(Phys. Rev. B 97, 085146 (2018)): ETRI-Wiliam & Marry U. 국제공동연구 [요약]
강상관 모트 절연체는 불균일한 것이 특징이다. 그런데 그 불균일한 물질로 소자를 만들면 재현성이 떨어질 수 있기 때문에 이 것이 문제가 되어 왔다. 모트 절연체 VO2 물질로 나노급으로 불균일성을 조사한 결과, 그 불균일한 정도는 나모급 정도로 작았 다. 또 소자를 만들어서 온도를 올리면서 MIT 현상을 일으키고 다시 온도를 낮추면서 측정한 결과 같은 나노급 이미지를 보여 주었다. 그래서 비록 나노급 불균일성이 있다고 해도 소자에서 재현성이 있음을 처음으로 밝혔다.
It is generally believed that in first-order phase transitions in materials with imperfections, the formation of phase domains must be affected to some extent by stochastic (probabilistic) processes. The stochasticity would lead to unreliable performance in nanoscale devices that have the potential to exploit the transformation of physical properties in a phase transition. Here we show that stochasticity at nanometer length scales is completely suppressed in the thermally driven metal-insulator transition (MIT) in sputtered vanadium dioxide (VO2) films. The nucleation and growth of domain patterns of metallic and insulating phases occur in a strikingly reproducible way. The completely deterministic nature of domain formation and growth in films with imperfections is a fundamental and unexpected finding about the kinetics of this material. Moreover, it opens the door for realizing reliable nanoscale devices based on the MIT in VO2 and similar phase-change materials
[본문 설명]
It is generally believed that in first-order phase transitions in materials with imperfections, the formation of phase domains must be affected to some extent by stochastic processes. The stochasticity would lead to unreliable performance in nanoscale devices that have the potential to exploit the transformation of physical properties in a phase transition. Here we show that stochasticity at nanometer length scales is completely suppressed in the thermally driven metal-insulator transition (MIT) in sputtered vanadium dioxide (VO2) films. The nucleation and growth of domain patterns of metallic and insulating phases occur in a strikingly reproducible way. The completely deterministic nature of domain formation and growth in films with imperfections is a fundamental and unexpected finding about the kinetics of this material. Moreover, it opens the door for realizing reliable nanoscale devices based on the MIT in VO2 and similar phase-change materials. To investigate the above mentioned, we image the patterns formed by coexisting metallic and insulating domains in the thermally driven MIT in a sputtered VO2 film using the technique of scattering-type scanning near-field infrared microscopy (S-SNIM). Fig. 6 shows the S-SNIM images obtained in the same spatial region on heating or cooling run through the phase coexistence regime of VO2 sample. It can be seen that the patterns are static and stable in time. Moreover, phase domains nucleate and grow reproducibly in separate thermal runs through the MIT. Deterministic factors that are “quenched,” or frozen into the film at the time of growth, alone dictate nucleation and domain patterns.
<그림 3> Near-field infrared amplitude images of the same region at different temperatures are displayed. Higher infrared amplitude corresponds to metallic regions, while lower signals correspond to insulating regions. The signals are normalized to the average signal of the completely insulating 329 K image (not shown). (a) and (b) show separate heating runs. (c) and (d) show separate cooling runs.
Figure 3(b)-(h) shows that the nucleation occurs at precisely the same location in each unidirectional, monotonic temperature excursion across the MIT in a given time interval with a probability (P_N) proportional to e^((-∆G_local^*)⁄(k_B T)) [Fig. 7(h)].
This pertains to homogeneous nucleation, which occurs in the homogeneous bulk, as well as heterogeneous nucleation, which occurs at an imperfection such as an atomic defect or a grain boundary. In heterogeneous nucleation, the free energy of forming the critical nucleus can be altered significantly, thereby reducing the barrier to nucleation. Nevertheless, as long as a barrier to nucleation exists, the process is expected to be inherently stochastic. Thus, it is quite surprising that stochastic nucleation is not observed in our experiments.
Thus, we have shown that phase-domain nucleation and propagation are completely deterministic processes in the thermally driven MIT in a sputtered VO2 film. In such films, quenched disorder can be used to reliably control the spatial distribution and propagation of phase domains. Interestingly, nanoscale spatial inhomogeneity in the ultrafast optically driven MIT suggests that our conclusion can be generalized to the MIT driven by optical pulses. Our work provides a platform for further meaningful exploration into reliable nanoscale VO2 electronic and photonic devices.
<그림 4> (a) Macroscopic thermal hysteresis loop measured via infrared transmission through the film-substrate system (upper panel) and dc resistance of the film (lower panel). (b)–(e): S-SNIM images demonstrating the nucleation sites on heating (b) and (c), and cooling (d) and (e) of the same area as shown in Near-field infrared amplitude images. (f), (g) Local hysteresis width (∆T_c) and local phase-equilibrium temperature (T_c^avg) respectively, for the area shown in (b)–(e). White (blue) circles in (b)–(g) serve to guide the eye to some of the nucleation sites which occur on heating (cooling). (h) Schematic of the free-energy landscape for a domain of characteristic linear dimension r for different types of nucleation sites. Here we make the distinction between heterogeneous nucleation and “barrierless nucleation.” Barrierless nucleation is a special case of heterogeneous nucleation where, unlike the more general case, the nucleation barrier is completely removed, and nucleation occurs deterministically.
3) Imaging the Nanoscale Phase Separation in Vanadium Dioxide Thin Films at Terahertz Frequencies
(Nature Communications 9, 3604 (2018)) : ETRI-Columbia 국제 공동연구
[요약]
20nm의 측정영역 초고속 펨토초 측정 기술로 대표적인 MIT 물질인 VO2에서 TeraHertz (THz) 와 Near Infrared (NIR) 두 영역에서 온도를 올리면서 MIT를 측정한 결과 THz 영역에서 나노급 불균일성이 존재함을 실험적으로 보이고 Dynamic MeanField Theory으로 그 정당성을 밝혔다.
Vanadium dioxide (VO2) is a material that undergoes an insulator–metal transition upon heating above 340 K. It remains debated as to whether this electronic transition is driven by a corresponding structural transition or by strong electron–electron
correlations. Here, we use apertureless scattering near-field optical microscopy to compare nanoscale images of the transition in VO2 thin films acquired at both mid-infrared and terahertz frequencies, using a home-built terahertz near-field microscope. We observe a much more gradual transition when THz frequencies are utilized as a probe, in contrast to the assumptions of a classical first-order phase transition. We discuss these results in light of dynamical mean-field theory calculations of the dimer Hubbard model recently applied to VO2, which account for a continuous temperature dependence of the optical response of the VO2 in the insulating state. In Fig. 5, we show the key experimental data of this work, temperature-dependent images at THz and MIR frequencies of the near-field response of a 100 nm thick VO2 film grown on sapphire. The top row of images are taken at THz frequencies, and the images in the bottom row are taken in the MIR with a commercial scattering near-field optical microscopy (SNOM) using a 10 μm CO2 laser source. The MIR images reveal that upon heating, the VO2 sample
<그림 5> SNOM images of the VO2 IMT. The images are taken at THz (top row) and MIR (bottom row) frequencies during a heating cycle. In all images, the signal at every temperature is normalized to the average signal obtained on gold (bright red region in the upper right or right of the image, for THz and MIR respectively). The dashed yellow line denotes the boundary between VO2 and gold regions. The THz and MIR data are S2 and S3, which is the detected signal demodulated at the second and third harmonic of the tip tapping frequency, respectively. Low near-field signal (blue) is measured in insulating regions, while high near-field signal (red) corresponds to a metallic state. The scales are different for the THz and MIR images to highlight the transition from insulator to metal in both cases. Scale bar, 2 μm.
insulating to metallic signal levels through the same temperature region. A histogram representation of the pixel intensity in each image, as shown in Fig. 5, elucidates this distinction.
At temperatures in the middle of the area-averaged transition, the MIR histograms are bimodal. There is an abrupt change in MIR near-field signal between metallic and insulating domains, represented by the separation between the two peaks in the histograms. The pixels in the THz images, on the other hand, are distributed according to a single Gaussian at all temperatures;
there is no clear separation in THz near-field signal between insulating and metallic domains. We plot the mean of each THz histogram as a function of temperature in Fig. 9c as circles, connected by a dashed line as a guide to the eye. The error in the parameter estimation of the fitted histogram is dominated by random noise in the THz-SNOM measurement, which is approximately 5% of the metallic near-field signal level. The THz near-field signal appears to evolve continuously with temperature.
Thus, using a novel THz-SNOM with 130 nm spatial resolution, we find that the nature of the domain formation through the phase transition in VO2 thin films appears homogeneous and continuous at THz frequencies. Moreover, MIR near-field images reveal that the local reflectivity of the insulating or metallic state is changing with temperature below and above Tc. The dimer Hubbard model can provide a framework for understanding of a continuously varying electronic response as revealed by the THz and MIR near-field images that can be consistent with a first order transition.
<그림 6> Histogram analysis of the VO2 SNOM images at THz and MIR frequencies. (a) Pixel intensity histograms of selected THz images shown in Fig. 5. The signal level S is shown normalized to that obtained on gold (SM). (b) Same, for MIR images. (c) Peak signal level as a function of temperature in the THz (circles) and MIR (diamonds) extracted from single or bi-modal Gaussian fits to the histograms. The error bars (s.d.) in the THz are derived from parameter estimation in a non-linear least-squares fit, and are limited by the random noise in the THz near-field signal measurement. The MIR error bars are smaller than the symbols and so are omitted. In the MIR case, there are two peaks at intermediate temperatures due to the bimodal nature of the pixel intensity distribution.