Analysis of temperature dependent current-voltage and capacitance-voltage characteristics of an Au / V 2 O 5 / nSi Schottky diode

Electronic properties of Au/V2O5/n-Si Schottky device have been investigated by temperature dependent current‒voltage (I–V) and capacitance‒voltage (C–V) measurements ranging from 300 K to 150 K. Ideality factor (n) and barrier height ( ) for the Schottky device were obtained from I–V characteristics as 2.04 and 0.83 eV at 300 K and 6.95 and 0.39 eV at 150 K respectively. It was observed that in presence of inhomogeneity at metal– semiconductor interface, the ideality factor increases and barrier height decreases with the decrease of temperature. The Richardson constant value was estimated as 137 A‒cm−2‒K−2 from modified Richardson plot, which is closer to the known theoretical value of n-Si where mean value of barrier height (φ , and its standard deviation (σ0) were estimated using double Gaussian distribution (DGD) analysis. Different device parameters, namely, built-in potential, carrier concentration, image force lowering and depletion width were also obtained from the C–V‒T measurements. First time use of V2O5 thin-film as an interfacial layer (IL) on Au/V2O5/n-Si Schottky diode was successfully explained by the thermionic emission (TE) theory. The interesting result obtained in this present work is the V2O5 thin-film reduced its conducting capability with decreasing temperature, while it shows a totally insulating behaviour below 150 K.


I. Introduction
Metal-oxide-semiconductor (MOS) based Schottky devices have great importance due to their unique electrical, optical and structural properties where the interfacial layer (IL) at metal-semiconductor (MS) contacts plays a dominant role in the device performance, stability and reliability.[1,2,3] The control of interface property has great importance in many device applications.[4,5] Attempts have so far been made to use different materials as an IL to improve the Schottky diode performance.Taşçıoğlu et.al.[6] reported that organic IL in MS structures improved the Schottky diode properties.A PVA film doped with different concentrations of nickel (Ni) and zinc (Zn) as IL between metal and semiconductor was also reported.[7] Further, Yṻksel et al. [8] used spin coated perylene-monoimide (PMI) organic semiconductor on n-Si wafer to formed a Schottky barrier diode.Of late, high work function based transition metal oxides (TMO) such as Molybdenum trioxide (MoO3) [9] and Tungsten trioxide (WO3) [10] have been used as an IL for Schottky device applications.
Vanadium pentoxide (V2O5) is another prominent transition metal oxide due to its high work function with semiconducting and light transparent properties which shows charge generation/recombination and transportation in many device applications.Therefore, it has emerged as one of the most important semiconductors for different device applications namely, organic light emitting diodes [11], photogenerated charges [12], organic field effect transistors [13], gas sensors [14], Schottky diodes [15] and especially in photovoltaics.
[ 16,17] In recent work,V2O5 thin films have been used as hole conducting buffer layers for heterojunction solar cell and interdigited back contacted solar cells that achieved 15.7% and 19.7% conversion efficiency respectively.[18,19] However, room temperature current-voltage measurements are not sufficient to fully understand the current conduction phenomenon through the V2O5 layer.Therefore, low temperature I-V and C-V measurements are necessary to get information on the charge transport mechanism through V2O5 thin-film.[20,21] In this present study, Vanadium pentoxide has been used as an IL between gold and n-type silicon to form a metal-oxide-semiconductor Schottky barrier diodes (SBDs).
Fabricated Au/V2O5/n-Si SBDs were characterized by I-V measurement under reverse and forward bias conditions and by C-V measurement, both carried out at temperatures ranging from 300 K to 150 K. Different electrical parameters namely ideality factor (n), barrier height ( ), series resistance (Rs), activation energy (Ea), Richardson constant (A * ), built-in potential (Vbi), carrier concentration (ND) image force lowering ( ) and depletion width (WD) are estimated.

II. Experimental
Double side polished n-type float zone silicon (100) wafers of 2.5 Ω-cm resistivity and 280 μm thickness were used as substrates.The wafers were cleaned by standard RCA cleaning and dipped in 1% HF acid for one minute for native oxide removal.After cleaning, the wafers were loaded into the thermal evaporation chamber.V2O5 powder, procured from Sigma Aldrich with a purity of 99.9% was used for the deposition process.When a base pressure of 10

i. Current-voltage characteristics
The thermionic I-V characteristics of Au/V2O5/n-Si Schottky diodes at forward and reverse bias can be expressed by [22]: where is the saturation current derived from the straight line fitting at a zero bias and is expressed by: * exp ϕ 2 where A, , q, V, k and T is the diode area, barrier height (BH), electron charge, forwardbias voltage, Boltzmann constant and operation temperature in Kelvin scale respectively.A * is the Richardson constant which is expressed as * * and its estimated is 120 A-cm −2 -K −2 for n-type Silicon.[23] The ideality factor of a Schottky barrier diode is described the deviation of experimental I-V data from the TE theory.Using the definition, ideality factor can be expressed as: The ideality factor is one of the most important parameters which indicate the formation of Schottky barrier uniformity.For an ideal diode n equals to 1 but in practice it is greater than 1 for real Schottky diode.However, here the value of n is greater than 1 due to the presence of interface thin layer of V2O5 film and the barrier inhomogeneity at MS interface.On the other side, barrier height can be obtained from Eq. ( 2) as: Figure 2 shows the reverse and forward bias I-V characteristics of Au/V2O5/n-Si SBDs at different temperature ranging from 300 K to 150 K. From the intercept and slope of lnI vs. V plot give the experimental value of and n at each temperature respectively.The value of ϕ and n for Au/V2O5/n-Si SBDs varies from 0.39 eV and 6.95 (at 150 K) to 0.83 eV and 2.04 (at 300 K).Therefore the values of n and depend on the temperature, where decreases and n increases with the decrease of temperature as shown in figure 3.
Other corresponding ideality factors and barrier heights at different temperature are calculated and listed in Table 1.A decrease in evaluated experimental barrier height and an increase in ideality factor with the decrease of temperature may be associated with the inhomogeneity of barrier height in SBDs.Moreover, the presence of V2O5 thin film between metal and semiconductor can affect the inhomogeneity of Schottky barrier diodes.
Subsequently, this IL layer can also affect the current conduction through the MS junction.
The current transport mechanism across the metal-semiconductor junction is a temperature dependent process wherein electrons are able to transport through the low barrier at the lower temperature and high barrier at the higher temperature.So when temperature is deceased the band gap (Eg) of V2O5 thin film is increased.[24] As a consequence the charge carriers will not have enough energy to overcome the BH at low temperature.Therefore, current conduction through the V2O5 layer is reduced at lower temperature range and at below 150 K the charge transportation is become zero.The current-voltage characteristic of Au/V2O5/n-Si SBDs at 145 K is also shown in the inset of figure 2. However, many research groups [6,7,8] have reached up to 80 K using IL of different organic or polymer materials.
Again, in order to determine Schottky parameters such as series resistance (Rs), ideality factor and barrier height from the temperature dependent forward bias I-V characteristics we have employed the model of Cheung and Cheung [25].According to this method current-voltage characteristics due to TE theory of SBDs can be expressed as: - * ϕ 6 Figure 5 shows a plot of H(I) vs.I at temperatures range from 300 K to 150 K. Again this plot also follows a straight line with intercept equal to nϕ and slope of this plot also provides a second estimation of Rs that can be used to check the consistency of Cheung's approach.The values of Rs and , obtained from the H(I) vs.I plot, are 6.61Ω and 1.10 eV at 300 K and 13.56 Ω and 0.55 eV at 150 K respectively.The Rs value evaluated from the plot of dV/d(lnI) vs.I is almost identical to those from the plot of H(I) vs.I implying the consistency and validity of these models (shown in figure 6).The current transport phenomenon is also dominated by the current flowing through the defect states present in V2O5 layer [26].It should be noted that the presence of defect state in oxide layer is reduced with decrease of temperature.As a consequence the current conduction through this defect state is decreased with the decrease of temperature.As a result the series resistance value is increased with decrease of temperature.Therefore, the voltage drop across the series resistance is decreased due to the presence of the V2O5 interface layer.So the main benefits of putting theV2O5thin-film as an interface layer on Au/n-Si SBDs is to get low series resistance.This low value of series resistance can help for achieving maximum fill factor (FF) of a solar cell.Our calculated Rs value is quite low as compare to other IL used in Au/n-Si SBDs.[8,27] Table In order to evaluate the activation energy, Richardson constant and homogeneous barrier height, the Eq. ( 2) can be rewritten as [28]: * ϕ 7 A conventional activation energy or Richardson plot is shown in figure 7, where the variation of ln(I0/T 2 ) vs. 1000/T is found to be linear in the temperature range from 300 K to 150 K.
From the linear fit of Richardson plot, activation energy (Ea) and Richardson constant (A * ) value are calculated.The value of Ea is 0.041 eV obtained from the slope of Richardson plot which is smaller as compared to the actual band gap of silicon.The Richardson constant is also measured from the intercept and found to be A * =8.69×10 −11 A-cm −2 -K −2 .The calculated value is not close to the known theoretical value of n-type Si.The above abnormal result reveals that the potential fluctuation in Richardson constant may be due to the presence of V2O5 and the inhomogeneity at the MS interface.Consequently, current conduction mechanism through the V2O5thin-films in Au/V2O5/n-Si SBDs may be explained by classical TE theory.Some authors [29, 30,31] have treated as a discrete regions or "patches" of lower barrier height to explain the observed barrier inhomogeneity between MS junction.In this case the current transport across the Schottky diode may be affected by the presence of barrier inhomogeneity.Figure 8 shows a variation of zero-bias barrier height vs.
ideality factor (n) for Au/V2O5/n-Si SBDs.It is observed that zero-bias BH increases linearly with the decrease of ideality factor which is mainly due to the presence of same barrier inhomogeneity at the interfaces in real SBDs.The extrapolation of vs. n plot to n = 1 has given a homogeneous barrier height is approximately 0.89 eV for Au/V2O5/n-Si Schottky diode.
Thus above abnormal behaviour of ideality factor and barrier height can be explained using an analytical potential fluctuation model based on inhomogeneous barrier height at the MS interface.Therefore, let us assume the double Gaussian distribution [32] of apparent BH ( ap) and apparent ideality factor (nap) with a mean value of BH ( and standard deviations ( ) can be expressed by following relations [33]: It is further assumed that the mean value of barrier height and standard deviations are linearly bias dependent on Gaussian parameters such as = + ρ2V and standard deviation as σ0 = σ0s + ρ3V where ρ2and ρ3 are voltage coefficients which is depends on temperature.We have attempted to draw a vs. 1000/T plot (figure 9) to get an evidence of the DGD of BH and evaluated the values of mean barrier height and standard deviation at zero biasare1.22eV and 0.15 respectively.Furthermore the plot of ((1/nap) -1) vs. 1000/T is shown in figure 9 and consequently ρ2 and ρ3, obtained from the intercept and slope of this plot, are estimated as 0.024 and -0.099respectively.Now, the conventional Richardson plot can be modified by combining with Eq. ( 2) and Eq. ( 8) as follows: Thus, the modified Richardson plot according to Eq. ( 10

ii. Capacitance-voltage characteristics
The temperature-dependent capacitance-voltage (C-V) measurement is an another important method that can provide information about the built-in-voltage, Fermi energy, carrier density, image force lowering and barrier height of SBDs.In metal-semiconductor junctions, the depletion region formed at the interface behaves like a bias-dependent capacitor, whereas junction capacitance is a reverse-bias phenomenon and the diffusion capacitance is seen under forward biased condition.The capacitance-voltage response of Au/V2O5/n-Si Schottky device was measured at the 1MHz frequency.The capacitance of the Schottky diode can be expressed as [34]: where ND, Vbi, V, εs and ε0is donor concentration, built-in-voltage at zero bias, reveres bias voltage, dielectric constant and vacuum permittivity (ε0 =8.85×10 -12 F/m) respectively.[35] The temperature-dependent C -2 vs. V characteristics are presented in figure 11.The linear behavior of the C -2 vs. V curves can be entirely explained on the basis of SBDs device.In the first step, we evaluated built-in voltage (Vbi) and donor concentration (ND) from the C -2 vs. V plot.
Using Vbi and ND, the barrier height ( ), Fermi energy (EF), image force lowering ( ) and effective carrier density (NC) can be calculated from the following relation: [36,37] 12 where V0 is the intercept of C -2 with voltage axis and is related to Vbi.EF is the Fermi energy is given by: 14 where NC is effective carrier density of states, * = 0.98m0.[38]  * is the effective mass of electron and m0is the rest mass of electron.Also is image force lowering which can express as [30]: The values of C-V, EF, NC and are calculated and shown in Table 2.The calculated value of C-V is a function of temperature and it decreases with increase of temperature.The obtained barrier height from the C-V measurement is higher than I-V measurement.This discrepancy between -and -(shown in figure 12) due to the existence of excess capacitance and barrier inhomogeneity at the MS junction can be explained by the Gaussian distribution of barrier height with mean value of BH and standard deviation.Thus, this existence of barrier inhomogeneity or excess capacitance is responsible for V2O5thin-filmuse as an interface layer between gold and Silicon.Also, it affects the temperature dependent I-V and C-V measurements as well.The depletion width (WD) is also obtained from the following expression: [39] 2 17 The value of depletion width is decreased with an increase of temperature (shown in

IV. Conclusion
The reversed and forward bias I-V-T and C-V-T characteristics of Au/V2O5/n-Si SBDs were measured in the temperature range from 300 K to 150 K. Experimental results show the interfacial Vanadium pentoxide layer at metal-semiconductor interface plays an important role to determinate the electrical parameters of Schottky device.Also, current conduction through the V2O5 thin film is totally stopped at a temperature less than 150 K.
Evaluating the experimental I-V-T results reveal a decrease in and an increase in n with

5 Figure 4
Figure 4 shows the dV/d(lnI) vs.I curves at different temperatures.This plot follows a linear behavior where its slope gives the series resistance and intercept gives the ideality factor.The value of Rs and n obtained from the figure 4 varies from 7.45 Ω and 2.30 at 300 K to 12.43 Ω and 7.70 at 150 K respectively.Using the estimated ideality factor, we obtained the barrier height and series resistance by another equation of Cheung and Cheung, defined as: ) should give a straight line.The slope directly yields mean BH and intercept (=lnAA * ) at the ordinate determining A * for a given diode area A. In figure 10 shows the modified Richardson plot of ln(I0/T 2 ) ˗ (qσ0) 2 /2(kT) 2 vs. 1000/T gives (T=0) and A * as 1.19 eV and 137A-cm −2 -K −2 for Au/V2O5/n-Si SBHs.It is clear that the value of =1.19 eV obtained from the modified Richardson plot is approximately same as the value of =1.22 eV obtained from the ap vs. 1000/T plot.Hence experimental electronic parameters of Au/V2O5/n-Si SBDs have been successfully explained on the basis of the TE theory with a DGD of BH.

Where 4 .
82 10 * 15 the decrease of temperature due to inhomogeneities in presence of V2O5 interface layer.The value of Rs also determined from Cheung's function is also depends on temperature.The conventional Richardson plot ln(I0/T 2 ) vs. 1000/T gives the activation energy of 0.041 eV and Richardson constant A * of 8.69×10 −11 A-cm −2 -K −2 which is much smaller than the known theoretical value of a n-type Si but the modified Richardson plot gives a closer value of Richardson constant of 137 A-cm −2 -K −2 , well as a correct value for the barrier height of 1.1 eV.The effect of IL in electrical parameters such as built-in-voltage, carrier concentration, image force lowering, depletion width, Fermi energy and barrier height were also obtained from temperature dependent capacitance-voltage characteristics.In summary it can be conclude that the use of a V2O5 thin-film as an IL in Au/n-Si Schottky junction exhibits a good rectifying behaviour and the current transport across interface layer is exponentially dependent on barrier height and defect states.Therefore, V2O5 thin-film is a compatible material for Metal-Oxide-Semiconductor and solar cell applications.

Figure 6 :
Figure 6: Variation of series resistance with temperature determined from the Cheung and Cheung equation.

Figure 7 :
Figure 7: Richardson plots of the ln(I0/T 2 ) vs. 1000/T for Au/V2O5/n-Si SBDs at the temperature range from 300 K to 150 K.

Figure 8 :
Figure 8: Zero-bias barrier height vs. the ideality factor of Au/V2O5/n-Si Schottky diode at different temperatures.

Figure 12 :
Figure 12: Variation of barrier height with temperature calculated from current-voltage and capacitance-voltage measurement.

Table 2 )
due to existence of excess capacitance at the MS junction.Above experimental values of Vbi, ND and WD are reported here for expected behavior of Au/V2O5/n-Si Schottky junctions.

Table 2
Temperature-dependent electrical parameters determined from C-V measurements of the Au/V2O5/n-Si Schottky barrier diode.