- Email: [email protected]
Growth and characterization of Bi-doped PbS thin films prepared by hot-wall epitaxy
N
J. . . . . . . .
CRYSTAL G R O W T H
ELSEVIER
Journal of Crystal Growth 181 (1997) 367-373
Growth and characterization of Bi-doped PbS thin films prepared by hot-wall epitaxy S. Abe a'*, K. M a s u m o t o a, K.
Suto b
a The Research Institute Jbr Electric and Magnetic Materials, Yag(vama-minami, Sendai 982, Japan bDepartment of Materials Science, Faculty of Engineering, Tohoku University, Sendai 980-77, Japan
Received 20 April 1997
Abstract We have investigated Bi-doped PbS thin films (Bi ~< 5.23 mol%) prepared by hot-wall epitaxy. Infrared absorption by free carrier was observed on the transmission measurement by FT-IR. The relation between the absorption coefficient and the carrier concentration n is expressed as follows: c~= 4.53 x 10 17 nl.O3 at 10 pm. The high carrier concentration is obtained above 1019 cm- 3 with n-type conductivity due to the doping of Bi in the films. Optical absorption edge shifts toward the larger frequency in proportion to the carrier concentration. The measurements of full-width at half-maximum (FWHM) of the X-ray diffraction patterns suggest that it is important to suppress the amount of Bi concentration in PbS thin films below about 1.5 mol% for obtaining excellent crystallinity.
1. Introduction Lead chalcogenide solid solution semiconductors are expected to be applied to tunable laser diodes, which operate at the mid-infrared wavelength region around 3 x 10 6 m. They are considered to be utilized mainly in advanced measurement systems for detecting hydrocarbon pollutants in the air [1] and in a new optical-fiber communication system over super-long distances, which has not yet been developed [2]. For practical use of these laser diodes, it is required to operate them at around room temperature. So far, many efforts have been made to fabricate laser diodes
* Corresponding author.
[3-5], but this has not yet been realized. As a proper cladding layer which is lattice matched with the PbS-active layer, we proposed a new quaternary solid solution semiconductor Pbt x(Cal-ySry)xS [6] and Pbl xCaxSt-sS% [7]. For fabricating the laser diodes with double-hetero structure employing these materials, it is important to investigate the n-PbS/p-Pbl_x(Cal ySry)xS or n-PbS/p-Pbl-xCa~S1 )£% hetero interface. Controlling carrier concentrations with deviations from stoichiometry has not been used very much in IV VI compounds recently because of the belief that impurity dopants would diffuse more slowly than vacancies [8, 9]. Therefore, it is important to investigate impurity doping for controlling the type of conductivity. So far, concerning the Bi-doped PbS, which is expected to have n-type conductivity,
0022-0248/97/$17.00 ~(" 1997Elsevier Science B.V. All rights reserved PII S 0 0 2 2 - 0 2 4 8 ( 9 7 ) 0 0 3 0 1 - 1
S. Abe et al. ,./Journal o/Crystal Growth 181 (1997) 367 373
368
some studies were reported on bulk substances [10-13] and detailed properties are not clear for thin films. So, in this study, we examine growth and characterization of Bi-doped PbS thin films prepared by hot-wall epitaxy.
2. Experimental procedure Bi-doped PbS thin films were grown on cleaved BaF2 (1 1 1) substrates by hot-wall epitaxy. About 10 g of PbS and 0.2, 0.025, 0.01 and 0.001 g of Bi2S3 (99.99% pure) are used as the source materials and S (99.9999% pure) is used as a reservoir material. The reservoir temperature is precisely controlled at around room temperature by circulating water in a water jacket. The PbS compound is synthesized from the element with 6 N purity and is vapor transported in quartz ampoule under a partial pressure of S regulated by the reservoir, which corresponds to a minimum total pressure ( P m i n ) [14]. Through this vapor transport, the atomic ratio between Pb and S in vapor phase is kept equal (Pb/S = 1). Before the growth of Bi-doped PbS thin films, thermal etching of the substrate is performed for 0.6h at 773K. The substrate, the wall, the
10 4 -
source and the reservoir temperatures are kept constant at 643, 833, 833 and 393 K, respectively, through this study. The Bi concentrations in PbS thin films are determined by electron-probe microanalysis (EPMA). The full-width at half-maximum (FWHM) of the X-ray diffraction pattern is determined by Cu K~t radiation. The thickness of the films are measured by surface-texture analysis system (DECTAK 3030ST). The carrier concentrations of the films are measured by the van der Pauw method. The optical absorption edge at room temperature of the films are evaluated by Fourier-transform infrared spectroscopy (FT-IR).
3. Results and discussions Fig. 1 shows the absorption coefficients of Bidoped PbS thin films at room temperature as a function of wavelength of the incident light (2). The dotted lines indicate the ~ b dependences. The absorption coefficient is obtained from the measurement of the transmittance T by FT-IR and derived from the following expression [15]: T = (1
-
R) 2
exp(-:~d),
a:8.1 x 1019 cm -3 b:6.7 x I0~9cm -3
/ I
..2
O
f
<
10 Wavelength / ~ m Fig. 1. The a b s o r p t i o n coefficients of Bi-doped PbS thin films at r o o m t e m p e r a t u r e as a function of the w a v e l e n g t h of the incident light•
S. Abe et al. / Journal o f Crystal Growth 181 (1997) 367-373
where R is the bulk reflectance and d is the thickness. The growth of these epitaxial films were performed under excess sulfur vapor pressure from the stoichiometric growth condition and these films have mirror-like appearance. An increase in the amount of S in the vapor generates vacancies in the cation sublattice. In this condition, doped Bi is expected to occupy sites in the cation sublattice and to be singly ionized donors. The absorption coefficients increase with increasing the carrier concentration and exhibiting power-law dependence against the wavelength variation. Fig. 2 shows the index number b as a function of Bi concentration. The number b is derived from the results of Fig. 1. The index number remains around at 2 when the Bi concentration is below about 1.5 m o l % and rapidly decrease with increasing the concentration above about 1.5 mot%. According to the classical expression [16], absorption coefficients vary in proportion to 22 . However, in general, the collision with the semiconductor lattice resulting in scattering by acoustic phonons [17], optical phonons [18] and ionized impurities [19] lead to an absorption increasing as 21'5 , 2 2.5 and 2 3 or 2 3.5, respectively [16]. In this study, it is known that the variation of the absorption
369
coefficient corresponds to classical expression on the Bi concentration range below about 1.5 mol%. Fig. 3 shows the absorption coefficients at 10 gm of Bi-doped PbS thin films as a function of the carrier concentration at r o o m temperature. F r o m the experimental results, the relation between the absorption coefficient ~ and the carrier concentration n is expressed as follows: = 4.53 x 10-17n 1°3 . It is clear that the absorption coefficient is linearly dependent on the carrier concentration and the free carrier causes this infrared absorption. On the other hand, H a r m a n and Strauss reported [20] = 3.95 x 10 19n1.14 at 10.6tam on nondoped PbS bulk crystal. For the coefficient of carrier concentration n in both equations, the Bi-doped sample has a larger value, of about twice that for the nondoped one. It seems reasonable to understand the difference between both the coefficients that the periodic potential in the crystal is disordered by doping Bi into PbS and the relaxation time ~ is shortened. Fig. 4 shows the F W H M of Bi-doped PbS thin films as a function of the Bi concentration. The film thickness is 3.0-6.7gm. In PbS-Bi2S3 system,
2.2
2.0.
..~ 1.8.
e-
1.6.
1.4.
1.2. I
I
I
I
I
I
0
1
2
3
4
5
Bi concentration / mol % Fig. 2. The index number b as a function of Bi concentration.
370
S. Abe et al. / Journal o f Crystal Growth 181 (1997) 367 373
at
10
10 4 ,
e-
8 e-
cz= 4.53x lOqTn 1"°3 i <
Cartier concentration / cm 3 Fig. 3. The absorption coefficients at 10 gm of Bi-doped PbS thin films as a function of the carrier concentration at room temperature.
0.45 0.40
(444) 0.35 0.30 0.25 0.20 0.15 0.10
i
I
I
I
L
i
0
1
2
3
4
5
Bi concentration / tool % Fig. 4. FWHM of Bi-doped PbS thin films as a function of the Bi concentration. it is k n o w n that i n t e r m e d i a t e phase l I of 6 Pbl-xBi2x/3SBi2S3 with o r t h o r h o m b i c structure exist at 82 m o l % P b S - 1 8 m o l % BizS3 k n o w n as h e y r o v s k i t e [21]. In this study, the diffraction pattern exhibited that P b S BizS3 solid solution
s e m i c o n d u c t o r has a NaC1 structure up to 5.23 m o l % of Bi c o n c e n t r a t i o n , which is the m o s t heavily d o p e d s a m p l e in this study.These epitaxial films are g r o w n in the (1 1 1) direction, a n d the ( 4 4 4 ) p e a k on the X-ray (Cu Kc0 diffraction
S. Abe et al. / Journal o f Crystal Growth 181 (1997) 367-373
pattern is selected for calculating their lattice constants. The lattice constants of Bi-doped PbS thin films slightly decrease with increasing the Bi concentration because of the difference in the radii between Pb and Bi. They vary only from 0.5933 to 0.5935 nm in this study. In the Bi concentration range below about 1.5 tool%, F W H M nearly keep constant at around 0.13 ° and excellent crystallinity is retained. On the other hand, in the concentration range above about 1.5 tool%, F W H M rapidly increases with increasing the Bi concentration. It is deduced that the crystallinity of PbS thin films become worse in the relatively high-concentration range compared with that in the range of low concentration. Consequently, it is important to suppress the amount of Bi concentration in PbS thin films below about 1.5 mol% for obtaining excellent crystallinity. Fig. 5 shows the carrier concentration of Bidoped PbS thin films at room temperature as a function of the Bi concentration. These films have n-type conductivity and have high carrier concentration above 1019 c m 3. In the low Bi concentration range below about 1.5mo1% , the carrier concentration increase monotonously in proportion to the concentration, whereas the carrier
371
concentration saturate with the 10 20 c m - 3 order in the high Bi concentration range above about 1.5 mol%. An increase in the amount of S in the vapor is equivalent to an increase in the number of vacant sites filled by Bi. In such sites, Bi exhibits donor properties, contributes one electron per dopant atom to the conduction band [22]. From the relation between the Bi concentration and the carrier concentration, activation rate, which is the rate of the amount of activated ion as a donor to the amount of doped ion, is estimated and decrease exponentially with increasing Bi concentration. In this study, the activation rate varies from 2.2%-0.5%. Fig. 6 shows the optical absorption edge (Eg)opt of Bi-doped PbS thin films at room temperature as a function of Bi concentration. The optical absorption edge is determined by linear extrapolation to zero of absorbance squared, which is derived from the absorption edge in the transmission measurement by FT-IR. The optical absorption edge vary almost the same as the variation of carrier concentration in Fig. 5 and have the tendency to saturate with the value below 0.6 eV. As the reason for increasing the optical absorption edge in proportion to the Bi concentration, it can be considered
f E
~9
1 0 2o
e-. O ",,7.
8e8
I
I
I
I
I
I
0
I
2
3
4
5
6
Bi concentration / mol % Fig. 5. The carrier concentration of Bi-doped PbS thin films at room temperature as a function of the Bi concentration.
372
S. Abe et al. / Journal o f C~stal Growth 181 (1997) 367-373
>
0.6-
at R.T.
L)
t~ ©
0.4 0
I
I
1
2
t
3
4
5
Bi concentration / mol % Fig. 6. The optical absorption edge (Eg)opt of Bi-doped PbS thin films at room temperature as a function of Bi concentration.
0.60 0.58
>
0.56 •
/
Experiment
/
~
0.54 0.52 0.50 o 0.48
-
0.46
.
e",
0.44
-
©
0.42 0.40
I
10 t9
1 0 2°
Carrier concentration / cm
-3
Fig. 7. The optical absorption edge (Eg)opt Of Bi-doped PbS thin films at room temperature as a function of the carrier concentration.
s. Abe et al. / Journal of Crystal Growth 181 (1997) 367 373 from the results of Figs. 5 a n d 6 t h a t the Burstein shift, which is o b s e r v e d in the case of the high carrier c o n c e n t r a t i o n , a n d increase of the energy b a n d g a p due to the m i x e d crystallization between P b S a n d Bi2S3, whose energy b a n d g a p is larger t h a n t h a t of PbS. H o w e v e r , the c o n t r i b u t i o n of the m i x e d crystallization to the increase of energy b a n d gaps is negligible in this c o n c e n t r a t i o n range acc o r d i n g to the e v a l u a t i o n of the energy b a n d gaps in a first a p p r o x i m a t i o n . Fig. 7 shows the o p t i c a l a b s o r p t i o n edge (E0)opt of B i - d o p e d P b S thin films at r o o m t e m p e r a t u r e as a function of the carrier c o n c e n t r a t i o n . (O) a n d (A) indicates the e x p e r i m e n t a l results a n d the calc u l a t e d Burstein shift, respectively. T h e index n u m b e r b of these films is a b o u t 2 with excellent crystallinity. T h e a m o u n t of Burstein shift is calc u l a t e d e m p l o y i n g the following expression [-23]: (Eg)opt- Eg : (1 + ~ p p ) ( E f - 4 k T ) . In the calculation, we use the value as the relative carrier effective m a s s of PbS, m, = 0.1167mo a n d mp = 0.1101too [24]. B o t h e x p e r i m e n t a l a n d calc u l a t e d values increase g r a d u a l l y in p r o p o r t i o n to the carrier c o n c e n t r a t i o n a n d the higher values are o b t a i n e d on the e x p e r i m e n t a l results. As the r e a s o n for the difference between b o t h the results, the following m a t t e r s are considered. In the experim e n t a l results, the carrier effective m a s s mn seems to be e n l a r g e d in p r o p o r t i o n to the carrier concentration, which has a l r e a d y been r e p o r t e d on P b T e s u b s t a n c e [25], arised from the n o n p a r a b o l i c i t y of b a n d at high c a r r i e r c o n c e n t r a t i o n a n d the n o n p a r a b o l i c i t y is n o t t a k e n into a c c o u n t in the c a l c u l a t i o n of Burstein shift.
373
References [1] K. Masumoto, N. Koguchi, S. Takahashi, T. Kiyosawa, I. Nakatani, Sci. Rep. Nat. Res. Inst. Metals 6 (1985) 101. [2] S. Shibata, M. Horiguchi, K. Jinguji, S. Mitachi, T. Kanamori, T. Manabe, Electron. Lett. 17 (1981) 775. [3] N. Koguchi, T. Kiyosawa, S. Takahashi, J. Crystal Growth 81 (1987) 400. [4] A. Ishida, N. Sakurai, K. Aikawa, H. Fujiyasu, Solid State Electron. 37 (1994) 1141. [5] N. Koguchi, S. Takahashi, Appl. Phys. Lett. 58 (1991) 799. [6] S. Abe, K. Masumoto, K. Suto, J. Crystal Growth 173 (1997) 104. [7] S. Abe, K. Masumoto, K. Mochizuki, K. Suto, J. Crystal Growth 144 (1994) 54. [8] J.N. Walpole, R.L. Guldi, Physics of I V V I Compounds and Alloys, Gordon and Breach, London, 1974. [9] D.L. Partin, Semiconductors Semimetals 33 (1991) 311. [10] M.S. Seltzer, J.B. Wagner Jr., J. Phys. Chem. Solids 24 (1963) 1525. [11] K.R. Zanio, J.B. Wagner, J. Appl. Phys. 39 (1968) 5686. [12] J. Bloem, Phil. Res. Rep. 11 (1956) 273. [13] J. Bloem, F.A. Kr6ger, H.J. Vink, Rep. Bristol Conf. on Defects in Crystalline Solids, vol. 1954, 1955, p. 273. [14] K. Igaki, N. Ohashi, Bull. Univ. Osaka Pref. A 11 (1962) 89. [15] R.B. Barmes, M. Czerny, Phys. Rev. 38 (1931) 338. [16] J.I. Pankove, Optical Processes in Semiconductors, Dover, New York, 1971, p. 74. [17] Y. Fan, M. Becket, in: Semiconductor Materials, Butterworth, London, 1951, p. 132. [18] S. Visvanathan, Phys. Rev. 120 (1960) 376. [19] H.Y. Fan, W.G. Spitzer, R.J. Collins, Phys. Rev. 101 (1956) 566. [20] T.C. Harman, A.J. Strauss, J. Electron. Mater. 5 (1976) 621. [21] B. Salanci, G.H. Moh, Neues Jahrb. Mineral. Abhandl. 112 (1969) 63. [22] V.A. Zykov, T.A. Gavrikova, S.A. Nemov, Fiz. Tech. Poluprovodn. 29 (1995) 309. [23] E. Burstein, Phys. Rev. 93 (1954) 632. [24] H. Preier, Appl. Phys. 20 (1979) 189. [25] T.E. Thompson, P.R. Aron, B.S. Chandrasekhar, D.N. Langenberg, Phys. Rev. B 4 (1971) 518.
J. . . . . . . .
CRYSTAL G R O W T H
ELSEVIER
Journal of Crystal Growth 181 (1997) 367-373
Growth and characterization of Bi-doped PbS thin films prepared by hot-wall epitaxy S. Abe a'*, K. M a s u m o t o a, K.
Suto b
a The Research Institute Jbr Electric and Magnetic Materials, Yag(vama-minami, Sendai 982, Japan bDepartment of Materials Science, Faculty of Engineering, Tohoku University, Sendai 980-77, Japan
Received 20 April 1997
Abstract We have investigated Bi-doped PbS thin films (Bi ~< 5.23 mol%) prepared by hot-wall epitaxy. Infrared absorption by free carrier was observed on the transmission measurement by FT-IR. The relation between the absorption coefficient and the carrier concentration n is expressed as follows: c~= 4.53 x 10 17 nl.O3 at 10 pm. The high carrier concentration is obtained above 1019 cm- 3 with n-type conductivity due to the doping of Bi in the films. Optical absorption edge shifts toward the larger frequency in proportion to the carrier concentration. The measurements of full-width at half-maximum (FWHM) of the X-ray diffraction patterns suggest that it is important to suppress the amount of Bi concentration in PbS thin films below about 1.5 mol% for obtaining excellent crystallinity.
1. Introduction Lead chalcogenide solid solution semiconductors are expected to be applied to tunable laser diodes, which operate at the mid-infrared wavelength region around 3 x 10 6 m. They are considered to be utilized mainly in advanced measurement systems for detecting hydrocarbon pollutants in the air [1] and in a new optical-fiber communication system over super-long distances, which has not yet been developed [2]. For practical use of these laser diodes, it is required to operate them at around room temperature. So far, many efforts have been made to fabricate laser diodes
* Corresponding author.
[3-5], but this has not yet been realized. As a proper cladding layer which is lattice matched with the PbS-active layer, we proposed a new quaternary solid solution semiconductor Pbt x(Cal-ySry)xS [6] and Pbl xCaxSt-sS% [7]. For fabricating the laser diodes with double-hetero structure employing these materials, it is important to investigate the n-PbS/p-Pbl_x(Cal ySry)xS or n-PbS/p-Pbl-xCa~S1 )£% hetero interface. Controlling carrier concentrations with deviations from stoichiometry has not been used very much in IV VI compounds recently because of the belief that impurity dopants would diffuse more slowly than vacancies [8, 9]. Therefore, it is important to investigate impurity doping for controlling the type of conductivity. So far, concerning the Bi-doped PbS, which is expected to have n-type conductivity,
0022-0248/97/$17.00 ~(" 1997Elsevier Science B.V. All rights reserved PII S 0 0 2 2 - 0 2 4 8 ( 9 7 ) 0 0 3 0 1 - 1
S. Abe et al. ,./Journal o/Crystal Growth 181 (1997) 367 373
368
some studies were reported on bulk substances [10-13] and detailed properties are not clear for thin films. So, in this study, we examine growth and characterization of Bi-doped PbS thin films prepared by hot-wall epitaxy.
2. Experimental procedure Bi-doped PbS thin films were grown on cleaved BaF2 (1 1 1) substrates by hot-wall epitaxy. About 10 g of PbS and 0.2, 0.025, 0.01 and 0.001 g of Bi2S3 (99.99% pure) are used as the source materials and S (99.9999% pure) is used as a reservoir material. The reservoir temperature is precisely controlled at around room temperature by circulating water in a water jacket. The PbS compound is synthesized from the element with 6 N purity and is vapor transported in quartz ampoule under a partial pressure of S regulated by the reservoir, which corresponds to a minimum total pressure ( P m i n ) [14]. Through this vapor transport, the atomic ratio between Pb and S in vapor phase is kept equal (Pb/S = 1). Before the growth of Bi-doped PbS thin films, thermal etching of the substrate is performed for 0.6h at 773K. The substrate, the wall, the
10 4 -
source and the reservoir temperatures are kept constant at 643, 833, 833 and 393 K, respectively, through this study. The Bi concentrations in PbS thin films are determined by electron-probe microanalysis (EPMA). The full-width at half-maximum (FWHM) of the X-ray diffraction pattern is determined by Cu K~t radiation. The thickness of the films are measured by surface-texture analysis system (DECTAK 3030ST). The carrier concentrations of the films are measured by the van der Pauw method. The optical absorption edge at room temperature of the films are evaluated by Fourier-transform infrared spectroscopy (FT-IR).
3. Results and discussions Fig. 1 shows the absorption coefficients of Bidoped PbS thin films at room temperature as a function of wavelength of the incident light (2). The dotted lines indicate the ~ b dependences. The absorption coefficient is obtained from the measurement of the transmittance T by FT-IR and derived from the following expression [15]: T = (1
-
R) 2
exp(-:~d),
a:8.1 x 1019 cm -3 b:6.7 x I0~9cm -3
/ I
..2
O
f
<
10 Wavelength / ~ m Fig. 1. The a b s o r p t i o n coefficients of Bi-doped PbS thin films at r o o m t e m p e r a t u r e as a function of the w a v e l e n g t h of the incident light•
S. Abe et al. / Journal o f Crystal Growth 181 (1997) 367-373
where R is the bulk reflectance and d is the thickness. The growth of these epitaxial films were performed under excess sulfur vapor pressure from the stoichiometric growth condition and these films have mirror-like appearance. An increase in the amount of S in the vapor generates vacancies in the cation sublattice. In this condition, doped Bi is expected to occupy sites in the cation sublattice and to be singly ionized donors. The absorption coefficients increase with increasing the carrier concentration and exhibiting power-law dependence against the wavelength variation. Fig. 2 shows the index number b as a function of Bi concentration. The number b is derived from the results of Fig. 1. The index number remains around at 2 when the Bi concentration is below about 1.5 m o l % and rapidly decrease with increasing the concentration above about 1.5 mot%. According to the classical expression [16], absorption coefficients vary in proportion to 22 . However, in general, the collision with the semiconductor lattice resulting in scattering by acoustic phonons [17], optical phonons [18] and ionized impurities [19] lead to an absorption increasing as 21'5 , 2 2.5 and 2 3 or 2 3.5, respectively [16]. In this study, it is known that the variation of the absorption
369
coefficient corresponds to classical expression on the Bi concentration range below about 1.5 mol%. Fig. 3 shows the absorption coefficients at 10 gm of Bi-doped PbS thin films as a function of the carrier concentration at r o o m temperature. F r o m the experimental results, the relation between the absorption coefficient ~ and the carrier concentration n is expressed as follows: = 4.53 x 10-17n 1°3 . It is clear that the absorption coefficient is linearly dependent on the carrier concentration and the free carrier causes this infrared absorption. On the other hand, H a r m a n and Strauss reported [20] = 3.95 x 10 19n1.14 at 10.6tam on nondoped PbS bulk crystal. For the coefficient of carrier concentration n in both equations, the Bi-doped sample has a larger value, of about twice that for the nondoped one. It seems reasonable to understand the difference between both the coefficients that the periodic potential in the crystal is disordered by doping Bi into PbS and the relaxation time ~ is shortened. Fig. 4 shows the F W H M of Bi-doped PbS thin films as a function of the Bi concentration. The film thickness is 3.0-6.7gm. In PbS-Bi2S3 system,
2.2
2.0.
..~ 1.8.
e-
1.6.
1.4.
1.2. I
I
I
I
I
I
0
1
2
3
4
5
Bi concentration / mol % Fig. 2. The index number b as a function of Bi concentration.
370
S. Abe et al. / Journal o f Crystal Growth 181 (1997) 367 373
at
10
10 4 ,
e-
8 e-
cz= 4.53x lOqTn 1"°3 i <
Cartier concentration / cm 3 Fig. 3. The absorption coefficients at 10 gm of Bi-doped PbS thin films as a function of the carrier concentration at room temperature.
0.45 0.40
(444) 0.35 0.30 0.25 0.20 0.15 0.10
i
I
I
I
L
i
0
1
2
3
4
5
Bi concentration / tool % Fig. 4. FWHM of Bi-doped PbS thin films as a function of the Bi concentration. it is k n o w n that i n t e r m e d i a t e phase l I of 6 Pbl-xBi2x/3SBi2S3 with o r t h o r h o m b i c structure exist at 82 m o l % P b S - 1 8 m o l % BizS3 k n o w n as h e y r o v s k i t e [21]. In this study, the diffraction pattern exhibited that P b S BizS3 solid solution
s e m i c o n d u c t o r has a NaC1 structure up to 5.23 m o l % of Bi c o n c e n t r a t i o n , which is the m o s t heavily d o p e d s a m p l e in this study.These epitaxial films are g r o w n in the (1 1 1) direction, a n d the ( 4 4 4 ) p e a k on the X-ray (Cu Kc0 diffraction
S. Abe et al. / Journal o f Crystal Growth 181 (1997) 367-373
pattern is selected for calculating their lattice constants. The lattice constants of Bi-doped PbS thin films slightly decrease with increasing the Bi concentration because of the difference in the radii between Pb and Bi. They vary only from 0.5933 to 0.5935 nm in this study. In the Bi concentration range below about 1.5 tool%, F W H M nearly keep constant at around 0.13 ° and excellent crystallinity is retained. On the other hand, in the concentration range above about 1.5 tool%, F W H M rapidly increases with increasing the Bi concentration. It is deduced that the crystallinity of PbS thin films become worse in the relatively high-concentration range compared with that in the range of low concentration. Consequently, it is important to suppress the amount of Bi concentration in PbS thin films below about 1.5 mol% for obtaining excellent crystallinity. Fig. 5 shows the carrier concentration of Bidoped PbS thin films at room temperature as a function of the Bi concentration. These films have n-type conductivity and have high carrier concentration above 1019 c m 3. In the low Bi concentration range below about 1.5mo1% , the carrier concentration increase monotonously in proportion to the concentration, whereas the carrier
371
concentration saturate with the 10 20 c m - 3 order in the high Bi concentration range above about 1.5 mol%. An increase in the amount of S in the vapor is equivalent to an increase in the number of vacant sites filled by Bi. In such sites, Bi exhibits donor properties, contributes one electron per dopant atom to the conduction band [22]. From the relation between the Bi concentration and the carrier concentration, activation rate, which is the rate of the amount of activated ion as a donor to the amount of doped ion, is estimated and decrease exponentially with increasing Bi concentration. In this study, the activation rate varies from 2.2%-0.5%. Fig. 6 shows the optical absorption edge (Eg)opt of Bi-doped PbS thin films at room temperature as a function of Bi concentration. The optical absorption edge is determined by linear extrapolation to zero of absorbance squared, which is derived from the absorption edge in the transmission measurement by FT-IR. The optical absorption edge vary almost the same as the variation of carrier concentration in Fig. 5 and have the tendency to saturate with the value below 0.6 eV. As the reason for increasing the optical absorption edge in proportion to the Bi concentration, it can be considered
f E
~9
1 0 2o
e-. O ",,7.
8e8
I
I
I
I
I
I
0
I
2
3
4
5
6
Bi concentration / mol % Fig. 5. The carrier concentration of Bi-doped PbS thin films at room temperature as a function of the Bi concentration.
372
S. Abe et al. / Journal o f C~stal Growth 181 (1997) 367-373
>
0.6-
at R.T.
L)
t~ ©
0.4 0
I
I
1
2
t
3
4
5
Bi concentration / mol % Fig. 6. The optical absorption edge (Eg)opt of Bi-doped PbS thin films at room temperature as a function of Bi concentration.
0.60 0.58
>
0.56 •
/
Experiment
/
~
0.54 0.52 0.50 o 0.48
-
0.46
.
e",
0.44
-
©
0.42 0.40
I
10 t9
1 0 2°
Carrier concentration / cm
-3
Fig. 7. The optical absorption edge (Eg)opt Of Bi-doped PbS thin films at room temperature as a function of the carrier concentration.
s. Abe et al. / Journal of Crystal Growth 181 (1997) 367 373 from the results of Figs. 5 a n d 6 t h a t the Burstein shift, which is o b s e r v e d in the case of the high carrier c o n c e n t r a t i o n , a n d increase of the energy b a n d g a p due to the m i x e d crystallization between P b S a n d Bi2S3, whose energy b a n d g a p is larger t h a n t h a t of PbS. H o w e v e r , the c o n t r i b u t i o n of the m i x e d crystallization to the increase of energy b a n d gaps is negligible in this c o n c e n t r a t i o n range acc o r d i n g to the e v a l u a t i o n of the energy b a n d gaps in a first a p p r o x i m a t i o n . Fig. 7 shows the o p t i c a l a b s o r p t i o n edge (E0)opt of B i - d o p e d P b S thin films at r o o m t e m p e r a t u r e as a function of the carrier c o n c e n t r a t i o n . (O) a n d (A) indicates the e x p e r i m e n t a l results a n d the calc u l a t e d Burstein shift, respectively. T h e index n u m b e r b of these films is a b o u t 2 with excellent crystallinity. T h e a m o u n t of Burstein shift is calc u l a t e d e m p l o y i n g the following expression [-23]: (Eg)opt- Eg : (1 + ~ p p ) ( E f - 4 k T ) . In the calculation, we use the value as the relative carrier effective m a s s of PbS, m, = 0.1167mo a n d mp = 0.1101too [24]. B o t h e x p e r i m e n t a l a n d calc u l a t e d values increase g r a d u a l l y in p r o p o r t i o n to the carrier c o n c e n t r a t i o n a n d the higher values are o b t a i n e d on the e x p e r i m e n t a l results. As the r e a s o n for the difference between b o t h the results, the following m a t t e r s are considered. In the experim e n t a l results, the carrier effective m a s s mn seems to be e n l a r g e d in p r o p o r t i o n to the carrier concentration, which has a l r e a d y been r e p o r t e d on P b T e s u b s t a n c e [25], arised from the n o n p a r a b o l i c i t y of b a n d at high c a r r i e r c o n c e n t r a t i o n a n d the n o n p a r a b o l i c i t y is n o t t a k e n into a c c o u n t in the c a l c u l a t i o n of Burstein shift.
373
References [1] K. Masumoto, N. Koguchi, S. Takahashi, T. Kiyosawa, I. Nakatani, Sci. Rep. Nat. Res. Inst. Metals 6 (1985) 101. [2] S. Shibata, M. Horiguchi, K. Jinguji, S. Mitachi, T. Kanamori, T. Manabe, Electron. Lett. 17 (1981) 775. [3] N. Koguchi, T. Kiyosawa, S. Takahashi, J. Crystal Growth 81 (1987) 400. [4] A. Ishida, N. Sakurai, K. Aikawa, H. Fujiyasu, Solid State Electron. 37 (1994) 1141. [5] N. Koguchi, S. Takahashi, Appl. Phys. Lett. 58 (1991) 799. [6] S. Abe, K. Masumoto, K. Suto, J. Crystal Growth 173 (1997) 104. [7] S. Abe, K. Masumoto, K. Mochizuki, K. Suto, J. Crystal Growth 144 (1994) 54. [8] J.N. Walpole, R.L. Guldi, Physics of I V V I Compounds and Alloys, Gordon and Breach, London, 1974. [9] D.L. Partin, Semiconductors Semimetals 33 (1991) 311. [10] M.S. Seltzer, J.B. Wagner Jr., J. Phys. Chem. Solids 24 (1963) 1525. [11] K.R. Zanio, J.B. Wagner, J. Appl. Phys. 39 (1968) 5686. [12] J. Bloem, Phil. Res. Rep. 11 (1956) 273. [13] J. Bloem, F.A. Kr6ger, H.J. Vink, Rep. Bristol Conf. on Defects in Crystalline Solids, vol. 1954, 1955, p. 273. [14] K. Igaki, N. Ohashi, Bull. Univ. Osaka Pref. A 11 (1962) 89. [15] R.B. Barmes, M. Czerny, Phys. Rev. 38 (1931) 338. [16] J.I. Pankove, Optical Processes in Semiconductors, Dover, New York, 1971, p. 74. [17] Y. Fan, M. Becket, in: Semiconductor Materials, Butterworth, London, 1951, p. 132. [18] S. Visvanathan, Phys. Rev. 120 (1960) 376. [19] H.Y. Fan, W.G. Spitzer, R.J. Collins, Phys. Rev. 101 (1956) 566. [20] T.C. Harman, A.J. Strauss, J. Electron. Mater. 5 (1976) 621. [21] B. Salanci, G.H. Moh, Neues Jahrb. Mineral. Abhandl. 112 (1969) 63. [22] V.A. Zykov, T.A. Gavrikova, S.A. Nemov, Fiz. Tech. Poluprovodn. 29 (1995) 309. [23] E. Burstein, Phys. Rev. 93 (1954) 632. [24] H. Preier, Appl. Phys. 20 (1979) 189. [25] T.E. Thompson, P.R. Aron, B.S. Chandrasekhar, D.N. Langenberg, Phys. Rev. B 4 (1971) 518.