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Contract No. DE-AC36-08GO28308

A Novel Way to Characterize 

Metal-Insulator-Metal Devices 

via Nanoindentation

Preprint

Prakash Periasamy and Corinne E. Packard

Colorado School of Mines and National Renewable 

Energy Laboratory 

Ryan P. O’Hayre

Colorado School of Mines

Joseph J. Berry, Philip A. Parilla, 

and David S. Ginley

National Renewable Energy Laboratory

Presented at the 37th IEEE Photovoltaic Specialists Conference 

(PVSC 37)

Seattle, Washington

June 19-24, 2011

Conference Paper

NREL/CP-5200-50727

July 2011
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1

A NOVEL WAY TO CHARACTERIZE METAL-INSULATOR-METAL DEVICES 

VIA NANOINDENTATION

Prakash Periasamy1,2, Ryan P. O’Hayre1, Joseph J. Berry2, Philip A. Parilla2, David S. Ginley2, and Corinne E. Packard1,2

1

Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, Colorado, USA 2

National Renewable Energy Laboratory, Golden, Colorado, USA

ABSTRACT

Metal-Insulator-Metal (MIM) devices are crucial 

components for applications ranging from optical 

rectennas for harvesting sunlight to infrared detectors. To 

date, the relationship between materials properties and 

device performance in MIM devices is not fully 

understood, partly due to the difficulty in making and 

reproducing reliable devices. One configuration that is 

popular due to its simplicity and ease of fabrication is the 

point-contact diode where a metal tip serves as one of the 

metals in the MIM device. The intrinsic advantage of the 

point-contact configuration is that it is possible to achieve 

very small contact areas for the device thereby allowing 

very high-frequency operation. In this study, precise 

control over the contact area and penetration depth of an 

electrically conductive tip into a metal/insulator 

combination is achieved using a nanoindenter with in-situ 

electrical contact resistance measurement capabilities. A 

diamond probe tip, doped (degeneratively) with boron for 

conductivity, serves as the point contact and second 

‘metal’ (b-Diamond) of the MIM diode. The base layer 

consists of Nb/Nb2O5 thin films on Si substrates and 

serves as the first metal /insulator combination of the MIM 

structure. The current-voltage response of the diodes is 

measured under a range of conditions to assess the 

validity and repeatability of the technique. Additionally, we 

compare the results of this technique to those acquired 

using a bent-wire approach and find that Nb/Nb2O5/bDiamond MIM devices show an excellent asymmetry (60-

300) and nonlinearity values (~6-9. This technique shows 

great promise for screening metal-insulator combinations 

for performance without the uncertainty that stems from a 

typical bent-wire point-contact.

INTRODUCTION

Although many exciting developments had been achieved 

in the PV arena, significant advances are still required 

from this and other clean energy technologies in order to 

compete with fossil fuels. In particular, transformative 

concepts that can provide dramatically higher efficiencies 

are increasingly desired. One such PV concept is the 

‘optical rectenna’ (antenna + rectifier), in which sunlight is 

captured as a wave via an antenna and subsequently 

converted directly to DC power using a suitable rectifier. 

The calculated conversion efficiency of such a rectenna 

device operating in the visible region is as high as 90% [1]. 

Despite remarkable potential gains in efficiency, rectenna 

technology is still in the nascent stage because of 

significant challenges, including the fabrication of a 

suitable ultra-high frequency rectifier [2, 3]. To ensure high 

rectification efficiency in the visible light (300-750 THz) 

regime, the rectifier should have (a) a fast electron 

transport mechanism, (b) device area of nanometer scale 

(to eliminate high-frequency capacitive leakage), and (c) 

efficient coupling with the antenna and other circuitry in 

the rectenna.

The Metal-Insulator-Metal (MIM) diode is a candidate 

rectifier device for rectenna applications due to the 

possibility of ultra fast rectifying electron transport via 

asymmetric tunneling. Various groups [4-8] have studied 

MIM devices over the last 5 decades. To date, the 

relationship between materials properties and device 

performance in MIM devices is not fully understood, partly 

due to the difficulty in making and reproducing reliable 

devices. One configuration that is popular due to its 

simplicity and ease of fabrication is the point-contact diode 

where a metal tip serves as one of the metals in the MIM 

device. The intrinsic advantage of the point-contact 

configuration is that it is possible to achieve very small 

contact areas for the device thereby allowing very highfrequency operation. The present authors have used a 

bent-wire point-contact MIM diode configuration to identify 

promising MIM combinations [9]. Major limitations with the 

bent-wire approach include the uncertainty in the resulting 

diode area and the lack of control on the penetration depth 

of the bent-wire into the insulator. This makes it difficult for 

quantitative analysis to compare MIM devices with 

different material combinations. In the present study, these 

challenges are overcome through the use of a 

nanoindenter with in situ electrical contact resistance 

measurement capabilities, which enables precise control 

over the contact area and depth of penetration of an 

electrically conductive tip into a metal-insulator 

combination. I-V measurements are repeatedly and 

reproducibly acquired at a range of fixed penetration 

depths.

EXPERIMENTAL DETAILS

A nanoindenter with nanoscale electrical contact 

resistance (nanoECR) capability is used in this study. The 

probe is a 3-sided pyramid-shaped boron-doped 

(degenerately) diamond tip. In this study, Nb/Nb2O5 bilayer 

thin films (on a Si substrate) are the metal 1/insulator 

combination of the MIM structure [9]. The Nb film (90 nm 

thick) was sputter deposited and anodically oxidized in 1M 

H2SO4 solution to grow Nb2O5 (15 nm thick). Contacting 

the boron-doped diamond tip (b-Diamond) on to the 
2

bilayer completes the MIM (Nb/Nb2O5/b-Diamond) diode 

as schematically shown in Fig. 1.

Figure 1 Schematic representation of MIM diode with 

the B-Diamond Indenter.

Figs. 2 and 3 show schematics of typical load function 

and load-displacement curves, respectively. A typical 

measurement involves contacting the b-Diamond tip to the 

insulator and then increasing the contact force to the 

desired value (loading). While holding at this point, the I-V 

is measured. Then unloading proceeds and the tip is 

retracted away.

Figure 2 Typical loading function used while 

measuring I-V curves with the nanoindenter.

The performance metrics of the devices are characterized 

via asymmetry and nonlinearity. Asymmetry is defined by 

the ratio of forward to reverse current. Nonlinearity is given 

by (dI/dV)*(V/I), where V and I are the voltage and current, 

respectively [7].

Figure 3 Typical Load-Displacement curve showing 

the maximum load and displacement.

RESULTS AND DISCUSSION

Fig. 4 shows the current-voltage (I-V) plot of Nb/Nb2O5/bDiamond MIM diodes for several tests that varied the 

displacement of the indenter into the oxide. The current 

magnitude increases with the increase in displacement 

(inset in Fig. 4) as expected because of reduced tunneling 

distance and increased device (contact) area. Figs. 5(a) 

and (b) show the asymmetry and nonlinearity curves as a 

function of displacement, respectively. The device shows 

excellent asymmetry and nonlinearity metrics. While 

asymmetry increases with displacement, nonlinearity is 

almost independent of displacement.

A very similar trend is noted in bent-wire point-contact 

experiments as a function of displacement (although in the 

bent-wire case, quantitative determination of contact area 

and pressure is not possible [9,10]). In a previous work by 

several of this study’s authors, different metal bent-wires 

were used as metal 2 and several I-Vs were taken for 

each of the wires [9]. In such studies, it was observed that 

the nonlinearity is the most reliable performance metric of 

the two considered here to characterize different MIM 

devices; nonlinearity was found to be sensitive to different 

metal combinations, but independent of the bent-wire 

penetration depth. In this present study, Fig. 6 further 

corroborates that nonlinearity is independent of 

displacement and thus an excellent, geometry-insensitive 

direct measure of MIM performance. Further results will be 

reported on the reproducibility of I-V curves using bDiamond tip in both constant-force and constantdisplacement modes. This technique provides a platform 

upon which different metal-insulator combinations will be 

characterized and analyzed quantitatively.
3

Figure 4. I-V curves of Nb/Nb2O5/B-Diamond as a function of penetration depth. Upper inset shows a zoomed 

view of the I-V to show the trend in current with increasing penetration depth (indicated by the arrow). Reported 

penetration depth (hmax) values are obtained from the load-displacement curves as shown in Fig. 3. The error on 

the reported hmax values is ± 2 nm.

2.5 nm

7.5

12
4

Figure 5: (a) Asymmetry and (b) Nonlinearity as a function of penetration depth. The device shows excellent 

asymmetry (60-300) and nonlinearity (6-9) values above 3V. Asymmetry increases with penetration depth 

whereas nonlinearity is independent of penetration depth in this range.

CONCLUSIONS

To summarize, characterizing MIM diodes via 

nanoindentation shows significant promise as a facile 

and systematic approach. For the first time we have 

reported MIM diodes fabricated using a nanoindenter 

with excellent asymmetry (60-300) and nonlinearity 

values (~6-9). Nonlinearity is found to be independent of 

penetration depth.

ACKNOWLEDGEMENTS

This work was supported by the U.S. Department of 

Energy under Contract No. DE-AC36-08-GO28308 with 

the National Renewable Energy Laboratory, as part of 

the Laboratory Directed Research and Development 

Program. This material is partially based upon work 

supported by the National Science Foundation through 

the Renewable Energy Materials Research Science and 

Engineering Center under Grant No. DMR-0820518.

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a Rapid Screening Technique”, Advanced Materials, 

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(a) (b)