Influence of the Chalcogen Element on the Filament Stability in CuIn

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Functional Inorganic Materials and Devices

Influence of the chalcogen element on the filament stability in CuIn(Te,Se,S)/AlO filamentary switching devices 2

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Tareq Ahmad, Wouter Devulder, Karl Opsomer, Matthias M. Minjauw, Umberto Celano, Thomas Hantschel, Wilfried Vandervorst, Ludovic Goux, Gouri Sankar Kar, and Christophe Detavernier ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 13, 2018

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ACS Applied Materials & Interfaces

Inuence of the chalcogen element on the lament stability in CuIn(Te,Se,S)2/Al2O3 lamentary switching devices ∗,†

Tareq Ahmad,

Celano,

‡

Wouter Devulder,

‡

Thomas Hantschel,

Kar,

†Ghent

‡

‡

Karl Opsomer,

‡ ‡

Wilfried Vandervorst,

Matthias Minjauw,

‡

Ludovic Goux,

and Christophe Detavernier

†

Umberto

Gouri Sankar

†

University, Dept. of Solid State Sciences, Krijgslaan 281 (S1), 9000 Ghent

‡IMEC,

Kapeldreef 75, 3001 Leuven, Belgium

E-mail: [email protected]

Abstract In this paper, we report on the use of CuInX2 (X = Te, Se, S) as a cation supply layer in lamentary switching applications. Being used as absorber layers in solar cells, we take advantage of the reported Cu ionic conductivity of these materials to investigate the eect of the chalcogen element on lament stability. In-situ XRD showed material stability attractive for Back-End-Of-Line in semiconductor industry. When integrated in 580 µm diameter memory cells, more volatile switching was found at low compliance current using CuInS2 and CuInSe2 compared to CuInTe2 , which is ascribed to the natural tendency for Cu to diuse back from the switching layer to the cation supply layer, because of the larger dierence in electrochemical potential using Se or S. Low-current and scaled behavior was also conrmed using C-AFM. Hence, by varying the chalcogen element, a method is presented to modulate the lament stability.

Keywords: chalcogenide, CBRAM, conductive bridge random access memory, selector, volatile lament, thermal stability, electrical functionality, AFM ACS Paragon Plus Environment

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Introduction Currently, ash memory is the most widely used non-volatile semiconductor memory technology in the world, due to its versatility in applications and standardized production processes. 1 Despite the best eorts, scaling of ash introduces problems in memory state retention. Furthermore, the writing process demands high power and voltages. Among many emerging memory types, such as Magnetic Random Access Memory (MRAM) and Phase Change Memory (PCM), Conductive Bridging Random Access Memory (CBRAM) specically adresses the aforementioned problems. 2 Its working principle is based on the formation and dissolution of a nano-sized conductive lament inside an electronically nonconductive switching layer (SL), resulting in two or more distinctive memory states of the memory cell. 3,4 A typical CBRAM cell consists of a solid electrolyte sandwiched between an active electrode (AE) and an inert counter electrode (CE). The AE provides the cations needed for the formation of the lament, often consisting of elements such as Cu and Ag. 5 Tellurides such as Cu0.6 Te0.4 6 can be used as AE, which can be additionaly doped to improve the electrical and thermal stability. 7,8 Materials used as switching layer for lament formation and dissolution typically include suldes (Cu2 S, 9 GeS2 10 ), selenides (Ge0.4 Se0.6 11 ) and oxides (e.g. Al2 O3 12 , SiO2 13 and Ta2 O5 14 ). While chalcogenide switching layers are reported as excellent Cu and Ag ion conductors compared to metal oxides, metal oxides are often preferred due to their ease-of-integration. 15 An important drawback of two-terminal resistive switching memories is that they require a selection component (selector) in series to address these cells during read-out. 5 During read-out of a cell in a high resistive state (HRS), the combined leakage currents from neighbouring cells programmed to a low resistive state (LRS), can lead to falsely believing the addressed device being in an LRS. Hence, these leakage currents should be suppressed, by using a suitable selector in series with the memory cell and an appropriate read-out voltage scheme. 16,17 The insights provided in this paper, can possibly stimulate further research into selectors based on a volatile lament inside a dedicated switching layer. Here, we studied the applicability of CuInX2 (X = Te, Se, S) chalcogenides as AE in lamentary switching devices. These I-III-VI2 chalcogenides have long found applications in solar cells due to their benecial optical absorption coecients and bandgap. 1822 There are several reports on Cu diusion inside these materials, 23,24 and it is suggested that ACS Paragon Plus Environment

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the mobile Cu can contribute to solar cell stability by suppressing the creation of local defects. 25 These reports of Cu diusion provided the inspiration to investigate these materials as potential AE in lamentary switching devices, as the Cu mobility might improve switching characteristics. 26 Note that the In present in these materials probably does not contribute to the formation of the lament, as it migrates much slower than Cu, on time scales order of magnitudes higher than those encountered in CBRAM. 27,28 Secondly, these materials crystallize in a single phase with melting points above the required temperatures for Back-End-Of-Line (BEOL), i.e. 400◦ C, making them attractive for integration. In this work, we examine the thermal stability of thin layers of CuInX2 and the electric functionality as AE in macroscopic CBRAM cells. We show that by proper selection of the chalcogen element a transition from non-volatile to more volatile switching can be obtained. To mitigate current parasitics that can arise with these large cells, electrical characteristics at the nanoscale were probed using conductive AFM, where the high resistance of the tip provides a natural compliance and hence eliminates possible parasitic eects. 29,30

Experimental Section Thin lms of CuInX2 (X = Te, Se, S) were deposited using magnetron sputter deposition. 50 nm of CuInTe2 was deposited by co-sputtering from elemental Cu, Te and In targets using DC magnetron sputtering in a commercial Balzers sputtering tool with a base pressure better than 5·10−7 mbar. The substrates were mounted onto a caroussel, which rotates consecutively in front of the elemental targets, ensuring a homogenous deposition. 30 nm of CuInSe2 and CuInS2 were stationary sputtered from single stoichiometric targets using an RF plasma in a dierent sputtering chamber with a base pressure better than 8·10−6 mbar. The thickness of the layers was conrmed by X-ray reectivity measurements. The stoichiometry of the lms (except that of CuInSe2 ) was measured using X-ray uorescence (XRF) using a Mo source after calibration by Rutherford backscattered spectroscopy (RBS). In RBS, there is an overlap in signal of the scattered ions between In-Te and Cu-Se. Hence, for CuInTe2 and CuInSe2 , no direct XRF-RBS calibration is available. In the case of CuInTe2 , seperate Cu-In and Cu-Te calibrations were available, which combined can be used to quantify this layer. An estimate for the composition ACS Paragon Plus Environment


of CuInSe2 was obtained from the X-ray photo-electron spectrscopy (XPS) signal. This yielded atomic percentages of 25:25:50 for CuInTe2 , 23:21:56 for CuInSe2 and 26:28:46 for CuInS2 . Typically with XPS, there can be a deviation from the calculated composition up to 10%. With RBS-XRF, the largest contribution to composition deviation stems from the used calculation in XRF, resulting in possible deviations up to 1%. XPS was used to probe the oxidation state of Cu in these alloys, using a Theta Probe XPS from Thermo Scientic, operating at a base pressure of 10−10 mbar with an Al X-Ray source. Spectra were calibrated against the adventitious C 1s line at 284.6 eV. Dedicated sputter steps using an Ar+ gun were performed to remove surface contamination and assess bulk properties. The thermal stability of the layers was investigated using in-situ X-ray diraction (XRD). In this way, phase separation and crystallization can be dynamically investigated while the temperature of the sample is ramped up at a xed heating rate. When an alloy segregates in two or more phases during integration, this results in an inhomogenuous supply layer and might result in unpredictable electrical functionality. For these measurements, thin lms were deposited on Al2 O3 (20 nm) / Si substrates. A Bruker D8 Discover XRD was used, which was adapted for automated measurements. After the heating chamber was evacuated and purged, a constant He ow was used. The samples were heated at a rate of 0.5◦ C/s. A stationary Vantec linear detector registered an XRD pattern in a 2θ range of 20◦ every 4 seconds. Layer adhesion and uniformity were investigated with a FEI Quanta 200F FEG Scanning Electron Microscope (SEM) at an acceleration voltage of 20 kV, for both as deposited and annealed samples. Samples were annealed in a He atmosphere at a rate of 0.5◦ C/s up to 400◦ C, after which the sample was allowed to cool down again. The roughness of the layers was investigated using a Bruker Dimension Edge Atomic Force Microscope (AFM) in air in intermittent contact mode with Bruker RTESP-300 AFM probes. The roughness was determined by measuring the root mean square (RMS) height deviations from the mean height plane. To assess the electrical functionality of these layers as AE in macroscopic CBRAM cells, 580 µm diameter memory dot cells were used. They were fabricated by sputtering the active CuInX2 layers through a shadow mask onto blanket Al2 O3 (3 nm) / n+ Si substrates; Paragonswitching Plus Environment here, the Al2 O3 layer served as ACS dedicated layer for Cu. Afterwards, a Pt Top

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Electrode (TE) was sputtered for an improved electrical contact. Current-voltage characteristics were recorded using a home-built automated IV probe system, wich was connected to a Keithley 2601A SMU. At all times, a sweep rate of 0.5V/s was used; the maximum current allowed to the system (i.e. the compliance current Icc applied by the measurement device to restrict the amount of Cu allowed into the switching layer), was varied between 10 µA and 100 µA. The cells were cycled in two steps. First, a positive double voltage sweep was applied to the Pt TE in order to drive Cu into the Al2 O3 layer and subsequently set the cell. Immediately afterwards, a negative double sweep was applied to the Pt TE to disrupt the Cu lament(s) and reset the cell; this was then repeated for a number of cycles. Conductive AFM (C-AFM) measurements were performed on blanket samples in a reverse stack conguration, i.e. rst the AE was deposited on a TiN/n+ Si substrate and afterwards, the switching layer was deposited. A 3 nm thin layer of Al2 O3 was grown using Plasma-Enhanced ALD at 100◦ C. In this way, the Al2 O3 switching layer can be accessed directly by the AFM tip. The samples were then loaded in a Bruker D3100 AFM in air, using durable B-doped full diamond tips (FDT) 31 for optimum electrical contact, reproducability and suitable tip resistance.

Results and Discussion Thermal stability In gure 1, in-situ XRD data of CuInTe2 , CuInSe2 and CuInS2 deposited on 20 nm Al2 O3 , are shown. On the abscissa, the continuously changing temperature is shown, while the 2θ range is depicted on the ordinate. The resulting intensities are plotted in colourscale values: a redder colour denotes a higher intensity. In gure 1 (a), the (112) peak of CuInTe2 in a body-centered tetragonal crystal structure is visible. 32 The same (112) crystallographic plane is also observed in the CuInSe2 and CuInS2 XRD patterns, but it is shifted to higher 2θ values; 33,34 indicating a decrease in interplane distance from CuInTe2 to CuInS2 because of the decreasing size of the chalcogen atoms. A full ex-situ pattern is shown in the supporting information. The onset for full crystalization takes place at signicantly lower temperatures in the case of CuInTe2 compared to CuInSe2 and CuInS2 . All three materials crystallize in single phases with melting points above 400◦ C, which might be benecial for electrical ACS switching Paragoncharacteristics, Plus Environmentwhere phase separation would


result in device-to-device variability.

Figure 1: (a-c) in-situ XRD patterns of (a) CuInTe2 , (b) CuInSe2 and (c) CuInS2 as function of temperature. After deposition, layer uniformity and adhesion of these layers were investigated using SEM and AFM; the results are shown in the supplementary information. As deposited, all samples have a smooth appearance. After heating to 400◦ C, the roughness of CuInTe2 has slightly increased, but also for these samples, no indications for delamination were observed.

Electrical functionality in lamentary switching devices Secondly, the electrical characteristics of these materials as cation supply layer are investigated in 580 µm diameter dot memory cells. In gure 2, the resulting I-V curves are shown ACS Paragon Plus Environment

when the cells are cycled 10 times; for clarity, only the forming, subsequent set and the

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last cycle are shown. Figure 2(a) shows typical I-V curves of CuInTe2 with a compliance current ICC of 10 µA. Before set, the current measured through the cell is very low. Then the voltage is ramped; when a sucient positive voltage is applied to the top electrode, a Cu lament is able to form in the switching layer and the cell is set. The current is then limited by the compliance current to avoid overgrown laments. When the voltage is brought back to 0V, compliance current levels are maintained, typical for non-volatile memory behavior. During reset, initially, the current rises rapidly before dropping drastically to a leakage current level, corresponding to a thermally dominated reset process. 35 Fig. 2(b) shows typical switching behavior of CuInTe2 memory cells with ICC = 100 µA. After the forming cycle, cell programming voltages are lower due to a larger template path formed during the rst sweep and hence, less energy is required to move Cu through the switching layer. This is dierent from Fig. 2(a), where the programming voltages do not dier much upon cycling, which we ascribe to the lower amount of copper passed through the device and hence the amount of damage inicted to the switching layer. Fig. 2(c) depicts the I-V characteristics of CuInSe2 , at a compliance current of 10 µA. Here, more volatile switching behavior is typically found in the rst cycles during sweeping: when applying positive voltages to the TE and returning to 0V in a double sweep, the current has a non-Ohmic characteristic, i.e. the current already starts to drop before reaching 0V, and the lament is not maintained. This enhanced lament dissolution can also clearly be observed in the reset cycle: compared to Fig. 2(a-b), there are less large reset currents during the negative sweep. To further substantiate this claim, a larger number of cells were cycled and the resistance of the cells in each cycle at 0.1V was extracted, before and after set. The cumulative distributions of these data are plotted in gure 3. In Fig. 3(a), cumulative distributions for cells programmed with a compliance current of 10 µA are shown. While the HRS resistance of these cells is roughly the same, LRS resistance among the studied materials varies considerably. The amount of cells programmed to a deep LRS decreases from CuInTe2 to CuInSe2 . In the case of CuInSe2 , this might indicate that (a part of) the lament is already dissolved before applying negative voltages to the top electrode. After a few cycles, more non-volatile behavior is noted, which might indicate that the formed lament was too thick to be dissolved by Cu diusion. Fig. 2(d) depicts typical voltage sweeps for CuInSe2 based CBRAM with ICC = 100 µA. ACS Paragon Environment Here, non-volatile switching is always foundPlus compared to Fig. 2(c), which is related to


the higher compliance current and consequently a stronger lament which is less easily dissolved. During the negative sweep, high currents are passed through the device, which result in abrupt thermal resets. In Fig. 2(e) and 2(f), I-V curves of CuInS2 with a compliance current ICC of 10 µA and 100 µA are shown, respectively. When programmed with ICC = 10 µA, switching of these cells shows an increased volatile behavior compared to CuInTe2 and CuInSe2 AE. This means that in these cases, the lament is even more easily dissolved, before a negative voltage is applied to the TE. When a reset voltage is applied to the TE, a very minor reset current is observed. Only in very few cases, a full reset cycle was observed, indicating a non-dissolved lament, but the volatile behavior was restored in the following cycle. In the inset in Fig. 2(e), a zoom-in of the current between 0V and 1V is shown, plotted in linear scale. At these voltages, the measured current is not completely Ohmic, but rather exponential, which can indicate that the lament is only partly dissolved at this point, by an electromigration process. 26,36 The negative sweep is then used to completely dissolve the lament by an electrochemical process. In gure 2(f), typical I-V cycles of CuInS2 , programmed with a ICC of 100 µA, are shown. The leakage current for cells cycled at ICC = 100 µA increases drastically after the rst forming sweep. In gure 3(a), a clear decrease in cells with a very low LRS can be observed from CuInTe2 to CuInS2 . As there is still a discrepancy in resistance in HRS and LRS for CuInS2 based cells, it indicates that a negative sweep on the TE is still necessary to completely reset the cell and remove residual Cu. Figure 3(b) shows cumulative distributions for cells programmed with a compliance current of 100 µA. Here, the behavior of all materials is CBRAM-like, with median resistive windows of over 3 orders of magnitude for all materials. Several other papers have been published about volatile lamentary bridges, but these mainly discuss Ag-based laments. Using Ag as lament source, it is possible to get a bipolar selector, although the engineered stack is asymmetric. 37 By selecting a suitable programming scheme, i.e. by rst using high forming currents as to deposit some Ag permanently in the switching layer, the cells show volatile lamentary behavior when both positive and negative voltages are applied to the AE. This is because a local deposit is created at the CE, which can be used as an ion source when negative voltages are applied ACS Paragon Environment at the AE. We have not encountered such Plus bipolar selector behavior in our cells. Also

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a Ag-Te/TiO2 stack was reported as a lamentary selector, wherein the Te content was varied. 38 As the Te content increased and the Ag decreased, more volatile behavior was noted. Hence, it was concluded that a lower amount of the active species enhances volatility. In our cells, the Cu content is quite low, which can be benicial for the volatility of our cells. 6 In conclusion, it is deduced from these observations that a low programming current is benecial for volatile switching. However, it can not be the only factor, as volatile switching was not observed for CuInTe2 cation supply layers. Hence, there is also a need for a driving factor for Cu to go back to the cation supply layer. The increased formation energy of the Cu-S (-56.0 kJ/mol) and Cu-Se (-42.7 kJ/mol) bonding, compared to Cu-Te (-25.1 kJ/mol) 39,40 can play a decisive role. Once a lament is formed, Cu prefers to bond with S and Se in the cation supply layer, and hence diuses back to this layer. For low compliance currents, and hence, small laments, this can lead to a disruption of the lament. Furthermore, when observing the dierent IV curves in Fig. 2 more closely, it can be seen that the current does not always go to zero at zero volts. The origin of this eect can be found in cell capacitance and the nanobattery eect: 41,42 there is a built-in eld present in the cell, mainly because of the Nernst potential, a diusion potential and the Gibbs-Thomson potential due to the nanoscopic size of the lament. Next, the electronic band structure can also cause an internal eld, due to the Metal-Insulator-Metal structure. These can lead e.g. to the formation of countercharges (such as (OH)− ), which aid to dissolve the lament. 42

Probing the functionality at the nanoscale using conductive AFM The macroscopic dot cells used in combination with a source-measure unit in this work, are suitable for materials screening due to their ease-of-implementation. However, two unwanted contributions to the lament formation process are present, related to cell size and electronic equipment. The diameter of these cells, 580 µm, is much larger than those of cells integrated in BEOL, and hence a substantial increase in parasitic electrode capacitance can be expected. 30,43 Secondly, the SMU is only able to impose a compliance current at time scales larger than those encountered in CBRAM. 28,44 Between the formation of the lament and setting the compliance current, large current overshoots may exist. This may ACS Paragon Plus Environment

lead to overgrown laments, irrespective of the used compliance current. To mitigate these


Figure 2: (a-c-e) IV curves of CuInX2 macroscopic 580µm diameter dot cells with compliance current ICC of 10 µA and (b-d-f) ICC of 100 µA. In the inset of (e) a close-up of the electrical behavior in linear scale between 0V and 1V is plotted.

Figure 3: (a-b) Cumulative distribution of HRS and LRS resistance measured at 0.1V from DC sweeps for cells programmed with a 10 µA and 100 µA compliance current respectively. ACS Paragon Plus Environment

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problems, here, we have opted to use conductive AFM to probe the electrical properties of these materials at the nanoscale. For these measurements, blanket samples were used: all the functional layers were deposited in a reverse stack, i.e. rst the AE was deposited and afterwards, a 3 nm thin layer of Al2 O3 was grown on top. The tip is then used as a nano-sized inert counter electrode which is able to form, visualize and dissolve laments very locally. 45,46 The typically higher resistance of doped diamond tips compared to metal tips in conductive AFM (up to several MΩ) provides a natural compliance and low current operation. 29 During I-V sweeps, the tip was held xed at a certain location on the sample; this was then repeated for a number of cycles at several sample locations. When the sample is biased positively with respect to the sample, Cu ions are injected into the SL, and a lament is able to form. Examples of the resulting I-V data in linear scale are shown in gure 4. Both CuInSe2 and CuInS2 AE show consistent volatile lamentary switching when only positive sweeps were applied: during retrace, the measured current is already very low at around 1V and does not show Ohmic behavior around 0V, which would indicate the existence of a non-volatile lament. In the case of CuInTe2 , the situation is dierent. While the current drops close to zero at 1V, we still needed to apply a negative voltage to remove residual Cu. However, no large reset currents were measured. To conrm these statements, the cumulative distributions of the forward and backward resistances extracted at 0.1V from DC cycling at several spots are shown in gure 4(b). The memory window of CuInTe2 is larger than those of CuInSe2 and CuInS2 . For the latter, the forward and backward resistances completely overlap, which indicates that almost no Cu remains in the switching layer. In fact, the resistances lie very close to the readout limit of the used amplier. In the case of CuInTe2 , a resistive window up to 2 orders of magnitude can be seen. This indicates that a more conductive path is still present at 0.1V, and a subsequent reset sweep is necessary. The results obtained with C-AFM are in line with results obtained from macroscopic cells, and show that the laments in Se and S based cells are more volatile. We found no distinct dierence between forming and set voltages, in line with the macroscopic results in gure 2(a-c-e). However, a clear shift in the set voltage was observed between dierent materials. This is depicted in gure 5. As the Al2 O3 layer is in all cases of the same thickness, this indicates that a larger voltage over the active layer is needed ACS Paragon Plus Environment to extract Cu; this fraction increases from CuInTe 2 to CuInS2 .


Figure 4: (a) Typical I-V curves taken with conductive AFM. (b) Cumulative distribution of HRS and LRS resistance measured at 0.1V from DC sweeps.

Figure 5: Boxplot of the set voltages for the studied materials extracted from DC IV sweeps using C-AFM. The results obtained with AFM conrm the need for low currents and a suitable driving force (i.e., the tendency of Cu to move back to the AE by forming energetically more favourable bonds) for volatile lamentary switching. Upon applying a positive voltage on the AE, Cu is oxidized from the cation supply layer and starts to form a lament through reduction in the SL. 47 As long as these voltages are high enough to overcome the internal elds, Cu+ is continuously reduced to maintain the lament. When the applied voltage is lowered, for a critical treshold voltage, the tendency for Cu to go back to the switching layer to bind with the more energetically favourable S, Se and Te atoms wins ACS Paragon Plus Environment

over the chemical oxidation and reduction of Cu by the applied electric eld. From the

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moment a gap appears in the lament, the tendency to go back to the supply layer is further enhanced by internal elds, arising from dierent potentials. 42 For elements such as S and Se, the bonding and elds are strong enough for the lament to dissolve more quickly compared to Te, and hence, more volatile switching is found. This oers a way to tune lament stability using CuIn(Te, Se, S)2 /Al2 O3 by changing the chalcogen element. Other ways of tuning lament stability might be achieved by e.g. varying the switching layer. 48 To understand the inuence of the bonding better, the nature of these bonds were studied with XPS.

XPS analysis of Cu oxidation state In many cases, the oxidation state of an element in a bonding can be inferred from the shift in the binding energy of the photo-electron. However, after a core-electron is ejected, the system will relax by screening of the created hole by other orbitals. In the case of Cu, this screening is very ecient, which masks any possible shift in binding energy. 49 This is shown on the abscissa of gure 6. However, it is still possible to acquire some insights in the chemical bonding of Cu by also measuring the Cu L3 M45 M45 Auger line, which is plotted on the ordinate of gure 6. As such, a Wagner plot can be constructed. The dotted lines in the gure have a slope of -1 and are lines of constant Auger parameter α = Ek (Cu L3 M45 M45 ) + Eb (Cu 2p3/2 ). This parameter has the advantage being insensitive to charging of the sample. Based on the most common oxidation numbers of In (+II) and Te, Se or S (-II), we expect Cu to be in the +I oxidation state. The outer electron shell of Cu+ is completely full, and screening will happen non-locally, i.e. by polarization of the orbitals of the ligands. Larger atoms are more easily polarizable, and hence, the screening of Cu by Te is very ecient. A Se atom is a larger atom than a S atom, and consequently screening of Cu by Se is more ecient than S. As there is no shift in binding energy compared to elemental Cu for all these alloys, this indicates that a dierence in the Auger parameter is related to a dierence in ionicity. 49 The Auger parameter of Cu in CuInTe2 is located close to that of elemental Cu, indicating that the bonding is quite metallic. In the case of CuInSe2 and CuInS2 , the Auger parameter is shifted to lower values, indicating that the bonding is more ionic. This illustrates the inuence of the chalcogen element on bonding and hence the switching behavior. ACS Paragon Plus Environment

We have linked the increased volatility of Cu to an increased formation energy and


Figure 6: Wagner plot of the kinetic energy of the Cu L3 M45 M45 Auger electron versus the bonding energy of the Cu 2p3/2 photo-electron for Cu, CuInTe2 , CuInSe2 and CuInS2 . Dotted lines of constant α are also plotted. nanobattery eect in the cation supply layer. More in general, the driving force for Cu to return to the supply layer is determined by the dierence in the electrochemical potential of Cu, which is linked to the activity of Cu in the respective alloys, surface energy, electric elds and the Gibbs free energy of the alloys. From Fig. 2 and Fig. 4 it can be seen that the volatility is the lowest in CuInTe2 based cells. Hence, the electrochemical potential dierence between the AE and the lament is also the lowest. As CuInS2 and CuInSe2 are more volatile compared to CuInTe2 , the net driving force will be higher, which is caused by a subsequently larger dierence in electrochemical potential. This is shown schematically in gure 7.

Conclusions In summary, we have characterized the CuInX2 (x = Te, Se, S) system as cation supply layer in lamentary switching applications. In-situ XRD was performed on these samples, which showed that all of the materials crystallize in a single phase. The nature of the bonding in these materials was investigated with XPS. Here, it was shown that the bonding of Cu in CuInTe2 was metallic in nature, in contrast with Cu in CuInS2 and CuInSe2 , which showed more ionic bonds. Afterwards, these materials were integrated as active electrode in dot Pt/AE/Al2 O3 (3 nm)/n+ Si memory cells. More volatile switching was found in CuInS2 and CuInSe2 based memory cells compared to CuInTe2 , for a current compliance ACS Paragon Plus Environment

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Figure 7: Proposed driving mechanism for Cu to diuse back to the supply layer after forming. The electrochemical potential dierence is the largest in the case of CuInS2 , resulting in the highest driving force. of 10 µA. In contrast, no volatile behavior was found for a current compliance of 100 µA. To conrm low-current operation and to avoid parasitic overshoots, conductive AFM was performed on reverse stack blanket layers. Also in this case, CuInS2 and CuInSe2 AE show volatile switching while for CuInTe2 , a reset sweep was necessary to fully remove residual Cu, although reset currents were negligibly small. A suitable explanation for the increase of lament volatility moving from Te to S based AE is ascribed to diusion of Cu from the lament to the active electrode. This diusion is particularly pronounced in the case of CuInS2 and CuInSe2 . Hence by proper selection of the chalcogen element, a modulation in lament stability is possible.

Acknowledgments T.A. acknowledges funding by FWO Vlaanderen through the Doctoral (PhD) grant strategic basic research (SB), UGENT-GOA-01G01513 and Hercules AUGE/09/014. Olivier Janssens is acknowledged for the SEM measurements.

Supporting Information

SEM micrographs of the investigated samples, before and after

anneal; AFM topography of the investigated samples, before and after anneal

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Figure 8: Table of Contents gure.


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Influence of the Chalcogen Element on the Filament Stability in CuIn

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