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Surface potential measurement of self-assembled InAs dots by scanning Maxwell stress microscopy
Physica E 7 (2000) 373–376
www.elsevier.nl/locate/physe
Surface potential measurement of self-assembled InAs dots by scanning Maxwell stress microscopy Ichiro Tanakaa; b; c; d; ∗ , I. Kamiyab; d; 1 , H. Sakakic; d , M. Fujimotoa a Department
of Materials Science and Chemistry, Faculty of Systems Engineering, Wakayama University, 930 Sakaedani, Wakayama 640-8510, Japan b Research Center for Advanced Science and Technology, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan c Institute of Industrial Science, University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106-8558, Japan d Quantum Transition Project, Japan Science and Technology Corporation, Japan
Abstract We have imaged surface potential of InAs self-assembled quantum dots on GaAs (0 0 1) surface by scanning Maxwell stress microscopy. Simultaneously obtained constant tip–sample capacitance image and surface potential image were compared with atomic force microscope (AFM) image, and was found that the surface potential on large dots of about 100-nm diameter is ∼ 610 mV which is 30 – 40 mV lower than that on regular size dots. This result well coincides with previously reported Schottky barrier heights which were estimated from current versus voltage measurements with the same type of samples using conductive probe AFM. ? 2000 Elsevier Science B.V. All rights reserved. PACS: 61.16.Ch; 73.30.+y; 73.61.Ey Keywords: Quantum dots; Scanning Maxwell stress microscopy; Schottky barrier
1. Introduction Semiconductor nanostructures such as quantum wires and dots are promising materials for future device applications [1,2]. Among various types of nanos∗ Correspondence address: Department of Materials Science and Chemistry, Faculty of Systems Engineering, Wakayama University, 930 Sakaedani, Wakayama 640-8510, Japan. Tel.: +81-734-57-8238; fax: +81-734-57-8272. E-mail address: [email protected] (I. Tanaka) 1 Present address: Non-Equilibrium Laboratory, Mitsubishi Chemical Corporation, 1000 Kamoshida-cho, Aoba-ku, Yokohama 227-8502, Japan.
tructures, self-assembled quantum dots (SAQDs) have been widely studied since they can be formed spontaneously by lattice-mismatched epitaxy without expensive lithographic processes [3]. The properties of SAQDs have been extensively studied on an ensemble of the dots by optical and electronic measurements. On the other hand, scanning probe microscopies (SPM) have been used for electronic and optical investigations of individual InAs SAQDs grown on GaAs (0 0 1) surfaces [4 – 6]. We already reported simultaneous imaging of topography and conductance of InAs SAQDs with nano-scale resolution by atomic force microscopy (AFM) using conductive probes
1386-9477/00/$ - see front matter ? 2000 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 9 4 7 7 ( 9 9 ) 0 0 3 4 4 - 6
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[7]. We observed di erence in the local conductance between the InAs dots and the wetting layer. Also, current versus voltage (I –V ) measurements on the SAQDs of various sizes have shown that larger dots are more conductive than the smaller ones, and that the surface-barrier height of the large dots of 70 –90 nm diameter is lower than that of the regular dots by ∼ 50 meV. This modi cation has been attributed to local lowering of surface potential due to possible positively charged surface states on InAs SAQDs [8]. In this study, we adopted scanning Maxwell stress microscopy (SMM) in order to directly measure the surface potential on InAs SAQDs [9]. Keeping the local tip–sample capacitance, it is possible to image surface potential and topography at the same time by SMM. We obtained a surface potential map which well agrees with our previous results from I –V measurements. 2. Experimental procedure We prepared InAs dots which comprise regular size dots of about 20 nm diameter at a density of ∼ 500 m−2 , and large ones of 50 –100 nm diameter at a density of ∼ 10 m−2 , on an n+ -GaAs (0 0 1) substrate by molecular beam epitaxy. The substrate was thermally cleaned under an As4 ux, and a 200-nm-thick Si-doped (n = 2:1 × 1017 cm−3 ) bu er layer was grown at a substrate temperature of about ◦ 580 C. Then, the substrate temperature was lowered ◦ to 470 C where nominally 2.4 monolayer InAs was deposited onto the surface. After growth, the sample was detached from the molybdenum block under nitrogen ow in order to minimize surface oxidation, and transferred through air into a high-vacuum chamber of a commercially available SPM system (SPI3800, Seiko Instruments Inc.). All measurements were performed at room temperature under a pressure of less than 3 × 10−7 Torr. First, the topographic image was acquired by constant force AFM, then the surface potential and the constant tip–sample capacitance images were simultaneously obtained by SMM where an AC voltage of 4 kHz with peak-to-peak voltage of 10 V was applied between the sample and Au-coated tip. Comparing the topographic AFM image with the constant tip–sample capacitance and surface potential images, one can determine the surface potential on SAQDs of various sizes.
3. Results and discussions The SMM tip, which is at the free end of a cantilever, and the sample form a small capacitor with a ∼ 5 nm gap. When AC voltage of frequency ! is applied between the tip and sample, the cantilever oscillates at a frequency of 2! because of the Coulomb force between the two electrodes that is attractive at both polarity. Therefore, adjusting the gap distance and keeping the 2! component of the cantilever oscillation, we can obtain a constant tip–sample capacitance image. If the sample surface has a non-zero built-in potential, however, frequency component of ! appears in the cantilever oscillation, because the attractive force between the two electrodes will not be symmetric. In order to make this ! component zero, an o set bias voltage is applied to the sample during the measurement. Thus, this sample bias voltage corresponds to the local surface potential. Fig. 1(a) shows the topographic AFM image of the SAQDs, and Fig. 1(b) the constant tip–sample capacitance image by SMM taken just after Fig. 1(a). Since the tip is scanned above the surface with ∼ 5 nm gap in SMM measurements, a higher lateral resolution is expected with the AFM image where the tip is in contact with the surface. Thus, the regular size SAQDs appear somewhat blurred in the constant tip–sample capacitance image. However, the SAQDs of ∼ 50 nm diameter or larger are clearly observed in the constant capacitance image as indicated by the arrows labelled a–d. Figs. 2(a) and 2(b) are the simultaneously measured constant tip–sample capacitance image and surface potential image, respectively. Comparing these two images, one can see that the surface potential on the largest dot of nearly 100-nm diameter is ∼ 610 mV, and that on the regular size dot region is 640 – 650 mV. We have already reported the simultaneous imaging of topography and conductance of InAs SAQDs grown on a GaAs n+ -bu er layer by AFM with conductive probe, and that the local conductance is higher on larger InAs SAQDs [7]. We also performed I – V measurements using the conductive AFM probe on the SAQDs of various sizes grown by the same processes explained in the experimental section [8]. The obtained I –V characteristics are shown in Fig. 3. In this gure, the current obtained under forward bias condition on large dots of 70 –90 nm diameter is
I. Tanaka et al. / Physica E 7 (2000) 373–376
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Fig. 1. Surface topography of InAs SAQDs: (a) constant force image by AFM; (b) constant tip–sample capacitance image by SMM.
Fig. 2. Simultaneously measured (a) constant tip–sample capacitance image and (b) surface potential image of InAs SAQDs by SMM.
indicated by circles, and that on small dots of 30 – 40 nm diameter by squares. We observe that on the larger dots, (1) the current is over two orders of magnitude higher than that on the smaller ones, and (2) the current increases more rapidly with increasing forward bias voltage. For example, the current on the large dots that is 200 times larger than that on the small ones at a forward bias of 0.1 V becomes 1000 times larger at 0.3 V. These characteristics cannot be explained by
the di erence in their size, and hence the electronic properties at the surface should be taken into account to explain the results. The sample surface is covered by a monolayer of InAs which is called the wetting layer and SAQDs of various sizes. Since the sample was taken into air for transfer, the wetting layer is expected to be oxidized with a few monolayers of GaAs underneath. Therefore, the wetting layer region is likely to show
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I. Tanaka et al. / Physica E 7 (2000) 373–376
measurement, the correspondence between the dot size and the local surface potential is observed easily. Recently, tunneling spectroscopy of InAs deposited on n-type (n = 3 × 1017 cm−3 ) GaAs (0 0 1) wafer suggested decrease of the depletion layer width with increasing amount of deposited InAs [11]. They observed SAQDs of more than 50-nm-diameter with density of 30 m−2 by AFM on the sample which had the thinnest depletion layer. This result is also consistent with our model because the depletion layer becomes thinner for lower Schottky barriers. 4. Conclusion Fig. 3. Current versus forward-bias voltage characteristics measured on InAs SAQDs of various sizes.
electronic properties similar to that of oxidized GaAs surfaces, where high density of negative charges strongly pin Fermi level at mid-gap, resulting in the formation of Schottky barrier. On the other hand, it is well known that an electron accumulation layer is formed on the InAs surfaces due to dense positive surface states [10]. It is therefore expected that similar positive charges exist on InAs SAQDs which locally lower the Schottky barrier by canceling the e ect of negative charges on the surrounding wetting layer region, and that this cancellation is more e ective on larger dots because of more positive charges. In fact, from the I –V characteristics in Fig. 3, the Schottky barrier heights are estimated to be 600 ± 20 and 650 ± 20 meV for large and small dots, respectively. The surface potential of SAQDs measured by SMM supports the above explained model in which possible positive charges on InAs SAQDs locally modify Schottky barrier heights since the measured potential of 610 mV for 100-nm-scale SAQDs and that of 640 – 650 mV for regular size SAQDs region well agree with the Schottky barrier heights estimated from the I –V characteristics in Fig. 3. SMM images can provide us with the spatial distribution of the surface potential, and hence, compared with the I –V
We have directly measured surface potential of InAs SAQDs on n-GaAs (0 0 1) surface by SMM, and found that the surface potential is 30 – 40 mV lower on large dots than on the surrounding regions. This result agrees well with our previous reports. References [1] H. Sakaki, Surf. Sci. 267 (1992) 623. [2] H. Sakaki, Solid State Commun. 92 (1994) 119. [3] D. Leonard, M. Krishamurthy, C.M. Reaves, S.P. Denbaars, P.M. Petro , Appl. Phys. Lett. 63 (1993) 3203. [4] B. Legrand, B. Grandidier, J.P. Nys, D. Stievenard, J.M. Gerard, V. Thierry-Mieg, Appl. Phys. Lett. 73 (1998) 96. [5] M.E. Rubin, G. Medeiros-Ribeiro, J.J. O’Shea, M.A. Chin, Y.E. Lee, P.M. Petro , V. Narayanamurti, Phys. Rev. Lett. 77 (1996) 5268. [6] Y. Toda, M. Kourogi, M. Ohtsu, Y. Nagamune, Y. Arakawa, Appl. Phys. Lett. 69 (1996) 827. [7] I. Tanaka, I. Kamiya, H. Sakaki, N. Qureshi, S.J. Allen Jr., P.M. Petro , Appl. Phys. Lett. 74 (1999) 844. [8] I. Tanaka, I. Kamiya, H. Sakaki, J. Cryst. Growth (1999) in press. [9] H. Yokoyama, K. Sato, T. Inoue, Molec. Electron. Bioelectron. 3 (1992) 79. [10] S. Kawaji, Y. Kawaguchi, J. Phys. Soc. Japan 21(Suppl.) (1966) 336. [11] T. Takahashi, M. Yoshita, I. Kamiya, H. Sakaki, Appl. Phys. A 66(Suppl.) (1998) S1055.
www.elsevier.nl/locate/physe
Surface potential measurement of self-assembled InAs dots by scanning Maxwell stress microscopy Ichiro Tanakaa; b; c; d; ∗ , I. Kamiyab; d; 1 , H. Sakakic; d , M. Fujimotoa a Department
of Materials Science and Chemistry, Faculty of Systems Engineering, Wakayama University, 930 Sakaedani, Wakayama 640-8510, Japan b Research Center for Advanced Science and Technology, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan c Institute of Industrial Science, University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106-8558, Japan d Quantum Transition Project, Japan Science and Technology Corporation, Japan
Abstract We have imaged surface potential of InAs self-assembled quantum dots on GaAs (0 0 1) surface by scanning Maxwell stress microscopy. Simultaneously obtained constant tip–sample capacitance image and surface potential image were compared with atomic force microscope (AFM) image, and was found that the surface potential on large dots of about 100-nm diameter is ∼ 610 mV which is 30 – 40 mV lower than that on regular size dots. This result well coincides with previously reported Schottky barrier heights which were estimated from current versus voltage measurements with the same type of samples using conductive probe AFM. ? 2000 Elsevier Science B.V. All rights reserved. PACS: 61.16.Ch; 73.30.+y; 73.61.Ey Keywords: Quantum dots; Scanning Maxwell stress microscopy; Schottky barrier
1. Introduction Semiconductor nanostructures such as quantum wires and dots are promising materials for future device applications [1,2]. Among various types of nanos∗ Correspondence address: Department of Materials Science and Chemistry, Faculty of Systems Engineering, Wakayama University, 930 Sakaedani, Wakayama 640-8510, Japan. Tel.: +81-734-57-8238; fax: +81-734-57-8272. E-mail address: [email protected] (I. Tanaka) 1 Present address: Non-Equilibrium Laboratory, Mitsubishi Chemical Corporation, 1000 Kamoshida-cho, Aoba-ku, Yokohama 227-8502, Japan.
tructures, self-assembled quantum dots (SAQDs) have been widely studied since they can be formed spontaneously by lattice-mismatched epitaxy without expensive lithographic processes [3]. The properties of SAQDs have been extensively studied on an ensemble of the dots by optical and electronic measurements. On the other hand, scanning probe microscopies (SPM) have been used for electronic and optical investigations of individual InAs SAQDs grown on GaAs (0 0 1) surfaces [4 – 6]. We already reported simultaneous imaging of topography and conductance of InAs SAQDs with nano-scale resolution by atomic force microscopy (AFM) using conductive probes
1386-9477/00/$ - see front matter ? 2000 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 9 4 7 7 ( 9 9 ) 0 0 3 4 4 - 6
374
I. Tanaka et al. / Physica E 7 (2000) 373–376
[7]. We observed di erence in the local conductance between the InAs dots and the wetting layer. Also, current versus voltage (I –V ) measurements on the SAQDs of various sizes have shown that larger dots are more conductive than the smaller ones, and that the surface-barrier height of the large dots of 70 –90 nm diameter is lower than that of the regular dots by ∼ 50 meV. This modi cation has been attributed to local lowering of surface potential due to possible positively charged surface states on InAs SAQDs [8]. In this study, we adopted scanning Maxwell stress microscopy (SMM) in order to directly measure the surface potential on InAs SAQDs [9]. Keeping the local tip–sample capacitance, it is possible to image surface potential and topography at the same time by SMM. We obtained a surface potential map which well agrees with our previous results from I –V measurements. 2. Experimental procedure We prepared InAs dots which comprise regular size dots of about 20 nm diameter at a density of ∼ 500 m−2 , and large ones of 50 –100 nm diameter at a density of ∼ 10 m−2 , on an n+ -GaAs (0 0 1) substrate by molecular beam epitaxy. The substrate was thermally cleaned under an As4 ux, and a 200-nm-thick Si-doped (n = 2:1 × 1017 cm−3 ) bu er layer was grown at a substrate temperature of about ◦ 580 C. Then, the substrate temperature was lowered ◦ to 470 C where nominally 2.4 monolayer InAs was deposited onto the surface. After growth, the sample was detached from the molybdenum block under nitrogen ow in order to minimize surface oxidation, and transferred through air into a high-vacuum chamber of a commercially available SPM system (SPI3800, Seiko Instruments Inc.). All measurements were performed at room temperature under a pressure of less than 3 × 10−7 Torr. First, the topographic image was acquired by constant force AFM, then the surface potential and the constant tip–sample capacitance images were simultaneously obtained by SMM where an AC voltage of 4 kHz with peak-to-peak voltage of 10 V was applied between the sample and Au-coated tip. Comparing the topographic AFM image with the constant tip–sample capacitance and surface potential images, one can determine the surface potential on SAQDs of various sizes.
3. Results and discussions The SMM tip, which is at the free end of a cantilever, and the sample form a small capacitor with a ∼ 5 nm gap. When AC voltage of frequency ! is applied between the tip and sample, the cantilever oscillates at a frequency of 2! because of the Coulomb force between the two electrodes that is attractive at both polarity. Therefore, adjusting the gap distance and keeping the 2! component of the cantilever oscillation, we can obtain a constant tip–sample capacitance image. If the sample surface has a non-zero built-in potential, however, frequency component of ! appears in the cantilever oscillation, because the attractive force between the two electrodes will not be symmetric. In order to make this ! component zero, an o set bias voltage is applied to the sample during the measurement. Thus, this sample bias voltage corresponds to the local surface potential. Fig. 1(a) shows the topographic AFM image of the SAQDs, and Fig. 1(b) the constant tip–sample capacitance image by SMM taken just after Fig. 1(a). Since the tip is scanned above the surface with ∼ 5 nm gap in SMM measurements, a higher lateral resolution is expected with the AFM image where the tip is in contact with the surface. Thus, the regular size SAQDs appear somewhat blurred in the constant tip–sample capacitance image. However, the SAQDs of ∼ 50 nm diameter or larger are clearly observed in the constant capacitance image as indicated by the arrows labelled a–d. Figs. 2(a) and 2(b) are the simultaneously measured constant tip–sample capacitance image and surface potential image, respectively. Comparing these two images, one can see that the surface potential on the largest dot of nearly 100-nm diameter is ∼ 610 mV, and that on the regular size dot region is 640 – 650 mV. We have already reported the simultaneous imaging of topography and conductance of InAs SAQDs grown on a GaAs n+ -bu er layer by AFM with conductive probe, and that the local conductance is higher on larger InAs SAQDs [7]. We also performed I – V measurements using the conductive AFM probe on the SAQDs of various sizes grown by the same processes explained in the experimental section [8]. The obtained I –V characteristics are shown in Fig. 3. In this gure, the current obtained under forward bias condition on large dots of 70 –90 nm diameter is
I. Tanaka et al. / Physica E 7 (2000) 373–376
375
Fig. 1. Surface topography of InAs SAQDs: (a) constant force image by AFM; (b) constant tip–sample capacitance image by SMM.
Fig. 2. Simultaneously measured (a) constant tip–sample capacitance image and (b) surface potential image of InAs SAQDs by SMM.
indicated by circles, and that on small dots of 30 – 40 nm diameter by squares. We observe that on the larger dots, (1) the current is over two orders of magnitude higher than that on the smaller ones, and (2) the current increases more rapidly with increasing forward bias voltage. For example, the current on the large dots that is 200 times larger than that on the small ones at a forward bias of 0.1 V becomes 1000 times larger at 0.3 V. These characteristics cannot be explained by
the di erence in their size, and hence the electronic properties at the surface should be taken into account to explain the results. The sample surface is covered by a monolayer of InAs which is called the wetting layer and SAQDs of various sizes. Since the sample was taken into air for transfer, the wetting layer is expected to be oxidized with a few monolayers of GaAs underneath. Therefore, the wetting layer region is likely to show
376
I. Tanaka et al. / Physica E 7 (2000) 373–376
measurement, the correspondence between the dot size and the local surface potential is observed easily. Recently, tunneling spectroscopy of InAs deposited on n-type (n = 3 × 1017 cm−3 ) GaAs (0 0 1) wafer suggested decrease of the depletion layer width with increasing amount of deposited InAs [11]. They observed SAQDs of more than 50-nm-diameter with density of 30 m−2 by AFM on the sample which had the thinnest depletion layer. This result is also consistent with our model because the depletion layer becomes thinner for lower Schottky barriers. 4. Conclusion Fig. 3. Current versus forward-bias voltage characteristics measured on InAs SAQDs of various sizes.
electronic properties similar to that of oxidized GaAs surfaces, where high density of negative charges strongly pin Fermi level at mid-gap, resulting in the formation of Schottky barrier. On the other hand, it is well known that an electron accumulation layer is formed on the InAs surfaces due to dense positive surface states [10]. It is therefore expected that similar positive charges exist on InAs SAQDs which locally lower the Schottky barrier by canceling the e ect of negative charges on the surrounding wetting layer region, and that this cancellation is more e ective on larger dots because of more positive charges. In fact, from the I –V characteristics in Fig. 3, the Schottky barrier heights are estimated to be 600 ± 20 and 650 ± 20 meV for large and small dots, respectively. The surface potential of SAQDs measured by SMM supports the above explained model in which possible positive charges on InAs SAQDs locally modify Schottky barrier heights since the measured potential of 610 mV for 100-nm-scale SAQDs and that of 640 – 650 mV for regular size SAQDs region well agree with the Schottky barrier heights estimated from the I –V characteristics in Fig. 3. SMM images can provide us with the spatial distribution of the surface potential, and hence, compared with the I –V
We have directly measured surface potential of InAs SAQDs on n-GaAs (0 0 1) surface by SMM, and found that the surface potential is 30 – 40 mV lower on large dots than on the surrounding regions. This result agrees well with our previous reports. References [1] H. Sakaki, Surf. Sci. 267 (1992) 623. [2] H. Sakaki, Solid State Commun. 92 (1994) 119. [3] D. Leonard, M. Krishamurthy, C.M. Reaves, S.P. Denbaars, P.M. Petro , Appl. Phys. Lett. 63 (1993) 3203. [4] B. Legrand, B. Grandidier, J.P. Nys, D. Stievenard, J.M. Gerard, V. Thierry-Mieg, Appl. Phys. Lett. 73 (1998) 96. [5] M.E. Rubin, G. Medeiros-Ribeiro, J.J. O’Shea, M.A. Chin, Y.E. Lee, P.M. Petro , V. Narayanamurti, Phys. Rev. Lett. 77 (1996) 5268. [6] Y. Toda, M. Kourogi, M. Ohtsu, Y. Nagamune, Y. Arakawa, Appl. Phys. Lett. 69 (1996) 827. [7] I. Tanaka, I. Kamiya, H. Sakaki, N. Qureshi, S.J. Allen Jr., P.M. Petro , Appl. Phys. Lett. 74 (1999) 844. [8] I. Tanaka, I. Kamiya, H. Sakaki, J. Cryst. Growth (1999) in press. [9] H. Yokoyama, K. Sato, T. Inoue, Molec. Electron. Bioelectron. 3 (1992) 79. [10] S. Kawaji, Y. Kawaguchi, J. Phys. Soc. Japan 21(Suppl.) (1966) 336. [11] T. Takahashi, M. Yoshita, I. Kamiya, H. Sakaki, Appl. Phys. A 66(Suppl.) (1998) S1055.