The more stable form of Ge2Sb2Te5 is a layered compound of two units of GeTe
! 45! unit cell of Ge2Sb2Te5 contains 9 layers of atoms, along the [0001] direction, starting and ending with a Te layer, which implies that there should be Te-Te bonds between every two unit-cells. This Te-Te bond is a weak van der Waals interaction, which can play a significant role in the microstructural evolution during amorphization. Another way of looking at this stable structure is to imagine an ordered layer of Ge vacancies between every two unit-cells. Zhang et al., found that vacancies in Ge-Sb-Te films annealed at higher temperatures (closer to equilibrium structure) are more ordered than vacancies in Ge-Sb-Te films annealed at lower temperatures 5. Generalizing these observations we can comment that not all the intrinsic Ge vacancies in Ge2Sb2Te5 nanowires form ordered layers (instead some of them exist as random intrinsic vacancies) if they are not synthesized at conditions close to the thermodynamic equilibrium, as is the case with VLS process- a kinetically controlled growth process. Hence, VLS grown single- crystalline Ge2Sb2Te5 nanowiresmay bethought of as having a large concentration of intrinsic Ge vacancies, some of them forming ordered vacancy planes, and the others random.
Figure 2.4: Equilibrium structure of Ge2Sb2Te5 calculated through density functional theory. Spacing of 3.31 Ao corresponds to van der Waals interaction between Te atoms in those planes. This structure was obtained by relaxing the super-cell structure proposed by Sun et.al, 6 using conjugate gradient algorithm 7. Calculations were performed by Xiaofeng Qian and Ju Li.
! 46!
Figure 2.5 shows the structural characterization of the as-grown Ge2Sb2Te5
nanowires using VLS process described above used for in situ TEM analysis. Selected
area diffraction (SAD) (Figure 2. 5(A)) patterns obtained in the [0001] (c-axis) zone, and HRTEM images (Figure 2. 5(B)), suggest that these nanowires are defect-free single-
crystals 8. A quick comment on the diffraction pattern is that the planes that are in the
zone of [0001] (prismatic planes, or the planes that contain the c-axis) are the ones that show up on the SAD pattern, and hence structural dynamics involving only these planes can be observed during device operations. The six-fold symmetry in the SAD patterns
comes from the six-fold symmetry of the c-axis1.
Figure 2.5: Structural characterization of Ge2Sb2Te5 nanowires in the pristine state. (A)
SAD of the VLS grown Ge2Sb2Te5 nanowires confirming single crystallinity as well as
hcp structure. (B) HRTEM image of the single crystalline nanowires. The arrow indicates
the growth axis. (C) Schematic atomistic picture of the hcp Ge2Sb2Te5 nanowire when
viewed through the c-axis, as is done in the TEM (atomic projection on the viewing
plane) , . Blue arrow represents the growth direction .
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1!It should be noted that the 3-fold symmetry of the [111] axis in cubic structures also shows a 6-fold symmetry in SAD. Fourier transforms always show inversion symmetry.!
a1=1
3[2110] a2 = 1
! 47! 2.4. In situ TEM microstructural studies on GST: Results
A train of voltage pulses with increasing amplitude (programming) separated by 2 seconds was applied on GST nanowire devices, while the structural changes were simultaneously recorded using dark field TEM imaging. Steady-state resistances were measured one second after the application of every voltage pulse using a d.c. bias of 0.02 V. The programming curve (Figure 2.6(A)) shows an initial dip in the value of resistance above 5 V, and a subsequent rise of resistance towards amorphization above 6.7 V. This is a general feature in the programming curve observed across all the devices that were tested. Figure 2.6 (B-J) shows DF images of structural evolution recorded during programming at certain points on the programming curve (indicated in Figure 2.6 (A)). During the initial stages of programming, i.e. upto 5.8 V on the programming curve, DFTEM images show a development of dislocation line contrast (Figure 2.6 (B-E)). Upon increasing the voltage above 5.8 V, it can be observed that these dislocations move in the direction of polarity with the carrier-wind force driving them (Ge2Sb2Te5 is a p- type semiconductor, hence carrier-wind force is hole-wind force). Beyond 6.5 V the dislocation mobility reduced, followed by formation of highly entangled network of dislocations (Figure 2.6 (G and H)). Further accumulation of dislocations in this region results in jamming of the dislocations, and subsequent addition of dislocations to the jammed region increases the resistance of the device, eventually collapsing the structure leading to amorphization (Figure 2.6 (I)). Hence, a huge cloud of dislocations (Figure 2.6 (J)) precedes the amorphous mark (Figure 2.6 (I)). To confirm the polarity dependence of dislocation motion, the nanowire device was programmed with voltage pulses applied with a reversed polarity (Figure 2.6(K)).
! 48! Figure 2.6: Real-time structural evolution of Ge2Sb2Te5 nanowire device during its
operation.(A to J) represents “forward-bias”, (K to T) represents “reverse-bias”, i.e. a reversed polarity. (A) Programming curve under forward bias. Arrows on programming curve are representative points where DF-TEM images are reported (from B to I). (B to I) Snapshots of dark-field DF-TEM images obtained from the movie during electrical switching: (B-E) individual dislocation formation (F-I), dislocations moving in the direction of the white arrow. (G,H) correspond to points where resistance dips. DF-TEM images show evolution of a dislocation cloud. Following the resistance dip regime, amorphization occurred at the dislocation-jamming region (red arrow) in (I). (J) Larger area DF-TEM image of the nanowire device after the amorphization. (K) Programming curve when polarity is reversed. (L-S) Snapshots from a movie recorded during the reverse-bias. The dislocation cloud behind the jamming region was first relieved (L-O); move towards the negative bias, and subsequently jam elsewhere (P-S). (T) Larger area DF-TEM image of the nanowire device after the “reverse-bias” amorphization . Scale bar; (B-I and L-S) 100 nm. (J and T) 500 nm.
Initially, voltage pulses of pulse width 500 ns were applied to the nanowire upto 5 V when the accumulated dislocation cloud during the previous programming event is
! 49! slowly relieved (Figure 2.6 (L-O)). Later on, when voltage pulses of 800 ns were applied the relieved dislocation cloud moved in the opposite direction– until jamming and amorphization take place (Figure 2.6 (P-S)). These set of experiments show the creation of dislocations, their propagation in the direction of hole-wind force, jamming-transition at a region of local in homogeneity, and eventual structural collapse- during programming.
To better visualize the amorphization switching process, we sculpted a notch in a nanowire suspended over the trench (see Figure 2.7(A)) to be able to localize the phase- change process at the notch. Notch is a morphological inhomogeneity at which both the stress and heat (from the voltage pulse) is concentrated, and hence is a very likely region for dislocation jam to take place. DFTEM analysis of a programmed notched nanowire device (Figure 2.7(A)), clearly shows a high density of dislocations predominantly on the positive-electrode side of the notch, reconfirming the polarity dependence of dislocation flow. Also important to appreciate is the fact that the notch– as intended– acted as a geometrical constriction, arresting the flow of the mobile dislocations and allowing for imaging the dislocation jam.
! 50!
Figure 2.7: Dislocation jamming observed on Ge2Sb2Te5 notched-nanowire device. (A)
Dislocation contrast is seen mostly on the positive polarity side of the notched-nanowire device. The negative polarity side is relatively clean. (B) HRTEM analysis of the formed dislocation.