Electron. Mater. Lett., Vol. 0, No. 0 (0000), pp. 1-6 DOI: 10.1007/s13391-014-3325-1 2D Like Photonic Crystal Using In 2 O 3 -SiO x 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 1 Department of Electronics and Communication Engineering, National Institute of Technology, Nagaland, Chumukedima, Dimapur-797103, India 2 Department of Electronics and Communication Engineering, National Institute of Technology Agartala, Jirania, Tripura (West) 799055, India 3 Department 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: ) Abstract: 2D like photonic crystal was fabricated with the help of GLAD synthesized In 2O 3-SiO x heterostructure nanocolumnar arrays. Different dielectric media like air and water were used to demonstrate the optical characteristics and band gap of the crystal. Nearly 33 nm red shift of the band gap was observed for wet sample as compared to dry. Broad band UV-Vis absorption has been observed for the dry In 2O 3-SiO x heterostructure nanocolumnar arrays, which decreases in wet condition. The device shows low current conduction at lower humidity, which enhances at higher humidity condition due to the absorption of water molecules from the environment by the porous surface. The device possesses 5.6 10 3 ma/cm 2 current at 10%, which increases to 1.4 10 1 ma/cm 2 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. *Corresponding author: aniruddhamo@gmail.com KIM and Springer 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 electronbeam 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 In 2 O 3 -SiO x dielectric heterostructure nanocolumnar arrays. In this paper, we use the In 2 O 3 -SiO x heterostructure nanocolumns 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.
2 N. Khelchand Singh et al. 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 heterostructure 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, JSM7600 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 UVvisible 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 Fig. 1. (a) Top view of FEG-SEM images of the In O -SiO nanocolumns grown on the Si substrate at 85 GLAD, (b) TEM image of a typical nanocolumn, (c) schematic of the In O -SiO heterostructure nanocolumns with periodic array and lattice constant of the photonic crystal. 2 2 3 3 x x 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
N. Khelchand Singh et al. 3 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) In 2 O 3 -SiO x heterostructure nanocolumnar samples, is displayed in Fig. 2(a). The authors have reported the absorption in the UV region (270-300 nm) of the In 2 O 3 columnar arrays. [8] No significant absorption has been produced by the In 2 O 3 column into the visible region (more than 300 nm). [8] But after introducing the SiO x columns beneath the In 2 O 3, 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 In 2 O 3 -SiO x 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 nanocolumns 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 In 2 O 3 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 In 2O 3-SiO x nanocolumns and color changes (b) (αhν) 2 versus hν plot, (c) reflection Spectrum of In 2O 3-SiO x nanocolumns.
4 N. Khelchand Singh et al. 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 = 2dn eff, for normal incidence (1) where, λ is the peak wavelength of reflected light, d = lattice constant of the 2D crystal (90.76 nm), and n eff is the effective refractive index of the sample. The effective refractive index of the crystal in dry condition can be estimated from the following formula [11] n eff = fn air + a(1 f)[( n In2 O 3 V In2 O 3 )/V NC +(n SiOx V SiOx)/V NC ] (2) where, f = 0.52 is the void fraction of the porous structures for an ideal simple cubic crystal, a is the volume fraction correction coefficient (introduced into the equations due to different volume nanocolumns and irregular shaped nanoholes in between). V In2 O 3, V SiOx, V NC are the volume of the In 2 O 3 nanocolumn, SiO x nanocolumn and heterostructure In 2 O 3 - SiO x nanocolumn respectively. [From the FEG-SEM topview 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 I In2 O 3 = 254 nm, l SiOx= 100 nm and l NC = I In2 O 3 +l SiOx = 354 nm and also the volume of the cylindrical shape nanocolumns were V In2O3 = 5.22 10 5 nm 3, V = 2.05 10 5 nm 3, and V NC =7.27 10 5 nm 3 SiOx ]. n air = 1 and n = 2.2, [12] = 2.1 [13] In2 O 3 n SiOx are the refractive index of air, In 2 O 3, and SiO x materials, respectively. When the water penetrates into nanoporous structure, the effective refractive index of the crystal becomes n * eff = fn w + a(1 f) [( n In2 O 3 V In2 O 3 )/V NC +( n SiOx V SiOx /V NC ] (3) where, n w = 1.33 is the refractive index of water. The photonic bandgap shift, λ in the peak reflected wavelength Fig. 3. (a) Schematic diagram of the experimental setup, (b) I-V characteristics of In 2O 3-SiO x heterostructure nanocolumnar devices (c) Schematic representation of carrier conduction mechanism at Ag/In 2O 3 Schottky junction.
N. Khelchand Singh et al. 5 due to the water penetration can be calculated as λ = * 2d( n eff n eff ) = 2df( n w n air ). 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 In 2 O 3 -SiO x nanocolumns 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/cm 2 (at 10% humidity), increases gradually to 9.1 10 2 ma/cm 2, 1.3 10 1 ma/cm 2 and 1.4 10 1 ma/cm 2 under 2 V biasing at 67%, 71% and 99% humidity respectively. The Ag produced Schottky contact [14] with In 2 O 3 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 In 2 O 3 (inherently n-type) and accumulates at its surface. [16] Then, the In 2 O 3 conduction band is bent to downward at the surface of the Ag/In 2 O 3, due to the accumulation of electrons (schematically shown in Fig. 3(c)), which tends to lower the barrier height at the Ag- In 2 O 3 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 In 2 O 3 -SiO x heterostructure nanocolumnar 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 In 2 O 3 /SiO x sample, which forced to bend the semiconductor conduction band at its surface and accumulated more electrons. 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