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Injection and thermostimulated currents in MnIn2S4 single crystals
Solid State Communications, Vol. 81, No. 8, pp. 693-695, 1992. Printed in Great Britain.
0038-1098/92 $5.00 + .00 Pergamon Press plc
I N J E C T I O N A N D T H E R M O S T I M U L A T E D C U R R E N T S IN Mnln2S4 S I N G L E CRYSTALS N.N. Niftiev and O.B. Tagiev Experimental Design Office 'Registr" of the Institute of Physics of the Academy of Sciences of Azerbaijan Republic, 33 Narimanov Pr., Baku 370143, Azerbaijan
(Received 1 July 1991 by D. Van Dyck) MnIn2S4 single crystals have been obtained by chemical transport reaction. Their dark static current-voltage characteristics, temperature dependences of the electroconductivity (cr(T)), thermostimulated conductivity (TSC) and thermostimulated depolarization (TSD) currents have been investigated in the temperature region from 77 to 310 K. The energy depth of the trapping level E,, the trap concentration N, and the cross-section of the traps S, have been determined. The mechanism of the transport of the current in the nonlinear region of the current-voltage characteristic has been found to be attributed to a monopolar and a binary injection.
T H E T E R N A R Y compounds of A"B~"XVl-type (A" = Mn, Fe, Co, Ni; B TM = Ga, In; X = S, Se, Te) are representatives of magnetic semiconductive classes. Recently, great interest was expressed in these compounds due to the possibility of a wide application for semiconductive devices [1-5]. Based on the above compounds, photosensitive Schottky diodes were produced [2]. A linear dichroism was found in Mnln2Te4 [3], and in [4] it was shown that the physical properties of these compounds can be controlled by introduction of different impurities. The Mnln2S4 single crystals are A"B~"XaVJ-type compounds, the photoelectric properties of which are not yet studied practically. The present paper shows the results of the investigation of the temperature dependence of electroconductivity (a(T)), currentvoltage (l-U) characteristics, thermostimulated currents (thermostimulated conductivity (TSC) and thermostimulated depolarization (TSD) currents) in Mnln2S4 single crystals. The MnlneS4 single crystals were obtained by chemical transport reaction. In this case crystalline iodine ( 4 m g c m -3) was used as a carrier. The temperature of the hot and cold bands were T, = 800°C and T2 = 700°C, respectively. By X-ray analysis it has been found that the single crystals obtained have a scheelite structure with lattice parameters a = 10.71/k [5]. The Mnln2S 4 crystals have the n-type conductivity. Indium contacts were attached to the opposite surfaces of the sample (sandwich structure). The distance between the electrodes varied from 50 to 300pm. The I-U characteristics for In-Mnln2Sa-In structures at different temperatures are shown in
Fig. 1. The following regions are revealed in the I-U characteristics: a linear region (I ~ U), a quadratic region (I ~ U 2) and a cubic region (I ~ U3). The above regions are revealed at all temperatures under study (242 to 310K). To clarify the mechanism of the current transport in the nonlinear region, the I-U characteristics with different interelectrode distances (L) were investigated. It was found that the current density is related to the interelectrode distances followingj ~ L 3 and j ~ L 5 respectively for the quadratic and cubic regions, [6, 7]. On the other hand, the condition 0 ,~ 1 is valid for the trapping factor value (0 = no/n, = N,/gN, exp E , - E , / k T where no is a free-carrier concentration) determined from the formula j
g OU2 = ~ 0 / a 0 L3 •
(1)
The validity of the above relation for the I - U quadratic region allows to conclude that the existence of the quadratic region is attributed to monopolar injection [9]. It should be noted that a cubic region as the observed one is usually observed in semiconductors and dielectrics, and according to [7, 8] it can be due to either monopolar injection with exponentionally distributed trapping levels or to binary injection. In the case of exponentionally distributed trapping levels, the index (n) in the region of a sharp current increase was found to increase as the temperature decreases [8]. The condition T~ = lT = const (where Tk is a characteristic temperature, l = n - 1, and Tis a temperature at which the measurements were carried out) should be fulfilled in this case. As it was mentioned
693
694
INJECTION AND T H E R M O S T I M U L A T E D CURRENTS IN Mnln2S4 10'
Vol. 81, No. 8 -5
-11 ¢
10"
t
~
t
-12
-6 t
"7
10'
uIE
10'o
~0 -13
-7
-14
-8
-t5
10°
10
-9
i 4
3
lO' 10~ U(V;--
5
I031T( K - ' ) - . -
Fig. 1. Dark current-voltage characteristics of Mnln2S4 single crystals at different temperatures T (K): 1 242; 2 251; 3 256; 4 264; 5 270; 6 276; 7 280; 8 288; 9 296; 10 308.
Fig. 2. The temperature dependence of the trapping factor (1) and conductivity (2) of MnIn2S4 single crystals.
above, a cubic region is observed at all temperatures investigated (i.e. T~ ¢ const). With increasing light intensity, the voltage at the transition from a quadratic region to the region of a sharp current increase also increases. Taking into account the above-mentioned, one can conclude that with the high injection levels the charge-carrier transfer is not limited by a monopolar injection but obeys the binary injection law (dielectric regime) [8]. The depth and the concentration of the traps were determined from the temperature dependence of the trapping factor O(T) and the conductivity a(T) (Fig. 2). The values of the trapping factor in this case were determined from the formula (1) and we plotted in a (lg 0, 103/T) diagram (Fig. 2, curve !). The trapping depth (0.59eV) was determined from the slope of the straight lines whereas the concentration of the traps (1.70 x 10~Scm 3) was determined from the intersection with the ordinate at I03/T = 0 (0 = N,/gN,). The obtained trapping depths values (both from a(T) and O(T) dependences) nearly coincide. This indicates that the monopolar injection and the electro-conductivity are attributed to one and the same level, and according to [9] one can conclude that a weak compensation takes place in MnIn2S4 single crystals. To obtain more detailed information on trapping levels in Mnln2S4, thermostimulated conductivity (TSC) and thermostimulated depolarization (TSD) were investigated. Figure 3 shows the curves of TSC (curves 1; 2) and TSD (curves 3; 4) for MnIn2S4 single
crystals. As seen in Fig. 3 (curves 1 and 2), the TSC maximum increases with growing heat rate and shifts toward the high temperatures. The TSC curves cover the temperature region from 77 to 170K. A large half-width of the TSC peak (45 K) indicates that the relatively shallow trapping levels in the forbidden gap of single crystals studied are not discrete. For a more precise determination of the trapping level parameters one should know the type of trapping level. The analysis of the shape of TSC and TSD peaks shows [10] that the following condition is fulfilled for
4 2
4
2
o 2"---
~2
i
,/
1
i
100
150
i
20O T(K)
25O
3OO
1
0 35O
."
Fig. 3. TSC curves at two heat rate values (curve 1 0.25Ks-~; curve 2 0.60Ks-~), and TSD curves (curves 3 and 4) at a polarizing field of 4.5 × 104Vcm ~ and an heat rate o f 0 . 2 0 K s ~.
Vol. 81. No. 8
INJECTION
AND
THERMOSTIMULATED
all peaks observed: 6 > e-.’ (I+?)
where
6
=
T, -
T,
Tz -
T, ’
T,, is the temperature of the TSC (TSD) maximum; T, and Tz are the temperatures corresponding to a half of TSC (TSD) maximum intensity from the side of low and high temperature at which the TSC (TSD) reaches its maximum value. This indicates the presence of an intensively repeated trapping (bimolecular recombination). The trapping depth was determined by Bube’s method [I I], Garlic-Gibson’s method [ 121, the method of different heat rates [13] and from the formulae considering the shapes of the TSC and TSD curve [14-161, while the concentration and cross-section of the traps were determined from [ 161. The average values of all the above parameters are respectively: E,,
=
0.21 + 0.03eV;
E,?
=
0.38 IfI 0.02eV;
E,,
=
0.59 + 0.02eV;
N,,
=
2.2 x 10’5cm-‘;
N,:
=
2.6 x 1O”cm-‘;
N,,
=
5.2 x lO”cm~-‘;
S,,
=
4.4 x 10~‘8cm2;
S,,
=
1.7 x lO~“cm’;
S,,
=
2.3 x 10..“cm’.
It should be noted that the 0.59eV levels are also revealed from the 0(T) dependence. This fact indicates once again that the injection and the TSD are attributed to one and the same level, while the I-U quadratic region in In-MnIn>S,-In structures is due to a monopolar injection. Thus, on the base of thorough investigations of the I-U characteristics, o(T), TSC and TSD, it was shown that the current transport mechanism in the nonlinear region is due to monoplanar and binary (at a high level) injection. The trapping levels in MnIn,S, are the rapid ones (bimolecular recombination mechanism) and the trappind depths, the
IN MnIn,S,
695
and the concentration
of the traps are
CURRENTS
cross-sections determined.
REFERENCES 1.
Z.
Metfessel
provodniki
2.
R.N. Bekimbetov, Fiz. Tekhn.
3.
Mittis,
Magnityne
Yu.V.
Polupr.
polu-
Rud & M.A. Tairov,
1987 21, 1051 (1987).
R.N. Bekimbetov, G.A. Medvedkin, Prochukhan, Yu.V. Rud & M.A. Tairov, v Zhurn.
4.
& D.
405 pp, Mir (1972).
Tekhn.
V.D. Pisma
Fiz. 13, 1040 (1987).
G.K. Averkieva, R.N. Bekimbetov, N.N. Konstantinova, L.V. Kradinova, V.D. Prochukhan, Yu.V. Rud & M.A. Tairov, Neorgan. Mater. 24, 59 I (1988). T. Kanomata, H. Ido & T. Kaneko, J. Phys. Sot. Japan.
34, 554 (1973).
& P. Mark, Inzhektsionnye Toki v Telakh 473 pp, Mir (1973). K. Kao & V. Huang, Perenos Elektronov v Tverdykh Telakh Part I, 350 pp, Mir (1984). R.K. Willardson & A.C. Reer, Semiconductors and Semimetals, 261 pp, Academic Press, New M. Lampert Tverdykh
9. 10.
York (1970). G.G. Roberts & W. Tred, Phys. Rev. 180, 785 (1969). P.G. Litovchenko & V.I. Ustianov, Aktualnie Voprosy
11. 12.
Fiziki
Poluprovodnikovykh
Priborov,
p. 153, Mokslas, Wilnius (1960). R. Bube, Photoconductivity of Solids, 586 pp, Moscow ( 1962). G.F.J. Garlic & A.F. Gibson, Proc. Phys. Sot.
14.
A60, 574 (1948). A.G. Mines, Deep Impurities in Semiconductors, 562 pp, Mir, Moscow (1977). Ch.B. Lushik, Dokl. AN SSSR, Ser. Fiz. 101, 4
IS. 16.
l(953). Ya.A. Piasta, Mikroelektronika 3, 178 (1974). G.A. Bordovski, Termostimulirovannye Toki
13.
Kak Metod Opredeleniya Parametrov Lovushek, In Collection: Fotoprovodyashie Okisly Svintsa,
pp. 87-l 10, (1976).
0038-1098/92 $5.00 + .00 Pergamon Press plc
I N J E C T I O N A N D T H E R M O S T I M U L A T E D C U R R E N T S IN Mnln2S4 S I N G L E CRYSTALS N.N. Niftiev and O.B. Tagiev Experimental Design Office 'Registr" of the Institute of Physics of the Academy of Sciences of Azerbaijan Republic, 33 Narimanov Pr., Baku 370143, Azerbaijan
(Received 1 July 1991 by D. Van Dyck) MnIn2S4 single crystals have been obtained by chemical transport reaction. Their dark static current-voltage characteristics, temperature dependences of the electroconductivity (cr(T)), thermostimulated conductivity (TSC) and thermostimulated depolarization (TSD) currents have been investigated in the temperature region from 77 to 310 K. The energy depth of the trapping level E,, the trap concentration N, and the cross-section of the traps S, have been determined. The mechanism of the transport of the current in the nonlinear region of the current-voltage characteristic has been found to be attributed to a monopolar and a binary injection.
T H E T E R N A R Y compounds of A"B~"XVl-type (A" = Mn, Fe, Co, Ni; B TM = Ga, In; X = S, Se, Te) are representatives of magnetic semiconductive classes. Recently, great interest was expressed in these compounds due to the possibility of a wide application for semiconductive devices [1-5]. Based on the above compounds, photosensitive Schottky diodes were produced [2]. A linear dichroism was found in Mnln2Te4 [3], and in [4] it was shown that the physical properties of these compounds can be controlled by introduction of different impurities. The Mnln2S4 single crystals are A"B~"XaVJ-type compounds, the photoelectric properties of which are not yet studied practically. The present paper shows the results of the investigation of the temperature dependence of electroconductivity (a(T)), currentvoltage (l-U) characteristics, thermostimulated currents (thermostimulated conductivity (TSC) and thermostimulated depolarization (TSD) currents) in Mnln2S4 single crystals. The MnlneS4 single crystals were obtained by chemical transport reaction. In this case crystalline iodine ( 4 m g c m -3) was used as a carrier. The temperature of the hot and cold bands were T, = 800°C and T2 = 700°C, respectively. By X-ray analysis it has been found that the single crystals obtained have a scheelite structure with lattice parameters a = 10.71/k [5]. The Mnln2S 4 crystals have the n-type conductivity. Indium contacts were attached to the opposite surfaces of the sample (sandwich structure). The distance between the electrodes varied from 50 to 300pm. The I-U characteristics for In-Mnln2Sa-In structures at different temperatures are shown in
Fig. 1. The following regions are revealed in the I-U characteristics: a linear region (I ~ U), a quadratic region (I ~ U 2) and a cubic region (I ~ U3). The above regions are revealed at all temperatures under study (242 to 310K). To clarify the mechanism of the current transport in the nonlinear region, the I-U characteristics with different interelectrode distances (L) were investigated. It was found that the current density is related to the interelectrode distances followingj ~ L 3 and j ~ L 5 respectively for the quadratic and cubic regions, [6, 7]. On the other hand, the condition 0 ,~ 1 is valid for the trapping factor value (0 = no/n, = N,/gN, exp E , - E , / k T where no is a free-carrier concentration) determined from the formula j
g OU2 = ~ 0 / a 0 L3 •
(1)
The validity of the above relation for the I - U quadratic region allows to conclude that the existence of the quadratic region is attributed to monopolar injection [9]. It should be noted that a cubic region as the observed one is usually observed in semiconductors and dielectrics, and according to [7, 8] it can be due to either monopolar injection with exponentionally distributed trapping levels or to binary injection. In the case of exponentionally distributed trapping levels, the index (n) in the region of a sharp current increase was found to increase as the temperature decreases [8]. The condition T~ = lT = const (where Tk is a characteristic temperature, l = n - 1, and Tis a temperature at which the measurements were carried out) should be fulfilled in this case. As it was mentioned
693
694
INJECTION AND T H E R M O S T I M U L A T E D CURRENTS IN Mnln2S4 10'
Vol. 81, No. 8 -5
-11 ¢
10"
t
~
t
-12
-6 t
"7
10'
uIE
10'o
~0 -13
-7
-14
-8
-t5
10°
10
-9
i 4
3
lO' 10~ U(V;--
5
I031T( K - ' ) - . -
Fig. 1. Dark current-voltage characteristics of Mnln2S4 single crystals at different temperatures T (K): 1 242; 2 251; 3 256; 4 264; 5 270; 6 276; 7 280; 8 288; 9 296; 10 308.
Fig. 2. The temperature dependence of the trapping factor (1) and conductivity (2) of MnIn2S4 single crystals.
above, a cubic region is observed at all temperatures investigated (i.e. T~ ¢ const). With increasing light intensity, the voltage at the transition from a quadratic region to the region of a sharp current increase also increases. Taking into account the above-mentioned, one can conclude that with the high injection levels the charge-carrier transfer is not limited by a monopolar injection but obeys the binary injection law (dielectric regime) [8]. The depth and the concentration of the traps were determined from the temperature dependence of the trapping factor O(T) and the conductivity a(T) (Fig. 2). The values of the trapping factor in this case were determined from the formula (1) and we plotted in a (lg 0, 103/T) diagram (Fig. 2, curve !). The trapping depth (0.59eV) was determined from the slope of the straight lines whereas the concentration of the traps (1.70 x 10~Scm 3) was determined from the intersection with the ordinate at I03/T = 0 (0 = N,/gN,). The obtained trapping depths values (both from a(T) and O(T) dependences) nearly coincide. This indicates that the monopolar injection and the electro-conductivity are attributed to one and the same level, and according to [9] one can conclude that a weak compensation takes place in MnIn2S4 single crystals. To obtain more detailed information on trapping levels in Mnln2S4, thermostimulated conductivity (TSC) and thermostimulated depolarization (TSD) were investigated. Figure 3 shows the curves of TSC (curves 1; 2) and TSD (curves 3; 4) for MnIn2S4 single
crystals. As seen in Fig. 3 (curves 1 and 2), the TSC maximum increases with growing heat rate and shifts toward the high temperatures. The TSC curves cover the temperature region from 77 to 170K. A large half-width of the TSC peak (45 K) indicates that the relatively shallow trapping levels in the forbidden gap of single crystals studied are not discrete. For a more precise determination of the trapping level parameters one should know the type of trapping level. The analysis of the shape of TSC and TSD peaks shows [10] that the following condition is fulfilled for
4 2
4
2
o 2"---
~2
i
,/
1
i
100
150
i
20O T(K)
25O
3OO
1
0 35O
."
Fig. 3. TSC curves at two heat rate values (curve 1 0.25Ks-~; curve 2 0.60Ks-~), and TSD curves (curves 3 and 4) at a polarizing field of 4.5 × 104Vcm ~ and an heat rate o f 0 . 2 0 K s ~.
Vol. 81. No. 8
INJECTION
AND
THERMOSTIMULATED
all peaks observed: 6 > e-.’ (I+?)
where
6
=
T, -
T,
Tz -
T, ’
T,, is the temperature of the TSC (TSD) maximum; T, and Tz are the temperatures corresponding to a half of TSC (TSD) maximum intensity from the side of low and high temperature at which the TSC (TSD) reaches its maximum value. This indicates the presence of an intensively repeated trapping (bimolecular recombination). The trapping depth was determined by Bube’s method [I I], Garlic-Gibson’s method [ 121, the method of different heat rates [13] and from the formulae considering the shapes of the TSC and TSD curve [14-161, while the concentration and cross-section of the traps were determined from [ 161. The average values of all the above parameters are respectively: E,,
=
0.21 + 0.03eV;
E,?
=
0.38 IfI 0.02eV;
E,,
=
0.59 + 0.02eV;
N,,
=
2.2 x 10’5cm-‘;
N,:
=
2.6 x 1O”cm-‘;
N,,
=
5.2 x lO”cm~-‘;
S,,
=
4.4 x 10~‘8cm2;
S,,
=
1.7 x lO~“cm’;
S,,
=
2.3 x 10..“cm’.
It should be noted that the 0.59eV levels are also revealed from the 0(T) dependence. This fact indicates once again that the injection and the TSD are attributed to one and the same level, while the I-U quadratic region in In-MnIn>S,-In structures is due to a monopolar injection. Thus, on the base of thorough investigations of the I-U characteristics, o(T), TSC and TSD, it was shown that the current transport mechanism in the nonlinear region is due to monoplanar and binary (at a high level) injection. The trapping levels in MnIn,S, are the rapid ones (bimolecular recombination mechanism) and the trappind depths, the
IN MnIn,S,
695
and the concentration
of the traps are
CURRENTS
cross-sections determined.
REFERENCES 1.
Z.
Metfessel
provodniki
2.
R.N. Bekimbetov, Fiz. Tekhn.
3.
Mittis,
Magnityne
Yu.V.
Polupr.
polu-
Rud & M.A. Tairov,
1987 21, 1051 (1987).
R.N. Bekimbetov, G.A. Medvedkin, Prochukhan, Yu.V. Rud & M.A. Tairov, v Zhurn.
4.
& D.
405 pp, Mir (1972).
Tekhn.
V.D. Pisma
Fiz. 13, 1040 (1987).
G.K. Averkieva, R.N. Bekimbetov, N.N. Konstantinova, L.V. Kradinova, V.D. Prochukhan, Yu.V. Rud & M.A. Tairov, Neorgan. Mater. 24, 59 I (1988). T. Kanomata, H. Ido & T. Kaneko, J. Phys. Sot. Japan.
34, 554 (1973).
& P. Mark, Inzhektsionnye Toki v Telakh 473 pp, Mir (1973). K. Kao & V. Huang, Perenos Elektronov v Tverdykh Telakh Part I, 350 pp, Mir (1984). R.K. Willardson & A.C. Reer, Semiconductors and Semimetals, 261 pp, Academic Press, New M. Lampert Tverdykh
9. 10.
York (1970). G.G. Roberts & W. Tred, Phys. Rev. 180, 785 (1969). P.G. Litovchenko & V.I. Ustianov, Aktualnie Voprosy
11. 12.
Fiziki
Poluprovodnikovykh
Priborov,
p. 153, Mokslas, Wilnius (1960). R. Bube, Photoconductivity of Solids, 586 pp, Moscow ( 1962). G.F.J. Garlic & A.F. Gibson, Proc. Phys. Sot.
14.
A60, 574 (1948). A.G. Mines, Deep Impurities in Semiconductors, 562 pp, Mir, Moscow (1977). Ch.B. Lushik, Dokl. AN SSSR, Ser. Fiz. 101, 4
IS. 16.
l(953). Ya.A. Piasta, Mikroelektronika 3, 178 (1974). G.A. Bordovski, Termostimulirovannye Toki
13.
Kak Metod Opredeleniya Parametrov Lovushek, In Collection: Fotoprovodyashie Okisly Svintsa,
pp. 87-l 10, (1976).