Electrical Properties of the Ag/n-ZnO/p-Si Structures Using Current-Voltage Measurements

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Optoelectronics, Applied Optics and Microelectronics

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Department of Advanced Technologies, University of Mohaghegh Ardabili

Namin, Ardabil, Iran Aug. 17-19, 2019

Electrical Properties of the Ag/n-ZnO/p-Si Structures Using Current-Voltage Measurements

Şükrü KARATAŞ

Physics Department / Faculty of Sciences and Arts, Kahramanmaraş¸ Sütçü Imam University, Kahramanmaraş, Turkey 46100, [email protected]

Abstract- In this work, nanostructure ZnO films were prepared by sol–gel method using spin coating technique. We investigated electrical properties of Ag/n-ZnO/p-Si Structures in dark and under 20, 60 and 100mW/cm2 white light (visible light) illuminations using current–voltage (I–V) measurements. The ZnO/p-Si diode exhibits a non-ideal behavior due to the interfacial layer and the series resistance. The values of main parameters such as ideality factors, barrier heights and series resistances obtained from different methods decreased with increasing illumination.

It is seen that the values of main parameters obtained from different methods are in agreement with each other.

Keywords: Electrical properties, ZnO, I-V measurements, Sol-gel method, İdeality factor, Barrier height

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1. Introduction

The starting point of semiconductor industry was the invention of the first semiconductor transistor at Bell Lab in 1947. Zinc oxide (ZnO) is one of the most important multifunctional semiconductor oxides because of its physical properties, such as resistivity control over the range 10⁻³–10⁵ Ωcm, high transparency in the visible range.

ZnO is a typical II-VI semiconductor material with a wide bandgap of 3.37 eV at room temperature, and a large exciton binding energy of 60 meV [1-7]. Thus, during the last decade, ZnO epilayer and various ZnO nanostructures have been grown by various techniques. The electrical properties of metal–oxide-semiconductor interfaces have been widely studied, both for their basic physical properties and for their technological applications to electronic devices.

In this work, nanostructure ZnO thin films were prepared to cover metal-semiconductor.

That is, we investigated electrical properties of ZnO/p-Si structure under white light (visible light) illuminations using current–voltage (I–V) measurements. The light intensity dependence of the main parameters such as ideality factor (n), zero-bias barrier height ( bo) and series resistance (RS) obtained from current–voltage (I–V) measurements. The experimental results showed that the values of main electrical parameters were found to be strongly light intensity.

2. Experimental procedure

The zinc oxide thin film was prepared using cadmium acetate dihydrate (Cd (CH3COO)2·2H2O), methoxy ethanol and monoethanolamine. For this, 0.5 M of the zinc acetate dehydrate (Zn (CH3COO)2·2H2O) was firstly dissolved in 2-metoxyethanol for 2 h at room temperature and then, the monoethanolamine was added to this solution. The molar ratio of monoethanolamine to zinc acetate dehydrate was taken as 1.0. The prepared mixtures were stirred using a magnetic stirrer at 60 ◦C for about 30 min to obtain clear homogeneous solution and then sol was kept for aging for 18 h prior to film deposition. After ageing of the gel solution, any precipitation in gel solution was not observed. p-type single crystal silicon with a thickness of 600 m, and a resistivity of 5–10 cm was used for the diode fabrication. The ohmic contact was formed by evaporating Al metal in pressure 10^-7 Torr on the back of Si wafer. After cleaning process, ZnO films were deposited on p-type silicon by sol–gel spin coating method. The Ag contact was formed by evaporating Ag metal onto ZnO films using PVD-HANDY/2S-TE (Vaksis Company) vacuum thermal evaporation at a pressure of 4.5×10⁻⁵ Torr. In order to carry out electrical measurement, the contacts were formed by evaporation of circular dots with diameter of about 2.0 mm and 100 nm thicknesses (contact area of diode 3.14×10⁻² cm²). The electrical characterizations of the diodes were performed using KEITHLEY 2400 Semiconductor characterization system

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and a FYtronix Solar IV characterization system (FYtronix Solar system has been shoved in Figure 1).

Figure 1. Fytronix electrical characterization system 3. Results and discussion

Fig. 2 shows the current–voltage characteristics (I–V) of the ZnO/p-Si structure under different illumination conditions at room temperature. As can be shown in the Fig. 2, the current curve increases exponentially with applied forward bias voltage at any illumination conditions, and the I–V characteristics of device show the nonlinear behaviour. According to thermionic emission (TE) theory, the relation between the voltage

(V) and current (I) can be written as [7-10]; 



 



 



 

 

 



 



 

kT qV nkT

I qV

I _oexp 1 exp

(1) where V is the applied voltage drop across the junction, q is the electronic charge, k is Boltzmann’s constant, T is the absolute temperature, and I0 is the reverse saturation current given by;

exp ^bo

2

* 



 



 



 kT

T q AA

I_o (2)

where A is the diode area (3.14x10^-2 cm²), A* is the effective Richardson constant (32 Acm⁻² K⁻² for p-Si) Si [7], bo is the barrier height, and n is the diode ideality factor which is a measure of conformity of the diode to pure thermionic emission. Using Eq. (1) and Eq. (2), the ideality factor and barrier height (SBH) can show as below, respectively;

) (lnI d

dV kT

n  q (3) ln

2

*



 



 



o

bo I

T AA q

kT ₍₄₎

In Fig.2 and using Eq. (3) and Eq. (4), from the intercept and slope of lnI-V curves give the experimental value of n and bo at room temperature respectively. As can be seen

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from Fig. 2, the obtained ideality factor values for Ag/n-ZnO/p-Si structure decreased with the increasing illumination intensity.

Figure 2. The I-V curves of the Ag/ZnO/p-Si structure at various illumination intensity.

From Fig.2, the values of the ideality factor (n) and the barrier height (bo) obtained from I-V measurements were found as 7.78; 7.51; 7.12; 6.94 and 0.765; 0.757; 0.749;

0.741 eV in dark and under 20, 60 and 100mW/cm² white light illuminations, respectively.

The experimental shoved that the barrier heights and ideality factors have been decreased to be dependent on illumination intensity. Also, these values are given in Table 1.

Table 1. The ideality factors and barrier heights at various illuminations Illumination

intensity (mW/cm²)

Dark 20 60 100

İdeality Factor

(n) 7.78 7.51 7.12 6.9 4 5.E-09

5.E-08 5.E-07 5.E-06 5.E-05 5.E-04

-2.3 -1.8 -1.3 -0.8 -0.3 0.2 0.7 1.2 1.7 2.2

C urr ent ( A )

Voltage (V)

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Namin, Ardabil, Iran Aug. 17-19, 2019 Barrier Height

(eV) 0.765 0.75 7

0.74 9

0.7 41

Furthermore, the values of series resistance have obtained using Cheung method [12].

Fig. 3 shows series resistance-current (RS-I) curves of Ag/n-ZnO/p-Si structure at various illuminations obtained from the Cheung method. The method modified by Cheung method [12] can be written as follows:



 



 



 



 

 ln _* ₂

q ) kT

( AAT

n I V I

H (5)

and

bo

S

n

IR I

H ( )   

(6)

Where RS is the series resistance, b is the barrier height [7,11,12] obtained from intercept of H(I)-I plots and q is the elementary charge. The effect of series resistance (RS) on the forward bias I-V dates is seen Fig. 3.

Figure 3. The H(I)-V curves of the Ag/ZnO/p-Si structure at various illumination intensity.

From Fig.3 and using Eqs.5 and 6, the values of the series resistance (RS) and the barrier height (bo) obtained from H(I)-I curves were found as 30349; 5877; 5734; 5145 and 0.621; 0.487; 0.431; 0.406 eV in dark and under 20, 60 and 100mW/cm² white light

2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

0.E+00 2.E-05 4.E-05 6.E-05

H(I) (V)

Current (A)

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illuminations, respectively. As can be seen Table.2 and in Fig.3, the obtained results indicate that the values of series resistance are decreased with the increasing illumination level due to the charges being generated from the valance band to conductance band and this causes the downward curvature in the forward bias I–V plots under light illuminations. The decrease in barrier heights and series resistance values with illumination density is associated with an improvement of the space charge layer due to the transfer of photogenerated electrons and holes.

Table 2. The series resistances and barrier heights at various illuminations Illumination

intensity (mW/cm²)

Dark 20 60 100

Series

resistance ( ) 2034 9

587 7

573

4 5145 Barrier Height

(eV)

0.62 1

0.4 87

0.4

31 0.406

4. Conclusion

In this study, we investigated electrical properties of Ag/ZnO/p-Si heterostructure in dark and under 20, 60 and 100mW/cm² white light illuminations using current–voltage (I–

V) measurements. The main parameters such as ideality factors, barrier heights and series resistance were obtained by different methods. The results show that the main electrical parameters such as the BH, n, and RS values were found to depend on the illumination intensity. The decrease of the barrier heights with illumination density is associated with an improvement of the space charge layer due to the transfer of photo-generated electrons and holes.

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Electrical Properties of the Ag/n-ZnO/p-Si Structures Using Current-Voltage Measurements

Electrical Properties of the Ag/n-ZnO/p-Si Structures Using Current-Voltage Measurements

C urr ent ( A )

Voltage (V)

n

IR I

H ( )   

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characteristics,” Appl. Phys. Lett. Vol. 49, pp. 85-87, 1986.