Zener Tunneling Breakdown in Phase-Change Materials Revealed by Intense Terahertz Pulses

We have systematically investigated the spatial and temporal dynamics of crystallization that occur in the phase-change material ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ upon irradiation with an intense terahertz (THz) pulse. THz-pump–optical-probe spectroscopy revealed that Zener tunneling induces a nonlinear increase in the conductivity of the crystalline phase. This fact causes the large enhancement of electric field associated with the THz pulses only at the edge of the crystallized area. The electric field concentrating in this area causes a temperature increase via Joule heating, which in turn leads to nanometer-scale crystal growth parallel to the field and the formation of filamentary conductive domains across the sample.

Figure 1

(a) Experimental setup. The Au antenna enhances the THz electric field over the $5\text{−}\mu \mathrm{m}$ gap. (b) Measured temporal profile of the pulse. (c) Electric field enhancement in the gap. White arrows indicate the electric field directions. (d)–(g) Microscope images of spatial reflectivity changes ($\mathrm{\Delta }R/R$) after THz irradiation as a function of pulse number. The images were obtained by averaging over 450 pulses (225 pulses before and after the corresponding center pulse) and dividing the result with the data obtained without THz excitation.

Figure 2

(a) Comparison of Raman spectra for point $B$ in Fig. 1 (red line), crystalline GST (black line), and amorphous GST (blue line). Arrows indicate the mode positions; the curves are offset for clarity. (b) Spatial map of intensity ratio of Raman signals at (i) 105 and (ii) $148{\mathrm{cm}}^{-1}$. Since Au has a constant signal over a broad region, the ratio is about unity.

Figure 3

(a) Time-resolved reflectivity change for point $B$ [Fig. 1] for different THz electric field strengths (black, $E_o=50\mathrm{kV}/\mathrm{cm}$; blue, $E_o=100\mathrm{kV}/\mathrm{cm}$; red, $E_o=150\mathrm{kV}/\mathrm{cm}$) and optical fluence of $0.12\mathrm{mJ}/{\mathrm{cm}}^2$ (yellow). The inset shows the data for ramp-up or ramp-down in THz field. (b) THz intensity dependence of the peak reflectivity modulation $\mathrm{\Delta }{r}_{\max }/r$ at the time delay of 25 ps in (a). No change was observed for the amorphous region because the electric field enhancement at the probe spot is significantly lower than the electric field enhancement near the antenna. While the latter reaches the crystallization temperature, the temperature of amorphous region at the probe spot increases only by about 3 K [35, 38]. The scale of $\mathrm{\Delta }{T}_L$ is on the right. The data were measured by varying $E_o$ as follows: 150, 120, 100, 80, 50, 90, 110, 130, 60, 70, and $140\mathrm{kV}/\mathrm{cm}$. Because the data line up on a single curve, the sample condition was not altered during the measurements and the repeatability is confirmed. (c) Electric field enhancement in the $5\text{−}\mu \mathrm{m}$ gap during the actual crystal growth indicated in Fig. 1. The electric field is dramatically enhanced at the edge of the crystalline area with a conductivity of $1\times {\text{10\hspace{0.17em}\hspace{0.17em}}}^3\mathrm{S}/\mathrm{cm}$.

Figure 4

THz pulse number dependence of the crystal volume fraction $f(P)$ obtained experimentally at the different locations marked as points $A$ (red square), $B$ (green square), and $C$ (blue square) in Fig. 1. The experimental $f(P)$ is obtained by dividing the reflectivity $\mathrm{\Delta }R(P)$ by the saturated maximum value: $\mathrm{\Delta }{R}_{\max }=\mathrm{\Delta }R(P=20000)$. The red, green, and blue solid curves are fits of the Avrami equation to the experimental values at points $A$, $B$, and $C$ ranging from $P_A$, $P_B$, and $P_C$ to $P_m$. The black curve is a fit using the averaged data measured at the points $A$, $B$, $C$ for $P>P_m$. $P_A=1.1\times {10}^4$, $P_B=1.2\times {10}^4$, $P_C=1.4\times {10}^4$, $P_{\mathrm{av}\mathrm{g}}=1.5\times {10}^4$, $K_A=2.0\times {10}^{-4}$, $K_B=6.6\times {10}^{-5}$, $K_C=1.5\times {10}^{-6}$, $K_{\mathrm{av}\mathrm{g}}=1.1\times {10}^{-8}$, $n_A=0.90$, $n_B=1.05$, $n_C=1.67$, $n_{\mathrm{av}\mathrm{g}}=2.48$. Inset: Mechanism of the crystal growth under applied electric field.