1 Introduction

Magnesium Oxide (MgO) is a face-centered cubic crystal consisting of magnesium (Mg) elements in group IIA and oxygen (O) elements in group VIA [1]. MgO is an ionic crystal due to the high bond strength and strong electrostatic forces between ions. It is a metal oxide material with high melting point, low electrical conductivity and hardness, high optical transmittance in the visible region of approximately 90% and a wide band gap of 3.5–7 eV [2,3,4,5,6]. It has many properties such as high electrical resistance, chemical inertness, low electron affinity, low thermal conductivity, high optical transparency, good heat resistance and mechanical durability [5, 7]. Due to the non-toxicity and chemical and physical stability of MgO material, it is widely used for properties such as antibacterial, antimicrobial and photocatalytic.

Due to its optical, electronic, magnetic, thermal, mechanical and chemical properties, MgO is a prominent material used in the production of resistant and high temperature insulating materials, electroluminescence, plasma and fluorescence display panels, heat-resistant glass composites, photovoltaic devices, capacitors, gas sensor and absorber material applications. [1, 3, 5, 8,9,10].

Many techniques have been used in MgO thin film studies such as sequential ionic layer absorption reaction method, laser evaporation method, electron beam evaporation method, ion beam-assisted deposition, arc deposition, sputtering and chemical vapor deposition (CVD) [2, 5, 11,12,13]. Raj et al. produced MgO thin films with different techniques such as laser evaporation, arc deposition, electron beam evaporation, sputtering technique and CVD, and determined the material properties of MgO thin films produced with different techniques [2]. Şenaslan et al. deposited MgO thin films on Si and glass substrates using the RF sputtering method using MgO targets and examined the effect of operating pressure on their structural and optical characteristics [7]. Càceres et al. obtained MgO thin films at different substrate temperatures using MgO or Mg target materials using the RF magnetron sputtering technique and examined their structural and optical characteristics [11]. Ho et al. grew [111] and [200] oriented 260 nm thick MgO thin films on Si wafer via sol–gel method and examined the optical and electrical properties of the produced thin films [13]. Güney et al. examined the structural and optical characteristics of MgO nanostructure grown at varied thicknesses between 279 and 1443 nm using the SILAR method [14].

As mentioned above, researchers have produced MgO thin films using different techniques and focused on studying their structural and optical properties. In our study, the structural, optical and morphological properties as well as the electrical properties of MgO thin films produced using the RF magnetron sputtering technique were determined. This reliable method enables the production of high-quality films with precise thickness control, density, and homogeneity [15, 16]. The aim of this study was to reveal the crucial electrical output parameters of MgO material produced by this technique, which are significant for optoelectronic device applications.

Metal–semiconductor (MS) structure form the basis of semiconductor-based circuit components such as diodes, capacitors and transistors [17, 18]. Metal insulator/oxide semiconductor (MIS/MOS) structures are created by depositing thin films between the metal and semiconductor. Metal oxides and polymers are used as thin films in these structures. The presence of interlayer film has a significant effect on basic electrical parameters like ideality factor (n), series resistance (Rs), barrier height (Φb) and interface trap/state density (Dit/Nss) of these devices [19, 20].

In this study, firstly, to determine the structural, morphological and optical properties of the MgO film AFM, XRD and UV–Vis spectrometer techniques were used. Then, the electrical properties of the Au/MgO/n-Si device were investigated using current–voltage (I-V) measurements. Thus, the electric parameters of the device were extracted.

2 Experimental details

Magnesium Oxide (MgO) thin film structures were deposited with RF magnetron sputtering technique on n-type Si and glass substrates at room temperature. The substrates were cleaned with ultrasonic wet bath in acetone, isopropyl and deionized water, respectively. Immersion time in liquids for cleaning is 5 min each. After the wet cleaning procedure, the substrates dried with pure nitrogen gas and loaded into deposition chamber. The system evacuated under 10–6 Torr. MgO (2inch dia., 99,99% pure) alloy target positioned far from the sample holder about 10 cm. Thin film deposition process was performed at room temperature, 5 mtorr Ar pressure, 100 W power, 5 rpm rotation speed. Profilometer (Veeco- Dektak 150) System was used to survey the thickness of the MgO thin film, and its thickness was found to be 100 nm. Structural analysis of the sample was performed by X-ray diffractometer (XRD) (Bruker D8 Advance (λ = 1.54178 Å) with CuKα1 X-ray source). The morphological analysis was investigated over a 5 × 5 µm2 area with NanoMagnetics Instruments atomic force microscopy (AFM) system. The optical properties of MgO nanostructure were analyzed with UV–Visible Spectrometer (UV–Vis) (Lambda 2S UV–VIS).

The back contact fabrication of the sample from which current (I)-voltage (V) measurements will be taken was completed with 150 nm Au. After the back/ohmic contacts were coated, they were annealed at 350 °C for 3 min in a vacuum. Afterwards, the front Schottky contacts were coated with Au at room temperature, in a circular geometry (1 mm dia.), at 150 nm. I–V measurements were performed with Keithley 4200 Semiconductor Analysis System.

3 Results and discussion

3.1 Structural and morphological analysis of MgO film

The XRD patterns of the structure scanned in the range of 20°–70° with the Bruker D8 Advance XRD measurement system are shown in Fig. 1. Diffraction peaks of the deposited thin film MgO structure were observed at 36.29° (111) and 43.55° (200) and it was determined that these peaks had a cubic crystal structure (JCPDS No: 00-045-0946). As can be seen in Fig. 1, the strong diffraction peak for MgO belongs to (111) at 36.29°.

Fig. 1
figure 1

XRD pattern of MgO thin film

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Sherrer equation was used when determining the grain size of MgO thin film nanostructures [8]:

$$\text{D}=\frac{{k\lambda }}{Bcos\theta }$$
(1)

Here, \(\uplambda\) is the wavelength of the X-ray used, B is the full with half maximum of the MgO structure at 36.29° (111), \(\uptheta\) is the Bragg angle and k is the constant: 0.9. The observed peak position (2θ), calculated grain size (D) and full with half maximum (FWHM) values (B) are given in Table 1.

Table 1 The determined grain size (D) and FWHM (B) at main peak region
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Surface roughness is a significant measurement for the analysis of thin films used in a wide range of applications including semiconductor electronics and optics. Additionally, surface morphology is of great importance in terms of mechanical, physical, electrical and dielectric properties of thin films. Surface roughness plays an important role on the dielectric strength and hence the operation of the capacitor [21]. The surface roughness significantly affects the conduction of electric current. As surface roughness decreases, the resistance also decreases. The decreased of surface roughness can minimize the scattering of charge carriers [22]. Moreover, the optical transmittance mainly depends on surface roughness. The increase of surface roughness causes an increase of absorbance and a decrease of transmittance [23].

The two-dimension (2D) surface morphology, root mean square (RMS) and the grain size values determined from AFM analysis. Figure 2 shows the 2D (5 × 5 µm2) AFM image of the MgO film. Table 1 also contain the RMS and grain size (D) values that obtained from AFM analysis of the MgO thin films.

Fig. 2
figure 2

AFM image of MgO thin film

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3.2 Optical properties of MgO film

Optical spectroscopy techniques such as ultraviolet–visible (UV–VIS) and infrared (IR) are utilized for characterize the opto-electronic characteristics of materials. UV–VIS and infrared spectroscopy describe the part of the electromagnetic spectrum between 100 nm (vacuum UV) and 20,000 nm (IR). UV–VIS spectrophotometer is used to measure the light transmitted, absorbed and reflected by the material depending on the wavelength of the electromagnetic radiation [24, 25]. In this study, optical properties like optical band gap, absorption coefficient, extinction coefficient, refractive index and optical conductivity of deposited structure were determined with the transmission and absorption data.

Figure 3(a) and (b) indicate the optical transmittance (T) and absorbance (A) spectrum of MgO film between 300–1000 nm, respectively. In Fig. 3(a), the optical transmittance increases with increasing wavelength and positioned a maximum level at ~ 500 nm. Then, the transmittance value decreases with increase in the wavelength, Moreover, this spectrum showed that the transmittance value of the MgO film was about 90% in visible light. This result indicates that MgO film is highly transparent. From Fig. 3(b), it is seen that the maximum optical absorbance of the film occurs in the UV region, while its value is low and relatively constant in the VIS/near-infrared (NIR) region. In addition, the absorbance value decreases with increase in wavelength. At a higher photon energy, light with a lower wavelength is absorbed. Similar results obtained for the MgO thin film have been reported in previously published articles [25,26,27,28,29].

Fig. 3
figure 3

Optical (a) transmission (b) absorbance spectrum of MgO thin film as a function of wavelength (λ)

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The absorption coefficient (α) expresses the decrease in the intensity of electromagnetic radiation as it passes through a material. From the transmission (T) or absorbance (A) data, the absorption coefficient (α) is calculated using the relations given below [30,31,32,33,34], respectively.

$${\alpha }=\frac{1}{\text{d}}\text{ln}\left(\frac{1}{\text{T}}\right){or\, \alpha }=\frac{2.303*\text{A}}{\text{d}}$$
(2)

Here, d is the MgO film thickness (~ 100 nm). The plot of α as a function of wavelength (λ) is illustrated in Fig. 4. It is seen that the absorption coefficient exhibits similar behavior to absorbance and its value decreases with increasing wavelength. The value of α in the UV region was found to be ~ 105 cm−1. This high absorption coefficient indicates that the MgO film has high UV absorption. Additionally, the value of absorption coefficient gives information about the direct and indirect transitions between the valence band and the conduction band of material. For α˃104 cm−1, this means that the direct transition takes place in material. However, the indirect transition takes place for α < 104 cm−1 [29,30,31,32,33]. The obtained α values are greater than 104 cm−1. This indicates that the prepared MgO film exhibits direct electronic transition, that is, it has a direct band gap. [35,36,37,38].

Fig. 4
figure 4

Variation of absorption coefficient (α) as a function of λ

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At the same time, the value of optical band-gap (Eg) was calculated from the absorption coefficient. Thus, the Eg value of the prepared film was estimated using the following relationship presented by Tauc [39].

$${\alpha h\nu }={{\alpha }}_{0}{\left({h\nu }-{\text{E}}_{\text{g}}\right)}^{\text{m}}$$
(3)

where the α0 is a constant and hν is the energy of the incident photon. m is a constant parameter that depends on the electronic transition type of material, and it takes values such as 1/2, 3/2, 2 and 3. m = 1/2 and m = 3/2 represent allowed and forbidden direct transitions, respectively. m = 2 and m = 3 are the values taken for the allowed and forbidden indirect transition, respectively. For MgO film exhibited allowed direct transition (m = 1/2), (αhν)2 is plotted against photon energy (hν). The Tauc plot of (αhν)2 versus hν is shown in Fig. 5. The intercept on the hν-axis of linear part of this plot gives the magnitude of the Eg. The calculated direct optical band gap value was obtained 3.98 eV. The Eg value is consistent with reported literature value of MgO thin film structures [35,36,37,38].

Fig. 5
figure 5

The variation of (αhν)2 with hν

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3.3 Current–voltage characteristics of Au/MgO/n-Si device

To determine the device parameters like reverse saturation current (I0), ideality factor (n), barrier height (Φb), series resistance (Rs) and interface state density (Nss) of the prepared Au/MgO/n-Si device, the thermionic emission (TE) method was used. The basis of TE theory is based on current–voltage (I–V) measurements. Here in this method, for the condition V-IRs ≥ 3kT/q, the current flowing through a Schottky diode at forward bias can be given by [17, 19]:

$$\text{I}={\text{I}}_{0}\left[(\text{exp}\left(\frac{\text{q}\left(\text{V}-{\text{IR}}_{\text{s}}\right)}{\text{nkT}}\right)-1)\right]$$
(4)

The I0 defined by the equation below is extracted from the straight-line intercept of Ln(I) curve at V = 0.

$${\text{I}}_{0}={\text{AA}}^{*}{\text{T}}^{2}\text{exp}\left(-\frac{\text{q}{\upphi }_{\text{b}}}{\text{kT}}\right)$$
(5)

where A*, which is equal to 112 A/cm2 K2 for n-Si [17], is the effective Richardson constant and Φb is the barrier height. The n expressed by the equation below is obtained from the slope of Ln(I)–V curve.

$$\text{n}=\frac{q}{kT}\left(\frac{\text{dV}}{\text{d}\left(\text{Ln I}\right)}\right)$$
(6)

The forward and reverse bias Ln(I)–V plot of Au/MgO/n-Si device at room temperature is illustrated in Fig. 6. As seen in this figure, the I–V characteristic curve of the device exhibits non-linear behavior. This results from the interface states and series resistance. Especially, series resistance is responsible for the downward bending of the forward-bias I–V curve. The value of Φb0 and n calculated from Eqs. (5) and (6), respectively, is presented in Table 2. As shown in this table, the magnitude of n is greater than 1. The reason for this is the series resistance effect, the presence of interlayer, barrier inhomogeneities and interfacial traps [40,41,42,43,44].

Fig. 6
figure 6

Ln(I)-V plot of Au/MgO/n-Si device

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Table 2 Critical electrical parameters of the fabricated device and their comparison with relevant literature studies
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Norde approach [45] was used as an alternative method to determine the Φb and Rs values of the diode. In this method, Norde’s function is described as F(V) and given as follows,

$$\text{F}\left(\text{V}\right)=\frac{\text{V}}{\upgamma }-\frac{kT}{q}\left[\text{ln}\left(\frac{\text{I}\left(\text{V}\right)}{{\text{AA}}^{{}*}{\text{T}}^{2}}\right)\right]$$
(7)

where γ refers an integer greater than the obtained value of n. The value of Φb and Rs were computed utilizing following equations.

$${\Phi }_{\text{b}}=\text{F}\left({\text{V}}_{\text{min}}\right)+\frac{{\text{V}}_{\text{min}}}{\upgamma }-\frac{kT}{q}$$
(8)
$${\text{R}}_{\text{s}}=\frac{\text{kT}(\upgamma -\text{n})}{{\text{qI}}_{\text{min}}}$$
(9)

Here F(Vmin) is the minimum value of the F(V) curve. Vmin and Imin are the minimum voltage and current values corresponding to F(Vmin). Figure 7 indicates the F(V)–V plot. It is seen that this plot shows the minimum value corresponding to F(Vmin). The value of Фb and Rs is given in Table 2. It is found that the Φb values obtained by the TE and Norde methods are very close to each other [46,47,48,49,50]. Similarly, as shown in Table 2, a comparison of the electrical parameters from our study with those from similar studies reveals that the results are consistent [51,52,53].

Fig. 7
figure 7

F(V)–V plot

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Additionally, Ohm’s law (junction resistance, Rj, = dV/dI) was used to extract the parameters of shunt resistance (Rsh) and Rs of the diode [41, 43, 47]. Schottky diodes must have very high Rsh and very small Rs. The high Rsh leads to a decrease in leakage current. In this approach, the value of Rsh and Rs correspond to the calculated junction resistance value at the same reverse and forward bias, respectively. Figure 8 shows the plot of the junction resistance (Rj) versus voltage. The computed value of Rsh and Rs at −3.5 V and + 3.5 V were 0.39 MΩ and 3.96 kΩ, respectively.

Fig. 8
figure 8

Rj–V plot

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Interface states/traps are one of the important parameters that degrade semiconductor device performance. The interface state density (Nss) derived from the forward I–V data is given by [54],

$${\text{N}}_{\text{ss}}\left(\text{V}\right)=\frac{1}{\text{q}}\left[\frac{{\upvarepsilon }_{\text{i}}}{\updelta }\left(\text{n}\left(\text{V}\right)-1\right)-\frac{{\upvarepsilon }_{\text{s}}}{\text{W}}\right]$$
(10)

where εi and εs are the permittivity of the interlayer material and the semiconductor, respectively. Here δ is the interlayer thickness. Also W is the width of the depletion layer. For the n-type semiconductor, surface state energy (Ess) with respect to the bottom of the conduction band is defined as,

$${E}_{c}-{E}_{ss}={q} \left({\Phi}_{{e}}-{V}\right)$$
(11)

The effective barrier height, Φe, is described as,

$${\Phi }_{\text{e}}-{\Phi }_{\text{b}}=\left(1-\frac{1}{\text{n}\left(\text{V}\right)}\right)\left(\text{V}\right)$$
(12)

Figure 9 demonstrates the energy distribution profile of the interface/surface state density of the diode. It is clear that the energy increases with the decrease of the Nss value. Also, the Nss curve rises exponentially from the middle region band-gap of the n-Si to the bottom of the conduction band [50, 54,55,56,57,58,59].

Fig. 9
figure 9

Plot of Nss vs. Ec–Ess

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4 Conclusion

In this study, both the MgO structure deposition and the Au/MgO/n-Si device fabrication were performed by RF magnetron sputtering method. The observed diffraction peaks correspond to mainly (111) and (200) crystal planes of MgO, respectively. From UV–Vis spectroscopy, it has been observed that the MgO film has an average of 90% high transmittance in the visible region. The optical energy band gap value of the MgO film was found 3.98 eV. In addition, the electrical parameters like n, Φb, Rs and Nss were calculated by from I–V data. The n value was found 3.79. This value indicates that the I–V characteristics of the device differ from the ideal diode I–V behavior. Moreover, it is determined that the Φb values extracted from the TE and Norde methods are very close to each other. All analyzes show that the MgO thin film formed by RF sputtering technique can be utilized in electronic and optoelectronic device applications.