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Crystallization of amorphous AgI studied by optical absorption of excitons
Journal of Non-Crystalline Solids 23 (1977) 181-186 © North-Holland Publishing Company
CRYSTALLIZATION OF AMORPHOUS Agl STUDIED BY OPTICAL ABSORPTION OF EXCITONS T. ARAI * and T.P. MARTIN Max-Planck-lnstitut fiir Festk6rperforschung, Stuttgart, Federal Republic of Germany
Received 6 June 1976 Revised manuscript received 9 July 1976
Exciton absorption spectra are reported for Agl measured during crystallization from the amorphous state. No well-defined absorption band could be observed in the disordered structure obtained by evaporating AgI onto a substrate at 80 K. An absorption band began to appear as the f'flms were annealed to successively higher temperatures. The position of the absorption peak indicates that the crystallization of disordered AgI into a stable zinc blende structure occurs only after passing through an unstable wurtzite form which more closely resembles the amorphous structure. The activation energy for the initial crystallization process was found to be 2.0 X 10 - 2 eV.
1. Introduction
Exciton absorption bands should be well defined even in a disordered system if the inhomogeneity o f the system averaged over the volume o f the exciton is small. Evidence for this is given by the observation o f a single absorption band for the relatively weakly bound excitons in mixed crystals such as A g I - C u I [1]. On the other hand, for the tightly bound excitions in KC1-KBr, the local inhomogeneity is so strong that the absorption band breaks up into two components [2]. The shape and intensity o f the excition absorption should also be a measure of the extent and degree o f disorder in amorphous systems. Many researchers have discussed this problem [3]. However, there have been few attempts to measure the change in exciton absorption due to the variation o f the degree o f disorder. Olley has investigated such a .phenomenon on AszSe 3, Se and PbI 2 using the ion-bombardment technique [4]. More recently, Heidrich and Kramer have measured the optical absorption spectra o f amorphous T1C1 and T1Br [5]. By observing changes in the absorption as the films were annealed, they were able to draw conclusions concerning the crystallization process. * Permanent address: Institute for Optical Research, Kyoiku University, Shinjuku-ku 3, Tokyo, Japan. 181
182
T. Arai, T.P. Martin / Crystallization of amorphous Agl
In this paper we have chosen to measure the exciton absorption spectra of AgI as it is transformed from an amorphous form to its stable, room-temperature, crystalline form. The exciton of crystalline AgI is neither tightly nor weakly bound (radius ~20 A) and is, therefore, sensitive to both long- and short-range order of lattice. From the results of X-ray diffraction experiments, it is known that AgI can be produced in an amorphous form by evaporation in vacuum onto a cold substrate [6]. Subsequent heating of the substrate results in crystallization at about 200 K. We report here the relation between the exciton absorption spectrum and the degree of crystallization of the AgI film. From another point of view AgI is an appropriate material for investigation: it is known to have both wurtzite and zinc blende modifications. During the past few years several investigations have indicated that the hexagonal wurtzite lattice may more closely approximate the structure of amorphous Ge and Si than does the zinc blende lattice. Rudee and Howie [7] were even able to conclude that their X-ray diffraction data for amorphous Si is consistent with the presence of 14,8, wurtzite microcrystals. In this paper we will also present evidence that tends to support this point of view.
2. Experimental and results Disordered films of AgI were produced by flash evaporation from a tantalum boat onto a quartz substrate cooled to 80 K. About 100 mg of high-purity (99.99%) AgI was flash heated for about 2 s at a pressure of 5 × 10 - 6 torr using a 150 A current at 5 V. The optical transmission spectra were measured in the region between 360 and 550 nm using a Jobin-Yvon-HRS1 spectrometer with a holographic grating. An RCA 6903 photomultiplier was used as detector. The slit widths were 150 ~tm, giving a resolution of 0.12 nm. Fig. 1 shows the block diagram of the experimental system. The evaporation chamber was joined to the sample chamber using 10 mm diameter pipe to avoid excessive heating of the substrate and contamination of the sample chamber wall. The angle between the evaporation direction and the optical axis was 45 ° . The measurement was carried out as follows. After evaporation at 80 K, the sample was quickly warmed to a temperature appropriate for annealing. After an interval of 30 min the sample was again quickly cooled to 80 K and the absorption spectrum measured. This anneal-measure cycle was repeated until no further changes in absorption could be observed for a given annealing temperature. Then the sample was warmed to room temperature. After 12 h the sample was again cooled to 80 K and the absorption measured. These procedures were repeated for three different annealing temperatures. The temperature during annealing or measuring was held constant within 2 K by means of a heater and a plug controlling the supply of liquid nitrogen to the sample. The absorption spectrum, measured at various stages of the annealing process is shown in fig. 2. One troublesome property of films prepared in this way was the presence
T. Arai, T.P. Martin / Crystallization of amorphous Agl
183
:1Detector (RCA) 6903
/.5*
' L3
Evapration~ ' \ chamber /* ( tantalum ~ boat \
Wz "~,-sample chamber cryostat - it-substrate
/
Wl
~/Exit slit
I Spectro- J
=
|
meter ~ J) -i --~,g J-y H R S l J ~ I ~ L ' - - Halogentamp l"nwonce II I Slit | Aperture Chopper (1 KHz)
Fig. 1. Block diagram of the experimental system.
I
1
I
I
I
I
1
2C
QJ"/
[
== 1.5 O
/"
J
,
U
j
O
.>_
I 0.5
I
2.2
I
24
I
2.6
I
,/
I
2.8 3.0
I
3.2
I
3/*
Photon Energy (eV) Fig. 2. Absorption spectra of an Agl film measured at 80 K after (a) no annealing, (b) annealing l(or 30 min at 220 K, (c) annealing for 2 h at 220 K, (d) annealing for 10 h at 300 K.
184
T. Arai, T.P. Martin / Crystallization of amorphous Agl
of a broad background light loss probably caused by iodine released by a partial thermal decomposition of AgI during the evaporation process. The additional light loss disappeared when the sample reached a temperature near 225 K. At this temperature the vacuum deteriorated for several minutes, indicating that the solid iodine, reaching a vapor pressure of 5 × 10 -4, began to vaporize. Because of the additional absorption due to the iodine, we cannot discuss the absolute value of the absorption coefficient. The shape of the transmission spectrum before and after vaporization of the iodine did not change except in the value of its background. Thus, we can determine the absorption intensity due to exciton excitation by subtracting the background absorption from the measured values. Absorption spectra obtained in this way are shown in fig. 3. In the first stage of heat treatment, the exciton absorption band shows strong asymmetry with the peak at about 2.953 eV (417.9 nm). This peak position agrees with the main peak position of the exciton band for crystalline AgI with wurtzite structure/3-modification), but we observe no peak or shoulder corresponding to the second peak for this modification (2.988 eV at liquid He temperature). There is, however, strong asymmetry toward the higher energy side. The peak position of the exciton absorption spectrum measured after the sample was subjected to heat treatment for a long time at room temperature was observed to be about 2.925 eV (421.8 nm). This peak position agrees with the exciton absorption peak of a zinc blende (3,-modification) AgI crystal. The asymmetry of the spectrum still remains, indicating that a small amount of the wurtzite AgI remains. The sample subjected to heat treatment for a short time at room temperature shows two peaks, i.e. one at around 2.927 eV and another at around 2.943 eV.
I
I
I
I
I
3.0
3.1
3.2
I""
0.5 Z Ill O 0,4
i
._I
03 O.. O W (12
ILl
0.1
0
2.7
2.B
2.9
3.3
PHOTON ENERGY (eV)
Fig. 3. Absorption spectra, with background subtracted, of an AgI film measured at 80 K after (a) annealing for 30 min at 220 K, (b) after annealing for 2 h at 220 K, (c) after annealing for 10 h at 300 K.
T. Arai, T.P. Martin / Crystallization of amorphous Agl
185
3. Discussion We have observed that upon annealing our disordered films from 80 to 200 K an exciton absorption peak appears at a frequency expected for wurtzite structure, in spite of the fact that the stable modification of AgI for temperatures below 408 K is zinc blende. Only when the films are annealed at room temperature does the exciton peak shift to a value corresponding to the zinc blende form. That is, the crystallization of the disordered AgI into a stable zinc blende structure is not continuous but occurs with the help of an intermediate wurtzite form. Non-equilibrium intermediate states in a crystallization process have been reported by Queisser [8] and by Heidrich und Kamer [5]. In our case the reason for the appearance of the wurtzite modification can be understood from the fact that the hexgonal wurtzite lattice more closely approximates the structure of a tetrahedrally bonded random network model o f an amorphous lattice [7]. Cardona has reported that an AgI film evaporated onto a substrate at high temperature (420 K) transforms from the wurtzite to the zinc blende structure when the sample is cooled to room temperature, but a film evaporated onto a substrate at room temperature has wurtzite structure [1]. This means that the phase transition from wurtzite to zinc blende does not occur at room temperature. The present experiments indicate that the phase transition from wurtzite to zinc blende occurs gradually at room temperature. This difference may come from the large residual stress in the present film. During the first stages of annealing, the exciton absorption band was strongly asymmetric with additional absorption at higher energies. This type of asymmetry
\"4.
0.5
k k
I
o.1
"X \
•~
~
a~
x\\
0.05
0
30
60
g0
120
TIME (minl Fig. 4. Fraction of Agl film in the amorphous state (1 - A) as a function of the annealing time at (a) 178 K, (b) 200 K, (c) 220 K.
T. Arai, T.P. Martin / Crystallization of amorphous Ag!
186
10C
v
E
LU :E I'--
10
I
4
I
5
I
6
l I T X 103 (K -1) --~
Fig. 5. Time necessary for (1 - A) to fall to lie as a function of the reciprocal temperature. From the slope of this line the activation energy for crystallization is determined as 2.0 × 10 -2 eV.
could be explained by a breakdown of momentum conservation in the microcrystalline material. The relaxed selection rules result in dipole activation of the entire density of exciton states which extends to higher energy. A similar effect has been considered theoretically by Onodera and Toyozawa [9]. They found that the absorption line of an exciton acquires a high-energy tail in substitutional binary solid solutions. However, no quantitative comparison can be made since the disorder in our samples is of a different nature. The speed of the macroscopic crystallization dA/dt is given by
dA/dt = C(1 - A ) exp(-E/kT) , where T is the annealing temperature, E the activation energy for crystallization (i.e. the height of the potential barrier between atomic positions in the amorphous and the crystalline state), C the frequency factor and A the fraction of the sample in the crystalline state. It is assumed that the value of A is proportional to the integrated absorption intensity of the exciton band. The value 1 - A is shown as a function of annealing time in fig. 4. The parameters in the figure denote the annealing temperature. The activation energy for crystallization is determined from the temperature variation of the slope of these curves. Fig. 5 shows the time necessary for 1 - A to fall to lie as a function of the reciprocal temperature. The determined activation energy is 2.0 X 10 -2 eV (220 K).
References [1] [2] [3] [4] [5] [6] [71 [8] [91
M. Cardona, Phys. Rev. 129 (1963) 69. H. Mahr, Phys. Rev. 122 (1961) 1464. J. Tauc, Res. Bull. 5 (1970) 721. J.A. Olley, Solid State Commun. 13 (1973) 1437. K. Heidrich and B. Kramer, Phys. Stat. Sol. (b) 72 (1975) 579. W. Riihl, Z. Phys. 143 (1956) 605. M.L. Rudee and A. Howie, Phil. Mag. 25 (1972) 1001. H.J. Queisser, Z. fiir Phys. 52 (1958) 507. Y. Onodera and Y. Toyozawa, J. Phys. Soc. Japan 24 (1968) 341.
CRYSTALLIZATION OF AMORPHOUS Agl STUDIED BY OPTICAL ABSORPTION OF EXCITONS T. ARAI * and T.P. MARTIN Max-Planck-lnstitut fiir Festk6rperforschung, Stuttgart, Federal Republic of Germany
Received 6 June 1976 Revised manuscript received 9 July 1976
Exciton absorption spectra are reported for Agl measured during crystallization from the amorphous state. No well-defined absorption band could be observed in the disordered structure obtained by evaporating AgI onto a substrate at 80 K. An absorption band began to appear as the f'flms were annealed to successively higher temperatures. The position of the absorption peak indicates that the crystallization of disordered AgI into a stable zinc blende structure occurs only after passing through an unstable wurtzite form which more closely resembles the amorphous structure. The activation energy for the initial crystallization process was found to be 2.0 X 10 - 2 eV.
1. Introduction
Exciton absorption bands should be well defined even in a disordered system if the inhomogeneity o f the system averaged over the volume o f the exciton is small. Evidence for this is given by the observation o f a single absorption band for the relatively weakly bound excitons in mixed crystals such as A g I - C u I [1]. On the other hand, for the tightly bound excitions in KC1-KBr, the local inhomogeneity is so strong that the absorption band breaks up into two components [2]. The shape and intensity o f the excition absorption should also be a measure of the extent and degree o f disorder in amorphous systems. Many researchers have discussed this problem [3]. However, there have been few attempts to measure the change in exciton absorption due to the variation o f the degree o f disorder. Olley has investigated such a .phenomenon on AszSe 3, Se and PbI 2 using the ion-bombardment technique [4]. More recently, Heidrich and Kramer have measured the optical absorption spectra o f amorphous T1C1 and T1Br [5]. By observing changes in the absorption as the films were annealed, they were able to draw conclusions concerning the crystallization process. * Permanent address: Institute for Optical Research, Kyoiku University, Shinjuku-ku 3, Tokyo, Japan. 181
182
T. Arai, T.P. Martin / Crystallization of amorphous Agl
In this paper we have chosen to measure the exciton absorption spectra of AgI as it is transformed from an amorphous form to its stable, room-temperature, crystalline form. The exciton of crystalline AgI is neither tightly nor weakly bound (radius ~20 A) and is, therefore, sensitive to both long- and short-range order of lattice. From the results of X-ray diffraction experiments, it is known that AgI can be produced in an amorphous form by evaporation in vacuum onto a cold substrate [6]. Subsequent heating of the substrate results in crystallization at about 200 K. We report here the relation between the exciton absorption spectrum and the degree of crystallization of the AgI film. From another point of view AgI is an appropriate material for investigation: it is known to have both wurtzite and zinc blende modifications. During the past few years several investigations have indicated that the hexagonal wurtzite lattice may more closely approximate the structure of amorphous Ge and Si than does the zinc blende lattice. Rudee and Howie [7] were even able to conclude that their X-ray diffraction data for amorphous Si is consistent with the presence of 14,8, wurtzite microcrystals. In this paper we will also present evidence that tends to support this point of view.
2. Experimental and results Disordered films of AgI were produced by flash evaporation from a tantalum boat onto a quartz substrate cooled to 80 K. About 100 mg of high-purity (99.99%) AgI was flash heated for about 2 s at a pressure of 5 × 10 - 6 torr using a 150 A current at 5 V. The optical transmission spectra were measured in the region between 360 and 550 nm using a Jobin-Yvon-HRS1 spectrometer with a holographic grating. An RCA 6903 photomultiplier was used as detector. The slit widths were 150 ~tm, giving a resolution of 0.12 nm. Fig. 1 shows the block diagram of the experimental system. The evaporation chamber was joined to the sample chamber using 10 mm diameter pipe to avoid excessive heating of the substrate and contamination of the sample chamber wall. The angle between the evaporation direction and the optical axis was 45 ° . The measurement was carried out as follows. After evaporation at 80 K, the sample was quickly warmed to a temperature appropriate for annealing. After an interval of 30 min the sample was again quickly cooled to 80 K and the absorption spectrum measured. This anneal-measure cycle was repeated until no further changes in absorption could be observed for a given annealing temperature. Then the sample was warmed to room temperature. After 12 h the sample was again cooled to 80 K and the absorption measured. These procedures were repeated for three different annealing temperatures. The temperature during annealing or measuring was held constant within 2 K by means of a heater and a plug controlling the supply of liquid nitrogen to the sample. The absorption spectrum, measured at various stages of the annealing process is shown in fig. 2. One troublesome property of films prepared in this way was the presence
T. Arai, T.P. Martin / Crystallization of amorphous Agl
183
:1Detector (RCA) 6903
/.5*
' L3
Evapration~ ' \ chamber /* ( tantalum ~ boat \
Wz "~,-sample chamber cryostat - it-substrate
/
Wl
~/Exit slit
I Spectro- J
=
|
meter ~ J) -i --~,g J-y H R S l J ~ I ~ L ' - - Halogentamp l"nwonce II I Slit | Aperture Chopper (1 KHz)
Fig. 1. Block diagram of the experimental system.
I
1
I
I
I
I
1
2C
QJ"/
[
== 1.5 O
/"
J
,
U
j
O
.>_
I 0.5
I
2.2
I
24
I
2.6
I
,/
I
2.8 3.0
I
3.2
I
3/*
Photon Energy (eV) Fig. 2. Absorption spectra of an Agl film measured at 80 K after (a) no annealing, (b) annealing l(or 30 min at 220 K, (c) annealing for 2 h at 220 K, (d) annealing for 10 h at 300 K.
184
T. Arai, T.P. Martin / Crystallization of amorphous Agl
of a broad background light loss probably caused by iodine released by a partial thermal decomposition of AgI during the evaporation process. The additional light loss disappeared when the sample reached a temperature near 225 K. At this temperature the vacuum deteriorated for several minutes, indicating that the solid iodine, reaching a vapor pressure of 5 × 10 -4, began to vaporize. Because of the additional absorption due to the iodine, we cannot discuss the absolute value of the absorption coefficient. The shape of the transmission spectrum before and after vaporization of the iodine did not change except in the value of its background. Thus, we can determine the absorption intensity due to exciton excitation by subtracting the background absorption from the measured values. Absorption spectra obtained in this way are shown in fig. 3. In the first stage of heat treatment, the exciton absorption band shows strong asymmetry with the peak at about 2.953 eV (417.9 nm). This peak position agrees with the main peak position of the exciton band for crystalline AgI with wurtzite structure/3-modification), but we observe no peak or shoulder corresponding to the second peak for this modification (2.988 eV at liquid He temperature). There is, however, strong asymmetry toward the higher energy side. The peak position of the exciton absorption spectrum measured after the sample was subjected to heat treatment for a long time at room temperature was observed to be about 2.925 eV (421.8 nm). This peak position agrees with the exciton absorption peak of a zinc blende (3,-modification) AgI crystal. The asymmetry of the spectrum still remains, indicating that a small amount of the wurtzite AgI remains. The sample subjected to heat treatment for a short time at room temperature shows two peaks, i.e. one at around 2.927 eV and another at around 2.943 eV.
I
I
I
I
I
3.0
3.1
3.2
I""
0.5 Z Ill O 0,4
i
._I
03 O.. O W (12
ILl
0.1
0
2.7
2.B
2.9
3.3
PHOTON ENERGY (eV)
Fig. 3. Absorption spectra, with background subtracted, of an AgI film measured at 80 K after (a) annealing for 30 min at 220 K, (b) after annealing for 2 h at 220 K, (c) after annealing for 10 h at 300 K.
T. Arai, T.P. Martin / Crystallization of amorphous Agl
185
3. Discussion We have observed that upon annealing our disordered films from 80 to 200 K an exciton absorption peak appears at a frequency expected for wurtzite structure, in spite of the fact that the stable modification of AgI for temperatures below 408 K is zinc blende. Only when the films are annealed at room temperature does the exciton peak shift to a value corresponding to the zinc blende form. That is, the crystallization of the disordered AgI into a stable zinc blende structure is not continuous but occurs with the help of an intermediate wurtzite form. Non-equilibrium intermediate states in a crystallization process have been reported by Queisser [8] and by Heidrich und Kamer [5]. In our case the reason for the appearance of the wurtzite modification can be understood from the fact that the hexgonal wurtzite lattice more closely approximates the structure of a tetrahedrally bonded random network model o f an amorphous lattice [7]. Cardona has reported that an AgI film evaporated onto a substrate at high temperature (420 K) transforms from the wurtzite to the zinc blende structure when the sample is cooled to room temperature, but a film evaporated onto a substrate at room temperature has wurtzite structure [1]. This means that the phase transition from wurtzite to zinc blende does not occur at room temperature. The present experiments indicate that the phase transition from wurtzite to zinc blende occurs gradually at room temperature. This difference may come from the large residual stress in the present film. During the first stages of annealing, the exciton absorption band was strongly asymmetric with additional absorption at higher energies. This type of asymmetry
\"4.
0.5
k k
I
o.1
"X \
•~
~
a~
x\\
0.05
0
30
60
g0
120
TIME (minl Fig. 4. Fraction of Agl film in the amorphous state (1 - A) as a function of the annealing time at (a) 178 K, (b) 200 K, (c) 220 K.
T. Arai, T.P. Martin / Crystallization of amorphous Ag!
186
10C
v
E
LU :E I'--
10
I
4
I
5
I
6
l I T X 103 (K -1) --~
Fig. 5. Time necessary for (1 - A) to fall to lie as a function of the reciprocal temperature. From the slope of this line the activation energy for crystallization is determined as 2.0 × 10 -2 eV.
could be explained by a breakdown of momentum conservation in the microcrystalline material. The relaxed selection rules result in dipole activation of the entire density of exciton states which extends to higher energy. A similar effect has been considered theoretically by Onodera and Toyozawa [9]. They found that the absorption line of an exciton acquires a high-energy tail in substitutional binary solid solutions. However, no quantitative comparison can be made since the disorder in our samples is of a different nature. The speed of the macroscopic crystallization dA/dt is given by
dA/dt = C(1 - A ) exp(-E/kT) , where T is the annealing temperature, E the activation energy for crystallization (i.e. the height of the potential barrier between atomic positions in the amorphous and the crystalline state), C the frequency factor and A the fraction of the sample in the crystalline state. It is assumed that the value of A is proportional to the integrated absorption intensity of the exciton band. The value 1 - A is shown as a function of annealing time in fig. 4. The parameters in the figure denote the annealing temperature. The activation energy for crystallization is determined from the temperature variation of the slope of these curves. Fig. 5 shows the time necessary for 1 - A to fall to lie as a function of the reciprocal temperature. The determined activation energy is 2.0 X 10 -2 eV (220 K).
References [1] [2] [3] [4] [5] [6] [71 [8] [91
M. Cardona, Phys. Rev. 129 (1963) 69. H. Mahr, Phys. Rev. 122 (1961) 1464. J. Tauc, Res. Bull. 5 (1970) 721. J.A. Olley, Solid State Commun. 13 (1973) 1437. K. Heidrich and B. Kramer, Phys. Stat. Sol. (b) 72 (1975) 579. W. Riihl, Z. Phys. 143 (1956) 605. M.L. Rudee and A. Howie, Phil. Mag. 25 (1972) 1001. H.J. Queisser, Z. fiir Phys. 52 (1958) 507. Y. Onodera and Y. Toyozawa, J. Phys. Soc. Japan 24 (1968) 341.