT TREATMENT IN VACUUM
When Ge powder was heated in vacuum of ,-~ 10 - 2 Torr, the ESR spectra showed the behavior as shown in fig. 1, where the line width AH (peak-topeak value of the first derivative) and spin density are plotted as a function of isochronal heating temperature. Measurements were carried out at both pressure of ~ 1 0 -2 Torr and 1 atm. Similar behavior was observed for other samples heated in the range 80-10 -4 Torr. For the sample heated in 10-2 Torr at 550 °C and sealed off in ! 0-2 Torr, the line width was nearly independent of the temperature between 300 and 77°K. In this temperature range, the height of the signal was inversely proportional to temperature, obeying Curie's law. The narrow line has been known to be broadened beyond detection upon admission of air or oxygen in a reversible wayX-3). Our result, however, indicates that there is the remaining signal which does not disappear by * On the p o w d e r o f the a g a t e m o r t a r , an E S R line at g = 2.0077 can be obs e rve d after heat t r e a t m e n t , but its influence is h a r d l y detected in the present experiment.
696
I. WATANABE, N. T A N A K A AND T. SHIMIZU
admission of air, in addition to a disappearing one. The latter signal reappears by pumping. The 9 values of the spectra as a function of heating temperature are shown in fig. 2. Since open symbols below 400°C in this figure are considered to be the remaining signal as is evident from fig. l b, the g value of the disappearing signal is concluded as 2.0026_+ 0.0002, which agrees with the well-known value of the narrow line within the experimental error. On the other hand, the 9 value for the remaining signal gradually decreases from 2.0034_+ 0.0002 to 2.0026 + 0.0002 as the heating temperature increases. Such a change of 9 value can be explained if the remaining signal is made up from various centers with different 9 values, as will be discussed later. The remaining signal was detected for all three types of Ge samples and
{a) ~8
~
o
6 o
~lOZTorr ofmosphere
o
• • •
~6 (.9
o ".~ o~E,,,, \',
4
~
0
•
5
-- ~c__ ~ . ~
15
-
I
I
J
,~oo
5oo
6oo
(b)
,.
I
zoo
a ii/ D \'it
-o
/ 0
X
I0
O~
/
7,
i 0A
o
a /
/
\k © o
/
\\
\
\
\
5 O.
Q\\
co
0 300
I
40O
,
,
500 600 Hea'l'ing Temperature (°C)
"~ 700
Fig. 1. ESR line-width (a) and spin density (b) versus isochronal (10min) heating temperature for Ge powder heated in ~ 1 0 2 Torr. Measurements were made at 10 -2 Torr and 1 atm after cooling to room temperature. The three types of symbol correspond to the three series of measurements, respectively.
ESR S T U D Y O N H E A T - T R E A T E D
Ge
697
SURFACES
also for GaAs, but for Si it was not detected. Both signal intensities, and hence the relative intensities between them depend on the ways of sample preparation, such as heating temperature, degree of vacuum or duration time in air after heating. The line shape for the resonance line changes remarkably with heating temperature. A typical spectrum heated at 625 °C and the change in the line
2.0036 o ,5 o n
:~
34
-~Z-C - ~ . . . . . . . . . . .
32
:
~lO'2Torr
-~
• • l atmosphere
30
I
~.............
26
o 2 . 0 0 2 4
ill
500
A
Q.... -~........... o--e....... A 0 0 O
~
Ii
400
500
l
'
600
I l
700
Heating Temperature (°C) Fig. 2. g value of E S R signals versus isochronal (10 min) heating temperature for Ge p o w d e r heated in ~ 1 0 -2 Torr. Measurements were made at N 10-2 T o r r and 1 atm. The three types of symbols correspond to the three series of measurements, respectively.
shape is shown in fig. 3, where a parameter S i S L indicates a ratio of measured signal intensity S to the Lorentzian intensity SL defined by S L = (rt/V"3)
A (AH) 2 ,
where A is the peak-to-peak height of a signal. If S is Gaussian, the ratio S/SL is about 0.29, therefore, the shape of the lines is intermediate between Gaussian and Lorentzian in the low temperature region. But in the high temperature region, the spectra show a longer tailing compared with the Lorentzian. 3.2.
HEAT TREATMENT IN AIR
The isochronal heating curve for the sample heated in air is somewhat different from that for the vacuum heated one as is shown in fig. 4. The result clearly indicates that there are two heating stages for a paramagnetic
698
[. WATANABE, N. TANAKA AND T. SHIMIZU
center. The center at the first stage (300-475°C) has a 9 value of 2.0034+ 0.0002, A H = 5 _ + I gauss and a maximum intensity at 350-400°C, and is annealed out at near 475°C. The line width for this center is rather narrow, compared with that for the 9 =2.0034 line observed in the case of vacuum heating in a similar temperature region. At the second stage (475-700°C),
o ~10"=Tort • • atmosphere
S/$L
5!,
300
4 0
500
600
HeatinTemperat g u(°C) re
700
(a)
t!=,j~/('Pr°t°n Piarker~j=t
i
i
L(
5G
(b) Fig. 3. (a) Line s h a p e versus isochronal (10 min) heating temperature for Ge powder heated in ~ 10 2 Torr. M e a s u r e m e n t s were m a d e at ~ 10 -2 T o r r a n d 1 a t m . S is the m e a sured intensity a n d SL is the Lorentzian intensity. T h e two types of s y m b o l c o r r e s p o n d to the two series o f m e a s u r e m e n t s , respectively. (b) Typical E S R s p e c t r u m m e a s u r e d in 1 arm for the sample heated at 625°C.
ESR STUDY
ON HEAT-TREATED
Ge
699
SURFACES
a resonance line appears at 9=2.0027_+0.0002 with a similar temperature dependent line width as is observed for the vacuum heated case. The signal intensity was almost identical in the measurement between vacuum and atmosphere for both centers.
30
25
CH,
b × 20
~15 c
c
H2
c~ir *
300
400
.
v
i
i
500 600 H e a t i n g Temperature ( ° c )
h.,,
700
Fig. 4. ESR spin density versus isochronal (10 min) heating temperature for Ge powder heated in air, H2, 10_2 Torr and CH4 atmosphere. Measurements were made at ~ 10-2 Torr after heating.
3.3. HEAT TREATMENTS IN METHANE AND IN HYDROGEN As for the origin of the narrow line, Miller and Haneman 6) have proposed CH 4 contamination among other gases as stated in the introduction. In order to examine their proposal, we tried to detect the effect of gaseous CH4 and H2. Gaseous CH 4 was prepared by heating a mixture of sodium acetate with caustic soda which was fixed in the vacuum system. The ESR intensities in the presence of the ambient of CH 4 and of H2 during isochronal heating are shown in fig.4 together with other data. The measurements were carried out after pumping the CH4 or H2 gas to ,,, 10- 2 Torr. The behavior of the g value and line width for both samples was similar to that for the vacuum heated sample. Upon exposure to air, the signal intensities were
700
i. WATANABE, N. TANAKA AND T. SHIMIZU
decreased except for low temperature heating but the remaining signals were also observed for both samples. 4. Discussion
From the experimental results described in the above section, the paramagnetic centers on the Ge surface heated in ~ 10- 2 Torr and their properties are summarized as follows. At low temperature heating (300-475°C), a g = 2.0034 center is produced. This center is considered to be not affected by the presence of oxygen during heating because it can be also observed in the case of air heating with similar intensity. At intermediate and high temperature heating above 400°C, there are two types of paramagnetic center. One is responsible for the disappearing signal (g=2.0026), which can be detected only in vacuum during the measurement. The other is responsible for the remaining signal, which is still present after introduction of air. The g value of this signal is considered to be 2.0029_+0.0002, since it has the maximum intensity at around 475°C, where its g value is 2.0029 (see figs. l b and 2), and furthermore, it is not produced in the presence of air during heat treatment. The slight increase of the line width at around 425°C in fig. la can be understood by the appearance of this g =2.0029 center. The line shape of the remaining signal has a longer tailing in comparison with the Lorentzian curve around 600°C, where the ratios S/SL are greater than I (fig. 3). Such an extreme tailing can not be expected from a single resonance line. Thus we propose that the 9=2.0027 line detected in the case of the air heating between 470 and 700°C is still present in addition to the g = 2.0029 line. The observed g shift in fig. 2 is consistent with the above statement. The production rate of the disappearing signal is largely affected by the atmospheric condition during the heat treatment. When the Ge powder is heated in a CH 4 atmosphere, the intensity of the ESR signal increases to about twice, compared with the vacuum heating. On the other hand, heating in a HE atmosphere also gives rise to similar results as the vacuum heating, but the signal intensity is reduced to about half, compared with the vacuum heating. The decrease by H 2 is explained as follows; if the disappearing signal is due to unpaired electrons on carbon rings, as suggested by Miller and Haneman 6), hydrogen in the CH4 molecule may have to be separated from the carbon during heating. Accordingly, the formation of the carbon ring is considered to be rather difficult, if the partial pressure of hydrogen during heating is high. Thus the disappearing signal is explained well on the basis of the suggestion by Miller et al.6). The presence of air during heating may place restriction on the formation of carbon rings as a result of the oxidation of carbons. The hardly detectable intensity of the dis-
ESR STUDY ON HEAT-TREATED
Ge
SURFACES
701
appearing signal tbund for the air heating will correspond to this remark. As for the remaining signal, a similar result has been found in ESR from coke deposits on silica aluminaS), where the intensity was reduced by 2 by the admission of air. In order to obtain further information about the remaining signal, here, one test was made. When the powder heated at 475°C in vacuum was rinsed with an etching solution made up from HF, HNO3 and H 2 0 in the ratio 1 : 1 : 1, the ratio of the intensity of the total signal to the remaining signal was decreased as a function of etching time (fig. 5). The decrease of the disappearing signal was also observed, if the powder was
Z%o 3
0
I I
~
I 3
i 4
i $
Etching Time (rain) Fig. 5.
Ratio of the spin density of total signal Sv to remaining signal Sa as a function of etching time for Ge powder heated at 475°C for 10 rain.
brought into contact with distilled water or acetone. As a result of these experiments, we propose that the disappearing center by exposure to air is attributed to unpaired electrons associated with carbons on or near the surface. On the other hand, the 9=2.0029 center is situated at some depth below the surface. It is well known that the carbon resonance is exchange narrowed up to about 550°C after which a broadening takes over. In our case, the spectra show such a feature. The maximum surface density of the disappearing center is ( l _ 0 . 5 ) x 1012 spins/cm 2, which correspond to a (1_+0.25)x 102A separation between the two centers if the paramagnetic center is distributed uniformly on the surface. The distance of 102A seems to be too large for causing the exchange narrowing. Therefore, we suppose that the paramagnetic center does not spread over the surface but may be localized at special contaminated positions.
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1. W A T A N A B E , N . T A N A K A A N D T. S H I M I Z U
Acknowledgements The authors wish to acknowledge Messrs. S. Y a m a k o s h i a n d N. T o k u d a for their c o l l a b o r a t i o n at the early stage. O u r t h a n k s are also due to Messrs. M. K a n o h a n d H. H a r a for their experimental assistance.
References 1) H. Kusumoto and M. Shoji, J. Phys. Soc. Japan 17 (1962) 1678. 2) K. A. Miiller, P. Chan, R. Kleiner, D. W. Ovenall and M. J. Sparnaay, J. Appl. Phys. 35 (1964) 2254. 3) P. Chan and A. Steinemann, Surface Sci. 5 (1966) 267. 4) T. Arizumi, T. Mizutani and K. Shimakawa, Japan. J. Appl. Phys. 8 (1969) 1411. 5) T. Mizutani, K. Shimakawa and T. Arizumi, Japan. J. Appl. Phys. 9 (1970) 1478. 6) D. J. Miller and O. Haneman, Surface Sci. 19 (1970) 45; 24 (1971) 639. 7) R. C. Henderson, R. B. Marcus and W. J. Polito, J. Appl. Phys. 42 (1971) 1208. 8) C. P. Poole, Jr., E. N. Dicarlo, C. S. Noble, J. F. Itzel, Jr. and H. H. Tobin, J. Catalysis 4 (1965) 518.