CENTROS E INSTITUTOS INTERNACIONALES

Figure 6.1. Structural analysis and transport measurements on GeTe nanowires with defects pre-induced by irradiation with 2MeV He+ ions at different dosages (A) Bright field TEM image showing stacking faults/APBs and dislocation loops induced randomly in a nanowire irradiated with a dosage of 45 µC/cm2 (Inset) DFTEM image of ordered set of boundaries created during electrical pulsing in a representative device, illustrating the difference between random nature of defects created by ion irradiation, and ordered nature of them created by electrical pulsing. (B,C) HRTEM images of a nanowire before ion-irradiation, and after irradiation with a dosage of 100 µC/cm2 showing defect tetrahedra. (d) BFTEM image of a nanowire ion-irradiated with large fluences (1800 µC/cm2). (E,F,G) Zoomed in, DFTEM images of different regions marked in (D), all showing lot of intersecting defects, a structural feature that corresponds to electron localization. 5 n m 5 n m No dosage 5"nm 2 n m 2 n m 2"nm 100"μC/cm2 Defect"(SF)" tetrahedra 5 0 n m 5 0 n m 5 0 n m 5 0 n m ! ! ! ! APBs/SFs Dislocation" loops 45"μC/cm2 50"nm A 2 0 0 n m 2 0 0 n m 200"nm 1800"μC/cm2 D 20 nm ! ! ! E 20 nm ! ! F B C E F G 20 nm ! G 20"nm !

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Upon irradiating as-grown single-crystalline defect-free GeTe nanowires (Figure 6.1B) with He+ ions at modest dosages (40-100 µC/cm2), we observe the formation of dislocation loops, 2D defects (stacking faults/anti-phase boundaries) and defect tetrahedra– formed due to vacancy/interstitial supersaturation (Figure 6.1A, C)14. It is important to identify that these defects are spatially created throughout the nanowire in a random fashion, unlike the ordered set of defects created during the application of low- amplitude voltage pulses (inset of Figure 6.1A) 12. For irradiation at higher dosages (>1800 µC/cm2 or 1016 ions/cm2), however, we observe that entire nanowire is replete with intersecting 2D defects (as illustrated in different regions of a representative nanowire in Figure 6.s 1D-G), hinting that carrier localization effects may dominate transport at this stage12.

To determine whether dirty-metallic and/or insulating states were engineered in as-grown metallic nanowire devices by pre-inducing defects, we performed temperature dependent resistivity measurements on the nanowire devices after exposure at every dosage; and resistivity was evaluated by subtracting the contact resistance measured in a multiple probe configuration (inset of Figure 6.2A) from the total device resistance, followed by multiplying with an appropriate geometric factor (ρ=RA/L, ρ is the resistivity of the material, R is the resistance, A is the cross sectional area of the device, and L is the length of the device) . In the metallic state, the resistivity increases linearly with temperature at higher temperatures, but saturates to a constant value (ρ0, saturation resistivity) below a certain temperature when defect scattering dominates over phonon scattering 12. ρ0, hence, depends on the defect density, and conversely can be used as a measurable metric for defect density. As illustrated for representative devices in Figure

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6.2A, ρ0 increases with increasing dosage (upto ~1015 ions/cm2), indicating an increasing

pre-induced defect density.

Another quantity which is sensitive to the defect density in metallic state, is the slope of temperature-resistivity plots in the linear regime (temperature coefficient of

resistivity, TCR), which generally decreases with increasing defect density 15,16. In all our

devices, however, TCR shows an initial increase (upto 50 µC/cm2), followed by a

subsequent ‘expected’ decrease (Figure 6.2B). TCR in the metallic phase, apart from depending on defect density, also depends upon the carrier concentration and effective

mass, as described in ref. 16. The carrier effective mass increases with increasing defect

density. Also, we confirmed through plasmonic spectroscopy17 (Figure 6.2C), a

reduction in the hole-carrier concentration upon ion irradiation, and pre-introduction of defects. Both the decrease in hole concentration as well as increase in carrier effective mass contribute towards an increase in TCR, and this explains its initial rise. Furthermore, the decrease in hole concentration suggests an intrinsic Ge vacancy

condensation in the creation of extended defects by knock on damage18.

At higher dosages (>1016 ions/cm2), structurally corresponding to intersecting

APBs spatially spread across the entire nanowire (Figure 6.1D-G), resistivity of all the tested devices shows a non-linear decrease with increasing temperature, demonstrating a transformation from a metallic state to a dirty metallic or an insulating state. The exact dosage at which this happens varies from device to device. As illustrated in Figure 6.2D and E, representative device, NW3, undergoes transformation to an insulating state

demonstrating variable range hopping (VRH) conduction (inset Figure 6.2D) at 1.1x1016

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transformation at 2.2x1016 ions/cm2, demonstrating a power law conduction (σ~T0.5)– a characteristic of metals showing weak localization effects19. Thus, pre-inducing defects using ion-irradiation is an ideal recipe to engineer GeTe in stable insulating or dirty metallic.

Figure 6.2: (A) Saturation resistivity (ρ0) plots as a function of dosage on four representative nanowires (NW 1,2,3,4), showing an increase in ρ0 with dosage, in the metallic state. (Inset) Scanning electron microscope image of a representative multiple probe nanowire devices on which transport measurements were performed. (B) Temperature coefficient of resistivity (TCR) plots as a function of dosage on four representative nanowires (NW1,2,3,4), showing an initial increase in TCR followed by a subsequent decrease with dosage in the metallic state. (C)Plasmonic spectroscopy data obtained from 15 nanowires before and after ion irradiation showing a shift in plasmonic peak, and hence a decrease in hole carrier concentration; and this can explain the initial increase in TCR (D) Temperature-resistance plots for NW3 at 700 µC/cm2 (magenta) and 1800 µC/cm2 (green), signifying a metal-insulator transition. (Inset) Variable range hopping (VRH) conduction behavior observed at 1800 µC/cm2, confirming an insulating state. (E) Temperature-resistance plots for NW4 at 1800 µC/cm2 (orange) and 3600 µC/cm2 _{(blue), signifying a metal-dirty metal transition. (Inset) Power law conduction} behavior observed for NW2 and 4 at 3600 µC/cm2 confirming dirty-metallic nature. (F)

NW3 showed a stable value of resistance at 200oC, and this means that a stable insulating

state for operational purposes has been engineered as a starting state for switching.

NW#3 _NW#4 2"μm A B D E 0 50 Dosage!(μC/cm2) C 200oC, NW 3 F

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These starting defect-engineered states showed no change in resistance at 200oC for ~36 hours (Figure 6.2F), suggesting that they are stable for operational purposes. It is important to note here that insulating phase obtained via electrical pulsing was not stable beyond 70oC; and this shows the role of homogenously pre-induced defects in stabilizing the insulating phase.