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Polycrystallisation of amorphous thin films of Zn3P2
Vacuum/volume
Pergamon PII :SOO42-207X(98)00029-3
Polycrystallisation
50/number
I-2/pages 97 to 98/1998 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0042-207X/98 $19.00+.00
0
of amorphous thin films of
L Bryja, M Ciorga, K Jezierski, A Bohdziewicz Wyb. Wyspianskiego 27, Wrodaw, Poland
and J Misiewicz,
lnsrirureof Physics, TechnicalUniversity,
The amorphous thin films of Zn,P, were elaborated by evaporation in vacuum on cold substrates. The polycrysralline samples were obtained by annealing in vacuum for several hours in temperatures T = IOO250°C. The evolution of physical properties of layers vs. annealing temperature were studied by means of transmission and reflectivity measurements in wide energy range E = 1.38-5.5 eV. The rare of short and long range ordering of ions in crysral lattice after annealing were considered. 0 1998 Elsevier Science Ltd. All rights reserved
Introduction Zn,P, belongs to the group of II-V semiconductors. It is a very promising material for application as solar cells and infrared detectors due to its optical and photoelectrical properties.’ The fundamental absorption edge is located at 1.5 eV and the minority carrier diffusion length is equal approximately 10 pm. Thin films seemed to be the much better for the fabrication of cheap and effective solar cells. It is connected with high absorption above energy gap and much more easier preparation of thin films than monocrystals. Recently polycrystalline thin films of Zn,P, have been layered on hot substrates by several methods.“’ The authors reported good quality of layers even comparable with the quality of monocrystals. The disadvantage of these methods is the very big loss of material which much easier to put down on cold than hot substrates. The purpose of our work was : (1) The elaboration of amorphous thin films of Zn,P, by evaporation on cold substrates and then polycrystallisation of layers obtained by annealing in vacuum ; (2) The study of evolution of physical properties of thin films as a function of the annealing temperature. Experiment and results The amorphous thin films of Zn,P, were prepared by direct evaporation from polycrystals, grown in our laboratory by use of a closed-tube physical and chemical vapour transport system. Since zinc phosphide vaporises and condenses without dissociation no special precaution was used. The evaporation was made at pressures of lo-’ Pa by heating material in a quartz crucible at temperatures of T= 55t37OO”C. The molybdenum wire was used as a heater. The A&Ox plate crystals were used as
substrates. They were carefully cleaned, annealed in vacua and then cooled down to room temperature. The deposition rate of the amorphous films was 0.24.8 nm s. The samples have thickness from d = 0.1 pm to d = 2 pm. Materials obtained in this way were then annealed in vacua in temperatures up to 250°C for about 10 h. The annealing in higher temperatures caused slow evaporation of layers which increased with the increase of temperature. The EDAX measurements show very good stoichiometry of layers. The elaborated samples were studied in two self-made fullycomputerised optical set-ups with high resolution double grating monochromator in lock-in technique with the high sensitivity GaAs based photomultiplier used as a detector. In the first set-up transmission and reflectivity measurements, in fundamental absorption edge region E = 1.38-2.2 eV, were performed. The transmission and reflection coefficients are presented in Fig. 1. It is clearly seen that the absorption strongly decrease under annealing even after applying quite low temperatures. This fact is the evidence of the strong zinc and phosphide ions ordering at least in short range, because it is well known that polycrystalline materials have a higher energy gap than the amorphous ones. This fact is also the reason for the strong increase of the interferences in both transmission and reflectivity spectra for thin films after annealing. The annealing of the samples at higher temperatures than T = 200°C did not change the studied spectra much. The increase of the temperature above 300°C destroyed the layers. In the second set-up reflectivity measurements were performed in a wide energy region E = 2-5.5 eV. In Fig. 2 the spectra of reflection coefficient for the same sample as in Fig. 1 are presented and compared with those for monocrystals. One can see that the 97
L Sryja et a/: Amorphous
thin films of Zn,P, 0.60
0.80
0.60
............
amorphous
- - - -
annealed in 100°C
-
annealed in 200°C
1 ! 0.40
.-z 2 .E 0.40 I!
............
amorphous
- - - -
annealed in 100°C
-
annealed in 200°C
II
8
3 S d
z
0.20
0.20
0.00
0.00
I
1.40
1so
I .60
Energy
1.70
I
1.40
1.80
I
1.50
I
I
1.60 Energy [eV]
I
I 1.70
I
1 .a0
WI
Figure 1. Transmission (a) and reflection (b) coefficient measurements of the amorphous and annealed thin film of Zn,P, in the vicinity of fundamental absorption edge.
0.55
amorphous 0.50
____
annealed at 200°C
..-.........
monocrystalline
sample, are not observed in thin films before or after annealing. This fact, and strong changes of optical spectra in fundamental absorption edge region (see Fig. l), we interpret as being due to the strong ordering of ions in short range and very weak in long range. This explanation is based on the well known observation that the optical transition in f point are due to the nearest ions interactions, whereas the optical transitions in L and X points are due to the long range interactions.
*.-*. **._ _.:.*I’ “--...-.. ::” ..+*
.2 0.45 .z ti g
Conclusions The studies performed in our work delivered two essential informations, one technological and the other experimental :
d 0.40
0.35
0.30
I
2.0
2.5
’
I
3.0
’
I
3.5
’
I
’
4.0
f
4.5
’
5.0
Energy WI Figure 2. Reflection coefficient of the amorphous and annealed thin film of Zn,P2 in visible and ultraviolet regions. In the monocrystal the optical transitions
in L (E = 2.7 eV) and X (E = 4.6) are observed.’
Technological: The evaporation of amorphous thin films of Zn,P, on cold substrates and following polycrystallisation by annealing in uacuo is an effective method to obtain good quality polycrystalline layers. Experimental: The transmission and reflectivity measurements are very useful methods of studies of the annealing process for Zn,P, amorphous layers. This gives us information about position of energy gap and short and long ordering in crystal lattice.
References 1. Misiewicz, J., Bryja, L., Jezierski, K., Szatkowski, J., Mirowska, N., Gumienny, Z. and Placzek-Popko, E., Microelectron. J., 1994, 25, xX111.
changes of reflectivity coefficient are not so strong in this region as in the fundamental absorption edge region. It is also seen that the reflection coefficient of the monocrystal is much bigger. The optical transitions in L (E - 2.72 V) and X (E = 4.6 eV) points at the border of Brillouine Zone, clearly visible in the monocrystal
98
2. Toikazu, Suda and Kazuhiko Kakishita, J. Appl. Ph~?s.. 1992, 71, 3039. 3. Bryja, L., Jezierski, K. and Misiewicz, J., Thin Solid Films, 1993, 229, 11. 4. Jarzabek, B., Weszka, J., Burian, A. and Pocztowski, G., Thin Solid Films, 1995,279,203. 5. Jezierski, K., Misiewicz, J. and Krolicki, F., Phys Stat. Sol. (a), 1989, 112.
Pergamon PII :SOO42-207X(98)00029-3
Polycrystallisation
50/number
I-2/pages 97 to 98/1998 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0042-207X/98 $19.00+.00
0
of amorphous thin films of
L Bryja, M Ciorga, K Jezierski, A Bohdziewicz Wyb. Wyspianskiego 27, Wrodaw, Poland
and J Misiewicz,
lnsrirureof Physics, TechnicalUniversity,
The amorphous thin films of Zn,P, were elaborated by evaporation in vacuum on cold substrates. The polycrysralline samples were obtained by annealing in vacuum for several hours in temperatures T = IOO250°C. The evolution of physical properties of layers vs. annealing temperature were studied by means of transmission and reflectivity measurements in wide energy range E = 1.38-5.5 eV. The rare of short and long range ordering of ions in crysral lattice after annealing were considered. 0 1998 Elsevier Science Ltd. All rights reserved
Introduction Zn,P, belongs to the group of II-V semiconductors. It is a very promising material for application as solar cells and infrared detectors due to its optical and photoelectrical properties.’ The fundamental absorption edge is located at 1.5 eV and the minority carrier diffusion length is equal approximately 10 pm. Thin films seemed to be the much better for the fabrication of cheap and effective solar cells. It is connected with high absorption above energy gap and much more easier preparation of thin films than monocrystals. Recently polycrystalline thin films of Zn,P, have been layered on hot substrates by several methods.“’ The authors reported good quality of layers even comparable with the quality of monocrystals. The disadvantage of these methods is the very big loss of material which much easier to put down on cold than hot substrates. The purpose of our work was : (1) The elaboration of amorphous thin films of Zn,P, by evaporation on cold substrates and then polycrystallisation of layers obtained by annealing in vacuum ; (2) The study of evolution of physical properties of thin films as a function of the annealing temperature. Experiment and results The amorphous thin films of Zn,P, were prepared by direct evaporation from polycrystals, grown in our laboratory by use of a closed-tube physical and chemical vapour transport system. Since zinc phosphide vaporises and condenses without dissociation no special precaution was used. The evaporation was made at pressures of lo-’ Pa by heating material in a quartz crucible at temperatures of T= 55t37OO”C. The molybdenum wire was used as a heater. The A&Ox plate crystals were used as
substrates. They were carefully cleaned, annealed in vacua and then cooled down to room temperature. The deposition rate of the amorphous films was 0.24.8 nm s. The samples have thickness from d = 0.1 pm to d = 2 pm. Materials obtained in this way were then annealed in vacua in temperatures up to 250°C for about 10 h. The annealing in higher temperatures caused slow evaporation of layers which increased with the increase of temperature. The EDAX measurements show very good stoichiometry of layers. The elaborated samples were studied in two self-made fullycomputerised optical set-ups with high resolution double grating monochromator in lock-in technique with the high sensitivity GaAs based photomultiplier used as a detector. In the first set-up transmission and reflectivity measurements, in fundamental absorption edge region E = 1.38-2.2 eV, were performed. The transmission and reflection coefficients are presented in Fig. 1. It is clearly seen that the absorption strongly decrease under annealing even after applying quite low temperatures. This fact is the evidence of the strong zinc and phosphide ions ordering at least in short range, because it is well known that polycrystalline materials have a higher energy gap than the amorphous ones. This fact is also the reason for the strong increase of the interferences in both transmission and reflectivity spectra for thin films after annealing. The annealing of the samples at higher temperatures than T = 200°C did not change the studied spectra much. The increase of the temperature above 300°C destroyed the layers. In the second set-up reflectivity measurements were performed in a wide energy region E = 2-5.5 eV. In Fig. 2 the spectra of reflection coefficient for the same sample as in Fig. 1 are presented and compared with those for monocrystals. One can see that the 97
L Sryja et a/: Amorphous
thin films of Zn,P, 0.60
0.80
0.60
............
amorphous
- - - -
annealed in 100°C
-
annealed in 200°C
1 ! 0.40
.-z 2 .E 0.40 I!
............
amorphous
- - - -
annealed in 100°C
-
annealed in 200°C
II
8
3 S d
z
0.20
0.20
0.00
0.00
I
1.40
1so
I .60
Energy
1.70
I
1.40
1.80
I
1.50
I
I
1.60 Energy [eV]
I
I 1.70
I
1 .a0
WI
Figure 1. Transmission (a) and reflection (b) coefficient measurements of the amorphous and annealed thin film of Zn,P, in the vicinity of fundamental absorption edge.
0.55
amorphous 0.50
____
annealed at 200°C
..-.........
monocrystalline
sample, are not observed in thin films before or after annealing. This fact, and strong changes of optical spectra in fundamental absorption edge region (see Fig. l), we interpret as being due to the strong ordering of ions in short range and very weak in long range. This explanation is based on the well known observation that the optical transition in f point are due to the nearest ions interactions, whereas the optical transitions in L and X points are due to the long range interactions.
*.-*. **._ _.:.*I’ “--...-.. ::” ..+*
.2 0.45 .z ti g
Conclusions The studies performed in our work delivered two essential informations, one technological and the other experimental :
d 0.40
0.35
0.30
I
2.0
2.5
’
I
3.0
’
I
3.5
’
I
’
4.0
f
4.5
’
5.0
Energy WI Figure 2. Reflection coefficient of the amorphous and annealed thin film of Zn,P2 in visible and ultraviolet regions. In the monocrystal the optical transitions
in L (E = 2.7 eV) and X (E = 4.6) are observed.’
Technological: The evaporation of amorphous thin films of Zn,P, on cold substrates and following polycrystallisation by annealing in uacuo is an effective method to obtain good quality polycrystalline layers. Experimental: The transmission and reflectivity measurements are very useful methods of studies of the annealing process for Zn,P, amorphous layers. This gives us information about position of energy gap and short and long ordering in crystal lattice.
References 1. Misiewicz, J., Bryja, L., Jezierski, K., Szatkowski, J., Mirowska, N., Gumienny, Z. and Placzek-Popko, E., Microelectron. J., 1994, 25, xX111.
changes of reflectivity coefficient are not so strong in this region as in the fundamental absorption edge region. It is also seen that the reflection coefficient of the monocrystal is much bigger. The optical transitions in L (E - 2.72 V) and X (E = 4.6 eV) points at the border of Brillouine Zone, clearly visible in the monocrystal
98
2. Toikazu, Suda and Kazuhiko Kakishita, J. Appl. Ph~?s.. 1992, 71, 3039. 3. Bryja, L., Jezierski, K. and Misiewicz, J., Thin Solid Films, 1993, 229, 11. 4. Jarzabek, B., Weszka, J., Burian, A. and Pocztowski, G., Thin Solid Films, 1995,279,203. 5. Jezierski, K., Misiewicz, J. and Krolicki, F., Phys Stat. Sol. (a), 1989, 112.