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Self organized InAs quantum dots grown on patterned GaAs substrates
Microelectronic Engineering 83 (2006) 1573–1576 www.elsevier.com/locate/mee
Self organized InAs quantum dots grown on patterned GaAs substrates Matthias Schramboeck *, W. Schrenk, T. Roch, A.M. Andrews, M. Austerer, G. Strasser Zentrum fu¨r Mikro- und Nanostrukturen, TU Wien, Vienna A-1040, Austria Available online 17 February 2006
Abstract GaAs substrates were patterned using holographic lithography and wet chemical etching. InAs quantum dots (QDs) grown on these substrates showed an alignment and an improved homogeneity compared to QDs grown on unpatterned GaAs. This was confirmed by atomic force microscopy and photo luminescence measurements. The use of an InGaAs stressor layer was also studied and could significantly improve the formation of QDs. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Quantum dots; Patterned substrates; Holographic lithography; InAs
1. Introduction During the last years, semiconductor nanostructures like quantum wires and quantum dots (QDs) have attracted an increasing attention. Especially QDs have been under intense investigation as they offer the ultimate limit in carrier confinement with discrete atomic-like energy states which can be exploited by novel devices like QD lasers. Using lattice mismatched material systems such as Si/Ge or In(Ga)As/GaAs, it is possible to grow QDs in a high density. However, QDs grown using the Stranski-Krastanow growth mode usually show an undesired size fluctuation which leads to a broadening of the optical linewidth. Furthermore, for device applications it is desirable to have more control over the size and the density of the QDs and to have control over the lateral position of the QDs on the substrate. To achieve this, a variety of methods have already been studied, ranging from atomic force microscopy based direct positioning of QDs [1] to overgrowth on pre-patterned substrates. Especially the growth of QDs on patterned substrates has shown promising results [2,3]. Among the lithographic techniques for pattern prep* Corresponding author. Tel.: +43 1 58801 36227; fax: +43 1 58801 36299. E-mail address: [email protected] (M. Schramboeck).
0167-9317/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2006.01.108
aration, the most common is electron beam lithography followed by either plasma enhanced or wet chemical etching [4,5]. In the following, we investigate holographic lithography (HL) as a method for pattern preparation. As a maskless technology, HL offers the possibility to transfer grid patterns in the nanometer range fast and efficiently onto the substrate. It has already shown promising results for pattern preparation for subsequent overgrowth [6,7]. In this work we show that QDs grown on patterned substrates improve the alignment and the homogeneity of the QDs. We also show that photoluminescence (PL) measurements conducted on QDs grown on patterned substrates show a higher intensity and a narrower linewidth than PL measurements conducted on QDs grown under the same growth conditions on unpatterned substrates. 2. Experiment The experimental setup for the HL comprises a HeCd Laser operating at a wavelength of 325 nm as an ultraviolet (UV) light source. The period of the pattern is determined by the incident angle between the laser optics and the substrate. To get a grid pattern, the sample was rotated 90° after the first exposure. For the overgrowth, samples with a period of 230 nm were fabricated. The holes have a diameter of 170 nm. After developing, a wet etch was performed, using H3PO4:H2O2:H2O =3:1:75, which resulted
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M. Schramboeck et al. / Microelectronic Engineering 83 (2006) 1573–1576
in an etch depth of 25 nm. The photoresist was then removed with acetone and isopropanol in an ultrasonic bath. As sample cleaning is an important issue regarding regrowth on pre-patterned samples, prior to overgrowth, any remnants of resist were removed by exposing the samples to an oxygen plasma. After a 19% HCl etch the patterned substrates were rinsed sufficiently long under DI water and a fresh layer of oxide was grown on top. They were then loaded into the molecular beam epitaxy (MBE) chamber. Sample oxides were removed by thermal desorption under an As4 overpressure. In situ reflection high energy electron diffraction (RHEED) was used to monitor the substrate surface. After oxide desorption a GaAs buffer layer ranging from 20 to 40 nm was grown and subsequently, 0.53 nm of InAs surface dots were deposited at 500 °C. For the atomic force microscopy (AFM) measurements performed on the sample, a dimension 3100 atomic force microscope (Veeco-Digital Instruments, Santa Barbara, CA, USA) operated in tapping mode has been used. The result is shown in Fig. 1. In one direction ridges are clearly visible, along which the InAs QDs align. These ridges result from the patterning and their period is still 230 nm. Furthermore, it is noticeable that the QDs nucleate not on top of the ridges or on the bottom of the holes but on the sidewalls of the ridges where the surface step density is greatest, which is a behaviour that has already been reported [8]. The fact that the ridges are only visible in one direction can be explained with the way the oxide is removed from the samples. Sample oxides were removed by thermal desorption under an As4 overpressure. During oxide removal the sample surface is monitored using
Fig. 1. AFM micrograph of HL patterned GaAs overgrown with 40 nm GaAs and 0.53 nm InAs. The 230 nm pattern was modified utilizing growth conditions to form ridges instead of the original 30 nm holes. Note that the InAs QDs nucleate on the ridge sidewalls.
RHEED but the 3-D nature of the patterned surface complicates using this tool for the oxide removal. Prior overgrowths also showed that especially in the holes of the patterns the removal of the oxide is often difficult because of inhomogeneous oxide thickness after surface patterning or a different surface kinetics due to the morphology. Successful oxide removal is accomplished by an optimization of these two parameters. This, however, must be balanced with the diffusion kinetics at the surface. High temperatures and low As4 pressures lead to fast diffusion on the surface which can, through the elimination of surface steps, smooth out the surface pattern into ridges oriented orthogonal to the fast diffusion direction [1 1 0]. So along the [1 1 0] direction the original 230 nm pattern was eliminated, while orthogonal to the [1 1 0] direction the 25 nm deep holes were reduced to only 2 nm high trenches. By growing an In0.2Ga0.8As layer directly on the patterned substrates before growing the GaAs buffer layer, the ordering of the QDs can be improved significantly. InGaAs is expected to be formed in the holes of the patterns thus forming a strain-modulated InGaAs layer. This InGaAs stressor layer can work as a template to order the QDs [9]. Fig. 2 shows an AFM scan of a HL pre-patterned substrate overgrown with a 20 nm In0.2Ga0.8As layer followed by a 10 nm GaAs buffer layer and 0.56 nm of InAs grown at 500 °C. An alignment of the InAs QDs according to the underlying pattern can be clearly observed. Also the size distribution of the QDs could be considerably improved. This is illustrated by the histograms shown in Fig. 3 where the distribution of QD height is plotted. Figs. 3(b) and (c) show the QD height distribution from
Fig. 2. AFM scan of HL patterned GaAs overgrown with 20 nm In0.2Ga0.8As, 10 nm GaAs and 5.6 nm InAs. Due to the strain coming from the InGaAs layer, the QDs align according to the underlying pattern.
M. Schramboeck et al. / Microelectronic Engineering 83 (2006) 1573–1576
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Fig. 3. Histogramms of QD height grown on: (a) unpatterned GaAs, (b) patterned GaAs and (c) on patterned GaAs with an InGaAs stressor layer. The histogramms were fitted to a Gaussian distribution.
the AFM scans shown in Figs. 1 and 2. Fig. 3(a) shows the height distribution of QDs grown on unpatterned GaAs at 500 °C with an InAs deposition of 0.54 nm as a reference. The histograms were fitted to a Gaussian distribution. The QDs grown on the unpatterned GaAs show a broad height distribution with two main peaks at 1.6 nm and 5.5 nm. This can be considerably improved by growing on patterned GaAs were only as single peak at 5.2 nm remains and the nucleation of the smaller QDs is noticeably reduced as shown in Fig. 3(b). The size uniformity of the QDs is further improved by growing the InGaAs stressor layer as is demostrated in Fig. 3(c). Furthermore, it is noticeable that the peak is now shifted to smaller QD heights, which hints to a different nucleation behaviour of the QDs due to the InGaAs layer. Also the optical emission properties of QDs grown on patterned and unpatterned substrates were investigated. Fig. 4 shows low-temperature (10 K) PL spectra of QDs grown under the same growth conditions on patterned and unpatterned areas of the same substrate. A stack of 10 layers of QDs was grown at 530 °C with an InAs deposition of 0.63 nm. The GaAs spacer layer between the layers of QDs was 10 nm to ensure vertical alignment. Before
QD growth an InGaAs stressor layer was grown. The signal from the patterned area clearly shows a higher intensity and a narrower linewidth (50 meV) than the signal from the unpatterned area (linewidth of 62 meV), which we attribute to the increased homogeneity of the QDs. The peak energy (1.25 eV) is slightly shifted to higher energies compared to the peak energy (1.22 eV) measured on the natural QDs which is probably due to the larger size of the natural QDs which was also shown in Fig. 3. The small peak at 1.16 meV comes from the laser used in our measurement setup. 3. Discussion The QDs grown on patterned GaAs substrates show an improved homogeneity, which was confirmed by AFM measurements. A noticeable alignment could be demonstrated by growing an InGaAs stressor layer prior to QD growth. The PL measurements performed on QDs grown on patterned and unpatterned GaAs showed an improved performance regarding linewidth and intensity on the patterned region of the GaAs. However, the investigated technique showed limitations regarding pattern diameter and pitch. Both have to be decreased to improve the alignment of the QDs. Thus we expect an improved performance by using other techniques like e-beam lithography and nanoimprinting. Also the use of focused ion beam (FIB) direct writing will be further investigated. Acknowledgments This work was partly supported by the European projects ANSWER, SANDiE, and IR-ON, the Austrian agencies GMe and FWF, and the MNA Network. References
Fig. 4. Low-temperature (10 K) PL spectra from QDs grown on patterned and unpatterned regions of the same substrate.
[1] S. Ohkouchi, Y. Nakamura, H. Nakamura, K. Asakawa, Physica E 21 (2004) 597. [2] G.S. Kar, S. Kiravittaya, M. Stoffel, O.G. Schmidt, PRL 93 (2004) 1922. [3] Z. Zong, G. Bauer, Appl. Phys. Lett. 84 (2004) 1922. [4] S. Watanabe, El Pelucchi, B. Dwir, M.H. Baier, K. Leifer, E. Kapon, Appl. Phys. Lett. 84 (2004) 2907.
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M. Schramboeck et al. / Microelectronic Engineering 83 (2006) 1573–1576
[5] H. Heidemeyer, U. Denker, C. Mu¨ller, O.G. Schmidt, PRL 91 (2003) 196103. [6] Seung-Chang Lee, K.J. Malloy, S.R.J. Brueck, J. Appl. Phys. 90 (2001) 4163. [7] H. Lee, J.A. Johnson, J.S. Speck, P.M. Petroff, J. Vac. Sci. Technol. B 18 (2000) 2193.
[8] W. Seifert, N. Carlsson, A. Petersson, L.-E. Wernersson, L. Samuelson, Appl. Phys. Lett. 68 (1996) 12. [9] Y. Nakamura, O.G. Schmidt, N.Y. Jin-Phillip, S. Kiravittaya, C. Mu¨ller, K. Eberl, H. Gra¨beldinger, H. Schweizer, J. Cryst. Growth 242 (2002) 339.
Self organized InAs quantum dots grown on patterned GaAs substrates Matthias Schramboeck *, W. Schrenk, T. Roch, A.M. Andrews, M. Austerer, G. Strasser Zentrum fu¨r Mikro- und Nanostrukturen, TU Wien, Vienna A-1040, Austria Available online 17 February 2006
Abstract GaAs substrates were patterned using holographic lithography and wet chemical etching. InAs quantum dots (QDs) grown on these substrates showed an alignment and an improved homogeneity compared to QDs grown on unpatterned GaAs. This was confirmed by atomic force microscopy and photo luminescence measurements. The use of an InGaAs stressor layer was also studied and could significantly improve the formation of QDs. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Quantum dots; Patterned substrates; Holographic lithography; InAs
1. Introduction During the last years, semiconductor nanostructures like quantum wires and quantum dots (QDs) have attracted an increasing attention. Especially QDs have been under intense investigation as they offer the ultimate limit in carrier confinement with discrete atomic-like energy states which can be exploited by novel devices like QD lasers. Using lattice mismatched material systems such as Si/Ge or In(Ga)As/GaAs, it is possible to grow QDs in a high density. However, QDs grown using the Stranski-Krastanow growth mode usually show an undesired size fluctuation which leads to a broadening of the optical linewidth. Furthermore, for device applications it is desirable to have more control over the size and the density of the QDs and to have control over the lateral position of the QDs on the substrate. To achieve this, a variety of methods have already been studied, ranging from atomic force microscopy based direct positioning of QDs [1] to overgrowth on pre-patterned substrates. Especially the growth of QDs on patterned substrates has shown promising results [2,3]. Among the lithographic techniques for pattern prep* Corresponding author. Tel.: +43 1 58801 36227; fax: +43 1 58801 36299. E-mail address: [email protected] (M. Schramboeck).
0167-9317/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2006.01.108
aration, the most common is electron beam lithography followed by either plasma enhanced or wet chemical etching [4,5]. In the following, we investigate holographic lithography (HL) as a method for pattern preparation. As a maskless technology, HL offers the possibility to transfer grid patterns in the nanometer range fast and efficiently onto the substrate. It has already shown promising results for pattern preparation for subsequent overgrowth [6,7]. In this work we show that QDs grown on patterned substrates improve the alignment and the homogeneity of the QDs. We also show that photoluminescence (PL) measurements conducted on QDs grown on patterned substrates show a higher intensity and a narrower linewidth than PL measurements conducted on QDs grown under the same growth conditions on unpatterned substrates. 2. Experiment The experimental setup for the HL comprises a HeCd Laser operating at a wavelength of 325 nm as an ultraviolet (UV) light source. The period of the pattern is determined by the incident angle between the laser optics and the substrate. To get a grid pattern, the sample was rotated 90° after the first exposure. For the overgrowth, samples with a period of 230 nm were fabricated. The holes have a diameter of 170 nm. After developing, a wet etch was performed, using H3PO4:H2O2:H2O =3:1:75, which resulted
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M. Schramboeck et al. / Microelectronic Engineering 83 (2006) 1573–1576
in an etch depth of 25 nm. The photoresist was then removed with acetone and isopropanol in an ultrasonic bath. As sample cleaning is an important issue regarding regrowth on pre-patterned samples, prior to overgrowth, any remnants of resist were removed by exposing the samples to an oxygen plasma. After a 19% HCl etch the patterned substrates were rinsed sufficiently long under DI water and a fresh layer of oxide was grown on top. They were then loaded into the molecular beam epitaxy (MBE) chamber. Sample oxides were removed by thermal desorption under an As4 overpressure. In situ reflection high energy electron diffraction (RHEED) was used to monitor the substrate surface. After oxide desorption a GaAs buffer layer ranging from 20 to 40 nm was grown and subsequently, 0.53 nm of InAs surface dots were deposited at 500 °C. For the atomic force microscopy (AFM) measurements performed on the sample, a dimension 3100 atomic force microscope (Veeco-Digital Instruments, Santa Barbara, CA, USA) operated in tapping mode has been used. The result is shown in Fig. 1. In one direction ridges are clearly visible, along which the InAs QDs align. These ridges result from the patterning and their period is still 230 nm. Furthermore, it is noticeable that the QDs nucleate not on top of the ridges or on the bottom of the holes but on the sidewalls of the ridges where the surface step density is greatest, which is a behaviour that has already been reported [8]. The fact that the ridges are only visible in one direction can be explained with the way the oxide is removed from the samples. Sample oxides were removed by thermal desorption under an As4 overpressure. During oxide removal the sample surface is monitored using
Fig. 1. AFM micrograph of HL patterned GaAs overgrown with 40 nm GaAs and 0.53 nm InAs. The 230 nm pattern was modified utilizing growth conditions to form ridges instead of the original 30 nm holes. Note that the InAs QDs nucleate on the ridge sidewalls.
RHEED but the 3-D nature of the patterned surface complicates using this tool for the oxide removal. Prior overgrowths also showed that especially in the holes of the patterns the removal of the oxide is often difficult because of inhomogeneous oxide thickness after surface patterning or a different surface kinetics due to the morphology. Successful oxide removal is accomplished by an optimization of these two parameters. This, however, must be balanced with the diffusion kinetics at the surface. High temperatures and low As4 pressures lead to fast diffusion on the surface which can, through the elimination of surface steps, smooth out the surface pattern into ridges oriented orthogonal to the fast diffusion direction [1 1 0]. So along the [1 1 0] direction the original 230 nm pattern was eliminated, while orthogonal to the [1 1 0] direction the 25 nm deep holes were reduced to only 2 nm high trenches. By growing an In0.2Ga0.8As layer directly on the patterned substrates before growing the GaAs buffer layer, the ordering of the QDs can be improved significantly. InGaAs is expected to be formed in the holes of the patterns thus forming a strain-modulated InGaAs layer. This InGaAs stressor layer can work as a template to order the QDs [9]. Fig. 2 shows an AFM scan of a HL pre-patterned substrate overgrown with a 20 nm In0.2Ga0.8As layer followed by a 10 nm GaAs buffer layer and 0.56 nm of InAs grown at 500 °C. An alignment of the InAs QDs according to the underlying pattern can be clearly observed. Also the size distribution of the QDs could be considerably improved. This is illustrated by the histograms shown in Fig. 3 where the distribution of QD height is plotted. Figs. 3(b) and (c) show the QD height distribution from
Fig. 2. AFM scan of HL patterned GaAs overgrown with 20 nm In0.2Ga0.8As, 10 nm GaAs and 5.6 nm InAs. Due to the strain coming from the InGaAs layer, the QDs align according to the underlying pattern.
M. Schramboeck et al. / Microelectronic Engineering 83 (2006) 1573–1576
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Fig. 3. Histogramms of QD height grown on: (a) unpatterned GaAs, (b) patterned GaAs and (c) on patterned GaAs with an InGaAs stressor layer. The histogramms were fitted to a Gaussian distribution.
the AFM scans shown in Figs. 1 and 2. Fig. 3(a) shows the height distribution of QDs grown on unpatterned GaAs at 500 °C with an InAs deposition of 0.54 nm as a reference. The histograms were fitted to a Gaussian distribution. The QDs grown on the unpatterned GaAs show a broad height distribution with two main peaks at 1.6 nm and 5.5 nm. This can be considerably improved by growing on patterned GaAs were only as single peak at 5.2 nm remains and the nucleation of the smaller QDs is noticeably reduced as shown in Fig. 3(b). The size uniformity of the QDs is further improved by growing the InGaAs stressor layer as is demostrated in Fig. 3(c). Furthermore, it is noticeable that the peak is now shifted to smaller QD heights, which hints to a different nucleation behaviour of the QDs due to the InGaAs layer. Also the optical emission properties of QDs grown on patterned and unpatterned substrates were investigated. Fig. 4 shows low-temperature (10 K) PL spectra of QDs grown under the same growth conditions on patterned and unpatterned areas of the same substrate. A stack of 10 layers of QDs was grown at 530 °C with an InAs deposition of 0.63 nm. The GaAs spacer layer between the layers of QDs was 10 nm to ensure vertical alignment. Before
QD growth an InGaAs stressor layer was grown. The signal from the patterned area clearly shows a higher intensity and a narrower linewidth (50 meV) than the signal from the unpatterned area (linewidth of 62 meV), which we attribute to the increased homogeneity of the QDs. The peak energy (1.25 eV) is slightly shifted to higher energies compared to the peak energy (1.22 eV) measured on the natural QDs which is probably due to the larger size of the natural QDs which was also shown in Fig. 3. The small peak at 1.16 meV comes from the laser used in our measurement setup. 3. Discussion The QDs grown on patterned GaAs substrates show an improved homogeneity, which was confirmed by AFM measurements. A noticeable alignment could be demonstrated by growing an InGaAs stressor layer prior to QD growth. The PL measurements performed on QDs grown on patterned and unpatterned GaAs showed an improved performance regarding linewidth and intensity on the patterned region of the GaAs. However, the investigated technique showed limitations regarding pattern diameter and pitch. Both have to be decreased to improve the alignment of the QDs. Thus we expect an improved performance by using other techniques like e-beam lithography and nanoimprinting. Also the use of focused ion beam (FIB) direct writing will be further investigated. Acknowledgments This work was partly supported by the European projects ANSWER, SANDiE, and IR-ON, the Austrian agencies GMe and FWF, and the MNA Network. References
Fig. 4. Low-temperature (10 K) PL spectra from QDs grown on patterned and unpatterned regions of the same substrate.
[1] S. Ohkouchi, Y. Nakamura, H. Nakamura, K. Asakawa, Physica E 21 (2004) 597. [2] G.S. Kar, S. Kiravittaya, M. Stoffel, O.G. Schmidt, PRL 93 (2004) 1922. [3] Z. Zong, G. Bauer, Appl. Phys. Lett. 84 (2004) 1922. [4] S. Watanabe, El Pelucchi, B. Dwir, M.H. Baier, K. Leifer, E. Kapon, Appl. Phys. Lett. 84 (2004) 2907.
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M. Schramboeck et al. / Microelectronic Engineering 83 (2006) 1573–1576
[5] H. Heidemeyer, U. Denker, C. Mu¨ller, O.G. Schmidt, PRL 91 (2003) 196103. [6] Seung-Chang Lee, K.J. Malloy, S.R.J. Brueck, J. Appl. Phys. 90 (2001) 4163. [7] H. Lee, J.A. Johnson, J.S. Speck, P.M. Petroff, J. Vac. Sci. Technol. B 18 (2000) 2193.
[8] W. Seifert, N. Carlsson, A. Petersson, L.-E. Wernersson, L. Samuelson, Appl. Phys. Lett. 68 (1996) 12. [9] Y. Nakamura, O.G. Schmidt, N.Y. Jin-Phillip, S. Kiravittaya, C. Mu¨ller, K. Eberl, H. Gra¨beldinger, H. Schweizer, J. Cryst. Growth 242 (2002) 339.