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Optical and electrical properties of PbTe films grown by laser induced evaporation of pressed PbTe pellets
Applied Surface Science 254 (2007) 1215–1219 www.elsevier.com/locate/apsusc
Optical and electrical properties of PbTe films grown by laser induced evaporation of pressed PbTe pellets Ali M. Mousa a, J.P. Ponpon b,* a
Material Research Unit, Department of Applied Sciences, University of Technology, P.O. Box 3051, Baghdad, Iraq b InESS, umr 7163, Laboratoire commun Universite´ Louis Pasteur et CNRS, B.P. 20, F67037 Strasbourg, France Received 9 May 2007; received in revised form 2 July 2007; accepted 20 July 2007 Available online 26 July 2007
Abstract A simple and cheap method has been developed for the deposition of lead telluride thin films on glass substrates by pulsed Nd:YAG laser evaporation of lead telluride pellets made of high purity Pb and Te powders. Preliminary characterization of the crystallographic and optical properties of the films has been performed as a function of the substrate temperature. The influence of deposition conditions on the sheet resistance of these thin films has been studied. Both deposition temperature, nitrogen pressure during deposition, and addition of Ga and As impurities in the source pellets have been considered. # 2007 Elsevier B.V. All rights reserved. PACS : 81.15.Fg; 61.10.Nz; 78.66.Li; 73.50. h; 73.20.Hb Keywords: Lead telluride; Laser ablation; Structural properties; Optical properties; Electrical properties
1. Introduction Lead telluride PbTe is a narrow direct band gap semiconductor (0.32 eV) with optical and thermoelectric properties which make it attractive to prepare optical and electro-optical devices for applications such as IR detectors or tunable laser diodes, either on single crystals or on polycrystalline thin films. Bulk properties of PbTe have been studied for a long time [1]. Several techniques have been used to grow thin PbTe films: hot wall [2], RF magnetron sputtering [3], thermal evaporation [4,5], and laser evaporation [6–8]. Doping and impurity behaviour in PbTe single crystals and thin films have long been investigated. Intrinsic defects play a major role on transport properties. As-grown crystals are generally p-type, with carrier concentration up to 1020 cm 3 [9]. As Pb vacancies are acceptors and Te vacancies are donors, stoichiometric deviation due to excess tellurium produces p-type conduction. Conversely, n-type doping can be obtained by adding Pb or impurities such as Cu, In, Ga, Sb, Br [10] or Bi
* Corresponding author. E-mail address: [email protected] (J.P. Ponpon). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.07.148
[11]. Gallium has been intensively studied as a n-type dopant [9,12–14]. It can be noticed that Tl, which also belongs to group III, is an acceptor. In the same way, addition of arsenic to the melt has been used to dope PbTe crystals or epitaxial layers p-type up to a few 1018 cm 3 [15,16]. Formation of p–n junctions for example has been achieved by using Bi and Tl doping [17]. On the other hand, influence of the atmosphere on electrical properties has also been demonstrated. Adsorption and diffusion of oxygen along grain boundaries in polycrystalline samples introduce p-type channels which play a major role in transport properties [18–20]. Introduction of hydrogen during thermal evaporation of PbTe improves the structural quality by compensation of defects; introduction of nitrogen creates donor centres [21]. We have recently prepared thin lead telluride layers by a simple and cheap method using laser ablation of PbTe pellets pressed from high purity Pb and Te powders [22]. The first aim of this paper is to present the crystallographic and optical properties of the deposited films as a function of the substrate temperature. The second goal is to highlight the influence of the deposition conditions on the electrical properties of these films in order to evaluate the possibility of doping the material during such a deposition process. Especially the effect of deposition
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temperature, nitrogen pressure and addition of Ga or As impurities into the pellets have been investigated by means of sheet resistance measurements. 2. Sample preparation Lead telluride films typically 250 nm thick (as determined by using Newton interference fringes on the sharp edge of the films) were deposited in a vacuum chamber (230 mm in diameter and 250 mm height) under a residual pressure of 1.33 10 3 Pa (a few films were deposited under 0.27–1.33 Pa N2 atmosphere to study the possibility of nitrogen doping). The chamber was equipped with a pulsed Nd:YAG laser (wavelength = 1.05 mm, pulse duration = 300 ns and fluence = 9 J/ cm2) with assorted optics. Both substrate and pellets holder were made from stainless steel, and the temperature of the substrate could be monitored up to 673 K. The source holder rotated after each pulse so as to expose a new position, keeping the distance between the source and the substrate constant (4 cm). During the deposition, the laser beam was focused on to the target by a 20 mm focal length lens. The evaporation sources were made of pellets pressed from high purity powders (99.999%) with equal percentage in weight of the constituents lead and tellurium. The crystallographic properties of the deposited films were determined by the X-ray diffraction technique using Cu Ka radiation and scanning 2Q in the range 20–408. Transmittance and reflectance of films prepared using different experimental conditions were measured at normal incidence. The transmission and reflectivity of the films were measured at room temperature in the range 0.4–3.5 mm using a Lambda-9 spectrophotometer. The UV reflectance spectrum was obtained by means of a Philips UV/visible spectrophotometer. Both doped and undoped PbTe films were deposited. Films thicker than 250 nm were obtained by using several laser pulses. However, the resistivity of such samples increased with thickness. This unexpected behaviour was probably due to the presence of interfacial defects (structural defects between the successive layers and surface contamination by adsorbed gases, especially oxygen) resulting from the layer by layer deposition mode. Ga or As doping was achieved by introducing the impurity of interest into the pellets which where crushed and cold
Fig. 1. X-ray diffraction spectrum of a PbTe film deposited at 373 K.
Fig. 2. X-ray diffraction spectrum of a PbTe film deposited at 573 K.
pressed several times before the final pressing. Up to 30 mg gallium and 50 mg arsenic were introduced into 6 g pellets. The effect of nitrogen was studied by depositing films under different nitrogen pressure. The electrical characterization of the films was performed by means of sheet resistance measurement at room temperature using a four point probe apparatus (FPP 500: Veeco instruments NC model). The resistivity was simply obtained by calculating the value of sheet resistance times film thickness. 3. Results 3.1. Structural properties Figs. 1–3 depict the X-ray spectra of PbTe films deposited on substrates held at 373, 573 and 673 K, respectively. The strong peak present at 2u = 27.588 can be observed from 373 K. It decreases significantly for 673 K deposition temperature while at the same time a peak situated at 2u = 31.298 increases. From ASTM tables, the 2u = 27.588 peak can be related to the (1 1 1) Te plane or to the (2 0 0) PbTe plane. The 2u = 31.298 peak corresponds to the (1 1 1) Pb plane. The preferential orientation of the (2 0 0) direction as temperature increases from 323 to 573 K as well as the vanishing of the 2u = 31.298 line indicates nucleation of PbTe. Increasing the substrate temperature up to 673 K results in the increase of the (1 1 1) peak. This reveals evaporation of tellurium from the PbTe film. The high vapor
Fig. 3. X-ray diffraction spectrum of a PbTe film deposited at 673 K.
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Fig. 4. Spectral dependence of the transmission of PbTe films as a function of substrate temperature.
Fig. 6. UV reflectivity of PbTe films as a function of substrate temperature.
pressure and the low vaporization heat of tellurium cause this preferential Te loss.
range the results are close to those previously reported for thin polycrystalline PbTe films [23]. The refractive index and the absorption coefficient have been deduced from the transmission and reflectivity measurements. As could be expected, they show drastic changes when the substrate temperature increases from 573 to 673 K. The band gap was obtained by extrapolation to zero of the linear part of the curve representing the square of the absorption coefficient versus photon energy. It was estimated to 0.30 0.02 eV, independent of the substrate temperature in the range 323–573 K.
3.2. Optical properties The transmission in the range 0.4–3.5 mm of PbTe films prepared on substrates held at temperatures up to 673 K is reported in Fig. 4. Up to 573 K the change in transmission is low. A strong increase can be observed for deposition at 673 K. This behaviour is in agreement with the hypothesis of tellurium evaporation at this temperature. The same applies for reflectivity curves in the range 0.4–2.5 mm (Fig. 5). The difference remains low up to 573 K substrate temperature. For deposition at 673 K a drastic change occurs: the reflectivity remains nearly constant over the whole wavelength range. As a result, for wavelengths higher than 1.2 mm it is much lower than the reflectivity of samples prepared at lower temperature. The UV reflectivity of samples prepared at temperature not higher than 573 K is reported in Fig. 6. For substrate temperature in the range 373–573 K, the curves show a peak close to 220 nm and a wide band at 280–320 nm. A third band at 360 nm is also probable. These peaks or bands do not appear for the lower substrate temperature (323 K), indicating low crystallisation of the film, if any. In the investigated wavelength
First the influence of temperature on the resistivity of 250 nm films deposited from undoped and stoichiometric PbTe pellets was investigated. Starting from 0.16 V cm for deposition on a substrate held at 300 K, the resistivity increases up to 1.2 and 2.2 V cm for depositions at 573 and 673 K, respectively. In Fig. 7 is reported the evolution of the PbTe sheet resistance as a function of gallium doping and substrate temperature. The sheet resistance increases up to a gallium concentration of 0.08% in weight in the PbTe source and then decreases for higher gallium amount. This behaviour is typical
Fig. 5. Spectral dependence of the reflectivity in the range 0.4–2.5 mm of PbTe films as a function of substrate temperature.
Fig. 7. Evolution of the resistivity of 250 nm thick films as a function of gallium concentration and substrate temperature.
3.3. Electrical properties
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of n-type doping of the p-type PbTe material: first, with increasing the gallium concentration the resistivity increases due to the compensation effect of the Ga donor states. Then for sufficient Ga concentration the material turns n-type and the resistivity decreases when further increasing the gallium concentration. The curves in Fig. 7 show that the gallium concentration corresponding to full compensation is about 5 1019 cm 3. This value is in good agreement with the concentration of acceptor defects generally found in asdeposited PbTe single crystals as indicated above [9]. The evolution of resistivity with arsenic concentration shows an unexpected behaviour considering that arsenic should behave like an acceptor dopant in PbTe. Not only the resistivity increases with arsenic concentration, but the values reached are significantly higher than the ones reported so far (up to 10 times). 4. Discussion From the X-ray diffraction experiments performed as a function of substrate temperature, it can be inferred that in the range 373–573 K PbTe films nucleate with a preferential orientation along the (2 0 0) plane. Increasing the substrate temperature up to 673 K leads to evaporation of Te from the deposited films, thus increasing the intensity of the peak corresponding to the lead (1 1 1) plane. It can be noticed that preparation of films using source pellets containing an excess of lead also gives results indicating the preferential grow of the lead (1 0 0) plane and the suppression of the PbTe (2 0 0) direction. For equal Pb and Te percentage in the pellets, transmission and reflectivity give results in good agreement with the previous ones: the change is very low up to 573 K substrate temperature and a drastic change is observed at 673 K. From UV reflectivity it appears that the crystalline structure only appears at 373 K. Increasing the lead percentage leads to a destruction of the structure, in agreement with the XRD results. Similar results are obtained for the refractive index and the absorption coefficient: from 373 up to 573 K the change is low. At 573 K the absorption coefficient reaches its minimum and the refractive index reaches its minimum. As long as the deposition temperature is not higher than 573 K, these values at a wavelength of 2 mm are close to that already published for thin PbTe films [24]. The temperature evolution is in good agreement with previous results published for films deposited by thermal evaporation [6]. A larger change occurs at a deposition temperature of 673 K. This behaviour is related to the crystalline modification of the layers resulting from tellurium evaporation. The electrical results performed on the layers as a function of temperature, pellet composition and various doping conditions are briefly discussed in the following. First, the resistivity increase of undoped layers with increasing deposition temperature can be attributed to structural changes in the layers, such as grain size growth, which is likely to occur with increasing temperature. The resistivity increase then results from the reduction of the associated grain
boundaries density. These boundaries constitute preferential conductive ways due to the presence of defects or impurities like oxygen creating p-type channels. For the highest temperatures, Te loss from the film, as previously reported [23], can account for a compensation effect due to the creation of donor-type Te vacancies. As already noticed, PbTe deposited layers are generally p-type. Owing to the deposition process used, this is likely to be the case in the present work: actually, the high vapor pressure and the low vaporization heat of Te produces a preferential Te vaporization from the pellet, then leading to the formation of a Te rich p-type layer on low temperature substrates. For the same reason, increasing the substrate temperature above 673 K leads to Te loss from the film itself and then to a compensation effect by the increased number of Te vacancies. Increasing the lead percentage in the pellets produces an increase of the sheet resistance, indicating that a compensation effect occurs. This result is in agreement with the fact that the as-deposited layers are preferentially p-type and that n-type doping can be obtained by adding more Pb in the layers. Incorporation of nitrogen into the layers during deposition on a substrate maintained at low temperature has a similar effect. The sheet resistance increases by a factor of five when rising the N2 pressure in the vacuum chamber from 0.27 up to 1.3 Pa. As nitrogen acts as a donor in PbTe, the sheet resistance increase again results from a compensation effect. The fact that the position of the resistivity maximum versus gallium amount (corresponding to full compensation and type inversion) does not change significantly with temperature is in agreement with the former assumption that the resistivity variation with temperature is due to structural changes involving the density of grain boundaries while conduction inside the grains is not or very slightly modified by the temperature change. Actually, no doping effect results from introduction of arsenic in the layers. This can be explained by taking into account that arsenic can react with tellurium to form arsenic telluride As2Te3. This compound can be deposited by evaporation to give amorphous layers whose resistivity at room temperature is about 4000 V cm, much higher than that of PbTe [25]. Therefore, the resistivity increase of the PbTe layers when increasing the arsenic amount results from the creation of a high resistivity compound into the films and by Te loss due to the formation of arsenic telluride occurring during laser ablation. Again a compensation effect due to tellurium shortage takes place. 5. Conclusion We have shown the possibility to prepare thin films of PbTe using laser ablation of pressed pellets made of lead and tellurium powders. Equal Pb and Te percentage in the pellets and substrate temperature of about 473–573 K produce thin films presenting optical and electrical properties comparable to those of films obtained by other methods. Donor doping can be achieved by using gallium or nitrogen. Introduction of lead excess also gives n-type doping. On the other hand, p-type
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doping by arsenic is probably hindered by the existence of an arsenic telluride compound which can be formed during laser ablation. References [1] M. Cardona, D.L. Greenaway, Phys. Rev. 133 (1964) A1685. [2] S. Guimara˜es, S. de C.F. da Silva, J.M.K. de Assis, Acta Microcoscopica 12 (2003) 189. [3] A. Jdanov, J. Pelleg, Z. Dashevsky, R. Shneck, Mater. Sci. Eng. B106 (2004) 89. [4] U.P. Khainar, P.H. Pawar, G.P. Bhavsar, Cryst. Res. Technol. 37 (2002) 1293. [5] W. Feng, Y. Yan, Vacuum 42 (1991) 1045. [6] V.A. Mikhailov, F.N. Putilin, D.N. Trubnikov, Appl. Surf. Sci. 86 (1995) 64. [7] A. Jacquot, B. Lenoir, M.O. Boffoue´, A. Dauscher, Appl. Phys. A69 (1999) S613. [8] M. Baleva, M. Surtchev, Vacuum 69 (2003) 419. [9] E. Skipetrov, E. Zvereva, B. Koralev, E. Slyn’ko, Phys. Stat. Sol. (b) 241 (2004) 120.
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[10] C. Herring, J. Appl. Phys. 32 (1961) 2146. [11] A.Y. Ueta, G. Springholz, F. Schinagl, G. Marschner, G. Bauer, Thin Solid Films 306 (1997) 320. [12] Y.A. Ugai, A.M. Samoilov, M.K. Sharov, O.B. Yatsenko, B.A. Akimov, Inorg. Mater. 38 (2002) 17. [13] Z. Feit, D. Eger, A. Zemel, Phys. Rev. B 31 (1985) 3903. [14] B.A. Akimov, V.A. Bogoyavlenskii, V.A. Vasil’kov, L.I. Ryabova, D.R. Khokhlov, Phys. Solid State 47 (2005) 160. [15] J.O. McCaldin, Technical Report NAA-SR-Memo-6921, 1961. [16] M.G. Astles, C. Pickering, M.L. Young, J. Electron. Mater. 8 (1979) 603. [17] W. Tamura, A. Yasuda, K. Suto, O. Itoh, J.-I. Nishizawa, J. Electron. Mater. 32 (2002) 39. [18] L.N. Neustroev, V.V. Osipov, Sov. Phys. Semicond. 20 (1986) 34. [19] K.S. Bhat, V.D. Das, Phys. Rev. B32 (1985) 6713. [20] D.E. Bode, H. Levinstein, Phys. Rev. 96 (1954) 259. [21] M.M. Kachabekov, A.E. Yunovich, Sov. Phys. Semicond. 22 (1988) 514. [22] A.M. Mousa, J.P. Ponpon, Eur. Phys. J. Appl. Phys. 34 (2006) 1. [23] M. Bartkowski, M. Jolochoski, M. Subotowicicz, J. Bartkowska, Thin Solid Films 62 (1979) 55. [24] E. Lasser, H. Levinstein, Phys. Rev. 96 (1954) 47. [25] K. Weisser, H. Brodsky, Phys. Rev. B1 (1970) 791.
Optical and electrical properties of PbTe films grown by laser induced evaporation of pressed PbTe pellets Ali M. Mousa a, J.P. Ponpon b,* a
Material Research Unit, Department of Applied Sciences, University of Technology, P.O. Box 3051, Baghdad, Iraq b InESS, umr 7163, Laboratoire commun Universite´ Louis Pasteur et CNRS, B.P. 20, F67037 Strasbourg, France Received 9 May 2007; received in revised form 2 July 2007; accepted 20 July 2007 Available online 26 July 2007
Abstract A simple and cheap method has been developed for the deposition of lead telluride thin films on glass substrates by pulsed Nd:YAG laser evaporation of lead telluride pellets made of high purity Pb and Te powders. Preliminary characterization of the crystallographic and optical properties of the films has been performed as a function of the substrate temperature. The influence of deposition conditions on the sheet resistance of these thin films has been studied. Both deposition temperature, nitrogen pressure during deposition, and addition of Ga and As impurities in the source pellets have been considered. # 2007 Elsevier B.V. All rights reserved. PACS : 81.15.Fg; 61.10.Nz; 78.66.Li; 73.50. h; 73.20.Hb Keywords: Lead telluride; Laser ablation; Structural properties; Optical properties; Electrical properties
1. Introduction Lead telluride PbTe is a narrow direct band gap semiconductor (0.32 eV) with optical and thermoelectric properties which make it attractive to prepare optical and electro-optical devices for applications such as IR detectors or tunable laser diodes, either on single crystals or on polycrystalline thin films. Bulk properties of PbTe have been studied for a long time [1]. Several techniques have been used to grow thin PbTe films: hot wall [2], RF magnetron sputtering [3], thermal evaporation [4,5], and laser evaporation [6–8]. Doping and impurity behaviour in PbTe single crystals and thin films have long been investigated. Intrinsic defects play a major role on transport properties. As-grown crystals are generally p-type, with carrier concentration up to 1020 cm 3 [9]. As Pb vacancies are acceptors and Te vacancies are donors, stoichiometric deviation due to excess tellurium produces p-type conduction. Conversely, n-type doping can be obtained by adding Pb or impurities such as Cu, In, Ga, Sb, Br [10] or Bi
* Corresponding author. E-mail address: [email protected] (J.P. Ponpon). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.07.148
[11]. Gallium has been intensively studied as a n-type dopant [9,12–14]. It can be noticed that Tl, which also belongs to group III, is an acceptor. In the same way, addition of arsenic to the melt has been used to dope PbTe crystals or epitaxial layers p-type up to a few 1018 cm 3 [15,16]. Formation of p–n junctions for example has been achieved by using Bi and Tl doping [17]. On the other hand, influence of the atmosphere on electrical properties has also been demonstrated. Adsorption and diffusion of oxygen along grain boundaries in polycrystalline samples introduce p-type channels which play a major role in transport properties [18–20]. Introduction of hydrogen during thermal evaporation of PbTe improves the structural quality by compensation of defects; introduction of nitrogen creates donor centres [21]. We have recently prepared thin lead telluride layers by a simple and cheap method using laser ablation of PbTe pellets pressed from high purity Pb and Te powders [22]. The first aim of this paper is to present the crystallographic and optical properties of the deposited films as a function of the substrate temperature. The second goal is to highlight the influence of the deposition conditions on the electrical properties of these films in order to evaluate the possibility of doping the material during such a deposition process. Especially the effect of deposition
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temperature, nitrogen pressure and addition of Ga or As impurities into the pellets have been investigated by means of sheet resistance measurements. 2. Sample preparation Lead telluride films typically 250 nm thick (as determined by using Newton interference fringes on the sharp edge of the films) were deposited in a vacuum chamber (230 mm in diameter and 250 mm height) under a residual pressure of 1.33 10 3 Pa (a few films were deposited under 0.27–1.33 Pa N2 atmosphere to study the possibility of nitrogen doping). The chamber was equipped with a pulsed Nd:YAG laser (wavelength = 1.05 mm, pulse duration = 300 ns and fluence = 9 J/ cm2) with assorted optics. Both substrate and pellets holder were made from stainless steel, and the temperature of the substrate could be monitored up to 673 K. The source holder rotated after each pulse so as to expose a new position, keeping the distance between the source and the substrate constant (4 cm). During the deposition, the laser beam was focused on to the target by a 20 mm focal length lens. The evaporation sources were made of pellets pressed from high purity powders (99.999%) with equal percentage in weight of the constituents lead and tellurium. The crystallographic properties of the deposited films were determined by the X-ray diffraction technique using Cu Ka radiation and scanning 2Q in the range 20–408. Transmittance and reflectance of films prepared using different experimental conditions were measured at normal incidence. The transmission and reflectivity of the films were measured at room temperature in the range 0.4–3.5 mm using a Lambda-9 spectrophotometer. The UV reflectance spectrum was obtained by means of a Philips UV/visible spectrophotometer. Both doped and undoped PbTe films were deposited. Films thicker than 250 nm were obtained by using several laser pulses. However, the resistivity of such samples increased with thickness. This unexpected behaviour was probably due to the presence of interfacial defects (structural defects between the successive layers and surface contamination by adsorbed gases, especially oxygen) resulting from the layer by layer deposition mode. Ga or As doping was achieved by introducing the impurity of interest into the pellets which where crushed and cold
Fig. 1. X-ray diffraction spectrum of a PbTe film deposited at 373 K.
Fig. 2. X-ray diffraction spectrum of a PbTe film deposited at 573 K.
pressed several times before the final pressing. Up to 30 mg gallium and 50 mg arsenic were introduced into 6 g pellets. The effect of nitrogen was studied by depositing films under different nitrogen pressure. The electrical characterization of the films was performed by means of sheet resistance measurement at room temperature using a four point probe apparatus (FPP 500: Veeco instruments NC model). The resistivity was simply obtained by calculating the value of sheet resistance times film thickness. 3. Results 3.1. Structural properties Figs. 1–3 depict the X-ray spectra of PbTe films deposited on substrates held at 373, 573 and 673 K, respectively. The strong peak present at 2u = 27.588 can be observed from 373 K. It decreases significantly for 673 K deposition temperature while at the same time a peak situated at 2u = 31.298 increases. From ASTM tables, the 2u = 27.588 peak can be related to the (1 1 1) Te plane or to the (2 0 0) PbTe plane. The 2u = 31.298 peak corresponds to the (1 1 1) Pb plane. The preferential orientation of the (2 0 0) direction as temperature increases from 323 to 573 K as well as the vanishing of the 2u = 31.298 line indicates nucleation of PbTe. Increasing the substrate temperature up to 673 K results in the increase of the (1 1 1) peak. This reveals evaporation of tellurium from the PbTe film. The high vapor
Fig. 3. X-ray diffraction spectrum of a PbTe film deposited at 673 K.
A.M. Mousa, J.P. Ponpon / Applied Surface Science 254 (2007) 1215–1219
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Fig. 4. Spectral dependence of the transmission of PbTe films as a function of substrate temperature.
Fig. 6. UV reflectivity of PbTe films as a function of substrate temperature.
pressure and the low vaporization heat of tellurium cause this preferential Te loss.
range the results are close to those previously reported for thin polycrystalline PbTe films [23]. The refractive index and the absorption coefficient have been deduced from the transmission and reflectivity measurements. As could be expected, they show drastic changes when the substrate temperature increases from 573 to 673 K. The band gap was obtained by extrapolation to zero of the linear part of the curve representing the square of the absorption coefficient versus photon energy. It was estimated to 0.30 0.02 eV, independent of the substrate temperature in the range 323–573 K.
3.2. Optical properties The transmission in the range 0.4–3.5 mm of PbTe films prepared on substrates held at temperatures up to 673 K is reported in Fig. 4. Up to 573 K the change in transmission is low. A strong increase can be observed for deposition at 673 K. This behaviour is in agreement with the hypothesis of tellurium evaporation at this temperature. The same applies for reflectivity curves in the range 0.4–2.5 mm (Fig. 5). The difference remains low up to 573 K substrate temperature. For deposition at 673 K a drastic change occurs: the reflectivity remains nearly constant over the whole wavelength range. As a result, for wavelengths higher than 1.2 mm it is much lower than the reflectivity of samples prepared at lower temperature. The UV reflectivity of samples prepared at temperature not higher than 573 K is reported in Fig. 6. For substrate temperature in the range 373–573 K, the curves show a peak close to 220 nm and a wide band at 280–320 nm. A third band at 360 nm is also probable. These peaks or bands do not appear for the lower substrate temperature (323 K), indicating low crystallisation of the film, if any. In the investigated wavelength
First the influence of temperature on the resistivity of 250 nm films deposited from undoped and stoichiometric PbTe pellets was investigated. Starting from 0.16 V cm for deposition on a substrate held at 300 K, the resistivity increases up to 1.2 and 2.2 V cm for depositions at 573 and 673 K, respectively. In Fig. 7 is reported the evolution of the PbTe sheet resistance as a function of gallium doping and substrate temperature. The sheet resistance increases up to a gallium concentration of 0.08% in weight in the PbTe source and then decreases for higher gallium amount. This behaviour is typical
Fig. 5. Spectral dependence of the reflectivity in the range 0.4–2.5 mm of PbTe films as a function of substrate temperature.
Fig. 7. Evolution of the resistivity of 250 nm thick films as a function of gallium concentration and substrate temperature.
3.3. Electrical properties
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of n-type doping of the p-type PbTe material: first, with increasing the gallium concentration the resistivity increases due to the compensation effect of the Ga donor states. Then for sufficient Ga concentration the material turns n-type and the resistivity decreases when further increasing the gallium concentration. The curves in Fig. 7 show that the gallium concentration corresponding to full compensation is about 5 1019 cm 3. This value is in good agreement with the concentration of acceptor defects generally found in asdeposited PbTe single crystals as indicated above [9]. The evolution of resistivity with arsenic concentration shows an unexpected behaviour considering that arsenic should behave like an acceptor dopant in PbTe. Not only the resistivity increases with arsenic concentration, but the values reached are significantly higher than the ones reported so far (up to 10 times). 4. Discussion From the X-ray diffraction experiments performed as a function of substrate temperature, it can be inferred that in the range 373–573 K PbTe films nucleate with a preferential orientation along the (2 0 0) plane. Increasing the substrate temperature up to 673 K leads to evaporation of Te from the deposited films, thus increasing the intensity of the peak corresponding to the lead (1 1 1) plane. It can be noticed that preparation of films using source pellets containing an excess of lead also gives results indicating the preferential grow of the lead (1 0 0) plane and the suppression of the PbTe (2 0 0) direction. For equal Pb and Te percentage in the pellets, transmission and reflectivity give results in good agreement with the previous ones: the change is very low up to 573 K substrate temperature and a drastic change is observed at 673 K. From UV reflectivity it appears that the crystalline structure only appears at 373 K. Increasing the lead percentage leads to a destruction of the structure, in agreement with the XRD results. Similar results are obtained for the refractive index and the absorption coefficient: from 373 up to 573 K the change is low. At 573 K the absorption coefficient reaches its minimum and the refractive index reaches its minimum. As long as the deposition temperature is not higher than 573 K, these values at a wavelength of 2 mm are close to that already published for thin PbTe films [24]. The temperature evolution is in good agreement with previous results published for films deposited by thermal evaporation [6]. A larger change occurs at a deposition temperature of 673 K. This behaviour is related to the crystalline modification of the layers resulting from tellurium evaporation. The electrical results performed on the layers as a function of temperature, pellet composition and various doping conditions are briefly discussed in the following. First, the resistivity increase of undoped layers with increasing deposition temperature can be attributed to structural changes in the layers, such as grain size growth, which is likely to occur with increasing temperature. The resistivity increase then results from the reduction of the associated grain
boundaries density. These boundaries constitute preferential conductive ways due to the presence of defects or impurities like oxygen creating p-type channels. For the highest temperatures, Te loss from the film, as previously reported [23], can account for a compensation effect due to the creation of donor-type Te vacancies. As already noticed, PbTe deposited layers are generally p-type. Owing to the deposition process used, this is likely to be the case in the present work: actually, the high vapor pressure and the low vaporization heat of Te produces a preferential Te vaporization from the pellet, then leading to the formation of a Te rich p-type layer on low temperature substrates. For the same reason, increasing the substrate temperature above 673 K leads to Te loss from the film itself and then to a compensation effect by the increased number of Te vacancies. Increasing the lead percentage in the pellets produces an increase of the sheet resistance, indicating that a compensation effect occurs. This result is in agreement with the fact that the as-deposited layers are preferentially p-type and that n-type doping can be obtained by adding more Pb in the layers. Incorporation of nitrogen into the layers during deposition on a substrate maintained at low temperature has a similar effect. The sheet resistance increases by a factor of five when rising the N2 pressure in the vacuum chamber from 0.27 up to 1.3 Pa. As nitrogen acts as a donor in PbTe, the sheet resistance increase again results from a compensation effect. The fact that the position of the resistivity maximum versus gallium amount (corresponding to full compensation and type inversion) does not change significantly with temperature is in agreement with the former assumption that the resistivity variation with temperature is due to structural changes involving the density of grain boundaries while conduction inside the grains is not or very slightly modified by the temperature change. Actually, no doping effect results from introduction of arsenic in the layers. This can be explained by taking into account that arsenic can react with tellurium to form arsenic telluride As2Te3. This compound can be deposited by evaporation to give amorphous layers whose resistivity at room temperature is about 4000 V cm, much higher than that of PbTe [25]. Therefore, the resistivity increase of the PbTe layers when increasing the arsenic amount results from the creation of a high resistivity compound into the films and by Te loss due to the formation of arsenic telluride occurring during laser ablation. Again a compensation effect due to tellurium shortage takes place. 5. Conclusion We have shown the possibility to prepare thin films of PbTe using laser ablation of pressed pellets made of lead and tellurium powders. Equal Pb and Te percentage in the pellets and substrate temperature of about 473–573 K produce thin films presenting optical and electrical properties comparable to those of films obtained by other methods. Donor doping can be achieved by using gallium or nitrogen. Introduction of lead excess also gives n-type doping. On the other hand, p-type
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