Investigation of temperature dependent barrier height of Au/ZnO/Si schottky diodes

In this study, temperature dependent current-voltage (I-V) measurements have been performed to investigate the inhomogeneity in the temperature dependent barrier heights of Au/ZnO/Si Schottky barrier diode in the temperature range 150 – 400K. The room temperature values for ideality factor and barrier height were found to be 2.9 and 0.60 eV respectively indicating the inhomogenity in the barrier heights of grown samples. The Richardson plot and ideality factor verses barrier height graph were also drawn to verified the discontinuity between Au and ZnO. This barrier height inhomogenity was explained by applying Gaussian distribution model. The extrapolation of the linear Φap (n) plot to n= 1 has given a homogeneous barrier height of approximately 1.1 eV. Φap versus 1/T plot was drawn to obtain the values of mean barrier height for Au/ZnO/Si Schottky diode (1.1 eV) and standard deviation(δs) (0.02 V) at zero bais.


Introduction
ZnO is a material of interest among the research community due to its wide band gap of 3.37 eV and large exciton binding energy (60 meV) at room temperature [1,2]. In order to utilized ZnO in practicle appications, metal contacts are neccessory. Therefore the study of characteristics parameters such as barrier height and ideality factor of metal-semiconductor Schottky diodes has fudamental importance. The performance and relaibility of these devices especially depend on the formation of insulator layer between metal and semiconductor interface, inhomogenities of Schottky barrier contacts and series resistance of diode [3]. Generally Schottky contacts show temperature dependent behavior as evidenced by the parameters such as ideality factor, barrier height and series resistance [4]. It is observed in the literature that I-V characteristics normally deviate from the ideal thermionic model because both, ideality factor and barrier height showed strong temperature dependence behavior [5][6][7]. This abnormal behavior of Schottky diodes was attributed to series resistance and inhomogenites between metal and semiconductor [8,9]. Different models have been employed to understand the abnormal behavior but the subject is still under hot debate. Therefore the study of Schottky contacts of different metals on ZnO is an active area of research and needs to be further investigated.
In this study temperature dependence I-V measurements have been performed on Au/ZnO/Si Schottky diodes in the temperature range of 150 to 400 K to investigate the discontuniuty present between Au and ZnO. The ideality factor, series resistance and barrier height show strong dependence of temperature. There was also correlation between ideality factor and barrier height which showed the inhomogenity in the barrier height. The inhomogenity in barrier height was explained using Gaussian distribution model of barrier heights.

Experimental
ZnO layers were grown on 3 inch diameter p-type silicon (001) wafer (NA ~ 1.2 x 10 15 cm -3 ) by means of Molecular Beam Epitaxy (MBE). The ZnO was grown under following conditions: the substrate temperature was set at 703 K while oxygen was supplied from an RF atomic source and Zn was evaporated from -Zn-Knudsen cell maintained at 562 K filled with Zn beads having a purity of 99.9999%. The pressure of the chamber during the growth was ~ 1× 10 -4 mbar mainly due to the pressure required to operate the oxygen RF atomic source and growth time was 7 hours. The oxygen RF atomic source was operated at 300W. For electrical measurements, gold Schottky contacts with diameter 78 mm 2 were fabricated by electron beam evaporation. I-V measurements of Schottky diodes were carried out by the following equipment (manufacturer): DLTS (DLS-83D Hungry) Fig. 1 shows the semi-logarithemic forwar biased I-V characteristic of the Au/ZnO/Si Schottky barrier diodes in the temperature range 150 to 400 K. Current in the Schottky barrier diode due to the thermionic emission, neglecting series resistance is given as [10],

Results and Discussion
Where n is the ideality factor and S Ι is the reverse saturation current given by the relation, Where A is the contact area, A * is the Richardson constant and its value is 32 AK -2 cm -2 for ZnO [11].
T is temperature in Kelvin, k is the Boltzmanns constant (k=1.38× 10 -23 J/K or 8.617 × 10 -5 eV/K), q is the electric charge and Β φ is the barrier height The ideality factor and barrier height shows a strong dependence of temperature as shown in fig.2. The ideality factor decreases and barrier height increases with temperature. Room temperature n and ɸB values are found to be 2.9 and 0.60 eV respectively. The value of ideality factor attributed to interface states between gold and ZnO, image force lowering and barrier height inhomogenity [12]. Such results have been already published by many researchers [13]. Such temperature dependence is an obvious disagreement with the reported negative temperature coefficient of the Schottky barrier height. The reason of this disagreement may be the barrier height inhomogenity. The barrier height inhomogenity can be also determined by plotting the graph between ideality factor and barrier height. Fig. 3 shows a plot of experimental BH and ideality factor. Fig. 3  factor plot of n=1 has given homogenious barrier height of 1.1 eV. Therefore decreases of ideality factor and increases of barrier height at high temperature clearly demonstrated that there is discontinuity at Au/ZnO interface. This barrier height inhomogeneity can be explained by using Gaussian distribution model of barrier height. According to this model the expression of barrier height can be written as [14][15][16] Фap=Фbo-qδs 2 /2kT where Фap is the apparent BH which can be measured experimentally, Фbo is the mean BH and δs is the standard deviation of the BH distribution. The standard deviation is the measure of barrier height inhomogenity.
The temperature dependence of δs is usually small and can be neglected. In this model the observed variation of ideality factor with temperature is given by [17] (1/nap -1) = -ρ1 + q ρ2/2kT where nap is the apparent ideality factor and ρ1 and ρ2 are voltage coefficients which may depend on temperature and they quantify the voltage deformation of the BH distribution Again, the plot of nap versus 1/2kT is a straight line that gives voltage coefficients ρ1 and ρ2 from the intercept and slope, respectively (shown in fig. 5). The values of ρ1 = 0.55 and ρ2 = -0.006 V were obtained from the experimental nap versus 1/2kT plot (Fig. 5). As the inhomogenity of the interface is depend upon the value of δs. The lower value of δs corresponds to more homogeneous barrier heights.

Conclusion
In this study, temperature dependent current-voltage (I-V) measurements have been performed to investigate the inhomogeneity in the temperature dependent barrier heights of Au/ZnO/Si Schottky barrier diode in the range 150-400K. The room temperature values for ideality factor and barrier height found to be 2.9 and 0.60 eV respectively indicating the inhomogenity in the barrier heights of grown samples. This barrier height inhomogenity was explained by applying Gaussian distribution model.