Control of Epitaxial Growth Plane of Rh Thin Films on A-Plane Sapphire by Sputter Deposition

Abstract

We have expected that the epitaxial growth of (111)Rh and (001)Rh films with a thickness of 100 nm can be controlled on (11 2 0) sapphire by changing sputtering parameters using an ultrahigh-vacuum sputtering system, because the lattice mismatches between the two-dimensional superlattice cells of (111)Rh/(11 2 0) sapphire and (001)Rh/(11 2 0) sapphire systems are the same. Thus, the effect of rf sputtering power (Prf) on each epitaxial growth was examined, and the crystal quality, surface morphology, resistivity and temperature coefficient of resistivity (TCR) of the prepared Rh films were evaluated. As a result, it is revealed that both the (111)Rh and (001)Rh films can be grown epitaxially on (11 2 0) sapphire by adjusting only the values of Prf (deposition rate: Rd) under the condition of the substrate temperature (Ts) of 500 °C. In addition, it is confirmed that the average surface roughness of these epitaxially grown Rh films is below approximately 1.2 nm, and resistivity of approximately 5.6 ×10-6 Ω cm is obtained.

Effect of Substrate Temperature on Epitaxial Orientation of Rh Thin Films Sputtered on A-Plane Sapphire

“…Experimental procedure is the same as the previous paper. 2) As mentioned above, it is expected that the epitaxial orientation of the Rh films may also be controlled by changing the values of T s during the film deposition instead of that of R d (P rf ) in the previous paper, because the effects of the low-deposition-rate condition on the film growth process are equivalent to an increase in T s as reported in the literature. 3) Whereas, under the high-deposition-rate condition, it seems that an explicit transformation of the preferential orientational plane of Rh films is difficult to achieve by changing only T s , because the (111)Rh plane, which is the closest-packed plane of the fcc structure, has the minimum surface energy and tends to grow faster, irrespective of T s , under the high-deposition-rate condition.…”

“…On the other hand, as pointed out in our previous paper, 2) because the sputtered atoms can migrate for a relatively long time on the substrate to arrive at suitable lattice sites under the low-deposition-rate condition, this effect is equivalent to an increase in T s . In addition, as described in the previous paper, 2) if we assume that the two-dimensional superlattice cell of (111)Rh and that of (11 " 2 20) sapphire consist of 14 Â 17 unit cells and 5 Â 11 unit cells, and that the twodimensional superlattice cell of (001)Rh and that of (11 " 2 20) sapphire consist of 24 Â 13 unit cells and 7 Â 6 unit cells, respectively, the lattice mismatches for the two combinations of two-dimensional superlattice cell are approximately the same. Therefore, because the difference in film/substrate interfacial energy between the systems of (111)Rh/(11 " 2 20) sapphire and (001)Rh/(11 " 2 20) sapphire is small, it is expected that the epitaxial growth plane of Rh films on (11 " 2 20) sapphire may be controlled by changing T s instead of P rf as discribed in our previous paper.…”

“…It is well known that electrical properties of dynamic random-access memory (DRAM) and ferroelectric randomaccess memory (FeRAM) capacitors depend strongly on the crystallinity of bottom electrodes. Then, in our recent papers, 1,2) we have examined to realize the epitaxial growth of Rh films as a candidate of the bottom electrodes by adjusting rf sputtering power (P rf ), which is equivalent to the deposition rate (R d ), on A-plane (11 " 2 20) sapphire and (001)MgO under the same condition of T s ¼ 500 C. As a result, it is revealed that (111)Rh film on (11 " 2 20) sapphire is grown epitaxially under the high-deposition-rate condition of R d ¼ 16:0 nm/min (P rf ¼ 50 W), while (001)Rh film on (11 " 2 20) sapphire is grown epitaxially under the low-deposition-rate condition of R d ¼ 1:0 nm/min (P rf ¼ 10 W). 2) Since this implies that epitaxial growth plane of ferroelectric thin film may also be controlled on the same Rh bottom electrodes, the intentional control of the epitaxial orientation of Rh films is supposed to be very interesting not only from the viewpoint of the DRAM and FeRAM applications, but also from that of thin film materials science.…”

“…Therefore, because the difference in film/substrate interfacial energy between the systems of (111)Rh/(11 " 2 20) sapphire and (001)Rh/(11 " 2 20) sapphire is small, it is expected that the epitaxial growth plane of Rh films on (11 " 2 20) sapphire may be controlled by changing T s instead of P rf as discribed in our previous paper. 2) Thus, in the present study, we examined the effects of T s on the epitaxial growth process of Rh thin films on A-plane (11 " 2 20) sapphire with a corundum structure. Experimental procedure is the same as the previous paper.…”

“…Thus, it is concluded that the epitaxial orientation of Rh film on (11 " 2 20) sapphire can be controlled from a mixed state of the epitaxially grown (111) and (001)Rh planes to a single state of the epitaxially grown (001)Rh plane by adjusting only T s instead of R d under the condition of R d ¼ 1:0 nm/ min, using the same ultrahigh-vacuum sputtering system as that used in our previous study. 2)…”