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Control of the crystal structure and electrical transport in undoped PbTe films grown by pulsed laser deposition
Journal of Crystal Growth 432 (2015) 19–23
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Control of the crystal structure and electrical transport in undoped PbTe films grown by pulsed laser deposition I.S. Virt a,b, Y. Tur a, I.O. Rudyi c, I.Ye. Lopatynskyi c, M.S. Frugynskyi c, I.V. Kurilo c, E. Lusakowska d, B.S. Witkowski d, G. Luka d,n a
Drogobych State Pedagogical University, Drogobych, Ukraine University of Rzeszow, Rzeszow, Poland c National University “Lviv Polytechnic”, Lviv, Ukraine d Institute of Physics PAS, Warsaw, Poland b
art ic l e i nf o
a b s t r a c t
Article history: Received 6 July 2015 Received in revised form 8 September 2015 Accepted 9 September 2015 Communicated by: P. Rudolph Available online 18 September 2015
Lead telluride (PbTe) undoped films with various thicknesses (40–1800 nm) were grown by pulsed laser deposition (PLD) on different single crystal substrates (KCl, Si) and at different substrate temperatures (30 °C, 200 °C). Structural and electrical investigations of the so-obtained films have been carried out. The growth conditions leading to the films having different properties that could be controlled in a possibly wide range were identified. The film crystal structure varied from pseudo-amorphous to a highly ordered one. The films exhibited semiconducting behavior except the case of the thinnest, metallic-like layers. Electrical transport properties of the films with different structural quality were affected by changes of the grain boundary-related potential barrier height whereas donor level-related activation energies remained unchanged. & 2015 Elsevier B.V. All rights reserved.
Keywords: A1. Crystal morphology A1. Crystal structure A3. Pulsed laser deposition B1. Tellurides
1. Introduction Narrow-gap semiconductors of AIVBVI compounds and their solid solutions have been the subject of detailed studies for many years [1,2]. An increased interest in AIVBVI compounds is due to their use in thermoelectric devices [3,4]. Numerical studies have shown that doped AIVBVI semiconductor compounds and their solid solutions can be regarded as the most promising for applications in infrared technology [5]. In particular, undoped PbTe single crystals and films possess unique physical properties. Namely, they reveal a stabilization of the Fermi level EF in the middle of the band gap. The EF stabilization reduces concentrations of the charge carriers close to the intrinsic values [6]. Currently, there is no clear opinion on the causes and mechanisms of the band structure changes of lead and tin telluride alloys. Likewise, in many studies the transport properties of single crystals of lead chalcogenides were investigated, but very little is known about the properties of the thin films, especially undoped ones. Electrical properties of PbTe films doped with thallium, silver or gallium were investigated in Refs. [7,8]. Moreover, thermoelectric properties were measured mainly on doped lead telluride [9], whereas very few studies were made on undoped thin n
Corresponding author. Tel.: þ 48 221163315. E-mail address: [email protected] (G. Luka).
http://dx.doi.org/10.1016/j.jcrysgro.2015.09.008 0022-0248/& 2015 Elsevier B.V. All rights reserved.
films. So far, the main emphasis was put on obtaining high structural quality or epitaxial PbTe films [10] and nanostructures [11]. However, for optimal device applications, especially industrial ones, where a compromise between material quality, fabrication costs, and device performance often has to be made, the ability to prepare films with properties controlled in a possibly wide range is an important issue. This motivates our present study. PbTe thin films can be produced using various deposition methods: the method of a “hot wall”, magnetron sputtering as well as a thermal deposition method. PbTe thin films obtained by pulsed laser deposition (PLD) were reported in Refs. [8,12,13]. The PLD method is advantageous compared to thermal methods of continuous deposition of semiconductor thin films. During the PLD process, the presence of a large fraction of excited atoms and ions allows to lower the temperature of the epitaxial growth. A high rate of nucleation allows to deposit extremely thin solid films (a few nanometers thick). Furthermore, since a single pulse vaporizes a small mass of the target material, the film thickness can be precisely controlled by the number of laser pulses. In this paper we study the conditions for obtaining undoped PbTe films with various structural properties, ranging from amorphous to mosaic crystal structure. We also carry out a detailed analysis of the film structural evolution during the PLD growth. We will explain the films structural self-organization (formation mechanisms) during the
20
I.S. Virt et al. / Journal of Crystal Growth 432 (2015) 19–23
growth. These investigations will be complemented by electrical and surface morphology studies of the deposited films. In particular, a relationship between oriented crystal growth and electrical transport properties will be discussed.
2. Experimental The polycrystalline PbTe target was prepared from the synthesis of 5N (99.999%) purity Pb and Te elements, which were placed with the atomic ratio of 1:1 in a quartz container. Structural analysis and uniformity of the PbTe target was carried out by X-ray diffraction (XRD) measurements (DRON-3.0 X-ray diffractometer with CuKα (λ ¼1.54 Å) monochromatic radiation). PbTe films were grown using a Nd:YAG3 þ laser with the following parameters: the laser wavelength of 1064 nm, the maximum energy in the pulse Emax E 0.4 J, the power density of 4 108 W/cm2, the pulse duration Δτ E10 ns, and the pulse repetition frequency of 1 Hz [14]. The pressure inside the PLD chamber was 1 10 5 mmHg and the substrate temperatures were 30 °C and 200 °C. The films were grown on KCl(001) and Si (111) single crystal substrates [15]. The distance from the target to the substrates was 2 cm. PbTe films having different thicknesses were deposited at 200 °C by applying different pulse numbers (deposition times) and keeping other growth parameters constant [16]. The film thicknesses were 40, 80, 120, 400 and 1800 nm (710 nm) which was estimated based on thickness measurements of the films grown on Si substrates. Structural properties of PbTe films grown on KCl and Si substrates were investigated by transmission and reflection high energy electron diffraction (THEED and RHEED) methods, respectively, by using an EG-100A electron diffractometer and accelerating voltages of 60–80 kV. The surface morphology of PbTe films was studied by the atomic force microscopy (AFM) (Nanoscope III, Veeco) and by the scanning electron microscopy (SEM) (Hitachi SU-70). Electrical measurements of the PbTe films (grown on c-sapphire) were carried out at temperature range of 77–300 K using a liquid nitrogen cryostat.
Fig. 1. XRD powder diffraction pattern of the synthesized PbTe target material.
3. Results and discussion The results of X-ray diffraction studies of the synthesized PbTe target material showed no peaks in the diffraction patterns corresponding to the precipitation of any components of compounds or binary alloys. The resulting one-phase polycrystalline material has a cubic structure. A typical diffraction pattern of PbTe is shown in Fig. 1. The presence of characteristic peaks in the PbTe diffraction pattern shows the fcc structure of NaCl-type. Calculated experimental interplanar distances dhkl, obtained from X-ray data for PbTe, are in a good agreement with the standard XRD data for PbTe [17]. The calculated value of the lattice constant was 6.4296 Å. This value is close to the standard value 6.443 Å [17]. Fig. 2 shows THEED patterns and their intensity spectra for PbTe films grown on KCl substrates at temperature of 200 °C and having thicknessses of 40, 80, 400, and 1800 nm. From the positions of the respective diffraction peak maxima, the interplanar distances dhkl and the lattice constants a were calculated. The a values were determined pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi from dhkl ones using the following for2
2
2
mula, a ¼ dhkl h þ k þ l . They are summarized in Table 1. All the films are polycrystalline and reveal a cubic structure. The films with thicknesses up to 80 nm have a polycrystalline structure without a preferable crystallite orientation. We observe very weak diffraction peaks of (440) and (600) with additional (311) and
Fig. 2. THEED patterns (a) and the respective intensity spectra (b) of PbTe films with different thicknesses, grown on KCl substrates at temperature of 200 °C.
(400) peaks. Thicker films, having 120 and 400 nm, reveal the preferential orientation of crystallites-textured polycrystalline structure. 1800 nm thick PbTe films showed the appearance of the
I.S. Virt et al. / Journal of Crystal Growth 432 (2015) 19–23
21
Table 1 Experimental values of the interplanar distances dhkl for the PbTe target and films with different thicknesses grown on KCl. The values were obtained from XRD (Fig. 1) and THEED (Fig. 2) data. hkl
200 311 220 400 420 422 a (Å) Δa/atarget (%)
dhkl (Å) Target
dhkl (Å)
dhkl (Å)
dhkl (Å)
dhkl (Å)
dhkl (Å)
t ¼40 nm
t ¼80 nm
t¼ 120 nm
t¼ 420 nm
t ¼1800 nm
3.1884 2.2643 1.8528 1.6047 1.4360 1.3089 6.4296 –
3.215 2.265 1.852 1.601 1.433 1.314 6.407 0.35
3.210 2.269 1.858 1.598 1.433 1.309 6.403 0.40
3.212 2.265 1.867 1.600 1.428 1.318 6.408 0.33
3.213 2.282 1.857 1.605 1.436 1.312 6.421 0.13
3.232 2.274 1.857 1.608 1.439 1.312 6.427 0.04
Table 2 Texture coefficients of PbTe films, having different thicknesses, with respect to the PbTe target powder XRD data, taken as a reference. hkl
200 311 220 400 420
Standard (target)
1 1 1 1 1
Film thickness 40 nm
80 nm
400 nm
1800 nm
0.31 0.30 1.74 1.48 0.85
0.41 0.57 2.13 0.67 1.21
0.18 0.31 1.22 1.91 1.37
0.28 0 0 3.34 1.38
Fig. 3. Thickness dependence of the PbTe film lattice constant on the film thickness. The films were deposited on KCl substrates at Tsub ¼200 °C.
mosaic structure. In this case, only four high intensity peaks are observed. THEED patterns of the films with lower thicknesses consist of a relatively large number of peaks. Since the lattice constant of the KCl crystal is equal to 6.27 Å, for lower PbTe film thicknesses (40–400 nm) we observe a compressive effect of the substrate on the PbTe lattice parameters. For thicker films, the lattice constant values approach the value for the target material, atarget (see Fig. 3). We studied the effect of the film thickness on the crystalline orientation of the deposited films using a concept of the texture coefficient Tc(hkl) [18–20]. The Tc(hkl) values were calculated for each (hkl) plane direction using the formula: IðhklÞ=I 0 ðhklÞ T C ðhklÞ ¼ P 1 IðhklÞ=I 0 ðhklÞ N
Fig. 4. Conductivity versus temperature data for PbTe films with various thicknesses. The inset shows the activation energy values, E1 and E2, calculated from fitting the respective linear parts of the relationships.
N
where Tc(hkl) is the texture coefficient for a (hkl) peak, I(hkl) is the measured intensity of a (hkl) peak, I0(hkl) is the relative intensity of the (hkl) peak taken from the JCPDS data [11] or from a given reference sample (XRD powder diffraction data of the target, in our case), and N is the number of peaks. The Tc values calculated for the five main (hkl) diffraction peaks of the PbTe films with various thicknesses are shown in Table 2. In case of the PbTe target, Tc(hkl)¼1 for all (hkl) planes which reflects a random orientation of crystallites. For all the deposited films, the texture coefficients of (200) and (311) planes are less than 1 indicating an absence of crystallites in these plane directions. In case of the films having thicknesses of 40 nm and 80 nm, the highest Tc values are obtained for (220) planes. Therefore, at this growth stage the predominant crystal orientation is the (220) one. However, this orientation competes with the (400) and (420) ones. For thicker films, i.e. 400 and 1800 nm thick, the (400) plane orientation
becomes predominant. This is associated with the highest Tc values, 1.91 and 3.34, and means an increased number of (400) crystallite planes in the thicker layers. Electrical transport properties of the investigated films were analyzed based on temperature-dependent conductivity measurements. The measurement results are shown in Fig. 4 as conductivity σ vs. inverse temperature T 1 data in a semi-log scale. The thinnest films, having 40 nm, reveal the lowest conductivities at room temperature (RT), 4 10 4 (Ω cm) 1. Thicker layers have conductivities 1 10 2 (Ω cm) 1 at RT, that are weakly dependent on the film thickness. The conductivity of the 40 nmthick PbTe film is almost independent of the measurement temperature. This indicates a metallic-like σ(T) behavior and may be related to a non-stoichiometric PbTe growth at the initial growth stage. The most probable non-stoichiometry source is excess of Pb. Thicker layers show both activation type σ(T) behavior and much
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I.S. Virt et al. / Journal of Crystal Growth 432 (2015) 19–23
higher (i.e. above one order of magnitude) conductivities. This suggest, firstly, that in the thinnest layers the electrical transport occurs not in the whole film plane. Instead, there might exist some layer parts (conduction paths) richer with Pb. Secondly, the excess Pb atoms affect the conductivity of the thicker layers, i.e these atoms are intrinsic donors. To investigate this, we determined activation energy values from σ(T 1) data using an expression, σ (T) e Ei/kT, where Ei (i¼1,2,…) is the activation energy of a given process, k is the Boltzmann constant, and T is absolute temperature. The Ei values were obtained from fitting linear parts of the ln σ vs. T 1 data, and are summarized in the inset of Fig. 4. In case of the thinnest PbTe layer, the Ei value is close to zero. For all other samples, we can distinguish the respective two activation regions, separated from one another by temperatures Tp. At temperatures lower than Tp, the activation energies E2 are almost independent of the the film thickness, and they are E20 meV. E1 values, however, are thickness-dependent and they decrease as the film thickness increases. For 80 nm-thick PbTe films, E1 ¼145 meV whereas for the thickest film (d¼ 1800 nm), E1 ¼47 meV. The E1 values may be associated with potential barriers at grain boundaries. The higher the film thickness, the better the structural quality. Therefore, potential barriers for electron transport should become lower. E2 values, on the other hand, might be related to PbTe defect (donor) states that give an additional energy level in the bandgap. The energy level value is, within the fit uncertainty, independent of the structural quality of the films. Oriented growth conditions also depend on the type and structure of the substrate. The substrate temperature and the type of a substrate affect the nature of growth and the structure of the films. This is also the case of the PbTe films grown by PLD on different substrates and different temperatures, which is illustrated in Fig. 5. The figure shows diffraction patterns for PbTe films deposited on KCl and Si substrates at different temperatures (30 °C and 200 °C). The films have a thickness of about 1 μm. There is a partial orientation of crystallites (a textured polycrystal) on the cleavage surface of KCl (Fig. 5a). By increasing the substrate temperature to Tsub ¼ 200 °C, the films with a more perfect structure can be grown, i.e. along with the textured polycrystal structure the mosaic crystal structure is observed (Fig. 5b). On the Si substrates at Tsub ¼30 °C (Fig. 5c), the PbTe film with a polycrystalline structure is formed. When Tsub ¼200 °C (Fig. 5d), the film shows a textured polycrystalline structure. The substrate temperature significantly affects the process of growth-oriented films. Restricted mobilities of adatoms on the surface of the substrate at low temperature inhibit the process of growth-oriented films (film crystallites). The calculated interplanar distances dhkl of the films grown on KCl substrates at Tsub ¼30 °C and Tsub ¼200 °C are shown in Table 3. Based on the determined dhkl values, the lattice constants of the films have been calculated: a ¼6.442 Å for Tsub ¼ 30 °C and a ¼6.426 Å for Tsub ¼200 °C. By comparing these values to the calculated lattice parameter of the target material, atarget ¼ 6.4296 Å, we observe that the increased substrate temperature leads to the PbTe film lattice constant value more close to the target value, under the given conditions of the film formation. The observed lattice mismatch is þ0.19% and 0.05% for the PbTe films grown at Tsub ¼30 °C and Tsub ¼ 200 °C, respectively. These values are comparable to (or even less than) the values reported e.g. in Ref. [10], where the mismatch values ranged from 2.5% to 1.5%. Finally, we investigated the surface morphology of PbTe films grown at 30 °C and 200 °C. The SEM and AFM images are shown in Fig. 6. The substrate temperature of 30 °C (Fig. 6a) results in a formation of grains with different sizes, ranging from several nanometers to several tens of nanometers. Additionally, the grains are separated by large grain boundaries. They are, most probably, electrically insulating which manifests itself by a lighter spaces
Fig. 5. (a, b) THEED patterns of PbTe films deposited on KCl at substrate temperature of 30 °C and 200 °C, respectively. (c, d) – RHEED patterns of PbTe films deposited on Si at substrate temperature of 30 °C and 200 °C, respectively. (e) – THEED intensity spectra of PbTe films grown on KCl at the substrate temperatures of 30 °C and 200 °C. The film thickness is about 1 μm.
Table 3 Interplanar distances dhkl for the PbTe target and films ( E 1 μm thick) grown on KCl at different temperatures. The values were obtained from the XRD (Fig. 1) and THEED (Fig. 4) data. hkl
200 311 220 400 420 422 a (Å) Δa/atarget (%)
dhkl (Å) Target
dhkl (Å)
dhkl (Å)
Ts ¼ 30 °C
Ts ¼ 200 °C
3.1884 2.2643 1.8528 1.6047 1.4360 1.3089 6.4296 –
3.215 2.265 1.852 1.601 1.433 1.314 6.442 þ 0.19
3.210 2.269 1.858 1.598 1.433 1.309 6.426 0.05
between the grains. In contrast, the films grown at 200 °C have grains with a more comparable size, 50–100 nm (Fig. 6b). Some of the grains are apparently larger than others. Their average size is about 100–150 nm. The AFM image (Fig. 6c) confirms the presence of a quite uniform grain morphology with additional, lowerdensity and larger-size grains randomly distributed at the surface. Despite this, the root-mean-square (RMS) surface roughness, measured by the AFM, is 3.3 nm. This is a relatively small value taking into account a high film thickness ( E1 μm).
I.S. Virt et al. / Journal of Crystal Growth 432 (2015) 19–23
23
Fig. 6. (a, b) SEM images of the surface morphology of PbTe films grown on Si at the substrate temperature of 30 °C and 200 °C, respectively (c) AFM image of the surface of the PbTe film grown on Si at 200 °C.
4. Conclusions Undoped PbTe films were obtained by the PLD method using a synthesized target source material. The films possessed crystal structure that ranged from pseudo-amorphous to a highly crystalline mosaic one, depending on the growth conditions. The RMS surface roughness of the E1 μm thick PbTe films grown at 200 °C was relatively low (3.3 nm). Electrical investigations indicated two types of conduction mechanisms in the so-obtained PbTe films. One of them was related to a reduction of potential barriers for a charge transport in thicker layers. The second one, independent of the layer thickness, was associated with an intrinsic donor state (level) located E0.02 eV below the conduction band edge.
References [1] A. Popescu, L.M. Woods, Appl. Phys. Lett. 97 (2010) 052102. [2] J. Niua, H. Shena, X. Li, W. Xu, H. Wang, L.S. Li, Colloids Surf. A: Physicochem. Eng. Asp. 406 (2012) 38. [3] T.C. Harman, D.L. Spears, M.J. Manfra, J. Electron. Mater. 25 (1996) 1121.
[4] Y. Gelbstein, Z. Dashevsky, M.P. Dariel, Physica B: Condens. Matter 363 (2005) 196. [5] J. Wang, J. Hu, P. Becla, A.M. Agarwal, L.C. Kimerling, J. Appl. Phys. 110 (2011) 083719. [6] T. Keiber, F. Bridges, B.C. Sales, H. Wang, Phys. Rev. B 87 (2013) 144104. [7] H. Yin, Q. Wang, G. Chen, Chem. Eng. J. 236 (2014) 131. [8] A.M. Mousa, J.P. Ponpon, Appl. Surf. Sci. 254 (2007) 1215–1219. [9] Q. Zhang, H. Wang, Q. Zhang, W. Liu, B. Yu, H. Wang, D. Wang, G. Ni, G. Chen, Z. Ren, Nano Lett. 12 (2012) 2324. [10] M. Baleva, L. Bozukov, E. Tzukeva, Semicond. Sci. Technol. 8 (1993) 1208. [11] A.F. Craievich, G. Kellermann, L.C. Barbosa, O.L. Alves, Phys. Rev. Lett. 89 (2002) 235503. [12] M. Baleva, D. Bakoeva, J. Mater. Sci. Lett. 5 (1986) 37. [13] A. Jacquot, B. Lenoir, M.O. Boffou´e, A. Dauscher, Appl. Phys. A 69 (1999) S613. [14] I.O. Rudyi, I.V. Kurilo, M.S. Frugynskyj, M. Kuzma, J. Zawislak, I.S. Virt, Appl. Surf. Sci. 154–155 (2010) 206. [15] ІS. Virt, T.P. Shkumbatyuk, I.V. Kurilo, I.O. Rudyi, T.Ye Lopatinskyi, L.F. Linnik, V. V. Tetyorkin, A.G. Phedorov, Semiconductors 44 (2010) 544. [16] I.S. Virt, I.O. Rudyj, I.V. Kurilo, I.Ye Lopatynskyi, L.F. Linnik, V.V. Tetyorkin, P. Potera, G. Luka, Semiconductors 47 (2013) 997. [17] Joint Committee on Powder Diffraction Standards, Powder Diffraction File, Card no: 77-0246. [18] R.S. Singh, S. Bhushan, A.K. Singh, S.R. Deo, J. Nanomater. Biostruct. 6 (2011) 403. [19] C.S. Barret, T.B. Massalski, Structure of Metals, Pergamon Press, Oxford, 1980. [20] S. Ilican, M. Caglar, Y. Caglar, Mater. Sci.-Pol. 25 (2007) 709.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Control of the crystal structure and electrical transport in undoped PbTe films grown by pulsed laser deposition I.S. Virt a,b, Y. Tur a, I.O. Rudyi c, I.Ye. Lopatynskyi c, M.S. Frugynskyi c, I.V. Kurilo c, E. Lusakowska d, B.S. Witkowski d, G. Luka d,n a
Drogobych State Pedagogical University, Drogobych, Ukraine University of Rzeszow, Rzeszow, Poland c National University “Lviv Polytechnic”, Lviv, Ukraine d Institute of Physics PAS, Warsaw, Poland b
art ic l e i nf o
a b s t r a c t
Article history: Received 6 July 2015 Received in revised form 8 September 2015 Accepted 9 September 2015 Communicated by: P. Rudolph Available online 18 September 2015
Lead telluride (PbTe) undoped films with various thicknesses (40–1800 nm) were grown by pulsed laser deposition (PLD) on different single crystal substrates (KCl, Si) and at different substrate temperatures (30 °C, 200 °C). Structural and electrical investigations of the so-obtained films have been carried out. The growth conditions leading to the films having different properties that could be controlled in a possibly wide range were identified. The film crystal structure varied from pseudo-amorphous to a highly ordered one. The films exhibited semiconducting behavior except the case of the thinnest, metallic-like layers. Electrical transport properties of the films with different structural quality were affected by changes of the grain boundary-related potential barrier height whereas donor level-related activation energies remained unchanged. & 2015 Elsevier B.V. All rights reserved.
Keywords: A1. Crystal morphology A1. Crystal structure A3. Pulsed laser deposition B1. Tellurides
1. Introduction Narrow-gap semiconductors of AIVBVI compounds and their solid solutions have been the subject of detailed studies for many years [1,2]. An increased interest in AIVBVI compounds is due to their use in thermoelectric devices [3,4]. Numerical studies have shown that doped AIVBVI semiconductor compounds and their solid solutions can be regarded as the most promising for applications in infrared technology [5]. In particular, undoped PbTe single crystals and films possess unique physical properties. Namely, they reveal a stabilization of the Fermi level EF in the middle of the band gap. The EF stabilization reduces concentrations of the charge carriers close to the intrinsic values [6]. Currently, there is no clear opinion on the causes and mechanisms of the band structure changes of lead and tin telluride alloys. Likewise, in many studies the transport properties of single crystals of lead chalcogenides were investigated, but very little is known about the properties of the thin films, especially undoped ones. Electrical properties of PbTe films doped with thallium, silver or gallium were investigated in Refs. [7,8]. Moreover, thermoelectric properties were measured mainly on doped lead telluride [9], whereas very few studies were made on undoped thin n
Corresponding author. Tel.: þ 48 221163315. E-mail address: [email protected] (G. Luka).
http://dx.doi.org/10.1016/j.jcrysgro.2015.09.008 0022-0248/& 2015 Elsevier B.V. All rights reserved.
films. So far, the main emphasis was put on obtaining high structural quality or epitaxial PbTe films [10] and nanostructures [11]. However, for optimal device applications, especially industrial ones, where a compromise between material quality, fabrication costs, and device performance often has to be made, the ability to prepare films with properties controlled in a possibly wide range is an important issue. This motivates our present study. PbTe thin films can be produced using various deposition methods: the method of a “hot wall”, magnetron sputtering as well as a thermal deposition method. PbTe thin films obtained by pulsed laser deposition (PLD) were reported in Refs. [8,12,13]. The PLD method is advantageous compared to thermal methods of continuous deposition of semiconductor thin films. During the PLD process, the presence of a large fraction of excited atoms and ions allows to lower the temperature of the epitaxial growth. A high rate of nucleation allows to deposit extremely thin solid films (a few nanometers thick). Furthermore, since a single pulse vaporizes a small mass of the target material, the film thickness can be precisely controlled by the number of laser pulses. In this paper we study the conditions for obtaining undoped PbTe films with various structural properties, ranging from amorphous to mosaic crystal structure. We also carry out a detailed analysis of the film structural evolution during the PLD growth. We will explain the films structural self-organization (formation mechanisms) during the
20
I.S. Virt et al. / Journal of Crystal Growth 432 (2015) 19–23
growth. These investigations will be complemented by electrical and surface morphology studies of the deposited films. In particular, a relationship between oriented crystal growth and electrical transport properties will be discussed.
2. Experimental The polycrystalline PbTe target was prepared from the synthesis of 5N (99.999%) purity Pb and Te elements, which were placed with the atomic ratio of 1:1 in a quartz container. Structural analysis and uniformity of the PbTe target was carried out by X-ray diffraction (XRD) measurements (DRON-3.0 X-ray diffractometer with CuKα (λ ¼1.54 Å) monochromatic radiation). PbTe films were grown using a Nd:YAG3 þ laser with the following parameters: the laser wavelength of 1064 nm, the maximum energy in the pulse Emax E 0.4 J, the power density of 4 108 W/cm2, the pulse duration Δτ E10 ns, and the pulse repetition frequency of 1 Hz [14]. The pressure inside the PLD chamber was 1 10 5 mmHg and the substrate temperatures were 30 °C and 200 °C. The films were grown on KCl(001) and Si (111) single crystal substrates [15]. The distance from the target to the substrates was 2 cm. PbTe films having different thicknesses were deposited at 200 °C by applying different pulse numbers (deposition times) and keeping other growth parameters constant [16]. The film thicknesses were 40, 80, 120, 400 and 1800 nm (710 nm) which was estimated based on thickness measurements of the films grown on Si substrates. Structural properties of PbTe films grown on KCl and Si substrates were investigated by transmission and reflection high energy electron diffraction (THEED and RHEED) methods, respectively, by using an EG-100A electron diffractometer and accelerating voltages of 60–80 kV. The surface morphology of PbTe films was studied by the atomic force microscopy (AFM) (Nanoscope III, Veeco) and by the scanning electron microscopy (SEM) (Hitachi SU-70). Electrical measurements of the PbTe films (grown on c-sapphire) were carried out at temperature range of 77–300 K using a liquid nitrogen cryostat.
Fig. 1. XRD powder diffraction pattern of the synthesized PbTe target material.
3. Results and discussion The results of X-ray diffraction studies of the synthesized PbTe target material showed no peaks in the diffraction patterns corresponding to the precipitation of any components of compounds or binary alloys. The resulting one-phase polycrystalline material has a cubic structure. A typical diffraction pattern of PbTe is shown in Fig. 1. The presence of characteristic peaks in the PbTe diffraction pattern shows the fcc structure of NaCl-type. Calculated experimental interplanar distances dhkl, obtained from X-ray data for PbTe, are in a good agreement with the standard XRD data for PbTe [17]. The calculated value of the lattice constant was 6.4296 Å. This value is close to the standard value 6.443 Å [17]. Fig. 2 shows THEED patterns and their intensity spectra for PbTe films grown on KCl substrates at temperature of 200 °C and having thicknessses of 40, 80, 400, and 1800 nm. From the positions of the respective diffraction peak maxima, the interplanar distances dhkl and the lattice constants a were calculated. The a values were determined pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi from dhkl ones using the following for2
2
2
mula, a ¼ dhkl h þ k þ l . They are summarized in Table 1. All the films are polycrystalline and reveal a cubic structure. The films with thicknesses up to 80 nm have a polycrystalline structure without a preferable crystallite orientation. We observe very weak diffraction peaks of (440) and (600) with additional (311) and
Fig. 2. THEED patterns (a) and the respective intensity spectra (b) of PbTe films with different thicknesses, grown on KCl substrates at temperature of 200 °C.
(400) peaks. Thicker films, having 120 and 400 nm, reveal the preferential orientation of crystallites-textured polycrystalline structure. 1800 nm thick PbTe films showed the appearance of the
I.S. Virt et al. / Journal of Crystal Growth 432 (2015) 19–23
21
Table 1 Experimental values of the interplanar distances dhkl for the PbTe target and films with different thicknesses grown on KCl. The values were obtained from XRD (Fig. 1) and THEED (Fig. 2) data. hkl
200 311 220 400 420 422 a (Å) Δa/atarget (%)
dhkl (Å) Target
dhkl (Å)
dhkl (Å)
dhkl (Å)
dhkl (Å)
dhkl (Å)
t ¼40 nm
t ¼80 nm
t¼ 120 nm
t¼ 420 nm
t ¼1800 nm
3.1884 2.2643 1.8528 1.6047 1.4360 1.3089 6.4296 –
3.215 2.265 1.852 1.601 1.433 1.314 6.407 0.35
3.210 2.269 1.858 1.598 1.433 1.309 6.403 0.40
3.212 2.265 1.867 1.600 1.428 1.318 6.408 0.33
3.213 2.282 1.857 1.605 1.436 1.312 6.421 0.13
3.232 2.274 1.857 1.608 1.439 1.312 6.427 0.04
Table 2 Texture coefficients of PbTe films, having different thicknesses, with respect to the PbTe target powder XRD data, taken as a reference. hkl
200 311 220 400 420
Standard (target)
1 1 1 1 1
Film thickness 40 nm
80 nm
400 nm
1800 nm
0.31 0.30 1.74 1.48 0.85
0.41 0.57 2.13 0.67 1.21
0.18 0.31 1.22 1.91 1.37
0.28 0 0 3.34 1.38
Fig. 3. Thickness dependence of the PbTe film lattice constant on the film thickness. The films were deposited on KCl substrates at Tsub ¼200 °C.
mosaic structure. In this case, only four high intensity peaks are observed. THEED patterns of the films with lower thicknesses consist of a relatively large number of peaks. Since the lattice constant of the KCl crystal is equal to 6.27 Å, for lower PbTe film thicknesses (40–400 nm) we observe a compressive effect of the substrate on the PbTe lattice parameters. For thicker films, the lattice constant values approach the value for the target material, atarget (see Fig. 3). We studied the effect of the film thickness on the crystalline orientation of the deposited films using a concept of the texture coefficient Tc(hkl) [18–20]. The Tc(hkl) values were calculated for each (hkl) plane direction using the formula: IðhklÞ=I 0 ðhklÞ T C ðhklÞ ¼ P 1 IðhklÞ=I 0 ðhklÞ N
Fig. 4. Conductivity versus temperature data for PbTe films with various thicknesses. The inset shows the activation energy values, E1 and E2, calculated from fitting the respective linear parts of the relationships.
N
where Tc(hkl) is the texture coefficient for a (hkl) peak, I(hkl) is the measured intensity of a (hkl) peak, I0(hkl) is the relative intensity of the (hkl) peak taken from the JCPDS data [11] or from a given reference sample (XRD powder diffraction data of the target, in our case), and N is the number of peaks. The Tc values calculated for the five main (hkl) diffraction peaks of the PbTe films with various thicknesses are shown in Table 2. In case of the PbTe target, Tc(hkl)¼1 for all (hkl) planes which reflects a random orientation of crystallites. For all the deposited films, the texture coefficients of (200) and (311) planes are less than 1 indicating an absence of crystallites in these plane directions. In case of the films having thicknesses of 40 nm and 80 nm, the highest Tc values are obtained for (220) planes. Therefore, at this growth stage the predominant crystal orientation is the (220) one. However, this orientation competes with the (400) and (420) ones. For thicker films, i.e. 400 and 1800 nm thick, the (400) plane orientation
becomes predominant. This is associated with the highest Tc values, 1.91 and 3.34, and means an increased number of (400) crystallite planes in the thicker layers. Electrical transport properties of the investigated films were analyzed based on temperature-dependent conductivity measurements. The measurement results are shown in Fig. 4 as conductivity σ vs. inverse temperature T 1 data in a semi-log scale. The thinnest films, having 40 nm, reveal the lowest conductivities at room temperature (RT), 4 10 4 (Ω cm) 1. Thicker layers have conductivities 1 10 2 (Ω cm) 1 at RT, that are weakly dependent on the film thickness. The conductivity of the 40 nmthick PbTe film is almost independent of the measurement temperature. This indicates a metallic-like σ(T) behavior and may be related to a non-stoichiometric PbTe growth at the initial growth stage. The most probable non-stoichiometry source is excess of Pb. Thicker layers show both activation type σ(T) behavior and much
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higher (i.e. above one order of magnitude) conductivities. This suggest, firstly, that in the thinnest layers the electrical transport occurs not in the whole film plane. Instead, there might exist some layer parts (conduction paths) richer with Pb. Secondly, the excess Pb atoms affect the conductivity of the thicker layers, i.e these atoms are intrinsic donors. To investigate this, we determined activation energy values from σ(T 1) data using an expression, σ (T) e Ei/kT, where Ei (i¼1,2,…) is the activation energy of a given process, k is the Boltzmann constant, and T is absolute temperature. The Ei values were obtained from fitting linear parts of the ln σ vs. T 1 data, and are summarized in the inset of Fig. 4. In case of the thinnest PbTe layer, the Ei value is close to zero. For all other samples, we can distinguish the respective two activation regions, separated from one another by temperatures Tp. At temperatures lower than Tp, the activation energies E2 are almost independent of the the film thickness, and they are E20 meV. E1 values, however, are thickness-dependent and they decrease as the film thickness increases. For 80 nm-thick PbTe films, E1 ¼145 meV whereas for the thickest film (d¼ 1800 nm), E1 ¼47 meV. The E1 values may be associated with potential barriers at grain boundaries. The higher the film thickness, the better the structural quality. Therefore, potential barriers for electron transport should become lower. E2 values, on the other hand, might be related to PbTe defect (donor) states that give an additional energy level in the bandgap. The energy level value is, within the fit uncertainty, independent of the structural quality of the films. Oriented growth conditions also depend on the type and structure of the substrate. The substrate temperature and the type of a substrate affect the nature of growth and the structure of the films. This is also the case of the PbTe films grown by PLD on different substrates and different temperatures, which is illustrated in Fig. 5. The figure shows diffraction patterns for PbTe films deposited on KCl and Si substrates at different temperatures (30 °C and 200 °C). The films have a thickness of about 1 μm. There is a partial orientation of crystallites (a textured polycrystal) on the cleavage surface of KCl (Fig. 5a). By increasing the substrate temperature to Tsub ¼ 200 °C, the films with a more perfect structure can be grown, i.e. along with the textured polycrystal structure the mosaic crystal structure is observed (Fig. 5b). On the Si substrates at Tsub ¼30 °C (Fig. 5c), the PbTe film with a polycrystalline structure is formed. When Tsub ¼200 °C (Fig. 5d), the film shows a textured polycrystalline structure. The substrate temperature significantly affects the process of growth-oriented films. Restricted mobilities of adatoms on the surface of the substrate at low temperature inhibit the process of growth-oriented films (film crystallites). The calculated interplanar distances dhkl of the films grown on KCl substrates at Tsub ¼30 °C and Tsub ¼200 °C are shown in Table 3. Based on the determined dhkl values, the lattice constants of the films have been calculated: a ¼6.442 Å for Tsub ¼ 30 °C and a ¼6.426 Å for Tsub ¼200 °C. By comparing these values to the calculated lattice parameter of the target material, atarget ¼ 6.4296 Å, we observe that the increased substrate temperature leads to the PbTe film lattice constant value more close to the target value, under the given conditions of the film formation. The observed lattice mismatch is þ0.19% and 0.05% for the PbTe films grown at Tsub ¼30 °C and Tsub ¼ 200 °C, respectively. These values are comparable to (or even less than) the values reported e.g. in Ref. [10], where the mismatch values ranged from 2.5% to 1.5%. Finally, we investigated the surface morphology of PbTe films grown at 30 °C and 200 °C. The SEM and AFM images are shown in Fig. 6. The substrate temperature of 30 °C (Fig. 6a) results in a formation of grains with different sizes, ranging from several nanometers to several tens of nanometers. Additionally, the grains are separated by large grain boundaries. They are, most probably, electrically insulating which manifests itself by a lighter spaces
Fig. 5. (a, b) THEED patterns of PbTe films deposited on KCl at substrate temperature of 30 °C and 200 °C, respectively. (c, d) – RHEED patterns of PbTe films deposited on Si at substrate temperature of 30 °C and 200 °C, respectively. (e) – THEED intensity spectra of PbTe films grown on KCl at the substrate temperatures of 30 °C and 200 °C. The film thickness is about 1 μm.
Table 3 Interplanar distances dhkl for the PbTe target and films ( E 1 μm thick) grown on KCl at different temperatures. The values were obtained from the XRD (Fig. 1) and THEED (Fig. 4) data. hkl
200 311 220 400 420 422 a (Å) Δa/atarget (%)
dhkl (Å) Target
dhkl (Å)
dhkl (Å)
Ts ¼ 30 °C
Ts ¼ 200 °C
3.1884 2.2643 1.8528 1.6047 1.4360 1.3089 6.4296 –
3.215 2.265 1.852 1.601 1.433 1.314 6.442 þ 0.19
3.210 2.269 1.858 1.598 1.433 1.309 6.426 0.05
between the grains. In contrast, the films grown at 200 °C have grains with a more comparable size, 50–100 nm (Fig. 6b). Some of the grains are apparently larger than others. Their average size is about 100–150 nm. The AFM image (Fig. 6c) confirms the presence of a quite uniform grain morphology with additional, lowerdensity and larger-size grains randomly distributed at the surface. Despite this, the root-mean-square (RMS) surface roughness, measured by the AFM, is 3.3 nm. This is a relatively small value taking into account a high film thickness ( E1 μm).
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Fig. 6. (a, b) SEM images of the surface morphology of PbTe films grown on Si at the substrate temperature of 30 °C and 200 °C, respectively (c) AFM image of the surface of the PbTe film grown on Si at 200 °C.
4. Conclusions Undoped PbTe films were obtained by the PLD method using a synthesized target source material. The films possessed crystal structure that ranged from pseudo-amorphous to a highly crystalline mosaic one, depending on the growth conditions. The RMS surface roughness of the E1 μm thick PbTe films grown at 200 °C was relatively low (3.3 nm). Electrical investigations indicated two types of conduction mechanisms in the so-obtained PbTe films. One of them was related to a reduction of potential barriers for a charge transport in thicker layers. The second one, independent of the layer thickness, was associated with an intrinsic donor state (level) located E0.02 eV below the conduction band edge.
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