2D like photonic crystal using In2O3-SiOxheterostructure nanocolumn arrays and humidity sensing

Electron. Mater. Lett., Vol. 10, No. 5 (2014), pp. 975-980

2D Like Photonic Crystal Using In2O3-SiOx Heterostructure Nanocolumn

Arrays and Humidity Sensing

Naorem Khelchand Singh,1 Bijit Choudhuri,

2 Aniruddha Mondal,

2,* Jay Chandra Dhar,1

Tamal Goswami,2 Saptadip Saha,

2 and Chitralekha Ngangbam

3

1Department of Electronics and Communication Engineering, National Institute of Technology, Nagaland, Chumukedima, Dimapur-797103, India

2Department of Electronics and Communication Engineering, National Institute of Technology Agartala, Jirania, Tripura (West) 799055, India

3Department of Electronics and Communication Engineering, National Institute of Technology Manipur, Imphal (West) 795001, India

(received date: 19 November 2013 / accepted date: 14 February 2014 / published date: 10 September 2014)

Abstract: 2D like photonic crystal was fabricated with the help of GLAD synthesized In2O3-SiOx heterostructurenanocolumnar arrays. Different dielectric media like air and water were used to demonstrate the optical charac-teristics and band gap of the crystal. Nearly 33 nm red shift of the band gap was observed for wet sample as com-pared to dry. Broad band UV-Vis absorption has been observed for the dry In2O3-SiOx heterostructurenanocolumnar arrays, which decreases in wet condition. The device shows low current conduction at lowerhumidity, which enhances at higher humidity condition due to the absorption of water molecules from the envi-ronment by the porous surface. The device possesses 5.6 × 10−3 mA/cm2 current at 10%, which increases to 1.4 ×10−1 mA/cm2 at 99% humidity under applied potential of 2 V. The sample shows the color alteration from black(dry) to brown (wet) due to changes in its effective refractive index.

Keywords: GLAD, photonic crystal, heterostructure, FEG-SEM, TEM, humidity sensor

1. INTRODUCTION

The research on optical devices has been inspired by the

existing structures of the lives in nature like butterfly wings,

beetle cuticles, fish, and peacock feathers. The different

colors in a single body of insects were described with the

physical phenomenon of light.[1] The changes in color of

biological species under different environmental conditions

have further extended the idea of making electronic sensors

for a wide range of applications.[2] The basic structures of the

insect’s body surface, which able to show the color changes

can be fabricated by using the modern nanotechnology.[3]

The artificial structure of the exo-skeleton of the Hercules

beetle that acts as three dimensional (3D) photonic crystals

(PC) was fabricated by Kim et al.,[4] which show the color

changes under dry and wet conditions. The two dimensional

(2D) photonic band gap crystals consist of periodic dielectric

arrays. The nanosized holes in the layer between two dielectric

columns can be occupied with mediums of different refractive

indices. Depending on the effective refractive index of the

system, the variation in the visible colors can be obtained.

Therefore, the 2D photonic crystal has advantages in

developing the sensor, which can change the color that can

be visualized to human eye by simply altering the refractive

medium into the nanoholes. The symmetric periodic arrays

of dielectric columns can be considered as an ideal 2D PC.

However, the challenge of fabricating the structures is that

the lattice constant of the photonic crystal must be comparable

to the wavelength of light. To meet the requirements, it needs

state-of-art nanolithography techniques, such as electron-

beam lithography and x-ray lithography. The techniques are

not viable for batch production and therefore costlier. To

overcome the limitation of 2D PC fabrication, the nanoparticles

assisted optical lithography techniques have been used to

serve the purposes.[5] The oblique angle deposition (OAD)[6]

and glancing angle depositions (GLAD) techniques have

been employed to fabricate the 1D photonic crystal using

period arrangements of different dielectrics in one direction.

But there is no report on the fabrication of 2D like PC with

the help of GLAD technique using In2O3-SiOx dielectric

heterostructure nanocolumnar arrays.

In this paper, we use the In2O3-SiOx heterostructure nano-

columns to fabricate 2D PC, with nearly periodic arrays of

the columns. The different dielectric media (air and water)

were used into the nanoholes to demonstrate the optical

characteristics and the changes in the band gap of the crystal.

DOI: 10.1007/s13391-014-3325-1

*Corresponding author: [email protected]©KIM and Springer

976 N. Khelchand Singh et al.

Electron. Mater. Lett. Vol. 10, No. 5 (2014)

The color alteration of the crystal was observed. The current

conduction through the crystal at different humidity conditions

has been described.

2. EXPERIMENTAL PROCEDURE

2.1 Fabrication of In2O3-SiOx heterostructure 2D PC,

Schottky contact and characterization

GLAD was employed to fabricate In2O3-SiOx hetero-

structure nanocolumnar arrays by evaporating 99.999%

pure (MTI USA) SiO and In2O3 inside the chamber of e-beam

evaporator (15F6, HHV India) on n-type Si<100> substrate

at a base pressure of ~2 × 10−5 mbar. The substrates were put

on the substrate holder at a perpendicular distance of 24 cm

from the evaporation source. The substrates were used at a

constant azimuthal rotation of 120 rpm and at an orientation

of 85° with respect to the perpendicular line between the

source material and the planar substrate holder for column

synthesis. The deposition rate of 1.2 A°s−1 was kept constant

for both SiOx and In2O3 (250 nm each), which were monitored

by a quartz crystal. The positions of the substrates were kept

unaltered for all the depositions of SiOx and In2O3 for the

formation of In2O3-SiOx heterostructure nanocolumns. Ag

has been evaporated through the aluminum mask hole of

area 1.77 × 10−6 m2, on the top of nanocolumns to form the

Schottky contact.

The samples were characterized by field emission gun

scanning electron microscopes (FEG-SEM) (JEOL, JSM-

7600 F) and transmission electron microscopy (TEM)

(JEOL, JEM-2010). The optical absorption measurement

was done on the dry as well as the wet samples by a UV-

visible near-infrared spectrophotometer (Lambda 950, Perkin

Elmer) using specular reflection. The current (I)-Voltage (V)

characteristics of the samples were investigated by using a

Keithley 236 source-measure unit through Ag contact, under

different humid conditions.

3. RESULTS AND DISCUSSION

3.1 2D PC and lattice constant

Figure 1(a) shows the top view FEG-SEM images of the

In2O3-SiOx nanocolumns grown on the Si substrate at 85°

GLAD. The white dashed circles between nanocolumns

indicate the presence of nanoholes. The average top diameter

of the columns was calculated ~51.51 nm and the diameter

of the nanoholes was averagely 39.25 nm. Figure 1(b) shows

the TEM image of a typical nanocolumn. The deposited

nanocolumns are unsymmetrical, having the bottom and top

width ~25 nm and ~15 nm respectively. The individual

nanocolumns are coupled to each other. This may be the

reason of difference in top diameter of the nanocolumns

measured from TEM compared to FEG-SEM image. The

bottom of the nanocolumn consists of SiOx, and the top

consists of In2O3, which is clearly seen from the difference in

color contrast of TEM image of the nanocolumn. The lighter

portion of the nanocolumn is SiOx of length ~100 nm and

comparatively the darker portion of the nanocolumn is In2O3

of length ~250 nm. The nanoholes marked in the Fig. 1(a)

are not in ideal periodic arrangement, and are the collection

of small and large sized holes. The competitive growth mode

process during the GLAD deposition is the reason of

formation of under grown nanocolumns.[7] Therefore, the

area of shadowing region will be altered on the sample and

hence the periodicity, as well as the gap between two

nanocolumns. The formation of perpendicular nanocolumnar

arrays all over the sample finally produced the holes. Due to

the high surface mobility of In2O3 under extremely shadowing

condition in GLAD,[8] SiOx columns were grown beneath, to

serve as seed layers for In2O3. So, separate In2O3-SiOx

nanocolumnar arrays were grown on the Si substrate, which

produced nanoholes in the sample. Figure 1(c) shows the

schematic of the In2O3-SiOx heterostructure nanocolumns,

which is periodic along X-Y direction. For the nanocolumn

spacing, this crystal can have photonic bandgap in the XY

Fig. 1. (a) Top view of FEG-SEM images of the In2O3-SiOx nanocol-umns grown on the Si substrate at 85° GLAD, (b) TEM image of atypical nanocolumn, (c) schematic of the In2O3-SiOx heterostructurenanocolumns with periodic array and lattice constant of the photoniccrystal.

N. Khelchand Singh et al. 977


plane. Then, by considering the four neighboring nanocolumns,

the said PC may be considered as square lattice and the

lattice constant was calculated as d = 90.76 nm. Therefore,

the incident photon can be reflected from the two consecutive

dielectric nanocolumns separated by the distance of d in the

XY plane to produce the diffraction pattern, similar to Bragg’s

diffraction from two successive planes of a natural crystal.

3.2 Optical propreties and determination of band gap:

Optical absorption measurements done at room temperature

on the dry and wet (water) In2O3-SiOx heterostructure nano-

columnar samples, is displayed in Fig. 2(a). The authors

have reported the absorption in the UV region (270 - 300 nm)

of the In2O3 columnar arrays.[8] No significant absorption has

been produced by the In2O3 column into the visible region

(more than 300 nm).[8] But after introducing the SiOx

columns beneath the In2O3, the light absorption is enhanced

in the visible region beyond the 300 nm (Fig. 2(a)). Therefore,

a broad band UV-Vis absorption has been produced by the

In2O3-SiOx heterostructure nanocolumnar arrays. The dry

sample shows enlarged absorption as compared to wet

sample. In case of dry sample, there are nanoholes between

two existing nanocolumns filled with air (r.i.: 1). The

incident photons then easily penetrated through the holes

and suffered multiple scattering at the wall of the nano-

columns and get absorbed.[9] In wet condition, the nanoholes

are filled by the water (r.i.: 1.33), which reflects the incident

photons from the surface of the sample and consequently,

less penetration of the incident photons into the nanoholes

and hence the absorption by the samples.

The color of the sample changes from black (dry) to

brown in wet condition, displayed inset Fig. 2(a). The fact

may be explained due to the changes of effective refractive

index of the sample in wet environment. The optical band

gap of the dry and wet samples were then estimated 3.77 eV

and ~3.39 eV respectively from (αhν)2 versus hν plot (α is

theabsorption coefficient, hν is the photon energy) (Fig.

2(b)). The band gap at ~3.77 eV is due to the main band

transition of the In2O3 material,[8] which shifted to lower

energy ~3.39 eV under wet condition. Therefore, ~0.38 eV

(~33 nm) red shift of the band gap of the sample was

observed. Figure 2(c) shows the enhancement in reflection

from the wet heterostructure nanocolumn compared to dry

sample. The reflection peaks at ~331 nm and ~364 nm were

Fig. 2. (a) Optical absorption of the In2O3-SiOx nanocolumns and color changes (b) (αhν)2 versus hν plot, (c) reflection Spectrum of In2O3-SiOx

nanocolumns.



observed for dry and wet samples respectively. So, the

related band gap shift (red shift) of ~33 nm was determined

from reflection, which cross-verified the translocation of the

band gap calculated from the absorption spectrum of the

sample. Further, the reflected wavelength from the film can

be determined using Bragg’s equation[10]

λ = 2dneff, for normal incidence (1)

where, λ is the peak wavelength of reflected light, d = lattice

constant of the 2D crystal (90.76 nm), and neff is the effective

refractive index of the sample. The effective refractive index

of the crystal in dry condition can be estimated from the fol-

lowing formula[11]

neff = fnair + a(1 − f )[( × )/VNC

+ (nSiOx× VSiOx

)/VNC] (2)

where, f = 0.52 is the void fraction of the porous structures

for an ideal simple cubic crystal, a is the volume fraction cor-

rection coefficient (introduced into the equations due to dif-

ferent volume nanocolumns and irregular shaped nanoholes

in between). , VSiOx, VNC are the volume of the In2O3

nanocolumn, SiOx nanocolumn and heterostructure In2O3-

SiOx nanocolumn respectively. [From the FEG-SEM top-

view images, the diameter of the nanocolumn was found to

be 51.51 nm and from typical TEM images, the length of

nanocolumns were calculated as = 254 nm, lSiOx=

100 nm and lNC = + lSiOx= 354 nm and also the volume

of the cylindrical shape nanocolumns were = 5.22 ×

105 nm3, = 2.05× 105 nm3, and VNC = 7.27 × 105 nm3].

nair = 1 and = 2.2,[12] = 2.1[13] are the refractive

index of air, In2O3, and SiOx materials, respectively. When

the water penetrates into nanoporous structure, the effective

refractive index of the crystal becomes

n*eff = fnw + a(1 − f) [( × )/VNC

+ ( × /VNC] (3)

where, nw = 1.33 is the refractive index of water. The

photonic bandgap shift, ∆λ in the peak reflected wavelength

nIn2O3VIn2O3

VIn2O3

IIn2O3

IIn2O3

VIn2O3

VSiOx

nIn2O3nSiO

x

nIn2O3VIn2O3

nSiOx

VSiOx

Fig. 3. (a) Schematic diagram of the experimental setup, (b) I-V characteristics of In2O3-SiOx heterostructure nanocolumnar devices (c)Schematic representation of carrier conduction mechanism at Ag/In2O3 Schottky junction.

N. Khelchand Singh et al. 979


due to the water penetration can be calculated as ∆λ =

.

Therefore, the bandgap shift from the dry state to wet state

can be estimated as ∆λ = 31 nm, which is closely related

with the experimental value of ~33 nm. The reflective

wavelengths in dry and wet states are ~332 nm (experimental

~331 nm) and ~363 nm (experimental ~364 nm), respectively,

calculated by fitting the correction coefficient as a = 1.26.

3.3 Current-Voltage relationship of In2O3-SiOx nano-

columns at various humidity levels

The current-voltage (I-V) characteristics of the nanocolumns

were measured with a voltage sweeping mode at various

humid points. In this configuration, one electrode is applied

with the sweeping voltage bias, and the other electrode is

grounded. The sample loaded inside the humidity chamber

(home-made), where the humidity was controlled by the

amount of vaporized water droplets, from outside the

chamber (The schematic diagram along with the original

experimental setup is displayed in Fig. 3(a)). The measured

I-V characteristics of the sample are shown in Fig. 3(b) at

different humid conditions ranging from 10% to 99%. The

levels of humidity were measured with the help of standard

sensor (TECXTRA-HR201). The bias was applied at the

two different electrodes through the probes from outside the

chamber, using Keithley (2400) I-V source measure unit.

The device conductivity increases from lower to higher

humidity conditions (Fig. 3(a)). The device produces a very

low current of 5.6 × 10−3 mA/cm2 (at 10% humidity), increases

gradually to 9.1 × 10−2 mA/cm2, 1.3 × 10−1 mA/cm2 and

1.4 × 10−1 mA/cm2 under 2 V biasing at 67%, 71% and 99%

humidity respectively. The Ag produced Schottky contact

with In2O3[14] and under forward bias, a large number of

majority carriers ionize the interface states,[15] which tends to

increase the barrier height. The thermionic emission of

carriers was dominated and therefore, results in the lower

conductivity of the device at low humid conditions. At high

humid conditions, the polar water molecules were absorbed

at the porous surface of the sample, which attracts the large

number of electrons from In2O3 (inherently n-type) and

accumulates at its surface.[16] Then, the In2O3 conduction

band is bent to downward at the surface of the Ag/In2O3, due

to the accumulation of electrons (schematically shown in

Fig. 3(c)), which tends to lower the barrier height at the Ag-

In2O3 junction and may allow high carrier conduction by

thermionic as well as tunneling process. Finally, the absorption

of water molecules at the porous surface of the sample

increases the device conductivity and therefore, indicates the

enhancement of humidity in precise manner.

4. CONCLUSIONS

We have successfully fabricated the 2D like PC with the

help of GLAD synthesized In2O3-SiOx heterostructure nano-

columnar arrays. The lattice constant between two consecutive

dielectric nanocolumns was found to be d = 90.76 nm, which

is used to produce the diffraction pattern. The optical band

gap of the dry and wet samples were 3.77 eV and ~3.39 eV

respectively. The related band gap shifting was ~33 nm,

which were calculated from absorption and reflection

respectively, closely related to the theoretically calculated

shift of ~31 nm from dry state to wet state. The dry sample

shows the enlarged absorption due to the presence of

nanoholes between two existing nanocolumns filled with air

(r.i.: 1) due to the easy penetration of the photon into the hole

and hence, multiple scattering of the incident photon at the

wall of the columns. In case ofthe wet sample, the nanoholes

are filled by water (r.i.: 1.33) and consequently, less penetration

of the incident photons into the nanoholes which produces

less absorption. The color changes have been observed from

black to brown for dry to wet condition respectively. At low

humidity (10%) conditions, the device shows low current

conduction. The junction current was produced mainly by

the thermionic emission process of the carriers over the

junction barrier height. As the humidity level increases

(99%), more water molecules (polar) were absorbed at the

surface of the porous In2O3/SiOx sample, which forced to

bend the semiconductor conduction band at its surface and

accumulated more electrons. The enormous carriers at the

Ag/In2O3-SiOx junction produced the large device current.

The maximum current 1.4 × 10−1 mA/cm2 was produced at

99% humidity under the applied voltage of 2 V. Therefore,

the change in color as well as current conductivity of In2O3-

SiOx nanocolumns was observed by simply changing the

effective refractive index of the medium, which may be used

for the fabrication of good electronic sensor.

ACKNOWLEDGEMENTS

The authors are grateful to Dr. Ardhendu Saha of Electrical

Department, NIT, Agartalafor providing the absorption

measurement, Dr. Syed Arshad Hussain of Tripura University,

Department of Physics for providing the I-V measurement

facility, Dr. Kalyan Kumar Chattopadhyay of Jadavpur

University for providing the TEM measurement facility. The

authors are also thankful to SAIF, IIT Bombay, India for

FEG-SEM measurement, Department of Science and

Technology, Govt. of India, TEQIP- II and NIT Agartala for

financial support.

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2D like photonic crystal using In2O3-SiOxheterostructure nanocolumn arrays and humidity sensing

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