RESISTIVITY OF THIN SILVER LAYERS DURING HEAVYION IRRADIATIONVlada Teodosić Citation: Applied Physics Letters 9, 209 (1966); doi: 10.1063/1.1754713 View online: http://dx.doi.org/10.1063/1.1754713 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/9/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Evolution and tailoring of plasmonic properties in Ag:ZrO2 nanocomposite films by swift heavy ionirradiation J. Appl. Phys. 109, 044311 (2011); 10.1063/1.3555593 Resistance switching properties of planner Ag/Li:NiO/Ag structures induced by swift heavy ion irradiation J. Appl. Phys. 105, 073704 (2009); 10.1063/1.3093683 Commissioning of a conformal irradiation system for heavy-ion radiotherapy using a layer-stackingmethod Med. Phys. 33, 2989 (2006); 10.1118/1.2219771 Enhancement of the Performances of AG/BSCCO2223 Tapes by HeavyIon Irradiation AIP Conf. Proc. 711, 620 (2004); 10.1063/1.1774622 Treatment planning for the layer-stacking irradiation system for three-dimensional conformal heavy-ionradiotherapy Med. Phys. 29, 2823 (2002); 10.1118/1.1521938

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Comparisons were made with p+ -p-p+ ohmic sam­ples having the same orientation, configuration, and preparational techniques as above. When there was no obvious evidence of spurious injection, no meas­urable AR was detected and no arc erosion streaks resulted by using voltages up to 50 kV/cm and peak current densities >10,000 A/cm2

• Under these ex­treme conditions, in similar tests, p+ -p-n+ samples blow themselves apart. Samples with n+ -n-n+ struc­tures emit AR and RR in fields -20 kV/cm at 77°K. There was no electrical evidence of hole injection to account for the RR, but under high field condi­tions it is difficult to prove that none occurs. How­ever, even if we concede a minute amount of hole injection this does not explain AR.

One possible explanation for the present effects is that when a large pulse of electrons is injected it decreases the impedance and therefore the voltage drop of the sample near that end. At the same time, holes are injected through the ohmic contact to maintain sample charge neutrality. Thus the two regions move towards one another at their respec­tive velocities and the field becomes concentrated where they meet. Depending on the sharpness of their wave fronts the initial gradient may be in­creased over a small region of the sample by a factor of a hundred or so. After the initial surge, the volt­age at this point could be expected to decrease somewhat but maintain a relatively high field region as long as the current waveform increases.

Knepper and Melngailis7 measured the static potential distribution across p+ -p-n+ InSb laser diodes. They found the highest potential was near the p+ contact. It was associated with hole injection which resulted from the necessity of maintaining sample charge neutrality. This is reasonable in the present case in view of the AR dependence on both Na and the injected current density. Such a poten­tial distribution might explain the presently ob­served high field effects very near the p+ electrode.

However, it does not explain AR and, at higher fields, arcing and chips blown from samples at positions well removed from the p+ electrode.

There is an additional explanation for the AR, assuming that one or both of the above mechanisms are valid and can account for a moderate field in­crease over a significant portion of the sample. If a small number (-e- lO

) of the injected electrons ac­quired energies > 10 times the average energy of the hot carrier distribution, then electron-hole pair production and energetic photon emission would be expected. Considering all losses and inefficiencies of the process, such radiation could be detected by the present apparatus. Such an explanation could then explain why a small amount of AR and RR has been observed in n+ -n-n+ samples. It may also help to explain hot majority carrier infrared radiation from GaAs by Chang, Liu, and Prager.s

I t is probable that all of the above mechanisms play a role in the observed phenomena at various times during each pulse. A more detailed study of the emission efficiency spectrum and measurements of the potential distribution are in progress to help clarify these mechanisms.

The author thanks J. L. Scales of Harry Diamond Laboratories and N. N. Winogradoff of the National Bureau of Standards for many valuable discussions.

1 A. G. Chynoweth and K. G. McKay, Phys. Rev. 102, 369 (1956). 2E. Kamieniecki, Phys. Stat. Sol. 6, 877 (1964). 3L. W. Davies, Phys. Rev. Lett. 4, 11 (1960), also L. W. Davies

and A. R. Storm, Jr., Phys. Rev. 121, 381 (1961). 4]. D. Van Wyk, Solid-State Electron. 8, 803 (1965). 5B. S. Blaisse, A. Van Den Boogaart, and F. Erne, "Problems

of Low Temperature Physics and Thermodynamics," Proc. of International Institute of Refrigeration (Pergamon Press, 1958), p.333.

6E. I. Dupont Co., Wilmington, Delaware: Freon Products Division, Technical Bulletin EL-8.

7R. W. Knepper and I. Melngailis, Solid-State Research 3, 5 (1965), Lincoln Laboratory Report, Lexington, Mass.

S K. K. N. Chang, S. G. Liu, and H.]. Prager, Appl. Phys. Let­ters 8, 196 (1966).

RESISTIVITY OF THIN SILVER LAYERS DURING HEAVY-ION IRRADIATION

(Ag ions; resistivity decrease -+ increase; E)

In this Letter, our first results on changes of re­sistivity of thin silver layers during silver ion irradia-

*On leave from the Electrical Engineering Faculty, University of Beograd, Yugoslavia.

Vlada Teodosic* Institute of Physics, University of Aarhus

Aarhus, Denmark (Received 30 June 1966)

tion will be described. For all thicknesses, ranging from 150 to 400 A, and for all energies of incoming ions ranging from 20 to 500 ke V, a common type of dependence exists. At the beginning of the irradia-

209

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Volume 9, Number 5 APPLIED PHYSICS LETTERS 1 September 1966

tion, the resistivity of a thin silver layer decreases, reaches a minimum, and then increases, as shown in Fig. 1.

The investigation was performed to study the "recoil-nuclei" part of neutron damage. Since 1954, many investigators have used ion beams to investi­gate this recoil-nuclei part of the total neutron dam­age. I - 3 With this method, it is possible to gain more control over the experiments; also, the duration of the experiments is cut down more than 105 times compared to neutron irradiations. As a consequence of the very short ranges of the ions, this type of damage investigation is restricted to thin layers only.

The theory of the conduction of electrical cur­rent states that the simplest cases would be those metals which possess only one conduction band and, further, which have a cubic lattice or which are isotropic (polycrystalline).4 In such cases, the cur­rent conductivity u is a scalar, which simplifies calculations; in all other cases it is a tensor. Alkali metals are not very convenient to handle, but it is a well-known fact that the noble metals differ very little from the "single conduction band" metals. This is the reason for silver being used in this ex­periment.

With silver in a neutron flux (of thermal as well as of fast neutrons), the possible interactions are elastic and inelastic scattering and (n, y) capture. Therefore, all the recoil nuclei are silver isotopes. To simulate this situation, the beam of silver ions was used.

Thin silver layers used in our experiments were produced in a vacuum better than 10-5 mm Hg by evaporation of the very high-purity silver ("for evaporation") at evaporation rates ranging from 5 to

~ 1.00

0.85~-:..z;;...L.----'-----L..-....i...­a 100 200 300 350 Q (Ve)

Fig. 1. The experimentally determined change in resistivity of a thin silver layer as a function of the dose of the impinging silver ions. The beam current was 0.1 /LA, the thickness of the silver layer 150 A, and the energy of the impinging ions 20 keV.

210

loA/sec on glass substrates. During evaporation both the substrate and the vacuum chamber were at room temperature. After evaporation the thin layers were stored for at least one week on dry air. At the vacuum and evaporation rates used in our experiment, residual gases can contaminate thin metal layers, and the oxygen content can be as large as 1-5%;5 but for silver, recent investigations showed no trend towards a difference between the "thin film" density and the density of the bulk.6

Silver irradiations were performed from 20 to 500 keY. For energies up to 80 keY, the Aarhus separator of Scandinavian type was used,7 and for energies from 100 to 500 keY, the Aarhus heavy­ion accelerator.8 During irradiations, the vacuum in the machines was also better than 10-5 mm Hg, and the beam current, ranging from 0.1 to 0.3 #LA, would not produce a significant rise in temperature. By using the sweeping7.8 of the ion beam together with suitable masking, it was possible to obtain a reasonable uniform bombardment of the entire area (20 X 5 mm) of the thin layer.

As was mentioned before, a common type of dependence, resistivity vs dose, exists. With differ­ent thicknesses of thin silver layers and/or different energies of impinging ions, some change in the posi­tion of the minimum or in the size of the relative decrease occurs, but in a qualitative way, all curves are very similar to each other.

Experiments are continued in this area, and a full report will appear at a later date. The subsequent increase in resistivity (after the above-mentioned minimum has been reached) is also very interesting, but before it can be interpreted, more accurate measurements should be performed. In this Letter, only the initial part of the dependence, resistivity vs dose, was described. In Figs. 2 and 3 are shown data concerning the influence of energy and thick­ness upon the change of resistivity. It is evident that the relative decrease of resistivity is larger for smaller thicknesses, and vice versa. For thicknesses larger than 500 A, it is very difficult to recognize any decrease, and for these layers it looks as if no min­imum exists.

The theory of the conductivity of the electrical current in a thin layer, started by Fuchs,9 and fur­ther devloped by Sondheimerlo and Lucas,11 states that the conductivity u is a function not only of its thickness d, but also of the probabilities P and Q for "specular" reflection of conducting electrons on the upper and lower boundary surfaces of the thin layer, respectively, i.e.,

1 u == -= u(d, P, Q) . . . . . . (1)

p

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Volume 9, Number 5 APPLIED PHYSICS LETTERS 1 September 1966

Specular reflection is defined by the following re­lations, connecting velocities "before" (without asterisk) and "after" (with asterisk) reflection (if the thin layer is parallel to the xy plane):

Vx = Vx* ; Vy = V/ ; Vz = -Vz* . . . . .. . (2)

2.0

1.5 0=210'&

• b=JIOA • c=390A

0.7L....L_-L..--L.-L....L...L.Ju..u_--L.--I.-L..1....LLL.LJL.:---I.--I-L.....L..L..

o 1 2 5 10 2 5 10' 2 5 QUJC)

Fig. 2. The experimentally determined changes in resistiv­ity of thin silver layers as functions of the doses of the imping­ing silver ions. The beam current was 0.3 p.A, the energy of the impinging ions 120 keY, and the thickness (as parameter) varies from 100 A to 500 A.

2.0

o l.S a=210A

b=310A o

c= 390A

10 2 S

Fig. 3. The experimentally determined changes in resistiv­ity of thin silver layers as functions of, the doses of the im­pinging silver ions. The beam current was 0.3 p.A, the energy of the impinging ions 400 keY, and the thickness (as parameter) varies from 100 A to 500 A.

Lucas12 showed that it is possible to change these probabilities (P, Q) by an extra evaporation (after the thin layer was being formed), and thus change the resistivity of this thin layer.

Our explanation of the decrease of resistivity at the beginning of the irradiation is that also the above-mentioned probabilities can be changed by heavy-ion irradiation. As shown before, the relative decrease of resistivity goes up as the thickness di­minishes. The Mathiesen rule states that the total resistivity Ptot can be shown as

Ptot = Pideal + Plrnperfection + Pthlckness . . . . .. (3)

and Pthlckness goes up as the thickness diminishes,9-11 i.e., the ratio Pthickness/Ptot goes up.

For some changes of the probability (P, Q), the corresponding change of resistivity apihickness goes up as the thickness decreases; as aPlrnperfection does not depend upon the thickness at all, aptot also goes up as the thickness decreases. In our opinion, the data in Figs. 2 and 3 give the positive answer that the initial decrease of the resistivity Ptot is affected by changing probabilities (P, Q), i.e., this effect is a "surface-type" effect. With higher doses, an increase of the resistivity predominates, produced partly by an increase of Pirnperfection and partly by an increase of Pthlckness as a consequence of a diminishing of the thickness produced by sputtering.

The amount of material deposited during the ion bombardment is small compared to the film thick­ness (100 ILC/cm2 = 10 A), and with energies of incoming ions used in our experiment, sputtering predominates. Thus it is impossible to explain the before-mentioned decrease of resistivity as being a consequence of the increase of the thickness caused by the deposition of the "new" silver from the ion heam.

The Fuchs-Sondheimer-Lucas theory uses a sim­plification, stating that a reflection on the boundary surfaces can be either full-specular or full-diffuse. Thus, the probabilities (P, Q) are, in a sense, inte­gral values which explain the roughness of the boundary surfaces. Our experiments show that during heavy-ion irradiation, the surfaces became more smooth so that the overall coefficients (P, Q) change towards a more specular case. An attempt to develop a new theory is in progress; in this theory, assumptions closer to the real situation will be used. The probability function, which has to replace co­efficients (P, Q), depends on the roughness of the boundary surfaces also, so a change of roughness will produce a corresponding change of Pthlckness,

thus also changing Plot.

211

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Volume 9, Number 5 APPLIED PHYSICS LETTERS I September 1966

The author is indebted to Professor Karl Ove Nielsen for stimulating interest in this work and for valuable discussions. The experiments were per­formed at the laboratories of the Institute of Phys­ics, University of Aarhus. The vacuum chamber, used for evaporation, is placed at the disposal of the Institute of Physics by the Danish State Research Foundation. The author also thanks the people in the accelerator department of the Institute of Phys­ics for very kind hospitality. A Danish State Fellow­ship made possible the author's stay in Denmark.

I U. Croatto and G. Giacomello, Acta 45th Congress of SIPS, Naples, 1954.

2R. M. Lemmon, F. Mazzetti, F. L. Reynolds, and M. Calvin, J. Am. Chern. Soc. 78, 6414 (1956).

3S. Ascoli and F. Cacace, Nucl. Instr. Meth. 38, 198 (l965). 4 A. H. Wilson, A Theory of Metals, (Cambridge Univ. Press,

1954), p. 197. 5 H. L. Caswell in Physics of Thin Films, vol. I, (Academic Press,

N. Y., 1963), p. 52. 6 A. R. Wolter,]. Appl. Phys. 36, 2377 (l965). 7K. O. Nielsen and V. Toft, to be published. "E. Bliigh, P. Dahl, H. E. Jliirgensen. and K. O. Nielsen. "A

600-kV Heavy-Ion Accelerator with Magnet for Isotopical Sepa­ration." Preprint. 1965. To be published.

"R. Fuchs. Proc. Cambridge Phil. Soc. 34, 100 (1938). 10E. H. Sondheimer. Adv. Phys. 1, I (l952). 11 M. S. P. Lucas,]. Appl. Phys. 36, 1632 (l965). 12 M. S. P. Lucas, Appl. Phys. Letters 4, 73 (1964).

HOLOGRAPHIC DIFFRACTION GRATINGS*

Nicholas George and J. W. Matthews California Institute of Technology

Pasadena. California (Received 10 June 1966; in final form 1 August 1966)

From a careful evaluation of a very simple hologram, the photographically produced grating, we have been able to obtain quantitative data on image reconstruction efficiency as a function of processing procedures and illumina­tion parameters such as orientation angle and wavelength. Experimental results are presented which show the de­pendence of efficiency on exposure, both before and after bleaching of the emulsion layer. In addition, the orientation sensitivity of thick-emulsion holograms is studied, and it is shown that maximum diffracted power occurs when the Bragg reflection condition is satisfied.

We have produced diffraction gratings of mod­erate quality by recording a laser-generated inter­ference pattern on high-resolution film. This technique can be viewed as a special case of the two-beam hologram, where both the object beam and reference beam are plane waves. The experi­mental apparatus used to make the diffraction gratings is similar to that reported by Leith and Upatnieks. 1 Earlier work on interferometric methods for the photographic production of gratings is re­ported by J. M. Burch and D. A. Palmer,2 and by A. K. Rigler and T. P. Vogl,3 and the principal methods of production of gratings are described by G. R. Harrison.4

A wide range of grating spacings is available with the two-beam holographic method as both the wavelength and the angular separation of the two beams can be changed. The grating spacing or distance between adjacent fringes in the plane of

*This work was supported in part by the Electronics Division of the Air Force Office of Scientific Research.

212

the emulsion surface is given by

A d = ---,.--.....,....,.--,-::-----,.-2 sin (_(J2_-_

2_(J_l) sin (_(Jl_+_2_(J_2)

where 01 and O2 are the angles which beams (1) and (2) make with the film plate and A is the wavelength of the source. The range of grating spacings avail­able using the more prominent laser lines are shown in Table I for beam separation angles of 1° and 160°, although the beam separation angle can be varied continuously to zero degrees if one desires to make a relatively coarse grating using a relatively short

Table I. Lines/mm with Symmetrical Beam Orientation.

.3440 J.t 50.8

5725.6

.4880 J.t 35.8

4034.8

.6328 J.t 27.6

3110.6

10.6 J.t 1.7

185.8

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