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Role of substrate temperature on MoO3 thin films by the JNS pyrolysis technique for P–N junction diode application
Materials Science in Semiconductor Processing 43 (2016) 104–113
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Role of substrate temperature on MoO3 thin films by the JNS pyrolysis technique for P–N junction diode application M. Balaji, J. Chandrasekaran n, M. Raja Department of Physics, Sri Ramakrishna Mission Vidyalaya College of Arts and Science, Coimbatore 641020, Tamil Nadu, India
art ic l e i nf o
a b s t r a c t
Article history: Received 15 October 2015 Received in revised form 29 November 2015 Accepted 9 December 2015
Molybdenum trioxide (MoO3) thin films were prepared at different substrate temperatures from 350 to 500 °C by the jet nebulizer spray (JNS) pyrolysis technique. The effect of the substrate temperature on the structural, optical and electrical properties of MoO3 films was characterized. The XRD pattern exposed that the crystallite size of the films increases with the increase in the substrate temperature. The SEM images showed the conversion of nanorods to sub-microsized plate-like structures by increasing the substrate temperature. The EDX analysis confirmed the presence of Mo and O elements. The UV–vis results revealed that the band gap obtained shows a decreasing trend on increasing the substrate temperature. The FTIR spectra confirmed the formation of MoO3. The dc electrical studies portrayed the minimum activation energy of 0.064 eV obtained for higher substrate temperature. The P–N diode of p-Si/n-MoO3 was fabricated at the substrate temperature of 500 °C. The diode parameters such as ideality factor (n), barrier height (Φb) and reverse saturation current (I0) values were calculated in darkness and under different light sources (Halogen and Metal halide lamps). & 2015 Elsevier Ltd. All rights reserved.
Keywords: Substrate temperature MoO3 thin films JNS pyrolysis technique P–N diode
1. Introduction Remarkable attention in research during the last few years has been accorded to transition metal oxide, especially molybdenum trioxide (MoO3) owing to its physical and chemical properties. MoO3 shows attractive structural, optical and electronic properties, which favor its use in a wide range of applications such as optoelectronics, gas sensors, catalysts, and additives in paints. Additionally, MoO3 films are relevant to P–N diode applications owing to their high ionic conductivity with wide optical band gap and well controllable electrical conductivity, which is almost in the range of an insulator to a semiconductor. The semiconducting behavior of the n-type MoO3 films could give better results in photovoltaic applications [1]. The previous reports hold the effect of temperature as an important factor in that (i) it changes the coloration in MoO3. The results of the coloration of transition metal oxides are from electron delocalization among the multi-valence states [2], (ii) which formats the different phases of MoO3. The basic crystal phases are the unique layered orthorhombic MoO3, the metastable monoclinic MoO3 and hexagonal MoO3, out of which the orthorhombic phase is thermodynamically stable one [3]. The phases differ in their vibrational and optical properties; they have varying refractive indices and optical band gap energy values [4]. n
Corresponding author. E-mail address: [email protected] (J. Chandrasekaran).
http://dx.doi.org/10.1016/j.mssp.2015.12.009 1369-8001/& 2015 Elsevier Ltd. All rights reserved.
Nowadays there are a number of preparation methods such as spray pyrolysis [5], sol–gel [6], thermal evaporation [7], sputtering [8–10], chemical vapor deposition [11,12], CO2 continuous-wave laser evaporation [13], molecular beam epitaxy [14], etc. Among them, the spray pyrolysis technique is considered the simplest and low-cost method. In this work, we prepared MoO3 thin films for different substrate temperatures (350–500 °C) using the JNS pyrolysis technique [15], which is more effective to coat the uniform thin films. It has economically attractive setups. Finally, the prepared films were characterized by structural (XRD and SEM), optical (UV–vis and FTIR) and electrical properties (I–V).
2. Experimental conditions 2.1. Silicon wafer cleaning The p-Si/n-MoO3 diode was prepared using a one side polished p-type Si wafer. The most essential area in the fabrication of the diode is wafer cleaning because the contaminated surface leads to very poor efficiency. There may be many impurities such as dust, grease, metallic impurities and organic residues on the surface, which may result in a number of imperfections at the interface. The cleaning steps for silicon wafers were carefully done as follows: ● The wafer was degreased for 10 min in boiling acetone and ethanol.
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● A piranha solution (H2O2 þ H2SO4 in the ratio of 2:1) was prepared for removing the organic residues off substrate. ● An HFþ H2O (1:10) solution was prepared and used to eliminate the native oxide from the polished surface on the Si substrate. ● The wafer was thoroughly rinsed in deionized water after each cleaning step. 2.2. MoO3 thin films and diode fabrication The precursor material ammonium hepta molybdate (AHM) ((NH4)6Mo7O24 4H2O) was purchased from Sigma-Aldrich with 99.98% purity. The precursor solution was prepared with 0.05 M of AHM in 30 ml of deionized water and stirred well for an hour at ambient temperature. 3 ml of the prepared solution was sprayed on the well-cleaned glass substrates (2 2.5 cm2 size) and heated at the different temperatures of 350, 400, 450 and 500 °C in air. For the diode fabrication, 1.5 ml solution was sprayed on the p type silicon wafer (1 1 cm2 size) at 500 °C. After the respective metal oxide film was deposited, the silver (Ag) paste (ELTECK Corporation) was used to make better ohmic contact at both surfaces of the Si wafer. The Ag paste was thoroughly mixed with Amyl acetate to make a gel and applied to both sides of the wafers. The device was then dried at room temperature for 5 h. The silver paste has good adhesion, high electrical conductivity, low sheet resistance (40.02 Ω/cm) and good solderability. The experimental setup of the JNS pyrolysis method is very simple, with which we can obtain uniformly coated thin films. The deposition parameters of the JNS pyrolysis is shown in Table 1 [15]. The formation of MoO3 from the precursor solution is given as the following equation,
(NH4 )6Mo7O24 ⋅4H2 O + 2H2 O
→
(350 − 500 ° C)
7MoO3 + 6NH3 + 9H2 O
(1)
The thickness of the film was measured by the stylus profilometer (Mitutoyo SJ 301). Structural studies were carried out using the X-ray diffractometer (XRD, XPERT-PRO) with CuKα1 radiation of wavelength 1.5406 Å at a generator setting of 30 mA and 40 kV in the 2θ range from 20° to 70°. The morphological (SEM) and chemical composition (EDX) features of the prepared thin films were investigated using the scanning electron microscopy (JEOL JEM 2100). The optical studies were carried out by the UV–visible spectrophotometer (Perkin Elmer Lambda 35) in the wavelength range from 300 to 900 nm. The functional group of the MoO3 films was analyzed by the FTIR spectrometer (Alpha-T FTIR Spectrometer). The DC electrical conductivity and diode studies of the MoO3 thin films were performed on the Keithley Electrometer 6517B.
Fig. 1. Thickness variation of the MoO3 thin films for different substrate temperatures.
3.2. Structural characterization by XRD The study of the varying substrate temperature is the main stage, which will increase the crystallinity, stoichiometry and other structural related results of oxide films. Fig. 2(a)–(d) shows the XRD pattern of the MoO3 films deposited at different substrate temperatures of 350, 400, 450 and 500 °C in the atmosphere. In Fig. 2, the obtained diffraction peaks at 2θ (°) are around 23.20, 25.51, 27.18, 33.66, 38.76, 52.64, 58.79 and 67.28 with the corresponding (h k l) planes of (1 1 0), (0 4 0), (0 2 1), (1 1 1), (1 3 1), (2 1 1), (0 8 1) and (2 6 1), respectively. At the low temperature of 350 °C, the peaks corresponding to the orthorhombic phase of MoO3 started appearing and again the increasing substrate temperature improved the crystallinity of α-MoO3 with the highly oriented planes of (0 4 0) and (1 3 1), which are well matched with JCPDS card (No. 35-0609). However, other phases are not observed in the MoO3 films prepared by spraying in the temperature range of 350–500 °C. Table 2 shows the micro structural properties of the MoO3 films deposited at different substrate temperatures. The hkl and d values for the MoO3 films deposited at different temperatures agree well with the values found in the JCPDS card. The crystallite size (D) was calculated from the full width at half maximum (FWHM) of the prominent XRD peaks using the Debye–Scherrer relation [16,17]
D= 3. Results and discussion 3.1. Thickness variation of the MoO3 films The thickness of the MoO3 films decreased from 273 to 206 nm on increasing the substrate temperature from 350 to 500 °C, which is shown in Fig. 1. The results show that in the JNS pyrolysis technique, the higher substrate temperatures played an effective role on the thin layered films. Table 1 Deposition conditions. S. no.
Parameters
Values
1 2 3 4 5
Nozzle to substrate distance Substrate temperature Flow rate Carrier gas pressure Time of Spray
5 cm 350–500 °C 0.5 ml/min 3.5 Kg/cm2 6 minute
kλ β cos θ
(2)
where D is the crystallite size, k is the shape factor (k¼0.94), β is the full width at half maximum of the diffraction peak, θ is the diffraction angle and λ is the wavelength of the X-ray radiation. The micro strain (ε), dislocation density (δ) and stacking fault (SF) values can be obtained using the following relations [17]
ϵ=
β cos θ 4
(3)
δ=
1 D2
(4)
⎡ ⎤ 2π 2 ⎥β SF = ⎢ 1/2 ⎣ 45(3 tan θ ) ⎦
(5)
The variation of microstructural parameters like crystallite size, micro strain and dislocation density with deposition temperature is shown in Fig. 3a and b. The crystallite size of preferred orientation
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Fig. 2. XRD pattern of the MoO3 thin films for different substrate temperatures of (a) 350 °C, (b) 400 °C, (c) 450 °C and (d) 500 °C.
Table 2 Micro structural properties of MoO3 films deposited at different substrate temperature. Substrate temperature °C
Diffraction angle 2θ (deg)
hkl
Inter planar distance Å
FWHM (radians)
crystallite size (D) nm
Micro strain (ε) Dislocation density ( 1014 lines/m2) Stacking fault 10 2 ( 10 3 lines 2 m 4)
350
23.2189 25.5104 27.1931 33.6605 38.7607 67.2823 23.2031 25.5053 27.1933 38.7553 52.6028 58.0443 67.2430 23.2082 25.5142 27.1880 38.7692 52.6481 58.7993 67.2807 23.2462 25.5845 27.2212 38.8627 52.6774 67.4068
110 040 021 111 131 261 110 040 021 131 211 081 261 110 040 021 131 211 081 261 110 040 021 131 211 261
3.83094 3.49179 3.27941 2.66266 2.32324 1.39161 3.83352 3.49247 3.27939 2.32355 1.73990 1.58908 1.39233 3.83268 3.49127 3.28002 2.32274 1.73851 1.57046 1.39164 3.82651 3.48184 3.27609 2.31737 1.73761 1.38934
0.0034 0.0034 0.0026 0.0034 0.0043 0.0069 0.0026 0.0034 0.0026 0.0043 0.0051 0.0034 0.0069 0.0026 0.0034 0.0026 0.0034 0.0069 0.0043 0.0060 0.0034 0.0026 0.0026 0.0034 0.0051 0.0043
41.261 41.439 55.442 42.225 34.275 24.273 55.013 41.439 55.442 34.275 30.056 46.220 24.268 55.013 41.440 55.442 42.845 22.546 37.112 27.741 41.263 55.260 55.446 42.858 30.065 38.865
0.8407 0.8371 0.6256 0.8215 1.0120 1.4291 0.6305 0.8371 0.6257 1.0121 1.1541 0. 7505 1.4294 0.6305 0.8371 0.6257 0.8096 1.5385 0.9347 1.2504 0.8407 0.6277 0.6256 0.8094 1.1538 0.8925
400
450
500
5.8739 5.8234 3.2532 5.6086 8.5121 16.972 3.3043 5.8235 3.2532 8.5124 11.070 4.6810 16.980 3.3042 5.8233 3.2533 5.4474 19.672 7.2604 12.995 5.8733 3.2747 3.2528 5.4443 11.063 6.6202
0.1708 0.1802 0.1402 0.2128 0.2921 0.8061 0.1280 0.1801 0.1402 0.2921 0.4474 0.3302 0.8054 0.1281 0.1802 0.1402 0.2337 0.5971 0.4189 0.7053 0.1709 0.1353 0.1403 0.2341 0.4480 0.5054
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Fig. 3. Microstructural properties of (a) grain size and micro strain and (b) dislocation density of the MoO3 thin films for different substrate temperatures.
(0 4 0) varies in the range from 41.44 to 55.26 nm as the substrate temperature of MoO3 film deposition increases from 350 to 500 °C [18,19]. It shows that the crystallite size reaches a maximum value of 55.26 nm at 500 °C [20]. The micro strain and dislocation density are found to decrease up to 500 °C. And the stacking fault values also decrease up to 500 °C as shown in Table 2.
3.3. Surface morphological analysis by SEM The surface structure and grain arrangement were analyzed by the SEM pictures profile results. The surface morphology of the MoO3 films deposited at 350, 400, 450 and 500 °C are shown in Fig. 4(a)–(d). Nanorods of randomly oriented surface feature are
Fig. 4. SEM images of the MoO3 thin films for different substrate temperatures of (a) 350 °C, (b) 400 °C, (c) 450 °C and (d) 500 °C.
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observed for the MoO3 film deposited at the temperature of 350 °C. The nanorods are changed into sub-microsized plate-like structures owing to increasing the temperature from 350 to 500 °C. In Fig. 4b and c, some pin holes and agglomeration on the surfaces of the films are observed. Randomly oriented surface morphology with sub-microsized plate-like structure is observed on the surface of the MoO3 films at 500 °C (Fig. 4d). These morphological changes with varying deposition temperature cause the formation of microplates, which may be attributed to the aggregation or fusion of small particles at higher temperatures. From these morphological results in the present study on the JNS pyrolysis technique it is observed that different surface structures of MoO3 films can be prepared by varying the substrate temperature.
Table 3 Atomic percentage of the MoO3 films prepared at different substrate temperatures. Substrate temperature °C
350 400 450 500
Atomic ratio (%) Mo
O
24.63 25.99 26.82 28.48
75.37 74.01 73.18 71.52
3.4. Elemental analysis by EDX The stoichiometry analysis of the elements present in the MoO3 thin film was carried out by the EDX result. Fig. 5 shows the EDX pattern of the MoO3 film deposited at the substrate temperature of 500 °C. From the EDX spectrum, it is observed that the prepared MoO3 has closer stoichiometric ratio of Mo and O but with oxygen deficiency. The ratio of Mo:O is found to be 26.48:73.52. This is a clear evidence that the oxygen deficiency gives lower resistivity to MoO3 films heated at 500 °C in the present study. The atomic percentage of the MoO3 films prepared at different substrate temperatures are presented in Table 3. EDX result confirms that the samples are composed of Mo and O with some secondary impurities of Si, Na and Ca, which presented owing to glass substrate. Interestingly, it is noted that the atomic percentage of oxygen decreases as temperature increases in substrate. 3.5. Optical results by UV–vis Fig. 6(a)–(d) shows the absorbance spectra in the wavelength region of 300–900 nm for the MoO3 films deposited at different substrate temperatures of 350, 400, 450 and 500 °C. However, the absorbance value increases steeply in the UV region i.e. less than 350 nm. It is observed that as the substrate temperature increases, the absorbance increases up to 450 °C in the UV–visible region because of the deep bluish coloration of the film, surface roughness and grain boundary [21–23] then the absorbance decreases at 500 °C, It is known that oxygen defects can lead to increased roughness or pore oxides and also may be due to the sudden change to light bluish color, increase in the packing density of the films, decrease in thickness owing to the shrinking of the diameter of the spray droplets [17], hasty variation of the surface morphology (Fig. 4d) and crystallite size (Table 2). The minimum absorbance is recorded for the MoO3 film deposited at a temperature of 500 °C.
Fig. 5. EDX result of the MoO3 thin film for the substrate temperature of 500 °C.
Fig. 6. UV–vis results of the MoO3 thin films for different substrate temperatures of (a) 350 °C, (b) 400 °C, (c) 450 °C and (d) 500 °C.
The variation of (αhν)2 versus (hν) for the MoO3 films deposited at different temperatures is shown in Fig. 7(a)–(d). The straight line portion indicates that the optical transition is direct in nature. The band gap energy is calculated using the following relation [24]
αhυn = B(hυ − Eg )
(6)
where α is the absorption co-efficient, hυ is the photon energy, B is the constant, Eg is the optical band gap energy and n is a number that characterizes the transition process (direct allowed transition n¼ 2 and indirect allowed transition n ¼1/2). The direct allowed band gap energy value has been determined by extrapolating the vertical straight line portion of the plot to the
Fig. 7. Band gap energy of MoO3 thin films for different substrate temperatures of (a) 350 °C, (b) 400 °C, (c) 450 °C and (d) 500 °C.
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These results show that the MoO3 film deposited at different substrate temperatures has variations in optical absorbance and band gap values.
3.6. Functional group analysis by FTIR
Fig. 8. IR spectra of the MoO3 thin films for different substrate temperatures of (a) 350 °C, (b) 400 °C, (c) 450 °C and (d) 500 °C.
energy axis. The intercept on the energy axis gives the values of the band gap energy 2.97, 2.94, 2.88 and 2.50 eV for the MoO3 films heated at 350, 400, 450 and 500 °C, respectively, which are in good agreement with the reported values of 2.75–2.95 eV [20,22].
The FTIR spectra of the MoO 3 thin film samples in the range of 4000-400 cm 1 are shown in Fig. 8. The results are presented in Fig. 8 for the deposition temperature from 350 to 500 °C. The samples show the peaks in the region of 1000-400 cm 1 corresponding to the stretching and bending vibrations of metal-oxygen characteristic bonds. The peaks between 500 and 600 cm 1 correspond to the vibration of the Mo–O bond. In all the spectra, some very weak peaks appear at 900 and 1000 cm 1. The vibrational analysis shows that the characteristic region for the MoO stretching mode occurs around 450, 514, 608, 700-730, 800–840 and 900–1000 cm 1 [25–27]. A broad band at 3580 cm 1 and a small band at 1605 cm 1 correspond to the stretching and bending vibrations, respectively, of the hydrogen bonded –OH group in water molecules. As the latter is attributed to the unique MoO bonds in polycrystalline α-MoO3 [28], its presence indicates that in the studied MoO3 films α-modification could exist. The fact that the basic transmittance bands are comparatively narrow is an evidence of the polycrystalline material. Moreover, other functional groups are not involved.
Fig. 9. I–V characteristics of the MoO3 thin films for different substrate temperatures of (a) 350 °C, (b) 400 °C, (c) 450 °C and (d) 500 °C.
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Fig. 10. ln s versus 1000/T plots for the MoO3 thin films for different substrate temperatures of (a) 350 °C, (b) 400 °C, (c) 450 °C and (d) 500 °C.
Fig. 12. Schematic diagram of the p-Si/n-MoO3 diode.
film for the constant voltage of 10–100 V was studied at different temperatures from 30 to 150 °C for the MoO3 films (350–500 °C) as shown in Fig. 9(a)–(d). The electrical conductivity (s) can be calculated using the given formula [29], Fig. 11. Activation energy of the MoO3 thin films for different substrate temperatures.
3.7. Electrical characterization 3.7.1. DC conductivity of MoO3 thin films The electrical conductivity was carried out using the Keithley electrometer 6517B two probe setup. The current flow through the
⎛ I ⎞ ⎛ d⎞ σ= ⎜ ⎟×⎜ ⎟ ⎝ V ⎠ ⎝ A⎠
(7)
where I is the current, V is the applied voltage, d is the inter-probe distance and A is the cross-sectional area of the film. As the temperature increases, the conductivity of the film for constant voltages (10–100 V) is found to increase as shown in Fig. 10(a)–(d) (i.e., 1000/T (K) versus ln s (S/cm)). This is due to an increase in the
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Fig. 13. I–V characteristics of the p-Si/n-MoO3 diode in darkness and under different light illuminations.
Table 4 Diode parameters obtained from I–V characteristics for p-Si/n-MoO3 diode in darkness and under different illuminations. Source of light
Barrier width (Φb) eV
Ideality factor (n)
Current (I) A
Dark Halogen lamp Metal halide lamp Halogenþ Metal halide
0.75 0.59 0.58 0.51
7.19 5.16 6.15 5.99
2.74 10 10 1.68 10 7 2.40 10 6 3.09 10 6
⎛ −E ⎞ σ = σ0 exp⎜ a ⎟ ⎝ KBT ⎠
Fig. 14. ln J versus voltage (a) in darkness and under different light sources of (b) halogen lamp (c) Metal halide and (d) Halogenþ Metal halide lamp.
grain size with a rise in temperature. This property of conductivity confirms the semiconducting nature of MoO3 films. The electrical values measured in the present work show a trend similar to that of the reported values [30]. It is clearly revealed that the electrical properties of MoO3 thin films are well improved by the substrate temperature. The activation energy (Ea) can be calculated by the Arrhenius equation,
(8)
where s is the conductivity, s0 is a constant, Ea is the activation energy, KB is the Boltzmann constant and T is the absolute temperature. The average activation energy of MoO3 thin films for the different substrate temperatures of 350–500 °C is shown in Fig. 11. The average activation energy values vary from 0.169 to 0.0639 eV for the different substrate temperatures from 350 to 500 °C. The minimum activation energy is obtained at 500 °C. The values obtained match the reported values well [22,31]. 3.7.2. I–V characterization of p-Si/n-MoO3 diode Based on the results of structure, morphology and conductivity, we prepared the P–N diode at the substrate temperature of 500 °C by the JNS pyrolysis. The formation of the P-N junction diode is carried
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out by the p-type Si and n-type MoO3. The p-Si/n-MoO3 diode schematic structure is illustrated in Fig. 12. The diode characteristics were studied in darkness and under different light sources of halogen and metal halide lamps, which caused the diode behavior to differ owing to their different illuminations (Halogen-45 mW/cm2, Metal halide-77 mW/cm2, halogenþmetal halide-100 mW/cm2 were calculated using the lux meter, and the distance between the diode and the lamps was 0.3 m). The I–V characteristics of the p-Si/n-MoO3 diode in darkness and under different lamps are shown in Fig. 13(a)– (d). The semi logarithmic plot for the current density (ln J) versus voltage (V) is shown in Fig. 14. The forward bias direction corresponds to the negative top electrode of the device. The forward to reverse current is in the range of þ 4 to 4 V. The characteristics of p-Si/n-MoO3 show good rectifying nature in darkness and under different illuminations. The rectifying voltages for darkness and different lamps of the hetero-junction are found to be 1.9, 2.1, 1.6 and 1.2 V; and beyond these voltages, the forward bias current increases as the applied voltage increases. But the reverse bias current shows a poor saturation current. This is nearly a good result for an ideal P–N diode, which has zero resistivity in forward bias and infinite resistivity in reverse bias. According to the thermionic emission theory (TE), the current through the diode can be calculated as follows [32],
SEM analysis revealed that nanorods with randomly oriented surface feature were observed for the MoO3 film deposited at the low temperature of 350 °C. Nanorods were changed into sub-microsized plate-like structures owing to the increasing temperature from 350 to 500 °C. Randomly oriented surface morphology with sub-microsized plate-like structures was observed on the surface of the MoO3 films at 500 °C. The EDX analysis showed the presence of the same Mo:O in the film as in the starting spray solution. Through the UV analysis, the absorbance was found to increase up to 450 °C and decrease at 500 °C. The band gap of MoO3 films decreased from 2.97 to 2.50 eV as the substrate temperature increased. The FTIR spectra of the MoO3 thin film samples in the range 4000–400 cm 1 confirmed the formation of the product. The electrical studies showed that the conductivity increased when the substrate temperature increased. The minimum activation energy value of 0.064 eV was obtained at the higher substrate temperature of 500 °C. From p-Si/ n-MoO3 diode characterization studies, the diode behavior was seen to change extremely owing to darkness and different lamps. The minimum ideality factor 5.16 was observed for the p-Si/n-MoO3 diode under the illumination of the halogen lamp.
⎛ qV ⎞ I = I0 exp⎜ − 1⎟ ⎝ nKT ⎠
Acknowledgments
(9)
where I0 is the reverse saturation current, q is the electron charge, V is the applied voltage, n is the ideality factor, K is the Boltzmann constant and T is the absolute temperature. The ideality factor (n) and the reverse bias saturation current (I0) of the diode are determined from the slope and the intercept of the semi-logarithmic forward bias J–V plot for VZ3kT/q using Eq. (9) and the ideality factor n and the barrier height Φb can be calculated as follows [33]
q dV n= KT d(ln I )
Φb =
(10)
KT ⎛ AA*T2 ⎞ ⎟ ln⎜ q ⎝ I0 ⎠
(11) *
where A is the active area of the diode and A is the Richardson constant. The ideality factor (n) values and barrier height (Φb) values are shown in Table 4. For an ideal P–N diode, the ideality factor value is unity (i.e., n¼ 1) but experimentally we obtained higher ideality factor (n) values of the p-Si/n-MoO3 diode in darkness and under different light illuminations in the range of 5–7 than unity (n41). This is a non-ideal behavior, which may be due to the existence of an interfacial thin native oxide layer of SiO2 and a wide distribution of barrier inhomogeneities [34]. It may also be due to series resistance and nonlinear metal-semiconductor contact [35]. Another reason may be the abnormalities of the inorganic film thickness and nonuniformity of the interfacial charges [36]. Compared to the metal halide lamp the halogen lamp gives minimum ideality factor. The variation of current in the diode of p-Si/n-MoO3 owing to different lights is also very relevant to photo-detector applications.
4. Conclusion Molybdenum trioxide thin films were prepared on glass substrates at different substrate temperatures from 350 to 500 °C by the JNS pyrolysis technique. The structural studies by the XRD analysis showed the polycrystalline nature of the MoO3 films with orthorhombic structure. All the films showed preferential orientation along with the (040) plane. The crystallite size varied from 41.44 to 55.26 nm when the substrate temperature increased. The
The authors are gratefully thanked to Sophisticated Test and Instrumentation Centre (STIC), Cochin University, Cochin, for supporting SEM studies for this work.
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Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Role of substrate temperature on MoO3 thin films by the JNS pyrolysis technique for P–N junction diode application M. Balaji, J. Chandrasekaran n, M. Raja Department of Physics, Sri Ramakrishna Mission Vidyalaya College of Arts and Science, Coimbatore 641020, Tamil Nadu, India
art ic l e i nf o
a b s t r a c t
Article history: Received 15 October 2015 Received in revised form 29 November 2015 Accepted 9 December 2015
Molybdenum trioxide (MoO3) thin films were prepared at different substrate temperatures from 350 to 500 °C by the jet nebulizer spray (JNS) pyrolysis technique. The effect of the substrate temperature on the structural, optical and electrical properties of MoO3 films was characterized. The XRD pattern exposed that the crystallite size of the films increases with the increase in the substrate temperature. The SEM images showed the conversion of nanorods to sub-microsized plate-like structures by increasing the substrate temperature. The EDX analysis confirmed the presence of Mo and O elements. The UV–vis results revealed that the band gap obtained shows a decreasing trend on increasing the substrate temperature. The FTIR spectra confirmed the formation of MoO3. The dc electrical studies portrayed the minimum activation energy of 0.064 eV obtained for higher substrate temperature. The P–N diode of p-Si/n-MoO3 was fabricated at the substrate temperature of 500 °C. The diode parameters such as ideality factor (n), barrier height (Φb) and reverse saturation current (I0) values were calculated in darkness and under different light sources (Halogen and Metal halide lamps). & 2015 Elsevier Ltd. All rights reserved.
Keywords: Substrate temperature MoO3 thin films JNS pyrolysis technique P–N diode
1. Introduction Remarkable attention in research during the last few years has been accorded to transition metal oxide, especially molybdenum trioxide (MoO3) owing to its physical and chemical properties. MoO3 shows attractive structural, optical and electronic properties, which favor its use in a wide range of applications such as optoelectronics, gas sensors, catalysts, and additives in paints. Additionally, MoO3 films are relevant to P–N diode applications owing to their high ionic conductivity with wide optical band gap and well controllable electrical conductivity, which is almost in the range of an insulator to a semiconductor. The semiconducting behavior of the n-type MoO3 films could give better results in photovoltaic applications [1]. The previous reports hold the effect of temperature as an important factor in that (i) it changes the coloration in MoO3. The results of the coloration of transition metal oxides are from electron delocalization among the multi-valence states [2], (ii) which formats the different phases of MoO3. The basic crystal phases are the unique layered orthorhombic MoO3, the metastable monoclinic MoO3 and hexagonal MoO3, out of which the orthorhombic phase is thermodynamically stable one [3]. The phases differ in their vibrational and optical properties; they have varying refractive indices and optical band gap energy values [4]. n
Corresponding author. E-mail address: [email protected] (J. Chandrasekaran).
http://dx.doi.org/10.1016/j.mssp.2015.12.009 1369-8001/& 2015 Elsevier Ltd. All rights reserved.
Nowadays there are a number of preparation methods such as spray pyrolysis [5], sol–gel [6], thermal evaporation [7], sputtering [8–10], chemical vapor deposition [11,12], CO2 continuous-wave laser evaporation [13], molecular beam epitaxy [14], etc. Among them, the spray pyrolysis technique is considered the simplest and low-cost method. In this work, we prepared MoO3 thin films for different substrate temperatures (350–500 °C) using the JNS pyrolysis technique [15], which is more effective to coat the uniform thin films. It has economically attractive setups. Finally, the prepared films were characterized by structural (XRD and SEM), optical (UV–vis and FTIR) and electrical properties (I–V).
2. Experimental conditions 2.1. Silicon wafer cleaning The p-Si/n-MoO3 diode was prepared using a one side polished p-type Si wafer. The most essential area in the fabrication of the diode is wafer cleaning because the contaminated surface leads to very poor efficiency. There may be many impurities such as dust, grease, metallic impurities and organic residues on the surface, which may result in a number of imperfections at the interface. The cleaning steps for silicon wafers were carefully done as follows: ● The wafer was degreased for 10 min in boiling acetone and ethanol.
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● A piranha solution (H2O2 þ H2SO4 in the ratio of 2:1) was prepared for removing the organic residues off substrate. ● An HFþ H2O (1:10) solution was prepared and used to eliminate the native oxide from the polished surface on the Si substrate. ● The wafer was thoroughly rinsed in deionized water after each cleaning step. 2.2. MoO3 thin films and diode fabrication The precursor material ammonium hepta molybdate (AHM) ((NH4)6Mo7O24 4H2O) was purchased from Sigma-Aldrich with 99.98% purity. The precursor solution was prepared with 0.05 M of AHM in 30 ml of deionized water and stirred well for an hour at ambient temperature. 3 ml of the prepared solution was sprayed on the well-cleaned glass substrates (2 2.5 cm2 size) and heated at the different temperatures of 350, 400, 450 and 500 °C in air. For the diode fabrication, 1.5 ml solution was sprayed on the p type silicon wafer (1 1 cm2 size) at 500 °C. After the respective metal oxide film was deposited, the silver (Ag) paste (ELTECK Corporation) was used to make better ohmic contact at both surfaces of the Si wafer. The Ag paste was thoroughly mixed with Amyl acetate to make a gel and applied to both sides of the wafers. The device was then dried at room temperature for 5 h. The silver paste has good adhesion, high electrical conductivity, low sheet resistance (40.02 Ω/cm) and good solderability. The experimental setup of the JNS pyrolysis method is very simple, with which we can obtain uniformly coated thin films. The deposition parameters of the JNS pyrolysis is shown in Table 1 [15]. The formation of MoO3 from the precursor solution is given as the following equation,
(NH4 )6Mo7O24 ⋅4H2 O + 2H2 O
→
(350 − 500 ° C)
7MoO3 + 6NH3 + 9H2 O
(1)
The thickness of the film was measured by the stylus profilometer (Mitutoyo SJ 301). Structural studies were carried out using the X-ray diffractometer (XRD, XPERT-PRO) with CuKα1 radiation of wavelength 1.5406 Å at a generator setting of 30 mA and 40 kV in the 2θ range from 20° to 70°. The morphological (SEM) and chemical composition (EDX) features of the prepared thin films were investigated using the scanning electron microscopy (JEOL JEM 2100). The optical studies were carried out by the UV–visible spectrophotometer (Perkin Elmer Lambda 35) in the wavelength range from 300 to 900 nm. The functional group of the MoO3 films was analyzed by the FTIR spectrometer (Alpha-T FTIR Spectrometer). The DC electrical conductivity and diode studies of the MoO3 thin films were performed on the Keithley Electrometer 6517B.
Fig. 1. Thickness variation of the MoO3 thin films for different substrate temperatures.
3.2. Structural characterization by XRD The study of the varying substrate temperature is the main stage, which will increase the crystallinity, stoichiometry and other structural related results of oxide films. Fig. 2(a)–(d) shows the XRD pattern of the MoO3 films deposited at different substrate temperatures of 350, 400, 450 and 500 °C in the atmosphere. In Fig. 2, the obtained diffraction peaks at 2θ (°) are around 23.20, 25.51, 27.18, 33.66, 38.76, 52.64, 58.79 and 67.28 with the corresponding (h k l) planes of (1 1 0), (0 4 0), (0 2 1), (1 1 1), (1 3 1), (2 1 1), (0 8 1) and (2 6 1), respectively. At the low temperature of 350 °C, the peaks corresponding to the orthorhombic phase of MoO3 started appearing and again the increasing substrate temperature improved the crystallinity of α-MoO3 with the highly oriented planes of (0 4 0) and (1 3 1), which are well matched with JCPDS card (No. 35-0609). However, other phases are not observed in the MoO3 films prepared by spraying in the temperature range of 350–500 °C. Table 2 shows the micro structural properties of the MoO3 films deposited at different substrate temperatures. The hkl and d values for the MoO3 films deposited at different temperatures agree well with the values found in the JCPDS card. The crystallite size (D) was calculated from the full width at half maximum (FWHM) of the prominent XRD peaks using the Debye–Scherrer relation [16,17]
D= 3. Results and discussion 3.1. Thickness variation of the MoO3 films The thickness of the MoO3 films decreased from 273 to 206 nm on increasing the substrate temperature from 350 to 500 °C, which is shown in Fig. 1. The results show that in the JNS pyrolysis technique, the higher substrate temperatures played an effective role on the thin layered films. Table 1 Deposition conditions. S. no.
Parameters
Values
1 2 3 4 5
Nozzle to substrate distance Substrate temperature Flow rate Carrier gas pressure Time of Spray
5 cm 350–500 °C 0.5 ml/min 3.5 Kg/cm2 6 minute
kλ β cos θ
(2)
where D is the crystallite size, k is the shape factor (k¼0.94), β is the full width at half maximum of the diffraction peak, θ is the diffraction angle and λ is the wavelength of the X-ray radiation. The micro strain (ε), dislocation density (δ) and stacking fault (SF) values can be obtained using the following relations [17]
ϵ=
β cos θ 4
(3)
δ=
1 D2
(4)
⎡ ⎤ 2π 2 ⎥β SF = ⎢ 1/2 ⎣ 45(3 tan θ ) ⎦
(5)
The variation of microstructural parameters like crystallite size, micro strain and dislocation density with deposition temperature is shown in Fig. 3a and b. The crystallite size of preferred orientation
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Fig. 2. XRD pattern of the MoO3 thin films for different substrate temperatures of (a) 350 °C, (b) 400 °C, (c) 450 °C and (d) 500 °C.
Table 2 Micro structural properties of MoO3 films deposited at different substrate temperature. Substrate temperature °C
Diffraction angle 2θ (deg)
hkl
Inter planar distance Å
FWHM (radians)
crystallite size (D) nm
Micro strain (ε) Dislocation density ( 1014 lines/m2) Stacking fault 10 2 ( 10 3 lines 2 m 4)
350
23.2189 25.5104 27.1931 33.6605 38.7607 67.2823 23.2031 25.5053 27.1933 38.7553 52.6028 58.0443 67.2430 23.2082 25.5142 27.1880 38.7692 52.6481 58.7993 67.2807 23.2462 25.5845 27.2212 38.8627 52.6774 67.4068
110 040 021 111 131 261 110 040 021 131 211 081 261 110 040 021 131 211 081 261 110 040 021 131 211 261
3.83094 3.49179 3.27941 2.66266 2.32324 1.39161 3.83352 3.49247 3.27939 2.32355 1.73990 1.58908 1.39233 3.83268 3.49127 3.28002 2.32274 1.73851 1.57046 1.39164 3.82651 3.48184 3.27609 2.31737 1.73761 1.38934
0.0034 0.0034 0.0026 0.0034 0.0043 0.0069 0.0026 0.0034 0.0026 0.0043 0.0051 0.0034 0.0069 0.0026 0.0034 0.0026 0.0034 0.0069 0.0043 0.0060 0.0034 0.0026 0.0026 0.0034 0.0051 0.0043
41.261 41.439 55.442 42.225 34.275 24.273 55.013 41.439 55.442 34.275 30.056 46.220 24.268 55.013 41.440 55.442 42.845 22.546 37.112 27.741 41.263 55.260 55.446 42.858 30.065 38.865
0.8407 0.8371 0.6256 0.8215 1.0120 1.4291 0.6305 0.8371 0.6257 1.0121 1.1541 0. 7505 1.4294 0.6305 0.8371 0.6257 0.8096 1.5385 0.9347 1.2504 0.8407 0.6277 0.6256 0.8094 1.1538 0.8925
400
450
500
5.8739 5.8234 3.2532 5.6086 8.5121 16.972 3.3043 5.8235 3.2532 8.5124 11.070 4.6810 16.980 3.3042 5.8233 3.2533 5.4474 19.672 7.2604 12.995 5.8733 3.2747 3.2528 5.4443 11.063 6.6202
0.1708 0.1802 0.1402 0.2128 0.2921 0.8061 0.1280 0.1801 0.1402 0.2921 0.4474 0.3302 0.8054 0.1281 0.1802 0.1402 0.2337 0.5971 0.4189 0.7053 0.1709 0.1353 0.1403 0.2341 0.4480 0.5054
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Fig. 3. Microstructural properties of (a) grain size and micro strain and (b) dislocation density of the MoO3 thin films for different substrate temperatures.
(0 4 0) varies in the range from 41.44 to 55.26 nm as the substrate temperature of MoO3 film deposition increases from 350 to 500 °C [18,19]. It shows that the crystallite size reaches a maximum value of 55.26 nm at 500 °C [20]. The micro strain and dislocation density are found to decrease up to 500 °C. And the stacking fault values also decrease up to 500 °C as shown in Table 2.
3.3. Surface morphological analysis by SEM The surface structure and grain arrangement were analyzed by the SEM pictures profile results. The surface morphology of the MoO3 films deposited at 350, 400, 450 and 500 °C are shown in Fig. 4(a)–(d). Nanorods of randomly oriented surface feature are
Fig. 4. SEM images of the MoO3 thin films for different substrate temperatures of (a) 350 °C, (b) 400 °C, (c) 450 °C and (d) 500 °C.
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observed for the MoO3 film deposited at the temperature of 350 °C. The nanorods are changed into sub-microsized plate-like structures owing to increasing the temperature from 350 to 500 °C. In Fig. 4b and c, some pin holes and agglomeration on the surfaces of the films are observed. Randomly oriented surface morphology with sub-microsized plate-like structure is observed on the surface of the MoO3 films at 500 °C (Fig. 4d). These morphological changes with varying deposition temperature cause the formation of microplates, which may be attributed to the aggregation or fusion of small particles at higher temperatures. From these morphological results in the present study on the JNS pyrolysis technique it is observed that different surface structures of MoO3 films can be prepared by varying the substrate temperature.
Table 3 Atomic percentage of the MoO3 films prepared at different substrate temperatures. Substrate temperature °C
350 400 450 500
Atomic ratio (%) Mo
O
24.63 25.99 26.82 28.48
75.37 74.01 73.18 71.52
3.4. Elemental analysis by EDX The stoichiometry analysis of the elements present in the MoO3 thin film was carried out by the EDX result. Fig. 5 shows the EDX pattern of the MoO3 film deposited at the substrate temperature of 500 °C. From the EDX spectrum, it is observed that the prepared MoO3 has closer stoichiometric ratio of Mo and O but with oxygen deficiency. The ratio of Mo:O is found to be 26.48:73.52. This is a clear evidence that the oxygen deficiency gives lower resistivity to MoO3 films heated at 500 °C in the present study. The atomic percentage of the MoO3 films prepared at different substrate temperatures are presented in Table 3. EDX result confirms that the samples are composed of Mo and O with some secondary impurities of Si, Na and Ca, which presented owing to glass substrate. Interestingly, it is noted that the atomic percentage of oxygen decreases as temperature increases in substrate. 3.5. Optical results by UV–vis Fig. 6(a)–(d) shows the absorbance spectra in the wavelength region of 300–900 nm for the MoO3 films deposited at different substrate temperatures of 350, 400, 450 and 500 °C. However, the absorbance value increases steeply in the UV region i.e. less than 350 nm. It is observed that as the substrate temperature increases, the absorbance increases up to 450 °C in the UV–visible region because of the deep bluish coloration of the film, surface roughness and grain boundary [21–23] then the absorbance decreases at 500 °C, It is known that oxygen defects can lead to increased roughness or pore oxides and also may be due to the sudden change to light bluish color, increase in the packing density of the films, decrease in thickness owing to the shrinking of the diameter of the spray droplets [17], hasty variation of the surface morphology (Fig. 4d) and crystallite size (Table 2). The minimum absorbance is recorded for the MoO3 film deposited at a temperature of 500 °C.
Fig. 5. EDX result of the MoO3 thin film for the substrate temperature of 500 °C.
Fig. 6. UV–vis results of the MoO3 thin films for different substrate temperatures of (a) 350 °C, (b) 400 °C, (c) 450 °C and (d) 500 °C.
The variation of (αhν)2 versus (hν) for the MoO3 films deposited at different temperatures is shown in Fig. 7(a)–(d). The straight line portion indicates that the optical transition is direct in nature. The band gap energy is calculated using the following relation [24]
αhυn = B(hυ − Eg )
(6)
where α is the absorption co-efficient, hυ is the photon energy, B is the constant, Eg is the optical band gap energy and n is a number that characterizes the transition process (direct allowed transition n¼ 2 and indirect allowed transition n ¼1/2). The direct allowed band gap energy value has been determined by extrapolating the vertical straight line portion of the plot to the
Fig. 7. Band gap energy of MoO3 thin films for different substrate temperatures of (a) 350 °C, (b) 400 °C, (c) 450 °C and (d) 500 °C.
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These results show that the MoO3 film deposited at different substrate temperatures has variations in optical absorbance and band gap values.
3.6. Functional group analysis by FTIR
Fig. 8. IR spectra of the MoO3 thin films for different substrate temperatures of (a) 350 °C, (b) 400 °C, (c) 450 °C and (d) 500 °C.
energy axis. The intercept on the energy axis gives the values of the band gap energy 2.97, 2.94, 2.88 and 2.50 eV for the MoO3 films heated at 350, 400, 450 and 500 °C, respectively, which are in good agreement with the reported values of 2.75–2.95 eV [20,22].
The FTIR spectra of the MoO 3 thin film samples in the range of 4000-400 cm 1 are shown in Fig. 8. The results are presented in Fig. 8 for the deposition temperature from 350 to 500 °C. The samples show the peaks in the region of 1000-400 cm 1 corresponding to the stretching and bending vibrations of metal-oxygen characteristic bonds. The peaks between 500 and 600 cm 1 correspond to the vibration of the Mo–O bond. In all the spectra, some very weak peaks appear at 900 and 1000 cm 1. The vibrational analysis shows that the characteristic region for the MoO stretching mode occurs around 450, 514, 608, 700-730, 800–840 and 900–1000 cm 1 [25–27]. A broad band at 3580 cm 1 and a small band at 1605 cm 1 correspond to the stretching and bending vibrations, respectively, of the hydrogen bonded –OH group in water molecules. As the latter is attributed to the unique MoO bonds in polycrystalline α-MoO3 [28], its presence indicates that in the studied MoO3 films α-modification could exist. The fact that the basic transmittance bands are comparatively narrow is an evidence of the polycrystalline material. Moreover, other functional groups are not involved.
Fig. 9. I–V characteristics of the MoO3 thin films for different substrate temperatures of (a) 350 °C, (b) 400 °C, (c) 450 °C and (d) 500 °C.
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Fig. 10. ln s versus 1000/T plots for the MoO3 thin films for different substrate temperatures of (a) 350 °C, (b) 400 °C, (c) 450 °C and (d) 500 °C.
Fig. 12. Schematic diagram of the p-Si/n-MoO3 diode.
film for the constant voltage of 10–100 V was studied at different temperatures from 30 to 150 °C for the MoO3 films (350–500 °C) as shown in Fig. 9(a)–(d). The electrical conductivity (s) can be calculated using the given formula [29], Fig. 11. Activation energy of the MoO3 thin films for different substrate temperatures.
3.7. Electrical characterization 3.7.1. DC conductivity of MoO3 thin films The electrical conductivity was carried out using the Keithley electrometer 6517B two probe setup. The current flow through the
⎛ I ⎞ ⎛ d⎞ σ= ⎜ ⎟×⎜ ⎟ ⎝ V ⎠ ⎝ A⎠
(7)
where I is the current, V is the applied voltage, d is the inter-probe distance and A is the cross-sectional area of the film. As the temperature increases, the conductivity of the film for constant voltages (10–100 V) is found to increase as shown in Fig. 10(a)–(d) (i.e., 1000/T (K) versus ln s (S/cm)). This is due to an increase in the
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Fig. 13. I–V characteristics of the p-Si/n-MoO3 diode in darkness and under different light illuminations.
Table 4 Diode parameters obtained from I–V characteristics for p-Si/n-MoO3 diode in darkness and under different illuminations. Source of light
Barrier width (Φb) eV
Ideality factor (n)
Current (I) A
Dark Halogen lamp Metal halide lamp Halogenþ Metal halide
0.75 0.59 0.58 0.51
7.19 5.16 6.15 5.99
2.74 10 10 1.68 10 7 2.40 10 6 3.09 10 6
⎛ −E ⎞ σ = σ0 exp⎜ a ⎟ ⎝ KBT ⎠
Fig. 14. ln J versus voltage (a) in darkness and under different light sources of (b) halogen lamp (c) Metal halide and (d) Halogenþ Metal halide lamp.
grain size with a rise in temperature. This property of conductivity confirms the semiconducting nature of MoO3 films. The electrical values measured in the present work show a trend similar to that of the reported values [30]. It is clearly revealed that the electrical properties of MoO3 thin films are well improved by the substrate temperature. The activation energy (Ea) can be calculated by the Arrhenius equation,
(8)
where s is the conductivity, s0 is a constant, Ea is the activation energy, KB is the Boltzmann constant and T is the absolute temperature. The average activation energy of MoO3 thin films for the different substrate temperatures of 350–500 °C is shown in Fig. 11. The average activation energy values vary from 0.169 to 0.0639 eV for the different substrate temperatures from 350 to 500 °C. The minimum activation energy is obtained at 500 °C. The values obtained match the reported values well [22,31]. 3.7.2. I–V characterization of p-Si/n-MoO3 diode Based on the results of structure, morphology and conductivity, we prepared the P–N diode at the substrate temperature of 500 °C by the JNS pyrolysis. The formation of the P-N junction diode is carried
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out by the p-type Si and n-type MoO3. The p-Si/n-MoO3 diode schematic structure is illustrated in Fig. 12. The diode characteristics were studied in darkness and under different light sources of halogen and metal halide lamps, which caused the diode behavior to differ owing to their different illuminations (Halogen-45 mW/cm2, Metal halide-77 mW/cm2, halogenþmetal halide-100 mW/cm2 were calculated using the lux meter, and the distance between the diode and the lamps was 0.3 m). The I–V characteristics of the p-Si/n-MoO3 diode in darkness and under different lamps are shown in Fig. 13(a)– (d). The semi logarithmic plot for the current density (ln J) versus voltage (V) is shown in Fig. 14. The forward bias direction corresponds to the negative top electrode of the device. The forward to reverse current is in the range of þ 4 to 4 V. The characteristics of p-Si/n-MoO3 show good rectifying nature in darkness and under different illuminations. The rectifying voltages for darkness and different lamps of the hetero-junction are found to be 1.9, 2.1, 1.6 and 1.2 V; and beyond these voltages, the forward bias current increases as the applied voltage increases. But the reverse bias current shows a poor saturation current. This is nearly a good result for an ideal P–N diode, which has zero resistivity in forward bias and infinite resistivity in reverse bias. According to the thermionic emission theory (TE), the current through the diode can be calculated as follows [32],
SEM analysis revealed that nanorods with randomly oriented surface feature were observed for the MoO3 film deposited at the low temperature of 350 °C. Nanorods were changed into sub-microsized plate-like structures owing to the increasing temperature from 350 to 500 °C. Randomly oriented surface morphology with sub-microsized plate-like structures was observed on the surface of the MoO3 films at 500 °C. The EDX analysis showed the presence of the same Mo:O in the film as in the starting spray solution. Through the UV analysis, the absorbance was found to increase up to 450 °C and decrease at 500 °C. The band gap of MoO3 films decreased from 2.97 to 2.50 eV as the substrate temperature increased. The FTIR spectra of the MoO3 thin film samples in the range 4000–400 cm 1 confirmed the formation of the product. The electrical studies showed that the conductivity increased when the substrate temperature increased. The minimum activation energy value of 0.064 eV was obtained at the higher substrate temperature of 500 °C. From p-Si/ n-MoO3 diode characterization studies, the diode behavior was seen to change extremely owing to darkness and different lamps. The minimum ideality factor 5.16 was observed for the p-Si/n-MoO3 diode under the illumination of the halogen lamp.
⎛ qV ⎞ I = I0 exp⎜ − 1⎟ ⎝ nKT ⎠
Acknowledgments
(9)
where I0 is the reverse saturation current, q is the electron charge, V is the applied voltage, n is the ideality factor, K is the Boltzmann constant and T is the absolute temperature. The ideality factor (n) and the reverse bias saturation current (I0) of the diode are determined from the slope and the intercept of the semi-logarithmic forward bias J–V plot for VZ3kT/q using Eq. (9) and the ideality factor n and the barrier height Φb can be calculated as follows [33]
q dV n= KT d(ln I )
Φb =
(10)
KT ⎛ AA*T2 ⎞ ⎟ ln⎜ q ⎝ I0 ⎠
(11) *
where A is the active area of the diode and A is the Richardson constant. The ideality factor (n) values and barrier height (Φb) values are shown in Table 4. For an ideal P–N diode, the ideality factor value is unity (i.e., n¼ 1) but experimentally we obtained higher ideality factor (n) values of the p-Si/n-MoO3 diode in darkness and under different light illuminations in the range of 5–7 than unity (n41). This is a non-ideal behavior, which may be due to the existence of an interfacial thin native oxide layer of SiO2 and a wide distribution of barrier inhomogeneities [34]. It may also be due to series resistance and nonlinear metal-semiconductor contact [35]. Another reason may be the abnormalities of the inorganic film thickness and nonuniformity of the interfacial charges [36]. Compared to the metal halide lamp the halogen lamp gives minimum ideality factor. The variation of current in the diode of p-Si/n-MoO3 owing to different lights is also very relevant to photo-detector applications.
4. Conclusion Molybdenum trioxide thin films were prepared on glass substrates at different substrate temperatures from 350 to 500 °C by the JNS pyrolysis technique. The structural studies by the XRD analysis showed the polycrystalline nature of the MoO3 films with orthorhombic structure. All the films showed preferential orientation along with the (040) plane. The crystallite size varied from 41.44 to 55.26 nm when the substrate temperature increased. The
The authors are gratefully thanked to Sophisticated Test and Instrumentation Centre (STIC), Cochin University, Cochin, for supporting SEM studies for this work.
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