Bi-directional phase transition of Cu/6H SiC( ) system discovered by positron beam study

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1 Applied Surface Science 194 (2002) Bi-directional phase transition of Cu/6H SiC( ) system discovered by positron beam study J.D. Zhang a,*, H.M. Weng b, Y.Y. Shan a, H.M. Ching a, C.D. Beling a, S. Fung a, C.C. Ling a a Department of Physics, University of Hong Kong, Pokfulam Road, Hong Kong b Department of Modern Physics, University of Science and Technology in China, Hefei, Anhui, PR China Abstract The slow positron beam facility at the University of Hong Kong has been used to study the Cu/6H SiC( ) system. The S E data show the presence of the Cu/SiC interface buried at a depth of 30 nm. Keeping the beam energy fixed and sweeping the sample temperature, sharp discontinuities are noted in the S-parameter at both 17 and 250 K. The S-parameter transitions, which are in opposite directions, are indicative of sharp free volume changes that come as a result of the sudden changes in the structure at the Cu/SiC interface accompanying some phase transition. Energy dispersive X-ray spectroscopy (EDXS) room temperature scans reveal the presence of O in addition to Cu, C, Si at the interface, and thus copper oxide phases should be considered in interpreting this new phenomenon. It is suggested that TEM investigation together with temperature dependent X- ray diffraction spectroscopy may be able to shed further light on the nature of this interesting bi-directional phase transition. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Cu/SiC; Phase transition; VEPFIT; EDXS 1. Introduction Recently, metal/semi-insulation structures have attracted increasing interest from those involved in the ultra-large-scale integration (ULSI) processing technology. Compared to Al and other alloys, Cu offers several intrinsic properties such as high thermal conductivity, high melting point and low thermal expansion coefficient [1,2]. On the other hand, SiC in its semi-insulating form presents itself as the obvious substrate material in ULSI SiC technology, required for high-temperature and high-power device applications [3]. The tendency of SiC to react with * Corresponding author. address: zhangjd@hkusua.hku.hk (J.D. Zhang). metals to form carbides and/or silicides at potential device operating temperatures (below 600 8C) has been widely discussed by Porter and Davis [4]. Unfortunately these authors did not study the Cu/SiC system. Other workers have only researched on techniques for depositing Cu on SiC from the materials science point of view [1,2]. In this paper, the positron beam facility at the University of Hong Kong has been used to study the Cu/SiC interface via the Doppler broadening S-parameter by fixing the mean positron implantation depth close to the position (30 nm) of the Cu/SiC interface. On varying the sample temperature, a bi-directional phase transition was seen to occur, the first transition at 20 K shows a drop in S-parameter with increasing temperature and the second at 245 K showing the reverse trend /02/$ see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S (02)

2 J.D. Zhang et al. / Applied Surface Science 194 (2002) Experimental The SiC wafer was purchased from Cree, USA Wafer Technology and had a 1 mm thick n-type epitaxial layer with a carrier concentration of 1: cm 3 and a Si( ) terminated face. The substrate carrier concentration was given as 2: cm 3.A30 5 nm thick layer of Cu was thermally deposited on the Si terminated face in a vacuum of 10 7 mbar. The positron beam experiment consisted of implanting positrons of controlled energy into the Cu on the epitaxial SiC layer. The intensity of the slow positron beam was about e þ s 1, and its diameter was 6 mm. The incident-beam energy was varied from 0.5 to 5 kev in steps of 500 ev and from 5 to 18 kev in steps of 1 kev. A total of counts were collected under the annihilation photo-peak for each positron energy using an HP Ge detector with a resolution of 1.4 kev (FWHM) at 514 kev. The line shape was characterized in the standard way by the low electron momentum fraction parameter S which was calculated in the normal way by dividing the central region of the 511 kev peak by the total peak counts. The sample was mounted on the cold head of a Janus 10 K He closed cycle fridge, and the temperature was controlled by a Lakeshore 330 controller. 3. Results and discussion In Fig. 1, the low electron momentum fraction S is plotted against positron beam energy E for the sample with the temperature ranging from 10 to 275 K. The general behaviour of the S-parameter data over the 0 15 kevenergy range is that S drops from the surface value to the value pertaining to the bulk SiC epilayer of Notably, though, at about 3 4 kev, there is a cusp-like feature in the otherwise exponentiallike fall in S that is strongly suggestive of the presence of an interface. Two-layer VEPFIT analyses [5] (the first layer being the Cu overlayer and the second being the SiC bulk) of the data confirm the location of the interface at nm as expected. All the data are well fitted with a bulk SiC diffusion length of 150 nm. Fig. 1. The low electron momentum fraction S-parameter plotted as a function of the positron implanting energy E for the Cu/n-6H SiC( ) system. The numbers against the arrows show the absolute temperature at which each S E scan was made. The inset shows the 300 K data fitted with 3-layer VEPFIT model as described in the text. The interfacial (layer 2) S-parameter is

3 280 J.D. Zhang et al. / Applied Surface Science 194 (2002) Also noted from Fig. 1 is the fact that S E measurements generally fall along two separate branches in the implantation energy range from 1 to 8 kev. It is clearly the cusp region, resulting from positrons annihilating at the Cu/SiC interface, that is responsible for the pulling up of the upper branch to higher S values. It is interesting to note the temperatures corresponding to the lower and upper branches. For upper branch one has 10, 12, 250, 275, 300 K, while for the lower branch one has the continuous range 13, 15, 25, 50, 100, 150, 200, 225, 245, 248 and 249 K. The abrupt nature of the transitions around 20 and 250 K is made clearer by fixing the beam energy at 4 kev (where positrons are implanting maximally into the interfacial region) and plotting the value of S as a function of sample temperature. Such a plot is shown in Fig. 2 where the reversible and bi-directional nature ðhi! Lo! HiÞ of the transitions are now clearly seen. To check to see just how abrupt the low temperature transition in S was, K temperature scans in both directions were made. The results are shown in Fig. 3. The problems faced here was that the temperature sensor in our equipment was about 10 cm away from the sample and the thermal response time of the cold head was not well known. To overcome this problem, we steadily increased the time of sweep. On the first sweep ðlo! HiÞ 30 min intervals were considered and the transition temperature appears at 22.5 K. On the second sweep ðhi! LoÞ made with 60 min intervals the transition temperature was significantly lower at 12.5 K. The final sweep ðlo! HiÞ made with 10 h intervals did not follow the data of the earlier Lo! Hi sweep suggesting that the thermal response time was in the order of a few hours. Moreover these hysteresis curves tend to suggest a low temperature transition point perhaps closer to 15 K rather than 20 K. It is believed that the most plausible explanation of the observed Hi Lo Hi bi-stability in S is that of a reversible phase transition. A sudden change of S can really only be caused by a sudden structural change in some component of the Cu/SiC system. The question arises as to which component this is? Here some digression is necessary to try to ascertain more information about our system. From the VEPFIT analyses carried out on the data it is difficult to be absolutely sure if the structural change is occurring in the Cu overlayer (or phases deriving from the Cu in the overlayer see below) or from the Cu/SiC interface itself since both the 2- and 3-layer fittings could be Fig. 2. A clear view of the bi-directional phase transition in the Cu/n-6H SiC( ) system as seen by plotting the low electron momentum fraction S-parameter against the sample temperature keeping the positron implantation energy fixed at 4 kev.

4 J.D. Zhang et al. / Applied Surface Science 194 (2002) Fig. 3. The low electron momentum fraction S-parameter plotted against the sample temperature for low temperature phase transition in the Cu/n-6H SiC( ) system. The experiment was started at 10 K with 30 min time intervals between collecting the data. This was followed sequentially by the 60 min and 10 h data. carried out with equally good chi-squares. In both the cases the primary interface was taken at a depth of 30 nm, however in the 3-layer model the interface was produced from a layer of finite width stretching from 28 to 32 nm deep, having a 1 nm diffusion length to simulate close-to-complete positron trapping at the terminated SiC surface. When two layers were considered, the effect required that both the overlayer S-parameter and the diffusion length vary with the sample temperature. However for the 3-layer model, it was sufficient solely for the S-parameter of the interfacial layer to vary with the positron diffusion length and S-parameter fixed at 30 2 nm and 0:45 0:01, respectively, in the overlayer. (In all VEPFIT analyses 2- and 3-layer the bulk and surface S parameters were fixed at 0.47 and 0.515, respectively.) EDXS scans of the Cu film revealed that oxygen was present in the film suggesting that Cu n O phases were existing on the surface of the sample because copper cannot easily react with Si or C. Copper oxide phases should thus be considered as possible candidates in interpreting this new phenomenon, as also should the SiO 2 layer terminating the Si( ) surface of the SiC, since the positron analysis makes it unclear as to where exactly the location of the structure undergoing the reversible phase transition is. However strong arguments can be put forward supporting the view that phase transitions that we are seeing are in fact coming from the Si( ) terminating SiC interface and not from the Cu/Cu n O overlayer structure. We argue that the Si( ) terminating SiC interface is not likely to be very transmitting towards positrons since there are bound to be oxygen bonds at the interface that will lead to non-epitaxial structures above it. Positrons will most likely experience grain boundaries and open volume sites at the interface. The 2-layer concept, relying as it does on a freely transmitting interface, although capable of giving a reasonable VEPFIT result is thus highly improbable. This is supported by the fact that the 2-layer model requires two varying and correlated parameters for a good fit, whilst the 3-layer model just requires one. The positron data have been helpful in leading us to conclude that we have a phase transition in a structure very close to the SiC termination. This interfacial structure may be of the general form Cu n O/SiO m /SiC. It would be desirable to carry out temperature dependent X-ray diffraction studies on this type of sample to

5 282 J.D. Zhang et al. / Applied Surface Science 194 (2002) study whether this phase transition can be seen by another method, and to see if the structural change is associated with the overlayer. If the phase transition is really closely associated with the SiC termination, then X-ray diffraction may be unable to detect it. TEM analysis would also be very helpful to find out more about the crystalline phases present in the sample for full elucidation of the effect reported in this paper. References [1] P. Yih, D.D.L. Chung, J. Mater. Sci. 31 (1996) 399. [2] M.K. Lee, H.D. Wang, J.J. Wang, Solid-State Electron. 41 (1997) 695. [3] H. Morkoc, S. Strite, G.B. Gao, J. Appl. Phys. 76 (1994) [4] L.M. Porter, R.F. Davis, Mater. Sci. Eng. B34 (1995) 83. [5] A. van Veen, A.C. Kruseman, H. Schut et al., Mater. Sci. Forum (1997) 76.

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