578_Generation of Electricity in GaN Nanorods Induced by Piezoelectric Effect

8/3/2019 578_Generation of Electricity in GaN Nanorods Induced by Piezoelectric Effect

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Generation of electricity in GaN nanorods induced by piezoelectric effect

W. S. Su and Y. F. Chena

Department of Physics, National Taiwan University, Taipei, Taiwan 10617, Republic of China

C. L. Hsiao and L. W. TuDepartment of Physics and Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University,Kaohsiung, Taiwan 80424, Republic of China

Received 4 December 2006; accepted 10 January 2007; published online 8 February 2007

Conversion of mechanical energy into electric energy has been demonstrated in GaN nanorods. The

measurement was achieved by deflecting GaN nanorods with a conductive atomic force microscope

PtIr tip in contact. The mechanism relies on the coupling between piezoelectric and semiconducting

properties in GaN nanorod, which creates a strain field and drives the charge flow across the

nanorod. The result shown here opens up an opportunity for harvesting electricity from wasted

mechanical energies in the ambient environment, which may lead to the realization of self-powered

nanodevices. 2007 American Institute of Physics. DOI: 10.1063/1.2472539

For the investigation of nanoscale science and technol-

ogy, it will be very attractive if one can develop a self-

powered nanodevice without depending on a battery. In this

way, one may be able to create wireless devices with a por-table integrated system. It will be even more alluring if the

nanodevice can harvest electricity from the wasted energy in

the ambient environment. Here we made an initial attempt to

convert mechanical energy into electric energy in GaN nano-

rods. It is well known that nitride semiconductors have the

wurtzite crystal structure, which has the two important char-

acteristics, including the noncentral symmetry and polar sur-

face. They lead to pronounced piezoelectric properties.1,2

We

demonstrated that with the usage of a moving atomic force

microscope AFM tip to deflect GaN nanorod, it is possibleto generate an output current. The underlying physics can be

well understood in terms of the coupling between piezoelec-

tricity and semiconducting property of GaN nanorod. Be-sides, we showed that the formation of the Schottky barrier

at the interface between the AFM tip and GaN nanorod as

well as the polarity of GaN nanorod can significantly influ-

ence the current output due to the bended nanorod.

The samples studied here were grown on Si 111 wafersby plasma-assisted molecular beam epitaxy Veeco-AppliedEpi 930. The Ga source is 7N5 pure metal in a conventionaleffusion cell. 6N pure N2 is further purified through a nitro-

gen purifier Aeronex and then fed into a plasma generator.The Si substrate was degreased and then lightly etched with

diluted HF. A reconstructed 77 reflection high-energy

electron diffraction pattern was obtained for the Si substrate

after thermal treatment at 800 C to ensure excellent surfacecondition. After low temperature buffer layer deposition at

500 600 C, high temperature GaN growth started at

720 C. Self-assembled vertical nanorods can be clearly

seen in the field emission scanning electron microscope

SEM images, as shown in Fig. 1a. Detailed studies withvarious growth parameters have been published elsewhere.

3

The AFM system we used was a standalone SMENA appa-

ratus NT-MDT in contact mode. The silicon tips of contactmode produced by NENOSENORS were coated by PtIr. The

thickness of the PtIr coating is about 23 nm. The force con-

stant is about 2.8 N/m. The tip was scanned over the top of

the GaN nanorod, and the height of the tip was adjusted

according to the surface morphology and local contacting

force. The thermal vibration of the nanorod at room tempera-ture was negligible. For the electric contact at the bottom of

the nanorod, Ohmic contacts were formed by depositing in-

dium drops to the samples, and annealing the samples at

240 C for 10 min. No external voltage was applied in any

stage of the experiment.

Figure 1b illustrates our experimental measurement forobtaining output current by deforming a piezoelectric nano-

rod with a conductive AFM tip. When the AFM tip was

scanned over the aligned nanorod, both the topography

feedback signal from the scanner and the correspondingcurrent images were recorded simultaneously; a typical result

is shown in Figs. 1c and 1d. In the image, sharp outputcurrent peaks were observed. These peaks are about several

times the noise level with a magnitude of 0.03 nA. It isinteresting that the location of the current peak can be corre-

aElectronic mail: [email protected]

FIG. 1. Color online a Scanning electron microscopy image of GaNnanorods. b Experimental setup and procedures for generating electricity

by deforming a piezoelectric nanorod with a conductive atomic force mi-

croscopy AFM tip. The AFM scans across a nanorod in contact mode. c

Topography image and d corresponding output current image of the nano-

rod arrays. e Line profiles from the topography and output current images

across a nanorod.

APPLIED PHYSICS LETTERS 90, 063110 2007

0003-6951/2007/906 /063110/3/$23.00 2007 American Institute of Physic90, 063110-1Downloaded 14 Mar 2008 to 140.117.109.242. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp
http://dx.doi.org/10.1063/1.2472539http://dx.doi.org/10.1063/1.2472539http://dx.doi.org/10.1063/1.2472539http://dx.doi.org/10.1063/1.2472539


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lated well with the site of the nanorod. It implies that the

output current is indeed due to the bended GaN nanorod. In

order to exclude the possibility that the output current may

arise from friction or contact potential, we have performed

the similar experiment for a metal film. It is found that there

is no detectable signal. The energy output generated by the

bended GaN rod can be roughly estimated as follows. The

full width at half maximum of the current peak is about

36 nm. The tip was scanned over a single nanorod at a scan-ning velocity of 12 042 nm/s. Therefore, the lifetime of the

output current is 18 ms. The nanorod can be approximated as

a resistor Rn and a capacitor Cn. The lifetime of the output

current across the load RL, which is 1.75 M for GaN film,

is = RL +RnCn. For the experiment studied here, the resis-

tance of the nanorod Rn is negligible, and the equivalent

capacitance of the nanorod is therefore equal to /RL, which

has a magnitude of about 10 nF. The electric power gener-

ated by the bended nanorod W CV02/ 2 Ci0R

2/ 2

1.5681017 J, where i0 is the peak value of the outputcurrent. The obtained W represents the generated electric

energy by one single nanorod in one single event. For a

typical resonant vibrational frequency of 5 MHz for GaNnanorod with a diameter of 10 nm and length of 1 m,4

and

the nanorod density of 1010/cm2, the estimated output power

density is about 0.75 W/cm2, which is quite feasible for a

nanomaterial based device.

As shown in Fig. 1e, when the tip starts to deflect thenanorod, the current output is observable. It sharply in-

creases and then decreases, and it drops to zero when the

deflection of the nanorod approaches its maximum. This be-

havior is in stark contrast with that of ZnO nanorod,5

in

which the output voltage is detectable only when the deflec-

tion of the nanowire reaches its maximum. It drops to zero,

when the nanowire is released by the AFM tip. To resolve

this subtle difference, let us have a more detailed examina-

tion of the electric field induced by the bended nanorod as

well as the interface consisting of the bended nanorod and

metallic contacts. In addition to its magnitude, the direction

of the generated piezoelectric field of the bended nanorod is

also an important factor to influence the current flow. Before

determining the direction of the piezoelectric field, we need

to know the polarity of the nanorod, which can be judged

from its resistence to H3PO4 etching.6

As shown in Figs. 2aand 2b, the SEM images of the surface morphologies be-fore and after etching are compared. It clearly shows that all

nanorods disappear after etching, which indicates that the

base area is Ga polar for its resistance to the etching, while

the nanorods are likely to be N polar for their dissolution by

the H3PO4. With the outer surface being stretched and theinner surface compressed as shown in Figs. 2c and 2d, thedeflected GaN nanorod by AFM tip will create a strain field.

The created electric field Ez is along the nanorod z directioninside the volume through the piezoelectric effect, Ez =z/d,

where d is the piezoelectric coefficient along the nanorod

direction. If the GaN nanorod owns the N polar, which

means the nitrogen atomic layer being the top-terminating

layer, the piezoelectric field direction is parallel to the z axis

at the outer surface and antiparallel to the z axis at the inner

surface. With respect to the ground base contact, the electric

potential of the compressed surface is negative, denoted as

V, while that of the stretched side is positive, denoted as V+.

At present, we did not know the exact magnitude for thevoltage V, which have to be calculated based on the ionic

charges induced by the piezoelectric effect and the surface

charges caused by the boundaries.

In our measurement, the bottom contact was an Ohmic

contact formed by depositing indium drops to the GaN film,

and annealing the sample at 240 C for 10 min. Because the

electron affinity of GaN is 4.2 eV,7

and the work function of

In is 4.12 eV, there is no barrier at the interface, and the

GaNIn contact is Ohmic. The work function of PtIr is

5.5 eV,8 and the electron affinity of GaN is 4.2 eV.7 There-fore, the PtIrGaN contact is a Schottky barrier and domi-

nates the charge conduction, as shown in Fig. 2f. We arenow ready to understand the current output, as shown in

Fig. 1e. When the AFM tip is in contact with the stretchedside of the nanorod, because the PtIrGaN contact is nega-

tively biased, which corresponds to a forward-biased

Schottky diode. Therefore, the output current is sharply in-

creased as shown in Figs. 2e and 2f as well as in Fig. 1e.When the AFM tip is in contact with the compressed surface

of potential V+, the PtIrGaN interface in this case corre-

sponds to a reverse-biased Schottky diode, and little current

flows across the interface as shown in Figs. 2g and 2f, as

well as in Fig. 1e. We therefore can see that the variation ofthe output current can be understood quite well in terms of

the coupling between piezoelectricity, semiconducting prop-

erties, and the characteristic of a Schottky diode.

To further probe the property of the current output in-

duced by the bended GaN nanorod, we have performed the

measurements for the AFM tip with different scanning

speeds. It was found that the magnitude of the output current

increases with the scanning speed, as shown in Fig. 3. To

quantitatively understand this behavior, let us consider the

detailed energy transfer process. When the tip sweeps across

the nanorod, the top of the nanorod will move with a speed

similar to that of the tip, and the kinetic energy will transfer

into the elastic energy of the bended nanorod. Under thefirst-order approximation, the elastic energy is proportional

FIG. 2. Scanning electron microscopy image of GaN nanorods. a Before

etching and b After etching. c Schematic definition of a nanorod and the

coordination system. d Corresponding longitudinal piezoelectricity in-

duced electric field Ez distribution in the nanorod. e and g Contactsbetween the AFM tip and the semiconductor GaN nanorod at two reversed

local contact potentials positive and negative. f Reverse and forward

biased Schottky rectifying behaviors.

063110-2 Su et al. Appl. Phys. Lett. 90, 063110 2007

Downloaded 14 Mar 2008 to 140.117.109.242. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp


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to the square of the displacement of the deflection S, and

therefore the displacement S is linearly proportional to the

scanning speed. Because the displacement S is linearly pro-

portional to the induced potential V according to the previous

reports,

4

V is thus also linearly proportional to the scanningspeed. The variation of the current output in terms of the

scanning speed should therefore follow the expression de-

scribing the current flow in a Schottky diode,

I= IseqV/kT 1, 1

where q is the magnitude of electronic charge, V is the ap-

plied voltage, k is the Boltzman constant, and T is the tem-

perature. Indeed, as we can see that the dependence of the

current output on the scanning speed can be described quite

well by Eq. 1 with q/kT replaced by a fitting parameter.This result therefore provides an additional evidence to sup-

port our proposed interpretation as shown above.

In summary, we have conclusively demonstrated the

conversion of mechanical energy into electric energy in GaN

nanorods. The obtained result can be well interpreted in

terms of the coupling between piezoelectricitivity, semocon-

ducting properties, as well as the characteristic of a Schottkycontact. In addition, the estimated output power is quite fea-

sible for the application in nanomaterial based devices. In

view of a wide range of applications based on nitride semi-

conductors, including full color displays, blue lasers, as well

as high temperature and high power microelectronics, our

study shown here should be very useful and timely.

This work was supported by the National Science Coun-

cil of the Republic of China.

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FIG. 3. Color online Variation of current output vs scanning speed of the

bended GaN nanorod. The red line is plotted according to the characteristics

of the current flow in a Schottky diode given by Eq. 1.

063110-3 Su et al. Appl. Phys. Lett. 90, 063110 2007

Downloaded 14 Mar 2008 to 140.117.109.242. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

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578_Generation of Electricity in GaN Nanorods Induced by Piezoelectric Effect