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Annealing behaviour of optical properties and structure of r.f.-sputtered InSe films
Thin Solid Films, 199 ( 1991 ) 215 222
215
ELECTRONICS AND OPTICS
A N N E A L I N G B E H A V I O U R OF OPTICAL PROPERTIES A N D S T R U C T U R E OF R . F . - S P U T T E R E D lnSe FILMS S. SHIGETOMI AND H. OHKUBO Department o['Physics, Kurume University, 1635 Mii-rnachi Kurume, Fukuoka 830 ( Jupan ) IKARI Department qf Electrical Engineering, M(vazaki University, 1-1 Gakuen-kibanadai, Mo'azaki 889-21 ( Japan )
T.
(Received April 11, 1990: revised September 18, 1990: accepted October 8, 1990)
Indium selenide (InSe) films were prepared by the r.f. sputtering technique in a range of thicknesses from 0.25 to 1.5 ~m. The annealing effects on the optical transition and structure were investigated by using optical absorption and X-ray diffractorneter measurements. The results of X-ray diffractometer measurements showed that the as-deposited film of 0.25 ~tm thickness at a substrate temperature T~ of 80 °C has an amorphous structure, while for thicknesses over 0.55 pm the asdeposited films (190°C < ~ < 280°C) small crystallites have been induced in the amorphous structure. The optical properties of the as-deposited film of 0.25 I-tm thickness could be understood from the model of amorphous solids proposed by Mott and Davis. All films annealed at 400 °C were crystalline and the c axis in the rhombohedral phase was oriented perpendicular to the substrate plane. A comparison with similar investigations of evaporated InSe films was made. The crystalline structure in the sputtered films was found to be the same as those in the flash evaporated films. The optical transition of crystalline films was also dominated by the direct interband transition, the same as that of InSe single crystal.
1. INTRODUCTION
Indium selenide (InSe) belongs to the III-VI group of semiconductors characterized by a layer structure. In recent years, InSe films have attracted attention as a new material for application to solar cells. Photovoltaic effects have been investigated on the heterostructure diode using InSe films by Ando and Katsui 1. An absolute quantum efficiency of about 18~o is obtained in the wavelength range 1.1-1.25 pm. The fundamental electrical or optical properties of evaporated InSe films have also been studied by using electrical conductivity and optical absorption measurements. The conductivity a obeys the law In a ~ T - 1/4, indicating a variable-range hopping in localized states near the Fermi level 2. Chaudhuri et al. 3 have found that the dependence of the absorption coefficient ~ on the photon energy hv is described by the relation (~hv) 1/2 ~: hv and is understood in terms of the model of amorphous solids proposed by Mott and Davis 4. Moreover, the results of 0040-6090/91,/$3.50
,'~!)ElsevierSequoia/Printedin The Netherlands
216
S. SHIGETOMI, H. OHKUBO, T. IKARI
the X-ray diffraction analysis indicate that the c axis of crystalline lnSe films after annealing is oriented perpendicular to the substrate plane s. In all previous work, InSe films were prepared by vacuum evaporation. In this paper, we form InSe films by using an r.f. sputtering technique and present the results of a systematic investigation of annealing effects on the structure and optical properties of sputtered lnSe films. 2. EXPERIMENTAL DETAILS
InSe films were prepared by a conventional r.f. sputtering technique. Pyrex glass was cleaned chemically and fixed in the substrate holder. A thermocouple was attached to the substrate holder, in order to measure the temperature of the substrate during sputtering. The target was obtained from a stoichiometric mixture (Ins0S%o) of indium (99.999°o pure) and selenium (99.99901; pure). The InSe target was sputtered on Pyrex glass substrates in argon gas ( 1 x 10-- 3 Torr). The r.f. input power (at 13.56 MHz) was 200 W. The sputtering time was varied from 2 to 25 min, and the thickness of sputtered films ranged from 0.25 to 1.5 I.tm. The substrate temperature increased gradually from 80 to 280 "C with the sputtering time and was not controlled. The substrate temperature was obtained from the maximum temperature during sputtering. Isochronal annealing of sputtered films was performed in a vacuum of 2 x 10- 6 Tort for 30 rain at temperatures up to 500 C . In order to compare the optical properties and structure between in the sputtered films and in the single crystal, lnSe single crystals were grown by the conventional Bridgman method. The single crystal was obtained from undoped stoichiometric melts. The optical absorption spectra were measured using a grating m o n o c h r o m a t o r with an attached S-1 photomultiplier and PbS photoresistor. The output signal was detected by a lock-in amplifier. The X-ray diffractometer measurements were employed to study the structure of the films. The K~t radiation from a copper target tube was used as an X-ray source. All optical and diffraction spectra were measured at room temperature. 3. RESULTS AND DISCUSSION
The X-ray diffraction spectra of as-deposited films with four different thicknesses (0.25, 0.55, 0.83 and 1.5 lam) are shown in Fig. 1. The spectra from the c plane of InSe single crystal and the glass substrate are also illustrated for comparison in the figure. For the spectrum of the film of 0.25 btm thickness, there are significant differences from that of the glass substrate. A broad peak appears at about 20 = 28". The film of 0.25 ~m thickness does not exhibit the crystalline peaks and shows the diffraction pattern of an a m o r p h o u s structure. In addition to the weaker broad peak, the films with thicknesses in the range 0.55-1.5 gm have two crystalline peaks corresponding to the (111) and (222) reflections of the InSe crystal, indicating the existence of small crystallites in the amorphous films. One of the most significant factors in the transformation from the amorphous state to the
EFFECT OF ANNEALING
ON R.F.-SPUTTERED
InSe
217
m t rature
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t~ t,,Z
x 1/2
(Ill)
I
t I
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11l
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20 30 40 DIFFRACTION ANGLE 20 (deg.)
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Fig. 1. X-raydiffractionspectra of as-depositedInSefilmswithdifferentthicknesses.
amorphous state including the small crystallites is the variation of the substrate temperature caused by r.f. sputtering. The annealing behaviour of the X-ray diffraction spectra of a film of 0.55 ~tm thickness is shown in Fig. 2. At an annealing temperature of 300 °C, the broad peak disappears completely and the (111) and (222) diffraction peaks are dominant. These intensities increase with increasing annealing temperature, indicating that the grains of the crystal in the film have grown with increasing annealing temperature and this film attains the polycrystalline state. When the annealing temperature reaches 400 °C, the (333) and (444) diffraction peaks appear in addition to the (111) and (222) peaks, and its pattern is the same as that from the c plane of InSe single crystal. The values of the d spacings of the lattice plane for different thicknesses after annealing at 400 °C are shown in Table I. These values are in good agreement with the d spacings of the rhombohedral phase observed in InSe single crystal. This observation indicates conclusively that the sputtered films after annealing at 400 °C form with the c axis of crystalline InSe oriented perpendicular to the substrate plane. The as-deposited film of 0.25 lam thickness exhibited the diffraction pattern of an amorphous structure, as shown in Fig. 1. The general features of the density of states in amorphous solids can be understood from the model proposed by Mott and
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Davis a. The optical absorption coclticient :4 of a m o r p h o u s s e m i c o n d u c t o r s obeys the relation
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(I)
where hv is the photon energy and Eo is the optical gap. Figure 3 shows the relation (2hvl~ e rs. hv for as-deposited tilms of different thicknesses. For the film of 0.25 lam thickness, e q n . ( l ) holds over a much wider range of hr. However. the relation between (21n') ~ e and hv for the lilms in the thickness range 0.55 1.5 t~m deviates from linearity. This behaviour reveals the transformation from the a m o r p h o u s state to the
EFFECT OF ANNEALING ON R.F.-SPUTTERED
InSe
219
2000 thickness 0.25 jam
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amorphous state including the small crystallites because two crystalline peaks exist in these films, as shown in Fig. 1. The value of E o for the film of 0.25 pm thickness can be determined from the intercept with ~ = 0 in the (~hv) 1/2 vs. hv plot. The obtained value o f E 0 is 1.42eV which almost agrees with that for lnSe films fabricated by flash evaporation (E0 = 1.3 e V ) 6. Figure 4 shows the isochronal annealing behaviour of ~ at hv = 2.25 eV. The annealing was carried out at temperatures higher than ~. The absorption coefficient gradually decreases with the increase in annealing temperature up to 250 °C and suddenly decreases with increasing temperature between 300 and 400 °C. However, increases for annealing at temperatures higher than 450 °C. In combination with the result of the X-ray diffraction measurements in Fig. 2, the variation in ~ with the annealing temperature between 300 and 400 °C indicates the transformation from the polycrystalline state to the crystalline state of the rhombohedral type. For annealing at temperature above 450°C, ~ increases with increasing annealing temperature. This may suggest that the decomposition of the films occurs at these higher annealing temperatures. Since the films after annealing at 400°C have the crystalline structure, a comparison of the optical transitions between the sputtered film and the single crystal was carried out. The absorption spectrum of the single crystal is dominated by the indirect and direct interband transitions at hv = 1-3 eV. The spectra of the crystalline films in the region of the investigated thickness were affected by the multiple reflection in the indirect transition region, and we could not estimate the
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100 200 300 400 500 600 ANNEALING TEMPERATURE (°C)
Fig. 4. Annealing behaviour of the optical absorption coefficient at hv - 2.25 eV. b a n d edge of the indirect transition. The a b s o r p t i o n spectra of the crystalline films with 0.83 a n d 1.5 p.m thicknesses axe a l m o s t uninfluenced by the effect of the multiple reflection for hv a b o v e 2.0 eV. The a b s o r p t i o n coefficient ~ of the crystalline films is 'controlled by the direct f o r b i d d e n t r a n s i t i o n a n d obeys the r e l a t i o n ; ~hv = A ( h v - Eg) 3/2
(2)
where A is c o n s t a n t a n d Eg is the direct energy gap. E q u a t i o n (2) holds over a m u c h wider range o f h v for the films a n n e a l e d at 400 ~'C as s h o w n in Fig. 5. The s p e c t r u m of 3200 400 °C anneat thickness 0.83 pm
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Fig. 5. Dependence of(~hv)23 on hv for tilms annnealed at 400' C and single-crystal InSe.
EFFECT OF ANNEALING ON R.F.-SPUTTERED
InSe
221
the single crystal is also illustrated for comparison in the figure. The electric field vector of the incident light was propagated parallel to the layer. The value of Eg = 1.34 eV for the single crystal is obtained from the intersection of the straight line with the abscissa and almost agrees with that given by Andriyashik et al. (E, = 1.293 eV)S.The values for E~ in the films of 0.83 ~tm and 1.5/am thickness are estimated as 1.40 eV and 1.32 eV respectively and are nearly the same as that of the single crystal. From the results of the above observation, it is concluded that the optical absorption in the region of hv from 2.0 to 2.5 eV for the films annealed at 400 °C is dominated by the direct interband transition in the same manner as that of the single crystal. Since the annealing effects on the structure and optical properties in the evaporated InSe have already been investigated, a comparison of the properties between the evaporated and the sputtered films may be valuable. The as-evaporated films have an amorphous structure at substrate temperatures T~ below about 150°C1. The optical properties of these films are interpreted according to the density of states model proposed by Mott and Davis. The optical gap of asevaporated films was obtained as 0.4-0.63 eV (conventional evaporation) 9 and 1.3 eV (flash evaporation) 6. As shown in Fig. 1, the structure of the as-sputtered film of 0.25/am thickness (T~ = 80 °C) is amorphous. The feature of the density of states in the energy band for this film in Fig. 3 can be understood by using the same model for evaporated films. The obtained value ofE o = 1.42 eV almost agrees with that for the film fabricated by flash evaporation. The degree of disorder and number of defects in the bonding structure are known to depend on E o 3. Therefore the amorphous structure prepared by sputtering is almost the same as that formed by flash evaporation. At T~in the range 150-350 °C, the as-evaporated films exhibit the polycrystalline lnSe structure 1. Furthermore, crystalline films are formed by subsequent annealing at 350 °C for 5-8 h. Also, the films formed by flash evaporation have the crystalline state of the rhombohedral type at about 340 °C 5 The as-sputtered films at T~ = 190-280 °C in Fig. I form the amorphous state including the small crystallites. The films attain the polycrystalline state as a result of subsequent annealing at 300 °C (Fig. 2). The crystalline films of the rhombohedral type are formed at an annealing temperature of 400 °C, as shown in Fig. 2 and Table I. The value of T~(190 °C) on crystallization of sputtered films almost agrees with that (150 °C) of the evaporated films. However, there are discrepancies in the structure between the sputtered (amorphous state including the small crystallites) and evaporated (polycrystalline state) films. One possible reason might be that T~of the films was not controlled during the sputtering. The final crystalline films have the rhombohedral structure of the same type in both the sputtered and the flashevaporated films. 4. CONCLUSION The X-ray diffraction and optical absorption of sputtered InSe films were measured as a function of thickness and annealing temperature. These results provided good evidence for the existence of the stoichiometric InSe film. A
222
S. SH1GET()MI. H. OHKUBO, T. IKARI
comparison of the properties between the evaporated and sputtered films were carried out. The optical properties of the amorphous structure formed by sputtering and by evaporation were summarized in terms of the same model of the density of states given by Mott and Davis. Both of lhe crystalline films after annealing were characterized by the rhombohedral structure of the same type. Furthermore, the optical absorption of sputtered crystalline films could be explained by the direct interband transition in the same manner as that of the single crystal. We see that the r.f. sputtering method is a useful technique for obtaining lnSe films. ACKNOWLEDGMENTS
The authors would like to thank Professor Yutaka Koga and Professor Shigenobu Shigetomi of Kurume University for their suggestions and encouragement. REFERENCES l 2 3 4 5 6 7
9
K. A n d o and A. Katsui, Yh#l SoIM Films'. 76 I 1981 ) 141. D . V . K . Sastry and P. J. Reddy, Th#z Solid Film.~, ltL5 (1983) 139, S. Chaudhuri, S. K. Biswas and A. Choudhury, S¢~IM S*ute Commlol., 53 (1985) 273. N . F . Mott and E. A. Davis, Elecm~*~ic Pmce,~.s'e,~' itz Nop>crrstalline .,~tatcriat~', Clarendon, Oxford. 1971. H. H a s h i m o l o , H. Nishimura and 1-|. Suzuki, Jl?t;. J..4ppl. Ph r.s., 2# ( 1981 ) 1163, 1. W a t a n a b e and T. Y a m a m o t o , Jpu..1. ,4ppl. P/ms'.. 24 (1985) 1282. J. 1. Pankove, Optical Proce.sses it; Senliconductor,s, Prentice-Hall, Englewood Cliffs, N J, 1971. M. V. Andriyashik, M. Y. Sakhnovskii. V. B. Trimofeev and A. S. Yakimova, Phy.s, ,Slutu,s Solidi. 2,~¢ (1968) 277. S . K . Biswas, S. Chaudhuri and A. C h o u d h u r y , t'/ly,~. Ntott,.s' .S'o[&tl', 1(15 ( [ 988) 467.
215
ELECTRONICS AND OPTICS
A N N E A L I N G B E H A V I O U R OF OPTICAL PROPERTIES A N D S T R U C T U R E OF R . F . - S P U T T E R E D lnSe FILMS S. SHIGETOMI AND H. OHKUBO Department o['Physics, Kurume University, 1635 Mii-rnachi Kurume, Fukuoka 830 ( Jupan ) IKARI Department qf Electrical Engineering, M(vazaki University, 1-1 Gakuen-kibanadai, Mo'azaki 889-21 ( Japan )
T.
(Received April 11, 1990: revised September 18, 1990: accepted October 8, 1990)
Indium selenide (InSe) films were prepared by the r.f. sputtering technique in a range of thicknesses from 0.25 to 1.5 ~m. The annealing effects on the optical transition and structure were investigated by using optical absorption and X-ray diffractorneter measurements. The results of X-ray diffractometer measurements showed that the as-deposited film of 0.25 ~tm thickness at a substrate temperature T~ of 80 °C has an amorphous structure, while for thicknesses over 0.55 pm the asdeposited films (190°C < ~ < 280°C) small crystallites have been induced in the amorphous structure. The optical properties of the as-deposited film of 0.25 I-tm thickness could be understood from the model of amorphous solids proposed by Mott and Davis. All films annealed at 400 °C were crystalline and the c axis in the rhombohedral phase was oriented perpendicular to the substrate plane. A comparison with similar investigations of evaporated InSe films was made. The crystalline structure in the sputtered films was found to be the same as those in the flash evaporated films. The optical transition of crystalline films was also dominated by the direct interband transition, the same as that of InSe single crystal.
1. INTRODUCTION
Indium selenide (InSe) belongs to the III-VI group of semiconductors characterized by a layer structure. In recent years, InSe films have attracted attention as a new material for application to solar cells. Photovoltaic effects have been investigated on the heterostructure diode using InSe films by Ando and Katsui 1. An absolute quantum efficiency of about 18~o is obtained in the wavelength range 1.1-1.25 pm. The fundamental electrical or optical properties of evaporated InSe films have also been studied by using electrical conductivity and optical absorption measurements. The conductivity a obeys the law In a ~ T - 1/4, indicating a variable-range hopping in localized states near the Fermi level 2. Chaudhuri et al. 3 have found that the dependence of the absorption coefficient ~ on the photon energy hv is described by the relation (~hv) 1/2 ~: hv and is understood in terms of the model of amorphous solids proposed by Mott and Davis 4. Moreover, the results of 0040-6090/91,/$3.50
,'~!)ElsevierSequoia/Printedin The Netherlands
216
S. SHIGETOMI, H. OHKUBO, T. IKARI
the X-ray diffraction analysis indicate that the c axis of crystalline lnSe films after annealing is oriented perpendicular to the substrate plane s. In all previous work, InSe films were prepared by vacuum evaporation. In this paper, we form InSe films by using an r.f. sputtering technique and present the results of a systematic investigation of annealing effects on the structure and optical properties of sputtered lnSe films. 2. EXPERIMENTAL DETAILS
InSe films were prepared by a conventional r.f. sputtering technique. Pyrex glass was cleaned chemically and fixed in the substrate holder. A thermocouple was attached to the substrate holder, in order to measure the temperature of the substrate during sputtering. The target was obtained from a stoichiometric mixture (Ins0S%o) of indium (99.999°o pure) and selenium (99.99901; pure). The InSe target was sputtered on Pyrex glass substrates in argon gas ( 1 x 10-- 3 Torr). The r.f. input power (at 13.56 MHz) was 200 W. The sputtering time was varied from 2 to 25 min, and the thickness of sputtered films ranged from 0.25 to 1.5 I.tm. The substrate temperature increased gradually from 80 to 280 "C with the sputtering time and was not controlled. The substrate temperature was obtained from the maximum temperature during sputtering. Isochronal annealing of sputtered films was performed in a vacuum of 2 x 10- 6 Tort for 30 rain at temperatures up to 500 C . In order to compare the optical properties and structure between in the sputtered films and in the single crystal, lnSe single crystals were grown by the conventional Bridgman method. The single crystal was obtained from undoped stoichiometric melts. The optical absorption spectra were measured using a grating m o n o c h r o m a t o r with an attached S-1 photomultiplier and PbS photoresistor. The output signal was detected by a lock-in amplifier. The X-ray diffractometer measurements were employed to study the structure of the films. The K~t radiation from a copper target tube was used as an X-ray source. All optical and diffraction spectra were measured at room temperature. 3. RESULTS AND DISCUSSION
The X-ray diffraction spectra of as-deposited films with four different thicknesses (0.25, 0.55, 0.83 and 1.5 lam) are shown in Fig. 1. The spectra from the c plane of InSe single crystal and the glass substrate are also illustrated for comparison in the figure. For the spectrum of the film of 0.25 btm thickness, there are significant differences from that of the glass substrate. A broad peak appears at about 20 = 28". The film of 0.25 ~m thickness does not exhibit the crystalline peaks and shows the diffraction pattern of an a m o r p h o u s structure. In addition to the weaker broad peak, the films with thicknesses in the range 0.55-1.5 gm have two crystalline peaks corresponding to the (111) and (222) reflections of the InSe crystal, indicating the existence of small crystallites in the amorphous films. One of the most significant factors in the transformation from the amorphous state to the
EFFECT OF ANNEALING
ON R.F.-SPUTTERED
InSe
217
m t rature
v )-
t~ t,,Z
x 1/2
(Ill)
I
t I
10
11l
11(222)
1, x '200
I] II
-° l
JL
single crystal I
I
20 30 40 DIFFRACTION ANGLE 20 (deg.)
50
Fig. 1. X-raydiffractionspectra of as-depositedInSefilmswithdifferentthicknesses.
amorphous state including the small crystallites is the variation of the substrate temperature caused by r.f. sputtering. The annealing behaviour of the X-ray diffraction spectra of a film of 0.55 ~tm thickness is shown in Fig. 2. At an annealing temperature of 300 °C, the broad peak disappears completely and the (111) and (222) diffraction peaks are dominant. These intensities increase with increasing annealing temperature, indicating that the grains of the crystal in the film have grown with increasing annealing temperature and this film attains the polycrystalline state. When the annealing temperature reaches 400 °C, the (333) and (444) diffraction peaks appear in addition to the (111) and (222) peaks, and its pattern is the same as that from the c plane of InSe single crystal. The values of the d spacings of the lattice plane for different thicknesses after annealing at 400 °C are shown in Table I. These values are in good agreement with the d spacings of the rhombohedral phase observed in InSe single crystal. This observation indicates conclusively that the sputtered films after annealing at 400 °C form with the c axis of crystalline InSe oriented perpendicular to the substrate plane. The as-deposited film of 0.25 lam thickness exhibited the diffraction pattern of an amorphous structure, as shown in Fig. 1. The general features of the density of states in amorphous solids can be understood from the model proposed by Mott and
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Davis a. The optical absorption coclticient :4 of a m o r p h o u s s e m i c o n d u c t o r s obeys the relation
~hv = c o n s t a n t × ( h v
I~,,)2
(I)
where hv is the photon energy and Eo is the optical gap. Figure 3 shows the relation (2hvl~ e rs. hv for as-deposited tilms of different thicknesses. For the film of 0.25 lam thickness, e q n . ( l ) holds over a much wider range of hr. However. the relation between (21n') ~ e and hv for the lilms in the thickness range 0.55 1.5 t~m deviates from linearity. This behaviour reveals the transformation from the a m o r p h o u s state to the
EFFECT OF ANNEALING ON R.F.-SPUTTERED
InSe
219
2000 thickness 0.25 jam
/
-~x.2°
0.55pm
/
100(3
/
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I
I
I
1.5
2.0 2.5 3.0 PHOTON ENERGY h9 (eV) Fig. 3. Dependenceof(~hv) 1'2 on photon energyhv for as-deposited InSe filmswith different thicknesses.
amorphous state including the small crystallites because two crystalline peaks exist in these films, as shown in Fig. 1. The value of E o for the film of 0.25 pm thickness can be determined from the intercept with ~ = 0 in the (~hv) 1/2 vs. hv plot. The obtained value o f E 0 is 1.42eV which almost agrees with that for lnSe films fabricated by flash evaporation (E0 = 1.3 e V ) 6. Figure 4 shows the isochronal annealing behaviour of ~ at hv = 2.25 eV. The annealing was carried out at temperatures higher than ~. The absorption coefficient gradually decreases with the increase in annealing temperature up to 250 °C and suddenly decreases with increasing temperature between 300 and 400 °C. However, increases for annealing at temperatures higher than 450 °C. In combination with the result of the X-ray diffraction measurements in Fig. 2, the variation in ~ with the annealing temperature between 300 and 400 °C indicates the transformation from the polycrystalline state to the crystalline state of the rhombohedral type. For annealing at temperature above 450°C, ~ increases with increasing annealing temperature. This may suggest that the decomposition of the films occurs at these higher annealing temperatures. Since the films after annealing at 400°C have the crystalline structure, a comparison of the optical transitions between the sputtered film and the single crystal was carried out. The absorption spectrum of the single crystal is dominated by the indirect and direct interband transitions at hv = 1-3 eV. The spectra of the crystalline films in the region of the investigated thickness were affected by the multiple reflection in the indirect transition region, and we could not estimate the
220
s. SHIGETOMI, H. OHKUBO, T. IKAR1
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o
~ o "z~,z~£
thickness 0.25 pm 0.55 pm 083pro 1.5 um
.o..'\\
E t3
o zx t3 •
10 5 El
10 ~
I
0
I
I
I
I
I
100 200 300 400 500 600 ANNEALING TEMPERATURE (°C)
Fig. 4. Annealing behaviour of the optical absorption coefficient at hv - 2.25 eV. b a n d edge of the indirect transition. The a b s o r p t i o n spectra of the crystalline films with 0.83 a n d 1.5 p.m thicknesses axe a l m o s t uninfluenced by the effect of the multiple reflection for hv a b o v e 2.0 eV. The a b s o r p t i o n coefficient ~ of the crystalline films is 'controlled by the direct f o r b i d d e n t r a n s i t i o n a n d obeys the r e l a t i o n ; ~hv = A ( h v - Eg) 3/2
(2)
where A is c o n s t a n t a n d Eg is the direct energy gap. E q u a t i o n (2) holds over a m u c h wider range o f h v for the films a n n e a l e d at 400 ~'C as s h o w n in Fig. 5. The s p e c t r u m of 3200 400 °C anneat thickness 0.83 pm
2400
1.5 pm
v
.C
single crystat
6 80o
/, J'
,?',"/,,0
,",,"" t
10
I 20
I
3.0
PHOTON ENERGY h~ (eV)
Fig. 5. Dependence of(~hv)23 on hv for tilms annnealed at 400' C and single-crystal InSe.
EFFECT OF ANNEALING ON R.F.-SPUTTERED
InSe
221
the single crystal is also illustrated for comparison in the figure. The electric field vector of the incident light was propagated parallel to the layer. The value of Eg = 1.34 eV for the single crystal is obtained from the intersection of the straight line with the abscissa and almost agrees with that given by Andriyashik et al. (E, = 1.293 eV)S.The values for E~ in the films of 0.83 ~tm and 1.5/am thickness are estimated as 1.40 eV and 1.32 eV respectively and are nearly the same as that of the single crystal. From the results of the above observation, it is concluded that the optical absorption in the region of hv from 2.0 to 2.5 eV for the films annealed at 400 °C is dominated by the direct interband transition in the same manner as that of the single crystal. Since the annealing effects on the structure and optical properties in the evaporated InSe have already been investigated, a comparison of the properties between the evaporated and the sputtered films may be valuable. The as-evaporated films have an amorphous structure at substrate temperatures T~ below about 150°C1. The optical properties of these films are interpreted according to the density of states model proposed by Mott and Davis. The optical gap of asevaporated films was obtained as 0.4-0.63 eV (conventional evaporation) 9 and 1.3 eV (flash evaporation) 6. As shown in Fig. 1, the structure of the as-sputtered film of 0.25/am thickness (T~ = 80 °C) is amorphous. The feature of the density of states in the energy band for this film in Fig. 3 can be understood by using the same model for evaporated films. The obtained value ofE o = 1.42 eV almost agrees with that for the film fabricated by flash evaporation. The degree of disorder and number of defects in the bonding structure are known to depend on E o 3. Therefore the amorphous structure prepared by sputtering is almost the same as that formed by flash evaporation. At T~in the range 150-350 °C, the as-evaporated films exhibit the polycrystalline lnSe structure 1. Furthermore, crystalline films are formed by subsequent annealing at 350 °C for 5-8 h. Also, the films formed by flash evaporation have the crystalline state of the rhombohedral type at about 340 °C 5 The as-sputtered films at T~ = 190-280 °C in Fig. I form the amorphous state including the small crystallites. The films attain the polycrystalline state as a result of subsequent annealing at 300 °C (Fig. 2). The crystalline films of the rhombohedral type are formed at an annealing temperature of 400 °C, as shown in Fig. 2 and Table I. The value of T~(190 °C) on crystallization of sputtered films almost agrees with that (150 °C) of the evaporated films. However, there are discrepancies in the structure between the sputtered (amorphous state including the small crystallites) and evaporated (polycrystalline state) films. One possible reason might be that T~of the films was not controlled during the sputtering. The final crystalline films have the rhombohedral structure of the same type in both the sputtered and the flashevaporated films. 4. CONCLUSION The X-ray diffraction and optical absorption of sputtered InSe films were measured as a function of thickness and annealing temperature. These results provided good evidence for the existence of the stoichiometric InSe film. A
222
S. SH1GET()MI. H. OHKUBO, T. IKARI
comparison of the properties between the evaporated and sputtered films were carried out. The optical properties of the amorphous structure formed by sputtering and by evaporation were summarized in terms of the same model of the density of states given by Mott and Davis. Both of lhe crystalline films after annealing were characterized by the rhombohedral structure of the same type. Furthermore, the optical absorption of sputtered crystalline films could be explained by the direct interband transition in the same manner as that of the single crystal. We see that the r.f. sputtering method is a useful technique for obtaining lnSe films. ACKNOWLEDGMENTS
The authors would like to thank Professor Yutaka Koga and Professor Shigenobu Shigetomi of Kurume University for their suggestions and encouragement. REFERENCES l 2 3 4 5 6 7
9
K. A n d o and A. Katsui, Yh#l SoIM Films'. 76 I 1981 ) 141. D . V . K . Sastry and P. J. Reddy, Th#z Solid Film.~, ltL5 (1983) 139, S. Chaudhuri, S. K. Biswas and A. Choudhury, S¢~IM S*ute Commlol., 53 (1985) 273. N . F . Mott and E. A. Davis, Elecm~*~ic Pmce,~.s'e,~' itz Nop>crrstalline .,~tatcriat~', Clarendon, Oxford. 1971. H. H a s h i m o l o , H. Nishimura and 1-|. Suzuki, Jl?t;. J..4ppl. Ph r.s., 2# ( 1981 ) 1163, 1. W a t a n a b e and T. Y a m a m o t o , Jpu..1. ,4ppl. P/ms'.. 24 (1985) 1282. J. 1. Pankove, Optical Proce.sses it; Senliconductor,s, Prentice-Hall, Englewood Cliffs, N J, 1971. M. V. Andriyashik, M. Y. Sakhnovskii. V. B. Trimofeev and A. S. Yakimova, Phy.s, ,Slutu,s Solidi. 2,~¢ (1968) 277. S . K . Biswas, S. Chaudhuri and A. C h o u d h u r y , t'/ly,~. Ntott,.s' .S'o[&tl', 1(15 ( [ 988) 467.