- Email: [email protected]
Optical and electrical properties of indium monosulfide (InS) thin films
Vacuum 63 (2001) 441–447
Optical and electrical properties of indium monosulfide (InS) thin films M.A.M. Seyam* Physics Department, Faculty of Education, Ain Shams University, Roxy, Cairo, Egypt Received 15 July 2000; received in revised form 25 January 2001; accepted 25 January 2001
Abstract Indium monosulfide (InS) thin films were prepared by thermal evaporation onto quartz and glass substrates held at 473 K during the deposition process. The structural investigations showed that the obtained films have an amorphous nature. Energy dispersion X-ray spectroscopy analysis showed that they are stoichiometric.The optical constants, the electrical resistivity, as well as the space charge limited currents were studied. The obtained results indicate that InS thin films have nondirect allowed optical transitions with an energy gap of 1.94 eV. The thermal activation energy of the charge carriers from the electrical resistivity measurements was found to be 0.84 eV, which was confirmed by the space charge limited current technique. The trap density Nt was found to be 5.88 1021 m@3. r 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction Indium monosulfide (InS) which belongs to groups III–IV compounds is a wide gap semiconductor with high photo-conductive and luminescent properties, which make this compound a promising optoelectronic material [1–3]. The electrical and optical properties of InS grown from In melt, indicate that it is always an n-type conduction with a carrier concentration of B1021 m@3 and with indirect and direct band gaps at room temperature located at 1.9 and 2.4 eV, respectively, were reported [4–8]. However, to our knowledge there is no study on the space charge limited currents in InS thin films but there is some interest paid to its optical and electrical properties. *Fax: +20-24552138. E-mail address: [email protected] (M.A.M. Seyam).
Therefore, it is worthy to investigate the optical properties, and the space charge limited currents as well as the resistivity of InS thin films in order to establish the basic conduction processes and to quantify the conduction and trapping parameters.
2. Experimental techniques All thin films of different thickness (from 250 to 800 nm) of InS, used in this work were thermally evaporated onto quartz and soda lime glass substrates, using a high vacuum coating unit (Edwards type E 306 A). The substrates were fixed to a rotatable holder to obtain films of uniform thickness. These substrate lie at a distance of 0.25 m above the evaporator.
0042-207X/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 3 6 3 - 3
442
M.A.M. Seyam / Vacuum 63 (2001) 441–447
The deposition rate was held constant at 10 nm s@1 and the temperature substrate was kept at 473 K during the deposition process. When the vacuum chamber is pumped down to 10@6 Torr, InS granules (Aldrich 99.999%) are allowed to evaporate. The film thickness and the deposition rates are controlled using a quartz thickness monitor (FTM4 Edwards). After deposition the films remain in the vacuum chamber for 1 h to avoid any oxidation problems. A multiple beam Fizeau fringes method [9] has been applied for measuring the actual thickness of the samples under test. Structural studies were made using an X-ray diffractometer (philips PM 822203) with Cu Ka radiation. The stoichiometry of the obtained samples was examined by an energy dispersion X-ray spectroscopy (EDS) unit attached to a scanning electron microscope (JEOL5400). Both the transmission T and the reflection R of InS thin films were measured for each sample in the spectral range 300–2500 nm (using UV-3101PC spectrophotometer, Schimadzu, Japan). The measured values T, R and the film thickness were used to determine the optical constants by applying a computer program [10], based on Murmann’s exact equations [11]. The electrical resistivities of InS thin films of different thickness were measured in vacuum of 10@6 Torr in the temperature range of 300–500 K by the two probe method using a high impedance electrometer (Keithley 617). Aluminum electrodes were prepared by evaporating high-purity Al (99.999%), through a suitable mask, onto the films. The ohmic nature of the Al-contacts was examined by the linear I–V characteristics through the above-mentioned temperature range. The linearity in both directions indicates the ohmic nature of the contacts under test.
800 nm are shown in Fig. 1 with various annealed temperatures and as grown the diffraction patterns show an amorphous structure. After annealing at 473 K for 1 h in vacuum, the diffraction patterns show that InS thin films after being annealed still have the amorphous state as shown in Fig. 1. EDS analysis of InS thin films showed that the deposited films at temperature 473 K were homogenous and stoichiometric. The composition of InS films being 50.76% S and 49.24% In from the data obtained by EDS analysis. Fig. 2 represents the EDS spectrum of InS thin films and the analyses are given in Table 1 in the same figure. As shown, the obtained InS films are stoichiometric.
Fig. 1. X-ray diffraction patterns of InS thin films of thickness 800 nm. [(a) as deposited, (b) annealed at 473 K for 1 h]).
3. Results and discussion 3.1. Film structure and composition The X-ray diffraction patterns of thermal evaporated InS films on a glass substrate at 473 K in the thickness range between 250 and
Fig. 2. EDX spectrum of InS thin films and their analysis in Table 1.
M.A.M. Seyam / Vacuum 63 (2001) 441–447
3.2. The optical properties Both the spectral distribution of transmission TðlÞ and reflection RðlÞ of InS films were measured for each sample in the spectral range 300–2500 nm as shown in Fig. 3. In this figure, it is observed that when the film thickness increases the number of maximal extremes and minimal extremes increase in both transmission T and reflection R; respectively. It is also clear from Fig. 3 that at longer wavelengths (l > 1000 nm) the films become transparent and no light is scattered as T þ R ¼ 1 for all samples. The refractive index (n) and the absorption index (k) at a given wavelength can be calculated, knowing film thickness as well as the transmission
443
(T), reflection (R) at the same wavelength using a computer program [10], based on Murmann’s exact equation [11]. The dispersion curves of nðlÞ and kðlÞ for InS films are illustrated in Figs. 4 and 5, respectively. The values of nðlÞ and kðlÞ shown in these figures represent the mean values determined for films of different thicknesses. Taking into account the experimental error in measuring T and R to be 71%, the error in the calculated values of n and k was estimated to be 72.2% and 72.3%, respectively. The high-frequency dielectric constant eN of InS thin films at an infinite wavelength could be calculated using the relation [12]: ½n2 @1@1 ¼ ½n2N @1@1 ½1@l20 l@2 ;
Table 1 The values of the activation energy, DE of InS thin films of different thickness t (nm)
DE (eV)
250 300 400 520 635 800
0.860 0.840 0.880 0.850 0.840 0.790
Fig. 4. The dispersion curve of refractive index nðlÞ for InS films in the spectral range 300–2500 nm.
Fig. 3. The spectral distributions of transmission TðlÞ and reflection RðlÞ of InS of different thickness in the spectral range 300– 2500 nm.
444
M.A.M. Seyam / Vacuum 63 (2001) 441–447
Fig. 5. The dispersion curve of absorption index kðlÞ for InS films in the spectral range 300–800 nm.
Fig. 7. The dependence of e2 ð¼ 2nkÞ on photon energy (hn) for an amorphous InS thin films.
transitions with Egind ¼ 1:9 eV in agreement with others predicted before [5–8].
3.3. The electric properties Fig. 6. The relation between ½n2 @1@1 and l@2 for InS thin films.
where nN is the refractive index of an empty lattice of infinite wavelength, n is the refractive index of InS and l is the wavelength at which n is measured. Fig. 6 illustrates the plot of ½n2 @1@1 versus l@2 , as shown it yields a straight line, the extrapolation of this straight line yield to ½n2N @1@1 . Hence the high-frequency dielectric constant eN ð¼ n2N Þ of InS thin films was found to be 5.76. For amorphous semiconductors, it is appropriate to illustrate e2 ð¼ 2nkÞ as a function of photon energy (hn) to obtain the nondirect energy gap for the material under test. It is obvious from Fig. 7 that the relation yields a straight line indicating the existence of a nondirect allowed optical transition. Extrapolating the straight part of this relation toward lower photon energy, the point of interception with the hn axis yields the value corresponding to energy gap (Egnond ). It was found to be 1.94 eV. Our results are in fair agreement with those mentioned before for InS [13,14]. Takarabe et al. [14] have predicted the existence of indirect
3.3.1. The temperature dependence of the dark resistivity The temperature dependence of the dark resistivity r of InS thin films of different thickness prepared by a thermal evaporation technique on glass substrates held at 473 K during the deposition process is represented in Fig. 8. It can be seen that the dark resistivity decreases with increasing temperature. A decrease in the resistivity was also observed with the increase in the film thickness. In addition, one distinct linear part was observed in the log r against 1000/T representation for each thickness. Applying the relation r ¼ r0 exp (DE=kT) for each linear part, the activation energy DE can be obtained, over the temperature range (300–500 K), the values of DE due to the deep levels are given in Table 1. These values are higher than the value given before by Takarabe et al. [14]. As shown in Table 1, DEoðEg =2Þ, indicating the presence of states formed during the preparation process. These are believed to be caused by a deficiency of In as shown by EDX in Fig. 2. Analysis of the EDX pattern in Fig. 2 indicates that the percentage composition of In is 49.24%.
M.A.M. Seyam / Vacuum 63 (2001) 441–447
3.3.2. The space charge limited current Figs. 9a and b show the current voltage (I–V) characteristics carried out for the amorphous InS thin films of different thickness (400 and 635 nm as example), in the sandwich configuration (Al–InS–
445
Al) at different temperatures (303, 333, 353 and 373 K). It can be seen that the I–V curve, for each film, is composed of three regions. Region I characterizes the low field region, while region II characterizes the high field region, and region III describes the superquadratic region, these three regions can be represented by the following equation: I ¼ AV þ BV n þ CV s ;
Fig. 8. Temperature dependence of the resistivity of InS thin films.
where A, B and C are temperature-dependent constants, n equals to 2 and s > 2 [15]. Such a dependence suggests that the overall current consists of three components, one ohmic in nature meanwhile the others are more related to the space charge limited and superquadratic flow. In the ohmic region the injected free carrier concentration is lower than the thermal generated free carrier density. For each film, the slope of the
Fig. 9. I–V characteristics of InS thin films with different temperature: [(1) 303 K, (2) 333 K, (3) 353 K and (4) 373 K] of different thickness (400 and 635 nm as example).
446
M.A.M. Seyam / Vacuum 63 (2001) 441–447
curve in region II has n ¼ 2. The cross-over voltage Vx , the voltage at which the two regions intercept, was estimated for each film. The voltage Vx depends upon the nature of the film through the relation [16] Vx ¼ n0 et2 =ey; where n0 is the concentration of the charge carriers, e is the electronic charge, t is the film thickness, e is the dielectric constant and y is the ratio between the free carriers concentration n0 and the trapped carriers nt . A plot between Vx and t2 is shown in Fig. 10, where a straight line is obtained, the slope of which, n0 e=ey, equals 8.2 1012 V/m2. Regarding Lampert theory for single injection of carriers and the modified Child’s law the current density J can be expressed as J ¼ 1:125 emyðV 2 =t3 Þ; where m is the carrier mobility. A plot between J and V 2 =t3 for amorphous, InS thin films yields a straight line. Fig. 11. represents the above relation with slope equals 2 10@22 A m/V2. Substituting by the mobility of the charge carriers m, the value of y can be estimated and it was found to be 3.21 10@14. In other words, the trapped carriers play a significant role in conduction. The trap energy Et
Fig. 11. A plot between the current density (J) and (V 2 =t3 ) for InS thin films.
and density of traps Nt of amorphous InS thin films were found to be 0.80 eV and 5.88 1021 m@3, respectively. The value of the trap energy Et of amorphous, InS thin films is consistent with the corresponding values of activation energy DE ¼ 0:84 eV of amorphous, InS thin films obtained from the resistivity measurements (Section 3.3.1).
4. Conclusion A stoichiometric InS thin film had been deposited onto soda lime glass and quartz substrates were obtained by thermal evaporation at a temperature of 473 K with deposition rate 10 nm s@1. The obtained films in the thickness range between 250 to 800 nm have an amorphous nature. The films exhibited the absorption coefficient of an order of 105 cm@1 and the energy band gap of 1.94 eV. These properties are suitable for solar cells. InS films always showed an n-type conduction with activation energy 0.84 eV and the density of charge carriers was found to be 1021 m@3 from the space charge limited currents analysis.
References Fig. 10. Thickness (t) dependence of the cross-over voltage Vx of InS thin films
[1] Gasanly NM, 1997;101:11,797.
Aydinli
A.
Solid
State
Comm
M.A.M. Seyam / Vacuum 63 (2001) 441–447 [2] Asikainen T, Ritala M, Leskela M. Appl Surf Sci 1994;82/ 83:122. [3] Rehwald W, Harbeke G. J Phys Chem Solids 1965;26:1309. [4] Hogg JHC, Duffin WG. Phys Status Solidi 1966;18:755. [5] Nishino T, Taniguchi K, Hamakawa Y. Solid State Commun 1976;19:635. [6] Deiseroth RWaHJ. Z Kristallogr 1995;210:360. [7] Nishino T, Taniguchi K, Hamakawa Y. Jpn J Appl Phys 1974;13:1921. [8] Nishino T, Hamakawa Y. Proceedings of 13th International Conference on the Physics of Semiconductors, Rome, North-Holland, Amsterdam, 1976, p. 427. [9] Volklein F, Kessler E. Thin Solid Films 1987;155:197.
447
[10] El-Nahass MM, Soliman HS, El-Kadry N, Morsy AY, Yagmour S. J Mater Sci Lett 1988;7:1050. [11] Agiev LA, Shkllyarevskii IN. J Prekl-Spekt 1972;76: 380. [12] Moss TS. Optical properties of semiconductors. Butterworths Scientific Publication Ltd., London, 1959. [13] Takarabe K, Ishino TN. J Phys Chem Solids 1983;44(7):681. [14] Takarabe K, Wakamura K, Ito E. Solid State Commun 1983;46:215. [15] Couts TJ. Electrical conduction in thin metal films. Amsterdam: Elsevier, 1974. [16] Lampert MA, Mark P. Current injection in solids. New York: Academic Press, 1970 (Chapters 4 and 5).
Optical and electrical properties of indium monosulfide (InS) thin films M.A.M. Seyam* Physics Department, Faculty of Education, Ain Shams University, Roxy, Cairo, Egypt Received 15 July 2000; received in revised form 25 January 2001; accepted 25 January 2001
Abstract Indium monosulfide (InS) thin films were prepared by thermal evaporation onto quartz and glass substrates held at 473 K during the deposition process. The structural investigations showed that the obtained films have an amorphous nature. Energy dispersion X-ray spectroscopy analysis showed that they are stoichiometric.The optical constants, the electrical resistivity, as well as the space charge limited currents were studied. The obtained results indicate that InS thin films have nondirect allowed optical transitions with an energy gap of 1.94 eV. The thermal activation energy of the charge carriers from the electrical resistivity measurements was found to be 0.84 eV, which was confirmed by the space charge limited current technique. The trap density Nt was found to be 5.88 1021 m@3. r 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction Indium monosulfide (InS) which belongs to groups III–IV compounds is a wide gap semiconductor with high photo-conductive and luminescent properties, which make this compound a promising optoelectronic material [1–3]. The electrical and optical properties of InS grown from In melt, indicate that it is always an n-type conduction with a carrier concentration of B1021 m@3 and with indirect and direct band gaps at room temperature located at 1.9 and 2.4 eV, respectively, were reported [4–8]. However, to our knowledge there is no study on the space charge limited currents in InS thin films but there is some interest paid to its optical and electrical properties. *Fax: +20-24552138. E-mail address: [email protected] (M.A.M. Seyam).
Therefore, it is worthy to investigate the optical properties, and the space charge limited currents as well as the resistivity of InS thin films in order to establish the basic conduction processes and to quantify the conduction and trapping parameters.
2. Experimental techniques All thin films of different thickness (from 250 to 800 nm) of InS, used in this work were thermally evaporated onto quartz and soda lime glass substrates, using a high vacuum coating unit (Edwards type E 306 A). The substrates were fixed to a rotatable holder to obtain films of uniform thickness. These substrate lie at a distance of 0.25 m above the evaporator.
0042-207X/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 3 6 3 - 3
442
M.A.M. Seyam / Vacuum 63 (2001) 441–447
The deposition rate was held constant at 10 nm s@1 and the temperature substrate was kept at 473 K during the deposition process. When the vacuum chamber is pumped down to 10@6 Torr, InS granules (Aldrich 99.999%) are allowed to evaporate. The film thickness and the deposition rates are controlled using a quartz thickness monitor (FTM4 Edwards). After deposition the films remain in the vacuum chamber for 1 h to avoid any oxidation problems. A multiple beam Fizeau fringes method [9] has been applied for measuring the actual thickness of the samples under test. Structural studies were made using an X-ray diffractometer (philips PM 822203) with Cu Ka radiation. The stoichiometry of the obtained samples was examined by an energy dispersion X-ray spectroscopy (EDS) unit attached to a scanning electron microscope (JEOL5400). Both the transmission T and the reflection R of InS thin films were measured for each sample in the spectral range 300–2500 nm (using UV-3101PC spectrophotometer, Schimadzu, Japan). The measured values T, R and the film thickness were used to determine the optical constants by applying a computer program [10], based on Murmann’s exact equations [11]. The electrical resistivities of InS thin films of different thickness were measured in vacuum of 10@6 Torr in the temperature range of 300–500 K by the two probe method using a high impedance electrometer (Keithley 617). Aluminum electrodes were prepared by evaporating high-purity Al (99.999%), through a suitable mask, onto the films. The ohmic nature of the Al-contacts was examined by the linear I–V characteristics through the above-mentioned temperature range. The linearity in both directions indicates the ohmic nature of the contacts under test.
800 nm are shown in Fig. 1 with various annealed temperatures and as grown the diffraction patterns show an amorphous structure. After annealing at 473 K for 1 h in vacuum, the diffraction patterns show that InS thin films after being annealed still have the amorphous state as shown in Fig. 1. EDS analysis of InS thin films showed that the deposited films at temperature 473 K were homogenous and stoichiometric. The composition of InS films being 50.76% S and 49.24% In from the data obtained by EDS analysis. Fig. 2 represents the EDS spectrum of InS thin films and the analyses are given in Table 1 in the same figure. As shown, the obtained InS films are stoichiometric.
Fig. 1. X-ray diffraction patterns of InS thin films of thickness 800 nm. [(a) as deposited, (b) annealed at 473 K for 1 h]).
3. Results and discussion 3.1. Film structure and composition The X-ray diffraction patterns of thermal evaporated InS films on a glass substrate at 473 K in the thickness range between 250 and
Fig. 2. EDX spectrum of InS thin films and their analysis in Table 1.
M.A.M. Seyam / Vacuum 63 (2001) 441–447
3.2. The optical properties Both the spectral distribution of transmission TðlÞ and reflection RðlÞ of InS films were measured for each sample in the spectral range 300–2500 nm as shown in Fig. 3. In this figure, it is observed that when the film thickness increases the number of maximal extremes and minimal extremes increase in both transmission T and reflection R; respectively. It is also clear from Fig. 3 that at longer wavelengths (l > 1000 nm) the films become transparent and no light is scattered as T þ R ¼ 1 for all samples. The refractive index (n) and the absorption index (k) at a given wavelength can be calculated, knowing film thickness as well as the transmission
443
(T), reflection (R) at the same wavelength using a computer program [10], based on Murmann’s exact equation [11]. The dispersion curves of nðlÞ and kðlÞ for InS films are illustrated in Figs. 4 and 5, respectively. The values of nðlÞ and kðlÞ shown in these figures represent the mean values determined for films of different thicknesses. Taking into account the experimental error in measuring T and R to be 71%, the error in the calculated values of n and k was estimated to be 72.2% and 72.3%, respectively. The high-frequency dielectric constant eN of InS thin films at an infinite wavelength could be calculated using the relation [12]: ½n2 @1@1 ¼ ½n2N @1@1 ½1@l20 l@2 ;
Table 1 The values of the activation energy, DE of InS thin films of different thickness t (nm)
DE (eV)
250 300 400 520 635 800
0.860 0.840 0.880 0.850 0.840 0.790
Fig. 4. The dispersion curve of refractive index nðlÞ for InS films in the spectral range 300–2500 nm.
Fig. 3. The spectral distributions of transmission TðlÞ and reflection RðlÞ of InS of different thickness in the spectral range 300– 2500 nm.
444
M.A.M. Seyam / Vacuum 63 (2001) 441–447
Fig. 5. The dispersion curve of absorption index kðlÞ for InS films in the spectral range 300–800 nm.
Fig. 7. The dependence of e2 ð¼ 2nkÞ on photon energy (hn) for an amorphous InS thin films.
transitions with Egind ¼ 1:9 eV in agreement with others predicted before [5–8].
3.3. The electric properties Fig. 6. The relation between ½n2 @1@1 and l@2 for InS thin films.
where nN is the refractive index of an empty lattice of infinite wavelength, n is the refractive index of InS and l is the wavelength at which n is measured. Fig. 6 illustrates the plot of ½n2 @1@1 versus l@2 , as shown it yields a straight line, the extrapolation of this straight line yield to ½n2N @1@1 . Hence the high-frequency dielectric constant eN ð¼ n2N Þ of InS thin films was found to be 5.76. For amorphous semiconductors, it is appropriate to illustrate e2 ð¼ 2nkÞ as a function of photon energy (hn) to obtain the nondirect energy gap for the material under test. It is obvious from Fig. 7 that the relation yields a straight line indicating the existence of a nondirect allowed optical transition. Extrapolating the straight part of this relation toward lower photon energy, the point of interception with the hn axis yields the value corresponding to energy gap (Egnond ). It was found to be 1.94 eV. Our results are in fair agreement with those mentioned before for InS [13,14]. Takarabe et al. [14] have predicted the existence of indirect
3.3.1. The temperature dependence of the dark resistivity The temperature dependence of the dark resistivity r of InS thin films of different thickness prepared by a thermal evaporation technique on glass substrates held at 473 K during the deposition process is represented in Fig. 8. It can be seen that the dark resistivity decreases with increasing temperature. A decrease in the resistivity was also observed with the increase in the film thickness. In addition, one distinct linear part was observed in the log r against 1000/T representation for each thickness. Applying the relation r ¼ r0 exp (DE=kT) for each linear part, the activation energy DE can be obtained, over the temperature range (300–500 K), the values of DE due to the deep levels are given in Table 1. These values are higher than the value given before by Takarabe et al. [14]. As shown in Table 1, DEoðEg =2Þ, indicating the presence of states formed during the preparation process. These are believed to be caused by a deficiency of In as shown by EDX in Fig. 2. Analysis of the EDX pattern in Fig. 2 indicates that the percentage composition of In is 49.24%.
M.A.M. Seyam / Vacuum 63 (2001) 441–447
3.3.2. The space charge limited current Figs. 9a and b show the current voltage (I–V) characteristics carried out for the amorphous InS thin films of different thickness (400 and 635 nm as example), in the sandwich configuration (Al–InS–
445
Al) at different temperatures (303, 333, 353 and 373 K). It can be seen that the I–V curve, for each film, is composed of three regions. Region I characterizes the low field region, while region II characterizes the high field region, and region III describes the superquadratic region, these three regions can be represented by the following equation: I ¼ AV þ BV n þ CV s ;
Fig. 8. Temperature dependence of the resistivity of InS thin films.
where A, B and C are temperature-dependent constants, n equals to 2 and s > 2 [15]. Such a dependence suggests that the overall current consists of three components, one ohmic in nature meanwhile the others are more related to the space charge limited and superquadratic flow. In the ohmic region the injected free carrier concentration is lower than the thermal generated free carrier density. For each film, the slope of the
Fig. 9. I–V characteristics of InS thin films with different temperature: [(1) 303 K, (2) 333 K, (3) 353 K and (4) 373 K] of different thickness (400 and 635 nm as example).
446
M.A.M. Seyam / Vacuum 63 (2001) 441–447
curve in region II has n ¼ 2. The cross-over voltage Vx , the voltage at which the two regions intercept, was estimated for each film. The voltage Vx depends upon the nature of the film through the relation [16] Vx ¼ n0 et2 =ey; where n0 is the concentration of the charge carriers, e is the electronic charge, t is the film thickness, e is the dielectric constant and y is the ratio between the free carriers concentration n0 and the trapped carriers nt . A plot between Vx and t2 is shown in Fig. 10, where a straight line is obtained, the slope of which, n0 e=ey, equals 8.2 1012 V/m2. Regarding Lampert theory for single injection of carriers and the modified Child’s law the current density J can be expressed as J ¼ 1:125 emyðV 2 =t3 Þ; where m is the carrier mobility. A plot between J and V 2 =t3 for amorphous, InS thin films yields a straight line. Fig. 11. represents the above relation with slope equals 2 10@22 A m/V2. Substituting by the mobility of the charge carriers m, the value of y can be estimated and it was found to be 3.21 10@14. In other words, the trapped carriers play a significant role in conduction. The trap energy Et
Fig. 11. A plot between the current density (J) and (V 2 =t3 ) for InS thin films.
and density of traps Nt of amorphous InS thin films were found to be 0.80 eV and 5.88 1021 m@3, respectively. The value of the trap energy Et of amorphous, InS thin films is consistent with the corresponding values of activation energy DE ¼ 0:84 eV of amorphous, InS thin films obtained from the resistivity measurements (Section 3.3.1).
4. Conclusion A stoichiometric InS thin film had been deposited onto soda lime glass and quartz substrates were obtained by thermal evaporation at a temperature of 473 K with deposition rate 10 nm s@1. The obtained films in the thickness range between 250 to 800 nm have an amorphous nature. The films exhibited the absorption coefficient of an order of 105 cm@1 and the energy band gap of 1.94 eV. These properties are suitable for solar cells. InS films always showed an n-type conduction with activation energy 0.84 eV and the density of charge carriers was found to be 1021 m@3 from the space charge limited currents analysis.
References Fig. 10. Thickness (t) dependence of the cross-over voltage Vx of InS thin films
[1] Gasanly NM, 1997;101:11,797.
Aydinli
A.
Solid
State
Comm
M.A.M. Seyam / Vacuum 63 (2001) 441–447 [2] Asikainen T, Ritala M, Leskela M. Appl Surf Sci 1994;82/ 83:122. [3] Rehwald W, Harbeke G. J Phys Chem Solids 1965;26:1309. [4] Hogg JHC, Duffin WG. Phys Status Solidi 1966;18:755. [5] Nishino T, Taniguchi K, Hamakawa Y. Solid State Commun 1976;19:635. [6] Deiseroth RWaHJ. Z Kristallogr 1995;210:360. [7] Nishino T, Taniguchi K, Hamakawa Y. Jpn J Appl Phys 1974;13:1921. [8] Nishino T, Hamakawa Y. Proceedings of 13th International Conference on the Physics of Semiconductors, Rome, North-Holland, Amsterdam, 1976, p. 427. [9] Volklein F, Kessler E. Thin Solid Films 1987;155:197.
447
[10] El-Nahass MM, Soliman HS, El-Kadry N, Morsy AY, Yagmour S. J Mater Sci Lett 1988;7:1050. [11] Agiev LA, Shkllyarevskii IN. J Prekl-Spekt 1972;76: 380. [12] Moss TS. Optical properties of semiconductors. Butterworths Scientific Publication Ltd., London, 1959. [13] Takarabe K, Ishino TN. J Phys Chem Solids 1983;44(7):681. [14] Takarabe K, Wakamura K, Ito E. Solid State Commun 1983;46:215. [15] Couts TJ. Electrical conduction in thin metal films. Amsterdam: Elsevier, 1974. [16] Lampert MA, Mark P. Current injection in solids. New York: Academic Press, 1970 (Chapters 4 and 5).