An in situ electrical measurement technique via a conducting diamond tip for nanoindentation in silicon

An in situ electrical measurement technique via a conductingdiamond tip for nanoindentation in silicon

S. Ruffell,a) J.E. Bradby, and J.S. WilliamsDepartment of Electronic Materials Engineering, Research School of Physical Sciences andEngineering, Australian National University, Canberra, Australian Capital Territory 0200, Australia

O.L. WarrenHysitron Inc., Minneapolis, Minnesota 55344

(Received 18 December 2005; accepted 28 April 2006)

An in situ electrical measurement technique for the investigation of nanoindentationusing a Hysitron Triboindenter is described, together with details of experiments toaddress some technical issues associated with the technique. Pressure-induced phasetransformations in silicon during indentation are of particular interest but are not fullyunderstood. The current in situ electrical characterization method makes use ofdifferences in electrical properties of the phase-transformed silicon to better understandthe sequence of transformations that occur during loading and unloading. Here, electriccurrent is measured through the sample/indenter tip during indentation, with a fixed orvariable voltage applied to the sample. This method allows both current monitoringduring indentation and the extraction of current-voltage (I-V) characteristics at variousstages of loading. The work presented here focuses on experimental issues that mustbe understood for a full interpretation of results from nanoindentation experiments insilicon. The tip/sample contact and subsurface electrical resistivity changes dominatethe resultant current measurement. Extracting the component of contact resistanceprovides an extremely sensitive method for measuring the electrical properties of thematerial immediately below the indenter tip, with initial results from indentation insilicon showing that this is a very sensitive probe of subsurface structural and electricalchanges.

I. INTRODUCTION

Silicon is a technologically important material and isthe workhorse of the semiconductor industry. There hasbeen substantial interest in its mechanical properties,most particularly the phase transformations that occurunder high-pressure loading and on subsequent pressurerelease. High-pressure diamond anvil and indentation ex-periments have revealed the following behavior. Onloading, a transformation from crystalline Si-I or a-Si toa conducting metallic phase (Si-II for the case of c-Si)occurs at a critical pressure of ∼11 GPa.1–3 For diamondanvil experiments, this phase has been observed to trans-form to high-pressure phases (Si-III and Si-XII) duringpressure release as the metallic phase is unstable belowpressures of ∼8 GPa.1 Indentation experiments have re-vealed further possible phase transformations on unload-ing. For fast unloading, the metallic phase transforms to

a-Si, but on slow unloading it transforms to a mixture ofhigh-pressure polycrystalline phases (Si-III and Si-XII).4–6 This pressure-induced crystallization is of greatinterest and workers have investigated the transformationpathways,7–14 but the exact mechanisms behind the phe-nomena are not understood. In situ characterization tech-niques are attractive approaches to complementing exsitu measurements such as Raman microspectroscopyand cross-sectional transmission electron microscopy(TEM) to study these phase transformations.

Electrical characterization has previously been per-formed by indenting between metal contacts spaced afew microns apart: current flows between these contactswhen they are electrically connected with metallic Si-II.11 More recently Bradby et al.9 developed an in situelectrical characterization method that is sensitive tothe Si-I-to-Si-II phase transformation by utilizing theSchottky-to-ohmic transformation of the indented area ofreverse-biased evaporated Al contacts on Si during in-dentation loading. A sudden increase in current flowthrough the contact occurs concurrently with this phasetransformation. Both techniques have proven to be

a)Address all correspondence to this author.e-mail: [email protected]

DOI: 10.1557/JMR.2007.0100

J. Mater. Res., Vol. 22, No. 3, Mar 2007 © 2007 Materials Research Society578

effective in monitoring the Si-I-to-Si-II phase transfor-mation where the conductivity of the silicon increases bymany orders of magnitude. However, these methods lackthe sensitivity required to detect the presence of high-pressure crystalline phases formed during unloading be-cause the technique relies on the ratio of the resistance ofthe indented material to the resistance of the rest of thesilicon to be of similar magnitude to the ratio of theindented area to the Al contact area (i.e., >106). Thissensitivity problem can be overcome, and some experi-ments have reported the measurement of contact resis-tance in situ via a conducting indenter tip but lack de-tail.15,16

In this article, we describe a method that measurescurrent flowing directly through a conductive diamondnanoindenter tip and across the tip/sample contact. Thissystem is extremely sensitive to conductivity changes inthe subsurface material below the indenter tip, wherebysmall changes in conductivity produce observablechanges in the electrical signature. In addition, the setupallows the applied voltage to be scanned at any point onthe load-unload curve to extract I-V characteristics. Thisprovides a means to investigate electrical barrier poten-tials among the different phases of silicon at variouspoints during the load/unload cycle. Careful consider-ation of a number of factors that control the measuredelectrical signal is required to interpret the data, and thisarticle addresses these issues to provide a foundation forinterpreting electrical data from nanoindentation experi-ments.

II. EXPERIMENT

All indentations were performed using a Hysitron Tri-boindenter (Minneapolis, MN) fitted with a Berkovichconducting diamond tip that is boron-doped to a nominalresistivity of 3.3 �-cm. The maximum load achievableon this instrument is 10,000 �N, resulting in a penetra-tion in silicon of ∼170 nm and ensuring the onset of apressure-induced phase transformation. The use of a con-ducting diamond tip ensures that the electrical measure-ments are representative of typical indentations per-formed using nonconducting diamond tips.

Figure 1(a) is a schematic of the through-tip electricalmeasurement setup that has been incorporated into theHysitron instrument. A voltage is applied to the samplestage to which samples are electrically connected, andcurrent flows through the sample and tip, with the latterheld at ground potential. An external Keithley 6487 pi-coammeter/voltage source (Cleveland, OH) provides thevoltage, measures the circuit current (sensitivity ∼10 pA)and transfers this to a personal computer with Keithleydata-logging software. The electrical measurements aremade in two modes. One tracks the current flowingthrough the indenter tip during indentation with a fixed

voltage applied across the sample-tip system. The secondsweeps the voltage and measures the current at each volt-age step, allowing current-voltage (I-V) characteristics tobe extracted at any point in the load/unload cycle.

FIG. 1. (a) Schematic of the in situ electrical measurement system onthe Hysitron Triboindenter. The sample is electrically connected to theconductive stage to which a voltage is applied and current flowthrough the sample and tip is measured. (b) The equivalent circuitdiagram of the series arrangement of resistances and rectifying junc-tions. The components enclosed with a dashed line are those extracontributions that are associated with indentation in Si, compared tothe simple case of Au.

S. Ruffell et al.: An in situ electrical measurement technique via a conducting diamond tip for nanoindentation in silicon

J. Mater. Res., Vol. 22, No. 3, Mar 2007 579

Two samples were used in this study. The first was athermally evaporated 800-nm-thick Au film of ∼5 mmdiameter on a silicon substrate. In this case, the Si wasacting only as a mechanical support for the Au film. TheAu layer was electrically connected to the stage via con-ducting silver paste so that current did not flow throughthe silicon substrate (the resistance between the stage andthe evaporated metal was ∼2 �). This sample allowed thetip conductivity and tip/sample contact properties to beinvestigated while removing complications caused by theknown material property changes in the case of siliconindentation. Figure 1(b) shows that the tip resistance andtip/Au contact resistance are the only resistances thatneed to be considered for this case. Additional circuitcomponents need to be added when indenting silicon [seeFig. 1(b)], as described in Sec. II. A. The second sampletype (for investigation of the indentation in Si) wascleaved from a Czochralski grown Si(100) substrate thatwas p-doped with boron to a resistivity of 0.006 �-cmwith a 7-�m epilayer also p-doped to a resistivity of 5�-cm. The 100-nm-thick aluminum contacts wereevaporated to the back and front sides of the samples,making ohmic contacts to the highly doped substrate(without sinter annealing) and, when required, to the epi-layer following sinter annealing at 550 °C for 3 min.These contacts merely serve as a means to electricallyconnect the sample to the indenter stage, whereby currentcan be measured through the sample, as indicated in Fig.1(a). Indentation is performed at a location on the samplesurface away from these contacts.

The remainder of this experimental section is dividedinto subsections that outline technical issues relevant inthe measurement. Each section includes a description ofthe relevant experiments performed.

A. Circuit resistances

Current flows through the following series resistances[see Fig. 1(b)]: the resistance of the tip; the tip/samplecontact; the phase transformed/stressed material (if any);and the resistance of the rest of the sample, includingspreading resistance contributions originating from thesmall tip/sample contact area (∼104–105 nm2). The inter-faces between these components add rectifying or ohmiccontact resistances. It should be noted that resistive heat-ing during these measurements is negligible. Anyincrease in temperature would result in changes in themechanical response of the silicon. Load/unload cyclesperformed with and without electrical measurementswere identical.

For the silicon samples, which measure ∼10 × 10 mm2,the 7-�m-thick epilayer (resistivity 5 �-cm) has a resis-tance of 3.5 × 10−3 �, and the 500-�m-thick substratehas a resistance of 3 × 10−4 � (resistivity 0.006 �-cm).As will be shown in Sec. III, the measured resistancesof the complete circuit are usually orders of magnitude

greater than these values, indicating that contact resis-tances and resistance changes under the indenter domi-nate the overall conductivity. Contacts at variouspositions on the front and back of the samples connectedto the indenter stage verify that these external metal-semiconductor contacts are not dominant, because meas-ured electrical currents are unaffected by the formation,location, and size of the ohmic contact.

The indenter tip is shaped on the end of a rectangular(diamond) parallelepiped of dimensions ∼1 × ∼0.3 ×∼0.3 mm. Using the quoted resistivity of 3.3 �-cm,17 theresistance of the tip is ∼300 �. At around room tempera-ture, the resistivity of diamond has a strong temperaturedependence, so this resistance can vary significantly(∼50%) with room temperature variations. However,even with such variations the tip resistance remains smallcompared to the resistance of the rest of the circuit.

The calculated spreading resistance18 [R � (�/2�r)tan−1(2t/r)] where, � is resistivity of the layer, r is theradius of the contact area, and t is the layer thickness)contribution originating from current spreading from asmall contact area into the epilayer and substrate ofthe silicon can be on the order of total measured resis-tance values (see Sec. III), particularly for low loads andvery small contact areas. Hence, it is necessary to takeaccount of this contribution to extract material resistancechanges. At the maximum load of 10,000 �N, we notethat the contact radius is ∼480 nm and a spreading resis-tance of ∼30 k� is obtained, which may be negligiblysmall for many measurements. However, at low loadsthis component becomes nonnegligible and even domi-nant: at a contact radius of 100 nm, for example, thespreading resistance is equal to ∼100 k�. Such effectsare not relevant for the case of Au because the very lowresistivity of Au determines that spreading resistancecontributions are <200 � due to the high conductivity ofAu compared to Si. Thus, in the case of Au, this leavesonly the contact resistance (between the diamond tipand the Au) as the dominant contribution to measuredcurrent/resistance.

To summarize, indentation in Au was performed underconditions that allow the contact resistance to be meas-ured directly from I-V data. In the case of Si, for loadsthat result in metallic Si-II, both the contact resistancebetween the tip and Si-II and that between Si-II and therest of the sample can be investigated with correction forthe spreading resistance.

B. Contact area

The contact resistance can become the dominant resis-tance in the complete circuit, particularly as the load in-creases and the spreading resistance becomes negligible.

Tip area functions were generated by indentation intofused quartz (a standard procedure for mechanical testing


J. Mater. Res., Vol. 22, No. 3, Mar 2007580

by indentation19). This provides a measured area, “pro-jected” onto the sample surface, as a function of contactdepth. The effective area that determines the magnitudeof current, however, is the “surface” area of the tip incontact with the indented material. This area increaseswith loading, thus varying the contact resistance duringthe loading/unloading cycle. For nonideal tips, details ofthe exact shape are required to calculate this area as afunction of contact depth. In addition to this, it is ex-tremely difficult to quantify how the material deformsaround the tip during indentation. For example, the vol-ume of extruded material that remains in contact with thetip is important for quantifying contact resistance.

Hence, to determine whether measured changes in cur-rent with load are only from contact area changes or fromsubsurface conductivity changes, current-versus-timedata need to be normalized by the contact area.

C. Pressure-induced phase transformations/conductivity changes

The interpretation of the electrical measurements forsilicon requires the consideration of other factors. First,conductivity of the silicon directly under the indenter tipboth before and after phase transformations is pressure-dependent20 and will vary with indentation load. Also,different phases of silicon can introduce significant bar-rier potentials with respect to the surrounding material.

In contrast to Si, indentation into the Au films causesno phase transformations, and it is therefore assumed thatthere are no electrical property changes. The main con-tributing factor that varies during indentation is the con-tact area between the tip and the Au. Hence, indentationinto these relatively soft layers (Au hardness 0.3–0.6 GPa19,21; Si hardness ∼11 GPa19,22) allows investi-gation of the electrical properties of the diamond tip andcontact resistance, while removing artifacts resultantfrom material compression and transformation beneaththe tip.

D. Sample surface

Because the measured current can be influenced oreven dominated by the tip/sample contact, the surfacestate of the silicon samples may be significant. Siliconexposed to the atmosphere readily forms a surface oxide(1.5–2 nm thick). Upon loading, therefore, the diamondtip is not in direct contact with the silicon, and currenthas to flow through this thin native oxide as well as overelectrical barriers between both the silicon and oxide andthe oxide and diamond tip.

To investigate this effect, a silicon sample was cleanedin organic solvents and placed in dilute hydrofluoric acid(10:1 H2O to 48% HF) to strip this oxide immediatelybefore mounting in the indenter for indentation. Current-voltage curves taken at various loads were compared

with those taken from a silicon sample still containing anative oxide.

E. Tip wear

Finally, it is necessary to consider changes in the tipgeometry with time. It has been found that the diamondindenter tips wear significantly when indenting and sur-face scanning hard materials such as silicon.23 Tip blunt-ing reduces the tip penetration depth for a particular load,and changes in the tip shape will produce variations inthe contact area, thus affecting the extraction of resistiv-ity data from the electrical measurements.

Tip area functions can be extracted via indentationsinto fused quartz, the mechanical properties of which arewell known, and this was done over the period of thisstudy to monitor tip wear. It should be noted, however,that the Triboindenter can be run in a scanning atomicforce microscopy (AFM) mode in which the indenter tip isscanned across the sample surface to map the topography.This mode of operation has been found to cause a substan-tial increase in tip wear compared to the indentationmode23 and was used during the time period of this study.

Although tip wear causes variations in the absoluteelectrical current measurements, this can be minimizedby not running the indenter in scanning mode23 and byregularly performing tip area function measurements sothat the changes in surface contact area can be monitoredand accounted for (see Sec. III).

III. RESULTS AND DISCUSSION

A. Contact area and resistivity

Figure 2 shows the I-V characteristics extracted at in-creasing indentation loads in the Au films (maintaining

FIG. 2. I-V curves extracted at maximum loads of 50, 500, 1000, and2000 �N (intermediate loads are omitted for clarity) from indentationsin an 800-nm evaporated Au film. The tip/sample contact exhibits fairohmic behavior over the range of loads. The measured resistance isextracted by applying linear least-squares fit to the data.



penetration depths of <80 nm to prevent mechanical in-fluences from the underlying Si substrate as the Au thick-ness was ∼800 nm). The Au–diamond contact exhibitsfair ohmic behavior over the range of indentation loads.As expected, the absolute current increases with load asthe contact area increases. The measured resistance canbe extracted from a linear fit to the measured I-V data.

Figure 3 shows I-V characteristics extracted duringindentation in silicon at loads of >5000 �N. This load issufficient to cause the Si-I-to-Si-II phase transformation,as indicated by the observation of a pop-in event in theload–unload curves (see inset of Fig. 3). At these loads,the tip is in contact with transformed Si-II and extrudedSi-II, which immediately transforms into another lessconducting phase. The pressure release in this case israpid, suggesting that the Si-II transforms to a-Si.4–6 Janget al.24 mapped the extruded material around indents viaRaman microspectroscopy and concluded it was a-Si orpossibly nanocrystalline Si. Gogotsi et al.25 reported ob-serving a-Si, Si-IV, and Si-III/XII in the extruded mate-rial. In situ electrical measurements by Bradby et al.9

showed evidence that the extruded material is certainlyless conductive than Si-II even if it is not a-Si. For theI-V characteristic measurements in Fig. 3, the tip is ef-fectively in electrical contact only with Si-II because theextruded material will be much less conductive. The re-sistance indicated from Fig. 3 is ohmic, suggesting thatthe contact between the tip and Si-II and the interface

between Si-II and the surrounding Si-I material is resis-tive, without the complicating influence of potential bar-riers between different phases.

The measured resistance during indentation in Au andSi (Figs. 2 and 3) was extracted by applying a linear fitto the I-V data. For the case of Si, the spreading resis-tance calculated from the contact area has to be sub-tracted, leaving the measured contact resistance. Thisspreading resistance is plotted in the inset of Fig. 4 as afunction of contact radius.

The contact resistivity should be constant during load-ing if we assume that the materials in contact, in this casethe diamond tip and Au or the tip and Si-II, have thesame electrical contact properties as the contact area in-creases. Contact resistivity is calculated by multiplyingthe measured resistance by the surface contact area. Thesurface area of contact for an ideal Berkovich tip can becalculated using the contact depth extracted from theload/unload curves. Using this approach, Fig. 4 showsthe calculated contact resistivity versus contact depth forindentations performed in Au and Si. The resistivity isnot constant, and, as expected because of the rounding ofreal Berkovich tips, the extracted surface contact area isoverestimated, as indicated by the increasing resistivity.

Assuming that the contact resistivity does not changewith increasing load, the surface contact area functioncan be extracted from Au indentation resistance data byforcing a constant contact resistivity. This is plotted inFig. 5 using estimates of the contact resistivities forAu/diamond taken from the literature (see below). This

FIG. 3. I-V curves extracted from indentation in Si at loads of 5000,6000, and 10,000 �N (curves measured at loads of 7000, 8000, and9000 �N are omitted for clarity). These loads ensure that the Si underthe indenter tip has transformed to Si-II, because the load is greaterthan the elastic–plastic transition (marked by an arrow on the inset,which shows a load-versus-depth curve for a typical load/unloadcycle). These I-V curves are determined by both the spreading resis-tance from the small contact area and the tip/Si-II contact resistivity.The system exhibits fair ohmic behavior and resistance is extracted bya linear least-squares fit to the data. It should be noted, however, thatthe slight “S” shape that is resultant from the small contact area willintroduce a systematic error in the resistance extracted from the linearfit for all of the curves.

FIG. 4. Calculated contact resistivity (i.e., measured resistance mul-tiplied by surface area of contact) for indents in Au and Si (loadsgreater than that for the Si-I-to-Si-II phase transformation) calculatedassuming an ideal Berkovich tip geometry. The resistivity increaseswith contact depth, indicating that the area is overestimated. The re-sistance is corrected for spreading contributions due to the small areaof contact. The contact radius is extracted from the projected areaassuming a circular contact area. The spreading resistance as a func-tion of contact radius is shown in the inset (arrows mark loads of 5000and 10,000 �N and the corresponding contact radius in Si).



curve for Au produces a reasonable fit for the measuredresistances extracted from the indentation in Si (Fig. 5)provided that a different contact resistivity is used (9.9 ×109 �-nm2 rather than 6 × 109 �-nm2 for Au). There isno evidence to suggest that the contact resistivities fordiamond/Au and diamond/Si-II should be equal.

Comparison of the measured contact resistivity to pub-lished data is not straightforward due to the difficulty inestimating the real contact area in our experiments. How-ever, the contact resistances measured for Au with thediamond tip, despite the uncertainty in contact area, areon the order of published values for metals evaporatedonto chemical vapor-deposited diamond films.26,27 Thetip has a resistivity of 3.3 �-cm, which corresponds to aboron concentration of ∼1021 cm−3.28,29 For a Au-Ti con-tact on CVD diamond, doped to a boron concentration of7 × 1020 cm−3, the contact resistivity has been found to be∼6 × 109 �-nm2.26 We have used this normalizing valuein Fig. 5 for Au.

B. Oxide stripping and indentation

Figure 6 shows I-V characteristics taken from anetched and nonetched epi-Si sample at various loads be-low 1000 �N. The curves from the sample that had noHF treatment lie approximately on top of each other. Thecontact with the tip and sample is rectifying, but forward-biased current is limited in some way, possibly by abarrier imposed by the need for carriers to tunnel throughthe oxide. Jeffery et al.30 investigated the effect of ex-ternal stress (applied by an indenter) on the breakdownfield strength of the native oxide on silicon. The electri-cal breakdown voltage decreases with increased loading;however, we see no such effect here, with the breakdownvoltage remaining constant for the loads up to 1000 �N.

As the indentation load exceeds ∼1000 �N, the I-V char-acteristics follow the same (increasing) trend as for theoxide-etched sample. This suggests that at high loads theoxide may be ruptured or deformed, allowing breakdown and increased current to flow.

The native oxide does not appear to have an effect onthe load/unload curves when loading to 10,000 �N. Theywere found to be identical for samples with and withoutthe native oxide. For lower loads, variations in the posi-tion of the pop-in event and the shape of load/unloadcurves have been reported to vary with oxide thick-ness,31,32 although the pop-in is still associated with thephase transformation to Si-II. Thus, at low loads, caremust be taken in interpreting the electrical measurements(and mechanical response) because they are dominatedby surface oxide effects unless the oxide is removed.

C. Tip area functions

Figure 7 shows two area function files for the conduc-tive diamond tip used in this study that were taken

FIG. 5. Surface area versus contact depth required to normalize thecontact resistivities for indentation in Au and Si. Resistivity values of6 × 109 �-nm2 and 9.9 × 109 �-nm2 are used for Au and Si, respec-tively. The rate of increase of surface contact area with depth is thesame for indentation in both materials, as expected if the contact areafunction of the tip is controlling this increase.

FIG. 6. I-V curves extracted at indentation loads up to 1000 �N insilicon: (a) in a silicon sample immediately etched in HF to remove theoxide prior to indentation; (b) in an identical silicon sample withoutoxide stripping.



approximately 2 months apart. Shown in the inset are I-Vcurves extracted at a maximum load of 10,000 �N usingthe same indent conditions but at the two times corre-sponding to the area function files. The measured currentincreases, at the same load, as the tip wears and becomesblunter.

As long as care is taken to minimize tip wear andfrequent area functions (both projected area and surfacearea) are measured, this variation in tip geometry can beaccounted for in the electrical measurements.

D. Indentation in silicon

Figure 8(a) shows the current versus time for an in-dentation in crystalline silicon for which the load/unloadcurve is shown in Fig. 8(b). A voltage of +1 V wasapplied to the sample stage, and loading up to 10,000 �Nwas completed in 30 s with unloading performed at thesame rate (the load versus time is also plotted on thesame graph). Before the pop-in event at ∼1200 �N onloading, 10 pA to 10 nA of current is measured [notvisible on the linear scale in Fig. 8(a)], but this currentincreases sharply to the �A range at loads close to pop-in. On unloading, an increase in current is observed at asimilar load to the beginning of the elbow on the unload-ing portion of the load/unload curve. Sensitivity to theseloading curve features in electrical measurements has notbeen observed previously. This elbow is thought to be aphase transformation to a-Si, possibly containing tracevolumes of crystalline Si-III and Si-XII.4,33,34

By applying a low load where no silicon deformationhas occurred, it is possible to extract electrical (I-V) char-

acteristics before and after phase changes induced at highloads. The low load is simply a means to use the indentertip to measure I-V characteristics without altering theunderlying material properties. Figure 9(a) shows I-Vcurves extracted before and after the 10,000 �N loadingcycle by sweeping the voltage while holding at a load of100 �N. First, the contact between the indenter tip andsilicon is rectifying. Because we have shown that thecontact between the tip and Au and the tip and metallicSi-II are ohmic (Figs. 2 and 3), indicating that the heavilydoped diamond tip behaves more like a metal, a rectify-ing contact implied that the tip forms a Schottky-likecontact with untransformed low-doped silicon. Second,substantial differences in the I-V curves are observed,particularly for reverse characteristics, as shown in Fig.9(b), which is a plot of the ratio of the currents before andafter loading. This behavior indicates that differences inthe resistivity and/or barrier potentials within the in-dented material can be detected by our present technique.

Finally, electrical measurements are sensitive to all ofthe phase transformations that silicon undergoes during

FIG. 7. Area functions extracted by indentation in fused quartz. Eachwere taken approximately 2 months apart (�, beginning of study; �,end of study). The inset shows I-V curves extracted at a load of 10,000�N during indentation into a silicon sample at times corresponding tothe area functions. These curves both have a slight asymmetry, indi-cating that a slight electric potential barrier exists at maximum load.This barrier height varies with tip wearing, which is observable in thelarger difference between I-V curves in forward bias.

FIG. 8. (a) Current and load versus time for a 10,000-�N indentationin a silicon sample. Substantial current is measured with the “pop-in”(marked by A) feature on the load-versus-penetration depth curveshown. (b) An increase in current is also measured at the onset of the“elbow” (marked by B) on the unloading portion of the indentation.



indentation, including the formation of Si-II during load-ing and mixtures of Si-III/Si-XII and a-Si during unload-ing. The difference in the conductivity of the indentedsilicon before and after indentation can also be readilyprobed, with large differences observable in the electricalsignature. Some of these features have been observedelsewhere,15,16 and more detailed results and discussionof electrical measurement during indentation into c-Siwill be reported elsewhere.35

IV. CONCLUSION

A new in situ electrical characterization technique forthe study of nanoindentation has been described. The

method uses a doped diamond tip for indentation, allow-ing through-tip/sample current measurements during in-dentation. Several technical issues associated with thetechnique have been considered, and experiments havebeen performed to quantify their effects on electricalmeasurements. Indentation experiments on deposited Aufilms provide a simple case in which the mechanicaldeformation induced during indentation is not expectedto modify the electrical properties of the material. Hence,for Au, the measured current variation with loading sim-ply reflects the changing contact area. Estimates for thiscontact area during indentation are nontrivial due to ma-terial extrusion, and further effort is required so that theexact area is known throughout an indentation cycle.When indenting silicon (the more complicated case) athigh loads, the major material in contact with the tip ismetallic Si-II, but extruded material is insulating a-Si anddoes not play a role in electrical measurements. Hence,for Si, material extrusion is not expected to increase theelectrical contact area. For both Au and Si, our data areconsistent with a constant contact resistivity that is con-stant with load after accounting for the geometry of theindenter tip.

Initial results from indentation in c-Si demonstrate theexcellent sensitivity of the present electrical measure-ment technique to subtle changes in the silicon beneaththe indenter tip. Work is continuing on the study of in-dentation-induced phase transformations in silicon usingthis in situ method in combination with other ex situtechniques. To date, this approach is proving to be ex-tremely promising for obtaining improved understandingof transformation pathways and mechanisms in nanoin-dented silicon. We anticipate that the technique will beapplicable to other systems that experience changes inelectrical resistivity under indentation.

ACKNOWLEDGMENTS

The authors would like to thank Hysitron Inc., withwhom we are developing this measurement technique.This work was funded by the Australian Research Coun-cil and WRiota PTY Ltd.

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An in situ electrical measurement technique via a conducting diamond tip for nanoindentation in silicon