- Email: [email protected]
GaAs self assembled quantum dots
Available online at www.sciencedirect.com
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 1439–1442 www.elsevier.com/locate/nimb
Ion beam studies of InAs/GaAs self assembled quantum dots E. Alves a,b,*, S. Magalha˜es a, N.P. Barradas a,b, N.V. Baidus c, M.I. Vasilevskiy c, B.N. Zvonkov d b
a Instituto Te´cnico Nuclear ITN, EN10, 2686-953 Sacave`m, Portugal CFN da Universidade de Lisboa, Avenue Gama Pinto 2, 1649-003 Lisboa, Portugal c Universidade do Minho, Departamento de Fı´sica, 4170-057 Braga, Portugal d N. I. Lobachevskii State University, Nihznii Novgorod, Russia
Received 21 September 2007; received in revised form 13 December 2007 Available online 28 December 2007
Abstract Self assembled InAs/GaAs quantum dots (QD’s) emit in the wavelength range (1.3–1.55 lm) revealing an enormous potential to become the active elements of low threshold lasers and light emitting diodes for communication systems. However, the luminescence is dramatically quenched at room temperature (and even below) due to the defects in the GaAs matrix which open non-radiative recombination paths. In this study we combine Rutherford backscattering/channelling (RBS-C) and high resolution X-ray diffraction (HRXRD) to study the structural properties of the InAs/GaAs structures. The InAs/GaAs QD heterostructures were grown by atmospheric pressure metal organic vapour phase epitaxy. Channelling measurements reveal a good crystalline quality along the main axial directions with minimum yields in the range of 4–6% through the entire capping layer. An increase on the dechannelling rate was observed in the region where the InAs quantum dots were buried. The channelling results also give evidence for the presence of defects preferentially oriented. Detailed angular scans in a structure with a 28 nm cap allowed the study of the In orientation with respect to the GaAs matrix and a perfect alignment was found along the growth direction. The strain in the dots shifts the angular curves along the tilt directions. Ó 2008 Elsevier B.V. All rights reserved. PACS: 82.80.Yc; 68.65.Ac; 78.67.Pt Keywords: Channelling; InAs quantum dots; Defects
1. Introduction A new class of optoelectronic devices is emerging based on the development of low dimensional structures. The space localization of carriers in these systems brings new challenges to solid state physics creating an enormous expectation of new technological breakthroughs. The zero dimensional confinement achieved in quantum dots (QD) is very much appealing both from the applications (photo* Corresponding author. Address: Instituto Te´cnico Nuclear ITN, EN10, 2686-953 Sacave`m, Portugal. Tel.: +351 219946086; fax: +351 219941525. E-mail address: [email protected] (E. Alves).
0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.12.078
detectors and laser diodes) and fundamental research point of view. Among the interesting properties are the low and temperature-independent threshold currents and the sharp optical transitions [1,2]. The growth of self assembled QDs is driven by the strain fields resulting from the lattice mismatch between the dots and the surrounding matrix. In the case of the InAs dots (lattice constant 0.605 nm) in GaAs (lattice constant 0.565 nm) the in-plane strain is 7%. The first layers grow strained and above the critical thickness the strain accumulated induces the spontaneous formation of the islands, the so called Stranski–Krastanov growth mode. The transition from the two dimensional growth to the three dimensional happen around 1–2 monolayers of InAs [3]. The growth
1440
E. Alves et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1439–1442
conditions allow the control of the shape, composition, size and strain distribution in the dots which have a strong impact on the electronic properties of the nanostructures. However, defects created during the growth, in particular the over layer cap, can degrade the operating characteristics of the QD systems. Recently a new process to reduce the number of defects present in the cap and consequently the optical quenching was proposed [4]. In this process the growth of the cap is interrupted and tetrachloromethane inserted in the reaction chamber before finishing the growth. In this work we will combine the channelling and high resolution X-ray diffraction techniques to study the strain and defects present in InAs/GaAs QD’s structures. Channelling is a well established technique offering the possibility to study defects and lattice distortions at nanometer scale. The results reveal an increase of the dechannelling at the depth where the InAs QD’s were embedded. The detailed angular scans along the major tilted axial directions show asymmetric In dips.
2. Experimental details InAs/GaAs self aligned quantum dots (SAQD) heterostructures were grown by atmospheric pressure metal organic vapour phase epitaxy (AP MOVPE). Two kinds of samples were prepared with different cap thicknesses. One sample were grown in standard conditions [5] with the quantum dots overgrown by an InxGa1 xAs quantum well (QW) layer at 520 °C (x = 0.2–0.3, the thickness 2– 3 nm) and covered with a 30 nm thick GaAs capping layer (CL), as illustrated in Fig. 1. The other samples (with both n and p types of doping) were grown slightly differently. After the deposition of 2–3 nm of InGaAs (QW) and 5 nm of GaAs onto the QD layer, the growth was interrupted and tetrachloromethane was introduced into the reactor at 580 °C for 20 s. Finally, the rest of the capping layer (with variable thickness) was grown at a temperature somewhat higher than for other samples. The crystalline quality, composition and strain were accessed both with high resolution X-ray diffraction
(HRXRD) and Rutherford backscattering/channelling spectrometry (RBS/C). The RBS/C measurements were performed with 1 mm collimated 4He+ beam with 2.0 MeV. The samples were mounted in a computer controlled two-axis goniometer with an accuracy of 0.01°. The backscattered particles were detected by two surface barrier silicon detectors placed at 160° and close to 180° with respect to the beam direction (Cornell geometry) and with 13 and 16 keV energy resolutions, respectively. The strain in the layers was studied performing detailed angular scans of the <1 0 1> and <1 1 1> directions along the (0 1 0) and (1–10) planes, respectively. The XRD studies were done with a double-crystal diffractometer with a resolution of the order of 30 arc sec. 3. Results and discussion The presence of the InAs dots and the depths and thickness of the In doped layer was measured by grazing incidence RBS analysis. The spectra were simulated using the NDF code to model the presence of the quantum dots [6]. The results are shown in Fig. 2 and the best results were obtained assuming a thickness of 28(3) nm for the cap layer (we did a self-consistent analysis enforcing the same energy calibration on the two angles and considering bulk densities) and 4 nm for the QD height. In this case, the volume fraction of the QDs is 38 vol.% and the surface roughness (or inhomogeneity of the cap layer thickness) is around 3.5 nm. Small deviations from these values also give good fits but our results are in excellent agreement with the AFM analyses of similar samples [5]. For the sample with a thicker GaAs cap the In signal is shadowed by the Ga and As signals. Fig. 3 shows the random, (1–10) plane and <1 1 1> axis aligned spectra where is visible the kink due to the dechannelling in the region of the buried InAs QD’s. The continuous line is the result of the simulation Energy(KeV) 1500 400
1600
1700
θinc= 75°, αscat=160°
300
GaAs
Yield (counts)
200 100
In
0
θinc= 4 °, αscat=160°
1500 GaAs 1000 500
In
0 260
280
300
Chennel
Fig. 1. Schematic diagram of the sample structure used in these studies.
Fig. 2. RBS spectra and simulated results (continuous curve) obtained for two incident angles.
E. Alves et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1439–1442 GaAs Depth (nm) 600
400
200
0
700
Yield (counts)
600
E=2.0 MeV
(1-10) plane random <111> axis
500
In
400 300
Wind1 Wind2
200
Step
100 0 300
400
500
600
Channel
Fig. 3. Random, <1 1 1> and (1–10) aligned RBS spectra obtained for a sample with a 227 nm GaAs cap. The step close to the interface of the GaAs/InAs is only visible along the axial direction. The windows used for the angular scans are indicated as well as the region where the In signal would appear.
obtained considering a cap thickness of 227 nm. The presence of the QD’s does not change the results of the simulation. The angular scans (Fig. 4) performed along the plane (1–10) through the <1 1 1> axis, integrating the counts in the windows corresponding to the two regions shown in Fig. 3, reveals a large difference on the minimum yield of the curves. A similar behaviour was observed for the <1 0 1> axis. On the other hand the kink visible in the <1 1 1> direction is missing in the (1–10) plane indicating a preferential orientation of the defects responsible for the dechannelling. This effect was reported by others [7] and in most of the cases was attributed to the presence of dislocations. To confirm this possibility we studied the
1441
energy dependence of the dechannelling process. The results obtained for the <1 1 1>, <1 0 1> and (1–10) directions are plotted in Fig. 5 for the regions immediately before and after the kink observed in the channelled spectra for the axial directions as we did for the angular scans. The data do not show any strong dependence with beam energy except for the <1 1 1> axis where a small increase is observed. According the analytical model developed by Quere´ [8] we expect a E1/2 dependence for the scattering by dislocations and a E 1/2 for the scattering by point defects. Taking this into account we can conclude that we must have different type of defects in the sample. Other possibility to explain the kink is the presence of stacking faults (extra or missing planes) close to the InAs dot layer since the dechannelling cross section for this type of defects is independent of the energy [9]. In a recent work Sears et al. [10] observed a large number of defects associated with the presence of InAs QD’s. The defects were grouped in two major classes: Stacking faults and V-shaped defects formed by pairs of edge dislocations running from the QD layer. It was also concluded that dislocations were in the {1 1 1} planes which could explain the channelling visibility along the <1 1 1> perpendicular directions. In order to find the structural correlation of the InAs QD’s with the GaAs matrix we did detailed angular scans through the <0 0 1>, <1 0 1> and <1 1 1> tilt axis in the sample with the 28 nm GaAs cap. The scan directions were parallel to the (0 1 0) for the <1 0 1> direction and (1–10) plane for the <1 1 1> direction. The results obtained for In and two regions in the GaAs are shown in Fig. 6. The regions chosen in the GaAs correspond to the substrate and the region containing the InAs quantum dots and the cap. Naturally we could not separate the As in the QD’s from the As in the cap but its amount is not enough to influence the results. For the <0 0 1> the angular curves (not shown) overlap but a shift was found for the tilt directions. It is also visible in the <1 0 1> axis some asymmetry in the curves. Our results are different from those found by
1. 0 <111>
Wind1
wind1 <110> wind2 <110> wind1 <111> wind 2 <111> wind1 (1-10) wind 2 (1-10)
1.0
0. 6
Normalised Yield
Normalized Yield
1.2
Wind2
0. 8
0. 4
0. 2
0.8
0.6
0.4
0.2
0. 0 -2
-1
0
1
2
Angle (degree) Fig. 4. Angular scans thought the <1 1 1> axis along the (1–10) plane for the sample with the thicker GaAs cap. The windows used correspond to the region just before and after the step observed in the aligned spectra of Fig. 3.
0.0 1.0
1.5
2.0
2.5
Energy (MeV)
Fig. 5. Energy dependence of the minimum yield for the directions indicated in the figure.
1442
E. Alves et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1439–1442
lattice parameter of the strained InAs dots and further studies are planned using synchrotron radiation.
<101>
1.0 0.8
4. Conclusions 0.6
Grazing incidence RBS and ion channelling results reveal the formation of strained InAs QD’s in GaAs. The high resolution RBS spectra allowed us to extract a value of 3 nm for the height of the dots and a volume fraction of the order of 50%. The energy dependence of the dechannelling cross section point for the presence of multiple defect types in the structure. The detailed angular scans through the major tilt axis suggest the presence of strain in the InAs quantum dots.
0.4
Normalized Yield
0.2 0.0 0.7 <111>
0.6 0.5
Acknowledgement
0.4
One of us (E. Alves) acknowledges Fundacßa˜o Calouste Gulbenkian for financial support.
0.3 0.2
References
GaAs (Substrate)
0.1
GaAs (QD layer) In
0.0 -2
-1
0
1
2
Angle (degrees) Fig. 6. Angular scans for the sample with the 28 nm GaAs cap thought the <1 1 1> and <1 0 1> axis along the (1–10) and (0 1 0) planes, respectively. The windows used for the GaAs correspond to the region containing the dots and the substrate.
Selen et al. [11] and Wang et al. [12] who observed a narrowing of the In curve compared to the GaAs. These authors assume a slight displacement of the In atoms from the regular sites or the lattice distortion to explain their data. This is not the case in our InAs/GaAs QD’s structure where the shift clearly indicates the presence of strain due to the lattice mismatch. Moreover the shape of the In curves seems to be the convolution of two curves with different widths. This could be due to the presence of a fully strain InAs wetting layer and a partial relaxed InAs QD’s. To obtain an average picture of the strain state of the dots we need to simulate the flux distribution in the channels to obtain the exact value of the kink angle as well as the correct value of its lattice parameter [13]. The development of the flux code [14] to model the QD’s layer is in progress. On the other hand due to the small dimension of the dots we could not resolve the X-ray signal to measure the
[1] T. Steiner (Ed.), Semiconductor Nanostructures for Optoelectronic Applications, Artech House Inc., 2004. [2] H. Saito, K. Nishi, S. Sugou, Appl. Phys. Lett. 73 (1998) 2742. [3] M. Sugawara (Ed.), Self-Assembled InGaAs/GaAs quantum dots, Semiconductors and Semimetals, Vol. 60, Academic Press, 1999. [4] N.V. Baidus, A. Chahboun, M.J. Gomes, M.I. Vasilevskiy, P.B. Demina, E.A. Uskova, B.N. Zvonkov, Appl. Phys. Lett. 87 (2005) 053109. [5] I.A. Karpovich, B.N. Zvonkov, N.V. Baidus, S.V. Tikhov, D.O. Filatov, Trends in Nanotechnology Research, Nova Science Publishers, 2004, Chap. 8. [6] N.P. Barradas, Nucl. Instr. and Meth. B 261 (2007) 435. [7] M. Mazzer, A.V. Drigo, F. Romanato, G. Salviati, L. Lazzarini, Selective ion-channeling study of misfit dislocation grids in semiconductor heterostructures: theory and experiments, Phys. Rev. B 56 (1997) 6895. [8] Y. Que´re´, Phys. Status Solidi 30 (1968) 713. [9] L.C. Feldman, J.W. Mayer, S.T. Picraux, Materials Analysis by Ion Channeling: Submicron Crystallography, Academic Press, Inc., 1982, ISBN 0-12-252680-5. [10] K. Sears, J. Wong-Leung, H.H. Tan, C. Jagadish, J. Appl. Phys. 99 (2006) 1135032. [11] L.M. Selen, L.J. van Ijzendorn, M.J.A. de Voigt, P.M. Koenrad, Phys. Rev. B 61 (2000) 8270. [12] H.Y. Wang, C.P. Lee, H. Niu, C.H. Chen, S.C. Wu, J. Appl. Phys. 100 (2006) 103502. [13] K. Lorentz, N. Franco, E. Alves, I.M. Watson, R.W. Martin, K.P. O’Donnell, Anomalous ion channeling in ALInN/GaN bilayers: determination of the strain state, Phys. Rev. Lett. 97 (2006) 85501. [14] P.J.M. Smulders, D.O. Boerma, Nucl. Instr. and Meth. B 29 (1987) 471.
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 1439–1442 www.elsevier.com/locate/nimb
Ion beam studies of InAs/GaAs self assembled quantum dots E. Alves a,b,*, S. Magalha˜es a, N.P. Barradas a,b, N.V. Baidus c, M.I. Vasilevskiy c, B.N. Zvonkov d b
a Instituto Te´cnico Nuclear ITN, EN10, 2686-953 Sacave`m, Portugal CFN da Universidade de Lisboa, Avenue Gama Pinto 2, 1649-003 Lisboa, Portugal c Universidade do Minho, Departamento de Fı´sica, 4170-057 Braga, Portugal d N. I. Lobachevskii State University, Nihznii Novgorod, Russia
Received 21 September 2007; received in revised form 13 December 2007 Available online 28 December 2007
Abstract Self assembled InAs/GaAs quantum dots (QD’s) emit in the wavelength range (1.3–1.55 lm) revealing an enormous potential to become the active elements of low threshold lasers and light emitting diodes for communication systems. However, the luminescence is dramatically quenched at room temperature (and even below) due to the defects in the GaAs matrix which open non-radiative recombination paths. In this study we combine Rutherford backscattering/channelling (RBS-C) and high resolution X-ray diffraction (HRXRD) to study the structural properties of the InAs/GaAs structures. The InAs/GaAs QD heterostructures were grown by atmospheric pressure metal organic vapour phase epitaxy. Channelling measurements reveal a good crystalline quality along the main axial directions with minimum yields in the range of 4–6% through the entire capping layer. An increase on the dechannelling rate was observed in the region where the InAs quantum dots were buried. The channelling results also give evidence for the presence of defects preferentially oriented. Detailed angular scans in a structure with a 28 nm cap allowed the study of the In orientation with respect to the GaAs matrix and a perfect alignment was found along the growth direction. The strain in the dots shifts the angular curves along the tilt directions. Ó 2008 Elsevier B.V. All rights reserved. PACS: 82.80.Yc; 68.65.Ac; 78.67.Pt Keywords: Channelling; InAs quantum dots; Defects
1. Introduction A new class of optoelectronic devices is emerging based on the development of low dimensional structures. The space localization of carriers in these systems brings new challenges to solid state physics creating an enormous expectation of new technological breakthroughs. The zero dimensional confinement achieved in quantum dots (QD) is very much appealing both from the applications (photo* Corresponding author. Address: Instituto Te´cnico Nuclear ITN, EN10, 2686-953 Sacave`m, Portugal. Tel.: +351 219946086; fax: +351 219941525. E-mail address: [email protected] (E. Alves).
0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.12.078
detectors and laser diodes) and fundamental research point of view. Among the interesting properties are the low and temperature-independent threshold currents and the sharp optical transitions [1,2]. The growth of self assembled QDs is driven by the strain fields resulting from the lattice mismatch between the dots and the surrounding matrix. In the case of the InAs dots (lattice constant 0.605 nm) in GaAs (lattice constant 0.565 nm) the in-plane strain is 7%. The first layers grow strained and above the critical thickness the strain accumulated induces the spontaneous formation of the islands, the so called Stranski–Krastanov growth mode. The transition from the two dimensional growth to the three dimensional happen around 1–2 monolayers of InAs [3]. The growth
1440
E. Alves et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1439–1442
conditions allow the control of the shape, composition, size and strain distribution in the dots which have a strong impact on the electronic properties of the nanostructures. However, defects created during the growth, in particular the over layer cap, can degrade the operating characteristics of the QD systems. Recently a new process to reduce the number of defects present in the cap and consequently the optical quenching was proposed [4]. In this process the growth of the cap is interrupted and tetrachloromethane inserted in the reaction chamber before finishing the growth. In this work we will combine the channelling and high resolution X-ray diffraction techniques to study the strain and defects present in InAs/GaAs QD’s structures. Channelling is a well established technique offering the possibility to study defects and lattice distortions at nanometer scale. The results reveal an increase of the dechannelling at the depth where the InAs QD’s were embedded. The detailed angular scans along the major tilted axial directions show asymmetric In dips.
2. Experimental details InAs/GaAs self aligned quantum dots (SAQD) heterostructures were grown by atmospheric pressure metal organic vapour phase epitaxy (AP MOVPE). Two kinds of samples were prepared with different cap thicknesses. One sample were grown in standard conditions [5] with the quantum dots overgrown by an InxGa1 xAs quantum well (QW) layer at 520 °C (x = 0.2–0.3, the thickness 2– 3 nm) and covered with a 30 nm thick GaAs capping layer (CL), as illustrated in Fig. 1. The other samples (with both n and p types of doping) were grown slightly differently. After the deposition of 2–3 nm of InGaAs (QW) and 5 nm of GaAs onto the QD layer, the growth was interrupted and tetrachloromethane was introduced into the reactor at 580 °C for 20 s. Finally, the rest of the capping layer (with variable thickness) was grown at a temperature somewhat higher than for other samples. The crystalline quality, composition and strain were accessed both with high resolution X-ray diffraction
(HRXRD) and Rutherford backscattering/channelling spectrometry (RBS/C). The RBS/C measurements were performed with 1 mm collimated 4He+ beam with 2.0 MeV. The samples were mounted in a computer controlled two-axis goniometer with an accuracy of 0.01°. The backscattered particles were detected by two surface barrier silicon detectors placed at 160° and close to 180° with respect to the beam direction (Cornell geometry) and with 13 and 16 keV energy resolutions, respectively. The strain in the layers was studied performing detailed angular scans of the <1 0 1> and <1 1 1> directions along the (0 1 0) and (1–10) planes, respectively. The XRD studies were done with a double-crystal diffractometer with a resolution of the order of 30 arc sec. 3. Results and discussion The presence of the InAs dots and the depths and thickness of the In doped layer was measured by grazing incidence RBS analysis. The spectra were simulated using the NDF code to model the presence of the quantum dots [6]. The results are shown in Fig. 2 and the best results were obtained assuming a thickness of 28(3) nm for the cap layer (we did a self-consistent analysis enforcing the same energy calibration on the two angles and considering bulk densities) and 4 nm for the QD height. In this case, the volume fraction of the QDs is 38 vol.% and the surface roughness (or inhomogeneity of the cap layer thickness) is around 3.5 nm. Small deviations from these values also give good fits but our results are in excellent agreement with the AFM analyses of similar samples [5]. For the sample with a thicker GaAs cap the In signal is shadowed by the Ga and As signals. Fig. 3 shows the random, (1–10) plane and <1 1 1> axis aligned spectra where is visible the kink due to the dechannelling in the region of the buried InAs QD’s. The continuous line is the result of the simulation Energy(KeV) 1500 400
1600
1700
θinc= 75°, αscat=160°
300
GaAs
Yield (counts)
200 100
In
0
θinc= 4 °, αscat=160°
1500 GaAs 1000 500
In
0 260
280
300
Chennel
Fig. 1. Schematic diagram of the sample structure used in these studies.
Fig. 2. RBS spectra and simulated results (continuous curve) obtained for two incident angles.
E. Alves et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1439–1442 GaAs Depth (nm) 600
400
200
0
700
Yield (counts)
600
E=2.0 MeV
(1-10) plane random <111> axis
500
In
400 300
Wind1 Wind2
200
Step
100 0 300
400
500
600
Channel
Fig. 3. Random, <1 1 1> and (1–10) aligned RBS spectra obtained for a sample with a 227 nm GaAs cap. The step close to the interface of the GaAs/InAs is only visible along the axial direction. The windows used for the angular scans are indicated as well as the region where the In signal would appear.
obtained considering a cap thickness of 227 nm. The presence of the QD’s does not change the results of the simulation. The angular scans (Fig. 4) performed along the plane (1–10) through the <1 1 1> axis, integrating the counts in the windows corresponding to the two regions shown in Fig. 3, reveals a large difference on the minimum yield of the curves. A similar behaviour was observed for the <1 0 1> axis. On the other hand the kink visible in the <1 1 1> direction is missing in the (1–10) plane indicating a preferential orientation of the defects responsible for the dechannelling. This effect was reported by others [7] and in most of the cases was attributed to the presence of dislocations. To confirm this possibility we studied the
1441
energy dependence of the dechannelling process. The results obtained for the <1 1 1>, <1 0 1> and (1–10) directions are plotted in Fig. 5 for the regions immediately before and after the kink observed in the channelled spectra for the axial directions as we did for the angular scans. The data do not show any strong dependence with beam energy except for the <1 1 1> axis where a small increase is observed. According the analytical model developed by Quere´ [8] we expect a E1/2 dependence for the scattering by dislocations and a E 1/2 for the scattering by point defects. Taking this into account we can conclude that we must have different type of defects in the sample. Other possibility to explain the kink is the presence of stacking faults (extra or missing planes) close to the InAs dot layer since the dechannelling cross section for this type of defects is independent of the energy [9]. In a recent work Sears et al. [10] observed a large number of defects associated with the presence of InAs QD’s. The defects were grouped in two major classes: Stacking faults and V-shaped defects formed by pairs of edge dislocations running from the QD layer. It was also concluded that dislocations were in the {1 1 1} planes which could explain the channelling visibility along the <1 1 1> perpendicular directions. In order to find the structural correlation of the InAs QD’s with the GaAs matrix we did detailed angular scans through the <0 0 1>, <1 0 1> and <1 1 1> tilt axis in the sample with the 28 nm GaAs cap. The scan directions were parallel to the (0 1 0) for the <1 0 1> direction and (1–10) plane for the <1 1 1> direction. The results obtained for In and two regions in the GaAs are shown in Fig. 6. The regions chosen in the GaAs correspond to the substrate and the region containing the InAs quantum dots and the cap. Naturally we could not separate the As in the QD’s from the As in the cap but its amount is not enough to influence the results. For the <0 0 1> the angular curves (not shown) overlap but a shift was found for the tilt directions. It is also visible in the <1 0 1> axis some asymmetry in the curves. Our results are different from those found by
1. 0 <111>
Wind1
wind1 <110> wind2 <110> wind1 <111> wind 2 <111> wind1 (1-10) wind 2 (1-10)
1.0
0. 6
Normalised Yield
Normalized Yield
1.2
Wind2
0. 8
0. 4
0. 2
0.8
0.6
0.4
0.2
0. 0 -2
-1
0
1
2
Angle (degree) Fig. 4. Angular scans thought the <1 1 1> axis along the (1–10) plane for the sample with the thicker GaAs cap. The windows used correspond to the region just before and after the step observed in the aligned spectra of Fig. 3.
0.0 1.0
1.5
2.0
2.5
Energy (MeV)
Fig. 5. Energy dependence of the minimum yield for the directions indicated in the figure.
1442
E. Alves et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1439–1442
lattice parameter of the strained InAs dots and further studies are planned using synchrotron radiation.
<101>
1.0 0.8
4. Conclusions 0.6
Grazing incidence RBS and ion channelling results reveal the formation of strained InAs QD’s in GaAs. The high resolution RBS spectra allowed us to extract a value of 3 nm for the height of the dots and a volume fraction of the order of 50%. The energy dependence of the dechannelling cross section point for the presence of multiple defect types in the structure. The detailed angular scans through the major tilt axis suggest the presence of strain in the InAs quantum dots.
0.4
Normalized Yield
0.2 0.0 0.7 <111>
0.6 0.5
Acknowledgement
0.4
One of us (E. Alves) acknowledges Fundacßa˜o Calouste Gulbenkian for financial support.
0.3 0.2
References
GaAs (Substrate)
0.1
GaAs (QD layer) In
0.0 -2
-1
0
1
2
Angle (degrees) Fig. 6. Angular scans for the sample with the 28 nm GaAs cap thought the <1 1 1> and <1 0 1> axis along the (1–10) and (0 1 0) planes, respectively. The windows used for the GaAs correspond to the region containing the dots and the substrate.
Selen et al. [11] and Wang et al. [12] who observed a narrowing of the In curve compared to the GaAs. These authors assume a slight displacement of the In atoms from the regular sites or the lattice distortion to explain their data. This is not the case in our InAs/GaAs QD’s structure where the shift clearly indicates the presence of strain due to the lattice mismatch. Moreover the shape of the In curves seems to be the convolution of two curves with different widths. This could be due to the presence of a fully strain InAs wetting layer and a partial relaxed InAs QD’s. To obtain an average picture of the strain state of the dots we need to simulate the flux distribution in the channels to obtain the exact value of the kink angle as well as the correct value of its lattice parameter [13]. The development of the flux code [14] to model the QD’s layer is in progress. On the other hand due to the small dimension of the dots we could not resolve the X-ray signal to measure the
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