Applied Surface Science 6a/fil (1992)448-453 Nurth-Holland
applled surface science
STM light emisslon spectroscopy of Au fi|m S. U s h i o d a , Y. U e h a r a a n d M. K u w a i t a r a Re,earth IIIslilltlt" of Eh'cmcof ~mltuolhxltfo~l, Tqho&~t IJnivt'r~to'. Se~utai 9811, J*lpmt Recei~,,ed20 November ltJyl: accepted for publication 3 December ITS01
We have measured the STM light emission (STM-LE) spectra of a Au film evapurated on a prism-coupler surface, The STM was p}aeed in an ultrahlgh-vacuumchamber and the speclra were me,tsurcd bolh oil 111,2lip side ~ntl through lhe prism. The prism-side emission has a ~ingh:narrow peak whose ptlsition n|ove~ with 1he emission ~lngle.This emission is definitely due to surface plasmon polariton~ (SPP) al tile Aa/vacuum interface, The lip-side emission spectrum contains three pe:tks between 1.5 and 1.9 eV. Their origin cannot be delermincd experimenlally, ~dlhough Ihey ;~rc believed to arise from Iocnlized stlrface plasmons (LSP), The STM-LE spectnl are extremely sensitive to the surt2tc¢ I~otldalOllSof Ih¢ tip and Ihe sample.
I. introduction Light emission from the gap region between the tip and the sample surface of the scanning tunneling microscope ( ST M ) was first reported by Gimzewski et al.  and Coombs ct al.  in 1988. The light emission process can be understood as inverse photoemission (|PE) or as cathode-ray luminescence (CL) excited by the tunneling current. Since the cross section of the incident electron beam (tunneling current) is on a nanometer scale, the spectrum of the emitted light reflects the materials properties of the sample immediately under the S T M tip. Thus. once we learn all the intermediate steps in the light emission process and determine what information is contained in the spectrum, we can obtain not only the geometrical information but also the materials properties of the sample with an extremely high spatial resolution. Let us first review the examples of S T M light emission (STM-LE) spectroscopy reported in the literature. Gimzewski ct al.  useo an ]r tip against S i ( l l l ) and T a polycrystalline samples, and measured the light emission intensity at a fixed photon energy of 9.5 eV as a function of the bias voltage (isoehromat spectroscopy). The plot of emission intensity versus bias voltage resem-
blcs the inverse photoemission (IPE) spectra of the respective samples. Although there are spectral features that do not appear in the corresponding IPE spectra, they considered the tunneling process as injection of electrons into the sample and interpreted the light emission as an inverse photoemission process. Coombs et al.  measured the emission spectra of tlte S T M also with an Ir tip for a A g film sample. They measured the emission spectra in the visible range for varying bias voltages between 1 and 4 V, and found two peaks in the spectra at 1.9 and 2.4 eV. These peak positions do not depend on the bias voltage. From the energies of these spectral features, they argued that the emission was from the localized surface plasmon (LSP)  of the tip-sample gap region excited by the tunneling current. Coombs et al.  also scanned the S T M tip position and measured the light emission intensity as a fi',~etion of the sample surface location. From this measurement they concluded that there is a correspondence between the emission intensity and the geometrical profile of the sample surface. They suggested that, by using this new phenomenon of light emission from the STM, one should be able to dcvclop a new spectroscopy with a nanometer-seale spatial resolution.
016%4332/92/$05.00 i~ 1992 - Elsevier Science Publishers B.V. All rights reserved
S. U.quoda et uL / STAr ~ght emission spectro~cop~' of Au flint
More recently, STM-LE spectroscopy has been applied to semiconductor ~urfaees with small regions of different materials properties. Abraham et al.  measured the emission intensity from G a A s / A I G a A s heterostructures as a function of tile S T M tip position, and demonstrated that a surface map of different materials can be generated from the emission intensity. The main emission process is the recombination of injected electrons with holes in p-type GaAs. The emission efficiency was found tc~ be considerably higher from these semiconductors than from metal samples. Alvaradu et al.  lound that injected electixJns diffuse before emitting light, thus limiting the spatial resolution. They could detect a quantum well of a few nanomctcrs in width from the light intensity measurements. A very important finding of tbeir work is that this method can be uscd to dctcrminc the thermalization length and diffusion length of electrons. They also found that the emission intensity depends on the hole concentration in the sample. A new design of the S T M specifically adapted to S T M - L E spectroscopy has been reported by Bcrndt et al. . They improved the light collection efficiency of the system and demonstrated some examples of S T M - L E spectroscopy on CdS and topographical mapping of a Ag film on St(Ill). T h e main thrust of our work has bccn to investigate diflerent light emission processes involved in the STM and to determine the information content of the spectrum. We have earlier mea~nred the angle dependence of the emission intensity for a thin A u film deposited on a prismcoupler . The experimental configuration was similar to the present one illustrated in fig. 1. From this experiment we found that strongest light is emitted in the direction in which the wavevector conservation is satisfied between the surface p l a s m o n p o l a r i t o n I S PP) at the A u / v a c u u m interface and the emitted photon in the coupler prism. Thus we concluded that SPF is the intermediate state in the light emission process on the prism side. By combining the results of a theoretical analysis  with this experimental evidence, we concluded that localized surface
Fig. I. E~:perimental~etup of STM-LE spectruscnpy in UHV. The inset shllws the sample geometry wilh a prism-coupler. plasmons (LSP) are initially excited in the gap between the tip and the sample surface, arid then they decay into SPP, light in vacuum on the tip side, or gets dissipated in the metal sample. Thus the emission on the tip side is due to direct decay of LSP. In this paper we report the first measurements of the emission spectra from a Au film through a prism-coupler and on the tip side into vacuum. We have measured the emission spectra for different angles into the coupler prism, and for different sample and tip conditions. We will disCUSS the emission mechanism based on the observed spectra.
2. Experiment The experimental setup is illustrated in fig. 1. The Au film (25 nm thick) was evaporated in low vacuum 1.1O-" Torr) on the flat surface of a hemispherical prism made of BK-7 glass . This sample with the prism was placed on the sample holder of a STM. The S T M was home-made after the design described by Lyding et al. lift]. ]t uses concentric tubular piezoelectric elements to drive the tip. Mechanically sharpened Pt tips or electrochemically prepared W tips were used. T h e whole STM with the sample was placed on a rotation table in an ultrahigh-vacuum ( U H V ) chamber, l'he base pressure of the U H V chamber is about 1 × 10 -~ Torr.
S. UshiocM ¢l aL / SZ~I light em~sion spl~tro~col, v o f A u ]ilm
Vs - -2 SV
. . . . .
I Vs ~ -4.OV
Vs ~ 5OV !
1.5 1.8 2.1 24 2.7
the details of tile spectra depend on the polarity of the bias voltage, the overall features are similar for both directions of the tunneling current. Wc show this series of spectra, because they represent a complete set of data taken in a single run. When the bias voltage is - 2 . 5 V, there are peaks at 1.57 and 1.73 eV. As the bias voltage is increased (made more negative) to - 3 . 2 V, a new peak appears at 1.9 eV and its intensity increases with the bias voltage. The overall intensity increases with the bias voltage up to - 4 V, but starts to decrease above that value. Fig. 3 shows the emission spectra measured through the prism-coupler at 41 °, 43°, and 45° from the surface normal. The sample blas was fixed at - 4 . 0 V with respect to the Pt tip, and the current was held constant at 100 hA. We see that very strong emission occurs at 43° from the surface normal. Takeuchi et al.  measured the emission angle depeudenee of the intensity for unresolved spectra and found that it is sharply peaked at 43°. The rt;sult in fig. 3 shows that the spectrally integrated intensity is indeed peaked at
E n e r g y [eV] Fig. 2. Emission spectra on the: |i D side for differem bias vahages. The Au film sample is negativelybiased with respect la the Pt alp. -- 41 ~ The emi:~cd light was collected through two lenses and the view port of the U H V chamber into a spectrograph, The collection angle was controlled by an iris to + l ° about the central dii~:~tion [of mu~l of the ll,easuremcnts. The spectra were detected and recorded digitally by a multichaonel eptieal detector. Fig. 2 shows the emission spectra into vacuum on the tip side for different bias voltages between the Pt tip and the Au sample. The light of all polarizations emitted between 69° and 88 ° from the surface normal was collected. These spectra are corrected for the energy-dependent sensitivity of the detector. The STM was operated in a constant-current mode with the current fixed at 160 nA. The sample was negatively biased with respect to the Pt tip. Thus the tunneling electrons move from the Au sample to the Pt tip. Although
E n e r g y [eVJ Fig. 3. Emission spectF~~hmugh the prism for thr~e emission angles. The Au film sample is negativelybiased at 4.0 V wilh respect to the Pt tip.
S, Usinoda er al. / STM light emi~sion spectroscop)"of At~film
43 ° in agreement with Takeuchi et al. This is the first time that the emission spectra are measured for Au. T h e new fact found in fig. 3 is that the emission peak shifts with [he angle. The peak position takes a minimum at 1.9 eV for 43 °. To explore the changes that occur in the spe¢ira for different tip-sample conditions, we measured them for various tip-sample preparations, Figs. 4a and 4b compare the spectra before and after collision of the tip with the sample. T h e spectra were taken on the prism side at 43 ° emission angle. The bias voltage was - 4 . 5 V and the current was 100 hA. The difference shown in fig. 4 is not completely reproducible, but this kind of chan~e is typical, We often found that the emission intensity increased after tip-sample collisions. We see that in addition to the overall intensity change the peak position changes after a collision. Figs, 5a and 5b indicate a typical change in the spectra before and after cleaning of the tip. In this measurement an electrochemically prepared W tip was used, and the chamber pressure was 10 -h Torr, higher than the ordinary operating pressure. The bias voltage was + 3 V with the sample positive, and the current was held constant at 20 nA. The spectra were taken on the
(a) &, "i v
E n e r g y leVI Fig. 5. Comparison of the emission spectra before and ~fter tip cleaning. The emission was measured through the pri.~mat 43~.
prism side at 43 ° emission angle with a collection angle of + 12°, After taking the spectrum of fig. 5a, the W tip was cleaned by applying + 10 V with respect to the chamber ground for 1 rain. Then the spectrum of fig. 5b was taken. Note that these spectra were taken in poor vacuum. In contrast, all other spectra shown in this paper were taken in vacuum at 10 ~ T o r t or better. We see a clear change in the emission spectrum. The peak at 1.6 e V disappears almost completely after cleaning and a new peak appears at 1.9 eV. T h e new spectrum after cleaning is very similar to the bottom spectrum of fig. 3. O u r experience indicates that the spectra, particularly on the prism side, arc extremely sensitive to the conditions of the tip as weft as the sample surface. This level of surface sensitivity will be very powerful for a surface probe, once all the causes of spectral changes are rigorously understood.
3. D i s c u s s i o n
Enerl~y [eV] Fig 4. Compari~n of the emission speclra before ~md after a ~llision of the tip with the sample. The emission was mc~lsured through the prism at 43~.
In the work by Abraham et al.  and AIvarado et al. , the emission intensity wss measured through a filter, and the emission was presumed to be concentrated aboat the band gap
s. Ushiolhl el ill / STM lighl emi3shm spectto~COl~' o]'Au film
transitions, but no spectrum was reported. Takeucld et al.  also measured the integrated intensity and no spectral distribution. Thus the published S T M - L E spectra are so far limited to those of evaporated Ag films , CdS single crystals , and the present results for evaporated Au fihns. O n the theoretical side, there have been work by Johansson and Moareal , TsukaOa et aL , and Uehara ct al. [8l to describe the S T M - L E spectra. The theory by Johansson and Mnnrea| [I I] is based on the theory of light emission from m e t a l - i n s u lato r - metal ( M I M ) light-emitting tun. uel junctions (LETJ). Their theory is very similar to ours [7,8,13]. [n both these theories the fluctuations in the tunneling current become the source of radiation [14,151. T h e shape of the emission spectrum is mainly determined by the electromagnetic resonances of the materials and the geometry. This information is contained in the electromagnetic Green's function. The elementary excitations involved in these resonances are LSP and SPP when the sample is a metal. SPP is the normal mode of a fiat m e t a l / v a c u u m interface and its spectrum is determined by the dielectric function of the metal alone. In contrast, the LSP spectrum depends on the local geometry of the tunneling gap, and it is sensitive to the curvature of ~.he tip and the local sample surface. Tsukada et al.  obtained the power spectrum of current fluctuations for the S T M that consist~ of a W tip and a A g sample from the electronic wavefunctions of the two metals. T h e i r spectrum of current fluctuations does not show prominent features seen in the emission spectra by Coombs et al.  and in our present data of Figs 2 and 3. T h eir current fluctuation spectrum is not expected to reproduce the observed emission spectra, when it is inserted in our light-emi~ sion formula [7.13,14]. According to our theory [7,8,13], the emission on the tip side is due to direct decay of LSP. T h u s the spectra shown in fig. 2 reflect the spectrum of LSP. We see at least three recognizable and reproducible peaks in these spectra, and they are more complex than those on the prism side. _The emission on the prism side arises from the SPP that has a definite dispersion relation. The
: 41 ° /" 43 ° / ' / 45 °
k I [104em -1 ] Fi~'. h. Dispersion curve nf SFP al lhc Au/vacuum interface and Ihe wavevector conselx~ltion calves for tilrt:e emission angles in the prism.
SPP dispersion relation at the A u / v a c u u m interface is shown by" a solid curve in fig. 6. The dashed straight lines correspond to the waveveetar conservation relation between SPP and the light in the prism for different emission angles. We see that the SPP dispersion curve crosses the 43 ° emission line between t.6 and 1,9 eV. Thus one quantum of SPP can be converted to a single photon conserving tile wavcvcctor along the surface: i.e,, 43 ° is the direction of wavevector matching. This is indeed the spectral region of strong emission at 43 ° as seen in fig. 3. Thus it is clear that the single-peaked speetr~!m seea on the prism side is due to the decay of SPP, as suggested by Takeuehi et al. . As we have demonstrated in figs. 4 and 5, tile emission spectra are very sensitive to the conditions of the sample and tip surfaces. Thus at this stage it is difficult to argue about tile definite origins of the peaks seen on the tip side (fig. 2). Some of these peaks must correspond to the normal modes (LSP) of the tip-sample gap, while others may arise from the image field states ill. It is also possible that some of them are due to adsorbed atoms on the surface. In an earlier preliminary experiment we have looked at the
S. Ushioda el aL / STM light e~t#~iolt ~p¢clro~copyof All filln
emission spectra of Au with a Pt tip in air. Then the emission was much weaker than seen in the presen! measurements and not reproducible. We found that fast changes occur daring a measurement in air. The fact that the STM-LE spectra are very sensitive to the sample and tip conditions is a blessing, when we apply this method as a probe of materials properties with a nanometer resolution. However, at the pfr2scnt stage of nnderstanding of the emission mechanisms, this sensitivity makes it difficult to interpret the spectra, except for the case of prism-coupled emission. In this case the cmis,sion angle dependence gives a definite clue to pin down the emission to SPP.
4. Conclusion We have measured the STM light emission spectra of an evaporated Au film with Pt and W tips in U H V . The method of prism-coupled light emission allows a definite assignment of the emission channel through the prism to surface plasmon polaritons (SPP). The tip-side emission is due t~l the decay of localized surface plasmons (LSP), according to our theory, but such an assignment cannot he made experimentally at this point. We have measured the STM-LE Ibr different tip-sample conditions and demonstrated that the spectra are very sensitive to the surface conditions, This fact is a potential merit in future applications of STM-LE spectroscopy in surface science.
Acknowledgements We would like to acknowledge valuable discussions with Dr. M, Tsukada of University, of Tokyo. and thank Dr. K. Takeuchi of Hewlett-Paekard
Laboratories Japan for his contributions in building the STM. We would like to thank Dr. S.F. Alvarado of IBM ZiJrich Research Laboratory for sharing his experience with STM-¢xeited luminescence from semiconductors. This research was supported financially by grants from the Casio Science Promotion Foundation and The Mitsubishi Foundation. and a Grant-in-Aid for Science from the Ministry of Education, Science and Culture.
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