The Effect of Doping Thiourea In CdO Thin Films For Electronic Applications

Cadmium oxide (CdO) and Thiourea (Th) doped CdO thin films were deposited on soda lime glass (SLG) and crystalline p-type Si (100) substrates for various Th doping concentrations (0.3, 0.5, 0.7 at.%) using spin coating method. Some structural parameters such as the crystallite size, lattice constant, dislocation d ensity (δ) and strain in the films were obtained from XRD analysis in which the polycrystalline structure with cubic nature and (111) prefential orientation was confirmed. CdO thin film has not shown any change in crystal phase after Th doping. The optical study emerged that the Th doping caused important changes in the transmittance, absorbance and reflectance spectra. A maximum optical transmittance (above 80%) have been obtained for 0.3% Th doped CdO thin films. Th doped CdO/p-Si heterojunctions exhibited low rectifying character and were not found to be the light sensitive.


Introduction
Semiconducting nanostructures are of great importance for device technology due to unique optical and electrical properties. The efforts have been made for the development of semiconductors for years. Compound semiconductors that are made from two or more elements have widespread applications. Transparent conducting oxides (TCOs) are an example of compound semiconductors [1][2][3]. TCOs have attracted a great attention of scientists in optoelectronic applications such as solar cells, phototransistors, flat panel field and manufacturing of gas-sensing.
CdO shows n-type semiconductor property, which might be due to oxygen (O) vacancies (VO) and Cd interstitials (Cdi). CdO exhibits some features as the moderate indirect (1.98 eV) band gap, low resistivity of 10 -2 -10 -3 Ω cm, relatively high electrical conductivity (10 2 -10 3 S/cm) and rock salt crystal structure. [7]. This material stands out with synthesis methods as sol-gelspin coating [8], successive ionic layer adsorption and reaction (SILAR) [9], spray pyrolysis [10], vacuum evaporation [11], radio frequency magnetron sputtering [12], chemical bath deposition [13]. Sol-gel method suggests some attractive features, such as lowcost equipment, easy control of growth and amenability to large area coverage. Relatively small band gap of CdO is one of its disadvantages for the photovoltaic applications. CdO acts like a degenerate semiconductor (close to metal) at high level of doping. The absorption edge can be shifted by free carriers in the conduction band (Burstein-Moss efect) [14,15]. Ganesh et al. [16] prepared Nitrogen (N) doped (1-20%) cadmium oxide thin films on the glass substrates in their work using sol-gel spin coating method. They obtained the optical direct and indirect band gaps varying with the doping concentration in the range of 3.92-3.98 eV and 3. 24-3.49 eV, respectively. In addition, several research groups reported that increasing doping concentrations decreased the band gap. Kabir et al. [17] carried out a work on strontium (Sr) doped cadmium oxide (CdO) thin films by using spray pyrolysis technique. It is seen that the band gap of CdO decreases from 2.67 to 2.49 eV, as Sr quantity in CdO increases (1%, 2%, 3% to 4%).
Generally, there are studies in which single or co-doping of elements to TCOs are made in literature. But, Cho et al. [18] reported the synthesis of ZnO nanostructures by organosulfur compound thiourea (SC(NH2)2) with sulfur (S). Scanning electron microscopy (SEM) images show that the shape of the nanostructure changes with the concentration of thiourea. Density functional theory (DFT) suggests that sulfur atoms prefer to be found at the hexagonal vertices of ZnO, leading to a change in charge distribution. From the photoluminescence (PL) spectra, the emission peak shows a slight blue-shift with increasing the concentration of thiourea, visible light emission where there is a concept of interest in visible light emitting device applications. Blue-shifted or enhancement visible light emission is ascribed to the change of S doping in ZnO nanostructures [19,20].
Recently, intense attention has been paid to understanding TCO-charge transport. Metallic oxide films might show rectifying property while brought into contact with a semiconductor.
Therefore, CdO thin films as an electron transport layer are used at p-n heterojunctions, depositing on p-type silicon (Si) semiconductor. Research should include the mechanisms that are needed to explain the charge transport for construction of optoelectronic nanotechnology.
To the best of our knowledge, no an experimental investigation has been performed on Thiourea (Th) doped cadmium oxide (CdO) thin films.
In the present work, we report the growth of Th doped CdO on Si and glass substrates by solgel method. Th doped CdO thin film microstructure, crystal structure, optical and electrical properties are examined in detail with increasing doping level.

Experimental Details
To prepare CdO thin films, 2-methoxyethanol (C3H8O2), monoethanolamine (MEA) (C2H7NO), Cadmium acetate dihydrate (CAD) (Cd(CH3COO)2.2H2O) were used. The materials (precursor, solvent and stabilizer) were adjusted so that the molarity of the solution was 0.5 molar. CAD was dissolved in 2-methoxyethanol (C3H8O2). The explanation of all the processing operations is presented in our previous work [24]. Molecular Formula of Thiourea is given as CH4N2S or H2NCSNH2. Fig. 1 shows a schematic representation of Thiourea. In fact, by adding Thiourea (Th), sulfur is added to CdO. CdO thin films with different proportions of Thiourea (Th0.03Cd0.99O, Th0.05Cd0.95O, Th0.07Cd0.93) were prepared. The solutions were coated on the ultrasonically cleaned soda-lime glass substrates with spin coating method. The spin speed was adjusted at 1500 rpm at a fixed spinning time of 60 s.
Three samples obtained were thermally annealed at 1h in furnace, when the temperature reached 400 °C. p-Si (100) substrate was used to obtain the heterojunction. Chemically cleaned samples were exposed to pure nitrogen gas. The front face of p-type Si substrates was coated with Th doped CdO and the annealing process was applied. Coating/annealing parameters of soda-lime glass substrates were continued and then, to obtain Al/Th:CdO/p-Si device structures, circular shaped Al contact was made on Th doped CdO thin films. The atomic force microscopy (AFM) was used to measure the surface morphology of the CdO thin films. Crystallographic data of the CdO thin films are routinely characterized using x-ray diffraction (XRD, Rigaku) with Cu Kα radiation. The transmittance, absorbance and reflectance data were were obtained by the aid of UV-vis spectrophotometer (Shimadzu UV-VISNIR 3600). The current-tension (I-V) relationship for Th:CdO/p-Si heterojunctions was supplied by a Keithley 2400 voltage source. Raman spectroscopy was evaluated to obtain the information about molecular vibrations.  Th doping, S ratio is specified in CdO thin films with 0.5% and 0.7% Th doping. It is also noteworthy that the increasing Th ratio is parallel to the S ratio. The texture coefficient (TC(hkl)) value represents the growth surface and is given as follows

Morphological, Structural and Optical Characteristics
where N represents the diffraction peak number, I(hkl) is the observed relative intensity of the plane (hkl), Io(hkl) is the standard intensity. The (111) and (200) preferential plane growths show dominant peak intensity as in earlier reports [28,29]. Table I contains the texture coefficients for CdO films, depending on the amount of doping Th. Since CdO thin films include randomly oriented grains, the texture coefficient is important. The texture coefficient values close to 1 indicate the presence of randomly oriented grains [30]. It is observed that the Th doping changes partially the texture coefficient values. It is seen that the textured CdO surface is especially dominant along (100) plane. The surface texture of the thin films reflects the mismatch of grain boundary. This is associated with porosity and plays an important role in the gas sensing mechanism [31].
The full width at half maximum (FWHM) is used as a tool to evaluate the peak intensity. The values of FWHM is given in Table I Where. 0.94 correlation coefficient is used.  , λ represent the Bragg angle and the incident xray wavelength (1.5406 A°), respectively. Table II shows that the nm-scaled crystal size for the crystal plane (200) decreases with increasing Th concentration. The observed pattern is contrary to the previous report on Sr-doped CdO [17]. The decreasing nature of the XRD peaks with Th is due to the decreasing crystallite size [33]. The change in crystal boundaries also has an effect on crystal size. In addition, dislocation density (δ) and strain (ε) (considering the crystallite size along c-axis) are some of other structural parameters, given as The values are given for the crystal plane (200) in Table II. The dislocation density (δ) is in order of 10 14 lines/m 2 while it is obtained in 10 15 lines/m 2 value for spray pyrolized strontium (Sr) doped cadmium oxide (SCO) thin films in Ref [17]. Both the δ and ε values increase with increasing Th doping concentration. Small dislocation density is associated with the crystalline quality or high degree of crystallinity of the film.
The lattice constant (a) for cubic structured CdO is given as [35]: The lattice constant values of samples annealed 400 o C for (200) preferential peak are give in  with increasing amount of additive element. [36,37]. The effect of S -2 ions, which can replace the oxygen vacancies or Cd +2 ions, can be considered. It is relevant to state that transmittance reduction due to increased doping is not related to scattering. that starts with a bump turns into a peak as the wavelength decreases. Similar behavior was also observed in the study examining the optical characteristics of Indium-doped cadmium oxide thin films prepared by Ganesh et al. [33]. Touch technique can be used to obtain the optical band gap (Eg) [38]: where  absorption coefficient, Bop is a constant coefficient, n is an exponent (index) associated with the character of the optical transition and h is the photon energy. This index follows the allowed direct, forbidden direct, allowed indirect, forbidden indirect transition as 1/2, 3/2, 2 and 3, respectively. The value of n is determined as ½ for CdO with direct transition.  (Table II). The obtained bandgap values were found within the standard range of Eg (CdO) (2.2-2.6 eV) [39,40]. The bandgap of CdO thin film was found to be blue-shifted as Th incorporation level increases (band-gap widening-BGW). This is attributed to Burstein-Moss (BM) band gap widening on varying doping of Th from 0.3% to 0.7% [41,42]. The variation in band gap is due to more O-vacancies resulting from the presence of S content in Th doped CdO nanostructures. Moss-Burstein BGW could be resulted by the variation of carrier concentration. The band gap change is phenomenologically affected by the carrier concentration. The band gap for a semiconductor consisting of conduction electrons (Nel) is given by the following equation [42]: The Δ BM g E is related to the band-gap widening (BGW) [43].
Where γ is equal to  The exponential edge region is characterized by Urbach-Martienssen rule: where Eu and 0  are Urbach energy and a constant, respectively. Fig. 6 shows the plots of ln(α) vs. photon energy (hν) for CdO thin films with Th (0.3%, 0.5%, 0.7%). The obtained values are given in Table II. The absorption is due to localized-to-extended-states transitions in the exponential tail. Urbach energy decreases as Th-content in CdO thin films increases, and then it increases. for Th doped CdO thin films. The absorption index is given as following relation: As it is seen, the absorption index (extinction coefficient) shows a similar character to the absorption plot. The absorption index also changes depending on Th doping concentration. The exciton coefficient (k) constitutes the imaginary part of the complex refractive index (n): The refractive index also behaves like reflectance curves and the characteristics change depending on the concentration. Morover, the refractive index first increases and then decreases with increasing wavelength. In particular, the region of reduction is known as normal dispersion and the region of increase as abnormal dispersion region [46]. The dispersion is given considering the single oscillator model [48]: where Ed is the dispersion energy due to oscillator and represents to the band-to-band electronic transitions, E0 is the oscillator's energy. The variation of 1/(n 2 -1) vs. (hν) 2 for the Th:CdO thin films is given in Fig. 8. The values of E0 and Ed are determined from the region of straight line. As seen in Table II Also, the static refractive index (n0) can be found as the Ed to E0 ratio (Ed/E0) and is given as The values of static refractive index are were found to be 1.
Dielectric with low dissipation factor is required to minimize power losses for high energy efficiency. Fig. 9 shows the variation of tanδ vs. λ. The dissipation factor decreases as the doping Th increases.
Bulk and surface energy loss functions (BELF and SELF) are derived by [51]: Fig . 10a,b shows the variation of the BELF and SELF vs.  h . The probability of electrons to be excited in bulk and surface is characterized in REELS analysis. The bulk energy loss takes smaller values than the surface energy loss. It is seen that the energy losses decrease with increasing Th concentration.
Analysis of the vibration modes is associated with transport properties and phase orientation. respectively. Peaks appear to be centered at larger wavelengths compared to the literature [53][54][55]. Additional peaks are might be related to the vibration modes, which can be attributed to the internal stress due to Th and surface influence. The conduction mechanism of the heterojunction is described by thermionic emission theory (TET) [56]: (21) where, A, A * , k, q, n, T and Rs have well-recognized meanings as characterized in our previous work [57]. V -JRsA is electrical potential loss across the heterojunction (voltage drop). J0 is the saturation current density and given by below equation:

Current-Voltage (I-V) Characteristics of Th:CdO/p-Si Heterojunction Diodes
The values of Io for Th doped CdO/p-Si heterojunction diodes are in order of 10 -6 A (Table   III), which is initially decreased, then increased with increasing Th doping. The saturation current flow of the diode seems to vary by Th doping. The barrier height ( b  , BH) and ideality factor (n) are expressed by below equations: The b  and n are found from the intercept and the slope of the straight-line region of the log I vs. V plot, respectively. The values of b  were determined to be 0.64 eV, 0.63 eV and 0.64 eV for Si diodes based on Th:CdO thin films with Th doping (0.3%, 0.5%, 0.7%), respectively, in dark conditions. Also, it is seen that the ideality factor increases as Th doping increases. The ideal factor (>1) is attributed to the presence of other current transport mechanisms [58]. The determined values were obtained from a specific (linear) region of the forward bias I-V curves. However, Norde proposed a method that allows to find diode parameters from data that takes into account whole forward I-V data [59]: where  is a dimensionless quantity. The Norde function, which allows to find the b  and series resistance (Rs) is rewritten as follows:  Table III. The Rs value is high for Al/p-Si heterojunction with 0.7% Th:CdO thin film layer.
In the forward bias region of Fig. 14a-c, while the capacitance do not show any variation, it decreases with increasing frequency in the reverse bias region. This is attributed to the presence of interfacial states that cannot follow a.c. (alternating current) signal. It seems that the doping Th partially affects the capacitance values.

Conclusions
CdO thin films with different Th-doping contents could be obtained by a simple and costeffective sol-gel spin coating method on to glass and p-Si (100) substrates. The AFM results show that the surface is composed of nano-clusters with the roughness in the order of nm for 0.5% Th doped CdO sample. SEM (EDX) results confirm the presence of the element sulfur (S) in the thiourea and the Cd/O in the CdO. While XRD results correspond to dominat preferential orientation with peaks (111) and (200), the doping Th has no effect on the crystal character, affecting some structural parameters. The increasing Th contribution increases the optical band gap. Th:CdO/p-Si heterojunction exhibits rectifying property, including a low rectification ratio in dark condition. However, it is seen that Th:CdO/p-Si diodes do not show any response to the illumination. All devices show relative capacitance changing depending on the voltage and frequency in the order of nF, which is partially affected by the Th contribution.