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tungsten in the field electron microscope
SURFACE
SCIENCE
45 (1974) 649-656 o North-Holland
Publishing Co.
DEPENDENCE OF WORK FUNCTION ON COVERAGE FOR ALUMINUM/TUNGSTEN
IN THE
FIELD ELECTRON MICROSCOPE A. J. MELMED
and J. J. CARROLL
I~t~t~t~ for ~ute~~ffls Research, NQtion~~ Bureau of Sfa~dar~, Washi~to~, D.C. 20.234, U.S.A.
and R. MQCLEWSKI institute
for Experimental
Physics,
University
of Wroclaw,
Received 5 April 1974; revised manuscript
Wroclaw,
Poland
received 27 June 1974
Aluminum adsorption on tungsten has been investigated by field electron microscope techniques. Changes in average work function and work functions of the (001) plane and (Ill) region were determined as a function of aluminum deposition. The average work function exhibits a minimum of about (4.11 -f: 0.04) eV (relative to an assumed vatue of 4.50 eV for the tungsten substrate) and reaches a value of (4.21 i 0.04) eV for thick deposits. The (001) plane and (111) region exhibit compiicated work function dependences on aluminum coverage. Some quaIitative observations of aluminum on tungsten surface diffusion are also reported.
1. Introduction Aluminum adsorption on tungsten was investigated by Neumann”) using field electron microscope techniques2). He measured the dependence of the average work function on deposition time and found that it decreased monotonically from 4.50 eV (assumed to be the clean substrate value) to a value of (4.2020.05) eV, which remained constant for further deposition. Mitchell and Mitchella) measured the work function of aluminum films of unspecified thickness evaporated onto polyc~stalline tungsten ribbon, using an electron retarding potential technique. They reported a work function of (4.25 kO.05) eV relative to an assumed work function of (4.54 F F0.02) eV for their substrate. Eastment and Mee*), using a photoelectric method, measured the work function of aluminum films on quartz and aluminum substrates. For thick annealed polycrystalline films on quartz their result was (4.28kO.01) eV. In the present paper field electron microscope techniques were used to determine the dependence on aluminum coverage of the average work function and also the work functions of the (001) plane and (I 11) region of 649
650
A. J. MELMED,
J. J. CARROLL
AND
tungsten (field-ion microscopy of thermally shows no plane development for the (111) plane measurements were made because showed unusual work function behavior in
R. MI$CLEWSKI
annealed tungsten emitter tips direction). These single crystal preliminary qualitative-studies the two regions.
2. Experimental These investigations were done in a relatively simple glass field electron microscope (FEM), originally built to investigate surface diffusion, containing a heatable tungsten tip equipped with potential leads for temperature determination and an aluminum evaporator. The microscope was connected to a glass bulb in which titanium was evaporated from time to time, and this in turn was connected to a metal sputter-ion pumped vacuum system. The entire apparatus was baked for several hours at about 350°C and after cooling to room temperature achieved a base pressure of less than 1.30 x lo-‘Pa. The methods of construction and operation of our aluminum evaporator evolved from several unsuccessful attempts to achieve a vapor source which would provide many reproducible doses of aluminum. A cylindrical tube, approximately 1.5 mm in diameter and 5 mm long, was made using 0.06 mm thick tantalum sheet. Five 1 mm long cuts, parallel to the cylinder axts and non-uniformly spaced around the periphery, were made at one end of the cylinder. The tantalum in between the cuts was then bent outward to form five small tabs. The other end of the cylinder was crimped closed, and the resulting container was then put inside a helical coil of 0.25 mm diameter tungsten wire, the tabs serving as stops. Three turns of the tungsten coil enclosed the tantalum container. The tungsten wire extended about 7 mm from each end of the coil and was spotwelded to a degassable V-shaped support, made with 0.50 mm diameter tungsten wire, at each end. The entire unit was degassed in high vacuum, and then a piece of 99.9999% pure aluminum about 1 x 1 x 2 mm in size was placed inside the container, and the entire unit was installed in the FEM envelope in such manner that the container was approximately vertical with open end up, and was located directly below the field-emitter tip. The separation of emitter tip and the top of the container was about 5 mm. After bakeout of the apparatus the aluminum evaporator was degassed without, however, melting the aluminum. Several hours later, when the system had reached base pressure, the aluminum was melted by passing current through the tungsten coil surrounding its container. This invariably caused aluminum to wet the tantalum container and to flow out of the container and onto adjacent parts of the tungsten coil. Thereafter, smaller currents through the tungsten coil were sufficient to cause evaporation of
DEPENDENCE
OF WORK
FUNCTION
ON COVERAGE
FOR
Al/W
651
aluminum, mainly from the aluminum which had previously flowed onto the coil, until this supply was depleted, which typically amounted to about 500 doses. The container was then heated to a higher temperature to again cause aluminum to flow from the container onto the tungsten coil, which regenerated the evaporator for further lower-temperature evaporations. In this manner we achieved a reasonably localized aluminum source which, however, was only approximately reproducible with regard to successive evaporations. After the source was replenished, the amount of aluminum evaporated in each dose (corresponding to heating the source for 15 set) increased. Consequently, several experimental runs were normalized with respect to dosage by assuming that one prominent work function feature, for example, the deep minimum in the case of the (001) plane data consistently occurred at the same dosage, and scaling all other points accordingly. The experiments consisted of (1) qualitative observations of aluminum surface diffusion on the tungsten field emitter tip at various aluminum coverages, during which interesting changes in the electron emission from the (001) planes and (111) regions were noted, and (2) measurements of the change in work function as a function of aluminum doses, determined by the Fowler-Nordheim method2), or more precisely the change in FowlerNordheim slope, for the entire field emission area (average work function) and for the (001) plane and the (111) region. Direct measurements of the total field electron current as a function of emitter voltage were used to determine changes in the average work function, whereas the single crystal plane determinations were done using an external photomultiplier to measure light output from the phosphor screens) at the field electron image of the (001) plane or (111) regions (with all other emission regions masked) as a function of emitter voltage. In order to account for the changes in phosphor efficiency as the emitter voltage was varied, frequent calibration was deemed to be necessary. Calibrations) consisted of simultaneously measuring the total electron emission current and the photomultiplier output current with the entire field electron image in view. The ratio of photomultiplier current and electron emission current as a function of the emitter voltage was plotted and this was used as a calibration curve. Calibration was done before and after each single crystal plane experimental run. Data was rejected if any difference in calibration was detected. An average work function of 4.50 eV was assumed for the clean tungsten field emitter, and the field factor, /3, for the (001) plane and for the (111) region was assumed to beequal to the p value for the average electron emission. The region of the field emission pattern used for the (001) measurements was about 90% of the relatively dark region commonly considered to represent field electron emission from the (001) plane of a clean tungsten emitter. The
652
A. J. MELMED,
J. J. CARROLL
AND R. MFCLEWSKI
region examined was circular with a diameter of about 2 mm. A similar size region was examined for the (111) measurements. Its location was determined by finding the position giving minimum photomultiplier current near the center of the triangle whose corners were { 112) planes. Although the inside of the microscope was not initially coated with an optically reflecting metal film, a coating of aluminum, estimated to be a few tens of nanometers thick, was deposited during the course of the experiments and this probably reduced errors due to light losses6). Thermal equilibration of the deposited aluminum for the average work function determinations was accomplished by heating the emitter tip during deposition. In view of the results of our preliminary experiments, discussed in the next section, we decreased the tip temperature progressively as the number of aluminum doses increased. For the first two doses the tip temperature was maintained at about 700-750 “C, and was then gradually reduced to about 100°C for succeeding doses. In an effort to achieve approximate linearity of coverage as a function of dosage a different equilibration procedure was used for the (001) plane and (111) region measurements. Evaporation of aluminum was done with the tip nominally at room temperature. Prior to each Fowler-Nordheim measurement, however, the tip temperature was raised to about 100°C and maintained at that temperature for 5 sec. Thus, the coverage-dosage relationship is diIferent for the average work function than for the single crystal plane work function determinations. 3. Results and discussion The preliminary experiments in which qualitative observations were made of surface diffusion for various amounts of deposited aluminum demonstrated that the aluminum mobility markedly increased with increasing coverage. Multilayer surface diffusion occurred in the absence of the electron emission field at temperatures near room temperature. This observation has important implications when the difference between our average work function results and those of ref. 1 are discussed below. Fig. 1 shows the results of our determinations of average work function as a function of aluminum dosage. The average work function exhibits a minimum of (4.11_+0.04) eV and reaches a constant value of (4.21 kO.04) eV for heavier dosages. The results of Neumanni) do not show a minimum. His data show the work function monotonically decreasing from 4.50 eV to a constant value of (4.2OkO.05) eV. We think that the absence of a minimum was due to the use of an excessively high equilibration temperature for high coverages, that is, 600~SOOK. For such high temperatures we would expect, on the basis of our surface diffusion observations, that some maximum de-
DEPENDENCE 4.6
,
,
,
,
,
OF WORK ,
FUNCTION
,
,
A
0
4
,
A
OF
FOR
,
ALUMINUM
653
Al/W I
A
n
12
6 NUMBER
,
ON COVERAGE
16
20
’
A
23
DOSES
Fig. 1. Average work function; &, for the entire tungsten field emitter calculated from measured Fowler-Nordheim slopes as a function of the number of Al doses. The initial? was assumed to be 4.50 eV. Data points indicated by the symbols 0, A, 0 represent three separate experimental runs.
posit thickness or density at the emitter tip was reached, beyond which surface diffusion to emitter shank regions precluded further accumulation at the tip. Thus, the work function remained constant at the value corresponding to the maximum deposit thickness, despite further evaporations. Our heavy dosage constant value of (4.21 ItO.04) eV for the average work function agrees well with the limiting value of (4.2OkO.05) eV found by Neumann l), and also with the values of (4.25 kO.05) eV reported by Mitchell and Mitchells), and (4.28 + 0.01) eV by Eastment and Mee4). It is important to note in comparing these results that the measurements of Neumann and the present authors were relative to an assumed work function of 4.50 eV for the initial W substrate, Mitchell and Mitchell assumed a value of 4.56 eV for their W substrate, and Eastment and Mee required no assumption about substrate work function. One difference between the experimental observations reported by Mitchell and Mitchells) and the present work should be noted. They report an inability to completely remove aluminum from their specmens by heating. We believe that our specimens were relieved of deposited aluminum at the end of each sequence of measurements by flashing to >23OOK. This is based on the reproducibility of our Fowler-Nordheim data for the cleaned surface at the start of each sequence. For eight such measurements the deviation in the Fowler-Nordheim slope was f 1.02%.
654
A. J. MELMED,
J. 3. CARROLL
AND
R. MQCLEWSKI
,
0
2
4
6
8
NUMBER
IO
OF ALUMINUM
I2
16
DOSES
Fig. 2. Work function, #J, for the (001) plane of tungsten calculated from measured Fowler-Nordheim slopes as a function of the number of Al doses. Initial 4 value obtained assuming field factor for (001) plane equal to that for the determination of 6 Data points indicated by the symbols 0, n represent two separate experimental runs.
NUMBER
OF ALUMINUM
DOSES
Fig. 3. Work function, #J, for the (111) region of tungsten calculated from measured Fowler-Nordheim slopes as a function of the number of Al doses. Initial 4 value obtained assuming field factor for (I 1I) region equal to that for & determinations. Data points indicated by the symbols 0, n represent two separate experimental runs.
Figs. 2 and 3 respectively show the results for the (001) plane and the (11 I) region measurements. The clean surface values were obtained by assuming that the average p factor prevailed. The actual aluminum coverages are probably not linear functions of the number of aluminum doses, due to the fact that the aluminum mobility increases with coverage. The (001) results, in fig. 2, show that a distinct work function minimum occurs at about 4.26 eV and that the thick layer work function reaches a value of about 4.65 eV. A rather shallow second minimum also reproducibly occurred, as seen at about 16 ahtminum doses. The curve for the (I 1 I) region, in fig. 3, shows a
DEPENDENCE
OF WORK
FUNCTION
ON COVERAGE
FOR
Al/W
655
Fig. 4. Field electron micrographs: (a) clean tungsten, 3900 V;(b) samespecimen, 3900 V, after exposure to 6.4 Al doses followed by a 15 set, about 100°C annealing treatment. The (001) plane in the 6 o’clock position shows a near minimum I$. Also, the (111) region appears to second maximum in $ versus Al does. The Al source was located below and to the right.
complicated behavior. A sharp minimum at about 4.60 eV and two maxima at about 4.95 and 4.80 eV occurred, and the thick film work function leveled off at about 4.36 eV. These work function features were clearly visible in the field electron emission patterns. The most pronounced effect was the work function minimum in the (001) region, and this is illustrated in the micrograph of fig. 4b. Relative to the micrograph the aluminum source was located below and to the right, thus for the low equilibration temperature (1OO’C) used, the coverage is greatest in the regions of the field emitter corresponding to the part of the emission patterns between about 2 o’clock and 6 o’clock. The (001) measurements were made using the (001) plane at the 6 o’clock location and the (I 11) measurements were made using the (111) region at the 3 o’clock location. In the absence of absolute coverage information it is difficult to interpret meaningfully the shapes of the various curves that we have determined. Even if such information had been obtained, non-speculative interpretation would be difficult, if not impossible at present, due to several theoretical deficiencies including the inability to distinguish between surface electronic and geometric effects. One geometric effect is peculiar to the field emission technique, namely the j factor, or field factor which varies with both microscopic and macroscopic geometry. It would seem useful, therefore, to make measurements of work function vs coverage for aluminum on tungsten by a technique (photoelectIon emission, for example) which does not require the use of an electric field.
656
A. J. MELMED,
J. J. CARROLL
AND R.MJ$CLEWSKI
Acknowledgements
The authors wish to thank Mr. J. Smit for his generous assistance with the experimental work. Special thanks are expressed to the International Research and Exchanges Board, IREX, New York, N.Y., for supporting the visit of R. Meclewski to NBS during parts of 1970 and 1971, and to the U.S. PL-480 program for enabling further research cooperation. References 1) H. Neumann, Ann. Physik (Leipzig) 21 1968) 414. 2) R. H. Good and E. W. Miiller, in: Hundbuch der Physik, Vol. 21 (Springer, Berlin, 1956). 3) E. W. J. Mitchell and J. W. Mitchell, Proc. Roy. Sot. (London) A 210 (1951) 70. 4) R. M. Eastment and C. H. B. Mee, J. Phys. F 3 (1973) 1738. 5) M. K. Wilkinson, J. Appl. Phys. 24 (1953) 1203. 6) W. P. Dyke and W. W. Dolan, in: Advances in Electronics and Electron Physics, Vol. 8, Ed. L. Marton (Academic Press, New York, 1956) p. 113.
SCIENCE
45 (1974) 649-656 o North-Holland
Publishing Co.
DEPENDENCE OF WORK FUNCTION ON COVERAGE FOR ALUMINUM/TUNGSTEN
IN THE
FIELD ELECTRON MICROSCOPE A. J. MELMED
and J. J. CARROLL
I~t~t~t~ for ~ute~~ffls Research, NQtion~~ Bureau of Sfa~dar~, Washi~to~, D.C. 20.234, U.S.A.
and R. MQCLEWSKI institute
for Experimental
Physics,
University
of Wroclaw,
Received 5 April 1974; revised manuscript
Wroclaw,
Poland
received 27 June 1974
Aluminum adsorption on tungsten has been investigated by field electron microscope techniques. Changes in average work function and work functions of the (001) plane and (Ill) region were determined as a function of aluminum deposition. The average work function exhibits a minimum of about (4.11 -f: 0.04) eV (relative to an assumed vatue of 4.50 eV for the tungsten substrate) and reaches a value of (4.21 i 0.04) eV for thick deposits. The (001) plane and (111) region exhibit compiicated work function dependences on aluminum coverage. Some quaIitative observations of aluminum on tungsten surface diffusion are also reported.
1. Introduction Aluminum adsorption on tungsten was investigated by Neumann”) using field electron microscope techniques2). He measured the dependence of the average work function on deposition time and found that it decreased monotonically from 4.50 eV (assumed to be the clean substrate value) to a value of (4.2020.05) eV, which remained constant for further deposition. Mitchell and Mitchella) measured the work function of aluminum films of unspecified thickness evaporated onto polyc~stalline tungsten ribbon, using an electron retarding potential technique. They reported a work function of (4.25 kO.05) eV relative to an assumed work function of (4.54 F F0.02) eV for their substrate. Eastment and Mee*), using a photoelectric method, measured the work function of aluminum films on quartz and aluminum substrates. For thick annealed polycrystalline films on quartz their result was (4.28kO.01) eV. In the present paper field electron microscope techniques were used to determine the dependence on aluminum coverage of the average work function and also the work functions of the (001) plane and (I 11) region of 649
650
A. J. MELMED,
J. J. CARROLL
AND
tungsten (field-ion microscopy of thermally shows no plane development for the (111) plane measurements were made because showed unusual work function behavior in
R. MI$CLEWSKI
annealed tungsten emitter tips direction). These single crystal preliminary qualitative-studies the two regions.
2. Experimental These investigations were done in a relatively simple glass field electron microscope (FEM), originally built to investigate surface diffusion, containing a heatable tungsten tip equipped with potential leads for temperature determination and an aluminum evaporator. The microscope was connected to a glass bulb in which titanium was evaporated from time to time, and this in turn was connected to a metal sputter-ion pumped vacuum system. The entire apparatus was baked for several hours at about 350°C and after cooling to room temperature achieved a base pressure of less than 1.30 x lo-‘Pa. The methods of construction and operation of our aluminum evaporator evolved from several unsuccessful attempts to achieve a vapor source which would provide many reproducible doses of aluminum. A cylindrical tube, approximately 1.5 mm in diameter and 5 mm long, was made using 0.06 mm thick tantalum sheet. Five 1 mm long cuts, parallel to the cylinder axts and non-uniformly spaced around the periphery, were made at one end of the cylinder. The tantalum in between the cuts was then bent outward to form five small tabs. The other end of the cylinder was crimped closed, and the resulting container was then put inside a helical coil of 0.25 mm diameter tungsten wire, the tabs serving as stops. Three turns of the tungsten coil enclosed the tantalum container. The tungsten wire extended about 7 mm from each end of the coil and was spotwelded to a degassable V-shaped support, made with 0.50 mm diameter tungsten wire, at each end. The entire unit was degassed in high vacuum, and then a piece of 99.9999% pure aluminum about 1 x 1 x 2 mm in size was placed inside the container, and the entire unit was installed in the FEM envelope in such manner that the container was approximately vertical with open end up, and was located directly below the field-emitter tip. The separation of emitter tip and the top of the container was about 5 mm. After bakeout of the apparatus the aluminum evaporator was degassed without, however, melting the aluminum. Several hours later, when the system had reached base pressure, the aluminum was melted by passing current through the tungsten coil surrounding its container. This invariably caused aluminum to wet the tantalum container and to flow out of the container and onto adjacent parts of the tungsten coil. Thereafter, smaller currents through the tungsten coil were sufficient to cause evaporation of
DEPENDENCE
OF WORK
FUNCTION
ON COVERAGE
FOR
Al/W
651
aluminum, mainly from the aluminum which had previously flowed onto the coil, until this supply was depleted, which typically amounted to about 500 doses. The container was then heated to a higher temperature to again cause aluminum to flow from the container onto the tungsten coil, which regenerated the evaporator for further lower-temperature evaporations. In this manner we achieved a reasonably localized aluminum source which, however, was only approximately reproducible with regard to successive evaporations. After the source was replenished, the amount of aluminum evaporated in each dose (corresponding to heating the source for 15 set) increased. Consequently, several experimental runs were normalized with respect to dosage by assuming that one prominent work function feature, for example, the deep minimum in the case of the (001) plane data consistently occurred at the same dosage, and scaling all other points accordingly. The experiments consisted of (1) qualitative observations of aluminum surface diffusion on the tungsten field emitter tip at various aluminum coverages, during which interesting changes in the electron emission from the (001) planes and (111) regions were noted, and (2) measurements of the change in work function as a function of aluminum doses, determined by the Fowler-Nordheim method2), or more precisely the change in FowlerNordheim slope, for the entire field emission area (average work function) and for the (001) plane and the (111) region. Direct measurements of the total field electron current as a function of emitter voltage were used to determine changes in the average work function, whereas the single crystal plane determinations were done using an external photomultiplier to measure light output from the phosphor screens) at the field electron image of the (001) plane or (111) regions (with all other emission regions masked) as a function of emitter voltage. In order to account for the changes in phosphor efficiency as the emitter voltage was varied, frequent calibration was deemed to be necessary. Calibrations) consisted of simultaneously measuring the total electron emission current and the photomultiplier output current with the entire field electron image in view. The ratio of photomultiplier current and electron emission current as a function of the emitter voltage was plotted and this was used as a calibration curve. Calibration was done before and after each single crystal plane experimental run. Data was rejected if any difference in calibration was detected. An average work function of 4.50 eV was assumed for the clean tungsten field emitter, and the field factor, /3, for the (001) plane and for the (111) region was assumed to beequal to the p value for the average electron emission. The region of the field emission pattern used for the (001) measurements was about 90% of the relatively dark region commonly considered to represent field electron emission from the (001) plane of a clean tungsten emitter. The
652
A. J. MELMED,
J. J. CARROLL
AND R. MFCLEWSKI
region examined was circular with a diameter of about 2 mm. A similar size region was examined for the (111) measurements. Its location was determined by finding the position giving minimum photomultiplier current near the center of the triangle whose corners were { 112) planes. Although the inside of the microscope was not initially coated with an optically reflecting metal film, a coating of aluminum, estimated to be a few tens of nanometers thick, was deposited during the course of the experiments and this probably reduced errors due to light losses6). Thermal equilibration of the deposited aluminum for the average work function determinations was accomplished by heating the emitter tip during deposition. In view of the results of our preliminary experiments, discussed in the next section, we decreased the tip temperature progressively as the number of aluminum doses increased. For the first two doses the tip temperature was maintained at about 700-750 “C, and was then gradually reduced to about 100°C for succeeding doses. In an effort to achieve approximate linearity of coverage as a function of dosage a different equilibration procedure was used for the (001) plane and (111) region measurements. Evaporation of aluminum was done with the tip nominally at room temperature. Prior to each Fowler-Nordheim measurement, however, the tip temperature was raised to about 100°C and maintained at that temperature for 5 sec. Thus, the coverage-dosage relationship is diIferent for the average work function than for the single crystal plane work function determinations. 3. Results and discussion The preliminary experiments in which qualitative observations were made of surface diffusion for various amounts of deposited aluminum demonstrated that the aluminum mobility markedly increased with increasing coverage. Multilayer surface diffusion occurred in the absence of the electron emission field at temperatures near room temperature. This observation has important implications when the difference between our average work function results and those of ref. 1 are discussed below. Fig. 1 shows the results of our determinations of average work function as a function of aluminum dosage. The average work function exhibits a minimum of (4.11_+0.04) eV and reaches a constant value of (4.21 kO.04) eV for heavier dosages. The results of Neumanni) do not show a minimum. His data show the work function monotonically decreasing from 4.50 eV to a constant value of (4.2OkO.05) eV. We think that the absence of a minimum was due to the use of an excessively high equilibration temperature for high coverages, that is, 600~SOOK. For such high temperatures we would expect, on the basis of our surface diffusion observations, that some maximum de-
DEPENDENCE 4.6
,
,
,
,
,
OF WORK ,
FUNCTION
,
,
A
0
4
,
A
OF
FOR
,
ALUMINUM
653
Al/W I
A
n
12
6 NUMBER
,
ON COVERAGE
16
20
’
A
23
DOSES
Fig. 1. Average work function; &, for the entire tungsten field emitter calculated from measured Fowler-Nordheim slopes as a function of the number of Al doses. The initial? was assumed to be 4.50 eV. Data points indicated by the symbols 0, A, 0 represent three separate experimental runs.
posit thickness or density at the emitter tip was reached, beyond which surface diffusion to emitter shank regions precluded further accumulation at the tip. Thus, the work function remained constant at the value corresponding to the maximum deposit thickness, despite further evaporations. Our heavy dosage constant value of (4.21 ItO.04) eV for the average work function agrees well with the limiting value of (4.2OkO.05) eV found by Neumann l), and also with the values of (4.25 kO.05) eV reported by Mitchell and Mitchells), and (4.28 + 0.01) eV by Eastment and Mee4). It is important to note in comparing these results that the measurements of Neumann and the present authors were relative to an assumed work function of 4.50 eV for the initial W substrate, Mitchell and Mitchell assumed a value of 4.56 eV for their W substrate, and Eastment and Mee required no assumption about substrate work function. One difference between the experimental observations reported by Mitchell and Mitchells) and the present work should be noted. They report an inability to completely remove aluminum from their specmens by heating. We believe that our specimens were relieved of deposited aluminum at the end of each sequence of measurements by flashing to >23OOK. This is based on the reproducibility of our Fowler-Nordheim data for the cleaned surface at the start of each sequence. For eight such measurements the deviation in the Fowler-Nordheim slope was f 1.02%.
654
A. J. MELMED,
J. 3. CARROLL
AND
R. MQCLEWSKI
,
0
2
4
6
8
NUMBER
IO
OF ALUMINUM
I2
16
DOSES
Fig. 2. Work function, #J, for the (001) plane of tungsten calculated from measured Fowler-Nordheim slopes as a function of the number of Al doses. Initial 4 value obtained assuming field factor for (001) plane equal to that for the determination of 6 Data points indicated by the symbols 0, n represent two separate experimental runs.
NUMBER
OF ALUMINUM
DOSES
Fig. 3. Work function, #J, for the (111) region of tungsten calculated from measured Fowler-Nordheim slopes as a function of the number of Al doses. Initial 4 value obtained assuming field factor for (I 1I) region equal to that for & determinations. Data points indicated by the symbols 0, n represent two separate experimental runs.
Figs. 2 and 3 respectively show the results for the (001) plane and the (11 I) region measurements. The clean surface values were obtained by assuming that the average p factor prevailed. The actual aluminum coverages are probably not linear functions of the number of aluminum doses, due to the fact that the aluminum mobility increases with coverage. The (001) results, in fig. 2, show that a distinct work function minimum occurs at about 4.26 eV and that the thick layer work function reaches a value of about 4.65 eV. A rather shallow second minimum also reproducibly occurred, as seen at about 16 ahtminum doses. The curve for the (I 1 I) region, in fig. 3, shows a
DEPENDENCE
OF WORK
FUNCTION
ON COVERAGE
FOR
Al/W
655
Fig. 4. Field electron micrographs: (a) clean tungsten, 3900 V;(b) samespecimen, 3900 V, after exposure to 6.4 Al doses followed by a 15 set, about 100°C annealing treatment. The (001) plane in the 6 o’clock position shows a near minimum I$. Also, the (111) region appears to second maximum in $ versus Al does. The Al source was located below and to the right.
complicated behavior. A sharp minimum at about 4.60 eV and two maxima at about 4.95 and 4.80 eV occurred, and the thick film work function leveled off at about 4.36 eV. These work function features were clearly visible in the field electron emission patterns. The most pronounced effect was the work function minimum in the (001) region, and this is illustrated in the micrograph of fig. 4b. Relative to the micrograph the aluminum source was located below and to the right, thus for the low equilibration temperature (1OO’C) used, the coverage is greatest in the regions of the field emitter corresponding to the part of the emission patterns between about 2 o’clock and 6 o’clock. The (001) measurements were made using the (001) plane at the 6 o’clock location and the (I 11) measurements were made using the (111) region at the 3 o’clock location. In the absence of absolute coverage information it is difficult to interpret meaningfully the shapes of the various curves that we have determined. Even if such information had been obtained, non-speculative interpretation would be difficult, if not impossible at present, due to several theoretical deficiencies including the inability to distinguish between surface electronic and geometric effects. One geometric effect is peculiar to the field emission technique, namely the j factor, or field factor which varies with both microscopic and macroscopic geometry. It would seem useful, therefore, to make measurements of work function vs coverage for aluminum on tungsten by a technique (photoelectIon emission, for example) which does not require the use of an electric field.
656
A. J. MELMED,
J. J. CARROLL
AND R.MJ$CLEWSKI
Acknowledgements
The authors wish to thank Mr. J. Smit for his generous assistance with the experimental work. Special thanks are expressed to the International Research and Exchanges Board, IREX, New York, N.Y., for supporting the visit of R. Meclewski to NBS during parts of 1970 and 1971, and to the U.S. PL-480 program for enabling further research cooperation. References 1) H. Neumann, Ann. Physik (Leipzig) 21 1968) 414. 2) R. H. Good and E. W. Miiller, in: Hundbuch der Physik, Vol. 21 (Springer, Berlin, 1956). 3) E. W. J. Mitchell and J. W. Mitchell, Proc. Roy. Sot. (London) A 210 (1951) 70. 4) R. M. Eastment and C. H. B. Mee, J. Phys. F 3 (1973) 1738. 5) M. K. Wilkinson, J. Appl. Phys. 24 (1953) 1203. 6) W. P. Dyke and W. W. Dolan, in: Advances in Electronics and Electron Physics, Vol. 8, Ed. L. Marton (Academic Press, New York, 1956) p. 113.