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Effect of GaAs polycrystal on the size and areal density of InAs quantum dots in selective area molecular beam epitaxy
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Journal of Crystal Growth 297 (2006) 38–43 www.elsevier.com/locate/jcrysgro
Effect of GaAs polycrystal on the size and areal density of InAs quantum dots in selective area molecular beam epitaxy J.C. Lina,, R. Hogga, P. Frya, M. Hopkinsona, I. Rossb, A. Cullisb, R. Kolodkac, A. Tartakovskiic, M. Skolnickc a
EPSRC National Centre for III-V Technologies, Department of Electronics and Electrical Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK b Department of Electronic and Electrical Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK c Department of Physics and Astronomy, University of Sheffield, Hounsfield Road, Sheffield S3 7RH, UK Received 2 June 2006; received in revised form 9 August 2006; accepted 13 September 2006 Communicated by K.H. Ploog Available online 15 November 2006
Abstract Selective growth of self-assembled InAs/GaAs quantum dots (QDs) is achieved by molecular beam epitaxy (MBE) utilizing dielectric masks on GaAs substrates. We find that polycrystalline deposits on the mask due to non-ideal growth selectivity between the mask and the epitaxy induce a significant modification of the QD height and areal density in their neighborhood. The results show an effective method to achieve selective area QD growth by using a dielectric mask and altering the degree of selectivity through control over the MBE growth conditions in a pulsed deposition mode. r 2006 Elsevier B.V. All rights reserved. PACS: 68.37.Hk; 68.37.Ps; 78.67.Hc; 81.15.Hi Keywords: A3. Molecular beam epitaxy; A3. Polycrystalline deposition; B2. Semiconducting III–V materials
1. Introduction Growth challenges for self-assembled quantum dot (QD) structures include controlling both the spatial position and the emission energy of many or single QDs. Large scale control of many dots (i.e. the tuning of emission energy to form active and passive areas) would find application in a wide range of opto-electronic devices, while single QD placement technologies allow the fabrication of devices which make full use of the atom-like nature of QDs. Selective area epitaxy (SAE) employs dielectric masks on the semiconductor surface to locally modify growth parameters on the epitaxial surface close to the mask. It is traditionally applied to growth using metal organic sources such as metal-organic vapor phase epitaxy (MOVPE) and chemical beam epitaxy (CBE), where a Corresponding author. Tel.: +44 114 2225355; fax: +44 114 2225145.
E-mail address: j.c.lin@sheffield.ac.uk (J.C. Lin). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.09.027
change in alloy composition and thickness is usually observed due to both the transport of material deposited upon the mask to the epitaxial growth regions and the diffusion of reagents within the gas phase to the epitaxial regions [1]. Both MOVPE [2–6] and CBE [7–9] have been applied to the SAE of self-assembled QDs. Most of the existing high-quality InGaAs QD structures are grown by molecular beam epitaxy (MBE). The availability of demonstrable nano-scale control and the in-situ growth monitoring tools in MBE make it a highly attractive technology for QD growth, as QD layer deposition rate, temperature, and thickness are critical to the QD density, size, and composition. It is comparatively difficult to obtain growth selectivity between a dielectric mask and epitaxial area by MBE, as a polycrystalline deposit tends to form on the mask [10]. Methods have been developed to increase the selectivity, such as using atomic hydrogen [11] and periodic supply epitaxy (PSE) [10,12]. The PSE technique that we have employed in this work
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interrupts the group III adatom material supply periodically while maintaining the As pressure within the MBE reactor, allowing the adatoms to either migrate off the mask or re-evaporate back into the MBE chamber. This effectively reduces the overall growth rate. The difference between Ga sticking coefficients on the epitaxy and on the SiO2 mask under various growth temperature has also been studied [13,14]. At low temperatures (o570 1C) both coefficients are close to unity; however, at elevated temperatures a substantial difference has been observed. Therefore, provided the growth is carried out using a high temperature and a low overall growth rate, a high selectivity is achievable. In this paper, we report the selective area MBE growth of QDs focused on the relationship between the GaAs polycrystal-covered dielectric mask and the InAs QD distribution and emission energy on the GaAs (0 0 1) surface. We show how the QD areal density, QD height, and, emission energy are altered by the presence of a GaAs polycrystal-covered mask, and discuss the origin of this effect. 2. Experimental procedure There are two experiments described in this paper. All samples in the both experiment sets were fabricated with identical dielectric mask pattern on GaAs (0 0 1) surface by plasma enhanced chemical vapor deposition, electron beam lithography, and reactive ion etching. The first set used 100 nm SiO2 layer and the second set used 120 nm Si3N4 layer as dielectric mask. The mask pattern consisted of a number of cells consisting of a series of 1.3 mm long parallel mask stripes oriented along the [1 1 0] orientation. The mask stripe widths and spacings were varied for each cell, with widths varying from 0.5 to 4 mm, and spacings varied from 1 to 4 mm. Similar results were obtained for all combinations of mask width and spacing. Unless stated otherwise all results discussed in the following are for a 2 mm mask width and 4 mm spacing cell. Solvent cleaning, oxygen plasma aching, and a short immersion into a diluted hydrofluoric acid solution were applied before the samples were transferred to a VG Semicon V90 H MBE system. The detail of the two samples grown for first experiment is described in the followings. The first sample of the set was an uncapped QD structure where the QDs were grown after the deposition of 200 nm of GaAs. The second sample was identical to the first except a second 200 nm GaAs cap was deposited to allow PL studies. The 200 nm GaAs layers were grown in 40 supply-interrupt PSE cycles. Each supply period deposits 5 nm of GaAs at 590 1C at a rate of 0.2 nm/ s, followed by an interrupt of 2 min at 620 1C. Four 1 nm AlAs marker layers were inserted within the GaAs at the same growth temperature before every 10 PSE cycles. For both samples, after the buffer layer growth the substrate temperature was set to 500 1C, the InAs QD deposition temperature. A 250 s interrupt period was waited while the
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samples were cooling down. The QDs were then formed by depositing 2.5 monolayers (MLs) of InAs at the rate of 0.05 ML/s. The growth procedure for the first, uncapped sample was completed at this stage and was then crashcooled to room temperature. For the second, capped sample, immediately after the deposition, another 10 nm GaAs layer was grown at 500 1C. It is then followed by another interrupt of 250 s during which the substrate temperature was reset to the growth temperature for the final, second 200 nm GaAs capping layer and AlAs marker layers. Both samples were crash-cooled to room temperature immediately after their respective growth procedures were completed. These growth parameters gave significant GaAs polycrystal coverage on the dielectric mask. The second set of five samples was fabricated for comparing QD areal density distribution under various degrees of GaAs polycrystalline deposition on the mask. All five samples are uncapped samples and the growth procedures were identical to the uncapped sample in the first experiment except that no AlAs marker layers were inserted, as the Al content makes the polycrystals more difficult to be removed with PSE [12]. The PSE growth supply and interrupt temperatures as well as the PSE interrupt durations were varied on the five samples, and these growth parameters together with SEM polycrystal images and QD areal density changes are illustrated later in a figure.
Fig. 1. (a) A SEM image of the uncapped sample; (b) Higher magnification image of the outlined area of (a); (c) is at the same magnification as (b), but at a point on the epitaxial ridge 20 mm further into the mask pattern (i.e. toward the right). Compared to (b), lower density of QDs is observed in (c).
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3. Result and discussion Fig. 1(a) shows a typical plan view scanning electron microscope (SEM) image on the uncapped sample of the first experiment. Fig. 1(b) is a higher magnification image of the outlined area of Fig. 1(a). Fig. 1(c) is at the same magnification as Fig. 1(b), but is at a relative position 20 mm further into the mask pattern (in the direction marked A to A0 ). The polycrystal-covered mask is visible in the upper right part of Fig. 1(a). For the epitaxial regions, a significant modification of the QD areal density is observed. On the epitaxial area close to the mask pattern, a high QD areal density is evident within 1 mm of the mask edge. However, this high areal density QD region decreases in density along the epitaxial ridge edge in the mask pattern. The increase of QD areal density is a result of the high availability of atomic steps due to surface morphology in this region, especially in the area around the ends of mask stripes where the shallow non-planar epitaxial surfaces (41 from the surface orientation far from the ridges, observed under cross-sectional SEM in the orientation B to B0 ) between mask stripes join the flat surface outside the mask pattern and form multiple facets. It can also be seen that QDs self-align along the [1 1 0], [1 0 0], [0 1 0] edges of the epitaxial layers in agreement with previous observations [15–17]. This alignment of QDs is maintained along the whole length of the epitaxial mesa. While Fig. 1 shows the QD areal distribution between the mask stripes in the pattern, Fig. 2 from a macroscopic view shows the study of the QD areal distribution outside the pattern on the same sample. The areal QD density is measured by SEM in directions both parallel ([1 1 0] orientation) and orthogonal to the mask stripes. Except for the 1 mm region adjacent to the mask where a high QD areal density is observed, the areal density around the mask is low in general. The QD density is observed to increase from this low value (2 109 cm2) quickly and
Fig. 2. QD areal density outside the mask region increases rapidly with the distance away from the mask pattern, tending towards a saturation value given by the background QD areal density (2 1010 cm2) at 30 mm from the mask.
Fig. 3. The average height and number of QDs as a function of distance into the mesa between parallel mask stripes in the mask cell with mask width 2 mm and mask spacing 4 mm. Each point has a sampling area taken from a rectangular region of 0.7 4 mm as schematically shown at the bottom of the figure.
then tends to a constant background density of 2 1010 cm2 at a distance around 30 mm. Fig. 3 shows the QD areal density and average height obtained by atomic force microscopy (AFM) as a function of distance along the epitaxial ridge between the mask stripes. The further into the mask cell, the greater are the reductions in both average QD height and QD areal density. In terms of QD areal density, the general trend observed above is that for epitaxy on any area, (except for the 1 mm region immediately adjacent to the polycrystal-covered mask), lower QD areal densities is observed in the region closer to the mask (Fig. 2) or in the region more surrounded by the mask (as in the case of Fig. 3). The QD areal density reduction is a result of the reduction in the InAs coverage reduction close to the mask area, and this implies some In adatoms around the pattern during growth were missing from the area, being either withdrawn or repelled away. The compositional change of In content in the structures around the pattern can be revealed by optical studies. Micro-photoluminescence (mPL) has been conducted on the capped sample and confirmed the presence of In on the polycrystal-covered mask. Fig. 4(a) shows a schematic indicating the location of mPL spectra shown in Figs. 4(b) and (c). The emission spectra were obtained at 7 K using excitation at 633 nm and a power of 0.2 mW focused onto a region with diameter in the order of 1 mm on the capped sample. Around 100 mm from the mask pattern (far away) the emission spectrum is characterized by bulk GaAs emission at 820 nm, very weak wetting layer emission at 875 nm and broad QD emission peaking at 1060 nm. As the spot is translated along the epitaxial ridge, a significant reduction in QD emission intensity, blue-shift of the peak
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STRIPE CENTER Far away
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Fig. 4. (a) Schematically shows the points where the mPL laser spots are focused on. (b) A series of mPL scans between mask stripes with various distances into the mask cell. (c) mPL scan comparison when the laser spot is in the center of the mask cell, but one on the epitaxy and the other on the polycrystal-covered mask.
QD emission wavelength, and concurrent increase in wetting layer emission intensity are observed. This is consistent with the reduction in QD areal density observed by AFM. Furthermore, the QD emission is observed to shift to shorter wavelengths as the excitation area is translated along the epitaxial ridge. This is also in agreement with our structural measurements where a reduction in QD height was observed with increasing
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distance along the epitaxial ridge. Fig. 4(c) shows PL spectra in the 800–940 nm range, where the InGaAs wetting layer like response occurs, obtained at locations shown schematically in Fig. 4(a). For excitation on the center of the epitaxial ridge, a peak at 860 nm is observed, which is attributed to the InAs wetting layer. For excitation on the polycrystal-covered mask right next to it, similar emission at 880 nm is observed, indicating the presence of an InGaAs region on the polycrystal-covered dielectric mask. A possibility that the missing In adatom from the epitaxial area is transported to the (1 1 1) B sidewall of the mesa was ruled out by cross-sectional scanning transmission electron microscopic (STEM) studies of the capped sample, as the contrast that might indicate an InAs layer on the (1 1 1) B side wall is not observed. The mPL and STEM studies agree with our AFM and SEM observation of the uncapped sample and suggest the In adatom transportation onto the mask is due to the presence of polycrystals. To assess how the polycrystal on the dielectric mask affect the QD distribution in the SA-MBE, QD areal densities around the mask patterns on samples with various degree of polycrystal deposit were investigated. Fig. 5 shows the growth parameters, SEM images of resulted polycrystalline deposit, and the trend of QD areal density changes around the pattern of the five samples labeled from A to E. It is noted that the high PSE growth and interrupt temperature is mandatory in order to reduce the polycrystalline deposit. For the QD areal density study, these densities were obtained by AFM at same locations on each sample, around the pattern group that has 4 mm mask stripe with and 2 mm spacing. A series of 1 1 mm AFM scans were performed along the center of epitaxial mesas, which are between two innermost dielectric mask stripes inside the pattern group. The trends of QD areal density changes shown in Fig. 5 are normalized against the QD background areal densities obtained on locations far away from the pattern. From sample labeled A–E in Fig. 5, as less polycrystalline deposit were formed on the mask, the higher the relative QD areal density at the center of the pattern compared to background density. It is therefore evident that the GaAs polycrystal is the key factor that alters the InAs QD distribution around the dielectric mask pattern in SA–MBE. The deposition of InAs on a planar growth surface to realize self-assembled QDs has been described by a segregation based mechanism where a critical indium composition of the surface layer is required to trigger the onset of QD formation [18,19]. Failing to achieve the critical In surface concentration on the epitaxial area inhibits the formation of QDs and prevents their ripening. The reduction in InAs QD height areal density observed in our experiments is therefore attributed to the effective InAs coverage reduction. It is interesting to note that this is in the opposite trend to selective area MOVPE growth reports, where an increase in effective InAs coverage is expected close to the mask region [1,20].
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Fig. 5. The PSE growth parameters, SEM images of resulted polycrystalline formation on the Si3N4 mask and relative QD areal density (normalized to background density) around the mask pattern with 4 mm mask width and 2 mm spacing.
It is apparent that different regions of the sample are competing for the deposited indium and our results indicate the GaAs polycrystals on the dielectric mask acts as a very effective sink for In adatoms. A hierarchy exists that the GaAs polycrystal and epitaxial ridge edges are both highly efficient at incorporating indium. For the GaAs polycrystal a large number of dangling bonds exist and a high density of step edges exist, and thus it is the most preferred site for In incorporation. Due to the presence of step edges, the non-planar epitaxial regions appear to be next most likely to incorporate indium. The planar epitaxial regions are the most inefficient, leading to a denuded InAs composition and hence QD density and size. This effect provides a novel SAE mechanism for QD structures. The non-selective growth of GaAs and formation of a polycrystal-covered dielectric mask results in the selective formation of self assembled QDs on the epitaxially grown material. 4. Summary In summary, we have discussed the SAE of InAs/GaAs self assembled QDs. We find a significant modification to the QD height and areal density in the region of a polycrystal-covered dielectric mask. The change in QD emission energy and intensity due to the change in QD size and areal density may be utilized for fabricating optically active and passive region on the substrate in single step growth process. We have also discussed the origin of this
phenomenon, that is, the transport of segregated indium to the polycrystal-covered mask, and its use as a new method for obtaining selective area growth of QD structures. Acknowledgment J. Lin acknowledges support from Engineering and Physical Science Research Council (EPSRC), United Kingdom under grant GR/R65534/01. The work was also supported in part by EPSRC grant GR/S76076/01 and GR/R65626/01. References [1] M. Gibbon, J.P. Stagg, C.G. Cureton, E.J. Thrush, C.J. Jones, R.E. Mallard, R.E. Pritchard, N. Collis, A. Chew, Semicond. Sci. Tech. 8 (1993) 998. [2] F. Nakajima, Y. Ogasawara, J. Motohisa, T. Fukui, J. Appl. Phys. 90 (2001) 2606. [3] F. Nakajima, Y. Miyoshi, J. Motohisa, T. Fukui, Appl. Phys. Lett. 83 (2003) 2680. [4] C.-K. Hahn, J. Motohisa, T. Fukui, Appl. Phys. Lett. 76 (2000) 3947. [5] H.J. Kim, J. Motohisa, T. Fukui, Appl. Phys. Lett. 81 (2002) 5147. [6] J. Motohisa, F. Nakajima, T. Fukui, Appl. Surf. Sci. 190 (2002) 184. [7] J. Lefebvre, P.J. Poole, J. Fraser, G.C. Aers, D. Chithrani, R.L. Williams, J. Crystal Growth 234 (2002) 391. [8] P.J. Poole, J. Lefebvre, J. Fraser, Appl. Phys. Lett. 83 (2003) 2055. [9] D. Chithrani, R.L. Williams, J. Lefebvre, P.J. Poole, G.C. Aers, Appl. Phys. Lett. 84 (2004) 978. [10] F.E. Allegretti, T. Nishinaga, J. Crystal Growth 156 (1995) 1. [11] T. Sugaya, Y. Okada, M. Kawabe, Jn. J. Appl. Phys. 31 (1992) L713. [12] G. Bacchin, T. Nishinaga, J. Crystal Growth 191 (1998) 599.
ARTICLE IN PRESS J.C. Lin et al. / Journal of Crystal Growth 297 (2006) 38–43 [13] S.C. Lee, K.J. Malloy, S.R.J. Brueck, J. Appl. Phys. 90 (2001) 4163. [14] S.C. Lee, K.J. Malloy, L.R. Dawson, S.R.J. Brueck, J. Appl. Phys. 92 (2002) 6567. [15] T.I. Kamins, R. Stanley Williams, Appl. Phys. Lett. 71 (1997) 1201. [16] R. Tsui, R. Zhang, K. Shiralagi, H. Goronkin, Appl. Phys. Lett. 71 (1997) 3254.
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[17] R. Zhang, R. Tsui, K. SHiralagi, D. Convey, H. Goronkin, Appl. Phys. Lett. 73 (1998) 505. [18] A.G. Cullis, D.J. Norris, M.A. Migliorato, M. Hopkinson, Phys. Rev. B 66 (2002) 081305. [19] A.G. Cullis, D.J. Norris, M.A. Migliorato, M. Hopkinson, Appl. Surf. Sci. 244 (2005) 65. [20] S. Mokkapati, P. Lever, H.H. Tan, C. Jagadish, Appl. Phys. Lett. 86 (2005) 113102.
Journal of Crystal Growth 297 (2006) 38–43 www.elsevier.com/locate/jcrysgro
Effect of GaAs polycrystal on the size and areal density of InAs quantum dots in selective area molecular beam epitaxy J.C. Lina,, R. Hogga, P. Frya, M. Hopkinsona, I. Rossb, A. Cullisb, R. Kolodkac, A. Tartakovskiic, M. Skolnickc a
EPSRC National Centre for III-V Technologies, Department of Electronics and Electrical Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK b Department of Electronic and Electrical Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK c Department of Physics and Astronomy, University of Sheffield, Hounsfield Road, Sheffield S3 7RH, UK Received 2 June 2006; received in revised form 9 August 2006; accepted 13 September 2006 Communicated by K.H. Ploog Available online 15 November 2006
Abstract Selective growth of self-assembled InAs/GaAs quantum dots (QDs) is achieved by molecular beam epitaxy (MBE) utilizing dielectric masks on GaAs substrates. We find that polycrystalline deposits on the mask due to non-ideal growth selectivity between the mask and the epitaxy induce a significant modification of the QD height and areal density in their neighborhood. The results show an effective method to achieve selective area QD growth by using a dielectric mask and altering the degree of selectivity through control over the MBE growth conditions in a pulsed deposition mode. r 2006 Elsevier B.V. All rights reserved. PACS: 68.37.Hk; 68.37.Ps; 78.67.Hc; 81.15.Hi Keywords: A3. Molecular beam epitaxy; A3. Polycrystalline deposition; B2. Semiconducting III–V materials
1. Introduction Growth challenges for self-assembled quantum dot (QD) structures include controlling both the spatial position and the emission energy of many or single QDs. Large scale control of many dots (i.e. the tuning of emission energy to form active and passive areas) would find application in a wide range of opto-electronic devices, while single QD placement technologies allow the fabrication of devices which make full use of the atom-like nature of QDs. Selective area epitaxy (SAE) employs dielectric masks on the semiconductor surface to locally modify growth parameters on the epitaxial surface close to the mask. It is traditionally applied to growth using metal organic sources such as metal-organic vapor phase epitaxy (MOVPE) and chemical beam epitaxy (CBE), where a Corresponding author. Tel.: +44 114 2225355; fax: +44 114 2225145.
E-mail address: j.c.lin@sheffield.ac.uk (J.C. Lin). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.09.027
change in alloy composition and thickness is usually observed due to both the transport of material deposited upon the mask to the epitaxial growth regions and the diffusion of reagents within the gas phase to the epitaxial regions [1]. Both MOVPE [2–6] and CBE [7–9] have been applied to the SAE of self-assembled QDs. Most of the existing high-quality InGaAs QD structures are grown by molecular beam epitaxy (MBE). The availability of demonstrable nano-scale control and the in-situ growth monitoring tools in MBE make it a highly attractive technology for QD growth, as QD layer deposition rate, temperature, and thickness are critical to the QD density, size, and composition. It is comparatively difficult to obtain growth selectivity between a dielectric mask and epitaxial area by MBE, as a polycrystalline deposit tends to form on the mask [10]. Methods have been developed to increase the selectivity, such as using atomic hydrogen [11] and periodic supply epitaxy (PSE) [10,12]. The PSE technique that we have employed in this work
ARTICLE IN PRESS J.C. Lin et al. / Journal of Crystal Growth 297 (2006) 38–43
interrupts the group III adatom material supply periodically while maintaining the As pressure within the MBE reactor, allowing the adatoms to either migrate off the mask or re-evaporate back into the MBE chamber. This effectively reduces the overall growth rate. The difference between Ga sticking coefficients on the epitaxy and on the SiO2 mask under various growth temperature has also been studied [13,14]. At low temperatures (o570 1C) both coefficients are close to unity; however, at elevated temperatures a substantial difference has been observed. Therefore, provided the growth is carried out using a high temperature and a low overall growth rate, a high selectivity is achievable. In this paper, we report the selective area MBE growth of QDs focused on the relationship between the GaAs polycrystal-covered dielectric mask and the InAs QD distribution and emission energy on the GaAs (0 0 1) surface. We show how the QD areal density, QD height, and, emission energy are altered by the presence of a GaAs polycrystal-covered mask, and discuss the origin of this effect. 2. Experimental procedure There are two experiments described in this paper. All samples in the both experiment sets were fabricated with identical dielectric mask pattern on GaAs (0 0 1) surface by plasma enhanced chemical vapor deposition, electron beam lithography, and reactive ion etching. The first set used 100 nm SiO2 layer and the second set used 120 nm Si3N4 layer as dielectric mask. The mask pattern consisted of a number of cells consisting of a series of 1.3 mm long parallel mask stripes oriented along the [1 1 0] orientation. The mask stripe widths and spacings were varied for each cell, with widths varying from 0.5 to 4 mm, and spacings varied from 1 to 4 mm. Similar results were obtained for all combinations of mask width and spacing. Unless stated otherwise all results discussed in the following are for a 2 mm mask width and 4 mm spacing cell. Solvent cleaning, oxygen plasma aching, and a short immersion into a diluted hydrofluoric acid solution were applied before the samples were transferred to a VG Semicon V90 H MBE system. The detail of the two samples grown for first experiment is described in the followings. The first sample of the set was an uncapped QD structure where the QDs were grown after the deposition of 200 nm of GaAs. The second sample was identical to the first except a second 200 nm GaAs cap was deposited to allow PL studies. The 200 nm GaAs layers were grown in 40 supply-interrupt PSE cycles. Each supply period deposits 5 nm of GaAs at 590 1C at a rate of 0.2 nm/ s, followed by an interrupt of 2 min at 620 1C. Four 1 nm AlAs marker layers were inserted within the GaAs at the same growth temperature before every 10 PSE cycles. For both samples, after the buffer layer growth the substrate temperature was set to 500 1C, the InAs QD deposition temperature. A 250 s interrupt period was waited while the
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samples were cooling down. The QDs were then formed by depositing 2.5 monolayers (MLs) of InAs at the rate of 0.05 ML/s. The growth procedure for the first, uncapped sample was completed at this stage and was then crashcooled to room temperature. For the second, capped sample, immediately after the deposition, another 10 nm GaAs layer was grown at 500 1C. It is then followed by another interrupt of 250 s during which the substrate temperature was reset to the growth temperature for the final, second 200 nm GaAs capping layer and AlAs marker layers. Both samples were crash-cooled to room temperature immediately after their respective growth procedures were completed. These growth parameters gave significant GaAs polycrystal coverage on the dielectric mask. The second set of five samples was fabricated for comparing QD areal density distribution under various degrees of GaAs polycrystalline deposition on the mask. All five samples are uncapped samples and the growth procedures were identical to the uncapped sample in the first experiment except that no AlAs marker layers were inserted, as the Al content makes the polycrystals more difficult to be removed with PSE [12]. The PSE growth supply and interrupt temperatures as well as the PSE interrupt durations were varied on the five samples, and these growth parameters together with SEM polycrystal images and QD areal density changes are illustrated later in a figure.
Fig. 1. (a) A SEM image of the uncapped sample; (b) Higher magnification image of the outlined area of (a); (c) is at the same magnification as (b), but at a point on the epitaxial ridge 20 mm further into the mask pattern (i.e. toward the right). Compared to (b), lower density of QDs is observed in (c).
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3. Result and discussion Fig. 1(a) shows a typical plan view scanning electron microscope (SEM) image on the uncapped sample of the first experiment. Fig. 1(b) is a higher magnification image of the outlined area of Fig. 1(a). Fig. 1(c) is at the same magnification as Fig. 1(b), but is at a relative position 20 mm further into the mask pattern (in the direction marked A to A0 ). The polycrystal-covered mask is visible in the upper right part of Fig. 1(a). For the epitaxial regions, a significant modification of the QD areal density is observed. On the epitaxial area close to the mask pattern, a high QD areal density is evident within 1 mm of the mask edge. However, this high areal density QD region decreases in density along the epitaxial ridge edge in the mask pattern. The increase of QD areal density is a result of the high availability of atomic steps due to surface morphology in this region, especially in the area around the ends of mask stripes where the shallow non-planar epitaxial surfaces (41 from the surface orientation far from the ridges, observed under cross-sectional SEM in the orientation B to B0 ) between mask stripes join the flat surface outside the mask pattern and form multiple facets. It can also be seen that QDs self-align along the [1 1 0], [1 0 0], [0 1 0] edges of the epitaxial layers in agreement with previous observations [15–17]. This alignment of QDs is maintained along the whole length of the epitaxial mesa. While Fig. 1 shows the QD areal distribution between the mask stripes in the pattern, Fig. 2 from a macroscopic view shows the study of the QD areal distribution outside the pattern on the same sample. The areal QD density is measured by SEM in directions both parallel ([1 1 0] orientation) and orthogonal to the mask stripes. Except for the 1 mm region adjacent to the mask where a high QD areal density is observed, the areal density around the mask is low in general. The QD density is observed to increase from this low value (2 109 cm2) quickly and
Fig. 2. QD areal density outside the mask region increases rapidly with the distance away from the mask pattern, tending towards a saturation value given by the background QD areal density (2 1010 cm2) at 30 mm from the mask.
Fig. 3. The average height and number of QDs as a function of distance into the mesa between parallel mask stripes in the mask cell with mask width 2 mm and mask spacing 4 mm. Each point has a sampling area taken from a rectangular region of 0.7 4 mm as schematically shown at the bottom of the figure.
then tends to a constant background density of 2 1010 cm2 at a distance around 30 mm. Fig. 3 shows the QD areal density and average height obtained by atomic force microscopy (AFM) as a function of distance along the epitaxial ridge between the mask stripes. The further into the mask cell, the greater are the reductions in both average QD height and QD areal density. In terms of QD areal density, the general trend observed above is that for epitaxy on any area, (except for the 1 mm region immediately adjacent to the polycrystal-covered mask), lower QD areal densities is observed in the region closer to the mask (Fig. 2) or in the region more surrounded by the mask (as in the case of Fig. 3). The QD areal density reduction is a result of the reduction in the InAs coverage reduction close to the mask area, and this implies some In adatoms around the pattern during growth were missing from the area, being either withdrawn or repelled away. The compositional change of In content in the structures around the pattern can be revealed by optical studies. Micro-photoluminescence (mPL) has been conducted on the capped sample and confirmed the presence of In on the polycrystal-covered mask. Fig. 4(a) shows a schematic indicating the location of mPL spectra shown in Figs. 4(b) and (c). The emission spectra were obtained at 7 K using excitation at 633 nm and a power of 0.2 mW focused onto a region with diameter in the order of 1 mm on the capped sample. Around 100 mm from the mask pattern (far away) the emission spectrum is characterized by bulk GaAs emission at 820 nm, very weak wetting layer emission at 875 nm and broad QD emission peaking at 1060 nm. As the spot is translated along the epitaxial ridge, a significant reduction in QD emission intensity, blue-shift of the peak
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STRIPE CENTER Far away
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Fig. 4. (a) Schematically shows the points where the mPL laser spots are focused on. (b) A series of mPL scans between mask stripes with various distances into the mask cell. (c) mPL scan comparison when the laser spot is in the center of the mask cell, but one on the epitaxy and the other on the polycrystal-covered mask.
QD emission wavelength, and concurrent increase in wetting layer emission intensity are observed. This is consistent with the reduction in QD areal density observed by AFM. Furthermore, the QD emission is observed to shift to shorter wavelengths as the excitation area is translated along the epitaxial ridge. This is also in agreement with our structural measurements where a reduction in QD height was observed with increasing
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distance along the epitaxial ridge. Fig. 4(c) shows PL spectra in the 800–940 nm range, where the InGaAs wetting layer like response occurs, obtained at locations shown schematically in Fig. 4(a). For excitation on the center of the epitaxial ridge, a peak at 860 nm is observed, which is attributed to the InAs wetting layer. For excitation on the polycrystal-covered mask right next to it, similar emission at 880 nm is observed, indicating the presence of an InGaAs region on the polycrystal-covered dielectric mask. A possibility that the missing In adatom from the epitaxial area is transported to the (1 1 1) B sidewall of the mesa was ruled out by cross-sectional scanning transmission electron microscopic (STEM) studies of the capped sample, as the contrast that might indicate an InAs layer on the (1 1 1) B side wall is not observed. The mPL and STEM studies agree with our AFM and SEM observation of the uncapped sample and suggest the In adatom transportation onto the mask is due to the presence of polycrystals. To assess how the polycrystal on the dielectric mask affect the QD distribution in the SA-MBE, QD areal densities around the mask patterns on samples with various degree of polycrystal deposit were investigated. Fig. 5 shows the growth parameters, SEM images of resulted polycrystalline deposit, and the trend of QD areal density changes around the pattern of the five samples labeled from A to E. It is noted that the high PSE growth and interrupt temperature is mandatory in order to reduce the polycrystalline deposit. For the QD areal density study, these densities were obtained by AFM at same locations on each sample, around the pattern group that has 4 mm mask stripe with and 2 mm spacing. A series of 1 1 mm AFM scans were performed along the center of epitaxial mesas, which are between two innermost dielectric mask stripes inside the pattern group. The trends of QD areal density changes shown in Fig. 5 are normalized against the QD background areal densities obtained on locations far away from the pattern. From sample labeled A–E in Fig. 5, as less polycrystalline deposit were formed on the mask, the higher the relative QD areal density at the center of the pattern compared to background density. It is therefore evident that the GaAs polycrystal is the key factor that alters the InAs QD distribution around the dielectric mask pattern in SA–MBE. The deposition of InAs on a planar growth surface to realize self-assembled QDs has been described by a segregation based mechanism where a critical indium composition of the surface layer is required to trigger the onset of QD formation [18,19]. Failing to achieve the critical In surface concentration on the epitaxial area inhibits the formation of QDs and prevents their ripening. The reduction in InAs QD height areal density observed in our experiments is therefore attributed to the effective InAs coverage reduction. It is interesting to note that this is in the opposite trend to selective area MOVPE growth reports, where an increase in effective InAs coverage is expected close to the mask region [1,20].
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Fig. 5. The PSE growth parameters, SEM images of resulted polycrystalline formation on the Si3N4 mask and relative QD areal density (normalized to background density) around the mask pattern with 4 mm mask width and 2 mm spacing.
It is apparent that different regions of the sample are competing for the deposited indium and our results indicate the GaAs polycrystals on the dielectric mask acts as a very effective sink for In adatoms. A hierarchy exists that the GaAs polycrystal and epitaxial ridge edges are both highly efficient at incorporating indium. For the GaAs polycrystal a large number of dangling bonds exist and a high density of step edges exist, and thus it is the most preferred site for In incorporation. Due to the presence of step edges, the non-planar epitaxial regions appear to be next most likely to incorporate indium. The planar epitaxial regions are the most inefficient, leading to a denuded InAs composition and hence QD density and size. This effect provides a novel SAE mechanism for QD structures. The non-selective growth of GaAs and formation of a polycrystal-covered dielectric mask results in the selective formation of self assembled QDs on the epitaxially grown material. 4. Summary In summary, we have discussed the SAE of InAs/GaAs self assembled QDs. We find a significant modification to the QD height and areal density in the region of a polycrystal-covered dielectric mask. The change in QD emission energy and intensity due to the change in QD size and areal density may be utilized for fabricating optically active and passive region on the substrate in single step growth process. We have also discussed the origin of this
phenomenon, that is, the transport of segregated indium to the polycrystal-covered mask, and its use as a new method for obtaining selective area growth of QD structures. Acknowledgment J. Lin acknowledges support from Engineering and Physical Science Research Council (EPSRC), United Kingdom under grant GR/R65534/01. The work was also supported in part by EPSRC grant GR/S76076/01 and GR/R65626/01. References [1] M. Gibbon, J.P. Stagg, C.G. Cureton, E.J. Thrush, C.J. Jones, R.E. Mallard, R.E. Pritchard, N. Collis, A. Chew, Semicond. Sci. Tech. 8 (1993) 998. [2] F. Nakajima, Y. Ogasawara, J. Motohisa, T. Fukui, J. Appl. Phys. 90 (2001) 2606. [3] F. Nakajima, Y. Miyoshi, J. Motohisa, T. Fukui, Appl. Phys. Lett. 83 (2003) 2680. [4] C.-K. Hahn, J. Motohisa, T. Fukui, Appl. Phys. Lett. 76 (2000) 3947. [5] H.J. Kim, J. Motohisa, T. Fukui, Appl. Phys. Lett. 81 (2002) 5147. [6] J. Motohisa, F. Nakajima, T. Fukui, Appl. Surf. Sci. 190 (2002) 184. [7] J. Lefebvre, P.J. Poole, J. Fraser, G.C. Aers, D. Chithrani, R.L. Williams, J. Crystal Growth 234 (2002) 391. [8] P.J. Poole, J. Lefebvre, J. Fraser, Appl. Phys. Lett. 83 (2003) 2055. [9] D. Chithrani, R.L. Williams, J. Lefebvre, P.J. Poole, G.C. Aers, Appl. Phys. Lett. 84 (2004) 978. [10] F.E. Allegretti, T. Nishinaga, J. Crystal Growth 156 (1995) 1. [11] T. Sugaya, Y. Okada, M. Kawabe, Jn. J. Appl. Phys. 31 (1992) L713. [12] G. Bacchin, T. Nishinaga, J. Crystal Growth 191 (1998) 599.
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