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Structural peculiarities of disordered gallium antimonide produced by high-pressure rapid quenching from the liquid
JOURNAL
Journal of Non-Crystalline Solids 135 (1991)255-258 North-Holland
~1~I~[~
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Letter to the Editor
Structural peculiarities of disordered gallium antimonide produced by high-pressure rapid quenching from the liquid S.V. P o p o v a a, G . G . S k r o t s k a y a a, V.I. L a r c h e v a, G. Z e n t a i b, L. Pogfiny b, V.N. D e n i s o v c a n d B.N. M a v r i n c " Institute of High Pressure Physics, USSR Academy of Sciences, 144092 Troitsk, Moscow Region, USSR /' Central Research Institute for Physics, PO Box 49, H-1525 Budapest, Hungary ' Institute of Spectroscopy, USSR Academy of Sciences, 142092 Moscow Region, USSR
Received 8 October 1990 Revised manuscript received 2 April 1991
The microstructure of bulk samples of GaSb, obtained by rapid quenching from the liquid, was investigated by scanning electron microscope (SEM) and by Raman scattering. The Raman data showed that some spots observed in the samples by SEM were Sb, thereby proving that partial decomposition of the originally homogeneous GaSb takes place under a particular temperature and pressure treatment.
1. Introduction It was shown recently that some amorphous metallic alloys and tetrahedrally bonded semiconductors can be prepared by rapid quenching of their liquids under high pressure [1]. Results show that, under certain pressure and temperature conditions, GaSb can be produced in the amorphous state. R a m a n spectra, temperature and heat of crystallization, some other physical properties and short-range order were investigated [1]. In this p a p e r the microstructure and R a m a n spectra of amorphous and of partially crystalline GaSb were studied.
2. Experimental A toroid type high pressure chamber was utilized for sample preparation [2]. Bismuth (at pressures of 2.55, 2.69 and 7.7 GPa) and tin (at 9.2 GPa) were used for the calibration of the chamber. The temperature was measured by a c h r o m e l - a l u m e l thermocouple located outside
the heater wall. The signal from the thermocouple was fed to an amplifier and then into an oscillograph in order to estimate the average cooling rate. This was found to be 8 × 10 2 K s - I . The original samples (cylindrical shape with a diameter of 2 m m and a height of 1 mm) were cut from a single crystal of GaSb. They were placed into the graphite heater of the high pressure chamber, heated and pressed. They were then rapidly quenched from the liquid state at constant pressure. The experiments were carried out in the 7.7-9.2 G P a pressure range. The maximum temperatures were between 1100 K and 1400 K. If the t e m p e r a t u r e was less than about 1200 K, the samples contained a mixture of crystalline and amorphous phases. The measure of disorder in the GaSb samples was determined by X-ray diffraction [3] and by R a m a n scattering. R a m a n spectra of the samples were obtained by a multichannel spectrometer [4] with the p-polarized excitation of an Ar-ion laser source (A = 514.5 nm, P < = 15 mW) in grazing incidence geometry. The structure and microcomposition of the disordered GaSb were investigated by a scanning electron
0022-3093/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved
LETTER TO THE EDITOR
256
S.V. PopotJa et al. / Structural peculiarities of disordered GaSb
Fig. 1. SEM picture of a polycrystalline GaSb sample.
microscope (SEM). For this purpose, a JEOL35-type scanning microscope equipped with an ORTEC and a PGT (Princeton Gamma Technol-
ogy) X-ray microanalyzator were used. For SEM studies all of the samples were polished: first mechanically, then by plasma etching in Ar.
Fig. 2. SEM secondary electron image of an Ar ion etched surface of an amorphous GaSb sample, produced by rapid quenching of the melt under high pressure.
LETTER TO THE EDITOR
S. I~ Popot'a et aL / Structural pecufiaritu,s of disordered GaSb
257
3. Results a-GaSb
Figure 1 is a SEM picture of a GaSb polycrystal. The regions of differing darkness correspond to the different microcrystals; the contrast arises from electron-channelling. The SEM secondary electron picture of amorphous GaSb (a-GaSb), obtained by rapid quenching from the melt under high pressure, is given by fig. 2. As mentioned before, although the samples were polished by Ar-ion etching, this did not give a completely smooth surface. Therefore, the picture shows regions of different contrast in the range of 5-10 ~m. The black lines in the 1 ~m range are microcracks and some of the black spots in the same range are partly holes and some partly show microinhomogeneities of the material itself. The backscattered electron image of another part of the same sample (fig. 3) clearly indicates that some of the cracks or microvoids extend into the bulk material well below the surface (black spots with a diameter of 2-5 ~m). From the different shadows, it can be seen that the material is non-homogeneous. In the Raman spectra of a-GaSb, three bands can be observed respectively at 52, 158 and 266
c-
;
a
v >if')
Z
klA F--
_z
-
~
<
160
L 200
a
i
300
400
FREQUENCY ( c m -1 ) Fig. 4. R a m a n spectrum (a) and reduced R a m a n spectrum (b) of an a-GaSb sample. The dash-dotted line gives the laser background in our spectrometers. The dashed curve is the R a m a n spectrum of an a-GaSb sample measured earlier. The difference between the two samples is most obvious below 200 cm
1.
cm-~ (fig. 4). The band at 266 cm-1 is close to the position of the TO band for GaSb single crystal [5]. The frequencies of the other bands (52
Fig. 3. Backscattered electron image of the same sample as in fig. 2, but the picture was taken on an another part of the surface.
LETTER TO THE EDITOR
258
s.v. Popova et al. / Structural peculiarities of disordered GaSb
and 158 cm 1) coincide with these acoustic vibrations (TA and 2TA) in the density of vibrational states [5-7]. The coincidence of these main bands in the density of vibrational states in the R a m a n spectra of GaSb crystal and of amorphous GaSb shows that the short range order is the same in amorphous and in crystalline GaSb. The broader band at 226 cm-1 as compared with the T O band of crystalline GaSb is due to non-homogeneous broadening associated with different lengths of G a - S b valence bonds in amorphous material.
4. Discussion We observed peculiarities of the a-GaSb Raman spectrum (fig. 4). The most intense band is the one at 52 cm-1. This band has not previously been observed in the R a m a n spectra of a-GaSb films [7]. The appearance of this band is a very important characteristic of the amorphicity of the substance. This band disappeared on crystallization because it is solely due to the structural disorder [8]. The other feature of this spectrum is the intensity of the 158 c m - I band greater than that observed earlier in R a m a n spectra of a-GaSb [7] (dashed line). In our case, the intensity increase of this band may be ascribed to inclusions of amorphous Sb. The R a m a n spectrum of amorphous Sb shows the most intensive band at the same wavelength. This can be attributed to the S b - S b valence bonds [9]. This assumption is further confirmed by the two narrow bands appearing at 115 and 150 c m - l . These bands are typical of crystalline Sb, and the enhancements of these bands could also be observed during the crystallization of an Sb sample [10]. As shown in ref. [9], the low frequency band in the region of 40-100 cm-1 was essentially less intensive than the band at 160 cm-1. Therefore, the main contribution to the intensity of the 52 c m - l band is due to the vibrations of a-GaSb.
It is possible that inclusions of a-Sb are formed in the process of rapid quenching of the liquid under high pressure because of the decomposition of GaSb. This decomposition is confirmed by R a m a n measurements. Nevertheless, one can also expect inclusions of metallic Ga in the a-GaSb samples studied. However the Ga inclusions cannot be observed by R a m a n scattering because the intensities of the metallic Ga bands at 189 and 200 c m - 1 are about twenty times weaker than the bands at the same place in a-GaSb judging from our estimation. Even so, we identified dark spots on the SEM picture (fig. 3) which can be attributed to G a enriched regions.
5. Conclusion These facts clearly indicate the partial decomposition of GaSb in the liquid under pressure.
References [1] V.V. Brazhkin, V.I. Larchev, S.V. Popova and G.G. Skrotzkaya, Phys. Scr. 39 (1989) 338. [2] LG. Khvostantsev, L.F. Vereschagin and A.P. Novikov, High Temp. High Press. 9 (1977) 637. [3] V.V. Aksenenkov, M.M. Aleksandrova, V.D. Blank, V.I. Larchev, S.V. Popova and G.G. Skrotskaya, Phys. Status Solidi (in press). [4] A.F. Goncharov, V.N. Denisov, B.N. Mavrin and V.B. Podobedov, Zh. Eksp. Teor. Fiz. 94 (1988) 321. [5] T. Sekine, K. Vchinokura and E. Matsumura, Solid State Commun. 18 (1976) 1337. [6] M.K. Farr, J.G. Taylor and S.K. Sinha, Phys. Rev. Bll (1975) 1587. [7] T.N. Krabach, N. Wada, M.V. Klein, K.C. Cadien and J.E. Greene, Solid State Commun. 45 (1983) 895. [8] M.H. Broadsky, in: Light Scattering in Solids, ed. M. Cardona, Vol. 8 (Springer, Berlin, 1975). [9] J.S. Lannin, Phys. Rev. B15 (1977) 3863. [10] J.S. Lannin, J.M. Galleja and M. Cardona, Phys. Rev. B12 (1975) 585.
Journal of Non-Crystalline Solids 135 (1991)255-258 North-Holland
~1~I~[~
()F
~0Ii~
Letter to the Editor
Structural peculiarities of disordered gallium antimonide produced by high-pressure rapid quenching from the liquid S.V. P o p o v a a, G . G . S k r o t s k a y a a, V.I. L a r c h e v a, G. Z e n t a i b, L. Pogfiny b, V.N. D e n i s o v c a n d B.N. M a v r i n c " Institute of High Pressure Physics, USSR Academy of Sciences, 144092 Troitsk, Moscow Region, USSR /' Central Research Institute for Physics, PO Box 49, H-1525 Budapest, Hungary ' Institute of Spectroscopy, USSR Academy of Sciences, 142092 Moscow Region, USSR
Received 8 October 1990 Revised manuscript received 2 April 1991
The microstructure of bulk samples of GaSb, obtained by rapid quenching from the liquid, was investigated by scanning electron microscope (SEM) and by Raman scattering. The Raman data showed that some spots observed in the samples by SEM were Sb, thereby proving that partial decomposition of the originally homogeneous GaSb takes place under a particular temperature and pressure treatment.
1. Introduction It was shown recently that some amorphous metallic alloys and tetrahedrally bonded semiconductors can be prepared by rapid quenching of their liquids under high pressure [1]. Results show that, under certain pressure and temperature conditions, GaSb can be produced in the amorphous state. R a m a n spectra, temperature and heat of crystallization, some other physical properties and short-range order were investigated [1]. In this p a p e r the microstructure and R a m a n spectra of amorphous and of partially crystalline GaSb were studied.
2. Experimental A toroid type high pressure chamber was utilized for sample preparation [2]. Bismuth (at pressures of 2.55, 2.69 and 7.7 GPa) and tin (at 9.2 GPa) were used for the calibration of the chamber. The temperature was measured by a c h r o m e l - a l u m e l thermocouple located outside
the heater wall. The signal from the thermocouple was fed to an amplifier and then into an oscillograph in order to estimate the average cooling rate. This was found to be 8 × 10 2 K s - I . The original samples (cylindrical shape with a diameter of 2 m m and a height of 1 mm) were cut from a single crystal of GaSb. They were placed into the graphite heater of the high pressure chamber, heated and pressed. They were then rapidly quenched from the liquid state at constant pressure. The experiments were carried out in the 7.7-9.2 G P a pressure range. The maximum temperatures were between 1100 K and 1400 K. If the t e m p e r a t u r e was less than about 1200 K, the samples contained a mixture of crystalline and amorphous phases. The measure of disorder in the GaSb samples was determined by X-ray diffraction [3] and by R a m a n scattering. R a m a n spectra of the samples were obtained by a multichannel spectrometer [4] with the p-polarized excitation of an Ar-ion laser source (A = 514.5 nm, P < = 15 mW) in grazing incidence geometry. The structure and microcomposition of the disordered GaSb were investigated by a scanning electron
0022-3093/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved
LETTER TO THE EDITOR
256
S.V. PopotJa et al. / Structural peculiarities of disordered GaSb
Fig. 1. SEM picture of a polycrystalline GaSb sample.
microscope (SEM). For this purpose, a JEOL35-type scanning microscope equipped with an ORTEC and a PGT (Princeton Gamma Technol-
ogy) X-ray microanalyzator were used. For SEM studies all of the samples were polished: first mechanically, then by plasma etching in Ar.
Fig. 2. SEM secondary electron image of an Ar ion etched surface of an amorphous GaSb sample, produced by rapid quenching of the melt under high pressure.
LETTER TO THE EDITOR
S. I~ Popot'a et aL / Structural pecufiaritu,s of disordered GaSb
257
3. Results a-GaSb
Figure 1 is a SEM picture of a GaSb polycrystal. The regions of differing darkness correspond to the different microcrystals; the contrast arises from electron-channelling. The SEM secondary electron picture of amorphous GaSb (a-GaSb), obtained by rapid quenching from the melt under high pressure, is given by fig. 2. As mentioned before, although the samples were polished by Ar-ion etching, this did not give a completely smooth surface. Therefore, the picture shows regions of different contrast in the range of 5-10 ~m. The black lines in the 1 ~m range are microcracks and some of the black spots in the same range are partly holes and some partly show microinhomogeneities of the material itself. The backscattered electron image of another part of the same sample (fig. 3) clearly indicates that some of the cracks or microvoids extend into the bulk material well below the surface (black spots with a diameter of 2-5 ~m). From the different shadows, it can be seen that the material is non-homogeneous. In the Raman spectra of a-GaSb, three bands can be observed respectively at 52, 158 and 266
c-
;
a
v >if')
Z
klA F--
_z
-
~
<
160
L 200
a
i
300
400
FREQUENCY ( c m -1 ) Fig. 4. R a m a n spectrum (a) and reduced R a m a n spectrum (b) of an a-GaSb sample. The dash-dotted line gives the laser background in our spectrometers. The dashed curve is the R a m a n spectrum of an a-GaSb sample measured earlier. The difference between the two samples is most obvious below 200 cm
1.
cm-~ (fig. 4). The band at 266 cm-1 is close to the position of the TO band for GaSb single crystal [5]. The frequencies of the other bands (52
Fig. 3. Backscattered electron image of the same sample as in fig. 2, but the picture was taken on an another part of the surface.
LETTER TO THE EDITOR
258
s.v. Popova et al. / Structural peculiarities of disordered GaSb
and 158 cm 1) coincide with these acoustic vibrations (TA and 2TA) in the density of vibrational states [5-7]. The coincidence of these main bands in the density of vibrational states in the R a m a n spectra of GaSb crystal and of amorphous GaSb shows that the short range order is the same in amorphous and in crystalline GaSb. The broader band at 226 cm-1 as compared with the T O band of crystalline GaSb is due to non-homogeneous broadening associated with different lengths of G a - S b valence bonds in amorphous material.
4. Discussion We observed peculiarities of the a-GaSb Raman spectrum (fig. 4). The most intense band is the one at 52 cm-1. This band has not previously been observed in the R a m a n spectra of a-GaSb films [7]. The appearance of this band is a very important characteristic of the amorphicity of the substance. This band disappeared on crystallization because it is solely due to the structural disorder [8]. The other feature of this spectrum is the intensity of the 158 c m - I band greater than that observed earlier in R a m a n spectra of a-GaSb [7] (dashed line). In our case, the intensity increase of this band may be ascribed to inclusions of amorphous Sb. The R a m a n spectrum of amorphous Sb shows the most intensive band at the same wavelength. This can be attributed to the S b - S b valence bonds [9]. This assumption is further confirmed by the two narrow bands appearing at 115 and 150 c m - l . These bands are typical of crystalline Sb, and the enhancements of these bands could also be observed during the crystallization of an Sb sample [10]. As shown in ref. [9], the low frequency band in the region of 40-100 cm-1 was essentially less intensive than the band at 160 cm-1. Therefore, the main contribution to the intensity of the 52 c m - l band is due to the vibrations of a-GaSb.
It is possible that inclusions of a-Sb are formed in the process of rapid quenching of the liquid under high pressure because of the decomposition of GaSb. This decomposition is confirmed by R a m a n measurements. Nevertheless, one can also expect inclusions of metallic Ga in the a-GaSb samples studied. However the Ga inclusions cannot be observed by R a m a n scattering because the intensities of the metallic Ga bands at 189 and 200 c m - 1 are about twenty times weaker than the bands at the same place in a-GaSb judging from our estimation. Even so, we identified dark spots on the SEM picture (fig. 3) which can be attributed to G a enriched regions.
5. Conclusion These facts clearly indicate the partial decomposition of GaSb in the liquid under pressure.
References [1] V.V. Brazhkin, V.I. Larchev, S.V. Popova and G.G. Skrotzkaya, Phys. Scr. 39 (1989) 338. [2] LG. Khvostantsev, L.F. Vereschagin and A.P. Novikov, High Temp. High Press. 9 (1977) 637. [3] V.V. Aksenenkov, M.M. Aleksandrova, V.D. Blank, V.I. Larchev, S.V. Popova and G.G. Skrotskaya, Phys. Status Solidi (in press). [4] A.F. Goncharov, V.N. Denisov, B.N. Mavrin and V.B. Podobedov, Zh. Eksp. Teor. Fiz. 94 (1988) 321. [5] T. Sekine, K. Vchinokura and E. Matsumura, Solid State Commun. 18 (1976) 1337. [6] M.K. Farr, J.G. Taylor and S.K. Sinha, Phys. Rev. Bll (1975) 1587. [7] T.N. Krabach, N. Wada, M.V. Klein, K.C. Cadien and J.E. Greene, Solid State Commun. 45 (1983) 895. [8] M.H. Broadsky, in: Light Scattering in Solids, ed. M. Cardona, Vol. 8 (Springer, Berlin, 1975). [9] J.S. Lannin, Phys. Rev. B15 (1977) 3863. [10] J.S. Lannin, J.M. Galleja and M. Cardona, Phys. Rev. B12 (1975) 585.