Temperature dependence of characteristic parameters of the Au/SnO2/n-Si (MIS) Schottky diodes

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Abstract

The variation in electrical characteristics of Au/SnO2/n-Si (MIS) Schottky diodes have been systematically investigated as a function of temperature by using forward bias current–voltage (IV) measurements. The main diode parameters, ideality factor n and zero-bias barrier height ΦB0, were found strongly temperature dependent and while the zero-bias barrier height ΦB0(IV) increases, the n decreases with increasing temperature. This behavior has been interpreted by the assumption of a Gaussian distribution of barrier heights due to barrier inhomogenities that prevail at the metal–semiconductor interface. The zero-bias barrier height ΦB0 vs q/(2kT) plot has been drawn to obtain evidence of a Gaussian distribution of the barrier heights, and values of Φ¯B0=1.101eV and σ0 = 0.158 V for the mean barrier height and zero-bias standard deviation have been obtained from this plot, respectively. Thus a modified ln(I0/T2)  (q2σ02)/2k2T2 vs q/(kT) plot has given mean barrier height Φ¯B0 and Richardson constant (A*) as 1.116 eV and 127.86 A cm−2 K−2, respectively. The A* value 127.86 A cm−2 K−2 obtained from this plot is in very close agreement with the theoretical value of 120 A cm−2 K−2for n-type Si. Hence, it has been concluded that the temperature dependence of the forward bias IV characteristics of the Au/SnO2/n-Si (MIS) Schottky diode can be successfully explained on the basis of a thermionic emission (TE) mechanism with a Gaussian distribution of the Schottky barrier heights (SBHs). In addition, we have reported a modification by the inclusion of both n and αχ0.5δ in the expression of I0 to explain the positive temperature dependence of ΦB0 against that of energy band-gap of Si. Thus, the values of temperature coefficient of the effective barrier height ΦBef(−3.64 × 10−4 eV/K) is very close agreement with the temperature coefficient of Si band-gap (−4.73 × 10−4 eV/K).

Introduction

The performance and reliability of metal–insulator–semiconductor (MIS) Shottky diodes especially depend on the formation of an insulator layer, active metal/semiconductor interface, the interface states distribution at the semiconductor, insulator interface, series resistance and inhomogeneous barrier heights [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. The interface properties, carrier transport mechanisms and the same structural parameters of type SBDs have been studied both experimentally and theoretically in past decades [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. The popularity of such studies does not assure uniformity of the results or of interpretation. The electrical characteristics of a Schottky barrier diode (SBD) with an interfacial insulator layer, such as SiO2, SnO2 and Si3N4, does not obey the ideal thermionic emission (TE) theory. The insulator layer (SnO2) is a material of growing importance for a wide variety of novel and special applications [24], [25]. Especially, the formation and characterization of SnO2 insulator layer on the Si and Schottky barrier formation between metal and semiconductor interface on a fundamental basis still remains a challenging problem. Various models have been proposed to describe the behavior of the Si/SnO2 interface and carrier transport across it [1], [5], [8], [26]. The SnO2 interfacial insulator layer plays a more significant role than serving as a contact and is likely to influence the current–voltage (IV) characteristics of the Si/SnO2 interface.

Until now, the literature has contained numerous reports on the current transport mechanism of MIS diodes [1], [3], [4], [5], [6], [7], [8], [9], [15], [16], [17], [18], [26], [27], [28], [29], [30], [31]. Card and Rhoderick [3] and Strikha [27] estimated the surface state density located at the insulator–silicon interface and examined effects of the surface states on the ideality factor of the forward bias IV characteristics. Some studies [5], [6], [7], [17], [22], [27], [28], [29] inspected the effects of surface states on the behavior of Schottky diodes and extracted the density distribution of surface states in the semiconductor band gap from the forward bias IV characteristics. In generally, analysis of the IV characteristics of SBDs based on TE theory usually reveal an abnormal decrease in the barrier height and increase in the n with decrease in the temperature [7], [18], [23], [30], [31], [32]. The decrease in the BH at low temperatures leads to a non-linearity in the activation energy ln(I0/T2) vs 1/T plot. Yu and Snow [33] observed that n for Schottky diodes depends on the forward bias voltage. Levine [34] also suggested that both ΦB and n should depend on the bias voltage. Hackam and Harrop [18] proposed that the ideality factor n should be included in the expression for the reverse saturation current I0. The analysis of the current–voltage (IV) characteristics of the Shottky barrier diodes (SBDs) only at room temperature or narrow range of temperature, do not give detailed information about their current-transport mechanisms or the nature of barrier formation at the metal–semiconductor (MS) interface. On the other hand, the temperature dependence of forward bias IV measurements allows us to understand different aspects of current-transport mechanism. For this purpose the current–voltage (IV) characteristics of Au/SnO2/n-Si (MIS) Schottky diodes have been systematically investigated at wide temperature range (200–350 K) by using forward bias current–voltage (IV) measurements.

In this study, for the first time, we report the forward-bias current–voltage characteristics and barrier parameters in Au/SnO2/n-Si diodes in the temperature range 200–350 K. The temperature dependence of barrier height (SBHs) characteristics of Au/SnO2/n-Si (MIS) diodes were interpreted on the basis of the existence of Gaussian distribution of the BHs around a mean value due to barrier height inhomogeneities prevailing at the metal–semiconductor interface. Also, we have reported a modification which is includes the ideality factor n and tunneling parameter αχ0.5δ in the expression of reverse saturation current I0.

Section snippets

Experimental procedure

The Au/SnO2/n-Si (MIS) diodes used in this work were fabricated using n-type (P-doped) single crystal silicon wafer with 〈1 1 1〉 surface orientation, 280 mm thick, 2″ diameter and 4 Ω cm resistivity. The Si wafer was degreased for 5 min in boiling trichloroethylene, acetone and ethanol consecutively and then etched in a sequence of H2SO4 and H2O2, 20% HF, a solution of 6HNO3:1HF:35H2O, 20%HF. Preceding each cleaning step, the wafer was rinsed thoroughly in deionised water of resistivity of 16  cm.

Temperature dependence of the forward bias IV characteristics

The current through a Schottky barrier diode (SBD) with the series resistance (Rs) at a forward bias, based on the thermionic emission (TE) theory, is given by the relation [1], [2]I=I0expq(V-IRs)nkT1-exp-q(V-IRs)kTwhere V is the applied voltage, the term IRs is the voltage drop across the Rs of diode, n is an ideality factor, T is the absolute temperature, k is the Boltzmann constant and q is the electronic charge and I0 is the reverse saturation current and expressed asI0=AAT2exp-qΦB0kTwhere

Conclusion

The current conduction mechanism across Au/SnO2/n-Si (MIS) Schottky diode have been investigated using forward bias IV measured in the temperature range of 200–350 K. It is found that while the zero-bias barrier height ΦB0(IV) increases, the ideality factor n decreases with increasing temperature. Therefore, for these Schottky diodes, the usual activation energy plot of ln(I0/T2) vs 1/T in accordance with the thermionic emission (TE) do not give a straight line due to the temperature

Acknowledgement

This work is partly supported by Turkish of Prime Ministry State Planning Organization Project Number 2001K120590 and Gazi University Scientific Research Project (BAP)-FEF.05/2005/53.

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