Radiation annealing of epitaxial silver films

+$L

1

0.88 lo-l5

9

wf7

which with oo= 50 nm gives R,-0.1, which accords quite well with Tochitskii and Belyavskii’s’” value of 0.25, considering the approximation in We Thus these results, although on glass, are particularly useful. Firstly they illustrate how specular the surface scattering in silver may be; to remove the specularity roughness has to be produced by overcoating at low temperaturessecondly they show how for films produced, annealed and 737

JR

Sambles, I Co&on

end D Jarvis: Radiation annealing of epitaxial silver films

measured in uhv the resistivity at low temperatures is readily dominated by grain-boundary scattering. This should be particularly true for epitaxial
Experimental Sample production. The silver samples were produced by evaporation in a vacuum of only 10m4 Pa. Using 99.9999% pure silver and a 99.99% pure tungsten basket filament films were produced by evaporation at a rate of 50 nm per minute on to air-cleaved mica and quartz substrates. For most of the experiments the substrates were held at a temperature of 350°C for half an hour prior to deposition, during deposition and for a few minutes afterwards. (The deposition rate and substrate temperature were chosen to give the best RRR after a lengthy series of experiments in which both were varied.) The OFHC copper heater plate was divided vertically by a screen of OFHC copper as illustrated in Figure 1. This allowed up to six equal thickness samples to be made at any one time while only one half were exposed to the radiation from the supplementary tungsten filament. Again for the vast majority of the experiments the tungsten filament annealing was performed after the sample had been allowed to cool slowly to room

Figure 1. Schematic diagram of radiation annealing arrangement

738

temperature. The filament temperature of 1000°C was maintained by a power of 100 W for a period of 100 s. Approximate sample thicknesses were controlled using a water cooled quartz crystal osciliator-aRhough more exact figures were deduced from the resistivity measurements themselves using’ 1

t= PL5-64.2 R 295 -R.,, xi

I

where p& and pi.2 are the intrinsic resistivities for high purity silver, I is the sample length and wits width. This formula assumes no temperature dependent deviations from Matthiessens rule. Strictly this is incorrect but to our desired accuracy of +2% this is no problem. !Sample characterixation. The crystal morphology is examined using RHEED, SEM and TEM of carbon replicas. For mica substrates RHEED gives typical streaked diffraction patterns with strong Kikuchi bands characteristic of flat crystals, while for the quartz substrates the crystal growth is very poor and the patterns are largely polycrystalline rings with some degree of fibre axis. Typical diffraction patterns are shown in Figures 2(a) and 2(b). The scanning electron micrographs indicate flat large grained samples in the case of mica with grain widths of order 1 pm-although this is a very variable and extremely difficult to estimate parameter for the thicker (> 400 nm) films. By contrast the films on quartz are very granular with grain size of order of the sample thickness. Resistance measurements. Most of the resistance measurements involved a very simple dip probe into liquid helium using the four point contact method. This gave only the residual resistivity ratio p295/p4,2.More detailed measurements were obtained using a conventional helium cryostat in which the sample temperature could be controlled over the range 1.5-300 K to a precision of better than 0.1 K. This system is particularly useful for examining in detail deviations from Matthiessen’s rule due to grainboundary and surface scattering3. However the main concern with the present work was with the influence of radiation annealing upon the residual resistivity ratio and consequently by far the majority of measurements involved the simple dip probe. Results The most significant result is the ratio of the mean resistivity ratios of the irradiated films on mica divided by the mean resistivity ratios of the identical thickness unirradiated films. The mean of these values, for films between 180 and 500 nm thick is 1.21 f0.09, the large spread being partially attributed to the vacuum conditions. It was observed that if the vacuum during evaporation rose to lo-’ Pa then the results showed a large scatter with very little consistency and, although working with a vacuum of 10e4 Pa or better avoids this problem to a large extent, still some scatter is observed. Clearly it is ofinterest to repeat these measurements in uhv conditions. Even in 10e4 Pa we were able to produce higher RRR’s than Larson and Boiko using 10m6 Pa. Typically with an RRR of 100 and a p,l., of 0.88 tD m2 one finds an apparent mean free path of 5.5 pm which is much greater than the film thickness. This suggests relatively specular surface scattering and, as estimated above, it indicates domination by grain-boundary scattering. Further evidence for grain-boundary scattering comes from the polycrystalhne films on quartz which never have a resistivity ratio

J R Sambles, I Codson and D Jarvist Radiation annealing

of epitaxial silver films

16.14

-- 20

-9_

x 10

3

--- 2

4, [

l-R,

1

0.88 x lo- I5

400~10-~

or K,=0.3 in accord with previous estimates; being somewhat higher than in the epitaxia! films. It would therefore appear that the changes in RRK found on radiation annealing are associated with an increase in mean grainsize of order 20‘;/,. This is very difficult to confirm from the micrographs because of the poor definition of the grainboundaries and the large range in grain size found in each tilm. Gcncra! indications are howcvcr in accord with the supposition of an incrcascd grain size. A significant test that surface scattering was changing littlc was achieved by exposing the silver films to H,S vapour when no significant resistivity ratio change was recorded. A further test on other influences of annealing was accorded by making the films at room tcmperaturc but with one set of samples continuously exposed to radiation during their production. These ,films gctae I/W tws~ .sfrikitl!l resulfs. The irradiated films on mica, with thickness less than 500 nm, had approximately twice the rcsistivity ratio of the unirradiated films-surprisingly those on quartz only showed an approximately 10% improvement. In these conditions, the diffraction patterns showed a high degree of twinning in both types of sample on mica, as shown in Figure 3, while the micrographs showed the annealed samples to have about twice the grain size.

Conclusions

Figure 2. RHEED patterns at 40 kV for: (a) a 200 nm thick film ofsilvcr. deposilcd al 50 nm min ’ wto a mica subslratc at 350 C. subscqucntly cooled IO room tcmpcraiurc and cxposcd IO radiation; (h) a 320 nm thick fihn ofsilvcr. deposited al 12 nm miC ’ onlo a quark substrate al 300 C. subsequently cooled to room tcmpcrature and exposed to radiation.

Resistivity ratio data on epitaxial films of silver on mica show conclusively that radiation from a tungsten filament held at - lOOO”C, typical of metallic evaporation temperatures, enhances grain growth while not significantly changing the RHEED patterns from the epitaxia! film. This suggests that perhaps epitaxia! growth temperatures are not only functions ofsubstrutc temperature and deposition rate but also of radiation llux arriving from the filament. The effect will be particularly important for metals which are evaporated using high power filaments, c.g. aluminium. This may not only partially explain the large scatter in apparent epitaxia! temperatures but it may provide a simple means for improving crystalline quality in thin metal films. A further tentative conclusion is that even well-annealed largegrained epitaxia! films may have their low temperature resistivities dominated by grain-boundary and not surface scattering. Evidence has already been obtained which shows that this is the case for epitaxial gold films on mica12. It is somewhat surprising to find that much the same is true for silver which is prone to atmospheric

attack.

References

Figure

&posited

3. RHEED pattern at 40 kV for a 265 nm thick film of silver, at 50 mn min ’ on 10a mica substrate at room tcmpcraturc.

higher than 25, even for a 700 nm film. Taking the grain size as the thickness for a 400 nm film of RKK 20 we then have from equation (2)

film

’ D B Tanner and D C Larson, PItvs NW. 166, 652 (196X). ’ D C Larson and B T Boiko, .4/7[jl P1ty.sLctr, 5, 155 (I 964). 3 1 Coulson. D J Jarvis and J R Samblcs. to be oublishcd. 4 M S P Lucas, &/>I Pkys hr. 4, 73 (1964). ’ K L Chopra and M R Randlett. J Appl Phys. 38, 3144 (1967). ” C Pariset and J P Chauvincau. Sur/‘Sci. 78, 478 (197X). ’ D Schumacher and D Stark. SW/‘&. 123, 384 (1982). ” K Fuchs, Proc Co,,th Phil So<, 34, I00 (lY38). ‘) A F Mayadas and M Schatzkcs, P/I!*s KU: f3, 1, 1383 (1970). “’ E T Tochitskii and N M Belyavskii. P/IFS Sr Sol (u). 61. k21 (1980). ” A von Bassewitz and C von Minnigerode. % Phys, 1X1, 368 (1964). ” J R Sambles, KC Elsom and D J Jarvis, Phil Trays H Sot Lund A, 304, 365 (1982). 739