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Composition dependence of the energy gap of ZnxMn1 − xIn2Se4 and optical absorption spectroscopy
January 1996
ELSEWIER
Materials Letters 26 (1996) 47-50
Composition dependence of the energy gap of ZnxMn, _,In,Se, and optical absorption spectroscopy Jorge Luengo *, N.V. Joshi Centro de Optica, Facultad de Ciencias, Universidad de Los Andes, Mhida, Venezuela
Received 31 July 1995; accepted 14 August 1995
Abstract Composition dependence of the band gap of Zn&In, -&Se, alloys has been studied at 300 K and 77 K for the first time. It was found that for smaller concentrations of Mn the band gap varies linearly with composition and for higher concentrations it
shows quadratic dependence. The same trend of dependence was observed for both temperatures. Absorption spectra do reveal the structure located on the higher energy side of the main band gap which has been interpreted on the basis of valence band structure, like, crystal fildd and spin-orbit interactions. Keywords: ZnIu,Se,; Manganese doped, Optical energy gap; Semiconductors; Optical properties; Semimagnetic
1. Introduction
ZnInzSe4 is a ternary semiconducting compound and belongs to the AnB~‘Xv’ defect chalcopyrite family. It crystallizes in the thiogallate structure [ 1] and possesses Si space group. It is common that these compounds have some percentage of vacancies of Zn sites and naturally, disordler is created which causes the appearance of a large number of electronic levels in the energy gap both very close to the bottom of the conduction band and in the mid gap region. In recent years much interest has been aroused [2-91 because of its intrinsic disorder related properties such as optical and photoconducting [ 561. Electra-optical memory effect [ 31 is particularly notable as it is directly related to the defect states which originated due to the intrinsic defects [ 61. * Corresponding author. 0167-577x/96/$12.00 8 11996Elsevier Science B.V. All rights reserved SSD10167-577x(95)a~o211-1
Because of the high density of states near the band edges, the precise determination of the band gap with optical absorption spectroscopy becomes rather difficult and there was contradiction about its nature (direct or indirect) and also different values for it were reported [ 71. Therefore, a careful optical absorption study of this compound is required. Moreover, it forms an alloy with Mn in a wide range of concentrations and makes a useful family of diluted semimagnetic semiconductors. In the present system, manganese plays a double role. First, it occupies partially vacant Zn sites and reduces the defect states and secondly, it forms a semiconducting alloy whose composition controls the band gap and lattice spacing, two key parameters used in the technology of devices. In spite of the potential importance of this system, due attention to it has not been given so far. The purpose of the present work, therefore, is to prepare better quality materials and examine their optical properties.
48
J. Luengo, N. V. Joshi /Materials
2. Experimental Samples of the quaternary alloy, Zn,Mn, -&Se, were prepared for various compositions (0.0
Letters 26 (1996) 47-50
is clear that the plot does not indicate the presence of a single phase but reveals the superposition of two edges. Naturally, in this region absorption spectroscopy is not
20
I
N,
-z 2
15 u
.
J
e’
10
)
5 0 1.3
1.4
1.5
1.6
1.7
U!J 1.8
1.9
2.0
hv (rV) Fig. 1. A plot of (&I u)’ vs hv for samples of lower Mn concentration. The structure on the higher energy side is clearly visible. Curves a, b, a’ and b’ correspond to x = 0 and x = 0.1 for 77 K and 300 K respectively.
25 20 we :
15
.x 10
5 0 1.3
1.4
1.5
1.6
1.7
1.11
1.9
1 0
hv (rV)
Fig. 2. A plot similar to Fig. 1 but for higher concentrations of Mn. The superposition of the curves is obvious as the band gap shows parabolic variation. Curves c, d, e, c’, d’ and e’ correspond tax= 0.8, 0.9 and 1.O for 77 K and 300 K respectively.
3. Results and discussion The optical absorption spectra were recorded at 300 K and 77 K for various compositions and from the obtained data the values of ( cubV) * were estimated and plotted against hv to examine the nature of the bandgap. The plots for lower concentrations of manganese are shown in Fig. 1, while Fig. 2 shows the behaviour at higher concentrations. It is worth mentioning that for x = 0.4 to 0.6 at% the materials do not exist in a single phase and X-ray crystallographic study shows the presence of two phases. Fig. 3 shows the plot of (&v)* vs hv for compositions x = 0.4,0.5 and 0.6 recorded at 77 K. Similar results have also been observed at 300 K. It
20-
1.3
1.4
1.5
1.6
Enrrgy
1.7
1.8
1.S
I
(CA/)
Fig. 3. A plot similar to Fig. 1 for concentration Two-phase behavior is visible.
x=0.4,0.5
and 0.6.
49
J. Luengo, N. V. Joshi / Materials Letters 26 (I 996) 47-50 1.90
an adequate technique for determination of the band gap and hence this reg,ion is excluded for analysis and discussion. It is clear from Fig. 1 that the material has a direct band gap. There are some contradictions in the literature which might have originated due to the high density of states situated very close to the bottom of the conduction band [ 71. Absorption spectra do not show any peak at the lower energy side of the main edge indicating the absenoe of the impurity states (either donor or acceptor) in the sample. In general, optical absorption [ 71 and photoconductivity [ 111 spectra do show a remarkable peak which is associated with the high density of acceptor states, a common feature in defect compounds. This might have been due to different growth techniques. In the chemical transport method, even though it provides a monocrystal of a reasonable size, the presence of the impurity of the transport agent (chlorine or iodine atom) is unavoidable and the role of these states is reflected on the lower energy side of the band gap. Moreover, it has been found that acceptor states give rise to a strong peak on the low energy side of the absorption spectra and this makes the analysis of the band gap even more difficult. The clear absorption edge observed in the present investigation is of fundamental importance in examining the composition dependence of the band gap. Lack of it has frequently forced one to estimate the band gap by other inadequate techniques such as photoconductivity [ 3,9]. With the help of the (crh~)~ vs hv plot, the energy gap of ZnIn,Se, is found to be 1.88 eV and 1.775 eV at 77 K and 300 K. The previously reported values lie in the region 1.9 1 to 1.93 eV at 77 K and 1.81 to 1.83 eV at 300 K [ 3,5]. The values obtained in the present investigation are slightly below the previous ones, however, they are consistent with the composition and temperature variation. The sharp absorption edge observed in the present investigation, absence of any structure on the lower energy side originating from the acceptor or an impurity state and a systematic variation of the band gap with temperature provide confidence in the present results. On the higher energy side of the main absorption edge, there exist two well-resolved peaks A and B for both temperatures and they are located from the band edge at 0.08 eV and 0 102 eV respectively at 77 K. The detailed structure is observed only at lower concentration of Mn. For x = 0.3 to 0.8, the structure is not clear,
1.85
771
1.80
L ._
1.75 s t 0 w
1.70FT
5 r 3001
::
1.65
f
1.60
: a
1.55
"r I-
:‘::.
0
0.1
0.2
0.3
0.4
0.5
Com~osiiion.
Fig. 4. Composition
, .7
0.6
1 0.8
0 0.9
1.0
x
variation of the band gap for 77 K and 300 K.
however for x= 0.9 and MnIn,Se,, peaks are again revealed (see Fig. 2). For the last sample the peaks are separated from the band edge by 0.085 eV and 0.12 eV. The peak on the high energy side originates from the upper valence band structure. Unfortunately, the precise band structure calculation of crystal field and spinorbit splitting has not been carried out for ZnIn,Se, and hence the interpretation of this structure is based on earlier experimental data. A photoconductivity study [ 81 reveals that the crystal field (c.f.) splitting is 0.075 eV, very close to the observed peak A and therefore, this peak could be attributed to the transition from this state (c.f.) to the bottom of the conduction band. Peak B is located approximately 0.12 eV from the main band edge, and corresponds to spin-orbit splitting. It is worth mentioning that these peaks were not reported before by optical absorption spectroscopy. MnIn,Se, is a ternary semiconducting compound which has received very little attention and only structural and magnetic properties have been reported [ 93. No data are available on optical properties and there is no theoretical or experimental work on valence band structure, therefore, a precise assignment for these peaks is not possible. However, considering the general trends of the splitting in the alloys of ternary compounds, we tentatively assigned peak A ( 1.78 eV) and peak B ( 1.81 eV) to the crystal field and spin-orbit splitting, respectively. The variation of the band gap as a function of concentration is shown in Fig. 4 both at 77 K and 300 K. For x = 0 to 0.3, the band gap variation is linear and can be expressed by the equation (the best fit was obtained by using program Mathematics) : Eg= 1.8836-0.364x
(at77
Eg= 1.7729-0.366x
(at 300 K).
K),
(la) (lb)
50
.I. Luengo, N.V. Joshi /Materials
It is worth mentioning that the rate of variation of the band gap with composition, tildx remains the same for 300 K and 77 K; this property can be used to monitor the band gap for different compositions and temperatures. Fig. 4 shows that the band gap variation is not linear in the range x = 0.7 to 1 but it shows quadratic dependence and the relation is given by the expression: E,=3.7983-5.138x+3.0.?
(at77K),
(2a)
E,=3.4489-4.474x+2.6x?
(at 300 K).
(2b)
Here also the nature of the variation of the band gap for different temperatures remains the same, which can be understood with the help of Fig. 4. Fig. 1 and 2 show the systematic variation of the band gap as a function of concentration of Mn. The role of 3d states in manganese-based alloys in optical absorption spectroscopy [ lo] and photoconductivity [ 111 was widely studied. The frequently observed feature of DMS (diluted magnetic semiconductors) systems is that the transitions corresponding to the 3d states are observed in the optical absorption spectra. However, the present investigation does not reveal any 3d transitions. This suggests that the first d-d* transition may lie on the higher energy side of the absorption edge. A careful photoluminescence study of ZnInzSed : Mn confirms this view and the ?,-6A1 transition lies at about 2.1 eV [ 121. Thus, in the present alloy there is no interference from d* states of Mn.
Letters 26 (1996) 47-50
This fact has important consequences on optical properties. Mn can be used to tune the band gap, adjust the lattice parameter and reduce the density of acceptor states. Moreover, the optical absorption process is clean. In short, ZnJkIn, _ .&Se4 is an adequate material for optical and electro-optical devices.
References [ 1 ] P. Manta, F. Raga and A. Spiga, Nuovo Cimento B 119 ( 1974) 15. [2] S. Sgariazzo and A. Serpi, Phys. Rev. B 41 ( 1990) 7718. [3] J. Filipowicz, N. Romeo and L. Tar&one, Solid State Commun. 38 (1980) 619. [4] E. Nowark, B. Neumann and B. Steiner, Phys. Stat. Sol. 13 A (1992) K13. [5] E. Fortin and F. Raga, Solid State Commun. 14 ( 1974) 847. [6] E. Grilli, M. Guzzi and R. Molteni, Phys. Stat. Sol. 37 ( 1976) 399. [7] R. Trykozko and J. Filipowicz, Japan. J. Appl. Phys. 119 (1980) 153. [ 81 N.M. Mekhtiev, Z.Z. Gruseinov and E. Yu Salaev, Sov. Phys. Semicond. 18 (1984) 677. [9] G. Doll, A. Anghel, J.R. Baummann, E. Bucher, A.P. Ramlrez and K.J. Range, Phys. Stat. Sol. 1126 (1991) 237. [ lo] N.V. Joshi and L. Mogollbn, Progr. Crystal Growth Character. 110 (1989) 65. [ 1 l] N.V. Joshi, Photoconductivity: arts, science and technology (Marcel Dekker, New York, 1990). [ 121 C. Benecke and M.E. Guumlich, in: The luminescence of wide band gap II-Mn-VI semimagnetic semiconductors in diluted magnetic systems, ed. M. Jain (World Scientific, Singapore, 1992).
ELSEWIER
Materials Letters 26 (1996) 47-50
Composition dependence of the energy gap of ZnxMn, _,In,Se, and optical absorption spectroscopy Jorge Luengo *, N.V. Joshi Centro de Optica, Facultad de Ciencias, Universidad de Los Andes, Mhida, Venezuela
Received 31 July 1995; accepted 14 August 1995
Abstract Composition dependence of the band gap of Zn&In, -&Se, alloys has been studied at 300 K and 77 K for the first time. It was found that for smaller concentrations of Mn the band gap varies linearly with composition and for higher concentrations it
shows quadratic dependence. The same trend of dependence was observed for both temperatures. Absorption spectra do reveal the structure located on the higher energy side of the main band gap which has been interpreted on the basis of valence band structure, like, crystal fildd and spin-orbit interactions. Keywords: ZnIu,Se,; Manganese doped, Optical energy gap; Semiconductors; Optical properties; Semimagnetic
1. Introduction
ZnInzSe4 is a ternary semiconducting compound and belongs to the AnB~‘Xv’ defect chalcopyrite family. It crystallizes in the thiogallate structure [ 1] and possesses Si space group. It is common that these compounds have some percentage of vacancies of Zn sites and naturally, disordler is created which causes the appearance of a large number of electronic levels in the energy gap both very close to the bottom of the conduction band and in the mid gap region. In recent years much interest has been aroused [2-91 because of its intrinsic disorder related properties such as optical and photoconducting [ 561. Electra-optical memory effect [ 31 is particularly notable as it is directly related to the defect states which originated due to the intrinsic defects [ 61. * Corresponding author. 0167-577x/96/$12.00 8 11996Elsevier Science B.V. All rights reserved SSD10167-577x(95)a~o211-1
Because of the high density of states near the band edges, the precise determination of the band gap with optical absorption spectroscopy becomes rather difficult and there was contradiction about its nature (direct or indirect) and also different values for it were reported [ 71. Therefore, a careful optical absorption study of this compound is required. Moreover, it forms an alloy with Mn in a wide range of concentrations and makes a useful family of diluted semimagnetic semiconductors. In the present system, manganese plays a double role. First, it occupies partially vacant Zn sites and reduces the defect states and secondly, it forms a semiconducting alloy whose composition controls the band gap and lattice spacing, two key parameters used in the technology of devices. In spite of the potential importance of this system, due attention to it has not been given so far. The purpose of the present work, therefore, is to prepare better quality materials and examine their optical properties.
48
J. Luengo, N. V. Joshi /Materials
2. Experimental Samples of the quaternary alloy, Zn,Mn, -&Se, were prepared for various compositions (0.0
Letters 26 (1996) 47-50
is clear that the plot does not indicate the presence of a single phase but reveals the superposition of two edges. Naturally, in this region absorption spectroscopy is not
20
I
N,
-z 2
15 u
.
J
e’
10
)
5 0 1.3
1.4
1.5
1.6
1.7
U!J 1.8
1.9
2.0
hv (rV) Fig. 1. A plot of (&I u)’ vs hv for samples of lower Mn concentration. The structure on the higher energy side is clearly visible. Curves a, b, a’ and b’ correspond to x = 0 and x = 0.1 for 77 K and 300 K respectively.
25 20 we :
15
.x 10
5 0 1.3
1.4
1.5
1.6
1.7
1.11
1.9
1 0
hv (rV)
Fig. 2. A plot similar to Fig. 1 but for higher concentrations of Mn. The superposition of the curves is obvious as the band gap shows parabolic variation. Curves c, d, e, c’, d’ and e’ correspond tax= 0.8, 0.9 and 1.O for 77 K and 300 K respectively.
3. Results and discussion The optical absorption spectra were recorded at 300 K and 77 K for various compositions and from the obtained data the values of ( cubV) * were estimated and plotted against hv to examine the nature of the bandgap. The plots for lower concentrations of manganese are shown in Fig. 1, while Fig. 2 shows the behaviour at higher concentrations. It is worth mentioning that for x = 0.4 to 0.6 at% the materials do not exist in a single phase and X-ray crystallographic study shows the presence of two phases. Fig. 3 shows the plot of (&v)* vs hv for compositions x = 0.4,0.5 and 0.6 recorded at 77 K. Similar results have also been observed at 300 K. It
20-
1.3
1.4
1.5
1.6
Enrrgy
1.7
1.8
1.S
I
(CA/)
Fig. 3. A plot similar to Fig. 1 for concentration Two-phase behavior is visible.
x=0.4,0.5
and 0.6.
49
J. Luengo, N. V. Joshi / Materials Letters 26 (I 996) 47-50 1.90
an adequate technique for determination of the band gap and hence this reg,ion is excluded for analysis and discussion. It is clear from Fig. 1 that the material has a direct band gap. There are some contradictions in the literature which might have originated due to the high density of states situated very close to the bottom of the conduction band [ 71. Absorption spectra do not show any peak at the lower energy side of the main edge indicating the absenoe of the impurity states (either donor or acceptor) in the sample. In general, optical absorption [ 71 and photoconductivity [ 111 spectra do show a remarkable peak which is associated with the high density of acceptor states, a common feature in defect compounds. This might have been due to different growth techniques. In the chemical transport method, even though it provides a monocrystal of a reasonable size, the presence of the impurity of the transport agent (chlorine or iodine atom) is unavoidable and the role of these states is reflected on the lower energy side of the band gap. Moreover, it has been found that acceptor states give rise to a strong peak on the low energy side of the absorption spectra and this makes the analysis of the band gap even more difficult. The clear absorption edge observed in the present investigation is of fundamental importance in examining the composition dependence of the band gap. Lack of it has frequently forced one to estimate the band gap by other inadequate techniques such as photoconductivity [ 3,9]. With the help of the (crh~)~ vs hv plot, the energy gap of ZnIn,Se, is found to be 1.88 eV and 1.775 eV at 77 K and 300 K. The previously reported values lie in the region 1.9 1 to 1.93 eV at 77 K and 1.81 to 1.83 eV at 300 K [ 3,5]. The values obtained in the present investigation are slightly below the previous ones, however, they are consistent with the composition and temperature variation. The sharp absorption edge observed in the present investigation, absence of any structure on the lower energy side originating from the acceptor or an impurity state and a systematic variation of the band gap with temperature provide confidence in the present results. On the higher energy side of the main absorption edge, there exist two well-resolved peaks A and B for both temperatures and they are located from the band edge at 0.08 eV and 0 102 eV respectively at 77 K. The detailed structure is observed only at lower concentration of Mn. For x = 0.3 to 0.8, the structure is not clear,
1.85
771
1.80
L ._
1.75 s t 0 w
1.70FT
5 r 3001
::
1.65
f
1.60
: a
1.55
"r I-
:‘::.
0
0.1
0.2
0.3
0.4
0.5
Com~osiiion.
Fig. 4. Composition
, .7
0.6
1 0.8
0 0.9
1.0
x
variation of the band gap for 77 K and 300 K.
however for x= 0.9 and MnIn,Se,, peaks are again revealed (see Fig. 2). For the last sample the peaks are separated from the band edge by 0.085 eV and 0.12 eV. The peak on the high energy side originates from the upper valence band structure. Unfortunately, the precise band structure calculation of crystal field and spinorbit splitting has not been carried out for ZnIn,Se, and hence the interpretation of this structure is based on earlier experimental data. A photoconductivity study [ 81 reveals that the crystal field (c.f.) splitting is 0.075 eV, very close to the observed peak A and therefore, this peak could be attributed to the transition from this state (c.f.) to the bottom of the conduction band. Peak B is located approximately 0.12 eV from the main band edge, and corresponds to spin-orbit splitting. It is worth mentioning that these peaks were not reported before by optical absorption spectroscopy. MnIn,Se, is a ternary semiconducting compound which has received very little attention and only structural and magnetic properties have been reported [ 93. No data are available on optical properties and there is no theoretical or experimental work on valence band structure, therefore, a precise assignment for these peaks is not possible. However, considering the general trends of the splitting in the alloys of ternary compounds, we tentatively assigned peak A ( 1.78 eV) and peak B ( 1.81 eV) to the crystal field and spin-orbit splitting, respectively. The variation of the band gap as a function of concentration is shown in Fig. 4 both at 77 K and 300 K. For x = 0 to 0.3, the band gap variation is linear and can be expressed by the equation (the best fit was obtained by using program Mathematics) : Eg= 1.8836-0.364x
(at77
Eg= 1.7729-0.366x
(at 300 K).
K),
(la) (lb)
50
.I. Luengo, N.V. Joshi /Materials
It is worth mentioning that the rate of variation of the band gap with composition, tildx remains the same for 300 K and 77 K; this property can be used to monitor the band gap for different compositions and temperatures. Fig. 4 shows that the band gap variation is not linear in the range x = 0.7 to 1 but it shows quadratic dependence and the relation is given by the expression: E,=3.7983-5.138x+3.0.?
(at77K),
(2a)
E,=3.4489-4.474x+2.6x?
(at 300 K).
(2b)
Here also the nature of the variation of the band gap for different temperatures remains the same, which can be understood with the help of Fig. 4. Fig. 1 and 2 show the systematic variation of the band gap as a function of concentration of Mn. The role of 3d states in manganese-based alloys in optical absorption spectroscopy [ lo] and photoconductivity [ 111 was widely studied. The frequently observed feature of DMS (diluted magnetic semiconductors) systems is that the transitions corresponding to the 3d states are observed in the optical absorption spectra. However, the present investigation does not reveal any 3d transitions. This suggests that the first d-d* transition may lie on the higher energy side of the absorption edge. A careful photoluminescence study of ZnInzSed : Mn confirms this view and the ?,-6A1 transition lies at about 2.1 eV [ 121. Thus, in the present alloy there is no interference from d* states of Mn.
Letters 26 (1996) 47-50
This fact has important consequences on optical properties. Mn can be used to tune the band gap, adjust the lattice parameter and reduce the density of acceptor states. Moreover, the optical absorption process is clean. In short, ZnJkIn, _ .&Se4 is an adequate material for optical and electro-optical devices.
References [ 1 ] P. Manta, F. Raga and A. Spiga, Nuovo Cimento B 119 ( 1974) 15. [2] S. Sgariazzo and A. Serpi, Phys. Rev. B 41 ( 1990) 7718. [3] J. Filipowicz, N. Romeo and L. Tar&one, Solid State Commun. 38 (1980) 619. [4] E. Nowark, B. Neumann and B. Steiner, Phys. Stat. Sol. 13 A (1992) K13. [5] E. Fortin and F. Raga, Solid State Commun. 14 ( 1974) 847. [6] E. Grilli, M. Guzzi and R. Molteni, Phys. Stat. Sol. 37 ( 1976) 399. [7] R. Trykozko and J. Filipowicz, Japan. J. Appl. Phys. 119 (1980) 153. [ 81 N.M. Mekhtiev, Z.Z. Gruseinov and E. Yu Salaev, Sov. Phys. Semicond. 18 (1984) 677. [9] G. Doll, A. Anghel, J.R. Baummann, E. Bucher, A.P. Ramlrez and K.J. Range, Phys. Stat. Sol. 1126 (1991) 237. [ lo] N.V. Joshi and L. Mogollbn, Progr. Crystal Growth Character. 110 (1989) 65. [ 1 l] N.V. Joshi, Photoconductivity: arts, science and technology (Marcel Dekker, New York, 1990). [ 121 C. Benecke and M.E. Guumlich, in: The luminescence of wide band gap II-Mn-VI semimagnetic semiconductors in diluted magnetic systems, ed. M. Jain (World Scientific, Singapore, 1992).