- Email: [email protected]
Pattern formation in PbTe multilayer films
Surface Science 454–456 (2000) 823–826 www.elsevier.nl/locate/susc
Pattern formation in PbTe multilayer films C. Teichert *, B. Jamnig, J. Oswald Department of Physics, University of Leoben, Franz Josef Str. 18, A-8700 Leoben, Austria
Abstract Atomic force microscopy is used to study the surface morphology evolution of PbTe films that have been grown by hot-wall epitaxy on BaF (111) substrates. We found that the morphology strongly depends on the doping type of 2 the films. For film thicknesses of about 1500 nm, Te rich films (p-type) show uniform pyramids with hexagonal bases whereas Pb rich films (n-type) exhibit irregularly shaped crystallites with (111) terraces on top. By growing npn multilayers we observe the formation of uniform triangular pyramids in the second n-layer. The arrangement of the pyramids can be controlled by varying the thickness of the p-layer. In addition to this pattern formation, the insertion of the p-layers reduces the surface roughness compared with that of the single n-layers. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Atomic force microscopy; Epitaxy; Lead telluride; Self-assembly; Semiconducting films; Surface structure, morphology, roughness, and topography
1. Introduction Multilayers of IV–VI compound semiconductors have recently attracted considerable interest because of their structural and electronic properties. In PbSe/Pb Eu Te superlattices, for 1−x x instance, a novel vertical stacking mechanism has been found that gives direct control of the size of the self-organized PbSe quantum dots [1]. Sandwiches of differently doped PbTe films have been used as model systems to study the magnetic field induced metal-to-insulator transition in wide quantum wells [2]. Little has been known so far about the influence of the doping type and the layer sequence on the surface morphology in the latter samples. Here, we use ex situ atomic force microscopy (AFM ) to investigate the growth morphology of p- and n-doped single PbTe films and * Corresponding author. Fax: +43-3842-402-760. E-mail address: [email protected] (C. Teichert)
how the morphology can be controlled by growing multilayers of varying sequences of n- and p-layers.
2. Experimental The PbTe films were grown by hot-wall epitaxy (HWE ) [3]. The HWE system is equipped with two PbTe sources with a slight excess of Pb, which results in the growth of n-type layers. The p-doping in the p-source is achieved by compensation via an additional Te flux from a separate Te source. The films are grown on freshly cleaved BaF (111) substrates. These — fluorine termi2 nated — substrates show atomically flat terraces with lateral extensions up to several tens of micrometers. The terraces are separated by single cleavage steps with a height of 0.36 nm, corresponding to the height of a F–Ba–F triple layer. The preferential orientation of the step edges is
0039-6028/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 00 ) 0 02 5 6 -9
824
C. Teichert et al. / Surface Science 454–456 (2000) 823–826
parallel to the 1: 10 direction. Occasionally, multiatomic cleavage steps with heights up to several nanometers are observed. These substrates are preheated at 520°C for 15 min to allow predominant growth of PbTe islands in the (111) orientation [4]. During growth the substrate temperature is 270°C. Growth rates of 30 nm/min and 60 nm/min have been used for p- and n-films, respectively. The growth rates are calibrated by measuring film thicknesses of single layers using an infrared spectrometer. The morphology evolution has been studied for a sample series of varying film thicknesses and varying multilayer sequences using a Digital Instruments Multi-Mode AFM with a 125 mm×125 mm lateral scan area. The measurements have been performed under ambient conditions in tapping mode using sharpened silicon tips.
3. Results and discussion The morphology in the early growth stage of single PbTe films on BaF (111) is dominated by 2 the transition from (100) oriented to (111) oriented single crystalline PbTe islands [4]. Above 100 nm we observe spiral growth that is due to the 4.2% lattice mismatch between PbTe and BaF , in 2 agreement with earlier scanning tunneling microscopy observations of PbTe grown by molecular beam epitaxy on BaF (111) [5]. Here, we focus 2 on the morphology evolution in the micrometer thickness range. Fig. 1 shows large-scale AFM images of a 1.5 mm thick n-doped film and a 1.7 mm thick p-doped film. Comparing both films of similar thickness we see striking differences in the surface morphology. The n-type film ( Fig. 1a) shows plate-like crystallites that are on average 15 nm high and are bounded by (111) terraces on top. The bases of the crystallites have close-packed
1: 10-oriented edges. However, due to the onset of crystallite coalescence the bases are very irregularly shaped. The root-mean-square (rms) roughness of the film is 5 nm (averaged over three independent 25 mm×25 mm areas). Contrary to the random array of crystallites in the n-layer, the p-doped film (Fig. 1b) exhibits a rather uniform array of pyramid-like hillocks. The
Fig. 1. 25 mm×25 mm AFM images of a 1.5 mm thick n-doped PbTe film (a) and a 1.7 mm thick p-doped PbTe film (b) grown by HWE on BaF (111). Gray-scale range in both images is 2 50 nm.
bases of the pyramids are bounded by 1: 10 edges. They have the shape of slightly truncated triangles with the long edges in the range of 2.5 mm. The pyramids have an average height of 10–15 nm and their side-walls are very shallow with tilt angles of less than 1°. The corresponding rms roughness is 4.5 nm. In order to investigate how these different growth morphologies interfere with each other in doping multilayers, we grew a series of npn PbTe films with constant n-layer thickness of 1.5 mm and increasing thickness of the inserted p-layer. In Fig. 2 the morphology of the second n-layer is shown for p-layer thicknesses of 0.2 mm (Fig. 2a), 0.5 mm ( Fig. 2b), and 1.8 mm ( Fig. 2c). Obviously, the insertion of the p-layers results in a remarkable
C. Teichert et al. / Surface Science 454–456 (2000) 823–826
Fig. 2. 25 mm×25 mm AFM images of npn PbTe multilayer films. The thickness of the n-layers is kept constant at 1.5 mm, whereas the p-layer thickness is 0.2 mm (a), 0.5 mm (b), and 1.7 mm (c). Gray-scale range is 50 nm in all images.
change in the surface morphology. For a 0.2 mm p-layer there are still plate-like crystallites present, that are surrounded by pyramids with triangular
825
base planes. The length of these edges is about 400 nm. The pyramids are preferentially arranged on a hexagonal grid with an average center-tocenter distance of 600 nm, as has been determined from the corresponding two-dimensional power spectrum of the image. The orientation of the triangular bases is rotated by 180° with respect to the truncated triangles of the single p-layer. By comparing the orientation of the triangular bases to the multiatomic cleavage edges of the substrate — that are {111: } faceted — we can determine the facet type of the pyramids. According to this, the pyramids are bounded by {mmn} type facets (m
1: 10/{001} step edges of the {558} facets are visible, as well as growth spirals on top of the plate-like crystallites. With the p-layer thickness increased to 0.5 mm ( Fig. 2b), the tetrahedron-like pyramids are becoming predominant. Only a few plate-like crystallites are left, that are surrounded by a halo that is not covered with pyramids. For even thicker p-layers (e.g. Fig. 2c) these plate-like crystallite vanish completely. The morphology of the top n-layer is now dominated by fairly uniform triangular structures of 25 nm height with straight step edges. Line scans reveal stacks of several nanometers high triangular plates. As determined from two-dimensional power spectral density analysis of the AFM image, the triangular structures are arranged on a hexagonal grid with an average separation of 3.5 mm. It should be mentioned that the size uniformity of the structures can be improved by starting with a p-layer (that is already more uniform) or by growing doping multilayers of increasing layer numbers [7]. However, there is preferential nucleation of the tetrahedron-like crystallites at the pre-existent, multiatomic steps of the BaF (111) substrate, resulting in an increased 2
826
C. Teichert et al. / Surface Science 454–456 (2000) 823–826
4. Conclusions
Fig. 3. Surface roughness evolution for the npn PbTe films as a function of the thickness of the inserted p-layer at a fixed n-layer thickness of 1.5 mm (dots). The upper x-axis denotes the overall film thickness. For comparison, rms values are also given for single n-layers of 3 mm and 4.25 mm thickness (asterisks).
crystallite density at the step edges that disturbs locally the hexagonal ordering of the crystallites. In addition to the pattern formation in the npnlayers, we observe a distinct change of the surface roughness due to the insertion of the p-layers. Fig. 3 shows the evolution of the rms roughness of the npn multilayers as a function of the p-layer thickness. Although the overall thickness of the multilayer film (see upper x-axis in Fig. 3) is increasing, there is a continuous decrease of the rms roughness from 12 nm (no p-layer) to 4.5 nm (1.25 mm p-layer). This reduction is clearly due to the insertion of the p-layer, because single n-layers of comparable thickness show an increase of the rms roughness with increasing thickness (see Fig. 3). There are two possible explanations for the observed pattern formation and reduction of surface roughness in the npn multilayers. On the one hand, the pattern formation can be strain mediated, i.e. driven by thermodynamics, as has been found in heteroepitaxial superlattices [8,9]. On the other hand, it can be driven by kinetics, i.e. due to the change in growth conditions between the pand n-layers. Such a growth manipulation has succeeded in metal homoepitaxy [10]. In order to decide between the two hypotheses, further studies, including variations of the growth rates of both doping types, are in progress [7].
The morphology of PbTe single layers and multilayers grown by HWE on BaF (111) has 2 been studied using AFM. In the micrometer thickness range, we found distinct differences in the resulting morphologies between Te rich (p-type) and Pb rich (n-type) films. For npn multilayers, we observe the formation of uniform pyramids with triangular base planes in the second n-layer. The arrangement of the pyramids can be controlled by varying the thickness of the p-layer. In addition to the pattern formation, the insertion of the p-layers results in a reduction of surface roughness compared to that of the single n-layers.
Acknowledgements This work was partly supported by FWF, No. ¨ NB, No. 6566, Austria. We P10510-NAW and O are grateful to P. Hosemann for technical assistance.
References [1] G. Springholz, V. Holy, M. Pinczolitz, G. Bauer, Science 282 (1998) 734. [2] P. Ganitzer, A. Homer, M. Lucyshyn, B. Jamnig, J. Oswald, D.K. Maude, J.C. Portal, Physica B 256–258 (1998) 600. [3] B. Liautard, M. Muller, S. Dal Corso, G. Brun, J.C. Tedenac, A. Obadi, C. Fau, S. Charar, F. Gisbert, M. Averous, Phys. Status Solidi A 133 (1992) 411. [4] H. Clemens, Ph.D. Thesis, Leoben, 1987. [5] G. Springholz, A.Y. Ueta, N. Frank, G. Bauer, Appl. Phys. Lett. 69 (1996) 2822. [6 ] M. Pinczolitz, G. Springholz, G. Bauer, Appl. Phys. Lett. 73 (1998) 250. [7] C. Teichert, K. Sorschag, in preparation. [8] C. Teichert, M.G. Lagally, L.J. Peticolas, J.C. Bean, J. Tersoff, Phys. Rev. B 53 (1996) 16 334. [9] J. Tersoff, C. Teichert, M.G. Lagally, Phys. Rev. Lett. 76 (1996) 1675. [10] G. Rosenfeld, R. Servaty, C. Teichert, B. Poelsema, G. Comsa, Phys. Rev. Lett. 71 (1993) 895.
Pattern formation in PbTe multilayer films C. Teichert *, B. Jamnig, J. Oswald Department of Physics, University of Leoben, Franz Josef Str. 18, A-8700 Leoben, Austria
Abstract Atomic force microscopy is used to study the surface morphology evolution of PbTe films that have been grown by hot-wall epitaxy on BaF (111) substrates. We found that the morphology strongly depends on the doping type of 2 the films. For film thicknesses of about 1500 nm, Te rich films (p-type) show uniform pyramids with hexagonal bases whereas Pb rich films (n-type) exhibit irregularly shaped crystallites with (111) terraces on top. By growing npn multilayers we observe the formation of uniform triangular pyramids in the second n-layer. The arrangement of the pyramids can be controlled by varying the thickness of the p-layer. In addition to this pattern formation, the insertion of the p-layers reduces the surface roughness compared with that of the single n-layers. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Atomic force microscopy; Epitaxy; Lead telluride; Self-assembly; Semiconducting films; Surface structure, morphology, roughness, and topography
1. Introduction Multilayers of IV–VI compound semiconductors have recently attracted considerable interest because of their structural and electronic properties. In PbSe/Pb Eu Te superlattices, for 1−x x instance, a novel vertical stacking mechanism has been found that gives direct control of the size of the self-organized PbSe quantum dots [1]. Sandwiches of differently doped PbTe films have been used as model systems to study the magnetic field induced metal-to-insulator transition in wide quantum wells [2]. Little has been known so far about the influence of the doping type and the layer sequence on the surface morphology in the latter samples. Here, we use ex situ atomic force microscopy (AFM ) to investigate the growth morphology of p- and n-doped single PbTe films and * Corresponding author. Fax: +43-3842-402-760. E-mail address: [email protected] (C. Teichert)
how the morphology can be controlled by growing multilayers of varying sequences of n- and p-layers.
2. Experimental The PbTe films were grown by hot-wall epitaxy (HWE ) [3]. The HWE system is equipped with two PbTe sources with a slight excess of Pb, which results in the growth of n-type layers. The p-doping in the p-source is achieved by compensation via an additional Te flux from a separate Te source. The films are grown on freshly cleaved BaF (111) substrates. These — fluorine termi2 nated — substrates show atomically flat terraces with lateral extensions up to several tens of micrometers. The terraces are separated by single cleavage steps with a height of 0.36 nm, corresponding to the height of a F–Ba–F triple layer. The preferential orientation of the step edges is
0039-6028/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 00 ) 0 02 5 6 -9
824
C. Teichert et al. / Surface Science 454–456 (2000) 823–826
parallel to the 1: 10 direction. Occasionally, multiatomic cleavage steps with heights up to several nanometers are observed. These substrates are preheated at 520°C for 15 min to allow predominant growth of PbTe islands in the (111) orientation [4]. During growth the substrate temperature is 270°C. Growth rates of 30 nm/min and 60 nm/min have been used for p- and n-films, respectively. The growth rates are calibrated by measuring film thicknesses of single layers using an infrared spectrometer. The morphology evolution has been studied for a sample series of varying film thicknesses and varying multilayer sequences using a Digital Instruments Multi-Mode AFM with a 125 mm×125 mm lateral scan area. The measurements have been performed under ambient conditions in tapping mode using sharpened silicon tips.
3. Results and discussion The morphology in the early growth stage of single PbTe films on BaF (111) is dominated by 2 the transition from (100) oriented to (111) oriented single crystalline PbTe islands [4]. Above 100 nm we observe spiral growth that is due to the 4.2% lattice mismatch between PbTe and BaF , in 2 agreement with earlier scanning tunneling microscopy observations of PbTe grown by molecular beam epitaxy on BaF (111) [5]. Here, we focus 2 on the morphology evolution in the micrometer thickness range. Fig. 1 shows large-scale AFM images of a 1.5 mm thick n-doped film and a 1.7 mm thick p-doped film. Comparing both films of similar thickness we see striking differences in the surface morphology. The n-type film ( Fig. 1a) shows plate-like crystallites that are on average 15 nm high and are bounded by (111) terraces on top. The bases of the crystallites have close-packed
1: 10-oriented edges. However, due to the onset of crystallite coalescence the bases are very irregularly shaped. The root-mean-square (rms) roughness of the film is 5 nm (averaged over three independent 25 mm×25 mm areas). Contrary to the random array of crystallites in the n-layer, the p-doped film (Fig. 1b) exhibits a rather uniform array of pyramid-like hillocks. The
Fig. 1. 25 mm×25 mm AFM images of a 1.5 mm thick n-doped PbTe film (a) and a 1.7 mm thick p-doped PbTe film (b) grown by HWE on BaF (111). Gray-scale range in both images is 2 50 nm.
bases of the pyramids are bounded by 1: 10 edges. They have the shape of slightly truncated triangles with the long edges in the range of 2.5 mm. The pyramids have an average height of 10–15 nm and their side-walls are very shallow with tilt angles of less than 1°. The corresponding rms roughness is 4.5 nm. In order to investigate how these different growth morphologies interfere with each other in doping multilayers, we grew a series of npn PbTe films with constant n-layer thickness of 1.5 mm and increasing thickness of the inserted p-layer. In Fig. 2 the morphology of the second n-layer is shown for p-layer thicknesses of 0.2 mm (Fig. 2a), 0.5 mm ( Fig. 2b), and 1.8 mm ( Fig. 2c). Obviously, the insertion of the p-layers results in a remarkable
C. Teichert et al. / Surface Science 454–456 (2000) 823–826
Fig. 2. 25 mm×25 mm AFM images of npn PbTe multilayer films. The thickness of the n-layers is kept constant at 1.5 mm, whereas the p-layer thickness is 0.2 mm (a), 0.5 mm (b), and 1.7 mm (c). Gray-scale range is 50 nm in all images.
change in the surface morphology. For a 0.2 mm p-layer there are still plate-like crystallites present, that are surrounded by pyramids with triangular
825
base planes. The length of these edges is about 400 nm. The pyramids are preferentially arranged on a hexagonal grid with an average center-tocenter distance of 600 nm, as has been determined from the corresponding two-dimensional power spectrum of the image. The orientation of the triangular bases is rotated by 180° with respect to the truncated triangles of the single p-layer. By comparing the orientation of the triangular bases to the multiatomic cleavage edges of the substrate — that are {111: } faceted — we can determine the facet type of the pyramids. According to this, the pyramids are bounded by {mmn} type facets (m
1: 10/{001} step edges of the {558} facets are visible, as well as growth spirals on top of the plate-like crystallites. With the p-layer thickness increased to 0.5 mm ( Fig. 2b), the tetrahedron-like pyramids are becoming predominant. Only a few plate-like crystallites are left, that are surrounded by a halo that is not covered with pyramids. For even thicker p-layers (e.g. Fig. 2c) these plate-like crystallite vanish completely. The morphology of the top n-layer is now dominated by fairly uniform triangular structures of 25 nm height with straight step edges. Line scans reveal stacks of several nanometers high triangular plates. As determined from two-dimensional power spectral density analysis of the AFM image, the triangular structures are arranged on a hexagonal grid with an average separation of 3.5 mm. It should be mentioned that the size uniformity of the structures can be improved by starting with a p-layer (that is already more uniform) or by growing doping multilayers of increasing layer numbers [7]. However, there is preferential nucleation of the tetrahedron-like crystallites at the pre-existent, multiatomic steps of the BaF (111) substrate, resulting in an increased 2
826
C. Teichert et al. / Surface Science 454–456 (2000) 823–826
4. Conclusions
Fig. 3. Surface roughness evolution for the npn PbTe films as a function of the thickness of the inserted p-layer at a fixed n-layer thickness of 1.5 mm (dots). The upper x-axis denotes the overall film thickness. For comparison, rms values are also given for single n-layers of 3 mm and 4.25 mm thickness (asterisks).
crystallite density at the step edges that disturbs locally the hexagonal ordering of the crystallites. In addition to the pattern formation in the npnlayers, we observe a distinct change of the surface roughness due to the insertion of the p-layers. Fig. 3 shows the evolution of the rms roughness of the npn multilayers as a function of the p-layer thickness. Although the overall thickness of the multilayer film (see upper x-axis in Fig. 3) is increasing, there is a continuous decrease of the rms roughness from 12 nm (no p-layer) to 4.5 nm (1.25 mm p-layer). This reduction is clearly due to the insertion of the p-layer, because single n-layers of comparable thickness show an increase of the rms roughness with increasing thickness (see Fig. 3). There are two possible explanations for the observed pattern formation and reduction of surface roughness in the npn multilayers. On the one hand, the pattern formation can be strain mediated, i.e. driven by thermodynamics, as has been found in heteroepitaxial superlattices [8,9]. On the other hand, it can be driven by kinetics, i.e. due to the change in growth conditions between the pand n-layers. Such a growth manipulation has succeeded in metal homoepitaxy [10]. In order to decide between the two hypotheses, further studies, including variations of the growth rates of both doping types, are in progress [7].
The morphology of PbTe single layers and multilayers grown by HWE on BaF (111) has 2 been studied using AFM. In the micrometer thickness range, we found distinct differences in the resulting morphologies between Te rich (p-type) and Pb rich (n-type) films. For npn multilayers, we observe the formation of uniform pyramids with triangular base planes in the second n-layer. The arrangement of the pyramids can be controlled by varying the thickness of the p-layer. In addition to the pattern formation, the insertion of the p-layers results in a reduction of surface roughness compared to that of the single n-layers.
Acknowledgements This work was partly supported by FWF, No. ¨ NB, No. 6566, Austria. We P10510-NAW and O are grateful to P. Hosemann for technical assistance.
References [1] G. Springholz, V. Holy, M. Pinczolitz, G. Bauer, Science 282 (1998) 734. [2] P. Ganitzer, A. Homer, M. Lucyshyn, B. Jamnig, J. Oswald, D.K. Maude, J.C. Portal, Physica B 256–258 (1998) 600. [3] B. Liautard, M. Muller, S. Dal Corso, G. Brun, J.C. Tedenac, A. Obadi, C. Fau, S. Charar, F. Gisbert, M. Averous, Phys. Status Solidi A 133 (1992) 411. [4] H. Clemens, Ph.D. Thesis, Leoben, 1987. [5] G. Springholz, A.Y. Ueta, N. Frank, G. Bauer, Appl. Phys. Lett. 69 (1996) 2822. [6 ] M. Pinczolitz, G. Springholz, G. Bauer, Appl. Phys. Lett. 73 (1998) 250. [7] C. Teichert, K. Sorschag, in preparation. [8] C. Teichert, M.G. Lagally, L.J. Peticolas, J.C. Bean, J. Tersoff, Phys. Rev. B 53 (1996) 16 334. [9] J. Tersoff, C. Teichert, M.G. Lagally, Phys. Rev. Lett. 76 (1996) 1675. [10] G. Rosenfeld, R. Servaty, C. Teichert, B. Poelsema, G. Comsa, Phys. Rev. Lett. 71 (1993) 895.