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Near band edge photoluminescence in Sb2S3
Journal of Luminescence 39 (1988) 175—180 North-Holland, Amsterdam
175
NEAR BAND EDGE PHOTOLUMINESCENCE IN Sb2S3 1, K. TAKIYAMA and T. ODA T. FUJITA, K. KURITA Department of Applied Physics and Chemistry, Faculty of Engineering Hiroshima
University, Higashi-Hiroshima 724, Japan
Received 5 October 1987 Revised 23 November 1987 Accepted 1 December 1987
Photoluminescence spectra in Sb 2S3 have been investigated for the first time. The photoluminescence composed of many peaks is observed near the indirect fundamental gap at 2 K. From the study of sample, excitation intensity and temperature dependences of photoluminescence and the time decay spectra, it is shown that the photoluminescence is caused by three different origins. Models of the origins are proposed. It is also shown that the photoluminescence spectra are very sensitive to the deviation from stoichiometry.
1. Introduction Photoluminescence (PL) spectra have been widely used to study the electronic states in semiconductors and insulators. In V—VI cornpound semiconductors, extensive investigations of PL spectra have been performed in As-chalcogenides (As2S3 and As2Se3), in which broad and largely Stokes-shifted PL bands have been observed near the mid-gap position [1]. The large band width and Stokes shift point to strong exciton—phonon interactions in these materials. A model, which is similar to the self-trapped exciton in ionic crystals [2], has bee~iproposed as the origin of the PL band [3,4]. However, it is not clear whether the broad and largely Stokes-shifted PL is the common feature in V—VI compounds or not, because very little is known about PL spectra in other substances. Recently we have investigated the fundamental absorption edge in orthorombic Sb2S3 [5,6]. It has been shown that the lowest energy gap is indirect (the top of the valence band is at F roint and the bottom at about the X point) [5] and at high temperature the absorption tail obeys the Urbach rule [6]. These features of the absorption spectra 1
Present address: Technical Headquarters, Hiroshima Technical Institute, Mitsubishi Heavy Industries Ltd., Hiroshima 733, Japan.
are similar to those in As2Se3 [7], but the steepness parameter a in the Urbach rule is larger than that in As2Se3. This suggests that the exciton—phonon interaction in Sb2S3 is weaker than that in As2Se3, because a is inversely proportional to the strength of the exciton —phonon interaction [8]. Further, the dielectric property of Sb2S3 is quite different from that in As-chalcogenides; the static dielectric constant parallel to the b-axis is very large (e ~ 100) [9]. Thus we may expect that the PL spectrum in Sb2S3 is different from those in As-chalcogenides. In this work, we studied the PL spectra in Sb2S3 single crystals for the first time. The PL spectra composed of many fairly sharp peaks were observed near the absorption edge, which is different from those in As-chalcogenides. The origins of the PL spectra will be discussed.
2. Experimental procedure Single crystals of Sb2S3 were grown from a vapor phase in a sulfur atmosphere. The starting materials of crystal growth were zone-refined Sb2 S3 ingots which were synthesized by a direct reaction of Sb and S in an evacuated quartz ampoule [10]. To investigate the effects of defects introduced by the deviation from stoichiometry on PL, the growth
0022-2313/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
176
T Fujita eta!.
/
Near band edge photoluminescence in Sb,S
1
Table I Impurity contents in starting Sb2S3 ingots determined spectrochemically Ingot
Concentration (ppm) P
#1 #2 #3
30 30 30
Mg
Ca
Cr
Mn
Fe
—
—
0.5
0.3
—
—
—
—
1
5
—
0.3
10 5 1
was performed under three different sulfur pressures (2, 7 and 40 Torr). It has been pointed out that crystals grown under these conditions are Sb-rich, stoichiometric and S-rich, respectively [11]. The impurity contents in the starting Sb2S3 estimated spectrochernically by means of an inductively coupled plasma spectrometer are listed in table 1. PL spectra were measured by a system cornbined with a 30 cm double monochromator, a cooled photomultiplier (Hamamatsu R943-02) and a photon counting unit. The spectral resolution was about 1 A 0.3 meV). The excitation source was a 1 mW He—Ne laser. Observations were performed either by a backscattering geometry or by a right-angle geometry, but notable differences between these could not be observed. The intensity of the exciting light was measured by a calibrated thermopile. Because of the anisotropic crystal structure [12], we measured the PL spectra for polarizations parallel and perpendicular to the b-axis, but the degree of polarization of PL spectra defined by P=(111 —I~)/(J +I~) was nearly constant (P 0.3) in the entire spectral region investigated. Thus, in the following we will show the spectra obtained without polarizer. Time decay spectra were measured by a photon counting technique. The light source was a conventional nitrogen discharge lamp of about 8 ns pulse width. Because of the weak PL intensity, the emitted light was dispersed by interference filters with about 50 A band pass. Samples were immersed in pumped liquid helium. (—
* 2 listed in table 1. The absorption spectra due to the indirect allowed exciton [5] are also shown for comparison. Here, E ii b and E II c represent the polarization of light. The position of the mdirect is exciton (1.749 eV) is indicated by the arrow. As shown in the figure, there were 7 distinct peaks and one shoulder. The peak having the largest peak energy lies at 1.7316 eV which is smaller by about 17 meV than the threshold energy of the Is indirect exciton. Hereafter these are designated in alphabetical order from the peak with higher energy. The estimated peak energies are listed in table 2. The reproducibility of the peak energy was better than 0.3 meV. Among these the peaks A, E and G were located with an equal spacing of about 19 meV. This value is close to the LO phonon energies observed in the infrared reflectivity spectra [13]. Thus it may be reasonable that these peaks are phonon replicas of peak A. It was found that the PL is sensitive not only to the vapor pressure of sulfur during crystal
—
3. Experimental results and discussion Figure 1 shows a typical PL spectrum observed for the stoichiometric crystal grown from the ingot
a -
B C H
F
A
E D
I
1.66
I
I
il/c
I
PHOTON ENERGY (eV)
E//b-
500
I
1.78
Fig. 1. Typical PL spectrum at 2 K. The absorption spectra are shown for comparison. The arrow indicates the threshold energy of the is indirect exciton.
T. Fujita eta!. / Near band edge photoluminescence in Sb
2S3
177
Table 2 Peak energies of PL spectra at 2 K. L~E is the energy difference between peaks A, B and C and their phonon replicas Peak
Energy (cv)
A B C D E F G
1.7316 1.7288 1.725 1.7172 1.7125 1.7026 1.6932
~E (meV)
19.1
>-
38.4
Z ILl
I.-
H PL notalso 1.686 be detected in the 39 spectral region of B1 couldbut B2 growth, 1.709 1.690 to the starting 19.8 38.8 Sb2S3. Any other
1.1
1.9 eV. Figure 2 shows the PL spectra observed for the stoichiometric samples grown from the ingots listed in table 1. Curves 1—3 corresponds to ingots * 1— * 3, respectively. The overall feature of curve 1 resembles that of curve 2, but peak C is more
z
Bi
______________________________
2
____________________________
—
1.67
1.69 1.71 1.73. PHOTON ENERGY (eV)
Fig. 3. PL pressures spectra observed the crystal grown under ent vapor of sulfur.inCurves 1—3 correspond to differsulfur pressures of 2, 7 and 40 Torr, respectively.
clearly seen in curve 2. For curve 3, the major peaks are A, E, F, and G, while peaks B and C are The PL spectra observed for the samples grown
J~
under different vapor pressure of sulfur are shown in fig. 3. The starting Sb2 S3 used was * 2. Curves
(0
0
1.67
1.69 1.71 PHOTON ENERGY (eV)
1.73
0
Fig. 2. Sample dependence of PL spectra 1—3 correspond with the samples grown from the ingots * 1—3 which are listed in table 1.
1—3 are the spectra for the stoichiometric, S-rich observed only as a weak shoulder. and Sb-rich crystals, respectively. As seen in the figure, the PL spectra showed a marked dependence on the growth conditions. For the Sb-rich sample, a broad PL spectrum, in which the structures observed in the stoichiometric samples are almost missing, was observed. For the S-rich sample, the most dominant peak was B and new peaks labeled B1 and B2 were observed at 1.709 and 1.690 eV, respectively. Since the energy differences between peaks B and B1 and between B3 and B2 are very close to the LO phonon energy, these new peaks can be assigned to be due to the LO phonon replicas of peak B. Peak A was observed as a weak shoulder on the high energy side of B. At the same time, the intensity of peaks E and G decreased notably.
178
F. Fujita et a!.
/ Near band
3
dences of peaks A, B and C are shown in fig. 5. It was found that the intensity of these peaks decreases markedly above K. The slope of the experimental points about above6 this temperature was well approximated by a straight line. Therefore the observed thermal quenching of PL may
2 15W/cm
be ascribed to a thermal activation process in which the by temperature variation of PL intensity is expressed
~77W/cm2~
>I—
edge photoluminescence in Sb, S
z
1(T) =I(0)/(1 +A .exp(—~/kT)). ~
~/cm2x2
I— w
z 0
1.67
1.69 1.71 PHOTON ENERGY (eV)
1.73
Fig. 4. PL spectra observed under different excitation intensOles.
~ andrespectively. A are thermal energy and aHere, constant, By activation the least-squares fitting the solid of eq.lines (1) inwith the the figures. experimental Similar temperature results, we estimated ~ to be 3.3 + 0.2, 2.4 + 0.3 and 3.4 + 0.2 meV for peaks A, B and C, respectively. The calculated temperature variations are shown by —
observed.
When the temperature was raised. The intensity of PL reduced rapidly. The temperature depen-
—
—
dependences of PL intensity were observed in the other peaks. The time decay spectrum measured in the spectral region at around the peaks A—C is plotted in fig. 6. The time resolution was 30 ps. Because of
Figure 4 shows the PL spectra for the stoichiometric crystal observed under different excitation intensities. The maximum intensity was 1.5 W/cm2. Note that curves 3 and 4 are enlarged by two and five times, respectively. When the excita-
tionand intensity was with reduced, theH. intensity ofalmost peaks A, creased compared itsintensity phonon replicas, C and and At Bthe largely minimum deexcitation (curve 4), these were missing and H became a distinct peak. It was found that the intensity of peaks A and B is nearly proportional to the excitation intensity, whereas peak C varied with about 0.7 powers of the excitation intensity. Further, the peak energy of C shifted to the low energy side with decreasing excitation intensity; the difference of peak energy between curves 1 and 4 was about 1 meV. Again the peak separation between C and H was 38 meV being about twice the LO phonon energy. Thus it is reasonable that peak H is the two-LO-phonon replica of C. Any appreciable change of peak positions of A, B, E, F, and G could not be
(1)
1
-
C
0.11 001
z ~‘)
VT
01
Lii
A 0.1
0.01 0.05
0.15 1/1 (K~)
0.25
Fig. 5. Temperature dependences of the intensity of peaks A, B and C. The solid lines are calculated curves according to eq. (1).
F Fujita et a!.
/ Near band
_____________________________________ 1
~
1
j
2
c~J°1]30
70~
~ 102
10
• .
0
~
•I~
-
100 102
I
10_i TIME
I
I
I
100
101
(ms)
Fig. 6. Time decay spectra of photoluminescence observed in the spectral region of peaks A—C at 2 K. The initial part of the decay curve is shown in the insert,
the poor spectral resolution, we could not measure the decay curves corresponding to these peaks separately. The decay curve was composed of fast and slow components. evidently the decay curve for the slow component is non-exponential. In the insert of the figure, the fast component measured with 40 ns time resolution is shown. This time decay spectrum could be decomposed into two components again; both of these were approximated well by an exponential decay with a decay time of 7 and 0.4 p.s, respectively. Note that we neglected the contribution of the non-exponential decay (the slowest component) in the decomposition, because its intensity is much weaker than the fast ones as shown in the figure. These three decays can be attributed to peaks A, B and C. The non-exponential decay curve like the slowest one has been usually observed for donor—acceptor pair recombination in many semiconductors [14]. Then one of the peaks A—C may be due to this recombination process. In the other spectral regions, similar decay curves were observed, but the relative intensity between the fast and slow components was somewhat different from those shown in this figure. As described so far, the PL spectra observed for the stoichiometric crystals are essentially cornposed of three components, the peaks A, B and C and their LO phonon replicas. Since the minimum energy gap in Sb2S3 is indirect [5] and the impur-
edge photoluminescence in Sb
2 S3
179
ity concentrations in the samples used are rather high, it may be reasonable that these components are due to extrinsic origins rather than an intrinsic one In fact in indirect gap semiconductors with exciton donor—acceptor recombination is moderateorpurity, extrinsic PLpair induced by a bound dominant, because of a small radiative recombination probability of the indirect free exciton [15].In the following we will concern ourselves with the origins of components A—C. For peak C its peak energy shifted to the low energy side with decreasing excitation intensity. This shift is caused by the rapid decrease of intensity in the higher energy part than the peak, compared with the low energy side of the peak. Part of this decrease may be induced by the reduction of B whose intensity is nearly proportional to the excitation intensity. However, because of the large separation between the peak positions of B and C (— 4 meV), compared to the half width of B (— 2.3 meY) which was estimated from curve 2 in fig. 3, the contribution of B to the peak shift can be neglected. The peak shift associated with the change of excitation intensity like this is characteristic of the PL spectrum induced by a donor—acceptor pair recombination [15].Thus it is likely that peak C is due to the no-phonon line of this recombination. This assignment is consistent with the non-exponential time decay shown in fig. 6. Peak H is the two LO phonon replica of the donor—acceptor pair luminescence. The small thermal activation energy as well as the small energy difference between the peak energy and the absorption threshold suggests that both donor and acceptor states are very shallow. However, the impurities which are responsible for donor and acceptor could not be identified, because any favorite element like donor or acceptor was not found in chemical analyses. Peak A was observed in all stoichiometric crystals grown from the different starting Sb 2S3 whose impurity contents are given in table 1, but almost missing in the non-stoichiometric samples, while peak B appeared in both stoichiometric and S-rich crystals, but disappeared in Sb-rich samples. Their intensities were nearly proportional to the excitation intensity and any appreciable peak shift associated with the reduction of excitation
180
T. Fujita et a!.
/ Near band
intensity could not be found. Two exponential time decays shown in the insert in fig. 6 can be attributed to peaks A and B, because the nonexponential decay is due to peak C. This suggests that both peaks A and B are caused by a monomolecular recombination of an electron and a hole. From these facts, we can conclude that peak A is attributed to an exciton bound to an impurity contained commonly in the samples and peak B is due to a bound exciton associated with a defect being characteristic to the S-rich specimens. As listed in table 1, the common impurity in the starting Sb2S3 is P which is isoelectronic with Sb. We suppose that peak A and its phonon replicas are connected with the exciton bound to P. The observed thermal activation energy (3.3 meV) may be the binding energy of the bound exciton. For peak B, Sb vacancies or interstitial S atoms, which may be the most likely defect in S-rich crystals, may play an important role in the binding of excitons, but details on the origin are not clear at present.
In summary, we have observed photolurninescence spectra composed of many fairly sharp peaks near the absorption edge. These components were attributed to three different extrinsic origins, one donor—acceptor pair and two bound excitons. No intrinsic PL spectra could not be detected. However, the results shown here strongly suggest that the intrinsic free exciton luminescence can be observed in more pure crystals than those used in the
edge photoluminescence in Sb
2 S3
present study. This is markedly different from the situation in As-chalcogenides, in which only the broad and largely Stokes-shifted PL bands due to the localized center have been observed.
References [1] R.A. Street, Adv. Phys. 25 (i976) 397. [2] MN. Kabler and D.A. Patterson, Phys. Rev. Lett. 19 (1967) 652. [3] K. Murayama and M.A. Bosch, Phys. Rev. B23 (1981) 6810. [4] L.H. Robins and M.A. Kastner, Phil. Mag. B50 (1984) 29. [5] T. Fujita, K. Kurita, K. Takiyama and T. Oda, J. Phys. Soc. Jpn. 56 (1987) 3734. [6] K. Kurita, T. Fujita, K. Takiyama and T. Oda, Phys. Lett. A, 126 (1987) 141. [7] R.S. Sussmann, TM. Searle and 1G. Austin, Phil. Mag. B44 (1981) 665. [8] M. Schreiber and Y. Toyozawa, J. Phys. Jpn. 52 (1983) 318.
[9] AS. Orlyukas and I.P. Grigas, Soy. Phys. Crystallogr. 19 (1975) 547. [10] P. Bohac and P. Kaufmann, Mat. Res. Bull. 10 (1975) 613. [11] I.P. Grigas and A.S. Karpus, Soy. Phys. Sol. St. 9 (1968) 2270. [12] P. Bayliss and W. Nowacki, Z. Kristallogr. 135 (1972) 308. [13] J. Petzelt and J. Gngas, Ferroelectrics 5 (1973) 59. [14] D.G. Thomas, J.J. Hopfield and W.M. Augustyniak, Phys. Rev. 140 (1965) A202. [15] E.W. Williams and H. B. Bebb, Semiconductors and Semimetals, Vol. 8, eds. R.K. Willarson and A. C. Beer (Academic Press, New York, London, 1973) p. 335.
175
NEAR BAND EDGE PHOTOLUMINESCENCE IN Sb2S3 1, K. TAKIYAMA and T. ODA T. FUJITA, K. KURITA Department of Applied Physics and Chemistry, Faculty of Engineering Hiroshima
University, Higashi-Hiroshima 724, Japan
Received 5 October 1987 Revised 23 November 1987 Accepted 1 December 1987
Photoluminescence spectra in Sb 2S3 have been investigated for the first time. The photoluminescence composed of many peaks is observed near the indirect fundamental gap at 2 K. From the study of sample, excitation intensity and temperature dependences of photoluminescence and the time decay spectra, it is shown that the photoluminescence is caused by three different origins. Models of the origins are proposed. It is also shown that the photoluminescence spectra are very sensitive to the deviation from stoichiometry.
1. Introduction Photoluminescence (PL) spectra have been widely used to study the electronic states in semiconductors and insulators. In V—VI cornpound semiconductors, extensive investigations of PL spectra have been performed in As-chalcogenides (As2S3 and As2Se3), in which broad and largely Stokes-shifted PL bands have been observed near the mid-gap position [1]. The large band width and Stokes shift point to strong exciton—phonon interactions in these materials. A model, which is similar to the self-trapped exciton in ionic crystals [2], has bee~iproposed as the origin of the PL band [3,4]. However, it is not clear whether the broad and largely Stokes-shifted PL is the common feature in V—VI compounds or not, because very little is known about PL spectra in other substances. Recently we have investigated the fundamental absorption edge in orthorombic Sb2S3 [5,6]. It has been shown that the lowest energy gap is indirect (the top of the valence band is at F roint and the bottom at about the X point) [5] and at high temperature the absorption tail obeys the Urbach rule [6]. These features of the absorption spectra 1
Present address: Technical Headquarters, Hiroshima Technical Institute, Mitsubishi Heavy Industries Ltd., Hiroshima 733, Japan.
are similar to those in As2Se3 [7], but the steepness parameter a in the Urbach rule is larger than that in As2Se3. This suggests that the exciton—phonon interaction in Sb2S3 is weaker than that in As2Se3, because a is inversely proportional to the strength of the exciton —phonon interaction [8]. Further, the dielectric property of Sb2S3 is quite different from that in As-chalcogenides; the static dielectric constant parallel to the b-axis is very large (e ~ 100) [9]. Thus we may expect that the PL spectrum in Sb2S3 is different from those in As-chalcogenides. In this work, we studied the PL spectra in Sb2S3 single crystals for the first time. The PL spectra composed of many fairly sharp peaks were observed near the absorption edge, which is different from those in As-chalcogenides. The origins of the PL spectra will be discussed.
2. Experimental procedure Single crystals of Sb2S3 were grown from a vapor phase in a sulfur atmosphere. The starting materials of crystal growth were zone-refined Sb2 S3 ingots which were synthesized by a direct reaction of Sb and S in an evacuated quartz ampoule [10]. To investigate the effects of defects introduced by the deviation from stoichiometry on PL, the growth
0022-2313/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
176
T Fujita eta!.
/
Near band edge photoluminescence in Sb,S
1
Table I Impurity contents in starting Sb2S3 ingots determined spectrochemically Ingot
Concentration (ppm) P
#1 #2 #3
30 30 30
Mg
Ca
Cr
Mn
Fe
—
—
0.5
0.3
—
—
—
—
1
5
—
0.3
10 5 1
was performed under three different sulfur pressures (2, 7 and 40 Torr). It has been pointed out that crystals grown under these conditions are Sb-rich, stoichiometric and S-rich, respectively [11]. The impurity contents in the starting Sb2S3 estimated spectrochernically by means of an inductively coupled plasma spectrometer are listed in table 1. PL spectra were measured by a system cornbined with a 30 cm double monochromator, a cooled photomultiplier (Hamamatsu R943-02) and a photon counting unit. The spectral resolution was about 1 A 0.3 meV). The excitation source was a 1 mW He—Ne laser. Observations were performed either by a backscattering geometry or by a right-angle geometry, but notable differences between these could not be observed. The intensity of the exciting light was measured by a calibrated thermopile. Because of the anisotropic crystal structure [12], we measured the PL spectra for polarizations parallel and perpendicular to the b-axis, but the degree of polarization of PL spectra defined by P=(111 —I~)/(J +I~) was nearly constant (P 0.3) in the entire spectral region investigated. Thus, in the following we will show the spectra obtained without polarizer. Time decay spectra were measured by a photon counting technique. The light source was a conventional nitrogen discharge lamp of about 8 ns pulse width. Because of the weak PL intensity, the emitted light was dispersed by interference filters with about 50 A band pass. Samples were immersed in pumped liquid helium. (—
* 2 listed in table 1. The absorption spectra due to the indirect allowed exciton [5] are also shown for comparison. Here, E ii b and E II c represent the polarization of light. The position of the mdirect is exciton (1.749 eV) is indicated by the arrow. As shown in the figure, there were 7 distinct peaks and one shoulder. The peak having the largest peak energy lies at 1.7316 eV which is smaller by about 17 meV than the threshold energy of the Is indirect exciton. Hereafter these are designated in alphabetical order from the peak with higher energy. The estimated peak energies are listed in table 2. The reproducibility of the peak energy was better than 0.3 meV. Among these the peaks A, E and G were located with an equal spacing of about 19 meV. This value is close to the LO phonon energies observed in the infrared reflectivity spectra [13]. Thus it may be reasonable that these peaks are phonon replicas of peak A. It was found that the PL is sensitive not only to the vapor pressure of sulfur during crystal
—
3. Experimental results and discussion Figure 1 shows a typical PL spectrum observed for the stoichiometric crystal grown from the ingot
a -
B C H
F
A
E D
I
1.66
I
I
il/c
I
PHOTON ENERGY (eV)
E//b-
500
I
1.78
Fig. 1. Typical PL spectrum at 2 K. The absorption spectra are shown for comparison. The arrow indicates the threshold energy of the is indirect exciton.
T. Fujita eta!. / Near band edge photoluminescence in Sb
2S3
177
Table 2 Peak energies of PL spectra at 2 K. L~E is the energy difference between peaks A, B and C and their phonon replicas Peak
Energy (cv)
A B C D E F G
1.7316 1.7288 1.725 1.7172 1.7125 1.7026 1.6932
~E (meV)
19.1
>-
38.4
Z ILl
I.-
H PL notalso 1.686 be detected in the 39 spectral region of B1 couldbut B2 growth, 1.709 1.690 to the starting 19.8 38.8 Sb2S3. Any other
1.1
1.9 eV. Figure 2 shows the PL spectra observed for the stoichiometric samples grown from the ingots listed in table 1. Curves 1—3 corresponds to ingots * 1— * 3, respectively. The overall feature of curve 1 resembles that of curve 2, but peak C is more
z
Bi
______________________________
2
____________________________
—
1.67
1.69 1.71 1.73. PHOTON ENERGY (eV)
Fig. 3. PL pressures spectra observed the crystal grown under ent vapor of sulfur.inCurves 1—3 correspond to differsulfur pressures of 2, 7 and 40 Torr, respectively.
clearly seen in curve 2. For curve 3, the major peaks are A, E, F, and G, while peaks B and C are The PL spectra observed for the samples grown
J~
under different vapor pressure of sulfur are shown in fig. 3. The starting Sb2 S3 used was * 2. Curves
(0
0
1.67
1.69 1.71 PHOTON ENERGY (eV)
1.73
0
Fig. 2. Sample dependence of PL spectra 1—3 correspond with the samples grown from the ingots * 1—3 which are listed in table 1.
1—3 are the spectra for the stoichiometric, S-rich observed only as a weak shoulder. and Sb-rich crystals, respectively. As seen in the figure, the PL spectra showed a marked dependence on the growth conditions. For the Sb-rich sample, a broad PL spectrum, in which the structures observed in the stoichiometric samples are almost missing, was observed. For the S-rich sample, the most dominant peak was B and new peaks labeled B1 and B2 were observed at 1.709 and 1.690 eV, respectively. Since the energy differences between peaks B and B1 and between B3 and B2 are very close to the LO phonon energy, these new peaks can be assigned to be due to the LO phonon replicas of peak B. Peak A was observed as a weak shoulder on the high energy side of B. At the same time, the intensity of peaks E and G decreased notably.
178
F. Fujita et a!.
/ Near band
3
dences of peaks A, B and C are shown in fig. 5. It was found that the intensity of these peaks decreases markedly above K. The slope of the experimental points about above6 this temperature was well approximated by a straight line. Therefore the observed thermal quenching of PL may
2 15W/cm
be ascribed to a thermal activation process in which the by temperature variation of PL intensity is expressed
~77W/cm2~
>I—
edge photoluminescence in Sb, S
z
1(T) =I(0)/(1 +A .exp(—~/kT)). ~
~/cm2x2
I— w
z 0
1.67
1.69 1.71 PHOTON ENERGY (eV)
1.73
Fig. 4. PL spectra observed under different excitation intensOles.
~ andrespectively. A are thermal energy and aHere, constant, By activation the least-squares fitting the solid of eq.lines (1) inwith the the figures. experimental Similar temperature results, we estimated ~ to be 3.3 + 0.2, 2.4 + 0.3 and 3.4 + 0.2 meV for peaks A, B and C, respectively. The calculated temperature variations are shown by —
observed.
When the temperature was raised. The intensity of PL reduced rapidly. The temperature depen-
—
—
dependences of PL intensity were observed in the other peaks. The time decay spectrum measured in the spectral region at around the peaks A—C is plotted in fig. 6. The time resolution was 30 ps. Because of
Figure 4 shows the PL spectra for the stoichiometric crystal observed under different excitation intensities. The maximum intensity was 1.5 W/cm2. Note that curves 3 and 4 are enlarged by two and five times, respectively. When the excita-
tionand intensity was with reduced, theH. intensity ofalmost peaks A, creased compared itsintensity phonon replicas, C and and At Bthe largely minimum deexcitation (curve 4), these were missing and H became a distinct peak. It was found that the intensity of peaks A and B is nearly proportional to the excitation intensity, whereas peak C varied with about 0.7 powers of the excitation intensity. Further, the peak energy of C shifted to the low energy side with decreasing excitation intensity; the difference of peak energy between curves 1 and 4 was about 1 meV. Again the peak separation between C and H was 38 meV being about twice the LO phonon energy. Thus it is reasonable that peak H is the two-LO-phonon replica of C. Any appreciable change of peak positions of A, B, E, F, and G could not be
(1)
1
-
C
0.11 001
z ~‘)
VT
01
Lii
A 0.1
0.01 0.05
0.15 1/1 (K~)
0.25
Fig. 5. Temperature dependences of the intensity of peaks A, B and C. The solid lines are calculated curves according to eq. (1).
F Fujita et a!.
/ Near band
_____________________________________ 1
~
1
j
2
c~J°1]30
70~
~ 102
10
• .
0
~
•I~
-
100 102
I
10_i TIME
I
I
I
100
101
(ms)
Fig. 6. Time decay spectra of photoluminescence observed in the spectral region of peaks A—C at 2 K. The initial part of the decay curve is shown in the insert,
the poor spectral resolution, we could not measure the decay curves corresponding to these peaks separately. The decay curve was composed of fast and slow components. evidently the decay curve for the slow component is non-exponential. In the insert of the figure, the fast component measured with 40 ns time resolution is shown. This time decay spectrum could be decomposed into two components again; both of these were approximated well by an exponential decay with a decay time of 7 and 0.4 p.s, respectively. Note that we neglected the contribution of the non-exponential decay (the slowest component) in the decomposition, because its intensity is much weaker than the fast ones as shown in the figure. These three decays can be attributed to peaks A, B and C. The non-exponential decay curve like the slowest one has been usually observed for donor—acceptor pair recombination in many semiconductors [14]. Then one of the peaks A—C may be due to this recombination process. In the other spectral regions, similar decay curves were observed, but the relative intensity between the fast and slow components was somewhat different from those shown in this figure. As described so far, the PL spectra observed for the stoichiometric crystals are essentially cornposed of three components, the peaks A, B and C and their LO phonon replicas. Since the minimum energy gap in Sb2S3 is indirect [5] and the impur-
edge photoluminescence in Sb
2 S3
179
ity concentrations in the samples used are rather high, it may be reasonable that these components are due to extrinsic origins rather than an intrinsic one In fact in indirect gap semiconductors with exciton donor—acceptor recombination is moderateorpurity, extrinsic PLpair induced by a bound dominant, because of a small radiative recombination probability of the indirect free exciton [15].In the following we will concern ourselves with the origins of components A—C. For peak C its peak energy shifted to the low energy side with decreasing excitation intensity. This shift is caused by the rapid decrease of intensity in the higher energy part than the peak, compared with the low energy side of the peak. Part of this decrease may be induced by the reduction of B whose intensity is nearly proportional to the excitation intensity. However, because of the large separation between the peak positions of B and C (— 4 meV), compared to the half width of B (— 2.3 meY) which was estimated from curve 2 in fig. 3, the contribution of B to the peak shift can be neglected. The peak shift associated with the change of excitation intensity like this is characteristic of the PL spectrum induced by a donor—acceptor pair recombination [15].Thus it is likely that peak C is due to the no-phonon line of this recombination. This assignment is consistent with the non-exponential time decay shown in fig. 6. Peak H is the two LO phonon replica of the donor—acceptor pair luminescence. The small thermal activation energy as well as the small energy difference between the peak energy and the absorption threshold suggests that both donor and acceptor states are very shallow. However, the impurities which are responsible for donor and acceptor could not be identified, because any favorite element like donor or acceptor was not found in chemical analyses. Peak A was observed in all stoichiometric crystals grown from the different starting Sb 2S3 whose impurity contents are given in table 1, but almost missing in the non-stoichiometric samples, while peak B appeared in both stoichiometric and S-rich crystals, but disappeared in Sb-rich samples. Their intensities were nearly proportional to the excitation intensity and any appreciable peak shift associated with the reduction of excitation
180
T. Fujita et a!.
/ Near band
intensity could not be found. Two exponential time decays shown in the insert in fig. 6 can be attributed to peaks A and B, because the nonexponential decay is due to peak C. This suggests that both peaks A and B are caused by a monomolecular recombination of an electron and a hole. From these facts, we can conclude that peak A is attributed to an exciton bound to an impurity contained commonly in the samples and peak B is due to a bound exciton associated with a defect being characteristic to the S-rich specimens. As listed in table 1, the common impurity in the starting Sb2S3 is P which is isoelectronic with Sb. We suppose that peak A and its phonon replicas are connected with the exciton bound to P. The observed thermal activation energy (3.3 meV) may be the binding energy of the bound exciton. For peak B, Sb vacancies or interstitial S atoms, which may be the most likely defect in S-rich crystals, may play an important role in the binding of excitons, but details on the origin are not clear at present.
In summary, we have observed photolurninescence spectra composed of many fairly sharp peaks near the absorption edge. These components were attributed to three different extrinsic origins, one donor—acceptor pair and two bound excitons. No intrinsic PL spectra could not be detected. However, the results shown here strongly suggest that the intrinsic free exciton luminescence can be observed in more pure crystals than those used in the
edge photoluminescence in Sb
2 S3
present study. This is markedly different from the situation in As-chalcogenides, in which only the broad and largely Stokes-shifted PL bands due to the localized center have been observed.
References [1] R.A. Street, Adv. Phys. 25 (i976) 397. [2] MN. Kabler and D.A. Patterson, Phys. Rev. Lett. 19 (1967) 652. [3] K. Murayama and M.A. Bosch, Phys. Rev. B23 (1981) 6810. [4] L.H. Robins and M.A. Kastner, Phil. Mag. B50 (1984) 29. [5] T. Fujita, K. Kurita, K. Takiyama and T. Oda, J. Phys. Soc. Jpn. 56 (1987) 3734. [6] K. Kurita, T. Fujita, K. Takiyama and T. Oda, Phys. Lett. A, 126 (1987) 141. [7] R.S. Sussmann, TM. Searle and 1G. Austin, Phil. Mag. B44 (1981) 665. [8] M. Schreiber and Y. Toyozawa, J. Phys. Jpn. 52 (1983) 318.
[9] AS. Orlyukas and I.P. Grigas, Soy. Phys. Crystallogr. 19 (1975) 547. [10] P. Bohac and P. Kaufmann, Mat. Res. Bull. 10 (1975) 613. [11] I.P. Grigas and A.S. Karpus, Soy. Phys. Sol. St. 9 (1968) 2270. [12] P. Bayliss and W. Nowacki, Z. Kristallogr. 135 (1972) 308. [13] J. Petzelt and J. Gngas, Ferroelectrics 5 (1973) 59. [14] D.G. Thomas, J.J. Hopfield and W.M. Augustyniak, Phys. Rev. 140 (1965) A202. [15] E.W. Williams and H. B. Bebb, Semiconductors and Semimetals, Vol. 8, eds. R.K. Willarson and A. C. Beer (Academic Press, New York, London, 1973) p. 335.