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Nanoscale morphology of multilayer PbTe/CdTe heterostructures and its effect on photoluminescence properties
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 135601 (http://iopscience.iop.org/0957-4484/26/13/135601) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 169.230.243.252 This content was downloaded on 20/03/2015 at 21:38
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Nanotechnology Nanotechnology 26 (2015) 135601 (7pp)
doi:10.1088/0957-4484/26/13/135601
Nanoscale morphology of multilayer PbTe/ CdTe heterostructures and its effect on photoluminescence properties G Karczewski1, M Szot1, S Kret1, L Kowalczyk1, S Chusnutdinow1, T Wojtowicz1, S Schreyeck2, K Brunner2, C Schumacher2 and L W Molenkamp2 1 2
Institute of Physics, Polish Academy of Science, al. Lotników 32/46, 02-668 Warszawa, Poland Physikalisches Institut (EP III), Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany
E-mail: [email protected] Received 16 July 2014, revised 14 November 2014 Accepted for publication 19 November 2014 Published 9 March 2015 Abstract
We study nanoscale morphology of PbTe/CdTe multilayer heterostuctures grown by molecular beam epitaxy on hybrid GaAs/CdTe (100) substrates. Nominally, the structures consist of 25 repetitions of subsequently deposited CdTe and PbTe layers with comparable thicknesses of 21 and 8 nm, respectively. However, the morphology of the resulting structures crucially depends on the growth temperature. The two-dimensional layered, superlattice-like character of the structures remains preserved only when grown at low substrate temperatures, such as 230 °C. The samples grown at the slightly elevated temperature of 270 °C undergo a morphological transformation to structures consisting of CdTe and PbTe pillars and columns oriented perpendicular to the substrate. Although the pillar-like objects are of various shapes and dimensions these structures exhibit exceptionally strong photoluminescence in the near infrared spectral region. At the higher growth temperature of 310 °C, PbTe and CdTe separate completely forming thick layers oriented longitudinally to the substrate plane. The observed topological transformations are driven by thermally activated atomic diffusion in the solid state phase. The solid state phase remains fully coherent during the processes. The observed topological transitions leading to the material separation in PbTe/CdTe system could be regarded as an analog of spinodal decomposition of an immiscible solid state solution and thus they can be qualitatively described by the Cahn–Hillard model as proposed by Groiss et al (2014 APL Mater. 2 012105). Keywords: nanoscale morphology, multilayer structures, photoluminescence (Some figures may appear in colour only in the online journal) 1. Introduction
dimensional (1D) percolation network and subsequently into an array of zero-dimensional (0D) PbTe quantum dots (QDs). The separation process proceeds in a fully coherent solid and is driven by thermally activated bulk diffusion. The PbTe/ CdTe material system is thus regarded as a prototype system for self-organized QD formation based on solid-state material separation [1–8]. The present research can be considered as a continuation and expansion of the 2D morphological study of Groiss et al [1] to the 3D, bulk-like case. The investigations of Groiss et al
Morphological transformations of a single, two-dimensional (2D) lead telluride (PbTe) layer embedded in a cadmium telluride (CdTe) matrix have been recently studied by realtime transmission electron microscopy (TEM) [1]. In the almost completely immiscible PbTe/CdTe material system, both constituents tend to separate causing structural, morphological transformations of the as-grown structure. Initially, the PbTe layer undergoes transformations into a one0957-4484/15/135601+07$33.00
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Nanotechnology 26 (2015) 135601
were devoted to morphological changes of a single thin layer of PbTe embedded in CdTe. Here, by steady-state, crosssectional TEM we study the morphology of quasi-bulk samples of a thick multilayer stack of PbTe/CdTe layers. In particular, we study the influence of the growth temperature on the morphology of the quasi-bulk samples. The multilayer structures were grown such that the volumes of the PbTe and CdTe deposits were nominally comparable. We show that only at the relatively low growth temperatures (here, 230 °C) the PbTe/CdTe superlattices (SLs) preserve their 2D-like character. At an intermediate growth temperature of 270 °C the material separation process leads to aggregation of PbTe deposits initially to large QDs and subsequently to long PbTe pillars surrounded coherently by CdTe. Surprisingly, the pillar- or column-like structures are oriented perpendicularly to the sample surface. When the growth temperature becomes high enough (over 310 °C) the two materials tend to separate completely and CdTe desorbs from the sample surface. We show that the growth temperature and thus the morphology of the material plays a crucial role in controlling the photoluminescence (PL) properties of the samples. PbTe belongs to the group of IV–VI narrow-band gap semiconductors. Its direct energy gap is located at the L-point of the Brillouin zone and at liquid helium temperatures it is equal to 0.187 eV. Because of the large value of the dielectric constant, ε ∼ 1000, and low effective masses, the carrier mobility in PbTe is high, approaching 104 cm2 (V−1 s−1) at room temperature and 106 cm2 (V−1 s−1) at cryogenic temperatures [9–11]. Auger recombination of excited carriers in PbTe is small in comparison to other semiconductors with similar band gap which makes PbTe a very attractive candidate for mid-infrared optoelectronic devices such as laser diodes and mid-infrared detectors [11–13]. CdTe, on the other hand, belongs to the group of II–VI wide-band gap semiconductors with an energy gap of 1.59 eV at liquid He temperatures and much lower dielectric constant of ε = 10.2. Both PbTe and CdTe crystallize in a cubic crystal structure and have closely matched lattice constants of 0.6462 and 0.6480 nm, respectively [14]. However, the crystal structures of the two materials are significantly different. In CdTe each atom has four nearest neighbors resulting in the zinc-blende structure with the space group F-43m (T2d). The less covalent PbTe has the rock-salt structure with the space group Fm3m (O5h) and sixfold coordinated atoms. Because of the latticetype mismatch the two materials are unable to form an alloy [1–8]. It is this immiscibility that leads to material separation in the PbTe/CdTe system. The immiscibility of PbTe and CdTe is employed in the formation of high-quality nanostructures. Most of the reports concern 0D structures, i.e., PbTe QDs embedded in a CdTe matrix. The dots are formed in two steps. Firstly, a few atomic monolayers of PbTe are deposited on a high quality CdTe substrate and subsequently capped by a CdTe layer. In the second step the layered structure is annealed for a few minutes in an inert atmosphere. The result of the anneal is the transformation of the PbTe layer into an array of QDs. The dots are coherently embedded in the crystal matrix of CdTe, almost free of intermixing, with highly symmetric shapes and
a height-to-width ratio close to 1 [2–8]. Such PbTe QDs in a CdTe matrix display strong mid-infrared optical emission [4–7]. Recently, such PbTe QDs were introduced into a CdTe p–n junction resulting in the first demonstration of PbTe/ CdTe electroluminescence diodes emitting in the mid-infrared spectral region up to the room temperature [8]. Photo- and electroluminescence measurements showed, however, an unusual temperature dependence of the optical emission intensity. At low temperatures the PL intensity significantly weakens in contrast to the PL intensity of III–V and II–VI QDs for which the PL intensity stays constant in the low temperature region. 2D PbTe/CdTe nanostructures—quantum wells (QWs) and SLs—have attracted less attention so far [15, 16]. Despite the crystal-structure mismatch an intense emission from QWs was observed in the mid-infrared region. The energy of the emission peak showed a blue-shift upon decreasing the QW width and increase of temperature, indicating that the emission results from electron–hole recombination in a type I QW. A type I band ordering is also confirmed by x-ray photoemission spectroscopy [17]. No reports on classical or quantum transport in PbTe/CdTe QWs have been reported so far. Most probably the lack of transport data is caused by the low quality of the PbTe/CdTe interfaces. In spite of the large dielectric constant of PbTe, screening of charged defects located at interfaces in ultra-thin QWs may still be insufficient to assure high carrier mobility. In addition, the carrier concentration in PbTe/CdTe QWs appears to be surprisingly low, taking into consideration that in typical, nominally undoped PbTe bulk samples it is as high as 1018 cm−3. The reduction of the carrier concentration is most probably caused by the capture of mobile carriers by dangling bonds at the QWs surfaces and/or by surface potential fluctuations. A vital requirement for the successful study of electron transport in this kind of structures is an improvement in the crystalline and thus the electrical properties of the PbTe/CdTe interfaces.
2. Experimental The investigated PbTe multilayer structures were grown by molecular beam epitaxy (MBE) on (100) oriented CdTe/GaAs hybrid substrates [18]. The CdTe buffer layer was about 4 μm thick. In order to observe the effect of the growth temperature on morphological properties of the investigated structures they were grown at three significantly different temperatures of 230, 270 and 310 °C. With the exception of the growth temperature, the studied PbTe/CdTe structures were grown with exactly the same conditions and the same design parameters. All three structures were constructed using 25 repetitions of PbTe and CdTe layers; each PbTe layer was deposited for 60 s and each CdTe layer for 180 s. The resulting structures were capped by depositing CdTe for 300 s. Immediately after the deposition of the cap the substrate heater was switched off and its temperature began to decrease. The entire time of growth of the SL and the cap was 1 h 45 min. The molecular fluxes of elemental Pb, Cd and Te 2
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were supplied from standard effusion cells. Beam equivalent pressures (BEP) for Cd and Te were 1.1 × 10−6 and 1.0−6 Torr, respectively. Thus, the CdTe layers were deposited using a slight Cd overpressure. Using a Pb-flux BEP of 6.6 × 10−7 Torr and the unchanged Te-flux the PbTe layers were grown with a Te overpressure. The growth was monitored in situ by reflection high energy electron diffraction (RHEED). The temperature dependence of RHEED patterns observed in our experiments did not differ from those reported in [4]. Pure (2 × 1) RHEED pattern characterizes the PbTe growth at 230 °C. When the growth precedes at 270 or 310 °C strong (022) and (024) spots appear immediately upon deposition of the first PbTe layer. The spots originate from (111)-oriented PbTe domains and they stay clearly resolved during the entire growth, similarly as was reported in [4]. Structural investigations were carried out with high resolution x-ray diffraction (HRXRD) measurements performed with a Philips X’Pert MRD laboratory diffractometer at Cu Kα1 wavelength. The as-grown samples were studied by TEM. The specimens were prepared by a mechanical prethinning followed by Ar ion milling. The conventional and high-resolution transmission electron microscopy observations were performed in a TITAN CUBED 80–300 microscope with aberration correction of objective lens, operating at 300 kV. Scanning transmission electron microscopy (STEM) was done with the use of high angle annular dark field detector. Elemental mapping was performed by energydispersive x-ray (EDX) in scanning TEM mode. We studied the temperature dependence of PL of the PbTe/CdTe heterostructures. The PL was excited by the first harmonic (1064 nm) of the radiation emitted by a pulsed YAG:Nd laser with a single pulse duration of 6 ns and repetition frequency of 10 Hz. The energy of laser excitation was 1.17 eV, i.e., above the band gap of PbTe but below the band gap of CdTe. A HgCdZnTe infrared diode detector was used to detect the infrared PL signal in the wavelength range from 2 to 6 μm.
Figure 1. θ–2 θ diffractogram of the intended 25 × (60 s PbTe/180 s CdTe) superlattice sequence at different substrate temperatures (indicated next to the graph). All graphs show a peak surrounded by oscillations with different periods corresponding to the layer thicknesses. In addition the weak CdTe(002) is visible in all graphs on the left side of the main peak. The graphs are shifted by 1000 cps each for clarity.
structures grown at elevated temperatures do not show the SL-like character. The HRXRD spectra indicate that the 270 °C and 310 °C structures consist of rather thick films, without any additional 2D periodicity. The positions of the central lines correspond to the vertical lattice constants of the films of 0.6462 and 0.6456 nm for 270 °C and 310 °C structures, respectively. The former value perfectly agrees with the lattice constant of unstrained PbTe. The later indicates that the material is under tensile strain in horizontal directions. From the clearly visible fringes on both sides of the central peak one can determine the thickness of the films: 570 and 207 nm, respectively indicating a strong reduction of the effective growth rate with increasing substrate temperature. The peculiarities of the XRD patterns become obvious in light of the TEM data. The TEM image of the 230 °C structure is shown in figure 2(a). The dark strips correspond to CdTe layers, the bright ones to the PbTe deposits as confirmed by EDX microanalysis. The distribution maps of Cd, Te and Pb obtained by EDX confirming the SL-like character of the structure are shown in figures 2(c)–(e), respectively. However, the layers located closer to the substrate are less perfect than those located closer to the structure surface, as illustrated in more details in figure 3. Figure 3 presents a high resolution TEM image of a fragment of the structure located in the close vicinity of the CdTe substrate visible in the lefthand side lower corner of the picture. Figure 3. provides a clear evidence that although the structure nearby the substrate is not perfectly ordered, the interfaces between PbTe and CdTe layers are sharp on the atomic scale. The imperfection is due to diffusion during growth—the layers deposited first stayed at the growth temperature much longer (1 h and 45 min) than those deposited later. This means that the topological transformations reported by Groiss et al in [1] of initially 2D PbTe layers into a 1D percolation network and
3. Results and discussion In spite of the same structure design, the HRXRD patterns of structures grown at various temperatures are very different, as shown in figure 1. The only common features for all the patterns are the CdTe (002) diffraction peak at 27.5° and the central peak at around 27.6° ascribed to PbTe. The PbTe related peak dominates the diffraction spectra in spite of the fact, that the combined PbTe deposit is much thinner than the combined thickness of all CdTe layers. However, because of different Bravais lattices and very different coefficients of atomic scattering of x-rays for PbTe and CdTe, the (002) xray reflex for PbTe is expected to be about 2500 times more intense than the (002) reflex for CdTe. The x-ray diffraction pattern of the low-temperature sample (230 °C) exhibits a SL-like, 2D character with a central peak at about 27.6° and satellite peaks. The distance between the satellite peaks, 0.331°, is determined by the SL period of 28.2 nm. This value coincides well with the period expected from the growth parameters. Surprisingly, the two 3
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Figure 2. (a) Scanning transmission electron microscopy (STEM) image of the PbTe/CdTe multilayer structure grown at 230 °C. The growth proceeded from left to right. (b) Magnified STEM image of the area marked by white rectangle in figure (a). (c)–(e) Energy-dispersive x-ray (EDX) mappings over the marked area of elemental Cd, Te, and Pb, respectively.
The TEM image presented in figure 4 of the nominally identical structure (25 times 60 s of PbTe and 180 s of CdTe) grown, however, at a much higher temperature of 310 °C looks quite different. Although, all other growth parameters, as effusion cell temperatures, molecular fluxes etc were identical to those in the previous case, there is no sign of a 2D SL or any periodic ordering. Both constituents, PbTe and CdTe, were identified not only by the TEM imaging but also by detailed EDS study (not shown). They appear to be almost completely separated one from the other. The PbTe deposit forms a thick layer on the CdTe buffer. The CdTe layers deposited during the SL growth aggregate on the top of the PbTe layer and form pyramidal-shape inclusions of irregular sizes. The estimated average thickness of the CdTe inclusions at the sample surface is in the order of 80 nm, i.e., the volume of the CdTe visible on the surface is much lower than the amount of CdTe deposited in the 230 °C structure. In the latter case the total thickness of all CdTe layers and of the CdTe cap add up to about 560 nm. This means that about 480 nm of CdTe desorbed from the sample surface in spite of the fact that growth temperature of 310 °C is quite normal for CdTe epitaxy. On the other hand, the PbTe layer is about 200 nm thick, what is in perfect agreement with the total thickness of the PbTe deposit in the 230 °C structure. The TEM results suggest, that at 310 °C the CdTe migrates toward the sample surface and effectively desorbs from there. This would indicate that the sticking coefficient of CdTe on PbTe surface is very low in comparison to that on CdTe or GaAs surfaces. The most intriguing case of PbTe/CdTe structure is represented by a TEM cross-sectional image shown in figure 5. The structure was grown at the intermediate temperature of 270 °C. The expected 2D, SL-like horizontal structure is not recognizable. Instead, the structure is
Figure 3. High resolution TEM image of the PbTe/CdTe multilayer
structure grown at 230 °C. The image shows a fragment of the structure located nearby the CdTe substrate visible at the left-hand side lower corner of the figure. The PbTe layers are dark, the CdTe layers are bright.
then separate 0D QDs start even at temperatures as low as 230 °C. In spite of this slight disorder, the 2D character of the entire structure is obvious. The average thickness of PbTe layers calculated from TEM data is 8.0 nm and of CdTe layers 21.1 nm. The deposition rates for both materials are thus, 1.33 A s−1 and 1.17 A s−1, respectively. The SL period based on TEM results is equal to 29.1 nm, i.e., in fair agreement with 28.2 nm obtained from XRD measurements shown in figure 1. 4
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Nanotechnology 26 (2015) 135601
and subsequently into an array of 0D QDs. The transitions are caused by the fact that PbTe and CdTe are immiscible. Because of that the observed topological transitions can be regarded as spinoidal decomposition process and thus described by the Cahn–Hillard model [19, 20]. According to the Cahn–Hillard model the diameters of the 1D PbTe structures and 0D PbTe QDs are strictly correlated with the initial thickness of the 2D PbTe layer, d, by the formula: z = (3 − D) × d, where D is the dimensionality class (D = 0 for QDs). In the discussed case, the thickness of the unperturbed PbTe layers, d = 8.0 nm, is smaller than the thickness of the neighboring CdTe layers (21.1 nm). However, when PbTe undergo transition to 0D objects their expected diameters (z = 3 × 8 = 24 nm) exceed the thickness of CdTe layers. In result the PbTe QDs start to merge vertically over the entire structure thickness. Such process leads to the formation of vertical pillars and columns. The vertical pillar- and column-like structures visible in figure 5 are approximately ten times larger (diameters in the range of 80 nm), than the initial horizontal layers. Thus growing at higher temperatures (here: 310 °C) leads to formation of larger objects and finally to the full separation of the both materials, as shown in figure 4. The most striking feature of the 270 °C structure is its very strong PL emission in the entire temperature range from liquid He temperatures up to room temperatures. Figure 7 shows PL spectra for the 270 °C structure recorded as a function of temperature. At low temperatures strong PL signal is detected at about 4.95 μm (250 meV). The full width at half maximum (FWHM) of the spectrum is 0.4 μm, which corresponds to 17 meV in energy terms. With the increasing temperature the emission peak narrows down to 7 meV and shifts towards higher energies reaching 3.49 μm (355 meV) at T = 290 K as shown in figure 8. The FWHM narrowing with increasing temperature can be explained in terms of a suppression of the optical recombination in small PbTe inclusions. With increasing temperature, photoexcited carriers, mostly holes, will escape first from small localizing center, because the binding energy in smaller inclusions is lower than in larger ones. The carriers may be captured by neighboring larger PbTe inclusions. As a result, with growing temperature the luminescence from larger objects increases while the signal from smaller objects is quenched, leading to the observed narrowing of the luminescence line. Analogs effects are observed in ensembles of QDs [21]. The temperature coefficient of the peak emission, 0.44 meV K−1 is in agreement with those of the PbTe energy gap. It is interesting to note that the PL energy is much lower than in the case of reported PbTe QDs embedded in CdTe [2, 4–6]. This indicates that the observed PL originates from much larger objects in which the quantum confinement effect is weaker [6]. The temperature dependence of the peak emission shown in figure 8 is in very good agreement with theoretical prediction for strained PbTe quantum objects of the size of 80–100 nm [6]. At low temperatures the experimental data points deviate from the theoretically predicted line and tend to saturate at a higher energy. The saturation is probably caused by an effective collection of the excited
Figure 4. TEM bright field image of the PbTe/CdTe structure grown at 310 °C. EDX analysis (not shown) indicates that CdTe deposited by molecular beam epitaxy aggregates at the top of the thick PbTe layer. The both materials are completely separated.
dominated by perpendicular, with respect to the substrate plane, column- or pillar-like objects. In the vicinity of the CdTe substrate QDs are also visible. In the HRTEM image shown in figure 6 a fully developed CdTe QD appears as a bright hexagon with diameter of about 86 nm. The QD has a highly symmetric shape of a rhombo–cubo–octahedron with surface facets of {100}, {110} and {111} crystal directions. Such a shape is obtained when all three interface energies become equal and the QD is formed in thermal equilibrium [2]. Besides the highly symmetric QD and pillar-like objects, figures 5 and 6 reveal the presence of elongated dots. The CdTe QDs are visible as bright spots and the PbTe QDs as dark ones. However, in contrast to previous observations [2], in this case the QDs are elongated perpendicularly to the growth direction. Because of the unusual, intruding morphology of the 270 °C structure we repeated the growth several times, always obtaining very similar results. The peculiar cross-section of the PbTe/CdTe 270 °C structure results from the topological transformations observed and reported by Groiss et al [1] in the 2D case. Upon annealing at the growth temperature, the initially flat PbTe layers start to undergo transitions into a quasi 1D percolation network of multiply interconnected 1D PbTe wires 5
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Nanotechnology 26 (2015) 135601
Figure 5. (a) STEM image of the PbTe/CdTe multilayer structure grown at 270 °C (a). Magnified TEM image of the area marked by white
rectangle in figure (a). (c)–(e) Energy-dispersive x-ray (EDX) mappings over the marked area of elemental Cd, Te, and Pb, respectively.
Figure 7. Temperature dependence of the PL spectra of the PbTe/
CdTe structure grown at the intermediate temperature of 270 °C.
Figure 6. High resolution TEM image of the PbTe/CdTe multilayer structure grown at 270 °C. The substrate is visible at the top right hand side corner. A fully developed, highly symmetric CdTe, elongated CdTe and PbTe QDs as well as column-like structures are visualized.
carriers in conduction band minima and an increase of the quasi-Fermi level, i.e., the Burstein–Moss effect. It is interesting to note, that the 230 °C structure, which exhibits the 2D SL-like order does not show any room temperature PL signal. The lack of PL signal in the 2D structure confirms previous observation that the intensity of the PL signal strongly depends on the thickness of the PbTe QW and for wells thinner than about 10 nm the PL disappears at elevated temperatures [16]. The temperature quenching of the PL signal of the 230 °C structure is caused by the low binding
Figure 8. Energy of the peak PL line as a function of temperature for the PbTe/CdTe structure grown at 270 °C. 6
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Nanotechnology 26 (2015) 135601
energy of photoexcited holes in thin 2D PbTe layers. The PL signal of the high-temperature, 310 °C structure can be observed over the entire temperature range, however, its intensity is more than one order of magnitude weaker than those of the 270 °C structure. Summarizing, we showed that the growth temperature determines morphological and optical properties of PbTe/ CdTe multilayer structures. The thermal energy supplied to the structures during the MBE growth enables morphological transitions leading to formation of pillar-like structures at intermediate temperatures and at higher temperatures to complete separation of the two immiscible materials. The observed transitions are analog of the spinoidal decomposition process on a nanoscale.
[3] Leitsmann R, Ramos L E and Bechstedt F 2006 Phys. Rev. B 74 085309 [4] Koike K, Harada H, Itakura T, Yano M, Heiss W, Groiss H, Kaufmann E, Hesser G and Schäffler F 2007 J. Cryst. Growth 301 722 [5] Grois H et al 2007 Appl. Phys. Lett. 91 222106 [6] Schwarzl T, Kaufmann E, Springholz G, Koike K, Hotei T, Yano M and Heiss W 2008 Phys. Rev. B 78 165320 [7] Leitsmann R and Bechstedt F 2009 Phys. Rev. B 80 165402 [8] Hochreiner A, Schwarzl T, Eibelhuber M, Heiss W, Springholz G, Kolkovsky V, Karczewski G and Wojtowicz T 2011 Appl. Phys. Lett. 98 021106 [9] Springholz G, Bauer G and Ihninger G 1993 J. Cryst. Growth 127 302 [10] Suzuki N and Adachi S 1994 Japan. J. Appl. Phys. 33 193 [11] Rappl P H O, Closs H, Ferreira S O, Abramof E, Boschetti C, Moitsuke P, Ueta A Y and Bandeira I N 1998 J. Cryst. Growth 191 466 [12] Ishida A, Ohashi T, Wang S, Tsuchiya T, Ishino K, Inoue Y and Fujiyasu H 2002 Japan. J. Appl. Phys. 41 3655 [13] Schwarzl T, Heiss W and Springholz G 1999 Appl. Phys. Lett. 75 1246 [14] Koike K, Honden T, Makabe I, Yan F P and Yano M 2003 J. Cryst. Growth 257 212 [15] Yano M, Makabe I and Koike K 2004 Physica E 20 449 [16] Szot M et al 2008 Acta Phys. Pol. A 114 1391 [17] Si J, Jin S, Zhang H, Zhu P, Qiu D and Wu H 2008 Appl. Phys. Lett. 93 202101 [18] Karczewski G, Jaroszynski J, Barcz A, Kutrowski M, Wojtowicz T and Kossut J 1998 J. Cryst. Growth 184 814 [19] de Gennes P G 1985 Rev. Mod. Phys. 57 827 [20] Binder K and Franzl P 2001 Phase Transformations in Materials ed G Kostorz (Weiheim, Germany: Wiley,VCH) ch 6 [21] Karczewski G, Mackowski S, Kutrowski M, Wojtowicz T and Kossut J 1999 Appl. Phys. Lett. 74 3011
Acknowledgments This work was supported by the Foundation for Polish Science by the Master program. The TEM investigations have been supported by European Regional Development Fund through the Innovative Economy grant (POIG.02.01-00-14-032/08).
References [1] Groiss H, Daruka I, Koike K, Yano M, Hesser G, Springholz G, Zakharov N, Werner P and Schäffler F 2014 APL Mater. 2 012105 [2] Heiss W, Groiss H, Kaufmann E, Hesser H, Böberl M, Springholz G, Schäffler F, Koike K, Harada H and Yano M 2006 Appl. Phys. Lett. 88 192109
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Nanoscale morphology of multilayer PbTe/CdTe heterostructures and its effect on photoluminescence properties
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 135601 (http://iopscience.iop.org/0957-4484/26/13/135601) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 169.230.243.252 This content was downloaded on 20/03/2015 at 21:38
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 26 (2015) 135601 (7pp)
doi:10.1088/0957-4484/26/13/135601
Nanoscale morphology of multilayer PbTe/ CdTe heterostructures and its effect on photoluminescence properties G Karczewski1, M Szot1, S Kret1, L Kowalczyk1, S Chusnutdinow1, T Wojtowicz1, S Schreyeck2, K Brunner2, C Schumacher2 and L W Molenkamp2 1 2
Institute of Physics, Polish Academy of Science, al. Lotników 32/46, 02-668 Warszawa, Poland Physikalisches Institut (EP III), Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany
E-mail: [email protected] Received 16 July 2014, revised 14 November 2014 Accepted for publication 19 November 2014 Published 9 March 2015 Abstract
We study nanoscale morphology of PbTe/CdTe multilayer heterostuctures grown by molecular beam epitaxy on hybrid GaAs/CdTe (100) substrates. Nominally, the structures consist of 25 repetitions of subsequently deposited CdTe and PbTe layers with comparable thicknesses of 21 and 8 nm, respectively. However, the morphology of the resulting structures crucially depends on the growth temperature. The two-dimensional layered, superlattice-like character of the structures remains preserved only when grown at low substrate temperatures, such as 230 °C. The samples grown at the slightly elevated temperature of 270 °C undergo a morphological transformation to structures consisting of CdTe and PbTe pillars and columns oriented perpendicular to the substrate. Although the pillar-like objects are of various shapes and dimensions these structures exhibit exceptionally strong photoluminescence in the near infrared spectral region. At the higher growth temperature of 310 °C, PbTe and CdTe separate completely forming thick layers oriented longitudinally to the substrate plane. The observed topological transformations are driven by thermally activated atomic diffusion in the solid state phase. The solid state phase remains fully coherent during the processes. The observed topological transitions leading to the material separation in PbTe/CdTe system could be regarded as an analog of spinodal decomposition of an immiscible solid state solution and thus they can be qualitatively described by the Cahn–Hillard model as proposed by Groiss et al (2014 APL Mater. 2 012105). Keywords: nanoscale morphology, multilayer structures, photoluminescence (Some figures may appear in colour only in the online journal) 1. Introduction
dimensional (1D) percolation network and subsequently into an array of zero-dimensional (0D) PbTe quantum dots (QDs). The separation process proceeds in a fully coherent solid and is driven by thermally activated bulk diffusion. The PbTe/ CdTe material system is thus regarded as a prototype system for self-organized QD formation based on solid-state material separation [1–8]. The present research can be considered as a continuation and expansion of the 2D morphological study of Groiss et al [1] to the 3D, bulk-like case. The investigations of Groiss et al
Morphological transformations of a single, two-dimensional (2D) lead telluride (PbTe) layer embedded in a cadmium telluride (CdTe) matrix have been recently studied by realtime transmission electron microscopy (TEM) [1]. In the almost completely immiscible PbTe/CdTe material system, both constituents tend to separate causing structural, morphological transformations of the as-grown structure. Initially, the PbTe layer undergoes transformations into a one0957-4484/15/135601+07$33.00
1
© 2015 IOP Publishing Ltd Printed in the UK
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were devoted to morphological changes of a single thin layer of PbTe embedded in CdTe. Here, by steady-state, crosssectional TEM we study the morphology of quasi-bulk samples of a thick multilayer stack of PbTe/CdTe layers. In particular, we study the influence of the growth temperature on the morphology of the quasi-bulk samples. The multilayer structures were grown such that the volumes of the PbTe and CdTe deposits were nominally comparable. We show that only at the relatively low growth temperatures (here, 230 °C) the PbTe/CdTe superlattices (SLs) preserve their 2D-like character. At an intermediate growth temperature of 270 °C the material separation process leads to aggregation of PbTe deposits initially to large QDs and subsequently to long PbTe pillars surrounded coherently by CdTe. Surprisingly, the pillar- or column-like structures are oriented perpendicularly to the sample surface. When the growth temperature becomes high enough (over 310 °C) the two materials tend to separate completely and CdTe desorbs from the sample surface. We show that the growth temperature and thus the morphology of the material plays a crucial role in controlling the photoluminescence (PL) properties of the samples. PbTe belongs to the group of IV–VI narrow-band gap semiconductors. Its direct energy gap is located at the L-point of the Brillouin zone and at liquid helium temperatures it is equal to 0.187 eV. Because of the large value of the dielectric constant, ε ∼ 1000, and low effective masses, the carrier mobility in PbTe is high, approaching 104 cm2 (V−1 s−1) at room temperature and 106 cm2 (V−1 s−1) at cryogenic temperatures [9–11]. Auger recombination of excited carriers in PbTe is small in comparison to other semiconductors with similar band gap which makes PbTe a very attractive candidate for mid-infrared optoelectronic devices such as laser diodes and mid-infrared detectors [11–13]. CdTe, on the other hand, belongs to the group of II–VI wide-band gap semiconductors with an energy gap of 1.59 eV at liquid He temperatures and much lower dielectric constant of ε = 10.2. Both PbTe and CdTe crystallize in a cubic crystal structure and have closely matched lattice constants of 0.6462 and 0.6480 nm, respectively [14]. However, the crystal structures of the two materials are significantly different. In CdTe each atom has four nearest neighbors resulting in the zinc-blende structure with the space group F-43m (T2d). The less covalent PbTe has the rock-salt structure with the space group Fm3m (O5h) and sixfold coordinated atoms. Because of the latticetype mismatch the two materials are unable to form an alloy [1–8]. It is this immiscibility that leads to material separation in the PbTe/CdTe system. The immiscibility of PbTe and CdTe is employed in the formation of high-quality nanostructures. Most of the reports concern 0D structures, i.e., PbTe QDs embedded in a CdTe matrix. The dots are formed in two steps. Firstly, a few atomic monolayers of PbTe are deposited on a high quality CdTe substrate and subsequently capped by a CdTe layer. In the second step the layered structure is annealed for a few minutes in an inert atmosphere. The result of the anneal is the transformation of the PbTe layer into an array of QDs. The dots are coherently embedded in the crystal matrix of CdTe, almost free of intermixing, with highly symmetric shapes and
a height-to-width ratio close to 1 [2–8]. Such PbTe QDs in a CdTe matrix display strong mid-infrared optical emission [4–7]. Recently, such PbTe QDs were introduced into a CdTe p–n junction resulting in the first demonstration of PbTe/ CdTe electroluminescence diodes emitting in the mid-infrared spectral region up to the room temperature [8]. Photo- and electroluminescence measurements showed, however, an unusual temperature dependence of the optical emission intensity. At low temperatures the PL intensity significantly weakens in contrast to the PL intensity of III–V and II–VI QDs for which the PL intensity stays constant in the low temperature region. 2D PbTe/CdTe nanostructures—quantum wells (QWs) and SLs—have attracted less attention so far [15, 16]. Despite the crystal-structure mismatch an intense emission from QWs was observed in the mid-infrared region. The energy of the emission peak showed a blue-shift upon decreasing the QW width and increase of temperature, indicating that the emission results from electron–hole recombination in a type I QW. A type I band ordering is also confirmed by x-ray photoemission spectroscopy [17]. No reports on classical or quantum transport in PbTe/CdTe QWs have been reported so far. Most probably the lack of transport data is caused by the low quality of the PbTe/CdTe interfaces. In spite of the large dielectric constant of PbTe, screening of charged defects located at interfaces in ultra-thin QWs may still be insufficient to assure high carrier mobility. In addition, the carrier concentration in PbTe/CdTe QWs appears to be surprisingly low, taking into consideration that in typical, nominally undoped PbTe bulk samples it is as high as 1018 cm−3. The reduction of the carrier concentration is most probably caused by the capture of mobile carriers by dangling bonds at the QWs surfaces and/or by surface potential fluctuations. A vital requirement for the successful study of electron transport in this kind of structures is an improvement in the crystalline and thus the electrical properties of the PbTe/CdTe interfaces.
2. Experimental The investigated PbTe multilayer structures were grown by molecular beam epitaxy (MBE) on (100) oriented CdTe/GaAs hybrid substrates [18]. The CdTe buffer layer was about 4 μm thick. In order to observe the effect of the growth temperature on morphological properties of the investigated structures they were grown at three significantly different temperatures of 230, 270 and 310 °C. With the exception of the growth temperature, the studied PbTe/CdTe structures were grown with exactly the same conditions and the same design parameters. All three structures were constructed using 25 repetitions of PbTe and CdTe layers; each PbTe layer was deposited for 60 s and each CdTe layer for 180 s. The resulting structures were capped by depositing CdTe for 300 s. Immediately after the deposition of the cap the substrate heater was switched off and its temperature began to decrease. The entire time of growth of the SL and the cap was 1 h 45 min. The molecular fluxes of elemental Pb, Cd and Te 2
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were supplied from standard effusion cells. Beam equivalent pressures (BEP) for Cd and Te were 1.1 × 10−6 and 1.0−6 Torr, respectively. Thus, the CdTe layers were deposited using a slight Cd overpressure. Using a Pb-flux BEP of 6.6 × 10−7 Torr and the unchanged Te-flux the PbTe layers were grown with a Te overpressure. The growth was monitored in situ by reflection high energy electron diffraction (RHEED). The temperature dependence of RHEED patterns observed in our experiments did not differ from those reported in [4]. Pure (2 × 1) RHEED pattern characterizes the PbTe growth at 230 °C. When the growth precedes at 270 or 310 °C strong (022) and (024) spots appear immediately upon deposition of the first PbTe layer. The spots originate from (111)-oriented PbTe domains and they stay clearly resolved during the entire growth, similarly as was reported in [4]. Structural investigations were carried out with high resolution x-ray diffraction (HRXRD) measurements performed with a Philips X’Pert MRD laboratory diffractometer at Cu Kα1 wavelength. The as-grown samples were studied by TEM. The specimens were prepared by a mechanical prethinning followed by Ar ion milling. The conventional and high-resolution transmission electron microscopy observations were performed in a TITAN CUBED 80–300 microscope with aberration correction of objective lens, operating at 300 kV. Scanning transmission electron microscopy (STEM) was done with the use of high angle annular dark field detector. Elemental mapping was performed by energydispersive x-ray (EDX) in scanning TEM mode. We studied the temperature dependence of PL of the PbTe/CdTe heterostructures. The PL was excited by the first harmonic (1064 nm) of the radiation emitted by a pulsed YAG:Nd laser with a single pulse duration of 6 ns and repetition frequency of 10 Hz. The energy of laser excitation was 1.17 eV, i.e., above the band gap of PbTe but below the band gap of CdTe. A HgCdZnTe infrared diode detector was used to detect the infrared PL signal in the wavelength range from 2 to 6 μm.
Figure 1. θ–2 θ diffractogram of the intended 25 × (60 s PbTe/180 s CdTe) superlattice sequence at different substrate temperatures (indicated next to the graph). All graphs show a peak surrounded by oscillations with different periods corresponding to the layer thicknesses. In addition the weak CdTe(002) is visible in all graphs on the left side of the main peak. The graphs are shifted by 1000 cps each for clarity.
structures grown at elevated temperatures do not show the SL-like character. The HRXRD spectra indicate that the 270 °C and 310 °C structures consist of rather thick films, without any additional 2D periodicity. The positions of the central lines correspond to the vertical lattice constants of the films of 0.6462 and 0.6456 nm for 270 °C and 310 °C structures, respectively. The former value perfectly agrees with the lattice constant of unstrained PbTe. The later indicates that the material is under tensile strain in horizontal directions. From the clearly visible fringes on both sides of the central peak one can determine the thickness of the films: 570 and 207 nm, respectively indicating a strong reduction of the effective growth rate with increasing substrate temperature. The peculiarities of the XRD patterns become obvious in light of the TEM data. The TEM image of the 230 °C structure is shown in figure 2(a). The dark strips correspond to CdTe layers, the bright ones to the PbTe deposits as confirmed by EDX microanalysis. The distribution maps of Cd, Te and Pb obtained by EDX confirming the SL-like character of the structure are shown in figures 2(c)–(e), respectively. However, the layers located closer to the substrate are less perfect than those located closer to the structure surface, as illustrated in more details in figure 3. Figure 3 presents a high resolution TEM image of a fragment of the structure located in the close vicinity of the CdTe substrate visible in the lefthand side lower corner of the picture. Figure 3. provides a clear evidence that although the structure nearby the substrate is not perfectly ordered, the interfaces between PbTe and CdTe layers are sharp on the atomic scale. The imperfection is due to diffusion during growth—the layers deposited first stayed at the growth temperature much longer (1 h and 45 min) than those deposited later. This means that the topological transformations reported by Groiss et al in [1] of initially 2D PbTe layers into a 1D percolation network and
3. Results and discussion In spite of the same structure design, the HRXRD patterns of structures grown at various temperatures are very different, as shown in figure 1. The only common features for all the patterns are the CdTe (002) diffraction peak at 27.5° and the central peak at around 27.6° ascribed to PbTe. The PbTe related peak dominates the diffraction spectra in spite of the fact, that the combined PbTe deposit is much thinner than the combined thickness of all CdTe layers. However, because of different Bravais lattices and very different coefficients of atomic scattering of x-rays for PbTe and CdTe, the (002) xray reflex for PbTe is expected to be about 2500 times more intense than the (002) reflex for CdTe. The x-ray diffraction pattern of the low-temperature sample (230 °C) exhibits a SL-like, 2D character with a central peak at about 27.6° and satellite peaks. The distance between the satellite peaks, 0.331°, is determined by the SL period of 28.2 nm. This value coincides well with the period expected from the growth parameters. Surprisingly, the two 3
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Figure 2. (a) Scanning transmission electron microscopy (STEM) image of the PbTe/CdTe multilayer structure grown at 230 °C. The growth proceeded from left to right. (b) Magnified STEM image of the area marked by white rectangle in figure (a). (c)–(e) Energy-dispersive x-ray (EDX) mappings over the marked area of elemental Cd, Te, and Pb, respectively.
The TEM image presented in figure 4 of the nominally identical structure (25 times 60 s of PbTe and 180 s of CdTe) grown, however, at a much higher temperature of 310 °C looks quite different. Although, all other growth parameters, as effusion cell temperatures, molecular fluxes etc were identical to those in the previous case, there is no sign of a 2D SL or any periodic ordering. Both constituents, PbTe and CdTe, were identified not only by the TEM imaging but also by detailed EDS study (not shown). They appear to be almost completely separated one from the other. The PbTe deposit forms a thick layer on the CdTe buffer. The CdTe layers deposited during the SL growth aggregate on the top of the PbTe layer and form pyramidal-shape inclusions of irregular sizes. The estimated average thickness of the CdTe inclusions at the sample surface is in the order of 80 nm, i.e., the volume of the CdTe visible on the surface is much lower than the amount of CdTe deposited in the 230 °C structure. In the latter case the total thickness of all CdTe layers and of the CdTe cap add up to about 560 nm. This means that about 480 nm of CdTe desorbed from the sample surface in spite of the fact that growth temperature of 310 °C is quite normal for CdTe epitaxy. On the other hand, the PbTe layer is about 200 nm thick, what is in perfect agreement with the total thickness of the PbTe deposit in the 230 °C structure. The TEM results suggest, that at 310 °C the CdTe migrates toward the sample surface and effectively desorbs from there. This would indicate that the sticking coefficient of CdTe on PbTe surface is very low in comparison to that on CdTe or GaAs surfaces. The most intriguing case of PbTe/CdTe structure is represented by a TEM cross-sectional image shown in figure 5. The structure was grown at the intermediate temperature of 270 °C. The expected 2D, SL-like horizontal structure is not recognizable. Instead, the structure is
Figure 3. High resolution TEM image of the PbTe/CdTe multilayer
structure grown at 230 °C. The image shows a fragment of the structure located nearby the CdTe substrate visible at the left-hand side lower corner of the figure. The PbTe layers are dark, the CdTe layers are bright.
then separate 0D QDs start even at temperatures as low as 230 °C. In spite of this slight disorder, the 2D character of the entire structure is obvious. The average thickness of PbTe layers calculated from TEM data is 8.0 nm and of CdTe layers 21.1 nm. The deposition rates for both materials are thus, 1.33 A s−1 and 1.17 A s−1, respectively. The SL period based on TEM results is equal to 29.1 nm, i.e., in fair agreement with 28.2 nm obtained from XRD measurements shown in figure 1. 4
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and subsequently into an array of 0D QDs. The transitions are caused by the fact that PbTe and CdTe are immiscible. Because of that the observed topological transitions can be regarded as spinoidal decomposition process and thus described by the Cahn–Hillard model [19, 20]. According to the Cahn–Hillard model the diameters of the 1D PbTe structures and 0D PbTe QDs are strictly correlated with the initial thickness of the 2D PbTe layer, d, by the formula: z = (3 − D) × d, where D is the dimensionality class (D = 0 for QDs). In the discussed case, the thickness of the unperturbed PbTe layers, d = 8.0 nm, is smaller than the thickness of the neighboring CdTe layers (21.1 nm). However, when PbTe undergo transition to 0D objects their expected diameters (z = 3 × 8 = 24 nm) exceed the thickness of CdTe layers. In result the PbTe QDs start to merge vertically over the entire structure thickness. Such process leads to the formation of vertical pillars and columns. The vertical pillar- and column-like structures visible in figure 5 are approximately ten times larger (diameters in the range of 80 nm), than the initial horizontal layers. Thus growing at higher temperatures (here: 310 °C) leads to formation of larger objects and finally to the full separation of the both materials, as shown in figure 4. The most striking feature of the 270 °C structure is its very strong PL emission in the entire temperature range from liquid He temperatures up to room temperatures. Figure 7 shows PL spectra for the 270 °C structure recorded as a function of temperature. At low temperatures strong PL signal is detected at about 4.95 μm (250 meV). The full width at half maximum (FWHM) of the spectrum is 0.4 μm, which corresponds to 17 meV in energy terms. With the increasing temperature the emission peak narrows down to 7 meV and shifts towards higher energies reaching 3.49 μm (355 meV) at T = 290 K as shown in figure 8. The FWHM narrowing with increasing temperature can be explained in terms of a suppression of the optical recombination in small PbTe inclusions. With increasing temperature, photoexcited carriers, mostly holes, will escape first from small localizing center, because the binding energy in smaller inclusions is lower than in larger ones. The carriers may be captured by neighboring larger PbTe inclusions. As a result, with growing temperature the luminescence from larger objects increases while the signal from smaller objects is quenched, leading to the observed narrowing of the luminescence line. Analogs effects are observed in ensembles of QDs [21]. The temperature coefficient of the peak emission, 0.44 meV K−1 is in agreement with those of the PbTe energy gap. It is interesting to note that the PL energy is much lower than in the case of reported PbTe QDs embedded in CdTe [2, 4–6]. This indicates that the observed PL originates from much larger objects in which the quantum confinement effect is weaker [6]. The temperature dependence of the peak emission shown in figure 8 is in very good agreement with theoretical prediction for strained PbTe quantum objects of the size of 80–100 nm [6]. At low temperatures the experimental data points deviate from the theoretically predicted line and tend to saturate at a higher energy. The saturation is probably caused by an effective collection of the excited
Figure 4. TEM bright field image of the PbTe/CdTe structure grown at 310 °C. EDX analysis (not shown) indicates that CdTe deposited by molecular beam epitaxy aggregates at the top of the thick PbTe layer. The both materials are completely separated.
dominated by perpendicular, with respect to the substrate plane, column- or pillar-like objects. In the vicinity of the CdTe substrate QDs are also visible. In the HRTEM image shown in figure 6 a fully developed CdTe QD appears as a bright hexagon with diameter of about 86 nm. The QD has a highly symmetric shape of a rhombo–cubo–octahedron with surface facets of {100}, {110} and {111} crystal directions. Such a shape is obtained when all three interface energies become equal and the QD is formed in thermal equilibrium [2]. Besides the highly symmetric QD and pillar-like objects, figures 5 and 6 reveal the presence of elongated dots. The CdTe QDs are visible as bright spots and the PbTe QDs as dark ones. However, in contrast to previous observations [2], in this case the QDs are elongated perpendicularly to the growth direction. Because of the unusual, intruding morphology of the 270 °C structure we repeated the growth several times, always obtaining very similar results. The peculiar cross-section of the PbTe/CdTe 270 °C structure results from the topological transformations observed and reported by Groiss et al [1] in the 2D case. Upon annealing at the growth temperature, the initially flat PbTe layers start to undergo transitions into a quasi 1D percolation network of multiply interconnected 1D PbTe wires 5
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Figure 5. (a) STEM image of the PbTe/CdTe multilayer structure grown at 270 °C (a). Magnified TEM image of the area marked by white
rectangle in figure (a). (c)–(e) Energy-dispersive x-ray (EDX) mappings over the marked area of elemental Cd, Te, and Pb, respectively.
Figure 7. Temperature dependence of the PL spectra of the PbTe/
CdTe structure grown at the intermediate temperature of 270 °C.
Figure 6. High resolution TEM image of the PbTe/CdTe multilayer structure grown at 270 °C. The substrate is visible at the top right hand side corner. A fully developed, highly symmetric CdTe, elongated CdTe and PbTe QDs as well as column-like structures are visualized.
carriers in conduction band minima and an increase of the quasi-Fermi level, i.e., the Burstein–Moss effect. It is interesting to note, that the 230 °C structure, which exhibits the 2D SL-like order does not show any room temperature PL signal. The lack of PL signal in the 2D structure confirms previous observation that the intensity of the PL signal strongly depends on the thickness of the PbTe QW and for wells thinner than about 10 nm the PL disappears at elevated temperatures [16]. The temperature quenching of the PL signal of the 230 °C structure is caused by the low binding
Figure 8. Energy of the peak PL line as a function of temperature for the PbTe/CdTe structure grown at 270 °C. 6
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energy of photoexcited holes in thin 2D PbTe layers. The PL signal of the high-temperature, 310 °C structure can be observed over the entire temperature range, however, its intensity is more than one order of magnitude weaker than those of the 270 °C structure. Summarizing, we showed that the growth temperature determines morphological and optical properties of PbTe/ CdTe multilayer structures. The thermal energy supplied to the structures during the MBE growth enables morphological transitions leading to formation of pillar-like structures at intermediate temperatures and at higher temperatures to complete separation of the two immiscible materials. The observed transitions are analog of the spinoidal decomposition process on a nanoscale.
[3] Leitsmann R, Ramos L E and Bechstedt F 2006 Phys. Rev. B 74 085309 [4] Koike K, Harada H, Itakura T, Yano M, Heiss W, Groiss H, Kaufmann E, Hesser G and Schäffler F 2007 J. Cryst. Growth 301 722 [5] Grois H et al 2007 Appl. Phys. Lett. 91 222106 [6] Schwarzl T, Kaufmann E, Springholz G, Koike K, Hotei T, Yano M and Heiss W 2008 Phys. Rev. B 78 165320 [7] Leitsmann R and Bechstedt F 2009 Phys. Rev. B 80 165402 [8] Hochreiner A, Schwarzl T, Eibelhuber M, Heiss W, Springholz G, Kolkovsky V, Karczewski G and Wojtowicz T 2011 Appl. Phys. Lett. 98 021106 [9] Springholz G, Bauer G and Ihninger G 1993 J. Cryst. Growth 127 302 [10] Suzuki N and Adachi S 1994 Japan. J. Appl. Phys. 33 193 [11] Rappl P H O, Closs H, Ferreira S O, Abramof E, Boschetti C, Moitsuke P, Ueta A Y and Bandeira I N 1998 J. Cryst. Growth 191 466 [12] Ishida A, Ohashi T, Wang S, Tsuchiya T, Ishino K, Inoue Y and Fujiyasu H 2002 Japan. J. Appl. Phys. 41 3655 [13] Schwarzl T, Heiss W and Springholz G 1999 Appl. Phys. Lett. 75 1246 [14] Koike K, Honden T, Makabe I, Yan F P and Yano M 2003 J. Cryst. Growth 257 212 [15] Yano M, Makabe I and Koike K 2004 Physica E 20 449 [16] Szot M et al 2008 Acta Phys. Pol. A 114 1391 [17] Si J, Jin S, Zhang H, Zhu P, Qiu D and Wu H 2008 Appl. Phys. Lett. 93 202101 [18] Karczewski G, Jaroszynski J, Barcz A, Kutrowski M, Wojtowicz T and Kossut J 1998 J. Cryst. Growth 184 814 [19] de Gennes P G 1985 Rev. Mod. Phys. 57 827 [20] Binder K and Franzl P 2001 Phase Transformations in Materials ed G Kostorz (Weiheim, Germany: Wiley,VCH) ch 6 [21] Karczewski G, Mackowski S, Kutrowski M, Wojtowicz T and Kossut J 1999 Appl. Phys. Lett. 74 3011
Acknowledgments This work was supported by the Foundation for Polish Science by the Master program. The TEM investigations have been supported by European Regional Development Fund through the Innovative Economy grant (POIG.02.01-00-14-032/08).
References [1] Groiss H, Daruka I, Koike K, Yano M, Hesser G, Springholz G, Zakharov N, Werner P and Schäffler F 2014 APL Mater. 2 012105 [2] Heiss W, Groiss H, Kaufmann E, Hesser H, Böberl M, Springholz G, Schäffler F, Koike K, Harada H and Yano M 2006 Appl. Phys. Lett. 88 192109
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