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Strain relaxation of CdTe on Ge studied by medium energy ion scattering
Nuclear Instruments and Methods in Physics Research B 384 (2016) 1–5
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Strain relaxation of CdTe on Ge studied by medium energy ion scattering J.C. Pillet a,b,⇑, F. Pierre a,c, D. Jalabert a,d a
Univ. Grenoble Alpes, CEA, LETI, MINATEC campus, F38000 Grenoble, France CEA, LETI, Département Optique et Photonique, F38054 Grenoble, France c CEA, LETI, Service de Caractérisation des Matériaux et Composants, F38054 Grenoble, France d CEA-INAC/UJF-Grenoble 1 UMR-E, SP2M, LEMMA, Minatec Grenoble F-38054, France b
a r t i c l e
i n f o
Article history: Received 28 January 2016 Received in revised form 24 June 2016 Accepted 29 July 2016
Keywords: CdTe Ge Substrates M.E.I.S Channeling Blocking
a b s t r a c t We have used the medium energy ion scattering (MEIS) technique to assess the strain relaxation in molecular-beam epitaxial (MBE) grown CdTe (2 1 1)/Ge (2 1 1) system. A previous X-ray diffraction study, on 10 samples of the same heterostructure having thicknesses ranging from 25 nm to 10 lm has allowed the measurement of the strain relaxation on a large scale. However, the X-ray diffraction measurements cannot achieve a stress measurement in close proximity to the CdTe/Ge interface at the nanometer scale. Due to the huge lattice misfit between the CdTe and Ge, a high degree of disorder is expected at the interface. The MEIS in channeling mode is a good alternative in order to profile defects with a high depth resolution. For a 21 nm thick CdTe layer, we observed, at the interface, a high density of Cd and/or Te atoms moved from their expected crystallographic positions followed by a rapid recombination of defects. Strain relaxation mechanisms in the vicinity of the interface are discussed Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction The replacement of CdZnTe (CZT) substrate by a germanium (Ge) substrate is a planned solution for the manufacture of infrared detectors based on CdHgTe (CMT) layers. Indeed, the sizes of available substrates (typically from 20 cm2 to 80 cm2 for CZT compared to 40 cm2 to 180 cm2 for Ge) as well as the manufacturing costs justify this interest. However, the huge lattice mismatch (14%) between CMT {aHgTe (6.46 Å [1])
quality of several CdTe buffers with different thicknesses has been studied in a previous work [4] by high resolution X-ray diffraction (HRXRD) for CdTe layers between 21 nm and a few microns. The aim of this work is to accurately determine the defects and lattice deformation profiles in the vicinity of the CdTe/Ge interface. These structural properties were analyzed on a 21 nm thick sample by MEIS (Medium Energy Ion Scattering), which, thanks to its high depth resolution and sensitivity to defects in channeling mode, is well adapted to investigate this type of heterostructure. The MEIS technique is similar to the RBS (Rutherford Backscattering Spectrometry) technique. Both are based on measuring the kinetic energy and the scattering angle of charged particles, usually protons or alpha particles, elastically scattered by the nuclei of the sample. In favorable cases, one can separate the elements according to their atomic mass and determine their depth distribution. While RBS is implemented with beam energies ranging from 0.5 to 5 MeV and a surface barrier detector with an energy resolution of 12–15 keV, MEIS uses particles with energies between 50 and 500 keV and an electrostatic analyzer with an energy resolution of 0.3% and an accuracy of about 0.1% for the scattering angle. Because the MEIS uses lower energy incident particles, the analysis focuses on the few tens of nanometers of the sample while, due to the higher energy of incident particles, RBS probe the sample over a few microns. In the most favorable cases, the MEIS depth resolution can reach 0.25 nm whereas the RBS depth resolution is of the order of 10 to 20 nm.
2
J.C. Pillet et al. / Nuclear Instruments and Methods in Physics Research B 384 (2016) 1–5
Since the electrostatic analyzer allows the simultaneous measurement of the energy and the scattering angle, over a 20° range, of the scattered particles, MEIS is well suited for in-depth structural investigations at the nanometric scale. 2. Experimental procedure After an initial ex-situ Ge (2 1 1) substrate preparation including an etching of the Ge substrate in a preparation solution (H3PO4: H2O2: H2O) [5], the material is rapidly introduced into a dedicated mercury MBE system and the substrate is heated at 650 °C to remove the native oxide layer [6]. During this process, the substrate is exposed to an As flux in order to saturate the Ge dangling bonds [7]. The growth is carried out in a RIBER (Paris, France) EPINEAT machine consisting of a suitable reactor for 4 in wafers of IIVI materials and equipped with standard effusion cells for CdTe, Te, Zn and As. The standard deposition method requires a slight Te overpressure and includes, at the end of growth, an annealing step at 400 °C during 1 h. The MEIS study of the CdTe/Ge interface was performed on the 21 nm thick CdTe sample with a monoenergetic proton beam delivered by the MEIS facility at MINATEC Grenoble, and by using two different scattering schematics, called channeling and blocking. Channeling occurs when the incident ion beam is aligned with a crystallographic axis of monocrystalline layers or substrates. In this configuration, the scattered yield is drastically reduced except for scattering by atoms displaced from their normal lattice positions, thus allowing the determination of defect depth profiles. Blocking is observed when ions scattered from deeper atoms suffer a second scattering from atoms at shallower depth. As a result, the angular distribution of the scattering yield exhibits a minimum, called blocking dip, at the angular position corresponding to the atom responsible for the second scattering. Any change in atomic positions, induced by stress for instance, gives rise to a shift of the blocking dip. The accurate measurement of these shifts as a function of depth provides the strain profile within the layer. The defect profile was performed with a 200 keV H+ beam incident at 20° with respect to the sample normal and the scattered particles were analyzed around an angle of 125°. The channeling experiments are very sensitive to defect. As a consequence, the choice of a higher energy minimizes the dechanneling of the ion bean passing through the disordered regions of the sample since a 21 nm layer is already thick for a MEIS experiment. The MEIS spectra in the CdTe layer as well as in the Ge substrate were recorded with the incident ion beam aligned to the [1 1 1] direction of the crystals. This channeling direction was chosen on the stereographic projection of the (2 1 1) oriented face-centered cubic crystal along the [1 1 1] direction found at 19, 47° from the sample’s normal (Fig. 1). An experimental measurement of the pole figure was achieved by MEIS [8] to identify the crystallographic orientation of the sample by comparing the mapping of H+ ions scattered by the Cd and Te atoms (Fig. 2) and the stereographic projection mentioned above. The precise orientation of the sample is then refined by small angular scans around the considered channeling direction. The deformation profile was obtained by using a 100 keV H+ beam and precisely locating the [2, 1, 1] blocking direction in the CdTe layer. Generally speaking, blocking experiments are not very sensitive to defects. Thereby, the choice of a low energy for these experiments has the advantage of maximizing the depth resolution. This technique relies on the accurate angular localization of a selected blocking dip as a function of depth [9,10]. Indeed, any lattice crystal deformation due to strain causes a blocking dip displacement relative to its value in a relaxed crystal. A random incident beam orientation was chosen in order to maximize the spectrum yield.
Fig. 1. Stereographic projection of a [2 1 1] face centered cubic crystal. The scattering angle is given by: h scattering = 180° – (h incident–h polar). The area bounded by the blue zone corresponds to the experimental mapping shown in Fig. 2. Within this blue area, four crystallographic directions (red dots) are also indicated in the experimental mapping shown on Fig. 2 in order to guide the eyes. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. Mapping of H+ ions scattered by the Cd and Te atoms to identify the crystallographic directions of the sample (red).
3. Results and discussions To start with, the MEIS technique is employed to assess the crystalline quality in an ultrathin CdTe layer at the nanometer scale. The measurement is based on a comparison of the MEIS spectrum obtained by aligning the incident ion beam on a selected crystallographic direction of the sample to that of a spectrum realized in a random direction. The ratio of the channeling to the random yields is called vmin. For an amorphous material, its value is equal to 1 and drop to a few 102 for a perfect crystal. The evolution of this parameter as a function of depth in the sample provides a depth profile of the crystalline quality. Surprisingly, the localization of the [1 1 1] direction within the CdTe layer does not
J.C. Pillet et al. / Nuclear Instruments and Methods in Physics Research B 384 (2016) 1–5
correspond to that of the Ge substrate. Indeed, a shift in the axis orientations of about 2° is observed between the layer and the substrate. However, it should be noted that no in-plane rotation (with accuracy better than 0.1°) has been found between layer and substrate. This out of plane misorientation of the epitaxial layer can be attributed to an elastic relaxation at the border of atomic steps that steers the atomic columns during the first growth stages. As a result, the vmin profiles (Fig. 3) in the CdTe layer and in the substrate were recorded in two slightly different geometries. We can observe on this figure a vmin value of about 22% in Ge while the substrate has a very high crystalline quality. Its vmin value should reach about 4% or 5% if at the surface sample. The use of a 100 keV beam would have led to a much higher vmin value in the substrate minimizing the defect sensitivity in the layer. The two channeled spectra as well as the random spectrum are shown in Fig. 4. The vmin profile within the Ge substrate along the [1 1 1] direction evidences a flat shape indicating the absence of defects in Ge. It should be noted that the relatively high vmin value in Ge is due to beam dechanneling in the CdTe layer. By contrast, the vmin profile observed in the CdTe layer is more complex. In the vicinity of the CdTe/Ge interface, over a thickness of 5 nm, the channeling process deterioration indicates a high density of Cd and/or Te atoms moved by at least 0.1 Å from their expected crystallographic positions [11]. This highly disordered interfacial layer is followed by a rapid channeling yield improvement in the CdTe layer corresponding to the bending dislocations [12] over a thickness of about 12 nm. Finally, the observed degradation of the channeling yield in the near-surface region corresponds to an amorphous region of about 5 nm resulting from oxidation during post growth air exposure. Complementary to this defect profiling, the variation of strain in the same CdTe layer can also be measured by MEIS with high depth resolution along the 16 nm above the interface. The accurate measurement of the blocking dip angular positions, as a function of the energy of backscattered particles (Fig. 5), can be traced back to a deformation profile within the crystal. This technique usually uses the position of the blocking dip within the substrate, as far as possible from the layer/substrate interface, as a reference for a fully relaxed crystal. In the present case, this method cannot be employed successfully due to the layer misorientation observed in channeling mode. Nevertheless, X-ray diffraction performed on the same sample [4] gives the average strain across the layer thick-
ness. This value of about 0.3%, very weak compared to the huge lattice misfit, has been used to normalize the MEIS strain profile shown on Fig. 6. According to this result, we observe a rapid strain variation in the interfacial region. This corresponds to a zone with high defect density (about 5 nm thick), with a maximum strain value of about 0.9% at the interface followed by a weaker decrease along the next 11 nm (Fig. 6). The model describing the relaxation of epitaxial layers having a lattice mismatch with their substrates has been proposed by Frank [13] and Van der Merwe [14]. Basically, for mismatch below about 7%, the layer is elastically strained by the substrate until it reaches a critical thickness which depends on the couple of materials. Above this critical thickness, the system introduces misfit dislocations in the layer to partly relax this excess of elastic energy. At this stage, the system shares this stored energy between elastic deformation and plastic relaxation. Nevertheless, the density of threading dislocations present at the sample surface decreases with the thickness of deposited material. Indeed, under certain conditions, adjacent dislocations can recombine with each other [12]. This mechanism, highly dependent on the distance between defects, varies with the local dislocations density. In other words, one can quickly reduce a high density of defects, but must deposit large thicknesses of material to remove the last dislocations. For larger misfit values, the situation is less clear since the critical thickness drops to zero while the dislocation density at the layer/substrate interface increases. Then, any HRTEM observation of the interface becomes infeasible precisely because of the high degree of disorder. An alternative possibility is the use of ion channeling with a high depth resolution in order to quantify the disorder and the strain state in the vicinity of the interface. Considering first the evolution of the defects with respect to the distance to the layer/substrate interface, the X-ray results clearly indicate a quasi-exponential increase of their density when approaching the interface [4]. Then, if we look closer, the MEIS data reveal a saturation of this increase which reaches a maximum value at a distance of about 5 nm from the substrate. In this highly disordered interfacial region, about half of the atoms are moved from their crystallographic sites and the others remain aligned along atomic columns insuring the epitaxial relationships between layer and substrate. It should be noted here that, in the immediate vicinity of this interface between layers, the CdTe atomic columns can be curved with respect to their direction within the layer, due to the huge material misfit. If this steering angle exceeds the critical angle of channeling w1/2, the ions can no longer oscillate inside the crystallographic axis and then can be scattered by the nuclei. The critical angle can be calculated using the following relation given by Lindhard [15] considering a pure Coulomb potential: 1=2
w
Fig. 3. vmin profiles in the Ge substrate and the CdTe layer along the [1 1 1] direction.
3
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Z 1 Ze2 2pe0 Ed
ð1Þ
where Z1 is the atomic number of the incident ion, Z is the average atomic number of the considered material, e is the elementary charge, eo is the dielectric constant of vacuum, E is the kinetic energy of the ions and d the average distance between atoms along atomic rows. For a 200 keV H+ beam channeled in the [1 1 1] direction of CdTe the critical angle is about 1.6°. Considering the large difference of lattice parameter in this system, the high proportion of displaced atoms seen by MEIS in channeling mode is probably due to both, the presence of dislocations and to the distortion of the atomic columns. Considering now the strain state of the crystal, its evolution with the distance to the interface remains weak. The MEIS strain profile, performed in the interface’s vicinity, reveals only 0.7% of
4
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Fig. 4. MEIS spectra recorded on CdTe (21 nm)/Ge [2 1 1].
Fig. 6. Strain variation of the CdTe layer referenced from the CdTe/Ge interface (nm). These calculations are based on the evolution of the blocking dip position. Fig. 5. Evolution of the blocking dip position (centered at 123.7°) in the CdTe layer.
to a decrease of their density with increasing thickness only starts after the growth of a 5 nm thick highly disordered layer. 4. Conclusions
elastic relaxation over the first few nanometers above the substrate. The maximum strain value at the interface reaches at least 0.9%. Given the huge CdTe/Ge misfit of about 14%, it is clear that the plastic relaxation dominates the relaxation mechanism through introduction of dislocations directly at the layer/substrate interface. However, the recombination of adjacent dislocations leading
The relaxation of CdTe layers grown by MBE on a Ge substrate has been investigated by the MEIS technique to accurately characterize the strain relaxation at the interface. The measurements showed, in the first 16 nm, an elastic relaxation of about 0.7% leading to a maximum strain value equal to 0.9% at the layer/substrate interface.
J.C. Pillet et al. / Nuclear Instruments and Methods in Physics Research B 384 (2016) 1–5
In this system, the main relaxation mechanism consists in the introduction of misfit dislocations at the layer/substrate interface. At 5 nm above the substrate, the degree of disorder remains approximately constant with an equivalent proportion of atoms located in, and away from, their lattice sites. Only after the growth of this thin disordered layer, the mechanism of dislocations recombination appears leading to a fast and almost exponential decrease of their density. Acknowledgment The authors thank P. Gergaud and J.P. Zanatta for fruitful discussions. References [1] T. Skauli, T. Colin, J. Cryst. Growth 222 (2001) 719–725. [2] R.N. Jacobs, L.A. Almeida, J. Markunas, J. Pellegrino, M. Groenert, M. JaimeVasquez, et al., J. Electron. Mater. 37 (2008) 1480–1487.
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[3] J.F.C. Baker, M. Hart, Acta Crysallogr. A31 (1975) 364–367. [4] G. Badano, P. Gergaud, I.C. Robin, X. Baudry, B. Amstatt, F. Gemain, J. Electron. Mater. 39 (2010) 908–911. [5] T.D. Dzhafarov, F. Ongul, J. Phys. D-Appl. Phys. 38 (2005) 3764–3767. [6] J.P. Zanatta, P. Ferret, P. Duvaut, S. Isselin, G. Theret, G. Rolland, et al., J. Cryst. Growth 184–185 (1998) 1297–1301. [7] J.P. Zanatta, P. Ferret, G. Theret, A. Million, M. Wolny, J.P. Chamonal, et al., J. Electron. Mater. 27 (1998) 542–545. [8] D. Jalabert, Nucl. Instrum. Methods Phys. Res. B 270 (2012) 19–22. [9] D. Jalabert, Y. Curé, K. Hestroffer, Y.M. Niquet, B. Daudin, Nanotechnology 23 (2012) 425703. [10] D. Jalabert, J. Coraux, H. Renevier, B. Daudin, M.-H. Cho, K.B. Chung, D.W. Moon, J.M. Llorens, N. Garro, A. Cros, A. García-Cristóbal, Phys. Rev. B 72 (2005) 115301. [11] L.C. Feldman, J.W. Mayer, S.T. Picraux, Materials Analysis by Ion Channeling: Submicron Crystallography, Academic Press, New-York, 1982. [12] J.E. Matthews, A.E. Blakeslee, J. Cryst. Growth 27 (1974) 118–125. [13] F.C. Frank, J.H. van der Merwe, Proc. R. Soc. London Ser. A Math. Phys. Sci. 198 (1949) 205–216. [14] J.H. van der Merwe, J. Appl. Phys. 34 (1963) 117–122. [15] J. Lindhard, M. Scharff, H.E. Schiott, K. Dan, Vidensk. Selsk. Mat.-Fys. Medd. 33 (1963) 1.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Strain relaxation of CdTe on Ge studied by medium energy ion scattering J.C. Pillet a,b,⇑, F. Pierre a,c, D. Jalabert a,d a
Univ. Grenoble Alpes, CEA, LETI, MINATEC campus, F38000 Grenoble, France CEA, LETI, Département Optique et Photonique, F38054 Grenoble, France c CEA, LETI, Service de Caractérisation des Matériaux et Composants, F38054 Grenoble, France d CEA-INAC/UJF-Grenoble 1 UMR-E, SP2M, LEMMA, Minatec Grenoble F-38054, France b
a r t i c l e
i n f o
Article history: Received 28 January 2016 Received in revised form 24 June 2016 Accepted 29 July 2016
Keywords: CdTe Ge Substrates M.E.I.S Channeling Blocking
a b s t r a c t We have used the medium energy ion scattering (MEIS) technique to assess the strain relaxation in molecular-beam epitaxial (MBE) grown CdTe (2 1 1)/Ge (2 1 1) system. A previous X-ray diffraction study, on 10 samples of the same heterostructure having thicknesses ranging from 25 nm to 10 lm has allowed the measurement of the strain relaxation on a large scale. However, the X-ray diffraction measurements cannot achieve a stress measurement in close proximity to the CdTe/Ge interface at the nanometer scale. Due to the huge lattice misfit between the CdTe and Ge, a high degree of disorder is expected at the interface. The MEIS in channeling mode is a good alternative in order to profile defects with a high depth resolution. For a 21 nm thick CdTe layer, we observed, at the interface, a high density of Cd and/or Te atoms moved from their expected crystallographic positions followed by a rapid recombination of defects. Strain relaxation mechanisms in the vicinity of the interface are discussed Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction The replacement of CdZnTe (CZT) substrate by a germanium (Ge) substrate is a planned solution for the manufacture of infrared detectors based on CdHgTe (CMT) layers. Indeed, the sizes of available substrates (typically from 20 cm2 to 80 cm2 for CZT compared to 40 cm2 to 180 cm2 for Ge) as well as the manufacturing costs justify this interest. However, the huge lattice mismatch (14%) between CMT {aHgTe (6.46 Å [1])
quality of several CdTe buffers with different thicknesses has been studied in a previous work [4] by high resolution X-ray diffraction (HRXRD) for CdTe layers between 21 nm and a few microns. The aim of this work is to accurately determine the defects and lattice deformation profiles in the vicinity of the CdTe/Ge interface. These structural properties were analyzed on a 21 nm thick sample by MEIS (Medium Energy Ion Scattering), which, thanks to its high depth resolution and sensitivity to defects in channeling mode, is well adapted to investigate this type of heterostructure. The MEIS technique is similar to the RBS (Rutherford Backscattering Spectrometry) technique. Both are based on measuring the kinetic energy and the scattering angle of charged particles, usually protons or alpha particles, elastically scattered by the nuclei of the sample. In favorable cases, one can separate the elements according to their atomic mass and determine their depth distribution. While RBS is implemented with beam energies ranging from 0.5 to 5 MeV and a surface barrier detector with an energy resolution of 12–15 keV, MEIS uses particles with energies between 50 and 500 keV and an electrostatic analyzer with an energy resolution of 0.3% and an accuracy of about 0.1% for the scattering angle. Because the MEIS uses lower energy incident particles, the analysis focuses on the few tens of nanometers of the sample while, due to the higher energy of incident particles, RBS probe the sample over a few microns. In the most favorable cases, the MEIS depth resolution can reach 0.25 nm whereas the RBS depth resolution is of the order of 10 to 20 nm.
2
J.C. Pillet et al. / Nuclear Instruments and Methods in Physics Research B 384 (2016) 1–5
Since the electrostatic analyzer allows the simultaneous measurement of the energy and the scattering angle, over a 20° range, of the scattered particles, MEIS is well suited for in-depth structural investigations at the nanometric scale. 2. Experimental procedure After an initial ex-situ Ge (2 1 1) substrate preparation including an etching of the Ge substrate in a preparation solution (H3PO4: H2O2: H2O) [5], the material is rapidly introduced into a dedicated mercury MBE system and the substrate is heated at 650 °C to remove the native oxide layer [6]. During this process, the substrate is exposed to an As flux in order to saturate the Ge dangling bonds [7]. The growth is carried out in a RIBER (Paris, France) EPINEAT machine consisting of a suitable reactor for 4 in wafers of IIVI materials and equipped with standard effusion cells for CdTe, Te, Zn and As. The standard deposition method requires a slight Te overpressure and includes, at the end of growth, an annealing step at 400 °C during 1 h. The MEIS study of the CdTe/Ge interface was performed on the 21 nm thick CdTe sample with a monoenergetic proton beam delivered by the MEIS facility at MINATEC Grenoble, and by using two different scattering schematics, called channeling and blocking. Channeling occurs when the incident ion beam is aligned with a crystallographic axis of monocrystalline layers or substrates. In this configuration, the scattered yield is drastically reduced except for scattering by atoms displaced from their normal lattice positions, thus allowing the determination of defect depth profiles. Blocking is observed when ions scattered from deeper atoms suffer a second scattering from atoms at shallower depth. As a result, the angular distribution of the scattering yield exhibits a minimum, called blocking dip, at the angular position corresponding to the atom responsible for the second scattering. Any change in atomic positions, induced by stress for instance, gives rise to a shift of the blocking dip. The accurate measurement of these shifts as a function of depth provides the strain profile within the layer. The defect profile was performed with a 200 keV H+ beam incident at 20° with respect to the sample normal and the scattered particles were analyzed around an angle of 125°. The channeling experiments are very sensitive to defect. As a consequence, the choice of a higher energy minimizes the dechanneling of the ion bean passing through the disordered regions of the sample since a 21 nm layer is already thick for a MEIS experiment. The MEIS spectra in the CdTe layer as well as in the Ge substrate were recorded with the incident ion beam aligned to the [1 1 1] direction of the crystals. This channeling direction was chosen on the stereographic projection of the (2 1 1) oriented face-centered cubic crystal along the [1 1 1] direction found at 19, 47° from the sample’s normal (Fig. 1). An experimental measurement of the pole figure was achieved by MEIS [8] to identify the crystallographic orientation of the sample by comparing the mapping of H+ ions scattered by the Cd and Te atoms (Fig. 2) and the stereographic projection mentioned above. The precise orientation of the sample is then refined by small angular scans around the considered channeling direction. The deformation profile was obtained by using a 100 keV H+ beam and precisely locating the [2, 1, 1] blocking direction in the CdTe layer. Generally speaking, blocking experiments are not very sensitive to defects. Thereby, the choice of a low energy for these experiments has the advantage of maximizing the depth resolution. This technique relies on the accurate angular localization of a selected blocking dip as a function of depth [9,10]. Indeed, any lattice crystal deformation due to strain causes a blocking dip displacement relative to its value in a relaxed crystal. A random incident beam orientation was chosen in order to maximize the spectrum yield.
Fig. 1. Stereographic projection of a [2 1 1] face centered cubic crystal. The scattering angle is given by: h scattering = 180° – (h incident–h polar). The area bounded by the blue zone corresponds to the experimental mapping shown in Fig. 2. Within this blue area, four crystallographic directions (red dots) are also indicated in the experimental mapping shown on Fig. 2 in order to guide the eyes. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. Mapping of H+ ions scattered by the Cd and Te atoms to identify the crystallographic directions of the sample (red).
3. Results and discussions To start with, the MEIS technique is employed to assess the crystalline quality in an ultrathin CdTe layer at the nanometer scale. The measurement is based on a comparison of the MEIS spectrum obtained by aligning the incident ion beam on a selected crystallographic direction of the sample to that of a spectrum realized in a random direction. The ratio of the channeling to the random yields is called vmin. For an amorphous material, its value is equal to 1 and drop to a few 102 for a perfect crystal. The evolution of this parameter as a function of depth in the sample provides a depth profile of the crystalline quality. Surprisingly, the localization of the [1 1 1] direction within the CdTe layer does not
J.C. Pillet et al. / Nuclear Instruments and Methods in Physics Research B 384 (2016) 1–5
correspond to that of the Ge substrate. Indeed, a shift in the axis orientations of about 2° is observed between the layer and the substrate. However, it should be noted that no in-plane rotation (with accuracy better than 0.1°) has been found between layer and substrate. This out of plane misorientation of the epitaxial layer can be attributed to an elastic relaxation at the border of atomic steps that steers the atomic columns during the first growth stages. As a result, the vmin profiles (Fig. 3) in the CdTe layer and in the substrate were recorded in two slightly different geometries. We can observe on this figure a vmin value of about 22% in Ge while the substrate has a very high crystalline quality. Its vmin value should reach about 4% or 5% if at the surface sample. The use of a 100 keV beam would have led to a much higher vmin value in the substrate minimizing the defect sensitivity in the layer. The two channeled spectra as well as the random spectrum are shown in Fig. 4. The vmin profile within the Ge substrate along the [1 1 1] direction evidences a flat shape indicating the absence of defects in Ge. It should be noted that the relatively high vmin value in Ge is due to beam dechanneling in the CdTe layer. By contrast, the vmin profile observed in the CdTe layer is more complex. In the vicinity of the CdTe/Ge interface, over a thickness of 5 nm, the channeling process deterioration indicates a high density of Cd and/or Te atoms moved by at least 0.1 Å from their expected crystallographic positions [11]. This highly disordered interfacial layer is followed by a rapid channeling yield improvement in the CdTe layer corresponding to the bending dislocations [12] over a thickness of about 12 nm. Finally, the observed degradation of the channeling yield in the near-surface region corresponds to an amorphous region of about 5 nm resulting from oxidation during post growth air exposure. Complementary to this defect profiling, the variation of strain in the same CdTe layer can also be measured by MEIS with high depth resolution along the 16 nm above the interface. The accurate measurement of the blocking dip angular positions, as a function of the energy of backscattered particles (Fig. 5), can be traced back to a deformation profile within the crystal. This technique usually uses the position of the blocking dip within the substrate, as far as possible from the layer/substrate interface, as a reference for a fully relaxed crystal. In the present case, this method cannot be employed successfully due to the layer misorientation observed in channeling mode. Nevertheless, X-ray diffraction performed on the same sample [4] gives the average strain across the layer thick-
ness. This value of about 0.3%, very weak compared to the huge lattice misfit, has been used to normalize the MEIS strain profile shown on Fig. 6. According to this result, we observe a rapid strain variation in the interfacial region. This corresponds to a zone with high defect density (about 5 nm thick), with a maximum strain value of about 0.9% at the interface followed by a weaker decrease along the next 11 nm (Fig. 6). The model describing the relaxation of epitaxial layers having a lattice mismatch with their substrates has been proposed by Frank [13] and Van der Merwe [14]. Basically, for mismatch below about 7%, the layer is elastically strained by the substrate until it reaches a critical thickness which depends on the couple of materials. Above this critical thickness, the system introduces misfit dislocations in the layer to partly relax this excess of elastic energy. At this stage, the system shares this stored energy between elastic deformation and plastic relaxation. Nevertheless, the density of threading dislocations present at the sample surface decreases with the thickness of deposited material. Indeed, under certain conditions, adjacent dislocations can recombine with each other [12]. This mechanism, highly dependent on the distance between defects, varies with the local dislocations density. In other words, one can quickly reduce a high density of defects, but must deposit large thicknesses of material to remove the last dislocations. For larger misfit values, the situation is less clear since the critical thickness drops to zero while the dislocation density at the layer/substrate interface increases. Then, any HRTEM observation of the interface becomes infeasible precisely because of the high degree of disorder. An alternative possibility is the use of ion channeling with a high depth resolution in order to quantify the disorder and the strain state in the vicinity of the interface. Considering first the evolution of the defects with respect to the distance to the layer/substrate interface, the X-ray results clearly indicate a quasi-exponential increase of their density when approaching the interface [4]. Then, if we look closer, the MEIS data reveal a saturation of this increase which reaches a maximum value at a distance of about 5 nm from the substrate. In this highly disordered interfacial region, about half of the atoms are moved from their crystallographic sites and the others remain aligned along atomic columns insuring the epitaxial relationships between layer and substrate. It should be noted here that, in the immediate vicinity of this interface between layers, the CdTe atomic columns can be curved with respect to their direction within the layer, due to the huge material misfit. If this steering angle exceeds the critical angle of channeling w1/2, the ions can no longer oscillate inside the crystallographic axis and then can be scattered by the nuclei. The critical angle can be calculated using the following relation given by Lindhard [15] considering a pure Coulomb potential: 1=2
w
Fig. 3. vmin profiles in the Ge substrate and the CdTe layer along the [1 1 1] direction.
3
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Z 1 Ze2 2pe0 Ed
ð1Þ
where Z1 is the atomic number of the incident ion, Z is the average atomic number of the considered material, e is the elementary charge, eo is the dielectric constant of vacuum, E is the kinetic energy of the ions and d the average distance between atoms along atomic rows. For a 200 keV H+ beam channeled in the [1 1 1] direction of CdTe the critical angle is about 1.6°. Considering the large difference of lattice parameter in this system, the high proportion of displaced atoms seen by MEIS in channeling mode is probably due to both, the presence of dislocations and to the distortion of the atomic columns. Considering now the strain state of the crystal, its evolution with the distance to the interface remains weak. The MEIS strain profile, performed in the interface’s vicinity, reveals only 0.7% of
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Fig. 4. MEIS spectra recorded on CdTe (21 nm)/Ge [2 1 1].
Fig. 6. Strain variation of the CdTe layer referenced from the CdTe/Ge interface (nm). These calculations are based on the evolution of the blocking dip position. Fig. 5. Evolution of the blocking dip position (centered at 123.7°) in the CdTe layer.
to a decrease of their density with increasing thickness only starts after the growth of a 5 nm thick highly disordered layer. 4. Conclusions
elastic relaxation over the first few nanometers above the substrate. The maximum strain value at the interface reaches at least 0.9%. Given the huge CdTe/Ge misfit of about 14%, it is clear that the plastic relaxation dominates the relaxation mechanism through introduction of dislocations directly at the layer/substrate interface. However, the recombination of adjacent dislocations leading
The relaxation of CdTe layers grown by MBE on a Ge substrate has been investigated by the MEIS technique to accurately characterize the strain relaxation at the interface. The measurements showed, in the first 16 nm, an elastic relaxation of about 0.7% leading to a maximum strain value equal to 0.9% at the layer/substrate interface.
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In this system, the main relaxation mechanism consists in the introduction of misfit dislocations at the layer/substrate interface. At 5 nm above the substrate, the degree of disorder remains approximately constant with an equivalent proportion of atoms located in, and away from, their lattice sites. Only after the growth of this thin disordered layer, the mechanism of dislocations recombination appears leading to a fast and almost exponential decrease of their density. Acknowledgment The authors thank P. Gergaud and J.P. Zanatta for fruitful discussions. References [1] T. Skauli, T. Colin, J. Cryst. Growth 222 (2001) 719–725. [2] R.N. Jacobs, L.A. Almeida, J. Markunas, J. Pellegrino, M. Groenert, M. JaimeVasquez, et al., J. Electron. Mater. 37 (2008) 1480–1487.
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