Giant negative piezoresistive effect in diamond like carbon and

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Functional Inorganic Materials and Devices

Giant negative piezoresistive effect in diamond like carbon and diamond like carbon based nickel nanocomposite films deposited by reactive magnetron sputtering of Ni target Šar#nas Meskinis, Rimantas Gudaitis, Kestutis Šlapikas, Andrius Vasiliauskas, Arvydas #iegis, Tomas Tamulevi#ius, Mindaugas Andrulevi#ius, and Sigitas Tamulevicius ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17439 • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 16, 2018

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ACS Applied Materials & Interfaces

Giant negative piezoresistive effect in diamond like carbon and diamond like carbon based nickel nanocomposite

films

deposited

by

reactive

magnetron sputtering of Ni target Šarūnas Meškinis†*, Rimantas Gudaitis†, Kęstutis Šlapikas†, Andrius Vasiliauskas†, Arvydas Čiegis†, Tomas Tamulevičius†, Mindaugas Andrulevičius†, Sigitas Tamulevičius†, ‡ †

Kaunas University of Technology, Institute of Materials Science, K. Baršausko Str. 59, LT-51423 Kaunas, Lithuania

‡

Mads Clausen Institute, NanoSYD, Alsion 2, DK-6400 Sønderborg, Denmark

Author information *Corresponding author E-mail: [email protected] (Š.M.)

ACS Paragon Plus Environment

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Piezoresistive properties of hydrogenated diamond like carbon (DLC) and diamond like carbon based nickel nanocomposite (DLC:Ni) films were studied in the range of low concentration of nickel nanoparticles. The films were deposited by reactive high power pulsed magnetron sputtering of Ni target and some samples were deposited by direct current reactive magnetron sputtering for comparison purposes. Raman scattering spectroscopy, energy-dispersive X-ray spectrometry (EDS) and X-ray photoelectron spectroscopy (XPS) were used to study structure and chemical composition of the films. Four point bending test was applied to study piezoresistive properties of the films. For some samples containing less than 4 at.% Ni and for the samples containing no Ni (as defined by both EDS and XPS) giant negative piezoresistive effect was observed. The giant negative piezoresistive effect in DLC films deposited by reactive HIPIMS either DC magnetron sputtering of Ni target, was explained by possible clustering of the sp2 bonded carbon and/or formation of areas with the decreased hydrogen content. It was suggested that the tensile stress induced rearrangements of these conglomerations have resulted in the increased conductivity paths.

KEYWORDS: Giant negative piezoresistive effect; hydrogenated diamond like carbon; diamond like carbon containing Ni; Raman scattering; EDS; XPS.


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INTRODUCTION

Diamond like carbon (DLC) is amorphous allotrope of carbon1. Films of DLC consist of carbon atoms bonded by sp3 type bonds (like in diamond) and sp2 type bonds (like in graphite).1,2 In addition, hydrogenated DLC films may contain some amount of hydrogen.1 The sp3 bonding of DLC confers on it many of the beneficial properties of diamond itself, such as its high mechanical hardness, chemical and electrochemical inertness.1 The sp2 bonds (π states) control the electronic properties of diamond like carbon such as bandgap and electrical conductivity.3 DLC films received considerable interest of researchers due to interesting combination of the mechanical properties such as high hardness (up to 80% of the diamond hardness) and Young's modulus, very low friction coefficient, wear and corrosion resistance as well as biocompatibility. Concerning the electrical and optical properties of diamond like carbon, they can be varied in a wide range by choosing appropriate deposition conditions. Recently, strong piezoresistive effect in DLC films was reported.4-34 Piezoresistive gauge factor (GF) of diamond like carbon films was found to be up to 100. This value is comparable with the gauge factor of silicon that now is the main semiconducting piezoresistive material.4-10

Therefore, in combination with the

mentioned mechanical properties as well as chemical inertness, DLC became an attractive material for advanced sensors working in different liquid or gaseous harsh environments. Properties of diamond like carbon films can be additionally controlled by introducing nanoparticles of different metals.6,15-25,34 Particularly, for DLC films containing Ni (DLC:Ni) beneficial combination of a rather high GF (up to 20-30) and zero temperature coefficient of resistance was reported.6,17,22-25


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It should be mentioned as well that in some cases large gauge factors up to 1000 were reported for DLC films.5 However, till now there is no explanation of such huge piezoresistance in diamond like carbon films. According to 5, no clear correlation between the extra-large gauge factor of DLC films and structure nor optical bandgap was observed. Usually resistance of DLC films increases with tensile strain (deformation) applied, i.e. material shows positive piezoresistive effect. The exceptions are not numerous. It should be noted that the negative piezoresistive effect was reported for (100) orientation n-type crystalline silicon35, some silicon and germanium nanowires under high strain conditions36,37 and some nanocomposites38,39. In the case of the Si nanowires, giant negative piezoresistive effect was explained by increase of the mobility of charge carriers.36 In the case of the nanotube (nanowire) based nanocomposites, the negative piezoresistive effect was explained by formation of new conductivity paths as a result of the tensile stress applied.38 In the present research, for the first time giant negative piezoresistive effect was observed in diamond like carbon films that were deposited by reactive magnetron sputtering of Ni target. Possible mechanisms of such effect are considered.

EXPERIMENTAL SECTION

Sample fabrication techniques. Reactive magnetron sputtering of Ni target was used to deposit hydrogenated diamond like carbon (DLC) and hydrogenated diamond like carbon films including nickel nanoparticles (DLC:Ni). Two modes of deposition - high power pulsed magnetron sputtering (HIPIMS) and direct current (DC) magnetron reactive sputtering (for comparison purposes) were applied. Target diameter was 3”. The films were deposited on


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grounded substrates (no additional bias was applied), at room temperature. No additional heating was applied. Mixture of argon (sputtering gas) and acetylene (C2H2) (reactive gas) was used in the experiments. The deposition was performed at two fixed Ar flow rates (70 and 140 sccm) and varied C2H2 flux. In the HIPIMS mode pulse current in all cases was 20 A, target voltage was in 540-610 V range. During DC magnetron sputtering, target voltage was in 407-479 V range. Current was set at 0.2 A that was close to the average current used in the most HIPIMS deposition processes. Table 1 provides with the summary of the deposition conditions. Table 1. Deposition conditions used in the present research Experiment No

Magnetron Ar gas C2H2 Ar/C2H2 sputtering flux gas flux flux ratio mode (sccm) (sccm)

Pulse on time (ton) (µs)

Pulse period (µs)

Number of the piezoresistive samples studied

Ar70H2

HIPIMS

70

2

0.029

100

10000

1

Ar70H6

HIPIMS

70

6

0.086

100

10000

1

Ar70H10

HIPIMS

70

10

0.14

100

10000

1

Ar70H12

HIPIMS

70

12

0.17

100

10000

1

Ar70H15

HIPIMS

70

15

0.21

100

10000

2

Ar140H10

HIPIMS

140

10

0.071

100

10000

2

Ar140H20

HIPIMS

140

20

0.142

100

10000

2

Ar140H22

HIPIMS

140

22

0.16

100

10000

6

Ar140H25

HIPIMS

140

25

0.18

100

10000

12

Ar140H40

HIPIMS

140

40

0.29

200

10000

8

DAr140H10

DC

140

10

0.071

-

-

2

DAr140H7

DC

140

7

0.050

-

-

1

DAr140H5

DC

140

5

0.036

-

-

2


5


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Polycrystalline alumina (Al2O3) and monocrystalline p-type silicon were used as substrates for the deposition of DLC and DLC:Ni films. The films deposited on monocrystalline silicon were used for the chemical composition and structural studies. Resistivity of p-type silicon (Si(100)) was in 1÷10 Ω⋅cm, according to the data provided by the supplier. Piezoresistive and electrical properties of the films were studied for the samples deposited on polycrystalline alumina substrates. Samples used for the piezoresistive and electrical studies were produced during the same technological process. Thickness of the deposited films was controlled by a quartz microbalance and in all cases it was approximately 100 nm. In the same technological deposition process, usually two polycrystalline alumina samples were coated by the piezoresistive film. Deposition of the samples at the conditions where the giant negative piezoresistance effects was observed, was repeated several times to ensure reproducibility of the results (Table 1, column “Number of the piezoresistive samples studied”). Due to large overall number of the samples and process runs, only selected samples were studied by Raman scattering spectroscopy, energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). Taking into account different time necessary for Raman scattering, EDS and XPS measurements as well as their different complexity, the highest number of the samples was studied by Raman scattering spectroscopy. While only five (typical) samples were investigated by XPS. Strip-shaped DLC and DLC:Ni films were used to fabricate piezoresistors on polycrystalline alumina substrates. Al-based top electrodes were vacuum evaporated using a mask. Dimensions of the polycrystalline alumina substrates were 15x48 mm. Distance between the Al electrodes was 1.5 mm.


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Measurement technique. Two probe configuration employing a picoammeter Keithley 6487 was used to study resistance of the samples. It should be noted that two probe configuration, as compared to four probe, is less complex but additional transmission line measurements (TLM) are needed to eliminate possible influence of the contact resistance. It worth to mention as well that two probe configuration was applied for the measurements of electrical resistance in numerous reported studies on piezoresistive effect in DLC films.4-34 Electrical measurements were performed at relatively low electric fields. In such a case ohmic conduction and/or PooleFrenkel emission were reported to be the main current transport mechanism in DLC.40,41 Nevertheless, to ensure that there is no any remarkable influence of the sample’s contact resistance on the measurement results, some additional experiments have been done. Samples with a varied inter-electrode distance were fabricated to evaluate contact resistance by transmission line measurement (TLM) method. Four-point bending test was used to study gauge factor of the DLC and DLC:Ni films

42

.A

custom-made setup including bending equipment and a picoammeter Keithley 6487 was employed. The applied tensile strain ε = ∆L/L in the piezoresistive test was in the 0 ÷ 0.25 × 10− 3 range (load range 0÷13.73 N). The gauge factor was found according to: ‫= ܨܩ‬

∆ோ ଵ ோ

∙ఌ

(1),

where R is the nominal electrical resistance, ∆ܴ is the change of resistance due to applied strain, and ε is the strain. In our case, calculation of GF was based on the measurements of the resistance of samples deformed by 13.73 N (1400 gf) and 0.49 N (50 gf) load respectively. The method used enabled to ensure measurement accuracy not worse than 3%.42 For more information on the measurement techniques used for structural and chemical composition studies please see Supporting information.


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3. EXPERIMENTAL RESULTS

Structure and chemical composition of the DLC:Ni and DLC films is thoroughly explained in Supporting information. It should be mentioned that increase of C2H2/Ar gas flow ratio used during reactive magnetron sputtering deposition, resulted in the decreased Ni atomic concentration in the films. Oxygen content in the films depending on the deposition conditions varied within 5-20 at.% range. Analysis of the Raman scattering spectra (Figure S3) revealed that in all cases they are typical for diamond like carbon and the sp3/sp2 ratio carbon bond ratio increases with the increase of C2H2/Ar gas flow ratio applied during deposition. The sp3/sp2 ratio carbon bond ratio decreases with the decrease of C2H2/Ar gas flow ratio (or increase of the resultant atomic concentration of Ni in the films). Electrical and piezoresistive properties of DLC and DLC:Ni piezoresistors (Al/DLC/Al and Al/DLC:Ni/Al planar two electrode structures) were studied, using four point bending technique. Figure 1 shows dependence of the piezoresistive gauge factor of such piezoresistor on the acetylene and argon gas flow ratio used during reactive HIPIMS deposition. One can see that in the case of the samples deposited in the range of low gas flow ratios, gauge factor increases from 1.5 to ~3.2 with the C2H2/Ar gas flow ratio. This tendency and GF values are in good accordance with the gauge factor values reported for DC reactive magnetron sputtered DLC:Ni films.17 The initial growth of the gauge factor with the flow ratio applied is followed by the saturation, and finally negative values of the piezoresistive gauge are observed in the region of used high C2H2/Ar flow ratios. In the region of high gas flow ratio the increase of the negative gauge factor to the surprisingly high values (up to -3200) was registered.


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Figure 1. Gauge factor of DLC:Ni films Vs acetylene and argon gas flow ratio used during HIPIMS deposition. (Some process runs were repeated several times to ensure reproducibility of the effects. Thus for the samples No Ar140H22, Ar140H25, Ar140H40 (i.e. deposited at C2H2/Ar gas flow ratios 0.16, 0.18, 0.29) average values are presented. The error bars show dispersion of the results. Measurement error was within 3%. For the most samples it can’t be seen in the present graphic). Figure 2 shows correlation between the Ni amount in DLC:Ni films and the gauge factor values. One can see that for DLC:Ni samples, positive piezoresistive effect is observed in the range of relatively high Ni concentrations (4 - 25 at.%). The gauge factor varies in the range from 1 to 4. While for some samples containing less than 4 at.% Ni and for the samples containing no Ni (as defined by both EDS and XPS) large negative piezoresistive effect was observed (Figures 1, 2). As it was mentioned above, in some cases the negative gauge factor of the piezoresistor was huge – it was in the range (-1000 - -3000) (Figures 1, 2). It worth to note as well that at the same time for some samples containing similar amount of Ni (3.5 - 3.7 at.%), very different gauge factor values (from 3.2 to -1000) were measured.


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Figure 2. Gauge factor of DLC and DLC:Ni films Vs Ni atomic concentration. (Atomic concentration measured by EDS). All films studied were deposited by HIPIMS. Analysing D/G ratio as well as position of G peak in the Raman scattering spectra (Figure 3) we observed some correlation between the gauge factor (values of GF and sign of the piezoresistive effect) and structure of the DLC matrix.

In the case of the DLC:Ni films

exhibiting large D/G ratios, positive piezoresistive effect was detected. While in the case of DLC:Ni films with D/G ratio below 0.6, in all cases negative piezoresistive effect took place. The gauge factor increased with the decrease of D/G ratio (i.e. increased sp3/sp2 carbon bond ratio). Similarly in the case of DLC:Ni films showing G peak position below ~1534 cm-1, in all cases negative piezoresistive effect was observed. The negative gauge factor increased with the downshift of G peak - thus negative gauge factor value increased with the decreased sp2 nanocluster size. No correlation of the gauge factor with FWHM of G peak was observed (Figure S6). Thus, according to

43

no clear correlation between the gauge factor of DLC:Ni films and

structural disorder of diamond like carbon matrix was found.


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Figure 3. Gauge factor of DLC:Ni based piezoresistor Vs D/G peak area ratio (a) and G peak position (b). All films studied were deposited by HIPIMS. Linear current-voltage dependencies of the DLC:Ni piezoresistors and transmission line measurement experiments confirmed that the observed giant negative piezoresistive effect is not related to any contact phenomena that could take place, e.g., decreased width of the depleted layer in a Schottky contact due to the deformation (see Supporting information 4). The dependences of resistance of the samples on the applied load and time are presented in Figures 4, 5. Significant variation of the resistance recorded at zero strain (or no strain) can be seen in Fig.5c. However, variation of the resistance value at zero strain, recorded during multiple load process, decreases with the increased number of the measurement cycles. The variation can be


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significantly reduced by setting appropriate time of loading and unloading (more information can be found in Supporting information 5). We have shown that the observed giant piezoresistance effect is related to processes in the bulk of the DLC films (more information can be found in Supporting information 6).

Figure 4. The dependence of the resistance of DLC:Ni film on deformation force (a) and resistance Vs time curve during two load-unload cycles (b). Measurements done for the films with a gauge factor of the piezoresistor ~3.2 (set of the samples No Ar70H10).


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Figure 5. The dependence of the resistance of DLC:Ni film on deformation force (a) and resistance Vs time curve during two load-unload cycles (b) (set of the samples No Ar140H22, gauge factor of the piezoresistor was -1028). Piezoresistor endurance test (50 cycles) (set of the samples No Ar140H40) (c).

Gauge factor of the DLC films deposited by reactive DC magnetron sputtering of Ni target was studied for comparison purposes (Table 1, experiments No DAr140H10, DAr140H7, DAr140H5). In all cases, no Ni was found in these films by using EDS technique. For all DC sputtered samples, significant changes of the resistance with applied load corresponding to the gauge factor of -2000 (and higher) were observed. However in all cases dependence of the


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resistance Vs load was nonlinear. In most cases, hysteresis of the resistance Vs applied load curve was detected.

Figure 6. Gauge factor Vs resistance of DLC:Ni film in the range ≤2⋅104 Ω⋅cm (a), 105-⋅2.5⋅106 Ω⋅cm (b) and 5⋅106 -3⋅108 Ω⋅cm (c) Dashed lines are used to show prevailing tendencies.

Gauge factor of the DLC:Ni (and DLC) piezoresistors deposited by reactive HIPIMS was dependent on resistivity of the samples. It can be seen in Figure 6 that in the case of the films with resistivity below 2⋅104 Ω⋅cm, the positive gauge factor linearly increases with the logarithm of resistivity. It should be mentioned that such dependence of the gauge factor of DLC films on resistivity was reported in

12,34

. It is in good accordance with percolation theory44-47 as well as


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simplified theory of piezoresistive effect in DLC films and DLC films containing metal.5,11,15 In the case of the samples with higher resistivity, the gauge factor was negative and linearly increased with the resistivity (Figure 6). It should be mentioned that similar dependence of the gauge factor on resistance (resistivity) was already reported for nanocomposite with sp2 carbon based conductive phase - film of the overlapping graphene flakes.48 It was explained by increased area of the graphene flakes overlap.48 Typical Arrhenius plots of conductivity of DLC and DLC:Ni films showing different values of the gauge factor (negative and positive values) are presented in Figure 7. It can be seen that, despite very different values of the gauge factors as well as different sign of the piezoresistive effect, in all cases the same shape of the curves was observed. Thus, in all the cases similar current transport mechanisms may be expected. It should be mentioned that surprisingly resistance of the films in all cases increased with temperature. Usually decrease of the resistance with temperature for the undoped DLC films was reported.6,22,34

Figure 7. Typical Arrhenius plot of conductivity for the DLC and DLC:Ni films with different gauge factor. Data for the samples from the sets No Ar70H6, Ar140H25, Ar140H40 are presented. Ni atomic concentration was measured by EDS.


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Thus, summarizing results presented in Fig 6 and Fig. 7, similar current transport mechanism was observed for all the samples investigated. However, piezoresistive mechanisms are different. Influence of some changes in geometrical factors can be expected for the negative piezoresistive samples.

4. DISCUSSION

Summarizing the experimental results presented it must be pointed out that no clear differences in shape of the Raman scattering spectra for the samples studied was found. In addition, giant negative piezoresistive effect was found in the samples containing no Ni. Thus, materials characterization methods used in the present study cannot explain the unusual behaviour of the resistance of DLC and DLC:Ni films versus strain and finally the giant negative piezoresistive effect found. Analysis of the electrical properties revealed the same current transport mechanism for all samples studied. However, piezoresistance mechanisms appear to be different in the case of positive piezoresistive and negative piezoresistive samples. Taking into account circumstances described above, probably we should refer to the negative piezoresistive effect found in the carbon nanotube based nanocomposites. In these nanocomposites negative piezoresistive effect was explained by formation of the new conductivity paths due to the tensile stress applied.38,39 One can assume that in the case of the diamond like carbon films studied in our research, formation of the conductivity paths (higher conductivity filaments) embedded into higher resistance matrix can take place under some circumstances, too. It should be mentioned that increase of the resistance with temperature


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observed in the present study supports the proposed model as such behaviour was reported for carbon nanotube based nanocomposites.49,50 The linear increase of the negative piezoresistive gauge factor value with the resistivity (see Figure 6) is in good accordance with this model, too. Similar dependence of the gauge factor on resistance was reported for other type of nanocomposite containing sp2 carbon conductive nanoclusters (film of the overlapping graphene flakes).48 It should be pointed out that existence of the conductive filaments in diamond like carbon films was already considered while investigating resistive switching effects. Resistance switching effect (changes of resistance up to several orders ) was observed in numerous studies for both hydrogen free40,51-55 and hydrogenated DLC films.56-58 The effect was explained by formation and rupture of the conductive sp2 bonded carbon filaments in the predominantly sp3 bonded insulating carbon matrix. In the case of the hydrogenated DLC films, additional effect of the hydrogenation and dehydrogenation of the sp2 bonded carbon filaments may take place.58 It should be mentioned that resistance switching phenomena can be induced by generation of mechanical stress in the film as well.59,60 Thus, resistance switching and piezoresistive phenomena can be related. In such a way, the observed negative giant piezoresistive effect can be explained by taking into account three considered phenomena: i.e. negative piezoresistive effect studied in the carbon nanotube nanocomposites, resistance switching effect in amorphous carbon and mechanical stress induced resistance switching. In the present study, electric field applied during the measurements of piezoresistance was substantially lower than in the studies mentioned above. Thus formation of the conductive filaments cannot be explained simply by the effects of electric field during electrical measurements. It should be mentioned that in our case, similar current


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transport mechanisms were observed for all the samples investigated, irrespectively to their gauge factor value and sign (see Figure 7 and corresponding text). Therefore, it can be assumed that formation of the conductive filaments takes place during growth of the film. Thus, existence of the higher electrical conductivity areas that are larger than the graphite like carbon nanoclusters inserted into the DLC matrix can be assumed. It could be formations of the sp2 bonded carbon nanoclusters and/or areas with the decreased hydrogen content. Part of these conductive areas should form conductive filaments connecting electrodes of the sample. Relative volume of the conductive filaments should be low. This follows from the fact that according to the Raman scattering spectra, sp3/sp2 carbon bond ratio was higher in the case of the films exhibiting giant piezoresistive effect, despite possible existence of the sp2 bonded carbon filaments. It should be mentioned that in the case of the nanocomposites sometimes addition of very small amount of the conductive phase material could result in substantial reduction of the electrical resistivity. Particularly, in the case of the carbon nanotube and polymer nanocomposites, formation of the conductive network was reported for nanocomposite containing as low as 0.085 wt. % of the carbon nanotubes.64 As a result of the tensile deformation of such film, several processes affecting resistance of the film may take place along with the effects related to phenomena described in percolation theory44-47 based model of the piezoresistive effect in DLC films.5,11,15 The increased distances between the conductive areas along the film as well as possible deformation of the conductive areas should result in the increased resistance (Figure 8b, c). However, creation of the new conductivity paths (conductive filaments) may take place as a result of the decreased film thickness as it is shown in Figure 8d. Finally, creation of the new conductivity paths may result in the decreased resistance of the film.


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Figure 8. Possible mechanism of the negative giant piezoresistance: deformation of the thin film as a result of the tensile stress (a), increased distance between the conductive areas across the film (b), possible deformation of the conductive areas (c), formation of the new conductive path as a result (d). Formation of the conductive filaments mentioned above can be related to two important features of Ni. Ni is widely used as a catalyst promoting formation of the sp2 bonded carbon (see e.g.

61,62

). Therefore, it can be supposed that Ni activates formation of local filament-like areas

with the increased amount of the sp2 bonded carbon or graphite-like carbon nanoclusters. Nevertheless, it worth to mention that in the present study giant piezoresistive effect was observed for the samples containing no Ni or very small concentration of Ni (according to both EDS and XPS). Second important feature of nickel is its ferromagnetism. Ferromagnetic Ni target can change configuration of the magnetron’s magnetic field. As a result of such effects, inhomogeneity of a thin film deposited by magnetron sputtering of the ferromagnetic target was reported.63 Similarly, in our case use of the ferromagnetic Ni target can result in inhomogeneous distribution of the graphite like carbon nanoclusters as well as hydrogen content in DLC film and


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subsequent formation of the local filament-like areas of the increased electrical conductivity. This is valid for both used modes of deposition (HiPIMS and DC). Finally, according to our model disappearance of the giant piezoresistive effect in the DLC films containing higher amount of nickel can be explained by formation of the Ni or NiCx nanoclusters evenly dispersed in DLC matrix. In such a case, conductivity mechanism becomes similar to the case of other DLC films containing metal.

CONCLUSIONS

In conclusion, hydrogenated diamond like carbon based nickel nanocomposite films were deposited by reactive high power pulsed magnetron sputtering of Ni target. Effects of the reactive gas and transport gas ratio on structure and composition of the deposited films were studied. It was found that sp3/sp2 carbon bond ratio increases and sp2 clusters size decreases linearly or quasilinearly, Ni atomic concentration in the films decreases with the increase of C2H2/Ar gas flow ratio.

Oxygen content in the films varied in 5-20 at.% range. XPS study

revealed increasing oxidation of Ni with the decreased nickel content in the films. For DLC:Ni samples, positive piezoresistive effect was observed in the range of

relatively high Ni

concentrations (4 - 25 at.%). While for the samples containing less than 4 at.% Ni and samples containing Ni concentration below atomic resolution of EDS and XPS, the giant negative piezoresistive effect was found. Gauge factor values up to -3200 were observed. Correlation between the changes of gauge factor (values of GF and sign of the piezoresistive effect) and structure of the DLC matrix was found. In all cases for the DLC:Ni films exhibiting D/G ratio below 0.6 or with G peak position below ~1534 cm-1 in Raman scattering spectra, negative piezoresistive effect was observed. Value of the negative gauge factor increased with decreased


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sp2 nanocluster size. Gauge factor of DLC:Ni piezoresistors non-monotonically depended on the resistivity. In the case of the sample’s resistivity below 2⋅104 Ω⋅cm, the positive gauge factor linearly increased with the logarithm of resistivity. In the case of higher sample‘s resistivity, the negative gauge factor linearly increased with the resistivity as well. The giant negative piezoresistive effect in DLC films deposited by reactive HIPIMS either DC magnetron sputtering of Ni target was explained by possible clustering of the sp2 bonded carbon and/or formation of areas with the decreased hydrogen content. One can suggest that the tensile stress induced rearrangements of these conglomerations have resulted in the increased conductivity paths.

ACKNOWLEDGEMENTS

This research was funded by a grant (no. MIP-070/2013) from the Research Council of Lithuania.

SUPPORTING INFORMATION

Techniques used for measurement of the chemical composition and structure, structure and chemical composition of the DLC and DLC:Ni films, gauge factor Vs G peak FWHM, Currentvoltage dependencies of DLC:Ni piezoresistor and transmission line measurement plot. The dependences of resistance of DLC:Ni piezoresistor on the applied load and time. Study on possible influence of the DLC:Ni films surface.


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Table of Contents graphic


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Giant negative piezoresistive effect in diamond like carbon and

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