ECS Journal of Solid State Science and Technology, 4 (8) Q83-Q91 (2015) Q83

Duality in Resistance Switching Behavior of TiO2-Cu2ZnSnS4DevicePrashant K. Sarswat,a,z York R. Smith,a Michael L. Free,a,∗ and Mano Misraa,b

aDepartment of Metallurgical Engineering, University of Utah, Salt Lake City, Utah 84112, USAbDepartment of Chemical Engineering, University of Utah, Salt Lake City, Utah 84112, USA

The resistive switching behavior of TiO2 based electronic devices have been extensively investigated the last two decades due to theirlow cost and potential applications in high speed electronic devices. However, the randomness in switching behavior and uncertaintyin conducting filament formation often restricts their usage for long-term applications. A duality in current transport mechanism wasobserved when nano-architectural parameters of the TiO2 memory resistance device were altered. TiO2 nanotube-CZTS nanocrystalsbased devices exhibit Fowler-Nordheim quantum tunneling, whereas TiO2 nanocrystals-CZTS thin film based devices exhibit spacecharge limited conduction. Temperature dependent electrical studies indicate that polaronic transport is one of the phenomenaassisting in electrical conduction. Electronic band information suggests that tunneling between CZTS and TiO2 is indirect. A uniqueinterface state between two adjacent materials seems to be responsible for the negative differential resistance (NDR) behavior ofTiO2-CZTS diodes.© The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative CommonsAttribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in anymedium, provided the original work is properly cited. [DOI: 10.1149/2.0071508jss] All rights reserved.

Manuscript submitted March 11, 2015; revised manuscript received May 13, 2015. Published June 12, 2015.

Low cost electrical bistability due to nonlinear conducting behav-ior has been an exciting area of research in recent years.1–8 Several newdevices such as tunnel diodes, multivibrators, and organic bistable sys-tems have been investigated.1–7 Some of these devices possess uniqueI-V characteristics and non-conventional current transport mecha-nisms. Devices showing negative differential resistance (NDR) can beutilized for various low power applications such as radio frequency,logic and neuromorphic devices, fast switching, and high frequencyoscillators.1–7,9–11 Materials such as carbon nanotubes, graphene, andconducting polymers have been recently utilized for room temper-ature NDR behavior or memory resistance applications.1–7,9,10,12,13

Memory resistors can be fabricated using semiconductors and metaloxides such as Nb2O5, NiO, ZnO and TiO2.14 Thin film memory re-sistors offer advantages such as high switching speed and low energyelectroforming. The use of thin film TiO2 in a memory resistor hasdemonstrated potential dependent on/off switching when sandwichedbetween two platinum contacts.15 It is generally believed that a mem-oresistance effect in TiO2 nanostructures is based on the mobility ofvacancies or defects.15–17 The conduction mechanism is based on theformation and transport of conducting filament (CF). A conductingfilament generally consists of oxygen vacancies that form or breakdue to electric field driven migration. Because of randomness and un-certainty in CF’s formation, resistance-switching (RS) devices oftenperform randomly and thus have less control over operating parame-ters. Another report suggests that Joule heating is also a major causefor current controlled negative resistance behavior.18 This behaviorrelies on polaronic transport in TiO2. In polaronic transport, the hop-ping motion of small polarons up to certain temperatures influencesconduction. Several other mechanisms and switching modes such asion migrations, filament type valance charge memory (VCM) forma-tion, interface type VCM formation, and charge trapping /de-trappingare other possible explanations for resistance switching behavior.19

Another possible mechanism is tunneling that works on the principleof tunnel junction formation between highly doped p-type and n-typesemiconductors. In this paper, we have utilized highly doped TiO2

(n-type) and Cu2ZnSnS4 (CZTS) (p-type) semiconductors for M-I-S-M (metal-insulator-semiconductor-metal) junction fabrication. Suchdevice architecture provides enhanced flexibility to achieve desiredcurrent-voltage characteristics, more uniformity in switching, and pro-vides resistive switching that is based on electronic phenomena. Theuse of TiO2 nanotubular structure achieved by an anodization methodallows for robust control in wall thickness, tube diameter, and oxide

∗Electrochemical Society Active Member.zE-mail: prashant.sarswat@utah.edu

thickness.20 The 1D architecture provides an operating channel forcurrent flow and has been examined in many technical applicationssuch as solar energy conversion,21 energy storage,22 VOC sensing,23

and biomedical devices,24 for example. An emerging low cost alterna-tive p-type absorber material for photovoltaic cell fabrication, CZTS,possesses attractive features such as utilization of abundant metalsand a high absorbance coefficient.25–28 Although there are reports thatdiscuss CZTS-TiO2 nanostructures and dye sensitized solar cells,29,30

investigations of CZTS-TiO2 memory resistance behavior has yet tobe fully examined to date.

Materials and Methods

Synthesis of titania nanotube array/titania nanoparticles (T-NTA/TNP).— Titanium foils (0.2032 mm thick, ESPI Metals, G1grade) were briefly polished with Emory paper and degreased in ace-tone/isopropanol by sonication for 5–10 min and cut into coupons ofapproximately 1.5 × 1.5 cm. The Ti coupons were then subject to so-noelectrochemical anodization as previously reported.31 In summary,the Ti coupons were anodized at 60 V (D.C.) for 1 h in a fluorinatedethylene glycol electrolyte (0.5 wt.% NH4F + 2 wt.% H2O) wherethe electrolyte bath temperature was maintained at 30◦C. A Pt meshserved as the counter electrode. After sonoelectrochemical anodiza-tion, the anode was rinsed with DI water followed by a brief sonication(2–5 min) in DI water. The anodes were then dried in vacuum overnight at 110◦C. After drying, the anodes were annealed at 350◦C for2 h in air (∼2◦C/min ramp rate). Deposition of a titania nanoparticle(TNP) layer can be used to increase the surface area of T-NTA andhas been shown to improve the deposition of the other nanocrystals,for example.32 Following reported protocol,33 TNPs were depositedby soaking the annealed titania naotubes in a 0.2 M TiCl4 ethanolicsolution at 65–70◦C for 20 min followed by rinsing with ethanol anddeionized (DI) water and additional annealing at 450◦C for 2 h.

Deposition of CZTS layer.— Nanocrystals of CZTS were synthe-sized using a previously developed method34 and dispersed in hexane.Prior to depositing CZTS, the T-NTA/TNP sample was soaked in1, 2-ethanedithiol (Fluka, ≥90%) for 24 hours, which served as aligand and to anchor the CZTS to the T-NTA/TNP sample. After1, 2-ethanedithiol treatment, the sample was then rinsed with dryhexane and a dilute CZTS/hexane solution was spin coated overthe T-NTA/TNP. After spincoating, the T-NTA/TNP/CZTS samplewas washed with hexane followed by drying under vacuum (110◦C)overnight before use.

Growth of CZTS layer and TiO2 deposition.— Thin films (TF)of CZTS were grown on the fluorinated tin oxide (FTO) substrate

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Q84 ECS Journal of Solid State Science and Technology, 4 (8) Q83-Q91 (2015)

Figure 1. Raman spectrum of (a) TiO2 NT, where three distinct peaks corresponding to anatase phase of TiO2 can be seen. Raman spectrum of (b) CZTSnanocrystals. A strong peak at ∼ 336 cm−1 perfectly matches with earlier reported Raman spectra of CZTS crystals.

using electrodeposition-sulfurization method. Metallic layers weredeposited using an electrolyte containing ions of interest (Cu2+, Zn2+,and Sn2+).35 Such films were annealed in an evaporated sulfur envi-ronment during continuous supply of argon to obtain CZTS thin film.Other details regarding experiments and film characterizations can befound in earlier reports.34,35 TiO2 nanocrystalline films (NCF) werethen deposited over the CZTS film using a sol-gel method.36 Anneal-ing was performed at 325◦C for 1 h in an inert environment usingargon purging.

Instrumentation and Characterization

Raman spectra of thin films, nanotubes, and nanocrystals wererecorded using a R 3000 QE Raman spectrometer in backscatter-ing mode. Raman spectra measurements were recorded using a laserwith a wavelength of ∼785 nm and a power of ∼95 mW. The wave-length stability for the Raman spectrometer was less than 1 cm−1

over a 12 h period. Energy band gap measurements were done usingdiffuse reflectance data obtained from a UV-3600 UV-Vis-NIR spec-trophotometer (Shimadzu, Kyoto, Japan) equipped with UV-Probesoftware. Scanning electron microscopy was performed using a fieldemission SEM (Hitachi, S-4800). Primary electrical characterizationwas performed using a Gamry Instrument Reference 600 Potentiostatequipped with Virtual Front Panel software. For assembly of a diodedevice, silver contacts were made on top and bottom of the elec-trode, with an effective area of ∼8 mm2. A two-point probe methodwas adopted for all electrical characterization. High frequency signalmeasurements, memory retention, and endurance examination wasperformed using USB multichannel Oscilloscope, a function genera-tor, memory resistor, load resistance, and a data acquisition system.

Results and Discussion

Raman spectroscopy.— A Raman spectrum of TiO2 nanotube sub-strates was investigated prior to CZTS deposition (Figure 1a). A dis-tinct band, showing three strong peaks is visible corresponding toanatase phase of TiO2. Three modes were obtained at ∼ 395 cm−1

(B1g), 516 cm−1 (A1g) and 640 cm−1 (Eg) and they match earlierreported Raman spectra of TiO2.37 Raman spectroscopy was also con-ducted on the TiO2 substrate coated with CZTS crystals. It can be ob-served that TiO2 thin film coated with CZTS crystals (Figure 1b) showsthree distinct Raman peaks or shoulders at ∼ 338 cm−1, 370 cm−1

and 298 cm−1 that match with earlier reported CZTS Ramanspectra.27,34,35

Surface morphology examination.— Scanning electron micro-graphs of ordered TiO2 nanotubes and T-NTA coated withother nanocrystals are shown in Figure 2. For plain T-NTA(Figures 2a, 2b) it can be observed that the inner diameters of thenanotubes are 100∼130 nm, with a wall thickness of ∼10 nm and film

thickness of ∼ 10 μm (see inset). Figures 2c, 2d show TiO2 nanotubecoated with T-NP. It can be seen that the tubes are uniformly coatedwith T-NP throughout its axial direction, outside as well as inside thetubes. An estimated average thickness of nanocrystals was ∼5 nm.Figures 2e, 2f show a T-NT/T-NP/CZTS assembly. It can be seen thatmost of the CZTS nanocrystals or their agglomerates deposit on thetops of nanotubes, while the thickness of CZTS nanocrystals film de-creases along the length of the nanotubes. A representative diagramof the T-NTA/T-NP/CZTS assembly is shown in Figure 3.

Electrical characterization.— An interesting observation that aTiO2-CZTS system provides is the variety of architecture dependentI-V characteristics, which is investigated and discussed in this section.Mainly, the I-V characteristics of FTO/CZTS TF/TiO2 NCF and T-NTA/T-NP/CZTS assemblies were examined. A strongly non-linearI-V behavior was observed for both (FTO/CZTS TF/TiO2 NCF and T-NT/T-NP/CZTS) samples. For the FTO/CZTS TF /TiO2 NCF device(Figure 4a) it can be seen that during the positive direction voltagesweep, transition from low resistance to high resistance states occurs

Figure 2. (a) and (b) SEM images of TiO2 nanotubes; (c) and (d) SEM imagesof TiO2 nanotubes coated with TiO2 nanocrystals; (e) and (f) SEM image ofTiO2 nanotubes coated with TiO2 nanocrystals/CZTS crystals.

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ECS Journal of Solid State Science and Technology, 4 (8) Q83-Q91 (2015) Q85

Figure 3. A schematic diagram showing current transport across TiO2nanotube coated with TiO2 nanocrystals/CZTS crystals.

at a potential of ∼1.0 V. However during the reverse sweep, a highresistance state was maintained up to ∼ −1.4 V. There is a suddenchange in current during the positive direction sweep at this value. Theratio of specific resistivity for high resistance and low resistance statesis ∼3:1. Although the degree of symmetry is not high, the magnitudeof current drop from a low resistance state to the high resistance stateis ∼1 mA. Such a bidirectional switching behavior can be attributedto avalanche multiplication and barrier redistribution.19

In contrast, the transition from low resistance to high resistancestates, for the T-NTA/T-NP/CZTS assembly, occurs at a significantlylower potential (∼0.1 V) (See Figures 4b and 4c). An appreciablereduction in the applied potential required for NDR behavior in theT-NTA/T-NP/CZTS device can result in lower operating power de-mand. Subsequently, the T-NTA/T-NP/CZTS assembly was furthercharacterized. Various parameters (i.e., peak to valley current ratio(PVCR), diode ideality factor, and excess voltage) for a tunnel diodewere calculated based on standard models and are summarized andin Table I. Amongst one of the best performing T-NTA/T-NP/CZTSdiodes, the NDR region extends from 0.11 V to 0.32 V. The peakcurrent Ip was ∼ 0.0127 A, where the value of Iv (valley current) was∼0.0031 A. The PVCR at room temperature was ∼ 4.10. It is impor-tant to note that a large peak to valley ratio can be useful in embeddedcircuit application by reduction of standby power consumption. Largepeak to valley current ratios (∼5–6) have been reported for Si tun-nel diodes, Si/SiGe resonant tunnel diode and GaN/AlGaN tunneling

Table I. Different CZTS-TiO2 based tunnel diodes and their peakto valley current ratio (PVCR).

Devicenumber PVCR Remarks

1 4.10 CZTS crystals were soaked for longer timeover TiO2 NT coated substrate.

2 3.10 CZTS crystals were coated on TiO2 NTsubstrate using spin coating and film wasannealed for ∼15 minutes

3 3.03 CZTS crystals were coated on TiO2 NTsubstrate using spin coating and film wasannealed in air for ∼15 minutes

4 2.04 TiO2 film was grown over CZTS thin film

Figure 5. The effect of CZTS layer thickness on peak current.

structures.38,39 However, most of the techniques used to prepare suchdiodes require non-equilibrium conditions (vacuum based) or chemi-cal vapor deposition. Conversely, we utilized a low-cost solution basedapproach for diode fabrication. It is important to mention here thatthe thickness of CZTS layer and method of coating greatly affects allthese parameters.

The effect of CZTS layer thickness on peak/valley current was alsoexamined. An exponential decrease in peak current with an increasein CZTS layer barrier thickness is observed (Figure 5). It can be seenthat by increasing the thickness of the CZTS layer the peak currentchanges. Such a change is governed by a change in tunneling prob-ability, which is directly related with current density. Low tunnelingprobabilities (Tp) can be determined by the following expression:40

Tp = 16E(Eo − E)

E2o

e−2κW [1]

Figure 4. (a) Experimental I-V curve of TiO2 NC TF/CZTS TF (b) TiO2 NT-NC/CZTS NC tunnel diode and (c) TiO2 NT-NC/CZTS NC for extended potentialrange tunnel diode. The inset of Figure 4(a) shows the device assembly.

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Q86 ECS Journal of Solid State Science and Technology, 4 (8) Q83-Q91 (2015)

Here Eo represents the height of a single potential energy barrier, Eis energy of the particle, W is the barrier width and κ represents thewave vector.

In many NDR devices, NDR disappears during successive sweeps.In our case, as can be seen in Figure 4a, NDR behavior is maintainedin forward as well as reverse sweeps with some symmetry. Such anobservation suggests that the deep defects and charge traps are presentat interfaces but their effects are not dominant for NDR features. Itis worth mentioning that in such a configuration, electroforming doesnot dominate because NDR is reversible with some symmetry.

The current density of a tunnel diode can be expressed as theaddition of three different currents based on quantum mechanicalprocesses. The total current is an addition of p-n junction current(Idiode), the tunnel current (Ituneel), and the excess current related toparasitic tunneling via impurities (Iexcess):39,41–43

Itotal = Itunnel + Iexcess + Idiode [2]

where the tunnel current can be written as:

Itunnel = V

Roexp

[−

(V

Vo

)m][3]

where Ro is the ohmic region resistance, m is a parameter relating toRo and maximum NDR. Vo is ∼ 0.1 to 0.5 V, which is related to peakvoltage. Such values depend on parameters that include Fermi levelposition on both sides of junction, band edges, carrier concentration,and temperature.

The maximum NDR can be calculated using the following:

|RN DR(max)| = Ro

exp(

1+mm

)m

[4]

The peak voltage can be written as:

VP =(

1

m

) 1m

Vo [5]

The excess current can be written as:

Iexcess = V

Rv

exp

[V − Vv

Vex

][6]

where Rv and Vex are fitting parameters.The diode current can be written as:

Idiode = IS exp

[(V

ηVth

)− 1

][7]

where Is is the saturation current, Vth is kT/q, and η is the ideal-ity factor. For our best performing diode (T-NTA/T-NP/CZTS), Vex

∼3.55 V, Rv ∼ 70 ohms, m ∼ 3.9, η ∼ 2. The simulated I-V curvefor CZTS-TiO2 tunnel diode is plotted with experimental data inFigure 4b.Investigation and modeling of resistive switching behavior.—In or-der to explain memory resistance behavior, various models have beenproposed. Among the models are Schottky emission (ln I ∝√

V),thermionic emission (I ∝ V), Poole-Frenkel emission (ln I/V ∝√

V),space charge limited conduction (SCLC) (I ∝ V2), and Fowler-Nordheim quantum tunneling (ln I/V2 ∝V−1). These models are gen-erally utilized to explain non-linear memory behavior.44–48 Figures 6,7 show plots for these models and respective linear fittings. For thefirst voltage sweep, the Schottky emission and the SCLC modelsexhibits R2 model fit values of ∼0.86 and 0.59, respectively. ForPoole-Frenkel (P-F) emission and Fowler-Nordheim (F-N) quantum

Figure 6. Evaluation of conductivity mechanism for TiO2 NT-NC/CZTS NC diodes using various models.

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ECS Journal of Solid State Science and Technology, 4 (8) Q83-Q91 (2015) Q87

Figure 7. Evaluation of conductivity mechanism for TiO2 NC TF/CZTS TF diodes using various models.

tunneling models, the R2 values were found have values closer tounity (0.91–0.99). Subsequently, several devices were tested and fitto these models where the average R2 values obtained for F-N tun-neling and P-F mechanism were 0.98 ± 0.01 and 0.95 ± 0.03, re-spectively. Although the R2 values for both mechanisms are close, thestandard deviation for F-N tunneling is lower than for F-P mech-anism. These results suggest that F-N tunneling is the prevailingmechanism, in which charge carriers are injected into TiO2 insu-lating layer from an electrode by tunneling effect. Other mechanismssuch as Schottky barrier emission, P-F mechanism, and SCLC effectsmost likely represent leakage currents as these currents are not easilymodulated.

The same models where also fit for a FTO/CZTS TF /TiO2 NCFassembly, where R2 values of ∼0.86, 0.97, 0.88, and 0.89 for Schottkyemission, SCLC, P-F emission, and F-N quantum tunneling model,respectively, were observed (Figure 7). These results suggest that theSCLC is the dominant mechanism responsible for non-linear current-voltage behavior in the thin film nanoparticle assemblies. It is in-teresting to note the difference in conduction mechanism betweenthe two systems. One of the possible explanations is attributed to thenano-architectural differences between the two systems (i.e., nanopar-ticle/nanoparticle vs. nanopartile/nanotube), and their subsequenteffects on vacancy mobility and charge transport. Non-adiabaticmolecular dynamics simulation result suggest that the electron mobil-ity of a nanotube film is not superior to a nanocrystalline film.49 Themigration of oxygen vacancy toward subsurface sites is also differentfor both systems. It was reported that the electronic scattering rate ison the time scale of ∼ 500 fs, whereas phonon induced relaxation was

observed to be ∼50 times higher.49 Another factor for lower mobilityis the presence of excitonic states.50

Memory endurance characteristics of T-NT/T-NP/ CZTS.—High speedswitching experiments using triangular, square wave, and sinusoidalvoltage signal were carried out. Switching cycles were examined us-ing with a pulse width of ∼ 40 μs. Measurements were performedusing a USB multichannel Oscilloscope, a function generator, mem-ory resistor and a data acquisition system (See Figure 8a). Resistance(∼ �V/�I) was determined by selecting a linear section of the I-Vcurve. Figure 8b shows such an input and output signal chosen forendurance examination. The variation in switching current was moni-tored for more than 103 cycles and was found to be fairly stable, wherethe high resistance state is ∼ 103 times greater than the low resistancestate (Figure 8c). It can be seen that the ratio between the high andlow resistance remains relatively constant for several cycles. This in-dicates stability and differentiability of states, which are important formemory device applications. The switching speed was estimated tobe ∼100 ns. All these examinations indicate that non-volatile switch-ing at room temperature can be realized while applying appropriateelectric field impulses. When T-NTA/T-NP/CZTS was used, two dis-tinct stable and reversible states with some asymmetry were obtained(Figure 8b). The asymmetry can be attributed to the presence of asym-metrical potential barrier between trap and trap free regions. Such aset and reset switching can be viewed as trap filling and emptying,but the presence of an asymmetric barrier produces a hysteresis in thisprocess (using different polarity). In the case of SCLC, high-densitydefects induce mobility in thick dielectric layers causing accumula-tion of charge. In this situation, low insulator resistance prevents high

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Q88 ECS Journal of Solid State Science and Technology, 4 (8) Q83-Q91 (2015)

Figure 8. (a) A schematic diagram for high frequency signal measurements for CZTS-TiO2 diodes. (b) High speed input and output current response characteristicsTiO2 NT-NC/CZTS NC diode. (c) Memory endurance characteristics of TiO2 NT-NC/CZTS NC tunnel diode.

electric field formation that can provide movement of ionic defects.Hence, ionic switching is difficult. An abrupt jump in current canbe understood as a trap-filling limit (∼ 1.0 V). During an increaseof compliance current, the system becomes more conductive (at setstate). During the re-set process, emptying of trap states will be eas-ier and the reset voltage will be lower. Once de-trapping is finishedfor the first cycle, subsequent de-trapping occurs at higher voltagesbecause the carriers trapped at higher energy levels are preferred forde-trapping.Series connection of two tunnel diodes (T-NTA/T-NP/ CZTS).—Se-ries connection of tunnel diodes is often utilized in order to re-move spurious oscillation.51–53 Such a connection is also useful toincrease oscillator output power. Series connection of tunnel diodeswith a common voltage source results in multistable composite I-Vcharacteristics.51–53 For observation of multistable composite charac-teristics, it is essential that the diodes possess continuous imbricationsof multivalued range.51–53 Hence, we identified two regions, or stages,of diode behavior, one which has a peak current at ∼ 0.2 V, and anotherat ∼ 1.2 V (Figure 9). It can be seen that the piecewise characteristicsare similar to individual characteristics of a single tunnel diode. Itshould be noted that no internal biasing was applied. The nature of theI-V curve suggests that various stable states can be assigned to thesesets of diodes.

Temperature dependent electrical characterization (T-NTA/T-NP/CZTS).— Temperature dependent electrical characterization (SeeFigures 10a–10c) of CZTS-TiO2 tunnel diode was performed inorder to better understand the space charge activity and tunnelingmechanism.54–59 Peak and valley currents were observed to increasewhen the temperature was increased. Both peak and valley current

positions shift toward higher voltage bias when the temperature wasincreased. A shift of resonance toward a lower potential, when thetemperature decreases, can be attributed to lowering of external cir-cuit series resistance.54–59 It is believed that PVCR diminishes mainlydue to an increase in valley current at a much faster rate than thepeak current increases with an increase in temperature. An effect ofinelastic scattering, primarily caused by phonons and scattering at theinterface, is more prominent at higher temperatures. To examine thecontribution of the thermal activated transport, the Arrhenius equationwas utilized in both ohmic and NDR regions.54 Figure 10a shows alog (resistance) vs. inverse temperature plot for tunnel diode underdark conditions. Discontinued linearity is visible on the Arrheniusplot. This suggests that the resistivity is regulated by two differentconduction mechanisms. This is indicative that in CZTS/TiO2 tun-nel diode, polaronic transport is one of the phenomena assisting inelectrical conduction.60–62 In the case of polaronic transport, chargeflow is controlled by small polarons hopping (SPH) motion up toa certain temperature, after which, non-linear behavior is observed.Experimental activation energy (in a wide temperature range) can beestimated using following equation:54

Resistance = AeEa/RgT [8]

where A is a constant, Ea is the activation energy, Rg is the universalgas constant, and T is the absolute temperature. Using Equation 8,the activation energy calculated for the ohmic region was ∼0.33 eV,whereas in the NDR region, the activation energy was calculatedto be ∼0.23 eV. Taking into consideration the SPH mechanism, theconductivity can be written as:60–62

σ = BT −α exp

(− Ea

kB T

)[9]

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ECS Journal of Solid State Science and Technology, 4 (8) Q83-Q91 (2015) Q89

Figure 9. Series connection of two TiO2 NT-NC/CZTS NC tunnel diodes and their I-V characteristics.

where B is the pre-exponential factor, Ea is the activation energy,α = 1 (for adiabatic region), and kB is a Boltzmann constant. For thenon-adiabatic region, a value of α = 3/2 was used. Using Equation9, the activation energy was estimated to be ∼ 0.34 eV and 0.24 eVfor ohmic and NDR regions, respectively. Figure 10b and 10c showsadiabatic conductivity vs. inverse temperatureplot for tunnel diode inthe dark condition for α = 1 and α = 1.5 respectively. The least squarefit was applied for data (ohmic region) obtained using Equations 8 and9 and for the non-adiabatic region. The correlation coefficient value forthese fits was ∼0.972, 0.976 and 0.98, when Equation 8, Equation 9 (α= 1), and Equation 9 (α = 3/2) were utilized, respectively. Althoughfor the non-adiabatic region a correlation coefficient closer to unitywas obtained over others, there is not a large enough difference in thevalues to determine which SPH mechanism dominates in conduction.

At higher temperatures, the resistance in the ohmic region changesmore rapidly compared to the resistance in the NDR region. It shouldbe noted that the order of activation energy values are similar tothe reported values for TiO2 thin films.60 In TiO2–CZTS diodes, thenegative differential region was found to decrease with decrease intemperature. Both peak and valley currents were observed to increasewhen temperature is increased. Such behavior indicates that tunnelingand excess current depends on the CZTS band gap, which decreaseswhen temperature is increased. This shows that the negative differen-tial region decreases with decrease in temperature. This observation isconsistent with a Germanium or double barrier resonant tunnel diode,which exhibits ‘S-type’ characteristics.58

Investigation of conduction mechanism and evaluation of NDRbehavior.— The predominant mechanism believed for current flowin a disordered system due to localized charge carrier states is hop-ing conduction. Examining the conduction at different temperatures

suggests that two mechanisms are responsible for charge flow. In con-trast, one mechanism (Mott variable range hopping) dominates in asingle nanotube.16 The effect of TiO2 grain boundary density on con-duction is not taken into account; however, CZTS grains may havesome contribution on charge transport as well. Studies on a singleT-NTA shows ohmic (linear) behavior at room temperature.16 How-ever, for the CZTS TF/TiO2 NCF assembly, the cumulative effect ofband mismatch and TiO2 –oxygen vacancies contribute to nonlinearI-V characteristics. TiO2 is a complex system, which contains severalpoint defects including oxygen vacancies, titanium vacancies, titaniuminterstitials, electrons and holes. These defects are formed during for-mation, oxidation, or reduction of TiO2.63,64 Some of these defectsform due to intrinsic ionization of TiO2. These defects and spatiallymodulated ionic conductivity are major factors responsible for chargetransport in a polycrystalline-rutile TiO2 system, for example.63,64 Atlow temperature, electronic transport is dominant, however, ions alsocontribute to current flow at elevated temperatures.64 It is essentialto note, that ionic effects on charge transport are more prominentwithin p-n transitions compared to electronic effects. Band energeticssuggests that the Fermi level difference and concentration gradient isone of the factors required for charge transport for the CZTS-TiO2

system.30 Hence, the tunneling process for this system was investi-gated, which can be direct or indirect.65,66 It is well known that TiO2

anatase structure has an indirect band gap, while the conduction bandmaximum at the �-point and valence band minimum is located alongthe (100) direction.67,68 On the other hand, CZTS is a direct bandgap semiconductor and its conduction band minimum and the valenceband maximum are aligned and located at the �-point.69 This suggeststhat in our CZTS-TiO2 diode system, indirect tunneling is most likelythe tunneling mechanism. Indirect tunneling is phonon assisted andproceeds with intraband charge transport. The band edge positions of

Figure 10. (a) log (Resistance) vs inverse of temperature (b) ln (σT) vs. inverse of temperature and (c) ln (σT3/2) vs. inverse of temperature graphs for TiO2NT-NC/CZTS NC tunnel diode.

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Q90 ECS Journal of Solid State Science and Technology, 4 (8) Q83-Q91 (2015)

Figure 11. A schematic diagram showing band alignment of CZTS-TiO2system, Ef denotes the Fermi level.

CZTS and TiO2 form a type II band structure (Figure 11), suggestingthat this system can be understood as a cascade type system, whereboth the conduction band and the valence band of each semiconduc-tor are arranged in a stepwise manner. It can be seen that the energylevel of valence band CZTS and conduction band of TiO2 are nearone another. A difference in band offset and high doping causes thedepletion layer thickness to decrease.42,43 In order to create a tun-neling barrier, the thickness of the potential energy barrier should besufficiently small.42,43 The existence of occupied energy states withina semiconductor that allows electron tunneling towards empty statesis essential. Heavy doping of p-type and n-type regions minimizesband offset-mismatch and promotes tunneling. In most of the cases,the Fermi level on the p-side is, more or less, at the same energy levelas that on the n-side. In this case, the probability of filled states on oneside of the junction (which are at similar energy levels as the emptystate on the opposite side) is insignificant.42 Hence, in this case thecharge flow is low. However, during application of a reverse bias, thebarrier height increases above the open circuit potential. This causesa shift of the n-side levels to lower energy. It can be observed thatsome of the states in the CZTS valence band may lie at the same levelof the empty states of TiO2. Charge carriers can tunnel from CZTSto TiO2 and can cause reverse diode current transport. When reversebias increases, such an overlap between the conduction and valencebands increases. This results in a large reverse diode current flow.Time resolved terahertz spectroscopy70 studies suggest that electronsare highly mobile within titania nanoparticles; however the flow ofelectrons through a nanocrystalline film is not smooth due to imped-iment caused by backscattering, and/or disorder induced localizationof electrons. The energy level diagram for anatase TiO2 nanotubesuggests that Ti3+ defect ground state energy lies ∼ 1 eV below theconduction band edge. The impurity band of mixed defect states startfrom the conduction band edge and extends up to ∼ 2 eV belowthe conduction band edge. For anodic TiO2 nanotubes, some p-typedopants are also present in form of C atoms when synthesized in anorganic electrolyte.20 This doping makes TiO2 similar to a compen-sated semiconductor. The presence of these states will cause loweringof the Fermi level of T-NTA compared to titania nanocrystalline film,which has no C doping or substoichiometeric titania). Hence, therewill be more overlapping of conduction band edge of T-NTA and va-lence band edge of CZTS. Such an enhanced overlap will improvetunneling probability.

When a forward bias is applied, such an overlap between thevalance band and conduction band decreases due to the TiO2 conduc-tion band upward shift with respect to the p-side. During an increasein applied potential, the mismatch between valance band and con-

duction band starts increasing, which results in a decrease in current.A further increase in current causes charge transport due to conduc-tion, not due to tunneling. Under this condition, the device acts like aconventional diode. When consecutive measurements were done, neg-ative differential resistance observed at room temperature generallydegrades presumably due to activated filling of trap states. However,it was observed that a CZTS-TiO2 device maintains its NDR activityeven after several cycles (∼ 104) of operation.

Another possible mechanism is via a multi-island tunnelingsystem71 where charges are mainly transferred through nanocrys-tal chains by tunneling. Due to the trapping and detrapping effect,the multi-island tunneling systems show resistive switching behaviorwith NDR effect.72,73 Although it may be possible that such a mech-anism is taking place, our data and modeling analysis suggests F-Ntunneling mechanism under that conditions examined. As a result, fu-ture detailed studies are required to understand the resistive switchingbehavior of TiO2-CZTS systems.

Most memory resistors fabricated in this study show NDR be-havior and can be useful for low power resistive switching and logicapplications. An NDR device possessing low valley current densityreduces the requirement of the number of devices needed for one tran-sistor tunneling diode static random access memory. In most of thesecases, the required circuit topologies can be accomplished by a devicewhose PVCR ≥ 3.

Conclusions

The non-linear current-voltage (or NDR) relationship for de-vices using a combination of TiO2 and Cu2ZnSnS4 material hasbeen demonstrated. Both polycrystalline CZTS/TiO2 NCF and NCCZTS/TiO2 T-NTA demonstrate resistive switching behavior. This in-vestigation suggests that Fowler-Nordheim tunneling is a dominantmechanism for NC CZTS/TiO2 T-NTA resistive switching whereasthe space charge limited model explains CZTS/TiO2 NCF switchingbehavior. Memory retention and endurance examination suggests thatTiO2 T-NTA-CZTS NC based devices exhibit repeatability whereasTiO2 TF-CZTS TF based devices show variation in performance. Mi-gration from a low resistance state to high resistance state is possibleand reversible with some degree of asymmetry. Switching perfor-mance of device (CZTS NC/TiO2 T-NTA) was confirmed for 104

cycles. Series connection of two tunnel diodes was made, which re-sults in two successive on/off stages. Temperature dependent electricalcharacterization suggests that charge flow is controlled by small po-laron hopping (SPH) motion up to a certain temperature (T ∼ 454 K).The activation energy for ohmic conduction mechanism was estimatedto be ∼ 0.33 eV. Overall, the combination of TiO2 - Cu2ZnSnS4 com-posite demonstrates the possible applications for use in memory resis-tors while utilizing a facile synthesis process and abundant, low costmaterials.

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