Optimization and characterization of CuO thin films for P–N junction diode application by JNSP technique

Accepted Manuscript Title: Optimization and characterization of CuO thin films for P-N junction diode application by JNSP technique Author: P. Venkateswari P. Thirunavukkarasu M. Ramamurthy M. Balaji J. Chandrasekaran PII: DOI: Reference:

S0030-4026(17)30434-5 http://dx.doi.org/doi:10.1016/j.ijleo.2017.04.039 IJLEO 59081

To appear in: Received date: Revised date: Accepted date:

16-11-2016 10-4-2017 10-4-2017

Please cite this article as: P. Venkateswari, P. Thirunavukkarasu, M. Ramamurthy, M. Balaji, J. Chandrasekaran, Optimization and characterization of CuO thin films for P-N junction diode application by JNSP technique, Optik - International Journal for Light and Electron Optics (2017), http://dx.doi.org/10.1016/j.ijleo.2017.04.039 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

*Manuscript

Optimization and characterization of CuO thin films for P-N junction diode application by JNSP technique P. Venkateswaria,b, P. Thirunavukkarasua,*, M. Ramamurthyc, M. Balajic, J. Chandrasekaranc a

Department of Electronics, Sri Ramakrishna Mission Vidyalaya College of Arts and Science, Coimbatore 641020,

b

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Tamil Nadu, India Department of Electronics and Communication, Rathnavel Subramaniam College of Arts and Science, Coimbatore 641402, Tamil Nadu, India

Department of Physics, Sri Ramakrishna Mission Vidyalaya College of Arts and Science, Coimbatore 641020, Tamil Nadu, India

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Corresponding author: Ph: +91 8925556764

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E-mail: [email protected]

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Abstract:

The present work illustrates the optimization of substrate temperature, mole concentration and volume of the solution of copper oxide (CuO) thin films prepared by

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jet nebulizer spray pyrolysis (JNSP) technique. Such prepared CuO films were optimized and characterized by XRD, SEM, EDX, UV-vis and I-V. From XRD analysis the mole

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concentration, volume level and substrate temperature of the prepared CuO films were fixed as 0.20M, 5 ml & 4500C respectively and optimized for P-N diode application. The

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XRD pattern of the optimized CuO film reveals monoclinic structure. The surface morphological variations and elemental present were confirmed by SEM and EDX analysis. The optical properties were recorded by UV-vis spectrum and the minimum band gap value is observed as 1.63 eV for 450°C substrate temperature. The maximum conductivity value of the prepared CuO is recorded as 7.4 x 10-9 S/cm from I-V characterization. Using J-V, the diode parameters of p-CuO/n-Si prepared at 450°C with

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0.2 M and 5 ml were measured in dark and under illumination. The ideality factor (n) and barrier height (b) values of p-CuO/n-Si diode are 6.2 and 0.80 eV in dark and 4.6 and 0.81 eV under illumination. Keywords: CuO; Thin films; JNSP technique; P-N junction diode; ideality factor; barrier height; 1. Introduction In recent years, the transition metal oxides (TMOs) have attracted substantial interest owing to their potential applications such as supercapacitors, sensors, solar cells, 1 Page 1 of 26

photocatalysis and electrochromic devices [1-6]. Among them, copper oxide has two stable forms of tenorite (CuO) with the band gap from 1.2 to 1.9 eV and cuprite (Cu 2O) with the band gap from 2.0 to 2.2 eV [7,8]. Both CuO and Cu2O exhibit p-type semiconducting nature owing to copper ion vacancies in the structure. The three main advantages of copper oxide are (i) low cost and less toxicity, (ii) highly effective in visible light and (iii) high absorption property for oxygen molecules [9-12].

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Nowadays, the various methods are available to grow copper oxide thin films such as chemical deposition, sol-gel, spray pyrolysis, chemical vapour deposition, sputtering,

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plasma evaporation and pulsed laser deposition [8, 13-18]. From those methods, spray pyrolysis is very simple, economical and a suitable method for large area coating for

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many semiconducting thin films.

In this research work, CuO thin films were prepared using modified spray technique,

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named jet nebulizer spray pyrolysis (JNSP) technique [19-21]. The prepared CuO films were characterized by optical, structural and electrical properties. Finally, p-CuO/n-Si

2. Experimental details 2.1. Preparation of CuO thin films

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junction diode parameters were observed using the current-voltage (I-V) measurements.

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The precursor solution was prepared with different mole concentrations of 0.10, 0.15, 0.20 and 0.25 M of copper (II) acetate monohydrate (C4H6CuO4. H2O) and deionized

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water. CuO thin films were coated on well cleaned glass substrates (2 x 2 cm) under three conditions as follows (i) Initially, the CuO thin films were prepared by varying the volume of spray solution (3, 4, 5 and 6 ml) with fixed 400°C substrate temperature and 0.20 mole concentration for optimizing the volume level of the solution. (ii) Then by varying the molarity (0.10, 0.15, 0.20 and 0.25 M) with fixed 400°C substrate temperature and volume level as 5ml, the CuO films were prepared for optimizing the

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mole concentration, (iii) Finally, CuO thin films were prepared by varying the substrate temperatures (350, 400, 450 and 500°C) with optimized molarity as 0.2 M and volume level as 5 ml and the temperature is optimized. 2.2. Fabrication of p-CuO/n-Si junction diode Prior fabricating a diode, silicon (Si) wafer is cleaned well by piranha solution (3:1 of H2SO4:H2O2) to remove impurities like organic residues, dust and grease. Next the native oxides from the Si wafer is removed by H2O:HF (10:1) solution. 2.5 ml of precursor solution with 0.20 M was coated on n-type Si wafer (1 x 1 cm) at 450°C substrate

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temperature. Then the p-type CuO thin film coated on n-Si wafer, the silver (Ag) paste was applied on the both surfaces of p-CuO/n-Si junction diode for better contacts. The Ag applied diode is dried at ambient condition for about 6 hr. I-V measurements under dark and light illumination (650 W halogen lamps) was taken using KEITHLEY 6517B electrometer. 2.3. Characterization Technique

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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° was

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used to obtain the structure of the CuO thin films. The variations in surface morphology of the films were analyzed by the scanning electron microscope (JEOL/EOJSM-6390).

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The presence of elements (Cu and O) was confirmed by the energy dispersive analysis from X-ray spectroscopy (EDX) (BRUKER). The UV-visible spectrophotometer (Perkin

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Elmer Lambda 35) showed the optical properties in the wavelength range from 300 to 900 nm. Dc electrical properties and diode characterizations of the CuO films and p-CuO/n-Si junction diode was measured using Keithley Electrometer (6517-B).

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3. Results and Discussion 3.1. Structural Studies

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The uniform CuO thin films were prepared using (JNSP) technique by varying the volume of the spray solution; mole concentration and substrate temperature and their crystalline natures were studied through XRD. By fixing the substrate temperature and

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mole concentration as 400°C and 0.20 M, the volume of precursor solution was varied as 3, 4, 5 and 6 ml and thin films of CuO were coated and their XRD patterns of various volume of solution from 3 to 6 ml were displayed in Fig. 1a, which depicts that the crystallinity increases till 5 ml then decreases for 6 ml. The diffraction an angle (2θ in degree) with corresponding hkl planes coincides with JCPDS values (Card no. 45-0937)

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and indicates monoclinic structure.Fig. 1b shows the XRD pattern of CuO for different mole concentrations 0.10, 0.15, 0.20 and 0.25 M with constant volume 5 ml and substrate temperature 400°C. The intensity of the diffraction peaks increases with increasing mole concentration from 0.10 to 0.20 M then it decreases for 0.25 M. The diffraction peaks confirm the monoclinic structure.Fig. 1c elucidates the XRD pattern of CuO for different substrate temperature from 350 to 500°C for fixed 0.20 M and 5 ml. The Fig. 1c reveals that the crystallinity increases as increasing the substrate temperature till 450°C and then decreases. It also confirms the monoclinic structure with preferred orientations of (002) and (111) planes 3 Page 3 of 26

The micro structural properties of crystallite size (D) and micro strain (ε) of CuO thin films for the preferred orientation of (0 0 2) were calculated by the following relations (1, 2) [20, 21] (1)

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(2) Where k is the shape factor (k=0.94), λ is the wavelength of the X-ray radiation, θ is

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diffraction angle and β is the full width at half maximum.

The tables 1, 2 and 3, illustrates the maximum crystallite size and minimum microstrain

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values as 41.37 nm and 0.8385 x 10-3 lines-2 m-4 (5 ml of volume), 46.63 nm and 0.7439 x 10-3 lines-2 m-4 (0.20 M concentration), and 48.18 nm and 0.7200 x 10-3 lines-2 m-4 (450°C substrate temperature). Fig. 2a-c implies the variation of D and ε of CuO films for various

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volumes of solution, mole concentrations and substrate temperatures. The results revealed that the 5 ml of volume, 0.20 M of concentration and 450°C of substrate temperature

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provide good crystalline nature in the present study of JNSP technique. 3.2. Morphological and Elemental Analysis

The surface morphological changes of CuO films for various volume of solution, mole

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concentrations and substrate temperatures are shown in Fig. 3, 4 and 5. In Fig. 3a, the closely packed sub-microsized spherical shape grains were observed for 3

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ml of volume. When the volume of the solution is increased, the large variations in surface with randomly oriented sub-micro sized grains were observed (Fig 3c-d). Fig. 4a-d shows the surface morphology of different mole concentrations for CuO films. Fig. 4 shows the randomly oriented sub-micro sized morphological results, which is similar to Fig. 3. The surface morphological images of CuO for different substrate

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temperatures from 350 to 500°C are shown in Fig. 5a-d. Fig. 5a exposes the irregularly arranged sub - microsized flake-like structure of CuO film at 350°C. Above 350°C substrate temperature, the spherical shaped grains are observed with asymmetrical arrangement (Fig. 5c-d). Fig. 6a-d depicts the EDX results for elemental analysis at different substrate temperatures from 350-500°C. The presence of elements of Cu and O is confirmed from Fig. 6. 3.3. Optical Properties

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Fig.7a illuminates the absorbance spectra of CuO films for different substrate temperatures 350, 400, 450 and 500ºC between the wavelength range 300 - 900 nm. The absorbance value is decreased up to 450ºC then increased in the visible region. The variation of absorbance owes to increase in packing density and decrease in thickness of the film which is due to shrinking of spray droplets [22]. Using UV-vis spectra, the band gap energy (Eg) for CuO films is calculated using

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equation (3) [23], (3)

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Where α is the absorption co-efficient, hυ is the photon energy, B is the constant and Eg is

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the band gap energy. Fig. 7b displays the variation of (hν)2 vs. (hν) plots for the CuO films prepared at different substrate temperatures. The direct band gap energy values are obtained by extrapolating the vertical straight line portion of the plot to the photon energy

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axis (x-axis). The interception on photon energy axis gives the band gap energy values as 1.86, 1.81, 1.62 and 1.70 eV for different substrate temperatures 350, 400, 450 and

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500ºC respectively. The minimum band gap energy 1.62 eV [24] is obtained for the substrate temperature 450ºC. The difference in band gap energy is due to the change in crystallinity of the films with temperature [25].

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3.4. DC Electrical Properties

I-V measurements of CuO thin films prepared for various volume of solution, mole

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concentrations and substrate temperatures were taken at room temperature using Keithley electrometer through two probe. The current values were measured for different applied voltages from 10 to 100 V (Fig. 8a, 9a and 10a).The dc conductivity () for CuO films was calculated using the given equation (4) [20,21],



(4)

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where I is current, V is applied potential, d is inter-probe distance and A is crosssectional area of the film. Fig. 8a shows the I-V characteristics of CuO films for different volume concentrations. The obtained average conductivity values are 3.13 x 10-9, 3.44 x 10-9, 3.70 x 10-9 and 3.39 x 10-9 S/cm for volume of solution from 3 to 6 ml. The results revealed that the conductivity value is increased up to 5 ml and decreases as shown in Fig. 8b. Fig. 9a indicates the I-V characteristics for different mole concentrations. The average conductivity values of CuO films obtained as 3.35 x 10-9, 3.72 x 10-9, 1.20 x 10-8 and 1.13

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x 10-8 S/cm for different mole concentrations of 0.10, 0.15, 0.20 and 0.25 M. From the Fig. 9b, the maximum conductivity is observed for 0.20 M. The I-V characterization for different substrate temperatures is shown in Fig. 10a. By varying the substrate temperatures from 350 to 500ºC, the observed average conductivity values of CuO films are 2.98 x 10-11, 7.54 x 10-10, 7.36 x 10-9 and 1.63 x 10-9 S/cm. At the substrate temperature of 450ºC, the maximum conductivity value is found as shown in Fig. 10b.

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The variations in conductivity values may be attributed to the oxygen vacancies in the prepared CuO thin films [21,26-28]. The increasing of conductivity with the substrate

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temperature owing to the variation in morphology and the increasing of grain size up to 450ºC, which reduced the lattice dislocations and imperfections of the Cu–O matrix. This

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phenomena decreases the grain boundary volume associated with flow of charge carriers [29].

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3.5. I-V characterization of p-CuO/n-Si junction diode

The junction formation of the P-N diode is carried out by the p-type CuO and n-type Si. The P-N junction diode of p-CuO/n-Si was prepared based on the optimized results of

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volume of solution (5 ml), mole concentration (0.20 M) and substrate temperature (450ºC) by JNSP technique. The prepared p-CuO/n-Si junction diode parameters of

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ideality factor (n) and barrier height (b) were measured under darkness and illumination of light (halogen + metal halide - 100 mW/cm2) [20]. The reverse to forward bias current measurement is taken for the applied potential of -4 to +4 V. Fig. 11 shows the I-V

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characteristics of p-CuO/n-Si junction diode under dark and illumination. Fig. 12 illustrates the semi-log plot of the current density (ln J) vs voltage (V).From the Fig. 11, the p-CuO/n-Si junction diode exhibits a good rectifying nature in dark and under illumination.

Using the thermionic emission (TE) equations, the current density of the p-CuO/n-Si

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junction diode was calculated as follows (5) [21,30], (5)

where Jo is reverse saturation current density, q is charge of electron, V is applied potential, n is ideality factor, K is Boltzmann constant and T is absolute temperature. Using equation (5) for V ≥ 3kT/q, n and Jo values were obtained from the slope and interception of semi-log forward bias J-V plot. Thus, n and b were calculated by the following equations (6,7) [21,30],

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(6)



(7)

where A is the active area of prepared diode and A* is the Richardson constant. The n and b values under darkness is obtained as 6.2 and 0.80 eV and under light illumination as

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4.6 and 0.81 eV. The n value is unity (i.e., n = 1) for an ideal P-N diode but in the present work, the n values are obtained as more than unity. The obtained result of n value is

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comparable to the reported value (3.5), for a reactive magnetron sputtering deposited Ag/p-CuO/n-Si Schottky diode [31]. The results suggest the non-ideal behavior of

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prepared p-CuO/n-Si junction diode which may be owing to the presence of an interfacial thin native oxide layer (SiO2) and barrier inhomogeneities [20,21,30]. It may also be

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owing to series resistance and nonlinear metal-semiconductor contact [20,32]. Another reasons may be the abnormalities of the inorganic film thickness and non-uniformity of the interfacial charges [20,33].

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4. Conclusion

The CuO thin films have been prepared by JNSP technique and the conditions for volume

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of solution, mole concentrations and substrate temperatures were optimized. From the XRD results, it was observed that the prepared CuO film has monoclinic structure and also has the maximum crystallite size as 41.37, 46.63 and 48.18 nm for 5 ml of volume,

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0.20 M of molarity and 450 ºC of substrate temperature respectively. The surface morphological variations of sub-microsized grains of CuO were displayed by SEM analysis for different volume, molarity and substrate temperatures and the presence of elements such as Cu and O were confirmed by EDX results. UU-vis for various substrate temperatures was observed and it showed the minimum absorbance and minimum band

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gap energy of 1.62 eV for the substrate temperature of 450ºC. The dc electrical conductivity reveals that the 5 ml of volume, 0.20 M of molarity and 450ºC of substrate temperature has maximum conductivity values. Based on the optimized volume (5ml), molarity (0.20 M) and substrate temperature (450ºC), the p-CuO/n-Si junction diode was prepared by JNSP technique. The diode parameter of ideality factor (n) values were measured as 6.2 in darkness and under illumination as 4.6.The photoconduction nature of the prepared p-CuO/n-Si junction diode will be endorsed for photo-detector application.

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FIGURE CAPTIONS Fig. 1. XRD patterns fordifferent (a) voume concentrations, (b)mole concentrations and (c) substrate temperatures. Fig. 2. Microstructural properties of crytallite size and microstrain values fordifferent (a) voume concentrations, (b)mole concentrations and (c) substrate temperatures. Fig. 3 SEM images for different volume concentrations of (a) 3, (b) 4, (c) 5 and (d) 6 ml.

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Fig. 4 SEM images for different mole concentrations of (a) 0.10, (b) 0.15, (c) 0.20 and (d) 0.25 M.

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Fig. 5 SEM images for different substrate temperaturesof (a) 350, (b) 400, (c) 450and (d) 500°C.

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Fig. 6 EDX spectra for different substrate temperaturesof (a) 350, (b) 400, (c) 450 and (d) 500°C. Fig. 7 (a) Absorbance spectra and (b) Band gap energy for different substrate temperatures

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Fig. 8 (a) I-V characterization and (b) Average conductivity for different volume concentrations

M

Fig. 9 (a) I-V characterization and (b) Average conductivity for different mole concentrations Fig. 10 (a) I-V characterization and (b) Average conductivity for different substrate temperature

ed

Fig. 11 I-V characterization for p-CuO/n-Si junction diode in darkness and under illumination

ce pt

Fig. 12 Plots ln J vs. V for p-CuO/n-Si junction diode in darkness and under illumination TABLE CAPTIONS

Table 1 Microstructural properties for different volume concentrations Table 2 Microstructural properties for different mole concentrations Table 3 Microstructural properties for different substrate temperature

Ac

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

11 Page 11 of 26

Figure

M

an

us

cr

ip t

Figure 1

(b)

Ac ce p

te

d

(a)

(c)

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Ac ce p

te

d

M

an

us

cr

ip t

Figure 2

(c)

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Figure 3

(b)

(a)

cr

ip t

(a)

(b) (d)

Ac ce p

te

d

M

an

us

(c)

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(b)

(c)

(d)

Ac ce p

te

d

M

an

us

ip t

(a)

cr

Figure 4

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(b)

(c)

(d)

Ac ce p

te

d

M

an

us

ip t

(a)

cr

Figure 5

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Figure 6

(a)

us

cr

ip t

(b)

(d)

Ac ce p

te

d

M

an

(c)

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us

cr

ip t

Figure 7

(b)

Ac ce p

te

d

M

an

(a)

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us

cr

ip t

Figure 8

(b)

Ac ce p

te

d

M

an

(a)

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us

cr

ip t

Figure 9

(b)

Ac ce p

te

d

M

an

(a)

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us

cr

ip t

Figure 10

(b)

Ac ce p

te

d

M

an

(a)

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Ac ce p

te

d

M

an

us

cr

ip t

Figure 11

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Ac ce p

te

d

M

an

us

cr

ip t

Figure 12

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Table

Table 1 Micro Crystallite FWHM strain (ε) size (D) (Radians) (x 10-3 lines-2 nm m-4) 0.0039

37.72

0.9197

4

0.0038

38.73

0.8957

5

0.0035

41.37

0.8385

6

0.0036

40.39

0.8589

Ac ce p

te

d

M

an

us

cr

3

ip t

Volume variation (ml)

Page 24 of 26

Table 2

0.10

0.0038

38.75

0.8949

0.15

0.0036

40.76

0.8508

0.20

0.0031

46.63

0.7439

0.25

0.0032

45.05

0.7700

Ac ce p

te

d

M

an

us

cr

FWHM (Radians)

ip t

Micro Crystallite strain (ε) size (D) (x 10-3 lines-2 nm m-4)

Molarity Variation (M)

Page 25 of 26

Table 3

350

0.0036

40.88

0.8485

400

0.0034

43.31

0.8010

450

0.0030

48.18

0.7200

500

0.0033

44.03

0.7878

Ac ce p

te

d

M

an

us

cr

FWHM (Radians)

ip t

Micro Crystallite strain (ε) size (D) (x 10-3 lines-2 nm m-4)

Substrate Temperature (°C)

Page 26 of 26