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Light emission associated with tunneling phenomena
Journal of Luminescence 47 (1991) 131 136 North-Holland
131
Light emission associated with tunneling phenomena* S. Ushioda Research Institute of Electrical Communication, Tohoku University, Katahira, Aoba-ku, Sendat 980, Japan Received 12 June 1990 Revised 8 October 1990 Accepted 16 October 1990
Light emission mechanisms in metal-insulator metal (MIM) tunnel junctions and scanning tunneling microscopes (STM) have been investigated, and similarities and differences in these phenomena are discussed. In MIM structures fluctuations of the tunneling current and/or hot electrons excite surface plasmon polariton ~SPP) modes which decay into external light via surface roughness or a coupling prism. In the STM geometry the tunneling current is microscopically localized; thus the surface modes excited in this case can be the SPP of a flat metal surface (propagating modes) as in the case of MIM, the localized plasmon modes of surface microstructures, or atomically localized electronic levels. By a prism-coupled light emission experiment from a STM on a Au film, we have found that almost all of the light emitted through the coupler-prism arises from the fast mode of SPP at the metal surface. The main interest of these experiments lies in studying tunneling phenomena by optical meahs which allow precise energy and momentum measurements with very high sensitivity.
1. Introduction Light emission from metal—insulator—metal (MIM) tunnel junctions was originally discovered by Lambe and McCarthy [1] in 1976, while the discovery of light emission from a scanning tunneling microscope (STM) came very recently. The first observation of light emission from a STM was reported in 1988 by Gimzewski, Reihl, Coombs and Schlittler [2], and by Coombs, Gimzewski, Reihl and Sass [3]. In both MIM and STM electrons generate visible light in the tunneling process between two metal electrodes. In the MIM structure tunneling takes place across an insulating oxide layer between evaporated metallic films, while in the STM geometry tunneling occurs across a vacuum or air gap between the tip of a metallic probe and the sample metal surface, The purpose of this paper is to review the light emission phenomena in the electron tunneling process, to consider possibly different emission mechanisms involved in the two cases, and to *
This paper was originally submitted for presentation at the International Conference on Luminescence 1990 as an in vited paper.
0022 2313/91/803.50
©
1991
indicate what new physics may be learned by optical investigations of tunneling phenomena in general.
2. Light emission from MIM junctions Light emitting MIM tunnel junctions are usually made on a glass substrate by the evaporation of an Al film, formation of an insulating layer by oxidation of Al, and finally deposition of a counter electrode, often of Au or Ag. Its typical structure and dimensions are illustrated in fig. 1. When a bias voltage of V0 2—5 V is applied across the —
junction, a current typically of the order of 10 to 50 mA flows, and visible light is emitted from both sides of the junction with the shortest wavelength determined by Am,n~hc/eV0. Typical emission spectra of an Al oxide-Au junction are shown in fig. 2 for different bias voltages. From the many experiments performed since the original discovery of this phenomenon [4], it is well established that the dominant mechanism of light generation involyes excitation of surface plasmon polaritons (SPP) of the layered structure as the intermediate step.
Elsevier Science Publishers B.V. (North-Holland)
132
/
S. Ushioda
Light emission associated with tunneling phenomena
1—2mm I
AU
6
I
~
0~
ox~ooL
/
3 2
Fig. 1. MIM tunnel junction formed on a glass substrate. 0
The dispersion curves of the SPP modes of a typical junction consisting of glass-Al—oxide-Au is sketched in fig. 3. These are the electromagnetic normal modes of the layered structure consisting of three branches [5]. The branch that asymptotically approaches the vacuum light line is called the “fast mode” and its field strength is mostly localized at the Au/vacuum (air) interface. The branch that lies close to the glass light line is called the “intermediate mode” or “prism-Al mode”. This mode has the largest field strength at the Al/glass interface. The third mode which is called the “slow mode” has a very small phase velocity and a large wave vector in the visible frequencies. The energy of this mode is concentrated in the insulator layer where the strongest component of the electric field is normal to the junction. Since the motion of the tunneling electron is parallel to _________________________________ ~ 1 000 0
.
v0=2.9V
•
~/v~-2.7V
~n 800 0
/v.
~ 600 z
••
I
400
••
~200
1 .5
~
S
-
25V .
0
2.0 2.5 3. 0 PHOTON ENERGY (eV)
Fig. 2. Typical emission spectra of an Al oxide-Au MlMjunction for different bias voltages, v0.
2
4
6
k11 ( ~ ~ cm~) Fig. 3. Dispersion curves of surface plasmon polaritons (SPP) in a prism (glass) Al oxide Au structure. kt is the wave vector parallel to the surface. (After ref. [5]).
this field, it delivers most energy to the slow mode with the result that the slow mode is most strongly excited in the junction. We note that all the branches of SPP lie to the right of the light line in vacuum; thus, the wave vector of SPP parallel to the surface for any given frequency is greater than that of the light of the same frequency in vacuum. This makes one-to-one conversion of a SPP to an external photon impossible, making SPP nonradiative. To allow SPP to radiate, we must have some mechanism to break the wave vector conservation along the surface or to enable wave vector matching. Surface roughness or a grating on the surface can do this by breaking the translational symmetry parallel to the surface. Thus surface roughness at one or all of the interfaces in the junction allows light emission. Also a along MIM definite junctiondirections formed on [6].aThis grating mechanism emits light can operate on all three branches of SPP, if the lateral
scale can Aalso coupler-prism of roughness allow light orinemission a contact grating is by with appropriate. increasing the junction the wave vector of the light in it by a factor of its index of refraction. Wave vector matching by a couplerprism is possible only for the fast mode (in the simple structure illustrated in fig. 1) [7,8], and this occurs when the emission angle is 430 from the surface normal for a prism made of BK-7 glass.
S. Ushioda
/ Light emission
associated with tunneling phenomena
Both of these methods have been used in getting the visible light out ofthe junction. However, since microscopic surface roughness that is relevant is present at all practical surfaces, the role of surface roughness cannot be ignored even in the case of prism-coupled light emission [9]. The theory of light emission mechanisms from MIM tunnel junctions has been discussed by many authors [10-13]. They all agree in assuming that the electron tunnels inelastically and excite SPP or localized plasmons in the process, i.e. they are excited while the electron is in the process of tunneling rather than becoming a hot electron in the terminal electrode. Kirtley et al. [14] on the other hand argued that the electron tunnels elastically generating a population of hot electrons which in turn decay by exciting SPP. At present it is not clear what the relative importance of the two processes (elastic versus inelastic tunneling) is. On the other hand, it is now clear that most of the light originates from the slow mode both in roughened junctions and in prism-coupled junctions [15]. Thus the remaining issue is the excitation mechanism of the slow mode by tunneling electrons. Also there is a problem with the reproducibility of the emission spectrum from junction to junction. We as well as others have found that the emission spectrum, the absolute intensity, and the durability depend on the fabrication procedure of the junction, such as the method of oxide formation and the evaporation speed of the metal electrodes. However, no definitive procedure for reliably making an efficient light emitter has been found. There are conjectures that the crucial point is quality of the oxide layer, the but no correlation hasthebeen established between nature of the oxide and the light emitting properties of the junction. This is an issue that must be addressed in future research. Our group has concentrated on the study of light emission by prism-coupling [5,7—9,13].This geometry is illustrated in the inset of fig. 4. In this case the fast mode is radiative in the prism and wave vector matched emission occurs at 43°from the surface normal into the prism, as seen in fig. 4. We see indeed that there is a peak of emission at 43°.The emission seen at other angles is due to the scattering by surface roughness.
133
1 .0 ~ 0.8
______
Il) -~
~
0.4 ~ 0.2
J~de
______
°
o.o 0
30 80 Angle (deg)
90
Fig. 4. Angle dependence of light emitted by a prism-coupled MIM junction. The inset shows the prism-coupling geometry (After ref. [9]).
The theory of light emission by prism-coupling including the effect of surface roughness was developed by Takeuchi et al. [13]. According to this theory, the emitted power can be written as a sum of two terms, p(O) representing the direct emission that is present even in the absence of surface roughness and p( i) arising from the scattering of SPP by surface roughness: P°(k~°w) S~
2w4 cos2O
0 8~c
f
dz’ dz”
x d~(k~°~, w z, z~)*d~(k~~), w z, z’) xJ~(k~°~, w z’, z”),
(1)
pi(k~1o w) 2w8 cos~O~ S~512~5c9
‘7.,)
d2Q~dz’ dz”
d~(k~°t, U) Z, x d~.(k~°t, w~z,zm±)Im(k~°~— Q X
11)
xd~(Q ,W~Zm ,z~)* ~ d~(Q1, i,,, z”)J~(Q11, ~
where L.(Q —
,
eIo(1
~‘,
,
w z’, z”),
(2)
z”)
1~w/eV) ~i(z’, z”) 2 2S (1+Q2~2)3 2ir
(3)
134
S. Ushioda
/
Light emission associated with tunneling phenomena
In the above equations d~’s are the Fourier transforms of the electromagnetic Green functions of the junction geometry, Im(Q ) is the power spectrum of surface roughness of the mth interface, J.(Q~1,w~Z’, z”) is the power spectrum of the tunneling current and ~ is the correlation distance between tunneling electrons. 00 is the angle of emission measured from the surface normal, and is the difference of the dielectric constants across the mth interface. Other symbols are defined in ref. [12]. The solid curve in fig. 4 was calculated by eqs. (1~i and (2) for a fixed wavelength A 600 nm and varying emission angles 00. The angle dependence of emission is sufficiently well described by this theory. On the other hand the emission spectra are only in rough agreement with the result obtained from eqs. (1) and (2), assuming a certain reasonable power spectrum of surface roughness [9]. More recently, Takeuchi et al. [16] actually measured the power spectrum of surface roughness of a junction by STM and simultaneously measured the light emission spectrum. They numerically inserted the measured power spectrum of surface roughness into eq. (2) and compared the measured emission spectrum with the calculated result. They could obtain good agreement between the measured spectrum and the calculated spectrum due to surface roughness pW, but its relative size to pO) was found to be too small. Since they measured the surface roughness on the top electrode (Au) and assumed the same roughness exists at other interfaces, it is possible that Al/oxide and Au/oxide interfaces. The most they underestimated the size of roughness at the effective location of surface roughness in inducing light emission is at the oxide/metal interfaces where the amplitude of the slow mode is large. Thus if the surface roughness here is larger than that on the top electrode, one greatly underesti~,
ROTATIO
STAGE
4r~-MICROMETER
toRM.T. PRISM TIP
~XYZ~~CAL -
STAGE ___________________________ PIEZOELECTRIC CELL~ Au FILM
Fig. 5. Experimental setup to measure light emission from a
STM. This is the geometry used to observe the light through a hemicylindrical prism-coupler. (After ref. [17]).
al. [2]. They used an Ir tunnel tip on Si (1 1 1) and polycrystalline tantalum surfaces, and measured the intensity of 9.5 eV photons by a Geiger—Muller type detector as a function of the bias voltage between the tip and the sample. They interpreted the results by comparing them with the conventional inverse photoemission spectra of these samples. Immediately following the above work, Coombs et al. [3] reported the emission spectra of a STM in the visible frequencies, using an Ir tip and an evaporated Ag film. Their spectra are reproduced in fig. 6. They suggested that the two prominent peaks at 1.9 and 2.4 eV may correspond to resonances due to localized plasmon
2 4V
/\ QUANTUM
i 9V
CUTOFF
zLU z
TUNNEL
3 23
o
5—
3 00 2.71 VOLTAGE 249 2 33 212 2.02
mates the contribution from eq. (2). 3. Light emission from STM 1
A geometry for the observation of light emission from a STM is illustrated in fig. 5. The first observation of this effect was reported by Gimzewski et
FIBER
2
3
4
PHOTON ENERGY (eV)
Fig. 6. Emission spectra of STM in the visible region for different bias voltages. (After ref. [3].)
S. Ushioda
/
Light emission associated with tunneling phenomena
modes of the tip/surface region. Similar features had been seen in a theoretical calculation by Rendell and Scalapino [11] who considered light emission from tunnel junctions containing small Ag particles on an Al substrate. Stimulated by the above reports, we have measured light emission from a STM, using a prism-coupling method [17]. In this experiment a Au film (22.3 nm thick) was evaporated on the flat surface of a hemicyclindrical BK-7 prism and a Pt-Ir tip was brought close to the Au film at the center of the prism in air. Light emission was observed through the prism as a function of the emission angle. The STM was operated in a constant current mode without scanning in the lateral direction. The emission intensity was measured by a photomultiplier without spectral analysis. The result is plotted in fig. 7 which shows a very sharp peak at 43°from the film normal. There are two branches of SPP in the glass-Au film—air geometry, namely the fast mode and the Au-glass mode. As in the case of prism-coupled light emission from MIM junctions, the observed peak corresponds to wave vector matched emission into the prism by the fast mode at the surface of the Au film. The solid curve in fig. 7 is the calculated angle dependence based on eqs. (1) and (2) with the wavelength fixed at 700 nm. For this calculation the Green functions in these equations were adapted to correspond to the prism—Au film-air geometry [18]. We found that the contribution from the surface rough-
(/)
I—ii’~~
‘‘‘‘‘‘
~ 0.8 06 ~ 0.4
135
ness p~ is insignificant compared to the contribution p(O) from a flat surface in this case. From this experiment it is clear that almost all of the light emitted through the prism is coming from the fast mode which is a delocalized normal mode of a flat metal surface. Coombs et al. [3] observed the light from the tip side in UHV and conjectured that the origin of the light are local plasmon modes confined by surface microstructures and the tip. Thus, our finding from the prism side contradicts their conjecture. It is possible that the situation is different on the vacuum side and much of the radiation comes from localized modes there. Further work is in progress in our laboratory to clarify this point.
4. Interesting questions From what is known already, it is clear that SPP (delocalized propagating mode) plays an important role in light emission from both MIM and STM. In the case of STM it is most interesting to determine if and to what extent the local plasmons rather than the delocalized SPP are involved in the light emission process. Since the local plasmons are the normal modes of the geometry formed by the tip and surface microstructures, their lateral range is of the order of a few nanometers, and therefore, the emission spectrum and intensity will reflect the nature of the microstructure, allowing spectroscopic studies of such structures. On the other hand, even if the main origin of the light is the delocalized SPP the emission intensity can depend on the presence of microstructures, because such structures will scatter SPP which will not be radiative in their absence. Thus the observa-
<
tion bybetween dence Coombs the et al.light [3] intensity that showsand corresponsurface
w
topology can be explained by scattering of SPP. If one measures the emission through a prismcoupler as we did, one selectively observes the emission from the fast mode. However, if we measure emission from fastthemode as we scan the the STM, we may findthethat excitation
~ —
0.2 ~j.sj
_______________________
0
30 60 90 EMISSION ANGLE(DEG)
Fig. 7. Angle dependence oflight emission from a STM through a coupler-prism illustrated in fig. 5. (After ref. [17].)
.
efficiency of the fast mode depends on the microstructures. A preparation for this kind of experiment is under way in our laboratory.
136
S. Ushioda
/
Light emission associated with tunneling phenomena
With regard to MIM, one of the unexplored questions is the form of the current fluctuations J ..(Q
1 w z’, z”). So far we have used the form proposed by Laks and Mills [10] and achieved a reasonable degree of agreement with the experimental findings. Particularly interesting is that the shape of the emission spectrum is a fairly sensitive function of the phenomenologically assumed lateral correlation distance ~ between tunneling electrons. Thus it may be possible to determine this correlation distance from the observed optical spectra. This kind of development will lead to a new insight into the electron tunneling phenomena and open a new avenue for investigating electron correlations in metals. As better reproducibility of junctions and better precision of optical measurements become reality, it is desirable to re-examine the theory and attempt to formulate it from the first principles dealing with the coupling of tunneling electrons to SPP.
[3] J.H. Coombs, J.K. Gimzewski, B. Reihl and J.K. Sass, J. Microscopy 152references (1988) 325. [4] A useful set of for work published before 1984
,
[5] [6] [7] [8] [9] [10]
[11] [12] [13] [14]
[15] [16]
References [1] J. Lambe and S.J. McCarthy, Phys. Rev. Lett. 37 (1976) 923. [2] J.K. Gimzewski, B. Reihl, J.l-l. Coombs and R.R. Schlittler, Z. Phys. B 72 (1988) 497.
[17] [18]
is found in P. Dawson, D.G. Walmsley, H.A. Quinn and A.J.L. Ferguson, Phys. Rev. B 30 (1984) 3164. S. Ushioda, i.E. Rutledge and R.M. Pierce, Phys. Rev. B 34 (1986) 6804. JR. Kirtley, TN. Theis and J.C. Tsang, AppI. Phys. Lett. 37 (1980) 435. S. Ushioda, J.E. Rutledge and R.M. Pierce, Phys. Rev. Lett. 54 (1985) 224. K. Suzuki, J. Watanabe, A. Takeuchi, Y. Uehara and S. Ushioda, Solid State Commun. 69 (1989) 35. J. Watanabe, A. Takeuchi, Y. Uehara and S. Ushioda, Phys. Rev. B 38 (1988) 12959. B. Laks and D.L. Mills, Phys. Rev. B 20 (1979) 4962; 21 (1980) 5175; 22 (1980) 5723. R.W. Rendell and D.J. Scalapino, Phys. Rev. B 24(1981) 3276. L.C. Davis, Phys. Rev. B 16 (1977) 2482. A. Takeuchi, J. Watanabe, Y. Uehara and S. Ushioda, Phys. Rev. B 38 (1988) 12948. J. Kirtley, TN. Theis and J.C. Tsang, Phys. Rev. B 24 (1981) 5650. PD. Sparks and J.E. Rutledge, Phys. Rev. B 40 (1989) 7574. K. Takeuchi, Y. Lehara, S. Ushioda, N. Mikoshiba and S. Morita, J. Vac. Sci. Technol. A 8 (1990) 557. K. Takeuchi, Y. Uehara, S. Ushioda and S. Morita, Proc. STM’90, J. Vac. Sci. Technol. A, in press. A similar calculation was previously carried out by C. R. Reed for Ag films on a prism; see Ph.D. Thesis by C. R. Reed, Univ. ofCalif., Irvine (1986) p. 153. Note that labels a. and b. in fig. 4.7 are interchanged by mistake.
131
Light emission associated with tunneling phenomena* S. Ushioda Research Institute of Electrical Communication, Tohoku University, Katahira, Aoba-ku, Sendat 980, Japan Received 12 June 1990 Revised 8 October 1990 Accepted 16 October 1990
Light emission mechanisms in metal-insulator metal (MIM) tunnel junctions and scanning tunneling microscopes (STM) have been investigated, and similarities and differences in these phenomena are discussed. In MIM structures fluctuations of the tunneling current and/or hot electrons excite surface plasmon polariton ~SPP) modes which decay into external light via surface roughness or a coupling prism. In the STM geometry the tunneling current is microscopically localized; thus the surface modes excited in this case can be the SPP of a flat metal surface (propagating modes) as in the case of MIM, the localized plasmon modes of surface microstructures, or atomically localized electronic levels. By a prism-coupled light emission experiment from a STM on a Au film, we have found that almost all of the light emitted through the coupler-prism arises from the fast mode of SPP at the metal surface. The main interest of these experiments lies in studying tunneling phenomena by optical meahs which allow precise energy and momentum measurements with very high sensitivity.
1. Introduction Light emission from metal—insulator—metal (MIM) tunnel junctions was originally discovered by Lambe and McCarthy [1] in 1976, while the discovery of light emission from a scanning tunneling microscope (STM) came very recently. The first observation of light emission from a STM was reported in 1988 by Gimzewski, Reihl, Coombs and Schlittler [2], and by Coombs, Gimzewski, Reihl and Sass [3]. In both MIM and STM electrons generate visible light in the tunneling process between two metal electrodes. In the MIM structure tunneling takes place across an insulating oxide layer between evaporated metallic films, while in the STM geometry tunneling occurs across a vacuum or air gap between the tip of a metallic probe and the sample metal surface, The purpose of this paper is to review the light emission phenomena in the electron tunneling process, to consider possibly different emission mechanisms involved in the two cases, and to *
This paper was originally submitted for presentation at the International Conference on Luminescence 1990 as an in vited paper.
0022 2313/91/803.50
©
1991
indicate what new physics may be learned by optical investigations of tunneling phenomena in general.
2. Light emission from MIM junctions Light emitting MIM tunnel junctions are usually made on a glass substrate by the evaporation of an Al film, formation of an insulating layer by oxidation of Al, and finally deposition of a counter electrode, often of Au or Ag. Its typical structure and dimensions are illustrated in fig. 1. When a bias voltage of V0 2—5 V is applied across the —
junction, a current typically of the order of 10 to 50 mA flows, and visible light is emitted from both sides of the junction with the shortest wavelength determined by Am,n~hc/eV0. Typical emission spectra of an Al oxide-Au junction are shown in fig. 2 for different bias voltages. From the many experiments performed since the original discovery of this phenomenon [4], it is well established that the dominant mechanism of light generation involyes excitation of surface plasmon polaritons (SPP) of the layered structure as the intermediate step.
Elsevier Science Publishers B.V. (North-Holland)
132
/
S. Ushioda
Light emission associated with tunneling phenomena
1—2mm I
AU
6
I
~
0~
ox~ooL
/
3 2
Fig. 1. MIM tunnel junction formed on a glass substrate. 0
The dispersion curves of the SPP modes of a typical junction consisting of glass-Al—oxide-Au is sketched in fig. 3. These are the electromagnetic normal modes of the layered structure consisting of three branches [5]. The branch that asymptotically approaches the vacuum light line is called the “fast mode” and its field strength is mostly localized at the Au/vacuum (air) interface. The branch that lies close to the glass light line is called the “intermediate mode” or “prism-Al mode”. This mode has the largest field strength at the Al/glass interface. The third mode which is called the “slow mode” has a very small phase velocity and a large wave vector in the visible frequencies. The energy of this mode is concentrated in the insulator layer where the strongest component of the electric field is normal to the junction. Since the motion of the tunneling electron is parallel to _________________________________ ~ 1 000 0
.
v0=2.9V
•
~/v~-2.7V
~n 800 0
/v.
~ 600 z
••
I
400
••
~200
1 .5
~
S
-
25V .
0
2.0 2.5 3. 0 PHOTON ENERGY (eV)
Fig. 2. Typical emission spectra of an Al oxide-Au MlMjunction for different bias voltages, v0.
2
4
6
k11 ( ~ ~ cm~) Fig. 3. Dispersion curves of surface plasmon polaritons (SPP) in a prism (glass) Al oxide Au structure. kt is the wave vector parallel to the surface. (After ref. [5]).
this field, it delivers most energy to the slow mode with the result that the slow mode is most strongly excited in the junction. We note that all the branches of SPP lie to the right of the light line in vacuum; thus, the wave vector of SPP parallel to the surface for any given frequency is greater than that of the light of the same frequency in vacuum. This makes one-to-one conversion of a SPP to an external photon impossible, making SPP nonradiative. To allow SPP to radiate, we must have some mechanism to break the wave vector conservation along the surface or to enable wave vector matching. Surface roughness or a grating on the surface can do this by breaking the translational symmetry parallel to the surface. Thus surface roughness at one or all of the interfaces in the junction allows light emission. Also a along MIM definite junctiondirections formed on [6].aThis grating mechanism emits light can operate on all three branches of SPP, if the lateral
scale can Aalso coupler-prism of roughness allow light orinemission a contact grating is by with appropriate. increasing the junction the wave vector of the light in it by a factor of its index of refraction. Wave vector matching by a couplerprism is possible only for the fast mode (in the simple structure illustrated in fig. 1) [7,8], and this occurs when the emission angle is 430 from the surface normal for a prism made of BK-7 glass.
S. Ushioda
/ Light emission
associated with tunneling phenomena
Both of these methods have been used in getting the visible light out ofthe junction. However, since microscopic surface roughness that is relevant is present at all practical surfaces, the role of surface roughness cannot be ignored even in the case of prism-coupled light emission [9]. The theory of light emission mechanisms from MIM tunnel junctions has been discussed by many authors [10-13]. They all agree in assuming that the electron tunnels inelastically and excite SPP or localized plasmons in the process, i.e. they are excited while the electron is in the process of tunneling rather than becoming a hot electron in the terminal electrode. Kirtley et al. [14] on the other hand argued that the electron tunnels elastically generating a population of hot electrons which in turn decay by exciting SPP. At present it is not clear what the relative importance of the two processes (elastic versus inelastic tunneling) is. On the other hand, it is now clear that most of the light originates from the slow mode both in roughened junctions and in prism-coupled junctions [15]. Thus the remaining issue is the excitation mechanism of the slow mode by tunneling electrons. Also there is a problem with the reproducibility of the emission spectrum from junction to junction. We as well as others have found that the emission spectrum, the absolute intensity, and the durability depend on the fabrication procedure of the junction, such as the method of oxide formation and the evaporation speed of the metal electrodes. However, no definitive procedure for reliably making an efficient light emitter has been found. There are conjectures that the crucial point is quality of the oxide layer, the but no correlation hasthebeen established between nature of the oxide and the light emitting properties of the junction. This is an issue that must be addressed in future research. Our group has concentrated on the study of light emission by prism-coupling [5,7—9,13].This geometry is illustrated in the inset of fig. 4. In this case the fast mode is radiative in the prism and wave vector matched emission occurs at 43°from the surface normal into the prism, as seen in fig. 4. We see indeed that there is a peak of emission at 43°.The emission seen at other angles is due to the scattering by surface roughness.
133
1 .0 ~ 0.8
______
Il) -~
~
0.4 ~ 0.2
J~de
______
°
o.o 0
30 80 Angle (deg)
90
Fig. 4. Angle dependence of light emitted by a prism-coupled MIM junction. The inset shows the prism-coupling geometry (After ref. [9]).
The theory of light emission by prism-coupling including the effect of surface roughness was developed by Takeuchi et al. [13]. According to this theory, the emitted power can be written as a sum of two terms, p(O) representing the direct emission that is present even in the absence of surface roughness and p( i) arising from the scattering of SPP by surface roughness: P°(k~°w) S~
2w4 cos2O
0 8~c
f
dz’ dz”
x d~(k~°~, w z, z~)*d~(k~~), w z, z’) xJ~(k~°~, w z’, z”),
(1)
pi(k~1o w) 2w8 cos~O~ S~512~5c9
‘7.,)
d2Q~dz’ dz”
d~(k~°t, U) Z, x d~.(k~°t, w~z,zm±)Im(k~°~— Q X
11)
xd~(Q ,W~Zm ,z~)* ~ d~(Q1, i,,, z”)J~(Q11, ~
where L.(Q —
,
eIo(1
~‘,
,
w z’, z”),
(2)
z”)
1~w/eV) ~i(z’, z”) 2 2S (1+Q2~2)3 2ir
(3)
134
S. Ushioda
/
Light emission associated with tunneling phenomena
In the above equations d~’s are the Fourier transforms of the electromagnetic Green functions of the junction geometry, Im(Q ) is the power spectrum of surface roughness of the mth interface, J.(Q~1,w~Z’, z”) is the power spectrum of the tunneling current and ~ is the correlation distance between tunneling electrons. 00 is the angle of emission measured from the surface normal, and is the difference of the dielectric constants across the mth interface. Other symbols are defined in ref. [12]. The solid curve in fig. 4 was calculated by eqs. (1~i and (2) for a fixed wavelength A 600 nm and varying emission angles 00. The angle dependence of emission is sufficiently well described by this theory. On the other hand the emission spectra are only in rough agreement with the result obtained from eqs. (1) and (2), assuming a certain reasonable power spectrum of surface roughness [9]. More recently, Takeuchi et al. [16] actually measured the power spectrum of surface roughness of a junction by STM and simultaneously measured the light emission spectrum. They numerically inserted the measured power spectrum of surface roughness into eq. (2) and compared the measured emission spectrum with the calculated result. They could obtain good agreement between the measured spectrum and the calculated spectrum due to surface roughness pW, but its relative size to pO) was found to be too small. Since they measured the surface roughness on the top electrode (Au) and assumed the same roughness exists at other interfaces, it is possible that Al/oxide and Au/oxide interfaces. The most they underestimated the size of roughness at the effective location of surface roughness in inducing light emission is at the oxide/metal interfaces where the amplitude of the slow mode is large. Thus if the surface roughness here is larger than that on the top electrode, one greatly underesti~,
ROTATIO
STAGE
4r~-MICROMETER
toRM.T. PRISM TIP
~XYZ~~CAL -
STAGE ___________________________ PIEZOELECTRIC CELL~ Au FILM
Fig. 5. Experimental setup to measure light emission from a
STM. This is the geometry used to observe the light through a hemicylindrical prism-coupler. (After ref. [17]).
al. [2]. They used an Ir tunnel tip on Si (1 1 1) and polycrystalline tantalum surfaces, and measured the intensity of 9.5 eV photons by a Geiger—Muller type detector as a function of the bias voltage between the tip and the sample. They interpreted the results by comparing them with the conventional inverse photoemission spectra of these samples. Immediately following the above work, Coombs et al. [3] reported the emission spectra of a STM in the visible frequencies, using an Ir tip and an evaporated Ag film. Their spectra are reproduced in fig. 6. They suggested that the two prominent peaks at 1.9 and 2.4 eV may correspond to resonances due to localized plasmon
2 4V
/\ QUANTUM
i 9V
CUTOFF
zLU z
TUNNEL
3 23
o
5—
3 00 2.71 VOLTAGE 249 2 33 212 2.02
mates the contribution from eq. (2). 3. Light emission from STM 1
A geometry for the observation of light emission from a STM is illustrated in fig. 5. The first observation of this effect was reported by Gimzewski et
FIBER
2
3
4
PHOTON ENERGY (eV)
Fig. 6. Emission spectra of STM in the visible region for different bias voltages. (After ref. [3].)
S. Ushioda
/
Light emission associated with tunneling phenomena
modes of the tip/surface region. Similar features had been seen in a theoretical calculation by Rendell and Scalapino [11] who considered light emission from tunnel junctions containing small Ag particles on an Al substrate. Stimulated by the above reports, we have measured light emission from a STM, using a prism-coupling method [17]. In this experiment a Au film (22.3 nm thick) was evaporated on the flat surface of a hemicyclindrical BK-7 prism and a Pt-Ir tip was brought close to the Au film at the center of the prism in air. Light emission was observed through the prism as a function of the emission angle. The STM was operated in a constant current mode without scanning in the lateral direction. The emission intensity was measured by a photomultiplier without spectral analysis. The result is plotted in fig. 7 which shows a very sharp peak at 43°from the film normal. There are two branches of SPP in the glass-Au film—air geometry, namely the fast mode and the Au-glass mode. As in the case of prism-coupled light emission from MIM junctions, the observed peak corresponds to wave vector matched emission into the prism by the fast mode at the surface of the Au film. The solid curve in fig. 7 is the calculated angle dependence based on eqs. (1) and (2) with the wavelength fixed at 700 nm. For this calculation the Green functions in these equations were adapted to correspond to the prism—Au film-air geometry [18]. We found that the contribution from the surface rough-
(/)
I—ii’~~
‘‘‘‘‘‘
~ 0.8 06 ~ 0.4
135
ness p~ is insignificant compared to the contribution p(O) from a flat surface in this case. From this experiment it is clear that almost all of the light emitted through the prism is coming from the fast mode which is a delocalized normal mode of a flat metal surface. Coombs et al. [3] observed the light from the tip side in UHV and conjectured that the origin of the light are local plasmon modes confined by surface microstructures and the tip. Thus, our finding from the prism side contradicts their conjecture. It is possible that the situation is different on the vacuum side and much of the radiation comes from localized modes there. Further work is in progress in our laboratory to clarify this point.
4. Interesting questions From what is known already, it is clear that SPP (delocalized propagating mode) plays an important role in light emission from both MIM and STM. In the case of STM it is most interesting to determine if and to what extent the local plasmons rather than the delocalized SPP are involved in the light emission process. Since the local plasmons are the normal modes of the geometry formed by the tip and surface microstructures, their lateral range is of the order of a few nanometers, and therefore, the emission spectrum and intensity will reflect the nature of the microstructure, allowing spectroscopic studies of such structures. On the other hand, even if the main origin of the light is the delocalized SPP the emission intensity can depend on the presence of microstructures, because such structures will scatter SPP which will not be radiative in their absence. Thus the observa-
<
tion bybetween dence Coombs the et al.light [3] intensity that showsand corresponsurface
w
topology can be explained by scattering of SPP. If one measures the emission through a prismcoupler as we did, one selectively observes the emission from the fast mode. However, if we measure emission from fastthemode as we scan the the STM, we may findthethat excitation
~ —
0.2 ~j.sj
_______________________
0
30 60 90 EMISSION ANGLE(DEG)
Fig. 7. Angle dependence oflight emission from a STM through a coupler-prism illustrated in fig. 5. (After ref. [17].)
.
efficiency of the fast mode depends on the microstructures. A preparation for this kind of experiment is under way in our laboratory.
136
S. Ushioda
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Light emission associated with tunneling phenomena
With regard to MIM, one of the unexplored questions is the form of the current fluctuations J ..(Q
1 w z’, z”). So far we have used the form proposed by Laks and Mills [10] and achieved a reasonable degree of agreement with the experimental findings. Particularly interesting is that the shape of the emission spectrum is a fairly sensitive function of the phenomenologically assumed lateral correlation distance ~ between tunneling electrons. Thus it may be possible to determine this correlation distance from the observed optical spectra. This kind of development will lead to a new insight into the electron tunneling phenomena and open a new avenue for investigating electron correlations in metals. As better reproducibility of junctions and better precision of optical measurements become reality, it is desirable to re-examine the theory and attempt to formulate it from the first principles dealing with the coupling of tunneling electrons to SPP.
[3] J.H. Coombs, J.K. Gimzewski, B. Reihl and J.K. Sass, J. Microscopy 152references (1988) 325. [4] A useful set of for work published before 1984
,
[5] [6] [7] [8] [9] [10]
[11] [12] [13] [14]
[15] [16]
References [1] J. Lambe and S.J. McCarthy, Phys. Rev. Lett. 37 (1976) 923. [2] J.K. Gimzewski, B. Reihl, J.l-l. Coombs and R.R. Schlittler, Z. Phys. B 72 (1988) 497.
[17] [18]
is found in P. Dawson, D.G. Walmsley, H.A. Quinn and A.J.L. Ferguson, Phys. Rev. B 30 (1984) 3164. S. Ushioda, i.E. Rutledge and R.M. Pierce, Phys. Rev. B 34 (1986) 6804. JR. Kirtley, TN. Theis and J.C. Tsang, AppI. Phys. Lett. 37 (1980) 435. S. Ushioda, J.E. Rutledge and R.M. Pierce, Phys. Rev. Lett. 54 (1985) 224. K. Suzuki, J. Watanabe, A. Takeuchi, Y. Uehara and S. Ushioda, Solid State Commun. 69 (1989) 35. J. Watanabe, A. Takeuchi, Y. Uehara and S. Ushioda, Phys. Rev. B 38 (1988) 12959. B. Laks and D.L. Mills, Phys. Rev. B 20 (1979) 4962; 21 (1980) 5175; 22 (1980) 5723. R.W. Rendell and D.J. Scalapino, Phys. Rev. B 24(1981) 3276. L.C. Davis, Phys. Rev. B 16 (1977) 2482. A. Takeuchi, J. Watanabe, Y. Uehara and S. Ushioda, Phys. Rev. B 38 (1988) 12948. J. Kirtley, TN. Theis and J.C. Tsang, Phys. Rev. B 24 (1981) 5650. PD. Sparks and J.E. Rutledge, Phys. Rev. B 40 (1989) 7574. K. Takeuchi, Y. Lehara, S. Ushioda, N. Mikoshiba and S. Morita, J. Vac. Sci. Technol. A 8 (1990) 557. K. Takeuchi, Y. Uehara, S. Ushioda and S. Morita, Proc. STM’90, J. Vac. Sci. Technol. A, in press. A similar calculation was previously carried out by C. R. Reed for Ag films on a prism; see Ph.D. Thesis by C. R. Reed, Univ. ofCalif., Irvine (1986) p. 153. Note that labels a. and b. in fig. 4.7 are interchanged by mistake.