Exploring the subsurface atomic structure of the epitaxially grown phase-change material ${\mathbf{Ge}}_2{\mathbf{Sb}}_2{\mathbf{Te}}_5$

Scanning tunneling microscopy (STM) and spectroscopy (STS) in combination with density functional theory (DFT) calculations are employed to study the surface and subsurface properties of the metastable phase of the phase-change material ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ as grown by molecular beam epitaxy. The (111) surface is covered by an intact Te layer, which nevertheless permits the detection of the more disordered subsurface layer made of Ge and Sb atoms. Centrally, we find that the subsurface layer is significantly more ordered than expected for metastable ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$. First, we show that vacancies are nearly absent within the subsurface layer. Secondly, the potential fluctuation, tracked by the spatial variation of the valence band onset, is significantly less than expected for a random distribution of atoms and vacancies in the subsurface layer. The strength of the fluctuation is compatible with the potential distribution of charged acceptors without being influenced by other types of defects. Thirdly, DFT calculations predict a partially tetrahedral Ge bonding within a disordered subsurface layer, exhibiting a clear fingerprint in the local density of states as a peak close to the conduction band onset. This peak is absent in the STS data implying the absence of tetrahedral Ge, which is likely due to the missing vacancies required for structural relaxation around the shorter tetrahedral Ge bonds. Finally, isolated defect configurations with a low density of ${10}^{-4}{\mathrm{nm}}^{-2}$ are identified by comparison of STM and DFT data, which corroborates the significantly improved order in the epitaxial films driven by the buildup of vacancy layers.

Figure 1

(a) Large scale STM image of GST-225, $V=-0.3\mathrm{V}$, $I=100\mathrm{pA}$, and $T=300\mathrm{K}$; blue line marks the profile line shown in the inset. (b) Atomically resolved STM image with inset at larger magnification, $V=-0.5$ V, $I=100$ pA, and $T=300\mathrm{K}$; blue arrow marks a dark spot in the apparent atomic lattice. (c) DFT simulated STM images of GST-225 with Te termination including SOC, $z_{\mathrm{DFT}}=1.5\AA$, $V=-0.35\mathrm{V}$; the red (blue) arrow marks an exemplary darkest (less dark) spot in the apparent atomic lattice (see text); red triangle marks vacancy surrounded by three octahedrally bonded Te atoms. (d) DFT simulated STM image of GST-225 with Ge/Sb/Vc termination including SOC, $z_{\mathrm{DFT}}=2.3\AA$, and $V=-0.35\mathrm{V}$. (e) Same as (c), but at $z_{\mathrm{DFT}}=5.0\AA$. (f) Same as (d), but at $z_{\mathrm{DFT}}=5.0\AA$. The logarithmic color scale for (c)–(f) shows the integrated LDOS from $E_{\mathrm{F}}$ to $-0.35\mathrm{eV}$.

Figure 2

(a) Atomically resolved STM image of GST-225 after UHV transfer, $V=-300$ mV, $I=50$ pA, and $T=9$ K; colored points mark positions of spectra shown in (b). (b) Scaled $dI/dV\left(V\right)$ spectra recorded at the positions marked in (a) with zoom into the region of the valence band maximum (VBM) as inset, $V_{\mathrm{stab}}=-300$ mV, $I_{\mathrm{stab}}=50$ pA, and $T=9$ K. (c) Map of the voltage at VBM ($V_{\mathrm{VBM}}$) determined as the local maximum close to the VB onset in $d^{3}I/d{V}^3\left(V\right)$ curves (inset), same area as in (a). (d) Histogram of $V_{\mathrm{VBM}}$ (blue bars) with dashed, Gaussian fit curve of $\sigma$ width as marked. (e) Vertical cut through the simulated electrostatic potential $V_{\mathrm{eff}}(x,y,z)$ for randomly distributed bulk acceptors (red dots) at density $N_{\mathrm{A}}=3\times {10}^{26}{\mathrm{m}}^{-3}$. (f) Histogram of the potential values $V_{\mathrm{eff}}(x,y)$ at the surface resulting from multiple simulations (blue bars). Gaussian fit curve with marked $\sigma$ width is added as a dashed line.

Figure 3

(a) Relaxed atomic positions of metastable rocksalt phase of GST-225 of a bulk unit cell: Te atoms (gray), Sb atoms (red), and Ge atoms (orange). The atomic composition of the layers is marked. (b) Relaxed atomic positions in a slab of metastable GST-225 terminated by Te surface layers [space group P$\overline{1}$, same color code as in (a)]. The central bulklike area is shaded in gray. Note that all atomic positions have been relaxed. (c) Radial distribution functions of labeled interatomic distances deduced from several bulk-type calculations as displayed in (a). Panel (d) same as (c), but for the models with Te terminated surfaces as displayed in (b); arrow marks Ge-Te distance of tetrahedral Ge bonding; a Gaussian smoothing has been applied to ease visualization.

Figure 4

(a), (b) Atomic positions of the upper three layers of one of the slabs as in Fig. 3 (a) before and (b) after structural relaxation: Te atoms (gray), Sb atoms (red), and Ge atoms (orange); the coordination of Ge and Sb atoms is highlighted by surrounding, semitransparent polyhedra; octahedral Ge (gray), tetrahedral Ge (orange), and octahedral Sb (red); arrows mark atoms used in (d). (c) Calculated LDOS without SOC for $s+p+d$ orbitals integrated over the area of a tetrahedral (red line, tet) and an octahedral (black line, oct) Ge-Te bond. (d) Same as (c), but for the calculation with SOC and after integration over the volume of surface Te atoms, which are next to Ge atoms. The three tetrahedrally bonded Te atoms are marked by arrows in (b).

Figure 5

(a) Calculated LDOS in vacuum ($z_{\mathrm{DFT}}=7\AA$) integrated over the equally colored diamonds in the inset, SOC included. Inset: atomic structure of the upper three layers of the relaxed GST-225 slab [same as Fig. 4], Te atoms (gray, blue), Sb atoms (red), Ge atoms (orange); diamonds are centered above octahedrally bonded (red) and tetrahedrally bonded (green, blue) Te; orange pyramids surround tetrahedrally bonded Ge; blue Te atoms are under-coordinated and exhibit a peak at the CBM, too. (b) Spatially averaged $dI/dV\left(V\right)$ spectrum, $V_{\mathrm{stab}}=-300\mathrm{mV}$, $I_{\mathrm{stab}}=50\mathrm{pA}$, and $T=9\mathrm{K}$; blue lines mark the averaging interval used in (c). Inset: second derivative of the main curve with marked band gap $E_{\mathrm{gap}}$. (c) Map of $d^{2}I/d{V}^2\left(V\right)$ averaged over $V=0.46\text{–}0.6\mathrm{V}$ [blue lines in (b)]; minimum and maximum positions are marked by a green and a red circle, respectively. (d) Histogram of $d^{2}I/d{V}^2\left(V\right)$ values from (c). (e) $dI/dV\left(V\right)$ at the two extremal positions as marked in (c), $V_{\mathrm{stab}}=-300\mathrm{mV}$, $I_{\mathrm{stab}}=50\mathrm{pA}$, and $T=9\mathrm{K}$.

Figure 6

(a), (b) Simulated STM images of a Te terminated GST-225 slab with disordered subsurface layer, $z_{\mathrm{DFT}}=4\AA$, different $V$ as marked, without SOC; positions of subsurface Ge atoms (yellow crosses) and Sb atoms (red diamonds) are overlaid; lines mark profile lines shown in (c), (d). (c), (d) Profile lines as marked in (a), (b), respectively; note the three different scales, which are directly comparable; peaks are labeled for comparison with the experimental data in (e)–(h). (e), (f) Atomically resolved STM images of the identical area, $I=50\mathrm{pA}$, $T=300\mathrm{K}$, (e) $V=+0.6\mathrm{V}$, and (f) $V=-0.6\mathrm{V}$; black lines mark the position of the scaled profile lines in (g), (h). (g), (h) Profile lines along the lines in (e), (f), respectively, after scaling into $I_{\mathrm{STM}}$ according to Eq. (1); Gaussian fit curves (blue) with maxima marked $I_1-I_4$ are added.

Figure 7

(a)–(c) Simulated $I_{\mathrm{DFT}}(x,y)$ images in logarithmic scale at $z_{\mathrm{DFT}}=4\AA$, $V=-1.0\mathrm{V}$, for three different defect configurations as sketched in (d)–(f), respectively, without SOC; the white parallelogram marks the in-plane unit cell of the calculation (side length $1.68\mathrm{nm}$); the defects are located in the center of the in-plane unit cell; dashed lines show the position of cuts as displayed in (d)–(f). (d)–(f) Atomic structure of the defect configurations of a vertical cut along the dashed line marked in (a)–(c); atomic symbols are labeled in (d); note that some of the displayed atoms are not centered at the dashed cutting line of (a)–(c). (g)–(i) STM images of characteristic triangular protrusions, $V=-0.5\mathrm{V}$, $I=100\mathrm{pA}$, and $T=300\mathrm{K}$; black lines mark profile lines shown in (j)–(l). (j)–(l) Profile lines across the protrusions as marked in (g)–(i), respectively; arrows mark the largest maxima along the line.