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Direct current conductivity of evaporated cadmium selenide thin films
Physica B 270 (1999) 366}370
Direct current conductivity of evaporated cadmium selenide thin "lms R.M. Abdel-Latif* Department of Physics, Faculty of Science, Minia University, Minia 61519, Egypt Received 7 October 1998; received in revised form 31 March 1999
Abstract Current density}voltage characteristics have been obtained from thin "lms of CdSe, when sandwiched between ohmic gold and blocking aluminium electrodes. At low voltage, the current in the forward direction varies exponentially with voltage, and the transport mechanism is found to be dominated by recombination at the depletion region. At higher voltage, two distinct regions of ohmic and space-charge limited conduction dominated by a discrete trap level were observed. The thickness dependence in the square-law region has been found to con"rm the d\ law. 1999 Elsevier Science B.V. All rights reserved.
1. Introduction The study of the temperature dependence of ohmic and space-charge limited (SCL) currents in CdSe material has provided valuable insight into the mechanism of charge transport and carrier trapping in this material [1]. Evaporated cadmium selenide (CdSe) thin "lms have previously been investigated from the standpoint of various potential applications, such as solar cells [2], thin "lm transistors [3], gamma-ray detector [4], photoconductors [5], and photoluminescence [6,7]. In both crystal form and as thin "lms CdSe has consistently been reported to be an n-type semi-conductor, although recently Singh et al. [8] observed p-type conductivity in thin "lms prepared by electrosynthesis. The in#uence of substrate temperature on resistivity and Hall mobility has been studied by
* Corresponding author.
many authors [9}13]. Space-charge limited conduction (SCLC) has been observed in both sandwich and planar structures of thin polycrystalline CdSe "lms [8,10,14,15]. In the present work, a comprehensive study of the electrical properties of evaporated CdSe thin "lms has been undertaken in order to understand the role of charge carriers injected from the electrode and generated in the bulk, and to gain more insight into the conduction mechanism in this important semiconductor material.
2. Experimental The evaporation processes were carried out in a vacuum coating unit (Edward 306 A) at a pressure of about 10\ Pa. Pure aluminium (99.99%) was evaporated from a tungsten "lament onto a carefully cleaned glass substrate through a suitable mask to form the base electrode. Pure CdSe was evaporated
0921-4526/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 0 1 7 3 - 8
R.M. Abdel-Latif / Physica B 270 (1999) 366}370
through a mask from a molybdenum boat to form the "lm over the aluminium electrode. The evaporation source was mounted about 14 cm below the substrate holder assembly which supported the glass substrate. The glass substrate was kept at room temperature. After this a top electrode of gold was evaporated to form A1}CdSe}Au sandwich structure of active area 1;10\ m. The method of preparing CdSe thin "lms is similar to that reported previously [10,15]. The thickness of the CdSe "lm was measured using the multiple-beam Fizeau}Frings method giving a thickness in the range 1}3.4 lm. The CdSe "lms were mounted on electrically heated copper disc, the temperature of which could be held constant to within 0.2 K over the range of 303}373 K by means of a temperature control unit. Temperature was measured directly by means of an NiCr/Ni}Al thermocouple connected to a Keithley 871 digital thermometer mounted in close proximity to the specimen of interest. Electrical characterization was performed over a voltage range appropariate to the thickness of the sample, using a stabilized power supply model AP 6050A and a Keithley 610C electrometer.
3. Results and discussion The forward current density}voltage (J}<) characteristic of an ideal Schottky diode, when the barrier height is dependent on the applied bias may be written as [16]
gv J"J exp nK¹
1!exp
!gv K¹
,
(1)
where J is the reverse saturation current density, n the diode ideality factor, < is the forward bias, k the Boltzman constant, and ¹ is the absolute temperature. The above equation is valid for <(3k¹/q and even for reverse bias. Fig. 1 shows the ln J/[1!exp(qv/k¹)] versus < plot at di!erent temperatures under forward bias voltage <(0.6 V with CdSe thickness 1 lm. The values of n and J have been calculated from the slope and the intercept of the straight-line portion of the J}< plot, respectively, and are tabulated in Table 1. It is observed (Table 1) that with increasing temperature
367
Fig. 1. Forward (J}<) characteristics of CdSe thin "lm with temperature as variable and voltage below 0.6 V.
Table 1 The diode ideality factor and the reverse saturation current as a function of temperature Temperature ¹ (K)
Ideality factor n
Reverse saturation current J (Am)\
308 323 343 363 373
1.56 1.61 1.58 1.52 1.62
1.08;10\ 5.21;10\ 2.14;10\ 1.56;10\ 1.72;10\
J increases and n show very little variation. The values of n'1 may be attributed to the recombination of electrons and holes in the depletion region and also to the increased e!ect of the applied voltage [17]. It can be seen from the following discussion that in CdSe "lms, trapped charges are more numerous than free carriers and therefore, the trapped charge density determines the properties of the depletion region. Following Richardson's equation
J "AH¹ exp ! , K¹
(2)
368
R.M. Abdel-Latif / Physica B 270 (1999) 366}370
voltage <'0.6 V with CdSe thickness 1 lm. It is clear from the "gure that there are two distinct regions of conduction for each characteristic. In the "rst region the slope of the log J versus log < plot is approximately equal to 1, while in the second region above the well-de"ned transition voltage < , 1 the slope is approximately 2. These slopes are typical for ohmic conduction at voltage below < and 1 SCLC controlled by a single dominant trap level at voltage above < . 1 Assuming that the conduction is primarily via electrons, the characteristics of the ohmic region (Fig. 3) may be expressed as n qk< J" , d Fig. 2. Richardson plot for CdSe thin "lm.
(3)
where n represents the concentration of electrons in the conduction band due to thermal excitation, k the electron mobility, and d the "lm thickness. The SCLC may be expressed by the relation [18]
9 < J " ekh , 1!* 8 d
(4)
where e is the permittivity of CdSe and h is the ratio of free charge to trapped charge and is given by
N E h" exp ! R , (5) N K¹ where N is the total trap concentration in a single energy level E below the conduction band edge and N is the e!ective density of states in the con duction band and is given by expression [19] N "2 Fig. 3. Forward (J}<) characteristics of CdSe thin "lm with temperature as variable and voltage above 0.6 V.
where is the e!ective barrier height and AH is the e!ective Richardson constant, ln (J /¹) is plotted against 1/¹ in Fig. 2. The slope and intercept of the Richardson plot gives the barrier height " 0.78 eV and the Richardson constant AH"6.71; 10 Am\ K\, respectively. The current density}voltage (J}<) characteristics at di!erent temperatures for the Al}CdSe}Au device are shown in Fig. 3 for the forward bias
2pm K¹ h
(6)
where h is Planck's constant and m is the electron e!ective mass. For CdSe m "0.13m [20], where m is the free electron mass, and Eq. (6) then yields N "1.17; 10 m\ at room temperature. Fig. 4 shows the dependence on "lm thickness of the current density at room temperature in the SCL region. The slope of about !3.04 indicates that single carrier SCL conduction dominated by a single trap level is occurring. Fig. 3 and Eq. (4) yield a value of h"9.33;10\ at room temperature, 308 K.
R.M. Abdel-Latif / Physica B 270 (1999) 366}370
369
support to the hypothesis of the presence of a single dominant trapping level. In the analysis of the temperature dependence of the ohmic and SCLC results (Fig. 5) values of e"7.82;10\ Fm\ and k"3.82;10\ m V\ S\ for CdSe thin "lms prepared by a similar technique [15] were used. From the intercept of the plot shown in Fig. 5 in the SCLC region, the value of N can be evaluated and was found to be N "1.43;10 m\. The electron concentra tion in the conduction band obtained from the ohmic region of Fig. 5 was found to be 1.1; 10 m\. These values are consistent with the values of N "1.55;10 m\ and E "0.2 eV calculated by Singh et al. [8] for CdSe thin "lms.
Fig. 4. Thickness dependence of SCL current densities at 4 V for di!erent thicknesses of CdSe thin "lms at 303 K.
4. Summary and conclusions A detailed study of current density as a function of voltage at di!erent temperatures for Al}CdSe} Au thin "lms has been obtained. Results show that: (1) At low voltage, the current varies exponentially with voltage. (2) At high voltage two separate regions of ohmic current and SCLC were observed. The latter processes have been explained in terms of a single dominant trap level. Analysis of the results yielded the following parameters: the electron concentration in the conduction band n "1.1;10 m\, and the total trap concentration N "1.43; 10 m\ located at 0.18 eV below the conduction band edge.
References Fig. 5. Dependence of the current density on reciprocal temperature in the ohmic (lower curve) and in the space-charge (upper curve) region.
The temperature dependence of the current density is shown in Fig. 5 for both the ohmic and the SCLC region. It is evident from Eqs. (4) and (5) that such a plot should yield a straight line of slope (!E /k). The activation energies for both regions were the same (E "0.18 eV), which adds further
[1] F. Raoult, B. Fortin, Y. Collin, Thin Solid Films 182 (1989) 1. [2] H. Richter, Proceedings of the 20th IEEE Photovoltaics Specialists Conference, IEEE, New York, 1988, p. 1537. [3] A. Van Claster, A. Vervaet, I. De Rycke, J. De Baets, J. Van#eteren, J. Crystal Growth 86 (1988) 924. [4] M. Roth, Nucl. Instr. Meth. A 283 (1989) 291. [5] B. Emolovich, E.L. Shtrum, Thin Solid Films 22 (1974) 157. [6] A.K. Arora, A.K. Ramdas, Phys. Rev. B. 35 (1987) 4345. [7] R. Tenne, H. Mariette, C. Levy-Clement, R. Jager-Waldau, Phys. Rev. B. 36 (1987) 1204.
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R.M. Abdel-Latif / Physica B 270 (1999) 366}370
[8] K. Singh, D.N. Upadhyay, V.K. Panday, Indian J. Chem. A 28 (1989) 832. [9] D.S.H. Chan, A.E. Hill, Thin Solid Films 38 (1976) 163. [10] K.N. Charma, K. Barua, J. Phys. D. 12 (1979) 1729. [11] F.V. Shallcross, Trans. Metall. Soc. AIME 236 (1966) 309. [12] D.E. Brodie, J. Lacombe, Can. J. Phys. 45 (1967) 1353. [13] V. Snejdar, J. Jerhot, D. Berkova, Zboncak, Thin Solid Films 9 (1971) 97. [14] R.W. Glew, Thin Solid Films 46 (1977) 59.
[15] A.O. Oduor, R.D. Gould, Thin Solid Films 270 (1995) 387. [16] E.H. Rhoderik, R.H. Williams, Metal Semiconductor Contact, Clarendon Press, Oxford, 1988. [17] E.H. Rhoderick, Metal Semiconductor Contacts, Oxford University Press, Oxford, 1978. [18] M.A. Lampert, Rep. Prog. Phys. 27 (1964) 329. [19] R.H. Bube, Electronic Properties of Crystalline Solids, Academic Press, New York, 1974. [20] M.V. Kurik, Phys. Lett. A 24 (1967) 742.
Direct current conductivity of evaporated cadmium selenide thin "lms R.M. Abdel-Latif* Department of Physics, Faculty of Science, Minia University, Minia 61519, Egypt Received 7 October 1998; received in revised form 31 March 1999
Abstract Current density}voltage characteristics have been obtained from thin "lms of CdSe, when sandwiched between ohmic gold and blocking aluminium electrodes. At low voltage, the current in the forward direction varies exponentially with voltage, and the transport mechanism is found to be dominated by recombination at the depletion region. At higher voltage, two distinct regions of ohmic and space-charge limited conduction dominated by a discrete trap level were observed. The thickness dependence in the square-law region has been found to con"rm the d\ law. 1999 Elsevier Science B.V. All rights reserved.
1. Introduction The study of the temperature dependence of ohmic and space-charge limited (SCL) currents in CdSe material has provided valuable insight into the mechanism of charge transport and carrier trapping in this material [1]. Evaporated cadmium selenide (CdSe) thin "lms have previously been investigated from the standpoint of various potential applications, such as solar cells [2], thin "lm transistors [3], gamma-ray detector [4], photoconductors [5], and photoluminescence [6,7]. In both crystal form and as thin "lms CdSe has consistently been reported to be an n-type semi-conductor, although recently Singh et al. [8] observed p-type conductivity in thin "lms prepared by electrosynthesis. The in#uence of substrate temperature on resistivity and Hall mobility has been studied by
* Corresponding author.
many authors [9}13]. Space-charge limited conduction (SCLC) has been observed in both sandwich and planar structures of thin polycrystalline CdSe "lms [8,10,14,15]. In the present work, a comprehensive study of the electrical properties of evaporated CdSe thin "lms has been undertaken in order to understand the role of charge carriers injected from the electrode and generated in the bulk, and to gain more insight into the conduction mechanism in this important semiconductor material.
2. Experimental The evaporation processes were carried out in a vacuum coating unit (Edward 306 A) at a pressure of about 10\ Pa. Pure aluminium (99.99%) was evaporated from a tungsten "lament onto a carefully cleaned glass substrate through a suitable mask to form the base electrode. Pure CdSe was evaporated
0921-4526/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 0 1 7 3 - 8
R.M. Abdel-Latif / Physica B 270 (1999) 366}370
through a mask from a molybdenum boat to form the "lm over the aluminium electrode. The evaporation source was mounted about 14 cm below the substrate holder assembly which supported the glass substrate. The glass substrate was kept at room temperature. After this a top electrode of gold was evaporated to form A1}CdSe}Au sandwich structure of active area 1;10\ m. The method of preparing CdSe thin "lms is similar to that reported previously [10,15]. The thickness of the CdSe "lm was measured using the multiple-beam Fizeau}Frings method giving a thickness in the range 1}3.4 lm. The CdSe "lms were mounted on electrically heated copper disc, the temperature of which could be held constant to within 0.2 K over the range of 303}373 K by means of a temperature control unit. Temperature was measured directly by means of an NiCr/Ni}Al thermocouple connected to a Keithley 871 digital thermometer mounted in close proximity to the specimen of interest. Electrical characterization was performed over a voltage range appropariate to the thickness of the sample, using a stabilized power supply model AP 6050A and a Keithley 610C electrometer.
3. Results and discussion The forward current density}voltage (J}<) characteristic of an ideal Schottky diode, when the barrier height is dependent on the applied bias may be written as [16]
gv J"J exp nK¹
1!exp
!gv K¹
,
(1)
where J is the reverse saturation current density, n the diode ideality factor, < is the forward bias, k the Boltzman constant, and ¹ is the absolute temperature. The above equation is valid for <(3k¹/q and even for reverse bias. Fig. 1 shows the ln J/[1!exp(qv/k¹)] versus < plot at di!erent temperatures under forward bias voltage <(0.6 V with CdSe thickness 1 lm. The values of n and J have been calculated from the slope and the intercept of the straight-line portion of the J}< plot, respectively, and are tabulated in Table 1. It is observed (Table 1) that with increasing temperature
367
Fig. 1. Forward (J}<) characteristics of CdSe thin "lm with temperature as variable and voltage below 0.6 V.
Table 1 The diode ideality factor and the reverse saturation current as a function of temperature Temperature ¹ (K)
Ideality factor n
Reverse saturation current J (Am)\
308 323 343 363 373
1.56 1.61 1.58 1.52 1.62
1.08;10\ 5.21;10\ 2.14;10\ 1.56;10\ 1.72;10\
J increases and n show very little variation. The values of n'1 may be attributed to the recombination of electrons and holes in the depletion region and also to the increased e!ect of the applied voltage [17]. It can be seen from the following discussion that in CdSe "lms, trapped charges are more numerous than free carriers and therefore, the trapped charge density determines the properties of the depletion region. Following Richardson's equation
J "AH¹ exp ! , K¹
(2)
368
R.M. Abdel-Latif / Physica B 270 (1999) 366}370
voltage <'0.6 V with CdSe thickness 1 lm. It is clear from the "gure that there are two distinct regions of conduction for each characteristic. In the "rst region the slope of the log J versus log < plot is approximately equal to 1, while in the second region above the well-de"ned transition voltage < , 1 the slope is approximately 2. These slopes are typical for ohmic conduction at voltage below < and 1 SCLC controlled by a single dominant trap level at voltage above < . 1 Assuming that the conduction is primarily via electrons, the characteristics of the ohmic region (Fig. 3) may be expressed as n qk< J" , d Fig. 2. Richardson plot for CdSe thin "lm.
(3)
where n represents the concentration of electrons in the conduction band due to thermal excitation, k the electron mobility, and d the "lm thickness. The SCLC may be expressed by the relation [18]
9 < J " ekh , 1!* 8 d
(4)
where e is the permittivity of CdSe and h is the ratio of free charge to trapped charge and is given by
N E h" exp ! R , (5) N K¹ where N is the total trap concentration in a single energy level E below the conduction band edge and N is the e!ective density of states in the con duction band and is given by expression [19] N "2 Fig. 3. Forward (J}<) characteristics of CdSe thin "lm with temperature as variable and voltage above 0.6 V.
where is the e!ective barrier height and AH is the e!ective Richardson constant, ln (J /¹) is plotted against 1/¹ in Fig. 2. The slope and intercept of the Richardson plot gives the barrier height " 0.78 eV and the Richardson constant AH"6.71; 10 Am\ K\, respectively. The current density}voltage (J}<) characteristics at di!erent temperatures for the Al}CdSe}Au device are shown in Fig. 3 for the forward bias
2pm K¹ h
(6)
where h is Planck's constant and m is the electron e!ective mass. For CdSe m "0.13m [20], where m is the free electron mass, and Eq. (6) then yields N "1.17; 10 m\ at room temperature. Fig. 4 shows the dependence on "lm thickness of the current density at room temperature in the SCL region. The slope of about !3.04 indicates that single carrier SCL conduction dominated by a single trap level is occurring. Fig. 3 and Eq. (4) yield a value of h"9.33;10\ at room temperature, 308 K.
R.M. Abdel-Latif / Physica B 270 (1999) 366}370
369
support to the hypothesis of the presence of a single dominant trapping level. In the analysis of the temperature dependence of the ohmic and SCLC results (Fig. 5) values of e"7.82;10\ Fm\ and k"3.82;10\ m V\ S\ for CdSe thin "lms prepared by a similar technique [15] were used. From the intercept of the plot shown in Fig. 5 in the SCLC region, the value of N can be evaluated and was found to be N "1.43;10 m\. The electron concentra tion in the conduction band obtained from the ohmic region of Fig. 5 was found to be 1.1; 10 m\. These values are consistent with the values of N "1.55;10 m\ and E "0.2 eV calculated by Singh et al. [8] for CdSe thin "lms.
Fig. 4. Thickness dependence of SCL current densities at 4 V for di!erent thicknesses of CdSe thin "lms at 303 K.
4. Summary and conclusions A detailed study of current density as a function of voltage at di!erent temperatures for Al}CdSe} Au thin "lms has been obtained. Results show that: (1) At low voltage, the current varies exponentially with voltage. (2) At high voltage two separate regions of ohmic current and SCLC were observed. The latter processes have been explained in terms of a single dominant trap level. Analysis of the results yielded the following parameters: the electron concentration in the conduction band n "1.1;10 m\, and the total trap concentration N "1.43; 10 m\ located at 0.18 eV below the conduction band edge.
References Fig. 5. Dependence of the current density on reciprocal temperature in the ohmic (lower curve) and in the space-charge (upper curve) region.
The temperature dependence of the current density is shown in Fig. 5 for both the ohmic and the SCLC region. It is evident from Eqs. (4) and (5) that such a plot should yield a straight line of slope (!E /k). The activation energies for both regions were the same (E "0.18 eV), which adds further
[1] F. Raoult, B. Fortin, Y. Collin, Thin Solid Films 182 (1989) 1. [2] H. Richter, Proceedings of the 20th IEEE Photovoltaics Specialists Conference, IEEE, New York, 1988, p. 1537. [3] A. Van Claster, A. Vervaet, I. De Rycke, J. De Baets, J. Van#eteren, J. Crystal Growth 86 (1988) 924. [4] M. Roth, Nucl. Instr. Meth. A 283 (1989) 291. [5] B. Emolovich, E.L. Shtrum, Thin Solid Films 22 (1974) 157. [6] A.K. Arora, A.K. Ramdas, Phys. Rev. B. 35 (1987) 4345. [7] R. Tenne, H. Mariette, C. Levy-Clement, R. Jager-Waldau, Phys. Rev. B. 36 (1987) 1204.
370
R.M. Abdel-Latif / Physica B 270 (1999) 366}370
[8] K. Singh, D.N. Upadhyay, V.K. Panday, Indian J. Chem. A 28 (1989) 832. [9] D.S.H. Chan, A.E. Hill, Thin Solid Films 38 (1976) 163. [10] K.N. Charma, K. Barua, J. Phys. D. 12 (1979) 1729. [11] F.V. Shallcross, Trans. Metall. Soc. AIME 236 (1966) 309. [12] D.E. Brodie, J. Lacombe, Can. J. Phys. 45 (1967) 1353. [13] V. Snejdar, J. Jerhot, D. Berkova, Zboncak, Thin Solid Films 9 (1971) 97. [14] R.W. Glew, Thin Solid Films 46 (1977) 59.
[15] A.O. Oduor, R.D. Gould, Thin Solid Films 270 (1995) 387. [16] E.H. Rhoderik, R.H. Williams, Metal Semiconductor Contact, Clarendon Press, Oxford, 1988. [17] E.H. Rhoderick, Metal Semiconductor Contacts, Oxford University Press, Oxford, 1978. [18] M.A. Lampert, Rep. Prog. Phys. 27 (1964) 329. [19] R.H. Bube, Electronic Properties of Crystalline Solids, Academic Press, New York, 1974. [20] M.V. Kurik, Phys. Lett. A 24 (1967) 742.