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Probing individual nanostructures with STM-induced light emission
PERGAMON
Solid State Communications 117 (2001) 159±166
www.elsevier.com/locate/ssc
Probing individual nanostructures with STM-induced light emission S. Ushioda a,b,* a
Research Institute of Electrical Communication, Tohoku University, Sendai 980-8577, Japan b CREST Ð Japan Science and Technology Corporation (JST), Sendai 980-8577, Japan
Abstract We present the results of our latest work using the spectroscopy of light emitted by a sample under the scanning tunneling microscope (STM). For the quantum wells in AlGaAs/GaAs layered structures, we have demonstrated that the emission spectra from individual wells can be measured by this method and that the minority carrier diffusion length can be estimated in real space. Furthermore, we have discovered that the emission linewidth depends on the location even within an individual well. On the reconstructed Au(110)-(2 £ 1) surface, the emission from an atomically localized electronic transition in the valley between the atomic rows was found. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Quantum wells; A. Nanostructures; C. Scanning tunneling microscopy PACS: 63.50. 1 x; 78.30.Ly; 78.40.Pg
1. Introduction The purpose of this paper is to present the results of our latest work using scanning tunneling microscope light emission spectroscopy (STM-LES), and to provide some perspective for the future of this method. STM-LES is based on the discovery by Gimzewski et al. [1] and Coombs et al. [2] that visible light is emitted from the gap between the sample and the tip of the STM. Since the electron beam that tunnels from the STM tip to the sample is extremely narrow, in optimal cases atomic scale spatial resolution can be achieved on the location of the light source. Then by analyzing the emission spectra, one can obtain information on the electronic transitions with an extremely high spatial resolution afforded by the STM. This technique has been used to investigate many different phenomena in which a high spatial resolution is relevant; for a recent review see Ref. [3]. For example, when one applies STM-LES to the study of quantum nanostructures, one can identify an individual structure by STM * Research Institute of Electrical Communication, Tohoku University, Sendai 980-8577, Japan. Tel.: 181-22-217-5497; fax: 181-22-217-5500. E-mail address: [email protected] (S. Ushioda).
imaging, and then obtain the optical spectra from that speci®c structure. Thus, one can directly gain information on the size and shape dependence of the electronic transitions (quantum con®nement effect). In this paper we will ®rst discuss the physical mechanisms of light emission. We will then describe the experimental method, and present the latest results from our recent work. In the course of discussion we will attempt to assess the strengths and weaknesses of the method and to give some perspective for the future.
2. Light emission mechanisms To gain physical information from STM light emission (STM-LE) spectra, it is necessary ®rst to understand the emission mechanisms, which differ depending on the nature of the sample. Fig. 1 illustrates the energy diagrams of tunneling for metals and semiconductors (insulators). For simplicity of explanation, we depicted the case where the electron is injected from the STM tip to the sample, and the semiconductor sample is assumed to be p-type. The bias voltage V0 applied across the STM tip and the sample creates an electronic potential energy difference of eV0 (e is the elemental charge) between the Fermi levels of the tip metal and the sample. Thus when the electron tunnels
0038-1098/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0038-109 8(00)00438-5
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Fig. 1. Energy diagrams of light emission processes: (a) metallic sample; and (b) p-type semiconductor sample.
starting near the Fermi level across the vacuum gap, it releases the energy ,eV0 before it relaxes to the Fermi level on the opposite side. Most of this energy is released by phonon scattering, i.e. dissipated in the form of heat. Only one in about 10 3 ±10 4 electrons induces light emission, making the spectral measurement very dif®cult. In any sample system, the conservation of energy limits the maximum energy of emitted photons to hn # eV0
1
where h is the Planck constant and n is the frequency of the photon. This relation holds except in two cases that we are aware of. When both the tip and the sample are superconducting, the Cooper pairs tunnel between them, giving rise to an emission peak at 2eV0 [4]. In another situation, the maximum emission energy is noticeably reduced below eV0 when the capacitance of the target object (metallic particle) is on the order of 10 218 C. This situation occurs when the sample system shows the Coulomb blockade. The electron that tunnels to a semi-isolated small particle raises its Fermi level after tunneling, and hence the total energy available for photon emission is reduced by the energy equal to the rise in the Fermi level [5]. When the sample is a metal, the light emission mechanism is similar to that of metal±insulator±metal (MIM) tunnel junctions [6]. The theories of light emission for MIM were developed by several groups including Rendell et al. [7], Lax and Mills [8], and Takeuchi et al. [9]. According to these theories, light is emitted by the optical frequency ¯uctuation of the tunneling current. The current ¯uctuation can cause light emission through direct radiation and through excitation of surface plasmon polaritons (SPP), which in turn emit external photons by scattering from surface roughness of the junction [8±10]. The corresponding dielectric or macroscopic theories of light emission from STM were reported by Persson and Baratoff [11], Johansson et al. [12], Johannsson [13], Aizupurua et al. [14], and Uehara et al. [15]. These theories are based on the macroscopic dielectric functions of the tip and sample, similar to the theories of MIM light emission.
Hence they predict only the radiation from current ¯uctuations in the STM gap. In contrast to the situation for the MIM junction, there is no translational symmetry along the surface in the presence of the tip above the sample surface. The normal modes of electromagnetic radiation excited in the tip±sample gap are localized surface plasmons (LSP). Most of the emission occurs through excitation of LSP and its subsequent decay into a free photon or a SPP [16]. These theories are based on macroscopic parameters. Thus they do not contain emissions from microscopically localized electronic transitions. As we will see later, some of our experimental data cannot be explained within the framework of such theories, because we observe electronic transitions that involve localized wavefunctions at the atomic level. A microscopic theory of light emission by tunneling electrons has been reported by Tsukada et al. [17,18]. They calculated the spectra due to inelastic tunneling in which light emission occurs via LSP. They did not focus on light emission from transitions between atomically localized states. Since their theory is based on atomic orbitals, in principle, it contains atomically localized transitions. It will be useful to re-examine their theory with a focus on microscopic transitions. An extension of their theory that treats emission from atomically localized states would be very interesting. When the sample is a semiconductor, the emission spectrum corresponds to the transition across the band gap. The peak energy is given by hn Egap
2
where Egap is the band gap. The electron that is injected into the conduction band of a p-type semiconductor relaxes rapidly to the bottom of the conduction band, before it combines with a hole to emit light. There is no light emission, if the sample is n-type and the electron is injected by tunneling. In n-type semiconductors the hole must be injected to induce light emission. Thus STM light emission is different from photoluminescence (PL) in that appropriate minority carriers must be injected to induce light emission.
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Fig. 2. STM light emission spectroscopy system in UHV.
In PL the incident light automatically creates both electrons and holes, so that light emission by recombination occurs even in intrinsic semiconductors. However, the ®nal recombination process is identical in both cases. Hence the emission peak energy is given by Eq. (2) in both PL and STM light emission. In the sense that electrons are injected into a sample and the emitted photons are analyzed, STM light emission is analogous to inverse photoemission and cathodoluminescence. The difference lies in the energies of the injected electrons. In STM light emission the electron energy is only a few eV, while in inverse photoemission it is usually few tens of eV. The electron energy is much greater in the case of cathodoluminescence, lying in the range of keV. Because of the low energy of the tunneling electrons, they do not penetrate deep into the sample, allowing local probing of the surface. Moreover, they do not create electron±hole pairs when the bias voltage is suf®ciently low. This results in light emission only from the immediate region in which the low energy electrons can diffuse before combining with holes. Thus STM-LES can achieve nearly atomic spatial resolution under favorable conditions.
3. Experimental method In STM-LES one ®rst obtains a topographic image of surface structures by the usual scanning method. Then the STM tip is located over a structure of interest, and the emission spectrum from the target structure is measured by keeping the tip at a ®xed position over it. Since the emission intensity is extremely weak in most cases, one must collect the photons over an extended period of time. The typical quantum ef®ciency for light emission by tunneling electrons is on the order of 10 24. Thus, for a tunneling current of 1 nA (,6 £ 10 9 electrons/s), the total integrated number of photons is ,6 £ 10 5 photons/s. The usual quantum ef®ciency of detection is around 5 £ 10 22, and a typical solid angle of collection is on the order of 10 21. Hence the expected number of detected photon counts is
about 3 £ 10 3 photons/s over the whole spectral range of emission. Then if the spectrum is divided into 10 3 channels of a multichannel detector, each channel will count ,3 photons/s. This is a typical signal level in our system. To achieve a satisfactory level of signal-to-noise ratio, one needs to accumulate the photon counts for an extended period, typically several hundred seconds for each spectrum. Then the thermal drift of the tip position becomes a serious problem. We have solved this problem by developing a software-controlled servo-mechanism to compensate for thermal drift during the spectral measurement [19]. Since the tunneling process is sensitively in¯uenced by the surface conditions of the tip and sample, the measurement must be made in an ultrahigh vacuum (UHV) environment to obtain reproducible results. Fig. 2 shows a schematic diagram of our STM-LES system. It comprises a STM with its control electronics, light collection optics, a grating spectrograph, a multichannel optical detector, and a personal computer to record the spectral data. The UHV system that houses the STM is also equipped with sample surface preparation and characterization tools. An important part of this system that requires a special attention in design is the light collection optics and the detector system. Fig. 2 illustrates a system with lens optics for light collection. The lens system has the disadvantage of being bulky and the angle of collection cannot be freely adjusted in UHV. To solve this problem we have developed an alternative system where an optical ®ber is used to collect light in UHV [20]. Another approach for improved light collection was developed by Murashita and Tanimoto [21]. They used a glass ®ber with a conductive coating as the STM tip and collected the emitted light through this tip. The most dif®cult and irreproducible part of this probing technique lies in the fabrication of the STM tip. We found that some tips that can generate STM images with clear atomic resolution are not ef®cient in inducing light emission. We are not aware of dependable and reproducible ways of making tips with high light emission ef®ciency. Thus ®nding a good tip for light emission is purely based on trial and error. This situation needs to be improved before
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Fig. 3. Two experimental con®gurations for the measurements of STM light emission from the AlGaAs/GaAs quantum well structure: from the (001) surface and from the cleaved (110) surface.
STM-LES can become a more generally applicable probing method. When the light emission involves an intermediate process of LSP excitation, Ag tips are found to be most ef®cient in photon production. This is understandable in view of the fact that the damping of plasmons in Ag is much smaller than in other metals such as W and PtIr that are often used as tip materials. The imaginary part of the dielectric function of Ag in the visible range is much smaller than that of these other materials. We found that with the Ag tip, the emission ef®ciency can be enhanced by more than two orders of magnitude relative to W or PtIr tips [22]. This enhancement effect by a sharp Ag tip is related to the similar effect seen with Ag nanostructures in surface enhanced Raman scattering (SERS) [23]. Furthermore, in our latest experiment we found that with the Ag tip the emission ef®ciency increases with decreasing radius of curvature of the tip [24]. Since we could measure the tip radius only with the resolution of a scanning electron microscope (SEM), this result is meaningful only down to a radius of ,10 nm. What is happening below that level is not observable. According to the theories cited above, if the electromagnetic retardation effect is not taken into account, the light emission ef®ciency increases with increasing tip radius. However, when the radius of the tip becomes more than ,10 nm, the retardation effect becomes signi®cant and the light emission ef®ciency goes down [13]. Most of the tips usually used have the radius greater than 10 nm, lying on the decreasing slope of the tip radius vs. emission ef®ciency curve. We have found that this is the case for Ag tips; i.e. the ef®ciency of the tip increases with decreasing tip radius down to ,80 nm [24]. In the following two sections we present the results obtained by using STM-LES as a probe of electronic transitions in individual nanostructures of semiconductors and at speci®c atomic sites on a metallic surface.
4. Probing of individual quantum wells
Fig. 4. (a) STM light emission spectra of the three AlGaAs/GaAs QWs with different widths 2.0, 5.1, and 10.2 nm observed when the electrons are injected from the (001) surface. (b) Photoluminescence spectra of the same samples.
Photoluminescence (PL) is often used to characterize semiconductor quantum structures like quantum dots and quantum wells (QW). In the conventional PL measurements, luminescence is measured from a collection of these structures with a size and shape distribution. Thus, it is not possible to characterize speci®c individual structures. In contrast, with STM-LES one can probe individual structures by imaging them ®rst and setting the STM tip over them. This is the great advantage of this method. An early work on semiconductor quantum structures using STM light emission was reported by Alvarado et al [25]. They measured the integrated emission intensity as a function of the tip position and plotted the photon intensity maps of QWs. However, they did not measure the emission spectra. By analyzing the emission spectra at different
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Fig. 5. STM images of the cleaved (110) surface of AlGaAs/GaAs QWs. The irregular stripes extending from the top left to the bottom right are the steps created by cleaving.
points, more detailed information can be gained on the electronic transitions in individual structures. We have recently measured the STM light emission from the QWs of AlGaAs/GaAs [26,27]. Fig. 3 illustrates the geometry for these experiments. In the work reported in Ref. [26], the samples were multiquantum well structures of p-Al0.4Ga0.6As/p-GaAs. The three samples used in this experiment had an identical thickness of the Al0.4Ga0.6As barrier layers (20.4 nm) and differed in the thickness of
Fig. 6. STM light emission spectra of individual wells of widths 2.5, 5.1, 10.2, and 50 nm. The electrons were injected into the individual wells seen in Fig. 5.
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Fig. 7. STM light emission spectra obtained when the electrons were injected at different positions indicated by a, b, and c in the inset.
the GaAs wells, being 10.2, 5.1, and 2 nm, respectively. The electron was injected from the STM tip into the (001) surface, and the light emitted from the same surface was measured (see the inset of Fig. 3). Fig. 4a shows the STM light emission spectra of the three samples. All the spectra consist of a single emission peak, whose peak energy shifts to the high-energy side from 1.44 to 1.73 eV with decreasing well widths. The same peak shift was also observed in the PL spectra of the same three samples as shown in Fig. 4b. This result shows that the injected electrons thermalize to the bottom of the conduction band, before combining with holes. Thus the ®nal light emission process is the same for PL and STM light emission. However, the initial excitation process is different, and this fact is clearly observed when holes instead of electrons are injected into the p-type sample by changing the polarity of the bias voltage. No emission was observed for hole injection. More recently, we have measured the STM light emission from individual QWs by injecting electrons into the cleaved (110) surface of AlGaAs/GaAs QW structures [27]. Since the growth direction of QWs is [001], the cleaved (110) surface exposes the cross-sections of QWs. Thus the electron can be injected into individual wells targeted by looking at the STM image of the surface. Two samples of quantum well structures were used in this experiment. One had alternating GaAs/Al0.38Ga0.62As layers of thickness 47 and 45 nm, respectively. The other had GaAs wells of widths ranging from 2.5 to 10.2 nm separated by Al0.38Ga0.62 As barriers of 40 nm thickness. All the layers were Be doped (p-type) to a level of ,6 £ 10 18 cm 23. Fig. 5 shows the STM images of the cleaved (110) surface of the two samples. We see well-de®ned stripes in the STM images re¯ecting the AlGaAs/GaAs QW structures. GaAs wells are
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Fig. 8. STM light emission spectra measured at three different points in a single GaAs well.
seen as bright bands in the STM images. The cleavage induced surface steps also appear as irregular black and white bands running from the top left to the bottom right. Fig. 6 shows the STM light emission spectra from individual GaAs QWs. The spectra were measured by locating the STM tip over individual wells. A single emission peak was observed for respective wells of different well widths, and the peak energy shifted to the high-energy side with decreasing well width. No emission peak was observed for negative sample bias ranging up to 23 V (corresponding to
hole injection). This means that photon emission occurred by radiative recombination of the minority carriers (electrons) injected from the tip rather than the impact ionization of electron±hole pairs. By comparison with theoretical calculations, these peaks were identi®ed with the transition between the electronic ground state of the GaAs well to the heavy hole state of the valence band. The emission intensity drastically decreased as the tip crossed the interface from the GaAs well into the AlGaAs barrier by only a few nm. Fig. 7 shows three of the STM light emission spectra that were measured by moving the tip between two GaAs wells separated by an AlGaAs barrier. When the spectrum is measured by locating the tip over one well, we do not see any emission from other wells (Fig. 7a and c). On the other hand, the emission from wells on both sides was barely observed when the tip was located over the middle of the AlGaAs barrier (Fig. 7b). These results mean that the thermalization length of hot electrons injected from the tip is comparable to or shorter than 20 nm. Moreover, the emission intensity was almost the same for the wells with 50 and 10.2 nm width, and then decreased drastically for narrower wells, as seen in Fig. 6. Thus we see that at low sample bias employed in this work, most of the injected hot electrons thermalize within about 10 nm, and subsequently recombine radiatively. The small emission features of Fig. 7b originate from the electrons that reach the wells of either side after thermalization. To investigate the difference in local conditions within a well, we have measured the emission linewidths at different spots inside a single well. Fig. 8a shows the STM image of a GaAs well with 5.1 nm width sandwiched by AlGaAs barriers. The GaAs well is seen as a slightly light band. The cleavage induced surface step also appears as an irregular dark band running from the top left to the bottom right. Fig. 8b shows three of the STM light emission spectra
Fig. 9. Atomic image of the reconstructed Au(110)-(2 £ 1) surface.
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Fig. 10. Emission spectra: (a) when the tip is located over the row of atoms; and (b) when the tip is located over the valley between the rows. The dotted curve is the calculated result based on the macroscopic dielectric theory.
that were measured by injecting electrons at three different points A, B, and C in the well of Fig. 8a. Although the peak energy of emission is almost identical for the three points, the spectral linewidth changes when the tip is moved only by ,20 nm. This result demonstrates that this technique is a sensitive probe of extremely localized electronic and optical properties of quantum structures. We note here that the measurements were carried out with a relatively small current of 0.5 nA. To insure that one does not modify the sample surface, it is essential to use as small a current as possible. We always examine the sample surface after spectroscopic measurements to con®rm that no sample damage was caused. The work described above is only the beginning of this class of experiments that are possible with STM-LES. Since one can obtain atomic scale spatial resolution, one can investigate electronic transitions very close to the walls of the well and gain information on the behavior of electrons near the wall. One can also investigate electronic transitions in the vicinity of defects and impurities. Thus there are many interesting opportunities to take advantage of this new experimental technique.
5. Probing of speci®c atomic sites on a metallic surface In our recent experiment on the reconstructed Au(110)(2 £ 1) surface (Fig. 9), we succeeded in identifying a localized electronic transition at different atomic sites [28]. This surface was previously studied by Berndt et al. using STM light emission [29]. They obtained the photon intensity map of this surface that shows the corrugation in photon intensity, but did not measure the emission spectra. We measured the STM light emission spectra, using a low temperature STM housed in a UHV chamber. The sample temperature was 80 K. The STM tip was made of polycrystalline tungsten and had the radius of ,50 nm. The exposure
165
time for all the reported spectra was 200 s, during which the estimated thermal drift distance was 0.038 nm. Figure 9 shows a topographic image of the Au(110) surface with the image of atoms on the (2 £ 1) reconstructed rows. The observed structure corresponds to the missing row model of the Au(110) surface [30]. On this surface we measured the emission spectra at two nonequivalent sites, namely over the row and between the rows of the Au atoms. Fig. 10 shows the emission spectra from the two sites: (a) over the atomic row and (b) between the rows. These spectra were measured at a constant current of 2 nA for a bias voltage of 2.3 V. The current direction was from the sample to the tip; i.e. electron injection into the sample. We con®rmed that no surface damage was caused during the optical measurement by rescanning the surface after the spectra were measured. The cut-off photon energy was at hnmax 2.3 eV as expected from Eq. (1). When the tip was over the atomic row (Fig. 10a), the spectrum had a single broad peak at 2.09 eV. When the tip was located over the valley between the rows (Fig. 10b), the spectrum showed a peak at 1.91 eV in addition to the peak at 2.09 eV. To understand the difference in the observed spectra over the atomic row and over the valley, comparisons were made with a theoretical calculation based on the macroscopic dielectric functions of the metals involved [15]. This theory predicts the spectra of emission mediated by LSP. The dotted curve in Fig. 10 is the calculated spectrum. As clearly seen in Fig. 10a the calculated spectrum reproduces the measured spectrum very well when the tip is located over the atomic row. From this agreement we concluded that the spectrum measured over the atomic row arises from the LSP between the sample and the tip. On the other hand, the extra peak at 1.91 eV seen in the spectrum from the valley between the rows cannot be accounted for by the macroscopic dielectric theory. Since the lateral extent of the LSP is much greater than the distance between the top and the valley of the atomic rows (,0.4 nm), the emission spectrum originating from the LSP should not depend on the spatial difference between these two separate sites. Hence the peak at 1.91 eV observed only when the tip is over the valley (Fig 10b) must be due to a more localized excitation than the LSP. Although we do not show the data here, we measured the spectra over the row and between the rows at a bias voltage of 2.5 V [28]. These spectra indicated that the peak at 1.91 eV appears independent of the bias voltage when there is an atomic scale depression under the tip. The exact origin of this emission peak is not clear at present, but it must arise from a localized state of the reconstructed Au surface. More recent work suggests that this peak is due to impact ionization of the d-electrons caused by the tunneling electron followed by recombination of the d-holes with the electrons near the Fermi surface. The d-band lies about 2 eV below the Fermi level, and the wavefunction is tightly localized. Our result indicates that it extend into vacuum between the rows, but not over the rows.
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This experiment demonstrates that the STM light emission spectroscopy can reach atomic scale spatial resolution. This new spectroscopic capability will serve as a very powerful tool in exploring detailed site dependent phenomena at solid surfaces, such as catalysis, molecular adsorption, and defect incorporation in crystal growth. 6. Summary and future perspective We have presented the results of our latest work using STM-LES, and demonstrated the power of this technique. The uniqueness of this spectroscopic method lies in the fact that light emission spectra from an individual nanostructure or a speci®c site can be obtained by taking advantage of the high spatial resolution and the imaging capability of the STM. There are many exciting possibilities for future work. For example, one should be able to obtain the spectra from single molecules, single nanoclusters, single surface defects and impurities, surface steps, etc. Then this method can contribute unique information that has not been accessible before. The current stage of the development of this technique is still near the beginning, and further improvements are required to develop it into a generally applicable spectroscopic technique. The most important development that is required is a dependable and reproducible method for fabrication of the STM tip and a method for collecting and detecting light with a higher ef®ciency. Acknowledgements The work presented here has been carried out as a cooperative effort with the staff and students of the author's group. I would like to acknowledge their contributions, in particular by Y. Uehara, K. Sakamoto, T. Tsuruoka, M. Iwami, and R. Arafune. Our research is supported by Japan Science and Technology Corporation through the CREST Program. References [1] J.K. Gimzewski, B. Reihl, J.H. Coombs, R.R. Schlitter, Z. Phys. B 72 (1988) 497. [2] H. Coombs, J.K. Gimzewski, B. Reihl, J.K. Sass, R.R. Schlittler, J. Microsc. 152 (1988) 325. [3] R. Berndt, in: R. Wiesendanger (Ed.), Scanning Probe Microscopy, Springer, Berlin, 1998, p. 97 (chap. 5).
[4] Y. Uehara, T. Fujita, M. Iwami, S. Ushioda, Solid State Commun. 116 (2000) 539. [5] Y. Uehara, S. Ohyama, K. Ito, S. Ushioda, Jpn. J. Appl. Phys. 35 (1996) 167. [6] J. Lambe, S. McCarthy, Phys. Rev. Lett. 37 (1976) 923. [7] R.W. Rendell, D.J. Scalapino, B. Muhlschlegel, Phys. Rev. Lett. 41 (1978) 1746. [8] B. Lax, D.L. Mills, Phys. Rev. B 20 (1979) 4962. [9] A. Takeuchi, J. Watanabe, Y. Uehara, S. Ushioda, Phys. Rev. B 38 (1988) 12948. [10] J. Watanabe, A. Takeuchi, Y. Uehara, S. Ushioda, Phys. Rev. B 38 (1988) 12959. [11] B.N.J. Persson, A. Baratoff, Phys. Rev. Lett. 68 (1992) 3224. [12] P. Johansson, R. Monreal, P. Apell, Phys. Rev. B 42 (1990) 9210. [13] P. Johansson, Phys. Rev. B 58 (1998) 10823. [14] J. Aizpurua, S.P. Apell, R. Berndt, Phys. Rev. B 62 (2000) 2065. [15] Y. Uehara, Y. Kimura, S. Ushioda, K. Takeuchi, Jpn. J. Appl. Phys. 31 (1992) 2465. [16] K. Takeuchi, Y. Uehara, S. Ushioda, S. Morita, J. Vac. Sci. Technol. 9 (1991) 557. [17] M. Tsukada, T. Schimizu, K. Kobayshi, Ultramicroscopy 42±44 (1992) 360. [18] T. Schimizu, K. Kobayashi, M. Tsukada, Appl. Surf. Sci. 60/ 61 (1992) 454. [19] K.J. Ito, K. Ito, Y. Uehara, S. Ushioda, Rev. Sci. Instrum. 71 (2000) 420. [20] R. Arafune, K. Sakamoto, S. Ushioda, submitted for publication. [21] T. Murashita, M. Tanimoto, Jpn. J. Appl. Phys. 34 (1995) 4398. [22] M. Iwami, Y. Uehara, S. Ushioda, Appl. Surf. Sci. (2000) (in press). [23] Surface Enhanced Raman Scattering, in: R.K. Chang, T.E. Furtak (Eds.), Plenum Press, New York, 1982. [24] M. Iwami, Y. Uehara, S. Ushioda, Jpn. J. Appl. Phys. 39 (2000) 4912. [25] S.F. Alvarado, Ph. Renaud, D.L. Abraham, Ch. SchoÈnenberger, D.J. Arent, H.P. Meier, J. Vac. Sci. Technol. B 9 (1991) 409. [26] T. Tsuruoka, Y. Ohizumi, S. Ushioda, Appl. Phys. Lett. 73 (1998) 1544. [27] T. Tsuruoka, Y. Ohizumi, R. Tanimoto, S. Ushioda, Appl. Phys. Lett. 75 (1999) 2289. [28] Y. Uehara, T. Fujita, S. Ushioda, Phys. Rev. Lett. 83 (1999) 2445. [29] R. Berndt, R. Gaisch, W.D. Schneider, J.K. Gimzewski, B. Reihl, R.R. Schlittler, M. Tschudy, Phys. Rev. Lett. 74 (1995) 102. [30] W. Moritz, D. Wolf, Surf. Sci. 88 (1979) L29.
Solid State Communications 117 (2001) 159±166
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Probing individual nanostructures with STM-induced light emission S. Ushioda a,b,* a
Research Institute of Electrical Communication, Tohoku University, Sendai 980-8577, Japan b CREST Ð Japan Science and Technology Corporation (JST), Sendai 980-8577, Japan
Abstract We present the results of our latest work using the spectroscopy of light emitted by a sample under the scanning tunneling microscope (STM). For the quantum wells in AlGaAs/GaAs layered structures, we have demonstrated that the emission spectra from individual wells can be measured by this method and that the minority carrier diffusion length can be estimated in real space. Furthermore, we have discovered that the emission linewidth depends on the location even within an individual well. On the reconstructed Au(110)-(2 £ 1) surface, the emission from an atomically localized electronic transition in the valley between the atomic rows was found. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Quantum wells; A. Nanostructures; C. Scanning tunneling microscopy PACS: 63.50. 1 x; 78.30.Ly; 78.40.Pg
1. Introduction The purpose of this paper is to present the results of our latest work using scanning tunneling microscope light emission spectroscopy (STM-LES), and to provide some perspective for the future of this method. STM-LES is based on the discovery by Gimzewski et al. [1] and Coombs et al. [2] that visible light is emitted from the gap between the sample and the tip of the STM. Since the electron beam that tunnels from the STM tip to the sample is extremely narrow, in optimal cases atomic scale spatial resolution can be achieved on the location of the light source. Then by analyzing the emission spectra, one can obtain information on the electronic transitions with an extremely high spatial resolution afforded by the STM. This technique has been used to investigate many different phenomena in which a high spatial resolution is relevant; for a recent review see Ref. [3]. For example, when one applies STM-LES to the study of quantum nanostructures, one can identify an individual structure by STM * Research Institute of Electrical Communication, Tohoku University, Sendai 980-8577, Japan. Tel.: 181-22-217-5497; fax: 181-22-217-5500. E-mail address: [email protected] (S. Ushioda).
imaging, and then obtain the optical spectra from that speci®c structure. Thus, one can directly gain information on the size and shape dependence of the electronic transitions (quantum con®nement effect). In this paper we will ®rst discuss the physical mechanisms of light emission. We will then describe the experimental method, and present the latest results from our recent work. In the course of discussion we will attempt to assess the strengths and weaknesses of the method and to give some perspective for the future.
2. Light emission mechanisms To gain physical information from STM light emission (STM-LE) spectra, it is necessary ®rst to understand the emission mechanisms, which differ depending on the nature of the sample. Fig. 1 illustrates the energy diagrams of tunneling for metals and semiconductors (insulators). For simplicity of explanation, we depicted the case where the electron is injected from the STM tip to the sample, and the semiconductor sample is assumed to be p-type. The bias voltage V0 applied across the STM tip and the sample creates an electronic potential energy difference of eV0 (e is the elemental charge) between the Fermi levels of the tip metal and the sample. Thus when the electron tunnels
0038-1098/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0038-109 8(00)00438-5
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Fig. 1. Energy diagrams of light emission processes: (a) metallic sample; and (b) p-type semiconductor sample.
starting near the Fermi level across the vacuum gap, it releases the energy ,eV0 before it relaxes to the Fermi level on the opposite side. Most of this energy is released by phonon scattering, i.e. dissipated in the form of heat. Only one in about 10 3 ±10 4 electrons induces light emission, making the spectral measurement very dif®cult. In any sample system, the conservation of energy limits the maximum energy of emitted photons to hn # eV0
1
where h is the Planck constant and n is the frequency of the photon. This relation holds except in two cases that we are aware of. When both the tip and the sample are superconducting, the Cooper pairs tunnel between them, giving rise to an emission peak at 2eV0 [4]. In another situation, the maximum emission energy is noticeably reduced below eV0 when the capacitance of the target object (metallic particle) is on the order of 10 218 C. This situation occurs when the sample system shows the Coulomb blockade. The electron that tunnels to a semi-isolated small particle raises its Fermi level after tunneling, and hence the total energy available for photon emission is reduced by the energy equal to the rise in the Fermi level [5]. When the sample is a metal, the light emission mechanism is similar to that of metal±insulator±metal (MIM) tunnel junctions [6]. The theories of light emission for MIM were developed by several groups including Rendell et al. [7], Lax and Mills [8], and Takeuchi et al. [9]. According to these theories, light is emitted by the optical frequency ¯uctuation of the tunneling current. The current ¯uctuation can cause light emission through direct radiation and through excitation of surface plasmon polaritons (SPP), which in turn emit external photons by scattering from surface roughness of the junction [8±10]. The corresponding dielectric or macroscopic theories of light emission from STM were reported by Persson and Baratoff [11], Johansson et al. [12], Johannsson [13], Aizupurua et al. [14], and Uehara et al. [15]. These theories are based on the macroscopic dielectric functions of the tip and sample, similar to the theories of MIM light emission.
Hence they predict only the radiation from current ¯uctuations in the STM gap. In contrast to the situation for the MIM junction, there is no translational symmetry along the surface in the presence of the tip above the sample surface. The normal modes of electromagnetic radiation excited in the tip±sample gap are localized surface plasmons (LSP). Most of the emission occurs through excitation of LSP and its subsequent decay into a free photon or a SPP [16]. These theories are based on macroscopic parameters. Thus they do not contain emissions from microscopically localized electronic transitions. As we will see later, some of our experimental data cannot be explained within the framework of such theories, because we observe electronic transitions that involve localized wavefunctions at the atomic level. A microscopic theory of light emission by tunneling electrons has been reported by Tsukada et al. [17,18]. They calculated the spectra due to inelastic tunneling in which light emission occurs via LSP. They did not focus on light emission from transitions between atomically localized states. Since their theory is based on atomic orbitals, in principle, it contains atomically localized transitions. It will be useful to re-examine their theory with a focus on microscopic transitions. An extension of their theory that treats emission from atomically localized states would be very interesting. When the sample is a semiconductor, the emission spectrum corresponds to the transition across the band gap. The peak energy is given by hn Egap
2
where Egap is the band gap. The electron that is injected into the conduction band of a p-type semiconductor relaxes rapidly to the bottom of the conduction band, before it combines with a hole to emit light. There is no light emission, if the sample is n-type and the electron is injected by tunneling. In n-type semiconductors the hole must be injected to induce light emission. Thus STM light emission is different from photoluminescence (PL) in that appropriate minority carriers must be injected to induce light emission.
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Fig. 2. STM light emission spectroscopy system in UHV.
In PL the incident light automatically creates both electrons and holes, so that light emission by recombination occurs even in intrinsic semiconductors. However, the ®nal recombination process is identical in both cases. Hence the emission peak energy is given by Eq. (2) in both PL and STM light emission. In the sense that electrons are injected into a sample and the emitted photons are analyzed, STM light emission is analogous to inverse photoemission and cathodoluminescence. The difference lies in the energies of the injected electrons. In STM light emission the electron energy is only a few eV, while in inverse photoemission it is usually few tens of eV. The electron energy is much greater in the case of cathodoluminescence, lying in the range of keV. Because of the low energy of the tunneling electrons, they do not penetrate deep into the sample, allowing local probing of the surface. Moreover, they do not create electron±hole pairs when the bias voltage is suf®ciently low. This results in light emission only from the immediate region in which the low energy electrons can diffuse before combining with holes. Thus STM-LES can achieve nearly atomic spatial resolution under favorable conditions.
3. Experimental method In STM-LES one ®rst obtains a topographic image of surface structures by the usual scanning method. Then the STM tip is located over a structure of interest, and the emission spectrum from the target structure is measured by keeping the tip at a ®xed position over it. Since the emission intensity is extremely weak in most cases, one must collect the photons over an extended period of time. The typical quantum ef®ciency for light emission by tunneling electrons is on the order of 10 24. Thus, for a tunneling current of 1 nA (,6 £ 10 9 electrons/s), the total integrated number of photons is ,6 £ 10 5 photons/s. The usual quantum ef®ciency of detection is around 5 £ 10 22, and a typical solid angle of collection is on the order of 10 21. Hence the expected number of detected photon counts is
about 3 £ 10 3 photons/s over the whole spectral range of emission. Then if the spectrum is divided into 10 3 channels of a multichannel detector, each channel will count ,3 photons/s. This is a typical signal level in our system. To achieve a satisfactory level of signal-to-noise ratio, one needs to accumulate the photon counts for an extended period, typically several hundred seconds for each spectrum. Then the thermal drift of the tip position becomes a serious problem. We have solved this problem by developing a software-controlled servo-mechanism to compensate for thermal drift during the spectral measurement [19]. Since the tunneling process is sensitively in¯uenced by the surface conditions of the tip and sample, the measurement must be made in an ultrahigh vacuum (UHV) environment to obtain reproducible results. Fig. 2 shows a schematic diagram of our STM-LES system. It comprises a STM with its control electronics, light collection optics, a grating spectrograph, a multichannel optical detector, and a personal computer to record the spectral data. The UHV system that houses the STM is also equipped with sample surface preparation and characterization tools. An important part of this system that requires a special attention in design is the light collection optics and the detector system. Fig. 2 illustrates a system with lens optics for light collection. The lens system has the disadvantage of being bulky and the angle of collection cannot be freely adjusted in UHV. To solve this problem we have developed an alternative system where an optical ®ber is used to collect light in UHV [20]. Another approach for improved light collection was developed by Murashita and Tanimoto [21]. They used a glass ®ber with a conductive coating as the STM tip and collected the emitted light through this tip. The most dif®cult and irreproducible part of this probing technique lies in the fabrication of the STM tip. We found that some tips that can generate STM images with clear atomic resolution are not ef®cient in inducing light emission. We are not aware of dependable and reproducible ways of making tips with high light emission ef®ciency. Thus ®nding a good tip for light emission is purely based on trial and error. This situation needs to be improved before
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Fig. 3. Two experimental con®gurations for the measurements of STM light emission from the AlGaAs/GaAs quantum well structure: from the (001) surface and from the cleaved (110) surface.
STM-LES can become a more generally applicable probing method. When the light emission involves an intermediate process of LSP excitation, Ag tips are found to be most ef®cient in photon production. This is understandable in view of the fact that the damping of plasmons in Ag is much smaller than in other metals such as W and PtIr that are often used as tip materials. The imaginary part of the dielectric function of Ag in the visible range is much smaller than that of these other materials. We found that with the Ag tip, the emission ef®ciency can be enhanced by more than two orders of magnitude relative to W or PtIr tips [22]. This enhancement effect by a sharp Ag tip is related to the similar effect seen with Ag nanostructures in surface enhanced Raman scattering (SERS) [23]. Furthermore, in our latest experiment we found that with the Ag tip the emission ef®ciency increases with decreasing radius of curvature of the tip [24]. Since we could measure the tip radius only with the resolution of a scanning electron microscope (SEM), this result is meaningful only down to a radius of ,10 nm. What is happening below that level is not observable. According to the theories cited above, if the electromagnetic retardation effect is not taken into account, the light emission ef®ciency increases with increasing tip radius. However, when the radius of the tip becomes more than ,10 nm, the retardation effect becomes signi®cant and the light emission ef®ciency goes down [13]. Most of the tips usually used have the radius greater than 10 nm, lying on the decreasing slope of the tip radius vs. emission ef®ciency curve. We have found that this is the case for Ag tips; i.e. the ef®ciency of the tip increases with decreasing tip radius down to ,80 nm [24]. In the following two sections we present the results obtained by using STM-LES as a probe of electronic transitions in individual nanostructures of semiconductors and at speci®c atomic sites on a metallic surface.
4. Probing of individual quantum wells
Fig. 4. (a) STM light emission spectra of the three AlGaAs/GaAs QWs with different widths 2.0, 5.1, and 10.2 nm observed when the electrons are injected from the (001) surface. (b) Photoluminescence spectra of the same samples.
Photoluminescence (PL) is often used to characterize semiconductor quantum structures like quantum dots and quantum wells (QW). In the conventional PL measurements, luminescence is measured from a collection of these structures with a size and shape distribution. Thus, it is not possible to characterize speci®c individual structures. In contrast, with STM-LES one can probe individual structures by imaging them ®rst and setting the STM tip over them. This is the great advantage of this method. An early work on semiconductor quantum structures using STM light emission was reported by Alvarado et al [25]. They measured the integrated emission intensity as a function of the tip position and plotted the photon intensity maps of QWs. However, they did not measure the emission spectra. By analyzing the emission spectra at different
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Fig. 5. STM images of the cleaved (110) surface of AlGaAs/GaAs QWs. The irregular stripes extending from the top left to the bottom right are the steps created by cleaving.
points, more detailed information can be gained on the electronic transitions in individual structures. We have recently measured the STM light emission from the QWs of AlGaAs/GaAs [26,27]. Fig. 3 illustrates the geometry for these experiments. In the work reported in Ref. [26], the samples were multiquantum well structures of p-Al0.4Ga0.6As/p-GaAs. The three samples used in this experiment had an identical thickness of the Al0.4Ga0.6As barrier layers (20.4 nm) and differed in the thickness of
Fig. 6. STM light emission spectra of individual wells of widths 2.5, 5.1, 10.2, and 50 nm. The electrons were injected into the individual wells seen in Fig. 5.
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Fig. 7. STM light emission spectra obtained when the electrons were injected at different positions indicated by a, b, and c in the inset.
the GaAs wells, being 10.2, 5.1, and 2 nm, respectively. The electron was injected from the STM tip into the (001) surface, and the light emitted from the same surface was measured (see the inset of Fig. 3). Fig. 4a shows the STM light emission spectra of the three samples. All the spectra consist of a single emission peak, whose peak energy shifts to the high-energy side from 1.44 to 1.73 eV with decreasing well widths. The same peak shift was also observed in the PL spectra of the same three samples as shown in Fig. 4b. This result shows that the injected electrons thermalize to the bottom of the conduction band, before combining with holes. Thus the ®nal light emission process is the same for PL and STM light emission. However, the initial excitation process is different, and this fact is clearly observed when holes instead of electrons are injected into the p-type sample by changing the polarity of the bias voltage. No emission was observed for hole injection. More recently, we have measured the STM light emission from individual QWs by injecting electrons into the cleaved (110) surface of AlGaAs/GaAs QW structures [27]. Since the growth direction of QWs is [001], the cleaved (110) surface exposes the cross-sections of QWs. Thus the electron can be injected into individual wells targeted by looking at the STM image of the surface. Two samples of quantum well structures were used in this experiment. One had alternating GaAs/Al0.38Ga0.62As layers of thickness 47 and 45 nm, respectively. The other had GaAs wells of widths ranging from 2.5 to 10.2 nm separated by Al0.38Ga0.62 As barriers of 40 nm thickness. All the layers were Be doped (p-type) to a level of ,6 £ 10 18 cm 23. Fig. 5 shows the STM images of the cleaved (110) surface of the two samples. We see well-de®ned stripes in the STM images re¯ecting the AlGaAs/GaAs QW structures. GaAs wells are
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Fig. 8. STM light emission spectra measured at three different points in a single GaAs well.
seen as bright bands in the STM images. The cleavage induced surface steps also appear as irregular black and white bands running from the top left to the bottom right. Fig. 6 shows the STM light emission spectra from individual GaAs QWs. The spectra were measured by locating the STM tip over individual wells. A single emission peak was observed for respective wells of different well widths, and the peak energy shifted to the high-energy side with decreasing well width. No emission peak was observed for negative sample bias ranging up to 23 V (corresponding to
hole injection). This means that photon emission occurred by radiative recombination of the minority carriers (electrons) injected from the tip rather than the impact ionization of electron±hole pairs. By comparison with theoretical calculations, these peaks were identi®ed with the transition between the electronic ground state of the GaAs well to the heavy hole state of the valence band. The emission intensity drastically decreased as the tip crossed the interface from the GaAs well into the AlGaAs barrier by only a few nm. Fig. 7 shows three of the STM light emission spectra that were measured by moving the tip between two GaAs wells separated by an AlGaAs barrier. When the spectrum is measured by locating the tip over one well, we do not see any emission from other wells (Fig. 7a and c). On the other hand, the emission from wells on both sides was barely observed when the tip was located over the middle of the AlGaAs barrier (Fig. 7b). These results mean that the thermalization length of hot electrons injected from the tip is comparable to or shorter than 20 nm. Moreover, the emission intensity was almost the same for the wells with 50 and 10.2 nm width, and then decreased drastically for narrower wells, as seen in Fig. 6. Thus we see that at low sample bias employed in this work, most of the injected hot electrons thermalize within about 10 nm, and subsequently recombine radiatively. The small emission features of Fig. 7b originate from the electrons that reach the wells of either side after thermalization. To investigate the difference in local conditions within a well, we have measured the emission linewidths at different spots inside a single well. Fig. 8a shows the STM image of a GaAs well with 5.1 nm width sandwiched by AlGaAs barriers. The GaAs well is seen as a slightly light band. The cleavage induced surface step also appears as an irregular dark band running from the top left to the bottom right. Fig. 8b shows three of the STM light emission spectra
Fig. 9. Atomic image of the reconstructed Au(110)-(2 £ 1) surface.
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Fig. 10. Emission spectra: (a) when the tip is located over the row of atoms; and (b) when the tip is located over the valley between the rows. The dotted curve is the calculated result based on the macroscopic dielectric theory.
that were measured by injecting electrons at three different points A, B, and C in the well of Fig. 8a. Although the peak energy of emission is almost identical for the three points, the spectral linewidth changes when the tip is moved only by ,20 nm. This result demonstrates that this technique is a sensitive probe of extremely localized electronic and optical properties of quantum structures. We note here that the measurements were carried out with a relatively small current of 0.5 nA. To insure that one does not modify the sample surface, it is essential to use as small a current as possible. We always examine the sample surface after spectroscopic measurements to con®rm that no sample damage was caused. The work described above is only the beginning of this class of experiments that are possible with STM-LES. Since one can obtain atomic scale spatial resolution, one can investigate electronic transitions very close to the walls of the well and gain information on the behavior of electrons near the wall. One can also investigate electronic transitions in the vicinity of defects and impurities. Thus there are many interesting opportunities to take advantage of this new experimental technique.
5. Probing of speci®c atomic sites on a metallic surface In our recent experiment on the reconstructed Au(110)(2 £ 1) surface (Fig. 9), we succeeded in identifying a localized electronic transition at different atomic sites [28]. This surface was previously studied by Berndt et al. using STM light emission [29]. They obtained the photon intensity map of this surface that shows the corrugation in photon intensity, but did not measure the emission spectra. We measured the STM light emission spectra, using a low temperature STM housed in a UHV chamber. The sample temperature was 80 K. The STM tip was made of polycrystalline tungsten and had the radius of ,50 nm. The exposure
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time for all the reported spectra was 200 s, during which the estimated thermal drift distance was 0.038 nm. Figure 9 shows a topographic image of the Au(110) surface with the image of atoms on the (2 £ 1) reconstructed rows. The observed structure corresponds to the missing row model of the Au(110) surface [30]. On this surface we measured the emission spectra at two nonequivalent sites, namely over the row and between the rows of the Au atoms. Fig. 10 shows the emission spectra from the two sites: (a) over the atomic row and (b) between the rows. These spectra were measured at a constant current of 2 nA for a bias voltage of 2.3 V. The current direction was from the sample to the tip; i.e. electron injection into the sample. We con®rmed that no surface damage was caused during the optical measurement by rescanning the surface after the spectra were measured. The cut-off photon energy was at hnmax 2.3 eV as expected from Eq. (1). When the tip was over the atomic row (Fig. 10a), the spectrum had a single broad peak at 2.09 eV. When the tip was located over the valley between the rows (Fig. 10b), the spectrum showed a peak at 1.91 eV in addition to the peak at 2.09 eV. To understand the difference in the observed spectra over the atomic row and over the valley, comparisons were made with a theoretical calculation based on the macroscopic dielectric functions of the metals involved [15]. This theory predicts the spectra of emission mediated by LSP. The dotted curve in Fig. 10 is the calculated spectrum. As clearly seen in Fig. 10a the calculated spectrum reproduces the measured spectrum very well when the tip is located over the atomic row. From this agreement we concluded that the spectrum measured over the atomic row arises from the LSP between the sample and the tip. On the other hand, the extra peak at 1.91 eV seen in the spectrum from the valley between the rows cannot be accounted for by the macroscopic dielectric theory. Since the lateral extent of the LSP is much greater than the distance between the top and the valley of the atomic rows (,0.4 nm), the emission spectrum originating from the LSP should not depend on the spatial difference between these two separate sites. Hence the peak at 1.91 eV observed only when the tip is over the valley (Fig 10b) must be due to a more localized excitation than the LSP. Although we do not show the data here, we measured the spectra over the row and between the rows at a bias voltage of 2.5 V [28]. These spectra indicated that the peak at 1.91 eV appears independent of the bias voltage when there is an atomic scale depression under the tip. The exact origin of this emission peak is not clear at present, but it must arise from a localized state of the reconstructed Au surface. More recent work suggests that this peak is due to impact ionization of the d-electrons caused by the tunneling electron followed by recombination of the d-holes with the electrons near the Fermi surface. The d-band lies about 2 eV below the Fermi level, and the wavefunction is tightly localized. Our result indicates that it extend into vacuum between the rows, but not over the rows.
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This experiment demonstrates that the STM light emission spectroscopy can reach atomic scale spatial resolution. This new spectroscopic capability will serve as a very powerful tool in exploring detailed site dependent phenomena at solid surfaces, such as catalysis, molecular adsorption, and defect incorporation in crystal growth. 6. Summary and future perspective We have presented the results of our latest work using STM-LES, and demonstrated the power of this technique. The uniqueness of this spectroscopic method lies in the fact that light emission spectra from an individual nanostructure or a speci®c site can be obtained by taking advantage of the high spatial resolution and the imaging capability of the STM. There are many exciting possibilities for future work. For example, one should be able to obtain the spectra from single molecules, single nanoclusters, single surface defects and impurities, surface steps, etc. Then this method can contribute unique information that has not been accessible before. The current stage of the development of this technique is still near the beginning, and further improvements are required to develop it into a generally applicable spectroscopic technique. The most important development that is required is a dependable and reproducible method for fabrication of the STM tip and a method for collecting and detecting light with a higher ef®ciency. Acknowledgements The work presented here has been carried out as a cooperative effort with the staff and students of the author's group. I would like to acknowledge their contributions, in particular by Y. Uehara, K. Sakamoto, T. Tsuruoka, M. Iwami, and R. Arafune. Our research is supported by Japan Science and Technology Corporation through the CREST Program. References [1] J.K. Gimzewski, B. Reihl, J.H. Coombs, R.R. Schlitter, Z. Phys. B 72 (1988) 497. [2] H. Coombs, J.K. Gimzewski, B. Reihl, J.K. Sass, R.R. Schlittler, J. Microsc. 152 (1988) 325. [3] R. Berndt, in: R. Wiesendanger (Ed.), Scanning Probe Microscopy, Springer, Berlin, 1998, p. 97 (chap. 5).
[4] Y. Uehara, T. Fujita, M. Iwami, S. Ushioda, Solid State Commun. 116 (2000) 539. [5] Y. Uehara, S. Ohyama, K. Ito, S. Ushioda, Jpn. J. Appl. Phys. 35 (1996) 167. [6] J. Lambe, S. McCarthy, Phys. Rev. Lett. 37 (1976) 923. [7] R.W. Rendell, D.J. Scalapino, B. Muhlschlegel, Phys. Rev. Lett. 41 (1978) 1746. [8] B. Lax, D.L. Mills, Phys. Rev. B 20 (1979) 4962. [9] A. Takeuchi, J. Watanabe, Y. Uehara, S. Ushioda, Phys. Rev. B 38 (1988) 12948. [10] J. Watanabe, A. Takeuchi, Y. Uehara, S. Ushioda, Phys. Rev. B 38 (1988) 12959. [11] B.N.J. Persson, A. Baratoff, Phys. Rev. Lett. 68 (1992) 3224. [12] P. Johansson, R. Monreal, P. Apell, Phys. Rev. B 42 (1990) 9210. [13] P. Johansson, Phys. Rev. B 58 (1998) 10823. [14] J. Aizpurua, S.P. Apell, R. Berndt, Phys. Rev. B 62 (2000) 2065. [15] Y. Uehara, Y. Kimura, S. Ushioda, K. Takeuchi, Jpn. J. Appl. Phys. 31 (1992) 2465. [16] K. Takeuchi, Y. Uehara, S. Ushioda, S. Morita, J. Vac. Sci. Technol. 9 (1991) 557. [17] M. Tsukada, T. Schimizu, K. Kobayshi, Ultramicroscopy 42±44 (1992) 360. [18] T. Schimizu, K. Kobayashi, M. Tsukada, Appl. Surf. Sci. 60/ 61 (1992) 454. [19] K.J. Ito, K. Ito, Y. Uehara, S. Ushioda, Rev. Sci. Instrum. 71 (2000) 420. [20] R. Arafune, K. Sakamoto, S. Ushioda, submitted for publication. [21] T. Murashita, M. Tanimoto, Jpn. J. Appl. Phys. 34 (1995) 4398. [22] M. Iwami, Y. Uehara, S. Ushioda, Appl. Surf. Sci. (2000) (in press). [23] Surface Enhanced Raman Scattering, in: R.K. Chang, T.E. Furtak (Eds.), Plenum Press, New York, 1982. [24] M. Iwami, Y. Uehara, S. Ushioda, Jpn. J. Appl. Phys. 39 (2000) 4912. [25] S.F. Alvarado, Ph. Renaud, D.L. Abraham, Ch. SchoÈnenberger, D.J. Arent, H.P. Meier, J. Vac. Sci. Technol. B 9 (1991) 409. [26] T. Tsuruoka, Y. Ohizumi, S. Ushioda, Appl. Phys. Lett. 73 (1998) 1544. [27] T. Tsuruoka, Y. Ohizumi, R. Tanimoto, S. Ushioda, Appl. Phys. Lett. 75 (1999) 2289. [28] Y. Uehara, T. Fujita, S. Ushioda, Phys. Rev. Lett. 83 (1999) 2445. [29] R. Berndt, R. Gaisch, W.D. Schneider, J.K. Gimzewski, B. Reihl, R.R. Schlittler, M. Tschudy, Phys. Rev. Lett. 74 (1995) 102. [30] W. Moritz, D. Wolf, Surf. Sci. 88 (1979) L29.