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InP selective area metal-organic vapor phase epitaxy for non-magnetic semiconductor spintronics
ARTICLE IN PRESS Journal of Crystal Growth 310 (2008) 4821–4825
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
InxGa1xAs/InP selective area metal-organic vapor phase epitaxy for non-magnetic semiconductor spintronics Masashi Akabori a,b,, Vitaliy A. Guzenko a,1, Thomas Scha¨pers a, Hilde Hardtdegen a a b
¨ lich-Aachen Research Alliance (JARA), Institute of Bio- and Nanosystems (IBN-1), Research Centre Ju ¨ lich, 52425 Ju ¨ lich, Germany Ju Center for Nano-Materials and Technology (CNMT), Japan Advanced Institute of Science and Technology (JAIST), 1-1, Asahidai, Nomi, Ishikawa 923-1292, Japan
a r t i c l e in fo
abstract
Available online 11 July 2008
We carried out selective area metal-organic vapor phase epitaxy (SA-MOVPE) of In-rich InxGa1xAs/InP modulation-doped heterostructures in an N2 atmosphere. The aim is to obtain wire structures which exhibit strong spin–orbit coupling without using etching of the heterostructures for their fabrication. The wire geometry was studied as a function of mask opening width. The transport properties were determined at 0.5 K and up to 10 T magnetic field. A clear growth enhancement was confirmed as the initial opening width decreases; thus, the InGaAs as well as the total thicknesses became larger. Moreover, we confirmed the top width saturates in some narrower wires due to geometrically limited growth by facets. Some narrower wires showed high resistivity, which might originate from structural deterioration in In0.77Ga0.23As due to the growth enhancement and/or non-uniformity of the parallel wires. On the other hand, wider wires showed Shubnikov–de Haas (SdH) oscillations, which exhibited SdH beating patterns. All in all it is possible to achieve strong spin–orbit coupling in InxGa1xAs/InP wires produced by SA-MOVPE. & 2008 Elsevier B.V. All rights reserved.
PACS: 81.07.Vb 73.63.Nm 71.70.Ej Keywords: A1. Nanostructures A3. Metelorganic vapor phase epitaxy A3. Selective epitaxy B2. Semiconductor III-V materials
1. Introduction Narrow gap III–V compound semiconductors such as InAs- and InSb-based materials have received much attention for nonmagnetic semiconductor spintronics, e.g. spin field effect transistors (spin-FETs) [1], because they exhibit high mobility as well as large spin-splitting. In this study we chose the InxGa1xAs/InP material system for this application. In the past we have reported on spin–orbit coupling in the respective two-dimensional electron gases [2] and later on etched wires having sub-micron width (but are still two-dimensional systems from a quantum-mechanical point of view) in the same material system [3]. If the wire widths are reduced towards one-dimensional systems, quantum wires are expected with spin–orbit coupling, which exhibit long spincoherence—an important characteristic for applications, since spin-FETs should show an improved output modulation. However, a top-down approach for obtaining wires may be detrimental to their characteristics, since they may be damaged by the dry etching procedure and may show roughness. As a technique for conductive wire fabrication without process damage, selective
area metal-organic vapor phase epitaxy (SA-MOVPE) has already been demonstrated for GaAs-based materials [4–6]. Moreover, there have been literature reports on SA-MOVPE of InGaAs/InP heterostructures for low-dimensional structures [7] and optical applications [8,9], but there are no reports on transport properties of conductive wires to our knowledge. In this report, we have chosen SA-MOVPE of In-rich InxGa1xAs/InP modulation-doped heterostructures in N2 atmosphere to fabricate conductive wire structures having sub-micron width. The aim is to achieve strong spin–orbit coupling in wires using a bottom-up fabrication approach. Up to now there have been no reports on SA-MOVPE behaviour of InGaAs/InP in N2 atmosphere. Due to the increased viscosity of the gas phase compared to hydrogen and the inert chemical nature of the carrier gas, a carrier gas exchange is expected to have an influence on desorption, adsorption and diffusion processes of growth species. Therefore it will be necessary to study the SA growth and wire geometry carefully so that the formation of quantum wires can be precisely controlled.
2. Experimental procedure Corresponding author at: Ju ¨ lich-Aachen Research Alliance (JARA), Institute of
Bio- and Nanosystems (IBN-1), Research Centre Ju¨lich, 52425 Ju¨lich, Germany. Tel.: +49 2461612731; fax: +49 2461612940. E-mail addresses: [email protected], [email protected] (M. Akabori). 1 Present address: Laboratory for Micro- and Nanotechnology, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland. 0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.07.020
Fig. 1(a) shows the schematic of a wire structure. We patterned a 100 nm-thick SiO2 layer on a (0 0 1) InP substrate by electronbeam lithography and wet chemical etching with different initial opening widths and a fixed length of 100 mm. Each width was
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Fig. 1. (a) Schematic of a wire structure, and (b) overview of the parallel wire structures. The wire width range is from 500 nm (wire A) to 1500 nm (wire H).
repeated 20 times in parallel in the same area with a fixed pitch of 2 mm. Growth conditions were used as described in Ref. [10]. An InxGa1xAs/InP heterostructure was grown with nominal thicknesses deduced from planar growth as follows: 35 nm i-InP followed by 10 nm n-InP, 20 nm i-InP spacer, 5 nm i-In0.77Ga0.23As channel, 30 nm i-In0.53Ga0.47As sub-channel, and capped by 120 nm i-InP. This asymmetric In0.53Ga0.47As/In0.77Ga0.23As/InP double heterojunction is very important for the spin-FET application, because it can enhance spin–orbit coupling due to the large structure inversion asymmetry [2]. After growth, we fabricated Au/Ni/AuGe/Ni ohmic contacts by electron-beam lithography and the lift-off technique. Fig. 1(b) shows an overview of the parallel wires after ohmic metallization with different wires (A–H) as well as a Hall-bar structure defined on the same chip. We carried out transport measurements in a He-3 cryostat with lock-in technique and magnetic fields up to 10 T. Typical currents and measurement temperature were 5–200 nA and 0.5 K, respectively.
/111SB facets similar to literature reports [7,8]. Moreover, there seems to be no growth on the facets because no overlap on mask edges was observed. Additionally, growth enhancement around the edge [8,9] was not observed. These results indicate that we can consider the electron system only on the (0 0 1) surface in the structures. The dependence of the actual geometry of parallel wires on initial opening width is shown in Fig. 2(b) and (c). It can be seen that the InGaAs thickness (sum of In0.53Ga0.47As and In0.77Ga0.23As layers) as well as the total thickness (height of wire) become larger as the initial opening width decreases, which is in agreement to former reports for growth with H2 carrier gas [7,8]. Especially, in A, B and C, the InGaAs thickness is more than twice as large as that of planar areas. Additionally, the top width of A and B is saturated, i.e. growth is geometrically limited by the /111SB facets. In order to understand the growth enhancement effect in more detail, SA-MOVPE growth was compared to planar growth. To this end cross-sections of the structures were defined and put in relation to the initial opening width as shown in Fig. 3. The repetition pitch of the structures was kept constant at 2 mm and the total layer thickness of the structures—if grown on unstructured substrates—would have been 220 nm (see Section 2). The line in Fig. 3 denotes the ratio of initial opening width to pitch. The more this ratio approaches 1, the more the growth will resemble planar growth. The open squares are experimental results and give the actually found cross-section area of the whole wire (Aw)—consisting mainly of InP—in relation to the area planar growth would have achieved (Ap ¼ 2 mm width 220 nm thickness). The filled squares are experimentally determined data for the cross-section area of selectively grown InGaAs (Aw-IGA) which is put in relation to the area planar growth would have achieved for the InGaAs (Ap-IGA ¼ 2 mm width 35 nm thickness, see Section 2). If all the material which does not incorporate onto the mask would diffuse to the opening, the area ratios would be equal to 1. If no surface diffusion would take place and all the material which is on the mask would evaporate or diffuse away to un-patterned areas on the sample, then the ratio with respect to the wire width would follow the dotted line in the diagram. We observe that the area ratios of the respective selectively grown layers to the planarly grown layers lie above the dotted line, which means that surface diffusion from the mask to the wire takes place and is responsible for growth enhancement. However, since there is a wire width effect and the area ratios are not equal to 1, re-evaporation also plays a major role—more strongly so for InP selective growth than for InGaAs growth. As the wire widths decrease, the area ratios only for the total wire areas approach the dotted line, indicative that reactive species for InP growth evaporate more strongly than for InGaAs growth. Since all area ratios are much smaller than 1, the main effect is the redistribution of the deposited material in the open spacing. Therefore, the observed growth enhancement as mentioned before mainly originates from the geometrical effect of /111SB facets, which inhibit growth. We note that these structural characteristics are useful for future precise control of wire structures.
3. Experimental results and discussion 3.2. Transport properties 3.1. Structural characterization Our main focus is on transport properties of SA-MOVPE-grown wires, but structural characterization is equally important to analyze and interpret transport properties. To this end we carried out scanning electron microscopy (SEM) to obtain actual wire geometries. Fig. 2(a) shows SEM cross-sections of wires A, D and G in which the InGaAs layers are encompassed by darker InP layers. We confirmed clear trapezoidal or triangular shapes with
Fig. 4 shows normalized zero-field resistance measurements of parallel wires as a function of initial opening width. Samples A, B and C exhibit a more than three times higher resistivity than the other samples. This might originate from structural deterioration when the critical thickness of the In0.77Ga0.23As channel layer is exceeded as a result of growth enhancement. Additionally, unintentional defects may be induced in narrower wires as well as the composition modulated in the InGaAs layers. The exceeded
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Fig. 2. (a) SEM image of wires A, D and G (initial opening width 640, 930 and 1410 nm, respectively), and dependences of the actual geometry of parallel wires on initial opening width in (b) InGaAs layers and (c) total wire growth. Scale bars in (a) represent 200 nm. It can be seen that the layer thicknesses surpass the design thickness for all initial opening widths and that growth is geometrically limited for initial opening widths below 830 nm initial opening width.
Fig. 3. Ratio of cross-sections of the structures to the initial opening width. The inset shows schematics about the definition of the areas. Since the area ratios do not exactly follow the dotted line, not following the dotted, some surface diffusion takes place and enhances the growth in the wire. Since still a clear dependence of area ratios on initial opening width is seen and the ratios areo1, not all material diffuses to the opening, some adatoms will re-evaporate.
critical thickness would lead to a drop of the electron mobility [11], and non-uniform parallel wires would lead to resistance variation. These explanations seem to be reasonable because the InGaAs thickness of wires A, B and C was more than twice as thick as the design value (5 nm) exceeding the critical thickness of the strained In0.77Ga0.23As, which is about 10 nm. Also, we confirmed slightly rough surfaces in wires A, B, and C by SEM (not shown). The composition modulation, which is a well-known effect during InGaAs SA-MOVPE [7–9], can also affect transport properties;
however, we have not yet confirmed the exact composition of the InGaAs layers, which is to be investigated in future. Fig. 5 shows magnetoresistance (MR) curves measured on wire samples (D–H) and on a Hall-bar. Shubnikov–de Haas (SdH) oscillations were observed on these samples. These layer structures should not exceed the critical thickness in In0.77Ga0.23As channel and the wires should be sufficiently uniform. Moreover, we could find SdH oscillations with beating patterns on samples F, G and H and on the Hall-bar. The beating patterns can be interpreted as an indication for strong spin–orbit coupling in the conductive channel; thus, we successfully obtained InxGa1xAs/InP wire structures with strong spin–orbit coupling. Additionally, we found a shift of first-node positions towards lower inverse magnetic fields, i.e. higher magnetic field from sample F to Hall-bar. The shift towards higher magnetic field corresponds to an increase of spin–orbit coupling. The data are shown in Table 1 together with a summary of transport properties extracted from the MR curves. For the channel thickness estimation, we simply used the InGaAs thickness as determined by Fig. 2b. For the distribution of the layer thicknesses with respect to channel and sub-channel layer thickness, the design ratio of the In0.77Ga0.23As channel to the In0.53Ga0.47As subchannel was taken into account (5 nm/35 nm ¼ 0.143). For mobility estimation we used normalized resistance values as shown in Fig. 4; however, the measurements were not done by 4 terminals but by 2 terminals; thus there should be a contact resistance contribution, and the estimated mobility of wires is lower than the actual mobility. The onset of SdH oscillations of each wire with respect to magnetic field seems to be lower than 0.5 T; therefore, the realistic mobility should be over 20,000 cm2/ V s. For spin–orbit coupling estimation, we used the relationship between the Landau level splitting and spin-splitting at the first node as follows [12]: 1 _oC 2
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi g m 2 2 2 1 _ oC þ ð2akF Þ2 _oC 2m0
(1)
where oC ¼ eB/m* is the cyclotron frequency, m* is the electron effective mass, g* is the effective gyro-magnetic factor, a is the pffiffiffiffiffiffiffiffiffiffiffi ffi strength of the spin–orbit coupling, and kF ¼ 2pNS is the Fermi
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Fig. 4. Normalized resistance of parallel wires. For initial opening widths o830 nm, the normalized resistance strongly increases, indicative of deterioration of structural, compositional or morphological wire quality.
Fig. 5. Magnetoresistance curves of parallel wires. For wires broader than 880 nm, clear beating patterns are observed. The position of the nodes shifts towards smaller fields as the initial opening width decreases, indicative of increasing spin–orbit coupling compared to the planar samples. Table 1 Summary of transport properties Sample
Wire width (nm)
Channel thickness (nm)
Electron concentration (1011 cm2)
Electron mobility (cm2/V s)
Spin–orbit coupling (1012 eV m)
D E F G H Hall-bar
770 880 1030 1290 1500 –
9.8 9.0 8.1 7.5 6.8 5.0
4.17 4.75 4.79 4.81 4.97 5.04
7500 10,200 15,400 13,200 13,500 94,300
– – 6.7 6.9 7.1 8.3
wavevector. We used m* ¼ 0.039m0 and g* ¼ 4 for the estimation according to our previous work [3]. In the narrower wire structures fabricated by etching, the spin–orbit coupling tends to be almost the same as for the original 2DEG structure [13,14].
Fig. 6. Channel thickness dependence of magnetoresistance curves in 500 nm wide wires. For the thinner design channel thickness (2 nm, the actual channel geometry should not exceed the critical thickness), a reduction in resistivity as well as Shubnikov de Hass oscillations with a beating pattern are observed. This result demonstrates that a proper knowledge of the growth enhancement effect is a prerequisite for the future preparation of wires towards one dimensionality, which still exhibit strong spin–orbit coupling.
Therefore, the shift might not originate from the decreasing wire width. A possible explanation might be due to growth enhancement, since the thicker In0.77Ga0.23As channel layer showed weaker spin–orbit coupling in our InxGa1xAs/InP 2DEGs [15]. Another possibility might be composition modulation, since InGaAs with lower indium content has a larger bandgap, which would basically correspond to weaker spin–orbit coupling. In the mean time, transport measurements were carried out on parallel wires which have a similar width to the wire sample A (about 500 nm), but with a reduced design thickness of the In0.77Ga0.23As channel of 2 nm. The reduced design channel layer thickness should lead to a wire structure in which the critical thickness of the actual strained channel is not yet exceeded according to the results of this study. Indeed the wires now exhibited lower normal resistance (1.7 kO) than the former wire sample A (12 kO), and SdH oscillations with beating patterns were observed as shown in Fig. 6. The details will be discussed elsewhere. Therefore, the improvement of design, which takes the results of the current study with respect to the growth enhancement into account, should lead to enhanced spin–orbit coupling and longer spin-coherence lengths in future one-dimensional wires.
4. Summary We carried out SA-MOVPE of InxGa1xAs/InP in N2 atmosphere to obtain wire structures with strong spin–orbit coupling without process damage. From SEM observations, we confirmed a clear growth enhancement depending on the pattern as well as geometrically limited growth in some narrower wires. The results are similar to typical SA-MOVPE results using H2 atmosphere. From transport measurements at 0.5 K, we found very high resistivity in some narrower wires, which might originate from structural deterioration in In0.77Ga0.23As due to the growth enhancement and/or non-uniformity. In wider wires, we confirmed clear SdH oscillations with beating patterns, which indicate strong spin–orbit coupling. Additionally, we observed SdH oscillations with beating patterns in 500-nm-wide wires if
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the design thickness of the In0.77Ga0.23As channel is decreased. In future it should be possible to achieve narrower wires with spin–orbit coupling by taking the growth enhancement for SAMOVPE in N2 carrier gas into account. Acknowledgement The authors thank K. Wirtz for his support in MOVPE and H. Kertz for his assistance during transport measurements. One of the authors (M.A.) was supported financially by JSPS Postdoctoral Fellowships for Research Abroad. References [1] S. Datta, B. Das, Appl. Phys. Lett. 56 (1990) 665. [2] Th. Scha¨pers, G. Engels, J. Lange, Th. Klocke, M. Hollfelder, H. Lu¨th, J. Appl. Phys. 83 (1998) 4324.
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[3] V.A. Guzenko, A. Bringer, J. Knobbe, H. Hardtdegen, Th. Scha¨pers, Appl. Phys. A 87 (2007) 577. [4] H. Asai, S. Yamada, T. Fukui, Appl. Phys. Lett. 51 (1987) 1518. [5] K. Kumakura, J. Motohisa, T. Fukui, Physica E 2 (1998) 809. [6] H. Takahashi, Y. Miyoshi, F. Nakajima, P. Mohan, J. Motohisa, T. Fukui, Appl. Surf. Sci. 216 (2003) 402. [7] Y.D. Galeuchet, P. Roentgen, V. Graf, J. Appl. Phys. 68 (1990) 560. [8] M. Suzuki, M. Aoki, T. Tsuchiya, T. Taniwatari, J. Crystal Growth 145 (1994) 249. [9] T. Tsuchiya, J. Shimizu, M. Shirai, M. Aoki, J. Crystal Growth 276 (2005) 439. [10] M. Hollfelder, S. Hon, B. Setzer, K. Schimpf, M. Horstmann, Th. Scha¨pers, D. Schmitz, H. Hardtdegen, H. Lu¨th, J. Crystal Growth 170 (1997) 103. [11] H. Hardtdegen, R. Meyer, M. Hollfelder, Th. Scha¨pers, J. Appenzeller, H. LøkenLarsen, Th. Klocke, Ch. Dieker, B. Lengeler, H. Lu¨th, J. Appl. Phys. 73 (1993) 4489. [12] B. Das, S. Datta, R. Reifenberger, Phys. Rev. B 41 (1990) 8278. [13] T. Kakegawa, M. Akabori, S. Yamada, J. Superconductivity 18 (2005) 229. [14] V.A. Guzenko, J. Knobbe, H. Hardtdegen, Th. Scha¨pers, A. Bringer, Appl. Phys. Lett. 88 (2006) 032102. [15] T. Scha¨pers, V. A. Guzenko, A. Bringer, M. Akabori, M. Hagedorn, H. Hardtdegen, Semicond. Sci. Tech., in press.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
InxGa1xAs/InP selective area metal-organic vapor phase epitaxy for non-magnetic semiconductor spintronics Masashi Akabori a,b,, Vitaliy A. Guzenko a,1, Thomas Scha¨pers a, Hilde Hardtdegen a a b
¨ lich-Aachen Research Alliance (JARA), Institute of Bio- and Nanosystems (IBN-1), Research Centre Ju ¨ lich, 52425 Ju ¨ lich, Germany Ju Center for Nano-Materials and Technology (CNMT), Japan Advanced Institute of Science and Technology (JAIST), 1-1, Asahidai, Nomi, Ishikawa 923-1292, Japan
a r t i c l e in fo
abstract
Available online 11 July 2008
We carried out selective area metal-organic vapor phase epitaxy (SA-MOVPE) of In-rich InxGa1xAs/InP modulation-doped heterostructures in an N2 atmosphere. The aim is to obtain wire structures which exhibit strong spin–orbit coupling without using etching of the heterostructures for their fabrication. The wire geometry was studied as a function of mask opening width. The transport properties were determined at 0.5 K and up to 10 T magnetic field. A clear growth enhancement was confirmed as the initial opening width decreases; thus, the InGaAs as well as the total thicknesses became larger. Moreover, we confirmed the top width saturates in some narrower wires due to geometrically limited growth by facets. Some narrower wires showed high resistivity, which might originate from structural deterioration in In0.77Ga0.23As due to the growth enhancement and/or non-uniformity of the parallel wires. On the other hand, wider wires showed Shubnikov–de Haas (SdH) oscillations, which exhibited SdH beating patterns. All in all it is possible to achieve strong spin–orbit coupling in InxGa1xAs/InP wires produced by SA-MOVPE. & 2008 Elsevier B.V. All rights reserved.
PACS: 81.07.Vb 73.63.Nm 71.70.Ej Keywords: A1. Nanostructures A3. Metelorganic vapor phase epitaxy A3. Selective epitaxy B2. Semiconductor III-V materials
1. Introduction Narrow gap III–V compound semiconductors such as InAs- and InSb-based materials have received much attention for nonmagnetic semiconductor spintronics, e.g. spin field effect transistors (spin-FETs) [1], because they exhibit high mobility as well as large spin-splitting. In this study we chose the InxGa1xAs/InP material system for this application. In the past we have reported on spin–orbit coupling in the respective two-dimensional electron gases [2] and later on etched wires having sub-micron width (but are still two-dimensional systems from a quantum-mechanical point of view) in the same material system [3]. If the wire widths are reduced towards one-dimensional systems, quantum wires are expected with spin–orbit coupling, which exhibit long spincoherence—an important characteristic for applications, since spin-FETs should show an improved output modulation. However, a top-down approach for obtaining wires may be detrimental to their characteristics, since they may be damaged by the dry etching procedure and may show roughness. As a technique for conductive wire fabrication without process damage, selective
area metal-organic vapor phase epitaxy (SA-MOVPE) has already been demonstrated for GaAs-based materials [4–6]. Moreover, there have been literature reports on SA-MOVPE of InGaAs/InP heterostructures for low-dimensional structures [7] and optical applications [8,9], but there are no reports on transport properties of conductive wires to our knowledge. In this report, we have chosen SA-MOVPE of In-rich InxGa1xAs/InP modulation-doped heterostructures in N2 atmosphere to fabricate conductive wire structures having sub-micron width. The aim is to achieve strong spin–orbit coupling in wires using a bottom-up fabrication approach. Up to now there have been no reports on SA-MOVPE behaviour of InGaAs/InP in N2 atmosphere. Due to the increased viscosity of the gas phase compared to hydrogen and the inert chemical nature of the carrier gas, a carrier gas exchange is expected to have an influence on desorption, adsorption and diffusion processes of growth species. Therefore it will be necessary to study the SA growth and wire geometry carefully so that the formation of quantum wires can be precisely controlled.
2. Experimental procedure Corresponding author at: Ju ¨ lich-Aachen Research Alliance (JARA), Institute of
Bio- and Nanosystems (IBN-1), Research Centre Ju¨lich, 52425 Ju¨lich, Germany. Tel.: +49 2461612731; fax: +49 2461612940. E-mail addresses: [email protected], [email protected] (M. Akabori). 1 Present address: Laboratory for Micro- and Nanotechnology, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland. 0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.07.020
Fig. 1(a) shows the schematic of a wire structure. We patterned a 100 nm-thick SiO2 layer on a (0 0 1) InP substrate by electronbeam lithography and wet chemical etching with different initial opening widths and a fixed length of 100 mm. Each width was
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Fig. 1. (a) Schematic of a wire structure, and (b) overview of the parallel wire structures. The wire width range is from 500 nm (wire A) to 1500 nm (wire H).
repeated 20 times in parallel in the same area with a fixed pitch of 2 mm. Growth conditions were used as described in Ref. [10]. An InxGa1xAs/InP heterostructure was grown with nominal thicknesses deduced from planar growth as follows: 35 nm i-InP followed by 10 nm n-InP, 20 nm i-InP spacer, 5 nm i-In0.77Ga0.23As channel, 30 nm i-In0.53Ga0.47As sub-channel, and capped by 120 nm i-InP. This asymmetric In0.53Ga0.47As/In0.77Ga0.23As/InP double heterojunction is very important for the spin-FET application, because it can enhance spin–orbit coupling due to the large structure inversion asymmetry [2]. After growth, we fabricated Au/Ni/AuGe/Ni ohmic contacts by electron-beam lithography and the lift-off technique. Fig. 1(b) shows an overview of the parallel wires after ohmic metallization with different wires (A–H) as well as a Hall-bar structure defined on the same chip. We carried out transport measurements in a He-3 cryostat with lock-in technique and magnetic fields up to 10 T. Typical currents and measurement temperature were 5–200 nA and 0.5 K, respectively.
/111SB facets similar to literature reports [7,8]. Moreover, there seems to be no growth on the facets because no overlap on mask edges was observed. Additionally, growth enhancement around the edge [8,9] was not observed. These results indicate that we can consider the electron system only on the (0 0 1) surface in the structures. The dependence of the actual geometry of parallel wires on initial opening width is shown in Fig. 2(b) and (c). It can be seen that the InGaAs thickness (sum of In0.53Ga0.47As and In0.77Ga0.23As layers) as well as the total thickness (height of wire) become larger as the initial opening width decreases, which is in agreement to former reports for growth with H2 carrier gas [7,8]. Especially, in A, B and C, the InGaAs thickness is more than twice as large as that of planar areas. Additionally, the top width of A and B is saturated, i.e. growth is geometrically limited by the /111SB facets. In order to understand the growth enhancement effect in more detail, SA-MOVPE growth was compared to planar growth. To this end cross-sections of the structures were defined and put in relation to the initial opening width as shown in Fig. 3. The repetition pitch of the structures was kept constant at 2 mm and the total layer thickness of the structures—if grown on unstructured substrates—would have been 220 nm (see Section 2). The line in Fig. 3 denotes the ratio of initial opening width to pitch. The more this ratio approaches 1, the more the growth will resemble planar growth. The open squares are experimental results and give the actually found cross-section area of the whole wire (Aw)—consisting mainly of InP—in relation to the area planar growth would have achieved (Ap ¼ 2 mm width 220 nm thickness). The filled squares are experimentally determined data for the cross-section area of selectively grown InGaAs (Aw-IGA) which is put in relation to the area planar growth would have achieved for the InGaAs (Ap-IGA ¼ 2 mm width 35 nm thickness, see Section 2). If all the material which does not incorporate onto the mask would diffuse to the opening, the area ratios would be equal to 1. If no surface diffusion would take place and all the material which is on the mask would evaporate or diffuse away to un-patterned areas on the sample, then the ratio with respect to the wire width would follow the dotted line in the diagram. We observe that the area ratios of the respective selectively grown layers to the planarly grown layers lie above the dotted line, which means that surface diffusion from the mask to the wire takes place and is responsible for growth enhancement. However, since there is a wire width effect and the area ratios are not equal to 1, re-evaporation also plays a major role—more strongly so for InP selective growth than for InGaAs growth. As the wire widths decrease, the area ratios only for the total wire areas approach the dotted line, indicative that reactive species for InP growth evaporate more strongly than for InGaAs growth. Since all area ratios are much smaller than 1, the main effect is the redistribution of the deposited material in the open spacing. Therefore, the observed growth enhancement as mentioned before mainly originates from the geometrical effect of /111SB facets, which inhibit growth. We note that these structural characteristics are useful for future precise control of wire structures.
3. Experimental results and discussion 3.2. Transport properties 3.1. Structural characterization Our main focus is on transport properties of SA-MOVPE-grown wires, but structural characterization is equally important to analyze and interpret transport properties. To this end we carried out scanning electron microscopy (SEM) to obtain actual wire geometries. Fig. 2(a) shows SEM cross-sections of wires A, D and G in which the InGaAs layers are encompassed by darker InP layers. We confirmed clear trapezoidal or triangular shapes with
Fig. 4 shows normalized zero-field resistance measurements of parallel wires as a function of initial opening width. Samples A, B and C exhibit a more than three times higher resistivity than the other samples. This might originate from structural deterioration when the critical thickness of the In0.77Ga0.23As channel layer is exceeded as a result of growth enhancement. Additionally, unintentional defects may be induced in narrower wires as well as the composition modulated in the InGaAs layers. The exceeded
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Fig. 2. (a) SEM image of wires A, D and G (initial opening width 640, 930 and 1410 nm, respectively), and dependences of the actual geometry of parallel wires on initial opening width in (b) InGaAs layers and (c) total wire growth. Scale bars in (a) represent 200 nm. It can be seen that the layer thicknesses surpass the design thickness for all initial opening widths and that growth is geometrically limited for initial opening widths below 830 nm initial opening width.
Fig. 3. Ratio of cross-sections of the structures to the initial opening width. The inset shows schematics about the definition of the areas. Since the area ratios do not exactly follow the dotted line, not following the dotted, some surface diffusion takes place and enhances the growth in the wire. Since still a clear dependence of area ratios on initial opening width is seen and the ratios areo1, not all material diffuses to the opening, some adatoms will re-evaporate.
critical thickness would lead to a drop of the electron mobility [11], and non-uniform parallel wires would lead to resistance variation. These explanations seem to be reasonable because the InGaAs thickness of wires A, B and C was more than twice as thick as the design value (5 nm) exceeding the critical thickness of the strained In0.77Ga0.23As, which is about 10 nm. Also, we confirmed slightly rough surfaces in wires A, B, and C by SEM (not shown). The composition modulation, which is a well-known effect during InGaAs SA-MOVPE [7–9], can also affect transport properties;
however, we have not yet confirmed the exact composition of the InGaAs layers, which is to be investigated in future. Fig. 5 shows magnetoresistance (MR) curves measured on wire samples (D–H) and on a Hall-bar. Shubnikov–de Haas (SdH) oscillations were observed on these samples. These layer structures should not exceed the critical thickness in In0.77Ga0.23As channel and the wires should be sufficiently uniform. Moreover, we could find SdH oscillations with beating patterns on samples F, G and H and on the Hall-bar. The beating patterns can be interpreted as an indication for strong spin–orbit coupling in the conductive channel; thus, we successfully obtained InxGa1xAs/InP wire structures with strong spin–orbit coupling. Additionally, we found a shift of first-node positions towards lower inverse magnetic fields, i.e. higher magnetic field from sample F to Hall-bar. The shift towards higher magnetic field corresponds to an increase of spin–orbit coupling. The data are shown in Table 1 together with a summary of transport properties extracted from the MR curves. For the channel thickness estimation, we simply used the InGaAs thickness as determined by Fig. 2b. For the distribution of the layer thicknesses with respect to channel and sub-channel layer thickness, the design ratio of the In0.77Ga0.23As channel to the In0.53Ga0.47As subchannel was taken into account (5 nm/35 nm ¼ 0.143). For mobility estimation we used normalized resistance values as shown in Fig. 4; however, the measurements were not done by 4 terminals but by 2 terminals; thus there should be a contact resistance contribution, and the estimated mobility of wires is lower than the actual mobility. The onset of SdH oscillations of each wire with respect to magnetic field seems to be lower than 0.5 T; therefore, the realistic mobility should be over 20,000 cm2/ V s. For spin–orbit coupling estimation, we used the relationship between the Landau level splitting and spin-splitting at the first node as follows [12]: 1 _oC 2
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi g m 2 2 2 1 _ oC þ ð2akF Þ2 _oC 2m0
(1)
where oC ¼ eB/m* is the cyclotron frequency, m* is the electron effective mass, g* is the effective gyro-magnetic factor, a is the pffiffiffiffiffiffiffiffiffiffiffi ffi strength of the spin–orbit coupling, and kF ¼ 2pNS is the Fermi
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Fig. 4. Normalized resistance of parallel wires. For initial opening widths o830 nm, the normalized resistance strongly increases, indicative of deterioration of structural, compositional or morphological wire quality.
Fig. 5. Magnetoresistance curves of parallel wires. For wires broader than 880 nm, clear beating patterns are observed. The position of the nodes shifts towards smaller fields as the initial opening width decreases, indicative of increasing spin–orbit coupling compared to the planar samples. Table 1 Summary of transport properties Sample
Wire width (nm)
Channel thickness (nm)
Electron concentration (1011 cm2)
Electron mobility (cm2/V s)
Spin–orbit coupling (1012 eV m)
D E F G H Hall-bar
770 880 1030 1290 1500 –
9.8 9.0 8.1 7.5 6.8 5.0
4.17 4.75 4.79 4.81 4.97 5.04
7500 10,200 15,400 13,200 13,500 94,300
– – 6.7 6.9 7.1 8.3
wavevector. We used m* ¼ 0.039m0 and g* ¼ 4 for the estimation according to our previous work [3]. In the narrower wire structures fabricated by etching, the spin–orbit coupling tends to be almost the same as for the original 2DEG structure [13,14].
Fig. 6. Channel thickness dependence of magnetoresistance curves in 500 nm wide wires. For the thinner design channel thickness (2 nm, the actual channel geometry should not exceed the critical thickness), a reduction in resistivity as well as Shubnikov de Hass oscillations with a beating pattern are observed. This result demonstrates that a proper knowledge of the growth enhancement effect is a prerequisite for the future preparation of wires towards one dimensionality, which still exhibit strong spin–orbit coupling.
Therefore, the shift might not originate from the decreasing wire width. A possible explanation might be due to growth enhancement, since the thicker In0.77Ga0.23As channel layer showed weaker spin–orbit coupling in our InxGa1xAs/InP 2DEGs [15]. Another possibility might be composition modulation, since InGaAs with lower indium content has a larger bandgap, which would basically correspond to weaker spin–orbit coupling. In the mean time, transport measurements were carried out on parallel wires which have a similar width to the wire sample A (about 500 nm), but with a reduced design thickness of the In0.77Ga0.23As channel of 2 nm. The reduced design channel layer thickness should lead to a wire structure in which the critical thickness of the actual strained channel is not yet exceeded according to the results of this study. Indeed the wires now exhibited lower normal resistance (1.7 kO) than the former wire sample A (12 kO), and SdH oscillations with beating patterns were observed as shown in Fig. 6. The details will be discussed elsewhere. Therefore, the improvement of design, which takes the results of the current study with respect to the growth enhancement into account, should lead to enhanced spin–orbit coupling and longer spin-coherence lengths in future one-dimensional wires.
4. Summary We carried out SA-MOVPE of InxGa1xAs/InP in N2 atmosphere to obtain wire structures with strong spin–orbit coupling without process damage. From SEM observations, we confirmed a clear growth enhancement depending on the pattern as well as geometrically limited growth in some narrower wires. The results are similar to typical SA-MOVPE results using H2 atmosphere. From transport measurements at 0.5 K, we found very high resistivity in some narrower wires, which might originate from structural deterioration in In0.77Ga0.23As due to the growth enhancement and/or non-uniformity. In wider wires, we confirmed clear SdH oscillations with beating patterns, which indicate strong spin–orbit coupling. Additionally, we observed SdH oscillations with beating patterns in 500-nm-wide wires if
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the design thickness of the In0.77Ga0.23As channel is decreased. In future it should be possible to achieve narrower wires with spin–orbit coupling by taking the growth enhancement for SAMOVPE in N2 carrier gas into account. Acknowledgement The authors thank K. Wirtz for his support in MOVPE and H. Kertz for his assistance during transport measurements. One of the authors (M.A.) was supported financially by JSPS Postdoctoral Fellowships for Research Abroad. References [1] S. Datta, B. Das, Appl. Phys. Lett. 56 (1990) 665. [2] Th. Scha¨pers, G. Engels, J. Lange, Th. Klocke, M. Hollfelder, H. Lu¨th, J. Appl. Phys. 83 (1998) 4324.
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