IEEE TRANSACTIONS ON ELECTRON DEVICES. ... Diode: Application as an Infrared and Optical ... novel application for a planarized STM-like structure that.
IEEE TRANSACTIONS ON ELECTRON DEVICES. VOL 36. NO. 11. NOVEMBER I Y X Y
2659
Proposed Planar Scanning Tunneling Microscope Diode: Application as an Infrared and Optical Detector THOMAS E. SULLIVAN, YOUNG KUK,
Abstract-Since the development of the scanning tunneling microscope (STM) in 1981, the STM has become one of the most powerful laboratory tools for atomic scale probing of material surfaces. In this paper, we propose a practical application for a planar STM-like structure incorporated on an integrated circuit. The receiving or detecting properties of the diode are tuned to selected infrared or visible wavelengths through controlled tunnel gap spacings. Diodes are configured to receive one wavelength reject incident signal for which the gap spacing is greater than some critical spacing s,, which can be determined by the frequency of the incident signal and gap parameters. With the selective reception characteristics of these diodes, coded infrared and optical signals can be received by an integrated circuit and can be converted to binary code compatible with the circuit logic.
I. INTRODUCTION HE development of the scanning tunneling microscope [l] has opened a new dimension in the control of point contact tunnel structures. These include any combination of metals and semiconductors operating through vacuum or insulator. This has made possible the measurement of the I-V characteristics for accurately controlled widths of the gap in tunnel junctions. In studies of tunneling characteristics [2], [3], the STM offers a number of advantages over the point contact diode. For example, the STM offers accurate positioning and tip-to-base spacing that is achieved through the use of piezo drive and feedback circuitry. Furthermore, STM experiments can be performed in ultrahigh vacuum on clean surfaces, thereby eliminating the possibility of tunneling through intermediate oxides. The STM has also been used as a direct detector and frequency mixing diode in the infrared [4]. This has demonstrated antenna properties similar to that first shown by the earlier point contact structures. In a recent paper, the authors and their co-workers have proposed the use of the STM to rectify infrared and optical frequencies and to measure an operational tunneling time [5]. They have reported preliminary results observing a cutoff response of the diode to 1.06-pm laser radia-
T
Manuscript received February 20 1989; revised April l , 1989. T. E. Sullivan is with the Department of Electrical Engineering, Temple University, Philadelphia, PA 19122. Y. Kuk is with AT&T Bell Laboratories, Murray Hill, NJ 07974. P. H. Cutler is with the Department of Physics, Pennsylvania State University, University Park, PA 16802. IEEE Log Number 8929872.
AND
P. H. CUTLER
tion, which is in reasonable agreement with the prediction that the tunneling time T is equal to v, is the frequency of the incident radiation, [6], [7]. The purpose of this paper is to propose and describe a novel application for a planarized STM-like structure that can be fabricated on an integrated circuit. This STM-like diode device acts both as an antenna for an incident laser signal and a rectifying circuit element. The device structure provides a high-speed room-temperature integratedcircuit laser receiver with an adjustable cutoff frequency for direct optical communication to silicon. In Sections I1 and 111, we review the role of asymmetry in producing rectification in the STM diode and the operational definition and measurement of a tunneling time. An analysis of the transit time in the device is given in Section IV. The proposed application of the STM diode is described in Section V. Modes of operation and device fabrication are discussed in Sections VI and VII. A summary is presented in Section VIII. 11. MECHANISMS OF RECTIFICATION In an STM, the current is carried by electrons tunneling across the classically forbidden vacuum barrier between the tip and the surface. Roughly speaking, tunneling involves three ingredients: 1) the initial and final states, 2) their occupation probabilities, and 3) the shape of the tunnel barrier. Correspondingly, the current asymmetry observed at fixed gap width s must originate from one or several of the three possible causes discussed below, namely material, geometrical, and thermal asymmetry. A . Material Asymmetry This effect is expected to be most pronounced for metal/ semiconductor STM’s. A potential model illustrating material or work function induced asymmetry is sketched in Fig. 1. When the electrodes have different work functions, the barrier shape will be asymmetric at zero bias. The barrier asymmetry is enhanced for forward and reverse bias. This results in different transmission probabilities for the same magnitude of bias voltage, as can be seen, for example, by inspection of the Wentzel-Kramers-Brillouin (WKB) transmission integral through a triangular barrier [8].
0018-9383/89/1100-2659$01.00 0 1989 IEEE
2660
IEFE T R A N S A C I ' I O N S ON E L F C T K O N DEVICES. V O L . 36. N O
I I, K O V k M B E K IOW
FORWARD BIAS
BASE
TIP / /
W(+)
-- E F
4
OCCUPIED STATES
E,
E,
w 0
.............
Fig. 2. A normally triangular tunnel barrier of a materially symmetric junction exhibits a bulging shape as indicated as a result of the polarity of the applied voltage.
(b)
Fig. 1. Schematic representation of asymmetric tunneling due to the material asymmetry of a junction such as tungstenisilicon ( W / S i ) .
B. Geometrical Asymmetry When the STM operates at atomic resolution, the nature of the tunneling phenomenon is such that the current passes predominantly through that sharp protrusion closest to the planar sample surface. In such conditions and even in the absence of any material asymmetry (e.g., W tip, W surface, and assuming no work function inhomogeneities), the shape of the tunnel barrier is asymmetric as a function of the applied bias field. This is due to the geometric asymmetry of the electrodes comprising the tunnel junctions. This effect is illustrated in Fig. 2. It is evident that the static electric field gradient is larger near the pointed apex of the tip than at the planar surface of the sample. This means that the vacuum tunnel barrier will tend to buckle inward or become thinner for forward bias and balloon outward or become thicker for reverse bias. The first observation of the geometrical asymmetry effect in STM was observed by Feenstra et al. [9]. More recently a detailed study of rectification in an STM was presented by Nguyen et al. [lo], [I 11. C. Thermal Asymmetry This effect will take place when there is a temperature difference between the two electrodes and hence a different electron occupation of the states involved in tunneling; these states are generally from a narrow region centered around the Fermi level. As schematically indicated in Fig. 3 , the forward current would exceed the backward one even for a planar-planar junction of identical materials if the tip temperature were higher than the sample surface. In the limit of a high temperature, one recovers a situation analogous to ordinary thermionic emission. Thermally assisted field emission, i.e., a combination of thermal and high-field effects, was in fact invoked as contributing to the mechanism of rectification in metal whisker diodes. It should be expected that, with laser irradiation, a differential heating of the tip and the base of the junction may occur causing a thermal asymmetry contribution to the tunneling current.
TIP
.-:.; ~
Fig. 3. Thermal asymmetry of the tunnel current caused by the tip temperature greater than the base temperature. The broader thermal spread of the electron distribution of the tip side should result in a higher forward bias current.
111. OPERATIONAL TUNNELING TIME A method has recently been proposed to use the nonlinear and rectification properties of an STM junction to measure a suitably defined electron tunneling or transit time through the vacuum barrier in the junction. The principle of the experiment is shown in Fig. 4. A linearly polarized laser beam is focused onto the junction [ 5 ] .The electric field component along the tip axis biases the junction and induces as tunneling across the junction at the laser frequency. Due to the rectifying properties described in Section I1 (see also [ 5 ]and [6]), a dc bias spontaneously develops and a dc current flows through the external circuit as in a detector. The strength of the rectified signal should, however, exhibit a dependence on the laser frequency because of the nonzero response time of the tunnel junction device. In effect, there are two different response times in the system. There is the collective response of the conduction electrons that establishes the ac bias when driven by the laser field. The other response time of the junction to the laser oscillation is an average "traversal time" for individual electrons crossing the vacuum barrier. The validity of this concept has been hotly debated for the past 50 years. In fact, it has been a controversy since the advent of quantum mechanics and the recognition that there can be particle tunneling through classically forbidden barrier regions. Basically the problem lies in the difficulty of defining and measuring the traversal time for the conceptually simple time-dependent scattering experiment in which an electron (wave packet) tunnels through a spatially localized barrier and is detected beyond the tunnel-
SULLIVAN et al.: PROPOSED PLANAR SCANNING TUNNELING MICROSCOPE DIODE Induced e - C u r r e n t
2661 C + D.Current
constants
DC Current
constant U
f
&,
1
Scutoff I
0
,
,
IO 20 30 GAP DISTANCE (ANGSTROMS)
(b) Fig. 4. Schematic representation of the preliminary experiment to determine the cutoff response of the STM diode. (b) Experimental data of the laser induced dc current as a function of gap distances indicates the onset of decreasing current for S, = 25 A for laser wavelength of 1.06 pm.
ing region [8]. In practice, most tunneling phenomena are observed either through a static or quasistatic current intensity that does not carry and direct time-resolved information, or through the spectroscopy of level splitting in multiple quantum-well systems (such as the NH, molecule that relates to the electron residence time in one well rather than to the time required to tunnel between wells. Recently, an elegant experiment to indirectly measure a traversal time in a wide but shallow tunnel barrier (i.e., a quantum-well structure) was conducted by Gueret et al. [ 191 in a purely static configuration involving the decrease of tunneling current caused by the barrier “widening” effect of an applied transverse magnetic field. The range of tunneling times accessible to this method unfortunately seems limited to the subpicosecond region. We have proposed a dynamical approach to probe tunneling times in which a natural time scale is provided by the laser as an integral part of the experimental arrangement. Indeed, the laser causes the tunneling and, at the same time, provides a “clock” to measure the duration of the event. It has been argued that, for a fixed spacing, the laser frequency is too high, and few electrons will be able to transfer from one electrode to the other during the half of the period when the electric field vector in the laser beam accelerates the tunneling electron [ 5 ] . This means that one should observe a cutoff in the strength of the rectified signal either 1) when the frequency is increased beyond a critical value vc while maintaining the tip-to-surface distance s fixed (Fig. 5(a)), or 2) when the gap width s is increased beyond a characteristic value se, while keeping the laser frequency constant (Fig. 5(b)). This latter method is the more feasible at the present time in view of the unavailability of lasers with a large visible tuning range.
(b) Fig. 5 . Expected behavior of the rectified signal intensity as a function of laser frequency for (a) fixed gap width s or (b) as a function of varying gap width for fixed laser frequency.
In a recent paper [6], [7], we reported the first data taken on an STM W/Si ( 1 1 1 ) junction illuminated by a 1.06pm yttrium-aluminum-garnet (YAG) laser. In this experiment the tungsten tip was held at a fixed distance ( 10 A ) from the silicon surface, the YAG laser was focused near the junction, and the resulting laser-induced current was measured. We note that while the laser beam irradiated the junction, no external bias was applied to the STM junction, so that the voltage and induced current that were measured in the junction were due solely to the laser field. The tip-to-base gap s was then progressively increased until the laser-induced current vanished. The dc rectified current as a function of gap width for fixed @er frequency indicated a cutoff distance of about 25 A for the 1.06-pm YAG laser line, as shown in Fig. 4(b). From method (1) described above, one can define the operational tunneling time 7 = v,’. If we assume, for simplicity, that the kinemetical equation s = velocity X time applies, then one can extract the corresponding velocity S , / T . When numbers are substituted, v is comparable to the Fermi velocity of an electron in the tip. It should be understood, however, that S c / 7 2: uFmeans only that the particle acts as if it obeys the kinematical equations of motion as the particle traverses the classically inaccessible region defining the barrier. The corresponding velocity S e / , = lo8 cm/s, where 7 is half the YAG laser period, is consistent with a traversal velocity of the order of Fermi velocities. It is tempting to think that this should be since the tunneling predominately involves electrons from states around the Fermi level. However, there is no a priori reason for an electron to act “classically” as it tunnels through a barrier.
-
+
IV. TRANSITTIME ANALYSIS In the following analysis, it is convenient to assume a simple classical model for the motion of the electron. An electron emitted at the tungsten tip due to the laser-in-
2662
1F.F.E T R A h S A C T I O i X S ON F L t ~ C T R O N DEVICES. V O L . 36. N O
I I . NOVEMBER l 9 X Y
duced field across the tunnel junction follows a “trajectory” from the emitting tip to the collecting base. As stated previously, it is assumed that the electron originates near the Fermi level in the metal and is emitted at a velocity approximately equal to the Fermi velocity uF.For A1 and W, uFis 2.03 and 1.68 X lo8 cm/s, respzctively. These metals are of technological interest since they are commonly used in VLSI circuit fabrication. Using the Fermi velocity for tungsten and the expression S = v F / r for the distance traveled by the tlectron, we obtain a critical cutoff distance of S, = 28 A which compares ably with the experimentally observed cutoff of 25 A
fava:-
as molecular-beam epitaxy (MBE), the formation of photodiodes on silicon is a formidable task involving questions of lattice and thermal mismatch, stoichiometric control, photolithographic definition, and etchability . Clearly, one approach to incorporate photodiodes on silicon or gallium arsenide circuits is to utilize compatible existing metal deposition techniques such as aluminum or tungsten deposition. ISTM diodes offer that inherent compatibility in that planar feature sizes on the order of the critical spacings for ISTM diodes are achievable with the recent advances in photolithography and reactive ion etching.
I61> VI. The importance of a critical cutoff distance for a given laser frequency will be discussed in the next section.
VI. MODESOF OPERATION In this section we discuss two modes of operation for an ISTM diode fabricated on an integrated circuit.
-
V. APPLICATIONS OF THE STM-DIODE Sensory devices operating in the infrared, visible, and ultraviolet frequencies form a broad class of detectors. They encompass a range of wavelength such that the interaction of this radiation with materials gives rise to a measurable photoeffect. Detectors operating in the ultraviolet, visible, or near infrared typically operate uncooled at room temperature (295 K ) . Detectors used in the longer wavelength regions (i.e., X 2 10-100 pm) require cooling to liquid-nitrogen or liquid-helium temperatures 1201. As shown in the early experiments with point contact diodes, these devices were useful for direct detection from dc to the visible while operating at room temperature. A second desirable characteristic of optical and infrared sensors is the existence of a distinct cutoff wavelength. This is especially important for optimizing detectors to a specific wavelength region. Semiconductor devices such as mercury cadmium telluride ( Hg, - ,Cd, Te ) have variable bandgaps depending on the Cd-to-Hg ratio. The STM diode, operating at 295 K, irradiated by the 1.06-pm YAG laser, demonstrated a characteristic cutoff distance [6], [7]. For different laser frequencies this implies, based on the transit time analysis of the previous section, that STM diodes arrays with variable fixed gaps would exhibit cutoff wavelengths proportional to the gap spacing. The STM diode therefore has the potential advantages of room-temperature operation and distinct cutoff frequency operation from dc to optical frequencies. The unusually broad response of this diode configuration has a wide range of applications in electrooptics. In this section we propose a novel use for an integrated scanning tunneling diode (ISTM) fabricated as part of an integrated circuit. Optical communication with silicon digital circuitry is a field of tremendous importance for optical computing and intelligent remote activation and sensing. Depending on the wavelength,region, signal reception on a silicon chip can be extremely difficult since the integration of an appropriate receiver can involve the fabrication of complex ternary or quaternary devices on an existing silicon circuit. With an appropriate deposition technology, such
A . Discrete Mode The discrete mode employs an array of two diodes with different critical spacings. The critical spacing is chosen as a function of the laser wavelengths used to optically communicate with the underlying silicon circuitry. The difficulty in controlling the critical spacings during fabrication affects the choice of wavelengths that can be employed. The use of closely spaced laser frequencies could preclude discrete diode switching. In the ISTM, diodes DI and D, have critical spacings of 95 and 18 A , respectively. In this example, when both diodes are illuminated by the laser bA = 3.39 pm, the critical distance for cut$€ is S 2 95 A . With diode spacings SDI,S,, < 95 A , both diodes respond to an incident laser signal of 3.39 pm. With AA = 3.39 pm illumination, diodes D , and D2 are O N . This corresponds to a binary 1 input. The same diode array shown in Fig. 6 responds differently to illumination at hB = 0.6328 pm. During illumination at 0.$328 pm the critical spacing for oy operation is S c 18 A . Clearly diode D , with S = 95 A is always OFF. Illumination at AB = 0.6328 pm turns D2 ON but keeps D , OFF.This state corresponds to binary 0. When the diode array is not illuminated, both diodes D1 and D2 are OFF. An encoded signal using two wavelengths (3.39 and 0.6328 pm) would be decoded by the ISTM array. The encoded signal is transmitted using sequential laser pulsers, e.g., AA, AA, AB, AA, AB, corresponding to a binary sequence of 1 , 1, 0, 1, 0.
B. Enable Mode In the ENABLE mode of operation, the same arr?y of ISTM diodes with critical spacings of 95 and 18 A are utilized in a different encoding scheme. In this mode, one of the laser signals is used as an ENABLE signal whose presence indicates to the underlying digital circuitry that an incoming coded signal is to be transmitted and decoded but, only during the presence of the ENABLE signal. In Fig. 7 the diode array is illuminated with the shorter wavelength AA = 0.6328 pm. With diode D,,OFF, and D2 ON, the array is illuminated by a longer wavelength signal AB = 3.39 pm. The presence and absence of longer wavelength signal switches diodes D, ON and OFF. The
SULLIVAN et al.: PROPOSED PLANAR SCANNING TUNNELING MICROSCOPE DIODE
2663
#
DISCRETE MODE X, = 3 3 9 p m
BASE
n
-
p-WELL
n-- SUBSTRATE
Fig. 8. Schematic representation of a planar STM-like diode on an integrated circuit. The diode acts as an antenna for the incident radiation. The diode provides a voltage reference that is used to switch the underlying silicon circuitry.
SEQUENTIAL LASER INPUT AA
Ae
AA
XA
tnnn n n I
1
0
0
I
Fig. 6. Discrete mode of diode operation. The generation of binary 1 and 0 are sensed using two discrete laser signals A, and AB.
L A S E R WAVELENGTH
0 6328pm
1
w y>
I
0
I
0
I
ENABLE
0
3 39pm
ON/OFF BINARY
BINARY OUTPUT
Fig. 7. ENABLE mode of operation using a pair of STM-like diodes. The 0.6328-pm signal acts as as enable signal, while the 3.39-pm signal provides the binary input.
signal corresponds to a binary 1 and 0. When the signal is on and a sequence of longer wavelength pulses are detected, the incident pulse is checked against an on-chip clock to discriminate between successively transmitted O’s, as shown in Fig. 7 .
ONIOFF
ENABLE
VII. DEVICEFABRICATION The task of incorporating ISTM diodes on an integrated circuit, as shown in Fig. 8 , is formidable due to the inherent difficulties in creating the plaaar arrays with critical spacings on the order of 25-100 A . There are a number of photolithographic approaches [21], [22] that may be used to fabricate such small features sizes, as, for ex-
ample, the use of A1F3 high-resolution electron-beam resists; etch patterns were reported with feature sizes on the order of 2 nm with 4nm pitch [23]. Planarization technologies have been used and are quite common in multi-level metallization circuit processes. Using dielectric deposition and etch-back procedures, planarized areas can be defined near existing aluminum metal lines that will serve as the sites for ISTM arrays. Finally, nanometer lithography using an STM was reported that prodvced regular line patterns with an average spacing of 160 A [24]. The critical feature sizes (25-100 ) required for the fabrication of ISTM arrays should be achievable in the near future, and experiments of this sort are now in progress.
A
VIII. SUMMARY The development of the STM in 1981 has brought about a powerful laboratory tool for atomic scale probing of material surfaces and for the study of tunneling phenomena in controlled point contact structures. In this paper, we have proposed a novel application for a diode based on the STM structure-specifically a patterned and etched planar point contact diode with specified tunnel or gap distances. We have shown how this diode is capable of rectifying an incident laser signal and discriminating between incident signals. We review preliminary experimental data that indicate the feasibility for this application. Two encoding schemes are presented in which the ISTM diode array functions as a circuit element in much the same way a traditional diode functions. The ISTM diode, however, is seen to potentially provide room-temperature detection from dc through the visible and the ability to discriminate between variable wavelength input signals. REFERENCES [I] G . Binnig, and H . Rohrer, “Scanning tunneling microscopy,” IBM J . Res. Develop., vol. 30, pp. 355-369, 1986. 121 J . A . Stroscio, R. M. Feenstra, and A. P. Fein, “Electronic structure of the S i ( 111) 2 X 1 surface by scanning-tunneling microscopy,” Phys. Rev. Lett., vol. 57, pp. 2579-2582, 1986.
R. J . Hamers, R . M . Tromp. and J . E . Demuth, "Surface electronic structure of S i ( I I I ) - ( 7 x 7 ) resolved in real space." Phjs. Rev. Lerr.. vol. 56. pp 1972-1975, 1986. L. Arnold. U'.Krieger. and H. Walther. "Laser-frequency mixing in the junction of a scanning tunneling microscope." A p p l . Phys. Lett., vol. 51. pp. 786-788. 1987. A. A . Lucas. "Use of a scanning tunneling microscope to rectify optical frequencies and measure an operational tunneling time," J . Vue. Sci. Techno/., vol. A6, no. 2, pp. 461-465, 1988. P . H . Cutler er al., "Experimental and theoretical results of rectification measurements in an STM-Part I." J . Phxs. (Paris), colloque C 6 . vol. 48. pp. 97-100, 1987. -. "Experimental and theoretical results of rectification measurements in an STM-Part 11." J . Phjs. (Paris), colloque C6, vol. 48. pp. 101-106, 1987. C . B . Duke, Tunneling in Solids. New York: Academic. 1969. R. M . Feenstra. J . A. Stroscio, and A . P. Fein. "Tunneling spectroscopy of the S i ( 111) 2 x I , surface." Surface Sci., vol. 181, pp. 295-306. 1987. H . Q. Nguyen. Ph.D. dissertation, The Pennsylvania State Univ., 1989. H. Q. Nguyen er al.. "Detection of infrared and visible radiation using STM and measurements of an operational tunneling time," I€€€ Tram. Elecrrori Dc\,ic.r.\. this issue, pp. 2671-2678. A. A. Lucas. A Moussiaux. M. Schmeits. and P. H. Cutler. "Geometrical asymmetry effect, of tunneling properties of point contact diodes." Cmimun. Ph!.\.. vol. 2 . pp. 169-174. 1977. A. A. Lucas and P. H . Cutler. "Themial field emission as a niechanism for infrared laser light detection in metal Hhisker diode." Solid Starc Commun.. \ o l . 13. pp. 361-365. 1973. T . E. Sullivan. P. H. Cutler. and A . A. Lucas. "The use of antenna theory to calculate the electric fields in a thermal field emission metal ahisker diodes." Sur$ice Sci . \oI. 62. pp. 155-471. 1977. N. M. Miskovsky. S . J . Sheperd. P . H. Cutler. T . E. Sullivan. and A . A. Lucas. "The importance of g e o m e t q . field, and temperature in tunneling and rectification beha\ ior of point contact junctions of identical metals." Appl. Plixs. Lc:r.. vol. 3 5 . pp. 560-562. 1979. N . M . Miskovsky. P. H . Cutler. T . E. Feuchtwang. S . J . Shepherd, and A. A. Lucas. "Effects of geometry an multiply-image interactions on tunneling and l-V characteristics of metal-vacuum-metal point contact junctions." Appl. Phys. Lerr., vol. 37. pp. 189-192. 1980. T . E . Feuchtwang. P. H . Cutler. N. M . MiskoLsk). and A. A. Lucas, "In quantum nieterolog) and fundamental physical constants," in ,VAT0 AS1 Series B 98. P. H . Cutler and A. A. Lucas. Eds. New York. Plenum. 1983. T . E . Harman, "Tunneling of a wave packet," J . Appl. Phys., vol. 3 3 , p p . 3427-3433. 1962. P. Gueret, A . Baratoff, and E. Marclay. "Effect of a transverse magnetic field on the tunnel current through thick and low semiconductor barriers," Europhys. Lerr., vol. 3. 367-372. 1987. R. J . Keyes. Ed.. Optical and Injrared Detrcrors: Topics in Applied Ph!..tics. New York: Springer-Verlag. 1977. R . E. Howard and D. E. Prober. VLSl €lecrronic~.\:Microsrrucrure Scienc,e. vol. 5 . New York: Academic. 1982. A. N . Broers. W . W . Molzen, J.* J . Cuomo, and N. 0 . Wittels, "Electron-beam fabrication of 80- A metal structures," Appl. Phys. Lerr., vol. 29, pp. 596-598. 1976. A. Murray. M . Issacson, and I . Adesida, "AIF,-A new very high resolution electron beam resist," Appl. Phys. L a . , vol. 45, pp. 589591. 1984. M . Ringger, H . R . Hidber. P. Schegle, P. Oelhafen, and H. J . Giintherodt, "Nanometer Iithogmphy with the scanning tunneling microscope," Appl. Phys. Lerr.. vol. 46, pp. 832-834, 1985.
Thomas E. Sullivan received the Ph.D. degree in physics from The Pennsylvania State Universit) in 1976. He was a Visiting Post-Doctoral Scholar in the Department of Engineering Sciences at The Pennsylvania State University in 1976. In 1979. he joined the RCA Laboratories Solid State Technology Center where he worked on VLSI process and device development. He held a number of technical and managerial positions at RCA prior to joining Temple University in 1987 where he is currently a Professor of Electrical Engineering
*
Young Kuk was born Februarq 5. 1953 in Seoul. Korea. He received the B.S. degree in phlsics from Seoul National Universit) in 1975 and the Ph.D. degree in physics from The Pennsylvania State University in 1981. In 1981, he joined AT&T Bell Laboratories. and he has been a Member of Technical Staff in the radiation physics department since then. He has written 33 contributed and invited papers. Dr. Kuk is a member of the American Physical Society, the American Vacuum Society. and the Bohemische Physical Society.
*
P. H. Cutler received the B . S . degree in 1948 and the Ph.D. degree in theoretical physics in 1958. both from The Pennsylvania State Unibersit] From 1950 to 1954. he b a s with Sylvania Electric Products Research Laboratories working in physical electronics and mass spectrometrq . From 1958 to 1960. he was with the Cavendish Laboratory doing post-doctoral studies in solid-state physics with Sir Neville Mott. In 1960. he v a s appointed Assistant Professor of Phjsics at Penn State and full Professor in 1966. From 1967 to 1968. he was a Fulbright Research Fellow at the Technical Uni\ersity. Copenhagen. He was a Visiting Scientist at ESA. Holland. in 1971 and 1972, and a visiting Professor of Physics at the University de Liege in 1977 and the Universitk de Paris in 1986. He was also a Visiting Felloh at Oxford University in 1985. His current research interests are electronic and transport properties in submicrometer structures. electrohydrodynamic theory of ion and cluster emission in EHD sources, metal-vacuum and junction tunneling with application to scanning tunneling microscopylspectroscopy, and theory and measurement of tunneling time in STM and other junction devices. He is the author or coauthor of more than 100 scientific papers. Dr. Cutler is a member of the American Vacuum Societ) and a Fellow of the American Physical Society.
